Environmental and Socioeconomic Impact of Copper Slag—A Review

: Copper slag is generated when copper and nickel ores are recovered from their parent ores using a pyrometallurgical process, and these ores usually contain other elements which include iron, cobalt, silica, and alumina. Slag is a major problem in the metallurgical industries as it is dumped into heaps which have accumulated into millions of tons over the years. Moreover, they pose a danger to the environment as they occupy vacant land (space problems). Over the past few years, studies have been conducted to investigate the copper slag-producing outlets to learn their behavior, as well as properties of slag, to have the knowledge of how to better reuse and recycle copper slag. This review article provides the environmental and socioeconomic impacts of slag, as well as a characterization of copper slag, with the aim of reusing and recycling the slag to beneﬁt the environment and economy. Recycling methods are considered an attractive technological pathway for reducing waste and greenhouse gas emissions, as well as promoting the concept of circular economy through the utilization of waste. These metal elements have value depending on their characteristics; hence, copper slag is considered as a secondary source of valuable metals. Some of the pyrometallurgical and hydrometallurgical processes to consider are physical separation, magnetic separation, ﬂotation, leaching, and direct reduction roasting of iron (DRI) . Some of the possible metals that can be recovered from the copper slag include Cu, Fe, Ni, Co, and Ag (precious metals).


Introduction
Numerous slags are produced as a second stream which is not desired in various metallurgical processes, e.g., copper slag (CS) from smelting and processing of copper ore [1,2]. The main mineral ore source for copper is chalcopyrite (CuFeS 2 ), with the smelting process contributing approximately 80% of the global copper production from around 21 million metric tons of ores being processed in 2018. The technology is always being improved due to the global importance of copper as a product. The metallurgical industries around the world are trying to find cost-effective and environmentally friendly ways to produce copper. Hence, technologies such as microbially driven metal leaching processes are used. Copper slag is subsequently produced as a byproduct which is solid waste and harmful to the environment but has the potential to yield viable metals if processed efficiently [3,4]. Waste management is one of the biggest challenges in the mining and processing industries, especially waste material such as copper slag. It is estimated that 2.2-3 tons of copper slag is generated per ton of copper produced, and this waste is mostly Crystals 2021, 11, 1504 2 of 16 disposed of near smelter sites [5,6]. There are a significant number of copper processing plants around the world, and this has resulted in the production of about 24.6 million tons per year of slag, which is regarded as waste [7][8][9]. The main questions related to this slag are as follows: What does the slag contain? Is the slag harmful to the environment? Does it affect the ecosystem? What is its socioeconomic impact? Most mineral processing sites dispose of slag around the smelter; however, with the development of regulations over the years, threshold limits have been put in place to regulate its disposal to avoid bringing harm to the environment. The mining sector has given much attention to this problem by focusing on research to determine how to process slag to mitigate this challenge.
Africa is known to be a continent rich in minerals, and Botswana is one nation specifically rich in base metals such as copper, nickel, and cobalt. This paper focuses on copper slag which is generated by pyrometallurgical concentration of copper ores to produce copper/nickel matte by Bamangwato Concession Limited (BCL) in Botswana. BCL is a copper ore-processing plant established in 1960 in Botswana, situated in a town called Selebi Phikwe; however, it went into liquidation in 2016. The focus of the company was to extract copper and nickel from chalcopyrite, chalcocite, and bornite copper ores. The mining operations followed the standard convectional route using pyrometallurgical processes to concentrate the valuables (in this case, copper) [10][11][12][13]. The processing stream produced valuable copper/nickel matte and waste (copper slag). The amount of slag accumulated from 1960 until the closure of the mine in 2016 was millions of tons, which served as the motivation for this study. Smelting of copper is a pyrometallurgical process that generates smelter slag (copper slag), which comprises metals related to copper, e.g., nickel, zinc, iron, and cobalt [14][15][16][17]. Over the years, many mining and mineral processing operations have accepted the responsibility of protecting the environment by following safe disposal regulations of the waste produced. Since the waste is of no use to the mine operations, it is disposed of on vacant land, and it has accumulated over the years into heaps/mountains of copper slag. This slag also contains heavy metals which have concentrated over time and can be harmful to the environment if not taken into consideration during disposal.
Slags have different characteristics depending on how they are cooled from the smelter. This determines their structure and the distribution of heavy metals, base metals, and other minerals in the phases of the slag ( Table 1). The world has taken a stand in viewing the economic benefits of the exploration of minerals, as well as the environmental considerations being implemented in the processing of these minerals to safeguard the environment and establish a sustainable process, leading to the introduction of sustainable metallurgy [15,18]. Silicates Glass fayalite [16] Iron has played a major role in the industrialization and global development of the modern world, including Botswana. Iron is an important metal due to its properties which make it relevant to the construction sector. Botswana is a nation that relies on the importation of most of its building materials such as iron, and it depends on the mining sector to generate this revenue. Most of the mines in Botswana are involved in mining diamonds, gold, copper, and nickel but not iron, as there is no primary iron ore source. The metal is associated with other ores such as copper and nickel. Another approach could be to use copper slag as a source of iron.
Botswana has been hit hard by the closure of copper and nickel mines, because this has resulted in thousands of people losing their jobs and the country being affected economically. A steel-making plant in Selebi Phikwe town was also closed due to the lack of scrap metal used as a feed material, mainly because of the scarce availability of scrap generated by the low population of Botswana, which is around 2.3 million. This article reviews the possibility of using slag accumulated from the closed mines for beneficiation and recovery of iron and copper from copper slags.
Previous research on slag supports this investigation, with researchers from the biggest mining sectors involving copper slag from as far as Chile and Turkey still investigating the reclamation of valuables from copper slag [8,20]. Botswana faces similar problems with high yields of slag produced over time, which have had a high impact on the environment [21]. The mining and the processing of ores for many years have led to high-grade ores being depleted; hence, a new approach using copper slag from BCL and Tati Nickel mines as a secondary source of raw materials can be considered [17]. There are several impacts associated with mining as shown in Figure 1; hence, it is more advantageous to process an already abundantly available slag as opposed to starting new mining activities. This brings into perspective the concept of diversification of the reclamation of valuable metals from slag.
This review article focuses on the negative impact of mining and processing minerals on the environment, and whether these effects are adequately regulated. A typical example is Asia, the continent with the largest production of copper slag in the world through producing 7.26 million tons of copper slag per annum [22,23]. Therefore, there is a need for utilization of this generated copper slag. North America, Europe, South America, Africa and Oceania produced 5.90, 5.56, 4.18, 1.23 and 0.45 million tons of copper slag per annum [24].

Case Studies on Copper Slag
Copper slag is perceived as a negative impact, and previous research has investigated its potential effect on different stakeholders. Many countries mine copper and nickel, which in turn generates millions of CS, and substantial funding has been invested to focus on finding sustainable ways of using CS to their benefit. Two case studies from different countries on the impact of and possible solutions for CS are presented below.

Chile
A good example is Chile, where copper smelters generate a large amount of CS, which causes similar problems to other copper-producing nations such as Botswana. The major problem is CS landfilling, which leads to environmental problems, but this can be tackled using modern pyrometallurgical processes. Consequently, the focus has been on minimizing and reusing the CS to benefit all stakeholders involved, subsequently leading to saving costs, environmental rehabilitation, and a source of economic benefit. Chile has conducted extensive tests on recovery and value addition using CS to address the abovementioned issues and processes, including highly intensive direct reduction to form different types of alloys such as Fe-Cu-C, which can be used in metal industries such as steel production. Some of these alloys obtained reflect sponge iron properties, which can be pelletized before magnetic separation to get iron. This in turn could be a viable approach toward the recovery of metals [8]. Thus, it I s worthwhile for Chile to take the necessary research steps toward utilizing CS to their advantage and benefit.
Chile has copper oxide ores which are processed using hydrometallurgy and output using flotation (smelter) [25]. This process also involves leaching of low-grade copper ores (sulfide-based). The focal leaching methods used over the years have involved heaping, dumping, and running of mine leaching, with them being separated on the basis of particle size in the leaching pad [26]. The authors of [27] stressed the importance of physicochemical characterization when studying and grading slags with respect to the possibility of reclaiming viable metals. Various experiments have been undertaken in the recovery of iron or iron/molybdenum using direct reduction; with the iron content remaining constant, the copper content is reduced intensively, indicating selective reduction [27].
dumping, and running of mine leaching, with them being separated on the basis of particle size in the leaching pad [26]. The authors of [27] stressed the importance of physicochemical characterization when studying and grading slags with respect to the possibility of reclaiming viable metals. Various experiments have been undertaken in the recovery of iron or iron/molybdenum using direct reduction; with the iron content remaining constant, the copper content is reduced intensively, indicating selective reduction [27].  Most of the phases in copper-and nickel-bearing ores are bornite, chalcopyrite, and chalcocite, which can be quantified using QEMSCAN. Copper slag following the production of copper and nickel includes phases such as Cu-Fe-S [19]. Table 1 shows the chemical composition of copper slag. The mechanical and physical properties of slags differ depending how the slag was cooled with most of slag having a unit weight ranging between 2800-3800 kg/m 3 , Bulk density between 144 × 162 Ib per cubic feet, hardness of 7 Moh, appearance being black, glassy, vesicular when granulated, abrasion loss of 24.1 and a specific gravity of 2.8-3.8 [22].

Zambia
Zambia, located in southern Africa, is another good example, and it suffers from similar problems to Chile and Botswana brought about by CS. A sample of slag from a dump (landfill) located at Nkana, Zambia was characterized, and it was realized that it contains a substantial quantity of cobalt. The Nkana dump includes 840 tons of slag with 0.66% Co processed per 100 tons of cobalt-bearing alloy composed of 5-14% Co [29]. This metal is used in various commercial and military applications due to its properties. Co is an element that can be attained in the form of rods, blocks, or powder; it is stable above 417 • C in its closed packed hexagonal form [30]. The monthly average report for the US in March 2021 indicated that the Co price increased by 7% from February, being 43% higher than March 2020. The London Metal Exchange (LME) also showed an increase in cash mean price by 11% from February, being 74% higher than March 2020 [30]. The prices for cobalt metal in March 2021 (USD per pound) were as follows: US, range of 23.00-26.25 at an average price of 24.75; LME, range of 22.96-23.95 at an average price of 23.80 [30]. This led stakeholders to build a 40 MW arc furnace in Chambishi with the sole purpose of recovering cobalt from CS. This extraction of cobalt has proven to be a success, producing about 100 tons of cobalt-bearing alloy (with 5-14% Co) [29]. Cobalt is a very expensive metal due to its properties and rarity.

Significance of This Review
Even with the attention given to this subject in the past, there has not been a dedicated review concerning metallurgical waste in Botswana. This review conducts an initial investigation, followed by documenting the progression, achievements, and challenges in the utilization of smelter slag. It focuses on the reclamation of valuable metals from smelter slag, exploring the different processing and recovery phases relevant to the metallurgical waste reclamation of mineral elements with reference to the properties of smelter slag. It goes on critically evaluate the environmental and socioeconomic impact of slag on relevant stakeholders, especially in Botswana.

Types of Metallurgical Slags
Slag can be classified as ferrous or nonferrous slag, with CS being part of the latter group. The former group comprises iron and steel slags with geochemical application, e.g., in road and ballast railroad construction in Europe and United States in the early 19th century. They still have significant use in engineering fill materials [31]. Botswana contributes to the production of copper slag in Africa, most of which is dumped on vacant land, thus accumulating into mountains that occupy viable land.

Ferrous (Fe) Slag
The process of recovering iron from its parent ore or secondary recyclable material (to produce steel or iron) produces ferrous slag. Slags are produced under high temperatures which can only be achieved in a furnace. Slags differ depending on the conditions used in the furnace and how it is subsequently cooled. The core purpose of a blast furnace is to reduce iron oxides into molten iron via the addition of coke (flux). The slag in the molten state needs to be cooled, and the way it is cooled determines its properties and commercial use; for example, in the recovery of copper, slow cooling of slag leads to a recovery of about 98.5% [32][33][34]. The molten slag solidifies during cooling in different ways depending on whether it is cooled under atmospheric conditions, by using measured quantities of water, by rapidly chilling in air (pelletized), or by quenching with high-pressure water to form granules. The authors of [33] illustrated different aspects brought about by the fast and slow cooling of slag.

Nonferrous Slag
The focus of this review is on nonferrous slags, which are mainly generated from hightemperature processes such as smelting (pyrometallurgical). Nonferrous slag is formed as a byproduct of metal recovery from nonferrous ores. Some nonferrous ores include chromite (FeCr 2 O 4 ) and chalcopyrite (CuS 2 ) [35], are also associated with other base metals. The ores of interest are copper ores and mostly sulfide ores such as pyrite (FeS 2 ). All these ores may be linked to one another because they are associated with different metals, but copper and nickel are the primary metals of interest for recovery as they do not have pure ores which exist on their own [36]. These sulfide ores generate concentrates that contain targeted metals (Cu, Ni, Zn, etc.) and subsequently produce slag as a byproduct, which is considered waste material. The authors of [37] investigated the production of nonferrous slag from parent ores. Several nonferrous ores are processed across the globe due to the high demand for target metals, giving rise to a high production of nonferrous slags. Some of the processing steps involved in treating these nonferrous ores (sulfide ores) involve concentrating, roasting, and smelting.
Cu and Ni, mostly sulfide ores, are processed using pyrometallurgical techniques that yield slag as a byproduct. Not all base metals exist in parent ores; hence, they are not all extracted from sulfide ores. This is evident with Cu, which can also be extracted from oxide and carbonate ores, whereby the optimal recovery technique is not pyrometallurgy. This paper mostly represents research on sulfide ores, such as chalcopyrite (CuS 2 ) from the BCL mine in Botswana, which is a parent ore of Cu and Ni. Such ores are associated with other metal elements, such as Co and Fe. This review focuses on CS which contains nontarget metal elements discarded from the concentrate, which is estimated to be between 45 and 55 million tons in Botswana [38]. Botswana also has similar copper/nickel-bearing ores which are sulfide in nature. BCL produced copper slag, which was disposed of near the mine since its inception (1960). The slag has since accumulated.
The relationship between the quantity of slag produced and the metal produced differs on the basis of the product. The authors of [39] estimated that approximately 0.6 tons of slag is produced for each ton of Pb by a smelter in France. According to [19], about 2.2 tons of slag is generated for each ton of Cu generated around the globe.
The pyrometallurgical processing of Cu involves the usage of water; hence, wastewater is also produced. Some mining operations recycle the water for further usage in the plant. This wastewater contains chemicals and minerals which also pose a potential threat to the environment when disposed of at dumpsites [40].

Uses of Metallurgical Slag
Multiple slags are generated as byproducts to processing minerals (metallurgical processes), and their characterization is an important aspect to determine the number of valuable metals they contain and what process routes can be taken to recover those metals. Across the globe, slag is generated at a rate of 2.2 tons per ton of copper generated, and this enormous rate gives rise to an annual figure of 24.6 million tons of slag. Characterization of the slag is important to determine its composition and the process route(s) for efficient recovery [7,41,42].
Characterization of feed materials allows determining the expected grade concentration and metal recovery, which is vital for the design of extraction flowsheets for the sustainable development and utilization of secondary ore resources [17,43,44].
In previous studies, the initial characterization of smelter slag was followed by the proposal of a process route to utilize the slag. The slag can be characterized using Xray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS), and light optical microscopy (LOM) in transmitted and reflected light [45][46][47]. Previous studies have indicated that copper slag behaves differently when subjected to different temperatures, thus exhibiting different properties [42,48]. BCL slag was characterized to analyze its composition. SEM images revealed Fe-Cu-Ni within the glass host due to the rapid cooling of slag. Fayalite, chromite, sulfide, and glass phases were also identified, with iron in the fayalite and glass phases at 12.85 and 85.19 abundance, respectively. XRF results indicated 35.4% Fe, 0.29% Cu, 0.28% Ni, and 0.14% Co [49].
The chemical composition of CS [27] revealed a high concentration of Fe as the main mineral constituent in all four phases. Phase 3 contained the least amount of Fe at 11.28%, compared to the other phases having a concentration between 50% and 65% Fe [21,50,51]. Thus, this significant amount of Fe can be taken into consideration for recovery. The authors of [52] discussed the importance of characterizing CS using X-ray, SEM, and EPMA. Chile is one of the largest miners of copper, with the slag revealed to contain 83-92% copper [53,54].
Chile is at the forefront of copper slag generation, which has triggered vast research on metallurgical slag utilization. Their slag was observed to be black with a glassy appearance. It is mainly granulated and porous with lower specific gravity, comprising regular shaped angular particles between 0.075 and 4.75 m 3 in size [55].

Slag as a Secondary Resource Ore
This paper investigates the sustainable reuse of metallurgical waste (copper slag) generated during the production of minerals. Waste management regulators across the world have encouraged such industries to find ways of reducing the generation of waste, as well as reusing and recycling this waste, due to environmental and socioeconomic concerns. This has led to a vast interest in metallurgical waste due to its magnitude produced by mineral-processing industries; accordingly, a previous study suggested that metallurgical waste can be treated as a secondary source of minerals or as a building material, meeting the standards of the regulatory authority of waste management [56,57]. Some of these metallurgical industries target certain minerals such as copper and nickel in copper/nickel smelting operations. Copper and nickel ores usually contain Fe and sulfide; hence, it is ideal to use smelting to recover copper and nickel. Copper and nickel were treated via flotation and smelting of the concentrate to yield 42,000 mt of nickel/copper matte per annum at the BCL mine [11]. Other minerals associated with copper ore are not of interest; therefore, during the recovery of target metasls, the undesired associated metals are lost in the slag and accumulate over time. Some of these metals include Co, REEs, and iron in the case of BCL smelter operations in Botswana. These metals are of great importance due to their properties [25,51,55,[58][59][60].
Several technologies have been improved over the years for the recovery of metals contained in CS, with one in particular being direct reduction-magnetic separation by adding Na 2 CO 3 and CaO to upgrade copper slag from 40.33% Fe total and 0.65% Cu to 94.3% and 86.5%, respectively [61]. The authors of [55] gave a good account of different process routes used to recover metallic elements from slag using a three-way approach.

Reuse and Recycling of Copper Slag
This review aimed to evaluate the potential of copper slag. Slag is formed via pyrometallurgical processes; hence, hydrometallurgy plays an important role in the study of recycling slag to find viable metallic elements. Direct reduction is one method used to recover iron from the slag by reducing Fe 3 O 3 and Fe 2 SiO 4 found in copper slag to metallic iron using coal. This involves using reduction temperatures of about 1300 • C over the course of 30 min, giving an output of 98.13% Fe [48]. Iron is a vital metal element that encourages the growth of humanity and national economies [62,63]. As iron is mostly sourced from its parent ore, using copper slag as an alternative source will subsequently benefit the environment, as it will revamp and rejuvenate the environment where slag was dumped and stored. The newly open land could then be reintroduced into the ecosystem and used for more productive activities such as farming and other industrial ventures.
Copper slag, as its name suggests, contains traces of copper, and most copper ores are associated with other metallic elements such as cobalt. Various studies have been conducted to recover copper and cobalt using magnetic separation and sulfuric acid leaching of copper slag containing Cu-Co-Fe. The sample was subjected to wet grinding to allow magnetic separation, producing 95.75% cobalt, while 87.8% of copper and 44.39% of iron remained in the tailings [12].
With the depletion of high-grade ores over the years due to the demand for minerals, different types of pyro and hydrometallurgical processes have been used to recover viable metals such Fe, Cu, Co, RREs, and Ni. Their characterization allows determining processing routes depending on the phases they are present in.
CS can also be feasibly processed by ammonia solution treatment to yield significant and viable yields of copper [64]. Using microwave treatment to treat CS slag to recover iron at high concentrations is another viable option [65]. For copper and nickel, leaching or flotation is typically used for recovery from CS [33] Botswana used to produce copper from Bamangwato Concession Limited [66], Khoemacau, and Tati Nickel mines. These mines also produced slag, which was disposed into heaps. The slag from BCL was produced from the smelter unit where nickel and copper matte were produced. Approximately 833,393 tons of slag was produced in 1998 with no plan for land reclamation at the slag deposition sites at the time [28]. The slag at BCL was rapidly cooled using water sprayed at high intensity, giving it a glassy dark shiny look. The outer layer of the slag had a tough appearance when handled. The disposed slag usually contained iron, copper, zinc, cobalt, nickel, silver, and gold [66]. All these metals have different demands; for example, iron has a high demand locally in Botswana and internationally, especially in China. The iron content in CS is sometimes more than that in the parent ore due to the concentration of iron over years of disposal [67]. The iron content in copper slag generated by pyrometallurgical processes is approximately 35% to 45% according to [68]. The iron content found in BCL copper slag that can be recovered is 35.4% [69]. Botswana suffered the recent closure of Pula Steel due to a lack of the main raw material, i.e., scrap iron; however, due to the small pollution of Botswana, this was not sustainable in that manner. Therefore, utilization of copper slag for recovering iron can be used to feed the steel plant, stimulating its resurrection. This can also help in the waste management of slag, thus reducing environmental pollution. Most slags contain 40 wt.% iron, and extraction of this iron from CS is ideal [70].

Slag for Construction
Slag possesses different properties and characteristics, which make it suitable for making construction materials depending on the processes the slag was subjected to [1,19]. Thus, characterization allows establishing the suitability of the slag by looking at its mineralogy and behavior under set conditions. Thus, the slag can be utilized instead of it being stockpiled over usable and potentially fertile land for farming [71,72].

Cu Slag in Concrete
Copper slag comprises pozzolanic characteristics due to it containing low amounts of CaO and other various oxides such as Al 2 O 3 , SiO 3 , and Fe 2 O 3 . This aspect of replacing components of concrete with slag can reduce the slag being dumped on vacant land, which causes problems for the environment and involved stakeholders. This reduces the costs of dealing with the slag to meet environmental regulations, as well as reducing costs for the development of cement, as the slag is of low cost or even free. In many ways, this is an advantage for concrete manufacturers and metallurgical industries [73,74]. CS slag, depending on the technique used to cool it, bears properties that can be used in the construction industry as it can be used as an aggregate [75]. CS slag is becoming popular around the globe as an auxiliary of sand in concrete, mostly in Singapore [76].
Depending on the processing route, different properties such strength-to-weight ratio determine the effective use of slag in concrete [1,77,78]. The authors of [79] further highlighted the use of CS as a fractional auxiliary replacement of fine aggregates to evade the discharge of carbon dioxide from cement in concrete. As previously highlighted, slag can be cooled slowly or quickly, which determines its granulation output and, thus, its suitability as a sustainable building material, e.g., as a fine aggregate in cement [75,80].

Environmental Impact of Slag
It is vital to test CS for the leaching of heavy metals, especially when used in valueadded products such as construction materials. CS disposed of in heaps of vacant land tends to be left alone with the assumption that it is waste; however, it is exposed to environmental conditions such as rainfall. This can prompt leaching of metals associated with the slag. The leached heavy metals can pollute drinking water. These heavy metals may include lead, arsenic, cadmium, iron, copper, and cobalt [71], which can affect the soil, water, animals, and people. Thus, review aimed to gather enough evidence to articulate the impact of slag on the environment and other relevant stakeholders.

Effect of Slag on People and Animals
The mining industry produces a concentrate and a byproduct (mainly slag), which contains heavy metals associated with the ore processed. These metals can be toxic and harmful to the environment. The environment can be exposed to these heavy metals in different ways, both natural and anthropogenic. CS contains various heavy metals such as lead, arsenic, copper, nickel, iron, and cobalt. Even though these metals are at low concentrations, following prolonged disposal, their concentration can increase over time with effects on the environment (plants, animals, and human beings) [39,81,82]. One of the most common effects on humans caused by the ingestion of copper is paralysis, which in most cases leads to death due to gastric hemorrhage. Furthermore, skin exposure can cause acute dermatitis [83]. The authors of [84] illustrated the toxicity of heavy metals toward plants and soil, as well as possible avenues for their use to decrease the level of toxicity to the environment [85].
Previous studies have been conducted in the Selebi Phikwe mining town, Botswana, whereby residents were given questionnaires regarding the environmental and socioeconomic impact of the mine (Table 2).
Slag, as discussed above, can be categorized as ferrous or nonferrous, which both have negative impacts on the environment if the disposal standards are not observed [86]. • Environmental policies showed that the waste was generalized instead of specified to mine waste, making it difficult to cap and control emissions and disposals by mine waste, leading to most mines using internal regulatory policies • Interviews showed that most of the polices were not adhered to • Field observations showed that the soil covered by the slag and around the slag changed in terms of texture and color due to the heavy metals present in the slag, while ponds of water were also discolored

Socioeconomic Impacts
Copper slag is perceived as waste material from the metallurgical processing of mineral ores and, hence, poses a risk to the environment. Therefore, reusing the slag for aspects of concrete and for reclamation of valuable metals from the slag will prove beneficial to the environment and, subsequently, the economy. This article reviews potential methods for the utilization of BCL smelter slag (copper slag). Instead of viewing copper slag as a waste product, it can be seen as having environmental and economic importance. Mines and plants have a responsibility toward the environment according to standards, making it less harmful after the recovery of heavy metals which can be of economic value. The less reactive or inert slag can be used for other purposes including construction as a replacement material in building materials such as cement [87].
In southern Africa, mining has been the backbone of many countries, most notably Botswana, where minerals such as copper and nickel are mined as a matte concentrate, yielding CS as the byproduct. In the early mining days of this industrial era, less research was conducted on the environmental implications brought about by the CS generated from the process. The mines were mostly focused on optimization and efficacy to produce better concentrates and, thus, more profit at the cost of the environment brought about by the handling and disposal of waste. Botswana was one of the top three mineral producers in Africa, along with the Democratic Republic of Congo and South Africa. The use of CS as a secondary source can provide over 10,000 jobs, which is critical for the sustainability of the mining industry [88].

Socioeconomic Impact of Metallurgical Slag in Botswana
CS has several harmful implications if not handled and disposed of correctly. As it has mainly been seen as waste over the years, less attention has been given to it in terms of research [89]. The authors of [90] highlighted the importance of keeping tabs on the concentration levels of heavy metals found in slag to avoid long-term environmental impacts. Another route to keep levels of heavy metals in line with environmental regulations is to utilize the CS sustainably.
Previous studies have illustrated that, over the years, high-grade ores have been used up, and waste such as CS is seen as a potentially rich secondary source of viable minerals. Characterization of CS is important to determine its behavior and mineral composition, thus allowing the identification of possible recovery routes for extraction. Iron ore is regarded as viable if contains at least 35% Fe, and CS such as that of BCL can meet this requirement. Iron is a very important element for economic growth. Copper is another important metal, and its current market price is the highest considering the last 10 years [22]. Copper and nickel are usually found in the sulfide phase, and previous research has indicated that the best possible recovery routes involve flotation and leaching, whereas iron can be recovered via direct reduction using coke or other reductants [8,91,92]. Table 3 shows a summary of possible process routes used in previous research for possible metals present in CS. Reduction [29,34,61,95] Africa is a continent blessed with abundant mineral deposits, which play an important role in its economic growth. Specifically, the mining and mineral processing sector accounts for a substantial share of the gross domestic product (GDP) around southern African, covered mostly by the sub-Saharan region. Due to the demand for viable minerals, the over-processing of mineral ores has subsequently led to severe environmental problems.