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Reduction of un-doped magnetite is developed topochemically with the formation of a dense iron shell. However, the reduction of alumina-doped magnetite to wüstite proceeds with the formation of a network-like structure which consists of criss-crossed horizontal and vertical plates of wüstite. Reduction of magnetite includes the conversion of Fe to Fe and the movement of iron cations from the tetrahedral sites on the {400} and {220} planes of magnetite to the octahedral sites on the {200} planes of wüstite. Alumina has a negligibly small solubility in wüstite. In the reduction of magnetite doped with AlO, rejected Al cations from wüstite diffuse to the magnetite-hercynite solid solution. Enrichment of the FeO-FeAlO solution with alumina in the vicinity of the reduction interface restricts the growth of {220} planes of wüstite and nucleation of {220} planes adjusted to the existing planes, preventing the merging of wüstite plates during the reduction process. Reduction of magnetite from the magnetite-hercynite solid solution practically stops when the Al content at the interface approaches the solubility limit. Wüstite in the separated plates is reduced further to iron. [ABSTRACT FROM AUTHOR]

The reduction of quartz was studied isothermally in a fluidized bed reactor using continuously flowing methane-hydrogen gas mixture in the temperature range from 1623 K to 1773 K (1350 °C to 1500 °C). The CO content in the off-gas was measured online using an infrared gas analyzer. The main phases of the reduced samples identified by XRD analysis were quartz and cristobalite. Significant weight loss in the reduction process indicated that the reduction products were SiO and CO. Reduction of SiO to SiO by methane starts with adsorption and dissociation of CH on the silica surface. The high carbon activity in the CH-H gas mixture provided a strongly reducing condition. At 1623 K (1350 °C), the reduction was very slow. The rate and extent of reduction increased with the increasing temperature to 1723 K (1450 °C). A further increase in temperature to 1773 K (1500 °C) resulted in a decrease in the rate and extent of reduction. An increase in the gas flow rate from 0.4 to 0.8 NL/min and an increase in the methane content in the CH-H gas mixture from 0 to 5 vol pct facilitated the reduction. Methane content in the gas mixture should be maintained at less than 5 vol pct in order to suppress methane cracking. [ABSTRACT FROM AUTHOR]

The gaseous reduction of magnetite ore concentrates was studied using CO-CO and CO-CO-H gas mixtures at different temperatures and gas compositions. The reduction of magnetite ore by CO-CO gas mixture was examined at temperatures 973 K to 1173 K (700 °C to 900 °C) at CO/CO ratio 80/20, and at varied CO/CO ratio from 60/40 to 85/15 at 1023 K (750 °C). In the reduction of magnetite ore by CO-CO-H gas mixture, temperature was 1173 K (800 °C) and hydrogen content changed from 5 to 25 vol pct at constant CO/CO ratio of 80/20. Reduction of magnetite ore did not go to completion in both CO-CO and CO-CO-H gas mixtures. Addition of H to the CO-CO gas mixture accelerated the reduction in the first 10 to 30 minutes of reaction. However, the degree of reduction by gas containing 5 to 25 vol pct H after 60 to 120 minutes of reaction was in the range 60 to 65 pct, while the degree of reduction by CO-CO gas (80 vol pct CO) after 120 minutes of reaction was close to 70 pct. Significant swelling of magnetite ore pellets was observed in the reduction by CO-CO gas mixture. Addition of H to the CO-CO gas mixture decreased swelling. Swelling of magnetite ore during the reduction was attributed to the breakout of iron layer caused by the increase of the inner pressure in the voids at the wüstite/iron phase boundary. [ABSTRACT FROM AUTHOR]

The reduction of magnetite of technical grade and magnetite doped with 3 mass pct AlO was studied in situ using high-temperature XRD (HT-XRD) analysis. Magnetite was reduced by CO-CO gas (80 vol pct CO) at 1023 K (750 °C). Reduction of magnetite doped with alumina occurred from the FeO-FeAlO solid solution which has a miscibility gap with critical temperature of 1133 K (860 °C). The degree of reduction of magnetite was derived using Rietveld refinement of the HT-XRD spectra; the compositions of the FeO-FeAlO solid solution and the concentrations of carbon in γ-iron were determined from the lattice constants of the solutions. The reduction of magnetite progressed topochemically with the formation of a dense iron shell. The reduction of alumina-containing magnetite started along certain lattice planes with the formation of a network-like structure. Reduction of alumina-containing magnetite was faster than that of un-doped magnetite; this difference was attributed to the formation of the network-like structure. Hercynite content in the FeO-FeAlO solid solution in the process of reduction of magnetite doped with 3 mass pct AlO increased from 5.11 to 20 mass pct, which is close to the miscibility gap at 1023 K (750 °C). The concentration of carbon in γ-Fe (0.76 mass pct) formed in the reduced sample of magnetite doped with 3 mass pct AlO was close to the equilibrium value with 80 vol pct CO to 20 vol pct CO gas used in the HT-XRD experiments. [ABSTRACT FROM AUTHOR]

Synthesis of silicon carbide (SiC) by carbothermal reduction of quartz in a CH-H-Ar gas mixture was investigated in a laboratory fixed-bed reactor in the temperature range of 1573 K to 1823 K (1300 °C to 1550 °C). The reduction process was monitored by an infrared gas analyser, and the reduction products were characterized by LECO, XRD, and SEM. A mixture of quartz-graphite powders with C/SiO molar ratio of 2 was pressed into pellets and used for reduction experiments. The reduction was completed within 2 hours under the conditions of temperature at or above 1773 K (1500 °C), methane content of 0.5 to 2 vol pct, and hydrogen content ≥70 vol pct. Methane partially substituted carbon as a reductant in the SiC synthesis and enhanced the reduction kinetics significantly. An increase in the methane content above 2 vol pct caused excessive carbon deposition which had a detrimental effect on the reaction rate. Hydrogen content in the gas mixture above 70 vol pct effectively suppressed the cracking of methane. [ABSTRACT FROM AUTHOR]

This article examines the influence of gas atmosphere on the synthesis of silicon carbide by carbothermal reduction of quartz. The quartz was crushed to <70 μm, uniformly mixed with graphite and pressed into pellets. Reduction was studied in isothermal and temperature-programmed reduction experiments in a tube reactor in argon, hydrogen and Ar-H gas mixtures. The concentrations of CO, CO, and CH in the off gas were measured online using an infrared gas analyzer. The samples after reduction were characterized by X-ray diffraction, scanning electron microscope, and LECO analyzer. The carbothermal reduction of quartz in hydrogen was faster than in argon. Formation of silicon carbide started at 1573 K (1300 °C) in argon, and 1473 K (1200 °C) in hydrogen. Synthesis of silicon carbide in hydrogen was close to completion in 270 minutes at 1673 K (1400 °C), 140 minutes at 1773 K (1500 °C), and 70 minutes at 1873 K (1600 °C). Faster carbothermal reduction rate in hydrogen was attributed to the involvement of hydrogen in the reduction reactions by directly reducing silica and/or indirectly, by reacting with graphite to form methane as an intermediate reductant. [ABSTRACT FROM AUTHOR]

Silicon feedstock for production of solar-grade silicon should be as pure as possible to decrease the cost of manufacturing of solar cells. Impurities in quartz, carbonaceous materials, electrodes, and refractories are mostly present in the form of oxides. These oxides can be reduced to volatile gaseous compounds in presence of SiO(g) and CO(g) atmosphere and potentially leave the furnace or stay in the condensed reaction products, metal, and slag. This work investigates the conditions under which volatile impurities report to the gas phase in laboratory experiments with lumpy and pelletized mixtures of SiO, SiC, and Si at 1923 K and 2123 K (1650 °C and 1850 °C), respectively, were carried out. The volatile compounds were generated by the reduction of quartz and collected in the form of condensate. The effects of the reaction temperature, quartz type, charge composition, pellets, and lumps on the composition of the condensate were studied. The trace elements in the charge input, reacting charge, and condensate were analyzed using inductively coupled plasma (ICP)-mass spectroscopy (MS) and X-ray diffraction (XRD). CO(g) and SiO(g), which are the major components in reduction reactions, formed four types of condensate: white, brown, green, and orange. The condensate constituents were amorphous SiO, 3C:SiC, Si, and α-quartz. Each impurity present in the quartz charge entered the gas phase during quartz reduction and was detected in the condensate. Al and Fe show limited volatility. The volatility of Mn, P, and B depends on the charge mix: a higher P enhances the concentration of these elements in the gas phase. Fluid inclusions, common in hydrothermal quartz, enhance the distribution of the contaminants to the gas phase. Industrial campaigns on Si and Fe-Si production confirm the experimental results. [ABSTRACT FROM AUTHOR]

Quartz and carbonaceous materials, which are used in the production of silicon as well as electrodes and refractories in the silicon furnace, contain trace elements mostly in the form of oxides. These oxides can be reduced to gaseous compounds and leave the furnace or stay in the reaction products-metal and slag. This article examines the behavior of trace elements in hydrothermal quartz and quartzite in the reaction of SiO with Si or SiC. Mixtures of SiO (quartz or quartzite), SiC, and Si in forms of lumps or pellets were heated to 1923 K and 2123 K (1650°C and 1850°C) in high purity graphite crucibles under Argon gas flow. The gaseous compounds condensed in the inner lining of the tube attached to the crucible. The phases present in the reacted charge and the collected condensates were studied quantitatively by X-ray diffraction (XRD) and qualitatively by Electron Probe Micro Analyzer (EPMA). Contaminants in the charge materials, reacted charge and condensate were analyzed by Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS). Muscovite in the mineral phase of quartz melted and formed two immiscible liquid phases: an Al-rich melt at the core of the mineral, and a SiO-rich melt at the mineral boundaries. B, Mn, and Pb in quartz were removed during heating in reducing atmosphere at temperature above 1923 K (1650°C). Mn, Fe, Al and B diffused from quartz into silicon. P concentration was under the detection limit. Quartzite and hydrothermal quartz had different initial impurity levels: quartzite remained more impure after reduction experiment but approached purity of hydrothermal quartz upon silica reduction. [ABSTRACT FROM AUTHOR]

This article examined the reduction/nitridation of rutile in the He-N, Ar-N, and He (Ar)-H-N gas mixtures, as well as pure nitrogen, in the temperature-programmed and isothermal experiments in a fixed-bed reactor. The extents of reduction and nitridation were determined from the off gas composition and LECO analysis. The off-gas composition was monitored using the infrared sensor (CO, CO, and CH) and dew point analyzer (HO). The phase composition of the reduced samples was analyzed using X-ray diffraction (XRD). The temperature and gas composition had a strong effect on the rutile reduction. The reduction was the fastest in the H-N gas mixture, followed by a reduction in nitrogen; the rate of reduction/nitridation in the He-N gas mixture was marginally higher than in the Ar-N gas. The rate of titania reduction/nitridation in the He (Ar)-H-N gas increased with the replacement of He (Ar) with hydrogen. The article also discusses the mechanisms of reduction/nitridation in different gas atmospheres. [ABSTRACT FROM AUTHOR]

The charge for silicomangansese production consists of manganese ore (often mixed with ferromanganese slag) dolomite or calcite, quartz, and in some cases, other additions. These materials have different melting properties, which have a strong effect on reduction and smelting reactions in the production of a silicomanganese alloy. This article discusses properties of Assman, Gabonese, and Companhia Vale do Rio Doce (CVRD) ores, CVRD sinter and high-carbon ferromanganese (HC FeMn) slag, and their change during silicomanganese production. The melting and reduction temperatures of these manganese sources were measured in a carbon monoxide atmosphere, using the sessile drop method and a differential thermal analysis/thermogravimetric analysis. Equilibrium phases were analyzed using FACTSage (CRCT, Montreal, Canada and GTT, Aachen, Germany) software. Experimental investigations and an analysis of equilibrium phases revealed significant differences in the melting behavior and reduction of different manganese sources. The difference in smelting of CVRD ore and CVRD sinter was attributed to a faster reduction of sinter by the graphite substrate and carbon monoxide. The calculation of equilibrium phases in the reduction process of manganese ores using FACTSage correctly reflects the trends in the production of manganese alloys. The temperature at which the manganese oxide concentration in the slag was reduced below 10 wt pct can be assigned to the top of the coke bed in the silicomanganese furnace. This temperature was in the range 1823 K to 1883 K (1550 °C to 1610 °C). [ABSTRACT FROM AUTHOR]

Highlights • Thermodynamic analysis outlines CO vol% of reducing gas for Ni & Co metallisation. • Morphological & compositional changes occur to silicate minerals during reduction. • Selective reduction obtains comparable Ni & Co metallisation but limits Fe to 20%. • Increasing temperature after 740 °C weakens Ni, Co and Fe metallisation degree. • Olivine recrystn traps more Ni, making it less reducible, & causes a drop in Ni yield above 740 °C. Abstract Reduction of an Australian garnieritic laterite ore by different CO-CO 2 gas mixtures was studied between 700 and 1000 °C. Two size ranges of the ore were examined: <53 μm and 53–200 μm. Metallic nickel, cobalt, and iron in reduced garnieritic laterite samples were leached out by a Br 2 -CH 3 OH solution; their concentrations in the solution were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis to determine reduction degree. The changes in mineral phases, morphology and the chemical composition of these phases in the reduced ore were examined by XRD and SEM/EDS. It is confirmed that in this ore, nickel occurs with magnesium and iron as silicate isomorphisms including chlorite, talc and their derivative phases/particles (olivine and pyroxene). The BSE micrographs at high magnifications found Fe-Ni alloy was formed during reduction and its grains dispersed within silicate particles. The morphological and compositional observations indicate that numerous pores and cracks were generated in dehydroxylated chlorite, talc, and pyroxene particles, making nickel readily attacked by gases. Dense structures, however, were formed for olivine particles, rendering nickel inside difficult to reduce. Current study suggests that reduction temperature and reducing gas compositions are key factors for selective reduction. The optimal conditions for nickel and cobalt production from this garnieritic laterite by selective reduction can be mediated at 740 °C in 60% CO, at which satisfactory metallisation of 91% Ni and 94% Co in the particles <53 μm and 85% Ni and 99% Co in the particles 53–200 μm but less than 20% Fe were achieved. [ABSTRACT FROM AUTHOR]

This study investigates the mechanisms of SiC whisker formation in the carbothermal reduction of quartz to SiC in different gas atmospheres. Reduction of quartz by graphite was studied in Ar, H 2 , and CH 4 –H 2 –Ar gas mixture in a laboratory fixed bed reactor. The reduction products were characterised by XRD, SEM and TEM. Whiskers were not formed in the carbothermal reduction of quartz in argon. Two types of SiC whiskers were observed in the carbothermal reduction of quartz in H 2 and CH 4 –H 2 –Ar gas mixture. In the process of reduction at 1400–1600 °C in H 2 and at 1200–1600 °C in CH 4 –H 2 –Ar gas mixture, whiskers with hexagonal shape with diameter 100–800 nm and length up to tens of microns were formed by the VLS mechanism under catalytic effect of iron. The whiskers with the characteristics of cylindrical shape and high aspect ratio were synthesized in CH 4 –H 2 –Ar gas mixture at 1400–1600 °C by VS mechanism. [ABSTRACT FROM AUTHOR]