Researchers at the University of British Columbia are studying the sequestration of CO2 by rock dumps. For those that believe that CO2 is the driver of climate change, waste dumps are good news! The principal is simple. Geologists have long shown that over the past 500 Million years the atmosphere had up to 25 times the CO2 in it that we have in ours today. One of the reasons why CO2 has decreased over time is through the gradual chemical weathering of rocks. Rain picks up CO2 from the atmosphere forming a weak acid. This acid then attacks exposed rock forming minerals like feldspar to form clay minerals as illustrated in this simplified CO2 consuming reaction:
Feldspar + Water + CO2 = Clay + HCO3 + SiO2
Magnesium and calcium silicate minerals common in basalts weather to form calcite CaCO3 a mineral that is a carbon sink. Rock weather at different rates but all rocks eventually break down. For example the weathering of carbonates is now also thought to play a big role in removing CO2 from the atmosphere.
To get at the rock of value, miners often have to remove rock in the way that doesn't have enough metal in it to be valuable. For decades miners called this valueless rock in the way "waste" and the neatly stacked piles of it "waste dumps". They are so named because the removal of waste rock represents a major cost in getting to the good stuff, the rock that the miners are after. This poorly conceived term has led people (sometimes on purpose) to confuse waste rock with pollution or garbage, when it is simply rock. But far from being garbage this waste rock, long thought valueless, could be a help to civilization for millenia to come. The rock in the waste piles has been blasted in order to remove it. The many broken surfaces of the blasted rock are now exposed to the air in the rock piles allowing for greater weathering and CO2 trapping.
Certainly different rocks have different capacities to be CO2 sinks but the process goes on for thousands of years, long after the people that mined the rock are long gone. Not only is the metal mined around for future generations but the CO2 sequestering capacity of the waste rock is as well! Maybe instead of accepting the label of unsightly "waste" we should be rethinking these rock piles as a positive legacy even beyond the valuable contribution of metal to our society that they represent.