Reaction characteristics of carbon-bearing pellets of Bayan Obo lean iron ores in a static magnetic field

Abstract

The use of low-grade, refractory and composite paragenetic mineral resources is necessary for overcoming the shortage of iron ore resources in China. As a solution to the treatment of such iron ores, the direct reduction of carbon-bearing pellets can ensure complete iron removal and the effective enrichment of other high-value elements. Thus, this technology enjoys a broad application prospect. However, there are several problems with low-temperature reduction, such as low iron ore reaction efficiency, long reaction time, and high energy consumption. To improve the low-temperature carbothermic reduction efficiency of iron ores, a static magnetic field with magnetic induction intensity of 1.0 T was introduced. An isothermal reduction experiment was conducted at 1223 K to study the low-temperature self-reduction characteristics of carbon-bearing pellets of Bayan Obo lean iron ores in the static magnetic field. Also, the acting mechanism of the magnetic field was explored from the perspective of the reduction process, reaction efficiency, phase composition, microstructure changes, and dynamic behavior of iron ores. The results showed that the magnetic field can increase the low-temperature reduction rate of carbon-bearing pellets of Bayan Obo lean iron ores. Under the conditions of reduction temperature of 1223 K, magnetic induction intensity of 1.0 T, and reduction time of 60 min, the reduction degree was 92.42%, 1.65 times that without a magnetic field. The magnetic field promoted the replacement of Ca²⁺ and Fe²⁺, so that the hard-to-reduce iron-bearing silicates were reduced in the order of Fe₂SiO₄ → (Ca, Na)FeSiO₄ → FeO → Fe. The magnetic field enabled loose minerals, more pores and cracks, and changes in the growth morphology and distribution position of metallic iron. Compared with the case under the non-magnetic condition, the metallic iron precipitated from the slag phase in a foliated shape, separated from the matrix iron oxides, and grew up at the junction of the slag phase and coke. The magnetic field significantly increased the interfacial chemical reaction rate of the carbothermic reduction of iron ores and reduced the internal diffusion resistance of gas in the product layer. Specifically, the interfacial chemical reaction rate increased by 138% and the internal diffusion coefficient increased by 309%. Therefore, the effect of the magnetic field on the internal diffusion resistance was the main cause for strengthening the low-temperature reduction of iron ores.