Synthesis of High-Purity Hydroxyapatite and Phosphoric Acid Derived from Moroccan Natural Phosphate Rocks by Minimizing Cation Content Using Dissolution-Precipitation Technique.

This study investigates, in the first part, the synthesis and purification of a poorly crystalline hydroxyapatite (HAp) using natural Moroccan phosphate (Boucraa region) as a raw material. Despite its successful preparation, the obtained HAp was contaminated by several metallic cations (mostly Cd, Pb, Sn, Ti, Mn, Mg, Fe, and Al) migrated from the natural rocks during the digestion process, inhibiting HAp application in several sectors. To minimize the existence of these elements, the dissolution-precipitation technique (DP) was investigated as a non-selective purification process. Following the initial DP cycle conducted on the precipitated HAp, the removal efficiency was approximately 60% for Al, Fe, Mg, Mn, and Ti and 90% for Cd and Pb. After three consecutive DP cycles, notable improvement in the removal efficiency was observed, reaching 66% for Fe, 69% for Mg, 73% for Mn, and 74% for Al, while Cd, Pb, and Ti were totally removed. In the second part of this study, the purified HAp was digested using sulfuric acid to produce high-quality phosphoric acid (PA) and gypsum (GP). The elemental analysis of the PA indicates a removal efficiency of approximately 89% for Fe and over 94% for all the examined cations. In addition, the generated GP was dominated by SO and CaO accompanied with minor impurities. Overall, this simple process proves to be practically useful, to reduce a broad spectrum of cationic impurities, and to be flexible to prepare valuable products such hydroxyapatite, phosphoric acid, and gypsum.