Can we use spoil heaps to absorb CO2?

Chris Smith asked Philip Broadwith...

Philip - It sounds completely crazy, but it's not actually. It is something that people are definitely looking at. There's a phenomenon called enhanced weathering. Certain types of rock - silicate rocks like basalt, olivine - carbon dioxide from the air dissolves in water on the surface, so either in rain or in the sea, that makes carbonic acid, it makes slightly acidic water. That acidic water can then start to dissolve the rocks and weather them away. That's a natural process of weathering. And as it does that, it creates carbonate. So it makes carbonate rocks elsewhere, or it dissolves the carbonate into the water, which goes into the ocean and actually reduces the acidity of the ocean, which could also be a good thing. So if we can speed that process up, we can turn it into a way of absorbing CO2 to speed it up. You basically increase the surface area of the rocks. So you grind them up very fine and you spread them out over farmland or over the beaches or anything like that. So there's people doing experiments with olivine, which is a magnesium silicate mineral. It's green and they grind it up and they spread it on bits of the beach and see what happens, see if it absorbs, see how fast it weathers, see how fast absorbs CO2 where it ends up. But, equally, there's people doing it on farmland and because of the way it weathers there, it can end up adding nutrients to the soil. So it has all sorts of benefits. Whether we can do that with slag from mines and whatever, it's entirely possible the risk that you have there is what else is in those slags? So maybe there's other metals, maybe there's arsenic, all sorts of contaminants.

Philip - These materials are not homogeneous. There's lots of different places where we've had slag from different processes or mine tailings from different mines. There may well be some of those that are suitable for doing this kind of thing. The other side of this obviously is that you have to smash the rocks up really small and that takes energy. So how much energy is associated with grinding the rocks up? Does that offset how much CO2 you're going to absorb? How fast is that process going to happen? So there's lots of things going on, but it's not a completely crazy idea.