Historical Background
In 1836, English chemist Edmund Davy observed that a by-product incidental to the production of potassium decomposed water and produced a gas which contained acetylene. In 1862, a German chemist, Wohler, discovered that acetylene could be produced from calcium carbide which he had made by heating a mixture of charcoal and an alloy of zinc and calcium to a very high temperature. Like Davy’s material, it decomposed water and yielded acetylene. He also
reported that the ignited gas produced a brilliant, smoky flame. But it was a French chemist, Berthellot, who in 1862 thoroughly described the reactions. Unfortunately, for the next thirty years only a few chemists observed the acetylene flame, and none of them saw any commercial potential.
However, with the development of the electric arc furnace, Thomas Willson, an electrical engineer in Spray, North Carolina, attempted to produce metallic calcium from lime and coal tar. Instead of calcium metal, he produced a dark molten mass which cooled to a brittle solid. When he discarded it in a stream, a large quantity of gas was suddenly liberated. On being ignited, the gas produced a bright but smoky flame. It was not the clean hydrogen flame which would have
been produced by the reaction of calcium and water, but obviously because of the soot, was a rich hydrocarbon. Repeating the smelt and analyzing the solid, which showed it to contain calcium carbide, Willson sent a specimen with a letter to Lord Kelvin in Glasgow on September 16, 1892. This dated document secured Willson the honor of being the first to produce calcium carbide on a commercially promising scale.
During the same time, others in French and German laboratories had been studying and describing carbides, but none were able to produce them on a commercial scale. Thus, as the result of an accident, the industrial possibilities of calcium and acetylene were recognized for the first time. The practicality of using acetylene as a means of illumination was demonstrated in 1892, and with the establishment of the Willson Illuminating Company in Spray, in the spring of 1895, the first factory to manufacture calcium carbide came into being.
Calcium carbide is produced in electric arc furnaces which attain temperatures of about 2760 to 3900°C (5000 to 7000°F). The arc established between two electrodes is used to heat a mixture of lime and coke, causing the following changes to occur:
CaO + 3C + CaC2 + CO Quicklime + coke yield calcium carbide +carbon monoxide
To obtain high quality acetylene, it is necessary to use quicklime that is essentially 99% pure, and low-ash coke. The phosphorus and sulfur levels of both must also be very low.
The solidified calcium carbide resembles dark brown or black or bluish black stone; its density is
2.24 times greater than water. It will not burn except at very high temperatures in the presence of oxygen. It is not affected by organic solvents and it is unaffected by shock. It can be stored indefinitely if sealed from air. It is odorless, but gives off a smell due to the presence of small amounts of acetylene produced by the interaction of moisture in the air. In the presence of that moisture, it slowly slakes to a dry lime.
Gas Production
The value of calcium carbide comes from the reaction which occurs when placed directly in contact with water according to the following equation:
CaC2 + 2 H20 + Ca(OH)2 + C2H2 Carbide + water yield slaked lime + acetylene
One kg (2.2 lb) of calcium carbide will produce 0.33 m3 (1 1.5 ft3) of acetylene at room temperature.