Influência da concentração de óxidos de ferro e variáveis de processo na produção de superfosfatos simples

Abstract

The single superphosphate (SSP) is manufactured by the direct reaction between phosphate concentrates and sulfuric acid in presence of water, in complex long-lasting reactions, whose results are highly influenced by the kinetic factors of the reaction medium. Approximately 95% of the phosphate concentrates used in Brazil originates from phosphate rocks extracted from mines of igneous origin, due to the country's geological vocation. The apatite, [Ca5(PO4)3(OH,F,Cl)], is mined from these deposits as it is the main source of phosphorus in igneous phosphate rocks. However, it is present in soils at low concentrations and thus associated to several contaminants. One of the main contaminants present in mines of igneous origin is iron, generally expressed as Fe2O3, but present in the forms of hematite, magnetite, goethite, among others, whose total removal during the rock’s concentration is technically and economically unfeasible. The high presence of iron oxides in SSP production results in the formation of water insoluble iron phosphates, which are partially insoluble in neutral ammonium citrate (NAC), possessing lower agronomic efficiency on most crops and usually bellow the established solubility specifications. With the objective of producing SSP using phosphate concentrates with high iron oxides concentrations, a Central Composite Design was created and executed enabling the joint verification of the effects of iron oxide concentrations associated to the usual kinetic variables of the SSP production, with utilization of phosphate concentrates containing up to 14.74% of Fe2O3. It was verified that by adopting certain kinetic conditions in association with the iron contents in the reaction medium it was possible to maximize the NAC solubility on SSP, with conversions nearing 100% achieved in the experiments. The high conversions reached partially offset the lower total P2O5 contents in the samples and demonstrated technical feasibility for the industrial implementation of the routes generated at the present work.