Last Updated 9/05
Links relating to acid mine drainage.
About Acid Mine Drainage: Acid
(H+ ions) is one of more important pollutants that can result
from mining, particularly sulfide ore mining for metals such as
lead, copper, zinc, and molybdenum. US states with a history of
mining of sulfide ores, such as Colorado, Arizona, Utah, and others,
face significant threats to their streams and wildlife as a result
of drainage produced by abandoned mines.
Acid mine drainage results from the decomposition of pyrite, usually
catalyzed by bacteria, to produce iron hydroxide and dissolved
H+ and SO4 -2 (sulfuric acid). Most acid mine drainage in the
US comes from coal mines or coal waste piles, which contain small
amounts of pyrite. Most of the remaining acid drainage comes from
pyrite in metal mines or wastes. .
Pyrite is a sulfide mineral (FeS2), similar to the sulfide minerals
galena (PbS) and chalcopyrite (CuFeS2) that are common ore minerals
of lead and copper, respectively. These minerals are commonly
all found in a given sulfide ore body, so acid mine drainage is
a particularly prevalent environmental consequence of sulfide
mining.
Metallic minerals are more soluble in acid water than in pure
water; thus acid formed by dissolution of pyrite dissolves more
pyrite, and this acid water then dissolves other sulfide minerals,
further acidifying the water, and so on, causing a feedback. In
addition, acid water moving downstream can mix with less acid
water, causing dissolved iron to precipitate as iron oxides, a
reaction that produces still more acid.
Because metals are more soluble in acid, the production of acid
water can cause metal ions to be dissolved from other minerals
besides sulfide minerals as well. Metals that are adsorbed to
the surface of clay minerals in rock or soil can also be leached
by acid water. These dissolved metals can cause health effects
separate from the effects of the acid itself. Dissolved metals
are a particular threat to aquatic animal life.
Sulfide ore deposits are natural bodies. Why are we not generally
concerned about acid drainage from these bodies until they are
mined? The reason is that the acid-forming process requires water
and air to come into contact with the mineral surface. In an intact
rock beneath the surface, water flow is restricted to limited
fractures in the rock, and it is typically poor in oxygen. Even
in a large body, there is limited surface area exposed to air
and water, so the acid-forming reactions are very slow, so there
is no significant pollution problem.
But if you remove some of that rock in the mine, you leave behind an open pit
or tunnel in contact with air, through which water can flow freely. Even more
significant is what happens to the ore that is mined. The reactions that produce
acid require a mineral surface on which to occur. The more surface area to the
reaction, the faster its rate. If you take a cube of something such as ore with
sides 1-inch square, you have 6 square inches of surface area. If you break
that cube into 8 cubes, each with 1/2-inch sides, you have 8*6*1/2*1/2 = 12
square inches. Do that again with each of the 8 cubes and you have 24 square
inches. The surface area increases geometrically as the grain size gets smaller.
In copper sulfide mines, for example, the mined rock is crushed into rock flour
so that the desired copper can be dissolved out of it by (you guessed it) acid.
The resulting copper-rich solution is processed to produce copper metal. The
rock flour remains, with all its other sulfide minerals such as pyrite. The
surface area of all the grains of rock flour is astronomically greater than
the surface area of the original, intact rock, so the flour is extremely reactive.
This waste is called tailings, and it is disposed of in tailing piles
or tailings ponds, from which acid drainage can potentially escape.
(179K GIF)
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