Nanometer-sized hematite was prepared via a two-step process. In the first step, FeSO₄·7H₂O was oxidized to Fe₂(SO₄)₃ by oxygen in an acidic solution. In the second step, the Fe₂(SO₄)₃ was reduced to nanosize hematite with sulfur vapor at 550 °C. The hematite has good thermal stability up to 500 °C and good colloid stability in water-based paint. Its properties satisfy the requirements of the international standard ISO 1248-A-I-1-a for an iron oxide red pigment. Graphical abstract: [Figure not available: see fulltext.]

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In order to understand the pre-reduction behaviour of fine hematite particles in the HIsarna process, change of morphology, phase and crystallography during the reduction were investigated in the high temperature drop tube furnace. Polycrystalline magnetite shell formed within 200 ms during the reduction. The grain size of the magnetite is in the order of magnitude of 10 µm. Lath magnetite was observed in the partly reduced samples. The grain boundary of magnetite was reduced to molten FeO firstly, and then the particle turned to be a droplet. The Johnson-Mehl-Avrami-Kolmogorov model is proposed to describe the kinetics of the reduction process. Both bulk and surface nucleation occurred during the reduction, which leads to the effect of size on the reduction rate in the nucleation and growth process. As a result, the reduction rate constant of hematite particles increases with the increasing particle size until 85 µm. It then decreases with a reciprocal relationship of the particle size above 85 µm.

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Suspension reduction kinetics of hematite ore particles at 1710 K to 1785 K was described by the Johnson-Mehl-Avrami-Kolmogorov model with Avrami exponent of 1.405. The apparent activation energy is 105.5 kJ mol⁻¹ with the rate determining step of nucleation and growth. The reduction degree of the hematite at the endpoint is a linear function of temperature and the logarithmic oxygen potential of the reacting gas. A peak function of reaction rate constant with particle size has been verified in this work, and the maximum value of the reaction rate is located at around 85 µm particle size. The influence of heat transfer on the reaction process has been evaluated. The results suggest that the heating-up process for large particles, 244 µm particles, for instance, cannot be ignored. It can retard the reaction rate compared to small particles. Normally, the reaction rate constant decreases linearly with the increase of ln[p(O₂)] of the reacting gas mixture. However, 95 vol pct CO₂ in the reacting gas can accelerate the reaction rate of thermal decomposition of hematite due to the emissivity of CO₂ gas. It results in a higher reaction rate of 110 µm particles in 95 vol pct CO₂-containing gas than that in other less CO₂-containing gases.

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For both the waste treatment of large quantities of blast furnace (BF) slag and carbon dioxide (CO₂) that are discharged in ironworks, mineral carbonation by BF slag was proposed in this decade. However, it has not been widely used due to its high energy consumption and low production efficiency. In this study, a microwave roasting method was employed to mineralize CO₂ with BF slag, and the process parameters for the sulfation and energy consumption were investigated. A mixture of BF slag and recyclable ammonium sulfate [(NH₄)₂SO₄] (mass ratio, 1 : 2) was roasted in a microwave tube furnace, and then leached with distilled water at a solid : liquid ratio of 1 : 3 (g mL^-1). Under the optimized experiment conditions (T = 340 °C, holding time = 2 min), the best sulfation ratios of calcium (Ca), magnesium (Mg), aluminum (Al), and titanium (Ti) were 93.3%, 98.3%, 97.5%, and 80.4%, respectively. Compared with traditional roasting, the production efficiency of this process was more than 10 times higher, and the energy consumption for mineralizing 1 kg of CO₂ could be reduced by 40.2% after simulation with Aspen Plus v8.8. Moreover, 236.1 kg of CO₂ could be mineralized by one ton of BF slag, and a series of by-products with economic value could also be obtained. The proposed process offers an energy-efficient method with high productivity and good economy for industrial waste treatment and CO₂ storage.

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In order to understand the thermal decomposition kinetics of hematite particles in inert atmosphere, thermogravimetriy was employed for isoconversional analysis. The kinetic triplet was estimated from the experimental data and the isothermal reaction kinetics was predicted. The results indicated that the thermal decomposition could be divided into two stages, of which the activation energies were 636 kJ/mol and 325 kJ/mol, respectively. The exponential form of pre-exponential factor, ln(A/s⁻¹), for the two stages were estimated to be 42.9±6.6 and 14.1±3.08. At last, the kinetic mechanism of the first stage was suggested to match Sestak-Berggren model as f(α)=(1−α)^1.38. The relatively slow reaction rate of the second stage was due to the slag formation during the reaction.

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High-temperature reduction processes of iron oxide particles suspension are promising in carbon emission abatement. Recently, researchers have contributed abundant knowledge of the reaction mechanism and kinetics of iron oxide particles above 1473 K, while there was very limited information 10 years ago. Although the understanding of the high-temperature reduction of iron oxide particles is still not comprehensive, a brief review of the academic reports is helpful for the future work on this topic. The high-temperature reduction of iron oxide suspension is characterized by having: rapid reaction, obvious thermal decomposition and melting process. Evaluation of the kinetic data shows that the reduction process of single particles is not rate-determined by the diffusion process at the studied temperatures. The reaction rate constant is within 10⁻²–10 s⁻¹ in these studies. Furthermore, comparing previous studies in iron oxide reduction field, the phase transformation and effect of gangue minerals to the reduction of iron oxide particles above 1473 K requires more input and research.

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Decreasing the preheating temperature is an effective step to control the energy consumption in the hot rolling process. In order to obtain the lowest preheating temperature to prepare enough thickness of oxide scale in the hot rolling process, the oxidation resistance of commercial steel samples with different Al and Si contents were investigated in this paper. The results indicate that both Al and Si based oxides form at the steel-oxides interface as diffusion barrier but Al provide stronger diffusion resistance than Si in the diffusion-controlling oxidation region. Meanwhile, a three-dimensional oxidation kinetic model has been adopted to depict the oxidation behavior of four types of commercial steel. The oxidation process of automotive steel sample containing with low alloy elements is kinetically determined by interface chemical reaction. Its activation energy is 55.2 ± 6.9 kJ/mol. As for silicon steel containing with relative high alloy elements, its controlling process is determined by diffusion step at low temperature and controlled by chemical reaction rate at high temperature. In order to obtain enough thickness of oxide scale, the lowest preheating temperature of different types steel range from high to low should be the steel with high content of Al (1180 °C), the steel with high content of Si and low content of Al (1130 °C) and the steel with high contents of Si and Al (1030 °C).

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An innovative electroslag remelting furnace with a water-cooled electrode was introduced to recycle the rejected electrolytic manganese metal (EMM) scrap. To clarify the desulfurization process in the rejected EMM scrap, a transient three-dimensional comprehensive numerical model was elaborated. Using the magnetic potential vector approach, the respective electromagnetic fields were calculated via the Maxwell equations. The Lorentz force and the Joule heating fields were derived as phase distribution functions and interrelated via the momentum and energy conservation equations as source terms, respectively. The molten manganese metal droplet motion, as well as the fluctuation of the slag–metal interface, was described by the volume-of-fluid (VOF) approach. Besides, the solidification was modeled via the enthalpy-based technique. A thermodynamic module was established to estimate the sulfur mass transfer rate between the molten manganese metal and the molten slag. Furthermore, a factor related to the magnitude and frequency of the alternating current and the physical properties of the melt was introduced to include the electro-emulsification phenomenon. An experiment has been carried out with a commercial-scale ESR device. The predicted values of the slag temperature and sulfur content in the final manganese ingot were found to agree reasonably with the corresponding measured data. Under continuous melting of the rejected EMM scrap, molten manganese metal droplets are formed at the domain inlet, grow, and fall down. Highly conductive molten manganese metal droplets significantly change distributions of the current streamline, the Joule heating, and the Lorentz force around and within it. Moreover, droplets are inclined to rotate and move inside the mold. With the renewal of the slag–manganese interface, sulfur in the molten manganese metal is constantly transferred to the molten slag. With the applied current ranging from 3000 to 4000 A, the average sulfur content of the manganese ingot dropped from 0.0447 to 0.0291 pct, and thus, the desulfurization rate rose from 55.3 to 70.9 pct.

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To improve the boron-removal efficiency of metallurgical-grade silicon by increasing the reaction rate, a combined method with the 30 mol pct CaO-23.3 mol pct SiO2-46.7 mol pct CaCl2 slag treatment and ammonia injection at 1723 K to 1823 K was proposed. For 1 hour and at 1823 K, the maximum removal efficiency of boron was 98 pct, and the final boron concentration in silicon decreased to 1.5 ppmw by the present method without the introduction of the iron catalyst. A kinetic model was also established to clarify the reaction mechanism and rate-limiting steps of this complicated boron-removal process. In this model, the rate-limiting step is the mass transfer of boron oxide at the interface between the slag and silicon phase.@en

A commercial solid oxide fuel cell with a Ni/YSZ anode was characterized under a pure methane atmosphere. The amount of deposited carbon increased with an increase in temperature but decreased when the temperature exceeded 700°C. The reactivity of carbon decreased with increasing deposition temperature. Filamentous carbon was deposited from 400 to 600°C, whereas flake carbon was deposited at 700 and 800°C. With increasing temperature, the intensity ratio of the D band over the sum of the G and D bands was constant at the beginning and then decreased with the transformation of the carbon morphology. The crystallite size increased from 2.9 to 13 nm with increasing temperature. The results also indicated that the structure of the deposited carbon was better ordered with increasing deposition temperature. In comparison with pure Ni powders, the interaction between the YSZ substrate and Ni particles could not only modify the carbon deposition kinetics but also reduce the temperature effect on the structure and reactivity variation of carbon.@en

Solar energy has received considerable attention over the past few decades, due to its importance as a green and renewable energy. Low-cost solar-grade silicon production is critical for the widespread use of solar cells. Conventional routes (e.g., modified Siemens process: chlorosilane and hot filament) have still dominated the production of solar-grade silicon. The metallurgical route offers benefits in the productivity and cost, but efficient removal of boron is one of the most daunting challenges in front of us. This paper reviews thermodynamic and kinetic properties (solubility, diffusivity, diffusion coefficients, mass transfer rate, and activity coefficient) of boron and recent research topics (slag treatment, solvent refining, gas injection, plasma treatment, and acid leaching) for boron removal.

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In this work, a novel low-cost and robust approach is developed to covert Ti-bearing blast furnace slag (Ti-slag) to the main feed stock of photoassisted NH3 selective catalytic reduction (Photo NH3-SCR) of NO with high performance. The CaTi1-xMnxO3-δ catalyst was directly extracted from the Ti-bearing slag with MnO2 in situ modification and Na2CO3 reformation followed by dilute hydrochloric acid leaching and water washing. Various methods were employed to characterize the structure changes of the slag in the preparation processes of MnO2 in situ doping, Na2CO3 reforming, and leaching. Mn element can be incorporated into the perovskite CaTiO3 with 0.16 to 1.79 wt % depending on the amount of MnO2 added in Ti-slag at 1500 °C. It is interesting to note that the amount of Mn incorporated into the perovskite decreases remarkably with increasing the additional amount of MnO2 in the in situ doped modification, which reaches a maximum of 9.54 wt % for the reformed slag of 5 wt % MnO2 addition. Moreover, most of the Si has been transferred to the sodium aluminum oxide instead of diopside, which is easily removed in the following leaching stage. Further semiquant crystalline analysis shows that there is 99 wt % of perovsvkite CaTi1-xMnxO3-δ with 1 wt % hexamagnesium manganese(IV) oxide in the slag after leaching with 5 wt % MnO2 addition, and the catalyst shows more than 93% NO removal efficiency at 300 °C in photoassisted NH3-SCR performance. The present study proposes a new route for the high value-added recycling of silicate minerals and wastes via structure-reforming with in situ optimization of secondary resources.@en

HIsarna is a promising ironmaking technology to reduce CO ₂ emission. Information of phase transformation is essential for reaction analysis of the cyclone reactor of the HIsarna process. In addition, data of density and volume of the ore particles are necessary for estimation of the residence time of the particles in the cyclone reactor. Phase transformation of iron ore particles was experimentally studied in a drop-tube furnace under simulated cyclone conditions and compared with thermodynamic calculation. During the pre-reduction process inside the reactor, the mineralogy of iron ore particles transforms sequentially from hematite to sub-oxides. The density changes of the particles during the melting and reduction can be predicted based on the phase composition and temperature. Therefore, density models in the studies were evaluated with reported experimental data of slag. As a result, a more reliable density model was developed to calculate the density of the formed slag containing mainly FeO–Fe ₂ O ₃ . The density and volume of the partially reduced ore particles or melt droplets were estimated based on this model. The results show that the density of the ore particles decreases by 15.1% at most along the progressive reduction process. Furthermore, the model results also indicate that heating, melting and reduction of the ore could lead to 6.63–9.37% swelling of the particles, which is mostly contributed by thermal expansion. It would result in corresponding variation in velocity of the ore particles or melt droplets during the flight inside the reactor.

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Ammonia gas blowing has been successfully applied to the silicon refining process for boron removal. Molten iron catalyst could effectively promote the boron removal ratio in the progress. It inspired the attempt of combining ammonia blowing with other refining treatments. Oxygen resources were introduced into the process in order to further decrease the boron content in silicon. However, neither slagging + ammonia blowing nor humidification + ammonia blowing shows a significant positive improvement in the silicon refining process, respectively. The experimental results implied that the introduction of oxygen sources into the ammonia system would decrease boron nitride activity or produce Si2N2O which retard the efficient generation of boron-bearing volatiles.@en

The increasing amount of silicon waste generated from the rapid developing photovoltaic industry calls for an economical silicon recycling process. The present work proposes a facile process with which silicon waste and ironmaking slag containing TiO₂ were used as raw materials to produce titanium silicides, a promising high added value material. The process was experimentally investigated in lab scale. The result shows that a high CaO/SiO₂ ratio in slag promotes the reaction. TiSi₂ and Ti₅Si₃ could be synthesized as principal products within 0.5 and 3 h with CaO/SiO₂ = 1.31 in mass, respectively. CaO-SiO₂ slag was produced as byproducts. Kinetic analysis indicates that silicon diffusion in slag is the rate-determining step of the reaction process. The reaction rate constant is around 1.0 × 10^-4 s, and the effective diffusion film thickness in slag side is around 10^-3 cm at the silicon-slag interface. Slag basicity is suggested to increase to 1.31 for a faster silicon diffusion and further promotion of the reaction rate.

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Hydrogen has received much attention in the development of direct reduction of iron ores because hydrogen metallurgy is one of the effective methods to reduce CO₂ emission in the iron and steel industry. In this study, the kinetic mechanism of reduction of hematite particles was studied in a hydrogen atmosphere. The phases and morphological transformation of hematite during the reduction were characterized using X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy. It was found that porous magnetite was formed, and the particles were degraded during the reduction. Finally, sintering of the reduced iron and wüstite retarded the reductive progress. The average activation energy was extracted to be 86.1 kJ/mol and 79.1 kJ/mol according to Flynn-Wall-Ozawa (FWO) and Starink methods, respectively. The reaction fraction dependent values of activation energy were suggested to be the result of multi-stage reactions during the reduction process. Furthermore, the variation of activation energy value was smoothed after heat treatment of hematite particles.

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A sustainable simultaneous synthesis method of TiB2-Ti5Si3 mixtures is developed as an alternative recovery route of silicon waste from the photovoltaics industry. TiO2-CaO-B2O3 ternary was reduced by molten silicon at 1500 °C, producing TiB2 within 3 h and Ti5Si3 within 10 h. The lab-scale experiments were designed to study the reaction kinetics of the reaction. SiO2 diffusion rate controlling kinetic model was derived to describe the reaction process. The reaction rate constant was recommended to be ∼5 × 10-4 s-1. B2O3 content in slag was suggested to have an insignificant effect on the reaction rate constant, whereas more titanium and boron could be reduced by silicon with higher B2O3 content in slag. Stability of the reaction interface was simulated using phase field model. A decreased interfacial tension in B2O3-containing system was indicated to be the reason for emulsification of phases.@en

Carbon deposition on nickel powders in methane involves three stages in different reaction temperature ranges. Temperature programing oxidation test and Raman spectrum results indicated the formation of complex and ordered carbon structures at high deposition temperatures. The values of I(D)/I(G) of the deposited carbon reached 1.86, 1.30, and 1.22 in the first, second, and third stages, respectively. The structure of carbon in the second stage was similar to that in the third stage. Carbon deposited in the first stage rarely contained homogeneous pyrolytic deposit layers. A kinetic model was developed to analyze the carbon deposition behavior in the first stage. The rate-determining step of the first stage is supposed to be interfacial reaction. Based on the investigation of carbon deposition kinetics on nickel powders from different resources, carbon deposition rate is suggested to have a linear relation with the square of specific surface area of nickel particles.

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Electroslag remelting (ESR) technology has been innovatively employed to recycle the rejected electrolytic manganese metal (EMM) scrap. For a better understanding of the refining process, a transient three-dimensional comprehensive numerical model was developed based on computational fluid dynamics (CFD). With the assistance of the magnetic potential vector, the Maxwell's equations were solved for representing the electromagnetic fields. The movement of the manganese droplets was described by the volume of fluid (VOF) approach. A reasonable agreement between the simulations and experimental data is obtained. With the continuous melting of the EMM scrap, manganese droplets are formed at the inlet of the simulation domain and fall down and through to the slag phase afterwards. The highly conductive manganese droplet significantly changes the distribution of the current streamline and the Lorentz force around it. The droplet tends to rotate and move along the horizontal direction. With the current ranges from 5000 A to 8000 A, the volume-average Joule hating density and the time-average melt rate increase from 1.97×10 7 W/m 3 to 5.25×10 7 W/m 3 and from 0.029 kg/s to 0.073 kg/s, respectively. The average equivalent diameter of the manganese droplet increases from about 3.02 mm to 3.77 mm. The average residence time however first decreases from 5.12 s to 4.86 s and then rise to 5.24 s. The more vigorous molten slag would increases the motion path as well as the motion time of the droplet in the slag layer. @en

Melting and reduction of fine iron ore particles in the gas environment of a HIsarna smelting cyclone is a critically important topic, but very limited information is currently available except for some experimental data from high temperature drop tube furnace (HTDF). This work discusses the equilibrium state of reacting iron ore in the HTDF environment by thermodynamic calculations to strengthen the understanding of the HIsarna process. The limit of reduction termination of the ore particles was estimated in the calculation for the thermal decomposition and topochemical gas reduction. The theoretical calculation results are compared with the experimental data from the previous studies. Furthermore, variation of slag composition and iron valence states were estimated theoretically to understand the effects of post combustion ratio value and hydrogen/carbon ratio on the equilibrium state of the ore particles in the reducing gas.

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