Renewable energy technologies and the global transition to net-zero emissions are driving unprecedented demand for critical minerals while simultaneously exposing vulnerabilities in global supply chains and domestic processing capabilities. Mine wastes represent a significant secondary resource, however, their low-grades, complex mineralogy and spatial variability present substantial technical and economic barriers to commercial-scale recovery. Analysis of 77 samples collected from a single mine waste dump demonstrates the considerable mineralogical and elemental heterogeneity that can exist within these secondary resources. Unlocking their value therefore requires a departure from conventional processing approaches developed for primary ores. Instead, flow sheets must be purpose-designed for heterogeneous secondary resources, targeting not only metal recovery but also the purity, consistency and material specifications required for critical mineral products. These processes must also be designed for the future operating environment of the minerals industry, where reagent availability, particularly sulfuric acid supply, may become increasingly constrained. This work explores hydrochloric acid (HCl) as an alternative hydrometallurgical platform for recovering critical minerals from complex mine waste rocks. HCl offers several potential advantages, including chloride complexation to enhance metal extraction, opportunities for selective downstream purification, avoidance of sulfate-rich secondary wastes, and the potential to recover and recycle acid within a more circular process. These attributes are increasingly relevant as sulfuric acid supply faces growing pressure from competing industrial demand and changes to traditional sulfur supply associated with decarbonisation. Comparative leaching of two complex mine waste materials also demonstrated substantially higher recovery from a mine waste rock using HCl relative to H2SO4. For one mine waste rock sample, HCl achieved approximately 98 per cent Cu, 87 per cent Co and 72 per cent Mn recovery, while only 16 per cent Mn was achieved for H2SO4 with negligible amounts of Cu and Co recovered. The opportunity, however, extends beyond extraction efficiency. Future critical-mineral projects must integrate reagent circularity, supply-chain resilience, environmental performance and sovereign capability into process design. Reprocessing mine wastes can simultaneously recover strategic resources, reduce long-term environmental liabilities and improve resource efficiency. Critical minerals are not bulk commodities, and their value cannot be assessed by tonnes and conventional economics alone. Unlocking secondary resources therefore requires a broader definition of value, one that recognises economic, environmental and strategic outcomes alongside metal recovery.