Implementation of Solvometallurgical Processing in the Recovery of Valuable Metals from a Sulfide Ore

Abstract

It has been demonstrated that the traditional hydrometallurgical method is still economically viable in several industrial applications such as Bayer, Boix, Platsol, Sherrit-Gordon, and so on. The conventional extraction technique of valuable metals from their ores using an aqua medium has several challenges. The following can be listed for the illustration of this: (1) Inorganic acids used during the leaching process have been proven to be non-environmentally friendly and ready to lead to non-selective processes in general, except in rare cases used in alkaline environments. (2) Special linings are required in the reactors used due to the corrosive impact of acids such as HCl and H₂SO_4, especially when leaching at high temperatures, rendering all processes costly. (3) Practically, using inorganic acids while leaching samples containing amorphous silicate phases leads to gel formation. Solvometallurgy overcomes these challenges by substituting the aqueous phase for other polar solvents, such as polar molecular organic or ionic solvents. The advantage of this substitution lies in the ability to manipulate metal ion distribution using solvents with varying solvation properties. This review examines the potential of solvometallurgical processes (solvoleaching) over conventional hydrometallurgy as viable alternatives for metal extraction from sulfide ores. It highlights the key distinctions between hydrometallurgy and solvometallurgy while emphasizing the potential economic and environmental advantages solvometallurgy offers.

1. Introduction

Sulfide minerals stand as a cornerstone of modern industry, serving as the primary source for a vast array of metals essential to our daily lives. Their economic significance cannot be overstated, positioning them as the planet’s most crucial group of ore minerals [1]. These minerals, compounds where sulfur combines with metals, are the origin of numerous elements, including those critical for advanced technologies and infrastructure. Among the most heavily processed sulfide minerals are Pyrite (FeS₂), Chalcopyrite (CuFeS₂), Sphalerite (ZnS), and Galena (PbS). The extensive processing of these minerals is driven by the global demand for the metals they contain. These minerals are extensively processed to extract refractory gold, copper, lead, and Zinc [2]. Two primary commercial methods exist for sulfide ore production: pyrometallurgy and hydrometallurgy.

Traditionally, pyrometallurgical processes, primarily roasting and smelting, have dominated the extraction of metals from these ores. This dominance stems from two key factors: the ease of sulfide ore concentration through flotation, which prepares the ore for efficient smelting, and the utilization of sulfur content within the ore as a fuel source during the smelting process [3]. However, the long-term sustainability of pyrometallurgy is increasingly challenged by environmental concerns and declining ore grades. The roasting and smelting of sulfide ores release substantial amounts of sulfur dioxide (SO₂), a significant air pollutant contributing to acid rain and respiratory problems. Stringent environmental regulations increasingly restrict SO₂ emissions, making pyrometallurgical operations more costly and complex [4]. In response to the limitations of pyrometallurgy, researchers shifted their focus to the hydrometallurgical processes that consisted of using aqueous media to dissolve valuable metals in the form of ions from their ores at relatively low temperatures [5]. These hydrometallurgical processes offer several advantages over pyrometallurgical processes, including that these processes are free SO₂ emission processes and involve lower energy consumption. They can be used to process lower-grade and more complex ores. However, hydrometallurgical processes are characterized by three primary stages: (1) leaching, where specific lixiviants, such as sulfuric acid or ammonia, dissolve the desired metal to generate pregnant solutions while selectively leaving unwanted elements behind, and, depending on the material and metal being treated, this step can be completed in several ways, including heap, dump, and agitated leaching under atmospheric conditions or under pressure (using autoclaves) using oxidized or reduced conditions [5,6,7]; (2) solution concentration and purification, including ion exchange, where resins are used to adsorb and purify the target metal, and solvent extraction, where immiscible organic solvents with aqueous phase are used to extract a particular metal ion from the aqueous solution selectively [5,6,7]; and (3) metal recovery, which is the final step, consisting of recovering metal from the purified solution in its solid form through electrowinning by using electrolysis to deposit pure metal at the cathode. It should be noted that during the hydrometallurgical processes, precipitation can be implemented to concentrate the metal or selectively remove undesirable components in the pregnant solution. Therefore, this can be achieved using chemical reagents to precipitate the metal as a solid compound. For instance, hydrogen can reduce metal ions in the solution and form pure metallic powders. In contrast, NaOH can be used to precipitate impurities from the pregnant solution [5,6,7].

Furthermore, the lower operating costs of hydrometallurgy make it an attractive process for low-grade ores and waste materials. In contrast, pyrometallurgy is reputed to be less efficient and less economically viable for low-grade ores. Hydrometallurgy thus plays a crucial role in modern extractive metallurgy, offering more sustainable and cost-effective metal extraction and production solutions [8]. However, several key issues can hinder its effectiveness, particularly when the concentration of valuable metals in the ore is low. The return on investment can be marginal or even negative, making the process economically unfeasible in some cases [9]. The costs include reagent consumption, energy input in the pre-treatment, such as roasting, and waste management. This challenge necessitates the development of more cost-effective hydrometallurgical techniques or green pre-treatment methods before leaching to enhance the metal dissolution. For instance, ferrooxidans can be applied to free the Au from a refractory sulfide mineral before cyanidation leaching [10].

Another significant hurdle arises during the leaching of sulfide minerals such as chalcopyrite, pentlandite, galena, sphalerite, etc., which are characterized by their refractoriness to reagents such as H₂SO₄, where passivation layers are formed on its surface. These layers often comprise solid sulfur and iron that precipitate like jarosite, preventing the leaching solution from accessing the metal ions within the mineral structure. This phenomenon negatively affects the dissolution rate and drastically reduces the overall metal recovery, particularly when using sulfate media [11]. Overcoming this passivation issue requires innovative approaches to promote the dissolution of values from sulfide minerals. However, pre-treatment can significantly enhance the recoveries of values from refractory sulfide ores [12]. Pre-treatment often involves thermal pre-treatment (roasting), pressure leaching, or bioleaching to decompose the host matrix, followed by metal extraction by leaching [12]. For the first thermal pre-treatment, the sample can be heated under oxygenated conditions at temperatures ranging from 300 to 1000 °C to generate an oxide mineral ready for direct acid leaching. The second requires using an autoclave and performing leaching using high pressures of oxygen gas (5–50 atm) at elevated temperatures close to 200 °C [12,13]. The third consists of bioleaching, where autotrophic bacteria are used to perform most of the bioleaching of sulfide minerals. For instance, bacteria such as Acidithiobacillus ferrooxidans, known as Thiobacillus ferrooxidans, are commonly used to leach sulfide minerals [12]. These mesophilic bacteria can optimally operate at temperatures ranging from 20 to 50 °C or (ideally 33–37 °C) at pH values between 1 and 2.5 [12]. Other leaching strategies to promote the dissolution of sulfide minerals require using alternative lixiviants or leaching in the presence of oxidizing, reducing, or complexing agents [12,13].

Furthermore, the leaching of complex minerals characterized by considerable amounts of silicate occurring as willemite or hemimorphite can lead to silica gel formation when leaching using H₂SO₄. This situation makes separating the sulfate solution containing the dissolved metal from the slurry difficult. The formation of silica gel during leaching can complicate downstream processing. This challenge was particularly observed during the dissolution of Cu from chrysocolla (CuSiO₃⋅2H₂O) by direct leaching with H₂SO_4, leading to silica gel formation [14]. Silica, a common gangue mineral in many ores, can dissolve and precipitate as a gelatinous substance during leaching, such as silica gel, and create severe problems. It can hinder the filtration of the pregnant leach solution, making it difficult to separate the metal-rich liquor from the solid residues [15]. Additionally, silica gel can contribute to the formation of “crud” in subsequent solvent extraction (SX) steps [16]. This complex mixture of organic and inorganic species interferes at the interface “solvent-aqueous phases”. The crud leads to an efficient extraction of metals from the leach solution to the organic phase, reducing the overall recovery and purity of the final product. Managing silica gel formation requires careful control of the leaching conditions, using silica depressants or implementing effective separation techniques [16]. Many researchers have proposed several approaches to avoid silica gel formation by performing microwave-assisted leaching or sulfuric acid pressure leaching, such as in the case of zinc silicate ore, which led to a non-gelatinous solution with an efficient Zn extraction rejecting silica while rendering good solid–liquid separation [15].

These drawbacks highlight the importance of ongoing research and innovation to address challenges and improve the efficiency and effectiveness of metal extraction processes. Solvometallurgy offers a promising alternative approach in such situations. This emerging field of extractive metallurgy utilizes non-aqueous or organic solvents for leaching and purification, which leads to new approaches to metal recovery [17]. Using organic solvents in solvometallurgy offers several key advantages over the conventional hydrometallurgical processes. However, despite noticeable differences in the leaching and/or separation steps, solvometallurgical processes rely on hydrometallurgical principles. This process can lead to an optimized flow sheet when it is combined with hydrometallurgy [18]. In solvometallurgy, the selection of solvents is judiciously tailored to dissolve the targeted metals selectively during the solvoleaching stage while leading to an efficient separation in the purification stages. This will enhance the efficient extraction of the targeted metal from the unwanted gangue materials and separate the targeted metal from the solvent containing the dissolved metal to the more polar organic phase [19]. Hydrometallurgical processes are widely used to process several complex and low-grade ores. However, they require large amounts of chemicals, leading to the generation of vast amounts of wastewater. In contrast to hydrometallurgy, solvometallurgical processes use non-aqueous solvent leaching (solvoleaching), non-aqueous solvent extraction, and non-aqueous electrodeposition to recover metals. Hence, to mitigate the hydrometallurgy drawback resulting in the production of large amounts of wastewater, solvometallurgy uses polar solvents such as ethylene glycol, methanol, or acetone instead, which leads to significant improvements in leaching and extraction efficiency [19]. Sun et al. (2022) revealed that ethylene glycol is an ideal organic solvent of choice mainly used in solvometallurgy due to its particular properties such as non-volatility, low toxicity, and low flammability. Ethylene glycol has typically been and is referred to as one of the greenest reagents. It is also referred to as an easily recyclable organic, making solvometallurgy one of the sustainable processes [19]. Some researchers have illustrated the effectiveness of using ethylene glycol in solvometallurgy as the polar solvent in the presence of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) as an oxidant, which leads to an effective recovery/separation of Cu from different resources [20]. It was demonstrated that using ethylene during the dissolution of Cu from chalcopyrite in non-aqueous solutions may lead to efficient Cu leaching since glycol in solvoleaching could decrease the interfacial tension due to a decrease in water activity and promote the formation of stable metal complex [21].

Furthermore, another advantage of solvometallurgy resides in treating particular ores containing soluble silicate, such as eudialyte. This type of mineral leads to the formation of silica gel when leaching using conventional hydrometallurgical methods. In solvoleaching, no silica gel will be observed when using acid-saturated neutral or basic extractants, chelating, halogens, or acidic extractants dissolved in either non-polar organic or acid-saturated polar solvents. The additive reagents percolate the mineral structure more efficiently than in hydrometallurgical processes, leading to the selective migration of target metal ions to the caring solvent such as LIX 63, D2EHPA, Cyanex 272, and so on, for Cu, Zn, and Co, respectively, depending on the target metal [14,22]. Performing a direct aqueous acid leaching on reach carbonate gangue material leads to high acid consumption and prevents efficient extraction of values from such gangue. However, the solvent leaching to such ore minerals leads to the selective dissolution of the target metal, leaving the carbonate gangue untouched and avoiding the co-dissolution of unwanted elements such as iron, which also contribute negatively to the consumption of acids. The solvoleaching can offer a higher selectivity than aqueous leaching using acids such as H₂SO₄, HNO₃, and HCl, mainly if it performs with chelating extractants such as the LIX family. The process was proven efficient with the extraction of uranium from low-grade uranium ores (U₃O₈ content of less than 1%) containing high carbonate minerals such as carnotite, autunite, torbernite and uranophane and calcite (CaCO₃) gangue material [14]. Furthermore, solvoleaching can also be extended to silicate ores because, in conventional hydro-leaching with acids, they are prompted to form silica gel, as mentioned above [14].

Other examples, such as those presented by Orefice and Binnemans (2021), revealed selectivity’s effectiveness when using non-aqueous solvents. For instance, using unscreened non-diluted D2EHPA, PC-88A, and Cyanex 923 at 165 °C to extract rare-earth elements over iron led to good selectivity. However, they pointed out that PC-88A and D2EHPA performed an excellent selective extraction of trivalent rare-earth ions over iron. At the same time, Cyanex 923 could lead to the extraction of 21% of iron together with 62% of neodymium (III) and 82% of dysprosium (III). However, PC-88A demonstrated high selectivity effectiveness since it led to a high separation factor α, between dissolved rare earth elements and ion (rare-earth elements)/iron of 241.1, while only α of 63.1 was achieved for D2EHPA and 21.2 for Cyanex 923. Therefore, solvent choice is critical to promote efficient selectivity [23]. According to Orefice and Binnemans (2021), most solvometallurgical processes can occur at low temperatures (<100 °C) in atmospheric leaching systems or at high temperatures (above 100 °C) in a controlled environment at high pressure, as in the case of hydrometallurgy [23].

This review aims to comprehensively analyze solvometallurgical processes as a compelling alternative to conventional hydrometallurgical techniques for both base and precious metal recovery from their ores. By examining its potential to enhance extraction efficiency, improve metal separations, and optimize underlying chemical mechanisms, this review seeks to demonstrate the viability and advantages of solvometallurgy within the broader landscape of extractive metallurgy over conventional extractive metallurgy. This exploration will delve into the specific applications and advancements in the field, ultimately contributing to a deeper understanding of its role in sustainable and efficient metal extraction.

2. Overview of Metals Extraction Methods

2.1. Hydrometallurgy

Hydrometallurgy is a branch of extractive metallurgy that involves using aqueous solutions to extract metals from ores. This process includes acidic or basic aqueous solutions for the dissolution of minerals. Unlike pyrometallurgy, which relies on high temperatures (≥300 °C) to extract metals, hydrometallurgy operates at temperatures ranging from ambient to less than 100 under atmospheric conditions and to a maximum of 200 °C when operating under pressure [24].

The foundation of modern hydrometallurgy was laid in the 1880s with the development of the Bayer and cyanidation processes. Various hydrometallurgical techniques such as heap, pressure, and in situ leaching are implemented to promote the recovery of values from their ores depending on metal occurrences [25].

The hydrometallurgical process consists of three primary stages: leaching, solution purification, and metal recovery. Leaching is the initial step where metals are dissolved from solid materials using chemical reagents. The choice of leaching agent depends on the metal’s nature and the ore’s composition [14]. Common types of leaching include acidic leaching, where sulfuric acid or hydrochloric acid is used to dissolve base metals such as copper, zinc, and nickel; alkaline leaching, which involves sodium hydroxide or ammonia solutions for extracting metals like aluminum and certain precious metals; and cyanide leaching, which is primarily used for gold and silver extraction, as cyanide solutions selectively dissolve these metals for recovery [26,27]. Solvent extraction is the next step after impurity removal during the hydrometallurgical processes of sulfide ores, which now contribute to more than 20% of the global metal supply, particularly in the case of copper. These processes are economically attractive due to their low capital and operational costs, ease of operation, and ability to produce high-purity electrolytic metals near the mine site [28]. Unlike traditional smelting methods, solvent extraction is viable across a broad range of production capacities, from small-scale operations to large-scale industrial applications, making it a practical and flexible solution for metal extraction. The separation and purification procedures through the solvent extraction (SX) rely on the differential distribution of metal ions between two immiscible phases: the solvent phase containing the metals to be separated (usually an aqueous or polar phase) and the extractant diluted in a diluent (typically an organic or apolar phase) where the metal must migrate. In aqueous systems, metal ions are frequently extensively hydrated, exhibiting strong hydrophilicity and insolubility in the organic phase. The coordination of metal ions enhances the transfer mechanism of metal ions between carrying phases characterized by the organic extractant resulting in the formation of hydrophobic complexes. Achieving excellent extraction efficiency and favorable separation factors is essential in solvent extraction. Batchu et al. [29] indicated that researchers should primarily concentrate on the composition of the organic phase to enhance separation and purification. Therefore, in the case of aqueous phases, these approaches include (1) the extractant structure’s modification to improve selectivity, (2) the addition of a second extractant to promote synergism (synergistic solvent extraction), (3) the addition of modifiers to prevent third phase formation and promote stripping, and (4) the change in diluents to improve the extraction rate. Nevertheless, promoting the metal’s ion transfer from the aqueous phase to the organic phase requires (1) adjustment of the pH, especially in the case of acidic extractants, to improve metal separation–purification [30] and (2) control of ionic strength by changing the salt concentration in the case of neutral and basic extractants due to the salting-out effect [31]. Complexing agents such as lactic acid and EDTA can be added to the aqueous phase to selectively complex metal ions to enhance the separation [32]. Out of all these constraints, stabilizing both the organic and aqueous phases optimizes the transfer of metal ions to the extraction phase. For challenges encountered in hydrometallurgy, such as the use of vast amounts of acids, resulting in large volumes of aqueous acidic solutions to handle them, and those related to differences in metal ion solvation observed during the SX, leading to distinct distribution properties, researchers have used the non-aqueous solvent technique as a palliative to the aqueous phase by replacing the aqueous phase with alternate polar solvents [33]. For illustration, Badihi et al. (2023) paraphrased other researchers to demonstrate the advantages observed in the implementation of non-aqueous solvent extraction (NASX) over traditional solvent extraction (SX) [33]. They revealed that after Batchu et al. (2020) compared the extraction of Nd (III) and Dy (III) from aqueous nitrate solutions and non-aqueous ethylene glycol, they discovered that ethylene glycol solutions had a more prominent separation factor than when used in aqueous solutions. Hence, when they used Cyanex923, Cyanex272, and tributyl phosphate as organic phases, they led to the extraction of 90%, 35%, and 20%, respectively, while only 70% was extracted with Cyanex923 in an aqueous system. Furthermore, they studied the separation of Y(III) and Eu(III) using a pure ethylene glycol system and 3 M trioctylmethylammonium chloride (LiCl) into the organic phase, including Cyanex923, 10 vol% 1-decanol, diluted in GS190. Results revealed that yttrium was separated from europium from EG feed solutions, reaching a separation factor of 46, which was impossible in the aqueous system where it was only 1 [33,34]. Li et al. (2021) compared the extraction of cobalt and samarium in both traditional SX and NASX. They realized that only 73.9% of Co(II) was extracted from the aqueous solution compared to over 99% obtained from the EG solution using a polar organic solvent (trioctylmethylammonium chloride) instead of water, reaching a separation factor of 3971 [17].

2.2. Solvometallurgy

While aqueous-based processes have been foundational in extractive metallurgy for nearly 150 years [25], the 21st century has witnessed a resurgence of interest in non-aqueous solvents, initially explored in the 1940s and 1950s under the theme of solvometallurgy. American researchers introduced this concept to recover uranium from low-grade uranium ores through leaching, using a non-aqueous phase consisting of acetone-HCl or alkyl phosphoric acids dissolved in kerosene [5,18]. The potential of solvometallurgy drives this shift [35], which utilizes non-aqueous solvents to solubilize and recover metals at ambient and lower temperatures, typically below 300 °C. Solvometallurgy employs a diverse range of non-aqueous solvents, including molecular organic and inorganic solvents, ionic liquids, and deep-eutectic solvents (DESs) [8]. These solvents contain at least 50 vol% solvents or are completely water-free. While ionic liquids and DESs are prominent in solvometallurgical leaching and metal production, solvent–water mixtures with water content below 50 vol% can also be utilized in specific applications. This transition towards non-aqueous systems offers potential advantages in terms of selectivity, efficiency, and the processing of complex metal-bearing materials.

The range of solvents employed in solvometallurgy is extensive, encompassing a variety of chemical species. Molecular organic solvents, familiar from traditional chemistry, offer a versatile platform for metal leaching. Ionic liquids (ILs), with unique properties such as high thermal stability and negligible vapor pressure, have emerged as a promising class of solvents for solvometallurgical applications. Deep-eutectic solvents (DESs), formed by combining two or more substances to create a mixture with a lower melting point than its components, provide a potentially greener and more cost-effective alternative. Even inorganic solvents like liquefied ammonia, concentrated sulfuric acid, and supercritical carbon dioxide find application in specialized solvometallurgical processes [35,36].

Historically, the terminology used to describe solvent phases in these systems has evolved. Alian et al. [37] initially referred to combining these solvents as the ‘polar phase’ and the organic phase as the ‘non-polar phase’ despite the latter’s potential polar characteristics. More recently, Batchu et al. [38] introduced the terms ‘more polar phase’ and ‘less polar phase’ to provide a more accurate and nuanced description of the relative polarities of the two solvent phases.

Solvometallurgy utilizing organic solvents presents several significant advantages. These solvents can be specifically designed to selectively extract target metals, resulting in greater efficiency and improved separation from unwanted gangue materials. Additionally, this method proves highly effective for processing ores that are challenging to treat using conventional hydrometallurgical techniques [18]. The advantages of solvometallurgy lie in its potential for reduced environmental impact by using closed-loop solvent systems, lower consumption of water, and reduced generation of aqueous waste streams. This is a crucial advantage in regions with scarce water resources or stringent environmental regulations [8]. Solvometallurgy encompasses a range of distinct techniques. The dissolution of metal values from solid materials using non-aqueous solvents constitutes a central focus of the current review. Non-aqueous solvent extraction, analogous to traditional solvent extraction but employing organic solvents, offers a powerful tool for separating and concentrating metals. Non-aqueous electrodeposition, the electrodeposition of metals from non-aqueous solutions, provides a means of recovering metals in a pure form.

2.2.1. Principle of Purification and Mechanism of Solvometallurgy

The differential distribution of metal ions between two immiscible organic phases is the foundation of solvometallurgy’s separation and purification principles. This approach, known as non-aqueous solvent extraction (NASX), stands apart by replacing the typical polar phase of water with a polar organic solvent. This substitution involves a re-evaluation of terminology. The traditional ‘aqueous-organic’ categorization becomes insufficient since the polar phase is no longer entirely aqueous.

Figure 1 compares hydrometallurgy and solvometallurgy processes, especially the dissolution and purification steps. In NASX, the polar phase consists of mineral acids dissolved in a water-miscible organic solvent, whereas the non-polar phase consists of water-immiscible organic solvents containing dissolved extractants. This shift to polar organic solvents in NASX allows for greater control over metal ion distribution and separation, potentially leading to enhanced selectivity and efficiency in metal recovery.

2.2.2. Organic Solvents

Solvometallurgy uses water-miscible (polar) organic solvents such as acetone, methanol, ethanol, isopropanol, and ethylene glycol. The efficiency and selectivity of the process can be improved by adding active components (acids, alkalis, oxidants, and ligands) to the solvent system. For example, acids and alkalis help adjust the leaching solution’s pH, affecting metal solubility and reaction rates. Oxidants can facilitate the breakdown of sulfide minerals, making the metals more accessible to the leaching solution. Conversely, ligaments can form complexes with specific metal ions, enhancing their solubility in the organic solvent and facilitating their extraction.

The selection of two immiscible liquid phases is essential in Non-Aqueous Solvent Extraction (NASX), as it directly impacts the efficiency of metal separation. The principle of “like dissolves like” governs solvent miscibility, meaning that solvents with similar polarity and hydrophobicity are more likely to be compatible.

A standard method for evaluating solvent compatibility is the Hansen solubility parameters [40], which consider factors such as polarity and hydrophobicity. One key measure of a solvent’s polarity is its dipole moment. Their miscibility can be estimated by comparing the chemical structures of two solvents and their differences in both polarity and hydrophobicity properties [41]. For instance, when molten hydrates inorganic salts significantly differ in polarity and hydrophobicity, they often form immiscible phases when combined with ionic solvents. This separation can benefit NASX, enabling selective metal extraction by controlling solvent interactions since less polar phases generally dilute hydrocarbon since they are hydrophobic [17].

2.2.3. Solvometallurgy of Sulfide Ore

Recent studies on the solvoleaching of metal sulfides have primarily focused on processing chalcopyrite (CuFeS₂), a common copper-bearing mineral. These studies have primarily focused on using ionic liquids (ILs) as solvents [24,25]. Because of their characteristics, ILs have shown their efficiencies in dissolving metal sulfides. While these investigations have provided valuable insights into the solvoleaching of chalcopyrite, they have also highlighted a significant knowledge gap. There is a lack of comprehensive understanding regarding solvoleaching processes for other types of Cu-Fe sulfides, such as bornite (Cu₅FeS₄) and covellite (CuS). These minerals, often found in association with chalcopyrite, can significantly contribute to the overall metal content of an ore deposit. Therefore, a more thorough investigation of their solvoleaching behavior is crucial for developing efficient and holistic metal recovery strategies. Furthermore, the exploration of alternative solvent systems beyond ILs is essential. While ILs offer certain advantages, their high cost and potential toxicity raise concerns about their widespread applicability. Investigating other organic solvents, such as deep eutectic solvents (DESs) or molecular organic solvents, could lead to more sustainable and economically viable solvoleaching processes. These alternative solvents may offer comparable or even superior performance in terms of metal extraction efficiency and selectivity while also addressing the limitations associated with Ils [42].

Kurniawam et al. [20] conducted an extensive study evaluating the effectiveness of solvoleaching in processing different metal sulfides, including CuS, Ni₃S₂, FeS, ZnS, PbS, and MoS₂. Utilizing diverse solvoleaching systems, specifically, D2EHPA + MnO₂ + H₂O, ammoniacal solvoleaching, TBP-HCl, and ethylene glycol (EG) with HCl/ChCl/NH₄Cl/FeCl₃, they demonstrated the successful dissolution of CuS, Ni₃S₂, FeS, ZnS, and PbS. At the same time, MoS₂ proved resistant to the process. The study revealed that the solvoleaching behavior of metal sulfides is governed by a combination of factors, including intrinsic metal properties, solvent solvating capacity, and the influence of active components. The inherent properties of target metals, including their distinct sulfide compounds and ionic forms, significantly impact their susceptibility to solvoleaching. Additionally, the solvating properties of the chosen solvents, particularly their affinity for specific metal ions or complexes, play a crucial role in determining leaching efficiency. Furthermore, the presence and nature of active components, such as oxidants, acids, and chloride sources, are pivotal in modulating the leaching process, enhancing metal dissolution and recovery [43].

Raghavan et al. [44] developed a solvometallurgical method for extracting copper from chrysocolla by leaching it with LIX 63 dissolved in kerosene and a small amount of aqueous ammonia solution. This approach effectively facilitated copper recovery, demonstrating the potential of solvometallurgical techniques for processing copper-bearing minerals. Matsui et al. [45] investigated the extraction of Zn(II) and Cd(II) from ethylene glycol (EG) solutions containing HCl, HBr, or alkali-metal chlorides or bromides using TOPO (tri-n-octylphosphine oxide) dissolved in toluene. Their study found that the presence of acids or salts enhances the extraction efficiency of Zn and Cd. However, optimal extraction occurs at a lower acid or salt concentration than conventional aqueous solution extraction. A similar trend was observed by Aoki [46], who studied the recovery of Mn(II) from EG solutions using TOPO. These findings suggest that metal halide complexes are more stable in ethylene glycol than water, improving extraction efficiency in non-aqueous environments. Kamariah et al. [47] successfully demonstrated the efficient and selective extraction of copper from high-grade chrysocolla ore using a novel solvometallurgical lixiviant. This lixiviant was formulated based on monoethanolamine (MEA) and ammonium chloride. The research included investigating the solubility of various ammonium salts within MEA. Ammonium chloride was incorporated explicitly into the MEA solution to enhance the solubility of the resulting copper complex and to provide counter anions. This strategic addition significantly improved the overall copper leaching yield, leading to a recovery of 88% after 4 h of leaching Cu from Chrysocolla at 100 °C. Furthermore, a study conducted by Latimer, Jr. [48] shows that the partitioning behavior of various metal salts—chlorides, bromides, nitrates, and thiocyanates of Co(II), Fe(III), Mo(V), Mo(VI), Sn(II), and Sn(IV)—was examined between a diethyl ether phase and a more polar phase. The polar phase consisted of either 2-aminoethanol, formamide, or hexanedinitrile. Results indicated that most of the metal salts preferred the more polar phase. However, a notable exception was observed with SnCl₄, which was preferentially distributed into the diethyl ether phase. This anomalous behavior was attributed to the inherent properties of diethyl ether, namely its low polarity and high hydrophobicity, as evidenced by its limited dipole moment and dielectric constant. These characteristics rendered diethyl ether a less effective solvent for most of the investigated metal salts, such as chlorides of Co(II), Fe(III), Mo(V), and so on, resulting in the observed disparities in metal distribution ratios.

3. Thermodynamic and Kinetic

3.1. Thermodynamic

Many studies have been conducted on the thermodynamics of complex formation between donor ligands and metal ions in both aqueous and non-aqueous solvents. The thermodynamic parameters related to metal–complex formation must be identified to assess the impact of solvation and the ligands’ electronic, structural, and steric characteristics on the complex stabilities and speciation. The correct interpretation of chemical processes in solutions and the advancement of practical applications depend on understanding the thermodynamic features of complex formation and the function of solvation. Determining metal speciation in non-aqueous solutions is crucial for developing separation technologies, such as liquid/liquid extraction methods, which frequently use supplementary organic ligands as selective complexing agents.

Solute distribution in solvent extraction systems is fundamentally governed by the interplay of forces between the solute and the molecules within both the aqueous and organic phases. These interactions are the driving factors behind the process’s efficiency and success. Consequently, the design of optimized solvent extraction systems necessitates a comprehensive grasp of the physicochemical properties characterizing both the aqueous electrolytes and the organic solvents [48].

The behavior of solutes within aqueous electrolytes is complex and influenced by many interactions. Dissolved ions and other species engage in complexation, ion pairing, and hydration, collectively determining the solute’s solubility and distribution. Furthermore, external factors like pH, temperature, and the presence of additional chemical species can significantly modulate these interactions [49]. Simultaneously, selecting an appropriate organic solvent is paramount in solvent extraction. The solvent’s ability to selectively remove the target solute from the aqueous phase is crucial. This is achieved by forming complexes, which facilitate the solute’s transfer. The effectiveness of this transfer is directly linked to the organic solvent’s physicochemical properties, including polarity, viscosity, and inherent solubility.

Engineers and researchers can optimize solvent extraction processes by understanding the physicochemical characteristics of both aqueous electrolytes and organic solvents. They can select appropriate solvent systems, adjust operating conditions, and design efficient extraction equipment to achieve desired separation efficiencies and yields. This comprehensive understanding is essential for successfully designing and operating solvent extraction systems in various industrial applications, including hydrometallurgy, chemical processing, and environmental remediation.

The equilibrium constant, also known as the stability constant, for a metal–ligand system, is represented by the intermediate reaction constant, K, for the following reaction:

ML_n−1 + L = ML_n

(1)

This constant measures the extent of metal ion complexation [50,51].

To determine the thermodynamic stability constant under standard state conditions, the various species’ activities are precisely utilized.

The following equation gives the standard free energy change associated with complex formation:

ΔG° = −RTlnK

(2)

where ΔG° represents the standard free energy change, R is the ideal gas constant, T denotes the absolute temperature, and K is the stability constant for the complexation reaction.

The enthalpy of complexation, ΔHn, can be determined either directly by reacting the metal and ligand in a calorimeter or indirectly by measuring lnK at various temperatures and applying the appropriate thermodynamic relationships (e.g., the van ‘t Hoff equation).

dInK/dT = −ΔHn/R

(3)

In solvent extraction investigations, the temperature variation approach is frequently utilized. It can provide accurate enthalpy values across the temperature range when the lnK vs. 1/T graph is linear.

The formation of complexes between hard cations and hard ligands (oxygen or nitrogen donors) often results in positive enthalpy (ΔH) and entropy (ΔS) changes. In these cases, the logarithm of the equilibrium constant (log K) is positive when TΔS > ΔH, as dictated by the Gibbs free energy equation (ΔG = ΔH − TΔS). These “entropy-driven” reactions are attributed to reduced ion hydration, leading to increased system disorder and positive entropy.

Dehydration contributes an endothermic enthalpy component (ΔH > 0) due to breaking metal–water bonds in hydrated species. Conversely, cation–ligand bond formation results in a negative enthalpy contribution (ΔH < 0), as well as a decrease in system disorder, leading to a negative entropy contribution (ΔS < 0). The observed overall enthalpy change is a composite of these dehydration and cation–ligand interaction effects. Positive ΔH and ΔS values indicate that dehydration plays a more significant role than the complexation step.

Hasseinzadeh and Hassanzadeh [52] investigated the thermodynamics of copper solvent extraction from a heap leach solution using CuPRO MEX-3302. They determined the enthalpy change (ΔH) to be 32.66 kJ/mol and the entropy change (ΔS) to be 248.92 J/K, resulting in a Gibbs free energy (ΔG) of −41.52 kJ/mol. The positive ΔH value confirmed the endothermic nature of the copper extraction process. Similar findings have been reported by Agarwa et al. [53] and Aziz et al. [54]. The positive ΔS value signifies increased disorder at the solid–liquid interface, indicating a favorable solvent affinity for copper extraction. Furthermore, the negative ΔG value demonstrates that the copper extraction process in this system is spontaneous when working at pH (0.1–2.7), CuPRO MEX-3302 concentration (5%–15% (v/v)), and temperature (25–70 °C).

3.2. Kinetic

Kinetics is a crucial aspect of any chemical process, as it influences the efficiency and optimization of extraction operations, particularly in solvent extraction. Understanding the kinetics of extraction provides valuable insights into the underlying extraction mechanisms, aiding in selecting and optimizing extractants for optimal performance.

Several factors affect the metal extraction rate from leach liquors when using an extractant. These factors include the concentration of the solvent, temperature, and the interfacial area where the leach liquor and extractant come into contact [52]. For instance, higher solvent concentrations generally lead to faster extraction rates due to the increased availability of extracting species. Similarly, elevated temperatures can enhance extraction kinetics by increasing the mobility of species involved in the extraction process. Maximizing the interfacial area between the leach liquor and extractant facilitates more efficient mass transfer, thereby enhancing extraction kinetics [55].

By studying extraction kinetics, researchers can gain insights into how these factors influence the metal extraction rate, allowing for the optimization of extraction processes to achieve the desired extraction efficiency and throughput. This more profound understanding of kinetics contributes to developing more efficient and effective extraction operations.

Depending on the interfacial area, involvement, and temperature of an acidic extractant, like a heterogeneous system, the Cu(II) extraction rate may be stated as follows at constant temperature:

$$rate=-\left[{\displaystyle \frac{d[{Cu}^{2+}{]}_a}{dt}}\right]=\left[{\displaystyle \frac{d[{Cu}^{2+}{]}_0}{dt}}\right]=K_f A[{Cu}^{2+}{]}^a[RH{]}^b[H^{+}{]}^c$$

(4)

In this context, the notations (a) and (o) distinguish the aqueous and organic phases, respectively. The symbol kₓ represents the rate constant for the forward extraction reaction. Additionally, the variables a, b, and c denote the reaction orders concerning the concentrations of [Cu²⁺], [RH], and [H⁺].

The flux (F) technique is used to analyze rates. To calculate the average mass flux (F) via the liquid–liquid interface, divide the quantity of Copper transported (Kmol) into the organic phase by the product of the extraction area (m²) and extraction time (s). Thus, the following is obtained:

$$F={\displaystyle \frac{m{t}_v}{A}}dt$$

(5)

which is proportional to [Cu²⁺]^a, [RH]^b, and [H⁺]^c, as shown below:

$$F=K_f{\left[{Cu}^{2+}\right]}^a{\left[RH\right]}^b{\left[H^{+}\right]}^c$$

(6)

Taking the logarithm, Equation (6) can be rewritten as follows:

$$Log\left(F\right)=Log\left(K_f\right)+a\times Log\left(\left[{Cu}^{2+}\right]\right)+b\times Log\left(\left[RH\right]\right)+c\times Log\left(\left[H^{+}\right]\right)$$

(7)

Maintaining two variable concentrations constant in experiments makes it possible to determine the reaction orders (a, b, and c) and the rate constant (kf) in liquid–liquid extraction processes. These experiments involve varying the concentration of a third variable and observing its effect on the extraction rate.

The orders concerning the two constant concentrations are determined based on the changes in the extraction rate as these concentrations are altered. This information is then used to construct a log-log plot of the flux functions (F) against the concentration of the third variable. The slope of this plot provides insight into the order concerning the third variable concentration [56].

Additionally, the intercept of the plotted data allows for the determination of the forward rate constant (kf). It is important to emphasize that while the mass transfer flux, representing the rate of mass transfer per unit interfacial area, is dependent on the interfacial area between the two liquid phases, the inherent rate of mass transfer is not. Consequently, the flux technique proves particularly advantageous for kinetic studies of liquid–liquid extraction. This method eliminates the need for precise interfacial area specifications, simplifying the analysis of experimental data. This approach offers a comprehensive understanding of the extraction kinetics, allowing for the optimization of extraction processes for enhanced efficiency and performance.

4. McCabe–Thiele Diagram

Researchers utilize extraction isotherms and McCabe–Thiele diagrams to evaluate extraction capacity and determine the optimal number of stages. An extraction isotherm depicts the equilibrium relationship between metal ion concentrations in the aqueous and organic phases. This is generated by contacting the two phases at varying organic-to-aqueous volume ratios [57].

Analyzing the extraction isotherm allows researchers to assess extraction efficiency and identify equilibrium conditions under different operating parameters. The McCabe–Thiele diagram, a graphical tool, determines the number of theoretical equilibrium stages required to extract or separate a target component from a feed mixture.

The necessary number of extraction stages can be visually determined by plotting the equilibrium data derived from the extraction isotherm onto the McCabe–Thiele diagram. This diagram aids in optimizing the design and operation of the extraction system, ensuring desired separation efficiency while minimizing energy consumption and operational costs [58]. The illustration of McCabe–Thiele diagram showing the number of stages is represented in Figure 2 [59].

5. Advantages of Solvometallurgy over Hydrometallurgy

Solvometallurgy offers distinct advantages over traditional hydrometallurgical methods. A significant aqueous phase’s inherent absence allows for precise water usage control, effectively reducing wastewater generation and associated treatment costs. Process intensification is achievable by integrating unit operations like leaching and solvent extraction, streamlining the overall process.

Furthermore, solvometallurgy addresses the challenges posed by carbonate-rich ores. Unlike acid leaching in hydrometallurgy, which necessitates significant acid consumption to dissolve carbonate gangue, solvoleaching selectively preserves these materials. This selectivity prevents the co-dissolution of undesirable metals, such as iron, and minimizes acid consumption.

Similarly, solvometallurgy mitigates issues associated with silica-rich ores. The reduced solubilization of silica in non-aqueous solvents minimizes silica gel formation, a common problem in hydrometallurgical processes due to the formation of silicic acid in water.

The effectiveness of solvometallurgical processes is further enhanced by leveraging principles from coordination chemistry and ion solvation in non-aqueous solvents [60]. This knowledge base allows for developing tailored solvent systems that optimize metal extraction and separation.

However, one challenge lies in the higher cost of organic solvents and extractants compared to water and mineral acids. To mitigate this issue, minimizing the number of reagents used and prioritizing the recycling of solvents and extractants after the procedure is essential. Environmental considerations underscore the necessity of solvent recycling to reduce resource consumption and minimize environmental impact.

6. Solvometallurgical Processing Flowsheet

A typical hydrometallurgical process involves leaching, solvent extraction (SX), and metal recovery through precipitation or electrodeposition to convert minerals into marketable salts or metals. These existing metallurgical techniques can be further enhanced by incorporating non-aqueous solvent extraction (NASX) and other solvometallurgical unit operations.

By integrating solvometallurgy, hydrometallurgical and pyrometallurgical processes can be optimized, offering potential technical and economic advantages. This makes solvometallurgy a valuable complement to conventional extraction methods, providing greater flexibility in metal recovery. Figure 3 presents a flowsheet illustrating the role of the solvometallurgical process in extractive metallurgy.

7. Conclusions

A typical hydrometallurgical process for converting minerals into commercial salts or metals often involves leaching, solvent extraction (SX), and metal recovery through precipitation or electrodeposition. Non-aqueous solvent extraction (NASX) and other solvometallurgical unit technologies can enhance conventional metallurgical processes, offering economic and technical advantages. Solvometallurgy becomes advantageous whenever it delivers such benefits.

The design of non-aqueous solvent extraction (NASX) systems starts by carefully selecting immiscible phases with varying polarity. The polarity and hydrophobicity characteristics of each phase primarily drive this selection. Some extractants employed in NASX systems have been discussed, utilizing polar molecular organic solvents to achieve metal extraction as the basis for the more polar phase.

Solvometallurgy represents a distinct and emerging area within extractive metallurgy, offering a complementary approach to both pyrometallurgy and hydrometallurgy. While sharing similarities with hydrometallurgical processes, solvometallurgy fundamentally replaces the aqueous phase with a non-aqueous solvent. It is important to note that conventional solvent extraction, due to a separate aqueous phase, does not fall under the classification of solvometallurgy.