High Na⁺ And Mg²⁺ Concentrated Saline Water Treatment With Via Membrane Distillation Crystallization

With the problem of resource shortages and environmental pollution around the world,the treatment and recycling of wastewater has attracted increasing attention.As a typical wastewater,and discharge directly not only causes environmental pollution,but also wastes salt resources.At present,for the treatment of highly saline wastewater,firstly remove the volatile organic gas from the wastewater,and then treat the treated highly saline water with evaporation crystallization.However,evaporation crystallization has the advantages of high energy consumption caused by the high operating temperature and recycled crystal product with poor property caused by the uncontrolled supersaturated.In view of the mentioned problems,this paper is about the treatment of high salt water(Na⁺?Mg²⁺//Cl^--H₂O)with membrane distillation crystallization.The treatment takes full advantages of making full use of low-quality heat source and high separation efficiency of the membrane,and implements the recovery of high purity water and excellent property crystal products,simultaneously.In this paper,the membrane distillation process of saline water,with high concentration of sodium chloride and magnesium chloride,was explored.The permeation flux of membrane was investigated for the solution with different magnesium chloride concentrations.It was found that the permeate flux gradually increased with the increase of the mass fraction of magnesium chloride in the feed solution.Moreover,other factors affecting the membrane permeation flux were investigated.It was found that the permeate flux gradually increased with the increase of flow rate of the feed solution,increased with increase of permeate side vacuum pressure,and increased with the operation temperature.In addition,the conductivity of the recycled water was all lower than 5.88?S/cm in the range of experimental investigation.Through the 200 h test of the membrane distillation for the PP hollow fiber membrane,the permeate flux of membrane remained relatively stable,and it was provided that the membrane used in the experiment has good recycling performance.The molecular simulation software Materials Studio was used to simulate the structure of highly saline solution.The hydration structure of the cation and coordination number of the ions were calculated by molecular trajectory.The results indicated that,hydration number of the first layer of Na cation and coordination number of Na⁺-Cl^-has no obvious change with the adding of the MgCl₂ and increased of the MgCl₂ percentage.However,hydration number of the first layer of Mg cation decreased and coordination number of Mg²⁺-Cl^-has obviously increased.It indicated that the Mg cation was more easily to push away the surrounding water molecular and coordinated with Cl anion.Moreover,the arrangement of water molecular around the Na cation was random.In addition,almost all the O atoms of the water molecular in the first hydration layer uniformly locate on the regularly spherical hydrate layer of Mg cation,and it indicated that the Mg cation possess a relatively weak interaction with the water.Moreover,Mg²⁺possesses a more stable cation hydration structure and the interaction with water molecular was stronger than Na nation.Moreover,the properties of NaCl crystals obtained by membrane distillation crystallization were investigated,and Mg/Na ions ratio and operation temperature had significant influence on the nucleation,size distribution,surface morphology and the purity of the terminal crystal products.The obtained crystal products has perfect cube shape,smooth surface and uniform size?the C.V.value was lower than 31.78?under the temperature lower than 70?and the W_MgCl2/(W_NaCl+W_MgCl2)lower than 7.5%.NaCl secondary nucleation agglomeration structure and the stock of helical structure caused the deposition of mother liquid.That caused the decreasing of the purity of NaCl products.By the washing with selective solvent?EtOH?,the purity of crystal products can be further improved,and the fraction of mass was above the 99.38%.In addition,the membrane,served as the heterogeneous nucleation interface,could produce crystal nucleus with uniform particle size,and may also cause the crystal to adhere to the surface of the membrane so as to cause membrane fouling.