Evolution of Alpha Phase Alumina in Agglomerates Upon Addition to Cryolitic Melts

Evolution of Alpha Phase Alumina in Agglomerates upon Addition to Cryolitic Melts by N. Peter Østbø Thesis submitted in partial fulfilment of the requirements for the degree Doktoringeniør Norwegian University of Science and Technology Department of Materials Technology and Electrochemistry May 2002 Adventure with Alumina Perkin Medal Lecture 1946 Francis C. Frary, Director of Alcoa Research Laboratory “Aluminum oxide and its hydrates present a variety of amazing contrasts. From the hardness of sapphire to a softness similar to that of talc, from an apparent density of over 200 pounds to one of about 5 pounds per cubic foot, from high insolubility and inertness to ready solubility in acids or alkalis and marked activity, the properties can be varied over wide limits. Some forms flow and filter like sand; others are viscous, thick, unfil- terable, or even thixotropic. Crystals may be of any size down to a fraction of a micron in diameter, with various allotropic forms, and there are also amorphous forms. Some varieties have a high adsorptive power, others none at all. Some are catalyti- cally active, others inactive. But they are all converted into α - alumina (corundum) if heated hot enough and long enough.” Cited in “Industrial Alumina Chemicals”, by Chanakya Misra [118] i Summary Rapid dissolution of alumina upon addition to the cryolitic melt is crucial for the modern Hall-Héroult process for aluminium production. The formation of slow - dissolving alumina agglomerates may be detrimental, and irregular dissolution kinetics may cause the loss of process control. So-called anode effects may subsequently ignite, which are a major source of green-house gases from the primary aluminium industry. A literature review and the study of the theory of sintering provides the background for discussing the present work. The most probable mass transport mechanism in the transition alumina-fluoride-moisture system studied here is surface diffusion. Surface diffusion is a non-densifying mass transport mechanism that will result in coarsening (alumina grain growth) but only weak interparticle bonding since no macroscopic shrinkage is involved. Rapid mass transport is known to result when there is a simultaneous phase transformation, and this is the case when transition alumina transforms to α-alumina, catalyzed by the presence of fluorides. The main experimental techniques used in the present work were powder X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Sup- porting techniques used have been specific area determination by the BET method and simple thermo-gravimetric techniques. An optical furnace was designed and built in order to study the dissolution of tablet alumina agglomerates. A preliminary agglomeration study of preformed cylindrical alumina samples served to map some of the most important mechanisms involved when alumina powder interacts with alumina-saturated cryolitic melt. The con- ditions at the alumina-melt interface were studied, but it is concluded that the experimental method could not provide the necessary parameter control in order to study the agglomeration mechanism in further detail. The tablet agglomerate study is the major experimental contribution of the present work. The experimental method provided good control of the sample chemistry and well defined temperature and time variables. It is concluded that liquid cryolitic melt (NaAlF4) provides an effective mass transport route for the transformation assisted growth of α-alumina platelets. The platelets that initially form will provide the limited mechanical strength necessary for agglomerate formation and their persistence in a cryolitic melt. Alumina agglomeration may therefore take place with only partial, initial phase transformation. It is concluded that differences in the agglomeration ii behavior of various qualities of alumina may be the rate determining prop- erty for alumina dissolution kinetics in cryolitic melts. Differences in the agglomeration behavior may be due to a number of physical properties of alumina. It is argued here that the fundamental, but difficult to measure, alumina nano-structure may be most important. The alumina nano-structure is correlated to secondary alumina properties such as the α-alumina content, specific surface area (BET) and moisture content (MOI, LOI). In this study an X-ray diffraction line profile analysis using the Warren-Averbach method shows that there is a significant dif- ference in the nano-structure of the two smelter grade alumina qualities under study. This may explain the different agglomeration behavior that is observed. An optical study of tablet agglomerate dissolution in cryolitic melt proved to be largely unsuccessful due to severe corrosion of the quartz crucibles used. However, a proposed mechanism for the tendency for disintegration of alumina agglomerates, thus dissolving as “snow-flakes” is supported. The temperature response time in the tablet alumina samples was studied in order to determine the experimental limit of the shortest time period possible in the experiments. The exothermal γ → α transformation is observed for secondary alumina samples containing adsorbed fluorides. An interesting effect of the carbon content in secondary alumina is also shown. The moisture content of smelter grade alumina is a function of the alumina quality, in particular the technology used for the calcination of the aluminium trihydrate precursor. In the current study the moisture content is shown to be a dynamic function of the ambient temperature and relative humidity. The moisture content is an important variable for the study of alumina agglomeration, and for the fluoride emission from the Hall-Héroult process. The kinetics of moisture desorption and absorption for various alumina qualities is studied. The desorption kinetics is concluded to be sig- nificantly different, while it is also shown that practical absorption kinetics is a function of the sample size and available surface area. iii Preface The present work was carried out at the Department of Materials Science and Electrochemistry (IME) at the Norwegian University of Science and Technology (NTNU), Trondheim, from January 1998 to October 2001. In addition to the compulsory courses in advanced materials technology and science, the work also included short courses on neutron spectroscopy at Kjeller, March 1998, and in electrochemistry in Southampton during the summer of 1998. Initially, a study of the mechanism of deformations of anodes during elec- trolysis was a part of the doctoral work, but that subject was discontinued mid-term. The work included a short research collaboration with Dr. Peter Witt at CSIRO in Melbourne, Australia over the course of a few weeks in March 1999, involving finite element modeling. The collaboration was ini- tiated by Prof. Jomar Thonstad, my supervisor, who spent his sabbatical year there. We found that some ceramic theory became a necessary part of the thesis in progress, and I became a student member of the American Ceramic Society (ACerS) at the end of 1999. Support and encouragement was found in Prof. Tor Grande and the “ceramics group” at NTNU, and a poster was presented during the ACerS meeting in St. Louis in May, 2000. Parts of the work described in this thesis was also presented at the conference on Molten Salts in Denmark, August 2000. The moisture content of smelter grade alumina became an important subject during the course of this work, and the experimental work reported in the final chapter of this thesis is the result of a collaboration with Senior Researcher Kjell Hamberg and his colleagues at the Hydro Research Center in Porsgrunn (HRE), who were also very hospitable during my stay there in the spring of 2001. Based on our results, work continues in Hydro Alu- minium and at the Alunorte alumina refinery in Brazil to further explore the effect of alumina structure and moisture content. In addition to the moisture study in chapter 8, supportive experimental work is also reported in chapter 7 on the thermal response in the tablet agglomerate samples studied in chapter 5. An introduction to the major experimental techniques used is given in Chapter 3, while chapter 4 serves as a partial reproduction of earlier results and forms the experimental basis for the research angle covered in the following chapters. iv An extensive literature review, chapter 2, was necessary in order to include discussions of relevant work in the vast research field of ceramics. The reader will find that the literature review lends a great deal to the current study of smelter grade alumina agglomeration. Together with the tablet agglomerate study in chapter 5, chapter 2, - and the accompanying bibliography - takes up most of the space in this thesis, partly reflecting that these are perhaps the most important contributions of this work. A study of alumina agglomeration during feeding could not be complete without a study of agglomerate dissolution, and, though mostly unsuccessful, a contribution to this field is reported in chapter 6. N. Peter Østbø v Acknowledgements The Research Council of Norway (NFR) and the aluminium industry in Norway cosponsored the work as a part of the PROSMAT research program. In particular, Bjørn P. Moxnes at Hydro Aluminium provided funding and support for some of the basic alumina analysis work in Ardal.˚ The Dept. of Materials Science and Electrochemistry at NTNU is also acknowledged for financial support towards the critical finale of this project. My participation at the ACerS meeting and conference in May, 2000, was sponsored by the Faculty of Chemistry and Biology. Professor emeritus Jomar Thonstad1 has been my supervisor during these years, and I am grateful for his support, advice and criticism. It was a great inspiration to be guided by him into the field of aluminium electrol- ysis, though also frustrating not to possess the same insight - especially when the project skewed off the traditional course. I also want to thank dr. ing. Sverre Rolseth2, who has been my co-supervisor for his encouragement and inspiration, not least when experiments failed.

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