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There exists an acidic pit known as the berkeley pit. I'll try to link it properly later.
Currently, 13 million gallons of water per day are processed for copper cementation, and it is thought 3 million gallons enter the pit via underground aquifers.
The fact water is moving underground contributes
to the acid levels, new acid enters the pit by going through the acidic rocks.
The proposed idea is this. try to freeze as much of the top of the pit water as possible. The area is below freezing naturally for about two months of the year, but I believe some gains can be made by freezing the thing year round.
A wall and windbreak must be contructed to reduce the rate at which cold air escapes, but gravity should be our friend. Since the rim of the pit is at elevation 5509' and the critical water level of 5410' is well below that, I believe the amount by which the nearby city of Butte would be chilled is minimal. if the windbreak is well designed, nearly all the "fog" should stay in the pit.
The freezing power should come from two sources: air liquefaction, and freezing the 13 million gallons per day after copper extraction. Bonus if we can process it further and maybe get some iron out, too.
Every hour of daylight above freezing, an amount of liquid air is dumped into the pit, enough to shroud the entire water suface in cryogenic fog. This will keep the existing ice from melting. The copper-depleted water can be frozen to well below freezing and shaped into spheres before being returned to the surface, these spheres can roll across the surface, evently distributing weight. Yes, the ice will crack in spots, but between the coldness of the ice balls and the liquid air the small cracks should mostly refreeze.
By keeping the copper-depleted water frozen year round, this should preserve the richness of the copper resource pumped from underneath the ice.
Currently the copper pump is located 150 feet below the surface, and water rises about 6-12 feet per year. If an ice sheet can be built at a rate faster than 13 feet per year, it would still take decades for the ice to be pushed all the way down to the point where copper is pumped up.
Meanwhile I suspect having the top of the pit frozen would reduce the chance of acid flowing into other aquifers. it should help water stay put.
And maybe if we do this for two decades or so, the problem will be more manageable even if some new technological advancement comes along.
--addendum:
another step can be taken to increase copper extraction and reduce the total volume of acidic water: drop "rocks" into the bottom of the pit.
this can be done as an alternative or in addition to freezing the pit over. Of course, the water treatment facilities would need to release extra water to offset any rocks dumped into the pit. Said "rocks" could be mine tailings (waste) encased in an acid-resistant shell of some kind. A shell might not even be needed, if it's all going to be toxic sludge anyway.
Berkeley_Pit
http://en.wikipedia.org/wiki/Berkeley_Pit
the site. [Madai, Feb 04 2013]
liquid air fog effect
http://www.youtube....watch?v=w2mj-Sq2oeo
the fog, on a smaller scale [Madai, Feb 04 2013]
source info
http://www.pitwatch.org/water.html
just some background [Madai, Feb 04 2013]
chemocline
http://www.pitwatch...Chemocline-2009.jpg
[Madai, Feb 04 2013]
Pitwatch FAQ
http://www.pitwatch.org/faq.html
[scad mientist, Feb 05 2013]
proposed icing-over
http://i.imgur.com/SGX40j2.jpg
if effective, the acidic part of the lake will never mix with the top 100 feet even after the ice melts. [Madai, Feb 06 2013]
[link]
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Is the intent to produce solid water which doesn't flow, rather than liquid water which does flow? If so, then this will not stop backflow into the aquifers. The reason there is no backflow now is that the surface is below the local water table. Pressure is therefore lower than the local water table pressure, causing water to flow always inward. Once the water level rises to the level of the local water table, water will flow through the pit, rather than just into it. Capping it with ice will not stop this. |
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Good point, Freefall, even if the pit is frozen, the water level cannot be allowed to rise above the critical level. |
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However, the ice should reduce the flows through the alluvium, leaving only the smaller bedrock flows to deal with. Once everything frozen, less fresh water should be entering the pit from the alluvial rock, keeping it in the freshwater wells. |
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This should also increase the concentration of copper in the unfrozen water, as it is no longer being diluted by return flows. |
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I'm adding an illustration of the chemocline. If the ice works, you'll have a new layer on top, with even higher PH and lower iron/copper concentrations. |
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Another thing I hope to acomplish by freezing the thing over year round is less dead birds. I am hoping that birds would opt not to try to land on an ice sheet shrouded in cryogenic fog. If I am wrong, naturally we'd have to halt the whole thing, |
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//13 million gallons of water per day are processed for copper cementation, and it is thought 3 million gallons enter the pit via underground aquifers.// Where does the other 10m gallons come from ? |
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It rains a lot where [Madai] is. |
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Maybe they just pour the water back into the pit after the "copper
cementation" process? That's what usually happens with the
water after the dissolved ions are precipitated out of solution,
during cementation. |
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The pitwatch FAQ answers a lot of those questions. |
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Yes, the 13 million gallons are put back into the pit after cementation. |
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There is about a foot of rain and 2 feet of evaporation each year. |
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Perhaps it would make more sense to actively extract
the metals (cementation, anode recovery and
flitration)
from the water and use the clean water for irrigation? |
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There will always be mineral leachates in the water,
though
concentration will reduce over time, to background
levels, as the aquifer and mine benches will still be
mineralised. |
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BP is stuck with a cleanup for a 19th century problem,
not of its
making. It will have to resolve before the water
overtops the
containment in about 7 years. |
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The Berkeley Pit is, unfortunately, not in an area that needs much irrigation. In fact the abundant groundwater is part of the problem. |
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Here's my line of thought. In 2022, they will slightly modify what they do now, pump up 13 million gallons, remove copper, release ~10 million gallons back to the pit and send ~3 million cleaned gallons down the Clark Fork river to offset the ~3 million that seeps in from bedrock. |
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If 3 million gallons translates to about 6 inches of pit water. 13 million is about 2 feet of pit water. |
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1000 foot deep pit means they could easily keep this up for 500 years even if no new acid enters the pit, each year, the copper concetration dropping just a little bit and thus them getting slightly less copper with each day's worth of pumping, until it's no longer cost effective. |
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But if the copper-depleted water remains frozen, it will not mix and dilute the copper-rich water. And if the water is needed for irrigation in the future, you can always go down and saw off a chunk of ice. |
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That pit will just keep coming up with new problems without end, it seems. |
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I visited there in 1970, when my pa's cousin's house was about a hundred yards from the edge of the pit. She and her husband had just finished building a new foundation for their house - in Whitehall, on the other side of the Continental Divide. The mining company paid all the expenses to have their house uprooted from its old spot, and trucked 40 miles over the mountains. |
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When you'd drive around Butte, on almost every block there would be a house or two sitting sagged - or collapsed completely - from the cave-in of the old mine shafts hidden below. So it was kind of a race - the pit (slow, but sure), the shafts (quick, unpredictable) or the movers (hold your breath - sometimes the moving process collapses a shaft underneath). |
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Why not just push the mountain into the pit and fill it in? Like filling any other slow area where you do not want water to accumulate. |
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