Selective recycling of critical metals from the dolomitic limestone waste through multi-step alkaline precipitation integrated with CO2 carbonation.

Waste valorization provides a sustainable pathway for metals resource management and environmental remediation. In this study, we presented a carbon-neutral hydrometallurgical process for selectively recycling critical metals from dolomitic limestone leachate, a waste stream of phosphate mining. By incorporating thermodynamic simulation with experimental works, we achieved high-purity recovery of magnesium, calcium, aluminum, iron, manganese, and titanium via stepwise alkaline precipitation integrated with CO2 carbonation. This process eliminates the need for redox agents or stabilizers. Thermodynamic simulations revealed that increasing temperature widened the pH gap between metal insolubilities, enabling selective precipitation at lower pH. Higher NaOH concentrations enhanced the supersaturation factor by 102-108 orders, thus accelerating precipitation. Experimental optimization of pH, temperature, and NaOH concentration enabled selective recycling of 97 % Ti, 99 % Al, 96 % Fe, 91 % Mn, 98 % Mg, and 90 % Ca at distinct pH thresholds 3, 4, 6, 7, 9, and 11, respectively. The NaOH effluent was repurposed to capture CO2 arising from the acid leaching step and to synthesize NaHCO3. The obtained NaHCO3 was further utilized for calcium recovery as CaCO3. Economic assessments supported the viability of this closed-loop process for recovering value-added products, including Mg(OH)2, Ca(OH)2, Fe3O4, Mn3O4, Al(OH)3, and TiO2. This work advances circular economy strategies by transforming mining waste into valuable metal resources while promoting carbon neutrality.