Membrane distillation treatment of high-salinity mine water

The main coal producing areas in western China have the problems of dry climate and water shortage, and the mine water produced by coal mining generally has the characteristics of high salinity and high hardness. At present, the zero-discharge treatment of high-salinity mine water generally has the problems of complex process and high operating cost. In order to shorten the treatment process and improve the treatment efficiency, in this study, the vacuum membrane distillation technology was applied to replace the existing multi-stage reverse osmosis membrane concentration technology for the high-salinity mine water concentration and desalination treatment process. The high-salinity mine water after a lime-soda softening treatment was treated with different concentration ratios, and the membrane fouling mechanism and membrane cleaning method during the concentration process were studied. The results showed that with the increase of the concentration ratio, the membrane flux and desalination rate of the polyvinylidene fluoride (PVDF) hollow fiber membrane gradually decreased, while the conductivity of the produced water increased slowly. When the concentration ratio was up to 27 times, the membrane flux was 8.87 L/(m²·h), the salt rejection rate was 99.7%, and the water conductance was 26.6 μS/cm. The membrane distillation process was inhibited by the membrane fouling caused by the scaling of calcium and magnesium ions in mine water, and the inorganic salt deposits could not be removed by physical methods such as backwashing and ultrasonic cleaning. After 96 hours of continuous operation, the PVDF membrane flux reduced from 12.85 L/(m²·h) to 5.21 L/(m²·h). The pretreatment process of hardness removal had obvious influence on the calcium and magnesium scaling in the membrane distillation process. When the hardness removal rate exceeded 95%, the desalting rate and membrane flux attenuation of PVDF membrane were obviously weakened. The membrane fouling caused by the calcium and magnesium precipitation could be effectively removed by the citric acid cleaning agent. Under the optimized conditions of citric acid concentration of 0.87 mol/L, the cleaning temperature of 38 ℃ and the cleaning time of 40 min, the membrane cleaning efficiency could reach 99.5%. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the microscopic morphology and structure of the original film, the contaminated film and the film after chemical cleaning. The results showed that the deposited pollutants on the surface of the cleaned film were significantly reduced, but the surface morphology and microscopic structure were irreversibly changed.