in which the oxygen is the active principle. In practice it is found necessary to use an excess of acid over that theoretically indicated.
For the best results an oil should be chosen containing under 2 per cent. impurities and a low percentage of free fatty acids. Lagos oil is best adapted to these requirements. The oil is melted by open steam from a jet introduced through the bung, the melted oil and condensed water running to the store tank through two sieves (about 1/8 inch mesh) to remove the fibrous material and gross impurities. The oil thus obtained contains fine earthy and fibrous material and vegetable albuminous matter which should be removed, as far as possible, since chemicals are wasted in their oxidation and they retard the bleaching. This is best done by boiling the oil for one hour with wet steam and 10 per cent. solution of common salt (2 per cent. dry salt on weight of oil used) in a lead-lined or wooden tank. After settling over night the brine and impurities are removed by running from a cock at the bottom of the vat and the oil is run out into the bleaching tank through an oil cock, situated about seven inches from the bottom.
The bleaching tank is a lead-lined iron tank of the approximate dimensions of 4 feet deep, 4 feet long and 3-1/2 feet wide, holding about 1-1/2 tons. The charge is one ton. A leaden outlet pipe is fixed at the bottom, to which is attached a rubber tube closed by a screw clip. A plug also is fitted into the lead outlet pipe from above. Seven inches above the lower outlet is affixed another tap through which the oil is drawn off.
The tank is further equipped with a wet steam coil and a coil arranged to allow thorough air agitation, both coils being of lead. A good arrangement is to use one coil to deliver either air or steam. These coils should extend as nearly as possible over the entire bottom of the tank and have a number of small downward perforations, so as to spread the agitation throughout the mass.
The temperature of the oil is reduced by passing in air to 110° F. and 40 pounds of fine common salt per ton added through a sieve. About one-half of the acid (40 pounds of concentrated commercial hydrochloric acid) is now poured in and this is followed by the sodium bichromate in concentrated solution, previously prepared in a small lead vat or earthen vessel by dissolving 17 pounds of bichromate in 45 pounds commercial hydrochloric acid. This solution should be added slowly and should occupy three hours, the whole mass being thoroughly agitated with air during the addition and for one hour after the last of the bleaching mixture has been introduced. The whole mixture is now allowed to settle for one hour and the exhausted chrome liquors are then run off from the lower pipe to a waste tank. About 40 gallons of water are now run into the bleached oil and the temperature raised by open steam to 150° to 160° F. The mass is then allowed to settle over night.
One such wash is sufficient to remove the spent chrome liquor completely, provided ample time is allowed for settling. A number of washings given successively with short periods of settling do not remove the chrome liquors effectually. The success of the operation depends entirely upon the completeness of settling.
The wash water is drawn off as before and the clear oil run to storage tanks or to the soap kettle through the upper oil cock.
The waste liquors are boiled with wet steam and the oil skimmed from the surface, after which the liquors are run out through an oil trap.
By following the above instructions carefully it is possible to bleach one ton of palm oil with 17 pounds of bichromate of soda and 85 pounds hydrochloric acid.
The spent liquors should be a bright green color. Should they be of a yellow or brownish shade insufficient acid has been allowed and more must be added to render the whole of the oxygen available.