Calcium Carbide for Acetylene Production: Grades, Yield & Safety

The short answer

Calcium carbide (CaC₂) is a hard, grey-black solid made from lime and coke in an electric-arc furnace. When it meets water it reacts to release acetylene gas, leaving hydrated lime behind. Commercial carbide yields roughly 280–300 litres of acetylene per kilogram, and it must be kept absolutely dry — it reacts with any moisture, even humidity in the air.

The governing reaction is:

How calcium carbide is made

Calcium carbide is produced by reacting burnt lime (calcium oxide) with a carbon source — coke — at around 2,000 °C in a submerged electric-arc furnace. It is one of the most energy-intensive chemical processes in industry, taking on the order of 3,000 kWh per tonne, which is why carbide cost and quality vary with the producer.

The molten carbide is tapped, cooled, crushed and screened into graded sizes for different uses — including acetylene generation.

How much acetylene does calcium carbide yield?

Chemically pure calcium carbide would give about 370 litres of acetylene per kilogram. Commercial carbide is only about 75–85% CaC₂, so real-world yield is lower — typically 280–300 litres per kilogram, which good plants approach by running an efficient generator and using clean, correctly-sized carbide.

IIGAS generators run at over 98% generator efficiency (92% guaranteed minimum) and can process low-cost carbide dust as well as graded lump, to cut feedstock cost without losing yield.

Carbide grades and sizes used in a plant

Carbide is screened into size grades — measured in millimetres — and the plant type decides which size it needs. A stationary carbide-to-water generator typically uses graded lump such as 25–50 mm; IIGAS plants accept a wide 4–80 mm range, and dust-carbide designs are available to use the cheapest feedstock. Consistent sizing matters: it controls how the carbide feeds and how fast it reacts, which keeps generation smooth and safe.

Impurities carbide brings into the gas

Carbide is never chemically pure, and its impurities pass into the acetylene: mainly phosphine (PH₃) and hydrogen sulphide (H₂S), with ammonia formed during generation and traces of arsine. These are exactly what the plant’s purification train removes — ammonia in a scrubber, then phosphine and sulphur compounds in the purifier — to take the gas to 98.5–99.5% purity, or 99.99% for ultra-high-purity acetylene. Cleaner carbide means an easier purification job and a better final gas.

Storing calcium carbide safely

Carbide’s biggest hazard is simple: it reacts with water — including atmospheric humidity — to release flammable acetylene. Safe storage is about keeping it dry and away from ignition.

• ✓Keep carbide in air- and water-tight metal drums, each holding no more than 250 kg, clearly labelled “Calcium Carbide — Dangerous If Not Dry”.
• ✓Store in a dry, well-ventilated area, raised off the floor, away from water, drains, steam lines and sources of ignition.
• ✓Open drums only when needed and reseal them; do not leave carbide exposed to humid air.
• ✓Keep dust down — do not charge carbide with more than about 15% dust into a lump-fed generator, and control fines to avoid rapid, uncontrolled reaction.
• ✓Any acetylene released around opened carbide is flammable — no smoking, sparks, or open flames in the carbide store.

What happens to the carbide after the reaction

The reaction leaves calcium hydroxide, Ca(OH)₂ — a lime slurry usually called carbide lime or carbide sludge. It is drained from the generator and, rather than being waste, can be reused in construction, soil treatment, effluent neutralisation and other uses.