Decomposition behavior of CaSO4 during potassium extraction from a potash feldspar-CaSO4 binary system by calcination

The extraction of potassium from a tablet mixture of K-feldspar ore and CaSO4 by roasting was studied with a focus on the effects of the decomposition behavior of CaSO4 on the potassium extraction process. The roasted slags were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and thermogravimetric (TG) analysis. The XRD analysis revealed that hydrosoluble mischcrystal K2Ca2(SO4)3 was obtained by ion exchange of Ca2 + in CaSO4 and K+ in KAlSi3O8. Meanwhile, the intermediate product, SiO2, separated from KAlSi3O8 and reacted with CaSO4 to decompose CaSO4. The SEM results showed that some blowholes emerged on the surface of the CaSO4 particles when they reacted with SiO2 at 1200 °C, which indicates that SO2 and O2 gases were released from CaSO4. The TG curves displayed that pure CaSO4 could not be decomposed below 1200 °C, while the mixture of K-feldspar ore and CaSO4 began to lose weight at 1000 °C. The extraction rate of potassium and decomposition rate of CaSO4 were 62% and 44%, respectively, at a mass ratio of CaSO4 to K-feldspar ore of 3:1, temperature of 1200 °C, tablet-forming pressure of 6 MPa, and roasting time of 2 h. The decomposition of CaSO4 reduced the potassium extraction rate; therefore, the required amount of CaSO4 was more than the theoretical amount. However, excess CaSO4 was also undesirable for the potassium extraction reaction because a massive amount of SO2 and O2 gas were derived from the decomposition of CaSO4, which provided poor contact between the reactants. The SO2 released from CaSO4 decomposition can be effectively recycled.