Levigation
The effect of levigation is based on the circumstance that bodies of greater density than water remain longer in suspension in that medium in proportion as the fineness of their particles increases. This treatment consequently enables the finer portions of a substance to be mechanically separated from the coarser. Levigation is extensively practised in colour works because it furnishes powder of finer grain than can be obtained by even the most careful grinding.
The appliances used for levigation may be of a very simple character, consisting only of several tubs or tanks, mounted in such a way that the liquid contained in one can be run off into the one next below. With this primitive plant, the material to be levigated is stirred up in the water in the uppermost tub and left to settle until the coarsest particles may be assumed to have settled down, whereupon the turbid water is drawn off into another tub, in which it is left to settle completely. When the clear liquid has been carefully drawn off, a fine sludge is left in the bottom of the tub, consisting of the fine particles of material mixed with water.
When a particularly fine powder is required, a single levigation does not always suffice, but the liquid in the second tub must be left to settle for a short time only, and then run into a third for complete subsidence.
Fig. 14.
A well-designed levigator for treating large quantities of powder is illustrated in [Fig. 14]. A stirrer R, driven by cone gearing, is arranged in a wooden or stone vat G. The levigating water enters close to the bottom of the vat, through the pipe W. When G is half full of water, the stirrer is set running, and the substance to be levigated is added. After a while, the water laden with the levigated powder begins to run off at A into the long narrow trough T1 provided, at the opposite end from A, with a number of perforations through which the water runs into the trough T2. From this it escapes through the perforations into the trough T3 and thence successively into T4 and T5, finally discharging into the large tank S.
The coarsest and heaviest of the water-borne particles deposit in the trough T1 finer particles settling down in T2, and so on in succession, until the water reaching the tank S contains only the very finest of all in suspension, these taking a long time to settle down to the bottom. The deposit in the upper troughs can be returned to the vat, whilst that in the lower ones will be fine enough to dry as it is. The residue in the vat is discharged through Z when the operation is finished.
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.
In the levigation process a few vessels of large size are preferable to a number of small ones. The nature of the material will determine whether any stirrers are required or not, these being unnecessary in the case of the pigmentary earths, which naturally remain a long time in suspension and therefore do not require stirring up.
The pulpy levigated material is taken out of the tubs, etc., drained (if necessary) and dried. The draining may be effected in bags, or—in large plants—filter presses or hydro-extractors. In these latter instances, pumps will be provided for feeding the sludge direct to the presses, and conveyors for delivering the pressed material to the drying-plant.