Improvement of Sinter Softening Property and Reducibility by Controlling Chemical Compositions

Increasing permeability of packed materials in a blast furnace and decreasing reducing agent rate at a blast furnace process take an important role to improve productivity. In order to increase permeability and decrease reducing agent rate, sinter softening property during reduction and sinter reducibility are important.Effects of sinter compositions (CaO, SiO2, Al2O3, MgO, FeO) on the sinter softening property during reduction and sinter reducibility were examined by samples produced from pot scale sintering. These compositions were controlled by lime stone, serpentine, and coke breeze content and varying iron ore source in raw sinter materials. And sinter softening property was estimated by vertical pressure drop of sinter packed bed (70 mmφ×100 mm) during sinter reduction with raising temperature to 1 600°C under vertical loading force (9.8×104 N/m2). This pressure drop is caused by melt formation from slag compositions in sinter above 1 000°C. And sinter reducibility was estimated as reduction degree by CO(30%)-N2 gas under constant temperature conditions (900°C, 1 100°C).Results were obtained as follows.(1) Low SiO2 and low CaO sinter had superiority of sinter softening property. That is small pressure drop caused by decreasing SiO2-CaO-FeO melt.(2) And low SiO2 and low CaO sinter had high reducibility at both 900°C and 1 100°C. It is considered that decrease of these compositions affects restriction of calcium silicate containing ferrous oxides.(3) High MgO and high FeO sinter using dolomite instead of serpentine and blended with high coke breeze also had superiority of sinter softening property. Adding MgO to phase of CaO-FeO-SiO2 raises melting temperature. The raising melting temperature is considered to make high pressure drop region to be narrow.(4) And high MgO and high FeO sinter had low reduction degree at 900°C but kept high reduction degree at 1 100°C. X-ray diffraction pattern for the sinter indicated that it contained much magnetite structure (magnetite and magnesioferrite) instead of hematite. This mineral composition is consistent with low reduction degree at 900°C. On the other hand, keeping high reduction degree at 1 100°C is considered to correspond not to promote silicate (FeO-2SiO2, FeO-SiO2-CaO) formation. By X-ray micro analyzer, it was found that calcium ferrite contained more Si and more Mg ion with increasing MgO in sinter. Increasing Si ion in calcium ferrite is considered to be the cause of restricting silicate formation.(5) Al2O3 content has little effect to sinter softening property.Summarized these results, low SiO2, low CaO, high MgO and high FeO sinter has a good softening property and high reducibility. These characteristics indicate good performance as raw material in a blast furnace.