Abstract:To address northern mixed light rare earths accounting for 2% of medium and heavy rare earths, effects of aluminum content, solution concentration, precipitation mode on formation and growth of crystal nucleus of Sm-Eu-Gd (SEG) carbonate were investigated. The results show that sediment is composed of Sm2(CO3)3?2.85H2O, Eu2(CO3)3?3H2O and Gd2(CO3)3?3H2O under the conditions including rare-earth concentration of 200 g/L, aluminum content of 0.018 g/L in SEG solution, crystal seed of 50%, and forward precipitation. Effect of aluminum hydrolysis can be eliminated if aluminum content is lower than 0.02 g/L, resulting in large size, high degree of crystallization, and easy filtration of Sm-Eu-Gd carbonate grains.