A N T I M O N Y : ITS HISTORY, CHEMISTRY, MINERALOGY ...

78 THE METALLURGY OF ANTIMONY.-
The product of oxidising roasting may be either the stable
tetroxide (Sbo04) or the volatile trioxide (Sb2O3). The latter method
is called " volatilising roasting " (Germ, verfiuchtigende Rostung), and
will be treated under the next heading.
The chemical changes which take place for the production of the
stable tetroxide are as follows :—"When antimony glance is heated not
much above 350° C, the trioxide is formed, with the evolution of
sulphur dioxide:
SbsS3 + 90=Sb2O3 + 3SO2.
A part of the trioxide is further oxidised to the pentoxide, Sb2O5,
which, combining with some of the trioxide, forms the tetroxide,
Sbo04. When oxides of metals are present, antimoniates are formed.
If the ore contains a large quantity of sulphides of foreign metals,
which would, under ordinary circumstances, form sulphates, if
they alone were calcined, antimoniates of the foreign metals are
formed.
The roasted mass should contain the tetroxide principally, if
the ore is comparatively pure and if the correct temperature
and proper quantity of air have been maintained throughout the
process of roasting. But ideal conditions are very seldom met
with; hence the product may contain antimony glass, undecomposed
sulphide, and antimoniates, arseniates, and sulphates of
foreign metals, if the ore contains these foreign sulphides and
arsenic compounds.
According to Bidon, the proper temperature for roasting antimony
glance is about 350° C.; if the temperature is too low, the
sulphide is not decomposed, and consequently antimony glass is
formed from the mechanically melting together of the sulphide and
the trioxide at a dark red-heat; and if the temperature is too high,
the mass begins to frit together, thereby preventing the penetration
of air to the interior of the mass, and both the sulphide and the
oxide are volatilised.
The fritting of the mass during roasting can be prevented by
constant and regular rabbling. The presence of gangue has a good
effect in obviating, partially at least, this difficulty. This accounts
for the fact that, the richer the ore is, the more difficult it is to
roast. But, with rich ores, it is impossible to prevent to some
extent the loss due to the liquation of the sulphide and the formation
of flue dust, which mainly consists of the trioxide, the tetroxide, the
sulphide, arsenic compounds, and carbonaceous matter. Towards
the end of roasting the temperature can "be raised a little, in order