It will be evident that the fineness of the product depends on the number and length of the troughs T, the larger these factors the more delicate will be the particles remaining in prolonged suspension in the liquid.
Many earth colours require no treatment beyond levigation to fit them for use in paints. This is the case with, e. g., the white clays; and certain grades of ferric oxide, which occur native in the state of fine powder, may also be included in this category. In many cases, however, if large quantities of a finely pulverulent mineral be stirred up with water and left to stand, the deposited solid matter forms such a highly coherent mass that it can only be distributed in water with difficulty, the fine particles adhering so firmly together that it is hardly possible to stir them up again completely in the liquid by means of a paddle.
Nevertheless, this can be easily effected by using a special appliance of the kind employed by starch manufacturers for a similar purpose, viz. the levigation of starch. This apparatus is designed in such a way that the pulpy charge of material is gradually and completely disseminated in the introduced liquid.
Fig. 15.
[Fig. 15] shows a device of this kind, consisting of a circular vessel provided with a step bearing for a vertical shaft driven by cone pinions. The lower part of the shaft is provided with a thread, on which a nut is adapted to travel up and down. By means of rods, this nut is connected to a wooden cross-bar provided with stiff bristles on its lower face. A horizontal handle is attached to the nut. The water is admitted through the pipe on the right.
In working the apparatus, the shaft is rotated and the handle held firmly, thus causing the nut and attached cross-bar to rise to the limit of its travel. The levigating liquid, mixed with the material under treatment, is then admitted, until the vessel is full, and when the solids have completely subsided, the clear liquid is drawn off, and the operation is repeated until a thick layer of sediment has accumulated on the bottom of the vessel.
To levigate this, the cross-arm carrying the bristles is lowered until it just touches the surface of the deposit, and a continuous stream of water is admitted through the pipe at the side. The bristles gradually disseminate the upper layers of the sediment in the water, which becomes turbid and is then drawn off into another vessel, cement-lined pits being used in the case of large quantities. When the brushes no longer encounter any of the sludge, the cross-arm is lowered sufficiently to stir up another layer; and in this way, large quantities of solid matter can be distributed in water. If the cross-arm is rotated at low enough speed, the coarser particles of material keep on settling down again, and the collecting vessels will receive only the finest particles.
In addition to the mechanical separation of coarse and fine particles, levigation accomplishes another purpose, namely that the prolonged contact of the treated material with water dissolves out any admixed soluble constituents which might affect the quality of the colour, the latter being left in a purified condition.
For successful levigation it is essential that the charge should be in a sufficiently fine condition at the outset. Clayey raw materials require no preliminary treatment other, perhaps, than passing them through a disintegrator, whereas hard, crystalline substances must first be ground in a wet mill, such as an edge-runner mill with stationary bed, into which the materials are fed with an admixture of water, provision being made for keeping the charge under the runners all the time. The crushed material is screened previous to levigation.