Publication
The disintegration of the carbon anodes during the electrolysis of alumina produces dust particles contaminating the electrolytic bath. This phenomenon, called dusting, is one of the most deleterious Al electrolysis failures, as it leads to a significant deterioration of the current efficiency, which causes severe economical losses for Al smelters. The mechanisms at work, related to carbon oxidations leading to the selective burning of the binder matrix that is responsible of the anode dusting, were reviewed through a detailed study of the literature. In the same manner, an overview of the "state of the art" knowledge, but also of the existing controversies in the field of raw material and anode manufacturing processes that affects the dusting, was delivered. The open questions concerning the root causes of dusting, as well as the ways to alleviate this phenomenon during the anode production, were considered. The process peculiarities of anodes made in China, as well as the atypical properties and behaviour of some anodes exported to the western world, were largely integrated into the frame of the experimental sections. Of particular interest was the fact that rather dense and highly calcined coke was used in the dry aggregate that does not contain recycled materials (baked scrap and butts). This situation results from the usage of a unique calcining process made in shaft (vertical) kilns, where the residence time exceeds those experienced in western rotary kiln/hearth type calcining plants by one order of magnitude. The effects of coke calcining and anode baking heat treatment temperature combinations were studied in a pilot scale simulating shaft and rotary kiln conditions. Representative types of green coke with different levels of volatiles and impurities (as S, V and Ca mainly) were selected for this study. The addition of Na contamination, in form of cryolite used as the main bath component was also examined in order to cover the aspects of the catalytic impact of contaminants present in the binder matrix on oxidation. As expected, the effects of baking were found to be of chief importance. However, the new element brought by this work is that not only the coke's sulphur content is important for its sensitivity to the negative impact of Na, but also its degree of calcination. Coke with a low calcining degree, measured by its real density/crystallite size, is showing a much better resistance to the catalytic effect of Na during the anode oxidation, and this especially when underbaking is a process issue. Recommendations on the optimum calcining heat treatment for western and Chinese calcining technologies are given as a conclusion of this part; this was done considering different scenarios of butts Na contaminations and baking issues in the anode production. The effects of desulphurization during calcining and especially during baking on the anode's physical, electrical and thermal properties were also addressed, so as to give a complete pic