A large variety of reagents are in use for bleaching technical oils intended for soaps, lubricants, paints, etc. In general, besides treatment with absorbent media, or exposure to air and light (olive oil), bleaching methods involve the use of oxidising agents such as sodium dichromate and acid, permanganate, hypochlorites, metallic per-salts, peroxides, etc.
Fat-hardening.—There is great demand in the arts for hard fats, and a great development of the fats and oils industry took place in the early years of the present century as a result of the invention of a successful method for converting the liquid un saturated fatty acids and glycerides into the corresponding solid saturated bodies. The change ("hardening," "hydrogenation") which consists in the addition of hydrogen to the unsaturated bonds, e.g., ides. The metal in general use is nickel; platinum and palladium are active catalysts, and by their use the reaction can be effected at much lower temperatures, but their high cost and susceptibility to poisoning by traces of impurities preclude their commercial use.
The usual process consists in passing hydrogen through the oil, which is intimately mixed with the catalyst ; this consists of finely divided metallic nickel, which, as a rule, has been deposited on an inert support, such as kieselguhr or fuller's earth. The reaction is conducted at temperatures between and C, according to conditions. After hydrogenation the oil has to be filtered to free it from powdered catalyst ; the nickel from the latter is recovered. A recent innovation, designed to allow of continuous working, and to obviate the tedious processes of filtration and recovery of the nickel from the oil-saturated residue, employs fine nickel turnings. The turnings are placed in perforated cages in the reaction vessel. The pre-heated oil and the hydrogen flow over them on the counter-current system. The oil delivered is free from nickel and does not require filtration ; when the activity of the catalyst becomes reduced, as a result of gradual poisoning, it is easily regenerated by electrolytic oxidation and subsequent reduction. The mechanism of oil-hydrogenation, and the way in which the nickel assists the reduction, has not been fully explained; it appears that the reduction takes place in stages, the most un saturated acids being first attacked. As the hardening process is always accompanied by a certain amount of deodorisation, more especially in the case of whale and fish oils, it has rendered pos sible the utilisation of these oils for the manufacture of hard soap, and has led to the substitution of cheaper vegetable oils, hydrogenated to the required consistency, for the more expensive tallow and lard.
The detection of hardened fats in a mixture is a difficult, if not an insoluble, problem. Indications may be furnished by the detec
tion of fatty acids of high molecular weight, e.g., behenic acid from hardened rape or marine animal oils, or of considerable proportions of iso-oleic acid, which is a transition product in the conversion of oleic into stearic acid.
Applications.—It is almost impossible to enumerate the many applications of the fats: in the table above the principal uses of some typical oils and fats are given. Almost all the best quality fats, including hardened whale oil, etc., can be used for edible purposes (margarine, cooking fats, salad oils, etc.), and some constitute a staple article of diet for native races, for in stance, palm oil in W. Africa, soya bean oil in Manchuria.
In the arts the fats are used for soap-making (q.v.), candle stearine manufacture (q.v.) and leather-dressing; the latter industry provides an outlet for fish-oils and other low grade fats. Linseed oil, tung oil, and to a less extent other drying oils, are employed as the basis of paints, varnishes and for linoleum. Many of the non-drying oils (e.g., neat's foot oil, sperm oil, etc.), are employed as lubricants, either alone or compounded with mineral oils (rape, castor oil), while lubricating greases are made from metallic soaps. Rape oil (colza) and other oils are still used as burning oils in remoter districts. Castor, and "tournant" olive oils provide the turkey-red oils used in dyeing textiles (see also GLYCERINE).
proved to be difficult of practical realisation. Many attempts had been made ; stearolactone had been prepared by the action of con centrated sulphuric acid on oleic acid, and palmitic acid by treat ment with potash, while the conversion into stearic acid itself had been effected by heating oleic acid under pressure with small quantities of iodine, or by subjecting it to the action of hydrogen under the influence of the silent electric discharge. None of these processes, however, proved economically successful on the com mercial scale. The problem was solved by the application of the general reaction discovered by Sabatier and Senderens, namely, that hydrogen could be assimilated by unsaturated compounds in the presence of a metallic catalyst. The principle was found to be readily applicable, not only to fatty acids, but also to the glycer In medicine.—Some of the oils possess peculiar properties for which they are valued medicinally. Castor (q.v.) and curcas oils have a strong purgative action, while chaulmoogra oil, and others of the same group, have recently attracted attention as specifics for leprosy. Lard, olive, almond and other oils find application in pharmacy as vehicles for various medicaments (see also COD LIVER OIL).