CC BY
14RAPID-HEATING-TYPE COMBUSTION SYNTHESIS OF METALLIC IRON: EFFECTS OF TEMPERATURE AND ATMOSPHERE
K. Abe*", A. Kurniawan", M. Sanada", T. Nomura", and T. Akiyama"
aHokkaido University, Sapporo, Hokkaido, 060-8628, Japan *e-mail: k_abe@eng.hokudai.ac.jp
DOI: 10.24411/9999-0014A-2019-10002
INTRODUCTION
CO2 emission from the iron and steel industry accounts for around 9% of total CO2 emission in the world [1]. Direct ironmaking method using carbon-infiltrated iron oxides has been focused on to reduce the amount of CO2 emission from the industry [2]. For reduction of the carbon-infiltrated goethite ore, the distance between iron ore and carbon is important; closer contact makes the reduction faster.
Goethite (a-FeOOH) based iron ore has been utilized as an alternative raw material to highgrade iron ore in the iron and steel industry. It is decomposed to Fe2O3 and H2O by mild calcination, and interestingly, nanopores form in the product Fe2O3 [3]. Carbon-infiltrated goethite ore has been prepared using tar vapor as a carbon source [4]. The ore was reduced at lower temperatures because carbon was deposited through the nanopores of the ore and close (nano-order) contact between ore and carbon was achieved [5].
Combustion synthesis (CS) is a simple and short-time method to synthesize a product using combustion reaction. We have already reported that carbon-infiltrated goethite ore had been obtained by tar impregnation/carbonization method and that metallic iron had generated from the carbon-infiltrated goethite ore after rapid-heatig-type CS experiment in an oxygen flow [6]. Metallic iron was partially obtained by the short-time experiments, however, the reduction degree is still not so high. The effects of temperature and oxygen concentration in the CS experiments on reduction degree were investigated in this study.
EXPERIMENTAL
Goethite-based iron ore (particle size: 1-2 mm, total Fe: 57 mass%, combined water: 8.8 mass%) was firstly calcined at 573 K for 24 h in a muffle furnace in air. Liquid carbon source was prepared from coal-tar (fixed carbon: 32 mass%, ash: 0.04 mass%, moisture: 0.15 mass%) and reagent toluene (99.5%). The coal tar and the toluene (1:1 in weight) were well-mixed using magnetic stirrer at 323 K for 1 h to make tar solution. The calcined goethite ore (3.0 g) and the tar solution (6.0 g) were put in an alumina crucible and it was heated at 773 K for 1 h under an Ar flow (0.5 L min-1) to get carbon-infiltrated goethite ore. After that, it was crushed into original ore size: 1-2 mm.
100 mg of the obtained carbon-infiltrated goethite ore were charged into a quarts tube (9 6) [6]. The ore was rapidly heated up to 973-1173 K at a heating rate of 20 K s-1 and was held at the temperatures for 10 s. O2/Ar gases (total flow rate; 1 L min-1, oxygen concentration; 15100 vol %) were flowed from the top of the tube during the heating. The gas flow was immediately changed to Ar after the finish of the heating process.
Phase identification of the samples was conducted using X-ray diffractometry (XRD; Miniflex, Rigaku, Tokyo, Japan). The surface and the sross-section of the samples were observed by scanning electron microscopy (SEM; JSM-7001FA, JEOL, Tokyo, Japan) with energy dispersive X-ray spectroscopy (EDS).
iSHS 2019
Moscow, Russia
Fig. 1. SEM-EDS images of the cross-section of the obtained carbon-infiltrated
goethite ore.
RESULTS AND DISCUSSIONS
Figure 1 shows the cross-sectional SEM and EDS observations of the carbon-infiltrated goethite ore. Carbon-infiltrated goethite ore with a thick carbon layer at the surface of the ore was successfully obtained by the tar treatment. Figure 2 shows the XRD patterns of the carbon-infiltrated goethite ores after the CS experiments at different heating temperatures. Despite the total heating time in the CS experiments was below one minute, reduction partially proceeded to metallic iron. Metallic iron was observed at all heating temperatures, and the reduction degree was the highest in the sample which was heated at 1073 K. Figure 3 shows the temperature changes during the CS experiments. Measurements of temperature changes during the experiments were conducted by an R-type thermocouple placed directly over the ore-bed. The temperatures measured during the experiments were much higher than the setting ones at every holding temperature. This meant carbon combustion successfully occurred at the surface of the ores. Combustion of the surface carbon started at around 973 K; sudden temperature increases were observed near the temperature. When the holding Fig. 2. XRD patterns of the carbon-temperature was higher (1123 or 1173 K), carbon infilitrated goethite ore after the rapid-combustion started before the temperature heating-type CS experiments at different holding and was completed before the heating temperatures.
process finished. This meant that almost all of the surface carbon disappeared during heating and ore-surface was exposed to oxygen, causing re-oxidation of the reduced ore at high holding temperatures. At 973 K, the temperature continued to increase until the end of the heating process, meaning carbon combustion was not completed during the experiment. Re-oxidation was ignored at this condition, however, reduction was not proceeded because the maximum temperature (1164 K) was much lower than the other conditions. At 1073 K, surface carbon remained during the experiment and the reached temperature was very high, resulting in the highest reduction degree.
Fig. 3. Temperature changes during the rapid-heating-type CS experiments. The dotted lines mean setting temperatures and the solid lines mean the measured temperatures.
Figure 4 shows the XRD patterns of the carbon-infiltrated goethite ores after the CS experiments at different oxygen concentrations. Metallic iron was not observed when the flowed gases had higher oxygen concentrations (50 and 100 vol % O2). Carbon combustion
completed in a moment at high oxygen concentration and iron ore was easily exposed to oxygen, resulting in the lower reduction degree. At lower oxygen concentrations, carbon combustion reaction continued until heating process finished. The iron ore was effectively reduced without re-oxidation at lower oxygen concentrations.
M M M¥ 100%02
M ..J.hiil. Y M M ..A -»..A ..A. 50%02/Ar
M M1 M M il t w M W I 25%0,/Ar 1 ...........T-------W
I M 1 M-sf vl 20%CyAr 1
I M W I 15%02/Ar ■ .. }
20 30 40 50 60 70 80 20 (degree)
Fig. 4. XRD patterns of the carbon-infilitrated goethite ore after the rapid-heating-type CS experiments flowing 15-100 vol % O2/A gases.
CONCLUSIONS
In this study, carbon-infiltrated goethite ore was produced by tar impregnation/carbonization method, and it was reduced via rapid-heating-type combustion synthesis process flowing oxygen gas. The surface carbon of the goethite ore worked not only as heat source for reduction of the ore but also as protective coat against oxidation. At higher temperatures and higher oxygen concentrations, carbon combustion completed early and oxidation of the ore by the flowing oxygen could not be prevented, resulting in lower reduction degree. At optimum condition, carbon combustion effectively promoted reduction of the ore and all of the surface carbon did not disappear, resulting in higher reduction degree.
1. International Energy Agency (IEA), CO2 emissions from fuel combustion, 2016.
2. C. Xu, D. Cang, A brief overview of low CO2 emission technologies for iron and steel making, J. Iron Steel Res. Int., 2010, vol. 17, no. 3, pp. 1-7.
3. H. Naono, R. Fujiwara, Micropore formation due to thermal decomposition of acicular microcrystals of a-FeOOH, J. Colloid Interface Sci., 1980, vol. 73, no. 2, pp. 406-415.
4. Y. Hata, H. Purwanto, S. Hosokai, J. Hayashi, Y. Kashiwaya, T. Akiyama, Biotar ironmaking using wooden biomass and nanoporous iron ore, Energy Fuels, 2009, vol. 23, no. 2, pp. 1128-1131.
5. S. Hosokai, K. Matsui, N. Okinaka, K. Ohno, M. Shimizu, T. Akiyama, kinetic study on the reduction reaction of biomass-tar-infiltrated iron ore, Energy Fuels, 2012, vol. 26, no. 12, pp. 7274-7279.
6. K. Abe, A. Kurniawan, K. Ohashi, T. Nomura, T. Akiyama, ultrafast iron-making method: carbon combustion synthesis from carbon-infiltrated goethite ore, ACS Omega, 2018, vol. 3, no. 6, pp. 6151-6157.
