GB745461A - Improvements in or relating to process for the manufacture of magnesium chloride - Google Patents

Abstract

Magnesium chloride is manufactured by calcining dolomite, dolomitic limestone or magnesite until at least the MgCO3 content thereof has been converted to MgO, preparing an aqueous slurry of the resulting calcine with a calcium-sulphate-containing material (gypsum, anhydrite or by-product calcium sulphate) and an alkali metal (specifically sodium or potassium) chloride in substantially the molar proportions MgO:MCl:CaSo4=1:2:1 (where M is an alkali metal), carbonating the slurry with a carbon dioxide-containing gas at between 5 DEG C. and the boiling-point e.g., 5-30 DEG C. completing the reaction by heating at between 40 DEG C. and the boiling point, filtering off the calcium carbonate, crystallizing and separating the alkali metal sulphate from the mother liquor and recovering magnesium chloride from the mother liquor. In an alternative method, the magnesia is slurried with water, carbonated at between 5 DEG C. and the boiling point until all of the magnesia has been converted to magnesium carbonate, MgCO3.3H2O, or bicarbonate and then adding the other reactants in the above-stated proportions, before heating at between 40 DEG C. and the boiling point and subsequently treating as before. Small amounts of alkali metal sulphate remaining in a concentrated magnesium chloride mother liquor may be removed by adding a quantity of calcium chloride stoichiometrically equivalent to the alkali metal sulphate and separating the so-formed precipitate of calcium sulphate and alkali metal chloride. To produce magnesia from a magnesium chloride solution, milk of dolomitic lime may be added so as to provide 1 mole of Ca(OH)2 for each mole of MgCl2 in the solution, and the insoluble Mg(OH)2 derived partly from the magnesium chloride and partly from the dolmitic milk of lime filtered off. In calcining the dolomite or &c., it may be half-burned (e.g. at 750-1000 DEG C.) and thus contain mainly magnesium oxide and calcium carbonate or dead-burned (e.g., at 1400-1600 DEG C.). For the carbonation, gases containing 2-100 per cent CO2 may be used, e.g., 8-12 per cent or 30-40 per cent, according to available sources. If the reaction is performed in a vessel with a conical bottom, calcium silicate and iron oxide from the calcine will settle and may then be discharged. After the removal of the finely divided calcium carbonate, the filtrate may be evaporated, preferably under reduced pressure and/or refrigerated to crystallize the solium sulphate, anhydrous crystals being formed above 35 DEG C and the decahydrate below 32 DEG C. The mother liquor is then concentrated to contain about 25 per cent magnesium chloride and the hexahydrate crystallized therefrom. This may be dehydrated stepwise in a hot-air oven or muffle furnace to MgCl2.2H2O, and finally to anhydrous MgCl2 in, e.g., a rotary kiln. Detailed examples are given.