Should We Recommend Building a
Copper Mine?
Student Manual
Mary Walczak
St. Olaf College
Linda Zarzana
American River College
Doug Williams
Kalamazoo College
Paul Charlesworth
Michigan Technological University
B
W. W Norton & Company
New York London
What Is the Role of a Copper Mine Consultant?
CREATING THE CONTEXT
Rancher Tom Sullivan owns a ten thousand acre ranch in central Colorado.
Last month the Copper Mining Company approached Tom to investigate the
possibility of buying and developing a portion of his land as a copper mine.
Tom was tired of ranching and welcomed the interest in his land because he
saw it as a way of decreasing his operation in a move toward retirement.
The local ranching interests had been declining over the last decade and
Tom thought that the Copper Mining Co. may provide a means for him to
make a profit on selling off some of his land.
Tom was hesitant, however. He had lived on this land since boyhood and
had some reservations about turning it over to a mining company. Tom did
agree to a feasibility study on the impact of the mine, provided it was
conducted by an impartial outfit. The mining company agreed to pay the
costs for the study. Together Tom and the Copper Mining Co. chose Earth
Explorations as the consulting firm to perform the study. The geology
division of Earth Explorations surveyed the land in an effort to find the
copper-rich areas of the ranch and collect samples for lab analysis. The
geological study indicated the northwest portion of Tom’s land contained
some copper-rich veins. Samples were collected from a square mile region in
this portion of the ranch.
You are part of the Earth Explorations team that will be performing the
feasibility study. Your team will analyze the collected ore samples,
investigate copper recovery and purification options and make a
recommendation to Tom Sullivan and the Copper Mining Co.
PREPARING FOR INQUIRY
Although Earth Explorations has consulted on mining projects before, as a
recent hire you do not have extensive mining background. To prepare for
the first team meeting review the information found at the Web sites below
or other resources specified by your instructor.
Copper mine process flow sheet
http://www.mine-engineer.com/mining/copperm.htm
http://www.mine-engineer.com/mining/small_copper.htm
Copper from mine to market
http://www.azcu.org/publicationsMineToMarket.php
Technology of copper mining
http://64.90.169.191/innovations/1998/11/astm.html
How do they make copper?
http://64.90.169.191/innovations/how/howdo3.htm
DEVELOPING IDEAS
In small groups discuss the feasibility study that Earth Explorations is
conducting for Tom Sullivan and the Copper Mining Co.
1.
Make a flow chart that illustrates the different steps in copper mining
and purification, noting the differences between sulfidic and oxidic ore
processing.
How Much Is the Land Worth?
REDOX REACTIONS AND ORE COMPOSITION ANALYSIS
The value of Tom Sullivan’s land depends on the amount of valuable metal
contained in the ore. If the amount of copper in the ore is quite high Tom
could bargain for a higher selling price for his land. If expensive metals, such
as silver, are also present, the land may be worth even more. If toxic
metals, such as lead, are present, the Copper Mining Co. may not be
interested in obtaining the land. On the other hand, a toxic material could be
processed into a salable product.
Earth Explorations will determine what minerals are expected in
copper rich deposits, analyze samples for copper and other
elements, and determine the economic and environmental
consequences of developing the mine.
What Minerals Would You Expect To Find?
CREATING THE CONTEXT
Processing copper-rich ore into metallic copper occurs industrially using at
least two production methods. The choice of method is driven largely by the
type of minerals found in the ore. In this exploration, Earth Explorations will
collect background information about the types of copper-containing
minerals that are present in the earth and learn the language of
describing these minerals. In addition, Earth Explorations will practice
balancing chemical reactions involved in the processing of copper.
The chemical reactions that occur during the processing of copper ore into
copper metal often involve changing the oxidation state of the copper atoms.
For example, the copper in Covellite (CuS) is in the +2 oxidation state and
must be reduced to an oxidation state of zero in copper metal. When this
change occurs another change must occur simultaneously that results in a
increase in oxidation number of another species.
PREPARING FOR INQUIRY
Copper is present in the earth’s crust primarily as sulfide and oxide minerals,
although some deposits of metallic copper have been found. One difference
between copper in minerals and native or metallic copper is the oxidation
state of the copper itself. Both the oxidation state of the copper and the kind
of mineral in which it is present determine the kind of processing required to
make copper that can be sold.
DEVELOPING IDEAS
Perform these exercises.
2. Copper-containing minerals are listed below in Table 1.
a. For each mineral assign oxidation numbers to all the elements
present.
b. In what oxidation states does copper tend to exist? What other
elements are usually parts of copper minerals?
c. For each mineral listed calculate the % Cu by mass.
d. Which minerals are richest in Cu? Choose the 5 minerals
containing the most copper by mass.
e. Of the five minerals with the most copper, use the specific
gravity (similar to density) to find which mineral has the most
copper in the smallest volume of rock.
Is there any Pb, Ag, Fe, or Zn in the Ore?
CREATING THE CONTEXT
Metals such as Fe, Zn, Ag, and Pb are often present along with the desired
copper in the ore. These other metals may either increase the worth of Tom
Sullivan’s land or decrease its worth. In addition, the presence of iron may
influence how an ore can be processed for extracting copper.
PREPARING FOR INQUIRY
Qualitative Identification of Metals (Method C)
3. Which metal ions (Fe, Zn, Ag, and Pb) readily precipitate in the presence
of chloride ion?
4. Why is it that Ag+ can be detected directly in the leachate by addition of
chloride ion but Pb2+ can’t?
5. Why is PbCO3 soluble in acid while PbSO4 is not?
6. Write a net ionic reaction for each step in the qualitative tests for Ag+,
Pb2+, Fe2+, Fe3+ and Zn2+.
What is the Price of Copper?
CREATING THE CONTEXT
The Copper Mining Co. will mine Tom Sullivan’s land only if it will be a
profitable venture for them. Since they approached Tom with the proposal,
we must assume that they have thought of this and already know the
approximate viability of the land. Earth Exploration’s job is to help Tom and
the Copper Mining Co. assess the viability and value of the land and make a
good judgment about the land’s value.
PREPARING FOR INQUIRY
Mining companies take data from many sources to assess profitability of any
project. One possible source of data in addition to print media is the Web.
There are many professionally run web sites giving up-to-the-minute
information about stocks, shares and metal prices around the world. This
information can be weighed against the cost of extraction discussed later to
create a profitability schedule.
A word of warning about information on the Web is in order. Currently, there
is no review of materials on the Web. There is nothing preventing completely
false information from appearing. Therefore, look at materials from the web
with a critical eye.
Use the following Web sites to determine the current value of copper:
The London Metal Exchange Web site provides the price of copper and many
other common metals on a per pound basis in U. S. dollars. It also provides
links to graphs of the fluctuations in metal prices although not all metals
may appear. There are links to other sites from here also.
http://metalprices.com
The mining-journal Ltd. is a large site dedicated to mining and so not only
provides access to prices but to many other pieces of information such as
books, CD-ROMs and magazines.
http://www.mining-journal.com
DEVELOPING IDEAS
Some things you might want to investigate are how metal prices fluctuate,
how other financial considerations affect prices and use this information in
constructing your feasibility study report. Take this information with you
into the next section. The variation in metal prices is crucial to any decision
the company might make.
What is the Cost of Extraction?
CREATING THE CONTEXT
You are probably already realizing the scope of a mining operation such as
the one proposed for Tom’s land. There are many costs associated with the
proposed mine. As with any industrial process, the cost of extraction and
processing of the ore must be weighed against the value of the copper
produced. The following exercise will help you to think about some of the
principles involved in making a profit through mineral resource development.
PREPARING FOR INQUIRY PREVIOUS ENVIRONMENTAL LAB
Your instructor will supply several different types of mining property
(chocolate chip cookies), mining equipment (toothpicks and paper clips)
together with a Cookie Mining Sheet, Cookie Mining Grid and $19 of Cookie
Mining Money. The object is to make a profit. You must use your Cookie
Mining Money to pay for the mining operation and reclamation. In return you
will receive money for the ore mined (chocolate chips). Your instructor will
write on a board or handout, how much the “mining property” and “mining
equipment” cost together with other rules. The player with the most money
at the end of the game wins.
DEVELOPING IDEAS
Now that you have an idea of the requirements for profitable mining we
would like to act as Tom’s consultants and assess what profit margins the
mining company can achieve and whether Tom is being offered a fair price
for his land.
1. In small groups discuss all the expenses that the company might incur.
One way to do this is to start at one point and rotate around the group, each
person writing down an idea and describing it to the others. Continue until
all the ideas have been exhausted and you have a substantial list.
2. Collect and group all the ideas into broad categories that you think the
mining company might use to provide an annual report to their
shareholders. Be sure that all group members are included in the discussion
and consensus on category titles is reached.
3. Each group presents and defends their categories before the whole class
with the goal of generating a consensus among the whole class about the
expense categories. We are now ready to begin looking at some data.
4. Your instructor will provide you with either tables of data or the internet
addresses for several on-line sites that list the individual mining expenses
and equipment requirements for a typical mine. Your job is to use the
discussion and data to generate an expense report for the land. If you know
the potential reserves of Tom’s land then you can calculate how much the
copper must sell for if the mine is to break even.
5. Use the copper prices you obtained in Exploration 2D to determine how
much copper the mining company must recover in order to equal the total
mining expenses incurred for the period. If you have already calculated the
operating costs you can use this value to determine any profit or loss for the
mining company and estimate whether you feel Tom is being offered a fair
price for his land.
THINKING FURTHER
The mining of any metal is not just a chemistry experiment; it is a complex
mix of science, engineering and economics. When the price of a metal is
high then the mine may be highly profitable; when it is low, the mine may
be forced to close. Classic examples of this were the copper mines on the
Keweenaw Peninsula. During the early nineteenth century the copper was
plentiful, easy to mine and process but more importantly it was in great
demand and very valuable. As a result the area contained more millionaires
per capita than any of the gold mining regions. With the decline in copper
usage and the drop in price due to imported copper, it became unprofitable
to mine the copper and so the mines slowly closed.
To give you some idea how much a mine costs to run, Canadian-based
Placer Dome, Inc. owns a gold mine called the Kanowna Belle in Western
Australia. The estimated unmined gold ore at Kanowna Belle is about 10.9
million tons of ore at a grade of about 4.2 grams of gold per ton (1.6 million
ounces of gold total). This mine produced 69,337 ounces of gold in 2002 at a
total cost of about $269 per ounce. For the mine to be profitable the price of
gold must be greater than the expenses of mining and processing the ore.
The price of gold in 2002 was about $309 per ounce, a profit of $40 per
ounce or $2,773,480 for the year.
How Is the Ore Processed?
THERMODYNAMICS AND SPONTANEITY
The decision regarding which type of processing is necessary for the ore
depends on identifying what type of minerals are present. Copper-containing
minerals can be classified as primary or secondary minerals or as sulfidic or
oxidic minerals.
Table 2 lists copper minerals, their chemical formulas, their primary or
secondary classification, and chemical type. All primary minerals are
sulfides. Secondary minerals can be sulfides or other compounds (e.g.,
oxides, carbonates, sulfates), all of which are grouped together as “oxides”
for our purpose of determining the appropriate processing method. The
processing of the different types of minerals depends largely upon whether
the mineral is sulfidic or oxidic.
Pyrometallurgy refers to a collection of processes that involve the use of
heat to reduce or change the mineral.
Hydrometallurgy refers to an aqueous based process to remove the copper
from the mineral.
How is Copper Extracted from Sulfidic Ores?
CREATING THE CONTEXT
About 90% of the world’s copper ore deposits contain primarily sulfidic
minerals. The percent copper by mass in these deposits is fairly low,
between 0.5 and 2%. The methods by which this copper ore is converted to
copper metal are generally referred to as smelting. Smelting takes the
impure ore and reduces the copper ion into an impure metal that is then
purified electrochemically. During the smelting process sulfur and iron ions
are oxidized.
PREPARING FOR INQUIRY
To effectively smelt an ore containing 0.5 to 2% copper by mass it is
necessary to first concentrate the ore. This copper enrichment is done by
first crushing the ore to small particles and then separating the copper
containing pieces through a process called froth floatation. The crushing is
performed in three steps. The primary crusher takes rocks from the mine
and breaks them into smaller rocks, on the order of 200 cm in diameter.
This initial crushing is followed by two successive grinding steps which
reduce the particles to ~3 cm and ~0.3 cm, respectively. The final size
required depends on the size of the copper ore grains in the rock - it
is most efficient to crush the rock small enough so that each particle
contains a single mineral grain.
Once the minerals are broken into small particles, the particles containing
copper sulfides are separated from the other particles. This is accomplished
by treating an aqueous slurry with a surfactant that preferentially adheres to
the surface of the copper sulfides and renders these particles hydrophobic.
The most commonly used surfactants are the xanthates, examples of which
are shown below.
The polar head group, drawn towards the left, adheres to the copper sulfide
mineral and the hydrophobic chain extends away from the surface. Air or
nitrogen gas is bubbled through this mixture and the hydrophobic particles
are carried along with the nonpolar gas. The copper bearing particles rise to
the top of the tank with the bubbles (float) and the other particles sink to
the bottom. This process is often referred to as froth floatation due to the
frothing caused by the gas forced through the solution. The copper rich
material is collected and dried. The material contains about 25-35% copper
by mass and can be conveniently and economically shipped away for
smelting, if desired.
Smelting is a general term for a process in which an ore is oxidized at high
temperature using gaseous oxygen. Smelting of copper ores usually consists
of two stages: matte smelting, and converting.
Matte smelting can be considered a preliminary oxidation phase in which the
Cu-Fe-S ore is subjected to an oxidizing atmosphere at temperatures on the
order of 1200°C. The amount of oxygen present is controlled carefully so the
desired product is obtained - a Cu-Fe-S “matte” that is richer in Cu than the
starting material (40-70%). The chemistry that occurs in the matte furnace
is complex but can be represented by equations such as:
All the above reactions are exothermic and the process itself is autothermal
meaning that, once started, the reactions themselves are sufficient to keep
the temperature at 1200°C.
The two layers that form - matte and slag - have different densities. The
lighter slag rises up in the furnace while the matte settles down at the
bottom. The matte and slag can be drawn off independently. Matte is
removed and placed in the converting furnace while the slag is sent to
stockpiling or copper recovery.
In the converting stage the enriched Cu-Fe-S matte is oxidized to form
“blister” Cu (99% pure). Oxygen-enriched air is blown into the molten matte
to convert the copper to blister Cu, the iron to a Fe-silicate slag, and the
sulfur to SO2. The converting phase is also done in two
chemically and physically distinct stages. First, in the slag formation stage,
the iron sulfides are converted into a slag material:
This stage is finished when the matte contains less than 1% Fe.
The copper making stage results in Cu2S being converted to Cu metal and
SO2. The reactions occurring in this stage are
The product “blister” copper contains 0.001-0.03% S and 0.1-0.8% O. The
SO2 bubbles thus represented give this impure copper its name. The blister
copper is poured from the converting furnace and taken to a fire refining
furnace to remove the SO2 and poured to make copper anodes which are
used in the final electrochemical purification step.
7. Divide the minerals listed in Table 2 into two categories: those that are
amenable to refining via pyrometallurgical techniques and those that are
not. For the two minerals that can be refined with pyrometallurgy,
calculate the amount of copper, in grams, that could be recovered from 1
ton of the pure mineral.
8. Calculate also the volume of oxygen gas at 1 atmosphere pressure
necessary to convert 1 ton of ore containing 1.25% Cu in the form of
chalococite to native copper.
DEVELOPING IDEAS
Industrial processes that involve spontaneous chemical reactions are lower
cost. There is no need to use energy to get the reaction to go under process
conditions. Earth Explorations will first investigate the thermodynamics of
the relevant reactions and then examine the economic
and environmental implications of the process.
ENVIRONMENTAL AND ECONOMIC CONSIDERATIONS IN PYROMETALLURGY
Once the copper has been extracted from the ore, it must be purified.
Purification requires an incredible amount of energy. The production of this
electrical energy can itself have environmental consequences. In addition to
energy used for smelting and purification, electricity is also used for
machinery and mining equipment.
A report from the WMC mining company provides information on the amount
of energy required in a typical mining operation (see Appendix E). Studying
the information shows that in 1997 a total of about 900 Megajoules of
electricity are required per ton of ore milled. This data is available on pages
18 and 19 of the performance section and in the site data section on pages
42 - 45. Knowing the total amount of energy per ton of ore and the
percentage of useable metal in the ore will give you an idea of the amount of
energy required per kilogram of pure metal.
9. The Nifty Copper Operation run by WMC in their 1996 - 1997 season
milled 0.705 million tons of ore using 313 Megajoules per ton of ore. Use
the data you calculated in Session 2 about the amount of copper present
in ores to determine the amount of energy required per kilogram of
copper produced.
10.
Using table data determine the costs per kg Cu associated with
pyrometallurgy.
Energy requirements for the extraction and purification of copper are huge
and a serious cause for environmental concern. Indeed, WMC acknowledged
this in their report by comparing energy production and consumption since
they are not the same. Historically, they have purchased all their electricity
but now generate their own with gas turbines in order to better control
consumption. WMC set a goal to reduce electricity consumption by all
operations and believed they achieved this. However, in doing so they
realized how much energy is lost during the generation of electricity. This is
clearly illustrated in the data and shows how improvements in energy
conversion are required when such large amounts of energy are being
used.
11.
List the environmental and economic considerations associated with
pyrometallurgical processing of copper-containing ores.
How is Copper Extracted from Oxidic Ores?
CREATING THE CONTEXT
Although most copper is recovered by pyrometallurgical methods, a
significant and growing amount of copper is recovered with
hydrometallurgical techniques. This approach is particularly well suited for
oxidic ores or sulfidic/oxidic ore mixtures with a significant fraction of oxidic
ore.
PREPARING FOR INQUIRY
Hydrometallurgical processing of copper ores is a three-step process. First,
the rock is sprayed with a dilute H2SO4 solution (known in the industry as
lixivant) that dissolves copper and other materials from the minerals. The
resulting copper-bearing leachate solution (known in the industry as
“pregnant leach solution” or PLS) is purified to remove other metal ions. The
purified solution serves as the electrolyte in an electrodeposition process in
which the Cu2+ ions are reduced to Cu metal.
In practice leaching is carried out by spraying sulfuric acid on a “heap” of
rock piled on top of a polyethylene mat. The heap is set on an inclined slope
and the solution trickles through the rock and eventually flows down the
slope to a holding pool. The liquid is recycled by spraying on the heap until
the copper content is high enough to send the solution on to the next step in
processing.
The suitability of hydrometallurgical processing for a particular ore body
depends on the nature of the minerals present. Table 3 presents the
common copper bearing minerals and describes how long it takes to remove
a significant fraction of the copper from the rock through leaching.
In general sulfidic materials take a longer time to leach than oxidic
materials.
The presence of Fe3+ in many ores facilitates the oxidation of sulfidic coppercontaining mineral in that Iron(III) can be reduced as the sulfides are
oxidized.
In this exploration, Earth Explorations examines the efficiency of leaching
ore samples from Tom Sullivan’s land. Answer these questions in preparation
for the laboratory:
DEVELOPING IDEAS
Leaching Study
In this exercise, teams will begin their investigation of an appropriate
hydrometallurgical leaching strategy for the copper ore that was analyzed
previously.
APPLYING YOUR IDEAS
Heap leaching is accomplished by piling broken pieces of ore in a large area
and sprinkling it with dilute sulfuric acid solution, known in the industry as
lixiviant. The solution trickles through the heap and runs down a sloped bed
and is collected at the bottom in a pool. The initial lixiviant solution sprayed
on the heap has the typical composition given below. The solution is recycled
and therefore contains some copper prior to being applied to the heap.
12.
The chemical reactions associated with leaching are described in words
below. Write the net ionic chemical reaction that is described.
(a)
Non-sulfide copper ores are easily leached by sulfuric acid.
For example, write a net ionic reaction for the leaching of
Tenorite by H2SO4.
(b)
An ore body contains many different minerals. When H2SO4
acts on heaped ore that contains FeS2 (pyrite) the sulfur’s
oxidation number is changed by oxygen from the air.
Describe what happens and what the new change on sulfur is
in the reaction.
Appendix A
QUALITATIVE IDENTIFICATION OF METALS
1. Qualitative test for Ag (for concentration ≥ 0.001 M or 0.1 g Ag+ /L)
To test a leachate or electrowinning solution for Ag+, place 2-3 mL of the
clear (filtered) solution in a small test tube and add 1-2 drops of 3 M HCl. If
present, Ag+ will appear as white, cloudy AgCl(s).
2. Qualitative test for Pb (for concentration ≥ 0.01 M or 2 g Pb
2+
/L)
This test is slightly more complicated than the Ag test, above, because any
Pb2+ leached from the ore will precipitate as PbSO4(s) in the presence of
sulfuric acid. To test for the presence of Pb2+ you will have to first convert
the PbSO4 to the less soluble PbCO3.
Adding acid to PbCO3 dissolves the precipitate and liberates CO2(g). The Pb2+
is now free in solution in the absence of sulfate, so you can perform the
usual chloride test for Pb.
(a) Place about 10 g of wet waste solids from the leaching operation
into a 250 mL Erlenmeyer flask with 100 mL of deionized water. Mix
well for a few minutes and collect the solids by vacuum filtration. Wash
the solids in the filter with several 30 mL portions of deionized water
(to remove residual sulfate ions).
(b) Return the washed solids to a 250 mL Erlenmeyer flask and add
100 mL of 0.1 M Na2CO3. Mix the solution for about 10-15 minutes and
collect the solids by vacuum filtration. Wash the solids in the filter with
several 30 mL portions of deionized water (to remove residual
carbonate ions).
(c) Place the filter paper with the filtered solids in the bottom of a 250
mL beaker and add 50 mL of 6 M HNO3. The acid will bubble as CO2(g)
is released and any PbCO3(s) in the sample dissolves.
(d) Filter or centrifuge 2-3 mL of the acidified solution in a small test
tube and add 1-2 drops of 3 M HCl. If present, Pb2+ will appear as
white, cloudy PbCl2(s).
3. Qualitative test for Fe3+, Fe2+, and Zn2+
(a) To test the leachate for the presence of Fe3+, place 1-2 mL of the
clear (filtered) solution in a small test tube and add 2-3 drops of 0.1 M
potassium thiocyante (KSCN) solution. The formation of a blood-red
complex indicates the presence of Fe3+ .
(b) To test for Fe2+, place 1 -2 mL of the filtered solution in a small
test tube and add 2- 3 drops of the Ferrozine solution. The formation
of a purple complex indicates the presence of Fe2+ .
(c) To test for Zn2+, place 2-3 mL of the filtered solution in a small test
tube and add 2- 3 drops of the Dithiozone solution. The formation of a
red complex indicates the presence of Zn2+ .