Abstract: The application markets for portable electronics, battery-operated electric vehicles, and large-scale energy-storage grids have been expanding rapidly for the past ten years, which has attracted massive attention to the investigation and development of batteries with high energy density, long cycle life, high safety, and low cost. A commonly used lithium-ion battery consists of intercalation-type materials, such as LiCoO2 as cathode and graphite as an anode. Owing to technical difficulties, including high cost, low stability, and the poor safety of Li, the large-scale application of the high-energy Li anode is still premature. A more common strategy than the one mentioned above for improving the energy density of Li-ion batteries is to develop a cathode material with high specific capacity and low cost, such as LiNi1–x–yCoxMnyO2 (NCM) and LiNi1–x–yCoxAlyO2 (NCA). Among the NCMs and NCAs, Co is more expensive and less abundant than Ni, Mn, and Al. Presently, high-nickel, low-cobalt NCMs, and NCAs have attracted huge attention as suitable cathodes for both academic and industrial purposes. LNiO2 can be regarded as the Ni content increasing to 100% for NMCs and NCAs, which stood as the “holy grail” of layered cathodes. This study aims to investigate the structural and electrochemical stability of LiNiO2 and B-doped LiNiO2. In this study, Ni(OH)2 was synthesized by a coprecipitation method using a continuous stirred tank reactor (CSTR). LiNiO2 and B-doped LiNiO2 were synthesized by high-temperature solid-state sintering. The crystal structure, surface morphology, and electrochemical performance were investigated by X-ray diffraction (XRD), Rietveld refinement, scanning electron microscopy (SEM), constant current charge–discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). XRD and Rietveld refinement results indicate that B-doping could slightly increase the lattice parameters and unit cell volume due to the occupancy of B in the tetrahedral site. Meanwhile, the LiO6 slab distance increases, consequently favoring the transportation of Li+ during (de)-intercalation. SEM images suggest that LiNiO2 and B-doped LiNiO2 consist of primary grains with a similar size, and the secondary particle in both samples has an average size of 10 μm. Long-term cycling data show that B-doping could improve capacity retention. The capacity retention at 40 mA·g?1 is 77.5% for the B-doped sample, whereas a value of 66.6% is obtained for LiNiO2. The dQ/dVvsV curves and EIS results suggest the suppression of impedance growth by B-doping.
Abstract: With the gradually increasing consumption of coal, oil, and natural gas and the increasing environmental pollution, recyclable secondary energy has become crucial to solving energy and environmental problems. Lithium-ion batteries have penetrated into all aspects of life. Its high energy density, high voltage platform, long life, and environment-friendly characteristics make it widely in-demand. Lithium-ion batteries are used in devices such as mobile phones, tablet computers, and electric vehicles, in which requirements of energy density, rate, and cycle performance are high. The high-capacity lithium-rich material can provide a reversible specific capacity higher than 250 mA·h·g–1 and an energy density of up to 600 W·h·kg–1, making it a positive electrode material. Being a scarce and strategic resource, the price of cobalt has considerably increased. The price fluctuation of cobalt directly affects the cost of the full battery. Drying conditions have a minor effect on most cathode materials and precursors and do not affect the size, morphology, and elemental distribution of their precursors. Thus, virtually no one has explored the effects of such drying conditions. Herein, we studied the drying conditions of cobalt-free lithium-rich cathode materials and explored the influence of drying condition on the morphology and electrochemical performance of cathode materials. Using sodium hydroxide, which is a transition metal sulfate, and ammonia as raw materials, a lithium-rich manganese-based cathode material (Li1.17Ni0.33Mn0.5O2) was prepared via coprecipitation followed by sintering at 900 ℃. The influence of the precursor drying temperature on the macro and micro morphology and electrochemical performance was studied. The results show that the precursor displays a clear macro sintering phenomenon, and particles appear after lithiation at a higher drying temperature. The precursor with the lower drying temperature did not display a macro sintering phenomenon, and no obvious particles appeared after lithiation. After 50 cycles, the remaining capacity of the high drying temperature was only 85%, which is a significant drop. The cathode material with the lower drying temperature did not decrease significantly in capacity after 50 cycles.
Abstract: All-solid-state lithium batteries are recognized as the next-generation energy storage batteries due to their high energy density and high security, to which researchers have paid more attention. All-solid-state lithium batteries are composed of solid materials, and the Li-ion solid electrolytes do not contain flammable and explosive organic solvents, which can enhance the safety of the battery. As important components, Li-ion solid electrolytes are widely studied in all-solid-state lithium batteries, which currently include Li-superionic solid electrolyte (LISICON), Na-superionic solid electrolyte (NASICON), garnet-type solid electrolyte, perovskite-type solid electrolyte, sulfide-type solid electrolyte, and polymer solid electrolyte. Li-ion solid electrolytes generally have the advantages of high Li-ion conductivity, low electronic conductivity, wide operating temperatures, wide electrochemical windows, and inhibition of lithium dendrite growth. Among the solid electrolytes, the perovskite-type solid electrolytes have a wide tolerance factor that allows most elements to dope into the ABO3 structure. Additionally, the perovskite-type Li-ion solid electrolytes are summarized into two types: (1) the three-component Li3xLa2/3?xTiO3 (LLTO, 0 < x < 1/6) and (2) the four-component (Li, Sr)(A, B)O3 (A = Zr, Hf, Ti, Sn; B = Nb, Ta). In this paper, the four-component Li2x?ySr1?xTi1?yNbyO3 (x = 3y/4, y = 0.25, 0.5, 0.6, 0.7, 0.75, 0.8) solid electrolytes were prepared by conventional solid-state reaction method. X-ray diffraction (XRD), scanning electron microscopy, alternating current impedance, and potentiostatic polarization methods were adopted to study the crystal structure, micromorphology, ion conductivity, and electronic conductivity, respectively. XRD analysis show the synthesized samples exhibit a cubic perovskite structure when y≤0.70 with almost no impurity phase formed. Li0.35Sr0.475Ti0.3Nb0.7O3 exhibits the highest ion conductivity of 3.62×10?5 S·cm?1, electronic conductivity of 2.55×10?9 S·cm?1 at 20 ℃, and activation energy of only 0.29 eV. The LiFePO4/Li half-cell was fabricated using Li0.35Sr0.475Ti0.3Nb0.7O3 as a separator, exhibiting a capacity of 93.9 mA·h·g?1 and a retention capacity of 90.72% after 100 cycles.
Abstract: ABO3-type perovskite oxides and A3B′B′′2O9-type composite perovskite oxides exhibit proton conduction from 200 ℃ to 1000 ℃. These high-temperature proton conductors have received considerable attention due to their promise as electrolytes in fuel cells, electrolytic hydrogen production, hydrogen separation, electrochemical reactors, sensors, etc. The Ba3Ca1+xNb2?xO9?δ composite perovskite-type solid electrolyte has stable chemical properties and corrosion resistance to CO2 and H2O, so it can be used in long-term electrochemical devices. Protons are incorporated into Ba3Ca1+xNb2?xO9?δ in a humid or hydrogen-containing atmosphere because of the reaction of H2O and oxygen vacancies in proton conductors. However, proton conductors also exhibit oxygen vacancy conduction in the high-temperature range. In addition, electron holes can be generated by an oxygen vacancy reaction with atmospheric oxygen, causing proton conductors to exhibit electron-hole conduction. Hence, more oxygen vacancies can be produced with more Ca2+ dopant in Ba3Ca1+xNb2?xO9?δ due to a lack of positive charge. Meanwhile, the proton and electron-hole concentrations increase with oxygen vacancies, and the conductivity of Ba3Ca1+xNb2?xO9?δ can be improved. However, the crystal structure of Ba3Ca1+xNb2?xO9?δ can be changed with Ca2+ doping, and changes in proton, oxygen vacancy, and electron-hole transport numbers, the ratio of protons, oxygen vacancies, and electron-hole conductivity to total conductivity respectively, are unknown with Ca2+ doping, with different effects of crystal structure for protons, oxygen vacancies, and electron-hole conduction. Ba3Ca1+xNb2?xO9?δ has high conductivity in a humid atmosphere, and the proton transport number with doping amount needs to be further studied. In this work, Ba3Ca1+xNb2?xO9?δ (x=0, 0.10, 0.18, and 0.30) with a composite perovskite phase was prepared using a solid-state reaction method. With the increase in Ca2+ doping amount, the conductivity of Ba3Ca1+xNb2?xO9?δ samples first increased and then decreased, and the conductivity of the sample with x=0.18 was the highest. The electron-hole transport number of Ba3Ca1+xNb2?xO9?δ under the atmosphere containing hydrogen was relatively low. Protons were mainly conductive carriers in Ba3Ca1+xNb2?xO9?δ below 750 ℃, while Ba3Ca1+xNb2?xO9?δ exhibited mainly oxygen vacancy conduction at 800 ℃. With the increase in dopant amount, the oxygen vacancy transport number of Ba3Ca1+xNb2?xO9?δ increased gradually, while the proton transport number decreased gradually.
Abstract: The electrodeposition of aluminum in ionic liquid has broad application prospects, and additives are an effective way to improve the performance of the aluminum coating. However, the relevant mechanism behind this remains to be clarified. In the present work, the effects of dichloromethane (DCM) and toluene (C7H8) on the microstructure, physicochemical properties, and aluminum electrodeposition with 1-butyl-3-methylimidazolium chloride/aluminum chloride ([BMIM]Cl/AlCl3) were studied using quantum chemistry and molecular dynamics simulation. It is found that DCM easily forms hydrogen bonds with anions and cations of ionic liquids. Since DCM is distributed between anion and cation, the distance between the anion and cation increases, and the interaction energy decreases. As a result, the diffusion ability of anions and cations is enhanced, and the aluminum complex anions tend to exist in the form of ${\rm{A}}{{\rm{l}}_2}{\rm{Cl}}_7^ - $. The viscosity of the system decreases, and the conductivity increases, so the electrochemical properties of the system are significantly improved, which are in good agreement with the experimental values. C7H8 is adsorbed on the protruding part of the electrode surface in a flat way, which plays a leveling role and results in a flat white coating. Alternatively, DCM easily interacts with the electroactive ion ${\rm{A}}{{\rm{l}}_2}{\rm{Cl}}_7^ - $, which makes it difficult to reduce this electroactive ion. At the same time, the concentration of the electroactive ion decreases as the concentration of the additive is increased, which leads to a large overpotential in the electrochemical process, resulting in a decrease in the electrode reaction rate that plays a role in grain refinement. Moreover, the interaction between DCM and cations is also strong, and they can be adsorbed on the protruding part of the electrode surface during the electrochemical process, playing a certain leveling role. Therefore, the addition of DCM can obtain specular gloss deposits. The effect of C7H8 on the interaction between anions and cations is not as good as DCM, which consequently results in inferior electrochemical properties compared to DCM. Therefore, DCM is more favorable for the electrodeposition of aluminum than the aromatic hydrocarbon C7H8.
Abstract: Manganese metal electrolysis is a typical nonlinear system far from the equilibrium state. In this case, nonlinear behaviors such as electrochemical oscillation and metal fractal occur in the electrode reaction process. The multiple valence state changes of manganese and the nonlinear coupling of multiple chemical reactions cause the electrolytic process to be unstable and unmanageable, and increase extra energy consumption. Therefore, a study regarding the physical and chemical processes of the electrode/solution interface will help in revealing the electrode reaction mechanism and elaborate the nonlinear behaviors of the interface reaction process. This should control the electrode reaction process more effectively and regulate the entire process more efficiently. This paper presents a new mode of chaotic current electrolysis by introducing a hyperchaotic circuit instead of the original direct current power supply. Galvanostatic polarization, anode polarization, the Tafel test, X-ray diffraction, and scanning electron microscopy were employed to analyze the relationship between the electrochemical oscillation behavior and anodic deposited manganese oxides on lead alloy anodes. Research results show that the potential oscillation behavior of the anode is suppressed to a certain extent. The average oscillation period was increased by 5.6 s, and the average oscillation amplitude was reduced by 38 mV compared with direct current polarization after 350 A·m?2 constant current polarization for 30 min. This would help to reduce the generation of anode slime and additional energy consumption during electrolysis. At the same time, the deposited MnO2 on the anode under hyperchaotic current had a dense and flat surface, which improved the oxygen evolution reaction activity and the corrosion resistance of the lead alloy anode. The comprehensive analysis demonstrated that the application of hyperchaotic current to manganese metal electrolysis could achieve effective regulation of anode electrochemical oscillation, providing a new insight for the further reduction in the energy consumption and pollution emission in the electrolysis process.
Abstract: Carbon residue in aluminum electrolytic cell is a kind of hazardous waste produced during the smelting and production process of the aluminum industry. Approximately 10 kg of carbon residue is produced for every ton of primary aluminum produced. China’s primary aluminum output was as high as 35.04 million tons in 2019, so its carbon residue production was about 350,000 tons. The accumulation of a large amount of carbon residue wastes electrolyte resources, as well as causes air, soil, and water pollution. Additionally, carbon residue was listed on the National Hazardous Waste List in 2016. Therefore, the treatment of carbon residue needs to be solved urgently. In this experiment, the characteristics of carbon residue were introduced, and it was used as the raw material to study the process feasibility of recovering carbon powder and cryolite by the roasting-water leaching process of carbon residue with Na2CO3 as the additive. Na2CO3 with a mass ratio of 2.5∶1 was mixed with carbon residue, placed in a crucible-resistance furnace, and then baked at 950 ℃ for 2 h. Test results show that the alumina, cryolite, and sub-cryolite in the carbon residue are consumed by Na2CO3, and the mixture after roasting consists of C, Na2CO3, NaF, and NaAlO2. After roasting, the mixture is leached for 1 hour with a pH of 14 and at a leaching temperature of 25 ℃. The purity of the recovered carbon powder after solid-liquid separation can reach 89%. The carbonation method is used to recover F? in the leachate to obtain powdered cryolite with qualified main components. Properly increasing the roasting temperature and extending the holding time can improve the separation efficiency of carbon and electrolyte. Research on economical and effective carbon residue treatment methods can not only solve the environmental pollution caused by carbon residue, but it can also have a profound impact on the sustainable development of society.
Abstract: The direct conversion of methane into methanol and other high value-added chemicals with low-energy consumption has always been an important goal and a major challenge for the sustainable chemical industry. In this paper, a three-dimensional (3D) ZnO/CdS/NiFe layered double hydroxide (LDH) shell/core/hierarchical nanowire arrays (NWAs) structure material was fabricated and utilized for photoelectrocatalytic oxidation of methane at room temperature under simulated sunlight. Results show that the ZnO/CdS/NiFe-LDH photoanode exhibites excellent photoelectrochemical performance and catalytic activity. The photocurrent density under the methane atmosphere reached 6.57 mA·cm?2 at 0.9 V (vs RHE). Yields of methane oxidation products, which mainly are methanol (CH3OH) and formic acid (HCOOH), catalyzed by the synthesized ZnO/CdS/NiFe-LDH composite are 5.0 and 6.3 times those of pure ZnO, respectively. The total Faraday efficiency of the two main products reach 54.87%. The deposition of CdS nanoparticles (NPs) significantly facilitates the absorption of visible light and promotes the separation of photo-generated carriers. The introduction of NiFe-LDH nanosheets with a three-dimensional porous structure improves the surface reaction kinetics of methane oxidation, acting as an excellent co-catalyst. It also effectively inhibites the production of O2?-, preventing O2?- from further oxidizing methanol and formic acid into CO2, which improves the selectivity of methanol and formic acid. Finally, this paper proposed a mechanism of the photoelectrocatalytic oxidation of methane to methanol and formic acid over 3D ZnO/CdS/NiFe-LDH composite material, which provides a new idea for the conversion of methane into high-value chemicals with low-energy consumption.
Abstract: Ionomer is an important part of the catalytic layer in proton exchange membrane fuel cell. Its main role is to conduct protons. We investigated the effect of ionomer on the durability of Pt/C catalyst using rotating disk electrode (RDE) under two modes: the real operating conditions (mode 1) and startup/shutdown conditions (mode 2). The structural changes of Pt/C catalyst after the durability test were analyzed by identical location transmission electron microscopy (IL-TEM). Results show that the addition of ionomer improved the durability of Pt/C catalysts. After the durability test of mode 1, the addition of ionomer reduced the change of half-wave potential (?E1/2) of oxygen reduction reaction from 23 to 11 mV, which is attributed to the growth of Pt particles rather than carbon corrosion. Ionomer delayed the decrease of electrochemical specific surface area (ECSA) of Pt/C catalyst, which is beneficial to the maintenance of Pt activity. After the durability test of mode 2, in addition to the growth of platinum particles, carbon corrosion occurred in the catalyst layer, and the growth of platinum particles was mainly due to carbon corrosion. The addition of ionomer reduced the ?E1/2 of oxygen reduction reaction from 25 to 5 mV. Furthermore, the growth of platinum particles and carbon corrosion can be clearly seen by IL-TEM, and the corrosion of the carbon support resulted in the loss and agglomeration of platinum. The average particle size of platinum in Nafion-containing catalyst increased from 2.7 to 3.76 nm, while that of Nafion-free catalyst increased from 2.44 nm to 4.19 nm.
Abstract: Owing to the large cross-section and wide solidification-temperature zone, bloom castings of medium- and high-carbon steels are prone to severe central shrinkage and macrosegregation defects. Flow control technologies such as nozzle injection mode and electromagnetic stirring, together with the casting speed, play a key role in the as-cast macrostructure and macrosegregation distribution in bloom castings achieving soundness and compositional homogeneity of the final as-rolled products. Based on the production process of a medium-carbon-steel bloom casting and its heavy section bars, various flow control modes have been adopted in the casting production to study their effects on the semiproduct solidification structure and the carbon distribution across the bloom section and the following rolled bars. The conventional nozzle with a single straight port shows that the equiaxed crystal ratio in the casting process can only increase from 6.06% to 11.71% with the M-EMS intensity changes from 0 to 800 A, in which a shrinkage cavity and macrosegregation exist along the centerline on the strand casting. While the novel five-port nozzle has been adopted, the equiaxed crystal ratio can reach 23.1% even with the M-EMS power off, and the center cavity index drops down to grade 1.0 or below, meeting the requirement of microvoid flaw detection for the bar products. Additionally, the carbon segregation across the bloom cross-section is observed to resemble an M-shaped curve with a maximum carbon segregation index in the columnar to equiaxed transition zone instead of the popular center region. For the heavy section bars rolled from bloom casting, a similar carbon distribution pattern as the cast is observed after hot working but with an even higher centerline segregation index. Therefore, considering the special quality requirements for the subsequent hot processing, the macrostructure and pattern and intensity of macro-segregation should be regulated from the beginning of casting with a reasonable flow control mode as mentioned in the study.
Abstract: 20CrMo alloy steel is commonly used to produce high-pressure pipes, gears, automobile parts, etc., and there are stringent requirements for its yield strength, tensile strength, and impact energy. In the actual production process, the existence of nonmetallic inclusions has an important impact on the properties of 20CrMo steel; therefore, studying the evolution of inclusions in the process is necessary. To further examine the evolutionary mechanism of inclusions in the overall production process, the evolution of nonmetallic inclusions in a 20CrMo alloy steel produced via the route of “Basic oxygen furnace (BOF)→Ladle furnace refining (LF)→ Vacuum cycle degassing process (RH)→ calcium treatment→ Continuous casting (CC)→ hot rolling” was studied. This process ensured a smooth production process and improved the mechanical properties of the products. Al2O3 was the main inclusions in the steel during LF and RH refining, which was up to 70%. After calcium treatment, CaS in inclusions increased to 59% and Al2O3 decreased to 21% due to the excessive mixing of calcium into the molten steel. Due to reoxidation during continuous casting, inclusions were transformed to CaO–Al2O3, with 50% Al2O3, 39% CaO, and 10% CaS. And the average diameter of inclusions also increased, which was detrimental to the mechanical properties of the steel. After cooling and solidification of the steel, CaO decreased to 5% and CaS increased to 57%. Inclusions in the steel were transformed into Al2O3–CaO–CaS, and a small amount of large-sized CaO–Al2O3 was also observed. During the rolling process of steel, the CaO content in inclusions further decreased while the CaS content and the diameter of inclusions increased. Moreover, two types of inclusions were observed in the hot-rolled plate, one being Al2O3–CaS compound inclusions, whose size was relatively small, and the other being CaO–Al2O3–CaS compound inclusions. Reasons for the formation of compound inclusions consisting of CaO–Al2O3 and CaS were also discussed.
Abstract: Cu-based alloys can be used as a selective laser melting (SLM) material for advanced engineering applications, such as aerospace, 5G mobile networks, and high-speed transportation. The mechanical properties and solidification microstructures of Cu alloys prepared using the casting technique differ from those prepared using the SLM technique, and SLM-built alloys can involve more complex microstructures and phase transformations developed in micromolten pools produced by high-power laser beams. However, nonequilibrium solidification microstructures and mechanical properties of SLM-built Cu–Sn alloys have seldom been studied in the literature. In this work, the Cu–5%Sn alloy was investigated using the SLM technique, along with cast Cu–Sn alloys for comparison. The high quality Cu-based alloy samples were fabricated using the SLM technique, with optimized processing parameters of 160 W laser power, 300 mm·s?1 scanning speed, and 0.07 mm line spacing. The samples exhibit a relative density of 99.2%, and virtually no pores and spheroidizing phenomena or warping defects were observed. The microstructural analysis of SLM-built Cu–5% Sn alloy reveals a nonequilibrium solidification feature under high cooling rates and rapid alternative thermal conditions during the SLM fabrication process, in which the α-Cu(Sn) solid solution is the major phase along with γ and δ phases. Columnar grains and reticular microstructures dominate the solidified SLM-built alloy, while segregated Sn appears in the boundaries of all levels within the alloys. The Sn-rich nanoparticles with super-lattice structures precipitates along the grain boundaries and inside the grains. With the combined effects of grain fining, super-lattice-structured nanoparticles precipitation, solid solution, and thermal residual stress, the SLM-built Cu–5%Sn alloy shows significantly enhanced mechanical properties, such as HV 133.83 Vickers hardness, 326 MPa yield strength, 387 MPa tensile strength, and 22.7% fracture extension. Such scientific information is very useful for improving the alloy composition design and optimizing the SLM processing parameters.
Abstract: Owing to insufficient converter heat, IF steel is produced via the BOF—LF—RH—CC process in the Xichang Steel & Vanadium Co.LTD of Pangang Group, Xichang, China. To explore the refining effect of IF steel produced via the RH forced and natural decarburization process, this work employed standard analysis methods such as production data statistics, total oxygen and nitrogen analysis, automatic scanning electron microscopy, scanning electron microscopy, and energy spectroscopy. The effects of different decarburization processes on the ladle slag oxidability and cleanliness of steel were investigated in detail. Compared with the natural decarburization process heats, results show that the forced decarburization process heats exhibit (1) lower average [O] content in molten steel after BOF and before RH, (2) a similar level of the [O] content in molten steel after decarburization with that of the natural decarburization process, and (3) 1.3% lower average T.Fe mass fraction in the ladle slag after RH treatment. To ensure the RH decarburization effect, the final carbon content increased and molten steel oxygen content reduced in the converter to the maximum extent. The forced oxygen blowing decarburization process was then used to compensate for the molten steel oxygen content during RH refining by increasing oxygen blowing properly, which can significantly decrease the ladle slag oxidability of IF steel. Both the natural decarburization and forced decarburization processes are ideal for controlling the T.O content of a hot–rolled sheet. Compared with the natural decarburization process, the forced decarburization process can effectively reduce the [N] content of IF steel, which is related to a more violent carbon–oxygen reaction in a vacuum chamber, resulting in a high volume of CO bubbles and a large gas–liquid reaction area. The decarburization process has no obvious influence on the type, size, and number of inclusions in the hot–rolled sheet of IF steel that mainly consist of Al2O3, Al2O3–TiOx, and other inclusions. The average sizes of the above three 4.5, 4.4, and 6.5 μm, respectively, according to the equivalent circle diameter of inclusions. In addition, more than 75% of inclusions are within 8 μm. During the RH refining process, reducing the [O] content in molten steel after RH decarburization to the maximum extent is beneficial to improve the cleanliness of molten steel.
Abstract: Increased Al2O3 content in blast furnace slag in China presents adverse effects in blast furnace smelting. In response to this problem and to improve fluidity of blast furnace slag, MgO pellets are added. Blast furnace can be operated smoothly, but on the other hand, higher MgO content is unfavorable to the pelletizing property and pellet roasting performance of raw materials. Five kinds of pellets containing high-magnesium magnetite, forsterite, dolomite, magnesite, and magnesia powder, respectively, were made to investigate the effect of MgO content and its occurrence on induration behavior and metallurgical performance of pellets. Results show that various magnesium-containing fluxes have different influences on the quality of green balls. Both magnesia powder and high-magnesium magnetite can improve the drop numbers of green balls, which is due to their chemical properties and specific surface areas. With fixed firing temperature and time, increasing MgO content leads to lowered compressive strength of the preheated and fired pellets, with the lowest impact from dolomite observed. With the oxidation degree elevation of preheated pellets, compressive strength of roasted pellets improves, which indicates that we can increase the preheating time in actual production to improve the roasting performance of magnesium magnetite pellets. Under the same source of MgO, increasing MgO content will lead to an increase in porosity of pellets, which presents a negative effect on the strength of pellets. For five kinds of fired pellets, increased MgO content improves the reduction swelling index, low temperature degradation indices, and reduction degree. The reduction swelling index and low temperature degradation indices of pellets with high-magnesium magnetite are observed to be better than those of other magnesium-containing pellets.
Abstract: Iron ore sintering is a process in which fuel, flux, and iron ore powders are mixed and sintered into a block under incomplete melting conditions. The flue gas from iron ore sintering process is one of the largest sources of nitrogen oxide (NOx) and dioxin emissions in industries. The V2O5–WO3/TiO2 (VWTi) catalyst can simultaneously remove NOx and dioxins, but the presence of the complex flue gas results in the deactivation of the catalysts. In response to this challenge, this study carried out experiments for ZnCl2, ZnO, and ZnSO4 poisoning over the VWTi catalyst via wet impregnation method. The effects of the different Zn species on the simultaneous removal of NOx and dioxins (chlorobenzene was used as the simulant for dioxins) by the VWTi catalyst were studied under simulated conditions of the iron ore sintering flue gas. The surface physicochemical properties of the fresh and poisoned catalysts were characterized to reveal the deactivation mechanism, and the regeneration experiments of the poisoned catalysts were investigated. Results showed that deactivation through catalytic denitrification and chlorobenzene (CB) catalytic degradation processes could be observed in different Zn-containing catalysts. The poisoning effect was more obvious with the increase of Zn content, and the effects of deactivation were as follows: ZnCl2>ZnO>ZnSO4. Results from physical and chemical analyses indicated that Zn species had a significant influence on the chemical environment of the active substances on the surface of the catalysts. Zn species caused a slight agglomeration of particles on the surface of the catalysts, a decrease in the number of surface acid sites, a reduction in the reducibility of surface V species, and a decrease in the chemisorbed oxygen ratio and the molar ratio of n(V5+)/n(V4+). The regeneration experiments confirmed that employing the dilute sulfuric acid solution washing method was effective for recovering the catalytic activity, whereas the water washing method failed to restore the catalytic activity. The mechanism of Zn salt poisoning is as follows: Zn2+ reacts with the acid sites V=O and V?OH on the surface of the catalyst to form V?O?Zn, which adversely affects the adsorption of NH3 and CB, resulting in the catalyst poisoning and deactivation. The ${\rm{SO}}_4^{2-} $ in ZnSO4 provides a new acidic site for the adsorption and transformation of NH3 and CB alleviating the poisoning effect. The Cl? in ZnCl2 produces HCl as a by-product after the reaction, resulting in more active sites poisoning on the surface of the catalyst and deepening the poisoning effect.
Monthly, started in 1955 Supervising institution：Ministry of Education Sponsoring Institution：University of Science and Technology Beijing Editorial office：Editorial Department of Chinese Journal of Engineering Publisher：Science Press Chairperson：Ren-shu Yang Editor-in-Chief：Ai-xiang Wu ISSN 2095-9389CN 2095-9389
