Alton T. Tabereaux

ANODE EFFECTS, PFCS, GLOBAL WARMING, AND THE ALUMINUM INDUSTRY

As concerns grow about the role that perfluorinated carbon compounds (PFCs) play in global warming, the aluminum industry is voluntarily exploring ways of reducing the volumes of CF4 and C2F6 generated during Hall-Héroult electrolysis. As these gases are produced only during anode effects, which are undesirable for other production reasons as well, aluminum producers are aggressively examining ways to reduce the number of anode effects that occur as well as their duration.

Reviewing advances in cathode refractory materials

Refractories for aluminum reduction cell cathodes are changing to meet the needs of new, energy-efficient long-life cathode designs. New refractories and modified versions of old refractories have been introduced, and information on their performance is becoming available. This article reviews some of the advantages and disadvantages of new and old cathode refractory materials.

Electrical Conductivity of Cryolitic Melts

A laboratory study was conducted to determine electrical conductivity of cryolitic melts by using a novel moving electrode technique. The newly-developed method proved to have distinct advantages over previously-used, classical techniques in accurately measuring the electrical conductivity of highly corrosive cryolitic melts. Results were obtained for a comprehensive range of cryolite ratio, alumina, calcium fluoride, magnesium fluoride, and lithium fluoride concentrations at different bath temperatures. The results were compared with that predicted by the Choudhary model. Differences in the electrical conductivity of low cryolite ratio and high concentrations of additives at low bath temperatures were found and discussed.

Lithium-Modified Low Ratio Electrolyte Chemistry for Improved Performance in Modern Reduction Cells

Modern aluminum reduction plants with large prebake reduction cells use essentially the same electrolyte chemistry, a low ratio bath containing 10 to 12 wt.% excess AlF3. Production results from modern reduction plants demonstrate that operating with low ratio bath contributes to a higher current efficiency, typically from 94 to 96%. Lithium is used as a bath additive to improve the performance, primarily in older cells, by increasing the electrical conductivity of the molten cryolitic bath and decreasing the bath operating temperature. Plant operational results were obtained using a combined lithium-modified low ratio (LMLR) bath in modern 180 kA prebake cells. The addition of lithium within a specific composition range was found to be beneficial in decreasing the cell voltage and unit energy consumption, and resulted in a significant improvement in the cell voltage stability. However, a decrease in current efficiencies was found to occur with the increase in LiF concentration, apparently as a consequence of the corresponding decrease in AlF3 content. The impact of the combined LMLR bath composition on electrolyte properties and cell performance is discussed.

Liquidus Curves for the Cryolite - AlF3 - CaF2 - Al2O3 System in Aluminum Cell Electrolytes

Measurements of cryolitic electrolyte freezing points in the quaternary system Na3AlF6 - AlF3 - CaF2 - A12O3 were conducted in order to resolve some of the apparent discrepancies reported by previous investigators. Rather than investigating just the boundaries of the quaternary system, a large number of tests were conducted in the “interior” of the defined system. This was done so that the results could be used to evaluate the freezing point of traditional bath chemistries. The freezing points of synthetic electrolyte samples were combined with accepted literature values for several of the binary boundaries to create a total data set covering the experimental range of 0 – 22% excess AlF3 (CR = NaF/AlF3 weight ratio = 1.50 to 0.90), 0 – 12% CaF2, and 0 – 5% A12O3. Five planes (one component held constant) in the experimental space were examined and fitted with isopleths for the electrolyte liquidus temperature. From the shape of the isopleths and using the complete data set, a single model for predicting freezing point temperatures was developed to cover the entire experimental range for the Na3AlF6 - AlF3 - CaF2 - Al2O3 electrolyte chemistry system.

Role of Heat Transfer and Interfacial Phenomena for the Formation of Carbon Oxides in Smelting Cells

Thermochemically CO should be the dominant product and various theories have been proposed to explain the electrochemical dominance of CO2. Following publication of the proposed correlation between current efficiency and cell gas composition by Pearson and Waddington [1], smelting operators have considered the presence of high amount of CO to be a direct indicator of poor cell performance. However substantial deviations occasionally occur in the gas composition [2, 3] yet the rigour of correlations and reaction mechanisms interpretation have not been questioned. As a consequence of anode gas composition trends associated with large multi-electrode smelting cells, and aided by supplementary data, the mechanistic interpretation for the formation of CO and CO2 during aluminium electrowinning has been re-analysed. The data indicates interfacial heat transfer to satisfy the entropic energy plays an important role in determining the proportions of the gas.

Sodium in Aluminum Metal of Operating Prebake Cells: Confirmation and New Findings

The concentration of sodium impurity in the aluminum metal pool of operating prebake cells has been shown to be an indicator of cell performance; in general, cells operating with higher current efficiency tend to have a higher sodium content in aluminum. The higher sodium content is due to the increase in sodium containing species in the electrolyte near the bath-metal interface as a consequence of reduced hydrodynamic stirring due to cell MHD forces, anode gases and disrupting cell operations. Correlations between the sodium content in aluminum metal and changes in potline amperage, ac-distance, cell operations, bath chemistry, etc. are discussed for individual cell studies and potlines. Correlations are demonstrated between sodium and other alkali/alkaline metals, (lithium, magnesium and calcium) in aluminum in equilibrium with the cryolite electrolyte.

In‐Situ Formation of Slots in Sederberg Anodes

Traditional carbon anode technology relies on the natural flow of gases from under the anodes during the aluminum reduction process. The anode gas bubbles generated on the bottom surface of carbon anodes during electrolysis are non-conductive and thus increase energy consumption as they increase the electrical resistance in cells. The use of single and multiple bottom anode slots across the entire bottom surface of prebake anodes is now a widely accepted practice to quickly divert anode gases into bottom slots to allow amperage creep. The slots have the potential to save about three percent of the energy required in the process depending on the number and design of slots. It has now been demonstrated in this work that vertical non-continuous slots can also be formed in-situ in self-baking VS Soderberg anodes by vertically inserting four rows/or layers of multiple aluminum plates into the top surface of the anode during charging anode carbon paste to cells. Extended plant tests confirm that these multiple slots significantly reduce the electrical resistance, lower the cell voltage, and thereby reduce the cell energy consumption in VS Soderberg cells; for example, the pot noise was found to be reduced 40%, (0.04–0.05 V) compared with traditional low noise Soderberg cells; pot noise was reduced by 80% (-0.200 V) when compared to high noise Soderberg cells.

The end of an era for Søderberg Technology in North and South America

In the 1970s twenty three Soderberg smelters located in North and South America had a primary aluminum capacity over 3 million tpy. The largest operating Soderberg smelter, Companhia Brasileira de Aluminio with a plant capacity of 475,000 tpy built the last Soderberg potline in 2007. Today there are only five Soderberg smelters operating with a capacity of less than 1 million tpy. Compared with prebake technology, Soderberg cells have higher production costs, they are more difficult to automate and they have the greatest environmental and health challenges. Health studies from the middle of the 1970s show a clear link between Soderberg tar fume exposure and the incidence of various types of cancer lead companies to propose a program of replacement. Starting in the late 1970s a number of programs and actions were taken to reduce PAH emissions and worker exposure to fumes, but in Canada they were always seen as stop-gap actions until replacement was completed.

Evaluation of Silicon Carbide Bricks

The combined experience and knowledge for testing and evaluating silicon carbide (SiC) bricks for their potential application in reduction cell linings are shared in this paper between a SiC brick manufacturer (AnnaWerk) and an aluminum producer (Reynolds Metals Company). Because of the recent introduction of new SiC bond phases and increase in global manufacture of SiC refractories, there is a need to conduct tests of SiC bricks to evaluate their effectiveness as a cathode lining material in reduction cells, but due to their inertness SiC bricks are very difficult to evaluate using standard laboratory refractory tests.