Extraction by Solvents.—The cakes obtained in the foregoing process still retain considerable proportions of oil, usually about o%. If it be desired to obtain larger quantities than are yielded by the methods described above, the extraction of the seeds must be performed by the use of volatile solvents. Extraction by car bon disulphide was first introduced in 1843 by Jesse Fisher of Birmingham. For several years the process made little advance, for the colour of the resulting oils was dark and the taste sharp. The oil retained traces of sulphur, causing a disagreeable smell in soaps made from it, and the blackening of substances with which it was used. The meal was so tainted with carbon disulphide that it was absolutely out of the question to use it as cattle-food. With improvement in the manufacture of carbon disulphide these draw backs have been surmounted to a large extent, and the practice of extraction with this solvent has especially gained extension for waste olive marc in France, Italy and Spain. Modern methods largely use petroleum ether (shale naphtha is more usual for the oil extraction of bone-fat) ; benzene and the non-inflammable hydrocarbons such as trichlorethylene, have been used, but have not proved so satisfactory, and in the latter case the risk of physiological action on the workmen, has militated against their wider utilisation. The methods of operation have been improved so that losses of solvent do not amount to more than I %. Good quality oils can be obtained, and the processes are able to com pete with expression, the choice of method resting mainly on considerations of the particular seed to be treated and the type of meal desired.
The apparatus employed on the large scale depends on the temperature at which the extraction is to be carried out. In the main two types of plant are differentiated, viz., for extraction in the cold, and for hot extraction. The seed is prepared as for pressing, but the grinding is coarser. In cold extraction the meal is placed in a series of closed vessels, and the solvent allowed to percolate by displacement (upward, in a recent process) on the "counter-current" system. The solution of extracted fat is then transferred to a steam-heated still, where the solvent is driven off and recovered by condensation to be used again. The last remnants of volatile solvent in the oil are driven off by a current of steam blown through the oil in the warm state. The hot extraction process is carried out in apparatus, the principle of which is exemplified in the well-known Soxhlet extractor. The comminuted seed, spread on trays, is placed inside a vessel con taining the solvent and fitted with a reflux condenser. On heating the solvent by means of a steam-coil or jacket the vapours rise through and around the meal, passing into the condenser, whence the solvent drops back as liquid on to the hot meal, percolating through it, and reaches the bottom of the vessel as a more or less saturated solution of oil in the solvent. The solvent is again evaporated, leaving the oil behind, the process proceeding con tinuously until the extraction is deemed finished. The oil solution
is run into a still and the oil freed from solvent as already de scribed. The solvent remaining in the meal is removed by a similar steaming process. It is true that on the European conti nent extracted meal, especially rape meal from good Indian seed and palm kernel meal, is somewhat largely used as cattle-food in admixture with press-cake; in England extracted meal is not so employed, but finds its proper use in manuring the land.
In special cases, such as the preparation of edible oils and fats, a further improvement in colour and greater purity is obtained by filtering the oils over absorbent materials such as charcoal, fuller's earth, etc. As in the case of coconut and palm-kernel oils, a preliminary purification in a current of steam may be employed. For refining oils and fats intended for edible purposes, only the foregoing methods, which may be summarized by the name of physical methods, can be used ; the only chemicals permissible are alkalies or alkaline earths to remove free fatty acids. The practice of refining by caustic soda has gained considerable extension; the fatty acids are neutralised by caustic soda and removed in the form of soaps by washing, but technically, the process is complicated by the formation of emulsions. Treatment with other chemicals renders the fats unfit for consumption. Hence, all bleaching and refining processes involving other means than those enumerated can only be used for technical oils and fats. There is no universal method of oil refining applicable to any and every oil and fat. Not only must each kind of fat be considered as a special problem, but frequently even varieties of one and the same fat are apt to cause the same difficulties as would a new individual. In many cases purification by sulphuric acid is still usefully applied. The oil is treated with a small pro portion of concentrated sulphuric acid; this acts on the suspended impurities, carbonising them to some extent, and causes them to coagulate in the form of a flocculent mass, which settles out carrying with it mechanically other impurities that have not been acted upon. This method is chiefly used in the refining of linseed and rape oils. After treatment with sulphuric acid or caustic soda the oils must be thoroughly washed; the water is allowed to settle out and the oils finally filtered.