The solid state reduction of chromite.
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Date
1989
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Abstract
High carbon ferrochromium serves as the main chromium
source for almost all chromium containing steel alloys. The
traditional method for the production of high carbon
ferrochromium via the reduction of chromite using coke in
electric arc furnaces, draws its considerable energy
requirement from electrical power.
The escalation in cost of electric power in South Africa
has motivated research into alternative, fuel fired,
reduction processes. One such process involves the partial
solid state reduction of chromite in coal fired rotary kilns
at temperatures between 1200 and 140rrc, prior to electric
smelting. such processes are currently operated on a
commercial scale and result in considerable savings in
electrical energy, despite slow reduction kinetics and low
reaction extents.
A large amount of research conducted in the past, . aimed at
establishing the fundamentals of the reduction process, has
not provided satisfactory answers to questions regarding the
mechanism of reduction. It was therefore necessary to
conduct further test work on the process to establish the
mechanism and factors limiting the rate and extent of
reduction.
Thermodynamic analysis of the reaction system indicates
that at temperatures above 1050C reduction of the ore will
proceed, and should reach an extent of approximately 90%
reduction at 1200C. Complete reduction should be achievable
at approximately 125ifc. However experimental results
indicate the persistence of a stable magnesiochromite spinel
under normal reducing conditions even at 140ifc. This limits
the degree of chromium reduction to approximately 65%.
Kinetic data from thermobalance studies and electron
microscope examination of the reduction product showed
independent reduction of iron and chromium. The rate of iron
reduction was found to be relatively rapid and to go to
completion, compared to that of chromium where the formation
of a relatively inert picrochromite- spinel solid solution
(MgO(Cr,AI)203) at the surface of the grain liinited the rate
and extent of reduction to approximately 65% in the case of
LG6 chromite.
These findings suggested that the only way in which the
kinetics of the process might be improved was through the
addition of a component capable of disrupting the spinel
layer at the surface of the chromite grain.
In this study, fluoride containing mixtures such as CaF2 -
NaF and fluorspar- feldspar- silica were successfully used
to accelerate the reaction. Such mixtures are commercially
interesting and highly effective even at low additions (4-
10%) .
The mechanism whereby such mixtures operate was shown to
involve the dissolution of all the spinel components in the
liquid flux phase. Following dissolution, rapid
i i
recrystallization of spinel (Mg . Al2 ~) occurs , simultaneous
to the transport of Fe 2+ and cr 3+ ions through the liquid to
a site where reduction can take place. The main effect of
this is to increase the rate and extent of chromium
reduction to the point where virtual total reduction can be
achieved in less than 90 min at temperatures as low as
1200C.
Although the reduction kinetics in the presence of such
solvent flux phases are still largely limited by the rate
of solid state diffusion, the disruption of the surface
enables faster overall diffusion rates to be achieved.
Ultimately as the particle size and separation between oxide
and reductant is increased, the rate of dissolution and
transport through the flux phase become rate limiting.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1989.
Keywords
Chromium ores., Chromite., Reduction (chemistry)., Theses--Chemical engineering.