Influence of Sulfur Species on Current Efficiency in the Aluminum Smelting Process
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O2 ¼ 1=2Al2 O3 þ 3=2CO:
INTRODUCTION
CURRENT efficiency is a vital indicator of energy efficiency in aluminum electrolysis. Power consumption is an important contributor (30 pct) to production costs in an aluminum smelter.[1] Even a 1 pct increase in current efficiency can substantially increase revenue. Efficient use of electricity is important not only with respect to budget but also from an environmental point of view. As aluminum smelters are known to be heavy industrial users of electricity (13 to 14 kWh/kg Al), the size of their CO2 footprint depends mainly on the source of energy production (e.g., 3.94 kg CO2/kg Al in the case of hydropower and 17.54 kg CO2/kg Al when using energy from coal).[2] Modern electrolysis cells can achieve current efficiencies of up to 96 pct.[3] The main mechanism for reduction in efficiency is via back reactions between dissolved metals in the electrolyte and dissolved CO2, producing dissolved alumina and CO gas.[4] Other contributors to current efficiency loss include the presence of impurities, electronic conductivity, metal short circuiting the anode, metal losses to the cell lining, and physical losses.[5] The total cell reaction and the back reaction are shown in Eqs. [1] and [2]: 1=2Al2 O3 þ 3=4C ¼ Al þ 3=4CO2
½1
RAUAN MEIRBEKOVA, Project Manager, and GUDRUN SAEVARSDOTTIR, Dean, are with the School of Science and Engineering, Reykjavik University 101, Reykjavik, Iceland. Contact e-mail: [email protected]; [email protected] GEIR MARTIN HAARBERG, Professor, and JOMAR THONSTAD, Professor Emeritus, are with the Department of Materials Science and Engineering, NTNU, 7491, Trondheim, Norway. Manuscript submitted July 8, 2015. Article published online February 4, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
½2
Reduction in current efficiency due to the presence of impurities mainly comes from noble impurities or impurities with different oxidation states that undergo cyclic redox reactions. Dissolved impurities nobler than aluminum will co-deposit at the cathode. These deposition reactions consume current, which would otherwise have contributed to aluminum production. Multivalent impurities are responsible for more loss in current efficiency than noble impurities of comparable molar concentrations. Phosphorus is a well-known example of such an impurity. The influence of phosphorus on current efficiency has been discussed by a number of authors,[6–11] who reported reductions in current efficiency of up to 1 pct for every 100 mg/kg (ppm) increase in phosphorus concentration in the electrolyte. To date, sulfur has not been recognized as a harmful element in aluminum cell operations. Its main recognized negative contribution is the environmental impact due to emissions of a range of sulfur-containing pollutant gases. Some of these gases contribute to local air pollution and acid rain.[12] Modest levels of sulfur have even been found to have a protective effect against air burn of carbon anodes and reactivity with CO2, as sulfur inhibits the catalysis of these reactions.[13–16] As a result, the presence o
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