Nature of influence exerted by Na2SiF6 on REE recovery from orthophosphoric acid solution in the course of CaSO4·0.5H2O crystallization

Abstract

Chemical and X-ray phase analyses were used to study the influence exerted by Na₂SiF₆ on the isomorphic inclusion of cerium into the structure of CaSO₄∙0.5H₂O precipitates formed from solutions of phosphoric acid hemihydrate (38 wt % P₂O₅). The poorly soluble suspensions of CaSO₄∙0.5H₂O precipitates can serve as adsorbents for cerium compounds, with CaSO₄∙0.5H₂O–NaCe(SO₄)₂∙H₂O and CaSO₄∙0.5H₂O–CePO₄∙0.5H₂O solid solutions formed. The introduction of Na₂SiF₆ makes the sorption properties CaSO4∙0.5H2O several times better because the Na₂SiF₆ phase is a source of sodium cations and creates the necessary Na: Ce ratio of 1: 1 for extracting cerium from the liquid phase into a precipitate in the form of a CaSO₄∙0.5H₂O–[NaCe(SO₄)₂∙H₂O + CePO₄∙0.5H₂O] solid solution. Under the industrial conditions in which extraction phosphoric acid is manufactured, a similar isomorphous capture of rare-earth elements of the cerium group (La–Sm) may occur in joint precipitation of CaSO₄∙0.5H₂O and Na₂SiF₆.