Low-temperature (NH₄)₂SO₄ roasting for sustainable recovery of aluminum and iron from coal gangue: process optimization and mechanism insight

The increasing global demand for aluminum (Al) and iron (Fe), along with the rapid depletion of high-grade ores, presents a critical challenge, driving the urgent search for alternative resources. Coal gangue is an abundant solid waste from coal mining which emerges as a compelling secondary resource due to its high Al and Fe content. In this study, we report a green and efficient process for the recovery of Al and Fe from coal gangue, featuring (NH₄)₂SO₄-assisted low-temperature roasting and subsequent water leaching. We systematically investigated the influence of key parameters and achieved optimal leaching efficiencies of 80.58 ± 1.99% for Al and 90.72 ± 1.55% for Fe. Mechanistic analysis revealed that the roasting process effectively disrupted the stable mineral matrix, converting Al and Fe into highly water-soluble sulfates (NH₄Al(SO4)₂ and Fe₂(SO₄)₃). Notably, this method immobilized sulfur as environmentally benign CaSO₄ and MgSO₄, thereby preventing SO₂ emissions. This acid-free, low-temperature method provides a sustainable and affordable way to turn coal gangue into valuable metals, thereby supporting a circular economy and reducing the environmental impact of the coal industry.