The behavior of impurity species in Hall-Heroult aluminum cells

Impurities in Hall-Heroult aluminum reduction cells have an important effect on the operation of the process. In this work, the inert anode constituent materials Ni, Fe, and Cu were reduced from alumina-saturated 3.0 CR cryolitic melts (commonly known as baths) into and onto liquid aluminum substrates. The transfer of these reducible impurity species was found to be incomplete. About 55% of the Ni, 63% of the Fe, and 13% of the Cu that were in the bath were reduced and alloyed into aluminum in non-electrolytic experiments. A majority of the Ni and Fe species that did not alloy with aluminum remained in the bulk of the bath while essentially all of the Cu species that remained in the bath was found in a greyish "skim" bath layer 1-2 mm thick around the metal. This incomplete transfer is due to the formation of reduced species upon reaction at the aluminum surface and/or with dissolved aluminum in the bath. Due to interfacial tensions and the presence of an aluminum oxide barrier at the bath/aluminum interface, these reduced species were not readily incorporated into the liquid aluminum.; Experiments showed that when the cell was electrolyzed at a cathodic current density of 0.9 A/cm{dollar}sp2{dollar}, the reduced impurity species on the aluminum surface can be incorporated into the liquid aluminum due to a coupling with the depositing aluminum. In the presence of an oxidizing atmosphere, or when anode gases are evolved during electrolysis, it was found that the reduced impurities can be reoxidized and be subsequently reduced again resulting in a gradual transfer of the impurities into the aluminum.; The critical finding of this work is that impurities dissolved from current candidate inert anodes are in a reduced form that permits reoxidation and subsequent reduction thereby resulting in a loss in current efficiency of the process. (Abstract shortened with permission of author.)...