What is Calcium Carbide? -

Calcium carbide, or calcium acetylide, is an inorganic chemical compound with the formula CaC₂. It is a dark grayish-black, crystalline solid, which is brittle and hard and has a rock-like appearance at room temperature. It is colorless and odorless when pure, but technical-grade calcium carbide tends to have a brown or gray color because of contamination and gives a garlic-like smell when exposed to air. It is insoluble in all solvents and is highly reactive with water to form acetylene gas (C₂H₂) and calcium hydroxide (Ca(OH)₂), a reaction that is strongly exothermic and can ignite the combustible acetylene gas. Calcium carbide is a dangerous good (hazard class 4.3) since it is highly reactive with water and can emit toxic gases such as phosphine from impurities.

The compound forms part of the acetylide class and is defined by a pair of triple-bonded carbon (C₂²⁻) atoms attached to a calcium ion (Ca²⁺). The carbon-carbon triple bond in CaC₂ is comparable to that of ethyne (acetylene), where the bond length is around 119.2 pm. Its molecular weight is 64.099 g/mol, while its density is 2.22 g/cm³, with a melting point of 2160°C and boiling point of 2300°C.

Structure of Calcium Carbide

Calcium carbide contains a distorted rock-salt crystal structure at room temperature, which is a derivative of the cubic sodium chloride (NaCl) structure. In the structure, calcium ions (Ca²⁺) are coordinated by the acetylide anion (C₂²⁻), composed of two carbon atoms connected by a triple bond. The calcium-acetylide ionic bond is the result of electron transfer from calcium, a group 2 alkaline earth metal, to the carbon atoms. The structure can be envisioned as such:

Chemical Formula: CaC₂
Bonding: Ionic, with one sigma bond and two pi bonds between the carbon atoms of the acetylide ion.
Crystal Form: Tetragonal, with calcium ions coordinated by acetylide units.

Preparation of Calcium Carbide

Industrial calcium carbide is prepared by an endothermic, high-temperature reaction between calcium oxide (lime, CaO) and carbon (usually in the form of coke) within an electric arc furnace. The reaction demands temperatures of around 2000–2200°C, which cannot be achieved by conventional combustion, so graphite electrodes are required for the furnace. The chemical reaction is:

CaO + 3C → CaC₂ + CO (ΔH = +460 kJ/mol)

Steps in Industrial Production:

Raw Materials: Calcium oxide (CaO, obtained from heating limestone, CaCO₃) and coke (carbon, C) are broken down to certain sizes (lime: 8–10 mm; coke: 8–25 mm) in order to facilitate effective reaction. The lime must have 92–95% CaO and below 1–2% CO₂, whereas coke must have 85–89% solid carbon and minimal moisture (<1%).
Heating: The molten mixture is then heated under an electric arc furnace up to 2000–2500°C, where the reaction takes place, yielding molten calcium carbide and carbon monoxide (CO) as a byproduct.
Cooling and Crushing: The tapped molten calcium carbide is cooled in water-cooled drums, solidifying. The solid product is crushed into lumps of a few millimeters to 50 mm.
Quality Control: The finished product usually has 80–85% by weight calcium carbide, with some impurities such as calcium oxide (CaO), calcium phosphide (Ca₃P₂), calcium sulfide (CaS), and calcium nitride (Ca₃N₂). The CaC₂ quality is determined through the measurement of the acetylene gas evolved during hydrolysis, with specifications such as 295–300 L/kg for coarser fractions.

Another, less widely used procedure is to react powdered calcium metal with carbon powder (e.g., activated charcoal) at 810°C under an inert atmosphere to produce a cleaner product free of the phosphine smell found in commercial-grade carbide.

Physical and Chemical Properties

Physical Properties:

Pure form: Colorless and crystalline; Technical grade: Grayish-black or brown due to the presence of impurities.
Odor: Odorless in pure form; technical grade has a garlic odor because of phosphine (PH₃) due to impurities in calcium phosphide reacting with water.
Density: 2.22 g/cm³
Melting Point: 2160°C
Boiling Point: 2300°C
Solubility: Does not dissolve in any known solvent; reacts with water and alcohol.

Chemical Properties:

Reactivity with Water: Calcium carbide reacts vigorously with water to give acetylene gas and calcium hydroxide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ The reaction is used as the source of acetylene production and is a strong exothermic reaction, which can cause the acetylene gas to ignite.
Reactivity with Nitrogen: Upon heating to 1000–1200°C, calcium carbide will react with nitrogen gas to produce calcium cyanamide (CaCN₂), an agricultural fertilizer: CaC₂ + N₂ → CaCN₂ + C
Reduction Agent: Being a superbase, calcium carbide is able to deprotonate alcohols and reduce metal oxides to their respective metals.
Reactivity with Acids: It reacts slowly with anhydrous acids to give acetylene and calcium salts.
Hazards: Impurities such as calcium phosphide can release phosphine (PH₃), a poisonous and flammable gas, upon hydrolysis. Calcium carbide also reacts with oxidizing agents and halogens (for example, chlorine at 245°C, bromine at 350°C).

Uses of Calcium Carbide

Calcium carbide has extensive industrial, agricultural, and specialty applications:

Production of Acetylene:

The major application of calcium carbide is the production of acetylene gas (C₂H₂) through reaction with water. Acetylene is an important raw material for:
Welding and Cutting: Oxy-acetylene torch generates a very high-temperature flame (up to 3500°C) to weld and cut metals.
Chemical Synthesis: Acetylene is utilized in the manufacture of vinyl chloride (for PVC), acetaldehyde, and acetic acid, which are all essential for plastics, resins, and synthetic rubber.
In China, calcium carbide-acetylene is a leading feedstock for the manufacture of PVC since it is cheaper than oil sources.

Calcium Cyanamide Production:

Calcium carbide interacts with nitrogen to form calcium cyanamide (CaCN₂), a fertilizer high in nitrogen content that dissociates when it comes into contact with water to form cyanamide (H₂NCN). This is commonly applied in agriculture to make soils fertile.

Steelmaking:

Desulfurization: Calcium carbide desulfurizes molten iron, enhancing the hardness and strength of steel.
Deoxidizer: It removes oxygen from molten steel, improving quality.
Fuel: It extends the scrap ratio for steelmaking as it serves as a fuel supplement

Carbide Lamps:

Historically, calcium carbide was utilized in carbide lamps, where water fell onto the carbide to create acetylene gas, which was burned with a brilliant, white flame. They were employed in mining (coal, copper, tin, and slate mines) and early car headlights but are now mostly obsolete because of safety issues in areas rich in methane. Contemporary applications are confined to specialized uses such as caving and mining in developing countries.

Agriculture:

Calcium carbide as a fruit ripening agent by producing acetylene, which induces the production of ethylene, a plant hormone responsible for ripening.

It is utilized to measure the content of soil moisture by reacting with water in a closed cylinder to form acetylene, whose pressure measures the level of moisture.

Other Uses:

Mole Repellent: Commercial calcium carbide is marketed as a mole repellent because phosphine gas has an objectionable odor.
Toy Cannons: Acetylene from calcium carbide drives bamboo and big-bang cannons.
Organic Synthesis: Calcium carbide is used as a solid source of acetylene in reactions such as the Kinugasa and Trofimov reactions to prepare heterocycles pyrroles and indoles.

Health and Safety Hazards

There are some health and safety hazards with calcium carbide:

Reactivity: Reaction with water or moisture results in flammable acetylene gas, a fire and an explosion risk. Impurities such as calcium phosphide can evolve toxic phosphine gas.
Health Effects: Inhalation, dermal exposure, or ingestion of calcium carbide or its products (e.g., phosphine) may lead to:
- Skin and eye irritation.
- Respiratory irritation, headache, dizziness, loss of memory, and neurological effects due to prolonged hypoxia.
- Severe exposures result in pulmonary edema or seizures.
Misuse in Farming: Application of calcium carbide for fruit ripening is problematic because of toxicity from impurities such as arsenic and phosphorus, which could be hazardous to health if there is residue on food.
- Precautions when Handling:
- Use protective clothing, gloves, and eye protection.
- Work in good ventilation or fume hoods.
- Store in sealed, dry containers away from moisture.
- Dispose of waste according to good practices to prevent environmental pollution.
Evidence indicates that supplementation with vitamin B12 can counteract some calcium carbide toxicity effects, for example, liver and kidney injury or hematologic complications like thrombocytopenia, through attenuation of oxidative stress and inflammation.

Historical Significance

Calcium carbide was first prepared by German scientist Friedrich Wöhler in 1862 and subsequently industrialized in 1892 by Thomas L. Willson and H. Moissan. Its discovery in the United States by Willson and James Turner Morehead in experiments with aluminum was a turning point during the Industrial Revolution, making it possible to produce acetylene for lighting and chemical syntheses. The cheap hydroelectric power available at Niagara Falls made mass production possible toward the end of the 19th century. The formation of the Union Carbide Corporation in 1898 further diversified its uses in petrochemicals and welding.

Global Production and Trends

Production: China manufactured 8.94 million tons of calcium carbide in 2005, with a production capacity of 17 million tons, due to its application in producing PVC. Conversely, consumption in the U.S., Europe, and Japan has been decreasing, while that in the U.S. was 236,000 tons a year during the 1990s.
Environmental Effects: Production creates a lot of dust and carbon monoxide, necessitating scrubbers for gas purification. It is energy-intensive, with 10–11 GJ/ton consumed in the 1980s, although newer closed furnaces minimize energy loss and allow recycling of CO.