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Source and control of wastewater from nitrogen fertilizer industry

来源: 作者: 阅读:79573 网友评论0 Updated: 2011-05-06 09:23 Source: Author: Read: 79573 2010 0 comments

China's nitrogen fertilizer industry began in the 1930s. Due to the limitation of the energy composition, China's natural gas resources are insufficient. The nitrogen fertilizer industry can basically only use coal coke as the main raw material, with coal head enterprises accounting for about 62%; natural gas-based nitrogen fertilizer companies account for 21%; Nitrogen fertilizer raw materials accounted for 17%. However, on the whole, China's nitrogen fertilizer production has been able to meet the needs of domestic agricultural production. China has changed from the world's largest nitrogen fertilizer importer to a nitrogen fertilizer exporter. Nitrogen fertilizer trade has steadily increased. From 9.99 million tons in 1980 to 25.32 million tons in 2001, the average annual growth rate was 7.3%. Nitrogen fertilizer exports have increased year by year, while imports have fallen year by year. China's nitrogen fertilizer technology and equipment have been greatly improved, energy consumption has been greatly reduced, and competitiveness has increased. However, during the production process of the nitrogen fertilizer industry, a large amount of industrial wastewater is discharged. The discharged wastewater contains cyanide, sulfide, phenol and other pollutants. Although enterprises have adopted corresponding control and treatment measures, there are still problems such as large wastewater discharge, high treatment costs, obsolete facilities, and poor treatment results. This will not only adversely affect the water environment of the surrounding areas, but also related to the sustainable development of the nitrogen fertilizer industry. Studying the current status of wastewater control in the nitrogen fertilizer industry, and finding out the key problems and solutions, have guiding significance for wastewater control in the nitrogen fertilizer industry.

1 Wastewater sources in the nitrogen fertilizer industry

There are many types of wastewater in the nitrogen fertilizer industry. For example, the raw material is divided into synthetic ammonia production wastewater with coal, oil, and gas as raw materials. The process is divided into gas generation, desulfurization, conversion, synthesis, fine drainage, and ammonia processing products. The nature of wastewater is divided into coal gas-containing cyanide wastewater, oil-gas carbon black wastewater, sulfur-containing wastewater and ammonia-containing wastewater. Among them, gas-making cyanide-containing wastewater and ammonia-containing wastewater have the greatest impact on the water environment.

1.1 Source of synthetic ammonia wastewater

There are three parts using coal and coke gas as raw materials: gas-making wastewater produced by gasification process production; wastewater produced by desulfurization process; ammonia-containing wastewater produced by copper washing process. Carbon black wastewater and cyanide-containing wastewater using oil as raw materials, desulfurization wastewater generated in the desulfurization process; low-pressure conversion condensate and methanation condensate generated in the process of removing organic sulfur, namely ammonia-containing wastewater; the wastewater from the process of synthesizing ammonia from gas is mainly The desulfurization wastewater generated in the desulfurization process and the ammonia-containing wastewater produced in the copper washing process, the condensate generated during the removal of organic sulfur is the ammonia-containing wastewater.

1.2 Sources of nitrogen fertilizer (ammonia processing) wastewater

The ammonium carbonate production wastewater is mainly the ammonia-containing wastewater produced by the tail gas washing tower; the urea production wastewater is mainly the desorbed liquid and the condensate that are ammonia-containing wastewater in the distillation and evaporation processes; the ammonium nitrate production wastewater is mainly the ammonia-containing wastewater produced in the vacuum evaporation process.

2 Wastewater control countermeasures

2.1 Implementation of Cleaner Production

2.1.1 For a production plant that uses natural gas as raw material and urea as its main product, the experience of a company in Cangzhou can be used for reference.

2.1.2 For the production plants that use coal as raw materials and urea and methanol as main products, they can learn from the experience of a private fertilizer company in Shijiazhuang. However, the problem of excessive discharge of sewage in the rainy season should be properly handled.

2.1.3 Implementation of the double armor process and the use of efficient fillers

The so-called "double methyl" process refers to the introduction of hydrazine and methanation technology into the ammonia synthesis plant, thereby eliminating the copper washing and regeneration process, so that there is no generation of dilute ammonia water.

Using high-efficiency packing to replace the blister absorption in the washing section of the carbonization comprehensive tower can maximize the gas-liquid contact and increase the mass transfer efficiency. Among the high-efficiency packings, the vertical sieve trays generally used in recent years are more economical and practical. The completion of the above two transformations is equivalent to completely eliminating the production of dilute ammonia water. As for the urea system, as long as the “double armor” process is implemented, the “zero” discharge of dilute ammonia water can be achieved.

2.1.4 Recovery of ammonium bicarbonate from waste ammonia

In the process of ammonia synthesis, the copper washing process discharges dilute ammonia water. After concentration, the ammonia nitrogen concentration is 18% to 20%. It is sent to the carbonization sub-tower to absorb CO 2 in the carbonized tail gas, and then pumped into the cleaning tower by the sub-tower. The scarring of the cleaning tower is dissolved, and the cleaning liquid from the cleaning tower is sent to the carbonization tower, which absorbs the pressurized CO 2 gas (CO 2 from the second stage of the decarbonization stage of the ammonia production process) sent by the compressor to generate ammonium bicarbonate. The product is crystallized and separated by centrifugation, and the mother liquor is recycled.

2.1.5 Dilute ammonia becomes waste

A low-temperature conversion furnace was added after the medium-temperature conversion furnace in the purification section. After the reform, the CO 2 content in the conversion gas was reduced from 3.5% to 1.5%. The copper washing regeneration gas produced in the refining section was reduced from 1000m 3 / h to 400. ~ 500 m 3 / h, correspondingly reducing the amount of dilute ammonia in copper washing. In order to further reduce the pollution caused by dilute ammonia washing in copper washing, a production device for producing ammonium sulfate using dilute ammonia and dilute H 2 SO 4 as raw materials can be established, and the dilute ammonia becomes waste.

2.2 Possible control measures

2.2.1 Cooling tower biological filter method

After the gas-making wastewater is precipitated in the sedimentation tank, it is sprayed and cooled in the upper part of the tower, and then enters the biochemical section in the middle of the tower for biochemical treatment. It is ventilated by an axial flow fan, and the hydrogen cyanide gas blown off is passed through the The biological section is degraded to reduce secondary pollution.

This method has high decyanation efficiency, simple equipment, no secondary pollution, and low cost, but it has large capital investment, slightly higher operating costs, and high management requirements. Suitable for medium-sized plants with large drainage and high cyanide concentration.

2.2.2 Cool Water Tower Recycling

After the gas-making wastewater is precipitated, it is pumped to the top of the cooling water tower and the water is sprayed downward. In order to distribute the water evenly and increase the contact area with the air, wooden grids or corrugated boards are installed in the tower. The air is blown off and escapes into the atmosphere. The spray water enters the pool under the tower and is pumped to the workshop for gas reuse. The cooling tower ventilation can be natural or mechanical.

The method has low price selection, convenient operation, easy to master, high cyanide removal efficiency and low processing cost. It is suitable for small factories in rural or urban neighborhoods with low cyanide content and low water content in wastewater.

The gas-making wastewater and the boiler dust-removing wastewater are mixed and discharged or reused after being precipitated, cooled, and filtered by slag. Slag filtration not only has the characteristics of biochemical treatment, but also has the function of sieving. If the inflow CN - concentration is large, the effluent CN -is difficult to meet the standard. Therefore, when setting a conventional pretreatment facility, add an appropriate amount of ferrite to oxidize. Part of the organic matter and CN - reduce the CN - concentration entering the filter to ensure the quality of the effluent.

This process has small investment, low operating cost, and cyanide removal efficiency can reach more than 90%. It is suitable for small-scale nitrogen fertilizer plants for small and medium-sized sewage treatment.

2.2.4 Chemical precipitation method

The chemical precipitation method is to add Mg 2+ and PO 4 3− to the ammonia nitrogen-containing wastewater, and make the three react to form MgNH 4 PO 4 6H 2 O. When n (Mg): n (N): n (P) = When 1.3: 1: 1.0 and pH value 9, the removal rate of ammonia nitrogen is the highest (up to 98%). The precipitated magnesium ammonium phosphate hexahydrate has a relatively high fertility and can be used for fertilization in nurseries.

The method has the advantages of simple process and ammonia nitrogen removal rate of more than 95%. Disadvantages: This method has large dosage and high cost. It is suitable for treating various concentrations of ammonia nitrogen wastewater, especially suitable for the treatment of high concentration ammonia nitrogen wastewater.

2.2.5 Ion exchange method

The process condensate from the ammonia plant is added from the upper part of the stripping tower, stripped with steam in the packed tower, and the exhaust gas is vented from the top of the tower. The CO 2 is removed, and the anions such as CO 3 2- and SO 4 2- are removed in an anion exchanger, and then further purified by an anion-cation mixed bed. The recovered demineralized water can be used for boiler make-up water or compressor, high-pressure pump Wait for the mainframe pump to seal the water. The steam used for stripping can be recovered steam or boiler exhaust steam.
To save gas consumption.

This method has high NH 4 + ion removal efficiency, simple equipment, and easy operation control. The removal rate of NH 3 -N wastewater containing NH 4 + at 10-50 mg / L can reach 93% -97%. It is suitable for small and medium-sized enterprises to treat NH 3 -N wastewater with medium or below concentration.

2.2.6 Treatment of NH 3 -N wastewater in chemical fertilizer plant by CASS method

The core structure of the CASS method is a reaction tank, without secondary sedimentation tanks and sludge return equipment. Generally, there are no adjustment tanks and primary sedimentation tanks. The facilities are compactly arranged, occupy less land, have low investment, operate stably, have a high substrate removal rate, and the remaining Small amount of sludge. Because the aeration is intermittent, the aeration time can be flexibly adjusted according to changes in water quality and quantity, thereby reducing costs. In addition, A / O method and A2 / O method can be implemented in a single tank to ensure that the wastewater is discharged to the standard. This method is suitable for large, medium and small sewage treatment projects.

The aerated biological filter process composed of immobilized microorganisms is mainly characterized by small equipment investment, low operating costs, simple operation management, and minimal amount of sludge generated by this treatment process, without increasing high investment in sludge disposal. And costs. Has been used in Shaanxi Xinghua Group, Lanzhou Petrochemical and landfill plants and other enterprises that produce high concentration ammonia nitrogen wastewater.

3 conclusions

Through investigation and data review, in order to solve the problem of excessive discharge of wastewater from the nitrogen fertilizer industry, five clean production countermeasures and seven feasible control measures were proposed. This will play a positive role in promoting the sustainable development of the nitrogen fertilizer industry.

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