Reaction Behavior of Iron Minerals and Metallic Iron Particles Growth in Coal-Based Reduction of an Oolitic Iron Ore

Enrichment Characteristics of Cr in Chromium Slag after Pre-Reduction and Melting/Magnetic Separation Treatment

Concentrating the chromium in chromium slag and improving the chromium–iron ratio is beneficial for the further utilization of chromium slag. In this paper, chromium slag obtained from a chromite lime-free roasting plant was used as the raw material. Pellets made of the chromium slag and pulverized coal were reduced at different pre-reduction temperatures and then separated by a melting separation process or magnetic separation process, respectively. The mass and composition of the metallized pellets before separation, along with the alloy and tail slag after separation, were comprehensively analyzed. The experimental results showed that the output yield of alloy, iron recovery rate, and chromium content in the alloy were all higher when using melting separation than when using magnetic separation, because of the further reduction during the melting stage. More importantly, a relatively low pre-reduction temperature and selection of magnetic separation process were found to be more beneficial for chromium enrichment in slag; the highest chromium–iron ratio in tail slag can reach 2.88.

The Growth Characteristics and Kinetics of Metallic Iron in Coal-Based Reduction of Jinchuan Ferronickel Slag

As the fourth-largest industry waste residue, after iron slag, steel slag, and red mud, in China, the comprehensive utilization of nickel slag is imminent. Coal-based reduction combined with magnetic separation was considered an efficient method to extract iron from nickel slag. During the coal-based reduction of Jinchuan ferronickel slag, the growth characteristics and kinetics of metallic iron were investigated in this paper. The metallisation rate and metal iron grain size gradually increased with the reduction temperature or the reaction time, and the coal-based reduction process was divided into the rapid formation period and the aggregation growth period of the metallic phase. The granularity distribution of metallic iron obeyed the Doseresp sigmoidal function, and the activation energy of grain growth at different stages were 52.482 ± 4.448 kJ·mol−1 and 26.426 ± 3.295 kJ·mol−1, respectively. Meanwhile, a mathematical growth model of the metallic iron grains was also established.

Mechanistic study on calcium ion diffusion into fayalite: A step toward sustainable management of copper slag

Copper slag, which contains Fe-rich fayalite (Fe₂SiO₄), is a valuable solid waste that warrants further research in order to recover iron. Calcium oxide (CaO) can significantly enhance iron recovery from copper slag; however, the associated mechanism has not yet been explored. In this study, we investigated the interaction between CaO and Fe₂SiO₄ to obtain detailed understanding of the role of CaO in enhancing iron recovery. The presence of CaO was found to accelerate the decomposition of Fe₂SiO₄ via an ion-exchange-like process. Specifically, CaO dissociated into Ca(II) and a Ca-deficient Ca_1-yO species at high temperatures. The Fe(II) ion at the M2 site of Fe₂SiO₄ was substituted by the released Ca(II) ion, resulting in the formation of [(Fe_(2-x)Ca_x)SiO₄]∙xFe(II). Subsequently, the substituted Fe(II) occupied the Ca vacancy in Ca_1-yO to form (Ca_(1-y)Fe(II)_y)O. The disproportionation of Fe(II) and the combination reaction between CaO and the SiO₂ separated from Fe₂SiO₄ led to the generation of the final products, viz. Fe₂O₃, Fe₃O₄, and CaSiO₃. This study explains the specific role of CaO in decomposing Fe₂SiO₄. It would not only provide theoretical guidance for iron recovery from copper slag but also present a new perspective on the recycling of valuable resources from many other smelting slags (e.g., iron slag, lead slag, and nickel slag).

Isothermal Coal-Based Reduction Kinetics of Fayalite in Copper Slag

The coal-based reduction of fayalite was characterized using thermogravimetric (TG) and differential TG methods with reduction temperatures from 1123 to 1273 K. The results of fayalite isothermal reduction indicate that the reduction process is divided two stages. The corresponding apparent activation energy E was gained using the isoconversional and model-fitting methods. At the first stage, the effect of temperature on the reduction degree was not clear, and the phase boundary chemical reaction was the controlling step, with an apparent activation energy E value of 175.32-202.37 kJ·mol^-1. At the second stage, when the temperature was more than 1123 K, the conversion degree and the reaction rate increased nonlinearly with increasing temperature, and two-dimensional diffusion, three-dimensional diffusion, one-dimensional diffusion, and phase boundary-controlled reaction were the controlling stages, with an apparent activation energy E ranging from 194.81 to 248.96 kJ·mol^-1. For the whole reduction process, the average activation energy E and pre-exponential factor A were 185.07-225.67 kJ·mol^-1 and 0.796-0.797 min^-1, respectively.

Kinetics of the Volume Shrinkage of a Magnetite/Carbon Composite Pellet during Solid-State Carbothermic Reduction

The volume shrinkage evolution of a magnetite iron ore/carbon composite pellet during solid-state isothermal reduction was investigated. For the shrinkage, the apparent activation energy and mechanism were obtained based on the experimental results. It was found that the volume shrinkage highly depended on the reduction temperature and on dwell time. The volume shrinkage of the pellet increased with the increasing reduction temperature, and the rate of increment was fast during the first 20 min of reduction. The shrinkage of the composite pellet was mainly due to the weight loss of carbon and oxygen, the sintering growth of gangue oxides and metallic iron particles, and the partial melting of the gangue phase at high temperature. The shrinkage apparent activation energy was different depending on the time range. During the first 20 min, the shrinkage apparent activation energy was 51,313 J/mol. After the first 20 min, the apparent activation energy for the volume shrinkage was only 19,697 J/mol. The change of the reduction rate-controlling step and the automatic sintering and reconstruction of the metallic iron particles and gangue oxides in the later reduction stage were the main reasons for the aforementioned time-dependent phenomena. The present work could provide a unique scientific index for the illustration of iron ore/carbon composite pellet behavior during solid-state carbothermic reduction.

Innovative methodology for recovering titanium and chromium from a raw ilmenite concentrate by magnetic separation after modifying magnetic properties

Raw ilmenite concentrate containing Cr can be either as a resource or as one kind of the most hazardous solid waste. In order to recover titanium and chromium from the raw concentrate which was separated from the Promenade deposit, Gaza province, Mozambique, an innovative technology using modification of magnetic property followed by magnetic separation was proposed. Magnetic property, phase and surface morphology of the sample before and after oxidizing roasting were firstly characterized by magnetism, chemistry, XRD and MLA analyses to interpret the mechanism of oxidizing roasting of the ilmenite. Then, these factors such as oxidizing roasting temperature, residence time and magnetic induction affecting on magnetic separation performance were examined and the optimum process parameters were determined. A commercial concentrate containing 47.94% TiO₂ and 0.23% Cr₂O₃ was obtained and the recovery of TiO₂ and Cr₂O₃ was 78.52% and 5.42%, respectively. The tailing obtained was preliminarily concentrated by a high-intensity magnetic separator and a rough chromite concentrate was gained. In order to further purify the rough one, reducing roasting was carried out to transform the minerals containing hematite into the minerals containing magnetite, followed by a low-intensity magnetic separation. The effects of these parameters such as temperature, carbon powder dosage, holding time and magnetic induction on magnetic separation performance were investigated and the optimal conditions were determined. A concentrate containing 28.65% Cr₂O₃ was obtained and the total recovery of Cr₂O₃ was 84.18%.

The stability, characteristics and magnetic properties of iron carbide from an oolitic hematite

Iron carbide (Fe₃C) is a magnetic material but it is not stable when it is prepared.