Calcium sulfite, or calcium sulphite, is a chemical compound, the

calcium salt Although most compounds are referred to by their IUPAC systematic names (following IUPAC nomenclature), traditional names have also been kept where they are in wide use or of significant historical interests. A Ac * Actinium(III) chlorid ...

of sulfite with the formula CaSO₃·x(H₂O). Two crystalline forms are known, the hemihydrate and the tetrahydrate, respectively CaSO₃·½(H₂O) and CaSO₃·4(H₂O). All forms are white solids. It is most notable as the product of flue-gas desulfurization.

Production

It is produced on a large scale by flue gas

desulfurization Desulfurization or desulphurisation is a chemical process for the removal of sulfur from a material. This involves either the removal of sulfur from a molecule (''e.g.'' A=S → A:) or the removal of sulfur compounds from a mixture such as oil refin ...

(FGD). When coal or other fossil fuel is burned, the byproduct is known as

flue gas Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produc ...

. Flue gas often contains SO₂, whose emission is often regulated to prevent acid rain. Sulfur dioxide is scrubbed before the remaining gases are emitted through the chimney stack. An economical way of scrubbing SO₂ from flue gases is by treating the effluent with Ca(OH)₂

hydrated lime Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has ma ...

or CaCO₃

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

. Scrubbing with limestone follows the following idealized reaction: : + → + Scrubbing with hydrated lime follows the following idealized reaction: : + → + The resulting calcium sulfite oxidizes in air to give gypsum: : + → The gypsum, if sufficiently pure, is marketable as a building material.

Uses

Drywall

Calcium sulfite is generated as the intermediate in the production of gypsum, which is the main component of

drywall Drywall (also called plasterboard, dry lining, wallboard, sheet rock, gypsum board, buster board, custard board, and gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thic ...

. A typical US home contains 7 metric tons of such drywall gypsum board.

Food additive

As a food additive it is used as a preservative under the E number E226. Along with other antioxidant sulfites, it is commonly used in preserving wine, cider, fruit juice, canned fruit and vegetables. Sulfites are strong reducers in solution, they act as

oxygen scavenger Oxygen scavengers or oxygen absorbers are added to enclosed packaging to help remove or decrease the level of oxygen in the package. They are used to help maintain product safety and extend shelf life. There are many types of oxygen absorbers ...

antioxidants Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants, ...

to preserve food, but labeling is required as some individuals might be

hypersensitive ''Hypersensitive'' is the second album by American rock band Ghost Machine. It was released on November 21, 2006 via Corporate Punishment Records. The original track listing of the album, displayed on sites like Amazon and AllMusic, was identic ...

Wood pulp production

Chemical wood pulping is the removal of cellulose from wood by dissolving the lignin that binds the cellulose together. Calcium sulfite can be used in the production of wood pulp through the sulfite process, as an alternative to the Kraft process that uses hydroxides and sulfides instead of sulfites. Calcium sulfite was used, but has been largely replaced by magnesium and sodium sulfites and bisulfites to attack the lignin.

Gypsum

There is a possibility to use calcium sulfite to produce

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...

by oxidizing (adding O₂) it in water mixture with the manganese ( Mn²⁺) cation or sulfuric acid catalyzers.

Structure

The structure of calcium sulfite has been examined by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. These studies show that sulfite anion adopts its characteristic pyramidal geometry. The Ca²⁺ centers have high coordination numbers, being bonded to the sulfite oxygen atoms and the water, giving calcium a coordination number of eight. The tetrahydrate crystallizes as a solid solution of Ca₃(SO₃)2(SO₄)^.12H₂O and Ca₃(SO₃)2(SO₃)^.12H₂O. The mixed sulfite-sulfate represents an intermediate in the oxidation of the sulfite to the sulfate, as is practiced in the production of

. This solid solution consists of a₃(SO₃)₂(H₂O)₁₂sup>2+ cations and either sulfite or sulfate as the anion.

Natural occurrence

Calcium sulfite(III) hemihydrate occurs in the nature as the rare mineral hannebachite.

References

{{Calcium compounds Calcium compounds Sulfites E-number additives