Maintenance of the Metastable State and Induced Precipitation of Dissolved Neodymium (III) in an Na2CO3 Solution

Rare earths dissolved in carbonate solutions exhibit a metastable state. During the period of metastability, rare earths dissolve stably without precipitation. In this paper, neodymium was chosen as a representative rare earth element. The effects of additional NaCl and CO2 on the metastable state were investigated. The metastable state can be controlled by adding NaCl to the Na2CO3 solution. Molecular dynamics studies indicated that the Cl− provided by the additional NaCl partially occupied the coordination layer of Nd3+, causing the delayed formation of neodymium carbonate precipitation. In addition, the additional NaCl decreased the concentration of free carbonate in the solution, thereby reducing the behavior of free contact between carbonate and Nd, as well as resulting in the delay of Nd precipitate formation. Consequently, the period of the metastable state was prolonged in the case of introduction of NaCl. However, changing the solution environment by introducing CO2 can destroy the metastable state rapidly. Introduction of CO2 gas significantly decreased the CO32− content in the solution and increased its activity, resulting in an increase of the free CO32− concentration of the solution in the opposite direction. As a result, the precipitation process was accelerated and the metastable state was destroyed. It was possible to obtain a large amount of rare earth carbonate precipitation in a short term by introducing CO2 into the solution with dissolved rare earths in the metastable state to achieve rapid separation of rare earths without introducing other precipitants during the process.