The aid of calorimetry for kinetic and thermal study: Application to the dissolution of Tunisian natural phosphates

Phosphoric acid, a non-renewable chemical, is used in different industries. Production of this chemical from natural phosphate can be done by two routes: wet process and thermal process. The nature of the natural phosphate, i.e., its chemical composition, plays an important role in the kinetics and thermodynamics of phosphoric acid production. Thus, the establishment of a kinetic model, based on reaction mechanism, for the dissolution of natural phosphate is cumbersome due to the presence of impurities. Besides, one should use an online analytical method because the dissolution reaction is fast. The dissolution of two natural phosphates with different percentages of phosphorus pentoxide (P₂O₅), phosphate samples (28 mass% of P₂O₅) from Gafsa region (Tunisia) and phosphate samples (18 mass% of P₂O₅) from Cheketma-Kasserine region (Tunisia), was studied from a kinetic and thermal aspect. Experiments were performed by using a Tian—Calvet calorimeter. Two acid solutions were used for the dissolution: one with phosphoric acid (S1) and the other a mixture of phosphoric and sulfuric acid (S2). For both natural phosphates, it was found that in case of using S1 solution the heat released due to the dissolution was lower than in case of using solution S2. This difference was explained by the precipitation of monohydrate sulfate calcium to its dihydrate form. By using a granulometry distribution lower than 500 μm, heat released during the dissolution of both phosphates by S1 was similar, i.e., − 230 J g⁻¹, and the same observation was done by using S2 solution, i.e., between − 300 and − 350 J g⁻¹. We have demonstrated that granulometry distribution plays an important role, and by using a granulometry lower than 120 μm for Cheketma-Kasserine region phosphate, the heat released during the dissolution was higher, i.e., − 400 J g⁻¹ with solution S2. Avrami model was found to describe the precipitation of calcium sulfate, and three distinguished domains were obtained by using Gafsa region phosphate compared to two domains with Cheketma-Kasserine region phosphate.