Lithium leach residue synthesis process of high crystallinity hydroxysodalite, NaX, NaA zeolites and solidification and migration of potentially toxic elements

3.1.1 Changes in structure and morphology at different alkali melting temperatures

The structure and morphology of the zeolite products at alkali melting temperatures (300–800 °C) were characterized using XRD and SEM, respectively, to monitor the type and quality of the products. Fig. 1 shows the change in the zeolite phase with increasing alkali fusion temperature. As the alkali melting temperature increases, the mullite in LLR undergoes a phase change reaction with alkali, and the original SiO₂ and Al₂O₃ hydrated cations at low temperatures come into contact with each other under the action of the alkali melt and condense into a white gel-like silica-aluminum anion (350 °C), and the elevated temperature induces an increase in the solubility of the gels, an increase in the concentration of liquid-phase silica-aluminate anions, and the hydrated anions undergo a renewed coordination polycondensation to form zeolite units, and crystallization and nucleation. When the alkali melting temperature is controlled at about 400 °C, the hydrated anions undergo coordination condensation to nucleate into particles and grow around the center to form an octahedral three-dimensional shape, which is the characteristic structural shape of NaX zeolites (Novembre et al., 2018, Muriithi et al., 2020). When the temperature was gradually increased to 500 °C, the hydrated anionic skeleton growing around the center was rearranged to form β-cage HS. Whose morphology showed alternating layers of molecular sieves from spherical clusters arranged into plate-like structures, with the length of molecular sieve crystals ranging from 500 to 600 nm and the width from 66 to 99 nm, which was consistent with the results of the HS prepared from part of the fly ash (Muriithi et al., 2020). When the temperature was further increased, the striated molecular sieves on the spherical surface dissolved (550 °C), became finer and shorter (600 °C), and formed a three-dimensional skeletal structure of β-cages connected by a double 4-membered ring (D4R) in the center of the crystalline cells, which gradually formed the cubic-structured NaA zeolite (700 °C) (Bukhari et al., 2015, Feng et al., 2018, Cao et al., 2020, Mamaghani et al., 2023).

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Fig. 1

Typical SEM variation images of zeolite samples synthesized with different alkali fusion temperatures(300 °C -800 °C), where NaX zeolite was synthesized at 400 °C, HS at 500 °C, and NaA zeolite at 800 °C.

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Fig. 2(a) shows the XRD patterns of the three synthesized zeolites. At the alkali melting temperature of 400 °C, a characteristic diffraction peak (d110 = 0. 633 nm) appeared in the XRD pattern, indicating that the system has reached the alkalinity required for the growth of zeolite, and the environment of the system began to be suitable for the growth of zeolite, and the diffraction peaks of the single-phase NaX zeolite appeared at 6 0.1°, 16.9°, 23.3°, and 26.7°, and the relative crystallinity was 51.6 %, respectively (Novembre et al., 2018). The characteristic diffraction peaks of the NaX zeolite weakened and shifted towards 13.9°, 24.4° and 34.47° as the alkali melting temperature was gradually increased, and the HS was formed at 500 °C (Prokof'ev and Gordina, 2014), when the relative crystallinity was 95. 2 %. As the temperature increases to 800 °C, the HS diffraction peaks shift to 7°, 10°, 13°, 24°, 27°, and 30°, which are categorized as typical characteristic diffraction patterns of A-type zeolites (Feng et al., 2018). Relative crystallinity of 78 %.

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Fig. 2

XRD images (a) and FTIR images (b) of zeolites synthesized at different alkali fusion temperatures, where 400 °C is NaX zeolite, 500 °C is HS, 800 °C is NaA zeolite.

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The above-mentioned XRD data are confirmed by the FTIR spectroscopic data. Fig. 2(b) shows the FTIR spectra of the three zeolites. The stretching and bending vibration peaks of adsorbed and zeolite water in the samples are shown at 3452 and 1612 cm⁻¹, the bands in the range of 1047–960 cm⁻¹ are attributed to the antisymmetric stretching vibration of TO₄ (T = Si or Al) (Criado et al., 2005, Yang et al., 2019, Lv et al., 2022). In addition to this, in the range 600–500 cm⁻¹ are the absorption bands of double 4-ring (D4R), through which the β-cage is connected in the zeolite framework. 565 cm⁻¹ is the characteristic peak of double 4-membered ring (D4R) in a typical A zeolite structure, and D4R is one of the most dominant secondary structural units (SBU) in the NaA zeolite structure, and the intensity of its absorption peak can be used as an important symbol to judge the growth of NaA zeolite (Prokof'ev and Gordina, 2014). All samples embodied absorption bands in the range of 800–600 cm⁻¹, which are zeolite species bonding bands, symmetric telescopic vibration peaks of T-O-T in the zeolite structure (Prokof'ev and Gordina, 2014), which corresponds to the 6-ring structure of the β-cage of the simple 4-ring, which is a typical feature of the SOD structure. Compared with the FTIR spectra of the zeolite product (HS) at 500 °C, the peak strength at 1016 cm⁻¹ of the zeolite product (NaX) at 400 °C, zeolite product (NaA) at 800 °C is significantly lower, which further suggests that there is more alternating condensation of Si-O and Al-O bonds in the HS, and a greater amount of aluminum ions penetrate into the TO4 tetrahedra (Palomo et al., 2014; Liu et al. 2018), which suggests that more zeolites were formed in the product at 500 °C, which is consistent with the results of scanning electron microscopy analysis.

3.1.2 BET surface area

The three zeolites were further characterized by N₂-BET and their adsorption isotherms and HK pore size distribution curves are shown in Fig. 3. The obtained isotherms indicate that all three zeolites are a microporous and mesoporous material, which can be categorized as class IV isotherms. It takes a long time to perform multilayer filling in the adsorption isotherm (region between 0.1 and 0.6P/P₀), while delayed condensation is observed at a P/P₀ value of about 0.9. The sharp capillary condensation step indicates a narrower pore size distribution for the three zeolites. Among the three zeolites, HS has the largest specific surface area of 181.1987 m² /g and NaX has a specific surface area of 95.8473 m² /g, both of which are of the same grade as reported zeolites(Chen et al., 2012, Muriithi et al., 2020, Mamaghani et al., 2023). The pore sizes of HS are about 9 Å, those of NaX zeolite are about 12 Å, and those of NaA zeolite are about 4 Å, which are consistent with the TEM result of Fig. 3 (c, d, e, f, g, h). The high-resolution images show that the crystal sizes of the three zeolites range from 9 to 18 nm. However, the pore-filling effect of NaA zeolite structure was found to be small, with a total surface area of only 41.38 m² /g, a microporous area of 11.93 m² /g, and an external surface area of 29.19 m² /g, which is of the same order of magnitude as that of the previously reported NaA zeolite (Jiang et al., 2015), which has a total surface area of 41.6 m² /g. The reason for the small specific surface area of NaA zeolite may be related to the finding by that in many of the small-pore zeolites where a molecular trapdoor effect occurs, where the center of the eight-membered ring (i.e., the entry point of the N₂ molecule) in NaA (Feng et al., 2018, Cheng et al., 2024) is partially blocked by its cations at 77 K, resulting in almost no measurable BET surface area.

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Fig. 3

TEM images of zeolites (c, d = NaX, e, f = HS g, h = NaA) and nitrogen adsorption–desorption isotherms(a) and HK pore size distribution curve of zeolite(b).

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3.1.3 Conversion rate of major elements Si, Al

The conversion rates of Si and Al in this zeolite synthesis process are important indicators of the effectiveness of the LLR recycling process. By analyzing the Si and Al elements in the three zeolite products, the conversion rates of Si and Al, the two main elements of the three zeolites (Table 2), were calculated to be greater than 90 % according to Equation (1), which demonstrates the high efficiency of the method of synthesizing lithium residue-based zeolites by the two-non-alkali fusion method. The significance of a high conversion rate is to ensure that the economy is maximized and secondary waste is minimized.

3.2.1 Leaching of toxic elements from zeolites

Consider the leaching risk of the main toxic elements Be, Tl, and the potentially toxic elements Pb, Hg, Cd, Cr, As, Ba, Cu, Ni, Co, Zn in the use of the product. The leaching risk at different concentrations was analyzed by pH-dependent experiments. And compared with the Chinese standard (GB/T 5085.3–2007) to determine whether there is a secondary environmental response hazard in the use of the three zeolite products.

As can be seen from Fig. 4, the Be concentration leaching from LLR at pH = 3.21 was as high as ∼57.99 μg/L, which was 3 times the standard limit value of 20 μg/L. The Tl concentration was as high as ∼57.59 μg/L, which was 11 times of the standard limit value of 5 μg/L. Which indicated that LLR was a hazardous waste. On the other hand, the three zeolites were lower than the standard limit value of leaching and did not show any hazardous characteristics, indicating that the zeolites prepared by alkali fusion of LLR can effectively avoid the leaching hazards of Be and Tl.

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Fig. 4

Leaching concentration of Be(a), Tl(b) at pH = 1, 3.21, 5, 6, 7, 11, 13 for HS, NaX zeolite, NaA zeolite, LLR, where safely limit Refer to the Chinese standard (GB/T 5085.3–2007).

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Meanwhile, the leaching concentration of toxic elements Tl in LLR was as high as ∼451.07 μg/L, which exceeded the standard by 90 times. The leaching concentration of Tl in HS and NaX zeolite after alkali fusion was very low in all other pH gradients, except for Tl leaching concentration exceeding 50 times in concentrated acid at pH = 1. Especially, the NaA zeolite under alkali fusion at 800 °C was not leached under any pH condition, indicating that the toxic elements Tl were better encapsulated and blocked by ion exchange at the elevated alkali fusion temperature.

Fig. 5 shows the leaching of other potentially toxic elements under a pH gradient. It was found that the leaching concentrations of toxic elements (Pb, Hg, Cd, Cr, As, Ba, Cu, Co) were higher in LLR than in the zeolite product at all pH gradients. In contrast, Ni and Zn were leached at concentrations comparable to those of LLR, suggesting that conversion of LLR to zeolite can significantly reduce the risk of harmful toxicity. At pH = 3.21, the leaching concentrations of toxic elements from all three zeolites were below the standard limits, while Ba, Hg, and Co in LLR exceeded the standard limits, showing the characteristics of hazardous waste. Meanwhile, it can be found from Fig. 5 (a, b, c) that the leaching concentration of Hg is higher under the neutral leaching agent condition, and HS zeolite has the lowest leaching concentration for all types of toxic elements. Combined with the scanning electron microscope data, it is found that HS zeolite has the highest relative crystallinity because zeolite can encapsulate and block the toxic elements well, and with the increase in the relative crystallinity of zeolite in the samples, the leaching concentration of the samples for the toxic elements decreases (Bukhari et al., 2015).

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Fig. 5

Leaching concentration of potentially toxic elements of HS (a), NaX (b), NaA (c), LLR(d) at pH = 1, 3.21, 5, 6, 7, 11, 13.

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In conclusion, the leaching concentration of toxic elements of zeolite prepared by the two-step alkali fusion method meets the Chinese GB/T 5085.3–2007 standard. The prepared HS has no secondary environmental risk reaction to the environment, the leaching rate is the lowest, and the relative crystallinity is more than 95 % under the condition of no templating agent and crystal seed, so it can realize the industrialized production and large-scale application.

3.2.2 Migration of toxic elements in production

The pH-dependent leaching experiments showed that among the three zeolite products, the leaching concentration of each toxic element in HS was the lowest, indicating that the toxic elements were best solidified in the HS. To further investigate the curing efficiency of toxic elements in zeolite and to analyze whether the zeolite production process will produce secondary environmental pollution, HS was used as an experimental object to analyze the heavy metal migration during the production process.

Fig. 6 shows the possible pathways of migration of various elements from the original LLR and the synthesized zeolite product in the production chain of zeolite. The zeolite synthesis process involves a hydrothermal treatment with a strongly alkaline solution at elevated temperatures, which breaks down the mullite and quartz hard crystalline phases in LLR, making Si and Al available for zeolite synthesis. At the same time, all other elements in the LLR are released and allowed to move (Feng et al., 2018).

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Fig. 6

An overview of toxic elements distribution and migration paths.

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Two steps of the experiment will produce waste liquids (Fig. 6 SI, SIII) to test the content of toxic elements in the effluent of the external environment. In addition, in the experiment there is an upper layer of clear liquid after crystallization in the auto-clave (Fig. 6 SII) (note that this part of the waste liquid is not dumped but goes directly to the next step of the drying experiments), and the content of toxic elements in the upper layer of the clear solution to verify the curing efficiency of toxic elements in zeolites.

It was found that the toxic elements content in the upper clear liquid changed with the change of crystallization time, so the crystallization time was set to 2 h, 4 h, 6 h, 12 h, and 24 h to examine the difference of cured toxic elements. The migration of toxic elements from LLR to zeolite was examined by analyzing the two waste steps as well as the upper clear liquid of the reactor to analyze the partitioning and quantitative distribution of these elements in the process wastewater and zeolite. In the following discussion, the concentration of the wastewater discharge is compared to the ceramic manufacturing emission standard limits to determine the environmental feasibility in the production step.

The LLR was cleaned twice with ultrapure water at a solid–liquid ratio of 1:20 (10 ± 2r/min for 5 min). From (Fig. 7 SI), it can be found that there will be precipitation of Be and Tl in the cleaning stage of the LLR, and the wastewater of the first cleaning exceeded the standard in a small amount of Be and Tl, and the content of Hg was ∼ 8.174 μg /L, it satisfied the standard discharge limit of ＜50 μg/L, and the rest of the toxic elements were lower than the standard limits (Pb ≤ 300 μg /L, Cd ≤ 70 μg/L, Cr ≤ 100 μg /L, As ≤ 500 μg /L). All kinds of toxic elements in the secondary cleaning wastewater meet the requirements of the emission standards, and some elements do not meet detection limits. After the completion of production, under the operating conditions of solid–liquid ratio of 1:20, the product zeolite was cleaned with ultrapure water (10 ± 2r/min, 5 min). From (Fig. 7 SIII), it can be found that Be, Tl, Cr, Zn, and As, leaching concentrations in the zeolite cleaning solution within 4 h of crystallization were relatively high, but could meet the limits of the emission standards.

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Fig. 7

Migration of toxic elements during the preparation process (where SI − LLR cleaning, SII − crystallization stage, SIII − zeolite product cleaning).

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In addition, we examined the content of toxic elements in the upper clear liquid within the auto-clave during the crystallization phase to determine the rate of solidification of toxic elements in the zeolite. From (Fig. 7 SII), it can be found that the toxic elements concentrations were higher than the SI and SIII stages at all crystallization times. Among them, the concentrations of Zn, Cr, Pb, As, and Be, were higher, with Zn maintained at ∼1300 μg/L, Cr reaching a maximum value of 778.6 μg/L at 12 h of crystallization, and Pb reaching a maximum value of 286 μg/L at 2 h of crystallization. In the experiments, the solid–liquid in the reactor was directly reacted and dried, i.e., captured and immobilized in the zeolite structure, so the solidification rate of the toxic elements was calculated by comparing the SII concentration with the SIII concentration, as shown in Table 3.

The toxic elements are classified into three groups based on the solidification rate, the first group(I) includes Tl, Be, Cd, Cu, Zn, Co, and Ba whose solidification rate is close to 100 %. This is due to zeolite owning to its unique physicochemical and surface (charge) properties, amphoteric ligand capacity and hollow “nanoball” structure (Baldermann et al., 2018), the ability of such toxic elements to exchange with adsorption sites in zeolites is dependent on short-range order alumino-silicates for some toxic elements such as Cs⁺, Ba²⁺, Co²⁺, Cu²⁺, Sr²⁺, and Zn²⁺, with unique physicochemical and surface (charge) properties, amphiphilic ligand capabilities and hollow “nanopore” structures (Wu et al., 2018, Baldermann et al., 2020). The second group (II) consists of Pb, Hg, and Cr, are 2-sex elements (Inoue et al., 2017), with immobilization rates of about 90 %, while the solidification rate of this group of toxic elements increases with crystallization time. For example, about 8 % or so of Pb enters the alkaline wastewater after crystallization and drying by washing, and the presence of Pb in the waste can be attributed to the formation of soluble anionic hydroxyl complexes (Huang et al., 2004), such as [Pb(OH)₄]^2- or [PbO₂]^2- (Zeng et al., 2019), most of the amphiphilic Pb(II) in LLR may be solidified in the zeolite lattice by forming hydrated anions with Si²⁺ in the presence of NaOH. The multivalent cations in the above two groups, due to their low solubility in supercritical water at high pH, the toxic elements in the LLR can be captured by physical/chemical adsorption, ion-exchange and physical encapsulation during the formation of amorphous or crystalline silica-aluminate. The heavy metal ions are adsorbed or encapsulated in the zeolite skeleton, which has a high immobilization rate (Bukhari et al., 2015). The third group (III) is the quasi-metal of As (Calugaru et al., 2018), which has a fixation rate lower than 90 %. Which may be attributed to the general negative electronegativity of the zeolite skeleton and the dissolution of arsenate complexes (AsO₄^3-) in LLR under strongly alkaline conditions during the hydrothermal reaction, thus the anionic form of the metal is difficult to be fixed in the lattice by the zeolite and thus prone to precipitation (Chen et al., 2023).

In our experiments, we do not use the pretreatment method of acid washing, i.e., we do not leach and filter out the toxic elements in the reaction mixture. The role of the alkaline additives is to provide enough OH^– to allow the silicon and aluminum in the lithium slag to form hydrated cations in the presence of OH^– and to be further converted into zeolite crystal nuclei, which grow around the deleterious elements (Bukhari et al., 2015). In other words, silica-aluminate anions form zeolites on the surface of these toxic elements and grow crystalline. Meanwhile, class I and II toxic elements mainly exist in free form, and the free form of harmful toxic elements are solidified in the zeolite lattice by physical adsorption as well as ion exchange, which can form insoluble minerals and realize that the toxic elements in the product are not easy to leach out at low pH (Feng et al., 2018). The mechanism is shown in Fig. 8.

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Fig. 8

Mechanism of toxic elements encapsulation during zeolite condensation growth map.

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