The use of alternative reductants in pyrometallurgical operations

Due to the rising concern about climate change in the last few decades, the metallurgical industry is moving toward greener practices. This move is driven by pressing concerns about the reduction of greenhouse gas emissions and environmental footprints of industrial activities. One of the main strategies in this transition is to employ alternative reductants in high temperature processes. Substituting traditional reductants like coal or coke with sustainable alternatives such as hydrogen or biochar minimizes the carbon emission and provides economic benefits in addition. In this thesis work, several non-fossil reductants were applied in high temperature processes for metal recovery from secondary sources. Leach residue of printed circuit board (PCB) was employed as reductant for solid-state reduction of hematite in DSC-TGA coupled with QMS. Hydrogen was utilized in the reduction of zinc leach residue while nickel and copper smelting slag reduction were treated with biochar on laboratory scale in a vertical fur-nace. The feasibility of adopting alternative reductants in ironmaking and pyrometallurgical treatment of secondary resources were determined in this study. The effect of the amount of reducing agent, reduction time, and temperature on the extent of reduction was investigated. The studies revealed that although PCB leach residue can be applied in reduction processes, it can only partially replace conventional reductants. PCB was also found to be effective at lower temperatures (< 1000 oC). Leach residue from zinc processing were reduced with hydrogen at temperatures of 1200 oC, 1250 oC, and 1300 oC using H2 and N2 gases to form the reducing gas atmosphere. The results showed that H2 is an effective reductant because reduction proceeded rapidly, forming speiss droplets within the slag already after 10 minutes. Nickel and copper smelting slags were reacted with biochar which were produced from hydrolysis lignin and black pellet biomass by pyrolysis at 600 and 1200 °C, and with metallurgical coke for comparison. Nickel reduction experiments were done at 1400 °C for 15, 30, and 60 min under inert gas atmosphere. The samples were quickly quenched and analyzed with Electron Probe X-ray Microanalysis. The results showed that the use of biochar resulted in faster reaction kinetics in the reduction process compared to coke. Copper slag reduction experiments were performed at 1250, 1300 and 1350 °C for 60 min in order to investigate the effect of temperature and the effect of time on reduction progress was studied at 1250 °C for 15, 30 and 60 min. The results revealed that reduction rapidly progresses to the formation of metal alloy within 10 min. Valuable metals like copper and nickel were reduced to the metal phase. Thermodynamic simulations were performed with FactSageTM at the experimental conditions and compared with results from the lab scale experiments. FactSageTM predictions were in agreement with the experiments.