Study of the solubility of sediments of certain lantanoids in carbonate solutions

The study examines the dissolution process for the precipitates of the rare-earth carbonates and phosphates with a change in the concentration parameters of the system. The objective of the study is to determine the nature of the dissolution process at variable composition of the studied solutions. As a result of the theoretical base analysis, the qualitative and quantitative content of rare-earth metals in red sludge and phosphogypsum is presented, which governs the interest in this problem under consideration. The available techniques for extraction of rare-earth metals from red sludge and phosphogypsum have also been considered, their advantages and disadvantages are presented. A promising carbonate-alkali method for the extraction of rare-earth metals has been considered. Thermodynamic parameters of the system have been calculated to determine the possibility of the process. During the experiment the factors affecting the dissolution process have been examined: the carbonate ion concentration, the nature of the solvent cation and rare-earth metal cation. Carbonates and phosphates of europium, holmium, and neodymium have been considered as the analyzed components. Carbonates and phosphates of neodymium, europium, and holmium have been dissolved in model solutions of K2CO3 with the concentrations of 0.2-4.5 mol/L until reaching equilibrium. The obtained results show the dependence of solubility of the precipitates of rare-earth metals on the solvent nature and concentration. The explored data are presented as solubility isotherms. On the basis of experimental data the values of the extraction degree of a rare-earth metal into the solution have been calculated. For neodymium, europium, and holmium carbonates, the maximum recovery degree αmax is 72.8 %, 81.2 %, 83.0 %, respectively. During the experiment with dissolution of lanthanide phosphates, the following results have been obtained: the maximal degree of neodymium extraction is 60.59%, while for europium it equals 51.66 % and 93.01 % for holmium.