Dissolution of Gibbsite and Its Transformation to Taranakite Depending on the Concentration of Phosphate Anions in the Solution

Structural changes of gibbsite at the binding of phosphate anions from NH4H2PO4 solutions of different concentrations (from 10(-6) to 2 mol P/L) have been studied using an X-ray diffractometer and an electron scanning microscope with a spectrometric microanalyzer. It has been shown that, beginning from a concentration of 10(-3) mol P/L, the binding of phosphate anions has been accompanied by the release of anionic aluminum-phosphate complexes into the liquid phase. The content of these complexes has increased with the phosphate solution concentration and the time of its contact with gibbsite. The treatment of gibbsite with a 1 M phosphate solution for two months has resulted in the pronounced dissolution of the gibbsite crystal surface, accompanied by the formation of ammonium taranakite (ammonium salt of complex aluminophosphoric acid). Successive changes in size and form of the initially formed ammonium taranakite crystal has occurred in a 2 M phosphate solution. The degradation of the layered structure of taranakite and the enrichment of degradation products with phosphorus and nitrogen have been revealed. The significant increase of the P/Al ratios in the degradation products compared to the corresponding ratio in the initially formed taranakite indicates the formation of new phosphate phases resulting from the transformation of taranakite under the impact of free phosphate anions in a high-concentration solution.