Chapter XXVIII: Waste Products and Their Disposal

The products which are of no direct value to the tanner and currier in the manufacture of leather, and which are nevertheless obtained in fairly large quantities, are of very varying characters. In the present chapter, the most important of them will be described, and some of their uses mentioned.

_Hair_ is removed from the skin of the animal in the process of depilation (p. 143) in the form of a wet sodden mass, containing a considerable amount of lime when the skin has been through the lime-pits.

As white hair is the more valuable, care should be taken in the unhairing to keep it separate from the coloured. It is washed first in plain water to get rid of as much of the lime as possible, and then in water containing a little acid. Hydrochloric acid is often used for this purpose, but sulphurous acid (p. 25) is preferable as it has a slight bleaching action on the hair. The acid neutralises and renders soluble the lime which still remains in the hair, so that it can be easily removed by washing with water. In many tanneries, hair-washing machines are used. The washed hair is dried by laying it out on frames; or preferably, the greater part of the water is first removed by a centrifugal drier, or by pressing, and the drying is completed in a drying room, the temperature of which is a few degrees higher than that of the outside air, and which is provided with a fan or some other appliance for mechanical ventilation. Tables of wire gauze on which the hair is spread, and through which the warm air of the room is drawn by a centrifugal fan, are the most effective.

Coloured hair is sometimes washed and treated like the white hair, but is usually sold direct to plasterers, in which case there is no necessity to remove all the lime and other impurities which the hair contains. A considerable amount of hair is also sold to iron founders, who use it in preparing cores and in loam-casting. The loose lime may be effectively beaten from dried hair by passing it through a disintegrator with one of the grates removed.

_Fleshings and Glue-stuff._--The various scraps of fat and flesh, more or less free from actual hide substance, are usually worked up for glue, though if they cannot be sold for a fair price it will pay to boil them in order to recover the fat they contain. If this is to be done, the fatty portions may be thrown out at the beam and not mixed with the fleshings as in the ordinary way. Before boiling, the fat is treated with sulphurous, sulphuric or hydrochloric acid, sufficient to neutralise the lime present. The boiling should be carried on very gently, so as to allow the fat to rise without emulsifying with the gelatinous matter. For boiling, open steam may be used, but in this case the size formed will have little value; on the other hand, if sulphurous acid has been used and a wooden vat with a copper steam-coil be employed, really good glue may be obtained, and the slight trace of bisulphite which it may contain will prevent its putrefaction. Except under special conditions it will not pay to make glue on a small scale in England, as its value depends much on its appearance, and the necessary plant is somewhat expensive. In some places, however, size can be sold to advantage. Fig. 98 shows a glue-boiling plant.

After separation of the fat by skimming, the clear size is run off from the residual matter into wooden cooling troughs about 5 feet long by 9 inches deep and 15 inches wide, in which it is allowed to set (Fig. 92, p. 425). Great care is required that both size and coolers are quite sweet and free from putrefaction, the coolers being frequently washed with sulphurous acid solution or fresh milk of lime. The jelly is cut out in blocks, and sliced into cakes of appropriate thickness by means of a series of frames like slate-frames which fit over the block of glue, and between which a wire or thin blade stretched on a saw-frame is inserted to cut the glue into sheets. In some factories a machine is used, with a series of parallel blades against which the glue-block is pushed. The sheets are afterwards separated by girls and laid to dry on nets, on which they are frequently turned. When dry, the cakes may be washed with warm water to remove any adhering dirt, but this causes some loss of weight, and in many cases it pays better to dry in a stove until quite hard, then grind in a disintegrator and sell as “size-powder,” in which appearance counts for little if the colour and strength of the size are good.

_Fat._--The fat, whether obtained in the manufacture of glue, or by boiling the fleshings and shavings for its recovery alone, is skimmed from the surface of the heated liquor, and should afterwards be freed from gelatinous matter by washing it with hot water in a tub and running off the upper layer after allowing the water to settle out. The fat thus obtained is a light-coloured grease of buttery consistence.

There are various other sources of waste fats which may be considered here. If glue is made from dried glue-stuff without previous treatment with acid, the fat skimmed off the pans, though dark in colour, will be neutral or alkaline, and a considerable additional quantity of fat and free fatty acids may be obtained by reboiling the “scutch” or refuse with open steam in lead pans with the addition of water and enough sulphuric acid to render the contents of the pan distinctly acid. This grease will be dark and of unpleasant smell from volatile fatty acids, but its odour may be to a considerable extent improved by blowing air and steam through it, and washing with water, or by heating to a temperature somewhat above the boiling-point of water for a considerable time. The same sort of treatment may be applied to the fat pressed out of sheepskins, and to that obtained by boiling currier’s shavings with water and a little acid.

Recovered fats may be separated into a tolerably firm grease suitable for use instead of tallow in currying, and an oil not unlike neatsfoot oil, by melting, allowing to cool slowly to a soupy consistency to promote the crystallisation of the harder fats, and forcing the mixture through flannel cloths in a filter press. The temperature at which the filtration should take place is generally 20-25° C. The oil is, of course, “tender,” or liable to solidify in cold weather; and the more so the higher the temperature at which filtration takes place. The tallow is obtained in cakes. If from fresh fleshings, it will be white and with little odour, but that from dried glue-stuff is usually brown and of unpleasant smell, while recovered grease from curriers’ shavings or “moisings” is always dark in colour.

If the fleshings are to be sold wet, they should be preserved in a sweet lime liquor; if to be dried, they are washed carefully in a fresh lime, spread on frames, and frequently turned over so that they may dry evenly and rapidly. Heat, if employed at all, is in most cases only used at the end of the drying operation, but some tanners dry from the first in a room the temperature of which is a few degrees higher than the normal, and which is provided with good ventilation. For the purposes of the glue manufacturer, the roundings and larger pieces are more valuable than the fleshings, and should be treated with correspondingly greater care by the beamsman and his assistants.

_Bate-Shavings_ are very valuable as sizing materials. They should be well washed in water, or with a very dilute solution of sulphurous acid, and are then laid out in thin layers to dry. They may also be partially dried by pressing between latticed boards in a screw or hydraulic press, and are then best finished as cakes. On the manufacture of sulphurous acid compare p. 25.

_Horns_ are usually kept until the “slough,” “pith,” or internal bone can be knocked out, having become loosened through drying and putrefaction. If kept dry, practically no longer time is required, and the smell and other annoyances incidental to storing in a damp place are avoided. The sloughs may be removed by steaming, but the horns are somewhat damaged by this treatment. The sloughs are principally ground for “bone-meal,” but some are boiled for glue, either without preparation, or after decalcifying with dilute hydrochloric acid.

The actual horn itself, which is quite incapable of making glue, is used chiefly in the manufacture of combs, buttons, and similar articles. The value of horns is to a considerable extent dependent on their size, small horns being unprofitable to work up for the articles above mentioned.

_Spent Tan._--The tan as it is obtained from the leaches after extraction has, naturally, no value for the tanner except as a fuel. Spent tan cannot be profitably sold as manure, as its worth in this respect is extremely small. In those places where white lead is still made by the Dutch process, oak-bark is used to cover up the earthen pots, and commands a good price. It is, however, essential that oak-bark only should be used, as many other tanning materials give off products which injure the colour of the white lead. The quantities of tan used for hot-beds, and for deadening the noise of traffic in the streets, are so small that they are of no practical account in the disposal of this product. Spent tan is not nearly so good as wood for the manufacture of paper, and an attempt to distil it and thereby obtain pyroligneous acid and wood-spirit did not result in any commercial success. On the Continent, fine-ground tan is usually pressed into briquettes for use as domestic fuel, but it would be hard to obtain a market for these in England.

On the whole, in spite of its low heating value, spent tan is best utilised as a fuel. For this purpose specially constructed furnaces are necessary on account of the dampness of the tan, and its low calorific value, which varies, however, with the particular materials: thus while oak-bark and valonia are only poor fuels, hemlock and myrobalans are much better on account of the resin and lignine they contain.

The first successful furnaces for raising steam with wet tan were introduced in the United States, and consisted of large arched combustion chamber with abundant grate-area, and with four or six feed-holes in the fire-brick top which formed a floor on which the spent tan was laid, and where to some extent it was dried by the waste heat. The flames and furnace gases were conducted under the boilers, the flue being very large and deep so as to collect the light ash which was drawn in great quantities from the furnace, and the gases then returned through the tubes of the boiler, afterwards passing down the sides and going to the chimney. The wet fuel was fed in through the firing holes alternately, so that only a part of the grate-space was covered at once with wet fuel; which was speedily ignited by the heat from other parts of the furnace, and especially from the vaulted arch.[187] The large grate-area was a necessity not only on this account, but because of the light weight of the fuel and its low calorific power, which involved the need of burning a large volume. Fig. 99 represents a furnace of similar principle constructed by Messrs. Huxham and Browns. Furnaces of this type are, the author believes, still largely in use in the United States, but in Germany “step-grates” sloping from the furnace-doors towards the back, are now preferred. In these the combustible material rests upon the flat surfaces of the grate, while the air enters by the spaces between the steps without the fuel being able to fall through. Fig. 100 represents the furnace on this principle constructed by the Moenus Co. of Frankfort.

[187] Detailed drawings and particulars are given in Jackson Schultz’s
‘Leather Manufacture in the United States,’ New York, 1876.

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.

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.

_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.

In the case of the waste liquors from a tannery, the use of these chemicals may often be avoided if sufficient care be taken in regulating the proportion of the various liquids which are to be mixed together and run into the settling tank. As tanning matter combines with lime and dissolved hide-substance to form a heavy brown insoluble precipitate, it is clear that if care be taken to have rather more waste lime-liquor mixed with the waste tan-liquors than is necessary to throw all the tan out of solution, a very considerable amount of purification of the effluent will have taken place without any cost whatever to the tanner. Hence, if the proportion of waste lime is small in comparison to that of the tanning liquors, an extra addition of lime may be necessary in order to precipitate the tannin.

The precipitation- or settling-tanks are usually square or rectangular vessels or pits, the size of which varies with the quantity of liquid to be treated, but the depth of which rarely exceeds six feet. They may be divided into two classes--the “intermittent,” and the “continuous.” In the former class the tank is filled with the mixed waste liquids, taking care that such a sufficiency of lime is present that the mixture is faintly alkaline to phenolphthalein paper, and is then allowed to rest until the suspended matter has settled down to the bottom of the tank, when the clear, or almost clear upper liquid is drawn off, the remainder being the “sludge”; some means must also be employed to prevent the passage of scum and floating matters. In the case of the intermittent process it is advisable to have two tanks, one of which is being filled while the other one is settling or being emptied. With the continuous process the liquids are run into the tank in the proportions calculated to give a maximum amount of purification, as described above, but as they enter very slowly the undissolved matter soon settles, and consequently the liquid may be continuously run out at the further end of the tank. This plan, though it does not yield such good results in the hands of unskilled workmen, is yet useful in many cases, as only one tank is absolutely necessary. It is desirable that in running off the tanks, the effluent should be taken as near the surface as possible, by means of a hinged pipe attached to a float, or some equivalent device; and care is required, as the tank gets low, to avoid the escape of any of the sludge.

For continuous settling the tanks are usually long and somewhat shallow rectangular ponds, into which the previously well-mixed precipitating liquid flows through a wooden trough fixed across one end and as long as the breadth of the tank, and perforated with holes to allow the uniform and quiet influx of the liquid, which finally escapes by a similar trough crossing the opposite end of the tank. In front of the exit-trough a “scum board” must be placed, which is a simple plank dipping slightly below the surface of the liquid, so as to prevent any oil, scum or other floating matter from passing out of the tank along with the clear effluent. Whether the intermittent or continuous system is employed, the effluent should in most cases be afterwards passed through a bacterial filter-bed, or treated by land filtration before it is allowed to flow into a stream or river. Tannery effluents are usually received into sewers without further treatment than mixing and settling to remove solid matter, and many authorities are satisfied with the removal of merely such coarse suspended matters as might choke the sewers. Where continuous precipitation-tanks are used, they must be emptied at frequent intervals, and the sludge run on to cinder-filters, to part with most of its water. These filters are conveniently placed at a lower level than the settling tanks, and it is generally necessary to return the effluent from them for further precipitation and settling. Several types of continuous settling tank with upward flow have been devised by Mr. Candy and others, which are very suitable for use where space is limited; but otherwise less costly constructions are often sufficient. Apart from the question of obtaining an effluent sufficiently good to satisfy the sanitary authority, the treatment of the sludge is one of the greatest difficulties in the purification of effluents. It is usually very bulky, easily putrescible, and therefore difficult to dry; it is of little value for manure; and if allowed to remain long wet, its smell is very offensive.

It has been mentioned that in most cases the liquid, and in every case the sludge, must be freed from solid undissolved matter by filtration. This may take place through open filters or through filter-presses. The open filters generally consist of a pit with an exit at the bottom for the filtered liquid. This pit is filled with either stones and sand, with clinker, ashes or coke. Most tanners use clinker and ashes, as they do not cost anything; and the material should be so arranged that while the lowest layers are very coarse, the surface of the filter-bed should be of the finest material. As soon as this has become covered with so thick a layer of solid matter that the filtration proceeds too slowly, the top surface of the filter may be removed with a rake (taking care to remove as little of the ashes or sand as possible), and burnt, or dried and used as manure. In some cases, filter-presses are used which are composed of grooved or perforated plates with cloths between them through which the liquid is forced by pressure. The solid matter remains behind in the form of a comparatively dry “cake.” The filter-cake, dried if desired, is sold as manure, for which it is in many ways very suitable. Although they work much more rapidly than do the open filters, the cloths so soon become rotten and have to be replaced, that the open ash-filter is on the whole the most convenient for the tanner’s use. It will be readily understood that apparatus of this kind, though very efficient on a small scale, is quite out of the question when many thousand gallons of liquid have to be filtered daily, and so can only be effectively applied to “sludge.”

No system of chemical precipitation has as yet proved entirely satisfactory. Undoubtedly a great deal of purification is effected by this means, but in most cases the “purified” liquid is still too impure to be turned into a stream, though for various reasons this is often permitted by the authorities.

A great advance was made in the purification of effluents when manufacturers were compelled by law to allow the effluent from the precipitation-tank to filter through land set apart for that purpose. In this case certain hardy cereals were sown on the land, which was watered as often as possible with the effluent. This latter, after soaking through the land, was drained off into the nearest stream. Although in many ways this treatment was satisfactory, it had the disadvantage of being very expensive, especially in the neighbourhood of large towns where the price of land is high, and, in addition to this, the conditions necessary for success were far from being correctly understood, so that the land often became “sewage-sick” or waterlogged, and ceased to purify the effluent. It was not until the researches of bacteriologists proved that the purification by land-filtration was mainly due to the bacteria in the soil, that any really satisfactory solution of the problem could be found, but the question has now been to a considerable extent simplified by the introduction of “bacterial treatment.”

Bacteria, considered from the point of view of their action on organic matter, are often classified as “anaerobic” and “aerobic,” though many species are capable of existing under both conditions (Cp. L.I.L.B.,