Carbon-free method and apparatus for producing manganese

Abstract

A method of producing manganese from a manganese ore by comminuting the ore into fines, converting manganiferous mineral in the comminuted ore into trivalent mangese oxide (Mn2O3) via a reductant-free

Description

The present invention concerns a method and apparatus for producing manganese from a manganese ore. Background of the Invention Manganese is the fourth most widely used metal after iron, aluminium and copper. Total annual global production of manganese ore usually ranges from about 50 to 60 Mt, with a peak in 2014 of 64 Mt, resulting in annual global production of about 20 Mt of manganese metal after its extraction. Manganese ores are found in two main contexts: primary ores comprising rhodochrosite ( / .e., manganese carbonate, MnCOa), and secondary ores created by weathering of these primary ores over geological time, in which the manganese is present as at least one of the mineral and mineraloid forms of manganese oxide and manganese oxyhydroxide. These oxides and oxyhydroxides include pyrolusite and ramsdellite ( / .e., polymorphs of manganese (IV) oxide, also known as manganese dioxide, MnOj), bixbyite ( / .e., manganese (III) oxide, MmCh), hausmannite ( / .e., manganese (11,111) oxide, Mn3O4), and manganite and groutite ( / .e., polymorphs of manganese (III) oxyhydroxide, MnO(OH)). Manganese (II) oxide (MnO) is rarely found in geological deposits. Commonly occurring manganese oxides and oxyhydroxides also include minerals with more complex crystal structures. The most prevalent of these are romanechite, hollandite and cryptomelane. Romanechite has edge-sharing MnOg octahedra forming 2x3 tunnels, which contain an ordered arrangement of barium cations and water molecules in a ratio of 1:2, with the overall formula (Ba,H20)2(Mn4+,Mn3+)5Oio. Hollandite is an anhydrous analogue of romanechite, which instead has edge-sharing MnOg octahedra forming 2x2 tunnels containing barium cations, with the overall formula Ba(Mn4+gMn3+2)Oig. Cryptomelane is isostructural to hollandite, but with tunnels containing potassium cations instead of barium cations. Both primary and secondary manganese ores may further comprise one or more gangue species, such as aluminosilicate minerals, which may include one or more clays and / or species of alumina and silica like quartz, for example. The most common gangue species in the primary ores are other carbonate minerals, particularly kutnohorite ( / .e., calcium-manganese carbonate, CaMnfCOsh) and the iron, calcium and magnesium carbonates. Because manganese is both more mobile and more soluble than iron, it is less easily precipitated. Manganese ores are therefore usually well separated in geological deposits from iron ores. However, some manganese oxides such as bixbyite and hausmannite may still comprise a minority of iron cations substituted for manganese cations. Secondary manganese ores may also comprise a small but significant admixture of iron present as at least one of the mineral and mineraloid forms of iron oxide and iron oxyhydroxide, including hematite ( / .e., FejOa), magnetite ( / .e., Fe3O4), goethite ( / .e., FeO(OH)) and limonite ( / .e., FeO(OH) • n(H2O)). Secondary manganese ores often also comprise siliceous manganese minerals, the most significant of which are braunite, which is a siliceous mineral comprising manganese (III) oxide with the overall chemical formula 3 MnjOa • MnSiOa, and rhodonite, which has the overall formula MnSiOs with substitutions of calcium cations for a minority of the manganese cations. Most manganese (about 90%) is used as an alloying element with iron in the production of austenitic manganese and / or stainless steels, although some manganese finds other uses, including as an alloying element with aluminium. Several different techniques exist in the prior art for extracting manganese from its ores. If the manganese is to be used as an alloying element with iron, the traditional extraction process comprises carbothermic reduction of manganese ore together with iron ore in a blast or electric arc furnace using coke or a similar source of carbon as a reducing agent. Initially, the carbon is partially oxidized into carbon monoxide, which reduces both iron oxides in the iron ore to elemental iron and higher manganese oxides in the manganese ore to manganese (II) oxide according to the sequence: MnOz MmOs MnsO4 -MnO [Eqn. 1] The actual chemical pathways by which these reductions occur are complex due to the formation of different molten and slag phases, the compositions of which depend on the relative proportions of manganese, iron, carbon and other constituents present, including gangue species in the manganese and iron ores and the use of fluxes such as lime, as does the temperature, which typically lies within a range of from about 1100 to about 1400 ’Celsius. For thermodynamic reasons, the final reduction of MnO to elemental manganese can only take place at higher temperatures in the presence of elemental carbon ( / .e., graphite). This results in an alloy called high-carbon ferromanganese, which has a typical composition of 76-80 % Mn, 12-15 % Fe, up to 7.5 % C and up to 1.2 % Si derived from gangue species in the ores. Complete oxidation of the carbon monoxide as the manganese and iron oxides are reduced results in the production of carbon dioxide gas. Manganese oxides may also be reduced to elemental manganese using silicon as a reducing agent to produce a low-carbon alloy with a high silicon content called silicomanganese, which has a typical composition of 65-68% Mn, 16-21 % Si and 1.5-2 % C. This silicothermic reduction is carried out in an electric submerged arc furnace charged with manganese-containing slag left over from the production of high-carbon ferromanganese and / or with manganese ore itself, which are mixed together with coke and quartz (i.e., SiOj) as a source of silicon. The contents of the furnace are fused by ohmic heating at a higher temperature (typically, about 1600 to 1650 ’Celsius) than for the carbothermic reduction of manganese ore, which therefore also consumes more energy. Carbon, present both in its elemental form (i.e., as graphite) and through the formation of silicon carbide, reduces the manganese (II) oxide to elemental manganese and the quartz to silicon according to the two equations: MnO (s) + C (sj Mn ( / ) + CO (g) [Eqn. 2a] SiOz (S) + 2 C (S) -> Si ( / ) + 2 CO (g) [Eqn. 2b] The reduced manganese and silicon transfer to a molten metal phase and the MnO and SiOz transfer to a solid slag phase. A dynamic equilibrium between the slag and metal phases establishes an equilibrium reaction derived from Eqns. 2a and 2b as: 2 MnO (S) + Si ( / ) -> 2 Mn ( / ) + SiOz (S> [Eqn. 2c] which is therefore equivalent to the direct reduction of manganese (II) oxide to elemental manganese by elemental silicon. Complete oxidation of the carbon monoxide produced by Eqns. 2a and 2b results in the production of carbon dioxide gas. To produce a ferromanganese alloy having both a low carbon and a low silicon content, manganese ore, a lime flux and coal, coke or a similar source of carbon are fused together in a furnace to produce a melt rich in manganese (II) oxide. When this melt is bought into contact with silicomanganese, the elemental silicon in the silicomanganese directly reduces the MnO in the melt to elemental manganese and the SiOz thus formed transfers to the slag phase. The carbon content of the low-carbon ferromanganese produced is substantially the same as that of the silicomanganese used. Complete oxidation of the elemental carbon which is consumed results in the production of carbon dioxide gas. Low-carbon ferromanganese may also be obtained by blowing oxygen through molten high-carbon ferromanganese, which oxidizes the carbon it contains to produce carbon dioxide as well. Thus in all stages of the process for producing high-carbon ferromanganese, silicomanganese and low-carbon ferromanganese, carbon is consumed and carbon dioxide is produced as a result. All the above techniques are used to produce manganese which is intended for use in alloying with iron. If, on the other hand, a purer form of manganese, not already alloyed with iron and / or silicon, is required for other uses, such as to alloy with aluminium, higher manganese oxides in the manganese ore, which are water-insoluble, are firstly reduced to MnO, which is water-soluble, before the reduced product is added to a dilute aqueous solution of sulphuric acid to dissolve the MnO therein and form a solution of manganese (II) sulphate. Gangue species also present in the reduced product are precipitated out by treating this solution with ammonia and other reagents, and these undesirable contaminants are removed by filtration, before the purified solution is then electrolysed to recover the manganese by electrowinning. The higher oxides in the manganese ore may be reduced to manganese (II) oxide by roasting the ore with carbon and silicon, or by injecting the ore with natural gas or with hydrogen derived from steam-methane reforming (SMR) at a temperature of about 850 ’Celsius. In all cases, however, fossil fuels are consumed and carbon dioxide is produced as a result. In view of the high energy consumption of the pyrometallurgical techniques described above, some attention has focussed more recently on finding commercially viable alternative hydrometallurgical techniques for reducing higher manganese oxides in manganese ores into water-soluble MnO without the need to use high temperatures, as described, for example, in R.N. Sahoo et al.: "Leaching of Manganese from Low Grade Manganese Ore using Oxalic Acid as Reductant in Sulfuric Acid Solution", Hydrometallurgy, Vol. 62, No. 3, pp. 157-163 (December 2001) and Widi Astuti et al.: "Reductive-Atmospheric Leaching of Manganese from Pyrolusite Ore using Various Reducing Agents", AIP Conference Proceedings, Vol. 2097, No. 1, p. 030117 etseg. (April 2019). In such hydrometallurgical techniques, an organic reducing agent, such as tannic and oxalic acids and reducing sugars like glucose and fructose, is used to reduce higher manganese oxides to MnO under acidic conditions at or near to ambient temperature. As a representative example of such a hydrometallurgical technique, pyrolusite ore may be reduced using glucose as a reducing agent in an acidic aqueous solution according to the equation: 12 MnOj (S) + CsHuOg (aq) 12 MnO (aqj + 6 H2O ( / > + 6 CO2 (gj [Eqn. 3] However, as Eqn. 3 shows, the concomitant oxidation of an organic reducing agent such as glucose when higher manganese oxides are reduced to MnO still results in the release of carbon dioxide gas. Using such an organic reducing agent to help extract manganese from its ores instead of using one or more of the established pyrometallurgical techniques is therefore only equivalent to burning biomass instead of a fossil fuel, with the same accompanying release of carbon dioxide. In summary, therefore, the existing techniques described above for extracting manganese from its ores all consume carbon in some form as at least part of the extraction process and result in the production of carbon dioxide gas. However, the present climate crisis demands that all the carbon dioxide thus produced, as well as any remaining carbon monoxide, should not be released into the environment, where they would contribute to global greenhouse gas emissions. If the manganese is extracted from primary ores comprising rhodochrosite, this problem is exacerbated because apart from the carbon dioxide produced by using a carbonaceous reductant, reduction of the ore to elemental manganese also releases carbon dioxide from the rhodochrosite which was previously mineralized in geological deposits. Several proposals have therefore already been made for how to mitigate these greenhouse gas emissions. One such proposal is that at least some of the carbon dioxide generated should be captured and stored. In such carbon capture and storage (CCS), a chemical sorbent is used to capture the carbon dioxide and produce a carbonated intermediate. This intermediate is then treated to regenerate the sorbent and release the carbon dioxide for storage and / or industrial usage (CCU). However, CCS and CCU both have the disadvantage that they represent an additional cost for the manganese producer to bear, making their adoption unattractive. As further background to the present invention, it is also known to beneficiate lower grade manganese ores using a combination of reductive roasting and magnetic separation techniques. Some examples of such techniques may be found in the following documents: V. Singh et al.: Review of Low Grade Manganese Ore Upgradation Processes", Mineral Processing and Extractive Metallurgy Review, Vol. 41, No. 6, pp. 417-438 (2020); R. Elliott etal.: “A. Review of the Beneficiation of Low-Grade Manganese Ores by Magnetic Separation", Canadian Metallurgical Quarterly, Vol. 59, No. 1, pp. 1-16 (2020); Yuanbo Zhang et al.: "Separation and Recovery of Iron and Manganese from High-Iron Manganese Oxide Ores by Reduction Roasting and Magnetic Separation Technique", Separation Science and Technology, Vol. 52, No. 7, pp. 1321-1332 (2017); and Lihua Gao et al. "Upgrading of Low-Grade Manganese Ore Based on Reduction Roasting and Magnetic Separation Technique", Separation Science and Technology, Vol. 54, No. 1, pp. 195-206 (2019). By way of further background, several alkali and alkaline earth metals are produced industrially by fusing and electrolysing a chloride salt of the metal in question. These metals include sodium. Application of an electrical current to sodium chloride in solid phase fuses (i.e., melts) the sodium chloride by ohmic heating. Subsequent electrolysis of the sodium chloride produces metallic sodium in liquid phase, with chlorine gas produced as a co-product. In order to lower the temperature at which the electrolysis takes place, the sodium chloride may be mixed with one or more other salts to form a eutectic mixture. Electrolysis of sodium chloride to produce elemental sodium typically occurs at a temperature of about 600 to 625 ’Celsius. The high-temperature chlorine gas produced as a coproduct is usually cooled, condensed and bottled. The liquid sodium produced in this way generally has a high purity of better than 99% and may therefore subsequently be used in other industrial processes. Moreover, the properties, behaviour and handling of liquid sodium are well known and understood from many decades of its use as a heat transfer fluid in the nuclear power industry. Object of the Invention It is therefore an object of the invention to provide a method and apparatus for producing manganese from a manganese ore without using any carbon or a carbon-containing reductant. Preferably, the method and apparatus should be able to treat both primary ores comprising rhodochrosite and secondary ores in which the manganese is present as at least one of the mineral and mineraloid forms of manganese oxide and manganese oxyhydroxide, for example. Description of the Invention Accordingly, in one aspect, the present invention provides a method of producing manganese from a manganese ore. The method comprises comminuting the manganese ore into fines and converting a manganiferous mineral in the ore into a trivalent manganese oxide using at least one of a reductant-free pyrometallurgical technique and an inorganic hydrometallurgical technique. The trivalent manganese oxide may comprise at least one of manganese (III) oxide (MnjOa) and manganese (11,111) oxide (Mn3O4). The method then comprises adding the trivalent manganese oxide to an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide, to produce a solid phase comprising both elemental manganese and other insoluble products at least comprising sodium oxide. The redox reaction is conducted in an inert atmosphere and at a temperature of less than 600 ’Celsius. The method then comprises separating at least some of the solid phase from the liquid sodium, and separating the elemental manganese from the other insoluble products. Before being subjected to the above method, the manganese ore may undergo one or more processes of beneficiation, which may comprise known processes, to increase the proportion of manganese compounds in the ore relative to iron compounds and / or relative to gangue. The method of the invention has at least the following advantages. Unlike the existing techniques for extracting manganese from manganese ore described above, the method of the invention consumes no carbon, and no carbon monoxide or dioxide are produced as a result. The method of the invention is therefore at least carbon-neutral and makes no contribution to global greenhouse gas emissions. Moreover, the method of the invention also produces sodium oxide, or sodium hydroxide derived from hydrating this sodium oxide, as a co-product. Since sodium oxide and hydroxide both have a high affinity for carbon dioxide, at least some of this co-product may therefore be used in a carbonation reaction with carbon dioxide gas to produce at least sodium carbonate. For example, the carbon dioxide may be captured from atmospheric air and / or from one or more other industrial processes, in order to mitigate greenhouse gas emissions. If so, the present invention can have a significantly negative carbon footprint overall. Since no carbon is used in the method of the invention, the carbon content of the manganese produced can be controlled to a very low level. Moreover, the method of the invention provides several different ways in which gangue species may be separated from manganese or from a manganese-containing compound derived from the ore without using any other chemicals, as described further below. For example, after the manganiferous mineral in the ore has been converted into a trivalent manganese oxide, any remaining aluminosilicate gangue species can be separated from this trivalent manganese oxide by subsequent magnetic separation, because these gangue species are diamagnetic whereas the trivalent manganese oxide is strongly paramagnetic. Therefore, unlike existing pyrometallurgical techniques for extracting manganese from its ore which use lime to react with and capture silicates in order to form a slag, the method of the invention does not consume any lime either. The thermal decomposition of limestone to produce this lime, with the accompanying release of carbon dioxide from the limestone, is thereby avoided entirely. Similarly, in comparison to existing hydrometallurgical techniques for extracting manganese from its ore which use ammonia to remove these gangue species from solution, the method of the invention also does not require the use of any ammonia. Since the Haber-Bosch process for the production of ammonia uses hydrogen which is produced from natural gas by SMR, the consumption of this fossil fuel and the associated production of carbon dioxide are therefore also avoided entirely. In comparison to the existing pyrometallurgical techniques for extracting manganese from its ore which use carbon monoxide, hydrogen or natural gas as a reductant, the greater reactivity of liquid sodium than these gases means that the solid-liquid phase reaction between the trivalent manganese oxide and the sodium can be carried out at a much lower temperature than these prior solid-gas phase reactions and is also significantly more exothermic, which therefore reduces the total quantity of heat required. On the other hand, in comparison to the existing hydrometallurgical techniques for extracting manganese from its ore, which all require a considerable time for the organic reducing agents to leach manganese ions from the ore into solution, the greater reactivity of sodium than these organic reducing agents makes the redox reaction between the trivalent manganese oxide and the liquid sodium proceed more quickly, which therefore reduces the total time required. The liquid sodium may be produced by electrolysis, for example from sodium chloride, with the coproduction of chlorine gas. Electricity for the electrolysis may be provided by a source of renewable energy, such as wind or solar, or come from nuclear power, and therefore need not generate any greenhouse gas emissions. The method of the invention therefore need not produce any greenhouse gases at all. At least some of the liquid sodium and chlorine gas may be produced together, for example, by fusing and electrolysing solid sodium chloride. This may be performed using a known technique, like the Downs process. Alternatively or additionally, at least some of the liquid sodium and chlorine gas may be produced separately from each other by firstly electrolysing an aqueous solution of sodium chloride ( / .e., brine) using the chlor-alkali process to produce an aqueous solution of sodium hydroxide and the chlorine gas. In some embodiments, the brine may be brine concentrate produced as a waste product by a desalination plant, in which case, the electrolysis may advantageously be conducted near the desalination plant and this waste can then be eliminated. If at least some of the brine is electrolysed in a standard type of membrane cell, then hydrogen gas may be produced as well. Alternatively or additionally, at least some of the brine may be electrolysed without co-production of hydrogen, using a technique such as that described in US patent appln. no. 2005 / 0026005 Al. Subsequently drying at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide then allows the solid sodium hydroxide to be fused and electrolysed to produce the liquid sodium, as well as hydrogen gas and oxygen gas. Fusing and electrolysing the solid sodium hydroxide may be performed using a known technique, like the Castner process. Whereas the Castner process is generally regarded as obsolete, fusing and electrolysing the solid sodium hydroxide by the Castner process after having produced the sodium hydroxide for this via the chlor-alkali process has the advantage that the two processes can be viewed as a combined electrolysis of equimolar amounts of sodium chloride and water. This is because the back-reaction of water in the Castner cell with the liquid sodium thus produced also produces hydrogen gas, as follows: Chlor-alkali process: 2 NaCI (aq) + 2 H2O-> 2 NaOH (aq) + CL + H2 [Eqn. 4a] Dry Electrolysis of melt: 2 NaOH ( )-> 2 Na ( ) + H2O + 1 / 2 O2 [Eqn. 4b] Castner 4s . process ~ | 4 / Water reacts with sodium: | Na ( ) + H2O NaOH ( ) + / 2 H2 [Eqn. 4c] I_________________________________________________________________________________________________________________________________________________________________________ Overall: 2 NaCI + 2 H2O 2 Na + Cl2 + 2 H2 + O2 [Eqn. 4d] The hydrogen gas thus produced may then be used, for example, as a fuel and / or to replace the use of SMR in the production of ammonia (as in the Haber-Bosch process). Alternatively or additionally, at least some of the hydrogen gas may be combusted with chlorine gas to produce hydrogen chloride. The oxygen gas is also a useful co-product which may be used, for example, in a steelmaking process by injecting it into molten iron. Moreover, unlike the existing techniques for extracting manganese from its ore, all of which produce undesirable by-products like carbon dioxide in addition to elemental manganese, the method of the invention also produces sodium oxide and / or hydroxide as a co-product, which is a useful industrial product in its own right, and electrolysing sodium chloride to produce liquid sodium produces chlorine gas as well, which may be used to produce hydrochloric acid and / or one or more chlorinated organic compounds. The electrical energy required to produce the liquid sodium is therefore shared between the production of several useful industrial products, and is not just consumed by the production of manganese. The method of the invention will now be described in greater detail. Comminution Firstly, the manganese ore is comminuted into fines to increase its surface area and to help separate mineral species in the ore from each other. Fines are generally considered to be less than about 6 or 7 mm across. Comminuting the ore may comprise a mechanical process, such as one or more of agitation, crushing, grinding, hammering, milling and rolling, as well as other similar processes. Alternatively or additionally, comminuting the ore may comprise a chemical process, such as one in which the ore is at least partially dissolved and / or manganiferous particles are precipitated out from solution, provided that such a chemical process does not reduce all the higher oxides of manganese in the ore to divalent manganese. Comminution produces fine ore particles with a range of different sizes. Smaller particles are more desirable because they have a greater surface area. This helps to accelerate the conversion of the manganiferous mineral in the ore into a trivalent manganese oxide, as well as the reaction of the resulting trivalent manganese oxide with the liquid sodium, described below. However, creating smaller particles with a greater surface area also requires more energy to be expended on comminuting the ore than would otherwise be expended on creating larger particles. The optimum size of particles will therefore be partly determined in any particular case by the most economic energy balance overall between comminuting the ore on the one hand and the subsequent conversion of the manganiferous mineral in the ore into a trivalent manganese oxide and reaction with liquid sodium on the other. This energy balance will in turn depend in any particular case on the specific energy requirements of the comminution device or devices which are used, the prior chemical composition of the manganese ore itself, and on the outcome of any beneficiation which the ore has undergone before comminution. If beneficiation has already resulted in finely comminuted ore particles (for example, having an average size of less than about 1 or 2 mm), then any further comminution will be unnecessary. Apart from increasing its surface area, comminuting the manganese ore into fines can also change one or more of the physicochemical properties of the ore in a desirable manner. For example, it may help to detach particles of the manganiferous mineral in the ore from particles of gangue species also present in the ore. Comminution may also be effective not only in releasing water which is trapped in the ore, thereby helping to dehydrate it, but may also contribute to dehydroxylating hydroxylated compounds, such as manganese oxyhydroxides and clays, which the ore may contain. The preferred range of particle sizes at the end of comminution can therefore be determined in any particular case from the best combination of overall energy expenditure with the desirable physicochemical properties of the resultant ore particles. Comminution may further comprise sieving, separating and recycling larger particles for further comminution into smaller particles. The method of the invention has the significant advantage that after comminution, the resulting fines do not need to undergo any agglomeration to prepare them for further processing. This is unlike the carbothermic reduction of manganese ore in a blast furnace, which requires manganese ore fines to be agglomerated, for example by being briquetting with the addition of a binder, nodulized with carbon, or sintered with other ingredients and pelletized into a particular range of sizes (typically about 10 to 20 mm) before the resulting pellets are introduced into the furnace. Since sintering and pelletizing typically involve heating the ore fines to 1200 ’Celsius or more, the method of the invention saves a considerable amount of energy, has no need to use any other raw materials, and generates no polluting combustion products, in comparison to the agglomeration techniques required for this traditional carbothermic reduction technology. As well as comminuting the manganese ore into fines, a manganiferous mineral in the ore is also converted into a trivalent manganese oxide using at least one of a reductant-free pyrometallurgical technique and an inorganic hydrometallurgical technique, before the trivalent manganese oxide is then added to liquid sodium. The pyrometallurgical technique will be described first. Pyrometallurgical Technique If the pyrometallurgical technique is used, during and / or after its comminution into fines, the manganese ore is heat-treated in atmospheric air to a temperature of from at least about 100 to at most about 600 ’Celsius, inclusive. This heat-treatment is performed without using any reductant, such as carbon or hydrogen. In order to reduce energy consumption, the ore is preferably heated to less than about 550 ’Celsius, more preferably to less than about 500 ’Celsius, even more preferably to less than about 450 ’Celsius, more preferably still to less than about 400 ’Celsius, and most preferably to less than about 350 ’Celsius. However, if the ore comprises a significant proportion, for example at least about 4% by weight, in some cases at least about 8%, and in other cases at least about 12% by weight, of rhodochrosite, for example because the ore is a primary ore, then the ore may preferably be heated to more than about 200 ’Celsius, because the conversion of rhodochrosite into a trivalent manganese oxide starts at about 200 ’Celsius. Depending on the initial chemical composition of the ore, the heat-treatment has several effects as follows. If the manganese ore comprises rhodochrosite, heat-treating the ore in this manner causes the MnCOa to react with atmospheric oxygen and produce bixbyite (MmOa) with the release of carbon dioxide, according to the equation: 2 MnCOa (S) + % O2 (gj MnjOa (S) + 2 CO2 (gj [Eqn. 5a] Since manganese carbonate does not have high thermal stability, this reaction has a relatively low activation energy of only about 17.9 kJ molflVInCOa)1 and is complete by about 300’Celsius. However, at temperatures above about 330 ’Celsius, the bixbyite starts to reabsorb the carbon dioxide. Therefore, if the ore comprises a significant proportion (for example, at least about 4% by weight, in some cases at least about 8% by weight, and in other cases at least about 12% by weight) of rhodochrosite, in some embodiments, the ore may preferably be heated to less than about 320 ’Celsius to avoid such reabsorption. If so, and the ore further comprises another manganiferous mineral apart from rhodochrosite which can be converted into a trivalent manganese oxide at a temperature of less than about 600 ’Celsius, this other manganiferous mineral may also be converted into a trivalent manganese oxide by subsequently using the hydrometallurgical technique as well. Some further reduction of the bixbyite to hausmannite may also start to occur under the stated conditions. If the ore comprises any iron, calcium and / or magnesium carbonates as gangue species, the iron and magnesium carbonates also thermally decompose at temperatures of less than about 600 ’Celsius into Fe20a and MgO respectively, releasing carbon dioxide, but the more thermally stable calcium carbonate remains unchanged as CaCO.3. Following the same pattern, kutnohorite thermally decomposes into hausmannite and calcite (CaCOs), releasing carbon dioxide gas. The thermal decomposition of kutnohorite in atmospheric air commences at about 500 ’Celsius. The thermal decomposition of siderite in atmospheric air starts at about 440 ’Celsius and is complete by about 540 ’Celsius. Thus the temperature of the heat-treatment can be selected to ensure the thermal decomposition of any of the aforementioned carbonate minerals, apart from calcium carbonate, based on prior analysis of the chemical composition of the ore. If the manganese ore comprises a carbonate mineral, in order to prevent the carbon dioxide gas released by the rhodochrosite and by any other associated carbonates from escaping, and thereby contributing to global greenhouse gas emissions, the ore is heated in atmospheric air closed off from their surrounding environment, and the carbon dioxide is captured. At least some of the captured carbon dioxide may then be reacted in a carbonation reaction with a portion of the sodium oxide also produced by the method of the invention, or with sodium hydroxide derived from hydrating at least some of this portion of sodium oxide. If the Mn2O3 and the carbon dioxide are produced in their stoichiometric ratios according to Eqn. 5a, then the amount of liquid sodium required to reduce all of this Mn2O3 to elemental manganese produces 1.5 times the amount of sodium oxide and / or hydroxide required for such a carbonation reaction with all the carbon dioxide produced by the reaction of Eqn. 5a. Thus all of the carbon dioxide produced by the reaction of Eqn. 5a can be mineralized by reacting it with a portion of the sodium oxide also produced by the method of the invention, whilst still leaving a third of this sodium oxide as an end-product. If the captured carbon dioxide reacts directly with a portion of the sodium oxide also produced by the method of the invention, then the product of this carbonation reaction is just anhydrous sodium carbonate, but with any possible admixture of gangue in the portion of sodium oxide still present. This may be acceptable, depending on the intended end use of the sodium carbonate. If, however, sodium hydroxide derived from hydrating at least some of this portion of sodium oxide in order to remove such gangue is instead reacted with the captured carbon dioxide, although the sodium hydroxide may be free of gangue, the products of this reaction may also be more complex. For example, at standard temperature and pressure (hereinafter, s.t.p.), this carbonation reaction would produce the following mixture of solid products in the following mole fractions at equilibrium: sodium hydrogencarbonate (NaHCO3) 55% anhydrous sodium carbonate (Na2CO3) 27% sodium carbonate monohydrate (Na2CO3 • H2O) 15% trona (Na3CO3HCO3 • 2H2O) 3% Such a mixture of products may again be acceptable, if, for example, it is to be used as an ingredient in the manufacture of soda-lime glass. In comparison to prior techniques for CCS / CCU, therefore, this carbonation reaction produces a commercially useful product, which unlike carbon dioxide, does not have to be sequestered or otherwise disposed of. Moreover, the manufacture of soda-lime glass has traditionally mined naturally occurring trona as a source of sodium carbonate. In comparison to mining trona, using the mixture of products produced by the above carbonation reaction in glassmaking instead has the advantage that it does not extract any new carbon dioxide from geological deposits. In other words, the mixture of products produced by the above carbonation reaction contains carbon dioxide from the rhodochrosite, which has already been mined to produce elemental manganese. Thus the carbon dioxide successively passes through two industrial processes ( / .e., both the production of manganese and glassmaking), whereby the total amount of carbon dioxide extracted from geological deposits to produce both the manganese and the glass is halved. Moreover, since this carbonation reaction is exothermic, in some embodiments, heat-treating the ore may comprise heating the ore with heat extracted from at least one of the carbonation reaction itself and the mixture of products produced thereby. This has the advantage that the heat generated by the carbonation reaction is then re-used, rather than being wasted. If the manganese ore comprises tetravalent manganese oxide, for example because the ore is a secondary ore, heat-treating the ore under the stated conditions reduces this to a trivalent manganese oxide. After this heat-treatment, the trivalent manganese may be present in manganese (III) oxide (MnjOa), manganese (11,111) oxide (MnsO4), or a mixture of both. For example, if the manganese ore comprises MnO? in the form of pyrolusite and / or ramsdellite, heat-treating the ore as described thermally decomposes the MnO? into MnjOa and oxygen, according to the equation: 2 MnO? (sj MnjOa (Sj + ½ Ch (g) [Eqn. 5b] This thermal decomposition starts at about 500 ’Celsius and is complete by about 600 ’Celsius. On the other hand, if the manganese ore already comprises one or more trivalent manganese oxides, the stated temperature range is not sufficiently high to reduce the trivalent manganese further into manganese (II) oxide if in the presence of atmospheric air without a reductant, such as carbon or hydrogen. Thus if the manganese ore initially comprises at least one of bixbyite and hausmannite, the trivalent manganese oxides in these minerals remain unreduced under the stated conditions. Conversely, however, if the manganese ore contains an admixture of magnetite (Fe3O4), heat-treating the ore in atmospheric air to within the stated temperature range may also result in at least partial oxidation of the magnetite into hematite (FejOs). If the manganese ore comprises water and / or any hydroxylated compounds, the ore is dehydrated, and hydroxylated compounds contained therein are dehydroxylated, preferably completely, and at least to a high degree. For example, if the manganese ore comprises romanechite and therefore comprises water, heat-treating the ore in this manner causes water molecules trapped within the tunnels of the romanechite to diffuse out and be driven off as water vapour. The heat-treatment also tends to convert the structure of the romanechite into hollandite as the water escapes. However, the thermal decomposition of hollandite itself and of other manganese oxides such as cryptomelane which are isostructural to hollandite, only starts at about 600 ’Celsius and is complete by about 700 ’Celsius, producing manganese (III) oxide and compounds such as baria (BaO) which contain the cations previously trapped within the tunnels. Similarly, the thermal decomposition of manganese silicates, such as braunite and rhodonite, only occurs at temperatures above 600 ’Celsius. For example, braunite thermally decomposes into hausmannite and silica at temperatures above about 700 ’Celsius, whereas tephroite (Mn2SiO4) decomposes on heating in atmospheric air into rhodonite, braunite and tridymite (which is a polymorph of SiCh) at temperatures of from about 800 to about 1100 ’Celsius. This is similar to the range of temperatures for the thermal decomposition of fayalite (Fe2SiO4), an iron analogue of tephroite, which decomposes in atmospheric air into iron (11,111) oxide and quartz from about 800 to about 1000 ’Celsius. Thus if the manganese ore initially contains a significant proportion, for example at least about 4% by weight, in some cases at least about 8% and in other cases, at least about 12% by weight, of at least one of a siliceous manganese mineral, such as braunite and / or rhodonite, and silica present as gangue, then these manganiferous minerals in the ore may be converted into a trivalent manganese oxide using the hydrometallurgical technique instead, as described below. The hydrometallurgical technique is also effective at converting hollandite and other manganese oxides such as cryptomelane which are isostructural to hollandite into a trivalent manganese oxide. Whether the pyrometallurgical technique or the hydrometallurgical technique is preferred for converting the manganiferous mineral in the ore into a trivalent manganese oxide may therefore depend at least in part on the initial chemical composition of the ore. Dehydroxylation also converts manganese oxyhydroxides, if any are present in the ore, into trivalent manganese oxides. Thus in another example, if the manganese ore comprises MnO(OH) in the form of manganite and / or groutite, heat-treating the ore in this manner thermally decomposes the MnO(OH) into Mn2O3 and water vapour, according to the equation: 2 MnO(OH) (S) -> Mn2O3 (S) + H2O (gj [Eqn. 5c] This conversion occurs in two stages, firstly by dehydroxylation of the manganite into a tetravalent manganese oxide, starting at about 300 ’Celsius and complete by about 400 ’Celsius, followed by reduction of the tetravalent manganese oxide thus produced, starting at about 500 ’Celsius and complete by about 600 ’Celsius, as stated above. Similarly, if the manganese ore contains any claylike aluminosilicate minerals as gangue, heat-treating the ore in this manner decomposes these minerals into their constituent oxides, silica (SiO2) and alumina (AI2O3), and / or compounds thereof. For example, halloysite pSiOz-AhOa-AHjO) and kaolinite (2SiO2-Al2O3-2H2O) are decomposed into metakaolinite pSiCh-ALOs). This releases their water of crystallization, which escapes as water vapour. Other metal oxyhydroxides are also decomposed into their respective oxides, releasing water vapour. For example, goethite (Fe(OH)3 or FeO(OH)-H2O) decomposes into ferric oxide (Fe2O3), and gibbsite (AI(OH)3) decomposes firstly into boehmite (AIOOH) and then into alumina (AI2O3). In these examples, the respective decomposition temperatures of the different minerals are dependent not only on the initial chemical composition of the ore and the crystal structure of each mineral component thereof, but also on the effects of comminuting the ore into fines. In other words, dehydration and dehydroxylation of the ore is a result of a combination of the effects of mechanical action (milling, etc.) on the ore and its heat-treatment. This is because the enthalpy of dehydroxylation decreases with decreasing particle size. Without any prior comminution into fines, dehydroxylation of any aluminosilicate minerals in the ore would occur over a range of about 400 to 600 ’Celsius. Goethite decomposes into ferric oxide over a range of about 210 to 340 ’Celsius. The decomposition of gibbsite firstly into boehmite, and then into alumina, exhibits enthalpy transitions firstly at 246 and 312 ’Celsius, and then at 542 ’Celsius. The decomposition of diaspore exhibits an enthalpy transition at 532 ’Celsius. Thus, even without any prior comminution, complete dehydration and dehydroxylation of aluminosilicate minerals in the ore can be achieved when it is heat-treated to within the stated temperature range. On the other hand, since the maximum temperature of 600 ’Celsius is also significantly less than the temperature required to reduce trivalent manganese oxides further into manganese (II) oxide in the presence of atmospheric air, considerable energy is still saved in comparison to existing pyrometallurgical techniques for extracting manganese from its ore, which unlike the present invention, instead aim to reduce the manganese in the ore to its lowest possible oxidation state. Moreover, if the manganese ore comprises a significant proportion of iron, heat-treating the ore to too high a temperature risks undesirably encouraging the comminuted ore particles to sinter. Thus if the ore initially contains a significant proportion of, for example at least about 4% by weight and in some cases, more than about 8% or even about 12% by weight, of an iron oxide, sintering the comminuted ore particles may be substantially avoided by heating the comminuted ore particles to less than about 600 ’Celsius, which is less than the Tammann temperature of Fe2O3 at 650 ’Celsius. Nonetheless, the heat-treated ore particles may still be further comminuted back into fines after their heat-treatment to reverse the effects of any possible sintering. In addition, if the manganese ore does comprise such a significant proportion of iron, the subsequent reaction of the trivalent manganese oxide with the liquid sodium should also be conducted with a reduced upper limit to its range of operating temperatures, as described further below. Depending on the intended use of the manganese produced, it may be desirable to mix the manganese ore with finely comminuted, dehydrated and dehydroxylated iron ore before the trivalent manganese oxide is added to the liquid sodium. Iron may be present in the iron ore as at least one of the mineral and mineraloid forms of iron oxide and iron oxyhydroxide, or the iron ore may be a siderite ore, in which the iron is present as iron carbonate. The manganese ore may be mixed with iron ore at one or more of several different possible stages before the reaction with the liquid sodium. For example, the manganese ore may be mixed with the iron ore before both are finely comminuted and heat-treated together. Alternatively or additionally, finely comminuted iron ore may be mixed with finely comminuted manganese ore before their mixture is then heat-treated. In either case, heat-treating the manganese ore may comprise heating the mixed iron and manganese ores in atmospheric air to a temperature of less than about 600 ’Celsius, to discourage undesirable sintering of the comminuted ore particles, and / or the heat-treated ore particles may be further comminuted back into fines after their heat-treatment to reverse the effects of any sintering. In another possible embodiment, the manganese ore and the iron ore may be comminuted into fines and heat-treated separately from each other, before the ores are then mixed together. In such a case, both the manganese ore and the iron ore may be separately heat-treated in atmospheric air to a temperature of from about 100 to about 600 ’Celsius, inclusive, to discourage undesirable sintering of the unmixed iron ores. However, whenever iron ore is mixed with the manganese ore, the subsequent reaction of the trivalent manganese oxide with the liquid sodium should also be conducted with a reduced upper limit to its range of operating temperatures, as described further below. Heat-treating the ore can also contribute to its comminution. For example, if the ore contains water, as the ore is heated to above the boiling point of water, water in the ore is driven off as steam. As the water vaporizes, if the water is trapped in pores within the ore, this can cause decrepitation ( / .e., fracturing) of the ore as the water expands into steam, thereby further increasing the surface area of the ore particles. Similarly, if the ore contains quartz (as is commonly the case), as the ore is heated to above the quartz inversion temperature of 573 ’Celsius, a small percentage increase in volume which accompanies the phase transition from a to p quartz at this temperature can help to split the ore apart. The comminution of the ore into fines and its heat-treatment therefore interact synergistically with each other. Comminuting the ore into fines on the one hand and heat-treating the ore on the other may also be at least partially combined, to discourage sintering of the ore particles as they are heated. In other words, some or all of the comminution of the manganese ore may be carried at the same time as when the ore is being heat-treated. For example, a dryer of the ore may comprise one or more comminution devices, such as moving parts for separating the ore particles as they are heated. On the other hand, some or all of the comminution does not have to be carried out near to or in the same location as the heat-treatment. For example, prior beneficiation of the ore in one location may already result in finely comminuted ore particles, which may then be delivered to a different location for heat-treatment, before the subsequent reaction of the trivalent manganese oxide with the liquid sodium. The manganese ore may be heat-treated by supplying it with heat from at least one of several different sources. Preferably, however, heat-treating the ore comprises heating the ore with heat extracted from at least one of: (i) cooling the liquid sodium before adding the trivalent manganese oxide to the liquid sodium; (ii) cooling the heat-treated ore back down before adding the trivalent manganese oxide to the liquid sodium; (iii) cooling the liquid sodium from which at least some of the solid phase has been separated; and (iv) cooling at least a component of the solid phase separated from the liquid sodium. The component of the solid phase from which heat is extracted may be any one or more of elemental manganese, sodium oxide and a gangue species, for example. All these techniques for heating the ore have the advantage that they re-use heat from other parts of the method of the invention which would otherwise have been lost to the environment, and that therefore none of them requires any more energy to be consumed than is already used to produce the liquid sodium for the subsequent reaction with the trivalent manganese oxide. Any of the above techniques for heating the ore may optionally be combined with one or more other known heating techniques. The maximum temperature of the pyrometallurgical technique of about 600 ’Celsius may easily be reached by electrical heating alone, which therefore avoids the need to burn any fuel, including any fossil fuel. If heating the ore comprises cooling the liquid sodium before adding the trivalent manganese oxide to the liquid sodium, liquid sodium produced in an electrolytic cell may be circulated as a heat transfer fluid (HTF) through and / or around a dryer containing the ore before the trivalent manganese oxide is added to the liquid sodium. Conveniently, the maximum temperature for the pyrometallurgical technique is about the same as or slightly less than the temperature of liquid sodium produced by fusing and electrolysing solid sodium chloride. If heating the ore comprises cooling excess liquid sodium remaining from the (exothermic) reaction between the trivalent manganese oxide and the liquid sodium, which in other words is the liquid sodium from which the elemental manganese and the other insoluble products of the reaction have been separated as a solid phase, this excess liquid sodium may similarly be circulated as a heat transfer fluid (HTF) through and / or around a dryer containing the ore before the trivalent manganese oxide is added to the liquid sodium. The total quantity of heat required to dehydrate the ore, dehydroxylate hydroxylated compounds contained therein and thermally decompose manganese carbonate and / or tetravalent manganese oxide in the ore into a trivalent manganese oxide, and therefore the temperature to which the ore particles should be heated, the dwell time of the particles at any given temperature, and the rate of heating them from their initial temperature, will all depend in any particular case on the original chemical composition of the manganese ore (after any beneficiation), and on the effects of comminuting the ore, including its degree of comminution and the temperature of the ore particles after comminution. For example, if the ore contained less water initially and / or has been more finely comminuted, it will require less heat to dry. Moreover, comminuting the manganese ore also tends to raise its temperature by converting mechanical energy into heat through friction. Ideally, for the subsequent reaction of the trivalent manganese oxide with the liquid sodium, any hydrous manganese compounds and / or manganese oxyhydroxides within the ore should be completely dehydrated and dehydroxylated, since any water or hydroxylated compounds remaining in the ore after the heat-treatment may otherwise react with the liquid sodium to produce a variety of different contaminants. However, complete dehydration and dehydroxylation of these manganese compounds requires more heat to be transferred to the ore than just a high degree of dehydration and dehydroxylation would require. The degree of dehydration and dehydroxylation to achieve can therefore be determined in any particular case from the overall balance between the quantity of heat required for dehydration and dehydroxylation on the one hand and the desired purities and percentage yields of the products from the subsequent reaction of the trivalent manganese oxide with the liquid sodium on the other. The latter will in turn depend on the prior chemical composition of the ore itself, as well as on the outcome of dehydration and dehydroxylation. Thus a high degree of dehydration and dehydroxylation of these manganese compounds, rather than their complete dehydration and dehydroxylation, may suffice in most cases. The thermal decomposition of manganese carbonate and / or tetravalent manganese oxide into one or more trivalent manganese oxides and the degree of dehydration and dehydroxylation of the comminuted ore particles may be measured by repeatedly weighing the ore at least during its heattreatment, and possibly also during its comminution, because the ore loses weight as the volatile components of the ore, carbon dioxide, oxygen and water vapour, are driven off. A relatively rapid decrease in weight is therefore indicative of the ongoing thermal decomposition of manganese carbonate and / or tetravalent manganese oxide into trivalent manganese oxide(s) and / or of continuing dehydration and dehydroxylation, whereas when the rate of weight-loss diminishes with ongoing comminution and / or heating, this indicates that these processes are nearly complete. The best conditions for heat-treating any particular batch of manganese ore may be determined by sampling the ore particles before heat-treatment, and performing a loss-on-ignition (LOI) test on each sample thus collected. Alternatively or additionally, the degree of thermal decomposition, dehydration and dehydroxylation achieved may be measured by sampling the ore during and / or after heat-treatment, and performing a similar LOI test on each sample thus collected. The hydrometallurgical technique for converting a manganiferous mineral in the ore into a trivalent manganese oxide will be described next. Hydrometallurgical Technique If the hydrometallurgical technique is used, comminuted ore particles are added to hot, concentrated hydrochloric acid, which causes the manganiferous mineral in the ore to dissolve in the acid. If the manganiferous mineral is an oxide, hydroxide and / or silicate, this produces gaseous chlorine and an acidic aqueous solution comprising manganese (II) chloride. For example, the dissolution of (a) pyrolusite, (b) manganite and (c) braunite each respectively proceeds according to the following equations: MnO2 (S) + 4 HCI (aq) -> MnCI2 (aqj + 2 H2O ( / ) + CI2 (g) [Eqn. 6a] MnO(OH) (s) + 3 HCI (aq) -» MnCI2 (aq) + 2 H2O M + / 2 Cl2 (g) [Eqn. 6b] 3 Mn2Oa • MnSiOa (S) + 20 HCI (aqj 7 MnCI2 (aq) + 10 H2O ( / ) + SiO2 (S) + 3 Cl2 (g) [Eqn. 6c] To allow a fair comparison of Eqn. 6c with Eqns. 6a and 6b, Eqn. 6c can be "normalized" to 1 mol of Mn by dividing it throughout by 7, as follows: 1 / 7 [3 Mn2O3 • MnSiOa] (s) + 20 / 7 HCI (aq) -> MnCI2(aq) + 10 / 7 H2O M + 1 / 7 SiO2 (s) + 3 / 7 Cl2 (g) [Eqn. 6c7] The ore is added to the hot, concentrated hydrochloric acid closed off from their surrounding environment and the gaseous chlorine is captured. Hot, concentrated hydrochloric acid also dissolves hollandite and other manganese oxides like cryptomelane which are isostructural to hollandite to produce the same reaction products, and releases the cations of group I and II metals, like potassium and barium, contained within the tunnels of these manganiferous minerals into solution. Whereas hydrochloric acid could also be used to dissolve carbonate minerals in the ore such as rhodochrosite, this would instead result in the production of carbon dioxide gas, which would contaminate the chlorine gas captured from other manganiferous minerals, such as from oxides, hydroxides and silicates. Accordingly, if the ore comprises carbonate minerals like rhodochrosite, for example because the ore is a primary ore, then this manganiferous mineral is preferably converted into a trivalent manganese oxide using the pyrometallurgical technique described above instead. Conversely, it may also be seen that whereas the temperature of the pyrometallurgical technique is insufficiently high to thermally decompose manganese silicates, such manganese silicates may be converted into a trivalent manganese oxide using the hydrometallurgical technique instead. The pyrometallurgical and hydrometallurgical techniques are therefore complementary to each other, and whether to use one or the other technique may be determined based on the initial chemical composition of the ore. Moreover, if the ore comprises a manganiferous mineral, such as rhodochrosite, which is better suited to the pyrometallurgical technique, and another manganiferous mineral, such as hollandite or braunite, which is better suited to the hydrometallurgical technique, then both techniques may be used to convert both types of manganiferous minerals in the ore into a trivalent manganese oxide. In such a case, however, the ore is preferably subjected to the pyrometallurgical technique before the heat-treated ore is then subjected to the hydrometallurgical technique. This has the advantage of avoiding the aforementioned contamination of chlorine gas captured in the hydrometallurgical technique by carbon dioxide released from carbonate minerals in the ore. Moreover, if the ore is subjected to the pyrometallurgical technique before the heat-treated ore is then subjected to the hydrometallurgical technique, the heat-treated ore is preferably comminuted after heat-treatment and before being subjected to the hydrometallurgical technique. This has the advantage that if the reactivity of the ore with the hydrochloric acid is reduced by reordering of its crystal structure as a result of its heat-treatment, then the reactivity of the ore with the hydrochloric acid may be increased again by comminution. More finely comminuted ore particles dissolve more readily in the hydrochloric acid. However, the comminuted ore particles continue to diminish in size as they dissolve. The hydrometallurgical technique therefore comprises comminuting the ore to some extent. How much the ore should be mechanically comminuted before being added to the hydrochloric acid may therefore be determined in any particular case by the most economic energy balance overall between such prior mechanical comminution of the ore and the subsequent dissolution of the manganiferous mineral(s) it contains. This energy balance is in turn determined in any particular case by the specific energy requirements of the comminution device or devices which are used, the prior chemical composition and structure of the ore, and the conditions of the dissolution process itself. The rate at which the manganiferous mineral dissolves in the hydrochloric acid is also affected by the initial temperature and concentration of the hydrochloric acid. In general, the rate at which the mineral dissolves in the hydrochloric acid increases with both the temperature and the concentration of the hydrochloric acid. The most preferable values for each of these two parameters can also be determined in any particular case by the most economic energy balance between the initial temperature and concentration of the hydrochloric acid, the rate of dissolution of the manganiferous mineral, and the final temperature of the acidic aqueous solution thus produced. Nonetheless, "hot" in this context means that the initial temperature of the hydrochloric acid is at least about 15 ’Celsius and preferably more than about 25 ’Celsius above ambient temperature, in order to increase the rate at which the ore dissolves therein. The initial temperature of the hydrochloric acid is also preferably above about 50 ’Celsius, more preferably above about 60 ’Celsius and most preferably above about 70 ’Celsius. This has the additional advantage that the solubility in the acidic aqueous solution of the chlorine gas which is produced diminishes at higher temperatures, as may be seen in Fig. 1. As Fig. 1 shows, which represents the situation at atmospheric pressure, above about 50 ’Celsius, the solubility of chlorine gas in liquid water is at most about 4.2 g kg^HjO). Moreover, at temperatures of 50 ’Celsius or more, the solubility of chlorine gas in hydrochloric acid is less than the solubility of chlorine gas in liquid water. If the captured chlorine gas is subsequently to be used to produce hydrochloric acid, it does not matter if the captured chlorine is contaminated by minor amounts of gaseous hydrogen chloride and water vapour resulting from partial evaporation and / or boiling of the hot hydrochloric acid in which the ore is dissolved. "Concentrated" in this context means that the initial concentration of the hydrochloric acid should preferably be at least about 2 M because at lower concentrations than this, the rate of dissolution of the ore in the hydrochloric acid may become unacceptably low. More preferably, the initial concentration of the hydrochloric acid should be at least about 3 M, more preferably still, at least about 4 M, and most preferably, at least about 5 M. This has the advantage that it reduces the amount of water present as a solvent in the resulting aqueous solution comprising manganese (II) chloride, and therefore the amount of water also present in an aqueous solution of sodium chloride formed by adding sodium oxide or hydroxide to it thereafter, as described below. Thus if this aqueous solution of sodium chloride is subsequently dried to recover the sodium chloride from it as a solid which is recycled for electrolysis, since water has a high specific heat capacity, this in turn reduces the amount of thermal energy required to evaporate or boil off the water of solution. On the other hand, the initial concentration of the hydrochloric acid may also have an upper limit greater than 5 M, for reasons explained shortly below. Dissolving the manganiferous mineral in the hydrochloric acid may be aided by stirring or otherwise mixing the ore particles into the hydrochloric acid. The rate of dissolution of the manganiferous mineral in the hydrochloric acid may be monitored and used to determine whether and / or when it has completely dissolved. For example, this may be done by measuring at least one of the flow rate of the chlorine gas thus produced and the pH and / or temperature of the resulting aqueous solution. After the manganiferous mineral has dissolved in the hydrochloric acid, the hydrometallurgical technique then comprises adding sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an alkaline aqueous solution and a precipitate comprising manganese (II) hydroxide. In the case of sodium oxide, this reaction proceeds according to the following equations, each respectively corresponding to one of Eqns. 6a to 6c': Na2O (sj + MnCI2(aq) + 2 H2O (<j -> Mn(OH)2 (Sj + 2 NaCI (aqj + H2O (<j [Eqn. 7a] Na2O (sj + MnCI2(aq) + 2 H2O (<j -> Mn(OH)2 (Sj + 2 NaCI (aq) + H2O (<j [Eqn. 7b] Na2O (sj + MnCI2(aq) + 10 / 7 H2O (<j -> Mn(OH)2 (Sj + 2 NaCI (aq) + 3 / 7 H2O (<j [Eqn. 7c] If sodium hydroxide is added to the acidic aqueous solution instead, Eqns. 7a to 7c all reduce to: 2 NaOH (s) + MnCI2 (aq) -» Mn(OH)2 (s) + 2 NaCI (aq) [Eqn. 7d] For example, at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, which is added to the acidic aqueous solution may be recycled from the sodium oxide produced by the redox reaction between the liquid sodium and the trivalent manganese oxide. The initial concentration of the hydrochloric acid to which the ore is added may have an upper limit which is chosen to ensure that the final concentration of sodium chloride in the alkaline aqueous solution formed by any of Eqns. 7a to 7d is not more than about 6 M. If an aqueous solution of sodium hydroxide is added to the acidic aqueous solution formed by adding the ore to the hydrochloric acid, then the initial concentration of this aqueous solution of sodium hydroxide should preferably also be chosen to ensure that the final concentration of sodium chloride in the alkaline aqueous solution thus formed is not more than about 6 M either. This is because, in either case, an aqueous solution of sodium chloride reaches saturation at a molarity not much above 6 M. For example, the saturation molarity of sodium chloride in water at 25 ’Celsius is 6.14 M, which rises to about 6.5 M at 70 ’Celsius. Contamination of the precipitate comprising manganese (II) hydroxide by solid sodium chloride can thereby be avoided. Precipitation of manganese (II) hydroxide commences at a pH above about 8.75. If the alkaline aqueous solution also comprises iron (II) ions, iron (II) hydroxide coprecipitates with the manganese (II) hydroxide. At least some of the precipitate comprising the manganese (II) hydroxide is then phase-separated from this alkaline aqueous solution, and at least some of the separated precipitate is dried and dehydroxylated to produce the trivalent manganese oxide. The phase-separation may be performed by at least one of settlement under gravity, filtration and centrifugation, for example. Drying and dehydroxylating the manganese (II) hydroxide in the precipitate does not require the application of heat and can instead occur spontaneously in atmospheric air. Preferably, the manganese (II) hydroxide is dried and dehydroxylated in dry air. The rate of dehydration and dehydroxylation may be increased and their thermal efficiency may be improved by passing sodium oxide (NajO), which is produced by the subsequent reaction of the trivalent manganese oxide with the liquid sodium, or sodium hydroxide derived from hydrating at least some of this sodium oxide, through an atmosphere to which the manganese (II) hydroxide is exposed as it dries and dehydroxylates. Sodium oxide is powerfully hygroscopic and therefore will absorb water vapour from this atmosphere to produce (solid-phase) sodium hydroxide. At atmospheric pressure and at temperatures above about 65 ’Celsius, this sodium hydroxide is anhydrous, which itself is also strongly hygroscopic. This reduces the partial pressure of water vapour within the atmosphere to which the manganese (II) hydroxide is exposed, thereby increasing the rate of vaporization of water from the manganese (II) hydroxide. The trivalent manganese oxide thus formed is y-MnjOa, which does not occur naturally as a mineral and may only be produced artificially, as in the present case. The pyrometallurgical technique, in contrast, if applied to p-MnOj (pyrolusite) produces a-MnjOa, which occurs naturally as the mineral bixbyite. Potassium and barium cations derived from manganiferous minerals like hollandite and cryptomelane remain in solution. Potassium cations may subsequently be removed using a known technique, such as that described in PCT patent publication no. WO 2008 / 106741. However, barium cations need not be removed if a supernatant remaining after the phase-separation is subsequently to be used as feed material for producing liquid sodium and gaseous chlorine by fusing and electrolysing solid sodium chloride because they help to lower the melting point of the solid sodium chloride. As may be seen from Eqn. 6c (and Eqn. 6c'), if the ore comprises a siliceous manganese mineral such as braunite, adding the ore to the hydrochloric acid produces a solid silica residue in addition to the acidic aqueous solution and gaseous chlorine which are produced. This is because silica is insoluble in aqueous solution under acidic conditions and remains so up to a pH above that at which the precipitation of manganese (II) hydroxide commences ( / .e, a pH >8.75 approx.). Therefore, in order to avoid the precipitate comprising the manganese (II) hydroxide from being contaminated with silica, for example if the ore comprises a siliceous manganese mineral and / or another silicate mineral as gangue, the hydrometallurgical technique may optionally comprise, after the ore has been added to the hydrochloric acid to create the acidic aqueous solution, but before adding sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution, phase-separating undissolved solids, which will therefore include this solid silica residue, from the acidic aqueous solution. This phase-separation may be carried out by at least one of settlement under gravity, filtration and centrifugation, for example. The silica in the undissolved solids phase-separated from the acidic aqueous solution is sufficiently reactive that if desired, these undissolved solids may subsequently be used, for example, as at least one of an ingredient with an alkaline activator in the manufacture of an alkaline-activated or geopolymer cement, and a pozzolan in a reaction with at least one of calcium oxide and magnesium oxide to produce a hydraulic cement. Moreover, if the manganese ore subjected to the hydrometallurgical technique comprises an aluminate mineral, the hydrometallurgical technique may optionally comprise, after phase-separating the undissolved solids from the acidic aqueous solution, but before producing the precipitate comprising the manganese (II) hydroxide, adding sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an aqueous solution with a pH of in a range of from about 5 to about 8.5, inclusive, and a precipitate comprising aluminium hydroxide, phase-separating this precipitate comprising aluminium hydroxide from the aqueous solution with the stated pH, and thereafter, continuing to add the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution with the pH in this range to produce the alkaline aqueous solution and the precipitate comprising manganese (II) hydroxide. This process may best be understood by reference to Fig. 2, which shows the solubility curves of silica, aluminium hydroxide and manganese (II) hydroxide in mol (H2O) plotted on a logarithmic scale on the y-axis or ordinate against pH plotted on a linear scale on the x-axis or abscissa. As may be seen from Fig. 2, at low pH, in the region of the graph labelled "A", silica is insoluble, whereas aluminium hydroxide and manganese (II) hydroxide are both sufficiently soluble to remain in solution. Silica may therefore be phase-separated from this acidic aqueous solution as described above. As sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is added to this acidic aqueous solution, the pH of the solution increases, passing into the region of the graph labelled "B". Here, the solubility of aluminium hydroxide drops, causing its precipitation at a pH in a range of from about 5 to about 8.5, whilst the manganese (II) hydroxide is still sufficiently soluble to remain in solution. The aluminium hydroxide may therefore be phase-separated from the aqueous solution with pH in this stated range. This phase-separation may be carried out by at least one of settlement under gravity, filtration and centrifugation, for example. Then, as more sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is added to the aqueous solution with pH in the stated range, the pH of the solution continues to increase and passes into the region of the graph labelled "C". Here, the solubility of the manganese (II) hydroxide finally becomes sufficiently low to precipitate out as well, and may therefore be phase-separated from this alkaline aqueous solution as described above. Thus silica and aluminium hydroxide, which are respectively derived from silicate and aluminate species in the ore, may each be separately eliminated from the precipitate comprising manganese (II) hydroxide using this process of sequential precipitation and phase separation, whereby contamination of the trivalent manganese oxide by either silica or alumina may be avoided. If desired, at least some of the precipitate comprising aluminium hydroxide may then be dried and dehydroxylated to produce alumina using a known technique, and the alumina thus produced may be used as feed material for producing aluminium by electrolysis. After the precipitate comprising manganese (II) hydroxide has been phase-separated from the alkaline aqueous solution, the supernatant remaining after this phase-separation essentially comprises an aqueous solution of sodium chloride with an excess of sodium hydroxide. In order to recover this sodium chloride, therefore, the hydrometallurgical technique may optionally further comprise after phase-separating at least some of this precipitate from the alkaline aqueous solution, adding further hydrochloric acid to the alkaline aqueous solution to neutralize the excess sodium hydroxide and produce an aqueous solution of sodium chloride with a pH of about 7. At least some of this aqueous solution of sodium chloride may then be used to produce liquid sodium and gaseous chlorine by electrolysis as described above. At least some of the liquid sodium thus produced may then be recycled to the redox reaction between the liquid sodium and the trivalent manganese oxide. Alternatively or additionally, at least some of the gaseous chlorine thus produced may be used to produce the hydrochloric acid in which the ore is dissolved initially. If so, the aqueous solution of sodium chloride may be used to produce liquid sodium and gaseous chlorine by a combination of the chlor-alkali and Castner processes as described above. If, however, the aqueous solution of sodium chloride is used to produce liquid sodium and gaseous chlorine by the Downs process, at least some of the aqueous solution of sodium chloride may be dried to produce water vapour, leaving solid sodium chloride for use in the Downs process, and at least some of the water vapour thus produced may be captured, after which at least some of the captured water vapour may be condensed to produce liquid water, which may then be used to produce the hydrochloric acid. Furthermore, at least some of the gaseous chlorine captured when the ore is initially added to the hydrochloric acid may also be used to produce hydrochloric acid. For example, at least some of the captured gaseous chlorine may be combusted with gaseous hydrogen to produce gaseous hydrogen chloride according to a respective one of the following equations: Ck (g) + H2 (gj -> 2 HCI (g; ½ Cl2 (g) + ½ H2 (g) -> HCI (g) 3 / 7 Cl2(g) + 3 / 7 H2 (g) 6 / 7 HCI (g) [Eqn. 8a] [Eqn. 8b] [Eqn. 8c] In each case, the gaseous hydrogen for this may be obtained via at least one of the chlor-alkali and Castner processes from the aqueous solution of sodium chloride which remains after the phaseseparation of the precipitate comprising manganese (II) hydroxide. At least some of the hydrogen chloride thus produced may then be contacted with liquid water to dissolve the hydrogen chloride therein, thereby producing the hydrochloric acid. The liquid water for this may also be recovered from the same aqueous solution of sodium chloride. For example, the liquid water may be obtained by capturing and condensing water vapour produced by drying an aqueous solution of sodium hydroxide produced from the aqueous solution of sodium chloride via the chlor-alkali process of Eqn. 4a before then fusing and electrolysing the resulting solid sodium hydroxide via the Castner process of Eqn. 4b. If the total number of mols of hydrogen chloride produced on the right-hand side of each of Eqns. 8a to 8c is used to make hydrochloric acid, then the hydrochloric acid thus produced may be added to 2 mols of hydrochloric acid per mol of Mn extracted from the ore to provide the total number of mols of hydrochloric acid required by the left-hand side of each respective one of Eqns. 6a to 6c', as follows: 2 HCI (aq) + 2 HCI (aq) 4 HCI (aq) HCI (aq) + 2 HCI (aq) 3 HCI (aq) 6 / 7 HCI (aq) + 2 HCI (aq) 20 / 7 HCI (aq) [Eqn. 9a] [Eqn. 9b] [Eqn. 9c] The additional 2 mols of hydrochloric acid per mol of Mn in each of Eqns. 9a to 9c may be obtained by recycling the 2 mols of aqueous solution of sodium chloride per mol of Mn remaining in the supernatant, as described above. Thus in each case, subject to real-world losses and inefficiencies, the stoichiometric ratio of hydrochloric acid to trivalent manganese oxide may be preserved and no waste or co-products are produced, except in two cases, as follows. The first is in the case of hydrous manganese compounds and / or manganese oxyhydroxides, where an excess amount of water is produced, which ultimately derives from hydroxyl groups in these manganiferous compounds. The second is in the case of siliceous manganese compounds and / or manganese silicates, where silica is also produced, which ultimately derives from the silica content of these siliceous manganese compounds and / or manganese silicates. These two exceptions are respectively exemplified by the ½ mol of water per mol of Mn remaining on the right-hand side of Eqn. 7b after ½ mol of water from the right-hand side of Eqn. 7b has been used to make ½ mol of hydrogen on the left-hand side of Eqn. 8b in the case of (b) manganite, and by the 1 / 7 mol of silica per mol of Mn remaining on the right-hand side of Eqn. 6c' in the case of (c) braunite. In contrast, as may be seen from the sequence of Eqns. 6a to 9a, no waste or co-products are produced in the case of (a) pyrolusite. Thus it may be seen that the hydrochloric acid to which the ore is initially added in the hydrometallurgical technique and the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, which is added to the acidic aqueous solution thus produced, may both be derived from other parts of the method of the invention and / or recycled. Optional Magnetic Separation of Para- and Diamagnetic Species During and / or after the manganiferous mineral in the ore has been converted into a trivalent manganese oxide using at least one of the pyrometallurgical technique and the hydrometallurgical technique, the method of the invention may optionally comprise magnetically separating a paramagnetic manganese compound derived from the ore, such as the trivalent manganese oxide, from one or more diamagnetic components of the ore, which are typically gangue species, such as silica and alumina. A magnetic separation technique which may be applied to the trivalent manganese oxide produced by the pyrometallurgical technique will be described first. This first magnetic separation technique comprises, after heat-treating the ore and before adding the trivalent manganese oxide to the liquid sodium, cooling the heat-treated ore back down to less than 100 ’Celsius, and then magnetically separating paramagnetic components of the ore at least comprising the trivalent manganese oxide from diamagnetic components of the ore. Heat-treating the manganese ore has two main effects. Firstly, it prepares the ore for the subsequent reaction of the trivalent manganese oxide with the liquid sodium by dehydrating and dehydroxylating any hydrous manganese compounds and / or manganese oxyhydroxides in the ore, preferably completely, and at least to a high degree. Secondly, however, it also allows easier magnetic separation of paramagnetic components of the ore from diamagnetic components thereof by increasing the proportion of trivalent manganese oxide present in the ore as manganese (III) oxide, manganese (11,111) oxide, or as a mixture of both. However, magnetic separation of the paramagnetic components of the ore from the diamagnetic components thereof can only be carried out at a temperature of less than about 100 ’Celsius because the magnetic susceptibilities of the paramagnetic components are significantly reduced at higher temperatures. This also applies to any iron (III) oxide which may be mixed in with the paramagnetic components of the ore because iron (III) oxide is a canted antiferromagnet. After heat-treating the ore, therefore, the heat-treated ore is cooled back down to less than about 100 ’Celsius, preferably less than about 75 ’Celsius, more preferably less than about 50 ’Celsius, and most preferably back down to ambient temperature. The temperature to which the ore should be cooled after its heat-treatment can be determined from the desired degree of separation of the paramagnetic components of the ore from the diamagnetic components of the ore. The time taken to cool the comminuted ore particles back down will be determined not only by their temperature immediately after heat-treatment, but also by the temperature gradient to which they are exposed in order to reach the target temperature for the ore particles before the magnetic separation. Because of the large surface area-to-volume ratio of the ore particles after comminution, the ore particles can be cooled relatively quickly to this desired temperature, just by being exposed to the ambient temperature of the environment. However, in order that this heat is not wasted, it is preferable to include a counterflow system within a dryer of the ore, whereby heat-treated ore particles leaving the dryer are brought into thermal, but not physical, contact with ore particles entering the dryer at or near to ambient temperature, such that heat is exchanged from the heat-treated ore to the ore entering the dryer. An example of such a counterflow system will be described below. Table 1 below gives the molar magnetic susceptibilities, Xmoicgs, at s.t.p. of some manganese oxides, of manganese (II) hydroxide and of elemental manganese, as well as those of all the other main species, which, depending on the initial composition of the ore, may also be present in the ore after its heattreatment: Substance Magnetic property Molar magnetic susceptibility, Xmoicgs / 106 cm3 mol1 Manganese (IV) oxide (MnO2) Paramagnetic + 2280 Manganese (III) oxide (Mn2O3) Paramagnetic +14100 Manganese (11,111) oxide (Mn3O4) Paramagnetic +12400 Manganese (II) oxide (MnO) Paramagnetic + 4850 Manganese (II) hydroxide (Mn(OH)2) Paramagnetic +13500 Manganese (Mn) Paramagnetic + 511 Iron (III) oxide (Fe2O3) Above 260 K &below 950 K Canted antiferromagnet + 3586 Above 950 K Paramagnetic > 0 (no data) Calcium carbonate (CaCO3) Diamagnetic -38.2 Magnesia (MgO) Diamagnetic -10.2 Baria (BaO) Diamagnetic -29.1 Silica (SiO2) Diamagnetic -29.6 Alumina (AI2O3) Diamagnetic -37 Table 1 Hollandite and other manganese oxides such as cryptomelane which are isostructural to hollandite, are also paramagnetic, with magnetic susceptibilities in a range of from about + 2700 to about + 3300. Precise values cannot be given for their magnetic susceptibilities, which depend on the occupancy rates of the 2x2 tunnels of MnOg octahedra by cations. Similarly, precise values cannot be given for the magnetic susceptibilities of siliceous manganese minerals like braunite and rhodonite because these depend on the level of substitution of manganese cations in their crystal structures by such other cations as silicon and calcium. However, their magnetic susceptibilities are comparable to that of hollandite. As Table 1 shows, all these manganese oxides are paramagnetic, whereas all the gangue species calcium carbonate, magnesia, baria, silica and alumina are diamagnetic. As may also be seen, however, the magnetic susceptibilities of manganese (III) oxide and of manganese (11,111) oxide, both of which contain trivalent manganese, as well as that of manganese (II) hydroxide, are significantly higher than those of manganese (IV) oxide, manganese (II) oxide and elemental manganese. 260 K is the Morin transition temperature and 950 K is the Neel temperature of iron (III) oxide (FejOa). Above the Morin transition temperature and below its Neel temperature, FejOa is a canted antiferromagnet with a magnetic susceptibility at s.t.p. between those of manganese (IV) oxide and manganese (II) oxide. Thus after heat-treating the ore, the strongly paramagnetic manganese (III) oxide and / or manganese (11,111) oxide therein can easily be magnetically separated from the diamagnetic gangue species like calcium carbonate, magnesia, baria, silica and alumina by bringing the heat-treated ore into proximity with a region of high magnetic field. The high degree of separation which can be achieved is significantly better than is typically obtained, for example, during the beneficiation of iron ore by magnetically separating iron (III) oxide in the iron ore from diamagnetic gangue species also contained therein because the magnetic susceptibility of iron (III) oxide is lower than that of the manganese oxides which contain trivalent manganese. Nonetheless, if any iron (III) oxide is present in the manganese ore after its heat-treatment, it will still tend to be separated from the diamagnetic gangue species in the ore, together with the more strongly paramagnetic manganese (III) oxide and / or manganese (11,111) oxide because it is a canted antiferromagnet. However, the degree of separation achieved may be lower than for these trivalent manganese oxide(s) because of iron (III) oxide's lower magnetic susceptibility. Thus the manganese / iron ratio of the paramagnetic components of the ore after magnetic separation may also be improved relative to the manganese / iron ratio of the ore before its magnetic separation. Moreover, if after the magnetic separation, some iron (III) oxide remains mixed in with the paramagnetic components of the manganese ore, the iron and manganese therein may still be separated from each other later in a manner to be described further below, in order to leave a substantially iron-free form of elemental manganese. Table 2 below gives the densities of all the main species, which, depending on the initial composition of the manganese ore, may be present in the ore after its heat-treatment. Precise values cannot be given for the densities of hollandite and of other manganese oxides like cryptomelane which are isostructural to hollandite, because these depend on the occupancy rates of the 2x2 tunnels of MnOg octahedra by cations. Similarly, precise values cannot be given for the densities of siliceous manganese minerals like braunite because these depend on the level of substitution of manganese cations by silicon cations in their crystal structures. However, they all have densities which are similar to that of manganese (III) oxide: Substance Density / gem'3 Manganese (III) oxide (MnjOa) 4.50 Manganese (11,111) oxide (Mn3O4) 4.86 Iron (III) oxide (FejOa) 5.25 Calcium carbonate (CaCOs) 2.71 Magnesia (MgO) 3.58 Baria (BaO) 5.72 Silica (SiOz) 2.65 Alumina (AI2O3) 3.99 Table 2 As Table 2 shows, all of these manganese oxides, as well as iron (III) oxide and baria, are significantly denser than all the other gangue species calcium carbonate, magnesia, silica and alumina. Thus the magnetic separation of the paramagnetic components of the ore from the diamagnetic components of the ore may be enhanced, and the speed and efficiency of their separation may be improved, by combining this magnetic separation with a density-based separation. This will also tend to increase the proportion of iron (III) oxide separated from the diamagnetic components of the ore along with the paramagnetic components of the ore, although if any baria is present, it will tend to reduce the proportion of baria separated from the paramagnetic components of the ore as well. However, this is unproblematic for reasons explained below. Moreover, magnetic and / or density-based separation of the same process stream may be repeated, in order to increase the degree of separation finally achieved. If any hollandite and / or other manganese oxides like cryptomelane which are isostructural to hollandite remain in the manganese ore after its heat-treatment, these will tend to behave like iron (III) oxide during the magnetic and / or density-based separation, because of their similar magnetic susceptibilities and densities, as will any siliceous manganese minerals like braunite and rhodonite. A second magnetic separation technique which may instead be applied to the manganese (II) hydroxide produced during the hydrometallurgical technique will now be described. As may be seen from Table 1, the magnetic susceptibility of manganese (II) hydroxide is similar to that of both the trivalent manganese oxides and significantly higher than those of manganese (IV) oxide, manganese (II) oxide and elemental manganese. Accordingly, the method of the invention may optionally comprise magnetically separating paramagnetic components of the precipitate produced from the alkaline aqueous solution during the hydrometallurgical technique and which at least comprises manganese (II) hydroxide from diamagnetic components of this precipitate. This magnetic separation may be performed by applying a magnetic field gradient to at least one of the alkaline aqueous solution with the precipitate still suspended or entrained therein, and the precipitate after it has been phase-separated from the alkaline aqueous solution. Alternatively or additionally, after this precipitate has been dried and dehydroxylated to produce y-MnjOa as the trivalent manganese oxide, the method of the invention may optionally comprise magnetically separating paramagnetic components of the precipitate at least comprising this trivalent manganese oxide from diamagnetic components of the dried and dehydroxylated precipitate. This is similar to the first magnetic separation technique described above which may be applied to the trivalent manganese oxide(s) produced using the pyrometallurgical technique, except that in this case, there is no need for the dried and dehydroxylated precipitate to be cooled first, because it is already at or near to ambient temperature. However, just as in the first magnetic separation technique described above which may be applied to the trivalent manganese oxide(s) produced using the pyrometallurgical technique, the paramagnetic components of the dried and dehydroxylated precipitate may also be magnetically separated from the diamagnetic components thereof based on their different densities. Reaction with Liquid Sodium After the manganiferous mineral in the ore has been converted into a trivalent manganese oxide, as well as after any possible magnetic and / or density-based separation, the trivalent manganese oxide is added to an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide. This may be done, for example, by introducing the trivalent manganese oxide into a bath of liquid sodium. As a result of the prior comminution of the ore and the conversion of a manganiferous mineral therein into a trivalent manganese oxide, as well as any possible magnetic and / or density-based separation, what is added to the liquid sodium consists chiefly of one or more trivalent manganese oxide(s), particularly bixbyite and / or hausmannite. However, depending on the initial chemical composition of the ore and the details of the conversion technique(s) which have been used, these trivalent manganese oxides may be accompanied by lesser amounts of one or more other manganese minerals. For example, if only the pyrometallurgical technique has been used, what is added to the liquid sodium may comprise manganese minerals with crystal structures comprising tunnels, like hollandite and cryptomelane, and siliceous manganese minerals like braunite. What is added to the liquid sodium may also comprise one or more trivalent iron oxides, such as hematite and magnetite, which may have been present in the ore originally and / or as a result, for example, of mixing the manganese ore with iron ore before the reaction with the liquid sodium. These different manganese and iron oxides are all reduced by the liquid sodium, as follows. The trivalent manganese oxides bixbyite and hausmannite are reduced by the liquid sodium to produce elemental manganese and sodium oxide (NajO). In the case of bixbyite, this reaction proceeds according to the equation: 6 Na ( / ) + MnjOa (S) 3 NajO (S) + 2 Mn (S)— 301 kJ [Eqn. 10] The reduction of bixbyite may initially occur by the same sequence as in Eqn. 1 above, but continues until elemental manganese is produced and the end-result is as represented by the righthand side of Eqn. 10. The reduction of hausmannite proceeds in a similar fashion until elemental manganese is produced as well. In other words, the initial oxidation state of these trivalent manganese oxide(s) is immaterial to the eventual outcome of the reaction. For manganese minerals with crystal structures comprising tunnels, like hollandite and cryptomelane, the liquid sodium attacks the MnOg octahedra, which are reduced to produce elemental manganese and NajO, in a similar fashion to the reaction of Eqn. 10. This process also releases the cations of group I and II metals, like potassium and barium, contained within the tunnels. If any of the manganese minerals comprises a minority of iron cations substituted for manganese cations, the reaction of the liquid sodium with such manganese minerals results in the production of elemental iron in addition to elemental manganese and sodium oxide. Any iron oxide(s) present in what is added to the liquid sodium as well are also reduced by the liquid sodium to produce elemental iron and sodium oxide. In the case of hematite, this reaction proceeds according to the equation: 6 Na ( / ) + FejOa (S) 3 NajO (S) + 2 Fe (S) — 438 kJ [Eqn. 11] The reduction of other iron oxides proceeds in a similar fashion until elemental iron is produced as well. In other words, the initial oxidation state of any iron oxide(s) present in what is added to the liquid sodium is immaterial to the eventual outcome of this reaction. If the weight ratio of manganese to iron is at least 2:1, the iron does not form a binary phase with the manganese, which remains as a-manganese from ambient temperatures up to about 707 ’Celsius. Since the solubility of manganese in liquid sodium is only about 0.01% by weight and the solubility of iron in liquid sodium is only a few ppm by weight, virtually all the elemental manganese, as well as any iron, produced by the reactions of Eqns. 10 and 11 remains in solid phase in the liquid sodium. The solubility of sodium oxide in liquid sodium is low but increases with temperature. At temperatures below about 400 ’Celsius, however, the solubility of sodium oxide in liquid sodium is less than about 0.1% by weight of oxygen. Virtually all the sodium oxide produced by these two reactions therefore remains in solid phase as well, leaving only a trace amount dissolved in the excess liquid sodium. (The exact amount remaining depends on the temperature at which the reactions of Eqns. 10 and 11 are carried out and on how much liquid sodium is present.) The insolubility of all the reaction products of Eqns. 10 and 11 in liquid sodium ensures that both these reactions are one-way. For any siliceous manganese minerals, such as braunite and / or rhodonite, which may remain in what is added to the liquid sodium, the liquid sodium also attacks the crystal structure of these minerals, which are reduced to produce elemental manganese, sodium oxide and silica (SiOz). For minerals like rhodonite, in which a minority of the manganese cations are substituted by calcium cations, this process also releases the calcium cations. At temperatures of from about 320 to about 350 ’Celsius, the silica thus produced, as well as any silica which may remain as gangue in what is added to the liquid sodium, can react in a Lux-Flood acid-base neutralization reaction with sodium oxide, such as that produced by the reactions of Eqns. 10 and 11, to produce sodium orthosilicate (Na4SiO4), as well as possibly also sodium metasilicate (NajSiOs), according to the equations: 2 NajO (S) + SiOz (Sj -> Na4SiO4 (S> [Eqn. 12a] NajO (sj + SiOz (s) Na2SiO3 (s) [Eqn. 12b] Sodium orthosilicate is produced as the only product of these two reactions if the molar ratio of Na2O to SiO2 exceeds 2:1, whereas a mixture of sodium orthosilicate and sodium metasilicate is produced at lower molar ratios than this. In either case, sodium ortho- and metasilicate are both soluble in liquid sodium upto a saturation concentration of at least about 5%, which increases with temperature, and therefore readily dissolve in the liquid sodium. A disproportionation reaction between the liquid sodium and silica to produce sodium metasilicate and elemental silicon according to the equation: 4 Na ( / ) + 3 SiO2 (s) 2 Na2SiO3 (S) + Si (S) [Eqn. 13] can also occur at higher temperatures of from about 520 to about 550 ’Celsius. However, the activation energy for the reaction of Eqn. 13 is also significantly higher than for the reactions of Eqns. 12a and 12b, and any elemental silicon thus produced will also tend to contribute to the reduction of the manganese oxide(s) and therefore be oxidized back to SiO2, leaving sodium orthosilicate, as well as possibly sodium metasilicate, as the only remaining siliceous reaction products dissolved in the liquid sodium. Cations of group I and II metals like potassium and barium, which are released by the reduction of manganese minerals isostructural to hollandite, and calcium cations released by the reduction of siliceous manganese minerals like rhodonite, behave as follows. The potassium cations are reduced by the liquid sodium to elemental potassium because the equilibrium oxide in the sodium-potassium system is sodium oxide (Na?O). The potassium metal thus produced is completely miscible with liquid sodium, so no potassium oxide is produced as a solid phase and the elemental potassium instead remains in solution with the liquid sodium as sodium-potassium alloy (NaK). In contrast, barium and calcium cations scavenge oxygen anions from the sodium oxide to produce baria (BaO) and calcia (CaO), respectively, which, like sodium oxide, both have very low solubilities in liquid sodium, so remain in solid phase. Thus after all these reactions, the reaction mixture comprises the excess amount of unreacted liquid sodium (possibly with sodium silicate(s) and a minor amount of potassium dissolved therein) and a solid phase, which is insoluble in the liquid sodium. This solid phase may at least partially precipitate out from the liquid sodium and / or be at least partially suspended in it as a colloid. The solid phase comprises elemental manganese and other insoluble products at least comprising sodium oxide. Depending on the initial chemical composition of the ore and on the temperature at which the redox reaction between the liquid sodium and the trivalent manganese oxide(s) has been carried out, the other insoluble products may further comprise one or more of elemental iron, baria, calcia and silica. In general, the redox reaction between the liquid sodium and the trivalent manganese oxide(s) can be carried out at any temperature of from about 390 K up to about 870 K. Below about 390 K, the liquid sodium risks freezing at the melting point of sodium, which at atmospheric pressure, is 98 ’Celsius (= 371 K). Above about 870 K, reacting the liquid sodium with the trivalent manganese oxide(s) risks producing ternary oxides like a-NaMnO? as the Na - Na?O - Mn system transitions to a Na -NaMnOj - Mn system. Such ternary oxides are generally soluble in liquid sodium, which both contaminates the liquid sodium and reduces the yield of elemental manganese. However, even at temperatures below this upper limit, there is still a possibility of the highly reactive sodium oxide from at least the reaction of Eqn. 10 producing small amounts of other compounds by undesirable side reactions with contaminants. The temperature of the reaction should therefore preferably be controlled to remain below about 820 K, more preferably below about 770 K, and most preferably below about 720 K. In particular, if the manganese compound(s) comprise a minority of iron cations substituted for manganese cations and / or if what is added to the liquid sodium also comprises one or more iron oxide(s), which may have been present in the ore originally or as a result of mixing the manganese ore with iron ore before the reaction with the liquid sodium, then above about 720 K, a reaction between the liquid sodium and such iron oxide(s) also risks producing ternary oxides like Na4FeO3, as the Na - Na?O - Fe system transitions to a Na - Na4FeO3 - Fe system. Therefore, if the manganese ore initially comprises, for example, at least about 4% by weight and in some cases, more than about 8% or even about 12% by weight, of iron oxide(s), and / or if the manganese ore has been mixed with iron ore before the redox reaction between the liquid sodium and the trivalent manganese oxide, then the temperature of this reaction should preferably be controlled to remain below about 720 K, more preferably below about 700 K, and most preferably below about 680 K. Moreover, in some embodiments, wherein what is added to the liquid sodium comprises at least about 4% by weight and in some cases, more than about 8% or even about 12% by weight, of at least one of a siliceous manganese mineral and silica as gangue, the redox reaction may be conducted at a temperature of at least about 320 ’Celsius, more preferably at least about 340 ’Celsius, and most preferably at least about 360 ’Celsius, in order to induce a reaction between the sodium oxide and silica derived from at least one of the siliceous manganese mineral and the gangue to produce at least sodium orthosilicate. In alternative embodiments, wherein what is added to the liquid sodium comprises at least about 4% by weight and in some cases, more than about 8% or even about 12% by weight, of at least one of a siliceous manganese mineral and silica as gangue, the redox reaction may instead be conducted at a temperature of less than about 300 ’Celsius, more preferably less than about 290 ’Celsius, and most preferably less than about 280 ’Celsius, to inhibit a reaction between the sodium oxide and the silica. In this way, it is possible to choose whether silica is removed from the other insoluble products by dissolving in the liquid sodium as sodium orthosilicate produced by the reaction of Eqn. 12a, as well as possibly also as sodium metasilicate produced by the reactions of Eqns. 12b and / or 13, or whether undissolved silica instead precipitates out along with the manganese and undissolved sodium oxide produced by the redox reaction of Eqn. 10. In the latter case, if the manganese is subsequently to be used as an alloying element (for example, with iron in the production of austenitic manganese and / or stainless steels), this silica may still be transferred to a slag phase during the alloying process. This may be considered advantageous because it allows the reaction of Eqn. 10 to be conducted at a lower temperature, thereby saving energy, and also avoids contamination of the liquid sodium with dissolved sodium silicate(s). Subject to the constraints imposed by the chemical composition of what is added to the liquid sodium and the above choice of reaction temperature, the reaction of Eqn. 10 is also preferably carried out at or above the middle of the range of available operating temperatures, to increase the rate of reaction. The rate of reaction may also be increased by stirring or otherwise mixing what is added to the liquid sodium into the liquid sodium. Regardless of the peak temperature reached by the reaction between the liquid sodium and the trivalent manganese oxide(s), it is also preferable that the resulting reaction mixture should be cooled thereafter and allowed to dwell at a temperature of less than about 500 K, more preferably less than about 450 K, and most preferably less than about 400 K, before at least some of the solid phase is separated from the liquid sodium. This is to ensure complete reduction by the liquid sodium of any manganese (II) oxide (MnO) produced by the sequence of Eqn. 1 into elemental manganese, because the Gibbs free energy of such a reaction, AG >Oat temperatures above about 505 K. The length of time that the reaction mixture should be allowed to dwell at this lower temperature can be determined by monitoring the temperature of the reaction mixture after it has been cooled and noting that when the temperature of the reaction mixture ceases to rise, the exothermic reduction of MnO by the liquid sodium is complete. To ensure that all the trivalent manganese oxide(s) from the ore are consumed, the amount of liquid sodium present should be in excess of the stoichiometric amount thereof required for the redox reaction between the liquid sodium and the trivalent manganese oxide(s). This excess amount of liquid sodium also absorbs some of the heat generated by the reaction of Eqn. 10, as well as possibly by the reaction of Eqn. 11. The excess amount of liquid sodium to use can therefore be determined from the desired temperature profile of these reaction(s), as described above. As may be seen from Eqn. 10, the reduction of the trivalent manganese oxide(s) by liquid sodium is exothermic, but not violently so. For example, if all the trivalent manganese oxide(s) are present as bixbyite, this reaction produces about -50 kJ of heat per mol of liquid sodium consumed. For comparison, the reaction of elemental sodium with oxygen in atmospheric air produces about -208 kJ of heat per mol of sodium consumed. The reduction of the trivalent manganese oxide(s) by the liquid sodium is therefore about 4 times less exothermic than the reaction of sodium with atmospheric oxygen, although as Eqn. 11 shows, if what is added to the liquid sodium also comprise iron oxide(s), the redox reaction becomes more exothermic. Since the reaction of Eqn. 10 is exothermic, the temperature of the reaction mixture will rise from its initial temperature when the comminuted and converted ore is introduced to the liquid sodium. The temperature of the reaction should therefore be controlled to remain within the desired range of operating temperatures by appropriate cooling, not just of the reagents before they are introduced to each other, but also of the reaction itself, as will now be described. As mentioned above, the liquid sodium may be produced by electrolysing fused sodium chloride in a Downs-type electrolytic cell. Since such an electrolytic cell operates at a temperature of about 600 to 625 ’Celsius, the liquid sodium thus produced typically leaves the cell at a similar temperature. Therefore, if the liquid sodium for the redox reaction is produced close to the reaction of Eqn. 10, the liquid sodium must firstly be cooled before the comminuted and converted ore is introduced to the liquid sodium, to prevent the reaction of Eqn. 10 from overheating. Preferably, the liquid sodium is cooled to a temperature of less than about 250 ’Celsius, and more preferably, less than about 200 ’Celsius. Liquid sodium has a molar heat capacity at constant pressure, Cp, of 31.5 J K1 mol1 at 400 K, and Eqn. 10 shows that 6 mol of liquid sodium are required to reduce 1 mol of MnjOa to produce 2 mol of manganese. For example, therefore, to cool 6 mol of liquid sodium from about 600 ’Celsius to about 175 ’Celsius, to a first-order approximation which assumes that the molar heat capacity of liquid sodium is constant over the stated temperature difference, AT, the quantity of heat, Q, which must be extracted from the liquid sodium is given by: Q = n(Naw) Cp(Naw) AT = 6 x 31.5 x (175 - 600) = - 80.3 kJ [Eqn. 14] where n denotes the number of mol of liquid sodium. For example, however, the thermal decomposition of MnOj into MnjOa and oxygen according to Eqn. 5b is an endothermic reaction requiring + 81.3 kJ. Therefore, approximately all the heat required for this thermal decomposition can be supplied by cooling the liquid sodium as described. The subsequent reaction of the trivalent manganese oxide(s) with the liquid sodium may, for example, be cooled by conducting the reaction in a reaction vessel surrounded by a heat exchanger in which a heat transfer fluid (HTF) circulates to extract heat from the reaction. Preferably, however, the reaction is cooled by maintaining a continuous flow of the excess liquid sodium through the reaction mixture, which can itself therefore be used as an HTF for extracting heat from the reaction. More preferably still, the excess liquid sodium circulates through the reaction mixture in a loop. In such a case, the liquid sodium from which at least some of the solid phase has been separated is cooled before being returned to the reaction, and the degree of cooling and / or the rate at which the liquid sodium is returned to the reaction mixture may be adjusted to keep the temperature of the reaction within its desired range of operating temperatures. Furthermore, cooling of the liquid sodium from which at least some of the solid phase has been separated may be combined with purifying the liquid sodium in a cold trap to remove dissolved contaminants remaining in the liquid sodium after removing undissolved contaminants. In some embodiments, the excess sodium may be pumped through the reaction mixture to encourage mixing of the comminuted and converted ore with the liquid sodium before the reaction products are formed. At least some of the heat extracted from the liquid sodium may, for example, then be transferred to the ore before it enters the reaction mixture, to help in heat-treating the ore. Carrying out the reaction with the liquid sodium in an inert atmosphere has the advantages of preventing the liquid sodium from reacting with atmospheric oxygen, and of inhibiting the formation of ternary oxides. For example, the inert atmosphere may consist of at least one of nitrogen and argon. Either nitrogen or argon, or both, may be produced on site by pressure swing adsorption (PSA) of atmospheric air. An on-site PSA generator of such inert gases may be powered, for example, using waste heat derived from cooling at least one of the comminuted and converted ore, fresh liquid sodium before it is introduced into the reaction mixture and the liquid sodium from which at least some of the solid phase has been separated, as described above. The inert atmosphere may be maintained at around atmospheric pressure or just above. A small positive pressurization of the inert atmosphere in which the reaction is carried out to above atmospheric pressure, for example by about 10 to 25%, is desirable to prevent ingress of air from the environment by leakage. The vapour pressure, p, in pascal of liquid sodium as a function of temperature, T, in kelvin from its melting point up to 700 ’Celsius is given by the following equation: logio p = 9.71- (5377 / T) [Eqn. 15] Table 3 below gives some representative examples of the values of this vapour pressure across the range of operating temperatures for the reaction of Eqn. 10: T / “Celsius 150 200 300 400 500 550 p / Pa 1.0 x 10-3 0.022 2.12 52.5 568 1502 Table 3 As Table 3 shows, the vapour pressure of liquid sodium over the range of operating temperatures for the reaction of Eqn. 10 is always less than about 2 kPa, or less than about 2% of standard atmospheric pressure. Thus if the reaction of Eqn. 10 is carried out at around atmospheric pressure or just above, the loss of liquid sodium from the reaction mixture by vaporization is negligible and the effect of any such vaporization on the pressure of the inert atmosphere in which the reaction is carried out is negligible as well. For safety and in order to maintain the pressure of the inert atmosphere similar to or slightly more than that of the surrounding environment, the pressure of the inert atmosphere should preferably be monitored, for example by means of a pressure gauge. To prevent any undesirable build-up of pressure, any reaction vessel in which the reaction of Eqn. 10 is carried out may be fitted with a pressure-relief valve, through which pressurised gas may be vented from the inert atmosphere to the surroundings. If so, the vented gases should preferably be cooled to below the dew point of sodium vapour before being released, to recover any sodium and to ensure that the remaining gases are harmless. On the other hand, if the pressure of the inert atmosphere above the reaction mixture needs to be increased for any reason, extra inert gas can instead be introduced into the reaction vessel along with the comminuted and converted ore. Solid Phase Separation from Liquid Sodium After the redox reaction between the liquid sodium and the trivalent manganese oxide, at least some of the solid phase is separated from the liquid sodium. Separation of the solid phase may, for example, comprise at least one of settlement under gravity, filtration (i.e., trapping) and centrifugation. For example, the present applicant's co-pending UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal"; applicant's ref: NE-P-GB 002), the entire contents of which is incorporated herein by reference, shows and describes an apparatus and method for separating undissolved contaminants, for example in the form of suspended or entrained particulates, from liquid metal, such as liquid sodium. In another example, a liquid sodium centrifuge is shown and described on pp. 29 to 33 of Summary of the APDA Sodium Technology Program by J.E. Meyers published under United States Atomic Energy Commission Contract No. AT (11-1)-865, Project Agreement No. 11 (June 1970), the entire contents of which is also incorporated herein by reference. If the phase separation comprises filtration, a filter substrate may be used which comprises, for example, a ceramic foam made, for example, of a titania- or zirconia-based ceramic, and / or a wire mesh or wool, made, for example, of one of the same materials as the inner surface of a vessel in which the redox reaction with the liquid sodium is conducted, such as grade 316 LN or 316 FR stainless steel or a titanium alloy having the composition described below. If the separated solid phase still contains manganese, in order to remove any residual liquid sodium from the separated solid phase, the degree of phase separation may be improved by flushing the separated solid phase with an inert gas which is hot enough for the liquid sodium to remain in liquid phase. Using such an inert gas prevents reoxidation of the elemental manganese and avoids the formation of more sodium oxide. If, on the other hand, the manganese has already been separated from the solid phase, the remaining insoluble products may simply be flushed with dry air, which just converts any residual liquid sodium into sodium oxide. Regardless of how the phase separation is carried out, phase separation of the same process stream may be repeated, in order to increase the degree of separation finally achieved. Liquid sodium recovered from the phase separation is preferably purified of dissolved contaminants, such as any residual sodium oxide, sodium hydride and / or sodium hydroxide dissolved therein, as well as other dissolved species, which may have been produced by the reduction of gangue, or carbon derived from stainless steel pipework used to transport the liquid sodium. The purification may be carried out using one or more know techniques, such as gettering, and hot and cold trapping. For example, gettering and / or trapping with zirconium, titanium and tantalum can be used to remove impurities. The recovered liquid sodium is then preferably recycled for reuse in the reaction of Eqn. 10, thereby maintaining the amount of excess liquid sodium used therein. If the reaction of the trivalent manganese oxide with the liquid sodium was conducted at a temperature above the onset temperature for the reactions of Eqns. 12a and 12b, then the liquid sodium may have at least sodium orthosilicate dissolved therein, as described above. If so, the liquid sodium with the sodium silicate(s) dissolved therein may be processed as described in the present applicant's co-pending UK patent application no. XXXXXXX.X ("Method and Apparatus for Producing an Alkaline Mixture comprising Sodium Silicate"; applicant's ref: NE-P-GB 009), the entire contents of which is incorporated herein by reference. Separation of Manganese from Other Insoluble Products The elemental manganese may be separated from the other insoluble products in at least one of two different ways, which will now be described. Manganese has a density, p(Mn) = 7.20 g cm'3. This is more than 3 times the density of sodium oxide (p(NajO) = 1.T1 g cm'3) and is also considerably more than the densities of any of the other minority species which might also be present after the reaction of Eqn. 10, as the densities of some of the gangue species listed in Table 2 show. Afirstway in which the elemental manganese may be separated from the other insoluble products is therefore based on their different densities. This density-based separation may be conducted before or after phase separation of the solid phase from the liquid sodium. Thus in alternative possible embodiments, either a "wet" separation of the different solid species may be carried out first, in which the manganese is separated from the other insoluble products whilst both are still suspended or entrained in liquid sodium, followed by separation of the manganese as a solid phase from the liquid sodium and separation of the other insoluble products as a solid phase from the liquid sodium in two different process streams, or separation of both the manganese and the other insoluble products as a solid phase from the liquid sodium in a single process stream may be carried out first, followed by a "dry" separation of the manganese from the other insoluble products. For example, in a "wet" density-based separation, a stream of liquid sodium with both the manganese and the other insoluble products suspended or entrained therein may be supplied to an inlet of a device functioning like a hydrocyclone (but which may instead be called a "natrocyclone"), in which denser manganese particles are directed to an underflow outlet thereof and particles of less dense other species are directed to an overflow outlet thereof. Alternatively, in a "dry" density-based separation, a stream of dry gas acting as a transport medium in which the manganese and the other insoluble products are suspended or entrained, and which may also be inert to avoid reoxidizing the manganese, may be supplied to an inlet of a cyclonic separator functioning in a similar manner to separate these different species from each other. Regardless of whether the manganese and the other insoluble products are subjected to "wet" or "dry" density-based separation from each other, this solid-species separation of the same process stream may be repeated, in order to increase the degree of separation finally achieved. A second technique for separating elemental manganese from the other insoluble products is as follows. This second technique may be used as well as or instead of a "wet" or "dry" density-based separation, but may only be carried out after phase separation of the solid phase from the liquid sodium. The second technique firstly comprises progressively adding at least some of the separated solid phase to liquid water. Since the separated solid phase may still be at an elevated temperature because of the exothermic nature of the reaction of Eqn. 10, it may firstly need to be cooled closer to ambient temperature, for example to less than about 100 ’Celsius. The elemental manganese in the separated solid phase is insoluble in the water and therefore remains undissolved. If the separated solid phase also comprises any elemental iron produced by the reaction of Eqn. 11 and / or silica derived, for example, from a siliceous manganese mineral like braunite and / or rhodonite in what is added to the liquid sodium, these are both insoluble in water as well and so remain undissolved along with the elemental manganese. In contrast, the sodium oxide in the separated solid phase readily dissolves in the water and hydrates to produce an aqueous solution of sodium hydroxide, according to the equation: NajO (sj + HjO ( / ) -> 2NaOH (aq) [Eqn. 16] Depending on the chemical composition of what is added to the liquid sodium, other species which may also be present in the separated solid phase after the reaction with the liquid sodium behave as follows. If what is added to the liquid sodium comprises any baria (BaO) and / or if any baria was formed by barium cations derived from a manganese mineral like romanechite and / or hollandite scavenging oxygen anions during the reaction with the liquid sodium, the baria also dissolves in the water and hydrates to produce an admixture of barium cations in the aqueous solution of sodium hydroxide. If the separated solid phase comprises any calcia (CaO) formed by calcium cations derived from a manganese mineral like rhodonite scavenging oxygen anions during the reaction with the liquid sodium, the calcia can also dissolve in the water to produce an admixture of calcium cations in the solution, even though calcia is only slightly soluble in water. This is for three reasons, as follows. Firstly, the proportion of any calcium cations in the separated solid phase is considerably less than the proportion of manganese cations therein. Secondly, the volume of water to which the separated solid phase is added can be made high relative to the amount of any calcia present. Thirdly, calcium hydroxide has inverse or retrograde solubility, which increases with decreasing temperature. Thus the temperature of the aqueous solution can be kept down as well, so that the concentration of calcium ions in solution remains below their saturation solubility. In general, either or both the temperature and the concentration of the aqueous solution of sodium hydroxide may be varied to suit the chemical composition of other species which may also be present in the separated solid phase, in order to increase their solubility. Thus, for example, if the separated solid phase comprises calcia, then the temperature of the aqueous solution can be decreased to increase the solubility of the calcia therein, as just described. If, in contrast, the separated solid phase comprises baria, which has normal or prograde solubility that increases with temperature, then the temperature of the aqueous solution can instead be increased to increase the solubility of the baria therein. Thus the amount of water to which the separated solid phase is added is not fixed, and can be determined, at least in part, by the chemical composition of other species which may be present in the separated solid phase as well, apart from manganese, sodium oxide and possibly also iron and / or silica. The reaction of Eqn. 16 is also significantly exothermic, with an enthalpy of reaction, AHreaction = - 239 kJ at s.t.p. Therefore, this reaction should preferably be controlled, for example by appropriate cooling, to keep the temperature of the aqueous solution substantially below its boiling point, for example at less than about 85 ’Celsius, more preferably at less than about 75 ’Celsius, and most preferably at less than about 65 ’Celsius if the solution is at atmospheric pressure. At least some of the heat extracted from firstly cooling the separated solid phase and / or from cooling the reaction of Eqn. 16 may then be used to contribute to another stage in the method if the invention, such as heat-treating the ore initially. Adding the separated solid phase to the water, rather than the other way round, helps to control the temperature of this reaction by ensuring that the final concentration of the aqueous solution is approached from below. Thus after the separated solid phase has been added to liquid water as described above, the elemental manganese, possibly with an admixture of iron and / or silica therein, remains undissolved in the water. These undissolved solids are then phase-separated from the aqueous solution of sodium hydroxide, for example by settlement under gravity, filtration and / or centrifugation, to recover the manganese from the aqueous solution. If, for example, the manganese is subsequently to be used as an ingredient in steelmaking, then any impurities (such as any residual silica) still remaining in the undissolved solids after this phase separation can still be removed by performing a further density-based separation of the manganese from these impurities or they can be transferred to a slag phase during the steelmaking process. The remaining aqueous solution of sodium hydroxide has many different possible uses. For example, at least some of it may be dried to produce solid sodium hydroxide, which may then be fused and electrolysed to produce more liquid sodium for reaction with the trivalent manganese oxide, so that the sodium is recycled in a closed loop. In another example, at least some of the remaining aqueous solution of sodium hydroxide may be reacted with hydrogen chloride to produce an aqueous solution of sodium chloride, at least some of which may then be dried to produce solid sodium chloride. The hydrogen chloride may be in the gaseous phase or in aqueous solution as hydrochloric acid. The solid sodium chloride may then be fused and electrolysed to produce more liquid sodium for reaction with the trivalent manganese oxide and chlorine gas, at least some of which may be used to produce the hydrogen chloride in a closed loop. This may be more appropriate if the aqueous solution of sodium hydroxide also comprises any barium and / or calcium cations, because the barium and / or calcium cations will remain present in the solid sodium chloride thus produced, allowing it to be electrolysed using a technique as described in US patent no. 3,020,221. Both alternatives, however, have the advantage of significantly reducing, and potentially eliminating, the consumption of sodium hydroxide and / or sodium chloride by electrolysis to produce the liquid sodium. In either case, the amount of water to which the separated solid phase is added to produce the aqueous solution of sodium hydroxide is preferably also minimized, in order to minimize the amount of energy required to dry the aqueous solution of sodium hydroxide or of sodium chloride, respectively. Reacting the aqueous solution of sodium hydroxide directly with hydrogen chloride in the gaseous phase instead of with hydrochloric acid helps to achieve this aim if the intention is to produce an aqueous solution of sodium chloride. Water consumption may also be reduced by capturing at least some of the water vapour produced by drying the aqueous solution of sodium hydroxide and / or of sodium chloride, condensing at least some of this captured water vapour, and using at least some of this captured and condensed water vapour as the liquid water to which the separated solid phase is added in order to recover the elemental manganese, as described above. Moreover, the remaining aqueous solution of sodium hydroxide also has a high affinity for carbon dioxide. At least some of this aqueous solution may therefore be used in a carbonation reaction with carbon dioxide gas to produce at least sodium carbonate, for example by reaction with carbon dioxide captured from the thermal decomposition of rhodochrosite, as described above. Alternatively or additionally, the carbon dioxide may be captured from atmospheric air and / or from one or more other industrial processes, in order to mitigate greenhouse gas emissions. If so, the present invention can have a significantly negative carbon footprint overall. If at least some of the remaining aqueous solution of sodium hydroxide is used in a carbonation reaction with carbon dioxide, this carbonation reaction can conveniently be used to precipitate out any barium and / or calcium cations also present in the aqueous solution of sodium hydroxide as insoluble barium carbonate and / or calcium carbonate, leaving the sodium ions in solution and thereby simultaneously purifying the sodium hydroxide. Alternatively or additionally, at least some of the aqueous solution of sodium hydroxide may be used as a reagent in one or more other chemical reactions because of its high overall pH and / or high sodium content. An example of such a use is described in the present applicant's co-pending UK patent application no. XXXXXXX.X ("Method and Apparatus for Producing Oxides of Calcium, Magnesium and Iron from Carbonate Mineral Ores without Burning Carbonaceous Fuels"; applicant's ref: NE-P-GB 005), the entire contents of which is incorporated herein by reference. Optional Magnetic Separation of Iron from Manganese As noted above, the solid phase separated from the liquid sodium may comprise elemental iron as well as elemental manganese. If the iron is present as a result of mixing the manganese ore with iron ore before the reaction with the liquid sodium, this admixture of iron may be desirable in view of an intended use of the manganese-iron mixture, for example in steelmaking. However, if the iron is derived from the manganese ore, for example as a result of a minority of iron cations being substituted for manganese cations in the ore, then in some cases, this admixture of iron may be undesirable or unacceptable. In some embodiments, therefore, the method may further comprise magnetically separating at least some of the iron from the manganese. This may be achieved as follows. As Table 1 shows, elemental manganese is only weakly paramagnetic, and is therefore only weakly attracted to regions of high magnetic field. In contrast, iron is strongly (and famously) ferromagnetic at temperatures below its Curie temperature of 1043 K, and is therefore powerfully attracted to regions of high magnetic field. Any iron in the solid phase separated from the liquid sodium may therefore be separated from the manganese therein by bringing both into proximity with a region of high magnetic field. Since the magnetic susceptibility of the ferromagnetic iron is many orders of magnitude greater than that of the paramagnetic manganese, a very high degree of their separation can be achieved. This optional magnetic separation of the iron from the manganese may be conducted at any point after the reactions of Eqns. 10 and 11. Thus it may be conducted when the solid phase comprising both elemental iron and elemental manganese is still entrained in the liquid sodium, and / or after the solid phase has been separated from the liquid sodium, and / or before, during and / or after the manganese is recovered from the separated solid phase as described above. Thus, for example, the iron may be magnetically separated from the manganese to produce an iron-enriched process stream and an iron-depleted process stream comprising the manganese, before the iron-depleted process stream is first added to liquid water and then the undissolved solids are phase-separated from the aqueous solution of sodium hydroxide thus formed to recover the manganese. In such cases, the iron-depleted process stream will also comprise the majority of the sodium oxide, as well as the majority of any baria, calcia and / or silica, all of which are diamagnetic. Nonetheless, the iron-enriched process stream may also be washed with or added to liquid water and then phase-separated as well, to remove any residual amounts of water-soluble diamagnetic species remaining in the iron-enriched process stream after this magnetic separation. Advantageously, the magnetic separation may be conducted when the solid phase comprising both the elemental iron and the elemental manganese is still hot from the reactions of Eqns. 10 and 11 because the maximum operating temperature for the reaction of Eqn. 11 is significantly below the Curie temperature of iron, whereas the magnetic susceptibility of the paramagnetic manganese will be further diminished by the elevated temperature, making the magnetic separation more effective. Preferably, therefore, the magnetic separation is conducted when the solid phase comprising both the iron and the manganese has a temperature of at least about 200 ’Celsius, more preferably at least about 250 ’Celsius, and most preferably at least about 300 ’Celsius. Alternatively or additionally, the iron may be magnetically separated from the manganese after the solid phase has been separated from the liquid sodium and has been added to liquid water, before the undissolved solids have been phase-separated from the aqueous solution of sodium hydroxide thus formed. In such a case, the aqueous solution may act as a transport medium for the undissolved solids comprising the iron and the manganese, and magnetically separating the iron from the manganese may comprise applying a magnetic field gradient to the aqueous solution of sodium hydroxide with the undissolved solids suspended or entrained therein. Moreover, the iron may be magnetically separated from the manganese after the undissolved solids, comprising both iron and manganese, have been phase-separated from the aqueous solution of sodium hydroxide thus formed. Regardless of how the magnetic separation of the iron from the manganese is conducted, magnetic separation of the same process stream may be repeated, in order to increase the degree of separation finally achieved. Thus after such an optional magnetic separation of the iron from the manganese, substantially pure elemental manganese and substantially pure elemental iron can both be obtained from the ironmanganese mixture. Any of the methods described herein may be carried out as a continuous, semi-batch or batch process. However, they are preferably carried out as a continuous process for reasons of economy and efficiency. Apparatus In a second aspect, the present invention also provides an apparatus for producing manganese from a manganese ore. The apparatus comprises a comminution device for comminuting the manganese ore into fines, a mineral-converting subassembly for converting a manganiferous mineral in the ore into a trivalent manganese oxide via at least one of a reductant-free pyrometallurgical technique and an inorganic hydrometallurgical technique, a gas-tight first reaction vessel, a solid-liquid sodium phase separator and a solid-species separator. The first reaction vessel is for reacting the trivalent manganese oxide in an inert atmosphere and at a temperature of less than 600 ’Celsius with an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide. The first reaction vessel comprises a first inlet for the trivalent manganese oxide, a second inlet for the liquid sodium, and an outlet for the liquid sodium and, entrained therein, a solid phase comprising both elemental manganese and other insoluble products at least comprising sodium oxide. The solid-liquid sodium phase separator is for separating at least some of the solid phase from the liquid sodium. The solid-species separator is for separating the elemental manganese from the other insoluble products. The comminution device may comprise one or more known types of comminution device, such as a rock crusher, grinder, hammer mill, ball, disc and / or rod mill, autogenous mill, roller mill, vibrator, and / or the like. In various embodiments, the mineral-converting subassembly may take one of several different forms described further below, which depend on whether the manganiferous mineral in the ore is converted into a trivalent manganese oxide via the pyrometallurgical technique and / or the hydrometallurgical technique. If after the manganiferous mineral in the ore has been converted into a trivalent manganese oxide, the trivalent manganese oxide has a significant admixture of iron oxide(s) and / or the trivalent manganese oxide comprise a significant minority of iron cations substituted for manganese cations, then the first reaction vessel may be made of steel of a type already used for containing and transporting liquid sodium, such as grade 316 LN or 316 FR stainless steel. In such a case, the inner surface of the reaction vessel may be provided with a sacrificial layer having the same chemical composition, for gradual corrosion by the highly reactive sodium oxide. Thus depending on the desired composition of other, minor elements mixed in with the elemental manganese eventually produced, the chemical constituents of the sacrificial layer will either be the same as those already present in the reaction mixture (i.e., Fe and / or Mn) or may be acceptable as minor constituents of the solid phase produced by the reaction (i.e., Cr, Ni, Mo). Alternatively, the reaction may be carried out in a reaction vessel made from or lined with titanium or a titanium alloy, such as one having a composition by weight of 98.8% Ti, 0.8% Ni and 0.4% Mo. Titanium and titanium alloy are found to be highly resistant to corrosion by sodium oxide across the entire range of operating temperatures for the reaction, by forming a stable surface passivation layer of titanium dioxide. The first reaction vessel may, for example, be a stirred tank reactor. In some embodiments, the first inlet of the first reaction vessel may comprise an airlock to prevent atmospheric oxygen from being introduced into the first reaction vessel whenever the vessel is fed with trivalent manganese oxide. If so, the airlock may have an interior for holding the trivalent manganese oxide and may also comprise first and second gas-tight doors, an inlet to the interior of the airlock for inert gas, and an outlet from the interior of the airlock for atmospheric air. The first gas-tight door connects the interior of the airlock with a surrounding environment of the first reaction vessel, and the second gas-tight door connects the interior of the airlock with an inside of the first reaction vessel. The inlet for inert gas and the outlet for atmospheric air each comprises a respective valve for opening and closing a respective one of the inlet and the outlet. Thus a charge of trivalent manganese oxide may be introduced into the interior of the airlock via the first door, and atmospheric air may be purged from within the interior of the airlock by opening and closing the valves to replace the air inside the airlock with inert gas, before the trivalent manganese oxide is released into the first reaction vessel via the second door. If the first inlet of the first reaction vessel does comprise such an airlock, the apparatus may further comprise a heat exchanger for transferring heat from the first reaction vessel to the inert gas upstream of the inlet to the airlock. Thus the inert gas can be heated to a temperature similar to that inside the first reaction vessel before it enters the first reaction vessel, which helps to prevent a build-up of pressure within the first reaction vessel, and to purge atmospheric air from within the airlock. In some embodiments of the apparatus, the solid-species separator may be a "dry" solid-species separator, wherein the solid-liquid sodium phase separator comprises an inlet for receiving the liquid sodium with the solid phase entrained therein from the outlet of the first reaction vessel, a first outlet for liquid sodium, and a second outlet for the solid phase, and the solid-species separator comprises an inlet for receiving the solid phase from the second outlet of the solid-liquid sodium phase separator, a first outlet for the elemental manganese, and a second outlet for the other insoluble products. In such embodiments, the solid-species separator may comprise a hydration vessel for reacting at least some of the sodium oxide in the solid phase with liquid water to produce an aqueous solution of sodium hydroxide, and a solid-aqueous phase separator for separating undissolved solids from the aqueous solution of sodium hydroxide. If so, the hydration vessel comprises the inlet of the solidspecies separator for receiving the solid phase from the second outlet of the solid-liquid sodium phase separator, a second inlet for the water, and an outlet for the aqueous solution of sodium hydroxide and, entrained therein, undissolved solids comprising the elemental manganese. The solid-aqueous phase separator comprises an inlet for receiving the aqueous solution of sodium hydroxide with the undissolved solids entrained therein from the outlet of the hydration vessel, a first outlet for the aqueous solution of sodium hydroxide, and a second outlet for the undissolved solids, which comprise the elemental manganese. In other embodiments of the apparatus, the solid-species separator may instead be a "wet" solidspecies separator, wherein the solid-species separator comprises an inlet for receiving the liquid sodium with the elemental manganese and other insoluble products entrained therein from the outlet of the first reaction vessel, a first outlet for liquid sodium with elemental manganese entrained therein, and a second outlet for liquid sodium with the other insoluble products entrained therein. If so, the solid-liquid sodium phase separator comprises first and second phase separating devices, wherein the first phase separating device comprises an inlet for receiving the liquid sodium with elemental manganese entrained therein from the first outlet of the solid-species separator, a first outlet for the liquid sodium, and a second outlet for the elemental manganese, and the second phase separating device comprises an inlet for receiving the liquid sodium with the other insoluble products entrained therein from the second outlet of the solid-species separator, a first outlet for the liquid sodium, and a second outlet for the other insoluble products. In either the "wet" or "dry" case, however, both the solid-species separator and those parts of the solid-liquid sodium phase separator which come into contact with the other insoluble products are preferably made of a material like the first reaction vessel, which is adapted to accommodate or withstand corrosion by sodium oxide, as described above. In both the "wet" or "dry" cases, the solidspecies separator may comprise a density-based separator, which separates the different solid species based on their different densities, such as an appropriate one of the different types of cyclonic separator mentioned above. In some embodiments of the apparatus, the first outlet for liquid sodium of the solid-liquid sodium phase separator may be connected to the second inlet of the first reaction vessel, so that the separated liquid sodium can be recycled back to the reaction with the trivalent manganese oxide. Some different possible embodiments of the mineral-converting subassembly will now be described, starting with those whereby the manganiferous mineral may be converted into a trivalent manganese oxide via the pyrometallurgical technique. Thus in some embodiments, the mineral-converting subassembly may comprise a high-temperature dryer for heat-treating the ore in atmospheric air to a temperature of from 100 to 600 ’Celsius, inclusive, to dehydrate the ore, dehydroxylate hydroxylated compounds contained therein and thermally decompose the manganiferous mineral into the trivalent manganese oxide. "High-temperature" in this context means that the dryer has an operating temperature above 100 ’Celsius. The high-temperature dryer may, for example, comprise a continuous tunnel dryer through which the comminuted ore is transferred, for example on a moving conveyor. Such a dryer may be compared and contrasted with a Dwight-Lloyd sintering machine, as follows. Like a Dwight-Lloyd sintering machine, the ore fines can be advanced through a hot zone of the dryer in order to heat-treat them as described, and the rate of advance may be similar to that in a Dwight-Lloyd sintering machine. However, unlike a Dwight-Lloyd sintering machine, the manganese ore is not mixed with any other ingredients, other than possibly with similarly comminuted iron ore, and is not fired or exposed to a flame front, and is only heated instead. In some embodiments, the high-temperature dryer may have an interior which is closed off from the surrounding environment, for example by means of one or more doors or curtains, so that the ore fines can be enclosed within the dryer whilst they are heat-treated, are not exposed to the surrounding environment and may instead be contained in their own atmosphere. Enclosing the ore fines whilst they are heat-treated so that they are contained in their own atmosphere allows this atmosphere to be controlled, and also helps to prevent gases like carbon dioxide which may be released from the ore as it is heat-treated from escaping into the surrounding environment. Enclosing the ore in this way also helps to maintain the thermal efficiency of its heat-treatment by reducing the loss of heat to the environment. In some embodiments, the dryer may contain an atmosphere to which the manganese ore is exposed during its heat-treatment, and the dryer may comprise means for reducing a pressure of this atmosphere to less than that of atmospheric air outside the dryer. Thus when ore enters or leaves the dryer, this pressure differential causes atmospheric air from outside the dryer to enter the dryer along with the ore, thereby hindering or preventing the escape of gases like carbon dioxide into the surrounding environment. If the high-temperature dryer can be rendered gas-tight to permit the thermal decomposition of the manganiferous mineral to be conducted closed off from its surrounding environment, in some embodiments, the apparatus may comprise a conduit for transporting sodium oxide, or solid sodium hydroxide derived therefrom, from downstream of the solid-liquid sodium phase separator through an atmosphere to which the manganiferous mineral is exposed within the high-temperature dryer. Alternatively or additionally, the high-temperature dryer may comprises an outlet for gaseous carbon dioxide produced by the thermal decomposition of the manganiferous mineral therein, and the apparatus may further comprise a carbonation vessel for reacting sodium oxide or hydroxide with at least some of the gaseous carbon dioxide to produce at least sodium carbonate. In such embodiments, the carbonation vessel comprises a first inlet for receiving the carbon dioxide from this outlet of the high-temperature dryer, a second inlet for receiving sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, from downstream of the solid-liquid sodium phase separator, and an outlet for at least the sodium carbonate. In either or both of these ways, carbon dioxide released by the thermal decomposition of a carbonaceous mineral like rhodochrosite within the high-temperature dryer can be captured and mineralized as sodium carbonate, which may subsequently be used in another industrial process, as noted above. If the apparatus comprises a conduit for transporting sodium oxide, or solid sodium hydroxide derived therefrom, through the atmosphere to which the manganiferous mineral is exposed within the high-temperature dryer, this exothermic carbonation reaction transfers heat directly from the sodium carbonate thus formed to the manganiferous mineral within the dryer, thereby aiding its thermal decomposition. In addition, if the sodium oxide or solid sodium hydroxide is still hot, for example from the redox reaction with the liquid sodium or from hydration of the sodium oxide, more heat is transferred to the manganiferous mineral within the dryer. The efficiency of this heat transfer process may be improved by ensuring that the atmosphere containing the manganiferous mineral is properly thermally insulated from the surrounding environment. Moreover, since heating of the mineral by the sodium oxide, hydroxide and / or carbonate is likely to occur mostly by convection and radiation, rather than by conduction, the rate of circulation of the atmosphere to which the manganiferous mineral is exposed may also be increased, for example, by being fan-driven, in order to increase the rate of convective heat transfer. The rate of radiative heat transfer may similarly be improved, for example, by at least partially enclosing this atmosphere within a reflective lining. If the apparatus comprises a carbonation vessel as noted above, the apparatus my also comprise a heat transfer pathway for transferring heat from at least one of the carbonation vessel and the sodium carbonate produced therein to the manganiferous mineral within the high-temperature dryer. In some embodiments, the apparatus may comprise a heat transfer pathway for transferring heat to the manganese ore within the high-temperature dryer from at least one of: (i) the liquid sodium before it enters the first reaction vessel; (ii) the manganese ore after it has been heat-treated and before it enters the first reaction vessel; (iii) the first reaction vessel itself; and (iv) at least one of the liquid sodium, the elemental manganese and the other insoluble products after they have left the first reaction vessel. This has the advantage that heat used to heat-treat the ore need not come from an external source. If so, the heat transfer pathway may contain the liquid sodium as a heat transfer fluid. For example, liquid sodium from the first reaction vessel may be circulated in pipework passing through and / or around the high-temperature dryer before being returned to the first reaction vessel. In some embodiments, the mineral-converting subassembly may comprise a first magnetic separator for magnetically separating paramagnetic components of the heat-treated ore at least comprising the trivalent manganese oxide from diamagnetic components thereof. The first magnetic separator comprises an inlet for receiving the heat-treated ore from the high-temperature dryer, a first outlet for the diamagnetic components and a second outlet for the paramagnetic components, this second outlet being upstream of the first inlet of the first reaction vessel. The first magnetic separator may be of a type normally used to magnetically separate ore from gangue in a known manner, and a series of such magnetic separators may be used to increase the separation efficiency. However, in any such embodiments, the first magnetic separator is adapted to maintain the heat-treated ore at a temperature of less than about 100 ’Celsius, preferably less than about 75 ’Celsius, more preferably less than about 50 ’Celsius, and most preferably at or around ambient temperature. Thus, for example, if the first magnetic separator is adapted to operate at or around ambient temperature, the high-temperature dryer may comprise a counterflow system as mentioned above, whereby heat-treated ore particles leaving the dryer are able to transfer at least some of their heat to ore particles entering the dryer at or near to ambient temperature by being brought into thermal, but not physical, contact with them. Embodiments of the mineral-converting subassembly will now be described, whereby the manganiferous mineral may be converted into a trivalent manganese oxide via the hydrometallurgical technique. Thus in some embodiments, the mineral-converting subassembly may comprise a gas-tight second reaction vessel, a solid-aqueous phase separator and a low-temperature dryer. The gas-tight second reaction vessel is for dissolving the manganiferous mineral therein in hot, concentrated hydrochloric acid and comprises a first inlet for receiving the manganiferous mineral, a second inlet for receiving the hydrochloric acid, a third inlet for receiving sodium oxide or hydroxide, and an outlet for an aqueous solution and a precipitate comprising manganese (II) hydroxide. The solid-aqueous phase separator is for separating at least some of the precipitate from the aqueous solution and comprises an inlet for receiving the aqueous solution and the precipitate from the outlet of the second reaction vessel, a first outlet for the precipitate and a second outlet for the aqueous solution. The low-temperature dryer is for drying and dehydroxylating at least some of the manganese (II) hydroxide in the precipitate to produce the trivalent manganese oxide and comprises an inlet for receiving the precipitate from the first outlet of the solid-aqueous phase separator, and an outlet for the trivalent manganese oxide, this outlet being upstream of the first inlet of the first reaction vessel. "Low-temperature" in this context means that the dryer has an operating temperature below 100 ’Celsius, preferably below about 75 ’Celsius, and more preferably below about 50 ’Celsius. In some embodiments, the low-temperature dryer may be able to be rendered gas-tight to allow an atmosphere within it to be better controlled. In some embodiments, the gas-tight second reaction vessel may further comprise an outlet for gaseous chlorine released when a manganiferous mineral dissolves therein in hydrochloric acid. In some embodiments, the apparatus may comprise a conduit for transporting sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, from downstream of the solid-liquid sodium phase separator to at least one of (i) through an atmosphere to which the manganese (II) hydroxide is exposed within the low-temperature dryer, and (ii) the third inlet of the second reaction vessel. If the conduit is arranged to transport sodium oxide or hydroxide through the atmosphere of the low-temperature dryer, then it should be adapted to transport sodium hydroxide therethrough in solid phase. Otherwise, it may be adapted to transport the sodium hydroxide in solid or aqueous phase. If the conduit is arranged to transport sodium oxide or hydroxide both through the atmosphere within the low-temperature dryer and to the third inlet of the second reaction vessel, then it should be arranged to transport the sodium oxide or hydroxide through the low-temperature dryer before it arrives at the third inlet of the second reaction vessel. In some embodiments, the mineral-converting subassembly may comprise a second magnetic separator for magnetically separating paramagnetic components of the precipitate from diamagnetic components thereof. If so, the second magnetic separator comprises an inlet for receiving the precipitate, this inlet being downstream of the third inlet of the second reaction vessel, a first outlet for the diamagnetic components and a second outlet for the paramagnetic components, this second outlet being upstream of the first inlet of the first reaction vessel. The second magnetic separator may separate the paramagnetic components from the diamagnetic components whilst both are still suspended or entrained in aqueous solution or after they have been phase-separated from it by the solid-aqueous phase separator. In the latter case, the second magnetic separator may separate the paramagnetic components from the diamagnetic components before, during and / or after the manganese (II) hydroxide in the precipitate is dried and dehydroxylated to produce the trivalent manganese oxide. The second magnetic separator is adapted to maintain the precipitate at a temperature of less than about 100 ’Celsius, preferably less than about 75 ’Celsius, more preferably less than about 50 ’Celsius, and most preferably at or around ambient temperature, so that the magnetic susceptibility of the paramagnetic components is not diminished in case the temperature of the precipitate has been raised in the low-temperature dryer. In some embodiments, the mineral-converting subassembly may comprise components as described herein whereby a manganiferous mineral in the ore may be converted into a trivalent manganese oxide via both the pyrometallurgical and the hydrometallurgical techniques, for example if the ore comprises two or more different types of manganiferous mineral, for which these different techniques are each respectively suited. If so, components of the mineral-converting subassembly which use the pyrometallurgical technique should be arranged upstream of components of the mineral-converting subassembly which use the hydrometallurgical technique, for reasons already explained above. In some embodiments, the apparatus may comprise a third magnetic separator for magnetically separating elemental iron in the solid phase from the elemental manganese therein. If so, the third magnetic separator comprises an inlet downstream of the outlet of the first reaction vessel for receiving the elemental iron mixed with the elemental manganese, a first outlet for the elemental iron and a second outlet for the elemental manganese. The third magnetic separator may separate the iron from the manganese whilst both are still suspended or entrained in liquid sodium or after they have been phase-separated from it. Other insoluble products may still be mixed in with the iron and manganese as well, or not. In any such embodiments, the third magnetic separator is adapted to maintain the elemental manganese at a temperature of at least 200 ’Celsius. This may be done, for example, by ensuring that the third magnetic separator is provided with suitable thermal insulation, to prevent the manganese from cooling down, if it is still hot from the redox reaction with the liquid sodium. Thus the magnetic susceptibility of the paramagnetic manganese is diminished by this raised temperature, which enhances the degree of its separation from the ferromagnetic iron. Any remaining diamagnetic gangue species which are separated from the iron along with the manganese can subsequently be separated from the manganese as well based on their different densities, for example. Brief Description of the Drawings Further features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and in association with the accompanying drawings, in which: Fig. 1 is a graph of the solubility of gaseous chlorine in liquid water at atmospheric pressure and as a function of temperature; Fig. 2 is a graph of the respective solubilities in liquid water of silica, aluminium hydroxide and manganese (II) hydroxide as a function of pH; Fig. 3 is a flow diagram of a first embodiment of a method of producing manganese from a manganese ore; Fig. 4 is a flow diagram of a second embodiment of such a method; Fig. 4A is a flow diagram of part of a first variant of the method of Fig. 4; Fig. 4B is a flow diagram of part of a second variant of the method of Fig. 4; Fig. 5 is a flow diagram of a third embodiment of a method of producing manganese from a manganese ore; Fig. 6 is a flow diagram of a fourth embodiment of such a method; Fig. 7 is a flow diagram of a fifth embodiment of such a method; Fig. 8 is a flow diagram of part of a sixth embodiment of such a method; Fig. 9 is a flow diagram of part of a seventh embodiment of such a method; Fig. 10 is a flow diagram of part of an eighth embodiment of such a method; Fig. 11 is a flow diagram of part of a ninth embodiment of such a method; Fig. 12A is a flow diagram of part of a tenth embodiment of such a method; Fig. 12B is a diagram of a chemical cycle in the tenth embodiment; Fig. 12C is a block diagram of the same chemical cycle as in Fig. 12A; Fig. 13 is a flow diagram of an eleventh embodiment of a method of producing manganese from a manganese ore; Fig. 14A is a flow diagram of a twelfth embodiment of such a method; Fig. 14B is a flow diagram of a thirteenth embodiment of such a method; Fig. 15 is a flow diagram of part of a fourteenth embodiment of such a method; Fig. 16 is a flow diagram of a fifteenth embodiment of such a method; Fig. 17 is a flow diagram of part of a sixteenth embodiment of such a method; Fig. 18 is a flow diagram of part of a seventeenth embodiment of such a method; Fig. 19 is a flow diagram of part of an eighteenth embodiment of such a method; Fig. 20 is a schematic diagram of a first embodiment of an apparatus for producing manganese from a manganese ore; Fig. 21 is a schematic diagram of a second embodiment of such an apparatus; Fig. 22 is a schematic diagram of a third embodiment of such an apparatus; Fig. 23 is a schematic longitudinal section through an embodiment of a dryer; Fig. 24 is a schematic diagram of an embodiment of a heat transfer pathway in the apparatus of Fig. 22; Fig. 25 is a schematic diagram of a fourth embodiment an apparatus for producing manganese from a manganese ore; Fig. 26 is a schematic diagram of an embodiment of an airlock; and Fig. 27 is a schematic diagram of a fifth embodiment an apparatus for producing manganese from a manganese ore. In the accompanying drawings and the associated description, the subscript "(Na)" attached to a chemical species as a state symbol denotes that the species is dissolved in liquid sodium. Detailed Description Fig. 3 shows a first embodiment of a method 500a of producing manganese from a manganese ore. In the method 500a, the manganese ore is comminuted 501 into fines, and a manganiferous mineral in the ore is converted into a trivalent manganese oxide using at least one of a reductant-free pyrometallurgical technique 502 and an inorganic hydrometallurgical technique 503, each as described above. The precise manner of converting the manganiferous mineral into the trivalent manganese oxide depends in any particular case on the initial chemical composition and crystal structure of the ore, as determined by prior analysis of the ore. The pyrometallurgical technique 502 comprises heat-treating the ore in atmospheric air to a temperature of from 100 to 600 ’Celsius, inclusive. This dehydrates the ore, dehydroxylates hydroxylated compounds contained therein and thermally decomposes the manganiferous mineral into the trivalent manganese oxide. Some further embodiments of both the pyrometallurgical technique 502 and the hydrometallurgical technique 503 applicable to particular cases are described below. As shown in Fig. 3, the temporal order or sequence of comminuting 501 the ore and converting 502, 503 a manganiferous mineral therein into a trivalent manganese oxide is not fixed. Thus, for example, initial comminution 501 may be followed by application of the pyrometallurgical technique 502, followed by further comminution 501 of the ore to alter its chemical activity, then by the hydrometallurgical technique 503, which may comprise further comminution 501 of the ore as the heat-treated ore dissolves in the hot, concentrated hydrochloric acid. However, if, as in this example, the hydrometallurgical technique 503 follows the pyrometallurgical technique 502, then before the heat-treated ore is added to the hydrochloric acid, the ore should be cooled back down to less than about 100 ’Celsius, more preferably less than about 75 ’Celsius, and most preferably less than about 50 ’Celsius, to avoid the hydrochloric acid from boiling when the ore is added to it. This may be achieved, for example, using a counterflow system of the type described below. On the other hand, the heat-treated ore does not have to be cooled all the way back to ambient temperature because some of the heat acquired by the ore when it is heat-treated can then be transferred to the hydrochloric acid. This therefore reduces the amount of energy needed to pre-heat the hydrochloric acid. After the ore has been comminuted 501 and a manganiferous mineral in the ore has been converted 502, 503 into a trivalent manganese oxide, the trivalent manganese oxide is then added 505 to an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide, to produce a solid phase comprising both elemental manganese and other products insoluble in the liquid sodium, at least comprising sodium oxide. This redox reaction is conducted in an inert atmosphere and at a temperature of less than 600 ’Celsius to inhibit the production of ternary oxides like a-NaMnOj. At least some of the solid phase is then separated 506 from the liquid sodium, and the elemental manganese produced by the redox reaction is separated 507 from the other insoluble products also present in the solid phase. As mentioned above, this separation 507 may be done in several different ways, some embodiments of which are described below. Gangue species also originally present in the manganese ore may be separated out to ensure that they do not end up with the elemental manganese in at least one of three different ways, which will now be described with reference to Figs. 4, 5 and 6, respectively. Fig. 4 therefore shows a second embodiment of a method 500b of producing manganese from a manganese ore. The method 500b comprises, in addition to the processes 501 to 503 and 505 to 507 described above in relation to Fig. 3, after comminuting 501 the ore and converting 502, 503 a manganiferous mineral in the ore into a trivalent manganese oxide, magnetically separating 504 the trivalent manganese oxide from diamagnetic gangue species also present in the ore, before the trivalent manganese oxide is added 505 to an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide. This magnetic separation 504 may be carried out successfully because unlike these gangue species, the trivalent manganese oxide is strongly paramagnetic, as described above in relation to Table 1. In this embodiment, separating 507 the elemental manganese from the other insoluble products comprises firstly adding 508a at least some of the solid phase separated 506 from the liquid sodium to liquid water, thereby hydrating the sodium oxide therein to produce an aqueous solution of sodium hydroxide, and then separating 508b solids which have not dissolved in the liquid water, and which therefore comprise the elemental manganese, from the aqueous solution of sodium hydroxide thus formed. Both of these processes 508a, 508b are conducted as described previously. Fig. 4A shows part of a first variant 500ba of the method 500b of Fig. 4, when the method 500b comprises the pyrometallurgical technique 502 and it is intended to magnetically separate 504 the trivalent manganese oxide from the diamagnetic gangue species immediately thereafter. In this case, before the magnetic separation 504, the heat-treated ore is firstly cooled back down 509 to less than about 100 ’Celsius. This may again be achieved, for example, using a counterflow system of the type described below. This is so that the magnetic susceptibilities of the paramagnetic components of the ore, which include the trivalent manganese oxide, as well as of any iron (III) oxide which may also be present in the ore, are not diminished by their increased temperature due to application of the pyrometallurgical technique 502. After the magnetic separation 504, the paramagnetic components of the ore, including the trivalent manganese oxide as well as any possible iron (III) oxide, may then either be passed to the hydrometallurgical technique 503 or added 505 directly to the liquid sodium. Fig. 4B shows part of a second variant 500bb of the method 500b of Fig. 4. In this case, in addition to magnetically separating 504 the trivalent manganese oxide from diamagnetic gangue species also present in the ore, the trivalent manganese oxide is separated 510 from these diamagnetic gangue species based on their different densities, as described above in relation to Table 2, in order to increase the degree of separation of these different magnetic species. The temporal order or sequence of this density-based separation 510 and the magnetic separation 504 is not set, and therefore the densitybased separation 510 may be carried out simultaneously with, before and / or after the magnetic separation 504. Moreover, this second variant 500bb of the method 500b of Fig. 4 may be used as well as or instead of the first variant 500ba of the method 500b of Fig. 4 described above in relation to Fig. 4A. Fig. 5 shows a third embodiment of a method 500c of producing manganese from a manganese ore, which also allows gangue species originally present in the ore to be separated from the ore, when it is being processed according to the method 500a of Fig. 3 and the method 500a comprises the inorganic hydrometallurgical technique 503. In the present embodiment, before the trivalent manganese oxide is produced, gangue species are separated 211, 511 from manganese (II) ions produced by the hydrometallurgical technique 503 in at least one of two ways, as follows. Firstly, they may be phase-separated 211 from the manganese (II) ions when the latter are still dissolved in aqueous solution, if the gangue species themselves are insoluble or have precipitated out, as described above in relation to Fig. 2. Secondly, after a precipitate comprising manganese (II) hydroxide has been produced from this aqueous solution, before, during and / or after the precipitate has been phase-separated 503c from the aqueous solution, but before at least some of the precipitated manganese (II) hydroxide is then dried and dehydroxylated 503d to produce the trivalent manganese oxide, paramagnetic components of the precipitate at least comprising the manganese (II) hydroxide may be magnetically separated 511 from diamagnetic components of the precipitate. This magnetic separation 511 is conducted by applying a magnetic field gradient to at least one of the aqueous solution with the precipitate suspended or entrained therein, and the precipitate after it has been phase-separated from the aqueous solution. The magnetic separation 511 may be carried out successfully because unlike diamagnetic gangue species, manganese (II) hydroxide is strongly paramagnetic, as described above in relation to Table 1. As shown in Fig. 5, either one of these separation techniques 211, 511 may be applied on their own to the manganese (II) ions, or first the gangue separation technique 211 and then the gangue separation technique 511 may both be used, in order to increase the degree of separation of gangue species from the manganese (II) hydroxide. In any case, regardless of which of these separation techniques 211, 511 is used, the manganese (II) hydroxide is then dried and dehydroxylated 503d to produce the trivalent manganese oxide, before the trivalent manganese oxide is added 505 to the liquid sodium as before. Thereafter, at least some of the solid phase produced by the redox reaction between the trivalent manganese oxide and the liquid sodium is separated 506 from the liquid sodium, and the elemental manganese in this solid phase is separated 507a from the other insoluble products based on their different densities in a "dry" separation process. In comparison to the manganese separation technique 508a, 508b used in the second embodiment, this has the advantage that after the manganese has been separated 507a, it still retains at least some of its heat from the redox reaction 505. This heat may therefore be transferred directly to a steel-making process, for example, along with the manganese, in a more energy-efficient manner. Fig. 6 shows a fourth embodiment of a method 500d of producing manganese from a manganese ore, which allows a silicate mineral originally present in the ore to be separated from the ore, when the ore is being processed according to the method 500a of Fig. 3. As mentioned previously, the silicate mineral may comprise at least one of a siliceous manganese mineral, such as braunite and / or rhodonite, and siliceous gangue, such as an aluminosilicate and / or silica, for example. If in the method 500d, heat-treated ore is transferred from the pyrometallurgical technique 502 to the redox reaction 505a between the liquid sodium and the trivalent manganese oxide, then the heat-treated ore may comprise a silicate mineral originally present in the ore which has been unaffected by the heat treatment of the pyrometallurgical technique 502, for example if the silicate mineral is rhodonite. If on the other hand, a precipitate comprising dried and dehydroxylated manganese (II) hydroxide is transferred from the hydrometallurgical technique 503 to the redox reaction 505a, then the precipitate may instead comprise a silicate modified by the hydrometallurgical technique 503 and which is derived from the silicate mineral originally present in the ore. In either case, in this embodiment, the redox reaction 505a is conducted at a temperature of at least 320 ’Celsius to induce a reaction between the sodium oxide produced by the redox reaction 505a and silica derived from the silicate mineral, to produce at least sodium orthosilicate, which dissolves in the excess liquid sodium. Next, the remaining solid phase produced by the redox reaction 505a is separated 507b into elemental manganese and other insoluble products according to their different densities in a "wet" separation process whilst both of them are still suspended or entrained in the liquid sodium. Thereafter, each of the two resulting process streams are phase-separated 506a, 506b into elemental manganese and liquid sodium on the one hand, and into other insoluble products and liquid sodium on the other. After the manganese has been separated 506a, it again retains at least some of its heat from the redox reaction 505a, which may therefore be transferred directly, along with the manganese, to a steelmaking process, for example. Meanwhile, the sodium orthosilicate remains dissolved in the excess liquid sodium in both process streams and is therefore separated from both the insoluble manganese and the other insoluble products during these subsequent phase separations 506a, 506b. As before, the phase separations 506a, 506b may both still be conducted as described in the present applicant's co-pending UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal"; applicant's ref: NE-P-GB 002), mentioned previously. If desired, the liquid sodium with the sodium orthosilicate dissolved therein from each of the phase separations 506a, 506b may subsequently be recombined and processed as described in the present applicant's co-pending UK patent application no. XXXXXXX.X ("Method and Apparatus for Producing an Alkaline Mixture comprising Sodium Silicate"; applicant's ref: NE-P-GB 009), also mentioned previously. Any combination of the gangue separation techniques described above in relation to Figs. 4 to 6 may be used to prevent gangue species originally present in the ore from ending up with the elemental manganese, subject to the following proviso. If the technique for removing a silicate mineral of Fig. 6 is to be used, then this silicate mineral should not have been completely removed already by using another one or more of the gangue separation techniques 504, 211, 511 which are applied upstream of the redox reaction 505a, unless at least some of the silicate mineral removed by such another gangue separation technique, or silica derived therefrom, is reintroduced back into this reaction mixture via another route. This ensures that if the technique of Fig. 6 is used, silica derived from the silicate mineral is still present in the reaction mixture for the redox reaction 505a when the trivalent manganese oxide is added 505 to the liquid sodium. Moreover, any of the techniques 507a, 507b, 508a, 508b for separating 507 the elemental manganese from the other insoluble products produced by the redox reactions 505, 505a may be used in combination with any of the different gangue separation techniques 504, 211, 510, 511 described above. Fig. 7 shows a fifth embodiment of a method 500e of producing manganese from a manganese ore, wherein the method 500e comprises the pyrometallurgical technique 502 and the manganese ore is a primary ore comprising rhodochrosite. In this case, therefore, as well as comminuting 501 the ore, the pyrometallurgical technique 502 comprises heat-treating 502a the ore in atmospheric air at a temperature of more than about 200 ’Celsius to thermally decompose the rhodochrosite into bixbyite, which is a trivalent manganese oxide, and carbon dioxide. However, this heat-treatment 502a is also conducted at a temperature which is not more than about 320 ’Celsius, to prevent reabsorption of the carbon dioxide by the bixbyite. Both the ore and the atmosphere in which the ore is heat-treated 502a are closed off from their surrounding environment, and the carbon dioxide released by the thermal decomposition of the rhodochrosite is captured 512. The heat-treated ore is then cooled 509 back down to less than about 100 ’Celsius, and paramagnetic components of the heat-treated ore, including the bixbyite, are magnetically separated 504 from diamagnetic components of the ore, before the paramagnetic components are added 505 to the liquid sodium. Thereafter, the method 500e continues with the same processes 506 to 508b already described above in relation to the method 500b of Fig. 4. Furthermore, in the present embodiment, the method 500e comprises bubbling the captured carbon dioxide through a portion of the aqueous solution of sodium hydroxide produced by adding 508a to liquid waterthe solid phase separated 506 from the excess liquid sodium. This results in an exothermic carbonation reaction 513, which produces an alkaline solution comprising sodium carbonate. The carbonation reaction 513 is cooled 514, and heat extracted by cooling 514 the carbonation reaction is recycled 515 and used to heat-treat 502a the manganese ore. Figs. 8 and 9 show other ways of providing heat used in the pyrometallurgical technique 502 to thermally decompose a manganiferous mineral into a trivalent manganese oxide, any of which may be used as well as or instead of the way used in the method 500e of Fig. 7, for example if the manganese ore does not comprise a carbonate mineral. Fig. 8 therefore shows part of a sixth embodiment of a method 500f of producing manganese from a manganese ore, wherein at least some of the liquid sodium used in the redox reaction 505 with the trivalent manganese oxide is produced by fusing and electrolysing 101b solid sodium chloride. Since this liquid sodium already has a temperature of about 600 ’Celsius after it has been produced by the electrolysis 101b, in the method 500f, the liquid sodium is cooled 516 substantially before the trivalent manganese oxide is added 505 to the liquid sodium, so that the exothermic redox reaction between the trivalent manganese oxide and the liquid sodium does not exceed the upper limit of its temperature range, which is also about 600 ’Celsius, in order to inhibit the production of ternary oxides like a-NaMnOj. Heat extracted by cooling 516 the liquid sodium is recycled 308 and used to heat-treat 502 the manganese ore. This may be done, for example, by circulating the hot liquid sodium after it has been produced by the electrolysis 101b through and / or around a dryer of the manganese ore, before the cooled liquid sodium then enters a reaction vessel for containing the redox reaction 505. After the redox reaction 505, the method 500f then comprises at least the further processes 506 and 507 already described above, which have not been reproduced in Fig. 8 for the sake of brevity. Fig. 9 shows part of a seventh embodiment of a method 500g of producing manganese from a manganese ore, wherein the method 500g again comprises the pyrometallurgical technique 502, but the processes 501 to 505 have also been omitted from Fig. 9 for brevity. In the method 500g, after the solid phase produced by the redox reaction 505 is phase separated 506 from the excess liquid sodium, the solid phase is cooled 517 substantially before it is added 508a to liquid water in order to hydrate at least the sodium oxide from the solid phase in the liquid water and produce an aqueous solution of sodium hydroxide having a molarity of less than 2.5 M. This cooling 517 is conducted to control the temperature of the exothermic hydration reaction. In addition, the aqueous solution of sodium hydroxide thus produced is also cooled 518 to maintain its temperature below about 85 ’Celsius and prevent it from boiling. The high-temperature heat extracted by cooling 517 the solid phase is recycled 308 and used to heat-treat 502 the manganese ore. Although not represented in Fig. 9, after the hydration reaction 508a, the method 500g further comprises at least separating 508b undissolved solids, which therefore comprise the elemental manganese, from the aqueous solution of sodium hydroxide. Fig. 10 shows an eighth embodiment of a method 500h of producing manganese from a manganese ore, wherein the method 500h comprises the inorganic hydrometallurgical technique 503. In this embodiment, the method 500h initially comprises comminuting 501 the manganese ore. The hydrometallurgical technique 503 then comprises adding 503a at least some of the ore to hot, concentrated hydrochloric acid to produce gaseous chlorine and an acidic aqueous solution comprising manganese (II) chloride. The ore and the hydrochloric acid are closed off from their surrounding environment and at least some of the gaseous chlorine evolved from the acid is captured 519. Next, the hydrometallurgical technique 503 comprises adding 503b sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an alkaline aqueous solution and a precipitate comprising manganese (II) hydroxide. At least some of this precipitate is phase-separated 503c from the alkaline aqueous solution, and at least some of the manganese (II) hydroxide in the precipitate is dried and dehydroxylated 503d to produce the trivalent manganese oxide. Thereafter, the trivalent manganese oxide is added to liquid sodium as before for the redox reaction 505a, which in this embodiment is conducted at a temperature of at least 320 ’Celsius, so that sodium oxide produced by the redox reaction 505a reacts with silica also present in the precipitate to produce sodium orthosilicate, which dissolves in the excess liquid sodium. The method 500h then proceeds with the further processes 506 and 507 previously described above, which have been omitted from Fig. 8 for the sake of brevity. The present embodiment also comprises the following two features. Firstly, the sodium oxide or hydroxide which is added 503b to the acidic aqueous solution is recycled 521 from a portion of the sodium oxide produced by the redox reaction 505a. Secondly, the method 500h also comprises using 520 at least some of the captured chlorine gas to make the hydrochloric acid to which the manganese ore is added 503a at the start of the hydrometallurgical technique 503. Since as mentioned above in relation to Eqns. 9a to 9c, the molar amount of chlorine which is captured 519 is not sufficient to make all the hydrochloric acid needed to dissolve the manganese mineral from the ore in the hydrochloric acid initially, the additional chlorine necessary for this is recovered from the supernatant which remains after the precipitate has been phase-separated 503c, as will now be described with reference to Fig. 11. Fig. 11 therefore shows part of a ninth embodiment of a method 500i of producing manganese from a manganese ore, wherein the method 500i again comprises the inorganic hydrometallurgical technique 503 and the manganese ore comprises both silica and aluminate gangues species. Fig. 11 focusses on processes which occur during the hydrometallurgical technique 503, so that processes occurring before or after the hydrometallurgical technique 503 (such as comminuting 501 the ore) are omitted from Fig. 11 for the sake of brevity. Nonetheless, these processes which are not shown in Fig. 11 are still present in the method 500i. In this embodiment, after adding 503a at least some of the ore to hot, concentrated hydrochloric acid and before adding 503b sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution formed thereby, undissolved solids are phase-separated 211a from this acidic aqueous solution. Since these undissolved solids comprise reactive silica, the method 500i comprises using 222 at least some of these undissolved solids as an ingredient in cement manufacture. Adding 503b sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution left behind by this phase-separation 211a then proceeds in two stages, 503bl and 503b2. Firstly, sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is added 503bl to the acidic aqueous solution to produce an aqueous solution with a pH of from 5 to 8.5, inclusive, and a precipitate comprising aluminium hydroxide. The precipitate comprising aluminium hydroxide is then phase-separated 211b from this aqueous solution and at least some of this precipitate is dried and dehydroxylated 223 to produce alumina. This is then used 224 as feed material for producing aluminium by electrolysis with an inert anode to avoid generating any greenhouse gas emissions. After phase-separating 211b this aluminium hydroxide-containing precipitate, the hydrometallurgical technique 503 then proceeds by continuing to add 503b2 more of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution with a pH of from 5 to 8.5 to produce an alkaline aqueous solution and the precipitate comprising manganese (II) hydroxide. This precipitate is phase-separated 503c from the alkaline aqueous solution and dried and dehydroxylated 503d to produce the trivalent manganese oxide, which is then added 505 to the liquid sodium as before. In addition, however, the method 500i also comprises adding 225 more hydrochloric acid to the alkaline aqueous solution left behind by this phase-separation 503c to neutralize the alkaline aqueous solution and produce an aqueous solution of sodium chloride. Since the hydrogen ions from the acid and the hydroxide ions from the alkaline aqueous solution are now in a molar ratio of 1:1, no excess molar amount of chlorine is required to produce this neutral solution than the molar amount of sodium present in the sodium oxide or sodium hydroxide which was already used to produce the precipitate comprising manganese (II) hydroxide. Thus the total amount of acid consumed may be generated by electrolysis from the same source of sodium chloride as the total amount of sodium oxide or sodium hydroxide consumed. Accordingly, the method 500i then comprises recycling 114 this neutral aqueous solution of sodium chloride to produce liquid sodium and gaseous chlorine by electrolysis. This chlorine is used to make the hydrochloric acid to which the manganese ore is added 503a initially, in addition to the chlorine which is captured 519. In other possible embodiments, as described shortly, at least some of the liquid sodium produced by this electrolysis may also be added to the redox reaction 505 between the liquid sodium and the trivalent manganese oxide. Fig. 12A therefore shows part of a tenth embodiment of a method 500j of producing manganese from a manganese ore, illustrating a way in which a neutral aqueous solution of sodium chloride which remains at the end of the hydrometallurgical technique 503 as just described may be recycled 114 by using a combination of the chlor-alkali and Castner processes. Thus in this embodiment, the neutral aqueous solution of sodium chloride is firstly electrolysed 201a to produce an aqueous solution of sodium hydroxide, as well as gaseous hydrogen and chlorine. The aqueous solution of sodium hydroxide is then dried 201b to produce solid sodium hydroxide and water vapour, and the solid sodium hydroxide is fused and electrolysed 201c to produce the liquid sodium, more gaseous hydrogen and oxygen. The gaseous chlorine captured 519 from when the ore is initially added 503a to the hydrochloric acid is used 520 and the gaseous chlorine produced by electrolysis 201a is also used 530 to make hydrogen chloride, by combusting 203b them together with the hydrogen gas produced by at least one of electrolysing 201a the aqueous solution of sodium chloride and electrolysing 201c the solid sodium hydroxide. Meanwhile, at least some of the water vapour produced by drying 201b the aqueous solution of sodium hydroxide is captured 215 and condensed 216, and then used as at least some of the liquid water in which the hydrogen chloride is dissolved 204 to produce the hydrochloric acid. Heat released by condensing 216 the captured water vapour can also be used to contribute to drying 201b the aqueous solution of sodium hydroxide. Figs. 12B and 12C both show a chemical cycle in the tenth embodiment, which illustrate how the recycling technique of Fig. 12A fits into the rest of the method 500j. Thus the method 500j comprises the same successive processes 503a, 503b, 503c and 503d of the hydrometallurgical technique 503 as described above in relation to Fig. 10, before the resulting trivalent manganese oxide is then added 505 to liquid sodium. In Figs. 12B-C, starting materials are contained in boxes edged with dashed lines and for the sake of example, Figs. 12B-C show how the method 500j treats pyrolusite (MnOj) as a starting material, so that the process 503a is represented by Eqn. 6a. However, similar chemical cycles could equally well be constructed to show how the method 500j would treat other manganese minerals instead, in which case, one of Eqns. 6b and 6c, for example, could be substituted for Eqn. 6a. In Figs. 12B-C, recycling 114 the aqueous solution of sodium chloride is represented by overall Eqn. 4d and the water cycle 201b, 215, 216, 204 of Fig. 12A has only been omitted from Figs. 12B-C for the sake of brevity. However, it may now be seen how the liquid sodium also produced by electrolysis 201c is recycled 529 to the redox reaction 505 with the trivalent manganese oxide. Moreover, the water vapour released when the manganese (II) hydroxide is dried and dehydroxylated 503d is captured by passing 528 sodium oxide from the redox reaction 505 through an atmosphere to which the manganese (II) hydroxide is exposed, to produce sodium hydroxide in solid phase, which is then also used as a reagent in Eqn. 7d to produce 503b the manganese (II) hydroxide. Since the amount of sodium oxide produced by the redox reaction 505 is 50% more than is required to produce this sodium hydroxide, this leaves some of the sodium oxide as an end-product, along with the elemental manganese produced by the redox reaction 505. In this embodiment, the molar quantity of sodium ions lost from the chemical cycle as a result is replaced by fusing and electrolysing 101b an equivalent molar quantity of solid sodium chloride to produce gaseous chlorine and liquid sodium. This liquid sodium is added to the redox reaction 505, along with the liquid sodium which is recycled 529 from electrolysis 201c. If desired, the additional gaseous chlorine thus produced, along with the excess sodium oxide remaining as an end-product, may then be used in another chemical process or reaction scheme, such as that described in the present applicant's co-pending UK patent application no. XXXXXXX.X ("Method and Apparatus for Producing Oxides of Calcium, Magnesium and Iron from Carbonate Mineral Ores without Burning Carbonaceous Fuels"; applicant's ref: NE-P-GB 005), mentioned above. Equally, the oxygen produced by electrolysis 201c, although it may just be safely vented to atmosphere, may instead be used, for example, in a steelmaking process by injecting it into molten iron. Fig. 13 shows an eleventh embodiment of a method 500k of producing manganese from a manganese ore, wherein the manganese ore comprises at least 4% by weight of an iron oxide. For the sake of example, in this embodiment, the iron oxide is FejOa and it is desired that elemental iron derived from this iron oxide should remain mixed in with the elemental manganese produced by the method 500k, so that their combined mixture may subsequently be used to make an austenitic manganese steel. Therefore, in this embodiment, as well as comminuting 501 the ore, a manganiferous mineral in the ore is converted into a trivalent manganese oxide using a reductant-free pyrometallurgical technique 502c, which comprises heat-treating the ore in atmospheric air to a temperature of from 100 to 600 ’Celsius. This pyrometallurgical technique 502c is used to convert the manganiferous mineral in the ore into a trivalent manganese oxide instead of using a hydrometallurgical technique as described herein because if the hydrometallurgical technique were used instead, the iron (III) ions in the FejOa would precipitate out as iron (III) oxyhydroxide at a lower pH than manganese (II) hydroxide and would therefore risk being separated from the trivalent manganese oxide produced by drying the manganese (II) hydroxide. On the other hand, if it were instead desired to separate the iron (III) ions from the trivalent manganese oxide as an unwanted contaminant, then in alternative possible embodiments, the hydrometallurgical technique could be used instead. After the heat-treatment 502c, the method 500k then comprises cooling 509 the heat-treated ore back down to less than 100 ’Celsius before diamagnetic gangue species are magnetically separated 504 from the strongly paramagnetic trivalent manganese oxide and the antiferromagnetic FejOa. This cooling 509 avoids the magnetic susceptibilities of the trivalent manganese oxide and the FejOa from being reduced by the elevated temperature of their prior heat-treatment 502c. Heat extracted from cooling 509 the heat-treated ore back down is also recycled 308 back to the heat-treatment 502c. Next, the heat-treated ore from which the diamagnetic gangue species have been magnetically separated is added to excess liquid sodium. In this case, however, the redox reaction 505b between the liquid sodium and the trivalent manganese oxide is conducted at a temperature of less than 450 ’Celsius to avoid forming ternary oxides like Na4FeO3 derived from the iron content of the ore. Thereafter, the method 500k further comprises phase-separating 506 the manganese, iron and other insoluble products from the liquid sodium, and separating 507 the iron and manganese from the other insoluble products, in any order and using one or more of the techniques described previously. For example, if the manganese is separated 507 from the other insoluble products based on their different densities, then the iron, which is of a similar density to the manganese, accompanies the manganese. As noted above, the temperature of the pyrometallurgical technique is insufficiently high to thermally decompose manganese silicates like braunite or rhodonite into a trivalent manganese oxide or to thermally decompose hollandite and other manganese oxides such as cryptomelane which are isostructural to hollandite. However, as also noted above, siliceous manganese minerals like braunite and rhodonite, as well as hollandite and other manganese oxides which are isostructural to hollandite, are significantly paramagnetic. Therefore, if the iron-containing manganese ore in the method 500k just described also comprises any of these manganiferous minerals, they would tend to remain mixed in with the trivalent manganese oxide and the FejOa after the magnetic separation 504. Accordingly, Figs. 14A and 14B respectively show two different embodiments of methods 500m and 500n of producing manganese from a manganese ore, wherein the manganese ore comprises at least 4% by weight of an iron oxide, but gangue species derived from siliceous manganese minerals like braunite and rhodonite or from manganese oxides which are isostructural to hollandite, including hollandite itself, may nonetheless still be removed from the process stream to arrive at the target metal(s) substantially uncontaminated by these gangue species. The methods 500m and 500n both initially proceed in the same manner as the method 500k of Fig. 13 in that they both comprise the same comminution 501, heat-treatment 502b, cooling 509 and magnetic separation 504. Figs. 14A and 14B show how some gangue species are separated out by these processes, but that hollandite, cryptomelane and braunite, for example, remain mixed in with the trivalent manganese oxide and FejOa even after the magnetic separation 504. In the method 500m of Fig. 14A, the temperature of the reaction 505c with the liquid sodium is controlled to remain below about 300 ’Celsius to prevent silica derived from the braunite from reacting with the liquid sodium and producing sodium silicate(s). In contrast, in the method 500n of Fig. 14B, the reaction 505d with the liquid sodium is conducted within a temperature range of from about 350 to about 450 ’Celsius to induce a reaction between the sodium oxide and silica derived from the braunite to produce at least sodium orthosilicate, whilst still avoiding the formation of ternary oxides like Na4FeO3 derived from the iron content of the ore. In both the methods 500m and 500n, potassium derived from cryptomelane dissolves in the excess liquid sodium, but in the method 500n of Fig. 14B, sodium orthosilicate, as well as possibly any sodium metasilicate also produced, dissolve in the excess liquid sodium as well. Thereafter, the methods 500m and 500n both comprise separating 506 the solid phase from the liquid sodium, wherein in both cases, the solid phase comprises barium oxide derived from hollandite and calcium oxide derived from rhodonite, for example, whereas the solid phase additionally comprises silica only in the case of the method 500m of Fig. 14A. The elemental manganese is then separated from the other insoluble products. For the sake of example, this separation comprises adding 508a the separated solid phase to liquid water, thereby dissolving the sodium oxide, barium oxide and calcium oxide therein, and phase-separating 508b undissolved solids comprising both elemental iron and elemental manganese from the aqueous solution thus formed. In the case of the method 500m of Fig. 14A, since these undissolved solids also comprise silica, a further density-based separation 507c is conducted to separate the silica from the much denser iron and manganese. Finally, an optional magnetic separation 523 may also be carried out in either embodiment to separate the ferromagnetic iron from the weakly paramagnetic manganese, depending on whether it is desired to keep the iron and manganese mixed together, for example to make an austenitic manganese steel, or not. Bearing this in mind, Fig. 15 shows part of a fourteenth embodiment of a method 500p of producing manganese from a manganese ore, wherein the manganese ore again comprises at least 4% by weight of an iron oxide, but it is desired to remove the iron produced by the redox reaction 505b from the elemental manganese as an unwanted contaminant. As before, the method 500p comprises the same comminution 501, heat-treatment 502b, cooling 509 and magnetic separation 504 before the redox reaction 505b, which have only been omitted from Fig. 15 for the sake of brevity. Since, in general, separating at least some of the solid phase from the liquid sodium and separating the elemental manganese from the other insoluble products of the redox reaction 505b may be carried out in any order, in contrast to the methods 500m and 500n of Figs. 14A and 14B, the method 500p comprises magnetically separating 523a the iron from the elemental manganese when the solid phase is still hot from the redox reaction 505b and has a temperature of at least 200 ’Celsius. This has the advantage in comparison to the methods 500m and 500n that the magnetic susceptibility of the manganese is significantly reduced by this elevated temperature, thereby enhancing the degree of separation achieved. The iron from this "wet" magnetic separation 523a is then phase separated 305a from the liquid sodium, and the manganese and the other insoluble products are phase separated 506 from the liquid sodium as well in a different process stream, either before or after the manganese is separated 507 from the other insoluble products using one of the techniques described previously. In contrast to the embodiments of Figs. 13 to 15, wherein an iron oxide is present as a naturally occurring constituent of the manganese ore, Fig. 16 shows a fifteenth embodiment of a method 500q, wherein before the trivalent manganese oxide is added to the liquid sodium, finely comminuted, dehydrated and dehydroxylated iron ore is mixed in with the manganese ore to increase its iron content as desired, for example in order to create an iron-manganese mixture equivalent to a low-carbon ferromanganese, suitable for making an austenitic manganese or stainless steel. Fig. 16 illustrates how this iron ore may be added to the manganese ore in at least one of several different ways, as follows, which are represented by dashed lines in Fig. 16. Firstly, the iron ore and manganese ore may be mixed together before their combined mixture is then comminuted 501 and heat-treated 502. Secondly, finely comminuted, dehydrated and dehydroxylated iron ore may be mixed in with the manganese ore after the manganese ore has itself been separately comminuted 501, heat-treated 502 and cooled 509 back down, but before diamagnetic gangue species are magnetically separated 504 from the combined ores. Thirdly, finely comminuted, dehydrated and dehydroxylated iron ore may be mixed with the manganese ore after diamagnetic gangue species have been magnetically separated 504 from the separately comminuted 501, heat-treated 502 and cooled 509 manganese ore. In the last two cases, in alternative possible embodiments and as Fig. 4 above makes clear, a manganiferous mineral in the ore may instead be converted into a trivalent manganese oxide using an inorganic hydrometallurgical technique as described herein. In all such cases, however, the subsequent reaction 505b of the mixed ores with the liquid sodium is conducted at a temperature of less than 450 ’Celsius to avoid a reaction between iron and sodium oxide forming unwanted ternary oxides like Na4FeO3. After the reaction 505b, the method 500q then comprises phase-separating 506 the manganese, iron and other insoluble products from the liquid sodium, and separating 507 the manganese and iron from the other insoluble products, in any order and using one or more of the techniques described previously. Fig. 17 shows part of a sixteenth embodiment of a method 500r, illustrating how the temperature of the reaction 505b may be controlled to remain below a certain level. Whereas in this embodiment, the temperature is controlled to remain below about 450 ’Celsius to prevent iron ions mixed in with the trivalent manganese oxide from reacting with sodium oxide to form unwanted ternary oxides like Na4FeO3, the same method is equally applicable to controlling the temperature of the redox reaction with the liquid sodium to any other level, such as to remain below about 600 ’Celsius to prevent manganese ions from reacting with sodium oxide to form other ternary oxides like a-NaMnO?, for example if iron ions are substantially absent from the trivalent manganese oxide, or to remain below about 300 ’Celsius to prevent any silica present from reacting with sodium oxide to form sodium silicate(s). Therefore, after comminuting 501 and converting 502, 503 a manganiferous mineral in the ore into a trivalent manganese oxide (which have again been omitted from Fig. 17 for brevity), the method 500r comprises reacting 505b the trivalent manganese oxide with liquid sodium in an inert atmosphere, as before. The temperature of the reaction 505b is controlled 313 by cooling 309c the liquid sodium from which the solid phase has been separated 506 following this reaction, and then returning 314 at least some of the cooled liquid sodium to the reaction in a loop or circuit. Before returning it to the reaction 505b, the liquid sodium is cooled 309c by being circulated through pipework of a heat exchanger surrounding and / or contained within a dryer of the ore. Heat from the liquid sodium is used to heat 308 the ore in the dryer to the desired temperature for its heat treatment 502b. Adjusting 315 a flow rate of the liquid sodium in this cooling circuit also allows the temperature of the reaction 505b to be controlled 313 to remain below 450 ’Celsius. The flow rate of the liquid sodium may be adjusted 315 by such things as an initial choice of how much liquid sodium in excess of the stoichiometric amount thereof required for the reaction 505b is used and / or an initial choice of the diameter of the pipework. Alternatively or additionally, a pumping rate for the liquid sodium around the cooling circuit may be varied. Following separation 506 of the solid phase from the liquid sodium, the iron and the other insoluble products are then separated from each other when the solid phase is still hot from the redox reaction 505b and has a temperature of at least 200 ’Celsius for the same reasons as explained above. This "dry" magnetic separation 523b may be contrasted with the "wet" magnetic separation 523a of the iron described above in relation to Fig. 15. Finally, the manganese is also separated 507 from the other insoluble products using one of the techniques described previously, such as based on their different densities. Fig. 18 shows part of a seventeenth embodiment of a method 500s of producing manganese from a manganese ore. The method 500s initially comprises comminuting the ore and converting a manganiferous mineral in the ore into a trivalent manganese oxide (which have only been omitted from Fig. 18 for the sake of brevity), before adding 505 the trivalent manganese oxide to an excess amount of liquid sodium. The solid phase produced thereby is then phase-separated 506 from the excess liquid sodium, as before. In this embodiment, the elemental manganese in the solid phase is then separated 507 from the other insoluble reaction products by adding 508a at least some of the separated solid phase to liquid water, and phase-separating 508b undissolved solids comprising the elemental manganese from the aqueous solution of sodium hydroxide thus formed, as described above. Fig. 18 then illustrates a first example of how, in such a case, the aqueous solution of sodium hydroxide thus produced may be recycled. Accordingly, in this embodiment, the aqueous solution of sodium hydroxide is firstly neutralized 403a by being injected with gaseous hydrogen chloride to produce an aqueous solution of sodium chloride. This aqueous solution of sodium chloride is then dried 110 to produce solid sodium chloride and water vapour. The solid sodium chloride is fused and electrolysed 101b via the Downs process to produce liquid sodium and gaseous chlorine. The liquid sodium is recycled 525 to the reaction 505 with the trivalent manganese oxide and the gaseous chlorine is used to make 203 the gaseous hydrogen chloride which is injected into the aqueous solution of sodium hydroxide. Meanwhile, the water vapour produced by drying 110 the aqueous solution of sodium chloride is captured 111 and condensed 112 into liquid water, which is also recycled 526 to provide the liquid water to which the solid phase produced by the reaction 505 is added 508a. Thus, it may be seen that sodium and chloride ions, and water, used in the method 500s can all be recycled and not consumed. In addition, and unrelated to this recycling, the method 500s also comprises the feature that the reaction mixture produced by the redox reaction 505 between the liquid sodium and the trivalent manganese oxide is cooled 524 to dwell at a temperature of less than 225 ’Celsius until the reaction is complete, before the solid phase produced by this reaction is separated 506 from the liquid sodium. This process ensures the complete reduction of any manganese (II) oxide (MnO) formed during the reduction of trivalent manganese oxide by the liquid sodium. Fig. 19 shows part of an eighteenth embodiment of a method 500t of producing manganese from a manganese ore. Like the method 500s of Fig. 18, the method 500t initially comprises comminuting the ore and converting a manganiferous mineral in the ore into a trivalent manganese oxide, which have again only been omitted from Fig. 19 for the sake of brevity, before the trivalent manganese oxide is added to liquid sodium. In the present embodiment, however, the trivalent manganese oxide comprises an admixture of iron ions, which, for the sake of example, are considered as an unwanted contaminant. The temperature of the reaction 505b between the iron and manganese oxides on the one hand and the liquid sodium on the other is therefore controlled to remain below about 450 ’Celsius, to avoid the formation of ternary oxides like Na4FeO3 from these iron ions. Once again, the solid phase produced by the reaction 505b is phase-separated 506 from the excess liquid sodium, before the solid phase is added 508a to liquid water. At this stage, however, the unwanted elemental iron produced by the reaction 505b from the admixture of iron ions is magnetically separated 523c from the other insoluble reaction products whilst they are still in aqueous phase. This aqueous-phase magnetic separation 523c of the iron may therefore be contrasted with the magnetic separation 523a of the iron whilst still suspended or entrained in liquid sodium, which was described above in relation to Fig. 15, and with the "dry" magnetic separation 523b of the iron described above in relation to Fig. 17. After this magnetic separation 523c, the iron is then phase-separated 527 from the aqueous solution of sodium hydroxide and the other insoluble products, including elemental manganese, are also phase-separated 508b from the aqueous solution of sodium hydroxide in two separate process streams. The manganese is then separated 507a from the other insoluble products based on their different densities, and the aqueous solution of sodium hydroxide from both process streams is recombined. Fig. 19 then illustrates an example of a second way in which this aqueous solution of sodium hydroxide may be recycled. Accordingly, in this embodiment, the aqueous solution of sodium hydroxide is dried 319 to produce solid sodium hydroxide and water vapour. The solid sodium hydroxide is fused and electrolysed 320 via the Castner process to produce liquid sodium, which is recycled 525a to the reaction 505b with the iron and manganese oxides. Meanwhile, the water vapour produced by drying 319 the aqueous solution of sodium hydroxide is captured 215 and condensed 216 into liquid water, which is also recycled 526 to provide the liquid water to which the solid phase produced by the reaction 505b is added 508a. Thus, it may be seen that both water and sodium ions used in the method 500t can be recycled and not consumed. Moreover, in alternative possible embodiments, some of the sodium used in the reaction with the trivalent manganese oxide may be produced via the Downs process and some of it may be produced via the Castner process, in which case, gaseous hydrogen also produced via the Castner process may be combusted with gaseous chlorine produced via the Downs process to produce at least some of the gaseous hydrogen chloride which is injected into the aqueous solution of sodium hydroxide in the manner of the method 500s of Fig. 18. Fig. 20 schematically shows a first embodiment of an apparatus 5a for producing manganese from a manganese ore. The apparatus 5a comprises a comminution device 2, a mineral-converting subassembly 54, a gas-tight first reaction vessel 130, a solid-species separator 180a and a solid-liquid sodium phase separator 190, which are connected to each other as shown. The comminution device 2 is for comminuting the manganese ore into fines. The mineral-converting subassembly 54 is for converting a manganiferous mineral in the ore into a trivalent manganese oxide via at least one of a reductant-free pyrometallurgical technique and an inorganic hydrometallurgical technique. Some different possible embodiments of the mineral-converting subassembly 54 will be described further below. The first reaction vessel 130 is for reacting the trivalent manganese oxide in an inert atmosphere and at a temperature of less than about 600 ’Celsius with an amount of liquid sodium which is in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide. It comprises a first inlet 131 for receiving the trivalent manganese oxide from the mineral-converting subassembly 54 and a second inlet 132 for liquid sodium. For example, the first reaction vessel 130 may contain a bath of liquid sodium to which the trivalent manganese oxide is added. Liquid sodium consumed in the reaction is replenished by fresh liquid sodium introduced via the second inlet 132, in order to maintain the level of liquid sodium in the first reaction vessel 130 roughly constant. The first reaction vessel 130 also comprises an outlet 134 for the liquid sodium and suspended or entrained therein, a solid phase comprising elemental manganese and other insoluble products at least comprising sodium oxide, produced by the redox reaction. The first reaction vessel 130 is sealed in a gas-tight manner from its surrounding environment to prevent the liquid sodium it contains from reacting with oxygen from atmospheric air. A head space in the reaction vessel 130 above the bath of liquid sodium is filled with an atmosphere consisting of one or more inert gases, such as nitrogen and / or argon. Either nitrogen or argon, or both, may be produced on site by pressure swing adsorption (PSA) of atmospheric air. An on-site PSA generator of inert gas may be powered using waste heat from the first reaction vessel 130 itself or from another source. To resist corrosion by the sodium oxide, the first reaction vessel 130, which is a stirred tank reactor, is made of a titanium alloy having the composition described above. Liquid sodium introduced into the first reaction vessel 130 establishes a gradient of concentration of dissolved species in the sodium bath, from the purest liquid sodium at the second inlet 132 to the highest concentration of dissolved species at the outlet 134. For safety, the first reaction vessel 130 may also comprise an overflow outlet for liquid sodium, as well as a pressure relief valve. This is preferably located at or near the top of the head space in the reaction vessel 130, not only to relieve excess pressure, but also to vent any light gases, such as hydrogen, which would tend to collect there. The solid-species separator 180a is for separating the elemental manganese from the other insoluble products. For the sake of example, in this embodiment, the solid-species separator 180a is a "wet" type of solid-species separator, which separates the elemental manganese from the other insoluble products whilst both are still suspended or entrained in liquid sodium, based on their different densities. It comprises an inlet 181 connected to the outlet 134 of the first reaction vessel 130 and two outlets 184,185, respectively for liquid sodium with elemental manganese entrained therein, and for liquid sodium with the other insoluble products entrained therein. The solid-species separator 180a may be a "natrocyclone" of the type mentioned above and should be made of materials suitable for containing and transporting liquid sodium. A series of such separators may be used to increase the separation efficiency. The solid-liquid sodium phase separator 190 is for separating at least some of the solid phase from the liquid sodium and comprises first and second phase separating devices 190a, 190b. The first 190a of these separates the elemental manganese from the liquid sodium, and the second 190b instead separates the other insoluble products from the liquid sodium. The present applicant's co-pending UK patent application no. 2417052.4, ("Apparatus and Method for Separating a Contaminant from Liquid Metal"; applicant's ref: NE-P-GB 002) mentioned above, shows and describes suitable phase separating devices 190a, 190b. In the apparatus 5a, liquid sodium from the respective outlets 194a, 194b of the first and second phase separating devices 190a, 190b is firstly cooled and then recycled back to the second inlet 132 of the first reaction vessel 130, to which fresh liquid sodium is also added in order to replace that consumed by the redox reaction. Fig. 21 schematically shows a second embodiment of an apparatus 5b for producing manganese from a manganese ore. The apparatus 5b again comprises a comminution device 2, a mineral-converting subassembly 54, a gas-tight first reaction vessel 130, a solid-liquid sodium phase separator 190 and a solid-species separator 180b, which are connected to each other as shown. In this embodiment, the mineral-converting subassembly 54 comprises a high-temperature dryer 4 for heat-treating the ore in atmospheric air and a first magnetic separator 560. For the sake of example, ore which is fed into the apparatus 5b comprises a carbonaceous mineral like rhodochrosite. Thus apart from water vapour produced by dehydrating and dehydroxylating the ore within the high-temperature dryer 4, carbon dioxide is also released by the thermal decomposition of the rhodochrosite therein to produce a trivalent manganese oxide according to Eqn. 5a. The first magnetic separator 560 is for magnetically separating diamagnetic components of the heat-treated ore from paramagnetic components thereof, the latter of which now therefore comprise the trivalent manganese oxide. The high-temperature dryer 4 can be rendered gas-tight, so that the thermal decomposition of the rhodochrosite can be conducted closed off from its surrounding environment and the carbon dioxide captured. In contrast to the apparatus 5a of Fig. 20, in the present embodiment, the solid-species separator 180b is a "dry" type of solid-species separator, which separates the elemental manganese from the other insoluble products based on their different densities, after they have both been separated from the liquid sodium by the solid-liquid sodium phase separator 190. In the apparatus 5b, liquid sodium recovered from the solid-liquid sodium phase separator 190 is also recycled back to the first reaction vessel 130, but the other insoluble products, which as a result of the action of the first magnetic separator 560, now consist mostly of sodium oxide, are transported by a conduit 137 from the second outlet 187 of the solid-species separator 180b through an atmosphere to which the rhodochrosite is exposed within the high-temperature dryer 4. These other insoluble products may be transported along the conduit 137 for example mechanically (e.g., using a screw conveyor) and / or at least partially under gravity, for example. Since they are in the form of a finely divided particulate, the sodium oxide therein can readily absorb both the water vapour and the carbon dioxide released by the rhodochrosite within the high-temperature dryer 4 to produce a mixture of products comprising sodium hydrogencarbonate, anhydrous sodium carbonate and so on, as described above. Moreover, since the sodium oxide is still hot from the redox reaction in the first reaction vessel 130 and both the hydration and the carbonation of this sodium oxide are strongly exothermic, once the thermal decomposition of the rhodochrosite has been initiated by an independent heat source, both the sodium oxide and the products of these hydration and carbonation reactions can transfer sufficient heat to the ore entering the dryer 4 to raise the temperature of the ore to between 200 and 320 ’Celsius, inclusive, which is sufficient for continuing thermal decomposition of the rhodochrosite within the dryer 4 without supplying any more heat from another source. Fig. 22 schematically shows a third embodiment of an apparatus 5c for producing manganese from a manganese ore. The apparatus 5c differs from the apparatus 5b of Fig. 21 in the following respects. In the present embodiment, the solid-species separator 180b again separates the elemental manganese from the other insoluble products after they have both been separated from the liquid sodium by the solid-liquid sodium phase separator 190. However, in the present embodiment, instead of being a density-based separator, the solid-species separator 180b comprises a hydration vessel 170 and a solid-aqueous phase separator 175. The hydration vessel 170 is for reacting at least some of the sodium oxide in the solid phase separated from the liquid sodium with liquid water to produce an aqueous solution of sodium hydroxide. It comprises an inlet 171, which is the inlet 181 of the solidspecies separator 180b, for receiving the solid phase from the second outlet 195 of the solid-liquid sodium phase separator 190, a second inlet 172 for the water, and an outlet 174 for the aqueous solution of sodium hydroxide and, entrained therein, undissolved solids comprising the elemental manganese. The solid-aqueous phase separator 175 is for separating these undissolved solids from the aqueous solution of sodium hydroxide. It comprisesan inlet 176 for receiving the aqueous solution of sodium hydroxide with the undissolved solids entrained therein from the outlet 174 of the hydration vessel 170, a first outlet 178 for the aqueous solution of sodium hydroxide and a second outlet 179 for the undissolved solids. As in the apparatus 5b of Fig. 21, the high-temperature dryer 4 can again be rendered gas-tight to permit the thermal decomposition of the rhodochrosite to be conducted therein closed off from its surrounding environment. In the present embodiment, however, the dryer 4 comprises an outlet 12 for gaseous carbon dioxide produced by this thermal decomposition, as well as for water vapour produced by drying and dehydroxylating the manganese ore within the dryer 4. The apparatus 5c further comprises a carbonation vessel 160 for reacting this gaseous carbon dioxide and water vapour with at least some of the aqueous solution of sodium hydroxide from the solid-aqueous phase separator 175 by bubbling the gases from the dryer 4 through the aqueous solution of sodium hydroxide. This carbonation reaction produces a mixture of products again comprising sodium hydrogencarbonate, anhydrous sodium carbonate and so on, as described previously. To this end, the carbonation vessel 160 comprises a first inlet 162 for receiving the gaseous carbon dioxide and water vapour from the outlet 12 of the high-temperature dryer 4, a second inlet 163 for receiving the aqueous solution of sodium hydroxide from the first outlet 178 of the solid-aqueous phase separator 175, and an outlet 166 for this mixture of products. Fig. 23 schematically shows an embodiment of the high-temperature dryer 4 in greater detail and having features which make it suitable for use in the apparatus 5c. The dryer 4 comprises an entry 6 and an exit 7 located in proximity to each other at one end of the dryer 4. The entry 6 and exit 7 may each be provided with spring-loaded doors and / or curtains to help maintain and control an atmosphere within the dryer 4. The atmosphere within the dryer 4 may also be held at a pressure below that of atmospheric air outside the dryer 4, to draw air in through the entry 6 and / or exit 7 and thereby hinder or prevent the release of gases from within the dryer 4 via the entry 6 and / or exit 7. Manganese ore 1 at or close to ambient temperature enters the dryer 4 via the entry 6 on a first conveyor 8a and travels towards a hot zone 9 located at an opposite end of the dryer 4 from the entry 6 and exit 7. The hot zone 9 comprises means for heating the ore 1, such as pipework 5, described further below. This establishes a temperature gradient from the entry 6 and exit 7, increasing along the dryer 4 towards the hot zone 9 at the opposite end thereof. A fan 10 mounted in a duct 11 extracts water vapour and other gases, such as carbon dioxide, driven off from the heated manganese ore through a vent 12 also located in the hot zone 9. This reduces the partial pressure of these gases within the dryer 4, which promotes the dehydration, dehydroxylation and thermal decomposition of the ore 1. The fan 10 can also be used to maintain the total pressure within the dryer 4 below that of the air outside the dryer 4 by overcompensating for the increased pressure of the atmosphere within the dryer 4 caused by its expansion on heating. The dryer 4 also comprises an inlet 13, whereby, for example, dry air or inert gas may be introduced into the dryer 4, to adjust the levels of oxygen and / or water vapour within the dryer 4. For example, inert gas may be introduced into the dryer 4 via the inlet 13 to reduce the level of oxygen within the dryer 4. When the manganese ore 1 reaches the hot zone 9, it falls under gravity onto a second conveyor 8b contained within the dryer 4 and is transported by this in the opposite direction back towards the exit 7. As the ore travels from the hot zone 9 back towards the exit 7, it radiates heat towards the manganese ore entering the dryer 4 on the first conveyor 8a. The dryer 4 may have a reflective lining to increase a rate of this radiative heat transfer. The radiative heat transfer may also be enhanced by convective heat transfer, by circulating the atmosphere within the dryer 4 from the second conveyor 8b towards the first conveyor 8a using appropriate ducting and / or one or more additional fans. Thus heat is transferred from the dehydrated and dehydroxylated ore leaving the dryer to fresh ore entering the dryer in a counterflow system within the dryer 4. When the ore 1 reaches the end of the second conveyor 8b, it falls under gravity through the exit 7 onto a third conveyor 8c and is transported by this towards the first inlet 131 of the first reaction vessel 130. Whereas in Fig. 23, the high-temperature dryer 4 is schematically represented as being only about twice as long as it is high, this is for illustrative and explanatory purposes only. In practice, the dryer 4 may be several tens of metres long in comparison to a height of only about one or two metres, so that the entry 6 and exit 7 are remotely located from the hot zone 9, and a significant temperature gradient can be established and maintained between the entry 6 and exit 7 at one end of the dryer 4 and the hot zone 9 at the other end. Moreover, whereas the embodiment of the high-temperature dryer 4 shown in Fig. 23 is suitable for use in the apparatus 5c, in another possible embodiment of the dryer 4 suitable for use in the apparatus 5b of Fig. 21, the dryer 4 may lack duct 11 and vent 12 and instead comprise entry and exit points for the conduit 137 to pass through the hot zone 9. In the apparatus 5b of Fig. 21, heat is transferred directly from solid-phase products being transported along the conduit 137 to the manganese ore within the dryer 4, as described above. However, in the apparatus 5c of Fig. 22, no such heat transfer takes place. Fig. 24 therefore schematically shows an embodiment of a heat transfer pathway, which is suitable for transferring heat to the manganese ore within the high-temperature dryer 4 of the apparatus 5c of Fig. 22, for example. In this embodiment, the apparatus 5c of Fig. 22 further comprises pipework 55, which takes liquid sodium from the first outlet 194 of the solid-liquid sodium phase separator 190 and circulates it around the hot zone 9 of the high-temperature dryer 4, before this liquid sodium is then routed to combine with fresh liquid sodium produced, for example, by fusing and electrolysing solid sodium hydroxide in a Castner cell. The liquid sodium from the first outlet 194 of the phase separator 190 may have a temperature from the exothermic reaction of Eqn. 10 of about 500 ’Celsius or more, whereas comminuted ore entering the dryer 4 via its entry 6 may have a temperature of only about 50 ’Celsius, for example. This liquid sodium is therefore able to transfer some of its heat to the ore within the hot zone 9 to raise the temperature of the ore to within a range of from about 200 to about 320 ’Celsius. This is sufficiently high to thermally decompose the rhodochrosite in the ore into bixbyite and carbon dioxide, but is not high enough for reabsorption of the carbon dioxide by the bixbyite. However, both the flow rate of the liquid sodium and the rate of transfer of the ore through the dryer 4 are chosen to ensure that the liquid sodium does not reach thermal equilibrium with the ore and instead remains hotter than the ore, so that when this liquid sodium combines and mixes with the fresh liquid sodium, it may still have a temperature of about 330 ’Celsius, similar to that of the fresh liquid sodium. Thus the combined liquid sodium enters the first reaction vessel 130 via its second inlet 132 at a temperature which is suitable for the reaction of Eqn. 10 and the manganese ore can simultaneously be heated by the pipework 55 to a sufficient temperature to cause the thermal decomposition of the rhodochrosite before the ore leaves the high-temperature dryer 4 via its exit 7. On the other hand, the counterflow system described above in relation to Fig. 23 also increases the temperature at which the ore arrives in the hot zone 9 and decreases the temperature at which the ore leaves the dryer 4 via its exit 7. Thus liquid sodium leaving the pipework 55 enters the first reaction vessel 130 at a higher temperature, and the heat-treated ore leaving the dryer 4 via its exit 7 enters the first magnetic separator 560 at a lower temperature, than if no such counterflow system were present. This ensures that the heat-treated ore is at a sufficiently low temperature, which for example in this embodiment may be about 70 ’Celsius, that the magnetic susceptibility of the paramagnetic bixbyite is not significantly diminished thereby and that it can still be magnetically separated from diamagnetic gangue species in the ore by the first magnetic separator 560. Although the apparatuses 5b and 5c of Figs. 21 and 22 both comprise a solid-liquid sodium phase separator 190, which separates excess liquid sodium from the solid phase produced by the redox reaction before elemental manganese is then separated from the other insoluble products in the solid phase, the same alternative possibilities for cooling the liquid sodium and heating the manganese ore within the high-temperature dryer 4 via pipework 55 may equally well be applied to an apparatus comprising a "wet" type of solid-species separator instead, as in the embodiment of Fig. 20. Thus the temperature of the reaction of Eqn. 10 may be controlled to remain within its desired range of operating temperatures by varying at least one of the flow rate of liquid sodium through the pipework 55, the cross-sectional area of the pipework 55 and the amount of liquid sodium in excess of the stoichiometric amount thereof required for the reaction of Eqn. 10. Fig. 25 schematically shows a fourth embodiment of an apparatus 5d for producing manganese from a manganese ore. The apparatus 5d again comprises a comminution device 2, a mineral-converting subassembly 54, a gas-tight first reaction vessel 130, a solid-liquid sodium phase separator 190 and a solid-species separator 180b, which are connected to each other as shown. In this embodiment, however, the mineral-converting subassembly 54 comprises components which are suitable for converting the manganiferous mineral in the ore into a trivalent manganese oxide via the hydrometallurgical technique. The mineral-converting subassembly 54 therefore comprises a gastight second reaction vessel 540, a solid-aqueous phase separator 545, a low-temperature dryer 550 and a second magnetic separator 570. The gas-tight second reaction vessel 540 is for dissolving the manganiferous mineral therein in hot, concentrated hydrochloric acid. It comprises a first inlet 541 for receiving the comminuted ore from the comminution device 2, a second inlet 542 for receiving the hydrochloric acid, a third inlet 543 for receiving sodium oxide or hydroxide, a first outlet 544 for an aqueous solution and a precipitate comprising manganese (II) hydroxide, and a second outlet 546 for gaseous chlorine. The first inlet 541 comprises an airlock 541a, which prevents gaseous chlorine escaping from the second reaction vessel 540 into the surrounding environment when the ore is introduced via the first inlet 541. In addition, the second and third inlets 542, 543 and the first and second outlets 544, 546 are each provided with respective valves, VI, V2, V3, V4, which keep the second reaction vessel 540 gas-tight when the respective reagents are introduced via the second and third inlets 542, 543 or when their products leave via the first and second outlets 544, 546, and which can also be used to regulate entry and exit of the respective reagents and products into and out of the reaction vessel 540. In an alternative possible embodiment, however, the first inlet 541 may double as the second outlet 546 for gaseous chlorine. The solid-aqueous phase separator 545 is for separating at least some of the precipitate from the aqueous solution and comprises an inlet 547 for receiving the aqueous solution and the precipitate from the first outlet 544 of the second reaction vessel 540, a first outlet 548 for the precipitate and a second outlet 549 for the aqueous solution. The low-temperature dryer 550 is for drying and dehydroxylating at least some of the manganese (II) hydroxide in the precipitate to produce the trivalent manganese oxide, and comprises an inlet 551 for receiving the precipitate from the first outlet 548 of the solid-aqueous phase separator 545, and an outlet 554 the trivalent manganese oxide. The second magnetic separator 570 is for magnetically separating paramagnetic components of the precipitate from diamagnetic components thereof. It is adapted to maintain the precipitate at a temperature of less than about 100 ’Celsius and comprises an inlet 571 for receiving the precipitate from the outlet 554 of the low-temperature dryer 550, a first outlet 574 for the diamagnetic components and a second outlet 575 for the paramagnetic components, which is connected to the first inlet 131 of the first reaction vessel 130. In other possible embodiments, however, the second magnetic separator 570 may be placed upstream of the low-temperature dryer 550, provided that it remains downstream of the third inlet 543 of the second reaction vessel 540, in which case, the outlet 554 of the low-temperature dryer 550 would be connected directly to the first inlet 131 of the first reaction vessel 130. During operation of the mineral-converting subassembly 54, the valves, VI, V2, V3, V4 and the airlock 541a may all be controlled to follow a method similar to that described above in relation to Fig. 10 or Fig. 11, for example. If the method of Fig. 11 is followed, solid phases respectively comprising silica or alumina, for example, may be directed from the first outlet 548 of the solid-aqueous phase separator 545 other than to the inlet 551 of the low-temperature dryer 550, and the aqueous solution may be recycled from the second outlet 549 of the phase separator 545 back to the second inlet 542 of the second reaction vessel 540, before the precipitate comprising manganese (II) hydroxide is eventually directed from the first outlet 548 of the phase separator 545 to the inlet 551 of the low-temperature dryer 550. At this time, the remaining supernatant, which is a neutral aqueous solution of sodium chloride, may be recycled from the second outlet 549 of the phase separator 545 to produce more liquid sodium and gaseous hydrogen and chlorine by electrolysis, as described above in relation to Fig. 12A, for example. The apparatus 5d further comprises a conduit 553 for transporting sodium oxide from the second outlet 187 of the solid-species separator 180b through an atmosphere to which the manganese (II) hydroxide is exposed within the low-temperature dryer 550. The sodium oxide may be transported along the conduit 553 for example mechanically (e.g., using a screw conveyor) and / or at least partially under gravity, for example. As the sodium oxide is in the form of a finely divided particulate, it readily absorbs water vapour released by the manganese (II) hydroxide as it dries and dehydroxylates and is hydrated thereby to produce solid-phase sodium hydroxide. This reduces the partial pressure of water vapour within the low-temperature dryer 550, thereby helping to dry and dehydroxylate the manganese (II) hydroxide. The conduit 553 then transports the sodium hydroxide thus formed to the third inlet 543 of the second reaction vessel 540. Since the sodium oxide is still hot from the redox reaction in the first reaction vessel 130, it may need to be cooled closer to ambient temperature before it enters the atmosphere to which the manganese (II) hydroxide is exposed within the low-temperature dryer 550. Heat recovered from the hot sodium oxide may be used in another process. For example, in an alternative possible embodiment, in which the mineral-converting subassembly 54 comprises components for converting different manganiferous minerals in the ore into one or more trivalent manganese oxides(s) via both the pyro-and hydrometallurgical techniques, the heat may be used in a high-temperature dryer thereof. Moreover, since hydration of the sodium oxide in the low-temperature dryer 550 is exothermic, the transport rates both of the precipitate through the dryer 550 and of the sodium oxide and hydroxide along the conduit 553 may also need to be regulated to ensure that the y-MnjOa produced by drying and dehydroxylating the manganese (II) hydroxide within the dryer 550 remains at a temperature of less than about 100 ’Celsius. This ensures that when the y-MnjOa enters the second magnetic separator 570, it is at a sufficiently low temperature that its magnetic susceptibility is not significantly diminished and that it can still be magnetically separated by the second magnetic separator 570 from any diamagnetic gangue species still remaining in the precipitate. Although the apparatus 5d of Fig. 25 comprises a solid-liquid sodium phase separator 190, which separates excess liquid sodium from the solid phase produced by the redox reaction before elemental manganese is then separated from the other insoluble products in the solid phase, the same possibilities of using sodium oxide in this solid phase to absorb water vapour within the low-temperature dryer 550 and / or as a reagent in the second reaction vessel 540 may equally well be applied to an apparatus comprising a "wet" type of solid-species separator instead, as in the embodiment of Fig. 20. Fig. 26 schematically shows an embodiment of an airlock 20 for preventing atmospheric oxygen from being introduced into the first reaction vessel 130 whenever the vessel is fed with the trivalent manganese oxide via the first inlet 131 thereof. The airlock 20 comprises two gas-tight doors 21, 22. The first such door 21 connects an interior 25 of the airlock 20 with the surrounding environment E, whereas the second such door 22 connects the interior 25 of the airlock 20 with an inside 135 of the first reaction vessel 130. The airlock 20 also comprises an inlet 23 to the interior 25 of the airlock 20 for inert gas and an outlet 24 from the interior 25 of the airlock 20 for atmospheric air. The inlet 23 and the outlet 24 each comprises a respective valve 23V, 24V for opening and closing a respective one of the inlet 23 and the outlet 24. To introduce a fresh charge of trivalent manganese oxide via the airlock 20 into the first reaction vessel 130, the first gas-tight door 21 is opened and the oxide particles are introduced into the interior 25 of the airlock 20, whilst the second gas-tight door 22 is kept closed. The first door 21 is then closed again and the interior 25 of the airlock 20 is flushed with inert gas by opening the valve 23V, and then also opening the valve 24V to purge atmospheric air from the airlock 20 via outlet 24. Both valves 23V, 24V are then closed again, before the second door 22 is opened to introduce the oxide into the first reaction vessel 130, whilst the first gas-tight door 21 is kept closed. The second gas-tight door 22 is then closed again, and the airlock 20 is ready to be used again, to introduce another charge of trivalent manganese oxide into the reaction vessel 130. To prevent the first reaction vessel 130 from being pressurized by inert gas which enters the reaction vessel 130 from the interior 25 of the airlock 20 together with the trivalent manganese oxide, by reason of the inert gas expanding as it heats up from the temperature of the surrounding environment E to the temperature inside 135 the first reaction vessel 130, the inert gas may be preheated before it is injected via inlet 23 into the airlock 20 to the same temperature as the temperature of the atmosphere of inert gas inside 135 the reaction vessel 130. This may be done using waste heat from the first reaction vessel 130 itself or from another source. For example, as shown in Fig. 13, the inert gas may be heated by liquid sodium from the first reaction vessel 130, via a heat exchanger HE6. Preheating the inert gas also has the advantage that it pressurizes the inert gas before it is injected into the airlock 20, thereby avoiding the need for the inert gas to be pumped into the airlock 20 and helping to purge the atmospheric air from the interior 25 of the airlock 20. Fig. 27 schematically shows a fifth embodiment of an apparatus 5e for producing manganese from a manganese ore, which comprises a third magnetic separator 580 as well as all the features of the apparatus 5a of Fig. 20. For the sake of example, manganese ore fed into the apparatus 5e comprises a significant proportion of iron cations, and it is desired to remove iron from the elemental manganese produced by the redox reaction as an unwanted contaminant. For example, the iron cations may be substituted for manganese cations in the manganiferous mineral and / or be present in the ore as one or more iron oxide(s) and / or oxyhydroxide(s). The third magnetic separator 580 is therefore for magnetically separating elemental iron in the solid phase produced by the redox reaction from the elemental manganese therein. It comprises an inlet 581 downstream of the outlet 134 of the first reaction vessel 130 for receiving the elemental iron mixed with the elemental manganese, a first outlet 584 for the elemental iron and a second outlet 585 for the elemental manganese. In this embodiment, the operating temperature of the first reaction vessel 130 is kept below about 450 ’Celsius to avoid the formation of ternary oxides like Na4FeO3. The elemental iron, elemental manganese and sodium oxide are all transported by excess liquid sodium from the reaction vessel 130 to the inlet 181 of the solid-species separator 180a, where the similarly dense iron and manganese are separated from the much lighter sodium oxide based on their different densities. As in the apparatus 5a of Fig. 20, the solid-species separator 180a is a "wet" separator, so that the iron and manganese, as well as the sodium oxide from which they are separated, remain entrained in liquid sodium as they leave the solid-species separator 180a via its respective outlets 184,185. Accordingly, the iron and manganese are transported by the liquid sodium from the outlet 184 to the inlet 581 of the third magnetic separator 580 still hot from the redox reaction. The third magnetic separator 580 is adapted to maintain the elemental manganese (as well as the elemental iron) at a temperature of at least 200 ’Celsius, so that the magnetic susceptibility of the paramagnetic manganese is diminished thereby, which enhances its degree of separation from the ferromagnetic iron. Since in this embodiment, the third magnetic separator 580 is also a "wet" type of separator, the iron and manganese remain entrained in liquid sodium as they leave the third magnetic separator 580 via its respective outlets 584, 585. Thus whilst the manganese is transported by the liquid sodium to the inlet 191a of the first phase separating device 190a, the iron is also transported by the liquid sodium to the inlet 191c of a third phase separating device 190c. Liquid sodium recovered by the third phase separating device 190c leaves via its outlet 194c and is recycled back to the first reaction vessel 130 in the same manner as it is from the respective outlets 194a, 194b of the first and second phase separating devices 190a, 190b. However, in alternative possible embodiments, the third magnetic separator 580 could instead be located downstream of the solid-liquid sodium phase separator 190, in which case, it would instead be a "dry" type of separator, or even upstream of the solid-species separator 180a, in which case, the iron would be magnetically separated from both the manganese and the sodium oxide before these are then separated from each other, for example based on their different densities or by hydrating the sodium oxide as described previously. Moreover, whereas the apparatus 5e comprises a "wet" type of solid-species separator 180a, in other possible embodiments, the third magnetic separator 580 may form part of an apparatus comprising a "dry" type of solidspecies separator instead and be located up- or downstream of that as well. In summary, therefore, the present invention a method and apparatus for producing manganese from a manganese ore without using any carbon or a carbon-containing reductant. Thus manganese can be extracted from its ores without producing any greenhouse gases, and sodium oxide, or sodium hydroxide derived from hydrating this sodium oxide, can be produced as a co-product. If this sodium oxide and / or hydroxide is then used to mineralize captured carbon dioxide, the invention can have a negative carbon footprint overall. 5 Whereas the present invention has been described above by reference to particular examples and embodiments, the scope of the invention should not be taken to be limited thereby and is instead defined by the appended claims.

Claims

1. A method (500a, 500b, 500ba, 500bb, 500c - 500k, 500m, 500n, 500p - 500t) of producing manganese from a manganese ore, wherein the method comprises:comminuting (501) the manganese ore into fines;converting (502, 502a, 502b, 503) a manganiferous mineral in the ore into a trivalent manganese oxide using at least one of a reductant-free pyrometallurgical technique (502) and an inorganic hydrometallurgical technique (503);adding (505, 505a, 505b, 505c, 505d) the trivalent manganese oxide to an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide, to produce a solid phase comprising both elemental manganese and other insoluble products at least comprising sodium oxide, wherein the redox reaction is conducted in an inert atmosphere and at a temperature of less than 600 ’Celsius;separating (506, 506a, 506b) at least some of the solid phase from the liquid sodium; and separating (507, 507a, 507b; 508a, 508b) the elemental manganese from the other insoluble products.

2. A method (500a, 500b, 500ba, 500bb, 500d, 500e, 500k, 500m, 500n, 500p - 500t) according to claim 1, wherein converting (502, 503) a manganiferous mineral in the ore into a trivalent manganese oxide comprises using a pyrometallurgical technique (502) comprising heat-treating (502, 502a, 502b) the ore in atmospheric air to a temperature of from 100 to 600 ’Celsius, inclusive, to dehydrate the ore, dehydroxylate hydroxylated compounds contained therein and thermally decompose the manganiferous mineral into the trivalent manganese oxide.

3. A method (500e) according to claim 2, wherein the manganese ore comprises a carbonate mineral, heat-treating (502) the manganese ore comprises heating (502a) the ore in atmospheric air closed off from their surrounding environment, and the method further comprises:capturing (512) at least some of the carbon dioxide gas released from the carbonate mineral during the heat-treatment; andreacting (513) at least some of the captured carbon dioxide in a carbonation reaction with a portion of the sodium oxide, or with sodium hydroxide derived from hydrating at least some of this portion of sodium oxide, to produce at least sodium carbonate.

4. A method (500e) according to claim 3, wherein the manganese ore comprises rhodochrosite, and heat-treating the ore (502, 502a, 502b) comprises heating (502a) the ore to a temperature of from 200 to 320 ’Celsius, inclusive.

5. A method (500e) according to claim 3 or claim 4, wherein heat-treating (502) the ore comprises heating (515) the ore with heat extracted (514) from at least one of the carbonation reaction (513) and the sodium carbonate.

6. A method (500ba, 500e, 500k, 500m, 500n, 500q) according to any one of claims 2 to 5, wherein heat-treating (502) the ore comprises heating (308) the ore with heat extracted from at least one of:(i) cooling (516) the liquid sodium before adding (505) the trivalent manganese oxide to the liquid sodium;(ii) cooling (509) the heat-treated ore back down before adding (505) the trivalent manganese oxide to the liquid sodium;(iii) cooling (309c) the liquid sodium from which at least some of the solid phase has been separated; and(iv) cooling (517) at least a component of the solid phase separated from the liquid sodium.

7. A method (500ba, 500e, 500k, 500m, 500n, 500q) according to any one of claims 2 to 6, further comprising, after heat-treating (502, 502a, 502b) the ore and before adding (505, 505a, 505b, 505c) the trivalent manganese oxide to the liquid sodium:cooling (509) the heat-treated ore back down to less than 100 ’Celsius; andmagnetically separating (504) paramagnetic components of the heat-treated ore at least comprising the trivalent manganese oxide from diamagnetic components of the heat-treated ore.

8. A method (500a, 500b, 500bb, 500c, 500d, 500h, 500i, 500j) according to any one of the preceding claims, wherein converting (502, 503) a manganiferous mineral in the ore into a trivalent manganese oxide comprises using a hydrometallurgical technique (503) comprising:adding (503a) at least some of the ore to hot, concentrated hydrochloric acid closed off from their surrounding environment to produce gaseous chlorine and an acidic aqueous solution comprising manganese (II) chloride;adding (503b) sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an alkaline aqueous solution and a precipitate comprising manganese (II) hydroxide;phase-separating (503c) at least some of the precipitate from the alkaline aqueous solution; anddrying and dehydroxylating (503d) at least some of the manganese (II) hydroxide to produce the trivalent manganese oxide;and the method further comprises capturing (519) at least some of the gaseous chlorine.

9. A method (500h) according to claim 8, wherein at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, which is added (503b) to the acidic aqueous solution is recycled (521) from the sodium oxide produced by the redox reaction (505, 505a, 505b, 505c) between the liquid sodium and the trivalent manganese oxide.

10. A method (500c) according to claim 8 or claim 9, further comprising magnetically separating (511) paramagnetic components of the precipitate at least comprising the manganese (II) hydroxide from diamagnetic components of the precipitate by applying a magnetic field gradient to at least one of:(i) the alkaline aqueous solution with the precipitate suspended or entrained therein; and(ii) the precipitate after it has been phase-separated from the alkaline aqueous solution.

11. A method (500bb) according to any one of claims 8 to 10, further comprising, after the manganese (II) hydroxide has been dried and dehydroxylated (503d), magnetically separating (504) paramagnetic components of the precipitate at least comprising the trivalent manganese oxide from diamagnetic components of the precipitate.

12. A method (500c, 500i) according to any one of claims 8 to 11, further comprising, after adding (503a) at least some of the ore to the hydrochloric acid and before producing the precipitate comprising manganese (II) hydroxide, phase-separating (211, 211a, 211b) undissolved solids from the aqueous solution.

13. A method (500i) according to claim 12, wherein the manganese ore comprises a silicate mineral, the undissolved solids phase-separated (211a) from the acidic aqueous solution comprisesilica, and the method further comprises using (222) at least some of the phase-separated undissolved solids as at least one of:(i) an ingredient with an alkaline activator in the manufacture of an alkaline-activated or geopolymer cement; and(ii) a pozzolan in a reaction with at least one of calcium oxide and magnesium oxide to produce a hydraulic cement.

14. A method (500i) according to claim 12 or claim 13, wherein the manganese ore comprises an aluminate mineral and the method further comprises, after phase-separating (211a) the undissolved solids from the acidic aqueous solution and before producing the precipitate comprising manganese (II) hydroxide:adding (503bl) sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an aqueous solution with a pH of from 5 to 8.5, inclusive, and a precipitate comprising aluminium hydroxide;phase-separating (211b) the precipitate comprising aluminium hydroxide from the aqueous solution with the stated pH; andthereafter, continuing to add (503b2) the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution with the stated pH to produce the alkaline aqueous solution and the precipitate comprising manganese (II) hydroxide.

15. A method (500i) according to claim 14, further comprising:drying and dehydroxylating (223) at least some of the phase-separated precipitate comprising aluminium hydroxide to produce alumina; andusing (224) the alumina as feed material for producing aluminium by electrolysis.

16. A method (500i, 500j) according to any one of claims 8 to 15, further comprising, after phaseseparating at least some of the precipitate from the alkaline aqueous solution:adding (225) further hydrochloric acid to the alkaline aqueous solution to neutralize the alkaline aqueous solution and produce an aqueous solution of sodium chloride;using (114) at least some of the aqueous solution of sodium chloride to produce liquid sodium and gaseous chlorine by electrolysis; and at least one of:(i) recycling (529) at least some of the liquid sodium thus produced to the redox reaction between the liquid sodium and the trivalent manganese oxide; and(ii) using (530) at least some of the gaseous chlorine thus produced to produce the hydrochloric acid.

17. A method (500h, 500j) according to any one of claims 8 to 16, further comprising using (520) at least some of the captured gaseous chlorine to produce the hydrochloric acid.

18. A method (500j) according to claim 17 when dependent on claim 16, wherein at least one of using (530) at least some of the gaseous chlorine produced by electrolysis and using (520) at least some of the captured gaseous chlorine, to produce the hydrochloric acid comprises:combusting (203b) at least one of at least some of the captured gaseous chlorine and at least some of the gaseous chlorine produced by electrolysis with gaseous hydrogen to produce gaseous hydrogen chloride; anddissolving (204) at least some of the hydrogen chloride thus produced in liquid water; wherein:(i) at least some of the gaseous hydrogen is produced by at least one of:electrolysing (201a) at least some of the aqueous solution of sodium chloride to produce an aqueous solution of sodium hydroxide; andfusing and electrolysing (201c) solid sodium hydroxide produced by drying (201b) at least some of this aqueous solution of sodium hydroxide; and(ii) at least some of the liquid water is produced by:drying (201b) at least some of the aqueous solution of sodium hydroxide to produce water vapour;capturing (215) at least some of the water vapour thus produced; and condensing (216) at least some of the captured water vapour.

19. A method (300d) according to any one of claims 8 to 18, wherein dehydrating and dehydroxylating (503d) at least some of the manganese (II) hydroxide comprises passing (528) at least some of the sodium oxide precipitated out from the liquid sodium, or sodium hydroxide derived from hydrating at least some of this sodium oxide, through an atmosphere to which the manganese (II) hydroxide is exposed.

20. A method (500bb) according to any one of the preceding claims, wherein the trivalent manganese oxide comprises an admixture of a diamagnetic gangue species, and the method further comprises separating (510) the trivalent manganese oxide from the diamagnetic gangue species basedon their different densities before adding (505, 505a, 505b, 505c) the trivalent manganese oxide to the liquid sodium.

21. A method (500d, 500h) according to any one of the preceding claims, wherein the trivalent manganese oxide comprises an admixture of a siliceous compound, and the redox reaction (505a) is conducted at a temperature of at least 320 ’Celsius to induce a reaction between the sodium oxide and silica derived from the siliceous compound to produce at least sodium orthosilicate.

22. A method (500a, 500b, 500bb, 500d) according to any one of the preceding claims, wherein the manganese ore comprises both rhodochrosite and another manganiferous mineral comprising at least one of a siliceous manganese mineral and a manganese mineral isostructural to hollandite, including hollandite itself, and the method comprises:converting at least the rhodochrosite into a trivalent manganese oxide using the pyrometallurgical technique (502); beforeconverting at least the other manganiferous mineral into a trivalent manganese oxide using the hydrometallurgical technique (503).

23. A method (500k, 500m, 500n, 500p, 500r, 500t) according to any one of the preceding claims, wherein the manganese ore comprises at least 4% by weight of an iron oxide and the redox reaction (505b, 505c, 505d) between the liquid sodium and the trivalent manganese oxide is conducted at a temperature of less than 450 ’Celsius.

24. A method (500q) according to any one of the preceding claims, further comprising, before adding (505) the trivalent manganese oxide to the liquid sodium, mixing (522) the manganese ore with finely comminuted, dehydrated and dehydroxylated iron ore, and wherein the redox reaction (505b) between the liquid sodium and the trivalent manganese oxide is conducted at a temperature of less than 450 ’Celsius.

25. A method (500m, 500n, 500p, 500r, 500t) according to any one of the preceding claims, wherein the solid phase comprises elemental iron, and the method further comprises magnetically separating (523) at least some of the elemental iron from the elemental manganese when the solid phase is still hot from the redox reaction (505b, 505c, 505d) and has a temperature of at least 200 ’Celsius.

26. A method (500r) according to any one of the preceding claims, comprising controlling (313) the temperature of the reaction between the liquid sodium and the trivalent manganese oxide to remain below a desired temperature by:cooling (309c) the liquid sodium from which at least some of the solid phase has been separated;returning (314) at least some of the liquid sodium thus cooled to the reaction between the liquid sodium and the trivalent manganese oxide; andadjusting (315) a rate at which the liquid sodium is returned to the reaction between the liquid sodium and the trivalent manganese oxide.

27. A method (500s) according to any one of the preceding claims, comprising cooling (524) a reaction mixture produced by the redox reaction (505) between the liquid sodium and the trivalent manganese oxide to dwell at a temperature of less than 225 ’Celsius until the reaction is complete, before separating (506, 506a, 506b) at least some of the solid phase from the liquid sodium.

28. A method (500b, 500e, 500g, 500m, 500n, 500s, 500t) according to any one of the preceding claims, wherein separating (507, 507a, 507b; 508a, 508b) the elemental manganese from the other insoluble products comprises:adding (508a) at least some of the separated solid phase to liquid water, thereby hydrating the sodium oxide therein to produce an aqueous solution of sodium hydroxide; andphase-separating (508b) undissolved solids comprising the elemental manganese from the aqueous solution of sodium hydroxide.

29. A method (500s) according to claim 28, further comprising:reacting (403a) at least some of the aqueous solution of sodium hydroxide, from which the undissolved solids comprising the elemental manganese have been phase-separated, with hydrogen chloride to produce an aqueous solution of sodium chloride;drying (110) at least some of the aqueous solution of sodium chloride to produce solid sodium chloride and water vapour;fusing and electrolysing (101b) at least some of the solid sodium chloride to produce liquid sodium and chlorine gas;recycling (525) at least some of the liquid sodium to react with the trivalent manganese oxide; andusing (203) at least some of the chlorine gas to produce the hydrogen chloride.

30. A method (500t) according to claim 28 or claim 29, further comprising:drying (319) at least some of the aqueous solution of sodium hydroxide, from which the undissolved solids comprising the elemental manganese have been phase-separated, to produce solid sodium hydroxide and water vapour;fusing and electrolysing (320) at least some of the solid sodium hydroxide to produce at least liquid sodium; andrecycling (525a) at least some of the liquid sodium to react with the trivalent manganese oxide.

31. A method (500s, 500t) according to claim 29 or claim 30, further comprising:capturing (111, 215) at least some of the water vapour produced by at least one of drying (110) at least some of the aqueous solution of sodium chloride and drying (319) at least some of the aqueous solution of sodium hydroxide;condensing (112, 216) at least some of the captured water vapour; andusing (526) at least some of the captured and condensed water vapour as the liquid water to which the separated solid phase is added (508a), thereby hydrating the sodium oxide therein to produce the aqueous solution of sodium hydroxide.

32. An apparatus (5a - 5e) for producing manganese from a manganese ore, wherein the apparatus comprises:a comminution device (2) for comminuting the manganese ore into fines;a mineral-converting subassembly (54) for converting a manganiferous mineral in the ore into a trivalent manganese oxide via at least one of a reductant-free pyrometallurgical technique and an inorganic hydrometallurgical technique;a gas-tight first reaction vessel (130) for reacting the trivalent manganese oxide in an inert atmosphere and at a temperature of less than 600 ’Celsius with an amount of liquid sodium in excess of the stoichiometric amount thereof required for a redox reaction between the liquid sodium and the trivalent manganese oxide, the first reaction vessel (130) comprising a first inlet (131) for the trivalent manganese oxide, a second inlet (132) for the liquid sodium, and an outlet (134) for the liquid sodium and, entrained therein, a solid phase comprising both elemental manganese and other insoluble products at least comprising sodium oxide;a solid-liquid sodium phase separator (190; 190a, 190b) for separating at least some of the solid phase from the liquid sodium; anda solid-species separator (180a, 180b) for separating the elemental manganese from the other insoluble products.

33. An apparatus (5b, 5c) according to claim 32, wherein the mineral-converting subassembly (54) comprises a high-temperature dryer (4) for heat-treating the ore in atmospheric air to a temperature of from 100 to 600 ’Celsius, inclusive, to dehydrate the ore, dehydroxylate hydroxylated compounds contained therein and thermally decompose the manganiferous mineral into the trivalent manganese oxide.

34. An apparatus (5c) according to claim 33, wherein:the high-temperature dryer (4) can be rendered gas-tight to permit the thermal decomposition of the manganiferous mineral to be conducted closed off from its surrounding environment and comprises an outlet (12) for gaseous carbon dioxide produced by the thermal decomposition of the manganiferous mineral therein; andthe apparatus (5c) further comprises a carbonation vessel (160) for reacting sodium oxide or hydroxide with at least some of the gaseous carbon dioxide to produce at least sodium carbonate, wherein the carbonation vessel (160) comprises a first inlet (162) for receiving the gaseous carbon dioxide from the outlet (12) of the high-temperature dryer (4), a second inlet (163) for receiving sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, from downstream of the solid-liquid sodium phase separator (190; 190a, 190b), and an outlet (166) for at least the sodium carbonate.

35. An apparatus (5b) according to claim 33 or claim 34, wherein:the high-temperature dryer (4) can be rendered gas-tight to permit the thermal decomposition of the manganiferous mineral to be conducted closed off from its surrounding environment; andthe apparatus (5b) further comprises a conduit (137) for transporting sodium oxide, or solid sodium hydroxide derived therefrom, from downstream of the solid-liquid sodium phase separator (190; 190a, 190b) through an atmosphere to which the manganiferous mineral is exposed within the high-temperature dryer (4).

36. An apparatus (5b, 5c) according to any one of claims 33 to 35, further comprising a heat transfer pathway (55, 8a, 8b) for transferring heat to the manganese ore within the high-temperature dryer (4) from at least one of:(i) the liquid sodium before it enters the first reaction vessel (130);(ii) the manganese ore after it has been heat-treated and before it enters the firstreaction vessel (130);(iii) the first reaction vessel (130); and(iv) at least one of the liquid sodium, the elemental manganese and the other insoluble products after they have left the first reaction vessel (130).

37. An apparatus (5c) according to claim 36, wherein the heat transfer pathway (55) is adapted to contain liquid sodium as a heat transfer fluid.

38. An apparatus (5b, 5c) according to any one of claims 33 to 37, wherein the mineral-converting subassembly (54) comprises a first magnetic separator (560) for magnetically separating paramagnetic components of the heat-treated ore at least comprising the trivalent manganese oxide from diamagnetic components thereof, wherein the first magnetic separator (560) is adapted to maintain the heat-treated ore at a temperature of less than 100 ’Celsius and comprises an inlet (561) for receiving the heat-treated ore from the high-temperature dryer (4), a first outlet (564) for the diamagnetic components and a second outlet (565) upstream of the first inlet (131) of the first reaction vessel (130) and for the paramagnetic components at least comprising the trivalent manganese oxide.

39. An apparatus (5d) according to any one of claims 32 to 38, wherein the mineral-converting subassembly (54) comprises:a gas-tight second reaction vessel (540) for dissolving the manganiferous mineral therein in hot, concentrated hydrochloric acid, the second reaction vessel (540) comprising a first inlet (541) for receiving the manganiferous mineral, a second inlet (542) for receiving the hydrochloric acid, a third inlet (543) for receiving sodium oxide or hydroxide, and an outlet (544) for an aqueous solution and a precipitate comprising manganese (II) hydroxide;a solid-aqueous phase separator (545) for separating at least some of the precipitate from the aqueous solution, the solid-aqueous phase separator (545) comprising an inlet (547) for receiving the aqueous solution and the precipitate from the outlet (544) of the second reaction vessel (540), a first outlet (548) for the precipitate and a second outlet (549) for the aqueous solution; anda low-temperature dryer (550) for drying and dehydroxylating at least some of the manganese (II) hydroxide in the precipitate to produce the trivalent manganese oxide, the low-temperature dryer (550) comprising an inlet (551) for receiving the precipitate from the first outlet (548) of the solid-aqueous phase separator (545), and an outlet (554) upstream of the first inlet (131) of the first reaction vessel (130) and for the trivalent manganese oxide.

40. An apparatus (5d) according to claim 39, comprising a conduit (553) for transporting sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, from downstream of the solid-liquid sodium phase separator (190; 190a, 190b) to at least one of:(i) through an atmosphere to which the manganese (II) hydroxide is exposed within the low-temperature dryer (550), wherein the conduit (553) is adapted to transport sodium hydroxide therethrough in solid phase; and(ii) the third inlet (543) of the second reaction vessel (540);wherein when the conduit (553) is arranged to transport sodium oxide or hydroxide through the atmosphere within the low-temperature dryer (550) and to the third inlet (543) of the second reaction vessel (540), the conduit (553) is arranged to transport the sodium oxide or hydroxide through the atmosphere within the dryer (550) before it arrives at the third inlet (543) of the second reaction vessel (540).

41. An apparatus (5d) according to claim 39 or claim 40, wherein the mineral-converting subassembly (54) comprises a second magnetic separator (570) for magnetically separating paramagnetic components of the precipitate from diamagnetic components thereof, wherein the second magnetic separator (570) is adapted to maintain the precipitate at a temperature of less than 100 ’Celsius and comprises an inlet (571) downstream of the third inlet (543) of the second reaction vessel (540) and for receiving the precipitate, a first outlet (574) for the diamagnetic components and a second outlet (575) upstream of the first inlet (131) of the first reaction vessel (130) and for the paramagnetic components.

42. An apparatus (5b, 5c, 5d) according to any one of claims 32 to 41, wherein:the solid-liquid sodium phase separator (190) comprises an inlet (191) for receiving the liquid sodium with the solid phase entrained therein from the outlet (134) of the first reaction vessel (130), a first outlet (194) for liquid sodium and a second outlet (195) for the solid phase; andthe solid-species separator (180b) comprises an inlet (181) for receiving the solid phase from the second outlet (195) of the solid-liquid sodium phase separator (190), a first outlet (186) for the elemental manganese, and a second outlet (187) for the other insoluble products.

43. An apparatus (5c) according to claim 42, wherein the solid-species separator (180b) comprises:a hydration vessel (170) for reacting at least some of the sodium oxide in the solid phase with liquid water to produce an aqueous solution of sodium hydroxide, wherein the hydration vessel (170) comprises the inlet (171, 181) of the solid-species separator for receiving the solid phase from the second outlet (195) of the solid-liquid sodium phase separator (190), a second inlet (172) for the water, and an outlet (174) for the aqueous solution of sodium hydroxide and, entrained therein, undissolved solids comprising the elemental manganese; anda solid-aqueous phase separator (175) for separating these undissolved solids from the aqueous solution of sodium hydroxide, wherein the solid-aqueous phase separator (175) comprises an inlet (176) for receiving the aqueous solution of sodium hydroxide with the undissolved solids entrained therein from the outlet (174) of the hydration vessel (170), a first outlet (178) for the aqueous solution of sodium hydroxide and a second outlet (179) for the undissolved solids.

44. An apparatus (5a, 5e) according to any one of claims 32 to 41, wherein:the solid-species separator (180a) comprises an inlet (181) for receiving the liquid sodium with the elemental manganese and other insoluble products entrained therein from the outlet (134) of the first reaction vessel (130), a first outlet (184) for liquid sodium with elemental manganese entrained therein, and a second outlet (185) for liquid sodium with the other insoluble products entrained therein; andthe solid-liquid sodium phase separator (190a, 190b) comprises first and second phase separating devices (190a, 190b), wherein the first phase separating device (190a) comprises an inlet (191a) for receiving the liquid sodium with elemental manganese entrained therein from the first outlet (184) of the solid-species separator (180a), a first outlet (194a) for the liquid sodium and a second outlet (195a) for the elemental manganese, and the second phase separating device (190b) comprises an inlet (191b) for receiving the liquid sodium with the other insoluble products entrained therein from the second outlet (185) of the solid-species separator (180a), a first outlet (194b) for the liquid sodium and a second outlet (195b) for the other insoluble products.

45. An apparatus (5a - 5e) according to any one of claims 42 to 44, wherein at least one of the first outlets (194,194a, 194b) for liquid sodium of the solid-liquid sodium phase separator (190,190a, 190b) is connected to the second inlet (132) of the first reaction vessel (130).

46. An apparatus (5e) according to any one of claims 32 to 45, comprising a third magnetic separator (580) for magnetically separating elemental iron in the solid phase from the elemental manganese therein, wherein the third magnetic separator (580) is adapted to maintain the elemental manganese at a temperature of at least 200 ’Celsius and comprises an inlet (581) downstream of the 5 outlet (134) of the first reaction vessel (130) for receiving the elemental iron mixed with the elemental manganese, a first outlet (584) for the elemental iron and a second outlet (585) for the elemental manganese.

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