Producing and Recycling Metals (VCE SSCE Chemistry): Revision Notes

Producing and Recycling Metals

Introduction to metal sources

Most metals in Earth's crust exist as mineral compounds rather than pure elements. Only platinum and gold are found naturally in their elemental state, as they are inert and unreactive.

Minerals are naturally occurring solid substances with a definite chemical composition, structure and properties. Common metal-containing minerals include oxides, sulfides and silicates.

When minerals are concentrated enough in rock to make extraction economically viable, that rock is called an ore. The concentration of minerals determines whether mining is worthwhile.

Metal production from ores

Obtaining pure metal from ore involves three main stages: mining, ore processing and metal extraction.

Mining

Mining extracts ore from Earth's crust using different techniques depending on the deposit location:

Surface mining removes vegetation, soil and bedrock to access ore deposits below the surface. Open-pit mining is a common surface technique where excavation creates an increasingly wide and deep pit. Large vehicles transport ore from the pit to processing facilities.

Underground mining involves digging vertical shafts into the ground, then creating horizontal tunnels to reach the ore. Ore travels via vehicles and conveyors to the shaft, where it's lifted to the surface for processing.

In-situ leaching mines certain metals (including rare earth elements and uranium) by injecting solutions into holes in the ore deposit. The solution dissolves minerals, then gets pumped to the surface where minerals are recovered. This method avoids conventional mining costs.

Ore processing

After mining, ore is processed to separate valuable minerals from waste rock. The processing method depends on the ore's nature and the mineral's properties (such as density and magnetism).

Ore is typically crushed into pebble-sized pieces or ground into fine powder. Some ores, like iron ore, need no further processing. Others require additional treatment based on their specific mineral properties.

Metal extraction

The extraction method depends on the metal's reactivity:

Electrolysis can extract all metals by passing electric current through the molten (liquefied) compound. However, this requires large amounts of electrical energy, making it expensive. Electrolysis is reserved for very reactive elements such as aluminium, sodium and potassium.

Smelting extracts less reactive metals like iron by heating ore at high temperatures with carbon. Iron is extracted from iron oxide ores through this process.

Roasting converts sulfide ores (containing copper, lead and zinc) to metal oxides by heating in air. The metals are then produced from oxides by smelting with carbon. Copper and zinc require further purification by electrolysis after smelting.

Environmental issues

Metal production contributes significantly to global environmental problems. The industry uses approximately 8% of total global energy supply and manufactures metals in large quantities.

Environmental impacts fall into four categories:

Land impacts:

• Vegetation clearing and soil erosion
• Large quantities of waste production
• Coarser waste stockpiled in long-term storage dumps
• Tailings (finely ground rock) stored in ponds
• Waste management described as one of the world's largest environmental issues

Water impacts:

• High water consumption
• Potential release of toxic substances into groundwater and surface water

Air impacts:

• Substantial greenhouse gas emissions, primarily carbon dioxide and sulfur dioxide
• Emissions from fossil fuel energy sources
• Carbon use in smelting processes
• Dust and particulates causing local ecosystem and health damage
• Contribution to global warming

Biodiversity impacts:

• Landscape and ecosystem degradation
• Decline in species number and variety in affected areas

Metal recycling and the circular economy

Linear and circular economies

Traditionally, economies operated on a linear economy model - a 'take-make-dispose' or 'take-make-waste' approach. In this system, natural resources become products that are ultimately thrown away after use.

However, there's growing momentum toward a circular economy model where manufacturers design products to be reusable and recyclable.

Reasons for transitioning to a circular economy include:

• Resources becoming scarcer with declining quality
• Rising raw material prices
• Increasing demand for manufactured goods from growing, prosperous populations
• Desire to minimise environmental impacts (air and water pollution, climate change, land degradation, biodiversity loss)

To achieve circular economy goals, processes and technologies must be developed with reusing and recycling in mind. For example:

• Electrical devices designed to last longer and be easier to repair
• Plastic products recycled to make new plastic items
• Metal recycling playing a key role in sustainability initiatives

Metal recycling benefits

Metals are ideal for recycling because they can be re-melted and reshaped without losing their properties, allowing repeated recycling. Once a metal-containing product is no longer needed, it can usually be reprocessed relatively easily, with recovered metals returned to production.

There's a long history of metal recycling with considerable infrastructure already in place. For example, more than 90% of the 180,000 tonnes of gold ever mined is still in use.

Metals used in pure form (gold, copper, platinum) have high recovery rates. Average recycling rates for precious metals exceed 50%, with large variations across applications:

• Over 90% of precious metals from chemical and oil-refining catalysts are recovered
• About 65% recycling rate for platinum in automotive catalysts

Aluminium and steel also have high recycling rates.

Highly reactive group 1 and 2 metals (sodium, potassium, calcium) are abundant in Earth's crust. Since their compounds have more uses than the metals themselves, these metals aren't usually extracted during recycling.

Metal recycling contributes to circular economy and environmental protection by:

• Saving up to 20 times the energy needed to extract metals from ores, with economic benefits from lower material costs
• Avoiding metal-containing waste in landfill, preventing loss of valuable raw materials and environmental damage from metal compounds leaching into water
• Reducing $\text{CO}_2$ emissions (lower energy consumption) and impacts on land and water from mining and extraction
• Reducing air pollution by approximately 80%, water pollution by 75%, and water use by 40%

Metal recycling helps close the loop in production processes, reducing raw material consumption and waste sent to landfill.

The recycling process

Metal recycling is called secondary production. Today, more aluminium and lead is produced by secondary production than from ores, and large quantities of steel and copper are also manufactured this way.

Metals recovered for recycling are called scrap metal or scrap, coming from three sources:

• Waste from initial metal manufacture and processing
• Waste from metal fabrication into products
• Discarded metal-based products

The recycling process has four steps:

1. Collection

Scrap metal yards serve as collecting centres for metals.

2. Preparation for recovery

Scrap metal is classified into two main groups:

• Ferrous (containing iron, such as steel) - magnetic and easily separated from mixed waste
• Non-ferrous (other metals)

Large sorting facilities use sensors based on X-ray and infrared scanning to identify metals and improve recovery rates. New separation methods are under development.

Recycled metal is compacted and shredded because small metal pieces can be melted using less energy than larger pieces.

3. Smelting

Scrap metal is fed into one of two smelter types:

• Primary smelter - also used for extracting metal from ores
• Secondary smelter - designed especially for recycled metal

4. Purification

After melting, impure metal is refined to ensure the final product is free of impurities. Different purification methods are used for different metals. Other chemicals may be added to molten metal to form alloys with desirable properties.

Recycling aluminium

More aluminium is used in manufacturing than any other metal except iron. It's employed in cans, foils, window frames and aeroplane parts. Aluminium is often alloyed with elements like copper, manganese and magnesium because pure aluminium isn't particularly strong.

Globally, approximately 50% of new aluminium products are made from recycled aluminium. Seventy-five per cent of all aluminium produced since commercial production started in 1886 is thought to still be in use today.

Aluminium is recycled at low cost using secondary smelters. Scrap metal is placed in a furnace heated with gas or oil burners. Molten aluminium is run off and solidified.

Recycling a single aluminium can may save about 95% of the energy needed to make a new one.

Challenges and opportunities

Despite relatively high recycling rates for some metals, there's room for improvement. Insufficient collection of consumer goods and inefficient recycling chain handling are major challenges.

Case study: Iron production

Modern society depends heavily on iron. Iron is blended with other transition metals and carbon to produce steel used for construction and many purposes.

Australia is the world's largest iron ore exporter. Massive deposits in Western Australia's Pilbara region contain iron as haematite ($\text{Fe}_2\text{O}_3$).

Mining

Iron ore is mined by open-pit methods. Coal and limestone are also mined for iron extraction.

Coal is converted to coke (a solid containing 80-90% carbon) by strongly heating coal in air-tight ovens for about 15 hours.

Limestone is sedimentary rock mainly composed of calcium carbonate ($\text{CaCO}_3$).

Iron extraction

Iron extraction from ore occurs at very high temperatures in a tall, bottle-shaped tower called a blast furnace.

Hot air is blasted into the furnace bottom while solid 'charges' (scoops) of iron ore, coke and limestone are added to the top.

Holes at the furnace base are opened and molten iron and slag are drained out and separated. The iron contains dissolved carbon. To make steel, molten iron is usually transferred to another furnace where oxygen is blown through the metal to reduce carbon content. Other elements are added to form the steel alloy.

Case study: E-waste recycling

E-waste describes discarded electrical or electronic devices. It's considered the fastest-growing source of hazardous waste globally, with tens of millions of tonnes generated annually. Less than 20% of e-waste is recycled; most ends up in landfill, potentially damaging the environment through leakage of hazardous substances like mercury, lead and cadmium.

A typical mobile phone contains over 40 different metals. Printed circuit boards in e-waste contain precious metals (gold, silver, platinum) and other metals (copper, iron, aluminium).

Processing approaches include melting circuit boards, burning cable sheathing to recover copper wire, and using acid to dissolve and separate valuable metals.

Recycling is essential for managing e-wastes, offering potential for reducing environmental damage and natural resource use. However, since e-wastes are complex material mixtures containing relatively small metal concentrations, processing can be challenging. Recycling likely needs encouragement through local authority regulation. Several countries have enacted legislation requiring electronic manufacturers to pay for product recycling and disposal.

The e-waste recycling industry is growing. One Japanese smelter extracts hundreds of thousands of tonnes of gold, silver, copper, palladium and other valuable metals each year from circuit boards of discarded appliances, computers and mobile phones.

Lithium-ion batteries: A challenge

Lithium-ion batteries are being widely adopted as portable energy sources. Electric car numbers powered by these batteries are expected to grow spectacularly. This raises environmental concerns because less than 9% of spent batteries are recycled (in Australia, less than 3%).

Batteries contain valuable metals like cobalt, nickel, copper and manganese. Recovering these metals has been the main recycling focus. Lithium metal is rarely recycled.