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State observer for the aluminum reduction process

The reduction of aluminum ore form alumina to metallic aluminum is done by dissolving the core in a cryolite bath at approximately 950 degrees C and passing DC current through the solution. Current flow through a typical cell can range from 75,000 to 200,000 Amperes, generating extremely strong electromagnetic fields. Common mode voltages of up to 800 VDC occur on reduction cell frames. Although the process is both complex and extremely energy intensive, it is generally only crudely instrumented because the hostile environment destroys any sensor placed in the bath. This paper describes a new approach to aluminum cell control employing the measurement of 'peripheral' process variables and the use of extensive modeling of the cell dynamics. A form of state observer is used to deduce critical process variables from the measurement of available related parameters. An experimental system is being installed at Century Aluminum and will be tested and refined over the coming year.