Fresh grease is now pumped into the lye or water remaining in the tank and the process repeated.
The glycerine water or first lye is run to the treatment tank, the fat skimmed off and neutralized with lime until it shows pink with phenolphthalein, after having been thoroughly boiled with steam. About 0.25 per cent. lime is the proper amount to add. The mixture is then allowed to settle and the supernatant mixture drawn off and run to the glycerine evaporator feed tank. The lime which holds considerable glycerine is filtered and the liquor added to the other. The evaporation is carried out in two stages. The glycerine water is first evaporated to about 60 per cent. glycerol, then dropped into a settling tank to settle out the calcium sulfate. The clear liquor is then evaporated to crude (about 90 per cent. glycerine) and the sediment filtered and also evaporated to crude.
As to the amount of saponifier to use on various stocks, this is best determined by experiment as to how high a percentage gives dark colored fatty acids. For good stock such as clean tallow, prime cottonseed oil, corn oil, cocoanut oil and stock of this kind 0.75 per cent. saponifier is sufficient. For poorer grades of tallow, house grease, poor cottonseed oil, etc., 1 per cent. saponifier is required and for poorer grade greases higher percentages. The percentage of fatty acids developed varies in various stocks, and also varies with the care that the operation is carried out, but is usually between 85 per cent.-95 per cent. Due to the water taken up in the saponification process there is a yield of about 103 pounds of fatty acids and glycerine for 100 pounds of fat.
The Twitchell reagent has undoubtedly caused a decided advance in the saponification of fats and oils and has been of great value to the soap manufacturer, because with a small expenditure it is possible to compete with the much more expensive equipment necessary for autoclave saponification. The drawback, however, has been that the reagent imparted a dark color to the fatty acids obtained, due to decomposition products forming when the reagent is made, and hence is not suitable for use in soaps where whiteness of color is desired.
There have recently been two new reagents introduced which act as catalyzers in splitting fats, just as the Twitchell reagent acts, but the fatty acids produced by the cleavage are of good color. The saponification, furthermore, takes place more rapidly. These are the Pfeilring reagent and Kontact reagent.
The Pfeilring reagent is very similar to the Twitchell reagent, being made from hydrogenated castor oil and naphthalene by sulfonation with concentrated sulfuric acid. It is manufactured in Germany and is being extensively used in that country with good success.
The Kontact or Petroff reagent, discovered by Petroff in Russia, is made from sulfonated mineral oils. Until very recently it has only been manufactured in Europe, but now that it has been found possible to obtain the proper mineral constituent from American petroleum, it is being manufactured in this country, and it is very probable that it will replace the Twitchell reagent because of the advantages derived by using it, as compared to the old Twitchell reagent.
The method and equipment necessary for employing either the Pfeilring or Kontact reagents is exactly the same as in using the Twitchell process.
AUTOCLAVE SAPONIFICATION.
While the introduction of the Twitchell process to a great extent replaced the autoclave method of saponification for obtaining fatty acids for soap making, the autoclave method is also used. This process consists in heating the previously purified fat or oil in the presence of lime and water, or water only, for several hours, which causes a splitting of the glycerides into fatty acids and glycerine. The advantage of autoclave saponification over the Twitchell process is that a greater cleavage of the fats and oils results in less time and at a slightly less expense. The glycerine thus obtained is also purer and of better color than that obtained by Twitchelling the fats.