MA Yanfang, XIANG Shaoji, CUI Zhenhua, LI Kanshe, ZHANG Zhihong

(1. Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China; 2. Qinghai Province Key Laboratory of Resources and Chemistry of Salt Lakes, Xining 810008, China; 3.Chemistry and Chemical Engineering College of Xi’an University of Science and Technology, Xi’an 710054, China; 4. Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing 100190, China; 5. Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032, China)

Abstract: Lithium carbonate (Li2CO3) was synthesised by adding sodium (Na) and magnesium (Mg)ions into a lithium chloride solution at different concentrations, followed by the addition of an appropriate sodium carbonate solution. Then, the morphology, purity and particle size of Li2CO3 crystals were investigated.The Na and Mg ions had negligible and remarkable effects, respectively, on the product purity; however they both greatly influenced its morphology. Their effects on the nucleation and growth rates, the radial distribution function and the diffusion behaviour of the synthesised Li2CO3 were investigated via molecular dynamics methods; the Na ions slowed down the crystal nucleation and growth rates, while the Mg ions accelerated them.Moreover, the Mg ions rendered the system short-range ordered and long-range disordered and also increased the diffusion coefficient. The results of this study showed that Mg ions are one of the most important factors influencing the purity and yield of Li2CO3.

Key words: sodium; magnesium; purity; particle size; diffusion coefficient; analytical kinetics

1 Introduction

Salt lake brines account for 61.8% of the global lithium (Li) resources[1-3]. As natural brines with high lithium content are important Li resources, the salt lake lithium extraction industry is emerging[4-7]. However,due to factors such as the low comprehensive utilization of salt lake resources and the inadequate impurity removal rate in the purification process, lithium carbonate (Li2CO3) products exhibit small particle size, large specific surface, severe agglomeration, mother liquor entrainment and peritectic nature. Their purity and specific impurities cannot meet the requirements of the Li battery industry; this severely restricts the development of the salt lake lithium extraction industry. The hightech field currently presents strict requirements pertaining to the Li2CO3purity and there are new morphology and particle size requisites as well[8-11]. Therefore, the role of the impurities in the Li2CO3crystallisation is particularly important.

Brines of interest for Li extraction have a complex solution chemistry with different concentrations of Na+,Mg2+, K+, Cl−, SO42−and B4O72−。The formation of ion pairs between the constituents of other oxyanion-bearing mineral phases, such as MgCO3⋅3H2O, has been shown to significantly change the ability of the mineral,meanwhile, dissolved ions can form direct interactions with different surfaces on a growing crystal, influencing the stability of the surface and grow[12].

Subsequent to the deep decontamination of the Lithium-rich brine in salt lakes, trace impurities, such as boron and Mg, in the raw material solution for Li precipitation affect the purity of the resulting Li2CO3crystals. In addition, as the Li extraction from salt lake brine for the Li2CO3precipitation used the Li+, Na+,Cl-, CO32-–H2O quaternary system, the effects of the Na ions on the solubility, morphology, particle size and purity of Li2CO3during the crystallisation of Li2CO3cannot be ignored. It is very necessary for molecular dynamics simulations on the crystallization of simulation Na and Mg ions on lithium carbonate crystallisation. Thus, the role of impurity ions in complex crystalline systems must be investigated.

2 Experimental

Lithium solutions with different (separate) concentrations of Na and Mg ions were prepared (Tables 1 and 2). As a raw material solution for research, these were added with a Na carbonate solution to prepare Li2CO3crystals. Then, the effects of the Na and Mg ions on the Li2CO3crystallisation process and its purity were investigated.

Table 1 Lithium solutions containing sodium (Na) ions

Table 2 Lithium solutions containing magnesium (Mg) ions

3 Results and discussion

3.1 Purity, morphology and particle size

Tables 3 and 4 list the lithium content and the purity of the various Li2CO3crystals prepared by the reaction.

Table 3 Li content and purity of the Li2CO3 crystals synthesized using Na

Table 4 Li content and purity of Li2CO3 crystals synthesized using Mg

These results show that the presence of the Na and Mg ions directly affected the Li2CO3purity, which decreased as their concentration increased. However,the influence of the Na ions was much smaller than that of the Mg ones.

Fig.1 compares the scanning electron microscopy (SEM) images of the Li2CO crystals prepared with different concentrations of Na and Mg ions in the raw material solution; the Mg ions directly influenced the product morphology, resulting in poor particle dispersion and a staggered distribution of large and small crystals. Lithium carbonate forms polycrystalline crystals, and Mg ions form a magnesium hydroxide colloid.The appearance of lithium carbonate was changed by complexation and adsorption.

The X-ray diffraction patterns of the Li2CO crystals (Fig.2) revealed that the increase in the Na and Mg ions concentrations gradually reduced the intensity of the Li2CO3diffraction peaks, indicating a decrease in the purity and Li content. The strongest absorption peak of lithium carbonate occurs at 31°-32°. As the content of sodium and magnesium ions in the system increased,the absorption peak gradually decreased. The reason is that the microcrystalline structure of lithium carbonate has changed. There are many defects in lattice structure.

Fig.3 illustrates the influence of Na and Mg ions on the particle size distribution of Li2CO3products,showing that the presence of Na ions slowed down the nucleation and growth rates. Moreover, the presence of Mg ions induced the crystal agglomeration, broadened the particle size distribution and reduced both the average particle size and bulk density. There was no clear regularity between the average particle size of the Li2CO3crystals and the Na and Mg contents, which needs further investigation.

Fig.1 Scanning electron microscopy images of the Li2CO3 crystals synthesized in the presence of (a) Na and (b) Mg ions

Fig.2 X-ray diffraction spectra of the Li2CO3 crystals synthesized in the presence of (a) Na and (b) Mg ions

Fig.3 Size distribution of the Li2CO3 crystals synthesized in the presence of (a) Na and (b) Mg ions

3.2 Molecular dynamics simulation

3.2.1 Simulation method

All the molecular dynamics simulations were performed with the AmberTools 18 package; the general AMBER force field and the TIP3P (transferable intermolecular potential with three points) water model were used for the ions and water, respectively. Subsequent to 2 000 cycles of energy minimization, the models(four simulation systems) were heated to 353 K within 200 ps in the NVT manner, followed by an NPT equilibration for 6 ns. All trajectories involved in the work of trajectories were visualized and analysed by the Visual Molecular Dynamics program; For simulation system, each system has about 6 000 atoms and about 2 000 water molecules. The number of other ions is given by the concentration. Force field is GAFF, and the distance cut-off for the dynamical bond was set to 1.9 Å[13–17].

The impurity effect in the crystallisation process can be divided into two categories: harmful impurities and insignificant impurity effects[18–20].However, the quantitative relationship between impurities and crystal properties is very lacking and therefore, the action mechanism of the impurities in the Li2CO3crystallisation must be simulated. Four systems were modelled to investigate the properties of Li2CO3crystals formed in the presence of Na and Mg ions. Table 5 summarises the simulation parameters used. The nucleation and growth rates were derived from the first-order kinetics of the free lithium ions concentration with time before and after, respectively the nucleation number stabilisation.

Table 5 Chemical composition of the four simulated systems

The initially established structural model of the system is shown in Fig.4.

3.2.2 Impurity effect on the nucleation and growth rates

Fig.4 Schematic illustration of the simulation box

Fig.5 Effect of the impurity (Na and Mg) concentration on the nucleation and growth rates of the Li2CO3 crystals

The simulations revealed that the nucleation and growth rates of Li2CO3crystals increase in the presence of Mg2+, with basically a linear relationship with concentration (Fig.5). The effect of Na ions was reversed.

Mg ions will limit Li-carbonate growth because of the formation of magnesium carbonate, causing solution supersaturation instability, and ventually affect the formation of lithium carbonate.

3.2.3 Radial distribution function and diffusion coefficient

The radial distribution function (RDF), also called pair correlation function, is the ratio between the atomic density in the system’s local area and the average density of the system. It can help to understand the structure of molecular systems[21], especially the interactions between two atoms. The first peak position ofg(r) corresponds to the closest distance between two atoms, whereas its height and sharpness express the degree of structural order between them[22]. The results show that the Li–Li and Li–Na RDFs indicate that the height has a short-range ordered and a long-range disordered structure, that is, there are a small number of peaks with varying heights and sharpness at close distances; however the peak height rapidly decreases with the increasing distance. At long distances, the RDF tends to be evenly distributed,i e,g(r) = 1. The atomic arrangement and distribution exhibit no regularity in amorphous systems and the corresponding RDF usually shows only short-range peak positions; as the value range gradually increases and tends to infinity, its value tends to 1 and the atomic density in the local area becomes equal to the overall density.

Figs.6 and 7 illustrate the RDFs of Li–Li and Li–Na, respectively, in the four simulated systems, showing obvious differences between Li–Li and Li–Na; in particular, Li–Li was more ordered and stable than Li–Na. Due to the isothermal process, the crystallisation was slow and the system structure changed relatively slowly as well; as a consequence, the change of the Li–Li RDF is not clear. The peak of the Li–Na RDF was reduced to a certain extent and the first peak distance was larger than that for Li–Li.

Fig.6 RDF of Li-Li

Fig.7 RDF of Li-Na

Fig.8 RDF of Li–Mg

Fig.8 displays the Li–Mg RDFs in system 3 and system 4. Due to the ordered structure of the crystal system, the RDFs presented peaks at both short and long distances and as the radius gradually increased approaching infinity, its value still fluctuated but did not tend to 1. The reported results show that Li–Mg had a poor order degree, the RDF had clear multiple peaks and the peaks were low, implying that the presence of Mg induced a certain degree of chaos in the system state, consequently increasing the disorder of the system particles and the number of Li–Mg peaks, which in turn, indicates that the crystallisation of Li and Mg was enhanced and could form relatively stable crystals.In addition, the SEM image (Fig.1(b)) confirms that the Mg ions were distributed only on the surface of the crystals formed by Mg itself.

The exact extraction method chosen for Li2CO3production depends on the composition of the aqueous solution[23].

The significantly lower peak of the Li–Mg curve at 2.5–3.0 Å with respect to those of the Li–Li and Li–Na curves suggests a smaller number of CO32–molecules around the Li and Mg atoms compared to those around Li–Li and Li–Na. The reduced binding probability of CO32–in the system with H atoms in H2O also confirms that Mg participated in the Li2CO3crystallisation.

The comparison between the peak intensities of the Li–Li, Li–Na and Li–Mg RDFs reveals that the magnitude of the intermolecular forces followed the order Li–Li > Li–Na > Li–Mg and that both were larger than the hydrogen bonds.

Fig.9 Mean square displacement of the four simulated systems

The diffusion coefficient (D) is an important index for measuring the diffusion capacity of particles and can be derived from their mean square displacement(MSD)[24], also called the squared particle displacement, which is calculated as follows:

whereri(t) is the position of theithparticle att,ri(0) is its initial position andNis the number of particles.As the real time interval for measuringDis extremely long in the case of the molecular translational motion, the limit at time (t) tends to infinity as follows:

By substituting the differential approximation in Eq.(2) with the ratio between MSD and time differential, that is, the slope (a) of the curve,Dcan be simplified as

Fig.9 shows the MSD of the Li ions in the four simulated systems at 353 K, along with the corresponding Einstein relationship coefficient andDcalculated[25–27]. The results showed that diffusion coefficient is closely related to the salt content and temperature of the system. Under constant temperature, Simulation system 1-4 corresponding the Li ionsDvalues were 6.17 × 10−9, 6.10 × 10−9, 7.27 × 10−9and 7.77× 10−9cm2/s respectively. The salinity influences the difficulty in diffusion. The salt content of the system has the greatest effect on the diffusion coefficient; as it increased, the diffusion of the Li ions was hindered andDsignificantly decreased, as shown in Fig.10. The Li ionsDof system2, with higher Na ions content, was the lowest among the simulated systems.

Fig.10 Diffusion coefficient of the four simulated systems

The diffusion rate was greater in the presence of Mg ions than that with Na ions, which suggests that the Mg ions destroyed the structure of the system solution to a greater extent. At the same time, the density of Na ions in the system was larger, resulting in stronger interactions with surrounding water molecules. The atoms became increasingly disordered and the value ofDincreased. This can be attributed to the difference of cations in the system.

4 Conclusions

The reported experimental study on the effects of Na and Mg ions on the Li2CO3crystallisation, especially on the crystal quality and morphology, in salt lake brines has revealed a smaller contribution of the Na ions compared with the Mg ions. When the Mg ion content increased seven times, the grade of Li2CO3decreased by twice of its initial value. Moreover, in the presence of Mg ions, the crystal morphology changed from rod-shaped to petal-shaped. The presence of the Na and Mg ions also broadened the Li2CO3particle size distribution and decreased the average particle size.

Through molecular dynamics simulations, the effects of Na and Mg impurity ions on the nucleation and growth rates of Li2CO3were further investigated; the Na ions slowed down the crystal nucleation and growth,whereas the Mg ions accelerated them and it was related to their concentration. Mg ions also participated in the crystallisation(MgCO3) along with the crystallisation of Li2CO3, while the Na ions did not crystallize and there was no precipitation of Na salts either, which is beneficial for the Li extraction from brine.

The presence of Mg ions made the Li–Mg RDF tending to be disordered and increased the diffusion coefficient. The diffusion rate was also greater compared with that in the presence of Na ions, which indicates that Mg ions can destroy the structure of the system solution to a greater extent. The comparison between the peak intensities of the Li–Li, Li–Na and Li–Mg RDFs revealed that the intermolecular forces followed the order Li–Li > Li–Na > Li–Mg and that were larger than the hydrogen bonds.

The results showed that the Na ions were adsorbed only on the surface of the Li2CO3crystals, while the Mg ions also formed Mg salt crystals. Therefore,the role of the Mg ions in the Li2CO system is far greater than that of Na ions, and therefore, the effects of impurities containing Mg ions on the Li2CO3crystallisation should be systematically investigated in future works.