## 4 Conclusions  
The use of copper slag for hydrogen production via thermolysis represents a compelling and innovative application of circular economy principles to the energy and mining sectors. Globally, more than 30 million tons of copper slag are generated annually, with major producers including Chile, Mexico, Australia, Spain, Namibia, India, and the United States. In northern Chile, particularly in the Atacama Desert, decades of intensive copper smelting have resulted in vast accumulations of copper slag, often stored in large heaps with limited reuse and increasing environmental concern. Although rich in metal oxides such as magnetite and fayalite, copper slag continues to be primarily used in low-value applications such as construction filling and abrasive blasting. Reimagining copper slag as a raw material for hydrogen production marks a significant paradigm shift. Unlike traditional methods such as water electrolysis, which rely on purified water and often require critical and expensive materials (e.g., platinum-based catalysts), the thermolysis of copper slag enables hydrogen generation using a readily available industrial by-product. This approach benefits from several unique advantages, such as the following: (i) extremely low material cost, as copper slag is abundant and often treated as waste; (ii) no requirement for additional reactants, with hydrogen being generated through redox reactions with water vapor; (iii) integration with concentrated solar power (CSP), allowing for carbon-free energy input and significantly lowering operational emissions; (iv) potential for metal recovery (e.g., magnetite, hematite) as a co-benefit, enhancing overall resource efficiency. In regions like the Atacama Desert, where both slag deposits and solar irradiance are abundant, this approach offers a unique opportunity to combine industrial waste recovery with renewable energy to create a local, sustainable pathway for hydrogen production. Moreover, the hydrogen generated through this method could be reintegrated into mining and metallurgical operations, powering microgrids, fueling equipment, or supporting broader decarbonization strategies. This closes material and energy loops, reinforcing circularity in traditionally linear sectors and contributing to the emergence of a low-carbon, green mining economy. Copper slag vaporization through solar thermolysis offers a cost-effective, low-emission, and circular alternative to conventional hydrogen production methods. Its success will depend on the continued optimization of reaction conditions, advances in reactor design, and integration with emerging clean technologies. Positioned at the intersection of waste recovery, renewable energy, and green hydrogen, this approach has the potential to contribute meaningfully to national and global clean energy transitions--particularly in resource-rich regions such as the Atacama Desert. **Author Contributions:** Conceptualization, M.F., S.L.-G. and F.M.G.M. ; methodology, M.F., S.L.-G., V.E.R.C. and F.M.G.M. ; validation, A.S. ([PERSON]), F.S., E.F., A.S. ([PERSON]), M.T. and N.T. ; investigation, M.F., N.T., S.L.-G., F.S., J.A.C.M. and F.M.G.M. ; writing--original draft preparation, M.F., S.L.-G. and F.M.G.M. ; writing--review and editing, M.F., A.S. ([PERSON]), F.S., S.L.-G., A.S. ([PERSON]), M.T., N.T. and F.M.G.M. ; visualization M.F., A.S. ([PERSON]) and F.M.G.M. ; supervision, F.M.G.M. and M.T. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this manuscript are available upon request from the corresponding author. **Acknowledgments:** The authors would like to thank the Programa de Doctorado en Energia Solar of the Universidad de Antofagasta, Chile.