Reaction of silicon carbide with fused coal ash. Final report. [4 refs

Abstract

The reaction of two grades of commercial silicon carbide (Carborundum KT and Super KT) with fused coal ash slags (acid and basic) in a synthetic coal combustion atmosphere have been studied. Computer calculations of chemical equilibria predicted a thermodynamic instability of the carbide in this environment. However, experimentally observed reaction rates were found to be exceedingly slow in either acid or basic ash slags. Both grades of silicon carbide were found to have equally good long term resistance of the SiC phase to corrosive attack by either sintered or fused ash at temperatures to 1250/sup 0/C (basic ash) and 1450/sup 0/C (acid ash). Super KT silicon carbode suffered extensive loss of free silicon from the entire outside surface of tubes exposed to both acid and basic ash slags in short times (less than 50 hr.). The effect was most pronounced in acid slags at temperatures above 1250/sup 0/C. The KT grade of SiC was much more resistant to this form of attack and suffered only a superficial loss of silicon in random locations. The loss of silicon occurs by the reaction of excess (free) silicon in the carbide body with particles of metallic iron in the slag to form lowmore » melting silicides and eutectics. Free iron is generated by the reduction of iron oxide in the fused coal ash slags on contact with silicon and silicon carbide. The attack can be minimized by limiting the service temperature to 1200/sup 0/C maximum and burning coals with a basic ash low in oxides of iron. In addition, silicon carbide grades such as KT in which the free silicon is not present is an interconnected matrix phase will have the best resistance to this form of selective attack.« less