Separation of Zr and Si in Zirconium Silicate by Sodium Hydroxide Sub-Molten Salt

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Introduction
Zirconium is a type of metal with important practical use.It is widely used in ceramics, chemical and arms industries, electronics, and other fields due to its excellent optical, electrical, thermal and chemical properties [1].Zircon sand is the main source of zirconium compounds.Zircon sand is the main mineral source of zirconium.The main component of zircon sand is zirconium silicate (ZrSiO 4 ), which belongs to the tetragonal crystal system, has a melting point of up to 2550 • C, and has extremely high chemical stability.The decomposition of ZrSiO 4 can only be achieved under extremely harsh reaction conditions, such as thermal decomposition, chlorination, and sintering [2,3].
The most important step of these processes is the separation of Zr and Si in ZrSiO 4 .During the process of thermal decomposition, ZrSiO 4 is decomposed into ZrO 2 and SiO 2 by high temperature electric furnace.The higher decomposition temperature causes expensive production costs, which limits the use of the process [4,5].In the chlorination process, ZrSiO 4 is chlorinated to form ZrCl 4 .However, this method requires specific equipment [6,7].During sintering, ZrSiO 4 reacts to produce zirconium oxychloride at a high temperature with sintering agents which include CaCO 3 , Na 2 CO 3 , and NaOH, etc.Compared with NaOH, CaCO 3 and Na 2 CO 3 have the disadvantages of needing a high sintering temperature and difficulty in treating their products, resulting in a complex process.Therefore, NaOH sintering is widely used in the industry at present [2,[8][9][10][11][12][13][14].
In the sintering process, ZrSiO 4 reacts with molten sodium hydroxide and decomposes into sodium zirconate (Na 2 ZrO 3 ) and sodium silicate (Na 2 SiO 3 ), which can be easily treated following sintering at high temperatures, so this sintering step is the most important.
In the sintering process, ZrSiO4 reacts with molten sodium hydroxide and decomposes into sodium zirconate (Na2ZrO3) and sodium silicate (Na2SiO3), which can be easily treated following sintering at high temperatures, so this sintering step is the most important.Zirconium oxychloride (ZrOCl2•8H2O), an important zirconium basic chemical, is obtained by washing, transformation and crystallization of NaZrO3 [8,10].However, this process has some disadvantages.For example, molten sodium hydroxide reacts with ZrSiO4 in open containers, which can produce a large amount of alkali vapor that threatens the safety of operators.A large amount of liquid produced in the washing process can become a potential pollutant.To solve these problems, it is imperative to develop a new technology to decompose ZrSiO4.
Sub-molten salt medium can effectively improve the extraction efficiency of minerals, and has been widely studied and developed in recent years.It is a medium between molten salt and ordinary electrolytes.It has the advantages of high chemical activity, a high boiling point, excellent fluidity and high activity of oxygen anion [15][16][17][18].In our previous study, to reduce reaction temperature and pollution, we applied sub-molten salt to decompose ZrSiO4.Compared with the sintering process, the process involving sub-molten salt helps to better recycle NaOH and provides more effective environmental protection.The decomposition kinetics of ZrSiO4 in sodium hydroxide sub-molten salt were studied, and the decomposition flow chart of ZrSiO4 in NaOH sub-molten salt solution was explored, as shown in Figure 1 [19].The product obtained owing to the decomposition of zircon sand is sodium zirconium silicate.Zirconium oxychloride is obtained by acidolysis, flocculation, filtration, and crystallization.However, in the process involving sub-molten salt, sodium hydroxide will be lost, production cost will be increased, and zirconium will be lost in the flocculation process, thus reducing the total yield of the process.When the decomposition product is sodium zirconate, the reaction product can be washed to remove sodium and silicon, omitting the acidolysis and flocculation process, shortening the process flow and increasing the total yield of the reaction.This study aims to investigate the phase change law of ZrSiO4 decomposition products in sodium hydroxide molten salt, to achieve effective separation of Zr and Si, and to explore the reaction process of ZrSiO4 decomposition.The effects of reaction temperature, reaction time, NaOH concentration, stirring speed, and alkali/mineral ratio on the phase This study aims to investigate the phase change law of ZrSiO 4 decomposition products in sodium hydroxide molten salt, to achieve effective separation of Zr and Si, and to explore the reaction process of ZrSiO 4 decomposition.The effects of reaction temperature, reaction time, NaOH concentration, stirring speed, and alkali/mineral ratio on the phase of the product were studied using the results of Raman, infrared, and X-ray crystal diffraction analyses.The variation rule of the phase of the product was obtained, which is beneficial to simplify the process of decomposition of zircon sand by NaOH sub-molten salt, and has important theoretical and practical significance for the optimization of a new clean production process of zirconium oxychloride.The reaction process was investigated, enriching the theoretical research of sub-molten salt and providing theoretical guidance for practical production.

Materials
The NaOH employed was of analytical grade and was provided by the Beijing Chemical Plant, China.The water used in the experiment was deionized water, obtained by the water super purification system (Milli-Q, Millipore, Burlington, MA, USA).The main element composition of zircon sand provided by Kingan Hi-tech.Co., Ltd., Nanchang, Jiangxi Province, China was obtained by X-ray fluorescence (XRF) (AXIOS, Arlington, VA, USA).The results are listed in Table 1.The particle size of zircon sand was 22.56 µm (d 50 ), which was measured by laser particle size analyzer (Mastersizer 2000, Malvern, UK), as shown in Figure 2. The zircon sand was characterized by X-ray diffraction (XRD, Philips 1140, Amsterdam, The Netherlands, Cu Kα radiation, 40 mA, and 30 kV) and its crystal structure was determined.The results are shown in Figure 3, and the main crystal phase is ZrSiO 4 .The scanning electron microscope (SEM, JSM-7610F, Tokyo, Japan) image of the particle is shown in Figure 4.It can be seen from Figure 4 that zircon sand is formed of blocky particles.
of the product were studied using the results of Raman, infrared, and X-ray crystal diffraction analyses.The variation rule of the phase of the product was obtained, which is beneficial to simplify the process of decomposition of zircon sand by NaOH sub-molten salt, and has important theoretical and practical significance for the optimization of a new clean production process of zirconium oxychloride.The reaction process was investigated, enriching the theoretical research of sub-molten salt and providing theoretical guidance for practical production.

Materials
The NaOH employed was of analytical grade and was provided by the Beijing Chemical Plant, China.The water used in the experiment was deionized water, obtained by the water super purification system (Milli-Q, Millipore, Burlington, MA, USA).The main element composition of zircon sand provided by Kingan Hi-tech.Co., Ltd., Nanchang, Jiangxi Province, China was obtained by X-ray fluorescence (XRF) (AXIOS, Arlington, VA, USA).The results are listed in Table 1.The particle size of zircon sand was 22.56 μm (d50), which was measured by laser particle size analyzer (Mastersizer 2000, Malvern, UK), as shown in Figure 2. The zircon sand was characterized by X-ray diffraction (XRD, Philips 1140, Amsterdam, The Netherlands, Cu Kα radiation, 40 mA, and 30 kV) and its crystal structure was determined.The results are shown in Figure 3, and the main crystal phase is ZrSiO4.The scanning electron microscope (SEM, JSM-7610F, Tokyo, Japan) image of the particle is shown in Figure 4.It can be seen from Figure 4 that zircon sand is formed of blocky particles.

Experimental Device and Process
All experiments were completed in an autoclave, and the structure of the autoclave is shown in Figure 5 [19].The autoclave was composed of a temperature control system, electric heating system, magnetic stirring system and cooling system.The volume of the inner tank was 1 L. The material of the inner tank in the autoclave was stainless steel lined with nickel.The temperature control system can adjust the heating power to maintain the reaction temperature constant with a precision of ±1 °C.

Experimental Device and Process
All experiments were completed in an autoclave, and the structure of the autoclave is shown in Figure 5 [19].The autoclave was composed of a temperature control system, electric heating system, magnetic stirring system and cooling system.The volume of the inner tank was 1 L. The material of the inner tank in the autoclave was stainless steel lined with nickel.The temperature control system can adjust the heating power to maintain the reaction temperature constant with a precision of ±1 • C. Sodium hydroxide and deionized water were added to the reactor to form a certain concentration of sub-molten salt, and zircon sand was added according to NaOH/ore ratio.Then, the autoclave was sealed and heated.When the temperature reached the set temperature, the timing started.After the reaction continued for a period of time, the autoclave with the cooling system was used to cool down.When the temperature in the autoclave was reduced to 90 °C, there was no pressure in the autoclave, and the reaction Sodium hydroxide and deionized water were added to the reactor to form a certain concentration of sub-molten salt, and zircon sand was added according to NaOH/ore ratio.Then, the autoclave was sealed and heated.When the temperature reached the set temperature, the timing started.After the reaction continued for a period of time, the autoclave with the cooling system was used to cool down.When the temperature in the autoclave was reduced to 90 • C, there was no pressure in the autoclave, and the reaction liquid was taken out.After hot filtration, the filter cake was washed five times with deionized water and dried at 105 • C for 12 h.By adding sodium hydroxide to increase the concentration of the filtrate to decompose the zircon sand, the recycling of sodium hydroxide was realized.The products in the reaction process were characterized by Fourier transform infrared (FTIR, Perkin-Elmer Spectrum One, Waltham, MA, USA, sample preparation using potassium bromide) spectroscopy, XRD, Raman spectroscopy (inVia, using a He Ne laser (532 nm)) and X-ray photoelectron spectroscopy (Thermofisher ESCALAB 250Xi, Waltham, MA, USA, Al Kα (hν = 1486.6eV) as the X-ray source).
The following method was used to obtain the conversion rate of zircon sand.First, the reaction product was washed with hot water to fully remove the soluble substance.Then, the washed solid was added to 3:1 hydrochloric acid (deionized water diluted four times with concentrated hydrochloric acid) (solid-liquid ratio 5:1), stirred at 50 • C for 4 h, and then filtered and washed.The obtained filter cake was unreacted zircon sand.The filter cake was dried and weighed to calculate the conversion rate of zircon sand.All the data in the study were the average of three parallel experiments.The conversion rate of zircon sand was calculated using the following equation: where m 1 is the mass of the residue (unreacted zircon sand), and m 2 is the total mass of the reacted zircon sand.

Effect of Initial NaOH Concentration
Sodium hydroxide not only acts as a reactant but also affects the properties of the sub-molten salt medium.Therefore, the concentration of initial NaOH is crucial to the decomposition of zircon sand.In order to obtain the influence of initial alkali concentration on the decomposition of zircon sand, the initial alkali concentrations of 50%, 60%, 70%, 75% and 80% by mass fraction were selected as the research object, and the influence of initial sodium hydroxide concentration on the reaction product and conversion rate was studied.In the experiments, the conditions of reaction temperature 245 • C, agitation speed 400 r/min, alkali-ore ratio 3:1 and reaction time 8 h were kept constant.
Figure 6 summarizes the effect of the initial NaOH concentration on the decomposition products of ZrSiO 4 .Compared with the XRD spectra of the reaction products, when the concentration of NaOH is ≤70%, the reaction products are all Na 2 ZrSiO 5 and unreacted ZrSiO 4 .As the concentration increases, Na 2 ZrO 3 appears in the product at 75% concentration; however, Na 2 ZrSiO 5 remains, the concentration increases to 80%, and almost all of the products are Na 2 ZrO 3 and Na 2 SiO 3 .According to the results shown in Figure 7, the products are extremely sensitive to NaOH concentration.When other conditions are determined, Na 2 ZrO 3 will appear in the reaction products only when the concentration is higher than a specific value.Therefore, considering the applicability, the concentration of NaOH is 80% when other factors are studied later.Figure 6 shows the relationship between concentration and conversion, indicating that the increase of concentration does not increase the conversion evidently.This is because the viscosity of the system increases with concentration, the mass transfer rate slows down, and the reaction rate decreases, which partly offsets the increase of the rate caused by the increase in concentration.
NaOH is 80% when other factors are studied later.Figure 6 shows the relationship between concentration and conversion, indicating that the increase of concentration does not increase the conversion evidently.This is because the viscosity of the system increases with concentration, the mass transfer rate slows down, and the reaction rate decreases, which partly offsets the increase of the rate caused by the increase in concentration.

Effect of Reaction Temperature
The effect of the reaction temperature on the decomposition rate of ZrSiO4 was investigated at 230 °C, 245 °C, 260 °C, and 275 °C, NaOH/ore mass ratio of 3:1, initial NaOH concentration of 80%, agitation speed of 400 r/min, and reaction time of 4 h, as shown in Figures 8 and 9.The results demonstrate that the conversion of ZrSiO4 increases with temperature.The reaction products are Na2ZrO3, Na2SiO3, and unreacted ZrSiO4 at 245-260 °C.When the temperature increases to 275 °C, most of the ZrSiO4 is decomposed; however, Na2ZrSiO5 appears in the product.Therefore, to avoid the formation of Na2ZrSiO5, the reaction temperature should not be extremely high.In the following investigation, the temperature of 260 °C was selected when the effect of reaction time on the reaction products was investigated.

Effect of Reaction Temperature
The effect of the reaction temperature on the decomposition rate of ZrSiO 4 was investigated at 230 • C, 245 • C, 260 • C, and 275 • C, NaOH/ore mass ratio of 3:1, initial NaOH concentration of 80%, agitation speed of 400 r/min, and reaction time of 4 h, as shown in Figures 8 and 9.The results demonstrate that the conversion of ZrSiO 4 increases with temperature.The reaction products are Na 2 ZrO 3 , Na 2 SiO 3 , and unreacted ZrSiO 4 at 245-260 • C. When the temperature increases to 275 • C, most of the ZrSiO 4 is decomposed; however, Na 2 ZrSiO 5 appears in the product.Therefore, to avoid the formation of Na 2 ZrSiO 5 , the reaction temperature should not be extremely high.In the following investigation, the temperature of 260 • C was selected when the effect of reaction time on the reaction products was investigated.
°C.When the temperature increases to 275 °C, most of the ZrSiO4 is decomposed; however, Na2ZrSiO5 appears in the product.Therefore, to avoid the formation of Na2ZrSiO5, the reaction temperature should not be extremely high.In the following investigation, the temperature of 260 °C was selected when the effect of reaction time on the reaction products was investigated.

Effect of Reaction Time
The effects of reaction time on the reaction products and conversion rate of ZrSiO4 were investigated at the conditions of reaction temperature 260 °C, agitation speed of 400 r/min, NaOH/ore mass ratio of 3:1, and initial NaOH concentration of 80%.The experimental results after reaction times of 2, 4, 5, and 8 h were obtained, as shown in Figures 10 and 11.

Effect of Reaction Time
The effects of reaction time on the reaction products and conversion rate of ZrSiO 4 were investigated at the conditions of reaction temperature 260 • C, agitation speed of 400 r/min, NaOH/ore mass ratio of 3:1, and initial NaOH concentration of 80%.The experimental results after reaction times of 2, 4, 5, and 8 h were obtained, as shown in Figures 10 and 11.
Figure 10 shows that the decomposition rate of ZrSiO 4 increases gradually with the reaction time.At the initial stage of the reaction, the conversion of ZrSiO 4 was low, and only Na 2 ZrO 3 and Na 2 SiO 3 existed in the product.Na 2 ZrSiO 5 gradually appeared in the product as the conversion increased.The concentration of Na 2 ZrO 3 and Na 2 SiO 3 increased with the decomposition rate.Simultaneously, the content of H 2 O also increases.Therefore, an important relationship between the reaction product and the content of Na 2 ZrO 3 , Na 2 SiO 3 , and H 2 O in the system is speculated to exist.
The effects of reaction time on the reaction products and conversion rate of ZrSiO4 were investigated at the conditions of reaction temperature 260 °C, agitation speed of 400 r/min, NaOH/ore mass ratio of 3:1, and initial NaOH concentration of 80%.The experimental results after reaction times of 2, 4, 5, and 8 h were obtained, as shown in Figures 10 and 11    Figure 10 shows that the decomposition rate of ZrSiO4 increases gradually with the reaction time.At the initial stage of the reaction, the conversion of ZrSiO4 was low, and only Na2ZrO3 and Na2SiO3 existed in the product.Na2ZrSiO5 gradually appeared in the product as the conversion increased.The concentration of Na2ZrO3 and Na2SiO3 increased with the decomposition rate.Simultaneously, the content of H2O also increases.Therefore, an important relationship between the reaction product and the content of Na2ZrO3, Na2SiO3, and H2O in the system is speculated to exist.

Effect of NaOH/Ore Mass Ratio
Section 3.3 demonstrates that the content of Na2ZrO3 and Na2SiO3 has an important influence on the reaction products, and the variation in the NaOH/ore mass ratio also affects the content of Na2ZrO3 and Na2SiO3 in the system.Therefore, the effects of NaOH/ore mass ratio on the decomposition products and conversion rate of ZrSiO4 were studied under the conditions of initial NaOH concentration of 80%, reaction temperature of 275 °C, agitation speed of 400 r/min and reaction time of 4 h.In Figure 12, the XRD spectra of

Effect of NaOH/Ore Mass Ratio
Section 3.3 demonstrates that the content of Na 2 ZrO 3 and Na 2 SiO 3 has an important influence on the reaction products, and the variation in the NaOH/ore mass ratio also affects the content of Na 2 ZrO 3 and Na 2 SiO 3 in the system.Therefore, the effects of NaOH/ore mass ratio on the decomposition products and conversion rate of ZrSiO 4 were studied under the conditions of initial NaOH concentration of 80%, reaction temperature of 275 • C, agitation speed of 400 r/min and reaction time of 4 h.In Figure 12, the XRD spectra of reaction products with NaOH/ore mass ratio of 3:1, 3.2:1, 3.5:1, 4:1, and 4.2:1 were obtained.The graph shows that the diffraction peak of Na 2 ZrSiO 5 decreases gradually with the increase of alkali/ore ratio.When the NaOH/ore ratio increases to 4.2:1, there is almost no Na 2 ZrSiO 5 in the product.According to the graph of conversion (Figure 13), ZrSiO 4 with a different NaOH/ore mass ratio is almost completely decomposed.

Optimum Technological Condition
According to the analysis of the factors affecting the products obtained during the decomposition of ZrSiO4 in NaOH sub-molten salt, the influence of each factor will be mutually restricted; thus, a series of experiments was conducted, and the reaction products that influence each factor are listed in Table 2.The table indicates that only Na2ZrSiO5 exists in the reaction product when the alkali concentration is 70%, and only when the concentration reaches 80% can Na2ZrO3 be formed in the product.Further, the optimum technological conditions were obtained: alkali concentration 80%, temperature 245 °C, alkali/ore ratio 4:1, reaction time 10 h, and stirring rate 400 r/min.

Optimum Technological Condition
According to the analysis of the factors affecting the products obtained during the decomposition of ZrSiO4 in NaOH sub-molten salt, the influence of each factor will be mutually restricted; thus, a series of experiments was conducted, and the reaction products that influence each factor are listed in Table 2.The table indicates that only Na2ZrSiO5 exists in the reaction product when the alkali concentration is 70%, and only when the concentration reaches 80% can Na2ZrO3 be formed in the product.Further, the optimum technological conditions were obtained: alkali concentration 80%, temperature 245 °C, alkali/ore ratio 4:1, reaction time 10 h, and stirring rate 400 r/min.

Optimum Technological Condition
According to the analysis of the factors affecting the products obtained during the decomposition of ZrSiO 4 in NaOH sub-molten salt, the influence of each factor will be mutually restricted; thus, a series of experiments was conducted, and the reaction products that influence each factor are listed in Table 2.The table indicates that only Na 2 ZrSiO 5 exists in the reaction product when the alkali concentration is 70%, and only when the concentration reaches 80% can Na 2 ZrO 3 be formed in the product.Further, the optimum technological conditions were obtained: alkali concentration 80%, temperature 245 • C, alkali/ore ratio 4:1, reaction time 10 h, and stirring rate 400 r/min.
The SEM image and XRD patterns of the product on the optimum technological conditions are shown in Figure 14.It can be seen from the figure that almost all ZrSiO 4 is decomposed, and the products are Na 2 ZrO 3 and Na 2 SiO 3 .Na 2 ZrO 3 consists of rod-shaped particles.Note: "ZS" refers to ZrSiO4, "NZ" refers to Na2ZrO3, "NZS" refers to Na2ZrSiO5.
The SEM image and XRD patterns of the product on the optimum technological conditions are shown in Figure 14.It can be seen from the figure that almost all ZrSiO4 is decomposed, and the products are Na2ZrO3 and Na2SiO3.Na2ZrO3 consists of rod-shaped particles.

Speculation of Reaction Pathway
The main reactions that take place between ZrSiO4 and NaOH are as follows:

Mechanism Analysis 3.6.1. Speculation of Reaction Pathway
The main reactions that take place between ZrSiO 4 and NaOH are as follows: (2) According to the previous discussion, under the experimental conditions for the reaction of ZrSiO 4 in NaOH sub-molten salt, at the beginning and end of the reaction an excess of NaOH is present.The XRD results of the reaction products demonstrate that the reaction products are Na 2 ZrO 3 , Na 2 SiO 3 , and Na 2 ZrSiO 5 .Therefore, it is speculated that the decomposition reaction of ZrSiO 4 in a sub-molten salt system may involve Equations ( 2) and (3).Na 2 ZrO 3 and Na 2 SiO 3 are the products in the early stage of the reaction.With the reaction proceeding, Na 2 ZrSiO 5 appears in the products.Therefore, it is presumed that Equation (3) represents the first reaction of ZrSiO 4 in NaOH sub-molten salt, Na 2 ZrO 3 and Na 2 SiO 3 are produced, and then Na 2 ZrSiO 5 is formed, that is, Equation ( 6), which is different from the alkali melting process.

Verification of Reaction Pathway
To verify our hypothesis, it is assumed that ZrSiO 4 is first decomposed into Na 2 ZrO 3 and Na 2 SiO 3 , that is, Equation (3) occurs first.Material proportions were simulated under the conditions of 70% alkali concentration, 4:1 NaOH/ore mass ratio, 260 • C reaction temperature, and 400 r/min stirring rate, as well as 80% alkali concentration, 4:1 NaOH/ore mass ratio, 260 • C reaction temperature, and 400 r/min stirring rate.It was then added to the reactor and continued to react for 4 h to obtain the product as shown in Figure 15.
The figure shows that all the products under the condition of 70% alkali concentration are Na 2 ZrSiO 5 , while most of the products under the condition of 80% alkali concentration are still Na 2 ZrO 3 and Na 2 SiO 3 ; only a very small amount of Na 2 ZrSiO 5 is produced.
According to the previous discussion, under the experimental conditions for the reaction of ZrSiO4 in NaOH sub-molten salt, at the beginning and end of the reaction an excess of NaOH is present.The XRD results of the reaction products demonstrate that the reaction products are Na2ZrO3, Na2SiO3, and Na2ZrSiO5.Therefore, it is speculated that the decomposition reaction of ZrSiO4 in a sub-molten salt system may involve Equations ( 2) and ( 3).Na2ZrO3 and Na2SiO3 are the products in the early stage of the reaction.With the reaction proceeding, Na2ZrSiO5 appears in the products.Therefore, it is presumed that Equation (3) represents the first reaction of ZrSiO4 in NaOH sub-molten salt, Na2ZrO3 and Na2SiO3 are produced, and then Na2ZrSiO5 is formed, that is, Equation ( 6), which is different from the alkali melting process.

Verification of Reaction Pathway
To verify our hypothesis, it is assumed that ZrSiO4 is first decomposed into Na2ZrO3 and Na2SiO3, that is, Equation (3) occurs first.Material proportions were simulated under the conditions of 70% alkali concentration, 4:1 NaOH/ore mass ratio, 260 °C reaction temperature, and 400 r/min stirring rate, as well as 80% alkali concentration, 4:1 NaOH/ore mass ratio, 260 °C reaction temperature, and 400 r/min stirring rate.It was then added to the reactor and continued to react for 4 h to obtain the product as shown in Figure 15.The figure shows that all the products under the condition of 70% alkali concentration are Na2ZrSiO5, while most of the products under the condition of 80% alkali concentration are still Na2ZrO3 and Na2SiO3; only a very small amount of Na2ZrSiO5 is produced.
ZrSiO4 is assumed to decompose into Na2ZrSiO5 at first, that is, Equation ( 2) occurs first.The final material ratio under the previous two experimental conditions is also simulated.The product obtained after adding NaOH to the reactor is shown in Figure 16.The figure also shows that the products under both conditions are still Na2ZrSiO5, indicating that sodium zirconium silicate does not react in sodium hydroxide sub-molten salt, which is different from the alkali fusion sintering method.ZrSiO 4 is assumed to decompose into Na 2 ZrSiO 5 at first, that is, Equation (2) occurs first.The final material ratio under the previous two experimental conditions is also simulated.The product obtained after adding NaOH to the reactor is shown in Figure 16.The figure also shows that the products under both conditions are still Na 2 ZrSiO 5 , indicating that sodium zirconium silicate does not react in sodium hydroxide sub-molten salt, which is different from the alkali fusion sintering method.
The discussion indicates that sodium zirconate and sodium silicate can continue to react to produce sodium zirconate in sodium hydroxide molten salt, while sodium silicate zirconate cannot react with sodium hydroxide.The reaction is rapid at low concentrations and slows down as the concentration increases.Thus, the decomposition process of ZrSiO 4 in NaOH sub-molten salt is first decomposed into Na 2 ZrO 3 and Na 2 SiO 3 , then the two react to produce Na 2 ZrSiO 5 , that is, first Equation (3) and then Equation (6).The discussion indicates that sodium zirconate and sodium silicate can continue to react to produce sodium zirconate in sodium hydroxide molten salt, while sodium silicate zirconate cannot react with sodium hydroxide.The reaction is rapid at low concentrations and slows down as the concentration increases.Thus, the decomposition process of ZrSiO4 in NaOH sub-molten salt is first decomposed into Na2ZrO3 and Na2SiO3, then the two react to produce Na2ZrSiO5, that is, first Equation (3) and then Equation ( 6).

Infrared Spectroscopy
Figure 17 shows the variations in the infrared spectra of products in sub-molten salts (reaction temperature of 260 °C, NaOH content of 70%, agitation speed of 400 r/min, and NaOH/ore mass ratio of 3:1) at different reaction times.The spectra of A show three characteristic fundamental IR bands of ZrSiO4 at approximately 445, 610, and 890 cm −1 attributed, respectively, to O-Si-O bending vibration, antisymmetric stretching vibration of SiO4 tetrahedron, and antisymmetric stretching vibration of SiO4 tetrahedron [20,21].The transmission intensity of all the three bands decreases with increasing reaction time, while new vibration peaks appeared at approximately 503, 879, 1043, and 1435 cm −1 .The absorption peaks at approximately 503 and 879 cm −1 are attributed to the vibration of Zr-O, and the absorption peaks at approximately 1043 and 1435 cm −1 are attributed to the vibration of Si-O-Si in sodium silicate [21][22][23].When the reaction time increased to 8 h, the absorption peak at 505 cm −1 disappeared, and at 570, 732, and 930 cm −1 , a new absorption peak representing the stretching vibration peak of Si-Onb appeared, which indicated the formation of isolated SiO4 tetrahedra [23].

Infrared Spectroscopy
Figure 17 shows the variations in the infrared spectra of products in sub-molten salts (reaction temperature of 260 • C, NaOH content of 70%, agitation speed of 400 r/min, and NaOH/ore mass ratio of 3:1) at different reaction times.The spectra of A show three characteristic fundamental IR bands of ZrSiO 4 at approximately 445, 610, and 890 cm −1 attributed, respectively, to O-Si-O bending vibration, antisymmetric stretching vibration of SiO 4 tetrahedron, and antisymmetric stretching vibration of SiO 4 tetrahedron [20,21].The transmission intensity of all the three bands decreases with increasing reaction time, while new vibration peaks appeared at approximately 503, 879, 1043, and 1435 cm −1 .The absorption peaks at approximately 503 and 879 cm −1 are attributed to the vibration of Zr-O, and the absorption peaks at approximately 1043 and 1435 cm −1 are attributed to the vibration of Si-O-Si in sodium silicate [21][22][23].When the reaction time increased to 8 h, the absorption peak at 505 cm −1 disappeared, and at 570, 732, and 930 cm −1 , a new absorption peak representing the stretching vibration peak of Si-O nb appeared, which indicated the formation of isolated SiO 4 tetrahedra [23].The Raman spectra of the products at different reaction times (reaction temperature of 260 °C, NaOH content of 70%, agitation speed of 400 r/min, and NaOH/ore mass ratio of 3:1) are shown in Figure 18.In the lower wavenumber region, the peaks at 358, 438, and

Raman Spectroscopy
The Raman spectra of the products at different reaction times (reaction temperature of 260 • C, NaOH content of 70%, agitation speed of 400 r/min, and NaOH/ore mass ratio of 3:1) are shown in Figure 18.In the lower wavenumber region, the peaks at 358, 438, and 1009 cm −1 (A 1g , ν 2 , and B 1g , symmetric bending) corresponding to Q 0 in ZrSiO 4 weaken as the reaction time increases [24][25][26][27].At the initial stage of the reaction, Raman peaks at 305 and 630 cm −1 assigned to ZrO 2 are observed, which proves that ZrO 2 was separated from ZrSiO 4 lattice in the decomposition process of ZrSiO 4 in NaOH sub-molten salt [24,26,27].With time, these peaks gradually disappear.When the reaction lasts for 3 h, a new vibration peak appears at 1050 cm −1 , which is the characteristic vibration peak of Si-O-Si.When 1050 characteristic peaks disappear, new vibration peaks at 865 and 930 cm −1 represent the formation of new Q 0 , which is the same as the result of infrared spectrum analysis [24].

Raman Spectroscopy
The Raman spectra of the products at different reaction times (reaction temperature of 260 °C, NaOH content of 70%, agitation speed of 400 r/min, and NaOH/ore mass ratio of 3:1) are shown in Figure 18.In the lower wavenumber region, the peaks at 358, 438, and 1009 cm −1 (A1g, ν2, and B1g, symmetric bending) corresponding to Q0 in ZrSiO4 weaken as the reaction time increases [24][25][26][27].At the initial stage of the reaction, Raman peaks at 305 and 630 cm −1 assigned to ZrO2 are observed, which proves that ZrO2 was separated from ZrSiO4 lattice in the decomposition process of ZrSiO4 in NaOH sub-molten salt [24,26,27].With time, these peaks gradually disappear.When the reaction lasts for 3 h, a new vibration peak appears at 1050 cm −1 , which is the characteristic vibration peak of Si-O-Si.When 1050 characteristic peaks disappear, new vibration peaks at 865 and 930 cm −1 represent the formation of new Q0, which is the same as the result of infrared spectrum analysis [24].

X-ray Photoelectron Spectroscopy
A chemical analysis of the product from ZrSiO 4 in NaOH sub-molten salt (reaction temperature of 260 • C, NaOH content of 80%, agitation speed of 400 r/min, NaOH/ore mass ratio of 3:1, and 1 h reaction time) was conducted using XPS, which is known to be a highly surface-sensitive technique.Figure 19 shows that the O 1s spectrum from ZrSiO 4 after its reaction in NaOH sub-molten salt leads to the attribution of four distinct chemical species of O atoms, by resolving the O 1s spectra into four spin-orbit pairs with BEs of 530.5, 532.5, 534.5, and 538.8 eV, respectively.Among these four O 1s BE components, the 530.51 eV can be assigned to the O 1s BE of O present in the ZrO 2 ; 532.50 eV, O 1s BE of O in the Si-O-Si; 534.5 eV, O 1s BE of O in the Na 2 ZrO 3 ; and 538.8 eV, O 1s BE of O in the Na 2 CO 3 , which is formed when sodium hydroxide absorbs carbon dioxide from the air [28][29][30].
In addition to the O 1s spectrum, the sample was also scanned for a Zr 3d signal.The BE of Zr 3d in Figure 20 can be fitted to two pairs of peaks of A1, A2 and B1, B2.The BEs of A1 and A2 are 188.5 eV and 185.0 eV, respectively, which correspond to the BE of Zr 3d in ZrO 2 [28,31].Further, ZrO 2 appears on the surface of ZrSiO 4 particles, which is consistent with the results of infrared and Raman analyses.In addition to the O 1s spectrum, the sample was also scanned for a Zr 3d signal.The BE of Zr 3d in Figure 20 can be fitted to two pairs of peaks of A1, A2 and B1, B2.The BEs of A1 and A2 are 188.5 eV and 185.0 eV, respectively, which correspond to the BE of Zr 3d in ZrO2 [28,31].Further, ZrO2 appears on the surface of ZrSiO4 particles, which is consistent with the results of infrared and Raman analyses.The BEs of B1 and B2 are 183.8eV and 187.6 eV, respectively, corresponding to the BE of Zr 3d in Na2ZrO3.The analysis indicates that the decomposition reaction of ZrSiO4 in NaOH sub-molten salt occurs as the zircon-oxygen polyhedron is separated from the silicate lattice under the action of sub-molten salt, which separates zirconium and silicon.Previous studies have demonstrated that there are several stable O 2− , which is Lewis alkaline, from the decomposition of OH -in the sub-molten salt system compared with conventional media.Therefore, when decomposing zircon sand in sub-molten salt, the highly active O 2− tends to attack the Zr atom in zircon sand, replacing the O in the crystal structure, promoting the formation of new Zr-O bonds, thus breaking the ring crystal structure.The zirconiumoxygen polyhedron is separated from silicate lattice, resulting in the separation of zirconium and silicon.The silicon-oxygen polyhedron and zirconium-oxygen polyhedron form chain-shaped sodium silicate and sodium zirconium, respectively, under the action of Na + .When the water content in the system increases, that is, the alkali concentration decreases, the Na2SiO3 produced will ionize under the action of water molecules to produce SiO3 2− .SiO3 2− will attack the Zr atom in Na2ZrO3 and enter the lattice of Na2ZrO3 to form a new island-like silicate (Na2ZrSiO5).With the increase of alkalinity, the solubility of Na2SiO3 decreases, SiO3 2− in the system decreases, and rate of Na2ZrSiO5 formation slows down, which is the same as the previous analysis results.Therefore, the reaction mechanism of NaOH sub-molten salt decomposition of ZrSiO4 is to decompose Na2ZrO3 and Na2SiO3 sodium under the action of O 2-and then combine them to form Na2ZrSiO5 The analysis indicates that the decomposition reaction of ZrSiO 4 in NaOH sub-molten salt occurs as the zircon-oxygen polyhedron is separated from the silicate lattice under the action of sub-molten salt, which separates zirconium and silicon.Previous studies have demonstrated that there are several stable O 2− , which is Lewis alkaline, from the decomposition of OH − in the sub-molten salt system compared with conventional media.Therefore, when decomposing zircon sand in sub-molten salt, the highly active O 2− tends to attack the Zr atom in zircon sand, replacing the O in the crystal structure, promoting the formation of new Zr-O bonds, thus breaking the ring crystal structure.The zirconiumoxygen polyhedron is separated from silicate lattice, resulting in the separation of zirconium and silicon.The silicon-oxygen polyhedron and zirconium-oxygen polyhedron form chainshaped sodium silicate and sodium zirconium, respectively, under the action of Na + .When the water content in the system increases, that is, the alkali concentration decreases, the Na 2 SiO 3 produced will ionize under the action of water molecules to produce SiO

Conclusions
The reaction parameters were investigated to control the products formed during the decomposition of ZrSiO4 in the NaOH sub-molten salt.The main conclusions are as follows: 1.The effects of alkali concentration, temperature, alkali/ore ratio, and reaction time on the product were explored.The product was extremely dependent on NaOH concentration.It was necessary to coordinate the relationship among NaOH concentration, temperature, alkali/ore ratio, and reaction time to strictly control the formation of Na2ZrSiO5.2. The optimum technological conditions were obtained: alkali concentration 80%, reaction temperature 245 °C, alkali/ore ratio 4:1, reaction time 10 h, stirring rate 400 r/min.Under these conditions, the product obtained was ensured to be Na2ZrO3, achieving effective separation of Zr and Si; further, a high decomposition rate of

Conclusions
The reaction parameters were investigated to control the products formed during the decomposition of ZrSiO4 in the NaOH sub-molten salt.The main conclusions are as follows: 1.The effects of alkali concentration, temperature, alkali/ore ratio, and reaction time on the product were explored.The product was extremely dependent on NaOH concentration.It was necessary to coordinate the relationship among NaOH concentration, temperature, alkali/ore ratio, and reaction time to strictly control the formation of Na2ZrSiO5.2. The optimum technological conditions were obtained: alkali concentration 80%, reaction temperature 245 °C, alkali/ore ratio 4:1, reaction time 10 h, stirring rate 400 r/min.Under these conditions, the product obtained was ensured to be Na2ZrO3, achieving effective separation of Zr and Si; further, a high decomposition rate of ZrSiO4 can be ensured.

Conclusions
The reaction parameters were investigated to control the products formed during the decomposition of ZrSiO 4 in the NaOH sub-molten salt.The main conclusions are as follows: 1.
The effects of alkali concentration, temperature, alkali/ore ratio, and reaction time on the product were explored.The product was extremely dependent on NaOH concentration.It was necessary to coordinate the relationship among NaOH concentration, temperature, alkali/ore ratio, and reaction time to strictly control the formation of Na 2 ZrSiO 5 .

2.
The optimum technological conditions were obtained: alkali concentration 80%, reaction temperature 245 • C, alkali/ore ratio 4:1, reaction time 10 h, stirring rate 400 r/min.these conditions, the product obtained was ensured to be Na 2 ZrO 3 , achieving effective separation of Zr and Si; further, a high decomposition rate of ZrSiO 4 can be ensured.

3.
The reaction mechanism was investigated and verified.The reaction process of ZrSiO 4 in sodium hydroxide sub-molten salt was obtained by XRD spectroscopy infrared spectroscopy, Raman spectroscopy, and XPS analysis.It is elucidated that ZiSiO 4 is decomposed to Na 2 ZrO 3 and Na 2 SiO 3 by reacting with NaOH, and then the reactions between Na 2 ZrO 3 and Na 2 SiO 3 result in the formation of Na 2 ZrSiO 5 .

Figure 3 .
Figure 3. XRD pattern of the zircon sand.

Figure 4 .
Figure 4. SEM image of the zircon sand.

Figure 4 .
Figure 4. SEM image of the zircon sand.

Figure 6 .
Figure 6.XRD pattern of products at different NaOH concentrations.

4 Figure 7 .
Figure 7. Effect of initial NaOH concentration on the conversion rate of ZrSiO 4 .

Figure 8 .
Figure 8. XRD pattern of products at different temperatures.

Figure 9 .
Figure 9.Effect of reaction temperature on the conversion rate of ZrSiO 4 . .

Figure 10 .
Figure 10.XRD pattern of products at different reaction times.

Figure 10 .Figure 11 .
Figure 10.XRD pattern of products at different reaction times.

Figure 11 .
Figure 11.Effect of reaction time on the conversion rate of ZrSiO 4 .

Figure 12 .
Figure 12.XRD pattern of products at different NaOH/ore ratio.

Figure 13 .
Figure 13.Effect of NaOH-ore ratio on the conversion rate of ZrSiO4.

Figure 13 .
Figure 13.Effect of NaOH-ore ratio on the conversion rate of ZrSiO 4 .

Figure 14 .
Figure 14.The SEM image and the X-ray diffraction pattern for the product.

Figure 15 .
Figure 15.XRD pattern of products at different conditions.

Figure 15 .
Figure 15.XRD pattern of products at different conditions.

Figure 16 .
Figure 16.XRD pattern of products at different conditions.

Figure 17 .
Figure 17.Infrared spectra of ZrSiO 4 and the products in the NaOH sub-molten salt with different reaction time.A: ZrSiO 4 , B: 1 h, C: 2 h, and D: 8 h.

Figure 17 .
Figure 17.Infrared spectra of ZrSiO4 and the products in the NaOH sub-molten salt with different reaction time.A: ZrSiO4, B: 1 h, C: 2 h, and D: 8 h.

Figure 18 .
Figure 18.Raman spectra of ZrSiO 4 and the products in the NaOH sub-molten salt with different reaction times.A: ZrSiO 4 B: 1 h C: 3 h D: 8 h.

Figure 19 .Figure 20 .
Figure 19.O 1s HRXPS spectra of the product obtained from ZrSiO 4 in the NaOH sub-molten salt with the reaction time of 1 h.Metals 2024, 14, x FOR PEER REVIEW 16 of 19

1 Figure 20 .
Figure 20.Zr 3d HRXPS spectra of the product obtained from ZrSiO 4 in the NaOH sub-molten salt with the reaction time of 1 h.

19 Figure 21 .
Figure 21.Schematic pathway of the reaction process to form Na2ZrO3 from ZrSiO4 in NaOH submolten salt.

Figure 22 .
Figure 22.Schematic pathway of the reaction process to form Na2ZrSiO5 from Na2ZrO3 in NaOH sub-molten salt.

Figure 21 . 19 Figure 21 .
Figure 21.Schematic pathway of the reaction process to form Na 2 ZrO 3 from ZrSiO 4 in NaOH sub-molten salt.

Figure 22 .
Figure 22.Schematic pathway of the reaction process to form Na2ZrSiO5 from Na2ZrO3 in NaOH sub-molten salt.

Figure 22 .
Figure 22.Schematic pathway of the reaction process to form Na 2 ZrSiO 5 from Na 2 ZrO 3 in NaOH sub-molten salt.

Table 2 .
Phase composition of reaction products with different reaction conditions.

Table 2 .
Phase composition of reaction products with different reaction conditions.

Table 2 .
Phase composition of reaction products with different reaction conditions.