by Bruce Golden, Regional Coordinator
Did you ever wonder why there’s such a wide range in the chemical make-up of coal mine drainage discharges? It’s because
there’s a great diversity in the mining environments in which mine drainage is formed. This multipart series of AMP will
explore, on an introductory level, the connection of mine drainage characteristics with the mining environments that created
it. It’s not meant to be rigorous, but to mainly get you thinking about the kinds of things that can happen underground that
are influential in the making of polluted water.
Part 5. Waste Coal Piles… Efficient AMD Factories
In the last three installments, I showed how the mineral pyrite is chemically reactive in the wet oxidizing environment we live in and depend on. Pyrite, as you’ll recall, is typically present in small percentages in coal and surrounding geologic strata. In its natural state, there is little opportunity for water and air to get to the pyrite, thus it’s protected from chemical reactions. Mining changes that big time.
Since the Abandoned Mine Drainage problems we currently deal with were created a long while ago, some dating back to the mid 1800s, it’s instructive to visit some of the mining processes of yesteryear to see if we can figure out how those processes have led to present day problems.
The first mining environment we’ll examine is waste coal piles. These man-made small black mountains go by a slew of colloquial names including gob piles, boney piles, slate dumps, slag heaps, and (as our friends in eastern PA’s anthracite region like to call them) culm banks. I once met a woman who called the pile close to her home Dmitri. I didn’t ask. Whatever we call them, they’re made up of the unwanted stuff discarded following the coal extraction process which most likely came from underground deep mines.
Thousands of waste piles accumulated over the course of mining in Pennsylvania. They are usually located in the vicinity of the old mine portals, as this was the most convenient and economical way of disposal. The piles are generally made up of a good bit of coal and a bunch of stuff that isn’t, including rocks called shale that are very commonly found adjacent to coal seams. (Note to my local newspaper: there’s no slate in waste coal. Please stop calling the piles slate dumps and the stuff in them slate. And while we’re at it, the orange stuff polluting streams isn’t sulfur. ) Why was so much coal discarded back in the old days? First, the separation techniques weren’t that great, and second, smaller pieces of coal had little economic value because the furnaces that burned coal required bigger chunks.
Anyway, throughout coal county it’s still fairly common to see these big, ugly, gray-black, steep-sided, highly eroded, loose-material, get-you-real-dirty, hard-to-walk-on piles with very little vegetation just sitting there as they have for decades. Their size varies greatly from one defunct mining operation to the next. Those associated with larger coal operations are really big and are measured in millions of tons. Sometimes, you’ll see pinkish-orangish streaks and patches in the piles. That’s the residue (sometimes called red dog) from the coal in the pile catching fire, sometimes spontaneously (that pesky oxidizing environment at work again) with a long slow burn that may have taken years to complete.
Because a waste coal pile’s origin is an accumulation of loose, relatively small material, it’s prone to shifts in the material. Think of mini landslides and being loose underfoot. Furthermore, because vegetation has trouble establishing and sustaining itself in this rather inhospitable growth medium, the piles are commonly devoid of most plant life. Without a good root system to stabilize the material, the material remains unstable over time. This is a perfect recipe for erosion. In fact, deep erosion gullies are a prominent feature of waste piles. This can result in siltation problems for adjacent streams. Those streams can also take on a decidedly orange color which, of course, is a very good indicator of AMD. Let’s see how that happens.
We’ll change gears and zero in on a small chunk of coal that might be found in a waste pile. For illustration’s sake, let’s assume it’s a perfect cube, say one inch on a side. That means that each face of our coal cube has an area of one square inch. Since cubes have 6 faces, our cube of coal has a surface area of 6 square inches. Now, the only pyrite contained in our coal cube having any opportunity to undergo that pyrite oxidation reaction we talked about several installments earlier is the pyrite located at the cube’s surface, on those 6 square inches. Water and oxygen can’t get to the pyrite located in the cube’s interior. The places on the cube’s surface where pyrite is exposed are the only places where oxygen, water, and pyrite can all team up and thus the only place that pyrite oxidation reaction can occur. Those 6 square inches of surface area on our cube puts an upper limit on the amount of pyrite that can oxidize on that piece of coal.
Okay, now let’s say we take our cube and carefully split it in two, making the split parallel to a cube face. We wind up with two rectangular solid chunks of coal. I think we can all agree that the original 6 square inches of surface area of the combined two pieces are still intact. In addition we have two new faces resulting from the split, each one having one square inch of surface area. So the two chunks of our carefully constructed mind experiment now have a combined surface area of 8 square inches. If we do a little ciphering, that comes out to a 1/3 increase in surface area with the opportunity of having exposed pyrite that can react. More exposed pyrite raises the limit of the amount of pollution that can be formed. Note: In the real world, we don’t see perfect cubes and often cracks, fissures, or fractures allow water to penetrate the interior regions of solid coal.
To generalize, when a solid is fractured or broken up into smaller pieces, the resulting surface area will increase. We’ve also seen that the pyrite reaction is dependent on having the pyrite exposed on a surface. Putting these facts together, it becomes clear that a waste coal pile, made up of lots and lots of relatively small broken-up chunks, is going to have an enormous amount of surface area with exposed pyrite which can be available to react when united with water and oxygen.
A waste coal pile is usually quite porous to water infiltration. This is particularly ominous when it comes to AMD formation. When it rains, water can easily find its way to a tremendous amount of exposed pyrite. On the pile’s outermost surface, the supply of atmospheric oxygen will be substantial. As water infiltrates into the pile’s interior regions atmospheric oxygen will become more scarce, yet water itself can transport oxygen in the form of dissolved oxygen. Even with available oxygen being the limiting factor, the initiating and most important reaction of AMD formation, the oxidation of pyrite, has tremendous opportunity to occur. Furthermore, pyrite not reacting during one rain will have the opportunity during the next, or the next, or the next after that. For piles that can interact with ground water, the situation is even worse: an almost continual source of water available as a reactant in pyrite oxidation (limited only by available oxygen).
The pyrite oxidation reaction occurs in copious amounts when it rains. The other AMD reactions described in previous installments, i.e. the ferrous to ferric oxidation and the hydrolysis of ferric ions, may also occur in or on the pile. The extent to which they occur will depend on the amount of available oxygen, the pH, and the presence of certain bacteria in the water. In addition, some complex chemical reactions (which are beyond the scope of this article) may result in the formation of highly reactive sulfate salts, easily visible as a light greenish-yellow solid. In essence, these solids are concentrated instant AMD, ready to dissolve during the next rain. No matter what, once the initiating pyrite oxidation reaction has occurred, the AMD express has left the station.
To sum things up, waste coal piles can be very prolific producers of AMD because of the tremendous amounts of exposed pyrite. Just add wind and rain with a dab of chemistry and you’ve got a pollution factory.
