Cold rolling wastewater is the wastewater produced in the production process of the cold rolling plant of iron and steel enterprises, and its main components are acid-containing wastewater, emulsions, and oils.
With the improvement of the water treatment process in recent years, cold-rolling wastewater has been discharged up to the standard. However, due to its large amount of water and complex components, its reuse and zero-discharge technologies have not been widely promoted. Even if a few projects have reused wastewater, it is recycling The usage rate is also not ideal.
According to the characteristics of different regions, many special discharge requirements have been set. In recent years, the new requirements of the state on the utilization of water resources, especially the restrictions on the amount of discharged water, have put forward strict requirements on the water utilization rate of production enterprises. Many enterprises are faced with the embarrassing situation of being able to handle the standard but unable to make comprehensive use. Therefore, recycling emission reduction technology is a technology urgently needed by production enterprises in recent years.
Cold rolling wastewater is generally discharged directly after treatment. There are a few domestic projects that reuse wastewater, but the recycling rate is generally designed to be around 35% to 50%, and the technology for further wastewater recycling has not been practically applied. However, many cold rolling mills across the country have put forward the requirement to further improve the recycling rate during the transformation process. This paper discusses the new technology of cold rolling wastewater reuse and emission reduction.
Conventional wastewater reuse and emission reduction technology and its characteristics
1.1 Conventional wastewater reuse and emission reduction technology
The wastewater reuse process shown in Figure 1 has been applied to the third cooling process of Benxi Steel’s cold rolling plant. The wastewater treatment station was put into use in 2016 and is now operating stably, and the recycling rate can be maintained at about 60%. Its technological process is as follows.
The effluent from the original collection tank is sent to the external MBR device after treatment by the pump, and the effluent of the secondary sedimentation tank is sent to the original pool with a pump for advanced treatment. The effluent from the raw pool is pumped to the ultrafiltration device to retain colloids and small suspended solids in the water. The ultrafiltration effluent is stored in the ultrafiltration production tank and is transported to the reverse osmosis device for desalination by a booster pump. The reverse osmosis device produces water into the reverse osmosis production tank and is transported to the circulating water station by the pump to be used as circulating water supplementary water. The concentrated water of reverse osmosis is stored in the concentrated water tank after being treated by the BAF device and then discharged up to the standard.
1.2 Problems existing in conventional recycling and emission reduction processes
The cold-rolling wastewater passes through the wastewater adjustment tank and is pumped to the pH adjustment/coagulation tank, were acid, coagulant, and flocculant are added to form larger flocs of oil and suspended particles in the wastewater.
The effluent enters the first-level flotation tank, and the flocs and suspended solid particles in the wastewater are carried to the surface of the tank to form scum removal. agent to make the wastewater flow to the secondary air flotation device to further remove the scum.
The effluent of the secondary air flotation device flows to the pH adjustment/intermediate pool, and the pH value is adjusted to neutral by adding acid and alkali.
The effluent from the MBR water absorption tank enters the external ceramic membrane device, which can completely separate the biological sludge and water, and flows into the original water tank of the reuse water system, and the other part flows into the coagulation/flocculation tank in front of the secondary sedimentation tank, where coagulant is added. and flocculants to form flocs.
The biological sludge and water are completely separated through the secondary sedimentation tank, and the fine flocs contained in the water are filtered through sand filtration and then flow into the original water tank of the reuse water system.
The water still contains a certain amount of COD, and the conductivity of the water is relatively high. The desalination treatment is carried out by UF and RO devices. The reuse rate of this process is generally not more than 70% due to the influence of the quality of the wastewater. It is easy to cause rapid fouling of the reverse osmosis membrane and eventually cause the paralysis of the recycling system.
Therefore, there is a need for a technology that can operate normally in the case of high COD, and can tolerate higher operating pressures to deal with high conductivity wastewater.
While increasing the recycling rate, cold-rolling enterprises are also faced with the dilemma of failing to meet the discharge standards after increasing the recycling rate. It is well known that reverse osmosis membranes only concentrate wastewater.
It will not reduce COD, ammonia nitrogen, total nitrogen and other pollutants. The concentrated water of reverse osmosis contains several times the concentration of pollutants in the influent. For example, when the recovery rate is 66%, when the COD of the influent is 30mg/L, the COD in the concentrated water can reach 90mg/L.
According to the “Water Pollutant Discharge Standard for Iron and Steel Industry” (GB13456-2012), the requirement for COD is 70mg/L. It can be seen that the higher the recycling rate, the greater the risk of excessive discharge.
The new technology of cold rolling wastewater reuse and emission reduction
2.1 A new dual-membrane process (ultrafiltration + reverse osmosis) that cannot be improved by conventional recycling technology
The reverse osmosis adopts the latest “LD technology”, that is, the antibacterial 34mi water inlet screen technology. The new LD combined with the antibacterial technology HYDRAblockTM uses new technology to introduce antibacterial function into the water inlet screen, so that the reverse osmosis membrane element has enhanced the ability to inhibit the growth of bacteria on the basis of reduced pressure and less fouling, and comprehensively improved the membrane Anti-pollution properties of components.
“LD technology” can solve the problem of high COD and electrical conductivity of cold rolling wastewater, and its anti-pollution performance is excellent. Even if it operates under high COD conditions, it can still ensure the improvement of its antibacterial performance, which can ensure that the system operates more stably and has a wider range of Water inlet screen, lower pressure loss, and denser aromatic polyamide separation skin have the advantages of high desalination, low-pressure drop, less fouling, easy cleaning, long cleaning cycle, long life, etc., reducing system cleaning costs, low-pressure drop Reduce energy consumption, thereby reducing the total cost of water production in the system. As shown in Figure 3:
2.2 Ozone multiphase oxidation + BAF technology launched for concentrated water pollutants exceeding the standard
1) Ozone Heterogeneous Oxidation Technology The ozone heterogeneous catalytic oxidation process consists of two parts including a catalytic oxidation cell and an ozone generation system, and the reaction core is a catalytic oxidation cell.
The wastewater enters the catalytic oxidation tank through the sewage inlet pipe, and the ozone oxidant produced by the ozone generation system enters uniformly through the diffusion device at the bottom of the tank. Under the action of catalysts, organic pollutants are oxidized and decomposed, and converted into harmless small molecules.
The transition metal-supported catalyst (catalystofsupportedtransitionmetal) uses a porous carbon-based carrier to be activated by Ni, Mn, and other transition metals at high temperature and is treated with a special pore structure adjustment to form a highly active supported heterogeneous catalyst.
The catalyst surface contains stable catalytic activity factors, which can induce ozone to form more active OH radicals greater than 103m2/g, the porosity is greater than 80%, the surface is rough, the pore structure is abundant, and the distribution is reasonable. Liquid-phase organics and gas-phase ozone oxidants are simultaneously adsorbed to the active adsorption sites on the surface of the solid-phase catalyst.
The change of the interface components changes the valence state of the ions, the movement of electrons, etc., the organic molecules form the intermediate state of free radicals, and the activation energy of the reaction is greatly reduced. CSTM catalyst is resistant to acid and alkali corrosion has high mechanical strength, is not easy to wear, and has a service life of more than five years.
The technical characteristics of CSTM catalyst are as follows:
(1) High oxidation efficiency and good effluent quality. The CSTM catalyst induces ozonolysis to generate OH, and forms free-state organic molecules, which reduces the activation energy of the reaction, and the oxidation reaction rate of organic substances is increased by more than 5 orders of magnitude compared with the direct oxidation reaction. The reaction can be completed in only 30 minutes, and the floor space of the structure is reduced by more than 1/2.
(2) The utilization rate of ozone is increased by 2 to 3 times, the equipment investment is low, and the operating cost is low. Compared with direct ozone oxidation, the ozone heterogeneous catalytic oxidation process degrades the same amount of organic matter, the ozone consumption is reduced by 60-70%, the equipment investment of the ozone generating device is reduced, and the operating cost is reduced by 50%.
(3) The catalytic efficiency is stable, and the catalyst has a long service life. The catalytic activity factor is fixed on the surface of the porous carrier through solid solution roasting, and the catalyst has a low dissolution rate, acid and alkali corrosion resistance, high mechanical strength, and a service life of more than five years.
(4) It can decolorize and deodorize organic matter at the same time, and does not produce secondary pollution. ·OH free radicals destroy the chromophoric groups in organic molecules non-selectively, and can also oxidize odorous substances such as hydrogen sulfide. The intermediate products in the oxidation process continued to react with ·OH, and the oxidation reaction was complete.
(5) The process is simple, the degree of automation is high, and the labor intensity is low. It is composed of an ozone generating system and an oxidation reaction system, and the technological process is simple. It adopts PLC or DCS automatic control, and the staff operation is less.
(6) The catalytic reaction device is made of stainless steel, ceramics, ABS and other corrosion-resistant materials, and the service life is more than five years in normal use.
(7) Diversified application methods. It can be processed alone or combined with other processes.
2) According to the process flow combining STRO and ozone catalytic oxidation + BAF technology
A new recycling and emission reduction process is added after the concentrated water tank of the conventional reuse water treatment system. The original reverse osmosis concentrated water tank is changed from discharge to further concentration and pumped to STRO. The recovery rate of STRO can be designed to be 65%, and STRO produces water. The reverse osmosis product water stored in the original reverse osmosis production tank is reused together with the reverse osmosis product water of the conventional reuse system.
The concentrated water of STRO is stored in the concentrated water tank and pumped to the ozone catalytic oxidation tank, which improves the biodegradability of the water and reduces part of the COD through ozone catalytic oxidation.
The effluent flows to the BAF through the biological bacteria to further reduce the COD, ammonia nitrogen and total nitrogen in the water. When the total nitrogen in the water is high, the BAF can be designed to operate in multi-stage series. Generally, the post denitrification method is used to reduce the total nitrogen in the water. BAF effluent can meet the discharge index requirements.
The above new reuse water treatment process can achieve a very high recycling rate of cold rolling wastewater and meet the national emission reduction requirements.
The zero-emission project has a number of application performances in coal chemical projects in the western region, but due to its characteristics of high energy consumption and high investment, there is no good operation case. Zero discharge treatment is generally not required for cold rolling wastewater treatment, but some regions have some special standards for wastewater discharge.
The general wastewater treatment process cannot meet the requirements, and the production enterprises are often forced to perform zero-discharge treatment of wastewater.
It is well known that chloride ions can only precipitate with silver or mercury, which is extremely difficult to separate from water. Most cold rolling mills use hydrochloric acid to pickle steel plates. Far from reaching the emission standard, only zero-emission treatment can be used.
It can also be seen that the emission standards in some places in my country are very unreasonable, and some pollutants have no effective treatment methods at all, but unreasonable emission standards have been formulated first. In order to meet this standard, a lot of energy is often wasted.
3.2 Zero-emission technologies
The only effective process for zero discharge now is to concentrate the water at a high multiple and then use the method of crystallization and evaporation to convert all the substances in the water into solid wastewater for treatment. In order to reduce the consumption of energy media in the process of crystallization and evaporation, multi-effect The concept of evaporation is widely used.
Multi-effect evaporator is referred to as MVR technology.
MVR (Mechanical Vapor Recompression, Mechanical Vapor Recompression Technology) utilizes the secondary steam generated by the compressor to compress the separator to increase the steam temperature, and the compressed steam is used as a heat source to supply heat for the heater, and so on.
By consuming a small amount of electric energy, the steam compressor converts low-grade steam into high-grade steam, and its steam utilization rate is equivalent to a 20-30-effect multi-effect evaporator.
However, even if the multi-effect evaporator is used for zero discharge, it is still a process of high energy consumption. The treatment cost per ton of water of the multi-effect evaporator is generally around 50-60 yuan per ton of water, which is very inconsistent with my country’s requirements for energy conservation and emission reduction.
With the stricter requirements for energy conservation and emission reduction in my country, the further reuse of cold rolling wastewater will be the general trend, and while improving the reuse rate, ensuring the discharge of concentrated water is also a new issue we are facing. The new technology of reuse and emission reduction mentioned above will be the core process of future cold rolling wastewater reuse projects.