Solubility measurements of synthetic neodymium monazite as a function of temperature at 2 kbars, and aqueous neodymium speciation in equilibrium with monazite

The solubility of synthetic NdPO4 monazite end-member was experimentally determined from 300 up to 800 °C, at 2000 bars in pure water, and in aqueous chloride or phosphate solutions. Both the classical weight-loss method and a new method based on isotope dilution coupled with thermal ionization mass spectrometer were used. In the range of temperature studied monazite showed a prograde solubility from 10−5.4 m at 300 °C up to 10−2.57 m at 800 °C. Experiments in H2O–H3PO4–NaCl–HCl solutions suggested Nd(OH)30 was the major species that was formed at high temperature and pressure. The equilibrium constants (log K) for the reaction: (NdPO4)S+3H2O⇔Nd(OH)30+H3PO40were determined at 650 and 800 °C, and are −6.3 and −5.5, respectively.At 300 °C and 2000 bars, it was shown that the dominant Nd-bearing species were Nd(OH)2+ in acid solution and Nd(OH)2+ in neutral conditions, while Nd(OH)30 species dominate in alkaline solutions.