GB208717A - Improvements in or relating to electrolytic refining of aluminium - Google Patents

Abstract

208,717. Aluminum Co. of America, (Assignees of Hoopes, W.). Dec. 21, 1922, [Convention date]. Void [Published under Sect. 91 of the Act]. Aluminium, obtaining. - The charge of an electric furnace for obtaining pure aluminium comprises a layer 14 of a relatively dense aluminium alloy constituting the anode, an intermediate layer 20 of fused fluorides, and an upper layer 21 of aluminium constituting the cathode. Refined aluminium is withdraw from time to time by a notch 26, provided with a refractory plug 26 and impoverished anode alloy is withdrawn by a tapping plug 29. Fresh alloy may be supplied through a carbon funnel let down into the anode layer. The furnace comprises a carbon base 13 having a bowllike cavity and enclosed together with a bottom heat-insulating layer 12 of bauxite in a steel shell 11. The shell 11 is provided with a water-cooled ring 19 surmounted by and insulated from a water cooled ring 18. An insulating refractory lining 17 is frozen on the shell walls by the cooled rings and an insulating refractory top, crust 22 which may be of both ingredients is formed above the cathode. The anode is connected to the positive supply conductor by a bus-bar 15 and distributor plates 16 and the cathode is connected to the negative supply conductor by graphite plates 23 and rods 24. The anode alloy may consist of aluminium and copper in the proportion of 60 to 40 and silicon up to 10 per cent. Iron though not advantageous may be present up to 10 per cent. The electrolyte preferably consists of aluminium fluoride 33 per cent, sodium fluoride 30 per cent, barium fluoride 35 per cent and alumina 2 per cent. The barium fluoride may be replaced wholly or in part by strontium fluoride. If alumina accumulates in the bath it may freeze on to the lining so as to choke the cell rendering it necessary to break away a portion of the lining and form a new one. To prevent this, a portion of the bath may be removed from time to time and replaced by material free from alumina. A pressure of 7 volts and current density between 900 and 1400, preferably 1200, amps per sq. foot of anode are employed, the working temperature being about 950‹ C. Aluminium containing not more than 0.15 per cent or even 0.1 per cent of impurities may be obtained, the impurities being iron silicon and copper. Such aluminium is said to be characterised by a high degree of softness, ductibilty, malleability and strength and by high resistance to attack by dilute hydrochloric and other acids. Its melting point is 659‹.5 + 0.3 C. Its specific resistance at 20‹ C. is 2.7 michroms per cur. cube. Its appearance is silvery and it retains its lustre for a long period. It is specially suitable for making collapsable tubes.