Abstract:

Aluminum dross and bauxite residue (red mud) are two of the most challenging waste streams generated across the aluminum value chain. During primary and secondary aluminum production, dross forms on the molten metal surface and is typically classified into white dross, rich in metallic aluminum, and black dross, which contains significantly less metal and is composed mainly of oxides and salts. While white dross can be treated effectively through pyrometallurgical routes to recover aluminum, the processing of black dross remains problematic. The low metal content and small particle size limit economic recovery, and poor management can lead to the release of hazardous gases such as ammonia or phosphine, as well as the leaching of soluble salts harmful to water systems.

Upstream in the aluminum supply chain, the residues produced during the Bayer extraction of alumina from bauxite presents a parallel challenge. Generated in significant quantities, they consist of complex mixtures of Fe, Al, Si, Ca, Na and Ti bearing oxides with high alkalinity, leading to storage constraints and long-term environmental risks, including caustic leachates. Additionally, the storage of bauxite residue in artificial damns has led to multiple accidents, posing a serious health and safety risks to neighboring communities. Despite decades of research, large-scale valorization pathways for red mud remain limited, leaving most of it stockpiled.

In this work, we propose a novel hydrometallurgical process, HYBRECOT (HYdrogen from Black dross and Red mud Eco Conversion Technology), capable of treating both waste streams simultaneously. The method exploits the intrinsic alkalinity and reactive aqueous chemistry of red mud to activate aluminum hydrolysis within black dross, enabling the co-neutralization of both residues while producing hydrogen and recoverable heat. In contrast to conventional black dross treatments that rely on concentrated sodium hydroxide, this synergistic approach significantly reduces chemical consumption, lowers environmental impacts, and promotes the formation of more stable mineral phases.

The study focuses primarily on the kinetic behavior of the coupled hydrolysis reactions and the influence of process parameters on hydrogen production rates. Particular attention is given to the effect of the type of additive, particle size distribution (PSD) of the dross, and composition of both wastes, which were systematically varied to quantify their impact on reaction kinetics. Several kinetic models were evaluated to identify the dominant rate‑controlling mechanisms, and the corresponding kinetic parameters (activation energies, reaction orders, and rate constants) were determined from experimental data. This kinetic comparison provides new insight into how the inherent chemistry of red mud enhances aluminum activation and how process conditions can be optimized for improved hydrogen yields and residue stabilization.

Overall, the HYBRECOT process offers a promising route for safer and more sustainable management of aluminum industry residues while enabling simultaneous hydrogen co‑production.

This work is funded by Novacium and the French National Association for Research and Technology (ANRT) under a CIFRE agreement.

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M. Diego Guerrero