A very largo proportion of the commercial saltpetre which is refined in this country is made artificially, by the mutual decomposition of nitrate of sodium (" chili saltpetre ") and chloride of potassium. The process originated with F. C. Hills about the year 1846, and has since been im proved by Anthon, Kuhlmann, and others. The reaction is exceedingly simple and direct :— KC1 NaNO, = KNO, NaCl.
Chloride of potassium usually contains about 8 per cent. of chloride of sodium, but, as will be apparent from the above equation, the presence of this substance is of very slight importance, as one of the products is chloride of sodium. The exact composition of the materials, however, must be ascertained beforehand, that the proper proportions may be used. The nitrate usually contains flora 95 to 97 per cent. of nitrate of sodium ; therefore quantities of both materials in slight excess of the equivalent proportions must be taken. The chloride of potassium is dissolved in water with the aid of steam, the solution standing at about 1.25. The nitrate of soda is then added, and the whole well agitated. The heat is kept up to boiling point by an underneath fire, or coil of pipes ; and as the decomposition proceeds, the chloride of sodium that forms and settles is fished out and placed upon iron drainers ranged alongside of the pans, that the mother liquors may run back into the solution. Evaporation is continued until a density of about 1.7 is attained. The liquors are then run off into a series of settlers, and left for a short time. When perfectly clear, they are transferred to crystallizing pans, where large crystals of nitrate separate out. These are somewhat impure, containing varying quantities of chlorides and other salts. They are accord ingly taken off, dissolved in hot water, the solution concentrated up to 1.6, and recrystallized. The product is now of very fair purity—about equal to East Indian " petre "—and is refined by the process already described. The chloride of sodium left upon the drainers is removed to washing pans, and digested with successive portions of hot water. The chloride being only slightly soluble, the saltpetre is thus entirely removed—or as nearly so as possible. The washed salt is then gently
dried. Though not well suited for decomposition with sulphuric acid in the Le Blanc soda process on account of its irregular and non-crystalline form, this article is sufficiently good for all agricul tural, fish-curing, and other purposes of a similar character.
The washings from the chloride and the mother' liquors are mixed together, concentrated, and used, so long as they are fairly pure, in the place of water for dissolving purposes. The constitu tion of these liquors varies of course very much with the material used, the chloride of potassium especially being of une,ertain character. Often a considerable amount of iodine is contained in them, which may be recovered. If the potassium chloride contains an appreciable amount of chloride of magnesium, it is cleared by addiog a sraall quantity of soda ash to the solution. Sometimes crude carbonate of potassium is employed in place of the chloride, but the latter forms the cheaper material.
The saltpetre industry is a very important one, about 35,000 tons per annum being manufac tured in this country and imported from other quarters. Of this quantity about 16,000 tons are produced artificially. The plant, as a rule, is of comparatively rough description, although new and better mechanical contrivances are now superseding the old methods. One of these consists of a complete apparatus for dissolving and agitating the first solutions, and forcing the liquors through a strainer, vvhich retains the chloride of sodium and other impurities, and allows the cleared liquor to pass to the crystallizers. By agitating the cooling solution, too, and thereby preventing the formation of large crystals, the nitrate may be obtained in a condition approaching the " salt petre flour " of the refining process, and in a sufficiently good state for most purposes without any after-purification.
The chief use of saltpetre is in the manufacture of explosives, fully five-sixths of the total consumption being applied to this purpose (see Explosive Agents). Minor uses are found in the curing of meat and fish, and in the preparation of certain diuretic medicines.