Studies on the transformation of calcium sulphate dihydrate to hemihydrate in the wet process phosphoric acid production

The influence of the process temperature from 85°C to 95°C, the content of phosphates and sulphates in the wet process phosphoric acid (about 22-36 wt% P₂O₅ and about 2-9 wt% SO₄^2-) and the addition of αCaSO₄·0.5H₂O crystallization nuclei (from 10% to 50% in relation to CaSO₄·2H₂O) on the transformation of calcium sulphate dihydrate to hemihydrate has been determined. The wet process phosphoric acid and phosphogypsum from the industrial plant was utilized. They were produced by reacting sulphuric acid with phosphate rock (Tunisia) in the DH-process. The X-ray diffraction analysis was used to determine the phase composition and fractions of various forms of calcium sulphates in the samples and the degree of conversion of CaSO₄·2H₂O to αCaSO₄·0.5H₂O and CaSO₄. It was found that the transformation of CaSO₄·2H₂O to αCaSO₄·0.5H₂O should be carried out in the presence of αCaSO₄·0.5H₂O crystallization nuclei as an additive (in the amount of 20% in relation to CaSO₄·2H₂O), at temperatures 90±2°C, in the wet process phosphoric acid containing the sulphates and phosphates in the range of 4±1 wt% and 27±1 wt%, respectively.