GB580367A - Process for manufacture of magnesium products - Google Patents

Abstract

Neutral hydrated magnesium carbonate is precipitated by aerating a magnesium bicarbonate solution with a relatively inert gas, e.g., decarbonated air or natural gas, at a rate insufficient to cause violent agitation, removing the suspension of neutral carbonate and reintroducing solid magnesium carbonate in amounts substantially, e.g., four to six times, greater than the amounts being precipitated so as to produce large particles e.g., 150 microns. The bicarbonate solution may be obtained from brines, or by crushing and calcining magnesium materials, e.g., dolomite, prucite, magnesite, serpentine, olivine, preferably selectively leaving calcium carbonate largely undecomposed; grinding preferably wet to 200 mesh, forming a magnesium hydroxide slurry; pretreating during or after grinding with flue <PICT:0580367/III/1> gas or bicarbonate solution to convert calcium oxide to rapid settling carbonate; diluting to, e.g., three per cent. solids; carbonating at normal or increased pressure; removing solids, e.g., by a hydro-separator or centrifuge, and clarifying. Magnesium carbonate may be recovered from the sludge solids by a continuous stream of water. Carbonation may be carried out in a series of tanks, the pressure rising to 20lbs. per square inch. Enriched carbon dioxide or high pressure carbonation may be used. As shown 2.5 per cent. magnesium bicarbonate solution 65 is aerated under normal conditions of temperature and pressure in three tanks 51, 52, 53 fitted with perforated bottom plates and baffles and supplied with successively greater amounts of air 57. Pentahydrate is deposited below 15 DEG C. and trihydrate at 15-40 DEG C. The suspension passes to the hydraulic thickener 62 and then to a centrifuging or dewatering stage. Sufficient thickened slurry is returned 64 to the aeration stage to maintain a content of 6-10 per cent. neutral solids. The evolved carbon dioxide is recovered at 67 for reuse. Only about 55-65 per cent. of the bicarbonate is decomposed and residual bicarbonate solution from the thickening and centrifuging stages is recycled 66 to the precarbonation dilution stage and/or the pretreatment stage. The magnesium carbonate may be ignited to oxide or dried and expanded at 250-400 DEG C. to produce basic carbonate. Specification 529,127 is referred to.