Applications of Diatomaceous Earth in Various Stages of Sulfuric Acid Production
Application as Catalyst Carrier
Core Role as Vanadium Catalyst Carrier
In sulfuric acid production, the most important application of diatomaceous earth is serving as the carrier for vanadium catalysts. The contact process is adopted for sulfuric acid manufacturing, where the core reaction is the oxidation of sulfur dioxide to sulfur trioxide under the action of vanadium catalysts. The composition system of vanadium catalysts is as follows: V₂O₅ acts as the active component (with a content of 7.5–8.5%), alkali metal sulfate (with a K₂O/V₂O₅ molar ratio of 2.5–3.0) serves as the promoter, and diatomaceous earth functions as the carrier.
Diatomaceous earth becomes an ideal carrier for vanadium catalysts mainly due to its unique microstructural advantages. Natural diatomaceous earth has a favorable specific surface area, pore volume, and pore size distribution, which can provide uniform adsorption sites for the active component. High-quality diatomaceous earth carriers can enhance the activity and thermal stability of vanadium catalysts, extending their service life by several years. In practical applications, vanadium catalysts (V₂O₅) supported on diatomaceous earth can operate stably for years without deactivation, which is of great significance for the continuous sulfuric acid production industry.
Impact of Carrier Performance on Catalytic Effect
The performance of diatomaceous earth carriers directly affects the activity and stability of vanadium catalysts. Studies have shown that the dispersion and pore structure of the carrier are the key factors influencing catalyst performance. Diatomaceous earth from different producing areas exhibits significant differences in carrier performance due to variations in geological origins and diatom species composition.
Taking major domestic diatomaceous earth producing areas as examples: diatomaceous earth from Changbai and Linjiang in Jilin Province, and Tengchong in Yunnan Province, has a SiO₂ content close to 90% with low impurity levels, making it an ideal raw material for producing high-purity filter aids. In contrast, diatomaceous earth from Linqu in Shandong Province, Xundian in Yunnan Province, and Sheng County in Zhejiang Province has relatively lower SiO₂ content but higher Al₂O₃ and Fe₂O₃ contents, along with a larger specific surface area (e.g., reaching 63.8 m²/g in Linqu, Shandong), which may offer unique advantages in certain catalytic applications.
New Modification Technologies for Enhancing Carrier Performance
To further improve the performance of diatomaceous earth carriers, various modification technologies have been developed in recent years. Electron beam bombardment modification technology removes hydroxyl impurities on the surface of diatomaceous earth through high-energy electron beams and introduces active sites, which can increase the loading capacity of iron-doped carbon nitride (FGCN) by 30%. This modification method not only improves the loading efficiency of the active component but also enhances the interaction between the carrier and the active component.
Dilute sulfuric acid ultrasonic treatment technology is another effective modification method. Drying diatomaceous earth is added to a dilute sulfuric acid solution and subjected to ultrasonic treatment under stirring conditions, which can increase the specific surface area of diatomaceous earth, thereby improving the catalytic activity of vanadium catalysts. Studies have indicated that the pores of modified diatomaceous earth are unblocked with larger pore sizes, which is conducive to increasing the mass transfer rate of sulfur dioxide and air in the pores of diatomaceous earth during sulfuric acid synthesis, as well as the contact area between reactants.
Application in Filtration Systems
Filtration Requirements in Sulfuric Acid Production
In the sulfuric acid production process, filtration is a key step to ensure product quality and the normal operation of equipment. Liquid sulfur filtration is a critical operation in sulfuric acid production, aiming to remove solid impurities from liquid sulfur and prevent these impurities from clogging the nozzles of sulfur burners, which would otherwise shorten the service life of sulfuric acid production units. The standard service life of a sulfuric acid plant is 24 months, but due to clogging issues, it may be reduced to less than 18 months, resulting in significant economic losses.
In the sulfuric acid process for titanium dioxide production, diatomaceous earth filter aids are mainly used in the titanium liquid filtration process to remove a small amount of extremely fine suspended solid particles and colloidal particles from the titanium liquid, ensuring that product quality requirements are met. This application fully demonstrates the chemical stability and excellent filtration performance of diatomaceous earth in strong acid environments.
Technical Advantages of Diatomaceous Earth Filter Aids
Diatomaceous earth filter aids have unique technical advantages. Their porous honeycomb structure is composed of hundreds of millions of tiny diatom skeletons, with a porosity of approximately 90% and a large specific surface area. This structural feature enables diatomaceous earth to effectively trap fine particles as small as 0.5 μm, achieving a good filtration effect.
The use of a diatomaceous earth filter aid in filtration operations consists of two steps:
Pre-coating stage: Before filtration, diatomaceous earth filter aid is pre-coated on the filter medium (filter cloth). Diatomaceous earth is formulated into a suspension at a ratio of 1:8–1:10 in a pre-coating tank, and then the suspension is pumped into the filter press through a pre-coating pump for repeated circulation (for about 12–30 minutes) until the circulating liquid becomes clear, forming a uniformly distributed pre-coating layer.
Body feed filtration stage: After the pre-coating layer is established, an appropriate amount of filter aid (body feed) is continuously added to the liquid to be filtered. Due to the high porosity and large specific surface area of diatomaceous earth, once fine particles enter the inner pores and complex pores of the filter aid, they are often trapped due to adsorption and other effects.
Technical Requirements for Filtration Applications
The use of diatomaceous earth filter aids in sulfuric acid environments must meet strict technical requirements:
Chemical stability: Diatomaceous earth must not chemically react with sulfuric acid (containing 20–30% H₂SO₄) or dissolve in it. Diatomaceous earth resources in China have a high SiO₂ content; after pickling and drying, the calcined diatomaceous earth has a SiO₂ content of up to 88% or more, a relative density of 2.1–2.5, and excellent acid resistance.
Mechanical strength: The particles are hard in texture and incompressible within the pressure range of filtration operations (1.0 MPa) to maintain high porosity.
Particle size distribution: Uniform particle size distribution with good suspension and dispersion properties in sulfuric acid solution.
Economy: Low cost, easy availability, abundant sources, and compliance with environmental protection requirements.
However, diatomaceous earth filter aids also have some limitations. Studies have found that the high silicon content of diatomaceous earth may cause premature degradation of filter cloth through abrasion, affecting filtration efficiency and equipment service life. In addition, cleaning the used diatomaceous earth filter cake is difficult and requires tools such as wooden sticks for removal; improper operation may damage the filter leaves.
Application in Drying and Auxiliary Processes
Application in Sulfuric Acid Drying Towers
In the drying stage of sulfuric acid production, the application of diatomaceous earth is mainly reflected in two aspects. First, in sulfuric acid drying towers, packed absorption towers are used, where concentrated circulating acid is employed to absorb moisture and produce 96% sulfuric acid; the circulating acid is cooled by water to remove the heat of absorption. Although diatomaceous earth is not directly used in the sulfuric acid drying process, it plays a role in related auxiliary processes.
Second, diatomaceous earth has a strong water absorption capacity and can absorb water equivalent to 2.5 times its own weight. This property allows it to convert harmful liquids such as sulfuric acid and phosphoric acid into a dry powder state, facilitating safe transportation and storage. In the waste acid treatment process, diatomaceous earth can be used as an adsorbent to effectively reduce the water content and fluidity of waste acid.
Waste Gas Treatment and Environmental Protection Applications
In the waste gas treatment stage of sulfuric acid production, diatomaceous earth can be used as an adsorbent for industrial waste gas treatment. Depending on the waste gas flow rate and pollutant concentration, the dosage of diatomaceous earth is usually 0.1–0.5 kg per cubic meter of waste gas. The porous structure and large specific surface area of diatomaceous earth enable it to effectively adsorb harmful gases and particulate matter in waste gas.
Against the backdrop of increasingly stringent environmental protection requirements, diatomaceous earth plays an important role in the clean production of the sulfuric acid industry. For example, in the treatment of oil-containing diatomaceous earth, aluminum sulfate produced during the stirring process has a flocculation and precipitation effect, which helps separate rolling oil from diatomaceous earth in oil-containing diatomaceous earth. This treatment method not only realizes the recycling of resources but also reduces environmental pollution.
Waste Acid Treatment and Recycling
The application of diatomaceous earth in waste acid treatment has important economic and environmental value. A large amount of waste acid liquid is generated during the acid leaching and purification process of diatomaceous earth. To reduce production costs and avoid environmental pollution, waste acid liquid can be recycled: high-concentration waste acid liquid can be used to produce diatomaceous earth water purifiers, while dilute acid can be used to produce calcium sulfate.
In the actual waste acid treatment process, diatomaceous earth can serve both as an adsorbent and a filter aid. On the one hand, its porous structure can adsorb organic matter and heavy metal ions in waste acid; on the other hand, as a filter aid, it can improve the efficiency of solid-liquid separation. Studies have shown that low-to-medium grade diatomaceous earth with a silica content of 50–60% produced during the sulfuric acid pickling and purification process, together with the generated waste acid liquid, can be used as raw materials to prepare sewage treatment agents.
Application Differences in Different Sulfuric Acid Production Processes
Application in Sulfur-burning Sulfuric Acid Process
The sulfur-burning sulfuric acid process is currently the most dominant sulfuric acid production process worldwide, featuring a short process flow, low equipment investment, and high product purity. In this process, the application of diatomaceous earth is mainly concentrated in the following stages:
Raw material pretreatment: In the molten sulfur filtration stage, diatomaceous earth is used as a pre-coating material to remove solid impurities from sulfur and prevent clogging of burner nozzles.
Catalyst carrier: In the sulfur dioxide conversion process, vanadium catalysts supported on diatomaceous earth are core equipment, directly affecting conversion efficiency and product quality.
Tail gas treatment: In the tail gas purification stage, diatomaceous earth can be used as an adsorbent to remove sulfur dioxide and acid mist from tail gas.
Application in Pyrite-burning Sulfuric Acid Process
The pyrite-burning sulfuric acid process is relatively complex, including multiple stages such as pyrite roasting, furnace gas purification, sulfur dioxide conversion, and sulfur trioxide absorption. In this process, the application characteristics of diatomaceous earth are as follows:
Furnace gas purification: In the furnace gas purification system, diatomaceous earth can be used as a filter aid to remove mineral dust and other solid impurities from furnace gas.
Waste heat utilization: In waste heat boilers and waste heat recovery systems, diatomaceous earth-based thermal insulation materials can be used for equipment insulation to improve thermal efficiency.
Sewage treatment: Sewage generated from pyrite-burning sulfuric acid production contains a large amount of iron ions and sulfuric acid; diatomaceous earth can be used for sewage treatment to remove pollutants through adsorption and filtration.
Application in Smelter Off-gas Sulfuric Acid Process
The smelter off-gas sulfuric acid process produces sulfuric acid using sulfur dioxide-containing off-gas generated during non-ferrous metal smelting. This process is characterized by complex raw material gas composition containing various impurities. The applications of diatomaceous earth in this process include:
Raw material gas purification: Since smelter off-gas contains harmful impurities such as arsenic, fluorine, and heavy metals, a multi-stage purification system is required. Diatomaceous earth filter aids can be used in the fine desulfurization and impurity removal processes.
Catalyst protection: Due to the high impurity content in raw material gas, which is prone to causing catalyst poisoning, higher requirements are imposed on the catalyst carrier. It is necessary to select diatomaceous earth with high purity and reasonable pore size distribution as the carrier to improve the anti-poisoning performance of the catalyst.
Wastewater treatment: Wastewater generated from smelter off-gas sulfuric acid production contains heavy metal ions; diatomaceous earth can be used as an adsorbent and filter aid for wastewater treatment.
Summary
Diatomaceous earth has a wide range of applications in sulfuric acid production: it is mainly used as the carrier for vanadium catalysts, leveraging its unique structure to enhance catalytic activity and service life, with modification technologies available to optimize its performance; as a filter aid, it is applied in the filtration of molten sulfur, titanium liquid, etc., effectively trapping fine impurities and requiring compliance with technical requirements such as acid resistance; it is also used in auxiliary processes such as waste acid treatment and waste gas adsorption, achieving environmental protection and resource recycling goals. In different sulfuric acid production processes (sulfur-burning, pyrite-burning, and smelter off-gas-based), diatomaceous earth is targeted at applications such as raw material pretreatment, furnace gas purification, and raw material gas fine desulfurization, respectively.
