Solid-state hydrogen reduction of metal oxide mixtures and an industrial by-product to produce metals and homogeneous alloys

Abstract

Conventional pyrometallurgical techniques for metal production rely heavily on carbon-based reduction and high-temperature operations such as melting, leading to significant CO₂ emissions and high energy consumption. Additionally, industrial by-products containing valuable metals remain underutilized for metallurgical applications, highlighting the need for more sustainable and efficient processes. To address these challenges, this thesis investigated a solid-state hydrogen reduction approach to produce metal alloy powders directly from mixed metal oxide mixtures. Copper–nickel and iron–nickel alloys were successfully synthesized using this method by reducing synthetic Cu₂O-NiO and Fe₂O₃-NiO powder mixtures, respectively, at 700 °C for 45 minutes in a horizontal tube furnace using hydrogen. Off-gas analysis results confirmed a complete reduction by monitoring the water vapor content. X-ray diffractometry confirmed the absence of residual oxides and the presence of solid phases consistent with the respective binary phase diagrams. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy revealed microscopic compositional fluctuations that remained within the variability typically observed in conventionally cast alloys. These findings demonstrate the potential of hydrogen-based processing as a low-carbon alternative for alloy production.

To improve chemical homogeneity at the microscopic scale and enable direct use of the alloy powder, iron–nickel powders obtained through hydrogen reduction of 50 wt% Fe₂O₃-NiO mixtures at 700 °C for 45 minutes were subjected to homogenization at 1100 °C under an argon atmosphere. Diffusion models constructed using the diffusion module in Thermo-Calc guided the selection of homogenization durations of 5, 10, and 15 hours. Experimental homogenization trials revealed an increasing chemical homogeneity over time. Furthermore, the 15-hour sample demonstrated a chemically uniform microstructure along with extensive neck growth and minimal porosity. X-ray diffractometry confirmed the absence of oxides after homogenization. In addition, energy-dispersive X-ray spectroscopy revealed impurities located along particle boundaries, identified as iron- and nickel-free oxide phases originating from the Fe₂O₃ feedstock. These inclusions did not interfere with the reduction or homogenization processes.

The reduction process was further scaled up and extended to multicomponent metal oxide systems to evaluate its robustness and applicability to industrial by-products. A 250 g batch of 50 wt% Fe₂O₃-NiO powder was reduced in a horizontal Fe-Cr-Al tube furnace by heating non-isothermally from room temperature to 700 °C over 30 minutes, followed by an isothermal reduction at 700 °C for 3 hours. The resulting alloy exhibited stable body-centred cubic and face-centred cubic phases, and melt extraction analysis confirmed only trace oxygen levels. X-ray diffraction peaks consistent with a nickel–hydrogen phase were observed in the product from one upscaling trial but disappeared after 30 days, indicating that the phase is thermodynamically unstable at room temperature. As a preliminary step towards more complex chemistries, reduction trials were conducted using 0.1 g of a synthetic oxide mixture composed of equal mass proportions of Fe₂O₃, NiO, Cu₂O, WO₃, CoO, and MoO₃. The reduction trials conducted at 700 °C for 45 minutes demonstrated that all constituent oxides in the synthetic mixture could be completely reduced under the established process conditions.

In small- and large-scale by-product reduction trials, involving 0.1 g and 250 g samples of roasted spent catalyst, selective reduction of WO₃ and NiO was achieved within an unreduced Al₂O₃-SiO₂ matrix by applying the same thermal profile used in the successful upscaling trial with the 50 wt% Fe₂O₃-NiO synthetic mixture. X-ray diffractometry confirmed complete reduction of WO₃ and NiO, and scanning electron microscopy showed tungsten- and nickel-rich sites embedded in the Al₂O₃-SiO₂ matrix. Mass spectrometry of the off-gas in all trials detected only water vapor, with no hazardous emissions, confirming the environmental safety of the process. These findings demonstrate that the approach not only offers a scalable and environmentally friendly route for alloy production from primary ores but also provides an effective means to recover metals from by-products, supporting a more sustainable and circular materials economy.

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