The influence of aluminium fluoride in hemi-dihydrate phosphoric acid processes

Bench-scale continuous crystallization experiments were performed with three different types of phosphate ores in a cascade of bench-scale crystallizers, thus simulating the calcium sulfate hemihydrate (CaSO₄·1/2H₂O) crystallization section in a two-filter hemi-dihydrate phosphoric acid process. The effect of aluminium fluoride in the system was investigated by the addition of an aluminium salt and the use of ores with different aluminium contents. Aluminium fluoride, which might affect the crystallization by its growth-retarding AIF^2-₅ complex, promotes the formation of DH (CaSO₄·2H₂O) modification and the formation of agglomerates of short crystals, and therefore influences the permeability of the crystal product formed. The aluminium distribution coefficient ([Al] in solid/[Al] in liquid) in calcium sulfate hemihydrate decreases strongly with increasing aluminium concentration in the liquid. Consequently, low aluminium distribution coefficients are encountered when an aluminium-rich phosphate ore is used and vice versa. Finally, from mass balance calculations, the maximum allowable aluminium concentrations in the phosphate ores are predicted for hemi(dihydrate) phosphoric acid processes. Above these concentrations, unwanted calcium sulfate dihydrate DH formation can take place but this does not always occur.