Abstract: A process for producing hydrogen and/or hemihydrate calcium sulphate from calcium sulphite. Waste solid containing calcium sulphite and other calcium salts is treated by the addition of aqueous sulphur dioxide solution or sulphuric acid solution to produce a liquid phase product. The liquid phase product may be separated from any remaining solid phase material and then subjected to the addition of sulphuric acid in a second step, wherein calcium sulphate hemihydrate is produced. The addition of sulphuric acid to the liquid phase results in the evolution of sulphur dioxide which may be used as such or in sulphur dioxide electrolysis cells in order to produce hydrogen and sulphuric acid. An especially unique feature of the process is that the sulphuric acid produced in generating hydrogen can be recycled to the step where the calcium sulphate crystals are produced.

Abstract: A process is provided for obtaining high quality high surface area zinc oxide from a roasted zinc sulphide concentrate by leaching with an ammoniacal ammonium carbonate solution. The process utilizes optional preliminary oxidation and water leaching steps to remove soluble impurities and an optional aqueous sulphur dioxide leaching step to selectively remove readily soluble zinc oxide. The process includes an improved source of roasted zinc sulphide concentrate. The process also includes an optional reduced pressure calcining step to produce zinc oxide with further increased surface area.

Abstract: A process is provided for obtaining high quality high surface area zinc oxide from a roasted zinc sulphide concentrate by leaching with an ammoniacal ammonium carbonate solution. The process utilizes optional preliminary oxidation and water leaching steps to remove soluble impurities and an optional aqueous sulphur dioxide leaching step to selectively remove readily soluble zinc oxide. The process also includes an optional reduced pressure calcining step to produce zinc oxide with further increased surface area.

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.

Abstract: A process is described for the controlled oxidation roasting of zinc bearing sulphidic ores and concentrates by adjusting the furnace temperature and residence time together with the oxygen availability to the sulphide retention requirements in the calcine. The obtained calcine is subsequently subjected to various physical and chemical process steps to separate the unreacted sulphides which are then dead roasted and treated for zinc recovery, according to one embodiment of the process. In another embodiment the separated, unreacted sulphides are returned to the controlled oxidation roasting step.The chemical separation process steps include sulphur dioxide leaching. In one embodiment zinc is preferentially dissolved as sulphite and the solution obtained is further treated for zinc recovery. In other embodiments sulphuric acid, or ammonical ammonium carbonate or similar leaching reagents are used for the preferential dissolution of the zinc oxide present in the calcine.