Drive through the wilds of Arizona and New Mexico and see those magnificent red sandstone cliff, standing proud in spite of thousands of years of erosion. It is one of my favorite sights. It is spectacular to behold. It tells me that landscapes can be the same for very long times.
Can we do the same with mining wastes?
The Pantheon in Rome is as it was when constructed 1,600 years ago. The secret is that the dome is of primitive cement. A lesson for long-term stability.
Today the lab sent me the results of a test to see what happens if I add cement to a nasty, clay tailings. I saw the material in its “natural” state, come straight from the metallurgical laboratory. The material was an ugly gray mess of sloppy consistency: a silty clay of high plasticity and obviously low strength.
“Add cement to the stuff,” I intoned.
We did. Five percent in this case, although I have recommended 2.5, 5.0 and 10.0 % addition tests. The results are pending.
The day after cement addition, the mix was still soft and slaked when water touched it. “That is a dumb idea,” some said.
Today, a week after five percent cement addition, the young engineer proclaimed: “It is rock hard.”
The point of this silly story (a true one) is that if you add cement to a filter-pressed tailings and even that produces a wet, sloppy tailings, the addition of a bit of cement gives you something that is rock-hard. Dare we hope that it is as hard as those cliffs of Arizona and as durable?
I do not know. More test data are needed to make a sound conclusion. You will have to test your own wastes. And consider the climate and the ability of the mine to afford the additional cost of lots of cement.
I am told that the Marlin Mine in Guatemala is to add cement to their filter-pressed tailings. They will backfill the open pit with this stable material. And continue to entertain NGOs who love trips to that beautiful part of the world.
Now the nice thing about filter-pressed, cement-stabilized tailings is that nobody can bleat about dam-break failure; nobody can fear the worst, for this will be a pile as stable as those old geomorphic cliffs of red muds deposited in old seas; and nobody can say that post-closure is not, in essence, forever.
Obviously this is expensive to do. But if the true cost of mining is reckoned up, and I include the cost to society of taking over the closed site and looking after it forever, then the cost of a bit of cement in the tailings is but a trivial cost.
This proposition is controversial. But that is what blogs are for: to postulate and expose the controversial (the future realities) and leave the rest to the fights of mines, society, regulators, and learned conferences and seminars. I bet there will be no papers on this topic at Paste 2013 in Bello Horizonte where the converted will convene.
That’s a really interesting idea – it even crossed my mind when thinking about Pebble… cool to hear you’re actually doing something with the idea, and that it’s being tried at a mine.
I’m thinking about your analogy to the beautiful rock bluffs – persisting over the long term.
This invites contemplation of what sort of geomorphic processes might operate on “lithified tailings” deposits.
The exact strength of the material really matters. If it’s strong enough that it can maintain healthy soil even on gully walls in a wide variety of possible future climates, that’d be really nice. Otherwise, it’s probably going to be vulnerable to becoming a badlands, the classic example of which is The Badlands in South Dakota. In a badlands, rainstorms liberate pulses of sediment, and if that sediment were rich in iron sulfide I imagine you’d see spikes in acid and metals, potentially leading to fish kills and other consequences.
Also, permeability and other controls on groundwater flow would be really important. In a future where the deposit has a few joints (cracks) running through it, might those become conduits for oxidizing water that bubbled to the surface in acidic springs? This sort of spring is quite common in the natural world, and “tailings rock” could be a supercharged source if it wasn’t an effective seal against oxidation.
That said, it seems like it could be a sure fix to catastrophic tailings failure. I bet itwould greatly increase the timeframe over which serious problems might arise. And it might make some mitigation measures easier.
Last note: 5% x lots of tailings is a lot of cement. Carbon intensity for cement is pretty high… over half the mass of CO2 is released per unit of cement produced (83%, <a href="http://www.iea.org/Textbase/npsum/tracking2007SUM.pdf"IEA 2007, p. 25). Back of the envelope: If Pebble were to be developed in this way, just the cement in its tailings facility would have a carbon footprint comparable to burning all the coal in the Chuitna deposit (~300 million tons coal is < 600 billion tons CO2 compared to 5% x 10 billion tons * 0.83 CO2/cement = 415 billion tons CO2 for Pebble tailings.) Chuitna is the largest proposed coal mine in Alaska – much larger than any other proposals anywhere near as far along.
What you just described is what we call paste fill underground. Tailings by themselves may not have large enough particles to gain optimum strength, so sand may be required. They also do not hold strength well in the presence of sulphides, particularly jarosite. You will need to do some tests.
Instead of adding pure cement you should use a mix of cement at say 2 pct and flyash at 2 to 3 pct. (type c) It’s cheaper, takes longer to set up but eventually attains the same strength. Lastly, consider the heatgenerated by hydration. Perhaps you can use it to grow tomatoes in winter!
You can see paste being made at several mines in Canada, but i would recommend kidd Creek in Timmins. If you go, tell the manager I said hi.
Forgot to mention how good this would br for themining industry. It would double the number of limestone quarries around the world!