| Given the situation of lack of water resources and fragile water environment in mining areas,coal mining enterprises have constructed the reverse osmosis reuse system of production wastewater,resulting in a large number of reverse osmosis brine.Its direct discharge will cause waste of water resources and seriously pollute the water environment in mining areas.The treatment of reverse osmosis brine has always been a technical difficulty and a research hotspot.Membrane distillation（MD）has broad application prospects in reducing the scale of reverse osmosis brine ascribe to that it breaks through the limitation of salinity,can be driven by industrial waste heat,and is convenient for modular application.However,membrane fouling and wetting in the process of MD treating coal mine brine will increase the energy consumption and reduce the stability of the process,which is the main reason why the MD process is rarely used.For prevention and control of the membrane fouling and wetting in MD treating coal mine brine,this study developed a high-performance hydrophobic membrane,revealed the mechanism of membrane fouling and wetting process,and explored prevention and control methods of membrane fouling and wetting.Firstly,the study began to develop a high-performance hydrophobic membrane.Utilizing polyvinyl alcohol（PVA）as raw material,an omniphobic electrospinning PVA membrane was prepared by glutaraldehyde crosslinking,nanoparticle coating,and impregnation fluorination.The mixture of chitosan and nanoparticles was coated on the commercial polyvinylidene fluoride（PVDF）membrane by spinning to construct a rough structure.Then,the PVDF membrane with a rough structure was modified by impregnation fluorination to prepare an omniphobic PVDF membrane.The preparation method of high-performance hydrophobic membranes was explored by comparing the surface structure,water flux,and hydrophobicity of the two kinds of omniphobic membranes.The results showed that although the preparation method of omniphobic PVDF membrane was simple,easy for large-scale production,and could enhance the hydrophobicity of the membrane,it would also significantly reduce the pure water flux.Differently,the water flux of the omniphobic PVA membrane was equivalent to that of the commercial PVDF membrane.Besides,the omniphobic PVA membrane had superhydrophobic and oil-repellent properties,which ascribed to the micro-nano rough structure and the low surface functional groups such as CF2 and CF3.The omniphobic PVA membrane showed better anti-scaling and anti-wetting properties than commercial PVDF membrane when treating 20%sodium chloride solution and 0.3 mmol/L surfactant solution.Secondly,the study aimed to solve the problem that the scale easily deposits on the membrane surface.The MD experiment of actual acid mine brine was carried out using a commercial PVDF membrane and omniphobic PVA membrane.The formation mechanism of membrane scale was revealed by analyzing the evolution of flux and permeated conductivity and the composition of membrane scale.An efficient pretreatment process for preventing and controlling membrane scale was explored.The results showed that the disposition of calcium sulfate and goethite on the membrane surface was the critical factor leading to the decline of the stability of the MD process.The omniphobic PVA membrane increased the energy barrier of heterogeneous nucleation of crystals,reduced the crystallization of inorganic salts on the membrane surface,delayed the rapid decline of flux,and increased the water recovery from 60%to 70%.However,after a concentration of 3.4 times,the membrane scale still reduced the flux by89.58%.In contrast,pretreatment could more effectively inhibit the formation of membrane scale.After adding sodium oxalate,there was almost no scale on the membrane surface.Similarly,after a concentration of 3.4 times,the flux was only reduced by 24.96%.It could be seen that the pretreatment technology based on the property that sodium oxalate could precipitate calcium and complex iron was an effective anti-scaling method in the MD process of mine brine.Thirdly,the study aimed to find a method to treat the concentrate produced by the MD process and the further recover pure water from the concentrate.The method of resource utilization of the concentrate was explored.The separation and utilization of ferric oxalate contained in the concentrate were studied.The feasibility of secondary MD of concentrate after separation of ferric oxalate was evaluated.The results showed that the MD concentrate had excellent photocatalytic degradation activity of methylene blue because it was rich in iron oxalate.In addition,iron oxalate in the concentrated solution could be converted into ferrous oxalate precipitation by visible light irradiation and then separated from the concentrate.The separated ferrous oxalate had a very excellent visible-light photocatalytic degradation effect on methylene blue in an aqueous solution when the initial solution p H value was 3.5.The MD process could further treat the concentrated solution after separating ferrous oxalate to further improve the recovery of water resources.Fourthly,the study aimed to prevent and control severe membrane wetting and inorganic-organic compound fouling in the MD process treating coal chemical brine.The MD experiment of actual coal gasification brine（CGB）was carried out using a commercial PVDF membrane.By the analysis of the morphology and composition of membrane fouling,the formation mechanism of membrane wetting and fouling was revealed.The results showed that the interaction of pollutants would aggravate the membrane fouling and wetting in the MD process of CGB.Surfactants and oily substances were the key factors causing membrane wetting.After membrane wetting,sodium chloride crystals entered the membrane pores and were deposited on the cold side,leading to membrane pore blockage.The interaction between humic acid（HA）like materials and sodium ions would lead to an organic-inorganic composite fouling layer wrapped with delicate crystals was formed on the membrane surface.Membrane wetting induced rapid deterioration of water quality.The synergistic effect of membrane pore blockage and membrane surface fouling reduced the flux by 92.80%.Based on the understanding of membrane fouling and wetting mechanism,the control effects of high-performance hydrophobic membrane and pretreatment on membrane wetting and fouling were explored.The results showed that the omniphobic PVA with super hydrophobicity and oil repellency avoided membrane wetting and membrane pore blockage but could not hinder the membrane surface fouling caused by the interaction between HA-like materials and sodium ions.After a concentration of 4.5times,the flux decreased by 49.44%.The pretreatment process composed of extraction and Fenton could synergistically and effectively remove typical organic substances such as surfactants,oily substances,and HA-like materials,avoid membrane wetting and membrane pore blockage,alleviate membrane surface fouling,and reduce the flux by only 32.52%under the same concentration multiple.The pretreatment process can effectively convert macromolecular organics in raw water into low molecular organics.The interactions between low molecular organics and calcium ions produced in the extraction process caused a fouling layer composed of large-size calcium sulfate crystals to cover the membrane surface.It had little effect on flux and hindered transmembrane transmission amphiphilic small molecular organics,preventing deterioration of permeate water quality.This study put forward the prevention and control methods of membrane fouling and membrane wetting,constructed the treatment process of mine brine and coal chemical brine with MD technology as the core,promoted the application of MD technology in zero discharge of coal mine brine,provided technical support for protecting water environment in coal mine area and improving water resource reuse rate,and finally promoted the high-quality development of coal industry. |
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