Permanent hardness of water is generally caused by sulphates of lime and magnesia, and more rarely by chlorides and nitrates. As none of these can be precipitated by lime, permanent hardness cannot be removed by Clark’s process, nor can it produce the injurious effect on limed hides which have been attributed to temporary hardness. Neither can the lime and magnesia present combine with the tannins if used for leaching, since they are already fixed by stronger acids, and at most can only act injuriously by slightly lessening the solubility of the tannins. Even this effect cannot be regarded as proved, though it deserves further investigation.[63] Permanent hardness is therefore of little moment as regards the ordinary uses of the tannery, though it has considerable influence in some of the processes of dyeing, and acts very injuriously where soap is used for scouring, as in the washing of sheep-skins for wool mats, since each part of lime reckoned as carbonate destroys at least twelve parts of pure soap (sodium stearate or oleate), producing a sticky and insoluble lime-soap which adheres to the fibre. In sole-leather tanning, permanent hardness is sometimes advantageous, especially if it be due to calcium and magnesium sulphates, and Vignon recommended that sulphuric acid should be added to the water before use in quantity sufficient to exactly neutralise the carbonates which cause temporary hardness, as magnesium and calcium sulphates are not injurious, but tend to plump the hides. It must be remembered, however, that the carbonic acid liberated may still have prejudicial effects on limed hides.
[63] Recent investigations by Nihoul (‘Influence de la nature de l’eau sur l’extraction des matières tannantes,’ Bulletin de la Bourse aux Cuirs de Liège, Sept. 1901) on the tanning waters of Belgium seem to show that permanent hardness is more injurious in the extraction of tannin than has generally been supposed.
Permanent hardness is most objectionable in waters employed for boiler-feeding, and calcium sulphate is especially so, as it becomes nearly insoluble in water at 150° C. or 55 lb. steam-pressure, and is deposited on the plates as a hard crystalline scale which has to be chipped off with a hammer. Where many boilers have to be worked with a hard water, it is much the most satisfactory to soften the water with caustic soda, or with lime and soda together before it comes into the boiler, but in cases where the plant required would be too costly, boiler-compositions are sometimes used with good effect, though considerable caution is advisable, since some of them affect the plates injuriously. The active constituent of many boiler-compositions is soda-ash or sodium carbonate, which acts by double decomposition with the calcium sulphate, forming sodium sulphate, and precipitating calcium carbonate as a sediment which is easily washed out. Most tanning materials, and even spent tan liquors, will prevent or lessen incrustation if mixed with the feed water, but sometimes corrode the plates if used too freely. This danger is lessened if they are used in conjunction with soda. Heavy mineral oils, either introduced in small quantity with the feed water, or painted on the sides of the boiler when cleaned, are useful in preventing the formation of a coherent scale.
The removal of permanent hardness from water is easily effected in most of the forms of apparatus employed for the softening of water by lime, by using a calculated quantity of sodium carbonate in addition. The reaction is represented in the case of calcium sulphate by the following equation—
| CaSO4 | + | Na2CO3 | = | CaCO3 | + | Na2SO4. |
The conversion of magnesium sulphate into carbonate may be similarly effected, but as the latter is somewhat soluble, an additional equivalent of lime must be used to precipitate it as hydrate. Magnesium salts, from their solubility, do not cause scale on boilers (though the chloride is apt to produce corrosion), but they are equally destructive of soap with the calcium salts. Caustic soda will remove temporary hardness, and after becoming converted into carbonate will further react on any permanent hardness present; and its use is therefore sometimes convenient in small softening plants, but it is not more effective, and considerably more costly than a suitable mixture of lime and sodium carbonate. Even with these, Archbutt states that the cost of softening permanent hardness is about ten times as great as that of removing temporary hardness with lime only.[64]
[64] Proceedings of Inst. of Mech. Engineers, 1898, pp. 404-54, in which much valuable information on water-softening is given.
As regards the influence of other impurities, our knowledge is far from complete, but the following are the most important matters likely to be present.
Mud under any circumstances is objectionable. It frequently contains organic slime and organisms which encourage the putrefaction of hides placed in it to wash or soften. It also almost invariably contains iron as one of its constituents, and hence stains leather and gives dark coloured liquors. It is not easily removed by filtration, as large filter-beds are expensive and difficult to keep in order, and much space is required to clear water by subsidence. Some mechanical filter which can be easily cleaned, and used under pressure, offers the best chance of success. The Pulsometer Company make one consisting of sponge tightly packed below a perforated piston. To cleanse the filter a stream of water is passed the reverse way, and the piston raised and worked up and down, either by hand or power, so as to loosen and knead the sponge. Filter-presses, in which cloths, or in some cases sand, are used as the filtering medium, are also well adapted for the purpose. If a water be softened by Clark’s or other process the precipitated chalk carries down the mud with it, together with most of the organisms.
Iron is always an objectionable impurity in the tannery, though it is less injurious to the quality than the appearance of the leather produced, and indeed German sole-leather tanners frequently put old iron in the handlers to darken the colour of the leather, and apparently, if not really, to quicken the tannage. It must not be present in waters used for dyeing. Iron oxide is frequently present as a mud merely, and in this case can be removed by filtration. It is rarely in solution in any other form than that of acid carbonate, since sulphate or chloride could not exist in presence of bicarbonate of lime. In this form, iron is precipitated at once by boiling or on the addition of lime, like the temporary hardness due to other bases, in the form of ferric hydrate, and more slowly by oxidation on exposure to the air. The mud produced by softening waters which contain iron must be completely removed by filtration, or subsidence, before the water is used for leaching, or the iron will redissolve in the acids of the liquors. Iron is not perceptibly injurious in the limes, but in the bates and wash-pits sometimes causes stains, which are scarcely visible till blackened by the tanning liquors. In presence of sulphur (from sulphide of sodium or the decomposition of sulphates by the sulphur-bacteria nearly always present in bates and soaks), the stains become bluish or greenish black, and a black deposit is frequently produced on the sides of the pit, in which the threads of sulphur-bacteria (Thiothrix) can often be recognised by the microscope. As ferric salts not only combine with the tannins, but are themselves tanning agents (see [p. 198]), they are rapidly absorbed by leather, and iron is always present in leather ash. (For detection and estimation see L.I.L.B., p. 218.)