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The One-stage Process. In the one-stage process, but one acid mixture is used. This mixture consists of 75 per cent sulphuric acid, and 25 per cent nitric acid. The usual charge for this process is in the ratio of one part toluene to twelve parts mixed acid. Each kilogram of toluene requires, therefore, 3 kilograms nitric acid and 9 kilograms sulphuric acid. The toluene is added to the acid in the one-stage process. The termination of the inlet pipe is in a kind of well at the bottom of the nitrator, and the toluene is nitrated before it can separate and spread about. During the addition of the toluene the temperature must not rise above 30° C. This addition requires from two to two and one-half hours. After the addition is complete, the temperature of the mixture is raised to 90 to 95° C , by means of the steam coils. This temperature is maintained for two hours, during which time the mononitrotoluene is nitrated to dinitrotoluene. The temperature is then further increased to 120°, and is " cooked " for an additional two hours. The nitration is now considered finished, and the charge is ready for separation, washing, and graining. In many chemical plants a chemist tests the charge of TNT, and the time of the final cooking is determined by the solidification point of the mixed TNT and acid. This principle of chemical control is adopted sometimes in the two- and three-stage processes as well as in the one-stage process. The yield of TNT by the one stage process is 1.9 kilos per kilo toluene used.
