Industrial by-product gypsum refers to solid waste generated as a by-product in industrial production processes, primarily composed of calcium sulfate (CaSO₄). It is typically produced as a by-product containing calcium sulfate through chemical reactions in certain chemical production processes. Industrial by-product gypsum is produced in large quantities, has a complex composition, and contains certain amounts of impurities and harmful substances. It requires appropriate treatment and processing before it can be utilized for resource recovery.
I. Main Characteristics of Industrial By-Product Gypsum
1. Complex Composition
In addition to the main component calcium sulfate, it may also contain other impurities and harmful substances, such as heavy metals, organic compounds, fluorides, chlorides, and trace amounts of radioactive substances.
2. Large production volume
In large-scale industrial production processes, such as phosphate fertilizer production and flue gas desulfurization in coal-fired power plants, the production volume of industrial by-product gypsum is substantial. According to relevant data, China is a major producer of by-product gypsum, accounting for approximately 75% of the world’s by-product gypsum production. In 2009, production capacity reached approximately 119 million tons, and by 2023, it had exceeded 300 million tons; desulfurization gypsum accounts for approximately two-thirds of the total, primarily resulting from the development of thermal power generation in China and environmental remediation efforts by coal-fired enterprises.
3. Significant differences in physical properties
Industrial by-product gypsum from different sources may exhibit significant variations in particle size, crystalline morphology, moisture content, and other physical properties.
II. Types, Sources, and Harmful Substance Composition of Industrial By-Product Gypsum
1. Phosphogypsum: A solid by-product generated during the production of phosphoric acid by treating phosphate ore with sulfuric acid. Its main component is calcium sulfate dihydrate, containing phosphates, sulfates, fluorides, heavy metals (such as manganese, cadmium, lead, mercury, etc.), organic compounds, and trace radioactive substances (such as radium), typically appearing in grayish-white, grayish-black, or grayish-yellow hues.
2. Desulfurization Gypsum: Its main component is the same as natural gypsum, calcium sulfate dihydrate, with a content of ≥93%. It is produced by grinding lime-limestone into a slurry, passing SO₂-containing flue gas through a scrubbing tower to remove SO₂, where the lime slurry reacts with SO₂ to form calcium sulfate and calcium sulfite. The calcium sulfite is then oxidized to form calcium sulfate, This is desulfurization gypsum. It contains calcium sulfite, excess chloride ions, and trace amounts of heavy metals (such as lead, cadmium, and mercury) and other harmful substances.
3. Titanium gypsum: Produced during the sulfuric acid process for titanium dioxide production, this waste residue is generated by adding lime (or calcium carbide slag) to neutralize large amounts of acidic wastewater, primarily composed of dihydrate gypsum. It contains impurities such as iron hydroxide, ferrous sulfate, aluminum hydroxide, and silica.
4. Boron gypsum: A by-product of boric acid production, primarily containing calcium sulfate with two molecules of water, followed by boron trioxide and other impurities.
5. Citric acid gypsum: A by-product generated during citric acid production when citric acid is acidified with sulfuric acid. It contains residual acids and organic matter. China is the world’s largest producer of citric acid, with a production volume exceeding one million tons in 2015, resulting in over 1.5 million tons of citric acid gypsum.
6. Fluorogipsum: A by-product generated during the production of hydrogen fluoride by acidifying fluorite with sulfuric acid. Its main component is anhydrous calcium sulfate, along with calcium fluoride, silicon dioxide, aluminum oxide, and trace amounts of residual hydrogen fluoride. China has abundant fluorite reserves, and hydrogen fluoride production has grown rapidly. In 2018, China produced 683.74 million tons of fluorite gypsum.
7. Salt gypsum: A waste residue generated in the salt industry through chemical reactions, primarily composed of calcium sulfate. It contains small amounts of chlorides and other impurities.
8. Sodium sulfate gypsum: A waste residue produced during the leaching of sodium sulfate from calcium sulfate mines using heap leaching. It contains small amounts of sodium sulfate and other impurities.
9. Tartaric acid gypsum: A waste residue generated during the production of tartaric acid.
In addition to the above, other types include lactic acid gypsum, monosodium glutamate gypsum, copper gypsum, nickel gypsum, and chromium gypsum, among others. These industrial by-product gypsums are produced in large quantities and have certain utilization value. However, due to the presence of harmful substances, they pose certain challenges in processing. Proper utilization of these by-product gypsums not only reduces environmental pollution but also conserves resources and promotes sustainable development.
III. Treatment Methods for Harmful Substances in Common Industrial By-Product Gypsum
1. Phosphogypsum
Water washing method: Through repeated washing, soluble fluorine, phosphorus, and surface-floating organic matter can be removed, reducing phosphorus and fluorine content. However, the wastewater generated must be treated again before discharge to prevent environmental pollution. This method is costly and does not meet energy-saving and emission-reduction requirements.
Lime neutralization method: Reacts with soluble phosphorus, fluorine, and other impurities in phosphogypsum to form insoluble substances that precipitate and separate. The amount of lime added is critical, with the pH of the slurry typically controlled around 7.
Screening method: Utilizing the uneven distribution of impurities in phosphogypsum, screening is used to remove larger particles, thereby reducing impurity content. However, this method is only effective when impurities are extremely unevenly distributed and is rarely applied in practice.
Chemical treatment method: Harmful impurities such as phosphorus in phosphogypsum are converted into usable materials through chemical reactions. For example, phosphogypsum is mixed with alkaline calcium materials, silicon-aluminum composite materials, and additives, then undergoes chemical reactions to produce phosphates and silicates beneficial to cement. This method is simple to operate, low-cost, and widely applied in the pretreatment of phosphogypsum for cement production.
Calcination method: During high-temperature calcination, the gases released from the decomposition of phosphorus pentoxide are volatilized and removed. Simultaneously, P₂O₅ reacts with certain active components in phosphogypsum to form stable, low-solubility phosphate compounds. High temperatures remove trace organic phosphorus, reduce impurity levels, improve phosphogypsum performance, remove free water and crystalline water, and lower viscosity.
Solidification/stabilization technology: Using calcium carbide slag or lime as an alkaline neutralizing agent for phosphogypsum, and polyferric sulfate or polyaluminum chloride as a directional solidification stabilizer, this method efficiently solidifies and stabilizes toxic and harmful components in phosphogypsum.
2. Desulfurization gypsum
Water washing method: This method removes soluble impurities such as chloride ions from desulfurization gypsum through water washing.
Chemical precipitation method: Add chemical precipitants to desulfurization gypsum to react with harmful substances such as heavy metal ions, forming insoluble precipitates for removal.
Solidification/stabilization method: Mix desulfurization gypsum with solidifiers and stabilizers to fix harmful substances in a stable solid matrix, reducing their mobility and bioavailability.
Landfill method: The desulfurization gypsum solid waste is disposed of in specialized landfills. The landfill must have leak-proof measures to prevent waste from leaking into groundwater.
3. Fluoride gypsum
Precipitation method: A precipitant is added to cause fluoride ions to react with the precipitant, forming insoluble fluoride precipitates, thereby removing them from the fluoride gypsum.
Adsorption method: Utilize the adsorption properties of adsorbents to remove fluoride from fluoride gypsum.
Calcination method: During high-temperature calcination, fluoride may be converted into insoluble or poorly soluble substances and thus fixed.
4. Citric acid gypsum
Water washing method: Remove part of the soluble impurities from citric acid gypsum through water washing.
Chemical precipitation method: Add chemical precipitants to citric acid gypsum to react with harmful substances such as heavy metal ions, forming insoluble precipitates for removal.
Solidification/stabilization method: Mix citric acid gypsum with solidifiers and stabilizers to fix harmful substances in a stable solid matrix, reducing their mobility and bioavailability.
5. Titanium gypsum
Water washing method: Remove some soluble impurities from titanium gypsum through water washing.
Chemical precipitation method: Add chemical precipitants to titanium gypsum to react with harmful substances such as heavy metal ions, forming insoluble precipitates for removal.
Solidification/stabilization method: Mix titanium gypsum with solidifiers, stabilizers, and other additives to fix harmful substances in a stable solid matrix, thereby reducing their mobility and bioavailability.
6. Salt gypsum
Water washing method: Remove part of the soluble impurities from salt gypsum through water washing.
Chemical precipitation method: Add chemical precipitants to salt gypsum to react with harmful substances such as heavy metal ions, forming insoluble precipitates for removal.
Solidification/stabilization method: Mix salt gypsum with solidifiers and stabilizers to fix harmful substances in a stable solid matrix, reducing their mobility and bioavailability.
7. Copper gypsum, nickel gypsum, chromium gypsum
Chemical precipitation method: Add chemical precipitants to copper gypsum, nickel gypsum, and chromium gypsum to react with heavy metal ions such as copper, nickel, and chromium, forming insoluble precipitates for removal.
Ion exchange method: Utilize the adsorption properties of ion exchange resins to remove heavy metal ions such as copper, nickel, and chromium from copper gypsum, nickel gypsum, and chromium gypsum.
Solidification/stabilization method: Mix copper gypsum, nickel gypsum, and chromium gypsum with solidifiers and stabilizers to fix harmful substances in a stable solid matrix, thereby reducing their mobility and bioavailability.
When treating harmful substances in industrial by-product gypsum, appropriate treatment methods or combinations of methods should be selected based on the specific type of gypsum, the type and content of harmful substances, and the intended subsequent use, to achieve harmless treatment and resource utilization of industrial by-product gypsum.
III. Main Resource Utilization Directions for Industrial By-Product Gypsum
1. Production of cement retarders
Phosphogypsum, desulfurization gypsum, fluoride gypsum, and titanium gypsum can replace natural gypsum as cement retarders, delaying cement setting time and facilitating concrete mixing, transportation, and construction. However, soluble phosphorus and fluorine impurities in phosphogypsum can affect cement performance, necessitating pretreatment methods such as staged calcination or gaseous sulfur reduction to remove impurities before use in cement production.
2. Production of Gypsum Building Materials
Utilizing gypsum’s excellent thermal insulation, lightweight, fire-resistant, and environmentally friendly properties, new wall materials and gypsum boards can be developed. By leveraging gypsum’s engineering properties, it can be utilized for plastering gypsum, self-leveling gypsum, and mine backfilling. Additionally, sulfur and calcium resources in gypsum can be used to produce calcium sulfate whiskers, ammonium sulfate, potassium sulfate, and sulfuric acid co-produced cement, among other chemical products, which have high purity and economic value.
3. Soil Conditioners
Phosphogypsum has a weak acidic nature and can be used to improve saline and alkaline soils, enhancing soil fertility.
4. Agricultural Applications
The Ca²⁺, Si⁴⁺, and S²⁻ ions present in desulfurization gypsum are beneficial mineral nutrients for plants, promoting the growth of crops in saline-alkali soils. It can serve as a soil conditioner to regulate soil nutrition, reduce soil acidity, and improve soil compaction.
5. Preparation of Chemical Products
Desulfurization gypsum can react with low-value ammonium carbonate to produce high-value, nutrient-rich ammonium sulfate fertilizer, while the resulting calcium carbonate can be used to manufacture cement.
Preparation of calcium sulfate whiskers: Calcium sulfate whiskers are prepared from calcium sulfate in fluorite gypsum for use in enhancing plastics, rubber, and other materials.
6. Extraction of Valuable Metals
Through hydrometallurgical methods, valuable metals such as copper, nickel, and chromium can be extracted from copper gypsum, nickel gypsum, and chromium gypsum.
7. Other applications
It can also be used as road construction materials, such as road base fillers and binder materials for the base layer, to improve road base performance and reduce cement consumption.