Dissolution of Calcium Silicates in Molten CaCl2

Electrodeposition of silicon in molten CaCl2 has been drawing increasing attention. Soluble silicate anions in the melt can be reduced and tailored into functional materials. To ensure a precise control of the electrochemical processes, a comprehensive knowledge on the soluble silicate anions in the melt is indispensable. Here, we clarify the formation behavior of soluble silicate anions by dissolving solid CaSiO3 and Ca2SiO4 in molten CaCl2 at temperatures from 1047 K to 1233 K (774 °C to 960 °C). It is found that Ca2SiO4 dissolves more and faster. The formation of Ca2SiO3Cl2 or Ca3SiO4Cl2 as an intermediate product is observed. Metasilicate (SiO32−) and orthosilicate (SiO44−) are detected as soluble anions in the melt when dissolving CaSiO3 and Ca2SiO4, respectively. Mass transfer of these ions plays an important role in determining the overall dissolution rate. The polymeric chain-like structure of SiO32− is the reason for the slower dissolution of CaSiO3 compared with that of Ca2SiO4, which dissolves as SiO44− with a completely isolated tetrahedral structure in the melt. Phase relationships in the CaCl2–CaSiO3 and CaCl2–Ca2SiO4 binary systems at the CaCl2-rich side are clarified. These results should be useful for manipulating the electrochemical reactions of Si (IV) anions on purposes. Supplementary Information The online version contains supplementary material available at 10.1007/s11663-022-02679-z.


I. INTRODUCTION
THE global photovoltaic (PV) market is growing rapidly in recent years. Despite the impact by Covid-19, approximately 175 GW of PV systems were newly installed worldwide in 2021, making the total cumulative installed capacity for PV reaching at least 942 GW at the end of 2021. [1] Silicon (Si) has been playing an important role in the PV market as the key material for PV cells. Si-based PV cells account for more than 95 pct of the global PV market. [2] As the second most abundant element in the crust, Si is widely distributed in rocks, gravels, dusts, and sands in the form of oxide or silicates. Generally, elemental Si is commercially prepared by carbothermic reduction of SiO 2 at temperatures higher than 1873 K (1600°C). This reaction is strongly endothermic, [3,4] and thus the production is energy-intensive with inevitable emission of a large amount of CO 2 . Si obtained by this process is called metallurgical grade Si (MG-Si), of which the purity is about 98 to 99 pct. MG-Si can be directly used to prepare various Si materials. However, in order to be used as a semiconductor for PV application, MG-Si must be purified into solar grade (SOG-Si) with a purity of at least 6N (99.9999 pct). At present, about 70 to 80 pct of SOG-Si is produced by the Siemens process based on chemical vapor deposition. Although the product is of high purity, the Siemens process is highly complex and extremely energy-intensive. Owing to these defects, development of a cost-effective technology is urgently needed for a sustainable production.
Many alternative technologies for extracting Si have been reported. Among them, molten salt electrolysis seems to be a promise one because of its advantages in cleanness and simplicity. [5,6] Electrodeposition of Si from fluoride-based molten salts has been investigated since the 1960s. Monnier et al. reported the electrodeposition of Si alloys or pure Si in molten Na 3 AlF 6 containing SiO 2 . [7,8] Cohen et al. was the first to report a successful plating of Si films onto an Ag coated Si wafer in molten LiF-KF-K 2 SiF 6 . [9,10] Elwell et al. and Boen et al. also obtained compact and smooth Si films on Ag or graphite substrates in molten LiF-NaF-KF-K 2 SiF 6 . [11,12] Recently, Yasuda et al. obtained relatively high-quality Si deposits in molten KF-KCl by using K 2 SiF 6 or SiCl 4 as the precursor. [13][14][15] Fluoride-based molten salts are useful in depositing Si films. However, due to the corrosive and harmful nature of fluorides, impurity control in the electrodeposited Si is difficult, and waste XIAOTIAN  fluorides may cause environmental pollution. Therefore, a fluoride-free electrolyte is desirable for the future development of this technology. Molten CaCl 2 with a large solubility of O 2À can be used as an alternative electrolyte to fluorides for electrodeposition of Si. [16][17][18] As demonstrated by Yang et al. (Figure 1), high-quality Si films can be synthesized in molten CaCl 2 -CaO-SiO 2 at 1123 K by electrodeposition. [19] Dong et al. prepared Si nanowires by electrolysis of CaSiO 3 dissolved in CaCl 2 -based molten salts. [20] Weng et al. also obtained Si nanowires on carbon cloth from SiO 2 dissolved in molten CaCl 2 -NaCl. [21] These Si products (films or nanowires) may have applications as functional materials in many fields.
The formation of soluble Si (IV) anions in molten CaCl 2 is the prerequisite condition for electrodeposition. Xiao et al. proposed a dissolution-electrodeposition mechanism during the electrochemical reduction of solid SiO 2 in molten CaCl 2 . [22,23] Ma et al. investigated the electrochemical reduction behaviors of silicates in CaCl 2 -based molten salts and revealed the ionic species of silicates by Raman spectroscopy. [24][25][26] However, knowledge on the fundamentals of the soluble Si (IV) anions is still far from sufficient for a precise control of the electrochemical processes. This work clarifies the formation behavior of Si (IV) anions in molten CaCl 2 from solid CaSiO 3 and Ca 2 SiO 4 , which are considered as proper precursors due to the cost and accessibility for a commercial process in the future. Solubility and dissolution rate at different temperatures were measured. Phase relationships of the CaCl 2 -CaSiO 3 and CaCl 2 -Ca 2 SiO 4 binary systems were clarified. Structure of soluble Si (IV) anions was characterized. These results should be useful for manipulating the electrochemical reactions of Si (IV) anions on purposes.

A. Materials
Reagent grade CaSiO 3 (> 98 pct), CaO (> 98 pct), SiO 2 (> 98 pct), and CaCl 2 (> 97 pct) were purchased from Greagent Co., Ltd. Prior to use, CaSiO 3 , CaO, and SiO 2 were calcined in a dry Ar atmosphere (200 mL/ min) at 1073 K (800°C) for 3 hours to remove residual moisture. CaCl 2 was kept in vacuum at 473 K (200°C) for 72 hours to remove moisture. In a pre-experiment, the molar fraction of O 2À in CaCl 2 was determined to be around 0.0007 by an acid-alkali titration method. [27] We considered it an acceptable low value and the effect on dissolution behaviors of silicates would not be significant. Ca 2 SiO 4 (c-type) was synthesized by heating the mixture of CaO and SiO 2 (molar ratio = 2:1) at 1673 K (1400°C) in air for 2 hours (Figure S-1). Figure 2 shows the distribution of particle size of CaSiO 3 or Ca 2 SiO 4 after calcination, measured by laser particle size analyzer (LPSA, Malvern, Mastersizer 2000/3000). 90 pct of CaSiO 3 particles were smaller than 47.7 lm, and 90 pct of Ca 2 SiO 4 particles were smaller than 59.7 lm.

B. Procedure
Intermediate product of reaction between CaCl 2 and CaSiO 3 or Ca 2 SiO 4 was first investigated. Mixture of powdery CaCl 2 and CaSiO 3 or Ca 2 SiO 4 (molar ratio = 1.5:1) placed in an alumina crucible (o.d. 30 mm, i.d. 26 mm, height 28 mm, 99.9 pct) was heated at temperatures from 873 K to 1047 K (600°C to 774°C) in Ar. Postreaction samples were rinsed with distilled water, followed by filtration to recover solid residues for analysis.    immersed in the bath. The melt outside the small crucible was sampled for chemical analysis after different reaction time by dipping an alumina rod (dia. 6 mm). After experiment, undissolved residues in the small crucible were also collected for analysis.
Cooling curves of homogeneous CaCl 2 -CaSiO 3 or CaCl 2 -Ca 2 SiO 4 melt were measured by a K-type thermocouple equipped with a temperature data logger. After melting 100 g of mixture of CaCl 2 and silicates and confirming complete dissolution of silicates, thermocouple wrapped by an Al 2 O 3 tube was immersed in the bath. By cooling the furnace at a rate of 5 K min À1 , variation of the temperature with time was measured by the thermocouple and recorded by a data logger at an interval of 2 seconds. Thermocouple was calibrated by measuring the cooling curves of pure chlorides (MgCl 2 , CaCl 2 , NaCl, and CsCl 2 ) beforehand (result shown in Figure S-2).

C. Analysis and Characterization
Solidified melt sample attached to the alumina rod was digested by an alkali fusion method to prepare solution for analysis. A mixture of Na 2 CO 3 (Sinopharm Chemical Reagent, > 99.8 pct) and H 3 BO 3 (Sinopharm Chemical Reagent, > 99.8 pct) with a mass ratio of 3:1 was used as an alkali flux. Melt sample (about 0.15 g) and alkali flux (about 0.30 g) were mixed evenly in a Pt crucible, which was heated to 1273 K (1000°C) at a rate of 4 K min À1 and kept for up to 40 minutes in air before cooling to room temperature at a rate of 5 K min À1 . Afterward, the Pt crucible was set in a polytetrafluoroethylene (PTFE) beaker containing 100 mL of 5 pct HNO 3 at 373 K (100°C) for 2 hours to completely dissolve the substances inside. Si concentration was measured by inductively coupled plasma mass spectrometer (ICP-MS, ThermoFisher, iCAP RQ).
In some cases, solidified melt sample was rinsed by distilled water, followed by filtration to recover insoluble residues. These residues after drying at 353 K (80°C ) for 24 hours were characterized by Powder X-ray diffractometer (PXRD, CuKa, Bruker, D8 Advance). In-situ Raman spectroscopy to evaluate the structure of Si(IV) anions was conducted by a spectrometer (WITec, Alpha300R) with a high-temperature stage (Linkam, TS1500) heated in a flowing Ar atmosphere.

III. RESULTS AND DISCUSSION
A. Formation of Intermediate Products Figure 4 shows the XRD patterns of the water-rinsed residues after reacting CaCl 2 with CaSiO 3 or Ca 2 SiO 4 at different temperatures. In the case of reacting CaSiO 3 [ Figure 4(a)], new phase was rarely found when the temperature was lower than 1045 K (772°C, melting point of CaCl 2 ), indicating that solid CaSiO 3 does not react with solid CaCl 2 . However, when raising the temperature to 1047 K (774°C), a new phase Ca 2 SiO 3 Cl 2 was detected in the residue after reaction for 20 hours, suggesting the following solid-liquid reaction It is known from these results that CaCl 2 reacts more readily with Ca 2 SiO 4 than with CaSiO 3 , which may bring different dissolution behaviors between the two silicates in molten CaCl 2 .
B. Solubility of CaSiO 3 and Ca 2 SiO 4 in Molten CaCl 2 Figure 5 shows the dissolution curves of solid CaSiO 3 and Ca 2 SiO 4 in molten CaCl 2 at various conditions. Raw data are listed in Supplementary Tables S-1 Table I, solubility increases as temperature rises, and Ca 2 SiO 4 dissolves more in molten CaCl 2 than CaSiO 3 at equal conditions. It is considered that Ca 2 SiO 4 is a precursor more suitable than CaSiO 3 for electrodepositing Si in molten CaCl 2 , because a large solubility of soluble Si (IV) can provide a wider range to design molten salt composition. In addition, as highlighted in Supplementary Figure S- Figure 6(b)], Ca 3 SiO 4 Cl 2 were detected regardless of the temperature. Although not dissolved, calcium silicates transformed into something else. These results confirm the findings in the previous section that intermediate products are formed when dissolving calcium silicates in molten CaCl 2 .

C. Thermodynamic Analysis of the Dissolution Process
The overall process of dissolving a solute A in a solution can be written as After reaching saturation, where DG o soln is the change of standard Gibbs free energy in dissolution; DH o soln and DS o soln are standard dissolution enthalpy and entropy, respectively; R is gas constant (= 8.314 J mol À1 K À1 ); a l A and a s A are the activities of A relative to pure liquid or solid,  . From the solubility data in Table I, Van't Hoff plots of dissolving solid CaSiO 3 and Ca 2 SiO 4 at temperatures from 1045 K to 1233 K (772°C to 960°C) are obtained, as shown in Figure 7. Accordingly, standard dissolution enthalpy and entropy can be calculated, as listed in Table II. Dissolution of both silicates in the temperature range are endothermic. A larger dissolution enthalpy for CaSiO 3 indicates that dissolution of CaSiO 3 is more difficult and requires more energy input.

D. Phase Relationships of the CaCl 2 -CaSiO 3 and CaCl 2 -Ca 2 SiO 4 Binary Systems
It is considered that CaSiO 3 or Ca 2 SiO 4 and CaCl 2 will form eutectic mixtures. Location of the liquidus lines of the CaCl 2 -CaSiO 3 and CaCl 2 -Ca 2 SiO 4 binary systems on both sides of a eutectic point can be estimated by the solubility of CaSiO 3 or Ca 2 SiO 4 in CaCl 2 at different temperatures and the cooling curves at different compositions. Figure 8 shows the measured cooling curves of homogeneous melts containing different amount of CaSiO 3 or Ca 2 SiO 4 . Because precipitation of solute during cooling will release heat and affect the cooling behavior, the inflection point on the cooling curve is regarded as the point on the liquidus line. 10 20    Note Enthalpy and entropy are assumed to be constant in the temperature range. Activity coefficients of solutes with reference to their pure liquid are assumed to be unity. Therefore, phase relationships in the CaCl 2 -CaSiO 3 and CaCl 2 -Ca 2 SiO 4 binary systems at the CaCl 2 -rich side can be clarified, as shown in Figure 9. Locations of liquidus lines in this work are at higher temperatures compared with a previous report by Stemmermann et al., [28] who employed differential thermal analysis (DTA) to determine the phase diagram. For instance, according to the Ca 2 SiO 4 -CaCl 2 phase diagram reported by Stemmermann et al., the solubility of Ca 2 SiO 4 in molten CaCl 2 is around 26.66 mass pct (19 mol pct) at 1079 K (822°C). However, our work indicates that only 8.90 mass pct (5.93 mol pct) of Ca 2 SiO 4 can dissolve at this temperature. We believe our results resting on chemical analysis are more accurate because the DTA method usually could not exclude the influence of solid-solid phase transitions with heat transfer.
Clarification of the liquid regions of the CaCl 2 -CaSiO 3 and CaCl 2 -Ca 2 SiO 4 binary systems is necessary to design the molten salt composition for an efficient electrodeposition of Si.

E. Dissolution Kinetics of CaSiO 3 and Ca 2 SiO 4 in Molten CaCl 2
As shown in Figure 5, Ar bubbling can greatly shorten the time to reach saturation when dissolving CaSiO 3 and Ca 2 SiO 4 in molten CaCl 2 , indicating that mass transfer in the melt plays an important role in the dissolution process. Assuming mass transfer in the melt is the rate-determining step for the whole procedure of dissolution, dissolution rate of Si (IV), r, can be expressed by where A is the sectional area of the small graphite crucible containing silicates. Here, the sectional area is taken as the dissolution interfacial area, as an approximation to simplify the calculation. k is the apparent mass transfer coefficient of Si (IV) in the melt (m sec À1 ); t is dissolution time (sec); V is the volume of molten CaCl 2 ; C t represents the instant concentration (g cm À3 ) of Si (IV) in the melt at time t; C s is the solubility (g cm À3 ) of Si (IV). Integration of Eq. [6] in the time range from t 0 to t leads to where C 0 is the concentration of Si (IV) at t 0 . Using the data in Figure 6, plots of À V A In C s ÀC t ð Þ C s ÀC 0 ð Þ against tÀt 0 for CaSiO 3 and Ca 2 SiO 4 are obtained, as shown in Figure S-5. Since mass transfer behavior may change dramatically when the melt is approaching saturation due to increased viscosity, only the data not exceeding 80 pct of solubility are used. It is shown that two variables are linearly related at all temperatures. The value of k is represented by the slope of the fitted line. Figure 10 shows the temperature dependence of apparent mass transfer coefficients of Si (IV) in the melt under Ar bubbling condition. In the case of dissolving CaSiO 3 , a sharp increase of k can be noticed as temperature rises from 1123 K (850°C) to higher, whereas temperature shows little influence when dissolving Ca 2 SiO 4 . The distinction in temperature dependence of mass transfer behavior is considered to be the different microstructures and the physical properties of the melts. Clarification of these issues needs more in-depth studies.

F. Characterization of Si (IV) Anions in the Melts
Some homogeneous melt samples saturated with CaSiO 3 or Ca 2 SiO 4 were water-rinsed to remove CaCl 2 . According to XRD characterization in Figure 11, the as-water-rinsed residues exhibited poor crystallinity. After heating at 1023 K (750°C) in Ar for 5 hours, crystalline CaSiO 3 or Ca 2 SiO 4 (b-type) was detected, indicating that the initial as-water-rinsed residues were amorphous CaSiO 3 and Ca 2 SiO 4 , respectively. Being amorphous is due to fast cooling of the melt samples after sampling. These results suggest that Si in the silicate-saturated melts may occur as SiO 3 2À or SiO 4 4À , the same as it does in the crystal lattice of solid CaSiO 3 or Ca 2 SiO 4 . Figure 12 shows the in-situ Raman spectra for pure molten CaCl 2 and the silicate-saturated melts at 850°C. According to Virgo et al., [29]  , and Cl À ) and diffusion of ions from interface to the bulk melt. These processes repeat until the melt becomes saturated with the silicate. Mass transfer of Si (IV) anions in the melt might be the rate-determining step. Difference in the structure between SiO 3 2À and SiO 4 4À in the melts explains the different dissolution behavior between solid CaSiO 3 and Ca 2 SiO 4 in molten CaCl 2 . As illustrated in Figure 13 SiO 3 2À has a polymeric chain-like structure formed by linking multiple basic units of [SiO 4 ] 4À tetrahedron in one direction with one bridging oxygen between two adjacent units. In comparison, SiO 4 4À occurs as an isolated unit without bridging oxygen sharing with others [ Figure 13(b)]. Therefore, SiO 4 4À anion with a smaller size has more freedom in moving and filling the spaces in molten CaCl 2 than SiO 3 2À , and thus Ca 2 SiO 4 dissolves more and faster than CaSiO 3 .

IV. CONCLUSION
In this contribution, dissolution behaviors of solid CaSiO 3 and Ca 2 SiO 4 in molten CaCl 2 in the temperature range from 1047 K to 1233 K (774°C to 960°C) were revealed. Major findings can be summarized as follows: