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Works in Progress Issue 11






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olivine weathering

Gravel costs money. Making olivine gravel costs maybe $20/ton. You'd need to dig up 2.3 tons of pure Mg silicate to potentially absorb 1 ton of CO2, and realistically speaking your "ore" won't be pure or react completely, so the correct ratio is >3.

Suppose you do that. Great, you exposed some fresh magnesium silicate to the CO2 in air, and now a very thin layer of carbonate will form on the surface as it very slowly reacts. If you crush it to fine particles and spread it over a large area, you can get it to actually react, but that involves transporting it to a grinder and then spreading it out, which would bring your cost to probably >$200 per ton of CO2 absorbed. Not great. (Plus, all this digging and grinding uses energy, and probably involves vehicles that burn fuel.)

The above link talks about the cost of electricity needed to grind up a ton of olivine. This is a weird approach because people already grind up a lot of rocks and we know a lot about how much that currently costs. You should always base cost estimates on the costs of the most similar existing things. (Why don't people do that?)

The article has a detailed analysis that comes up with a much lower cost. If you think that analysis goes wrong, I'd be curious to understand exactly where?

The article has a detailed analysis

I sure didn't see one! I saw some analysis of the cost of energy used for grinding up rock, with no consideration of other costs. Can you point me to the section with detailed analysis of the costs of mining, crushing, and spreading the rock, or the capital costs of grinders? A detailed analysis would have numbers for these things, not just dismiss them.

If you think that analysis goes wrong, I'd be curious to understand exactly where?

OK then.

What is quite certain is that the vast majority of that expense, both financially and in terms of energy, comes not from mining or crushing but from milling the crushed rock down to particle size.

Digging up and crushing olivine to gravel would be $20-30/ton. We know this from the cost of gravel and the availability of olivine deposits. That alone makes this uneconomical, yet the author just dismisses them as negligible next to the cost of milling. So either the dismissal is wrong, or the milling cost estimation is wrong, or both.

for an all-inclusive energy cost of 61 kWh ($9.15) per tonne of rock – about $7.32 per sequestered tonne of CO2

Why is the cost per ton of CO2 lower than the cost per ton of rock, when 1 ton of rock stores much less than 1 ton of CO2?

And the largest rock mills are large indeed; the biggest on the market can process tens of thousands of tonnes a day. It should be clear by now that capital expenditures, while not irrelevant, are small compared to the cost of energy

That's quite a non sequitur! We know what grinding rock to fine powder costs. Use those costs, not the cost of electricity.