Patent GB-874974-A (GB)
Title: Process for the manufacture of salt mixtures for the electrolytic production of magnesium
Inventor: BAUER ERICH
IPC: C25C3/04
Filed: 1958-10-22
Abstract: <PICT:0874974/III/1> <PICT:0874974/III/2> Magnesium chloride substantially free from oxide and water, suitable for electrolytic production of magnesium, is prepared by concentrating an aqueous solution of magnesium chloride (in which has been dissolved recycled by-product magnesium oxide) and spray drying the concentrated chloride solution in a stream of hot air entering 400 to 600 DEG C. and leaving at 300 to 350 DEG C. containing a steam content of 7 to 12% by volume.  The resulting salt, separated in a cyclone or filtering device, containing 2 to 3% by weight of magnesium oxide and 1 to 2% by weight of water is then melted whereby the magnesium oxide impurity deposits and the anhydrous magnesium chloride supernatant melt may be withdrawn for electrolysis since it contains 0.1 to 0.5% by weight of magnesium oxide.  The magnesium oxide deposited may be withdrawn and dissolved in the initial magnesium chloride solution. The hydrogen chloride-containing gases withdrawn from the cyclone are employed to preheat the air fed to the spray-drier.  They may then be recycled to heat and concentrate the initial magnesium chloride solution, or alternatively may be scrubbed with water to remove hydrogen chloride and entrained magnesium chloride.  The acid solution formed may be neutralized with chalk, dolomite or magnesite and the chloride solution recycled.  As shown in the Figures, Fig. 1, an initial magnesium chloride solution which may contain potassium chloride and sodium chloride, passes at 70 to 100 DEG C. into a mixing vessel 2 which is fed with re-cycled magnesium oxide from a line 3.  The solution so formed then passes to a concentrating tower 6 where it falls countercurrent hot air entering through line 8 at 70 to 150 DEG C.  The magnesium chloride solution containing 400 to 500 grams of magnesium chloride per litre passes through a filter 10, which removes undissolved magnesium oxide, to an overflow vessel 12 from which the solution passes to a spraying nozzle 14 in a spray tower 16.  The solution is atomized by compressed hot air fed from conduit 15 and spray dried by hot fresh air passing through pump 18 and a heat exchanger 7 and a heater 20 to enter the top of spray drying tower 16 at 400 to 600 DEG C.  The gases leaving the tower at 300 to 350 DEG C. and containing about 7 to 12% by volume of steam, and entrained magnesium chloride solids, pass to a cyclone separator 21 from which the impure magnesium chloride product is conveyed to melting vessels 23.  The effluent gases from the cyclone pass via an air preheater 7 back through line 8 to the initial concentrating tower 6.  The magnesium chloride crude is melted in the vessels 23, the supernatant purified chloride withdrawn through line 24 and the magnesium oxide precipitate is withdrawn from the bottom, flaked in rollers 25 and re-cycled through line 3 to vessel 2 for dissolution in the initial magnesium chloride solution. Alternatively as shown in Fig. 2, the crude magnesium chloride from the cyclone 21 passes to a melting vessel 23 and either the magnesium oxide precipitate or if desired the whole melt of chloride and oxide may be passed in the presence of carbon to an electrically heated chlorinating vessel 27, to which is fed a stream of chlorine through line 28.  Alternatively phosgene or hydrogen chloride may be used.  Effluent gases containing carbon monoxide and dioxide and hydrogen chloride leave through 29 and the purified salt melt is withdrawn at 24.  Gases escaping from cyclone 21, containing hydrogen chloride and fine magnesium chloride dust pass through heat exchanger 7 and enter an absorption tower 31 where they pass counter current to a water spray from line 32 thereby forming an acid magnesium chloride solution; this is pumped to a neutralizing tower 35 containing limestone, dolomite or magnesite whereby any free hydrogen chloride is converted to magnesium or calcium chloride whilst the carbon dioxide formed is removed through line 36.  The neutralized solution leaves the neutralizing tower 35 through line 37 and is passed through line 32 to the absorption tower 31.  Periodically magnesium chloride solution is withdrawn through line 38.