Timea Tordai
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 picture including the anode's density but also the butts' weight after usage of anodes during their cycle time in the pots. Optical light microscopy examinations of the porosity created on soft butts, related to permeable low density anodes, as well as on pilot anode cores oxidized in isobar laboratory conditions allowed laying emphasis on the prime importance of anode permeability in the dusting mechanisms. Some Chinese carbon plants are still using batch mixers with low temperatures and mixing intensity potential. As relatively low densification severity vibrators are also used, it felt adequate to study the role of porosity in the dusting propensity in an anode bench scale plant. For this purpose, a multi-factorial design of experiments was set up using two levels of pitching, mixing and forming intensities, and this at two levels of Na contamination and two baking temperatures. The levels of pitching, mixing (sigma blade versus high speed impeller mixers) and forming were selected in such a way that the difference in terms of apparent density reached a significant level (about 0.05 kg/dm3). Their individual impacts on the air and CO2 reactivity dust measured after laboratory oxidation tests were comparable to about 500 ppm Na contamination or 100 °C baking temperature differences, which is quite substantial for the anode behaviour in the pots. As the effect of the porosity differences on gas permeability exceeds one order of magnitude, it is obvious that the level of apparent density and therefore the pitching, mixing and forming conditions are of utmost importance for the anode dusting. The implications for equipment suppliers and plant designers are obvious: for the production of paste, efficient preheaters and intensive kneader mixers as well as controlled paste coolers need to be selected. Efficient forming machines working with vacuum conditions should be preferred and further developed in terms of densification severity and control efficiency. A parallel study on an ultra high intensive laboratory mixer, working with a mixture of fines and pitch only, i.e. producing after forming binder matrix electrodes, demonstrated the enormous potential left for porosity reduction of baked specimens. Substantial shrinkage of the dense green anodes during the baking step was observed on specimens produced under intensive mixing conditions. Concerning the dry aggregate recipe, bench scale trials with extreme ranges of fines content and fineness confirmed that, as long as the level of fineness is compensated by the fines content, optimum anode properties can be achieved with unchanged optimum pitch content. For a laboratory intensive impeller mixer, a range of fineness between 2500 and 5000 Blaine with a corresponding content between 42 % and 21 % provided anodes showing practically the same properties relevant for dusting. The conclusions drawn from the studies performed in the frame of this thesis allow a much better understanding on the traps of manufacturing world class quality anodes with zero dusting in a very different technological environment (as in China). In addition to this, the axis of development for western technology and equipment suppliers could be formulated based on the numerous quantitative pilot information gathered during this thesis work.
EPFL2007
