Sulfuric acid production's "double conversion, double absorption" process still generates SO₂-containing tail gas (typically 500-1000mg/m³). Direct emission easily causes acid rain, so tail gas treatment is essential for the contact process.
Raw Material Pretreatment: Removing Impurities to Ensure Subsequent Process Stability
The first step in the contact process for sulfuric acid production is raw material pretreatment, whose core goal is to remove impurities from raw materials to avoid catalyst poisoning, equipment corrosion, or substandard product purity in subsequent processes. The pretreatment methods vary significantly depending on the raw materials: if sulfur is used as the raw material, solid sulfur is first sent to a melting tank and melted into liquid sulfur at a temperature of 130-150°C.
Then, mechanical impurities (such as sediment and carbon particles) are removed through a filter, and fine particles are further separated by a cyclone separator to ensure the purity of sulfur entering the next step is ≥99.9%. If pyrite (main component FeS₂) is adopted, it needs to go through crushing and screening processes first to break pyrite into uniform particles of 8-15mm. At the same time, metal impurities such as iron filings are removed by a magnetic separator to prevent iron oxides generated during subsequent roasting from adhering to the inner wall of equipment or blocking pipelines. If the raw material is smelting flue gas (such as SO₂-containing flue gas produced in the smelting process of copper, lead, and zinc), it is necessary to first perform dust removal (using an electrostatic precipitator or bag filter to remove dust particles), demisting (removing water mist through a Venturi scrubber), and heavy metal removal (such as removing mercury, arsenic, etc. using activated carbon adsorption or chelating resin exchange method) to prevent impurities in the flue gas from affecting catalyst activity. The quality of raw material pretreatment directly determines the stability of subsequent processes. For example, if the arsenic content in pyrite is too high, it will cause permanent poisoning of the subsequent vanadium catalyst. Therefore, the pretreatment link must strictly control the impurity content, usually requiring the content of harmful elements such as arsenic and selenium in raw materials to be ≤0.05%.
Sulfur Dioxide Preparation: Core Reaction Link to Generate Key Process Raw Materials
Sulfur dioxide (SO₂) is the core intermediate raw material for sulfuric acid production via the contact process. The preparation link requires selecting the corresponding process route according to the type of raw material to ensure the output and purity of SO₂ meet the needs of subsequent oxidation. When sulfur is used as the raw material, the refined liquid sulfur is sent to a sulfur burner, mixed with compressed air that has been dried (using concentrated sulfuric acid for drying to avoid moisture affecting subsequent reactions) in proportion (air excess coefficient 1.05-1.1), and undergoes a combustion reaction at a high temperature of 800-1000°C: S + O₂ → SO₂ + heat.
The conversion rate of this reaction can reach over 99.8%, and the concentration of the generated SO₂ gas is approximately 10%-12% (volume fraction). Meanwhile, the released heat can be used to generate steam for energy recovery. If pyrite is used as the raw material, the pretreated pyrite particles are sent to a fluidized bed roaster (boiling furnace), and a roasting reaction is carried out with excess air at a temperature of 650-850°C: 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂ + heat. During the roasting process, the air flow rate must be controlled by a fan to keep the pyrite particles in a boiling state, ensuring sufficient reaction. The concentration of generated SO₂ is about 7%-9%, and the by-product iron oxide (slag) can be recovered as a raw material for ironmaking. For smelting flue gas raw materials, the pretreated flue gas is sent to a desorption tower, and the low-concentration SO₂ (usually 1%-5%) in the flue gas is concentrated to 8%-10% through dilute sulfuric acid desorption or pyrolysis process, meeting the requirement of SO₂ concentration for subsequent catalytic oxidation. Regardless of the raw material used, the generated SO₂ gas must be cooled by a waste heat boiler (from 800-1000°C to 300-400°C), and heat is recovered to generate medium-pressure steam, which not only reduces the heat resistance requirements of subsequent equipment but also realizes the recycling of energy.
Catalytic Oxidation of Sulfur Dioxide: Core of the Contact Process to Realize Conversion from SO₂ to SO₃
The catalytic oxidation of sulfur dioxide is the core link in the contact process for sulfuric acid production. Its essence is to oxidize SO₂ to sulfur trioxide (SO₃) under the action of a catalyst, and the conversion rate of this reaction directly determines the output of sulfuric acid and exhaust emission indicators. Currently, vanadium catalysts (main component V₂O₅, carrier SiO₂, promoters K₂SO₄ and Na₂SO₄) are widely used in industry due to their high activity, good selectivity, and long service life (usually 3-5 years). The reaction is carried out in a converter (multi-stage adiabatic fixed-bed reactor) using the "two-stage conversion and two-stage absorption" process: during the first conversion, the cooled SO₂ gas (containing O₂) enters the first catalyst bed of the converter, and the reaction occurs at a temperature of 400-450°C: 2SO₂ + O₂ ⇌ 2SO₃ + heat. Since this reaction is exothermic, the bed temperature will rise to 550-600°C, exceeding the optimal active temperature of the catalyst. Therefore, the gas must be cooled to 400-420°C through an intermediate heat exchanger before entering the second catalyst bed for further reaction. The total conversion rate of the first conversion can reach 90%-95%.
Subsequently, the gas containing SO₃ enters the first absorption tower (using 98.3% concentrated sulfuric acid to absorb SO₃) to remove most of the SO₃, avoiding the formation of acid mist during the subsequent cooling process. The unreacted SO₂ gas (concentration about 0.5%-1%) is heated again to about 400°C through a heat exchanger and enters the third and fourth catalyst beds of the converter for the second conversion, with the conversion rate further increased to over 99.5%. This process effectively controls the reaction temperature within the catalyst activity range (400-600°C) through segmented reactions and intermediate heat exchange, while avoiding the reverse reaction when SO₃ is mixed with unreacted SO₂ and O₂. In addition, the use of catalysts requires strict control of impurity content in raw materials. Elements such as arsenic, selenium, and fluorine will adhere to the catalyst surface, block the active centers, and cause catalyst deactivation. Therefore, the catalyst activity must be regularly tested, and when the conversion rate drops below 95%, the catalyst needs to be replaced.
Sulfur Trioxide Absorption: Avoiding Acid Mist Formation and Efficiently Preparing Sulfuric Acid
The absorption of sulfur trioxide (SO₃) is a key step in converting SO₃ generated by catalytic oxidation into sulfuric acid. Its core challenge is to avoid direct contact between SO₃ and water to form acid mist (SO₃ + H₂O → H₂SO₄, this reaction is highly exothermic and easily causes sulfuric acid vapor to condense into tiny droplets that are difficult to capture). Therefore, 98.3% concentrated sulfuric acid is commonly used as the absorbent in industry. This concentration of sulfuric acid has the highest absorption efficiency for SO₃ and is not prone to acid mist formation. The absorption process is carried out in an absorption tower (usually a packed tower or bubble cap tower): the SO₃ gas (temperature about 150-200°C) after the first conversion enters from the bottom of the absorption tower and contacts countercurrently with 98.3% concentrated sulfuric acid sprayed from the top of the tower. SO₃ dissolves in the concentrated sulfuric acid to form more concentrated sulfuric acid (concentration up to over 99.5%) or fuming sulfuric acid (sulfuric acid containing free SO₃, concentration expressed as mass fraction of SO₃, usually 20%-65%).
In the absorption tower, the spray density (usually 15-25m³/(m²·h)) and gas flow rate (0.5-1.0m/s) must be controlled to ensure sufficient gas-liquid contact. At the same time, a demister (such as a fiber demister) installed in the tower is used to remove sulfuric acid droplets entrained in the gas, avoiding corrosion of subsequent equipment. If dilute sulfuric acid (such as 70% concentration for metal pickling) needs to be produced, the concentrated sulfuric acid generated by absorption can be sent to a dilution tank, and demineralized water is slowly added under stirring conditions (it is strictly forbidden to add water directly to concentrated sulfuric acid to prevent boiling). The dilution temperature is controlled not to exceed 60°C, and the concentration is monitored in real-time by an online concentration meter. After reaching the target value, it is sent to the finished product storage tank. For the production of fuming sulfuric acid, a fuming sulfuric acid generation tower must be added after the absorption tower to further contact SO₃ gas with 98.3% concentrated sulfuric acid, so that the free SO₃ content meets the design requirements. The control of operating parameters in the absorption link is crucial. For example, if the absorbent temperature is too high, the solubility of SO₃ will decrease; if the temperature is too low, the solution viscosity will increase, affecting absorption efficiency. Therefore, the absorbent temperature is usually controlled at 40-60°C through an acid cooler. At the same time, the pressure of the absorption tower must be maintained at a slight negative pressure (-50 to -100Pa) to prevent SO₃ gas leakage.
Product Refining: Adjusting Concentration and Purity According to Downstream Demands
The core of the product refining link is to adjust the concentration and remove impurities of the sulfuric acid generated in the absorption link according to the different needs of downstream industries, ensuring that the product meets the corresponding industrial standards. The first is concentration adjustment: if the downstream demand is 98% industrial concentrated sulfuric acid (used in fertilizer production, such as manufacturing diammonium phosphate), the 99.5% concentrated sulfuric acid generated by absorption needs to be sent to a concentration tower, heated by introducing low-pressure steam (120-150°C) to evaporate part of the water, reducing the concentration to 98%. If the demand is 70% dilute sulfuric acid (used in metal pickling in the iron and steel industry to remove iron oxide on the steel surface), demineralized water must be added in proportion to the dilution tank, while turning on the stirring and cooling systems to control the temperature during dilution not to exceed 60°C, preventing sulfuric acid from decomposing at high temperatures or corroding equipment.
The second is impurity removal: different application scenarios have significantly different requirements for sulfuric acid purity. For example, ordinary industrial sulfuric acid requires iron content ≤0.01% and arsenic content ≤0.005%, while battery-grade sulfuric acid (used as electrolyte for lead-acid batteries) requires heavy metal (lead, mercury, cadmium) content ≤0.1ppm and chloride ion content ≤0.5ppm. For ordinary industrial sulfuric acid, filtration is usually used to remove mechanical impurities (such as filtration through polypropylene filter membranes), and hydrogen peroxide (H₂O₂) is added to oxidize and remove reductive impurities such as sulfurous acid (H₂SO₃). For battery-grade sulfuric acid, a deep refining process is required: first, activated carbon adsorption is used to remove organic impurities, then ion exchange resins (such as cation exchange resins to remove heavy metal ions, anion exchange resins to remove chloride ions and nitrate ions) are used for deep impurity removal, and finally, vacuum distillation is used to further improve purity, ensuring that the impurity content meets the battery-grade standards. In addition, quality testing must be carried out in the product refining link, including concentration testing (using densitometer method or titration method) and impurity content testing (using atomic absorption spectrometry or ion chromatography). After passing the test, the sulfuric acid must be stored in special storage tanks according to different concentration and purity levels (such as 98% concentrated sulfuric acid in carbon steel tanks, dilute sulfuric acid in FRP tanks, and battery-grade sulfuric acid in stainless steel tanks) to avoid mixed pollution of products of different grades.
Exhaust Gas Treatment: Controlling Pollutant Emissions to Meet Environmental Standards
Although the "two-stage conversion and two-stage absorption" process is adopted, a small amount of exhaust gas containing SO₂ (usually SO₂ concentration 500-1000mg/m³) is still generated during sulfuric acid production. Direct emission will cause air pollution (forming acid rain), so the exhaust gas treatment link is an indispensable environmental protection step in the contact process. Currently, there are three mainstream exhaust gas treatment technologies in industry: the first is the ammonia desulfurization process, which sends the exhaust gas to a desulfurization tower and contacts it countercurrently with ammonia water (concentration 15%-20%), resulting in reactions: SO₂ + 2NH₃·H₂O → (NH₄)₂SO₃ + H₂O, (NH₄)₂SO₃ + SO₂ + H₂O → 2NH₄HSO₃.
Then, air is introduced into the reaction solution to oxidize and generate ammonium sulfate: 2NH₄HSO₃ + O₂ → 2(NH₄)₂SO₄. Ammonium sulfate can be sold as nitrogen fertilizer to realize the resource utilization of pollutants. The SO₂ removal rate of this process can reach over 98%, and the exhaust emission concentration is ≤50mg/m³, meeting the requirements of China's "Integrated Emission Standard of Air Pollutants" (GB 16297-1996). The second is the lime milk desulfurization process, which uses lime milk (Ca(OH)₂ suspension) as the absorbent to react with SO₂ in the exhaust gas to generate calcium sulfite: SO₂ + Ca(OH)₂ → CaSO₃↓ + H₂O. Calcium sulfite is oxidized to generate gypsum (CaSO₄·2H₂O), which can be used in the production of building materials (such as gypsum boards).
This process has low cost but relatively low SO₂ removal rate (about 95%), suitable for small sulfuric acid production enterprises. The third is the activated carbon adsorption method, which passes the exhaust gas through an activated carbon adsorption tower. After SO₂ is adsorbed by activated carbon, high-concentration SO₂ gas is generated by desorption under heating conditions, which can be returned to the converter to participate in the reaction again, realizing the recycling of SO₂.
This process has no secondary pollution, but the cost of activated carbon replacement is high, suitable for enterprises with strict environmental requirements and high raw material costs. Regardless of the process adopted, the SO₂ emission concentration must be monitored in real-time through an online monitoring system after exhaust gas treatment to ensure stable compliance. At the same time, the by-products generated during the treatment process (such as ammonium sulfate and gypsum) must be disposed of in compliance to avoid secondary pollution. For example, the heavy metal content of gypsum must be tested and can be used only after meeting the building material standards.
In addition, some large-scale sulfuric acid production enterprises also adopt exhaust gas waste heat recovery technology, using the heat in the exhaust gas (temperature about 100-150°C) to heat demineralized water through a heat exchanger, generating low-pressure steam for production, further improving energy utilization efficiency and realizing the dual goals of environmental protection and energy conservation.
