Non Invasive estimation of aluminum concentration in Hall-Heroult reduction cells

The present best practice for the preparation of primary aluminum is by electrolysis of alumina in the traditional Hall-Heroult reduction cell. The process conditions in the electrolyte of this cell required for the reduction to proceed are sufficiently harsh to have precluded the implementation of in situ sensing of the electrolyte composition, specifically the concentration of the ionized alumina. This report reveals the theoretical basis for a non-invasive method for estimation of the ionized alumina concentration which does not require the use of any sensor in direct contact with the cell electrolyte. The proposed method can in principle be applied with equal efficacy to the so-called drained cathode cell designs and to cells having any anode composition, because only knowledge of the electrolyte conduction behavior is required a priori. For an operating cell, the proposed method requires only readily available electrical measurements and the facilities to process the acquired signals. The proposed method rests on the ability to identify certain characteristics of the transients in the reduction cell terminal voltages caused by the quasiperiodic introduction of alumina. It will be shown that these voltage transients manifest measurable properties, in a statistical sense, that should permit estimation of the ionized alumina concentration with a delay of one alumina feed cycle. The next logical step following the present work, consistent with the Aluminum Technology Roadmap [1], is to experimentally verify the predictions made here; no doubt practical refinements to the proposed approach will evolve during the course of experimentation. Successful verification of the proposed estimation method will permit the design of reduction cell control algorithms based directly on the mass balance of alumina in the electrolyte. This report assumes that the reader understands certain basic concepts important to the operation of electrolytic cells, and the Hall-Heroult cell in particular. References [2,3] provide such concepts in a manner accessible to the technically educated reader; reference [6] is a more thorough treatment