It may also be pointed out that the provision of a proper system for pressing or circulating leaches does not prevent their being pumped off as frequently as desired, though this is generally to be avoided, since when the leach is emptied of liquor, the material tends to settle into a compact mass, which is not easy to percolate, and which is liable to shrink from the sides of the pit, thus causing the very trouble which it is desired to avoid. There are some advantages in taking the first and strongest liquors off the material in a separate tank, and then finishing the exhaustion in the press leaches, since many materials swell, and pack tightly when they are first wetted, but on the whole the method hardly pays for its added cost.
The press-leach system as above described is well adapted for the requirements of tanners, as its first cost is very small in addition to that of the construction of the leaches themselves; it extracts the bark well, and saves much labour in pumping, and greatly lessens the tendency of the pumper to miss pits in the series, to save time, when the master’s eye is not on him. Another advantage which is often important, is that when the leaches are full, much more than a single liquor can be run from the head-leach without pumping on; and similarly when they are run down to their lowest level, much more than a single liquor can be pumped on to the worst leach before it overflows. As the leaches flow slowly in comparison to the rate at which liquors can be pumped by a good steam pump, it is very advantageous to allow the pump to discharge into a liquor-tank raised to such a height that the liquor can be run from it into any leach at a suitable rate for the circulation, and it also enables liquors to be pumped without waiting till room has been found for them in the leaches. Similar tanks are very useful in running liquors for the yard, and especially for the suspenders in a sole-leather yard, enabling circulation to be kept up during the night, and at other times when the pumps are not running. They may also be used as filters for the suspender liquors by fitting them with false bottoms covered with a layer of nearly spent tan. The liquors may be distributed to the different pits and leaches by means of canvas hose-pipes, or, what is often more convenient, by overhead troughs, carefully levelled, and fitted with discharge valves where required. The latter are conveniently made of lead in a hemispherical form, resting on an indiarubber washer supported by a light brass casting, or a suitably turned rebate in a block of wood. (Cp. [p. 457] and [Fig. 79].) Such valves if good indiarubber is used, wear well, and are absolutely tight.
Fig. 79.—Valve for Liquor-Troughs.
In England, leaches are usually sunk in the ground, and are frequently made of brick and cement, or of large Yorkshire flagstones. Such leaches are somewhat costly but very durable. Square wooden pits, puddled outside with clay, are also used, and last well with cold, or even warm liquors, but will not stand direct steaming, the wood gradually bending, and allowing the clay to leak into the liquor, causing black stains. The large round vats of thick pine, and often holding 10 or 12 tons, which are generally used in the United States, stand boiling much better, and are frequently supported above a tramway or conveyor, into which the spent bark can be discharged through a manhole in the bottom. If this method is adopted, it must be remembered that bark, and indeed most other tanning materials, will not run through a hole like corn, but must be cast into it, so that unless the vat is of great depth, it is simpler and almost as easy to cast over the top. If the manhole is used, a central hole must be made in the false bottom, and this must be surmounted by a copper pipe made in sections of two or three feet, and reaching to the top of the leach. When the pit is to be emptied, the top length is removed, and the tan shovelled down the hole until the second length is reached, and the process repeated. The central pipe serves also for the circulation of the liquor when the pits are boiled, and may be used as the ascending pipe for circulating on the press-leach system.
The question of the influence of temperature on extraction is discussed on [p. 344], but except where a pale colour is all important, it is generally profitable to use a moderate degree of heat in extraction. In the opinion of the writer (which is supported by a vast amount of careful experiment) only the nearly exhausted leaches should be heated, not merely to avoid discoloration, but to extract the maximum amount of tannin. In American tanneries the boiling is frequently done by copper coils fixed below the false bottoms of the vats, but such coils are very costly, and, where weak liquors only are to be heated, seem to present no advantage over a well-arranged system of heating by direct steam in which care is taken that dry steam only is used, and that all water condensed in steam pipes, and usually containing iron, is removed by effective steam-traps. If steam is blown into cold liquor through an open pipe, a very disagreeable rattling and vibration is produced, which is not only annoying, but is very injurious to the leaches. This evil may be avoided by the use of “silent boiling jets” on the principle of the steam-jet water-raiser; and, following a suggestion of the writer, these jets may be used at the same time to circulate the water through the tanning material of the nearly exhausted vat, and so wash out the last traces of tan. The simplest way to accomplish this is to lower the boiling jet, directed upwards, and connected with a movable steam-pipe, into the eye of the leach (which is preferably central) so that the heated water flows over its top, and percolates downwards through the material to be washed. Two forms of these boiling and mixing jets made by Messrs. Körting are shown in [Figs. 80 and 81].
Figs. 80 and 81.—Boiling and Mixing Jets.
Batteries of closed copper extractors, worked on the press system, and similar to those used in extracting sugar from beetroot, have frequently been advocated, but are very costly, and have no other advantage over open vats than that the liquor can be forced through the series by pressure, instead of circulating by gravity. No advantage is gained by boiling under pressure, since even boiling in open vats has been shown to destroy tannin, darken the colour of the liquor, and increase the amount of insolubles, and higher temperatures are still more injurious.