Understand your copper emissions

Part 4 of CarbonChain's 'High-carbon Commodities' blog series. 

Copper is a critical material in the energy transition. It’s heavily used for renewable technologies like solar panels and electric transport. However, copper also has a significant carbon footprint.

What is copper's carbon footprint?

Currently the world produces around 21 million tonnes of copper per year, contributing to around 0.2% to 0.3% of global greenhouse gas (GHG) emissions. 

While this is a relatively low contribution compared to aluminium or steel, it's set to greatly increase as demand for copper skyrockets; some research predicts demand for copper will more than double by 2050, exceeding supply.

Copper is also more than twice as carbon intensive as steel, emitting 4.1 tonnes of carbon dioxide equivalent (tCO₂e) per metric tonne of copper (compared to 1.9 tCO2e per tonne of steel).

Read on to understand your copper product emissions and how they might be reduced:

• What causes copper's emissions?
• How is copper cathode made? Drilling down into copper production emissions
• Which copper cathode production method is lower carbon?
• Is recycling copper lower carbon?
• How are manufacturers and producers reducing copper's carbon footprint?
• Calculate your copper emissions

What causes copper's emissions?

One of the key materials in the copper supply chain is copper cathode (copper with a purity of >99.95%). There are two key stages to copper cathode production, which contribute to its carbon intensity: mining copper ore, followed by processing into copper cathode. After this, copper cathode is semi-fabricated into products like rods, sheets and wires, which are then manufactured into finished goods. 

How is copper cathode made? Drilling down into copper production emissions:

Mining

There are two main types of copper ore found in the Earth’s crust: copper oxide ores and copper sulfide ores. Copper sulfide ores (where copper is found in compounds with iron and sulfur) are the most common source of copper.  Copper ore contains a low concentration of copper; around 0.5% to 2%, depending on where it is mined. The ore typically also contains trace amounts of valuable metals such as gold and silver. 

Emissions are mainly caused by diesel combustion in mining equipment like excavators and trucks. The lower the copper concentration in the ore, the more processing is subsequently needed, which increases the carbon intensity. The quality (Cu concentration) of copper ore is reducing over time, since the mining of higher grade ore has been prioritized.

Processing

One of two methods is used for refining the copper ore into copper cathode, depending on the type of ore. Sulfide ores are usually processed pyrometallurgically (using heat) and oxide ores hydrometallurgically (using aqueous solutions).

Method 1: Pyrometallurgy

This method accounts for 80% of primary copper production (copper from non-scrap sources). It's used for copper sulfide ores and involves three main steps: concentrating, smelting and electrorefining.

Concentrating:

First, the ore goes through an initial concentration step to increase the purity. The ore is crushed to a fine powder, mixed with reagents, and aerated to make the copper float to the top. The froth skimmed off the top contains around 30% Cu and similar amounts of iron and sulfur. The output of this process is known as copper concentrate and contains trace amounts of valuable metals.

Sulfide ores are concentrated on-site at the mine before being transported to a smelter or refinery, which may be off-site. 

The concentrating process is very emissions intensive as a lot of mechanical energy is required to crush the hard rock into a fine powder.

Smelting:

Smelting copper concentrate involves two main stages: heating and converting.

Silica and limestone are added to the Cu mixture, heated to high temperatures (around 1,200°C), and unwanted compounds, such as iron oxide, are removed as slag. This product is known as copper matte and contains around 60% Cu as copper sulfide (Cu₂S).

Next, oxygen-rich air is added, reacting with the copper sulfide to form copper and sulfur dioxide. 

Cu₂S + O₂ -> 2Cu + SO₂

While not a GHG, sulfur dioxide is a harmful air pollutant and so is captured and processed into sulfuric acid which can be sold, or used in hydrometallurgical copper processing.

This results in a product with around 99% Cu, known as blister copper.

Emissions from the smelting stage result from burning fossil fuels (such as coal or natural gas) to heat the furnace to such high temperatures.

Electrorefining: Finally, electrolytic refining is used to produce high quality copper cathodes (>99.99% Cu), as illustrated in the diagram below. This very high purity is required for electrical industries, where 45% of copper is used.

Electrorefining: The blister copper is cast into a large slab to be used as the anode, and a pure copper sheet is used as the cathode. The two electrodes are placed in a solution containing copper sulfate and sulfuric acid. A current is passed through the solution, releasing copper ions (Cu²⁺) from the copper anode, which move through the solution and collect on the pure copper cathode.

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The emissions intensity of the electrorefining stage depends on the source of the electricity used. Depending on the refinery, electricity may be generated on site or purchased from the local grid, and may be generated from fossil fuels or renewable sources.

Method 2: Hydrometallurgy

This method accounts for 20% of primary copper production. It's used for oxide ores and involves leaching, solvent extraction, and electrowinning.

As with pyrometallurgy, the oxide ores are first crushed to a fine powder, requiring lots of mechanical energy (typically diesel-powered machinery). Then the powder is leached. This involves applying dilute sulfuric acid to the surface, to create a weak copper sulfate solution, known as ‘pregnant liquor’ or pregnant leach solution (PLS). 

CuO + H₂SO₄ → CuSO₄ + H₂O

The pregnant liquor then goes through a process of Solvent eXtraction and Electro-Winning (SX-EW) to produce >99.99% copper cathode:

• SX: The pregnant liquor is purified by using an organic solvent (or extractant) which selectively reacts with copper over the other elements. The copper is then stripped from the organic solvent into a copper-bearing, electrolytic solution.
• EW: A current is applied to the solution (similar to electrorefining), reducing the copper ions to copper metal at the cathode. Sulfuric acid is produced at the anode.

Electrowinning: An anode of an inert metal such as lead, and a cathode such as stainless steel are placed in the solution and an electric current is applied. The solution contains dissolved copper ions (Cu²⁺) which are attracted to the negative anode, where they gain electrons forming copper metal. The negative O₂- ions are attracted to the positive anode and bubble to the surface as oxygen gas. 

As with electrorefining, the emissions from electrowinning depend on how the electricity was generated. 

The hydrometallurgical process is efficient. It does not require high temperatures, nor does it directly produce GHGs through its chemical reactions, and at each stage the solutions can be reused. 

Which copper cathode production method is lower carbon? 

Electrowinning alone is more energy intensive than electrorefining alone. However, the hydrometallurgical process overall is less energy intensive than the pyrometallurgical method, resulting in a significantly lower carbon footprint (up to three times less tCO₂e / t Cu in some cases). 

Yet, it’s not a simple case of choosing one method over the other. Different ores require different methods, and one method relies on a byproduct from the other. Reducing the GHG emissions from copper production will depend on reducing the emissions intensity of every process above.

Is recycling copper lower carbon? 

Copper is infinitely recyclable without loss in quality. Secondary copper (produced from recycled scrap) accounts for 40% of today’s copper production. 
The carbon intensity of producing secondary copper can vary greatly. It can require up to 90% less energy than primary copper, but there is some overlap in the carbon intensity ranges. A 2017 review found:

• Primary production: 1.1-8.5 tCO₂e/t copper
• Secondary production: 0.2-1.9 tCO₂e/t copper

Depending on the mix of the used copper products, the processing stages for recycling differ. Sometimes, simple remelting is enough, while overly contaminated copper and copper alloy scraps may require electrorefining, thereby increasing emissions. Recycling post-consumer scrap also involves collection and sorting processes.

How are manufacturers and producers reducing copper's carbon footprint?

The key ways to decarbonize copper production include:

1. Run on renewables: The share of renewables in the electricity used in copper production is a key determiner of carbon intensity. The Kamoa-Kakula Copper Mine in Lualabla, DRC, is powered by renewable hydro-generated electricity, Antofagasta’s copper mining sites use 100% renewable energy, while BHP, RIO and Vale are electrifying mining trucks.  
2. Boost scrap supply: Currently, there’s not enough copper scrap to meet demand. Companies should encourage the recyclability of copper end products to increase recycling rates and supply. Copper smelting and refining company LS MnM has partnered with SK Network to promote recycling in electronics in South Korea. JX Nippon Mining and Metals is targeting 25% recycled raw materials at its smelters by 2030, while acquiring Canadian electronic waste recycler, eCycle, to address future recycling demand.
3. Improve efficiency: Selecting more energy-efficient production technologies, for example flash smelting instead of shaft furnaces. For Sweden Boliden, investing in automation technology and on-demand ventilation systems achieves up to 25% energy savings.

Calculate your copper emissions

Meet the demand for sustainable copper products and prepare your business for carbon regulation. Whether a producer, trader, or procurer of copper, you can start calculating your embedded copper emissions and find ways to reduce them towards net zero.

CarbonChain's carbon accounting platform can help you accurately and automatically track your copper supply chain emissions.

Compare suppliers and assets, find lower-carbon options, and share your product carbon footprint and progress with your customers.

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