Where filtration fails to remove impurities, recourse may be had to the action of acids or alkalies upon them. There are several methods of applying mineral acids to the purification of oils. Thenard's process consists in gradually adding 1-2 per cent. of sulphuric acid to oil previously heated to 38° (100° F.), and mixing by constant agitation. When the action of the acid is complete, the oil, after 24 hours' rest, appears as a clear liquid, holding flocculent matter in suspension; a quantity of water heated to 60° (140° F.) equal to about of the oil is added, and the mixture is well agitated until it acquires a milky appearance. It is then allowed to settle for a few days, when the clarified oil rises to the surface, while the flocculent matter falls to the bottom with the acid liquid. The oil is then drawn off, and washed in another vessel by agitation with half its bulk of warm water ; but it requires to be filtered to make it perfectly clear. This process is largely used for refining linseed oil.
Cogan operates upon about 100 gal. of oil with about 10 lb. of sulphuric acid, previously diluted with an equal bulk of water. This mixture is added to the oil in three portions, the oil being well stirred for about an hour after each addition. It is then stirred for 2-3 hours to ensure perfect mixture. After being allowed to stand for 12 hours, it is transferred to a copper boiler with a perforated bottom, through which, steam enters and passes in a finely divided state through the oil, raising it to the temperature of 100° (212° F.). This is continued for 6-7 hours, and the oil is transferred to a cooler, shaped like an inverted cone, terminating in a short pipe, and provided with a stop-cock at the side, a little distance from the bottom. After standing till the liquids are separated, generally about 12 hours, the acid liquor is drawn off through the pipe at the bottom, and the clear oil by the stop-cock in the side of the cooler ; all below this tap is generally turbid, and is clarified by subsidence, or mixed with the next portion of oil.
These acid processes are efficient when well conducted, but too much or too little acid may spoil the product, because, as most of them depend for their action upon the fact that strong sulphuric acid chars organic substances by the removal from them of the elements of water, it chars the fibre in the oil first, but if more acid than necessary for this be present, it attacks the oil itself, and oil thus stained by charring cannot be completely decolorized again.
On this account, perhaps, more general preference seems to be accorded to alkaline processes. Evrard's, which is chiefly applied to colza- and rape-oils, is as follows. The oil, drawn cold, or at very slight heat, is well crutched up with a weak lye of soda or potash, and allowed to settle. Two layers soon form—a neutral oil floating on an alkaline liquid, a mixed emulsion intervening. The alkaline liquid is drawn off, and replaced by slightly alkaline water, and the whole is left to settle. This is repeated a few times with clear water, till the liquid at the bottom of the settlers is only slightly milky. The oil is drawn off and filtered, and is superior to oil purified by sulphuric acid, being much less corrosive to metal. The turbid residual waters are treated with acid, and give a greasy product fit for soap-making. A much simpler alkaline method adopted in Italy for
olive-oil is to add 400 grm. of ammonia, diluted with 800 grm. of water, to every 100 kilo. of oil, agitating thoroughly, allowing to stand for 3 days, and then decanting and filtering.
One of the most remarkable impurities in fats, arising from methods of preparation merely, is that of lime in bone-fat. This fat has the power of dissolving considerable quantities of lime-salts, especially phosphate and carbonate. No amount of subsidence or filtration will remove them, and their presence in a soap-copper is most objectionable. It is greatly to be desired, therefore, that English makers of this fat would follow the example of their American confreres, and boil their bone-grease, after removal from the extractors (p. 1449), with a weak solution of sulphuric acid, in lead-lined wooden tanks. This removes all the lime, in the form of sulphate, which deposits on the floor of the tank after due subsidence ; it also removes the gelatine and extraneous water entangled in the bone-fat, which cause the crude grease to froth greatly when heated.
A good example of the removal of resinous substances from oils is afforded by the process adopted for refining and bleaching cotton-seed-oil, an industry which has enormously developed both in England and the United States, within the last 15 years. When freshly expressed from new seed, this oil is of a•light-claret colour, which darkens by long keeping, in which case also, the oil becomes more viscid, probably from oxidation of some of its constituents. The colouring matter is almost entirely resin, which may be removed by agitation at about 60° (140° F.) with solution of carbonate of soda. It is found in practice, however, that a, much better result is obtained by the use of a caustic alkali—solution of soda, potash, or, in some rare cases, milk of lime. The amount of alkali thus employed depends entirely upon the quality of the crude oil, and is best determined by a pre liminary experiment upon a small scale. A solution of caustic soda at about 1.10 sp. gr. is a suitable strength. Agitation must be thorough, and may be effected by any convenient mechanical means. The process is a rapid one ; if the saponaceous liquid does not readily separate from the oil, the addition of a little brine will cause it to do so. The operation is often divided into two or three stages, and occasionally the refined oil is bleached by one of the oxidation processes, such as by chloride of lime. After all the refining, it should be washed with warm water, allowed to settle, and decanted, or filtered at as low a temperature as possible, especially if an oil be desired that will remain fluid at a low temperature. This process will answer well for any resin-contaiuing oil. The imperfect soap, after removal, is treated with enough mineral acid to remove all the soda, and the resulting mixture of resin, fatty acids, and neutral oil is distilled with superheated steam (see Candles) for the manufacture of fatty acids, the resin being left in the still as pitch. The chief seat of this industry in England is at Hull. In the United States, the quantity of resin is so small, that the " foots" from the cotton-seed-oil refineries are made into a curd soap (see Soap).