Solid-state reduction of chromium oxide by methane-containing gas
- PDF / 516,631 Bytes
- 7 Pages / 612 x 786 pts Page_size
- 11 Downloads / 223 Views
07/08/2004
5:45 PM
Page 609
Solid-State Reduction of Chromium Oxide by Methane-Containing Gas NATHANIEL ANACLETO and OLEG OSTROVSKI Reduction of chromium oxide, Cr2O3, was investigated in a fixed bed laboratory reactor in the temperature range 900 °C to 1200 °C using CH4-H2-Ar gas mixture. The extent and rate of reduction as functions of gas composition and temperature were determined by on-line off-gas analysis using a mass spectrometer. Samples at different stages of reduction were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The chromium oxide was reduced to chromium carbide Cr3C2 with a degree of reduction close to 100 pct. The rate of reduction increased with temperature and methane content in the reducing gas. Carbon monoxide, added to the reducing gas, strongly retarded the rate of Cr2O3 reduction. The hydrogen content had a slight effect on the reduction rate. High extent and rate of reduction by methane-containing gas in comparison with carbothermal reduction were attributed to high carbon activity in the reducing gas—15 to 50 (relative to graphite).
I. INTRODUCTION
HIGH carbon ferrochromium production is dominated by the electric-arc furnace (EAF) process. The EAF ferrochromium making requires high-quality lump chromium ore, coke, and approximately 4000 kWh of electrical energy per tonne of ferrochromium, which all drive the investigation into alternative ferrochromium production technologies.[1] In prereduction of chromium ore by hydrogen or carbon monoxide, only iron oxides are reduced. However, using methane-hydrogen gas, iron and chromium oxides can be converted to carbides.[2,3] Methane reduces chromium oxide by the following reaction:
This conclusion was mainly based on the experimental fact that the reaction rate was independent of the methane partial pressure in the range of 0.02 to 0.25 atm, and backed by the observation that nonisothermal reduction of chromium oxide by solid carbon in the hydrogen atmosphere was similar to that by methane-hydrogen gas mixture. Qayyum and Reave[3] modified the reduction mechanism, suggesting that under conditions when methane is stable (low CH4/H2 ratio) chromite is reduced by Reaction [7], which is similar to Reaction [1]: 3FeO # Cr2O3 17CH4 Fe3C 2Cr3C2 12CO 34H2 [7]
Cr2O3(s) 13/3CH4(g) 2/3Cr3C2(s) 3CO(g) 26/3H2(g) [1]
At high temperatures and CH4 /H2 ratios, when methane is unstable, the following reactions were suggested.[3]
The standard Gibbs free energy of this reaction derived using data from Knacke et al.[4] is
17CH4 17C 34H2
[8]
3FeO # Cr2O3 12H2 6Cr 3Fe 12H2O
[9]
6Cr 4C 2Cr3C2
[10]
3Fe C Fe3C
[11]
G ° 1,097,519.4 989.79T (J)
[2]
Reduction of chromium oxides by methane-hydrogen gas was studied by Read et al.[2] and Qayyum and Reave.[3] Read et al.[2] concluded that the role of methane was to supply carbon, which diffuses into the oxide. In accordance with their work,[2] the overall reduction Reaction [1] is a sum of Reactions [3] through [6]. 13CH4 13
Data Loading...
