Introduction to the multi-stage vacuum evaporation crystallization unit for titanium dioxide waste sulfuric acid

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Introduction to the Multi-Stage Negative Pressure Evaporation Crystallization Device for Titanium Dioxide Waste Sulfuric Acid (Qian Jun, Changzhou Tait Environment Equipment Engineering Co., Ltd., Changzhou, Jiangsu 213144). Author’s profile: Qian Jun is primarily engaged in the research and development as well as the promotion of treatment processes for the resource utilization of hazardous solid wastes such as waste acid. Author’s contact information: cztthb@vip.163.com Abstract: This paper provides a systematic overview of the processes and principles related to the multi-effect negative-pressure evaporation crystallization apparatus for treating titanium dioxide waste sulfuric acid. Drawing on years of practical experience in the recycling and disposal of waste acid, it offers a detailed description of the multi-stage negative-pressure evaporation crystallization technique for such waste acid. 1 Background 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 acidify water bodies or soil, causing harm to the ecological environment, but also result in a significant waste of resources. Waste sulfuric acid emitted by the steel industry or other sectors is characterized by high concentrations of metal ions and sulfuric acid, and it is currently classified as hazardous waste in various countries. The United States has included it in the Resource Conservation and Recovery Act, while our country has also listed it in the **List of Hazardous Wastes**. The main hazards caused by the illegal discharge of waste sulfuric acid include the 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 sulfuric acid seeps into the soil, it can cause calcification of the soil over time, disrupting its loose structure and thus affecting the growth of crops ； Hindering the growth of microorganisms in biological wastewater treatment ； Can poison 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, poisoning of organisms, and ultimately significant harm to human health. According to **environmental regulations, waste sulfuric acid is not allowed to be discharged directly. Currently, there are various methods for treating waste sulfuric acid both domestically and internationally. It is necessary to select the appropriate treatment technology based on the specific characteristics of the waste sulfuric acid in question, as well as the conditions of the enterprise itself. The commonly used methods at present mainly include the submerged combustion high-temperature crystallization method, vacuum concentration freeze crystallization method, iron filings addition method for producing metal sulfates, natural crystallization-diffusion dialysis method, and acid addition freeze crystallization method. However, the aforementioned treatment methods are tailored to the various components in waste sulfuric acid solutions, which limits the versatility of waste sulfuric acid treatment processes. Titanium dioxide is an important chemical raw material. There are two industrial production methods: the sulfuric acid method and the chlorination method. At present, China has a production capacity of over 2 million tons per year, which is achieved through the sulfuric acid method. The sulfuric acid method has advantages such as abundant raw material sources, mature technology, simple operation, and a wide range of available products. However, it also has disadvantages including long processes, complex operations, and high emissions of various types of waste, especially large amounts of waste sulfuric acid. Originally, the sulfuric acid process for producing 1 ton of titanium dioxide generated 8–10 tons of waste sulfuric acid with a w(H2SO4) content of 20%–25%. Currently, this amount has been reduced to around 5 tons. At the same time, 3–4 tons of ferrous sulfate and 0.2–0.3 tons of residue (titanyl sulfate) are produced as by-products; this residue also contains small amounts of sulfates of metal ions such as aluminum, manganese, calcium, and magnesium. Waste sulfuric acid generated in titanium dioxide production is currently the main source of such waste acid in China. If left untreated, it can cause severe environmental pollution. However, due to its low acidity and high impurity content, it does not meet the requirements for reuse in production processes and cannot be directly reused in these systems; failing to adopt appropriate treatment methods would result in a waste of resources. The current method for treating waste sulfuric acid from titanium dioxide production is generally evaporation concentration; the challenges lie in crystal scaling and blockages in the material flow channels, issues that are difficult to resolve. The main components of these crystalline scaling substances are salts of iron ions or calcium ions; for example, calcium sulfate is a substance that tends to form crystals and scale, and it has a low solubility. The sulfuric acid from titanium dioxide production easily precipitates during concentration, thereby blocking the channels in heat exchangers, and ferrous sulfate also exhibits significant crystallization behavior. Through evaporation and concentration, the concentrations of sulfuric acid and ferrous sulfate increase simultaneously. Since the solubility of ferrous sulfate decreases as the concentration of sulfuric acid rises, crystallization of ferrous sulfate must occur during the concentration process. Moreover, when the concentration of sulfuric acid reaches a certain level, ferrous sulfate will crystallize out as monohydrate ferrous sulfate. Currently, there are various issues with the processes used domestically and internationally to deal with waste sulfuric acid from titanium dioxide production. The main problem lies in the difficulty of determining the solubility of calcium sulfate and ferrous sulfate in sulfuric acid at specific temperatures. Coupled with the unique physical properties of these substances, it is necessary to remove them during the evaporation and concentration process; otherwise, it will affect the subsequent steps of this process. Currently, the waste sulfuric acid generated from titanium dioxide production is mainly treated using evaporation crystallization processes. In addition to the risk of clogging in the evaporators during this process, it also leads to significant losses of sulfuric acid and ferrous sulfate as resources. Therefore, it is of great significance to carry out scientific and rational treatment, disposal, and reuse of the waste sulfuric acid generated in the titanium dioxide industry, in order to reduce environmental pollution and ecological damage, promote the healthy economic development of this industry, achieve the resource utilization of waste, and foster an environment-friendly approach to its operation. For many years, the design firm has been committed to developing technologies and devices for the resourceful treatment and utilization of waste acid and wastewater. Through years of experience in engineering projects, combined with the design firm’s proprietary technologies and equipment for treating acidic wastewater, as well as new materials, processes, and technologies from home and abroad, numerous invention patents and utility model patents have been obtained for the device technologies and related specialized equipment used in the effective treatment of acidic wastewater. Over the past twenty-odd years, as the design firms have continued to improve and innovate in the treatment of acid wastewater, they have also come to realize that making full use of such wastewater to produce economically valuable products is of great practical significance, as it helps to reduce operating costs and minimize environmental pollution. The new device that uses a three-stage negative-pressure evaporation crystallization technique for treating titanium dioxide waste sulfuric acid is characterized by its ability to address the problem of evaporator clogging during the evaporation process of this waste acid, as well as the issue of significant waste of sulfuric acid and ferrous sulfate resources resulting from the evaporation crystallization process. This treatment method is a cyclic process that results in low emissions of waste materials, is environmentally friendly with minimal pollution, has low energy consumption, and is safe during operation. The recovered product has a high sulfuric acid concentration and high purity, and can be fully returned to the workshop for use directly ； It offers good economic and environmental benefits. This treatment method is simple, easy to implement, straightforward to maintain, and has low costs. Ultimately, complete recovery and treatment of the waste sulfuric acid from titanium dioxide production is achieved, enabling an environmentally friendly processing method with minimal pollution. Using this process to treat waste sulfuric acid is more suitable for dealing with such waste in the titanium dioxide production industry compared to other treatment methods; it offers advantages such as low investment costs, low operating expenses, environmental sustainability, energy efficiency, and high value in terms of the recyclable materials obtained. 2 Process description and characteristics of the equipment
2.1 Design reference materials
HG/T 6115-2022 “Methods for treatment and disposal of waste sulfuric acid generated during the production of titanium dioxide”
GB/T 36380-2025 “Specifications for treatment and disposal of industrial waste sulfuric acid”
GB/T 50815-2013 “Technical specifications for vacuum concentration treatment of dilute sulfuric acid”
GB/T 534-2024 “Industrial sulfuric acid”
GB 4920-1985 “Determination of sulfuric acid mist in exhaust gas from sulfuric acid concentration – Barium chromate colorimetric method”
T/CSAIA 005—2021 “Technical specifications for pollution control in the utilization and disposal of organic waste sulfuric acid”
GB/T 40124-2021 “Technical requirements and test methods for regenerated sulfuric acid”
HG/T 5026-2016 “Recovery of sulfuric acid in the chlor-alkali industry”
HJ 1335—2023 “Technical specifications for pollution control in the utilization and disposal of waste sulfuric acid”
GB 10531－2006 “Ferrous sulfate as a water treatment agent”
GB/T 664-2011 “Ferrous sulfate”
GB 10531－1989 “Industrial ferrous sulfate”
“Chemical Engineering Handbook – Sulfuric Acid Volume”
“Hazardous Chemicals – Sulfuric Acid Volume”
“Research on control standards for pollutants in the sulfuric acid industry”
“Standards for sulfuric acid used in various applications”
“Practical handbook on new processes, technologies, quality inspection standards, and analysis methods for sulfuric acid production, processing, and equipment installation”
“Design handbook for treatment of ‘three wastes’”
“Handbook of physical and chemical properties of chemicals – Inorganic volume”
“Industrial crystallization”
“Handbook of inorganic salt industry”
“Handbook of salt-making industry”
“Phase diagrams of water-salt systems and their applications”
“Principles of Chemical Engineering”
“Corrosion data and material selection handbook”

2.1.1 Design reference data:
1) Solubility of ferrous sulfate in water (g/100 g water): (Revised edition of “Handbook of Physical and Chemical Properties of Chemicals – Inorganic Volume”)
Temperature: °C  0 10 20 30 40 50 60
Ferrous sulfate  15.8 20.8 26.3 32.8 40.1 48.4 52.4
Temperature: °C  70 80 90 100
Ferrous sulfate  50.9 43.7 37.3 — — — —

2) Solubility of ferrous sulfate in water (g/100 g solution): (Sulfuric Acid Workbook)
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3) Solubility of ferrous sulfate heptahydrate in water (g/100 g water): (Derived from relevant textbooks and literature)
Temperature: °C  0 10 20 30 40 50 60
Ferrous sulfate heptahydrate  40 48 60 73.3 88.6 100.7 —

4) Solubility of ferrous sulfate in sulfuric acid: (Sulfuric Acid Workbook)
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5) Ferrous sulfate, sulfuric acid, and water:
Ferrous sulfate, sulfuric acid, and water
No.  827 Saturated solution, wt%
Temperature: °C  Solid state  Ferrous sulfate  Sulfuric acid  Water
11.4  82  6.4  62.1  280 FeSO4·H2O 8.75 30.6 66 0.598 0
5.35 37.75 56.98 0 2.73 44.83 52.44 0 1.13 51.57 48.38 0 0.65 56.43 58 0 18.7 2.97 78.33 100 17.98 2.79 76.23 100 17.62 8.24 74.14 100 17.28 11.04 71.68 100 16.78 13.28 69.94 100 15.98 16.08 67.94 100 14.81 19.16 66.11 100 14 21.17 64.83 100 9.95 28.38 61.67 100 6.63 5.94 57.46 100 3.83 42.19 53.98 100 2.99 44.97 52.04 100 1.62 50 47.53 100 0.92 56.31 42.77 100

6) Solubility of ferrous sulfate heptahydrate in sulfuric acid solutions of different concentrations (g/100 g sulfuric acid solution): (Provided by Tate Laboratory)
Sulfuric acid content: %  25 30 35 40 45 50 55
Ferrous sulfate heptahydrate  26 23 17 9 7 5 4
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7) Reference data on solubility of ferrous sulfate in sulfuric acid solutions: (Derived from research project data, institutional research findings, and related literature)
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8) Working principle of concentrating waste sulfuric acid containing iron (from titanium dioxide production, stainless steel manufacturing, and smelting):
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As can be seen from Figure 1: Based on the solubility curve of ferrous sulfate in sulfuric acid solutions within the FeSO4–H2SO4–H2O ternary system, at a constant temperature, the solubility of ferrous sulfate decreases as the sulfuric acid concentration increases ； When the sulfuric acid concentration is constant, its solubility increases rapidly as the temperature rises. File:///C:/Users/M17X/AppData/Local/Temp/ksohtml16704/wps6.jpg As can be seen from Figure 2, when the concentration of H2SO4 in the solution remains constant, the solubility of FeSO4 reaches a maximum value as the temperature increases; moreover, the temperature at which this maximum value occurs decreases as the amount of FeSO4 in the solution decreases. This is because in lower temperature ranges, the crystals of FeSO4 exist in the form of ferrous sulfate heptahydrate, and the solubility of ferrous sulfate heptahydrate increases as the temperature rises. In the higher temperature range, the crystals of ferrous sulfate appear in the form of ferrous sulfate monohydrate, and the solubility of ferrous sulfate monohydrate generally decreases as the temperature rises. Since the crystal particles of ferrous sulfate heptahydrate are large and easy to separate, after dilute sulfuric acid is concentrated to a certain degree, freezing crystallization is used to remove FeSO4, causing the majority of it to crystallize out in the form of ferrous sulfate heptahydrate (FeSO4·7H2O). This is the theoretical basis for concentrating dilute sulfuric acid and removing iron through freezing crystallization. 2.2 Process description and characteristics of the device 2.2.1 Explanation of process selection: a. Design scheme selection: The three-stage negative-pressure evaporation crystallization device, which was specifically designed taking into account the characteristics of the sulfuric acid derived from titanium dioxide production, is based on the physical properties of sulfuric acid as well as the solubility rules of ferrous sulfate in water (sulfuric acid). It also takes into account the increase in boiling point during the concentration of sulfuric acid. For this type of sulfuric acid, a treatment approach that combines a raw liquid freezing crystallization system, a three-effect negative-pressure evaporation crystallization system, and a two-effect negative-pressure evaporation crystallization system is employed, thereby creating a complete set of processing procedures. Negative pressure evaporation is primarily used to save energy, reduce steam consumption, and lower the boiling point elevation during the evaporation and concentration of sulfuric acid. Because the boiling point of sulfuric acid rises significantly during concentration, the boiling point of the 20% sulfuric acid solution is approximately 102℃ ； The boiling point of the recycled sulfuric acid with a concentration of 55% is 132°C (since the waste sulfuric acid contains high levels of metal sulfates, the boiling point of the solution can reach up to 155°C during evaporation and concentration). The conventional evaporation method cannot complete the concentration task. Considering the need to remove ferrous sulfate from the sulfuric acid waste resulting from titanium dioxide production to the greatest extent possible ； Therefore, the original solution was subjected to cold crystallization using a cold freezing crystallization system; after solid-liquid separation, the centrifugate obtained was processed through a three-effect negative-pressure evaporation crystallization system to increase the sulfuric acid concentration to 35%, followed by cold crystallization ； The centrifugate obtained after solid-liquid separation is subjected to a two-effect negative-pressure evaporation crystallization system to increase the sulfuric acid concentration to 55%, after which it is frozen and crystallized ； The filtrate generated after solid-liquid separation is discharged to the recovered sulfuric acid collection tank. b. Main characteristics of the production process: 1) All heating systems in the equipment feature high heat transfer efficiency and short heating times, owing 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. 2) Negative pressure evaporation concentration: The evaporation temperature of titanium dioxide waste sulfuric acid at normal pressure is relatively high, and it is highly corrosive; as a result, equipment maintenance is required frequently and its lifespan is short. This is the main reason for the high operating costs associated with the treatment of titanium dioxide waste sulfuric acid, as well as the high costs of daily equipment maintenance. This device employs a fully negative-pressure concentration process for the waste sulfuric acid from titanium dioxide production: under negative pressure, the evaporation temperature is low, which reduces corrosion of the equipment and pipeline materials, thereby ensuring continuous and stable operation. Due to the lower operating temperature, there are many favorable conditions and wide possibilities in terms of material selection for equipment and pipelines, which can reduce project costs. Since a negative-pressure process is employed throughout the entire treatment process, there is virtually no leakage of acidic gases, resulting in a significant improvement in both the operating environment and the factory environment. It ensures the hygiene requirements of the processing workshop, as well as its environmental protection requirements, while also reducing the demands on utility systems. 3) External heating evaporator combined with forced circulation mode: The waste sulfuric acid from titanium dioxide production tends to crystallize and form scale after being evaporated and concentrated to a certain extent; this can even block the material flow channels in the evaporator, leading to equipment damage. An externally heated evaporator is used in combination with a forced circulation mode. In terms of the 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 titanium dioxide waste sulfuric acid in the evaporator rises due to heating, while the relatively cooler titanium dioxide waste sulfuric acid in the separator descends, resulting in a strong circulation. This is further enhanced by the forced circulation mode ； Ensure that the material circulation speed is above 2 m/s. Under such high-speed and intense movement, the waste sulfuric acid from titanium dioxide production essentially eliminates the possibility of crystallization within the evaporator and blockage of the same, thereby ensuring stable operation of the equipment during normal production. 4) Simple process and low equipment investment: The process used in this device requires few pieces of equipment, resulting in low investment, and its operation is simple and easy. This device boasts numerous 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. 5) Obvious environmental benefits: In the process employed by this unit, considering the characteristics of waste sulfuric acid from titanium dioxide production, a non-condensable gas condenser is installed in the evaporation unit. This not only minimizes corrosion to the vacuum system but also purifies the non-condensable gases ； A recovery condenser has been installed in the crystallization and separation unit to collect, condense, and recover any acidic gases that may be generated from the crystallization kettle, centrifuge, filter press, centrifugal liquid tank, filtrate tank, etc. The entire system does not require an additional exhaust gas treatment tower, ensuring that the environment in the production workshop fully meets relevant environmental protection requirements and standards. It also maximally protects the health and occupational safety of the staff. c. Selection of evaporation mode: Evaporation (or concentration) refers to the process of evaporating and concentrating a solution that consists of non-volatile solutes and volatile solvents; it is achieved primarily by using heat to vaporize a portion of the solvent in the solution. Evaporation equipment is generally referred to as evaporators; they come in a wide variety of designs and types, and have a long history of development. Based on the operating mode, it can be classified into single-effect evaporation, multi-effect evaporation, and direct-contact evaporation ； Based on the fluid circulation method, it can be classified into non-circulating evaporation, natural circulation evaporation, forced circulation evaporation, wiped film evaporation, and centrifugal thin-film evaporation. During design, different evaporators are used for different materials. Proper application not only improves product quality but also helps save energy and reduce consumption, lowering production costs and enhancing economic efficiency. d. Selection of the number of evaporation stages: In multi-effect evaporation, the steam from one stage is used as the heating steam for the subsequent stage, thereby allowing multi-effect evaporation to reduce the consumption of raw steam. However, it’s not the case that the more effects, the better; the number of effects is limited by technical and economic constraints. 1) As the number of effect stages in multi-effect evaporation increases, the amount of steam required remains the same while the total evaporation volume decreases, resulting in lower operating costs. However, the more efficiency factors there are, the higher the cost of the equipment, and as the number of efficiency factors increases, the amount of steam that can be saved decreases. 2) Theoretically, if there are too many effective numbers, the evaporation process will be difficult to carry out. Generally, both the temperature of the heating steam in the first stage of multi-effect evaporation and the operating temperature of the condenser are limited; likewise, the theoretical total temperature difference for heat transfer in multi-effect evaporation (i.e., the difference between these two temperatures) is also restricted. Under specific operating conditions, as the number of effects increases, the sum of the temperature difference losses across each effect also increases, thereby reducing the total effective temperature difference. When there are too many effects, the effective total temperature difference is very small; the effective total temperature difference allocated to each effect becomes so small that it is not sufficient to ensure normal boiling in those effects, making the evaporation process difficult to carry out. The choice of the number of effectives mainly depends on the properties of the material to be evaporated. Long-term experience clearly shows that for ordinary electrolyte solutions, the boiling point elevation is relatively rapid, and 1–3 effects can be adopted ； For ordinary non-electrolyte solutions, the boiling point elevation is slow, so 3 to 6 effects can be used. Based on the characteristics of the sulfuric acid generated from titanium dioxide production, the physical properties of sulfuric acid, as well as the requirements of the client regarding the recovery of this acid, and through software simulations, experimental data analysis, and practical production experience from similar projects, a three-stage negative-pressure evaporation crystallization system was adopted to treat the sulfuric acid waste from titanium dioxide production. 2.2.2 Working Principle: A. Original liquid-cooled crystallization system: This system primarily uses a vacuum jacketed mixer for crystallization; the crystallization process takes place within the crystallization tank, with solid-phase crystallization occurring as a result of the cooling effect provided by the circulating coolant in the jacket of the tank ； The slurry is subjected to solid-liquid separation using a centrifuge; the resulting ferrous sulfate crystals are manually weighed, packaged, and stored in inventory ； The separated centrifugate enters a three-effect negative-pressure evaporation crystallization system or a temporary storage intermediate tank. This system is essentially a physical process for separating solutes from solvents in a solution. By cold crystallization, the solubility of ferrous sulfate is reduced, causing most of it to crystallize out in the form of ferrous sulfate heptahydrate, thereby achieving the separation of the solute from the solvent in the solution. B. Three-effect negative-pressure evaporation crystallization system: This system primarily utilizes three-effect negative-pressure evaporation in combination with forced circulation evaporation, thereby saving energy, reducing steam consumption, lowering the boiling point increase during the concentration of waste sulfuric acid from titanium dioxide production, and reducing treatment costs. Three-effect negative pressure evaporation involves leaving sufficient space in each separator for gas-liquid separation, with steam entering the next evaporator directly from the top of the separator. Since the cross-sectional area of the secondary steam pipeline in the separator is more than twice that of ordinary steam pipelines, there is no need for any bending as the steam flows to the next stage of evaporation; the distance is short, which greatly reduces steam resistance, increases flow rate, and improves heating efficiency. Furthermore, since the gas-liquid separation is carried out within the separator, heat loss from the extracted steam is reduced. The condensate water from the heating steam of the first-effect evaporator is sent to the hot water preheater through a steam trap; the condensate water then exits the hot water preheater, which prevents steam loss and eliminates the noise and pollution associated with steam traps. The heat from the evaporation condensate of each effect is utilized to preheat the titanium dioxide waste sulfuric acid solution multiple times. The concentrate that has reached the concentration required by the design enters the crystallization tank. Solid-phase crystallization occurs through circulating cooling with a refrigerant in the jacket of the crystallization tank ； The slurry is subjected to solid-liquid separation using a centrifuge; the resulting ferrous sulfate crystals are manually weighed, packaged, and stored in inventory ； The centrifugate obtained after solid-liquid separation enters a two-effect negative-pressure evaporation crystallization system for further evaporation and concentration. The three-effect negative-pressure evaporation process is an evaporation operation that consists of three sets of evaporators and separators combined together. In three-effect vacuum evaporation, it is required that both the operating pressure and the boiling point of the solution in the subsequent effects be lower than those in the preceding effect. The secondary steam from the preceding effect is used as the heating medium for the subsequent effects; thus, the evaporator in each subsequent effect acts as a condenser for the secondary steam from the preceding effect. As a result, only the first effect requires the use of raw steam. Final evaporation operates under vacuum, reducing the boiling point of the solution. Since the secondary steam from the previous stage is used as the heating steam for the next stage, the utilization rate of raw steam is improved, thereby enhancing economic efficiency. The entire system of this device makes full use of moisture and latent heat, reducing the consumption of raw steam and lowering operating costs. The total steam consumption of the evaporation system in this device is 1/3 that of conventional evaporation equipment, and its total operating power is also 1/3 of that of traditional evaporation equipment. The design concept of the entire system is environmentally friendly, scientific, as well as efficient and energy-saving ； It complies with the provisions of the **formulated Energy Conservation Law**. This system 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 sulfuric acid and ferrous sulfate under vacuum, causing the volatile substances and water in the solution to evaporate together; these substances are then condensed using cooling water in a condenser to form a condensed liquid ； As the volume of the solution decreases, the concentration of the non-volatile solutes in the solution, namely sulfuric acid and ferrous sulfate, increases, resulting in a sulfuric acid concentration that meets the design requirements. Then, through cold crystallization, the solubility of ferrous sulfate is reduced, causing most of it to crystallize out in the form of ferrous sulfate heptahydrate, thereby achieving the separation of the solute from the solvent in the solution. C. Two-effect negative-pressure evaporation crystallization system: This system primarily uses two-effect negative-pressure evaporation to enhance forced circulation evaporation. Two-effect negative pressure evaporation involves leaving sufficient space in each separator for gas-liquid separation, with steam entering the next evaporator directly from the top of the separator. Since the cross-sectional area of the exhaust pipeline from the separator is more than twice that of ordinary steam pipelines, there is no need for any bends when the steam moves to the next stage of evaporation; the distance is short, which significantly reduces steam resistance, increases flow rate, and improves heating efficiency ； Furthermore, since the gas-liquid separation is carried out within the separator, heat loss from the extracted steam is reduced. The condensate water from the heating steam of the first-effect evaporator is sent to the hot water preheater through a steam trap; the condensate water then exits the hot water preheater, which prevents steam loss and eliminates the noise and pollution associated with steam traps. The heat from the condensate of each effect evaporator is utilized to preheat the raw liquid multiple times. The concentrate that has reached the concentration required by the design enters the crystallization tank. Solid-phase crystallization occurs through circulating cooling with a refrigerant in the jacket of the crystallization tank ； The slurry is subjected to solid-liquid separation using a filter press; the resulting ferrous sulfate crystals are manually weighed, packaged, and stored ； The filtrate after solid-liquid separation is discharged to the sulfuric acid recovery tank. The two-effect evaporation process is an evaporation operation that consists of two sets of evaporators combined with a separator. In two-effect evaporation, it is required that both the operating pressure and the boiling point of the solution in the subsequent effect be lower than those in the preceding effect. The secondary steam from the previous effect is used as the heating medium for the subsequent effect; thus, the evaporator in the subsequent effect acts as a condenser for the secondary steam from the previous effect. Only the first effect requires the use of raw steam. Final evaporation operates under vacuum, reducing the boiling point of the solution. Since the secondary steam from the previous stage is used as the heating steam for the subsequent stage, the utilization rate of raw steam is increased, thereby improving economic efficiency. The two-effect negative-pressure evaporation system of this device makes full use of moist and latent heat, reducing the consumption of raw steam and thereby lowering operating costs. The total steam consumption of the two-effect negative-pressure evaporation crystallization system in this device is half that of conventional evaporation equipment, and its total operating power is also half that of traditional evaporation equipment. The design philosophy of the entire system is environmentally friendly, scientific, as well as efficient and energy-saving ； It complies with the provisions of the **formulated Energy Conservation Law**. This system is essentially a physical process for separating solutes from solvents in a solution. Its basic principle is to heat a solution containing solutes such as sulfuric acid and ferrous sulfate under vacuum to cause the water to evaporate, and then use cooling water in a condenser to convert it into a condensed liquid ； As the volume of the solution decreases, the concentration of the non-volatile solutes in the solution, namely sulfuric acid and ferrous sulfate, increases, resulting in a sulfuric acid concentration that meets the design requirements. Then, through cold crystallization, the solubility of ferrous sulfate is reduced, causing most of the ferrous sulfate to crystallize out as crystals containing crystal water, thereby achieving the separation of the solute from the solvent in the solution. 2.2.3 Brief introduction to the process flow: The three-stage negative-pressure evaporation crystallization method for treating titanium dioxide waste sulfuric acid takes into account the compositional characteristics of such waste sulfuric acid, the properties and boiling points of sulfuric acids at different concentrations, as well as their solubility patterns. It also considers the solubility and dissolution properties of ferrous sulfate in water (sulfuric acid). This method employs indirect steam heating and negative-pressure evaporation concentration; the gases generated during evaporation are condensed in a condenser to form a condensed liquid ； The waste sulfuric acid from titanium dioxide production is concentrated through multi-stage vacuum evaporation to achieve a concentration that meets the requirements for reuse in production. Taking advantage of the solubility characteristics of ferrous sulfate in sulfuric acid at different concentrations, the concentrate is cooled and crystallized using a refrigerant or cooling water. After most of the ferrous sulfate crystals precipitate, they are removed through solid-liquid separation, thereby maintaining a high level of purity in the recovered sulfuric acid. The three-stage negative-pressure evaporation crystallization system for treating titanium dioxide waste sulfuric acid consists of dozens of chemical equipment units, including a mother liquor freezing crystallization system, a three-effect negative-pressure evaporation crystallization system, and a two-effect negative-pressure evaporation crystallization system – these three independent chemical operation units together form a complete set of processes. The evaporation unit, which consists of an evaporator and a separator, employs negative pressure evaporation to achieve forced external circulation evaporation, offering high evaporation intensity and thermal efficiency ； The condensation system, which consists of a condenser and a condensate tank, uses a special type of condenser along with cooling water circulation for cooling. Crystallization is carried out entirely within the crystallization tank; after crystallization, the slurry is fed into a centrifuge or filter press to separate the solid and liquid phases, thereby extracting ferrous sulfate. The centrifugate or filtrate produced after solid-liquid separation is collected and fed into the next system for further concentration. The recovered sulfuric acid that has reached the concentration required for actual production is discharged into the recovered sulfuric acid collection tank. 3 Introduction to the main equipment The main factors considered when selecting the main equipment include: 1) the properties of the waste sulfuric acid from titanium dioxide production and the physical characteristics of its components in different directions ； ； 2) Engineering and technical requirements, such as processing capacity, amount of water evaporated, quantity and quality of condensate, quantity and quality of recovered sulfuric acid, area and height of the installation site, and whether production is continuous or intermittent ； 3) Use of heat sources and cooling media ； 4) The viscosity of the material as it changes due to variations in temperature, concentration, pressure, etc., during the evaporation process. In order to improve the evaporation capacity, processing capacity, and service life of the project for treating waste sulfuric acid from titanium dioxide production, and taking into account the actual investment required for the project, its daily operation, as well as the characteristics of the waste sulfuric acid, the design team selected specialized evaporators, separators, preheaters, condensers, and other equipment made of non-metallic materials such as graphite, which were developed specifically for waste acid wastewater treatment, along with related accessories. The core equipment of the three-stage negative-pressure evaporation crystallization system for treating waste sulfuric acid includes evaporators, separators, condensers, etc. Equipment made of non-metallic materials such as graphite exhibits excellent corrosion resistance against acidic media, as well as good thermal conductivity. Years of operational experience have shown that equipment constructed from non-metallic materials like graphite for use in acidic environments offers reliable performance, a long service life, and easy operation and maintenance, making it the only ideal choice. The evaporator and separator are the most critical devices in the waste sulfuric acid evaporation and crystallization system; they are products that have been carefully designed taking into account the characteristics of external circulation evaporation and concentration processes. The design team, through years of research and development and improvement, has created specialized evaporators and separators that have achieved satisfactory results in practical applications across various projects over the years. Given the high calcium ion content, complex composition in the waste sulfuric acid from titanium dioxide production, and the presence of calcium and magnesium ions that tend to form crystals and scale, a sedimentation tank is installed in each stage of the two-stage evaporation system of this unit. The upper clear liquid maintains the proper operation of the entire system, while the slurry formed at the bottom enters the crystallization vessel. Minimize crystallization and scaling in the evaporator to extend the cleaning cycle of the evaporation system. Since the separator contains boiling waste sulfuric acid, which is highly heated and corrosive, a specially designed corrosion-resistant separator is required. All separators in this project are special glass-lined separators designed for the treatment of waste acid and wastewater, which are patented products. The separator incorporates the advantages of the Levene-type separator; its specially designed length-to-diameter ratio increases surface flashing, allowing the problem of secondary steam entrainment to be resolved without the need for a demister. All separators are equipped with independent feed inlets, observation ports, and level control systems, which facilitate the monitoring and control of feed flow rates as well as the actual conditions during the concentration process. All the evaporators, preheaters, and condensers in this unit are equipped with specially designed high-temperature impregnated graphite heat exchangers, while the corrosion protection for the shell side is achieved through special treatment using steel lined with ECTFE. To prevent issues such as crystallization and scaling, as well as oxidation and hydrolysis, which can cause blockages in the material flow channels of the evaporator during the evaporation and concentration process, all evaporators used in this project are graphite evaporators designed specifically for the treatment of waste acid and wastewater – patented products. Steam distributors are installed on these evaporators to minimize the formation of overheated areas, ensuring uniform heating of the material during concentration and thus avoiding problems like crystallization, scaling, oxidation, and hydrolysis. All preheaters and coolers in this project utilize patented multi-channel, multi-process graphite heat exchangers designed specifically for the treatment of waste acid and wastewater. These exchangers make full use of the heat from the condensate, thereby minimizing the space required. All condensers in this project utilize specially designed block-hole graphite heat exchangers, equipped with gas-liquid separators at the bottom to ensure optimal condensation and gas-liquid separation performance. Since the gas-liquid separator and the condenser are integrated devices, the acidic gases evaporated from the condensate tank basically do not enter the vacuum system. This not only ensures the proper operation and longer service life of the vacuum system to the greatest extent possible, but also significantly improves the production environment. Thanks to the improvements made by the project design team to the equipment, as well as the excellent design of the entire system, it is one of the few companies in China that can utilize high-performing and stable water ring pumps and Roots water ring vacuum units for acid waste treatment projects. The crystallization process takes place entirely within the crystallization tank, where metal sulfate crystals are formed through cooling by the refrigerant or cooling water circulating in the jacket of the tank. After crystallization is complete, solid-liquid separation is carried out. During the separation process, a centrifuge or filter press is used to separate the slurry in its solid-liquid mixture state, thereby extracting and isolating the solid-phase metal sulfate crystals from the liquid slurry. Due to the high corrosivity of spent sulfuric acid, ordinary metal materials are not suitable for meeting production requirements; therefore, glass-lined reactors, fully automatic centrifuges with scrapers for bottom discharge, acid-resistant programmable diaphragm filter presses, and related supporting equipment were selected in accordance with the process requirements. For feed pumps, circulation pumps, discharge pumps, centrifugal liquid pumps, filtrate pumps, filter press pumps, etc., specially designed clamp-type chemical process centrifugal pumps or low-speed mixed-flow pumps are used; all the flow-through components of these pumps are made of (modified) ultra-high molecular weight polyethylene or F46 material. Equipped with double-end mechanical seals. It ensures a high level of safe system operation and long equipment maintenance cycles. Ensures the stable operation of the device and a long service life. For the evaporation vacuum pumps in both the three-effect negative pressure evaporation crystallization system and the two-effect negative pressure evaporation crystallization system, 2SK series two-stage water ring vacuum pumps made of S30408 stainless steel and equipped with mechanical seals are used ； The water ring pump is equipped with a built-in circulation tank and an additional cooler, which ensure stable vacuum levels in the system and minimize the need for maintenance on the equipment; this also reduces the impact on the production workshop environment to the greatest extent possible. For both the crystallization vacuum units in the three-effect negative-pressure evaporation crystallization system and those in the two-effect negative-pressure evaporation crystallization system, RPP series water-jet vacuum units are used. The use of polypropylene material ensures the corrosion resistance of these devices, and the built-in coolers enable effective control of the water temperature in the tank, thus maintaining a stable vacuum level and minimizing the need for maintenance on the equipment. Meanwhile, minimize the frequency of water changes as much as possible. Glass-lined storage tanks are used for all condensate tanks ； Polypropylene storage tanks are used for the centrifugate tank, filtrate tank, recovered liquid tank, and vapor-liquid separation tank. It not only meets the corrosion resistance requirements of actual production, but is also more practical in terms of vacuum resistance and temperature resistance, while being relatively cost-effective. 4 Proprietary intellectual property rights and patented technologies involved
4.1 Invention patents:
ZL202010484404.8 “Two-stage negative-pressure evaporation, concentration, purification process and device for food-grade waste phosphoric acid”
ZL202010485113.0 “Process and device for treating fluorine-containing waste hydrochloric acid”
ZL202010485194.4 “Three-stage negative-pressure evaporation and crystallization process and device for waste sulfuric acid generated in titanium dioxide production”
ZL201310351303.3 “Highly efficient and energy-saving triple-effect negative-pressure graphite evaporation and crystallization device for waste hydrochloric acid from pickling processes”
ZL201410102740.6 “Multi-pass block-type graphite heat exchanger”
ZL201721176680.8 “Device for recovering hydrochloric acid from acidic etching waste liquid and producing copper sulfate”
ZL201810863356.6 “Triple-effect negative-pressure countercurrent flash crystallization device for treating waste hydrochloric acid from pickling processes”
ZL201810863360.3 “Resource recovery device for waste hydrochloric acid from pickling processes using sulfonation method”
4.2 Utility model patents:
ZL202020969691.7 “Three-stage negative-pressure evaporation and crystallization device for waste sulfuric acid generated in titanium dioxide production”
ZL202020968522.1 “Treatment device for fluorine-containing waste hydrochloric acid”
ZL202020969695.5 “Two-stage negative-pressure evaporation, concentration and purification device for food-grade waste phosphoric acid”
ZL201821408180.7 “Specially designed chloride sulfonation reactor”
ZL201821227836.5 “Resource recovery device for waste hydrochloric acid from pickling processes using sulfonation method”
ZL201821227312.6 “Triple-effect negative-pressure countercurrent flash crystallization device for treating waste hydrochloric acid from pickling processes”
ZL201620446413.7 “Recovery and treatment device for waste acid from photoelectric material pickling processes”
ZL201620447833.7 “Concentration device for dilute sulfuric acid waste liquid containing peroxides and hydrogen peroxide”
ZL201620450057.6 “Recovery and treatment device for electronic-grade phosphoric acid from electronic waste acid liquid”
ZL20170826734.9 “Device for recovering hydrochloric acid from acidic etching waste liquid and producing copper sulfate”
ZL201620450271.1 “Triple-effect evaporation and crystallization device for high-salinity wastewater requiring no cooling water”
ZL201620450272.6 “3+1+1 negative-pressure evaporation and concentration device for dilute sulfuric acid”
ZL201320493878.4 “Highly efficient and energy-saving triple-effect negative-pressure graphite evaporation and crystallization device for waste hydrochloric acid from pickling processes”
ZL201120167001.7 “Recovery and treatment device for pickling waste liquid”
ZL201220341703.7 “Recovery and treatment device for pickling waste liquid”
ZL201220292311.6 “Special concentration and separation device for pickling waste liquid”
ZL201220606723.2 “Equipment for recovering iron salts from gold ore residues”
ZL201120166991.2 “Extraction equipment for tetramethylammonium hydroxide waste liquid”
ZL201420126530.6 “Multi-pass block-type graphite heat exchanger”
ZL201420125062.0 “Graphite evaporator used for evaporation and concentration”
ZL201420126500.5 “Graphite separator specifically designed for evaporation and concentration”
ZL201320366725.3 “Steel-lined graphite elbow”
ZL201220292308.4 “Specially designed block-type graphite condenser”
ZL201220291634.3 “Graphite blocks used in block-type heat exchangers”
ZL2012202923121.0 “Block-type graphite condenser”
ZL201120167006.X “Steel-lined graphite evaporator”
ZL201120166991.2 “Block-type heater”
4.3 Relevant standards drafted and revised with participation of the design unit:
GB/T 32125-2021 “Specifications for treatment and disposal of industrial waste hydrochloric acid”
GB/T 36380-2025 “Specifications for treatment and disposal of industrial waste sulfuric acid”
GB/T 37387-2019 “Specifications for treatment and disposal of industrial waste phosphoric acid”
GB/T 38102-2019 “Methods for treatment and disposal of fluosilicic acid waste liquid generated during wet-process phosphoric acid and phosphate fertilizer production”
GB/T 1621 “Industrial ferric chloride”
HJ 1335—2023 “Technical specifications for pollution control in utilization and disposal of waste sulfuric acid”
HG/T 5363-2018 “Methods for treatment and disposal of phosphorus-containing waste liquid”
HG/T 5811-2020 “Method for determining iron content in iron-containing chemical sludge”
HG/T 3251-2018 “Industrial crystalline aluminum chloride”
HG/T 5547-2019 “Technical specifications for mobile chemical cleaning waste liquid treatment devices”
HG/T 5967-2021 “Methods for treatment and disposal of waste hydrochloric acid generated in hot-dip galvanizing processes”
HG/T 6111-2022 “Methods for treatment and disposal of waste acid from steel wire rope pickling processes”
HG/T 6112-2022 “Methods for treatment and disposal of waste acid from carbon steel pickling processes”
HG/T 6115-2022 “Methods for treatment and disposal of waste sulfuric acid generated during titanium dioxide (titanium dioxide) production”
HG/T 6265-2024 “Methods for treatment and disposal of chromium-containing pickling waste liquid”
T/CSAIA 003—2021 “Technical requirements for preparation of water treatment agents from waste acid – Part 1: Polyferric sulfate”
T/CSAIA 005—2021 “Technical specifications for pollution control in utilization and disposal of organic waste sulfuric acid”
T/CPCIF 0285-2023 “Evaluation specifications for suppliers of evaporation and crystallization equipment in the petroleum and chemical industries”
T/CPCIF 0428-2025 “Composite aluminum salt solutions”; “Evaluation requirements for green factories in the waste acid treatment and disposal industry”; “Guidelines for environmental management of hazardous waste – Chemical waste acids”
5 Conclusion: The core aspect of the multi-stage negative-pressure evaporation and crystallization device for waste sulfuric acid generated in titanium dioxide production lies in fully understanding and analyzing the composition of substances within this waste sulfuric acid as well as the forms and mechanisms of scale formation. This analysis led to the conclusion that scale formation is not caused by crystallization of sulfate-related substances as traditionally believed ； Measures were taken to reduce or eliminate the supersaturation of crystalline scaling substances, thereby lowering the rate of sulfate scaling and the likelihood of scaling ； It resolved a complex and challenging engineering and production technology issue that has long plagued the industry in the processing of titanium dioxide waste sulfuric acid: fouling of heat exchangers and easy blockages. Secondly, the multi-stage negative-pressure evaporation crystallization device is simple in process and economical and practical for treating titanium dioxide waste sulfuric acid.