Patent GB-814754-A (GB)
Title: Method of production of aluminium by means of the electrolysis of fused salts, and apparatus for such electrolysis
IPC: C25C3/08
Filed: 1956-06-07
Abstract: 814,754. Fused-electrolyte apparatus. MONTECATINI SOC. GENERALE PER L'INDUSTRIA MINERARIA E CHIMICA, and VARDA, G. DE. June 7, 1956 [June 8, 1955], No. 17620/56. Class 41. A method of electrolytically producing aluminium from a fused salt bath which includes aluminium oxide, comprises circulating the bath during electrolysis through a closed " necklace " sequence of electrolysis cells 19 within a single furnace, the cells being electrically in series and being formed by bi-polar electrodes 17 the anode faces of which are restorable from the bath side, the topping of aluminium produced and the anodic restoration being effected separately for each cell, under each of which is a separate collecting chamber 21, the circulation being promoted by a difference in bath level obtained by lifting the bath within the furnace. By a " necklace " sequence of cells is meant at least two groups of cells which are mechanically and electrically in series. The apparatus comprises two furnaces, e.g. electrically in parallel, separated by a wall 9, each furnace comprising two parallel rows of cells 5<SP>1</SP>, 5<SP>11</SP> and 6<SP>1</SP>, 6<SP>11</SP> separated by walls.7, 8 respectively. The bath 20 circulates in the direction of arrows 2 against the direction of the electric current indicated by arrows 1, flowing in each furnace from a chamber 32 through the cells to a chamber 33. Each row of cells may have a slight slope in the direction of bath flow to assists circulation. Fused electrolyte is periodically delivered from the chamber 33 to the chamber 32, Fig. 6, by admitting an inert gas, e.g. gas produced by the electrolysis, at 44 to force electrolyte through a duct or ducts 34. Fresh alumina is delivered continuously or intermittently through one or more chambers, tanks or gaps at the ends or at intermediate positions 13 in the rows of cells. The cells 19, Fig. 3, are similar to those described in Specification 800,092 having inclined bi-polar electrodes 17 (the electrodes sloping in opposite. directions in each row), above chambers 21 in which aluminium 22 collects, each chamber 21 communicating laterally at 23 with a pocket 31, Fig. 1, which lies in the walls 7, 8. Other topping ducts lead from the chambers 21 to the outer wall of the furnace. The cells have gas outlets 28. The electrolyte flows through cells by passages 26 in electrically insulating blocks 25 surmounting the electrodes 17, and a oneway ball valve 41, e.g. of carbon coated with electrofused aluminium or magnesium oxide, is inserted in a duct or ducts 39 leading from the last, i.e. lowest, cell 36 to the chamber 33. Metal is taken from a pocket 31 to facilitate anode restoration as follows: the current is cut off and the appropriate cover 10 and tile 11 removed ; if this does not cause the electrolyte to solidify, plugs 30 are lowered to throttle the passages 26; a cover is removed from the pocket 31 and metal removed by suction until the bath 20 descends to a depth below a step of insulating material 24 which supports the electrode 17. The anode face of the electrode 17 has then applied to it a carbon portion or portions as described in Specification 800,092. Finally, some metal 22 is returned to the pocket 31 to restore the bath depth. The cells at the outer end of each row are connected bv a transverse duct through the walls 7, 8 which is of a size which ensures that the electric current passing through it is small, e.g. less than 3 per cent of the total current. Instead of flowing through passages 26, the bath may flow through conduits in the internal side walls of the cells. Metal nipples 16, e.g. of iron, serve to lead current to and from the furnace. Any electrode in either row of each furnace may be divided into an anode and a cathode insulated from each other and serving as the terminal electrodes of the furnace, or the terminal electrodes 14, Fig. 3, may be at the inner end of each row, i.e. adjacent the chambers 32, 33. The electrically insulating blocks 25 are good heat conductors, e.g. of electrofused magnesium oxide, and the tiles 11 should be light and porous to allow passage of gas, e.g. containing magnesia or asbestos, and they may be apertured. Powdered alumina or refractory bricks 4 line a metal casing 3 containing the cells, and fused or sintered magnesia or aluminium nitride 24 lines all surfaces which contact the electrolyte, except the graphite electrodes 17 which have their anode faces 18 of a carbonaceous material. Metal strips may hang from the cathode faces in the chambers 21 for good electrical contact, and heating means may be provided at the positions 13 and in the chambers 32, 33. Chambers similar to 32, 33 with means for raising the bath from the lower to the higher chamber may be additionally provided at the outer ends of the rows of cells, the transverse duct being omitted. The rows of cells may be U-form or S-form in plan view. In one furnace comprising two rows of cells, 12 cells in one row, 16 in the other, the volume of each cell is 50 litres and of each chamber 21 slightly more. The electrodes slope at 30 degrees to the vertical, are separated by 4 to 8 cms. and are 26 to 22 cms. thick, according to consumption. The vertical projection of the anode and cathode surfaces is 4800 square cms. giving 0.5 amp./square cm. current density at a voltage of less than 3.8 for each cell. Alumina is fed at each of two positions 13 at 11 to 13 gms./second and the bath flow is 30 to 150 c.c./ second, preferably 50 to 100 c.c./second. The magnitude of the difference in bath level for each furnace may be 5 decimetres. Specification 802,959 also is referred to.