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150 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

60% HNO 3 or the weight ratio H 2 SO 4 /HNO 3 = 40/60 the intercept on the ordinate

representing 20% water indicates a heat of dilution of ca. 115 B.t.u. per lb,

in agreement with the foregoing calculations. It can be seen from the curves that

as the nitration progresses the heats of dilution decrease and as the water content

of the nitrating acid increases. There is greater danger of violent or explosive re-

action at the early stages of nitration, owing to the 1ower specific heats of the more

concentrated acids, than later on in the reaction. (See also [17]).

The curves also indicate why it is comparatively safe to carry out rapid nitration

at relatively high temperatures with spent acid or with nitric acid alone.

MIXING THE ACIDS

The operation of mixing is carried out in iron vessels equipped with cooling

jackets and stirrer.

A schematic diagram of a mixing plant is given in Fig. 24. It consists of a large

vessel (1) for storing the acids, a measuring tank for the acids (2), a mixer (3), a

pressure-egg (4) and a vessel for the storage of the prepared nitrating mixture (5):

FIG. 24. Diagram of a plant for mixing acids.

Vessels and pipes for concentrated nitric acid are made of pure aluminium.

Those for sulphuric acid, oleum and the mixture of acids are made of iron. Steam

pipes must be laid close to oleum pipes and the whole should be coated with thermal

insulation. In this way the oleum pipes are heated, thus preventing the oleum from
freezing in winter.

Calculation of acid mixtures. A mixture of the composition:

HNO 3 A%

H 2 SO 4 B%