Lithium Processing in the Past and for the Future

Abstract

Lithium is a key element in the transition to carbon-free power generation. Over the last decade or so, there has been a surge in extracting lithium from its diverse natural sources, driven by a growing gap between its demand and production. Traditionally, lithium is extracted from salar brines; however, as the demand for this commodity has increased, processing from pegmatites and other types of brines and lithium-bearing clays is becoming more important. This paper revisits current technologies available to produce battery-grade lithium carbonate from diverse sources. We particularly discuss clay processing and the environmental issues associated with processing lithium from its natural sources. Plant data is required to make accurate environmental assessments concerning the processing of clay minerals. Uncertainties on the actual amount of lithium reserves exist, and it is unknown if, with the current data available, it is possible to close the gap between demand and supply of lithium.

1. Introduction

Humanity faces several problems associated with its growth and development. One of the key issues communities worldwide face nowadays is the planet’s overheating. There is a direct link between global warming and CO₂ emissions. Most of these emissions are due to the burning of hydrocarbons to produce electricity or to provide energy for transportation (by land, air, and water). To counteract these negative effects, it has been agreed to limit the generation of CO₂ emissions. Those limitations are signed in international treaties such as the UN Sustainable Development Goals, the Kyoto Protocol, and the Paris Agreement.

Among the key points in these treaties is the change from fossil fuel energy-driven systems to systems powered with renewable energy from distinct sources other than hydrocarbons [1]. To adhere to the reduction in greenhouse gas emissions, the transportation and automotive sectors are shifting towards the electrification of vehicles [2]. Under this scope, massive vehicle electrification and energy storage facilities are to be created. Fasel and Tran [3] suggested developing novel fusion reactors to supply electricity using lithium to produce deuterium and tritium. At the center of this technological shift, lithium becomes an attractive commodity [4].

Lithium resources are finite, and the amount of the metal that can be mined for its exploitation is determined by geological, technical, economic, social, and environmental constraints. To extract and utilize as much lithium as possible, it is imperative to decrease metal losses, as they are unstable and impact the overall lithium production [5,6]. In addition to mineral resources, the recycling of lithium-bearing materials should be considered as well. In this regard, Simon et al. [7] indicated the need to develop facilities to recycle battery wastes to recover lithium from LIB and NiMH types of batteries as they are more abundant in the market due to their overall performance and lightness, among other properties [8].

Lithium has been categorized as a critical or strategic element, in part because of its role in shifting from burning fossil fuels towards environmentally friendly electrification. Important research efforts were conducted in the second half of the past century to understand and develop lithium processing routes, yet such efforts stopped until recently, when revitalized interest in lithium production from its diverse natural sources has been driven by an aggressive agenda to develop hybrid vehicles, aiming at full vehicle electrification [9].

The sudden interest in the exploitation of lithium is an indicator of the demand for goods and devices that contain the commodity as an input [10]. The lithium market, along with that of magnesium, will expand because of the demands of the automotive industry [11].

Different estimations on the availability of lithium offer distinct figures on the actual amounts of metal that can be extracted from its natural sources. These diverse numbers are attributed to the fact that the rates of lithium demand and supply constantly change because of different factors [12]. Regardless of these differences, all forecasts agree on the exponential growth of the demand for lithium. As an example, it is estimated that under the current scenario, Europe’s demand for lithium would be fulfilled until the year 2030; after that year, lithium imports would be required [7].

To exploit lithium, currently, there are two main natural sources available for recovering its salts [9,13]:

(1)

From lithium-bearing brines (salars, geothermal brines, oil field brines);

(2)

From lithium ores that need thermal activation (rocks, clays).

As the demand for lithium increases, the fraction of lithium produced from brines has decreased, while shifting the interest of lithium producers to extract the metal from rock and clay deposits. Nearly 50% of current lithium extraction occurs from exploiting mineral ores; this percentage keeps growing [13,14,15]. On the other hand, Zhao et al. [16] estimated that lithium extraction from brines is about 64% of the overall production. Lithium in pegmatites and clay minerals represents global reserves of 29% and 7%, respectively.

Haddad et al. [17] noted that to decrease the gap between lithium supply and demand, novel technologies need to be developed to cut the CO₂ emissions associated with lithium extraction. Complementary to this, lithium producers need to heavily invest in finding new lithium deposits to exploit them [18].

Uncertainties associated with forecasts on the availability of natural sources of lithium may lead to technological developments that include the processing of waste lithium-based batteries for their recycling [19].

This paper explores the technological routes necessary to produce lithium in a suitable environmentally friendly form that copes with the increasing demand for this resource, as society changes its energy generation and storage.

2. Lithium Demand, Resources, and Availability

As mentioned, different reports provide distinct figures related to lithium availability, resulting in variable estimations of lithium reserves worldwide. Consequently, there are conflicting conclusions on whether the amount of available lithium resources is sufficient to meet the demands for lithium for the upcoming years [8]; for example, Kesler et al. [20] estimated that regardless of data scattering, there are about 32 MTonnes of lithium available from different mineral resources (rocks and brines). This figure suggests that there is enough lithium to cope with demand until the end of this century. Furthermore, as lithium-based batteries start to be recycled, the need for natural mineral sources of lithium will be reduced.

In contrast, Egbue & Long [2] considered that the discrepancies in estimating lithium resources and reserves can be attributed to different reasons:

• Estimations are made using distinct non-standardized methodologies;
• Information comes from distinct sources; some of these are privately funded studies [3] that do not necessarily reflect the reality of the lithium deposits.

Because of this, actual lithium accessibility is a concern for some researchers, as there is exponential growth in demand for this material [21,22], particularly in applications related to the automotive industry. Depending on the baseline used to forecast the accessibility of natural lithium resources, different scenarios present distinct constraints that are used to estimate the foreseeable exploitable lithium. According to Greim et al. [23], the known natural lithium resources can meet demand for the remainder of the century. If the production of batteries unexpectedly increases, then the balance between supply and demand can shift towards scenarios where a significant gap might develop, so to decrease such a gap, the incorporation of lithium recycling technologies will be required to keep up with production rates.

Thus, the supply chain for lithium batteries is compromised by the accuracy of the information used; furthermore, there are critical areas that need to be addressed before forecasting the availability of lithium. Among these areas are production capacity, trade, geopolitics, and resource availability. Based on this, it is likely that the demand for lithium will exceed the supply.

Perhaps this difference in opinion can be attributed to how some authors have not accounted for the available mineral deposits in forecasting actual demand for the distinct batteries that will be produced eventually [8].

Haddad et al. [17] estimated that the demand for lithium would be in the vicinity of 150,000 to 190,000 tonnes by 2025. Furthermore, these researchers also estimated that there are nearly 20 Mtonnes of lithium reserves worldwide that can be processed economically.

Data from the U.S. Geological Survey from 2013 to date [24,25,26,27,28,29,30,31,32,33,34,35,36] indicates that over this period, there is a total of 30 million metric tonnes of lithium resources worldwide that can be mined or somehow processed to produce batteries of different kinds to satisfy the demand for such goods. Over this time frame, the price of battery-grade lithium carbonate showed little increase from 2010 to 2015, and then it experienced higher increases until 2022, when it reached its maximum value at $71,100.00 USD per tonne of carbonate [37]. Figure 1 shows the evolution of these figures.

According to the U.S. Geological Survey, lithium production is concentrated in: Australia, Chile, China, and Argentina. Australia produces the most lithium, followed by Chile, China, and then Argentina. Lithium production in South America comes from brine processing. In the case of Australia and China, these producers, besides treating brines, also process pegmatite rocks. From 2014 to 2017, the production of lithium from these producers increased steadily; however, in 2018, Australia significantly increased its production capacity. Chile and China followed the same trend but at a lower rate compared to Australia. Argentina’s productivity remained steady until 2022, when there was a sharp increase in Argentina’s lithium production. During 2025, China surpassed Chile as the second lithium producer. Figure 2 shows these trends. Other minor lithium producers include Brazil, Canada, Portugal, and Zimbabwe.

Over the same time interval, the lithium market has changed as well. Figure 3 shows how rapidly the LIB battery market increased over the reference period. During 2014, 29% of the total lithium produced worldwide was destined for the manufacture of batteries. Ever since that year, the usage of lithium for batteries has constantly increased; in 2025, 88% of the total lithium production is designed to produce batteries. This growth is attributed to the increase in the manufacturing of portable electronic devices, as well as to the drive for creating electric-powered vehicles [4,38,39,40,41].

After batteries, lithium is used as additive in glass and ceramics making. Other applications for lithium are in fluxes, as an alloying element in aluminum alloys, in grease and in lubricants, among other minor applications. Figure 3 illustrates the market share for lithium over the last decade.

This is of particular interest, as it has been predicted that the availability of lithium will be required to comply with CO₂ abatement goals. As an example, estimates [39] of the relationship between supply and demand of lithium carbonate showed a potential shortage of lithium salts, risking the commitment made by the EU27 regarding full electrification of vehicles circulating in Europe. In this regard, estimates on the depletion of lithium mineral bodies seem to support the demand for this metal by 2050 [38].

Bulin [42] considered that the lithium supply is under low risk. Additionally, the role of China in the lithium trade has been considered, given its importance both as a producer and as a trader, and because of its amount of natural lithium resources.

Sun et al. [43] studied the flow of the lithium market from 1994 to 2015; Sun’s paper indicates that lithium trade takes place in different forms and between different trade partners. Mineral trade occurs in Australia, Chile, China, and the US, with Australia and Chile as the major exporters. Australia and China conducted the largest ore trade, whereas Chile and the US traded lithium from brines. Most of the lithium exports were concentrated in China, the EU, the US, Japan and South Korea. It is also found in [43] that the production of lithium has increased steadily over the last two decades, and the trend is to keep growing. The main applications of lithium compounds are lithium-ion batteries (LIB), glass and ceramics making, pharmaceuticals, and other industrial products [44].

In terms of batteries, two categories are identified. Primary-use batteries, which are discarded once the battery is exhausted: these batteries use metallic lithium as an anode. Secondary batteries are rechargeable and are used in the automotive industry and in portable electronic devices. These batteries use lithium compounds, mainly Li₂CO₃ and LiOH, among others, as electrodes and electrolytes [45]. Besides batteries, lithium will play a not-so-prominent role in other energy production technologies (solar PV, eolic, hydro, bioenergy, geothermal, hydrogen, and nuclear, among others) [46].

In glass and ceramics making, lithium carbonate is used as a flux and as a coating. Additionally, lithium provides gloss and lowers the fluidity of the glass. In terms of lubricants, lithium greases keep their lubricating properties at high temperatures while offering good resistance to water. In basic metallurgy, lithium is added to aluminum electrolysis to improve the electrical conductivity of the bath while lowering its melting point. Additionally, lithium hydroxide or lithium carbonate is used in continuous-casting fluxing powders. In polymer making, n-butyl lithium is used to start the polymerization of rubbers such as styrene–butadiene and polybutadiene. Additionally, organic lithium salts are used to obtain thermoplastic materials and rubbers in tire making [45].

It has been described [10,15,40,41,47,48,49,50,51,52,53,54,55] that lithium can be found in pegmatite rocks, as well as in some sedimentary rocks (clay mineral) deposits. Another important source of lithium is brines; they are found in sites where geothermal energy is used as a source of electricity generation, where salty lakes existed and in oil fields. Seawater has been considered as another natural source of lithium, as it is estimated that nearly 230 billion tons of lithium could be extracted from it [48,56,57,58].

However, it is also important to consider energy, water consumption, and hazardous emissions related to the processing of lithium-bearing minerals. Most of the processes in place to refine lithium consist of a combination of several processing steps that not only slow down the whole refining operation but also are energy- and water-demanding. Producing a tonne of lithium salts requires 60 MWh of energy and 70 m³ of water while generating up to 17 tonnes of CO₂ gas [17,59,60,61].

As these distinct natural sources of lithium differ in nature, so do the processing technologies developed up until now to extract lithium. Before revisiting the processing schemes available to recover lithium, it is important to understand the physicochemical properties of this metal.

3. Physicochemical Properties of Lithium

Lithium is the third chemical element in the periodic table, and it is the first metallic element in such array. Additionally, it is the first of the alkali metals group. Its atomic mass is 6.94 g/mole, and its electronic configuration is [He] 2s1. The melting point of lithium is 453.5 K (180.50 °C), and it boils at 1615 K (1342 °C); it is less dense than water (534 kg/m³). Its vapor pressure increases at low riskwith temperature [62,63,64], as seen in Figure 4.

The specific heat capacity of lithium is 3582 J/kg/K. The electronegativity of lithium on the Pauling scale is 0.98, whereas its atomic radius is 1.82 Å (un-bonded), and its covalent radius is 1.30 Å.

Table 1. Chemical properties of lithium, sodium and potassium. Data from [65].

Property	Li	Na	K
Atomic mass [g/mole]	6.94	22.99	39.09
Heat of formation of molecules from atoms [kJ/mole]	−113.8	−76.9	−52.7
Electronic affinity [kJ]	8.65 × 10−23	1.19 × 10−22	1.12 × 10−22
Electronegativity	1.0	0.9	0.8
Normal electrode potential [V]	3.038	2.710	2.920
Ionic radius [Å]	0.68	0.97	1.33
Covalent radius [Å]	1.58	1.92	2.38
Internuclear distance in molecule [Å]	2.67	3.08	3.91

compares some chemical properties of lithium, sodium, and potassium.

Because of these properties, lithium, as well as sodium and potassium, can easily react with the chemical elements in group VIIA (halogens). Lithium reacts readily with chlorine, fluorine, and iodine to form their respective salts; additionally, lithium reacts spontaneously with oxygen to form either lithium oxide or any salt containing oxygen, such as hydroxide or carbonate. This is exemplified in mineral species that contain lithium. There are nearly 75 mineral species bearing lithium [66]; of those, the principal species are shown in

Table 2. Principal mineral species bearing lithium. Data from [53,66].

Mineral Species	Chemical Formula	Element Content [mass %]													Typical Li Content [mass %]
		Li	Al	P	O	H	Na	F	K	Si	Fe	C	Mg	B
Amblygonite	(LiNa)AlPO4(FOH)	3.35	17.8	20.5	44.9	0.17	3.8	9.4							-
Zinnwaldite	KLiFeAl(AlSi3)O10(OHF)2	1.59	12.4		38.4	0.12		6.5	8.9	19.3	12.8				1.2–1.3
Zabuyelite	Li2CO3	18.8			65.0							16.26			-
Triphylite	LiFe2PO4	4.40		19.6	40.6						35.4				2.5–3.8
Spodumene	LiAl(SiO3)2	3.73	14.5		51.6					30.2					1.9–3.3
Petalite	LiAlSi4O10	2.27	8.8		52.2					36.7					1.6–2.2
Montebrasite	LiAl(PO4) OH	4.76	18.5	21.2	54.8	0.69									0.9–1.8
Nalipoite	NaLi2PO4	10.5		23.5	48.5		17.4								-
Lepidolite	K(Li Al)3(AlSi)4O10(FOH)2	3.51	31.9		32.4	0.34		6.4	6.6	18.9					1.4–1.9
Hectorite	Na0.3(Mg Li)3Si4O10(OH)2	4.68			43.2	0.45	1.6	8.6		25.2			16.4		0.36
Eucryptite	LiAlSiO4	5.51	21.4		50.8					22.2					2.3–3.3
Bikitaite	LiAlSi2O6 H2O	3.40	13.2		54.9	0.98				27.5					1.3–1.7
Jadarite	LiNaSiB3O7(OH)	3.16			58.4	0.46	10.5			12.8				14.8	0.09–0.1

, along with their stoichiometry and their chemical formula.

Figure 5 shows the amount of lithium that each of the species listed in Table 2 contains. From this figure, Zabuyelite is the mineral species that has the highest lithium content, as it is lithium carbonate. Spodumene and lepidolite contain 3.73 and 3.51 wt% of lithium, respectively. These two mineral species are the most common in pegmatite rock deposits as well as in some clays. Hectorite contains 4.68 wt% Li; this mineral species is also found in clay deposits next to spodumene. These contents are based on the stoichiometry of the mineral species. It is found that actual lithium deposits have lower metal concentrations than those shown in Table 2 [20,49,53].

It should be noticed in the data in Table 2 that oxygen is a major constituent in these mineral species; additionally, elements like sodium and potassium are present in many of these minerals. Silicon and aluminum are also present in these molecules; this indicates that lithium is bound to silicates, aluminates or silico-aluminates; this is of particular interest in the processing of clays.

Of the mineral species in Table 2, spodumene is the most used to extract lithium due to its relatively high Li concentration. In the case of zabuyelite, it was originally discovered at Lake Zabuye [67]. This mineral is a rare lithium carbonate polluted with sodium chloride [68]. It was believed that zabuyelite only occurred locally; however, it has been shown that this mineral occurs at different locations, and in some cases is bound to spodumene in pegmatite deposits [69]. More recently, there has been some interest in the exploitation of zabuyelite associated with clay minerals [70]. However, no significant developments on further exploration and exploitation of zabuyelite deposits are reported.

When considering processing alternatives to recover the lithium from the different mineral species cited in Table 2, it is necessary to look at data on the solubilities in water and the possible salts obtained upon leaching the minerals with specific reagents. Data on the Gibbs free energy [66] of diverse alkali salts is presented in Figure 6. Sulfate salts are more stable than carbonates, chlorides, and hydroxides formed from metals such as lithium, sodium, potassium, calcium, and magnesium. Sodium and potassium hydroxides are practically as stable as their chlorides. Lithium, calcium and magnesium hydroxides are more stable than their chlorides. Chlorides are the least stable salts. Of the sulfate salts, potassium sulfate is the most stable as it has the most negative value of Gibbs free energy; the second most stable salt is lithium sulfate. Thus, to recover the lithium contained in minerals or brines, it would be necessary to turn the system into sulfate chemistry to facilitate lithium extraction.

Another important piece of information necessary to process lithium salts is their solubility and that of other metal salts. Figure 7 shows the solubility in water as a function of temperature for diverse Li, Na, K, Ca and Mg salts [64,71,72,73,74,75]. The data in Figure 7 indicates that some of the compounds increase their solubility as temperature rises, while other compounds decrease their solubility when temperature rises. Lithium carbonate solubility in water at room temperature is about 1.5 wt% and decreases below 1 wt% above 60 °C. Calcium and magnesium salts are less soluble in water and are expected to precipitate along the lithium carbonate, polluting the resulting lithium product. These calcium and magnesium salts need to be removed before refining the final lithium carbonate.

Figure 8 shows the relationship between the Gibbs free energy and the solubility in water of distinct lithium, sodium and potassium salts at 25 °C (298 K). It can be noticed in Figure 8 that most of these compounds have solubilities higher than 100 g/L, with the exceptions of lithium carbonate and lithium fluoride. This plot confirms that sulfates are the most stable compounds, while iodides are the least stable ones. This plot also indicates that it is possible to precipitate lithium carbonate since it has limited solubility. The lithium carbonate precipitate can be obtained by using sodium or potassium carbonate combined with any lithium salt dissolved in water. This is of technological importance since it proves that mineral resources could be treated in such a way that sulfate salts are formed and then converted in such a fashion that the ultimate product should be lithium carbonate, which is a key component in manufacturing LIBs.

4. Lithium Extraction from Mineral Resources

The principal use of lithium nowadays is the manufacture of batteries for various purposes; besides these applications, lithium can be processed to obtain other products. Figure 9 shows a schematic based on Sohn’s work [76] of the different intermediate and end lithium products that are processed from natural resources. Figure 9 does not consider the recycling of waste batteries as they are processed according to their type.

The intermediate lithium products result from applying the physicochemical principles discussed in Section 3 of this manuscript. It is clear from Figure 9 that lithium raw materials are processed into either chloride, carbonate, or hydroxide before obtaining a final marketable product.

As will be discussed, the processes that are available for lithium extraction were developed with the idea of obtaining any of these salts. It is customary to obtain lithium chloride or lithium sulphate in solution, and from this point, do the proper operations to obtain lithium carbonate or lithium hydroxide.

Lithium can be extracted from either mineral deposits by conventional mining or from liquid sources (brines, seawater) [65]. In general terms, lithium minerals can be divided into two big groups: phosphates and silico-aluminate silicates [77]. In terms of commercial importance, spodumene, lepidolite, petalite, and amblygonite are the main lithium-bearing ores [77]. The ores that are economically suitable for lithium extraction are: spodumene, petalite, lepidolite, amblygonite, zinnwaldite, and eucryptite [78]. These ores are processed in facilities installed in different places, such as Greenbushes in Australia, Kings Mountain in the USA, Bernic Lake in Canada, Bikita in Zimbabwe, and Bald Hill in Australia [78].

As lithium producers move toward the exploitation of lithium minerals [79], pegmatite deposits are preferred due to their relatively high lithium contents. From pegmatites, spodumene ore (lithium silicate) is favored for lithium extraction in part because of its higher metal content and because it presents a relatively low processing cost [80].

Pegmatite rocks are characterized by a relatively high concentration of rare elements [81]. This feature can be attributed to the origin of pegmatites, which is believed to occur through the remelting of continental materials [82,83].

On the other hand, lepidolite, petalite and amblygonite belong to the silico-aluminate silicate type of minerals and are treated similarly to spodumene [45,49,65,77].

Lithium can also be extracted from mineral clays. Clays usually contain hectorite and other mineral species bearing lithium. Technology is under development to recover the metal from these natural sources [9,77]. The lithium-bearing clay minerals relate to volcanic activity. A key issue with these minerals is that, economically speaking, their processing is not proven yet; thus, uncertainties in their potential as a reliable source of lithium arise [84].

4.1. Beneficiation of Lithium-Bearing Ores

Ore dressing operations are used to increase the concentration of the valuable metal in an ore to recover as much of that target metal as possible in an economic way. In the case of lithium-bearing minerals, their physicochemical properties limit the use of beneficiation techniques [78] for distinct reasons:

(a)

There is a significant difference in the density of gangue minerals and ores like spodumene [85].

(b)

If spodumene is found in mica or clay minerals, its density does not change. Additionally, upon milling, spodumene particles are shaped into acicular particles that float away along with gangue minerals [86].

According to Sahoo et al. [87], lithium-bearing minerals are characterized by having a coarse-grained structure. This feature minimizes the need for excessive grinding; additionally, mineral species containing lithium have similar physicochemical properties to the gangue minerals associated with them. Because of this, careful selection of beneficiation processes should be considered.

Lithium minerals can be beneficiated by different techniques, among them, flotation, heavy-media separation, magnetic separation and electrostatic separation [87].

Results from some experimental work related to the use of these beneficiation techniques have been published.

For example, separation tests by electrostatic separation of lithium-bearing micas were reported [88]. It was possible to obtain good separation of micas containing lithium from felspar and quartz in a pegmatite sample. Furthermore, it was found that successful separation of lithium can be attained with mean particle sizes coarser than 500 microns. Iuga’s research [88] revealed that the magnitude of the forces arising from the application of the electric field to separate the lithium mineral species is proportional to the square root of the particle size. In spite of these findings, further test work is needed to fully understand the phenomena governing this type of separation, as well as to optimize process parameters to improve the separation dynamics of these lithium-bearing minerals.

In a different study, Botula’s group [89] tested the beneficiation of a sample of zinnwaldite mineral by means of magnetic separation. Magnetic separation of this mineral species was selected due to the iron content of the zinnwaldite. It was found that the zinnwaldite can be concentrated by means of magnetic separation in a dry separator utilizing a magnetic field in the range of 0.3 to 0.7 T. This set up allowed for a lithium recovery efficiency of 70%. However, as in the case of Iuga’s group report, further test work is required to optimize process parameters.

Amarante’s group [90] beneficiated a spodumene ore sample by means of heavy-media separation. This group used bromoform (density = 2890 kg/m³) to separate the light gangue from the spodumene mineral, whose density varies from 3150 to 3200 kg/m³.

Upon completion of the test work, it was possible to recover over 95% of the lithium, after several water cleaning steps. In spite of these findings, Amarante concluded that more test work on the selection of the heavy media and the configuration of the separation circuit needs to be performed to optimize this process.

In view of the problems associated with spodumene flotation, Sagzhanov et al. [91] reported results on experimental work combining heavy-media separation and flotation (froth and inverse) for the beneficiation of a low-grade lithium spodumene ore. To recover as much lithium as possible, Sagzhanov’s group increased the lithium content in an initial concentrate obtained by using heavy-media separation in a relatively coarse mineral. Heavy-media separation was conducted using an aqueous solution of sodium polytungstate (3Na₂WO₄ 9 WO₃ H₂O, density 3100 kg/m³). The enriched lithium concentrate was then floated using a combined anionic/cationic collector. The anionic collector was NaOL (sodium oleate), whereas the cationic collector was DAA (dodecyl amine acetate). This collector was applied to the flotation device at 1000 g/ton to a pulp with pH = 10. Sagzhanov’s group proposed that NaOL reacts with the aluminum in the spodumene molecule on the mineral’s surface and then DAA develops a complex with NaOL that adsorbs on the Stern layer of the system. These phenomena improve the efficiency of the flotation process. In addition to this mechanism, the mixed NaOL/DAA collector turns gangue species such as feldspar and quartz hydrophobic, thus enhancing the recovery of lithium.

In spite of these results, the authors claim that more experimentation in developing collectors and heavy media is needed to improve the recovery of lithium.

Like other ores, spodumene initially passes through comminution operations; these can be done by combining crushers and mills. It has been reported that crushing circuits with up to three stages have been used, combining the use of jaw and cone crushers. Depending on the final particle size achieved after crushing, a milling circuit can be implemented; ball and autogenous mills are typically used [45,65].

After comminution, the spodumene ore is sent to a concentration stage using froth flotation. Extensive experimental work [92] on the flotation of spodumene showed that to optimize lithium recovery, the pulp pH should be kept between 7.5 and 9.8, although a pH of 7.5 is preferred. Highly acidic or alkaline conditions yield extremely poor Li recoveries. Flotation cells should not be operated higher than 1040 rpm. Higher rotating speeds limit the selectivity of the flotation. The pulp temperature should be kept at 15 °C; if the pulp gets heated up to 50 °C, there is a reduction in the collector’s selectivity. The feedstock should have 18% solid content. Higher or lower solid concentrations hinder recovery values. The rough concentration should go through two cleaning stages to improve the grade of the spodumene obtained.

Once ready, the spodumene concentrate can be sent for lithium extraction using a combination of pyro- and hydrometallurgical processes.

Opoku et al. [93] revisited current industrial lithium mineral beneficiation practices worldwide. In their assessment, they found out the most common lithium mineral processed is spodumene; however, it is noted that there is a need to develop specific beneficiation practices to treat minerals such as zinnwaldite, lepidolite and petalite.

Coarse-particle minerals are suited for heavy-media separation in single or multiple separators. Minerals with fine particles are better suited for a combination of heavy-media separation and flotation. Basic and applied research efforts are directed at developing specific reactants to conduct these operations. The use of novel technologies like fluid bed flotation, among others, is also discernible.

Pre-treatment by ultrasound or calcination of lithium ores might improve the lithium recovery; however, the implementation of these processes presents additional costs.

Regarding the mineral processing of lithium-bearing clay minerals, the extreme difficulties in trying to concentrate this group of minerals have been extensively discussed [94,95]. The problems associated with clay minerals arise from their structural nature. As these minerals structure themselves by atomic layers, different bonding energies develop between the layered structure of clay minerals. Consequently, these minerals are highly hydrophilic, which limits the recovery of lithium. Another issue associated with clay minerals is that when ground into fine particles, they tend to form clusters that increase the viscosity of the pulp during flotation, decreasing the lithium recovery efficiency. Changes in the pulp pH plus the addition of collectors and other chemicals may provoke mineral particles to lump, which also decreases the efficiency of flotation recovery.

More research efforts are needed on this particular topic to develop a better understanding of concentration operations with these minerals to develop specific processes for their beneficiation.

4.2. Processing of Spodumene Ores

Spodumene ore (LiAl(SiO₃)₂) is an allotrope [79,96,97] that is found as α-spodumene, with a monoclinic crystal structure. Upon heating up to 1100 °C, α-spodumene fully transforms into β-spodumene, which has a tetragonal crystal lattice. As the α-to-β transition occurs, the density of the spodumene decreases from 3270 kg/m³ to 2450 kg/m³ [96].

The reduction in density allows lithium to be extracted by leaching. The α-spodumene has a compact structure and low reactivity, whereas the β-spodumene becomes porous with the reduction in density, resulting in higher surface area and enhanced reactivity; because of this feature, it is imperative to heat spodumene ore to 1100 °C to induce the β phase.

Additionally, the presence of γ-spodumene has been reported, which has a hexagonal structure. Thermodynamic assessment of the three spodumene phases reveals that γ-spodumene is a metastable phase [79,98,99] that can be obtained from 800 to 900 °C; above this temperature interval, γ-spodumene transforms into its β-allotrope [79]. It has been reported that when milled, α-spodumene enhances the formation of the γ-phase [79].

Under current industrial practice, the conditions are set for the coexistence of the β and γ phases; as indicated, with the increase in temperature, the γ phase transitions spontaneously into the β phase. Experiments controlling the amount of γ phase showed that the presence of γ-spodumene hinders the extraction of lithium [79], hence the importance of transforming all the α-spodumene into the β allotrope. This last claim contradicts findings by Abdullah et al. [100]. These researchers followed the spodumene phase changes during its calcination using a hot-stage XRD apparatus. They pointed out that lithium can be extracted from γ-spodumene if it is activated thermally and mechanically. In a different report, Kotsupalo et al. [101] confirmed that combining mechanical activation (milling) of a spodumene sample with heat treatment (β and γ spodumene) gives good lithium recoveries by using alkaline leaching of a spodumene ore.

These phase transformations result in volumetric changes. The γ phase is 8% larger than the α one; similarly, the β lattice is 26% larger than α and 17% larger than γ. Additionally, the transformations α to β and γ to β are endothermic, while that of α to γ is exothermic [79].

Spodumene calcination is highly endothermic, which means this stage consumes considerable amounts of energy; unfortunately, calcination also results in significant heat losses to the surroundings. Typically, spodumene calcination is carried out in rotary kilns. Recently, the use of fluid-bed technology has been proposed to decrease the heat losses inherent to the operation of the kiln [13]. It was found that the fluid bed offers better temperature distribution in comparison with the kiln; additionally, when roasting the spodumene between 1050 and 1070 °C, it was found that conversion of α-spodumene approximates 90% with holding times up to 40 min. This makes fluid-bed technology an attractive alternative to decrease heat losses while achieving good conversion rates in lithium extraction.

Once the β-spodumene is formed, lithium can be extracted by leaching. This operation can be done using either an acidic solution, an alkaline one, or the concentrate can be chlorinated. Figure 10 shows a flowsheet that demonstrates the generic processing of spodumene.

Research has been conducted on extracting lithium from spodumene ores. As an example, Han’s group [102] recovered lithium from spodumene samples. To do so, they roasted several spodumene concentrate samples with different salts (NaOH, Na₂CO₃, NaCl, Na₂SO₄, KOH, Ca(OH)₂, CaCl₂, CaSO₄, (NH₄)₂SO₄). The roasting temperature changed with each salt. They found out that roasting the concentrate with NaOH yielded better lithium recovery at 88% after water leaching of the calcine. Roasting was performed at 320 °C with a NaOH/spodumene mass ratio of 1.5/1. Lithium was recovered as hydroxide.

In a different study, Rosales’ group [103,104], recovered lithium from leaching β-spodumene with HF. In this process, the authors were able to recover over 90% of the metal upon leaching at 75 °C with an acidic solution with a concentration of HF of 7% by volume. Furthermore, the lithium in the pregnant solution was recovered by precipitation as carbonate.

Despite recovering lithium as carbonate, there is no report of the purity of the resulting carbonate, or if there is a need to further refine the lithium carbonate obtained by those authors [102,103,104].

Barbosa’s group [105,106] studied chlorine roasting of β-spodumene. They found that β-spodumene chlorination with pure chlorine gas is highly sensitive to process temperature and that the extraction of lithium is kinetically described by the shrinking core model. Full lithium extraction from β-spodumene can be achieved at 1100 °C after chlorinating for 2.5 h. This research also showed that impurities such as aluminum and silicon do not react with chlorine gas and are left as solid residues.

4.3. Processing of Lepidolite, Petalite, Amblygonite and Zinnwaldite Ores

After spodumene, lepidolite, petalite and zinnwaldite are the most common lithium-bearing minerals processed for the extraction of lithium.

Like spodumene processing, these minerals undergo an ore dressing stage followed by a calcination/roasting stage before leaching them. For these mineral species, alkaline leaching is the most used [10,40,45,49]. Figure 11 shows a general flow sheet for the extraction of lithium from these mineral species.

Depending on specific deposits, some deviations from the flowsheet in Figure 11 are observed. For example, lepidolite can also be leached in acidic media. Hien-Dinh’s [107] and Yan’s [108] groups treated lepidolite by combining the ore with sodium sulphate (Na₂SO₄); the mix is roasted and then water leached. The addition of sodium sulphate during roasting helps in breaking down the crystal structure of the lepidolite while facilitating the dissolution of the lithium into the leaching liquor.

To achieve better lithium recovery, additions of FeS [107] or K₂SO₄ [108], along with CaO [107,108], prevent the ore from fusing, significantly improving the rate of lithium extraction. The extraction efficiency of lithium is over 85% when roasting at least at 850 °C and leaching with a water/calcine mass ratio of 10. Luong et al.’s [109] results are in good agreement with Hien-Dinh et al. and Yan et al.; actually, Luong’s group achieved a lithium extraction efficiency of nearly 91% when leaching calcines roasted at 1000 °C over 3 h and leached at 85 °C with a water/calcine mass ratio of 15/1. In their experiments, Luong’s group used a Na₂SO₄/Li molar ratio of 2/1, while in [107], molar ratios of FeS/Li (2/1) and Ca/F (2/1) were used. Yan’s experiments [108] used mass ratios of lepidolite/Na₂SO₄/K₂SO₄/CaO of 1/0.5/0.1/0.1.

To complement these observations, it has been proven that lepidolite recovery can be improved by doing ore flotation using mixtures of alkyl amine, alkyl alcohols and alkali metal salts [110].

In another report, Liu’s group [111] reported on the lithium extraction from lepidolite after leaching the ore in sulfuric acid under atmospheric conditions. This group found that lepidolite leaching kinetics are described by the shrinking core model. The best results obtained by Liu et al. were obtained when using a sulfuric acid to lepidolite mass ratio of 1.2 to 1; leaching experiments were carried out at 138 °C. The ore was ground to a mean particle size of 180 μm and then leached for 10 h. The efficiency of this process yields 94.2% of lithium. This process does not require previous roasting of the ore.

Kuang’s group [112] recovered lithium from lepidolite acidic leaching residues. The residue treated by these researchers was leached in an alkaline medium (NaOH). Besides lithium, aluminum was also recovered; firstly, the lithium-bearing residue was leached using a molar ratio of Na₂O/Al₂O₃ of 1.7. Lixiviation was conducted at 136 °C for 1 h. Aluminum was precipitated as hydroxide, while lithium remained in solution. To recover the lithium, sodium phosphate was added, and lithium was recovered as lithium phosphate. The yield of Li as Li₃PO₄ was 87.98%, using a molar ratio Li⁺/PO₄³⁻ of 3/0.98.

Tian-ming et al. [113] revisited several extraction processes. They concluded that acidic leaching is the best way to recover lithium from its main ores, i.e., spodumene, lepidolite, zinnwaldite, and petalite.

4.4. Processing of Lithium-Bearing Clays

To explain the processing of lithium-bearing clays, it is necessary to understand the nature of this mineral group. Clay minerals are ubiquitous as they can be found anywhere; they are sedimentary rocks of fine grains and can be utilized in many different applications [114]. In general terms, clays are a form of silicate. More specifically, they belong to the phyllosilicates group whose basic structure consists of a bidimensional tetrahedral network linked to an octahedral one or to some specific cations. Structurally, silicates are constituted by a tetrahedral network that forms as oxygen and silicon bond together.

The chemical bond between silicon and oxygen cannot be defined as ionic or covalent. On the one hand, the ionic model allows us to determine some properties of the silicates. On the other hand, the covalent model explains the directionality of the tetrahedral bonds [115].

The basic tetrahedral units of silicate (SiO₄⁴⁻) can be found forming long chains, or they can be joined together through other ions. In the former, bidimensional layers are formed, leading to three-dimensional tetrahedral networks. In the latter case, the SiO₄⁴⁻ groups are isolated. As the dimensionality of the tetrahedra increases, oxygen atoms are shared between more structural units, thus increasing the Si:O ratio [115,116].

This basic difference allows for the creation of distinct types of silicates. Micas are a particular kind of silicate whose tetrahedral structure has an electrical charge of −1. To keep neutrality in such a molecule, a cation with a positive charge is needed; potassium is the most common cation found in micas. Potassium bonds ionically with the layered silicate structure. This leads to two kinds of structure, either tetrahedron–octahedron (T-O) or tetrahedron–octahedron–tetrahedron (T-O-T). Figure 12 illustrates how these layers form.

In clay’s structure, lithium can partially substitute for ions like Mg²⁺ and Fe²⁺; however, it cannot substitute for K⁺ or Na⁺ ions. Additionally, in arid or semiarid climates, lithium is dispersed throughout the clay fraction of sedimentary rocks. Thus, lithium clay deposits are associated with desertic climates where, at some point, volcanic activity leads to a combination of hydrothermal and intrusive activity [117].

Lithium-bearing clays are divided into two great groups: smectites and illites. The smectite group has a T-O structure, whereas the illite group exhibits T-O-T structures [16].

Early studies on these minerals by Foster [118,119,120] revealed some important features of clays bearing lithium. It was determined that differences in chemical composition occur by isomorphic substitution. The atomic positions that some cations occupy within the clay structure are due to their size and not due to their valence number [120]. Because of this, the chemical composition at the outermost tetrahedral layers is simple, as only silicon and aluminum are the proper size to occupy such tetrahedral sites. In contrast, cations such as Fe³⁺, Cr³⁺, Mg²⁺, Ca²⁺, Fe²⁺, Mn²⁺, K⁺, Na⁺, and Li⁺ have the proper size to occupy octahedral sites; this explains why the chemical composition of octahedral sites is more complex. Thus, octahedral occupancy and its charges, along with tetrahedral charge, depend on how the silicate structure accommodates additional positive charges [119], These arrangements can be done in two distinct manners: (1) by a positive charge placed in the octahedral layer that is neutralized by a negative charge within the tetrahedral layer, or (2) by equivalent negative charges from an unoccupied site in the same octahedral layer.

Sedimentary rocks are found as layers that occur in clay sediments that tend to concentrate close to pegmatite deposits [81]. The formation of these minerals is the result of hydrothermal metamorphism. The presence of lithium can be attributed to a series of concentration phenomena induced by the hydrothermal effects.

In the specific case of lithium, its proportion increases in minerals as the aluminum concentration within the octahedral or tetrahedral layers decreases [118]. These compositional changes are regarded as continuous cation replacement of Al³⁺ with Li⁺ within the octahedral layer. Additionally, Fe²⁺ cations in the octahedral layer have a similar effect on Li⁺ as Al³⁺ cations have. According to the number of sites vacated by the aluminum cations, a series of distinct Li-bearing species can develop, from simple micas with nearly no lithium content to polylithionite that bears a higher lithium concentration. This means that a series of interrelated species form as these cation exchanges take place; furthermore, if Fe²⁺ cations participate in these exchanges, species from siderophyllite to lepidolites can be formed. This means that a series of solid solutions containing lithium can be found in these kinds of minerals.

Starkey [121], reported on the distribution of lithium in clay minerals. He identified that lithium can be found in cookeite as well as in illites and in some mica minerals. This fact is important since micas may degrade by weathering and transform into illite-type minerals and eventually, upon further weathering, end up as a part of the smectite group of minerals, which have a central octahedral layer sandwiched by two tetrahedral layers [122]. Williams and Hervig [123] concluded that illite clays result from a dissolution/crystallization mechanism. This kind of transformation is important for determining where lithium is allocated within the clay structure. It is likely that during weathering, lithium is carried into the structure of the resulting illite or smectite, meaning that mica minerals can be considered as lithium carriers.

If the lithium concentration is low, it can be found within the mica structure in sixfold sites associated with cations such as Al³⁺, Fe²⁺ or Mg²⁺ [121]. On the other hand, minerals from the smectite group contain higher concentrations of lithium. Starkey [121] reported that smectites are formed by either hydrothermal alteration or by precipitation in high-alkali lake deposits. As an example, swinefordite mineral is formed because of the weathering of spodumene.

Summarizing Starkey’s remarks, lithium in clay minerals is present as part of their structure. Lithium is allocated within that structure during clay formation; additionally, it is unlikely that lithium is found on exchange sites. Hydrothermal alteration may have a role in incorporating lithium into the clay structure. What is more, lithium cannot be simply removed by water from the clay because it is at the end of the cation replacement series, so any other cation would be removed from the clay instead of the lithium.

Work by Tindle and Webb [124] with clay minerals validated Foster’s observations. These researchers found the same trends as Foster did. These researchers correlated the lithium concentration in zinnwaldite, petalite and some muscovite micas bearing lithium as a function of the silica contents in these minerals after analyzing over 400 mineral samples.

Tindle and Webb [124] found that in the case of aluminum lithium micas, the lithium content in the clay reduces considerably with the increase in the alumina concentration; on the contrary, in ferrous lithium micas, lower alumina concentrations result in lower lithium contents. These observations are in good agreement with the notion of cation exchange expressed in [118]. In addition, Tindle and Webb confirmed the phase relations and boundaries between the different lithium species that are found in mica minerals, as they were initially proposed [118].

The authors in [124] also proposed a complete set of correlations to determine the lithium content in clay minerals as a function of some oxides such as MgO, FeO, TiO₂ and even fluorine.

Similar estimations for determining the lithium content from microprobe analysis were developed [125]; there is good agreement between the correlations in [126] and those reported in [124]. This approach to estimating lithium contents through other compounds found in clay minerals gives rise to the development of a graphical system to classify micas. From this development [125], it is confirmed that lithium is allocated in the octahedral layer of the mineral species structure that contains that element. Tischendorf’s group also noted that lithium concentration increases from mica-type minerals to polythionite. Lithium concentration grows as iron and magnesium contents in the clay minerals decrease. If the concentration of either magnesium or iron increases, it leads to species ranging from syderophillite to the different zinnwaldites. The classification proposed in [125] is in good agreement with the ternary representation originated by Foster [118] and replicates elsewhere. Tischendorf representation allows for a better understanding of the relations that exist between the different elements present in clay minerals, particularly that of magnesium to lithium.

Of all correlations developed, the SiO₂ one is the most accurate since it provides the most acceptable values for lithium concentration; nonetheless, precise instrumental chemical analyses are necessary [127]. These phase relationships are shown schematically in Figure 13.

As depicted in Figure 13, there are some relationships between lepidolite, trilithionite, polylithionite and zinnwaldite. Lepidolite is a phyllosilicate mica that has a variable chemical composition [78] and polymorphic structure [16]. Additionally, lepidolite represents a series of solid solutions of aluminum-rich micas with trilithionite and polylithionite [16]. Polylithionite has higher lithium content, whereas trilithionite has a lower lithium concentration. These mineral species consist of two tetrahedral silicate layers, and between them, there is an octahedral layer. Zinnwaldite is a variation of lepidolite with high iron content that is low in lithium concentration [78].

Independent work on micas rich in Li, Fe and Mn [129,130] confirmed that Fe and Mn can substitute for Al and Li within the octahedral sites in Li-bearing micas. Brigatti’s research group found the actual mechanism that allows for the transformation of siderophyllite into polylithionite. Cationic exchange between octahedral and tetrahedral layers within the silicates is key in determining the presence and location of lithium in these minerals.

Calvet and Prost [131] showed that upon heating, lithium in montmorillonite clays exhibited two characteristics:

(1)

Unheated clays have fewer exchangeable cations;

(2)

Clay structure has local atomic sites of the trioctahedral kind.

In considering trioctahedral and trioctahedral-structured clay minerals, they exhibited similar solvation properties, and so, the presence of interlaminar cations is more important than the actual crystalline structure. Heated lithium-bearing montmorillonite shows the importance of the number of cations on the solvation properties of the clay surface.

Water molecules cannot reach the inner clay surface when there is less than 50% interlaminar cations. This feature is important in tailoring an adequate leaching scheme for the extraction of lithium from clay minerals.

Calvet and Prost [131] demonstrated that lithium ions move within the clay structure towards occupying octahedral sites; Starkey [121] also realized this fact. Consequently, negative charges could be located around the octahedral layer, facilitating the lithium exchange. However, a reduced fraction of lithium in unexchangeable form remains locked within the clay structure, leading to structural modifications creating a trioctahedral configuration as cation exchange occurs. This mechanism is closely related to clay swelling. Feng et al. [132] indicated that illite minerals have low activity because their interlayer is cation-saturated, and to promote cation exchange, alkali additions are needed. Montmorillonite clays offer higher resistance to acidic leaching [133].

Additionally, it is reported that lithium aluminosilicates found in pegmatites such as spodumene and lepidolite, among others, transform into clay-type minerals because of hydrothermal alteration [134]. One of these transformations occurs by dissolution of spodumene, resulting in cookeite [135], which is a mineral of the chlorite group that is a phyllosilicate with a T-O-T structure that is closely related to smectite clays [136].

Only a small fraction of the world’s clay deposits contains lithium; what is more, it is estimated that lithium-bearing clays represent approximately between 7% and 9% of the total lithium global resources [9,137]. Developments of lithium extraction from clay minerals seem to be in the area delimited by the western end of the Mexico—US border and in the Jadar region in Serbia [9].

The processing of clay-bearing lithium has been little investigated. Zhao’s [16] review of the subject matter reveals that these types of minerals could be an alternative and attractive source of lithium. However, it is reported that in clay minerals, species such as lepidolite and zinnwaldite, among others, are found. Zhao’s review suggests processing lithium-bearing clays as if they were not sedimentary rocks. Zhao’s work puts strong emphasis on the use of acidic leaching processing to extract lithium. It suggests the use of sulfuric acid solutions of different concentrations as the main leaching reagent; it also suggests the use of sulphating calcination. A third option to process lithium-bearing clays, according to Zhao, is to use alkaline reagents, mainly sodium hydroxide, either as a regent for calcination or during leaching. Depending on the metallurgical route selected, it is possible to obtain different lithium salts, such as LiF, Li₂CO₃ or Li₂SO₄, in aqueous solution.

Aylmore et al. [128] demonstrated how the lithium content in these minerals directly relates to the other chemical elements in these species. Aylmore’s work found similar relationships to those previously developed [124] between the lithium content in the clay minerals and other components in them. From these relationships among different mineral species, and to recover as much lithium as possible, it is suggested to remove quartz, feldspar, and similar minerals from the clay. To achieve such a goal, intensive grinding is suggested before the extraction process, such as leaching. It is recommended to grind until reaching a mean particle size in the order of 180 μm to expose lithium and facilitate its recovery.

In other studies [138,139], the leaching of pegmatite ore was compared with that of smectite-bearing lithium clay. In these studies, the authors found that there is no significant difference in processing either kind of mineral by sulfuric acid leaching. Their data analysis showed that the difference in terms of energy and reactant consumption, along with the overall costs of these operations, showed differences on the order of 10%, thus concluding that these differences are not as significant.

Amer [140] ground clay samples to different mean particle sizes, ranging from 40 to 250 microns. The ground material was leached in an autoclave between 180 and 250 °C with sulfuric acid in concentrations ranging from 4 to 8 M and a solid-to-liquid mass ratio of 5–15. It was found that the best lithium extraction into the leaching liquor was achieved with mineral particles of 40 microns treated with a 7 M sulfuric acid solution at 250 °C and 90 min of processing. The extracted lithium was then converted into lithium carbonate of unspecified purity but with a yield of 12 g/L of lithium sulphate.

The U.S. Bureau of Mines commissioned extensive research on the recovery of lithium [141,142,143] from a clay-bearing mineral from the McDermitt deposit in the state of Nevada. The mineral body from the McDermitt caldera has a varying lithium concentration ranging from 0.1 to 0.6 wt%. The main lithium species in this deposit was Hectorite, whose chemical formula is Na₀.₃₃(Mg, Li)₃Si₄O₁₀(F, OH)₂.

Davidson [141] attempted lithium extraction by chlorinating the lithium silicate in the mineral to produce soluble LiCl. Initial tests were conducted by injecting a stream of HCl onto a static bed of dry clay mineral from 500 to 850 °C for 30 min at a flow rate of 70 cm³/min of dry HCl. After chlorination, the resulting calcine was leached in water at 80 °C for up to 60 min. Results from this experimental set showed some lithium extraction was polluted with calcium. To limit the amount of calcium extraction, a second set of tests was carried out. This time, mixtures of HCl–water vapor were used for chlorination. The resulting calcine was water leached under the same conditions as in the first set, resulting in a decrease in the calcium extraction; unfortunately, the same was true for lithium extraction. The best result from this second test yielded a lithium extraction of 20%.

To improve this lithium recovery while hindering calcium extraction, Davidson tried chlorinating the clay mineral with HCl-H₂O (vapor)-CaCO₃ mixtures. Lithium recovery improved, but its extraction was still limited by the presence of free silica in the clay. This free silica reacted with some of the chlorides obtained to form insoluble chlorosilicates.

Lien [142,143] tried a different approach for treating the same mineral body as Davidson did. Lien added gypsum and limestone to the ground mineral to produce pellets of 6.5 mm in diameter. The pellets were calcined at 900 °C in a rotary furnace. From this operation, a calcine with lithium sulfate was obtained; this sulfate is a highly soluble salt in water. The calcine was then water leached up for 30 min; the resulting solution carries a mixture of lithium, sodium, potassium and calcium sulfates. Before producing the lithium carbonate, it is necessary to remove these other sulfates. The removal of calcium sulfate can be achieved by filtration operations.

Once the impurities are removed, the resulting solution is concentrated by evaporation to increase the lithium concentration. At this point, sodium carbonate is added to the lithium-rich liquor. Lithium carbonate is obtained from a cation exchange between lithium sulfate and sodium carbonate. This process offered an overall yield of 82%, recovering lithium carbonate with 99% purity.

In the United States, a project is currently under development in the Thacker Pass reserve in the state of Nevada; this project has been conducted by Lithium Americas Corporation [144]. This project has a lifespan of 85 years of operation and is designed to process up to 160,000 tonnes per year of clay minerals. The lithium minerals found in this development consist of clays from the illite and smectite groups, with lithium concentrations ranging from 2000 to 4000 ppm for the latter, while the former contains higher than 4000 ppm of lithium. Deeper sites in the prospected area have shown lithium concentrations up to 9000 ppm.

Another important development in this subject is in Mexico. Bacanora Minerals Limited commissioned several studies [145,146,147,148,149] to verify the presence and concentration of lithium in the area known as La Ventana. Studies on this site determined, in a preliminary way, the feasibility of installing an extraction plant to produce lithium carbonate from clay minerals. However, only inferred resources were determined [148], and further work was needed to determine the actual lithium reserves. The concentration of lithium reported in these works [145,147,148] is in good agreement among them; only minimal differences were found. It is believed that the lithium concentration at the La Ventana site ranges from 1800 to 4000 ppm, and the mineral feedstock is a mixture of calcite, quartz, K-feldspar, and montmorillonite with traces of swinefordite, along with smectites and illites with occasional occurrence of polylithionite and hectorite [145].

Servicio Geológico Mexicano has explored the entire country and has found the presence of lithium in 18 out of 32 states in Mexico [150]. Because of this occurrence and given the strategic value of lithium, in 2022, the Mexican government expropriated all lithium natural resources and created the state-owned company LitioMx, whose main objective is to commercialize the lithium supply chain by adding value from exploration through battery design and commercialization. To provide technical support for LitioMx, a thorough research program was commissioned by the Mexican government to develop an extractive process to recover the lithium from the clay minerals and to recover the lithium as battery-grade lithium carbonate. As a result of these efforts, a patent for this process has been issued [151].

The Mexican developed process (CIMAV process) is characterized by having extremely low water consumption and the energy usage is only a fraction of the energy used in processing spodumene ores. On top of that, the CIMAV process does not use acidic or alkaline solutions to leach out the lithium; it uses water leaching, thus decreasing its environmental impacts. Seven mineral bodies from distinct regions in Mexico with different mineralogical associations were treated. The lithium concentrations found in these minerals ranged from 800 ppm Li to 4000 ppm Li. In every case it was possible to recover battery-grade lithium carbonate (purity ≥ 99.5 wt%) with different recoveries. Leaner minerals offered lower yields, whereas more concentrated minerals resulted in better lithium recoveries.

Currently, a project to design, build and operate a pilot plant to produce lithium carbonate from lithium-bearing clay minerals is under development and it is expected to start operations by the end of 2026.

4.5. Processing of Brines

Brines can be defined as highly concentrated saline solutions [152]. The geological origin of brines is still unsolved [153]; many theories describing the origin of brines are available. Some of them include [153]:

○

Sedimentation: Brines originate from seawater;

○

Magmatic: Brines emerge from the Earth’s mantle;

○

Leakage: Brines stem from fluid inclusions in bedrock due to the presence of pressure gradients.

In addition, brines can be categorized into three main types: closed-basin, sedimentary-basin (oil field) and geothermal-basin [154]. In the case of the closed-basin type of brine, it is the most important as it is the principal source of lithium. Besides this, lithium recovery from these resources is the most economical to process and has the lowest environmental cost associated with their processing. Most of the lithium recovered from brines is obtained through evaporation in ponds; however, direct lithium technologies seem to be an attractive alternative to recover lithium from brines. This will be discussed.

Closed-basin brines have seven features that allow them to be recognized from other types of brines [154]:

(1)

Arid Climate: Contributes to the formation of salars. It also factors into the concentration of lithium in brines. Determines the rate of evaporation during the recovery of lithium.

(2)

Contains a salar: Closed-basin brines are associated with a salar or a salty lake. This feature is controlled by tectonics and climate. In salars, brines are found in shallow aquifers that contain mixtures of salts like halite (NaCl), gypsum (CaSO₄), and some carbonates, in addition to volcanic ashes and alluvial deposits (resulting from hydrothermal activity). Because of the presence of these species, lithium enrichment is found, although the enrichment is in the low lithium concentration range.

(3)

Associated hydrothermal or geothermal activity: This provides enough thermal energy to increase the lithium concentration through leaching and evaporation processes, besides inducing flows to transport the lithium from its source to the bulk of the brine. It may contribute to the alteration of clays.

(4)

Tectonic activity: Closed-basin brines occur in sites where tectonic drivers like extension, transtension, or orogenic loading occur.

(5)

Sources of lithium: Lithium may originate from high-silica rocks like ignimbrites and ashes, lithium-bearing clays, and other salar salt deposits.

(6)

Time: Refers to the period required to leach, transport, and concentrate the lithium. The mechanisms involved in these three stages are not well understood yet. However, evidence in North and South American brine sites indicates that brines are geologically young.

(7)

Hydrogeology: This accounts for the combination of flow phenomena involved in the recharging and transport mechanisms that are present between the brines and fresh waters. These phenomena control the mass flux of lithium from unsaturated to saturated regions and impact the determination of the size of the brine deposit and possible economic viability.

Sedimentary-basin lithium brines are associated with oil fields. These kinds of brines are found worldwide; however, these brines are more common in North America [154]. Collins [155] determined that brines associated with oil fields may contain up to 700 ppm of lithium; also, these brines have low magnesium concentrations. These brines are found in deep reservoirs [156] at temperatures below 100 °C. Additionally, it has been discussed that these brines result from mixing solutions of different origins [157,158]. Dissolution of NaCl and the concentration of calcium and magnesium may indicate the mixing of the brines [157].

Nonetheless, brines are located almost everywhere on the planet. Brines can be found in salt lakes, sedimentary basins, oil fields, geothermal fields, etc. [156]. A specific feature of brines is the amount of minerals and their concentration that they carry. Some of the mineral concentrations in brines can be, if not higher, at least like those found in ground deposits [159]. In the case of lithium, it can reach different concentrations, ranging from below 50 mg/L up to values higher than 325 mg/L [160]. The relatively high lithium concentration makes brines the most attractive source of lithium, economically speaking [161]. It has been reported [49] that the brines have distinct chemistry; sulfates, chlorides, or carbonates have been found in different brine sites [162]. Liu et al. [163] considered brines as complex systems comprised by eight components: Li⁺, Na⁺, K⁺, Mg²⁺, Cl⁻, SO₄²⁻, B₄O₇²⁻, CO₃²⁻ and water. Regardless of the specific chemistry of the brines, their processing is quite similar. An important parameter to keep in mind when processing the brines is the mass ratio that exists between the magnesium and lithium in solution [49,162,164,165,166,167]; for example, Mg/Li ratios from less than one to over 7000 have been reported [168]. The problem with magnesium is that its properties are very similar to those of lithium [169,170].

Table 3. Some properties of cations in brines. Data from [168,169,170].

Cation	Hydrated Ionic Radii at 25 °C [Å]	Hydration Number	Distance M-O [Å]	Ionic Radii/Coordination Number [Å]
Li+	3.82	4 5 6	2.08	0.59/4 0.60/4 0.76/6 0.79/6
Na+	3.58	4 5 6	2.35	0.99/4 1.02/5 1.02/6 1.07/6
K+	3.31	3 6 6–8	2.79	1.37/4 1.38/6 1.46/7
Ca2+	4.12	6 6–10	2.42	1.00/6 1.12/8
Mg2+	4.28	6	2.09	0.57/4 0.72/6

shows some of these properties.

According to the data in Table 3, the mean distance between Li⁺ and oxygen and that between Mg²⁺ and oxygen are quite similar; this value complicates Li-Mg separation. If the Mg/Li mass ratio becomes larger, then separating lithium from magnesium in solution becomes challenging. Additionally, it has been reported [171] that lithium in brines not only exists as a simple cation but has also been found as a complex ion.

Regarding the processing of brines, there are two major approaches to treat these resources. The first one is the traditional evaporation technology, followed by selective precipitation of impurities to finally precipitate the high-purity lithium carbonate. The second approach consists of a series of technologies known as Direct Lithium Extraction (DLE) [49,168,172,173]. Depending on the author, DLE technologies are defined as a group of four technologies [160] up to a group of seven technologies [173]. Regardless of these classifications, DLE technologies aim at being highly selective for lithium. Figure 14 illustrates how these distinct technologies are related.

As stated, brines are the main natural source of lithium. The main incentive to extract lithium from brines using evaporation technology is its low cost, as this processing relies on solar irradiation to induce the evaporation of the brine [55,152,159]. This technology is characterized by increasing the concentration of lithium in the brine by evaporating considerable volumes of water with solar exposure. Lithium and other elements like sodium, potassium, calcium, and magnesium are selectively precipitated by inducing pH changes or modifying some salts’ solubility using specific reagents.

A consequence of the precipitation technology is that it has extremely low production rates, and it takes up to two years to obtain valuable lithium salts from brines in South America [152]. Another important issue to consider with evaporating lithium production from brines relates to water usage. The brine itself is evaporated to concentrate the lithium, at the expense of losing large volumes of water; the remaining liquid is treated to recover lithium, and the final discharged solution is not suitable for consumption. Furthermore, considerable amounts of fresh water are needed in the final refining stages of lithium carbonate from brine, and the by-products of brine evaporation are unsuitable for that task [152]. To complicate matters, most lithium brine resources are commonly found in water-stressed locations [55,152,154,160,173,174].

Conventional brine processing involves transferring the brine into shallow ponds to evaporate the water with solar irradiation to increase the lithium concentration in the brine. The concentrated solution is then pumped into a lithium carbonate facility to convert the lithium from the brine into lithium carbonate or hydroxide. To do so, a series of procedures are followed [55] (borate–lithium coprecipitation, phosphate coprecipitation, aluminate precipitation) to obtain the desired salt. A basic flowsheet [49] for the processing of the brines is shown in Figure 15A. As seen in this figure, magnesium and calcium are separated from the brine by precipitating them as hydroxides in consecutive process stages. After the initial separation of these two elements, the remaining brine is re-evaporated to eliminate residual calcium and magnesium. Lithium in the brine increases its concentration as water from the brine is evaporated, and sodium carbonate is added to the lithium-rich solution to precipitate the lithium carbonate. This carbonate is then filtered and washed until it reaches the desired purity.

Another alternative to treat high-Mg/Li-mass-ratio brines proposes the use of aluminum chloride [167] to remove magnesium and other elements. The unwanted elements are precipitated as hydroxides, and then hydrochloric acid solutions are used to remove boron and to control the presence of CO₃²⁻ ions in aqueous solution. Sodium carbonate is added after concentrating the remaining brine, and the final product is lithium carbonate. This is shown in Figure 15B.

Alternatively, An et al. [162] developed a process using oxalates to treat brine samples from the Uyuni salar in Bolivia. In their processing, they removed calcium and magnesium as oxalate. Upon the elimination of these two elements, sodium carbonate is added to the remaining brine to precipitate battery-grade lithium carbonate. Figure 16 shows the flowsheet developed by the An’s group.

The use of different aluminum-bearing materials to recover the lithium from the brine has been reported. Liu et al. [175] mixed and ground aluminum powder with sodium chloride. The resulting powder was added to a synthetic solution with a high Mg/Li mass ratio. The solution consisted of chlorides. The solution and the powders were constantly stirred for half an hour, and then the leaching liquor was separated from the solid residue. This work showed that most of the magnesium was kept in the solid residue, while the lithium in solution was precipitated as a complex of Li, Al hydroxide. The authors claim that this processing allows for a successful separation of lithium from magnesium; however, the conversion of the Li-Al hydroxide to either lithium carbonate or hydroxide is not reported.

Paranthaman et al. [176], developed a double-layered micrometric Li-Al powder to separate the lithium from the magnesium in a brine. The authors claim, after conducting laboratory-scale tests, that this Li-Al hydroxide is highly selective for lithium and recovers over 90% of the lithium in the brine. Lithium is recovered as chloride, which needs to be converted into carbonate or hydroxide. The authors do not report on this matter.

Li et al. [177] tested Al-Fe and Al-Ca alloys to recover lithium from brines. Both aluminum alloys were mixed and ground with sodium chloride to obtain a fine powder. These mixtures were leached with synthetic lithium chloride solutions. Lithium from leaching experiments is recovered as LiCl∙2Al(OH)₃∙H₂O. Of the alloys tested, the Al-Ca alloy had better results than the Al-Fe alloy. Recovery of lithium of 94% is reported; however, these authors do not report on the purity of the lithium compound obtained nor on the conversion into lithium carbonate or hydroxide.

As reported, aluminum has been tested to recover the lithium in brines containing chlorides [162,167,175,176,177]. Liu et al. [166] treated synthetic lithium sulfate solutions to separate the lithium from such synthetic brine using aluminum-based powders. Aluminum powders were mixed and milled with sodium sulfate, and the resulting mix was then added to synthetic sulfate-based brine. It was found that a complex lithium–aluminum salt (Li₂Al₄(OH)12SO₄∙H₂O) was obtained from these experiments. Lithium recovery from this sulfate brine is not as efficient as that reported for the chloride brine. The higher the magnesium concentration in the sulfate brine, the more difficult lithium extraction is. Additionally, the method to convert the resulting lithium salt into lithium carbonate or lithium hydroxide is not reported.

Kaplan [178] tested AlCl₃, Al₂(SO₄)₃ and NH₄Al(SO₄)₂ to recover lithium from the Dead Sea samples. In either case, lithium precipitated from the samples as lithium aluminate. Lithium was separated from the aluminum by adding sulfuric acid to the aluminate, producing soluble lithium sulfate, and after adding ammonium sulfate, insoluble aluminum alum (NH₄Al(SO₄)₂ 12H₂O) precipitates. Kaplan also realized that the presence of magnesium hinders the recovery of lithium.

Regarding DLE technologies, they aim to be highly selective for lithium. DLE technologies are developed to overcome the low productivity of evaporative brine processing. DLE offers alternative lithium-bearing brine processes that are characterized by not relying on the evaporation of the brine. Instead, these processes are highly selective for capturing lithium; because of this, DLE can be applied to recover lithium from lean brines [179]. Additionally, brines from different origins can be treated by one or a combination of DLE technologies.

One of the DLE technologies developed for lithium extraction from brines is ion exchange. Ion exchange consists of using highly selective mesoporous material membranes that can capture lithium. Such materials have active sites that pick up the lithium ions present in the brines. Manganese- [56,180,181,182] and titanium-based materials have been utilized [168,183] to create ion exchangers. Membranes based on manganese tend to dissolve in acidic solutions, whereas titanium-based membranes do not have this problem [168]. Lithium can either be adsorbed on the membrane surface, or it can be absorbed by the structure of the membranes [152]. Nonetheless, after a certain time, the membranes tend to get polluted from residual elements present in brines; because of this, the membranes need to be cleaned or replaced with new ones [168].

An alternative to extracting lithium from brines is the use of ionic liquids. These compounds are defined as organic solvents comprising ions that have melting points below 100 °C [184,185,186]. Ionic liquids exhibit limited flammability and volatility. Those properties, along with their ionic nature, allow them to be used to extract specific ions such as lithium. The high selectivity of ionic liquids for lithium allows for the effective extraction of lithium in brines with high Mg/Li ratios [187].

Problems associated with the processing of lithium-bearing brines using ionic liquids are the pollution of the lithium-bearing solution with organic anions and the generation of pollutants to the environment [172,187]. Another drawback associated with ionic liquids is that they are expensive and have not been tested yet on either a pilot or commercial scale.

Electrochemical processes are also used to facilitate the extraction of lithium from brines. In particular, the use of electrodialysis with porous Mn-based membranes has been tested to extract lithium from brine solutions. However, after some time, the membranes need to be cleaned to enhance their ability to extract lithium, which delays the production of lithium salts.

4.6. Other Technologies

Active metals like aluminum, sodium, calcium, and magnesium, among others, are susceptible to being processed by molten salt electrolysis [188,189]. According to different reports [190,191], molten salt technology could be useful in processing lithium from its mineral sources to produce the salts required to make the electrolytes for lithium-based batteries.

Molten-salt processing consists of the decomposition of a compound in an ionic melt mixture under the application of an electrical current [192]. Due to this fact, molten-salt technology can be highly selective in extracting lithium from its ores. An advantage of these technologies is that they do not use water, so finely ground mineral particles can be directly treated. Additionally, the proper selection of the molten-salt system to treat the lithium-bearing minerals could improve significantly the efficiency of the lithium extraction.

Selection of the molten-salt system is paramount, since the electrolyte not only provides the conducting medium to reduce the lithium, but it also serves as a degasser; it also helps in removing undesired inclusions while promoting the metal recovery [193,194]. Typically, electrolytes consisting of LiCl–KCl mixtures are used in the electrolysis of lithium by molten salts [195,196,197]. This system is used due to its relatively low melting point, at its eutectic composition (41% KCl, 352 °C) [198]. Another reason to use this chloride mixture relates to its low cost and chemical stability [199].

Process parameters such as cell temperature, current density, anode to cathode distance, and applied voltage, among others, influence the performance of the lithium production cell [193,195].

Metallic lithium was produced by Sato et al. [196] between 400 and 440 °C in a laboratory cell utilizing 400 mA of current with an efficiency of 91% of lithium deposited. They used a LiCl–KCl electrolyte but realized that the electrolyte was consumed as lithium was deposited, needing to constantly feed LiCl to the system to maintain lithium production. To counteract this negative effect, they added lithium carbonate to the chloride mixture to reduce the LiCl consumption rate. It was found that lithium carbonate in moderate concentrations helps in the performance of this system [200].

Higher current densities being applied to the electrolytic cell result in increased lithium reduction kinetics; but on the negative side, carbon anodes (if used) are consumed at faster rates through the formation of CO₂ bubbles. Using current densities in the order of 0.3 to 0.4 A/cm² is suggested [195].

The gap between the cathode and anode should be kept between three and five centimeters. A larger separation of the electrodes reduces the efficiency of the electrolysis, whereas a shorter anode-to-cathode distance may result in lithium reacting with the chlorine gas emanating from the anode [195]. On the other hand, the best operating cell temperature for the LiCl-KCl system is 420 to 550 °C. The higher the cell temperature, the lower the operating voltage observed [195].

Takeda [197] mixed the LiCl-KCl and LiCl-KCl-CsCl mixtures with LiOH to produce metallic lithium. They ran their tests between 300 and 400 °C and used current densities of 0.3 and 0.05 A/cm². They found that this process is unviable due to the presence of excessive CO₂ generation at the cathode.

Another factor to consider relates to bubbling. If carbon cathodes are used, CO₂ gas is generated while lithium is reduced. The presence of this bubbling adds electrical resistance to the system, as the bubbles grow from small bubbles to a complete gas layer covering the entire anode surface. Laboratory testing has shown that electrolysis is highly sensitive to the presence of these bubbles [201]. The presence of the CO₂ bubbles is known as the anode effect. This demands an increase in power to refine the metal while reducing the productivity of the refining process [202,203]. To counteract these effects, research on the use of inert electrodes is required. Additionally, the voltage required for the reduction of lithium is −3.05 V. Calculations with HSC software V 10 [66] reveal that to reduce lithium electrolytically, one needs to apply up to 10 V. Hoh et al. [195] reported that to produce lithium by molten-salt electrolysis, one needs to supply 6 to 10 V and 80 A of electricity to overcome the distinct electrical resistances that occur in an electrolytic cell. This value also indicates that considerable amounts of energy are needed to refine lithium with this technology. To run lithium electrolysis, one is required to supply 28 kWh/kg Li produced [204]; this represents an important cost in this kind of lithium processing.

As mentioned, other problems associated with this processing scheme relate to the production of harmful CO₂ emissions, and oftentimes, the carbon reacts with elements present in the molten electrolyte to produce hazardous CFC chemicals [205].

Another issue to consider relates to the low density of lithium (534 kg/m³). Lithium floats on top of the denser electrolyte used to produce the metal [195]; therefore, it is necessary to have a cell design that allows for the rapid separation of lithium from the electrolyte. Providing the electrolysis cell with a diaphragm or a special collector to collect lithium has been proposed [204,206].

According to these statements, there are research opportunities in cell design [206] as well as in testing different electrolyte systems [207]. Operationally, extensive test work is needed in optimizing process parameters to reduce the processing costs associated with molten-salt electrolysis.

Another technology that needs to be explored for processing lithium from various mineral sources is mechanochemistry [208,209,210,211]. This processing induces chemical reactions in the solid state by supplying mechanical energy to the system under study. Mechanochemistry has been used in different industries; however, in mineral processing, it has not been extensively tested yet. The advantages associated with mechanochemistry are that it does not use water, and it is not expected to produce toxic emissions.

Some attempts to obtain lithium through the implementation of some sort of mechano-activated processes are available [100,212,213]. These works report on the combination of mechanical (milling) activation of spodumene [100], petalite [212], and lepidolite [213] with calcination and acidic leaching. In every case it is reported that lithium recovery improves after mechanical activation of the lithium-bearing minerals. However, there are no reports depicting a process to extract and/or recover lithium from its ore solely by mechanochemical synthesis.

Perhaps, the main issue associated with the adoption of mechanochemical processing of lithium minerals relates to the scaling up of the devices to carry out the production of the desired lithium salts and, in addition to this issue, the pollution that grinding media could potentially induce in the synthesis of the lithium minerals [214,215]. Other advantages of mechanochemical processing are the reduced waste generation, which is well-suited for circular economy strategies. However, as these technologies are not yet fully implemented at an industrial scale there is little knowledge on how to scale up these systems [215].

Deciding on whether a continuous or batch operation is required at an industrial scale needs to be supported by experimental data. Many milling devices are available on a laboratory scale; however, the results from them might not translate adequately to a larger scale.

In spite of this, conventional milling in rotating drums is a robust technology used in mineral processing that can be adequate to run some tests on a larger scale. However, these mills provide low-energy grinding and their performance cannot be directly compared with high-energy milling devices. This might be difficult for the transition from the laboratory scale to industrial mechanochemical processing of lithium-bearing minerals.

Another issue found with this technology relates to the pollution from the grinding media to the processing material. It is known that grinding media in ore dressing operations somehow pollute the minerals under processing as a consequence of the continuous grinding mechanics; the grinding media wears down and needs to be replaced. The main pollutant is iron. The concentration of iron can be increased by the wearing of the grinding media, which may impose additional difficulties on the extraction and refining of lithium. If milling is conducted with ceramic media such as zirconia or silica-based materials, this could represent significant processing costs.

These issues constitute significant technical challenges that need to be resolved as the demand for lithium consumption constantly increases. Furthermore, basic research on the applicability of mechanochemistry is needed to evaluate its feasibility for processing lithium-bearing minerals.

Another alternative worth investigating is that of biohydrometallurgy, which is based on the metal extraction by organic acids produced by live microorganisms like bacteria and fungi, among others [216].

In this regard, Duan et al. [217] leached lepidolite ore samples with P. chrysogenum and R. mucilaginosa fungi that were developed in a cultivation medium consisting of sucrose, NaNO₃, KH₂PO₄, MgSO₄ and 7H₂O. This medium was set at a pH of 7 and was processed in an autoclave for 20 min at 120 °C and 0.1 MPa of pressure. After this period concluded, the mineral was leached at 30 °C for 60 days at 160 rpm. However, no actual lithium recoveries were reported.

Rezza et al. [218] treated a spodumene ore sample with 6.7% content of Li₂O. To treat this mineral sample, a bioleaching scheme was developed. To do so, two culturing media were used to grow microorganisms; both of these media consisted of a mixture of glucose and ions such as Fe²⁺, K⁺ and Mg²⁺ in different proportions. Leaching was conducted after allowing the growth of the culture media for 48 h. Leaching experiments were conducted at 30 °C under constant agitation of 200 rpm for up to 12 days. Lithium was recovered from both culture media in solutions containing approximately 0.8 ppm Li (culture media 1) and nearly 1.0 ppm Li (culture media 2).

In another study [219], the results from bioleaching lepidolite ores using three different biological systems are reported: acidophilicibacteria, imicroscopicifungus and yeasts. It was found that in these systems Li can be extracted with these techniques; however, the lithium extraction mechanisms differ in each of these systems. In either case, the amount of microorganisms present has a strong influence on the rate of lithium extraction.

Marcincakova et al. [220], recovered lithium after bioleaching samples of lepidolite ores in solutions made of Rhodotorula rubra. The leaching experiments lasted for up to 50 days. Lithium was recovered in aqueous solutions containing up to 0.17% Li.

From these reports it appears that bioleaching may become an attractive technology for recovering lithium; however, the reaction rates reported are extremely slow, which may reduce its applicability; another issue to consider relates to the recoveries of lithium. The values of lithium recovery are extremely low. No mass and energy balances are presented; thus, it becomes difficult to have a real assessment of this kind of processing. This bioleaching technology has only been tested on a laboratory scale. Further developments are needed in this particular subject.

5. Environmental Impacts

Extracting lithium from brines, clays, or pegmatite rocks requires considerable amounts of energy. Water and chemical consumption also negatively impact the environmental footprint associated with lithium extraction. As the need to fulfill the demand for lithium increases, it will be necessary to explore and identify new lithium mineral deposits. Additionally, better characterization of current and future deposits is required [221,222]. This will provide data useful for making reliable environmental assessments of lithium salts production. It is necessary to make such evaluations from actual process data [59]. Information related to the consumption of chemicals, energy, and water usage may vary from site to site, even though the source of lithium is the same. Considering the extraction of lithium from brines, the environmental footprint changes due to different factors such as initial lithium concentration in the brines, energy, and water availability, among others. Life cycle analysis (LCA) is a very useful tool to assess and improve different productive systems. When used appropriately, LCA may help in deciding on the use or implementation of new technologies; this is because LCA heavily relies on the data gathered from production systems. This fact represents a serious concern for the proper application of LCA methodologies, as large data may induce different degrees of uncertainty. In addition, the nature of the data collected (measurements or modeling results) also influences the propagation of uncertainties [223,224]. Particular considerations must be taken when trying to incorporate unexpected events that might arise within a system [224].

LCA is determined by evaluating material and energy flows used in a specific productive process.

Life cycle analyses on lithium extraction from brines and pegmatites indicate that lithium extraction from brines has less impact than the processing of pegmatites [59,225]. Brine processing demands less water and energy usage. In the case of the processing of pegmatites, it has been reported that acidic leaching of calcines has the most negative impact. No mention is made of the impacts associated with the calcination stage [225].

In the case of lithium-bearing clay minerals [61], to date, an established commercial process to extract lithium from clay minerals is not known to convert it into lithium carbonate or lithium hydroxide. Nevertheless, extensive experimental work by Lithium Americas indicates that the lithium contained in clay minerals is extracted by sulfuric acid leaching. This processing has fewer negative impacts than pegmatite processing but is more impactful than brine processing. The environmental footprint associated with clay minerals relates to the use of reactants such as limestone and soda ash, among others, rather than the actual extraction process itself.

Production of lithium from distinct sources results in the production of different CO_{2 (eq)} emissions. These emissions arise from factors such as energy sources, mining methods, lithium origin, and concentration, among others. Additional factors impacting the generation of CO_{2 (eq)} emissions are the use of chemicals and water throughout the extraction processes used to produce lithium salts such as lithium carbonate or lithium hydroxide. The final product also influences the environmental footprint related to lithium processing.

Another factor to consider in assessing environmental impacts due to lithium processing relates to land usage [226,227]. The land usage concept considers direct and indirect land usage to produce lithium. Direct land usage includes the area used by processing plants, landfills for waste disposal, and storage of raw materials, and in general, all the surface area needed to produce lithium carbonate or lithium hydroxide. Indirect land usage refers to energy, materials (chemicals), and water usage in lithium salt production. Land usage is expressed in square meters per ton of lithium carbonate equivalent (LCE) produced per year.

Lithium extraction from pegmatites has moderate direct and indirect land usage per ton of LCE produced. On the contrary, evaporative brine processing requires more direct land usage due to the construction of the ponds used to evaporate the water from the brine. Brine processing has the lowest indirect land usage due to the little generation of solid waste, and the limited amount of chemicals used. DLE technologies to produce LCE from brines demand the lowest direct land usage of the current technologies available to produce LCE; however, the indirect land usage for DLE is the greatest.

Comparison of the environmental impacts of LCE produced from pegmatite rocks and those of brine production shows that such impacts vary according to the lithium source. Conventional evaporation technology uses the least amount of energy. In this regard, the main drawback of DLE technologies is energy usage. The use of solar panels should reduce the negative impacts associated with DLE [227]. Chemical consumption increases the indirect land usage indicator for DLE, despite consuming less water compared to pegmatite processing.

In the case of spodumene ore (pegmatite) processing, its main setback is the high production of CO_{2 (eq)} emissions related to its calcination [227].

Production of LCE from lithium-bearing clays is still under development; however, Mousavinezhad’s group [226] determined, from the data from Lithium Americas’ acidic leaching process, that the environmental impacts associated with processing such minerals could decrease with the installation of a sulfuric acid plant near the lithium processing facility. Another way to diminish such impacts would be to change the leaching stage at Lithium Americas.

In this regard, these authors do not agree with Mousavinezhad’s appraisal on the acidic leaching, as it has been proven that lithium from lithium-bearing clay can be extracted with water leaching [151]. Therefore, the elimination of the need for acid would significantly improve the environmental impact of processing clay minerals.

Furthermore, it was proven in the research conducted in Mexico that the process has major advantages over processing spodumene ores. CIMAV process utilizes 28 MWh/tonne of lithium in clay minerals, whereas spodumene processing uses 60 MWh/tonne. In terms of water usage, the CIMAV process consumes 3 m³/tonne, whereas spodumene processing consumes 77 m³/tonne. In terms of environmental footprint, the CIMAV process produces 11 tonnes of CO_{2 (eq)}, while spodumene processing generates 21 tonnes of CO_{2 (eq.)}. These figures show that the CIMAV process is a promising competitive option for producing battery-grade lithium carbonate. Further pilot testing is needed to confirm this data.

Results from the life cycle analyses conducted by Stamp et al. [228] revealed that there are no significant differences in the environmental impacts caused by processing lithium from brine or pegmatites. However, if lithium is to be processed from seawater [56,57,58] the environmental impacts would be much more significant. Another factor to consider in determining the environmental impacts of lithium processing relates to energy consumption, as this indicator is directly linked to process costs.

To complement the evaluation of the environmental impacts due to lithium extraction and production, waste generation in the form of gaseous emissions, solid residues and liquid effluents should be considered. As mentioned, operations that involve any sort of calcination produce CO₂ emissions. These emissions are inherent to the carbonate decomposition process; besides this, CO₂ is also produced from the combustion of the fuels used to heat up the furnaces used for calcination. These gaseous emissions strongly impact the production of greenhouse gases.

Regarding the liquid effluents, spent leaching solutions may require different treatment to neutralize the acidity or basicity that they might have due to the different chemicals used in the leaching operations. On this subject, it is convenient to implement water leaching as it can be reincorporated into the process stream, while lowering its environmental impacts.

In the case of brine processing, it is necessary to evaluate the costs associated with implementing water recovery systems to reincorporate water into another process or where it is needed.

The solid residues that are generated in lithium extraction are essentially silicates that can be stored as they are innocuous or they can be utilized as filling material in road making. Additionally, some sulfates and carbonates could be present in solid residues; these could be used as fertilizers.

6. Final Remarks

As we surveyed the data available on the lithium reserves and the processing alternatives that have been developed so far, more precise data regarding the actual lithium reserves is necessary. Despite having yearly data on the number of lithium reserves, it is necessary to precisely determine the actual figures on the sources of lithium. This data is required to determine the best processing route for treating specific mineral resources. In this context, the estimation of reserves from brines, pegmatites and clay-bearing lithium minerals is paramount.

Given the need to produce as much lithium as possible, the extraction of lithium from more diverse sources becomes imperative. It is believed that shifting from hydrocarbons to electrification would reduce the carbon footprint due to human activities. However, such a transition might not be as simple as portrayed since lithium extraction could demand significant amounts of energy and other resources such as fresh water.

In the case of brine processing, DLE technologies need to be tested at an industrial scale. Results from these diverse technologies at laboratory and in some instances at pilot scales seem promising. To have a full assessment of the DLE processes, industrial-scale data is required. This data would allow us to estimate actual environmental impacts, and production rates could be established in a way such that the gap between lithium demand and its consumption can be narrowed.

In the case of lithium-bearing clays, it is necessary to develop novel technologies to obtain and transform the lithium contained in them. Clay minerals could become increasingly important as lithium from brine and pegmatite deposits becomes depleted. Different mineralogical associations and variable lithium concentrations in the clay minerals are key factors to consider when deciding on a specific route to recover the lithium in them.

As the distinct natural mineral sources of lithium become depleted, the incorporation of energy-intensive processing might be required to recover the metal. Molten-salt technology seems to be a viable alternative for such an alternative. On top of that, recycling lithium from waste batteries could be considered. In this regard, it would be necessary to determine which types of batteries are easier to process at the lowest possible environmental and economic costs.

Additionally, it is important to address environmental impacts due to the processing of increasingly complex lithium minerals, as such impacts might be detrimental and perhaps not worth the efforts to produce the lithium required to fulfill the need for this commodity.

To be more specific, depending on the nature of a particular lithium resource available for its exploitation, a specific technology to recover lithium must be selected. This technology selection will allow us to work on specific process development and optimization that in turn would improve the lithium yield while keeping the environmental constraints in check as we move toward fulfilling the lithium needs. By deciding on a particular technology, bottlenecks related to the process logistics and others may be cut short, thus enhancing the performance of the processes used to extract lithium. Up to now, lithium recovery from brines (geothermal and continental) as well as the recovery from pegmatite ores have reached an acceptable maturation level. The key issue in dealing with brines is related to defining which of the DLE technologies investigated is the best. In this sense optimization of processes is necessary.

Clay processing still needs to be developed, as clay mineral deposits are heterogeneous. Of the existing natural lithium deposits, this is the least understood in terms of lithium processing; therefore, more basic research is required to try to find techno-ecological solutions for their treatment.