Crystallization separation of Na2CO3 and Na2MoO4 from the high-salinity ash slag produced by production process of Na2MoO4 based on the theory of phase equilibrium

Abstract

High-salinity ash slag containing Na2CO3 and Na2MoO4 produced from the production process of Na2MoO4 brings about the resource waste and environmental pollution issues. To realize the resource utilization of these high-salinity ash slag, the phase equilibrium data of the ternary Na2CO3–Na2MoO4–H2O system at 298.15 K and 323.15 K were determined using the isothermal dissolution equilibrium method, and the equilibrium solid phase was determined by X-ray diffraction. By comparing the phase diagrams of the ternary Na2CO3–Na2MoO4–H2O system at 298.15 K, 323.15 K, and 353.15 K, the change rules of the crystallization region were analyzed. The Pitzer and HW models were used to calculate the ionic interaction parameters of θMoO4–CO3 and ψNa–MoO4–CO3, which could be further used to predict the solubility curves of the ternary Na2CO3–Na2MoO4–H2O system. The calculation results were in good agreement with the experimental results. Furthermore, the phase diagrams of the ternary Na2MoO4–Na2CO3–H2O system at 298.15 K and 353.15 K were selected to design a process for the highly-efficient recovery of Na2MoO4·2H2O and Na2CO3·H2O from the high-salinity ash slag. The experimental results showed that the one-way recovery rate of Na2CO3 could reach 92.55%, with a purity of 99.62%, and that of Na2MoO4·2H2O is 45.61%, with a purity of 99.14%. These results were almost consistent with the theoretical calculation results. After multiple cycles, Na2CO3 and Na2MoO4 could be completely recovered.