Missouri University of Science and Technology
Acid Rock Drainage
Josh Huighe
Cameron Williams (Wrote Section IX)
MET 203 – Extractive Metallurgy
May, 2014
TABLE OF CONTENTS
Page
I. Introduction 3
II. Acid Rock Drainage Overview 4
III. Factors that Affect Acid Rock Drainage 6
IV. Environmental Issues Associated with Acid Rock Drainage 7
V. The Effects of Varying pH Values 8
VI. Metal Leaching Caused by Acid Rock Drainage 9
VII. Examples of Mines with Acid Rock Drainage Issues 10
i. Newmont Rain Facility in Elko County, Nevada 10
ii. Iron Mountain Mine near Redding, California 11
VIII. Iron Mountain Mine near Redding, California 13
IX. Outline of an Acid Rock Drainage Management Plan 14
i. General Overview 14
ii. Size Characterization 14
iii. Mine Description 15
iv. Design 16
v. Alternatives 16
vi. Analyzing and Selecting a Solution 17
X. Conclusion 18
References 19
I. Introduction
It is estimated that the average American baby born this year will use approximately three
million pounds of minerals, metals, and fuels in his or her lifetime, all of which must be obtained
from somewhere on Earth. As a result, mining is one of the largest and most important
industries in the world. Mined items are everywhere around us in our society and we would not
be able to live without them. Some mined metals may be very common such as the copper, zinc,
or nickel that are likely in your pocket right now in the form of coins or keys, however others
such as gold, silver, and platinum are less common. While not all are as commonly used as
others, all are obtained from mines in the ground none the less.
While mining may be done so frequently that most commercial mines have successful processes
in place to extract the metals from their common sulfide or oxide ores in a successful manner,
like all parts of life, issues still exist. One common issue with mining that will be discussed in
this paper is acid rock drainage (ARD). Acid rock drainage is the tendency of sulfide rock ores
to produce acidic water when exposed to oxygen and water. This acidic water can be a potential
issue for nearby wildlife, the environment, and the desired metals. The goal of any mine dealing
with sulfide ores is to contain or eliminate acid rock drainage as much as possible. This paper
will discuss what acid rock drainage is, how it works, what its potentially harmful effects are,
some examples of its impact on real mines, and how to control it.
II. Acid Rock Drainage Overview
Acid Rock Drainage refers to acid water that forms when sulfide minerals are exposed to water
and air. The acid that is produced during this naturally occurring reaction can be potentially
harmful to both the success of the mine, and to the wellbeing of the environment. An example of
the impact that acid drainage can have is seen in Figure 1 on the following page. The
yellow/orange colored water is very acidic and it can impact nearby wildlife as well as other
potential uses of the creek such as for drinking water. This acid is most commonly produced
through the formation of either SO4 or Fe(OH)3. Fe(OH)3 is commonly referred to as “yellow
boy” due to its bright yellow appearance in creeks and streams. Examples of the common
reactions that produce these acids are seen below. Equation 1 is when pyrite (commonly referred
to as fool’s gold due to its gold appearance) is reacted with water and oxygen. In the reaction,
S22- is oxidized to form hydrogen ions, sulfates (dissociated products of sulfuric acid in solution),
and soluble Fe2+. Equation 2 is the reaction that occurs when Fe2+ is allowed to further react with
air. Equation 3 is the sequential reaction of the products from Equation 2.
• Equation 1: 2FeS2(s) + 2H2O + 7O2 --> 4H+ + 4SO42- + 2Fe2+
• Equation 2: 4Fe2+ + O2 + 4H+ --> 4Fe3+ + 2H2O
• Equation 3: Fe3+ + 3H2O <--> Fe(OH)3 + 3H+
Equation 1 produces SO4 and H+ ions which can react to form H2SO4 which is known as sulfuric
acid. Sulfuric acid is a very strong acid that is soluble in water. It is highly corrosive and can be
harmful to humans as well as fish, plants, and other wildlife. Equation 3 produces Fe(OH)3
which is a strong acidic hydroxide that can appear in creeks and streams as a nasty yellow
substance known as “yellow boy” (Seen in Figure 1) as previously stated due to its lack of
solubility in water. For many mines, the appearance of “yellow boy” acts as an obvious sign that
they have a potentially serious problem that needs to be addressed in order to ensure the success
of their mine both now and in the future.
Figure 1: An example of acid drainage from a mine that is flowing down a nearby creek.
Image courtesy of wikipedia.org
While pyrite is a good mineral to use as an example for explaining acid rock drainage, many
other sulfides exist that have a similar reaction with water and oxygen. Table 1 on the following
page shows a common list of these sulfides. Any mining industry that works with sulfides has
the potential to encounter issues with acid drainage. While some minerals react more violently
than others, all mines must be conscious of it before it can become a large problem.
Table 1: Common Sulfide Minerals and Their Chemical Formulas
III. Factors that Affect Acid Rock Drainage
Some factors that affect the time needed for acid rock drainage to occur are the temperature, the
oxygen content, the water availability, the type of mineral, and whether or not microorganisms
are present. Temperature affects the acid drainage by affecting the formation of the sulfides that
are present. For example, in a low temperature environment, pyrite will be very poorly
crystallized. This will result in more acid drainage. In terms of the effect of oxygen content and
water availability, the more of each that is present, the more likely it is for acid to be produced.
Next, the type of mineral has an effect on acid formation because if a mineral has a larger
particle size it will be more exposed to oxidation and weathering. Other features of a mineral
that have an effect include pore size, permeability, and mineral composition. Finally,
microorganisms also have an effect on acid drainage. Colonies of bacteria and archaea can
Mineral Composition
Pyrite FeS2
Marcasite FeS2
Chalcopyrite CuFeS2
Chalcocite Cu2S
Sphalerite ZnS
Galena PbS
Cinnabar HgS
accelerate the decomposition of a metal. They can survive in harsh conditions, however they
only can survive in environments that have a water and oxygen supply. This is a problem
because if the conditions are right for acid to be formed, then microorganisms will be provided
with the necessary environment to survive as well.
IV. Environmental Issues Associated with Acid Rock Drainage
There are many issues associated with acid rock drainage. The environmental impact of ARD
hinges primarily on the size and sensitivity of water body that is affected, and the amount of
neutralization and dilution that occurs. For instance, the same volume of ARD would have a
much greater impact on the quality of water in a stream than it would in the ocean. The ocean
would be able to dilute the ARD more than the stream would. Also, salt water has stronger acid-
buffering capacity than fresh water. Basically, the ocean is able to naturally combat the low pH
levels to a greater extent.
One of the largest impacts of acid rock drainage is its effect on the ecosystem with which it
comes in contact. ARD could leak into the surrounding soil, a stream, a pond, or even the ocean.
Wherever it enters, it can have hostile impacts on the health of aquatic animals, insects, and
plants. Acid rock drainage is well known for the amount of fish that it kills due to the low pH
levels. It is also important to note that the metals (nickel, copper, lead, aluminum, manganese,
etc.) that dissolve and are associated with ARD are usually even more toxic to fish and aquatic
organisms than the acidity is.
Another problem caused by acid rock drainage is the corroding effect of the acid on various parts
of infrastructure. Most common issues are with bridge support structures, culverts, and pipelines.
Once ARD gets ahold of something metal it does not take long for corrosion to take place.
Corrosion is also a major source of metal contamination in drinking water. Some metals that are
possible causes of contamination are lead, copper and zinc. Metals which are frequently used in
household plumbing and that may be affected by a low pH are copper and zinc. This may cause
the water to become toxic and harmful to those drinking it. It is clear that this is a serious
problem that must be controlled.
V. The Effects of Varying pH Values
pH is an expression of hydrogen ion concentration in water. The term is used to specify the
degree of acidity or basicity of a solution, with a pH 7 being neutral. As the concentration of
hydrogen ions in the solution increases, acidity increases and pH gets lower (below 7). When pH
is above 7, the solution is considered basic.
When sulfide minerals are exposed to water and air, sulfuric acid is formed through a natural
chemical reaction. This acid lowers the pH value down to around zero. In certain circumstances,
the pH can even become negative if water evaporates off of the acidic pools. At a low pH,
microorganisms which cause ARD can start to thrive. These microscopic bacteria and archaea
accelerate the decomposition of the metal. In other words, they speed up the rate of sulfur
oxidation. The only way to limit these organisms is to cut off the water and oxygen supplies.
This is very hard to do considering waste rock is piled up outside and in constant contact with
wind and rain.
Interestingly, pH levels are not always a good way of indicating acid rock drainage because they
only indicate the concentration of hydrogen ions. When figuring out the extent of ARD, the key
is knowing the hydrogen ions remaining in solution after a streams natural buffering system is
complete. Streams have carbonates that will dissolve and cause high alkalinity to counter-act the
acid rock drainage. Eventually the ARD exhausts the buffering ability of water by neutralizing
the carbonates to form carbonic acid (H2CO3). When the buffering system is unable to react
anymore, usually at a pH of 4.2, the water way suddenly becomes acidic and it is too late to
reverse the damage. Measuring the excess of hydrogen ions over basic ions, "total acidity," is a
better measurement of ARD within a stream.
VI. Metal Leaching Caused by Acid Rock Drainage
Many times when there is acid rock drainage, there is also metal leaching. Through the same
reaction as ARD, toxic metals can be leached into the environment. The difference is that these
metals don’t need acid to dissolve, they can dissolve at neutral or basic pH levels. There are two
ways that a basic pH can occur: if the rock contains a lot of carbonate, or if the mineral
processing requires a high pH. For example, when extracting gold, cyanide is used. pH must be
kept high in this situation to avoid the formation of cyanide gas which can kill a person. Alkaline
(basic) water can cause arsenic, cadmium and selenium to dissolve. These are toxic and they can
commonly kill fish. If they are in drinking water, they can reduce growth and cause physical
deformities.
VII. Examples of Mines with Acid Rock Drainage Issues
i. Newmont Rain Facility in Elko County, Nevada:
The Newmont Rain Facility was a mining-milling-leaching operation for gold ore. At the
facility they open pit mined both ore and waste rock. In 1991 the facility on average mined just
5,500 tons of ore versus 29,500 tons of waste rock per day. Of the ore that was mined, the gold
concentration was only 0.01-0.15 ounces per ton of rock. This small concentration resulted in
just 55-825 ounces of gold being mined per day versus 7,500 tons of sulfides and 22,000 tons of
oxides being mined each day. While these numbers do not show a very profitable operation to
begin with, this facility also had a major issue with acid drainage. Prior to 1990 the sulfides,
oxides, and other waste rocks were disposed of together in waste dumps near the mine. The
waste dump was openly exposed to both air and other outside elements. Also, the location where
the dump was located was near a spot in the mountainous region in which snow had to be
displaced for transportation purposes. In order to not slow down the total operation of the mine,
the snow was removed from the roads and was thrown on the dump. This problematically
caused the melting snow to cause the sulfides to oxidize and produce acid. The mine produced
acid drainage containing red-brown precipitates that flowed into a nearby creek and spring. This
produced problems for the wildlife near the mine which included the death of several fish.
Water samples were taken at points near the spring and along the creek to measure the pH
present. At points near the waste rock dump, pH reading ranged from 2.37-3.21. These very
acidic values could be compared to those of lemon juice, beer, and vinegar. At points further
from the dump about 4,000 feet downstream, the levels were far better with pH values of 6.5-
8.64 which could be compared to milk, water, and human blood. While these values would not
create too much of a concern at this distance, the readings near the dump were a different story.
In 1991 the EPA visited the mine and threatened to shut down the operation. In order to stay
open, the mine came up with several common solutions. Their short term response was to build
a small pond to collect the flow from the dump. However, this just delayed the issue rather than
solving the problem. Eventually, the mine’s long term response was to encapsulate the sulfuric
waste rock within oxidized waste rock in order to prevent the acid drainage. This solution was
fairly successful. However, in 1995 the mine closed because they decided that the amount of
gold produced was not worth all of the work that went into the rest of the operation.
ii. Iron Mountain Mine near Redding, California:
The Iron Mountain Mine (Seen in Figure 2 on the following page) was a massive sulfide deposit
that was mined for iron, silver, gold, copper, zinc, and pyrite. At one point it was known as the
most productive copper mine in the United States. This title brought the mine a lot of publicity
and in return a lot of business throughout most of the 20th Century. The mine encompassed about
4,400 acres and it included several mines, a flotation mill, a loading station, and a large reservoir.
The major operation of the mine was to produce copper. However, the mine was originally
founded in the 1860’s after an large iron deposit was discovered. Surveyors noticed a bright red
color on the mountain which turned out to be an area full of oxidized iron ores. A mining
operation for pyrite followed this discovery.
This mine was very successful for many years, however eventually it was observed that the mine
had large amounts of acid drainage that did not meet government regulations. Some areas near
the mine contained water that had pH values as low as -3.6. This pH is about 1000 times more
acidic than battery acid. These high acidity levels were strong enough to not just cause problems
for wildlife, but the acid was also strong enough to leach harmful and toxic heavy metals such as
arsenic and lead from the mountain rather than just the desired metals. This problem was
eventually determined to be beyond the point of fixing. In 1963 the mine was closed and
eventually in 1983 the EPA designated the mine as a Superfund site. The area had a large
amount of fish kills and it required active maintenance in order to prevent the acid from also
contaminating nearby drinking water. It was determined that no future human habitation could
take place in the area. Recently in the year 2000 a settlement was reached between the
government and the mining company for a long term cleanup effort.
Figure 2: The Iron Mountain Mine reservoir and mining facilities.
Image courtesy of epa.gov
VIII. Government Regulations and the Impact of the EPA
Regulation of acid rock drainage falls largely under state requirements. However, the U.S.
Environmental Protection Agency (EPA) can and will get involved. Usually the state will
provide specific recommendations to combat ARD. There are no formal policies or regulations
that specifically address acid rock drainage. Though, the mining companies are required to set up
reclamation and operating plans just in case of an outbreak.
The EPA has compiled methods that are being used to predict acid formation. It starts with
sampling. The purpose of testing the rock is to classify and plan for waste disposal based on the
predicted quality from that material. Samples must be selected based on the type and volume of
the rock materials and also account for the variability of materials that will be uncovered when
mining. Researchers have agreed that collection of samples happen during the resource
evaluation and mine planning stage. This way the company will have time to prepare and know
what to expect. The next step is to start some static tests. Static tests predict drainage quality by
comparing the sample's maximum acid production potential (AP) to its maximum neutralization
potential (NP). If the difference between NP and AP is negative then there is a potential for the
waste to form acid. If it is positive then there is a much lower risk. After static tests, kinetic tests
should be run. These tests usually use a larger sample size and require a much longer time to run
than static tests. Kinetic tests provide information on the rate of sulfide mineral oxidation which
causes acid production, as well as an indication of drainage water quality. Once all of the tests
have been compiled, a prediction analysis is made. These tests classify waste based on their
likelihood of generating acid. The last step of predicting acid formation is modeling the acid
generation potential. Static tests yield information about a sample's ability to neutralize and
generate acid. Kinetic testing is more helpful in regard to estimating the rates of oxidation and
neutralization. Whenever these two tests are combined companies can create models which may
provide valuable information for planning purposes, and may have an important role in
understanding and predicting ARD.
IX. Outline of an Acid Rock Drainage Management Plan
Once a mine has been found to be generating acidic water that exceeds acceptable limits, an Acid
Rock Drainage Management Plan (ARD-MP) should be established. The purpose of this plan is
to help provide acceptable protocol for dealing with acid rock drainage. This plan is divided into
multiple parts.
i. General Overview
This section discusses the parts of a typical Acid Rock Drainage Management Plan (ARD-MP).
It will describe the objectives, scope, and contents of each section. This section also provides
some knowledge about the technical, scientific, and engineering issues involved and what needs
to be done to compile the necessary technical approaches to actually manage the acid rock
drainage at a site.
ii. Size Characterization
In this section of the mine's ARD-MP, as much as is relevant to the production and management
of acid rock drainage at the site is described.
An Environmental Impact Statement (EIS) for the mine should be established which shows that
the acid rock drainage is manageable at the site. Much of the information that is needed in this
section of a mine's ARD-MP should be in the mine's EIS.
An environmental impact statement (EIS), under United States environmental law is a document
required by the National Environments Policy Act (NEPA) for certain actions "significantly
affecting the quality of the human environment". An EIS is a tool for decision making. It
describes the positive and negative environmental effects of a proposed action, and it usually
also lists one or more alternative actions that may be chosen instead of the action described in the
EIS. Several state governments require that a document similar to an EIS be submitted to the
state for certain actions. If a mine is producing acid rock drainage it will require this document.
If there has not been an EIS for the mine, or the EIS was compiled and approved a long time ago,
a new EIS may need to be completed. This data will include information about: site and regional
location and topography; site and regional geology; geochemistry; climate; surface water;
groundwater; and the receiving environment that may be impacted by acid rock drainage.
iii. Mine Description
In this section of the ARD-MP the layout of the mine and its associated facilities are described.
This will involve text and figures that provide information about the mine workings; the
underground mine shaft and adits if this is an underground mine; the open pit and its slopes and
sequential development if this is an open pit mine; the heap leach pad if heap leaching is
undertaken; the tailings impoundment and its operation; the waste rock dumps; and the surface
water management facilities, including diversion swales, channels, and ditches, and any ponds
and water retention facilities that are on site.
The data about the rock and soils and how they will generate or be precluded from generating
acid rock drainage should be described in this section of your ARD-MP. There is no substitute
for test results from laboratory work aimed at establishing the acid generating potential of
potentially suspect soils and rock.
iv. Design
In this section of the mine's ARD-MP, it should describe the design of all facilities that may
generate acid drainage. This will involve documenting the layout of such facilities, the size, the
operation, and the closure works. This is not a design report. The objective is to provide a full
description that establishes that all options and alternatives have been considered. Next, it should
be established that the mine has selected the approach that minimizes acid drainage or that
enables them to best (most cost-effectively) manage acid drainage. It should provide enough
information to enable those charged with constructing, operating, managing, and closing the acid
generating facility to do so while staying with the initial intentions of the designer.
v. Alternatives
All design involves the identification and comparison of alternatives. Identify a range of
alternative approaches to facilities that will contain acid generating materials. Describe the
alternatives and document the process of identifying and comparing alternatives.
Basic decision making theory may be applied. In addition it is recommended that a value
engineering workshop be completed. Obvious alternative solutions include: encapsulation of
acid generating materials, direct acidic seepage to attenuating materials, and/or collect and treat
acidic seepage. But there are as many alternatives as there are mines and waste disposal
facilities.
vi. Analyzing and Selecting a Solution
Once a reasonable range of practical alternatives to limiting and controlling acid rock drainage
has been identified, the next step is to analyze the alternatives and select the best. This part of
the process, as documented in the ARD-MP, depends primarily on the criteria applicable to each
mine. If in the United States, it will probably be faced with National Pollutant Discharge
Elimination System (NPDES) discharge criteria. These are simple: do not discharge water from
the mine of the quality of which exceeds present limits-generally the discharge water should not
be acidic.
In Canada, it may be possible to adopt a more subtle approach: dilute by directing the discharge
to larger bodies of water, one of the many lakes or swiftly flowing rivers.
If you are in a primitive jurisdiction, it may even be enough to establish own criteria including
providing alternative sources of water to those whose primary source is affected by acidic
discharge. Assuming that the mine has established the discharge criteria and assuming they have
more than one approach that results in achievement of these discharge criteria it will probably
seek to implement the least cost approach. This goes without explanation. Why pay more to
achieve the same end?
X. Conclusion
Acid Rock Drainage is a problem that affects a large variety of mining industries. Anytime a
sulfide ore is in the presence of air and water at one of these mines, acid may be generated.
While it is impossible to completely eliminate the possibility of acid drainage, it is possible to
contain it. By decreasing the factors that affect it, and by establishing a plan for how to deal with
it, a mine can successfully operate without acid drainage becoming too large of a problem.
Many companies have fought with this issue in the past, but while it may be bad for business, the
real losers are the fish, plants, and other wildlife that live nearby. Environmental sustainability is
an important issue that all mines must deal with, and preventing acid drainage from damaging
the environment is one step in helping to ensure that the world is left in a useable condition for
future generations.
Figure 3: Photo of acid drainage in a small stream. Courtesy of usgs.gov
References
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D. K. Nordstrom, C. N. Alpers, C. J. Ptacek, D. W. Blowes (2000). "Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California". Environmental Science & Technology 34 (2): 254–258. doi:10.1021/es990646v
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