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. 2023 Jun 5;8(24):21450-21463.
doi: 10.1021/acsomega.3c00250. eCollection 2023 Jun 20.

Processing of a Zinc Leach Residue by a Non-Fossil Reductant

Minna Rämä  1 , Lassi Klemettinen  1 , Marja Rinne  1 , Pekka Taskinen  1 , Radosław Markus Michallik  2 , Justin Salminen  3 , Ari Jokilaakso  1
Affiliations

Processing of a Zinc Leach Residue by a Non-Fossil Reductant

Minna Rämä et al. ACS Omega. .

Abstract

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.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic of the experimental setup. R = rotameter.
Figure 2
Figure 2
Quasi-ternary isothermal section of the MgO-SiO2-FeOx system at 1300 °C. Four slag compositions with varying MgO additions, based on experimental results, are superimposed. The phase diagram contains constant CaO (21 wt %) and Na2O (6 wt %) concentrations. The black lines are calculated for 10–3 Pa oxygen partial pressure (reduction stage) and the red lines for 103 Pa (oxidation stage, liquidus line only).
Figure 3
Figure 3
SEM BSE microstructure images of the samples after a 40 min reduction at 1200 °C using (a) metallurgical coke, (b) biochar, and (c) 50:50 mixture of coke and biochar as a reductant.
Figure 4
Figure 4
Total amount of impurities (Ni, Co, Zn, Pb, Ag, Cu, S, Sb, and As) in wt % in (a) slag (EDS analysis) and (b) bulk material (ICP-OES analysis) after 20, 30, and 40 min of reduction with different reductants.
Figure 5
Figure 5
SEM BSE microstructure images of samples after reduction treatment at 1300 °C with (a) no MgO addition, 10 min, (b) 10 wt % MgO addition, 10 min, and (c) 10 wt % MgO addition, 60 min. The intermediate slag used had been produced with 32 mL/min O2 flow during oxidation.
Figure 6
Figure 6
Effect of MgO addition on (a) Zn and (b) Pb concentrations (intermediate slag produced with 32 mL/min O2 flow), and on the total amount of impurities (Ni, Co, Zn, Pb, Ag, Cu, S, Sb, and As), for the two intermediate slags; (c) 32 mL/min and (d) 65 mL/min O2 flow in slag (wt %) after 10, 30, and 60 min reduction times based on EPMA analyses.
Figure 7
Figure 7
Metal/speiss phases formed during the reduction stage; (a) Pb-rich droplets accompanied by Fe-As speiss after a 30 min treatment of a sample with 10 wt % MgO addition (32 mL/min O2 during oxidation); (b) Fe-As speiss formed at the top of the melt after a 10 min treatment of a sample with 10 wt % MgO addition (65 mL/min O2 during oxidation).
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