Purification of high iron wet-process phosphoric acid via oxalate precipitation method

Abstract

A method has been developed to reduce the iron content in phosphoric acid produced by the dihydrate wet process from open cast phosphate rock of Abu-Tartur mine in Egypt. In this method, oxalic acid was used to precipitate iron as ferrous oxalate dihydrate (FeC2O4.2H2O). The achieved removal efficiency of iron was about 91% due to the low solubility of the precipitated salt in dilute phosphoric acid medium. The maximum P2O5 losses were less than 1.0%. The optimum conditions of oxalate precipitation method were obtained using synthetic dilute phosphoric acid (28% P2O5, 390 g/L H3PO4) with iron content amounting 2.8% (w/w) Fe2O3 (26.3 g/L Fe). It was found that, the clarification time, reaction temperature, P2O5 concentration and oxalic acid dose were 2 h, 60 °C, 28% P2O5 and 7.5 g oxalic acid/100 g 28% P2O5 acid, respectively. These conditions were applied using wet-process phosphoric acid (26% P2O5). Firstly, ferric ions in the acid were reduced to ferrous ions using iron scrap, followed by oxalic acid addition. Mostly all iron ions in the solution are in the divalent state at Electro Motive Force (EMF) values less than 275 mV. The solubility of ferrous oxalate dihydrate was measured at different temperatures and phosphoric acid concentrations. A recovery study for oxalic acid from the precipitated FeC2O4.2H2O has performed.