The Schlakenatlas/Slag Atlas (Dusseldorf: Verlag Stahleisen M.B.H., 1981)
contains several viscosity plots for titania in blast furnace slags - see
in particular pages 201, 203, 207 and 213. The underlying data are of
varying age and may not be entirely reliable.

If I remember correctly a short tutorial on the subject that Ned Rehder
gave me some years ago, a slag needs to have viscosity under 6 poise to
tap freely. Blast furnace slags are basically CaO + Al2O3 + SiO2, assuming
that enough CaO was added as lime to take up all the gangue (if not, some
of the FeO in the charge will be sacrificed to mop up the excess SiO2 +
Al2O3). The plots in the slag atlas, especially those on p. 207, show that
the viscosity of these slags is marginal (6 or above) at 1400C. The
addition of TiO2 clearly DECREASES the viscosity, as Ian Freestone notes,
and thus makes slags flow more freely - see in particular figure 42 0n p.
207.

If TiO2 doesn't have adverse effects on slag viscosity, then why are
modern blast furnace operators so careful to exclude it from the charge?
The cut-off value for TiO2 in modern blast furnace feed is 1%. (At
Phalaborwa in South Africa, which is a copper mine, the magnetite in the
ore is segregated by Ti content - that under 1% is sold, that above is
dumped in huge heaps. They spray these with molasses to keep the finely
ground ore from blowing, so they act as a Mecca for flies !) The ore above
1% TiO2 can't be sold because in a modern blast furnace the TiO2 would be
reduced into the iron metal, as in the case cited by Peter Hutchinson -
not because of any effect on the viscosity of the slag.

In answer to James Brothers then, I think that the TiO2-slag question is a
red herring. If you are looking for technical reasons for failure, try
looking instead at the difficulties of achieving sufficiently high
fluidity in CaO/SiO2/Al2O3 slags (usually with some MgO as well) in cold
blast furnaces. There could be many reasons for failure to attain
sufficent temperature - damp charcoal, for example, poor furnace design
(leading to "bridging" of the mid-shaft by sintered ore) and lack of skill
on the part of the operators, as in charging incorrect proportions or
trying to tap too soon, resulting in the freezing of slag in the taphole.

----------------------
David J Killick
Associate Professor, Department of Anthropology
University of Arizona, Tucson, AZ 85721-0030
Phone (520)621-8685; FAX 621-2088
[log in to unmask]
http://www.mse.arizona.edu/faculty/killick.html




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