Reducing the incidence of anode effects at the Krasnoyarsk aluminum plant

Abstract

This article examines the reasons for the occurrence of anode effects on S-8BM electrolytic cells with self- baking anodes at the Krasnoyarsk Aluminum Plant. It discusses the possibility of reducing the frequency of anode effects based on data on the manner in which the electrical conductivity of the electrolyte is affected by changes in the content of elemental carbon in the electrolyte before and after cell maintenance. There is currently a school of thought which in essence states that the anode effect (AE) seen in aluminum produc- tion is a technological deviation that shifts the parameters of the electrolysis operation outside their optimum range of val- ues. Recent scientific findings on the anode effect and automated equipment that has been developed have made it possible to find ways of curtailing AEs and reduce the voltage associated with them (1-3). At the company RUSAL Krasnoyarsk (KrAZ) - the second-largest aluminum producer in the world - scheduled anode effects are an integral part of the mainte- nance of the electrolytic cells (4). However, reducing the number of AEs at KrAZ would alleviate the accompanying adverse environmental effects in the area of the plant and give it the opportunity to sell Emission Reduction Credits (ERCs) in accor- dance with the Kyoto Protocol. Features of the technology used at KrAZ to produce aluminum. More than 90% of the aluminum at KrAZ (about 910,000 tons) is made on electrolytic cells with a self-baking anode (SA). This was one of the world's most advanced tech- nologies when it was first introduced (1960-1970), mainly because it eliminated the need for the separate production of baked anodes. However, study of the basic principles behind the technology led to the conclusion that it should not be more wide- ly adopted. This conclusion was reached for the following reasons: the technology has a significantly adverse effect on the environment; it entails a large amount of time-consuming manual labor; the technology is difficult to mechanize; there is an alternative technology with excellent production indices. Today, plants that use SA technology are in need of modernization, mainly due to their large emissions of environ- mentally harmful substances: HF, CO, CO 2 , perfluocarbons (PFCs), alumina-bearing dust, benzopyrenes, etc. One of the main challenges that faced KrAZ in its modernization program - which was begun in 2000 - was to improve the efficiency indices by increasing the unit productivity of the main equipment (5). Implementation of the program did not have to lead to shortening of the service life of the electrolytic cells because of the heat imbalance that developed when current was increased from 155 to 172 kA. The accompanying increase in the amount of heat entering the bath was compensated for by making changes to several process parameters: