Phosphogypsum beneficiation

Msila, Xolani
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Three alternatives for the utilisation of the phosphogypsum from the Lowveld region of South Africa were investigated in this thesis, namely (a) its suitability for use as building material; (b) its conversion to saleable ammonium sulphate and precipitated calcium carbonate; and (c) the reaction mechanism, pathway and kinetics of its thermal reduction with graphitic carbon to calcium sulphide. The phosphogypsum contains activity concentrations of naturally occurring radioactive nuclides of uranium, thorium and their progenies that are lower than the 500 Bq/kg limit set for regulation in South Africa. The K-40 activity concentration is below the minimum detectable amount of 100 Bq/kg by gamma spectrometry. The calculated values for Hex, Hin and Iγ are: 2.12 + 0.59, 3.44 + 0.64 and 2.65 + 0.76 respectively. The calculated Raeq is 513 + 76 Bq/kg. The phosphogypsum is suitable for restricted use in building materials but a final decision for its use should be taken with the consideration of scenarios of use. The phosphogypsum was converted to ammonium sulphate and precipitated calcium carbonate in a Merseburg process that has been modified by omitting the CO2 scrubbing towers. This modification reduced the heating requirement in the gypsum conversion reactor by 35 °C. The calcium carbonate precipitated as calcitic, scalenohedral polymorphs with a mean size of 3.4+ 0.1 μm in diameter. A kinetic study of the reaction between phosphogypsum and graphitic carbon was carried out in the range 25 °C to 1200 °C using a TG-DSC analyser. The results showed that after the dehydration of the phosphogypsum to anhydrite the anhydrite converts to calcium sulphide via an intermediate compound, characterised by a mass loss of 11%. The average activation energy for the reduction of phosphogypsum with iv graphite was determined using the Ozawa-Flynn-Wall- (OFW) and Kissinger-Akahira-Sunose (KAS) methods to be between 330-370 kJ/mol. The reaction pathway for the thermal reduction of the phosphogypsum with graphite was studied using in situ Raman spectroscopy. The dehydration of the phosphogypsum to anhydrite was completed at about 142 °C. The dehydration was followed by the formation of the intermediate compound at about 900 °C. The intermediate compound, proposed to be a dehydrated Orschallite-type compound (Ca3[SO4][SO3]2), converted to CaS at about 1000 °C.