A process for stabilizing a sulfate and/or sulfide-rich waste material (comprising metal sulfide minerals) and sequestering CO 2 comprises exposing the material to a CO 2 -enriched gas mixture, reacting the CO 2 -enriche gas mixture with the metal sulfide minerals and forming a CO 2 -depleted gas mixture and a carbon-containing compound and at least one product selected from the group consisting of a purified metal or a metal-rich compound suitable for smelting or refining, sulfuric acid, sulfur and sulfurous acid, and system and apparatus therefor.