Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/802

742 OILS a calender. The seeds pass in succession between the first and second rollers in the series, then between the second and third, and so on till they are delivered by the lowest, sufficiently bruised, crushed, and ground. Pressing. With the least possible delay the meal is transferred from the heating kettles, so that the oil may be pressed out while the material still retains its heat. Measured quantities, say 10 to 12 tb of meal, are filled into woollen bags of strong, thick texture, sufficiently open and porous to allow free flow of the expressed oil, yet having consistency enough to resist rupture by the enormous pressure to which it is subjected. Each bag is further placed within "hairs," thick mats of horse-hair bound with leather. In some methods of working press-cloths not bags are used ; and the con struction of recent presses is such as to dispense altogether with the use of bags or other coverings. The essentials of proper oil-pressing are a slowly accumulating pressure, so that the liberated oil may have time to flow out and escape, a pressure that increases in pro portion as the resistance of the materials increases, and that main tains itself as the volume of material decreases through the escape of the oil. These essentials the Dutch or stamper press and the hydraulic press fulfil perfectly, and the prevalence of hydraulic pressure over the other and older method is only due to the greater convenience and ultimate economy of the power. Previous to the early years of the present century the Dutch press was almost ex clusively employed in Europe for pressing oil-seeds. It consists of two principal parts, an oblong rectangular box with an arrangement of plates, blocks, and wedges, and over it a framework with heavy stampers, the fall of which produces the pressure. The press box is made either of cast-iron or, according to the older method, of strongly-bound oaken planks. At each extremity of the box there is placed a bag of oil-meal between two perforated iron plates, under which are a perforated bottom and channels for conducting away the expressed oil. Next are inserted filling-up pieces of wood, two of which the speering-blocks are oblique or bevelled on one face, forming ways for the two wedges which press against them. Be tween the speering-blocks, and separated also by a filling piece, are inserted the two wedges, one being the ordinary or driving wedge by which the pressure is applied to the seed-bags, and the other an inverted or spring wedge, which is only driven down to loosen and free the various parts when the pressing operation is complete. The stamper which drives home the ordinary wedge is a heavy log of wood about 16 feet long by 8 inches square, and it falls about fif teen times a minute through a maximum distance of 22 inches by the action of a pair of cams or wypers fixed on a revolving shaft. As soon as the pressure is complete the stamper suspended over the inverted wedge is brought into action, and by a single heavy blow knocking the wedge out of its key-like position it frees the various parts of the apparatus for the removal of the pressed cakes. Ill a double stamper press about 12 cwt. of finished cake is made per day. Since the introduction of the Bramah press, numerous modifications have been invented with the special object of improv ing its convenience as an oil-press. The various forms devised for oil-extracting may be comprehended under standing or vertical presses and horizontal or lying presses, with specially-modified seed- boxes and press plates in each instance. The most primitive form of upright press, and one which still recommends itself for simplicity where great pressures are not essential, is a drum or box press, so called because on the platen are placed two circular metal tubs, one within the other, the inner perforated throughout for the escape of the oil. At the top of the press is secured a strong metal plate or table the same diameter as the inner box, and the seed is pressed by the working up of the ram carrying the box against the surface of this table. Within the perforated box the seed-bags are deposited with metal plates between them. Experience, however, has demon strated that the best presses are those provided with separate trays or seed-boxes for each bag, and the ordinary oil-press of the present day is fitted with four seed-boxes, and presses four separate cakes at one working. A convenient form is the double oil-press of Blundell, which admits of continuous working, one division being under pressure, while the other division is being emptied and recharged. The final pressure applied in ordinary practice on the seed-trays in an hydraulic press is equal to a weight of about 300 tons. This weight is allowed to remain on for seven minutes, and the whole operation of charging, pressing, and emptying a press maybe finished in ten minutes. A Blundell double press is capable of working off about 5 cwt. of seed per hour. A form of press of the most recent and improved construction, called the "pack press," dispenses with bags, seed-boxes, and hair-mats. Consolidated cakes of seed enclosed in press-cloths to the number of sixteen for one charge are simply placed between corrugated plates. To enable the press to be charged with sufficient rapidity, and to allow the large number of bags to occupy as small a vertical space as practicable when placed in the press, a moulding or form machine, such as was invented by John Bennie of Glasgow and patented in October 1880, or a similar machine, worked on a modified principle, patented by Francis Virtue, is employed. In these machines, by mechanical contrivances, the measured quantity of heated meal is placed in a trapeziform tray enclosed in a cloth and submitted to a pressure which reduces the thickness of the mass from about 3 inches to a little more than 1 inch and forms it into a solid cake, in which state it packs in small space in the press. The whole time occupied in filling and forming a cake and placing it in the press is not more than a quarter of a minute, and a set of three pack presses in a day of ten hours will work off nearly 9 tons of seed, yielding in the case of linseed about 108 cwt. of cake, and 54 cwt. of oil. It is claimed for the pack press that it extracts a much larger proportion of oil than that worked with seed-boxes. Horizontal presses are not much in favour in the United Kingdom, but in many Continental mills where two pressings are the rule a set of horizontal presses are kept for the first operation. The oil, during the process of pressure, works its way from the centre to the edge of the cake, whence it exudes. For this reason an oblong form is the most favourable for the easy separation of the oil, and the trapeziform shape oil-cakes usually present has been selected on account of the wedge -like steadiness the mass has under pressure, and the readiness with which the entire cake frees itself once it is moved the smallest distance from the thin end of the box or tray in which it is pressed. The edges to which the oil is pressed almost invariably retain a considerable proportion of oil. They are pared off, and the parings are returned to the edge-stones to be ground up and again pressed with fresh meal. The oil from the presses flows into the receiver tanks placed under the level -of the floor, from which it is pumped into the storage tanks, where it is permitted to settle and clarify. After mechanical impurities and water, &c. , have separated themselves, the oil is in some cases ready for the market, but for the most part it has to undergo a process of refining. Extraction by Solvents. The only method of obtaining vegetable oils which has come into practical competition with pressing is that in which the solvents bisulphide of carbon (CS 2 ), the light spirit of petroleum, and common ether are used. In ordinary pressing about 10 per cent, of oil remains in the finished cake, while by means of solvents practically the whole of the oil may be separated. Solvents might therefore be used for extracting that remaining percentage of oil from any oil-cake were such desirable, or they can be employed for treating fresh unpressed seed. As a matter of fact, it is desirable to leave a proportion of oil in the cake which is used for feeding purposes, because its food value depends to no small extent on the oil it contains, and the perfect separation of oil in the solvent process is a drawback, on account of the poverty in fatty matter of the exhausted meal. Extraction by the agency of bisulphide of carbon was first intro duced in 1843 by Jesse Fisher of Birmingham. Twelve years later a patent was secured by E. Deiss of Brunswick, but for several years afterwards the process made little advance. The colour of the oil produced was high, and its taste sharp ; it retained traces of sulphur, which showed themselves disagreeably in the smell of soaps made from it, and in the blackening of paints in which it was used ; and the meal left by the process was so tainted with evil-smelling car bon bisulphide that cattle would not taste it. These drawbacks have now been perfectly surmounted, and the process appears likely to come into extended use on the Continent. The seed for treatment with bisulphide of carbon is prepared as for pressing, except that it is not reduced to a fine meal, which would prevent the percolation of the solvent freely through the mass. It is only bruised and placed in a series of four or five up right cylinders, which are hermetically closed and provided with a complicated arrangement of pipes leading to and from each other in various ways, all being controlled by stop-cocks. Into the first cylinder A bisulphide of carbon is admitted from an overhead reservoir, till the whole mass is saturated with the fluid. After allowing the bisulphide to act on the contents of the cylinder for about fifteen minutes, communication between cylinders A and B is opened, the fluid from A passss into B, and a fresh supply of bisulphide comes from the reservoir into A. Again, after the lapse of fifteen minutes, the pipe leading into cylinder C is opened, and the fluid contents of B enter C. B is filled from A, which again is replenished from the reservoir. The extraction thus goes on, pure bisulphide of carbon always entering the weakest, most nearly exhausted cylinder, and passing on with gradually increasing per centage of oil to cylinders having more and more oil in their con tents, till at the end, in the most recently charged cylinder the bisulphide is fully saturated with oil and passes off to the distil ling apparatus, where the oil and the bisulphide are separated. When the contents of a cylinder have been fully extracted that vessel is isolated from the others, the remaining bisulphide of car bon is forced out by the admission of compressed air, and there after steam is run through the exhausted meal till not a trace of the solvent remains. The bisulphide, boiling at 46 C., is easily separated from the oil by simple distillation, and the last traces of the sulphur compound are removed by blowing steam through the oil in the distilling apparatus. Careful provision is made for the prevention of escape and for the recovery and proper storing of the bisulphide of carbon in use.