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444 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

The nitrator contents are drawn off to the filter tank (1) (Fig. 110). The tank is

lead-lined and equipped with a ventilating pipe for conducting away nitrogen oxide.

Filtration is most effective with a 20-cm layer of coarse-grained dinitronaphthalene

as a filter material, placed between perforated plates of sheet aluminium.

FIG. 110. Flow sheet of the nitration of nitronaphthalene to dinitro-

naphthalene (Pascal [20a]).

Nearly 60% of the spent acid (600 kg) of composition:

HNO 3 14.5%

H 2 SO 4 56.5%

H 2 O 29%

flows off from the dinitronaphthalene and is recycled to the nitration of naphthalene

to nitronaphthalene. The rest remains in the dinitronaphthalene and can be recov-

ered by washing the latter with the spent acid from mononitration (containing

only insignificant quantities of nitric acid and nitrogen oxides). The recovered acid

is mixed with the nitrating mixture for the nitration of naphthalene to nitronaph-
thalene.

The acid-washed dinitronaphthalene is washed six times with water, each time

the filter tank being filled with cold water up to the brim. Then dinitronaphthalene

is discharged through a side opening into double bottomed wooden vat (2), con-

taining water heated to 60°C. After six washings with hot water, the product is

discharged through a side opening and loaded into trucks.

The water content in washed dinitronaphthalene is 25%; the product is dried
in dryers, which may be of various construction, until the content of water falls

to 0.5%.

The two-stage nitration of naphthalene to dinitronaphthalene requires more
complex plant and is more labour consuming: nevertheless it is more economic

than single stage nitration, since the product obtained is of higher purity, more

easily freed from acid, and the consumption of nitric acid is lower.
The following data given by Pascal illustrate the superiority of two-stage nitra-

tion (Table 98).