Calcium carbide (CaC2) structure, properties, production, uses

The calcium carbide It is an inorganic compound formed by the elements calcium (Ca) and carbon (C). Its chemical formula is CaC_two. It is a solid that can be colorless to yellowish or grayish white, and even black depending on the impurities it contains.

One of its most important chemical reactions of CaC_two is what happens with water H_twoOr, in which acetylene HC≡CH forms. For this reason it is used to obtain acetylene in an industrial way. Due to the same reaction with water, it is used to ripen fruits, in false cannons and in naval flares..

The CaC reaction_two with water also produces a useful sludge to prepare clinker (a component of cement), which produces less carbon dioxide (CO_two) when compared to the traditional method of producing cement.

With nitrogen (N_two) calcium carbide forms calcium cyanamide, which is used as a fertilizer. The CaC_two it is also used to remove sulfur from certain metal alloys.

Some time ago the CaC_two It was used in so-called carbide lamps, but these are no longer very common because they are dangerous.

Article index

• 1 Structure
• 2 Nomenclature
• 3 Properties
  • 3.1 Physical state
  • 3.2 Molecular weight
  • 3.3 Melting point
  • 3.4 Boiling point
  • 3.5 Density
  • 3.6 Chemical properties
• 4 Obtaining
• 5 Uses
  • 5.1 In the production of acetylene
  • 5.2 In reducing CO2 emissions
  • 5.3 In obtaining calcium cyanamide
  • 5.4 In the metallurgical industry
  • 5.5 In various uses
  • 5.6 Discontinued use
• 6 Risks
• 7 References

Structure

Calcium carbide is an ionic compound and is made up of the calcium ion Ca^two+ and the carbide or acetylide ion C_two^two-. The carbide ion is made up of two carbon atoms joined by a triple bond.

The crystal structure of CaC_two is derived from the cubic (like sodium chloride NaCl), but as the C ion_two^two- is elongated the structure is distorted and becomes tetragonal.

Nomenclature

• Calcium carbide
• Calcium carbide
• Calcium acetylide

Properties

Physical state

Crystalline solid that when pure is colorless, but if it is contaminated with other compounds it can be yellowish white or grayish to black.

Molecular weight

64.0992 g / mol

Melting point

2160 ºC

Boiling point

The CaC_two boils at 2300 ºC with decomposition. The boiling point must be measured under an inert atmosphere, that is, without oxygen or moisture..

Density

2.22 g / cm³

Chemical properties

Calcium carbide reacts with water to form acetylene HC≡CH and calcium hydroxide Ca (OH)_two:

CaC_two + 2 h_twoO → HC≡CH + Ca (OH)_two

Acetylene is flammable, therefore CaC in the presence of moisture_two it can be flammable. However, when it is dry it is not.

Calcium carbide reacts with nitrogen N_two to form calcium cyanamide CaCN_two:

CaC_two + N_two → CaCN_two + C

Obtaining

Calcium carbide is produced industrially in an electric arc furnace starting from a mixture of calcium carbonate (CaCO₃) and carbon (C) that is subjected to a temperature of 2000 ° C. The reaction is summarized like this:

Thief₃ + 3 C → CaC_two + CO ↑ + CO_two↑

Or also:

CaO + 3 C → CaC_two + CO ↑

In an electric arc furnace an arc of electricity is produced between two graphite electrodes, which resist the high temperatures that are formed. A calcium carbide with 80-85% purity is obtained.

Applications

In the production of acetylene

Industrially, the reaction of calcium carbide with water is used to produce acetylene C_twoH_two.

CaC_two + 2 h_twoO → HC≡CH + Ca (OH)_two

This is the most important use of calcium carbide. In some countries, acetylene is highly valued, as it makes it possible to produce polyvinyl chloride, which is a type of plastic. In addition, acetylene is used to weld at high temperatures.

In reducing CO emissions_two

The remains obtained from obtaining acetylene starting from CaC_two (also called "calcium carbide sludge" or "calcium carbide residues") are used to obtain clinker or concrete.

Calcium carbide mud has a high content of calcium hydroxide (Ca (OH)_two) (about 90%), some calcium carbonate (CaCO₃) and has a pH greater than 12.

For these reasons it can react with SiO_two or Al_twoOR₃ forming a product similar to that obtained by the cement hydration process.

One of the human activities that produces the most CO emissions_two it is the construction industry. The CO_two is generated to the east detach from calcium carbonate during the reaction to form concrete.

Using calcium carbide sludge to replace calcium carbonate (CaCO₃) has been found to decrease CO emissions_two by 39%.

In obtaining calcium cyanamide

Calcium carbide is also used industrially to obtain calcium cyanamide CaCN_two.

CaC_two + N_two → CaCN_two + C

Calcium cyanamide is used as a fertilizer, because with the soil water it is converted into cyanamide H2N = C = N, which provides nitrogen to plants, an essential nutrient for them.

In the metallurgical industry

Calcium carbide is used to remove sulfur from alloys such as ferronickel. CaC is mixed_two with the alloy molten at 1550 ° C. Sulfur (S) reacts with calcium carbide and produces calcium sulfide CaS and carbon C:

CaC_two + S → 2 C + CaS

Sulfur removal is favored if mixing is efficient and the carbon content in the alloy is low. The calcium sulfide CaS floats on the surface of the molten alloy from where it is decanted and discarded..

In various uses

Calcium carbide has been used to remove sulfur from iron. Also as a fuel in the production of steel and as a powerful deoxidizer.

It is used to ripen fruit. Acetylene is generated from calcium carbide with water, which induces the ripening of fruits, such as bananas..

Calcium carbide is used in dummy guns to cause the loud bang noise that characterizes them. Here also the formation of acetylene is used, which explodes with a spark inside the device.

The CaC_two used to generate signals offshore in self-igniting naval flares.

Discontinued use

The CaC_two it has been used in so-called carbide lamps. The operation of these consists of dripping water on the calcium carbide to form acetylene, which ignites and in this way provides light.

These lamps were used in coal mines, but their use was discontinued due to the presence of the methane gas CH₄ in those mines. This gas is flammable and the flame from the carbide lamp can cause it to ignite or explode..

They were widely used in slate, copper and tin rock mines, and also in early automobiles, motorcycles and bicycles, as headlights or headlights.

Currently they have been replaced by electric lamps or even LED lamps. However, they are still used in countries like Bolivia, in the silver mines of Potosí.

Risks

Calcium Carbide CaC_two dry it is not flammable but in the presence of humidity it forms acetylene rapidly, which it is.

To extinguish a fire in the presence of CaC_two water, foam, carbon dioxide, or halogen extinguishers should never be used. Sand or sodium or calcium hydroxide should be used.

References

1. Ropp, R.C. (2013). Group 14 (C, Si, Ge, Sn, and Pb) Alkaline Earth Compounds. Calcium Carbides. In Encyclopedia of the Alkaline Earth Compounds. Recovered from sciencedirect.com.
2. Pohanish, R. P. (2017). C. Calcium Carbide. In Sittig's Handbook of Toxic and Hazardous Chemicals and Carcinogens (Seventh Edition). Recovered from sciencedirect.com.
3. Sun, H. et al. (2015). Properties of Chemically Combusted Calcium Carbide Residue and Its Influence on Cement Properties. Materials 2015, 8, 638-651. Recovered from ncbi.nlm.nih.gov.
4. Nie, Z. (2016). Eco-Materials and Life-Cycle Assessment. Case Study: CO_two Emission Analysis of Calcium Carbide Sludge Clinker. In Green and Sustainable Manufacturing of Advanced Material. Recovered from sciencedirect.com.
5. Crundwell, F.K. Et al. (2011). Refining Molten Ferronickel. Sulfur Removal. In Extractive Metallurgy of Nickel, Cobalt and Platinum Group Metals. Recovered from sciencedirect.com.
6. Tressler, R.E. (2001). Structural and Thermostructural Ceramics. Carbides. In Encyclopedia of Materials Science and Technology. Recovered from sciencedirect.com.
7. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.