NdFeB magnets are the most widely used rare earth permanent magnet materials at present. The increasing number of the waste NdFeB magnets and their high rare earth content motivate a search for technologies to allow their cost-effective and environmental-friendly recycling. In this study, removal of oxide inclusions from waste NdFeB magnets by supergravity technology was investigated and the separating conditions were optimized for maximum oxide removal. Under the optimized conditions of G = 800 and t = 15 min, the total oxygen of the sample decreased from 410 ppm to 28 ppm, with the oxide removal efficiency of 96.8%. The theoretical time to remove inclusions with different sizes was calculated by Stokes’ law, and the experimental phenomena were in good agreement with the calculated ones. The supergravity technology has been demonstrated highly efficient in removing oxide from waste NdFeB magnets for the recycling. A design for an industrial reactor was presented to pave the way for future commercial processing and utilization of waste NdFeB magnets.

With the continued exploitation and utilization of high-grade rare earth ores, it is increasingly important to extract rare earths from separated slag containing low-grade rare earth. The X-ray powder diffraction, scanning electron microscopy, electron probe micro-analyzer and confocal laser scanning microscopy were used to explore the influence of TiO₂ and cooling rate on the crystallization of CaO-SiO₂-TiO₂-10 wt% P₂O₅-8 wt% Nb₂O₅-5 wt% CeO₂-5 wt% CaF₂ slag system. In this study, the britholite was precipitated selectively as the Ce-enriched phase. When TiO₂ was added at less than 12 wt%, the britholite was promoted to crystallize meanwhile the Ca₂Nb₂O₇ was suppressed. However, CaTiSiO₅ inhibited the growth of britholite when the TiO₂ content exceeded 15 wt%. The non-isothermal crystallization kinetics had also been investigated for the TiO₂ content and cooling rate varied from 0–18 wt% and 10–40°C/min, respectively. The continuous cooling transformation diagram and the relative crystallinity of the primary crystals were also constructed. Based on the observation and measurement of crystallization process, the modified Avrami model was applied to determine the crystallization mode of britholite with 9 wt% TiO₂ addition. It was constant nucleation rate and one-dimensional growth with diffusion controlled. Considering the nucleation and growth of crystals, 20–30°C/min was preferred to be the reasonable parameter during cooling stage.

The defluidization behavior of cohesive metal particles with different sizes in high-temperature gas fluidization was studied experimentally and theoretically. Taking iron particles as an example, first, we theoretically assessed the variation in sintering neck size with the characteristic parameters (particle size d_p, temperature T, gas velocity u_g, and sintering time τ) in high temperature gas fluidization and found that larger particles can form a greater sintering neck and induce a shorter sintering time (collision contact time). In particular, the calculated results with different empirical correlations are quite different. Then, according to the microstructure observations, we assume that a stable sintering neck is formed between the metal particles when defluidization occurs. On the basis of this, a quantitative relationship between particle size and operating parameters (temperature and gas velocity) is established, where appropriate empirical correlations are selected by fitting the experimental results. Furthermore, it is demonstrated that the new model can successfully predict the defluidization behavior of other cohesive metal particles (Co, Ni, and Cu) with different sizes in high temperature gas fluidization.

Semi-coke is a product of low-temperature pyrolysis by low-rank coal, with a composition similar to that of anthracite for pulverized coal injection (PCI). Herein, we investigated the differences in grindability and combustibility between semi-coke and anthracite, analyzed the compositional and microstructural characteristics related to the performance of semi-coke, and assessed the impacts on grinding efficiency and blast furnace operation after replacing anthracite for injection with two types of semi-coke. Semi-coke is rich in high-hardness quartz that is tightly bound to the carbon matrix, making the semi-coke particles very hard, with a high Hardgrove grindability index (HGI) and high abrasion index. The addition of semi-coke reduced the grinding efficiency of the mill and afforded large-sized milled particles. The developed pore structure of semi-coke can enhance kinetic diffusion, and semi-coke is less ordered than coal, thereby providing more reactive sites for combustion reactions. These two reasons cause the ignition temperature of semi-coke to be significantly lower than that of coal. The addition of semi-coke increased the PCI ratio, decreased the fuel ratio, improved the permeability of the blast furnace, decreased the sulfur content in pig iron and carbon content in blast furnace dust. The difference in grinding productivity between semi-coke and coal widens as grinding time increases, suggesting that the HGI method may overestimate the actual grindability of semi-coke. The feasibility of reducing the grinding energy by optimizing particle sizes of semi-coke and improving the grindability of semi-coke by using selected pyrolytic coal and adjusting the pyrolysis temperature was proposed.

Titanium ore furnace protection is an important means to ensure the safe production of blast furnace. Based on the industrial test of titanium ore furnace protection in the hearth of a cast vertical blast furnace in China, this paper analyzes the influence of titanium ore furnace protection on the fuel consumption of blast furnace, explores the reasonable titanium load suitable for smelting, and analyzes the unsteady process of titanium deposition. The relationship between Ti content and C, Si, and S content in molten iron is clarified, and the reasonable titanium content range for controlling Ti (C, N) precipitation is established. It is found that the fuel ratio is reduced by about 5 kg/t during the industrial test, and the titanium load should be maintained at 6–8 kg/t. Since the titanium ore is put into the furnace, the titanium deposition process has a delay of 6–7 days, and the unsteady period of the whole furnace protection process is 10 days. The Ti content in molten iron will increase with the increase of C content and Si content, and decrease with the increase of S content. Under the condition of Ti (C, N) precipitation, when the Ti content in molten iron is greater than 0.074%, the Si content is greater than 0.42%, and the S content is less than 0.061%, a better furnace protection effect can be obtained.

The drying process of green pellet is the intermediate link of direct reduction in rotary hearth furnace, which can reduce energy consumption, prevent green pellet from bursting during drying and reduce pulverization rate. In this study, the effects on the drying rate of raw metallurgical dusts pellet at a hot air flow rate of 1 m/s and drying temperatures of 211°C, 254°C, 282°C and 314°C was investigated to clarify the drying characteristics of the green pellet. The results show that there were accelerated drying stage, constant drying stage and deceleration drying stage in the drying process of green pellet. The drying temperature had a significant effect on the dehydration rate during the drying process. The effective diffusion coefficient increased with the increase of drying temperature, and the activation energy of the whole drying process was 10.4 kJ/mol. Then the drying fitting models of Page (III), Lewis, Wang and Singh, and Weibull are used to describe the drying kinetics of green pellet. The fitting results show that the green pellet drying process is consistent with the Page (III) and Weibull models. Finally, the Weibull model was selected to compare the experimental values with the fitted values and the results indicate that the fitting model can well describe the actual drying process.

With the aim of developing an iron ore sinter with a better reducibility, the authors researched the co-firing of green pellets and a sinter mix in a packed bed. First, a physical simulation with a simulator for the charging equipment of a sintering machine demonstrated that the green pellets were distributed more strongly to the lowest part of the packed bed because their size is larger than that of the sinter mix. Pot tests based on the simulated pellet distribution proved that it is possible to produce a highly reducible sinter at higher productivity, particularly when a concentrate with a large Blaine surface area is used to produce rigid green pellets. Factors that could reduce product yield were also pointed out, i.e., diffusion of the melt from the sinter mix into the pellets and a larger local heat supply to the sinter mix than is suitable. These factors are thought to hinder the formation of thick and strong bonds in the co-fired sinter bed. A 450 kV class X-ray CT was used to visualize the internal structure of the sinter plugs, and revealed the plug made with the green pellets has higher porosity, which is important for better permeability, but also results in less bond formation, which reduces the yield in a co-fired plug. These findings revealed insights for fully enjoying the advantages of co-firing green pellets and sinter mix, as well as issues to be overcome in order to realize a successful co-firing process.

In this study, a new method of microwave-hydrogen synergistic reduction of vanadium titano-magnetite (VTM) was developed to carry out experimental research. Using the theory of the direct low-temperature reduction process, VTM from the area surrounding Chengde, China was used as raw material and H₂ was used as a reducing agent. The experimental results and the theoretical analysis proved that VTM can be feasibly reduced via microwave-hydrogen synergistic reduction at low temperature. In addition, H₂ reduction of iron titanium oxides was more difficult than that of iron oxides and required a higher reaction temperature. Under microwave heating conditions, increasing the temperature, reduction time, and H₂ proportion improved the metallization rate. When reducing for 40 min at 1100°C with 60% H₂, the metallization rate reached 92.2%. The reduction product had a porous, sponge-like structure, and it was primarily composed of Fe and Fe_9.64Ti_0.36 phases. This implies that the Fe_9.64Ti_0.36 phase may be the enriched phase of Mg, Ca, and Si. During the synergistic reduction process, the metallic iron that precipitated inside the particles migrated to the outer edge of the particles, and the titanium iron oxides that were difficult to reduce inside were coated with metallic iron.

The impeller blade structures are developed to maximize reagent utilization in the desulfurization process of molten iron by Kambara Reactor stirring method. Compared the flow structure and mixing effect of four impeller systems by numerical simulation. The distribution and motion behavior of desulfurizer particles are further discussed. In addition, the effect of rotation speed on mixing ability is illuminated. The results show that changing the central structure of the traditional impeller can effectively improve the flow field, changing the width of the asymmetrical impeller blades can enhance the involvement ability, and setting the downward angle on the basis of changing the width of asymmetrical impeller blades can obviously improve the mixing effect in the region under the impeller. The recommended rotation speed is 80 rpm, under actual operation conditions, since it may cause the overflow of molten iron when the rotation speed exceeds 80 rpm.

In this study, a thermodynamics model was developed for the basic oxygen steelmaking (BOS) process (combined top & bottom blown), which was used to calculate the transient carbon, silicon, manganese, and phosphorus contents, as well as their slag composition, for different input hot metal-carbon contents. The model considered three different zones, and the volume of different phases was calculated using the Equip module of the FactSage macro processing code. The results of the model showed that manganese, phosphorus, and iron oxidation rates were high for input hot metal with low carbon content (3%). Additionally, the model predicted transient metal composition, and metal bath temperature, which were similar to plant observations for input carbon percentages ranging from 4 to 4.5.

In response to the frequent problem of inconsistent quality of billet castings and their rolled products from each strand by a five-strand tundish, the flow field in tundish is optimized by presenting new flow control devices and conducting isothermal physical modelling along with numerical simulation. The results show that the dead volume fraction of the optimized case A6 is reduced from 27.74% to 19%, the stagnation time is prolonged from 12 s to 35 s, and the flow dynamic consistency for each strand is improved as well. In the subsequent industry production tests, the temperature difference of molten steel at the outlet of each strand is reduced to 1–5°C. The maximum difference of the as-cast equiaxed crystal rate among five strands is reduced from 5.67% to 2.7%, and the consistency of carbon segregation index is also improved with a basically identical appearance through the billet cross section. The maximum differences in oxygen and nitrogen contents for the rolled products of all strands are 2.7 ppm and 5.7 ppm respectively, which are lower than 5.0 ppm and 13.8 ppm before tundish optimization. The yield strength of rolled products is stabilized with much less divergence as compared to the products with the original tundish. Thus, it is believed that the reasonable flow field optimization to a multi-strand tundish not only will have a well-known positive impact on its tranditional metallurgical effect, but also may bring out an approaching identical steel quality from the same caster as we expected.

Heat transfer model is the basis for control and optimization of continuous casting of steel. In this paper, a three dimensional (3D) real-time heat transfer model has been presented for continuous casting blooms, especially for the initial and final casting stages. To be real-time, an algorithm based on inheritable variable non-uniform grid and variable time steps has been developed, accelerating the 3D heat transfer model by 110–200 times. Meanwhile, the discrete parameters including grid and time step have been optimized, and the influence of central processing units (CPUs) and programming languages has also been studied. Then the relative calculation time, which is the ratio of calculation time to physical time, has been reduced to 0.62 while numerical errors are within 0.74%. Further, to reduce the uncertainty of the model, the machine-dependent parameters appearing in the thermo-physical properties and the boundary conditions have been calibrated with measurements of surface temperature by thermometer and shell-thickness by nail-shooting, then the corresponding optimization problem of minimizing the errors between calculation and measurements has been solved by particle swarm optimization algorithm. After calibration, the model’s uncertainty has been obviously reduced. Finally, the calibrated model has been verified by online surface temperature measurement and it shows good agreement as the errors between calculation and measurements are less than ±10°C.

Internal cracks are one of the main defects limiting the quality of continuous casting blooms, and the effect of final electromagnetic stirring (F-EMS) on the internal crack is important. To study the effect of F-EMS on thermal stress, strain and cracking inside continuous casting billet, the liquidus and solidus temperature and tensile strength of the P91 continuous casting bloom were valued by thermal analysis and high-temperature tensile tests. A three-dimensional coupling model of electromagnetic field, flow and thermodynamics was established. The thermal stress, strain and cracking index of the zero ductility temperature to the liquid impenetrable temperature of continuous casting bloom were analyzed, and the results of industrial production were discussed. It is found that the temperature and thermal stress in the center (L₁), 1/3R (L₂), 2/3R (L₃) and R (L₄) are opposite and symmetrical. At Z=7 m (L₅), Z=8 m (L₆), Z=9 m (L₇) and Z=10 m (L₈), the temperature gradient is distributed in a centralized segmental manner. The farther the position is from the meniscus, the greater the thermal stress is and the less the influence of the temperature gradient is. When the final electromagnetic stirring (F-EMS) is added at the L₇ position, the thermal stress and strain have been decreased. An internal cracking index formula is proposed, the cracking index is smaller with the F-EMS than without F-EMS. In the action range of F-EMS, the cracking index of continuous casting bloom decreases, indicating that F-EMS reduces the cracking probability.

Scrap bundles offer numerous benefits, but their composition and quality can vary significantly, making accurate recognition of each bundle a key challenge. In this study, a new dataset called RSBL was created and the performance of popular CNN models was improved through transfer learning technology. This approach effectively enhances the recognition accuracy of scrap bundle models, and the most suitable model among the improved models for scrap bundles intelligent recognition application scenarios is determined through three sets of comparison experiments. The results demonstrated that MobileNet_V3_Large model improved by the transfer learning technique performed better in the scrap bundle intelligent recognition scenario, with one training epoch time of 62.54 s on the recognition of scrap bundles (RSBL)_10 dataset and average test accuracy of 99.5%; one training epoch time of 89.78 s on RSBL dataset and average test accuracy of 99.8%. Using the MobileNet_V3_Large model for scrap bundle recognition can improve the accuracy of the static and dynamic model in converter steelmaking.

The effect of brighteners on the deposition behavior of Zn–Ni alloys and their microstructure was investigated. Zn–Ni alloys were electrodeposited on Cu electrode at 10–5000 A·m⁻², 10⁵ C·m⁻², and 308 K. Although the degree of suppression of hydrogen evolution differed depending on the kind of brightener, the transition current density at which the deposition behavior shifted from normal to anomalous was practically the same in all the solutions containing brighteners. The current efficiency of the alloy deposition significantly decreased with the addition of brighteners, which had a suppression effect on the Zn deposition. Considering that the brighteners suppressed the Ni deposition more than the Zn deposition, the Ni content in the deposited films decreased with the addition of brighteners. When the brightener of a straight-chain polymer comprising a quaternary ammonium cation (PQ) that can suppress the diffusion of ZnO₂²⁻ and Ni ions in a solution was added, the Ni content in the deposited films increased with increasing current density in a high-current-density region. This was attributed to the fact that Zn, which was preferentially deposited over Ni earlier, reached the diffusion limitation of ZnO₂²⁻, and Ni deposition did not reach the diffusion-limited current density. When PQ and a quaternary ammonium salt with a benzene ring were added to the solution, the films obtained at the diffusion-limited current density of ZnO₂²⁻ exhibited smooth surfaces comprising fine crystals. With the addition of brighteners to increase the overpotential for deposition, the γ-phase (the intermetallic compound of Ni₂Zn₁₁) of the deposited films easily formed.

Plasma nitriding was performed to suppress hydrogen uptake and permeation in pure iron. The influence of the phase structure in the nitrogen compound layer on hydrogen uptake and permeation behaviors was examined. The phase structure in the nitrogen compound layer could be controlled by changing the gas composition in N₂–H₂ plasma. The nitrogen compound layers consisting of ε-Fe_2-3N and γ’-Fe₄N with a thickness of 4.9 µm, and of γ’-Fe₄N with a thickness of 2.0 µm were formed by plasma nitriding in this study. During the hydrogen permeation tests, plasma nitriding resulted in small permeation current values compared to the as-polished specimen when it reached the steady state. Especially, hydrogen uptake and permeation for the specimen with the nitrogen compound layer consisting of ε-Fe_2-3N and γ’-Fe₄N were extremely suppressed. Hydrogen uptake and permeation behaviors for the plasma-nitrided pure iron specimens are discussed from the perspective of the catalytic activity for hydrogen evolution and hydrogen diffusivity in the nitrogen compound layers.

We recently found the formation of periodically arrayed rows of very fine Fe₂Hf Laves phase by interphase precipitation on a eutectoid type reaction path: δ-Fe → γ-Fe+Laves in 9Cr ferritic alloys with a small addition of Hf. In the present work, the precipitate morphology and precipitation mode of Laves phase were investigated on the eutectoid path in Hf or Ta doped high Cr ferritic alloys. The precipitation mode was found to change from fibrous precipitation to interphase precipitation with raising the δ→ γ transformation kinetics. The transition would be related to the time availability for solute diffusion to grow the fibrous precipitates through the advancing interface boundary diffusion. The nucleation of interphase precipitation of Fe₂Hf phase was measured to be ~2 orders of magnitude faster than that of Fe₂Ta. A thermodynamical consideration suggests that the faster kinetics of the Fe₂Hf phase mainly derived from the higher chemical driving force for the nucleation.

The crack propagation behavior of nitrocarburized JIS SCM420 steel was investigated in a rotational bending fatigue test, focusing on crack stagnation behavior. The crack had clearly stagnated at a length of approximately 200 µm at the fatigue limit of 400 MPa, indicating that crack stagnation could control fatigue strength. The crack stagnation cannot be explained only by the change of the stress intensity factor, since the calculated value in this process increases with the depth from the notch. A large amount of plastic strain was observed around the tip of the crack by EBSD analysis. Because the stagnated position corresponds to the critical depth between the hardened and unhardened regions formed by nitrocarburizing, it can be easily deformed. Therefore, it is inferred that the crack stagnation in nitrocarburized JIS SCM420 steel can be explained by the plastic-induced closure mechanism.