Hydrogen reduction of wustite single crystals doped with Mg, Mn, Ca, Al, and Si

Abstract

In order to investigate the reduction mechanism of wustite in the presence of impurities usually met in the ironmaking industry, single crystals have been prepared with Mg, Mn, Ca, Al, and Si as dopants. The amounts of dopant in the lattice is around 4,4,2.5,0.5, and 0.01 mol pct, respectively, at 800 ‡C. For reduction with pure hydrogen, from 600 to 950 ‡C, Ca is the most efficient for accelerating the process at high degrees of reduction (75 pct) Mg and Mn are also active in this respect. Al has only a slowing down effect. Si also slows down the reaction at temperatures between 600 <T < 850 ‡C, but it becomes accelerating atT > 850 ‡C. In the presence of 20 torrs of water vapor in the gas, Mg and Mn are less efficient and unable to prevent the same slowing down of reaction observed with pure wustite at around 850 ‡C and classically called the ‡rate minimum‡. Our interpretation of these results is mainly based upon the observations of microstructures of partly reduced crystals which show a change in the texture of the iron produced which can be correlated with the reduction rate. These observations lead to a possible explanation in terms of the role of inclusions of impurity oxides on the sintering process of the metal, correlated with their ability to dissolve into the wustite lattice. However, this suggestion cannot apply in the case of Si at low temperatures, and this element is therefore supposed to play a role in the stages of reaction associated with the surface of the crystals.