Method and apparatus for producing liquid sodium

Abstract

A method of producing liquid sodium by reacting solid-phase sodium hydroxide in an anoxic environment with excess liquid sodium to produce sodium oxide and hydrogen gas, capturing hydrogen, separating

Description

The present invention concerns a method and apparatus for producing liquid sodium. Background of the Invention The present applicant's co-pending UK patent application no. 2417059.9 ("Carbon-Free Method and Apparatus for Producing Iron and Steel"; applicant's ref: NE-P-GB 001) describes a method and apparatus for producing iron from an iron ore and / or from a source of waste iron oxide, and the onward conversion of that iron into steel. The present applicant's co-pending UK patent application no. 2417063.1 ("Carbon-Free Method and Apparatus for Producing Manganese"; applicant's ref: NE-P-GB 008) describes a method and apparatus for producing manganese from a manganese ore. In each case, an amount of liquid sodium in excess of the stoichiometric amount thereof is used in a redox reaction between the liquid sodium and an oxide of iron or manganese, respectively, to precipitate out from the liquid sodium both the respective metal in elemental form and other insoluble products comprising sodium oxide. In each case, the liquid sodium may be produced by electrolysis, for example from sodium chloride, with the co-production of chlorine gas. In both cases, after the liquid sodium has been separated from the precipitated reaction products and the elemental metal has also been separated from the other insoluble products, the sodium oxide may be recovered by adding at least some of these other insoluble products to liquid water, thereby hydrating the sodium oxide therein to produce an aqueous solution of sodium hydroxide, and separating undissolved solids (typically, gangue mineral species like silica and alumina) as a solid phase from the aqueous solution of sodium hydroxide. This sodium hydroxide solution may then be used as a source for producing liquid sodium by electrolysis and / or in another, chemical process. For example, it may be dried to produce solid sodium hydroxide, which may then be fused and electrolysed using the Castner process to produce liquid sodium, together with gaseous hydrogen and oxygen. The present applicant's co-pending UK patent application no. 2417075.5 ("Industrial Chemical Process and Apparatus"; applicant's ref: NE-P-GB 007) describes an industrial chemical process and an apparatus for carrying out that process, in which a method for producing iron and / or manganese in elemental form from oxides thereof, as respectively described in the applicant's co-pending UK patent application nos. 2417059.9 and 2417063.1 just mentioned above, is integrated with a method for producing an oxide or hydroxide of at least one of calcium, magnesium and iron from another ore respectively comprising a carbonate mineral of at least one of calcium, magnesium and iron. In the latter method, chlorine gas produced by electrolysis is used to produce hydrogen chloride, and at least some of the hydrogen chloride is dissolved in liquid water to produce hydrochloric acid. The other ore comprising the carbonate mineral is then added to at least some of this hydrochloric acid to dissolve the carbonate mineral therein and produce carbon dioxide gas and an aqueous solution respectively comprising at least one of calcium chloride, magnesium chloride and ferrous chloride. Moreover, a first portion of the sodium oxide from the redox reaction, or sodium hydroxide derived from hydrating at least some of the first portion of sodium oxide, is reacted with at least some of this aqueous solution to produce an aqueous solution of sodium chloride and a precipitate respectively comprising at least one of calcium hydroxide, magnesium hydroxide and ferrous hydroxide. Any of these hydroxides may then be dehydroxylated to produce their respective oxides. Meanwhile, a second portion of the sodium oxide from the redox reaction, or sodium hydroxide derived from hydrating at least some of the first portion of sodium oxide, is used in at least one of: (i) a reaction with at least some of the hydrochloric acid to produce an aqueous solution of sodium chloride, and (ii) a reaction with at least some of the carbon dioxidegas to produce at least sodium carbonate. The aqueous solution of sodium chloride produced from the first portion of sodium oxide and / or from the second portion of sodium oxide may then be recycled back for electrolysis. For example, it may be dried to produce solid sodium chloride, which may then be fused and electrolysed using the Downs process to produce liquid sodium and chlorine gas again. The entire contents of each of these co-pending UK patent application nos. 2417059.9, 2417063.1 and 2417075.5 referred to above are incorporated herein by reference. In the integrated chemical process described in co-pending UK patent application no. 2417075.5, the molar ratio of input materials, namely of the respective oxides from the iron and / or manganese ore on the one hand to the carbonate mineral on the other, can be varied as desired. Thus if this molar ratio is chosen to be high, the total molar amount of sodium oxide produced by the redox reaction, or of sodium hydroxide derived from hydrating at least some of it, may be made to exceed the sum of the molar amounts of the first and second portions thereof which are consumed as described above, to leave an excess of sodium hydroxide. Thermodynamic calculations show that, excluding the electrical energy required for electrolysis, the integrated chemical process described in application no. 2417075.5 is exothermic overall. Furthermore, the redox reaction between the liquid sodium and an oxide of iron or manganese, as respectively described in application nos. 2417059.9 and 2417063.1, is also exothermic, so that the main energy input to the methods described therein is the energy required to produce the liquid sodium, for example by electrolysis. In all these cases, electricity for the electrolysis may be provided by a source of renewable energy, such as wind or solar, or come from nuclear power, and therefore need not generate any greenhouse gas emissions. If any of these processes are conducted at pilot scale, a modest initial capital investment, for example in solar panels and / or a wind turbine, with an electrical battery suitable for its storage, can provide this electricity at negligible operating cost thereafter. If, however, any of these processes are instead conducted at industrial scale, it is likely that the electricity would have to be purchased from a third-party electricity supplier instead. Whereas the unit cost of electricity from renewable sources is falling rapidly, it still varies widely by geographical location, and the unit cost of electricity from nuclear power remains high. It would therefore be desirable for conducting any of these processes at industrial scale to provide a chemical route for producing liquid sodium from sodium hydroxide as an alternative to producing liquid sodium by electrolysis, and which therefore uses thermal energy ( / .e., heat) instead of electrical energy to do so. If so, at least some of the heat required could be recovered from one or more of the exothermic reactions in these processes themselves. This would therefore reduce or eliminate the need to use electrolysis to produce liquid sodium for use in either of the methods described in copending UK patent application nos. 2417059.9 and 2417063.1, and would reduce the need to use electrolysis to produce liquid sodium in the integrated chemical process described in the applicant's co-pending UK patent application no. 2417075.5 in proportion to the molar ratio of the input materials. Object of the Invention It is therefore an object of the invention to provide a method and apparatus for producing liquid sodium from sodium hydroxide other than by electrolysis. Description of the Invention Accordingly, in one aspect, the present invention provides a method of producing liquid sodium from sodium hydroxide, wherein the method comprises reacting solid-phase sodium hydroxide in an anoxic environment with an amount of liquid sodium in excess of the stoichiometric amount thereof required to produce sodium oxide and hydrogen gas, capturing at least some of the hydrogen gas, phaseseparating at least some of the sodium oxide from the excess amount of liquid sodium, thermally decomposing at least some of the sodium oxide under vacuum to produce sodium vapour and oxygen gas, condensing at least some of the sodium vapour to produce liquid sodium, and recycling some of the liquid sodium thus produced to the reaction with the solid-phase sodium hydroxide. The solid-phase sodium hydroxide reacts with the liquid sodium in a stoichiometric ratio of 1:1 according to the equation: 2 NaOH (s) + 2 Na ( / ) 2 NazO (S) + Hz (g) [Eqn. 1] However, the thermal decomposition of the sodium oxide under vacuum produces two mols of sodium vapour for every mol of sodium oxide according to the equation: 2 NazO (S) -> 4 Na (g) + Oz (g) [Eqn. 2] Therefore, by summing Eqns. 1 and 2: 2 NaOH (S) -> 2 Na (g) + Hz (g) + Oz (g) [Eqn. 3] it may be seen that overall, this method decomposes the sodium hydroxide into its constituent elements and that no nett amount of liquid sodium is consumed. Thus subject to real-world losses and inefficiencies, the amount of liquid sodium produced by condensing the sodium vapour is double the stoichiometric amount thereof required by the reaction of Eqn. 1, so that half of it may be recycled back to this reaction, whilst recovering the same amount of liquid sodium from the sodium hydroxide. Accordingly, in a preferred embodiment of this method, the amount of liquid sodium recycled to react with the solid-phase sodium hydroxide is equal to the stoichiometric amount thereof required for that reaction, in which case, any real-world losses and inefficiencies just reduce the yield of liquid sodium recovered from the sodium hydroxide. At standard pressure, the reaction of Eqn. 1 is only mildly endothermic (AHreaction = + 11 kJ) and its Gibbs free energy, AG <0 at all temperatures above the melting point of sodium, so this reaction proceeds spontaneously with only mild cooling. On the other hand, at standard pressure, the thermal decomposition of sodium oxide only occurs at very high temperatures. Under vacuum, however, for example at 1 x 10-4 Pa, the reaction of Eqn. 2 can proceed at temperatures above about 540 ’Celsius. Such a vacuum may easily be generated using a turbomolecular pump, for example. This temperature is also less than the operating temperature of the Downs process, which is typically from about 600 to about 625 ’Celsius. As a guide, the enthalpy of vaporization of sodium oxide, AHvaporization (NazO) = + 611 kJ mol1 at standard pressure and a temperature of about 1000 K (= TH ’Celsius). However, the exothermic enthalpy of condensation of sodium vapour, AHCOndensation (Na) = - 97 kJ mol1 at standard temperature and pressure. Thus a significant proportion (> 30%) of the heat consumed by the thermal decomposition of sodium oxide under vacuum according to Eqn. 2 (= 2 x + 611 = + 1222 kJ) may be offset by the heat released by condensing the sodium vapour (= 4 x - 97 = - 388 kJ), making the total heat budget of this method = + 11 + 1222 - 388 = + 845 kJ. This compares very favourably with the theoretical minimum electrical energy needed to produce the same molar quantity of liquid sodium by fusing and electrolysing 2 mol of sodium chloride using the Downs process = 2[AHfUSion (Na) -△Hf (NaCI (sj)] = 2 [3 - (- 411)] = + 828 kJ. In addition, however, sodium chloride for the Downs process must firstly be heated from ambient to electrolysis temperature, which requires roughly an additional n Cp (NaCI (Sj) AT = 2 x 56 x 600 = + 67 kJ of heat, assuming that the heat capacity of sodium chloride at constant pressure, Cp (NaCI (Sj) = 56 J K1 across the temperature range, which makes the theoretical minimum total energy budget for electrolysis via the Downs process + 828 + 67 = + 895 kJ. In comparison to the Downs process, the method of the invention is therefore a favourable alternative method of producing liquid sodium from sodium hydroxide other than by electrolysis, with the additional advantage that it avoids the overproduction of chlorine gas as a co-product, if large quantities of liquid sodium are required to be produced. The method of the invention may also be viewed as a method of decomposing sodium hydroxide into its constituent elements other than by electrolysis, thus effectively providing a chemical alternative to the Castner process. In particular, even when subject to real-world losses and inefficiencies, the method of the invention is significantly more energy efficient than the Castner process because the theoretical maximum efficiency of the Castner process is only 50% due to the back-reaction of water produced at the anode with liquid sodium produced at the cathode when solid-phase sodium hydroxide is electrolysed. Thus a typical real-world electrical energy consumption to fuse and electrolyse 2 mol of anhydrous sodium hydroxide via the Castner process is approximately 2300 kJ, whereas the theoretical minimum without this back-reaction would instead be only 2[AHfUSion (Na) -AHf (NaOH (s))] = 2 [3 - (- 425.8)] = + 857.6 kJ. The reaction of Eqn. 1 is conducted in an anoxic environment to prevent the liquid sodium from reacting with atmospheric oxygen and closed off from the surrounding environment to capture the hydrogen gas it produces. For example, it may be conducted in a gas-tight reaction vessel. At start up, if the reaction vessel initially contains liquid sodium to which no sodium hydroxide has yet been added, a head space inside the reaction vessel above the liquid sodium may be occupied by an inert atmosphere, such as one consisting of at least one of nitrogen and argon. Either nitrogen or argon, or both, may be produced on site by pressure swing adsorption (PSA) of atmospheric air. After sodium hydroxide starts to be added to the liquid sodium and the production of hydrogen gas commences, the hydrogen rises rapidly through the inert atmosphere and collects above it in the top of the head space, from where it can be abstracted, because it is about 14 times less dense than the inert atmosphere. Blanketing the liquid sodium with an inert atmosphere in this way also has the advantage of reducing the partial pressure of hydrogen above the liquid sodium, and any minor contamination of the abstracted hydrogen by inert gas is of no consequence. The total pressure inside the reaction vessel may be maintained at around atmospheric pressure or just above by abstracting the hydrogen produced from the top of the head space via a valve. A small positive pressurization of the atmosphere in which the reaction is carried out to above atmospheric pressure, for example by about 10 to 25%, is desirable to prevent ingress of air from the environment by leakage. For safety and in order to maintain the pressure of the atmosphere inside the reaction vessel as desired, the pressure of the atmosphere should preferably be monitored, for example by means of a pressure gauge. To prevent any undesirable build-up of pressure, the reaction vessel may also be fitted with an auxiliary pressure-relief valve, in addition to the valve for abstracting hydrogen from it. The vapour pressure, p, in pascal of liquid sodium as a function of temperature, T, in kelvin from its melting point up to 700 ’Celsius is given by the following equation: logio p = 9.71- (5377 / T) [Eqn. 4] Table 1 below gives some examples of the values of this vapour pressure across a representative range of operating temperatures for the reaction of Eqn. 1: T / “Celsius 150 200 300 400 450 p / Pa 1.0 x 10-3 0.022 2.12 52.5 187 Table 1 As Table 1 shows, the vapour pressure of liquid sodium over this representative range of operating temperatures is always less than about 190 Pa, or less than about 0.19% of standard atmospheric pressure. Thus if the reaction of Eqn. 1 is carried out at around atmospheric pressure or just above, the loss of liquid sodium from the reaction mixture by vaporization is negligible and the effect of any such vaporization on the pressure of the atmosphere in which the reaction is carried out is negligible as well. Nonetheless, the hydrogen abstracted from the reaction vessel should preferably be cooled to below the dew point of sodium vapour to recover any sodium from it before it is used or transported elsewhere. The solubility of hydrogen in liquid sodium increases with temperature, but remains very low even at a temperature of 450 ’Celsius, for example, where it is less than about 100 ppm by weight of hydrogen. At temperatures above about 350 ’Celsius, however, any dissolved hydrogen also reacts with the liquid sodium to produce sodium hydride, which dissociates in the liquid sodium as Na+ and 1-1“ ions. The reaction of Eqn. 1 is therefore preferably conducted at a temperature of less than about 350 ’Celsius, more preferably less than about 340 ’Celsius, and most preferably less than about 330 ’Celsius, to avoid the formation of sodium hydride. The hydrogen gas released into the atmosphere above the liquid sodium will therefore reach an equilibrium with the hydrogen thus dissolved in the liquid sodium, according to the temperature of the reaction mixture and the corresponding partial pressure of the hydrogen gas above the liquid sodium. On the other hand, the endothermic reaction of Eqn. 1 is preferably maintained at a temperature above about 300 ’Celsius, more preferably above about 310 ’Celsius, and most preferably above about 320 ’Celsius, to enable complete dissolution of the sodium hydroxide in the liquid sodium. Separating at least some of the sodium oxide as a solid phase from the excess amount of liquid sodium may, for example, comprise at least one of settlement under gravity, filtration (i.e., trapping) and centrifugation. For example, the present applicant's co-pending UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal"; applicant's ref: NE-P-GB 002), the entire contents of which is incorporated herein by reference, shows and describes an apparatus and method for separating undissolved contaminants, for example in the form of suspended or entrained particulates, from liquid metal, such as liquid sodium. In another example, a liquid sodium centrifuge is shown and described on pp. 29 to 33 of Summary of the APDA Sodium Technology Program by J.E. Meyers published under United States Atomic Energy Commission Contract No. AT (11-1)-865, Project Agreement No. 11 (June 1970), the entire contents of which is also incorporated herein by reference. If the phase separation comprises filtration, a filter substrate may be used which comprises, for example, a ceramic foam made, for example, of a titania- or zirconia-based ceramic, and / or a wire mesh or wool, made, for example, of one of the same materials as the inner surface of the reaction vessel, such as titanium or a titanium alloy having the composition described below. To remove residual liquid sodium from the separated sodium oxide, the degree of phase separation may be improved by flushing the sodium oxide with an inert gas which is hot enough for the residual liquid sodium to remain in liquid phase. Using such an inert gas prevents the nett consumption of liquid sodium by the formation of more sodium oxide. However, it does not matter if some residual liquid sodium remains with the sodium oxide when this is subjected to thermal decomposition, since it will then just join the sodium vapour. In some embodiments, at least some of the heat released by condensing the sodium vapour may be recovered by condensing it at a temperature above (preferably, significantly above) that at which the reaction of Eqn. 1 is conducted, for example, at least 50 ’Celsius, more preferably at least 75 ’Celsius and most preferably at least 100 ’Celsius above the temperature for the reaction of Eqn. 1. Thus when some of the liquid sodium produced by condensing the sodium vapour is recycled back to the reaction of Eqn. 1, heat is also transferred to this mildly endothermic reaction, thereby offsetting the heat it consumes. More preferably still, the sodium vapour may be condensed at a temperature which is also above (and preferably, significantly above) that at which the sodium oxide is thermally decomposed, and the method further comprises transferring heat from the liquid sodium thus produced to the sodium oxide to contribute to its thermal decomposition before some of the liquid sodium is recycled back to the reaction of Eqn. 1. For example, the sodium vapour may be condensed at a temperature of from about 650 to about 750 ’Celsius (whereas the boiling point of liquid sodium at atmospheric pressure is 883 ’Celsius), the sodium oxide may be thermally decomposed at a temperature of from about 550 to about 650 ’Celsius, and the reaction of Eqn. 1 may be conducted at a temperature of from about 310 to about 340 ’Celsius. Because the heat released by condensing the sodium vapour (= -388 kJ) is very much greater than the heat consumed by the reaction of Eqn. 1 (= +11 kJ), this has the advantage of recovering as much heat as possible from the condensation of the sodium vapour, whilst also maintaining the reaction of Eqn. 1 at a steady temperature. In some embodiments, the solid-phase sodium hydroxide may be produced by the following process. Reacting liquid sodium in a redox reaction with an oxide of another metal, M, wherein the other metal, M, comprises at least one of iron and manganese, to precipitate out from the liquid sodium both the other metal, M, in elemental form and other insoluble products comprising sodium oxide. Separating the other metal, M, from the other insoluble products, and separating the other metal, M, and the other insoluble products as a solid phase from the liquid sodium. Adding at least some of the other insoluble products to liquid water, thereby hydrating the sodium oxide in the other insoluble products, 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. Separating undissolved solids as a solid phase from the aqueous solution of sodium hydroxide thus formed, and drying at least some of the aqueous solution of sodium hydroxide from which the undissolved solids have been separated to produce the solidphase sodium hydroxide. Accordingly, the source of the solid-phase sodium hydroxide used in the reaction of Eqn. 1 may be one of the methods described in the present applicant's co-pending UK patent application nos. 2417059.9 and 2417063.1. This has the advantage of allowing elemental sodium to be recovered from the sodium hydroxide thus produced without electrolysis. Moreover, since these methods are exothermic overall, excluding the energy required to produce the liquid sodium they consume, at least some of the excess heat they release can be used to contribute to recovering the sodium from the sodium hydroxide they produce. In such embodiments, the redox reaction may be conducted in an inert atmosphere to prevent the liquid sodium from reacting with atmospheric oxygen. The other metal, M, may be separated from the other insoluble products according to their different properties, such as different densities and / or magnetic susceptibilities. The other metal, M, and the other insoluble products may be separated from the liquid sodium in a manner similar to that described above for separating the sodium oxide produced by the reaction of Eqn. lfrom excess, unreacted liquid sodium. The other insoluble products are added to liquid water rather than the other way round, so that the final concentration of the aqueous solution of sodium hydroxide is approached from below. This avoids the aqueous solution from overheating and helps prevent the dissolution of gangue species therein. The undissolved solids may be separated as a solid phase from the aqueous solution of sodium hydroxide thus formed by any one or more of settlement under gravity, filtration and centrifugation, for example. If the source of the solid-phase sodium hydroxide for the reaction of Eqn. 1 is as just described, the method may further comprise using at least some of the liquid sodium produced by condensing the sodium vapour to react with the oxide of the other metal, M. This has the advantage that the electrical energy required to produce the liquid sodium consumed by the redox reaction with the oxide of the other metal, M, can then be reduced in proportion to how much liquid sodium produced by condensing the sodium vapour is used to react with the oxide of the other metal, M, instead. Nonetheless, in some embodiments, the method may further comprise producing liquid sodium and chlorine gas by electrolysis, and at least some of the liquid sodium thus produced may be used to react in the redox reaction with the oxide of the other metal, M, and / or with the solid-phase sodium hydroxide in the reaction of Eqn. 1. In such embodiments, however, the method may also comprise combusting at least some of the chlorine gas produced by electrolysis with at least some of the hydrogen gas produced by the reaction of Eqn. 1, to produce hydrogen chloride. The hydrogen chloride may then be used to produce hydrochloric acid, for example, for use in the integrated chemical process described in the applicant's co-pending UK patent application no. 2417075.5. If so, the method may further comprise using heat from the strongly exothermic combustion of the chlorine gas with the hydrogen gas to contribute to the endothermic thermal decomposition of the sodium oxide under vacuum. This is possible because producing hydrogen chloride in this way occurs at a much higher temperature than thermally decomposing the sodium oxide. It also avoids heat released by producing the hydrogen chloride from being wasted and reduces the need to cool this reaction by other means. In a second aspect, the present invention also provides an apparatus for producing liquid sodium from sodium hydroxide. The apparatus comprises a gas-tight reaction vessel, a solid-liquid sodium phase separator, a vacuum chamber and a sodium condenser. The gas-tight reaction vessel is for reacting the sodium hydroxide with an amount of liquid sodium in excess of the stoichiometric amount thereof required to produce sodium oxide and hydrogen gas, and for capturing the hydrogen gas thus produced. It comprises a first inlet for solid-phase sodium hydroxide, a second inlet for the liquid sodium, a first outlet for the liquid sodium with sodium oxide entrained therein, and a second outlet for the hydrogen gas. The solid-liquid sodium phase separator is for separating the sodium oxide from the liquid sodium. It comprises an inlet for receiving the liquid sodium with sodium oxide entrained therein from the first outlet of the reaction vessel, a first outlet for the liquid sodium and a second outlet for the sodium oxide. The vacuum chamber is for thermally decomposing the sodium oxide under vacuum therein to produce sodium vapour and oxygen gas. It comprises an inlet for receiving the sodium oxide from the second outlet of the solid-liquid sodium phase separator and an outlet for sodium vapour and oxygen gas. The sodium condenser is for condensing the sodium vapour and comprises an inlet for receiving the sodium vapour and oxygen gas from the outlet of the vacuum chamber, a first outlet for liquid sodium and a second outlet for the oxygen gas. In addition, the apparatus comprises a liquid sodium return path having a first inlet for receiving liquid sodium from the first outlet of the sodium condenser, and an outlet connected upstream of the second inlet of the gas-tight reaction vessel. The gas-tight reaction vessel may, for example, be a stirred tank reactor. It may be made from or lined with, for example, titanium or a titanium alloy, such as one having a composition by weight of 98.8% Ti, 0.8% Ni and 0.4% Mo. Titanium and titanium alloy are found to be highly resistant to corrosion by sodium oxide and sodium hydroxide, by forming a stable surface passivation layer of titanium dioxide. The reaction vessel is unlikely to suffer from hydrogen embrittlement for two reasons, firstly because the temperature of the reaction of Eqn. 1 is well above the temperature at which hydrogen embrittlement normally occurs and secondly because the hydrogen gas released into the reaction vessel by the reaction of Eqn. 1 is molecular and not atomic hydrogen. The solid-liquid sodium phase separator, and the vacuum chamber, as well as any associated pipework, may be made from or lined with materials similar to the gas-tight reaction vessel, which are resistant to corrosion by sodium oxide. The vacuum chamber may comprise means for generating the vacuum, such as a turbomolecular pump, as well as means for heating the sodium oxide, some examples of which are described below. The second outlet of the sodium condenser may, for example, be vented to atmosphere. In some embodiments, the liquid sodium return path preferably comprises a heat exchanger for transferring heat from liquid sodium produced in the sodium condenser to sodium oxide in the vacuum chamber. This therefore allows some of the heat consumed by thermally decomposing the sodium oxide in the vacuum chamber to be offset by heat released when condensing the sodium vapour in the sodium condenser. In some embodiments, the liquid sodium return path preferably also comprises a second inlet for receiving liquid sodium from the first outlet of the solid-liquid sodium phase separator, so that excess liquid sodium separated therein from the sodium oxide may be returned to the gas-tight reaction vessel as well and not wasted. In some embodiments, the apparatus of the invention may be part of a larger apparatus comprising further components, which are arranged to supply the gas-tight reaction vessel with solid-phase sodium hydroxide. If so, then the apparatus further comprises a redox reaction subassembly, a separation subassembly, a hydration vessel, a solid-aqueous phase separator and a caustic dryer. The redox reaction subassembly is for containing therein a redox reaction between liquid sodium and an oxide of another metal, M, wherein the other metal comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide. It comprises a first inlet for the oxide of the other metal, M, a second inlet for liquid sodium, and an outlet for liquid sodium and entrained therein, a solid phase comprising the other metal, M, in elemental form and other insoluble products comprising sodium oxide. The separation subassembly is for separating the liquid sodium, the other metal, M, in elemental form and the other insoluble products from each other. It comprises an inlet for receiving the liquid sodium with the solid phase entrained therein from the outlet of the redox reaction subassembly, a first outlet for liquid sodium, a second outlet for the other metal, M, in elemental form and a third outlet for the other insoluble products. The hydration vessel is for reacting at least some of the sodium oxide in the other insoluble products with liquid water to produce an aqueous solution of sodium hydroxide. It comprises a first inlet for receiving the other insoluble products from the third outlet of the separation subassembly, a second inlet for liquid water, and an outlet for the aqueous solution of sodium hydroxide and undissolved solids. The solid-aqueous phase separator is for separating the undissolved solids from the aqueous solution of sodium hydroxide, and comprises an inlet downstream of 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 caustic dryer is for drying the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour. The caustic dryer comprises an inlet for receiving the aqueous solution of sodium hydroxide from the first outlet of the solid-aqueous phase separator, a first outlet for water vapour, and a second outlet for solid sodium hydroxide which is connected upstream of the first inlet of the gas-tight reaction vessel. The redox reaction subassembly may comprise a vessel made of similar materials as the gas-tight reaction vessel in which the reaction of Eqn. 1 is conducted, which are resistant to corrosion by sodium oxide. It may also be closed off from the surrounding environment to prevent liquid sodium contained therein from reacting with atmospheric oxygen. It may therefore comprise an airlock to allow the oxide of the other metal, M, to be introduced into it from thesurroundingenvironment whilst avoiding the ingress of atmospheric oxygen. The construction of such a vessel and airlock are fully described in the present applicant's co-pending UK patent application no. 2417059.9 mentioned previously. The separation subassembly may comprise a solid-liquid sodium phase separator of the type described in the applicant's co-pending UK patent application no. 2417052.4 already mentioned above, as well as a solid species separator for separating different solid species from each other according to their different properties, such as different densities and / or magnetic susceptibilities. The solid-aqueous phase separator may comprise a settlement tank, filter and / or centrifuge, for example. The caustic dryer may comprise a so-called "multi-effect" evaporator, for example. For example, during operation of such an apparatus, the oxide of the other metal, M, may be introduced into the redox reaction subassembly as part of an ore of that metal which comprises at least that oxide, as well as one or more gangue mineral species, like aluminosilicates, for example. If so, the other insoluble products of the redox reaction conducted in the redox reaction subassembly will comprise these gangue mineral species and their derivatives, as well as sodium oxide produced by the redox reaction. Once all these other products which are insoluble in liquid sodium, have been separated by the separation subassembly from the excess, unreacted liquid sodium and from the elemental metal, M, the sodium oxide in these other insoluble products dissolves within the hydration vessel in the liquid water to produce the aqueous solution of sodium hydroxide, whereas the gangue mineral species and their derivatives do not. The solid-aqueous phase separator then separates the undissolved solids from the aqueous solution of sodium hydroxide, which the caustic dryer dries, to leave just solid sodium hydroxide, which is then supplied to the gas-tight reaction vessel in which the reaction of Eqn. 1 is conducted. In some embodiments, the second inlet of the redox reaction subassembly may also be connected downstream of the first outlet of the sodium condenser, so that liquid sodium condensed therein may be supplied to the redox reaction with the oxide of the other metal, M, which takes place therein. In some embodiments, the apparatus may further comprise an electrolytic subassembly for producing at least liquid sodium and chlorine gas by electrolysis, and a combustion chamber for combusting chlorine gas with hydrogen gas to produce hydrogen chloride. In such embodiments, the electrolytic subassembly 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, and the combustion chamber comprises a first inlet for receiving the chlorine gas from the second outlet of the electrolytic subassembly, a second inlet for receiving the hydrogen gas from the second outlet of the gas-tight reaction vessel in which the reaction of Eqn. 1 is conducted, and an outlet for hydrogen chloride. The first outlet of the electrolytic subassembly may be connected upstream of the second inlet of the redox reaction subassembly and / or upstream of the second inlet of the gas-tight reaction vessel. Thus hydrogen gas produced in the gas-tight reaction vessel can be used with chlorine gas produced by the electrolytic subassembly to produce hydrogen chloride, and liquid sodium produced by the electrolytic subassembly can be supplied as a reagent to one or both of the redox reaction and the reaction of Eqn. 1. Such embodiments may also comprise a heat transfer pathway for transferring heat from the combustion chamber to sodium oxide within the vacuum chamber. This has the advantage of allowing heat from combusting the chlorine gas with the hydrogen gas to be used for the thermal decomposition of the sodium oxide in the vacuum chamber, because this combustion reaction is strongly exothermic and occurs at a much higher temperature than the thermal decomposition of the sodium oxide. If so, the heat transfer pathway may be adapted to contain liquid sodium or a liquid sodium-potassium (NaK) alloy as a heat transfer fluid. This has the advantage of allowing liquid sodium circulating in any part of the apparatus to be diverted via this heat transfer pathway and used to transfer the heat. Brief Description of the Drawings Further features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and in association with the accompanying drawings, in which: Fig. 1 is a flow diagram of a first embodiment of a method of producing liquid sodium; Fig. 2 is a diagram of a chemical cycle in the first embodiment; Fig. 3 is a flow diagram of a second embodiment of a method of producing liquid sodium; Fig. 4 is a diagram of a chemical cycle in the second embodiment; Fig. 5A is a flow diagram of part of a third embodiment of a method of producing liquid sodium; Fig. 5B is a flow diagram of part of a fourth embodiment of a method of producing liquid sodium; Fig. 6 is a diagram of a chemical cycle in the third and fourth embodiments; Fig. 7A is a schematic diagram of a first embodiment of an apparatus according to the invention; Fig. 7B is a schematic diagram of part of a variant of the first embodiment; Fig. 8 is a schematic diagram of part of a second embodiment of such an apparatus; Fig. 9 is a schematic diagram of part of a third embodiment of such an apparatus; and Fig. 10 is a schematic diagram of an embodiment of a heat transfer pathway forming part of a fourth embodiment of such an apparatus. Detailed Description Fig. 1 shows a first embodiment of a method 700a of producing liquid sodium. The method 700a comprises reacting 701 solid-phase sodium hydroxide with an amount of liquid sodium in excess of the stoichiometric amount thereof required to produce sodium oxide and hydrogen gas. The reaction 701 is conducted in an anoxic environment to prevent the liquid sodium from reacting with atmospheric air, and closed off from the surrounding environment to allow at least some of the hydrogen gas to be captured 702. In this embodiment, the reaction 701 is maintained at a temperature of about 325 ’Celsius to enable the solid-phase sodium hydroxide to dissolve completely in the liquid sodium and to avoid the formation of sodium hydride. The method 700a then comprises phase-separating 703 at least some of the sodium oxide produced by the reaction 701 from the excess amount of liquid sodium, and thermally decomposing 704 at least some of the sodium oxide under vacuum to produce sodium vapour and oxygen gas. Next, at least some of the sodium vapour is condensed 705 to produce liquid sodium, some of which is recycled 706 back to the reaction 701 with the solid-phase sodium hydroxide, along with excess liquid sodium from the phase-separation 703. The temperature at which the sodium vapour is condensed 705 is above that of the reaction 701 of the liquid sodium with the solid-phase sodium hydroxide, so that the liquid sodium recycled 706 back to the reaction 701 also transfers heat to it, thereby offsetting the heat which this endothermic reaction consumes. Fig. 2 shows a chemical cycle 700 in the first embodiment of Fig. 1. As may be seen from Fig. 2, solidphase sodium hydroxide reacts 701 with excess liquid sodium in the top right-hand part of the cycle 700 to produce sodium oxide and hydrogen gas. The sodium oxide is thermally decomposed 704 into sodium vapour and oxygen gas, and the sodium vapour is condensed 705 into liquid sodium, in the bottom of the cycle. Some of this sodium leaves the cycle 700 at the left of Fig. 2 and the rest of the sodium is recycled 706 back to the reaction 701 with the solid-phase sodium hydroxide. If the chemical cycle 700 operates at its stoichiometric ratios and has 100% yield, half of the sodium leaves the cycle and half is recycled 706 back to the reaction 701. Thus the sodium, oxygen and hydrogen are all recovered from the solid-phase sodium hydroxide, and no liquid sodium is consumed overall. Fig. 3 shows a second embodiment of a method 700b of producing liquid sodium. The method 700b comprises reacting 303 an excess amount of liquid sodium in a redox reaction with an oxide of another metal, M, wherein the other metal comprises at least one of iron and manganese, to precipitate out from the liquid sodium both the other metal, M, in elemental form and other insoluble products comprising sodium oxide. The other metal, M, is separated 304b from the other insoluble products using a "wet" separation technique, in which excess, unreacted liquid sodium from the reaction 303 acts as a transport medium for the solid-phase reaction products. The other metal, M, and the other insoluble products are separated 304b from each other into two process streams according to their different densities and / or different magnetic susceptibilities. Each of the other metal, M, and the other insoluble products in a respective one of these two process streams are then phase-separated 305a, 305b from the excess, unreacted liquid sodium, which is returned to the reaction 303. The other insoluble products comprising the sodium oxide, which are hot from the exothermic reaction 303, are then cooled 309d significantly closer to ambient temperature, for example using a heat transfer fluid. At least some of the heat extracted from the other insoluble products may be used to help in subsequent thermal decomposition 704 of the sodium oxide. The method 700b further comprises adding 317 at least some of the other insoluble products to liquid water. This hydrates the sodium oxide in the other insoluble products, to produce an aqueous solution of sodium hydroxide having a final concentration of less than 2.5 M. The other insoluble products are added to the liquid water, rather than the other way around, so that this final concentration is approached from below. Helped by the prior cooling of the other insoluble products, this also allows the temperature of this solution to be controlled to remain below 85 ’Celsius. The method 700b then comprises separating 318 undissolved solids, such as silica and alumina, as a solid phase from the aqueous solution of sodium hydroxide, which is dried 319 to produce solid sodium hydroxide. At least some of the solid sodium hydroxide is then decomposed 700 into its constituent elements via the chemical cycle of Fig. 2 to produce liquid sodium, with gaseous hydrogen and oxygen as co-products, as described above. Accordingly, decomposition 700 of the solid sodium hydroxide comprises the operations 701 to 706, inclusive, described above, which include condensing 705 the sodium vapour into liquid sodium. By condensing 705 the liquid sodium at a suitable temperature for the reaction 303, in this embodiment of the method 700b, at least some of this liquid sodium is recycled 707 back to react 303 with more of the oxide of the other metal, M, again, without needing any further heating or cooling. A suitable temperature for the reaction 303 is described in the present applicant's copending UK patent application nos. 2417059.9 and 2417063.1. Fig. 4 shows a chemical cycle in the second embodiment of Fig. 2. As may be seen by comparing it with Fig. 2, the chemical cycle of Fig. 4 contains the chemical cycle 700 of Fig. 2, in which the molar amounts of its reagents have all been multiplied by a factor of 3. In addition, in comparison to Fig. 2, the sodium which leaves the chemical cycle 700 then reacts at the top left of Fig. 4 in a redox reaction 303 with the oxide of the other metal, M, according to the equation: 6 Na ( / ) + MjOajs) 3 NajO (S) + 2 M (S) [Eqn. 5] wherein M represents at least one of iron and manganese. The sodium oxide produced by the reaction of Eqn. 5 is then hydrated 317 by liquid water from the top right of Fig. 4 to produce an aqueous solution of sodium hydroxide, which is dried 319 to produce the solid-phase sodium hydroxide, which re-enters the chemical cycle 700 at the top of Fig. 4. It may thus be seen that the chemical cycle 700 provides an effective way of supplying the reaction of Eqn. 5 with liquid sodium, other than by electrolysis and without the co-production of chlorine gas. Figs. 5A and 5B respectively show parts of third and fourth embodiments of methods 700c, 700d of producing liquid sodium. The methods 700c, 700d both comprise producing 101b liquid sodium and chlorine gas by electrolysis, using the Downs process. In both cases, chlorine gas produced in this way is then combusted 203b with hydrogen gas produced by decomposing 700 solid-phase sodium hydroxide into its constituent elements as described herein, to produce hydrogen chloride. The hydrogen chloride may subsequently be dissolved in liquid water to produce hydrochloric acid, for example. In the method 700c of Fig. 5A, liquid sodium produced 101b by electrolysis is used 708a to react 303 with an oxide of another metal, M, wherein the other metal, M, comprises at least one of iron and manganese, as described above. In the method 700d of Fig. 5B, on the other hand, liquid sodium produced 101b by electrolysis is instead used 708b to react 701 with solid-phase sodium hydroxide to produce sodium oxide and hydrogen gas, as also described above. However, an alternative possible embodiment may comprise using 708a some of the liquid sodium produced 101b by electrolysis to react 303 with an oxide of another metal, M, and using 708b some of it to react 701 with solid-phase sodium hydroxide. The methods 700c, 700d both comprise all the features of the method 700b described above, which have not been reproduced in Figs. 5A and 5B and will not be described again here for the sake of conciseness. In addition, however, the method 700d of Fig. 5B comprises using 709 heat from combusting 203b the chlorine and hydrogen gases to contribute towards the thermal decomposition 704 of the sodium oxide under vacuum, because this combustion reaction 203b is strongly exothermic and occurs at a much higher temperature than the thermal decomposition 704 of the sodium oxide. This heat may be transferred, for example, using liquid sodium as a heat transfer fluid, which comes into thermal but not direct contact both with the combustion reaction 203b and with the sodium oxide. In an alternative possible embodiment, the method 700c of Fig. 5A may similarly comprise such a heat transfer process 709. Fig. 6 shows a chemical cycle in the third and fourth embodiments of Figs. 5A and 5B. As may be seen by comparing it with Fig. 4, the chemical cycle of Fig. 6 further comprises, in the bottom right of Fig. 6, producing 4 mol of liquid sodium and 2 mol of chlorine gas by fusing and electrolysing 101b 4 mol of sodium chloride using the Downs process. Half of the chlorine gas thus produced is combusted 203b with an equimolar amount of the hydrogen gas produced by the chemical cycle 700 in the middle of Fig. 6, to produce 2 mol of hydrogen chloride and leave the other half of the chlorine gas thus produced unconsumed. The 4 mol of liquid sodium produced by electrolysis is combined with an equimolar amount of liquid sodium produced by the chemical cycle 700 in the middle of Fig. 6 to make a total of 8 mol of liquid sodium, 6 mol of which is passed to the reaction of Eqn. 5 in the top left of Fig. 6 and 2 mol of which is used to react with an equimolar amount of solid sodium hydroxide in the chemical cycle 700. Thus only one third of the 3 mol of sodium oxide produced by the reaction of Eqn. 5 is consumed by the chemical cycle 700 in the middle of Fig. 6, leaving 2 mol of sodium oxide remaining at the top of Fig. 6. This remaining sodium oxide and the equimolar amount of hydrogen chloride produced at the right of Fig. 6 can both be used in other chemical reactions, whereby sodium chloride can be reconstituted for electrolysis using the Downs process, thus resupplying the bottom right of Fig. 6. It may therefore be seen that in comparison to supplying the redox reaction of Eqn. 5 with liquid sodium produced only by electrolysis, the whole chemical cycle of Fig. 6 reduces the electrical power consumption by one-third, by producing 2 mol of liquid sodium via the chemical cycle 700 instead, and that the co-production of chlorine gas is consequently also reduced by two-thirds from 3 mol to 1 mol per mol of the oxide of the other metal, M, consumed. Fig. 7A schematically shows a first embodiment of an apparatus 7a for producing liquid sodium, which is suitable for conducting the method 700a described above in relation to Fig. 1. The apparatus 7a comprises a gas-tight reaction vessel 710, a solid-liquid sodium phase separator 190, a vacuum chamber 720 and a sodium condenser 730, which are connected to each other as shown in Fig. 7A. The gas-tight reaction vessel 710 is for reacting the sodium hydroxide with an amount of liquid sodium in excess of the stoichiometric amount thereof required to produce sodium oxide and hydrogen gas, and for capturing the hydrogen gas thus produced. It comprises a first inlet 711 for solid-phase sodium hydroxide, a second inlet 712 for the liquid sodium, a first outlet 714 for the liquid sodium with sodium oxide entrained therein, and a second outlet 715 for the hydrogen gas. The solid-liquid sodium phase separator 190 is for separating the sodium oxide from the liquid sodium. It comprises an inlet 191 for receiving the liquid sodium with sodium oxide entrained therein from the first outlet 714 of the reaction vessel 710, a first outlet 194 for the liquid sodium and a second outlet 195 for the sodium oxide. The solid-liquid sodium phase separator 190 is constructed and functions as described in the applicant's co-pending UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal"; applicant's ref: NE-P-GB 002) mentioned above. The vacuum chamber 720 is for thermally decomposing the sodium oxide under vacuum therein to produce sodium vapour and oxygen gas. It comprises an inlet 721 for receiving the sodium oxide from the second outlet 195 of the solid-liquid sodium phase separator 190 and an outlet 724 for sodium vapour and oxygen gas, as well as means for generating the vacuum, such as a turbomolecular pump, and means for heating the sodium oxide until it vaporizes. The sodium condenser 730 is for condensing the sodium vapour and comprises an inlet 731 for receiving the sodium vapour and oxygen gas from the outlet 724 of the vacuum chamber 720, a first outlet 734 for liquid sodium and a second outlet 735 for the oxygen gas. The sodium condenser 730 is maintained at a pressure and temperature, such that the temperature is below the dew point of sodium at that pressure, but well above the freezing point of liquid sodium, whereby the oxygen may escape freely via the second outlet 735 once the sodium vapour has condensed inside the sodium condenser 730. The apparatus 7a further comprises a liquid sodium return path 740 having a first inlet 741 for receiving liquid sodium from the first outlet 734 of the sodium condenser 730, and an outlet 744, which is connected upstream of the second inlet 712 of the gas-tight reaction vessel 710. The liquid sodium return path 740 also comprises a second inlet 742 for receiving liquid sodium from the first outlet 194 of the solid-liquid sodium phase separator 190. During operation of the apparatus 7a, the temperature of the sodium condenser 730 is kept above that of the gas-tight reaction vessel 710, so that liquid sodium returning to the reaction vessel 710 via the liquid sodium return path 740 also transfers heat to the reaction vessel 710, thereby offsetting the cooling from the mildly endothermic reaction therein and maintaining it at a desired temperature. Fig. 7B schematically shows part of a variant of the first embodiment, in which the liquid sodium return path 740 comprises a heat exchanger HEI for transferring heat from liquid sodium produced in the sodium condenser 730 to sodium oxide in the vacuum chamber 720, before the liquid sodium arrives at the reaction vessel 710. Thus if the sodium vapour is condensed at a temperature significantly above the temperature for thermally decomposing the sodium oxide, a major portion of the heat released by the exothermic condensation of the sodium vapour may be used to contribute to the endothermic thermal decomposition of the sodium oxide inside the vacuum chamber 720. This heat transfer process may be performed in addition to transferring heat from the liquid sodium produced in the sodium condenser 730 to the reaction of Eqn. 1 as described above, because in such a case, the hierarchy of temperatures is T(sodium condenser 730) >T(vacuum chamber 720) >T(reaction vessel 710). Fig. 8 schematically shows part of a second embodiment of an apparatus 7b for producing liquid sodium, which is suitable for conducting the method 700b described above in relation to Fig. 3. The apparatus 7b comprises a redox reaction subassembly 130, a separation subassembly 180, 190a, a hydration vessel 170, a solid-aqueous phase separator 175, a caustic dryer 460 and an aqueous condenser 70. The redox reaction subassembly 130 is for containing therein a redox reaction between liquid sodium and an oxide of another metal, M, wherein the other metal comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide. It comprises a first inlet 131 for the oxide of the other metal, M, a second inlet 132 for liquid sodium, and an outlet 134 for liquid sodium and entrained therein, a solid phase comprising the other metal, M, in elemental form and other insoluble products comprising sodium oxide. The redox reaction subassembly 130 is constructed and functions as described in the applicant's co-pending UK patent application nos. 2417059.9 and 2417063.1 mentioned above. The separation subassembly 180, 190a is for separating the liquid sodium, the other metal, M, in elemental form and the other insoluble products from each other. It comprises a solid-liquid sodium phase separator 190a and a density-based separator 180. The solid-liquid sodium phase separator 190a comprises an inlet 191a for receiving the liquid sodium with the solid phase entrained therein from the outlet 134 of the redox reaction subassembly 130, a first outlet 194a for the liquid sodium and a second outlet 195a for the solid phase. It is constructed and functions as described in the applicant's co-pending UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal"; applicant's ref: NE-P-GB 002) mentioned previously. The first outlet 194a of the solid-liquid sodium phase separator 190a is connected upstream of the second inlet 132 of the redox reaction subassembly 130 to return the liquid sodium to the reaction therein. The density-based separator 180 comprises an inlet 181 for receiving the solid phase from the second outlet 195a of the solid-liquid sodium phase separator 190a, a second outlet 186 for the other metal, M, in elemental form and a third outlet 187 for the other insoluble products. The density-based separator 180 separates the other metal, M, from the other insoluble products according to their significantly different densities, because the other metal, M, is more than three times denser than other insoluble products like sodium oxide. For example, it may comprise a centrifuge. If the other metal, M, comprises iron, the iron may also be separated from the other insoluble products according to their significantly different magnetic susceptibilities. The hydration vessel 170 is for reacting at least some of the sodium oxide in the other insoluble products with liquid water to produce an aqueous solution of sodium hydroxide. It comprises a first inlet 171 for receiving the other insoluble products from the third outlet 187 of the density-based separator 180, a second inlet 172 for liquid water, and an outlet 174 for the aqueous solution of sodium hydroxide and undissolved solids. The solid-aqueous phase separator 175 is for separating the undissolved solids from the aqueous solution of sodium hydroxide, and comprises an inlet 162 downstream of the outlet 174 of the hydration vessel 170, a first outlet for the aqueous solution of sodium hydroxide, and a second outlet for the undissolved solids. The solid-aqueous phase separator 175 may comprise a settlement tank, filter and / or centrifuge, for example. The caustic dryer 460 is for drying the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour. It comprises an inlet 461 for receiving the aqueous solution of sodium hydroxide from the first outlet 178 of the solid-aqueous phase separator 175, a first outlet 464 for water vapour, and a second outlet 465 for solid sodium hydroxide. The aqueous condenser 70 is for condensing at least some of the water vapour produced by drying the aqueous solution of sodium hydroxide. It comprises an inlet 71 for receiving water vapour from the first outlet 464 of the caustic dryer 460 and an outlet 74 for liquid water. The outlet 74 of the aqueous condenser 70 is connected upstream of the second inlet 172 of the hydration vessel 170. In this embodiment, the caustic dryer 460 and the aqueous condenser 70 are both parts of a so-called "multi-effect" evaporator. As stated above, Fig. 8 only shows part of the apparatus 7b in this second embodiment. In addition, the apparatus 7b comprises the apparatus 7a of Fig. 7A. Thus, the second outlet 465 of the caustic dryer 460, indicated by the letter "A" in Fig. 8, is connected upstream of the first inlet 711 of the gastight reaction vessel 710, which is also indicated by the letter "A" in Fig. 7A. The part of the apparatus 7b show in Fig. 8 therefore supplies the apparatus 7a of Fig. 7A with solid-phase sodium hydroxide. Moreover, the second inlet 132 of the redox reaction subassembly 130, indicated by the letter "B" in Fig. 8, is connected downstream of the first outlet 734 of the sodium condenser 730, which is indicated by the letter "B" in Fig. 7A as well. Thus the apparatus 7a of Fig. 7A also supplies the part of the apparatus 7b show in Fig. 8 with liquid sodium. Fig. 9 schematically shows part of a third embodiment of an apparatus 7c for producing liquid sodium, which is suitable for conducting either of the methods 700c, 700d respectively described above in relation to Figs. 5A and 5B. The apparatus 7c comprises an electrolytic subassembly 10 and a combustion chamber 430. The electrolytic subassembly 10 in turn comprises a bank of Downs cells for producing liquid sodium and chlorine gas by electrolysis and which have an inlet 11 for sodium chloride in at least one of solid and molten phase, a first outlet 17 for liquid sodium, and a second outlet 15 for chlorine gas. The combustion chamber 430 is for combusting chlorine gas with hydrogen gas to produce hydrogen chloride and comprises a first inlet 431 for receiving the chlorine gas from the second outlet 15 of the electrolytic subassembly 10, a second inlet 432 for receiving the hydrogen gas and an outlet 434 for hydrogen chloride. As stated, Fig. 9 only shows part of the apparatus 7c in this third embodiment. In addition, the apparatus 7c comprises the apparatuses 7a, 7b of Figs. 7 and 8. Thus, the second inlet 432 of the combustion chamber 430, indicated by the letter "C" in Fig. 9, is connected downstream of the second outlet 715 of the gas-tight reaction vessel 710, which is also indicated by the letter "C" in Fig. 7A. The apparatus 7a of Fig. 7A therefore supplies the part of the apparatus 7c show in Fig. 9 with hydrogen. Moreover, the first outlet 17 of the electrolytic subassembly 10 is connected upstream of one or both of the second inlet 132 of the redox reaction subassembly 130 in Fig. 8 and the second inlet 712 of the gas-tight reaction vessel 710 in Fig. 7A. The electrolytic subassembly 10 can therefore supply at least one of the apparatuses 7a, 7b of Figs. 7 and 8 with liquid sodium produced by electrolysis. Fig. 10 schematically shows an embodiment of a heat transfer pathway 750 forming part of a fourth embodiment of an apparatus 7d, which also comprises the apparatus 7c shown in Fig. 9. The heat transfer pathway 750 contains a heat transfer fluid (HTF) and is arranged as shown in Fig. 10 to transfer heat from the combustion chamber 430 to sodium oxide within the vacuum chamber 720 via a heat exchanger HE2, which is in thermal contact with the sodium oxide therein. The apparatus 7d is therefore suitable for use in the method 700d described above in relation to Fig. 5B. The HTF in the heat transfer pathway 750 may be a molten salt, for example. In the present embodiment, however, the heat transfer pathway 750 is adapted to contain liquid sodium or a liquid sodium-potassium (NaK) alloy as the HTF. Liquid sodium circulating in any part of the chemical cycle of Fig. 6 may therefore be diverted via the heat transfer pathway 750 to transfer heat from the combustion chamber 430 to sodium oxide within the vacuum chamber 720. For example, liquid sodium from the first outlet 17 of the electrolytic subassembly 10 may be diverted via the heat transfer pathway 750 before arriving at the second inlet 712 of the gas-tight reaction vessel 710 and / or the second inlet 132 of the redox reaction subassembly 130. Whereas the present invention has been described above by reference to particular examples and embodiments, the scope of the invention should not be taken to be limited thereby and is instead defined by the appended claims.

Claims

1. A method (700, 700a - 700e) of producing liquid sodium from sodium hydroxide, the method comprising:reacting (701) solid-phase sodium hydroxide in an anoxic environment with an amount of liquid sodium in excess of the stoichiometric amount thereof required to produce sodium oxide and hydrogen gas;capturing (702) at least some of the hydrogen gas;phase-separating (703) at least some of the sodium oxide from the excess amount of liquid sodium;thermally decomposing (704) at least some of the sodium oxide under vacuum to produce sodium vapour and oxygen gas;condensing (705) at least some of the sodium vapour to produce liquid sodium; andrecycling (706) some of the liquid sodium thus produced to the reaction (701) with the solidphase sodium hydroxide.

2. A method (700a) according to claim 1, wherein the amount of liquid sodium recycled to the reaction (701) with the solid-phase sodium hydroxide is equal to the stoichiometric amount thereof required for that reaction.

3. A method (700a) according to claim 1 or claim 2, wherein the sodium vapour is condensed (705) at a temperature above that of the reaction (701) of the liquid sodium with the solid-phase sodium hydroxide.

4. A method (700b) according to any one of the preceding claims, wherein the solid-phase sodium hydroxide is produced by:reacting (303) liquid sodium in a redox reaction with an oxide of another metal (M), wherein the other metal (M) comprises at least one of iron and manganese, to precipitate out from the liquid sodium both the other metal (M) in elemental form and other insoluble products comprising sodium oxide;separating (304b) the other metal (M) from the other insoluble products;separating (305a, 305b) the other metal (M) and the other insoluble products as a solid phase from the liquid sodium;adding (317) at least some of the other insoluble products to liquid water, thereby hydrating the sodium oxide in the other insoluble products, 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;separating (318) undissolved solids as a solid phase from the aqueous solution of sodium hydroxide; anddrying (319) at least some of the aqueous solution of sodium hydroxide from which the undissolved solids have been separated (318) to produce the solid-phase sodium hydroxide.

5. A method (700b) according to claim 4, further comprising using (707) at least some of the liquid sodium produced by condensing (705) the sodium vapour to react (303) with the oxide of the other metal (M).

6. A method (700c) according to claim 4 or claim 5, comprising:producing (101b) liquid sodium and chlorine gas by electrolysis;combusting (203b) at least some of the chlorine gas thus produced with at least some of the hydrogen gas to produce hydrogen chloride; andusing (708a) at least some of the liquid sodium produced by electrolysis to react (303) with the oxide of the other metal (M).

7. A method (700d) according to any one of the preceding claims, comprising:producing (101b) liquid sodium and chlorine gas by electrolysis;combusting (203b) at least some of the chlorine gas thus produced with at least some of the hydrogen gas to produce hydrogen chloride; andusing (708b) at least some of the liquid sodium produced by electrolysis to react (701) with the solid-phase sodium hydroxide.

8. A method (700d) according to claim 6 or claim 7, further comprising using (709) heat from the combustion (203b) of the chlorine gas with the hydrogen gas to contribute to the thermal decomposition (704) of the sodium oxide under vacuum.

9. An apparatus (7a - 7d) for producing liquid sodium from sodium hydroxide, the apparatus comprising:a gas-tight reaction vessel (710) for reacting the sodium hydroxide with an amount of liquid sodium in excess of the stoichiometric amount thereof required to produce sodium oxide andhydrogen gas, and for capturing the hydrogen gas thus produced, wherein the reaction vessel (710) comprises a first inlet (711) for solid-phase sodium hydroxide, a second inlet (712) for the liquid sodium, a first outlet (714) for the liquid sodium with sodium oxide entrained therein and a second outlet (715) for the hydrogen gas;a solid-liquid sodium phase separator (190) for separating the sodium oxide from the liquid sodium, the solid-liquid sodium phase separator (190) comprising an inlet (191) for receiving the liquid sodium with sodium oxide entrained therein from the first outlet (714) of the reaction vessel (710), a first outlet (194) for the liquid sodium and a second outlet (195) for the sodium oxide;a vacuum chamber (720) for thermally decomposing the sodium oxide under vacuum therein to produce sodium vapour and oxygen gas, the vacuum chamber (720) comprising an inlet (721) for receiving the sodium oxide from the second outlet (195) of the solid-liquid sodium phase separator (190) and an outlet (724) for sodium vapour and oxygen gas;a sodium condenser (730) for condensing the sodium vapour, the sodium condenser (730) comprising an inlet (731) for receiving the sodium vapour and oxygen gas from the outlet (724) of the vacuum chamber (720), a first outlet (734) for liquid sodium and a second outlet (735) for the oxygen gas; anda liquid sodium return path (740) comprising a first inlet (741) for receiving liquid sodium from the first outlet (734) of the sodium condenser (730), and an outlet (744) connected upstream of the second inlet (712) of the gas-tight reaction vessel (710).

10. An apparatus (7a) according to claim 9, wherein the liquid sodium return path (740) comprises a heat exchanger (HEI) for transferring heat from liquid sodium produced in the sodium condenser (730) to sodium oxide in the vacuum chamber (720).

11. An apparatus (7a) according to claim 9 or claim 10, wherein the liquid sodium return path (740) further comprises a second inlet (742) for receiving liquid sodium from the first outlet (194) of the solid-liquid sodium phase separator (190).

12. An apparatus (7b) according to any one of claims 9 to 11, further comprising:a redox reaction subassembly (130) for containing therein a redox reaction between liquid sodium and an oxide of another metal (M), wherein the other metal (M) comprises at least one of iron and manganese, to produce at least the other metal in elemental form and sodium oxide, the redox reaction subassembly (130) comprising a first inlet (131) for the oxide of the other metal (M), a second inlet (132) for liquid sodium, and an outlet (134) for liquid sodium and entrained therein, a solid phasecomprising the other metal (M) in elemental form and other insoluble products comprising sodium oxide;a separation subassembly (180,190a) for separating the liquid sodium, the other metal (M) in elemental form and the other insoluble products from each other, the separation subassembly (180, 190a) comprising an inlet (191a) for receiving the liquid sodium with the solid phase entrained therein from the outlet (134) of the redox reaction subassembly (130), a first outlet (194a) for liquid sodium, a second outlet (186) for the other metal (M) in elemental form and a third outlet (187) for the other insoluble products;a hydration vessel (170) for reacting at least some of the sodium oxide in the other insoluble products with liquid water to produce an aqueous solution of sodium hydroxide, wherein the hydration vessel (170) comprises a first inlet (171) for receiving the other insoluble products from the third outlet (187) of the separation subassembly (180, 190a), a second inlet (172) for liquid water, and an outlet (174) for the aqueous solution of sodium hydroxide and undissolved solids;a solid-aqueous phase separator (175) for separating the undissolved solids from the aqueous solution of sodium hydroxide, wherein the solid-aqueous phase separator (175) comprises an inlet (176) downstream of 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; anda caustic dryer (460) for drying the aqueous solution of sodium hydroxide to produce solid sodium hydroxide and water vapour, the caustic dryer (460) comprising an inlet (461) for receiving the aqueous solution of sodium hydroxide from the first outlet (178) of the solid-aqueous phase separator (175), a first outlet (464) for water vapour, and a second outlet (465) for solid sodium hydroxide which is connected upstream of the first inlet (711) of the gas-tight reaction vessel (710).

13. An apparatus (7b) according to claim 12, wherein the second inlet (132) of the redox reaction subassembly (130) is connected downstream of the first outlet (734) of the sodium condenser (730).

14. An apparatus (7c) according to claim 12 or claim 13, 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; anda combustion chamber (430) for combusting chlorine gas with hydrogen gas to produce hydrogen chloride, the combustion chamber (430) comprising a first inlet (431) for receiving the chlorine gas from the second outlet (15) of the electrolytic subassembly (10), a second inlet (432) forreceiving the hydrogen gas from the second outlet (715) of the gas-tight reaction vessel (710), and an outlet (434) for hydrogen chloride;wherein the first outlet (17) of the electrolytic subassembly (10) is connected upstream of the second inlet (132) of the redox reaction subassembly (130).

15. An apparatus (7c) according to any one of claims 9 to 14, 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; anda combustion chamber (430) for combusting chlorine gas with hydrogen gas to produce hydrogen chloride, the combustion chamber (430) comprising a first inlet (431) for receiving the chlorine gas from the second outlet (15) of the electrolytic subassembly (10), a second inlet (432) for receiving the hydrogen gas from the second outlet (715) of the gas-tight reaction vessel (710), and an outlet (434) for hydrogen chloride;wherein the first outlet (17) of the electrolytic subassembly (10) is connected upstream of the second inlet (712) of the gas-tight reaction vessel (710).

16. An apparatus (7d) according to claim 14 or claim 15, further comprising a heat transfer pathway (750) for transferring heat from the combustion chamber (430) to sodium oxide within the vacuum chamber (720).

17. An apparatus (7d) according to claim 16, wherein the heat transfer pathway (750) is adapted to contain liquid sodium or a liquid sodium-potassium (NaK) alloy as a heat transfer fluid.T +44(0)30 0300 2000A

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