Effect of CO partial pressure on phosphogypsum decomposition kinetics

Phosphogypsum (PG) is a byproduct of phosphoric acid production and consists mainly of calcium sulfate (CaSO₄). PG management is a critical concern in the phosphoric acid industry. As a solution, PG is decomposed to extract sulfur dioxide (SO₂), which is recycled to produce sulfuric acid (H₂SO₄). Carbon monoxide (CO) decreases the temperature of PG decomposition, producing calcium monoxide (CaO) and calcium sulfide (CaS). This work aims to develop a kinetic model of PG decomposition under reductive conditions by considering the effect of CO partial pressure on the reaction scheme and kinetics. The tests performed with a thermogravimetric analyzer (TGA) at several CO partial pressures and a thermodynamic study indicate that PG decomposition produces CaS and CaO as parallel reactions, while the selectivity towards each of the products depends on the temperature and CO partial pressure. As part of the kinetic study, the kinetics triplet of CaS and CaO production were individually estimated. The activation energy for each reaction, CaS and CaO production, was defined as 259.0 and 140.3 kJ/mol, respectively. The overall kinetic model of PG decomposition with CO reveals that both diffusion and interface reaction (reaction on the surface and adsorption) are equally controlling steps. The PG reduction with CO is achieved by CO adsorption on the CaSO₄ surface.