FGD gypsum and phosphogypsum are both industrial by-product gypsum materials, but they come from different industries and usually require different processing routes. FGD gypsum is produced during flue-gas desulfurization and is often a relatively high-calcium-sulfate material after oxidation, washing and dewatering. Phosphogypsum is generated during phosphoric acid production and typically requires more extensive evaluation because it can contain soluble phosphorus, fluoride-related compounds, residual acidity, trace elements and naturally occurring radionuclides.

The main processing difference is that FGD gypsum projects often focus on moisture control, stable feeding, drying and particle-size control. Phosphogypsum projects may require those same steps plus source-specific impurity management, radiological evaluation, environmental review and application-specific pre-treatment before grinding.

FGD Gypsum and Phosphogypsum at a Glance

How FGD Gypsum Is Produced

FGD gypsum is generated when sulfur dioxide is removed from flue gas. In wet limestone- or lime-based flue-gas desulfurization systems, sulfur dioxide reacts with the absorbent and forms calcium sulfite. When the material is oxidized, calcium sulfite is converted into calcium sulfate dihydrate, commonly known as FGD gypsum.

The material is usually washed and dewatered before it is transported for reuse or storage. High-quality FGD gypsum can contain a high proportion of calcium sulfate dihydrate. One published comparison reports FGD gypsum with more than 90% calcium sulfate dihydrate and other measured elements below 0.1%, although actual composition varies with fuel source, absorbent, oxidation efficiency, washing performance and plant operation.

FGD gypsum is widely recognized as a type of synthetic gypsum. It can be used in gypsum board, cement, agricultural and other applications when it meets the quality requirements of the receiving market.

How Phosphogypsum Is Produced

Phosphogypsum is generated during wet-process phosphoric acid production. Phosphate rock reacts with sulfuric acid to produce phosphoric acid for fertilizer manufacture. Calcium from the phosphate rock combines with sulfate to form phosphogypsum.

Although phosphogypsum is mainly calcium sulfate dihydrate, it carries source-specific components from the phosphate rock and acid-production process. These may include soluble phosphorus, fluoride-related compounds, residual acidity, organic matter, soluble salts, silica, alumina, trace elements and naturally occurring radionuclides.

Phosphogypsum is therefore more complex than FGD gypsum for many reuse applications. The material must be assessed not only for gypsum content and moisture, but also for the impurities that may affect product quality, environmental performance and regulatory acceptance.

Recent reviews of synthetic gypsum identify phosphorus and fluorine-related impurities as notable characteristics of phosphogypsum. They also note that impurity levels can influence hydration behavior, water demand, material strength, salt-related moisture absorption and the suitability of products such as gypsum boards or cementitious materials.

Raw-Material Testing Differences

Both materials should be tested before process design, but phosphogypsum generally requires a broader test program.

Typical FGD gypsum test items

• Calcium sulfate dihydrate content

• Free moisture and total moisture

• Particle-size distribution and degree of agglomeration

• Chloride and soluble-salt content where relevant

• Residual sulfite and oxidation quality

• Residual carbonate, fly ash, silica or other non-gypsum components

• Bulk density, flowability and storage behavior

• Required final powder fineness and intended application

Typical phosphogypsum test items

• Calcium sulfate dihydrate content

• Free moisture, total moisture and seasonal moisture variation

• pH and residual acidity

• Soluble phosphorus and phosphate-related compounds

• Fluoride-related compounds

• Soluble salts, chloride and conductivity where relevant

• Organic matter and color-related impurities

• Silica, alumina, iron and other mineral impurities

• Trace elements and heavy metals where required

• Radionuclide content and radiological characteristics where required

• Particle-size distribution, bulk density and flowability

• Application-specific performance testing

The wider phosphogypsum test program does not mean every source is unsuitable. It means the processing route must be designed around actual material data. Two phosphogypsum sources can have very different reuse potential because phosphate-rock chemistry, acid-process conditions and storage history can differ.

Moisture and Material Handling

Both FGD gypsum and phosphogypsum can have significant free moisture. They are often generated in wet processes and may be supplied as filter cake, compacted material or stockpile feed.

FGD gypsum moisture is often influenced by washing and dewatering performance. The material may be relatively uniform when it is supplied directly from a controlled filtration process. Its main handling challenges are moisture-related lump formation, filter-cake flowability, storage conditions and stable feeding to the dryer or mill.

Phosphogypsum moisture can be more variable because the material may be taken from fresh production, long-term stacks, mixed storage zones or weather-exposed stockpiles. Older material can have uneven moisture, compaction, surface contamination and changed physical behavior.

For both materials, the plant should measure minimum, average and maximum moisture. The process must be designed for normal moisture variation, not only for the best sample received during laboratory testing.

Pre-treatment Differences

FGD gypsum pre-treatment

For FGD gypsum, the process commonly focuses on preparing a stable feed for powder production. Depending on the source condition, the plant may include:

• Covered storage to reduce rain exposure and moisture variation

• Lump breaking or deagglomeration of filter cake

• Screening and foreign-material removal

• Mechanical dewatering when required

• Thermal drying before grinding or integrated drying during grinding

• Blending to stabilize moisture or quality variation

If FGD gypsum is well washed, oxidized and dewatered, the route can be relatively direct: prepare the feed, reduce free moisture, grind, classify and store the finished powder.

Phosphogypsum pre-treatment

Phosphogypsum may require the same mechanical preparation steps, but it can also require impurity-management measures before grinding. The selected process depends on the final application and the actual impurity profile.

Potential pre-treatment steps may include:

• Controlled sampling and stockpile segregation

• Screening and removal of foreign materials

• Washing to reduce selected soluble impurities

• Neutralization or pH adjustment when required

• Filtration or dewatering after washing

• Physical separation, blending or other material-conditioning steps

• Thermal treatment where justified by the product route

• Drying and deagglomeration before grinding

Grinding should follow the treatment steps required to make the material suitable for its end use. Fine grinding cannot replace impurity control. If soluble phosphorus, fluoride-related compounds, acidity or other constituents remain outside the customer’s acceptance range, the project may need additional treatment, a different application or a different material source.

Drying Differences

Both FGD gypsum and phosphogypsum may need drying before conventional grinding. The main difference is the degree of variation and the interaction between moisture management and material quality.

FGD gypsum often needs drying because it is supplied as moist filter cake. The drying system is usually designed around free-water removal, stable feeding and the required final powder moisture. If production capacity is high, LM Vertical Roller Mill can integrate drying, grinding and classification in one system.

Phosphogypsum may have a similar or higher drying requirement, but its drying route must also consider impurities and end-use limitations. Drying improves handling but does not remove soluble phosphorus, fluoride-related compounds, acidity or other chemical constituents. For phosphogypsum, drying is one part of the process, not the complete solution.

Grinding Differences

FGD gypsum and phosphogypsum can both be ground into controlled powder after suitable preparation. The grinding principle is similar: feed the material steadily, reduce particle size, classify the powder and collect the finished product.

The difference lies in what must happen before the mill:

Suitable Grinding Mills

For both FGD gypsum and prepared phosphogypsum, mill selection should be based on feed moisture, target fineness, capacity, drying requirement and final application. MTW European Grinding Mill, LM Vertical Roller Mill and Raymond mill can be considered for suitable material.

MTW European Grinding Mill

MTW European Grinding Mill is suitable for small-to-medium capacity projects with prepared, relatively stable feed. It can be used for FGD gypsum after dewatering and drying as needed. It can also be used for phosphogypsum after the required material evaluation and pre-treatment have been completed.

For wet, sticky material, upstream preparation is important. MTW European Grinding Mill controls powder fineness but is not designed to replace major dewatering, impurity-treatment or high-moisture drying systems.

LM Vertical Roller Mill

LM Vertical Roller Mill is suitable for larger-capacity projects and material that benefits from integrated drying, grinding and classification. It is especially relevant for moist FGD gypsum and prepared phosphogypsum when a suitable heat source is available.

For high-volume phosphogypsum projects, LM Vertical Roller Mill can improve process integration after the source material has been evaluated and treated as needed. Its drying capability does not remove chemical impurities, so upstream treatment remains necessary when required by the final application.

Raymond Mill

Raymond mill can be considered for conventional gypsum powder production with moderate capacity requirements and dry, stable, pre-treated feed. It is generally more suitable for low-moisture FGD gypsum or adequately dried phosphogypsum than for wet filter cake or highly variable stockpile material.

Application Differences

FGD gypsum has an established reuse route in gypsum board, cement, selected building materials and some agricultural applications when material quality meets local requirements. The relatively high calcium sulfate content of many FGD sources can make it a practical substitute for natural gypsum after washing, dewatering and drying.

Phosphogypsum may also be used in cement, building materials, roads, soil-related applications and other industrial uses, but its reuse route must be selected more carefully. The material’s impurity profile, environmental performance and regulatory status can limit applications that may be available to FGD gypsum.

For both materials, final-use acceptance must be based on the receiving customer’s specification and local rules. A material suitable for cement use may not be suitable for gypsum board. A material accepted in one country may require additional evaluation or may be restricted in another.

How to Select the Right Process Route

A practical selection sequence is:

1. Identify whether the material is FGD gypsum or phosphogypsum and document the source process.

2. Collect representative samples from normal production and storage conditions.

3. Measure moisture, gypsum content, particle size, physical condition and source-specific impurities.

4. Define the intended final application and its acceptance criteria.

5. For FGD gypsum, evaluate washing quality, dewatering, drying, feeding and grinding requirements.

6. For phosphogypsum, complete additional chemical, environmental, radiological and regulatory evaluation where required.

7. Select necessary pre-treatment before choosing the final grinding system.

8. Set target fineness, finished-powder moisture and required production capacity.

9. Select MTW European Grinding Mill, LM Vertical Roller Mill or Raymond mill based on the prepared feed and process requirements.

10. Verify the finished powder through end-use trials and regular quality control.

Conclusion

FGD gypsum and phosphogypsum are both gypsum-containing industrial by-products, but their processing routes are different because their sources and impurity profiles are different. FGD gypsum processing often focuses on washing quality, dewatering, drying, stable feeding and controlled grinding. Phosphogypsum processing may require these steps plus impurity management, radiological assessment, compliance review and application-specific treatment.

For both materials, MTW European Grinding Mill is suitable for prepared small-to-medium capacity powder production. LM Vertical Roller Mill is suitable for larger-capacity projects and materials requiring integrated drying. Raymond mill is suitable for conventional powder production with dry, stable and adequately prepared feed.

The correct process should be selected from representative material analysis and final-use requirements. Grinding is essential for particle-size control, but successful reuse depends on the complete route from source evaluation and pre-treatment to finished-powder quality and verified end-use acceptance.