Production of calcium carbonate from phosphogypsum wastes

Phosphogypsum is a waste generated during the production of wet phosphoric acid from phosphate rocks. Numerous studies have attempted to find applications for phosphogypsum, but its composition is generally limiting its reuse. For instance, depending on the origin of the phosphaterocks it originates from, phosphogypsum can contain environmentally-problematic elements suchas radionuclides or fluoride. In this study, the potential of using a South African phosphogypsum waste for the production of calcium carbonate was investigated. The sample was collected from the Rustenburg plant of Omnia and was characterised by a number of analytical techniques. Two different processes of mineral carbonation were studied. The first method, which can be described as an indirect mineral carbonation route, involved the reaction between phosphogypsum and an aqueous NaOH solution to form Ca(OH)2 where after the Ca(OH)2was converted into CaCO3 using CO2. During the first step of this process, the impurities contained in the phosphogypsum sample was transferred to the Ca(OH)2 precipitate. Upon carbonation, a low grade CaCO3 product, containing more than 10% impurities (such as unreacted gypsum and portlandite) was obtained. Using the optimised reaction conditions, the effect varying the solid to liquid ratio and merging of the two steps applied during the indirect mineral carbonation route was studied.XRD analysis indicated that most of the carbonated products contained mainly two polymorphs of CaCO3, calcite and vaterite, in varying quantities. Only one set of reaction conditions was effective in both inducing a complete conversion of phosphogypsum into carbonate, and producing a single polymorph of CaCO3 (calcite). The obtained calcite was defined as high-grade, containing close to 100% CaCO3. Its particles were spherical and contained the rare earth elements originally contained in the untreated phosphogypsum. The second process used in this study, defined as the Merseburg process, involved the reaction between phosphogypsum and an (NH4)2CO3 solution. The effect of varying the reaction time and temperature was studied on the conversion of phosphogypsum into calcium carbonate. The reactions were studied at 3 different temperatures (25°C, 40°C and 65°C) for 1 to 4 hours. It was found that a temperature of 65°C was required to yield the complete conversion of phosphogypsum into high-grade (CaCO3> 99%) calcite. The time had no effect on the conversion rate at this temperature (65°C). The calcite particles formed had cube-like and plate-like structures and contained significant amounts of the rare earth elements contained in the untreated phosphogypsum as impurities. This study shows that both these processes are able to produce calcite particles which contain most of the rare earth elements (REE) originally present in the untreated phosphogypsum sample. The process involving the use of NaOH may not be suitable for large-scale industrial applications due to cost implications, whereas the Merseburg process may prove to be more favourable.Dissertation (MSc)--University of Pretoria, 2014.ChemistryMScUnrestricte