Removal of Fe(III) from sulfuric acid leaching solution of phosphate ores with bisphosphonic acids

Iron(III) in phosphate ore leach liquor harms the quality of phosphoric acid and phosphogypsum crystallization. In this paper, the selective separation of iron(III) was studied with two novel extractants, N, N-n-octylamine di(methylene phenylphosphonic acid) (OADMPPA) and N, N-n-hexadetamine di(methylene phenylphosphonic acid) (HADMPPA), from a mixed media of sulfuric acid and phosphoric acid. The effects of temperature, time, diluents, and the concentrations of sulfuric acid and extractant on the separation of iron were investigated. The mechanism of extraction was established by UV–Vis spectroscopy, FT-IR spectroscopy, slope analysis, and the equimolar series method. The results showed that the two new reagents have good extraction ability toward iron(III) even at higher sulfuric acid concentration. Both OADMPPA and HADMPPA can remove more than 99% of iron(III) from the sulfuric acid leach liquor of phosphate ore. The stripping of iron(III) from the organic phase after extraction with acidic phosphine (phosphorus) extractants was difficult. Oxalic acid was found to be a good stripping agent. The single stage stripping efficiency reached 62.8%, and the two-stage stripping efficiency could reach over 99%. The separation and recovery of iron(III) in phosphate ore leach liquor could be realized by our method without changing the traditional phosphoric acid production process, which is beneficial to the purification of phosphoric acid and the recovery of iron. •Two new extractants OADMPPA and HADMPPA were designed and synthesized.•They have high extraction rate and selectivity for Fe3+ in sulfuric acid medium.•Their extraction rates of Fe3+ were 40 times higher than common extractants.•The extraction mechanisms of Fe3+ with extractants were proved to be ion exchange.•Difficulty in stripping of Fe3+ from the loaded organic phase was resolved.