Electrolytic cells and their use

Abstract

The present invention provides an electrolytic cell suitable for electrolysing certain inorganic compounds of alkali and alkaline earth (Group 1 and Group 2) metals in their molten state. The cell (100) comprises a vessel (102) for containing a molten electrolyte, an anode (10) and a cathode (70) both inside the vessel (102), and a source (20) of a magnetic field having a gradient with a positive component in a direction (– x) from the anode (10) to the cathode (70), such that the strength of the magnetic field increases from the anode (10) to the cathode (70). The rest of the electrolytic cell (100), apart from the source of the magnetic field, including at least both the vessel (102) and the anode (10), consists of material having a magnetic susceptibility with an absolute value of less than 10-2. Thus the rest of the electrolytic cell, which is made of diamagnetic and / or paramagnetic material(s), does not affect the magnetic field between the anode (10) and the cathode (70) to any appreciable extent in comparison to the same magnetic field in free space. The source of the magnetic field between the anode (10) and the cathode (70) may either be a permanent magnet (20) located outside the vessel (102) adjacent to the cathode (70), or it may be provided by permanently magnetizing the cathode (70) itself. In the latter case, the cathode comprises an electrically conducting "hard" ferromagnetic material with a high Curie temperature. In the former case, the cathode instead consists of an electrically conducting material also having a magnetic susceptibility with an absolute value of less than 10-2, so as not to affect the magnetic field from the permanent magnet (20) located outside the vessel (102). In either case, if such a cell (100) is used to electrolyse a halide of an alkali metal, a halide of an alkaline earth metal except beryllium or a hydroxide of an alkali metal in their molten state, the magnetic field between the anode (10) and the cathode (70) inhibits a back-reaction between the electrolysis products formed at the anode (10) and cathode (70), thereby improving the energy efficiency of the cell. A plurality of such cells (100) may also be arranged with just one permanent magnet (20) as the source of the magnetic field between the anode (10) and cathode (70) inside the respective vessels (102) of two adjacent electrolytic cells (100), thereby halving the total number of magnetized components which are used.

Description

[0001] Electrolytic Cells and their Use

[0002] Field of the Invention

[0003] The present invention concerns electrolytic cells and the use of electrolytic cells to electrolyse certain inorganic compounds of alkali and alkaline earth (Group 1 and Group 2) metals in their molten state.

[0004] Background of the Invention

[0005] Several alkali and alkaline earth metals are produced on an industrial scale by electrolysing a chloride salt of the metal in question. 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. Subsequent 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 liquid 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 in 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 a hollow cylindrical cathode 70, which is open at both ends, are supplied with electrical current via current-carrying conductors 30a, 30b 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 liquid metal 50, which is produced at the cathode 70, and chlorine gas 60, which is produced at the anode 10. The liquid 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 liquid 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. In another example, electrolysis of 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).

[0007] Alkali and alkaline earth metals may also be produced by fusing and electrolysing different compounds of these metals than just their halides. For 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. If electrolysing pure 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. However, the electrolysis temperature may also be reduced significantly by mixing the sodium hydroxide in a eutectic mixture with potassium hydroxide, which has a eutectic temperature of only 170 “Celsius, as suggested in Castner's original patent.

[0008] 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.

[0009] It is well known that the above techniques for producing alkali metals and alkaline earth metals by electrolysis suffer from several inefficiencies. In particular, 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 multiplied by the high current through an electrolytic cell required for a reasonable rate of production of the electrolysis products, this 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 liquid metal with chlorine gas within the cell, and an overpotential caused by bubbles of chlorine gas adhering to the anode, which reduces the effective surface area of the anode available to conduct current.

[0010] Much attention has been paid in the prior art to addressing these various inefficiencies. Some examples of prior art documents which aim to reduce the back-reaction of electrolysis products include US patent no. 5 904 821 , European patent no. 3 033 443, and Qian-Wen Zhao et al . "Analysing and Optimizing the Electrolysis Efficiency of a Lithium Cell Based on the Electrochemical and Multiphase Model”, Royal Society Open Science, vol. 7, no. 191124 (2019). All of these prior art documents address this problem by varying the cell geometry, the design of a diaphragm for separating the electrolysis products (which is functionally equivalent to the mesh or grille 75 in Fig. 1) and / or the layout of the electrodes within the cell. However, it would be desirable if another way could be found to inhibit the back-reaction of electrolysis products more generally, which was completely independent of cell geometry or construction, and which could therefore be applied to any electrolytic cell used for electrolysing a range of inorganic compounds of alkali and alkaline earth metals.

[0011] By way of further background, it is also known that magnetic fields can affect electrochemical reactions. Some examples of review articles which describe the prior art in this field of magnetoelectrochemistry include Lorena M.A. Monzon & J.M.D. Coey: "Magnetic fields in electrochemistry: The Lorentz force. A mini-review”, Electrochemistry Communications, No. 42, pp. 38-41 (2014), and "Magnetic fields in electrochemistry: The Kelvin force. A mini-review”, Electrochemistry Communications, No. 42, pp. 42-45 (2014); V. Gatard, J. Deseure & M. Chatenet: "Use of magnetic fields in electrochemistry: a selected review”, Current Opinion in Electrochemistry, No. 23, pp. 96-105 (2020); and Songzhu Luo, Kamal Elouarzaki & Zhichuan J. Xu: "Electrochemistry in Magnetic Fields”, Angewandte Chemie Int. Ed., Vol. 61 , Issue 27 (July 2022). However, much of the prior art in the field of magnetoelectrochemistry is either concerned with or presupposes that magnetic fields are applied to aqueous- phase solutions, and very little attention seems to have been been paid in the art to the effects of magnetic fields on molten electrolytes, such as molten salts. Molten salts are fundamentally different from aqueous-phase solutions because the ions present in a molten salt are not solute species which are dissolved in another medium ( / '.e., a solvent) to create the solution. Instead, a molten salt is much better thought of as a quasi-crystal with intermediate-range ordering of ions and a large percentage (for example, as much as about 10%) of vacant lattice sites or "holes”.

[0012] Object of the Invention

[0013] It is therefore an object of the invention to provide an electrolytic cell having improved energy efficiency, and use of such an electrolytic cell to electrolyse certain inorganic compounds of alkali metals and alkaline earth metals in their molten state.

[0014] Description of the Invention

[0015] Accordingly, in one aspect, the present invention provides an electrolytic cell comprising a vessel for containing a molten electrolyte, an anode inside the vessel, a cathode inside the vessel, and a source of a magnetic field having a gradient with a positive component in a direction from the anode to the cathode, such that the strength of the magnetic field increases from the anode to the cathode. The rest of the electrolytic cell, apart from the source of the magnetic field, including at least both the vessel for containing the molten electrolyte and the anode, consists of material having a magnetic susceptibility with an absolute value of less than 10’2.

[0016] A magnetic susceptibility with an absolute value of less than 102means that the rest of the electrolytic cell, including both the vessel and the anode, consists of paramagnetic and / or diamagnetic material(s). If such a material is paramagnetic, its magnetic susceptibility will have a small positive value less than 10’2, whereas if the material is diamagnetic, its magnetic susceptibility will have a small negative value less than 10’2. The fact that the rest of the electrolytic cell consists of paramagnetic and / or diamagnetic material(s) ensures that nothing else in the cell, apart from the source of the magnetic field, affects the magnetic field between the anode and the cathode to any appreciable extent in comparison to the same magnetic field in free space. For example, if the vessel instead comprised a ferromagnetic material, at least some of the magnetic field would be diverted into the ferromagnetic material away from the space between the anode and the cathode, as in a transformer core.

[0017] The source of the magnetic field between the anode and the cathode may either be a permanent magnet located outside the vessel adjacent to the cathode inside the vessel or it may be provided by permanently magnetizing the cathode itself, in manners which will be described in greater detail below. Since in both cases, the magnetic field is provided by a permanent magnet, maintaining the magnetic field when the cell is being used for electrolysis does not consume any energy, after the source of the magnetic field has been initially magnetized.

[0018] The electrolytic cell of the invention is suitable for molten-phase electrolysis of halides and hydroxides of alkali metals, and halides of the alkaline earth metals except beryllium, for reasons explained below. The beneficial technical effects of an electrolytic cell according to the invention may best be understood by reference to the following two examples.

[0019] Example 1 : Electrolysis of molten sodium chloride

[0020] In this case, the species present in the cell comprise sodium cations and chloride anions, both present in the molten electrolyte, and two electrolysis products, namely liquid sodium formed at the cathode and diatomic chlorine gas formed at the anode. The sodium cations and the chloride anions are largely unaffected by the magnetic field between the anode and the cathode because the drift velocities of the sodium cations towards the cathode and of the chloride anions towards the anode are so low that the Lorentz force on each ion due to the magnetic field is swamped by the Lorentz force on each ion due to the electric field between the anode and the cathode, which is several orders of magnitude higher.

[0021] In contrast, the two electrolysis products are both electrically neutral, so do not feel any Lorentz force at all. They are therefore unaffected by the electric field between the anode and the cathode. The liquid sodium is paramagnetic and the diatomic chlorine gas is diamagnetic. The liquid sodium is therefore attracted to regions of stronger magnetic field, whereas the diatomic chlorine gas is repelled by regions of stronger magnetic field. The strength of the repulsive or attractive force, F, in each case, is given by: F = ±1 / 2p0K V VH2[Eqn. 1] where po is the permeability of free space, K is the volume susceptibility of the electrolysis product in question, V is its volume and H is the strength of the magnetic field applied to the electrolysis product. As is well known, the susceptibility of diamagnetic materials like diatomic chlorine is substantially independent of their temperature. In common with the other alkali metals and the alkaline earth metals except beryllium, the electrons in the liquid sodium are delocalized and exhibit Pauli paramagnetism, which is also independent of temperature up to the Fermi temperature, TF, of the paramagnetic material. In the case of sodium, which has a Fermi energy, EF = 3.23 eV, the Fermi temperature, TF = EF I ks (where ks is Boltzmann's constant) = 3.75 x 104K, which is well above the electrolysis temperature. The sodium cations also present in the liquid sodium are diamagnetic, but the paramagnetic susceptibility of the delocalized electrons outweighs the diamagnetic susceptibility of these sodium cations, which therefore leaves the liquid sodium with a nett paramagnetism. Thus in the case of both electrolysis products, the force experienced by the electrolysis products is dependent only on the size of the droplets of liquid sodium formed at the cathode or of the bubbles of diatomic chlorine gas formed at the anode, and on the gradient of the magnetic field strength as defined by Eqn. 1 at their respective locations. Therefore, since according to the invention, the magnetic field has a gradient with a positive component in a direction from the anode to the cathode, such that the strength of the magnetic field increases from the anode to the cathode, larger droplets of liquid sodium are attracted more to the cathode and larger bubbles of diatomic chlorine are repelled more from the cathode, which impedes the migration of droplets of liquid sodium towards the anode and of bubbles of diatomic chlorine towards the cathode. This therefore inhibits the back-reaction of liquid sodium with chlorine gas within the cell in proportion to the gradient of the magnetic field strength according to Eqn. 1 , which in turn increases the energy efficiency of the cell. Moreover, since liquid sodium is a good electrical conductor, any droplets of liquid sodium which adhere to the cathode as a result of their attraction to the cathode do not impede the flow of electrical current between the anode and the cathode. Meanwhile, the strength of the magnetic field at the surface of the anode can also be made sufficiently low in comparison to its strength at the surface of the cathode that its effect on bubbles of chlorine gas at the surface of the anode is negligible, such that there is no increase in the bubble overpotential at the anode either.

[0022] Example 2: Electrolysis of molten sodium hydroxide

[0023] In this case, the species present in the electrolytic cell comprise sodium cations and hydroxide anions, both present in the molten electrolyte, and the following immediate products of electrolysing the sodium hydroxide: liquid sodium formed at the cathode, and molecular water and diatomic oxygen gas, both of which are formed at the anode. In addition, even at the high temperature of the electrolysis, at least some of the water is able to dissolve in the molten sodium hydroxide and migrate through it until it reacts with liquid sodium formed at the cathode in a back-reaction with the liquid sodium to produce diatomic hydrogen gas as an additional electrolysis product at the cathode, as well as reconstituting further sodium cations and hydroxide anions in the electrolyte. These different species behave as follows in the presence of a magnetic field according to the invention. The sodium cations and hydroxide anions in the molten electrolyte are largely unaffected by the magnetic field and behave as the sodium cations and chloride anions do in the first example given above, for the same reasons as explained before. Water is diamagnetic. The liquid sodium formed at the cathode and the water formed at the anode in the present example therefore respectively behave just as the cathodic and anodic electrolysis products do in the first example, for the same reasons as explained there. This therefore inhibits the back-reaction of the water with the liquid sodium within the cell in proportion to the magnetic field strength as defined by Eqn. 1, thereby increasing the energy efficiency of the cell. Nonetheless, if any water is still able to migrate through the molten sodium hydroxide and react with liquid sodium, the diatomic hydrogen gas formed thereby is diamagnetic. Bubbles of hydrogen gas formed at the cathode are therefore repelled from the cathode in proportion to their size and the magnetic field strength. This encourages their detachment from the cathode, thereby reducing the bubble overpotential at the cathode. Moreover, the diatomic oxygen gas formed at the anode by the original electrolysis of the sodium hydroxide is quite strongly paramagnetic. Bubbles of oxygen gas formed at the anode are therefore attracted to the cathode, even at the distance of the anode, in proportion to their size and the magnetic field strength. This encourages their detachment from the anode, thereby reducing the bubble overpotential at the anode as well. Thus the energy efficiency of the cell is also increased by the reduction in the bubble overpotentials at both the anode and the cathode. Since gaseous hydrogen and oxygen are both very significantly less soluble in basic media like sodium hydroxide than in acidic media, the bubbles of hydrogen gas released from the cathode and of oxygen gas released from the anode do not dissolve in the molten electrolyte, but instead rise up out of the electrolyte under their own buoyancy, with no adverse effects on the energy efficiency of the cell.

[0024] Thus it can be seen from the above two examples that an electrolytic cell according to the invention provides beneficial technical effects when electrolysing both molten sodium chloride and molten sodium hydroxide. In fact, since all the alkali metals and alkaline earth metals except beryllium are paramagnetic, the same applies to these other metals in these two groups, subject to their being producible in liquid phase by molten electrolysis. (For example, it may not apply to calcium, which is typically produced at the cathode in solid phase by molten electrolysis or to barium, which tends to dissolve in the molten electrolyte.) Moreover, since all the other halogens are also diamagnetic like diatomic chlorine and all the halides and hydroxides of the alkali (Group 1) metals are diamagnetic as well, any of them may be constituted as a molten electrolyte in an electrolytic cell according to the invention without affecting the magnetic field between the anode and the cathode to any appreciable extent in comparison to the same magnetic field in free space. The same applies to the halides of the alkaline earth (Group 2) metals, but not to the hydroxides thereof, which generally decompose upon being heated and / or electrolysed into their respective oxides and water vapour before they can be melted. Thus the same principles as are illustrated by the two examples given above apply equally to other halides and hydroxides of alkali metals, as well as to the halides of the alkaline earth metals except beryllium, subject to the same consideration of the phase or state of the electrolysis products at their respective electrolysis temperatures, which are well known from the prior art.

[0025] In some circumstances, depending on the geometry of the cell and on other factors like the cell temperature and voltage, the inhibition of the back-reaction of the electrolysis products may be sufficiently great that a separator of electrolysis products or diaphragm like the mesh or grille 75 described above in relation to Fig. 1 may be omitted entirely from the cell. Since such a separator of electrolysis products is a major contributor to the internal resistance of an electrolytic cell which contains one, the energy efficiency of the cell may also be improved significantly thereby.

[0026] Some examples of materials which the vessel for containing the molten electrolyte may consist of include austenitic stainless steel, nickel or a nickel alloy, or a titanium alloy, all of which are either paramagnetic or may be chosen to have a Curie temperature below the temperature at which the electrolysis is conducted. For example, if the molten electrolyte is a chloride salt, adding from 2% to 10% aluminium to a 300 series austenitic heat-resistant steel has been found to be effective at improving its resistance to oxidation and corrosion by chloride salts, as well as its mechanical properties. If, on the other hand, the molten electrolyte is a hydroxide, the vessel may consist of Fe64Ni36iron-nickel alloy, which has a Curie temperature of only 503 K, well below the melting point, Tm= 596 K, of sodium hydroxide. Alternatively, the vessel may consist of a titanium alloy, for example one having a composition by weight of 98.8% Ti, 0.8% Ni and 0.4% Mo, which is found to be highly resistant to corrosion by sodium hydroxide. In any event, the vessel may be lined with refractory bricks to provide thermal insulation in a manner well known from the prior art, since such refractory bricks are diamagnetic. Preferably, the vessel is electrically as well as thermally insulated to avoid accidentally inducing any stray magnetic fields, even though materials such as austenitic stainless steel and titanium alloy are relatively poor electrical conductors.

[0027] Some examples of materials which the anode may consist of are as follows. If electrolysing a molten halide, the anode may consist of amorphous, glassy or graphitic carbon, which is diamagnetic, as in the prior art. If electrolysing a molten hydroxide, the anode, which in the prior art is typically made of iron or nickel, both of which are ferromagnetic, may be replaced with an anode consisting of an iron-nickel alloy such as Fe64Ni36, for the same reasons as described above. Copper bus bars and connectors for supplying an electrical current to the anode and cathode in a manner well known from the prior art may remain the same as in the prior art, since copper is diamagnetic.

[0028] In some embodiments, however, the electrolytic cell may further comprise a separator of electrolysis products or diaphragm located between the anode and the cathode for immersion in the molten electrolyte and which is porous to the molten electrolyte. If so, the separator should consist of material also having a magnetic susceptibility with an absolute value of less than 102for the same reasons as described above, to avoid disturbing the magnetic field between the anode and the cathode. Thus, unlike the mesh or grille 75 described above in relation to Fig. 1 , which is typically made of iron or low-carbon steel and which is therefore ferromagnetic, some suitable materials which the separator may instead consist of include those described above in relation to the vessel for containing the molten electrolyte, such as austenitic stainless steel, nickel or a nickel alloy, or a titanium alloy. If the molten electrolyte is a chloride salt, austenitic stainless steel may suffice. If the molten electrolyte is a hydroxide, the nickel separator traditionally used in a Castner cell, which has a Curie temperature of 627 K, may be replaced by one instead consisting of Fe64Ni36 iron-nickel alloy because of its lower Curie temperature, or consisting of a titanium alloy.

[0029] In some embodiments in which the electrolytic cell comprises a porous separator, the material of which the separator consists may also have an electrical resistivity of at least about 108Cm. Some examples of such materials which are porous to the molten electrolyte, but which are also resistant to corrosion by it, include titaniabased and zirconia-based ceramic foams. Since the separator in such an embodiment therefore consists of an electrically insulating material, the anode-cathode distance can be reduced, even to the extent that the separator may be allowed to touch at least one of the anode and cathode, thereby reducing the internal resistance of the cell and improving its energy efficiency without creating an electrical short circuit between the anode and cathode.

[0030] As mentioned above, the source of the magnetic field between the anode and cathode may either be a permanent magnet located outside the vessel of the electrolytic cell or it may be provided by permanently magnetizing the cathode itself. Thus, in some embodiments, the cathode may consist of an electrically conducting material having a magnetic susceptibility with an absolute value of less than 102and the electrolytic cell may further comprise a permanent magnet located outside the vessel adjacent to the cathode inside the vessel, on an opposite side of the cathode from the anode, and magnetized to provide the magnetic field. This has the advantage that the permanent magnet may comprise one or more of a range of different ferromagnetic materials, which do not also need to be electrically conducting or to be chemically resistant to corrosion by the molten electrolyte or to be able to retain their magnetization at the high temperatures inside the vessel. In such embodiments, the cathode may, for example, consist of amorphous, glassy or graphitic carbon (which is diamagnetic) to avoid disturbing the magnetic field between the anode and the cathode, but which also has good electrical conductivity. This is in contrast to the cathode 70 described above in relation to Fig. 1 or the cathode in a Castner cell, both of which are typically made of iron or low-carbon steel. Alternatively, if electrolysing molten magnesium chloride, for example, which is typically conducted at a temperature above the melting point of magnesium, Tm(Mg) = 650 “Celsius, and which is therefore also significantly above the Curie temperature of nickel at 354 “Celsius, the steel cathode traditionally used in a Norsk-Hydro-type cell for electrolysing molten magnesium chloride may be replaced by one instead consisting of nickel or of a nickel alloy having a Curie temperature well below this electrolysis temperature but which is still a good electrical conductor.

[0031] Alternatively, in some embodiments, the cathode may comprise an electrically conducting ferromagnetic material having a coercivity of at least about 1000 Arm1and a Curie temperature of at least about 600 K, more preferably at least about 700 K, even more preferably at least about 800 K, more preferably still at least about 900 K, and most preferably at least about 1000 K, and the ferromagnetic material may be permanently magnetized to provide the magnetic field between the anode and the cathode. This has the advantage that when the cathode is permanently magnetized, the highest strength of the magnetic field between the anode and the cathode is on a surface of the cathode. Some examples of suitable materials which the cathode may comprise in such embodiments are listed in Table 1 below:

[0032] (s.t.p. denotes standard temperature and pressure)

[0033] Table 1

[0034] A coercivity of about 1000 Amr1is generally considered as the boundary between "hard” magnetic materials, which can be permanently magnetized, and "soft” magnetic materials, which fail to retain their magnetization if temporarily magnetized. Electrically conducting ferromagnetic materials of low coercivity, such as the pure metals iron, cobalt and nickel, as well as some of their alloys like Permendur™, are therefore not suitable cathode materials for such embodiments. Similarly, ferromagnetic materials with high coercivity but which are not electrically conducting, such as ferrites, are not suitable for making a cathode in such embodiments either. Thus, if the cathode has traditionally been made of iron or low-carbon steel, both of which are "soft” magnetic materials, this should be replaced with a cathode comprising a "hard” magnetic material such as those listed in Table 1 above, which can retain at least some of their magnetization at the high temperatures inside the vessel. For example, Alnico 8 or 9, both of which have an electrical resistivity at s.t.p. of 5.3 x 107Qm, a Curie temperature of about 860 “Celsius and a maximum working temperature generally of about 550 “Celsius, may be used to make cathodes for electrolysing a eutectic mixture of sodium chloride and calcium chloride, which has a eutectic temperature of 504 “Celsius, to obtain liquid sodium and chlorine gas as the electrolysis products.

[0035] Prior art electrolytic cells for electrolysing molten electrolytes such as those described herein may comprise a thermal management system, such as a layer of thermal insulation and / or a heat exchanger. For example, the vessel of such a prior art cell may be lined with a layer of refractory bricks or the vessel may be wrapped in mineral wool as thermal insulation, or the cell may comprise a heat exchanger supplied with a heat transfer fluid such as water to conduct excess heat away from the cell. However, in embodiments of electrolytic cells according to the invention in which the source of the magnetic field is a permanent magnet located outside the vessel of the electrolytic cell, the permanent magnet outside the vessel should preferably be located as close as possible to the cathode inside the vessel, to maximise the strength of the magnetic field at the cathode. Therefore, in such embodiments, it is also preferable that nothing apart from the vessel itself, like a thermal management system, should be interposed between the permanent magnet outside the vessel and the cathode inside the vessel, so that the permanent magnet and the cathode can be separated from each other by only a few millimetres, equal to just slightly more than the thickness of the vessel. However, this also means that heat may be transferred from inside the vessel to the permanent magnet located outside the vessel. In some embodiments, therefore, the permanent magnet may have a Curie temperature of at least about 600 K, more preferably at least about 700 K, even more preferably at least about 800 K, more preferably still at least about 900 K, and most preferably at least about 1000 K. This has the advantage that the permanent magnet is then still able to retain at least some of its magnetization, despite heat being transferred to it from inside the vessel. Moreover, such embodiments may in some cases still comprise a thermal management system such as those just mentioned, which may be located, for example, outside the permanent magnet.

[0036] In some embodiments, regardless of whether the magnetic field is provided by a permanent magnet located outside the vessel or whether the cathode itself comprises a ferromagnetic material which is permanently magnetized to provide the magnetic field, the source of the magnetic field may be magnetized as a Halbach array arranged to concentrate the magnetic field on a surface of the source facing the anode. This has the advantage of increasing the percentage of the total magnetic field directed between the anode and the cathode, thereby also increasing the strength of the magnetic field on a surface of the cathode facing the anode.

[0037] In some embodiments, the cathode may be rotationally symmetrical about a central axis, as in the Downs cell shown in Fig. 1 or as in US patent no. 452 030 of Castner. In such embodiments, the magnetic field between the anode and the cathode is preferably a quadrupole field or a field of higher even polarity distributed evenly about the central axis. This has the advantage that the beneficial technical effects described above of having a positive magnetic field gradient which increases in a direction from the anode to the cathode are then also distributed more evenly about the central axis of the cathode.

[0038] If the cathode is rotationally symmetrical about a central axis, in some embodiments, the cathode may comprise a hollow cylinder open at at least one end thereof, as in the Downs cell shown in Fig. 1. If so and if the magnetic field is a quadrupole field or a field of higher even polarity distributed evenly about the central axis, the magnetic field may be arranged such that at least one magnetic pole of the magnetic field emerges from an inner curved surface of the hollow cylinder. In this manner, at least some of the magnetic field is then directed inside the hollow cylinder of the cathode.

[0039] In such a case, in some embodiments thereof, the anode may comprise a cylinder, which may be solid or hollow, and the cylinder of the anode may be located inside and concentric with the hollow cylinder of the cathode. This has the advantage that the geometry of the magnetic field can then be superimposed on the geometry of the anode and cathode, such that the strength of the magnetic field diminishes with the same rotational symmetry as the anode and cathode, inwardly towards the cylinder of the anode and according to the radial distance from the cathode, until it reaches zero along the central axis.

[0040] In some embodiments, if the cathode is rotationally symmetrical about a central axis, the cathode may instead comprise a centrally located cylinder, as in US patent no. 452 030 of Castner. If so and if the magnetic field is a quadrupole field or a field of higher even polarity distributed evenly about the central axis, then preferably, the magnetic field may instead be arranged such that at least one magnetic pole of the magnetic field emerges from an outer curved surface of the cylinder of the cathode. In this manner, at least some of the magnetic field is then directed outwardly from the cylinder of the cathode. In such embodiments, the cylinder of the cathode may be either solid or hollow.

[0041] In such a case, in some embodiments thereof, the anode may comprise a hollow cylinder open at at least one end thereof, and the hollow cylinder of the anode may be located around and concentric with the cylinder of the cathode. This has the advantage that the geometry of the magnetic field can then be superimposed on the geometry of the anode and cathode, such that the strength of the magnetic field diminishes with the same rotational symmetry as the anode and cathode, outwardly towards the hollow cylinder of the anode and according to the radial distance from the cathode.

[0042] In any embodiment in which the anode and the cathode both comprise concentric cylinders, during use, the anode and cathode may rotate relative to each other about the central axis. This has the advantage that since the orientation of the magnetic field varies in space between the anode and cathode because the field is a quadrupole field or a field of higher even polarity, the orientation of the magnetic field between the anode and cathode is time- averaged about the central axis by the relative rotation, whereby different regions on a surface of the anode facing the cathode experience the same average magnetic field over time. Such relative rotation may consist of the cathode remaining stationary whilst the anode rotates or the anode remaining stationary whilst the cathode rotates or both the anode and the cathode being in contrarotation. The choice of which of these alternatives to use may be made on the basis of convenience and of which of them uses the least energy according to which of the anode and cathode has the lower moment of inertia. The rate of rotation need not be particularly high and should not be so high as to cause any significant drag on the rotating anode and / or cathode from the molten electrolyte or as to introduce any turbulence or magnetohydrodynamic effects into the molten electrolyte. For example, the rate of rotation may be only one revolution every several minutes. The rotation may therefore be driven by a clockworktype mechanism, for example, having a very low power consumption.

[0043] In a second aspect, the present invention also provides a plurality of electrolytic cells according to the first aspect, in each of which the cathode consists of an electrically conducting material having a magnetic susceptibility with an absolute value of less than 102and the electrolytic cell further comprises a permanent magnet located outside the vessel adjacent to the cathode inside the vessel, on an opposite side of the cathode from the anode, and magnetized as a source of the magnetic field, wherein at least a respective one of the permanent magnets is arranged with a first magnetic pole thereof adjacent to the cathode inside a first one of the vessels and with another magnetic pole thereof of opposite polarity to the first magnetic pole adjacent to the cathode inside another one of the vessels. This configuration has the advantage that since the respective permanent magnet is shared by the respective cathodes in two different vessels, the total number of permanent magnets needed to provide a magnetic field adjacent to the cathode in each of the vessels may be halved in comparison to providing each cathode with a separate permanent magnet of its own. Furthermore, since magnetic poles always occur in dipole pairs of opposite polarity to each other, both poles of the respective permanent magnet are efficiently utilized to provide the magnetic fields in two different vessels, rather than just one of the poles being utilized to provide the magnetic field in only one vessel whilst the other pole is oriented outwardly from the same vessel towards free space.

[0044] In some embodiments comprising a plurality of electrolytic cells as just described, the respective one of the permanent magnets which is shared between two adjacent vessels may have a Curie temperature of at least about 600 K, more preferably at least about 700 K, even more preferably at least about 800 K, more preferably still at least about 900 K, and most preferably at least about 1000 K, and an interior of the first one of the vessels may be in thermal contact with an interior of the other one of the vessels via the respective one of the permanent magnets. This has the advantage that the permanent magnet is then still able to retain at least some of its magnetization, despite heat being transferred to it from inside one or both of the adjacent vessels.

[0045] In some embodiments comprising a plurality of electrolytic cells which share a permanent magnet between two adjacent vessels, in each of the electrolytic cells, the cathode may comprises a hollow prism open at at least one end thereof which is tetragonally or hexagonally symmetrical about a central axis, the anode may be located inside and coaxial with the hollow prism of the cathode, the magnetic field inside the cathode may be a quadrupole field or a sextupole field, respectively, distributed evenly about the central axis, and the plurality of electrolytic cells may be arranged in a square or hexagonal array, respectively. Such an arrangement has the advantage of providing a very efficient packing in space of the plurality of electrolytic cells, thereby allowing the density of the magnetic field across all of the cells to be maximised with the least number of permanent magnets.

[0046] Moreover, if the plurality of electrolytic cells is arranged as just described, in some embodiments thereof, the anode may comprise a cylinder which is cylindrically symmetrical about the central axis, and during use, the cylinder of the anode may rotate relative to the cathode about the central axis. This has the same advantage as described above that since the orientation of the magnetic field varies in space between the anode and the cathode because the field is a quadrupole field or a sextupole field, the orientation of the magnetic field between the anode and the cathode is time-averaged about the central axis by the rotation of the anode, whereby different regions on a surface of the anode facing the cathode experience the same average magnetic field over time. As mentioned before, the rate of rotation need not be particularly high and should not be so high as to cause any significant drag on the rotating anode from the molten electrolyte or as to introduce any turbulence or magnetohydrodynamic effects into the molten electrolyte. For example, the rate of rotation may be only one revolution every several minutes.

[0047] In a third aspect, the present invention also provides for use of an electrolytic cell as described herein to electrolyse an inorganic compound selected from the group consisting of a halide of an alkali metal, a halide of an alkaline earth metal except beryllium and a hydroxide of an alkali metal when the inorganic compound is in its molten state. An electrolytic cell as described herein is particularly suitable for electrolysing such inorganic compounds for the reasons explained above.

[0048] If the cathode comprises an electrically conducting ferromagnetic material having a coercivity of at least 1000 Am1and a Curie temperature of at least 600 K, and the ferromagnetic material is permanently magnetized as a source of the magnetic field, then the inorganic compound should preferably be electrolysed at an absolute temperature of not more than about 85%, more preferably not more than about 75%, and most preferably not more than about 65%, of the Curie temperature of the ferromagnetic material. Thus, for example, if the Curie temperature of the ferromagnetic material is 1100 K (= 827 “Celsius), then the inorganic compound should preferably be electrolysed at not more than about 935 K (= 662 “Celsius), more preferably not more than about 825 K (= 552 “Celsius), and most preferably not more than about 715 K (= 442 “Celsius). This has the advantage that the larger the difference in temperature between the temperature of the electrolysis and the Curie temperature of the ferromagnetic material, the more the ferromagnetic material is able to retain its magnetization. In practice, the maximum operating temperature of the ferromagnetic material may depend not only on its Curie temperature but also on the geometry of the magnetic field. Nonetheless, the ferromagnetic material may still be remagnetized from time to time as part of routine maintenance of the electrolytic cell.

[0049] Preferably, the alkali metal is selected from the group consisting of lithium, sodium and potassium, the alkaline earth metal is magnesium, and the halide is a chloride.

[0050] If the inorganic compound is magnesium chloride, the magnetic field may have to be provided by a permanent magnet located outside the vessel adjacent to the cathode inside the vessel because the temperature of the molten electrolyte may typically be from about 700 to about 750 “Celsius due to the requirement that the magnesium should be produced in molten phase and the fact that the melting point of magnesium, Tm(Mg) = 650 “Celsius. Thus it might be difficult to find a ferromagnetic material which has a high enough Curie temperature to be used as a cathode material. If so, the cathode may instead consist of nickel, nickel alloy 301 ( / '.e., Duranickel™) or an iron-nickel alloy having a Curie temperature which is at least about 10 K below the temperature of the molten electrolyte. Such materials have the advantage that unlike the steel cathode traditionally used in the Norsk-Hydro process for electrolysing magnesium chloride, the cathode will then not disturb the magnetic field from the permanent magnet because it has a magnetic susceptibility with an absolute value of less than 10’2. For example, the Curie temperature of nickel is only 354 “Celsius and the Curie temperature of nickel alloy 301 is less than about 100 “Celsius, depending on whether the alloy is annealed or age-hardened. All such materials also have the advantage that they have lower electrical resistivity than steel, whereby the energy efficiency of the electrolytic cell may be improved as well. For example, the electrical resistivity of age-hardened nickel alloy 301 is 4.3 x 107Cm at s.t.p., rising to only 6 x 107Cm at 700 “Celsius. If the cathode consists of an iron-nickel alloy, then the ironnickel alloy should preferably be chosen to have as high a nickel content as possible. This has at least two advantages, as follows. Firstly, the electrical resistivity of iron-nickel alloys falls as the nickel content of the alloy increases. For example, Fe64Ni36 ( / .©., Invar™), which has a Curie temperature of 280 “Celsius, has an electrical resistivity of 8.2 x 107Cm at s.t.p., whereas iron-nickel alloy 52 conforming to ASTM F-30 has an electrical resistivity of only 4.3 x 107Cm at s.t.p. and a Curie temperature of 530 “Celsius, which is still well below the temperature of the molten electrolyte. Another advantage of making the nickel content of the iron-nickel alloy as high as possible is that the magnetostriction of iron-nickel alloys also falls as the nickel content of the alloy increases. Thus when the magnetic field is applied to the cathode, the strain on the cathode is reduced as well. If the inorganic compound is sodium hydroxide, the anode may consist of an iron-nickel alloy having a Curie temperature at least about 10 K below the temperature of the molten electrolyte, regardless of whether the magnetic field is provided by magnetizing the cathode itself or by a permanent magnet located outside the vessel adjacent to the cathode inside the vessel. This has the advantage that unlike the nickel anode traditionally used in the Castner process for electrolysing sodium hydroxide, the anode will then not disturb the magnetic field because it has a magnetic susceptibility with an absolute value of less than 10’2. For example, since the melting point of sodium hydroxide, Tm(NaOH) = 323 “Celsius, the anode may consist of Fe64Ni36. An additional advantage of an iron-nickel alloy anode over a traditional pure nickel anode is that it also has a lower coefficient of thermal expansion than pure nickel. This allows the electrolytic cell to be assembled at ambient temperatures with reduced tolerances allowed for thermal expansion, whereby the anode-cathode distance may also be reduced, thereby reducing the internal resistance of the cell and improving its energy efficiency as well.

[0051] Moreover, if the inorganic compound is sodium hydroxide, in those embodiments in which the magnetic field is provided by a permanent magnet located outside the vessel adjacent to the cathode inside the vessel, the cathode may also consist of an iron-nickel alloy having a Curie temperature at least about 10 K below the temperature of the molten electrolyte, with the same advantages as just described.

[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; Fig. 2A is a schematic plan view of a first embodiment of an electrolytic cell;

[0054] Fig. 2B is a graph of magnetic field strength in a direction from the cathode to the anode in the electrolytic cell of Fig. 2A;

[0055] Fig. 3A is a schematic plan view of a second embodiment of an electrolytic cell;

[0056] Fig. 3B is a graph of magnetic field strength in a direction from the cathode to the anode in the electrolytic cell of Fig. 3A;

[0057] Fig. 4A is a schematic plan view of a third embodiment of an electrolytic cell;

[0058] Fig. 4B is a graph of magnetic field strength in a direction from the cathode to the anode in the electrolytic cell of Fig. 4A;

[0059] Fig. 5A is a schematic plan view of a fourth embodiment of an electrolytic cell;

[0060] Fig. 5B is a graph of magnetic field strength in a direction from the cathode to the anode in the electrolytic cell of Fig. 5A;

[0061] Figs. 6A, 6B and 6C, respectively, are schematic plan views of quadrupole, sextupole and octupole magnetic fields inside embodiments of hollow cathodes;

[0062] Fig. 7 is a schematic plan view of a gradient of the magnetic fields shown in Figs. 6A, 6B and 6C; Figs. 8A and 8B, respectively, are schematic plan views of quadrupole and sextupole magnetic fields around embodiments of central cathodes;

[0063] Fig. 9 is a schematic plan view of a gradient of the magnetic fields shown in Figs. 8A and 8B;

[0064] Fig. 10A is a schematic plan view of an embodiment of an anode and a cathode, wherein the anode and cathode are in rotation relative to each other;

[0065] Fig. 10B is a schematic plan view of another embodiment of an anode and a cathode, wherein the anode and cathode are in rotation relative to each other;

[0066] Fig. 11 A is a schematic plan view of a first embodiment of a series of electrolytic cells;

[0067] Fig. 11 B is a graph of magnetic field strength along the series of electrolytic cells of Fig. 11 A;

[0068] Fig. 12A is a schematic plan view of a second embodiment of a series of electrolytic cells;

[0069] Fig. 12B is a graph of magnetic field strength along the series of electrolytic cells of Fig. 12A;

[0070] Fig. 13 is a schematic plan view of a first embodiment of an array of electrolytic cells;

[0071] Fig. 14 is a schematic plan view of a second embodiment of an array of electrolytic cells;

[0072] Fig. 15A is a schematic plan view of a fifth embodiment of an electrolytic cell, wherein the anode is in rotation relative to the cathode;

[0073] Fig. 15B is a schematic plan view of a sixth embodiment of an electrolytic cell, wherein the anode is in rotation relative to the cathode; and

[0074] Fig. 16 is a flow diagram of an embodiment of use of an electrolytic cell according to the invention.

[0075] In the accompanying drawings, none of the schematic diagrams or their accompanying graphs are drawn to scale.

[0076] Detailed

[0077] Fig. 2A schematically shows a plan view of a first embodiment of an electrolytic cell 100. The electrolytic cell 100 of this first embodiment comprises a vessel 102 for containing a molten electrolyte, an anode 10 and a cathode 70. In this embodiment, the vessel 102 has a generally rectangular shape in plan, and the anode 10 and the cathode 70 are both planar and are arranged in parallel inside the vessel 102. For illustrative purposes only, other parts of the electrolytic cell 100, such as for introducing the electrolyte into the cell 100 equivalent to the inlet 104 shown in Fig. 1 , for capturing and directing the electrolysis products out of the cell 100 equivalent to the hood 80, outlet 50, lid 103 and outlet 106 shown in Fig. 1 and for supplying the anode 10 and cathode 70 with an electrical current, such as the current-carrying conductors 30a, 30b shown in Fig. 1 , are all omitted from Fig. 2A, which therefore only shows those parts of the electrolytic cell 100 for immersion in the molten electrolyte.

[0078] The vessel 102, the anode 10 and the other parts of the electrolytic cell 100 not shown in Fig. 2A as just mentioned all consist of material(s) having a magnetic susceptibility with an absolute value of less than 10’2. In contrast, the cathode 70 comprises an electrically conducting "hard” ferromagnetic material (in other words, one having a coercivity of at least 1000 Am’1) with a Curie temperature of at least 600 K. In this embodiment, this high- temperature-resistant ferromagnetic material is permanently magnetized with a simple dipole field having its north pole on a surface of the cathode 70 facing the anode 10 and its south pole on the opposite surface of the cathode 70, although the cathode 70 could equally well be magnetized with opposite polarity to have the south pole facing the anode 10 and the north pole on the opposite surface of the cathode 70 instead. In both cases, the dipole field emerges from the surface of the cathode 70 facing the anode 10 to provide a magnetic field having a gradient with a positive component in a direction from the anode 10 to the cathode 70, such that the strength of the magnetic field increases from the anode 10 to the cathode 70.

[0079] Fig. 2B schematically represents the strength of this magnetic field H in a direction x from the cathode 70 to the anode 10. The exact shape of the graph in Fig. 2B is arbitrary, but as may be seen from Fig. 2B, the magnetic field H has its maximum strength at the surface of the cathode 70 and diminishes towards the anode 10. Therefore, provided that the electrolysis temperature is sufficiently less than the Curie temperature of the ferromagnetic material of the cathode 70 for the cathode 70 to retain at least some of its magnetization, a paramagnetic electrolysis product is attracted to the cathode 70, whereas a diamagnetic electrolysis product is repelled from the cathode 70, according to their distance x from the cathode 70, with the greatest attraction or repulsion being at the surface of the cathode 70 itself. However, other parts of the electrolytic cell 100, including the vessel 102 and the anode 10, have hardly any effect on the magnetic field H, because they all consist of material(s) having a magnetic susceptibility with an absolute value of less than 10’2.

[0080] Fig. 3A schematically shows a plan view of a second embodiment of an electrolytic cell 100. The electrolytic cell 100 of this second embodiment comprises all the same components as the electrolytic cell 100 of the first embodiment described above in relation to Fig. 2A, but further comprises a separator 75 of the electrolysis products located between the anode 10 and the cathode 70 for immersion in the molten electrolyte, which is functionally equivalent to the mesh or grille 75 shown in Fig. 1. In this case, however, the separator 75 instead consists of material having a magnetic susceptibility with an absolute value of less than 10’2, like the vessel 102, the anode 10 and other parts of the electrolytic cell 100, except for the cathode 70. The separator 75 therefore has a negligible effect on the magnetic field H from the cathode 70 either. This is demonstrated by Fig. 3B, which shows that the magnetic field H in this second embodiment is unchanged from the magnetic field in the first embodiment shown in Fig. 2B.

[0081] Fig. 4A schematically shows a plan view of a third embodiment of an electrolytic cell 100. The electrolytic cell 100 of this third embodiment again comprises all the same components as the electrolytic cell 100 of the first embodiment described above in relation to Fig. 2A, but in this case, the cathode 70 instead consists of an electrically conducting material having a magnetic susceptibility with an absolute value of less than 102and the electrolytic cell 100 further comprises a permanent magnet 20 located outside the vessel 102 adjacent to the cathode 70 inside the vessel 102, on an opposite side of the cathode 70 from the anode 10, and magnetized as a source of the magnetic field H. Like the cathode 70 in the first and second embodiments, the permanent magnet 20 is magnetized with a simple dipole field having its north pole on a surface of the permanent magnet 20 facing the anode 10 and its south pole on the opposite surface of the permanent magnet 20. Again, however, the magnet 20 could equally well be magnetized with opposite polarity to have its south pole facing the anode 10 and its north pole on the opposite surface of the magnet 20 instead. In this embodiment, therefore, the dipole field emerging from the surface of the permanent magnet 20 facing the anode 10 again provides a magnetic field having a gradient with a positive component in a direction from the anode 10 to the cathode 70, such that the strength of the magnetic field increases from the anode 10 to the cathode 70. As may be seen from Fig. 4A, nothing apart from the vessel 102 is interposed between the magnet 20 outside the vessel and the cathode 70 inside the vessel, so that the magnet 20 and the cathode 70 are only separated from each other by just slightly more than the thickness of vessel 102. Since Fig. 4A is schematic only and is not drawn to scale, this is equal to only a few millimetres. In consequence, in this embodiment, the permanent magnet 20 also has a Curie temperature of at least 600 K, which allows it to retain at least some of its magnetization, despite heat being transferred to it from inside the vessel 102.

[0082] Fig. 4B schematically represents the strength of this magnetic field H in a direction x from the permanent magnet 20 to the anode 10. As before, the exact shape of the graph in Fig. 4B is arbitrary. However, as may be seen by comparing Fig. 4B with Figs. 2B and 3B, the magnetic field H in this case now has its maximum strength at the surface of the permanent magnet 20 rather than at the surface of the cathode 70. Nonetheless, because the permanent magnet 20 is located adjacent to the cathode 70, on an opposite side of the cathode 70 from the anode 10, the magnetic field still diminishes in a direction x towards the anode 10. Thus for electrolysis products formed between the anode 10 and the cathode 70, a paramagnetic electrolysis product is still attracted to the cathode 70, and a diamagnetic electrolysis product is still repelled from the cathode 70, according to their distance x from the cathode 70, with the greatest attraction or repulsion being at the surface of the cathode 70 facing the anode 10.

[0083] Fig. 5A schematically shows a plan view of a fourth embodiment of an electrolytic cell 100. The electrolytic cell 100 of this fourth embodiment comprises all the same components as the electrolytic cell 100 of the third embodiment described above in relation to Fig. 4A, but like the second embodiment of Fig. 3A, the present embodiment further comprises a separator 75 of the electrolysis products, which is located between the anode 10 and the cathode 70, for immersion in the molten electrolyte and which touches both the anode 10 and the cathode 70. As in the second embodiment, the separator 75 in this fourth embodiment also consists of material having a magnetic susceptibility with an absolute value of less than 10’2, so that the separator 75 has a negligible effect on the magnetic field H from the permanent magnet 20 located outside the vessel 102. This may be seen by noting that the magnetic field H in this fourth embodiment, which is shown in Fig. 5B, is unchanged from the magnetic field of the third embodiment shown in Fig. 4B. In addition, the anode-cathode distance in this fourth embodiment is significantly reduced in comparison to that of the third embodiment, thereby reducing the internal resistance of the cell. Nonetheless, the strength of the magnetic field at the surface of the anode 10 remains low because the permanent magnet 20 is now located outside the vessel 102 adjacent to the cathode 70. However, the material of which the separator 75 consists has an electrical resistivity of more than 108Qm, which makes the separator 75 an electrical insulator. Accordingly, the separator 75 does not create an electrical short circuit between the anode 10 and the cathode 70 even though it touches both of them. Whereas the cathode 70 in the embodiments of Figs. 2A and 3A and the permanent magnet 20 in the embodiments of Figs. 4A and 5A are all magnetized with a simple dipole field, in other possible embodiments, either the cathode 70 or the permanent magnet 20 could instead by magnetized as a Halbach array, arranged to concentrate the magnetic field on a surface of either the cathode 70 or of the permanent magnet 20, respectively, which faces the anode 10. If so, for ease of manufacture, distinct components of the Halbach array may each be magnetized separately and then assembled together to form the Halbach array, in a manner well known from the prior art.

[0084] For the sake of example and to help illustrate the principles of the invention, all the embodiments of Figs. 2A to 5A comprise an anode 10 and a cathode 70 which are both planar and are arranged in parallel to each other inside the vessel 102. However, in other possible embodiments, the anode and cathode may have a different geometry which still allows the same variation in the strength of the magnetic field between the anode and cathode. In particular, the cathode 70 may be rotationally symmetrical about a central axis A, and the magnetic field H may be a quadrupole field or a field of higher even polarity distributed evenly about the central axis A. Figs. 6A, 6B and 6C, respectively, therefore schematically show quadrupole, sextupole and octupole magnetic fields inside embodiments of hollow cathodes. In each of the cases illustrated in Figs. 6A, 6B and 6C, the hollow cathode has tetragonal symmetry ( / '.e., rotational symmetry of order 4), which gives it a square cross-section. In these figures, the magnetic field is shown as a vector field, in which the lengths of the arrows represent the strength of the magnetic field at the location of each arrow, the directions of the arrows represent the direction of the magnetic field from north to south poles thereof at the location of each arrow and a dotted open circle labelled "A” represents the central axis of each hollow cathode. It may therefore be seen that the strength of the magnetic field increases in a radial direction outwardly from the central axis A of the hollow cathode in each case. Thus if a rotationally symmetrical anode is located inside and coaxial with the hollow cathode, the magnetic field will diminish in strength in a radial direction towards the anode, and reaches zero along the central axis A. (In contrast to a quadrupole field or a field of higher even polarity, a dipole field inside such a hollow cathode is substantially uniform and has no magnetic field gradient along a line between the two poles if the field at each pole is of equal strength.)

[0085] In each of the embodiments represented in Figs. 6A, 6B and 6C, the source of the magnetic field may either be a plurality of permanent magnets located outside the vessel of the electrolytic cell or it may be provided by permanently magnetizing the cathode itself. In the former case, the source of the magnetic field may, for example, be an even number of dipole permanent magnets equal in number to the number of poles of the field and arranged around the outside of the cathode with alternating north and south poles facing inwardly towards the central axis A. Alternatively, in the latter case, the cathode may be assembled from separately magnetized planar components. For example, if the magnetic field is a quadrupole field, four planar cathode components may each be magnetized with a simple dipole field in the same manner as the cathode 70 in Figs. 2A and 3A, and then arranged with alternating north and south poles facing inwardly towards the central axis A, to create a hollow cathode with tetragonal symmetry. Fig. 7 shows another embodiment of a rotationally symmetrical hollow cathode 70, wherein the cathode 70 comprises a hollow cylinder open at at least one end thereof. Like the cathodes represented in Figs. 2A, 3A and 4A, Fig. 7 only shows that part of the cathode 70 for immersion in the molten electrolyte, which is the hollow cylinder of the cathode 70. The hollow cylinder of the cathode 70 is therefore cylindrically symmetrical, which may be considered a special case of rotational symmetry about the central axis, which is again represented in Fig. 7 by a dotted open circle labelled "A”. In Fig. 7, as in Figs. 6A, 6B and 6C, the magnetic field is a quadrupole field or a field of higher even polarity distributed evenly about the central axis A. Furthermore, the magnetic poles of this magnetic field emerge from an inner curved surface 71 of the hollow cylinder 70. Thus, the directions of the arrows in Fig. 7 represent the directions in which the strength of the magnetic field increases radially outwardly at the location of each arrow, which is the same as the direction in which the strength of the magnetic field increases in each of Figs. 6A, 6B and 6C. Thus if an anode comprising a cylinder is located with the cylinder of the anode inside and concentric with the hollow cylinder of the cathode 70 in Fig. 7, the magnetic field will diminish in strength in a radial direction towards the anode, until it reaches zero along the central axis A. If so, the anode and cathode

[0086] 70 will have the same geometry as in the Downs cell of Fig. 1 , but with a rotationally symmetrical magnetic field, like those in Figs. 6A, 6B and 6C, superimposed thereon, with the strength of the magnetic field increasing outwardly from the anode to the cathode.

[0087] In Fig. 7, as in Figs. 6A, 6B and 6C, the source of the magnetic field may either be a plurality of permanent magnets located outside the vessel of the electrolytic cell or it may be provided by permanently magnetizing the cathode itself. In either case, the most convenient way in which to create the magnetic field is for the source of the magnetic field to be magnetized as a Halbach array arranged to concentrate the magnetic field on the inner curved surface

[0088] 71 of the hollow cylinder of the cathode 70. For ease of manufacture, distinct components of the Halbach array may each be magnetized separately and then assembled together to form the Halbach array, in a manner well known from the prior art.

[0089] Figs. 8A and 8B, respectively, schematically show quadrupole and sextupole magnetic fields around the outside of embodiments of cathodes 70 both comprising hollow cylinders, which are therefore cylindrically symmetrical about a central axis, again represented in each case by a dotted open circle labelled "A”. (A dipole field of radial orientation around the outside of a cylindrically symmetrical cathode is substantially the same as that around a simple bar magnet, so is not reproduced here.) In both of the cases represented in Figs. 8A and 8B, the magnetic poles of the magnetic field emerge from an outer curved surface 72 of the cylinder of the cathode 70. Thus in Figs. 8A and 8B, the curved lines represent magnetic field lines between adjacent north and south poles, and the density of the curved lines represents the strength of the magnetic field. It may therefore be seen that the strength of the magnetic field increases in a radial direction inwardly towards the outer curved surface 72 of the cylinder of the cathode 70 in both cases. Thus if a rotationally symmetrical hollow anode is located around and coaxial with the cylinder of the cathode 70, the magnetic field will increase in strength in a radial direction from the anode to the cathode. This is represented by the direction of the arrows in Fig. 9, in which the centrally located cathode 70 instead comprises a solid, rather than a hollow cylinder. If so, the anode and cathode 70 will then have the same geometry as in an electrolytic cell of the type shown in US patent no. 452 030 of Castner, but with a rotationally symmetrical magnetic field, like those in Figs. 8A and 8B, superimposed thereon, with the strength of the magnetic field increasing inwardly from the anode towards the cathode.

[0090] Fig. 9 only differs from Figs. 8A and 8B in that in Fig. 9, the cathode 70 comprises a solid cylinder, whereas in both Figs. 8A and 8B, the cathode 70 instead comprises a hollow cylinder. The hollow cylinder of the cathode 70 in Figs. 8A and 8B allows the vessel of the electrolytic cell to be provided with an axial borehole inside the hollow cylinder of the cathode 70, in which a permanent magnet may be located outside the vessel as the source of the magnetic field, if the cathode 70 consists of an electrically conducting material having a magnetic susceptibility with an absolute value of less than 10’2. Alternatively, the solid cylinder of the cathode 70 in the embodiment of Fig. 9 may itself be magnetized as the source of the magnetic field instead. In either case, the most convenient way in which to create the magnetic field is for the source of the magnetic field to be magnetized as a Halbach array arranged to concentrate the magnetic field on the outer curved surface 72 of the cylinder of the cathode 70. For ease of manufacture, distinct components of the Halbach array may each be magnetized separately and then assembled together to form the array, in a manner well known from the prior art.

[0091] Fig. 10A schematically shows an embodiment of a cathode 70 having the same geometry as in Fig. 7 with an anode 10 which comprises a solid cylinder located inside and concentric with the hollow cylinder of the cathode 70, and Fig. 10B schematically shows an embodiment of a cathode 70 having the same geometry as in Fig. 9 with an anode 10 which comprises a hollow cylinder open at at least one end thereof which is located around and concentric with the cylinder of the cathode 70. In both embodiments, the anode 10 and the cathode 70 are placed in rotation relative to each other about the central axis A, which is represented in each of Figs. 10A and 10B by a pair of clockwise and anticlockwise arrows. However, only one of the anode 10 and the cathode 70 needs to be put in rotation relative to the surroundings of the electrolytic cell to achieve this relative rotation of the anode 10 and the cathode 70. Moreover, the handedness of the relative rotation ( / '.e., whether it is clockwise or anticlockwise) is immaterial to the time-averaging effect of the relative rotation, and may therefore be chosen, for example, to minimise energy consumption.

[0092] Sometimes, a plurality of similar electrolytic cells according to the invention may be placed in close proximity to each other, to increase the total amounts of the electrolysis products produced. If so, care should be taken to avoid the magnetic field associated with one cell from adversely affecting the performance of another, nearby cell. This applies regardless of whether the source of the magnetic field in each cell is a permanent magnet located outside the vessel of each cell or whether it is instead provided by permanently magnetizing the cathode inside each vessel. In either case, such an undesirable effect may be avoided in one of several different ways, as follows. Firstly, for example, each of the cells may be given a geometry which locates the source of the magnetic field associated with each cell near to or at the centre of each cell, such that the strength of the magnetic field diminishes to zero outside the periphery of each cell and there is negligible overlap between the magnetic fields of adjacent cells. This would apply, for example, to a cell with a geometry similar to that of the cell shown in US patent no. located near the periphery of each cell, for example if each cell has a geometry similar to that of the Downs cell of Fig. 1 or to those of the embodiments of Figs. 2A to 7 and 10A.

[0093] If so and if the cathode in each cell comprises a ferromagnetic material which is permanently magnetized as the source of the magnetic field in each cell, adverse magnetic effects on nearby cells may be avoided by arranging adjacent cells with the cathode inside a first one of the adjacent cells located with a first magnetic pole thereof adjacent to another magnetic pole of opposite polarity which belongs to the cathode inside a second one of the adjacent cells. Thus, for example, if the cathode inside the first cell is magnetized with a simple dipole field having its north pole facing the anode inside the first cell and therefore with its south pole facing the exterior of the first cell (as in the embodiments of Figs. 2A and 3A), then the adjacent, second cell should be arranged such that the cathode inside the second cell is also magnetized with a simple dipole field, but with the south pole thereof facing the anode inside the second cell and with the north pole thereof, which therefore faces the exterior of the second cell, adjacent to the south pole of the cathode inside the first cell.

[0094] Such an arrangement is represented in Fig. 11A, which schematically shows a first embodiment of a series 1 r of electrolytic cells, which comprises two adjacent cells 100a, 100b. As in previous drawings which depict other embodiments of the invention, only those parts of the electrolytic cells 100a, 100b for immersion in molten electrolyte are shown in Fig. 11 A. In the embodiment of Fig. 11A, each of the cells 100a, 100b comprises a respective vessel 102a, 102b, and inside each of the vessels 102a, 102b there is a respective anode 10a, 10b and two cathodes 70a, 70b and 70c, 70d, both of which are magnetized with a dipole field. The cathodes 70a, 70b inside the vessel 102a of a first one 100a of the cells both have their south poles facing the anode 10a, whereas the cathodes 70c, 70d inside the vessel 102b of a second one 100b of the cells both have their north poles facing the anode 10b. However, as may be seen from Fig. 11 A, the cathode 70b inside the first cell 100a therefore has its north pole adjacent to and facing the south pole of the cathode 70c inside the second cell 100b. Thus the magnetic fields from the cathodes 70b, 70c of the adjacent cells 100a, 100b complement each other, and the strength of the magnetic field in each cell may even be increased slightly by the poles of opposite polarity on the respective cathodes 70b, 70c being adjacent to and facing each other. The same pattern of adjacent poles of opposite polarity facing each other between adjacent cells may be repeated indefinitely along the series 1 r to achieve the same result.

[0095] Fig. 11 B schematically represents the strength of the magnetic field H in a direction x along the series 1 r of the electrolytic cells 100a, 100b, ... in Fig. 11A. The exact shape of the graph in Fig. 11 B is again arbitrary, but as may be seen from Fig. 11 B, the magnetic field H has its maximum strength at the surfaces of the cathodes 70a, 70b and 70c, 70d, and diminishes in strength towards the anode 10a, 10b, ... inside each vessel 102a, 102b, ... . Thus a paramagnetic electrolysis product is attracted to both cathodes 70a, 70b and 70c, 70d in each respective one of the vessels 102a, 102b, and a diamagnetic electrolysis product is repelled from both cathodes 70a, 70b and 70c, 70d in each respective one of the vessels 102a, 102b, according to the distances of the electrolysis products along the x axis from their nearest cathode, with the greatest attraction or repulsion being at the surface of each cathode 70a, 70b and 70c, 70d facing the anode 10a, 10b inside each vessel.

[0096] Alternatively, however, if the source of the magnetic field in each cell is a permanent magnet located outside the vessel of each cell, then a single permanent magnet may be used to provide the magnetic fields of two adjacent cells, as follows. In comparison to the embodiment of Fig. 11 A, this has the advantage of using half as many magnetized components per cell. Fig. 12A, therefore, schematically shows a second embodiment of a series 1s of electrolytic cells, in which a single permanent magnet 20b provides the magnetic fields of two adjacent cells 100a, 100b. As in Fig. 11 A, only those parts of the electrolytic cells 100a, 100b for immersion in the molten electrolyte are shown in Fig. 12A. In the embodiment of Fig. 12A, the permanent magnet 20b is arranged with its north magnetic pole adjacent to the cathode 70b of a first one 100a of the cells and with its south magnetic pole adjacent to the cathode 70c of another one 100b of the cells. Like the embodiment of Fig. 11 A, each of the cells 100a, 100b in the embodiment of Fig. 12A respectively comprises two cathodes 70a, 70b and 70c, 70d, such that the same pattern of alternating permanent magnets 20a, 20b, 20c, ... and electrolytic cell vessels 102a, 102b, ... may be repeated indefinitely along the series 1s. Nonetheless, as may be seen from Fig. 12A, each of the electrolytic cells 100a, 100b, ... in the series 1s still comprises a respective permanent magnet 20a, 20b, 20c, ... .

[0097] Fig. 12B schematically represents the strength of the magnetic field H in a direction x along the series 1s of the electrolytic cells 100a, 100b, ... in Fig. 12A. The exact shape of the graph in Fig. 12B is again arbitrary, but as may be seen from Fig. 12B, the magnetic field H has its maximum strength at the surfaces of the permanent magnets 20a, 20b, 20c, ... and diminishes towards the anode 10a, 10b, ... inside each vessel 102a, 102b, ... . Thus paramagnetic and diamagnetic electrolysis products are respectively attracted to and repelled from the cathodes inside each cell, according to the distances of the electrolysis products along the x axis from their nearest cathode.

[0098] The electrolytic cells 100a, 100b in the embodiment of Fig. 11 A may be in thermal contact with each other or not, depending, for example, on the separation between the vessels 102a, 102b and on whether or not one or both of the cells 100a, 100b comprises a thermal management system, such as those mentioned above. However, in the embodiment of Fig. 12A, since the permanent magnet 20b is preferably located as close as possible to the cathodes 70b, 70c, to maximise the strength of the magnetic field on the cathodes 70b, 70c, the respective interiors of the vessels 102a, 102b are likely to be in thermal contact with each other via the permanent magnet 20b. Thus in this embodiment, the permanent magnet 20b has a Curie temperature of at least 600 K, which allows it to retain at least some of its magnetization, despite heat being transferred to it from inside one or both of the vessels 102a, 102b.

[0099] For the sake of example, the embodiments of Figs. 11A and 12A are both one-dimensional series of electrolytic cells extending in the direction of the x axis in each case. However, in other possible embodiments, each of the electrolytic cells may comprise not just two cathodes perpendicular to the x axis, but a further two cathodes parallel to the x axis and positioned relative thereto inside the vessel of each cell to create a rectangle of cathodes with an anode having a rectangular cross-section located therein at a constant anode-cathode distance when immersed in the molten electrolyte, and wherein the magnetic field associated with each cell is a quadrupole field comparable to that described above in relation to Fig. 6A. In the case of the embodiments of both Figs. 11A and 12A, the onedimensional series of electrolytic cells may then be extended into a two-dimensional rectangular array by arranging further such series of electrolytic cells in parallel with each other, whilst observing the same principles as described above for alternating north and south poles between adjacent cells in a direction perpendicular to the x axis, so that the magnetic fields associated with adjacent cells complement each other.

[0100] Fig. 13, therefore, schematically shows a first embodiment of a plurality of electrolytic cells 100c, 100d, 100e, 100f arranged in a square array 1 q, which may be considered a special case of such a rectangular array. As before, only those parts of the electrolytic cells 100c - 10Of for immersion in the molten electrolyte are shown in Fig. 13. In each of the electrolytic cells 100c - 10Of, the cathode comprises a hollow prism open at at least one end thereof, which is tetragonally symmetrical about a central axis, giving the cathode a square cross-section, and a square anode is located inside and coaxial with the hollow prism of the cathode, with the same orientation as the cathode, so that the anode-cathode distance is constant. In each of the cells 100c - 10Of, both the anode and the cathode are rounded off at their edges parallel to their common axis, to minimise edge effects during electrolysis. Taking the electrolytic cell 100c as an example, it may be seen from Fig. 13 that the magnetic field within electrolytic cell 100c is provided by four planar permanent magnets 20d, 20e, 20f, 20g, which are arranged in a square outside the vessel 102c of electrolytic cell 100c. The permanent magnets 20d - 20g are each magnetized with a dipole field, which alternates in orientation around the cathode 70e of electrolytic cell 100c, so that the magnetic field inside the cathode 70e is a quadrupole field. Taking permanent magnet 20g as an example, it may also be seen that the dipole field of permanent magnet 20g contributes not only to the quadrupole field inside the cathode 70e of electrolytic cell 100c, but also to another quadrupole field inside the cathode 70f of the adjacent electrolytic cell 100f. Thus, as in the embodiment described above in relation to Fig. 12A, this single permanent magnet 20g contributes to the magnetic fields of the two adjacent electrolytic cells 100c, 10Of. This pattern repeats itself across the square array 1 q, which may be indefinite in extent, so that the total number of permanent magnets is halved, in comparison to providing each of the electrolytic cells with its own complement of four permanent magnets. Moreover, the arrangement of the permanent magnets each magnetized with a dipole field, which alternates in orientation around each cathode, ensures that the dipole fields act together to complete an array of magnetic circuits, represented in Fig. 13 by dashed lines, superimposed on the square array 1q of electrolytic cells 100c - 10Of in the manner shown.

[0101] Fig. 14 schematically shows a second embodiment of a plurality of electrolytic cells 100g, 10Oh, 10Oj, 100k instead arranged in a hexagonal array 1 h. As before, only those parts of the electrolytic cells 100g - 100k for immersion in the molten electrolyte are shown in Fig. 14. In each of the electrolytic cells 100g - 100k, the cathode comprises a hollow prism open at at least one end thereof, which is hexagonally symmetrical about a central axis, giving the cathode a hexagonal cross-section, and a hexagonal anode is located inside and coaxial with the hollow prism of the cathode, with the same orientation as the cathode, so that the anode-cathode distance is constant. Taking the electrolytic cell 100g as an example, it may be seen from Fig. 14 that the magnetic field within electrolytic cell 100g is provided by six planar permanent magnets 20, 20h, which are arranged in a hexagon outside the vessel 102g of electrolytic cell 100g. The permanent magnets 20, 20h are each magnetized with a dipole field, which alternates in orientation around the cathode 70g of electrolytic cell 100g, so that the magnetic field inside the cathode 70g is a sextupole field. Taking permanent magnet 20h as an example, it may also be seen that the dipole field of permanent magnet 20h contributes not only to the sextupole field inside the cathode 70g of electrolytic cell 100g, but also to another sextupole field inside the cathode 70h of the adjacent electrolytic cell 10Oh. Thus, as in the embodiments described above in relation to Figs. 12A and 13, this single permanent magnet 20h contributes to the magnetic fields of the two adjacent electrolytic cells 100g, 100h. This pattern repeats itself across the hexagonal array 1 h, which may be indefinite in extent, so that the total number of permanent magnets is halved, in comparison to providing each of the electrolytic cells with its own complement of six permanent magnets. Moreover, the arrangement of the permanent magnets each magnetized with a dipole field, which alternates in orientation around each cathode, ensures that the dipole fields act together to complete an array of magnetic circuits, represented in Fig. 14 by dashed lines, superimposed on the hexagonal array 1 h of electrolytic cells 100g - 100k in the manner shown.

[0102] Whereas in the embodiments of Figs. 13 and 14, the source of the magnetic field associated with each cell is the permanent magnets located outside the vessel of each cell, in other possible embodiments, the magnetic field associated with each cell may instead be provided by magnetizing the cathodes inside each cell as described previously.

[0103] Moreover, whereas in the embodiments of Figs. 13 and 14, both the cathode and the anode have prismatic crosssections, in other possible embodiments, the anode may instead be cylindrically symmetrical about the central axis of the cathode, so that the anode-cathode distance is not constant. However, in such embodiments, during use, the cylinder of the anode may rotate relative to the cathode about the central axis, so that the anode-cathode distance is averaged to a constant value over time. Fig. 15A, therefore, schematically shows a fifth embodiment of an electrolytic cell 100 in which the cathode 70 comprises a hollow prism open at at least one end thereof, which is tetragonally symmetrical about a central axis A, giving the cathode a square cross-section, and a cylindrical anode 10 is located inside and coaxial with the hollow prism of the cathode 70. In this embodiment, the anode 10 is in slow rotation about the central axis A as described previously, and is therefore also in rotation relative to the cathode 70. This relative rotation not only averages the magnetic field to which the anode is exposed over time, but also averages the anode-cathode distance over time. Such an embodiment of an electrolytic cell may be substituted for one or more of the electrolytic cells in the square array 1q of Fig. 13.

[0104] Similarly, Fig. 15B schematically shows a sixth embodiment of an electrolytic cell 100 in which the cathode 70 comprises a hollow prism open at at least one end thereof, which is hexagonally symmetrical about a central axis A, giving the cathode a hexagonal cross-section, and a cylindrical anode 10 is located inside and coaxial with the hollow prism of the cathode 70. In this embodiment, the anode 10 is in slow rotation about the central axis A as described above, and is therefore also in rotation relative to the cathode 70. This relative rotation not only averages the magnetic field to which the anode is exposed over time, but also averages the anode-cathode distance over time. Such an embodiment of an electrolytic cell may be substituted for one or more of the electrolytic cells in the hexagonal array 1h of Fig. 14. Whereas in the embodiments of Figs. 15A and 15B, the handedness of the rotation of the anode 10 is represented by a clockwise arrow in both cases, the handedness of the rotation is immaterial to its time-averaging effect and could equally well be anticlockwise instead.

[0105] Fig. 16 shows an embodiment of use 90 of an electrolytic cell according to the invention, which can be employed as a decision tree to help select suitable anode and cathode materials depending on the electrolysis temperature. The use 90 comprises electrolysing 91 an inorganic compound in its molten state, where the inorganic compound is selected from the group consisting of a halide of an alkali metal, a halide of an alkaline earth metal except beryllium, and a hydroxide of an alkali metal. If the electrolysis temperature is less than the Curie temperature of nickel, Tc (Ni) = 354 “Celsius, as is the case, for example, when electrolysing sodium hydroxide by the Castner process, then using a nickel anode is ruled out. In this case, Fe64Ni36, which has a lower Curie temperature, Tc (Fe64Ni36) = 230 “Celsius, may therefore be used as an anode material instead. If the magnetic field is provided by a permanent magnet located outside the vessel, then Fe64Ni36 may also be used as a cathode material, whereas if the magnetic field is instead provided by magnetizing the cathode itself, then a cathode material of lowest electrical resistivity is selected, in order to maximise the energy efficiency of the cell, subject to the requirement that the cathode material is also resistant to corrosion by the molten electrolyte.

[0106] If, on the other hand, the electrolysis temperature is more than about the melting point of magnesium, Tm(Mg) = 650 “Celsius, as is the case, for example, when electrolysing magnesium chloride, then using a permanently magnetized cathode is ruled out. If so, the magnetic field is provided by a permanent magnet located outside the vessel. In such a case or if the electrolysis temperature lies between the melting point of magnesium and the Curie temperature of nickel, then an anode made either of nickel or of amorphous, glassy or graphitic carbon may be used, and a cathode material of lowest electrical resistivity is selected, in order to maximise the energy efficiency of the cell, again subject to the requirement that the cathode material is also resistant to corrosion by the molten electrolyte.

[0107] Whereas the present invention has been described above by reference to particular examples and embodiments, the scope of the invention should not be taken to be limited thereby and is instead defined by the appended claims.

Claims

AMENDED CLAIMSreceived by the International Bureau on 16 May 2026 (16.05.2026).Claims1. An electrolytic cell (100; 100a - 100b; 100c - 100f; 100g - 100k) comprising:a vessel (102) for containing a molten electrolyte;an anode (10) inside the vessel;a cathode (70) inside the vessel; anda permanent source (20, 70) of a magnetic field (H) having a gradient with a positive component in a direction from the anode (10) to the cathode (70), such that the strength of the magnetic field increases from the anode (10) to the cathode (70);wherein the rest of the electrolytic cell, apart from the source (20, 70) of the magnetic field (H), including at least the vessel (102) and the anode (10), consists of material having a magnetic susceptibility with an absolute value of less than 102.

2. An electrolytic cell according to claim 1 , further comprising a separator (75) of electrolysis products located between the anode (10) and the cathode (70) for immersion in the molten electrolyte and which is porous to the molten electrolyte, wherein the separator (75) consists of material having a magnetic susceptibility with an absolute value of less than 102.

3. An electrolytic cell according to claim 2, wherein the material of the separator (75) has an electrical resistivity of at least 108Qm and the separator (75) is touching at least one of the anode (10) and the cathode (70).

4. An electrolytic cell according to any one of the preceding claims, wherein the cathode (70) consists of an electrically conducting material having a magnetic susceptibility with an absolute value of less than 102and the electrolytic cell further comprises a permanent magnet (20) located outside the vessel (102) adjacent to the cathode (70) inside the vessel (102), on an opposite side of the cathode (70) from the anode (10), and the magnetization of which provides the source of the magnetic field (H).

5. An electrolytic cell according to claim 4, wherein the permanent magnet (20) has a Curie temperature of at least 600 K.

6. An electrolytic cell according to any one of claims 1 to 3, wherein the cathode (70) comprises an electrically conducting ferromagnetic material having a coercivity of at least 1000 Am1and a Curie temperature of at least 600 K, and the ferromagnetic material has a permanent magnetization which provides the source of the magnetic field (H).

267. An electrolytic cell according to any one of the preceding claims, wherein the source (20, 70) of the magnetic field (H) has the magnetization of a Halbach array arranged to concentrate the magnetic field (H) on a surface of the source (20, 70) facing the anode (10).

8. An electrolytic cell according to any one of the preceding claims, wherein:the cathode (70) is rotationally symmetrical about a central axis (A); andthe magnetic field (H) is a quadrupole field or a field of higher even polarity distributed evenly about the central axis (A).

9. An electrolytic cell according to claim 8, wherein the cathode (70) comprises a hollow cylinder open at at least one end thereof; andat least one magnetic pole of the magnetic field (H) emerges from an inner curved surface (71) of the hollow cylinder.

10. An electrolytic cell according to claim 9, wherein the anode (10) comprises a cylinder; andthe cylinder of the anode (10) is located inside and concentric with the hollow cylinder of the cathode (70).

11. An electrolytic cell according to claim 8, wherein the cathode (70) comprises a cylinder; andat least one magnetic pole of the magnetic field (H) emerges from an outer curved surface (72) of the cylinder of the cathode (70).

12. An electrolytic cell according to claim 11, wherein the anode (10) comprises a hollow cylinder open at at least one end thereof; andthe hollow cylinder of the anode (10) is located around and concentric with the cylinder of the cathode (70).

13. Use of an electrolytic cell according to claim 10 or claim 12, wherein the use comprises rotating the anode (10) and the cathode (70) relative to each other about the central axis (A).

14. A plurality (1s, 1q, 1h) of electrolytic cells (100a - 100b; 100c - 100f; 100g - 100k) each according to claim 4, wherein at least a respective one (20b; 20g; 20h) of the permanent magnets is arranged with a first magnetic pole thereof adjacent to the cathode (70b; 70e; 70g) inside a first one (102a; 102c; 102g) of the vessels and with another magnetic pole thereof of opposite polarity to the first magnetic pole adjacent to the cathode (70c; 70f; 70h) inside another one (102b; 102f; 102h) of the vessels.

15. A plurality (1s, 1q, 1h) of electrolytic cells according to claim 14, wherein the respective one (20b; 20g;102c; 102g) of the vessels is in thermal contact with an interior of the other one (102b; 102f; 10Oh) of the vessels via the respective one (20b; 20g; 20h) of the permanent magnets.

16. A plurality of electrolytic cells (100c - 10Of; 100g - 100k) each according to claim 14 or claim 15, wherein in each of the electrolytic cells (100c - 10Of; 100g - 100k):the cathode (70) comprises a hollow prism open at at least one end thereof which is tetragonally or hexagonally symmetrical about a central axis (A);the anode (10) is located inside and coaxial with the hollow prism of the cathode (70);the magnetic field (H) inside the cathode (70) is a quadrupole field or a sextupole field, respectively, distributed evenly about the central axis (A); andthe plurality of electrolytic cells (100c - 10Of; 100g - 100k) is arranged in a square or hexagonal array (1q, 1h), respectively.

17. Use of a plurality (1 q, 1h) of electrolytic cells according to claim 16, wherein the anode (10) comprises a cylinder which is cylindrically symmetrical about the central axis (A); andthe use comprises rotating the cylinder of the anode (10) relative to the cathode (70) about the central axis (A).

18. Use (90) of an electrolytic cell according to any one of claims 1 to 12 to electrolyse (91) an inorganic compound selected from the group consisting of:a halide of an alkali metal;a halide of an alkaline earth metal except beryllium; anda hydroxide of an alkali metal,when the inorganic compound is in its molten state.

19. Use of an electrolytic cell according to claim 18, wherein the cathode (70) comprises an electrically conducting ferromagnetic material having a coercivity of at least 1000 Am1and a Curie temperature of at least 600 K, and the ferromagnetic material has a permanent magnetization which provides the source of the magnetic field (H), wherein the use comprises electrolysing (91) the inorganic compound at an absolute temperature of not more than 85% of the Curie temperature of the ferromagnetic material.

20. Use of an electrolytic cell according to claim 18 or claim 19, wherein:the alkali metal is selected from the group consisting of lithium, sodium and potassium;the alkaline earth metal is magnesium; andthe halide is a chloride.

21. Use of an electrolytic cell according to claim 18 to electrolyse (91) molten magnesium chloride, wherein the electrolytic cell comprises a permanent magnet (20) located outside the vessel (102) adjacent to the cathode (70) inside the vessel (102), on an opposite side of the cathode (70) from the anode (10), and the magnetization of which provides the source of the magnetic field (H), and the cathode consists of a material selected from the group consisting of:nickel;nickel alloy 301; andan iron-nickel alloy having a Curie temperature at least 10 K below the temperature of the molten electrolyte.

22. Use of an electrolytic cell according to claim 18 or claim 19, wherein the inorganic compound is sodium hydroxide and the anode consists of an iron-nickel alloy having a Curie temperature at least 10 K below the temperature of the molten electrolyte.

23. Use of an electrolytic cell according to claim 22, wherein the electrolytic cell comprises a permanent magnet (20) located outside the vessel (102) adjacent to the cathode (70) inside the vessel (102), on an opposite side of the cathode (70) from the anode (10), and the magnetization of which provides the source of the magnetic field (H), and the cathode consists of an iron-nickel alloy having a Curie temperature at least 10 K below the temperature of the molten electrolyte.

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