Key Takeaways & Executive Findings
- •• Achieved 95% total solid waste utilization in backfill material, with red mud at 40 wt%, demonstrating high-volume waste recycling. • Optimal backfill exhibited 7-day and 28-day compressive strengths of 4.4 MPa and 6.9 MPa, meeting typical underground support requirements. • Hazardous elements (Ni, Al, Cr6+, As) were effectively immobilized with efficiencies up to 97%, reducing environmental risks. • The approach offers significant cost savings and carbon emission reductions, promoting sustainable alumina production.
Abstract
The large-scale accumulation of industrial solid waste, including red mud and coal gangue, coupled with goafs left by underground mining activities, poses significant challenges to sustainable human development. In this study, red mud, coal gangue, and other solid wastes were used to prepare underground backfilling materials. The utilization rate of the total solid waste reached 95%, with red mud accounting for approximately 40wt% of the total. The unconfined compressive strength, setting time, and slump tests were conducted to evaluate the mechanical properties of the material. At the optimal ratio, the 7- and 28-d strengths reach 4.4 and 6.9 MPa, respectively. The initial and final setting times were 200 and 250 min, respectively, whereas the initial and 1-h slump exceed 250 and 210 mm, respectively. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed to explore the microstructure, phase composition, and chemical bonding within the material. Needle-like, clustered, and granular hydration products were observed, and the primary crystalline structures were identified as ettringite, gmelinite, C–A–S–H, and C–S–H. In addition, a thorough environmental risk assessment was conducted, complemented by detailed economic cost and carbon emission calculations. During the creation of backfill material, hazardous elements from solid waste are immobilized through adsorption, precipitation, and incorporation into the crystal lattice. The immobilization efficiencies for Ni, Al, Cr6+, and As were 97.03%, 94.32%, 86.43%, and 84.22%, respectively, at a pH of 8.49. Moreover, the use of solid waste as a raw material results in considerable cost savings and marked reduction in carbon emissions. This study innovatively promotes the green cycle of alumina production in the bauxite mining industry.
1. Introduction
Red mud (RM) is a typical byproduct of alumina extraction. The foremost global producers of alumina are China, Australia, Brazil, and India, where the manufacturing of each ton of alumina results in the generation of 1–1.5 metric tons of RM. The total global accumulation of RM is estimated to be 4 billion metric tons, with an annual increase of approximately 150 million metric tons [1]. Coal gangue (CG), a byproduct of coal mining and processing, is generated at a rate of 0.5–1 t for every ton of coal mined, depending on the mining method and coal quality. China, the United States, Russia, and India produce significant amounts of CG. According to a previous report [2], the total global accumulation of CG has surpassed 10 billion metric tons, with an annual increase of approximately 1 billion metric tons. In addition, with the development of industries, such as coal-fired power generation and steelmaking, the accumulation of solid waste [3], including fly ash (FM) and desulfurization gypsum (DG), continues to increase.
In recent decades, the pursuit of harmless and resourceful utilization of solid waste has become a significant research topic. Zhang et al. [4] achieved extraction efficiencies for Al (85.1%), Ce (82.4%), Gd (86.8%), Y (85.3%), and Sc (78.6%) via bioleaching using a 2:1 mass ratio of pyrite to RM over 22 d. Oliveira et al. [5] reported enhanced biphasic sulfide oxidation when gold nanoparticles were combined with a modified RM, which could serve as a promising catalyst for various chemical processes. Qin et al. [6] reported that the leaching efficiencies of Al, Li, and Ga from CG could reach 62.74%, 86.56%, and 46.18%, respectively, using delamination pretreatment and recycling strategies, whereas the leaching of Pb was notably inhibited. Koshy et al. [7] reported that geopolymers made from RM, CG, and FM attained strengths of approximately 5.7 MPa after 80°C curing. Binary mixtures of RM and CG cured at 800°C presented even higher strengths, reaching 7.3 MPa. Researchers have actively developed novel techniques for the sustainable utilization and disposal of RM, CG, and various solid wastes. However, many of these methods are not yet suitable for large-scale industrial applications [8].
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Jie Wang, Song Guo, Xiaoming Liu, Zengqi Zhang (2025). Utilization of red mud and coal gangue for underground backfill material: Hydration and environmental characteristics. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3144-4
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Frequently Asked Questions
What is the main objective of this study?
The study aims to develop an underground backfill material using red mud and coal gangue, achieving high solid waste utilization (95%) while ensuring mechanical strength and environmental safety.
What are the key mechanical properties of the developed backfill material?
At the optimal mix, the backfill material achieves 7-day and 28-day unconfined compressive strengths of 4.4 MPa and 6.9 MPa, respectively, with initial and final setting times of 200 and 250 minutes, and slump values exceeding 250 mm initially and 210 mm after 1 hour.
How does the material immobilize hazardous elements?
Hazardous elements such as Ni, Al, Cr6+, and As are immobilized through adsorption, precipitation, and incorporation into the crystal lattice of hydration products, achieving efficiencies of 97.03%, 94.32%, 86.43%, and 84.22%, respectively, at pH 8.49.
What are the environmental and economic benefits of this approach?
The use of solid waste as raw material reduces the need for virgin resources, lowers material costs, and significantly cuts carbon emissions, contributing to a green cycle in alumina production and mining.
What characterization techniques were used to analyze the material?
The study employed X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) to analyze the microstructure, phase composition, and chemical bonding, identifying hydration products like ettringite, gmelinite, C–A–S–H, and C–S–H.
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