Key Takeaways & Executive Findings
- •• The self-diffusion coefficient of Fe ions is consistently higher than that of Cr ions during solid-phase synthesis of iron chromite at 1473 K. • The activation energy for the formation of Fe(CrxFe1−x)2O4 spinel is 177.20 kJ·mol−1, with a pre-exponential factor of 610.78 min−1. • The diffusion couple method effectively reveals the diffusion and solid solution behavior of Fe2+, Cr3+, and Fe3+ ions in the reaction. • The findings provide kinetic parameters essential for optimizing chromium recovery from stainless-steel pickling sludge via pyrometallurgical processes.
Abstract
The enrichment of chromium in the magnetic iron chromite (Fe(CrxFe1−x)2O4) phase is crucial for the recovery and recycling of chromium in stainless-steel pickling sludge. The kinetics and reaction mechanism of the solid-phase reaction between Fe3O4 and Cr2O3 were investigated using the diffusion couple method at 1473 K. Not only the diffusion behavior of Fe2+ ions and Cr3+ ions was elucidated, but also the solid solution behavior of Fe3+ ions was discussed clearly. The microscopic morphology of the diffusion couple and the change in the concentrations of Fe and Cr cations across the diffusion layers were analyzed using scanning electron microscopy and energy dispersive spectroscopy. The self-diffusion coefficients of cations were calculated based on the concentration profiles of Fe and Cr, with the results indicating that the self-diffusion coefficient of the Fe ions was consistently higher than that of the Cr ions. Additionally, a mixture of Fe3O4 and Cr2O3 was annealed at 1373–1473 K for 1–5 h, and the kinetic parameters were calculated by studying the phase content of the product. The phase content of Fe(CrxFe1−x)2O4 in the product was determined by Rietveld refinement of X-ray diffraction data, revealing that an activation energy (E) of 177.20 kJ·mol−1 and a pre-exponential factor (B) of 610.78 min−1 of the solid-phase reaction that produced the Fe(CrxFe1−x)2O4 spinel.
1. Introduction
Stainless steel is widely used in architectural decoration, transportation, aerospace, energy generation, and environmental and structural engineering protection owing to its good corrosion resistance, mechanical characteristics, formability, and long service life [1–4]. However, an oxide layer is produced on the surface of stainless steel during hot rolling and annealing, which reduces its corrosion resistance and adversely affects its performance [5–6]. Therefore, to improve surface quality and enhance corrosion resistance, the oxide layer is usually cleaned by acid pickling [7–8]. This surface treatment method typically uses hydrochloric, hydrofluoric, and sulfuric acids for pickling the oxide layer. During the treatment process, the metals on the surface layer are dissolved in a pickling solution to produce a large amount of metal-containing wastewater [9–11], which is precipitated by calcium hydroxide to form sludge. Stainless-steel pickling sludge contains many metals such as Fe, Cr, and Ca, among which Cr(III) ions are easily oxidized into highly toxic Cr(VI) ions in the air, which causes severe environmental pollution problems [12–13]. Therefore, to effectively address the issue of environmental pollution caused by sludge storage, it is crucial to find a way to achieve the recovery and recycling of chromium in the sludge.
Several treatment methods have been proposed to recover and recycle chromium from sludge. Zhang et al. [14] introduced microbial-induced carbonate precipitation to immobilize chromium in stainless-steel pickling sludge, and the results showed that the bacteria-based biomineralization process has excellent detoxification work on stainless-steel pickling sludge. However, this method is more suitable for lower chromium leaching concentrations, and its application is relatively limited. The pyrometallurgical process can directly reduce valuable metals in sludge at high temperatures of 1573–1773 K by adding carbon to the sludge to form a Fe–Cr alloy [10,15]. However, carbon allocation leads to the production of CO2 gas during the treatment process, and the main CaSO4 and CaF2 in the sludge consequently produces volatile harmful gases, such as SO3, when the temperature is higher than 1473 K, which is contrary to green environmental protection. Therefore, the development of an efficient and environmentally friendly pollution-free treatment method is particularly important for the recovery of chromium from sludge.
Spinel-structured materials with the general formula AB2O4 are ceramic compounds that have garnered significant attention. In this formula, A(II) and B(III) represent cations. Depending on the characteristics of the constituent ions in the composition formula, these oxide materials can exhibit a wide range of physical properties, including good corrosion resistance, catalytic activity, electrical conductivity, dielectric properties, and magnetism [16–20].
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Yan Wang, Peiyuan Ni, Yuling Liu, Tengfei Deng (2025). Study on the ion behavior of solid-phase reaction synthesis of iron chromite at 1473 K. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3007-4
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Frequently Asked Questions
What is the main objective of this study?
The study aims to investigate the kinetics and reaction mechanism of the solid-phase reaction between Fe3O4 and Cr2O3 at 1473 K, focusing on the diffusion behavior of Fe2+ and Cr3+ ions and the solid solution behavior of Fe3+ ions, to enhance chromium recovery from stainless-steel pickling sludge.
What methods were used to analyze the reaction?
The diffusion couple method was employed, with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to analyze morphology and cation concentration profiles. X-ray diffraction (XRD) with Rietveld refinement was used to determine phase content and calculate kinetic parameters.
What were the key kinetic parameters obtained?
The activation energy (E) for the formation of Fe(CrxFe1−x)2O4 spinel was found to be 177.20 kJ·mol−1, and the pre-exponential factor (B) was 610.78 min−1.
How do the diffusion coefficients of Fe and Cr ions compare?
The self-diffusion coefficient of Fe ions was consistently higher than that of Cr ions, indicating faster diffusion of Fe in the solid-phase reaction.
What is the significance of this research for environmental protection?
By understanding the kinetics and mechanism of iron chromite formation, the study contributes to developing efficient and environmentally friendly methods for chromium recovery from stainless-steel pickling sludge, reducing pollution from toxic Cr(VI) ions.
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