1. Introduction
The present commercial process to extract and produce alumina from bauxite ore is the Bayer Process. In addition to producing CO
2 and being generally energy consuming, it also produces 170 million tons of red mud worldwide annually, per 2019 [
1,
2]. Depending on the quality of the mined bauxite ore, up to two times red mud is produced per mass of aluminum produced [
3]. Red mud is the main by-product of the Bayer Process and there are severe environmental issues concerning its disposal. The most practiced way to dispose of this very alkaline and heavy-metal-containing waste is to store it in ponds around the world. The ponds percolate into the local environments and can potentially flood large areas. Its alkalinity makes it damaging to agriculture and life, depending on groundwater, which threatens the ecosystems surrounding the deposits. Despite being hazardous, the red mud contains considerable amounts of useful minerals and metals such as iron, remnants of aluminum and rare earth elements. Red mud can contain up to 50 precent iron oxides and 10 percent aluminum oxides [
4]. These perfectly good raw materials are simply dumped in large deposits—a glaring example of how the linear economy and short-term cycles of supply and demand disregard long-term planning for conservation of natural resources for future generations [
4]. Because of the environmental challenges of the red mud, several solutions have been sought out in the past few years [
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16], and some of these are based on the Pedersen Process, which targets the prevention of red mud generation. The most important difference of the Pedersen process with the Bayer process is that it produces grey mud, a usable by-product, as opposed to red mud. The main by-product in the Pedersen Process is pig iron, which can further be used in foundries and steelmaking. It also produces CO
2 in the early stages of the process, although it is reused in later stages, which results in lower overall emissions [
13]. Even though the Pederson Process seems to be more eco-friendly than the Bayer Process, it is probably economically inferior and is not used commercially for this reason. However, due to the needs for sustainable development, the process is again under consideration to be revived in a modern form. The ENSUREAL Project is an example of previous research further exploring the use of the Pedersen Process. The project’s main objective is ensuring zero waste production of alumina in Europe. This optimalization of the Pederson Process also include extracting rare earth elements [
17].
The reduction of iron with hydrogen has been observed in recent studies [
18,
19,
20]. In bauxite, iron is mainly found in the oxide form called hematite (Fe
2O
3), while lower quantity of goethite, FeOOH, may co-exist. Throughout the reduction of the iron oxide with hydrogen, it will form magnetite (Fe
3O
4) before further reduction to metallic iron [
5]. Depending on the reduction temperature, the process forms different intermediate oxides. When reduction is performed at temperatures above 570 °C and below 1400 °C, it will form wüstite (FeO) before the metallic iron formation [
18]. When intending to increase the formation of metallic iron, the oxygen amount should be below 23.25% and above the initial wüstite formation temperature [
18]. The hydrogen reduction reactions are shown in reactions (1)–(3), with coherent change in enthalpy at 700 °C [
12]. The first reaction explains the formation of magnetite from hematite, while the two subsequent reactions explain the further reduction of magnetite to wüstite, and furthermore, wüstite is reduced to metallic iron.
From a thermodynamics point of view, chemical reactions (1) to (3) can occur at appropriate temperatures and H
2/H
2O ratios. For a typical industrial temperature of 700 °C, the H
2/H
2O ratio for chemical reactions (1) to (3) must be above 1 × 10
−5, 0.7 and 2.8, respectively. In bauxite residue (BR; dewatered red mud), there is a significant amount of iron, with higher content than the utilized bauxite ore in the Bayer Process, and in addition, this material has some undigested alumina, about 15–25 wt% Al
2O
3 [
11]. To further ensure eco-friendly metals and alumina production via bauxite residue valorization, the HARARE Project was started in 2021 and is a consortium of universities, research institutes and industrial companies from Norway, Greece, Germany and Belgium [
21,
22]. This project was established based on the ENSUREAL project achievements [
23]. The main innovative approach is that the bauxite residue is agglomerated and then reduced with hydrogen. Furthermore, the metallic iron is separated by magnetic separation and alumina and rare earth elements are recovered via hydrometallurgical approaches. Hydrogen use for the reduction step ensures no direct CO
2 emission from the process, as mostly H
2O is emitted in this step. In a process scheme of the Harare project, lime is used as an additive to agglomerate bauxite residue and form more digestible compounds of aluminum, leachable calcium aluminates. Lime addition is also of great importance when it comes to inhibiting the leaching of silicon and titanium of BR, which increases the purity of alumina in later processing [
24]. The present work presents the test of a new novel process on bauxite residue valorization by first hydrogen reduction, followed by alkaline leaching under specific conditions. No similar research on bauxite residue valorization was found in the literature to evaluate regarding the applied approach and obtained results.