Patent GB-208715-A (GB)
Title: Improvements in or relating to the production of aluminium
IPC: C25C3/24
Filed: 1923-12-20
Abstract: 208,715. Aluminum Go. of America, (Assignees of Hoopes, W. Frary, F. C., and Edwards, J. D.). Dec. 21, 1922, [Convention date]. Aluminium, obtaining. - Aluminium is obtained by electrolysis of a molten electrolyte with an anode of molten aluminium alloy containing silicon proportioned to maintain the mobility of the anode at a temperature below that causing material alteration of the electrolyte even after part of the aluminium has been removed. The working temperature is between 850‹ and 1100‹ C. and preferably about 950‹ C. The alloy should be of higher density than the fused electrolyte which is between 2.5 and 2.7. An alloy containing 25 per cent of copper is suitable having a density of 2.8 at 950‹ C. ; higher proportions of copper may be used when working at higher temperatures. The silicon may be between 2 and 32 per cent of the copper-silicon content, the copper being not less than 20 per cent of the whole. The following composition is suitable. Aluminium 30 per cent, copper 55 per cent, silicon 10 per cent, iron less than 5 per cent, titanium less than 1 per cent. If the iron and titanium content of the impoverished alloy is high, these metals may be removed together with silicon. The electrolyte consists of aluminium fluoride and other fluorides such as those of barium and sodium compounded to be fluid below the temperature of material volatilization of aluminium fluoride; proportions are given. An electrolytic furnace for carrying out the process comprises a steel shell 10, enclosing a heatinsulating layer 28 of bauxite, alumina, magnesia or refractory bricks and a bowl-shaped lining 29 formed by tamping into the shell a mixture of tar, pitch and powdered coke and then baking. The shell 10 is provided at the top with a waterjacket 11, surmounted by and insulated from a water-jacket 14. Water is circulated successively through the two jackets which are joined by a rubber tube. The two jackets are provided with a refractory lining 45. A part of the electrolyte is drawn up by capillary action between the aluminium layer and the lining 45 and spreads over the metal and solidifies forming a top crust. A top crust may alternatively be formed by dusting over the aluminium a layer of powdered alumina, carbon and magnesia. Further powdered material may be laid over the crust so formed. If the bath has become saturated with alumina, the alumina content may be reduced by breaking away a portion of the lining and allowing it to reform at the expense of the bath the excess alumina crystallising in corundum-like form. The alumina may also be reduced by removing a part of the bath and replacing it by alumina-free material; to do this the aluminium may be first removed. Excess alumina may also be removed by electrolysing the bath for a time with a carbon anode. An anode for this purpose is described which is identical with that described in Specification 208,714. The shell 10, is provided on the inside with radial contact vanes 31 and on the outside with contact plates, not shown, to which a polygonal bus-bar is bolted. The cathode connections consist of graphite cylinders 34 supported by copper or other metal rods 35 clamped to bus-bars 37, which may be on more than one level supported by a frame 38. A notch, not shown, is provided for removal of aluminium and a tap 12<a> for removal of impoverished alloy. The thickness of the electrolyte layer should be from 2¢ to 4 inches, and that of the aluminium layershould be not less than 2 inches so as to secure firm contact with the boundary crust and thus prevent volatilization. The current in the bath has a radial component which produces electromagnetic stirring. This, together with the use of a suitable current density, allows secondary reactions to proceed at the anode and cathode surfaces whereby impurities removed from the anode are precipitated thereon and deposition on the cathode of bath-components more electropositive than aluminium is prevented.