Wetting and Interfacial Reaction Investigations of Coke/Slag Systems and Associated Liquid Permeability of Blast Furnaces

Abstract

An interfacial study between metallurgical coke and synthetic slags representing typical slag chemistry of bosh regions of an operating blast furnace was conducted at 1 500°C using a horizontal laboratory furnace. The wetting characteristics of coke with slag was studied by measuring dynamic contact angle of sessile droplet of slag on coke substrate, while reaction kinetics of interface was monitored by measuring the amount of CO and CO 2 gases with the help of infrared analyzer. The interface chemistry was also examined by SEM/EDS/EPMA. The study demonstrated that coke wettability was strongly dependent both on the slag composition as well as coke chemistry such that low basicity (CaO/SiO 2 ) of slag, that is high amounts of oxides of silicon enhanced the wetting propensity of slag. The study further showed that the wettability of coke with slag was found to improve with increased extent of gasification, and was attributed to higher ash content of partially gasified cokes. The CO/CO 2 measurements of off gases from the reacting chamber were used to demonstrate that slag wettability with coke was primarily controlled by the kinetics of reduction of metal oxides at the slag/coke interface particularly that of silica. The study demonstrates that in addition to slag composition, coke properties could also influence the liquid permeability of lower zone of the blast furnace, and hence needs attention while optimizing the bosh slag composition for high productivity blast furnace operation.KEY WORDS: blast furnace; coke; slag composition; reactivity; wetting; slag-coke interaction; interfacial reaction. Where g SG , g LS and g LG are the surface and interfacial tensions at solid-gas, solid-liquid and liquid-gas interfaces, respectively. For a reactive system, the solid phase actively participates in the reaction at the interface, then the driving force (g SG Ϫg LS ) for wetting could increase due to the contribution of Gibbs free energy, which could be through mass transfer across the interface or formation of new phases at the interface. 8) As the slag/coke/gas is a reactive system, the contact angle and consequently the wetting behaviour will be influenced by the reaction of slag/coke carbon as well as in-situ reaction of coke ash/coke carbon. Consequently, slag and carbon compositions of interface will change continuously over the reaction duration, and hence wetting will be function of time often referred as dynamic wetting.In slag/carbonaceous systems, reduction of iron oxide and silica are known to be the main reactions affecting wettability. 4,5,9) On the other hand, the same oxides of silicon and iron are the main reducible oxides present in the bosh slag in lower part of the blast furnace. In addition, coke also contains oxides of iron and silicon which could also participate in reduction reactions. Therefore, it is imperative to look at the some of blast furnace reactions relating to silica and iron reduction phenomena. For example, direct reduction of molten iron oxide by solid carbon is known to occur acco...

Reactions Between Liquid CaO-SiO2 Slags and Graphite Substrates

“…The reduction of silica in the slag and the adverse effect of increasing slag basicity (decreasing silica activity) on the spreading of slag has been documented in several of these studies. [1,5,[7][8][9] Gas evolution during wetting of carbonaceous substrates with slag has also been described by several others. [1,3,4,8] Some industrial processes contain both a liquid metallic phase and a slag phase in contact with a graphite refractory, such as during slag refining of silicon.…”

“…[1,5,[7][8][9] Gas evolution during wetting of carbonaceous substrates with slag has also been described by several others. [1,3,4,8] Some industrial processes contain both a liquid metallic phase and a slag phase in contact with a graphite refractory, such as during slag refining of silicon. In slag refining of metals, proper wetting of graphite by liquid slag could prove to have a beneficial impact: The lifespan of graphite refractories could be prolonged by reacting with slag to protect the refractory from infiltration and reaction with the metallic phase.…”

“…[1][2][3][4][5][6][7][8][9][10] Most studies were in the context of ironmaking processes, and thus, they involved iron-bearing slags reacting and spreading on coal char, coke, and both natural and synthetic graphite substrates. [1][2][3][4][5][6][7][8] The reduction reaction of FeO by the carbon substrate has been found to play a major role in the wetting behavior in such systems. [3,4] Some studies focused specifically on the wetting behavior of nonferrous slags: Recently, Oh and Lee measured the wetting of CaO-SiO 2 -Al 2 O 3 -MgO slags on pressed coke substrates at 1773 K (1500°C); they attributed the process of slag wetting to the formation of SiC at the slag-graphite interface.…”

“…[10] Some studies made careful analyses of the kinetics of slag-graphite reactions, including measurements of the concentration of CO and CO 2 gas in the furnace off-gas using a mass spectrometer, and the change in slag composition with time. [4,7,8] Liquid slags react with carbon in surprisingly complex ways. The reduction of silica in the slag and the adverse effect of increasing slag basicity (decreasing silica activity) on the spreading of slag has been documented in several of these studies.…”

Slag Behavior in Gasifiers. Part II: Constitutive Modeling of Slag

“…In many cases, as Kang et al [16] observed, the fluidity of blast furnace slag is controlled by changing the slag chemistry. Kang et al [16] indicate that not only is the viscosity of slag important in these operations, but also coke/slag interactions, which could significantly impact the liquid permeability of the blast furnace. There is a great deal of similarity in the processes involving steel production and those of slag in coal gasification or combustion processes.…”