Sulfuric acid is widely used in chemical and steel industries. In many production processes, the utilization of sulfuric acid is low, and a large amount of sulfuric acid is discharged along with the acid-containing wastewater. If these wastewaters are discharged into the environment without being treated, they will not only acidify the water body or the soil, but also cause harm to the ecological environment and waste a lot of resources. In recent years, many countries have already established strict emission standards. At the same time, advanced governance technologies have also developed rapidly around the world.
Spent sulfuric acid and sulfuric acid wastewater contain not only acidity but also a large amount of impurities. According to the differences in waste acid, waste water composition and treatment objectives, the governance methods adopted at home and abroad can be roughly divided into three categories: recycling, comprehensive utilization and neutralization treatment.
1 Reuse of waste sulfuric acid
Sulfuric acid in waste sulfuric acid concentration is higher, can be recycled after treatment. The treatment is mainly to remove the impurities in the waste sulfuric acid while enriching the sulfuric acid. Treatment methods include concentration, oxidation, extraction, and crystallization.
1.1 Concentration method
The method is to heat and concentrate the waste dilute sulfuric acid so that the organics therein undergo oxidation, polymerization, etc., and change into a dark colloidal substance or suspended matter, which is filtered and removed, so as to achieve the dual purposes of removing impurities and concentrating dilute sulfuric acid. This type of method is widely used and mature. On the basis of the general application of the high-temperature concentration method, a more advanced low-temperature concentration method has been developed, which is described below.
1.1.1 high temperature concentration method
There is waste sulfuric acid produced in the production process of chloral aldehyde in Zibo Chemical Plant, in which the mass fraction of H2SO4 is 65% to 75%, the mass fraction of trichloroacetaldehyde is 1% to 3%, and the mass fraction of other organic impurities is 1%. The plant will filter its precipitate, directly heated with coal distillation, the colorless sulfuric acid recovered, H2SO4 mass fraction greater than 95%, no trichloraldehyde detection, and sedimentation by alkaline hydrolysis, distillation and filtration can be recycled Chloroform. The waste sulfuric acid treatment capacity of the plant is 4000t/a, and the profit of sulfuric acid recovery is 550,000 yuan/a[1].
The waste sulfuric acid concentration method of Japan's Kimura-Datong Chemical Machinery Co., Ltd. is a combination of leaching membrane evaporation with helium glass tubes and staged vacuum evaporation. The concentration of H2SO4 in waste sulfuric acid is concentrated from 10% to 40% to 95%. Divided into 3 segments, the first two segments were evaporatively concentrated with impervious graphite tube heaters, and the latter segment was enriched with a helium-encapsulated glass membrane evaporator. The H2SO4 mass fraction gradually increased in each segment, reaching 60% and 80%, respectively. 95%. Heating process using high temperature heat carrier, the temperature is 150 ~ 220 °C, the organic matter can be converted into insoluble materials, and then filtered to remove, the process to 2t / h on the scale, 5a well. The process is very adaptable and can be used for the treatment of waste sulfuric acid containing many organic impurities [2].
1.1.2 Low Temperature Concentration
The disadvantage of the high-temperature concentration method is that the strong corrosiveness of sulfuric acid and the acid mist are very harmful to the equipment and the operator, and the actual operation is very troublesome. Therefore, in recent years, an improved concentration method has been developed, called the vapor-liquid separation non-volatile solution concentration method (abbreviated as WCG method) [3].
The principle and process of the WCG method are as follows: The waste dilute sulfuric acid is pumped into the circulating concentration tower by the acid-proof pump in the storage tank, then heated by a heat exchanger, and then enters the mist generator and diffuser to force atomization and further forced vaporization. After the gas is highly defogged, it enters the gas purifier and is discharged after purification. The separated acid solution is returned to the circulation concentration column again, and after repeated cycles of concentration and distillation to reach the concentration requirements, the concentrated sulfuric acid storage tank is pumped. Concentrated sulfuric acid can be reused as a raw material for production.
The WCG enrichment device is mainly composed of a heat exchanger, a circulation concentration tower and an induced draft fan. The material of the heat exchanger is graphite, the material of the enrichment tower is composite polypropylene, and the pump and the induced draft fan are all acid-proof equipment.
Compared with the high-temperature concentration method, this method has a low evaporating temperature (50-60°C), low steam consumption, and low cost (approximately 30-60 RMB per ton of dilute sulfuric acid consumes electricity and steam). Shanghai Yinhua No.5 Plant produces dilute sulfuric acid (H2SO4 mass fraction of 20%) produced by dispersed dark blue H-GL, dilute sulfuric acid produced by dyestuff intermediates at Shanghai Yinhua No. 8 Plant, Wuhan Dyestuffs Factory, and Jining Dyestuffs Factory, and concentrated by WCG method. , have achieved significant results.
Concentration of dilute sulfuric acid with the WCG method should pay attention to the following points:
(1) If solids precipitate during the concentration process, heat transfer effects and separation of waste acids will be affected;
(2) The device is not closed, if there are volatile substances in waste acid, it will affect the working environment;
(3) The main material of the device is composite polypropylene, and the working temperature is limited by the main material and cannot exceed 80 °C;
(4) This method is only applicable to dilute sulfuric acid with H2SO4 mass fraction less than 60%.
1.2 Oxidation
The method has been used for a long time. The principle is to oxidize and decompose the organic impurities in the waste sulfuric acid with an oxidizing agent under proper conditions to convert it into carbon dioxide, water, nitrogen oxides, etc. to be separated from the sulfuric acid, so that the waste sulfuric acid is purified and recovered. . The commonly used oxidants are hydrogen peroxide, nitric acid, perchloric acid, hypochlorous acid, nitrates, ozone and the like. Each oxidizer has its advantages and limitations.
The Tianjin Dyestuffs No.8 Plant uses nitric acid as the oxidant to oxidize the nitric acid wastes [2,4]. The operation process is: Diluted waste acid to 30% H2SO4, so that the dinitrofluorene contained in it is maximum. To a limited extent, the waste acid is filtered through a filter tank and vacuum-filtered. The waste acid enters a rising-membrane tube evaporator and is concentrated at 112° C. and 88.1 kPa. The water vapor and acid are separated in a hydrocyclone (the H2SO4 mass fraction is about 70%), waste acid flows into a cast iron concentrator (280°C to 310°C, vacuum degree is 6.67~13.34kPa), steam is blown out with a jet pump so that the H2SO4 mass fraction reaches 93%, and then it flows into an enamel oxidation tank, adding Concentrated nitric acid (HNO3 mass fraction of 65%) was oxidized until the sulfuric acid appeared pale yellow. The nitric oxide gas generated during the reaction is absorbed by the alkaline solution.
Sulfuric acid is also highly oxidizing at high concentrations (H2SO4 mass fraction of 97% to 98%) and at high temperatures, and it can oxidize organic substances thoroughly. For example, in the treatment of phenylanthraquinone waste acid, disperse blue waste acid, and disperse yellow waste acid, the waste acid is heated to 320-330° C. to oxidize the organic matter, and part of the sulfuric acid is reduced to sulfur dioxide. In this method, the concentration and temperature of sulfuric acid are too high, a large amount of acid mist is generated, environmental pollution is caused, and at the same time, a certain amount of sulfuric acid is consumed, so that the yield of sulfuric acid is reduced. Therefore, its application is greatly limited.
1.3 Extraction
The extraction method is to fully contact the waste sulfuric acid with an organic solvent and transfer the impurities in the waste acid to the solvent. The requirements for the extractant are:
(1) It is inert to sulfuric acid and does not react with sulfuric acid and does not dissolve in sulfuric acid;
(2) The impurities in the spent acid have a high distribution coefficient in the extractant and sulfuric acid;
(3) It is cheap and easy to get;
(4) It is easy to separate from impurities, and the loss during stripping is small.
Common extractants include benzene (toluene, nitrobenzene, chlorobenzene), phenols (cresol oil, crude diphenols), halogenated hydrocarbons (trichloroethane, dichloroethane), isopropyl ether, and N -503 and so on.
Dalian Dye No. 8 Plant used chlorobenzene to perform primary extraction of waste sulfuric acid containing dinitrochlorobenzene and p-nitrochlorobenzene, which reduced the organic content in wastewater from 30,000 to 50,000 mg/L to 200 to 250 mg/L.[2] ]. Jinan Iron and Steel Coking Plant Co., Ltd. also used the inexpensive CI extractor and PI adsorbent to treat the plant's regenerated sulfuric acid with good results [5]. The process is to extract the regenerated sulfuric acid by CI extraction agent and then use PI adsorbent and activated carbon adsorption treatment to obtain pure regenerated sulfuric acid. To prevent corrosion, the extraction tank and sorption tank are lined with lead. The waste sulfuric acid treatment capacity of the plant is 500t/a, and 250t of sulfuric acid is recovered, worth 75,000 yuan.
Compared with other methods, the technical requirements of the extraction method are relatively high. It is not easy for the extractant to satisfy the above four requirements at the same time, and the operating cost is also high.
1.4 Crystallization
When the waste sulfuric acid contains a large amount of organic or inorganic impurities, it may be considered to select the method of crystal precipitation to remove impurities according to its characteristics.
For example, waste sulfuric acid discharged from the acid washing process of the Nanjing Rolling Mill contains a large amount of ferrous sulfate, which can be treated by a concentration-crystallization-filtration process [6]. The acid liquid after filtration to remove ferrous sulfate can be returned to the steel pickling process for continued use.
A certain chemical plant in Chongqing will use H2SO4 with a mass fraction of 17% of titanium white waste acid to concentrate and crystallize at normal pressure. The crystallization of the precipitated sulphate will be carried out after beating and washing. The filtrate is then concentrated and filtered under a vacuum of 93.4 kPa to obtain concentrated sulfuric acid having a H2SO4 mass fraction of 80% to 85%. The second filtration residue is also transferred to the beating step to recover ferrous sulfate [7].
2 Comprehensive utilization of waste sulfuric acid and waste water containing sulfuric acid
Waste sulfuric acid or waste water containing sulfuric acid discharged from the production, if it can no longer be directly used in the original process, can be used in other production processes where the quality of sulfuric acid is not required so as to save resources and reduce waste acid emissions. . In addition, some production processes using sulfuric acid as a raw material, if the strict requirements of impurities in sulfuric acid, can also be used directly as waste sulfuric acid or waste sulfuric acid slightly treated as raw materials.
For example, Belenkov.DA uses a waste acid solution containing 5.2 g/L of arsenic in a sulfuric acid plant, adding 8.78 g/L Cr2O3, 3.26 g/L ZnO, and 3.00 g/L CuCO3 to prepare a wood preservative solution. The pH of the solution is 1.7. Pine wood can effectively prevent the growth of mold after immersion in this solution [8]. Toth, Andras, Hungary, and others tried to use the sulfuric acid wastewater from a refinery mixed with lignite fly ash, and then added water and then mixed with bream cement to produce concrete with high strength, which can be used in paving and construction industries [9].
Shimko, IG uses sulphuric acid-containing exhaust scrubber water to react with Al(OH) 3 -containing sludge discharged from viscose fiber mills to produce Al2(SO4)3 as a coagulant for water treatment. The recovery of aluminum sulfate in this method is from 85% to 95% [10]. Wenzhou Dyeing Chemical Plant uses Alum slag and waste sulfuric acid as raw materials to produce industrial-grade aluminum sulfate. The process flow is shown in Figure 2 [11].
In addition, many sulfate industrial products can also be produced using waste sulfuric acid or sulfuric acid wastewater. For example, Mokanty and Bibhupada of India used the waste water containing sulfuric acid from a detergent factory to react with copper particles and copper shavings in a reaction tower, and the solution was crystallized and filtered to obtain copper sulfate crystals [12].
Jining No. 2 Chemical Plant uses waste sulfuric acid (H 2 SO 4 mass fraction of 20%) to react with rhodochrosite or pyrolusite to produce industrial-grade manganese sulfate. The process flow is as follows: rhodochrosite or pyrolusite is mixed with waste sulfuric acid to perform acid hydrolysis and acid Solution after the solution of hydraulic filter. The filter residue is discharged in the form of waste residue after beating and pressure filtration, and the washing solution returns to the acid hydrolysis process. The filtrate was subjected to removal of impurities, filtration, evaporative crystallization, centrifugal separation and drying to obtain product manganese sulfate [13].
Neutralization of waste sulfuric acid with ammonia can produce ammonium sulfate fertilizer. The organic impurities in the spent acid are generally removed after ammonium sulfate is prepared. The methods for removing impurities include extraction, oxidation, salting out, agglomeration, and ion exchange.
3 Neutralization of waste sulfuric acid and wastewater containing sulfuric acid
For wastewater with a low concentration of sulfuric acid and a large amount of water, since the value of the recovered sulfuric acid is not high, it is difficult to perform comprehensive utilization. Lime or spent caustic soda can be used to neutralize it to an emission standard or to facilitate subsequent treatment.
Taking the Shanghai Sulfuric Acid Plant as an example, the plant discharges 3,600 tons of sulfuric acid-containing wastewater every day, with a pH of 2.6, which also contains a small amount of arsenic, fluorine, and the like. The plant is neutralized with calcium carbide (mainly composed of Ca(OH)2), polyacrylamide is used as a coagulant, Rs is used as an oxidant, and the wastewater is treated with a neutralization-coagulation-precipitation-oxidation process. With acid, fluorine and arsenic were removed, and the effluent reached the discharge standard [14].
4 Conclusion
In addition to the above-mentioned several common methods, the treatment of waste sulfuric acid and waste water containing sulfuric acid includes electrolytic method, freezing method, pyrolysis method, dialysis method, and gas stripping method [16-19], but in China, the concentration recovery method and And processing method is still the most widely used method. In the production, the recovery or treatment method should be selected according to the concentration of waste sulfuric acid or sulfuric acid-containing waste water and the composition of the contained impurities. Especially for the waste sulfuric acid or sulfuric acid wastewater generated in the fine chemical industry, due to the extremely complex organic impurities, the concentration of sulfuric acid varies greatly, and the amount of treatment is not large, which requires more attention to select investment based on specific circumstances. Small, more effective method.
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