Application Practices of Multi-effect Evaporation in the Recycling and Utilization of Waste Acids

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Application practices of multi-effect evaporation in the resource utilization of waste acid ★ 2021 Sulfuric Acid Industry Annual Conference Qian Jun (Jiangsu Tait United Environmental Protection Technology Co., Ltd., Changzhou, Jiangsu 213144) Author introduction: Qian Jun, born in 1970, is from Changzhou, Jiangsu. He serves as the chief engineer and is mainly engaged in the design, research and development, and equipment manufacturing related to production processes, equipment, and devices for the resource utilization of hazardous solid wastes such as waste acid. Contact information for the author: cztthb@vip.163.com
1. Multi-effect evaporation
1.1 Evaporation
Evaporation refers to the process of heating a solution containing non-volatile impurities to boiling point, thereby partially removing those impurities through vaporization and thus concentrating the solution. Currently, evaporation is primarily used to concentrate and crystallize solutions of non-volatile substances in industries such as food, pharmaceuticals, and chemicals. In chemical production, there are four purposes for evaporation: the first is to obtain the product ； Second, after evaporating the solution to increase its concentration, it is cooled to induce crystallization, thereby obtaining a solid product ； Third, high-purity semi-finished products or solvents are obtained after removing impurities ； Fourth is to improve the environment or reduce various types of environmental pollution associated with it. An evaporator is a specialized device used to carry out evaporation operations. Therefore, there should be sufficient heating area within the evaporator to meet the requirements for heating and boiling of the solution. 1.2 Multi-effect evaporation: Multi-effect evaporation is a heat transfer process that involves operating several evaporators in series, thereby allowing the thermal energy of steam to be utilized multiple times and improving the efficiency of heat utilization. It is widely used in the treatment of aqueous solutions; it is a process involving boiling and condensation for heat exchange, and it features a high heat transfer coefficient. Multiple-effect evaporation is a series evaporation process in which the secondary steam from the previous effect is used as the heating steam for the next effect. In multi-effect evaporation, the operating pressure of each effect, as well as the corresponding heating steam temperature and solution boiling point, decrease sequentially. 1.3 Working principle of multi-effect evaporation: In evaporation processes, a large amount of secondary vapor is generated, and this vapor contains substantial latent heat; therefore, it should be recovered and utilized. If this secondary vapor is introduced into the heating chamber of another evaporator, as long as the operating pressure and boiling point in the latter are lower than those in the original evaporator, the introduced secondary vapor can still serve to provide heating. This approach is known as multi-effect evaporation. Each evaporator in multi-effect evaporation is called an effect. An evaporator to which heated steam is supplied is called the first effect; an evaporator that uses the secondary steam from the first effect as a heating agent is called the second effect, and so on. The purpose of using a multi-effect evaporator is to save on the consumption of heating steam. Theoretically, 1 kg of heated steam can evaporate approximately 1 kg of water. However, due to heat losses, and because the latent heat of vaporization of water in the separation chamber is greater than the latent heat of condensation in the heating chamber, in practice more steam is required to evaporate 1 kg of water than 1 kg. Based on experience, the economy of steam (U=W/D) is 0.91 for a single-effect system ； The double efficiency is 1.76 ； The three-effect value is 2.5; the four-effect value is 3.33 ； The five-effect rating is 3.71. It can be seen that as the exponent increases, the growth rate of W/D gradually decreases. For example, when changing from a single-effect to a double-effect system, heating steam can be saved by about 50% ； When changing from four-effect to five-effect, heating steam is only saved by 10%. However, as the effectiveness increases, the temperature difference loss in heat transfer rises, which leads to a **decrease in the production capacity of the evaporator** and a doubling of the equipment costs. When the efficiency factor increases to a certain level, the savings on steam costs resulting from this increase may not be worth the additional cost of the equipment. In industry, a trade-off must be made between operating costs and equipment costs to determine the most reasonable efficiency factor. The most commonly used configurations are 2–3 effects; at most, there can be 6 effects. 1.4 Advantages and disadvantages of multi-effect evaporation: Advantages of multi-effect evaporation: Multi-effect evaporators consume relatively little energy during operation. Compared to other ordinary evaporators, multi-effect evaporation yields excellent results. It can evaporate products in a very short amount of time, with highly satisfactory evaporation outcomes. Multi-effect evaporation systems have a long service life, which helps to reduce costs significantly. The cost of replacement is lowered since these systems can be used for a long time without the need for replacement, thereby potentially saving even more money. Disadvantages of multi-effect evaporation: The products produced by multi-effect evaporation are quite bulky, and they have many components, making disassembly and installation complicated. The working principle of multi-effect evaporation is complex, and it is necessary to monitor the temperature inside the separator constantly; therefore, someone needs to be present beside the machine. It is difficult for non-technical personnel to operate it. In multi-effect evaporation, as the number of effects increases, the temperature difference loss associated with heat transfer rises, which leads to a **decrease in the evaporation capacity**. The cost of the equipment also increases significantly, and too many effects require additional investment costs. In industry, a trade-off must be made between operating costs and equipment costs to determine a reasonable efficiency factor. 2. Waste Acids 2.1 Sources of Waste Acids Strong acids such as sulfuric acid, nitric acid, hydrochloric acid, and hydrofluoric acid are important basic chemical raw materials that are widely used in various sectors of the national economy, including metallurgy and steel production, petrochemicals, printing and textiles, paints and coatings, defense and military industries, pharmaceuticals and pesticides, as well as electronics and electrical equipment. Except for the fertilizer industry, which hardly generates waste acid, other industries produce large amounts of waste acid while using acids. For example, in the steel products industry, hydrochloric acid and sulfuric acid are used for surface cleaning during the production process. This generates waste acid containing iron, zinc, and manganese ions. On average, approximately 15 kg to 30 kg of waste acid is produced per ton of steel products cleaned ； In the stainless steel pickling industry, a three-stage cleaning process is generally used during production: sulfuric acid, a mixture of hydrofluoric acid and nitric acid, and nitric acid are employed to clean the surface of stainless steel products. This results in the generation of large amounts of waste acid containing iron ions, nickel ions, and chromium ions; typically, about 100 kg to 150 kg of waste acid is produced for every ton of stainless steel products cleaned ； The filtrate generated after the hydrolysis of titanium slurry and the washing liquid produced during the first washing in the titanium dioxide industry are both waste sulfuric acid. Generally, for every ton of titanium dioxide produced, approximately 8 tons of waste sulfuric acid with a concentration of about 20% is generated ； China’s printed circuit board industry generates approximately 60,000 tons of etching waste liquid per day, of which about 70% is acidic waste liquid ； The waste acid generated from cleaning aluminum profiles and aluminum foil using various inorganic acids amounts to about 9 million tons per year ； The chemical reactions involved in the organic chemicals industry, such as sulfonation, nitration, alkylation, and esterification, all require large amounts of sulfuric acid, generating waste sulfuric acid with a concentration of 10–90% ； The total amount of waste sulfuric acid generated in the graphene industry is even more astonishing: generally, to produce 1 ton of graphene, 40 to 50 tons of concentrated sulfuric acid with a 98% purity are required, resulting in 80 to 100 tons of high-concentration waste sulfuric acid (the amount generated for expanded graphite is more than twice as much), and 120 to 150 tons of low-concentration waste sulfuric acid. 2.2 Characteristics of waste acids According to industry research, among the waste acids generated in China, inorganic waste acids account for about 35%, while organic waste acids make up around 65%. Waste acids with a residual acid content of over 40% constitute approximately 46% of the total, and they exhibit the following characteristics: (1) Diverse sources: In addition to major industries such as titanium dioxide production, petrochemicals, and steel pickling, waste acids are also generated during the production of hundreds of other products, with the enterprises that produce these acids located throughout the country ； (2) Large volume: Estimates show that the chemical industry generates nearly 80 million tons of waste acid of various concentrations each year. Steel companies, metal processing, and pickling industries produce around 65 million tons of waste acid per year. Industries such as petroleum refining, mineral processing, battery production, military, and nuclear industries generate more than 50 million tons of waste acid annually. In China, the total amount of waste acid produced each year exceeds 200 million tons, which is an extremely large figure. The source of this figure remains quite vague; based on observations over the past 10 years, the amount of waste acid generated by the industries dealing with rare earths, quartz, quartz sand, graphene, and other new materials is several times higher than that produced by other industries, though it is not possible to obtain specific figures. The huge volume of production poses significant environmental threats and hazards ； (3) Uneven distribution: Although the total amount of waste acid is large, there are significant differences in the amount of waste acid produced by different companies; the annual output of waste acid varies from a few hundred tons to hundreds of thousands of tons per company ； (4) Varied composition: Waste acids from different sources vary greatly in type and concentration, and the impurities present are diverse with significant differences in their amounts. 2.3 Hazards of waste acid: Waste acid generated in any industry possesses characteristics such as high corrosivity and severe environmental pollution, and it is now classified as hazardous waste and managed accordingly by various countries. Our country has also included it in the **List of Hazardous Wastes**. The main hazards caused by the illegal discharge of waste acid include: corrosion of sewer pipes and other hydraulic structures such as reinforced concrete ； It causes crops to wither and affects the growth of aquatic plants ； When waste acid seeps into the soil, it can cause calcification of the soil over time, destroying its loose structure and thus affecting the growth of crops ； Hindering the growth of microorganisms in biological wastewater treatment ； Can kill fish ； Consuming water contaminated in this way by humans and animals can cause gastrointestinal inflammation, and even burns ； It causes severe pollution of water bodies, toxicity to organisms, and ultimately significant harm to human health. According to **environmental regulations, waste acid shall not be discharged directly without treatment. The harm caused by waste acids to groundwater is much greater than that of ordinary chemical wastewater. For example, as waste sulfuric acid seeps into the ground, it reacts with carbonates, sulfites, sulfides, etc. in rocks and soil to produce harmful gases such as sulfur dioxide and hydrogen sulfide, which are released into the air and cause air pollution. In industries that generate large amounts of waste acid, such as those in the non-ferrous metals and titanium dioxide sectors, the waste acid discharged contains high levels of heavy metals. These metals dissolve into rivers or groundwater, causing severe pollution to them. Irrigating crops with this polluted water further leads to serious contamination of the soil. According to investigations by the Ministry of Environmental Protection, nearly 20% of the soil in China has been polluted. The waste acid discharged from organic chemical industries contains large amounts of organic substances, some of which are carcinogenic and non-degradable. Some companies also make use of this waste acid, for example in the production of phosphate fertilizers, sulfuric anhydride, magnesium sulfate, and in paper mills where it is used to neutralize the alkaline solutions used in paper production in order to create organic fertilizers. If not treated in a scientific and effective manner, these harmful substances can end up in the soil along with the fertilizers, first contaminating surface water and then seeping underground or entering rivers via rainwater, thus contaminating river bottoms and groundwater. Even more concerning is the use of contaminated water for irrigation, as this leads to soil acidification; heavy metals are more active in acidic soil and are therefore more easily absorbed by plants. These organic or inorganic harmful substances then enter food through plants, posing a serious threat to food safety. 3. Practical Applications of Multi-effect Evaporation in the Recycling and Utilization of Waste Acids 3.1 Multi-effect Vacuum Evaporation Crystallization Technology for Waste Hydrochloric Acid 3.1.1 Working Principle The multi-effect vacuum evaporation crystallization system for waste hydrochloric acid relies on multi-effect vacuum evaporation of hydrochloric acid wastewater, thereby saving energy and reducing steam consumption. The technical advantages of counter-current flash evaporation also include: using the heat from the concentrated solution for flash evaporation, which increases the concentration of the recovered hydrochloric acid as well as the chloride content in the concentrated solution; it also reduces crystallization and scaling in the evaporator ； The washing cycle of the evaporator has been extended. Principle of acid separation: Utilizing the laws and characteristics regarding the solubility of hydrogen chloride in water at different pressures and temperatures, as well as the pressure and temperature differences among the various evaporation units in a multi-effect evaporation system, the hydrochloric acid pickling waste liquid first enters the last evaporation unit, which has a lower temperature and higher vacuum. The secondary steam generated during evaporation contains less hydrogen chloride; after being condensed by a condenser, this results in a dilute acid with a low hydrogen chloride concentration. The initially concentrated hydrochloric acid waste liquid enters the pre-evaporation unit, where it is re-evaporated under conditions of high temperature and low vacuum. Part of the hydrochloric acid flashes, producing secondary steam with a high hydrogen chloride concentration; after being condensed through heat exchange, this results in recovered hydrochloric acid with a high hydrogen chloride concentration. The multi-effect negative-pressure evaporation crystallization method for treating hydrochloric acid waste liquid is essentially a physical process for separating solutes from solvents in a solution. Its basic principle is to heat an aqueous solution containing solutes such as metal ions and hydrogen chloride under vacuum conditions, causing the volatile solute hydrogen chloride to evaporate together with water; this vapor is then condensed using cooling water in a condenser, resulting in dilute hydrochloric acid of high purity ； As the volume of the solution decreases, the concentration of the non-volatile solute metal ions in the solution increases, resulting in a supersaturated solution of metal salts. Then, by cooling, the solubility of the solution is reduced; in a supersaturated state of the solution, most of the metal salts precipitate as crystalline forms of those metal salts, thereby achieving the separation of the solute from the solvent in the solution. This process can recover over 90% of hydrogen chloride from hydrochloric acid waste liquid ； This causes all metal ions to precipitate in the form of metal salt crystals with crystalline water. 3.1.2 Process characteristics: The multi-effect negative-pressure evaporation crystallization method for treating hydrochloric acid waste liquid makes use of specialized graphite equipment designed for this purpose, which helps to improve the operational efficiency, service life, and corrosion resistance of the system ； The use of multi-effect negative pressure evaporation technology is primarily aimed at reducing the steam consumption and treatment costs associated with the processing of hydrochloric acid waste liquid ； Adopting a full negative pressure system ensures clean and safe production while reducing environmental pollution in the workshop. The specific features include the following: 1) Negative pressure evaporation concentration: Hydrochloric acid waste liquid has a high evaporation temperature at normal pressure, is highly corrosive, requires frequent equipment maintenance and has a short lifespan; these factors are the main reasons for the high operating costs associated with the treatment of hydrochloric acid waste liquid as well as the high costs of daily equipment maintenance. This method employs negative-pressure external circulation evaporation concentration for hydrochloric acid washing waste liquid: under negative pressure, the evaporation temperature is low, which reduces corrosion of the equipment and pipelines, thereby ensuring continuous and stable production. Due to the lower operating temperature, there are many favorable conditions and wide possibilities in terms of material selection for the equipment, which can reduce project costs. All processing operations are carried out under negative pressure, reducing the leakage of hydrogen chloride gas and significantly improving both the operating environment and the overall factory conditions. 2) External heating evaporator combined with forced circulation mode: Hydrochloric acid waste liquid tends to crystallize after being evaporated and concentrated to a certain extent, which can even block the material channels in the evaporator and cause equipment damage. An externally heated evaporator is used in combination with a forced circulation mode. In terms of process layout, the evaporator and the separator are arranged at different heights. Under the combined effect of gravity and temperature differences, as well as the system vacuum, the material inside the evaporator rises due to heating, while the relatively cooler material inside the separator descends, resulting in intense circulation; this is supplemented by the forced circulation mode ； Ensure that the material circulation speed is above 2 m/s. Under such high-speed and intense motion, it essentially eliminates the possibility of material crystallization within the evaporator as well as blockages in the evaporator, ensuring stable operation of the equipment during normal production. 3) The purity of the recovered regenerated acid is high: In the process of recovering hydrochloric acid, since metal ions are not volatile, the hydrogen chloride and water vapor evolved in the regenerated acid system are condensed into hydrochloric acid using a condenser; this acid contains virtually no metal ions, hence its purity is very high. Using it back in the production process has no adverse effects on the manufacturing process. 4) Simple process and low equipment investment: The process used in this method requires few pieces of equipment, resulting in low investment, and it is simple and easy to operate. This method boasts many advantages, including high evaporation efficiency, the ability to produce continuously and stably, simple operation, no need to add additional additives during the treatment process, corrosion-resistant and durable equipment and pipeline materials, low treatment costs, and significant environmental benefits. The multi-effect negative pressure evaporation crystallization method can be used for the recovery and treatment of hydrochloric acid waste streams from steel products, hydrochloric acid waste streams containing aluminum, and hydrochloric acid waste streams containing copper. The working principle of this device can also be widely applied to the evaporation crystallization processes for other volatile waste acids and high-salt wastewater. 3.2 Equipment for treating waste hydrochloric acid by sulfonation The technology for treating waste hydrochloric acid from metal products through sulfonation involves a combination of methods such as negative-pressure counter-current evaporation and concentration of waste hydrochloric acid, sulfonation reactions to produce sulfates, absorption of hydrogen chloride gas to generate acid, recrystallization of the crude sulfate, and concentration of the sulfate mother liquor. 3.2.1 Introduction to the process of the waste hydrochloric acid evaporation and concentration unit: The waste hydrochloric acid evaporation system utilizes three-effect negative-pressure evaporation for hydrochloric acid wastewater, thereby saving energy and reducing steam consumption. Considering the specific implementation of the entire project, it was decided to adopt a three-effect counter-current negative-pressure evaporation concentration process ； Ensure that the chloride salt concentrate enters the sulfate production system via the synthesis sulfonation reaction at a high concentration and at a high temperature. 3.2.2 Introduction to the process of producing sulfates via sulfonation reaction The working principle of the system for producing sulfates through sulfonation reaction is based on the chemical reaction equations in which sulfuric acid displaces hydrogen chloride, as well as the reaction between chlorides and sulfuric acid: HCl·H2O + H2SO4 → H2SO4·H2O + HCl↑ ； MeCL2+H2SO4→MeSO4+2HCL↑ ； Since the boiling point of sulfuric acid is higher than that of hydrochloric acid, an excess of sulfuric acid is added to the concentrated solution. Since sulfuric acid is more acidic than hydrochloric acid and has a higher boiling point, concentrated sulfuric acid first undergoes a displacement reaction with the hydrogen chloride in the concentrate, replacing all of the free hydrogen chloride in it with hydrogen chloride gas. It then undergoes a double decomposition reaction with the chlorides, resulting in the formation of corresponding sulfates and hydrogen chloride as a byproduct of this displacement reaction. Hydrogen chloride gas, after being absorbed by dilute hydrochloric acid (water), becomes recovered hydrochloric acid. The sulfate resulting from the completion of the reaction is added, in proportion, to pure water or sulfate mother liquor; after thorough dissolution, it is discharged into the crystallization tank. The sulfate crystal system is determined by the solubility of sulfates in water (or sulfuric acid). Solid-phase crystallization and precipitation occur through circulating cooling ； In the solid-liquid separation process, a centrifuge is used to separate the slurry in its solid-liquid mixture state, thereby extracting and isolating the solid sulfate crystals from the liquid slurry. The isolated crude sulfate is fed into the recrystallization system. The separated sulfate mother liquor, after being collected, is fed into the synthesis kettle to dilute the concentrated slurry resulting from the synthesis reaction, or it is returned to the crystallization kettle; there it is mixed with the concentrated sulfate slurry produced in the crystallization and synthesis kettles for further crystallization. 3.2.3 Brief description of the process for producing acid by absorbing hydrogen chloride gas: For the absorption system of hydrogen chloride gas, a three-stage falling-film absorption process combined with a tail gas absorption tower is employed. Using the dilute hydrochloric acid from the evaporation system as the absorption liquid not only increases the concentration of hydrochloric acid but also reduces production costs. The entire system employs full negative-pressure absorption. It ensures the hydrogen chloride content in the finished acid, reduces the leakage of hydrogen chloride gas, and achieves maximum clean production. 3.2.4 Process Introduction to the Recrystallization System for Crude Sulfates The recrystallization system for crude sulfates primarily employs a process of thermal dissolution followed by cold crystallization; this method enables the production of standard sulfate products while also removing impurities and surface sulfur from the crude sulfates. The mother liquor obtained after solid-liquid separation can be reused. Mother liquor with a sulfur content of 50% is either used as a diluent in the sulfonation reaction process or fed into the sulfate mother liquor concentration system for further concentration. 3.2.5 Process Introduction of the Sulfate Mother Liquor Concentration System: The sulfate mother liquor concentration system employs a single-effect evaporation concentration process, which utilizes high-vacuum evaporation for the sulfate mother liquor in order to save energy and reduce steam consumption. The concentrated solution, whose sulfuric acid content exceeds 60% after concentration, is fed into the sulfonation reaction system. After crystallization in the crude product crystallizer, solid-liquid separation is carried out; the separated liquid is sent to a mother liquor tank to be used for adding sulfuric acid, thereby reducing the amount of concentrated sulfuric acid required. After collection, the condensate enters the crude sulfate recrystallization system to be used as a solvent for the crude sulfate. The evaporation crystallization section of this treatment unit can handle hydrochloric acid waste liquid from metal products, while the combined freezing crystallization section can handle sulfuric acid waste liquid from metal products ； Its working principle enables the treatment of hydrochloric acid waste from steel products to produce ferrous sulfate and recover hydrochloric acid ； It can also be used to process acidic etching waste liquid to recover hydrochloric acid, as well as waste acids from the production of other metal products such as copper sulfate. After adjustments to the process and equipment materials, it can also be applied to the treatment of stainless steel mixed acid waste liquids, the separation and purification of sulfuric acid, phosphoric acid, and other volatile acids, as well as the purification of hydrochloric acid. 3.3 Treatment technology for dilute sulfuric acid using the 3+1+1 concentration method: The 3+1+1 (three-stage) negative-pressure concentration process for dilute sulfuric acid makes use of three-effect negative-pressure evaporation, single-effect negative-pressure evaporation, and high-vacuum low-temperature evaporation techniques in order to save energy and reduce steam consumption. (For projects with too low sulfuric acid concentrations (below 15%) or particularly large treatment volumes, four-effect concentration (35%) can be used in the preceding stage.) The boiling point of dilute sulfuric acid rises significantly during concentration; the boiling point of dilute sulfuric acid with a sulfuric acid content of 35% or less is always below 110 degrees℃ ； The finished acid, with a sulfuric acid content of 92.5%, has a boiling point as high as 300°C. A three-effect concentration unit is used to raise the concentration of dilute sulfuric acid to 60% ； The single-effect concentration unit increases the sulfuric acid concentration from 60% to 80%. The high-vacuum low-temperature distillation apparatus increased the sulfuric acid concentration from 80% to 92.5%. Meet the design requirements of the product. Main features: 1) The entire heating system ensures uniform steam heating; the material fluid evaporates in a liquid film flow pattern, which offers advantages such as high heat transfer efficiency and short heating time. It has advantages such as energy savings and reduced consumption, low steam usage, and low cooling water circulation volume. 2) The unique vacuum system design ensures the stable operation of the entire system as well as excellent processing performance. It significantly reduces steam energy consumption and eliminates the drawback of relying on high-temperature steam in traditional processes. 3) The entire evaporation process takes place under negative pressure, which not only ensures the hygiene standards required in the workshop where the material is produced but also meets environmental protection requirements. 4) The process operation mode features a fully automatic control system with a continuous feeding and discharging system. In the available engineering examples, the high-concentration sulfuric acid concentration section does not use expensive tantalum materials or silicon carbide heat exchangers; instead, cost-effective glass-lined equipment is employed to achieve concentration levels of over 90%. 3.4 Negative-pressure evaporation crystallization treatment technology for waste sulfuric acid (multi-stage solid-liquid separation). The negative-pressure evaporation crystallization unit is used to treat waste sulfuric acid (used in titanium dioxide, graphene, and stainless steel production). Based on the physical properties of sulfuric acid and metal sulfates, as well as the solubility patterns of these metal sulfates in water (sulfuric acid), this technology employs indirect steam heating and negative-pressure evaporation concentration; the gases generated during evaporation are condensed into a condensate through a condenser ； The sulfuric acid waste liquid is evaporated and concentrated to achieve the desired concentration of sulfuric acid and metal sulfate crystals for production. The concentrated solution is then cooled to cause most of the metal sulfates to crystallize out in the form of metal sulfate crystals, after which the wet metal sulfate crystals are obtained through solid-liquid separation. The centrifugate obtained after solid-liquid separation is collected and returned to the production workshop for use. (Demonstration process for treating titanium dioxide and stainless steel waste sulfuric acid): The waste sulfuric acid discharged from the production workshops first enters the original liquid cooling and crystallization system; after crystallization, solid-liquid separation is carried out to extract sulfates, and the centrifugate is sent to the evaporation system for evaporation and concentration. The concentrate, whose sulfuric acid concentration meets the design requirements after cyclic concentration, is sent to the concentrate cooling and crystallization system ； After crystallization is complete, the slurry is subjected to solid-liquid separation to extract sulfates; the centrifuged liquid is collected and returned to the production facility for reuse. (Different modes are adopted based on the properties of the spent sulfuric acid and the physical characteristics of its constituent components.) (Demonstration process for treating graphene waste sulfuric acid): The waste sulfuric acid discharged from the production workshop is sent to an evaporation system for evaporation and concentration. After cyclic concentration, when the sulfuric acid concentration reaches 60%, manganese ions are removed while the mixture is still hot; the centrifuged liquid is then further evaporated and concentrated until it reaches a supersaturated concentration of potassium ions, after which cooling and crystallization are used to remove the potassium ions. The centrifugate is further concentrated to meet the production requirements, after which it is cooled and crystallized for solid-liquid separation. The centrifuged liquid is collected and returned to the production workshop for reuse. 3.5 Hydrolytic purification and concentration treatment technology for nitrosyl sulfuric acid waste liquid. Nitrosyl sulfuric acid waste liquid refers to the waste liquid generated during the synthesis reaction using a mixture of concentrated sulfuric acid and concentrated nitric acid. Decomposes in water to sulfuric acid, nitric acid, nitrogen dioxide, and nitric oxide ； It dissolves in concentrated sulfuric acid without decomposition. It is highly corrosive and extremely toxic; contact with organic materials can cause combustion, and it reacts violently with reducing agents. Due to the special properties of nitrosyl sulfate waste liquid, safety issues can easily arise during transportation and transfer. The hydrolytic purification and concentration treatment process for nitrosyl sulfuric acid waste liquid makes use of hydrolytic purification to address the fact that this type of waste liquid decomposes in water to produce nitric acid and nitrogen dioxide. Under negative pressure, 50% of the weight of pure water (or the evaporation condensate from the concentration system) is added to the reactor; the nitrosyl sulfuric acid waste solution is then added slowly. After the addition is complete, the temperature is raised to remove most of the nitrogen dioxide from the waste solution. Nitrogen dioxide enters the neutralization tower and purification tower, where it is neutralized using a 10% sodium hydroxide solution. Purification. The dilute sulfuric acid after hydrolytic purification enters the concentration system. The single-effect negative-pressure concentration method is based on the characteristics and laws of sulfuric acid at different concentrations, as well as its solubility in water; it employs indirect steam heating and negative-pressure evaporation for concentration, with the gases generated during evaporation being condensed in a condenser to form a condensate ； Dilute sulfuric acid is evaporated and concentrated to achieve the sulfuric acid concentration specified for production. 3.6 Single-effect high-vacuum low-temperature concentration treatment technology for hydrogen peroxide-containing waste sulfuric acid: Process description: The single-effect high-vacuum low-temperature concentration process is used to treat dilute sulfuric acid containing hydrogen peroxide. Since hydrogen peroxide is present in the sulfuric acid, at normal pressure such waste sulfuric acid can reach a boiling point of 55°C quite easily, which may lead to bumping or even explosions; therefore, conventional evaporation concentration processes are completely unsuitable for concentrating and reusing this type of dilute sulfuric acid. This treatment technology features a single-effect high-vacuum low-temperature concentration unit specifically designed for dilute sulfuric acid containing hydrogen peroxide. It utilizes single-effect high-vacuum low-temperature concentration methods for such solutions, thereby saving energy, reducing steam consumption, and minimizing the increase in boiling point during the sulfuric acid concentration process. Ensure safety during the concentration process. Because the boiling point of sulfuric acid rises significantly during concentration, the boiling point of the 40% sulfuric acid solution is 115℃ ； The boiling point of the finished acid with a concentration of 80% sulfuric acid is 200°C. The conventional evaporation method cannot complete the concentration task. Therefore, a specially designed single-effect high-vacuum low-temperature concentration device is used. Increase the sulfuric acid concentration to 80% ； Meet the design requirements for product reuse. The main features of the single-effect high-vacuum low-temperature concentration technology for treating waste sulfuric acid containing hydrogen peroxide are as follows: 1) For the concentration of dilute sulfuric acid containing hydrogen peroxide, the key aspect is the decomposition of hydrogen peroxide. This device utilizes high vacuum and low temperatures to carry out the decomposition of hydrogen peroxide, thereby reducing the risk of boiling over or even explosion, and enhancing the safety and reliability of the concentration process for dilute sulfuric acid. 2) All the heating systems of the device feature high heat transfer efficiency and short heating time, thanks to the high average distribution coefficient of steam heating and its high reuse rate. It features energy savings and reduced consumption, with low steam usage and a low volume of cooling water circulation. 3) External heating evaporator combined with forced circulation mode: This approach uses an external heating evaporator in conjunction with a forced circulation system. In terms of process layout, the evaporator and the separator are arranged at different heights. Under the combined effect of gravity and temperature differences, as well as the vacuum conditions in the system, the material inside the evaporator rises due to heating, while the relatively cooler material in the separator moves downward. This creates a strong circulation effect, which is enhanced by the forced circulation mode ； Ensure that the material circulation speed is above 2 m/s. Ensure stable operation of equipment during normal production. 4) A negative pressure process is used throughout the evaporation process, which not only ensures the hygiene standards required in the workshop where the material is produced but also meets the environmental protection requirements there; simultaneously, it **reduces the evaporation temperature**. 3.7 High-vacuum low-temperature concentration treatment technology for phosphoric acid mixed-acid waste liquid: The high-vacuum low-temperature concentration method for treating such waste liquid takes into account the properties of substances such as phosphoric acid, nitric acid, acetic acid, hydrofluoric acid, and hydrogen peroxide, as well as their solubility in water. This method utilizes indirect steam heating and high-vacuum low-temperature concentration to evaporate the volatile acidic gases; these gases are then condensed in a condenser to yield recoverable acids ； The waste liquid is concentrated to achieve the specified phosphate concentration for production. This process design features a continuous feeding and intermittent discharging operation mode. Vacuum external circulation concentration is primarily used, firstly to lower the evaporation temperature ； The second is to increase the evaporation rate ； Third is to reduce energy consumption ； Fourth, it reduces the crystallization of materials, ensuring the normal operation and service life of the concentration process. Other notable features include: A. The use of high-vacuum low-temperature concentration to treat electronic phosphoric acid mixed waste liquid, which is technically reliable and economically efficient. B. Since a high vacuum significantly lowers the evaporation temperature, it extends the equipment’s service life and reduces the costs associated with its maintenance. C. Lower energy consumption enables the treatment system to operate continuously. 4. Conclusion on the Resource Recovery and Utilization of Waste Acid Liquids: Summary of the overall approach to the treatment, resource recovery, and utilization of waste acid liquids: Resource recovery and utilization are essential; clear classification is required, standards must be established, coordinated planning is needed, hierarchical management should be implemented, key areas should be given priority, demonstration projects should serve as a model, attention should be paid to outcomes, and progress should be made steadily. 4.1 Resource utilization is imperative. The way in which waste acid solutions are utilized in our country is rather crude, and the overall utilization rate is below the levels of advanced countries around the world; this leads to resource waste as well as environmental pollution. It is therefore essential to strengthen recycling efforts, and policy adjustments must be made to improve the overall planning. Strengthening macro-control, adopting measures such as laws and regulations, establishing long-term mechanisms, and addressing weaknesses are indeed the most effective and sustainable approaches at present to ensure that the resource-based treatment and utilization of waste acid solutions is carried out in a legal, reasonable, and appropriate manner. 4.2 Clear classification and standardized criteria: Due to the wide variety of sources of waste acid solutions and the fragmented nature of the industry, manufacturers that produce such waste acids are located across the country, with varying production processes. This results in diversity and complexity in the process technologies, overall layout, and engineering configurations for waste acid treatment. In practical work, identifying the common patterns among waste acid solutions of the same type and classifying the treatment processes for such solutions is a prerequisite and foundation for promoting the resource utilization of waste acids. On this basis, optimizing, simplifying, classifying, and standardizing aspects such as the scale of construction, layout, process flow, equipment, and pipelines is essential. 4.3 Unified deployment and hierarchical management: The resource utilization of waste acid solutions requires the joint efforts of all parties; it needs a unified approach, as well as clear and standardized work requirements, principles, and objectives. It is also necessary to define the responsibilities of those involved in the treatment of waste acid solutions, so that all parties are clear about their respective duties and objectives, can work together to support one another, and their enthusiasm can be mobilized. This helps to avoid situations of multiple management bodies and concurrent decision-making, and it is an important guarantee for the rapid and effective advancement of efforts to recycle waste acid solutions. 4.4 Focus on key areas and lead with demonstrations: Given the importance of recycling waste acid solutions and the challenges involved, it is necessary to focus on key areas and carry out demonstration projects. Focusing on key areas means concentrating efforts on standardized initiatives for projects related to the treatment and recycling of bulk, general-purpose, and repetitive acid waste streams, in order to achieve results as soon as possible and lay a foundation for further expansion. 4.5 Focus on outcomes and advance the systematic treatment and resource utilization of waste acid solutions in a steady manner. This is a systematic task that should be carried out in light of actual conditions, adapted to local circumstances, step by step, progressing from simpler to more complex approaches. To carry out the treatment, recycling, and utilization of waste acid solutions, it is necessary to always aim at objectives such as safety and environmental protection, resource utilization, energy conservation and emission reduction, as well as cost reduction. It is important to think creatively and seek truth from facts, using the scientific development concept as a driving force and technological innovation as a foundation, in order to advance this work in a proactive and cautious manner. Final thought: Under market conditions, it is about **steering rather than rowing**. By making comprehensive plans for the construction of various environmental infrastructure facilities, it is possible to make them more economical, **efficient, and environmentally friendly. Environmental infrastructure should be regulated from the following three aspects: first, creating markets. Establish the appropriate role of private enterprises in the field of environmental protection, set up a reasonable fee structure for this purpose, and define property rights systems under different market-based models. Second is to regulate the market. Comprehensively formulate plans for the construction of environmental protection facilities to avoid randomness in such construction during the marketization process ； Establish rules for access and fair competition for private enterprises, in order to prevent fraud and unfair competition ； By regulating pricing, it ensures that everyone can enjoy the facility services ； Strict supervision to prevent secondary environmental pollution. Third is to support the market. Through preferential policies in areas such as taxation, land use, and electricity consumption, as well as technical and information consultation services, relevant enterprises are supported to actively participate in the market. In this way, the comprehensive benefits of environmental infrastructure, such as economic, social, and environmental benefits, can be achieved in a harmonious unity. Let’s work together to create a better life in harmony with nature. 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