Li-Distribution in Compounds of the Li₂O-MgO-Al₂O₃-SiO₂-CaO System—A First Survey

Abstract

The recovery of critical elements in recycling processes of complex high-tech products is often limited when applying only mechanical separation methods. A possible route is the pyrometallurgical processing that allows transferring of important critical elements into an alloy melt. Chemical rather ignoble elements will report in slag or dust. Valuable ignoble elements such as lithium should be recovered out of that material stream. A novel approach to accomplish this is enrichment in engineered artificial minerals (EnAM). An application with a high potential for resource efficient solutions is the pyrometallurgical processing of Li ion batteries. Starting from comparatively simple slag compositions such as the Li-Al-Si-Ca-O system, the next level of complexity is reached when adding Mg, derived from slag builders or other sources. Every additional component will change the distribution of Li between the compounds generated in the slag. Investigations with powder X-Ray diffraction (PXRD) and electron probe microanalysis (EPMA) of solidified melt of the five-compound system Li₂O-MgO-Al₂O₃- SiO₂-CaO reveal that Li can occur in various compounds from beginning to the end of the crystallization. Among these compounds are Li_1−x(Al_1−xSi_x)O₂, Li_1−xMg_y(Al)(Al_3/2y+xSi_2−x−3/2y)O₆, solid solutions of Mg_1−(3/2y)Al_2+yO₄/LiAl₅O₈ and Ca-alumosilicate (melilite). There are indications of segregation processes of Al-rich and Si(Ca)-rich melts. The experimental results were compared with solidification curves via thermodynamic calculations of the systems MgO-Al₂O₃ and Li₂O-SiO₂-Al₂O₃.

1. Introduction

With respect to the development in electromobility as well as to the changes in circular energy systems, Li-ion batteries are of central importance. To safeguard raw material sources especially for critical elements such as Co, Ni and Li as key components of this technology, efficient recycling processes are essential. One of the most important routes to recycle these battery types is the pyrometallurgical processing, which can deal with a broad range of input material. While Co, Ni and other valuable heavy metals such as Cu report into the alloy melt, Li is transferred at least in major amounts into the slag phase of this process.

A resource and energy efficient recovery of Li from the slag could be accomplished, if Li were concentrated in specific Li-rich artificial mineral phases, which could then be separated after crystallization and cooling of the slag by means of mineral processing technologies, generating concentrates for following hydrometallurgical processing.

Previous research has shown that Li can be recovered in the form of the LiAlO₂ crystals through flotation from a remaining silicate slag matrix [1]. The hydrometallurgical processing of Li enriched silicate slag has also shown that Li recovery can reach 80–95% [2].

As long as the complete system, based on a Li₂O-Al₂O₃-SiO₂-CaO mixture, does not contain any other element, the results are promising. As soon as other elements are added, new phases start to crystallize.

Besides Li and Al, reporting from the Li-ion battery input into the slag, Si, Ca and often Mg (at least partly from dolomite as slag building component) are introduced as slag builders to ensure an optimized split between metal alloy melt, slag and dust phase in the pyrometallurgical process.

Until now, the thermodynamics of the overall process have not been investigated sufficiently and therefore for extended systems such as Li₂O-MgO-Al₂O₃-SiO₂-CaO this work serves as a starting point. Consequently, this should allow understanding some basic principles and giving further insights into these slag-systems. Additionally, a solid ground should be provided for further research on these slag systems, because in the future more complex slag systems, e.g., Mn-containing mixtures, should be investigated since they will represent future inputs to this recycling route especially for the NCM-type batteries.

The Umicore Battery Recycling Process is a vital pyrometallurgical process developed for the recovery of NiMH and spent lithium-ion batteries [3]. From the composition of a slag with the compounds Li₂O-MgO-Al₂O₃-SiO₂-CaO and high aluminum content, it is observed that Li is present in the slag in the form of the LiAlO₂ [2], which would facilitate subsequent recovery by flotation. At the same time, the spinel phase appears in all three Umicore slags, and, in one of the Umicore slags, Li is even partially dispersed in the spinel phase [3].

Even though spinel phases appear in different slags if bivalent ions such as those of Mg are present, there is little published research on the impact of spinel on the formation of separate LiAlO₂ crystals because of the scavenging of Al from the melt and the formation of Mg_1−(3/2y)Al_2+yO₄/LiAl₅O₈ solid solution.

In this study, three synthetic slags with different contents of MgO based on the Li₂O-MgO-Al₂O₃-SiO₂-CaO oxide system were prepared using pure chemical reagents. The degree of supercooling was then reduced by controlling and cooling the melt slowly to obtain thoroughly crystallized synthetic slags for research. The synthetic slags were then analyzed by X’Ray powder diffraction (PXRD) and electron probe microanalysis (EPMA) for mineralogical studies and finally compared to the solidification curves obtained by thermodynamic calculation. This served as a starting point for studying the influence of spinel formation and understanding important phase reactions in the five-component oxide system Li–Mg–Al–Si–Ca.

To increase the knowledge on the behavior of slag systems and the options to predict and stimulate the creation of artificial mineral phases, an interdisciplinary approach was taken, comprising thermodynamical modeling, pyrometallurgical processing, mineralogical analysis and prediction and testing of mineral processing technologies. In this paper, the focus is put on mineralogical analysis in connection with thermodynamic modeling.

2. Background

To better understand the results presented in this article, the existing information about the compounds of important binary and ternary systems containing Li₂O, MgO, Al₂O₃, SiO₂ and CaO is summarized. This information serves as the starting point to analyze and improve the existing data and develop respective thermodynamic modeling strategies.

2.1. Important Binary Phase Systems Containing Li

In the systems Li₂O-CaO and Li₂O-MgO, except for limited solid solution, no explicit phase reactions are reported (e.g., Konar et al. [4]).

In the system Li₂O-Al₂O₃, several stable lithium aluminate compounds are described: Li₅AlO₄, LiAlO₂ and LiAl₅O₈ [5,6]. Additionally, a high temperature compound LiAl₁₁O₁₇ at 0.8 < Al₂O₃ < 0.92 and >2200 °C is mentioned [5]. The compounds Li₂Al₄O₇ synthesized by Kale et al. [7] and Li₃AlO₃ were found to be instable by Kale et al. [7] and are not part of the data published by Konar et al. [5]. In this phase diagram, there is also a thermal barrier at the mole fraction of Al₂O₃ = 0.5 (LiAlO₂), so that at 0.18 < Al₂O₃ < 0.5 the resulting mixture is Li₅AlO₄/LiAlO₂ and at 0.5 < Al₂O₃ < 0.82 the resulting mixture is LiAlO₂/LiAl₅O₈. The two compounds important for this work, LiAlO₂ and LiAl₅O₈, both have polymorphs. According to Konar et al. [5], LiAlO₂ comprises a tetragonal γ-phase (high temperature) and a cubic α-phase (low temperature) modification and LiAl₅O₈ generally crystallizes in a spinel (high temperature) and a low temperature primitive cubic form [8]. According to Li et al. [9], LiAlO₂ comprises four polymorphs: a tetragonal γ-phase, a rombohedral α-phase, an orthorhombic β-phase and two phases of high temperature.

In the system MgO-Al₂O₃, the only binary compound is cubic MgAl₂O₄ (spinel) with the idealized composition at a mole fraction of Al₂O₃ = 0.5. At this ratio, there is also a thermal barrier. In the area of mole fraction 0 < MgO < 0.05 in the temperature range 1900–2800 °C, solid MgO can form a solid solution with Al₂O₃ [5]. The region of mole fraction 0.5 < Al₂O₃ < 0.96, particular important for this study, comprises a complete solid solution, so that an Al-rich melt can be in equilibrium with a spinel relatively enriched in Mg [10].

The system Li₂O-SiO₂ comprises the binary compounds Li₈SiO₆, Li₄SiO₄, Li₆Si₂O₇, Li₂SiO₃ and Li₂Si₂O₅ [11]. Additionally, the prediction shows two thermal barriers at the composition Li₄SiO₄ and Li₂SiO₃.

2.2. Important Ternary Phase Systems Containing Li

In the system Li₂O-MgO-Al₂O₃, three important primary crystallization fields can be predicted [5]: spinel (MgAl₂O₄), MgO and γ-LiAlO₂. Interesting isopleths are spinel-LiAl₅O₈, spinel-LiAlO₂, spinel-Li₂O and MgO-LiAlO₂. From this intersects, it can be concluded that a limited amount (i.e., maximum mole fraction = 0.31) of LiAlO₂ can be dissolved in MgO. Additionally, the compounds LiAl₅O₈ and MgAl₂O₄ can be combined to an ideal spinel solid solution [5].

The system Li₂O-Al₂O₃-SiO₂ contains the Li-bearing binary systems Li₂O-SiO₂ and Li₂O-Al₂O₃, as described in Section 2.1 [12]. With respect to the present work, the primary crystallization fields of LiAlO₂, LiAl₅O₈, eucryptite (LiAlSiO₄) and spinel are of particular interest. The compound LiAlO₂, described in Section 2.1, can additionally incorporate Si according to an substitution of Li⁺ + Al³⁺ = Si⁴⁺ + v (vacancy) so that the general formula is α (LiAl⁴⁺, vSi⁴⁺])O₂ and γ (Li, v)^Li[Al³⁺, Si⁴⁺]^MO₂ [12]. The compound eucryptite can be derived from SiO₂ via a substitution of Li⁺ + Al³⁺ = Si⁴⁺ + v [12] and crystallizes as quartz in the trigonal system, whereas a low temperature α-polymorph is disordered and a β-polymorph is ordered. Additionally, this compound can incorporate Mg and be broken up into the compounds LiAlO₂, Mg_0.5AlO₂ and SiO₂ [13]. The spinel crystallizes in a cubic system and can have a very variable chemistry with respect to the Al/Mg ratio and the solid solution with LiAl₅O₈ (see Section 2.1).

3. Materials and Methods

3.1. Materials

Chemicals

The chemicals used for producing synthetic slags are lithium carbonate (Merck, purum), calcium oxide (Sigma-Aldrich, reagent grade, St. Louis, MO, United States), silicon dioxide (Sigma-Aldrich, purum p.a., St. Louis, MO, United States), aluminum oxide (Merck) and magnesium oxide (98% wt.%, Roth, Karlsruhe, Germany). All chemicals ordered via Merck KGaA, Darmstadt, Germany.

3.2. Methods

3.2.1. Experiments

The chemical compositions of input materials for the synthesis of slags are listed in

Table 1. Calculated Theoretical Chemical Bulk Composition of the Samples According to the Weighed Quantities.

Sample	Li2CO3	CaCO3	SiO2	Al2O3	MgO
Content	%	%	%	%	%
1	22.87	22.40	16.36	32.97	5.40
2	22.51	21.55	16.08	32.17	7.70
3	22.32	21.24	15.63	31.54	9.27

. The chemicals were manually mixed in a mortar and grinded in a disc mill for 5 min. Each sample was placed in a Pt-Rh crucible and heated in a chamber furnace (Nabertherm HT16/17, Nabertherm GmbH, Lilienthal, Germany) in an air atmosphere. The heating regime is shown in Figure 1. A heating rate of 2.89 °C/min was first employed to reach 720 °C, which is the melting temperature of Li₂CO₃, and then a heating rate of 1.54 °C/min was used to aid in the decomposition of Li₂CO₃ and to reach the target temperature. Samples were kept at 1600 °C for 2 h. Thereafter, the samples were cooled to 500 °C at a cooling rate of 0.38 °C/min and quenched in water.

3.2.2. Chemical Bulk Analysis

The element content was determined with ICP optical emission spectrometry (ICP-OES 5100, Agilent, Agilent Technologies Germany GmbH & Co. KG, Waldbronn, Germany). Samples were melted with lithium tetra borate in a platinum crucible at 1050 °C, and then the samples were leached with dilute hydrochloric acid to measure the content of Al, Ca, Mg and Si. To measure other elements, the samples were mixed with nitric acid and digested at 250 °C and under a pressure of 80 bar in an autoclave (TurboWAVE, MLS, Leutkirch im Allgäu, Germany).

3.2.3. Mineralogical Investigation

An overview of the mineralogical composition was provided by powder X-Ray diffraction (PXRD), using a PANalytical X-Pert Pro diffractometer, equipped with a Co-X-Ray tube (Malvern Panalytical GmbH, Kassel, Germany). For identification of the compounds, the pdf-2 ICCD XRD database, the American Mineralogist Crystal Structure Database [14] and the RRUFF-Structure database [15] were assessed.

The analysis of single crystals and grains was carried out with electron probe microanalysis (EPMA). EPMA is a standard method to characterize the chemical composition in terms of single spot analysis or element distribution patterns, accompanied by electron backscattered Z (ordinal number) contrast (BSE(Z)) or secondary electron (SE) micrographs. To carry out EPMA measurements, the sample was prepared as polished block in epoxy resin, coated with carbon and characterized using a Cameca SX^FIVE FE Field Emission) electron probe, equipped with five wavelength dispersive (WDX) spectrometers (CAMECA SAS, Gennevilliers Cedex, France). The following elements/(lines) were used to quantify the measurement points: Na (Kα), Mg (Kα), Al (Kα), Si (Kα), K (Kα), Ca (Kα), Ti (Kα), Mn (Kα) and Fe (Kα). To calibrate the wavelength dispersive X-ray fluorescence spectrometers (WDRFA), an appropriate suite of standards and analyzing crystals was used. The reference materials were provided by P&H Developments Ltd. (Glossop, Derbyshire, UK) and Astimex Standards Ltd. (Toronto, ON, Canada). The beam size was set to 0, leading to a beam diameter of substantially below 1 µm (100–600 nm with field emitters of Schottky-type, e.g., Jercinovic et al. [16]). To evaluate the measured intensities, the X-PHI-Model was applied [17].

Lithium, one of the key elements in this study, cannot be directly analyzed since EPMA uses X-ray fluorescence to detect the elements in the sample and the extremely low fluorescence yield and long wavelength of Li Kα makes the direct determination of this element nearly impossible. With the reasonable assumption that other refractory light elements such as Be and B are not present in the investigated material and volatile elements and compounds such as F, H₂O, CO₂ or NO₃⁻ are effectively eliminated during the melt experiment, Li can be calculated using virtual compounds, as depicted in as described in Section 4.3.1. If necessary, the balanced Li concentration was included into the matrix correction calculation. To access the analytical accuracy with respect to Li-containing compounds, the international reference material spodumene (Astimex) and the in-house standard LiAlO₂ were used (

Table 2. Repeated Measurements on Two Li-Compounds. Spod, Spodumene; %StdDev, Percentage Standard Deviation of the Measured Points (pepeats: n = 5); R, Recovery; LiAl, LiAlO₂.

wt.%	Average Spod.	%StDev., Spod.	Ref. Spod.	R (%)	Average LiAl	%StDev., LiAl	Ref. LiAl	R (%)
Al	15.04	0.35	14.4	104	41.24	0.22	40.9	101
Mg	0.00	n. a.	0.0	n. a.	0.01	n. a.	0.0	n. a.
Ti	0.00	n. a.	0.0	n. a.	0.00	n. a.	0.0	n. a.
Mn	0.05	n. a.	0.0	n. a.	0.00	n. a.	0.0	n. a.
Fe	0.02	n. a.	0.0	n. a.	0.03	n. a.	0.0	n. a.
Ca	0.01	n. a.	0.0	n. a.	0.01	n. a.	0.0	n. a.
K	0.00	n. a.	0.0	n. a.	0.00	n. a.	0.0	n. a.
Si	28.71	0.56	30.0	96	0.01	n. a.	0.0	n. a.
Na	0.10	2.83	0.09	112	0.00	n. a.	0.0	n. a.

). Additionally, Li containing crystalline phases identified by X-ray diffraction (PXRD) could be referenced to the EPMA result.

3.2.4. Thermodynamic Modeling

For a better understanding of the experimental mechanisms investigated in the Li₂O-MgO-Al₂O₃-SiO₂-CaO system, the thermodynamic modeling of the phase behavior and the solidification in subsystems is of high relevance and hence applied in this work. Especially the knowledge of the phase behavior of the MgO-Al₂O₃ subsystem and the phases solidified at respective temperatures of certain component concentrations of the Li₂O-Al₂O₃-SiO₂ subsystem is important and contributes to the clarification and understanding of primary crystallization mechanisms figured out by the mineralogical characterization. On principal, based on already existing experimental data and thermodynamic studies, which are stated below, an optimized database for the subsystem was completed and applied to calculate the respective phase and solidification behavior. Generally, all calculations, i.e., for the binary MgO-Al₂O₃ and the ternary Li₂O-Al₂O₃-SiO₂ subsystems, were performed with the modified quasi-chemical model (MQM) [18,19,20] and the compound-energy formalism (CEF) [21], implemented in Factsage [22].

Specific insights into the database adaption regarding the two subsystems are presented subsequently. The thermodynamic database for the oxides such as MgO and Al₂O₃ comes from the FT oxide database [22] without any modification. Regarding the ternary subsystem Li₂O-Al₂O₃-SiO₂, the thermodynamic properties of SiO₂, Al₂O₃ and the mullite solid solution were used from the FT oxide [22] database without any modification. However, for the Gibbs energy of the Li₂O, the optimized value from [11] was integrated into the database. For compounds such as Li₂SiO₃, Li₄SiO₄, Li₆Si₂O₇, Li₂Si₂O₅-LT (low-temperature form) and Li₂Si₂O₅-HT (high-temperature form), the thermodynamic data were taken from [11]. The standard formation enthalpy of Li₈SiO₆ was optimized in this work with a value of 3, 521, 499.2 J/mol. Furthermore, for the binary compounds in the Li₂O-Al₂O₃, the standard formation enthalpy of the Li₅AlO₄ was optimized to 2,389,980 J/mol. The standard entropy of LiAl₁₁O₁₇ was optimized to a value of 350.55 Jmol⁻¹K⁻¹. The ternary compounds including the α- and β-eucryptite solid solutions, β-spodumene solid solution and α-LiAlO₂ solid solution were obtained from [12] without any modification. However, the Gibbs energy of the end member $G_{VaAl{O}_2}^0$ in the γ-LiAlO₂ solid solution was calculated with the assumption that the reciprocal energy of endmember is zero, while the other three endmembers were obtained directly from [12].

Based on these data, the CALPHAD calculations were performed for the subsystems, which are used for further explanations and discussions in connection with the new experimental findings in the next section.

4. Results

This section presents the measurement results of the melt experiments from PXRD and EPMA. First, three PXRD measurements from experiments with different Mg-concentration are compared (Section 4.2). In Section 4.3, an overview of the material with BSE(Z) micrographs and detailed spatially resolved quantitative point measurements and element distribution profiles recorded with EPMA are presented. Additionally, in Section 4.4, experimental findings are compared with thermodynamic model predictions for the relevant subsystems.

4.1. Bulk Chemistry of the Melt Experiments

The measurement results presented in

Table 3. Comparison of the Bulk Chemical Composition Measured with ICP-OES of the Four Melt Experiments, Given in Mole Percent. The bold emphazises the Li-loss which is important to see (Li is volatile).

	Raw Mix (Mole Fraction)			Product (Mole Fraction)			Recovery %
	V1	V2	V3	V1	V2	V3	V1	V2	V3
Al2O3	32.90	32.10	31.48	34.21	33.64	33.63	3.99	4.79	6.86
CaO	22.35	21.50	21.19	22.79	22.57	22.13	1.97	4.95	4.44
Li2O	22.81	22.46	22.28	20.00	18.69	17.57	−12.35	−16.77	−21.11
MgO	5.32	7.59	9.14	5.32	8.27	9.90	−0.11	9.03	8.32
SiO2	16.32	16.05	15.60	17.43	16.59	16.53	6.81	3.40	5.98
Na2O	0.3	0.3	0.3	0.3	0.23	0.2	−14.0	−21.4	−28.6

show that 14–20% of Li is lost during the melting and cooling of the material. The same applies for Na, which always appears as contaminant in open systems due to the overall availability (air, dust, skin, clothing, etc.).

4.2. PXRD Comparison of the Three Melt Experiments

The results of the PXRD measurements are presented in Figure 2, showing an overview of the diffractograms of all experiments (above) and three enlarged sections, showing important spinel and lithium aluminate diffraction peaks.

The overview of all XRD measurements show the compounds gehlenite, spinel, LiAlO₂ and eucryptite (Figure 2A), whereas eucryptite is at the detection limit (<2–5 wt.%). The enlarged section of the 2-theta region of the main spinel peaks gives an indication of the changing composition of the spinel with the change of the Mg content (Figure 2B). Because of the high Al-concentration, the main (100%) spinel peaks of all experiments lie between those of the standard spinel MgAl₂O₄ and an aluminum-dominated Mg_1−(3/2y)Al_2+yO₄. Additionally, there is an indication of increasing spinel content with rising Mg concentrations. The Li-Al-oxide peaks are best explained with the diffraction pattern of LiAlO₂ (ICCD PDF2 No. 00-038-1464). The comparison of the two main peaks of the three experiments gives a hint that the amount of crystalline LiAlO₂ could be negatively correlated with the amount of Mg in the melt because the highest main peak intensities were measured in the sample with the lowest Mg concentration (Figure 2C).

4.3. EPMA Results

The main compounds of the melt experiments, determined with EPMA, were:

• Spinel: Mg_1−(3/2y)Al_2+yO₄
• Lithium aluminate (LiAl): Li_1−x(Al_1−xSi_x)O₂
• Eucryptite-like lithium alumosilicate (ELAS): Li_1−xMg_y(Al)(Al_3/2y+xSi_2−x−3/2y)O₆
• Gehlenite-like calcium-alumosilicate (GCAS): Ca₂AL₂SiO₇ with minute amounts of Mg

The compound (GCAS) is an end member of the melilite-like calcium-alumosilicate (MCAS), which is used for this phase with higher amounts of ions in addition to Ca:

• Melilite-like calcium-alumosilicate (MCAS): (Na,Ca,Li)₂(Al,Mg,Li)(Al,Si)₂O₇, which according to the calculations (Section 4.3.3) can also be a potential host for Li

An overview recorded with BSE(Z) shows a matrix of bright Ca-alumosilicate (GCAS/MCAS) interspersed with dendritic or massive dark LiAl. Within this mixture, large idiomorphic or hypidiomorphic crystals of spinel can be identified (Figure 3 and Figure 4).

4.3.1. Lithium Aluminate (LiAl) and Lithium-Alumosilicate (ELAS)

The LiAl can be classified into two morphologic forms: massive and dendrite-like (Figure 3 and Figure 4A). A closer look into the massive LiAl reveals thin lath-shaped grains of ELAS or a corresponding melt (Figure 4B). A line scan over such a lath-shaped grain reveals a quite homogeneous composition with more or less sharp borders to the surrounding LiAl (Figure 4C). The ELAS can be described as a mixture of the virtual compounds LiAlO₂, Mg_0.5AlO₂ and SiO₂, as listed in

Table 4. Calculation of virtual compound ratios and average composition of the ELAS and the LiAl on Line 3, shown in Figure 4. Opt., calculated ideal composition; Meas., measured average; Mult., factor for multiplication of the virtual compounds; (Calc.), calculated values (Li, O); %StdDev, percentage standard deviation of the measured points (LiAl (Meas.), repeats, n = 23).

wt.%	Virtual Compounds			ELAS (Opt.)	ELAS, Meas.	ELAS %StDev.	LiAl (Opt.)	LiAl (Meas.)	LiAl %StDev.
	LiAlO2	Mg0.5AlO2	SiO2
Al	40.9	37.9	0.0	20.1	20.1	3.2	38.9	38.9	1.4
Mg	0.0	17.1	0.0	3.1	3.0	7.5	0.0	0.0	n. a.
Ti	0.0	0.0	0.0	0.0	0.0	n. a.	0.0	0.0	n. a.
Mn	0.0	0.0	0.0	0.0	0.1	n. a.	0.0	0.0	n. a.
Fe	0.0	0.0	0.0	0.0	0.1	n. a.	0.0	0.1	n. a.
Ca	0.0	0.0	0.0	0.0	0.0	8.9	0.0	0.0	n. a.
K	0.0	0.0	0.0	0.0	0.0	n. a.	0.0	0.0	n. a.
Si	0.0	0.0	46.7	23.2	22.4	3.6	2.5	2.5	18.3
Na	0.0	0.0	0.0	0.0	0.0	n. a.	0.0	0.0	n. a.
O (Calc.)	48.5	45.0	53.3	50.2	49.5	n. a.	49.0	49.1	n. a.
Li (Calc.)	10.5	0.0	0.0	3.4	3.4	n. a.	10.01	10.01	n. a.
Mult.	0.33	0.18	0.49	←  Multiplication factors for ELAS (Opt.)
Mult.	0.95	0	0.049	←  Multiplication factors for LiAl (Opt.)
Sum	100	100	100	100	98.7		100.4	100.6

.

Mult. depicts a factor to multiply the three components to generate an optimized ELAS or Li_1−xMg_y(Al)(Al_3/2y+xSi_2−x−3/2y)O₆ similar to the average measured concentrations (except for Li) on Line 3 (Figure 4). The Li value results from the multiplications and this was used to calculate the formula of the analyzed ELAS in the sample. In a similar manner, the Si-containing LiAl with the general formula Li_1−x(Al_1−xSi_x)O₂ can be calculated as a mixture of SiO₂ and LiAlO₂. The calculated formulas of the ELAS and the LiAl are:

ELAS: (Li_0.96Mg_0.24)(Al)(Al_0.45Si_1.55)O₆

LiAl: (Li_0.94)(Al_0.94Si_0.06)O₂

The Si concentration in the dendritic LiAl is distinctively lower as in the massive crystals (compare Table 4 and

Table 5. Calculation of Virtual Compound Ratios and Average Composition of the LiAl in the Dendrites (Dend.) Shown in the BSE(Z) Micrograph of Figure 3. Opt., Calculated Ideal Composition; Meas., Measured Average; Mult., Factor for Multiplication of the Virtual Compounds; (Calc.), Calculated Values (Li, O); %StdDev, Percentage Standard Deviation of the Measured Points (LiAl (Dend.) (Meas.), Repeats, n = 4).

wt.%	Virtual Compounds		LiAl (Opt.)	LiAl (Dend.) (Meas.)	LiAl (Dend.) % StDev.
	LiAlO2	SiO2
Al	40.9	0.0	40.3	40.3	0.4
Mg	0.0	0.0	0.0	0.0	n. a.
Ti	0.0	0.0	0.0	0.0	n. a.
Mn	0.0	0.0	0.0	0.0	n. a.
Fe	0.0	0.0	0.0	0.0	n. a.
Ca	0.0	0.0	0.0	0.0	n. a.
K	0.0	0.0	0.0	0.0	n. a.
Si	0.0	46.7	1.1	1.1	12.3
Na	0.0	0.0	0.0	0.0	n. a.
O (Calc.)	48.5	53.3	49.0	49.1	n. a.
Li (Calc.)	10.5	0.0	10.36	10.36	n. a.
Mult.	0.98	0.024	←  Multiplication factors for LiAl (Opt.)
Sum	100	100	100.8	100.9

). The calculated formula of the LiAl in this case is:

LiAl: (Li_0.97)(Al_0.97Si_0.03)O₂

4.3.2. Spinel

Spinel as the first crystallizing compound obeys the crystallization equilibrium inasmuch as the composition of the spinel with the highest Al content is connected with the corresponding Al-rich melt. The Mg concentrations in the measured profile (Figure 5B) are increasing from the center to the rim of the crystal. A look at the ratio of Mg/Al in a line scan through a spinel crystal starting at the center of the grain in the direction to the rim shows no increase within a first region. After this first region, the ratio increases. Closer to the rim, the ratio decreases sharply and directly at the rim (a few µm) the ratio development of the two elements is reversed again (Figure 5B).

4.3.3. Ca-Alumosilicate (GCAS/MCAS)

The matrix component of the melt experiments (e.g., Figure 3 or Figure 4A light grey sections) can be generally expressed as X₂YZ₂O₇, where X can be Na⁺ and Ca²⁺; Y can be Al³⁺, Mg²⁺ and Fe²⁺; and Z can be Al³⁺ and Si⁴⁺. The coordination of X is 8, and Y and Z are tetrahedral [23]. This Ca-alumosilicate compound generally is known as melilite. The investigated material comprises two types of Ca-alumosilicate:

• GCAS: High Al, low Si, very low Mg and virtually no Na
• MCAS: Low Al, high Si, ~3 wt.% Mg and 0.7–2.3 wt.% Na/Li is plausible

These two types are difficult to distinguish in the BSE(Z)-micrograph (Figure 4A) because of the almost same light grey shade (very similar mean atomic number). Because Na is not part of the initial materials (impurity), the concentration of the MCAS compound can be considered rather low and represents the eutectic residual melt. Nevertheless, this compound is interesting to assess a potential Li incorporation into the matrix of Ca-alumosilicate. In theory, Li⁺ can be present in 4 or 8 coordination, whereas the ionic radius is very similar to Mg (4-coordination) or Na (8-coordination) (e.g., the ionic radii are published by Shannon (1976) [24]). The MCAS possesses a lower total sum of the measured concentrations and excess Si when calculating the chemical formula of the MCAS using the general melilite based on seven oxygen atoms.

Table 6a depicts how a calculation of virtual Ca-alumosilicate (CAS) can be conducted using five virtual compounds, namely Li₂Si₃O₇, Na₂Si₃O₇, Ca₂Al₂SiO₇ Ca₂MgSi₂O₇ and Ca₃Si₂O₇, assuming (limited) solid solution between those compounds. The Li value resulting from the multiplications was used to calculate a chemical formula of the analyzed MCAS. According to this calculation, the Li₂Si₃O₇ makes up about 1 wt.% of the total composition (see Table 6b).

The calculated formulas of the GCAS and the MCAS are:

GCAS:   Ca_2.02(Al_1.96Mg_0.03)(Al_1.96Si)O₇

MCAS:   (Na_0.18Ca_1.67Li_0.15)(Al_0.52Mg_0.32Li_0.08)(Al_0.17Si_1.82)O₇

4.4. Comparison of Experimental Findings with Thermodynamically Modeled Subsystems

Based on the respective phases of interest, the relevant subsystems are MgO-Al₂O₃ and Li₂O-Al₂O₃-SiO₂. The modeled phase diagrams are presented in Figure 6 and Figure 7, respectively. In Figure 6, a comparison between the modeled phase equilibria and the experimental data is made. In Figure 6, the composition of the initial melt and the composition of different spinel grains from two line scans and several single spot measurements, analyzed experimentally at room temperature (RT), are presented in an overlay with the thermodynamic phase equilibrium data for the subsystem MgO-Al₂O₃. The composition of the initial melt is the starting point of the spinel crystallization. It can be seen that all measured spinel grains show a significant higher Mg concentration compared to the initial Mg concentration in the melt.

Figure 7 shows the thermodynamic calculated Li₂O-Al₂O₃-SiO₂ subsystem. The equilibrium solidification paths for the “raw mix” (Figure 7B) and the “product” (Figure 7A) composition are calculated and presented in the respective ternary phase diagram. Additionally, the average compositions of the single compounds, analyzed with EPMA at room temperature (see Table 4, Table 5 and Table 6b), and the bulk chemistry of the “raw mix” and “product” are visualized in an overlay with the modeling results in Figure 7.

The “raw mix” and the “product” composition is in the spinel solid solution area, which is concluded by the thermodynamic modeling results based on the subsystem. Hence, the thermodynamic modeled solidification predicts spinel as the primary crystallizing phase (see Figure 8). After decreasing the temperature continuously under assumed equilibrium conditions, the solidifications of different phases are shown in Figure 8, for the “product” (Figure 8A) and “raw mix” (Figure 8B) initial concentrations, respectively. The thermodynamic prediction of the subsystem solidification shows that spinel as primary crystal is formed in solid solution with high temperature LiAl₅O₈ for both initial compositions. With progressing solidification, low temperature LiAl₅O₈ is also crystallizing out of solution. This finding holds true for both initial compositions. With increasing solidification progress, LiAlO₂ and Li₂SiO₃ are formed with a very low amount of eucryptite, for the “raw mix” configuration, while, for the “product” composition (Figure 8A), eucryptite is formed in a higher amount without any LiAlO₂.

5. Discussion

The experimental investigation of solidified melt in connection with thermodynamic modeling of chemical reactions and solidification is an important tool to investigate how a slag system behaves and how it can be engineered. These investigations and the obtained results can serve as starting point for understanding efficient design of experiments to generate the desired phases. With a combination of thermodynamic calculation and mineralogical investigation, the probability that the artificial slag contains the desired phases can be maximized. Therefore, one purpose of the experiments carried out in this project was a first survey of the mineral compounds and the morphology of a solidified melt with the basic components Li₂O, MgO, Al₂O₃, SiO₂ and CaO with a melt composition in the primary crystallization field of spinel in the subsystem Li₂O, Al₂O₃ and SiO₂. Another purpose was to investigate the influence of the Mg content on the ratio of the mineral compounds. The results of these experiments are also intended to serve as a basis for further thermodynamic modeling. Additionally, the applicability of the combination of PXRD and EPMA to this research topic was assessed. This includes the calculation of the lithium containing mineral compounds on basis of the EPMA result without access to measured lithium concentrations. In the following, the different identified phases are discussed:

5.1. Spinel-Like Oxides

The experiments show idiomorphic phenocrysts of spinel as the first crystallizing compound with decreasing temperature. The spinel crystals are surrounded by massive hypidiomorphic crystallites of LiAl and melilite-like alumosilicate (GCAS/MCAS). Additionally, LiAl forms dendritic elongated structures of hypidiomorphic crystallites. The changes in the chemistry of a single spinel crystal (Figure 5B) can help to explain a part of the crystallization curve of the melt. This is also used to validate thermodynamic model predictions of the three-component subsystem. Starting in the primary crystallization field of spinel, an Al-rich Mg_1−(3/2y)Al_2+yO₄ starts to form. These crystals are in equilibrium with a corresponding melt (Figure 5B (blue area) and Figure 6). The EPMA reveals that, compared with the Mg/Al ratio of the melted material, all measured spinel grains are enriched in Mg. This observation shows the complex spinel behavior, which cannot be explained with the simple binary phase diagram MgO-Al₂O₃. Nevertheless, the measured Mg/Al ratio increases (Figure 5B, yellow area). This can be explained with the composition of the melt reaching the phase boundary between spinel and LiAl. Through scavenging of Al from the melt during formation of LiAl, the Mg concentration in the melt increases and therefore the spinel–melt equilibrium changes. At a later stage of the crystallization process, the Al concentration in the crystal increases again, an indication that now a solid solution between Mg_1−(3/2y)Al_2+yO₄ and Mg-free LiAl₅O₈ forms (Figure 5B, green area). At the end of the crystallization, the Mg concentration rises again (Figure 5B, red area). This is an indication that the crystallization leaves the crystallization path between LiAl₅O₈ and spinel in direction of the crystallization path between eucryptite (or ELAS) and spinel. Therefore, the crystallization of the spinel would no longer include the aluminum-rich LiAl₅O₈ and the relative Mg concentration of the crystallizing spinel would increase, although a part of the Mg is incorporated into the ELAS. The crystallization path concluded by experimental observations of the developing spinel composition is on principal in good correlation with the thermodynamically predicted solidification phases in the early stages (Figure 7 and Figure 8). However, for lower temperatures, the solidification predictions deviate from the experimental findings, which is due to non-equilibrium cooling conditions and hence phase generation. The modeled results show that small deviations in the initial concentration in the spinel solid solution field can result in strong deviations regarding appearing solid phases and solidification path behavior.

The PXRD patterns of the investigated melts with increasing Mg concentration show a displacement of the spinel main peak. The angular position of the main peak of these spinel variations is between the simple MgO × Al₂O₃ compound and a pattern of an Al-rich spinel with the formula Mg_0.39Al_2.41O₄ and weakly correlates with the Mg concentration as:

Mg_y = 20.198 × d₃₁₁ − 48.247

(1)

5.2. LiAl and ELAS

The formation of dendrites is an indication for rapid crystallization of LiAl in a small temperature interval from a supercooled melt and/or (macro)segregation (for macro segregation, see, e.g., Ahmadein et al. [25]). Due to the rather long cooling cycle (two days, Figure 1), undercooling may be improbable but cannot completely be excluded. Nevertheless, it is plausible that a segregation of an Al-Li-rich melt occurs from which the first generation of LiAl crystals forms. The Si concentrations of the dendritic LiAl is lower than in the massive LiAl, indicating a different origin, thus a different melt as well (compare LiAl in Table 4 and Table 5). A few parts of the massive LiAl contain small lath-shaped grains of ELAS. This compound can be derived from eucryptite and can contain up to 18 wt.% Mg_0.5AlO₂. These grains are an indication of segregation of Si- and Mg-rich phases (melt) from the Al-rich LiAl-melt, as described above. Interestingly, the representing point of this calculated compound is not located in the primary crystallization field of pure LiAlSiO₄. This is due to the lower calculated Li content because of the Li-free Mg_0.5AlO₂ compound.

5.3. Ca-Alumosilicate

The matrix of the material consists of slightly hypidiomorphic grains of Ca-alumosilicate. The morphology of the crystals indicates that the formation starts together or slightly after the beginning of the crystallization of LiAl, which itself often shows hypidiomorphic growth. The chemical composition of these Ca-alumosilicates starts with nearly ideal gehlenite (GCAS) with minute amounts of impurities such as Mg. The other type of Ca-alumosilicate (MCAS) incorporates higher amounts of impurities such as Na and Mg (Table 6b). Because of the presence of an alkaline element such as Na, the latter compound seems to represent the end of the crystallization, i.e., the residual eutectic melt. Interestingly, this compound delivers a total of distinctively less than 100 wt.% (element concentrations calculated as simple oxide compounds) and possesses an excess of Si after calculation of the melilite formula. This is an indication that another silicious component is present in the crystal structure. Because the sample contains no free SiO₂ (like quartz) and the analysis shows no additional element for calculation of a silicious component, incorporation of Li into the crystal or glassy structure as shown in Table 6b is plausible. After incorporation of a virtual compound Li₂Si₃O₇, a formula can be calculated indicating a consistent crystal-like chemistry or a stoichiometric glass.

The mineralogical characterization of a melt as presented above provides a basis for refining the thermodynamic model, showing the real assemblage of components and the real chemical composition of the compounds/phases. An example would be ELAS. The eucryptite compound, used for the thermodynamic calculation, is ideal LiAlSiO₄. EPMA reveals that the real eucryptite-like alumosilicate (ELAS) can be expressed with (Li_0.96Mg_0.24)(Al)(Al_0.45Si_1.55)O₆. This compound contains Mg, which has to be taken into account when using this compound to predict a crystallization curve. The same is valid for the lithium aluminate compound (LiAl, Li_1−x(Al_1−xSi_x)O₂) that contains Si. Another important property is the inherent potential kinetic inhibition of the phase reactions in the system of interest. The morphology of the slag including structure and habitus corresponds to the crystallizing reactions during the cooling process. Additionally, the total chemistry and the spatial resolved development of element ratios can be used to explain the solidification process.

6. Conclusions and Outlook

In this work, an experimental investigation of a Li₂O-MgO-Al₂O₃-SiO₂-CaO system was carried out in combination with thermodynamic modeling of relevant subsystems. Based on bulk chemistry analysis, PXRD and EPMA, the crystallization paths of various phases were reconstructed and explained. It was shown that spinel is always the primary crystallizate. Furthermore, depending on minute variations in the chemistry of the melt, the result of the thermodynamically predicted further phase development can be substantially different. In this case, comparing the solidification of the raw material and the product the unpredictable loss of Li during the melt experiment seems to offer the possibility of a complete suppression of the LiAlO₂ formation in favor of Mg_1−(3/2y)Al_2+yO₄/LiAl₅O₈ solid solution, although this was not observed in the experiments. The eucryptite and Li-silicate compound are the ends of the solidification in both scenarios. Nevertheless, a knowledge and/or control of all reaction parameters such as partial pressures of all elements (particularly, Li here) and compounds, grain size distribution, morphology and chemistry of the raw material is crucial to develop an efficient and reproducible slag modification process. The solidification route of the system could be qualitatively predicted by thermodynamic modeling of the Li₂O-Al₂O₃-SiO₂ subsystem with the result that minute variations of the initial chemistry can lead to different solidification paths.

Additionally, a relative Mg enrichment of spinel grains could be observed experimentally. Furthermore, the development of the composition in single spinel grains during spinel grains give an indication of the existing solid solution Mg_1−(3/2y)Al_2+yO₄/LiAl₅O₈, which was only predicted and not verified in the past.

The results presented in this article show that Li cannot be incorporated into a single early crystallizing compound in an easy way. The investigations show that Li is present in LiAl, ELAS and with higher uncertainty in spinel (as solid solution Mg_1−(3/2y)Al_2+yO₄/LiAl₅O₈) and MCAS. To modify this complex multi-component system (oxides of Li, Mg, Al, Si and Ca) to gain desired mineral compounds requires, besides experimental work (melt experiments, component printing and combinatorial thin film deposition), new thermodynamic modeling strategies even for higher component systems, especially with a good quantitative predictability for the phase fractions.

Furthermore, future research work will concentrate on the development of phase separation processes, predominantly by flotation for the main identified Li-bearing phases described in this paper (basic research on the way) and on the extension of component mixtures in the slag building process, advancing step by step into slag systems expected in melting processes of actual and future battery systems.