The Weight Loss in the Reaction Process between Siliceous Sand and Al in N₂ in a Presence of Carbon Material on Route of Gas Flow

Abstract

The compacts of the mixture of siliceous sand and Al were heated in N₂ atmosphere in a presence of carbon material on the route of gas flow. The weight change of the compacts was measured by thermogravimetric apparatus from a room temperature to 1500°C, and the phases in the compacts at various stages of heating were analyzed by using X-ray diffraction technique.
Nitriding of Al took place above about 500°C and its reaction rate increased at the melting point of Al (660°C), The exothermic reaction of AlN formation induced the reaction between siliceous sand and Al to form Si and α-Al₂O₃. AlN⋅Al₂O₃ was formed in the compacts heated at the temperature range of 660° to 1200°C. Si reacted with N₂ to form β′-Si₃N₄ above 1100°C and 15R-AlN was also formed in the compacts heated above 1300°C. Silica rich phases such as mullite, O′ and X phases were observed in the compacts which did not contain enough Al to reduce silica in the siliceous sand.
With increasing temperature, weight loss was observed and became considerable above 1400°C. From the phase change in the compacts, the weight loss was considered to be mainly caused by the evaporation of SiO formed through the following reaction:
Si₃N₄+Al₂O₃=3SiO+2AlN+N₂
Mullite, O′ phase and X phase disappeared from the compacts when they were heated at 1500°C for about 27h. They were considered to be reduced by CO gas which was formed by the reaction between carbon material and oxygen present in N₂ as impurity.
The weight loss kinetics at 1500°C followed parabolic rate law. The pore structure of the compacts seemed to have effect on the permeability of the gas phases.