Recovery of REEs, Zr(+Hf), Mn and Nb by H₂SO₄ leaching of eudialyte concentrate

In this study three hydrometallurgical methods are described for leaching of a eudialyte concentrate with H2SO4: (i) direct leaching, (ii) fast leaching and (iii) water leaching of dehydrated acid/concentrate mixture. It is demonstrated how to obtain a silica free solution, how parameter variations impact the properties of precipitated silica and which processes lead to losses of valuable components during leaching. Furthermore, the acid solubility of gangue minerals in the concentrate is analyzed and the resulting consequences in terms of leach solution contamination and acid consumption are discussed. The best result in terms of the average yield of value components (REEs, Zr(+Hf), Mn and Nb) of 86 % is obtained by direct leaching under mild conditions (cH2SO4=1 mol/L; TL=60 °C). However, released silicic acid does not precipitate and aggregates at pulp density ϱPD,L=100 kg/m3 by gelling. Fast leaching allows the efficient removal of silica at high solid-liquid ratios in the pre-treatment stage. Due to mass transfer limitations, high efficiency stirrers are crucial for achieving high yields in short reaction times. Dehydration of the acid/concentrate mixture before water leaching can be a good alternative if well-defined amount of acid is used; however, high energy input is needed.