Reduction Of Chromium Oxide Research Articles

Decarburization and Gas Formation During Sintering of Alloyed PM Steel Components

AbstractThis study investigates the delubrication, reduction, and decarburization processes of powder metallurgical steel alloys (CrM, CrL, AHC, Mo85, SintD 35) and an unalloyed steel during sintering in a pure hydrogen atmosphere. Utilizing in-situ FTIR gas phase analysis, components with ethylenebisstearamide (EBS) as a lubricant are analyzed. EBS decomposition in steel components yields CO, CO2, H2O, and CH4, with dominant CH groups observed in the 230 °C to 480 °C range. In the temperature range between 750 °C and 850 °C, where CO formation is expected due to the reduction of surface iron oxides, CH4 is present instead, indicating that an “internal getter effect” also occurs in pre-alloyed powders. In addition, with high carbon activity, the reduction of internal iron oxides and the reduction of chromium oxides also trigger an internal getter effect. Depending on the carbon potential, these processes cause a considerable reduction in the carbon content of the powder metallurgical components. The study therefore shows that the decarburization of powder metallurgical components during the heat treatment phases prior to sintering in a 100 pct hydrogen atmosphere is less due to the mechanism of delubrication, but rather to mechanisms of carbothermal reduction.

Thermodynamic features of the synthesis of high-density ceramic-metal materials “chromium oxide–chromium” from mixtures “chromium oxide–carbon” in the process of hot pressing

Obtaining ceramic-metal materials Cr2O3–Cr by reduction of chromium with carbon in the process of hot pressing makes it possible to get high-density materials for various functional applications. The thermodynamical study of the interaction of chromium oxide with carbon (soot) is of particular interest, since not only reduced chromium is formed during the synthesis, but also chromium carbides with different stoichiometry, which makes it possible to increase the density and, consequently, the strength of the composite as a whole. Using the partial reduction of chromium oxide with carbon, it is possible to synthesize ceramic-metal materials “chromium oxide–chromium” with different chromium content. The degree of occurrence of this reaction depends on the content of carbon monoxide in its products by lowering the concentration of the obtained pure chromium. This method enables obtaining a ceramic-metal material Cr2O3–22 wt% Cr of eutectic composition, which is characterized by special physical and mechanical properties.

THERMODYNAMIC LAWS OF CHROMIUM REDUCTION BY MIXTURES OF СН4 + Н2О AND СН4 + СО2

The development of a physico-chemical model of methane behavior in the processes of solid-phase reduction of chromium-containing raw materials will allow us to create the technological basis for the production of multicomponent sponge ligatures. The reduction of oxides with methane is accompanied by the deposition of soot carbon. The negative effect of carbon black, which is blocking the reaction surface, can be eliminated by adding carbon dioxide or water vapor to methane. A thermodynamic analysis of the reduction of chromium oxide with mixtures of CH4-CO2, CH4-H2O was carried out. The effect of the partial replacement of methane by carbon on the thermodynamic preferences of the process is analyzed. A physicochemical model of the behavior of the CH4 molecule in the recovery process is proposed. The thermodynamic features of reduction at various ratios of СН4 are considered: (Н2О, СО2, О2), as well as the composition of the mixture (Н2О+СО2+О2), which ensure the conditions of solid-phase reduction of oxides and the product of carbide destruction. The use of CH4 + H2O + O2 mixtures in the reduction of chromium oxide is thermodynamically less effective. The use of a mixture of СН4 + Н2О + СО2 has a very insignificant effect on the conversion of methane, and also reduces the thermodynamic preference for the reduction of Cr2O3 in comparison with СН4 + Н2О + О2 and СН4 + Н2О. However, it should be noted that in addition to CO and H2, carbon can be present in the reduction products, so the high reduction potential in this process variant (C+CO+H2) can be used to reduce oxides. The reduction of chromium oxide with mixtures of СН4 + Н2О + О2, СН4 + Н2О, and СН4 + Н2О + СО2 can be thermodynamically realized in the temperature range excluding the melting of the charge. As a result of this process, a carbide phase is formed, as well as a gas phase, which has a high reduction potential and can be used in further technological schemes. Meanwhile, it seems possible to control the carbon content in the sponge ligature by selecting the optimal composition of the source gas mixture.

A New Redox Technique for Depositing CrOx on the Surface of Pd(0) Nanoparticles: Catalysts for Selective Phenylacetylene Hydrogenation

A catalyst containing chromium oxide particles on the surface of palladium nanoparticles is synthesized for the first time using a redox technique for reduction with adsorbed hydrogen. The low-temperature reduction of chromium oxide and formation of palladium hydride is reported for the first time in such catalyst. A considerable difference between its catalytic properties in the selective hydrogenation of phenylacetylene and those of the initial palladium catalyst is shown.

On the in situ synthesis of (Fe,Cr)Al and (Fe,Cr)Al–Al2O3 nanostructured materials

In the present work, FeAl, (Fe,Cr)Al and (Fe,Cr)Al–Al2O3 intermetallic compounds were synthesized by mechanical alloying (MA), in which the intermetallic compounds were formed in one step at room temperature, and the nanocrystalline structure led to significant improvements of strength and ductility. Specifically, MA of a Fe50Al50 powder mixture led to the formation of the FeAl compound in 30 h with a grain size of about 80 nm. Increasing milling time to 100 h decreased the grain size to about 50 nm. In the next step, a Fe40Cr10Al50 power mixture was converted into a solid solution and then an intermetallic compound with a grain size of about 30 nm after 100 h of MA. Finally, the synthesis of nanocomposites with (Fe,Cr)Al intermetallic compound containing Al2O3 as the reinforcing phase was investigated. For this purpose, two different methods, the reduction of chromium oxide (Cr2O3) and hematite (Fe2O3), were examined. In the first method, a mixture of Fe–Al–Cr2O3 powder aluminum was milled and alumina phase was formed, but after 100 h of MA, the Cr2O3 phase was still observed. In the second method, a Cr–Al–Fe2O3 mixture was milled, and the heat of reaction immediately resulted in the formation of (Fe,Cr)Al nanoparticles with a grain size of about 29 nm, along with Al2O3 crystalline particles. The results of nanoindentation tests on these particles revealed that their hardness increases by chromium addition as well as Al2O3 reinforcing nanoparticles.

Investigations on Reaction Pathway and Microstructure Transformations during Electrochemical Reduction of Cr2O3 in Molten CaCl2

The reaction pathway and microstructure transformations in the electrochemical reduction of chromium oxide (Cr2O3) to chromium (Cr) metal in CaCl2 molten salt were investigated. Some partially electrolyzed products and Cr metal were prepared by terminating the reduction process after different time periods, which were further characterized by means of XRD and SEM et al. Calcium chromite (CaCr2O4) was chemically in-situ synthesized in CaCl2 molten salt using Cr2O3 and CaO as raw materials, and its cyclic voltammogram test and electrolysis process were examined. The reaction pathway in the electrochemical reduction of Cr2O3 was hence demonstrated to proceed through a number of well-defined reaction steps, one of which involved the chemical formation and decomposition of CaCr2O4. Meanwhile, the microstructures changed dramatically with the process progresses. An observation-based assumption was proposed to account for the different morphologies of cubic and nodular Cr particles formed at the early and later stages, respectively.

Reduction of chromium oxides in stainless steel dust

The recovery of metal oxides from stainless steel dust using C (graphite), SiFe, and Al as reductants was investigated under various conditions. The apparent distribution ratio of Cr (L′Crm/s) in the recovered metal and residual slag phases was defined as the major performance metric. The results show that the recovery ratio of metals increases as the ratio of CaO:SiO2 by mass in the residual slag increases to 1.17. The residual content of metals in the slag decreases as the Al2O3 content of the slag is increased from approximately 8wt% to 10wt%. The recovery ratio of Cr increases with increasing L′Crm/s, and a linear relationship between L′Crm/s and the activity coefficient ratio of CrO in the slag and the recovered metal phase is observed. The combination of C and SiFe or Al as the reducing agents reveals that Si is the more effective coreductant.

Comparative Study of Mechanical Activation Improved by High Energy Ball Mills in Chromium Oxide Reduction

The processes of high-energy milling and gained importance among the unconventional methods. In this work, we seek to compare the power supply two types of high energy mills (vibratory mill (SPEX) and planetary mill) with the variation of the milling power. The millings were carried out with a mixture of chromium oxide and aluminum metalic. The reduction of chromium oxide does not occur instantaneously, but gradually as the progress of milling with mechanical activation of powders, this mechanical activation occurs leading to the solid state reaction occurs. The results were obtained for thermal analysis of the samples. The energy released varies, exhibiting a maximum mechanical activation for the range of powers milling studied. The correlation between the energy mills can be made by identifying the milling power is reached at which the maximum in each mechanical activation mill and quantifying this activation.

THERMODYNAMIC ANALYSIS OF REACTIONS IN THE PROCESS OF MICRO-ARC STEEL CHROMINGTHERMODYNAMIC ANATHERMODYNAMIC ANALYSIS OF REACTIONS IN THE PROCESS OF MICRO-ARC STEEL CHROMINGLYSIS OF REACTIONS IN THE PROCESS OF MICRO-ARC STEEL CHROMING

It is shown that the existing chromizing processes are characterized by a number of shortcomings. It is proposed to use the microarc thermochemical treatment technology to form a carbide coating on the material surface. Thermodynamic calculations on the reducing reactions of chromium oxide which is the diffusion source are made. It is revealed that the most probable reaction behavior is the chromium oxide reduction by methane and carbon with the formation of atomic chrome, and a consistent formation of Cr3C2, Cr7C3, and Cr23C6 carbides with an increase in temperature. The obtained results are checked through experiments. As a result of the hardening processing, a surface layer on the basis of chromium carbides with the depth of 20–25 microns is produced under which there is a carbonized area of eutectoid concentration, followed by a baseline ferrite-pearlitic structure.

Cellular automata modeling of non-catalytic gas–solid reactions

Modeling of non-catalytic gas–solid reactions (NCGSRs) carried out using the cellular automata (CA). A computerized program based on the CA rules was developed in MATLAB. A porous medium with known porosity was constructed in two-dimensional at first using the Boolean variable and the CA rules. The process of gas diffusion and reaction through the porous medium was then simulated using the CA rules. The simulation results have been compared to experimental data of reduction of chromium oxide with methane (Khoshandam et al., 2006) [10] and a mathematical model based on the grain model. The simulation results using CA show better agreement with experimental data than the grain model results.

Utilization of ashes obtained from leather shaving incineration as a source of chromium for the production of HC-FeCr alloy

This study is aimed at presenting the possibility of recycling chromium contained in ashes from leather shaving incineration (ALSI) and indicating its utility as raw material for the production of commercial high carbon ferrochrome alloy (HC-FeCr). Initially, a thermodynamic analysis was carried out for simulating the process of chromium oxide reduction to provide the basis and the most favorable conditions for the experiments. At the laboratory level, experiments were performed to demonstrate the feasibility of using chromium bearing ashes for the production of HC-FeCr alloy. The results demonstrated the possibility of producing HC-FeCr alloy from the ALSI by carbothermal reduction at a temperature of 1600°C.

Reduction of Chromium from Al2O3–CaO–SiO2–CrOX Slags by Carbon Dissolved in Liquid Iron

The goal of this work is to investigate the reduction of chromium from a quaternary slag by carbon dissolved in liquid steel. Laboratory scale experiments were conducted to study the reduction of chromium oxides in the slag by carbon dissolved in the melt. These experiments were made under different conditions of slag basicity and amount of added carbon. Thermodynamic calculations based on Double Sublattice model were applied using the commercial software Thermo-Calc, with the IRSID database. The results obtained showed good correlation with practical and calculated results, making it possible to predict equilibrium conditions of the system and to determine the activities of chromium oxides in the slag.

Current efficiency studies for graphite and SnO 2-based anodes for the electro-deoxidation of metal oxides

Studies were performed investigating the electrochemical reduction of chromium oxide (Cr 2O 3) by electro-deoxidation by utilising either a graphite anode or a tin oxide (SnO 2) based anode. Potentiostatic electrolysis was performed at 3.0 V for both a graphite and for a SnO 2-based anode, and also 2.0 V for a graphite anode. The cathode reduction purity, anode mass change, anode potential relative to a glassy carbon pseudo-reference and current efficiency were measured and compared. The key observations are that substituting a SnO 2-based anode for a graphite anode led to greater current efficiencies for electro-deoxidation. This was attributed to the lack of contamination of the melt by carbon and the lower cathode potential due to the higher anodic potential when using tin oxide based anodes for the same applied voltage. The current efficiency was also found to decrease with both anode materials when higher anode surface areas or lower current densities were used. Again this was attributed to a decrease in anodic potentials and a corresponding increase in the cathodic potential resulting in a greater number of parasitic reactions occurring at the cathode.

Synthesis of chromium carbide by reduction of chromium oxide with methane

In the present work, production of the chromium carbide was investigated by reduction of chromium oxide with methane-containing gas mixture. The experiments were conducted on the chromium oxide powder and methane gas at different temperatures, times, and gas mixtures. X-ray diffraction (XRD) analysis was used to characterize the products at different stages of reduction. The morphology of the starting chromium oxide powder and Cr3C2 were studied by electron microscopy technique. The results showed that the minimum temperature and time for carbide formation in 30%-methane gas mixture is about 850 °C and 20 min, respectively. X-ray diffraction analysis showed that Cr3C2 is the only carbide product. The formation of chromium carbide in 30%-methane gas mixture was completed at 1000 °C and 60 min.

Modelling of non-catalytic gas–solid reactions – multicomponent non-equimolar counter diffusion of gaseous phase

Mathematical modelling of non-catalytic gas–solid reactions was carried out based on the grain model in terms of non-equimolar counter diffusion of gaseous phase in the solid matrix. By justifying the model parameters, the model can be used for reactions involving volume change between gaseous products and gaseous reactants. The grain model was used in the present work. The model was used for studying the reduction of chromium oxide with methane in the formation of chromium carbide. By justifying the diffusion coefficient of reactant methane through the product layer around the grains, the simulation results showed a good fit with experimental data. A MATLAB m-file has been developed to simulate the gas–solid reactions under unsteady state condition in the gas phase and by considering mass transfer resistances in gas phase outside the solid and through the product layer around the grains for different orders of reaction. Transport equations were solved numerically by using finite volume technique in the implicit form. After parameter justification, the model was used for studying effect of different parameters on the conversion of the reaction. The model shows a good fit to experimental data and can be used for studying the sensitivity analysis for such reactions.

Oxidation of barium on the surface of nanothick chromium oxide films grown on the (110) molybdenum substrate

AbstractAuger electron spectroscopy and work function measurements were used to investigate adsorption of barium onto the surface of nanothick chromium oxide films grown on the (110) molybdenum substrate. We prepared smooth 3‐monolayers (ML) thick pseudomorphic chromium oxide films and rough 2–3 ML chromium oxide films carrying three‐dimensional oxide nanoparticles (the Stranski‐Krastanov (SK) surface morphology). The interaction of barium with chromium oxide films of both types was found to lead to barium oxide formation and partial reduction of chromium oxide to chromium. When barium was deposited onto a 3 ML pseudomorphic chromium oxide film, the barium oxidation at room temperature was restricted to formation of only one monolayer of barium oxide. However, in the case of barium adsorption on chromium oxide film with the SK type surface morphology, the oxidation encompassed 3 monolayers of barium. A strong effect of metal substrate‐oxide film interface on the oxidation ability of nanothick oxide films was revealed. These results showed possibilities of tailoring surface properties of the barium oxide films by variation of their thickness and by substrate‐film interface modification.

Thermo-mechanical processing of Cu-Cr alloys prepared by using electro slag crucible melting

Cu-Cr ingots of Cr contents of up to 2 Wt % were prepared by melting copper scrap using electroslag crucible melting (ESCM). Chromium addition to copper was carried out by in-situ reduction of chromium oxide present in the slag. The Cu-Cr ingots produced were subjected to up to 90% rolling reduction using different combination of rolling and heat treatment. A range of microstructure and mechanical properties was obtained. Alloys with recrystallised microstructure exhibited lower strength and higher ductility. The alloys prepared by ESCM compare well with Cu-Cr alloys reported in the literature. The ESCM route is more attractive as it uses copper scrap for melting and in-situ reduction of chromium oxide for alloying of chromium. ESCM also enables preparation of alloy with high (up to 2%) Cr content.

UTILIZAÇÃO DE CINZAS DA INCINERAÇÃO DE APARAS DE COURO NA PRODUÇÃO EM LABORATÓRIO DE FeCr-AC

The tanning industry, during the leather tanning, produces shavings that, because of the method used, have chromium in its chemical composition. Volume reduction and energy utilization can be achieved by shavings incineration. The aim of this work is to show that the recycling of the metal contained in leather shaving ashes is possible as raw material for stainless steel production, relieving the environment of a dangerous residue. The elaboration of a ferroalloy with similar characteristics of the commercial HC-FeCr ferroalloy was investigated at the laboratory scale. Prior, a thermodynamic analysis was carried out, simulating the process of chromium oxide reduction in order to provide the basis and the most favorable conditions for the experiments. At the laboratory level, it was demonstrated that is viable the reutilization of chromium bearing ashes from the incineration of leather shavings for the production of HC-FeCr

Influence of oxygen partial pressure in sintering atmosphere on properties of Cr–Mo prealloyed powder metallurgy steel

Chromium is an attractive alloying element in low alloyed steels since it gives good hardenability at low cost. The drawback with using chromium in powder metallurgy (PM) grades is its high affinity for oxygen. Thermodynamic calculations show that the oxygen partial pressure should be <4 × 10−18 atm at 1120°C in order to have reducing conditions for a powder grade prealloyed with 3 wt-%Cr and 0·5 wt-%Mo, which is supported by sintering experiments. With graphite added to the powder grade, conditions are reducing during sintering at higher partial pressures of oxygen (up to 10−16 atm) due to favourable conditions locally in the material. Sintering of the powder grade with 0·35 wt-% graphite added at 1120°C for 30 min at reducing conditions leads to high mechanical properties, although kinetics is insufficient for complete reduction of chromium oxides. At 1250°C, kinetics is faster and practically all oxides are reduced after sintering for 30 min, with enhanced mechanical performance as result.

EAF Stainless Steel Refining - Part I: Observational Study on Chromium Recovery in an Eccentric Bottom Tapping Furnace and a Spout Tapping Furnace

An observational study of the electric arc furnace (EAF) process for austenitic stainless steel was performed. A comparison was made between two distinct EAF types: (1) an eccentric bottom tapping furnace (EBTF) and (2) a spout tapping furnace (STF). In order to study the slag evolution during the EAF process, per heat several slag samples were collected at consecutive process stages. They were subjected to electron probe microanalysis using energy dispersive spectroscopy (EPMA-EDS). Compositional and mineralogical data from 36 heats corroborate that both thermodynamic and kinetic conditions exert a strong influence on the final chromium oxide content of the slag. In the STF, chromium oxide reduction predominantly occurs during tapping, owing to the intimate mixing of steel and slag. A multivariate linear regression analysis reveals that the main parameters determining the overall chromium recovery are the slag basicity and the content of dissolved silicon of the steel. These parameters explain 70% of the observed variance in final chromium oxide levels and can be used as control parameters to improve the chromium recovery. For the EBTF, lower chromium recoveries are recorded due to the absence of sufficient mixing during tapping.