At present, the development of coal chemical industry mainly includes three industrial chains: coal coking, coal gasification and coal liquefaction.
According to this, coal chemical industry wastewater can be divided into three categories: coking wastewater, coal gasification wastewater and coal liquefaction wastewater.
Pretreatment biochemical treatment advanced treatment high salt water treatment.
Pretreatment generally includes oil separation, acidification, precipitation, air flotation, sand filtration, solvent extraction and so on. Each method has its own advantages and disadvantages. Now solvent extraction is mainly used.
In the extraction process, the complexing extractant is phased out, solvent extraction, which does not need acid-base blending, has recently started to rise and gradually occupy the market.
According to different water quality conditions, the pretreatment should take targeted technical measures.
The content of H2S, NH3 and phenol in the phenol containing acid wastewater produced by the direct coal liquefaction project is high. The concentration of H2S, NH3 and phenol can reach the biochemical treatment range by adopting double tower stripping and isopropyl ether extraction.
The pretreatment can also remove or decompose the macromolecular refractory organics in the wastewater in advance. For example, n-octanol and cyclohexane are used as extractants to extract the refractory organics in the coking wastewater. The biodegradability of the wastewater increases from 0.09 to 0.29, and the COD removal rate changes from 68.81% to 88.63%.
Biochemical treatment at home and abroad generally uses anoxic and aerobic biological treatment (A / O process).
However, due to the polycyclic and heterocyclic compounds in the coal chemical wastewater, the COD index of the effluent treated by aerobic biological method is difficult to reach the standard stably.
In order to solve these problems, some new treatment methods have emerged recently, such as biochar process (PACT), biological fluidized bed process (PAM), immobilized biotechnology, carrier flow bed biofilm process (CBR), anaerobic biological process, anaerobic aerobic biological process, etc.
Biochar process (PACT) can deal with toxic and harmful organic pollutants which are difficult to be degraded by organisms, and has a good effect on high concentration macromolecular organic compounds.
The degradation efficiency of the biological fluidized bed treatment (PAM) is doubled than that of the suspended growth activated sludge process, and it also has a strong ability of nitrification and ammonia nitrogen removal.
Immobilized biological technology is a new technology developed in recent years. The degradation capacity of quinoline, isoquinoline and pyridine of the domesticated dominant strains is 2-5 times higher than that of ordinary sludge, and the degradation rate of pyridine and other substances is more than 90%.
In order to make up for the limitation of biochemical treatment, the secondary effluent is treated.
It mainly includes coagulation sedimentation, flocculation sedimentation, multi-media filtration, activated carbon adsorption, membrane separation, and high-order oxidation process to ensure the quality of effluent water.
Advanced treatment technologies are commonly used, such as electrocatalytic oxidation, and other advanced treatment technologies include coagulation sedimentation, filtration, ozonation, activated carbon filtration and ultrafiltration.
Taking the wastewater from crushed coal pressurized gasification as an example, the biological combination technology is widely used. The common advanced treatment methods include “ozone + BAF”, “Fenton + contact oxidation”, lab, etc.
BAF is the core process of advanced treatment at present. The process combination of “ozone + BAF” is more and more favored by new projects because of its reasonable process and good operation performance.
High salt water treatment
The waste water is reused in the system after being treated by double membrane. The concentrated brine evaporates, crystallizes and basks in salt, and evaporates and crystallizes are the main difficulties in the research and development of coal chemical waste water treatment process.
The evaporator can generally increase the salt content in the waste water to more than 20%. The salt brine discharged from the evaporator is usually sent to the evaporation pond for natural evaporation and crystallization, or to the crystallizer, where the crystallization is dried to solid, and then transported to the landfill area for burial.
The first several stages of coal chemical wastewater treatment process are relatively mature, and the current technical difficulties are mainly concentrated on the final evaporation crystallization and salt separation, and the main problems are as follows:
- Serious corrosion and pollution blocking problems affect the continuous and stable operation of the evaporation device. Calcium, magnesium ions, sulfate ions, carbonate ions, silicates, etc. in sewage will produce calcium sulfate, calcium carbonate, etc. in the evaporation crystallization process, which will adhere to form a scale layer and easily pollute and block equipment and pipelines.
- The operation cost is high. The steam consumption of multi effect evaporation and the chemical consumption of mechanical recompression process are the main consumption costs of the two technologies.
- Large construction investment. Due to the strong corrosiveness of concentrated brine under high temperature, the high requirements for equipment and material selection lead to the increase of equipment and material costs.
Without considering the cost, it is possible for the existing technology to realize zero discharge of coal chemical wastewater.
However, considering the cost factor and the maturity of relevant technologies, it is necessary to design the treatment process reasonably, choose which technology combination to adopt according to the effluent quality and site conditions, what process route to adopt, and use various methods to jointly treat coal chemical wastewater in sections, which is the basic development direction of coal chemical wastewater treatment technology.
At the same time, coal chemical wastewater treatment should continue to develop unit technology, such as the front-end water treatment process, to find a suitable process route according to the water quality, to reduce the cost of refinement; evaporation crystallization process cost accounts for a large proportion in the whole process, so it is urgent to develop a crystallization method suitable for high COD and high salt wastewater in coal chemical industry; do a good job in quality and salt separation, develop the value of by-products, and reduce the crystallization risk Waste disposal expenses.
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