Method and apparatus for electrochemical reduction of a solid feedstock

Abstract

The method, apparatus and product relate to the electrochemical reduction of a solid feedstock (20) to produce a product. A container (2) is filled with a fused salt (6), and one or more anodes (14) contact the fused salt. A cathode (18) is loaded with feedstock and engages with a transport apparatus (22, 36, 40) which locates and moves the cathode past the anodes(s), while the cathode and the feedstock contact the fused salt. As the cathode moves past the anodes(s), a voltage applied between the cathode and the anode(s) electrochemically reduces the solid feedstock to form the product.

Claims

1. An apparatus for electrochemical reduction of a solid feedstock to form a solid product, comprising; a container for a fused salt; an anode assembly comprising one or more anodes assembled with an anode support in which, during operation of the apparatus, the one or more anodes contact the fused salt; a cathode loadable with the solid feedstock at a feedstock loading station before immersion in the fused salt, the cathode comprising an electrically-conductive, horizontally-oriented tray for carrying the solid feedstock; and a cathode transport apparatus configured for locating and moving the cathode so that, in use, the cathode and the solid feedstock contact the fused salt; the cathode transport apparatus configured for moving the cathode from a loading position to an unloading position, in which the loading position is spaced from the unloading position, the anode assembly being positioned between the loading position and the unloading position; in which the cathode transport apparatus is configured to lower the cathode, carrying the solid feedstock, into the container and into the fused salt at the loading position, is also configured to then move the cathode past the anode assembly, below the anode assembly, and is also configured to then raise the cathode, carrying the solid product, out of the container and out of the fused salt at the unloading position; the apparatus being couplable to a power supply for applying a potential between the one or more anodes and the cathode such that the solid feedstock loaded on the cathode is reduced to form the solid product as the cathode transport apparatus moves the cathode past the anode assembly.

2. The apparatus according to claim 1, in which the cathode transport apparatus raises the cathode out of the container at an unloading position into a vessel containing an inert atmosphere.

3. The apparatus according to claim 1, in which the container comprises a base and the cathode transport apparatus moves the cathode between the anode assembly and the base of the container, and in which optionally either the cathode or a cathode assembly comprising the cathode and a cathode support contacts the base of the container.

4. The apparatus according to claim 1, in which the anode assembly comprises an array of two or more carbon anodes which are spaced from each other in a horizontal direction.

5. The apparatus according to claim 1, in which the position of the anode or each anode is adjustable to control the spacing between the anode or each anode and the cathode.

6. The apparatus according to claim 1, in which the container comprises a side wall, and an opening is defined between the side wall and the anode assembly, and in which the cathode transport apparatus comprises a cathode support for supporting the cathode such that, when the cathode is positioned in the container for electrochemical reduction of the feedstock, the cathode support extends through the opening and an upper end of the cathode support extends out of the fused salt.

7. The apparatus according to claim 6, in which the side wall is one of two parallel side walls, and the opening is one of two openings, each defined between a respective one of the side walls and the anode assembly, and in which the cathode support is one of two cathode supports for supporting the cathode, each extending through a respective one of the openings during electrochemical reduction.

8. The apparatus according to claim 7, in which at least one of the cathode supports is electrically conductive, for the application of a cathodic potential to the cathode.

9. The apparatus according to claim 6, in which the cathode transport apparatus comprises a drive apparatus for engaging with the cathode support so as to move the cathode support along the opening between the side wall and the anode assembly, and to move the cathode past the anode assembly.

10. The apparatus according to claim 9, in which the drive apparatus comprises a cathode support rail extending along a side of the opening between the loading position and the unloading position, adjacent to the side wall, and in which the cathode support engages with the rail to locate the cathode in position.

11. The apparatus according to claim 10, in which the cathode support and the rail are electrically conductive and are in electrical contact with each other, and in which a cathodic potential is applied to the cathode by supplying a voltage to the electrically-conductive rail.

12. A method for electrochemical reduction of a solid feedstock, comprising the steps of; providing an apparatus of claim 1; loading the cathode with a solid feedstock; and moving the cathode past the anode assembly while passing a current between the cathode and the one or more anodes so as to reduce the feedstock.

13. A method for converting an aluminium production cell that uses the Hall-Hroult process into a cell for reduction of a solid feedstock by electrolysis in a fused salt that uses an apparatus of claim 1, comprising the steps of removing anodes adjacent to each end of the cell in order to allow space for a loading position for loading into the cell cathodes carrying the solid feedstock, and an unloading position for removing cathodes carrying reduced feedstock, and installing a cathode transport apparatus for moving the cathodes from the loading position to the unloading position past the remaining anodes of the cell.

14. The apparatus according to claim 11, wherein the cathode support and its respective rail are in electrical contact with each other by means of a sliding contact.

15. The apparatus according to claim 6, in which the cathode support is electrically conductive, for the application of a cathodic potential to the cathode.

16. The apparatus according to claim 1, in which the cathode is inert in the presence of the feedstock, the product and the fused salt.

17. The apparatus according to claim 7, in which the cathode transport apparatus comprises a drive apparatus for engaging with the cathode supports so as to move the cathode supports along the openings between the side walls and the anode assembly, and to move the cathode past the anode assembly.

18. The apparatus according to claim 17, in which the drive apparatus comprises a cathode support rail extending along a side of each opening between the loading position and the unloading position, adjacent to the respective side wall beside each opening, and in which each cathode support engages with a respective rail to locate the cathode in position.

19. The apparatus according to claim 18, in which at least one of the cathode supports and its respective rail are electrically conductive and are in electrical contact with each other, and in which a cathodic potential is applied to the cathode by supplying a voltage to the electrically-conductive rail.

20. The apparatus according to claim 19, wherein at least one of the cathode supports and its respective rail are in electrical contact with each other by means of a sliding contact.

21. The apparatus according to claim 6, in which the cathode transport apparatus comprises a drive apparatus for engaging with the cathode support so as to move the cathode support along the opening between the side wall and the anode assembly, and to move the cathode past the anode assembly from the loading position to the unloading position.

22. The apparatus according to claim 7, in which the cathode transport apparatus comprises a drive apparatus for engaging with the cathode supports so as to move the cathode supports along the openings between the side walls and the anode assembly, and to move the cathode past the anode assembly from the loading position to the unloading position.

Description

SPECIFIC EMBODIMENTS AND BEST MODE OF THE INVENTION

(1) Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic side view of an electro-reduction apparatus according to a first embodiment of the invention, with a side wall of the fused-salt container removed to show the structure inside the container;

(3) FIG. 2 is a transverse section through the apparatus of FIG. 1;

(4) FIG. 3 is a three-quarter view of a loaded cathode and two cathode supports of the embodiment of FIGS. 1 and 2;

(5) FIG. 4 is a transverse section of an apparatus according to a second embodiment of the invention;

(6) FIG. 5 is a schematic plan view of the electro-reduction apparatus of the first embodiment;

(7) FIG. 6 is a three-quarter view of a cathode, loaded with feedstock;

(8) FIG. 7 is a schematic transverse section of an electro-reduction apparatus according to a third embodiment of the invention, illustrating side-loading of a cathode;

(9) FIG. 8 is a transverse section of a conventional aluminium smelter cell;

(10) FIG. 9 is a plan view of the anode arrangement in a conventional aluminium smelter cell;

(11) FIG. 10 is a plan view of the aluminium smelter cell of FIG. 9, with the end anodes removed;

(12) FIG. 11 is a plan view of an aluminium smelter cell showing a frame for supporting the anodes;

(13) FIG. 12 is a plan view of an aluminium smelter pot-room, with cells arranged end-to-end;

(14) FIG. 13 is a plan view of an aluminium smelter pot-room with cells arranged side-by-side; and

(15) FIG. 14 is a plan view of the aluminium smelter pot-room of FIG. 13 modified for continuous solid-phase feedstock electro-reduction according to an embodiment of the invention.

(16) FIGS. 1 and 2 show longitudinal and transverse sections of an electro-reduction apparatus according to a first embodiment of the invention. FIG. 5 shows a plan view of the apparatus. The apparatus comprises a container 2 comprising a base 4 and a peripheral wall extending upwardly from the base for containing a fused salt 6. The container is rectangular in plan, the peripheral wall comprising two parallel side walls 8 and two parallel ends walls 10. The length of the cell between the end walls is greater than its width between the side walls.

(17) An anode assembly 12 comprising an array of rectangular carbon anodes 14 is suspended from a beam (not shown in FIGS. 1 and 2) such that a lower end of each carbon anode is immersed in and contacts the fused salt. Current flows from the anodes through anode conductors 16.

(18) Cathodes 18 in the form of electrically-conductive trays loadable with feedstock 20 are supported by cathode supports 22, which hold the cathodes in a horizontal orientation and extend upwardly from each end of the cathode. The trays are of stainless steel and have a peripheral lip, or upstanding flange or wall, to retain a layer of the feedstock on the cathode. The trays are perforated to allow the fused salt to flow through the trays during electro-reduction. The feedstock is in the form of porous pellets or particles formed by agglomeration or moulding of the feedstock in powder form, followed by sintering to increase the strength of the pellets or particles.

(19) The apparatus comprises a plurality of cathodes which can be loaded into the fused salt for electro-reduction one after the other at a loading station 24 at one end of the container. During electro-reduction the cathodes move in a horizontal direction past the stationary anodes, between the anodes and the base of the container, to an unloading station 26 at the other end of the container.

(20) Each cathode is generally rectangular in plan, and its longer dimension extends across the width of the container. A cathode support 22 at each end of the cathode extends upwardly, out of the fused salt and through an opening defined between a side wall 8 of the container and the anode assembly 12. An upper end 28 of each cathode support is cranked outwardly, away from the anodes, and rests on a rail 30 which is fixed in position above a side wall of the container. The length of the cathode support is such that when the upper end of the cathode support rests on the fixed rail, the cathode is suitably positioned for electro-reduction below the anodes.

(21) Each fixed rail extends alongside or parallel to a side wall 8 of the container. As in the first embodiment illustrated in FIGS. 1 and 2, the rail may be held by a support structure (not shown) above the container side wall. Alternatively, as shown in the embodiment of FIG. 4, the rail may be secured to an upper edge of the side wall (the same numbering is used for similar components in the various embodiments described herein, where appropriate).

(22) As shown in FIGS. 2, 3 and 4 (but omitted from FIG. 1), each cathode support comprises a block 31 of a ceramic heat insulator (for example alumina) positioned so as to fill at least part of the opening defined between the side wall 8 of the container and the anodes 14 when the cathode is in position for electro-reduction beneath the anodes. The size of each heat-insulating block is determined such that the insulation is substantially continuous along the opening when a row of cathodes is in position beneath the anodes during electro-reduction. The length of each block is therefore equal to or less than the desired spacing of the cathodes during electro-reduction.

(23) FIG. 1 shows a schematic illustration of a cathode loading apparatus 32 and an cathode unloading apparatus 34. At the loading apparatus, cathodes filled with feedstock are engaged with cathode supports and suspended from a pair of loading rails 34. Each cathode is then lowered into the fused salt at the loading position 24 until the upper ends 28 of the cathode supports 22 rest on the cathode-support rails 30 of the electro-reduction apparatus.

(24) At the other end of the container, the unloading apparatus 34 comprises an unloading vessel or shroud 38, filled with an inert gas such as argon. At the unloading position 26, the cathode supports of a cathode can engage with a pair of unloading rails 40 of the unloading apparatus, which raise the cathode, now filled with reduced feedstock, into the shroud vessel 38. It may be desirable to unload the reduced feedstock into an inert atmosphere at this stage to prevent undesired re-oxidation of the electro-reduction product in air. The feedstock may then be cooled in the inert atmosphere and washed to remove any salt attached to the product.

(25) The anode assembly 12 is positioned between the loading position and the unloading position, and electro-reduction of the feedstock occurs as the cathodes are moved from the loading position to the unloading position, beneath the anodes. During this process, the cathodes are cathodically connected to a power source (not shown). This is achieved in the embodiment by making the cathode support rail an electrical conductor, and coupling the conductive rail to the cathode voltage of the power supply. Each cathode support is also electrically conductive and its upper portion, which contacts the cathode support rail, makes a sliding electrical contact with the support rail. Thus, the required cathodic current is supplied to each cathode from the cathode support rail.

(26) In the embodiment, the cathode support rail is fixed and the cathode supports engage with a conveyer system, or chain drive system, (not shown) to drive the cathode supports along the cathode support rail, in sliding contact with the rail, from the loading position to the unloading position.

(27) In a preferred embodiment of the invention, the fused salt is a mixture of calcium chloride and calcium oxide at a temperature of about 900 C. The anodes are of carbon, and each anode is mounted in the anode assembly such that its vertical height can be adjusted, in order to control the spacing between each anode and the cathodes passing beneath it. The cathode trays are of a non-magnetic material, to avoid undesirable effects of magnetic fields, and are of a material which resists corrosion in the electro-reduction environment. Suitable materials include stainless steel and titanium. The cathode supports may be of a similar material to the cathodes but should additionally be insulated from the fused salt (at least where the cathode supports contact the fused salt) in order to avoid stray electrical currents. Thus, for example, the cathode supports may be sheathed in a ceramic sheath, for example of alumina or boron nitride.

(28) As shown in FIG. 1, some or all of the cathodes may be provided with a scoop 40 extending below the cathode so as to be positioned in contact with or in close proximity to the base of the container. Such scoops may advantageously serve to dislodge any high-density contaminants from the base of the cell as the cathode is moved from the loading position to the unloading position.

(29) As shown in the embodiment of FIG. 4, it may be desirable to increase the thermal insulation of the container by allowing a layer 42 of the fused salt to solidify, or freeze, on the side walls of the container. In that case, the cathode supports should be shaped so as to be positioned sufficiently far from the side wall to avoid contact with the frozen salt layer. The frozen salt layer may advantageously protect the wall of the container from corrosion as well as providing thermal insulation.

(30) FIG. 8 is a cross-section of a conventional aluminium production apparatus, or pot, for implementing the Hall-Hroult method. The apparatus comprises a container 100. The base 102 of the container is of carbon and forms the cathode, fed with electricity by collector bars 104. An anode assembly 106 supports an array of rectangular carbon anodes 108. The vertical height of each anode is adjustable. The container is covered by a pot cover 110 and an alumina bin 112 is positioned above the container for feeding additional alumina into the container as required.

(31) During electrolysis, the container contains a layer of fused salt 114 (cryolite and alumina) in contact with the anodes and floating on top of a layer of molten aluminium 116. The aluminium is in contact with the carbon base of the container and acts as the cathode. Electrolysis of the alumina dissolved in the fused salt continuously produces aluminium metal, which can be tapped from the container in known manner.

(32) During electrolysis, a crust 118 forms on top of the fused salt, which helps to thermally insulate the melt.

(33) FIG. 9 shows a schematic plan view of the anodes of the aluminium cell of FIG. 8.

(34) In a preferred aspect, the present invention provides a method for modifying an existing aluminium cell, including cells of this type, for the electro-reduction of a solid feedstock. An aluminium cell does not require loading or unloading positions as described above in relation to FIGS. 1 to 5, and therefore the array of anodes covers the whole of the area of the cell as shown in FIG. 9. Anodes at the end of the aluminium cell may then be removed, as shown in FIG. 10, on converting the cell for reduction of a solid feedstock, so as to provide cathode loading and unloading positions 24, 26.

(35) A further feature of the aluminium cell is that the anode assembly is typically supported as shown in the schematic plan view of FIG. 11 by a beam extending longitudinally above the central axis of the cell, supported at each end by a substantial A-frame. On converting an aluminium cell to a cell for continuous electro-reduction of a solid feedstock, it may be advantageous to retain the anode supporting frame and to load and unload the cathodes, carrying the solid feedstock and solid product, over the side wall of the cell as shown in FIGS. 6 and 7 (the loading direction is indicated by an arrow in each Figure). Once the cathode and cathode supports have passed over the side wall of the container and are in position above the loading position, they can be lowered until the cathode supports come into contact with the cathode support rails. Advantageously, in order to assist with loading and unloading, in particular over a side wall of the container, the cathode support rails should be positioned as low as possible, or in other words at a minimum elevation above the surface of the fused salt. Conveniently, as shown in FIG. 7, the cathode support rails may therefore be mounted on or recessed into the tops of the side walls of the container.

(36) A conventional aluminium production facility, or pot-room, typically comprises many individual electrolysis cells. In some cases, the cells are arranged in a row end-to-end, as shown in FIG. 12. The row of cells can then be converted to operation with a solid feedstock by removing the anodes at each end of each cell, as described above. The cathodes can then be loaded and unloaded over the side wall of each container.

(37) In other cases, the aluminium cells in a pot-room are arranged side-by-side. Even if the anodes at each end of each cell are removed, there may then be no space to load and unload the cathodes for solid feedstock reduction. The A-frames at the ends of each aluminium cell for supporting the anode assembly may prevent loading the cathodes from the ends of the cells. In this case, to covert an aluminium pot-room for electro-reduction of solid feedstock, every third aluminium cell may be removed from the pot-room as shown in FIGS. 13 and 14. FIG. 13 shows an aluminium pot-room, from which two of the cells 150 need to be removed. In FIG. 14 these cells 150 have been removed, and the endmost anodes of the remaining cells removed, to allow side access to each cell for cathode loading and unloading.

(38) In a cell for continuous electro-reduction of a solid feedstock, it may be desirable to maintain a substantially steady state for the electro-reduction reaction. In this way, the feedstock loaded onto each successive cathode may experience the same reduction conditions and produce product of the same quality.