Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt 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).
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 often contains SO
2, 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 SO
2 from flue gases is by treating the effluent with Ca(OH)
2 hydrated lime or CaCO
3 limestone.
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:
- 2 + → 2
The gypsum, if sufficiently pure, is marketable as a building material.
Uses
Water treatment
Used in some shower filters to remove chlorine due to its reducing properties and slow dissolution in water.
Drywall
Calcium sulfite is generated as the intermediate in the production of gypsum, which is the main component of
drywall. 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 antioxidants 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, 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 by oxidizing (adding
Oxygen) it in water mixture with the manganese (
Manganese) cation or
sulfuric acid Catalysis.
Structure
Ca3(SO3)2aq12.png|Structure of the Ca3(SO3)2(H2O)122+ cage in calcium sulfite tetrahydrate.
CaSO30.5aq.png|Structure of anhydrous CaSO3.
X-ray crystallography shows that anhydrous calcium sulfite has a complicated polymeric structure.
The tetrahydrate crystallizes as a solid solution of Ca
3(SO
3)2(SO
4)
.12H
2O and Ca
3(SO
3)2(SO
3)
.12H
2O. The
sulfite sulfate represents an intermediate in the oxidation of the sulfite to the sulfate, as is practiced in the production of
gypsum. This solid solution consists of Ca
3(SO
3)
2(H
2O)
122+ cations and either sulfite or sulfate as the anion.
Natural occurrence
Calcium sulfite(III) hemihydrate occurs in the nature as the rare mineral hannebachite.
See also
