urbaf2

NITRO DERIVATIVES OF BENZENE 247

by a wire gauze, which retains the dinitrobenzene crystals, while the liquor runs

into an outlet pipe, through a separator where any crystals which have been carried

over are retained. The dinitrobenzene in the vessel is thoroughly washed by re-

peated rinsing with water, which is then pumped away.

The dinitrobenzene, which is now well washed and free from the sodium sulphite

and sodium nitrite formed during sulphitation, is melted and introduced to a special

vessel of 15 m^3 capacity, where it is separated from residual water by decantation.

It is tested to ensure that washing has been fully effective, by boiling a sample with

water, followed by cooling and filtering. The filtrate should not decolorize a solu-

tion of KMnO 4 during a 10-min test period.

The tinal product is dried at 135°C under reduced pressure in the molten state

in a drying vat of 16 m^3 capacity.

The product is cast in blocks. From the quantities of raw materials given above,

4100 kg of dinitrobenzene, melting at 88-89ºC, are obtained.

Nitration in two stages in one nitrator. The Griesheim (I. G. Farbenindustrie) process

for the manufacture of dinitrobenzene differs from that of Höchst since in the latter the

whole nitration starting from benzene is carried out in one cast iron nitrator of 8 m^3

capacity. The nitrator is equipped with a stirrer, rotating at 110 r.p.m., a jacket

having a cooling area of 8 m^2 and a lead cooling coil of 16 m^2 area.

2200 kg of the waste acid from the previous batch are left in the nitrator and

to this 1900 kg of nitrating mixture is added, the composition of which is as follows:

HNO 3 88%

H 2 SO 4 11%

H 2 O 1%

2200 kg of benzene are run in during a period of 4 hr while the temperature

rises from 25°C to 40°C. Then the temperature is allowed to rise gradually to 60°C

over a period of 1 hr. Then the stirrer is stopped, the contents are allowed to settle

and the waste acid is run off to a lead lined washing vessel (A) of 12 m^3 capacity,

where it is washed with benzene or nitrobenzene.

To the crude nitrobenzene left in the nitrator 5000-6000 kg of a nitrating mix-

ture are then added gradually over 3 hr, the temperature rising from 25°C to 40°C.

The mean composition of the mixture is:

HNO 3 33%

H 2 SO 4 67%

The temperature should rise to 90°C and be maintained there for 1 hr. A sample is then

tested by steam distillation for the presence of nitrobenzene. The amount of nitrating

mixture used depends on the actual amount of HNO 3 in it and on the result of the test.

When the batch has settled the waste acid is run into a washing tank (A) where

the acid from three or four nitrations has been collected and where the whole is

washed with nitrobenzene.

Next the dinitrobenzene is run to a tile-lined stainless steel washing tank (B)

of 12 m

3
capacity, where it is washed with 4000 l. of hot water and then with a
further 4000 l. containing sufficient NaOH to give a neutral reaction. Finally it