Calcining (Burning) Ochre
In many places ochre is only put through a very simple mechanical preparation before being sold for pigment, namely left to dry in the air so that most of the uncombined water evaporates. No matter how this drying process is protracted, however, it is impossible to get rid of all the water in this way, a certain proportion being retained by the hygroscopic action of the ferric hydroxide, and to expel this the mass must be heated to above 100° C. Drying is usually succeeded by pulverising and sifting the loose earthy mass, which is then ready for sale.
When the ochre contains sand or stones, this treatment is not sufficient, and levigation is necessary. No particular trouble is involved, the mineral being fairly heavy as the result of its content of ferric hydroxide. A simple method of treatment suffices to improve the value of the ochre considerably, and enables a grade that is not particularly bright-coloured in its natural condition to be converted into products of very handsome tone and various shades. This treatment consists in heating the raw ochre to a definite temperature, during which process the colour changes progressively, and any desired tone can be obtained by suddenly cooling the hot mass.
The reason for this phenomenon is that the higher the temperature, the larger the amount of water driven off from the ferric hydroxide, until finally, when a very high temperature has been reached, the whole of the water is expelled, and the ferric hydroxide is transformed into ferric oxide. The hydroxide is brown, whereas the oxide, provided the temperature has not been raised too high, exhibits the characteristic colour known as “iron red.”
Consequently, the colour of moderately calcined ochre ranges through a whole scale from brown to red; and the higher the temperature employed, the redder the tone. If the heating be protracted after all the hydroxide has become oxide, the latter undergoes molecular change, increasing considerably in density and altering in colour; and after very prolonged heating, the colour finally becomes violet.
The calcination, or burning, of ochre is ordinarily performed in a very crude manner. The mineral is crushed to the size of peas, and spread out on an iron plate which is made red-hot. As soon as the ochre has reached the desired shade of colour, it is dropped into a tub of water and then crushed to powder. The calcination requires great experience on the part of the operator, because so long as the product is hot, it has quite a different colour from that assumed on complete cooling. Since only comparatively small quantities of ochre can be treated in this way, and the operation unnecessarily increases the cost of the product, owing to the large consumption of fuel, it is highly desirable to employ a simple calcining apparatus capable of treating large quantities.
Such an apparatus may consist of an iron drum, mounted with a gentle slope inside a furnace, from which it projects at both ends. A shaft carrying a sheet metal worm is rotated inside the drum; and the whole apparatus is very similar to an Archimedean screw.
When the iron drum is raised to a strong red heat, and small quantities of ochre are fed continuously into the upper end of the drum, the rotation of the worm will push the material forward, and contact with the glowing sides of the drum will produce the necessary calcination, the degree of which can be modified by altering the speed at which the worm is turned. The calcined product is discharged at the lower end of the drum, either into a vessel of water, or, if only moderate heating has been applied, direct into a collector.
Fig. 28.
[Fig. 28] represents an apparatus designed by Halliday for the dry distillation of wood waste; but, with slight structural modifications, it can also be used for calcining ochre. The material to be heated is introduced, in small pieces, into the feed hopper B, and is carried downward, by the worm C, into the red-hot drum A, through which it is propelled by the worm D until it drops out, at F, into the tank G. The length of time the material is subjected to calcination depends on the speed at which the worm D is run. The pipe E carries off the water vapour expelled from the charge.
In order to obtain a uniform product when ochre is calcined in an apparatus constructed on this principle, it is necessary that the material introduced should be fairly regular in size, a condition which is easily fulfilled by squeezing the freshly dug ochre between fluted rollers, and then passing it over a series of screens, each grade being then calcined separately.
Moreover, the apparatus is only suitable for calcining at medium temperatures; and when highly calcined products are in question, the operation is best performed in fire-clay cylinders, or in thick cast-iron drums, similar to gas retorts, built into a furnace.
Other devices for calcining ochre will be described later.