Part V – May 1969 - Papers - Dissolution of Alumina in Carbon-Saturated Liquid Iron

The rate of dissolution of alumina in carbon-saturated liquid iron has been studied experimentally in a system where alumina was in the form of a cylindrical rod immersed in an iron bath contained in a graphite crucible. Data obtained consisted of the concentrations of aluminum in the melt as a function of time. In the case of static experiments, the data are shown to agree with theoretical prdictions based on the diffusion of aluminum.. The rate of dissolution was greatly increased by the rotation of the alumina rod. It is concluded that the diffusion of aluminum from the alumina/metal interface is the rate-controlling step. In the past, thermodynamic investigations of systems encountered in ferrous process metallurgy have received widespread attention. More recently, considerable work has been devoted to the study of kinetics associated with these systems in an effort to determine their rate controlling mechanisms. The alumina-iron system is of great importance in ferrous metallurgy. Yet information concerning kinetics of reaction in this system is seriously limited. The present study was made in order to establish the rate-controlling step for dissolution of solid alumina in liquid iron. LITERATURE REVIEW A number of papers concerning dissolution of solid metals in liquid metals have been reported in the literat~re. Generally, for these simple systems, dissolution is controlled by mass transfer of the dissolving species. Complex systems involving dissolution of solid metal carbides and oxides in liquid metals and slags have been studied to a much lesser extent. Skolnick5,6 reported on the reaction between liquid cobalt and poly-crystalline cylinders of tungsten carbide, in which the cylinders were dissolved while being rotated about their longitudinal axes at various speeds and temperatures. As a result of unexpected preferential grain boundary attack by the liquid cobalt, large errors in the measured dissolution rates occurred because of loss of tungsten carbide grains to the liquid cobalt. Nevertheless, it was possible to establish that the liquid Co-W carbide reaction was not controlled by mass transfer. In a similar approach, cooper7 was able to show that artificial sapphire rods, (alumina single crystals) dissolving in lime-alumina-silica slags obeyed a mechanism of mass transfer control. Here, again, the rods were rotated at various speeds and temperatures, and the process was followed as a function of these variables. Forster and Knacke8 took a practical approach to reaction between slags and refractories. By blowing argon through refractory cylinders of silica, silli-manite, or dolomite and directing the gas to rise along the slag-refractory interface, it was possible to increase the rate of mass transfer. Although the method was admittedly crude, it nevertheless permitted an evaluation of the relative stabilities of refractories with respect to slag attack. Data were interpreted on the basis of mass transfer control. EXPERIMENTAL TECHNIQUE Apparatus. An illustration of the apparatus used in this study is shown in Fig. 1. The furnace consisted of a Morganite recrystallized alumina tube wound with a molybdenum coil. A secondary molybdenum heater was mounted around the upper half of the primary coil to aid in controlling the thermal gradient within the furnace. The primary heater tube was 3 in. in ID and 30 in. long. A reducing mixture of 95 pct N and 5 pct H was maintained around the heating elements. Thermal insulation was provided by alumina powder. The chamber within the primary combustion tube contained a boron nitride block near the top to assist in controlling the thermal gradient to the furnace and also to provide a bearing surface for the rotating graphite shaft. The outside diameter of the graphite shaft was $ in. A separate threaded graphite specimen holder was screwed into the end of the shaft. The holder contained a tapered hole drilled into the end to guide the oxide specimens as they were pressed into it for mounting. Additional guidance for the rotating graphite shaft was furnished by a water-cooled bronze bushing attached to the top of the furnace. A steel clamp was fastened to the upper end of the graphite shaft and rested on a thrust bearing; the shaft and clamp were driven by a dc motor through a set of gears. Two O-rings located immediately above the bronze bushing maintained a gas-tight seal about the graphite shaft. The lower half of the alumina tube housed the crucible and charge, which were placed on a 3/4-in. diam movable alumina support tube. With this arrangement, charges could be inserted into or removed from the furnace while the hot zone was maintained at or above 1000°C. To control the temperature of the furnace, the thermocouple was mounted inside the support tube and in contact with the crucible bottom. Stray electric fields in the furnace were of sufficient intensity to cause erratic indications by the thermocouple. By enclosing the thermocouple protection tube in a molybdenum sheath and grounding this shield, the problem was eliminated. Output of the thermocouple went to an automatic continuous balance controller. Procedure. A typical run was as follows. First, electrolytic iron was premelted in graphite crucibles and cast into graphite molds with the same configura-