Metallurgical sludges – Bio/leaching and heavy metals recovery (Zn & Cu)
This research was carried out in order to develop a technology to demonstrate the metallurgical residues as a potential secondary source for heavy metals (Cu and Zn). Three different (based on their age of generation and deposition) zinc leach residues (ZLR1, ZLR2 & ZLR3) and zinc purification residue (ZPR) were collected from a Zn-metallurgical industry located in Brazil. The characterization of ZLRs and ZPR were examined for their mineralogical, physico-chemical, bulk chemical features. Fractionation of heavy metals and liquid-solid partitioning with respect to pH were also determined. Geo-chemical modelling was done to understand the mechanisms affecting the mineral solubilities of these residues.
Following the above, the residues were subjected to (bio)leachability tests to optimize the maximal extraction of heavy metals. The effects of experimental factors such as temperature, leachant concentration, pulp density and agitation speed have been optimized in shake flasks. The mass transfer kinetics of these solid-fluid heterogeneous leaching processes were examined by shrinking core kinetic models. Later, the recovery of Zn (ZLRs) and Cu (ZPR) from the polymetallic acidic leachates were investigated. The selective recovery of metals from the acidic leachates was achieved by metal sulfide precipitation (MSP). MSP process parameters such as initial pH and metal – sulfide dosage were also optimized for the selective recovery. The metal sulfide precipitates were characterized for mineralogy, purity and particle size distribution. Finally, hydrometallurgical flow charts for the selective recovery of Cu and Zn were proposed.
The results reveal that the ZLRs contain significant concentration of Zn (2.5% to 5%), Pb (1.7% to 2.3%) and metals such as Mn, Cu, and Al in detectable fractions. The ZPRs contain high concentration of Cu (47%), Zn (28%), Cd (9%) and Pb (5%). Fractionation with acetic and nitric acid suggest that both the leach and purification residues are hazardous wastes, releasing higher concentration of Pb and Cd into the environment, than the permissible concentration suggested by U.S. EPA. Leaching of metals from the residues is highly pH dependent. Heavy metals leaching (Zn & Cu) is high at low pH and the release of metals was decreased with increase in pH. Sulfated and carbonated mineral phases were predicted to be the solubility controlling minerals.
The leaching of Zn from ZLRs was highly influenced by temperature and acid concentration. The leaching kinetics of ZLRs results state that more than 92%, 85% and 70% of zinc can be extracted from ZLR1, ZLR2 and ZLR3 by H2SO4 (1.5 M) leaching (at 80 °C for 6 hours with a pulp density 2%, while the agitation speed was maintained 250 RPM). The sulfuric acid leaching kinetics of ZLRs follow the shrinking core diffusion model. The activation energy required to leach zinc from the ZLR1, ZLR2 and ZLR were estimated to be 2.24 Kcal/mol, 6.63 Kcal/mol and 11.7 Kcal/mol respectively, by Arrhenius equation. Order of the reaction with respect to the sulfuric acid concentration was also determined as 0.2, 0.56, and 0.87 for ZLR1, ZLR2 and ZLR3, respectively. Selective precipitation of Zn (as sphalerite) from the leachates was achieved by the combination of hydroxide and sulfide precipitation. Biohydrometallurgy is also as effective as the chemical hydrometallurgy for the selective Zn recovery from the ZLRs. Cu leaching from ZPR was highly influenced by solid to liquid phase ratio and agitation speed, suggesting that the mass transfer depends on the diffusion. The leaching of Cu from the ZPR also follows the shrinking core diffusion model and requires apparently 2.9 Kcal/mol activation energy throughout the leaching process. More than, more than 50%, 70% and 60% of the total Cd, Cu and Zn can be leached from ZPR by 1M H2SO4 with 2% pulp density continuously shaken at 450 rpm at 80 °C. Covellite was selectively recovered from the acid multi-metallic (Cd, Cu & Zn) leachates were investigated by optimizing the initial pH and Cu to sulfide ratio.
In conclusion, these hazardous metallurgical residues can be seen as potential alternative resource for Zn and Cu. Not only the capital costs and environmental issues associated with the storage/disposal of these ZLRs & ZPR but also the gradual depletion of high grade sulfidic ores (for Zn and Cu) can be addressed. The study also leaves a perspective of investigating the leached ZLR & ZPR, for the selective leaching and recovery of Pb. Bioleaching and biorecovery of the heavy metals from these residues are interesting to investigate for future applications.