Basic Research On Reduction Of Metal Oxides With Cold Plasma Hydrogen

Reduction of oxides with hydrogen in place of carbon is considered as a green process, which is in accord with the continuable development policy. If hydrogen is applied as a reductant in metallurgical process, besides a solution to low-cast hydrogen source and its safe storage, it's necessary to find out a novel way to enhance the reduction of oxides with hydrogen at low temperature. In this paper, the reduction of oxides with hydrogen cold plasma was investigated. In terms of thermodynamics, the comparative research on reduction with plasma hydrogen and molecular one had been carried out. The effects of cold plasma on the reduction with hydrogen were explored. All done in this work are directive for the application of hydrogen reduction.On the basis of brief review of previous research work and the analysis of the chemical characteristics of cold plasma, a wide range of oxides with different reducibility, CuO, Fe₂O₃ and TiO₂, were used to be reduced in this study. The cold plasma hydrogen was generated by a DC pulsed electric field.The reduction of metal oxide Fe₂O₃ to metal Fe with cold hydrogen plasma was realized under 1500Pa ,490℃, but this reduction did not happen for using molecular hydrogen. The reaction path was as follows: Fe₂O₃→Fe₃O₄→Fe. As the reduction proceeded, the reaction started to accelerate. The reason might be that more active hydrogen species, which are of better reducing potential, participated in the reduction with the modification of the plasma sheath on the sample surface. The results of an additional experiment with a sample placed on a small insulting flake confirmed the above explanation. From this, it could be assumed that ionic and atomic hydrogen species were all involved in reduction and the sample potential was important. This provides a base for the design of the industrial equipments and technologic process. Between 390℃ and 530℃, the reaction temperature had no obvious influence on the reduction. At a high temperature of 680℃,a pressure of 1850Pa and the treatment time of 15min, Fe₂O₃ to Fe with hydrogen cold plasma(the discharge conditions are voltage-500V and current-0.3A.) was realized and only a few of Fe and FeO were detected when using molecular hydrogen. The plasma hydrogen is obviously more reactive than molecular one. With the increase of discharge voltage, gas pressure and the ratio of pulse duty, the thickness of reduced layer also increased. Thishad a close relation to the density of active plasma hydrogen species. Only could oxides placed on the cathode be reduced with cold plasma hydrogen generated by DC glow discharge.The reduction of CuO to metallic Cu with cold hydrogen plasma produced by a DC pulsed glow discharge was investigated under a pressure of 450Pa and a reduction temperature of 200 °C. The same reduction had not been achieved when using molecular hydrogen. The reaction proceeded by the sequential reduction of CuO(CuO-^Cu2O^* Cu). Similar to the reduction of ferric oxide, the thickness of the reduced layer increased with the reduction time and was influenced by the change of plasma sheath on the sample surface. Between 160"C and 300°C, the reduction of CuO with hydrogen cold plasma was independent of treatment temperature. It was also found that the inherent characteristics of the product metal had significant influence on the reductions.The reduction of refractory oxide TiO2 to T12O3 with hydrogen cold plasma generated by a DC pulsed glow discharge was realized at 2500Pa, 960°C and 60min. Only a few of TiioOig and TigOn were detected for using molecular hydrogen. Through more experiments, it might be possible for Ti2O3 to be further reduced. The present experimental technique is not suited to producing metallic titanium. It might be related to the reaction kinetics and the concentration of active hydrogen species on the sample surface. Further investigations should be required for the complete reduction of TiO2.In cold hydrogen plasma with moderate pressures, the main chemically active species are H, H+,H+2 and H+3.The density of monatomic hydrogen is greater that of ionic one. The order of the reducibility for these species is H+>H2+>H3+ >H. Though the densities of ionic hydrogen species are less, their reducing potentials are much higher in terms of thermodynamics. More ionic species in plasma are highly advantageous to the reductions of refractory oxides. The reduction ability for monatomic hydrogen was also discussed. It could reduce stable oxides such as Cr2O3, MnO and SiO2 to produce metals at the reduced temperature. This chapter is instructive to reduce refractory oxides with plasma hydrogen and to understand the mechanism on oxides reduction at lower temperature.Based on the above experimental results and plasma chemistry, the steps involved in the reduction of oxides with cold plasma hydrogen and their mathematical descriptions were analyzed in detail. The rate-limiting step was also discussed. When...