Calcium Oxide, CaO

Calcium Oxide, CaO, also known as caustic lime or quicklime, may be formed by the direct combination of calcium and oxygen. The molecular heat of formation is about 152 Cal. It can also be obtained by the calcination of the nitrate or the carbonate. For the preparation on the commercial scale the carbonate is used, and " lime-burning " is one of the oldest of chemical processes.

Among the chief sources of the carbonate is, first and foremost, limestone of varying degrees of purity. A limestone consisting of nearly pure carbonate gives a rich or " fat " lime. Limes containing aluminates and silicates are hydraulic limes. Dolomitic limestone does not give a satisfactory product, and is not used except for special purposes - for example, the lining of certain furnaces. Chalk may also be burnt, and in certain places in Holland and America sea- shells are used for instance, in Baltimore, where the oyster-canning industry is carried on. Another source is the spent lime from paper-pulp mills, caustic-soda works, and beet-sugar factories.

Although the modern limekiln differs greatly in efficiency from the ancient one, and is a much more elaborate structure, the principle of the method of lime-burning has remained essentially the same. The carbonate is heated strongly to drive off the carbon dioxide, the usual working temperature being from 800° to 1000° C. The escape of the gas is facilitated by the presence of steam, which dilutes it and so reduces its partial pressure. A lower temperature may thus be employed and an economy of fuel effected. In the older process the limestone was damped, but in the modern ones steam is injected into the kilns. In the simplest form of kiln, known as the flare kiln, the limestone in small pieces is supported over the fire on an arch made of larger lumps, the kiln itself being also constructed of limestone. The same form may be built of brick or stone and the inside lined with fireclay. The process is intermittent, the kiln being emptied and recharged after each batch.

A more economical type is the continuous or running kiln. A long steel shaft, of a height three to four times the diameter, and lined with firebrick, is necessary. Instead of steel, a reinforced-concrete shell has recently been recommended in America. The limestone and fuel may be fed in at the top in alternate layers. The fire is started at the bottom and more of the charge introduced at the top. If lime uncontaminated with ash is required, calcination must be effected by the hot gases from furnaces at the sides. As fuel in the latter case, wood, coal, oil, natural gas, or producer gas may be employed.

For the highest economy in fuel and labour, and for uniform and thorough burning, it is best to use a rotary kiln such as is adopted in cement-burning. This method was first proposed in 1885, but in spite of its advantages it is only in very recent times that it has met with much approval, because it produces lime in small pieces - from dust to pieces of 2 inches in diameter - and builders are prejudiced in favour of large lumps. Spent lime sludge may be fed into the rotary kiln whilst still wet, like cement slurry.

Calcium oxide as ordinarily obtained is a white amorphous mass with a density which varies between 3.15 and 3.30 according to the temperature at which it has been prepared.3 Hare stated that he had fused it in the oxyhydrogen flame, but probably the lime was not a very pure specimen. It may be fused in the electric furnace giving on cooling a milky crystalline mass of density 3.4. According to Day and Shepherd the density of fused lime is 3.316 at 25° C. and the hardness 3.4. Moissan obtained crystals in the form of both cubes and needles. Brugelmann obtained transparent cubical crystals of 2 mm. side and density 3.251, by slowly heating calcium nitrate with the addition of 0.25 – 0.6 per cent, of calcium hydroxide.

The high melting-point of lime makes it difficult to determine its value with accuracy. Ruff and his collaborators found that under reduced pressure it had not melted at 2450° C., but that it volatilised readily above 2000° C. Kanolt gave the melting-point as 2572° C.±3°. The boiling-point of lime at 760 mm. is near 2850° C. Considerable volatilisation takes place at 1500° C. The latent heat of fusion is 490 small calories per gram.

The molecular heat of calcium oxide at 2552° C. is 14.8 cal. or specific heat 0.242. Laschtschenko studied the change of specific heat with temperature at lower temperatures. He found the value 0.172 cal. at 190° C., 0.181 at 376° to 400° C., 0.190 at 415° C., and thereafter a gradual increase to 0.193 between 590° and 680° C. He concluded that the fused lime had undergone transformation between 400° and 415° C., and that the heat of transformation was 0.280 Cal. per gram-molecule. In connection with this an observation of Moissan's might be mentioned. He mounted some cubical crystals of lime in Canada balsam, and found, six months later, that they had broken into fragments which polarised light. He regarded this as evidence of dimorphism. Other indications have also been obtained of a transition point between 400° and 430° C., from a fine-grained porous form showing slight double refraction at low temperatures, to a cubic form at high temperatures.

The reactivity of lime with non-metals and metals has been studied by Moissan. Fluorine reacts with calcium oxide in the cold with the production of heat and light. Calcium fluoride is formed and oxygen given off. Other non-metals require the aid of heat. Chlorine at 300° C. will partially replace oxygen. Sulphur, arsenic, silicon, boron, and titanium also react. Carbon in the electric furnace forms first calcium carbide and carbon monoxide, and then, in the presence of excess of lime, the latter is reduced by the carbide producing metallic calcium. Many of the metals, even platinum, reduce lime to a greater or less extent at a sufficiently high temperature. Anhydrous calcium oxide will not react with the different acid gases, hydrochloric acid, carbon dioxide, sulphur dioxide, etc., in the cold, but on heating, reaction takes place.

Vignon found that in the presence of lime, carbon reacted with steam at a lower temperature than when alone, producing hydrogen, methane, and ethylene, the proportion varying with the amount and rate of flow of the steam. This probably has a bearing on the question of the formation of natural petroleum.

When heated in the oxyhydrogen flame, calcium oxide emits a brilliant white light, the well-known limelight. Under the influence of the cathode rays it gives an orange-yellow fluorescence. Schmidt obtained a phosphorescent calcium oxide by grinding up with a small quantity of one of the following substances: sodium chloride, fluoride, and phosphate, calcium and magnesium fluoride, lithium phosphate, potassium hydrogen phosphate, and potassium borate, adding a nitric acid solution of bismuth, copper, manganese, or lead, and heating the whole to redness. It then phosphoresced after exposure to light. The position of brightest phosphorescence was at a shorter wave-length than in calcium sulphide and selenide.

Calcium oxide dissolves in fused calcium chloride, one molecule of the oxide saturating seven of the chloride.

By heating the double carbonate of calcium and lithium an isomorphous mixture of lime and lithia, crystallising in regular octahedra, can be obtained.

A double oxide of calcium and lead is formed by dissolving lead oxide in boiling lime-water and allowing to crystallise. The solution blackens wool.