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Abstract:Optimization of the acid leaching process for Mongolian apatite-based ore containing rare-earth elements (REEs) was studied. The ore contained approximately 10% of REEs as total rare earth oxides, and the major impurities were Ca (33% as CaO) and Fe (23% as Fe 2 O 3 ). Fe bearing minerals could be removed by passing the sample through a wet high-intensity magnetic separator before leaching. After magnetic separation, basic leaching tests were conducted to investigate the influence of the acid type and concentration, temperature, and the pH on the REE leaching level and kinetics. Hydrochloric acid was found to be the most effective leaching agent, leaching more than 90% of REEs in an hour. However, the concentrations of Ca ions in the leachate were also high, which would complicate recovery of the REEs. Therefore, to reduce the amount of Ca ions in the leachate, a two-stage leaching procedure was attempted. In stage 1, the sample was preleached using 1.0 M hydrochloric acid to dissolve Ca. In stage 2, the solid residue of stage 1 was leached using 2.0 M hydrochloric acid to dissolve REEs. Consequently, this two-stage leaching significantly reduced the Ca concentration in the final leachate without affecting the leaching levels of REEs.

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Mineral carbonation of CO 2 with fine-grained waste cement was investigated using NH 4 Cl as a recyclable extracting agent. The amount of calcium extracted with NH 4 Cl was not as high as with more commonly used extraction agents such as HCl and CH 3 COOH. NH 4 Cl also exhibited high selectivity in the calcium extraction process, such that calcium ions comprised over 99% of the leaching solution. Another positive benefit of using NH 4 Cl was that precipitation of calcium carbonate by CO 2 injection was possible without the addition of basic reagents. Moreover, the NH 4 Cl regenerated during carbonation can be reused for calcium extraction. However, test results using regenerated NH 4 Cl solution in a cyclic fashion revealed that the process was not perfectly cyclic, but rather the calcium amount after precipitation increased as the cycle proceeded. The geochemical computer simulation PHREEQC was utilized to gain better insight into the cyclic mineral carbonation processes using NH 4 Cl solution. The simulation was based on thermodynamic equilibrium so that the amount of Ca in the solution fluctuated between specific values in a periodic fashion, unlike the experimental results of calcium accumulation in the extraction solution. One reason for this phenomenon was the kinetic/thermodynamic balance controlled by the amount of Ca 2+ and CO 2 present in the solution. However, it was feasible to use a geochemical model to evaluate the mineral carbonation process with the correction factors since the deviation between the experimental and the simulation results remained fairly constant throughout the cycle.

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