FULLER'S EARTH PROCESS FOR BLEACHING TALLOW.
From one to two tons of tallow are melted out into the bleaching tank. This tank is jacketed, made of iron and provided with a good agitator designed to stir up sediment or a coil provided with tangential downward opening perforations and a draw-off cock at the bottom. The coil is the far simpler arrangement, more cleanly and less likely to cause trouble. By this arrangement compressed air which is really essential in the utilization of the press (see later) is utilized for agitation. A dry steam coil in an ordinary tank may be employed in place of a jacketed tank, which lessens the cost of installation.
The tallow in the bleaching tank is heated to 180° F. (82° C.) and ten pounds of dry salt per ton of fat used added and thoroughly mixed by agitation. This addition coagulates any albumen and dehydrates the fat. The whole mass is allowed to settle over night where possible, or for at least five hours. Any brine which has separated is drawn off from the bottom and the temperature of the fat is then raised to 160° F. (71° C).
Five per cent. of the weight of the tallow operated upon, of dry Fuller's earth is now added and the whole mass agitated from twenty to thirty minutes.
The new bleached fat, containing the Fuller's earth is pumped directly to a previously heated filter press and the issuing clear oil run directly to the soap kettle.
One of the difficulties experienced in the process is the heating of the press to a temperature sufficient to prevent solidification of the fat without raising the press to too great a temperature. To overcome this the first plate is heated by wet steam. Air delivered from a blower and heated by passage through a series of coils raised to a high temperature by external application of heat (super-heated steam) is then substituted for the steam. The moisture produced by the condensation of the steam is vaporized by the hot air and carried on gradually to each succeeding plate where it again condenses and vaporizes. In this way the small quantity of water is carried through the entire press, raising its temperature to 80°-100° C. This temperature is subsequently maintained by the passage of hot air. By this method of heating the poor conductivity of hot air is overcome through the intermediary action of a liquid vapor and the latent heat of steam is utilized to obtain the initial rise in temperature. To heat a small press economically where conditions are such that a large output is not required the entire press may be encased in a small wooden house which can be heated by steam coils. The cake in the press is heated for some time after the filtration is complete to assist drainage. After such treatment the cake should contain approximately 15 per cent. fat and 25 per cent. water. The cake is now removed from the press and transferred to a small tank where it is treated with sufficient caustic soda to convert the fat content into soap.
Saturated brine is then added to salt out the soap, the Fuller's earth is allowed to settle to the bottom of the tank and the soap which solidifies after a short time is skimmed off to be used in a cheap soap where color is not important. The liquor underneath may also be run off without disturbing the sediment to be used in graining a similar cheap soap. The waste Fuller's earth contains about 0.1 to 0.3 per cent. of fat.