Environmental Considerations in the Zero-waste Valorisation of Bauxite Residue : A Life Cycle Perspective
Abstract: Bauxite residue, also known as red mud, is produced in large quantities as a result of alumina refining (the first stage in aluminium production), and is one of the world’s most abundant and important industrial wastes. As demand for aluminium continues to increase and space to store this residue diminishes, the potential to utilise bauxite residue as a secondary resource is increasingly being considered by the alumina industry. Bauxite residue can be used as a source of iron, aluminium, titanium oxide, scandium and rare earth oxides, or utilised for its bulk properties to create cement clinkers or inorganic polymers. Achieving any of these uses however requires a series of complex valorisation processes, which in turn require inputs of energy and materials. Some bauxite residues also contain trace amounts of naturally occurring radionuclides.The EU Horizon 2020 MSCA-ETN REDMUD project was set up to investigate the valorisation of bauxite residue in an integrated manner. The ultimate aim of the REDMUD project is to develop environmentally-friendly, zero-waste, integrated processes for extracting valuable materials from bauxite residue and/or utilising it at high volume. This thesis presents the environmental perspective on this aim, taking a life cycle view; that is, taking into account the upstream and downstream impacts, in addition to the direct impacts, which may result from diverting bauxite residue from landfill to the proposed valorisation processes. This involves using Life Cycle Assessment (LCA) approaches to understand the environmental balance between the impact avoided through landfill diversion and the substitution of conventional materials, and the impacts incurred by the use of materials and energy in the valorisation processes themselves. Importantly, the potential ionising radiation impact from naturally occurring radionuclides is also considered from a life cycle perspective for the first time.A new life cycle impact assessment method for assessing the impacts of naturally occurring radionuclides was developed. In addition, two pieces of research software, designed to overcome the current shortcomings in LCA software with respect to streamlined and prospective LCA studies of emerging technologies are presented as part of this thesis.The potential hotspots of environmental impact in a single step valorisation process, the production of high bauxite residue content inorganic polymers, were identified. The results identify the high temperature processing of bauxite residue, in order to transform it into a reactive precursor capable of forming solid inorganic polymers, as a hotspot of environmental impact across a range of environmental impact measures. The production of alkaline activating solutions (the other reagent in the polymerisation reaction) also represented a hotspot of environmental impact. These hotspots were used to identify possible future research directions for this process, which have the potential to reduce the environmental impact of this valorisation process.Finally it was shown that even in the absence of a detailed and quantified system description, qualitative approaches based on life cycle thinking can be usefully applied to identify important aspects on both sides of the environmental balance between the impacts avoided and the impacts incurred in waste valorisation. Chemical reaction products, chemical synthesis, thermal and mechanical energy are highlighted as potential sources of environmental impact. A case study, looking at the combined extraction of iron and production of inorganic polymers from bauxite residue was used to demonstrate the validity of these qualitative approaches. This study also demonstrated that combining the extraction of iron and inorganic polymers is vital in order to yield a net environmental benefit in terms of climate change.This thesis provides an initial step on the road towards the environmentally sustainable valorisation of bauxite residue, as well as the analytical tools and additional impact assessment measures required to ensure that this journey can be continued, both within the REDMUD project and beyond.
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