BLEACHING AND TREATMENT OF RAW MATERIALS INTENDED FOR SOAP-MAKING.
_Palm Oil--Cotton-seed Oil--Cotton-seed "Foots"--Vegetable
Oils--Animal Fats--Bone Fat--Rosin._
Having described the most important and interesting oils and fats used
or suggested for use in the manufacture of soap, let us now consider
briefly the methods of bleaching and treating the raw materials, prior
to their transference to the soap-pan.
_Crude Palm Oil._--Of the various methods suggested for bleaching palm
oil, the bichromate process originated by Watts is undoubtedly the best.
The reaction may be expressed by the following equation, though in
practice it is necessary to use twice the amount of acid required by
theory:--
K_{2}Cr_{2}O_{7} + 14HCl = 2KCl + Cr_{2}Cl_{6} + 7H_{2}O + 6Cl.
6Cl + 3H_{2}O = 6HCl + 3O.
The palm oil, freed from solid impurities by melting and subsidence, is
placed in the bleaching tank, and washed with water containing a little
hydrochloric acid. Having allowed it to rest, and drawn off the liquor
and sediment (chiefly sand), the palm oil is ready for treatment with
the bleaching reagent, which consists of potassium bichromate and
commercial muriatic acid. For every ton of oil, 22 to 28 lb. potassium
bichromate and 45 to 60 lb. acid will be found sufficient to produce a
good bleached oil.
The best procedure is to act upon the colouring matter of the oil three
successive times, using in the first two treatments one-third of the
average of the figures just given, and in the final treatment an
appropriate quantity which can be easily gauged by the appearance of a
cooled sample of the oil.
The potassium bichromate is dissolved in hot water and added to the
crude palm oil, previously heated to 125° F. (52° C.), the requisite
amount of muriatic acid being then run in and the whole well agitated by
means of air. The bright red colour of the oil gradually changes to dark
brown, and soon becomes green. The action having proceeded for a few
minutes, agitation is stopped, and, after allowing to settle, the green
liquor is withdrawn.
When sufficiently bleached the oil is finally washed (without further
heating) with hot water (which may contain salt), to remove the last
traces of chrome liquor.
If the above operation is carried out carefully, the colouring matter
will be completely oxidised.
It is important, however, that the temperature should not be allowed to
rise above 130° F. (54° C.) during the bleaching of palm oil, otherwise
the resultant oil on saponification is apt to yield a soap of a "foxy"
colour. The bleached oil retains the characteristic violet odour of the
original oil.
It has been suggested to use dilute sulphuric acid, or a mixture of this
and common salt, in the place of muriatic acid in the above process.
_Crude Cotton-seed Oil._--The deep colouring matter of crude cotton-seed
oil, together with the mucilaginous and resinous principles, are removed
by refining with caustic soda lye.
The chief aim of the refiner is to remove these impurities without
saponifying any of the neutral oil. The percentage of free fatty acids
in the oil will determine the quantity of caustic lye required, which
must only be sufficient to remove this acidity.
Having determined the amount of free acidity, the quantity of caustic
soda lye necessary to neutralise it is diluted with water to 12° or 15°
Tw. (8° or 10° B.), and the refining process carried out in three
stages. The oil is placed in a suitable tank and heated by means of a
closed steam coil to 100° F. (38° C.), a third of the necessary weak
caustic soda lye added in a fine stream or by means of a sprinkler, and
the whole well agitated with a mechanical agitator or by blowing a
current of air through a pipe laid on the bottom of the tank.
Prolonged agitation with air has a tendency to oxidise the oil, which
increases its specific gravity and refractive index, and will be found
in the soap-pan to produce a reddish soap. As the treatment proceeds,
the temperature may be carefully raised, by means of the steam coil, to
120° F. (49° C.).
The first treatment having proceeded fifteen minutes, the contents of
the tank are allowed to rest; the settling should be prolonged as much
as possible, say overnight, to allow the impurities to precipitate well,
and carry down the least amount of entangled oil. Having withdrawn these
coloured "foots," the second portion of the weak caustic soda solution
is agitated with the partially refined oil, and, when the latter is
sufficiently treated, it is allowed to rest and the settled coloured
liquor drawn off as before. The oil is now ready for the final
treatment, which is performed in the same manner as the two previous
ones. On settling, a clear yellow oil separates.
If desired, the oil may be brightened and filtered, after refining to
produce a marketable article, but if it is being refined for own use in
the soap-house, this may be omitted.
The residue or "foots" produced during the refining of crude
cotton-seed oil, known in the trade as "mucilage," may be converted into
"soapstock" as mentioned in the preceding chapter, or decomposed by a
mineral acid and made into "black grease" ready for distillation by
superheated steam.
_Vegetable Oils._--The other vegetable oils come to the soap-maker's
hand in a refined condition; occasionally, however, it is desirable to
remove a portion of the free fatty acids, which treatment also causes
the colouring matter to be preciptated. This is effected by bringing the
oil and a weak solution of caustic lye into intimate contact. Cocoa-nut
oil is often treated in this manner. Sometimes it is only necessary to
well agitate the oil with 1-1/2 per cent. of its weight of a 12° Tw. (8°
B.) solution of caustic soda and allow to settle. The foots are utilised
in the soap-pan.
_Animal Fats._--Tallows are often greatly improved by the above alkaline
treatment at 165° F. (73° C.). It is one of the best methods and
possesses advantages over acid processes--the caustic soda removes the
free acid and bodies of aldehyde nature, which are most probably the
result of oxidation or polymerisation, whereas the neutral fat is not
attacked, and further, the alkaline foots can be used in the production
of soap.
_Bone fat_ often contains calcium (lime) salts, which are very
objectionable substances in a soap-pan. These impurities must be removed
by a treatment with hydrochloric or sulphuric acid. The former acid is
preferable, as the lime salt formed is readily soluble and easily
removed. The fat is agitated with a weak solution of acid in a
lead-lined tank by blowing in steam, and when the treatment is complete
and the waste liquor withdrawn, the last traces of acid are well washed
out of the liquid fat with hot water.
_Rosin._--Several methods have been suggested for bleaching rosin; in
some instances the constitution of the rosin is altered, and in others
the cost is too great or the process impracticable.
The aim of these processes must necessarily be the elimination of the
colouring matter without altering the original properties of the
substance. This is best carried out by converting the rosin into a
resinate of soda by boiling it with a solution of either caustic soda or
carbonated alkali. The process is commenced by heating 37 cwt. of 17°
Tw. (11° B.) caustic soda lye, and adding 20 cwt. of rosin, broken into
pieces, and continuing the boiling until all the resinate is
homogeneous, when an addition of 1-1/2 cwt. of salt is made and the
boiling prolonged a little. On resting, the coloured liquor rises to the
surface of the resinate, and may be siphoned off (or pumped away through
a skimmer pipe) and the resinate further washed with water containing a
little salt.
The treatment with carbonated alkali is performed in a similar manner. A
solution, consisting of 2-3/4 cwt. of soda ash (58°), in about four
times its weight of water, is heated and 20 cwt. of rosin, broken into
small pieces, added. The whole is heated by means of the open steam
coil, and care must be taken to avoid boiling over. Owing to the
liberation of CO_{2} gas, frothing takes place. A large number of
patents have been granted for the preparation of resinate of soda, and
many methods devised to obviate the boiling over. Some suggest mixing
the rosin and soda ash (or only a portion of the soda ash) prior to
dissolving in water; others saponify in a boiler connected with a trap
which returns the resinate to the pan and allows the carbonic-acid gas
to escape or to be collected.
With due precaution the method can be easily worked in open vessels,
and, using the above proportions, there will be sufficient uncombined
rosin remaining to allow the resultant product to be pumped into the
soap with which it is intended to intermix it, where it will be finally
saponified thoroughly.
The salt required, which, in the example given, would be 1-1/2 cwt., may
be added to the solution prior to the addition of rosin or sprinkled in
towards the finish of the boiling. When the whole has been sufficiently
boiled and allowed to rest, the liquor containing the colouring matter
will float over the resinate, and, after removal, may be replaced by
another washing.
Many other methods have been suggested for the bleaching, refining and
treatment of materials intended for saponification, but the above
practical processes are successfully employed.
All fats and oils after being melted by the aid of steam must be allowed
to thoroughly settle, and the condensed water and impurities withdrawn
through a trap arrangement for collecting the fatty matter. The molten
settled fatty materials _en route_ to the soap-pan should be passed
through sieves sufficiently fine to free them from suspended matter.