Iron ore reduction with hydrogen-containing low-pressure inductively-coupled rf plasma

Kurzfassung

Due to traditional coal-coke-based processes in iron- and steelmaking, which generate an average of 1.92 tons of carbon dioxide emissions per ton of crude steel cast and a total production of 1.9 billion tons [1], the iron and steel industry is responsible for approximately 10% of the global carbon dioxide emissions [2]. For the decarbonisation of the ironmaking process, sustainable alternatives like hydrogen reduction are investigated. Hydrogen as reductant is promising since it offers thermodynamic and kinetic advantages and the production of water as byproduct instead of carbon dioxide [3]. Hydrogen in its plasma state has further advantages including the electron-impact activation of hydrogen molecules through vibrational excitation and dissociation in combination with local heating of the reduction interface [4]. Approaches with arc discharges including melting of the processed iron ores are investigated but also non-thermal applications are feasible [5,6]. Some of the current investigations include plasma diagnostics but the knowledge about the correlation between plasma parameters and reduction rates is limited. To investigate the impact of plasma parameters on reduction rates, experiments on hydrogen plasma reduction of iron ores are conducted with simultaneous monitoring of plasma parameters in an inductively-coupled radiofrequency (RF) plasma setup at low pressures implying low process rates but offering enhanced accessibility to plasma diagnostics. The plasma reactor is a modified Gaseous Electronics Conference (GEC) RF Reference Cell with standard upper Inductively-Coupled Plasma (ICP) flange [7]. The 5-turn-coil antenna placed over a quartz window is powered by a RF generator at 13.56 MHz with powers up to 300 W. The base pressure of the system is below 10^-6 mbar, operating pressures are in the order of 10^-1 mbar in an argon-hydrogen gas mixture with ratio 90:10. The samples are kept on a heating plate approximately 5 cm below the RF coil and can be heated up to several hundred °C. A differentially pumped mass spectrometer is connected to the process chamber for analysis of the species participating in the reduction process. A VI-sensor is integrated in the setup to analyse the electrical circuit. Microwave interferometry is applied to measure the line-averaged electron density in the plasma chamber during the reduction process. Further plasma parameters are evaluated by means of Optical Emission Spectrometry (OES). An actinometric approach for simultaneous evaluation of the dissociation degree of hydrogen and the mean electron energy in the plasma is applied [8]. The iron ore samples are prepared by a drop-casting method [9]. Hematite powder (grain size <= 10 µm, purity 99.9 %) is dispersed in isopropanol or distilled water and this mixture is drop-cast and dried on quartz wafers with an area of 10 mm x 20 mm. Alternatively, the cleaned wafers are immersed in a suspension and the solvent is evaporated slowly. The resulting samples contain hematite powder masses of several hundred µg. Sample analysis is performed by means of gravimetry, energy-dispersive x-ray spectrometry (EDS) and x-ray diffractometry (XRD). For quantitative analysis of the sample composition, XRD measurements with Co-K-alpha radiation and subsequent quantitative phase analysis with Rietveld refinement are performed. After processing times below one hour, pure iron is observed in the samples. Preliminary results indicate that complete removal of the hematite phase is achievable within a few hours of processing. In the first reduction experiments, measured electron densities were in the range between 1 and 3·10^11 /cm^3, dissociation degrees were estimated from 0 % to 4 % and mean electron energies from 4.5 to 6.0 eV. In further experiments, by variation of processing conditions like the sample temperature, the gas pressure and the plasma power, the dependence of reduction rates on these settings and simultaneously measured corresponding plasma parameters like the electron density, the mean electron energy and the content of reactive species is investigated. The results of these experiments will be presented and discussed in the contribution.