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.
Draining and Drying
The levigated colour earths form a stiff pulp containing a large quantity of water, which can be eliminated in various ways. Usually, the mass is dried by spreading it out thinly on boards and leaving it exposed to the air until it has become solid; or else it is only left long enough to acquire the consistence of a thick paste, which is then shaped into cones or blocks, which are allowed to dry completely in an airy place. If the colours are to be sold in the form of powder, the dried lumps are crushed.
To accelerate drying, the pulp may be put through a hydro-extractor, or dried in hot-air stoves or rooms. As, however, this last method entails special appliances and also expenditure, this acceleration is only resorted to when rendered necessary by special conditions.
The Hydro-extractor.—When a substance is set in rapid rotation, it tends to fly away from the centre at which the rotational force is applied. The centrifugal force thus coming into action increases with the velocity of rotation and with the distance of the substance from the axis of rotation.
The centrifugal hydro-extractor consists, therefore, of a vessel in rapid rotation; and if a liquid be introduced into such vessel, it is projected with considerable force against the peripheral walls. If the peripheral surface be perforated, the liquid portion of a charge consisting of liquid and solid matters will be ejected through the perforations, while the solid matter remains inside. As a rule, a few minutes’ treatment in a hydro-extractor is sufficient to separate the water from a thin pulp so completely that the solid residue is in an almost completely dry state. A hydro-extractor which, though of an old pattern, is well adapted for the purposes of the colour-maker, is shown in [Fig. 16].
Fig. 16.
The drum A, which revolves easily on a vertical axis, is of metal, and is provided with a large number of fine perforations on its peripheral surface. It can be rotated at high speed by means of the crank f and pinions d, e, or by the fast-and-loose pulley a b connected with a source of power. To prevent any of the charge from being projected over the rim of the drum, the upper edge is turned over so as to leave only a comparatively small opening at the top. The lower end of the drum shaft carries a strong steel spindle, which must be carefully machined and enable the drum to revolve as easily as possible. This is essential, because even small machines require a comparatively large amount of motive power—which is not surprising in view of the high speed at which the drum has to revolve in order to perform its functions.