Filtering of precipitating silica

A process for recovering a water-soluble magnesium salt from a reaction mixture containing a water-soluble magnesium salt, silica, and metal salt impurities comprising reacting the magnesium oxide of an aqueous slurry of asbestos tailings with a mineral acid, there being employed sufficient asbestos tailings that the reaction mixture contains at least 5 percent by weight of asbestos tailings in excess of the stoichiometric amount required for the reaction of the magnesium oxide contained in the asbestos tailings with the mineral acid said excess asbestos tailings being present and recovering the magnesium salt in solution by filtration to eliminate silica and metal salt impurities, the excess asbestos tailings in the reaction mixture facilitating the filtration of the precipitated silica in forming a mat on the membrane of the filter, the mat being permeable to the solution and retaining the silica and metal salt impurities.