Page:Popular Science Monthly Volume 83.djvu/225

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 The crystallizing tanks. Refined nitrate in the tanks in the foreground; tanks recently filled with caldo in the background.

tanks, known as chullladores, where the use of wheat flour, stable manure, or other substances, causes the precipitation of the miscellaneous soluble impurities, except ordinary salt, which have been dissolved out with the nitrate. From this purification process the solution goes to the crystallizing tanks, or bateas, which are placed ten or twelve feet above the ground to permit free circulation of air and promote cooling and evaporation. Thus dryness which figures in the origin and preservation of the caliche also has an equally great value in the process of manufacture. As the solution cools and the water evaporates, the nitrate begins to crystallize on the surface, so a "stirring boy," or rayandero, is employed to break up the film and make it settle. Five or six days are necessary to complete the crystallizing process. A large plant may have 300 or more bateas, capable of holding more than 1,000,000 gallons of caldo, and yielding at each full charge as much as 2,500 tons of nitrate.

When crystallization has gone as far as it will, a valve in the bottom of the batea is opened and the liquid is drawn off, leaving behind a thick layer of glistening white crystals. This is the nitrate or salitre of commerce, being 95 per cent, or more of pure nitrate of soda; the remainder is largely water and salt. The liquid which is drawn off, known as agua vieja, or mother liquor, still contains a large amount of nitrate in solution, and is used over and over again in the boiling tanks. In fact, no water is ever thrown away, the only loss being that which passes into steam from the boiling tanks and evaporates from the crystallizing