46
S. Nicol et al.
throughout the process and will be entirely removed from the slag before copper
is fully recovered. The thermodynamic calculations indicated that the copper in the
slag is strongly associated with the sulphur in the slag, preventing the recovery of
copper until a significant quantity of other metal species has been recovered from
the slag. Discussions of exact equilibrium concentrations in the slag are not justified
given the uncertainty in the models.
The deep reduction of slags (where iron is reduced) has also been suggested for
copper slags. For copper slags, the base slag is an iron silicate, SiO 2 –FeO, and the
removal of iron requires the addition of another oxide to maintain a molten slag
during the reduction process. For the lead slag investigated, the base slag is a sodium
silicate, with neither of these species reduced under deep reduction conditions. The
binary phase diagram, shown in Fig. 3, shows that for the current 32/18 mass ratio
of SiO 2 to Na 2 O (65/35 mol ratio), the slag liquidus is below 900 °C, which allows
for operability when iron oxide is reduced. Another advantage is that the reduced
iron has very limited solubility in the lead and will form solid particles which can
grow in the slag [4]. This means that they can either be recovered at the bottom of the
slag, or the slag can be crushed and magnetically separated if desired, but they will
not contaminate the lead. However, this iron could also cause operational problems
in the reduction process and requires significantly more methane to reduce than the
small amount of lead in the slag.
Fig. 3 Sodium silicate phase diagram
S. Nicol et al.
throughout the process and will be entirely removed from the slag before copper
is fully recovered. The thermodynamic calculations indicated that the copper in the
slag is strongly associated with the sulphur in the slag, preventing the recovery of
copper until a significant quantity of other metal species has been recovered from
the slag. Discussions of exact equilibrium concentrations in the slag are not justified
given the uncertainty in the models.
The deep reduction of slags (where iron is reduced) has also been suggested for
copper slags. For copper slags, the base slag is an iron silicate, SiO 2 –FeO, and the
removal of iron requires the addition of another oxide to maintain a molten slag
during the reduction process. For the lead slag investigated, the base slag is a sodium
silicate, with neither of these species reduced under deep reduction conditions. The
binary phase diagram, shown in Fig. 3, shows that for the current 32/18 mass ratio
of SiO 2 to Na 2 O (65/35 mol ratio), the slag liquidus is below 900 °C, which allows
for operability when iron oxide is reduced. Another advantage is that the reduced
iron has very limited solubility in the lead and will form solid particles which can
grow in the slag [4]. This means that they can either be recovered at the bottom of the
slag, or the slag can be crushed and magnetically separated if desired, but they will
not contaminate the lead. However, this iron could also cause operational problems
in the reduction process and requires significantly more methane to reduce than the
small amount of lead in the slag.
Fig. 3 Sodium silicate phase diagram