The apparatus (figure 4-1) consists of a trough of internal dimensions 150 mm x
150 mm x 500 mm, open at the top. The trough is constructed of square-meshed gauze
(preferably stainless steel) with a mesh width of about 1.5 mm and a wire thickness of
1.0 mm supported on a frame made from, for example, 15 mm wide, 2 mm thick steel bars.
The gauze at each end of the trough may be replaced by 1.5 mm thick, 150 mm x 150 mm
stainless steel plates. The trough should be rested on a suitable support. Fertilizers
with a particle size distribution such that a significant amount falls through the mesh
of the trough should be tested in a trough of smaller mesh gauze, or alternatively in a
trough lined with gauze of a smaller mesh. During initiation sufficient heat should be
provided and maintained to establish a uniform decomposition front. Two alternative
heating methods are recommended, namely:
An electrical heating element (capacity 250 W) enclosed
in a stainless steel box is placed inside and at one end of the trough
(figure 4-2). The dimensions of the
stainless steel box are 145 mm x 145 mm x 10 mm, and the wall thickness
is 3 mm. The side of the box which is not in contact with the fertilizer
should be protected with a heat shield (insulation plate 5 mm thick).
The heating side of the box may be protected with aluminium foil or
a stainless steel plate.
A steel plate (thickness 1 mm to 3 mm) is placed inside
one end of the trough and in contact with the wire gauze (figure 4-1). The plate is heated by means
of two burners which are fixed to the trough support and are capable
of maintaining the plate at temperatures between 400oC
and 600o C, i.e. dull red heat.
4.2.3 To prevent heat transport along the outside
of the trough, a heat shield consisting of a steel plate (2 mm thick)
should be installed at about 50 mm from the end of the trough where
the heating takes place.
4.2.4 The life of the apparatus may be prolonged
if it is constructed of stainless steel throughout. This is particularly
important in the case of the gauze trough.
4.2.5 Propagation may be measured using thermocouples
in the substance and recording the time at which a sudden temperature
rise occurs as the reaction front reaches the thermocouple.