NITRE-BEDS.
The most important prerequisite in the construction of nitre-beds in such manner as to yield nitre in the shortest possible time, is a good supply of thoroughly rotted manure of the richest kind, in the condition usually called mould, or black earth. It is believed that in every vicinity a considerable supply of such manure may be found, either ready prepared by nature, or by the farmer and gardener for agricultural and horticultural purposes. To make the bed, a floor is prepared of clay, well rammed, so as to be impervious to water. An intimate mixture is then made of rotted manure, old mortar coarsely ground, or wood ashes (leached ashes will do), together with leaves, straw, small twigs, branches, &c. to give porosity to the mass, and a considerable quantity of common earth, if this has not been sufficiently added in the original manure-heap. The mixture is thrown somewhat lightly on the clay floor, so as to form a porous heap four or five feet high, six or seven wide, and fifteen feet long. The whole is then covered by a rough shed to protect from weather, and perhaps protected on the sides in some degree from winds. The heap is watered every week with the richest kinds of liquid manure, such as urine, dung-water, water of privies, cess-pools, drains, &c. The quantity of liquid should be such as to keep the heap always moist, but not wet. Drains, also, should be so constructed as to conduct any superfluous liquid to a tank, where it is preserved and used in watering the heaps. The materials are turned over to a depth of five or six inches every week, and the whole heap turned over every month. This is not always done, but it hastens very much the process of nitrification. During the last few months of the process, no more urine, nor liquid manure of any kind, must be used, but the heaps must be kept moist by water only. The reason of this is, that undecomposed organic matter interferes with the separation of the nitre from the ley. As the heap ripens, the nitre is brought to the surface by evaporation, and appears as a whitish efflorescence, detectible by the taste. When this efflorescence appears, the surface of the heap is removed, to the depth of two or three inches, and put aside under shelter, and kept moist with water. The nitre contained is thus considerably increased. When the whitish crust again appears, it is again removed until a quantity sufficient for leaching is obtained. The small mound which is thus left is usually used as the nucleus of a new heap. By this method it is believed that an abundant supply of nitrified earth, in a condition fit for leaching, may be obtained by autumn or early winter.I have spoken thus far of the method of preparing a single heap, or nitre-bed, such as any farmer or gardener may prepare with little trouble. But where saltpetre is manufactured on a large scale, as in the saltpetre plantations, many such beds are made and symmetrically arranged, so as to economize space; all under the same roof, with regularly arranged drains, all leading to a large cistern. In such plantations everything may be carried on with more economy, and with correspondingly increased profits.
LEACHING.
When the process of nitrification is complete, the earth of the heaps must be leached. Manufacturers are accustomed to judge roughly of the amount of nitre in any earth by the taste. A more accurate method is by leaching a small quantity of the earth, and boiling to dryness, and weighing the salt. There is much diversity of opinion as to the per centage of nitre necessary to render its extraction profitable. The best writers on this subject vary in their estimates from fifteen pounds to sixty pounds of salt per cubic yard of nitrified earth. The high price of nitre with us at present would make a smaller per centage profitable. This point, however, will soon be determined by the enterprising manufacturer. In the process of leaching, in order to save fuel, we must strive to get as strong a solution as possible, and at the same time to extract all or nearly all the nitre. These two objects can only be attained by repeated leachings of the same earth, the ley thus obtained being used on fresh earth until the strength of the ley is sufficient. A quantity of nitrified earth is thrown into a vat, or ash-tub, or barrel, or hogshead with an aperture below, closely stopped and covered lightly with straw. Water is added, about half as much in volume as the earth. After stirring, this is allowed to remain twelve hours. Upon opening the bung, about half the water runs through containing, of course, one-half the nitre. Pure water, in quantity half as much as first used, is again poured on, and after a few moments run through. This will contain one-half the remaining nitre, and therefore one-fourth of the original quantity. Thus the leys of successive leachings become weaker and weaker, until, after the sixth leaching, the earth is considered as sufficiently exhausted. The exhausted earth is thrown back on the nitre-beds, or else mixed with black earth to form new beds. The leys thus obtained are used upon fresh earth until the solution is of sufficient density to bear an egg. It then contains about a pound of salt to a gallon of liquid.CONVERSION.
The ley thus obtained contains, besides nitrate of potash (nitre), also nitrate of lime and magnesia, and chlorides of sodium and potassium. The object of the next process is to convert all other nitrates into nitrate of potash. This is done by adding wood ashes. The potash of the ashes takes all the nitric acid of the other nitrates forming the nitrate of potash (nitre), and the lime and magnesia are precipitated as an insoluble sediment. Sometimes the ashes is mixed with the nitrified earth and leached together, sometimes the saltpetre ley is filtered through wood ashes, sometimes the ley of ashes is added to the saltpetre ley. In either case the result is precisely the same.CRYSTALLIZATION.
The ley thus converted is then poured off from the precipitate, into copper or iron boilers. It still contains common salt (chloride of sodium) in considerable, and some other impurities in smaller, quantities. It is a peculiarity of nitre, that it is much more soluble than common salt in boiling water, but much less soluble in cold water. As the boiling proceeds, therefore, and the solution becomes more concentrated, the common salt is, most of it, precipitated in small crystals, as a sandy sediment, and may be raked out. Much organic matter rises as scum, and must also be removed. When the concentration has reached almost the point of saturation, the boiler must be allowed to cool. This is known by letting fall a drop of the boiling liquid upon a cold metallic surface; if it quickly crystallizes, it is time to stop the boiling. It is now poured into large receivers and left to cool. As the ley cools, nearly the whole of the nitre separates in the form of crystals, which sink to the bottom. These are then removed, drained by throwing them in baskets, and dried by gentle beat. The mother-liquor is either thrown back into the boilers, or else used in watering the heaps. The product thus obtained is the crude saltpetre of commerce. It still contains fifteen to twenty-five per cent. of impurities, principally common salt (chloride of sodium), chloride of potassium and organic matter. In this impure form it is usually brought to market.There is still another process, viz: that of refining, by which the whole of the impurities is removed. This is seldom done by the manufacturer, but by a separate class, called the refiners.
