Reducing Iron Oxide Content in Phosphoric Acid Using Polyacrylates and Phosphonic Acids

Phosphoric acid is the second most produced and consumed inorganic acid in the world and the most important intermediary in the production of phosphate fertilizers. The quality and agronomic utilization of phosphate fertilizers are highly dependent on the levels of contaminants present on the phosphoric acid, especially iron oxides. The presence of iron oxides in phosphate fertilizers results in the formation of iron phosphates possessing lower agronomic efficiency and a reduced commercial value. The wet-process phosphoric acid uses phosphate rocks or concentrates as a source of P₂O₅. Most phosphate rocks in Brazil originate from igneous mines, with high concentrations of iron oxides, as opposed to most mines in operation in the world, with sedimentary origins and lower contaminant contents. Iron content is a limiting factor in the economic and technical feasibility of phosphate mines, and the complete removal of iron oxides during the industrial concentration of phosphate rock is unfeasible due to phosphate losses. To improve the quality of the phosphoric acid produced from such ores and to enable the usage of phosphate rock concentrates with higher levels of contaminants, this study aimed to evaluate the effectiveness of two compounds for removing the iron oxides from the acid: (i) a mixture of polyacrylates associated with phosphonic acids and (ii) a solution with 60% of diethylenetriamine penta (phosphonic methylene) (60% DTPMP). The mixture of polyacrylates associated with phosphonic acids showed limited effectiveness in removing contaminants from the phosphoric acid. The DTPMP solution, however, proved effective as an iron chelating agent in phosphoric acid, whereas the Fe₂O₃ content on a dry basis was reduced by 40% and the P₂O₅/Fe₂O₃ ratio was increased by 59%. The optimal reactional conditions were <i>T</i> = 55 °C, 14 h of precipitation time, and 5.57% of chelating reagent in relation to the initial phosphoric acid mass (w/w). A total of 0.58% of Fe₂O₃ was removed from the acid, a yield comparable to the 0.55% observed in other benchmark studies.