The Principles of Leather Manufacture

by H. R. Procter

Heating the weakest leach in the press-leach system promotes the even circulation of the liquor, since the warm weak liquor is much lighter than the colder and stronger liquors in the forward leaches, and so floats on the top, and presses the stronger liquor uniformly downwards. It also has the advantage that the liquors are cooled before they are strong enough for the yard, while in tanneries where all the leaches are heated, expensive tubular coolers are often employed. As the liquor cools, much of the colouring matters and reds dissolved in the hot liquor separate, and are filtered out by the tanning material, so that much brighter and lighter coloured liquors are obtained.

Fig. 82.—Sprinkler-Leach.

Sprinkler-Leaches, [Fig. 82], were formerly used in many tanneries and extract factories, especially in the United States. They were introduced by Allen and Warren, and yield a liquor which is at first very strong, but which becomes very rapidly weaker as the running is continued. These leaches are similar in principle to the mashing-tub and sparger of the brewer, but the process is not well adapted for tanners’ use, as the material is left too much exposed to the air, which is apt to cause oxidation and loss of tannin. It is also extremely difficult to completely exhaust the material without using an impracticably large volume of water. Sprinkler-leaches are arranged so as to spray the liquor, or water, on to the top of the solid material which is to be extracted at such a rate that it flows out just as rapidly as it flows into the vat. Some idea of the great amount of oxidation and consequent loss of tannin which takes place in this form of extractor may be obtained when it is remembered that this same method is now used for the destruction of sewage matter by spraying it on to beds of coke so that it may be mixed with as much air as possible before it is attacked by the bacteria of the coke-beds (see [p. 473]), and also to oxidise weak alcohol to acetic acid in the “quick vinegar process.”

So far as extraction is concerned, there is no difference in principle between the methods adopted by the tanner and the extract manufacturer, though the latter usually works on a larger scale, and not unfrequently, in order to increase his output, or the gravity of his extract, employs a higher temperature. This is probably justified by practical considerations in the manufacture of extracts from very low-grade materials, such as oakwood, which only contains 2 to 3 per cent. of tanning matter, or even of chestnut wood which is somewhat stronger, but it is one of the causes why decoloration of the battery liquor is generally necessary.

Dried blood is chiefly used as the decolorising agent, but a paste of blood-albumen has been recently placed on the market, which is said to be free from several of the disadvantages attending the use of the crude material.

The liquor to be decolorised is run into a mixing vat fitted with a steam coil capable of raising the temperature of the liquid to at least 80° C., and usually provided with a simple rotary stirring gear. The liquor, as run into the mixing vat, must not have a temperature of more than 48° C. (118° F.) nor a strength of more than about 20° Bkr. (sp. gr. 1·020).

The blood or albumen dissolved in a little water, is added to the contents of the vat, which are then well mixed, and the temperature is raised to 70° C. when the albumen coagulates and carries down much of the colouring matter. The solution is run into another tank where the precipitate is allowed to settle, and the clear liquor is then drawn off for the evaporation. The muddy portion, about 8 inches in depth, is pumped through filter-presses (which can be cheaply constructed of wood), the clear liquors going to the evaporators and the press-cakes being dried for manure.

In addition to blood-albumen, several other substances, such as lead acetate (sugar of lead), salts of alumina, casein and other albuminous matters have been employed in the decoloration of extracts, but they are by no means so efficient as albumen.

Decolorising always causes a loss of tanning matter, some of this being carried down with the precipitated colouring matter; and is for this reason to be dispensed with whenever its use is not really necessary. It may often be avoided by careful extraction at moderate temperatures, and this is especially to be aimed at in the case of strong tanning materials, which easily yield battery liquors of much greater strength than 20° Bkr., and which thus, if they can be sent direct to the evaporator, save cost in evaporation, which is often an important consideration.