Deoxidation studies : The interaction of vitreous silica with molten aluminium and systems of aluminium, manganese, iron and silicon.

The interaction of aluminium with silica and silicates isthermodynamically inevitable and leads to reaction effects in many systems. The resultant reactions can continue over a wide temperature range including both liquid and solid state systems such as molten aluminium handling units and the heat treatment of aluminium deoxidised steel. The development of the reaction product layer under controlled conditions has allowed the reaction rate constants for the reaction between liquid aluminium and vitreous silica to be calculated as a function of temperature. These determinations show that the aluminium reduction of the silica is almost entirely controlled and maintained by the transport of the metallic species through the porous matrix of a solid oxide reaction product. At temperatures between 760°C and 860°C the reaction product layer is porous as a result of volume changes of phases formed during reaction and the reaction rate increases with temperature. At intermediate temperatures the reaction progress is interrupted and the reaction rate indeterminate probably as a consequence of the formation of a compact matrix resulting from volume changes associated with polymorphic changes of the alumina formed. There is also some speculation in relation to a spinel phase formed between silica and alumina in the reaction front at intermediate temperatures. At temperatures above 1110°C the rate of reaction becomes a function of the sintering of the alumina in the product layer with the formation of a fissured matrix and temperature, where the process is additionally controlled by the composition of saturated liquidus of the metallic system involved. It is clear that the morphologies of the reaction product layer dictate the variation in the rates of reaction observed over the different temperature ranges. It is proposed that alloying the liquid aluminium with silicon up to an equivalent of 15 atomic percent does not affect the progress of the reaction at all. Equilibrium saturation of liquid aluminium with iron or manganese effectively reduces the solubility of silicon at specific temperatures thereby reducing rates of counter-diffusing of silicon and aluminium and hence the rate of reaction.