Abstract: Processes for the selective leaching of magnesium-containing solids, which contain calcium and/or heavy metals, and the selective precipitation and crystallization of the leached magnesium are provided. The leaching is conducted with aqueous sulphur dioxide at elevated temperatures with feed pulp densities which are sufficiently high to produce a leach liquor which is substantially saturated with magnesium. The selective crystallization and precipitation processes which preferably follow the selective leaching process, utilize controlled cooling, heating and pH conditions to yield a high purity magnesium sulphite product.