Phosphoric acid production by attacking phosphate rock with recycled hexafluosilicic acid

Abstract

During the classical production of phosphoric acid, the reaction between sulfuric acid and phosphate rock generates suspension. The latter is subsequently filtered and concentrated by evaporation process resulting in fluorine or acid gas. In this paper, we present a new procedure of phosphoric acid production by using recycled hexafluosilicic acid instead of sulfuric acid.Fluorine recycling process involves several steps starting with the bubbling of the resulting acid gas through hexafluosilicic acid solutions followed by a pre-concentration step before attacking the natural phosphate rock. The advantages of such procedure lies in decreasing the sulfuric acid quantities needed for the phosphoric acid production as well as in the environmental benefits from the recycling process of the harmful fluorine gas.In this work, phosphate rock was attacked by a mixture of hexafluosilicic acid (20% by weight) and recycled phosphoric acid (25% in P2O5). The cake obtained by vacuum filtration of the slurry was characterized by X-ray diffraction (XRD), Differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The main peaks obtained by XRD were attributed to chukrovite (Ca4AlSiSO4F13·12H2O), gypsum, and silica quartz. However, the calcium content in the produced phosphoric acid has to be maintained lower than 3.5% in order to avoid the formation of cloudy phosphoric solutions by the formation of CaF2 which affects the filtration of the mixture.