An idea of vacuum suction through a porous graphite tube has been applied to maintain direct contact between FeO bearing molten slag and graphite by continuous removal of CO gas generated by the reduction. In this way examination of relative importance of the direct and indirect reductions has become possible. The primary slag had the composition of 20.5%Li₂O-38.4%CaO-41.4%SiO₂ (molar ratio 1:1:1). The initial concentration of FeO in the molten slag was varied between 1.5 and 12.5%. Experiments were made at 1300°C under argon gas stream. The rotation speed of the porous graphite tube was kept constant at 100 rpm.
The net effect of vacuum suction is the resultant of two opposing effects of enhancement by interfacial agitation by CO bubble evolution and interference by gas bubbles around the graphite tube. These two effects balance each other when the initial FeO concentration is below about 2%. For concentrations high than 2%, the rate of reduction is larger when the gas is removed by vacuum suction. The positive net effect indicates that the indirect reduction by CO gas is not so important as the direct reduction by graphite.
A possible mechanism has been proposed to explain how the direct reduction can be responsible for the reduction of iron oxide in molten slag by graphite.