An experimental study of the dissolution stoichiometry and rates of a natural monazite as a function of temperature from 50 to 230 °C and pH from 1.5 to 10

Steady state dissolution rates of Manangotry monazite ((Ca0.04La0.21Ce0.43Pr0.05Nd0.15Sm0.02Gd0.01Th0.13)P0.90Si0.09O4) were determined in open system titanium mixed flow reactors at pH = 2 at temperatures from 50 to 229 degreesC, and at pH 1.6, 2.6, and 10 at 70 degreesC. Dissolution rates at 70 degreesC and pH 2, 6, and 10 were determined from closed system experiments. All dissolution rates are calculated from release rates of Cc into solution as measured by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Measured pH=2 monazite dissolution rates increase from 4.3 x 10(-17) to 1.2 x 10(-14) mol/cm(2)/s with increasing temperature from 50 to 229 degreesC. This is consistent with an apparent activation energy of 10.3 kcal/mol. Measured and estimated 70 degreesC monazite dissolution rates reach a minimum at near neutral pH; dissolution rates were found to be 1 X 10(-16), 4 X 10(-18), and 4 X 10(-17) mol/cm(2)/s at pH = 2, 6, and 10, respectively. The light rare earth elements (REE) (La, Ce, Pr, and Nd) and uranium were found to be released in approximately stoichiometric quantities in all experiments. Sm/Ce and Gd/Ce ratios in the outlet solutions were found to be slightly higher than those of the dissolving monazite at all pH and temperatures. In contrast, thorium release stoichiometry depended on pH and temperature. Th was released at close to stoichiometric proportions at basic conditions, but was released to solution in far lower proportions at pH 2, likely due to the precipitation of a Th-rich secondary phase. (C) 2002 Elsevier Science B.V. All rights reserved.