The effect of calcium sulfate crystallization and the crystal modification on aqueous REE stability in Ca saturated REE-Ca-SO4-H2O systems

Impurity removal is a key stage in the hydrometallurgical processing of rare earth elements (REE), which is performed after water leaching of the acid baked REE concentrate. Lime neutralization is deemed a preferred process due to its high efficiency in the removal of impurities and low reagent cost. However, some drawbacks such as co-precipitation of REE with calcium sulfate dihydrate (CaSO4·2H2O, CSD), which results in a significant REE loss, renders this process inefficient. A feasible route to eliminate or minimize the REE loss associated with the lime neutralization process is to use a crystal habit modifier to alter the crystalline morphology and surface properties of CSD and subsequently reduce the portion of REE loss to the CSD. In this study, cetryltrimethylammonium bromide (CTAB) and polyacrylic acid (PAA) as cationic and anionic modifiers were employed at different concentrations of 0.2, 2 and 5 g/L to reduce the REE loss to CSD in a titration process simulating the impurity removal step. In addition, the effect of temperature and Ca(OH)2 pulp density on the co-precipitation process and morphology changes were investigated. It was observed that the cationic modifier (CTAB) reduced the fraction of REE that co-precipitate with CSD and 2 g/L CTAB was chosen as the optimum dosage of CTAB. However, the anionic modifier (PAA) deteriorated the process by helping an average of about 70% of REE to co-precipitate with CSD at low pH values (1-5). ζ-potential analysis on the precipitates revealed a more negative surface charge in the precipitates obtained from the PAA-containing solution and a less negative surface charge in the precipitates from CTAB-containing solution compared to the no-additive experiments that explains the mechanism of co-precipitation of positively charged trivalent REE ions with CSD crystals. Increasing the precipitation process temperature from 25 to 50 °C resulted in a slight increase in the fraction of REE loss and further increase in the temperature up to 70 °C resulted in a lower REE loss compared to 50 °C. Similarly, increasing pulp density of the Ca(OH)2 slurry from 0.5 to 10% showed an increase in the fraction of REE loss to CSD by changing the supersaturation of the system. Effect of all the parameters under investigation on the morphology changers of CSD crystals were studied by HR-SEM and the elemental composition of the precipitates were probed by XPS analysis.