Calcium carbide is a chemical compound with the chemical formula of CaC2. The material is colorless, but most samples appear black through to grayish white lumps, depending on the grade. Its main use industrially is in the production of acetylene and calcium cyanamide.
Production
Calcium carbide is produced industrially in an electric arc furnace loaded with a mixture of lime and coke at approximately 2000 °C. This method has not changed since its invention in 1888:
CaO + 3C → CaC2 + CO
The high temperature required for this reaction is not practically achievable by traditional combustion, so the reaction is performed in an electric arc furnace with graphite electrodes. The carbide product produced generally contains around 80% calcium carbide by weight. The carbide is crushed to produce small lumps that can range a few mm up to 50 mm. The impurities are concentrated in the finer fractions. The CaC2 content of the product is assayed by measuring the amount of acetylene produced on hydrolysis. As an example the British and German standards for the content of the coarser fractions are 295 L/kg and 300 L/kg respectively. Impurities present in the carbide include phosphide, which produces phosphine when hydrolysed.
Crystal structure
Pure calcium carbide is a colourless solid. The common crystalline form at room temperature is a distorted rock salt structure with the C22− units lying parallel.
Applications
Production of acetylene
The reaction of calcium carbide with water was discovered by Friedrich Wöhler in 1862.
CaC2 + 2H2O → C2H2 + Ca(OH)2
Production of calcium cyanamide
Calcium carbide reacts with nitrogen at high temperature to form calcium cyanamide:
CaC2 + N2 → CaCN2 + C
Calcium cyanamide is used as fertilizer. It is hydrolysed to cyanamide, H2NCN.
Steelmaking
Calcium carbide is used:
- in the desulfurisation of iron (pig iron, cast iron and steel)
- as a fuel in steelmaking to extend the scrap ratio to liquid iron depending on economics.
- as a powerful deoxidizer at ladle treatment facilities.
