The Principles of Leather Manufacture

by H. R. Procter

Fig. 77.—Section of Leach-Bottom.

As a strong liquor cannot be made by the use of a single leaching-pit, a series of pits are now always employed, and it is the leaching, systematic or otherwise, which determines how much of the total tannin will be thrown away and lost in the “spent tan.” In the case of properly extracted materials the “spent tan” will not contain more than one per cent. of tanning matter, but the degree of extraction which is profitable is dependent on the tanning material employed and the class of leather to be produced.

The system of leaches now considered to be the best is based on the “continuous” process of extraction. Of its different forms, the “press-leach” is the simplest and in most cases is all that is required.

Fig. 78.—Plan and Section of Battery of Press-Leaches.

A plan and vertical section of the leaches is shown in [Fig. 78]. Assuming that the leaches have been working for some time and that the liquor in the strongest leach has been run off to the tan-pits, or in the case of manufacturing extracts to the decolorising tanks or evaporator, the last vat in the series is now filled with water or spent liquor, which may be heated by steam if desired, and this water, which completes the exhaustion of the material in this vat, forces the liquor forward in the whole series, so that it gets stronger and stronger as it passes from vat to vat. The very weak liquor remaining in the last vat is now pumped into a spare pit, or on to the next stronger vat, pressing the liquor forward as before; the vat is emptied of the spent material and refilled with new, and now becomes the head leach; and the strongest liquor is pressed on to it by running water or weak liquor on the weakest vat.

As regards the construction of such a “battery” of leaches, details will differ according to whether the usual English square sunk pits, or the American form of circular tub-leaches is employed. In the former case the vertical spouts connected with the space under the false bottoms are usually made of wood, like the old fashioned “eye,” and placed at one side or corner of each pit, and connected with the top of the next pit by a short trough which may be open above or covered as preferred. Both eyes and cross troughs must be of ample size, so as not to check the running of the liquor, and for a set of six or eight leaches, the bottom of the cross trough should be at least 10 or 12 inches below the actual top of the leach, which should not be filled with material above that level. The object of this is to allow of a sufficient fall from the first to the last leach. Means must be provided for the temporary closing of the cross-trough between the vats which form the first and last leach. On a very small scale, this may be done with a plug; sliding wooden doors are convenient, but difficult to keep tight. A hinged or sliding door held against an indiarubber facing by a wedge or toggle-joint would seem a practicable device.

If round tub-leaches are employed, the vertical connection may be similarly made with a wooden trough, but copper tubes are almost essential for the cross connections. If a vertical copper eye in the centre of the leach be provided for boiling, or for emptying the leach ([p. 334]), it may be utilised for the upflow by connecting it with the cross pipe with a thin copper pipe of large diameter, which must be movable for the purpose of casting the leach. A joint like that of a stove-pipe will probably prove sufficiently tight, but if necessary may be made tighter by rolling an indiarubber ring over it.

Six to eight leaches is generally a sufficient number to form a press-leach “battery.” If more are connected in one series it will usually be necessary to assist the circulation, either by pumping an intermediate leach, or by one or more pumps on the Holbrook system, in which a power-driven pump of simple construction is fitted in the eye of the leach. It is hardly necessary to note that the liquor must run downward through the leaches, and up through the vertical pipes, in order to prevent mixture of the weaker with the stronger liquor.

Several additions and modifications to the system have been made with a view of obviating the so-called “channel difficulty.” There is always a fear on the part of some tanners that the liquid in the leaches may push the material aside and form channels through it, thus preventing proper extraction of the tanning matter. In the author’s opinion this evil has been greatly exaggerated, as, unless the liquid be pumped from the leaches at a very rapid rate while they are in circulation, it is not at all easy for the formation of such channels to take place. In any case it can be entirely avoided by turning over the material in the leaches occasionally, so as to lighten it somewhat and rearrange it a little.