Phosphogypsum: From the Dilemma of 600 Million Tons of Stockpiles to the Path of Green Resource Utilization

In our science article published on July 16, we provided a detailed overview of the phosphate chemical industry chain. You can revisit it here: https://www.kelewell.de/en/post/the-phosphate-chemical-an-integrated-value-chain.In that article, we discussed phosphoric acid as a fundamental chemical material. Currently, there are two main production methods used both domestically and internationally: the wet process and the thermal process.

• The wet process uses strong inorganic acids (mainly sulfuric acid) to decompose phosphate rock to produce phosphoric acid. This is known as wet-process phosphoric acid or extraction phosphoric acid, and it is widely used in the manufacture of high-efficiency fertilizers.

  
  

• The thermal process involves reducing fluorapatite in high-grade phosphate ore at high temperatures to elemental phosphorus (yellow phosphorus), which is then oxidized and hydrated to produce phosphoric acid.

At present, approximately 90% of phosphoric acid on the market is produced via the wet process. The main chemical reaction of this process is:

Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O → 5CaSO₄·2H₂O + 3H₃PO₄ + HF↑

This equation shows that the wet-process method generates large amounts of phosphogypsum as a by-product—mainly composed of calcium sulfate dihydrate.

What is Phosphogypsum?

Phosphogypsum (PG) refers to the solid waste residue generated when phosphate rock is treated with sulfuric acid during the production of phosphoric acid. Its main component is calcium sulfate dihydrate (CaSO₄·2H₂O).For every 1 ton of phosphoric acid produced, about 4 to 6 tons of phosphogypsum are generated. Phosphogypsum is generally powdery, appearing gray-white, gray-yellow, or light green. It has a bulk density of 0.733–0.88 g/cm³, contains 10–30% free moisture, and is acidic with a pH range of 1.9–5.3.

Its crystal structure is mainly needle-like or plate-like, and the surface contains various attached substances. Particle sizes range from 40 to 200 μm, and distribution is approximately normal. It contains complex chemical components, including residual organic and inorganic phosphorus, fluorides, fluorine, potassium, sodium, and other inorganic substances.

Due to these impurities, phosphogypsum must undergo modification and impurity removal before it can be effectively utilized as a resource.

Impurities in Phosphogypsum and Associated Issues

Insoluble impurities include:

• Quartz

• Undecomposed apatite

• Insoluble and eutectic P₂O₅

• Fluorides

• Phosphates and sulfates of aluminum and magnesium

Soluble impurities include:

• Water-soluble P₂O₅

• Slightly soluble fluorides and sulfates

Additionally, phosphogypsum may contain arsenic, copper, zinc, iron, manganese, lead, cadmium, mercury, and radioactive elements. These are generally present in trace amounts and are mostly insoluble solids, so their harmful effects are negligible.However, fluorides, free phosphoric acid, P₂O₅, and phosphates are major contributors to environmental pollution during stockpiling.

Phosphorus exists in soluble, eutectic, and insoluble forms:

• Soluble phosphorus: Mainly H₃PO₄, H₂PO₄⁻, HPO₄²⁻; H₃PO₄ is the most detrimental to gypsum.

• Eutectic phosphorus: Reduces hydration rate, acts as a set retarder in construction materials, increases water demand.

• Insoluble phosphorus: Mostly inert, negligible effect.

Fluorine exists as:

• Soluble fluorine (e.g., NaF): Promotes crystal growth, weakens structure, reduces strength.

• Insoluble fluorine (e.g., CaF₂, Na₂AlF₆, Na₂SiF₆): Inert, low impact on gypsum performance.

Organic impurities include:

• Ethylene glycol monoethyl ether acetate

• Methoxypentane

• Isothiocyanomethane

These are more prevalent in finer particles and coat gypsum crystals, leading to increased water demand and weaker bonding in cementitious applications. They also affect the color and appearance.

China's Situation

China discharges approximately 80 million tons of phosphogypsum annually, with cumulative stockpiles exceeding 600 million tons.Stockpiling not only occupies vast land but also poses risks such as dam failure and overflow, while soluble impurities such as phosphorus and fluorine may contaminate soil and water systems. The high cost of storage wastes social resources and has become a major bottleneck for the phosphate chemical industry.

Methods for Removing Impurities

Impurities in phosphogypsum affect its usability. Pre-treatment is necessary to convert it into a reusable secondary resource. Major approaches include:

1. Physical Methods:

• Washing, flotation, ball milling, screening, aging.

• Water washing is the most common and effective for removing soluble impurities and organics. However, the wastewater must be treated to avoid secondary pollution.

2. Thermal Treatment:

• High-temperature calcination removes eutectic phosphorus.

• Converts phosphorus into inert forms.

• Simple process, no secondary pollution.

• However, the resulting product is no longer calcium sulfate dihydrate, limiting its applications.

3. Chemical Methods:

• Addition of reagents to convert impurities into compounds.

• CaO neutralization removes soluble phosphorus and fluorine and adjusts pH.

• Raising pH helps convert soluble species into inert salts.

Comprehensive Utilization of Phosphogypsum

Globally, phosphogypsum resource utilization is under active research.

• USA: With abundant natural gypsum and vast land, 98% of PG is stockpiled.

• Japan & Germany: Low PG output and limited gypsum reserves make them prioritize reuse:

  • Japan: Nearly 100% utilization – 60% in gypsum construction materials, 30% as cement retarders, and the rest in medical/food industries.

  • Germany: 95% used mainly in cement retarders.

Properly treated phosphogypsum can be converted into high-value products, generating environmental, social, and economic benefits.

Main application sectors:

• Construction

• Agriculture

• Roads

Applications

1. Construction Materials

Technology for using phosphogypsum in building materials is mature:

• As an air-setting binder, used in gypsum boards, blocks, paper-faced boards, and gypsum fibers.

• Most products are suitable for dry indoor use only due to poor water resistance.

By calcining and purifying PG into β-hemihydrate gypsum, it can be used in:

• Boards, blocks, plasters, self-leveling floors.

New direction: modification and blending with alkaline materials to develop hydraulic binders for broader applications, including:

• Lightweight

• Fire-resistant

• Sound-insulating

Also used as a cement set retarder and in co-production with sulfuric acid.

2. Transportation Infrastructure

PG can be processed into road base material:

• Washed and neutralized to eliminate harmful substances.

• Combined with cement and stabilizers to form strong, dense roadbeds.

• Higher compressive and alkali resistance.

• Lower cost than traditional materials.

3. Mine Backfilling & Ecological Restoration

Modified PG is used to:

• Fill abandoned underground cavities (strength >0.2 MPa).

• Restore open-pit mines.

Large-scale adoption of PG backfilling is already occurring in dozens of Chinese mines.

4. Agriculture

PG improves saline-alkali soils:

• Supplies phosphorus, sulfur, and calcium.

• Acidic nature adjusts soil pH.

• Reduces aluminum leaching, toxic element accumulation.

• Improves soil structure and fertility.

• Mitigates problems from long-term fertilizer and herbicide use.

5. Chemical Industry

PG can be converted into:

• Calcium sulfate whiskers

• Ammonium sulfate

• Potassium sulfate

These products offer high purity and value:

• Calcium sulfate whiskers: fibrous single crystals with:

  • High strength

  • Toughness

  • Insulation

  • Heat resistance

Used as reinforcing agents, fillers, filters, insulation, and cable sheaths. Preparation methods include:

• Atmospheric acidification

• Autoclaving

• Ion exchange

Conclusion

As global demand for phosphate fertilizer continues to grow, the challenge of phosphogypsum disposal will intensify.Solving the problem of large-scale disposal is essential—not only for environmental reasons but also for the sustainable development of the phosphate industry.

Future efforts should focus on:

• Industrial application of low-cost impurity removal

• Cross-sector integration (construction, roads, agriculture)

• Policy incentives to support enterprise-level recycling

Let us work together to promote solid waste recycling and advance green chemical development!