CA6943 Desulphurisation Gypsum Powder

Desulfurization Gypsum Description

Desulfurization Gypsum (FGD gypsum), primarily composed of calcium sulfate dihydrate (CaSO₄·2H₂O, ≥85%), is a material with defined physicochemical properties that differ from natural gypsum. Its crystal structure conforms to the standard monoclinic system (space group C2/c), while controlled precipitation in flue gas scrubbers produces acicular crystals ranging from 10–150 μm. The material contains up to 15% free moisture (surface‐adsorbed water) and exhibits 20.9% stoichiometric crystalline water. The dehydration process begins at 100–120°C owing to trace levels of calcium sulfite hemihydrate (≤0.5% CaSO₃·½H₂O), which increases the yield of hemihydrate. Chemically, the material shows retrograde solubility comparable to that of natural gypsum (0.24 g/100 mL at 20°C), with the dissolution rate modified by dissolved ions. Chloride (≤600 mg/kg) moderately increases the solubility, whereas water‐soluble MgO (≤0.1%) and Na₂O (≤0.06%) alter the crystal surface energy. The FTIR analysis records O–H stretching bands at 3540 cm⁻¹, 3400 cm⁻¹, and 1685 cm⁻¹, and sulfate vibrations at 1140 cm⁻¹ (ν₃) and 670 cm⁻¹ (ν₄); a weak shoulder at 970 cm⁻¹ indicates minor sulfate reduction. Thermogravimetric analysis identifies two mass loss steps: evaporation of free moisture below 80°C (up to 15%) and release of crystalline water peaking at 145°C. X-ray diffraction confirms the predominance of dihydrate phases (d-spacing 7.63 Å) with a minor presence of bassanite (CaSO₄·0.5H₂O, d-spacing 6.02 Å) as a result of limited dehydration. The material does not absorb moisture under relative humidities below 70% but displays a higher tendency to cake than natural gypsum due to soluble alkali salts. Its measured whiteness (L* >92) and low radioactivity (<1 Bq/g for ²²⁶Ra) reflect rigorous control of impurities in modern FGD systems.

Desulfurization Gypsum Applications

Desulfurization gypsum is employed as an alternative to mined gypsum in various industrial applications. In construction, the material is subjected to calcination at 150–160°C to generate high-purity β-hemihydrate, which is then used in gypsum plasterboards, drywall and self-levelling floor screeds. Its consistent particle dimensions and low chloride content (<600 ppm) contribute to enhanced binding performance and reduced corrosion risk. In the cement industry, a dosage of 4–5% is applied to regulate the early hydration kinetics of C₃A phases. In agriculture, the product is used as a soil amendment for sodic clay soils by supplying soluble calcium (15–18% CaO) to displace exchangeable sodium and providing sulphur (≥16% SO₃) without significantly altering soil pH. Additional uses include serving as a raw material for sulphuric acid manufacture via thermal decomposition at temperatures exceeding 900°C, and as a filler in thermoplastic composites (PP/PE) due to its high whiteness (L* >92) and low oil absorption. In wastewater treatment, the material functions as a coagulant aid for phosphate removal through calcium phosphate precipitation. The controlled levels of calcium sulfite hemihydrate (<0.5%) and soluble alkalis (Na₂O <0.06%) ensure that the product complies with ASTM C471/C472 building material standards and FDA 21 CFR §184.1230 agricultural limits.

Desulfurization Gypsum Packaging

Our products are packaged in customised cartons in various sizes in accordance with material dimensions. Small items are packed in PP boxes, while larger items are transported in bespoke wooden crates. We ensure adherence to prescribed packaging procedures and employ appropriate cushioning materials to provide adequate protection during transportation.

Packaging: Carton, Wooden Box, or Customised.

Kindly review the packaging details provided for your reference.

Manufacturing Process

1.       Testing Method

(1)    Chemical Composition Analysis – Samples are analysed using techniques such as GDMS or XRF to verify compliance with defined purity requirements.

(2)    Mechanical Properties Testing – Tensile strength, yield strength and elongation are measured to assess material performance.

(3)    Dimensional Inspection – The thickness, width and length are measured to confirm that they meet prescribed tolerances.

(4)    Surface Quality Inspection – Defects such as scratches, cracks or inclusions are detected through visual and ultrasonic examination.

(5)    Hardness Testing – The material’s hardness is determined to confirm uniformity and mechanical reliability.

Please refer to the testing procedures of SAM for detailed information.

Desulfurization Gypsum FAQs

Q1. How does the impurity profile differ from natural gypsum in performance-critical applications?

FGD gypsum typically contains crystalline silica at levels up to 0.5%, compared with 1–8% in natural gypsum, and exhibits reduced heavy metal concentrations due to controlled synthesis. It includes trace amounts of calcium sulfite hemihydrate (≤0.5%) and soluble alkali compounds (Na₂O ≤0.06%, MgO ≤0.1%), which increase the setting time in cement formulations by approximately 8–12% relative to natural gypsum.

Q2. What maximum temperature can be applied during storage to prevent phase transformation?

Temperatures exceeding 65°C during prolonged storage initiate dehydration to β-hemihydrate and alter hydration kinetics. Consequently, storage should be maintained at ≤50°C with a relative humidity of ≤70% to preserve dihydrate crystallinity. Given that each 0.1% increase in CaSO₃·½H₂O reduces the dehydration onset temperature by about 3°C, temperature control is critical.

Q3. How do water-soluble alkalis impact the durability of calcined products?

Na₂O concentrations above 0.04% induce sodium sulphate formation, which may lead to efflorescence in set plasters. Similarly, MgO levels exceeding 0.08% reduce the efficiency of β-hemihydrate conversion by 5–7% during calcination because of the formation of magnesium silicate. Combustion analysis is therefore conducted to confirm that alkali levels remain within specified limits prior to processing.

Related Information

1.       Common Preparation Methods

Desulfurization gypsum is produced through flue gas desulphurisation (FGD) systems in coal-fired power plants. In this process, sulphur dioxide (SO₂) in exhaust gases reacts with a calcium carbonate (CaCO₃) slurry within wet scrubbers. The procedure begins with the absorption of SO₂ into an aqueous suspension at a pH of 5.0–5.8 and temperatures of 50–60°C, forming calcium sulphite (CaSO₃) via the reaction: SO₂ + CaCO₃ + ½H₂O → CaSO₃·½H₂O + CO₂. Thereafter, forced oxidation is applied by injecting compressed air with a stoichiometric excess of at least 30% to convert sulphite into sulphate, as shown by the reaction: 2CaSO₃·½H₂O + O₂ + 3H₂O → 2CaSO₄·2H₂O. Crystallisation occurs over 8–12 hours under controlled supersaturation (relative saturation 1.05–1.15), which results in acicular dihydrate crystals measuring 10–150 μm. The slurry is filtered using a vacuum belt system to achieve a free moisture content of 8–15%, followed by counter-current washing with demineralised water to lower the chloride content to ≤600 mg/kg and to reduce soluble alkalis (Na₂O ≤0.06% and MgO ≤0.1%). Critical process control includes an oxidation efficiency of >99.5% (conversion to sulphate), limestone purity of >95% CaCO₃, and the use of sulphite suppression additives (e.g. thiosulphate catalysts). The filter cake is subsequently dried in rotary dryers at temperatures between 50 and 65°C, maintained below the dehydration threshold in order to preserve CaSO₄·2H₂O crystallinity (≥85%) while limiting hemihydrate formation (≤0.5% CaSO₃·½H₂O). Final product validation is performed using XRF for elemental composition (CaO ≥32.0%, SO₃ ≥45.5%), XRD for phase purity (dihydrate d-spacing 7.63 Å), and ion chromatography for anion compliance.