Technospheric Mining of Critical and Strategic Metals from Non-Ferrous Slags

Abstract

The technosphere consists of material stocks accumulated by human activities, which can include processing residue, such as slag. Various smelting processes generate slag, and some valuable elements are concentrated in this by-product. In this review, the extraction of critical and strategic metals from non-ferrous slags is discussed. Critical and strategic metals are materials that are vital for the nation’s economy and defence, as well as its industries, and have common features, such as expected shortfalls, increasing demand, and few substitutions. There are several definitions, methods, and classifications of critical and strategic elements by different organisations. In this study, reports from seven institutions around the world are summarised, and a list of recommended critical and strategic metals is presented. Non-ferrous slags contain a considerable amount of critical and strategic elements, and research on technology and process development using both pyro- and hydrometallurgical methods is very attractive. When it comes to the extraction of values from slag and the development of technology, it is not only important to consider the economic aspect but also to ensure the processes are low in emissions and energy consumption but high in efficiency and recycling.

1. Introduction

The extraction of metals from ores using high-temperature processing, or pyrometallurgy, produces slag as a by-product. Slag is a fusible or molten mass formed when flux, on being subjected to heat, reacts with impurities in the ore. Slag floats on top of matte and can be collected separately [1]. It can normally be divided into two groups: ferrous slags and non-ferrous slags [2]. Other slags, such as those generated from municipal waste, are excluded from this study to avoid presenting an overwhelming amount of information. The focus of this review is non-ferrous slag from metallurgical operations.

Slag is regarded as waste and has applications in brick, cement, and construction [1]; however, these is much attention due to the valuable metals it contains. Slag may cause environmental concerns for its release of potentially toxic elements; however, utilising slags as a secondary source will reduce the possibility of environmental exposure [2,3,4,5,6,7].

While there is abundant information available on copper slag, the characteristics of other slags such as nickel, tin, and vanadium slags are relatively less known, and their considerations as a potential secondary source of valuable metal warrant detailed characterisation and analysis of physical and chemical properties both for metal extraction and environmental purposes [2,7]. Subsequently, the recovery of valuable metals from ferrous and non-ferrous slag sources has been reviewed from the perspective of repurposing and reprocessing the material [6]. Ferrous slags have been effectively utilised for various applications such as cement production, road construction, and others [6]. Non-ferrous slags also have the potential to be utilised in those applications; however, they contain critical and strategic metals, which makes them more valuable as secondary sources for these metals. This review will focus on the contents of critical and strategic elements in non-ferrous slags and the extraction methods to recover them from the slag. Projected shortfalls, increasing demand, and limited substitutions make metals critical and strategic, and it is hard to secure the supply even though these metals are essential.

As mentioned, there are a few general review articles on slag, and these include the following: characterisation [2,7,8], recovery of values from slag [3,6,9], the environmental aspect [2], bioleaching potential [10,11], each commodity such as copper [12], vanadium [13], titanium [14], tungsten [15], and cobalt [4]. There has been no literature published that focuses specifically on the critical and strategic metals, particularly those contained in non-ferrous slags, and on the potential of critical and strategic metal extraction from these materials. In this review paper, the concept of technospheric mining and its application to the extraction of critical and strategic metals from non-ferrous slags are discussed. This paper also details the different critical and strategic elements found in non-ferrous slags and the processes studied and applied to extract them.

2. Research Methodology

This review paper is devoted to exploring the technospheric mining of critical and strategic metals from non-ferrous slags by introducing the terms, reviewing the literature database, and exploring relevant technologies in hydro- and pyrometallurgy.

The references in this paper were searched through ‘Google Scholar’ using keywords and were carefully chosen to reflect relevance to the topic. Publications on technospheric mining and the technosphere were used to provide a summary of the concept, except for dissertations. Most reports were chosen to introduce the classifications and methodologies of critical and strategic metals from each institute. There are also other countries or organisations available, for example India and Korea, that provide reports on critical and strategic metals. However, only seven geologically representative institutions were chosen for this review.

An extensive literature review was conducted on the recovery of critical and strategic metals from each slag using various methods. In terms of extraction methods, hydrometallurgical and pyrometallurgical methods such as leaching or roasting were the key technologies to focus on over mineral processing and bio-treatment methods unless specifically relevant and useful to restrict the scope. Some slags have limited literature available. On the other hand, copper slag had extensive literature; thus, strict and careful criteria were used to select the relevant literature; for example, the extraction of critical and strategic metals in metallic or semi-metallic forms was the main focus of choosing the literature.

3. Technosphere and Technospheric Mining

The technosphere contains materials, known as technospheric stocks, which are generated and accumulated by human and technological intervention. These stocks may include waste repositories and by-products, such as urban and industrial waste, process residue, mine waste, process effluents, slag, and tailings [16,17]. Researchers from the 1990s introduced, defined, and classified technospheric stocks and their status based on several factors relating to material flow and composition [17,18,19,20,21]. They can also be categorised based on location, type of stock, management, and state of the stocks. The subgroups of technospheric stocks are shown in

Table 1. Sub-classification of technospheric stocks.

Technospheric Stocks	Status	References
In-use stocks (In-use mining)	Active	[17]
Landfills (Landfill mining)	Controlled inactive
Tailings (Tailing mining)
Slags (Slag mining)
Hibernating stocks (Hibernation mining)	Uncontrolled inactive
Dissipated stocks (Dissipation mining)
Urban technosphere	Active	[22]
Rural technosphere	Uncontrolled inactive
Subterranean technosphere	Controlled inactive
Marine technosphere	Uncontrolled inactive
Aerial technosphere	Uncontrolled inactive

[17,22]. The concept of mining the technosphere refers to technospheric mining.

Technospheric mining is a term and concept referring to recovering minerals or metals from material stockpiles established by human activity and anthropogenic processes [17]. It is an umbrella concept that incorporates various terminologies common in a circular economy, such as resource recycling and urban mining. Johansson et al. [17] suggested a taxonomy in technospheric mining to address a better understanding of this newly developing field, providing a more holistic approach to reprocessing or repurposing unwanted, end-of-life, or waste materials. For example, the taxonomy also described and reviewed landfill mining [23] and urban mining [24]. There are a number of publications now highlighting the application and importance of technospheric mining and the development of technologies to recover valuable metals from technospheric stocks [25,26,27,28].

Some of the technospheric stocks, such as tailing, slag, and metallurgical by-products, have drawn attention since they contain a significant amount of critical and strategic metals [28,29]. However, accumulated tailings or slags potentially contain materials that are hazardous for humans and the environment and can cause tragic accidents when managed inappropriately. Thus, employing the technosphere as a secondary source of resources can contribute to mitigating those environmental problems. Therefore, mining the technosphere is a desirable way to reach sustainable development in social and environmental sections, along with obtaining critical and strategic metals from secondary sources.

Due to projected shortfalls, increasing demand, and limited substitutions of critical and strategic metals, the mining industry needs to pay attention to the exploitation of secondary resources that are already extracted and stored as waste to secure critical and strategic metals. Advanced technology, economic and political advocacy, and the enormous amount of technospheric stock support technospheric mining in becoming a new business model. Technospheric mining not only provides secondary sources for critical and strategic metals but also assists in achieving a circular economy that promotes sustainable development, especially in the environment. Overall, technospheric mining can provide economic, environmental, and industrial advantages.

4. Critical and Strategic Metals

Critical metals are essential for the economy and trade, and strategic metals are needed for a nation’s defence and its economy [30]. There are multiple definitions, methodologies, and classifications of critical and strategic metals from various institutes [31]. Although several publications on critical and strategic materials or metals have significantly escalated since the 2010s [32], the concept of strategic and critical raw materials publicly appeared in 1939 with a concern for securing those metals that are required in case of war but are unobtainable within the country and thus need to be imported from other countries. The list of critical and strategic materials in 1939 from the United States included antimony, asbestos, bauxite, camphor, cobalt, cryolite, ferrous alloys, iodine, mercury, opium, quinine, platinum, potash, rubber, and tin [33,34], of which some are still critical and strategic in society and others are not anymore.

It is worthwhile to note that criticality is a degree of how critical each element or metal is; for example, from the measure of 0 to 100, rather than the binary state of ‘critical’ or ‘non-critical’. Particularly, critical metals refer to those that have increasing demand with scarce supply and, therefore, become more important in the industry [35]. Similar to critical metals, some metals that are essential for a nation’s defence are classified as strategic, and they share distinct features: (1) Strategic metals are concentrated in a few countries, causing resource nationalism: (2) They are highly essential in the high-technology industry, for example, solar panels, wind turbines, and superalloys: (3) They are irreplaceable, which means few replacements for the strategic metals are available [35,36]. Several articles [37,38] mentioned the terms ‘strategic’ and ‘strategic metals’ but did not clarify what the terms mean or the methodology.

Critical and strategic metals are getting more attention with the increasing number of publications and reports, yet there is no comprehensive definition that is agreed on by the institutes or governments around the globe since the geological, political, infrastructural, and environmental situations and factors are divergent [31,32]. A conundrum in this situation is whether it is possible to have a globally defined, long-term classification and methodology for critical and strategic metals. As technology and industry evolve, the factors in the methodology change rapidly, which inevitably results in the alteration of critical and strategic metals. Jim et al. [31] also argued that having a comprehensive and concrete methodology would be ideal, although the amount of work for assessment can be incredibly large; thus, simplifying the assessment can reduce the work and, at the same time, the precision of the result as well.

Representatively, national organisations and governments, in alphabetical order, the Australian Government (AG), the British Geological Survey (BGS), the Chinese Government (CG), the European Commission (EC, referred to as EU), the Ministry of Mines from Government of India (GI), the Organisation for Economic Co-operation and Development (OECD), and the United States Department of the Interior from the United States of America (US DoI, referred to as the USA), released publications on critical and strategic materials and their strategies to secure those valuable metals.

Table 2. Classification of critical and strategic metals.

Metals	AG [40]	BGS [43]	CG [41]	EU [39]	GI [44]	OECD [36]	USA [42]
Aluminium (Al)	X	X	X	X	X	X	X
Antimony (Sb)	X	X	X	X			X
Arsenic (As)	X	X		X		X	X
Barium (Ba)		X		X 1			X
Beryllium (Be)	X	X		X			X
Bismuth (Bi)	X	X		X			X
Cadmium (Cd)		X				X
Carbon * (C)	X	X	X	X	X	X	X
Chromium (Cr)	X	X	X		X	X	X
Cobalt (Co)	X	X	X	X	X	X	X
Copper (Cu)	X	X	X	X	X	X
Fluorspar (F)		X	X				X
Gallium (Ga)	X	X		X		X	X
Germanium (Ge)	X	X		X		X	X
Gold (Au)		X	X			X
Hafnium (Hf)	X			X		X	X
HREEs	X	X	X	X	X	X	X
Indium (In)	X	X			X	X	X
Iron (Fe)		X	X		X	X
Lead (Pb)		X			X	X
Lithium (Li)	X	X	X	X	X	X	X
LREEs	X	X	X	X	X	X	X
Magnesium (Mg)	X	X		X	X	X	X
Manganese (Mn)	X	X		X		X	X
Molybdenum (Mo)	X	X	X		X	X
Nickel (Ni)	X	X	X	X	X	X	X
Niobium (Nb)	X	X		X	X	X	X
PGMs	X	X		X		X	X
Rhenium (Re)	X	X
Selenium (Se)	X	X				X
Silicon metal (Si)	X			X	X
Silver (Ag)		X				X
Strontium (Sr)		X		X	X
Tantalum (Ta)	X	X		X			X
Thorium (Th)	X	X
Tin (Sn)	X	X	X			X	X
Titanium (Ti)	X	X		X	X	X	X
Tungsten (W)	X	X	X	X			X
Uranium (U)		X	X
Vanadium (V)	X	X		X	X	X	X
Zinc (Zn)	X	X				X	X
Zirconium (Zr)		X	X		X	X	X
HREEs = Heavy Rare Earth Elements	Dysprosium, Erbium, Europium, Gadolinium, Holmium, Lutetium, Terbium, Thulium, Ytterbium, Scandium, Yttrium
LREEs = Light Rare Earth Elements	Cerium, Lanthanum, Neodymium, Praseodymium, Samarium
PGMs = Platinum Group Metals	Iridium, Palladium, Platinum, Rhodium, Ruthenium

* Carbon is regarded in several forms, for example, BGS included graphite and diamond, EU Commission included coking coal and natural graphite, OECD mentioned natural graphite, and AG, CG, GI, and US DoI chose graphite. ¹ baryte instead of barium.

shows the critical and strategic metals selected by the institutions listed above. X in Table 2 means the institute chose the element as critical and/or strategic. Among metals described by several organisations, those shared in the list in Table 2 are classified as critical and strategic. This table, however, excludes non-metals such as borate (BO₃³⁻ or B₄O₇²⁻), helium (He), fluorine (F), phosphorus (P), and natural rubber [39,40,41], while acknowledging that the non-metals are vital for many industries. The purpose of Table 2 is to compare the common critical and strategic metals that were chosen from the seven institutes, regardless of the geological factors. For instance, all institutes categorised REEs as critical and strategic, whereas only two institutes consider cadmium (Cd) as critical and strategic. Metals classified as critical and strategic by two institutes out of seven can be considered low priority globally, for example, rhenium (Re), silver (Ag), thorium (Th), uranium (U), as well as Cd. Some metals that were chosen by only one institute were eliminated from Table 2, considering their criticality not to be as globally significant as others. The metals that were chosen only once were the following: caesium (Cs), rubidium (Rb), tellurium (Te) by the USA [42], and mercury (Hg) by BGS [43].

The Australian government initiated the Critical Minerals Strategy, which entails the vision of critical minerals by 2023, and became the global supplier of those minerals. The Critical Minerals List provides not only the critical minerals that are crucial to the technologies necessary for achieving net zero emissions as well as energy security, but also the strategic materials that are regarded as a ‘watchlist’ to ensure there is no disruption on the supply chain [40]. Among the metals and materials listed in Table 2, the strategic materials classified by Australia are Al, Cu, Ni, P, Sn, and Zn.

The British Geological Survey published a report on the supply risk of chemical elements and its methodology. The supply risk index rates each element from 4.5 to 9.5. The element with the highest index is Rare Earth Element (REE) with a rate of 9.5, and the lowest-rated metal was gold with 4.5, indicating incredibly insignificant risk. The critical metals chosen by the BGS alone are shown in Table 2. A few metals, for example, mercury (Hg) and uranium (U), were excluded from the list because they were chosen by only one institute [43].

The Chinese government released National Mineral Resources Planning in 2016 and classified 24 minerals as critical and strategic within three categories (energy, metal, and non-metal). However, this government report was only available in Chinese; thus, the alternative publication in English was used to obtain the list of critical minerals from the Chinese government [41].

The European Commission has published a report on critical raw materials every three years since 2011, and the latest report in 2023 has been mostly referenced. Based on market fluctuations, progress of technology, and production status, the list changes every three years in the reports. The critical raw metals (CRMs) in Table 2 are notably vital in Europe because of their high demand in various industries and the advancement of green and high technology. The two main parameters used by the EU in the methodology to assess the criticality of metals since 2011 are economic importance (EI) and supply risk (SR), which comprise several factors. The EU Commission is trying to ensure the consistency of the assessment throughout the years [39].

The Ministry of Mines of the Government of India published Critical Minerals for India [44]. The report included a global overview, five factors of the value chain of critical minerals, the methodology of classification, and so on. An interesting and meaningful exercise by the Government of India that was included in this report was a comparison of critical minerals chosen by different ministries in India through inter-ministerial consultation. For example, the Department of Science and Technology and the Department of Pharmaceuticals notably chose different minerals as critical; hence, a broad spectrum of critical minerals was reviewed and chosen [44].

The Organisation for Economic Co-operation and Development has 38 countries from around the globe as members, building international standards and policies and solving various issues together [45]. Hence, the OECD’s view on critical and strategic materials is significant since the organisation represents a broad perspective regardless of geological factors. In terms of critical metals, the OECD is mentioned in the case study of critical metals and mobile devices for sustainable material management. The organisation acknowledges that the definition of critical metals is subject to change over time due to factors such as geography and technology [46].

In 2020, the Energy Act of 2020 was passed by the United States Congress to boost clean energy and climate policy in the US. Following the Energy Act of 2020, the Secretary of the DoI, who is also the director of the US Geological Survey (USGS), published the draft list of critical minerals in the Federal Register. This 2021 list was consulted by the Department of Agriculture, Commerce, Defence, and Energy, and the United States Trade Representative [42]. In 2022, the Final List of Critical Minerals was published [47]. The recent reports on the critical materials from the Department of Defence (DoD) and Energy (DoE) referred to the list from the DoI [48,49]. Even if a commodity has no supply risk or trade exposure and then a single producer in the US becomes unable to produce the commodity, there is a chance to be exposed to the supply risk, which was categorised as Single Point of Failure (SPOF). With the SPOF method, Be, Ni, and Zr are included in the draft list of critical minerals. In cases where there was insufficient data for a quantitative assessment of a certain commodity, a qualitative evaluation of supply and demand was conducted. As a result, caesium (Cs), rubidium (Rb), Sc, and the REEs remained on the list [50].

5. Recovery of Critical and Strategic Metals from Non-Ferrous Slags

As demand for critical and strategic metals keeps increasing from various industries, securing primary and secondary sources of critical and strategic materials and having feasible extraction technologies are crucial for nations’ economies and defence. Hence, in the following chapter of this paper, we would like to introduce the potential of extracting critical and strategic metals from the by-product of the smelting process, non-ferrous slags. Indeed, because the recovery of critical and strategic metals from slag materials is still in its infancy, the available literature is limited. Thus, tin, copper, nickel, vanadium, and titanium slags were studied. Also, it is worth noting that the emphasis on base metals will be less than that on non-base metals since the production and supply of base metals from the primary source are more stable than the other critical and strategic metals.

5.1. Tin Slag

Tin slag is a by-product of the tin smelting process. Tin slag has drawn broad attention since it contains a significant amount of critical and strategic metals, representatively Nb and Ta [51]. It can be considered a secondary source of Nb and Ta, as there are a limited number of primary ores of those metals [52]. Several studies [52,53,54,55,56,57] have attempted to recover valuable metals from tin slag using pyro- and hydrometallurgical methods. The chemical composition of tin slag, including the critical and strategic elements, is shown in Table 3. Tin slag became a feasible candidate for secondary sources of Ta when the market price increased [58], and it can happen any time soon for other valuable metals or commodities as well.

As shown in Table 3, tin slags contain not only Sn, Nb, and Ta but also critical and strategic metals such as Ti, W, V, Cr, Ni, and REEs. The figures in Table 3 are mostly average concentrations of samples to avoid including all the samples from the same countries and regions in each paper [52,59]. The concentrations of Nb and Tb vary significantly depending on the origin of the sample: Nb 70.89 mg/kg to 14.1% and Ta 1.75 mg/kg to 20.8% as oxides. Thus, many sources actually have a high grade of Nb and Ta: Nb + Ta > 8% high, 5% < Nb + Ta < 8% low, and Nb + Ta < 5% extremely low [58]. Also, the concentrations of tin slags from Smelterskop and Elandsberg, South Africa are ancient samples from 1650 to 1850 CE. The geologists estimated that 180,000 tonnes of tin were mined in these regions before 1905 [59]. Thus, considering the grade of Sn in these regions is 11.10% SnO to 25.28 SnO₂, it might be feasible to extract Sn from the historical tin slags in South Africa.

Table 3. Compositions (%) of critical and strategic elements in tin slags.

Origin (Region)	Nb2O5 [Nb]	Ta2O5 [Ta]	(Ta, Nb)2O5	TiO2 [Ti]	SnO2 [Sn]	MgO [Mg]	Al2O3 [Al]	MnO [Mn]	WO3 [W]	V2O5 [V]	Cr2O3 [Cr]	Ni2O3 [Ni]	Ce + La	References
N/A	9.35	2.6	11.95	5.9	-	5.3	6.56	-	-	-	-	-	-	[52]
N/A	5.2	7.5	12.7	1.3	0.7	-	11.2	3.7	-	-	-	-	-	[53]
Nigeria	14.1	20.8	34.9	-	0.37	1.2	5.1	1.7	-	-	-	-	-	[55]
Malaysia	3.43	3.05	6.48	7.97	1.33	-	10.42	0.7	1.4	-	-	-	-	[56,60]
	4	4	8	11	0.5	0.5	9	0.5	8	0.5	-	-	-	[61]
Indonesia	-	-	2.7	13.3	0.8	-	8.0	0.4	0.0	-	-	-	-	[57,60]
Indonesia	0.33	0.64	0.97	11.92	-	-	11.7	-	-	-	-	-	-	[62,63]
(Bangka)	-	-	-	2.38	1.78	-	0.52	-	-	-	-	-	1.7	[63]
Spain	-	-	18.7	7.3	0.1	-	6.9	1.7	0.1	-	-	-	-	[57,60]
South Korea	-	-	10.8	17.8	0.8	-	4.7	0.7	0.1	-	-	-	-	[57,60]
Thailand	-	-	24.9	15.5	0.7	-	7.0	2.1	1.0	0.14	0.65	-	-	[57,60]
DR Congo	-	-	12.7	1.3	0.7	-	11.2	3.7	0.2	-	-	-	-	[57,60]
England	-	-	-	-	16.44	2.21	10.60	0.68	7.31	-	-	-	-	[64]
Zimbabwe	[29.9]	[27.3]	-	[0.16]	-	-	[0.17]	[2.95]	[0.41]	[0.06]	[0.03]	[0.13]	-	[65]
Australia	5.5	10	15.5	9	-	-	-	-	-	-	-	-	-	[62]
South Africa	7	5	12	2	-	-	-	-	-	-	-	-	-	[62]
(Smelterskop)	70.89 *	1.75 *	72.64 *	1.4	25.28	3.09	11.86	0.11	0.34	0.11	0.045	20.41 *	456.9 *	A, [59]
(Elandsberg)	0.1	-	0.1	8.61	11.19 ᵃ	2.37	7.34	0.19	-	0.13	0.13	0.02 ᵇ	-	A, [59]
Brazil	-	-	-	0.8	-	6.2	11.8	0.7	-	-	-	-	-	[66]
Brazil	[0.3]	[0.5]	-	[1.0]	[2.0]	[4.0]	[2.0]	[0.6]	-	-	-	-	-	[51]

* is mg/kg, not %. A is the ancient sample. ᵃ and ᵇ represent different forms of oxide; in this case, ᵃ is SnO and ᵇ is NiO.

Table 3. Compositions (%) of critical and strategic elements in tin slags.

Origin (Region)	Nb2O5 [Nb]	Ta2O5 [Ta]	(Ta, Nb)2O5	TiO2 [Ti]	SnO2 [Sn]	MgO [Mg]	Al2O3 [Al]	MnO [Mn]	WO3 [W]	V2O5 [V]	Cr2O3 [Cr]	Ni2O3 [Ni]	Ce + La	References
N/A	9.35	2.6	11.95	5.9	-	5.3	6.56	-	-	-	-	-	-	[52]
N/A	5.2	7.5	12.7	1.3	0.7	-	11.2	3.7	-	-	-	-	-	[53]
Nigeria	14.1	20.8	34.9	-	0.37	1.2	5.1	1.7	-	-	-	-	-	[55]
Malaysia	3.43	3.05	6.48	7.97	1.33	-	10.42	0.7	1.4	-	-	-	-	[56,60]
	4	4	8	11	0.5	0.5	9	0.5	8	0.5	-	-	-	[61]
Indonesia	-	-	2.7	13.3	0.8	-	8.0	0.4	0.0	-	-	-	-	[57,60]
Indonesia	0.33	0.64	0.97	11.92	-	-	11.7	-	-	-	-	-	-	[62,63]
(Bangka)	-	-	-	2.38	1.78	-	0.52	-	-	-	-	-	1.7	[63]
Spain	-	-	18.7	7.3	0.1	-	6.9	1.7	0.1	-	-	-	-	[57,60]
South Korea	-	-	10.8	17.8	0.8	-	4.7	0.7	0.1	-	-	-	-	[57,60]
Thailand	-	-	24.9	15.5	0.7	-	7.0	2.1	1.0	0.14	0.65	-	-	[57,60]
DR Congo	-	-	12.7	1.3	0.7	-	11.2	3.7	0.2	-	-	-	-	[57,60]
England	-	-	-	-	16.44	2.21	10.60	0.68	7.31	-	-	-	-	[64]
Zimbabwe	[29.9]	[27.3]	-	[0.16]	-	-	[0.17]	[2.95]	[0.41]	[0.06]	[0.03]	[0.13]	-	[65]
Australia	5.5	10	15.5	9	-	-	-	-	-	-	-	-	-	[62]
South Africa	7	5	12	2	-	-	-	-	-	-	-	-	-	[62]
(Smelterskop)	70.89 *	1.75 *	72.64 *	1.4	25.28	3.09	11.86	0.11	0.34	0.11	0.045	20.41 *	456.9 *	A, [59]
(Elandsberg)	0.1	-	0.1	8.61	11.19 ᵃ	2.37	7.34	0.19	-	0.13	0.13	0.02 ᵇ	-	A, [59]
Brazil	-	-	-	0.8	-	6.2	11.8	0.7	-	-	-	-	-	[66]
Brazil	[0.3]	[0.5]	-	[1.0]	[2.0]	[4.0]	[2.0]	[0.6]	-	-	-	-	-	[51]

* is mg/kg, not %. A is the ancient sample. ᵃ and ᵇ represent different forms of oxide; in this case, ᵃ is SnO and ᵇ is NiO.

Nb and Ta are the main products that can be extracted from tin slags. As shown in

Table 4. Extraction methods of critical and strategic metals from tin slags (Condition and recovery given for bolded method).

Samples	Methods (Pyrometallurgical, Hydrometallurgical and Combined)	Optimum Conditions	Recovery (Grade)	References
Tin slag	- (Pre-treatment) Leaching - Chlorination - Carbochlorination	LGC (Low grade concentrate) 1000 °C, 24 h	Nb2O5 98.4% Ta2O5 99.4%	[53]
Tin slag	- Hydrochloric acid leaching - Carbochlorination - Carbon-tetrachloride chlorination	20% Charcoal 900 °C, 40 min	Nb2O5 79.8% Ta2O5 64.5% TiO2 74.3% Converted	[52]
Tin slag (Thailand, Zaire, Indonesia)	- Successive acid and base leaching (HF, HCl and NaOH) - Carbochlorination/halogenation (Cl2 + N2 + CO)	Leaching NaOH 2 N HCl 2 N HF 1.4 N, 70 °C, 1 h Solid/Liquid: 50 g/L Carbochlorination HGC, 500 °C, 24 h	Nb2O5 99.71% Ta2O5 98.36%	[54]
Zimbabwe tin slag	In situ carbothermic reduction	Reducing agents: Collie coal, 1275 °C, 30 min	NbC 31.81% Nb2O5 48.11% TaC 12.49% Ta2O5 10.37%	[65]
Tin slag (Congo, Indonesia, Malaysia, South Korea, Spain, Thailand)	Acid-Base-Acid leaching	HCl 2 M HF 1.2 M NaOH 2 M 70 °C, 4 h, Solid/Liquid: 50 g/L	(Ta + Nb)2O5 85.6% (63.3%)	[57]
Malaysian tin slag	- Alkali pre-treatment (Caustic leaching, alkali pugging, caustic fusion) - Hydrochloric acid leaching	Alkali processing Slag/NaOH: 1/2, 250 °C, 4 h Acid Leaching HCl: 3.25 N, 50 °C, 15 min	Nb2O5 86.5% (10.6%) Ta2O5 95.5% (10.4%)	[56]
Indonesian tin slag	Roasting Water-quenching Leaching (HF/HCl and NaOH)	Roasting 900 °C, 2 h Leaching HF 8%, 2 h HCl 6 M, 2 h NaOH 10 M, 20 h	Nb2O5 91% (1.34%) Ta2O5 87.5% (0.66%)	[67]
	Roasting Water-quenching Successive leaching (NaOH and HClO4)	−200 + 250 mesh Solid/Liquid: 1/10 Roasting 900 °C, 2 h Leaching NaOH 8 M, 2 h, 25 °C	Ce (4.35%) La (1.45%) TiO2 (3.23%) Sn (6.49%)	[63]
	Roasting Water-quenching Alkali and acid leaching	Roasting 800 °C, 1 h −100 + 150 mesh Leaching Solid/Liquid: 1/10 NaOH 5 M, 20 min HCl 5 M, 50 min 25 °C	Nb2O5 (1.11%) Ta2O5 (1.56%) TiO2 (21%)	[62]

, several studies [52,53,54] showed that pyrometallurgical methods, such as chlorination and carbochlorination, were mostly adapted for the recovery of Nb and Ta. For the hydrometallurgical methods, successive acid and alkali leaching were studied [55,56,57,67]. Conventional methods of extracting Nb and Ta from tin slags are hydrofluoric acid leaching or successive leaching of hydrofluoric and sulphuric acids, followed by carbochlorination. So, the most suitable process in the past would have been a combination of pyro- and hydrometallurgical methods. However, they are less utilised due to economic and environmental issues these days [53]. In June 2019, the latest study by [57], showing the best outcomes of grade and recovery of Nb and Ta, developed the acid–alkali–acid leaching process presented in Figure 1. Although a greater recovery of Nb and Ta is desirable, the grade of the product is outstanding compared to pyrometallurgical methods such as carbochlorination and roasting.

Tin slags also contain other critical and strategic metals, such as Ti and REEs, in addition to Sn. Since current studies mostly focus on recovering Nb and Ta from tin slags, a knowledge gap exists in the recovery of other critical and strategic metals. Hence, exploring a suitable process to extract other critical and strategic metals from tin slags is a beneficial and desirable provision for the mineral industry. Other research gaps and current drawbacks of utilising tin slags include the following: (1) low grade and recovery of critical and strategic metals; (2) separation of valuable metals that have chemical similarities; and (3) silica gelation. Since the latest study [57] was able to produce a commercial grade product (63.3%), the research is progressing with technology. It will be favourable to examine green reagents, for instance, organic acids.

5.2. Copper Slag

Copper slag is a by-product of copper production. As much as copper mines have been active around the world, an enormous amount of copper slag has been accumulated in dumps. Copper slag is known as a suitable material for construction, geotechnical applications, road pavements, and so on due to its physical and chemical properties [68,69]. However, this review attempts to highlight the potential economic benefits of copper slag as a great secondary source of valuable metals since copper slag contains a significant amount of critical and strategic metals, as shown in

Table 5. Compositions (%) of critical and strategic elements in copper slags (F: Furnace slag, C: Converter slag, A: Ancient slag).

Origin (Region)	Co	Ni	Al2O3 (Al)	MgO (Mg)	MnO (Mn)	TiO2 (Ti)	V	Cr	Mo	Sr	Y	Sb	Sn	References
Germany	0.013	0.012	(7.27)	(3)	(0.26)	(0.42)	0.16	0.026	0.021	0.03	33 ppm	-	-	F, [71]
Turkey	0.023	-	-	-	-	-	-	-	-	-	-	0.021	-	[72,73]
	0.51	35 ppm	-	-	-	-	-	-	-	-	-	-	-	A, [74,75]
	0.095	0.065	-	-	-	-	-	-	-	-	-	-	-	F, C, [76]
	0.104	0.015	2.4	2.82	(0.042)	-	-	-	-	-	-	-	-	F, [77]
	0.36	0.045	-	-	-	-	-	-	-	-	-	-	-	C, [78]
	0.43	0.035	-	-	-	-	-	-	-	-	-	-	-	[79]
Poland	0.145	-	5.73	4.99	-	-	0.123	-	0.077	-	0.026 (ƩREE)	-	-	F, [80,81]
	0.063	-	12.45	6.48	-	-	0.012	-	0.0296	-	0.031 (ƩREE)	-	-	F, [80,81]
	-	-	14.7	2.42	0.35	0.69	-	-	-	-	-	-	-	A, [81,82]
	5.8	0.4	-	-	-	-	-	-	-	-	-	-	-	C, [83]
China	0.16	0.53	(3.90)	(3.53)	-	-	-	-	-	-	-	-	-	[5]
	0.49	0.27	1.36	1.34	-	-	-	-	-	-	-	-	-	C, [84]
	0.60	0.23	-	-	-	-	-	-	-	-	-	-	-	C, [85]
	4.09	0.043	(3.22)	(2.15)	(0.14)	(0.19)	-	0.075	0.01	0.012	-	-	0.14	[86]
Congo	0.72	-	(2.56)	(2.53)	-	-	-	-	-	-	-	-	-	[87]
	1.7	-	6.0	5.6	-	-	-	-	-	-	-	-	-	F, [88]
	0.789	79 ppm	8.12	3.36	0.074	0.53	-	0.185	0.017	-	-	8.61 ppm	19 ppm	[89]
Namibia	0.051	73 ppm	5.22	3.09	0.2	0.22	-	-	-	-	-	0.073	-	[90]
South Africa	0.032	0.024	6.61	4.52	0.13	0.78	0.03	0.11	68 ppm	0.023	0.04 (ƩREE)	-	15 ppm	[91]
Finland	0.04	0.05	3.8	1.0	(0.07)	-	-	-	0.07	-	-	0.04	0.1	F, C, [92]
Chile	-	-	(0.31)	-	-	-	-	-	0.3	-	-	-	-	[93]
	-	-	25.75	1.15										F, [94]
	-	-	2.72	0.83	-	-	-	-	-	-	-	-	-	F, C, [95]
India	0.48	1.98	(0.08)	(2.65)	-	-	-	-	-	-	-	-	-	C, [96,97]
	0.19	0.23	(1.58)	(0.97)	-	-	-	-	-	-	-	-	-	F, [98]
Italy	-	-	4.66	0.89	1.57	0.19	-	-	-	-	-	-	-	A, [99]
N/A	0.1	0.027	-	1.035	-	-	-	0.024	-	-	-	0.043	-	F, C, [100]
	0.44	<0.1	(0.83)	(0.38)	-	(10 ppm)	-	0.16	<0.1	-	-	50 ppm	<0.5	C, [101]

[70].

There are two forms of copper slag, which vary based on their cooling processes. Air-cooled slag and water-quenched slag show different properties. Water-quenched slag has a more granular form, like sand. Thus, it can be suitable for use in the construction industry. Air-cooled slag is slow-cooled and can be suitable for extracting valuable metals since it may contain various forms of minerals due to crystallisation with enough time [6,68,69]. However, slow-cooled slags tend to have more dense structures than fast-cooled slags, so it is sometimes harder to liberate valuable metals from slow-cooled slags.

Every ton of copper production generates 2.2 tonnes of copper slag [8]. Total reserves of copper in the world are estimated at 1 billion tonnes in 2024, and the global mine production of copper in 2023 was 22 million tonnes [102], which would have generated 44 million tonnes of slag. The largest production occurs in Chile, which was 5 million tonnes in 2023. Chile is followed by Peru, Congo, China, the USA, Russia, Indonesia, Australia, Zambia, Mexico, Kazakhstan, Canada, Poland, and others in terms of copper mine production [102]. As is shown in the active production of copper, a significant amount of copper slag keeps being dumped worldwide. It would be beneficial to recover valuable metals from copper slag for those countries that have significant amounts of dumped slag and will constantly generate slag in the future.

The amount of copper slag that is dumped in known and unknown places would be immeasurable. For example, around 2.5 million tonnes of ancient Turkish copper slag from the 13th century have been stored in the northern part of Turkey. The ancient slag also was adopted to develop the process by researchers. Based on the chemical analysis, this slag has 0.5% of cobalt, which can produce around 12,500 tonnes of cobalt [74,75], which is equivalent to 337 million US dollars (US $27,000 per tonne in June 2024) [103]. There is a literature [104] that reports a pilot-test work performed on a copper reverberatory furnace slag in Zambia and the production of cobalt from the copper slag. Around 20 million tonnes of dumped slag that contained 0.66% cobalt were processed through the furnace, which commenced in 2001.

Methods to recover critical and strategic metals from copper slags are presented in

Table 6. Extraction method for critical and strategic metals from copper slags.

Samples	Methods	Optimum Conditions	Recovery (Grade)	References
Turkish Copper Slag	Sulphurisation Roasting Water leaching	−100 mesh (150 µm) Sulphurisation: Closed system, gas mixture; 6 g H2S + 25 g H2O, 130 °C, 1 h, solid-gas interaction, Roasting: 600 °C, 6 h, Water leaching	Co 98.8%	[106]
	Roasting Water leaching	−100 mesh (150 µm), Roasting: 200 °C, Sulphating agent; 1.5 stoichiometric amount H2SO4, 1 h, muffle furnace, Water leaching	Co 96%	[107]
	Pre-roasting Roasting Water leaching	−63 µm pyrite, various for slag Pre-roasting: 550 °C, 1 h. Roasting: 550 °C, 3 h, pyrite/slag 0.25 Water leaching: 15 min, ambient temperature, pulp density 10%	Co 98.4% Ni 22.2%	[108]
	Roasting Leaching (Microwave acid digestion)	−150 µm, Roasting: Slag: Pyrite: Copper concentrate 3:6:6 (220 g), iron powder 2.2 g, 600 °C, 5 h, closed medium Leaching (Microwave acid digestion): 0.2 g dried sample, 1450 W, 37% HCl 2.5 mL + 65% HNO3 7.5 mL	Co 98.4%	[73]
	Acid roasting Leaching	−0.1 mm, Acid roasting: H2SO4, 2 h, 150 °C, acid/slag 3:1 Leaching: Hot water, 70 °C, 1 h	Co 87%	[76]
	Flotation Leaching	−150 + 300 mesh Flotation: Collector; X−231 100 g/t, Na2S 300 g/t Leaching: HCl 150 g/t, 2 h, 80 °C, S/L 1/10	Flotation tail Sn 73% (2.58%)	[109]
Turkish ancient copper slag	Flotation Roasting Leaching	−0.1 mm, Flotation: Na2S 300 g/t, Aero 211 200 g/t, pH 7~7.2, Pulp density 15%, 1300 rpm Roasting: 500 °C, 1 h, pyrite: slag 3:1, Leaching: H2SO4 10 g/L	Co 86.5%	[74]
	Leaching	H2SO4 120 g/L, 2 h, 60 °C	Co 90%	[75]
Indian copper slag	Roasting Leaching Precipitation	−75 µm, Roasting (1st and 2nd): 1 stoichiometric amount of H2SO4, 150 °C, 1 h/650 °C, Leaching with water Precipitation (iron removal): NH3 or CaO, pH 2.5–3	Co 95% Ni 95%	[96]
	Pressure leaching	Pulp density 10%, H2SO4 0.35 N, 130 °C, 0.59 MPa, 4 h, 840 rpm	Co >95% Ni >95%	[97]
	Reductive roasting Leaching	Reductive roasting: 10% furnace oil, 850 °C, 2 h Leaching: 1.25 stoichiometric amount of FeCl3, 2 h	Co 80% Ni 95%	[98,110]
Chinese copper slag	Reductive—sulfidising reduction Magnetic separation Leaching	Reductive—sulfidising reduction: Product—Cu-Co-Fe matte (Start material), Magnetic separation: 40 mT, Feed rate 100 mL/min Leaching: H2SO4 1.15 stoichiometric, 1 h, 80 °C, 500 rpm, 10% (w/v)	Co 95.57%	[111]
	Reduction-roasting	Reduction-roasting: MoSi2 electric furnace (12 kW), 1350 °C, 1 h, Reducing agent; coke powder 7%, Curing agent; pyrite 30%	Co 95%	[84]
	Reduction smelting	Reduction smelting: 1350 °C, 2.5 h, reducing agent; activated carbon 5%, Modifiers; TiO2 5%, CaO 4%	Co 94.02% (1.76%)	[85]
	Leaching Precipitation	−180 µm, Leaching (20g slag): H2SO4 17 g, oxidant; NaClO3 2.5 g, 95 °C, 3 h Neutralisation (Precipitation): Ca (OH)2 4.3 g, pH 2, 1 h	Co 98%	[86]
German Copper slag	Sorting Roasting Leaching Precipitation Complexation Solvent extraction	−100 µm Sorting: Crystalline: Vitreous 3:1 Leaching: 37% HCl, 70 °C, 1 h, pH 2.5 (start), Solid/liquid 100 g/L Precipitation: NH4HCO3, pH 7 Precipitate complexation: NH4OH + H2O2, pH 10 Solvent extraction: H2SO4	Various products	[71]
Congolese copper slag	Oxidising leaching Solvent extraction Precipitation Solvent extraction	−100 µm Oxidising leaching: 500 kg/t H2SO4 + 62.5 L/t H2O2, 70 °C, pH 2.5, 2 h, S/L 10% Solvent extraction: Copper—12% v/v LIX 984 in Kerosene, pH 2.5, A/O 1:1 Precipitation: Iron—300 g/L CaCO3, pH 3.5, 85 °C Solvent extraction: Zinc and Cobalt—20% v/v D2EPHA in kerosene, 25 °C, pH 3.5, A/O 1:1 Cobalt stripping—H2SO4, 25 °C, pH 2.5, Zinc stripping—H2SO4, 25 °C, pH 1	Co 90%	[87]
Polish copper slag	Reduction roasting Electrolytic dissolution Ammoniacal leaching Electrowinning	Reduction roasting: electric furnace, 80–90 V, 900–950 A, fluxing agents; limestone 8.6% + dolomite 8.6%, reducing agents; coke breeze 2.7% + pig iron 2.0%, 1425–1570 °C Electrolytic dissolution: ammonia-ammonium chloride electrolyte, NH3 + NH4+ 2.5 M, Cl− 1 M, pH 10.1, circulation rate 15 dm3/h, 50 °C, anodic current density 350 A/m2 Ammoniacal leaching (for anode slime from previous stage): NH3 + NH4Cl Electrowinning: copper—pH 0, cobalt—pH 6	Co (92%)	[83]
Chilean copper slag	Reduction (Slag cleaning) Water quenching Drying Milling Reduction (Removal of carbon contamination)	Reduction (Slag cleaning): 800 g power mixture; SiO2, CaO, Al2O3 and MgO, Induction furnace (10 kHz, 15 kW), Reductant; graphite, Pure argon atmosphere, Modifier; TiO2 11 wt% Water quenching Drying: Muffle furnace, 100 °C Milling: −75 µm Reduction (Removal of carbon contamination): Muffle furnace, 8 h, 700 °C	Recovery Co/Recovery Fe ratio 1.5	[112]
Copper converter slag	Curing Leaching Cementation Precipitation	−75 µm, Curing: H2SO4, 2 h, 85 °C Leaching: 1.2 times of stoichiometric amount H2SO4, 90 min, 75 °C Cementation (Cobalt and Zinc): Cementation reagent; Manganese, Mn/Co molar ratio 7, initial pH 3, 45 min Precipitation (Cobalt sulphide): initial pH 4, 90 min, 100 °C, Precipitate reagent; Barium sulphide, BaS/Co molar ratio 3	Co 98%	[101]

. The main valuable metals are Cu, Co, and Ni. The extraction of base metals from copper slag was excluded since base metals’ criticality is lower than that of critical and strategic metals [43,105]. To recover Co from copper slags, roasting and leaching were adopted, along with sulphurisation, flotation, precipitation, and so on. More details on the generation, characteristics, utilisation, environmental impacts, and recycling technology of copper slag, regardless of the critical and strategic elements, can be referred to in [8,9,12].

Several attempts have been made to recover Co from copper slag, mainly by hydrometallurgy approaches. They are showing remarkable results, considering that traditional methods mostly include roasting or other pyrometallurgical methods. After leaching, researchers also adapted separation and purification methods, such as solvent extraction and precipitation. Figure 2 is the process flowsheet to recover Co selectively using leaching with sulphuric acid and sodium chlorate as an oxidant and precipitation with calcium hydroxide, and it produced 98% recovery of Co, which is an outstanding result considering it was conducted under atmospheric pressure [86].

Another feasible process is shown in Figure 3. Banza and colleagues investigated a combined process of hydrometallurgical methods to extract Co, Cu, and Zn. Leaching was conducted in a sulphuric acid medium with hydrogen peroxide as an oxidant. Then, Fe was separated through precipitation, and Cu was extracted by solvent extraction. Finally, Co and Zn were recovered from the solution by solvent extraction with D2EPHA, and they were stripped separately. The overall recovery of Co was 90%, which indicates it is promising to take into consideration for adapting hydrometallurgical methods to upgrade the copper slag for the extraction of critical and strategic metals [87].

A study was also conducted to recover Sn from copper slag. As shown in Table 2, Sn is one of the critical and strategic metals in copper slag. The process includes size reduction and classification, flotation, and leaching. In flotation, Cu was recovered as concentrate and Sn remained in tailing. In the leaching test, the recovery of Sn was not as efficient as that of Cu since the process was focused on Cu extraction. Consequently, there is still a research gap in the recovery of Sn from copper slag [109]. Including the processes that were mentioned above, hydrometallurgical methods are listed in Table 6 to recover critical and strategic metals from copper slag [97,98,101,113].

As one of the hydrometallurgical methods for extracting critical and strategic elements from copper slag, bioleaching has been studied for the past decades. Bioleaching is considered an environmentally friendly and sustainable process and is a gradually evolving technology [114]. In bioleaching, microorganisms play a key role as extractants [115]. A recent study by Mikoda et al. (2019) studied the feasibility of bioleaching assisted by Acidithiobacillus thiooxidans. The optimal conditions for this study were a particle size of 0.25–0.5 mm and a pulp density of 1% for 28 days. With these conditions, high recoveries of Co, Mo, REEs, and V were achieved: shaft furnace slag—Co 100%, Mo 44%, REE 70%, and V 70%; and granulated slag—Co 95%, Mo 70%, REE 99%, and V 93% [80]. Another study from the same research group suspected that the bioleaching of historical copper slag could create up to $135 worth of metals from a ton of slag. Hence, extracting values from historical copper slag can be an economically and environmentally positive option [116]. A broader perspective has been implemented by Lee and Pandey (2012), who reviewed the bioprocessing of solid wastes, including copper slag, for metal extraction [10]. Using a mixed culture that contains Acidithiobacillus and Leptospirillum with 10 g/L sulphur (S) and 4.4 g/L ferrous sulphate (Fe²⁺) at a pulp density of 10 g/L and a pH of 1.5, in 42 days, 100% recovery of Ni was achieved [117]. Mehta and Pandey [118] recovered 64% of cobalt and 50% of nickel from copper converter slag in 80 days with the optimum conditions of pH 2, pulp density 1/20, and particle size −75 µm using Thiobacillus ferrooxidans. Despite significant research efforts aimed at developing bioleaching processes, most of the experiments have been limited to the laboratory scale [10]. Since the new extraction processes should minimise the impacts on the environment and pursue economic profit, bioleaching is considered a green technology for the mining industry.

Pyrometallurgical methods tend to be used frequently owing to effective sulphurisation or reduction. Thus, there have been various implementations of pyrometallurgical methods on copper slag. A recent study by Yuksel and Teğin [73] reported a huge improvement in pyrometallurgy by reducing the temperature for roasting, which saves energy as well as the processing budget. A total of 98.4% of Co from copper slag was recovered by roasting at 600 °C for 5 h with a 3:6:6 ratio of slag: pyrite: copper concentrate and iron powder [73], whereas other studies [84,85] required 1350 °C to extract around 95% Co. Depending on the methods, Co remains in the depleted slag after roasting [84], or a cobalt-bearing alloy can be produced after reduction [85]. In these cases, recoveries of Co were 95% and 94.02%, respectively. There is also a study to identify the most suitable modifier to selectively recover Co from copper slag when the reductant is graphite. It turned out to be more titanium dioxide than calcium oxide or calcium fluoride [112].

One of the greatest achievements of exploiting slag as a secondary source is the pilot test work that was later conducted at a plant in Zambia in 2001. A 40 MW DC arc furnace was built to extract cobalt from the 20 Mt reverberatory furnace slag dump. The feed was specifically blended with slag, lime, and coal, and the tapping temperature on average was 1500 °C [104]. This example indicates that it is economical and feasible to extract valuable metals from slags, along with providing a chance to clean slag if required.

Various flowsheets combining the pyrometallurgical and hydrometallurgical methods have been widely explored to extract critical and strategic metals from copper slag. A few cases of studies in pyrometallurgy required water leaching after roasting, so they were categorised as combined methods [5,106,107,108]. A novel process was developed by Yin, Xing [111] that includes smelting, magnetic separation, and leaching with sulphuric acid. Through reductive-sulphurising smelting, a Cu–Co–Fe matte was produced and then processed under a magnetic separator. Magnetic concentrate possessed cobalt-bearing phases, which include Co–Fe alloy, and non-magnetic substances had copper-bearing phases. Subsequently, sulphuric acid leaching at 80 °C for 1 h was conducted for magnetic substances. The overall recovery of Co was 95.57% after this series of processes [111]. A research group in India investigated various methods to extract Co and Ni from copper slag: roasting with acid, water leaching and precipitation [96], pressure leaching [97], reductive-roasting, and leaching with ferric chloride [110]. The conditions of these methods are listed in Table 6. These methods showed remarkable results, with recoveries exceeding 95% of Co and Ni. Several attempts have also been made to recover Mo from copper slag by a sequence of selective reduction, roasting, and leaching with sulphuric acid [93] and by oxidative roasting followed by sulphuric acid leaching [119]. The former study achieved enrichment of Mo by 8.9 (final/initial grade: 2.67/0.3), and the latter recovered 80% of Mo. One of the most comprehensive flowsheets for processing copper slag is presented in Figure 4. It encompassed from reduction roasting to electrowinning and produced a 92% purity of Co [83]. It indicates that a combined flowsheet of hydro- and pyrometallurgical methods for extracting critical and strategic metals such as Co can produce high-purity concentrates. However, the feasibility, profitability, and viability of the process should still be assessed before applying to industry.

5.3. Nickel Slag

Nickel slag is a smelting waste from nickel production [120]. Slag is usually stored in dumps since they are considered as waste materials [121]. Furthermore, nickel slag contains critical and strategic metals, for example, Ni and Co, as shown in Table 7. One of the potential problems with dumped slags is that they may contain potentially toxic elements (PTEs). In case of rain, PTEs can be leached out from the slag and exposed to the environment. Therefore, the extraction of valuable metals from nickel slags is not only a great secondary source of critical and strategic metals but is also able to make the slag cleaner for the environment.

Nickel slag can be categorised based on its production stages and cooling methods: (1) furnace or converter slag and (2) slow-cooled or fast-cooled slag. They behave differently in a chemical reaction and have dissimilar crystal structures. In the pyrometallurgical process, the slag from the smelting phase is called furnace or smelter slag, and the other slag from the converting phase is distinguished as converter slag. Usually, converter slag contains a higher concentration of valuable metals than furnace slag since converting deals with a higher grade of product. For example, Ni in converter slag is 2.87–4.80%; however, the primary ores contain 1.05–2.3% of Ni [122]. For this reason, converter slag is normally going back to the circuit for processing.

Due to cooling time and methods, slow-cooled and fast-cooled slags show different crystallisations. Slow-cooled slag contains well-developed crystalline, fayalite, and minor spinel. On the other hand, fast-cooled slags are more homogeneous in terms of metal distribution and contain amorphous iron silicate glass [7].

Nickel slag contains critical and strategic metals such as Ni, Co, Ti, Al, Mg, Mn, V, and Cr, as shown in Table 7. Nickel slag also has base metals, for example, Fe, Cu, Si, etc.; however, they were excluded from Table 7 because they are regarded as less critical due to their stable supply. In recent years, there has been an increasing interest and attempt to recover Ni and Co from nickel slag in the industry. Hence, this review mostly focuses on the extraction of Ni and Co from nickel slag based on the publications.

Table 7. Compositions (%) of critical and strategic elements in nickel slags (SC: Slow-cooled slag, FC: Fast-cooled slag, C: Converter slag, F: Furnace slag).

Origin (Region)	Ni	Co	Ti	Al	Mg	Mn	V	Cr	References
Brazil	0.08	73 ppm	0.04	1.99	17.2	0.29	85 ppm	0.76	[120]
	0.38	0.02	-	0.68	5.76	-	-	1.11	[123]
	1.05	0.243	-	1.66	0.8	0.05	-	-	SC, [122,124]
China	1.48	0.6	-	-	-	-	-	-	C, [125]
	0.9	0.12	-	1.4	5.83	-	-	-	F, [126]
Canada	0.28	0.13	0.14	3.63	1.87	0.04	-	-	F, [127]
	0.66	0.21	0.16	2.62	1.54	0.04	-	-	SC, [7]
	0.31	0.14	0.1	3.73	2.0	0.03	-	-	FC, [7]
	0.19	0.11	0.21	3.97	2.1	0.05	-	-	SC, [7]
	0.05	0.1	0.02	0.78	0.49	0.02	-	-	FC, [7]
Poland	0.55	0.05	0.64 (TiO2)	11.47 (Al2O3)	2.76 (MgO)	0.26 (MnO)	-	-	[128]
	0.02	77 ppm	0.53 (TiO2)	16.4 (Al2O3)	5.52 (MgO)	1.16 (MnO)	-	-	[128]
	82 ppm	5 ppm	0.09 (TiO2)	2.79 (Al2O3)	10.06 (MgO)	0.22 (MnO)	-	-	[128]
N/A	3.64	1.05	-	1.63	0.29	-	-	-	C, [129]
	0.27	0.11	-	3.84	3.81	-	-	-	F, [129]
	0.32	0.16	0.12	2.86	2.57	0.05	0.01	0.05	[121]
	0.16	0.05	0.13	3.64	1.22	0.03	0.01	0.05	[121]
	0.12	0.02	-	6.92 (Al2O3)	8.49 (MgO)	0.34 (MnO)	-	-	F, [130]

Table 7. Compositions (%) of critical and strategic elements in nickel slags (SC: Slow-cooled slag, FC: Fast-cooled slag, C: Converter slag, F: Furnace slag).

Origin (Region)	Ni	Co	Ti	Al	Mg	Mn	V	Cr	References
Brazil	0.08	73 ppm	0.04	1.99	17.2	0.29	85 ppm	0.76	[120]
	0.38	0.02	-	0.68	5.76	-	-	1.11	[123]
	1.05	0.243	-	1.66	0.8	0.05	-	-	SC, [122,124]
China	1.48	0.6	-	-	-	-	-	-	C, [125]
	0.9	0.12	-	1.4	5.83	-	-	-	F, [126]
Canada	0.28	0.13	0.14	3.63	1.87	0.04	-	-	F, [127]
	0.66	0.21	0.16	2.62	1.54	0.04	-	-	SC, [7]
	0.31	0.14	0.1	3.73	2.0	0.03	-	-	FC, [7]
	0.19	0.11	0.21	3.97	2.1	0.05	-	-	SC, [7]
	0.05	0.1	0.02	0.78	0.49	0.02	-	-	FC, [7]
Poland	0.55	0.05	0.64 (TiO2)	11.47 (Al2O3)	2.76 (MgO)	0.26 (MnO)	-	-	[128]
	0.02	77 ppm	0.53 (TiO2)	16.4 (Al2O3)	5.52 (MgO)	1.16 (MnO)	-	-	[128]
	82 ppm	5 ppm	0.09 (TiO2)	2.79 (Al2O3)	10.06 (MgO)	0.22 (MnO)	-	-	[128]
N/A	3.64	1.05	-	1.63	0.29	-	-	-	C, [129]
	0.27	0.11	-	3.84	3.81	-	-	-	F, [129]
	0.32	0.16	0.12	2.86	2.57	0.05	0.01	0.05	[121]
	0.16	0.05	0.13	3.64	1.22	0.03	0.01	0.05	[121]
	0.12	0.02	-	6.92 (Al2O3)	8.49 (MgO)	0.34 (MnO)	-	-	F, [130]

The concentrations of Ni and Co in nickel slag are from 82 ppm to 3.64% and from 5 ppm to 1.05%, respectively. The chemical composition of critical and strategic metals in nickel slag from various references is shown in Table 7. The maximum Ni grade is 3.64%, from converter slag, which is higher than the primary ore’s grade. The highest grade of Co in nickel slag is 1.05%, from converter slag [129]. Nickel slag also contains a reasonable amount of Ti and Cr which implies that nickel slag can be a secondary source of these other valuable metals.

There have been several studies in the literature on nickel slag reporting leaching behaviour [120,121,127,129], selective recovery of valuable metals [122,125,126], characterisation, and mineralogy [7,124,128], and environmental effects [130]. There has been plenty of research on the characteristics of nickel slag in the past, and recently, research has been focusing on the extraction of valuable metals from nickel slag as well. However, there is a limited number of publications on nickel slag.

Ettler, Kvapil [120] investigated the leaching behaviour of metals in nickel slag. The optimum pH was 3 in the range between pH 3 and 12. In this range, chromium shows a U-shaped leaching pattern. Nickel, cobalt, and magnesium have a decreasing trend in concentration from pH 3 to 12, with a low concentration at around pH 12. However, this study focused on the exposure of potentially toxic elements (PTEs) to the environment rather than an extraction method of valuable metals from nickel slag [120]. It is seldom appropriate to apply the result for the recovery of valuable metals to this study since the leaching test was to evaluate the environmental implications when these metals were dissolved. Thus, further studies of leaching behaviour in the extraction process for production purposes are required for a better understanding of the leaching mechanism.

Extraction processes, including the pyrometallurgical methods, were also studied. Huang et al. [125] achieved more than 97% recovery for nickel and cobalt using leaching with 0.3 M sulphuric acid and 600 kPa oxygen partial pressure at 200 °C for 80 min. Pan et al. [126] conducted selective reduction and magnetic separation to recover nickel and copper and demonstrated when the basicity increases, the enrichment of nickel improves as well. As a result, the author recovered 82.2% of nickel with a 3.25% grade.

The optimum method for the recovery of critical and strategic metals from nickel slag is high-pressure and -temperature leaching. There have been attempts to dissolve the target metals at atmospheric pressure, but it was unsuccessful due to several drawbacks, such as slow oxidation of iron, slow filtration, and high acid consumption [121]. Thus, current studies are focusing on high-pressure oxidative leaching, which provides selectivity towards Ni and Co with more than 90% of the recovery.

The common parameters of high-pressure temperature oxidative leaching from various research groups are a choice of acid and temperature. Weak sulphuric acid is a by-product of nickel production; as a result, nickel production companies have a significant amount of acid stored. It is desirable to utilise sulphuric acid to process the slag. One of the reasons that weak acid should be applied is that under atmospheric pressure and temperature, above 0.05 M of sulphuric acid reduces selectivity towards Ni and Co in nickel slag [121]. Thus, employing high-pressure and -temperature leaching with a slightly increased acid concentration is consistent with the literature as shown in

Table 8. Extraction methods of critical and strategic metals from nickel slags.

Samples	Methods	Optimum Conditions	Recovery (Grade)	References
Chinese slag	High-pressure oxidative acid leaching	0.3 M H2SO4, 200 °C, 80 min, 600 kPa, −150 + 74 µm, Solid/liquid 1 g/7 mL	>97% Ni, Co	[125]
Brazilian slag	High-pressure oxidative acid leaching	20% Acid, 250° C, 2 h, 500 kPa, Solid/liquid 25%	>99% Ni, Co	[124]
	High-temperature pressure oxidative acid leaching	20% Acid, 250 °C, 2 h, 200–300 kPa, Solid/liquid 25%, Acid consumption 38.5 kg H2SO4/t slag Oxygen consumption 84 kg O2/t slag	>99% Ni, Co	[122]
Nickel slag	Oxidative pressure acid leaching	H2SO4, Acid/slag 0.3, 250 °C, 1 h, 250 kPa	>90% Ni, Co	[129]
Canadian slag	Aqueous sulphur dioxide leaching	Aqueous SO2 1 M SO2 (aq.), 200 mL/min, 70 min Leaching 900 mL 1 M SO2 (aq.) + 9 g slag, 600 rpm, −106 + 75 µm	Co 77% Ni 35%	[127]
Nickel slag	Sulphuric acid leaching	H2SO4 0.1 M, 20 h, Solid/liquid 1/10	Co 75% Ni 65%	[121]
Chinese slag	Selective reduction Magnetic separation	Selective Reduction 5% coal, 1200 °C, 20 min Magnetic Separation 131.34 kA/m, 75 µm	Ni 82.2% (3.25%)	[126]
Nickel slag	Magnetic separation	Hand magnet (Low-intensity magnetic separator), −8 + 4.7 mm	Ni 65.92% (1.31%)	[130]

. The temperature of the leaching experiment is mostly 250 °C as shown in Table 8. A high temperature, especially 250 °C, ensures that iron and aluminium do not dissolve to leach the solution at this temperature [7]. With high-pressure and -temperature leaching, an over 90% recovery of Ni and Co could be extracted in several publications which shows a promising result of the hydrometallurgical method.

For further development of a feasible process, removing impurities such as Fe, Cu, Si, etc., in other words, improving the grade of Ni and Co, will be one of the major difficulties since target metals in starting materials are low grade. Based on the results of magnetic separation, the combination of high-pressure oxidative leaching and magnetic separation may be an option; however, there is no literature on this combination. Another way to enhance the selectivity of valuable metals can be complexation leaching, which employs a complexing agent in leaching.

Another challenge for the recovery of Ni and Co in nickel slag will be the separation of those metals since they exist together in the leach solution. So far, solvent extraction seems the optimum way to separate Ni and Co using Cyanex as an extractant [131,132,133]. Since Cyanex 272 is effective in extracting cobalt from nickel, Mubarok and Hanif (2016) [131] used an artificial solution containing only Co and Ni without impurities to separate them. As a result, the authors extracted 99.08% of Co with 10.16% of Ni at pH 5, Cyanex 272 concentration 20% (v/v), and at room temperature. However, from [132,133], it was confirmed again that Cyanex 272 is the optimal extractant and the most commercially used in the industry for the separation of Co and Ni. Further studies on separation focusing on the leach solution from nickel slag are required.

5.4. Vanadium Slag

This chapter discusses the recovery of critical and strategic metals from vanadium slag, including the chemical compositions of vanadium slags, extraction methods, and so on. Notably, most of the literature and sources of vanadium slag are from China. Several studies on the extraction of critical and strategic metals from vanadium slag have increased since the 2010s, which shows the potential of vanadium slag as a secondary source.

Vanadium is widely exploited in ferrous and non-ferrous alloys and is mostly consumed in the iron and steel industry due to its outstanding physical properties, such as fatigue resistance, hardness, and so on [13,134]. Vanadium can be found in vanadium–titanium magnetite, vanadium slag, vanadium stone coal, steel slag, spent catalyst, and fly ashes [135,136,137]. The main source of vanadium comes from vanadium titano-magnetite as a form of slag. Smelting the vanadium titano-magnetite ore enriches vanadium into solid slag, which is called vanadium slag [138]. Vanadium is mostly associated with ferrovanadium spinel (FeO·V₂O₃) as a main phase for both vanadium–titanium magnetite and vanadium slag [137]. Vanadium slag contains a significant amount of chromium, titanium, and vanadium.

Table 9. Compositions (%) of critical and strategic elements in vanadium slags.

Origin (Region)	V2O5 (V)	Cr2O3 (Cr)	TiO2 (Ti)	Al2O3 (Al)	MnO (Mn)	MgO (Mg)	References
China (Sichuan)	(6.85)	(5.66)	(5.94)	-	(4.81)	(2.10)	[139]
	(8.57)	-	(8.08)	-	(7.05)	(4.06)	[135]
	16.17	1.18	11.97	3.85	8.43	1.95	[140]
	14.3	4.4	7.4	2.1	8.5	3.7	[136]
	8.55	2.29	10.98	-	8.45	3.07	[141]
(Chengde)	13.72	9.19	10.45	1.30	6.73	1.17	[142]
(Liaoning)	(3.3~5.2)	(3.4~5.7)	-	-	-	-	[143]
(Panzhihua)	(7.15)	(2.19)	(6.86)	(1.55)	(5.92)	(1.38)	[144]
	(8.15)	(2.77)	(7.75)	(1.79)	(6.91)	(2.73)	[145]
(Hebei)	10.20	4.15	11.03	2.42	5.22	1.60	[146]
	14.42	9.45	11.19	-	7.03	-	[147]
China	12.22	5.14	10.82	2.22	5.72	2.95	[148]
	15.44	-	11.45	-	6.67	3.53	[149]
	14.08	-	10.63	1.67	6.50	3.25	[150]
	(12.0)	(6.48)	(0.02)	(0.04)	-	-	[151]
N/A	10.45	3.54	9.75	1.1	5.22	2.15	[152]

shows the chemical compositions of vanadium slags from various origins in China.

Countries that possess vanadium in the world are Australia, Russia, China, South Africa, the USA, and others [13]. In 2023, China mined 68,000 tonnes, while Russia, South Africa, and Brazil produced 35,500 tonnes all together: 20,000 tonnes, 9100 tonnes and 6400 tonnes, respectively. These countries combined have produced approximately 103,500 tonnes of vanadium in 2023, which makes up the world’s total production [102,153].

A traditional method to extract V from vanadium slag is sodium salt roasting and water leaching. However, this process can recover only 65 to 85% of the V during leaching due to its poor transfer efficiency [146]. Other well-known methods are calcification roasting, acid leaching, and direct alkaline leaching. Calcification roasting was introduced due to limited access to the sodium slag in Russia, while direct alkaline leaching was suggested to overcome the disadvantages of pyrometallurgical processes [137]. Thus, as can be seen in

Table 10. Extraction methods of critical and strategic metals from vanadium slags.

Samples	Methods	Optimum Conditions	Recovery (Grade)	References
China (Sichuan)	Sulphating roasting Reduction leaching Selective oxidisation Precipitation	Sulphating roasting: H2SO4 40 wt%, 160 °C, 2 h Reduction leaching: Na2SO3 2%, S/L 1 g/4 mL, 95 °C, 1 h Selective oxidisation: oxidant; CrO3 6 g/100 mL, (NH4)2SO4 16 g/L, pH 2.0, 95 °C, 2 h - Filtration; 1.5 H, 105 °C - Calcination for vanadium; 550 °C, 2.5 h Precipitation: pH 3.2~3.8 by Na2CO3 (Fe removal), pH 8.5 by NaOH at 90 °C (Cr)	V2O5 (99.1%) Cr2O3 (98.9%)	[138]
	Electro-oxidation leaching	−200 mesh, H2SO4 40 wt%, S/L 250 g/L, 75 °C, 4 h, anode current 0.4 A, operating potential 2.8~3.0 V, electrode spacing 20 mm, M (V slag:MnSO4) 2.5:1	V 75.64%	[135]
	Microwave roasting Leaching	−75 µm, Microwave roasting: 2450 MHz, 350 °C, 1 h Leaching: H2SO4 250 g/L, 1 h, 90 °C, 350 rpm	V 94%	[140]
	Calcification roasting Acid leaching	−75 µm, Calcification roasting: M (V slag:CaO) 16:1, muffle furnace, 850 °C, 2 h Acid leaching: H2SO4 15%, 55 °C, S/L 1:10, 150 rpm	V 93%	[141]
	Direct roasting Soda leaching	−75 µm, Direct roasting: Na2CO3 160 g/L, 850 °C, 1 h Soda leaching: 95 °C, 150 min, S/L 1:10	V > 90%	[154]
China (Liaoning)	Salt roasting Water leaching Solvent extraction Precipitation Roasting	Roasting: muffle furnace, sodium salt Water leaching Solvent extraction: LK-N21, mixing 15 min, 20 °C Precipitation: Ammonium salt Roasting: high temperature to produce vanadium pentoxide	V2O5 98.5% (99.5%)	[143]
China (Panzhihua)	Salt roasting Electro-oxidation leaching	Roasting: sodium salt Electro-oxidation leaching: current density 1000 A/m2, S/L 1 g/4 mL, 80 °C, 40 min	V 93.67%	[144]
	Salt roasting Water leaching	Roasting: NaCl 30% + Na2CO3 20%, 700 °C, 2 h Leaching: 95 °C, 3 h	V 96% Cr 91%	[145]
China (Hebei)	Leaching (Electrochemical decomposition)	−200 mesh, NaOH 40 wt%, 120 °C, slot current density 750 A/m2, M(alkali:ore) 4:1, 1000 rpm, 6 h	V 95% Cr 90%	[146]
	Roasting	−200 mesh, NaOH-NaNO3 binary melt reaction system, liquid/solid 4:1, base/salt 1:1, 400 °C, 6 h, O2 flow 0.5 L/min	V 93.7% Cr 88.2%	[155]
China	Roasting	NaOH-added pellet, 700 °C, R(Na/Cr) 7.67, 15 min	V 99.2%	[148]
	Calcification roasting Ammonium carbonate leaching	−74 µm, Calcification roasting: V/Ca molar ratio 1:1.1, muffle furnace, 900 °C, 2 h Ammonium carbonate leaching: (NH4)2CO3 600 g/L, 70 min, 80 °C, S/L 1:20	V 96%	[149]
	Roasting Leaching	−200 mesh, Roasting: No additives, 950 °C, 2.5 h Leaching: Na2CO3 160 g/L, 95 °C, 1 h, S/L 1/10 palacios	V 94.13%	[150]
	Salt roasting Leaching Solvent extraction Stripping	Roasting: Sodium salt Water leaching Solvent extraction: LK-N21 15 vol%, pH 5.0, 15 min Stripping: NaOH 0.1 M, 45 °C, 15 min	V > 90% Cr > 90% in raffinate	[151]
N/A	Roasting (Liquid oxidation)	−200 mesh, NaOH-NaNO3 binary molten salt medium, 375 °C, 1 h, 700 rpm, O2 flow 0.3 L/min, liquid (NaOH-NaNO3)/ore 4:1, M(NaOH/NaNO3) 2.2:1.8	V 90% Cr 80%	[156]
	Roasting (Liquid oxidation)	KOH 75 wt%, 240 °C, 1 h, liquid (KOH mass)/ore 4:1, 700 rpm, O2 flow 1 L/min	V 95% Cr 93%	[152]

, several attempts from predominantly China have been made to overcome the drawbacks and to eventually achieve a higher recovery of V as well as Cr with various methods, such as electrochemical decomposition [146], microwave roasting—leaching [140], and electro-oxidation leaching [135].

All the roasting methods above showed more than 90% recovery of V, and some of them included a minimum of 80% of Cr. The mixture of sodium chloride and sodium carbonate achieved especially 96% of V and 91% of Cr [145]. In the case of NaOH–NaNO₃ binary molten salt medium, the roasting temperature was dropped from 850 °C to 375~400 °C by the traditional method [156]. With potassium hydroxide, Li et al. (2012) tested three reaction media, such as sub-molten KOH, sub-molten NaOH, and NaOH–NaNO₃ binary molten salt medium. They also dropped the roasting temperature to 200~450 °C [152]. Yan et al. (2015) put no additives for roasting at 950 °C and subsequently conducted soda roasting, which still presented a 94.13% recovery of V [150]. These attempts indicate that it is promising to establish a novel roasting method without toxic emissions.

Even without sodium salt roasting, Wang et al. (2018) established a clean metallurgical process with combined methods of pyro- and hydrometallurgy and produced V with 99.1% purity and Cr with 98.9% purity [138]. Furthermore, there have been efforts to adapt modern technology to vanadium processing. For example, electrochemistry, such as electro-oxidation, has been introduced in leaching. This method can break the silicon layer and expose more V. Hence, there is a possibility to develop further and apply this method to the industry [135,144,146].

It is interesting to note the attempts to extract valuable metals from the residue of slag processing [157,158,159]. The residue of slag processing is called ‘V–Cr-bearing reduced (reducing) residue’, and is from wastewater after roasting and neutralisation of vanadium slag [157]. V–Cr-bearing reduced slag contains 5.1% V and 13.4% Cr [158]. The methods for extracting V from V–Cr-bearing reduced residue include selective oxidation and alkaline leaching [157] and a comprehensive approach: leaching with sulphuric acid, selective oxidation, and precipitation of vanadium and chromium [158]. These methods cover separation and purification as well, showing great recoveries of V and Cr of 98.7% and 99.4%, respectively [159]. This case shows the fast development of utilising secondary waste. Therefore, developing processes to recover critical and strategic metals from secondary and tertiary sources should continue regardless of market status or metal price to the material cycle as much as possible.

5.5. Titanium Slag

In this chapter, titanium slag will be discussed briefly. Its chemical compositions are listed in

Table 11. Compositions (%) of critical and strategic elements in titanium slag.

Sample (Origin)	TiO2	Ti2O3	Al2O3	MgO	MnO	V2O5	Zn	References
Ti-bearing slag (China)	22.58	-	13.04	6.93	-	-	-	[160]
	16.9	4.06	13.76	8.48	0.53	-	-	[161]
	17.58	3.86	14.08	7.86	-	0.21	-	[162]
	10.4	-	12.5	10.6	-	-	-	[163]
Ti-bearing slag (N/A)	38.2	-	10.4	9.7	0.75	0.92	-	[164]

, and its extraction methods are reviewed below.

Titanium slag is produced by a conventional method of titanium production. Ilmenite ore is upgraded to titanium slag and then to synthetic rutile [165]. Accordingly, many publications on titanium slag deal with high grades of titanium, ranging from around 70% to more than 90% as a main product [166,167,168]. Thus, ‘titanium-bearing blasting furnace slag’ (also called titanium-bearing slag) as a by-product from vanadium–titanium magnetite ore or titanomagnetite ore is more appropriate in this study [169].

The main difference between titanium slag and titanium-bearing slag is the grade of titanium. The latter shows a lower grade, less than 30%, of TiO₂ in the slags [14] and around 1% of V₂O₅, as shown in Table 11. Much literature focuses on the upgrade and extraction of Ti from titanium-bearing slag, such as the removal of impurities [164], the enrichment of anosovite by adding SiO₂ [160], and various oxidation conditions [161,170,171], as well as novel technology such as microwave treatment, super-gravity separation, and ultrasonic processes [14].

Even though Ti is considered one of the most abundant elements in the crust [165], it is critical and strategic for many applications. Thus, studies on the characteristics, mineralogy, and processing of titanium-bearing slag would be highly beneficial for waste management and process development. Another important critical and strategic element in the titanium-bearing slag is V, and Han et al. [162] presented the flowsheet of two-stage oxidation, alkali leaching and acid leaching, to extract Fe, V, and Ti.

6. Conclusions

This review attempted to present the potential of non-ferrous slag as a viable source of critical and strategic metals needed for high-technology and clean energy applications. Slag is considered waste and is mostly dumped in heaps after smelting. Non-ferrous slag materials such as tin slag, copper slag, nickel slag, vanadium slag, and titanium slag contain significant amounts of critical and strategic elements, making them potential secondary resources for such metals. Numerous studies have been conducted to extract critical and strategic metals from slag. However, most are still at the laboratory scale and are yet to be proven at pilot plant levels. In addition, their commercial viability is yet to be assessed for economic and environmental benefits. Technology development, as applied to resource recovery from slag, involves applications of mineral processing, hydrometallurgical, or pyrometallurgical techniques. As the importance of and the demand for critical and strategic metals continues to rise, it will soon be inevitable to consider extracting valuable metals in waste and by-products, such as slag, to augment the global demand for supplies. Extracting values from waste is categorised under technospheric mining, a concept that promotes a circular economy in the mining and resource industries. Until then, developing green and sustainable technologies for metal extraction and filling the knowledge gaps in the recovery of critical and strategic elements from non-ferrous slag should be actively pursued and promoted.