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 SO{4}-- 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. 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.
All the above decolorizing materials are insoluble and hydrophobe, and
act in virtue of their finely divided conditions, which causes them to
have a large specific surface; but there is another type or branch of
substances, whose effect is due to surface action of rather a different
type. These are the hydrophile gels. In a gelatine sol the colloid
particles have largely adsorbed the colouring matters which it is
desired to remove. This adsorption, which is after all only an
equilibrium, is reduced by introducing another very strong adsorbent.
This latter, by adsorption from the continuous phase, reduces the
adsorption of colouring matters by the gelatine particles. In the case
under discussion another lyophile colloid is introduced, and after
bringing about such an action is removed by appropriate means. The use
of albumin has long been known for such a purpose, its special advantage
being that after its admixture and adsorptive action, it may easily be
removed by raising the temperature above 70° C., when coagulation takes
place, and by subsequent mechanical filtration. The coagulated albumin
takes down the adsorbed colouring matters. Albumin has been used in this
way not only for gelatine and glue liquors, but also for tanning
extracts (Part I., Section III.) and other commercial preparations. Into
this class of decolorizing agents fall the insoluble inorganic gels
which have been advocated by W. Gordon Bennett, _e.g._ alumina cream.
Freshly precipitated alumina hydrate is a colloid gel with very
considerable adsorptive powers. It has also the advantage that it is
quite insoluble, easily removed in filtration, and has a powerful
adsorptive action upon other objectionable impurities, especially the
poisonous metals, arsenic, copper, zinc and lead. Its use is an
undoubted advantage when in addition to the other clarifying agents and
adsorbents. It is conceivable, in some cases, that when alum is employed
as clarifying agent in an alkaline gelatine liquor, some alumina may be
formed, and as such contribute to the total effect.