Notes · Environmental ScienceUK · A-Levels

Mineral resources

This chapter examines the minerals and metals extracted from the lithosphere, from how deposits form to how they are mined, processed and, eventually, run down. It covers the concentration of ore deposits, the methods and impacts of extraction, the control and mitigation of those impacts including acid mine drainage and site restoration, and the strategies for securing future supplies through recycling, substitution and more efficient use.

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

T·0999 / 16

Exam profile

AO1 · Describe how mineral deposits form and how they are extracted, processed and their impacts controlledAO2 · Apply the concepts of ore grade and recycling to data and unfamiliar contextsAO3 · Analyse the environmental impacts of extraction and evaluate strategies for securing sustainable supplies

Operators:describeexplaincalculateanalyseevaluateassess

basic level

AS-Level expects you to describe how ores form, the main extraction methods and impacts, and the idea of recycling and reserves.

higher level

The full A-Level requires quantitative work with ore grade and recycling and the evaluation of strategies to control impacts and secure future supply.

Depth

Text

Contents · 4 sections

Mineral resources

Minerals, ores and how deposits form#

FoundationAQA 7447 3.2.3

Types of mineral deposit

Fig. 1Ore deposits form by different geological processes, each concentrating minerals in a characteristic way.

A mineral is a naturally occurring inorganic solid with a definite chemical composition, and an ore is a rock or mineral deposit from which a useful substance, usually a metal, can be extracted profitably. The crucial idea is concentration: most elements are present in ordinary rock at very low levels, and it is only where geological processes have concentrated them far above their average crustal abundance that extraction is worthwhile. The degree of concentration is the ore grade, the proportion of the useful mineral in the ore, and it largely decides whether a deposit can be worked.

Ore deposits form by a range of geological processes that concentrate particular elements. Igneous processes concentrate minerals as a magma cools and dense crystals settle, or as late fluids fill fractures. Hydrothermal processes, in which hot, mineral-rich water circulates through rock and deposits metals as it cools, form many important metal veins. Sedimentary processes concentrate minerals through weathering, transport and deposition, and secondary enrichment concentrates a metal when weathering removes the surrounding material or redeposits the metal at depth. Knowing that different processes form different deposits explains why particular ores are found in particular geological settings.

The distinction between reserves and resources is important for judging supply. The resource is the total amount of a mineral present in the Earth's crust, most of it too dilute or too deep to extract; the reserve is the portion that can be extracted economically with current technology and prices. Reserves are therefore not fixed: they grow when new deposits are found, when prices rise so that lower-grade ore becomes worth mining, or when technology improves, and they shrink as ore is used up. This is why simple predictions that a metal will run out by a certain date are unreliable.

Because the richest, most accessible deposits are mined first, the average grade of the ore being worked tends to fall over time, so more rock must be moved and processed to obtain the same amount of metal. This rising effort has consequences for energy use, cost and environmental impact, all of which grow as grade falls, and it is one of the main arguments for recycling and for using minerals more efficiently, considered at the end of the chapter.

\text{ore grade} = \frac{\text{mass of the useful mineral}}{\text{total mass of ore}} \times 100

Ore grade

The percentage of the useful mineral in the ore; a higher grade means less rock must be processed per unit of metal.

Worked example

Calculating and interpreting ore grade

A copper ore contains 8 tonnes of copper in every 1000 tonnes of ore. Calculate the ore grade and comment on how the grade affects the amount of waste rock produced.

Apply the equation
$\text{ore grade} = \dfrac{8}{1000} \times 100$ .
$\text{ore grade} = \frac{8}{1000} \times 100 = 0.8\%$
Interpret
Only 0.8% of the ore is copper, so about 992 tonnes of every 1000 become waste rock and tailings.
Comment
A low grade means a great deal of rock must be moved and processed per tonne of metal, increasing energy use, cost and environmental impact.

Result: The ore grade is 0.8%, so almost all the rock mined becomes waste, raising the impact of extraction.

Typical mistakes

Treating reserves as a fixed quantity; they change with price, technology and new discoveries.
Confusing the resource (all the mineral present) with the reserve (the part that is economic to extract now).

Active revision

Explain why a rise in the market price of a metal can increase its reserves without any new deposit being discovered.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Exploration, extraction and processing#

StandardAQA 7447 3.2.3

From ore to metal

Fig. 2The stages from finding a deposit to producing metal, each using energy and generating waste.

Before a deposit can be worked it must be found and assessed. Exploration uses geological mapping, geophysical surveys (measuring properties such as magnetism, density and conductivity), geochemical sampling of soils and streams, and drilling to locate a deposit and estimate its size and grade. This assessment decides whether the deposit is a reserve worth developing, because mining requires very large investment and the deposit must be rich and large enough to repay it.

The method of extraction depends chiefly on how deep the deposit lies. Surface or opencast mining, in which the overlying rock and soil (the overburden) are stripped away to reach a shallow deposit, is cheaper and safer for workers and can recover a high proportion of the ore, but it disturbs a very large area of land. Sub-surface or deep mining, using shafts and tunnels to reach a deep deposit, disturbs far less of the surface but is more expensive and dangerous and recovers less of the ore. The choice is therefore a trade-off between cost, safety, recovery and the area of land affected.

Once extracted, the ore must be processed to obtain the useful mineral. It is usually crushed and ground to free the mineral grains, then concentrated by physical or chemical separation to remove the unwanted rock (gangue), and finally the metal is extracted from the concentrate, for example by smelting or by electrolysis. Each step uses energy and, in the case of low-grade ores, must handle enormous volumes of rock, which is why the energy cost of primary metal production is so high and why it rises as ore grades fall.

Processing generates large quantities of waste. The unwanted rock removed during concentration forms tailings, often a fine, wet slurry stored behind dams, and smelting can release gases such as sulfur dioxide. Managing these wastes safely is a major part of a mine's operation and cost, and their potential to cause pollution, considered next, is one of the chief environmental concerns of mineral extraction.

Worked example

Choosing an extraction method

A copper deposit lies within 40 m of the surface and is spread over a wide area. Recommend an extraction method and justify your choice.

Consider the depth
At only 40 m the overburden is shallow, so it can be stripped economically; deep mining would be unnecessarily expensive.
Consider recovery and cost
Opencast mining recovers a high proportion of a wide, shallow deposit at lower cost and with greater worker safety.
Recommend with a caveat
Opencast mining is appropriate, but it will disturb a large area, so a restoration plan should be required as a condition.

Result: Opencast mining suits a shallow, wide deposit for cost, recovery and safety, but requires a restoration commitment.

Typical mistakes

Assuming opencast mining is always worse; it recovers more ore and is safer for workers, though it disturbs more land.
Forgetting that processing, not just mining, produces large volumes of waste (tailings) and uses much energy.

Active revision

A shallow but low-grade ore body underlies farmland. Discuss the factors that would decide whether it is mined by opencast methods.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Environmental impacts of mining and their control#

StandardAQA 7447 3.2.3

Acid mine drainage

Fig. 3Acid mine drainage: exposed sulfides oxidise to sulfuric acid and metals that poison streams for years.

Mineral extraction has a wide range of environmental impacts. It takes and disturbs land, destroying habitats and displacing wildlife; it produces spoil and tailings that must be stored; it generates dust and noise; and it uses large amounts of energy and water. Because so much rock must be handled, especially for low-grade ores, these impacts are large in scale. Recognising the full range, from the direct loss of land to the indirect emissions from the energy used, is the basis for judging and controlling them.

Water pollution is among the most serious impacts, and acid mine drainage is its classic form. When sulfide minerals, such as iron pyrite, are exposed to air and water during mining, they oxidise to form sulfuric acid and release dissolved metals. The resulting acidic, metal-rich water drains from the mine and its waste into streams, where the low pH and toxic metals kill aquatic life and can persist long after the mine has closed. The mechanism, the exposure and oxidation of sulfides, is worth learning because it explains why the problem continues for so long and is so hard to remedy.

These impacts can be controlled and mitigated at every stage. Dust can be suppressed with water, noise limited by working hours and barriers, and tailings dams engineered and monitored to prevent collapse; acid drainage can be reduced by keeping sulfides away from air and water, by neutralising the acid with lime, and by treating the water in constructed wetlands before it is released. Prevention is more effective than treatment, so careful design and management of the waste at the outset avoids problems that would otherwise last for decades.

When a mine closes, restoration returns the land to a productive or natural use. The land is regraded, the stored topsoil is replaced, and the site is replanted for agriculture, forestry, recreation or wildlife; former quarries and pits may become lakes or nature reserves. Restoration is now often a legal condition of permission and is paid for by a bond set aside in advance, so that the cost falls on the operator rather than on the public. Evaluating whether restoration truly compensates for the disturbance, and how to fund it, is a legitimate focus for higher-level answers.

Worked example

Explaining a persistent pollution problem

A stream below an abandoned mine is orange, acidic and lifeless. Explain the cause and suggest a treatment.

Identify the cause
Sulfide minerals in the mine waste are exposed to air and water and oxidise, forming sulfuric acid and releasing iron and other metals, which colour and acidify the water.
Explain the persistence
As long as sulfides remain exposed, oxidation continues, so the drainage goes on for decades regardless of whether the mine is working.
Suggest a treatment
Neutralise the acid with lime and pass the water through a constructed wetland to remove metals before it reaches the stream, and cap the waste to exclude air and water.

Result: Continuing oxidation of exposed sulfides causes lasting acid drainage; neutralising the acid and treating the water reduces it.

Typical mistakes

Describing acid mine drainage without the oxidation of sulfides, which is the essential step.
Forgetting the indirect impacts of mining, such as the emissions from the large amounts of energy used.

Active revision

Explain why a metal mine can continue to pollute a river with acidic water for decades after it has closed, and suggest one way to reduce this.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Securing future mineral supplies and sustainability#

AdvancedAQA 7447 3.2.3

Energy saved by recycling metals

Fig. 4Approximate energy saved by recycling instead of primary production (illustrative values): recycling metals saves most of the energy.

Because minerals are finite and non-renewable, and the richest ores are being used first, securing future supplies is a central concern. Several strategies extend how long supplies last. Exploiting lower-grade ores and new sources (including deposits that were previously uneconomic) adds to reserves but increases the energy, cost and impact of extraction. Using minerals more efficiently, so that less metal is needed for the same job, reduces demand. Substituting a scarce material with a more abundant one, for example replacing a scarce metal with a plastic or a common alloy, can relieve pressure on a particular resource.

Recycling is often the most effective strategy, because it returns metal to use without mining new ore and usually at a large saving of energy. Recycling a metal avoids the exploration, extraction and, above all, the energy-intensive extraction of the metal from its ore; for many metals the energy needed to recycle is only a small fraction of that needed to produce the metal from ore. Recycling also reduces the land disturbance, waste and pollution of primary production. Its limits are that not all products are easy to collect and separate, some metals are dispersed or contaminated in use, and recycling is not entirely free of energy or impact.

The energy savings from recycling differ between metals but are frequently very large, which is why recycling is favoured on both resource and climate grounds. Recycling aluminium, for instance, saves the great majority of the energy needed to extract it from its ore, and recycling steel and copper also saves substantial energy. These savings mean that recycling reduces greenhouse-gas emissions as well as conserving the ore, linking mineral management to the energy and climate chapters.

Judging the sustainability of mineral use means weighing supply security against cost and environmental protection. No single measure is sufficient: lower-grade ores extend supply but at rising impact, efficiency and substitution reduce demand but have limits, and recycling saves energy and ore but cannot recover everything. A sustainable strategy combines them, prioritising efficiency and recycling to reduce the need for primary extraction, and reserving the mining of new, lower-grade deposits for the demand that cannot be met otherwise. This mirrors the reduce-reuse-recycle logic developed fully in the sustainability chapter.

\text{percentage energy saved} = \frac{E_{\text{primary}} - E_{\text{recycled}}}{E_{\text{primary}}} \times 100

Energy saved by recycling

Compares the energy to produce a metal from ore with the energy to recycle it; a higher percentage means a greater saving.

Worked example

Calculating the energy saved by recycling

Producing 1 tonne of aluminium from ore needs about 200 units of energy; recycling the same tonne needs about 10 units. Calculate the percentage of energy saved by recycling.

Find the saving
Energy saved = $200 - 10 = 190$ units.
Express as a percentage of the primary energy
$\dfrac{190}{200} \times 100$ .
$\frac{190}{200} \times 100 = 95\%$
Interpret
Recycling saves about 95% of the energy, which conserves the ore and greatly reduces the greenhouse-gas emissions of production.

Result: Recycling saves about 95% of the energy of primary production, benefiting both resources and climate.

Typical mistakes

Presenting recycling as completely free of energy and impact; it saves a large fraction of energy but is not costless.
Assuming lower-grade ores are a simple solution; extracting them raises energy use, cost and environmental impact.

Active revision

Explain why recycling aluminium is strongly favoured on both resource-conservation and climate grounds.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Mineral resources

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

Mineral resources

Key points

Calculating and interpreting ore grade

Key points

Choosing an extraction method

Key points

Explaining a persistent pollution problem

Key points

Calculating the energy saved by recycling

Sources

Mineral resources

Key points

Calculating and interpreting ore grade

Key points

Choosing an extraction method

Key points

Explaining a persistent pollution problem

Key points

Calculating the energy saved by recycling

Sources