Abstract: A process for treating "brine mud" as produced in brine purification processes and for the coproduction of a calcium sulphate and magnesium carbonate which comprises(i) carbonating a slurry of "brine mud" in sodium chloride brine with carbon dioxide to produce a slurry of solids comprising calcium carbonate and a liquor comprising magnesium bicarbonate and sodium chloride.(ii) separating the slurry produced by the carbonation stage (i) into moist solid calcium carbonate and a liquor comprising magnesium bicarbonate and sodium chloride.(iii) heating the liquor from (ii) to produce a slurry of solids comprising magnesium carbonate, a liquor comprising sodium chloride, and gaseous carbon dioxide.(iv) separating the slurry from (iii) into moist hydrated magnesium carbonate and a liquor comprising sodium chloride and(v) drying the moist hydrated magnesium carbonate.