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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“Acid Rock Drainage”. Quantification of Metal Loads by Tracer Injection and Synoptic Sampling in Daisy Creek and the Stillwater River, Park County, Montana, August 1999. http://pubs.usgs.gov/wri/wri004261/images/cover.jpg

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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