Although, in former ages, this substance was held in low estimation as a pigment, attempts have been made in recent times to convert it, by suitable treatment, into a more valuable product; and these attempts have been crowned with success, affording another instance of how a high commercial value can be imparted to a waste product by proper manipulation.
(c) Calcining Ferric Oxide
In order to obtain a series of tones of colcothar, it is subjected to repeated calcination, but not by itself, since it would require an extremely large quantity of fuel to effect any change of tone in view of the very high temperature the material has already been exposed to in the sulphuric acid plant. If, however, salt be added, then a variety of tones can be obtained without recourse to any particularly high temperature. It is frequently stated that the only effect of the presence of salt is to keep the calcining temperature uniform, inasmuch as the salt volatilises at a strong red heat, and when that temperature is reached, the whole mass cannot get any hotter until the whole of the salt has passed off, all the heat applied being consumed in transforming the salt into the state of vapour.
As a rule, however, the amount of salt added does not exceed 6% of the weight of the charge to be calcined; and this quantity does not seem to be sufficient to keep the temperature at a uniform level through the several hours required for the calcining process. The author is therefore of opinion that the salt also has a chemical action on the material during the calcination.
As already mentioned, colcothar is by no means pure ferric oxide, but always contains basic ferric sulphate. Now, it is feasible that some reaction may take place between the basic sulphate and the sodium chloride at calcination temperature, with the formation of caustic soda, which, being a far more powerful base than ferric oxide, deprives the latter of sulphuric acid, sodium sulphate being formed. The chlorine of the salt combines with the iron to form ferric chloride, which volatilises at a glowing heat.
According to this hypothesis, therefore, the addition of common salt in the calcination of colcothar is less for the purpose of maintaining a uniform temperature within certain limits than for decomposing the basic ferric sulphate present and inducing the formation of a product consisting entirely of pure ferric oxide. The various tones obtained are due to the varying length of exposure to the heat.
The following method is pursued in the conversion of colcothar into iron pigments on a manufacturing scale. The crude colcothar from the sulphuric acid plant is ground, as finely as possible, in ordinary mills, and the resulting soft powder is intimately mixed with salt, 2, 4 or 6% being the usual proportions added. The calcination is ordinarily continued for six hours in the case of the mixture containing the largest amount of salt; but only two hours, or even one, for the other mixtures.
The operation is carried on in earthenware pipes, a large number of which (up to sixty) are built into a furnace. The latter is fired very carefully, the temperature being raised only very gradually, since experience has shown that much better coloured products are obtained in this way than by raising the mass quickly to a high temperature.
When incandescent ferric oxide is allowed to cool down with unrestricted access of air, the colour is not nearly so bright as when air is excluded during the cooling. Since air has no action on ferric oxide, this remarkable phenomenon cannot be due to the presence of the air, but probably to the influence exerted by the rapid change of temperature on the arrangement of the finest particles of the oxide. Nevertheless, some manufacturers hold that rapid cooling, with restricted access of air, improves the colour.
To exclude air from the ferric oxide during calcination, the open ends of the pipes are flanged and covered with close-fitting plates, which are luted with clay. The expansion of the internal air as it grows hot would burst the pipes unless a means of escape were provided, which consists in leaving small vent holes in the cover plates.