Effect of quaternary basicity on reduction behavior of iron-bearing dust pellets

The treatment of iron-bearing dusts and sludges by the rotary hearth furnace process has the advantage of sufficient utilization of valuable metals and a high impurity removal rate, but the lower strength of the metallized product needs to be addressed. The effects of quaternary basicity R₄ (w(CaO + MgO)/w(SiO₂ + Al₂O₃)) on the reduction behavior and physical and chemical properties of metallized pellets, including phase composition, compressive strength, microstructure and soft melting area, were investigated with FactSage thermodynamic software and experiments. The strength of metallized pellets depended on the gangue composition, such as CaO, MgO, Al₂O₃ and SiO₂, due to the altered chemical composition, physical phase composition, microscopic morphology and stability of the slag phase. The reduction of carbon-bearing pellets was significantly promoted by suitable basicity. The lower basicity (R₄ < 1.4) facilitated the formation of low melting point iron-containing compounds from SiO₂ and Al₂O₃ with FeO, resulting in increased liquid phase generation, but lower metallization rate, due to the hindered precipitation and growth of iron grains. Interestingly, the higher basicity (R₄ > 1.8) also increased the amount of liquid phase and improved the strength of the pellets, due to the granular iron crystals bonded into sheets. Notably, the main component of the liquid phase in high-basicity conditions was calcium ferrite. Although the additional amount of liquid phase was beneficial to the strength of the metallized pellets, calcium disilicate was formed at R₄ = 1.6, resulting in a reduction in the compressive strength of the pellets to 1521.9 N/pellet.