SECTION IV.—CLARIFICATION AND DECOLORIZATION
After the raw material has been appropriately prepared and an aqueous extract or gelatine sol obtained therefrom, there are certain refinements necessary before the weak sol is evaporated. These purifying processes include (1) clarification, (2) decolorization, and (3) bleaching. Whilst most manufacturers have more or less successfully solved the problems involved in these processes, the practical methods that are in common use have been evolved and elaborated in a purely empirical way, and the underlying principles have been very imperfectly recognized, and indeed often confused and misunderstood. Hence it is even yet not uncommon to find these terms rather loosely used, and it is one aim of this section to define and distinguish these various operations in principle as well as in practice.
Clarification consists essentially in the removal of suspended matters, with the consequent production of a sol or gel which is bright, clear, and apparently homogeneous. Bleaching consists essentially in destroying the colouring matters of the sol by chemical action, such as oxidation or reduction. Decolorization involves the removal rather than the destruction of colouring matters, and does not therefore imply a chemical action in the ordinary sense.
Clarification may be now considered more particularly. It is necessary in this connection to consider what is meant by "suspended matter." The modern view is that the difference between a true solution and a muddy liquor or an emulsion is one chiefly of degree. If the particles of matter in suspension or emulsion (the disperse phase) be reduced in size they eventually merge into colloidal sols which are sometimes analogously named "suspensoids" and "emulsoids," if further reduced in size into "suspensides" and "emulsides," and with further reduction into true solutions. On this view not only suspensions and emulsions, but also sols, solutides and solutions are all heterogeneous. Now in practice the clarifying of a gelatine sol involves only the removal of the particles which are evident to sight. What is needed is that the product should make a sol or gel which to the naked eye appears to be optically clear both to reflected and to transmitted light. If desired, the limit could be expressed in terms of dispersity or specific surface. Now it is a comparatively easy matter to remove the coarser substances which often pass into the sol, e.g. undissolved portions of raw material or the insoluble portions, such as the hair, the grain (hyaline layer), and the elastic fibres of skin gelatine material, and the fibres which even remain in extracting acidulated bones. A more difficult proposition is the removal of still finer particles which may be almost said to be in colloidal solution, but which at any rate are so large that they cause a visible opalescence or even a turbidity of the gelatine sol. A more difficult task also is the removal of minute particles of grease, which are an exceedingly common cause of turbidity and which are often very effectively emulsified in the sol.
Now at this stage it is necessary to point out that besides the difference in the size of the particles of the disperse phase, there is another important difference involved, viz. that the particles of a colloid sol carry an electric charge owing to the adsorption of electrically charged ions of the electrolytes (salts, acids or alkalies) present. If this charge be removed the colloid is precipitated (coagulated, flocculated) and is then filtered off with comparative ease. This precipitation can be brought about by a reduction or elimination of the potential difference between the disperse phase and the continuous phase. The electric charge given by the adsorbed ions may be reduced by dilution, for dilution causes a lessened adsorption of the charging ions. Hence the well-known practical fact that it is more satisfactory to filter a dilute gelatine sol. Further, the electric charge may be reduced also by causing the adsorption of an ion of opposite charge. This is the principle underlying the precipitation (of any colloid) by adding electrolytes. It is essential here to consider which ions are most likely to be adsorbed, and also to bear in mind what charge they carry. Now the hydrion (H+) of acids and the hydroxyl ion (OH-) of alkalies are most strongly adsorbed, so that to precipitate a negative sol, acid is very effective, whilst with a positive sol an alkali is an appropriate precipitant. Further, it is known that organic ions are usually more strongly adsorbed, hence when precipitating from an alkaline sol (negative sol), one should preferably select an inorganic or mineral acid rather than an organic acid. Thus in clarifying an alkaline gelatine sol, hydrochloric or sulphuric acid is to be preferred to acetic or lactic acid. Again, it is necessary to remember that a divalent ion carries twice the charge of a univalent ion, hence the precipitating power of an electrolyte depends upon the valency of the ion whose electric charge is opposite to that on the sol (Hardy's valency rule). Thus a negative sol is most easily precipitated by a monobasic acid. Thus hydrochloric acid is better than sulphuric, on account of the stabilizing effect of the divalent SO4— ion on a negative sol. In such a sol, also, the valency rule indicates that the multivalent kations, e.g. iron, Fe+++; chromium, Cr+++; and aluminium, Al+++, should have great precipitating and clarifying effect. This of course is known to be the case, aluminium salts having long been used. The rule indicates, also, that aluminium chloride would be better than the sulphate or than potash alum. Another feature of precipitation worthy of mention is the phenomenon of "acclimatization." This describes the fact that when the precipitating reagent is added very slowly, or a little at a time, a larger amount must be used, and the slower the addition the greater the excess required. Hence in precipitating matters from an alkaline gelatine sol the acid, if practicable, should be added all at once. In any case it is clear that one should aim at filtering a gelatine sol when it is near the iso-electric point, which is stable enough for gelatine itself, but a point of instability for many undesired impurities. Yet another phenomenon of colloid chemistry is concerned, viz. "protection." The particles it is desired to precipitate not only adsorb ions of electrolytes, but also the gelatine sol itself, and the particles, thus covered by a layer of a stable emulsoid sol, attain much of the stability of this gelatine sol. Unfortunately for gelatine manufacturers, gelatine possesses very great powers as "protective colloid," and this no doubt greatly enhances the practical difficulty of obtaining a clear and bright sol or gel. Here again dilution of the sol reduces the adsorption and correspondingly reduces, to some extent, the difficulty.
With regard to the turbidity or opalescence in a gelatine sol due to minute globules of grease, the case presents some analogy to the coarser colloid solutions, but the analogy has its limits, for an emulsion of grease is not an emulsoid sol. Doubtless the grease globules exhibit adsorptive phenomena, in which case the valency rule comes into force; the gelatine, also, by lowering interfacial tension, assists in protecting the emulsion; but grease emulsions are certainly stabilized in alkaline media (hence the detergent effect of soap, soda, borax, etc.), and it is undoubtedly easier to separate the emulsion by making the medium acid. Hence the practical fact that an acid sol is more easily clarified from grease than an alkaline or even than a neutral one.
The next stage in clarification is the separation of precipitated matters and of the coalesced particles of grease. This may be attained by the two processes usual in such a problem of chemical engineering, viz. sedimentation and filtration. After precipitation, therefore, the sol should be allowed to stand for some hours, during which time the precipitate not only flocculates but also settles to the bottom, and the globules of grease coalesce further and rise to the top, from which they may be skimmed off. Sedimentation alone is both too slow and too incomplete to be sufficient for proper clarification, and in these days it is always supplemented by the use of the filter-press. This well-known appliance can easily be adapted to the local requirements of the manufacturer. As speed of working is an essential requirement it is necessary to have a large filtering surface, and this may be done either by increasing the number of plates in the press or by increasing the area of the plates used. The large plates, however, are often cumbrous and inconvenient, and if of metal are very heavy. The plates may be constructed of well-seasoned wood, or in the case of alkaline gelatine and glues, even of iron. The framework is in any case usually iron. Acid gelatines and glues may have wooden plates, but "acid-proof" alloys are sometimes used to make them. Where it is essential to filter quickly two presses may be arranged in parallel, thus doubling the active filtering surface. When it is essential to obtain the highest possible clarity, two presses may be worked in series, which, in effect, means that the sol is filtered twice. In using the filter press for gelatine and glue it is most necessary to observe the most scrupulous cleanliness, and the plates must be frequently washed and sterilized. Rideal recommends weak chlorine water or bleaching powder solution for this purpose.
The process of decolorization, by which colouring matters are removed without being chemically altered or destroyed, usually precedes or takes place concurrently with the filtration. The underlying principle of this operation is adsorption. The colouring matters are usually in colloidal solution and most frequently are emulsoids, hence they are substances which are known to be exceedingly susceptible to positive adsorption. It is probable, also, that in a gelatine sol are particles which cause turbidity, though not coloured, and which are capable of being adsorbed. Hence the adsorption of colouring matters not only makes the sol more colourless, but in all probability makes it brighter and clearer. Further, decolorization by adsorption probably also involves the removal of the last traces of emulsified grease. It will be clear, therefore, that in the improvement in brightness and colour of a gelatine sol, adsorption fulfils a triple usefulness. The ordinary processes of dyeing fabrics or leather are adsorption processes, and the decolorization of gelatine sols consists essentially of the same process, except that the concentration of the dyestuff is much less, and the liquor remaining, instead of the adsorbent, is the primary consideration.
Decolorization of gelatine sols may be effected by any substance with a large specific surface (see p. [201]). Indeed, a great variety of adsorbents are actually used in practice, and each factory has its favourite material or mixture, and its favourite mode, place, and time of application, determined partly by the nature of the adsorbent and partly by the precise form of apparatus used. Amongst the adsorbents which have received special favour are sand, kieselguhr, asbestos, animal charcoal, wood pulp fibre, albumin and alumina. Sand is very effective, but a comparatively large weight is needed, and its cleansing for repeated use is troublesome. On the other hand, it may be completely renovated by ignition. Kieselguhr is a very powerful adsorbent, and only a little will do much good; it is, however, hardly sufficient alone. Animal charcoal has great specific surface, but its pores are very small for viscous liquors, and its use is less suitable in the case of gelatine than in the decolorization of liquors which may be boiled. Wood pulp fibre is a very popular decolorizing material, not only in gelatine but also in other trades. Its short, woolly fibres give a clarifying as well as a decolorizing effect. It may thus act as a mechanical filter for suspended matter and grease, as well as an adsorbent for colouring matters present as sols. Its two functions, however, are often confused. It may be regenerated for repeated use by careful washing, and special pulp-washing machines are manufactured and sold for the purpose. Detergents are usually employed in the wash waters. Asbestos is also a good adsorbent, and its long fibres make it much less liable to non-operating "channels" and "bursts." It also has the advantage that, if desired, it may be regenerated by ignition. It forms a very useful mixture with pulp fibre.