Ochres as By-Products
In the manufacture of certain chemicals, substances of divergent composition are obtained which are sold under the name of ochre and are used as painters’ colours. Whereas ochre, properly so-called, consists of either ferric hydroxide or ferric oxide in association with clay, lime, etc., the products now under consideration are basic ferric salts composed of varying quantities of ferric oxide in combination with certain proportions of sulphuric acid.
These ochres are obtained as by-products in the manufacture of green vitriol from pyrites, and in alum manufacture; and, according to their origin, they are classed as vitriol ochre, so-called alum sludge, and pit ochre. All the basic ferric sulphates of which they are composed form fairly large crystals, and, therefore, in most cases, the covering power is small. On this account the products are of low grade and are put on the market at low prices, for which reason they are largely used in making cheap paints.
Vitriol Ochre.—Commercial green vitriol is, for the most part, manufactured from native sulphides of iron. When many of these sulphides are piled in heaps and left to the action of the air, oxygen is gradually absorbed and green vitriol is formed which is dissolved out by rain and is collected in large clarifying tanks.
In the case of pyrites, however, the mineral must first be roasted in a current of air, since otherwise its conversion into green vitriol would only proceed in a very sluggish manner. In any event, the aqueous solution of ferrous sulphate has to be concentrated, by evaporation, to the point at which the green vitriol crystallises out.
Both in the clarifying-tanks and—still more so—in the evaporating-pans, a rusty-looking sediment forms at the bottom, consisting of basic ferric sulphate. This originates in the partial oxidation of the ferrous oxide (first formed) while the pyrites is exposed to the air, and since the quantity of sulphuric acid present is insufficient to saturate all the ferric oxide, basic salts are produced.
The yellow-brown sludge deposited in the pans during the concentration of crude green vitriol liquor, constitutes the product termed vitriol ochre, which contains varying amounts of ferric oxide, sulphuric acid and water, according to the quantity of ferric oxide resulting from the oxidation of the pyrites and the character of the latter, e. g.:—
| Ferric oxide | 65–70% |
| Sulphuric acid | 14–16% |
| Water | 13–16% |
Although the colour of these ochres is not particularly handsome, they can be transformed, by firing, into colours of fairly good quality. As this subject will be more thoroughly gone into when dealing with the preparation of the red iron pigments, the applicability of these ochres will only be casually referred to here. During the burning process, these ochres, of course, part with the whole of their contained water; and by protracted, high calcination, the whole of the sulphuric acid can also be expelled, so that finally nothing but pure ferric oxide is left.
Alum Sludge.—Solutions of crude alum always contain a certain amount of ferric oxide which settles down at the bottom of the pans during concentration. This sludge, too, consists of basic ferric sulphate, but is inferior in covering power to vitriol ochre, the crystals being of coarser grain. On the other hand, the ochreous sediment from the alum concentrating-pans has the valuable property of being readily transformable into red-brown to pure red tones by burning. For this reason, particular attention has been devoted to this sludge in a number of alum works.