AIR BLEACHING OF PALM OIL.
The method of conducting this process is identical with the chrome process to the point where the hydrochloric acid is to be added to the oil. In this method no acid or chrome is necessary, as the active bleaching agent is the oxygen of the air.
The equipment is similar to that of the former process, except that a wooden tank in which no iron is exposed will suffice to bleach the oil in. The process depends in rapidity upon the amount of air blown through the oil and its even distribution. Iron should not be present or exposed to the oil during bleaching, as it retards the process considerably.
After the impurities have been removed, as outlined under the chrome process, the temperature of the oil is raised by open steam to boiling. The steam is then shut off and air allowed to blow through the oil until it is completely bleached, the temperature being maintained above 150° F. by occasionally passing in steam. Usually a ton of oil is readily and completely bleached after the air has been passed through it for 18 to 20 hours, provided the oil is thoroughly agitated by a sufficient flow of air.
If the oil has been allowed to settle over night, it is advisable to run off the condensed water and impurities by the lower cock before agitating again the second day.
When the oil has been bleached to the desired color, which can be determined by removing a sample and cooling, the mass is allowed to settle, the water run off to a waste tank from which any oil carried along may be skimmed off and the supernatant clear oil run to the storage or soap kettle.
In bleaching by this process, while the process consumes more time and is not as efficient in bleaching the lower grade oils, the cost of bleaching is less and with a good oil success is more probable, as there is no possibility of any of the chrome liquors being present in the oil. These give the bleached oil a green tint when the chrome method is improperly conducted and they are not removed.
Instead of blowing the air through it, the heater oil may be brought into contact with the air, either by a paddle wheel arrangement, which, in constantly turning, brings the oil into contact with the air, or by pumping the heated oil into an elevated vessel, pierced with numerous fine holes from which the oil continuously flows back into the vessel from which the oil is pumped. While in these methods air, light and heat act simultaneously in the bleaching of the oil, the equipment required is too cumbersome to be practical.
Recent investigations[1] in bleaching palm oil by oxygen have shown that not only the coloring matter but the oil itself was affected. In bleaching palm oil for 30 hours with air the free fatty acid content rose and titer decreased considerably.
Olive Oil, which comes from the fruit of the olive trees, varies greatly in quality, according to the method by which it is obtained and according to the tree bearing the fruit. Three hundred varieties are known in Italy alone. Since the larger portion of olive oil is used for edible purposes, a lower grade, denatured oil, denatured because of the tariff, is used for soap manufacture in this country. The oil varies in color from pale green to golden yellow. The percentage of free acid in this oil varies greatly, though the oil does not turn rancid easily. It is used mainly in the manufacture of white castile soap.
Olive oil foots, which is the oil extracted by solvents after the better oil is expressed, finds its use in soap making mostly in textile soaps for washing and dyeing silks and in the production of green castile soaps.
Other oils, as poppy seed oil, sesame oil, cottonseed oil, rape oil, peanut (arachis) oil, are used as adulterants for olive oil, also as substitutes in the manufacture of castile soap, since they are cheaper than olive oil.
Cottonseed Oil is largely used in the manufacture of floating and laundry soaps. It may be used for toilet soaps where a white color is not desired, as yellow spots appear on a finished soap in which it has been used after having been in stock a short time.
Corn Oil and Soya Bean Oil are also used to a slight extent in the manufacture of toilet soaps, although the oils form a soap of very little body. Their soaps also spot yellow on aging.
Corn oil finds its greatest use in the manufacture of soap for washing automobiles. It is further employed for the manufacture of cheap liquid soaps.
Fatty Acids are also used extensively in soap manufacture. While the soap manufacturer prefers to use a neutral oil or fat, since from these the by-product glycerine is obtained, circumstances arise where it is an advantage to use the free fatty acids. Red oil (oleic acid, elaine) and stearic acid are the two fatty acids most generally bought for soap making. In plants using the Twitchell process, which consists in splitting the neutral fats and oils into fatty acids and glycerine by dilute sulphuric acid and producing their final separation by the use of so-called aromatic sulphonic acids, these fatty acids consisting of a mixture of oleic, stearic, palmitic acids, etc., are used directly after having been purified by distillation, the glycerine being obtained from evaporating the wash water.
Oleic acid (red oil) and stearic acid are obtained usually by the saponification of oils, fats and greases by acid, lime or water under pressure or Twitchelling. The fatty acids thus are freed from their combination with glycerine and solidify upon cooling, after which they are separated from the water and pressed at a higher or lower temperature. The oleic acid, being liquid at ordinary temperature, together with some stearic and palmitic acid, is thus pressed out. These latter acids are usually separated by distillation, combined with the press cake further purified and sold as stearic acid.
The red oil, sometimes called saponified red oil, is often semi-solid, resembling a soft tallow, due to the presence of stearic acid. The distilled oils are usually clear, varying in color from light to a deep brown. Stearic acid, which reaches the trade in slab form, varies in quality from a soft brown, greasy, crumbly solid of unpleasant odor to a snow white, wax-like, hard, odorless mass. The quality of stearic acid is best judged by the melting point, since the presence of any oleic acid lowers this. The melting point of the varieties used in soap manufacture usually ranges from 128° to 132° F. Red oil is used in the manufacture of textile soaps, replacing olive oil foots soap for this purpose, chlorophyll being used to color the soap green. Stearic acid, being the hard firm fatty acid, may be used in small quantities to give a better grade of soap body and finish. In adding this substance it should always be done in the crutcher, as it will not mix in the kettle. It finds its largest use for soap, however, in the manufacture of shaving soaps and shaving creams, since it produces the non-drying creamy lather so greatly desired for this purpose. Both red oil and stearic acid being fatty acids, readily unite with the alkali carbonates, carbon dioxide being formed in the reaction and this method is extensively used in the formation of soap from them.