Kinetics of static immersed leaching of low-grade sea-type evaporites based on theoretical and experimental investigation of unsteady-state mass transfer

Abstract

In the immersed leaching process of low-grade sea-type evaporate wastes, the relative proportions of the solute contents in the mining brine continuously change over time, which makes the composition of the brine from leaching unpredictable. This strange phenomenon has been lacking a reasonable explanation for many years. To investigate this issue, the immersed dissolution and diffusion experiments of four typical sea-type evaporites NaCl, Na2SO4·10H2O, MgCl2·6H2O, and MgSO4·7H2O were performed in laboratory and pilot scale, respectively. Based on Fick’s second law, Stokes–Einstein equation, Thomas equation, Jones–Dole equation etc., a mathematical model was established to describe the mass transfer process. The results show that the unsteady-state mass transfer of each salt component in static water has three stages: linear fast mass transfer before the first 720 h, nonlinear unsteady-state mass transfer from 720 h to 7000 h, and pseudo-linear slow mass transfer after 7000 h. In every stage, the diffusion of Na+ and Cl− was faster than that of Mg2+ and SO42−, respectively, but their local diffusion rates were changing at different magnitudes, and their local viscosity and local mass transfer coefficients also varied to a great extent and underwent dynamic changes. At a distance of 60 mm from the evaporite-brine interface, the saturation degree reached by NaCl is approximately 1.23, 1.50, and 1.58 times that of Na2SO4, MgCl2, and MgSO4 when the diffusion time is 720 h, respectively. The local mass transfer coefficients of NaCl can reach approximately 1.48, 3.01, and 2.85 times that of Na2SO4, MgCl2, and MgSO4, respectively. The aforementioned results reveal that the unconstant change of the ratios of solute contents of the leaching brine resulted from the dynamic change in the local mass transfer rate of each component. The difference in concentration between Na+ and Mg2+ ions becomes more apparent when the diffusion distance is below 90 mm within the first 120 h of the diffusion time. Under the comprehensive influence of these factors, when NaCl or MgSO4·7H2O is present in the mixed evaporites, as the leaching time increases, the Na+/Mg2+ ratio changes continuously in the leaching brine owing to variations in the dissolution rate and magnitude of changes of components such as NaCl and MgSO4. Therefore, brines with different solute content ratios can be obtained as the leaching process proceeds.