The production of precipitated calcium carbonate from industrial gypsum wastes
Abstract
Precipitated calcium carbonate (PCC) is a material of great interest due to its large range of applications in polymer composites, as rubber filler, additive for plastics, paints and paper, and in pharmaceuticals. PCC is also known as purified or synthetic calcium carbonate and has the same chemical formula (CaCO3) as other types of calcium carbonate such as limestone, marble and chalk. CaCO3 crystallizes in several different schemes of atomic arrangements, called polymorphs. The typical morphologies of CaCO3 polymorphs are generally classified as rhombohedral calcite, spherical vaterite and needle-like aragonite. The application of PCC is mainly determined by a number of
strictly defined parameters, such as purity, particle morphology, and structure. On industrial scale, limestone rock is the preferred raw material for the production of PCC. However, gypsum is an industrial solid waste product generated in various industrial processes such as phosphoric, hydrofluoric, citric and boric acid production, treatment of waste from desulphurisation of
flue gases from coal-fired power stations, ore smelting, and acid mine water treatment. Large stockpiles of gypsum waste exist with little or no usage and commercial applications. The objective of the research was to investigate the possibility of producing high-purity PCC from calcium sulphide (CaS), an intermediate product in the process of the recovery of elemental sulphur from waste gypsum. At first, the suitability of a direct aqueous CaS carbonation (one-step) process was tested. Although only a low-grade CaCO3 product (86-88 mass% as CaCO3) could be produced,
experimental results on the characteristics of CaS in the presence of CO2 in the CaS-H2O-CO2 system showed that the reaction proceeded in two distinct stages. In the first stage, CaS dissolution took place, with H2S stripping occurring in the second stage. Calcium carbonation and the resulting precipitation of CaCO3 were concurrent with the CaS dissolution and the H2S stripping reactions. Because the production of high-purity CaCO3 could not be obtained via the direct aqueous CaS carbonation process, a two-step or indirect carbonation process route was also developed and tested. In
the first step, either CO2 gas or H2S gas was used to induce CaS dissolution. This was followed by the separate carbonation of the solubilized calcium (in the form of Ca(HS)2 solution). The indirect process using CO2 as ‘CaS dissolution catalyst’ produced two separate CaCO3 products of different grades, i.e. a low-purity CaCO3 product (< 90 mass% as CaCO3) in the first step and a high-purity CaCO3 product (> 99 mass% as CaCO3) in the second step. Importantly, the H2S-based process was successful in producing a single CaCO3 product, which formed in the second step and was of high purity (> 99 mass% as CaCO3). The effects of various process conditions and experimental techniques were applied in order to control the morphology, structure and characteristics of the formed PCC. The control of the purity and the crystal structure of the carbonate products derived from waste gypsum in a mineral carbonation process were demonstrated. The indirect carbonation process when H2S gas was used for CaS dissolution, yielded only one carbonate product in the form of a high-purity CaCO3 (> 99 mass% as CaCO3) product, which was often made up of two polymorphs, calcite and vaterite, in varying proportions. Approximately 0.62 kg of the high-grade CaCO3 was produced from
every 1 kg CaS processed, while 0.43 kg residue was generated. The use of calcium-rich solid wastes (gypsum in this study) as primary material in replacement to mined limestone for the production of PCC could not only alleviate waste disposal problems but could also convert significant volumes of waste materials into marketable commodities.
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