Processing of mining waste containing phosphogypsm and sodium sulphate for recovery of calcium carbonate ,sodium carbonate and rare earth metals

Abstract

Waste gypsum from the fertilizer industry, sodium sulphate and magnesium sulphate rich brines from the mining industry and power stations can be utilized for the recovery of valuable products, as an alternative to stockpiling as waste dumps, or stored in brine ponds respectively. Waste gypsum can be used for the recovery of nano calcium carbonate, sulphur, and rare earth elements (REEs) (cerium, praseodymium, lanthanum, neodymium, and samarium). Sodium sulphate can be processed into sodium carbonate, sulphur, and sodium bisulphide. Magnesium sulphate can be processed to magnesium hydroxide, and in combination with sodium sulphide, sodium bisulphide and sodium sulphate can be produced. The proposed project allowed the generation of profits from waste as sodium carbonate has a value of R7 000/t, nano calcium carbonate a value of R14 000/t, magnesium hydroxide a value of R7 000/t and the global REEs market of R139.4 billion in 2023. Rare earth elements are used for magnets and due to its use in the electronic industry, and due to limited resources, it is of strategic value to the Western Word. Rare earth elements in phosphogypsum amounted to 4.48 mg/g gypsum and were collected with the crude CaCO3. Rare earth elements were present in lower concentrations in AMD, ranging from were 0.7 to 9.1 mg/L. The Mintek Pyrosim software model was used to predict the products formed under the conditions that were studied experimentally. A muffle furnace was used to thermally reduce CaSO4 to CaS and BaSO4 to BaS. OLI and beaker studies were used to determine which compound will precipitate out and which compounds/elements will stay in solution. The pyrosim studies and the muffle furnace studies both showed that CaSO4 and BaSO4 can be converted to CaS and BaS, respectively, through reduction with coal at 1000 °C. Less energy was needed for the conversion of BaSO4 into BaS (1 480 MJ/t coal) than for the conversion of CaSO4·2H2O into CaS (3 657 MJ/t coal). Furthermore, it was showed that CaS can be used to produce Na2CO3, CaCO3 and nano CaCO3. Acid mine drainage (AMD), a notorious kind of pollution associated with both active and abandoned mining sites, needs to be treated with the aim of achieving net zero waste. The ROC (Reverse Osmosis/Cooling) process can be used for the treatment of AMD though neutralization with Na2CO3 for the removal of metals, desalination with reverse osmosis (RO), freeze-crystallization for recovering Na2SO4 from the RO brine, and processing of Na2SO4 via Na2S to its raw material, Na2CO3. The conditions needed for the processing of Na2S to Na2CO3 were investigated. It was found that: (i) Na2S can be reacted with CO2 to form NaHCO3(aq) and NaHS(aq), (ii) the latter two compounds can be separated though freeze-crystallization as NaHCO3 has a lower solubility at 0°C, and (iii) NaHCO3 can be converted into Na2CO3 though heattreatment. OLI simulations also confirmed that NaHCO3 and NaHS were formed when Na2S was reacted with CO2. It was showed that Mg(OH)2 can be produced as a single solid compound when reacted with Na2S, as the other product, Na2SO4, has a high solubility. Mg(OH)2 will also form when reacted with CaS or BaS, but this approach will have the disadvantage that the solid, Mg(OH)2 is mixed with other solids, namely CaSO4·2H2O or BaSO4. The formation of Mg(OH)2 was also predicted by OLI simulations. Magnesium oxide can be formed from Mg(OH)2 through heating. These developments will contribute to achieve zero waste generation during solid waste treatment. Hydrogen sulphide can be converted to S by using the Claus process. In this study, H2S gas was produced when CaS or Na2S was contacted with CO2. This H2S was converted to S by contacting it with Fe3+ through OLI software. The effect of Fe3+ concentration was shown by stepwise by increment of the Fe2(SO4)3 dosage from 0 to 3000 mmol/L. It was noted that FeS2 formed when Fe2(SO4)3/H2S mole ratio was 0.5 with further dosing of Fe2(SO4)3, the formed FeS2 was converted to mainly S8. H2S was also oxidized with O2. OLI software showed that 50 mmol O2 was needed for oxidation of H2S to S8. The study also focused on the formation of CaSO4·½H2O from CaSO4·2H2O. It was noted that CaSO4·½H2O formed at temperatures of 240 °C and 180 °C when 60 min and 120 min reaction time was allowed, respectively. In the same section, gypsum and the reductant were determined if they should be processed as a powder or as a solid in the form of a brick or a tile. It was noted that hard gypsum formed when anhydrite was contacted with water in the presence of activated carbon.