GB757908A - Process for producing aluminium electrolytically by amalgam metallurgy - Google Patents

Abstract

757,908. Production of aluminium. REYNOLDS METALS CO. June 19, 1953 [June 24, 1952], No. 17078/53. Class 82(1). [Also in Group XXXVI] Aluminium is produced by electro-depositing the metal in a moving mercury or. aluminium amalgam cathode from a molten electrolyte containing anhydrous aluminium chloride together with one or more halogen compounds of ammonium, alkali metals and alkaline earth metals until the aluminium concentration reaches 2 per cent by weight, removing the cathodic mixture from the cell, heating it to at least 250‹C. and then cooling to cause growth of crystals of aluminium sufficiently coarse to permit separation by their buoyancy, mechanically separating the crystals from the mother liquid which then contains not more than 0À2 per cent by weight of aluminium and is returned to the cell as the cathode, distilling off residual and alloyed mercury from the crystals, and melting the crystals either simultaneously with the start of distillation or after this has removed the free mercury. The chloride addition to the aluminium chloride may be of ammonium, alkali metal, or alkaline earth metal and the mercury cathode receives from 0À25-2 per cent aluminium. The mercury vaporized forms mercury chloride either in the cell or during the chlorination of bauxite utilizing the evolved chlorine, and is electrolysed to deposit mercury at the mercury cathode. The cathode mixture may be heated to about 600‹C. in a vessel having a mercury tower to provide sufficient pressure to prevent boiling to encourage growth of coarse crystals of impurities due to the presence of Fe Cl 3 , Si Cl 4 , Ti Cl 4 , V Cl 3 in the aluminium chloride, which are then mechanically removed as by centrifuges, presses, a ceramic filter and/or flotation. Iron alone may be removed by a magnetic iron filter. To avoid loss of aluminium amalgam, the impurity crytals are washed with mercury and the resulting liquid is returned for use as the cathode. The impurity-free amalgam containing some suspended aluminium is then cooled to cause crystallization and the aluminium crystals are removed by mechanical means and/or flotation as in the case of the impurities. Instead of heating the whole of the amalgam part of it may be heated below, up to, or above the temperature of a saturated solution and cooled by adding amalgam direct from the cell, and crystals may be separated in two stages if desired, followed by rapid cooling of the mother liquid to cell temperature in a heat exchanger. Typically a 80-14-6 percentage mixture of aluminium, sodium and potassium chlorides is electrolysed with a hard carbon or tungsten anode at 120‹-150‹C. by means of an applied voltage of 2À5 to give 1 per cent aluminium in the mercury cathode. The cathode mixture after passage through a heat exchanger 2 is then heated to 450‹C. in a pre-heater 4 before passage into a vessel 5 in furnace 6. The impurities are retained by a ceramic filter 7 and are conveyed by a power-driven screw 15 to a mercury column in a tower 18, from the upper surface of which column the impurity crystals are removed by a similar screw 21, mercury being supplied by a pipe 20 to wash these crystals and to ensure a downward circulation of mercury. The impurities are distilled at 24 and the freed mercury is supplied to the pipe 20. The aluminium amalgam after passage through the filter 7 is fed to the inner compartment 8 of the heat exchanger where it is cooled to 200‹C. to form aluminium crystals which are removed from the surface of the mother liquid by an extrusion press 9. The extruded rod which also forms a seal for the press is then passed to a distillation unit 12x where it is melted and the adhered and trapped mercury is distilled off into a condenser 14. If due to insufficient crystallization no rod is extruded from the press, drive to the ram is interrupted and the expelled liquid returned to the crystal growth chamber as by automatic electrical controls for the ram drive and a valve in a liquid return circuit. If an extrusion press is used to remove impurities, especially silicon, the extruded rod should preferably pass into a mercury bath at a temperature not above 200‹C.