The Utilization of the Copper Smelting Slag: A Critical Review

Abstract

As the primary method of obtaining metallic copper resources from copper ore, pyrometallurgical smelting usually produces a large amount of copper slag, which has good physical properties and contains many valuable metals. Therefore, how to fully recycle and utilize it has become a research direction that has received much attention in recent years. To better understand the ‘artificial’ ore of copper slag, this article reviews the copper smelting process, the sources and properties of copper slag, and its resource potential. It introduces the method of recovering valuable metals from copper slag, including pyrometallurgical impoverishment, hydrometallurgical recovery, mineral processing methods, etc. Furthermore, it lists some applications of copper slag in the field of materials, primarily in cement, concrete aggregates, and glass-ceramic materials. Finally, based on the sustainable development background, copper slag’s efficient recycling is prospected. However, scalable, eco-friendly recovery technologies remain limited and warrant further research.

1. Introduction

Copper is one of the most important metal resources globally. It has excellent thermal conductivity, electrical conductivity, and flexibility, and is widely used in the electronic equipment, transportation, and defense industries. It is a crucial tool for promoting national development and social progress [1,2]. Therefore, with the progress of science and technology, the demand for copper resources worldwide is also increasing, as shown in the ISCG data. In 2023, global refined copper production increased by about 3.8% compared with 2022, and world copper production was expected to increase by about 4.6% in 2024 [3]. Copper production continues to rise, indicating that global demand for copper resources in the world is still increasing. The main methods for extracting metal copper resources from copper ores are pyrometallurgy and hydrometallurgy. The final product of these two methods can be refined copper, a basic copper raw material with few impurities and high purity. Due to the ongoing extraction of copper resources over many years, the grade of copper ore available for mining has significantly decreased, consequently impacting the purity of refined copper to some extent. Due to the declining grade of copper ore and the reduced purity of refined copper, coupled with increasing global demand, meeting the current requirements for copper resources presents significant challenges in the metallurgical and mining industries.

Recycling metal copper from waste copper resources is a promising sustainable development method. It can not only effectively alleviate the pressure on the ecological environment caused by waste copper resources but also alleviate the urgent need for metal copper resources to a certain extent. This growing emphasis on recycling is evident in recent production trends. For instance, In January 2024, the production of refined copper resources increased by 4.6% compared with 2022, and the production of secondary refined copper increased by 9% year-on-year [4]. This data shows that the utilization of recycled copper resources has gradually attracted people’s attention. However, whether it is copper ore or waste copper resources, pyrometallurgy and hydrometallurgy remain the methods of obtaining metal copper resources [5,6]. Due to the deep historical background and relatively mature and complete technical system of pyrometallurgical copper smelting, the production of pyrometallurgical refined copper accounts for approximately 80% of the world’s copper production, i.e., concentration followed by smelting and refining [7,8,9,10]. A large amount of copper slag is often produced in the matte smelting and refining process of pyrometallurgical copper smelting; copper slag differs from other common smelting slags (such as iron, lead, or nickel slags) in terms of its chemical composition, mineral phases, and residual metal content, which gives it a distinct potential for resource recovery. These copper slags also contain a variety of valuable metals [11,12]. Therefore, if the copper slag is not handled properly, it will not only occupy a significant amount of land resources, but also the metal ions will cause irreversible damage to the natural environment [13,14]. Due to its favorable chemical and physical properties, copper slag is widely regarded as a suitable material for resource recovery and reuse. In this case, it will be conducive to the sustainable development of the metal industry and contribute to the protection of the ecological environment. With the increase in copper production, the production and accumulation of copper slag are also rising. Therefore, research on the comprehensive recovery of copper slag is urgent and of great practical significance.

To address this issue from the perspective of environmental protection and resource efficiency, this paper systematically introduces the formation mechanism of copper slag during the copper smelting process. It then analyzes the current status and physicochemical properties of copper slag, which is often referred to as an “artificial ore” due to its content of recoverable valuable metals and ore-like behavior. Based on these features, the paper reviews advanced recovery technologies for valuable metals in copper slag from both domestic and international research, as well as its utilization in various fields such as construction materials, agriculture, and environmental remediation. Finally, the paper discusses future trends and potential in the comprehensive utilization of copper slag, aiming to provide a theoretical reference for its sustainable and efficient valorization.

2. Methodology

A detailed PRISMA-style flow diagram as illustrated in Figure 1 has been added to illustrate the literature selection process. Initially, over 300 records were identified from Web of Science, Scopus, and CNKI using keywords related to copper slag and associated recovery/utilization techniques. After removing duplicates and irrelevant literature, 180 records were screened. Three inclusion criteria were strictly applied: relevance to the topic, peer-reviewed publication quality, and sufficient technical content. Ultimately, 112 studies were included in the analysis and categorized into four thematic groups: slag composition (27), recovery technologies (41), utilization pathways (31), and environmental aspects (13).

3. Generation and Properties of Slag

3.1. Copper Smelting Process

The copper smelting process is divided into two main stages: pyrometallurgy and hydrometallurgy, as illustrated in Figure 2. Pyrometallurgy includes pretreatment, smelting (using a Reverberatory, Noranda, Flash, or Closed Blast Furnace to produce matte and slag), conversion, and refining (yielding cathode copper), with melting as a distinct step. Hydrometallurgy involves leaching (extracting copper with acid or ammonia), separation (via solvent extraction and back extraction), and electrodeposition (electrowinning to produce cathode copper) without the production of slag [15,16]. The two methods have their advantages and disadvantages (

Table 1. Comparison between copper smelting in the hydrometallurgy and pyrometallurgical method.

	Main Process Flow	Advantages	Disadvantages
Pyrometallurgy	Smelting—Converting—Refining	High smelting efficiency, Mature technology	High cost, Poor environmental friendliness
Hydrometallurgy	Leaching—Extracting—Electrodeposition process	Can handle low-grade copper ore and multi-impurity metal copper, relatively environmentally friendly	Long cycle

). Table 1 compares the main technologies used in copper slag treatment, based on data and trends reported in recent studies [16]. Pyrometallurgical technology is mature, and the output is high but not environmentally friendly. Hydrometallurgy is suitable for low-grade ores but involves prolonged leaching and separation steps, resulting in low throughput. The primary reason for the limited proportion of wet copper production is that its production capacity cannot meet the urgent needs of copper in various international industries, and its technical system still has a lot of room for progress [17,18,19]. The traditional pyrometallurgical smelting process includes a blast furnace, reverberatory furnace, and electric furnace smelting. With the progress of technology, some advanced pyrometallurgical copper smelting processes, such as flash furnace smelting, Isa smelting, Noranda smelting, etc., have been widely used worldwide [20,21]. These processes have characteristics and are suitable for different raw materials and production needs. The specific choice of method for copper smelting depends on the type of raw materials processed, the purity requirements of the required metals, and the equipment and process conditions.

3.2. Present Situation of Copper Slag

According to the ISCG report, the world’s refined copper production in 2023 totaled 26,351 kt, of which Asia’s refined copper production (15,362 kt) accounts for 60% of the world, followed by the Americas (3896 kt) and Europe (3812 kt) [4]. As the world’s largest producer and consumer of refined copper, China’s refined copper production will reach 12.99 million tons in 2023 [22]. However, the high yield of refined copper also represents an increase in copper slag production [23]. In the process of copper smelting, for every ton of refined copper produced, 2.2 tons of copper slag will be produced [24,25]. Due to the differences in copper smelting activities between continents and countries, there have been no accurate statistics on copper slag production in recent years. Therefore, according to the data above, global copper slag production is expected to be about 57.8 million tons in 2023, with China accounting for half of this total. With the increasing production of copper slag and the large-scale industrial application of copper slag recovery not yet realized, the accumulation of copper slag is growing, posing severe challenges to land resources and the ecological environment. Therefore, it is urgent to examine the effective resource utilization of copper slag globally, not just in China.

3.3. Properties

Copper slag is generally black or dark green, with physical properties such as a density of about 3.8 g/cm³, a hardness of 7 Mohs scale, a melting point of about 1200 °C, and good stability [24]. Unlike the characterization of copper slag, its chemical composition is influenced by the composition of the copper concentrate, the smelting process, and the additives used during the smelting process. Hence, the composition of copper slag produced by different smelting methods also varies [26,27]. As shown in

Table 2. Chemical composition of copper metallurgical slag.

Methods of Smelting	Slag Mass Fraction/%
	Cu	FeO	Fe3O4	SiO2	S	Al2O3	CaO	MgO
Imperial smelt furnace	0.4–2.8	33–42	5–9	32–34	-	3–8	3–10	1–4
Outokumpu flash smelting (without impoverishment)	1.5	44.4	11.8	26.6	1.6	-	-	-
Outokumpu flash smelting (impoverishment)	0.78	44.06	-	29.7	1.4	7.8	0.6	-
Inco smelting	0.9–1.7	44–54	10–12	32–35	1.1	4–5	1.5–2.5	1.4–2.2
Noranda smelting process	2.5–5	40–52	15–29	21–25	1.7	5.0	0.5–2	1.0–1.5
Vanukov smelting	Cu	FeO	Fe3O4	SiO2	S	Al2O3	CaO	MgO
Baiyin smelting	0.45	35	3.15	35	0.7	3.3	8	1.4
Teninte converter smelting	1.1–4.6	43–65	12–29	16–28	0.8	5–10	1–2	5–10
Isasmelt	0.7–2.0	36.6–45	6–8	31–34	0.8	3.64	1.5–4.4	1.0–2.0
Ausmelt	0.65	34	7.5	31	2.8	7.5	5.0	-
Mitsubishi process smelting	0.6–2.4	38–58	-	30–35	0.6	2–6	5–8	-

[28], copper slag often contains a large amount of iron (27%–35%) and silicon (30%–40%), mainly hematite, magnetite, and some silicate minerals. In addition, it also contains precious metals such as copper (0.5%–2%) and zinc (2%–3%), mainly in the form of metal elements, sulfides, or oxides, and is encapsulated by other minerals. Because copper slag contains many metal elements and silicates, it exhibits good physical properties and can be utilized in various fields [29,30]. At the same time, if the metal elements in copper slag can be fully recycled, it can bring considerable economic benefits and alleviate environmental pressure to a certain extent [31].

4. Valuable Metal Recycling

The methods of recovering valuable metals from copper slag include pyrometallurgical impoverishment, hydrometallurgical recovery, physical beneficiation, etc. [32,33,34]. In general, the copper and other metal content in the copper slag after impoverishment is still high, so it can be recovered by other beneficiation methods after impoverishment. The recovery method is selected based on the expected recovery of the metal and mineral composition [35,36].

4.1. Pyrometallurgical Impoverishment

In the process of copper smelting at high temperatures and low oxygen content, the generation of magnetite is inevitable [37]. The formation of magnetite (Fe₃O₄) in the slag increases the slag’s viscosity and affects the copper matte’s sinking rate. To address this, pyrometallurgical “impoverishment” is applied as a post-smelting treatment to reduce the copper content in slag. This process aims to decrease slag viscosity and improve matte separation by converting Fe₃O₄ into lower-valence oxides and facilitating settling. The principle of the method is to minimize the Fe₃O₄ in the copper slag to iron oxide by adding a reducing agent and then take measures such as remelting, vulcanization reduction, and blast stirring to reduce the copper content in the slag, improve the smelting efficiency, reduce the viscosity of the copper slag, and increase its fluidity (Figure 3). As shown in Figure 3, the reduction of Fe₃O₄ plays a central role in enhancing copper recovery by decreasing slag viscosity and facilitating matte separation. Pyrometallurgical impoverishment can be further divided into electric furnace impoverishment, vacuum impoverishment, reflecting furnace impoverishment, and so on [38,39].

The main object of impoverishment in the reverberatory furnace is the copper slag generated during the converter-blowing process. Its principle is to spray fly ash, oil, and natural gas into the melt through the tuyere to reduce Fe₃O₄, thereby causing the sedimentation and separation of copper matte and slag [40,41], by using the reflection of the top of the furnace to concentrate the heat on the furnace burden so that the impurities in the copper slag are oxidized or volatilized. It offers the advantages of simple operation and low cost, with ample space in the furnace, and a large slag processing capacity. However, the thermal efficiency of the reflecting furnace is low, and flue gas will be produced during the impoverishment process. If these flue gases are not treated, they will pollute the environment. At the same time, during the reduction of Fe₃O₄, it is easy to form a furnace structure in the furnace, thereby reducing the lifespan of the reverberatory furnace. In summary, to improve treatment efficiency, reduce environmental impact, and develop new lining materials with high-temperature resistance and corrosion resistance, it is necessary to prolong the service life of the reflecting furnace in the future [42,43].

Electric furnace impoverishment is the most common method of pyrometallurgical copper slag impoverishment, which can process all types of slag components. Increasing the furnace temperature and reducing the viscosity of copper slag, the density difference is used to settle the copper matte and reduce the copper content in the slag. In the process of electric furnace impoverishment, the stirring generated by the current flowing between the electrodes can promote the agglomeration of copper particles in the slag, and the agglomeration effect is affected by the type of current [44]. Through comparison, it was found that direct current has a faster rate of impoverishment on copper slag than alternating current, and the recovery rate of copper in slag is higher when direct current is applied [45]. Moreover, under the action of an electric field, direct current can accelerate the sedimentation of copper ions, with a sedimentation rate of 2–3 times that of gravity alone, which helps to recover fine copper particles from the slag [46]. The biggest advantage of electric furnace impoverishment is that it recovers metals such as lead, cobalt, and zinc, which are easily soluble in acids. Moreover, it also has advantages such as a wide range of use, low pollution, and easy control. However, the electric furnace impoverishment equipment is expensive, the electrode material consumption is rapid, and the cost is high, making it only suitable for targeted impoverishment. Therefore, the electric furnace impoverishment of copper slag requires further study to reduce energy consumption and equipment costs in the future.

Vacuum impoverishment is to impoverish copper slag under vacuum conditions, which can quickly reduce the content of iron oxide in slag, reduce the melting point, viscosity, and density of slag, and promote the settlement and separation of matte slag [47]. Du Qingzhi (1995) conducted a vacuum impoverishment test on the slag produced by the Noranda smelting process and analyzed its physical and chemical properties [48]. By heating the slag to 800–1000 °C under vacuum conditions, the impurities in the copper slag are volatilized or reduced, thereby purifying the copper. The results showed that vacuum impoverishment can reduce the copper content of 1/3 to 2/3 of the slag layer from over 5% to below 0.5%. However, due to the difficulty of the vacuum impoverishment operation and the complexity of the equipment, there are few studies on this topic at this stage, and it is currently impossible to realize the industrial application. Simplifying the equipment and process of vacuum impoverishment will be the main research direction in the future. These pyrometallurgical impoverishment methods are implemented after matte smelting to further recover copper from slag by promoting phase separation. Each technique presents different operational advantages and limitations depending on the slag composition and facility scale. Furthermore, since valuable metals remain in the depleted slag, their subsequent utilization also deserves further attention.

4.2. Mineral Processing Method Recovery

The recovery of copper from copper slag typically employs the flotation method, which can be divided into main methods based on the different copper phases (such as sulfide copper and oxide copper): the direct flotation method and the sulfide flotation method [49,50]. Copper sulfide can be recovered through direct flotation, and pH adjusters, collectors, and foaming agents are often added during the flotation process [51]. Among them, the collector is a crucial factor affecting the flotation recovery rate. Xanthates, as the most used collector in the copper sulfide flotation industry at present, have the principle of generating chemical adsorption on the mineral surface through covalent bonds with Cu (II), making it easier for Cu to float [52]. Similarly, xanthate collectors have also been applied in the flotation of copper slag flotation.

Table 3. The influence of different conditions on copper flotation from copper slag.

Collector	Frother	Depressant	Sulfurizing Agent	pH	Cu Recovery	Particle Size μm
SBX	MIBC	HEC			82%
PAX	Pine oil			8–9	96%
PAX + FC7245 or FC 4146	Flotanol CO7		Na2S		~70%–80%, up to 90% after regrinding	d80 = 45
PAX + FC7245 or FC 4146	Flotanol CO7		Na2S		~80%–85%, >90% after regrinding	d80 = 45
PAX + FC7245 or FC 4146	Flotanol CO7		Na2S		5%–15%	d80 = 45
BX	Pine oil (Terpineol)			10	80%	<74
PAX + AERO 3477	Pine oil (Terpineol)			11	80%	d80 = 48
SIPX/DTP(40:160 g/t)	A65, MIBC	CMC		9	85%	d80 = 75
KBX	Pine oil (Terpineol)				73%	d90 = 43

shows the effects of some xanthate collectors on copper slag flotation under different conditions [53]. Shamsi et al. (2016) found that adding a secondary collector during the flotation process can significantly improve the copper recovery rate [54]. When using a single collector (pentyl xanthate) flotation system, the copper recovery rate is only 62.23%. When in a composite collector flotation system composed of pentyl xanthate and sodium isopropyl xanthate, the recovery rate of copper can be increased to 80.27%.

The floating rate of copper in distinct phases of copper slag also varies, with secondary copper sulfide, natural copper, and copper sulfide having the fastest floating rate [55]. However, the natural floatability of copper oxide, encapsulated copper, and some copper sulfide with other minerals is poor, requiring sulfurization before flotation. The vulcanization principle involves adding a vulcanizing agent to form a metal sulfide film on the surface of the mineral, which alters the floatability of the mineral and enhances its adsorption of collector ions (Figure 4). As shown in Figure 4, vulcanizing agents such as sodium sulfide convert the mineral surface to a hydrophobic sulfide layer, thereby enhancing flotation performance for poorly floatable copper species. Jin et al. (2009) conducted sulfide flotation experiments on a complex copper slag in Yunnan [56]. Using sodium sulfide as a sulfide agent, under the conditions of grinding fineness of −0.074 mm accounting for 90.6%, collector KM-109 dosage of 162 g/t, and sodium sulfide dosage of 3.4 kg/t, the copper recovery rate can reach 86%. In addition, the particle size of copper slag, the dosage of reagents, and the pH of the slurry are also the main factors affecting the efficiency of copper flotation [57]. If the particle size is too fine, non-selective aggregation may occur, and the capture rate of bubbles on the ore particles may be reduced. If the reagent is excessive, the stability of bubbles may be reduced, and the adsorption force between minerals and bubbles may be weakened. PH can affect the potential of mineral surfaces and the adsorption of reagents.

Magnetic separation is usually used to recover iron from copper slag. However, the content of single magnetite in copper slag is low, typically combined with silicate minerals and other gangue minerals to form a whole, as shown in Figure 5 [58]. Therefore, the recovery rate of iron by the direct magnetic separation of copper slag is often not ideal. Kim et al. (2013) first crushed and ground the copper slag below 1 mm, followed by carbon thermal reduction at 1255 °C for 90 min [59]. The resulting product was then subjected to a second crushing to 75–104 μm, followed by magnetic separation. Under a magnetic field intensity of 0.2 T, the iron recovery rate in the copper slag could reach 72%. Luo et al. (2022) conducted a study on the behavior of element distribution in the copper slag magnetic separation process, and the results showed that the directional enrichment of lead and zinc in copper slag in magnetic separation tailings can be achieved through the grinding and magnetic separation process [60]. At the same time, under the conditions of grinding fineness of 5.12 μm and magnetic field strength of 0.1 T, magnetic separation concentrate containing 51.82% Fe, 0.95% Zn, and 0.087% Pb can be obtained. This conclusion further demonstrates that most magnetic iron minerals in copper slag form interconnected bodies with silicate minerals, and single grinding makes it challenging to achieve effective dissociation, thereby affecting the magnetic separation recovery rate. In response to this situation, Sun (2021) employed sodium roasting to remove silicon from copper smelting slag [61]. NaOH was used as the sodium agent, and under the conditions of a roasting temperature of 650 °C, a mineral alkali mass ratio of 1:1.75, and roasting time of 180 min, silicon in the copper slag was removed in the form of sodium silicate, with a desilication rate of 94.5%. On this basis, it will be a very valuable research direction to remove silicon and sodium from copper slag and then carry out beneficiation and enrichment to recover valuable metals from it.

In summary, the flotation method separates valuable metal minerals from gangue minerals through the action of reagents exhibiting high selectivity and recovery. The use of agents may cause pollution to the environment and require wastewater treatment. The magnetic separation method is relatively environmentally friendly; however, it primarily relies on the magnetic difference between minerals, and the separation effect for non-magnetic or weakly magnetic minerals is limited. In actual production, the flotation and magnetic separation methods are often selected or combined based on the specific properties of the copper slag and the recovery targets. For example, magnetic separation can recover magnetic minerals, and non-magnetic minerals can be recovered by flotation thereby improving the overall recovery rate and economic benefits [62]. This can fully leverage the advantages of the two methods, compensate for their shortcomings, and achieve efficient resource utilization and effective environmental protection.

4.3. Hydrometallurgical Recovery

Hydrometallurgical recovery of valuable metals from copper slag is achieved using different leaching agents or methods. According to the different types of leaching agents, leaching can be divided into acid, ammonia, chlorination, and bioleaching [63,64].

Acid leaching is currently the most used method for recovering valuable metals from copper slag, mainly divided into two categories: direct acid leaching and indirect acid leaching. In the acid leaching process of copper slag, temperature, sulfuric acid concentration, time, and stirring rate are the main factors affecting the leaching rate [65]. Bulut (2006) conducted comparative experiments on direct acid leaching and acid-roasting hot water leaching of copper slag and studied the dissolution efficiency of copper and cobalt in these two leaching methods [66]. During the experiment, it was found that the residual amount after acid roasting and hot water leaching was reduced by 20% compared to direct acid leaching. It can be intuitively concluded that the hot water leaching rate of acid roasting is significantly higher than that of direct acid leaching, and the leaching rate is also affected by roasting time and temperature. In addition, due to the complex composition of copper slag, the degree of damage to its mineral-associated bodies may not be high under atmospheric acid leaching conditions, thereby affecting the metal leaching rate. Given this, Karimov et al. (2019) conducted oxygen-pressure acid leaching experiments on copper smelting dust using a high-temperature and high-pressure reactor [67]. The purpose of the experiment was not only to extract valuable metals but also to study the conditions under which a small amount of arsenic carried in copper slag can be separated. The experimental results show that under the condition of 160 °C, the oxygen partial pressure was 0.3 Mpa, the leaching time was 2 h, the concentration of H₂SO₄ was 130 g/dm³, the extraction rates of copper, zinc, and arsenic were 93%, 96%, and 99%, respectively. He et al. (2018) conducted acid-leaching experiments on black copper slag under oxygen pressure conditions [68]. The results showed that under the conditions of sulfuric acid concentration of 180 g/L, 140 °C, a liquid-solid ratio of 8 mg/L, and a stirring speed of 600 rpm, Cu and As leaching rates can reach 97.59% and 95.42%, respectively. The biggest advantage of copper slag acid leaching is its high leaching rate for valuable metals, but valuable metals such as copper, zinc, and iron are soluble in acid, making subsequent separation and purification difficult [69]. Moreover, acids have strong corrosiveness and require high equipment requirements. Improper waste liquid treatment can cause environmental pollution. Therefore, how to achieve selective and efficient dissolution of valuable metals in copper slag under the premise of environmental friendliness has great research prospects.

Ammonia leaching was originally used to leach low-grade oxidized copper ore and later gradually applied to the leaching of valuable metals from copper slag. Its principle is that metal ions react with ammonia to form soluble ammonia complexes, allowing metal ions to leach from the ore sample. In the process of ammonia leaching, temperature, ammonia concentration, PH value, and stirring rate are the main factors affecting the efficiency of ammonia leaching [70]. Bidari et al. (2015) investigated the leaching behavior of copper slag in ammonia solution [71]. The experimental results showed that temperature and stirring rate had the most significant effects on copper leaching behavior. Under the optimal conditions (70 °C, ammonia concentration 2 mol/L, pH 10.5, 900 rpm, leaching time 4 h), 78% of copper was obtained from the test sample. In addition, the leaching efficiency of copper slag in the ammonia-ammonium carbonate system is higher because only ammonia dissolved in water can form complexes with metal ions, and ammonium carbonate can significantly increase the concentration of free ammonia in the liquid phase. Yu et al. (2012) conducted a kinetic study on the leaching of copper using an ammonia-ammonium carbonate system using flotation tailings from depleted electric furnace slag [72]. The copper grade in such flotation tailings was lower and more difficult to recover. The experimental results show that the leaching rate of copper is not high when only ammonia is used to leach copper. Still, the leaching rate of copper can be significantly improved by adding a small amount of ammonium carbonate in the leaching process. However, the effect of ammonium carbonate dosage exceeding 20% of the leaching tailings mass on copper leaching is no longer significant. Moreover, the particle size of the sample is also a crucial factor affecting the leaching rate. Aracena et al. (2019) took copper slag with an average particle size of 5.5 mm, used a 1.2 m high leaching column with a cross-sectional diameter of 7.5 × 10⁻² m, and set the drip rate to 8.0 L/(h · m²)⁻¹ [73]. The ammonia leaching experiment of copper in converter slag was carried out under the ammonium hydroxide concentration of 2.4 mol/L and pH 10.5. In the converter slag, the leaching rate of copper in the slag can reach 87.7%, with almost no impurities. During the leaching process, Cu ions mainly come from copper oxides and react with the hydrated hydrogen ions produced by the hydrolysis of ammonium hydroxide to achieve leaching. The main reactions are shown in formulas 1–5 [73]. As an effective method for extracting copper from copper slag, it has the advantages of high selectivity, mild operation, and environmental friendliness. However, its limitations, such as a slow reaction speed, ammonia volatilization, and complex waste liquid treatment, must also be addressed in practical applications.

2Cu₂O + 2H₃O⁺ → 3Cu²⁺ + 2H₂O + Cu(OH)₂ + 4e

(1)

Cu(OH)₂ + 2H₃O⁺ → Cu²⁺ + 4H₂O

(2)

4H₃O⁺ + 4e + O₂ → 6H₂O

(3)

2Cu²⁺ + 8NH₃ → 2(Cu(NH₃)₄²⁺)

(4)

Cu₂O + 8NH₄OH + 1/2O₂ → 2(Cu(NH₃)₄²⁺) + 6H₂O + 4OH⁻

(5)

The principle of chlorination leaching involves using chlorides to convert metal ions in copper slag into soluble chloride ions, which then are dissolved in the leaching solution. The target metal is subsequently obtained by precipitation, filtration, and other methods. Dimitrijević et al. (2017) investigated the leaching of copper in chloride media, utilizing HCl as a leaching agent to extract copper from copper smelting slag in a solution composed of H₂O₂ and HCl [74]. The experimental results showed that the leaching rate of copper could reach 73% after a leaching agent concentration of 2 mol/L, HCl concentration of 1 mol/L in the solution, hydrogen peroxide concentration of 3 mol/L, and room temperature leaching for 2 h. Chlorination leaching can be achieved not only by adding chloride but also by adding chlorine gas. For instance, Yu et al. (2023) utilized chlorination leaching to treat the matte slag generated from lead-antimony smelting, thereby separating and recovering valuable metals such as copper, antimony, lead, and silver [75]. The experimental method involved placing the copper slag in a mixed solution of sulfuric acid and hydrochloric acid, stirring it in a reactor, and introducing chlorine gas during the stirring process. Under the conditions of both sulfuric acid and hydrochloric acid concentrations of 2.0 mol/L, a leaching temperature of 90 °C, and a leaching time of 20 min, the leaching rates of copper and antimony reached 99.52% and 99.36%, respectively. In general, the chlorination leaching method of copper slag has certain advantages in the treatment of refractory copper slag, such as high-sulfur copper slag, but there are also some shortcomings, such as poor selectivity and high toxicity [76]. Therefore, it is necessary to conduct research in the direction of environmental protection and multi-type metal recycling.

Unlike the chemical leaching method, the bioleaching principle uses microorganisms’ metabolic activity to dissolve metals from copper slag. Commonly used microorganisms include Thiobacillus oxidans and iron-oxidizing bacteria. These microorganisms oxidize some copper slag metals into soluble forms through oxidation reactions [77]. Panda et al. (2015) used a combination of acidophilic bacteria composed of iron-oxidizing bacteria and sulfur-oxidizing bacteria to study the leaching of copper slag and conducted a control experiment on the flotation of copper slag [78]. The flotation experiment results showed that the recovery rate of copper was only 42%–46%. However, under the conditions of pH 1.5, a bacterial inoculation amount of 15%, initial Fe (II) concentration of 3 g/L, slurry concentration of 7%, and leaching time of 12 days, the copper leaching rate could reach 96%. The reason for the better leaching effect compared to flotation can be attributed to the fact that copper in copper slag is mostly contained in iron olivine and silicate. During leaching, iron olivine and bornite in the slag are decomposed by microorganisms. The decomposition formula is as follows [78]:

Fe₂SiO₄ → SiO₄²⁻ + 2Fe³⁺

(6)

SiO₄²⁻ + 4H⁺ → H₄SiO₄

(7)

H₄SiO₄ → H₂SiO₃ + H₂O

(8)

Cu₅FeS₄ + O₂ + H⁺ → 5Cu²⁺ + 2Fe²⁺ + 2H₂O

(9)

Behera et al. (2022) applied surface-active agents extracted from fermented lactobacilli to copper slag flotation to reduce the content of SiO₂ and Al₂O₃ in copper slag [79]. Subsequently, iron-oxidizing bacteria were used for the biological leaching of copper slag, and the experimental results showed that 37.90% of SiO₂ and 22.82% of Al₂O₃ could be removed during the flotation process. In subsequent leaching experiments, the recovery rates of Cu, Ni, and Co reached 74.0%, 60.0%, and 33.4%, respectively. Although bioleaching is environmentally friendly, it has problems such as slow reaction speed, long treatment time, and harsh conditions in the microbial culture process.

In summary, different leaching methods have certain advantages and limitations, as shown in

Table 4. Comparison of various leaching methods.

	Acid Leaching	Ammonia Leaching	Chlorination Leaching	Bioleaching
Advantages	Simplicity of operation, Fast reaction rate, Wide application range	Strong complexation ability, Low corrosivity, Environmentally friendly	Without elevated temperature required, Recoverable precious metals	Environmentally friendly, Low energy consumption and cost
Disadvantages	Strong corrosion, Environmental pollution	Long leaching time, Limited scope of application, Excessive cost	Equipment corrosion, Low safety, Impact on the environment	Difficulty in microbial culture, Harsh terms, Long period

, which offers a comparative overview of leaching methods under various chemical and operational conditions as a reference for selecting appropriate recovery strategies. The selection of the leaching process should consider the characteristics of copper slag, the recovery rate of the target metal, environmental impact, and economic cost.

4.4. Other Recycling Methods

With the development of science and technology, the application of combined processes and the exploration of new processes have been utilized in the comprehensive recovery research of copper slag. The photothermal method can be used to treat copper slag. The principle is to convert olivine into magnetite, copper oxide, and sulfide into copper nodules by concentrating the elevated temperatures generated by solar energy. The copper slag treated by solar energy is crushed and ground before magnetic separation. The experiment can obtain 85% of the magnetic components composed of magnetite, and 7.5–18 kg/t of copper can be recovered from the slag. Lan et al. (2018) proposed a method for the rapid separation of copper and iron phases in copper slag at low temperatures under high gravity [80]. Firstly, carbon was used as a reducing agent to reduce ferric oxide in copper slag to ferrous oxide, which was then combined with silicon dioxide to form a slag under low-temperature conditions, facilitating the separation of copper and iron. The experimental results showed that after separation for 3 min at a gravity coefficient of 1000 and a temperature of 1200 °C, the recovery rate of copper could reach 97.47%. Shibayama et al. (2010) used a pyrometallurgical and hydrometallurgy process to recover metallic copper from copper slag [81]. Firstly, the As in the copper slag was removed by pyrometallurgy, followed by acid leaching in H₂SO₄. Finally, the copper in the solution was extracted using a LIX-84I extractant. The experimental results showed that more than 90% of As in the copper slag was separated in the form of As₂O₃, and more than 90% of copper could be extracted and recovered.

In summary, the new beneficiation process and the combined process are also of great significance and value for recovering valuable metals in copper slag. They are conducive to resource utilization and environmental protection and can bring economic benefits and promote technological innovation. However, future research in this field should be based on the advantages and disadvantages of the existing process to have more practical significance. Among the current methods, pyrometallurgical treatment is more suitable for large-scale industrial applications due to its mature technology and high processing capacity. At the same time, hydrometallurgical and combined processes offer better selectivity and environmental performance. Therefore, selecting an appropriate method should be based on slag characteristics, economic considerations, and environmental regulations.

5. Material Application

Nowadays, according to the sufficient hardness, density, and stability of copper slag, it has been applied to the fields of concrete, cement, asphalt, and glass ceramics (Figure 6). Copper slag is most widely used in building materials because it does not require complicated pretreatment. In addition, the iron separated from copper slag by the carbothermal reduction of copper slag can be used to make steel, and the remaining slag can be used to make glass-ceramics [82].

5.1. Concrete

Silicate is one of the important components of concrete and a key material that affects its hardness and strength. Since copper slag contains a large amount of silicate and iron, it can be considered a raw material for concrete mortar. In recent years, copper slag has been fully studied for the manufacture of concrete [83]. Sharma (Sharma and Khan 2017) found that self-compacting concrete (SCC) has certain limitations [84]. Due to the need for a large amount of dehydrating agents and cement during the manufacturing process, the cost increases. Therefore, experiments determined that using 60% copper slag to replace ordinary sand and gravel in concrete can reduce costs and significantly improve the durability performance of SCC. Shirdam et al. (2019) [85] studied the feasibility of using copper slag as concrete aggregate from an environmental perspective. They found that concrete with good workability, strength, and durability can be obtained when copper slag aggregate reaches 20% [85]. Lori et al. (2019) studied the application effect of copper slag in permeable concrete [86]. When 60% of the dolomite aggregate in the concrete was replaced by copper slag, its compressive strength, flexural strength, and tensile strength were increased by 31%, 19%, and 18%, respectively. The porosity and permeability were also improved. The permeability and compressive strength of concrete with different admixtures are shown in Figure 7 [86]. Ambily et al. (2015) studied the technical feasibility of using copper slag instead of ultra-high-performance concrete fine aggregates [87]. The experiment shows that the addition of copper slag can make the strength of the concrete produced greater than that of ordinary ultra-high-performance concrete. After completely replacing standard sand with copper slag, the compressive strength of the concrete after 28 days decreases by 15%–25%, and the decrease in flexural and tensile strength is also close to this range. The experimental results show that this type of concrete has considerable application prospects and the advantage of a lower cost. In summary, using copper slag to produce concrete enables waste reuse, reduces the need for sand and gravel raw materials, and lowers costs. This approach is conducive to sustainable development. Additionally, the concrete produced from copper slag exhibits higher durability and strength. Copper slag often contains harmful metals such as arsenic. Although the degree of metal migration will be reduced and the harm can be curbed after it is used to produce concrete, the market acceptance of concrete produced still needs to be improved [88].

5.2. Cement

Copper slag, including converter slag generated during matte converting, has the advantages of good wear resistance, stability, and fluidity and has the potential to be used as fine aggregate [89,90]. Moreover, the SiO₂ in copper slag can react weakly with Ca(OH)₂ in cement to form colloidal substances such as hydrated calcium silicate under specific molecular forces [26]. Therefore, the incorporation of copper slag makes the internal structure of concrete denser and can also improve the strength of concrete to a certain extent. In addition, copper slag has good mechanical properties and can be used as a suitable substitute in the construction industry. Nazer et al. (2013) conducted comparative tests on the compressive and flexural properties of copper slag and ordinary cement mortar [91]. The test results showed that the mortar prepared by mixing copper slag and cement had better compressive and flexural properties than ordinary cement mortar. Pavez et al. (2019) conducted an experimental study on replacing cement with various proportions of copper slag, and the results indicated that ideal-performance cement mortar can be produced when the replacement amount of copper slag is 5% [92]. In summary, copper slag as a substitute for cement and concrete aggregates has extensive potential for application. While saving costs, it can also achieve performance similar to ordinary cement and reduce the impact of copper slag accumulation on the environment. However, challenges such as the long-term stability of heavy metals within slag-containing materials, limited market acceptance, and the cost of processing treatments remain obstacles to large-scale industrial adoption. Addressing these issues is essential to realize the full potential of copper slag in construction applications.

5.3. Asphalt

Copper slag has some prerequisites for replacing asphalt aggregates, such as good wear resistance and shape characteristics. Due to the presence of various oxides in copper slag, it has good adhesion and can enhance adhesion and compactness between particles when used as asphalt aggregates [93]. Raposeiras et al. (2021) studied the effect of different particle sizes of copper slag on replacing asphalt aggregates, and the results showed that the water resistance and service life of asphalt mixtures increased with the increase of copper slag particle size [94]. It can be a suitable substitute for asphalt aggregates, reduce the use of raw materials by 15%–20%, and reduce the accumulation of copper slag. Ziari et al. (2019) studied the effect of different proportions of copper slag on replacing warm mix asphalt aggregates [95]. The experimental results showed that the proportion of copper slag was 20%, significantly improving the tensile strength, elastic modulus, and flow coefficient of asphalt mixtures. Modarres et al. (2019) conducted control experiments on copper slag and limestone as hot mix asphalt aggregates, and the results showed that at the same content (6%), the performance of copper slag aggregates was better than that of limestone aggregates [96]. In general, using copper slag as an asphalt material can reduce the resource utilization of waste, reduce the exploitation of natural resources, improve the pavement performance, and prolong the pavement’s service life at the same time, which has important environmental, economic, and social significance.

5.4. Glass-Ceramics

The main components, such as silicon dioxide and calcium oxide in copper slag, are also used as the main raw materials to produce glass ceramics, so the copper slag can be used to make glass-ceramics [97]. However, due to the high Fe²⁺ content in copper slag, it is easy to cause the coloring of microcrystalline glass. Therefore, the first step is separating the iron when using copper slag to prepare glass materials. The carbon thermal reduction method is currently used for copper-iron separation before preparing glass [98]. Yang et al. (2014) studied the synthesis of microcrystalline glass using iron-rich copper slag [99]. Firstly, the copper slag was subjected to high-temperature reduction for iron removal, followed by secondary melting, crushing, and screening of the iron-removed copper slag. Finally, the microcrystalline glass was obtained by heating at a 20 K/min heating rate for 60 min. In the experiment, it was found that CaO/SiO₂ in copper slag has an impact on the iron recovery rate and the iron concentration in the glass. When CaO/SiO₂ is 0.42, the microcrystalline glass produced has the best performance: porosity of 0.11%, water absorption of 0.04%, density of 2.75 g/cm, and hardness of 85.75 HBa. Mohamed et al. (2015) conducted an experimental study on the preparation of microcrystalline glass using iron-rich copper slag [100]. Under the conditions of using a copper slag content of 32 wt%, a heating rate of 10 °C/min, a reaction temperature of 950 °C, and a soaking time of 25 min, microcrystalline glass with a relative density of 0.18 and a compressive strength of 9 Mpa can be obtained. Sarfo et al. (2017) also used a carbon thermal reduction process to separate iron from copper slag [101]. The reducing agents used in the experiment were carbon, calcium oxide, and aluminum oxide as fluxes. Under the conditions of a carbon content of 12.5%, reaction temperature of 1420 °C, and reaction time of 75 min, the metal recovery rate could reach 90%. The reduced copper slag had low density and high hardness, making it an excellent raw material for microcrystalline glass.

After the iron is separated from copper slag, the secondary treatment of the reduced copper slag is the most important thing in producing glass ceramics. Blenau et al. (2023) used a carbon reduction fiber stretching process to recycle copper slag [102]. Firstly, the copper slag was subjected to carbon thermal reduction to separate iron from the copper slag. Then, the reduced copper slag was subjected to fiber stretching. The research results showed that the iron separated by carbon thermal reduction can be directly used to produce steel, and the reduced copper slag can be used as a glass raw material through fiber stretching. Glass-ceramic is a special glass material with the characteristics of a microcrystalline structure and uniform microstructure. Compared with ordinary glass, glass ceramics usually have higher tensile strength, better optical properties, and higher chemical stability. Therefore, the production of glass ceramics from copper slag can not only reduce the impact of copper slag on the environment but also reduce the use of raw materials for the production of glass ceramics [103]. However, from a cost perspective, a too complicated process flow may lead to high costs when producing glass ceramics with copper slag.

5.5. Other Applications

In addition to the application fields of copper slag materials mentioned above, Erdenebold et al. (2018) conducted an experimental study on the recovery of pig iron from copper slag by the reduction melting method [104]. The results showed that under the condition of using activated carbon as the reducing agent and reaction temperature of 1600 °C for 25 min, no iron phase was present in the unmelted slag, which has good economic feasibility. Fan et al. (2018) prepared an electromagnetic interference shielding material using copper slag, adding 45 wt% copper slag to the cement matrix [105]. The test results showed that the modified material can attenuate the intake of electromagnetic energy from 500 MHz to 1.5 GHz by about 60%. Gyurov et al. (2017) prepared amorphous silica gel using copper slag by the following steps [106]:

• Copper slag oxidizes for 2 h at temperatures above 800 °C.
• Oxidized slag undergoes hydrothermal treatment with NaOH (140 g/L) at 190 °C for 3 h.
• Heat filtration.
• Change pH to hydrolyze liquid silicate into gel.
• Dry at 80 °C to obtain amorphous silica gel.

Xu et al. (2024) conducted experimental research on how copper slag can produce battery-grade FePO₄·2H₂O [107]. Firstly, iron was leached by acid leaching, followed by evaporation. Then, Na₂HPO₄·12H₂O was added to generate FePO₄·2H₂O. Then, under an HCl concentration of 2.5 mol/L and 180 °C for 5 h, FePO₄·2H2O was obtained with a purity of 99.47%. Life cycle assessment showed that for every 1 kg of iron phosphate produced from copper slag, 4.71 kg equivalent CO₂ emissions were reduced.

In general, copper slag as a production material has certain practical significance regarding secondary resource utilization, cost conditions, and innovation. However, the harmful metals in the slag and the production cost must also be considered. Therefore, it is essential to combine the above factors to further research copper slag materials. Although extensive experimental studies have demonstrated the feasibility of using copper slag in building materials, industrial-scale application remains limited due to raw material variability, potential heavy metal leaching, and limited market acceptance.

5.6. Large-Scale Implementation and Technical Limitations

Although copper slag has shown potential in construction and building materials, its industrial-scale utilization remains limited. In some countries, pilot projects have evaluated its use as an asphalt aggregate and in cement products. However, large-scale applications are still uncommon due to concerns about long-term durability, regulatory approval, and environmental safety [108,109,110].

One of the main barriers to high-value applications lies in the variability of its chemical composition. In particular, the presence of impurities such as sulfur can significantly impact product performance. High sulfur content can reduce thermal stability and affect coloration in glass-ceramic products, as sulfur-containing compounds influence the optical and thermal properties of chalcogenide glasses [111].

These technical constraints, particularly related to chemical composition and product performance, remain key challenges for the large-scale utilization of copper slag across various industries.

6. Conclusions and Outlook

Figure 8 summarizes the current situation of the comprehensive utilization of copper slag in this article. Improving the comprehensive utilization rate of copper slag requires in-depth research and exploration of valuable metal recovery, reuse of secondary slag, direct utilization of copper slag, and separation of valuable metals during the direct utilization process.

• The low recovery rate of valuable metals in copper slag results from their complex embedding with gangue minerals, limitations of single-method recovery, and high costs outweighing profits. Recent studies focus on efficient, cost-effective, and environmentally sustainable recovery techniques driven by economic and environmental needs as a key future research direction.
• The pyrometallurgical impoverishment of copper slag mainly targets the recovery of copper from copper slag and has almost no effect on other valuable metals, which has certain limitations. The differences in raw ore properties and copper smelting processes can lead to changes in the properties of copper slag. If flotation recovery is used, the reagents usually need to be replaced, and there are issues such as the cost of reagents and whether the reagents are toxic. The single magnetic separation method can only separate part of the iron phase from the copper slag, and the magnetic separation concentrate is usually accompanied by some gangue minerals, which affects the magnetic separation recovery rate. The magnetic flotation combined process can treat the most valuable metals in copper slag. However, at present, the demand for copper slag treatment is significant, and it cannot be widely applied in industry in the short term. In particular, integrating magnetic–flotation technology with environmentally friendly leaching agents could offer a promising direction for future research and provide a more sustainable and efficient approach to copper slag treatment.
• Copper slag has a wide range of material fields but mainly focuses on cement, concrete aggregates, and the production of glass-ceramic materials. Domestic and foreign research scholars have made abundant achievements in these two areas, but there is a lack of feasible solutions for large-scale industrial production. In the future, the direct recycling and utilization of copper slag needs to be considered from the perspective of industrial practice.
• The comprehensive recovery of copper smelting slag is a promising research topic that can improve resource utilization efficiency while reducing environmental pollution. However, further technological research and the strengthening of large-scale applications are still needed to achieve effective comprehensive recycling and sustainable development.