What is calcium carbide? CaC₂ explained: how it is made from lime and coke its properties, IS 1040 grades, gas yield and uses

Calcium carbide is the grey-black, stony solid that acetylene is made from. Its formula is CaC₂: one calcium atom bonded to a pair of carbon atoms. It is produced by melting lime and coke together in an electric arc furnace, and when it meets water it gives off acetylene gas and leaves calcium hydroxide, the wet lime of every acetylene plant. One kilogram of good commercial carbide gives about 300 litres of acetylene. This page explains what the material is, how it is made, its properties, how the Indian standard grades it, what happens when it reacts with water and what it is used for.

Carbide is a man-made rock. Burnt lime and coke are heated in an electric furnace until they melt together, and the result, once cooled and broken up, is calcium carbide. It is safe while dry. Add water and it fizzes, gets hot and gives off acetylene, a gas that burns with a very hot flame and is used for welding and cutting metal. What is left is wet lime. Carbide must be kept in sealed steel drums, away from water, because even damp air slowly turns it into gas and lime. It must never be used to ripen fruit.

The commercial product is never pure. A typical technical carbide carries about 12 % free lime, small amounts of free carbon, silica, ferrosilicon, alumina and, importantly for acetylene quality, about 9 g/kg of calcium sulphide and around 1 g/kg of calcium phosphide. Those last two become hydrogen sulphide and phosphine in the gas, which is why acetylene plants need a purifier; the arithmetic is on phosphine in acetylene and Indian carbide.

The process has not changed in principle since the industry began in the 1890s. Limestone is burnt to quicklime, mixed with coke, and fed into an electric arc furnace, where large carbon electrodes carry the current into the charge.

1. Lime. Limestone of 97–98 % CaCO₃ is burnt to quicklime. Phosphorus in the lime should be below 0.01 %, because most of it reports to the carbide and ends up as phosphine in the acetylene.
2. Carbon. Metallurgical coke, typically screened to 10–20 mm, supplies the carbon; low ash and low sulphur are what a carbide maker pays for.
3. Reaction. In the arc zone the lime and carbon react: CaO + 3C → CaC₂ + CO. The reaction absorbs about 112 kcal per mole of carbide, about 1,750 kcal per kg, so the furnace is a large electrical load. The reaction is brisk at 1,600 °C and slow at 1,300 °C.
4. Tapping. Liquid carbide is tapped from the furnace at 1,840–2,070 °C, measured by radiation pyrometry (Vapnuk, 1963), into cast-iron moulds.
5. Cooling, crushing, grading. The pigs are cooled, crushed in jaw or gyratory crushers, screened into graded sizes and packed into sealed drums. Ferrosilicon is removed magnetically during screening.

Energy use depends on how hard the furnace is run. In one 6,000 kW furnace making 275 l/kg carbide, Givreanu (1959) measured about 5,000 kWh per tonne at 2,600 kW load, falling to 2,850 kWh per tonne at 5,000 kW. The carbon monoxide given off is collected from closed furnaces and burnt as fuel. There is no refining step in a carbide furnace: whatever impurities the lime and coke bring, the carbide keeps.

The Indian standard for carbide is IS 1040, Calcium carbide, technical. The edition held and read in full is the 1987 Third Revision; it has been revised as IS 1040:2024, whose text was not available for this page. The 1987 edition sets:

The gas limits in clause 5.3 apply to acetylene straight from the carbide, not to cylinder gas. Dissolved acetylene in cylinders is governed by IS 308:2024, which caps phosphorus compounds at 0.01 % and sulphur compounds at 0.02 %; closing that gap is the purifier's job. Grading, sieving and the yield tables of other national standards are covered in calcium carbide grades, sizes and gas yield.

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

By mass, 64 g of carbide and 36 g of water give 26 g of acetylene and 74 g of calcium hydroxide. The reaction releases 31.0 kcal per mole, which is 484 kcal per kg of pure carbide and about 429 kcal per kg of commercial carbide yielding 300 l/kg. With no cooling, that heat would raise the products by about 730 °C, which is why an acetylene generator holds a large excess of water.

Pure CaC₂ would give 346.89 litres of dry acetylene per kg at 0 °C and 760 mm (C.P.I., 1968), which is about 366 litres at 15 °C. Commercial carbide of about 82 % CaC₂ gives about 300 l/kg. KIGL's published schedule for its generators is:

Smaller pieces give less gas because more of their surface has already reacted with moisture in the air. The same air reaction is why an open drum can lose a large share of its gas in days; see carbide storage. Each kilogram of carbide also leaves about 1.17 kg of dry lime, which is the subject of the calcium hydroxide data sheet.