KINETICS OF THE JAROSITE/HEMATITE CRYSTAL TRANSITION IN A SIZE CLASSIFIED CRYSTALLIZER

Abstract

The crystallization kinetics of hydronium jarosite have been studied in the area of the Fe₂O₃-SO₃-H₂O phase diagram where hematite is the stable phase. Hydronium jarosite has been shown to be a kinetically favored intermediate to hematite over a wide range of chemical and thermal conditions. A model useful for predicting the crystal size distribution as a function of temperature, free acid and iron concentrations, and residence time has been developed. Hydronium, sodium, and potassium jarosite have been shown to convert, via a solid phase reaction, to hematite. A model useful for predicting the conversion rate as a function of temperature, free acid concentration, and particle size has been developed. A predictive model, the growing core model, has been developed. It is useful for predicting the crystal size distribution and the product split between hydronium jarosite and hematite when both crystallization and conversion are occurring simultaneously. The cardinal assumption in this model is that crystal growth and conversion occur at separate cites on the crystal surface simultaneously. The model, with only one adjustable constant, has been verified with experimental results. The effect of double draw off (DDO) operation in this system has been demonstrated. It has been shown, via the growing core model and experimental results, that DDO operation can produce a high iron, low sulfur product. Without DDO operation, this high product quality can only be achieved through higher operating temperature, high neutralization rates, or very large vessel size.