An Electroanalytical Alumina Sensor for Use in Aluminum Production

Improved control of dissolved alumina concentration in the molten cryolite of Hall-Heroult cells is an important need in the aluminum smelting industry. A major incentive is avoiding "anode effect" that occurs when the cryolite bath's alumina content becomes too low (e.g., less than 2.2 w/o) to support normal electrolysis. Anode effect reflects production and accumulation of perfluorocarbons at the anode surface - these insulating gases cause it to dewet while driving voltages to between 30 and 50 volts. Anode effect thus results in the emission of greenhouse gases CF4 and C2F6 while raising process energy requirements. Inadequate knowledge of melt alumina content also leads to alumina overfeeding. The resultant cell bottom "muck" accumulation causes production interruptions. Numerous attempts have been made - with poor success - to develop in situ electroanalytical alumina sensors based on consumable carbon-based electrodes. The objective of this proposal is to develop a viable in situ electroanalytical alumina sensor employing an inert nonconsumable electrode material where the reaction is oxygen evolution. Based on work conducted by Oxley Research, Inc., and an evaluation of literature data, it has been concluded that certain metals could provide the required corrosion resistance, if the appropriate precautions are taken. Strong environmental and cost incentives exist for developing an online sensor for determining alumina concentration in cryolite melts used in aluminum production. Because no in situ sensor currently is available, operators now rely on external analysis of samples periodically withdrawn from the melt. An electroanalytical sensor would provide an electrical signal directly proportional to dissolved alumina concentration. This signal could be used in an automatic control loop to adjust the feed rate of alumina to the bath. The sensor could be incorporated with automatic or semiautomatic alumina feed equipment as part of a bath composition control package. A sizable world market for such a control package would be expected to develop rapidly, because it would be logical to fit every aluminum reduction pot with these units.