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

sulfite Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid ( sulfurous acid) is elusive, its salts are w ...

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 Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide () from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste incineration. Methods ...

Production

It is produced on a large scale by flue gas desulfurization (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 Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but ac ...

. Sulfur dioxide is scrubbed before the remaining gases are emitted through the

chimney A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typ ...

stack. An economical way of scrubbing SO₂ from flue gases is by treating the effluent with Ca(OH)₂ hydrated lime 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 w ...

. 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 Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are wide ...

, 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

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.

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 The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A ...

, as an alternative to the

Kraft process The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chip ...

that uses hydroxides and

sulfides Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds la ...

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 drywa ...

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

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...

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