title: Conversion of calcium sulphide to calcium carbonate during the process of recovery of elemental sulphur from gypsum waste.
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).
authors.affiliation: DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, PO Box 395, Pretoria 0001, South Africa; Centre for Research and Continued Engineering Development, North-West University, Pretoria, South Africa. Electronic address: mdebeer@csir.co.za.
authors.forename: M
authors.initials: M
authors.lastname: de Beer
authors.affiliation: Department of Environmental, Water and Earth Science, Faculty of Science, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa.
authors.forename: J P
authors.initials: JP
authors.lastname: Maree
authors.affiliation: Centre for Research and Continued Engineering Development, North-West University, Pretoria, South Africa.
authors.forename: L
authors.initials: L
authors.lastname: Liebenberg
authors.affiliation: Industrial Mineralogy Laboratory, Council for Geoscience, Private Bag X112, Pretoria 0001, South Africa. Electronic address: fdoucet@geoscience.org.za.
authors.forename: F J
authors.initials: FJ
authors.lastname: Doucet
mesh_terms: D002119:Calcium Carbonate / Q000737:chemistry*; D017610:Calcium Compounds / Q000737:chemistry*; D002133:Calcium Sulfate / Q000737:chemistry*; D007220:Industrial Waste / Q000032:analysis*; D066192:Manufacturing Industry; D013440:Sulfides / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*; D018505:Waste Management
chemical_list: D017610:Calcium Compounds; D007220:Industrial Waste; D013440:Sulfides; C033907:calcium sulfide; D013455:Sulfur; D002119:Calcium Carbonate; D002133:Calcium Sulfate
keywords: Calcium carbonate; Calcium sulphide; Carbonation; Gypsum waste; Valorisation