[Haichuan Chemical Salary News] Application of Membrane Integration Technology in the Recycling of High-Salt Wastewater from Coal Chemical Industry
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The membrane combination and integration process technology developed by the Hangzhou Water Treatment Technology Research and Development Center is used for separating salts, concentrating wastewater, crystallizing salt, and producing acids and bases from high-salt wastewater in the coal chemical industry. The application results show that the membrane integration process can significantly reduce evaporation volume, lower the investment required for evaporators and simplify the process of crystallization and salt separation. The bipolar membrane electrodialysis system can replace the evaporation-crystallization unit, converting liquid salts into acids and bases for reuse, thereby enabling the resource utilization of high-salt wastewater from coal chemical industries and making \"zero liquid discharge\" economically feasible. The coal chemical industry is characterized by high pollution levels and high water consumption. The severe shortage of water resources has become a major constraint on the development of this industry. Advanced wastewater reuse technologies and \"zero discharge\" technologies play a crucial role in promoting the sustainable development of coal chemistry. The key to achieving the resource utilization of coal chemical industry wastewater lies in the effective treatment of highly saline wastewater. Membrane technology is an efficient, energy-saving, and easy-to-operate liquid separation technique. Compared with traditional water treatment methods, it offers advantages such as better treatment results, the possibility of reusing wastewater, and the recovery of useful components, making it an effective technology for wastewater resource utilization. Hangzhou Water Treatment Technology R&D Center Co., Ltd. (hereinafter referred to as Hangshui) leverages its technical advantages and years of experience in membrane-based water treatment projects. Taking into account the characteristics of the high-salinity wastewater generated in the coal chemical industry, it has developed an efficient membrane combination process suitable for the resource recovery of such wastewater, which has been successfully applied in actual engineering projects.
1 Membrane concentration separation technology: Membrane technology is a technique that uses external energy or chemical potential differences as driving forces to separate certain substances from liquids. Based on the driving force, membrane technologies can be divided into diffusion dialysis membranes, pressure-driven membranes, potential difference-driven membranes, etc. The membrane technologies currently used primarily for concentration and separation include nanofiltration membrane technology, reverse osmosis (RO) technology, disk-stack reverse osmosis technology, forward osmosis technology, membrane distillation technology, and electrically driven membrane technology. 1.1 Nanofiltration membrane separation technology: Nanofiltration membranes were developed based on reverse osmosis membranes; their pore size is in the nanometer range, and their retention efficiency lies between that of reverse osmosis membranes and ultrafiltration membranes, with a retention molecular weight of 200. 1,000. Typically, the surface of nanofiltration membranes is negatively charged, and it has different Donnan potentials for ions with varying charges and valencies. In the field of high-salt wastewater treatment, the selectivity of nanofiltration can be utilized to separate monovalent and divalent salts as well as to concentrate solutions of higher-valent salts.
1.2 Membrane distillation technology: Membrane distillation is a non-isothermal physical separation technique that combines traditional distillation processes with membrane separation technology; it utilizes hydrophobic microporous membranes, with the vapor pressure difference across the membrane serving as the driving force for mass transfer. After the steam molecules on the hot side pass through the membrane pores, they condense and accumulate on the cold side; this is a membrane process involving phase change, along with the transfer of heat and mass. Compared to other separation processes, the advantages of membrane distillation include: (1) it can achieve 100% retention of non-volatile solutes in liquids such as ions, macromolecules, and colloids ； (2) The operating temperature is lower than that of conventional distillation, and the operating pressure is much lower than that in the reverse osmosis process ； (3) Compared with traditional distillation equipment, it has no problem of evaporator corrosion and a smaller size. Membrane distillation can handle aqueous solutions with extremely high concentrations, and it holds great potential for concentration. Moreover, it is the only membrane process that can directly separate crystalline products from solutions at present. However, membrane distillation technology is still in the research and development stage, with few practical applications; there are no such applications in the field of zero-discharge treatment of high-salt wastewater from coal chemical industries.
1.3 Osmotic reverse osmosis technology: Osmotic reverse osmosis is a physical phenomenon that occurs widely in nature; it relies on the chemical potential difference or osmotic pressure difference between two solutions as the driving force, enabling water to move spontaneously from areas with higher chemical potential (the side with lower osmotic pressure) to areas with lower chemical potential (the side with higher osmotic pressure). By taking advantage of the spontaneous movement of water in forward osmosis technology, along with a driving solution that is easy to recycle, this technology can be applied to seawater desalination and the re-concentration of concentrated brine. Due to the hydrophilicity of the forward osmosis membrane material, no high pressure is required for operation, which effectively reduces membrane fouling. It is suitable for wastewater treatment in situations where reverse osmosis technology cannot be applied, and it also helps to save on the costs associated with membrane cleaning as well as minimize environmental pollution caused by chemical cleaning agents. Currently, osmotic process technology is still under research and optimization, and there are few application examples of this technology in achieving zero discharge through the reuse of wastewater.
1. 4-tube membrane reverse osmosis technology: The 4-tube membrane reverse osmosis system uses a special type of separation membrane that possesses strong resistance to contamination and can operate at higher pressures. Its reverse osmosis membrane and hydraulic flow guide plates are stacked together; compared to traditional reverse osmosis systems, it features wider channels, shorter flow paths, and greater turbulence. Disc-tube reverse osmosis is suitable for the treatment of wastewater with high salt content and high organic levels, such as landfill leachate. It is widely used in the treatment of landfill leachate, and the mass fraction of the concentrated solution obtained is generally 5%–8%, usually not exceeding 10%.
1. 4-tube membrane reverse osmosis technology: The 4-tube membrane reverse osmosis system uses a special type of separation membrane that possesses strong resistance to contamination and can operate at higher pressures. Its reverse osmosis membrane and hydraulic flow guide plates are stacked together; compared to traditional reverse osmosis systems, it features wider channels, shorter flow paths, and greater turbulence. Disc-tube reverse osmosis is suitable for the treatment of wastewater with high salt content and high organic levels, such as landfill leachate. It is widely used in the treatment of landfill leachate, and the mass fraction of the concentrated solution obtained is generally 5%–8%, usually not exceeding 10%.
1.5 Electrodriven membrane technology: Electrodriven membrane technology refers to a membrane technique that separates and concentrates salt ions from a solution by electromigrating ions. The core component of an electrically driven membrane device is the cation-anion exchange membrane, which exhibits selective permeability to ions in the solution. Based on their structure, ion exchange membranes can be further divided into heterogeneous membranes and homogeneous membranes. In addition to the aforementioned conventional electrically driven membrane technologies, there are also the selective electrically driven membrane devices and bipolar membrane devices developed by Hangshui. Selective electrodriven membrane devices can achieve the separation and concentration of monovalent and divalent ions ； Bipolar membrane devices can convert liquid salts into acids and bases for direct recycling.
2. Resource utilization treatment processes for high-salinity wastewater from coal chemical industries: The water quality characteristics of high-salinity wastewater in coal chemical industries are related to factors such as production processes and raw materials; therefore, the water quality of such wastewater varies significantly among different companies. For different types of high-salt wastewater from coal chemical industries, different process routes should be adopted to achieve water reuse and zero liquid discharge through salt crystallization, under conditions that are technically and economically feasible. The water quality in high-salt wastewater from coal chemical industries is complex, with multiple salts present in large quantities simultaneously; therefore, the use of membrane technology for salt separation becomes particularly crucial during treatment. The technologies employed by Hangshui for the resource recovery of salts include selective nanofiltration and selectively driven membrane technologies.
When wastewater contains both sulfates and chlorides, with the mass fraction of sulfates not exceeding 8%, nanofiltration technology is an appropriate choice for achieving effective separation of sulfates and chlorides; subsequent crystallization processes can then be used to recover sulfate crystals and chloride crystals separately； When the concentrations of sulfate and chloride in the wastewater are comparable and the mass fraction of chloride ions is at least 1%, a selectively driven membrane is suitable for separating the monovalent salts; further concentration and evaporation crystallization then enable the recovery and utilization of the different salt components. Both nanofiltration technology and selectively electro-driven membrane technology can be used for the deep purification of single salts, thereby minimizing the residual amount of impurity salts in the crystals of the salt to be recovered. For coal chemical enterprises with high demands for acids and bases, bipolar membranes can be used to replace or partially replace the evaporation and crystallization units, thereby converting the concentrated liquid salts into acids and bases for reuse. Compared with the evaporation crystallization process, bipolar membranes can save a large amount of energy. The bipolar membrane technology developed for water treatment has been successfully applied in projects such as Sinopec’s.
3. The nanofiltration pre-salting + membrane concentration + crystallization salting process developed by Hangshui. The process flow of the nanofiltration pre-salting + membrane concentration + crystallization salting process developed by Hangshui is shown in Figure 1.