Method and apparatus for producing oxides of calcium, magnesium and iron from carbonate mineral ores without burning carbonaceous fuels

Abstract

The present invention provides a method and apparatus (2a) for producing at least one of calcium oxide, magnesium oxide and an iron oxide from an ore comprising a carbonate mineral of at least one of calcium, magnesium and iron, such as limestone and siderite ores. The method comprises producing liquid sodium and chlorine gas by electrolysis. The liquid sodium is oxidised to sodium oxide and the chlorine is used to make hydrochloric acid. The sodium oxide may be produced in a redox reaction with an oxide of another metal to produce the other metal in elemental form as well. The ore is added to the hydrochloric acid to dissolve the carbonate mineral therein, which produces gaseous carbon dioxide 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 is captured. The aqueous solution is reacted with at least some of the sodium oxide, or sodium hydroxide derived therefrom, 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 precipitate is then phase separated from the aqueous solution of sodium chloride and the hydroxide(s) in the separated precipitate are thermally decomposed to produce water vapour and a solid end-product comprising the desired oxide(s). Sufficient heat for this thermal decomposition is transferred to the separated precipitate from at least one of the electrolysis products, the exothermic processes by which the precipitate is produced, their respective products and the products of the thermal decomposition itself. Thus the invention consumes no carbonaceous fuel, including any fossil fuel, and has electricity as its only energy requirement. If the electricity is produced from a source of renewable energy, it produces no greenhouse gas emissions. The apparatus (2a) comprises an electrolytic subassembly (10), a sodium oxide or hydroxide-producing subassembly (13), a hydrochloric acid-producing subassembly (12), a reaction subassembly (14), a solid-aqueous phase separator (16), a kiln (20), and a heat transfer pathway (23) for transferring enough heat to the precipitate in the kiln (20) to cause the thermal decomposition from at least one of: a product of the electrolytic subassembly (10), the other subassemblies (12, 13, 14) and their respective products, and a product of the thermal decomposition. The invention is intended to replace the traditional calcination process, including for the production of cement clinker, and can also be used to convert siderite ores into one or more iron oxides suitable for ironmaking. The captured carbon dioxide may be reacted with a second portion of the sodium oxide, or with sodium hydroxide derived therefrom, to produce sodium carbonate, thereby also providing a replacement for the Solvay process. In some embodiments, the method can even have a negative carbon footprint overall.

Description

[0001] Method and Apparatus for Producing Oxides of Calcium, Magnesium and Iron from Carbonate Mineral Ores without Burning Carbonaceous Fuels

[0002] Field of the Invention

[0003] The present invention concerns a method and apparatus for producing at least one of calcium oxide, magnesium oxide and an iron oxide from an ore respectively comprising a carbonate mineral of at least one of calcium, magnesium and iron. Such minerals include calcite and aragonite ( / .e., polymorphs of calcium carbonate, CaCOs), magnesite ( / .e., magnesium carbonate, MgCOs), dolomite ( / '.e., calcium-magnesium carbonate) and siderite ( / '.e., iron carbonate, FeCOs). 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) (COs)2) and minerals in each of the solid solution series from calcite, magnesite and siderite to rhodochrosite ( / '. e., manganese carbonate, MnCOs), wherein each of the solid solutions contains more of the calcium, magnesium and / or iron cations, respectively, than of the manganese cations. The ore may further comprise one or more gangue species, such as aiuminosiiicate minerals, which may include one or more ciays and / or species of alumina and silica, like quartz, for example.

[0004] Background of the Invention

[0005] Calcium oxide, also known as quick lime, has traditionally been extracted from limestone (a rock or ore composed almost entirely of calcite) by heating the limestone in a lime kiln to decompose the limestone into calcium oxide and carbon dioxide gas. This industrial process, called calcination, has been around for many centuries and proceeds according to the equation:

[0006] CaC03(S) -> CaO(s)+ CO2(g) [Eqn. 1]

[0007] This thermal decomposition 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. 1 is highly positive, so that the limestone must be heated to a temperature above about 900 “Celsius before the Gibbs free energy 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 limestone in the lime kiln until the limestone reaches the required temperature. Apart from the energy consumed, burning fossil fuel in this way generates its own very significant carbon dioxide emissions, which are additional to the carbon dioxide also liberated from the limestone according to Eqn. 1.

[0008] Calcining limestone in this way is a major component of cement production, which is responsible for at least 8% of all anthropogenic carbon dioxide emissions. Lime made by the same calcination 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 today. Although data for aggregated carbon dioxide emissions from the production of lime for such other purposes are not readily available, this suggests that cement production and the production of lime for other purposes are together responsible for over 10% of all anthropogenic carbon dioxide emissions in total. In view of this and the present climate crisis, it is widely recognized that the carbon dioxide produced during the extraction of lime from limestone by calcination should somehow be mitigated. Several proposals have therefore already been made for how to achieve this.

[0009] For example, it has been proposed that at least some of the carbon dioxide generated may be captured and stored, rather than being released into the environment, where it would contribute to global greenhouse gas emissions. In such carbon capture and storage (CCS), a chemical sorbent is used to capture the carbon dioxide and produce a carbonated intermediate. This intermediate is then treated to regenerate the sorbent and release the carbon dioxide for storage and / or industrial usage (CCU). However, CCS and CCU both have the disadvantage that they represent an extra cost for the lime producer to bear, making their adoption unattractive. Another existing proposal is to replace at least some of the clinker in ordinary Portland cement (CPC) produced by calcining limestone with one or more supplementary cementitious materials (SCMs), such as a mixture of uncalcined limestone and calcined clays. Although this proposal has the potential to reduce the amount of carbon dioxide generated by cement production significantly, the cement production process itself remains unchanged, so that fuel is still consumed and carbon dioxide is still released by calcining the limestone which remains. Such a technique is also inapplicable to the production of lime for purposes other than cement production, such as for basic oxygen steelmaking. On the other hand, little or no attention seems to have been paid in the prior art to fundamentally modifying or replacing the calcination process itself. In contrast, the present invention considers that the traditional technique for calcining limestone should now be viewed as a primitive and obsolete technology, which should be replaced.

[0010] Moreover, whereas the above background description has focused on the extraction of lime from limestone and the carbon dioxide this produces, in practice, the same calcination technique has traditionally also been used to produce dolomitic lime from dolomite, by the thermal decomposition of dolomite to release carbon dioxide therefrom. Dolomite is a mineral composed of both calcium carbonate and magnesium carbonate, and dolomitic lime is therefore a mixture of calcium oxide and magnesium oxide. Pure dolomite has the formula CaMg(CC>3)2. However, naturally occurring dolostone (a rock or ore comprising dolomite) is typically composed of pure dolomite mixed with calcite in varying proportions and with various different mineral structures. The thermal decomposition of naturally occurring dolomite at atmospheric pressure is therefore more complex as well, and depends not only on the chemical composition and structure of the dolostone, but also on the process conditions. For example, dolomite from Zelatowa mine in Poland decomposes into dolomitic lime and carbon dioxide at a temperature of about 643 “Celsius when heated at a rate of 2.5 “Celsius per minute, but at a temperature of about 861 “Celsius when heated at the higher rate of 15 “Celsius per minute, as described in M. Olszak-Humienik & M. Jablonski: "Thermal Behavior of Natural Dolomite”, Journal of Thermal Analysis and Calorimetry, Vol. 119, pp. 2239-2248 (2015). A similar calcination technique can also be used to decompose magnesite ( / '.e., magnesium carbonate) into magnesia ( / '.e., magnesium oxide) and carbon dioxide. Whereas the thermal decomposition of pure magnesite can be carried out at a temperature as low as about 350 “Celsius at atmospheric pressure, the magnesium oxide thus produced is so reactive that it tends to reabsorb carbon dioxide from atmospheric air. In practice, therefore, the calcination of magnesite is conducted either at a higher temperature of from about 450 to about 900 “Celsius, to produce a less reactive "light-burnt” product, or at a temperature above about 900 “Celsius, to produce a more inert "dead-burnt” product, which do not suffer from this problem and which are distinguishable from each other by their crystalline microstructures. The "light-burnt” product is more useful as a reagent in other chemical processes, whereas the "dead-burnt” product is more useful as a refractory material.

[0011] In addition, 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, with the accompanying release of carbon dioxide from the iron carbonate. The thermal decomposition of naturally occurring siderite is also complex, and the decomposition depends not only on the chemical composition and structure of the siderite, but also on the process conditions. For further details, see, for example, Y.H. Luo et al. "Thermal Decomposition Behaviour and Kinetics of Xinjiang Siderite Ore”, Mineral Processing and Extractive Metallurgy, Transactions of the Institutions of Mining and Metallurgy: Section C, Vol. 125, Issue 1, pp. 17-25 (2016).

[0012] The chemical reduction of iron carbonate by hydrogen directly into iron has therefore been proposed as an alternative technique for reducing the quantity of carbon dioxide emitted by the production of iron from siderite ores, by avoiding such a thermal decomposition. In this respect, see, for example, G. Baldauf-Sommerbauer et al. "Sustainable Iron Production from Mineral Iron Carbonate and Hydrogen”, Green Chemistry, Vol. 18, pp. 6255- 6265 (2016). However, in order to be able to use hydrogen to extract iron from siderite ores, the hydrogen must itself first be produced, for example by electrolysing water into hydrogen and oxygen. If the electricity for this comes from a source of renewable energy, such as wind or solar, or from nuclear power, this does not generate any greenhouse gases. On the other hand, producing hydrogen by electrolysis using such zero-carbon electricity (commonly called "green” hydrogen) currently only accounts for about 4% of total hydrogen production.

[0013] At present, therefore, about 96% of all hydrogen produced is still derived from fossil fuels (commonly called "blue” hydrogen). This breaks down into about 48% produced from steam-methane reforming (SMR), 18% from the gasification of coal, and 30% from the partial oxidation of hydrocarbons by other means. All of these processes also produce carbon dioxide, possibly along with other greenhouse gases like carbon monoxide, which must therefore be captured and stored if they are not to be released into the environment. When compared to the thermal decomposition of siderite into one or more iron oxides which can be more easily processed, direct reduction of iron carbonate by hydrogen therefore just pushes the problem with producing carbon dioxide and other greenhouse gases in ironmaking one step up the supply chain. Moreover, whatever the source of the hydrogen, technological problems with storing and transporting the hydrogen remain.

[0014] Accordingly, it would be desirable if any replacement for the traditional calcination process were effective not only at producing lime from limestone, but also at producing dolomitic lime from dolomite, as well as magnesia from magnesite. It would also be desirable if an alternative technique could be found for converting siderite ores into an iron oxide suitable for ironmaking, without calcining the siderite. However, a minority of the calcium and / or magnesium cations in calcite, dolomite and magnesite are often substituted by iron and / or manganese cations, which produces minerals like manganoan calcite and ankerite. Similarly, a minority of the iron cations in siderite ores are often substituted by magnesium and / or manganese cations to produce minerals in the solid solution series from siderite to magnesite and from siderite to rhodochrosite, respectively. It would therefore also be desirable if any replacement for the traditional calcination process could accommodate such cation substitutions as well.

[0015] 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.

[0016] It is also known that calcium hydroxide, magnesium hydroxide and iron hydroxides can 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 occurs at a lower temperature than the thermal decomposition of the corresponding carbonates. At atmospheric pressure, calcium hydroxide decomposes into calcium oxide and water vapour at about 512 “Celsius. Also at atmospheric pressure, magnesium hydroxide decomposes into magnesium oxide and water vapour at about 350 “Celsius. At atmospheric pressure, the thermal decomposition of, for example, pure ferric hydroxide (Fe(OH)s) into ferric oxide (Fe2O3) and water vapour occurs at about 475 “Celsius, whereas ferrous hydroxide (Fe(OH)2) may be thermally decomposed in the absence of oxygen into ferrous oxide (FeO) and water vapour at a lower temperature of from about 150 to about 200 “Celsius (depending on the rate of removal of the water vapour). Also at atmospheric pressure, manganese hydroxide (Mn(OH)2) decomposes in the absence of oxygen into manganese monoxide (MnO) and water vapour at about 220 “Celsius. In contrast, in the presence of oxygen, manganese hydroxide spontaneously oxidises into manganese (III) oxide (M^Os) and / or manganese dioxide (MnO2 or manganese (IV) oxide) and water vapour at ambient temperatures. Object of the Invention

[0017] It is therefore an object of the invention to provide a method and apparatus for producing at least one of calcium oxide, magnesium oxide and an iron oxide from an ore respectively comprising a carbonate mineral of at least one of calcium, magnesium and iron, without burning any carbonaceous fuel, including any fossil fuel.

[0018] Description of the Invention

[0019] Accordingly, in one aspect, the present invention provides a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide. The method firstly comprises producing liquid sodium and chlorine gas by electrolysis. The method then comprises oxidising at least some of the liquid sodium to produce at least sodium oxide, and using at least some of the chlorine gas to produce hydrogen chloride, at least some of which is dissolved in liquid water to produce hydrochloric acid. The method then comprises adding an ore comprising a carbonate mineral of at least one of calcium, magnesium and iron to the hydrochloric acid to dissolve the carbonate mineral therein and produce gaseous carbon dioxide 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 dioxide is captured. The method then comprises reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the 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 (Fe(OH)2). At least some of the precipitate is phase separated from this aqueous solution of sodium chloride, and at least some of the calcium hydroxide, magnesium hydroxide and ferrous hydroxide in the separated precipitate is 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. The thermal decomposition comprises transferring sufficient heat to the separated precipitate to cause the thermal decomposition of the respective hydroxide(s) from at least one of the aforementioned processes, their respective products and the products of the thermal decomposition itself.

[0020] This implies that the thermal decomposition may be conducted by transferring enough heat to the separated precipitate to cause the thermal decomposition from at least one of the electrolysis products, the exothermic processes by which the precipitate is produced, their respective products, as well as possibly also from at least one of the products of the thermal decomposition itself. These may therefore include any one or more of: (I) a product of the electrolysis, such as the liquid sodium or a gaseous electrolysis product;

[0021] (ii) oxidising at least some of the liquid sodium in an exothermic reaction, and / or a product thereof;

[0022] (ill) producing the hydrogen chloride and / or hydrochloric acid;

[0023] (iv) hydrating at least some of the sodium oxide, and / or sodium hydroxide derived from hydrating at least some of the sodium oxide; (v) reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride; and

[0024] (vi) at least one of the water vapour and the solid end-product produced by the thermal decomposition.

[0025] The heat may be transferred to the separated precipitate in one of several different ways, some of which are described below. In some embodiments, however, the method may comprise using the liquid sodium as a heat transfer fluid to transfer heat to the separated precipitate.

[0026] Producing the liquid sodium and chlorine gas by electrolysis may comprise 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 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 the liquid sodium, hydrogen gas and oxygen gas. In some embodiments, some of the liquid sodium may also 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.

[0027] The invention exploits the fact that the total quantity of heat released by exothermic processes in the method of the invention exceeds the heat required to thermally decompose the hydroxide(s) in the separated precipitate. The method of the invention has at least the following advantages.

[0028] Firstly, the method described herein comprises a well-known sequence of chemical reactions, which is: dissolving a carbonate mineral in hydrochloric acid, reacting the resulting aqueous solution with sodium oxide, or with sodium hydroxide derived from hydrating at least some of this sodium oxide, and then phase separating the resulting precipitate from the aqueous solution of sodium chloride thus formed. For example, this reaction sequence can be seen being performed in a laboratory context at the following weblink: The fact that this reaction sequence is well known is an advantage of the present invention. However, the invention does not just consist in using this reaction sequence by itself, but rather in how the precursors to this reaction sequence are first produced, how the resulting precipitate is then processed, and in transferring sufficient heat to this precipitate to cause the thermal decomposition, wherein the heat is transferred to the precipitate from one or more of the processes by which the precipitate is formed and their respective products, and / or from at least one of the products of the thermal decomposition itself.

[0029] Because of this heat transfer process, in comparison to the traditional calcination process, the above method completely eliminates the need to burn any carbonaceous fuel, including any fossil fuel. Since it consumes no carbonaceous fuel, it also generates no carbon dioxide emissions from burning such fuel. Since the carbonate mineral comprises mineralized carbon dioxide, dissolving the carbonate mineral in hydrochloric acid necessarily releases this carbon dioxide as gas. However, since the carbonate mineral is dissolved in the hydrochloric acid closed off from their surrounding environment and the carbon dioxide is captured, no greenhouse gases are emitted. The captured carbon dioxide may, for example, be reacted with some more of the sodium oxide produced by oxidising the liquid sodium, or with sodium hydroxide derived from hydrating some more of this sodium oxide, as described further below, to produce sodium carbonate, which is both harmless and a useful industrial product in its own right.

[0030] The only energy which the method of the invention consumes is electricity for the electrolysis. However, this electricity may be provided by a source of renewable energy, such as wind or solar, or come from nuclear power, and therefore need not generate any greenhouse gas emissions either. Thus the method of the invention is able to generate no greenhouse gas emissions at all. In fact, since the oxide(s) produced by the method described herein have a high affinity for carbon dioxide, with which they can themselves be reacted, the method of the invention can have a negative carbon footprint overall. For example, calcium oxide produced by the method of the invention may be used as an ingredient in the manufacture of non-hydraulic lime mortar, which absorbs carbon dioxide from atmospheric air as it hardens.

[0031] Apart from the initial electrolysis, the only significantly endothermic process in the method of the invention is the thermal decomposition of the hydroxide(s) in the separated precipitate into their respective oxide(s) and water vapour. This thermal decomposition proceeds according to the equation:

[0032] Q(OH)2(S) — > QO (S) + H2O <g) [Eqn. 2] wherein Q represents at least one of calcium, magnesium and iron. For example, when Q = Ca, the reaction of Eqn. 2 is considerably less endothermic than the reaction of Eqn. 1 and therefore occurs at a temperature which is very significantly below the temperature at which calcium carbonate decomposes into calcium oxide and carbon dioxide. The reaction of Eqn. 2 then has a Gibbs free energy which becomes negative at a temperature which is about 400 degrees lower than the temperature required for the calcination reaction of Eqn. 1. The heat required for the thermal decomposition of Eqn. 2 is therefore readily supplied by one or more of the precursors to the reaction sequence described above, the processes for their production, the reaction sequence itself, and / or the products of the thermal decomposition, the total heat from which collectively more than exceeds the heat consumed by the reaction of Eqn. 2 and which can therefore also be used for other purposes, such as for drying an aqueous solution of sodium hydroxide produced by hydrating the sodium oxide.

[0033] In particular, since the liquid sodium produced by electrolysis has a temperature of at least about 330 “Celsius if produced by fusing and electrolysing solid sodium hydroxide, and of up to about 625 “Celsius if produced by fusing and electrolysing solid sodium chloride, the quantity of heat carried by the liquid sodium is considerable. At least some of this heat may therefore be extracted and used to contribute towards the thermal decomposition. Oxidising at least some of the liquid sodium to produce the sodium oxide is a significantly exothermic reaction, so that a considerable amount of heat can be extracted from this reaction and / or from the sodium oxide thus formed. Dissolving hydrogen chloride in water to produce the hydrochloric acid is also exothermic, allowing heat to be extracted from this process too. Hydrating at least some of the sodium oxide and reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride are also significantly exothermic processes. Heat can therefore be extracted from at least one of these processes as well. Thus the total quantity of heat available from the different exothermic processes in the method of the invention easily exceeds the total quantity of heat required for the thermal decomposition of the respective hydroxide(s). Heat may also be recycled by cooling at least one of the solid end-product and the water vapour down towards ambient temperature. Thus the carbonate mineral may be converted into its corresponding oxide(s) without burning any carbonaceous fuel, including any fossil fuel, by using heat from one or more of these different sources.

[0034] The method of the invention also has the advantage that both the liquid sodium and the chlorine gas produced by the electrolysis are used as precursors to the sequence of reactions which produce the precipitate. Thus the method of the invention has excellent atom economy overall. Oxidising at least some of the liquid sodium to produce sodium oxide may also produce another industrially useful co-product, by simultaneously reducing another oxide in a redox reaction. In addition, the electrolysis may produce hydrogen gas as a valuable co-product. The electrical energy required for the electrolysis can therefore be shared between the production of several different industrial products and need not be consumed just by converting the carbonate mineral into its corresponding oxide(s). This makes the method described herein highly competitive with the traditional calcination process. Apart from the electrical energy needed for the electrolysis, the only raw materials which the method of the invention needs in addition to the carbonate mineral ore itself are sodium chloride ( / '. e. , common salt) as feed material for the electrolysis, and the water used to make the hydrochloric acid. However, both of these two additional ingredients can be regenerated from the method of the invention in a closed loop, as described in the present applicant's UK patent application no. 2417075.5 ("Industrial Chemical Process and Apparatus”; applicant's ref: NE- P-GB 007), the entire contents of which is incorporated herein by reference. Thus, subject to real-world losses and inefficiencies, no nett quantity of salt or water need be consumed in addition to the ore comprising the carbonate mineral itself.

[0035] The method described herein has the further advantage that the calcium, magnesium and iron all follow the same reaction pathway as each other. Thus the same method can be used not only to extract lime from limestone, dolomitic lime from dolomite, and magnesia from magnesite, but also to convert siderite-bearing ores into one or more iron oxides suitable for ironmaking. These iron oxides may comprise Fe2O3, FeO and / or Fe3O4, depending on the conditions used for the thermal decomposition. For example, the ferrous hydroxide (Fe(OH)2) may be oxidised in an oxygenated atmosphere to ferric hydroxide (Fe(OH)3), which is then thermally decomposed into ferric oxide (Fe2C>3) and water vapour. In another example, the ferrous hydroxide may be thermally decomposed at a lower temperature and in the absence of oxygen into water vapour and ferrous oxide (FeO), which may then at least partially disproportionate into FesO4 and metallic iron. Whereas the first of these two examples produces only a single oxide, the second example may be preferred since the thermal decomposition needs less heat, and if the solid end-product is subsequently to be used as feed material for ironmaking, it is also in a lower oxidation state, requiring less energy for its reduction to elemental iron.

[0036] The method of the invention will now be described in greater detail.

[0037] Electrolysis

[0038] At least some of the liquid sodium and chlorine gas may be produced together, for example, by fusing and electrolysing solid sodium chloride. This may be performed using a known technique, like the Downs process. Alternatively or additionally, at least some of the liquid sodium and chlorine gas may be produced separately from each other by firstly electrolysing an aqueous solution of sodium chloride ( / '.e., brine) using the chlor-alkali process to produce an aqueous solution of sodium hydroxide and the chlorine gas. In some embodiments, the brine may be brine concentrate produced as a waste product by a desalination plant, in which case, the electrolysis may advantageously be conducted near the desalination plant and this waste can then be eliminated. If at least some of the brine is electrolysed in a conventional 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 A1 . Subsequently drying at least some of the aqueous solution of sodium hydroxide to produce solid sodium hydroxide then allows the solid sodium hydroxide to be fused and electrolysed to produce the liquid sodium, as well as hydrogen gas and oxygen gas. Fusing and electrolysing the solid sodium hydroxide may be performed using a known technique, like the Castner process. Whereas the Castner process is generally regarded as obsolete, fusing and electrolysing the solid sodium hydroxide by the Castner process after having produced the sodium hydroxide for this via the chlor-alkali process has the advantage that the two processes can be viewed as a combined electrolysis of equimolar amounts of sodium chloride and water. This is because the back-reaction of water in the Castner cell with the liquid sodium thus produced also produces hydrogen gas, as follows:

[0039] Chlor-alkali process: 2 NaCI <aq) + 2 H2O — > 2 NaOH <aq) + CI2 + H2 [Eqn. 3a]

[0040] I Dry

[0041] Overall: 2 NaCI + 2 H2O 2 Na + CI2+ 2 H2+ O2[Eqn. 3d]

[0042] The hydrogen gas thus produced may then be used, for example, as fuel in a hydrogen economy and / or in a process for the manufacture of ammonia (such as in the Haber-Bosch process). The oxygen gas is also a useful co-product and may be used, for example, in a steelmaking process by injecting it into molten iron. In all cases, whether electrolysing solid sodium chloride, brine or solid sodium hydroxide, as already mentioned, electricity for the electrolysis may be provided by a source of renewable energy and therefore need not create any greenhouse gas emissions.

[0043] If the liquid sodium is produced by the Castner process, it has a temperature immediately thereafter of from about 300 to about 330 “Celsius, and may therefore be oxidised without first being cooled significantly. If on the other hand the liquid sodium is produced by electrolysing solid sodium chloride, it already has a temperature immediately thereafter of at least about 600 “Celsius. Therefore, to make the oxidation more controllable, in such a case, the liquid sodium is preferably cooled significantly before being oxidised. However, at least some of the heat extracted from the liquid sodium may be transferred to the separated precipitate to contribute towards the thermal decomposition of the hydroxide(s) therein. This has the advantage that the heat which the liquid sodium carries immediately after it has been produced is re-used, rather than being wasted.

[0044] Oxidation of Liquid Sodium

[0045] As also mentioned, oxidising at least some of the liquid sodium to produce sodium oxide is highly exothermic. Although this oxidation may in principle be carried out by reacting the liquid sodium with atmospheric oxygen, the oxidation is preferably carried out as part of another process which produces a desired co-product. For example, oxidising at least some of the liquid sodium may comprise reacting the liquid sodium in an exothermic redox reaction with an oxide of another metal to produce the other metal in elemental form and the sodium oxide. If so, the method described herein can then be combined with another industrial process, wherein the liquid sodium is used to extract the other metal in elemental form from its ore. This has the advantage that the energy consumed by the initial electrolysis is then split between two integrated industrial processes, namely, the production of at least one of calcium oxide, magnesium oxide and an iron oxide from a carbonate mineral ore and the production of the other metal from its own ore. Moreover, since the other metal can then potentially be extracted from its own ore without consuming any fossil fuels and since the only energy which is consumed by the two integrated processes is the electricity for the electrolysis, the other metal can then be extracted from its ore with a zero or even negative carbon footprint as well.

[0046] For example, in some embodiments, the other metal may comprise iron and the redox reaction may be conducted at a temperature of less than 450 “Celsius and in an inert atmosphere (such as in an atmosphere consisting of at least one of nitrogen and argon) to produce sodium oxide and elemental iron, according to the equation:

[0047] 6 NaM+ Fe2O3<S) — > 3 Na2O(s)+ 2 Fe(s)[Eqn. 4a]

[0048] Conducting this redox reaction at a temperature of less than 450 “Celsius and in an inert atmosphere has several advantages, including that it is effective at preventing the formation of ternary oxides, such as Na4FeO3. As is well known, iron has traditionally been extracted from iron ore by reducing the iron ore in a blast furnace at temperatures well in excess of 1200 “Celsius using coke ( / '.e., carbon) derived from coal as a reducing agent to produce elemental iron and gaseous carbon dioxide. In comparison thereto, such embodiments of the invention therefore have the advantages that in addition to a much lower operating temperature, the consumption of fossil fuel and the concomitant production of carbon dioxide can be completely eliminated from the ironmaking process. Further details concerning such a technique are contained in the present applicant's 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.

[0049] In another example, in some embodiments, the other metal may comprise manganese and the redox reaction may be conducted in a similarly inert atmosphere and at a temperature of less than 600 “Celsius to produce sodium oxide and elemental manganese, according to the equation:

[0050] 6 Na <4 + Mn2Oa (S> — > 3 Na2O (S> + 2 Mn (S> [Eqn. 4b]

[0051] Conducting this redox reaction at a temperature of less than 600 “Celsius and in an inert atmosphere also has several advantages, including that it is effective at preventing the formation of ternary oxides like o-NaMnO2. Manganese has traditionally been extracted from its ores using a variety of different techniques, all of which use a carbonaceous compound, such as a fossil fuel, as a reducing agent. In comparison thereto, therefore, such embodiments of the invention at least have the advantage that they also avoid the concomitant production of carbon dioxide. Further details concerning such a technique are contained in the present applicant's 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 also incorporated herein by reference.

[0052] The enthalpy of reaction of Eqn. 4a, Hreaction = - 438.3 kJ at s.t.p., whereas that of Eqn. 4b, AHreaction = - 301 kJ at s.t.p. Thus at least some of the heat produced by either of these reactions may also be transferred to the separated precipitate to contribute towards the thermal decomposition. This has the advantage that the heat from oxidising at least some of the liquid sodium is re-used, rather than being wasted. Even if some of the heat from the reaction of Eqn. 4a or 4b is re-used in this way, the reaction products will still have a temperature immediately thereafter of up to about 450 “Celsius in the case of Eqn. 4a and up to about 600 “Celsius in the case of Eqn. 4b and will carry with them at least some of the heat from the reaction. Heat may therefore also be extracted from one or more reaction products and used to contribute to the thermal decomposition of the hydroxide(s) in the precipitate.

[0053] Production of Hydrochloric Acid

[0054] At least some of the chlorine gas may be used to produce the hydrogen chloride in one or more of several different ways. For example, if at least some of the liquid sodium is produced by a combination of the chlor-alkali and Castner processes as described above, at least some of the chlorine gas thus produced may be combusted with at least some of the hydrogen gas also produced by this electrolysis to produce at least some of the hydrogen chloride. Since the enthalpy of formation of hydrogen chloride, AHf = - 92.3 kJ mol1, this combustion process is exothermic. At least some of the heat it generates may therefore be extracted from it and transferred to the separated precipitate to contribute towards the thermal decomposition. In another example, if at least some of the chlorine gas is produced by fusing and electrolysing solid sodium chloride and the thermal decomposition of the hydroxide(s) in the precipitate is performed in the absence of oxygen, the method may further comprise capturing at least some of the water vapour produced by the thermal decomposition. In such a case, at least some of the chlorine gas thus produced may be reacted with at least some of the captured water vapour in a reverse Deacon reaction (hereinafter, RDR) at a temperature of from about 450 to about 750 “Celsius, inclusive, to produce a mixture of gases at least comprising hydrogen chloride and oxygen. At least some of the mixture of gases may then be immediately contacted with liquid water to dissolve the hydrogen chloride therein, thereby producing the hydrochloric acid and a stream of tail gases. The enthalpy of solution of hydrogen chloride in liquid water is about -75 kJ mol1at 50 “Celsius and an absolute pressure of one atmosphere. Therefore, heat is also extracted 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 therewith. Further details concerning such a process are contained in the present applicant's 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 also incorporated herein by reference. At least some of the heat thus extracted may then be transferred to the separated precipitate to contribute towards the thermal decomposition.

[0055] Dissolution of Carbonate Mineral

[0056] Dissolving the carbonate mineral in the hydrochloric acid involves two reactions, which proceed according to the following two equations:

[0057] QCO3 (s) + 2 HCI (aq) — > H2CO3 (aq) + QCI2 (aq) [Eqn. 5a]

[0058] H2CO3 (aq) — > CO2 (g) + H2O [Eqn. 5b] wherein Q represents at least one of calcium, magnesium and iron. In the case of pure calcium carbonate ( / .e., when Q = Ca), for example, the reactions of Eqns. 5a and 5b are mildly exothermic overall, with a total enthalpy of reaction, AHreaction = - 15.2 kJ at s.t.p. If the carbonate mineral is pure magnesite ( / .e., when Q = Mg), the reactions of Eqns. 5a and 5b are slightly more exothermic overall, with a total enthalpy of reaction, AHreaction = - 45 kJ at s.t.p. There is no unique value for the total enthalpy of reaction of Eqns. 5a and 5b if the carbonate mineral comprises dolomite, because the total enthalpy of reaction will depend in any particular case on the relative proportions of calcium carbonate and magnesium carbonate in the mineral, as well as on the mineral's structure. However, the reactions of Eqns. 5a and 5b for dolomite remain exothermic overall, with a total enthalpy of reaction having a value between the above two extremes when Q = Ca and Q = Mg. In the case of pure iron carbonate ( / .e. , when Q = Fe), the reactions of Eqns. 5a and 5b are similarly exothermic overall, with a total enthalpy of reaction, AHreaction = - 28.8 kJ at s.t.p.

[0059] Preferably, the ore comprising the carbonate mineral is comminuted to increase its surface area before it is added to the hydrochloric acid, in order to increase the rate at which the carbonate mineral dissolves in the hydrochloric acid. Comminuting the ore can also help to separate out gangue in the ore from the carbonate mineral. Comminution may comprise one or more of agitation, crushing, grinding, hammering, milling and rolling the mineral, as well as other similar processes. These may be carried out using one or more known types of comminution device, such as a rock crusher, grinder, hammer mill, ball, disc and / or rod mill, autogenous mill, roller mill, vibrator, and / or the like.

[0060] Comminution produces ore particles with a range of different sizes. Smaller ore particles are more desirable because they have a greater surface area to volume ratio. However, comminuting the ore also tends to raise its temperature by the conversion of mechanical energy into heat through friction. The increased temperature of the ore alters the thermodynamic equilibria of Eqns. 5a and 5b, depending on the ore's composition. Creating smaller ore particles with a greater surface area also requires more energy to be expended on comminution than would otherwise be expended on creating larger particles. The optimum size of the ore particles therefore depends in any particular case on the most economic energy balance overall between comminuting the ore and dissolving the carbonate mineral it contains in the hydrochloric acid. This energy balance is in turn determined in any particular case by the specific energy requirements of the comminution device or devices which are used, the prior chemical composition and structure of the ore, and the conditions of the dissolution process itself. Comminution may further comprise sieving, separating and recycling larger particles for further comminution into smaller particles.

[0061] The rate at which the carbonate mineral dissolves in the hydrochloric acid is also affected by the initial temperature and concentration of the hydrochloric acid. In general, the rate at which the carbonate mineral dissolves in the hydrochloric acid increases with both the temperature and the concentration of the hydrochloric acid. The most preferable values for each of these two parameters can also be determined in any particular case by the most economic energy balance between the initial temperature and concentration of the hydrochloric acid, the rate of dissolution of the carbonate mineral, and the final temperature of the aqueous solution thus produced.

[0062] In some embodiments, therefore, the hydrochloric acid may be at above ambient temperature, for example above about 50 “Celsius, more preferably above about 60 “Celsius, in order to increase the rate at which the carbonate mineral dissolves therein. If so, however, the temperature of the hydrochloric acid should also be at least about 15 “Celsius, more preferably about 23 “Celsius, and most preferably about 30 “Celsius, below its boiling point at the prevailing pressure to avoid its undesirable evaporation or boiling, due to the mildly exothermic nature of the reaction of Eqns. 5a and 5b. This is undesirable because any hydrogen chloride gas vaporised from the hydrochloric acid would tend to contaminate the captured carbon dioxide. For example, at atmospheric pressure, the initial temperature of the hydrochloric acid should preferably be less than about 85 “Celsius, more preferably less than about 77 “Celsius, and most preferably less than about 70 “Celsius.

[0063] However, dissolving the carbonate mineral in the hydrochloric acid closed off from their surrounding environment has the additional advantage that the carbonate mineral can then be dissolved in the hydrochloric acid at above atmospheric pressure, in which case, the temperature of the hydrochloric acid can also be increased, without risking its undesirable evaporation or boiling. For example, increasing pressurization of the atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid can be achieved at least in part by the release of the carbon dioxide gas itself. The rate of dissolution of the carbonate mineral in the hydrochloric acid is then increased as well, because increasing the temperature of the hydrochloric acid in turn increases the reaction kinetics. On the other hand, the solubility of the carbon dioxide released from the carbonate mineral decreases with increasing temperature. The increased temperature of the hydrochloric acid therefore also helps to prevent an undesirable reduction in the rate at which carbon dioxide is evolved as a result of the increased pressure. For example, if the carbonate mineral is dissolved in the hydrochloric acid under a mild pressurization of up to about 2, 3 or even 5 atmospheres, the temperature of the hydrochloric acid can also be increased to above 100 “Celsius in order to maintain the rate at which carbon dioxide is evolved, but without any substantial evaporation or risk of boiling.

[0064] Moreover, unlike hydrogen chloride gas, the solubility of carbon dioxide in aqueous solution is negligible at low pH, as Fig. 1 shows. Thus dissolution of the carbon dioxide released from the carbonate mineral may be suppressed by maintaining the reaction mixture of Eqns. 5a and 5b at a pH of less than about 7, and more preferably less than about 6, until the carbonate mineral is fully dissolved. This may be done, for example, by having more of the hydrochloric acid present than a stoichiometric amount thereof required by the reaction of Eqn. 5a. Thus the temperature of the hydrochloric acid can be increased to increase the rate of dissolution of the carbonate mineral therein and the total vapour pressure above the hydrochloric acid can be increased as well to reduce the rate at which water and hydrogen chloride gas evaporate from the hydrochloric acid as a result of its increased temperature, such that the rate of evaporation of hydrogen chloride gas from the acid can be kept at or below its rate of evaporation at atmospheric pressure, whilst still suppressing dissolution of the carbon dioxide.

[0065] Dissolving the carbonate mineral in the hydrochloric acid may be aided by stirring or otherwise mixing the ore particles into the hydrochloric acid.

[0066] The rate of dissolution of the carbonate mineral in the hydrochloric acid may be monitored and used to determine whether and / or when the carbonate mineral has completely dissolved. For example, this may be done by measuring at least one of the flow rate of the carbon dioxide thus produced and the pH and / or temperature of the resulting aqueous solution.

[0067] Optional Separation of Gangue

[0068] As noted above, the ore comprising the carbonate mineral may also comprise one or more gangue species, such as aluminosilicate minerals, which may include one or more clays, silica and / or alumina, for example. If so, these aluminosilicate minerals are much less soluble in hydrochloric acid than the carbonate mineral. Therefore, after the carbonate mineral has dissolved in the hydrochloric acid, the method may optionally comprise phaseseparating undissolved gangue species from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride thus formed, before reacting sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, with at least some of this aqueous solution to produce a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. In comparison to the traditional calcination process, this has the advantage that the proportion of such gangue species in the solid end-product may thereby be reduced. For example, the undissolved gangue species may be separated from the aqueous solution by at least one of settlement under gravity, filtration and centrifugation. Any undissolved gangue species not removed from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride are likely to end up in the solid end-product. However, any undissolved gangue species remaining in this aqueous solution can then act as nucleation sites for subsequent formation of the precipitate. In general, the presence of gangue species in the solid end-product presents no particular problem, since they would also be present if the solid end-product had been produced using the traditional calcination process instead. If, on the other hand, it is desired to increase the proportion of aluminosilicate minerals which do dissolve in the hydrochloric acid, this can be done by simultaneously increasing both the temperature of the hydrochloric acid and the total vapour pressure above the hydrochloric acid, as described above. This therefore gives great flexibility in controlling the level of aluminosilicate minerals in the solid end-product.

[0069] If the ore comprises a siliceous mineral such as quartz, undissolved gangue species phase-separated from the aqueous solution may comprise silica. If so, this silica is sufficiently reactive that in some embodiments, if desired, at least some of the phase-separated gangue may subsequently be used, for example, as at least one of an ingredient with an alkaline activator in the manufacture of an alkaline-activated or geopolymer cement, and a pozzolan in a reaction with at least one of calcium oxide and magnesium oxide to produce a hydraulic cement.

[0070] In some cases, the ore may also comprise an aluminate mineral as gangue. If so, the undissolved gangue species may be phase-separated from the aqueous solution at a pH of about 4.5 or less, which is too acidic for the precipitation of any aluminium hydroxide which may have been produced by dissolving such an aluminate mineral. In such cases, however, the method may further comprise, after phase-separating the undissolved gangue species from this acidic aqueous solution and before producing the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide, adding sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an aqueous solution with a pH of from about 5 to about 7.5, inclusive, which is within the range to precipitate out aluminium hydroxide. The precipitate comprising aluminium hydroxide may then be phase-separated from the aqueous solution with pH in the stated range, before more of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is added to the aqueous solution with pH in the stated range to produce a more alkaline aqueous solution and the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. As before, this phase-separation may be carried out by at least one of settlement under gravity, filtration and centrifugation, for example.

[0071] This process may best be understood by reference to Fig. 2, which shows the solubility curves of silica, aluminium hydroxide, ferrous hydroxide, magnesium hydroxide and calcium hydroxide in mol (H2O) plotted on a logarithmic scale on the y-axis or ordinate against pH plotted on a linear scale on the x-axis or abscissa. As may be seen from Fig. 2, at low pH, in the region of the graph labelled "A”, silica is insoluble, whereas aluminium hydroxide, ferrous hydroxide, magnesium hydroxide and calcium hydroxide are all sufficiently soluble to remain in solution. Silica may therefore be phase-separated from this acidic aqueous solution as described above. As sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is added to this acidic aqueous solution, the pH of the solution increases, passing into the region of the graph labelled “B”. Here, the solubility of aluminium hydroxide drops, causing its precipitation at a pH in a range of from about 5 to about 7.5, whilst ferrous hydroxide, magnesium hydroxide and calcium hydroxide are all still sufficiently soluble to remain in solution. The aluminium hydroxide may therefore be phase-separated from the aqueous solution with pH in this stated range. Then, as more sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is added to the aqueous solution with pH in the stated range, the pH of the solution continues to increase and passes into the region of the graph labelled “C”, where first ferrous hydroxide and then magnesium hydroxide and calcium hydroxide start to precipitate out.

[0072] In some embodiments, if desired, at least some of the phase-separated precipitate comprising aluminium hydroxide may then be dried and dehydroxylating to produce alumina, and at least some of the alumina may be used as feed material for producing aluminium by electrolysis.

[0073] Reaction with Sodium Oxide or Hydroxide, Precipitation and Phase Separation

[0074] Whether or not any undissolved gangue species are phase-separated from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride as just described, at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, is then reacted with this aqueous solution to produces an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. In the case of sodium oxide, this reaction proceeds according to the equation:

[0075] Na20 (S) + QCI2 <aq) + H2O — > Q(OH)2 (s) + 2 NaCI <aq) [Eqn. 6] wherein Q represents at least one of calcium, magnesium and iron, and the water on the left-hand side of Eqn. 6 comes from the right-hand side of Eqn. 5b. The reaction of Eqn. 6 is significantly exothermic, mostly due to the hydration of the sodium oxide. It has similar enthalpies of reaction AHreaction ~ - 230 kJ at s.t.p. regardless of whether Q = Ca, Q = Mg, or Q is a mixture thereof, and an enthalpy of reaction AHreaction = - 257 kJ at s.t.p. when Q = Fe. As noted above, since oxidising at least some of the liquid sodium to produce the sodium oxide is also highly exothermic, the sodium oxide, may have a temperature immediately after its oxidation of, for example, up to about 450 “Celsius, and still carry with it at least some of the heat produced in the oxidation. Similarly, even if the sodium oxide is then cooled significantly, if at least some of this sodium oxide is then hydrated to produce sodium hydroxide for reaction with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, the sodium hydroxide would also be heated significantly by the exothermic nature of this hydration reaction. The sodium oxide, or the sodium hydroxide derived therefrom, is therefore preferably cooled significantly before either one of them is introduced to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. The temperature of the subsequent reaction is preferably also controlled by extracting heat from it, in order to avoid substantial evaporation or a risk of boiling of the resulting aqueous solution of sodium chloride. However, at least some of the heat extracted by cooling the sodium oxide, or the sodium hydroxide derived from hydrating it, and / or the heat extracted by controlling the temperature of the reaction of Eqn. 6 may be transferred to the separated precipitate, to contribute to the thermal decomposition of the hydroxide(s) it contains. Thus the heat derived from cooling the sodium oxide, or the sodium hydroxide derived therefrom, as well as the heat generated by the reaction of Eqn. 6, can be re-used, rather than being wasted.

[0076] If, for example, the sodium oxide has been produced in a redox reaction between the liquid sodium and an oxide of another metal to produce the other metal in elemental form and the sodium oxide, then this sodium oxide may have an admixture of gangue, such as one or more aluminosilicate minerals, which may be derived from an ore comprising the oxide of the other metal. As already noted above, if the solid end-product were produced by the traditional calcination process, it would contain such gangue species anyway. The sodium oxide with the gangue mixed therein may therefore be added directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, in which case, the gangue from the ore comprising the oxide of the other metal would join any remaining gangue instead derived from the ore comprising the carbonate mineral. However, just as the gangue derived from the ore comprising the carbonate mineral can be removed by phase separating it from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride as described above, so the gangue derived from the ore comprising the oxide of the other metal may also be removed by a technique which includes hydrating the sodium oxide to produce sodium hydroxide, before this sodium hydroxide is then added to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. The technique by which the gangue may be removed from the sodium oxide is as follows.

[0077] The hot sodium oxide from the redox reaction, with the gangue still mixed therein, is firstly cooled closer to ambient temperature (for example, to less than about 100 “Celsius). Both the sodium oxide and the gangue are then progressively adding to liquid water. The sodium oxide readily dissolves in the water, whereas gangue species like aluminosilicates do not. The sodium oxide is hydrated to produce an aqueous solution of sodium hydroxide, according to the equation:

[0078] Na2O (S) + H2O t) — > 2NaOH <aq) [Eqn. 7]

[0079] The quantity of water to which the sodium oxide and gangue are added should be enough to create an aqueous solution, in which the final concentration of sodium hydroxide after they have been added to it is less than about 2.5 M ( / '. e., less than about 100 g dm3), more preferably less than about 2 M. If the final concentration of sodium hydroxide is kept below about this level and if the temperature of the aqueous solution is also kept down, the sodium hydroxide will not react with gangue species like aluminosilicates to any appreciable extent. Adding the sodium oxide and gangue to the water, rather than the other way round, ensures that the final concentration of the aqueous solution is approached from below. As indicated above, the hydration reaction of Eqn. 7 is significantly exothermic, with an enthalpy of reaction, AHreaction = - 239 kJ at s.t.p. Therefore, this reaction should be controlled, for example by appropriate cooling, to keep the temperature of the aqueous solution substantially below its boiling point, for example at less than about 85 “Celsius, more preferably at less than about 75 “Celsius, and most preferably at less than about 65 “Celsius if the solution is at atmospheric pressure. At least some of the heat extracted from firstly cooling the hot sodium oxide and gangue and / or from cooling the reaction of Eqn. 7 may then be used to contribute to the thermal decomposition of the hydroxide(s) in the separated precipitate. Once the sodium oxide and gangue have been added to the water, the undissolved gangue species may be phase separated from the aqueous solution, for example by settlement under gravity, filtration and / or centrifugation, to leave just the aqueous solution of sodium hydroxide. This separation should be carried out soon after the reaction of Eqn. 7, so that the undissolved gangue species do not have time to start reacting with the sodium hydroxide. The remaining aqueous solution of sodium hydroxide may then be added directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. However, since this will also tend to reduce the concentration of the aqueous solution of sodium chloride thus formed, the aqueous solution of sodium hydroxide may instead first be dried to produce solid sodium hydroxide, which is then added to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride.

[0080] Thus by phase separating gangue from the sodium oxide as just described, and by phase separating gangue derived from the ore comprising the carbonate mineral from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, before these two are reacted together, the method described herein can produce a purer solid end-product than the traditional calcination process.

[0081] Preferably, the final concentration of the aqueous solution of sodium chloride formed by reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating this sodium oxide, with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride should be less than the saturation concentration of this aqueous solution of sodium chloride. This helps to ensure that the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide is not contaminated by sodium chloride. The final concentration of the aqueous solution of sodium chloride may be controlled to remain below its saturation concentration in one of several different ways. For example, if the sodium oxide is added directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, without firstly being hydrated to produce sodium hydroxide, the final concentration of the aqueous solution of sodium chloride depends only on the initial concentration of the hydrochloric acid used to dissolve the carbonate mineral. In such a case, the initial concentration of the hydrochloric acid should preferably be at least about 0.5 M because at lower concentrations than this, the rate of dissolution of the carbonate mineral in the hydrochloric acid may become unacceptably low. More preferably, the initial concentration of the hydrochloric acid should be at least about 3 M, more preferably still, at least about 4 M, and most preferably, at least about 5 M. This has the advantage that it reduces the amount of water present as a solvent in the resulting aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, and therefore the amount of water also present in the aqueous solution of sodium chloride after the sodium oxide has been added. Thus if this aqueous solution of sodium chloride is subsequently dried to recover the sodium chloride from it as a solid which is recycled for electrolysis, since water has a high specific heat capacity, this in turn reduces the amount of thermal energy required to evaporate or boil off the water of solution.

[0082] On the other hand, the initial concentration of the hydrochloric acid should preferably not be more than about 6 M. This is because after the sodium oxide reacts with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, the resulting aqueous solution of sodium chloride reaches saturation at a molarity not much above 6 M. For example, the saturation molarity of sodium chloride in water at 25 “Celsius is 6.14 M, which rises to about 6.5 M at 70 “Celsius. Contamination of the precipitate by solid sodium chloride can thereby be avoided. Moreover, at atmospheric pressure, the initial concentration of the hydrochloric acid should preferably be not more than about 20% by weight or about 6 M because an aqueous solution of hydrogen chloride with a molarity less than this always boils at a temperature above 100 “Celsius under one atmosphere of pressure. The boiling point of the hydrochloric acid therefore remains far enough above its preferred maximum initial temperature at atmospheric pressure of 85 “Celsius to avoid any substantial evaporation or risk of boiling of the hydrochloric acid or of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride.

[0083] If, on the other hand, the sodium oxide is first hydrated to produce sodium hydroxide, which is then added to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, the final concentration of the aqueous solution of sodium chloride depends not only on the initial concentration of the hydrochloric acid, but also on the initial concentration of the sodium hydroxide. If the sodium hydroxide is solid but not anhydrous or if it is in aqueous solution, this tends to dilute the final concentration of the aqueous solution of sodium chloride. Thus in comparison to when the sodium oxide is added directly to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride without firstly being hydrated, the initial concentration of the hydrochloric acid can be increased to compensate for this dilution of the final concentration of the aqueous solution of sodium chloride, without also increasing the risk of the precipitate being contaminated by solid sodium chloride.

[0084] After the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, has reacted with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, at least some of the resulting precipitate is phase separated from the aqueous solution of sodium chloride. The precipitate may be separated from the aqueous solution of sodium chloride by at least one of settlement under gravity, filtration and centrifugation, for example. Even if the final concentration of the aqueous solution of sodium chloride is less than its saturation concentration, the separated precipitate should preferably also be washed with hot water to remove any remaining aqueous solution of sodium chloride from the separated precipitate, apart from any possible admixture of solid sodium chloride. The hydroxide(s) in the separated precipitate may then be subjected to thermal decomposition to produce the solid end-product and water vapour. Referring back to Fig. 2, it may be seen that the solubility curve of magnesium hydroxide overlaps those of both ferrous hydroxide and calcium hydroxide. Therefore, co-precipitation of magnesium hydroxide with either ferrous hydroxide or with calcium hydroxide at any given pH is hard to avoid. However, the solubility curves of ferrous hydroxide and calcium hydroxide are sufficiently differentiated that if desired, a similar process of sequential precipitation and phase separation to that described above in relation to the precipitation of different gangue mineral species at different pHs may also be carried out to precipitate out ferrous hydroxide separately from calcium hydroxide. Thus in some embodiments, reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the 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, and phase separating at least some of this precipitate from the aqueous solution of sodium chloride, may respectively comprise the following sequence of operations. Firstly, add sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to precipitate out ferrous hydroxide at a pH of from about 8 to about 12, and phase separate at least some of the ferrous hydroxide from the aqueous solution of sodium chloride. Then, continue to add sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution of sodium chloride to precipitate out calcium hydroxide at a pH of more than about 12.5, and phase separate at least some of the calcium hydroxide from the aqueous solution of sodium chloride.

[0085] This process may best be understood by referring to Fig. 2 again. In the region of this graph labelled “C”, ferrous hydroxide and magnesium hydroxide both become increasingly insoluble as more sodium oxide or hydroxide is added and the pH of the solution increases, whereas calcium hydroxide is still sufficiently soluble to remain in solution. The ferrous hydroxide and / or magnesium hydroxide may therefore be phase-separated as described above. As more sodium oxide or hydroxide is added, the pH of the solution continues to increase, passing into the region of the graph labelled “D”. Here, the solubility of calcium hydroxide drops, causing its precipitation at a pH above about 12.5. If magnesium hydroxide has already co-precipitated out and been removed with ferrous hydroxide, the calcium hydroxide may then be phase-separated from the aqueous solution at this high pH. If, however, no ferrous hydroxide was present in the aqueous solution initially, precipitation of magnesium hydroxide may induce co-precipitation of calcium hydroxide at a somewhat lower pH, and both may then be phase-separated from the aqueous solution.

[0086] If the carbonate mineral ore comprises siderite, the ferrous hydroxide which is precipitated is derived from iron (II) ions originally present in the siderite ( / '.e., iron (II) carbonate (FeCOs)). However, care should be taken to avoid these iron (II) ions from being oxidised into iron (III) ions by, for example, atmospheric oxygen. This is because iron (III) ions precipitate out as ferric hydroxide (Fe(OH)s) at a pH of about 7 to 8, which as Fig. 2 shows, would risk contamination of the precipitate by aluminosilicate gangue species. In contrast, this can be avoided if the iron (II) ions instead precipitate out as ferrous hydroxide (Fe(OH)2) at a higher pH of about 11 to 12, which also allows the ferrous hydroxide to be thermally decomposed at a lower temperature. This can be achieved because the carbonate mineral is dissolved in hydrochloric acid closed off from their surrounding environment, so that an anoxic atmosphere can be maintained above the aqueous solution thus formed.

[0087] After the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide has been phase-separated from the alkaline aqueous solution, the supernatant remaining after this phaseseparation essentially comprises an aqueous solution of sodium chloride with an excess of sodium hydroxide. It is desirable, for reasons of economy and efficiency, that at least some of this sodium chloride should be recovered and recycled. In some embodiments, therefore, the method of the invention may optionally comprise, after phaseseparating at least some of the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide from the aqueous solution of sodium chloride, adding further hydrochloric acid to this aqueous solution to reduce its pH and produce a neutral aqueous solution of sodium chloride at least some of which may be used as feed material for producing the liquid sodium and chlorine gas by electrolysis.

[0088] Thermal Decomposition

[0089] The thermal decomposition may be carried out either in an oxygenated atmosphere or in the absence of oxygen, depending on the initial composition of the separated precipitate and on the desired composition of the solid endproduct, as described further below. Immediately after the thermal decomposition, the solid end-product and the water vapour will both be hot. The solid end-product may be allowed to remain hot, for example if it mainly comprises calcium and / or magnesium oxide intended for use as an ingredient in the manufacture of cement clinker, or if it mainly comprises iron oxide, which is to be used as an ingredient in ironmaking. Alternatively, it may be cooled down towards ambient temperature, in which case, at least some of the heat extracted by cooling the solid end-product may be transferred to the separated precipitate, to contribute to the thermal decomposition. The water vapour may also be allowed to remain hot, in which case, if the thermal decomposition is carried out in the absence of oxygen, it may be captured and supplied as a reagent to an RDR, as described above. Alternatively, it may be cooled down towards ambient temperature and possibly also condensed, in which case, at least some of the heat extracted by cooling the water vapour, as well as possibly also from condensing it, may be transferred to the separated precipitate, to contribute to the thermal decomposition.

[0090] The crystalline microstructure of the calcium oxide, magnesium oxide and / or iron oxide(s) in the solid end-product may vary, depending on how the precipitate has been separated from the aqueous solution of sodium chloride and on the conditions for the subsequent thermal decomposition. "Optimizing Reactivity of Light-Burned Magnesia through Mechanical Milling” by A. Khalil et al., Ceramics International, Vol. 45, No. 17, Part B, pp. 22821-22828 (2019) describes how the chemical activity of "light-burnt” magnesia produced by the traditional calcination process can be altered by mechanical milling, such that the chemical activity increases or decreases, depending on the duration of the milling. In some embodiments, therefore, wherein the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium, the method described herein may further comprise altering the chemical activity of the solid end-product by milling. Thus the method described herein can be used to produce a solid end-product having a desired chemical activity, without the need to inject any more heat into the solid endproduct. For example, varieties of magnesia corresponding to the "light-burnt” and "dead-burnt” products produced by the traditional calcination process can also be produced by the method described herein.

[0091] Optional Carbonation

[0092] Apart from producing the solid end-product as described above, the method of the invention also comprises dissolving the carbonate mineral in the hydrochloric acid closed off from their surrounding environment and capturing the gaseous carbon dioxide released from the carbonate mineral. At least some of the captured carbon dioxide can then be sequestered and / or mineralized. For example, at least some of the captured carbon dioxide may be mineralized by reacting it either with sodium oxide to produce sodium carbonate, according to the equation:

[0093] CO2 <g) + Na20 (S) — Na2CO3 <S) [Eqn. 8] or with sodium hydroxide to produce sodium carbonate and / or sodium hydrogencarbonate, according to the equations:

[0094] The carbonation reactions of Eqns. 8, 9a and 9b are all highly exothermic and therefore release a large quantity of heat. Comparing the enthalpy of reaction of Eqn. 1 with the enthalpy of reaction of Eqn. 8 shows that the nett reaction which can be written by adding Eqns. 1 and 8 together:

[0095] CaC03(S) + Na20(s)— > CaO(s)+ Na2CO3<S) [Eqn. 10] is actually also exothermic, with an enthalpy of reaction, AHreaction = - 142.7 kJ at s.t.p. (as may be derived from the standard enthalpies of formation of the reagents and their products). Similarly, comparing the enthalpy of reaction of Eqn. 1 with the enthalpies of reaction of Eqns. 9a and 9b shows that the nett reactions which can be written by adding Eqn. 1 to either Eqn. 9a or 9b:

[0096] CaC03(S) + 2 NaOH(s)-> CaO(s)+ Na2CO3<S) + H2OM[Eqn. 11 a] and CaCOs (S> + NaOH (S> — > CaO (S> + NaHCOs (S> [Eqn. 11 b] are only mildly endothermic, with enthalpies of reaction, AHreaction = + 7.1 kJ and + 47 kJ, respectively, at s.t.p. (as may also be derived from the standard enthalpies of formation of the reagents and their products). In the case when Q = Mg, the corresponding values are - 219.9 kJ for Eqn. 10, - 70.4 kJ for Eqn. 11 a and - 30.9 kJ for Eqn. 11 b, all of which are exothermic. And when Q = Fe, the corresponding values are - 246 kJ for Eqn. 10, - 75.8 kJ for Eqn. 11 a and - 56.3 kJ for Eqn. 11 b, all of which are also exothermic. Therefore, according to Hess's Law, it should be possible to decompose a carbonate mineral of at least one of calcium, magnesium and iron into its corresponding oxide(s) and carbon dioxide, and to mineralize the carbon dioxide this releases by producing sodium carbonate or sodium hydrogencarbonate, whilst consuming very little, if any, energy. In other words, it makes little sense to expend a large quantity of heat on extracting an oxide such as lime from a carbonate mineral ore such as limestone using the traditional calcination process, if much or all of that heat is going to be released again by mineralizing the carbon dioxide with either sodium oxide or sodium hydroxide.

[0097] However, the nett reactions of Eqns. 10, 11 a and 11 b all suffer from the problem that the two starting reagents in each case, namely the carbonate mineral on the one hand and either sodium oxide or sodium hydroxide on the other, are both in their solid state at s.t.p., and therefore will not react with each other unless at least one of them is firstly mobilized. In the case of the reaction of Eqn. 10, this could be done, for example, by hydrating the sodium oxide to make anhydrous sodium hydroxide (which would just convert Eqn. 10 into Eqns. 11 a and 11b) and then dissolving the anhydrous sodium hydroxide of Eqns. 11 a and 11 b in liquid water to make an aqueous solution of sodium hydroxide. However, even before the anhydrous sodium hydroxide is dissolved in water, the Gibbs free energy of Eqn. 11 a ~ 0 at s.t.p., so that the reagents and products of Eqn. 11 a are in thermodynamic equilibrium with each other, and the Gibbs free energy of Eqn. 11 b is positive at s.t.p., so that converting the starting reagents into the desired end products cannot be achieved without changing the reaction conditions significantly.

[0098] The present invention solves this problem by using electrical energy instead of heat to produce the precursors for a sequence of reactions which decomposes the carbonate mineral into its corresponding oxide(s) and carbon dioxide, and which is exothermic overall. Thus the carbon dioxide is liberated from the carbonate mineral and can subsequently react with sodium oxide or with sodium hydroxide to produce sodium carbonate and / or sodium hydrogencarbonate as in Eqns. 8, 9a or 9b without requiring any additional energy input. Moreover, since these carbonation reactions are all highly exothermic, at least some of the heat they release may also be transferred to the separated precipitate to contribute to the thermal decomposition. Furthermore, the same electrolysis and oxidation of the liquid sodium, as well as possibly also hydration of the sodium oxide this produces, can be used to produce the sodium oxide, or the sodium hydroxide derived therefrom, not only for reaction with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, but also for reaction with the carbon dioxide liberated from the carbonate mineral.

[0099] In some embodiments, therefore, the sodium oxide, or the sodium hydroxide derived therefrom, which is reacted with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, may be or be derived from a first portion constituting only about half of the total number of mols of the sodium oxide produced by oxidising at least some of the liquid sodium. In practice, this first portion may constitute between about 5 / 12 and about 7 / 12, inclusive, of the total number of mols of the sodium oxide because of the presence of gangue, at least in the ore comprising the carbonate mineral, and if the sodium oxide has been produced by a redox reaction between the liquid sodium and an oxide of another metal, also in the ore comprising the oxide of the other metal. In such embodiments, the method may further comprise reacting at least some of the captured carbon dioxide with a second portion of the sodium oxide, or with sodium hydroxide derived from hydrating at least some of this second portion of the sodium oxide, in a carbonation reaction to produce at least sodium carbonate, and transferring heat to the separated precipitate from at least one of this carbonation reaction and the sodium carbonate thus produced. Some ways in which this carbonation reaction may be carried out are described below. This technique therefore differs from applying an existing CCS or CCU technology to the traditional calcination process, in that the captured carbon dioxide is instead permanently mineralized as sodium carbonate. This has the advantage that since sodium carbonate is an important industrial product in its own right, mineralizing the carbon dioxide in this way is more economically attractive than CCS or CCU.

[0100] As described above, if the sodium oxide has been produced by a redox reaction with an oxide of another metal to produce the other metal in elemental form and the sodium oxide, the first portion of sodium oxide may be hydrated to remove gangue derived from the ore comprising the oxide of the other metal, in order to control the purity of the solid end-product. Similarly, therefore, in order to control the purity of the sodium carbonate produced by the carbonation reaction, so the second portion of sodium oxide may also be hydrated to remove gangue derived from the same source by using the same technique, before the sodium hydroxide from this hydration reaction is introduced into the carbonation reaction.

[0101] Dividing the total number of mols of the sodium oxide into the first and second portions thereof may be carried out in time or in space, or in a combination of both. In other words, at least some of the sodium oxide may be divided into the first and second portions temporally by producing the first portion first and then the second portion thereafter (or vice versa), spatially by producing the first and second portions at the same time as each other and then separating them in space, or by a combination of the two. Since in some embodiments, the method may comprise hydrating at least one of the first and second portions of sodium oxide to produce sodium hydroxide, hydration of the first and / or second portions of sodium oxide may be conducted either before or after dividing at least some of the total number of mols of sodium oxide into the first and second portions.

[0102] In some embodiments, hydrating at least one of the first and second portions of sodium oxide to produce sodium hydroxide may comprise passing at least one of the first and second portions of sodium oxide through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed. Since sodium oxide is powerfully hygroscopic, it absorbs water vapour from the calcium hydroxide, magnesium hydroxide and / or ferrous hydroxide in the separated precipitate to produce solid-phase sodium hydroxide. According to Le Chatelier's principle, this encourages the thermal decomposition of the hydroxide(s) in the separated precipitate by reducing the partial pressure of water vapour within the atmosphere to which the precipitate is exposed. Moreover, since the sodium oxide may still be hot from the oxidation of the liquid sodium, heat may also be transferred from the sodium oxide to the separated precipitate, thereby contributing to the thermal decomposition of the respective hydroxides therein. Hydrating the sodium oxide in this manner is also strongly exothermic, which contributes more heat to the thermal decomposition of the respective hydroxides. Thus heat from hydrating the sodium oxide, as well as possibly also from the oxidation of the liquid sodium, can be effectively and efficiently recycled. Thereafter, if desired, the solid-phase sodium hydroxide thus formed may still be added to liquid water to produce an aqueous solution of sodium hydroxide, from which gangue species may be phase-separated as described above, before a first or second portion of the resulting sodium hydroxide is respectively either added to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, or used in the carbonation reaction.

[0103] In some embodiments, the carbonation reaction may comprise passing the second portion of the sodium oxide or solid sodium hydroxide derived therefrom through an atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid. This has the advantage that since the sodium oxide or the solid sodium hydroxide derived therefrom may already be hot, either from the oxidation of the liquid sodium or from the hydration of the sodium oxide, the rate of reaction of the captured carbon dioxide with the sodium oxide or with the solid sodium hydroxide is thereby increased. Moreover, since the sodium oxide or the solid sodium hydroxide is also likely to be a finely divided precipitate, it can readily react with and absorb the carbon dioxide. This therefore reduces the partial pressure of carbon dioxide in the atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid, which according to Le Chatelier's principle, in turn pulls Eqn. 5b to the right, thereby increasing the rate of dissolution of the carbonate mineral.

[0104] Alternatively, the captured carbon dioxide may be transported from a gas-tight reaction vessel, in which the carbonate mineral is dissolved in the hydrochloric acid, to another reaction vessel, in which the carbonation reaction may be carried out instead. In such a case, the carbonation reaction may comprise bubbling the carbon dioxide through an aqueous solution of sodium hydroxide derived from hydrating at least some of the second portion of the sodium oxide, for example. In another example, the other reaction vessel may instead comprise a fluidized bed reactor containing the hot second portion of the sodium oxide, into which the gaseous carbon dioxide is introduced. In the latter case, consumption of the carbon dioxide by the second portion of sodium oxide creates a pressure difference between the two reaction vessels, which helps to propel the gaseous carbon dioxide from where it is produced to where it is consumed.

[0105] The atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid may just be ordinary atmospheric air, since the solubility of oxygen in aqueous solution is negligible and also decreases with increasing temperature, whereas the other main constituents of atmospheric air, nitrogen and argon, are both inert. Any water vapour or carbon dioxide also present in the atmospheric air will form a dynamic equilibrium with the reaction mixture of Eqns. 5a and 5b at the respective partial pressures of carbon dioxide and water prevailing at the temperature of the liquid-vapour system. The air can therefore act as a carrier gas for the carbon dioxide released from the carbonate mineral.

[0106] In any event, if the second portion of sodium oxide reacts directly with the captured carbon dioxide, then the product is just anhydrous sodium carbonate, according to the reaction of Eqn. 8, but with any possible admixture of gangue in the second portion of sodium oxide still present. This may be acceptable, depending on the intended end use of the sodium carbonate. If, however, sodium hydroxide derived from hydrating at least some of the second portion of sodium oxide in order to remove such gangue is instead reacted with the captured carbon dioxide, although the sodium hydroxide may be free of gangue, the products of this reaction may also be more complex. This is because both the reactions of Eqns. 9a and 9b are thermodynamically favoured at s.t.p. This will also apply if a small proportion of water vapour which has evaporated from the hydrochloric acid is mixed in with the captured carbon dioxide. Reacting the captured carbon dioxide with the sodium hydroxide at s.t.p. would produce the following mixture of solid products in the following mole fractions at equilibrium: sodium hydrogencarbonate (NaHCOs) 55% anhydrous sodium carbonate (Na2CO3) 27% sodium carbonate monohydrate (Na2CC>3 ■ H2O) 15% trona (Na3CO3HCO3 ■ 2H2O) 3%

[0107] Such a mixture of products may again be acceptable, if, for example, it is to be used as an ingredient in the manufacture of soda-lime glass. However, these mol fractions also change with temperature. Therefore, if a purer form of sodium carbonate is desired instead, in some embodiments, at least some of the captured carbon dioxide may be reacted with at least some of the solid sodium hydroxide derived from the second portion of sodium oxide at afinal temperature of from about 310 to about 400 “Celsius, inclusive. In this temperature range, such a reaction produces anhydrous sodium carbonate as the only solid-phase product when it reaches equilibrium, together with a reduced amount of gaseous carbon dioxide and water vapour, according to the equation:

[0108] 2 NaOH (S) + 2 CO2 (g> — > Na2CO3 <S) + CO2 (g> + H2O (gj [Eqn. 12]

[0109] This is equivalent to the reaction of Eqn. 9a on its own, without the reaction of Eqn. 9b, at s.t.p. The reduced amount of carbon dioxide and the water vapour thus produced then form a dynamic equilibrium with the reaction mixture of Eqns. 5a and 5b, as described above. Such an embodiment therefore has the advantage that it can be used to produce sodium carbonate as the only solid-phase product of the carbonation reaction, and without any admixture of gangue.

[0110] Unlike the reaction of Eqn. 8, which tends to reduce the total pressure of the atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid by reducing the partial pressure of carbon dioxide therein, the reaction of Eqn. 12 also has the advantage that it has little or no effect on the total pressure of the atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid. This is because the carbon dioxide removed from the atmosphere by the reaction of Eqn. 12 is replaced by the equimolar amount of water vapour which is also produced. Thus if the carbonate mineral is dissolved in the hydrochloric acid at above atmospheric pressure, as described above, the carbonation reaction of Eqn. 12 may also be carried out at the same pressure, and if so, this will have little if any effect on the rate of reaction of Eqn. 12, for the same reason.

[0111] The carbonation reaction of Eqn. 12 should not be allowed to exceed about 400 “Celsius because at this temperature, the reaction of Eqn. 12 starts to reverse, so that the anhydrous sodium carbonate it produces starts to decompose in the presence of water vapour back into sodium hydroxide and carbon dioxide. Whereas the reaction of Eqn. 12 should reach a final temperature above about 310 “Celsius, the initial temperature of the solid sodium hydroxide should preferably be either near to the bottom of this temperature range or somewhat cooler, since sodium hydroxide melts at 323 “Celsius. On the other hand, since the carbonation reaction is strongly exothermic, the desired final temperature can easily be reached and the heat extracted from the reaction can be used to ensure that the top of the range of operating temperatures is not exceeded. The sodium carbonate thus produced also has a melting point well above the top of the desired temperature range for this reaction. Furthermore, even if the temperature of the carbonation reaction is controlled in this way, the sodium carbonate it produces will also have a temperature of up to about 400 “Celsius and will carry with it at least some of the heat generated during the reaction. Heat may therefore also be extracted from the sodium carbonate after its production to cool it down towards ambient temperature, and at least some of the heat extracted may be used to contribute to the thermal decomposition of the hydroxide(s) in the separated precipitate.

[0112] Thus some embodiments of the invention may comprise a carbonation reaction which is able to produce pure sodium carbonate as a co-product. Therefore, apart from being a method of producing an oxide from a carbonate mineral, which also produces sodium carbonate as a co-product, these embodiments may instead be seen as a method of producing sodium carbonate, which produces at least one of calcium oxide, magnesium oxide and iron oxide as a co-product. Until now, sodium carbonate has generally been produced industrially by one of two main processes. The first is the ammonia-soda, or Solvay, process. The second is by the thermal decomposition of either naturally occurring nahcolite ( / '.e., sodium hydrogencarbonate, NaHCOs) or naturally occurring trona (NasCOsHCOs ■ 2H2O), both of which are mined. Such embodiments of the invention may therefore also be considered as a replacement for both of these existing processes.

[0113] The Solvay process includes as one of its process steps the calcination of limestone described above, and uses ammonia as a reaction intermediary, which has potential problems not just as a contaminant of the sodium carbonate, but also as a pollutant of the environment. The Solvay process 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. In comparison to the Solvay process, therefore, the method described herein at least has the advantages that it avoids the calcination of limestone completely, the use of hazardous ammonia, and the overproduction of calcium chloride.

[0114] Mining naturally occurring trona or naturally occurring nahcolite has the disadvantage that it extracts mineralized carbon dioxide from geological deposits, which is then released as a gas by their thermal decomposition to produce sodium carbonate. In comparison to mining trona or nahcolite, therefore, sodium carbonate produced as described above has the advantage that it does not extract any new carbon dioxide from geological deposits. In other words, sodium carbonate produced in this way contains carbon dioxide from the carbonate mineral, which has already been mined to produce the oxide derived from the carbonate mineral. Thus the total amount of carbon dioxide extracted from geological deposits to produce both this oxide and the sodium carbonate is effectively halved. Further Optional Method Features

[0115] As mentioned above, at least some of the sodium chloride required for the electrolysis, as well as at least some of the water used to make the hydrochloric acid, may be regenerated from the method of the invention in a closed loop. For example, if the electrolysis comprises electrolysing solid sodium hydroxide, at least some of the water vapour produced by drying the aqueous solution of sodium hydroxide may be captured and condensed, and then used as at least some of the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid. In another example, at least some of the aqueous solution of sodium chloride produced by reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride may be used as feed material for the electrolysis. For example, at least some of the aqueous solution of sodium chloride thus produced may itself be electrolysed. Alternatively or additionally, at least some of it may be dried to produce solid sodium chloride, which is then electrolysed. In either case, this has the advantage that the total sodium chloride consumption can be reduced by up to a half. In the latter case, at least some of the water vapour produced by drying the aqueous solution of sodium chloride may be captured and condensed, and then used as at least some of the liquid water in which the hydrogen chloride is dissolved to produce the hydrochloric acid. Apart from reducing the amount of water consumed, recycling water which has been condensed from drying either the aqueous solution of sodium hydroxide or the aqueous solution of sodium chloride has the following additional advantage. If the hydrochloric acid is produced by a technique which includes an RDR as described above, the liquid water can then already be at a sufficient temperature to prevent unreacted chlorine gas and oxygen produced by the RDR from dissolving in the water along with the hydrogen chloride gas. In other words, heat is recycled along with the water. Further alternative ways in which salt and / or water may be regenerated from the method described herein are described in the present applicant's UK patent application no. 2417075.5 ("Industrial Chemical Process and Apparatus”; applicant's ref: NE-P-GB 007), whereby no nett amount of salt or water need be consumed at all.

[0116] As mentioned above, the carbonate mineral may be dissolved in the hydrochloric acid at above atmospheric pressure to allow the temperature of the hydrochloric acid to be increased as well without any substantial evaporation from the hydrochloric acid or risk of it boiling. In some embodiments, this increased pressure may be maintained even after the carbonate mineral has dissolved in the hydrochloric acid. This then allows the precipitation reaction of Eqn. 6 to be carried out at a higher temperature as well, possibly even above 100 “Celsius, whilst still suppressing vaporization of water from the aqueous solution of sodium chloride formed by the reaction of Eqn. 6. This therefore avoids possible saturation of the aqueous solution of sodium chloride as a result of such evaporation or from it boiling, until after the precipitation has occurred. For example, at atmospheric pressure, a saturated aqueous solution of sodium chloride boils at 109 “Celsius. Carrying out the precipitation reaction at a higher temperature has several advantages, as follows. Firstly, since the solubility of sodium chloride increases with temperature, the risk of the resulting precipitate being contaminated with sodium chloride is thereby reduced. However, when Q = Ca, the calcium hydroxide also precipitates out more easily, since calcium hydroxide has inverse or retrograde solubility, which decreases with increasing temperature. When Q = Mg, although magnesium hydroxide has normal or prograde solubility, which increases with increasing temperature, the solubility of magnesium hydroxide in aqueous solution is considerably lower than that of calcium hydroxide, even at temperatures approaching the boiling point of water. Both calcium hydroxide and magnesium hydroxide may therefore be precipitated out from the aqueous solution of sodium chloride, even at such an increased temperature. When Q = Fe, ferrous hydroxide is even less soluble than magnesium hydroxide, and therefore readily precipitates out at a higher temperature as well.

[0117] Secondly, once the precipitate has been phase separated from the aqueous solution of sodium chloride, carrying out the reaction of Eqn. 6 at a higher temperature also reduces the amount of additional heat required to thermally decompose the hydroxide(s) in the separated precipitate into the solid end-product and water vapour, because the precipitate is already at a temperature well above ambient, and possibly even above 100 “Celsius.

[0118] Thirdly, after the precipitate has been phase separated from the aqueous solution of sodium chloride, if the remaining aqueous solution of sodium chloride is then dried to produce solid sodium chloride, carrying out the reaction of Eqn. 6 at a higher temperature also decreases the amount of heat required to evaporate or boil off the water of solution because this remaining aqueous solution of sodium chloride is still at a higher temperature as well. If the water vapour thus produced is then captured and condensed, and used to dissolve the hydrogen chloride gas to make the hydrochloric acid in which the carbonate mineral is dissolved, with suitable thermal insulation, much of the heat can be preserved in a closed loop.

[0119] More particularly, in some embodiments, the hydrochloric acid may have an initial temperature, before dissolving the carbonate mineral, which is at least about 15 “Celsius below the boiling point of the acid at an initial pressure at which dissolving the carbonate mineral in the hydrochloric acid is started. Dissolving the carbonate mineral in the hydrochloric acid may then be continued at increasing pressure until reaching a final pressure. If so, reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride may be conducted at the final pressure and at increasing temperature until reaching a final temperature above the boiling point of the aqueous solution of sodium chloride at the initial pressure and below its boiling point at the final pressure. After phase separating the resulting precipitate, the remaining aqueous solution of sodium chloride may then be dried by reducing the pressure of an atmosphere above the aqueous solution of sodium chloride to at or below the initial pressure. Such a procedure has the advantage that it is a highly thermally efficient way of drying the aqueous solution of sodium chloride which remains after separating the precipitate therefrom. Dissolving the carbonate mineral in the hydrochloric acid may be carried out adiabatically, or substantially adiabatically, resulting in a small temperature increase accompanying the increase in pressure, as a result of the mildly exothermic nature of the reactions of Eqns. 5a and 5b.

[0120] If such embodiments also comprise a carbonation reaction with at least some of the captured carbon dioxide as described above, this carbonation reaction is preferably carried out during and / or after dissolving the carbonate mineral in the hydrochloric acid, but before reacting at least some of the sodium oxide, or sodium hydroxide derived therefrom, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. This therefore ensures that carbon dioxide released by dissolving the carbonate mineral in the hydrochloric acid does not dissolve in the (neutral) aqueous solution of sodium chloride. Moreover, at least some of the heat from the carbonation reaction may also contribute towards increasing the temperature at which the sodium oxide, or sodium hydroxide derived therefrom, reacts with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride.

[0121] As noted above, the carbonate mineral may comprise a minority of one or more cation substituents. If the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium (for example, if the carbonate mineral is calcite, magnesite or dolomite), these are most likely to be iron and / or manganese cations, substituted for some of the calcium and / or magnesium cations in the carbonate mineral. If so, the iron and / or manganese cations also dissolve in the hydrochloric acid and thereafter follow the same reaction pathway as the calcium and / or magnesium cations. They would therefore end up in the solid end-product as well, unless somehow removed. The presence of iron and / or manganese in the solid end-product presents no particular problem, as they would also be present if the solid end-product had been produced via the traditional calcination process instead. However, in some embodiments, if desired, the proportion of iron and / or manganese cations substituted for the calcium and / or magnesium cations in the solid end-product may be reduced as described further below.

[0122] If the carbonate mineral instead has a majority of iron cations (for example, if the carbonate mineral is siderite), the most likely substituents are magnesium and / or manganese cations substituted for iron cations in the siderite. These also both dissolve in the hydrochloric acid and subsequently follow the same reaction pathway as the iron. They would therefore end up in the solid end-product as well, unless somehow removed. Once again, this presents no particular problem, since in this case, the solid end-product can still be treated using traditional beneficiation techniques for hematite and magnetite ores. For example, if the solid end-product contains a particularly high proportion of manganese oxide, the mixture of iron and manganese oxides in the solid end-product may subsequently be used as feed material for the production of austenitic manganese and / or stainless steels. However, in some embodiments, if desired, the proportion at least of the magnesium cations, and possibly also of the manganese cations, substituted for the iron cations in the solid end-product may be reduced as also described further below. The hydroxides and oxides of calcium and magnesium are all diamagnetic. T able 1 below gives the molar magnetic susceptibilities, Xmoicgs, at s.t.p. of manganese hydroxide and of some manganese oxides, all of which are paramagnetic:

[0123] Table 1

[0124] Ferrous hydroxide (Fe(OH)2) and ferrous oxide (FeO) are also both paramagnetic, whereas ferric oxide (Fe2C>3) is a canted antiferromagnet at temperatures above its Morin transition temperature (= 260 K) and below its Neel temperature (= 950 K). Accordingly, if the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium and a minority of cations comprising at least one of iron and manganese, the proportion of iron and / or manganese cations in the solid end-product may be reduced in one of two ways, as follows.

[0125] In some embodiments, the method may comprise phase separating the precipitate from the aqueous solution of sodium chloride in the absence of oxygen, and then magnetically separating at least one of ferrous hydroxide and manganese hydroxide from at least one of calcium hydroxide and magnesium hydroxide in the separated precipitate in the absence of oxygen and at a temperature of less than about 90 “Celsius, more preferably less than about 70 “Celsius, and most preferably less than about 50 “Celsius, before thermally decomposing the hydroxide(s) of at least one of calcium and magnesium, either in an oxygenated atmosphere or in the absence of oxygen as desired, to produce the solid end-product. Since manganese hydroxide readily oxidises, even at ambient temperature, in the presence of oxygen into manganese (III) oxide (M^Os) and / or manganese dioxide (MnO2) and water vapour, phase separating the precipitate from the aqueous solution of sodium chloride in the absence of oxygen and then carrying out the magnetic separation in the absence of oxygen as well ensures that the manganese hydroxide is not converted into manganese dioxide, which as Table 1 shows, has significantly lower magnetic susceptibility than manganese hydroxide. Moreover, since the magnetic susceptibility of paramagnetic materials is inversely proportional to their temperature, carrying out the magnetic separation at a temperature of less than about 70 “Celsius ensures that both the ferrous hydroxide and the manganese hydroxide retain sufficient magnetic susceptibility to be separated from the diamagnetic calcium hydroxide and / or magnesium hydroxide. Thus the proportion of any iron and / or manganese cations in the solid end-product may both be reduced.

[0126] Alternatively, the method may comprise conducting the thermal decomposition of the hydroxide(s) in the separated precipitate in an oxygenated atmosphere at a temperature of from about 600 “Celsius to about 800 “Celsius, cooling the solid end-product down thereafter, and then magnetically separating at least one of ferric oxide (Fe2Os) and manganese (III) oxide (M^Os) from at least one of calcium oxide and magnesium oxide in the solid end- product at a temperature of less than about 90 “Celsius, more preferably less than about 70 “Celsius, and most preferably less than about 50 “Celsius. Exposing the manganese hydroxide in the separated precipitate to an oxygenated atmosphere causes it to oxidise into manganese dioxide (MnO2) and water vapour. Transferring heat to the manganese dioxide when still in the presence of oxygen and within the stated temperature range then causes the manganese dioxide to thermally decompose into manganese (III) oxide and release oxygen gas according to the equation:

[0127] 4 MnO2 (S) — 2 M^Os <S) + 02 <g) [Eqn. 13]

[0128] This has the advantage that whereas manganese dioxide and manganese (III) oxide are both paramagnetic, as Table 1 shows, the magnetic susceptibility of manganese (III) oxide is more than 6 times greater than that of manganese dioxide. If the separated precipitate comprises any ferrous hydroxide, thermal decomposition converts this into ferric oxide (Fe2O3), as described above. In such a case, therefore, if a significant proportion (for example, more than about 4% by weight) of ferrous hydroxide is present, the thermal decomposition should preferably be carried out at a temperature of less than about 650 “Celsius, which is the Tammann temperature of Fe2O3. This avoids sintering the ferric oxide produced by the thermal decomposition, which otherwise might interfere with the magnetic separation, and also has the advantage of reducing the total quantity of heat required for the thermal decomposition. Although the magnetic susceptibility of ferric oxide remains roughly constant or increases somewhat with temperature at temperatures well below its Neel temperature, conducting the magnetic separation at a temperature of less than about 90 “Celsius ensures that the manganese (III) oxide retains sufficient magnetic susceptibility to be separated from the diamagnetic calcium oxide and / or magnesium oxide, along with the ferric oxide. Moreover, whereas the magnetic susceptibility of pure ferric oxide is less than that of manganese (III) oxide, the magnetic susceptibility of any ferric oxide present may be enhanced in the presence of basic oxides such as calcium oxide and magnesium oxide by the formation of their respective ferrites during the thermal decomposition. Thus the proportion of any iron and / or manganese cations in the solid end-product may both be reduced.

[0129] If, on the other hand, the carbonate mineral has a majority of iron cations and a minority of cations comprising both magnesium and manganese, the proportion at least of the magnesium cations, and possibly also of the manganese cations, in the solid end-product may be reduced in one of two ways, as follows.

[0130] In some embodiments, the method may comprise conducting the thermal decomposition of the hydroxide(s) in the separated precipitate in an oxygenated atmosphere at a temperature of from about 600 “Celsius to about 650 “Celsius, cooling the solid end-product down thereafter, and then magnetically separating magnesium oxide from both ferric oxide and manganese (III) oxide in the solid end-product at a temperature of less than about 90 “Celsius, more preferably less than about 70 “Celsius, and most preferably less than about 50 “Celsius. Carrying out the thermal decomposition of the respective hydroxide(s) in an oxygenated atmosphere within the stated temperature range converts manganese hydroxide in the separated precipitate into manganese (III) oxide, as described above, whilst also avoiding sintering the ferric oxide produced by the thermal decomposition, which otherwise might interfere with the magnetic separation. Conducting the magnetic separation at a temperature of less than about 90 “Celsius ensures that the manganese (III) oxide retains sufficient magnetic susceptibility to be separated along with the ferric oxide from the diamagnetic magnesium oxide. Such a method is therefore similar to the method just described above, in that both comprise conducting the thermal decomposition in an oxygenated atmosphere, although in a more restricted temperature range of from about 600 “Celsius to about 650 “Celsius to avoid sintering the ferric oxide, cooling the solid end-product down thereafter, and magnetically separating a diamagnetic oxide from a paramagnetic oxide in the solid end-product at a temperature of less than about 90 “Celsius. In this case, after the magnetic separation, the solid end-product contains a mixture of ferric oxide and manganese (III) oxide, in which the proportion of magnesium cations has been reduced. Manganese is commonly used as an alloying element in most steels, and at particularly high proportions of from about 11 % to about 15% in austenitic manganese steels and of up to about 19% in some stainless steels. Thus if the siderite-bearing ore has a particularly high proportion of manganese cations substituted for the iron cations, the solid end-product may subsequently be used as feed material for the production of austenitic manganese and / or stainless steels, for example.

[0131] Alternatively, the method may comprise conducting the thermal decomposition of the hydroxide(s) in the separated precipitate at a temperature of more than 220 “Celsius in the absence of oxygen, and magnetically separating magnesium oxide and manganese monoxide from metallic iron and iron oxide(s) in the solid end-product by applying a magnetic field gradient to the solid end-product at a temperature of more than about 160 “Celsius, more preferably more than about 190 “Celsius, and most preferably more than about 220 “Celsius, and at less than about 330 “Celsius, more preferably less than about 300 “Celsius, and most preferably less than about 270 “Celsius. Conducting the thermal decomposition of the respective hydroxide(s) at more than 220 “Celsius in the absence of oxygen thermally decomposes the manganese hydroxide in the separated precipitate into manganese monoxide (i.e., manganese (II) oxide, MnO) and water vapour. The ferrous hydroxide in the separated precipitate also thermally decomposes into ferrous oxide (FeO) and water vapour. However, since ferrous oxide is metastable at temperatures below 850 K, the ferrous oxide then at least partially disproportionates into iron (II, III) oxide (FesO^ and metallic iron, creating a mixture of iron and iron oxide(s) in the solid end-product.

[0132] As can be seen from Table 1 , the magnetic susceptibility of manganese monoxide is considerably less than that of manganese (III) oxide and is also less than that of ferrous oxide, which has a magnetic susceptibility of + 7200 x 106cm3mol’1at s.t.p. The magnetic susceptibility of manganese monoxide is also less than those of both iron (II, III) oxide, which is ferrimagnetic, and metallic iron, which is ferromagnetic. Carrying out the magnetic separation when the paramagnetic manganese monoxide is still hot reduces its magnetic susceptibility still further than at s.t.p. However, conducting the magnetic separation at a temperature which is also well below the Curie temperatures of both iron (11, 111) oxide (= 858 K) and metallic iron (= 1043 K) ensures that the magnetic susceptibility of these two substances is not similarly affected. Thus the diamagnetic magnesium oxide and the weakly paramagnetic manganese monoxide may both be separated from the ferromagnetic iron and ferrimagnetic iron (II, III) oxide in the solid end-product. Although the magnetic susceptibility of the paramagnetic ferrous oxide is also reduced by carrying out the magnetic separation when the solid end-product is still hot, at least some of the ferrous oxide may still be separated along with the metallic iron and the iron (11,111) oxide since the Neel temperature of ferrous oxide (= 198 K) is higher than that of manganese monoxide (= 116 K). Applying a magnetic field gradient to the solid end-product helps to achieve this. Thus, after the magnetic separation, in this case, the solid endproduct contains a mixture of iron and iron oxide(s), in which the proportion of both magnesium and manganese cations has been reduced, making the solid end-product useful as feed material for the production of iron and / or low-manganese steels.

[0133] Thus the method described herein can be used not only to produce lime from limestone, dolomitic lime from dolomite and magnesia from magnesite, wherein the lime, dolomitic lime and magnesia are substantially uncontaminated by iron and / or manganese, but also to produce iron oxide(s) suitable for ironmaking from a range of different siderite-bearing ores, wherein the iron oxide is substantially uncontaminated by magnesium. Furthermore, the proportion of manganese in the iron oxide(s) may also be adjusted as desired according to how the thermal decomposition is carried out and how the solid end-product is subsequently treated.

[0134] In all of the above embodiments wherein the magnetic separation follows the thermal decomposition of the hydroxide(s) in the separated precipitate, the thermal decomposition should preferably be conducted without sintering the separated precipitate, to allow the magnetic separation to be carried out more effectively.

[0135] As mentioned above, the sequence of reactions which transform the carbonate mineral into its corresponding oxide(s) and carbon dioxide are exothermic overall, and the total quantity of heat available from these processes, their respective products, and the products of the thermal decomposition itself more than exceeds the heat required for the thermal decomposition. Moreover, if at least some of the captured carbon dioxide is then reacted in a carbonation reaction with a second portion of the sodium oxide, or with sodium hydroxide derived therefrom, this carbonation reaction is also strongly exothermic. Any of this excess heat may therefore be used in one or more other processes, as desired. For example, in some embodiments, the method may comprise transferring at least some of the heat not consumed by the thermal decomposition to a sodium compound which is fused and electrolysed to produce the liquid sodium. Some examples of such compounds include sodium chloride, sodium hydroxide, and an electrolyte comprising a mixture of both sodium chloride and aluminium chloride. This has the advantage of reducing the quantity of electrical energy consumed by the electrolysis, thereby making the complete process more energy efficient overall. For example, liquid sodium leaving an electrolytic cell immediately after it has been electrolysed may transfer heat via a heat exchanger to such a sodium compound entering the same electrolytic cell. In another example, the electrolytic cell may be provided with a device as described in the present applicant's 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. Alternatively or additionally, in some embodiments, the method described herein may comprise transferring at least some of the heat not consumed by the thermal decomposition to raw mix for manufacturing cement clinker, wherein the raw mix comprises the solid end-product of the thermal decomposition as an ingredient. This has the advantage of reducing the total quantity of heat required to produce the cement clinker, which may therefore be done without the need to burn any fossil fuel.

[0136] Any of the methods described herein may be carried out as a continuous, semi-batch or batch process. However, they are preferably carried out as a continuous process for reasons of economy and efficiency.

[0137] Apparatus

[0138] In a second aspect, the present invention also provides an apparatus for producing at least one of calcium oxide, magnesium oxide and an iron oxide from an ore comprising a carbonate mineral of at least one of calcium, magnesium and iron. The apparatus comprises an electrolytic subassembly, a sodium oxide or hydroxide- producing subassembly, a hydrochloric acid-producing subassembly, a reaction subassembly, a solid-aqueous phase separator and a kiln. 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 sodium oxide or hydroxide-producing subassembly is for producing sodium oxide or hydroxide from at least some of the liquid sodium and an oxidant, and comprises a first inlet for the oxidant, a second inlet for receiving liquid sodium from the first outlet of the electrolytic subassembly, and an outlet for sodium oxide or hydroxide. The hydrochloric acid-producing subassembly is for producing hydrochloric acid from at least some of the 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 reaction subassembly comprises a gas-tight reaction vessel for dissolving therein a carbonate mineral of at least one of calcium, magnesium and iron in hydrochloric acid, a first inlet for receiving an 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 sodium oxide or hydroxide from the outlet of the sodium oxide or hydroxide-producing subassembly, and an outlet for an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. The 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 reaction subassembly, a first outlet for the precipitate, and a second outlet for the aqueous solution of sodium chloride. The kiln is for thermally decomposing at least some of the calcium hydroxide, magnesium hydroxide and ferrous hydroxide in 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. The kiln comprises an inlet for receiving the precipitate from the first outlet of the phase separator and an outlet for the solid end-product. The apparatus also comprises a heat transfer pathway for transferring sufficient heat to the precipitate in the kiln to cause the thermal decomposition from at least one of the aforementioned subassemblies, their respective products and the products of the thermal decomposition itself. The heat transfer pathway may take one of several forms, some embodiments of which are described below. In some embodiments, the heat transfer pathway may contain the liquid sodium as a heat transfer fluid. In some embodiments, the heat transfer pathway may comprise a heat pipe, for rapid and effective heat transfer. In some embodiments, the kiln may comprise a further outlet for the water vapour produced by the thermal decomposition, as well as the outlet for the solid end-product.

[0139] In some embodiments, the sodium oxide or hydroxide-producing subassembly may comprise a conduit for transporting at least some of the sodium oxide through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed within the kiln. Such a conduit allows water vapour produced by the thermal decomposition to be absorbed by the sodium oxide, thereby reducing the partial pressure of water vapour within the kiln. This exothermic hydration reaction and the sodium hydroxide it produces also transfer heat directly to the separated precipitate, thereby aiding the thermal decomposition of the hydroxide(s) it contains. Additionally, if the sodium oxide is still hot from the exothermic oxidation of the liquid sodium, more heat is introduced directly into the kiln thereby.

[0140] In some embodiments, if the oxidant comprises an oxide of another metal from an ore of the other metal, the sodium oxide or hydroxide-producing subassembly may comprise an oxidation vessel, a hydration vessel and a second solid-aqueous phase separator. The oxidation vessel is for oxidising the liquid sodium in a redox reaction with the oxide of the other metal to produce at least the other metal in elemental form and the sodium oxide, and comprises the first and second inlets of the sodium oxide or hydroxide-producing subassembly and an outlet for a solid phase at least comprising the sodium oxide. The hydration vessel is for hydrating the sodium oxide with water to produce an aqueous solution of sodium hydroxide, and comprises a first inlet for receiving the solid phase from the outlet of the oxidation vessel, a second inlet for water and an outlet for the aqueous solution of sodium hydroxide and undissolved solids. The second solid-aqueous phase separator is for separating the undissolved solids from the aqueous solution of sodium hydroxide, and comprises a first inlet for receiving the aqueous solution of sodium hydroxide and undissolved solids from the outlet of the hydration vessel, a first outlet for the aqueous solution of sodium hydroxide and a second outlet for the undissolved solids. The first outlet of the second phase separator provides the sodium hydroxide to the third inlet of the reaction subassembly. Such embodiments therefore have the advantage that if the oxidant has an admixture of gangue, for example if the oxidant comprises an oxide of another metal from an ore of the other metal which also comprises gangue, sodium hydroxide from which the gangue has been removed can still be supplied to the reaction subassembly. However, in some such embodiments, the sodium oxide or hydroxide-producing subassembly may also comprise a dryer for drying the aqueous solution of sodium hydroxide from which the gangue has been removed to produce solid sodium hydroxide and water vapour, before the solid sodium hydroxide is supplied to the reaction subassembly. If so, the dryer may comprise an inlet for receiving the aqueous solution of sodium hydroxide from the first outlet of the second phase separator, a first outlet for the solid sodium hydroxide and a second outlet for the water vapour, wherein the first outlet of the dryer is connected upstream of the third inlet of the reaction subassembly. In some embodiments, the reaction subassembly may comprise a device for separating undissolved gangue species from an aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, which is produced by dissolving the carbonate mineral in hydrochloric acid within the gas-tight reaction vessel. For example, the separation device may be contained within the gas-tight reaction vessel and may comprise means for filtering, sieving and / or straining this aqueous solution to remove the undissolved gangue species before the first portion of sodium oxide or sodium hydroxide derived therefrom is added to the aqueous solution thus treated, within the same reaction vessel. Alternatively, however, the separation device may be distinct from the gas-tight reaction vessel and may instead comprise an inlet for receiving the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride with the undissolved gangue species therein from an outlet therefor of the gas-tight reaction vessel, a first outlet for the aqueous solution from which the undissolved gangue species have been removed and a second outlet for the gangue species. If so, the separation device may comprise a settlement tank, filter and / or centrifuge, for example. In such cases, the reaction subassembly may also comprise a second, precipitation vessel comprising an inlet for receiving the aqueous solution from which the undissolved gangue species have been separated from the first outlet of this separation device. Such a precipitation vessel is for reacting at least some of the sodium oxide or sodium hydroxide derived therefrom with this aqueous solution from which the undissolved gangue species have been separated. It therefore comprises the third inlet of the reaction subassembly for receiving the sodium oxide or sodium hydroxide derived therefrom from the outlet of the sodium oxide or hydroxide-producing subassembly, as well as the outlet of the reaction subassembly for the aqueous solution of sodium chloride and the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide.

[0141] In some embodiments, the gas-tight reaction vessel may further comprise a second outlet for gaseous carbon dioxide produced by dissolving the carbonate mineral in the hydrochloric acid within the gas-tight reaction vessel, and the apparatus may further comprise a carbonation vessel for reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the sodium oxide, with the gaseous carbon dioxide thus produced to produce at least sodium carbonate. If so, the carbonation vessel may comprise a first inlet for receiving carbon dioxide from the second outlet of the gas-tight reaction vessel, a second inlet for receiving sodium oxide or hydroxide from the outlet of the sodium oxide or hydroxide-producing subassembly, and an outlet for at least the sodium carbonate. For example, such a carbonation vessel may comprise a fluidized bed reactor containing hot sodium oxide, into which the carbon dioxide is introduced. In another example, the carbonation vessel may instead contain an aqueous solution of sodium hydroxide through which the carbon dioxide is bubbled. In any such embodiments, the apparatus also comprises a heat transfer pathway for transferring heat from the carbonation vessel to the precipitate within the kiln. Thus heat from the exothermic carbonation reaction can contribute towards the thermal decomposition of the hydroxide(s) in the separated precipitate.

[0142] Alternatively or additionally, in some embodiments, the apparatus may comprise a conduit for transporting sodium oxide or solid sodium hydroxide from the outlet of the sodium oxide or hydroxide-producing subassembly through an atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid within the gas-tight reaction vessel. This has the advantage that the atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid can then be controlled by means of the carbonation reaction.

[0143] In some embodiments, the kiln may be such that it can be rendered gas-tight to permit the thermal decomposition of the hydroxide(s) in the separated precipitate to be conducted in the absence of oxygen. This has the advantage that the respective compositions of both products of the thermal decomposition can then be controlled. For example, the inlet and outlets of the kiln may be provided with respective valves to keep the kiln gas-tight when the separated precipitate is introduced via the inlet or when the water vapour and solid end-product leave the kiln via their respective outlets. Conducting the thermal decomposition of the respective hydroxide(s) in the absence of oxygen may then be achieved in one of several different ways. For example, the kiln may be evacuated or purged with deoxygenated air or with water vapour before the separated precipitate is introduced into the kiln.

[0144] In some embodiments, the second outlet of the kiln for the solid end-product may be connected upstream of an inlet of a rotary kiln for the manufacture of cement clinker. In such a case, the kiln which produces the solid endproduct may replace or substitute for a precalciner in a cement plant which produces, for example, ordinary Portland cement. This has the advantage that cement can then be made in a manner which avoids the traditional calcination process, without requiring any alteration to the composition of the resulting cement clinker.

[0145] Brief Description of the Drawings

[0146] 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:

[0147] Fig. 1 is a graph of the solubility of gaseous carbon dioxide in liquid water at atmospheric pressure and as a function of pH;

[0148] Fig. 2 is a graph of the respective solubilities in liquid water of silica, aluminium hydroxide, ferrous hydroxide, magnesium hydroxide and calcium hydroxide as a function of pH;

[0149] Fig. 3 is a flow diagram of a first embodiment of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0150] Fig. 4 is a flow diagram of a second embodiment of such a method;

[0151] Fig. 5 is a flow diagram of a third embodiment of such a method;

[0152] Figs. 6A and 6B are flow diagrams of first and second embodiments, respectively, of precipitation and separation techniques, each of which may form part of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide as described herein;

[0153] Fig. 7 is a flow diagram of a fourth embodiment of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0154] Fig. 8 is a flow diagram of a fifth embodiment of such a method;

[0155] Fig. 9 is a flow diagram of a sixth embodiment of such a method; Fig. 10 schematically represents an embodiment of a method of oxidising liquid sodium to produce sodium oxide, which may form part of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide as described herein;

[0156] Fig. 11 is a flow diagram of an embodiment of a method of producing hydrochloric acid, which may form part of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide as described herein;

[0157] Fig. 12 is a graph schematically representing the temperature of some of the processes in a seventh embodiment of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0158] Fig. 13 is a graph schematically representing the temperature of other of the processes in the seventh embodiment of such a method;

[0159] Fig. 14 is a flow diagram of part of an eighth embodiment of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0160] Fig. 15 is a flow diagram of part of a ninth embodiment of such a method;

[0161] Fig. 16 is a graph schematically representing the temperature and pressure of some of the processes in a tenth embodiment of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0162] Figs. 17 A and 17B are flow diagrams of parts of eleventh and twelfth embodiments, respectively, of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide, wherein the carbonate mineral comprises a majority of calcium and / or magnesium cations;

[0163] Figs. 18A and 18B are flow diagrams of parts of thirteenth and fourteenth embodiments, respectively, of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide, wherein the carbonate mineral comprises a majority of iron cations;

[0164] Fig. 19 is a schematic diagram of a first embodiment of an apparatus for producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0165] Fig. 20 is a schematic diagram of a first embodiment of a heat transfer pathway which may form part of the apparatus of Fig. 18;

[0166] Fig. 21 is a schematic diagram of a second embodiment of an apparatus for producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0167] Fig. 22 is a schematic diagram of a third embodiment of such an apparatus;

[0168] Figs. 23A and 23B are schematic diagrams of parts of fourth and fifth embodiments, respectively, of an apparatus for producing at least one of calcium oxide, magnesium oxide and an iron oxide;

[0169] Fig. 24 is a schematic diagram of a second embodiment of a heat transfer pathway which may form part of the apparatus of Figs. 22 and 23B;

[0170] Fig. 25 is a schematic diagram of a third embodiment of a heat transfer pathway which may form part of the apparatus of Fig. 23A; and

[0171] Fig. 26 is a schematic diagram of part of a sixth embodiment of an apparatus of the invention connected to an inlet of a rotary kiln for manufacturing cement clinker. Detailed Description

[0172] Fig. 3 shows a first embodiment of a method 200a of producing at least one of calcium oxide, magnesium oxide and an iron oxide. The method 200a comprises fusing and electrolysing 101 b solid sodium chloride to produce liquid sodium and chlorine gas. The liquid sodium is then oxidised 202 to produce sodium oxide and the chlorine gas is used 203 to produce hydrogen chloride, for example by reacting the chlorine gas in an RDR with high- temperature water vapour captured from the thermal decomposition 209, as described further below in relation to Fig. 11. The hydrogen chloride is then dissolved 204 in liquid water to produce hydrochloric acid. Next, an ore comprising a carbonate mineral of at least one of calcium, magnesium and iron is added 205 to the hydrochloric acid to dissolve the carbonate mineral therein and produce gaseous carbon dioxide and an aqueous solution 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, so that the carbon dioxide released from the carbonate mineral can be captured 206. The sodium oxide is then added 207 to 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 comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. This precipitate is phase separated 208 from the aqueous solution of sodium chloride and the hydroxide(s) in the separated precipitate are thermally decomposed 209 to produce water vapour and a solid end-product comprising at least one of calcium oxide, magnesium oxide and an iron oxide. In this embodiment, the thermal decomposition is conducted by transferring 210 heat to the separated precipitate from the two significantly exothermic reactions by which the liquid sodium is oxidised 202 and then the sodium oxide is reacted 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride.

[0173] Fig. 4 shows a second embodiment of a method 200b of producing at least one of calcium oxide, magnesium oxide and an iron oxide. Parts of the method 200b which are the same as parts of the method 200a described above in relation to Fig. 3 are labelled in Fig. 4 by the same reference numerals as in Fig. 3 and will not be described again for the sake of brevity. In this embodiment, however, the liquid sodium and chlorine gas are instead produced by electrolysing 201 a an aqueous solution of sodium chloride to produce an aqueous solution of sodium hydroxide and the chlorine gas, drying 201 b the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, and fusing and electrolysing 201c the solid sodium hydroxide to produce the liquid sodium, hydrogen gas and oxygen gas. The chlorine gas is used 203 to produce hydrogen chloride, for example by combusting it together with the hydrogen gas produced by at least one of electrolysing 201 a the aqueous solution of sodium chloride and electrolysing 201c the solid sodium hydroxide.

[0174] Fig. 5 shows a third embodiment of a method 200c of producing at least one of calcium oxide, magnesium oxide and an iron oxide. Again, parts of the method 200c which are the same as parts of the methods 200a and 200b described above in relation to Figs. 3 and 4 are labelled in Fig. 5 by the same reference numerals as in Figs. 3 and 4 and will not be described again for the sake of brevity. However, in this embodiment, the sodium oxide is produced 202 by reacting the liquid sodium in a redox reaction with an oxide of another metal to produce the other metal and the sodium oxide, as described further below in relation to Fig. 8. The sodium oxide therefore has an admixture of gangue as a result of this redox reaction. Transferring heat to the separated precipitate in order to thermally decompose the hydroxide(s) it contains therefore comprises cooling 210 the sodium oxide and the gangue to a temperature of less than 100 “Celsius, adding 317 the cooled sodium oxide and gangue to liquid water to hydrate the sodium oxide as described above, and then phase separating 318 the undissolved gangue from the aqueous solution of sodium hydroxide, before the aqueous solution of sodium hydroxide is then reacted 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Similarly, after the carbonate mineral has dissolved 205 in the hydrochloric acid, undissolved gangue species are phase-separated 211 from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride before reacting 207 this aqueous solution with the sodium hydroxide. Thus gangue species, such as aluminosilicate minerals, derived either from an ore comprising the oxide of the other metal or from an ore comprising the carbonate mineral can be excluded from the separated precipitate. This makes the method 200c particularly suitable for producing a purer form of the solid end-product. Moreover, whereas the method 200c may in principle be used to produce at least one of calcium oxide, magnesium oxide and an iron oxide, in this embodiment, the carbonate mineral consists only of at least one of calcium oxide and magnesium oxide, and the method 200c further comprises altering 212 the chemical activity of the solid end-product by milling it.

[0175] Fig. 6A shows a first embodiment of a precipitation and separation technique 200d, which may form part of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide as described herein. For example, the technique 200d of Fig. 6A may take the place of the operations 205, 211 , 207 and 208 in the method 200c of Fig. 5 described above or in the method 200g of Fig. 8 described below. Unlike the methods 200c and 200g, in which undissolved gangue species are separated 211 from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride in a single operation before enough sodium oxide or hydroxide is reacted 207 with this aqueous solution to precipitate out at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide, respectively, the technique 200d shown in Fig. 6A breaks this phase separation 211 of gangue species down into multiple operations, each of which removes different gangue species from the aqueous solution. Thus, the precipitation and separation technique 200d comprises, after the carbonate mineral has been dissolved 205 in the hydrochloric acid, separating 211 a undissolved gangue species from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride at an acidic pH of less than about 4.5. Sufficient sodium oxide or hydroxide is then added 207a to this aqueous solution to raise its pH until it falls within a range of from about 5 to about 7.5, which is still insufficiently high to precipitate out any calcium hydroxide, magnesium hydroxide or ferrous hydroxide, respectively. However, within this pH range, aluminium hydroxide can precipitate out. Thus if, for example, the carbonate mineral ore originally comprised both siliceous and aluminate gangue species, the siliceous gangue is precipitated out first as silica, which is separated out 211 a at an acidic pH of less than about 4.5, whereas the aluminate gangue is precipitated out thereafter in the pH range of from about 5 to about 7.5 as aluminium hydroxide, which is then separated out 211 b from the aqueous solution, before more sodium oxide or hydroxide is added 207b to increase the pH to an alkaline value which is sufficiently high to precipitate out calcium hydroxide, magnesium hydroxide and / or ferrous hydroxide, respectively. The Q hydroxide is then separated out 208 from this alkaline aqueous solution as described previously.

[0176] In this embodiment, by way of example, the silica which is separated out 211 a first is sufficiently active that it is then used 222 as an ingredient in cement manufacture. Also by way of example in this embodiment, the aluminium hydroxide which is separated out 211 b after the pH of the aqueous solution has been raised is then dried and dehydroxylated 223 to produce alumina, which is subsequently used 224 to produce elemental aluminium by electrolysis. The alumina is electrolysed using an inert anode rather than a carbon anode, to avoid emitting any carbon dioxide. In addition, since it is also desirable to recover the sodium and chloride ions which remain dissolved in the alkaline aqueous solution after the Q hydroxide has been separated out 208, more hydrochloric acid is then added 225 to this alkaline aqueous solution until its pH returns to a neutral value. At least some of this neutral aqueous solution of sodium chloride is then used 114 as feed material for producing the liquid sodium and chlorine gas by electrolysis.

[0177] Fig. 6B shows a second embodiment of a precipitation and separation technique 200e, which may form part of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide as described herein. The technique 200e of Fig. 6B may be used instead of or in addition to the technique 200d of Fig. 6A described above. For example, the technique 200e of Fig. 6B may take the place of the operations 207 and 208 in any of the methods 200a, 200b, 200c respectively of Figs. 3 to 5 described above, or the operations 207 and 208 in either of the methods 200f, 200g respectively of Figs. 7 and 8 described below, or it may take the place of the operations 207b, 208 and 225 in the precipitation and separation technique 200d of Fig. 6A. Whereas the technique 200d of Fig. 6A allows siliceous and aluminate gangue species to be separated from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride independently of each other, the technique 200e of Fig. 6B allows ferrous hydroxide and calcium hydroxide to be separated from this aqueous solution independently of each other instead. The precipitation and separation technique 200e therefore comprises adding 207c sufficient sodium oxide or hydroxide to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to precipitate out ferrous hydroxide at a pH of from about 8 to about 12, before phase separating 208a at least some of this ferrous hydroxide from the aqueous solution of sodium chloride thus formed. The technique 200e then comprises continuing to add 207d sufficient of the sodium oxide or hydroxide to this aqueous solution of sodium chloride to precipitate out calcium hydroxide at a pH of about 12.5 or more, before phase separating 208b at least some of this calcium hydroxide from the aqueous solution of sodium chloride as well. Thus if, for example, the carbonate mineral has a majority of cations comprising at least one of calcium and magnesium (for example, if the carbonate mineral is calcite, magnesite or dolomite), if desired, any iron cations also present in the ore may be treated as gangue and separated out accordingly. If on the other hand, the carbonate mineral has a majority of iron cations (for example, if the carbonate mineral is siderite), if desired, any calcium cations also present in the ore may be treated as gangue and similarly separated out. Since co-precipitation of magnesium hydroxide with ferrous hydroxide or calcium hydroxide cannot be avoided (see the above description of Fig. 2), if the carbonate mineral ore originally comprised any magnesium cations, either or both of the ferrous hydroxide which is separated out 208a from the aqueous solution first and the calcium hydroxide which is separated out 208b thereafter may have an admixture of magnesium hydroxide. Moreover, if a minority of the calcium, magnesium and / or iron cations in the carbonate mineral were originally substituted by manganese cations, this would also result in the co-precipitation of manganese hydroxide with either or both of the ferrous hydroxide and calcium hydroxide. However, as shown in Fig. 6B, if the ferrous hydroxide has an admixture of magnesium hydroxide and / or manganese hydroxide, these may be processed by conducting the subsequent thermal decomposition in one of two ways 209c, 209d, which are respectively described in greater detail below with reference to Figs. 18A and 18B. As also shown in Fig. 6B, if the calcium hydroxide has an admixture of magnesium hydroxide and / or manganese hydroxide, these may be processed by conducting the subsequent thermal decomposition in the manner 209b described in greater detail below with reference to Fig. 17B. Moreover, in the present embodiment, by way of example and as in the precipitation and separation technique 200d of Fig. 6A, after the calcium hydroxide has been separated out 208b, more hydrochloric acid is then added 225 to the supernatant solution until its pH returns to a neutral value, and at least some of this neutral aqueous solution of sodium chloride is then used 114 as feed material for producing the liquid sodium and chlorine gas by electrolysis.

[0178] Fig. 7 shows a fourth embodiment of a method 200f of producing at least one of calcium oxide, magnesium oxide and an iron oxide. Parts of the method 200f which are the same as parts of the method 200a described above in relation to Fig. 3 are labelled in Fig. 7 by the same reference numerals as in Fig. 3 and will not be described again for the sake of brevity. In this embodiment, however, the sodium oxide, which is reacted 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, is only a first portion constituting between 5 / 12 and 7 / 12, inclusive, of a total number of mols of the sodium oxide produced by oxidising 202 the liquid sodium produced by the electrolysis 101 b. In addition, the method 200f comprises passing 213 a second portion of the sodium oxide thus produced through an atmosphere in which the carbonate mineral is dissolved 205 in the hydrochloric acid, where this second portion of the sodium oxide reacts 214a with the captured carbon dioxide in a carbonation reaction to produce sodium carbonate. Heat is also transferred 210 to the separated precipitate from this carbonation reaction 214a and / or the resulting sodium carbonate to aid the thermal decomposition 209.

[0179] Fig. 8 shows a fifth embodiment of a method 200g of producing at least one of calcium oxide, magnesium oxide and an iron oxide. Parts of the method 200g which are the same as parts of the method 200c described above in relation to Fig. 5 are labelled in Fig. 8 by the same reference numerals as in Fig. 5 and will not be described again for the sake of brevity. In this embodiment, however, after the sodium oxide is hydrated 317 as described above, and undissolved gangue is phase separated 318 from the aqueous solution of sodium hydroxide thus produced, the remaining aqueous solution of sodium hydroxide is then dried 319 to produce solid sodium hydroxide. A first portion of this solid sodium hydroxide constituting between 5 / 12 and 7 / 12, inclusive, of a total number of mols of the sodium oxide produced by oxidising the liquid sodium is reacted 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, and a second portion of the solid sodium hydroxide is passed 213a through an atmosphere in which the carbonate mineral is dissolved 205 in the hydrochloric acid. There, the second portion of solid sodium hydroxide reacts 214b at a final temperature of from 310 to 400 “Celsius, inclusive, to produce sodium carbonate, a reduced amount of gaseous carbon dioxide and water vapour, as described above. The temperature of the reaction products is controlled by transferring 210 heat to the separated precipitate to aid its thermal decomposition 209.

[0180] Whereas in the embodiment of Fig. 8, both the first and second portions of sodium oxide are hydrated 317 to produce an aqueous solution of sodium hydroxide, in other alternative possible embodiments, just one of the first and second portions of sodium oxide may be hydrated in this way by firstly dividing the sodium oxide produced by oxidising 202 the liquid sodium into first and second portions thereof, before hydrating just one or other of these two portions. Furthermore, whereas the embodiments of Figs. 5 and 8 both comprise phase-separating 211 undissolved gangue species from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, as well as hydrating 317 the sodium oxide, in other possible embodiments, either one of these two processes 21 1 , 317 may be omitted. For example, if it is desired to produce a purer version of sodium carbonate, whereas the presence of some gangue species in the solid end-product may be acceptable, just the second portion of sodium oxide need be hydrated 317, whereas hydrating 317 the first portion of sodium oxide and phase-separating 211 undissolved gangue species from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride may both be omitted.

[0181] Fig. 9 shows a sixth embodiment of a method 200h of producing at least one of calcium oxide, magnesium oxide and an iron oxide. Parts of the method 200h which are the same as parts of the methods 200a and 200f respectively described above in relation to Figs. 3 and 7 are labelled in Fig. 9 by the same reference numerals as in Figs. 3 and 7 and will not be described again for the sake of brevity. In this embodiment, only the second portion of sodium oxide produced by oxidising 202 the liquid sodium is hydrated to produce sodium hydroxide. In this case, however, the second portion of sodium oxide is hydrated by passing 226 it through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed 209. Instead of forming an aqueous solution of sodium hydroxide, the second portion of sodium oxide reacts in a different hydration reaction 317a with the water vapour produced by the thermal decomposition 209 to produce anhydrous sodium hydroxide in solid phase. Since the second portion of sodium oxide is still hot from the oxidation 202, heat is transferred 210 from this second portion of sodium oxide to the hydroxide(s) in the separated precipitate, as represented in Fig. 9 by the arrow labelled "I”. However, heat generated by the exothermic hydration reaction 317a is also transferred 210 from the solid sodium hydroxide formed thereby to the hydroxide(s) in the separated precipitate, as represented in Fig. 9 by the arrow labelled "II”. Thereafter, the solid sodium hydroxide is passed 213a through an atmosphere in which the carbonate mineral is dissolved 205 in the hydrochloric acid, where the solid sodium hydroxide reacts 214b with the captured carbon dioxide in a carbonation reaction to produce sodium carbonate. Heat generated by this exothermic carbonation reaction is transferred 210 to the hydroxide(s) in the separated precipitate as well, as represented in Fig. 9 by the arrow labelled "III”. In other possible embodiments to that shown in Fig. 9, the first portion of sodium oxide could alternatively or additionally be passed 226 through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed 209 to produce solid sodium hydroxide as well, for subsequent reaction 207 with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Moreover, whereas gangue species may be removed by hydrating either or both the first and second portions of sodium oxide to produce an aqueous solution of sodium hydroxide, as in the embodiment of Fig. 8, and either or both the first and second portions of sodium oxide may be hydrated to produce sodium hydroxide in solid phase, as in the embodiment of Fig. 9, these two ways of hydrating the sodium oxide are not mutually exclusive. In other possible embodiments, therefore, hydrating either or both the first and second portions of sodium oxide to produce solid sodium hydroxide, as in the embodiment of Fig. 9, may precede further hydrating the solid sodium hydroxide thus formed to produce an aqueous solution of sodium hydroxide, as in the embodiment of Fig. 8, in order to remove gangue species therefrom, or vice versa. Furthermore, whereas in the embodiments of Figs. 5, 7, 8 and 9, the liquid sodium and chlorine gas are produced by fusing and electrolysing 101 b solid sodium chloride as in the embodiment of Fig. 3, in alternative possible embodiments, at least some of the liquid sodium and chlorine gas may instead be produced by electrolysing 201 a an aqueous solution of sodium chloride, drying 201 b the aqueous solution of sodium hydroxide thus produced, and then fusing and electrolysing 201c the resulting solid sodium hydroxide, as in the embodiment of Fig. 4.

[0182] Fig. 10 shows an embodiment of how the liquid sodium may be oxidised 202 to produce sodium oxide, which may form part of any one of the methods of producing at least one of calcium oxide, magnesium oxide and an iron oxide described herein. In this embodiment, oxidising 202 the liquid sodium comprises reacting 303 the liquid sodium in a redox reaction with an oxide of another metal to produce the other metal and the sodium oxide. For example, the other metal may be iron, in which case, the oxide of the other metal may be ferric oxide (Fe2C>3) present in an iron ore, such as a hematite-bearing ore from a banded iron formation. In another example, the other metal may be manganese, in which case, the oxide of the other metal may be manganese (III) oxide (M^C ) derived from a manganese ore, such as an ore containing one or more of pyrolusite, braunite and hausmannite. If the other metal comprises iron, the redox reaction 303 is conducted at a temperature of less than 450 “Celsius and in an inert atmosphere, as described in the present applicant's UK patent application no. 2417059.9 ("Carbon-Free Method and Apparatus for Producing Iron and Steel”; applicant's ref: NE-P-GB 001), already mentioned above. If the other metal comprises manganese, the redox reaction 303 is conducted at a temperature of less than 600 “Celsius and in an inert atmosphere, as described in the present applicant's UK patent application no. 2417063.1 ("Carbon-Free Method and Apparatus for Producing Manganese”; applicant's ref: NE-P-GB 008), also mentioned previously.

[0183] Fig. 11 shows an embodiment of a method 100 of producing hydrochloric acid, whereby the chlorine gas may be used 203 to produce hydrogen chloride, and then the resulting hydrogen chloride may be dissolved 204 in liquid water to produce hydrochloric acid. This method 100 may form part of any one of the methods of producing at least one of calcium oxide, magnesium oxide and an iron oxide described herein, in which at least some of the liquid sodium is produced by fusing and electrolysing 101 b solid sodium chloride. In the method 100, the thermal decomposition 209 is also performed 209a in the absence of oxygen. For example, if the thermal decomposition is performed 209a in an atmosphere of deoxygenated air, the remaining nitrogen and argon are both inert, and any other trace gases, such as carbon dioxide, which may also be in the deoxygenated air are present in such small amounts as can be neglected, in which case the deoxygenated air just acts as a carrier gas for the high- temperature water vapour produced by the thermal decomposition 209a. The method 100 then further comprises capturing 103 this water vapour and reacting it 104 with at least some of the high-temperature chlorine gas produced 101b by fusing and electrolysing solid sodium chloride in an RDR at a temperature of from 450 to 750 “Celsius, inclusive, to produce a mixture of gases at least comprising hydrogen chloride and oxygen. This mixture of gases is then immediately contacted 105 with liquid water having a temperature of from about 50 to about 70 “Celsius, inclusive, to dissolve the hydrogen chloride therein, thereby producing the hydrochloric acid and a stream of tail gases. Heat is extracted 106 from the hydrochloric acid thus produced, to maintain its temperature within approximately the same range until the stream of tail gases is no longer in contact therewith. Further features and advantages of this method 100, including how the stream of tail gases may be processed and recycled, are described in the present applicant's UK patent application no. 2417058.1 ("Method and Apparatus for Producing Hydrochloric Acid”; applicant's ref: NE-P-GB 006), already mentioned above.

[0184] Figs. 12 and 13 are both graphs plotting temperature on the y-axis or ordinate against the progress of a sequence of successive processes on the x-axis or abscissa. Whereas in both graphs, the ordinate is marked by a linear temperature scale, the abscissa does not represent time but instead represents an ordered sequence, in which each process has been assigned an equal portion of the abscissa for illustrative purposes only. In practice, however, each such process may last a different length of time from each other. In the embodiment of Figs. 12 and 13, solid sodium chloride is firstly fused and electrolysed at a temperature of about 610 “Celsius to produce liquid sodium and chlorine gas. The liquid sodium is then cooled by extracting heat from it until it reaches point A on both graphs, which as Fig. 13 illustrates, lies above the melting point of liquid sodium, Tm(Na), at 98 “Celsius. The liquid sodium is then oxidised to produce sodium oxide by reacting it in a redox reaction with ferric oxide (Fe2O3), as described above. Since this reaction is exothermic, the temperature of the reaction mixture rises until it reaches point B on both graphs. However, enough heat is extracted from this redox reaction to ensure that point B remains below the temperature at which ternary oxides of sodium and iron, such as Na4FeO3, can start to form, T (ternary oxides form), at 447 “Celsius. The sodium oxide is then divided into a first portion and a second portion. Fig. 12 illustrates what happens to the first portion of sodium oxide, as represented in Fig. 12 by 1st Na2O (Sj, whereas Fig. 13 illustrates what happens to the second portion of sodium oxide, as represented in Fig. 13 by 2nd Na2O (S).

[0185] Referring firstly to Fig. 12, in this embodiment, the first portion of sodium oxide is firstly cooled towards ambient temperature before being introduced into an aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. This produces an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. Since this reaction is also exothermic, the temperature of the reaction mixture rises until it reaches point C on the graph. However, enough heat is extracted from the reaction mixture to ensure that point C remains below the temperature at which the aqueous solution of sodium chloride can boil, Tb (NaCI (aq)), which is at about 110 “Celsius, depending on the concentration of the aqueous solution of sodium chloride. The precipitate is then phase separated from the aqueous solution of sodium chloride, during which some heat may be lost to the environment. Thereafter, heat is injected into the separated precipitate to thermally decompose the hydroxide(s) in contains into one or more oxides and water vapour by raising the temperature of the precipitate until it reaches point D on the graph. Enough heat is injected into it to ensure that point D lies above the minimum temperature at which the precipitate decomposes, Tmin (Q(OH)2 decomposes), which in this embodiment, is represented on the graph as being at about 350 “Celsius. The oxide(s) are then cooled back down towards ambient temperature by extracting heat from them as well. Meanwhile, the aqueous solution of sodium chloride from which the precipitate has been separated is also injected with heat until it reaches point E on the graph. Enough heat is injected into it to ensure that point E lies above the boiling point of the aqueous solution of sodium chloride, Tb (NaCI <aq)). The water of solution therefore boils off to leave solid sodium chloride, which then starts to cool back down towards ambient temperature by losing heat to the environment.

[0186] Next referring to Fig. 13, in this embodiment, the second portion of sodium oxide is firstly cooled, before being hydrated to produce solid anhydrous sodium hydroxide. In this case, the hydration is performed by reacting the hot sodium oxide in a solid-vapour phase reaction (for example, in a fluidized bed reactor) with water vapour captured from another process, such as from the thermal decomposition itself. Since this reaction is also exothermic, the temperature of the sodium hydroxide thus formed rises until it reaches point F on the graph. However, enough heat is extracted from this hydration reaction to ensure that point F remains below the temperature at which the sodium hydroxide can melt, Tm(NaOH), at 323 “Celsius. The sodium hydroxide may then lose a small quantity of heat to its environment before reacting with captured carbon dioxide to produce sodium carbonate. Since once again, this reaction is exothermic, the temperature of the sodium carbonate thus formed rises until it reaches point G on the graph. However, enough heat is extracted from this carbonation reaction to ensure that point G remains below the temperature at which the carbonation reaction can start to reverse, T (carbonation starts to reverse), at about 400 “Celsius. The sodium carbonate is then cooled down towards ambient temperature by extracting heat from it. The total quantity of heat extracted from the different processes represented in Figs. 12 and 13 exceeds the quantity of heat injected into the separated precipitate to cause the thermal decomposition of the hydroxide(s) therein.

[0187] Figs. 14 and 15 both show examples of ways in which at least some of the sodium chloride required for the initial electrolysis and the water used to make the hydrochloric acid can be regenerated from the method of the invention in a closed loop. Thus Fig. 14 schematically represents part of an eighth embodiment of a method 200I of producing at least one of calcium oxide, magnesium oxide and an iron oxide, in which the liquid sodium and chlorine gas are produced by electrolysing 201 a an aqueous solution of sodium chloride, drying 201b the aqueous solution of sodium hydroxide thus produced, and then fusing and electrolysing 201c the resulting solid sodium hydroxide, as in the embodiment of Fig. 4. In the present embodiment, however, at least some of the water vapour produced by drying 201 b the aqueous solution of sodium hydroxide is captured 215, condensed 216, and then used 217 as at least some of the liquid water in which the hydrogen chloride is dissolved 204 to produce the hydrochloric acid. Heat released by condensing 216 the captured water vapour can also be used to contribute to drying 201 b the aqueous solution of sodium hydroxide. Furthermore, in the method 200I, at least some of the hydrogen gas produced by fusing and electrolysing 201c the solid sodium hydroxide is combusted 203b together with the chlorine gas produced by electrolysing 201a the aqueous solution of sodium chloride to produce the hydrogen chloride which is dissolved 204 in the liquid water. The rest of the method 200I not shown in Fig. 14 then comprises using the liquid sodium and hydrochloric acid thus produced in one or more of the manners already described above.

[0188] Fig. 15 schematically represents part of a ninth embodiment of a method 200j of producing at least one of calcium oxide, magnesium oxide and an iron oxide. In this embodiment, parts of the method 200I not shown in Fig. 15 comprise producing the solid end-product in one or more of the ways already described above. However, the method 200j further comprises, after phase separating 208 the precipitate from the aqueous solution of sodium chloride, drying 110 at least some of the remaining aqueous solution of sodium chloride to produce water vapour. This water vapour is then captured 111 , condensed 112, and used 113 as at least some of the liquid water in which the hydrogen chloride is dissolved 204 to produce the hydrochloric acid. Heat released by condensing 112 the captured water vapour can also be used to contribute to drying 110 the aqueous solution of sodium chloride. Moreover, in the method 200j, the solid sodium chloride left by drying 110 the remaining aqueous solution of sodium chloride is also used as feed material for producing 101 b the liquid sodium and the chlorine gas by electrolysis.

[0189] Fig. 16 is a graph schematically showing the temperature and pressure of some of the processes in a tenth embodiment of a method of producing at least one of calcium oxide, magnesium oxide and an iron oxide, in which the processes involved follow a cycle on the pressure-temperature graph of Fig. 16 back to its starting point. In Fig. 16, the starting point for this cycle is marked by the letter “S”. At this point, by way of example only, the hydrochloric acid is at an initial pressure of 1 atmosphere and has an initial temperature, before dissolving 205 the carbonate mineral, of about 70 “Celsius. This increases the rate of dissolution of the carbonate mineral therein, compared to if the hydrochloric acid were instead at ambient temperature. However, it is also more than 15 “Celsius below the boiling point of the hydrochloric acid at atmospheric pressure, at which dissolving 205 the carbonate mineral in the hydrochloric acid starts. Excessive evaporation from the hydrochloric acid or a risk of it boiling are therefore avoided. The carbonate mineral then continues to dissolve 205 in the hydrochloric acid at increasing pressure as a result of the gaseous carbon dioxide being released from the carbonate mineral closed off from their surrounding environment into a gas-tight reaction vessel in which the reaction 205 is carried out, until a final pressure is reached, which in this embodiment is about 2 atmospheres. This increasing pressure is also accompanied by a small increase in temperature of the reaction mixture to about 78 “Celsius as a result of the mildly exothermic nature of the reactions of Eqns. 5a and 5b. However, the increased pressure on the hydrochloric acid suppresses its evaporation in spite of its increased temperature, which also helps to prevent the dissolution of the carbon dioxide therein. At this point, if desired, undissolved gangue species may be phase-separated 211 from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride thus formed, whilst the temperature and pressure of the reaction mixture remain substantially unchanged.

[0190] Next, sodium oxide is reacted 207 with this aqueous solution at the same final pressure. Since no more gas is released, the pressure remains constant, but the highly exothermic nature of the reaction of Eqn. 6 causes the temperature of the reaction mixture to increase until it reaches a final temperature of about 115 “Celsius. This is above the boiling point of the aqueous solution of sodium chloride thus formed at the initial pressure of 1 atmosphere, but is also below the boiling point of the aqueous solution of sodium chloride at the final pressure of about 2 atmospheres. Then, after phase separating 208 the precipitated hydroxide from this aqueous solution of sodium chloride whilst the temperature and pressure remain substantially unchanged, the pressure above the aqueous solution of sodium chloride is reduced back down to the initial pressure. This may be done, for example by reacting the carbon dioxide captured inside the gas-tight reaction vessel in a carbonation reaction 214a with a second portion of the sodium oxide, as described above. The boiling point of the aqueous solution of sodium chloride at the prevailing pressure therefore drops below the temperature of the aqueous solution of sodium chloride as the pressure reduces, as represented in Fig. 16 by the letter “R”. This has the effect of drying 110 the aqueous solution of sodium chloride by causing it to boil. Water of solution released from the aqueous solution of sodium chloride as vapour as the solution boils is captured 111 and condensed 112 by extracting heat from it until its temperature is reduced back to the initial temperature of about 70 “Celsius again. The condensed water is then used 113 as at least some of the liquid water in which the hydrogen chloride is dissolved 204 to produce the hydrochloric acid, as described above in relation to Fig. 15. Moreover, if the molarity of the hydrochloric acid is correctly chosen initially, such that the aqueous solution of sodium chloride is close to its saturation concentration when the pressure above the aqueous solution of sodium chloride is reduced, the sodium chloride also reaches saturation in the solution and begins to crystallize out as the water of solution boils off 110. Thus the increased pressure caused by the release of gaseous carbon dioxide from the carbonate mineral according to the reactions of Eqns. 5a and 5b, some of the heat generated by the exothermic reaction of Eqn. 6, and the reduction in pressure caused by the mineralization of the carbon dioxide according to the reaction of Eqn. 8, can all be used to recover both water and solid salt from the aqueous solution of sodium chloride thus formed in a highly energy-efficient manner. Nonetheless, the reaction of Eqn. 6 is sufficiently exothermic that more heat can still be extracted from the reaction mixture to maintain its temperature below the boiling point of the aqueous solution of sodium chloride at the final pressure, and this extracted heat can still be used in one or more other processes, such as to help the thermal decomposition 209 of the hydroxide(s) in the separated precipitate.

[0191] Figs. 17A and 17B are flow diagrams of parts of eleventh and twelfth embodiments of respective methods 200k, 200m of producing at least one of calcium oxide, magnesium oxide and an iron oxide, when the carbonate mineral comprises a majority of calcium and / or magnesium cations. In the embodiment of Fig. 17A, parts of the method 200k not shown in Fig. 17A comprise producing the aqueous solution of sodium chloride and the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide in one or more of the ways already described above. However, the method 200k further comprises phase separating 208c the precipitate from the aqueous solution of sodium chloride in the absence of oxygen, and magnetically separating 218 at least one of ferrous hydroxide and manganese hydroxide from at least one of calcium hydroxide and magnesium hydroxide in the separated precipitate also in the absence of oxygen and at a temperature of less than 90 “Celsius, before thermally decomposing 209 the hydroxide of at least one of calcium and magnesium to produce the solid end-product. In the embodiment of Fig. 17B, parts of the method 200m not shown in Fig. 17B comprise producing the solid end-product in one or more of the ways already described above. However, the method 200m further comprises, after phase separating 208 the precipitate from the aqueous solution of sodium chloride, conducting the thermal decomposition 209b of the hydroxide(s) in the separated precipitate in an oxygenated atmosphere at a temperature of from 600 to 800 “Celsius, inclusive, cooling 219 the solid end-product down thereafter, and then magnetically separating 220a manganese (III) oxide from at least one of calcium oxide and magnesium oxide in the solid end-product at a temperature of less than 90 “Celsius.

[0192] Figs. 18A and 18B are flow diagrams of parts of thirteenth and fourteenth embodiments of respective methods 200n, 200p of producing at least one of calcium oxide, magnesium oxide and an iron oxide, when the carbonate mineral comprises a majority of iron cations. In both embodiments, parts of these methods 200n, 200p not shown in Figs. 18A and 18B comprise producing the solid end-product in one or more of the ways already described above. The method 200n is similar to the method 200m in that it further comprises, after phase separating 208 the precipitate from the aqueous solution of sodium chloride, conducting the thermal decomposition 209c of the hydroxide(s) in the separated precipitate in an oxygenated atmosphere, cooling 219 the solid end-product down thereafter, and then magnetically separating 220b magnesium oxide from ferric oxide and manganese (III) oxide in the solid end-product at a temperature of less than 90 “Celsius. However, in this case, the thermal decomposition 209c is carried out in a more restricted temperature range of from 600 to 650 “Celsius, inclusive, to avoid sintering the ferric oxide. In addition, the method 200n comprises transferring 221 at least some of the heat extracted by cooling 219 the solid end-product down, to at least one of the solid sodium chloride and the solid sodium hydroxide which are fused and electrolysed 101 b, 201c. In contrast, the method 200p further comprises, after phase separating 208 the precipitate from the aqueous solution of sodium chloride, conducting the thermal decomposition 209d at a temperature of more than 220 “Celsius in the absence of oxygen, and magnetically separating 220c magnesium oxide and manganese monoxide from metallic iron and iron oxide(s) in the solid endproduct by applying a magnetic field gradient to the solid end-product at a temperature of more than 160 “Celsius and less than 330 “Celsius.

[0193] Fig. 19 schematically shows a first embodiment of an apparatus 2a for producing at least one of calcium oxide, magnesium oxide and an iron oxide. In this embodiment, the apparatus 2a comprises an electrolytic subassembly 10 for producing liquid sodium and chlorine gas by fusing and electrolysing solid sodium chloride, a hydrochloric acid-producing subassembly 12, a sodium oxide-producing subassembly 13, a reaction subassembly 14, a solid- aqueous phase separator 16 and a kiln 20. The subassemblies 10, 12, 13, 14, the phase separator 16 and the kiln 20 are connected together as shown in Fig. 19. As Fig. 19 also shows, the sodium oxide-producing subassembly 13 comprises an oxidation vessel 130 having first and second inlets 131 , 132 and an outlet 134, and the reaction subassembly 14 comprises a gas-tight reaction vessel 140 having first, second and third inlets 141 , 142, 143 and an outlet 144. The first inlet 131 of the oxidation vessel 130 is for receiving an oxidant into the oxidation vessel 130, the second inlet 132 is for receiving liquid sodium from the electrolytic subassembly 10 and the outlet 134 is for supplying sodium oxide to the reaction vessel 140. The first inlet 141 of the reaction vessel 140 is for receiving an ore comprising a carbonate mineral into the reaction vessel 140, the second inlet 142 is for receiving the hydrochloric acid from the hydrochloric acid-producing subassembly 12, the third inlet 143 is for receiving the sodium oxide from the sodium oxide-producing subassembly 13, and the outlet 144 is for an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. This aqueous solution and the precipitate are phase separated from each other by the phase separator 16, before the separated precipitate is heated inside the kiln 20 to thermally decompose the hydroxide(s) it contains. The first inlet 141 of the reaction vessel 140 comprises an airlock 141 a, which prevents gaseous carbon dioxide escaping from the reaction vessel 140 into the surrounding environment when the ore is introduced via the first inlet 141. In addition, the second and third inlets 142, 143 and the outlet 144 are each provided with respective valves, V1 , V2, V3, which keep the reaction vessel 140 gas-tight when the respective reagents are introduced via the second and third inlets 142, 143 or when their products leave via the outlet 144, and which can also be used to regulate entry and exit of the respective reagents and products into and out of the reaction vessel 140. The apparatus 2a also comprises a heat transfer pathway 23, schematically represented in Fig. 19 by dashed and dotted lines. In the illustrated embodiment, the heat transfer pathway 23 transfers heat to the separated precipitate within the kiln 20 from at least one of the liquid sodium which leaves the electrolytic subassembly 10, the hydrochloric acid-producing subassembly 12, the sodium oxide-producing subassembly 13, the gas-tight reaction vessel 140 and the solid end-product which leaves the kiln 20 via an outlet 25 for the same. However, in other possible embodiments, some of which are described below, the heat transfer pathway 23 may comprise alternative or additional arrangements.

[0194] The apparatus 2a may be operated as follows. Liquid sodium and chlorine gas are produced in the electrolytic subassembly 10. The liquid sodium passes to the oxidation vessel 130, where it is oxidised to produce sodium oxide, and the chlorine gas passes to the hydrochloric acid-producing subassembly 12, where it is used to produce hydrochloric acid. The hydrochloric acid is then introduced into the gas-tight reaction vessel 140 by opening and then closing valve V1 and the ore comprising the carbonate mineral is introduced thereto as well via the airlock 141 a of inlet 141. The carbonate mineral then dissolves in the hydrochloric acid to produce an aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, whilst also releasing carbon dioxide, which is trapped above this aqueous solution. Whilst this solution still has an acidic pH, the carbon dioxide is removed from reaction vessel 140 to prevent its reabsorption into aqueous solution at higher pH, as described above in relation to Fig. 1 . This may be done in one of several ways, some examples of which are described below in relation to Figs. 23A and 23B. As a result of the initial release of the carbon dioxide from the carbonate mineral, the pressure inside the reaction vessel 140 increases, which may also be accompanied by a small increase in temperature, as mentioned above in relation to Fig. 16. When the carbon dioxide is removed from reaction vessel 140, this increased pressure may still be maintained, if it is desired to follow the procedure described above in relation to Fig. 16. This may be done, for example, by mechanically reducing an internal volume of reaction vessel 140 (e.g., using a piston) when the carbon dioxide is removed from reaction vessel 140 or by displacing the carbon dioxide with another gas, for example.

[0195] Valve V2 is then opened and closed to introduce sodium oxide from the oxidation vessel 130 into the reaction vessel 140 and produce an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. If desired, this may again be done at the same elevated pressure. Due to the exothermic nature of this reaction, the temperature inside the reaction vessel 140 rises significantly. Heat is therefore extracted from the reaction vessel 140 to suppress this temperature rise. However, since no more gas is released, the pressure remains constant. Valve V3 is then opened and closed to transfer this aqueous solution and the precipitate (still under pressure, if desired), to the solid-aqueous phase separator 16, where they are separated. Thereafter, if the aqueous solution of sodium chloride has been pressurized, the pressure above it may be reduced back down to the initial pressure, to aid its separation into solid sodium chloride and water vapour, as described previously in relation to Fig. 16.

[0196] Fig. 20 schematically shows a first embodiment of a heat transfer pathway 23a, which may be used in the apparatus 2a of Fig. 19. The heat transfer pathway 23a comprises first, second and third heat exchangers, HE1 , HE2 and HE3. In this embodiment, the first heat exchanger HE1 transfers heat from the hot solid end-product leaving the kiln 20 via its outlet 25 to the cooler precipitate from the first outlet 164 of the solid-aqueous phase separator 16 as the precipitate enters the kiln 20 via its inlet 21. This part of the heat transfer pathway 23a is represented by the clockwise dashed and dotted loop in the bottom right-hand corner of Fig. 19. This first transfer of heat raises the temperature of the precipitate to a first intermediate temperature, Ti. Next, the second heat exchanger HE2 transfers heat from the hotter sodium oxide leaving the oxidation vessel 130 via its outlet 134 to the precipitate in the kiln 20 before the sodium oxide enters the reaction vessel 140 via its third inlet 143. This part of the heat transfer pathway 23a is represented by the dashed and dotted line in Fig. 19 from the oxidation vessel 130 to the kiln 20. This second heat transfer raises the temperature of the precipitate further to a second intermediate temperature, T2 > T1. Finally, the third heat exchanger HE3 transfers heat from the even hotter liquid sodium leaving the electrolytic subassembly 10 via its first outlet 17 to the precipitate in the kiln 20 before the liquid sodium enters the oxidation vessel 130 via its second inlet 132. This part of the heat transfer pathway 23a is represented by the dashed and dotted line in Fig. 19 from this outlet 17 to the kiln 20. This final heat transfer is sufficient to cause the thermal decomposition of the hydroxide(s) in the precipitate within the kiln 20 and also raises the temperature of the decomposition products to a final temperature, T3 > T2, which is greater than the thermal decomposition temperature of the respective hydroxide(s). Referring back to Fig. 12, it may be seen that these three successive transfers of heat to the precipitate respectively occur during the three heat extraction phases shown in the graph of Fig. 12 when this graph is read from right to left, and that these three heat extraction phases have successively higher peak temperatures. Whereas after the thermal decomposition, the hot solid end-product is cooled back down by passing through the first heat exchanger HE1 before it leaves the kiln 20, the water vapour leaves the kiln 20 at the higher temperature, T3, and may therefore be recycled back to the hydrochloric acidproducing subassembly 12 for use as a reagent in an RDR to produce hydrochloric acid, as described above.

[0197] Fig. 21 schematically shows a second embodiment of an apparatus 2b for producing at least one of calcium oxide, magnesium oxide and an iron oxide. The apparatus 2b differs from the apparatus 2a of Fig. 19 in the following respects. Firstly, the sodium oxide or hydroxide-producing subassembly 13 comprises a conduit 133 for transporting at least some of the sodium oxide produced in the oxidation vessel 130 through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed within the kiln 20. Sodium oxide transported along conduit 133 therefore absorbs water vapour from the hydroxide(s) in the kiln 20 as they are thermally decomposed. Consequently, the apparatus 2b also differs from the apparatus 2a of Fig. 19 in that the kiln 20 does not have an outlet 24 for water vapour, which having been absorbed by the sodium oxide, is instead carried away along conduit 133 by the solid sodium hydroxide thus formed. Thereafter, the conduit 133 transports this sodium hydroxide to the third inlet 143 of the reaction vessel 140. The sodium oxide and hydroxide may be transported along conduit 133 for example mechanically (e.g., using a screw conveyor) and / or at least partially under gravity, for example. The apparatus 2b of Fig. 21 may comprise a heat transfer pathway similar to that 23a of Fig. 20 with the following difference. In the present case, since heat is transferred directly from the sodium hydroxide formed in the kiln 20 to the hydroxide(s) in the separated precipitate, this direct transfer of heat replaces the second heat exchanger HE2 in the heat transfer pathway 23a.

[0198] Fig. 22 schematically shows a third embodiment of an apparatus 2c for producing at least one of calcium oxide, magnesium oxide and an iron oxide. The apparatus 2c differs from the apparatus 2a of Fig. 19 in several respects, as follows. Firstly, it comprises a sodium oxide or hydroxide-producing subassembly 137, which is adapted to situations in which the oxidant comprises an oxide of another metal, such as iron and / or manganese, derived from an ore of the other metal. It therefore comprises an oxidation vessel 130, a hydration vessel 170 and a second solid-aqueous phase separator 175. The oxidation vessel 130 is for oxidising the liquid sodium in a redox reaction with the oxide of the other metal to produce at least the other metal in elemental form and sodium oxide. The oxidation vessel 130 comprises the first and second inlets 131 , 132 of the sodium oxide or hydroxide-producing subassembly 137 and an outlet 134 for a solid phase at least comprising the sodium oxide, but which may also comprise one or more of: the other metal in elemental form, gangue mineral species, and further reaction products, such as from reactions between gangue species and the liquid sodium and / or sodium oxide. This solid phase may be suspended or entrained in unreacted liquid sodium if the liquid sodium is supplied in excess of a stoichiometric amount thereof required for the redox reaction, but in other possible embodiments, the sodium oxide or hydroxide- producing subassembly 137 may further comprise a solid-liquid sodium phase separator for separating the solid phase from the unreacted liquid sodium. A solid-liquid sodium phase separator suitable for this is described in the present applicant's UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal”; applicant's ref: NE-P-GB 002), the entire contents of which is incorporated herein by reference. The hydration vessel 170 is for hydrating the sodium oxide with water to produce an aqueous solution of sodium hydroxide. It comprises a first inlet 171 for receiving the solid phase from the outlet 134 of the oxidation vessel 130, a second inlet 172 for water and an outlet 174 for the aqueous solution of sodium hydroxide and undissolved solids. The second solid-aqueous phase separator 175 is for separating the undissolved solids from the aqueous solution of sodium hydroxide. It comprises a first inlet 176 for receiving the aqueous solution of sodium hydroxide and undissolved solids from the outlet 174 of the hydration vessel 170, a first outlet 178 for the aqueous solution of sodium hydroxide and a second outlet 179 for the undissolved solids. The first outlet 178 of this second phase separator 175 provides the sodium hydroxide to the third inlet 143 of the reaction subassembly 14.

[0199] In other possible embodiments, the sodium oxide or hydroxide-producing subassembly 137 may also comprise means for separating the other metal in elemental form from the sodium oxide or hydroxide. For example, if the metal in elemental form comprises iron, this may comprise a magnetic separator for separating the iron, which is ferromagnetic, from the sodium oxide and / or gangue species, such as silica and / or alumina, all of which are diamagnetic. This separation may be conducted before the solid phase is passed to the hydration vessel 170, in which case, the solid phase in the hydration vessel 170 may already consist chiefly of sodium oxide, as well as possibly some gangue. Or it may be conducted after the solid phase is passed to the hydration vessel 170, in which case, the other metal in elemental form remains undissolved in the aqueous solution of sodium hydroxide formed in the hydration vessel 170, as described above, and is instead separated from the aqueous solution of sodium hydroxide by the second solid-aqueous phase separator 175, along with any other undissolved solids. The other metal in elemental form may then be separated from the other undissolved solids thereafter. Various techniques for separating the metal in elemental form from other solids are described in the present applicant's UK patent application nos. 2417059.9 ("Carbon-Free Method and Apparatus for Producing Iron and Steel”; applicant's ref: NE-P-GB 001) and 2417063.1 ("Carbon-Free Method and Apparatus for Producing Manganese”; applicant's ref: NE-P-GB 008), both mentioned above.

[0200] Moreover, in other possible embodiments, the sodium oxide or hydroxide-producing subassembly 137 may also comprise a conduit 133 similar to that described above in relation to Fig. 21 and located between the outlet 134 of the oxidation vessel 130 and the first inlet 171 of the hydration vessel 170. In such a case, the quantity of water introduced to the hydration vessel 170 via the second inlet 172 thereof may be reduced by the same amount as the amount of water vapour absorbed by the sodium oxide transported along conduit 133 from the hydroxide(s) in the separated precipitate within the kiln 20.

[0201] A second way in which the apparatus 2c of Fig. 22 differs from the apparatus 2a of Fig. 19 is in that the reaction subassembly 14 comprises a separation device 145 and a precipitation vessel 150, as well as the gas-tight reaction vessel 140. In this embodiment, the gas-tight reaction vessel 140 comprises first and second inlets 141 , 142 and an outlet 144 as described above in relation to Fig. 19, but does not comprise the third inlet 143 of the reaction subassembly 14, which is instead provided as an inlet 152 of the precipitation vessel 150. The separation device 145 is for separating undissolved gangue species from an aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride produced by dissolving the carbonate mineral in the hydrochloric acid within the gas-tight reaction vessel 140. The separation device 145 comprises an inlet 147, which is connected to the outlet 144 of the reaction vessel 140, a first outlet 148 for this aqueous solution and a second outlet 149 for the undissolved gangue species. The precipitation vessel 150 comprises a first inlet 151, which is connected to the first outlet 148 of the separation device 145, the second inlet 152 mentioned above for receiving the aqueous solution of sodium hydroxide from the first outlet 178 of the second solid-aqueous phase separator 175, and an outlet 154 for an aqueous solution of sodium chloride and a precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide.

[0202] The reaction subassembly 14 may be operated as follows. The carbonate mineral is dissolved in hydrochloric acid in the reaction vessel 140, and whilst the aqueous solution formed thereby inside reaction vessel 140 still has an acidic pH, the carbon dioxide is removed to prevent its reabsorption into aqueous solution at higher pH, as described above in relation to Fig. 19. Valve V3 is then opened to transfer the aqueous solution and undissolved gangue species to the separation device 145, where the undissolved gangue species are separated from the aqueous solution, before the aqueous solution is transferred to the precipitation vessel 150. Here, the aqueous solution of sodium hydroxide from the second solid-aqueous phase separator 175 is added to it to produce the aqueous solution of sodium chloride and the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. In other possible embodiments, the same principle may be repeated by providing further separation devices and precipitation vessels connected in series and operating at different pHs to allow the separation of different gangue species and different hydroxides from each other, as described above in relation to Figs. 6A and 6B. Moreover, in either of the apparatuses 2a, 2c, the outlet 124 of the hydrochloric acid-producing subassembly 12 may also be connected to the second outlet 165 of the first solid-aqueous phase separator 16 to allow more hydrochloric acid to be added to the alkaline aqueous solution which remains after separating the desired hydroxide(s), to produce a neutral aqueous solution of sodium chloride suitable for recycling to the electrolytic subassembly 10.

[0203] Thus the apparatus 2c of Fig. 22 allows gangue mineral species in the ore comprising the carbonate mineral to be separated from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, and also allows an ore comprising an oxide of another metal, such as iron and / or manganese, to be used as the oxidant to produce sodium oxide from the liquid sodium, but for the other metal in elemental form produced by this redox reaction, as well as possibly gangue derived from this other ore, to be separated from the sodium oxide, so that these other chemical species are not added to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride either, when producing the aqueous solution of sodium chloride and the precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide in the reaction subassembly 14.

[0204] A third way in which the apparatus 2c of Fig. 22 differs from the apparatus 2a of Fig. 19 is that the hydrochloric acid-producing subassembly 12 comprises a combustion chamber 430 and an absorber 440. The combustion chamber 430 is for combusting chlorine gas with hydrogen gas to produce hydrogen chloride. It therefore comprises a first inlet 431 for chlorine gas, a second inlet 432 for hydrogen gas, and an outlet 434 for the hydrogen chloride. The first inlet 431 of the combustion chamber 430 is equivalent to the first inlet 121 of the hydrochloric acid-producing subassembly 12 and is therefore connected downstream of the second outlet 15 of the electrolytic subassembly 10. The second inlet 432 of the combustion chamber 430 may receive hydrogen from the electrolytic subassembly 10 as well. In this embodiment, the electrolytic subassembly 10 is adapted to electrolyse an aqueous solution of sodium chloride, at least some of which may be recycled to the electrolytic subassembly 10 from the outlet 165 of the solid-aqueous phase separator 16. For example, if the electrolytic subassembly 10 comprises 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 the aqueous solution of sodium hydroxide thus formed 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 this solid sodium hydroxide to produce liquid sodium, hydrogen gas and oxygen gas, then the second inlet 432 of the combustion chamber 430 may be connected downstream of an outlet for hydrogen gas from at least the second type of electrolytic cell. The absorber 440 is for contacting the hydrogen chloride with liquid water to produce the hydrochloric acid. It 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. The second inlet 442 and the outlet 444 of the absorber 440 are respectively equivalent to the second inlet 122 and the outlet 124 of the hydrochloric acid-producing subassembly 12.

[0205] Producing hydrogen chloride by combusting hydrogen with chlorine in the combustion chamber 430 and producing hydrochloric acid by absorbing the hydrogen chloride thus formed in water in the absorber 440 are both exothermic processes. Whereas the temperature of the hydrochloric acid inside the absorber 440 has to be maintained below its boiling point, the combustion reaction inside the combustion chamber 430 can be conducted at a significantly higher temperature, allowing heat to be transferred from the combustion chamber 430 to the separated precipitate inside the kiln 20.

[0206] Figs. 23A and 23B schematically show two possible ways in which gaseous carbon dioxide may be removed from reaction vessel 140 in either of the apparatuses 2a, 2c described above. Fig. 23A schematically shows a first embodiment of such an apparatus 2d, which comprises a carbonation vessel 160 having a first inlet 162 for carbon dioxide gas, a second inlet 163 for a second portion of sodium oxide and / or hydroxide and an outlet 166 for at least sodium carbonate produced by reacting the carbon dioxide in a carbonation reaction with the second portion of sodium oxide and / or hydroxide. This second portion of sodium oxide and / or hydroxide is supplied to the carbonation vessel 160 from the outlet 134, 178 of the sodium oxide or hydroxide-producing subassembly 13, 137. In the apparatus 2d, the reaction vessel 140 further comprises a second outlet 146 for carbon dioxide gas, which is connected to the inlet 162 of the carbonation vessel 160. The outlet 146 is provided with a valve V4, which allows the transfer of carbon dioxide from the reaction vessel 140 to the carbonation vessel 160 to be regulated. Referring to the description given above in relation to Fig. 19, valve V4 may therefore be opened and closed after valve V1 is closed, but before valve V2 is opened. The apparatus 2d further comprises a heat transfer pathway 23c (described below in relation to Fig. 25) for transferring heat produced by this exothermic carbonation reaction from the carbonation vessel 160 to the precipitate within the kiln 20.

[0207] Fig. 23B schematically shows a second embodiment of such an apparatus 2e, comprising a conduit 167 for transporting the second portion of sodium oxide or of solid sodium hydroxide from the outlet 134, 178 of the sodium oxide or hydroxide-producing subassembly 13, 137 through an atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid within the gas-tight reaction vessel 140. The conduit 167 is therefore provided with a first valve V5 for admitting the sodium oxide or solid sodium hydroxide to the reaction vessel 140 and a second valve V6, which allows solid sodium carbonate produced by a carbonation reaction between the carbon dioxide and the sodium oxide or solid sodium hydroxide to be removed from the reaction vessel 140. Solid-phase reagents and products may be transported along conduit 167 from valve V5 to valve V6 for example mechanically (e.g., using a screw conveyor) and / or at least partially under gravity, for example. Valve V5 may therefore be opened and closed after valve V1 is closed, to admit the sodium oxide or solid sodium hydroxide to the reaction vessel 140, and valve V6 may be opened and closed after solid sodium carbonate has been produced by this carbonation reaction, to remove this sodium carbonate from the reaction vessel 140 before valve V2 is opened. Heat produced by this exothermic carbonation reaction within the reaction vessel 140 may be transferred to the precipitate within the kiln 20 using a suitable heat transfer pathway. Alternatively or additionally, however, heat may also be transferred to the precipitate within the kiln 20 from the solid sodium carbonate after it has left the reaction vessel 140, by bringing the hot sodium carbonate into thermal but not physical contact with the precipitate contained therein, as described below in relation to Fig. 24.

[0208] Fig. 24 therefore schematically shows a second embodiment of a heat transfer pathway 23b, which may be used in the apparatuses 2c of Fig. 22 and 2e of Fig. 23B. The heat transfer pathway 23b comprises fourth, fifth and sixth heat exchangers, HE4, HE5 and HE6. The fourth heat exchanger HE4 transfers heat to the precipitate entering the kiln 20 via its inlet 21 from hot, solid sodium hydroxide produced in hydration vessel 170 by hydrating a second portion of sodium oxide from the oxidation vessel 130 with steam. This first heat transfer raises the temperature of the precipitate to a first intermediate temperature, T4. Next, the fifth heat exchanger HE5 transfers heat from hotter sodium carbonate produced by transporting the solid sodium hydroxide along conduit 167 through the reaction vessel 140. This part of the heat transfer pathway 23b is represented by the dashed and dotted line in Fig. 19 from the reaction vessel 140 to the kiln 20. This second heat transfer raises the temperature of the precipitate further to a second intermediate temperature, T5 > T4. Referring back to Fig. 13, it may be seen that these two successive transfers of heat to the precipitate respectively occur during the last two heat extraction phases shown in the graph of Fig. 13 when this graph is read from left to right, and that the second of these two heat extraction phases (carbonation) has a higher peak temperature than the first of them (hydration).

[0209] Since the electrolytic subassembly 10 in the apparatus 2c produces liquid sodium by fusing and electrolysing solid sodium hydroxide, liquid sodium leaving this electrolytic subassembly 10 via its first outlet 17 is not hot enough to heat the precipitate in kiln 20 above Ts. This is unlike the liquid sodium produced by the electrolytic subassembly 10 in the apparatus 2a of Fig. 19. However, since the hydrochloric acid-producing subassembly 12 in apparatus 2c comprises a combustion chamber 430, in which the combustion of chlorine with hydrogen can reach temperatures above 2 000 “Celsius, the heat transfer pathway 23b comprises a branch 231 for raising the temperature of the liquid sodium from electrolytic subassembly 10 by extracting heat from the combustion chamber 430, before this liquid sodium is then used to heat the precipitate to a temperature > Ts. In other words, in this case, part of the heat transfer pathway 23b contains liquid sodium, which is used as a heat transfer fluid to transfer heat from the combustion chamber 430 to the precipitate inside the kiln 20.

[0210] Finally, therefore, the sixth heat exchanger HE6 transfers heat to the precipitate in the kiln 20 from the now even hotter liquid sodium from the branch 231 of heat transfer pathway 23b, before the liquid sodium then enters the oxidation vessel 130 via its second inlet 132. This part of the heat transfer pathway 23b is represented in Fig. 19 by the dashed and dotted line from the hydrochloric acid-producing subassembly 12 to the kiln 20. This final heat transfer is sufficient to cause the thermal decomposition of the hydroxide(s) in the precipitate within the kiln 20 and also raises the temperature of the decomposition products to a final temperature, Te > Ts, which is greater than the thermal decomposition temperature of the respective hydroxide(s). After this thermal decomposition, therefore, steam leaving the kiln 20 via its first outlet 24 can be recycled back to the hydration vessel 170 to produce more solid sodium hydroxide and the hot solid end-product which leaves the kiln 20 via its second outlet 25 can, for example, be supplied directly to an inlet of a rotary kiln for manufacturing cement clinker.

[0211] Fig. 25 schematically shows a third embodiment of a heat transfer pathway 23c, which may be used in any of the apparatuses 2a, 2b and 2c if any of them comprises the apparatus 2d of Fig. 23A instead of the apparatus 2e of Fig. 23B. The heat transfer pathway 23c comprises a section 232, which is arranged to extract heat from the carbonation vessel 160, and a seventh heat exchanger HE7. The heat transfer pathway 23c also contains a heat transfer fluid HTF, which transfers the heat from the carbonation vessel 160 to the precipitate in the kiln 20 via the seventh heat exchanger HE7. The seventh heat exchanger HE7 therefore takes the place of the fifth heat exchanger HE5 in the heat transfer pathway 23b of Fig. 24. However, as described above in relation thereto, the HTF may, for example, again be liquid sodium derived from the electrolytic subassembly 10.

[0212] Fig. 26 schematically shows part of a sixth embodiment of an apparatus 2f as described herein, wherein the outlet 25 of kiln 20 is connected to and therefore upstream of an inlet 241 of a rotary kiln 240 for manufacturing cement clinker. The apparatus 2f may comprise any combination of features of the apparatuses 2a - 2e described above and any arrangement of heat transfer pathway as described above. Thus if the apparatus 2f is supplied with a carbonate mineral having a majority of cations comprising at least one of calcium and magnesium (for example, if the carbonate mineral is calcite, magnesite or dolomite), the apparatus 2f may substitute for a pre-calciner of the carbonate mineral by supplying the solid end-product produced at outlet 25 to the rotary kiln 240 without emitting any carbon dioxide. Moreover, if the solid end-product leaves the kiln 20 at an elevated temperature, considerable energy may also be saved in manufacturing cement clinker in rotary kiln 240, raw mix for which may therefore be heated without the need to burn any fossil fuel, for example by using excess heat from the apparatus 2f and / or electrical heating.

[0213] If, on the other hand, an apparatus as described herein is supplied with a carbonate mineral having a majority of iron cations (for example, if the carbonate mineral is siderite), in other possible embodiments, the outlet 25 of the kiln 20 may instead be connected upstream of the first inlet 131 of the sodium oxide or hydroxide-producing subassembly 13, 137. This connection may be made directly or indirectly via a magnetic separator. In either case, iron oxide produced at the outlet 25 of kiln 20 can then act as a feedstock for the production of iron in elemental form in oxidation vessel 130.

[0214] Regardless of how the outlet 25 of kiln 20 is connected, the inlet 21 and both of the outlets 24, 25 of kiln 20 may each be provided with respective open-and-shut valves to allow the kiln 20 to be rendered gas-tight. This permits the thermal decomposition of the hydroxide(s) in the separated precipitate to be conducted in the absence of oxygen. In such embodiments, the outlet 24 for water vapour from the kiln 20 may then be connected upstream of a respective inlet of the hydrochloric acid-producing subassembly 12 to allow pure and high-temperature water vapour from kiln 20 to be used as a reagent in an RDR with chlorine gas also supplied to the hydrochloric acidproducing subassembly 12 via the second inlet 15 thereof.

[0215] In summary, therefore, the present invention provides a method and apparatus for producing oxides of calcium, magnesium and iron from carbonate mineral ores of the same metals, whereby, for example, lime may be extracted from limestone, dolomitic lime from dolomite, magnesia from magnesite and iron oxide(s) from siderite ores, all without burning any carbonaceous fuel, including any fossil fuel, and with electricity as its only energy requirement. Thus the invention can replace the traditional calcination process, including for the production of cement clinker, and can also be used to convert siderite ores into one or more iron oxides suitable for ironmaking. In some embodiments, carbon dioxide captured from a carbonate mineral ore treated by the invention may be reacted with sodium oxide which the invention also produces, and / or with sodium hydroxide derived therefrom, to produce sodium carbonate, thereby providing a replacement for the ammonia-soda, or Solvay, process as well.

[0216] 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

Claims1. A method (100, 200a - 200k, 200m, 200n, 200p) of producing at least one of calcium oxide, magnesium oxide and an iron oxide, comprising: producing (101 b; 201 a, 201 b, 201c) liquid sodium and chlorine gas by electrolysis; oxidising (202, 303) at least some of the liquid sodium to produce at least sodium oxide; using (203) 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; adding (205) an ore comprising a carbonate mineral of at least one of calcium, magnesium and iron to the hydrochloric acid to dissolve the carbonate mineral therein and produce gaseous carbon dioxide 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; reacting (207) at least some of the sodium oxide, or sodium hydroxide derived from hydrating (317) at least some of the 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, 208c) at least some of the precipitate from the aqueous solution of sodium chloride; and thermally decomposing (209, 209a, 209b, 209c, 209d) at least some of the calcium hydroxide, magnesium hydroxide and ferrous hydroxide to produce water vapour and a solid end-product respectively comprising at least one of calcium oxide, magnesium oxide and an iron oxide; wherein the thermal decomposition (209, 209a, 209b, 209c, 209d) comprises transferring (210) sufficient heat to the separated precipitate to cause the thermal decomposition of the respective hydroxide(s) from at least one of the aforementioned processes (101b; 201 a, 201b, 201c; 202, 303; 203, 204; 317), their respective products and the products of the thermal decomposition (209, 209a, 209b, 209c, 209d) itself.

2. A method (200a - 200 k, 200m, 200n, 200p) according to claim 1 , wherein: oxidising (202) at least some of the liquid sodium comprises reacting (303) at least some of the liquid sodium in an exothermic redox reaction with an oxide of another metal (M) to produce the other metal in elemental form and the sodium oxide; the other metal (M) comprises at least one of iron and manganese; and the redox reaction (303) is conducted in an inert atmosphere and below a temperature for formation of a ternary oxide of sodium with the other metal (M).

3. A method (200h) according to claim 1 or claim 2, further comprising: passing (226) at least some of the sodium oxide through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed (209, 209a, 209b, 209c, 209d), whereby said at least some of the sodium oxide reacts in a hydration reaction (317a) with the water vapour to produce solid sodium hydroxide; and transferring (210) heat to the separated precipitate from at least one of the hydration reaction (317a) and the solid sodium hydroxide produced thereby.

4. A method (200c, 200g) according to any one of claims 1 to 3, wherein the sodium oxide, or solid sodium hydroxide derived from hydrating (317a) at least some of the sodium oxide, has an admixture of gangue and the method further comprises: cooling (210) the sodium oxide, or the solid sodium hydroxide derived therefrom, and the gangue to a temperature of less than 100 “Celsius; adding (317) at least some of the cooled sodium oxide or hydroxide and gangue to liquid water, thereby hydrating the sodium oxide or hydroxide to produce an aqueous solution of sodium hydroxide having a final concentration of less than 2.5 M and a temperature of less than 85 “Celsius and undissolved gangue; and phase separating (318) the undissolved gangue from the aqueous solution of sodium hydroxide.

5. A method (200f, 200g, 200h) according to any one of the preceding claims, wherein the sodium oxide, or the sodium hydroxide derived therefrom, which is reacted (207) with the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride, is or is derived from a first portion constituting between 5 / 12 and 7 / 12, inclusive, of a total number of mols of the sodium oxide, and the method further comprises: reacting (214a, 214b) at least some of the captured carbon dioxide with a second portion of the sodium oxide, or with sodium hydroxide derived from hydrating at least some of the second portion of sodium oxide, in a carbonation reaction to produce at least sodium carbonate; and transferring (210) heat to the separated precipitate from at least one of the carbonation reaction (214a, 214b) and the sodium carbonate.

6. A method (200f, 200h) according to claim 5, wherein the carbonation reaction (214a, 214b) comprises passing (213, 213a) the second portion of sodium oxide or solid sodium hydroxide derived therefrom through an atmosphere in which the carbonate mineral is dissolved (205) in the hydrochloric acid.

7. A method (200g, 200h) according to claim 6, wherein the carbonation reaction (214a, 214b) comprises reacting (214b) at least some of the captured carbon dioxide with solid sodium hydroxide to produce the sodium carbonate, a reduced amount of gaseous carbon dioxide and water vapour at a final temperature of from 310 to 400 “Celsius, inclusive.

8. A method (200c, 200d, 200e, 200g) according to any one of the preceding claims, further comprising phase-separating (211, 211 a) undissolved gangue species from the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride before reacting (207) at least some of this aqueous solution with sufficient of the sodium oxide or sodium hydroxide derived therefrom to produce a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide.

9. A method (200d) according to claim 8, wherein the ore comprises a siliceous mineral, the undissolved gangue species phase-separated (211, 21 1 a) from the aqueous solution comprise silica, and the method further comprises using (222) at least some of the phase-separated gangue species as at least one of:(I) an ingredient with an alkaline activator in the manufacture of an alkaline-activated or geopolymer cement; and(ii) a pozzolan in a reaction with at least one of calcium oxide and magnesium oxide to produce a hydraulic cement.

10. A method (200d) according to claim 8 or claim 9, wherein the ore comprises an aluminate mineral, the undissolved gangue species are phase-separated (211a) from the aqueous solution at a pH of 4.5 or less, and the method further comprises, after phase-separating (211 a) the undissolved gangue species from this acidic aqueous solution and before producing the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide: adding (207a) sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the acidic aqueous solution to produce an aqueous solution with a pH of from 5 to 7.5, inclusive, and a precipitate comprising aluminium hydroxide; phase-separating (211 b) the precipitate comprising aluminium hydroxide from the aqueous solution with pH in the stated range; and thereafter, continuing to add (207b) the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution with pH in the stated range to produce an alkaline aqueous solution and the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide.

11. A method (200d) according to claim 10, further comprising: drying and dehydroxylating (223) at least some of the phase-separated precipitate comprising aluminium hydroxide to produce alumina; and using (224) at least some of the alumina as feed material for producing aluminium by electrolysis.

12. A method (200e) according to any one of the preceding claims, wherein reacting (207) at least some of the sodium oxide, or sodium hydroxide derived from hydrating (317) at least some of the sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrouschloride to produce an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide, and phase separating (208, 208c) at least some of this precipitate from the aqueous solution of sodium chloride respectively comprise: adding (207c) sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride to precipitate out ferrous hydroxide at a pH of from 8 to 12, inclusive; phase separating (208a) at least some of the ferrous hydroxide from the aqueous solution of sodium chloride; thereafter, continuing to add (207c) sufficient of the sodium oxide, or sodium hydroxide derived from hydrating at least some of this sodium oxide, to the aqueous solution of sodium chloride to precipitate out calcium hydroxide at a pH of 12.5 or more; and phase separating (208b) at least some of the calcium hydroxide from the aqueous solution of sodium chloride.

13. A method (200d, 200e) according to any one of the preceding claims, further comprising, after phaseseparating (208, 208c) at least some of the precipitate comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide from the aqueous solution of sodium chloride: adding (225) further hydrochloric acid to this aqueous solution to reduce its pH and produce a neutral aqueous solution of sodium chloride; and using (114) at least some of the neutral aqueous solution of sodium chloride as feed material for producing (101b) the liquid sodium and chlorine gas by electrolysis.

14. A method (200j) according to any one of the preceding claims, further comprising: drying (110) at least some of the aqueous solution of sodium chloride to produce water vapour; capturing (111 ) at least some of the water vapour thus produced; condensing (112) at least some of the captured water vapour to produce liquid water; and using (113) at least some of the liquid water thus produced (112) as at least some of the liquid water in which the hydrogen chloride is dissolved (204) to produce the hydrochloric acid.

15. A method (100) according to any one of the preceding claims, wherein the thermal decomposition (209) is performed (209a) in the absence of oxygen, and the method further comprises: capturing (103) at least some of the water vapour produced by the thermal decomposition; reacting (104) at least some of the chlorine gas produced by fusing and electrolysing solid sodium chloride 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 (106) heat from the hydrochloric acid thus produced, to maintain its temperature substantially constant until the stream of tail gases is no longer in contact therewith.

16. A method (200I) according to any one of the preceding claims, wherein: producing (101 b; 201a, 201b, 201c) liquid sodium and chlorine gas by electrolysis comprises electrolysing (201 a) 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 (201 b) 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; using (104, 203) 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 (201 a) an aqueous solution of sodium chloride and fusing and electrolysing (201c) at least some of the solid sodium hydroxide, to produce the hydrogen chloride; and the method further comprises transferring (210) heat to the separated precipitate from at least one of the combustion (203b) and the hydrogen chloride.

17. A method (200a - 200k, 200m, 200n, 200p) according to any one of the preceding claims, wherein: the carbonate mineral is dissolved (205) in the hydrochloric acid at a pressure above atmospheric pressure; and the hydrochloric acid is present in excess of a stoichiometric amount thereof required to dissolve the carbonate mineral and has a temperature above 50 “Celsius and at least 15 “Celsius below the boiling point of the hydrochloric acid at that pressure.

18. A method (200a - 200k, 200m, 200n, 200p) according to any one of the preceding claims, wherein: the hydrochloric acid has an initial temperature, before dissolving the carbonate mineral, at least 15 “Celsius below the boiling point of the hydrochloric acid at an initial pressure at which dissolving (205) the carbonate mineral in the hydrochloric acid is started (S); dissolving (205) the carbonate mineral in the hydrochloric acid is continued at increasing pressure until reaching a final pressure; reacting (207) at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the sodium oxide, with at least some of the aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride is conducted at the final pressure and at increasing temperature until reaching a final temperature above the boiling point of the aqueous solution of sodium chloride at the initial pressure and below the boiling point of the aqueous solution of sodium chloride at the final pressure; and the method further comprises, after phase separating (208) at least some of the precipitate from the aqueous solution of sodium chloride, drying (110) the aqueous solution of sodium chloride by reducing (R) the pressure of an atmosphere above the aqueous solution of sodium chloride to at or below the initial pressure.

19. A method (200k) 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 a minority of cations comprising at least one of iron and manganese, and the method further comprises: phase separating (208c) the precipitate from the aqueous solution of sodium chloride in the absence of oxygen; and magnetically separating (218) at least one of ferrous hydroxide and manganese hydroxide from at least one of calcium hydroxide and magnesium hydroxide in the separated precipitate in the absence of oxygen and at a temperature of less than 90 “Celsius, before thermally decomposing (209) the hydroxide of at least one of calcium and magnesium to produce the solid end-product.

20. A method (200m, 200n) according to any one of claims 1 to 18, wherein the carbonate mineral has a composition selected from the group consisting of:(I) a majority of cations comprising at least one of calcium and magnesium and a minority of cations at least comprising manganese; and(ii) a majority of iron cations and a minority of cations comprising both magnesium and manganese; and the method further comprises: conducting the thermal decomposition (209b, 209c) of the hydroxide(s) in the separated precipitate in an oxygenated atmosphere at a temperature of from 600 to 650 “Celsius, inclusive; cooling (219) the solid end-product down thereafter; and magnetically separating (220a, 220b) a diamagnetic oxide from a paramagnetic oxide in the solid endproduct at a temperature of less than 90 “Celsius.21 . A method (200p) according to any one of claims 1 to 18, wherein the carbonate mineral has a majority of iron cations and a minority of cations comprising both magnesium and manganese, and the method further comprises: conducting the thermal decomposition (209d) of the hydroxide(s) in the separated precipitate at a temperature of more than 220 “Celsius in the absence of oxygen; and magnetically separating (220c) magnesium oxide and manganese monoxide from metallic iron and iron oxide(s) in the solid end-product by applying a magnetic field gradient to the solid end-product at a temperature of more than 160 “Celsius and less than 330 “Celsius.

22. A method (200n) according to any one of the preceding claims, further comprising transferring (221) at least some of the heat not consumed by the thermal decomposition (209, 209a, 209b, 209c, 209d) to at least one of: a sodium compound which is fused and electrolysed (101 b, 201c) to produce the liquid sodium; andraw mix for manufacturing cement clinker, wherein the raw mix comprises the solid end-product as an ingredient thereof.

23. An apparatus (2a - 2f) for producing at least one of calcium oxide, magnesium oxide and an iron oxide, the apparatus 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 sodium oxide or hydroxide-producing subassembly (13, 137) for producing sodium oxide or hydroxide from at least some of the liquid sodium and an oxidant, and comprising a first inlet (131) for the oxidant, a second inlet (132) for receiving liquid sodium from the first outlet (17) of the electrolytic subassembly (10), and an outlet (134, 178) for sodium oxide or hydroxide; a hydrochloric acid-producing subassembly (12) for producing hydrochloric acid from at least some of the 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 reaction subassembly (14) comprising a 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 receiving an 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 sodium oxide or hydroxide from the outlet (134, 178) of the sodium oxide or hydroxide-producing subassembly (130), and an outlet (144, 154) for an aqueous solution of sodium chloride and a precipitate respectively 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, wherein the phase separator (16) comprises an inlet (161) for receiving the aqueous solution of sodium chloride and the precipitate from the outlet (144, 154) of the reaction subassembly (14), a first outlet (164) for the precipitate, and a second outlet (165) for the aqueous solution of sodium chloride; a kiln (20) for thermally decomposing at least some of the calcium hydroxide, magnesium hydroxide and ferrous hydroxide in 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, wherein the kiln (20) comprises an inlet (21) for receiving the precipitate from the first outlet (164) of the phase separator (16) and an outlet (25) for the solid endproduct; and a heat transfer pathway (23, 23a, 23b, 23c) for transferring sufficient heat to the precipitate in the kiln (20) to cause the thermal decomposition from at least one of the aforementioned subassemblies (10, 12, 13, 137, 14), their respective products and the products of the thermal decomposition itself.

24. An apparatus (2a - 2f) according to claim 23, wherein the heat transfer pathway (23, 23a, 23b, 23c) is adapted to contain liquid sodium as a heat transfer fluid.

25. An apparatus (2b) according to claim 23 or claim 24, wherein the sodium oxide or hydroxide-producing subassembly (13) comprises a conduit (133) for transporting at least some of the sodium oxide through an atmosphere in which the hydroxide(s) in the separated precipitate are thermally decomposed within the kiln (20).

26. An apparatus (2c) according to claim 24 or claim 25, wherein the oxidant comprises an oxide of another metal from an ore of the other metal, and the sodium oxide or hydroxide-producing subassembly (13, 137) comprises: an oxidation vessel (130) for oxidising the liquid sodium in a redox reaction with the oxide of the other metal to produce at least the other metal in elemental form and the sodium oxide, wherein the oxidation vessel (130) comprises the first and second inlets (131 , 132) of the sodium oxide or hydroxide-producing subassembly (13, 137) and an outlet (134) for a solid phase at least comprising the sodium oxide; a hydration vessel (170) for hydrating the sodium oxide with water to produce an aqueous solution of sodium hydroxide, and comprising a first inlet (171) for receiving the solid phase from the outlet (134) of the oxidation vessel (130), a second inlet (172) for water and an outlet (174) for the aqueous solution of sodium hydroxide and undissolved solids; and a second solid-aqueous phase separator (175) for separating the undissolved solids from the aqueous solution of sodium hydroxide, comprising a first inlet (176) for receiving the aqueous solution of sodium hydroxide and undissolved solids from the outlet (174) of the hydration vessel (170), a first outlet (178) for the aqueous solution of sodium hydroxide and a second outlet (179) for the undissolved solids, wherein the first outlet (178) of the second phase separator (175) is upstream of the third inlet (143, 152) of the reaction subassembly (14).

27. An apparatus (2c) according to any one of claims 24 to 26, wherein the reaction subassembly (14) comprises a separation device (145) for separating undissolved gangue species from an aqueous solution comprising at least one of calcium chloride, magnesium chloride and ferrous chloride produced by dissolving the carbonate mineral in the hydrochloric acid within the gas-tight reaction vessel (140).

28. An apparatus (2d) according to any one of claims 24 to 27, wherein the gas-tight reaction vessel (140) further comprises a second outlet (146) for gaseous carbon dioxide produced by dissolving the carbonate mineral in the hydrochloric acid within the gas-tight reaction vessel (140), and the apparatus further comprises: a carbonation vessel (160) for reacting at least some of the sodium oxide, or sodium hydroxide derived from hydrating at least some of the sodium oxide, with the gaseous carbon dioxide thus produced to produce at least sodium carbonate, wherein the carbonation vessel (160) comprises a first inlet (162) for receiving carbon dioxide from the second outlet (146) of the gas-tight reaction vessel (140), a second inlet (163) for receiving sodiumoxide or hydroxide from the outlet (134, 178) of the sodium oxide or hydroxide-producing subassembly (13, 137), and an outlet (166) for at least the sodium carbonate; and a heat transfer pathway (23b) for transferring heat from the carbonation vessel (160) to the precipitate within the kiln (20).

29. An apparatus (2e) according to any one of claims 24 to 28, further comprising a conduit (167) for transporting sodium oxide or solid sodium hydroxide from the outlet (134, 178) of the sodium oxide or hydroxide- producing subassembly (13, 137) through an atmosphere in which the carbonate mineral is dissolved in the hydrochloric acid within the gas-tight reaction vessel (140).

30. An apparatus (2a, 2b, 2c) according to any one of claims 24 to 29, wherein the kiln (20) can be rendered gas-tight to permit the thermal decomposition of the hydroxide(s) in the separated precipitate to be conducted in the absence of oxygen. 31 . An apparatus (2f) according to any one of claims 24 to 30, wherein the second outlet (25) of the kiln (20) for the solid end-product is connected upstream of an inlet (241) of a rotary kiln (240) for manufacturing cement clinker.

Images