Wear Mechanisms of Carbon-Based Refractory Materials in Silicomanganese Tap Holes—Part I: Equilibrium Calculations and Slag and Refractory Characterization

Silicomanganese (SiMn) as an alloy supplies silicon and manganese to the steelmaking industry. It is produced through carbothermic reduction in a submerged arc furnace. The slag and metal are typically tapped through a single-level tap hole at 50 K (50 C) below the process temperature of 1873 K to 1923 K (1600 C to 1650 C). In one tapblock refractory design configuration, the tap hole is installed as a carbon tapblock and rebuilt during the life of the lining using carbon-based cold ramming paste. The carbon tapblock lasts for a number of years and ramming paste only for months. The purpose of the study presented here was to determine to what extent chemical reactions between carbon-based refractory and slag or metal in the tap hole of a SiMn furnace can contribute to wear of tap-hole refractory. The results of the study are reported in two parts. In Part I, the results of thermodynamic calculations of the potential for chemical reaction between carbon-based refractory material and slag or metal are reported. The results were tested experimentally using pure graphite and synthetic SiMn slag (produced from pure oxides). The paper also reports the composition, microstructure, and phases of industrial SiMn slag, and commercially available carbon block and cold ramming paste refractory materials. These compositions were used in predicted equilibria of refractory–slag reactions. Thermodynamic calculations suggest that reaction between SiMn slag and carbonbased tap-hole refractory is possible, and experiments with nominally pure materials support this. However, practical refractory materials are by no means pure materials, and contain secondary phases and porosity which can be expected to affect reaction with slag. Such reactions are examined in Part II.National Research Foundation of South Africa (Grant TP2011070800005).http://link.springer.com/journal/116632016-04-30hb201

Investigation of freeze linings in copper-containing slag systems: Part II. Mechanism of the deposit stabilization

A major industrial problem in high-temperature liquid reaction systems is the attack of furnace components by chemically aggressive molten reactants. Freeze-lining technologies involving the deliberate formation of controlled frozen deposits are increasingly being applied to extend the range of liquid bath compositions and process temperatures that can be used; this has resulted in significant increases in process performance and productivity. It has been widely assumed that the interface between the stationary frozen layer and the agitated molten bath at steady state consists of the primary phase, which stays in contact with the bulk liquid at the liquidus temperature, T . It has been shown in the current laboratory-based studies through the use of a cold finger technique that, at steady state and in selected ranges of process conditions and bath compositions, the phase assemblage present at the deposit/liquid interface is not that of the primary phase alone. The microstructural observations clearly demonstrate that the temperature of the deposit/liquid bath interface, T , can be lower than the liquidus temperature of the bulk liquid, T . These observations point to a significant change in the mechanism and behavior of the systems. To explain this phenomenon, it is proposed that the steady-state thickness of freeze linings is not the result of equilibrium freezing but rather represents a state of dynamic equilibrium that is critically dependent on the relative rates of crystallization, mass, and heat transfer processes, occurring close to and at the deposit interface. The mechanisms taking place in the boundary liquid layer involve both partial crystallization/remelting and continuous removal of solids. This finding has important implications for the design of the high-temperature industrial reactors and selection of ranges of melt chemistries and conditions that can be used. This finding means that temperatures below the liquidus can be selected for some processes, resulting potentially in significant savings of energy and increases in throughput of pyrometallurgical reactors. The findings are generic and are not limited to the specific chemical systems reported in the article