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412 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

xylene is partly nitrated by the residual nitric acid still present in the spent acid).

Then over a period of 3-4 hr, 1950 kg of xylene and about 3000 kg of a ni-

trating mixture :

HNO 3 28%

H 2 SO 4 56.5%

H 2 O 15.5%

are run in, the initial temperature of 15°C rising to 35°C by the end of the

procedure.

Then over half an hour the rest of the acid is introduced so that the total quantity

of the fresh nitrating mixture amounts to 4150 kg. The mixture in the nitrator is

then stirred for another half an hour and conveyed to a separator where it is allowed

to remain at rest for 2 hr so as to separate the nitro compound from the spent acid.

Part of the spent acid, having an average composition:

HNO 3 0.3%

NO 2 0.4%

H 2 SO 4 70%

H 2 O 29%

is sent to a concentration plant, while the rest is recycled to the nitrator.

The nitroxylene is washed with water until free from acid (test with Congo

paper), then it is mixed with 150 1. of 30% sodium hydroxide solution at 70°C.

Thus any nitroxylenols resulting from oxidation are removed.

The nitroxylene is then mixed with 50 kg of sodium carbonate and steam is

passed through the mixture to remove unnitrated volatile compounds. The sodium

carbonate dissolves in water and the aqueous solution thus formed is separated

from nitroxylene.

The nitroxylene is further washed, this time twice with a 2.5-3% solution of

NaOH at a temperature of 60°C and finally with water until quite free from alkalis.

From 100 kg of xylene 125 kg of nitroxylene can be obtained, which is 88.5%

of the theoretical yield.

Purification of TNX

If trinitro-m-xylene has been prepared from pure m- xylene simple washing

with hot water is sufficient to obtain a product of high purity. Otherwise, the remov-

al of oily components by hot centrifugation is necessary. Additional purification

by washing the product in the centrifuge with organic solvents (alcohol, benzene)

may be applied. In consequence of the higher solubility of nitro derivatives of

o- and p- xylene, mainly these compounds are removed.

A sodium sulphite wash may also be applied. According to Kravchinskii [3],
the agitation of trinitro-m-xylene with a 7-9% solution of sodium sulphite at 70°C

for 4 hr results in the complete removal of oily products. However, losses caused

by this process may be considerable (15-25%), so that this method of purification
does not always pay.

Drying. The trinitro-m-xylene, purified as above, and containing 13-18%