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158 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

sets of blades or bars, mounted on two axes, which rotate either in the opposite

or in the same direction, sometimes at different speeds. This type of stirrer is used

for the nitration of cellulose and is often mounted in small nitrators in which ad-

ditional internal cooling (or heating) would be difficult to apply.
The types of stirrers mentioned are particularly useful when the contents of the

nitrator are highly viscous or semi-liquid, for example a mixture of acid and a

crystalline product.

For liquids of low viscosity a spiral screw stirrer may be used (Fig. 30). Circu-

lation can be facilitated by surrounding the stirrer (I) with a sheet metal cylinder

FIG. 32. Diagram of a nitrator with a propeller stirrer.

(2), provided with double walls through which cooling water or steam circulates

for additional cooling or heating,

One rather unusual design comprises a mixing system with two external chambers

(Fig. 31). The nitrator is connected with both sides of each chamber, in each of which

a propeller stirrer is rotated at 200-400 r.p.m. The speed of rotation and cross-

sections of the chambers are so selected as to enable the whole of the nitrator con-

tents to make one round within one minute.

Propeller stirrers with the propeller located inside a sheet metal cylinder (Fig.

32) or in the space delineated by a heating coil are frequently used. The propeller

draws the liquid from the top of the cylinder, forcing it downwards. In the space

between the cylinder and the nitrator walls the liquid flows in the opposite direc-

tion. If a well designed propeller is used and a suitable rotation speed applied,

circulation of the liquid is sufficient to secure adequate mixing. The system is also

used for continuous nitrators, not only for nitration of aromatic compounds, but

also for nitration of alcohols, for example glycerol.

Mixing by means of a draw-lift turbine is the most modern and efficient method

(Fig. 33). The turbine is rotated at high speed (several hundred r.p.m.) and powered

by an electric motor with a worm reduction gear. The turbine draws the liquid