Calcium sulfite 0.5-water
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Product Description
Calcium sulfite 0.5-water, or calcium sulfite hemihydrate (CaSO₃·½H₂O), is a white, nearly water-insoluble crystalline solid in which one water molecule is shared between every two CaSO₃ units. Best known as the principal precipitate formed when limestone slurries scrub sulfur dioxide from power-plant flue gases, this hemihydrate also finds niche uses as a food preservative (E 226) that scavenges oxygen in wines and juices, as a chlorine-removing agent in shower filters, and as a set-retarding additive in cement where it yields AFm phases instead of the ettringite produced by gypsum. Its crystal water participates directly in thermal chemistry, generating reactive SO₂⁻ radicals upon mild heating, while its low solubility and specific lattice make it the preferred solid phase in many industrial desulfurization systems.
Synthesis and Production
Primary Industrial Method (Flue Gas Desulfurization - FGD):
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Calcium sulfite hemihydrate is primarily produced as a by-product in semi-dry or wet flue gas desulfurization processes.
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In the dense-phase tower desulfurization process, SO₂-containing flue gas reacts with a powdered desulfurizing agent (e.g., Ca(OH)₂ or CaO) in the presence of moisture:
SO₂ + Ca(OH)₂→CaSO₃⋅0.5H₂O + H₂O -
The process involves controlled pH (6.5–7), temperature, and gas-solid contact to favor the formation of the hemihydrate form.
Laboratory Synthesis:
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Can be prepared by reacting calcium chloride with sodium sulfite:
CaCl₂ + Na₂SO₃→CaSO₃⋅0.5H₂O↓ + 2NaCl -
Partial dehydration of higher hydrates (e.g., CaSO₃·2H₂O) is also possible but less common.
Uses and Applications
| Sector | Application |
|---|---|
| Environmental | Main by-product of FGD systems; can be oxidized to CaSO₄ for use in gypsum-based products. |
| Construction | Limited use as a cement retarder (≤2% replacement of gypsum); must be controlled to avoid expansion issues. |
| Food Industry | Used as a preservative and antioxidant (E 226) in dried fruits, wine, etc. (max 0.2 g/kg SO₂ equivalent). |
| Paper & Textile | Acts as a bleaching agent and dechlorinator. |
| Chemical Industry | Intermediate in the production of calcium sulfate or other sulfur compounds. |
Chemical and Physical Properties
Physical Properties:
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Appearance: White or greyish-white crystalline powder.
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Solubility: Sparingly soluble in water (~0.0043 g/100 mL at 25°C).
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Thermal Behavior:
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Loses water of crystallization around 200°C.
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Oxidizes to CaSO₄ between 400–560°C.
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Decomposes to CaO and CaS above 600°C in inert atmospheres.
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Chemical Properties:
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Stability: Relatively stable but oxidizes slowly in moist air.
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Reactivity:
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Oxidizes in air:
2CaSO₃⋅0.5H₂O + O₂→2CaSO₄ + H₂O -
Reacts with acids to release SO₂.
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Structural Properties:
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Crystal System: Monoclinic (space group *C2/c*).
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Water Role: Zeolitic; water molecules are shared between two CaSO₃ units.
Safety and Handling
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Hazards:
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May cause eye and respiratory irritation.
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Dust can be harmful if inhaled.
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Storage:
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Store in a dry, sealed container to prevent oxidation and moisture absorption.
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Disposal:
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Should be oxidized to calcium sulfate (gypsum) before landfilling to avoid environmental release of SO₂.
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