Key Takeaways & Executive Findings
- •• Microwave fluidization roasting significantly increases the specific surface area of limonite, enhancing CO contact and accelerating phase transformation from FeO(OH) to α-Fe2O3 and then to Fe3O4. • The water in limonite and newly formed magnetite exhibit strong microwave absorption, activating the reduction roasting process. • The process yields an iron concentrate with Fe grade of 59.26wt% and recovery of 90.07wt% after magnetic separation. • Kinetic analysis reveals a diffusion-controlled mechanism (G(α) = α2) with activation energy of 20.34 kJ/mol, providing a basis for process optimization.
Abstract
As a refractory iron ore, the clean and efficient beneficiation of limonite is crucial for ensuring a sustainable long-term supply of iron metal. In this study, the microwave fluidization magnetization roasting of limonite was explored. The micromorphology, microstructure, and mineral phase transformation of the roasted products were analyzed using a scanning electron microscope, an automatic surface area and porosity analyzer, an X-ray diffractometer, and a vibrating sample magnetometer. Kinetic analysis was also conducted to identify the factors limiting the roasting reaction rate. Microwave fluidization roasting significantly increased the specific surface area of limonite, increased the opportunity of contact between CO and limonite, and accelerated the transformation from FeO(OH) to α-Fe2O3 and then to Fe3O4. In addition, the water in the limonite ore and the newly formed magnetite exhibited a strong microwave absorption capacity, which has a certain activation effect on the reduction roasting of limonite. The saturation magnetization and maximum specific magnetization coefficient increased to 23.08 A·m2·kg−1 and 2.50 × 10−4 m3·kg−1, respectively. The subsequent magnetic separation of the reconstructed limonite yielded an iron concentrate with an Fe grade of 59.26wt% and a recovery of 90.07wt%. Kinetic analysis revealed that the reaction mechanism function model was consistent with the diffusion model (G(α) = α2), with the mechanism function described as k = 0.08208exp[−20.3441/(RgT)]. Therefore, microwave fluidization roasting shows significant potential in the beneficiation of limonite, offering a promising approach for the exploitation of refractory iron ores.
1. Introduction
Steel is the backbone of construction, bridges, and transportation infrastructure, playing a crucial role in driving economic and industrial activities [1–3]. Iron metal, the primary raw material for steelmaking, has long held the top position in global metal production and consumption. According to the World Steel Association, world iron and steel production in 2023 reached as high as 1309 million and 1892 million tons, respectively. Iron ore is the primary source of metallic iron; with the increasing depletion of high-quality iron ore resources, the mining of refractory iron ores has become increasingly important in maintaining a regular supply of iron ore [4–5]. High-quality iron ore can be processed into qualified iron concentrate through methods such as gravity separation, magnetic separation, flotation, or even just crushing and screening. For refractory iron ores, additional processing is necessary to effectively enrich the iron content [6–8].
Limonite is a mineral aggregate formed by the weathering of iron sulfide minerals, iron carbonate minerals, and iron-rich silicate minerals [9]. It primarily consists of iron oxides (hematite and goethite) and is a refractory iron ore widely distributed in supergene environments [10]. Its high crystalline water content, loose structure, and susceptibility to sliming make it difficult to process goethite using conventional beneficiation methods [11]. Furthermore, the high content of impurities, such as phosphorus, aluminum, and silicon dioxide, contributes to the difficulty in obtaining qualified iron concentrate [12]. Magnetic separation is the most conventional method for the preenrichment of iron ores; however, limonite is a weak magnetic mineral, and direct magnetic separation is not an optimal approach for this iron ore. Transforming limonite into a strong magnetic mineral through heat treatment to enhance the magnetic separation effect is an effective method for the production of qualified iron concentrate. Fluidization roasting, also known as suspension roasting [13], fluidized bed roasting [14], and flash roasting [15], is currently the main research direction to improve the heat and mass transfer efficiency of the roasting process and has been successfully applied in industrial production [16–17]. In this process, the material is in a fluidized state in the roaster. Liu et al. [11] conducted fluidization roasting and magnetic separation on limonite by utilizing H2 as the reducing gas. Roasting at 798 K and H2 concentration of 20vol% for 10 min and subsequent weak magnetic separation produced an iron concentrate with an Fe grade of 59.92wt% and a recovery of 87.26wt%. Tang et al. [18] and Sun et al. [19] employed suspension magnetization roasting, regrinding,
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Xinran Zhu, Yuangan Chen, Xu Liu, Yongsheng Sun, Yuexin Han (2025). Microwave fluidization magnetization roasting of limonite ores: Phase transformation, microstructure and kinetics. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3018-1
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Frequently Asked Questions
What is microwave fluidization magnetization roasting?
It is a novel beneficiation technique that combines microwave heating with fluidization to enhance the reduction roasting of limonite, transforming weak magnetic iron minerals into strongly magnetic magnetite for efficient magnetic separation.
How does microwave fluidization roasting improve limonite beneficiation?
It increases the specific surface area of limonite, improves contact with reducing gas CO, accelerates phase transformation to magnetite, and leverages the microwave absorption of water and magnetite to enhance the reduction process, resulting in higher iron recovery and grade.
What are the key kinetic findings of the study?
The roasting reaction follows a diffusion model (G(α) = α2) with a rate constant k = 0.08208exp[−20.3441/(RgT)], indicating that diffusion is the rate-limiting step.
What are the practical implications of this research?
The findings offer a promising approach for the efficient exploitation of refractory iron ores like limonite, potentially improving the sustainability of iron supply through cleaner and more efficient processing.
What iron grade and recovery were achieved after magnetic separation?
The process yielded an iron concentrate with an Fe grade of 59.26wt% and a recovery of 90.07wt%.
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