Utilization Potential of Iron Oxide By-Product from Serpentinite Carbonation

Mineral carbonation is a naturally occurring process to capture CO2. Research at Åbo Akademi University (ÅAU) has tried to speed up the process in order to find a suitable option for future CO2 capture and storage. Mineral carbonation is a safe way to store CO2 since no leakage is expected in a time range of hundred thousand years. This study was focused on the possibilities to use the iron by-product that is extracted from different suitable rocks suggested for use in the in-direct gas solid route developed at ÅAU.Two different serpentinites from Finland and Lithuania were studied. Serpentinite is a magnesium silicate rich rock which is found to be suitable for mineral carbonation. Magnesium will be extracted by allowing serpentinite react with ammonium sulphate. This reaction will make it possible for magnesium to form sulphates which can be dissolved in water. Magnesium hydroxide will precipitate as pH of the solution is increased. The hydroxides are in next step carbonated to magnesium carbonate by carbon monoxide.Iron content in the studied rocks is approximately 10 wt%, which makes it interesting to study the possibility to utilize iron as a by-product from mineral carbonation. The focus of this report was held on the characterization of the chosen rock types as well as finding a suitable iron-rich product which could be used for iron and steel making. Goethite has until today been precipitated in the process route developed at ÅAU. Attempts were therefore done in order to precipitate magnetite instead, which has some benefits with respect to energy needs comparing to other iron oxides. Increased temperature and sufficient time for iron to form magnetite seems to be two important factors to achieve optimum magnetite precipitation.Twelve precipitation experiments were conducted and two of the final iron precipitates were found to contain visible amounts of magnetic material. This indicates that it is possible to precipitate magnetite instead of goethite as the developed process proposes. Precipitations were done at different temperatures, with/without nitrogen gas supply and with ammonia/sodium hydroxide to increase solution pH. Iron will precipitate at pH 8-10 while magnesium will precipitate at higher pH. Taking advantage of this, it becomes possible to extract iron and magnesium separately.Analyses of the presence of ferrous and ferric ions in the starting solutions were done by the ferrozine method. Some differences could be found between the Lithuanian and Finnish serpentinite. The Finnish serpentinite, unlike the Lithuanian serpentinite, resulted in an iron rich initial solution containing both ferrous and ferric ions. This can partly explain why it seemed to be somewhat easier to precipitate magnetite from the Finnish serpentinite than from the Lithuanian serpentinite, which seemed to contain only ferrous ions.Reduction tests by TGA/DTA showed a large weight decrease upon reduction of two different iron precipitates. Hydroxide compounds are expected to form upon precipitation. Decomposition of these compounds will result in water release when the precipitates are heated. This will contribute to the weight loss. Both samples containing iron precipitate seemed to be reduced to metallic iron upon reduction.More precipitation experiments together with more reduction tests are needed to optimize the stage in the ÅAU process where iron is precipitated. In order to be able to do this efficiently, it will be important to find an even more suitable analysis method to analyze the presence of different iron compounds in the iron precipitates than used in this study. Analyses of the product obtained immediately after the solid/solid reaction with ammonium sulphate and serpentinite could give more information about the reactions of the iron. This could also give suggestions for how iron should be further treated, and show differences occurring when different types of minerals are used.Validerat; 20130620 (global_studentproject_submitter