Copper, a sustainable material for the energetic transition

Copper, the first metal used by humanity, is at the heart of the current energetic transition. Thanks to its excellent thermal and electrical conductivity, it is indeed particularly valuable in the electrical and electronics industries, which account for more than half of its total consumption. Copper is indeed essential for manufacturing cables, wires, electric motors, transformers, switches, and circuit boards. Beyond these, it is integral to the production of industrial valves, fittings, instruments, plain bearings, molds, heat exchangers, and various pumps used in machinery and transportation. It ranks third in global production and usage among engineering metals, following iron and aluminum. The environmental impact of industrial materials must be evaluated from two key perspectives. First is the assessment of energy and resource consumption during the entire production process, from ore extraction to final product manufacturing. Second is the analysis of the product’s life cycle and reuse rate. Copper is recognised as a sustainable material. A testament to its durability is the Zenghouyi Chime Bells, depicted on the cover, a musical instrument comprising 65 bronze bells, remained intact after being buried underground for 2,430 years.

1. Copper and human civilisation

The discovery and use of new materials have always driven the progress of human civilization. Pottery, as the first man-made material, marked the end of the Stone Age, transitioning humanity to a more organized, though still primitive, era. During this period, mankind gradually developed new materials and invented a material with better performance and wider use than ceramic, the bronze.  Bronze agricultural tools accelerated the development of agriculture, while bronze weapons ushered in the Cold Weapon era. The Bronze Age is recognized as the second period in the Three-Age System (Stone Age, Bronze Age, and Iron Age), a classification proposed by Danish archaeologist Christian Jürgensen Thomsen in 1836 [2].

In the ancient Near East, the Bronze Age (4,500BC–1,000 CE) began with the rise of Sumerian civilization and spanned the first 4,000 years of human history. This era was defined by the widespread use of bronze—an alloy of copper with tin, lead, antimony, or arsenic—for making tools and weapons. In China, the Bronze Age reached its peak during the Xia, Shang, and Zhou dynasties, continuing into the Spring and Autumn and Warring States periods. A notable example from this era is the Simuwu Rectangle Tripod, one of the largest extant bronze vessels in the world [2].

1.1 Basic properties of copper

Copper, with the chemical symbol Cu (from Latin Cuprum) and atomic number 29, is a transition metal found in the Earth’s crust and oceans. Copper constitutes approximately 0.01% of the Earth’s crust, though in concentrated deposits, its content can reach 3% to 5%. Most naturally occurring copper exists in the form of copper minerals or compounds. Copper physical properties are summarized in Table 1 [3]. It exhibits excellent electrical and thermal conductivity, along with low hardness and high plasticity, allowing it to be easily processed into products of various shapes for electrical and thermal applications.

Purple copper, also known as pure copper, has high electrical conductivity and plasticity but relatively low strength and hardness. When copper is alloyed with different elements, it forms various copper alloys with distinct properties. For example:

Cu-Ni (copper-nickel alloy), also known as white copper, where nickel is the primary alloying element.

Cu-Zn (copper-zinc alloy), commonly known as brass, where zinc is the primary alloying element.

All other copper alloys, including those with tin and other elements, are collectively referred to as bronze.

Table 1 Main Physical and Mechanical Properties of Copper [3].

1.2 Copper and cultural heritage

Copper is renowned for its distinctive, beautiful color. Its corrosion resistance and ease of processing make it a favored material for iconic statues that symbolize the history and culture of cities and nations.

Figure 1 depicts the famous bronze bull statue on the Wall Street, designed by Italian artist Arturo Di Modica. Standing 3.4 meters tall, 4.8 meters long, and weighing 3.2 tonnes, the bull symbolizes “strength and courage,” representing the prosperity and wealth associated with Wall Street [4]. (similarly, the Chinese have long admired the bull’s vigor, reflected in the idiom “Niu Qi Chong Tian”, meaning “at the peak of one’s power” or “unstoppable energy.” This cultural parallel highlights the shared recognition of the bull’s enduring strength.)

Figure 2 illustrates a traditional Chinese medicine practitioner treating a patient in Sanfangqixiang, Fuzhou, China, emphasizing the long-standing heritage of Chinese medicine.

1.3 Copper role as currency

Over 2,000 years ago, metallic currencies—primarily gold, silver, and copper coins—emerged in China during the Spring and Autumn Period and the Warring States Period. The Spring and Autumn Period marked a critical transition from slavery to feudalism in Chinese history. As the commodity economy expanded, the demand for currency increased significantly. Advancements in metal casting technology enabled large-scale production of metal coins. This era was characterized by a diverse range of currencies and a fragmented coinage system. As the Zhou Dynasty’s power waned, individual states established independent economic systems, each minting its own currency. This led to the long-term coexistence of multiple currency systems and types.

Figure 3 displays bronze coins exhibited at the Shanxi Museum in China. These coins were used following the unification of currency during the Qin Dynasty and remained in circulation for over 2,000 years until the Qing Dynasty. Figure 4 showcases bronze coins from various periods of the Qing Dynasty. After the founding of the People’s Republic of China in 1949, several versions of the Chinese yuan were issued, with coins often made from copper-nickel alloys.

1.4 Copper in daily life and religion

Copper is a metal deeply intertwined with human life and culture. Since the Bronze Age over 2,000 years ago, people have used bronze to craft a variety of artifacts, including:

Copper tripod: Originally used for cooking food, it later became an important ceremonial vessel, particularly for sacrifices and feasts during the Shang and Zhou Dynasties.

• Copper pot with engraved figures
• Copper seals
• Copper weapons
• Copper statues
• Copper washing utensils [5]
• Copper columns
• Copper lotus-shaped candle-stick holders
• Copper lion-shaped incense burners
• Copper armillary spheres

Copper remains one of the most commonly used materials in daily life. Figure 5 shows a copper tap, illustrating its practical use while Figure 6 depicts a copper Buddha statue from Longkou, Shandong Province, China, highlighting its artistic and cultural significance.

2. Smelting and processing of copper and copper alloys

2.1 Smelting

More than 200 copper-bearing minerals exist in nature, but only 20 are of industrial importance. These include primarily copper sulfide minerals and, to a lesser extent, copper oxide minerals, as outlined in Table 2 [6].

Table 2 Important copper minerals [6].

The development of copper smelting technology has evolved over centuries and can be classified into three major categories: pyrometallurgy, hydrometallurgy, and electrometallurgy. Currently, the extraction of copper from sulfide ores is predominantly carried out via pyrometallurgy, which accounts for approximately 85% of global copper production.

Pyrometallurgical processing involves increasing the copper content from its natural state of a few percent to around 20–30% through separation. Copper concentrates are then smelted in closed furnaces, such as blast furnaces, reverberatory furnaces, electric furnaces, or flash furnaces, to produce (also known as ice copper). This molten matte is transferred to a converter, where it is refined into crude copper. The crude copper undergoes further refining in another reverberatory furnace, occurs. It is then cast into anode plates for electrolysis, resulting in electrolytic copper with a purity of up to 99.9%. This process is efficient, with a copper recovery rate of up to 95%, and adaptable to various ore types. However, it is not environmentally friendly due to the emission of sulfur dioxide during the smelting and blowing stages. In recent years, processes such as the Baiyin process, Noranda process, and Japan’s Mitsubishi method have contributed to making pyrometallurgy more continuous and automated [6][7].

Modern hydrometallurgical (wet) copper smelting methods include sulfated roasting-leaching-electrowinning, leaching-extraction-electrowinning, and bacterial leaching. These methods are particularly suited for heap leaching, tank leaching, or in situ leaching of low-grade, complex ores, oxidized copper ores, and copper-containing waste. Wet smelting technology is gradually being adopted, and copper production using this method is expected to reach 20% of the total by the end of the century. One major advantage of wet smelting is its ability to significantly reduce the cost of copper extraction [6][7].

2.2 Refining

Common refining methods include zone refining, electrolytic refining, electron beam refining, and ion exchange. Among them, electrolytic refining is the most mature and widely adopted method.

In electrolytic refining, the anode consists of pure copper obtained through fire refining, while the cathode is a thin copper sheet (original pole piece) produced by electrolysis. The electrolyte is an acidified copper sulfate solution. When a direct current is applied, copper at the anode dissolves electrochemically, releasing copper ions that migrate and deposit onto the cathode. Impurities either settle in the anode sludge or remain in the electrolyte, facilitating the separation of copper from other metals. Figure 7 illustrates the flow of the electrolytic refining process [6].

Oxidation reaction at the anode:

Here, M’ represents metals such as Ni, Pb, and As, which are more electronegative than copper. Due to their low concentrations, these metals dissolve preferentially into the electrolyte. Copper, being the primary component of the anode, undergoes oxidation to form Cu²⁺ ions. Oxidation reactions involving H₂ O cannot occur at the anode due to the higher electrode potential compared to copper. Precious metals like Ag, Au, and Pt, with more positive electrode potentials, do not dissolve and instead settle as part of the anode sludge.

Reduction reaction at the cathode:

The standard electrode potential of hydrogen is more negative than that of copper. Additionally, the overvoltage at the copper cathode further lowers the electrode potential of hydrogen. As a result, under normal conditions, only copper deposits at the cathode, while hydrogen does not. Similarly, negatively charged metals with lower standard potentials than copper do not precipitate at the cathode [6].

Figure 8 shows a high-purity copper plate produced at the cathode. The main drawback of this method is the use of sulfuric acid, which necessitates stringent environmental protection measures.

The electron beam refining (E-beam) method uses an electron beam to bombard raw copper under vacuum, generating high temperatures to melt the metal. In this process, impurities with low saturation vapor pressure are vaporized under vacuum. Directional solidification concentrates impurities with low solute distribution coefficients at the top, enabling further purification. While this method is more environmentally friendly than electrolytic refining, it is also more costly and less efficient.

2.3 Melting

Copper and copper alloys used in industry are primarily shaped into rods, wires, plates, foils, tubes, and other forms. To achieve these shapes, electrolytic copper plates must be melted at high temperatures. In some cases, alloying elements are added to produce copper alloy liquids, which are then cast into billets of various shapes.

The commonly used furnaces for melting copper and copper alloys are power-frequency core induction furnaces and medium-frequency induction furnaces. Power-frequency core induction furnaces are primarily used for large-scale continuous casting, while medium-frequency induction furnaces are suitable for producing multiple types of billets in small batches.

Copper alloy melting can cause significant environmental issues, particularly when the alloy contains low-melting-point or volatile elements. For example, brass, an alloy of copper and zinc (Cu-Zn), is highly valued for its excellent mechanical and wear-resistant properties and is used in precision instruments, ship components, gun cartridges, coins, and more. Brass typically contains 30%-40% zinc.

Since the melting point of copper is 1083°C and the boiling point of zinc is approximately 907°C, the melting temperature of brass exceeds zinc’s boiling point, producing a large volume of zinc vapor that contributes to serious environmental pollution. Therefore, factories must install appropriate environmental protection equipment, such as fume extraction systems, above the melting furnaces.

2.4 Casting

Currently, most copper billets are produced through continuous casting or semi-continuous casting. Depending on the product requirements, the following casting methods are commonly used:

1. Downward Vertical Semi-Continuous Casting
   This method is used to produce round billets (ingots), square billets, thick slabs, and hollow round billets. It allows for a wide range of billet sizes and offers high productivity. However, the casting pit typically requires a depth of about 6 meters. Once the billet reaches a certain length, the process must be paused, and the billet removed. The crystallizer must also be cleaned before the next casting cycle. Both the top and bottom of the billet require trimming.
2. Horizontal Continuous Casting
   This method is used for producing round billets, strips, and hollow round billets. It features continuous casting, where cutting saws trim the billets to the desired lengths. However, this method is unsuitable for casting billets with large cross-sections.
3. Upward Continuous Casting
   This technique is used to produce round billets, strips, and hollow round billets. It allows for continuous casting, where billets can be directly rolled. However, it is also unsuitable for large cross-sections, and due to the anti-gravity nature of the process, the internal densification of the billets is slightly inferior.

Figure 9 shows a photograph of horizontal continuous casting of tin-phosphor bronze strips. Figure 10 depicts vertical semi-continuous casting of a copper-chromium-zirconium alloy round billet.

2.5 Forming

Copper has a face-centered cubic structure, allowing it to deform plastically with ease. This makes it possible to process copper and its alloys into various shapes such as plates, strips, foils, rods, wires, and tubes.

One notable application is copper foil, which serves as the anode collector in lithium-ion batteries used in new energy vehicles and electronic devices. Copper foil is a critical component, accounting for 5-8% of the total cost of a lithium battery. It functions as both the carrier of the anode active material and the collector and conductor of electron flow. Therefore, the tensile strength, elongation, density, surface roughness, thickness uniformity, and appearance quality of electrolytic copper foil have a significant impact on the production process and electrochemical performance of lithium-ion batteries. The typical thickness of lithium-ion battery copper foil ranges from 7 to 20 μm, but it is reduced to  4 to 12 μm in new devices. In a single vehicle, the total mass of copper foil can exceed 10 kg. Thinner copper foil not only reduces the  weight but also decreases internal resistance, thereby enhancing battery performance. Figure 11 shows the winding process of copper foil for lithium battery.

In addition, silver-copper alloy wires are extensively used in electronic industry, as discussed in section 3.2. Figure 12 depicts the continuous drawing process of copper-silver alloy wire, along with a sample.

3. Applications of copper in key industries

3.1 Heat transfer

Power plants use various condensers to condense vapor from turbines. Similarly, in refrigeration systems, condensers are essential for condensing refrigerants such as ammonia and freon in vapor-compression refrigeration systems. This process is also employed in desalination equipment and air conditioners used in daily life. Refrigerants absorb heat in the evaporator and release it in the condenser. The outer surface of the condenser tube is connected to a heat sink, as shown in Figure 13. Copper is widely used for condenser tubes due to its excellent thermal conductivity, weldability, mechanical strength, and ductility. Its surface seals easily, reducing the risk of refrigerant leaks. As a result, copper has become the preferred material for condenser tubes, with around 2 million tonnes used globally each year. Figure 14 shows an example of a copper condenser tube. To enhance heat transfer efficiency by increasing the surface area, the inner surface of the copper tube is often serrated.

3.2 Electrical conductivity

Copper-silver alloy wires, with silver as the primary alloying element, possess exceptional electrical conductivity, mechanical strength, abrasion resistance, fusion-weld resistance, and thermal stability. Enameled wires made directly or coated with insulating layers (as shown in Figure 12) are extensively used in consumer electronics such as headphones, mobile phones, computer voice coils, and semiconductor bonding wires. In audio and video transmission, these wires provide high-fidelity signal transmission, exhibiting excellent resistance to high-frequency signal attenuation. This ensures clear audio and video transmission and meets the growing demand for high-speed data transfer. In bonding wire applications, copper-silver alloy wires offer a cost-effective alternative to gold wires, combining high electrical conductivity with superior mechanical strength. Their excellent ductility and resistance to breakage enable stable connections in high-speed bonding equipment, significantly reducing the risk of wire failure. These characteristics make copper-silver alloy wires an ideal material for high-performance electronics, balancing reliability and cost-effectiveness, and advancing the development of lightweight, high-performance electronic devices.

With the rapid development of integrated circuit technology, lead frames—critical components in integrated circuits—must continually improve in performance. As the chip carrier in integrated circuits, the lead frame facilitates electrical connections between the internal circuit leads of the chip and external leads using bonding materials like gold, aluminum, and copper wires. It acts as a bridge connecting internal circuits with external connections. Lead frames are essential in most semiconductor integrated circuits, serving as a vital material in the electronic information industry.

Copper alloys are the material of choice for lead frames due to their excellent electrical and thermal conductivity, high strength, and hardness. Their composition includes Cu-Fe, Cu-Ni-Si, and Cu-Cr systems. Figure 15 shows a rolled copper alloy strip used for lead frames.

A high-speed railway consists of three main components: the overhead contact line, the train, and the track. The overhead contact line, which delivers power to the train, must possess high electrical conductivity, strength, abrasion resistance, and resistance to electrical corrosion. Its performance is crucial to the safety and efficiency of high-speed railway operations, as power failure is a common cause of accidents. For example, China’s first high-speed railway accident occurred at the contact point between the overhead contact line of the Wuhan-Guangzhou high-speed railway and the train’s pantograph. Currently, copper alloys are the only viable materials for constructing the overhead catenary systems used in high-speed railways. For railways operating at speeds exceeding 350 km/h, such as those in China, these systems require higher tension, a greater safety margin, and superior overall performance. Figure 16 shows the Cu-Cr-Zr (copper-chromium-zirconium) overhead contact lines used in the Beijing-Shanghai high-speed railway.

3.3 Corrosion and wear resistance function

Copper and its alloys exhibit excellent corrosion resistance, making them ideal for use in liquid transmission pipes and valves in various industrial applications, including ships, warships, condensers, and valves. Copper alloys are the preferred material for propeller thrusters due to their superior resistance to seawater corrosion, biofouling, high mechanical strength, and resistance to cavitation in marine environments (Figure 17).

In addition, bearing shells (or bushings)—which are the contact surfaces between sliding bearings and shaft journals—are crucial components. Bearing shells have a smooth, circular shape and are typically made from materials with excellent wear resistance, high compressive strength, and good machinability, such as bronze and antifriction alloys. Figure 18 depicts a large copper alloy integral bearing shell [8].

4. Recycling and reuse of copper and copper alloys

The environmental protection and reuse of copper during smelting, processing, and usage can be divided into two key areas: the extraction of precious metals from anode sludge during smelting and electrolysis, and the recycling and reuse of scrap copper.

4.1 Recovery of precious metals from anode sludge [9]

Anode sludge from copper electrorefining accounts for approximately 1% of the total output and contains valuable precious metals such as gold, silver, platinum, and palladium, making it highly valuable for recycling. Efforts to recover these metals focus on minimizing secondary pollution and ensuring compliance with emission standards for the “three wastes” (waste gas, waste liquid, and solid waste). The goal is to foster circular use of copper resources while reducing pollution emissions, thus supporting the sustainable development of the copper industry.

The hydrometallurgical process for recovering precious metals like gold and silver involves the following steps:

1. Precious metal enrichment: Impurities are removed to create optimal conditions for comprehensive recovery. Key pollutants include roasting dust from anode sludge, SO₂ emissions, sulfuric acid mist, and arsenic-containing waste alkali generated during copper extraction.
2. Gold extraction: Gold is leached from the solution and then reduced, with sulfuric acid mist and waste acid being the primary pollutants.
3. Silver extraction: Silver is leached and recovered as silver powder. The main pollutant is the alkaline waste solution (pH ~13), which contains high concentrations of sodium sulfite.

4.2 Recycling and reuse of scrap copper

With the rapid development of the circular economy, recycled copper has become an essential resource. Two primary methods for producing recycled copper are:

1. Direct utilization method: Scrap copper is directly melted to produce various grades of copper alloys or refined copper.
2. Electrorefining method: Scrap copper is processed through pyrometallurgy into anode copper, which is then electrolytically refined into high-purity copper, with valuable elements recovered during the process.

Direct utilization of scrap copper can save over 80% of the energy compared to copper production through smelting ore and electrolytic refining, and around 50% compared to traditional electrolytic refining alone. However, the electrolytic process poses significant environmental risks. Therefore, properly sorted and separated scrap copper should be directly melted to minimize metal losses and reduce environmental pollution. The main recycling processes include:

1. Sorting: this involves crushing, cleaning, and degreasing copper materials to separate non-metallic contaminants and non-copper metals like iron, aluminum, and stainless steel. This step ensures different grades of copper are categorized correctly for further processing.
2. Separation: surface contaminants such as coatings, plating, and soldering materials are removed. Low melting-point brazing materials, plating metals, and paint films from waste electromagnetic wires are also recovered. This purification improves the quality and usability of recycled copper, while ensuring the safe disposal or repurposing of hazardous materials.
3. Utilization: the sorted and purified copper is smelted in metallurgical or induction furnaces to produce high-quality oxygen-free copper rods or various copper alloys. These alloys are used to manufacture simple brass, complex lead-free brass, corrosion-resistant brass (aluminum and tin brass), tin phosphor bronze, white copper, and leaded brass, preserving quality and ensuring effective recycling.

According to the China Renewable Innovation Alliance, China produced approximately 3.3 million tonnes of recycled copper in 2019, 3.25 million tonnes in 2020, 3.65 million tonnes in 2021, 3.75 million tonnes in 2022, and 3.95 million tonnes in 2023. The recycled copper accounts for about a quarter of the total electrolytic copper production in China. Globally, tonnes annually in recent years, accounting for 30.7% of total electrolytic copper production, of which the United States is about 2 million tons per year, second only to China in recycled copper production.

This paper was made possible through the generous support of various individuals and organizations. I would like to extend my sincere gratitude to all those who contributed to this work. Special thanks go to Mr. Wang Yongru, Senior Engineer at Ningbo Jintian Copper (Group) Co., Ltd., for providing the pictures and content for Section 4.2, and to Prof. Tang Yuejin from Huazhong University of Science and Technology, Mr. Zhang Zhongtao, Senior Engineer at Golden Dragon Precision Copper Tube Group Co., Ltd., and Prof. Fu Ying from Zhongke Jingyi (Dongguan) Material Science and Technology Co. for supplying additional images. I am also grateful to Assoc. Prof. Zhang Yubo and Dr. Li Guoliang, at the School of Materials Science and Engineering, Dalian University of Technology, for contributing part of the text, and to Mr. Wang Yongru for his meticulous proofreading.

 

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References and notes

Cover image. Zenghouyi Chime Bells of Hubei Provincial Museum [1]. [Source: Hubei Provincial Museum (hbww.org.cn)]

[1] The Zenghouyi chime bells were unearthed in 1978 from the tomb of Marquis Yi of Zeng in Suizhou, Hubei Province, dating back to the early Warring States period. The bell frame is 7.48 meters long and 2.65 meters high. The full set consists of 65 bells, arranged in three tiers and eight groups, suspended on an L-shaped bronze and wooden frame. The upper tier contains three groups of 19 niu bells, while the middle and lower tiers have five groups of 45 yong bells, and a special bo bell gifted to Marquis Yi by King Hui of the Chu State. The bells and the frame feature 3,755 inscribed characters, detailing numbering, historical records, musical notation, and theories of musical scales. Each bell can produce two distinct tones, with a complete chromatic scale in the central tonal range, allowing it to play music in five-tone, six-tone, or seven-tone scales.

[2] Zheng’an County Museum, China https://www.zabwg.cn/home/87/show.

[3] 360 Wenku.

[4] 360 Baike.

[5] Ancient Chinese often used bronze as a mirror, as referenced in New Book of Tang, authored by Ouyang Xiu and Song Qi:
“Using bronze as a mirror, one can correct one’s attire; using history as a mirror, one can understand the rise and fall of dynasties; using people as a mirror, one can discern personal gains and losses. I have always kept these three mirrors to prevent mistakes. Now that Wei Zheng has passed, I have lost one mirror.”

[6] Zhang, Y, Chen, X, Tian, B. et al. Copper and Copper Alloy Smelting, Processing and Application [M]. Beijing: chemical industry press, 2016.

[7] W. G, King. M, Schlesinger. M, Biswas. A. K, Extractive Metallurgy of Coppe [M], Beijing: Chemical Industry Press, 2006.

[8] Li, D., Research and Application of Casting Process for Large-sized Axial Tile Bushings, Annual Meeting of Foundry of Northeastern Three Provinces and Four Cities, Shenyang, 2017.

[9] Wang, F. and Wu, H. Comprehensive Utilisation and Management of Three Wastes in the Process of Scrap Copper Refining and Recycling of Precious Metals, Environmental Protection and Circular Economy [J], 2010, China Knowledge Network http://www.cnki.net.