Industrial process of preparing magnesia of high purity

Claim 1

In an industrial process of preparing magnesia of high purity from a magnesium-containing starting material containing impurities which comprises (a) dissolving the starting material in aqueous hydrochloric acid to produce an acidic solution of the starting material, and (b) subjecting the acidic solution of the starting material to a multi-step treatment wherein the impurities are precipitated from the solution and the precipitate is separated from the residual purified solution, said multi-step treatment comprising (1) a first step in the purification treatment including precipitating impurities as hydroxides and separating the precipitate from the residual magnesium chloride solution, and (2) a second treatment step including precipitating calcium present in the magnesium chloride solution by adding sulfate ions containing material and separating the precipitate, said calcium originating from the starting material and/or from material added for promotion of precipitation during the first step of the treatment, and (c) thermally decomposing the magnesium chloride in the purified solution to obtain magnesia, and recovering the magnesia, the improvement of first concentrating the magnesium chloride solution in the second treatment step to a content of more than 350 grams MgCl 2 /liter and then adding thereto the sulfate ions containing material to precipitate the calcium ions from the concentrated solution as calcium sulfate.