My dog and I were out for our usual morning walk. It had been a while since we last had a discussion about politics and stuff. Most of our recent conversations have involved rabbits. There sure are a lot of them in my neighbourhood this year.
So it was surprising when she asked: "What is a tailing pond?"
It was a bit out of the blue but I replied: "It is a large underwater storage facility for the excess rock that is pulled out of a mine pit. Why do you ask?"
"Well, I have a tail, you see ..." she said.
"No. It has nothing to do with tails" I said with a chuckle. She tends to get confused about the multiple meanings that humans sometimes attach to words.
"So why do mines need to store the excess rock?" she asked.
"Consider what a mine is. It is a giant hole in the ground. It can be an open pit mind which literally looks like a hole in the ground. Or it can be a tunnel carved into hard rock." I replied "In any case, you have to move all of the stuff that is in the way to get to the stuff that you want - the minerals or ore body.
"More importantly, the elements that you are after - say copper or gold - are mixed in with all of these other minerals. You get them out by separating them. Tailings are the stuff that is leftover after you separate the elements that you want." I added.
"Is it a bit like when I bury one of my bones?" she asked.
"Almost. Except a better analogy would be when you dig up one of your bones. You are moving all of the top soil aside temporarily so that you can get to the thing that you want. In your case, it is the bone that you are after and all you need to do is clean off the dirt. In the case of the mine, it the ore that you are after but you must remove it from the rest of the rock. The leftovers are tailings." I said.
"You know, I can never remember where I bury my bones and I don't fill the holes back in." she said.
"Yeah. I know. I have to do it for you. That is one of the concerns people generally have about mines. Who is going to clean up after the mining company has finished digging out all of the useful rock? There is legislation that says that the company has to clean up but lots of people don't think that they will." I said.
"Oh" she said "So where do these tailing ponds come into play?"
"Well, sometimes the excess rock that the miners dig up contains specific types of minerals. For example, iron pyrite ..."
"Oooo. I know what that is! It's fool's gold. We used to find it at the dog park, right?" she said.
"... yes, we did. I am surprised that you remember. It is a mineral composed of iron and sulphide ions and it is very common." I said.
"When it is in the ground, it is not really much of a problem. It can't really go anywhere. More importantly, when it is deep in the ground, it isn't exposed to air or water so it doesn't decompose. But when someone digs it up, then it can easily be exposed to oxygen. It starts to react and will eventually give an acidic solution."
"Is this another one of your chemistry lessons?" she asked. "You know how I feel about chemistry."
"Well, mining really is a combination of engineering and chemistry, so it is kind of hard to avoid. But to keep it simple, when the rock that is deep underground is brought to the surface, it needs to be treated to remove the elements that the miners are after. For example, they might float away the gold and copper." I explained.
"This treatment process might alter the rock a little bit or maybe even a whole lot. The result is that the tailings are not simply dug up rock but a little bit more complex. When they are impounded in a tailing pond, they might also undergo further reactions. Generally, though, the tailings settle out as sediment on the bottom of the pond leaving fairly pure water above.
"The sediment can be rock that, over time, generates acids or bases. It can contain elements such as arsenic or lead which are now available when they used to be buried deep in the ground. The result is that tailing ponds are a strange and complex mixture of chemical compounds."
"You know, this is all too complicated for me" she said. "Let's just look for rabbits."
