Unlocking the Value of Phosphogypsum Waste: Steam Calcination for Efficient Decomposition and Resource Recovery

Phosphogypsum is a waste by-product of phosphate fertilizer production. It contains calcium sulfate (CaSO₄.xH₂O), which can be decomposed to produce calcium oxide (CaO) and sulfur dioxide (SO₂). Steam calcination is a process that uses steam to decompose CaSO₄. This study represents the first comprehensive investigation of steam calcination applied to phosphogypsum, comparing its performance to pure CaSO₄ at various temperatures (1050–1200 °C) and steam molar ratios (0, 4%, and 30%). The results showed that steam calcination can significantly lower the calcination temperature of both CaSO₄ and phosphogypsum. The presence of steam can significantly accelerate the decomposition reaction of pure CaSO₄ and phosphogypsum when compared to decomposition in nitrogen or air. It was found that the decomposition reaction in steam follows the contracting cylinder reaction model in the case of CaSO₄ and the first-order reaction model for the phosphogypsum. Additionally, the activation energy for the steam calcination of CaSO₄ and phosphogypsum were found to be 421.2 kJ/mol and 418.6 kJ/mol, respectively, which are lower than the activation energy for the calcination in oxyfuel combustion products (O₂, CO₂, SO₂, and H₂O gases) 531.4 kJ/mol demonstrating its potential for enhanced energy efficiency. As a result, steam calcination emerges as a promising sustainable solution for valorizing phosphogypsum while reducing energy consumption and greenhouse gas emissions.