Study On The Wetting Behaviors During The Iron Ore Sintering

In China, blast furnace?BF? technology currently occupies the dominant position in ironmaking process, and most of the BF utilize sinter with a small amount of pellet as the raw materials. Thus, high-quality sinter is crucial essential for smooth running of smelting operation in BF and for producing high-quality molten iron. After spring up in 1960 s, the calcium ferrites?CF? sintering theory gradually became the mainstream theory of sintering technology and made significant contribution. As the main binding phase of fluxed sinter, the structure and property of CF is critical to the quality of iron ore sinter. For the past few years, the iron and steel enterprises faced with the stresses, such as the decreasing of production profit, the stringency of environmental protection and pollutant discharge, the cruel competing with other enterprises, which caused by excess production capacity. Hence, in order to cut down the cost of raw materials, many iron and steel enterprises positively utilize low grade ore with much harmful elements and gangue to deal with these stresses, which results in the frequent fluctuation of the chemical composition and size distribution of raw materials. Finally it influences the liquid phase formation, assimilation and solidification phenomenon in the sintering process, further influences the sinter quality. Therefore, maintain the quality of sinter has became a big complication under condition of complex raw materials.This study is supported by National Natural Science Foundation of China?NSFC No. 51104192 and 51522403?, and aim to deepen the understand of liquid phase formation, assimilation and solidification phenomenon by studying the wetting behavior of calcium ferrite series slag with gangue and flux during the sintering process. In this study, dispensed drop method is used to measure the relationship between apparent contact angle of the slag and time. At the same time, interfacial structure and spreading mechanisms were also discussed. The main conclusions includes:1) For the wetting behavior of CF-based slag, CF2 and C2 F with Al₂O₃, the equilibrium contact angles ranged from 10o to 30o, namely all the slag were wetting well with Al₂O₃. Dissolution of the substrate was the driving force during the spreading process. The wettability of Al₂O₃ by CF slag was improved by adding a certain amount of MgO, SiO₂ or TiO₂.2) For the wetting behavior of CF-based slag with Al₂O₃, except for CFM system, the slag was divided into three layers after wetting. Layer I near the slag-substrate interfacial was composed of lump Fe2O3 solid solution. Layer III at the surface of slag was composed of un-continuous lath-shaped CaAl2O4 crystal. Layer II between Layers I and III was composed of calcium ferrite solid solution which containing a certain amount of Al₂O₃. For CFM system, the main body of the slag was a CF solid solution containing Al₂O₃ and MgO. At the same time, lump CaAl2O4 and Fe2O3 solid solution uniformly distributed in the main body of the slag.3) The slag was divided into three layers after Al₂O₃ wet by CF2. Layer I at the upside of the slag was composed of interlaced lump CaAl2O4 crystal, lamellar Fe2O3 solid solution and rod Ca2?Fe,Al?2O5 crystal. Layer III at the slag-substrate interfacial was a dissolving transition layer of Al₂O₃. Layer II between Layers I and III was composed of large lump Fe2O3 solid solution.4) After Al₂O₃ wet by C2 F, a dissolving transition layer of Al₂O₃ was formed at the slag-substrate interfacial. The main body of the slag was uniformly composed of Ca2?Fe,Al?2O5 and lump Fe2O3 solid solution. At the same time, rows of small lump or strip CaO?Al₂O₃ distributed in the main body of the slag.5) For the wetting behavior of CF-based slag with MgO, the equilibrium contact angles ranged from 4o to 11o, namely all the slag were wetting well with MgO. For the wetting behavior of CF2 with MgO, the apparent contact angle first quickly decreased from 14.5o to 8.6o and then continuous slowly decreased. The system did not attained a equilibrium state finally. For the wetting behavior of C2 F with MgO, the slag quickly diffused into MgO after melted so that no droplet formed during the wetting process.6) For the wetting behavior of CF-based slag with MgO, except for CFM system, the wetting process was divided into four stages: dissolving driving, interim, interface chemical reaction driving, and equilibrium. After wetting, the slag was divided into two layers on basis of mineral composition: Layer I near the slag-substrate interfacial was plentiful of MgO and Fex Oy; Layer II was composed of interlaced CF and C2 F.7) For the wetting behavior of CF2 with MgO, most of the slag diffused into the substrate and formed a thick diffusion layer. The main body of the slag was composed of calcium ferrite crystal, and a little CF2 generated between the calcium ferrite crystal. On the other hand, FexOy, which diffused into the substrate, reacted with MgO and generated iron-magnesium spinel and monoxide?MgO·FeO?.8) For the wetting behavior of C2 F with MgO, all the slag diffused into the substrate and formed a thick diffusion layer?centimeters?. The main body of the diffusion layer contained MgO and a little FexOy, and plenty of irregularity component containing FexOy, MgO, CaO inlaid the main body.9) For the wetting behavior of CF-based slag with SiO₂, the apparent contact angle was small than 40o for the whole wetting process, namely CF-based slag was wetting well with SiO₂. After melted, the slag was first spread driven by dissolution, then the spreading width maintained constant and the slag diffused into the substrate. Finally, all of the slag diffused into the substrate, and the wetting process translated into the diffusion process. After wetting process, a radiate diffusion layer edge formed in the substrate. The main body of the diffusion layer was a black crisp phase, and the diffusion layer edge was a thin off-white CaO·SiO₂ layer.10) For the wetting behavior of CF2 with SiO₂, the apparent contact angle was small than 20o, namely CF2 was wetting well with SiO₂. During the wetting process, SiO₂ dissolved into the slag nubble by nubble. After wetting process, a pure SiO₂ layer formed at the outer edge of the slag; the main body of the slag composed of Fe2O3, 2CaO·SiO₂ and SFC solid solution. At the same time, plenty of tiny lump Fe2O3 solid solution and lump or rod SiO₂ distribute in the main body.11) For the wetting behavior of C2 F with SiO₂, the apparent contact angle was small than 25o, namely CF2 was wetting well with SiO₂. Dissolution was the driving force of the wetting process. After wetting process, a pure SiO₂ layer formed at the outer edge of the slag; the main body of the slag composed of Fe2O3, 2CaO·SiO₂ and SFC solid solution. At the same time, lots of cross SiO₂ nubble relative uniformly distributed in the main body. Some of the SiO₂ nubble tidily arranged a row, and the other random distributed in the slag.