Phosphoric acid production by attacking phosphate rock with recycled hexafluosilicic acid

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.