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varying rates of speed, the coarse particles will be left in the first tank, the medium particles in the second tank, the fine particles in the third tank, and the very fine in the fourth or last tank.
It will be seen that levigation, while effective, is a very cheap process; for it only requires a cheap material, water, and the cheapest kind of colour plant, tanks, for carrying it out.
The details of the plant required or used in levigating at any particular works depends upon many factors, such as the position of the works, whether situated in the centre of a town, in a wide valley, or on a hill side. The facilities for obtaining the requisite supply of water is also a factor in determining the arrangement of the plant.
In Fig. 28 is shown in plan and elevation a plant suitable for
Fig. 28.-Levigating plant. levigating ochres, umbers, &c. It consists of 9 tanks, 8 of which are arranged in 2 series of 4, while the ninth is an odd tank. Another good arrangement would be one of 10 tanks, 9 arranged in 3 sets of 3, the tenth being an odd one.
In the odd tank, A, the crude material is thoroughly mixed with water; in Cornwall, Derbyshire, and a few other mining districts, this tank is known as the “buddle;" in this the very heavy stuff remains while the current of water, which is continually passing through, washes away the finer particles. From the buddle the water flows into the first settling tank 1; this being large, the current becomes retarded, and some of the material it contains settles out; from tank No. 1 the water flows into tank No. 2; this is made, or should be made, rather larger than tank No. 1, so that the current being spread over a larger
surface becomes slower, and, therefore, has less force, thus allowing the finer particles to settle out. From No. 2 the water flows to No. 3 tank, which is larger still; and, finally, to No. 4 tank, which is yet larger, so that very fine particles of pigment can settle out; when tank No. 1 is full the current from the buddle is diverted into the second series of tanks, while the colour or pigment in the tanks of the first series is settling out; when this is completed the water in these tanks is run off, and the pigment dug out, when the tanks are ready to be filled again. By having a set of four settling tanks, four qualities of ochres, or siennas, or umbers, may be obtained. When the second series of tanks are full, the current is again sent through the first series. By having three series a more perfect system can be adopted ; the current of water is sent through the first series until these are full, then through the second series while the material in the first is settling out; when the second is full, the current is diverted to the third series ; by this time the colour in the first will have settled out, and can, as explained above, be collected ; when the first lot of tanks are emptied of their contents they are ready to be refilled by diverting the current from the third set of tanks. Thus the three operations of filling, settling, and emptying can go on concurrently in a complete manner.
The tanks should be arranged, as shown in the drawing, one above the other, so that the water can run from one to the other; and the last tank of the series should be of such a size that it will take a day to fill it.
Fig. 29 is the plan of a levigating and drying plant which has been devised and constructed by Messrs. Follows & Bate. In this there are two sets of four settling tanks, T, T, T, T, between which is placed a drying kiln. The colour to be levigated is first thrown into à levigating mill, L, which is shown in Fig. 30, where it is ground under water; from this mill the wet colour is carried by the water into the buddle tank,
T, from thence it flows to either set of tanks as may be required. When one set of tanks is full, the current is diverted to the second set, and the colour in the first then settles out; when this settling is completed, the clear top liquor is run down the gutters, G, to the drain, D, and so away. The settled colour is then placed on the drying kiln, K, which consists of a large shallow tank placed above the flue of a stove, M; as shown by the arrows, the construction of the flue is such that the stove gases have to take a circuitous course before they pass away by the flue, F, to the works' chimney, thereby giving them every opportunity of parting with their heat to the wet colour and drying it more quickly and more efficiently.
In ultramarine making, where levigation forms an important part of the finishing process, the last tank is either made very large or a large number of small ones are provided, as the fine ultramarine takes a week to settle.
When space is available it is a good plan to have a set of large storage tanks; into these is thrown the wet pigment taken out of the settling tanks, and here it remains for some time; a further settling takes place, and the pigment becomes drier; this effects an economy of both time and fuel in the complete drying during the final stage. This saving of fuel is a matter of some importance in dealing with such cheap natural pigments as china clay, umber, &c.
The strength of the current of water is a matter that requires attention ; it too strong, it will carry over some of the coarse material from the buddle to the settling tanks, and will prevent the fine material from settling in the end tanks ; on the other hand, too gentle a current will not extract the whole of the valuable material from the crude stuff in the buddle; this is a detail which the operator can easily arrange.
If only small tanks are required they may be made of wood; large tanks may be built of stone flags, or of brick, if flagstones of sufficient size are not available. If bricks are used the inside of the tank should have a smooth surface, so as to facilitate the ready removal of the colour which has settled out. case arrangements should be provided for running off the clear top liquor from the settled pigment; this may be done by providing in each tank a set of holes kept stopped by plugs, which are removed when it is desired to run the water away. Or the water may be syphoned off by means of syphons provided for that purpose.
The amount of water required to levigate a pigment is a variable amount, depending on the nature of the sample of colour under treatment and on the plant used, so that no definite rules can be laid down.
The size of the tanks can be varied to suit the required output of colour, and is a point which every colour-maker must settle for himself, remembering, first, the deposited colour will contain about half its weight of water, and will therefore be heavier than the dry material ; second, that the total volume of the tanks must be much larger than that of the material which settles out from them. Another point is to make the tanks sufficiently strong to bear the pressure of the water, &c., they contain, which is great; thus a tank, 20 feet long x 5 feet broad and 4 feet deep will hold 20 x 5 x 4 = 400 cubic feet of water, or 400 x 62.35 lbs.
= 11:13 tons, which is the pressure exerted by the water on the bottom of the tank.
In some cases, before levigating, the material is ground, and in such cases the grinding is usually done under water; for this purpose special mills are made.
Such a mill made by Messrs. Follows & Bate is shown in Fig. 30. It consists essentially of a pair of edge runners working on a bed at the bottom of an iron cylinder or tank. The tank is kept full of water by suitable means, the grinding
Fig. 30.-Grinding mill. being, therefore, done under water. In the lower portion of the tank is a sliding door, which is used to run off the sludge, dirt, and grit. Running-off cocks are provided at the top of the tank and also about half way down. The top tap can be, and is commonly, used for running off the finer portions of the ground colour into the settling tanks; by regulating the speed flow through the tank a very fine grade of colour can be floated into the settling tanks. The sludge door will only require opening at the end of a day's work for the removal of the coarser portions of the colour. The machine is very efficient in use and is capable of dealing with the hardest of natural