Life Cycle Assessment (LCA) has been used to assess the environmental sustainability of the chlor-alkali production in Europe. The three current technologies applied nowadays are mercury, diaphragm, and membrane cell technology. Despite, having achieved higher energy efficiencies since the introduction of membrane technology, energy consumption is still one of the most important issues in this sector. An emerging technology namely oxygen-depolarised cathodes (ODC) is suggested as a promising approach for reducing the electrolysis energy demand. However, its requirement of pure oxygen and the lack of production of hydrogen, which could otherwise be valorised, are controversial features for greener chlorine production. The aim of this work is to evaluate and compare the environmental profiles of the current and emerging technologies for chlorine production and to identify the main hot spots of the process. Salt mining, brine preparation, electrolysis technology and products treatment are included inside the system boundaries. Twelve environmental impact categories grouped into natural resources usage and environmental burdens are assessed from cradle to gate and further normalised and weighted. Furthermore, hydrogen valorisation, current density and allocation procedure are subjected to sensitivity analysis. Results show that the electrolysis stage is the main contributor to the environmental impacts due to energy consumption, causing 99.5-72% of these impacts. Mercury is the less environmentally sustainable technology, closely followed by diaphragm. This difference becomes bigger after normalisation, owing to hazardous waste generated by mercury technique. Conversely, best results are obtained for ODC instead of membrane scenario, although the reduction in energy requirements is lesser than expected (7%).

Currently, the chlor-alkali sector is shared by three main electrolysis technologies: mercury, membrane and diaphragm cell. As the energy demand of the process is one of its main drawbacks, new technological improvements are emerging such as the replacement of the standard hydrogen-evolving cathode in membrane technology by an oxygen-depolarised cathode (ODC). In this sense, the environmental impacts of novel techniques must be analysed over their entire life cycle to assess properly their integration opportunities. This work develops a life cycle assessment (LCA) model to describe the chlor-alkali European industry. The multi-functional production of chlorine, sodium hydroxide and hydrogen is studied from cradle to gate, including salt production, products treatment and waste management within the system boundaries. While the worst scenario results mercury technique, ODC technology emerges as the most environmentally sustainable *Manuscript Click here to view linked References 2 process. The results suggest the importance of considering every process included, especially salt production and brine preparation, which can involve up to 20% of the total environmental impacts. In fact, taken as reference membrane scenario, results demonstrated that the environmental profile can be reduced by up to 18% when lower energy demanding processes for salt production and NaOH concentration were selected. This improvement percentage overcomes the competitive advantage shown by ODC versus membrane technology (7%). This model is a useful tool not only for the comparative assessment of the environmental sustainability of the different chlor-alkali installations, but also to guide and support the decision-making process in the introduction of emergent technologies in the sector. HIGHLIGHTS  Exhaustive Life Cycle Assessment model to describe the European chlor-alkali sector  Environmental sustainability of current and emerging electrolysis technologies  Importance of every life cycle stage is remarked, especially salt production  Success of emergent technology is challenged by the lack of hydrogen production  Tool to support decision-making process in the introduction of emergent techniques

Our current economic model is experiencing increasing demand and increasing pressure on resource utilisation, as valuable materials are lost as waste. Moving towards a circular economy and supporting efficient resource utilisation is essential for protecting the environment. The chlor-alkali industry is one of the largest consumers of salt, and efforts have been made to reduce its electricity use. Furthermore, KCl mining wastes have received increasing attention because they can be transformed into value-added resources. This work studies the influence of using different salt sources on the environmental sustainability of the chlor-alkali industry to identify further improvement opportunities. Rock salt, solar salt, KCl waste salt, vacuum salt and solution-mined salt were studied. Membrane cells in both bipolar and monopolar configurations were studied and compared to the emergent oxygen-depolarised cathode (ODC) technology. Life cycle assessment (LCA) was applied to estimate the cradle-to-gate environmental impacts. The natural resource (NR) requirements and the environmental burdens (EBs) to the air and water environments were assessed. The total NR and EB requirements were reduced by 20% when vacuum salt was replaced with KCl. Moreover, the environmental impacts estimated for the monopolar membrane using KCl were comparable to those generated for the bipolar membrane using VS. The difference between the monopolar and bipolar scenarios (17%) was slightly higher than that between the bipolar and ODC technologies (12%). This work demonstrates the importance of studying every life cycle stage in a chemical process and the environmental benefit of applying a circular economy, even in energy intensive industries such as the chloralkali industry.

Abstract:The EU Emissions Trading System (EU ETS), which is a cornerstone of the EU's policy to combat climate change, has been criticised by its effects on the competitiveness of intensive energy demanding industries, and in particular, of the chlor-alkali sector. The main chlorine application in Europe is the production of polyvinyl chloride (PVC) from ethylene dichloride (EDC) as intermediate. Since chlorine is mainly traded in terms of derivatives, the aim of this work is to assess the vulnerability of the European chlor-alkali industry to chlorine replacement by imported EDC. An Energetic, Economic and Environmental Sustainability Assessment (EEESA) methodology is proposed based on the main variables affecting EDC production. Moreover, the influence of the EU ETS compensation measures and the emission allowance price in the current (mercury, diaphragm and membrane) and emergent (oxygen-depolarized cathodes (ODC)) technologies is studied. The most vulnerable scenarios become mercury and diaphragm technologies due to energy consumption. However, the salt price dependency on the quality requirements substantially influences the EEESA results. This analysis also shows the importance of hydrogen valorisation, whose major impact is observed in ODC scenario.

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