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.