Electric vehicles using lithium-ion batteries (LIBs) are gaining more and more attention as a promising option to reduce the greenhouse gases emissions associated with transport which causes climate change issues. To reach our needs in terms of transport electrification, the expected growth rate for lithium raw materials used in LIBs is estimated at 10% for Li2CO3 and 14.5% for LiOH by 2025 [1]. Nowadays, lithium is extracted from brines, mainly located in Chile, Argentina, and Bolivia, and from pegmatite deposits, mainly located in Australia [2, 3]. Several countries around the world, including Canada, are also looking at the possibility of extracting Li from Li-rich pegmatite deposits, in which the main Li-bearing mineral is spodumene, LiAlSi2O6 [1]. The production of Li2CO3 and/or LiOH from spodumene ore includes the conversion of α-spodumene to β-spodumene by calcination at high temperatures (at least 1,000°C for 30 min) followed by a roasting step in the presence of H2SO4 [1, 4]. The reaction between βspodumene and sulfuric acid is based on a substitution of the Li atom by a H atom inside the structure of spodumene, leading to the production of an aluminosilicate residue [4]. Besides, The production of Li generates huge amounts of residues (with approximatively 10 tons and around of 20–40 tons per 1 ton of lithium carbonate and hydroxide, respectively) [2, 5], which are usually disposed of in landfills or open air-storage and may potentially generate environmental problems [2, 5]. To reduce the environmental footprint of Li production from spodumene ore, exhaustive mineralogical and physico-chemical characterization have been conducted on these aluminosilicate residues to identify options for their valorization. At this point, the production of zeolites from these aluminosilicate residues is an interesting valorization avenue, considering their high contents in alumina and silica [2].
Zeolites are microporous crystalline aluminosilicates, belonging to the tectosilicate family. They are composed of TO4 tetrahedra (T = Si or Al) with oxygen atoms connecting neighboring tetrahedra. Some silicon (Si(IV)) atoms are replaced by aluminum (Al(III)) atoms, making the crystal lattice deficient in positive charges. The amount of Al in the structure can vary over a wide range, i.e. according to an Si/Al ratio ranging from 1 to infinity [6]. The structure of zeolites contains open cavities in the form of channels and cages occupied by water molecules and by exchangeable cations such as K+, Na+, Ca2+ and Mg2+, which make it possible to maintain a neutral crystal lattice [7]. Zeolites are characterized by the presence of channels, cavities and cages of molecular dimensions [7, 8], giving them interesting ion-exchange and sorption properties [7]. Synthetic zeolites A and 13X, which are zeolite with a low Si/Al molar ratio (1-1.5), are the most commercialized and are of great importance in industry, since they have a very high ion-exchange capacity [6]. Zeolite A is widely used in detergents as a replacement for sodium tripolyphosphate (STPP) [6, 7, 9, 10] to remove Ca2+ and Mg2+ ions contained in water. More than 1 Mt/year of zeolite A are used in detergents and cleaning products [7]. Zeolites A are also widely used for the sorption of contaminants present in mine effluents such as acid mine drainage and NH4+ from wastewater [7] or for CO2 gas sorption [11], as well as in trapping moisture in double glazing [12]. Zeolites X are characterized by high adsorption and selective sorption capacity due to their unique crystal structure [13]. Zeolites 13X are added in some detergents, because of their higher Mg2+ ion-exchange capacity compared to zeolites A [14]. Moreover, zeolites 13X have a higher adsorption capacity for CO2 [13, 15], in gas drying [6] and for the selective separation of CO2 from a CO2-N2 gas stream at a medium or industrial scale [16].
The synthesis of zeolites from pure chemical or residual materials by hydrothermal process features a multiphase reaction-crystallization process including, at least one liquid as well as amorphous and crystalline solid phases [17]. The process consists of mixing solutions of aluminates and silicates, allowing the formation of a gel, which is maintained at temperatures ranging from 30 to 180°C for several minutes to days to favor nucleation and crystallization processes [2, 18–20]. Synthetic zeolites are produced under hydrothermal conditions, requiring a large quantity of water, and high alkalinity [2, 10]. The use of pure aluminate and silicate for the production of zeolites are very expensive in addition to being unsustainable materials [17]. Therefore, several studies have been carried out to use more sustainable, cost effective and environmentally friendly raw material such as kaolinite [11, 21], coal fly ash [18, 22–25], bagasse fly ash [26], waste bauxite tailings [27], lithium slag [5, 14] and Kfeldspar [28], while very few studies were performed with residues from Li extraction [2]. Zeolitization steps from these raw materials involve the dissolution of Si and Al using an alkaline solution, the formation of a geopolymer, a crystalline nucleation and crystal growth of zeolites [18, 20, 29]. Several processes, including conventional hydrothermal process, hydrothermal process assisted by calcination/fusion pre-treatment step, microwave heating or ultrasonic treatment, have been developed to produce zeolites from secondary materials [18, 20].
Hydrothermal conventional processes are widely used and considered as a simple and costeffective technique, although they result in low zeolite production yield. The incomplete dissolution of the Si and Al from the initial product and the presence of unreacted elements in the final zeolite are among the drawbacks of this approach [2, 18, 29]. To produce pure zeolites from these materials and improve the efficiency of the process, some pre-treatments should be considered. Pre-treatment with inorganic acid (HCl) remove some impurities, such as iron oxides [27]. Calcination, at temperature between 500 and 900°C, can also be used to remove hydroxyl groups from kaolinite clay material, allowing the conversion of kaolin into metakaolin [9, 11, 30, 31], which is amorphous and therefore a more reactive structure [9]. A roasting at 800°C was efficient to remove the unburned carbon and volatile compounds to synthesize zeolite X [22] and pure zeolite Na-P and Na-X from fly ash [32]. An alkaline fusion-assisted hydrothermal (AFH) method has been used to efficiently solubilize Si and Al from fly ash, favoring zeolite formation in the subsequent hydrothermal treatment [18, 33, 34]. Indeed, fusion of fly ash with alkaline hydroxides (usually NaOH powder) converts the fly ash into very soluble alkali salts: silicate and aluminate [18, 33, 35]. Molten NaOH or Na2CO3 are usually used to activate and treat fly ash, while the addition of Na improves nucleation and crystallization processes [25]. A patented process has been developed to produce zeolite A from aluminosilicate residues originating from Li extraction using a conventional hydrothermal process assisted by alkaline fusion [36]. Operating factors such as alkali/sample ratio, temperature and melting time influence the type and crystallinity of zeolite synthesized by AFH process. Insufficient alkali concentration leads to a weak extraction of aluminosilicates from fly ash [34], while an excess of NaOH leads to sodalite formation [23]. NaOH/Lougkou oil shale ash ratio of 1.2/1 was identified as optimal for the production of NaX zeolite type faujasite using AFH method at 600°C for 1 h [37]. In addition, an increase of NaOH/high silicon fly ash from 1.2/1 to 2/1 showed a slightly increase in NaA and faujasite (NaX and NaY) zeolite crystallinity using AFH process at 600°C for 1.5 h [38]. At a melting temperature of 400˚C, it was found that the alkaline fusion reaction is incomplete [27], while most of the studies are performed at 500°C [27, 39], 550°C [22, 40, 41] or 600°C [23, 37, 38]. A previous study showed that melting time plays a crucial role in the type of zeolite to be produced and short fusion time (< 1 h) are not enough to allow a complete digestion of the fly ash [23]. Dissolution of Si and Al is improved by AFH process, which makes it possible to obtain purer zeolites compared to those obtained by the conventional process due to the diminution of unreacted elements in the final product [18]. However, the higher energy needed during fusion step (temperature between 500 and 600°C) is the major drawback of this process [18, 29]. Finally, regarding the two processes, when assisted by microwave heating and ultrasound irradiation, studies showed that the zeolites synthesis time decreases significantly by microwave heating compared to conventional heating [18, 31, 42, 43]. Besides, ultrasound irradiation is also used to accelerate the dissolution of Si and Al of the amorphous aluminosilicates of fly ash [18, 44]. Despite its simplicity, low energy consumption and diminution of retention time, there is no pilot or industrial application of ultrasound-assisted and microwave processes [18, 43, 45].
The performances of zeolite synthesis process (e.g., type and crystallinity of the zeolite produced) can be influenced by several factors including the composition of the reaction mixture, the alkalinity, the Si/Al ratio, the solid/liquid ratio, the crystallization time as well as the aging time and temperature [17, 18, 20]. It has been noticed that a higher synthesis temperature (> 100°C) is suitable for crystal growth, while lower temperatures favor the nucleation step [46]. For example, it has been mentioned that crystal growth rate and high crystallinity of zeolite A was obtained with higher crystallization temperature of 100°C [46]. Besides, another study showed that crystallization temperature lower than 80°C resulted in the production of low crystalline phases, while at 100°C, pure zeolite A was formed from liquid crystal display waste glass and sandblasting waste [8]. However, a gradual decrease in zeolite A crystallinity was observed at temperatures greater than 120°C [8]. The synthesis of pure zeolites X required a longer crystallization time (around 48 h) at 90°C, and increasing the crystallization temperature up to 120°C for 24 h increased the amount of impurities like zeolite P [26]. Maximum crystallinity of zeolite NaX produced from fly ash was achieved after 15 h of crystallization at 90°C, while no to few crystallization occurred after 3 and 7 h, respectively [47]. Increasing crystallization time from 12 to 16 h at 110°C led to an improvement of crystallinity and purity of zeolite X, while after 18 h, a conversion of zeolite X to zeolite P seemed to occur [13]. Regarding the aging step, this period preceding crystallization step, is a crucial step to synthesize a desired zeolite. Partial dissolution or depolymerization of the silica (SiO2) is one of the important steps occurring during the period of aging [19]. A better crystallinity of zeolite A synthesized from kaolin was obtained at an aging temperature of 50°C. This temperature increases the dissolution of Al and Si, reaching its critical concentration for nucleation [9]. In addition, longer aging time allows the production of zeolites with higher crystallinity [48]. Zeolitization efficiencies are also affected by the dissolution step, where S/L ratio plays a crucial role. Increasing S/L ratio led to a lower degree of crystallization. For example, a S/L ratio lower than 5 g/L improves Al and Si dissolution, therefore increasing the zeolitization efficiency. However, the large amount of chemicals used limits their application at industrial scale [49].
The aim of the present study is to compare the performances of three different processes to produce zeolites from aluminosilicate residues originating from Li extraction. To do so, zeolite was synthesized using: i) a conventional hydrothermal process (Process_1), ii) a conventional hydrothermal process assisted by calcination (Process_2) and iii) a conventional hydrothermal process assisted by alkaline fusion (Process_3). An exhaustive characterization of the produced zeolite as well as commercial zeolites A and 13 X was done to determine the most performant approach to convert aluminosilicate residues into value-added by-products.