Theoretical Considerations
Reduction reactions
The higher oxides of manganese which
predominate in manganese ores, namely MnO3, Mn2O3, and Mn3O4,
are relatively unstable and decompose spontaneously in the presence of carbon and carbon
monoxide, well below smelting temperatures. It is well known that MnO3
dissociates at quite a low temperature of around 400oC forming Mn2O3,
and that this oxide in turn, dissociates below 1000oC to give Mn2O4,
which in the presence of CO gets converted to MnO.
The reduction reaction which takes
precedence is the formation of the carbide Mn7C3:
7MnO+10C -> Mn7C3+7CO3
(a) for which AGo = 0 at 1257oC whereas for the reaction: MnO+C
Mn+CO (b) the standard free energy AGo does not become negative until a
temperature of 1420oC is reached. This means that manganese metal cannot appear
in this system until liquid state conditions have been reached, and by this time,
manganese carbide and iron/iron carbide would be present in the system. That is why in the
blast furnace, manganese oxide is not reduced until it trickles through (with probably
some FeO and SiO2 in solution) the incandescent coke in the bosh and hearth of
the furnace.
One of the major difficulties in the
smelting of manganese alloys is the relative volatility of Mn metal, and if the smelting
temperature is increased to push reaction (b) to the right, excessive losses of manganese
by volatilisation will ensue. Although the boiling point (b.p.) of Mn is 2036oC,
it has an appreciable vapour pressure at temperatures readily attainable in submerged-arc
furnaces, for example the vapour pressure at 1600oC is 0.05 atm. Lowering the
partial pressure of CO, which is used to assist chrome reduction reactions with solid
carbon at high temperature, will merely increase the volatilisation of manganes.
Slag composition
Slag composition and conditions also have
to be taken into consideration. The oxides FeO and MnO form a continuous series of
solutions, both solid and liquid, and when SiO2 is present, two manganese
silicates can be formed (MnSiO2 and Mn2SiO4), as well as
fayalite (Fe2SiO4). This means that in the FeO-MnO-SiO2
ternary system, slags containing these oxides and silicates can be formed in the
temperature range 1250 to 1350oC, and MnO is much more difficult to reduce from
a liquid slag. To do so, it is necessary to displace MnO as the base in the silicate by
adding dolomitic lime, which increases the free-running temperature of the slag and
onsequently the smelting temperature, leading inevitably to increased volatilisation
losses. There are, therefore, two routes open to the ferro-manganese producer:
Operation with a high basicity slag with
a ratio (CaO+MgO)/(SiO2) of 1.3 or somewhat higher and a high smelting
temperature, allows cutting slag losses down to 5-6% of the total Mn input, but incurring
volatilisation losses of around 15 % of the manganese.
The other method is to operate at as low
a temperature as is consistent with the necessary fluidity of the slag, thereby carrying
out most of the reduction up to the carbide-forming stage as per the scheme of reaction
(a) and allowing the balance of the MnO (often amounting to 40 to 45% of the total Mn) to
pass unreduced into the slag.
In this process, very little flux is
added and in the case of some ores, none at all. Operating data are available to support
the fact that the energy consumption can be reduced by 30% or more by adopting this
method, and the volatilisation losses can be cut down to a negligible proportion,
especially by the use of new techniques. The rich slag containing around 50% MnO, is
granulated or crushed and it constitutes an ideal raw material for the manufacture of
silico-manganese. A typical % composition could be MnO: 52, SiO2: 32, FeO: 1.0,
Al2O2: 4.5,CaO: 8, MgO: 1.5, and Others: 1.0. Being predominantly a
silicate, it has practically no carrying capacity for phosphorus, and it will be noted
that the Mn/Fe ratio is over 50.
Furnace Design
There are certain special requirements
for the production of Fe-Mn. In the first place,the resistance of the charge is
considerably lower in ferro-manganese operation than it is for instance in making
ferro-silicon - possibly by as much as 30 to 35%. Then, to operate the smelting zone at a
lower temperature, the energy must be dissipated through a larger volume and the
electrodes worked at a low current density (3 to 4 A/cm2). Because of the low
restistance it is, on the other hand, necessary to operate at relatively low voltages and,
therefore, high currents, to ensure deep electrode penetration so that much of the
manganese fume are condensed in the upper layers of the charge. This means that a
ferro-manganese furnace (compared with ferro-silicon and ferro-chrome furnaces) will be
larger in diameter, have larger electrodes, and will be deeper from the hearth to the top
of the side walls. Relevant data reported for a 13.2 mVA furnace similar in capacity to
the one available at FAP, Joda and operating at 11.15 mW are presented below:
Molten metal at FAP, Joda
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Diameter inside the brick work: 9.1m
Depth from the hearth to the top of the
side walls: 3.35m
Electrode diameter: 1.42m
Electrode current density: 3.7A/cm2
Voltage between electrodes: 132.4
Power factor: 0.82
Power density averaging over the whole
hearth area: 172kW/m2 (equivalent to 430kW/m2 for ferro-silicon
practice).
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