SEMI-BATCH PRECIPITATION OF CALCIUM SULFATE DIHYDRATE FROM CALCITE AND SULFURIC ACID

The present work focuses on the physical and chemical aspects of a semi-batch precipitation of gypsum by injecting a calcite suspension to a sulfuric acid solution from industrial waste. The morphology of the precipitated crystals evolves from a needle-like shape to a platelet-like shape when temperature is increased from 25 to 90 degrees C; when the initial concentration of the acid is increased from 15 to 30 wt%; or when the inlet flow of the suspension of calcite is decreased from 20 down to 5ml/min. The mean particle size increases from about 15 mu m to about when temperature is increased from 25 to 90 degrees C and to about 160 mu m when the inlet flows is decreased to 5 ml/min. A decrease in the speed of the propeller below 400 rpm is to be avoided as it induces the apparition of an undesired small sized crystal population to the detriment of the desired big sized crystal population. Apparent density of synthesized powders is affected by none of the parameters investigated but the inlet flow of the suspension which, being decreased from 20 to 5 ml/min, leads to an increase in the apparent density from 0.3 to 0.5. For each experiment, pH in water solution is about 8. Investigations are conducted on the influence of three calcite batches: two high purity calcites A1 and A2 from Urgonien limestone and a calcite B from Corbieres limestone with high magnesium content. The presence of magnesium is suspected to lower the crystal size by inhibiting crystal growth, as mean particle size and apparent density are found to be smaller with a magnesium-rich calcite than with a high-purity calcite. Seeding experiments were conducted but did not show any enhancement in the mean particle size and in the apparent density.