In America, preference is given to petroleum-spirit of low boiling-point, as being cheaper and less dangerous, though less rapid in its action. The materials there principally dealt with are the residues from the rendering of animal fats (as tallow and lard), containing 12-15 per cent. of extractable grease. The patents relating to this industry in America are very numerous, but those taken out by Adamson seem to be most largely adopted. The process lasts 24-36 hours, and the products are used mainly as lubricants, being unfit for making good soap. "Pomace " or castor cake, and greasy cotton-waste, are similarly treated.
A very great disadvantage of petroleum-spirit, however, is that whereas the high sp. gr. of carbon bisulpbide enables it to be kept covered with water (and even collected from the mouths of the condenser-worms under water), and thus protected from all risk of explosion by contact of its vapour with air and flame, petroleum-spirit, which is, bulk for bulk, about half as heavy as the bisulphide, cannot be so pro tected from accident and loss by evaporation. Its vapour-density also is much less.
Recovery of Waste Grease.— A description of the means suited to the recovery of grease from fabrics and liquids which have served their purpose may fitly be introduced here.
In the recovery of oil from oily cloths, the latter are first soaked with water and soda crystals in a tub, and steam is thrown in ; the mixture of alkaline water and oil thus obtained (the latter saponifies to a slight extent), is run off into an open tank ; this operation is repeated as often as appears necessary. The oloths are then squeezed nearly dry between rollers ; the liquor is received in a tank, where it accumulates, and separates on standing, the oil collecting on the surface, while the dirty water remains below. The oil is ladled out into casks, and used for making blacking, or refined for other purposes. The oloths are finally dried by suspension in a chamber heated by coke fires in braziers. Offensive odours are generated, and the whole arrangement should be enclosed, and provided with means for conveying the vapours into a tall chimney.
The recovery of grease from the waste shoddy of woollen-mills, from cotton waete used for wiping oily machinery, from scrapings from fat-melting establishments, and in short from almost any mixed mass of grease and other matters, is now effected on a large scale by means of carbon bisulphide. The method of applying this solvent now generally adopted is to introduce the greasy matter very lightly and loosely into an airtight iron cylinder provided with a false bottom, taking care to ensure the porosity of the mass, and its freedom from more than a very small percentage of water. The hisulphide, pumped in from below, because it diminishes in sp. gr. as it dissolves the fatty matters, rises through the cylinder, dissolves out the grease, and flows away at the top into a still. It is then distilled off by steam, and condensed, running with the condensed steam into an underground reservoir, where it lies covered with water till required again. The greasy matters are left in the still, whence they can be drawn off. When all the grease has bean extracted from the
material, the bisulphide retained by the latter is drained off; the injection of free steam then drives off the last traces of bisulphide into a condensing-worm, and thence to the underground reservoir. The loss of solvent in each operation is trifling, but the most stringent precautions are necessary to prevent leakage. An arrangement of this character has been described and illustrated under Carbon Bisulphide (see pp. 605-6, Fig. 473).
Soap-suds contain a large quantity of fat combined with alkali as a soap. The recovery of this fat is now largely accomplished, especially in the woollen-manufacturing districts of England, from the suds leaving woollen-mills, which contain, in addition to the soap employed in washing or fulling the wool, a large quantity of fat derived from the wool itself. These are first carefully strained and then settled to remove extraneous matters. From the settling-tank, the liquid flows into several large tanks, where, in succession, the " breaking" or decomposition of the soap is effected. Steam is first injected to raise the temperature to about 49° (120° F.), and sulphuric acid is then added, in such proportion as to leave the (broken) liquor feebly sour to the taste. This preliminary heating is not adopted in all cases, as the same end (the facilitation of the breaking) is attained by keeping the suds for 3-4 days. But this is a most objectionable proceeding, on account of the offensive putrefactive odours emitted ; it also requires the provision of much additional storage room. In the breaking, the acid combines with the soda, and liberates the grease, which partially floats, and partially settles to the bottom of the tank with earthy and other impurities. Time is allowed for the complete separation of the grease ; the liquid is then run off through drains, and the scum or " magma" is put into canvas filters on wooden frames. From these, the liquid portion escapes into the drains, and a blackish-grey greasy mass remains upon the filters. This mass is wrapped up in cloths, and first subjected to cold expression to free it completely from water. To recover the grease, the packages, ait they leave the cold press, are transferred to a hot press, where steam is introduced at ordinary pressure. The grease is thus melted out, and, with condensed water and some dirt, escapes at the bottom into a sunken tank, about 2 ft. long, 1 ft. wide, and 3 ft. deep. Here the oil separates from the dirty condensed water, floats, and is ladled out while still liquid. The dirty water is let out at intervals into a large underground tank, where more grease separates. The oil ladled out is transferred to a lead-lined vessel, and treated with strong sulphuric acid to remove any water that may be in it, after which, it is barrelled for sale. The residue in the hot press, called " sud-cake," is treated with carbon bisulphide to extract any remaining traces of grease, and is then used for manure-making. The waste liquor, neutralized with lime, or made slightly alkaline, may be beneficially used for irrigating pasture.