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NTRODUCTION

ALUMINUM is produced commercially using the Hall–He´roult process. It is an electrochemical process in which aluminum is reduced from smelter grade alumina dissolved in a cryolite-based molten salt electrolyte (bath). The reduction cells are connected in series under relatively high electrical current. Today the largest cells are operating at 600,000 Amperes.[1] Modern cells use point feeding technology to replenish alumina as it is consumed by the electrolytic process. A relatively small amount of alumina, 1 to 5 kg depending on the technology,[2] is added periodically at the point breaker/feeder locations. Figure 1 depicts the top view of a modern high amperage cell, with feeder hole locations distributed along the center channel. The historical trend in the primary aluminum industry has been to increase the cell size and amperage over time. Most of the recent incremental amperage increase at existing sites has been accompanied by increasing the anode size, which collaterally reduces the bath volume and mass available to dissolve alumina.[3] Anode size increase is often achieved by extending the anode length towards the sidewalls of the cells. However, in many cases, the center channel width is also being reduced.

PASCAL LAVOIE, Chief Engineer, Light Metals Research Centre, MARK P. TAYLOR, Professor, Materials and Chemical Engineering; and JAMES B. METSON, Professor, Faculty of Science, are with the University of Auckland, Auckland, New Zealand. Contact e-mail: [email protected]. Manuscript submitted on November 26, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B

This not only reduces the total bath volume, but more importantly considerably reduces the bath volume and potentially circulation rate, in the dissolution zone. Moreover, the rise in energy prices has increasingly forced smelters to value higher energy efficiency over production, usually achieved through reducing the power input into the cells. This reduces the inter-polar distance, also reducing the bath volume in the cell.[4] A second trend in reduction technology focuses on economies of scale by building larger cells capable of operating at higher amperage. As cells get bigger, the number of point feeders has been increased, but not proportionally. For example, AP technology* over 30 *Originally designed and commercialized by Aluminum Pechiney. AP technology is a series of electrolysis cell designs and related equipment now owned by Rio Tinto Alcan.

years evolved from a ratio of 50 kA per point feeder at 180 kA to over 120 kA per point feeder at 600 kA. This means that as the cell size has increased, more alumina must be fed at each feeder location per unit time and into a smaller bath mixing volume. Failing to replenish the alumina concentration in the electrolyte or a subsequent failure to transport this enriched electrolyte into the inter-polar gap results in anode effects, a phenomenon where passivation of the anodes leads to decomposition of other fluoride species, forming CF4 and C2F6 gases.[5] Additionally, mounting evidence s