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

摘要

The solubility of synthetic NdPO4 monazite end-member was experimentally determined from 300 up to 800 degrees 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 degrees C up to 10(-2.57) m at 800 degrees C. Experiments in H2O-H3PO4-NaCl-HCl solutions suggested Nd(OH)(3)(0) was the major species that was formed at high temperature and pressure. The equilibrium constants (log K) for the reaction: (NdPO4)(S) + 3H(2)O double left right arrow Nd(OH)(3)(0) + H3PO40 were determined at 650 and 800 degrees C, and are -6.3 and -5.5, respectively. At 300 degrees 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)(3)(0) species dominate in alkaline solutions.