The Krebitz process which has been used to some extent in Europe is based upon the conversion of the fat or oil into lime soap which is transformed into the soda soap by the addition of sodium carbonate. To carry out the process a convenient batch of, say, 10,000 pounds of fat or oil, is run into a shallow kettle containing 1,200 to 1,400 pounds of lime previously slaked with 3,700 to 4,500 pounds of water. The mass is slowly heated with live steam to almost boiling until an emulsion is obtained. The tank is then covered and allowed to stand about 12 hours. The lime soap thus formed is dropped from the tank into the hopper of a mill, finely ground and conveyed to a leeching tank. The glycerine is washed out and the glycerine water run to a tank for evaporation. The soap is then further washed and these washings are run to other tanks to be used over again to wash a fresh batch of soap. About 150,000 pounds of water will wash the soap made from 10,000 pounds of fat which makes between 15,000 and 16,000 pounds of soap. The first wash contains approximately 10 per cent. glycerine and under ordinary circumstances this only need be evaporated for glycerine recovery.
After extracting the glycerine the soap is slowly introduced into a boiling solution of sodium carbonate or soda ash and boiled until the soda has replaced the lime. This is indicated by the disappearance of the small lumps of lime soap. Caustic soda is then added to saponify the fat not converted by the lime saponification. The soap is then salted out and allowed to settle out the calcium carbonate. This drops to the bottom of the kettle as a heavy sludge entangling about 10 per cent. of the soap. A portion of this soap may be recovered by agitating the sludge with heat and water, pumping the soap off the top and filtering the remaining sludge.
While the soap thus obtained is very good, the percentage of glycerine recovered is greatly increased and the cost of alkali as carbonate is less. The disadvantages are many. Large quantities of lime are required; it is difficult to recover the soap from the lime sludge; the operations are numerous prior to the soap making proper and rather complicated apparatus is required.
DISTILLATION OF FATTY ACIDS.
The fatty acids obtained by various methods of saponification may be further improved by distillation.
In order to carry out this distillation, two methods may be pursued, first, the continuous method, whereby the fatty acids are continually distilled for five to six days, and, second, the two phase method, whereby the distillation continues for 16 to 20 hours, after which the residue is drawn off, treated with acid, and its distillate added to a fresh charge of fatty acids. The latter method is by far the best, since the advantages derived by thus proceeding more than compensate the necessity of cleaning the still. Better colored fatty acids are obtained; less unsaponifiable matter is contained therein; there is no accumulation of impurities; the amount of neutral fat is lessened because the treatment of the tar with acid causes a cleavage of the neutral fat and the candle tar or pitch obtained is harder and better and thus more valuable.
The stills are usually built of copper, which are heated by both direct fire and superheated steam. Distillation under vacuum is advisable. To begin the distilling operation, the still is first filled with dry hot fatty acids to the proper level. Superheated steam is then admitted and the condenser is first heated to prevent the freezing of the fatty acids, passing over into same. When the temperature reaches 230 deg. C. the distillation begins. At the beginning, the fatty acids flow from the condenser, an intense green color, due to the formation of copper soaps produced by the action of the fatty acids on the copper still. This color may easily be removed by treating with dilute acid to decompose the copper soaps.
In vacuum distillation, the operation is begun without the use of vacuum. Vacuum is introduced only when the distillation has proceeded for a time and the introduction of this must be carefully regulated, else the rapid influence of vacuum will cause the contents of the still to overflow. When distillation has begun a constant level of fatty acids is retained therein by opening the feeding valve to same, and the heat is so regulated as to produce the desired rate of distillation. As soon as the distillate flows darker and slower, the feeding valve to the still is shut off and the distillation continued until most of the contents of the still are distilled off, which is indicated by a rise in the temperature. Distillation is then discontinued, the still shut down, and in about an hour the contents are sufficiently cool to be emptied. The residue is run off into a proper receiving vessel, treated with dilute acid and used in the distillation of tar.
In the distillation of tar the same method as the above is followed, only distillation proceeds at a higher temperature. The first portion and last portion of the distillate from tar are so dark that it is necessary to add them to a fresh charge of fatty acids. By a well conducted distillation of tar about 50 per cent. of the fatty acids from the tar can be used to mix with the distilled fatty acids. The residue of this operation called stearine pitch or candle tar consists of a hard, brittle, dark substance. Elastic pitch only results where distillation has been kept constant for several days without interrupting the process, and re-distilling the tar. In a good distillation the distillation loss is 0.5 to 1.5% and loss in pitch 1.5%. Fatty acids which are not acidified deliver about 3% of pitch. Very impure fats yield even a higher percentage in spite of acidifying. For a long time it was found impossible to find any use for stearine pitch, but in recent years a use has been found for same in the electrical installation of cables.