Dissolution behavior of MgO–pyrochlore composites in acidic solutions

The dissolution behavior of sintered MgO–pyrochlore (Nd2Zr2O7 was chosen for this study) composites in HNO3 and H2SO4 solutions has been studied. The dissolution in 11 M HNO3 and 7.9 M H2SO4 at 60 °C resulted in a selective dissolution of MgO. It was found that initially the fraction of dissolved MgO increases linearly with dissolution time. Magnetic bar stirring enhanced the mass transfer rate and resulted in a higher dissolution rate of MgO compared with the static and ultrasonic dissolution. It also provided mechanical forces to completely disintegrate the undissolved Nd2Zr2O7 porous matrix into residual powder. Both MgO and Nd2Zr2O7 can be dissolved in boiling concentrated (18 M) H2SO4, and Nd2Zr2O7 dissolves incongruently. The dissolution of Mg2+ and Nd3+ followed first order kinetics, but Zr4+ precipitated out due to low solubility in concentrated H2SO4.