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.
The drum is enclosed in a casing of somewhat larger diameter, which may be of any convenient material. The bottom of the casing is preferably tapered slightly downward, and is covered, at its lowest part—below the bearing of the drum—with a sieve communicating with a pipe through which the ejected liquid is drained off.
When a liquid, containing solid matter, is fed into the drum, which is already running at high speed, the liquid is thrown, by the centrifugal force, against the peripheral surface of the drum and escapes through the perforations, leaving the solid matter behind. Where large crystals are in question, as for instance in sugar factories, the centrifugal machine can be employed without any additional precautions, the liquid being expelled and the crystals being practically dried by keeping the machine running a short time longer. In the case of the pulp obtained by levigating colours, however, this procedure would result in failure, because the fine solid particles would be ejected along with the liquid and the drum would be left quite empty.
In this case it is therefore necessary to provide means for retaining the solid matter in the drum, and allow only the water to escape, with which object the drum is lined with a bag of closely woven fabric, open at the top and fitting snugly against the inner surface of the drum. When the drum is first started, the ejected liquid is milky, no fabric being sufficiently close to retain all the extremely fine solid particles present. In a very short time, however, the liquid will begin to run away perfectly clear, this occurring as soon as the pores in the fabric have become so far obstructed by the projected solids as to allow water alone to pass through. The milky water is then returned to the feed tank and run slowly into the machine. The water is very quickly expelled, and the colour remains in the drum as a stiff paste, of sufficient consistence to be moulded into lumps of any desired shape. The use of the hydro-extractor may be particularly recommended when ample motive power is available and accelerated draining is desirable.
Fig. 17.
[Fig. 17] illustrates a modern type of hydro-extractor with bottom discharge and suspended drum, the shaft of which is coupled directly to an electro-motor.
Filter-presses.—Whereas the hydro-extractor is only used in particular cases for the purpose of the earth-colour manufacturer, the filter-press enjoys more extensive application. Every filter-press is composed of a number of closely fitting press frames, held together by the pressure of a screw. These frames, when assembled, form chambers provided with inlet and outlet openings. Suitably shaped and stitched filter-cloths are secured inside the chambers, and the sludge to be filtered is run into the press from a high-level tank. The water passes through the filter-cloths and runs off, whilst the colour earth gradually fills the chambers. When draining is completed, the press is taken apart and emptied. In this way the earths are obtained in the form of more or less dry cakes, which are then put through further treatment or dried.
Fig. 18.
[Fig. 18] shows a Dehne filter-press suitable for the earth-colour manufacturer. Wood internal fittings are often used, because wood does not affect the shade of the colours; but, wherever the nature of the materials admits, iron presses are to be preferred on account of their greater durability and the certainty of the joints continuing tight. The finer the grain of the levigated colour, the more difficult the expulsion of the water; but as a rule, a pressure of 115–195 inches, water-gauge, will be sufficient.
If the sludge be run into the press from a tank at sufficient height, two charges can be worked in a day, but the cakes will not be as firm as butter of medium hardness. It is better to pump the charge into the press by means of a special diaphragm pump. The drainage is then incomparably quicker, the cakes will be formed in about an hour and will also be drier. A good deal, however, depends, naturally, on the nature of the earth colour.
If the colour contains acid, alkali or salts, the filter-cloths can be washed by flushing the press with water under pressure. The cloths are made of specially fine cotton fabric. The press-runnings, which are never quite clear, are collected in a clarifying tank, where they are treated with lime and kieserite, whereby gypsum is formed, and the mass is put through a filter-press, which retains the solids and leaves the effluent clear.
Fig. 19.
Filter-cloths which have become choked by use are spread on a table and scrubbed with water, or else washed in a special machine ([Fig. 19]), consisting of a rotary drum, with belt drive, the rotation circulating the water in the interior trough and enabling it to extract the dirt from the cloths. The flow and discharge of the water are controlled by valves, and the water may be warmed by admitting steam into the machine. The size of the washer depends on that of the filter-cloths.
From the press, the cakes of colour are conveyed to the drying-plant, usually by the aid of automatic machinery.
Drying Appliances.—The stiff paste or cakes from the hydro-extractor or filter-press can be shaped, but require to be dried before they are put on the market. Drying is a wearisome operation, the finely divided material taking a very long time to dry completely, even during the summer months, whilst in winter it is almost impossible to get certain colours—such as ferric oxide colours and levigated clay—quite dry in the air, the inside of the lumps remaining soft and pasty after lying for months.
The only way in which this troublesome delay in the completion of the operation can be overcome is by artificial drying; but as the employment of artificial heat entails expense, it is necessary to carry on the process with the smallest possible outlay, in view of the low commercial value of most earth colours.
Long experience has convinced the author that the arrangement of the drying-rooms in many colour works is based on entirely wrong principles, and that a great portion of the heat furnished by the fuel is wasted. For this reason the description of a properly arranged drying-room will be welcomed by a number of readers.
It is a well-known fact that hot air is lighter than cold. Consequently, when a room is artificially heated, the highest temperature will be found just under the roof or ceiling, and articles placed in that part of a heated room will dry much faster than those near the floor. If the drying-room is heated by an ordinary stove, articles placed on a fairly low level will only dry very slowly, because the hot air flowing from the stove tends to ascend.
In order, therefore, to utilise the entire space of the drying-room, it is necessary to place the heating apparatus in such a position that the whole of the room will be warmed as uniformly as possible. The stove should therefore be situated in a chamber underneath the drying-room proper.
Fig. 20.
Because air that is already saturated with moisture cannot take up any further quantity, care must be taken to remove the damp air continuously from the drying-room, and to replace it by dry air. This may be effected by suitably designed ventilation, on the lines shown in [Fig. 20], which represents a drying-room arranged in such a way as to provide for all the above-mentioned contingencies, and ensure continuous drying.
The heating apparatus is located in the cellar, and consists preferably of a slow-combustion stove comprising a cast-iron cylinder, with an air inlet A (with sliding regulator T), for the air of combustion, and a shoot F at the top, through which the stove is fed with fuel—preferably coke, on account of its great heating power.
The stove is surrounded by an iron or brick shell M, having two flues R and R1 leading to the chambers I and II, where they terminate in register cowls K, which can be adjusted, by turning the handles h, so that when the slots o in K coincide with corresponding slots in the end of the pipe, the maximum amount of hot air from the stove is delivered into the drying-chambers; and, by suitably adjusting the cowls and the draught through the fire-door T, it is possible to regulate the temperature of the chambers to within one degree of the thermometer scale. When only one of the drying-chambers is required to be heated, the register in the other is closed, and the whole of the hot air is delivered to the first one. With this arrangement none of the heat is wasted, and the contents of one chamber can be dried while those of the other are being removed and replaced.
The moisture-laden air from the drying-chambers can be led direct into the stove chimney. When coke is used, the flue gases consist almost entirely of carbon dioxide. If the vent pipes are led from the top of the drying-chambers into the chimney, the hot gases ascending the latter induce a strong draught in the chambers and carry off the moist air into the open. These pipes, also, are fitted with registers, which, when suitably adjusted, assist in the maintenance of a uniform drying temperature.
The colours to be dried are spread on trays laid on suitable racks in the drying-chambers; and, by carefully planning out the available space, a very large quantity of colour can be quickly and completely dried in a comparatively small plant. The cost of the fuel is so small as to be more than counterbalanced by the saving of time.
The heating arrangements in drying-rooms are capable of improvement in many respects, especially where steam is at disposal; and in such cases, it is better to substitute steam heating for a fire. It will then be necessary to put in a good fan, or other device, to ensure the removal of the moist air. An excessive room temperature—above, say, 50° C. (122° F.)—is not only superfluous, but in many cases injurious, because, apart from the fact that some colours change in shade when over-warmed, an unduly high temperature causes the surface layers to dry very quickly and form a crust which prevents the escape of water vapour from the interior of the material.
Another form of drying-plant for earth colours is the drying-floor, a large room with a rammed concrete or stone floor, intersected with brick flues (about one foot square), covered with iron or concrete slabs and conveying hot flue gases from a furnace. These floors are particularly suitable where there is a possibility of utilising an existing supply of hot flue gases.
Drying-tunnels are specially adapted where large amounts of material have to be dried. The tunnels are built of brick and provided with a rail track on which the trucks carrying a series of trays laden with colour are run. As the trucks move slowly forward, they are met by a current of hot air which dries the charge. The tunnel is kept filled with laden trucks, each fresh one introduced pushing a finished one out at the further end.
In many cases, drying troughs are also useful. These are long, semicircular, jacketed troughs of boiler plate, hot air or steam being passed through the jacket space. A worm conveyor keeps the contents moved forward, turned over and mixed to facilitate drying.
Mention may finally be made of vacuum drying-cupboards, which are heated, air-tight chambers, for the material, in which the air is partially exhausted, thus increasing the rate of evaporation of the water and causing the materials to dry quickly at a much lower temperature than otherwise.