Transcript Processing—Mineral Resources

Mineral Resources
Chapter 12
© 2011 Pearson Education, Inc.
You will learn
• What makes minerals a resource for people
• The roles played by exploration, mining, and
processing in providing people with mineral
resources
• The environmental concerns associated with
mineral resource production
• How negative environmental impacts can be
avoided or mitigated
• The future challenges of mineral resource use
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Mineral Use in
the U.S.
Figure 12-6
Each year, mineral consumption in
the United States amounts to nearly
21,700 kilograms (47,800 lb) per
person.
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Mineral Resources
Naturally occurring solid materials in or on Earth’s crust
from which we can currently or potentially extract a useful
commodity
• Example: Lead—Pb—widely distributed in small amounts
• Ave. concentration in Earth’s crust is 12.5 ppm, or 0.00125%
• Cube of ave. crust 100 m/side = 3 million tonnes (6.6 billion lb)
with an ave. Pb content of 0.00125%
• Cube will have over 3.7 tonnes (8200 lb) of Pb
• At 2008 ave. price of $1.31 per pound
• Pb recovered from this cube could equal more than $10,000,
equivalent to approx. 0.3 cents/ton of crust processed
• Difficult to process amounts this low
• Pb concentrations 1000s of times greater than the crustal
average are needed if it is to be recovered economically
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Making Mineral Deposits
• Ore deposits—mineral deposits with high enough
concentrations of valuable elements to allow
profitable mining and recovery of a saleable
commodity
Ore deposits form by:
• crystallization processes in igneous rocks
• surface processes that concentrate heavy metalbearing minerals in sediments such as beach
sands
• chemical precipitation on the seafloor
• interaction of water-rich fluids and crustal rocks
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Types of Ore Deposits—Igneous
• Hot oceanic crust heats ocean water, which dissolves
metals (e.g., Cu, Pb, Zn) distributed through the crust
• Metal-bearing hot water migrates to the ocean floor to form
springs
• Dissolved metals react with sulfur, create sulfide minerals
• Sink downward and accumulate on the seafloor
• (e.g., ore deposits in ancient Cu mines on the island of
Cyprus)
• Magmas that solidify in the shallow crust may release hot
fluids rich in H2O and dissolved metals
• These fluids precipitate sulfide minerals containing metals
such as copper
• (e.g., Bingham Canyon Cu deposit in Utah)
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Figure 12-8 The Bingham
Canyon Mine in Utah
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Types of Ore Deposits—
Sedimentary
• Sediments deposited on the ocean floor commonly contain
seawater in the pores between sediment grains
• This salty water becomes part of the sedimentary sequence
and can contain dissolved metals, especially Pb and Zn
• Mineral deposits on the
seafloor form when metalbearing water escapes
along permeable pathways
(e.g., faults) and emerges
on the seafloor
• (e.g., Pb and Zn deposits
at the Red Dog mine in
Alaska)
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Types of Ore Deposits—
Metamorphism
• During metamorphism rocks may
dehydrate and release H2O-rich fluids
containing Au and other metals
• Metal-bearing waters can escape
to shallower crustal levels—and
localize along permeable
structures
• Precipitate quartz and
disseminated Au
•(e.g., California Mother Lode
region)
Figure 12-10 A Gold-Bearing Vein
in the Mother Lode Region, CA.
(a) This gold-bearing white quartz
vein is exposed underground in the
Lincoln Mine. (b) Gold in quartz.
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Ore Deposits That Can Form in
Stratovolcanoes
• Deposits containing Cu, Pb, Zn, Au,
and Ag commonly form within or near
stratovolcanoes. The Au and Ag
deposits tend to be veins
• The Pb, Zn, and Cu deposits in
carbonate rock tend to be lens-like or
podlike in shape
• Largest Cu-rich deposits form in the
interior of the volcano near the top
• Ore deposits that extend deep into
Earth’s crust need to be mined by
underground methods, and need to
be of a relatively higher grade—that
is, have a higher concentration of
valuable minerals—to be profitable.
Figure 12-11
The large copper deposit at the top of
the intrusion can be mined by open-pit
methods. Such ore deposits range from
hundreds to thousands of meters.
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Finding, Mining, and Processing
Mineral Resources
• Steps before mining:
• More exploration to determine whether site contains an ore
deposit
• Permission to mine must be obtained from one or more
government regulatory agencies
• If the deposit can be mined in an environmentally responsible
manner, permits will be approved and mining can start
• Mining commonly recovers material that needs to be
processed to separate minerals containing useful elements
• This step is called beneficiation. The separated minerals
are then further processed in another step, called
metallurgy, to separate useful elements from their mineral
hosts
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Finding Mineral Resources—
Exploration
• Geologists identify areas that may be favorable for mineral
deposits
• Fieldwork to make more detailed geologic maps
• Extensively sample surface materials (e.g. rocks, soils,
vegetation)
• Surface samples are analyzed by geochemical techniques
• Subsurface relations are investigated with geophysical
techniques
• If a mineral deposit is found by initial exploration efforts it is called
a prospect until it is shown to contain ore that can be profitably
mined
• To determine whether a mineral deposit contains ore:
• Combination of trenching/drilling
• Significant infrastructure needed to support this stage of exploration
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Mineral Exploration:
Drill Rigs
Figure 12-14 Mineral Exploration Drill Rig
This drill rig cuts and collects rock chips at a
gold prospect in Australia. The white bags are
full of rock chips recovered from specific
intervals underground. These are the samples
that are analyzed to determine the amount of
gold that is present.
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Mining Mineral Resources
• Mines recover ore using a variety of technically complex and highly
mechanized operations, and large facilities are needed to support them
• Facilities include roads, buildings, power systems, and water systems
• Open-pit mines commonly excavate and process large amounts of
rock.
• Waste rock from open-pit
mines must be processed
• These dumps are huge piles
of processed rock—classified
as toxic waste
Figure 12-17 Open-Pit Mines Produce
Huge Quantities of Waste Rock
The hill in the background is all waste rock
from the Bingham Canyon mine in Utah.
(The area in the foreground has already
been reclaimed.)
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Underground Mining
• Underground methods are used to mine deposits extending deep
into the subsurface. Ore deposits can be accessed in a variety of
ways:
• Adits = vertical shafts driven into the side of slopes
• Declines = inclined from the surface downward
• Process much less ore than open-pit mines
• Waste rock may be disposed of underground
• The Mining Law passed by Congress in 1872 allowed prospectors
to obtain the right to exploit the mineral resources of an area by
staking a claim—physically marking the corners of the area
• Physical disturbances such as prospect pits, trenches, exploration
shafts or adits, and small waste rock dumps are common
• Environmental consequences can be significant
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Underground Mining (cont.)
Figure 12-18
(a) This diagram of a zinc mine in
Tennessee shows the basic components of
an underground mine.
(b) Underground mine operations can
use large equipment like this scaler,
which removes loose materials left on the
walls and roof after blasting
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Abandoned Mine Land
Figure 12-19
The waste rock dump at this abandoned mine in Colorado is the
gray pile of loose rock at upper left.
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Processing—Mineral Resources
• Typical copper ore contains the valuable Cu-bearing sulfide mineral
chalcopyrite along with other minerals such as quartz, mica, and the iron-sulfide
mineral pyrite
• Beneficiation separates and concentrates the valuable minerals. The key
steps in this process, milling and flotation, produce a waste material called
tailings
• Milling
• Milling grinds the ore into particles the
size of silt or fine sand
• The objective is to break the ore down
into separate mineral grains
• The mills that do this are cylinders
containing steel balls that grind the ore
• Water is mixed with the ore in the mill
• Slurry—ground-up ore suspended in
water removed for further processing
Figure 12-21 Milling
The large cylinders (ball mills) hold steel
balls that grind the ore as the mill rotates.
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Flotation
• Flotation concentrates valuable
minerals—separated from nonvaluable
minerals
• Vats—contain the slurry from the mill and
special bubble-forming reagents
• Specific sulfide minerals such as chalcopyrite
will selectively adhere to a bubble and float to
the surface, where they can be collected
• Mineral-bearing bubbles are dewatered/
filtered
• Leaving behind a concentrate of valuable
minerals
• Tailings—minerals left behind in the slurry
after flotation (e.g., quartz, pyrite)
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Tailings Disposal
• Tailings are the principal focus of the environmental concerns
• Commonly contain large amounts of sulfide minerals (e.g., pyrite)
• Oxidation of these creates acidic conditions—degrade soil and water quality
• For large mines like that at Bingham Canyon, Utah, the associated tailings
ponds can be huge, over thousands of acres in area and about 100 meters
deep.
Figure 12-23 The waste after milling and flotation, called tailings, is pumped to a disposal
area (a). The disposal area can be very large and collect pools of water on its surface
where the tailings are less permeable (b).
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Heap Leach Operations
for the Recovery of Gold
• Some metals—are removed from
rocks and concentrated by leaching
• Ores are piled in large heaps
• Specially formulated chemical
solutions percolate through them
• Dissolving the metals they
contain
• The metal-bearing solutions are
collected from the base of the
heap
• Processed to recover the metals
• Acidic (for Cu) and cyanidebearing (for Au) solutions are
restricted from entering
groundwater by impermeable
liners of synthetic or natural clay
Figure 12-24 Gold ore is being placed in a
pile (heap) for leaching at this mine in
Ghana. Solutions will be sprinkled on the
surface that dissolve gold as they percolate
through the heap. The solutions are trapped
above an impermeable liner (the black
sheet) at the base of the heap, and
collected for processing to remove the
dissolved gold
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Recovering Metals—from Ore
Concentrate
Metallurgy removes desired elements from the valuable minerals
that are concentrated by beneficiation
• Smelting—melting sulfide minerals—common metallurgical
technique
• Smelters—heat the ore to its melting point
• Molten metals sink to the bottom and are removed
• Impurities (mostly Fe, SiO2) rise to the top; cool to glassy slag
• Large dark piles of slag—commonly mark the location of smelters
• Because it is glassy, slag is not highly reactive in the environment
• Some does contain metals (e.g., Pb, As) that need proper
disposal
• Smelters release gases (SO2) and particulates to the atmosphere
• Slag can be used for abrasives, base material for railroads, and
even in traps on golf courses
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Environmental Concerns
• Physical disturbances
• Exploration—trenches and roads
• Extraction—open-pit mines—deep excavations—large waste rock
dumps
• Beneficiation—waste materials (tailings) or smelting (slag)
• Underground mining—adits, declines, waste piles
• Steps for mitigation—reclamation
• Reshaping of the land surface to resist erosion
• Covering it with soil
• Planting new vegetation to help stabilize the land surface
• At mine closure, it is common practice to demolish, salvage, or
otherwise remove all mine support facilities
• Reclamation is capable of changing large piles of bare rocks,
tailings, or slag into stable, vegetated landscapes.
• Does not restore the land to its pre-mining conditions
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Environmental Concerns (cont.)
• Surface water quality
• Surface and ocean waters can be degraded by:
• Accidental spills of toxic chemicals
• Erosion of waste materials
• Discharge of contaminated water from mines or related
facilities.
• Spills
• Accidental spills of toxic chemicals from storage or processing
facilities
• Modern facilities are surrounded by berms to contain potential
spills
• International Cyanide Management Code—best practices include:
• Strong, impermeable barriers at the base of the leach pads
• Effective collection systems for the leach solutions
• Rinsing, physical isolation, detoxification of heap leach pads
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Environmental Concerns (cont.)
• Erosion
• Erosion of waste materials can affect surface water quality
• Metal-bearing materials can be eroded into bodies of water
• Materials react with water and oxygen to release metals
• Dissolved metals are more bioavailable to organisms
• Current mining operations may not dispose of waste rocks or
tailings where they can be eroded into surface bodies of water
• Proper disposal requires placement outside active floodplains
• Reclamation that stabilizes the waste rock or tailings
• Many of these wastes can oxidize—generate acidic soils or waters
• Add materials (e.g., lime) to neutralize acidity
• Cover the rock with topsoil to promote vegetation growth
• Revegetation and surface contouring to control runoff are
additional reclamation steps that will inhibit water infiltration,
stabilize slopes, and prevent erosion
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Discharge of Acid Rock Drainage
• Water that collects in mines or drains through them can become acidic
and contaminated with toxic metals. Where this water is discharged to
the surface, it can degrade nearby surface water quality.
• This happens especially where the ore deposit is rich in sulfide
minerals. The sulfide mineral that has the greatest effect on water
quality is pyrite (iron sulfide).
• Pyrite (FeS2) oxidizes (with bacteria) to form Fe oxides + sulfuric acid.
FIGURE 12-28 Generation of Acid Rock Drainage Reaction of oxygen with pyrite,
catalyzed by certain bacteria, produces sulfuric acid that can mix with and contaminate
surface and groundwater.
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Acid Rock Drainage (ARD)
• Acid rock drainage (ARD)—acidic water
• Produced by the oxidation of pyrite (and other sulfide minerals)
• Dissolves metals such as copper, zinc, and silver
• Must be properly treated and disposed of
• Prevention of ARD, avoiding oxidation of sulfide minerals
(e.g., pyrite)
• Disposal of pyrite-bearing wastes in appropriate places
• With impermeable materials at their base; inhibit water infiltration
• Other prevention techniques include:
• Flooding underground mine openings
• Capping pyrite-bearing rock in mines with impermeable coatings
• Filling unused mine openings with material that neutralizes acid
(e.g., limestone)
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ARD Being Released from a Mine Adit
FIGURE 12-30 This ARD, flowing from a small underground mine
in Colorado, contains high levels of dissolved metals including Cu,
Fe, Al, Zn, and As.
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Soil Quality
Acidic soils prevent plant
growth—leave surface
vulnerable to erosion
Hypothesis:
Pb in ore minerals (e.g., galena)
may be less bioavailable than
the type of lead people are
exposed to elsewhere
(e.g., Pb in gasoline or in paint)
Figure 12-35 Blood and Soil
Lead Levels in Mining
Communities
Mining communities have high
soil lead levels—but blood lead
levels have been low, along with
low health issues.
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Soil Remediation Techniques
• Add chemicals—make elements
of concern less mobile and
bioavailable
• Reactions with these chemicals
form new minerals in the soil
• Keep the elements of concern
from being dissolved in passing
water
• Phytoremediation—grow
plants that take up elements of
concern
• Harvesting these plants
decreases toxic element
content of the soil
Figure 12-33 A Repository Design
for Metal-Bearing Soil
Repositories physically isolate metalbearing soil from interacting with the
environment.
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Air Quality
• The principal concerns are dust generated at mine sites or
blown off tailings ponds and the emissions from smelter
operations
• Dust
• Drilling, blasting, hauling, and crushing rocks—all
create dust
• Water spray systems and vacuums—used to diminish
dust
• Smelter emissions
• Controlling emissions = biggest challenge at smelters
• Sulfur dioxide reacts with water to form sulfuric acid
• Contaminates soils and water and kills vegetation
• Smelters also emitted concentrations of metals (e.g., Pb)
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Mineral Resources in the Future
• Environmental challenges:
•
•
•
•
Proper waste rock disposal
Sound tailings pond construction
Prevention of ARD
Control of smelter emissions
• Other issues:
• Rapidly increasing demand for mineral resources
• Role of recycling in meeting mineral commodity
needs
• Application of sustainability concepts to use of
minerals
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Recycling
• Extend the use of a finite resource
• Diminish environmental consequences of mineral
resource development and production
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Sustainability and Mineral Resource Use
• Mineral resources are considered nonrenewable
• Consumption may lead to shortages of some mineral
commodities
• Substitute materials for needed minerals
• Use less mineral-intensive technologies
• More carefully use and recycling (conserve)
• How can sustainability concepts be applied to nonrenewable
resources like a mined ore deposit?
• Such conversion is a major contribution that mineral resource
development can make to a sustainable future for society as a
whole
• The mineral resource capital, an ore deposit in this case, is
converted to another form of capital that can provide
sustainable benefits to society
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SUMMARY
• Concentrations of valuable minerals may form where
valuable minerals precipitate from metal-bearing waters in
the shallow crust or on the seafloor. Ore is the part of a
mineral deposit that at current or foreseeable commodity
prices can be mined at a profit.
• Ore deposits are uncommon geologic features. Exploration
can be expensive and time-consuming, involving surface
and subsurface sampling and observations.
• Mining removes ore by underground or open-pit methods.
Mining also removes waste rock surrounding the ore. The
amount of waste rock is commonly two or three times the
amount of ore in open-pit mines, but it can be less than the
amount of ore in underground mines.
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SUMMARY (cont.)
• Ore is processed by milling and flotation to separate and
concentrate valuable minerals. The leftover nonvaluable
minerals are a waste product called tailings.
• Leaching removes valuable metals from some types of ores.
Leach solutions soak through the ore, dissolve the metal,
and carry it to processing facilities where the metal is
removed.
• The process of smelting recovers metals from the minerals
concentrated by milling and flotation. The solid waste from
smelting is commonly an iron- and silicon-rich material
called slag. Smelters are a source of gas (particularly sulfur
dioxide) and particulate emissions to the atmosphere.
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SUMMARY (cont.)
• Exploration, mining, and mineral processes lead to physical
disturbances of the landscape, many of which can be
reclaimed.
• Mining and mineral processing can affect water quality.
Containment structures and proper reclamation prevent
erosion of wastes, such as tailings, that could contaminate
surface water bodies.
• Acid Rock Drainage (ARD) can be prevented by various
methods and treated by the addition of neutralizing
materials.
• Dust, generated during mining operations, can be controlled
by water spraying, and sound reclamation prevents dust
generation from tailings ponds.
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SUMMARY (cont.)
• Sulfur dioxide emissions from smelters react with water to
form acid rain, which acidifies surface water (lakes and
streams) and ultimately the soil itself.
• Population growth and expanding economies will
significantly increase demand for mineral resources in
years to come.
• Recycling can help sustain mineral resources, but is not
efficient enough to replace mining or new mineral resource
production altogether.
• Sustainability can come partly from mineral resource
production if the financial capital derived from production is
satisfactorily converted to other forms of capital that can
sustain society after the mineral resources are depleted.
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