Acid Leaching for Carbon Sequestration with Prospects for Critical Metal Recovery

Reducing carbon emissions and supplying critical metals for the green energy transition are essential to limiting global warming to below 1.5 °C. Here, we propose heap leaching ultramafic material with sulfuric acid followed by sequential precipitation for nickel recovery and carbon sequestration. Ultramafic materials were subjected to benchtop-scale acid leaches using 0.08–0.8 M H2SO4. The leachates were combined with NaOH to precipitate Ni (oxy)hydroxide minerals and then with NaOH + Na2CO3 to precipitate Mg carbonate minerals. The carbon sequestration potential of each sample and the efficiency of each experimental stage were calculated to estimate carbon sequestration using these techniques. The acid leaching released 2–57% of the Mg2+ along with 6–64% of the Ni2+ into the leachate, depending on acid strength and tailings mineralogy, while carbonation precipitated up to 95% of the dissolved Mg2+ and 90% of the CO32– + HCO3– in solution, depending on initial Mg2+ concentrations. Techno-economic analysis demonstrates that treatment cost and Ni production are the most important factors determining the profitability of the process, with CO2 removal being less important due to the relatively low value of carbon credits compared to Ni, but still contributing to the sustainability of this process.