(A) Direct combination of fatty acids with alkali.
(B) Treatment of fat with definite amount of alkali and no separation of waste lye.
(C) Treatment of fat with indefinite amount of alkali and no separation of waste lye.
(D) Treatment of fat with indefinite amount of alkali and separation of waste lye.
(A) Direct Combination of Fatty Acids with Alkali.—This method consists in the complete saturation of fatty acids with alkali, and permits of the use of the deglycerised products mentioned in chapter ii., section 2, and of carbonated alkalies or caustic soda or potash. Fatty acids are readily saponified with caustic soda or caustic potash of all strengths.
The saponification by means of carbonated alkali may be performed in an open vat containing a steam coil, or in a pan provided with a removable agitator.
It is usual to take soda ash (58°), amounting to 19 per cent. of the weight of fatty acids to be saponified, and dissolve it in water by the aid of steam until the density of the solution is 53° Tw. (30° B.); then bring to the boil, and, whilst boiling, add the molten fatty acids slowly, but not continuously.
Combination takes place immediately with evolution of carbonic acid gas, which causes the contents of the vat or pan to swell, and frequently to boil over. The use of the agitator, or the cessation of the flow of fatty acids, will sometimes tend to prevent the boiling over. It is imperative that the steam should not be checked but boiling continued as vigorously as possible until all the alkali has been absorbed and the gas driven off.
The use of air to replace steam in expelling the carbonic acid gas has been patented (Fr. Pat. 333,974, 1903).
A better method of procedure, however, is to commence with a solution of 64° Tw. (35° B.) density, made from half the requisite soda ash (9-1/2 per cent.), and when this amount of alkali has all been taken up by the fatty acids (which have been added gradually and with continuous boiling), the remaining quantity of soda ash is added in a dry state, being sprinkled over each further addition of fatty acid.