THE INFLUENCE OF ALUMINUM 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 c rystallizers, thus simulating the calcium sulfate hemihydrate (CaSO4 . 1/2H(2)O) crystallization section in a two-filter hemi-dihydrate phos phoric acid process. The effect of aluminium fluoride in the system wa s investigated by the addition of an aluminium salt and the use of ore s with different aluminium contents. Aluminium fluoride, which might a ffect the crystallization by its growth-retarding AlF52- complex, prom otes the formation of DH (CaSO4 . 2H(2)O) modification and the formati on of agglomerates of short crystals, and therefore influences the per meability of the crystal product formed. The aluminium distribution co efficient ([Al] in solid/[Al] in liquid) in calcium sulfate hemihydrat e decreases strongly with increasing aluminium concentration in the li quid. Consequently, low aluminium distribution coefficients are encoun tered when an aluminium-rich phosphate ore is used and vice versa. Fin ally, 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 calci um sulfate dihydrate DH formation can take place but this does not alw ays occur.