Waste sulfuric acid treatment technology

Thread Content

Waste sulfuric acid treatment technology: Sulfuric acid is widely used in industries such as chemicals and steel manufacturing. In many production processes, the utilization rate of sulfuric acid is very low, and a large amount of it is discharged along with acid-containing wastewater. If these wastewater are discharged into the environment without treatment, they not only cause acidification of water bodies or soil, thereby harming the ecological environment, but also result in a significant waste of resources. In recent years, many places have established strict emission standards; at the same time, advanced treatment technologies have also been developing rapidly around the world. Waste sulfuric acid and sulfuric acid wastewater contain a large amount of impurities in addition to being acidic. Based on the differences in the composition of waste acids and wastewater as well as the treatment objectives, the treatment methods currently used domestically and internationally can be roughly divided into three categories: recycling and reuse, comprehensive utilization, and neutralization treatment. 1 Recycling of spent sulfuric acid: The sulfuric acid concentration in spent sulfuric acid is high, and it can be recycled after treatment. The treatment mainly involves removing impurities from the waste sulfuric acid, while also concentrating the sulfuric acid. Treatment methods include concentration, oxidation, extraction, and crystallization. 1.1 Concentration method: In this method, during the heating and concentration of waste dilute sulfuric acid, the organic substances present undergo reactions such as oxidation and polymerization, turning into dark-colored gelatinous substances or suspensions which are then filtered out. This approach achieves both the purpose of removing impurities and concentrating the dilute sulfuric acid. Such methods are widely used and the technology is well-developed. Building on the widespread use of high-temperature concentration methods, more advanced low-temperature concentration methods have been developed, which will be introduced below respectively. 1.1.1 High-temperature concentration method: During the production of trichloroacetaldehyde at Zibo Chemical Plant, waste sulfuric acid is generated; the mass fraction of H2SO4 in this waste is 65%–75%, the mass fraction of trichloroacetaldehyde is 1%–3%, and the mass fraction of other organic impurities is 1%. After precipitation and filtration in the plant, distillation is carried out using coal as a heating source; the recovered concentrated sulfuric acid is colorless and transparent, with a H2SO4 mass fraction of over 95%, and no trichloroacetaldehyde is detected. Chloroform can be recovered from the precipitate through alkaline treatment, distillation, and filtration. The plant handles 4,000 tons of waste sulfuric acid per year, and the recovery of sulfuric acid generates a profit of 550,000 yuan per year [1]. The waste sulfuric acid concentration method used by Japan’s Kimura-Daido Chemical Machinery Company combines rising film evaporation in glass-lined tubes with staged vacuum evaporation to increase the mass fraction of H2SO4 in the waste sulfuric acid from 10%–40% to 95%. The process can be divided into three stages: in the first two stages, evaporation and concentration are carried out using heat exchangers made of impermeable graphite tubes, while in the final stage, rising film evaporators with glass-lined tubes are used for concentration. In each stage, the mass fraction of H2SO4 increases gradually, reaching 60%, 80%, and 95% respectively. The heating process uses a high-temperature heat carrier at a temperature of 150–220°C, which converts organic substances into insoluble materials that can then be removed by filtration. This process has been tested on a pilot scale of 2 t/h and has operated successfully for 5 years. This process has strong adaptability and can be used for the treatment of waste sulfuric acid containing various organic impurities [2]. 1.1.2 Low-temperature concentration method: The disadvantages of the high-temperature concentration method are that the strong corrosiveness of sulfuric acid and the acid mist pose significant risks to equipment and operators, making actual operation quite troublesome. Therefore, an improved concentration method has been developed in recent years, known as the vapor-liquid separation type non-volatile solution concentration method (abbreviated as WCG method) [3]. The principle and process of the WCG method are as follows: Waste dilute sulfuric acid is pumped from a storage tank using an acid-resistant pump into a circulation concentration tower for concentration; thereafter, it is heated in a heat exchanger before being fed into a misting device and a diffuser to be forced to form mist and further vaporized. The separated gas undergoes thorough demisting before entering a gas purifier, from where it is released after purification. The separated acid is sent back to the circulation concentration tower, where it undergoes repeated cycles of concentration and distillation. Once the desired concentration is achieved, it is pumped into the concentrated sulfuric acid storage tank. Concentrated sulfuric acid can be reused as a raw material in production. Its process flow is shown in Figure 1. The WCG concentration unit mainly consists of a heat exchanger, a circulation concentration tower, and an induced draft fan. The heat exchanger is made of graphite, the concentration tower is made of composite polypropylene, and both the pumps and exhaust fans are acid-resistant equipment. Compared with the high-temperature concentration method, this method has a lower evaporation temperature (50–60°C), less steam consumption, and lower costs (the cost of electricity and steam required to concentrate each ton of dilute sulfuric acid is approximately 30–60 yuan). The dilute sulfuric acid (with a mass fraction of H2SO4 of 20%) produced by Shanghai Dye and Chemical Factory No. 5 during the production of Disperse Dark Blue H-GL, as well as the dilute sulfuric acid generated by Shanghai Dye and Chemical Factory No. 8, Wuhan Dye Factory, and Jining Dye Factory during the production of dye intermediates, were all concentrated using the WCG method, yielding significant results. When concentrating dilute sulfuric acid using the WCG method, the following points should be noted: (1) If solids precipitate during the concentration process, it will affect heat transfer and the separation of waste acid ； (2) The device is not airtight; if there are volatile substances in the waste acid, it will affect the working environment ； (3) The main material of the device is composite polypropylene; the operating temperature is limited by this material and must not exceed 80℃ ； (4) This method is only applicable to dilute sulfuric acid with a mass fraction of H2SO4 less than 60%. 1.2 Oxidation method: This method has been in use for a long time. Its principle is to use an oxidizing agent under appropriate conditions to oxidize and decompose the organic impurities present in waste sulfuric acid, converting them into carbon dioxide, water, nitrogen oxides, etc., which are then separated from the sulfuric acid, thereby purifying and recovering the waste sulfuric acid. Common oxidizing agents include hydrogen peroxide, nitric acid, perchloric acid, hypochlorous acid, nitrates, ozone, etc. Each oxidizing agent has its advantages and limitations. Tianjin Dye Factory No. 8 uses nitric acid as an oxidant to treat the waste acid resulting from anthraquinone nitration [2,4]. The process involves diluting the waste acid to a mass fraction of 30% H2SO4 in order to maximize the precipitation of anthraquinone; after vacuum filtration in a filter tank, the waste acid enters a rising-film tubular evaporator, where it is concentrated at 112°C and 88.1 kPa. In a rotary liquid separator, water vapor and acid are separated (at this point, the mass fraction of H2SO4 is approximately 70%). The waste acid then flows into a cast-iron concentration vessel, where it is heated to 280–310°C under a vacuum pressure of 6.67–13.34 kPa. A jet pump is used to remove water vapor, raising the mass fraction of H2SO4 to 93%. Finally, the mixture is transferred to an enamel-lined oxidation tank, where concentrated nitric acid (with a mass fraction of 65%) is added for oxidation until the sulfuric acid turns light yellow. The nitric oxide gas produced in the reaction is absorbed by an alkaline solution. Sulfuric acid also exhibits strong oxidizing properties at high concentrations (with a mass fraction of H2SO4 ranging from 97% to 98%) and at high temperatures; it can oxidize organic substances quite thoroughly. For example, when treating benzanthracene waste acid, disperse blue waste acid, and disperse yellow waste acid, the waste acid is heated to 320–330°C to oxidize the organic substances, with part of the sulfuric acid being reduced to sulfur dioxide. This method is highly restricted in its application due to the high concentration and temperature of sulfuric acid, which results in the generation of large amounts of acid mist that causes environmental pollution. Additionally, it consumes a certain amount of sulfuric acid, reducing the yield of this acid. 1.3 Extraction method: The extraction method involves bringing an organic solvent into full contact with waste sulfuric acid, thereby transferring the impurities in the waste acid into the solvent. The requirements for the extractant are: (1) it must be inert to sulfuric acid, not react chemically with it, and not dissolve in it ； (2) The impurities in the spent acid have very high distribution coefficients in the extractant and sulfuric acid ； (3) Low price and easy to obtain ； (4) It is easy to separate from impurities, resulting in low loss during back-extraction. Common extractants include benzenes (toluene, xylene, chlorobenzene), phenols (creosote, crude diphenol), halohydrocarbons (trichloroethane, dichloroethane), isopropyl ether, and N-503, among others. Dalian Dye Factory No. 8 used chlorobenzene for primary extraction of waste sulfuric acid containing ***chlorobenzene and p-nitrochlorobenzene, reducing the organic content in the wastewater from 30,000–50,000 mg/L to 200–250 mg/L [2]. The coking branch of Jinan Iron and Steel Plant also achieved good results in treating the recycled sulfuric acid at the plant using inexpensive C-I extractants and P-I adsorbents [5]. This process involves extracting and separating recycled sulfuric acid using a C-I extractant, and then subjecting it to adsorption treatment with a P-I adsorbent and activated carbon to obtain pure recycled sulfuric acid. To prevent corrosion, the extraction tank and adsorption tank are lined with lead. The plant handles 500 tons of waste sulfuric acid per year, recovering 250 tons of sulfuric acid worth 75,000 yuan. Compared with other methods, the extraction method requires higher technical standards; it is not easy for the extractant to meet all four of the aforementioned requirements, and the operating costs are also high. 1.4 Crystallization method When waste sulfuric acid contains large amounts of organic or inorganic impurities, the crystallization precipitation method can be considered for removing these impurities, depending on their properties. If the waste sulfuric acid emitted during the pickling process at Nanjing Steel Mill contains large amounts of ferrous sulfate, a treatment process of concentration-crystallization-filtering can be employed [6]. The acid solution from which ferrous sulfate has been filtered out can be returned to the steel pickling process for further use. A chemical plant in Chongqing concentrated titanium white waste acid with a 17% H2SO4 mass fraction at atmospheric pressure, allowed the crystals to precipitate and mature, and then filtered them; the filter residue, after being pulped and washed, became the recovered ferrous sulfate. The filtrate is then concentrated and crystallized under a vacuum of 93.4 kPa through filtration, yielding concentrated sulfuric acid with a mass fraction of 80%–85% of H2SO4. The residue from the second filtration is also sent to the pulping process for the recovery of ferrous sulfate [7]. 2 Comprehensive utilization of waste sulfuric acid and sulfuric acid-containing wastewater: Waste sulfuric acid or sulfuric acid-containing wastewater generated in production processes, which can no longer be used directly in those same processes, can be considered for use in other production processes where lower standards regarding the quality of sulfuric acid are required. This approach helps to conserve resources while reducing the amount of waste acid discharged. Additionally, in some production processes that use sulfuric acid as a raw material, if strict requirements are not imposed on the impurities in the sulfuric acid, waste sulfuric acid can be used directly or after minor treatment. For example, Belenkov.D.A used waste acid from a sulfuric acid plant containing 5.2 g/L of arsenic, and by adding 8.78 g/L of Cr2O3, 3.26 g/L of ZnO, and 3.00 g/L of CuCO3 to it, created a solution for preserving wood. The pH of this solution was 1.7, and pine wood treated with this solution could effectively prevent the growth of mold [8]. Toth, Andras and others from Hungary attempted to use sulfuric acid wastewater from oil refineries in combination with lignite fly ash; after adding water, this mixture was then mixed with Blent cement to produce concrete with high strength, which can be used for road construction and building applications [9]. Shimko, I.G. produced Al2(SO4)3 by reacting wastewater containing sulfuric acid with sludge containing Al(OH)3 discharged from viscose fiber factories, using it as a coagulant for water treatment. In this method, the recovery rate of aluminum sulfate is 85%–95% [10]. Wenzhou Dyeing and Chemicals Factory uses alumina slag and waste sulfuric acid as raw materials to produce industrial-grade aluminum sulfate; the process flow is shown in Figure 2 [11]. Furthermore, many sulfate-based industrial products can also be produced using waste sulfuric acid or sulfuric acid wastewater. For example, Mokanty and Bibhupada in India used sulfuric acid-containing wastewater from detergent factories to react with copper particles and copper shavings in a reaction tower; after crystallization and filtration of the solution, copper sulfate crystals could be obtained [12]. Jining No. 2 Chemical Plant uses waste sulfuric acid (with a mass fraction of 20% H2SO4) to react with rhodochrosite or pyrolusite in order to produce industrial-grade manganese sulfate. The process flow is as follows: rhodochrosite or pyrolusite is mixed with waste sulfuric acid for acidolysis, and the material resulting from this acidolysis is then filtered under pressure. The filter residue is discharged as waste after being pulped and filtered under pressure, while the wash liquid is returned to the acidolysis process. The filtrate is processed by removing impurities, filtering, evaporation crystallization, centrifugal separation, and drying to produce the product manganese sulfate [13]. Ammonia can be used to neutralize waste sulfuric acid to produce sulfate ammonium fertilizer. Organic impurities in spent acid are generally removed after the production of sulfuric acid an. The methods for removing these impurities include extraction, oxidation, salting out, coagulation, and ion exchange. 3. Neutralization of spent sulfuric acid and sulfuric acid-containing wastewater: For wastewater with a very low sulfuric acid concentration but large volumes, it is difficult to carry out comprehensive utilization due to the low value of sulfuric acid that can be recovered; instead, lime or waste alkali can be used for neutralization to bring it to emission standards or to facilitate further treatment. Taking the Shanghai Sulfuric Acid Plant as an example, this plant discharges 3,600 tons of wastewater containing sulfuric acid per day, with a pH of 2.6; it also contains small amounts of arsenic, fluorine, and other substances. The wastewater is treated using calcium carbide sludge (with Ca(OH)2 as its main component) for neutralization, polyacrylamide as a coagulant, and Rs as an oxidant, through a process of neutralization-coagulation sedimentation-oxidation. This approach not only neutralizes the acidity but also removes substances such as fluorine and arsenic, ensuring that the effluent meets the discharge standards [14]. 4 Conclusion Besides the aforementioned common methods, there are also electrolytic treatment, freezing method, pyrolysis, dialysis, air stripping, and others for treating waste sulfuric acid and sulfuric acid-containing wastewater [16–19]. However, in China, concentration and recovery methods as well as neutralization treatment remain the most widely used approaches. In production, the recovery or treatment method should be selected based on the concentration of waste sulfuric acid or sulfuric acid-containing wastewater, as well as the composition of the impurities present. Especially for the waste sulfuric acid or sulfuric acid wastewater generated by the fine chemical industry, where the organic impurities present are extremely complex, the concentration of sulfuric acid varies greatly and the volume of waste to be treated is small, it is even more important to choose methods that require less investment while still yielding good results, based on the specific circumstances.