GB720261A - Distillation of calcium or magnesium - Google Patents

Abstract

720,261. Distilling metals; calcium, magnesium, alkali metals. LORD PRESIDENT OF THE COUNCIL. Jan. 5, 1948 [Jan. 3, 1947], No. 316/47. Class 82 (1). Calcium or magnesium containing as impurity alkali metal other than lithium are purified by distilling in two stages, in the first of which distillation is carried out in the presence of an inert gas at an absolute pressure substantially greater than the vapour pressure of calcium or magnesium at the temperature of distillation but less than the vapour pressure at that temperature of the said alkali metal so that the alkali metal is distilled and condensed on a surface, the second stage being carried out in the absence of inert gas so that the calcium or magnesium distils under its own vapour pressure only and condenses on a lower condensing surface. The Figure shows a stainless steel vessel 1 containing crude calcium or magnesium 2, the vessel being immersed in a molten salt bath 3. Cover 5 carries a hollow cylinder 6 with water-cooled flange 9 and internal fins 8, which assist in cooling the condenser surface, and a plate 7 on which the calcium or magnesium is to be condensed. Between the plate 7 and the wall of the vessel is an annular space through which alkali metal vapour rises to condense on the cooled flange 9. Cooling of the condensing surfaces is effected by air or other cooling medium from duct 11 passing over the fins 8. Duct 12 connects the interior of the vessel with vacuum pump, pressure gauge and inert gas supply. A perforated plate 17 controls the radiation of heat to the condenser. A suitable plate is one having 3000-4000 perforations of “ inch diameter occupying 15-25 per cent of the total plate area. In operation the salt bath is heated to about 870‹ C. and alkali metal distils in an argon atmosphere at a pressure of about 20 mm. mercury condensing on the cooled flange 9 maintained at a temperature of about 250‹ C. The first stage is completed when the lower condensing surface shows a sudden drop in temperature. The still is then evacuated and calcium or magnesium distils and condenses on the plate 7 which attains, with calcium, a temperature of about 400‹ C. A rapid drop of this temperature indicates the completion of distillation. Specifications 270,060, [Class 82 (1)], and 698,861 are referred to.