Influence of Chemical Compositions of Slag and Graphite on the Phenomena Occurring in the Graphite/Slag Interfacial Region V. SAHAJWALLA, A.S. MEHTA, and R. KHANNA Silica reduction reactions taking place in the slag/carbon interfacial region were investigated for synthetic/natural graphite in the temperature range 1500 °C to 1700 °C. Two silica-rich blast furnace slags, with low levels of iron oxide, were used in this study. Silica concentration in these slags, labeled as 1 and 2, was 30.80 pct and 36.80 pct with a respective basicity of 1.67 and 1.22. Reaction rate investigations were supplemented with wettability measurements on these systems with an aim to probe a possible interdependence between wetting characteristics and reaction rates of silica reduction. Wettability and slag/carbon reactions were studied in a horizontal tube resistance furnace in argon atmosphere, using the sessile drop approach. While the contact angles were measured by recording live images of the assembly with a charge-coupled device camera, the volumes of CO and CO 2 evolved were obtained from an analysis of off-gases with the help of a mass spectrometer. Reaction rates for silica reduction showed a wide variation for different systems. Synthetic graphite showed nonwetting behavior with both slags. Natural graphite, however, showed dynamic wetting with slag 2, resulting in low contact angles. This is attributed to the difference in the deposition of Si-based reaction products in the interfacial region, which in turn influences wettability. Temperature had a significant effect on both the wettability and silica reduction rate of the graphite/slag system. Activation energies for silica reduction in slags 1 and 2 with natural graphite were estimated to be 253 and 241 kJ/mol, respectively. Chemical composition of carbonaceous materials and slags were found to play a very important role in dictating overall reaction rates and wetting characteristics. I. INTRODUCTION AN understanding of slag/carbon interactions is of great significance due to their extensive application in a number of metallurgical processes, especially in the blast furnace ironmaking process. One of the major advancements in blast furnace technology has been the injection of pulverized coal through tuyeres to partially replace metallurgical coke as a source of heat and reductant. High injection rates for the pulverized coal (PCI), desirable for increasing the efficiency of the furnace, can seriously affect furnace stability with an increasing amount of unburnt char entering the blast furnace burden. [1,2,3] After the unburnt char leaves the raceway, it can either be partially consumed within the furnace or escape along with off-gases from the top of the furnace. The accu- mulation of unconsumed char within a blast furnace is a major concern because it can adversely affect the permeabil- ity in the burden and therefore the stable operation of the furnace. The consumption of the unburnt residual char can take place through reactions with the metal, the slag, and the gaseous phases. The interfacial phenomena between the slag and carbon are expected to play a major role in determin- ing the consumption of unburnt char because they could dic- tate the kinetics of reduction reactions. Wetting character- istics also affect the capture and the assimilation of the unburnt char by the slag phase. Blast furnace slags mainly contain iron oxide, silica, alumina, lime, and magnesia. In the temperature range of interest, 1500 °C to 1700 °C, iron oxide and silica are the two main reducible oxides present in the slag. In addition, the ash impurities present in the char also contain iron oxide and silica to varying degrees and could participate in reduction reactions. Current understand- ing of interactions between blast furnace slags and carbona- ceous materials, especially the influence of silica, is far from complete. In this article, we report wettability and reactivity measurements on synthetic/natural graphite with two silica- rich slags. Only a limited amount of work has been reported in the literature on the wettability of solid carbonaceous materials by molten slag. Nakashima and Mori [4] have reviewed wet- tability studies on a variety of materials including refracto- ries and graphite by liquid slags. They have reported that, while liquid slags wetted most refractories, they did not wet graphite. Towers [5] has observed that the contact angle of a blast furnace slag on graphite decreases with time due to the reduction of silica at the interface. The addition of small amounts of SiC to graphite and an increase in the sulfur con- tent of the slag have also been found to improve the wetta- bility. The reduction of silica and the formation of SiC at the interface were believed to be responsible for the wetting of graphite by such a slag. In our earlier work, [6] we have also observed that the wettability of various carbonaceous materials (synthetic graphite, natural graphite, and coal-chars) by blast furnace slags was strongly influenced by the com- position of the slag and the carbonaceous material. This METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 35B, FEBRUARY 2004—75 V. SAHAJWALLA, Professor, and R. KHANNA, Research Associate, are with the School of Materials Science and Engineering, The University of New South Wales, Sydney NSW 2052, Australia. Contact e-mail: veena@unsw.edu.au A.S. MEHTA, formerly Ph.D. Student, School of Materials Science and Engineering, The University of New South Wales, is with Tristor Steering and Suspension Australia Ltd., Sydney, Australia NSW 2204. Manuscript submitted January 2, 2003.