A pyrolysis method for fluorosilicates

Abstract

This invention relates to a pyrolysis method for fluorosilicates, belonging to the field of fluorochemical technology. The pyrolysis method includes the following steps: mixing fluorosilicates and alkali metal fluorides in a molar ratio of 1:(0.5-4) and granulating to obtain composite particles; then, the composite particles are subjected to static pyrolysis to prepare SiF4; the particle size of the composite particles is less than 10 mm. This invention mixes and granulates fluorosilicates and alkali metal fluorides. At high temperatures, the alkali metal fluorides achieve trace solid dissolution with fluorosilicates, increasing the melting point of fluorosilicates. During granulation, the alkali metal fluorides are uniformly distributed around the fluorosilicate particles. Since the melting point of the alkali metal fluorides is higher than that of fluorosilicates, the fluorosilicate particles reaching their melting point are blocked by the alkali metal fluorides, preventing localized melting and improving pyrolysis efficiency. Furthermore, the alkali metal fluorides are pyrolysis products of fluorosilicates and, as additives, do not introduce other impurities.

Description

Technical Field

[0001] This invention relates to a pyrolysis method for fluorosilicates, belonging to the field of fluorochemical technology. Background Technology

[0002] Sodium fluorosilicate is an inorganic compound, belonging to the category of coordination salts (complex salts), with the chemical formula Na₂SiF₆. It is mainly used as a glass and enamel whitening agent, flux, and agricultural pesticide. Pyrolysis of Na₂SiF₆ yields NaF and high-value SiF₄ gas, an important silicon source. Further reactions produce single-crystal silicon, realizing the high-value utilization of Na₂SiF₆. However, during pyrolysis (Na₂SiF₆ = 2NaF + SiF₄, pyrolysis temperature 400℃~700℃, melting point of Na₂SiF₆ is 630℃), melting easily occurs, leading to two problems. First, the formed liquid and corrosive melt adheres to the equipment lining, causing corrosion and, upon cooling, solidifying on the lining surface, causing equipment failure. Second, the resulting melt is a mixture of Na₂SiF₆ and NaF, resulting in low pyrolysis efficiency (<80%), failing to meet the required pyrolysis process parameters. Therefore, developing a pyrolysis process that does not produce melt during pyrolysis and has a pyrolysis efficiency of ≥90% has become a challenge.

[0003] Chinese invention patent CN110683548B discloses a method for the efficient production of silicon tetrafluoride and sodium fluoride using sodium fluorosilicate. This method employs a fluidized bed reactor, where sodium fluorosilicate is pyrolyzed at 400℃ to 700℃ for 15 to 180 minutes. However, this process still suffers from unfavorable melting conditions during pyrolysis, and the finer NaF fractions produced during pyrolysis are carried away by the negative pressure under the dynamic conditions of the fluidized bed, clogging the filter and causing equipment malfunction. Summary of the Invention

[0004] The purpose of this invention is to provide a pyrolysis method for fluorosilicates to solve the problem of low pyrolysis efficiency caused by the generation of melts during the pyrolysis process of fluorosilicates in the prior art.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A pyrolysis method for fluorosilicates includes the following steps: mixing fluorosilicates and alkali metal fluorides in a molar ratio of 1:(0.5-4) to granulate, obtaining composite particles, and then subjecting the composite particles to static pyrolysis to prepare SiF4; the particle size of the composite particles is less than 10 mm.

[0007] This invention prepares composite particles from fluorosilicates and alkali metal fluorides. At high temperatures, the alkali metal fluorides achieve a trace solid solution with the fluorosilicates. Since alkali metal fluorides are ionic compounds with high melting points, they increase the melting point of the fluorosilicate-alkali metal fluoride composite. The composite particles consist of alkali metal fluorides uniformly distributed around the fluorosilicate particles. Because the melting point of the alkali metal fluorides is higher than that of the fluorosilicates, the fluorosilicate particles reaching their melting point are blocked by the alkali metal fluorides, preventing localized melting and improving pyrolysis efficiency. Furthermore, the alkali metal fluorides are pyrolysis products of fluorosilicates and, as additives, do not introduce other impurities. Under a molar ratio of 1:(0.5-4), the alkali metal fluorides effectively block the molten fluorosilicates. Static pyrolysis avoids blockage caused by the alkali metal fluorides produced during the reaction. Dynamic pyrolysis of the composite particles prepared in this invention would produce melt, reducing pyrolysis efficiency.

[0008] The molar ratio of fluorosilicate to alkali metal fluoride was further optimized to 1:(0.5-2). When granulation was carried out at this molar ratio, pyrolysis not only did not produce melt, but also achieved higher pyrolysis efficiency.

[0009] To better encapsulate fluorosilicates with alkali metal fluorides, preferably, the particle size of the fluorosilicates is 10–100 μm; and the particle size of the alkali metal fluorides is 20–100 μm.

[0010] To ensure pyrolysis efficiency, the composite particles are preferably 1–8 mm in size. More preferably, they are 1–5 mm in size. This particle size results in higher pyrolysis efficiency.

[0011] Preferably, the static pyrolysis is microwave pyrolysis. The electromagnetic waves of microwave pyrolysis can penetrate the interior of the raw material particles, ensuring that the fluorosilicate undergoes pyrolysis without melting. Furthermore, microwave pyrolysis is a static process, ensuring that the pyrolysis product, alkali metal fluorides, will not be blown away by the negative pressure system, preventing blockage.

[0012] Preferably, the mixed granulation involves mixing fluorosilicate and alkali metal fluoride in a solvent, followed by sand milling and granulation.

[0013] Preferably, the grinding medium is zirconium beads with a particle size of 0.4-0.6 mm.

[0014] Preferably, the grinding time is 30-60 minutes.

[0015] Preferably, the granulation is one of spray granulation or fluidized bed granulation.

[0016] Preferably, the fluorosilicate is one or more of sodium fluorosilicate, calcium fluorosilicate, potassium fluorosilicate, and magnesium fluorosilicate. These are common types of fluorosilicates used for pyrolysis.

[0017] Preferably, the alkali metal fluoride is one or more selected from sodium fluoride, calcium fluoride, potassium fluoride, and magnesium fluoride. It is a product of the pyrolysis of conventional fluorosilicates.

[0018] Preferably, the static pyrolysis temperature is 400–700°C. This temperature is the conventional operating temperature for fluorosilicate pyrolysis. Attached Figure Description

[0019] Figure 1 This is a flowchart of the pyrolysis method for sodium fluorosilicate according to Example 1 of the present invention;

[0020] Figure 2 This is the diffraction pattern of the melt produced by the pyrolysis of sodium fluorosilicate in Comparative Example 3 of this invention. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0022] I. Specific embodiments of the pyrolysis method for fluorosilicates of the present invention are as follows:

[0023] Example 1

[0024] The pyrolysis method for sodium fluorosilicate in this embodiment adopts the following steps:

[0025] 188g of sodium fluorosilicate powder (10-100μm) and 21g of sodium fluoride powder (20-100μm) were mixed at a molar ratio of 1:0.5. After thorough mixing, 1045g of water was added to form a solution. The solution was then milled with 0.4-0.6mm zirconium beads for 60min to form a homogeneous suspension. This suspension was then spray-granulated to form 1mm spheres at 120℃ and 150mL / h. The spheres were then placed in a microwave oven and statically pyrolyzed at 600℃ for 2h to obtain NaF and SiF4. The flow chart is shown below. Figure 1 As shown.

[0026] Example 2

[0027] The pyrolysis method for sodium fluorosilicate in this embodiment adopts the following steps:

[0028] 188g of sodium fluorosilicate powder (10-100μm) and 168g of sodium fluoride powder (20-100μm) were mixed at a molar ratio of 1:4. After mixing, 2300g of water was added to form a solution. The solution was then milled with 0.4-0.6mm zirconium beads for 30min to form a homogeneous suspension. The suspension was then granulated by boiling to form 5mm spheres. The binder for the boiling granulation was polyvinyl alcohol, and the hot air temperature was 110℃. The spheres were then placed in a microwave oven and statically pyrolyzed at 700℃ for 1h to obtain NaF and SiF4.

[0029] Example 3

[0030] The pyrolysis method for sodium fluorosilicate in this embodiment adopts the following steps:

[0031] 188g of sodium fluorosilicate powder (10-100μm) and 42g of sodium fluoride powder (20-100μm) were mixed at a molar ratio of 1:1, and 2008g of water was added. The mixture was then milled with 0.4-0.6mm zirconium beads for 45min to form a homogeneous suspension. The suspension was then spray-granulated to form 3mm spheres at 120℃ and 150mL / h. The spheres were then placed in a microwave oven and statically pyrolyzed at 600℃ for 1.5h to obtain NaF and SiF4.

[0032] II. Comparative Example

[0033] Comparative Example 1

[0034] The pyrolysis method for sodium fluorosilicate in this comparative example adopts the following steps:

[0035] 188g of sodium fluorosilicate powder (10-100μm) was placed in a microwave oven and statically pyrolyzed at 600℃ for 1.5h to obtain NaF and SiF4.

[0036] Comparative Example 2

[0037] The pyrolysis method for sodium fluorosilicate in this comparative example adopts the following steps:

[0038] 188g of sodium fluorosilicate powder (10-100μm) and 168g of sodium fluoride powder (20-100μm) were mixed at a molar ratio of 1:4. 2300g of water was added, and the mixture was milled with 0.4-0.6mm zirconium beads for 30min. The mixture was then spray-granulated to form 10mm spheres at 120℃ and 150mL / h. The spheres were then placed in a microwave oven and statically pyrolyzed at 700℃ for 2h to obtain NaF and SiF4.

[0039] Comparative Example 3

[0040] The pyrolysis method for sodium fluorosilicate in this comparative example adopts the following steps:

[0041] 188g of sodium fluorosilicate powder (10-100μm) and 42g of sodium fluoride powder (20-100μm) were mixed at a molar ratio of 1:1, 2008g of water was added, and the mixture was milled with 0.4-0.6mm zirconium beads for 45min. Then, it was spray-granulated to form 3mm spheres at 120℃ and 150mL / h. The spheres were then placed in a fluidized bed and statically pyrolyzed at 600℃ for 1.5h to obtain NaF and SiF4.

[0042] III. Experimental Examples

[0043] This experimental example measures the pyrolysis products of Examples 1-3 and Comparative Examples 1-3.

[0044]

[0045]

[0046] Where M1 is the mass of Na2SiF6 fed, M2 is the mass of the pyrolyzed material, M3 is the mass of NaF fed, 188 is the relative molecular weight of Na2SiF6, 104 is the relative molecular weight of SiF4, 84 is the relative molecular mass of NaF, and w is the decomposition rate of Na2SiF6.

[0047] The decomposition rate (i.e., pyrolysis efficiency) of Na₂SiF₆ in Example 1 was determined to be 94%. The purity of NaF was 52%, the purity of SiF₄ was 90%, and the yield was 68%. No melt was produced during this pyrolysis process. Comparative Example 1 differed from Example 1 only in that it did not involve mixing with sodium fluoride or milling, but melt was produced during pyrolysis. Its Na₂SiF₆ decomposition rate was only 29%, the purity of NaF was 15%, the purity of SiF₄ was 76%, and the yield was 65% (the SiF₄ yield is the ratio of the actual collected SiF₄ to the theoretical value of SiF₄ produced by the decomposition of sodium fluorosilicate).

[0048] In Example 2, the Na₂SiF₆ decomposition rate was 92%. The purity of NaF was 94.2%, and the purity of SiF₄ was 85%, with a yield of 73%. No melt was produced during this pyrolysis process. Comparative Example 2 differs from Example 2 in that the granulated spheres had a particle size of 10 mm, and molten agglomerates were produced during pyrolysis. In Comparative Example 2, the Na₂SiF₆ decomposition rate was 57%, the NaF purity was 73%, the SiF₄ purity was 72%, and the yield was 67%.

[0049] In Example 3, the decomposition rate of Na₂SiF₆ was 95%. The purity of NaF was 95%, and the purity of SiF₄ was 85%, with a yield of 70%. No melt was produced during this pyrolysis process. The difference between Comparative Example 3 and Example 3 lies in the pyrolysis method after granulation. Comparative Example 3 used fluidized bed pyrolysis, achieving a Na₂SiF₆ decomposition rate of 43%, a NaF purity of 42%, a SiF₄ purity of 67%, and a yield of 65%. Pyrolysis produced a melt consisting of sodium fluoride and sodium fluorosilicate. The diffraction pattern of this melt is shown below. Figure 2 As shown.

Claims

1. A method for pyrolysis of a fluorosilicate salt, characterized in that, The method comprises the following steps: The fluorosilicate and the alkali metal fluoride are mixed and granulated at a molar ratio of 1:(0.5-4) to obtain composite particles, and then the composite particles are prepared into SiF4 through static pyrolysis.

2. The method of claim 1, wherein the fluorosilicate is selected from the group consisting of sodium fluorosilicate, potassium fluorosilicate, ammonium fluorosilicate, and mixtures thereof. The particle size of the fluorosilicate is 10-100 μm, and the particle size of the alkali metal fluoride is 20-100 μm.

3. The method of claim 1, wherein the fluorosilicate is selected from the group consisting of sodium fluorosilicate, potassium fluorosilicate, ammonium fluorosilicate, and mixtures thereof. The particle size of the composite particles is 1-8 mm.

4. The method of pyrolysis of fluosilicate salt according to any one of claims 1-3, characterized in that, The static pyrolysis is microwave pyrolysis.

5. The method of pyrolysis of fluosilicate salt according to any one of claims 1-3, characterized in that, The mixing and granulating are performed by mixing the fluorosilicate and the alkali metal fluoride in a solvent, sand milling and granulating.

6. The method of claim 5, wherein the fluorosilicate is selected from the group consisting of sodium fluorosilicate, potassium fluorosilicate, ammonium fluorosilicate, and mixtures thereof. The medium for the sand milling is zirconium beads, and the particle size of the zirconium beads is 0.4-0.6 mm.

7. The method of claim 1, wherein the fluorosilicate is selected from the group consisting of sodium fluorosilicate, potassium fluorosilicate, ammonium fluorosilicate, and mixtures thereof. The molar ratio of the fluorosilicate to the alkali metal fluoride is 1:(0.5-2).

8. The method of pyrolysis of fluosilicate salt according to claim 1 or 2, characterized in that, The fluorosilicate is one or more of sodium fluorosilicate, calcium fluorosilicate, potassium fluorosilicate and magnesium fluorosilicate.

9. The method of pyrolyzing fluosilicate salt according to claim 1 or 2, characterized in that, The alkali metal fluoride is one or both of sodium fluoride and potassium fluoride.

10. The method of pyrolysis of fluosilicate salt according to any one of claims 1-3, characterized in that, The temperature of the static pyrolysis is 400-700 ℃.

Images