Hydrogen reduction of oxide blends: a sustainable pathway for synthesis of ferroalloys

The paper considers carbothermal solid state reduction of manganese, titanium and aluminium oxides in argon, helium and hydrogen. The difference in reduction in helium and argon was reflected by different diffusion coefficients of gaseous reactants and products, which are much higher in helium than in argon. When carbothermal reduction took place in hydrogen, it was involved in the reduction process by reducing oxides to suboxides and forming methane.Manganese and titanium oxides were reduced to carbide Mn7C3 and oxycarbide Ti(OxC1‐x) correspondingly, while products of alumina reduction included carbide Al4C3 and vapours of Al and Al2O, which were re‐oxidised to Al4O4C outside the reactor and deposited in the lower temperature zone. Gas atmosphere had a profound effect on the extent and rate of reduction. This effect was different in reduction of different oxides. Reduction of manganese oxides was the fastest in hydrogen, and faster in helium than in argon. Reduction of titania in argon and helium proceeded with about the same rate and was much faster in hydrogen than in the inert atmospheres. The rate and extent of alumina reduction in hydrogen and helium were higher than in argon, although no significant difference was observed in alumina reduction in hydrogen and helium. This reflects differences in reduction mechanisms, which are discussed in the paper.

In this work, the high-temperature reduction of nickel oxide and reoxidation of metallic nickel in the YSZ–NiO anode material have been studied. As-sintered material (of mode 1) manufactured by tape casting underwent two treatment modes: one-time reduction (mode 2) in pure hydrogen for 4 h at 600 °C under a pressure of 0.15 MPa; reduction/oxidation (redox) for five cycles in hydrogen/air atmospheres at 600 °C (mode 3). The material of mode 1 exhibited a bending strength of about 209 MPa. This level is high enough for anode materials. Significant changes have been detected in the microstructure of the material reduced in hydrogen (mode 2). Agglomerates of particles (average size about 5 µm) with a predominance of the zirconium phase as well as regions of small particles (1 µm) with uniformly distributed both the nickel and zirconium phases were revealed. The intergranular fracture micromechanism prevailed in the specimens tested. The material strength (40 MPa) did not satisfy the requirements for anode materials. Despite too “rigid” redox treatment mode in pure hydrogen and air at 600 °C (mode 3), the cermet is not inferior in strength (85 MPa) to those fabricated using other techniques. On the specimen fracture surface, a porous core and dense outer layers have been revealed. An increased amount of nickel in the surface layer is probably a consequence of the microstructural transformation due to the diffusion of nickel from the subsurface region towards the surface. The redox treatment caused the specimen core degradation due to forming large pores, their coalescence into cracks, and loss of material integrity. In contrast to the mixed fracture micromechanism noted in the specimen surface layer, the intergranular one in the core was revealed. The reached level of electrical conductivity and a positive effect of the treatment on the material strength indicate the potential of the tape cast material, especially, in the case of reduction in Ar–5 vol% H2 gas mixture instead of pure hydrogen.

This work is aimed at studying the high-temperature reduction of nickel oxide and reoxidation of metallic nickel in a fine-grained YSZ–NiO anode material. The YSZ–NiO anode substrates were manufactured by tape casting. As-sintered material (of mode 1) was undergone to two treatment mode: one-time reduction in pure hydrogen for 4 h at 600 °C under a pressure of 0.15 MPa (mode 2); reduction/oxidation (redox) for five cycles in hydrogen/air atmospheres at 600 °C (mode 3). As-sintered material (mode 1) showed the strength under three-point bending about 209 MPa that is high enough for anode materials. Significant changes in the microstructure of the material have been detected after reduction in hydrogen (mode 2). It consisted of agglomerates of particles (average size about 5 µm) with a predominance of the zirconium phase as well as regions of small particles (1 µm) with uniformly distributed both the nickel and zirconium phases. The intergranular fracture micromechanism prevailed in the specimens tested. The material exhibited strength of about 40 MPa that did not satisfy the requirements for anode materials. Despite too “rigid” mode of redox cycling in pure hydrogen and air at 600 °C (mode 3), according to the level of strength (85 MPa), the cermet is not inferior to those obtained using other techniques. On the specimen fracture surface, dense outer layers and a porous core have been revealed. An increased amount of nickel in the surface layer is probably a consequence of the redistribution due to the diffusion of nickel from the subsurface region towards the surface. The redox treatment led to the degradation of the specimen core by forming large pores, their coalescence into cracks, and loss of material integrity. The intergranular fracture micromechanism in the specimen core was noted, in contrast to the mixed one revealed in the surface layer. The obtained value of electrical conductivity (7.4·105 S/m) and a certain positive effect of this treatment on strength indicate the potential of the tape cast material in the case of the optimization of its redox treatment mode, in particular, by reduction in Ar–5 vol% H2 gas mixture instead of pure hydrogen.

A new kinetic model for hydrogen reduction of metal oxides under external gas diffusion controlling condition

Hydrogen reduction process for zinc-bearing dust treatment: Reduction kinetic mechanism and microstructure transformations in a novel and environmentally friendly metallurgical technique

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

The behavior of tungsten oxides in the presence of copper during hydrogen reduction

Reduction and carburization of iron oxides for Fischer–Tropsch synthesis

I have previously used a temperature-programmed reduction in hydrogen to identify surface oxides on nickel base alloys [1]. The reduction onset temperature of oxides depends on the cation-oxygen bond strength [2-6]. Sufficiently large differences in the reduction onset temperatures make identification of oxides possible [7-9]. The amount of water vapour produced as a result of the reduction is directly proportional to the amount of the oxide reduced. The high sensitivity of the thermal conductivity detectors (TCDs) used in gas chromatography allows detection of the minute amounts of water vapour evolved during the reduction of surface oxide on samples with small surface area [1]. The quantity of the oxides is determined by measuring the area under the peak of the detector signal curve, which is directly proportional to the water vapour content. Thin films, consisting of iron oxide and chromium oxide mixtures, their solid solutions or iron-chromium spinel form during oxidation of Fe-Cr alloys below 500°C [10-12]. In this work surface oxides were identified on an Fe-12Cr alloy oxidized in air at 400 and 500 ° C, the composition of the oxide films were calculated and the oxidation rates of the alloy components were compared. Samples of a commercial (Middelburg Steel and Alloys, South Africa) 3CR12 alloy (11.5% Cr, 0.6% Ni, 0.25% Ti, 0.4% Si, 0.9% Mn, 0.02% C and 0.015% N) were oxidized at 400 and 500°C in air. The 10 mmx 10 mm x 7 mm samples were polished before oxidation with 1/~m diamond paste. In the experiments the following were used: 99.9% pure electrolytic iron (Metallurg Co., South Africa), 99.5% pure electrolytic 2 to 3 mm chromium particles (BDH Chemical Co., UK), 99% pure ferric oxide (Cerac Co., USA), 410L water-atomized stainless steel (0.071% C, 0.86% Si, 0.49% Mn, 0.024% S, 12.3% Cr and 0.012% O; Glidden Metals, USA), and 99.99% pure hydrogen, containing < lp.p.m, oxygen and having a dew point below - 72 ° C. The analysis technique was described in detail in [1]. A specimen was placed in a quartz reactor with a hydrogen stream of 4dm3h ~ The reactor temperature increased at a rate of 15°Cmin ~. A

Assessment of copper-vanadium oxide on mixed alumina-titania supports as sulphur dioxide sorbents and as catalysts for the selective catalytic reduction of NO x by ammonia

One of the major obstacles in the incorporation of nanomaterials in high technology is the lack of new processes for the bulk production of the materials with compositions tailored to suit the application. Oxides can potentially be reduced to metals, intermetallics or alloys by hydrogen or natural gas. The formation of homogeneous alloys and intermetallics by this method has been confirmed by a number of experimental studies. The kinetics of hydrogen reduction of pure oxides of transition metals as well as complex tungstates, molybdates, titanates, aluminates and chromate were investigated by thermogravimetry. The formation of homogeneous alloys and intermetallics was confirmed by these studies. Arrhenius activation energies of the reduction reactions could be linked to the stabilities of the complex oxides. The products were found to have nanograin structure. Bulk processing through hydrogen reduction route was examined in the case of iron molybdate using a fluidized bed reactor.

Treating bauxite residue as an alternative source of metals for iron and aluminium industry is an approach to promote circular economy in metal industries. Reduction of metal oxides with H2-based process is an important step on decarbonisation of metal industry. In this study bauxite residue pellets were prepared and were reduced with different H2-H2O gas compositions at different temperatures which yielded with various degrees of reduction. The bauxite residue pellets were made from a mixture of bauxite residue and Ca(OH)2 powders and sintered at 1150°C. Hydrogen reduction was carried out on the oxide pellets using a resistance furnace at elevated temperatures in controlled reduction atmosphere of H2-H2O gas mixtures which resulted in reduction of iron oxides in the pellets. Unreduced and reduced pellets were subsequently heated to 1400°C to study their smelting behaviour using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) technique to investigate the evolution of phases related to slag formation via smelting. Equilibrium module of Factsage™ ver 8.1 was utilised to analyse results of thermal analysis. It was observed that Tricalcium phosphate-β (CaP2O8), Bredigite (Ca7Si4MgO16), Rankinite (Ca3Si2O7), and calcium alumino ferrite (Ca[Al,Fe]6O10) were formed during thermal analysis as intermediate phases. The initial slag formation for sintered and reduced pellets occurred at 900°C which mainly contains calcium and small amount of strontium component. The slag formation rate increases significantly starting from 1100°C when iron oxides started to form initial molten slag phase which then followed by other oxides dissolution in the system. Gas formation was observed at 1180°C and it was found that the gas to be released from unreduced pellets is O2 meanwhile the gas to be released from reduced pellets is SO2 gas. Gas formation rate for both pellets start to increase at 1280°C when the remaining chemically bonded gas start to be released from the system.

Kinetic isotope effects in nitrous oxide thermal decomposition and reduction reactions were investigated in flowing N2O/He and N2O/He/H2 systems. The intramolecular distribution of stable isotopes, isotopomer, was measured and evaluated using an isotope ratio mass spectrometer equipped with a modified collector system. Thermal decomposition and reduction of nitrous oxide were carried out using a flow tube reactor made of quartz glass in the temperature range of 1173–1373 K. Both thermal decomposition and hydrogen reduction of nitrous oxide proceeded by first‐order kinetics. Isotopomer ratios (δ15Nbulk, δ15Nα, δ15Nβ, and δ18O) of residual nitrous oxide were found to follow a Rayleigh model based on batch distillation. Residual nitrous oxide was depleted slightly in 15N during thermal decomposition but slightly enriched in 15N during hydrogen reduction. Regarding the nitrogen isotopomers, results showed that the central nitrogen atom, Nα, was enriched in 15N during both destruction reactions of nitrous oxide. Although the N2O destruction reactions proceeded at high temperature, large isotopic fractionations of nitrogen isotopomers were observed. Enrichment factors for nitrous oxide as a result of thermal decomposition and reduction reactions were evaluated.

Treating bauxite residue as an alternative source of metals for iron and aluminum industry is an approach to promote circular economy in metal industries. Reduction of metal oxides with a H2-based process is an important step for the decarbonization of metal industry. In this study, bauxite residue (BR) pellets were prepared and were reduced with different H2-H2O gas compositions at different temperatures, which yielded with various degrees of reduction. The bauxite residue pellets were made from a mixture of bauxite residue and Ca(OH)2 powders and sintered at 1150 °C. Hydrogen reduction was carried out on the oxide pellets using a resistance furnace at elevated temperatures in controlled reduction atmosphere of H2-H2O gas mixtures, which resulted in the reduction of iron oxides in the pellets. Unreduced and reduced pellets were subsequently heated to 1400 °C to study their sintering behavior during H2 reduction using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) techniques to investigate the evolution of phases related to slag formation. Equilibrium module of Factsage™ was utilized to analyze results of thermal analysis. Both chemical and physical changes that occurred during the sintering process of the H2-reduced BR pellets were successfully detected by TG-DTA analysis, and the initial slag- and gas-phase formation were detected to occur from 900 °C and 1180 °C, respectively. One of the most notable chemical reactions to occur during the analysis was formation of mayenite at 810 °C, which is easily leachable, providing potential for recovery of alumina.

Introduction/purpose: Hydrogen is the most abundant element in the universe (75 % by mass) and the lightest element (with a density of 0.00082 g/cm3 ) which consists of only one proton and one electron. Because of its presence in many different forms such as gaseous hydrogen, its plasma species, water, acid, alkaline, ammonia and hydrocarbons, it has various applications in different industrial disciplines. Methods: Different hydrometallurgical and pyrometallurgical methods are considered in order to point out many different processes such as formation of hydrogen, reduction of metallic oxides and chlorides, and electrochemical reactions such as hydrogen overvoltage and the spillover effect. Ultrasonic spray pyrolysis enables the formation of very fine aerosols which can be used for the production of metallic powders. Results: Hydrogen formation was observed during the dissolution of metallic alloys with hydrochloric acid. The reduction of metallic oxides and metallic chlorides by hydrogen leads to the formation of metallic powders. Metallic powders were collected by a new developed electrostatic precipitator. Conclusion: Hydrogen can be applied in different reduction processes for the production of metallic powders. Recycling processes can be used for the formation of hydrogen. A new research strategy for powder production is proposed combining recycling of the black mass of used Li-Ion batteries, ultrasonic spray pyrolysis, and hydrogen reduction.