Kurt Aasly Properties and behavior of quartz for the silicon process Thesis for the degree of philosophiae doctor Trondheim, August 2008 Norwegian University of Science and Technology Faculty of Engineering Science and Technology Department of Geology and Mineral Resources Engineering NTNU Norwegian University of Science and Technology Thesis for the degree of philosophiae doctor Faculty of Engineering Science and Technology Department of Geology and Mineral Resources Engineering © Kurt Aasly ISBN 978-82-471-1163-5 (printed ver.) ISBN 978-82-471-1164-2 (electronic ver.) ISSN 1503-8181 Doctoral theses at NTNU, 2008:236 Printed by NTNU-trykk Preface The work presented in this PhD-thesis was carried out at the Department of Geology and Mineral Resources Engineering at NTNU from June 2003 until May 2008. The project was mainly funded as a four-year PhD position by the Department of Geology and Mineral Resources Engineering. However, Elkem AS funded the analytical and experimental work and made traveling to conferences etc. possible. Elkem AS also co-sponsored the purchase of necessary analytical equipment. The PhD-project was carried out in close cooperation with other researchers in the Strategic University Program “The value chain from mineral deposit to beneficiated product with emphasis on Quartz”. However, this particular PhD- project was not funded by this SUP, but the cooperation with several PhD- candidates through the SUP gave additional motivation and contribution to the results of this PhD. First of all, I want to thank my supervisors Professor Terje Malvik at the Department of Geology and Mineral Resources Engineering and Dr. ing. Edin Myrhaug at Elkem AS for their encouragement and support, and for their contributions with ideas for the analytical work and investigations. Paper IV is the result of cooperation with Prof. Jens Götze at the department of Mineralogy at TU-Bergakademie Freiberg. He helped with carrying out the cathodoluminescence microscopy and spectroscopy investigations as well as discussing the results. I want to thank him for welcoming me to Freiberg and sitting down with me for the week it took to carry out the CL investigations. All the discussions with my fellow PhD-candidates, both technical discussions and social discussions have been of great importance. Special thanks to Kari Moen and Bjørn E. Sørensen for many important discussions around quartz and technical questions. Thanks to Trond Brenden-Veisal and Halvard Tveit at Elkem AS for many good discussions and contributions to the thesis work through the reference group. i Arild Monsøy and Kjetil Eriksen at the IGB preparation lab are thanked for their important help with preparing the polished thin sections and thick sections as well as preparing samples for the shock-heating experiments. Kjell Kvam and Torill Sørløkk were important for carrying out the XRD- and DTA analysis. Thanks to Steinar Prytz at SINTEF Materials and Chemistry for help with shock heating of quartz in the ASEA induction furnace. Thanks to Elkem AS for giving me the chance to spend my first few months to finish my thesis before I started working fully on my new challenges. Finally and most importantly, I would like to thank my wife Kari and two little daughters Sofie and Guro for being there in the everyday life and being patient and supportive through my PhD work. I love you! Trondheim, May 2008 Kurt Aasly ii Abstract This PhD-thesis is a result of the study on important properties of quartz as a raw material for the metallurgical production of ferrosilicon and silicon metal. This includes defining mechanical properties important for the size reduction experienced during transport and storage and thermo-mechanical properties of quartz that is important for how the quartz reacts to the high temperatures experienced as it is charged on the furnace. Additionally, softening properties of quartz have been briefly discussed in some of the papers. Another important goal has been to test analytical and experimental methods for investigating the various properties. The investigations of important factors for the mechanical properties of ores and industrial minerals have been carried out as a literature study. The mining operation and transport from mine to smelter has been discussed and several factors that are significant for achieving best possible mechanical properties of the quartz have been identified. The most important factors are related to production in the mine and processing plant, which should be carefully planned to minimize the amount of blast-induced damage in the rock and thus achieve the best possible mechanical strength of the raw material. The amount of fines can be minimized by controlling the handling of the raw materials during the transport and storage. It is especially important to avoid high drops, both high single drops and accumulated height of all the drops in total. Investigations of the thermo-mechanical properties of quartz have been carried out by using different experimental and characterization methods. The petrographic investigations of the raw materials by polarized light microscopy have been important. Thermo-mechanical investigations have been high-temperature microthermometry and shock heating of quartz samples in an induction furnace with subsequent investigations of the heated material. The subsequent investigation included polarized- and fluorescence light microscopy of polished thin sections, cathodoluminescence microscopy and spectroscopy and x-ray diffraction. Combining high-temperature microthermometry and shock-heating investigations has proved to provide useful knowledge about the effects of high temperatures on quartz. iii Results from earlier research have been confirmed showing that mica is the cause of the effects seen in the temperature interval 900 – 1000 °C. This has been shown by the total absence of tridymite in the samples and the fact that mica has been seen in the unheated reference samples. Cathodoluminescence microscopy and spectroscopy was used to investigate sample from shock-heating experiments and corresponding reference samples. These investigations show that cathodoluminescence is a useful tool for petrographic investigations of quartz. The shock-heated samples showed a significant change in cathodoluminescence characteristics that need to be investigated further to understand the cause of these changes. A spotted red luminescence was seen in two of the samples indicating the formation of cristobalite or the transition phase within these samples. Cristobalite has been shown in samples after heating to different temperatures in the interval 1250 to 1550 °C, although in different amount in the different types of quartz. However, the transformation rates seem to be more similar after prolonged heating at the highest temperature. Experiments also indicate that the quartz- cristobalite transformation may be a cause of the disintegration of quartz at high temperatures. This is related to the severe volume expansion as the quartz transforms to cristobalite via the amorphous intermediate transition phase. The last paper presented in the thesis presents investigations of two furnaces that have been producing ferrosilicon and silicon metal respectively. The results from these investigations show that cristobalite is formed relatively rapidly inside the furnace, however, indicating that the cristobalite formation takes place earlier at a distance from the electrode, than closer to the electrode. Another surprising discovery was to find tridymite in the ferrosilicon furnace. The investigations of silica phase transformation by shock-heating experiments did not show any tridymite even in the same type of quartz as used in the ferrosilicon furnace. The cause of the tridymite formation seen in this furnace is probably related to the total impurity content in the furnace as well as the long retention time of the quartz in the rather inactive parts of the furnace where the tridymite was found. iv Table of contents Preface i Abstract iii Contributions to the papers ix Part one: 1. Introduction ...................................................................................................................1 1.1 Objectives and Scope............................................................................................2 1.2 Outline of the thesis ..............................................................................................2 1.2.1 Part one.............................................................................................................2 1.2.2 Part two ............................................................................................................3 2. Background ...................................................................................................................5 2.1 The ferrosilicon and silicon metal production......................................................5 2.1.1 The Carbothermic Process of silicon................................................................5 2.1.2 Important properties of quartz..........................................................................6 2.1.3 Previous work on important properties of the quartz .......................................9 2.1.4 Quartz behavior in the furnace .........................................................................9 2.2 Quartz and quartz deposits.................................................................................12 2.2.1 Quartz.............................................................................................................12