The wet-process for phosphoric acid production

Abstract

Project Purephos was initiated by Foskor, to investigate some aspects of the phosphoric acid industry, in particular, the effect of impurities in phosphate rock, commonly found by their clients, on the crystallisation of calcium sulphate dihydrate and the produced phosphoric add. The saturation and supersaturation curve for Palfos 88P rock was determined. This will assist producers to optimise, and understand the impact of the free sulphate level on the crystallisation of calcium sulphate dihydrate, and the co-precipitation, and thus loss, of P20s. The influence of sodium, potassium, magnesium and the aluminium/fluoride ratio were investigated. It was found that although sodium and potassium promote crystal growth, the filtration rates could decrease with elevated levels, as a result of the precipitation of hexafluorosilicates. Elevated levels of magnesium and aluminium increase acid viscosity and thus reduce filtration rates. The effect of aluminium can however be reduced, by controlling the aluminium/fluoride ratio. This study attempts to readdress the misconception that igneous rock is more difficult to process, and show that by using high-grade igneous rock, under controlled conditions, the benefits of igneous rock can be enjoyed.