Carbon consumption and current efficiency in hall-héroult cell

In the aluminum industry, the carbon consumption (CC) expressed as the weight in kilograms of carbon used per kilogram of aluminum produced is one of the most common factors for evaluating reduction cell performance in terms of raw material consumption. According to the well-accepted electrochemical Reaction with the exclusive formation of carbon dioxide and the main back reaction responsible for a loss of current efficiency, the expected relationship between the carbon consumption and the cell current efficiency is given. To this theoretical value, carbon losses due to carbon dioxide reactivity, air oxidation, and dust formation should be added to obtain the usual industrial net carbon consumption values. Linear regression analysis performed on these values give a least-squares slope equal to 0.685 plus or minus 0.06. This value is twice as large as the one expected from the given equation, and suggests a completely different back reaction mechanism to account for the greater carbon consumption dependence on the current efficiency. The simplest approach consistent with this observation is to consider the direct reaction of the aluminum present in the electrolyte with the carbon of the anode to form Al4C3 and because of the instability of the carbide in the dioxide atmosphere, its complete decomposition by carbon dioxide.