Roasting Sulphurous Iron-Ore. [Volume: 8, Issue: 4, 1889, pp. 208-218]

216

UNITED STATES ASSOCIATION OF

[VOL. 8,

differences between the results of the first three heats and of the
last two.
The following are analyses of some clinkers taken from roasters
working on Cornwall ore, showing the same effect:
Per Cent. Sulphur Sulphur Per Cent. Total
Total as as Sulphur as
Sulphur. Sulphide. Sulphate. Sulphate.
1.397 1.224.173 12.380
1.380 1.245.135 9.782
1.873 1.777.096 5.125
These indicate plainly that the fusion of ores in roasting may
prove prejudicial to good results. The viscid coating of fused iron
oxide, silica, and lime, or whatever may, in various cases, go to
make up the impervious exterior, is a barrier both to ingress of air
and egress of sulphur in any form. If the heat in any roasting
process must be carried to so high a point, it should be gradually
raised, and sintering should take place only after continued heating
at a lower temperature in abundant air. I have frequently found
pure sulphur condensed in the cavities of cold clinkers, where it had
been vaporized from pyrites, but was unable to escape. Clinkering
ore quickly is equivalent to roasting without air, and even more,
for the sulphur that heat alone would vaporize is imprisoned in
the mass. Furthermore, it prevents any great amount of the sulphur left from being in the form of sulphate.
Ledebur states that desulphurization will be the more complete
the smaller the pieces of ore, the more freely air has access, and the
higher the temperature on roasting. Small pieces mean greater surface exposure and less requirement for penetration of heat and
air. And we have seen that air is absolutely needed for anything
like a complete desulphurization. But our results do not bear out
the asserted necessity of a high heat for thorough roasting in all
cases.
The practical application of all these conditions in the roasting of
ores is a matter of no little difficulty. A proper roaster must be
one in which the heat is under control, the ingress of air ample, and
the egress of the products of combustion prompt. A gas-roaster
seems to approach most nearly to these conditions. The Gjers kiln