An experimental study of the solubility of rare earth chloride salts (La, Nd, Er) in HCl bearing water vapor from 350 to 425 °C

The solubilities of the rare earth chlorides REECl3, where REE = (La, Nd, Er), were measured in HCl bearing water vapor from 350 to 425 °C with water partial pressures ranging from 8 to 170 bar. Solubility data were fit to the Pitzer-Pabalan quasi-chemical model in order to extract thermodynamic parameters for the formation of the gaseous REE-chloride-water clusters REECl3(H2O)n. The data show that the solubility of the REE chlorides are orders of magnitude higher than salts such as NaCl or CuCl at low water fugacities, despite their sublimation energies being substantially higher. This enhanced solubility is likely due to the high enthalpy associated with binding a single water molecule to form the species REECl3(H2O), with derived enthalpies ranging from −378 to −465 kJ/mol. Addition of further water molecules to form higher order clusters (n > 1) involves enthalpy changes of ~−20 kJ/mol, and are in effect thermodynamically suppressed over the temperature range 350–425 °C. Despite the enhanced solubility of small REECl3 water clusters, simulations of boiling processes demonstrate that the REE show highly conservative behavior, partitioning strongly into the dense aqueous phase. Not surprisingly, the presence of phosphates in the system makes this effect even more pronounced, completely immobilizing the REE. This would reduce transport in both the vapor and aqueous phase to negligible levels. However, we suggest that vapor phase transport of the REE may play a significant role in systems having a relatively low partial pressure of water (below the saturation point), where the relatively high stability of the first hydrated REE chloride clusters (REECl3(H2O) and REECl3(H2O)2) will give a preference for gas transport of the REE relative to other elements. This can likely happen in systems involving a gas/melt exchange in fumarolic exhalations, where water vapor discharges at relatively low (close to atmospheric) pressures.