Estudo da redução do teor de óxidos de ferro na produção industrial de ácido fosfórico

Abstract

Phosphoric acid is the second most produced and consumed inorganic acid in the world and the most important chemical intermediate in the production of phosphate fertilizers, serving as raw material for all high concentration phosphates. The agronomic potential of fertilizers produced with phosphoric acid is highly dependent on its contaminant levels, especially iron oxides. The presence of various iron oxides on phosphate rocks or concentrates can generate fertilizers containing iron phosphates, which are partially soluble on neutral ammonium citrate (NAC) and water-insoluble, therefore of lesser agronomic and commercial value. Approximately 95% of the phosphate rocks used in Brazil originates from igneous mines, on which iron oxides are the main contaminants and its total removal during the rocks` beneficiation is technical and economically unfeasible. Aiming at improving the phosphoric acid quality for fertilizer production, as well as enabling the use of phosphate concentrates with high contaminant levels as raw material for its production, the present work had as objective the development of routes enabling the removal of iron oxides from the acid. Preliminary bench tests of 43 different purification routes led to 4 potential processes, which were mapped through the Central Composite Design (CCD) technique. The experimental designs results showed that up to 75% of the Fe2O3 was removed from phosphoric acid by the usage of aminotris(methylenephosphonic acid) on the defined temperature and precipitation time. To validate the developed route, a pilot test followed by two industrial trials conducted on a 900 t P2O5/day industrial site, on which were consumed 50 metric tons of test reagent. The trials indicated technical and industrial viability for the iron oxides removal thus leading to consequent purification of phosphoric acid.