Electrochemical apparatuses and methods of operating them

Abstract

The present invention provides an electrochemical apparatus (300a) comprising an electrolytic cell (100) for producing an electrolysis product (50, 60) and a reaction vessel (200) for consuming at least some of the electrolysis product (50, 60) as a reagent in an exothermic chemical reaction. The electrolytic cell (100) comprises an electrochemical device, itself comprising an electrode (10) and a conduit (20). The electrode (10), which is configured as an electrode (10, 70) of the electrolytic cell (100), has a first part for immersion in an electrolyte, a second part for making electrical contact with a current-carrying conductor, and a third part between the first and second parts for making thermal contact with a heat transfer fluid (HTF). The conduit (20) is arranged to bring the heat transfer fluid (HTF) into thermal contact with the third part of the electrode (10). The electrode (10) is made of carbon, which has reduced electrical resistivity and thermal conductivity at temperatures above ambient. Therefore, if the third part of the electrode (10) is heated by means of the HTF, its electrical resistivity and thermal conductivity are reduced. The former enables more efficient flow of electrical current between the first and second parts of the electrode and therefore reduces the total internal resistance of the electrolytic cell (100). The latter reduces the flow of heat between the first and second parts of the electrode (10) and therefore helps to prevent heat loss from the electrolytic cell (100) via the electrode (10) to the current-carrying conductor. Both effects make the electrolytic cell (100) more energy efficient. The apparatus (300a) further comprises a product transfer pathway (110) arranged to transfer the electrolysis product (50, 60) from the electrolytic cell (100) to the reaction vessel (200), and a first heat exchanger (210) arranged to transfer heat (Q) from the reaction vessel (200) to the heat transfer fluid (HTF) in the conduit (20). The electrochemical device may therefore be used to inject waste heat from the exothermic chemical reaction into the electrolytic cell (100). This reduces the electrical power consumption of the electrolytic cell and also makes the combination of the electrolytic cell (100) with the exothermic chemical reaction supplying the heat (Q) more energy efficient overall. The invention also provides methods of operating such an electrochemical apparatus. The invention is particularly suited to producing Group 1 and Group 2 metals by electrolysis with increased energy efficiency.

Description

[0001] Electrochemical Apparatuses and Methods of Operating them

[0002] Field of the Invention

[0003] The present invention concerns electrochemical apparatuses comprising electrolytic cells and methods of operating such apparatuses. An electrochemical device as described herein is particularly suitable for use in electrolytic cells for producing alkali and alkaline earth (Group 1 and Group 2) metals by electrolysis.

[0004] Background of the Invention

[0005] Several alkali and alkaline earth metals are produced industrially by fusing and electrolysing a chloride salt of the metal in guestion. These metals include lithium, sodium, magnesium and calcium. Application of an electrical current to the chloride salt of the selected metal in solid phase fuses ( / '.e., melts) the chloride salt by ohmic heating. Subseguent electrolysis of the fused salt produces the selected metal, with chlorine gas produced as a co-product. In order to lower the temperature at which the electrolysis takes place, the chloride salt of the selected metal may be mixed with one or more other salts to form a eutectic mixture. For example, electrolysis of lithium chloride to produce elemental lithium typically occurs at about 420 to 450 “Celsius, electrolysis of sodium chloride to produce elemental sodium at about 600 to 625 “Celsius, electrolysis of magnesium chloride to produce elemental magnesium at about 700 to 750 “Celsius, and electrolysis of calcium chloride to produce elemental calcium at about 800 “Celsius. In the cases of lithium, sodium and magnesium, the metal is produced in liguid phase, whereas in the case of calcium, which has a melting point above the temperature for electrolysis of the chloride salt, calcium metal is produced in solid phase instead.

[0006] By way of example, electrolysis of fused sodium chloride is traditionally carried out in an electrolytic cell based on the design of Downs, originally described in US patent no. 1 501 756. An example of such a Downs cell is shown in Fig. 1. As shown therein, an electrolytic cell 101 comprises a vessel 102 having a lid 103 comprising an inlet 104 through which electrolyte 40 can be introduced into the cell 101. A centrally located anode 10 surrounded by an annular cathode 70 are each supplied with electrical current via current-carrying conductors 30a, 30b connected to the cathode 70, and of opposite polarity to the anode 10 (not shown). The current fuses the electrolyte 40 by ohmic heating and electrolyses it into liguid metal 50 and chlorine gas 60. The liguid metal 50, being less dense than the fused electrolyte 40, floats to the top of the electrolyte 40, where it is captured by a hood 80 and siphoned off via an outlet 105. Bubbles of chlorine gas 60 rise from the anode 10 to another outlet 106 in the lid 103. An iron or steel mesh or grille 75 located between the anode 10 and the cathode 70 allows the electrolyte 40 to flow therethrough, but inhibits the back-reaction of liguid metal 50 formed at the cathode 70 with chlorine gas 60 formed at the anode 10. The anode 10 is typically made of amorphous, glassy or graphitic carbon because carbon is generally unreactive with the chlorine gas 60 evolved from the anode 10, whereas the cathode 70 is typically made of iron or steel. The electrolyte 40 is typically sodium chloride mixed in a eutectic mixture with calcium chloride and / or sodium carbonate to lower the temperature for electrolysis to about 600 to 625 “Celsius, as mentioned above.

[0007] In another example, electrolysis of fused magnesium chloride can be conducted in an electrolytic cell operating on the same general principles but having a different layout, in which fumes of magnesium vapour are extracted under vacuum from the cell. A review of the production of magnesium metal by electrolysing magnesium chloride can be found in "The Chemistry and Electrochemistry of Magnesium Production” by G.J. Kipouros & D.R. Sadoway in Advances in Molten Salt Chemistry, Vol. 6, edited by G. Mamantov, C.B. Mamantov & J. Braunstein, Elsevier, Amsterdam, pp. 127-209 (1987).

[0008] Alkali and alkaline earth metals may also be produced by fusing and electrolysing different compounds of these metals than just their halides. For example, magnesium may be produced by fusing and electrolysing magnesium oxide in a mixture with magnesium fluoride and lithium fluoride, as described in "Molten Salt Electrolysis of Magnesium Oxide Using a Liquid-Metal Cathode for the Production of Magnesium Metal” by T.-H. Lee et al., Metallurgical and Materials Transactions B, Vol. 51, pp. 2993-3006 (2020). In another example, sodium may also be produced by fusing and electrolysing solid sodium hydroxide. This has traditionally been carried out in an electrolytic cell based on the design of Castner, originally described in US patent no. 452 030. In such a Castner cell, the geometry of the anode and cathode are reversed in comparison to that in a Downs cell, with an annular anode surrounding a centrally located cathode. Although the cathode in a Castner cell may be made of iron or steel, the anode is typically made of nickel or a nickel-containing alloy, which is highly resistant to corrosion by the caustic sodium hydroxide. The electrolysis is generally conducted at a temperature of about 330 “Celsius, just above the melting point of the sodium hydroxide, so that the electrolyte remains solid in a region around where the cathode enters the cell, thereby preventing the molten electrolyte from leaking out of the cell.

[0009] The Castner process for producing sodium by electrolysing sodium hydroxide is generally regarded as obsolete, because the back-reaction of water produced in the Castner cell with the liquid sodium produced at the cathode lowers the overall efficiency of this process. Whereas some attempts have been made in the prior art to address this problem (see, for example, US patent no. 4 276 145) the Castner process has therefore been almost entirely superseded by the Downs process for producing sodium by electrolysing sodium chloride instead.

[0010] Several other attempts have also been made in the prior art to improve on these different techniques. The suggested improvements generally fall into one of two categories. Firstly, there are those which vary the composition of the electrolyte itself. Some examples of prior art in this first category are US patent nos. 464 097, 2 850 442 and 3 020 221 , and US patent application no. 2001 / 045365. Secondly, there are those which vary the design of the electrolytic cell, for example by varying the cell geometry and / or the layout of the electrodes within the cell. Some examples of prior art in this second category are US patent nos. 2 893 940, 4 584 068 and 5 904 821 , European patent no. 3 033 443, and "Analysing and Optimizing the Electrolysis Efficiency of a Lithium Cell Based on the Electrochemical and Multiphase Model” by Qian-Wen Zhao et al., Royal Society Open Science, vol. 7, no. 191124 (2019). However, little attention seems to have been paid in the prior art to the basic thermodynamic aspects of producing alkali and alkaline earth metals by electrolysis, whereby the energy efficiency of the overall process may be improved.

[0011] In particular, it is well known that whereas in theory, electrode potentials of, for example, the reduction of sodium and the oxidation of chlorine imply a total potential difference for the electrolysis of sodium chloride of only 4.07 V, in practice, the electrolysis of sodium chloride to produce sodium metal is typically carried out industrially at a voltage of from about 6.5 to about 7.5 V. When this voltage is multiplied by the high current through an electrolytic cell required for a reasonable rate of production of the electrolysis products, the overvoltage translates into a significant energy loss. The excess energy is consumed not only by ohmic heating of the electrolyte and to overcome the internal resistance of the electrolytic cell, but also by such other inefficiencies as the back-reaction of the electrolysis products within the cell, and an overpotential caused by bubbles of gas adhering to the anode, which reduces the effective surface area of the anode available to conduct current. Whereas some attention has been paid in the prior art to addressing these other inefficiencies, the energy consumed by ohmic heating of the electrolyte and the internal resistance of the electrolytic cell seem to be generally accepted in the prior art as being inevitable, and not to have been addressed.

[0012] For example, heat can be applied to the exterior of an electrolytic cell to help fuse the electrolyte, which therefore reduces the excess voltage required to melt it by ohmic heating. Although this possibility is mentioned in European patent no. 3 033 443, no consideration seems to have been given therein to the possibility of using waste heat from another process to fuse the electrolyte. Moreover, it seems to be generally accepted in the prior art that it is more efficient to thermally insulate an electrolytic cell from its environment, which can be done relatively easily, and to fuse the electrolyte by ohmic heating, which rules out the possibility of using waste heat from another source to fuse the electrolyte and therefore increases the total amount of electrical energy required for its electrolysis. However, whereas the exterior of an electrolytic cell can be thermally insulated from its environment relatively easily, heat is also lost from within the cell via its electrodes, which are both electrically and thermally conducting. Because the surrounding air is thermally insulating, this heat is mostly lost by transmission via the electrodes to the current-carrying conductors to which the electrodes are connected. In the traditional design of electrolytic cell due to Downs, this problem is more acute in the case of the anode than it is for the cathode because, as Fig. 1 shows, the anode has a solid, cylindrically symmetrical shape, whereas the cathode has an annular shape, open at both ends. This gives the anode a much higher ratio than the cathode between the end cross-sectional areas of each electrode for carrying current and the surface area of the same electrode in contact with the fused electrolyte. European patent no. 3 033 443 recognizes this problem, but addresses it by changing the geometry of the anode.

[0013] Finally, it should also be noted that in common with other metals, the electrical resistivity of iron, steel and nickel increases with temperature. In contrast, as Fig. 2 shows, the electrical resistivity of carbon falls from ambient temperature to reach a minimum at about 500 “Celsius, above which it starts to rise again. The electrical resistivity of carbon then does not reach the same value as it has at room temperature until above about 1200 “Celsius. Fig. 2 is a graph depicting both the electrical resistivity and the thermal conductivity of carbon over a range of temperatures from room temperature up to about 1000 “Celsius. Since graphite has a laminar structure consisting of planes of carbon atoms, the physical properties of a single crystal of graphite differ significantly, depending on the angle at which they are measured relative to the atomic planes ( / '. e., parallel, perpendicular or obliquely thereto). Thus the values of electrical resistivity and thermal conductivity shown in Fig. 2 are in both cases the bulk values for carbon which is isotropic or substantially isotropic, for example as a result of it being amorphous, glassy or polycrystalline, and which are therefore independent of the angles at which they are measured.

[0014] Superimposed on Fig. 2 are the temperatures typically used for the electrolysis of the chlorides of lithium, sodium, magnesium and calcium, as well as that for sodium hydroxide. As can be seen, these temperatures all lie at or near to the minimum electrical resistivity of carbon, as well as at significantly reduced values for the thermal conductivity of carbon in comparison to its value at room temperature. For example, as Fig. 2 shows, the thermal conductivity of carbon is about 55 W K1nr1at 600 “Celsius. For comparison, the thermal conductivity of iron at the same temperature is about 50 W K1rm1. Also taking account of the change in specific heat capacity of carbon over the same temperature range as depicted in Fig. 2, the thermal diffusivity of isotropic carbon also drops significantly in line with its thermal conductivity.

[0015] Further background prior art may be found in European patent application nos. 0 050 680 and 2 350 353, UK patent application no. 2 037 549, US patent no. 4 256 918 and Canadian patent no. 1 181 792.

[0016] Object of the Invention

[0017] It is therefore an object of the invention to provide an apparatus comprising an electrolytic cell and a method of operating such an apparatus to improve the energy efficiency of the electrolytic cell.

[0018] Description of the Invention

[0019] Accordingly, in one aspect, the present invention provides an apparatus comprising an electrolytic cell for producing an electrolysis product, wherein the electrolytic cell comprises an electrochemical device, itself comprising a carbon electrode configured as an electrode of the electrolytic cell and a conduit for a heat transfer fluid (HTF). The carbon electrode has a first part for immersion in an electrolyte, a second part for making electrical contact with a current-carrying conductor, and a third part between the first and second parts for making thermal contact with a heat transfer fluid (HTF). The conduit is arranged to bring the HTF into thermal contact with the third part of the electrode. The apparatus further comprises a reaction vessel which consumes at least some of the electrolysis product in an exothermic chemical reaction, a product transfer pathway arranged to transfer the electrolysis product from the electrolytic cell to the reaction vessel, and a first heat exchanger arranged to transfer heat from the reaction vessel to the heat transfer fluid in the conduit.

[0020] Such an apparatus at least has the advantage that heat generated by the exothermic reaction can then be transferred to the electrolytic cell by the HTF and used to help fuse the electrolyte in the cell. For example, if the electrolyte comprises sodium chloride, and the electrolytic cell produces liquid sodium and chlorine gas as electrolysis products, the liquid sodium may be used as a chemical reducing agent and / or the chlorine gas may be used in the production of hydrochloric acid and / or chlorinated organic compounds. Heat generated by any of these chemical reactions may then be transferred back to the electrolytic cell via the HTF to contribute to heating the electrolyte.

[0021] In some embodiments, the HTF may comprise at least one of a reagent and a product of the exothermic reaction. This at least has the advantage that the reagent and / or product of the reaction carry away with them at least some of the heat generated by the reaction, which avoids transferring the heat from the reagent and / or product to another HTF with a possible accompanying loss of some of the heat.

[0022] Thus the electrochemical device of the electrolytic cell serves a dual function: firstly, as an electrode for supplying electricity from a current-carrying conductor to an electrolyte, and secondly, as a heat exchanger for transferring heat between the electrolyte and an HTF, via the electrode itself. This can be done if an HTF is brought into contact with the third part of the electrode. The third part of the electrode then effectively acts as a pin fin heat exchanger around which the HTF can circulate, and which therefore may be subjected to thermal analysis using known techniques. For example, if the HTF is at a temperature above ambient temperature but below that within an electrolytic cell which contains the electrolyte, the rate of heat loss from the cell via the electrode can be reduced. If, on the other hand, the HTF is at a temperature above that within the cell, heat can be transferred into the cell via the electrode. Since the first part of the electrode comes into direct contact with the electrolyte, this is more efficient than transferring heat into the cell through an outer surface of the cell.

[0023] Moreover, since the electrode is made of carbon, as it is heated up from ambient temperature by the HTF, both its thermal conductivity and its electrical resistivity drop. Thus the electrode becomes more thermally insulating as it heats up, reducing the loss of heat from an electrolytic cell in which the electrochemical device is configured as one of the cell's electrodes. The electrode also becomes more electrically conducting as it heats up, which reduces the voltage drop across the electrode and therefore also reduces the electrical energy lost through ohmic heating of the electrode by electricity supplied via the current-carrying conductor. In addition, since the third part of the electrode, whereby heat can be introduced into the electrolytic cell, lies between the first part of the electrode which is immersed in the electrolyte and the second part which makes electrical contact with the current-carrying conductor, the temperature gradient from the electrolyte to the electrode, and therefore the rate of heat loss from the cell via the electrode to the current-carrying conductor, are reduced as well. This further advantageous effect, which results from the device's geometry, is independent of and additional to the advantageous effects which result from the electrode being made of carbon.

[0024] The electrochemical device permits waste heat from another process to be introduced directly into an electrolyte inside an electrolytic cell which includes the device, without having to be transferred across an outer surface of the electrolytic cell from its environment. The heat transfer process is accordingly more efficient, and the electrical energy required for ohmic heating of the electrolyte can thereby be reduced. The electrochemical device also simultaneously reduces the internal resistance of the electrolytic cell, as the following idealised example illustrates. An isotropic carbon electrode in the shape of a right circular cylinder having a length of 1 m and a radius of 0.25 m has a resistivity of 10.5 x 106Qm at a uniform temperature throughout the electrode of 25 “Celsius, which can be reduced to 7.5 x 106Qm by heating the electrode to a uniform temperature of 500 “Celsius. The resistance of the electrode along its length is therefore reduced from 5.35 x 105Q at 25 “Celsius to 3.82 x 105Q at 500 “Celsius. For a current of 40 kA passing along the electrode, which is typical of the currents used to electrolyse alkali and alkaline earth metal chlorides, the voltage drop along the length of the electrode is therefore reduced from 2.13 V at 25 “Celsius to 1.53 V at 500 “Celsius, and the corresponding rate of energy loss through ohmic heating of the carbon electrode is reduced from 85.6 kW to 61.1 kW. (In practice, the voltage drop along the electrode will be less than in this idealised example in the cases of both the unheated and the heated electrode because the electrode may have a more complicated geometry than just a right circular cylinder and the current is conducted to the electrolyte not just through one end of the electrode, but also through its sides. However, the percentage reduction in the voltage drop and the corresponding percentage reduction in the rate of energy loss which can be achieved will remain the same.)

[0025] Usually, the carbon electrode will be made of amorphous, glassy or graphitic carbon which is isotropic or substantially isotropic. However, similar improvements can also be achieved with a carbon electrode even if the carbon it is made of is spatially ordered (for example, if the electrode is a single graphite crystal), provided that the orientation of the graphite layers is taken into account when the electrode's shape is designed. Although the carbon electrode is most conveniently generally cylindrical or mushroom-shaped as shown in Fig. 1 , it may in principle have any shape, such as an annular shape, provided that the spatial relationship between the first, second and third parts of the electrode is maintained as described herein.

[0026] The electrochemical device also has the advantage that it can be used to quickly and easily control the temperature of an electrolyte, by changing the temperature of the HTF, for example during electrolysis, but without any need to adjust the supply of electrical current to the cell. For example, rapid cooling of the interior of an electrolytic cell may be achieved by reducing the temperature of the HTF, which cannot be done just by altering the supply of electrical current to the cell. This may be considered desirable, for example, in an emergency situation, or to help combat an "anode effect” which occurs when the potential difference across the cell increases suddenly as a result of polarisation of the anode, with a corresponding drop in the rate of electrolysis and an increase in the temperature inside the cell instead. If the interior of an electrolytic cell is rapidly cooled in this manner, the concomitant cooling of the carbon electrode by the HTF also has the effect of increasing the thermal conductivity of the electrode, which therefore enhances the rate of cooling.

[0027] If the HTF is electrically insulating and does not react with amorphous, glassy or graphitic carbon (for example, if the HTF is a mineral oil or a synthetic oil), it can be brought into direct contact with the carbon electrode. Nonetheless, in such a case, the conduit for the HTF should be both thermally and electrically isolated from the electrode to prevent leakage of heat and electrical current from the electrode via the conduit. This may be achieved in one of several ways. For example, a thermally and electrically insulating buffer may be interposed between the electrode and the conduit. Alternatively or additionally, the conduit may itself be made of a material which is both thermally and electrically insulating. There are many suitable materials which are both thermally and electrically insulating, including many synthetic polymers, which may be used to make the thermally and electrically insulating buffer and / or the conduit.

[0028] However, many heat transfer fluids, such as molten salts and liquid metals, are either electrically conducting or react with amorphous, glassy or graphitic carbon, or both. For example, if the HTF is liquid sodium or liquid sodiumpotassium alloy (NaK), even though liquid sodium and NaK are not particularly reactive with carbon, they are both highly electrically conductive. Therefore, even if the conduit itself is electrically isolated from the electrode, if the HTF is electrically conducting, the HTF would provide an alternative pathway for conduction of electrical current from the current-carrying conductor eventually to earth, which is clearly undesirable. Moreover, if the HTF reacts with carbon, it would attack the carbon electrode, which is equally undesirable. In either case, therefore, the carbon electrode should be protected from the HTF. This may be achieved, whilst still allowing heat to be exchanged between the electrode and the HTF, by providing the electrochemical device with an interface layer made of a thermally conducting and electrically insulating material between and in thermal contact with both the third part of the electrode and an interior of the conduit for the HTF. For example, the interface layer may take the form of a collar or grommet around the third part of the carbon electrode.

[0029] Preferably, the material of the interface layer should have an electrical resistivity of at least about 108Qm and a thermal conductivity of at least about 10 W K1rm1. This has the advantage of ensuring a high level of thermal conductivity between the electrode and the HTF, whilst still preventing the leakage of electrical current from the electrode to the HTF. Some examples of such materials include aluminium nitride (AIN), aluminium oxide (AI2O3), beryllium oxide (BeO), boron nitride (BN), silicon nitride (SisN^ and synthetic diamond (C), as well as composite materials comprising at least two of the aforementioned materials. If the material of the interface layer has a similar coefficient of thermal expansion to the carbon electrode, the interface layer may be deposited or grown directly onto the third part of the carbon electrode using a known deposition or growth technique. If not, however, the interface layer is preferably separate from, but in thermal contact with, the third part of the carbon electrode, for reasons explained below. In the latter case, the interface layer may be grown or deposited onto a support or substrate made of a material with high thermal conductivity and having a similar coefficient of thermal expansion to the material of the interface layer, but which need not itself be electrically insulating.

[0030] The appropriate thickness of the interface layer to use in any particular case will depend on the dimensions and geometry of the carbon electrode, and in particular on the surface area of the third part of the electrode in thermal contact with the HTF. The thickness of the interface layer can therefore be calculated from the thermal conductivity and electrical resistivity of the material from which the interface layer is made, the desired rate of heat transfer and total electrical resistance to be provided by the interface layer, and the dimensions and geometry of the electrochemical device, including those of the third part of the electrode and of the interface layer itself. Examples will be given below.

[0031] Many of the above-mentioned materials from which the interface layer may be made are more resistant to chemical attack than amorphous, glassy or graphitic carbon, and may therefore be used in direct contact with a molten-salt HTF such as a eutectic mixture of sodium nitrate and potassium nitrate or a ternary mixture of sodium nitrate, potassium nitrate and lithium nitrate. In some cases, however, the HTF may react with the material of the interface layer as well, particularly at high temperatures. This may occur, for example, if the HTF is a molten halide or a liquid metal. In such cases, therefore, to protect the interface layer from attack by the HTF, the conduit may comprise a portion between and in thermal contact with both the interface layer and the interior of the conduit, made of a material which is itself resistant to corrosion by molten halides and / or liquid metals. The material from which the portion of the conduit is made should also be thermally conducting to ensure the transfer of heat between the carbon electrode and the HTF, and may also be electrically conducting. Therefore, whereas the interface layer electrically isolates the carbon electrode from both the conduit and the HTF, the portion of the conduit which is between and in thermal contact with both the interface layer and the interior of the conduit protects the interface layer from chemical attack by the HTF, whilst still allowing heat transfer through the interface layer between the electrode and the HTF. Examples of such materials which are resistant to corrosion by molten halides and / or liquid metals, but which are also thermally and electrically conducting, include several different grades of stainless steel and various other structural alloys which are well known to those skilled in the art of liquid metal engineering and / or the use of molten halides as HTFs. A review of materials resistant to corrosion by a range of molten halides when used as HTFs can be found in Engineering Database of Liquid Salt Thermophysical and Thermochemical Properties by Manohar S. Sohal et al., publication no. INL / EXT-10-18297 of Idaho National Laboratory, prepared for the U.S. Department of Energy Office of Nuclear Energy under DOE Idaho Operations Office Contract no. DE- AC07-05ID14517 (June 2013), the entire contents of which is incorporated herein by reference.

[0032] If the electrochemical device comprises an interface layer, and especially if the conduit also comprises a portion between and in thermal contact with both the interface layer and the interior of the conduit, then the third part of the carbon electrode, the interface layer and this portion of the conduit may be subject to different rates of thermal expansion, due to the different thermal expansion coefficients of the different materials from which they are made. If the electrochemical device is subject to large changes in temperature, as may be expected, one or more of these components may then be at risk of cracking, as a result of strain induced by differential thermal expansion of these different components in contact with each other. Cracking of the interface layer is likeliest because the interface layer may be both thin and made of a stiff and brittle material. This is a potentially serious problem because apart from reducing the effectiveness of the interface layer for conducting heat, it also risks breakdown of the electrical isolation it provides between the carbon electrode and the HTF. Graphitic carbon has a coefficient of thermal expansion linearly of about 4 to 8 x 106K1. For example, therefore, a cylindrical carbon electrode having a diameter of 0.25 m can be expected to expand widthwise by between about 0.5 and 1 mm if subjected to a temperature increase of 500 “Celsius. This would clearly put a significant strain on an interface layer only 1 mm thick if it had a significantly different coefficient of thermal expansion, and would probably be more than enough to crack it. However, this problem may be solved by providing the device with a thermal expansion joint as follows.

[0033] In some embodiments, therefore, the interface layer may be separate from both the third part of the electrode and the portion of the conduit between the interface layer and the interior of the conduit. This allows for relative movement between the interface layer on the one hand and the third part of the electrode and the portion of the conduit on the other. In such a case, the interface layer may form a hollow tube around the third part of the electrode and comprise an even number of at least four abutting elongate sections. Each of the elongate sections has a wider end and an opposing narrower end, wherein the wider end of each section is wider in a circumferential direction of the interface layer than the narrower end of the same elongate section. The elongate sections are arranged around the third part of the electrode with their respective wider and narrower ends alternating with each other. The interface layer also comprises a flange at each of its opposing ends, overlapping an outer surface of the conduit. The separation between these opposing flanges is greater than the thickness of the conduit by an amount which allows each of the elongate sections to slide independently of each other parallel to the electrode. In such embodiments, the device further comprises two thermally and electrically insulating washers around the electrode, each abutting a respective one of the flanges, and at least one resilient member around the electrode, abutting a respective one of the washers, and arranged to apply a compressive force to the wider ends of the elongate sections of the interface layer. For example, the compressive force may be applied by also abutting the at least one resilient member against a static buffer and / or against an exterior surface of the electrolytic cell. The washers and the at least one resilient member are able to slide freely along the electrode, whereby adjacent ones of the elongate sections are induced to slide antiparallel to each other.

[0034] With this arrangement, the compressive force applied by the at least one resilient member keeps the elongate sections of the interface layer in contact with each other and with both the third part of the electrode and the portion of the conduit between the interface layer and the interior of the conduit. Meanwhile, this portion of the conduit prevents leakage of HTF between the elongate sections of the interface layer. In addition, any strain on the interface layer due to thermal expansion of the third part of the electrode is converted into a movement of adjacent ones of the elongate sections of the interface layer antiparallel to each other, as a result of the compressive force applied to the interface layer by the at least one resilient member. This movement of the elongate sections induces a corresponding strain in the at least one resilient member, reducing the compressive force applied by the at least one resilient member to the interface layer by an amount determined by the Young's modulus of the at least one resilient member. If subsequent thermal contraction of the third part of the electrode reduces the strain on the interface layer, the reduced compressive force applied by the at least one resilient member to the interface layer allows the elongate sections to move back in the opposite directions to their movement when heated. Although the thickness of the interface layer is unchanged by such movement, both the inner and outer diameters of the hollow tube formed by the interface layer can be freely adjusted by such movement, which therefore accommodates the strain induced by thermal expansion and contraction of the third part of the electrode and significantly reduces the risk of the interface layer cracking, in comparison to if the interface layer were instead formed as a single component.

[0035] The compressive force applied to the interface layer by the at least one resilient member need only be enough to overcome static friction between the elongate sections of the interface layer, as well as between the interface layer and the carbon electrode, and between the interface layer and the conduit. The size of this compressive force can be adjusted by altering the Young's modulus, geometry and dimensions of the at least one resilient member in a known fashion. Static friction between the elongate sections of the interface layer, as well as between the interface layer and the other components it is in contact with, may be reduced by lubricating the elongate sections of the interface layer with powdered graphite.

[0036] Thermal expansion and contraction of the conduit can be accommodated by providing the conduit with one or more thermal expansion joints of a known type, such as a bellows-type thermal expansion joint. If such a thermal expansion joint is located in the conduit in proximity to the arrangement described above, it can therefore also accommodate the thermal expansion and contraction of the third part of the electrode and the corresponding movement of the elongate sections of the interface layer as a result.

[0037] If the first part of the electrode is to be immersed in an electrolyte which may react with carbon, the electrochemical device may further comprise an electrically and thermally conducting sheath around and in contact with the first part of the electrode, wherein the sheath is made of a material which does not react with the electrolyte and which has a similar linear coefficient of thermal expansion to that of carbon, such as within 25%, more preferably 10%, and most preferably 5%, thereof. The sheath therefore protects the first part of the electrode from being corroded by the electrolyte, whilst still being able to expand and contract with the electrode as they both change temperature, without inducing stresses in either the sheath or the electrode. For example, if the electrolyte is sodium hydroxide, the sheath may be made of a material which is resistant to corrosion by sodium hydroxide, such as a nickel- containing alloy like Inconel® or Hastelloy®, several different grades of which have coefficients of thermal expansion which are either very similar to or the same as that of carbon. In order to minimize its electrical and thermal resistance, the thickness of the sheath need not be more than is sufficient to maintain its structural integrity and to protect the electrode from being attacked by the electrolyte, such as less than about 2 mm, and preferably about 1 mm.

[0038] Whereas the electrochemical device in the apparatus of the invention is most suitable for use in an electrolytic cell for producing alkali and alkaline earth metals by electrolysis, wherein the carbon electrode is configured as an anode of the cell, this electrochemical device may in principle be used in any electrochemical apparatus employing at least one carbon electrode, and the carbon electrode of the device may be configured as either an anode or a cathode of the electrochemical apparatus, or the electrodes of two such devices may be respectively configured one as an anode and one as a cathode in the same electrochemical apparatus. For example, the device may also be used in a Hall— Heroult electrolytic cell for electrolysing cryolite in the production of aluminium from alumina (AI2O3), or in an electrolytic cell using the FFC Cambridge process to produce metals such as titanium and tantalum from their respective oxides.

[0039] If the carbon electrode of a device as described herein is configured as an anode of an electrolytic cell, the anode may be at least partially surrounded by a cathode of the electrolytic cell in an arrangement such as that in the Downs cell of Fig. 1 , or the anode may instead at least partially surround the cathode, as in a Castner cell. Preferably, however, an arrangement in which a carbon electrode of such a device is centrally located within the cell and is at least partially surrounded by an electrode of opposite polarity has the advantage that heat may then be introduced into the heart of the electrolytic cell and diffuse outwardly from there within the cell, aided by convection of the electrolyte. Moreover, if the electrolytic cell produces a gas as an electrolysis product at the carbon electrode, controlled heating and / or cooling of the carbon electrode may also be effective in reducing the bubble overpotential, by helping to dislodge bubbles of gas adhering to the electrode.

[0040] In some embodiments, wherein the electrolytic cell produces liquid metal as an electrolysis product, the conduit may be arranged to bring at least some of the liquid metal thus produced into thermal contact with the third part of the carbon electrode. Thus the liquid metal produced by the electrolytic cell may be used as the heat transfer fluid (HTF). For example, if a quantity of the liquid metal produced by the electrolytic cell needs to accumulate before it can be used in another process, the liquid metal produced by the cell can be brought into thermal contact with the interior of the cell via the electrochemical device, whereby the temperature of the liquid metal may be maintained as it accumulates. In other words, the electrolytic cell and a storage tank for liquid metal produced by the cell may be thermally coupled with each other via this device, thereby creating a single, combined thermal mass with greater thermal inertia, which can be maintained at the same temperature with less heat lost to the environment, than if the electrolytic cell and the storage tank for the liquid metal were to remain as two separate thermal masses.

[0041] In some embodiments in which the apparatus comprises an electrolytic cell which produces liquid metal as an electrolysis product, the apparatus may further comprise a thermally insulated storage tank for storing the liquid metal, and a supply route arranged to transfer the liquid metal from the electrolytic cell to the storage tank. In such an apparatus, the conduit is arranged to bring at least some of the liquid metal thus produced into thermal contact with the third part of the carbon electrode as the HTF, and the storage tank is arranged to provide the conduit with the liquid metal. This at least has the advantage that the electrolytic cell and the storage tank are thermally coupled to each other via the HTF in the conduit, thereby creating a single, combined thermal mass with greater thermal inertia. In some embodiments, the conduit may also be arranged to return the liquid metal back to the storage tank in a loop.

[0042] In some embodiments, the reaction vessel may be adapted for consuming at least some of the liquid metal from the storage tank as a reagent in the exothermic chemical reaction, the product transfer pathway may be arranged to transfer the liquid metal from the storage tank to the reaction vessel, and the first heat exchanger may be arranged to transfer at least some of the liquid metal in excess of a stoichiometric amount thereof required for the exothermic chemical reaction, which has been heated by the reaction, from the reaction vessel to the storage tank. Thus liquid metal accumulated in the storage tank, which is used to thermally couple the electrolytic cell and the storage tank together, may also be used to transfer heat from the reaction vessel back to the storage tank, thereby maintaining the temperature within the electrolytic cell within a desired range, even as at least some of the liquid metal is consumed by the reaction.

[0043] In some embodiments, such an apparatus may further comprise a valve located in the product transfer pathway to regulate a flow of the liquid metal from the storage tank to the reaction vessel. This has the advantage that if the liquid metal is produced by the electrolytic cell at a different rate from that at which it is consumed by the exothermic chemical reaction, the liquid metal may be accumulated in the storage tank until there is sufficient of it to be supplied to the reaction vessel at the desired rate of reaction.

[0044] In some embodiments, the apparatus may further comprise a container for an endothermic process and a second heat exchanger arranged to transfer heat to the endothermic process from the electrolysis product in the product transfer pathway. Thus if the quantity of heat carried by the electrolysis product is more than would allow the exothermic chemical reaction to remain within a desired temperature range for the reaction, the electrolysis product may be cooled before it is supplied to the reaction.

[0045] In a second aspect, the present invention also provides a method of operating an apparatus as described herein when the electrolytic cell thereof contains an electrolyte. The method comprises immersing the first part of the carbon electrode in the electrolyte, supplying the electrolyte with electricity via the second part of the carbon electrode, supplying the electrolyte with heat via the third part of the carbon electrode, supplying an electrolysis product from the electrolytic cell to an exothermic chemical reaction, and obtaining from the exothermic chemical reaction at least some of the heat supplied to the electrolyte within the cell via the third part of the carbon electrode.

[0046] In some embodiments, the method may comprise using at least one of a reagent and a product of the exothermic reaction as a heat transfer fluid to transfer heat from the exothermic chemical reaction to the electrolytic cell, which has the advantage already described above that it avoids transferring heat from the reagent and / or product to another substance.

[0047] In some embodiments, the method may further comprise controlling a temperature within the electrolytic cell by altering a quantity of the heat supplied to the electrolyte via the third part of the carbon electrode. This at least has the advantage that the temperature within the cell may thus be controlled both quickly and easily without having to alter the supply of electricity to the electrolytic cell.

[0048] Preferably, the temperature within the electrolytic cell is thus controlled to lie within a range of from about 300 to about 1200 “Celsius, more preferably from about 300 to about 1000 “Celsius, and most preferably from about 300 to about 850 “Celsius. In such a range of temperatures, both the thermal conductivity and the electrical resistivity of the carbon electrode are less than their corresponding values at ambient temperature, giving all of the advantages described above. In either case, the method may comprise extracting heat from the electrolysis product before supplying the electrolysis product to the exothermic chemical reaction, for example to help prevent the reaction from overheating.

[0049] In some embodiments, the exothermic chemical reaction may comprise reducing a metal oxide with the electrolysis product. For example, if the electrolysis product is liquid sodium, the exothermic reaction may comprise reacting the liquid sodium in a redox reaction with one or more oxides of iron and / or manganese to produce sodium oxide and at least one of iron and manganese in elemental form, respectively.

[0050] In some embodiments, the method may comprise using liquid metal produced by the electrolytic cell as a heat transfer fluid to supply the electrolyte with heat via the third part of the carbon electrode. This at least has the advantage that the temperature of the liquid metal immediately after it has been produced is the same as the temperature within the electrolytic cell and therefore it requires little additional heating to be raised to a higher temperature, in comparison to an alternative heat transfer fluid heated from ambient temperature.

[0051] In some embodiments, the method may comprise fusing and electrolysing in the electrolytic cell a solid-phase halide, oxide or hydroxide of at least one of an alkali metal and an alkaline earth metal to produce the respective metal as the electrolysis product. As noted above, the electrolysis of, for example, sodium hydroxide, and all of the chlorides of lithium, sodium, magnesium and calcium, occur in a range of temperatures, within which both the thermal conductivity and the electrical resistivity of the carbon electrode are significantly reduced in comparison to their values at ambient temperature, therefore making such a method highly suited to producing alkali and alkaline earth metals by electrolysis of such solid-phase compounds.

[0052] Brief Description of the Drawings

[0053] 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 schematic diagram showing a partially cut-away, isometric view of an electrolytic cell of the prior art;

[0054] Fig. 2 is a graph of the electrical resistivity and thermal conductivity of isotropic carbon as a function of temperature;

[0055] Fig. 3 is a schematic diagram of a first embodiment of an electrochemical device;

[0056] Fig. 4A is a schematic diagram of a second embodiment of such a device;

[0057] Fig. 4B is a cross-section through the embodiment of the device of Fig. 4A, looking in the direction of the arrows labelled A-A' in Fig. 4A;

[0058] Fig. 5A is a schematic diagram of a third embodiment of an electrochemical device;

[0059] Fig. 5B is a cross-section through the embodiment of the electrochemical device of Fig. 5A, looking in the direction of the arrows labelled B-B' in Fig. 5A;

[0060] Fig. 6A is a schematic diagram showing a partially exploded view of a fourth embodiment of an electrochemical device comprising a thermal expansion joint;

[0061] Fig. 6B is a first end view of an interface layer in the thermal expansion joint of Fig. 6A, looking in the direction of the arrow labelled C in Fig. 6A; Fig. 6C is a second end view of the interface layer in the thermal expansion joint of Fig. 6A, looking in the direction of the arrow labelled C in Fig. 6A;

[0062] Fig. 6D is a schematic diagram of three elongate sections of the interface layer in the thermal expansion joint of Fig. 6A;

[0063] Fig. 7A is a schematic diagram of a fifth embodiment of an electrochemical device;

[0064] Fig. 7B is a partial section through the device of Fig .7A in the direction of the arrows labelled F-F' in Fig. 7A;

[0065] Fig. 8 is a schematic diagram of a first embodiment of an apparatus according to the invention;

[0066] Fig. 9 is a schematic diagram of an apparatus not part of the claimed invention;

[0067] Fig. 10 is a schematic diagram of a second embodiment of an apparatus according to the invention;

[0068] Fig. 11 is a schematic diagram of a third embodiment of such an apparatus;

[0069] Fig. 12 is a schematic diagram of a fourth embodiment of such an apparatus;

[0070] Figs. 13A and 13B are flow diagrams of a first embodiment of a method of operating an apparatus according to the invention;

[0071] Fig. 14 is a flow diagram of a second embodiment of such a method;

[0072] Fig. 15 is a flow diagram of a third embodiment of such a method;

[0073] Fig. 16 is a flow diagram of a fourth embodiment of such a method; and

[0074] Fig. 17 is a flow diagram of a fifth embodiment of such a method.

[0075] Detailed

[0076] Fig. 3 schematically shows a first embodiment of an electrochemical device 1a which may form part of the apparatus of the invention. The electrochemical device 1 a comprises a carbon electrode 10 and a conduit 20. The carbon electrode 10 has a first part 12 for immersion in an electrolyte 40, such as the electrolyte in the electrolytic cell 101 shown in Fig. 1, a second part 14 for making electrical contact with a current-carrying conductor 30c, and a third part for making thermal contact with a heat transfer fluid (HTF). The third part 16 is located between the first and second parts 12, 14. In this embodiment, the HTF is an electrical insulator and does not react with the carbon of electrode 10, so can be brought into direct contact with the electrode 10 without creating a path for electrical current to flow to earth and without corroding the electrode 10. (For example, the HTF may be a synthetic or mineral oil.) The conduit 20 carries the HTF through its interior 22 to bring the HTF into contact with the third part 16 of the electrode 10. A pair of resilient seals 28a, 28b inhibit leakage of the HTF from within the interior 22 of the conduit 20 at locations where the electrode 10 enters and leaves the conduit 20. During operation, electrical current from the current-carrying conductor 30c is conducted from the second part 14 of the electrode 10 through the third part 16 thereof to the first part 12, where it meets the electrolyte in the electrolytic cell. The third part 16 of the electrode 10 is heated by the HTF to substantially above ambient temperature, which simultaneously increases the electrical conductivity of the electrode, reduces its thermal conductivity and also reduces the temperature difference between the third part 16 of the electrode 10 and the electrolyte in the electrolytic cell, thereby reducing the outflow of heat from the cell via the electrode 10 and reducing the total internal resistance of the cell at the same time. If the temperature difference is negative (in other words, if the HTF is hotter than the electrolyte), heat will be transferred into the electrolytic cell instead.

[0077] Figs. 4A and 4B schematically show a second embodiment of an electrochemical device 1 b. The electrochemical device 1b comprises a carbon electrode 10 and a conduit 20 as in the device 1 a of the first embodiment, which are structured and function the same as in the device 1a, and will therefore not be described again in further detail. In this case, however, the heat transfer fluid is electrically conducting. (For example, the HTF may be a molten salt or a liquid metal.) Accordingly, the device 1b further comprises an interface layer 90 made of a thermally conducting and electrically insulating material, which is located between and in thermal contact with both the third part 16 of the electrode 10 and the interior 22 of the conduit 20. The interface layer 90 therefore electrically isolates the HTF from the electrode 10. The electrochemical device 1 b also comprises a pair of resilient seals similar to those in the device 1a of Fig. 3, to inhibit leakage of the HTF from within the interior 22 of the conduit 20 at locations where the electrode 10 enters and leaves the conduit 20. However, these resilient seals have been omitted from Fig. 4A for improved clarity.

[0078] In this example, the HTF is not chemically reactive with the material from which the interface layer 90 is made, and can therefore be brought into direct contact with it. The material from which the interface layer 90 is made has an electrical resistivity greater than 108Qm and a thermal conductivity above 10 W K1rm1. However, it also has a coefficient of thermal expansion which is similar to that of graphitic carbon, and is therefore deposited directly onto the carbon electrode 10 during its manufacture.

[0079] In this embodiment, the interface layer 90 has the shape of a hollow, right circular cylinder. Suppose, for example, that it has dimensions of height, h = 0.5 m, inner radius, ri = 0.1 m and thickness, w = 1 mm = 0.001 m. The electrical resistance, R, of the interface layer for electrical conduction normal to the cylindrical axis is then given by:

[0080] R = p In (r2 / ri) 1 2nh [Eqn. 1] where p is the resistivity of the material from which the interface layer is made, the subscripts 1 and 2 respectively denote the inner and outer surfaces of the interface layer, and therefore r2= ri + w = 0.101 m. If, for example, the material from which the interface layer is made has a resistivity, p = 1012Qm, then Eqn. 1 implies that R = 3 x 109Q. Thus, if the electrode 10 is at an electrical potential, V = 7 V and the conduit 20 and the HTF it contains are both at ground potential, the leakage current, I, through the interface layer = V / R = 2.2 x 109A. For comparison, the current conducted through the electrolytic cell across the same potential difference may be as high as about 45 kA.

[0081] With the same interface layer geometry, the rate, dQ / dt, of heat transfer by the interface layer for thermal conduction normal to the cylindrical axis is then given by: dQ / dt = 2rrAh (Ti - T2) I In (r2 / ri) [Eqn. 2] where A is the thermal conductivity of the material from which the interface layer is made, and Ti - T2 is the difference in temperature between the inner and outer surfaces of the interface layer, respectively. If, for example, the material from which the interface layer is made has a thermal conductivity, A = 30 W K1nr1and the temperature difference between the inner and outer surfaces of the interface layer T1 - T2 = 10 °C, then Eqn. 2 implies that dQ / dt = 94.6 kW. For comparison, the total power consumption, P = I V of an electrolytic cell operating adiabatically ( / '. e., without any heat transfer) at 7 V d.c. and a current of 40 kA is 280 kW.

[0082] Thus it may be seen from this example that an interface layer only 1 mm thick is sufficient to electrically isolate the HTF from the carbon electrode 10, whilst still achieving a high rate of heat transfer between them. A higher rate of heat transfer may clearly be obtained with a larger temperature difference across the interface layer, or if heat is dissipated within the electrolytic cell at a higher rate, for example as a result of fusing the electrolyte. The above calculations may be carried out in reverse order to derive the thickness of the interface layer from desired values for the leakage current through the interface layer and for the rate of heat transfer, as well as from the resistivity and thermal conductivity of the material from which the interface layer is made. Eqn. 2 above may be integrated with respect to time to obtain the total quantity, Q, of heat transferred. Eqns. 1 and 2 may also be adapted to other interface layer geometries, although the forms of Eqns. 1 and 2 given above are those of a common and simple geometry, which is therefore also one of the most useful.

[0083] Figs. 5A and 5B schematically show a third embodiment of an electrochemical device 1c. The electrochemical device 1 c comprises a carbon electrode 10, a conduit 20 and an interface layer 90 as in the device 1 b of the second embodiment, which are structured and function the same as in the device 1 b, and will therefore not be described again in greater detail. In this case, however, not only is the heat transfer fluid electrically conducting, but the HTF is also chemically reactive with the material from which the interface layer 90 is made, and therefore cannot be brought into direct contact with it. Accordingly, the conduit 20 of the device 1c further comprises a portion 24 between and in thermal contact with both the interface layer 90 and the interior 22 of the conduit 20. The entire conduit 20, including the portion 24 thereof, is made of a thermally conducting material, such as stainless steel, which is resistant to corrosion by the HTF. Thus, during operation, heat is transferred between the HTF and the third part 16 of the electrode 10 across both the portion 24 of the conduit 20 and the interface layer 90, but the interface layer 90 is protected from chemical attack by the portion 24 of the conduit 20. Since the HTF is completely enclosed within the conduit 20 by the portion 24 thereof, there is no need for a resilient seal between the conduit 20 and the interface layer 90, in the manner of those in the devices 1 a, 1 b.

[0084] Figs. 6A to 6D schematically show a fourth embodiment of an electrochemical device 1d. The electrochemical device 1d also comprises a carbon electrode 10, a conduit 20 having a portion 24, and an interface layer 90. The carbon electrode 10 and the conduit 20 are like those in the device 1c of the third embodiment, and are structured and function the same as in the device 1c, and will therefore not be described again in greater detail. However, in this case, the interface layer 90 has a coefficient of thermal expansion which is significantly different from that of the carbon from which the electrode 10 is made, and is therefore structured differently to prevent it from cracking. In this embodiment, the interface layer 90 is separate from the third part 16 of the electrode 10 and from the portion 24 of the conduit 20. It is therefore able to move relative to both the third part 16 of the electrode 10 and the portion 24 of the conduit 20.

[0085] The interface layer 90 forms a hollow tube around the third part 16 of the electrode 10 and comprises an even number of abutting, similarly shaped elongate sections 92, which in this embodiment are eight in number. Each of these elongate sections 92 has a wider end 92w and an opposing narrower end 92n, as may best be seen in Fig. 6D, which shows three of the elongate sections 92 removed from the device 1 d. The wider end 92w of each of the elongate sections 92 is wider than the narrower end 92n of the same elongate section 92 in a circumferential direction of the interface layer 90, in other words, in a direction around the electrode 10. The elongate sections 92 are arranged around the third part 16 of the electrode 10 with their respective wider 92w and narrower 92n ends alternating with each other, in the manner shown in Fig. 6D. Since there are an even number of elongate sections 92, the diameter of the interface layer 90 is constant along its length when the elongate sections 92 are arranged in this manner.

[0086] With the elongate sections 92 thus arranged, together they form a flange 94a, 94b at each of the opposing ends of the interface layer 90. Each of the flanges 94a, 94b overlaps an outer surface 26 of the conduit 20, as may best be seen in the view of Fig. 6A. The separation, s, between the opposing flanges 94a, 94b, which is defined by the separation between the opposing narrower ends 94n of the elongate sections 92, is greater than the thickness, t, of the conduit 20 by an amount, 2u = s - 1, which allows each of the elongate sections 92 to slide independently of each other in the directions of the arrows labelled C and C in Fig. 6A, until they are prevented from sliding any further by one of the flanges 94a, 94b abutting the outer surface 26 of the conduit 20. The electrochemical device 1d further comprises two thermally and electrically insulating washers 18a, 18b around the electrode 10, each abutting a respective one of the flanges 94a, 94b, and a pair of resilient members 35a, 35b around the electrode 10, each abutting a respective one of the washers 18a, 18b. The resilient members 35a, 35b are arranged to apply a compressive force to the wider ends 92w of the elongate sections 92 of the interface layer 90. I n this embodiment, this is achieved by compressing the washers 18a, 18b, the resilient members 35a, 35b and the interface layer 90 between an exterior surface of the electrolytic cell 100 in which the first part 12 of the electrode 10 is inserted and a static buffer 36, with one of the resilient members 35a abutting the exterior of the electrolytic cell 100, and the other resilient member 35b abutting the buffer 36. Since the washers 18a, 18b and the two resilient member 35a, 35b are able to slide freely along the electrode 10, adjacent ones of the elongate sections 92 are induced to slide antiparallel to each other, as indicated by the arrows labelled C and C in Fig. 6D. This pushes the elongate sections 92 together and keeps them in contact with each other.

[0087] During operation, thermal expansion of the third part 16 of the electrode 10 would tend to push the elongate sections 92 apart by increasing the internal diameter, d, of the central bore 96 through the interface layer 90, visible in Figs. 6B and 6C. However, rather than separating from each other, since the elongate sections 92 are kept in contact by the compressive force applied to their wider ends 92w by the resilient members 35a, 35b, the elongate sections 92 are instead caused to slide antiparallel to each other in the same directions as this compressive force is applied to them. Referring to Figs. 6B and 6C, this movement of the elongate sections 92 increases both the internal diameter, d, and the external diameter, e, of the interface layer 90, whilst the thickness, w = e - d, of the interface layer 90 remains the same and the separation, s, between the two flanges 94a, 94b decreases. Thus a widthwise expansion of the third part 16 of the electrode 10 is converted into a longitudinal motion of the elongate sections 92.

[0088] The different possible materials from which the interface layer 90 may be made are all stronger in compression than when subjected to a tensile stress. Thus, whereas thermal expansion of the third part 16 of the electrode 10 tends to compress the elongate sections 92 between the expanded electrode 10 and the portion 24 of the conduit 20, this does not cause them to fracture. On the other hand, since there are at least four elongate sections 92, none of them is subjected to a tensile stress which is great enough to fracture them. This is in contrast to if the interface layer 90 were a single component, in which case, diametric expansion of the interface layer 90 would result in a corresponding tensile stress in a circumferential direction. Meanwhile, in the electrochemical device 1 d, diametric expansion of the interface layer 90 is passed on to the conduit 20, where it can be absorbed by a thermal expansion joint of a known type, such as a bellows-type thermal expansion joint. In all cases, however, total movement of the components of the device 1d is only of the order of 1 mm over a range of temperatures of several hundred “Celsius, but is sufficient to avoid cracking.

[0089] Figs. 7A and 7B schematically show a fifth embodiment of an electrochemical device 1e. The electrochemical device 1e comprises a carbon electrode 10, a conduit 20 and an electrically and thermally conducting sheath 85. In this case, the electrode 10 has a different geometry from that shown in Figs. 3 to 6, and instead has the shape of a hollow tube or cylinder. Nonetheless, the electrode 10 still has a first part 12 for immersion in an electrolyte 40, a second part 14 for making electrical contact with a current-carrying conductor 30c, and located between the first and second parts 12, 14, a third part 16 for making thermal contact with an HTF flowing through conduit 20. The first part 12 of the electrode 10 is enclosed within the sheath 85, so that when the first part 12 is immersed in an electrolyte, the electrode 10 does not come into direct contact with the electrolyte, but is instead protected from it by the sheath 85, which covers the interior, exterior and end surfaces of the first part 12 of the hollow cylinder of electrode 10, in the manner shown in Fig. 7B. However, since the sheath 85 is in contact with the entire surface of the first part 12 of the electrode 10 and is made of a material which is both electrically and thermally conducting, both heat and electrical current can still flow between the electrode 10 and the electrolyte 40. Moreover, since the thickness, w, of the sheath 85 is only about 1 mm, its contributions to the internal electrical resistance and volumetric heat capacity of the device 1e are negligible.

[0090] In this embodiment, the sheath 85 is made of a grade of nickel-containing alloy resistant to corrosion by sodium hydroxide, which, when combined with the shape of the electrode 10, makes the electrochemical device 1e suitable for use as an anode in a Castner cell. The sheath 85 has a linear coefficient of thermal expansion of about 6 x 106K’1, ensuring that as the carbon electrode 10 changes temperature, the sheath 85 expands and contracts with it at the same rate, thereby avoiding stressing either the electrode 10 or the sheath 85.

[0091] The third part 16 of the electrode 10 comprises two portholes or windows 15a, 15b arranged as shown in Fig. 7A. These allow the HTF in conduit 20 to flow not just around the exterior of electrode 10, but also through the interior of the hollow cylinder of electrode 10, thereby ensuring rapid heat transfer between the electrode 10 and the HTF. The device 1e also comprises a pair of resilient seals similar to those in the devices 1 a, 1b of Figs. 3, 4A and 4B, to inhibit leakage of the HTF from within the interior 22 of the conduit 20 at locations where the electrode 10 enters and leaves the conduit 20. However, these resilient seals have been omitted from Fig. 7A for improved clarity.

[0092] Fig. 8 schematically shows a first embodiment of an apparatus 300a according to the invention, comprising an electrolytic cell 100 and a reaction vessel 200. The electrolytic cell 100 includes an electrochemical device such as the devices 1 a, 1 b, 1c, 1d, 1e described above in relation to Figs. 3 to 7B. In the present embodiment, the carbon electrode 10 of the electrochemical device is configured as an anode of the cell 100 and the anode 10 surrounds a cathode 70 of the cell 100 as in a Castner cell. The electrolytic cell 100 produces at least two electrolysis products 50, 60, such as a liquid metal 50 at the cathode 70 and a gas 60 at the anode 10. The reaction vessel 200 contains an exothermic chemical reaction which consumes at least some of one of these electrolysis products 50, 60 as a reagent. For example, the exothermic reaction may consume the liquid metal 50 as a reducing agent in a redox reaction or the gas 60 as an oxidant. The electrolytic cell 100 is connected to the reaction vessel 200 via a product transfer pathway 110, which transfers a respective one of the electrolysis products 50, 60 from the cell 100 to the reaction vessel 200. The apparatus 300a also comprises a heat exchanger 210, which transfers a quantity of heat, Q, from the exothermic reaction in the reaction vessel 200 to the heat transfer fluid in the conduit 20. With the apparatus 300a, heat from the exothermic reaction can therefore be used to reduce the amount of electrical energy consumed by the electrolytic cell 100.

[0093] For example, suppose that the electrolytic cell 100 operates at a temperature of about 330 “Celsius to produce liquid sodium by fusing and electrolysing solid sodium hydroxide. The exothermic reaction in the reaction vessel 200 may, for example, be a redox reaction which reacts the liquid sodium from the cell 100 with one or more iron oxides to produce elemental iron and sodium oxide. Alternatively or additionally, the exothermic reaction may comprise one or more downstream processes, such as hydrating at least some of the sodium oxide thus produced to produce sodium hydroxide, and / or reacting at least some of this sodium hydroxide with carbon dioxide to produce sodium carbonate. The quantity of heat, Q, extracted from the exothermic reaction in the reaction vessel 200 via the heat exchanger 210 is carried away by the HTF in the conduit 20. The exothermic reaction may be maintained at a desired equilibrium temperature by adjusting a rate of flow of the reagents into the reaction vessel 200, a rate of flow of the products from the exothermic reaction out of the reaction vessel 200 and / or a rate of flow of the HTF through the heat exchanger 210. For example, if the exothermic reaction is the redox reaction mentioned above, the equilibrium temperature may be maintained at about 400 “Celsius. When the HTF arrives at the carbon electrode 10 at a similar temperature, it therefore heats up the electrode 10. By referring back to Fig. 2, it can be seen that this simultaneously increases the electrical conductivity of the electrode 10 and reduces its thermal conductivity. The former reduces the total internal resistance of the cell 100 and the latter reduces the rate of heat lost via the electrode 10 from within the cell 100 to a current carrying conductor to which the second part 14 of the carbon electrode 10 is connected. The increased temperature of the electrode 10 also injects heat into the cell 100, by creating a temperature difference between the third part 16 of the electrode 10, which is in thermal contact with the HTF in the conduit 20, and the interior of the electrolytic cell 100. In comparison to a conventional Castner cell, therefore, the electrolytic cell 100 may be operated without the need to apply another source of external heating to the cell 100, in order to melt the sodium hydroxide. Nonetheless, the interior of the cell 100 may be maintained at a lower temperature than the equilibrium temperature of the exothermic reaction in the reaction vessel 200 because fusing and electrolysing sodium hydroxide is an endothermic process.

[0094] Whereas in Fig. 8, the conduit 20 is shown open at both ends, the HTF may instead circulate through the conduit 20 in a closed loop by joining these two ends together. The apparatus 300a may further comprise a pump for pumping the HTF through the conduit 20. Since some liquid metals are effective as heat transfer fluids, the HTF may be a liquid metal such as liquid sodium. If so, the apparatus 300a may comprise an electromagnetic pump for pumping the liquid metal around the conduit 20. Moreover, the liquid metal produced at the cathode 70 of the cell 100 may itself be used as the HTF in the conduit 20. In other words, if an amount of liquid metal is produced in the cell 100 which is in excess of a stoichiometric amount thereof required for the exothermic reaction in the reaction vessel 200, the excess amount of liquid metal may subsequently be removed from the reaction vessel 200 via the conduit 20, taking away with it the heat, Q, from the reaction. If the conduit 20 is a closed loop, this excess amount of liquid metal may therefore circulate in the conduit 20, with only the stoichiometric amount thereof which is consumed in the exothermic reaction being replaced by more liquid metal newly produced at the cathode 70 of the electrolytic cell 100.

[0095] The liquid metal removed from the reaction vessel 200 via the heat exchanger 210 may be cleaned before it enters the conduit 20 by separating it from other products of the exothermic reaction using at least one of several different possible separation techniques. For example, if the liquid metal is sodium, one or more solid-phase products of the exothermic reaction may be separated from the liquid sodium using at least one of settlement under gravity, filtration ( / '. e., trapping) and centrifugation. For example, the present applicant's co-pending UK patent application no. 2417052.4 ("Apparatus and Method for Separating a Contaminant from Liquid Metal”; applicant's ref: NE-P- GB 002), the entire contents of which is 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. Fig. 9 schematically shows an apparatus 300b which although not part of the claimed invention, is helpful to its understanding. The apparatus 300b comprises an electrolytic cell 100 and a thermally insulated storage tank 150, wherein the electrolytic cell 100 includes an electrochemical device, such as the devices 1 b, 1c, 1 d, 1e described above in relation to Figs. 3 to 7B. In this example, the carbon electrode 10 of the device is again configured as an anode of the cell 100 but the anode 10 is surrounded by a cathode 70 of the cell 100 as in a Downs cell. The electrolytic cell 100 produces at least two electrolysis products 50, 60, such as a liquid metal 50 at the cathode 70 and a gas 60 at the anode 10. The electrolytic cell 100 is connected to the storage tank 150 by a supply route 120, which conveys the liquid metal 50 produced at the cathode 70 from the electrolytic cell 100 to the storage tank 150, where the liquid metal 50 therefore accumulates. The conduit 20 of the electrochemical device circulates the liquid metal 50 as a heat transfer fluid from the storage tank 150 to the carbon electrode 10 and back to the storage tank 150 again in a closed loop. This therefore thermally couples the interior of the electrolytic cell 100 via the carbon electrode 10 with the bulk quantity of liquid metal 50 accumulated in the storage tank 150. This simultaneously helps to maintain the temperature of the liquid metal 50 in the storage tank 150 closer to the temperature it had when it was initially produced, and also reduces the outflow of heat from the cell 100 via the carbon electrode 10. The apparatus 300b is therefore more energy efficient than if the electrolytic cell 100 and the storage tank 150 were thermally isolated from each other, apart from being connected by only a slow trickle of newly produced liquid metal 50 from the cell 100 via the supply route 120 to the storage tank 150. Additionally, the apparatus 300b exhibits the same benefits also derived from heating the carbon electrode 10, including those of increasing the electrical conductivity and reducing the thermal conductivity of the electrode 10.

[0096] Whereas in the apparatus of Fig. 9, the conduit 20 circulates the liquid metal 50 around the carbon electrode 10 in a closed loop from and then back to the storage tank 150, in an alternative possible arrangement, the third part 16 of the carbon electrode 10 could be inside the storage tank 150 itself, in which case, the storage tank 150 would be functionally equivalent to the conduit 20. In other words, the apparatus shown in Fig. 9 is one of several different possible geometries whereby the interior of an electrolytic cell 100 may be thermally coupled via a carbon electrode 10 to a bulk quantity of liquid metal 50 produced by the cell 100 and which is accumulated in a storage tank 150.

[0097] Fig. 10 schematically shows a second embodiment of an apparatus 300c, which combines the features of the apparatuses 300a of Fig. 8 and 300b of Fig. 9. Thus the apparatus 300c comprises an electrolytic cell 100, a thermally insulated storage tank 150 and a reaction vessel 200. As before, the electrolytic cell 100 includes an electrochemical device such as those described above in relation to Figs. 3 to 7B, and produces at least two electrolysis products 50, 60, such as a liquid metal 50 at the cathode 70 and a gas 60 at the anode 10. Also as before, the storage tank 150 accumulates the liquid metal 50 produced in the cell 100 and the reaction vessel 200 contains an exothermic chemical reaction which consumes at least some of one of these electrolysis products 50, 60, in particular the liquid metal 50, as a reagent.

[0098] As described above in relation to Fig. 9, the electrolytic cell 100 and the storage tank 150 are connected to each other by a supply route 120, which conveys the liquid metal 50 produced in the cell 100 to the storage tank 150. The liquid metal 50 accumulated in the storage tank 150 circulates in a loop through the conduit 20 as a heat transfer fluid from the storage tank 150 to the carbon electrode 10 of the cell 100 and back to the storage tank 150 again. The storage tank 150 is also connected to the reaction vessel 200 via a product transfer pathway 110 and a heat exchanger 210. The product transfer pathway 110 transfers liquid metal 50 produced in the cell 100 from the storage tank 150 to the reaction vessel 200, where some of it is consumed in the exothermic chemical reaction. The heat exchanger 210 conveys an excess amount of liquid metal 50 not consumed by the exothermic reaction, together with a quantity of heat, Q, generated by the reaction, back in a loop to the storage tank 150 again, from where the heat, Q, can then enter the HTF circulating in the conduit 20. Thus, as in the embodiment of Fig. 8, liquid metal 50 produced by the cell 100 is transferred from the cell 100 to the reaction vessel 200, and a quantity of heat, Q, is transferred from the reaction vessel 200 back to the cell 100, although in contrast to the embodiment shown in Fig. 8, both transfers now occur via the storage tank 150. As described above, the quantity of heat, Q, transferred from the reaction vessel 200 back to the cell 100, and therefore the temperature of the exothermic reaction, can be controlled by adjusting the rates of flow of the liquid metal 50 into and / or out of the reaction vessel 200, via the product transfer pathway 110 and the heat exchanger 210, respectively.

[0099] The apparatus 300c of Fig. 10 has the advantage that it may be used to accommodate a situation in which the rate of production of the electrolysis products 50, 60 by the cell 100 is different from the rate of consumption of at least some of one of these products as a reagent by the exothermic chemical reaction in the reaction vessel 200. Thus, for example, suppose that the electrolytic cell 100 takes 1 hour to produce a stoichiometric amount of liquid metal 50 required for the exothermic reaction, but that the reaction goes to completion in only 10 minutes. Therefore, with the apparatus 300c of Fig. 10, liquid metal 50 produced by the cell 100 may be allowed to accumulate in the storage tank 150 continuously, but the product transfer pathway 110 may comprise a valve V1 as shown in Fig. 10, which is only opened for a ten-minute period each hour to allow liquid metal 50 from the storage tank 150 to circulate through the reaction vessel 200, but which remains closed otherwise. Thus when the valve V1 is open, some of the liquid metal 50 is consumed by the reaction, thereby depleting the level of liquid metal 50 in the storage tank 150, whilst also reheating the unconsumed liquid metal 50 remaining in the storage tank 150 with the quantity of heat, Q, extracted from the reaction via the heat exchanger 210. Therefore, whereas liquid metal 50 may circulate continuously through the conduit 20, it may only circulate via the product transfer pathway 110 into the reaction vessel 200 and back via the heat exchanger 210 into the storage tank 150 again for a ten-minute period each hour. In such a case, the exothermic chemical reaction may be operated as a batch-wise process.

[0100] Fig. 11 schematically shows a third embodiment of an apparatus 300d. The apparatus 300d is similar to the apparatus 300a of Fig. 8, in that it also comprises an electrolytic cell 100 producing at least two electrolysis products 50, 60, and a reaction vessel 200 containing an exothermic chemical reaction, which consumes at least some of one of these electrolysis products 50, 60 as a reagent. As before, the electrolytic cell 100 comprises an electrochemical device as described elsewhere herein. The cell 100 and the reaction vessel 200 are therefore arranged and function as described previously in relation to Fig. 8, although in Fig. 11 , the electrolytic cell 100 is represented by way of example as a Downs cell, rather than as a Castner cell. Moreover, in the present embodiment, the apparatus 300d further comprises a container 250 for an endothermic process, which extracts a first quantity of heat, Qi, via a second heat exchanger 260 from the electrolysis product 50, 60 flowing through the product transfer pathway 110. Thus the electrolysis product 50, 60 arrives at the reaction vessel 200 at a temperature which is significantly lower than that at which it left the electrolytic cell 100.

[0101] For example, suppose that the electrolytic cell 100 operates at a temperature of about 600 “Celsius to produce liquid sodium by fusing and electrolysing solid sodium chloride, and that the reaction vessel 200 contains one or more iron oxides which are reduced by the liquid sodium from the Downs cell 100 in a redox reaction to produce elemental iron and sodium oxide. This reaction is exothermic. However, it is also desirable that a temperature of the reaction should be kept below about 450 “Celsius to inhibit the formation of ternary oxides. The reaction can therefore be prevented from overheating by reducing the temperature of the liquid sodium leaving the Downs cell 100, for example to about 200 “Celsius, before it enters the reaction vessel 200 by transferring the first quantity of heat, Qi, to the endothermic process in the container 250 via the second heat exchanger 260. The endothermic process may comprise one or more of several different things. For example, the endothermic process may comprise using the first quantity of heat, Qi, to help thermally decompose at least one of calcium hydroxide, magnesium hydroxide and an iron hydroxide to produce their respective oxide(s) and water vapour.

[0102] Nonetheless, a second quantity of heat, Q2, may still be extracted from the exothermic reaction inside the reaction vessel 200 via the first heat exchanger 210 and carried away by the HTF in the conduit 20. The exothermic reaction may therefore be maintained at an equilibrium temperature of, for example, about 400 “Celsius by adjusting the rate of flow of the reagents into the reaction vessel 200, the rate of flow of the products from the exothermic reaction out of the reaction vessel 200 and / or the rate of flow of the HTF through the first heat exchanger 210. The second quantity of heat, Q2, may then be used to reduce the rate of heat outflow from the electrolytic cell 100 via the carbon electrode 10, in the manner described above. The second quantity of heat, Q2, may be either the same as or different from the first quantity of heat, Qi. Whereas in Fig. 11 , the conduit 20 is shown open at both ends, the HTF may instead circulate through the conduit 20 in a closed loop by joining these two ends together.

[0103] Fig. 12 schematically shows a fourth embodiment of an apparatus 300e, which combines the features of the apparatuses 300c of Fig. 10 and 300d of Fig. 11. Thus the apparatus 300e comprises an electrolytic cell 100, a thermally insulated storage tank 150, a reaction vessel 200 and a container 250 for an endothermic process. As before, the electrolytic cell 100 comprises an electrochemical device as described elsewhere herein. As described above in relation to Fig. 10, the storage tank 150 stores liquid metal 50 produced by the cell 100 before at least some of the liquid metal 50 from the storage tank 150 is consumed by the exothermic reaction in the reaction vessel 200. A valve, V1 , regulates the supply of liquid metal 50 from the storage tank 150 to the reaction vessel 200 as previously described. As also described above in relation to Fig. 11 , the endothermic process in the container 250 extracts a first quantity of heat, Qi, via a second heat exchanger 260 from the liquid metal 50 flowing through the product transfer pathway 110 before the liquid metal 50 reaches the reaction vessel 200. Thus the liquid metal 50 arrives at the reaction vessel 200 at a temperature which is significantly lower than that at which it left the storage tank 150. Nonetheless, the liquid metal 50 is heated by the exothermic chemical reaction in the reaction vessel 200 and an excess amount of liquid metal 50 not consumed by the reaction carries away with it a second quantity of heat, Q2, via the first heat exchanger 210 back to the storage tank 150. Here, the second quantity of heat, Q2, can be returned to the cell 100 via the carbon electrode 10 by circulating in the conduit 20 as a heat transfer fluid. The second quantity of heat, Q2, may be either the same as or different from the first quantity of heat, Qi.

[0104] Figs. 8 and 10 to 12 show various different embodiments of an apparatus comprising an electrolytic cell, wherein the electrolytic cell comprises an electrochemical device as described herein, such that the carbon electrode of the device is configured as an electrode of the electrolytic cell. These embodiments respectively have some features in common and some which are different. However, these features may be combined in other possible ways than those described above to provide other apparatuses which still remain within the scope of the appended claims.

[0105] Fig. 13A and 13B shows a first embodiment of a method 400a of operating an apparatus according to the invention when the electrolytic cell thereof contains an electrolyte. The method 400a comprises immersing 401 the first part 12 of the carbon electrode 10 of the electrochemical device in the electrolyte, supplying 402 the electrolyte with electricity via the second part 14 of the carbon electrode 10, and supplying 403 the electrolyte with heat via the third part 16 of the same electrode in the manner described above. The method 400a also comprises supplying

[0106] 405 an electrolysis product from the electrolytic cell to an exothermic chemical reaction, and at least some of the heat supplied 403 to the electrolyte via the third part 16 of the carbon electrode 10 is obtained 406 from the exothermic chemical reaction. In this way, at least some of the heat generated by the exothermic reaction is recycled to increase the energy efficiency of the electrolysis, rather than being lost to the environment. The heat may be obtained 406 from the exothermic chemical reaction by means of an HTF which is brought into thermal but not physical contact with the exothermic reaction, for example, by being circulated in a pipe which passes through or around a reaction vessel in which the exothermic reaction takes place. Preferably, however, as shown in Fig. 13B, at least one of the reagents and / or products of the reaction may be used 407 as the HTF. In other words, if one or more of the products of the reaction or one or more of the reagents which are present in excess of a stoichiometric amount thereof required for the reaction and which therefore remain unconsumed by the reaction, are removed from the reaction as it proceeds or when it has gone to completion, these reagents and / or products will carry away with them at least some of the heat generated by the reaction. This has the advantage of avoiding heat transfer from the reagents and / or products of the reaction to another substance at less than 100% efficiency.

[0107] Fig. 14 shows a second embodiment of a method 400b of operating such an apparatus. Like the method 400a of Figs. 13A and 13B, the method 400b also comprises immersing 401 the first part 12 of the carbon electrode 10 of the electrochemical device in the electrolyte, supplying 402 the electrolyte with electricity via the second part 14 of the carbon electrode 10, supplying 403 the electrolyte with heat via the third part 16 of the same electrode, supplying 405 an electrolysis product from the electrolytic cell to an exothermic chemical reaction, and obtaining

[0108] 406 from the exothermic chemical reaction at least some of the heat supplied 403 to the electrolyte via the third part 16 of the carbon electrode 10. In the present embodiment, the method 400b further comprises controlling 404 a temperature within the electrolytic cell by altering a quantity of the heat supplied 403 to the electrolyte via the third part 16 of the electrode 10. The temperature within the cell may be controlled 404 by varying at least one of the rate of flow and the temperature of the HTF in the conduit 20 of the electrochemical device. For example, the temperature within the cell may be controlled 404 in this manner to lie within a range of from about 300 to about 1200 “Celsius. In this range, as may be seen from Fig. 2, both the thermal conductivity and the electrical resistivity of the carbon electrode 10 are less than their corresponding values at ambient temperature, giving the energy savings described above for electrolysis performed by the cell. In another example, the temperature within the cell may be rapidly cooled using the same technique, without any need to alter the supply of electrical current to the cell, for example in order to combat an "anode effect”.

[0109] Fig. 15 shows a third embodiment of a method 400c of operating an apparatus according to the invention when the electrolytic cell thereof contains an electrolyte. The method 400c is similar to the method 400a of Figs. 13A and 13B, but further comprises extracting 408 heat from the electrolysis product before it is supplied to the exothermic chemical reaction. If the temperature of the electrolysis product were greater than a desired temperature range for the exothermic reaction, the exothermic reaction would risk overheating when the electrolysis product was supplied to the reaction. Extracting 408 heat from the electrolysis product before it is supplied to the exothermic chemical reaction, for example to reduce its temperature from above the desired temperature range for the exothermic reaction, can therefore help to prevent this from happening.

[0110] The exothermic chemical reaction may comprise reducing a metal oxide with the electrolysis product. For example, if the electrolysis product is liquid sodium, the exothermic chemical reaction may comprise reducing one or more iron oxides in a redox reaction to produce metallic iron and sodium oxide, as described in the present applicant's co-pending UK patent application no. 2417059.9 ("Carbon-Free Method and Apparatus for Producing Iron and Steel”; applicant's ref: NE-P-GB 001), the entire contents of which is incorporated herein by reference. In another example, if the electrolysis product is liquid sodium, the exothermic chemical reaction may comprise reducing a trivalent manganese oxide in a redox reaction to produce elemental manganese and sodium oxide, as described in the present applicant's co-pending UK patent application no. 2417063.1 ("Carbon-Free Method and Apparatus for Producing Manganese”; applicant's ref: NE-P-GB 008), the entire contents of which is also incorporated herein by reference. In other examples, the exothermic chemical reaction may comprise one or more downstream processes, such as hydrating at least some of the sodium oxide thus produced to produce sodium hydroxide, and / or reacting at least some of this sodium hydroxide with carbon dioxide to produce sodium carbonate.

[0111] Fig. 16 shows a fourth embodiment of a method 400d of operating an apparatus according to the invention when the electrolytic cell thereof contains an electrolyte. According to this embodiment, when the first part 12 of the carbon electrode 10 of the device is immersed in the electrolyte and the electrolyte is supplied 402 with electricity via the second part 14 of the carbon electrode 10 and is also supplied 403 with heat via the third part 16 of the same electrode, the electrolytic cell produces 409a a liquid metal, such as liquid sodium, as an electrolysis product. The liquid metal is then used 409b as an HTF to supply 403 the electrolyte with heat via the third part 16 of the carbon electrode 10. If the liquid metal thus produced 409a is also supplied 405 as a reagent to an exothermic chemical reaction, it may also be used to obtain 406 heat from the chemical reaction by carrying away with it at least some of the heat generated by the reaction, in the manner described above in relation to Fig. 13B. Fig. 17 shows a fifth embodiment of a method 400e of operating an apparatus according to the invention when the electrolytic cell thereof contains an electrolyte. In this case, electricity supplied 402 to the electrolyte via the second part 14 of the carbon electrode 10 of the device and heat supplied 403 to the electrolyte via the third part 16 of the same electrode are used to fuse and electrolyse 410 in the electrolytic cell a solid-phase halide, oxide or hydroxide of at least one of an alkali metal and an alkaline earth metal, to produce the respective metal as one of the products of the electrolysis. As may be seen from Fig. 2, the method of the invention is particularly suited to producing alkali and alkaline earth (Group 1 and Group 2) metals by electrolysis because electrolysis of several of their compounds, and in particular of their chlorides, occurs in a temperature range over which both the thermal conductivity and the electrical resistivity of the carbon electrode 10 are significantly less than their corresponding values at ambient temperature. Whereas Figs. 13 to 17 show various different embodiments of a method of operating an electrolytic cell respectively having some features in common and some which are different, these features may be combined in other possible ways than those described above, whilst still remaining within the scope of the appended claims.

Claims

Claims1 . An apparatus (300a, 300c, 300d, 300e) comprising: an electrolytic cell (100) for producing an electrolysis product (50, 60), wherein the electrolytic cell (100) comprises an electrochemical device (1 a - 1e), itself comprising: a carbon electrode (10) configured as an electrode (10, 70) of the electrolytic cell (100) and having a first part (12) for immersion in an electrolyte, a second part (14) for making electrical contact with a current-carrying conductor (30c), and a third part (16) between the first and second parts (12, 14) for making thermal contact with a heat transfer fluid (HTF); and a conduit (20) arranged to bring the heat transfer fluid (HTF) into thermal contact with the third part (16) of the electrode (10); wherein the apparatus (300a, 300c, 300d, 300e) further comprises: a reaction vessel (200) for consuming at least some of the electrolysis product (50, 60) as a reagent in an exothermic chemical reaction; a product transfer pathway (110) arranged to transfer the electrolysis product (50, 60) from the electrolytic cell (100) to the reaction vessel (200); and a first heat exchanger (210) arranged to transfer heat (Q) from the reaction vessel (200) to the heat transfer fluid (HTF) in the conduit (20).

2. An apparatus (300c, 300e) according to claim 1 , wherein the heat transfer fluid (HTF) comprises at least one of a reagent and a product of the exothermic reaction.

3. An apparatus (300a, 300c, 300d, 300e) according to claim 1 or claim 2, wherein the electrochemical device (1 b, 1c, 1d) further comprises an interface layer (90) made of a thermally conducting and electrically insulating material between and in thermal contact with both the third part (16) of the electrode (10) and an interior (22) of the conduit (20).

4. An apparatus (300a, 300c, 300d, 300e) according to claim 3, wherein the material of the interface layer (90) has an electrical resistivity of at least 108Qm and a thermal conductivity of at least 10 W K1rm1.

5. An apparatus (300a, 300c, 300d, 300e) according to claim 3 or claim 4, wherein the conduit (20) comprises a portion (24) between and in thermal contact with both the interface layer (90) and the interior (22) of the conduit (20), made of a thermally conducting material which is resistant to corrosion by molten halides and / or liquid metals.

6. An apparatus (300a, 300c, 300d, 300e) according to claim 5, wherein: the interface layer (90) is separate from the third part (16) of the electrode (10) and from the portion (24) of the conduit (20); the interface layer (90) forms a hollow tube around the third part (16) of the electrode (10) and comprises an even number of at least four similarly shaped elongate sections (92) abutting each other; each of the elongate sections (92) has a wider end (92w) and an opposing narrower end (92n), wherein the wider end (92w) of each respective one of the elongate sections (92) is wider in a circumferential direction of the interface layer (90) than the narrower end (92n) of the same elongate section (92); the elongate sections (92) are arranged around the third part (16) of the electrode (10) with their respective wider (92w) and narrower (92n) ends alternating with each other; the interface layer (90) comprises a flange (94a, 94b) at each of its opposing ends, overlapping an outer surface (26) of the conduit (20); the separation (s) between the opposing flanges (94a, 94b) is greater than the thickness (t) of the conduit (20) by an amount (2u) allowing each of the elongate sections (92) to slide independently of each other parallel to the electrode (10); and the device (1 d) further comprises: two thermally and electrically insulating washers (18a, 18b) around the electrode (10), each abutting a respective one of the flanges (94a, 94b); and at least one resilient member (35a, 35b) around the electrode (10), abutting a respective one of the washers (18a, 18b), and arranged to apply a compressive force to the wider ends (92w) of the elongate sections (92) of the interface layer (90); wherein the washers (18a, 18b) and the at least one resilient member (35a, 35b) are able to slide freely along the electrode (10); whereby adjacent ones of the elongate sections (92) are induced to slide antiparallel to each other.

7. An apparatus (300a, 300c, 300d, 300e) according to any one of the preceding claims, wherein the electrochemical device (1 e) further comprises an electrically and thermally conducting sheath (85) around and in contact with the first part (12) of the electrode (10), wherein the sheath is made of a material which is resistant to corrosion by sodium hydroxide and has a linear coefficient of thermal expansion of from 3 x 106to 10 x 106K1.

8. An apparatus (300a, 300c, 300d, 300e) according to any one of claims 3 to 6, wherein the electrolytic cell (100) is adapted to produce liquid metal (50) as an electrolysis product, and the conduit (20) is arranged to bring at least some of the liquid metal (50) into thermal contact with the third part (16) of the electrode (10) as the heat transfer fluid (HTF).

9. An apparatus (300c, 300e) according to claim 8, further comprising: a thermally insulated storage tank (150) for storing the liquid metal (50); anda supply route (120) arranged to transfer the liquid metal (50) from the electrolytic cell (100) to the storage tank (150); wherein the storage tank (150) is arranged to provide the conduit (20) with the liquid metal (50).

10. An apparatus (300c, 300e) according to claim 9, wherein: the reaction vessel (200) is adapted for consuming at least some of the liquid metal (50) from the storage tank (150) as a reagent in the exothermic chemical reaction; the product transfer pathway (110) is arranged to transfer the liquid metal (50) from the storage tank (150) to the reaction vessel (200); and the first heat exchanger (210) is arranged to transfer at least some of the liquid metal (50) in excess of a stoichiometric amount thereof required for the exothermic chemical reaction and heated thereby from the reaction vessel (200) to the storage tank (150).

11. An apparatus (300c, 300e) according to claim 10, wherein the product transfer pathway (110) further comprises a valve (V1) to regulate a flow of the liquid metal (50) from the storage tank (150) to the reaction vessel (200).

12. An apparatus (300d, 300e) according to any one of the preceding claims, further comprising: a container (250) for an endothermic process; and a second heat exchanger (260) arranged to transfer heat (Q) to the endothermic process from the electrolysis product (50, 60) in the product transfer pathway (110).

13. A method (400a, 400b, 400c) of operating an apparatus (300a, 300c, 300d, 300e) according to any one of the preceding claims when the electrolytic cell (100) thereof contains an electrolyte (40), the method comprising: immersing (401) the first part (12) of the carbon electrode (10) in the electrolyte (40); supplying (402) the electrolyte (40) with electricity via the second part (14) of the carbon electrode (10); supplying (403) the electrolyte (40) with heat via the third part (16) of the carbon electrode (10); supplying (405) an electrolysis product (50, 60) from the electrolytic cell (100) to an exothermic chemical reaction; and obtaining (406) from the exothermic chemical reaction at least some of the heat supplied (403) to the electrolyte (40) via the third part (16) of the carbon electrode (10).

14. A method (400a) according to claim 13, comprising using (407) at least one of a reagent and a product of the exothermic reaction as a heat transfer fluid (HTF) to transfer heat from the exothermic chemical reaction to the electrolytic cell (10).

15. A method (400b) according to claim 13 or claim 14, further comprising controlling (404) a temperature within the electrolytic cell (100) by altering a quantity of the heat supplied (403) to the electrolyte (40) via the third part (16) of the carbon electrode (10).

16. A method (400b) according to claim 15, wherein the temperature within the electrolytic cell (100) is thus controlled (404) to lie within a range of from 300 to 1200 “Celsius.

17. A method (400c) according to any one of claims 13 to 16, further comprising extracting (408) heat from the electrolysis product (50, 60) before supplying (405) the electrolysis product (50, 60) to the exothermic chemical reaction.

18. A method (400b, 400c) according to any one of claims 13 to 17, wherein the exothermic chemical reaction comprises reducing a metal oxide with the electrolysis product (50, 60).

19. A method (400d) according to any one of claims 13 to 18, comprising using (409b) liquid metal (50) produced (409a) by the electrolytic cell (100) as a heat transfer fluid (HTF) to supply (403) the electrolyte (40) with heat via the third part (16) of the carbon electrode (10).

20. A method (400e) according to any one of claims 13 to 19, comprising fusing and electrolysing (410) a solid-phase halide, oxide or hydroxide of at least one of an alkali metal and an alkaline earth metal in the electrolytic cell (100) to produce the respective metal as the electrolysis product (50, 60).

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