Life Cycle Analysis on the Extraction of Ca(OH)₂ from Industrial Waste Products for Use in a Passive-DAC System for CO₂ Capture

Abstract

Climate change mitigation is a problem that requires a two-pronged approach, which is a reduction in Greenhouse gas emissions and the large-scale removal of potent greenhouse gases such as Carbon dioxide (CO₂). Passive Direct Air Capture (Passive DAC) using alkaline minerals presents as a less energy and cost-intensive approach for carbon dioxide removal, particularly when using porous materials produced with alkaline minerals derived from industrial by-products like LD-slag from the steel making industry. This study evaluates the environmental feasibility of extracting alkaline minerals, specifically Ca(OH)₂, from steel-making by-products (LD-slag) for application in passive DAC systems, using a cradle to gate Life Cycle Assessment (LCA) with a functional unit of 1 tonne of captured CO₂. Two extraction routes for Ca(OH)₂: route 1 using hydrochloric acid (HCl) and route 2 using ammonium chloride (NH₄Cl) were modelled across different environmental impact categories, including climate change, acidification, toxicity, and resource depletion. Results indicate that the NH₄Cl pathway consistently exhibits about 6-7 times higher impacts than the HCl route across all categories. For both pathways, upstream chemical production, particularly NaOH, HCl, and ammonia, dominates the overall footprint, outweighing impacts from Ca(OH)₂ extraction itself. Transport sensitivity analysis highlights the influence of supply-chain distance, while renewable electricity substitution yields only marginal improvements. In all modelled scenarios, total life-cycle emissions exceed the CO₂ captured, underscoring the trade-offs inherent in Ca(OH)₂ production for DAC applications. This ultimately implies that the extraction of Ca(OH)₂ from Industrial waste products for the capture of CO₂ in a passive DAC system is not a viable solution to address industrial waste and reduce atmospheric CO₂ levels.