About the year 1670, the importation of saltpetre from the East Indies (where it is obtained as a natural product, being {276} disengaged by a kind of efflorescence from the surface of the soil) had so increased as to affect the home manufacture, which has since gradually declined and become extinct. The manufacture of saltpetre from sources of the kind above mentioned, is not followed in this country at the present day, and it will be unnecessary to indicate here the process employed in France, Sweden, Germany, and other countries for obtaining it by the decomposition of animal refuse, the more especially as full accounts are given in Knapp’s Technology, Ure’s Dictionary of Arts and Manufactures, and other standard chemical works; we shall therefore confine our attention to an account of the processes which have been proposed for obtaining nitrate of potash by the decomposition of nitrate of soda and other sources.

The first of these processes is that of adding nitrate of lime to a solution of sulphate of potash; sulphate of lime is precipitated, and nitrate of potash obtained in solution, which, on evaporation yields crystals of that salt.

Mr. Hill’s method of manufacturing nitrate of potash is by decomposing nitrate of soda by means of muriate of potash. For this purpose the nitrate of soda is put into a suitable vessel, made of wrought or cast iron, and dissolved in as much water as is required, and then the equivalent quantity of muriate of potash is added; decomposition ensues, with the formation of nitrate of potash and muriate of soda; the greater portion of the latter is separated during evaporation, as it is equally soluble at all temperatures. The nitrate crystallizes on the cooling of the solution. Specimens of this nitre were shown at the Great Industrial Exhibition.

Mr. Rotch’s processes for converting nitrate of soda into nitrate of potash are as follows:—

First process with American potashes, (caustic).—In a suitable round-bottomed iron boiler, he dissolves 2000 lbs. of the ashes in 1000 quarts of water, and then applies heat for three hours, at the end of which time the solution ought to be of a density of 45° Baumè, (sp. gr. 1.453). In a similar boiler he dissolves 1300 lbs. of nitrate of soda in 1200 quarts of water, {277} applying the heat as before, until the solution becomes of the density of 45° Baumé. Both solutions are then allowed to stand for twelve hours to cool and settle. They should be heated to from 175° to 200° Fah., and then both poured into a third vessel or crystallizing pan, when the double decomposition will take place, and the crystals of nitrate of potash be deposited, this first deposition giving from 700 to 900 lbs. of good merchantable saltpetre.

Care must be taken not to let the heat fall below 85°, at which the crystals form; and the better and more regularly the heat is kept up, the speedier will be the deposition of the crystals. The mother-liquor should then be poured off, and the crystals collected and thrown into the centrifugal drying machines, where they may be washed with weak mother-liquors. The portion of nitrate of potash that is left in the mother-liquor may be obtained by cry­stal­li­za­tion as before.

Second process with carbonate of potash (Pearlash).—The pearlash is dissolved in water, and the solution brought to a density of 40° Baumé (sp. gr. 1.384). This will cause whatever sulphate of potash may be contained in it to be deposited. The solution should then be left to stand for five or six days, after which it should be poured off, and diluted with water, until its density becomes 15° Baumé (sp. gr. 1,116). Caustic lime should then be added in the proportion of one-fourth of the weight of the original quantity of carbonate employed. It should then be poured off from the carbonate of lime formed, heated and mixed with the solution of nitrate of soda, as above described. The precise proportions that the caustic alkali should bear to the nitrate of soda, are forty-eight parts of the former to eighty-six parts of the latter. The materials to be used should be tested, so as to enable the just proportions to be arranged according to the formula just given. The patentee states that by this means a nitre is produced which is equal to the Bengal saltpetre, after the latter has gone through the expensive process of refining.

A Stockholm manufacturer says:—“On dissolving nitrate of {278} soda in excess of caustic potash solution, and evaporating to 28° or 32° Baumé (sp. gr. 1.241 or 1.285), the chief part of the saltpetre crystallizes, contaminated by the magnesia which is precipitated, and a small quantity of carbonate of lime. In order to obtain the whole of the saltpetre, the solution must be concentrated to 45° or 50° Baumé (sp. gr. 1.453 or 1.530). Here however, a difficulty arises; the cast iron crystallizing vessels are not impermeable to the liquor, which, whatever the thickness of the vessels, oozes through them, thus occasioning great loss. The saltpetre which still remains in solution after cry­stal­li­za­tion in the caustic solution at 30° Baumé (sp. gr. 1.263), cannot be collected, and if it be employed in the manufacture of soap, this will be found to contain so large a proportion of saltpetre, that it deliquesces and falls to pieces in a few days.”

“A method employed in the Russian manufactories is first to dissolve the fine pearlash, and the nitrate of soda in the relative proportions of water required for their mutual decomposition, or rather with an excess of potash in such a quantity of water that the resulting product remains dissolved at 50° Reaumur. The solution is then allowed to settle, whereby the carbonates of lime and magnesia are deposited, after which the liquor is run off into wooden crystallizing vessels. As soon as the temperature is lower than 50° Reaumur, the principal part of the nitrate of potash crystallizes. The cry­stal­li­za­tion must now be very attentively watched, for as soon as the soda begins also to crystallize, the mother-liquors should be run off into other vessels, where a small quantity of nitrate of potash will crystallize, though the principal part will be soda. The nitrate of potash and the soda must then be purified by new cry­stal­li­za­tions. The salts formed from the mother-liquors must be redissolved with the nitrate of potash or the soda, according to which of the two most predominates.”

Messrs. Crane and Jullion patented in 1848 the following method of manufacturing the nitrates of potash and soda:—The oxides of nitrogen evolved in the process of manufacturing oxalic acid, are mixed with oxygen gas or atmospheric air, and {279} made to pass slowly through a chamber or other apparatus containing an alkali placed on trays (similar to the lime in a dry lime purifier), the mixed gases combine with the alkali, forming a nitrate of potash or soda, whichever alkali may have been employed.