Baotou mixed-type rare earth (RE) ore is one of the three typical RE minerals in China and the most important light RE resource. At present, about 90% of Baotou mixed-type RE concentrate is smelted and separated by sulfuric acid method. In this sulfuric acid treatment, the total concentration of RE in the purification liquid obtained after neutralization and impurity removal by MgO is low, and the dissolution behavior of RE2(SO4)3 crystal in the solution system (MgSO4-H2SO4-H2O) is not clear enough and need to be deeply studied. The mixed RE2(SO4)3 crystal was obtained from Baotou sulfuric acid roasted RE concentrate as raw material through water leaching and heating crystallization. The re-solubility and recovery performance of the mixed RE sulphate crystal in MgSO4-H2SO4-H2O system were studied by the control variable method. The effects of equilibrium pH, free water and crystal water, grinding and magnesium sulfate on the dissolution behavior of mixed RE2(SO4)3 crystals were investigated. The results showed that: (1) The mixed RE2(SO4)3 crystal obtained from recrystallization of RE sulphate leaching solution was mainly composed of La2(SO4)3∙9H2O, Ce2(SO4)3∙9H2O and Nd2(SO4)3∙2H2O with a small amount of impurities such as Fe(Fe2O3, 1.17%), P(P2O5, 1.58%), and Ca(CaO, 0.46%). The crystal could truly reflect the dissolution behavior of the mixed RE2(SO4)3 crystal in the purification and removal liquid system with low ionic strength; (2) With pH of the re-dissolved solution rising from 1.0 to 1.8, the concentration of total RE (in terms of RE oxides, REO) in the dissolution solution decreased from 20 to 18 g·L-1, which was lower than the concentration required by industrial production of more than 30 g·L-1; (3) The dissolution equilibrium time of RE2(SO4)3 crystal for drying to remove free water was shortened from 45 to 15 min, and the total concentration of RE in the re-dissolved solution was maintained at about 25 g·L-1. The total RE concentration of re-dissolved solution after roasting for removing the combined water showed an inverted "V" change, first increasing to 34.1 g·L-1 and then decreasing to 25 g·L-1 and tending to equilibrium. (4) Grinding the mixed RE2(SO4)3 crystal could only improve its dissolution rate, which reduced the dissolution equilibrium time from 45 min to less than 20 min. It could not improve the re-dissolution recovery performance of the mixed RE2(SO4)3 crystal, and the total RE concentration of the re-dissolved liquid remained at about 18 g·L-1. (5) According to the coordination chemical properties of RE in H2SO4-H2O system, RE in the neutralization and removal impurity solution system (MgSO4-H2SO4-H2O) mainly existed in the forms of RESO4+ and RE3+, and that indicated that increasing the concentration of SO42- within the appropriate concentration range could improve the saturation concentration of RE. The results of the magnesium sulfate concentration gradient experiment also verified this conclusion. When the concentration of MgSO4 (in terms of MgO) increased from 0 to 49.17 g·L-1, the total RE concentration of the re-dissolved solution increased from 18.1 to 36.4 g·L-1, and when MgSO4 concentration was more than 15 g·L-1, RE concentration in the re-dissolved solution could be realized more than 30 g·L-1. The main reasons for the low RE solubility in MgSO4-H2SO4-H2O system included : (1) In the sulfuric acid treatment, during the neutralization and removal of impurities, the change of solution system resulted in the reduction of anionic types (SO42- and H2PO4-) and the number of anions; (2) The presence of free water and bound water in the mixed RE2(SO4)3 crystal reduced its dissolution activity. It was determined that increasing the MgSO4 concentration was the key factor to significantly improve the recovery performance of mixed RE2(SO4)3 crystals. Through recrystallization of RE2(SO4)3 leaching liquid could realize the separation the most of RE and the impurities Fe, P, Al and H+, and the mixed RE2(SO4)3 crystals obtained for secondary dissolution to get RE2(SO4)3 solution with a low content of impurities. Then only a small amount of alkaline was needed for deeply removing impurities in this could obtain pure RE2(SO4)3 solution for extraction and separation, which could improve the efficiency of impurity removing, and reduced the RE loss. The results might offer new ideas for recycling MgSO4 wastewater and develop a new process of "re-crystallization and re-dissolution" for separating and recover RE from the leaching solution. © 2022, Youke Publishing Co., Ltd. All right reserved.
