When the foreign salt is not fully deposited, the liquor is clarified, and more water is added, sufficient to form ten parts, including that which has already been poured upon it. The liquor is removed, when it is clear and less heated, and put into copper vessels, where it is agitated to prevent crystallization, and to effect the pulverization of the saltpetre.
The saltpetre obtained by this process is not sufficiently pure. The purification is completed by washing it with water saturated with nitre, which dissolves the foreign substances. This washing is completed in a vessel, the bottom of which has been pierced with holes. The nitre, however, is left some hours in contact with the water, when the latter is permitted to run out. When the solution is of the same degree of concentration as that of the saturated water, the operation is finished. The nitre is dried for use.
The old process of refining saltpetre is thus described: Put into a copper, one hundred pounds of nitre, and fourteen gallons of water; let it boil gently half an hour, removing the scum as it forms; then stir it, and before it settles put it into filtering bags, which must be suspended from a rack. Put under the filters glazed earthen pans, to receive the liquor; in which place sticks for the crystals to form on. In two or three days, it will all crystallize.
In some saltpetre works, sulphate of potassa is used with advantage. This salt is furnished in abundance, by the combustion of a mixture of nitre and sulphur, in the manufacture of oil of vitriol. It forms the residue after the combustion. It is likewise produced in the preparation of nitric acid, in the decomposition of nitrate of potassa, by sulphuric acid. It may, therefore, be obtained in quantity, from the oil of vitriol manufacturers, and the aquafortis distillers. It is usually called vitriolated tartar.
It is known that sulphuric acid forms, with lime, an almost insoluble compound, called sulphate of lime, or gypsum; and hence, when sulphate of potassa is mixed with a solution of nitrate of lime, nitrate of potassa is formed, which remains in solution, and sulphate of lime is precipitated. The same effect takes place with all earthy nitrates. For the application of sulphate of potassa, in this way, we are indebted to M. Berard. It might be advantageously employed in decomposing the calcareous nitrate of the nitre-caves of the western country.
M. Longchamp has recommended the use of sulphate of soda, or Glauber's salt, for decomposing the muriate of lime, which exists occasionally in impure nitre. These two salts reciprocally decompose each other; sulphate of lime is precipitated, and muriate of soda remains in solution. The latter is separated by evaporating the nitrous solution.
M. de Saluces (Mémoire de l'Académie des Sciences de Turin, Année, 1805 à 1808,) has proposed a new process for purifying nitre. It consists in filtering it through argillaceous earth, or clay. Although the process is highly spoken of, yet we can see no particular advantage it possesses.
Chaptal observes, that the process mostly in use is that of dissolving 2000 pounds of crude saltpetre in a copper boiler, in 1600 lbs. of water. As the solution is made by the heat, the scum, which forms, is taken off. Twelve ounces of glue, dissolved in ten pints of boiling water, and mixed with four pails full of cold water, are then added. This addition cools the solution. As to the manipulations of the process, they have been given. The principal thing to be attended to, is to separate the marine salt, which is done during the boiling.
To pass this saltpetre through a second operation, in order the more to purify it, it is again dissolved, in the proportion of 2000 pounds, in one-fourth of its weight of water. Heat is applied. The scum is separated; a solution of 8 ounces of glue in one or two pails full of water is then added. After the solution becomes clear, it is suffered to cool, and at the expiration of five days, it will crystallize, or form in a mass, which is then exposed to the air six or eight weeks to become completely dry.
In treating of the formation of nitre in France, Bottée and Riffault (Traité de l'Art de Fabriquer la Poudre à Canon,) consider it under the following heads: