Industrial chemical process and apparatus

Abstract

A method may comprise producing liquid sodium and chlorine gas by electrolysis. The liquid sodium may be redox reacted with iron and / or manganese oxide in an ore to produce iron and / or manganese and s

Description

Field of the Invention The present invention concerns an industrial chemical process and an apparatus for carrying out that process. An example of a known industrial chemical process is the ammonia-soda, or Solvay, process for the production of sodium carbonate, illustrated in Fig. 1. The chemical process of the present invention integrates two methods, which interact synergistically with each other. One is a method for producing iron and / or manganese in elemental form from oxides thereof derived from their respective ores. The other is a method for producing an oxide or hydroxide of at least one of calcium, magnesium and iron from another ore respectively comprising a carbonate mineral of at least one of calcium, magnesium and iron. The first method is comparable to and is intended to replace existing techniques for extracting iron and / or manganese in elemental form from their respective ores, which release carbon dioxide. The other method is comparable to and is intended to replace existing techniques for the thermal decomposition of such carbonate mineral ores to produce at least one of calcium oxide, magnesium oxide and an iron oxide, which also release carbon dioxide. Background of the Invention Two major industrial processes - steelmaking and cement production - are each responsible for producing about 8% of all anthropogenic carbon dioxide emissions. In cement production, a carbonate mineral from an ore such as limestone is thermally decomposed into its corresponding oxide, such as lime, releasing carbon dioxide. Lime made by the same process for purposes other than its use in cement production, such as in agriculture, water purification, steelmaking or in the manufacture of other construction materials, makes lime the most widely used alkali in the world. Although data for aggregated carbon dioxide emissions from the production of lime for such other purposes are not readily available, this suggests that steelmaking, cement production and the production of lime for other purposes are together responsible for about 20% of all anthropogenic carbon dioxide emissions in total. However, the present climate crisis demands that this carbon dioxide should be captured and stored or otherwise mitigated. The predominant constituent element of all grades of steel is iron. Traditionally, iron has been extracted from iron ore by heating the iron ore in a blast furnace at temperatures above about 1200 ’Celsius, with coke or a similar source of carbon being used as a reducing agent to produce elemental iron. Initially, the carbon is partially oxidized into carbon monoxide, which reduces iron oxides in the iron ore according to the sequence: Fe2O3 -» Fe3O4 -» FeO -> Fe [Eqn. 1] Complete oxidation of the carbon monoxide as the iron oxides are reduced results in the production of carbon dioxide gas. Manganese is the fourth most widely used metal after iron, aluminium and copper. 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, all of which result in the production of carbon dioxide in some way. For example, 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, in a manner similar to that described above for extracting iron from iron ore, which therefore also results in the production of carbon dioxide gas. Ordinary Portland cement (OPC), which currently accounts for over 95% of all cement manufactured, is made by a chemical reaction between a cocktail of several ingredients, the main one of which, accounting for more than four-fifths of the starting materials, is a carbonate mineral consisting of calcium and / or magnesium carbonate. The carbonate mineral is heated until it thermally decomposes into its corresponding oxide(s) and carbon dioxide gas. This process, called calcination, has been around for many hundreds of years. Taking calcium carbonate as an example, its thermal decomposition proceeds according to the equation: CaCO3 (s) CaO (S) + CO2 (gj [Eqn. 2] The reaction of Eqn. 2 is highly endothermic, and therefore requires a large quantity of heat to proceed. At standard temperature and pressure (hereinafter, s.t.p.), the Gibbs free energy of the reaction of Eqn. 2 is highly positive, so that the calcium carbonate must be heated to a temperature above about 900 ’Celsius before the Gibbs free energy of this reaction becomes negative and the reaction can proceed. This heat is usually supplied by burning a fossil fuel, such as natural gas, which is injected into the carbonate mineral until it reaches the required temperature. Apart from the energy consumed, burning fossil fuel in this way generates its own very significant carbon dioxide emissions, in addition to the carbon dioxide which is liberated from the carbonate mineral according to Eqn. 2. In view of this and the present climate crisis, it is widely recognized that the carbon dioxide produced during cement manufacture should somehow be mitigated. Several proposals have therefore already been made for how to achieve this. One such proposal is to replace OPC with an alternative type of cement manufactured by a different process which produces lower carbon dioxide emissions. Amongst the most promising alternatives to OPC are alkaline activated and geopolymer cements. Alkaline activated and geopolymer cements generally comprise at least two main constituents: a reactive aluminosilicate and an alkaline activator. Such alkaline activators include sodium silicate, which is a generic name for a range of compounds with the general formula n(Na2O) • SiCh, wherein n does not have to be an integer number, such as sodium metasilicate (Na2SiO3) (when n = 1), sodium orthosilicate (Na4SiO4) (when n = 2), and sodium pyrosilicate (NagSijO?) (when n =3 / 2). Sodium silicate does not occur naturally in geological deposits because it is water-soluble and must therefore be manufactured. However, current methods for producing sodium silicate are highly energy intensive and generate their own carbon dioxide emissions. For example, sodium silicate can be produced by fusing ( / .e., melting) quartz sand ( / .e., silica, SiCh) and sodium carbonate (Na2CO3) together at a temperature of between about 1350 and about 1450 ’Celsius, according to the equation: SiOz (sj + n Na2CO3 (s) n(Na2O) • SiO2 (s) + CO2 (gj [Eqn. 3] which therefore requires a large quantity of heat to fuse the starting materials and also releases carbon dioxide from the sodium carbonate. Progressive depletion of high-grade iron ores, in which the iron is present as at least one of the mineral and mineraloid forms of iron oxide and iron oxyhydroxide, has made it increasingly attractive instead to mine iron ores, in which the iron is present mostly as siderite ( / .e., iron carbonate). However, such siderite ores are difficult to beneficiate using traditional techniques of magnetic and / or density-based separation of the siderite from gangue species in the ores because of their low iron content. The ore is therefore usually subjected to a thermal decomposition of the siderite into one or more iron oxides which can be more easily processed. This thermal decomposition proceeds in a similar manner to the reaction of Eqn. 2, with the accompanying release of carbon dioxide from the iron carbonate. Sodium oxide, and its hydrate, sodium hydroxide, are well known to have a high affinity for carbon dioxide. Therefore, in principle, carbon dioxide generated by any one or more of steelmaking, cement production, the production of lime for other purposes, the manufacture of sodium silicate and the thermal decomposition of siderite ores can be captured and mineralized by reacting the captured carbon dioxide with sodium oxide and / or with sodium hydroxide to produce at least sodium carbonate. Currently, almost all sodium carbonate for industrial use is either mined as trona or is produced by the ammonia-soda, or Solvay, process of Fig. 1. Mining trona has the disadvantage that it extracts mineralized carbon dioxide from geological deposits, which if the trona is then decomposed in an industrial process such as glassmaking, releases the carbon dioxide it contains, thereby adding to global greenhouse gas emissions. The Solvay process has at least the disadvantages that it includes the calcination of limestone described above as one of its process steps, it uses hazardous ammonia as a reaction intermediary, and it also produces large quantities of calcium chloride as a by-product, whereas the global market for calcium chloride is less than about 10% of that for sodium carbonate. 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 solid sodium chloride fuses ( / .e., melts) the sodium chloride by ohmic heating. Subsequent electrolysis of the sodium chloride produces metallic sodium in liquid phase and gaseous chlorine. Both the liquid sodium and the chlorine gas thus produced generally have a high purity of better than 99% and may therefore subsequently be used in other industrial processes. However, the high-temperature chlorine gas is usually cooled, condensed and bottled. On the other hand, 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. It is also known that calcium hydroxide, magnesium hydroxide and iron hydroxides can all be thermally decomposed into calcium oxide, magnesium oxide and iron oxides, respectively, and water vapour. In general, the thermal decomposition of each of these hydroxides requires less heat and occurs at a lower temperature than the thermal decomposition of the corresponding carbonates. Object of the Invention It is therefore an object of the invention to provide an industrial chemical process and an apparatus for carrying out that process, which mitigate the production of carbon dioxide, in comparison to existing techniques for producing iron and / or manganese from their respective ores and existing techniques for calcining an ore comprising a carbonate mineral of at least one of calcium, magnesium and iron to produce at least one of calcium oxide, magnesium oxide and an iron oxide, respectively. It is another object of the invention to provide an industrial chemical process and an apparatus for carrying out that process, wherein the process integrates a method for producing iron and / or manganese from their respective ores with a method for producing an oxide or hydroxide of at least one of calcium, magnesium and iron from another ore respectively comprising a carbonate mineral of at least one of calcium, magnesium and iron. Description of the Invention Accordingly, in one aspect, the present invention provides a method comprising producing liquid sodium and chlorine gas by electrolysis, and reacting at least some of the liquid sodium in a redox reaction with an oxide of another metal, M, from an ore of that other metal, wherein the other metal, M, comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide, separating the other metal from the sodium oxide, and dividing at least some of the sodium oxide into a first portion and a second portion. The method also comprises using at least some of the chlorine gas to produce hydrogen chloride and dissolving at least some of this hydrogen chloride in liquid water to produce hydrochloric acid. The method then comprises adding another ore comprising a carbonate mineral of at least one of calcium, magnesium and iron to at least some of the hydrochloric acid thus produced to dissolve the carbonate mineral therein and produce carbon dioxide gas and an aqueous solution respectively comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. The carbonate mineral is dissolved in the hydrochloric acid closed off from their surrounding environment and at least some of the carbon dioxidegas is captured. The method also comprises reacting at least some of the first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the first portion of sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, to produce an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. The method then comprises phase-separating at least some of this precipitate from the aqueous solution of sodium chloride thus produced. The method further comprises using at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the second portion of sodium oxide, in at least one of: (i) a reaction with at least some of the hydrochloric acid to produce an aqueous solution of sodium chloride; and (ii) a reaction with at least some of the captured carbon dioxide gas to produce at least sodium carbonate. In some embodiments, the method may further comprise thermally decomposing at least some of the separated precipitate to produce water vapour and a solid end-product comprising at least one of calcium oxide, magnesium oxide and an iron oxide. At least some of the aqueous solution of sodium chloride produced by the method of the invention may be recycled as a feedstock in one or more ways to produce at least some of the liquid sodium and chlorine gas by electrolysis. This has the advantage of reducing the nett consumption of sodium chloride. Some examples of such recycling are described below. In embodiments of the invention which also produce water vapour, at least some of the water vapour may be captured and recycled as a feedstock in one or more ways to produce at least some of the liquid water in which the hydrogen chloride is dissolved. This has the advantage of reducing the nett consumption of water. Some examples of such recycling are described below. In some of the embodiments described below, enough sodium chloride and / or enough water may be recycled, such that no sodium chloride and / or water needs to be consumed as a starting ingredient. In such embodiments, the method of the invention therefore becomes a chemical cycle, wherein the sodium chloride and / or water act only as intermediaries. In some embodiments, at least some of the liquid sodium and the chlorine gas may be produced by at least one of: (i) fusing and electrolysing solid sodium chloride, and (ii) electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas. In some embodiments, some of the liquid sodium may be produced by electrolysing a molten electrolyte comprising sodium chloride and aluminium chloride using a consumable aluminium anode and a solid electrolyte to separate the molten electrolyte from the liquid sodium. Examples of such an electrolytic technique are described in US patent nos. 4,846,943 and 6,402,910 and US patent application publication no. 2002 / 0088719 A. The solid electrolyte may, for example, be so-called P-alumina, although other sodium-ion conducting solid electrolytes are also known. In such embodiments, gaseous aluminium chloride is produced alongside the liquid sodium. The gaseous aluminium chloride may subsequently be used, for example, in the manufacture of chlorinated organic compounds and / or as an ingredient in the production of sodium aluminium chloride (NaAICI4), as described, for example, in US patent no. 9,567,232. The sodium aluminium chloride may then be used, for example, in the manufacture of sodium-aluminium rechargeable batteries. In those embodiments which comprise such an electrolytic technique, the method described herein further comprises consuming the carbonate mineral and the oxide of the other metal, M, in a molar ratio of from 0.75 to 1.75 mol, inclusive, of the carbonate mineral per mol of the oxide of the other metal, and producing the liquid sodium in a molar ratio of from 5 to 10.5 mol, inclusive, of the liquid sodium per mol of the oxide of the other metal. Moreover, in such embodiments, the first and second portions of sodium oxide together constitute between 7 / 12 and 12 / 12, inclusive, of a total number of mols of the sodium oxide, the first portion and the second portion of sodium oxide each constitutes between 5 / 12 and 7 / 12, inclusive, of the total number of mols of the first and second portions, at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the second portion of sodium oxide, is used only in a reaction with at least some of the hydrochloric acid to produce an aqueous solution of sodium chloride, and at least some of a third portion of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the third portion of sodium oxide, is used in a reaction with at least some of the captured carbon dioxide gas to produce at least sodium carbonate. Such embodiments have at least the following two advantages. Firstly, the total number of mols of sodium oxide produced in the redox reaction with the oxide of the other metal, M, can be increased without increasing the co-production of chlorine and / or hydrochloric acid until enough sodium oxide is produced not only to react with all the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to produce the precipitate, but also to react with at least some of the hydrochloric acid to produce an aqueous solution of sodium chloride, and react with all the captured carbon dioxide gas to produce at least sodium carbonate. A second advantage of these embodiments is that such an electrolytic technique also consumes less energy than producing an equimolar amount of liquid sodium from sodium chloride using either one of the other two electrolytic techniques (i) and (ii) mentioned above. For example, fusing and electrolysing 3 mol of solid sodium chloride according to the equation: 3 NaCI (s) 3 Na M + 3 / 2 Cl2 (g) [Eqn. 4] is an endothermic process with an enthalpy change, AH (Eqn. 4) = AHf (products) - AHf (reagents), subject to real-world losses and inefficiencies, of 3(2.41) - (3(-411.2)) = +1241 kJ, where AHf represents the standard enthalpy of formation of the respective species involved. For comparison, fusing and electrolysing an electrolyte comprising both sodium chloride and aluminium chloride using a consumable aluminium anode may be represented overall by the equation: Al (sj + 3 NaCI (sj 3 Na ( / ) + ½ AI2Clg (g) [Eqn. 5a] because the gaseous aluminium chloride thus formed dimerises. This, in contrast, is a mildly exothermic process with an enthalpy change, AH (Eqn. 5a), subject to the same assumptions, of 3(2.41) + ½(-1291) - (3(-411.2)) = -49.2 kJ. However, since elemental aluminium does not occur naturally in geological deposits, for a fair comparison with AH (Eqn. 4), production of the aluminium consumed in Eqn. 5a should also be accounted for. This may be represented by the equation: / 2 AI2O3 -» Al (s) + % O2 (g) [Eqn. 5b] assuming that the aluminium is produced by electrolysis using an inert anode. The enthalpy change, AH (Eqn. 5b), subject to the same assumptions, is 0 - ½(-1676) = +838 kJ. The nett enthalpy change of Eqns. 5a and 5b therefore equals AH (Eqn. 5a) + AH (Eqn. 5b) = -49.2 + 838 = +788.8 kJ. Although this is endothermic overall, the total energy saved in comparison to Eqn. 4, AH (Eqns. 5a and 5b) -AH (Eqn. 4) = +788.8 - (+1241) = -452.2 kJ or -36%. The method of the invention produces at least the following products: iron and / or manganese in elemental form, gaseous oxygen, and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide, at least some of which may subsequently be thermally decomposed to produce water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide. If the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium (for example, if the carbonate mineral comprises at least one of calcite, magnesite and dolomite), then the solid end-product will consist mostly of calcium oxide and / or magnesium oxide, which may be used, for example, in cement manufacture. If, on the other hand, the carbonate mineral has a majority of iron cations (for example because the carbonate mineral is siderite), then the solid end-product will consist mostly of iron oxide, which may be recycled as a reagent with the liquid sodium to produce iron in elemental form. The method of the invention requires only the following two starting materials: an iron and / or manganese ore of any of the types described below, and another ore comprising a carbonate mineral of at least one of calcium, magnesium and iron. In some embodiments, the method of the invention may also consume common salt ( / .e., sodium chloride) as an extra ingredient, in which case, the method of the invention can produce at least one of sodium oxide, sodium hydroxide and sodium carbonate as a co-product. In some embodiments, the method of the invention may also consume water as an extra ingredient, in which case, the method of the invention can produce hydrogen as a co-product. However, the method of the invention consumes no fossil fuels and produces no greenhouse gas emissions. If the metal produced in elemental form is iron, the iron may be present in its ore as at least one of the mineral and mineraloid forms of iron oxide and iron oxyhydroxide, including hematite and maghemite ( / .e., polymorphs of FejOa), magnetite ( / .e., Fe3O4), goethite and lepidocrocite ( / .e., polymorphs of FeO(OH)) and limonite ( / .e., FeO(OH) • n(H2O)). Such ores include not just traditional iron ores like banded iron formations, but also red bauxite, which is an aluminium ore having a high iron content, which may therefore be considered as another iron ore within the context of the present invention. In either case, the iron ore is comminuted into fines, and dried and dehydroxylated, before the ore thus treated is introduced to the liquid sodium. If the metal produced in elemental form is manganese, the manganese ore may be a primary ore comprising rhodochrosite ( / .e., manganese carbonate, MnCOa) and / or a secondary ore created by weathering of a primary manganese ore 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. Such oxides and oxyhydroxides of manganese include pyrolusite and ramsdellite ( / .e., polymorphs of manganese (IV) oxide, also known as manganese dioxide, MnOz), bixbyite ( / .e., manganese (III) oxide, Mn2O3), hausmannite ( / .e., manganese (11,111) oxide, Mn3O4), and manganite and groutite ( / .e., polymorphs of manganese (III) oxyhydroxide, MnO(OH)). However, commonly occurring manganese oxides and oxyhydroxides also include minerals with more complex crystal structures, the most prevalent of which are romanechite, hollandite and cryptomelane. In all cases, the manganese ore is comminuted 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 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 pyrometallurgical technique may comprise 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. The hydrometallurgical technique may comprise adding at least some of the ore to hot, concentrated hydrochloric acid to produce gaseous chlorine and an acidic aqueous solution comprising manganese (II) chloride, 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, phase-separating at least some of this precipitate from the alkaline aqueous solution, and drying and dehydroxylating at least some of the manganese (II) hydroxide to produce the trivalent manganese oxide. In any event, regardless of whether such a pyrometallurgical or hydrometallurgical technique is used, the trivalent manganese oxide is then introduced to the liquid sodium. Some of the carbonate minerals which may be present in the other ore include calcite and aragonite ( / .e., polymorphs of calcium carbonate, CaCOs), magnesite ( / .e., magnesium carbonate, MgCOa), dolomite ( / .e., calcium-magnesium carbonate) and siderite ( / .e., iron carbonate, FeCOa). However, they also include other carbonate minerals, in which a minority of the calcium, magnesium and / or iron cations may be substituted by manganese cations, including ankerite ( / .e., Ca (Mg, Fe, Mn) (CChh) and minerals in each of the solid solution series from calcite, magnesite and siderite to rhodochrosite ( / .e., manganese carbonate, MnCOa), wherein each of the solid solutions contains more of the calcium, magnesium or iron cations, respectively, than of the manganese cations. In many cases, both the ore of the metal produced in elemental form and the ore comprising a carbonate mineral may each comprise one or more gangue mineral species, such as aluminosilicates, which may include one or more clays, and / or species of alumina and silica, like quartz, for example. The ore of the metal produced in elemental form may further comprise a siliceous mineral of this metal itself. For example, if the metal produced in elemental form is iron, its ore may comprise one or more iron silicates, like minnesotaite (Fe2+3Si4Oio(OH)2) and stilpnomelane. If the metal produced in elemental form is manganese, its ore may comprise one or more siliceous manganese minerals, like braunite and rhodonite, for example. In some embodiments described below, therefore, the method of the invention may also produce a mixture comprising sodium silicate as a co-product, wherein the sodium silicate is derived from the silica content of a siliceous mineral also present in the ore, which may be a gangue mineral species and / or a siliceous mineral of the metal produced in elemental form. The metal produced in elemental form may be separated from the sodium oxide in at least one of several different ways. For example, it may be separated from the sodium oxide based on their significantly different densities, p (p(Fe) = 7.87 g cm'3 and p(Mn) = 7.20 g cm'3, whereas p(NazO) = 1.T1 g cm'3). Alternatively or additionally, if the metal produced in elemental form comprises iron, the iron may be separated from the sodium oxide magnetically. On the other hand, if the metal produced in elemental form comprises manganese, the manganese may be separated from the sodium oxide for example by adding both of them to liquid water, thereby hydrating the sodium oxide therein to produce an aqueous solution of sodium hydroxide, and then separating undissolved solids comprising the manganese from the aqueous solution of sodium hydroxide, such as by settlement under gravity, filtration and / or centrifugation. The relative amounts of the oxide of the other metal and of the carbonate mineral consumed by the method of the invention may take any value. For example, the oxide of the other metal and the carbonate mineral may be consumed in a molar ratio of 1:1. In some embodiments, however, the method of the invention comprises consuming the carbonate mineral and the oxide of the other metal in a molar ratio of from about 1.25 to about 1.75 mol, inclusive, of the carbonate mineral per mol of the oxide of the other metal, producing the liquid sodium in a molar ratio of from about 5 to about 7 mol, inclusive, of the liquid sodium per mol of the oxide of the other metal consumed, and wherein the first portion of sodium oxide constitutes between about 5 / 12 and about 7 / 12, inclusive, of a total number of mols of the sodium oxide. If so, the method preferably comprises consuming the carbonate mineral and the oxide of the other metal in a molar ratio of about 1.5 mol of the carbonate mineral per mol of the oxide of the other metal, producing the liquid sodium in a molar ratio of about 6 mol of the liquid sodium per mol of the oxide of the other metal consumed, and wherein the first portion of sodium oxide constitutes about half the total number of mols of the sodium oxide. This has the advantage that the carbonate mineral and the oxide of the other metal are then consumed and the liquid sodium is also produced in their stoichiometric ratios, thereby maximizing the atom economy of the method. In such a case, the method can produce 2 mol of iron and / or manganese and 1.5 mol of the solid end-product comprising at least one of calcium oxide, magnesium oxide and an iron oxide per mol of the oxide of the other metal consumed, as well as at least 0.75 mol of oxygen, the last of which depends on the molar amounts of salt and / or water also consumed as ingredient(s). For example, with the same stoichiometric ratios, if the method also consumes 3 mol of salt, it can produce up to 1.5 mol of sodium carbonate and 1.5 mol of chlorine gas. In another such example, if the method also consumes 1.5 mol of water per mol of oxide of the other metal consumed, the method can produce 1.5 mol of hydrogen as another product and a total of 1.5 mol of oxygen. Thermodynamic calculations show that, excluding the electrical energy required for electrolysis, the method of the invention is exothermic overall. For example, the total negative energy budget (excluding electrolysis but including thermally decomposing the phase-separated precipitate from one or more hydroxide(s) into their respective oxides) when the carbonate mineral and the oxide of the other metal are consumed and the liquid sodium is produced in their stoichiometric ratios, and when 3 mol of salt are also consumed per mol of oxide of the other metal but no water is added as an extra ingredient, is between about -1.2 and about -1.7 MJ per mol of oxide of the other metal consumed, depending on whether the other metal, M, is iron or manganese or a mixture of both, and on whether the cations, Q, in the carbonate mineral are calcium, magnesium, iron, or a mixture thereof. The electrical energy required to fuse and electrolyse 6 mol of solid sodium chloride at the same stoichiometric ratio is about +5 MJ per mol of iron oxide consumed, assuming an electrolytic cell efficiency of only 80%. Therefore, even if none of the heat generated by the method of the invention is recovered, the total energy consumption, including the electrical energy required for electrolysis, is highly competitive, not only with traditional techniques for the production of iron and / or manganese, for calcining carbonate minerals and the Solvay process, but also if more recently developed pyrometallurgical techniques, such as the direct reduction of iron by hydrogen ( / .e., hydrogen-DRI), are taken into account. This is because in this example, the total energy consumption is split between producing iron and / or manganese, the solid end-product comprising at least one of calcium oxide, magnesium oxide and an iron oxide, as well as 1.5 mol of sodium carbonate and the other gaseous coproducts mentioned above. Moreover, the present applicant's co-pending UK patent application no. 2417067.2 ("Electrochemical Device, Electrolytic Cells and Methods of Operating them, and Apparatuses Comprising such Cells"; applicant's ref: NE-P-GB 004), the entire contents of which is incorporated herein by reference, shows and describes an apparatus and method whereby at least some, and preferably most, of the heat generated by the exothermic reactions of the method described herein may be recovered and recycled to the electrolysis, thereby also significantly reducing the amount of electrical energy required for electrolysis. The standard enthalpies of formation, AHf, of the necessary starting materials and of the corresponding products in the method of the invention suggest that according to Hess's Law, the method of the invention has a theoretical minimum total energy consumption, including both the electrical energy required for electrolysing solid sodium chloride and thermal decomposition of the phase-separated precipitate, of from about +0.94 MJ to about +1.1 MJ per mol of the oxide of the other metal, M, consumed, again depending on whether the other metal is iron or manganese or a mixture of both, and on whether the cations, Q, in the carbonate mineral are calcium, magnesium, iron, ora mixture thereof, when the carbonate mineral and the oxide of the other metal are consumed and the liquid sodium is produced in their stoichiometric ratios and when no water or salt are added as extra ingredients (i.e., as in the embodiment described below in relation to Figs. 7A-B). However, this also assumes that the starting materials and products are all in their standard states at s.t.p. and that the method operates with no wastage and at 100% thermodynamic efficiency. An optional feature of the method of the invention is hydrating either or both of the first and second portions of sodium oxide. This hydration has the advantage that it may be used to remove gangue species which might be mixed in with the sodium oxide. Whereas sodium oxide readily dissolves in water to produce an aqueous solution of sodium hydroxide, gangue species like silica and alumina do not. Thus if the ore of the other metal contained any such gangue species originally, which become mixed in with the sodium oxide during the reaction of the liquid sodium with the oxide of the other metal, these gangue species may be separated from the first and / or second portions of sodium oxide before the first and second portions enter into subsequent stages of the method described herein. Hydrating the first and / or second portions of sodium oxide therefore gives great flexibility in controlling the purity both of the sodium carbonate and of the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide which are produced by the method described herein. For example, if a purer form of this precipitate is desired, gangue species may be removed from the first portion of sodium oxide by hydrating it to produce an aqueous solution of sodium hydroxide and then separating the undissolved gangue species from this aqueous solution, for example, by settlement under gravity, filtration and / or centrifugation. The remaining aqueous solution of sodium hydroxide, from which the gangue species have now been removed, may then be added directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Insoluble gangue species originally present in the carbonate mineral may previously also have been removed from this aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride by a similar phase separation process, comprising at least one of settlement, filtration and centrifugation. In such a case, therefore, the gangue species will also be absent from the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. If, on the other hand, the first portion of sodium oxide is not hydrated first, and is instead added directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, and / or this aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride is not firstly subjected to a phase separation process such as settlement, filtration and / or centrifugation, the gangue species will remain mixed in with the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. This may be acceptable if, for example, this precipitate is then to be used as an ingredient in cement manufacture. Similarly, if a purer form of sodium carbonate is desired, gangue species may be removed from the second portion of sodium oxide by hydrating it to produce an aqueous solution of sodium hydroxide and then separating the undissolved gangue species from this aqueous solution, again, for example by settlement under gravity, filtration and / or centrifugation. The remaining aqueous solution of sodium hydroxide, from which the gangue species have now been removed, may then be dried to produce anhydrous sodium hydroxide, which may be carbonated at a temperature of from about 310 to about 400 ’Celsius, inclusive, to produce the purer form of sodium carbonate. If, on the other hand, the second portion of sodium oxide is not hydrated before it reacts directly with carbon dioxide to produce sodium carbonate, the gangue species will remain mixed in with the sodium carbonate thus produced. This may be acceptable if, for example, the sodium carbonate is then to be used as an ingredient in the manufacture of soda-lime glass. Thus the respective purities of the sodium carbonate and of the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide may both be controlled by hydrating one or both of the first and second portions of sodium oxide. The purity of the precipitate may also be controlled by subjecting the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to a phase separation technique, such as settlement under gravity, filtration and / or centrifugation, before the first portion of sodium oxide is added thereto. If the metal produced by the method of the invention is iron, in principle, the iron and sodium oxide produced by reacting at least some of the liquid sodium with the iron oxide from the iron ore could also be separated from each other by adding the iron and sodium oxide to water, in order to hydrate the sodium oxide, before and / or after separating the sodium oxide into the first and second portions thereof. However, this would tend to leave water-insoluble gangue species like silica and alumina originally present in the iron ore mixed in with the insoluble iron, which is undesirable. Preferably, therefore, the purity of the iron produced by reacting the liquid sodium with the iron oxide from the iron ore may also be controlled by instead conducting a magnetic and / or density-based separation of the iron and the sodium oxide from each other after they are produced. Their magnetic separation is possible because not only the sodium oxide, but also gangue species like silica and alumina, are all diamagnetic, whereas iron is ferromagnetic. Thus such magnetic separation causes these gangue species to be separated out from the iron along with the sodium oxide, leaving the iron substantially free of gangue. Their density-based separation is also possible because iron is denser than all the other main species produced by the reaction of the iron ore with the liquid sodium, and is nearly 3.5 times denser than sodium oxide. Following such magnetic and / or density-based separation of the iron, water-insoluble gangue species like silica and alumina may then be separated from the sodium oxide by hydrating one or both of the first and second portions of the sodium oxide as described above. Some embodiments of the method described herein may further comprise thermally decomposing at least some of the phase-separated precipitate in the absence of oxygen to produce water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide, and capturing at least some of the water vapour thus produced. If so, using at least some of the chlorine gas to produce hydrogen chloride and dissolving at least some of the hydrogen chloride in liquid water to produce hydrochloric acid may comprise reacting at least some of the chlorine gas produced by electrolysis with at least some of this captured water vapour in a reverse Deacon reaction (hereinafter, RDR) at a temperature of from about 450 to about 750 ’Celsius, inclusive, more preferably from about 500 to about 700 ’Celsius, and most preferably from about 550 to about 650 ’Celsius, to produce a mixture of gases at least comprising hydrogen chloride and oxygen, immediately contacting at least some of this mixture of gases with liquid water to dissolve the hydrogen chloride therein, thereby producing the hydrochloric acid and a stream of tail gases, and extracting heat from the hydrochloric acid thus produced, to maintain its temperature substantially constant, preferably within about 20 ’Celsius, and more preferably within about 10 ’Celsius of its initial temperature, until the stream of tail gases is no longer in contact with it. The present applicant's co-pending UK patent application no. 2417058.1 ("Method and Apparatus for Producing Hydrochloric Acid"; applicant's ref: NE-P-GB 006), the entire contents of which is incorporated herein by reference, describes such a technique for producing hydrochloric acid. A particularly remarkable feature of such embodiments is that the temperature at which the (forward) Deacon reaction reverses (about 600 ’Celsius) is almost identical to the temperature at which solid sodium chloride can be fused and electrolysed. Moreover, fusing and electrolysing solid sodium chloride to produce the stoichiometric amount of liquid sodium for reaction with the oxide of the other metal also produces exactly twice the stoichiometric amount of chlorine required to produce the stoichiometric amount of hydrochloric acid for reaction with the carbonate mineral. However, the theoretical maximum yield of hydrogen chloride gas from the RDR is 50% at the temperature at which the (forward) Deacon reaction reverses. In other words, at this temperature, the RDR can produce the stoichiometric amount of hydrogen chloride needed to produce the hydrochloric acid for reaction with the carbonate mineral. Thus, subject to real-world losses and inefficiencies, exactly the right amount of chlorine required to produce the hydrochloric acid for reaction with the carbonate mineral can be produced by this electrolysis. Moreover, since the chlorine gas, and the water vapour captured from the thermal decomposition of the separated precipitate, are both already either at or near to the temperature at which the (forward) Deacon reaction reverses, both of these reagents for the RDR require either no or hardly any extra heat input to reach the desired range of operating temperatures for this reaction. Preferably, therefore, in some embodiments, the method comprises supplying a total number of mols of chlorine gas to the RDR which is substantially equal to (for example, within about 15% of) the stoichiometric amount of hydrochloric acid required for reaction with the carbonate mineral. In such embodiments, the total number of mols of water vapour supplied to the RDR should be at least as great as the total number of mols of chlorine gas supplied, in order to preserve the stoichiometry of the RDR. At or around the temperature at which the (forward) Deacon reaction reverses, this may be achieved by supplying approximately half the total molar amount of chlorine gas required to the RDR from fusing and electrolysing solid sodium chloride and supplementing this with an approximately equal amount of unreacted chlorine recycled from the reaction, and by supplying approximately half the total molar amount of water vapour required which is captured from the thermal decomposition of the separated precipitate and supplementing this with an approximately equal amount of unreacted water vapour also recycled from the RDR. This may in turn be achieved by recycling at least some of the stream of tail gases from the RDR, from which the oxygen has been separated, back to the reaction in a loop. However, the oxygen may be easily separated from the stream of tail gases, for example by distillation, because the boiling point of oxygen is so much lower than that of both chlorine and water. Moreover, the stream of tail gases from which the oxygen has been separated may be at least partially reheated by exchanging heat to it from hot chlorine gas produced by fusing and electrolysing the solid sodium chloride, but which is not supplied to the RDR. Thus per mol of the oxide of the other metal consumed, the nett reaction proceeds according to the following equation: Water vapour captured from thermal decomposition: 3A H2O (gj |Unreacted chlorine 4b 4b t 3 Cl2 (g) + 3 H2O (g; -> 3 HCI (gj + 3 / 2 Cl2 (g) + % O2 (g; + V2 H2O (gj [Eqn. 6a] Chlorine from 4 / electrolysis: 3 / 2Cl2(g) Unreacted water vapour This has the advantage of maximizing the atom economy of the method described herein. In addition, since it is already known that the actual yield of hydrogen chloride from the RDR can be improved by supplying an amount of water vapour to this reaction which is in excess of the stoichiometric amount, more preferably still, the number of mols of unreacted water vapour recycled to the reaction may also be significantly in excess of the number of mols of water vapour captured from the thermal decomposition of the separated precipitate. In such a case, Eqn. 6a then becomes: Water vapour captured from thermal decomposition: 3A H2O (g> |Unreacted chlorine 4b 4b t 3 CI2 (g) + (3 + e) H2O (gj -> 3 HCI (g> + 3A CI2 (g) + % O2 (g) + (V2 + e) H2O (gj [Eqn. 6b] Chlorine from 4s 4s 4 / electrolysis: 3 / 2Cl2(g) Unreacted water vapour wherein e represents the excess number of mols of unreacted water vapour recycled to the reaction. As an alternative or in addition to the RDR described above, if the method described herein comprises thermally decomposing at least some of the phase-separated precipitate to produce water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide, then in some embodiments, the method may further comprise capturing at least some of the water vapour thus produced, and electrolysing it to produce hydrogen gas and oxygen gas. If so, using at least some of the chlorine gas to produce hydrogen chloride may comprise combusting at least some of the hydrogen gas produced by this electrolysis with at least some of the chlorine gas to produce the hydrogen chloride. The water vapour may be electrolysed by firstly condensing the captured water vapour (with optional recovery of its enthalpy of condensation) and then electrolysing the liquid water thus produced, for example using a proton exchange membrane. Alternatively or additionally, the water vapour may be electrolysed without a phase change by performing high-temperature steam electrolysis (HTSE) on it, for example using a solid oxide electrolysis cell. This has the advantage that the captured water vapour is already either at or near to the temperature at which such HTSE may be conducted. In both cases, however, the captured water vapour is already sufficiently pure to be electrolysed without any prior treatment. Moreover, since the subsequent combustion of the hydrogen gas with the chlorine gas is strongly exothermic and occurs at high temperature, heat may also be extracted from this reaction and used to contribute to powering the electrolysis. If producing liquid sodium and chlorine gas by electrolysis comprises electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas, the method of the invention may further comprise capturing at least some of the water vapour produced by drying the aqueous solution of sodium hydroxide, condensing at least some of the water vapour captured from drying the aqueous solution of sodium hydroxide to produce liquid water, and using at least some of the liquid water thus produced as the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid. This has the advantage of significantly reducing the nett amount of water consumed. In embodiments in which the method of the invention comprises capturing and condensing at least some of the water vapour produced by drying the aqueous solution of sodium hydroxide to produce liquid water, and using at least some of the liquid water thus produced as the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid, the method may further comprise adding liquid water to the liquid water thus produced, wherein a number of mols of the liquid water added is at least equal to a number of mols of the hydrogen gas produced by electrolysis. This has the advantage that the liquid water added replenishes the hydrogen ions converted to hydrogen gas by electrolysis, which can be used to maintain the molarity of the hydrochloric acid within a desired range. As an alternative or in addition to producing at least some of the hydrochloric acid by a technique which includes a reverse Deacon reaction as described above, in some embodiments, using at least some of the chlorine gas to produce hydrogen chloride may comprise combusting at least some of the chlorine gas with hydrogen gas to produce the hydrogen chloride. In particular, if producing liquid sodium and chlorine gas by electrolysis comprises electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas, then the method of the invention may further comprise combusting at least some of the chlorine gas with at least some of the hydrogen produced by at least one of electrolysing an aqueous solution of sodium chloride and electrolysing at least some of the solid sodium hydroxide, to produce the hydrogen chloride. Whereas it is well known that chlorine can be combusted with hydrogen to produce hydrogen chloride, such a technique of producing at least some of the hydrogen chloride has the advantage that it only consumes hydrogen gas produced as a co-product of producing liquid sodium and chlorine gas by electrolysis, which therefore avoids consuming hydrogen produced by another route, such as by steam-methane reforming (SMR), which would otherwise also result in producing carbon dioxide. Furthermore, because combusting gaseous chlorine and hydrogen is exothermic, heat may be extracted from this reaction as well and supplied to another process, such as to thermally decomposing the separated precipitate. As mentioned above, at least some of the aqueous solution of sodium chloride produced by at least one of (i) reacting at least some of the first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the first portion of sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, and (ii) reacting at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the second portion of sodium oxide, with at least some of the hydrochloric acid, may be used as a feedstock for the electrolysis. In some embodiments, therefore, producing liquid sodium and chlorine gas by electrolysis may comprise electrolysing at least some of the aqueous solution of sodium chloride produced by at least one of these two reactions. This has the advantage of significantly reducing the amount of new sodium chloride required for the electrolysis. Alternatively or additionally, in some embodiments, the method may further comprise drying at least some of the aqueous solution of sodium chloride produced by at least one of these two reactions, to produce solid sodium chloride and water vapour, in which case, producing liquid sodium and chlorine gas by electrolysis may comprise fusing and electrolysing at least some of the solid sodium chloride thus produced. This also has the advantage of significantly reducing the amount of new sodium chloride required for the electrolysis. If the method does comprise drying at least some of the aqueous solution of sodium chloride produced by at least one of these two reactions, the method may further comprise capturing at least some of the water vapour produced by drying the aqueous solution of sodium chloride, condensing at least some of the water vapour captured from drying the aqueous solution of sodium chloride to produce liquid water, and using at least some of the liquid water thus produced as the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid. This also has the advantage of significantly reducing the nett amount of water consumed. In embodiments in which the other metal comprises iron, reacting at least some of the liquid sodium in a redox reaction with the iron oxide may comprise conducting this reaction in an inert atmosphere and at a temperature of less than about 450 ’Celsius with an amount of the liquid sodium in excess of the stoichiometric amount thereof required for the reaction, then separating the iron and other insoluble products at least comprising the sodium oxide as a solid phase from the excess liquid sodium. The inert atmosphere may consist of at least one of nitrogen and argon, for example. Such a technique has at least the following advantages. Firstly, the formation of ternary oxides, such as Na4FeO3, is thereby avoided. Secondly, unreacted liquid sodium in excess of the stoichiometric amount thereof required for this reaction may be used as a transport medium for the insoluble reaction products. Thirdly, the excess liquid sodium may also be circulated as a heat transfer fluid through this strongly exothermic reaction, in order to extract heat from it. At least some of the heat extracted may then be transferred to another process, such as to thermally decomposing the separated precipitate. The present applicant's co-pending UK patent application no. 2417059.9 ("Carbon-Free Method and Apparatus for Producing Iron and Steel"; applicant's ref: NE-P-GB 001), the entire contents of which is incorporated herein by reference, describes a method and apparatus, wherein liquid sodium is used as a chemical reducing agent in such a reaction with iron oxide from an iron ore, in which the iron is present as at least one of the mineral and mineraloid forms of iron oxide and iron oxyhydroxide, to produce iron and sodium oxide. Separating the iron and the other insoluble products as a solid phase from the liquid sodium may, for example, comprise at least one of settlement under gravity, filtration ( / .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 also incorporated herein by reference, describes an apparatus and method for separating undissolved contaminants, for example in the form of suspended or entrained particulates, from a liquid metal, such as liquid sodium, whereby the insoluble reaction products may be separated from the excess liquid sodium used as a transport medium. In another example, a liquid sodium centrifuge is shown and described on pp. 29 to 33 of Summary of the APDA Sodium Technology Program byJ.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. Regardless of how this 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. In some embodiments, at least some of the oxygen produced by the method described herein may be used in a steelmaking process. For example, the oxygen may be injected into molten iron in a basic oxygen furnace. Moreover, if the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium, then at least some of the precipitate produced by the method described herein may be thermally decomposed to produce calcium and / or magnesium oxide, which may then be used as a flux in the same or another steelmaking process. In embodiments in which the other metal, M, comprises iron, at least some of the elemental iron produced by the method described herein may also be used to make steel. This has the advantage that steelmaking may then be collocated with the production of iron by the method of the invention. For example, the iron may be melted by at least one of ohmic heating, as in an arc furnace, and induction heating, as in an induction furnace, and alloyed with carbon and possibly also with other elements as desired to produce steel of an intended composition. If the electricity required to melt the iron comes from a source of renewable energy, such as wind or solar, or from nuclear power, then no greenhouse gases are produced by this process. Since the other metal, M, produced by the method described herein is in the form of a finely divided particulate, it is particularly suited for use in powder metallurgical techniques, such as hot isostatic pressing ("hipping"), cold pressing followed by sintering, and so on. Therefore, if the other metal, M, comprises iron, at least some of the elemental iron produced by the method described herein may be used to make steel by adding powdered carbon to the iron, mixing the powdered carbon and iron together to produce a resulting mixture, and then subjecting the resulting mixture to a powder metallurgical process. This has the advantage that the powdered mixture does not have to be melted to make the steel and only has to be heated to above theTammann temperature of the iron. Wastage from powder metallurgical steelmaking techniques is considerably lower as well. Moreover, since finely divided iron produced by the method described herein is not produced from molten iron either, steel made by combining the method of producing iron described herein with a subsequent powder metallurgical process is therefore energetically significantly cheaper than steel made using prior art steelmaking techniques, even including those which already comprise a powder metallurgical process. In embodiments in which the other metal comprises manganese, reacting at least some of the liquid sodium in a redox reaction with the trivalent manganese oxide may comprise conducting this reaction in an inert atmosphere and at a temperature of less than about 600 ’Celsius with an amount of the liquid sodium in excess of the stoichiometric amount thereof required for the reaction, then separating the manganese and other insoluble products at least comprising the sodium oxide as a solid phase from the excess liquid sodium. The inert atmosphere may be as described above. Such a technique has at least the following advantages. Firstly, the formation of ternary oxides, such as a-NaMnOj, is thereby avoided. Secondly, unreacted liquid sodium in excess of the stoichiometric amount thereof required for this reaction may be used as a transport medium for the insoluble reaction products. Thirdly, the excess liquid sodium may also be circulated as a heat transfer fluid through this exothermic reaction, in order to extract heat from it. At least some of the heat extracted may then be transferred to another process, such as to thermally decomposing the separated precipitate. The present applicant's co-pending UK patent application no. 2417063.1 ("Carbon-Free Method and Apparatus for Producing Manganese"; applicant's ref: NE-P-GB 008), the entire contents of which is incorporated herein by reference, describes a method and apparatus, wherein liquid sodium is used as a chemical reducing agent in a redox reaction with a trivalent manganese oxide derived from a manganese ore of the types described above to produce elemental manganese and sodium oxide. As before, the insoluble reaction products may be separated from the excess liquid sodium using at least one of several different techniques, such as by settlement under gravity, filtration and / or centrifugation, including that 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), already referred to above. In some embodiments which produce manganese in elemental form, at least some of the manganese may be used as an alloying element with iron to produce an austenitic manganese or stainless steel. If the manganese is used as an ingredient in steelmaking in this manner, then any impurities (such as any residual silica) still remaining in the undissolved solids after the phase separation can be transferred to a slag phase during the steelmaking process. If the ore of the other metal comprises a siliceous mineral, in some embodiments, the redox reaction between the liquid sodium and the oxide of the other metal may be conducted at a temperature of at least about 320 ’Celsius to induce a reaction between the sodium oxide and silica derived from the siliceous mineral to produce at least sodium silicate, which dissolves in the liquid sodium. The siliceous mineral may comprise the other metal itself and / or it may be silica and / or another silicate mineral present as gangue. In such embodiments, the redox reaction should also be conducted at a temperature below that at which the liquid sodium can react with the oxide of the other metal to produce a ternary oxide thereof, because such ternary oxides are generally soluble in liquid sodium, which would both contaminate the liquid sodium and reduce the yield of the metal produced in elemental form. Thus, for example, if the other metal is manganese, the reaction should be conducted at a temperature of less than about 600 ’Celsius, whereas if the other metal is iron, the reaction should be conducted at a temperature of less than about 450’Celsius. If the other metal comprises a mixture of both iron and manganese, then the reaction should be conducted at a temperature of less than about 450 ’Celsius. In such embodiments wherein the ore of the other metal comprises a siliceous mineral, at least some of the liquid sodium with the sodium silicate dissolved therein is then separated from the undissolved metal and other insoluble products, and oxidized by reacting it with at least one of oxygen and water to produce a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide. Once again, the liquid sodium with the sodium silicate dissolved therein may be separated from the undissolved metal and the other insoluble products using at least one of several different techniques, such as by settlement under gravity, filtration and / or centrifugation, including that 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. The present applicant's co-pending UK patent application no. 2417073.0 ("Method and Apparatus for Producing an Alkaline Mixture comprising Sodium Silicate"; applicant's ref: NE-P-GB 009), the entire contents of which is also incorporated herein by reference, describes a method and apparatus, whereby an alkaline mixture comprising sodium silicate can be produced from an ore comprising a siliceous mineral and at least one of iron and manganese as the other metal. In some embodiments, the method may further comprise using at least some of the mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide as at least one of an alkaline activator for an alkaline activated or geopolymer cement, and a reagent with carbon dioxide to produce a composition comprising sodium carbonate and silica. If at least some of the mixture is used as an alkaline activator, since this mixture is produced as a co-product of several other industrial products, the method of the invention therefore provides a low-energy alternative method for producing such an alkaline activator. If at least some of the mixture is used as a reagent with carbon dioxide to produce a composition comprising sodium carbonate and silica, the method of the invention can have a negative carbon footprint overall. The carbon dioxide may be captured from atmospheric air and / or derived from one or more industrial processes. In some embodiments, if the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium (for example, if the carbonate mineral comprises at least one of calcite, magnesite and dolomite), then at least some of the phase-separated precipitate, or at least one of calcium oxide and magnesium oxide derived from thermally decomposing at least some of the separated precipitate, may be used as at least one of an ingredient in cement manufacture and a reagent with carbon dioxide to produce a substance comprising at least one of calcium carbonate and magnesium carbonate. The carbon dioxide may be captured from atmospheric air and / or derived from one or more industrial processes. Moreover, if the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium, in some embodiments, some of the phase-separated precipitate may be used as an ingredient in the manufacture of non-hydraulic lime mortar. Non-hydraulic lime mortar sets and hardens by absorbing carbon dioxide from atmospheric air in a carbonation reaction to produce at least calcium carbonate. Therefore, in the last two cases, the method of the invention can have a negative carbon footprint overall. Alternatively or additionally, some of the separated precipitate may be used as a reagent with a pozzolan to produce a hydraulic cement. If, on the other hand, the carbonate mineral has a majority of iron cations (for example, if the carbonate mineral is siderite), in some embodiments, the method may further comprise thermally decomposing at least some of the phase-separated precipitate to produce water vapour and a solid end-product comprising iron oxide, and using at least some of this solid end-product as a reagent with at least some of the liquid sodium. Before using it as a reagent with the liquid sodium, at least some of the solid end-product may possibly also be beneficiated, for example using magnetic and / or density-based separation techniques, to increase the proportion of iron oxide therein. In other words, the method described herein can be used to convert siderite ores into a solid end-product, which can itself then be processed for reaction with liquid sodium by the same method to produce iron. In embodiments in which the method of the invention produces sodium carbonate, at least some of the sodium carbonate produced may be used as at least one of an ingredient in the manufacture of at least one of soda-lime glass and borosilicate glass, a reagent with carbon dioxide and water to produce a substance comprising at least sodium hydrogencarbonate, and a sorbent for capturing carbon dioxide from a stream of flue gases. At present, about half of all sodium carbonate consumed is used in the manufacture of soda-lime glass. Traditionally, much of the sodium carbonate used to manufacture soda-lime glass comes from mining trona. However, in comparison to mining trona, using sodium carbonate produced by the method described herein in glassmaking has the advantage that it does not extract new carbon dioxide from geological deposits. In other words, the sodium carbonate produced by the method of the invention contains carbon dioxide from the carbonate mineral, which has already been mined to produce the oxide or hydroxide derived from the carbonate mineral. Thus the carbon dioxide successively passes through two industrial processes ( / .e., the production of this oxide or hydroxide and glassmaking), whereby the total amount of carbon dioxide extracted from geological deposits to produce both the oxide or hydroxide derived from the carbonate mineral and the glass is halved. In comparison to using sodium carbonate made by the Solvay process, using sodium carbonate produced by the method described herein has at least the advantages that it avoids the calcination of limestone, the use of hazardous ammonia, and the overproduction of calcium chloride. If at least some of the sodium carbonate produced by the method described herein is used as a reagent with carbon dioxide and water to produce a substance comprising at least sodium hydrogencarbonate, the method of the invention can have a negative carbon footprint overall. The carbon dioxide may be captured from atmospheric air and / or it may derive from one or more industrial processes. If at least some of the sodium carbonate produced by the method described herein is used as a sorbent for capturing carbon dioxide from a stream of flue gases, no new greenhouse gas emissions are created, since the sodium carbonate used as a sorbent is regenerated. For example, sodium carbonate produced by the method of the invention may be used as a sorbent for capturing carbon dioxide from a stream of flue gases in the manner described in "Carbon Dioxide Capture from Flue Gas using Dry Regenerable Sorbents" by D.A. Green eto / ., Topical Report, DOE Cooperative Agreement No. DE-FC26-00NT40923, Research Triangle Institute, NC, USA (November, 2004). Alternatively or additionally, if the method produces sodium carbonate, at least some of the sodium carbonate may be stored in a salt mine from which a greater volume of salt has been extracted. This exploits the fact that the volume of sodium carbonate produced by the method of the invention is always less than the volume of sodium chloride the method consumes. If so, storage of the sodium carbonate in such a mine can be used for long-term sequestration of the carbon dioxide from the carbonate mineral. If the sodium carbonate is also used as a reagent with more carbon dioxide and water to produce a substance comprising at least sodium hydrogencarbonate, a salt mine containing sodium carbonate produced by the method described herein then becomes a suitable site for storage of captured carbon. This has the advantage that the overall result is to replace and restore previously mined reserves of trona. In embodiments in which the method of the invention comprises producing liquid sodium and chlorine gas by electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas, the method may further comprise using at least some of the hydrogen thus produced as at least one of an ingredient in a process for the manufacture of ammonia (such as the Haber-Bosch process), a fuel to produce electricity in an electrochemical cell, and a fuel to produce heat by combustion with oxygen. Using hydrogen produced by the method described herein has the advantage that at present, about 96% of all hydrogen produced is still derived from fossil fuels (commonly called "blue" hydrogen). Moreover, since in some embodiments, the method of the invention can produce a molar amount of hydrogen which is up to 1.5 times the molar amount of the other metal, M, produced, the method described herein also has the advantage that it can meet a greatly increased demand for hydrogen for use as a fuel, for example. The only effect of increasing the amount of hydrogen produced by the method described herein is a corresponding increase in the amount of oxygen produced as well. However, any excess oxygen can just be safely vented to atmosphere. The method of the invention may be carried out as a continuous, semi-batch or batch process. However, it is preferably carried out as a continuous process for reasons of economy and efficiency. In a second aspect, the present invention also provides an apparatus for carrying out the method of the invention. The apparatus comprises an electrolytic subassembly, a redox reaction subassembly, a separation subassembly, a hydrochloric acid-producing subassembly, a carbonate dissolution subassembly, a solid-aqueous phase separator and a caustic transfer pathway. The electrolytic subassembly is for producing at least liquid sodium and chlorine gas by electrolysis, and comprises an inlet for sodium chloride in at least one of solid, molten and aqueous phase, a first outlet for liquid sodium, and a second outlet for chlorine gas. The redox reaction subassembly comprises a first gastight reaction vessel for containing therein a redox reaction between liquid sodium and an oxide of another metal, M, from an ore of the other metal, wherein the other metal, M, comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide. The redox reaction subassembly further comprises a first inlet for the oxide of the other metal, M, a second inlet for receiving liquid sodium from the first outlet of the electrolytic subassembly, and an outlet for liquid sodium and entrained therein, a solid phase comprising the other metal, M, in elemental form and sodium oxide. The separation subassembly is for separating the liquid sodium, the other metal, M, in elemental form and the sodium oxide from each other, and comprises an inlet for receiving the liquid sodium with the solid phase entrained therein from the outlet of the redox reaction subassembly, a first outlet for liquid sodium, a second outlet for the other metal, M, in elemental form and a third outlet for sodium oxide or hydroxide. The hydrochloric acid-producing subassembly is for producing hydrochloric acid from chlorine gas and liquid water, and comprises a first inlet for receiving chlorine gas from the second outlet of the electrolytic subassembly, a second inlet for liquid water and an outlet for hydrochloric acid. The carbonate dissolution subassembly comprises a second gastight reaction vessel for dissolving therein a carbonate mineral of at least one of calcium, magnesium and iron in hydrochloric acid. The carbonate dissolution subassembly further comprises a first inlet for another ore comprising the carbonate mineral, a second inlet for receiving hydrochloric acid from the outlet of the hydrochloric acid-producing subassembly, a third inlet for receiving a first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this first portion of sodium oxide, from the third outlet of the separation subassembly, and an outlet for an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. The solid-aqueous phase separator is for separating at least some of this precipitate from the aqueous solution of sodium chloride, and comprises an inlet for receiving the aqueous solution of sodium chloride and the precipitate from the outlet of the carbonate dissolution subassembly, a first outlet for the precipitate, and a second outlet for the aqueous solution of sodium chloride. The caustic transfer pathway is for transferring a second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this second portion of sodium oxide, from the third outlet of the separation subassembly to at least one of (i) a neutralization vessel for reaction with hydrochloric acid from the outlet of the hydrochloric acid-producing subassembly, and (ii) a carbonation vessel for reaction with carbon dioxide gas produced in the carbonate dissolution subassembly. The apparatus may further comprise a kiln for thermally decomposing at least some of the separated precipitate into water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide. In some embodiments, the kiln may be gas-tight to allow this thermal decomposition to be carried out in the absence of oxygen. If so, the kiln may comprise an inlet for receiving the precipitate from the first outlet of the solid-aqueous phase separator, a first outlet for water vapour, and a second outlet for the solid end-product. In such embodiments, the hydrochloric acid-producing subassembly may comprise a gas-phase reactor for reacting chlorine gas with water vapour in a reverse Deacon reaction at a temperature of from about 450 to about 750 ’Celsius to produce a mixture of gases at least comprising hydrogen chloride and oxygen, and an absorber for contacting this mixture of gases with liquid water to produce hydrochloric acid and a stream of tail gases. In such cases, the gas-phase reactor may comprise a first inlet for receiving the chlorine gas from the second outlet of the electrolytic subassembly, a second inlet for receiving the water vapour from the first outlet of the kiln, and an outlet for the mixture of gases. The absorber may comprise a first inlet for receiving the mixture of gases from the outlet of the gas-phase reactor, a second inlet for receiving the liquid water, a first outlet for the hydrochloric acid and a second outlet for the stream of tail gases. The water vapour from the thermal decomposition is therefore already at a high temperature for the RDR. Moreover, since the kiln is gas-tight and the thermal decomposition is carried out in the absence of oxygen, the water vapour can be supplied to the gasphase reactor uncontaminated by oxygen. In some embodiments, the electrolytic subassembly may comprise a first type of electrolytic cell, such as a chlor-alkali type of cell, for electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, a caustic dryer for drying an aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and a second type of electrolytic cell, such as a Castner type of cell, for fusing and electrolysing solid sodium hydroxide to produce liquid sodium, hydrogen gas and oxygen gas. The first type of electrolytic cell may comprise an inlet for the aqueous solution of sodium chloride, a first outlet for the aqueous solution of sodium hydroxide and a second outlet for chlorine gas. The caustic dryer may comprise an inlet for receiving the aqueous solution of sodium hydroxide from the first outlet of the first type of electrolytic cell, a first outlet for water vapour and a second outlet for solid sodium hydroxide. The second type of electrolytic cell may comprise an inlet for receiving the solid sodium hydroxide from the second outlet of the caustic dryer, a first outlet for oxygen gas, a second outlet for hydrogen gas and a third outlet for liquid sodium. In such embodiments, the apparatus may further comprise a condenser for condensing water vapour, wherein the condenser comprises an inlet for receiving the water vapour from the first outlet of the caustic dryer and an outlet for water in liquid phase connected upstream of the second inlet of the hydrochloric acid-producing subassembly. Thus water obtained by drying the aqueous solution of sodium hydroxide may be recycled to help produce the hydrochloric acid. Alternatively or additionally, in such embodiments, the hydrochloric acid-producing subassembly may comprise a combustion chamber for combusting chlorine gas with hydrogen gas to produce hydrogen chloride, and an absorber for contacting hydrogen chloride with liquid water to produce hydrochloric acid. The combustion chamber may comprise a first inlet for receiving the chlorine gas from the second outlet of the first type of electrolytic cell, a second inlet for receiving the hydrogen gas from at least one of a hydrogen outlet of the first type of electrolytic cell and the second outlet of the second type of electrolytic cell, and an outlet for hydrogen chloride. The absorber may comprise a first inlet for receiving the hydrogen chloride from the outlet of the combustion chamber, a second inlet for receiving the liquid water, and an outlet for the hydrochloric acid. Thus hydrogen produced by the co-electrolysis of water in either or both of the first and second types of electrolytic cell may be used to help produce the hydrochloric acid. If the electrolytic subassembly does comprise the first type of electrolytic cell, this electrolytic cell may comprise an inlet for receiving the aqueous solution of sodium chloride from at least one of the second outlet of the solid-aqueous phase separator and the outlet of the neutralization vessel. Thus the aqueous solution of sodium chloride produced by this phase separator and / or the neutralization vessel may be recycled for electrolysis by the first type of electrolytic cell. In some embodiments, the electrolytic subassembly may comprise a third type of electrolytic cell, like that described in US patent no. 1 501756 of Downs, for fusing and electrolysing solid sodium chloride. If so, the apparatus may further comprise a dryer for drying an aqueous solution of sodium chloride, wherein the dryer comprises an inlet for receiving the aqueous solution of sodium chloride from at least one of the second outlet of the solid-aqueous phase separator and the outlet of the neutralization vessel, a first outlet for water vapour and a second outlet for sodium chloride in solid phase. In such cases, the third type of electrolytic cell may comprise an inlet for receiving the solid sodium chloride from the second outlet of the dryer. Thus sodium chloride in solid phase obtained by drying the aqueous solution of sodium chloride may be recycled for electrolysis by the third type of electrolytic cell. In such embodiments, the apparatus may further comprise a condenser for condensing water vapour, wherein the condenser comprises an inlet for receiving the water vapour from the first outlet of the dryer and an outlet for water in liquid phase connected upstream of the second inlet of the hydrochloric acid-producing subassembly. Thus water obtained by drying the aqueous solution of sodium chloride may be recycled to help produce the hydrochloric acid. 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 diagram of the ammonia-soda, or Solvay, process; Fig. 2A is a diagram of a first embodiment of a method according to the invention; Fig. 2B is a block diagram of the method of Fig. 2A; Fig. 3A is a diagram of a chemical cycle in a second embodiment of a method of the invention; Fig. 3B is a block diagram of the same chemical cycle as in Fig. 3A; Fig. 4A is a diagram of a chemical cycle in a third embodiment of a method of the invention; Fig. 4B is a block diagram of the same chemical cycle as in Fig. 4A; Fig. 4C is a diagram of a chemical cycle which is a variant of the third embodiment; Fig. 4D is a block diagram of the same chemical cycle as in Fig. 4C; Fig. 5A is a flow diagram of a first water / hydrogen-ion cycle in the chemical cycles of Figs. 4A-D; Fig. 5B is a flow diagram of a second water / hydrogen-ion cycle in the chemical cycles of Figs. 4A-D; Fig. 6A is a diagram of a chemical cycle in a fourth embodiment of a method of the invention; Fig. 6B is a block diagram of the same chemical cycle as in Fig. 6A; Fig. 7A is a flow diagram of a third water / hydrogen-ion cycle in the chemical cycle of Fig. 6A; Fig. 7B is a flow diagram of a fourth water / hydrogen-ion cycle in the chemical cycle of Fig. 6A; Fig. 8A is a diagram of a chemical cycle in a fifth embodiment of a method of the invention; Fig. 8B is a block diagram of the same chemical cycle as in Fig. 8A; Fig. 9A is a diagram of a chemical cycle in a sixth embodiment of a method of the invention; Fig. 9B is a block diagram of the same chemical cycle as in Fig. 9A; Fig. 10A is a diagram of a chemical cycle in a seventh embodiment of a method of the invention; Fig. 10B is a block diagram of the same chemical cycle as in Fig. 10A; Fig. 11A is a diagram of part of a chemical cycle in an eighth embodiment of a method of the invention; Fig. 11B is a block diagram of the same part of the chemical cycle as in Fig. 11A; Fig. 12A is a diagram of a chemical cycle in a ninth embodiment of a method of the invention; Fig. 12B is a block diagram of the same chemical cycle as in Fig. 12A; Fig. 13 is a schematic diagram of a first embodiment of an apparatus according to the invention; Fig. 14 is a schematic diagram of an embodiment of a separation subassembly in an apparatus of the invention; Fig. 15 is a schematic diagram of a second embodiment of an apparatus according to the invention; Fig. 16 is a schematic diagram of an embodiment of an electrolytic subassembly and an embodiment of a hydrochloric acid-producing subassembly in an apparatus of the invention; Fig. 17 is a schematic diagram of a third embodiment of an apparatus according to the invention; Fig. 18 is a schematic diagram of an embodiment of a subassembly for producing a mixture comprising sodium silicate in an apparatus of the invention; and Fig. 19 is a conceptual diagram summarizing the raw materials and main products of some embodiments of the method described herein, and examples of some uses of those products. 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 Figs. 2A-B both show a first embodiment of a method 400a according to the invention. In Figs. 2A-B, starting materials are contained in boxes edged with dashed lines, M represents Fe and / or Mn, Q represents at least one of Ca, Mg and Fe, and x represents a variable molar quantity. In the embodiment of Figs. 2A-B, for illustrative purposes only, the starting materials are consumed in their stoichiometric ratios, so that 1.5 mol of the carbonate mineral, QCO3, are consumed per mol of trivalent metal oxide, M2O3, 6 mol of liquid sodium are produced by electrolysis per mol of M2O3 consumed, and all of the sodium oxide produced is divided 401 into the first and second portions thereof in a ratio of 1:1. However, these stoichiometric ratios are unlikely to be achieved in practice, because of the presence of gangue species, such as silica or alumina, mixed in with one or more of the starting materials. Thus the molar ratio of the starting materials is more likely be in a range of from about 1.25 to about 1.75 mol, inclusive, of the carbonate mineral per mol of M2O3, and from about 2.5 to about 3.5 mol, inclusive, of the sodium chloride which is used to produce the 6 mol of liquid sodium, per mol of the M2O3 consumed. The method 400a firstly comprises producing 101b, 201c liquid sodium and chlorine gas by electrolysis. In this embodiment, by way of example, 3 mol of liquid sodium are produced per mol of M2O3 by fusing and electrolysing 101b solid sodium chloride and 3 mol of liquid sodium per mol of M2O3 are instead produced by fusing and electrolysing 201c solid sodium hydroxide. Fusing and electrolysing 101b the solid sodium chloride also produces 1.5 mol of chlorine gas, whereas fusing and electrolysing 201c the solid sodium hydroxide also produces 1.5 mol of hydrogen gas, which are then combusted 203b together to produce hydrogen chloride. The hydrogen chloride is then dissolved 204 in liquid water to produce hydrochloric acid. Fusing and electrolysing 201c the solid sodium hydroxide also produces 1.5 mol of oxygen gas as a co-product. The combined 6 mol of liquid sodium are then reacted in a redox reaction 303 with the oxide of the other metal, M, from an ore of that other metal, wherein the other metal, M, comprises at least one of iron and manganese, to produce at least the other metal, M, in elemental form and sodium oxide. The other metal, M, is then separated 304 from the sodium oxide. In addition, another ore comprising a carbonate mineral of at least one of calcium, magnesium and iron is added 205a to the hydrochloric acid to dissolve the carbonate mineral therein and produce 205b carbon dioxide gas and an aqueous solution respectively comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. The carbonate mineral is dissolved in the hydrochloric acid closed off from their surrounding environment and the carbon dioxide gas is captured. Next, the sodium oxide from the redox reaction 303 is divided 401 into a first portion and a second portion. The first portion of sodium oxide, is reacted 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to produce an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. This precipitate is then phase-separating 208 from the aqueous solution of sodium chloride thus produced. In this embodiment, by way of example, the second portion of sodium oxide, is reacted 214a with the captured carbon dioxide to produce sodium carbonate. Thus the method 400a consumes the following starting materials: 3 mol of sodium chloride, 3 mol of sodium hydroxide and x mol of water, in addition to 1 mol of the trivalent metal oxide, M2O3, and 1.5 mol of the carbonate mineral, QCO3. The products of the method 400a are 2 mol of the other metal, M, in elemental form, 1.5 mol of Q hydroxide, 1.5 mol of sodium carbonate and 1.5 mol of oxygen. However, the method 400a also leaves 3 mol of aqueous solution of sodium chloride and x mol of water per mol of M2O3 remaining. Figs. 3A-B both show a second embodiment of a method 400b of the invention, wherein the 3 mol of aqueous solution of sodium chloride and x mol of water remaining from the method 400a are recycled to provide the 3 mol of sodium chloride and x mol of water consumed at the start of the method 400a. The method 400b of Figs. 3A-B therefore differs from the method 400a of Figs. 2A-B in two respects, as follows. Firstly, the method 400b further comprises drying 110 the remaining aqueous solution of sodium chloride, capturing 111 the water vapour produced by drying 110 this aqueous solution, condensing 112 the captured water vapour to produce liquid water, and using 113 this liquid water as the liquid water in which the hydrogen chloride is dissolved 204 to produce the hydrochloric acid. This therefore eliminates the need to add any water 402, as in the method 400a. Secondly, the method 400b also comprises using the solid sodium chloride left from drying 110 the remaining aqueous solution of sodium chloride as the solid sodium chloride which is fused and electrolysed 101b to produce 3 mol of the liquid sodium. This therefore also eliminates the need to consume any sodium chloride, as in the method 400a. The method 400b therefore becomes a chemical cycle in which no 5 nett amount of salt or water is produced or consumed. Tables 1A and IB below respectively summarize the starting materials and products of the chemical cycle 400b of Figs. 3A-B: Starting Materials Examples of Reagents No. of mol consumed per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 per mol of M2O3 Metal ore Hematite (Fe2O3) 1 159.7 30.4 Bixbyite (Mn2O3) 157.9 35.1 Carbonate mineral ore Calcite (CaCO3) 1.5 150 55.4 Magnesite (MgCO3) 126.5 42.7 Dolomite (CaCO3-MgCO3) 126.5 to 150 42.7 to 55.4 Siderite (FeCO3) 173.7 45.7 Caustic soda Sodium hydroxide (NaOH) 3 120.0 56.3 Table 1A Main Products* No. of mol produced per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 @ s.t.p. per mol of M2O3 Examples of end uses Elemental metal 2 Fe 111.6 15.9 Steelmaking Mn 109.8 15.3 Hydroxide from carbonate mineral 1.5 Ca(OH)2 111 50.2 Cement making, steelmaking, glassmaking Mg(OH)2 87.5 37.1 Ca(OH)2- Mg(OH)2 87.5 to 111 37.1 to 50.2 Fe(OH)2 134.9 39.7 Ironmaking Sodium carbonate (Na2CO3) 1.5 159 62.6 Glassmaking, soap powders, papermaking Oxygen (O2) 1.5 48 33.6 x 103 Steelmaking (* / .e., ignoring any gangue species which may also be present in one or more of the raw materials) 10 Table IB The theoretical total energy consumption of the embodiment of Figs. 3A-B could be calculated from the standard enthalpies of formation, AHf, of the reagents and products in Tables 1A and IB, which according to Hess's Law, is equal to AHf (products) - AHf (reagents), subject to real-world losses and inefficiencies, and assuming that the reagents and their products are all in their standard states at s.t.p. However, such a calculation would not give a realistic view of this total energy consumption because the sodium hydroxide consumed in both the methods 400a and 400b is water-soluble, so does not occur in geological deposits and must be manufactured instead. Further embodiments described below therefore take account of any energy consumed in such manufacture by avoiding the consumption of a nett amount of sodium hydroxide. In general, the method of the invention comprises producing both liquid sodium and chlorine gas by electrolysis. 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. 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, such as with calcium chloride or strontium chloride, as described, for example, in US patent no. 2,850,442 or US patent appln. no. 2001 / 0045365 Al. 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 gaseous chlorine. 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. To lower the temperature at which the electrolysis takes place, the sodium hydroxide may be mixed with potassium hydroxide to form a eutectic mixture, as described, for example, in Castner's original US patent no. 452,030. Whereas the Castner process is generally regarded as obsolete, fusing and electrolysing 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 (aqj + 2 H2O -> 2 NaOH (aqj + CI2 + H2 [Eqn. 7a] Dry Electrolysis of melt: 2 NaOH 2 Na ( / ) + H2O + ½ O2 [Eqn. 7b] Castner 4s . process "| 4 / Water reacts with sodium: | Na + H2O-> NaOH + ½ H2 [Eqn. 7c] I_________________________________________________________________________________________________________________________________________________________________________ Overall: 2 NaCI + 2 H2O 2 Na + Cl2 + 2 H2 + O2 [Eqn. 7d] Below, a third embodiment of the method of the invention, wherein all of the liquid sodium and chlorine gas are produced by fusing and electrolysing solid sodium chloride will firstly be described in relation to Figs. 4A-B, and then a fourth embodiment of the method of the invention, wherein only half of the liquid sodium and chlorine gas are produced by fusing and electrolysing solid sodium chloride will be described in relation to Figs. 6A-B. In this fourth embodiment, the other half of the liquid sodium and chlorine gas are instead produced by electrolysing an aqueous solution of sodium chloride to produce an aqueous solution of sodium hydroxide and the other half of the chlorine gas, drying the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and then fusing and electrolysing the solid sodium hydroxide to produce the other half of the liquid sodium. Figs. 4A-B both show a chemical cycle 400c in a third embodiment of a method according to the invention. In Figs. 4A-B, starting materials are again contained in boxes edged with dashed lines, M, Q and x respectively represent the same as in Figs. 2A to 3B, and for illustrative purposes only, the starting materials are consumed in their stoichiometric ratios, as was described above in relation to the first embodiment of Figs. 2A-B. In the present embodiment, all 6 mol of the liquid sodium are produced by fusing and electrolysing 101b solid sodium chloride. 3 mol of this solid sodium chloride are added as one of the starting materials, whereas the other 3 mol of the solid sodium chloride are recycled by drying 110 the aqueous solution of sodium chloride, which is produced by reacting 207 the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride with the first portion of sodium oxide. In this embodiment, the second portion of sodium oxide is hydrated 317b, before sodium hydroxide derived therefrom is reacted with carbon dioxide captured from the reaction 205a, 205b between the hydrochloric acid and the carbonate mineral. Hydrating 317b the second portion of sodium oxide and carbonating 214b the sodium hydroxide which this produces proceed as follows. The second portion of sodium oxide is added to a quantity of water which is sufficient to produce an aqueous solution of sodium hydroxide, in which the final concentration of sodium hydroxide after the second portion of sodium oxide has been added to it is less than about 2.5 M (i.e., less than about 100 g dm-3). The temperature of this hydration reaction, which is strongly exothermic, is also controlled to remain below about 85 ’Celsius. At such temperatures and concentrations, gangue species like silica and alumina mixed in with the second portion of sodium oxide do not react with the aqueous solution of sodium hydroxide to any appreciable extent. They therefore precipitate out and can be separated from the aqueous solution of sodium hydroxide, for example by settlement under gravity, filtration and / or centrifugation. The aqueous solution of sodium hydroxide is then dried using heat extracted from the hydration reaction and / or from the subsequent carbonation, which is also strongly exothermic, to produce solid anhydrous sodium hydroxide. This anhydrous sodium hydroxide is then reacted 214b with the captured carbon dioxide at a temperature of from about 310 to about 400 ’Celsius, inclusive. In this temperature range, the anhydrous sodium hydroxide reacts with the carbon dioxide to produce anhydrous sodium carbonate as the only solid-phase product, and water vapour. Thus, a relatively pure form of sodium carbonate can be produced by firstly removing the gangue species like silica and alumina following the hydration reaction. The water vapour also produced by the carbonation reaction 214b is combined with the water vapour driven off by drying the aqueous solution of sodium hydroxide. This combined quantity of water vapour is condensed to extract heat from it for drying the aqueous solution of sodium hydroxide, and the condensed water is recycled back to dissolve the second portion of sodium oxide. Overall, these hydration 317b and carbonation 214b reactions proceed according to the equations represented in Fig. 2A, in which 1.5 mol of water circulates in a closed cycle, but no nett quantity of water is added to or removed from the chemical cycle 400c of Figs. 4A-B as a whole. In contrast, in the embodiment of Figs. 4A-B, the first portion of sodium oxide is not hydrated and is just added 207 directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Thus gangue species like silica and alumina mixed in with the first portion of sodium oxide are transferred to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride and precipitate out from it along with the Q hydroxide. This may be acceptable if, for example, the precipitate is subsequently to be used as an ingredient in cement manufacture. Tables 2A and 2B below respectively summarize the starting materials and products of the chemical cycle 400c of Figs. 4A-B. A notable feature of Tables 2A and 2B is that the volume of sodium carbonate produced per mol of M2O3 consumed, equal to 62.6 cm3, is significantly less than the volume of salt consumed per mol of M2O3 consumed, equal to 80.8 cm3. Therefore, even in an extreme case in which none of the sodium carbonate finds an end use, all of it may still be stored in one or more salt mines from which the salt has been extracted. In such a case, storage of the sodium carbonate would constitute sequestration of the carbon dioxide from the carbonate mineral. Since the number of mols of sodium carbonate produced by the method of the invention is at most half the number of mols of sodium chloride consumed, this also applies to any other possible embodiments in which a nett quantity of sodium carbonate is produced. Raw Materials Examples of Reagents No. of mol consumed per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 per mol of M2O3 Metal ore Hematite (Fe2O3) 1 159.7 30.4 Bixbyite (Mn2O3) 157.9 35.1 Carbonate mineral ore Calcite (CaCO3) 1.5 150 55.4 Magnesite (MgCO3) 126.5 42.7 Dolomite (CaCO3-MgCO3) 126.5 to 150 42.7 to 55.4 Siderite (FeCO3) 173.7 45.7 Salt Halite (NaCI) 3 175.3 80.8 Table 2A Main Products* No. of mol produced per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 @ s.t.p. per mol of M2O3 Examples of end uses Elemental metal 2 Fe 111.6 15.9 Steelmaking Mn 109.8 15.3 Oxide from carbonate mineral 1.5 CaO 84 25.6 Cement making, steelmaking, glassmaking MgO 60.5 16.9 CaO-MgO 60.5 to 84 16.9 to 25.6 FeO 107.8 18.8 Ironmaking Sodium carbonate (Na2CO3) 1.5 159 62.6 Glassmaking, soap powders, papermaking Chlorine (Cl2) 1.5 106.4 33.3 x 103 Chlorinated organic compounds, etc. Oxygen (O2) 0.75 24 16.8 x 103 Steelmaking (* / .e., ignoring any gangue species which may also be present in one or more of the raw materials) Table 2B 10 The theoretical total energy consumption of the embodiment of Figs. 4A-B may be calculated from the standard enthalpies of formation, AHf, of the reagents and products in Tables 2A and 2B, which according to Hess's Law, is equal to AHf (products) - AHf (reagents), subject to the same assumptions as described previously. For example, if M = Fe and Q. = Ca, AHf (products) - AHf (reagents) = +1219 kJ per mol of Fe2O3 consumed, but will be less if M and Q. take different values. Figs. 4C-D both showa chemical cycle 400c', which is a variant of the third embodiment shown in Figs. 4A-B. In this variant, the RDR 104 of the chemical cycle 400c is replaced by steam electrolysis 411 of the water vapour captured from the thermal decomposition 209 of the separated precipitate, followed by combustion 203b of the hydrogen gas thus produced with chlorine gas from the molten salt electrolysis 101b. However, since the starting materials and products of the chemical cycle 400c' are the same as those in the chemical cycle 400c, the theoretical total energy consumption of this variant is also the same as that of the chemical cycle shown in Figs. 4A-B. As mentioned previously, in another possible variant having the same starting materials and products and the same theoretical total energy consumption, the HTSE 411 could be replaced by condensing the water vapour captured from the thermal decomposition 209 and then electrolysing the liquid water thus produced. As described above, the chemical cycles of Figs. 4A-D both include a closed cycle for water which circulates round the reactions for hydrating 317b the second portion of sodium oxide and then carbonating 214b the sodium hydroxide thus produced. Apart from this, the chemical cycles 400c, 400c' of Figs. 4A-D also comprise first and second water / hydrogen-ion cycles 405, 406, which are respectively shown in Figs. 5A and 5B. In Figs. 5A and 5B, as well as in Figs. 7A and 7B described below, boxes edged with continuous lines represent processes, boxes edged with dashed lines represent reagents, and Q represents at least one of Ca, Mg and Fe, as before. In the first water / hydrogen-ion cycle 405 of Fig. 5A, a relatively small number of mols of water / hydrogen ions undergoes a relatively large change in temperature, AT, over the cycle, such that 500 <AT <600 ’Celsius approximately, which also involves three changes of phase, represented by ¢, from gas (g) to liquid (£) to solid (s) and then back to gas (g) again. In contrast, in the second water / hydrogen-ion cycle 406 of Fig. 5B, a relatively large number of mols of water / hydrogen ions undergoes a relatively small change in temperature, AT, over the cycle, such that AT = 0, which involves only two changes of phase, ¢, from gas (g) to liquid (£) and then back to gas (g) again. The ratio of the number of mols of water / hydrogen ions in the second cycle 406 to the number of mols of water / hydrogen ions in the first cycle 405 depends on the molarity of the hydrochloric acid, but may typically be, for example, a factor of about 10. Thus, although the first water / hydrogen-ion cycle 405 contains both the thermal decomposition 209 of the precipitate of Q hydroxide and either the RDR 104 or the HTSE 411 and combustion 203b, both of which, overall, are endothermic processes for producing hydrogen chloride from the high-temperature steam captured from the thermal decomposition 209, the vast majority of the water / hydrogen ions which circulate in the chemical cycles 400c, 400c' of Figs. 4A-D do not participate in any of these reactions because they instead follow the second water / hydrogen-ion cycle 406, which has a significantly smaller overall energy budget than the first water / hydrogen-ion cycle 405. Whereas the first and second water / hydrogen-ion cycles 405, 406 of Figs. 5A and 5B both involve water as a reagent, if conducted at the stoichiometric ratios, the two cycles 405, 406 are both closed cycles, which do not consume or produce any water. Therefore, subject to real-world losses and inefficiencies, neither cycle requires any water to be added to them as an ingredient. In addition to the water / hydrogen-ion cycles 405, 406, the chemical cycles 400c, 400c' of Figs. 4A-D also comprise a salt cycle, as a result of drying 110 the aqueous solution of sodium chloride to produce solid sodium chloride, which is recycled back for electrolysis 101b. This has the advantage of halving the amount of salt consumed. In other possible embodiments, drying 110 the aqueous solution of sodium chloride could be omitted, in which case no salt would be recycled. Instead, the electrolysis 101b could be supplied only with new salt and hydrogen chloride produced by at least one of the RDR 104 and the HTSE 411 and combustion 203b could be dissolved 105 in newly supplied water, rather than in water condensed 112 from drying 110 the aqueous solution of sodium chloride. However, recycling at least some of the salt is more desirable from an environmental point of view. Figs. 6A-B both show a chemical cycle 400d in a fourth embodiment of a method according to the invention. In Figs. 6A-B, starting materials are again contained in boxes edged with dashed lines, M, Q and x respectively represent the same as previously, and for illustrative purposes only, the starting materials are consumed in their stoichiometric ratios, as was described above in relation to the first embodiment of Figs. 2A-B. The fourth embodiment of Figs. 6A-B differs from the third embodiment of Figs. 4A-D in the following respects. Firstly, only 3 mol of the liquid sodium for reaction 303 with the oxide of the other metal, M, is produced by fusing and electrolysing 101b solid sodium chloride. The remaining 3 mol of the liquid sodium for this reaction 303 are instead produced by electrolysing 201a an aqueous solution of sodium chloride to produce an aqueous solution of sodium hydroxide, drying 201b this aqueous solution of sodium hydroxide to produce solid sodium hydroxide, and then fusing and electrolysing 201c the solid sodium hydroxide. The aqueous solution of sodium chloride which is electrolysed 201a is recycled from the aqueous solution of sodium chloride produced by reacting 207 the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride with the first portion of sodium oxide. Thus, as in the third embodiment of Figs. 4A-D, only 3 mol of sodium chloride are added to the chemical cycle 400d as one of the starting materials, and the rest of the sodium chloride used to produce the liquid sodium is recycled. Secondly, this embodiment also differs from the embodiment of Figs. 4A-D in that the hydrogen chloride used to produce 204 the hydrochloric acid is made by combusting 203b together gaseous hydrogen and chlorine produced by electrolysing 201a the aqueous solution of sodium chloride, rather than by reacting high-temperature water vapour and chlorine in an RDR and / or by combusting chlorine with hydrogen obtained from electrolysing water in liquid or vapour phase. In this embodiment of Figs. 6A-B, the hydrogen and chlorine which are combusted 203b together both come from the electrolysis 201a of the aqueous solution of sodium chloride. However, in other possible alternative embodiments, the hydrogen and chlorine which are combusted together could instead respectively come from the electrolysis 201c of the solid sodium hydroxide and from the electrolysis 101b of the solid sodium chloride, or from a combination of these different sources. Thirdly, the embodiment of Figs. 6A-B also differs from the embodiment of Figs. 4A-B in that neither the first nor the second portion of sodium oxide is hydrated before the first portion of sodium oxide reacts 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, or before the second portion of sodium oxide reacts 214a with carbon dioxide captured from the reaction 205a, 205b between the hydrochloric acid and the carbonate mineral, to produce sodium carbonate. Thus any gangue species mixed in with the second portion of sodium oxide will also remain mixed in with the sodium carbonate thus produced, which may be acceptable if, for example, the sodium carbonate is to be used as an ingredient in the manufacture of soda-lime glass. The present embodiment also differs from the embodiment of Figs. 4A-D in that it comprises 1.5 mol of water added 402 as a starting material. This additional starting material is the ultimate source of the additional oxygen and hydrogen which are produced by fusing and electrolysing 201c the solid sodium hydroxide. Tables 3A and 3B below respectively summarize the starting materials and products of the chemical cycle 400d of Figs. 6A-B. The contents of Tables 3Aand 3B may be compared and contrasted with those of Tables 2A and 2B above, in particular in relation to the entries for water, oxygen and hydrogen. As may be seen, the embodiment of Figs. 6A-B results in the concurrent electrolysis of 1.5 mol of water into 1.5 mol of hydrogen and 0.75 mol of oxygen, in addition to the 0.75 mol of oxygen produced in the embodiment of Figs. 4A-D. Raw Materials Examples of Reagents No. of mol consumed per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 per mol of M2O3 Metal ore Hematite (Fe2O3) 1 159.7 30.4 Bixbyite (Mn2O3) 157.9 35.1 Carbonate mineral ore Calcite (CaCO3) 1.5 150 55.4 Magnesite (MgCO3) 126.5 42.7 Dolomite (CaCO3-MgCO3) 126.5-150 42.7-55.4 Siderite (FeCO3) 173.7 45.7 Salt Halite (NaCI) 3 175.3 80.8 Water Water (H2O) 1.5 T1 T1 Table 3A Main Products* No. oi produ mol o mol ced per 1 M2O3 Mass / g per mol of M2O3 Volume / cm3 @ s.t.p. per mol of M2O3 Examples of end uses Elemental metal 2 Fe 111.6 15.9 Steelmaking Mn 109.8 15.3 Oxide from carbonate mineral 1.5 CaO 84 25.6 Cement making, steelmaking, glassmaking MgO 60.5 16.9 CaO-MgO 60.5-84 16.9-25.6 FeO 107.8 18.8 Ironmaking Sodium carbonate (Na2CO3) 1.5 159 62.6 Glassmaking, soap powders, papermaking Chlorine (Cl2) 1.5 106.4 33.3 x 103 Chlorinated organic compounds, etc. Oxygen (O2) 1.5 48 33.6 x 103 Steelmaking, etc. Hydrogen (H2) 1.5 3 33.4 x 103 Ammonia production, fuel cells (*i.e., ignoring any gangue species which may also be present in one or more of the raw materials) Table 3B The theoretical total energy consumption of the embodiment of Figs. 6A-B calculated from Tables 3A 5 and 3B according to Hess's Law as AHf (products) - AHf (reagents) = +1647 kJ per mol of Fe2O3 consumed if, for example, M = Fe and Q = Ca, but is less if M and / or Q take different values. Like the chemical cycles 400c, 400c' of Figs. 4A-D, the chemical cycle 400d of Figs. 6A-B also comprises several water / hydrogen-ion cycles, which are shown in Figs. 7A and 7B. Referring firstly to Fig. 7A, it may be seen that the chemical cycle 400d of Figs. 6A-B comprises three closed water / hydrogen-ion 10 cycles, which may be compared and contrasted to those shown in Figs. 5A and 5B as follows. The topmost water / hydrogen-ion cycle in Fig. 7A may be considered equivalent to the water / hydrogen-ion cycle 405 shown in Fig. 5A, in that a relatively small number of mols of water / hydrogen ions undergoes a relatively large change in temperature, AT, over the cycle, such that 300 <AT <500 ’Celsius approximately, which also involves three changes of phase, represented by ¢, from gas (g) to 15 liquid (£) to solid (s) and then back to gas (g) again. However, the topmost water / hydrogen-ion cycle in Fig. 7A consumes less energy than the water / hydrogen-ion cycle 405, firstly because the RDR 104 in cycle 405, which is an endothermic process, is replaced by the exothermic condensation 216 of the high temperature steam from the thermal decomposition 209 of the hydroxide precipitate in Fig. 7A, and secondly because AT over this cycle is less than in cycle 405. The water / hydrogen-ion cycle occupying the bottom two-thirds of Fig. 7A is equivalent to the water / hydrogen-ion cycle 406 shown in Fig. 5B, in that a relatively large number of mols of water / hydrogen ions undergoes a relatively small change in temperature, AT, over the cycle, such that △T = 0, which involves only two changes of phase, ¢, from gas (g) to liquid (£) and then back to gas (g) again. The water / hydrogen-ion cycle occupying the bottom two-thirds of Fig. 7A consumes more energy than the water / hydrogen-ion cycle 406 because of the brine electrolysis 201a it additionally comprises. However, this electrolysis 201a also produces the gaseous hydrogen which appears in the third water / hydrogen-ion cycle of Fig. 7A. This third water / hydrogen-ion cycle has no equivalent in the chemical cycle of Figs. 4A-B and is exothermic, firstly because the brine electrolysis 201a it contains has already been accounted for, and secondly because it comprises the exothermic combustion 203b of the gaseous hydrogen with chlorine to produce hydrogen chloride gas. Overall, therefore, the energy budget of the three water / hydrogen-ion cycles in Fig. 7A is substantially the same as the combined energy budget of the water / hydrogen-ion cycles 405, 406 in Figs. 5A and 5B. Referring next to Fig. 7B, it may also be seen that the chemical cycle 400d of Figs. 6A-B comprises a fourth sequence 407 of water / hydrogen ion reactions, which also has no equivalent in the chemical cycle 400c of Figs. 4A-B. Unlike the three water / hydrogen-ion cycles shown in Fig. 7A, the sequence 407 of water / hydrogen ion reactions of Fig. 7B is open, and if conducted at the stoichiometric ratio, both consumes water and produces an equal number of mols of hydrogen gas. Thus in Fig. 7B, box 402 with the legend "Add water" replenishes the hydrogen ions converted to gaseous hydrogen by fusing and electrolysing 201c solid sodium hydroxide. Subject to real-world losses and inefficiencies, the additional energy required by the sequence 407 of reactions in comparison to the combined energy budget for the water / hydrogen-ion cycles 405,406 shown in Figs. 5A and 5B is therefore equal to the amount of energy required to electrolyse the nett amount of water added 402 to the chemical cycle 400d of Figs. 6A-B to produce this hydrogen. Figs. 8A-B both show a chemical cycle 400e in a fifth embodiment of a method according to the invention, wherein starting materials are again contained in boxes edged with dashed lines and M, Q and x respectively represent the same as previously. The embodiment of Figs. 8A-B is the same as the embodiment of Figs. 6A-B except that in the present case, the first portion of sodium oxide is hydrated 317a, before sodium hydroxide derived therefrom is added 207 to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Hydrating 317a the first portion of sodium oxide in this case proceeds in a different manner from the hydration 317b of the second portion of sodium oxide described above in relation to Figs. 4A-B. Instead, in the present embodiment, the first portion of sodium oxide is passed through an atmosphere in which the thermal decomposition 209 of the precipitate of Q hydroxide occurs. Since sodium oxide is powerfully hygroscopic, it absorbs water vapour from the Q hydroxide to produce solid-phase sodium hydroxide. According to Le Chatelier's principle, this encourages the thermal decomposition 209 by reducing the partial pressure of water vapour within the atmosphere to which the Q hydroxide is exposed. Moreover, since the first portion of sodium oxide may still be hot from the redox reaction 303, heat may also be transferred from the first portion of sodium oxide to the Q hydroxide, thereby contributing to its thermal decomposition 209. At atmospheric pressure and at temperatures above about 65 ’Celsius, the sodium hydroxide thus formed is anhydrous, which itself is also strongly hygroscopic, thus continuing the thermal decomposition 209 in the same manner. The hydration 317a of the first portion of sodium oxide is also strongly exothermic, which contributes more heat to the thermal decomposition 209 of the Q hydroxide. Thus heat from the hydration 317a, as well as possibly also from the redox reaction 303, can be effectively and efficiently recycled. Thereafter, the solidphase sodium hydroxide is added 207 to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. 1.5 mol of water therefore cycles around the hydration 317a, precipitation reaction 207 and thermal decomposition 209 of the Q hydroxide. The starting materials and products of the chemical cycle 400e of Figs. 8A-B are the same as those respectively listed in Tables 3A and 3B above, so will not be repeated here. Like the chemical cycle 400d of Figs. 6A-B, a nett amount of 1.5 mol of water is added 402 to the chemical cycle 400e of Figs. 8A-B to replace that lost by the production of hydrogen and oxygen during electrolysis 201a, 201c. As in the embodiment of Figs. 6A-B, this water is introduced into the cycle during the hydrochloric acid production 204. Whereas in the chemical cycle 400e of Figs. 8A-B, gangue species like silica and alumina remain mixed in with the first portion of sodium oxide, in alternative possible embodiments, the hydration 317a of the first portion of sodium oxide may instead be carried out in the same manner as the hydration 317b of the second portion of sodium oxide described above in relation to Figs. 4A-B. If so, the insoluble gangue species mixed in with the first portion of sodium oxide precipitate out, and can be separated from the aqueous solution of sodium hydroxide which such a hydration reaction produces, for example by settlement under gravity, filtration and / or centrifugation. This aqueous solution of sodium hydroxide, without the gangue species mixed therein, can then be added 207 to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Preferably, however, the aqueous solution of sodium hydroxide is firstly dried using heat extracted from the hydration 317b to produce solid anhydrous sodium hydroxide. This ensures that the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride is not diluted when the sodium hydroxide is added to it. The water vapour driven off by drying the aqueous solution of sodium hydroxide can be condensed to extract heat from it for drying the aqueous solution of sodium hydroxide, and the condensed water can also be recycled to dissolve the first portion of sodium oxide. Similarly, in alternative possible embodiments to that described above in relation to Figs. 4A-B, the hydration 317b of the second portion of sodium oxide may instead be carried out in a manner similar to the hydration 317a of the first portion of sodium oxide described above in relation to Figs. 8A-B. If so, the second portion of sodium oxide may be passed through an atmosphere in which the thermal decomposition 209 of the precipitate of Q. hydroxide occurs, before the solid-phase sodium hydroxide which results is used in the carbonation reaction 214b. In such a case, hydrating the second portion of sodium oxide can then be used to give the resulting sodium hydroxide a temperature of from about 310 to about 400 ’Celsius, necessary for this anhydrous sodium hydroxide to produce anhydrous sodium carbonate as the only solid-phase product of the carbonation reaction 214b. Thus heat from both the redox reaction 303 and the hydration 317b can be effectively and efficiently recycled. Moreover, whereas in the embodiment described above in relation to Figs. 4A-B, at least some of the second portion of sodium oxide is hydrated to produce sodium hydroxide, and in the embodiment described above in relation to Figs. 8A-B, at least some of the first portion of sodium oxide is hydrated to produce sodium hydroxide, in other possible alternative embodiments, both the first and second portions of sodium oxide may be hydrated in either one of the ways described above to produce solid-or aqueous-phase sodium hydroxide, with or without gangue species mixed therein, in each case. At present, annual global production of crude steel is about 2 Gt ( / .e., 2 gigatonnes). Annual global production of cement is about 4.4 Gt. Annual global production of lime for purposes other than cement production is about 400 Mt. Therefore, the total annual global production of lime for all purposes, including during cement manufacture, is of the same order of magnitude as the annual global production of crude steel. In comparison, global consumption of sodium carbonate is only about 62 Mt annually, projected to rise to about 70 Mt by 2029, and annual global production of chlorine by the chlor-alkali process is about 58 Mt, with about 62 Mt of sodium hydroxide produced as a co-product. Therefore, the annual global demand for sodium carbonate and chlorine, although of the same order of magnitude as each other, is clearly not of the same order of magnitude as that for crude steel and cement. In each of the third, fourth and fifth embodiments of the method of the invention respectively described above in relation to Figs. 4A-D, 6A-B and 8A-B, for illustrative and explanatory purposes only, the molar amounts of sodium carbonate and chlorine produced are both equal to the molar amount of oxide produced from the carbonate mineral. However, these 1:1 molar ratios would lead to a considerable oversupply of both sodium carbonate and chlorine by the method of the invention, given the current levels of demand for these two chemicals globally, relative to the much greater demand for crude steel and cement described above. Therefore, sixth and seventh embodiments of the method will now be described, whereby the molar amounts of sodium carbonate and chlorine produced can both be tailored relative to the molar amounts of elemental metal and of the oxide from the carbonate mineral which are produced, in order to match the much lower levels of global demand for each of these two chemicals relative to the greater demands for crude steel and cement. Figs. 9A-B, therefore, both show a chemical cycle 400f in a sixth embodiment of a method according to the invention, wherein starting materials are again contained in boxes edged with dashed lines, M, Q and x respectively represent the same as before, and y represents a variable molar quantity independent of x. In this embodiment, in comparison to the embodiments described above in relation to Figs. 4A-D, 6A-B and 8A-B, the total molar amount of hydrochloric acid produced is increased by combusting together 203b increased amounts of the chlorine and hydrogen produced by electrolysis 101b, 201a, 201c. In addition, at least some of the second portion of sodium oxide is used to neutralize 403 some of the hydrochloric acid produced, instead of reacting with carbon dioxide captured from dissolving 205a, 205b the carbonate mineral in some other of the hydrochloric acid. The result of this neutralization reaction 403 is an aqueous solution of sodium chloride ( / .e., brine), which is then dried 110 to yield solid sodium chloride and water vapour. The water vapour is captured and added to that which is condensed 216 to produce the liquid water in which the hydrogen chloride gas is dissolved 204 to produce the hydrochloric acid. Thus no extra water needs to be added to this chemical cycle 400f to compensate for the increased amount of hydrochloric acid produced. The solid sodium chloride is also recycled back to be electrolysed 101b once again. Thus the nett amount of sodium chloride consumed is reduced, in comparison to the amount consumed in the chemical cycles 400c, 400c', 400d, 400e of Figs. 4A-D, 6A-B and 8A-B. This reduction in sodium chloride consumption is proportional to a reduction in the amount of sodium carbonate produced by reacting 214a some of the second portion of sodium oxide with the carbon dioxide captured from dissolving 205a, 205b the carbonate mineral in the hydrochloric acid. Although this means that the carbon dioxide liberated from the carbonate mineral when it dissolves in the hydrochloric acid is not mineralized as sodium carbonate, the carbon dioxide is still captured, rather than being released into the atmosphere, and may therefore be sequestered. Tables 4A and 4B below respectively summarize the starting materials and products of the chemical cycle 400f of Figs. 9A-B. The theoretical total energy consumption of the embodiment of Figs. 9A-B calculated from Tables 4A and 4B according to Hess's Law as AHf (products) - AHf (reagents) = +1092 kJ per mol of FejOa consumed if, for example, M = Feand Q = Ca, but is less if M and / or Qtake different values. Raw Materials Examples of Reagents No. of mol consumed per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 per mol of M2O3 Metal ore Hematite (Fe2O3) 1 159.7 30.4 Bixbyite (Mn2O3) 157.9 35.1 Carbonate mineral ore Calcite (CaCO3) 1.5 150 55.4 Magnesite (MgCO3) 126.5 42.7 Dolomite (CaCO3-MgCO3) 126.5-150 42.7-55.4 Siderite (FeCO3) 173.7 45.7 Table 4A Main Products* No. oi produ mol o mol ced per ' M2O3 Mass / g per mol of M2O3 Volume / cm3 @ s.t.p. per mol of M2O3 Examples of end uses Elemental metal 2 Fe 111.6 15.9 Steelmaking Mn 109.8 15.3 Oxide from carbonate mineral 1.5 CaO 84 25.6 Cement making, steelmaking, glassmaking MgO 60.5 16.9 CaO-MgO 60.5-84 16.9-25.6 FeO 107.8 18.8 Ironmaking Oxygen (O2) 1.5 48 33.6 x 103 Steelmaking, etc. Carbon dioxide (CO2) 1.5 44 33.4 x 103 Captured for sequestration ignoring any gangue species which may also be present in one or more of the raw materials) 5 Table 4B Whereas in the embodiment shown in Figs. 9A-B, for illustrative and explanatory purposes only, all of the gaseous chlorine and hydrogen produced by electrolysis 101b, 201a, 201c is combusted together 203b to produce an increased amount of hydrochloric acid, and all of the second portion of sodium oxide is used to neutralize 403 the increased amount of hydrochloric acid thus produced, so that no 10 sodium carbonate is produced at all, and all of the sodium chloride produced from this neutralization reaction is recycled 110 for electrolysis 101b, this only represents an extreme example. In this case, even though sodium chloride circulates as a reagent in the chemical cycle 400f of Figs. 9A-B, no nett amount of salt needs to be added to or is consumed by the cycle. In practice, however, a position between this extreme case and an opposite extreme, which is represented by the embodiments of 15 Figs. 4A-B, 6A-B and 8A-B, can also be adopted. In such an intermediate position, only some of the chlorine and hydrogen produced by electrolysis 101b, 201a, 201c is combusted together 203b to produce an increased amount of hydrochloric acid and / or only some of the second portion of sodium oxide is used to neutralize 403 some of the increased amount of hydrochloric acid thus produced, whereas some other of the second portion of sodium oxide is instead used to produce 214a, 214b sodium carbonate from carbon dioxide captured from dissolving 205a, 205b the carbonate mineral in some other of the hydrochloric acid. This has the result that a molar amount of sodium chloride which is twice the molar amount of sodium carbonate produced is added to the cycle to replace the sodium ions which leave the cycle as sodium carbonate and the chloride ions which remain either as uncombusted chlorine gas or in an excess amount of hydrochloric acid not consumed either by dissolving the carbonate mineral or by neutralizing some of the second portion of sodium oxide. For example, 97% of the second portion of sodium oxide could be used to neutralize some of the total amount of hydrochloric acid produced and 3% of it could instead be used to produce sodium carbonate from carbon dioxide captured from dissolving the carbonate mineral in some other of the hydrochloric acid. In such a case, 6% of the molar amount of sodium chloride would need to be added to the cycle relative to the molar amount of carbonate mineral consumed, since 2 mols of sodium chloride are consumed per mol of sodium carbonate produced. In this example, the cycle would only produce 3% of the molar amount of sodium carbonate and 3% of the molar amount of chlorine gas, relative to the molar amount of oxide produced from the carbonate mineral. Thus, the molar amounts of sodium carbonate and chlorine produced can both be tailored relative to the molar amounts of elemental metal and this oxide produced, in order to match the lower levels of global demand for sodium carbonate and chlorine relative to the much greater demand for crude steel and cement, and the nett molar amount of common salt consumed by the cycle is also reduced proportionally to match the global level of supply of salt, which currently stands at about 290 Mt annually. Whereas in the example just given, a percentage of 3% has been used as an example, the relative proportions of the second portion of sodium oxide which is used to produce 214a, 214b sodium carbonate and which is instead used to regenerate salt for electrolysis by neutralizing 403 some of the hydrochloric acid produced can be any percentages between the extremes represented by the embodiments of Figs. 4A-D, 6A-B and 8A-B (which all have 100% sodium carbonate production and 0% salt regeneration) and that represented in Figs. 9A-B (which has 0% sodium carbonate production and 100% salt regeneration). For example, the percentages may conveniently lie within a range of from about 1.5% to about 6%, inclusive, for sodium carbonate production and correspondingly from about 98.5% to about 94%, inclusive, for salt regeneration. In the embodiment of Figs. 9A-B, for improved clarity of these two diagrams and for ease of explanation, neither of the first and second portions of sodium oxide is hydrated to produce sodium hydroxide. However, in alternative possible embodiments, either or both of the first and second portions of sodium oxide may be hydrated, as respectively described above in relation to Figs 4A-B and 8A-B, for example in order to remove insoluble gangue species originating from the ore of the other metal, M, which have become mixed in with the sodium oxide during the reaction 303 of the liquid sodium with the oxide of the other metal, M. In such alternative possible embodiments, the chemical cycle will therefore comprise at least one water cycle as a result of the hydration 317a, 317b of the first and / or second portions of sodium oxide, similar to those described above in relation to Figs 4A-B and 8A-B. In the embodiment of Figs. 9A-B, all of the brine produced by the neutralization reaction 403 between at least some of the second portion of sodium oxide and some of the hydrochloric acid is dried 110 to produce solid sodium chloride, which is then electrolysed 101b to produce liquid sodium and chlorine gas. However, in alternative possible embodiments, at least some of this brine may instead be electrolysed 201a to produce gaseous chlorine and an aqueous solution of sodium hydroxide, the latter of which is then dried 201b, fused and electrolysed 201c to produce the liquid sodium. Figs. 10A-B therefore both show a chemical cycle 400g in a seventh embodiment of a method according to the invention, wherein starting materials are again contained in boxes edged with dashed lines and M, Q, x and y respectively represent the same as before. In the embodiment of Figs. 10A-B, in contrast to the embodiment of Figs. 9A-B, all of the brine produced by the neutralization reaction 403 between the second portion of sodium oxide and some of the hydrochloric acid is electrolysed 201a to produce chlorine gas and an aqueous solution of sodium hydroxide, the latter of which is then dried 201b, fused and electrolysed 201c to produce all the liquid sodium. The result is that none of the liquid sodium is produced by electrolysing solid sodium chloride, and water is also added 402 to the chemical cycle 400g of Figs. 10A-B to replace the gaseous hydrogen and oxygen which are produced by the simultaneous electrolysis of water. Tables 5A and 5B below respectively summarize the starting materials and products of the chemical cycle 400g of Figs. 10A-B. In the same way as the contents of Tables 3A and 3B were compared and contrasted above with those of Tables 2A and 2B, so the contents of Tables 5A and 5B may be compared and contrasted with those of Tables 4A and 4B, in particular in relation to the entries for water, oxygen and hydrogen. As may be seen, the embodiment of Figs. 10A-B results in the concurrent electrolysis of 3 mol of water into 3 mol of hydrogen and 1.5 mol of oxygen, in addition to the 1.5 mol of oxygen produced in the embodiment of Figs. 9A-B. The theoretical total energy consumption of the embodiment of Figs. 10A-B calculated from Tables 5A and 5B according to Hess's Law as AHf (products) - AHf (reagents) = +1948 kJ per mol of FejOa consumed if, for example, M = Fe and Q = Ca, but is less if M and / or Q take different values. Raw Materials Examples of Reagents No. of mol consumed per mol of M2O3 Mass / g per mol of M2O3 Volume / cm3 per mol of M2O3 Metal ore Hematite (Fe2O3) 1 159.7 30.4 Bixbyite (Mn2O3) 157.9 35.1 Carbonate mineral ore Calcite (CaCO3) 1.5 150 55.4 Magnesite (MgCO3) 126.5 42.7 Dolomite (CaCO3-MgCO3) 126.5-150 42.7-55.4 Siderite (FeCO3) 173.7 45.7 Water Water (H2O) 3 54 54 Table 5A Main Products* No. oi produ mol o mol ced per ' M2O3 Mass / g per mol of M2O3 Volume / cm3 @ s.t.p. per mol of M2O3 Examples of end uses Elemental metal 2 Fe 111.6 15.9 Steelmaking Mn 109.8 15.3 Oxide from carbonate mineral 1.5 CaO 84 25.6 Cement making, steelmaking, glassmaking MgO 60.5 16.9 CaO-MgO 60.5-84 16.9-25.6 FeO 107.8 18.8 Ironmaking Oxygen (O2) 3 96 67.2 x 103 Steelmaking, etc. Hydrogen (H2) 3 6 66.8 x 103 Ammonia production, fuel cells Carbon dioxide (CO2) 1.5 44 33.4 x 103 Captured for sequestration ignoring any gangue species which may also be present in one or more of the raw materials) 5 Table 5B Whereas in the embodiment of Figs. 10A-B, for illustrative and explanatory purposes only, all of the brine produced by using the second portion of sodium oxide to neutralize 403 some of the hydrochloric acid is electrolysed 201a, this is only an extreme example. In practice, an intermediate position between this extreme case and the opposite extreme represented by the embodiment of 10 Figs. 9A-B, wherein none of this brine is electrolysed, can be adopted instead. In such an intermediate position, only some of the brine thus produced is electrolysed 201a and some other of this brine is instead dried 110 to produce solid sodium chloride, which is then electrolysed 101b to produce liquid sodium and chlorine gas, as in the other embodiments described above in relation to Figs 6A to 9B. In such a case, the molar amount of hydrogen produced by electrolysis is reduced in comparison to the 15 embodiment of Figs. 10A-B and the nett molar amount of water consumed by the chemical cycle is also reduced in proportion to the reduction in how much hydrogen is produced by electrolysis. This has the advantage that it conveniently allows the amount of hydrogen produced to be tailored to meet the demand for hydrogen, for example from a hydrogen economy. As in the embodiment of Figs. 9A-B, in the embodiment of Figs. 10A-B, for improved clarity of these last two diagrams and for ease of explanation, neither of the first and second portions of sodium oxide is hydrated to produce sodium hydroxide. However, in alternative possible embodiments, either or both of the first and second portions of sodium oxide may be hydrated as described previously. In such cases, the chemical cycle 400g of Figs. 10A-B will comprise at least one additional water cycle as a result of the hydration 317 a, 317b of the first and / or second portions of sodium oxide, similar to those described above in relation to Figs. 4A-B and 8A-B. Thus, by varying how much of the second portion of sodium oxide is used to neutralize some of the hydrochloric acid and produce brine, instead of being used to produce sodium carbonate, as described above in relation to Figs. 9A-B, and by varying how much of this brine is then electrolysed, as described above in relation to Figs. 10A-B, the relative amounts of sodium carbonate and gaseous chlorine and / or hydrochloric acid produced on the one hand, and of hydrogen on the other, may both be adjusted independently of each other, as well as independently of the levels of production of elemental metal and of the oxide derived from the carbonate mineral, in order to meet the different levels of demand for these different products. Furthermore, since the total amount of oxygen produced as a co-product depends in part on the nett amount of hydrogen produced, whereas at least some of this oxygen may, for example, be used in steelmaking, any excess oxygen may, of course, just be safely vented to atmosphere. Table 6 below summarizes the theoretical total energy consumption in kJ per mol of FejOa consumed in each of the embodiments of Figs. 4A-D, Figs. 6A-B and 8A-B, Figs. 9A-B and Figs. 10A-B, under the same assumptions as stated above, including that the reagents and products are all in their standard states at s.t.p.: No. of mol of water -> consumed per mol of Fe2O3 No. of mol of salt 4 / consumed per mol of Fe2O3 0 1.5 3 0 +1092 (Figs. 9A-B) +1520 +1948 (Figs. 10A-B) 1.5 +1156 +1584 +2012 3 +1219 (Figs. 4A-D) +1647 (Figs. 6A-B and 8A-B) +2075 Table 6 In Table 6, by way of example, M = Fe and Q = Ca in each case, and numerical values which appear in italics are indicative only, as they are interpolated from the theoretical total energy consumptions of the illustrated embodiments. Thus prevailing market prices for the reagents and products in different possible embodiments, as well as the prevailing cost of energy, may be used to determine the most economic amounts of salt and water to consume per mol of M2O3. A particular embodiment may therefore be selected as being optimized to meet prevailing market conditions by varying these amounts independently of each other. In summary, therefore, the embodiments shown in Figs. 4A to 10B and described above in relation thereto demonstrate that the method of the invention comprises at least the following available options. The liquid sodium may be produced by fusing and electrolysing solid sodium chloride, by fusing and electrolysing solid sodium hydroxide, or by a combination of these two techniques in any proportion. The embodiment of Figs. 4A-D exemplifies a method in which all the liquid sodium is produced by fusing and electrolysing solid sodium chloride, as in a Downs cell, for example. The embodiment of Figs. 10A-B exemplifies a method in which all the liquid sodium is produced by fusing and electrolysing solid sodium hydroxide, as in a Castner cell, for example. The embodiments of Figs. 6A-B, 8A-B, and 9A-B all exemplify methods in which the liquid sodium is produced by a combination of 50% fusing and electrolysing solid sodium chloride with 50% fusing and electrolysing solid sodium hydroxide. However, these percentages are for illustrative purposes only and can be varied. At least some of the chlorine gas which is also produced by electrolysis may be used to make hydrogen chloride gas by a reverse Deacon reaction between high-temperature steam and high-temperature chlorine from electrolysing solid sodium chloride, as in the embodiment of Figs. 4A-B, or by combusting the chlorine with hydrogen, as in the other embodiments, or by a combination of both techniques in any proportion. At least some of the hydrochloric acid produced by dissolving this hydrogen chloride in water may be used to neutralize at least some of the second portion of sodium oxide to produce an aqueous solution of sodium chloride ( / .e., brine), at least some of which may be recycled for electrolysis. This brine may be firstly dried to produce solid sodium chloride which is then electrolysed, as in the embodiment of Figs. 9A-B, or it may be firstly electrolysed to produce an aqueous solution of sodium hydroxide, which is then dried to produce solid sodium hydroxide, as in the embodiment of Figs. 10A-B, or a combination of both techniques in any proportion. Either at least some of the second portion of sodium oxide may be hydrated to produce sodium hydroxide, as in the embodiment of Figs. 4A-B, or at least some of the first portion of sodium oxide may be hydrated to produce sodium hydroxide, as in the embodiment of Figs. 8A-B, or at least some of both portions may be hydrated, or neither, as in the embodiments of Figs. 6A-B, 9A-B, and 10A-B. At least some of the carbon dioxide produced by dissolving the carbonate mineral in the hydrochloric acid may be mineralized as sodium carbonate, as in the embodiments of Figs. 4A-B, 6A-B, and 8A-B, and any unmineralized carbon dioxide, which is still captured, may be sequestered instead, as in the embodiments of Figs. 9A-B, and 10A-B. Moreover, in other possible embodiments not illustrated in the drawings, when at least some of the sodium oxide is hydrated to produce sodium hydroxide and brine is also electrolysed, at least some of the sodium hydroxide produced by hydrating the sodium oxide may be electrolysed to produce more liquid sodium in a closed loop, and at least some of the aqueous solution of sodium hydroxide produced by electrolysing the brine may either be added to the aqueous solution comprising at least one of calcium oxide, magnesium oxide and iron oxide in a closed loop as well or used in a reaction with at least some of the captured carbon dioxide gas to produce at least sodium carbonate. In other words, when the sodium hydroxide is produced in more than one way and is also consumed in more than one way, how the sodium hydroxide is produced need not determine how it is consumed, or vice versa. Whereas the method of the invention has an iron and / or manganese ore and an ore comprising a carbonate mineral as its only essential ingredients, the method of the invention may also consume sodium chloride (i.e., common salt) and / or water as optional extra ingredient(s). The embodiment of Figs. 9A-B exemplifies a method in which no water or salt are consumed as well, the embodiment of Figs. 4A-B exemplifies a method in which only salt is consumed as an extra ingredient, the embodiment of Figs. 10A-B exemplifies a method in which only water is consumed as an extra ingredient, and the embodiments of Figs. 6A-B and 8A-B, both exemplify methods in which both salt and water are consumed as additional ingredients. In the embodiments described above, subject to real-world losses and inefficiencies, if the essential ingredients are consumed in their stoichiometric ratios, the total number of mols of sodium oxide and / or sodium carbonate produced and the number of mols of gaseous chlorine produced are both equal to half the number of mols of sodium chloride consumed. Also subject to real-world losses and inefficiencies, if the essential ingredients are consumed in their stoichiometric ratios, the number of mols of gaseous hydrogen produced is equal to the number of mols of water consumed. However, the number of mols of sodium chloride consumed and the number of mols of water consumed may be varied independently of each other. Thus the molar amounts of sodium carbonate and of chlorine produced can both be tailored, relative to the molar amounts of elemental metal and of the oxide or hydroxide derived from the carbonate mineral which are produced, to match the levels of demand for each of these two chemicals relative to the demands for the elemental metals and for these oxides or hydroxides, and the molar amount of hydrogen produced can also be varied independently of that, to match the demand for hydrogen from a hydrogen economy. Figs. 11A-B both show part of a chemical cycle 400h in an eighth embodiment of a method according to the invention. In contrast to the embodiments described previously, the chemical cycle 400h comprises producing some of the liquid sodium by electrolysing 408 a molten electrolyte comprising both sodium chloride (NaCI) and aluminium chloride (AICU) using a consumable aluminium anode and a solid electrolyte to separate this molten electrolyte from the liquid sodium thus produced. Although the molten electrolyte (NaCI + AICI3) is represented for ease of illustration in Fig. 12B by NaAICI4, the sodium chloride and aluminium chloride do not have to be in a 1:1 molar ratio and in fact form a eutectic mixture at a molar ratio of about 61.4 % AICI3 / (NaCI + AICI3) which melts at less than about 120 ’Celsius under one atmosphere of pressure. It should also be noted that production of the consumable aluminium anode does not form part of the present invention. However, in order to show where the aluminium comes from, Figs. 11A-B also include the electrolysis 409 of alumina using an inert anode, to represent the consumption of 0.5 mol of alumina this entails and the co-production of 0.75 mol of gaseous oxygen which results. In the illustrated embodiment, the number of mols of aqueous sodium chloride produced at El and E2 is sufficient to supply all the hydrochloric acid consumed at Fl and F2, as well as the corresponding number of mols of liquid sodium consumed at G. The hydrochloric acid consumed at Fl and F2 and the liquid sodium consumed at G can be recycled from El and E2 by electrolysing the sodium chloride produced at El and E2 in one or more of the ways already described above. In other words, the aqueous solution of sodium chloride produced at El and E2 may be firstly dried 110 to produce solid sodium chloride which is then fused and electrolysed 101b, as in the embodiment of Figs. 9A-B, or it may be firstly electrolysed 201a to produce an aqueous solution of sodium hydroxide, which is then dried 201b to produce solid sodium hydroxide, which is fused and electrolysed 201c, as in the embodiment of Figs. 10A-B, or a combination of both techniques in any proportion. Thus the only salt which is added to the cycle 400h is the solid sodium chloride consumed by the electrolysis 408 and the amount of salt added determines the amount of aluminium chloride produced as a coproduct. The amount of water added to the cycle 400h to produce the hydrochloric acid consumed at Fl and F2 depends on how the sodium chloride produced at El and E2 is electrolysed and varies in proportion to the molar amount of hydrogen which is also produced by electrolysis, as described previously. In other words, the part of the chemical cycle 400h shown in Figs. 11A-B may be inserted into any of the other embodiments described previously, whereby the method of the invention maybe integrated with the co-production of gaseous aluminium chloride. The amount of gaseous aluminium chloride thus produced may be varied in proportion to the amount of salt consumed by the electrolysis 408. However, in the example illustrated in Figs. 11A-B, it may be noted that the oxide of the other metal, M, and the carbonate mineral are now consumed in a molar ratio of 1:1, whereby production of the other metal, M, in elemental form increases by 50% relative to the other embodiments described above. Since consumption of the oxide of the other metal is thereby also increased by 50%, the amount of salt consumed by the electrolysis 408 is actually only 2 mol per mol of M2O3, as may be seen by multiplying all the molar quantities in Figs. 11A-B by %. Similarly, for a fair comparison with the theoretical energy consumptions given in Table 6, the energy consumed by the electrolysis 408 should be reduced proportionately. Thus, for example, if the part of the chemical cycle 400h shown in Figs. 11A-B is inserted in the embodiment of Figs. 9A-B, wherein no salt or water are consumed other than by the electrolysis 408, then the theoretical total energy consumption when M = Fe and Q = Ca under the same assumptions as before, is only +1215 kJ per mol of FejOa consumed, yielding 2 mol of Fe, 1 mol of CaO, 1 mol of Na2CO3, % mol of ALCIg and 1.5 mol of O2 from 1 mol of Fe2O3, 1 mol of CaCOs, 2 mol of NaCI and % mol of AI2O3 as raw materials. Figs. 12A-B both show a chemical cycle 400j in a ninth embodiment of a method according to the invention. The ninth embodiment of Figs. 12A-B is the same as the third embodiment of Figs. 4A-B except that in the present case, the redox reaction 303 between the liquid sodium and the oxide of the other metal is conducted at a temperature of at least about 320 ’Celsius to induce a reaction 601 between some of the sodium oxide thus produced and silica derived from a siliceous mineral in the ore of the other metal, to produce sodium silicate(s), which dissolve in the liquid sodium. The redox reaction 303 is also conducted at a temperature below that at which the liquid sodium can react with the oxide of the other metal to produce a ternary oxide thereof, as described previously. At least some of the liquid sodium with the sodium silicate(s) dissolved therein is then separated 305 from the undissolved metal and other insoluble products and oxidized 602 by reacting it with at least one of oxygen and water to produce a mixture comprising sodium silicate(s) and at least one of sodium oxide and sodium hydroxide. In the embodiment illustrated in Figs. 12A-B, by way of example, the liquid sodium with the sodium silicate(s) dissolved therein reacts 602 with oxygen to produce a mixture comprising sodium orthosilicate ( / .e., Na4SiO4) and sodium oxide. The oxygen for the reaction 602 may come, for example, from atmospheric air and / or be a co-product of the RDR 104. In the embodiment of Figs. 12A-B, the value of e depends on the amount of siliceous mineral present in the iron and / or manganese ore relative to the amount of the target metal in the ore. Specifically, in the example illustrated in Figs. 12A-B, e is defined by the following equation: e = 6R / (3-2R) where R = no. of mol (SiO2) / no. of mol (M2O3) [Eqn. 8a] [Eqn. 8b] and wherein M = Fe and / or Mn. Typically, therefore, if R <0.25, e may have a value of from about 0.1 to about 0.6, for example. However, the saturation solubility of sodium silicate(s) in liquid sodium, which determines how much of the sodium silicate(s) produced can dissolve in the liquid sodium, also places an effective upper bound on the value of e. This saturation solubility in turn depends on both the temperature of the liquid sodium and the amount of liquid sodium which is present in excess of the stoichiometric amount thereof required for reaction with the iron and / or manganese oxide(s) from the ore. For example, if the weight percentage of SiOj present in the ore which is introduced to the liquid sodium is about 4.5% of the total weight of the ore, this may correspond to a value of e = 0.29, but all of the sodium silicate(s) thus produced will only dissolve in the liquid sodium if the saturation solubility of the sodium silicate(s) in the liquid sodium allows e >0.29. If not and the percentage of SiOj present in the ore is more than this limit allows, any excess silica will remain undissolved in the liquid sodium. Moreover, if substantially all of the sodium silicate(s) are produced as sodium orthosilicate as in the illustrated embodiment, then the stoichiometry of the reactions involved implies that production of this sodium orthosilicate consumes 2e / (6 + 2e) of the total amount of liquid sodium consumed. Thus in the example given above where e = 0.29, this corresponds to about 8.9% of the total amount of liquid sodium consumed by both the redox reaction 303 of the liquid sodium with the oxide of the target metal from the ore and the reaction 601 of the sodium oxide thus produced with silica derived from a siliceous mineral also present in the ore as gangue and / or as a siliceous mineral of the target metal. As mentioned above, the ninth embodiment of Figs. 12A-B is the same as the third embodiment of Figs. 4A-B except as just described. However, in other possible embodiments, other aspects of the embodiment of Figs. 4A-B may also be varied in any manner as was already described above in relation to Figs. 6A to 11B, whilst also conducting the redox reaction 303 between the liquid sodium and the oxide of the other metal at a temperature of at least about 320 ’Celsius but below a temperature at which the liquid sodium can react with the oxide of the other metal to produce a ternary oxide thereof. In other words, any of the fourth to eighth embodiments of Figs. 6A to 11B may also be modified in a manner similar to how the third embodiment of Figs. 4A-B was modified to obtain the ninth embodiment of Figs. 12A-B. Thus any of these other embodiments may be similarly modified to induce a reaction 601 between some of the sodium oxide produced by the redox reaction 303 and silica derived from a siliceous mineral in the ore, and may then further comprise phase separation 305 from the undissolved metal and other insoluble products and oxidation 602 of at least some of the liquid sodium with the resulting sodium silicate dissolved therein to obtain a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide. Fig. 13 schematically shows a first embodiment of an apparatus 4a according to the invention. The apparatus 4a comprises an electrolytic subassembly 10, a redox reaction subassembly 13, a separation subassembly 18, a hydrochloric acid-producing subassembly 12, a carbonate dissolution subassembly 14, a solid-aqueous phase separator 16 and a caustic transfer pathway 410. The electrolytic subassembly 10 is for producing at least liquid sodium and chlorine gas by electrolysis, and comprises an inlet 11 for sodium chloride in at least one of solid, molten and aqueous phase, a first outlet for liquid sodium 17, and a second outlet 15 for chlorine gas. The redox reaction subassembly 13 comprises a first gas-tight reaction vessel 130 for containing therein a redox reaction between liquid sodium and an oxide of another metal, M, from an ore of the other metal, wherein the other metal, M, comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide. The redox reaction subassembly 13 further comprises a first inlet 131 for the oxide of the other metal, M, a second inlet 132 for receiving liquid sodium from the first outlet 17 of the electrolytic subassembly 10, and an outlet 134 for liquid sodium and entrained therein, a solid phase comprising the other metal, M, in elemental form and sodium oxide. The oxide of the other metal, M, may be introduced into the gas-tight reaction vessel 130 as a finely comminuted and dried and dehydroxylated ore. The first inlet 131 for the oxide of the other metal is therefore provided with an airlock 131a to prevent atmospheric air from entering the gas-tight reaction vessel 130 when the ore is introduced, which allows the redox reaction to be conducted in an inert atmosphere. If the other metal, M, comprises iron, the redox reaction may be conducted at a temperature of less than about 450 ’Celsius with an amount of liquid sodium in excess of the stoichiometric amount thereof required for the reaction. This prevents the formation of ternary oxides, such as Na4FeO3, and allows unreacted liquid sodium in excess of the stoichiometric amount thereof required for the reaction to be used as a transport medium for the insoluble reaction products. The separation subassembly 18 is for separating the liquid sodium, the other metal, M, in elemental form and the sodium oxide from each other. It comprises an inlet 191 for receiving the liquid sodium with the solid phase entrained therein from the outlet of the redox reaction subassembly 13, a first outlet 194 for liquid sodium, a second outlet 186 for the other metal, M, in elemental form and a third outlet 187 for sodium oxide. In the illustrated embodiment, the separation subassembly 18 comprises a solid-liquid sodium phase separator 190 connected in series with a solid-species separator 180. The solid-liquid sodium phase separator 190 is for separating the liquid sodium with the solid phase entrained therein into liquid sodium and the solid phase. This separator 190 may be as described in the 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. In this embodiment, the first outlet 194 of the separator 190 is connected back to the second inlet 132 of the redox reaction subassembly 13 so that excess liquid sodium can be circulated through the gastight reaction vessel 130 in a closed loop. Liquid sodium consumed by the redox reaction is replenished from the electrolytic subassembly 10. The solid-species separator 180 is for separating the other metal, M, in elemental form from other components of the solid phase, including sodium oxide, for example by using magnetic and / or density-based separation techniques. Thus if the other metal, M, comprises iron, this type of separation subassembly 18 is suitable for separating the iron, which is ferromagnetic and more dense, from the sodium oxide and other gangue species, such as silica and alumina, which are diamagnetic and less dense. The hydrochloric acid-producing subassembly 12 is for producing hydrochloric acid from chlorine gas and liquid water, and comprises a first inlet 121 for receiving chlorine gas from the second outlet 15 of the electrolytic subassembly 10, a second inlet 122 for liquid water and an outlet 124 for hydrochloric acid. Further details concerning different possible configurations of the hydrochloric acid-producing subassembly 12 will be described below in relation to Figs. 14 and 15. The carbonate dissolution subassembly 14 comprises a second gas-tight reaction vessel 140 for dissolving therein a carbonate mineral of at least one of calcium, magnesium and iron in hydrochloric acid. The carbonate dissolution subassembly 14 further comprises a first inlet 141 for another ore comprising the carbonate mineral, a second inlet 142 for receiving hydrochloric acid from the outlet 124 of the hydrochloric acid-producing subassembly 12, a third inlet 143 for receiving a first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this first portion of sodium oxide, from the third outlet 187 of the separation subassembly 18, and an outlet 144 for an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. Like the first inlet 131 of the first gas-tight reaction vessel 130, the first inlet 141 of the second gas-tight reaction vessel 140 is also provided with an airlock 141a. However, in this case, the airlock 141a is to prevent gases, in particular carbon dioxide, from escaping from within the second gas-tight reaction vessel 140 into the surrounding environment when the ore comprising the carbonate mineral is introduced into the second gas-tight reaction vessel 140. The second and third inlets 142, 143 and the outlet 144 of the second gas-tight reaction vessel 140 are also provided with respective valves VI, V2 and V3 to regulate entry of hydrochloric acid and entry of the first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this first portion of sodium oxide, into the second gas-tight reaction vessel 140, and the exit of reaction products therefrom, respectively. Thus hydrochloric acid may be introduced into the second gas-tight reaction vessel 140 by opening valve VI to dissolve the carbonate mineral therein and produce an aqueous solution of at least one of calcium, magnesium and ferrous chloride, after which valve VI may be closed and valve V2 may be opened instead to introduce the first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this first portion of sodium oxide, into the second gas-tight reaction vessel 140 and produce an aqueous solution of sodium chloride and a precipitate, respectively comprising at least one of calcium, magnesium and ferrous hydroxide. Valve V2 may then be closed and valve V3 may be opened instead to transfer this solution and the precipitate to the solid-aqueous phase separator 16. The solid-aqueous phase separator 16 is for separating at least some of this precipitate from the aqueous solution of sodium chloride. It comprises an inlet 161 for receiving the aqueous solution of sodium chloride and the precipitate from the outlet 144 of the carbonate dissolution subassembly 14, a first outlet 164 for the precipitate, and a second outlet 165 for the aqueous solution of sodium chloride. The caustic transfer pathway 410 is for transferring a second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this second portion of sodium oxide, from the third outlet 187 of the separation subassembly 18 to at least one of (i) a neutralization vessel 420 for reaction with hydrochloric acid from the outlet 124 of the hydrochloric acid-producing subassembly 12, and (ii) a carbonation vessel 160 for reaction with carbon dioxide gas produced in the carbonate dissolution subassembly 14. For ease of illustration, both the neutralization vessel 420 and the carbonation vessel 160 are represented by in Fig. 13a single element. However, either or both of these vessels 160, 420 may be present in alternative possible embodiments. If the neutralization vessel 420 is present, it comprises a first inlet 422 for receiving hydrochloric acid from the outlet 124 of the hydrochloric acid-producing subassembly 12, a second inlet 423 for receiving the second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this second portion of sodium oxide, from the third outlet 187 of the separation subassembly 18, and an outlet 426 for an aqueous solution of sodium chloride. If the carbonation vessel 160 is present, it comprises a first inlet 162 for receiving carbon dioxide from a second outlet 146 of the carbonate dissolution subassembly 14. The latter is provided with a fourth valve V4, which allows the transfer of carbon dioxide gas from the second gas-tight reaction vessel 140 to the carbonation vessel 160 to be regulated. Thus, for example, valve V4 may be opened when valve VI is opened and then closed again before valve V2 is opened, as described above. The carbonation vessel 160 further comprises a second inlet 163 for receiving the second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of this second portion of sodium oxide, from the third outlet 187 of the separation subassembly 18, and an outlet 166 for products of the carbonation reaction, such as sodium carbonate. Fig. 14 schematically shows an embodiment of a separation subassembly 18, which is an alternative to that shown in Fig. 13 and which may be incorporated into the apparatus 4a instead of that shown in Fig. 13 if the other metal, M, comprises manganese. In this case, the apparatus 4a further comprises a magnetic separator 580, but other elements of the apparatus 4a, apart from those shown in Fig. 14, are the same as those already depicted in Fig. 13 and have only been omitted from Fig. 14 for improved clarity. The magnetic separator 580 comprises an inlet 581 for receiving finely comminuted and dried and dehydroxylated ore. If the ore originally comprised a manganiferous mineral, this may be converted into a trivalent manganese oxide using at least one of a reductant-free pyrometallurgical technique and an inorganic hydrometallurgical technique, before the finely comminuted and dried and dehydroxylated ore is introduced into the magnetic separator 580. Since trivalent manganese oxides are strongly paramagnetic, they may be separated from diamagnetic gangue species, like silica and alumina, also present in the ore by bringing the ore into proximity with a region of high magnetic field. The magnetic separator 580 therefore also comprises a first outlet 584 for such diamagnetic species and a second outlet 585 for the trivalent manganese oxide(s), which is connected to the first inlet 131 of the redox reaction subassembly 13. In the redox reaction subassembly 13, the first inlet 131 of the first gas-tight reaction vessel 130 is again provided with an airlock 131a (also not shown in Fig. 14 for improved clarity) to prevent atmospheric air from entering the reaction vessel 130 when the trivalent manganese oxide(s) are introduced thereto. This allows the redox reaction to be conducted in an inert atmosphere. If the other metal, M, comprises manganese, the redox reaction may be conducted at a temperature of less than about 600 ’Celsius with an amount of liquid sodium in excess of the stoichiometric amount thereof required for the reaction. This prevents the formation of ternary oxides, such as a-NaMnOj, and allows unreacted liquid sodium in excess of the stoichiometric amount thereof required for the reaction to be used as a transport medium for the insoluble reaction products. The separation subassembly 18 in this alternative embodiment is again for separating liquid sodium, the other metal, M, in elemental form and the sodium oxide from each other. In this case, it comprises an inlet 191 for receiving the liquid sodium with the solid phase entrained therein from the outlet of the redox reaction subassembly 13, a first outlet 194 for liquid sodium, a second outlet 177 for the other metal, M, in elemental form and a third outlet 178 for sodium hydroxide. In this alternative embodiment, the separation subassembly 18 comprises a solid-liquid sodium phase separator 190, a hydration vessel 170 and a second solid-aqueous phase separator 175. The solid-liquid sodium phase separator 190 is constructed and functions as described above in relation to Fig. 13. The hydration vessel 170 is for reacting at least some of the sodium oxide from the first reaction vessel 130 with liquid water to produce an aqueous solution of sodium hydroxide. It therefore comprises a first inlet 171 for receiving the solid phase from the second outlet 195 of the solid-liquid sodium phase separator 190, a second inlet 172 for water, and an outlet 174 for the aqueous solution of sodium hydroxide and undissolved solids, including the other metal, M, in elemental form. The second solid-aqueous phase separator 175, which may for example comprise a settlement tank, filter and / or centrifuge, is for separating these undissolved solids from the aqueous solution of sodium hydroxide. It therefore comprises an inlet 176 connected to the outlet 174 of the hydration vessel 170, a first outlet 177 for the undissolved solids including the other metal, M, in elemental form, and a second outlet 178 for the aqueous solution of sodium hydroxide. The second outlet 178 of the second solid-aqueous phase separator 175 is connected to the third inlet 143 of the second gas-tight reaction vessel 140, and to the caustic transfer pathway 410, which supplies sodium hydroxide from the second outlet 178 to at least one of the neutralization vessel 420 and the carbonation vessel 160 in the same manner as the third outlet 187 of the separation subassembly 18 of Fig. 13 supplies them with sodium oxide, described above. Water for the hydration vessel 170 may be supplied to the second inlet 172 thereof from at least one of two sources, as follows. Firstly, it may be recycled from the carbonation vessel 160, as described above in relation to Figs. 4A-B. Alternatively or additionally, it may be recovered from an aqueous solution of sodium chloride supplied from at least one of the second outlet 165 of the first solid-aqueous phase separator 16 and the outlet 426 of the neutralization vessel 420, as described below in relation to Fig. 17. Fig. 15 schematically shows a second embodiment of an apparatus 4b according to the invention. The apparatus 4b is shown in Fig. 15 as having the same type of separation subassembly 18 as in Fig. 13, but in alternative possible embodiments, it could comprise a separation subassembly 18 as in Fig. 14. The apparatus 4b differs from the apparatus 4a of Figs. 13 and 14 in the following respects. Firstly, the apparatus 4b further comprises a kiln 20for thermally decomposing at least some of the separated precipitate into water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide. In this embodiment, the kiln 20 is gas-tight to allow this thermal decomposition to be carried out in the absence of oxygen. The kiln 20 therefore comprises an inlet 21 for receiving the precipitate from the first outlet 164 of the solid-aqueous phase separator 16, a first outlet 24 for water vapour, and a second outlet 25 for the solid end-product. Secondly, the hydrochloric acid-producing subassembly 12 in the apparatus 4b comprises a gas-phase reactor 30 and an absorber 40. The gas-phase reactor 30 is for reacting chlorine gas with water vapour in a reverse Deacon reaction at a temperature of from about 450 to about 750 ’Celsius to produce a mixture of gases at least comprising hydrogen chloride and oxygen. The gas-phase reactor 30 comprises a first inlet 31 for receiving chlorine gas from the second outlet 15 of the electrolytic subassembly 10, a second inlet 32 for receiving the water vapour from the first outlet 24 of the kiln 20, and an outlet 34 for the mixture of gases. The absorber 40 is for contacting this mixture of gases with liquid water to produce hydrochloric acid and a stream of tail gases. The absorber 40 therefore comprises a first inlet 41 for receiving the mixture of gases from the outlet 34 of the gas-phase reactor 30, a second inlet 42 for receiving liquid water, a first outlet 44 for the hydrochloric acid and a second outlet 45 for the stream of tail gases. Water for the absorber 40 may be supplied to the second inlet 42 thereof from one or more of the following sources. Firstly, it may be recovered from an aqueous solution of sodium chloride supplied from at least one of the second outlet 165 of the solid-aqueous phase separator 16 and the outlet 426 of the neutralization vessel 420, as described below in relation to Fig. 17. If the apparatus 4b comprises a separation subassembly as in Fig. 14, it may also be recycled from the carbonation vessel 160, as described above in relation to Figs. 4A-B. The stream of tail gases produced at the second outlet 45 of the absorber 40 may be processed as described in the present applicant's co-pending UK patent application no. 2417058.1 ("Method and Apparatus for Producing Hydrochloric Acid"; applicant's ref: NE-P-GB 006) and / or it may be recycled as described above. Fig. 16 schematically shows an embodiment of an electrolytic subassembly 10 and an embodiment of a hydrochloric acid-producing subassembly 12, which is an alternative to that shown in Fig. 15 and which may be incorporated into either one of the apparatuses 4a, 4b as well as or instead of the hydrochloric acid-producing subassembly 12 of Fig. 15. In the present embodiment, the electrolytic subassembly 10 comprises a first type of electrolytic cell 110, a caustic dryer 460 and a second type of electrolytic cell 120. The first type of electrolytic cell 110 is for electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, and may therefore be a chlor-alkali type of cell, for example. The caustic dryer 460 is for drying an aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour. The second type of electrolytic cell 120 is for fusing and electrolysing solid sodium hydroxide to produce liquid sodium, hydrogen gas and oxygen gas, and may therefore be a Castner type of cell, for example. The first type of electrolytic cell 110 comprises an inlet 111 for the aqueous solution of sodium chloride, a first outlet 114 for the aqueous solution of sodium hydroxide, a second outlet 115 for chlorine gas and a third outlet 116 for hydrogen gas. The inlet 111 of the first type of electrolytic cell 110 receives the aqueous solution of sodium chloride from at least one of the second outlet 165 of the solid-aqueous phase separator 16 and the outlet 426 of the neutralization vessel 420. The caustic dryer 460 comprises an inlet 461 for receiving the aqueous solution of sodium hydroxide from the first outlet 114 of the first type of electrolytic cell 110, a first outlet 464 for water vapour and a second outlet 465 for solid sodium hydroxide. The second type of electrolytic cell 120 comprises an inlet 123 for receiving the solid sodium hydroxide from the second outlet 465 of the caustic dryer 460, a first outlet 125 for oxygen gas, a second outlet 126 for hydrogen gas and a third outlet 127 for liquid sodium. The solid sodium hydroxide received from the second outlet 465 of the caustic dryer 460 may, for example, be a mixture of anhydrous sodium hydroxide and sodium hydroxide monohydrate. The third outlet 127 of the second type of electrolytic cell 120 provides the first outlet 17 for liquid sodium of the electrolytic subassembly 10, whereas the second outlet 115 of the first type of electrolytic cell 110 provides the second outlet 15 for chlorine gas of the electrolytic subassembly 10. The third outlet 116 of the first type of electrolytic cell 110 and the second outlet 126 are combined to provide a common outlet for hydrogen gas from the electrolytic subassembly 10 as well. The hydrochloric acid-producing subassembly 12 comprises a combustion chamber 430 for combusting chlorine gas with hydrogen gas to produce hydrogen chloride, and an absorber 440 for contacting hydrogen chloride with liquid water to produce hydrochloric acid. The combustion chamber 430 comprises a first inlet 431 for receiving chlorine gas from the second outlet 115 of the first type of electrolytic cell 110, a second inlet 432 for receiving hydrogen gas from the common outlet for hydrogen of the electrolytic subassembly 10, and an outlet 434 for hydrogen chloride. The first inlet 431 of the combustion chamber 430 provides the first inlet 121 of the hydrochloric acidproducing subassembly 12. Hydrogen not consumed by the production of hydrogen chloride in the combustion chamber 430 is branched off to a hydrogen outlet 416. The absorber 440 comprises a first inlet 441 for receiving the hydrogen chloride from the outlet 434 of the combustion chamber 430, a second inlet 442 for receiving liquid water, and an outlet 444 for the hydrochloric acid. The second inlet 442 and the outlet 444 of the combustion chamber 430 respectively provide the second inlet 122 and the outlet 124 of the hydrochloric acid-producing subassembly 12. In this embodiment, the water consumed in the absorber 440 is derived from the aqueous solution of sodium hydroxide dried by the caustic dryer 460. Thus the apparatus further comprises a condenser 70 for condensing water vapour, which has an inlet 71 for receiving water vapour from the first outlet 464 of the caustic dryer 460 and an outlet 74 for water in liquid phase, which is connected upstream of the second inlet 442 of the absorber 440. The caustic dryer 460 and condenser 70 may both be parts of a so-called "multi-effect" evaporator, for example. Whereas in Fig. 16, the inlet 11 of the electrolytic subassembly 10 is supplied with an aqueous solution of sodium chloride, in other possible embodiments, the electrolytic subassembly 10 may alternatively or additionally comprise a third type of electrolytic cell, such as a Downs type of cell, for fusing and electrolysing solid sodium chloride. If so, an apparatus according to the invention may comprise means for supplying the electrolytic subassembly 10 with solid sodium chloride. Fig. 17 therefore schematically shows a third embodiment of an apparatus 4c, in which the electrolytic subassembly 10 comprises such a third type of electrolytic cell 450 having an inlet 451 for receiving solid sodium chloride. The apparatus 4c is shown in Fig. 17 as having the same type of separation subassembly 18 as in Fig. 13, but in alternative possible embodiments, it could comprise a separation subassembly 18 as in Fig. 14. The apparatus 4c also differs from the apparatus 4a of Figs. 13 and 14 in that the apparatus 4c further comprises a dryer 60 for drying an aqueous solution of sodium chloride to produce sodium chloride in solid phase for electrolysis by this third type of electrolytic cell 450. The dryer 60 comprises an inlet 61 for receiving an aqueous solution of sodium chloride from at least one of the second outlet 165 of the solid-aqueous phase separator 16 and the outlet 426 of the neutralization vessel 420, a first outlet 64 for water vapour and a second outlet 65 for solid sodium chloride. The second outlet 65 of the dryer 60 is connected to the inlet 451 of the third type of electrolytic cell 450. Thus sodium chloride in solid phase obtained by drying the aqueous solution of sodium chloride in the dryer 60 is recycled to the electrolytic subassembly 10. In this embodiment, the water consumed by the hydrochloric acid-producing subassembly 12 is derived from the aqueous solution of sodium chloride dried by the dryer 60. Thus the apparatus 4c further comprises a condenser 70 for condensing water vapour, which has an inlet 71 for receiving water vapour from the first outlet 64 of the dryer 60 and an outlet 74 for water in liquid phase, which is connected to the second inlet 122 of the hydrochloric acid-producing subassembly 12. The dryer 60 and condenser 70 may be parts of a so-called "multi-effect" evaporator, for example. In other possible embodiments of an apparatus according to the invention, the electrolytic subassembly 10 may comprise other combinations of the first, second and third types of electrolytic cell mentioned above, in which case, water vapour derived from drying an aqueous solution of sodium hydroxide in the caustic dryer 460 may be combined with water vapour derived from drying an aqueous solution of sodium chloride in the dryer 60 to provide a single stream of water vapour to a condenser 70. Moreover, solid sodium hydroxide from the caustic dryer 460 and / or solid sodium chloride from the dryer 60 may be heated and / or fused using waste heat recovered from other parts of the apparatus described herein before being supplied to the electrolytic subassembly 10. This has the advantage of decreasing the electricity consumed by the electrolytic subassembly 10 for ohmic heating of the respective electrolytes. In all of the apparatuses 4a, 4b, 4c described above, the first outlet 194 of the solid-liquid sodium phase separator 190 is connected back to the second inlet 132 of the redox reaction subassembly 13 so that excess liquid sodium can be circulated through the gas-tight reaction vessel 130 in a closed loop. In other possible embodiments, excess liquid sodium does not have to be recirculated in this way, but doing so has the advantage that it allows heat to be extracted from the excess liquid sodium before it is recycled, whereby the temperature of the redox reaction may be controlled. However, the ore of the metal, M, introduced into the redox reaction subassembly 13 may often comprise a siliceous mineral, which may be a gangue mineral species, such as quartz, and / or a siliceous mineral of the metal produced in elemental form itself. If so, the redox reaction may be conducted at a temperature of at least about 320 ’Celsius to induce a reaction between the sodium oxide and silica derived from the siliceous mineral to produce at least sodium silicate, which dissolves in the liquid sodium, as described above in relation to Figs. 12A-B. Thus if excess liquid sodium is recirculated through the gas-tight reaction vessel 130, this dissolved sodium silicate may reach saturation solubility in the liquid sodium inside the reaction vessel 130 as the excess liquid sodium is recycled and as more ore comprising such a siliceous mineral is introduced thereto. Fig. 18 therefore schematically shows an embodiment of a subassembly 6 for producing a mixture comprising sodium silicate, which may be incorporated into any one of the apparatuses 4a, 4b, 4c. This subassembly 6 allows sodium silicate dissolved in the liquid sodium to be abstracted from the excess liquid sodium, whereby the amount of sodium silicate dissolved therein may be maintained below its saturation solubility inside the gastight reaction vessel 130 and a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide may be produced as a co-product. This subassembly 6 comprises a second reaction vessel 620, a precipitation tank 630 and a second solid-liquid sodium phase separator 640. The second reaction vessel 620 is for reacting liquid sodium having sodium silicate dissolved therein with at least one of oxygen and water to produce a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide. The second reaction vessel 620 therefore comprises a first inlet 621 for receiving the liquid sodium with sodium silicate dissolved therein from the first outlet 194 of the first solid-liquid sodium phase separator 190, a second inlet 622 for at least one of oxygen and water, and an outlet 624 for the mixture thus produced. Like the first solid-liquid sodium phase separator 190, the second solid-liquid sodium phase separator 640 may also be of a type as shown and 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 above. In the present embodiment, the first outlet 194 of the separator 190 is connected to a two-way valve V5, which is itself connected both to the first inlet 621 of the second reaction vessel 620 and to a first inlet 631 of the precipitation tank 630. The two-way valve V5 can therefore be operated to direct liquid sodium with sodium silicate dissolved therein from the first solid-liquid sodium phase separator 190 either to the second reaction vessel 620 or to the precipitation tank 630. In addition to the first inlet 631 thereof, the precipitation tank 630 also comprises a second inlet 632 for receiving crystals of solid sodium silicate and an outlet 634 for liquid sodium having both sodium silicate dissolved therein and solid sodium silicate entrained therein. The second solid-liquid sodium phase separator 640 comprises an inlet 641 for receiving liquid sodium with both sodium silicate dissolved therein and solid sodium silicate entrained therein from the outlet 634 of the precipitation tank 630, a first outlet 644 for solid sodium silicate and a second outlet 645 for liquid sodium with sodium silicate dissolved therein. The inlet 641 is connected to the outlet 634 of the precipitation tank 630 via an open / shut valve V6. Thus the inlet 641 is also downstream of the first outlet 194 of the separator 190 via the open / shut valve V6, the precipitation tank 630 and the two-way valve V5. The first outlet 644 is arranged to add solid sodium silicate to a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide which is produced in the second reaction vessel 620 and which comes out of the outlet 624 thereof. The first outlet 644 of the second separator 640 is also connected to the second inlet 632 of the precipitation tank 630. This allows a small portion of the solid sodium silicate from the first outlet 644 to be abstracted from a major portion thereof which is added to the alkaline mixture produced in the second reaction vessel 620, to provide nucleation sites for precipitation of solid sodium silicate in the precipitation tank 630. The second outlet 645 of the second separator 640 is connected upstream of the second inlet 132 of the redox reaction vessel 130. During operation, both the precipitation tank 630 and the second solid-liquid sodium phase separator 640 are maintained below a temperature at which sodium silicate in the liquid sodium received from the first outlet 194 of the first solid-liquid sodium phase separator 190 reaches saturation solubility. When valve V5 is positioned to direct liquid sodium with sodium silicate dissolved therein from the first separator 190 to the precipitation tank 630, valve V6 can be closed to allow the precipitation tank 630 to fill up, and crystals of solid sodium silicate are introduced into the precipitation tank 630 via the second inlet 632 thereof. Since the precipitation tank 630 is maintained below a temperature at which sodium silicate in the liquid sodium reaches saturation solubility, these crystals of solid sodium silicate act as nucleation sites for solid sodium silicate to precipitate out in the tank 630. Valve V6 is then opened again, the precipitation tank 630 drains into the second solid-liquid sodium phase separator 640, and the precipitated solid sodium silicate is separated from the liquid sodium with sodium silicate dissolved therein, but now at a lower concentration, by the second separator 640. This liquid sodium with a relatively lower concentration of sodium silicate dissolved therein leaves the second separator 640 via the second outlet 645 thereof and is returned to the redox reaction vessel 130 via the second inlet 132 thereof, where it is diluted down but also heated up by being mixed with fresh liquid sodium from the first outlet 17 of the electrolytic subassembly 10, and where more ore is introduced to the first reaction vessel 130 as well, via the first inlet 131 thereof. Meanwhile, solid sodium silicate continues to accumulate at the first outlet 644 of the second solid-liquid sodium phase separator 640. This process may be repeated until enough solid sodium silicate has accumulated at the first outlet 644 of the second solid-liquid sodium phase separator 640, whereupon the valve V5 is repositioned to direct liquid sodium with sodium silicate dissolved therein from the first solid-liquid sodium phase separator 190 to the second reaction vessel 620 instead. The recirculation of liquid sodium with sodium silicate dissolved therein from the first outlet 194 of the separator 190 back to the redox reaction vessel 130 therefore temporarily ceases. Instead, the liquid sodium with sodium silicate dissolved therein now reacts with at least one of oxygen and water inside the second reaction vessel 620 to produce a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide. Since the concentration of sodium silicate dissolved in the liquid sodium is limited by its saturation solubility at the temperature of the liquid sodium, this mixture comprises a majority of at least one of sodium oxide and sodium hydroxide and only a minority of sodium silicate. However, by adding the solid sodium silicate which has accumulated at the first outlet 644 of the second separator 640 thereto, an alkaline mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide in desired proportions can be obtained. Further details concerning such a procedure and the subassembly 6 can be found in the present applicant's co-pending UK patent application no. 2417073.0 ("Method and Apparatus for Producing an Alkaline Mixture comprising Sodium Silicate"; applicant's ref: NE-P-GB 009), mentioned above. Fig. 19 provides a conceptual summary of the raw materials and main products of some embodiments of the method described herein, as well as examples of some of the uses of those products in various different embodiments. In Fig. 19, arrows do not represent chemical equations, and chemical formulae do not represent specific quantities or relative amounts of the reagents to which they refer. Instead, solid arrows represent top-level processes, whereby raw materials are converted into products, which may then be transferred as inputs for corresponding downstream uses. The method described herein has only two necessary raw materials, shown in Fig. 19 in bold, which are an ore of iron and / or manganese and an ore comprising a carbonate mineral. The method has three necessary products, also shown in Fig. 19 in bold, which are iron and / or manganese, an oxide or hydroxide derived from the carbonate mineral ore, and oxygen. Common salt and water, which are both optional extra raw materials, but which are always present as intermediaries in this method, may be viewed conceptually as being similar to catalysts, which enable the two necessary raw materials to be converted into the three necessary main products. The salt is electrolysed, in molten phase and / or as an aqueous solution, to produce liquid sodium and gaseous chlorine. The liquid sodium is used to dissolve the iron ore and / or manganese ore, and the gaseous chlorine and water are used to make hydrochloric acid, which is used to dissolve the carbonate mineral. Introducing an excess amount of salt results in the production of an excess amount of chlorine and / or hydrochloric acid, and a corresponding amount of sodium oxide, hydroxide and / or carbonate. If at least some of the salt is electrolysed as an aqueous solution, simultaneous electrolysis of a corresponding amount of water occurs, in which case, water also has to be added as a raw material to replace the hydrogen and the increased amount of oxygen produced by this electrolysis. Whereas the following does not describe the actual chemical pathways involved, the method of the invention may be viewed conceptually as a sequence of reactions represented by the dashed arrows in Fig. 19. According to this sequence, an oxide ion is removed from the oxide of iron and / or manganese and transferred to the cations of the carbonate mineral to leave elemental metal and create a different oxide. A carbonate ion is removed from the carbonate mineral and transferred to the sodium from the salt to create the sodium carbonate. The chloride ion is removed from the salt by electrolysis to create gaseous chlorine and / or is transferred to hydrogen from the water to create the hydrochloric acid. The water is also split by electrolysis to leave the oxygen as a product, and may also create gaseous hydrogen and an increased amount of oxygen, depending on how much water is electrolysed. In summary, therefore, the present invention provides an industrial chemical process and an apparatus for carrying out that process. The chemical process of the invention integrates into a single, combined method: (i) a method of producing iron and / or manganese in elemental form from oxides thereof derived from their respective ores with (ii) a method of producing an oxide or hydroxide of at least one of calcium, magnesium and iron from another ore respectively comprising a carbonate mineral of at least one of calcium, magnesium and iron. This combined method has iron ore and / or manganese ore and the ore comprising the carbonate mineral as its only essential ingredients, with common salt and / or water as optional additional ingredients. The method produces iron and / or manganese, an oxide or hydroxide derived from the carbonate mineral and oxygen as its corresponding products. If the iron and / or manganese ore also comprises a siliceous mineral, in some embodiments, the method can also produce an alkaline mixture comprising sodium silicate as a coproduct. Salt and water act as intermediaries in the method of the invention to produce these endproducts, but are only consumed if supplied in excess. An amount of sodium oxide, hydroxide and / or carbonate determined by the amount of salt consumed may be produced as a co-product, along with a corresponding amount of at least one of chlorine and hydrochloric acid. An amount of hydrogen determined by the amount of water consumed may also be produced as a co-product, along with an increased amount of oxygen. The iron, manganese and oxygen may all be used in steelmaking. Any excess oxygen produced may be safely vented to atmosphere. If the carbonate mineral mostly comprises calcium carbonate and / or magnesium carbonate, the calcium oxide or hydroxide and / or magnesium oxide or hydroxide may be used as any one or more of an ingredient in cement manufacture, a flux in steelmaking and an ingredient in the manufacture of soda-lime glass. In comparison to traditional techniques for manufacturing cement, steel and soda-lime glass, all of which instead use calcium and / or magnesium carbonate as an ingredient, this has the advantage that since carbon dioxide has already been removed from these carbonate minerals by the method described herein, emissions of carbon dioxide from each of these manufacturing processes are thereby reduced proportionately. If, however, the carbonate mineral mostly comprises iron carbonate, the same method described herein may instead be used to convert siderite ores into iron oxide(s), which may then be fed back into the method of the invention to produce iron. Sodium carbonate produced by the method of the invention may be used in the manufacture of at least one of soda-lime glass and borosilicate glass, to mineralize carbon dioxide as sodium hydrogencarbonate, and / or as a sorbent for capturing carbon dioxide from flue gases. The chlorine and / or hydrochloric acid produced may be used in one or more other chemical processes, such as in the manufacture of chlorinated organic compounds, for example. Thus the method of the invention provides the most important ingredients required for the manufacture of each of steel and cement, and can also provide one or more of the major ingredients required for the manufacture of the most common types of glass. On the other hand, it consumes no fossil fuels at all and produces no greenhouse gas emissions. In some embodiments, in which the method of the invention has water as an additional ingredient, the hydrogen produced as a co-product may be used as a fuel and / or as an ingredient in the manufacture of ammonia, for example, which has the advantage that it avoids producing hydrogen by steam-methane reforming (SMR). The chemical reactions in the method of the invention are exothermic overall, and therefore require no nett input of heat. At least some of the heat generated by these chemical reactions may be reused to reduce the total energy required by the method of the invention. The only energy which the method of the invention needs is electricity for electrolysis. However, this may be supplied from a renewable source of energy, such as wind or solar, or come from nuclear power. In some embodiments, the method of the invention may even have a negative carbon footprint overall. 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 (400a - 400j) comprising:producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis;reacting (303) at least some of the liquid sodium in a redox reaction with an oxide of another metal (M) from an ore of the other metal, wherein the other metal (M) comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide;separating (304) the other metal from the sodium oxide;dividing (401) at least some of the sodium oxide into a first portion and a second portion;using (104, 203b) at least some of the chlorine gas to produce hydrogen chloride;dissolving (204) at least some of the hydrogen chloride in liquid water to produce hydrochloric acid;adding (205a, 205b) another ore comprising a carbonate mineral of at least one of calcium, magnesium and iron to at least some of the hydrochloric acid thus produced to dissolve the carbonate mineral therein and produce carbon dioxide gas and an aqueous solution respectively comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, wherein the carbonate mineral is dissolved in the hydrochloric acid closed off from their surrounding environment;capturing (206) at least some of the carbon dioxide gas;reacting (207) at least some of the first portion of sodium oxide, or sodium hydroxide derived from hydrating (317a) at least some of the first portion of sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to produce an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide;phase-separating (208) at least some of the precipitate from the aqueous solution of sodium chloride thus produced; andusing (214a, 214b; 403) at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating (317b) at least some of the second portion of sodium oxide, in at least one of:(i) a reaction (403) with at least some of the hydrochloric acid to produce an aqueous solution of sodium chloride; and(ii) a reaction (214a, 214b) with at least some of the captured carbon dioxide gas to produce at least sodium carbonate.

2. A method (400a - 400g) according to claim 1, comprising:consuming (205, 303) the carbonate mineral and the oxide of the other metal (M) in a molar ratio of from 1.25 to 1.75 mol, inclusive, of the carbonate mineral per mol of the oxide of the other metal;producing (101b, 201c) the liquid sodium in a molar ratio of from 5 to 7 mol, inclusive, of the liquid sodium per mol of the oxide of the other metal; andwherein the first portion of sodium oxide constitutes between 5 / 12 and 7 / 12, inclusive, of a total number of mols of the sodium oxide.

3. A method (400h) according to claim 1, wherein:producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis comprises producing some of the liquid sodium by electrolysing (408) a molten electrolyte comprising sodium chloride and aluminium chloride using a consumable aluminium anode and a solid electrolyte to separate the molten electrolyte from the liquid sodium, thereby producing some of the liquid sodium and gaseous aluminium chloride;and the method further comprises:consuming (205, 303) the carbonate mineral and the oxide of the other metal (M) in a molar ratio of from 0.75 to 1.75 mol, inclusive, of the carbonate mineral per mol of the oxide of the other metal;producing (101b, 201c, 408) the liquid sodium in a molar ratio of from 5 to 10.5 mol, inclusive, of the liquid sodium per mol of the oxide of the other metal; and wherein:the first and second portions of sodium oxide together constitute between 7 / 12 and 12 / 12, inclusive, of a total number of mols of the sodium oxide;the first portion and the second portion of sodium oxide each constitutes between 5 / 12 and 7 / 12, inclusive, of the total number of mols of the first and second portions;at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating (317b) at least some of the second portion of sodium oxide, is used only in a reaction (403) with at least some of the hydrochloric acid to produce an aqueous solution of sodium chloride; andat least some of a third portion of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the third portion of sodium oxide, is used in a reaction (214a) with at least some of the captured carbon dioxide gas to produce at least sodium carbonate.

4. A method (400c, 400j) according to any one of the preceding claims, further comprising: thermally decomposing (209) at least some of the separated precipitate in the absence of oxygen to produce water vapour and a solid end-product comprising at least one of calcium oxide, magnesium oxide and an iron oxide;capturing (103) at least some of the water vapour thus produced; andwherein using (104, 203b) at least some of the chlorine gas to produce hydrogen chloride and dissolving (204) at least some of the hydrogen chloride in liquid water to produce hydrochloric acid comprises:reacting (104) at least some of the chlorine gas produced by electrolysis (101b, 201a) with at least some of this captured water vapour in a reverse Deacon reaction at a temperature of from 450 to 750 ’Celsius, inclusive, to produce a mixture of gases at least comprising hydrogen chloride and oxygen;immediately contacting (105) at least some of the mixture of gases with liquid water to dissolve the hydrogen chloride therein, thereby producing the hydrochloric acid and a stream of tail gases; andextracting heat (106) from the hydrochloric acid thus produced, to maintain its temperature substantially constant until the stream of tail gases is no longer in contact therewith.

5. A method (400c, 400j) according to claim 4, wherein:a total number of mols of chlorine gas supplied to the reverse Deacon reaction (104) is within 15% of the stoichiometric amount of hydrochloric acid required for the reaction (205) with the carbonate mineral;a total number of mols of water vapour supplied to the reverse Deacon reaction (104) is at least as great as the total number of mols of chlorine gas supplied to the reverse Deacon reaction (104); andat least some of the stream of tail gases, from which the oxygen has been separated (116), is recycled (117) back to the reverse Deacon reaction (104) in a loop.

6. A method (400c') according to any one of the preceding claims, further comprising: thermally decomposing (209) at least some of the separated precipitate to produce water vapour and a solid end-product comprising at least one of calcium oxide, magnesium oxide and an iron oxide;capturing (103) at least some of the water vapour thus produced;electrolysing (411) at least some of this captured water vapour to produce hydrogen gas and oxygen gas; andwherein using (104, 203b) at least some of the chlorine gas to produce hydrogen chloride comprises combusting (203b) at least some of the chlorine gas with at least some of the hydrogen gas thus produced to produce the hydrogen chloride.

7. A method (400d - 400g) according to any one of the preceding claims, wherein producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis comprises:electrolysing (201a) an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying (201b) at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing (201c) at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas; andthe method further comprises:capturing (215) at least some of the water vapour produced by drying (201b) the aqueous solution of sodium hydroxide;condensing (216) at least some of the water vapour captured (215) from drying the aqueous solution of sodium hydroxide to produce liquid water; andusing (217) at least some of the liquid water thus produced as at least some of the liquid water in which the hydrogen chloride is dissolved (204) to produce the hydrochloric acid.

8. A method (400d, 400e, 400g) according to claim 7, further comprising adding (402) liquid water to the liquid water produced by condensing (216) at least some of the water vapour captured from drying (201b) the aqueous solution of sodium hydroxide, wherein a number of mols of the liquid water added (402) is at least equal to a number of mols of the hydrogen gas produced by electrolysis (201a, 201c).

9. A method (400a, 400b, 400d - 400g) according to any one of the preceding claims, wherein producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis comprises:electrolysing (201a) an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying (201b) at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing (201c) at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas; andusing (104, 203b) at least some of the chlorine gas to produce hydrogen chloride comprises: combusting (203b) at least some of the chlorine gas with at least some of the hydrogen gas produced by at least one of electrolysing (201a) an aqueous solution of sodium chloride and fusing and electrolysing (201c) at least some of the solid sodium hydroxide, to produce the hydrogen chloride.

10. A method (400d - 400g) according to any one of the preceding claims, wherein producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis comprises electrolysing (201a) at least some of the aqueous solution of sodium chloride produced by at least one of:(i) reacting (207) at least some of the first portion of sodium oxide, or sodium hydroxide derived from hydrating (317a) at least some of the first portion of sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride; and(ii) reacting (403) at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating (317b) at least some of the second portion of sodium oxide, with at least some of the hydrochloric acid.

11. A method (400b, 400c, 400c', 400f, 400j) according to any one of the preceding claims, further comprising:drying (110) at least some of the aqueous solution of sodium chloride produced by at least one of:(i) reacting (207) at least some of the first portion of sodium oxide, or sodium hydroxide derived from hydrating (317a) at least some of the first portion of sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride; and(ii) reacting (403) at least some of the second portion of sodium oxide, or sodium hydroxide derived from hydrating (317b) at least some of the second portion of sodium oxide, with at least some of the hydrochloric acid,to produce solid sodium chloride and water vapour; andwherein producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis comprises fusing and electrolysing (101b) at least some of the solid sodium chloride thus produced.

12. A method (400b, 400c, 400c', 400f, 400j) according to claim 11, further comprising:capturing (111) at least some of the water vapour produced by drying (110) the aqueous solution of sodium chloride;condensing (112) at least some of the water vapour captured from drying the aqueous solution of sodium chloride to produce liquid water; andusing (113) at least some of the liquid water thus produced as the liquid water in which the hydrogen chloride is dissolved (204) to produce the hydrochloric acid.

13. A method according to any one of the preceding claims, wherein:the other metal (M) comprises iron, the iron ore is comminuted into fines, and dried and dehydroxylated, before the ore thus treated is introduced to the liquid sodium;reacting (303) at least some of the liquid sodium in a redox reaction with the iron oxide comprises conducting the reaction in an inert atmosphere and at a temperature of less than 450 ’Celsius with an amount of the liquid sodium in excess of the stoichiometric amount thereof required for the reaction, then separating (305) the iron and other insoluble products at least comprising the sodium oxide as a solid phase from the excess liquid sodium.

14. A method according to claim 13, further comprising using at least some of the iron to make steel by:adding powdered carbon to the iron;mixing the powdered carbon and iron together to produce a resulting mixture; and subjecting the resulting mixture to a powder metallurgical process.

15. A method according to any one of claims 1 to 12, wherein:the other metal (M) comprises manganese, the manganese ore is comminuted 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 and an inorganic hydrometallurgical technique, before the trivalent manganese oxide is introduced to the liquid sodium; andreacting (303) at least some of the liquid sodium in a redox reaction with the trivalent manganese oxide comprises conducting the reaction in an inert atmosphere and at a temperature of less than 600 ’Celsius with an amount of the liquid sodium in excess of the stoichiometric amount thereof required for the reaction, then separating (305) the manganese and other insoluble products at least comprising the sodium oxide as a solid phase from the excess liquid sodium.

16. A method according to claim 15, further comprising using at least some of the manganese as an alloying element with iron to produce an austenitic manganese or stainless steel.

17. A method (400f) according to any one of the preceding claims, wherein the ore of the other metal (M) comprises a siliceous mineral, the redox reaction (303) is conducted at a temperature of at least 320 ’Celsius to induce a reaction (601) between the sodium oxide and silica derived from the siliceous mineral to produce at least sodium silicate, which dissolves in the liquid sodium, and at a temperature below that at which the liquid sodium can react with the oxide of the other metal to produce a ternary oxide thereof, at least some of the liquid sodium with the sodium silicate dissolved therein is separated (305) from the undissolved metal and other insoluble products, and oxidized (602) by reacting it with at least one of oxygen and water to produce a mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide.

18. A method according to claim 17, further comprising using at least some of the mixture comprising sodium silicate and at least one of sodium oxide and sodium hydroxide as at least one of:(i) an alkaline activator for an alkaline activated or geopolymer cement; and(ii) a reagent with carbon dioxide to produce a composition comprising sodium carbonate and silica.

19. A method according to any one of the preceding claims, wherein the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium, and the method further comprises using at least some of the separated precipitate, or at least one of calcium oxide and magnesium oxide derived from thermally decomposing at least some of the separated precipitate, as at least one of:(i) an ingredient in cement manufacture;(ii) a flux in steelmaking;(iii) an ingredient in the manufacture of soda-lime glass; and(iv) a reagent with carbon dioxide to produce a substance comprising at least one of calcium carbonate and magnesium carbonate.

20. A method according to any one of claims 1 to 18, wherein the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium, and the method further comprises using some of the separated precipitate as at least one of:(i) an ingredient in the manufacture of non-hydraulic lime mortar; and(ii) a reagent with a pozzolan to produce a hydraulic cement.

21. A method according to any one of claims 1 to 18, wherein the carbonate mineral has a majority of iron cations, and the method further comprises:thermally decomposing (209) at least some of the separated precipitate to produce water vapour and a solid end-product comprising an iron oxide; andusing at least some of the solid end-product as a reagent with at least some of the liquid sodium.

22. A method according to anyone of the preceding claims, wherein the method produces sodium carbonate and further comprises using at least some of the sodium carbonate as at least one of:(i) an ingredient in the manufacture of at least one of soda-lime glass and borosilicate glass;(ii) a reagent with carbon dioxide and water to produce a substance comprising at least sodium hydrogencarbonate; and(iii) a sorbent for capturing carbon dioxide from a stream of flue gases.

23. A method according to anyone of the preceding claims, wherein the method produces sodium carbonate and further comprises storing at least some of the sodium carbonate in a salt mine from which a greater volume of salt has been extracted.

24. A method according to any one of the preceding claims, wherein producing (101b; 201a, 201b, 201c; 408) liquid sodium and chlorine gas by electrolysis comprises:electrolysing (201a) an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, drying (201b) at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing (201c) at least some of the solid sodium hydroxide to produce at least some of the liquid sodium, hydrogen gas and oxygen gas; andthe method further comprises using at least some of the hydrogen gas as at least one of:(i) an ingredient in a process for the manufacture of ammonia;(ii) a fuel to produce electricity in an electrochemical cell; and(iii) a fuel to produce heat by combustion with oxygen.

25. An apparatus (4a, 4b, 4c) comprising:an electrolytic subassembly (10) for producing at least liquid sodium and chlorine gas by electrolysis, the electrolytic subassembly comprising an inlet (11) for sodium chloride in at least one of solid, molten and aqueous phase, a first outlet (17) for liquid sodium, and a second outlet (15) for chlorine gas;a redox reaction subassembly (13) comprising a first gas-tight reaction vessel (130) for containing therein a redox reaction between liquid sodium and an oxide of another metal (M) from an ore of the other metal, wherein the other metal (M) comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide, the redox reaction subassembly (13) further comprising a first inlet (131) for the oxide of the other metal (M), a second inlet (132) for receiving liquid sodium from the first outlet (17) of the electrolytic subassembly (10), and an outlet (134, 178) for liquid sodium and entrained therein, a solid phase comprising the other metal (M) in elemental form and sodium oxide;a separation subassembly (18) for separating the liquid sodium, the other metal (M) in elemental form and the sodium oxide from each other, the separation subassembly (18) comprising an inlet (181, 191) for receiving the liquid sodium with the solid phase entrained therein from the outlet (134, 178) of the redox reaction subassembly, a first outlet (194) for liquid sodium, a second outlet (186, 195a) for the other metal (M) in elemental form and a third outlet (187, 195b) for sodium oxide or hydroxide;a hydrochloric acid-producing subassembly (12) for producing hydrochloric acid from chlorine gas and liquid water, and comprising a first inlet (121) for receiving chlorine gas from the second outlet (15) of the electrolytic subassembly (10), a second inlet (122) for liquid water and an outlet (124, 44) for hydrochloric acid;a carbonate dissolution subassembly (14) comprising a second gas-tight reaction vessel (140) for dissolving therein a carbonate mineral of at least one of calcium, magnesium and iron in hydrochloric acid, a first inlet (141) for another ore comprising the carbonate mineral, a second inlet (142) for receiving hydrochloric acid from the outlet (124) of the hydrochloric acid-producing subassembly (12), a third inlet (143, 152) for receiving a first portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the first portion of sodium oxide, from the third outlet (187,195b) of the separation subassembly (18), and an outlet (144,154) for an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide;a solid-aqueous phase separator (16) for separating at least some of the precipitate from the aqueous solution of sodium chloride, the solid-aqueous phase separator (16) comprising an inlet (161)for receiving the aqueous solution of sodium chloride and the precipitate from the outlet (144, 154) of the carbonate dissolution subassembly (14), a first outlet (164) for the precipitate, and a second outlet (165) for the aqueous solution of sodium chloride; anda caustic transfer pathway (410) for transferring a second portion of sodium oxide, or sodium hydroxide derived from hydrating at least some of the second portion of sodium oxide, from the third outlet (187, 195b) of the separation subassembly (18) to at least one of:(i) a neutralization vessel (420) for reaction with hydrochloric acid from the outlet (124, 44) of the hydrochloric acid-producing subassembly (12); and(ii) a carbonation vessel (160) for reaction with carbon dioxide gas produced in the carbonate dissolution subassembly (14).

26. An apparatus (4b) according to claim 25, further comprising:a gas-tight kiln (20) for thermally decomposing at least some of the separated precipitate in the absence of oxygen into water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide, wherein the kiln (20) comprises an inlet (21) for receiving the precipitate from the first outlet (164) of the solid-aqueous phase separator (16), a first outlet (24) for water vapour, and a second outlet (25) for the solid end-product; and wherein the hydrochloric acid-producing subassembly (12) comprises:a gas-phase reactor (30) for reacting chlorine gas with water vapour in a reverse Deacon reaction at a temperature of from 450 to 750 ’Celsius to produce a mixture of gases at least comprising hydrogen chloride and oxygen, wherein the gas-phase reactor (30) comprises a first inlet (31) for receiving the chlorine gas from the second outlet (15) of the electrolytic subassembly (10), a second inlet (32) for receiving the water vapour from the first outlet (24) of the kiln (20), and an outlet (34) for the mixture of gases; andan absorber (40) for contacting the mixture of gases with liquid water to produce hydrochloric acid and a stream of tail gases, wherein the absorber (40) comprises a first inlet (41) for receiving the mixture of gases from the outlet (34) of the gas-phase reactor (30), a second inlet (42) for receiving the liquid water, a first outlet (44) for the hydrochloric acid and a second outlet (45) for the stream of tail gases.

27. An apparatus according to claim 25 or claim 26, wherein the electrolytic subassembly (10) comprises:a first type of electrolytic cell (110) for electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas,wherein the first type of electrolytic cell (110) comprises an inlet (111) for the aqueous solution of sodium chloride, a first outlet (114) for the aqueous solution of sodium hydroxide and a second outlet (115) for chlorine gas;a caustic dryer (460) for drying an aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, the caustic dryer (460) comprising an inlet (461) for receiving the aqueous solution of sodium hydroxide from the first outlet (114) of the first type of electrolytic cell (110), a first outlet (464) for water vapour and a second outlet (465) for solid sodium hydroxide; anda second type of electrolytic cell (120) for fusing and electrolysing solid sodium hydroxide to produce liquid sodium, hydrogen gas and oxygen gas, wherein the second type of electrolytic cell (120) comprises an inlet (123) for receiving the solid sodium hydroxide from the second outlet (465) of the caustic dryer (460), a first outlet (125) for oxygen gas, a second outlet (126) for hydrogen gas and a third outlet (127) for liquid sodium; andwherein the apparatus further comprises a condenser (70) for condensing water vapour, wherein the condenser (70) comprises an inlet (71) for receiving the water vapour from the first outlet (464) of the caustic dryer (460) and an outlet (74) for water in liquid phase connected upstream of the second inlet (122) of the hydrochloric acid-producing subassembly (12).

28. An apparatus according to any one of claims 25 to 27, wherein the electrolytic subassembly (10) comprises:a first type of electrolytic cell (110) for electrolysing an aqueous solution of sodium chloride to produce at least an aqueous solution of sodium hydroxide and at least some of the chlorine gas, wherein the first type of electrolytic cell (110) comprises an inlet (111) for the aqueous solution of sodium chloride, a first outlet (114) for the aqueous solution of sodium hydroxide and a second outlet (115) for chlorine gas;a caustic dryer (460) for drying an aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, the caustic dryer (460) comprising an inlet (461) for receiving the aqueous solution of sodium hydroxide from the first outlet (114) of the first type of electrolytic cell (110), a first outlet (464) for water vapour and a second outlet (465) for solid sodium hydroxide; anda second type of electrolytic cell (120) for fusing and electrolysing solid sodium hydroxide to produce liquid sodium, hydrogen gas and oxygen gas, wherein the second type of electrolytic cell (120) comprises an inlet (123) for receiving the solid sodium hydroxide from the second outlet (465) of the caustic dryer (460), a first outlet (125) for oxygen gas, a second outlet (126) for hydrogen gas and a third outlet (127) for liquid sodium; andwherein the hydrochloric acid-producing subassembly (12) comprises:a combustion chamber (430) for combusting chlorine gas with hydrogen gas to produce hydrogen chloride, the combustion chamber (430) comprising a first inlet (431) for receiving the chlorine gas from the second outlet (115) of the first type of electrolytic cell (110), a second inlet (432) for receiving the hydrogen gas from at least one of a hydrogen outlet (116) of the first type of electrolytic cell (110) and the second outlet (126) of the second type of electrolytic cell (120), and an outlet (434) for hydrogen chloride; andan absorber (440) for contacting hydrogen chloride with liquid water to produce hydrochloric acid, wherein the absorber (440) comprises a first inlet (441) for receiving the hydrogen chloride from the outlet (434) of the combustion chamber (430), a second inlet (442) for receiving the liquid water, and an outlet (444) for the hydrochloric acid.

29. An apparatus according to any one of claims 25 to 28, wherein:the electrolytic subassembly (10) comprises an electrolytic cell (110) for electrolysing an aqueous solution of sodium chloride; andthe electrolytic cell (110) comprises an inlet (111) for receiving the aqueous solution of sodium chloride from at least one of the second outlet (165) of the solid-aqueous phase separator (16) and the outlet (426) of the neutralization vessel (420).

30. An apparatus (4c) according to any one of claims 25 to 29, further comprising:a dryer (60) for drying an aqueous solution of sodium chloride, wherein the dryer (60) comprises an inlet (61) for receiving the aqueous solution of sodium chloride from at least one of the second outlet (165) of the solid-aqueous phase separator (16) and the outlet (426) of the neutralization vessel (420), a first outlet (64) for water vapour and a second outlet (65) for sodium chloride in solid phase; andwherein the electrolytic subassembly (10) comprises a third type of electrolytic cell (450) for fusing and electrolysing solid sodium chloride, comprising an inlet (451) for receiving the solid sodium chloride from the second outlet (65) of the dryer (60).

31. An apparatus (4c) according to claim 30, further comprising a condenser (70) for condensing water vapour, wherein the condenser (70) comprises an inlet (71) for receiving the water vapour from the first outlet (64) of the dryer (60) and an outlet (74) for water in liquid phase connected upstream of the second inlet (122) of the hydrochloric acid-producing subassembly (12).s

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