Calcium Carbide, CaC₂

Wohler observed that a substance which evolved acetylene on treatment with water, apparently Calcium Carbide, CaC₂, was obtained by the action of carbon on a zinc-calcium alloy. Calcium carbide may also be formed by the action of amorphous carbon on metallic calcium, calcium hydride, or calcium nitride, by heating the compound CaC₂.C₂H₂.4NH₃, produced by the absorption of acetylene by calcium ammonium, and by a secondary reaction when calcium chloride or a mixture of calcium chloride and calcium fluoride is electrolysed with carbon electrodes. It was the work of Moissan in Paris, and independently of Willson at Spray in North Carolina, U.S.A., that made possible the production of calcium carbide on a commercial scale by fusing together lime and carbon in the electric furnace.

The physical and chemical properties were first studied by Moissan. The pure compound forms transparent, colourless crystals, of density 2.22 at 18° C., but the commercial product is coloured by iron and other impurities. The crystalline structure is complex, but probably orthorhombic. The melting-point is above the temperature of fusion of platinum.

It was formerly stated that calcium carbide is produced from its elements with the absorption of heat, but, according to later investigations, it has a positive heat of formation of 13.15 Cal. As might be expected, therefore, it dissociates at a very high temperature, above 1475° C., calcium volatilising and leaving the carbon behind. The action is accelerated by certain salts, especially calcium fluoride.

Calcium carbide is insoluble in all known solvents. It is not reduced by hydrogen, but it reacts with the halogens between 250° and 300° C. with incandescence, burns in oxygen at red heat forming carbonate, and in sulphur at 500° C. forming calcium sulphide and carbon disulphide. With water and with dilute acids it produces acetylene. It reacts with gaseous hydrochloric acid at red heat, becomes incandescent when treated with oxidising agents, and reduces oxides and salts, forming lime and the free element or carbide. It also reduces a number of organic compounds to carbon.

At high temperatures calcium carbide absorbs nitrogen, forming calcium cyanamide, CaCN₂, and at 100° C. it absorbs acetylene, giving the compound CaC₂.C₂H₂.

The reaction by which carbide is produced is reversible.

CaO + 3C ⇔ CaC₂ + CO.

The heat absorbed in the reaction from left to right at ordinary temperatures is 111 Cal., with a negative temperature coefficient of 0.0033 Cal., per degree. Under the conditions of manufacture the pressure of carbon monoxide cannot be less than ⅓ atmosphere, which will be the amount formed by the action of atmospheric oxygen on the carbon. The temperature at which this pressure is reached is 1655°- 1720° C., calculated from the heat of reaction and from the pressure determinations at lower temperatures, by means of the equation 4.57 log₁₀ p₂/p₁=Q(1/T₁ - 1/T₂) where Q is the quantity of heat absorbed, and p₁ and p₂ the pressures at the absolute temperatures T₁ and T₂. This could not be verified experimentally, because, under the conditions of the experiment, the calcium carbide breaks up into its elements a little above 1475° C., but in practice this temperature must evidently be exceeded before carbide can be formed. If the carbide is cooled in an atmosphere of carbon monoxide there is a back reaction in the neighbourhood of the solidifying point, but the effect is limited to the surface in contact with the gas, and so is not important on the large scale.

On heating the carbide with carbon in the electric furnace the carbon is dissolved.

The manufacture of calcium carbide consists simply in the fusion of a mixture of lime and carbon at the high temperature of the electric furnace. Carbon or graphite electrodes are used. The furnaces in use may be divided into two classes - intermittent and continuous. Although the intermittent process, by which the carbide is gradually built up into a huge block during the operation of the furnace, produces the best quality of carbide, it is falling into disfavour because it is uneconomical to work. The furnace has to be allowed to cool, and is then dismantled in order to remove the carbide and to recharge.

The continuous furnace is made of sheet iron lined with carbon and mounted on wheels. It is open at the top for the admission of the charge. One electrode consists of several blocks of carbon hanging over the top of the furnace, the other forms the hearth or bottom of the furnace. An outlet near the base allows the molten carbide to be drawn off from time to time, whilst fresh charge is fed in at the top. The carbide is run into cast-iron trays and allowed to cool. To increase the fusibility of the product an excess of lime is used, so that there is a smaller volume of acetylene per unit weight than in the case of block carbide. There is also a gradual accumulation of slag and impurities which cling to the sides of the furnace and fill it up.

The Alby furnace, used at Odd a in Norway, which is of this type, can produce 7-8 tons of carbide per day and takes a current of 28,000 amperes at 50 volts. The temperature is between 2000° and 3000° C. At Niagara the Horry furnace is in use. It is in the form of a large revolving wheel by which the charge, as the operation is completed, is gradually carried past the fixed electrodes.

The lime and carbon are introduced in the form of lumps, 5-8 cm. in diameter, so the lime must be such as to calcine without falling into powder. The natural impurities should not exceed more than 4-5 per cent, of the burnt lime, and of this not more than 2 per cent, should consist of magnesium and aluminium oxides which make the carbide less friable. The formation of iron silicide should be avoided, as it injures the furnace and crushing apparatus and may be the cause of explosions. If the carbide is to be used for the production of acetylene, phosphorus is dangerous and sulphur disagreeable but not dangerous. The carbon may be in the form of anthracite, coke, retort carbon, or wood charcoal, but the ash content must be very low and the phosphorus absent.

As water power is desirable for the electric current requirements, production on the largest scale is in Scandinavia and America. There was, at first, over-production of carbide, owing to a too great optimism as to the value of acetylene as an illuminant, and many factories had to be closed again and still remain closed (1922). The carbide first produced contained too many impurities. Now, however, a better product is obtained, and there is an increasing demand due to the use of acetylene in oxyacetylene welding, and flares in marine work. More important still has been the use of carbide as a starting-point in the manufacture of cyanamide and other synthetic nitrogen products.

Other uses for it have also been proposed; for example, the formation of metallic alloys from oxides or chlorides, the carbide being used as a deoxidising flux with borax, sodium chloride, or calcium chloride, and the preparation of argon from the atmosphere by circulating air over a mixture of calcium carbide and chloride at 800° C. to absorb oxygen and nitrogen. The production of alcohol from carbide has apparently been tried successfully on a small scale in Switzerland. Its application as an explosive in mining operations by using carbide cartridges containing an air-space, so as to give an explosive mixture of air and acetylene, has also been suggested.

Instead of using water in the liquid form for the production of acetylene, the carbide may be heated with salts containing water of crystallisation.