The main characteristics of metallurgical wastewater are large volume of water, many types, and complex and variable water quality. According to the source and characteristics of wastewater classification, the main cooling water, pickling wastewater, washing wastewater (dust removal, gas or flue gas), flushing slag wastewater, coking wastewater and wastewater from the production of condensation, separation or overflow, and so on.
Reasonable treatment of coking wastewater produced by metallurgical industry, coking wastewater produced by different industries has its own characteristics, when treating it, it must be combined with the characteristics of coking wastewater and choose a suitable treatment process in order to achieve the best treatment effect.
Therefore, a simple analysis of the characteristics of the coking wastewater produced by the metallurgical industry needs to be carried out, the wastewater produced by the metallurgical industry has two main characteristics, one is its high degree of pollution; the other is that it is very difficult to degrade.
1
Coking wastewater and hazards
Coking wastewater, which is a kind of water used in the primary cooling of coke oven gas and coking production process or condensed from steam, is a kind of organic wastewater that is toxic and difficult to degrade.
This waste water has two main sources. On the one hand, it comes from the residual ammonia waste liquid, which is produced in the process of high temperature coal cracking.
On the other hand, it comes from phenol water, which is produced mainly during the gas purification process and is relatively less polluting.
Knowing that the two important components of coking wastewater are phenols, as well as nitrogen-containing compounds, and of course, some oxygen as well as sulfur-containing compounds.
In the coking wastewater, it contains a large amount of ammonia and nitrogen, which is unstable and easily reacts with airborne microorganisms in nitrification, producing a large amount of nitrogen trioxide and nitrogen dioxide, which are extremely harmful to human body.
2
Processes
At present, in the metallurgical industry, China mainly adopts biochemical method to treat the coking wastewater, which can effectively remove phenol and cyanide and other substances, but the treatment effect of substances containing ammonia and nitrogen is not good, and the discharge is difficult to reach the national unified standards.
1 Activated sludge method
This method uses activated sludge as the main body, and aeration treatment is carried out on the wastewater that has been pretreated with oil removal and leveling, and then solid-liquid separation is carried out, and the content of phenol in the coking wastewater treated by this method can be effectively reduced, containing only about 0.5 mg of phenol per liter of wastewater.
Using this method to treat coking wastewater, on the one hand, its treatment effect is better and can be flexibly adjusted to the degree of treatment, on the other hand, the problem of load increase can also be solved by increasing the reflux ratio in the process of treatment. However, this treatment process still has many shortcomings, such as: it basically has no removal effect, and this method in the aeration treatment, the aeration tank volume is large, the need for investment is also large, in addition, its inlet water load can not be too high, the adaptability to the impact load is also poor.
1 Fly ash treatment method
This process for treating coking wastewater uses fly ash as an adsorbent, which is a reusable material consisting of a variety of particles, some of these particles have porous surfaces and large surface areas and are capable of adsorbing a variety of debris, so it is used to treat the wastewater in depth, it has a high removal rate of phenolic substances and the right concentration conditions can ensure that only ammonia and nitrogen are substandard in the treated coking wastewater.
This process for treating coking wastewater has a wide range of raw materials, so its cost is low and its operation is simple, and at the same time it achieves the purpose of using waste materials to treat waste materials, so it is an important process worth developing in the future for treating coking wastewater.
3 Flue gas treatment process
Some metallurgical plants use flue gas to treat the remaining ammonia water, and the content of substances containing ammonia and phenols in the atmosphere is very small. This method and fly ash method have similarities and similarities, both achieve the purpose of waste and stop waste, and low cost, good treatment effect; However, this method still has certain limitations affecting its large-scale use, on the one hand, this method requires that the amount of ammonia contained in the waste water and flue gas to be consistent, which limits its use, on the other hand, the waste water also contains some difficult to deal with microorganisms. Their emission into the atmosphere may cause new pollution.
4 Anaerobic-Aerobic (AO) process
This process, also known as cyclic biological deoxygenation process, first denitrification occurs in the anoxic pond, then the sludge flow back alone, nitrification occurs in the aerobic pond, in both chemical reactions, the microorganisms involved as well as the reaction conditions are different, and the substrates converted are also different. The treatment efficiency of this method is improved, but the long residence time of the wastewater in the pond results in a poor degradation of the organic matter in it.
On the basis of this process, many researchers have improved this method, such as the formation of anaerobic - anoxic - aerobic AAO process, AOO process, etc., therefore, this is a more effective method in place, based on which also derived a variety of methods for the treatment of coking wastewater.
5 Catalytic wet oxidation method
This method uses air or oxygen as the oxidant, under the action of the catalyst to oxidize ammonia, nitrogen and organic pollutants to nitrogen and carbon dioxide, the removal rate of ammonia and nitrogen can be as high as 90% or more, and the treated water quality is far better than the biochemical treatment results. However, the catalyst used in this method is expensive, which increases the production cost of the metallurgical plant, so it cannot be the preferred method for metallurgical plants.
6 Supercritical water oxidation process
This is an emerging process for treating organic wastewater. Above the critical point of water, the physical and chemical properties of water will change, which greatly increases the solubility of organic matter in coking wastewater, and at the same time some small molecules of gas, such as methane, hydrogen, etc., can be arbitrarily miscible in supercritical water, while the solubility of salts in it is drastically reduced, allowing the inorganic salts in coking wastewater to be separated.
This technology reduces the resistance to dissolution between substances, allowing difficult to treat wastewater to be treated efficiently. This method has a wide source of raw materials and low cost, the equipment used for reaction is small in size and large in treatment capacity, the structure is simple and easy to operate, so it can be widely promoted, and it is fully closed in operation to avoid secondary pollution, so it is a process worth promoting in the future to treat coking wastewater.
In short, with the increasing concern for the environment, improving the efficiency of coking wastewater treatment becomes a problem that must be solved in the metallurgical industry, and only by solving the technical problems can we ensure the sustainable development of the metallurgical industry.