Reduction of Lead-Rich Slags with Coke in the Lead Blast Furnace
183
within the cavity, separating the slag from the coke. The presence of a gas phase has
been suggested as an essential part for the reduction of slag with coke, because the
gas-ferrying mechanism has been listed as one of the main reaction mechanisms in
both reduction of iron and lead from slags [16–18]. It can be summarised as follows:
1. Initial reduction of PbO generates a CO bubble:
PbO(slag) + C(s) → Pb(l) + CO(g)
(3)
2. Mass transfer of PbO through the slag to the gas/slag interface
3. Gaseous (indirect) reduction reaction at the gas/slag interface:
PbO(slag) + CO(g) → Pb(l) + CO 2 (g)
(4)
4. Diffusion of gaseous species in the gas phase surrounding the carbon
5. Boudouard reaction at the gas/carbon interface:
C(s) + CO 2 (g) → 2CO(g)
(5)
Due to the dynamic nature of the melting zone of the blast furnace, it is difficult for stable gas bubbles or films to form and hence to allow this mechanism take
place. However, on a localised scale within the pores of coke as just discussed, this
is definitely possible. The presence of lead in the pores is in this case not explained
because it gets trapped there as it is dripping down, but rather because it is preferentially formed in exactly those locations.
Outlook and Future Work
The results provided in this work are in the early stages of analysis. Further work
to verify some results as well as more accurate tests will be performed, such as
EPMA to confirm the exact composition of all the phases. Another aspect that will
be investigated is how the cooling procedure affects the formed structures. Because
the cooling was performed quite slowly in the current work, some surface phenomena
can be influenced by the cooling and give a distorted view of the high-temperature
reality. A set of duplicate experiments will be performed, in which one sample is
quenched to room temperature, and the other is quenched to 700 °C and then oven
cooled, like was performed in this study. Another important thing to note is that
this is essentially a batch process representation of a continuous process. This way
of representation has its limits, as the bottom of the crucible will build up in end
products for instance. Another consequence of this is that it is unknown whether
the phenomena observed here also hold when a continuous supply of “fresh” slag
is added. If the hypothesis of localised gas pockets proves valid, this could also
hold under these conditions, as the gas pockets would be shielded from the flow of
liquids. These issues and other aspects of the system will be studied by making small
modifications to the set-up to make it more closely resemble the blast furnace.
183
within the cavity, separating the slag from the coke. The presence of a gas phase has
been suggested as an essential part for the reduction of slag with coke, because the
gas-ferrying mechanism has been listed as one of the main reaction mechanisms in
both reduction of iron and lead from slags [16–18]. It can be summarised as follows:
1. Initial reduction of PbO generates a CO bubble:
PbO(slag) + C(s) → Pb(l) + CO(g)
(3)
2. Mass transfer of PbO through the slag to the gas/slag interface
3. Gaseous (indirect) reduction reaction at the gas/slag interface:
PbO(slag) + CO(g) → Pb(l) + CO 2 (g)
(4)
4. Diffusion of gaseous species in the gas phase surrounding the carbon
5. Boudouard reaction at the gas/carbon interface:
C(s) + CO 2 (g) → 2CO(g)
(5)
Due to the dynamic nature of the melting zone of the blast furnace, it is difficult for stable gas bubbles or films to form and hence to allow this mechanism take
place. However, on a localised scale within the pores of coke as just discussed, this
is definitely possible. The presence of lead in the pores is in this case not explained
because it gets trapped there as it is dripping down, but rather because it is preferentially formed in exactly those locations.
Outlook and Future Work
The results provided in this work are in the early stages of analysis. Further work
to verify some results as well as more accurate tests will be performed, such as
EPMA to confirm the exact composition of all the phases. Another aspect that will
be investigated is how the cooling procedure affects the formed structures. Because
the cooling was performed quite slowly in the current work, some surface phenomena
can be influenced by the cooling and give a distorted view of the high-temperature
reality. A set of duplicate experiments will be performed, in which one sample is
quenched to room temperature, and the other is quenched to 700 °C and then oven
cooled, like was performed in this study. Another important thing to note is that
this is essentially a batch process representation of a continuous process. This way
of representation has its limits, as the bottom of the crucible will build up in end
products for instance. Another consequence of this is that it is unknown whether
the phenomena observed here also hold when a continuous supply of “fresh” slag
is added. If the hypothesis of localised gas pockets proves valid, this could also
hold under these conditions, as the gas pockets would be shielded from the flow of
liquids. These issues and other aspects of the system will be studied by making small
modifications to the set-up to make it more closely resemble the blast furnace.