Recovery of Zr, Hf, Nb from eudialyte residue by sulfuric acid dry digestion and water leaching with H2O2 as a promoter

Following the initial hydrometallurgical extraction of rare earth elements from the eudialyte mineral, the residue is known to contain additional valuable metals. In this reported study, a two-stage acidic treatment was investigated to extract Zr, Hf, Nb from the eudialyte residue. The objective was to obtain high recovery yields with a low consumption of acid, so the proposed preprocessing step was sulfuric acid dry digestion, which was held above the boiling point of water. The treated residue was then leached with water and H2O2 was added to promote the leaching of Zr, Hf and Nb. It was found that the H2O2 increased the oxidation potential of the acidic system, which assisted the decomposition of the mineral phases that contained Zr, Hf and Nb. In addition, the H2O2 acted as a reaction promoter by providing O22- ions that formed complexes with the cations, shifted the reaction equilibrium and inhibited hydrolysis of Nb(IV). The QEMSCAN, XPS and SEM analysis were used to reveal the mineralogical characteristics of the solid samples. These characteristics demonstrated the chemical changes that occurred during extraction and helped to explain the mechanisms at work during the processes. The SEM analysis results showed that metals sulfate salts and quartz were present after the digestion process. The treated eudialyte residue leached the soluble salts into solution, whereas the quartz remained in the residue. After optimizing the treatment parameters by varying the process conditions it was found that the eudialyte extract produced 89.1% Zr, 81.2% Hf and 71.2% Nb and the most suitable process parameters were: digestion conditions: H2SO4/eudialyte residue: 25 ml/100 g, water/eudialyte residue: 20 ml/100 g, 110 °C, 4 h; leaching conditions: L/S: 3:1, H2O2/eudialyte residue: 10 ml/100 g, room temperature, 1 h.