Sunday, June 17, 2007

Carbon sequestration in mine tailings

Attention Grist and Wikipedia readers:

Please see this comment on the linking of this post from those websites (update Jan 2 2008).

We had a cool talk on Friday about carbon sequestration in mine tailings.

First, the background:
For the past couple billion years, the primary planetary thermostat is generally thought to have been the carbonate-silicate geochemical cycle. Weathering processes remove CO2 from the atmosphere with the following reaction (using diopside as an example mineral): CaMgSi2O6 + 2CO2 -> CaMg(CO3)2 + 2 SiO2. subduction and continental metamorphism reverse the reaction: CaMg(CO3)2 + 2 SiO2 -> CaMgSi2O6 + 2CO2. This works for all calcium and magnesium silicates, not just diopside. The metamorphic part is assumed to proceed at a constant rate. The weathering part is dependent on temperature and p CO2. So when CO2 and temperature are high, the reaction proceeds faster. As CO2 is removed from the atmosphere, the planet cools, and the reaction slows down. Without human interference, this cycle is expected to correct the current anthropogenic CO2 increase in about 0.1 Ma.

Now, the observation:
Scientists studying mine tailings in abandoned asbestos mines in Canada found that the tailings (which are primarily serpentine: Mg3Si2O5(OH)4) were covered in white crusts- various hydrated magnesium carbonate species. Detailed study ensued.

The details (which I’m skipping since it’s after midnight and I’m only up because the baby is) are in themselves pretty awesome, as they show how the scientists sourced the carbon and investigated the relative importance of biotic and abiotic processes in reacting the tailings. Their conclusions are that in the 20 years since the mines closed, the tailings had sequestered as much CO2 as the mining operations originally produced, making the mines carbon-neutral. Even better, the potential sequestration capacity of the unreacted serpentine in the tailings was substantially higher. Now they are trying to figure out how to accelerate the process, in the hopes that all mantle or mafic cumulate-based mines can sink carbon as a byproduct of production. In true lab style, they’ve evidently got labs full of tailings-packed tupperware reacting at all sorts of temperature, pH, and fluid activities.

We don’t currently mine enough nickel, chromite, or asbestos for this to be a stand-alone solution for all emissions, but in theory it could sequester up to about 0.1 Gt of carbon per year- substantially more than is used to operate the mines.

I love this kind of science, for several reasons. Firstly, it is one of those things that is bleeding obvious once observed, but still quite unexpected. If mine tailings weren’t reactive, we wouldn’t have mine pollution issues. So how could hundreds of millions of tons of powdered mafics fail to react with the atmosphere?

Secondly, it makes absolutists’ brains explode. Just when the left wing fruitcake faction of environmentalists thought that bird-killing windmills and nuclear power plants were the worst thing to come out of global warming, along comes this new discovery: What we really need to do to save the planet is to dig up more asbestos, nickel, and chromium.

More info:
The UBC mineral carbonation group
AGU abstract
American Mineralogist abstract (full article available for fee)

No comments: