Sulphitation and drying of TNT

There are various methods of purification of the TNT with sodium sulphite. French method 

In the French method the ground TNT is mixed with a 7.5% solution of Na2S03 at room temperature for 2 hr. The operation is carried out in wooden vats, 1.3 m high and 1.3 m in diameter (Fig. 78), using 750 kg of TNT and 750 1. of the solution at a time. Then the mixture in the vat is drained off to a vacuum filter, where the TNT is filtered off, washed with water four times, and sent to a drier. 

For drying, an iron vessel of construction similar to that of the nitrator, is charged with TNT (2250 kg). The TNT is melted by passing steam through the jacket of the drying vessel, after which dry air is passed from the bottom through the molten product. 

The vessel is covered by a hood with a ventilation pipe through which the moisture escapes (Fig. 93). Initially a temperature of 90°C is maintained, and toward the end of the operation it is raised to 105°C to let the rest of moisture escape. 

The water and liquors from washing and sulphitation of TNT contain some TNT both dissolved and suspended. For recovering the latter the water and wash liquor are conveyed to labyrinth tanks equipped with baffles barring the passage of suspended TNT, which settles at the bottom. In this way a certain amount of lower grade TNT may be recovered. 

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