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The action having proceeded for a few minutes, agitation is stopped, and, after allowing to settle, the green liquor is withdrawn.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.).