A description of the older apparatus for lixiviating black ash is only interesting to the alkali antiquarian. The ingenious method at present adopted was originally the invention of Shanks of St. Helens, and leaves little to be desired. It depends upon the different specific gravities of the water and liquor. The tanks vary in size with the experience and judgment of different manufacturers. Good dimensions may be taken to be 10 ft. long by 8 ft. wide and 7 ft. deep. They are usually arranged in sets of four—four tanks of the size named being sufficient for three hand ball furnaces, or a decomposition of 60 tons of sulphate per week—and are formed by placing partitions in one long tank. The sides, ends, and bottom are formed of :--inch iron plates well riveted together with angle-irons at all the corners. The bottom is sometimes fiat, sometimes assumes for each tank the shape shown in the drawings, sloping down to a drainer, or "well," which runs along the centre of the tank. In either case a lining of 4i. bricks, on edge, is given to the bottom, leaving w cross drainer, as shown in Fig. 249. Over both longitudinal and cross drains are laid loose sheets of iron, well perforated. In each drainer, reaching just below the false bottoms, are fixed two " jugs," one of Ilhich communicates with the next tank and the other with a spout running along the whole range of pipes, which conveys the strong liquor to the settlers. These jugs consist of metal pipes, 3 inches bore in the lower part, widening to 4 inches in the upper part—shaped in fact like a pump. By means of a plug and seat arranged just below the outlet pipe, or "nose," communication with the neighbouring tank or settlers can be made or cut off at will. The outlet pipes of the jugs along the front of the tanks—those by which the strong liquor runs to the settlers—are placed slightly below the level of the communications between the tanks. By a pipe running back from the fourth to the first tank the whole operation is made con tinuous, each one becoming in turn the " strong " tank, an intermediary, and the "weak," or exhausted tank. Water is supplied to the surface of the tanks by any convenient apparatus, and is heated, before it touches the liquor, or balls, by waste or other steam. Some manufacturers put the steam direct into the tank, a method causing loss through the temperature of the tank at that particular spot being raised too high, and the sulphides dissolved. Finally, in the drainer of each tank is fixed a pipe and cock to carry off the wasto liquors.
The plan of working is as follows :—The tanks are filled with lumps of ball—not too large—to within about a foot of the top, a layer of dry ashes being placed upon the bottoms. Water heated to about 100° F. is then run on, which, percolating through the mass of black ash, rises up the jugs, and that one which communicates with the settlers being open, finds its way out in the shape of strong soda liquor. At first this liquor will test about 50° Tw., but the strength speedily advances to 55° or even 60°, and then rapidly falls down to 40°- The plug is then placed in its seat, and the tank left to itself for awhile. After a quarter of an hour or so the plug is withdrawn and a second "running " of liquor taken off, now testing up to 48° or so. Each tank should bear a third tapping, the liquor never being allowed to go to the settlers below 38°. This outlet pipe is
then closed, and the communication between the first and second tanks opened, The liquor from the first tank flows over, percolates through the balls with which the tank is filled, and is drawn off to the settlers in the manner described. In the meantime a steady flow of water upon the balls in the first tank is kept up. This operation is repeated with all four tanks. By the time the last is reached, a sample of the liquor drawn from the jug of the first tank will be found to test not more than 1° or 2° Tw., showing that all the soda is, practically, dissolved out. The water is then turned upon the second tank, the first being shut off. The spent liquors are drawn off through the pipe at the bottom and run away, leaving a mass of insoluble residue—tank waste—about half filling the tank. This is shovelled out, the drainer cleaned, a fresh layer of ashes sprinkled over the hottom, and the tank is ready to receive a supply of broken ball and the liquor to be dissolved by the returned liquors from the fourth tank. Sometimes the weak tank is " run down," as it is called, to 0° Tw., but between 2° and 0°, the sulphides dissolve more rapidly than the carbonate of soda and spoil the liquor. Fig. 249 gives a plan of the tanks, ahowing the bottom drains and.false bottoms ; Fig. 250, elevation and sectional elevation through the line A, B, Fig. 249 ; Fig. 251, a section through the line C D. The last drawing shows n set of tanks aa at work. Nu. 1 tank is just filled with ball, and is receiving the liquor running round from No. 4. No. 2 is empty, No. 3 spent, No. 4 about half through its work.
Tho working of the tauks is an operation requiring considerable care and judgment, much of the success of the after processes depending upon the securing of good liquor. The most important points are, to keep the temperature as low as possible ; to take off the strong liquor speedily, that it may be kept from contamination, and not to allow the strength of the liquor running to the settlers to fall below 38° Tw. With regard to the first point, the water should not be run upon the tanks hotter than 32° (90° F.) in summer, and 42° (110° F.) in winter. The temperature of the mass in the tanks has always a tendency to rise owing to the hydration of the lime and other chemical reactions going on. If the liquora show any greater heat than 65° (150' F.), it is safe to conclude that the water has been run on too hot. Both strong and weak liquors, and tank waste should be tested daily—at least once on eaeh shift. The waste: should present no lumps of undissolved ball, and should be of a dirty green colour. It should be tested at any rate for soda, and from time to time should bo subje,cted to complete analysis. The amount of soluble aotla should not exceed O. l5 per cent. A fresh sample will give about the following composition :— Per cent. Per cent.
Calcium sulphide .. 37.0 Sodium carbonate 0.25hydrate .. .. 9 . 0 Iron, alumina, and magnesia 7.0 „ carbonate .. .. 16 . 0 „ sulphate .. 6 . 0 Silica, •Ire. 5.0 Sodium sulphide .. 0.5 It is of the greatest importance to keep both sulphide and carbonate of sodium as low as possible. A good manufacturer will not allow even as mueh of these salts as set down in the above analysis, 0.25 total soda being the point to be aimed at.