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 Having withdrawn these coloured "foots," the second portion of the weak caustic soda solution is agitated with the partially refined oil, and, when the latter is sufficiently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 CO2 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.