Effect of gas atmosphere on carbothermal reduction and nitridation of titanium dioxide

This article examined the reduction/nitridation of rutile in the He-N2, Ar-N2, and He (Ar)-H2- N2 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, CO2, and CH4) and dew point analyzer (H2O). 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 H2- N2 gas mixture, followed by a reduction in nitrogen; the rate of reduction/nitridation in the He- N2 gas mixture was marginally higher than in the Ar-N2 gas. The rate of titania reduction/ nitridation in the He (Ar)-H2-N2 gas increased with the replacement of He (Ar) with hydrogen. The article also discusses the mechanisms of reduction/nitridation in different gas atmospheres

Effects of annealing on microstructure and microstrength of metallurgical coke

Two metallurgical cokes were heat treated at 1673 K to 2273 K (1400 degrees celsius to 2000 degrees celsius) in a nitrogen atmosphere. The effect of heat treatment on the microstructure and microstrength of metallurgical cokes was characterized using X-ray diffraction, Raman spectroscopy, and ultramicroindentation. In the process of heat treatment, the microstructure of the metallurgical cokes transformed toward the graphite structure. Raman spectroscopy of reactive maceral-derived component (RMDC) and inert maceral-derived component (IMDC) indicated that the graphitisation degree of the RMDC was slightly lower than that of the IMDC in the original cokes; however graphitisation of the RMDC progressed faster than that of the IMDC during annealing, and became significantly higher after annealing at 2273 K (2000 degrees celsius). The microstrength of cokes was significantly degraded in the process of heat treatment. The microstrength of the RMDC was lower, and of its deterioration caused by heat treatment was more severe than IMDC. The degradation of the microstrength of cokes was attributed to their increased graphitisation degree during the heat treatment

A feasibility study of recycling of manganese furnace dust

This paper presents results of a feasibility study of recycling manganese furnace dust generated in production of ferromanganese and silicomanganese at Tasmanian Electrometallurgical Company, Australia. Dried manganese furnace dust contains about 20 wt% of carbon, in average 33.4 wt% of manganese and 1.3 wt% of zinc. Manganese in the dust is in the form of MnO, Mn3O4 and MnCO3; zinc is mainly in the form of ZnO and ZnSO4. Analysis of the zinc balance with dust recycling showed that to keep zinc intake at the acceptable level, it should be partly removed from the dust. In the reduction laboratory experiments, zinc oxide was reduced to zinc vapour by tar of the dust. Reduction of zinc oxide started at 800oC and zinc removal rate increased with increasing temperature; removal of zinc was close to completion at 1100oC. Optimal conditions for removing zinc from the dust include temperature in the range 1000-1150oC, inert gas atmosphere and furnace dust fraction in the furnace dust-manganese ore mixture above 60%. In the sintering of manganese ore with addition of manganese dust in the sintering pot, zinc was reoxidised and deposited in the sinter bed. Removal of zinc in the sintering pot tests was in the range 4-17%. Up to 30% zinc removal was achieved from the bottom layer of the sinter bed. It can be concluded that zinc removal will be low during the processing of manganese furnace dust in the sinter plant. The zinc removal rate will be the highest when pelletised manganese furnace dust is added to the bottom layer of the sintering bed

Microstrength, strength and microstructure of carbonaceous materials

The effect of heat treatment at 700-1500 ºC on the mechanical strength, micro strength and pore structure of carbonaceous materials, including coke, char and coals, were studied using tensile test, ultra micro indentation and image analysis. Strength of chars and pyrolysed coals was strongly enhanced by heat treatment at temperature below 1100 °C; strength of cokes was slightly degraded after heat treatment at 1500 °C. Mechanical strength of carbonaceous materials was demonstrated to be significantly affected by micro strength and porosity. Micro strength of chars and coals was significantly enhanced by heat treatment, whereas micro strength of cokes was only marginally increased by heat treatment. The major growth in the micro strength of chars and coals took place at annealing temperature below 1100 °C. Porosity of chars and coals significantly increased during annealing at temperatures below 1100 °C. Further increasing annealing temperature from 1100-1500 °C caused marginal porosity evolution in pyrolysed coals and chars. Porosity of cokes increased slightly in the temperature range of 1300-1500 °C

Reduction of quartz to silicon monoxide by methane-hydrogen mixtures

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 SiO2 to SiO by methane starts with adsorption and dissociation of CH4 on the silica surface. The high carbon activity in the CH4-H2 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 CH4-H2 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

Stability of Cementite formed from Hematite and Titanomagnetite Ore

The stability of cementite formed during the reduction of hematite and preoxidized titanomagnetite ores in a methane-hydrogen gas mixture was examined in the temperature interval 500oC to 900oC for the hematite ore and 300oC to 1100oC for titanomagnetite. Cementite formed from hematite ore was most stable at temperatures between 750oC to 770oC. Its decomposition rate increased with decreasing temperature between 750oC and 600oC and with increasing temperature above 770oC. Cementite formed from preoxidized titanomagnetite was most stable in the temperature range 700oC to 900oC. The rate of cementite decomposition increased with decreasing temperature between 700oC and 400oC and with increasing temperature above 900oC. Cementite formed from titanomagnetite ore was more stable than cementite formed from hematite under all conditions examined

Phase development in carbothermal reduction and nitridation of ilmenite concentrates

The phase development in the course of carbothermal reduction and nitridation of ilmentie concentrates and synthetic rutile was studied in temperature programmed reduction (623-1873 K) and isothermal reduction experiments. Ilmenites and synthetic rutile were reduced in a tube reactor with continuously flowing hydrogen-nitrogen mixture or pure nitrogen. The rate and extent of reduction were monitored by online off-gas analysis. Samples reduced to different extent were subjected to XRD and SEM/BSE analyses. Pseudorutile and ilmenite were the main phases in ilmenite concentrates; rutile was the main phase in synthetic rutile. Pseudorutile was first converted to ilmenite and titania which occurred at temperatures below 623 K; iron oxides in ilmenite were quickly reduced to metallic iron. Titania was reduced to titanium suboxides and further to titanium oxycarbonitride. Reduction of ilmenites and synthetic rutile in hydrogen-nitrogen mixture was much faster than in pure nitrogen. The rate of conversion of titanium oxides to oxycarbonitride was affected by iron content in the ilmenites. The rate of reduction increased with increasing iron content in ilmenite (decreasing grade) when ilmenites were reduced in the hydrogen-nitrogen gas mixture, but decreased with decreasing ilmenite grade in reduction experiments in nitrogen; reduction in nitrogen was the fastest for synthetic rutile. The difference in the reduction behaviour was attributed to different chemical compositions and morphologies of ilmenites of different grades