Kinetics of the Kola Apatite Rock Interaction With H3PO4

Abstract

Besides the production of fertilizers, Kola phosphate rock may be used as a source of lanthanides, strontium and fluorine. It implies the necessity to carry out a study of kinetics and mechanisms of the process in order to choose optimal conditions for the realization of the technological scheme 1]. The fluorapatite concentrate used had the following composition: ?Ln₂O₃ - 0.89; Y₂O₃ - 0.04; SrO - 2 80; CaO - 45.40; Fe₂O₃ -0.42; Al₂O₃ - 0.86; MgO - 0 10; F - 2.80; SiO₂ - 1.80; P₂ ^O ₅ - 39.40 weight %; molar ratio CaO : P₂O₅ = 1.5; the content of the apatite - 98.5%. The reaction of H₃PO₄ with fluorapatite was studied using a laboratory reactor with a stationary layer. The following parameters were varied: H₃PO₄ concentration (20, 30 and 50 weight % P₂O₅), temperature (20, 75, 150, 200 and 250°C) and time of contact (1–180 min.). A multimethod approach was used. X-ray diffraction, electron probe microanalysis and paper chromatography were applied to follow the bulk structural aspects of the apatite powders. It was shown that at the first stage of reaction a thin film of calcium monophosphate Ca(H₂PO₄)₂ H₂O is deposited on the apatite particles (avg. diameter 150 nm). The reaction is thought to proceed at the interphase solid/liquid and its kinetics may be described by the equation