The Principles of Leather Manufacture

by H. R. Procter

Fig. 99.—Huxham and Browns’ Furnace.

The essential conditions which are to be observed in the proper burning of the tan are a sufficiently large grate-area, a correct and sufficient supply of air, and a combustion-chamber of very high temperature. It is consequently not possible to burn tan very successfully in an ordinary Lancashire or Cornish boiler, since not only the grate-space is too limited, but the water of the boiler prevents the upper part of the furnace from attaining a high temperature; and it is therefore difficult to get the damp tan rapidly into vigorous combustion. The difficulty may to some extent be overcome by mixing the tan with a proportion of coal, and by closing the ash-pit and employing a forced draught unless the chimney is a very powerful one. In this way large quantities of tan may be burnt, but without effecting any great saving of coal. The heating power of the tan is improved by the partial removal of its water by pressing, and this is almost essential where a special furnace is not employed.

Fig. 100.—Moenus Step-grate Furnace.

The answer to the question as to whether tan should be used as fuel in the wet state in which it is obtained from the leaches, or whether it should be previously pressed, depends upon the nature and quantity of the tan. Where abundant quantities of a fairly good material such as hemlock bark are to be disposed of, the cost of pressing is an unnecessary expenditure; but if it is desirable to obtain the highest value from the fuel, or if the furnaces are not well constructed for burning very wet fuels, it will be profitable to press the tan. Hydraulic presses have been used for this purpose, but those now commonly employed consist of powerful rollers arranged in the same way as those of the valonia-crusher ([p. 322]). The pressure is given by levers loaded with weights or fitted with powerful springs. The liquid which runs from these presses is of little value, as it contains such large quantities of finely divided material that it is almost impossible to filter it, and if run upon the leaches it chokes them and prevents their proper circulation. Much of the cost of pressing is caused by the labour of feeding it to the press, and this may be greatly reduced by the use of mechanical conveyors ([p. 325]) from the leaches. A tan press is shown in [Fig. 101].

Fig. 101.—Tan Press.

Sewage and other Waste Liquids.—The waste liquors from the different liming, bateing, puering, tanning, washing and other soaking processes are, without any doubt, the most troublesome of any of the side-products which are obtained in the manufacture of leather. In former times they were simply run into the nearest stream, but nowaday the various sanitary authorities and other similar bodies will only permit comparatively pure waters to be turned into a public stream or watercourse.

Various methods of effecting the necessary purification of the waste liquors from tanneries have been proposed at different times, and have been used with varying degrees of success. These methods may be divided into three heads: precipitation, followed by filtration or sedimentation land-treatment; and bacterial purification.

The first of these depends on the power of certain substances, such as alumina and oxide of iron, to carry down organic matter with them if precipitated in solutions containing it. The method usually consists in adding a sufficient quantity of lime to render the waste liquid slightly alkaline, and then treating it with some crude salt of aluminium or of iron. By this means a precipitate of aluminium or iron hydrate is formed, which encloses within itself a considerable proportion of the organic matter of the liquid, and after settling to the bottom of the precipitation-tank is drawn off as “sludge.” Various chemicals are sold under fancy names, such as “alumino-ferric,” “ferrozone,” etc., and have a composition not very dissimilar to that of crude sulphate of iron or alumina. In some cases by-products, such as the acid liquors used in preparing iron articles for “galvanizing,” can be used with advantage.