Use of regenerative ammonium salt for mineral extraction and CO₂ mineralisation from Victorian brown coal fly ash
thesisposted on 03.03.2017, 00:37 by Hosseinidashtikhani, Seyyedehtahereh
In Victoria, brown coal combustion is the single largest source for energy, meeting greater than 85% of the electricity needs. However this produces approximately 1.3 million tonnes of fly ash annually, which is predominantly dumped into ash ponds. The brown coal fly ash is rich in alkali and alkaline earth metals and transition metals with little aluminium and silicon. Industrial wastes such as fly ash are being considered for CO₂ mineralisation as part of the strategy for carbon dioxide capture, storage and utilisation (CCSU). This process also converts the valueless wastes, into value-added products such as magnesium carbonate and calcium carbonate to replace dolomite in industrial applications. A cost-effective carbon capture process is fundamental for the sustainability of brown coal in the carbon-constrained future. With the continuous increase in the amount of fly ash generated, there is an increasing demand in the use of vast land for landfill, which simultaneously contaminates soil, ground and water. To date, the majority of studies on fly ash utilisation have focused on direct route (single stage leaching and carbonation in the same reactor) under high CO₂ partial pressure with a long reaction time. However, there are a limited number of studies on utilisation of brown coal fly ash through the indirect mineral carbonation route, i.e. using separate leaching and carbonation steps. The scope of this thesis includes, firstly establishing a closed-loop multi-step leaching-carbonation process using regenerative ammonium chloride as the leaching reagent. This process allows efficient extraction of magnesium and calcium from Victorian brown coal fly ash under relatively mild operating condition (<80°C). Secondly, optimising the leaching condition to achieve highest Mg²⁺ and Ca²⁺ extraction yields and detailed modelling on the kinetics of leaching reaction was also conducted. Thirdly, extensive investigations on chemical and morphological changes of weathered fly ash via a variety of analytical instruments. The motivation was to understand the weathering process in ash pond and how the weathering affects the mineral carbonation process. In this part, modification of ammonium chloride solution by combining with acid via pH control leaching was conducted so as to improve both the extraction yield and selectivity of magnesium and calcium over other metals. Thirdly, to clarify the competition of Mg²⁺ and Ca²⁺ ions in the leachate upon carbonation, a synthetic leachate was employed to examine the effects of variables including Mg²⁺/Ca²⁺ ratio, reaction time and temperature on the carbonation rate and precipitate composition. Based on the experimental data, a mathematical kinetic model was further developed to quantitatively clarify the interaction and competition between Mg²⁺ and Ca²⁺ cations in the leachate upon carbonation. Finally, a comprehensive energy and techno-economic analysis was conducted to identify the most promising process configurations in terms of energy consumption, product yield and cost.