Abstract:
The production of elemental sulphur and calcium carbonate (CaCO3) from gypsum waste can be achieved
by thermally reducing the waste into calcium sulphide (CaS), which is then subjected to a direct aqueous
carbonation step for the generation of hydrogen sulphide (H2S) and CaCO3. H2S can subsequently be converted
to elemental sulphur via the commercially available chemical catalytic Claus process. This study
investigated the carbonation of CaS by examining both the solution chemistry of the process and the
properties of the formed carbonated product. CaS was successfully converted into CaCO3; however, the
reaction yielded low-grade carbonate products (i.e. <90 mass% as CaCO3) which comprised a mixture
of two CaCO3 polymorphs (calcite and vaterite), as well as trace minerals originating from the starting
material. These products could replace the Sappi Enstra CaCO3 (69 mass% CaCO3), a by-product from
the paper industry which is used in many full-scale AMD neutralisation plants but is becoming
insufficient. The insight gained is now also being used to develop and optimize an indirect aqueous
CaS carbonation process for the production of high-grade CaCO3 (i.e. >99 mass% as CaCO3) or precipitated
calcium carbonate (PCC).
