Life cycle assessment of secondary aluminium refining
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The goal of this life cycle assessment is to investigate refining methods which makes it possible to utilise post-consumer aluminium scrap in production of aluminium products that have strict requirements to chemical purity. A further goal is to establish whether applying these methods are environmentally preferable compared to using primary aluminium to produce the same products. The investigation is based on a given scrap composition and given purity limits that must be achieved for the refined aluminium, related to six alloying elements. Based on the given requirements, a selection of refining processes which are able to refine the given scrap to meet the given limits are identified. Table A below gives the scrap composition and the desired range of removal to fit seven different alloy requirements. The scrap composition is given in weight%.Table A: Scrap composition and desired level of removal. Si Mg Fe Cu Mn ZnScrap composition 6.11 0.53 0.53 1.66 0.23 0.86Desired removal 10 ? 83 % - 6 - 57 % 45 - 100 % - 77 - 100 %Different types of electrolysis and fractional crystallisation are identified as possible refining processes. Electrolysis can in theory be used to eliminate any tramp element in the scrap, as it extracts the pure aluminium from the melt using an electric current. Unfortunately this is an expensive, energy consuming process which requires use of chemicals. Fractional crystallisation does not require any chemicals and generally has low energy use, but this is a slow method which currently is not continuous. The ability of fractional crystallisation to remove the various elements depends on their solubility in aluminium in solid and liquid state. Various studies show that this method is promising for removal of Si, Fe and Cu, mediocre for Mg and Zn and poor for Mn. Based on this, six different production scenarios for secondary scrap, to meet the set of alloy requirements, were developed. The life cycle impact assessment conducted for the different refining scenarios shows that energy use is very closely linked to the environmental burden associated with each of the production scenarios. From an environmental perspective low temperature, electrolysis and fractional crystallisation seem to be the best alternatives. Since the assessment is based on specific requirements set by the scrap composition and the given purity limits, an overall impression is that other possibilities to handle excess scrap should be investigated further. For example better sorting processes to separate tramp elements earlier in the production or development of alloys which are made with a motive for recycling. Such methods are likely to be more relevant when the use of aluminium has increased even further and more stable sources of scrap can be established.