Inhibition of calcium sulphate hemihydrate crystallization under simulated conditions of phosphoric acid evaporation

Nucleation and crystal growth of CaSO4 ∙ 0.5H2O (calcium sulphate hemihydrate) was studied under conditions simulating scale formation in phosphoric acid production evaporators. Phosphoric acid, calcium hydrogen phosphate, and sulphuric acid were mixed at 88°C. The induction time was determined using a turbidimeter across a range of supersaturation from 1.3–3.5. Crystallization equations, taking as input the induction time and supersaturation, were used to calculate surface energy, nucleation rate, free energy, and the critical nucleus radius of CaSO4 ∙ 0.5H2O with and without the addition of 75 mg/L of various scale inhibitors: polyacrylic acid (PAA) 8000 g/mol, poly(diallyldimethylammonium chloride) (PDADMAC) <100 000 g/mol, or polyethyleneimine (PEI) 60 000 g/mol. The results suggest that PEI is capable of inhibiting crystal growth through the impediment of crystal nucleation.