Heat treatment of carbon anodes for the aluminum industry

Aluminum is produced by the electrolytic reduction of alumina dissolved in molten cryolite. This process requires carbon anodes which are consumed during the electrolysis. The anodes contribute roughly with 15% to the total aluminum production cost. If the anode quality is poor the share can as well reach 25 %. It is known that the anode behavior during aluminum electrolysis is significantly influenced by the anode baking process. However, there is a lack of knowledge to predict the anode behavior through given baking parameters. In addition, process control of industrial furnaces is still to a large degree empirical. Often the emission of industrial furnaces causes problems as deposits can accumulate in the exhaust system which leads to fire incidents. This investigation has three major goals: The prediction of the anode behavior during electrolysis for a given heat treatment. The understanding of the process control of industrial scale open-top anode baking furnaces. The emission control of the open-top anode baking furnace. The baking parameters are defined in this thesis as the anode heat-up rate and the baking level. Through pilot baking it is shown that the baking level is basically determined by the final baking temperature although the soaking duration has some influence too. The anode baking level can be estimated with a calculation for any given baking curve. Therefore the impact of the temperature and soaking duration on the real density anode property needs to be determined through a pilot study. The optimum baking parameters regarding the anode behavior during electrolysis depend on the material. Some anode materials react more sensitive to the heat treatment than others do and are therfore more difficult to handle in the industrial baking process. Through pilot scale anode baking it is shown how to predict the anode behavior during electrolysis. Process control of the anode baking in industrial open-top furnaces is complex since the anode baking is indirectly controlled by the combustion process inside the flues. Through measurements the combustion process is analyzed in this thesis. The results show that a furnace operated at its physical limits is difficult to control regarding to emissions. However, the gained knowledge allows to optimize existing furnace process control systems. The furnace emissions are mainly caused by insufficient combustion of the pitch fumes formed during the pyrolysis of the anode binder pitch. It is shown that emission problems cannot always be solved by a well chosen control system setup. In such a situation combustion control is required in addition to temperature control. A technology has been developed to measure online the combustion situation of each flue. For this purpose 2-color pyrometers, together with an intelligent signal processing, are required. This technology is integrated since 10 months in an industrial baking furnace control system. The results are promising. The developed technology is an important step for furnace operation at maximum productivity and minimum emission. A patent has been applied for the combination of online combustion analysis concurrent with temperature measurement (swiss apply No. 01598/02 by Mauriz Lustenberger and Felix Keller, R&D Carbon Ltd. Sierre).