The interaction of carbon with metals at high temperatures produces carbides. For example, calcium carbide: Ca + 2C → CaC2. Of all carbides, it has the greatest practical value. Pure CaC2 is a well crystallized solid, colorless crystals are formed by Ca2 + and C2- ions. Its other names are calcium acetylenide or calcium carbon. The molar mass is 64.1 g / mol. It is not volatile and insoluble in any of the known solvents, but decomposes under the influence of water. Its density is 2.22 g / cm³. The melting point is 2160 ° C, and the boiling point is 2300 ° C. By the degree of exposure to the body, it is extremely hazardous (1st hazard class).
Calcium acetylenide was first obtained in 1862 by the German chemist Friedrich Weller while heating an alloy of zinc and calcium with coal. He also described the reaction of calcium carbon with H2O. Even with its traces (for example, with atmospheric moisture), calcium carbide reacts vigorously, with the release of a large amount of heat. But in case of lack of water, self-ignition of the resulting acetylene can occur. Calcium carbide reacts violently with dilute inorganic acids and aqueous alkali solutions. In this case, acetylene is also released. Being a strong reducing agent, CaC2 is able to reduce all metal oxides to carbides or pure metals.
Obtaining calcium carbide is more convenient not from calcium itself, but from its oxide. At high temperatures (from 2000 to 2300 ° C), it is restored. In this case, the metal and carbon combine: CaO + 3C → CO ↑ + CaC2. The process takes place in an electric arc furnace, where a mixture of quicklime and coke or anthracite is heated. The technical product is painted, has a grayish color, as it contains impurities in the form of free carbon, oxide, as well as sulfide, calcium phosphide and other chemical compounds. The mass fraction of CaC2 in it is 80–85%.
When carbides interact with water, either metal or acetylene is released. In a second embodiment, calcium carbide is reacted with water. Acetylene obtained as a result of a reaction that has industrial significance: 2H2O + CaC2 → C2H2 + Ca (OH) 2 is technical and has an unpleasant odor, as it contains a number of impurities (NH3, H2S, PH3 and others). Although in its pure form, it is a colorless gas with a characteristic faint odor, and it dissolves quite well in water. It is of great importance for the production of calcium cyanamide (it is a raw material in the production of cyanide compounds or used as fertilizer) from carbide by its interaction with nitrogen according to the reaction equation: N2 + CaC2 → CaCN2 + C.
In the past, CaC2 was widely used in carbide lamps, where it served as a source of acetylene flame. Currently, the use of such a light source is limited mainly by speleology, it is also used in beacons and lighthouses. Other important applications of CaC2 are chemical technologies, where it is a raw material. For example, in the production of organic synthesis products, the main one is synthetic rubber. Acetylene black, vinyl chloride, acetic acid, acrylonitrile, ethylene, acetone, artificial resins, and styrene are also obtained from calcium carbon. In metallurgy, it is used for metal deoxidation and desulfurization (reduction of oxygen and sulfur, respectively). Calcium carbide is used to make a plant growth regulator, a powder carbide reagent (acetylene welding).
Getting every ton of CaC2 requires about 3 thousand kW / h of electricity. Therefore, it is justified in the presence of low prices for it. However, overall global calcium carbon production is increasing. If in the 70s of the previous century up to 5 million tons were produced annually, then in our century this amount has approximately doubled. So, in China, the production of calcium carbide-based acetylene is the main source of raw materials for the chemical industry, in particular, for the production of polyvinyl chloride. Obtaining acetylene from its raw materials is more economical than using imported oil. Therefore, its production in China is growing. In 2005, it reached 8.94 million tons, and there was a real opportunity to produce 17 million tons.
Unlike China, in the USA, Europe and Japan, the consumption of calcium carbon is usually reduced. The level of its production in the United States in the 90s was only 236 thousand tons per year. In our country, calcium carbide is produced according to the specifications set forth in GOST 1460-81. Its main consumers are Ukraine, Uzbekistan, Romania and Slovakia. Due to the high energy intensity of the product and due to rising electricity prices, the consumption of calcium acetylide in Russia and exporting countries halved.