Behavior Of Slag System During Reduction Of Metallurgical Dust And Mud And Its Influence On Bonding Strength

The strength of metallized pellets is directly related to the distribution of metal phase,slag system behavior and porosity.The behavior of slag system in the reduction process has an impact on the structural densification and strength of the pellets.In this paper,the slag system behavior and its impact on the consolidation strength of metallized pellets prepared by direct reduction of metallurgical dust and mud are studied,and the causes of low strength and high powdering rate of dust and mud metallized pellets produced by rotary hearth furnace are discussed.Firstly,Fact Sage thermodynamics was used to simulate the distribution and formation conditions of low melting point slag phase of dust pellets in Ca O-Al₂O₃-Si O₂and Ca O-Mg O-Si O₂ systems of basic slag system,and the influence of FeO change on the distribution and formation conditions of low melting point slag phase of basic slag system.The results show that the content of Ca O in dust pellets is high and the alkalinity is large.Therefore,when the dust pellets based on basic slag system are roasted under the reduction condition of 1250°C,there is basically no slag phase melting or slag phase reaction with liquid phase formation.FeO can significantly reduce the temperature of liquid phase formation of basic slag system.With the increase of FeO content,the low melting point compounds formed in these systems have obvious melting effect on unmelted components such as Ca O and Al₂O₃.On this basis,the direct reduction experiment of metallurgical dust pellets was carried out to investigate the influence of the formation process of slag system and the behavior of alkaline materials in the formation process of slag system on the properties of slag system and the consolidation strengthening of pellets.It was found that the formation process of slag system of alkaline dust pellets was always accompanied by the participation of alkaline oxides and eventually formed the slag system of complex compounds containing calcium.Solid-state reaction was the basis for the formation of slag system.Alkaline calcium ferrite（Ca/Mg Fe₂O₄）was the initial product of solid-state reaction.A large number of intermediate products,dicalcium silicate（Ca₂Si O₄）,had good solid solubility,which provided conditions for the solid-state formation of calcium iron and calcium magnesium complex compounds.It is also one of the reasons for the expansion and pulverization of alkaline pellets at about 1000°C.FeO can promote the formation of liquid phase in the slag system of alkaline pellets at reduction temperature,which creates conditions for the large area migration,aggregation and bonding of the phase in the pellets,and the volume shrinkage of the liquid slag during the cooling crystallization process also significantly improves the strength of the pellets.Finally,through the visualization experiment,the influence of alkalinity change on slag composition and microstructure was investigated.It was confirmed that the increase of alkalinity would bring more Ca O and Mg O,promote the formation and development of solid phase reaction,and promote more FeO to participate in slag phase reaction,providing favorable conditions for the formation and development of liquid phase.However,Ca O would eventually inhibit the reduction of pellets,and a large number of uniform and aggregated reduced iron phases could not be formed to significantly improve the strength of pellets.