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TNT MANUFACTURE 359

The aim of dividing the operation into two stages is to achieve the most effec-
tive thermal control, since the conversion of toluene to mononitrotoluene is the most
exothermic of all the stages of the reaction, as already mentioned in the section

dealing with the theory of nitration. The operation is effected in 5-6 hr. 5000 kg

of MNT is obtained from each charge of the nitrator.
When nitration has been completed the reaction mixture is transferred by
compressed air to a cast iron separator with a capacity of 15 m^3 , where it is allowed
to remain at rest for 6 hr to separate into two liquid phases. Then the spent acid
is conveyed to a special cast iron tank, where it stays for a few days to recover the
temainder of the MNT, which is collected from the surface and added to the toluene
to be mononitrated. The spent acid, having a composition:

H 2 SO 4 70.0%

NO 2 0.5%

H 2 O 29.5%

is conveyed to denitration, which yields 67-68% sulphuric acid. Subsequently

this is concentrated by distillation to obtain a 96% product.

MNT thus prepared is washed with water in a wrought iron tank, lined with

sheet lead. Washing is effected by vigorous stirring until the product is free from

acid (test with Congo red paper). The neutralized MNT is transferred to a so-called

distillation unit (of cast iron), where it is mixed with a dilute solution of NaOH,

(ca. 1%), the latter being used in a quantity equal to 1 wt. % of MNT (i.e. about

50 kg of NaOH). Washing with alkali not only aims at a complete removal of acids,

but also it converts nitrocresols impurities into water soluble nitrocresolates.

Live steam is passed through the mixture of MNT and alkali. The unnitrated

toluene and benzene (the latter is a common impurity of toluene) are thus expelled.

The steam is introduced until MNT begins to distil, after all the hydrocarbons or

other volatile impurities have been expelled. The condensate, containing toluene

and MNT, is either recycled for nitration to liquid nitro compounds, e.g. for mining

explosives, or is used as a solvent for varnishes.

The alkali layer is separated by decantation and distilled. The MNT recovered

is added to the main product. The latter is conveyed from the distiller to a washing

tank where it is washed with water, then again with alkali and finally with a sodium

carbonate solution. The moist MNT is conveyed by means of compressed air to

a cast iron tank for temporary storage.

From 100 parts of toluene 140-144 parts of MNT are obtained by this method,

containing about 4% of m- nitrotoluene.

Dinitration. Dinitration is carried out in two stages. The first consists of the

following operations. The spent acid from trinitration, of the composition:

HNO 3 4-5%

NO 2 34%

H 2 SO 4 80%

H 2 O 11-13%