52a Mining, Metallurgy and the Environment

advertisement
5.5
Mining, Metallurgy,
and the Environment
Copper is one of those substances that you rarely think about but could not do
without. What would your life be like if all the copper suddenly disappeared? You
may have no running water, because the water pipes in your home are probably made
of copper. You also might have no electricity, since most of the wires that distribute
electricity are made of copper. Furthermore, all of your electronic devices could not
function without their copper components (Figure 1).
Figure 1 Copper is an important
component in computer systems
because it is such a good conductor
of electricity.
mineral a naturally occurring solid
that has a definite crystal structure and
chemical composition
ore rock containing a relatively high
proportion of a desirable mineral
Learning TIp
Base Metals
Do not let the term “base metals”
confuse you. Their name has nothing
to do with the bases that neutralize
acids. Rather, the word “base” has
a very ancient origin, and means
“common” or “of little value.”
what Is mining?
Copper rarely occurs in nature as a pure element. Instead, it is found in minerals. A
mineral is a solid substance found in Earth; usually a crystalline ionic compound, or
mixture of compounds (Figure 2). Occasionally, however, a mineral can be made up
of a single element such as gold, silver, or even copper. The chemical composition
of every mineral is unique. Azurite, Cu3(CO3)2 ~(OH)2, and bornite, Cu5FeS4, are
examples of minerals that contain copper ions.
Rock that contains enough of a valued resource that it can profitably be mined is called
an ore. Highly desired metals such as gold and silver are known as “precious metals.” Less
valuable metals—copper, nickel, and zinc—are sometimes called “base metals.”
To access the ore, mining companies must first excavate it out of the ground.
This usually involves either surface mining or underground mining. Surface mining
involves opening a large pit to expose mineral deposits (Figure 3). Surface or open-pit
mines extract ores that are close to Earth’s surface. Vast quantities of material must be
removed to obtain the minerals in the ore. Returning the site to its natural state once
mining operations cease is a huge environmental challenge.
In an underground mine, miners drill shafts deep into Earth to reach the ore.
Figure 2 Clockwise from lower left: native copper,
bornite, azurite, and a mixture of malachite and
azurite. Only native copper contains copper atoms.
The other minerals contain either Cu+ or Cu2+ ions.
Figure 3 The Diavik diamond mine, 300 km
northeast of Yellowknife, Northwest Territories,
is Canada’s largest diamond mine.
what Is metallurgy?
metallurgy the science and technology of
separating and refining metals from their
ores and subsequent processing
Once an ore is mined it must be processed to extract its valued components. Metallurgy
involves extracting metals from their ores and processing them into a useful form.
Metallurgy often involves a number of chemical and physical processes. Sometimes
the extraction and processing procedures are done where the ore is mined. In other
cases, the ore is shipped elsewhere for processing.
a Typical metallurgical process: extracting Copper
A typical ore sample from an operating mine contains surprisingly little metal. Copper
ore, for example, can contain as little as 0.5 % copper by mass. This means that every
1000 t of ore might contain only 5 t of copper. A huge quantity of rock needs to be
processed and discarded to extract the “good stuff.” The discarded materials from this
212
Chapter 5 • Chemical Processes
NEL
process are called tailings. The challenge for the metallurgical engineer is to design a
profitable process for separating copper from its ore, changing it to metallic copper,
and then purifying it. Figure 4 summarizes the key steps of processing a metal such
as copper from its ore into a finished product.
career lInK
What do metallurgical engineers do?
Where do they work? How do they use
chemistry? To find answers to these
questions, and to research this career,
chemical processing
physical processing
go t o n eLs on s c i en c e
mining and grinding the ore
ore (0.5 %–3.0 % copper)
mineral extraction (e.g., flotation)
concentrate (10 %–40 % copper)
metal extraction (e.g., flash smelting)
(70 %–80 % copper)
further purification (e.g., electrorefining)
(99.9 % copper)
metal
waste rock
Figure 4 Extracting and purifying copper metal from its ore involves both physical and chemical
processing, in several stages. A great deal of rock must be processed to extract a small quantity of metal.
pHysICal proCessInG
Ore fragments extracted from the mine can be as large as a small car. Huge grinding
machines crush them into small pieces. The ore is then concentrated by an ingenious
technology called flotation (Figure 5, next page). During flotation the crushed ore is
mixed with water, air bubbles, and a detergent-like ionic compound. This compound
breaks down into ions when it dissolves. The resulting anion is negatively charged
at one end and non-polar at the other. The negatively charged end is hydrophilic
(attracted to water); the non-polar end is hydrophobic (repelled by water).
NEL
5.5 Mining, Metallurgy, and the Environment
213
water
air bubble
froth containing
mineral-rich ore
hollow core
air in


mineral
anions
surrounding
the air bubble
tiny pieces
of ore
air
bubbles
waste sand and rock
crushed
ore in
stirrer
Figure 5 In a flotation cell, particles of metal-bearing ore attach to bubbles and float to the surface,
where they are skimmed off.
Figure 6 A flotation cell separates
powdered ore with high metal content
from worthless rock.
The charged hydrophilic ends of these anions attach themselves to copper-bearing
ore particles. The hydrophobic end enters the “skin” of air bubbles. Rising air bubbles
pull ore particles to the surface of the tank, where they can be skimmed off (Figure 6).
The concentrate collected from flotation contains from 30 % to 40 % copper.
CHemICal proCessInG
smelting the chemical process that
extracts a metal from its ore using
heat and chemicals
flash smelting a fairly new technology
for separating a metal from its ore by
heating ore in an atmosphere of almost
pure oxygen
power supply
e–
+
–
impure
copper
(70%-90 % pure)
e–
pure
copper
(99.99 % pure)
2+
Cu
negatively
charged ions
• reducedfuelcosts
• lesspollution
Flash smelting produces more concentrated sulfur dioxide than traditional smelting
because pure oxygen is used. This makes it economically feasible for the company to
convert sulfur dioxide into sulfuric acid, which they can sell to help offset costs.
FurTHer purIFICaTIon
Figure 7 Electricity can be used to
produce very pure copper. Industrialscale versions of this setup purify many
large sheets of copper at a time.
214
The goal of chemical processing is to extract copper from the ore concentrate and
convert it into pure copper metal. In the past, this was done using a process called
smelting. Smelting involves heating ore in air, together with other chemicals, to over
1000 °C in a furnace. The chemical reactions in the smelter produce liquid copper
metal. The copper sinks to the bottom of the smelter, where it is separated. Waste
materials are skimmed off and discarded.
In the process, any sulfide that may have been present in the ore is converted to
sulfur dioxide gas. In the early days of smelting, sulfur dioxide gas went up the furnace chimney untreated and into the air. The acid rain caused by these emissions devastated ecosystems in the surrounding area. Later, the installation of devices called
scrubbers prevented much of the sulfur dioxide emissions from escaping.
A new technology called flash smelting is becoming the preferred method of processing copper, particularly from sulfide-containing ores. Like traditional smelting, flash
smelting involves heating ore with other chemicals to high temperatures. However, rather
than using air, flash smelting is done in almost pure oxygen. As a result, the chemical
reactions in the smelter occur more vigorously. So much thermal energy is released
by these reactions that very little additional fuel is needed to keep the furnace hot.
This benefits the company in two ways:
Flash smelting can produce copper that is up to 90 % pure. This may be adequate for
water pipes, but not for electrical applications. Impurities decrease copper’s electrical
conductivity, so copper used in the electrical industry must be over 99 % pure. In
order to achieve this standard, copper is further purified, often using an electrical
process called electrolysis (Figure 7). In this process, a large sheet of impure copper
Chapter 5 • Chemical Processes
NEL
slowly breaks down into ions with the aid of an electric current. The pure copper ions
in the solution travel through the solution and collect on a thin sheet of pure copper.
The impurities fall to the bottom of the container.
Mini Investigation
recovering Copper
Skills: Predicting, Planning, Performing, Observing, Analyzing, Communicating
SKILLS
HANDBOOK
a1.1, a1.2
Electrolysis is used to produce highly pure metals. It can also be used to extract toxic
metal ions from wastewater. In this investigation, you will use electrolysis to extract
copper metal from a solution containing copper(II) ions, Cu2+ (aq).
Equipment and Materials: chemical safety goggles; lab apron; 250 mL beaker;
2 electrical leads with clips; power supply; 2 pencils with both ends sharpened;
100 mL of dilute copper(II) sulfate solution, CuSO4(aq) (0.1 mol/L)
Copper(II) sulfate is toxic. It is also a skin and eye irritant. Wash your hands
thoroughly after pouring the solution.
When disconnecting the power supply, pull on the plug rather than the cord.
1. Put on your chemical safety goggles and lab apron.
2. Connect one end of one electrical lead to the end of one pencil. Connect one end of
the other electrical lead to the end of the other pencil.
3. Connect the other ends of the leads to the terminals of the power supply.
4. Pour about 100 mL of the copper(II) sulfate solution into the beaker.
5. Place the free ends of the pencils into the copper sulfate solution so that their tips do
not touch.
6. Turn on the power supply and sent it to 6 V.
7. After 1 to 2 min, turn off the power supply and remove the pencils from the solution.
Observe their tips.
8. Dispose of your materials and clean up your workstation as directed. Wash your
hands.
A. What solid was produced at one of the tips?
B. What was the source of the solid?
T/I
T/I
C. Predict what changes you would observe in the blue colour of the solution if you left
the power on for a longer period of time. T/I
D. How would you know that all of the copper ions have been removed from the
solution? T/I
E. Design a process that will remove zinc ions from a zinc nitrate solution. If time
permits, and with the approval of your teacher, proceed with your plating procedure.
Record your observations. T/I
Unit tasK booKmarK
You might find the technique outlined
in this Mini Investigation useful as you
plan for the Unit Task described on
page 242.
The Impact of mining on the environment
Mining has been responsible for a large portion of Canada’s income over the years.
However, the mining and mineral processing industry also has a long history of environmental mishaps and human tragedy. Mining engineers, supervisors, and employees must
constantly be on the alert for potential problems. For example, large quantities of sodium
cyanide, NaCN, are used to extract gold out of ore. Because cyanide is so toxic, accidental
NEL
5.5 Mining, Metallurgy, and the Environment
215
web Link
To find out more about the effects
of mining—both positive and
negative—on Canada,
g o t o n e l so n sci e nce
Figure 8 The red mud that flowed out
of the holding pond in Hungary had a
very high pH and contained potentially
dangerous metal ions.
discharges can be devastating to local wildlife. Acidic oxide emissions from smelting
operations kill vegetation and make the surrounding regions barren.
Environmental disasters were common in the past, when there was little regulation of the mining industry. The situation is quite different today—at least in North
America. Mining companies must fund detailed environmental assessments before
they get permission to open a new mine. Moreover, plans for the closure of the mine
and rehabilitation of the site must be in place before the first shovel enters the ground.
Recent technological advances, together with stricter government regulations, have
greatly reduced the environmental impact of the mining industry. Despite these
advances, the risk of contamination of our air and water remains.
Not all countries have such strict mining regulations. In 2010, an environmental
disaster occurred near an aluminum mine in Hungary. Aluminum is obtained from
a mineral called bauxite. When bauxite is processed, it is crushed and mixed with a
solution of sodium hydroxide. The resulting basic mixture is commonly called “red
mud.” After the aluminum compounds are removed, the red mud is sometimes stored
in holding ponds. In October, 2010, a holding pond in Hungary burst its banks and
released a flood of corrosive sludge into nearby towns and villages (Figure 8). The
short-term effects were serious—at least eight people died—but the long-term effects
of heavy metal ions in the environment may be even more problematic.
Air Pollution
Mining operations are a source of many air pollutants. Surface mining, in particular,
is a major source of dust. Digging, blasting, hauling, and crushing the large quantities
of dirt and rock all produce rock dust. Wind sweeping across the overturned rubble
can spread this dust over a wide area.
Emissions of sulfur dioxide remain the single most important air pollutant of the
mining industry worldwide. A significant portion of acid precipitation in Canada
results from sulfur dioxide emissions from the United States. Thanks primarily to
improvements in technology and stricter emission standards, sulfur dioxide emissions
are declining (Figure 9). New, stricter emission standards will ensure that they continue
to decline into the future. This is not necessarily the case in other countries, however.
Sulfur Dioxide Emissions from the United States and Canada
Emissions (× 10 6 t)
30
25
20
15
10
5
0
1970
1975
1980
1985
1990
1995 2000
Years
United States
2005
2010
2015
2020
Canada
Figure 9 Sulfur dioxide emissions have fallen significantly in both Canada and the United States
since 1970. The data for 2010, 2015, and 2020 are predictions.
Water Pollution
Mining uncovers rock that has been buried deep in the ground for much of Earth’s history.
Freshly exposed rock is prone to erosion. This can result in a water quality problem called
acid mine drainage. Acid mine drainage is the outflow of unusually acidic water from a mine
or tailings dump site. The excess acidity is caused by the reaction of sulfide minerals such as
pyrite, FeS2, reacting with oxygen and water. The chemical equation for this reaction is
4 FeS2(s) 1 15 O2(g) 1 2 H2O(l) → 4 Fe31(aq) 1 8 SO422(aq) 1 4 H1(aq)
216
Chapter 5 • Chemical Processes
NEL
The hydrogen ions produced in the reaction make the water acidic. Iron(III) ions,
and the precipitates they form, give the polluted water a rusty colour (Figure 10).
Acidified groundwater can easily dissolve toxic metals, such as aluminum, from the
rock. These contaminants can harm aquatic life in nearby bodies of water.
One approach to dealing with acid mine drainage is to neutralize the acidity with
calcium carbonate (in limestone). This raises the pH of the water to more normal levels.
It also promotes the precipitation of metal ions, such as Fe3+(aq), as hydroxides.
Figure 10 The rusty colour of this
stream is caused by iron(III) hydroxide
precipitates.
5.5 Summary
•
•
•
•
Miningremovesmineralsfromtheground.
Metallurgyisthetechnologyofobtainingandrefiningmetals.
Flotationisusedtoconcentrateore.
Smeltingisthechemicalprocessusedinmetallurgytoextractametalfrom
its ore.
• Flashsmeltingismoreefficientthantraditionalsmelting.Muchoftheenergy
comes from its chemical reactions, and the by-products of flash smelting can
be captured and sold as valuable raw materials for other chemical processes.
• Acidicminedrainagecancontaminategroundwaterandaquaticecosystems.
5.5 Questions
1. Why is it necessary to concentrate ore before it is
chemically processed? K/u
2. Does flotation rely on a physical or chemical property of the
ore? Explain. K/u
3. Identify two ways in which flash smelting is a greener
technology than traditional smelting. K/u
4. How can groundwater passing through mine tailings be a
threat to a nearby aquatic ecosystem? K/u
5. The reaction of the mineral pyrite, FeS2, with air and
water is a natural process that can acidify groundwater in
undisturbed ground. Why do you think mining speeds up
this process? K/u T/I
6. The acidity caused by acid mine drainage can leach toxic
aluminum ions, Al3+(aq), out of rock. One way to treat
this problem is to add calcium oxide, CaO, or lime, to the
water. K/u T/I a
(a) Explain the effect this has on the pH of the water.
(b) Use the solubility table (Section 4.6, Table 1) to explain
how adding lime removes aluminum ions from water.
7. Future legislation will require further reductions in sulfur
dioxide emissions. To meet these targets, some mining
companies will have to update their processes. Some
companies may decide to close their mines or smelters,
rather than make costly upgrades. A mine closure can
devastate the economies of nearby towns. In your opinion,
should a government subsidy be used to keep these mines
operating? Give one argument for and one against. T/I a
8. The carbonyl process is used to produce highly pure
nickel. It occurs in two steps. First, impure nickel reacts
with carbon monoxide gas to produce a gas called nickel
carbonyl:
50 °c
Ni(s) + 4 CO(g) '''"
Ni(CO)4(g)
Nickel carbonyl vapour is then purified and injected into
a reaction vessel containing pellets of hot solid nickel at
about 200 °C. At this temperature, nickel carbonyl readily
breaks down:
200 °c
Ni(CO)4(g) '''"
Ni(s) + 4 CO(g)
K/u
T/I
(a) Classify each of these reactions as synthesis,
decomposition, or displacement.
(b) Research the hazards of this process. Why must it be
done in sealed vessels?
9. In 2007, Canada’s provincial ministers responsible for
mines encouraged the mining industry and Canada’s
Aboriginal peoples to develop a stronger working
relationship. Research specific examples that show how
T/I
a
this would benefit both groups.
go to n eLs on s c i en c e
NEL
5.5 Mining, Metallurgy, and the Environment
217
Download
Related flashcards
Create Flashcards