Copper, the first metal used by humanity, is at the heart of the current energy 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. Copper is recognised as a sustainable and highly recyclable material. A testament to its durability is the Zenghouyi Chime Bells [1], 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, the first man-made material, ushered humanity in a more organised, if still primitive, society. During this period, mankind gradually developed new materials and invented a material with better performance than ceramic: bronze, an alloy of copper with tin, lead, antimony or arsenic.  Bronze farming tools accelerated the development of agriculture, while new weapons appeared. The Bronze Age is recognised as the second period of the three-age system (Stone Age, Bronze Age and Iron Age), a classification proposed by the prehistorian Nicolas Mahudel in 1734 and then taken up by the Danish archaeologist Christian Jürgensen Thomsen in 1820 [2].

Traces of copper smelting have been found as far back as 5,000 BC, but the manufacture of bronze began around 2,700 BC, with the rise of Sumerian civilisation. In China, the Bronze Age also began more than 2,000 BC, reaching its peak during the Shang (1,570 BC – 1,046 BC) and Zhou (1,046 BC – 256 BC) dynasties. A notable example dating from the Shang dynasty is the Simuwu rectangular tripod, one of the world’s largest surviving bronze vessels [2].

1.1 Basic properties of copper

Copper, whose chemical symbol is Cu (from the Latin Cuprum) and atomic number 29, meaning that its nucleus contains 29 protons. In its natural state, it is a mixture of two isotopes, ⁶³Cu and ⁶⁵Cu, containing 34 and 36 neutrons respectively. Copper makes up around 0.01% of Earth’s crust by mass, but its content can reach 3 to 5% in concentrated deposits. Copper can sometimes be found in native metallic form, as was the case in Cyprus, from which the Latin word cuprum is derived. However, most copper is found in the form of minerals or compounds. It is also present in the oceans in dissolved form and in polymetallic nodules.

As a pure metal, sometimes called ‘purple copper’, copper has excellent electrical and thermal conductivity, as well as low hardness and high plasticity. This makes it easy to transform into products of various shapes for electrical and thermal applications. Its physical properties are summarised in Table 1 [3].

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

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.

1.2 Copper and cultural heritage

Copper is renowned for its distinctive, beautiful colour. Its corrosion resistance and ease of processing make it the material of choice for iconic statues that symbolise the history and culture of cities and nations.

Figure 1 shows the famous bronze bull statue on the Wall Street, designed by the Italian artist Arturo Di Modica. Standing 3.4 meters tall, 4.8 meters long, and weighing 3.2 tonnes, the bull symbolises ‘strength and courage’, representing the prosperity and wealth associated with Wall Street [4].

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

1.3 Copper role as currency

More than 2,000 years ago, metal coins, mainly gold, silver and copper, appeared in China during the Zhou dynasty. This period marked a crucial transition from slavery to feudalism in Chinese history. With the development of the raw materials economy, the demand for money increased significantly. Technological advances in metal smelting enabled metal coins to be produced on a large scale. As the power of the Zhou dynasty 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 (221 to 206 BC) and remained in circulation for over 2,000 years. Figure 4 showcases bronze coins from various periods of the Qing Dynasty (1644-1912)[4]. After the founding of the People’s Republic of China in 1949, several versions of the Chinese yuan were issued, with coins often being made from copper and nickel alloys.

1.4 Copper in daily life

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

• Tripod: Originally used for cooking food, it later became an important ceremonial vessel, particularly for sacrifices and feasts during the Shang and Zhou Dynasties.
• Pot with engraved figures
• Seals
• Weapons
• Copper statues
• Mirrors [5]
• Columns
• Lotus-shaped candle-stick holders
• Lion-shaped incense burners
• Armillary spheres (modelling the apparent movement of the stars)

Copper remains one of the most commonly used materials in daily life. Figure 5 shows a copper tap, illustrating its practical use. Its artistic and cultural significance is highlighted by Figure 6, which depicts a copper Buddha statue from Longkou, in the Chinese province of Shandong.

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.

An initial phase of mechanical separation of the minerals (crushing and sorting) increases the copper content from its natural state by a few per cent to around 20-30%.

In pyrometallurgical processing, the concentrated ores are smelted in closed furnaces to produce molten matte, also known as ice copper. This is a mixture of molten copper and impurities. Different types of furnace can be used. In a blast furnace, the ore is mixed with coke (purified coal), which produces heat while extracting the metal by chemical reduction. Depending on the nature of the ore, a reverberatory furnace can be used instead, where the heat is produced  outside the ore and transmitted by reverberation. These old techniques are tending to be replaced by flash furnaces, which are more energy-efficient thanks to a system for recovering the heat produced by the combustion of the sulphur contained in the ore.

This molten matte, which still contains more than 50% impurities, is then transferred to a converter, where it is refined into crude copper. This refining takes place in a reverberatory furnace, with controlled oxidation by blowing to extract iron and other impurities. This blister copper still contains around 1% impurities, which affect its electrical properties. It is then cast into anode plates for electrolysis, resulting in electrolytic copper with a purity of up to 99.9% (see section 2.2).

This process is efficient, with a copper recovery rate of up to 95%, and can be adapted to different types of ore. However, it is not very environmentally friendly, due to the sulphur dioxide emitted during the smelting and blowing stages. In recent years, processes such as the Baiyin process, the Noranda process and the Japanese Mitsubishi method have helped to make pyrometallurgy more continuous and automated [6],[7].

Modern hydrometallurgical methods extract copper by the wet process. The ore is leached with strong acids or bacteria (‘bacterial leaching’). The metal is then extracted by electrolysis from the resulting ain-si salt solution. These processes may be preceded by a process of roasting the ore by passing it through a stream of hot air (600°C) to eliminate volatile elements and convert sulphides into oxide.

These wet extraction techniques are particularly suited to the in-situ treatment of low-grade complex ores, oxidised copper ores and copper-bearing waste. They are gradually being adopted, and copper production by this method is approaching 20% of the total. A 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. The impurities remain in the electrolyte or form aggregates on the anode, known as anode sludge, which facilitates the separation of the copper from the other metals. Figure 7 illustrates the flow of the electrolytic refining process [6].

The anode extracts electrons from the solution, which can produce the following reactions:

The numbers E indicate the electrode potential, which characterises the energy required for each reaction. Reactions with the lowest electrode potential are favoured. M’ represents metals such as nickel, lead and arsenic, which have a lower electrode potential than copper and therefore dissolve more easily. In contrast, precious metals such as silver, gold and platinum, which have a higher electrode potential, dissolve very little. They accumulate and feed the anode sludge. Reactions decomposing water and sulphate ions cannot take place either, because the electrode potential is higher than that of copper.

The cathode emits electrons into the solution, which can produce the following reactions:

As electron transfer is in the opposite direction to that at the anode, reactions at the highest electrode potential are  favoured here. Metals M’ with a lower electrode potential than copper therefore tend to remain in solution. The same applies to hydrogen. Consequently, under normal conditions, only copper is deposited 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 more volatile than copper are vaporised under vacuum. Other impurities, known as impurities with low solute distribution coefficient, tend to concentrate preferentially in the upper liquid phase during solidification, which improves purification. While this method is more environmentally friendly than electrolytic refining, it is also more expensive and less efficient.

2.3 Melting

Copper and its alloys used in industry are mainly shaped into rods, wires, plates, sheets, tubes and other forms. To obtain these shapes, the electrolytic copper plates must be melted down, along with the alloying elements if required. The copper is then solidified into ingots or billets (small ingots with controlled shapes). Most copper billets are currently produced using continuous casting techniques, which involve continuously feeding a pouring basin with liquid metal and simultaneously extracting the solidified product from the other end.

The furnaces commonly used to melt copper and its alloys use electrical induction. The metal is heated by eddy currents induced by an oscillating magnetic field, in a similar way to microwave ovens or induction hotplates used in cooking.

Copper alloy melting can cause significant environmental issues when they contain volatile elements. This is the case with brass, an alloy of copper and zinc (Cu-Zn), highly prized for its excellent mechanical properties and resistance to wear. It is used in precision instruments, ship components, rifle cartridges, coins, etc.

Brass generally contains 30 to 40% zinc. As the melting point of copper is 1083°C and the boiling point of zinc is around 907°C, the melting temperature of brass exceeds the boiling point of zinc, producing a large volume of zinc vapour. This causes serious environmental pollution. Plants must therefore install appropriate protective equipment above the melting furnaces, such as fume extraction systems.

2.4 Casting

Depending on product requirements, the following casting methods are commonly used:

1. Downward vertical semi-continuous casting

This method is used to produce solid or hollow round billets, square billets or thick slabs. It enables a wide range of billet sizes to be obtained and offers high productivity. However, the casting pit generally requires a depth of around 6 metres. When the billet reaches a certain length, the process must be interrupted and the billet removed. The crystalliser where the metal solidifies must also be cleaned before the next casting cycle. The top and bottom of the billet require trimming.

2. Horizontal continuous casting

This method is used to produce solid or hollow round billets, or strips. It features continuous casting, where cutting saws trim the billets to the desired length. However, this method is unsuitable for casting billets with a large cross-section.

3. Upward Continuous Casting

This technique is used to produce solid or hollw round billets, or strips. It allows for continuous casting, as 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 can be plastically deformed with ease (a property related to its face-centered cubic cristalline structure). This makes it possible to process copper and its alloys into various shapes such as plates, strips, foils, rods, wires, and tubes.

A 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 this 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.

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 vapor-compression refrigeration systems, condensers are essential for condensing refrigerants such as ammonia and freon. Such systems are 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 this application 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 each year worldwide. 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. Electricity failures can be the cause of accidents as they bring a train to a standstill. For example, China’s first high-speed rail accident occurred on the Wuhan-Guangzhou line when the contact between the overhead line and the train’s pantograph failed.

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, 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) are cylindrical rings that provide sliding contact between a rotating shaft and its fixed housing. They are generally made from materials with excellent wear resistance, high compressive strength and good machinability, such as bronze and anti-friction alloys. Figure 18 shows a large integral bearing (including its support in a single piece) made of copper alloy [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. The aim is to foster the circular use of copper resources while reducing pollutant emissions, thus supporting the sustainable development of the copper industry.

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 minimising secondary pollution and ensuring compliance with emission standards for the three types of wastes: waste gas, waste liquid, and solid waste.

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 complete 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 use 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 use of copper scrap 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 categorised 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. Utilisation: 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 tons of recycled copper in 2019, 3.25 million tons in 2020, 3.65 million tons in 2021, 3.75 million tons in 2022, and 3.95 million tons in 2023. The recycled copper accounts for about a quarter of the total electrolytic copper production in China. The United States produce about 2 million tons per year, second only to China in recycled copper production. The total worldwide production of recycled copper approaches 10 million tons per year, one third of the copper production estimated as 28 million tons [10].

5. Messages to remember

• Copper was the first metal used by mankind. In the form of bronze, essentially an alloy of copper and tin, it has contributed to the development of civilisation through the manufacture of tools, weapons, coins, statues and other objects that are virtually unalterable.
• Copper metal is extracted from ores, mainly copper sulphides, by pyrometallurgy or hydrometallurgy. Pyrometallurgy uses high-temperature coal reduction, while hydrometallurgy is based on chemical processes in aqueous solution. Electrolysis provides the 99.9% purity required for most applications.
• Precious metals such as silver, gold and platinum can be extracted as by-products of electrolytic purification.
• Pure copper, which is highly malleable, is used for its exceptional thermal and electrical conduction properties. It therefore plays an essential role in the energy transition, through its use in heat exchangers, electrical transmission lines and lithium-ion vehicle batteries.
• Brass, an alloy of copper and zinc, is ideally suited to the manufacture of machined parts. Bronzes are appreciated for their inalterability.
• Copper is highly recyclable, with one third of its production from recycling, either by direct smelting or by pyrometallurgy and electrolytic refining.

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. [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. & 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.

[10] https://internationalcopper.org/sustainable-copper/about-copper/cu-demand-long-term-availability