Abstract
This article looks into process of supercritical-CO2 phosphogypsum (PG) conversion, which is production residue after making mineral fertilizers by "Kasphosphate" LLC. Nowadays, over 30 million tons of PG have been accumulated in Kazakhstan; its background radiation is low enough to recycle it into valuables such as construction materials, rare earth elements and strontium salts. Complete factorial experiment method was used to describe conversion process; conversion of PG into CaCO3 was optimized using simplex method. Conversion optimization outcomes show possibility of using fewer reagents and shortening conversion process compared to existing technologies. Conversion degree is 87% after 10 min at 33 °С. Conversion was carried out at semi-industrial supercritical equipment with maximum CO2 flow rate up to 5000 g/min and integrated CO2 recirculation system. Characteristics of initial PG samples and conversion products/semi-products were done using X-ray phase analysis, scanning electron microscopy, atomic absorption spectroscopy, ICP-MS, and gravimetric procedure. In order to obtain purer CaCO3 PG was cleared of water-soluble impurities beforehand. As conversion result, finely dispersed CaCO3 and Ca(HCO3)2 as well as Na2SO4 solution were obtained.
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