The suitability of
a non-fossil reductant in high-temperature treatment
of a zinc leach residue was studied in laboratory-scale experiments.
The pyrometallurgical experiments carried out at temperatures of 1200–1350
°C consisted of melting the residue under an oxidizing atmosphere
to produce an intermediate, desulfurized slag, which was further cleaned
of metals such as Zn, Pb, Cu, and Ag, using renewable biochar as a
reductant. The aim was to recover valuable metals and produce a clean,
stable slag for use as construction material, for example. The first
experiments indicated that biochar is a viable alternative to fossil-based
metallurgical coke. The capabilities of biochar as a reductant were
studied in more detail after optimizing the processing temperature
at 1300 °C and modifying the experimental arrangement by adding
rapid quenching of the sample (to a solid state in less than 5 s)
to the procedure. Modifying the slag viscosity by adding 5–10
wt % MgO was found to enhance the slag cleaning significantly. With
an addition of 10 wt % MgO, the target Zn concentration in slag (Zn <
1 wt %) was reached after as little as 10 min of reduction, and the
Pb concentration was also decreased relatively close to the target
value (Pb < 0.03 wt %). With an addition of 0–5 wt % MgO,
the target Zn and Pb levels were not reached within 10 min, but with
longer treatment times of 30–60 min, 5 wt % of MgO was enough
to decrease the Zn content in slag sufficiently. The lowest Pb concentration
achieved with an addition of 5 wt % MgO was 0.09 wt % after a 60 min
reduction time.