(682b) A Holistic Approach to Biomass Thermochemical Treatment and Red Mud Recycling.

It is estimated that over 4 billion tons of red mud are currently deposited in storage facilities worldwide, while 200 million tons of bauxite residue were produced globally in 2023 [1]. Red mud is the main by-product of the alumina refining process from bauxite ore, using sodium hydroxide for aluminum oxide extraction, producing a highly alkaline tailing[2]. Moreover, the bauxite residue can also exhibit radionuclidic activity [3]. Therefore, the dumping of red mud can cause severe environmental disruptions on disposal sites and surroundings, making it imperative to employ engineering structures to contain the residue. Thus, a new circular paradigm must be sought to tackle the red mud challenge. Rather than an unwanted residue, it is possible to face red mud as an important source of valuable materials. For instance, the major constituent of red mud, hematite, can be as high as 50 wt. % [4]. Other valuable metals, such as lanthanum, cerium, vanadium, and rare earth elements often add up to over 1000 ppm [5]. In addition, bauxite residue is also rich in oxides of aluminum, titanium, sodium, and calcium carbonate.

In this work, it is proposed the use of red mud as the oxidizing agent for biomass conversion, instead of air, carbon dioxide, or steam, as is customary. A gasification process that does not require the input of extra gases will yield a producer gas with higher HHV, given its undiluted nature. This will also result in a treated material in which iron is found in a reduced state, therefore recoverable by magnetic separation [6]. Given the high amounts of iron, alkali, and alkaline earth metal compounds in red mud, the bauxite residue is hypothesized to be catalytically active in both char conversion and tar reforming [7], [8]. This work aims to provide a holistic proof-of-concept of using red mud in biomass gasification with both gasification products and the treated red mud under evaluation.

Methods:

Bauxite residue samples used in this work were received from ETI Aluminum. Thermogravimetric experiments were performed to evaluate the mud-driven biomass conversion. Derived solid samples were collected and characterized by powder X-ray diffraction (XRD). Three samples were analyzed: 1) 10 ± 0.1 mg of pine sawdust; 2) 50 ± 0.5 mg of red mud, and 3) a mixture of 50 ± 0.5 mg of red mud with 10 ± 0.1 mg of pine sawdust. Thermogravimetric analyses were performed on a Netzsch STA 449 F3 apparatus. A 400 Nml/min nitrogen flow was employed as samples were heated from room temperature to 950 °C at a rate of 20 °C/min. A final 10-minute step was employed at 950 °C. Powder X-ray diffraction (XRD) was recorded with a PANalytical X’Pert PRO diffractometer with Cu Kα radiation (λ = 1.54 Å) at 40 kV/40 mA. samples were scanned from 5° to 80° (2θ) at a rate of 5° min^−1 with 0.01° step size.

Furthermore, packed bed reactor experiments using red mud are carried out at similar conditions to TGA runs. Biomass and red mud samples are mixed at a mass ratio of 5:1 and exposed to 85 vol% CO2 (in N₂) gas stream. SPA method cand GC analysis is used for product characterization [9]. The resulting roasted red mud will be analyzed via XRD to ensure that the reduction behavior observed in TGA experiments is still valid when increasing the scale.

Results:

The thermogravimetric analysis results for the three samples are shown in Figure 1a in the form of DTG curves. Three mass loss events can be found for the red mud sample: drying of physically adsorbed water at low temperatures (<150 °C), decomposition of aluminum hydroxides into alumina is observed up to 350 °C [10]. The decomposition of carbonates is another important mass loss step observed in the characteristic 650 - 775 °C range, which is supported by the XRD spectra for the Red mud – 950 °C (Figure 1b) [11]. No change in the Iron oxidation state is recorded when red mud alone is taken to the TGA. The pine sawdust sample exhibits a typical lignocellulosic biomass behavior, with drying and pyrolysis. When pine and red mud were analyzed together, a synergistic effect was observed in three extra mass loss events at temperatures higher than 650 °C. This mass loss is due to the biomass-driven reduction of iron. This phenomenon is confirmed by the XRD spectra for red mud + pine – 950 °C, where no peaks for Iron III are found, but peaks for zero-valent Iron and FeO arise.

Implications:

This paper will present an experimental evaluation of red mud as an oxidizing agent in biomass conversion and a catalyst for tar reforming. Furthermore, the reduction of the iron content will also be assessed, given its importance for iron recovery. The proposed proof-of-concept establishes a disruptive synergetic framework for the simultaneous treatment of red mud and the valorization of waste biomass.

Acknowledgments:

The work is part of the project “Abtomat - Utilization Of Aluminium Bearing Raw Materials For The Production Of Aluminium Metal, Other Metals, And Compounds” (project number 2021-05201) funded by Vinnova (Sweden´s Innovation Agency) via the ERA-NET Cofund on Raw Materials (ERA-MIN3) program. The authors would like to thank the project consortium partners for the discussions. We would also like to thank ETI Aluminyum for the samples.

Bibliography

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Figure 1. a) DTG for red mud (50 mg), pine sawdust (10 mg), and a mix of red mud (50 mg) with pine sawdust (10 mg); and b) XRD spectra for raw red mud, red mud after the TGA, and red mud + pine sawdust after TGA.