Reduction Behaviors of Pellets Under Different Reducing Potentials

Owing to the change of gas composition in top gas recycling-oxygen blast furnaces compared with traditional blast furnace, many attentions are attracted to the research on iron oxide reduction again. In order to study the influence of H2 and CO on the reduction behavior of pellets, experiments were conducted with H2-N2, CO-N2 or H2-CO gas mixtures at 1173 K by measuring the mass loss, respectively. It was found that the reduction degree increased with increasing the ratio of H2 or CO in the gas mixture, but the reduction with hydrogen was faster than that with carbon monoxide. The reduction degree could reach 96. 72% after 65 min for the reduction with 50% H2 + 50% N2, while it is only 53. 37% for the reduction with 50% CO+ 50% N2. The addition of hydrogen to carbon monoxide will accelerate the reduction because the hydrogen molecules are more easily chemisorbed and reacted with iron oxide than carbon monoxide. A scanning electron microscope was used to characterize the structures of reduced samples. Dense structure of iron was obtained in the reduction with hydrogen while the structure of iron showed many small fragments for the reduction with carbon monoxide. At the later stage of reduction with the gas mixtures containing carbon monoxide, the reduction curves showed a descending trend because the rate of carbon deposition caused by the thermal decomposition of carbon monoxide was faster than the rate of oxygen loss. Compared with the reduction with CO-N2 and H2-CO gas mixtures, H2 gas could enhance the carbon deposition while N2 gas would reduce this phenomenon. The results of X-ray diffraction and chemical analysis demonstrated that the carbons are mainly in the form of cementite (Fc3C) and graphite in reduced sample.