Method and device for purifying cyclic siloxane

Abstract

The invention relates to the technical field of organic silicon material purification, in particular to a method and a device for purifying cyclic siloxane. The purification method of cyclic siloxane provided by the invention combines a synergistic process of rectification pre-purification (efficient separation of components with different volatility degrees) and deep impurity removal through melt crystallization (by utilizing the difference between the melting points of D3 / D4 and the impurities, and through temperature-controlled fractional crystallization, the impurities cannot crystallize), breaks through the bottleneck of a single technology, and is suitable for industrial production. Specifically, after rectification pre-purification, through temperature control sweating of staged cooling crystallization and dynamic heating sweating, crystal breakage is reduced, and the impurity removal rate is increased. Meanwhile, tower kettle raffinate and crystallization mother liquor are classified and recycled and are returned to a cracking system, so that loss is reduced, and resource circulation is realized. The method not only solves the problem that the cyclic siloxane is difficult to purify to 99.99% or above by the existing rectification technology, but also can realize large-scale adaptation through the parallel design of the multi-stage crystallizers, is suitable for industrial production, and reduces the energy consumption by 30% or above compared with the traditional process.

Description

Technical Field

[0001] This invention relates to the field of organosilicon material purification technology, specifically a method and apparatus for purifying cyclic siloxanes. Background Technology

[0002] Industrially, the purification of cyclic siloxanes typically employs distillation technology. However, because the boiling points of the cyclic compounds and impurities are extremely close, distillation alone can only purify the cyclic compounds to approximately 99.8%. In many high-end applications (such as high-end silicone rubber and optical materials), cyclic siloxanes require a purity of over 99.99%, which existing distillation technologies struggle to meet. While melt crystallization offers advantages in separating high-melting-point, high-purity substances, its use alone suffers from low separation efficiency and limited throughput. Summary of the Invention

[0003] In view of this, the technical problem to be solved by the present invention is to provide a method and apparatus for purifying cyclic siloxanes. The method for purifying cyclic siloxanes provided by the present invention can extract hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane with a purity of more than 99.99% from a mixture of cyclic siloxanes.

[0004] This invention provides a method for purifying cyclic siloxanes, specifically for purifying hexamethylcyclotrisiloxane and / or octamethylcyclotetrasiloxane from a mixture of cyclic siloxanes. The cyclic siloxane mixture of this invention comprises 5 wt% to 10 wt% hexamethylcyclotrisiloxane, 80 wt% to 85 wt% octamethylcyclotetrasiloxane, and 8 wt% to 13 wt% decamethylcyclopentasiloxane.

[0005] In a first aspect of the present invention, the method for purifying the cyclic siloxane of the present invention includes the following steps:

[0006] S1) The cyclic siloxane mixture is purified by distillation of hexamethylcyclotrisiloxane, and the purified hexamethylcyclotrisiloxane is obtained at the top of the distillation column and the bottom product is obtained at the bottom of the distillation column.

[0007] S2) The distillation purified material of hexamethylcyclotrisiloxane obtained in step S1) is subjected to melt crystallization of hexamethylcyclotrisiloxane in a molten state to obtain hexamethylcyclotrisiloxane;

[0008] The specific process of melting and crystallizing the hexamethylcyclotrisiloxane is as follows: crystallizing is carried out by cooling the temperature at a rate of 0.1℃ / h to 3℃ / h to 5℃ to 20℃ below the melting point of the hexamethylcyclotrisiloxane, and then dynamic sweating is carried out at a rate of 0.1℃ / h to 3℃ / h within the range of 59℃ to 66℃.

[0009] A first aspect of the present invention involves the purification of hexamethylcyclotrisiloxane from a mixture of cyclic siloxanes. First, the cyclic siloxane mixture is subjected to distillation purification of hexamethylcyclotrisiloxane, yielding a purified hexamethylcyclotrisiloxane feed at the top of a distillation column and a bottom product at the bottom of the distillation column. In some embodiments of the present invention, the distillation purification of hexamethylcyclotrisiloxane is carried out in a distillation column; the distillation column is a high-efficiency packed column, and the cyclic siloxane mixture is introduced into the high-efficiency packed column for the distillation purification of hexamethylcyclotrisiloxane. The present invention purifies hexamethylcyclotrisiloxane by distillation under conditions of slightly negative or slightly positive gauge pressure and a reflux ratio of 1 to 10. Specifically, the distillation purification of hexamethylcyclotrisiloxane is carried out under conditions of -0.01 MPa to 0.05 MPa gauge pressure and a reflux ratio of 1 to 10; preferably, the distillation purification of hexamethylcyclotrisiloxane is carried out under conditions of -0.01 MPa to 0.05 MPa gauge pressure and a reflux ratio of 1 to 10. More specifically, the feed temperature for the distillation purification of hexamethylcyclotrisiloxane is 170°C to 175°C, and the reboiler temperature is 180°C to 185°C. This invention purifies a mixture of cyclic siloxanes with hexamethylcyclotrisiloxane by distillation under the specified distillation process parameters. The hexamethylcyclotrisiloxane can be purified to ≥98% and separated from the top of the distillation column, while octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane and high-boiling impurities are transferred to the bottom of the distillation column as bottom feed.

[0010] In a first aspect of the present invention, after obtaining the purified hexamethylcyclotrisiloxane by distillation, the purified hexamethylcyclotrisiloxane is subjected to melt crystallization in a molten state to obtain hexamethylcyclotrisiloxane. The melt crystallization of hexamethylcyclotrisiloxane according to the present invention includes static crystallization and dynamic sweating stages, which can yield hexamethylcyclotrisiloxane with a purity ≥99.99%. In some embodiments of the present invention, the purified hexamethylcyclotrisiloxane is fed into a static cooling crystallizer and cooled at a rate of 0.1℃ / h to 3℃ / h to 5℃ to 20℃ below the melting point of hexamethylcyclotrisiloxane for crystallization, allowing for initial crystallization; then, dynamic sweating is performed at a rate of 0.1℃ / h to 3℃ / h within a range of 59℃ to 66℃ to promote the enrichment of impurities in the liquid phase, resulting in a final product purity ≥99.99%. Preferably, the melt crystallization of the hexamethylcyclotrisiloxane specifically involves: cooling to 43℃~59℃ at a rate of 0.1℃ / h~3℃ / h and holding at that temperature for 1h~2h to induce crystallization, followed by dynamic sweating at a rate of 0.1℃ / h~3℃ / h within the range of 59℃~66℃. Dynamic sweating, as described in this invention, means that dynamic sweating is achieved by heating at a predetermined rate within any range of 59℃~66℃. For example, heating from 59℃ to 66℃ at a rate of 0.1℃ / h~3℃ / h completes dynamic sweating; another example is heating from 62℃ to 65℃ at a rate of 0.1℃ / h~3℃ / h. Subsequent descriptions of dynamic sweating follow the same principle and will not be repeated here. The material temperature after crystallization in this invention is lower than the lower limit temperature for subsequent dynamic sweating. After heating to the lower limit temperature for dynamic sweating using conventional techniques in the field, dynamic sweating can be carried out; the same principle applies to subsequent cases, and will not be repeated here.

[0011] In some embodiments of the present invention, the melt crystallization of the hexamethylcyclotrisiloxane specifically involves: cooling to 43°C~59°C at a rate of 0.1°C / h~3°C / h and holding at that temperature for 1h~2h to crystallize, followed by dynamic sweating at a rate of 1°C / h~3°C / h within the range of 61°C~66°C. In other embodiments of the present invention, the melt crystallization of the hexamethylcyclotrisiloxane specifically involves: cooling to 43°C~50°C at a rate of 1°C / h~3°C / h and holding at that temperature for 1h~2h to crystallize, followed by dynamic sweating at a rate of 1°C / h~3°C / h within the range of 61°C~66°C.

[0012] In a first aspect of the present invention, after the melt crystallization of hexamethylcyclotrisiloxane, the method further includes melting and discharging the solid material after melt crystallization to obtain hexamethylcyclotrisiloxane. The melting and discharging temperature of the present invention is such that the solid material after melt crystallization is in a molten state; preferably, the melting and discharging temperature is 73°C to 79°C.

[0013] In a second aspect of the present invention, the method for purifying the cyclic siloxane of the present invention includes the following steps:

[0014] a) The cyclic siloxane mixture is purified by distillation of hexamethylcyclotrisiloxane to obtain the overhead product at the top of the distillation column and the bottom product at the bottom of the distillation column.

[0015] b) The bottom material of the column described in step a) is purified by distillation of octamethylcyclotetrasiloxane to obtain purified octamethylcyclotetrasiloxane at the top of the distillation column.

[0016] c) The distilled and purified material of octamethylcyclotetrasiloxane obtained in step b) is subjected to melt crystallization of octamethylcyclotetrasiloxane in a molten state to obtain octamethylcyclotetrasiloxane.

[0017] The specific melting and crystallization of the octamethylcyclotetrasiloxane involves: cooling the temperature at a rate of 0.1℃ / h to 3℃ / h to crystallize at 5℃ to 20℃ below the melting point of the octamethylcyclotetrasiloxane, and then performing dynamic sweating at a rate of 0.1℃ / h to 3℃ / h within the range of 14℃ to 19℃.

[0018] The second aspect of the present invention involves the purification of octamethylcyclotetrasiloxane from a mixture of cyclic siloxanes. First, the mixture of cyclic siloxanes is purified by distillation of hexamethylcyclotrisiloxane, the distillation of hexamethylcyclotrisiloxane being the same as described above and will not be repeated here. After the distillation of hexamethylcyclotrisiloxane, the purified material of hexamethylcyclotrisiloxane is obtained at the top of the distillation column, and the bottom material is obtained at the bottom of the distillation column.

[0019] In the second aspect of the present invention, the distilled and purified material of hexamethylcyclotrisiloxane obtained is the same as that of the distilled and purified material of hexamethylcyclotrisiloxane in the first aspect. Alternatively, the distilled and purified material of hexamethylcyclotrisiloxane obtained can be melt-crystallized in the same way as in the first aspect to finally obtain hexamethylcyclotrisiloxane with a purity ≥99.99%, which will not be elaborated further.

[0020] In the second aspect of the present invention, the obtained bottom product is the same as that obtained in the first aspect. After obtaining the bottom product, it is subjected to distillation purification of octamethylcyclotetrasiloxane, and the purified octamethylcyclotetrasiloxane is obtained at the top of the distillation column. In some embodiments of the present invention, the distillation purification of octamethylcyclotetrasiloxane is carried out in a distillation column; the distillation column is a high-efficiency packed column, and the bottom product is introduced into the high-efficiency packed column for the distillation purification of octamethylcyclotetrasiloxane. The present invention purifies octamethylcyclotetrasiloxane by distillation under conditions of slightly positive or slightly negative gauge pressure and a reflux ratio of 1 to 10. Specifically, the distillation purification of octamethylcyclotetrasiloxane is carried out under conditions of -0.01 MPa to 0.05 MPa gauge pressure and a reflux ratio of 1 to 10; preferably, the distillation purification of octamethylcyclotetrasiloxane is carried out under conditions of -0.01 MPa to 0.05 MPa gauge pressure and a reflux ratio of 1 to 10. More specifically, the feed temperature for the distillation purification of octamethylcyclotetrasiloxane is 180°C to 185°C, and the reboiler temperature is 210°C to 215°C. This invention purifies a mixture of cyclic siloxanes by distillation of octamethylcyclotetrasiloxane under the specified distillation process parameters. The octamethylcyclotetrasiloxane is purified to ≥99.5% and separated from the top of the distillation column, while decamethylcyclopentasiloxane and high-boiling-point impurities are transferred to the bottom of the distillation column as bottom feed.

[0021] In a second aspect of the present invention, after obtaining the purified octamethylcyclotetrasiloxane by distillation, the purified octamethylcyclotetrasiloxane is subjected to melt crystallization in a molten state to obtain octamethylcyclotetrasiloxane. The melt crystallization of octamethylcyclotetrasiloxane according to the present invention includes static crystallization and dynamic sweating stages, which can yield octamethylcyclotetrasiloxane with a purity ≥99.99%. In some embodiments of the present invention, the purified octamethylcyclotetrasiloxane is fed into a static cooling crystallizer and cooled at a rate of 0.1℃ / h to 3℃ / h to 5℃ to 20℃ below the melting point of octamethylcyclotetrasiloxane for crystallization, allowing for initial crystallization; then, dynamic sweating is performed at a rate of 0.1℃ / h to 3℃ / h within a range of 14℃ to 19℃ to promote the enrichment of impurities in the liquid phase, resulting in a final product purity ≥99.99%. Preferably, the melting and crystallization of the octamethylcyclotetrasiloxane specifically involves: cooling to -2℃ to 13℃ at a rate of 0.1℃ / h to 3℃ / h and holding at that temperature for 1 h to 3 h to crystallize, followed by dynamic sweating at a rate of 0.1℃ / h to 3℃ / h within the range of 14℃ to 19℃.

[0022] In some embodiments of the present invention, the melt crystallization of the octamethylcyclotetrasiloxane specifically involves: cooling to -2℃ to 13℃ at a rate of 0.1℃ / h to 3℃ / h and holding for 1 h to 3 h to crystallize, followed by dynamic sweating at a rate of 1℃ / h to 3℃ / h within the range of 14℃ to 19℃. In other embodiments of the present invention, the melt crystallization of the hexamethylcyclotrisiloxane specifically involves: cooling to -2℃ to 13℃ at a rate of 1℃ / h to 3℃ / h and holding for 1 h to 2 h to crystallize, followed by dynamic sweating at a rate of 1℃ / h to 3℃ / h within the range of 14℃ to 19℃. In still other embodiments of the present invention, the melt crystallization of the hexamethylcyclotrisiloxane specifically involves: cooling to 4℃ to 8℃ at a rate of 1℃ / h to 3℃ / h and holding for 1 h to 2 h to crystallize, followed by dynamic sweating at a rate of 1℃ / h to 3℃ / h within the range of 14℃ to 19℃.

[0023] In a second aspect of the present invention, after the melt crystallization of octamethylcyclotetrasiloxane, the method further includes melting and discharging the solid material after melt crystallization to obtain octamethylcyclotetrasiloxane. The melting and discharging temperature of the present invention is such that the solid material after melt crystallization is in a molten state; preferably, the melting and discharging temperature is 27°C to 28°C.

[0024] The purification method for cyclic siloxanes provided by this invention employs a two-step process of "distillation + melt crystallization". Specifically, the cyclic siloxanes are first separated from low / high boiling point impurities by distillation in a high-efficiency packed tower (atmospheric pressure / slight negative pressure, reflux ratio), and purified to ≥98%. Then, trace impurities are further removed by static cooling crystallization and dynamic sweating, resulting in a final purity of ≥99.99%. This method solves the problem that existing distillation technologies are unable to purify cyclic siloxanes to above 99.99%. It is suitable for the preparation of high-purity cyclic siloxanes in the synthesis of organosilicon monomers, silicone oil, silicone rubber, and other production processes, and can also be applied in the electronics field.

[0025] The present invention also provides a purification apparatus for applying any of the purification methods described above, comprising a distillation unit and a melt crystallization unit;

[0026] The distillation unit includes a packed column, a bottom reboiler, and a top condenser; specifically, it includes a packed column, a bottom reboiler, a top condenser, a bottom circulation pump, a top reflux pump, and a top outflow pump.

[0027] The melt crystallization unit includes a melt crystallizer connected to the packed tower.

[0028] Specifically, the packed tower is equipped with a raw material inlet, a reflux port, a steam inlet, a top product outlet, and a bottom residue outlet; the melt crystallizer has static cooling crystallization and dynamic sweating functions, and is equipped with a material inlet, a cooling medium inlet and outlet, a heating medium inlet and outlet, a crystallization mother liquor outlet, a sweating liquid outlet, a crystal outlet, a mother liquor tank, a sweating liquid tank, and a product tank.

[0029] More specifically, the product outlet at the top of the packed tower is connected to the material inlet of the static cooling crystallization / dynamic sweating device. The product outlet of the distillation unit and the material inlet of the molten crystallization unit, as well as the connections between the outlets of each unit and the inlet of the next processing step, are all connected via material conveying pipelines. The purification apparatus provided by this invention also includes a centrifugal drying module, which comprises a centrifuge and a dryer.

[0030] This invention provides a method and apparatus for purifying cyclic siloxanes. The purification method combines pre-purification by distillation (efficiently separating components with different volatility) with deep impurity removal by melt crystallization (utilizing the difference in melting points between D3 / D4 and impurities, and controlling temperature-stage crystallization to prevent impurities from crystallizing), overcoming the bottleneck of single-technology approaches. Specifically, after pre-purification by distillation, staged cooling crystallization combined with dynamic temperature-controlled sweating reduces crystal breakage and improves impurity removal rate. Simultaneously, the residue from the distillation column and the mother liquor from crystallization are separately recovered and returned to the cracking system, reducing losses and achieving resource recycling. This invention not only solves the problem that existing distillation technologies struggle to purify cyclic siloxanes to over 99.99%, but also allows for large-scale adaptation through a multi-stage crystallizer parallel design, achieving a processing capacity of 1-10 tons / hour, suitable for industrial production, and reducing energy consumption by more than 30% compared to traditional processes. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the purification apparatus for cyclic siloxanes according to the present invention;

[0032] Figure 2 This is the gas chromatogram of D3 finally obtained in Example 1 of the present invention;

[0033] Figure 3 This is the gas chromatogram of D4 obtained in Example 3 of the present invention. Detailed Implementation

[0034] This invention discloses a method and apparatus for purifying cyclic siloxanes. Those skilled in the art can refer to this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The method and application of this invention have been described through preferred embodiments; those skilled in the art will clearly be able to modify or appropriately change and combine the method and application described herein without departing from the content, spirit, and scope of this invention to implement and apply the technology of this invention.

[0035] This invention purifies cyclic siloxanes from a mixed cyclic system, wherein the mixed cyclic system comprises hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5), and the cyclic siloxanes to be purified are D3 and D4. The purification method of this invention includes a distillation stage and a melt crystallization stage, as detailed below:

[0036] Distillation stage: A high-efficiency packed column is used, and the process is carried out under slightly positive or slightly negative pressure (gauge pressure range of -0.01~0.05MPa). The reflux ratio is controlled between 1 and 10. By utilizing the difference in relative volatility (boiling point), cyclic siloxanes (D3 and D4) are separated from low-boiling-point impurities (such as low-carbon hydrocarbons and low-carbon alcohols) and high-boiling-point impurities (such as cyclic derivatives, low-chain linear polysiloxanes, and high-boiling-point siloxanes) through this distillation process, purifying the cyclic siloxanes to ≥98%.

[0037] Melt crystallization stage: The cyclic siloxane material after distillation enters a static cooling crystallizer and is cooled at a specific rate to a specific temperature range below the melting point of the cyclic siloxane, so that the cyclic siloxane can initially crystallize. Then, dynamic sweating is carried out at a specific rate within a specific temperature range. The melt crystallization stage is used to separate and purify impurities with boiling points close to D3 that are difficult to separate by distillation. The cyclic siloxane product can crystallize, while impurities (metal ions, low-chain linear polysiloxanes, chloroalkanes, fatty alcohols, cyclic siloxane substituted derivatives, etc.) cannot crystallize. Through melt crystallization, the impurities are enriched in the liquid phase.

[0038] Melting and discharge: After the solid phase obtained in the melting and crystallization stage is melted into a molten state, it is discharged. Finally, cyclic siloxanes with a purity of ≥99.99% can be obtained without centrifugation, washing and drying.

[0039] This invention employs a purification apparatus including a distillation unit and a melt crystallization unit for the purification of cyclic siloxanes, such as... Figure 1 As shown, Figure 1This is a schematic diagram of the purification apparatus for cyclic siloxanes according to the present invention. In the diagram, 1 is the raw material feed line, T1 is the distillation column, E1 is the reboiler (heat medium includes steam, heat transfer oil, molten salt, electric heating, etc.), 2, 3, and 5 are the liquid phase circulation lines at the bottom of the distillation column, P1 is the liquid phase circulation pump at the bottom of the distillation column, 4 is the liquid phase discharge line at the bottom of the distillation column, 6 is the vapor phase pipe at the top of the column, E2 is the top condenser (cooling medium includes circulating water, low-temperature cold water, chilled brine, ethylene glycol aqueous solution, propylene glycol aqueous solution, etc.), 7 is the vacuum tube, 8 is the top condenser tube, P1 is the liquid phase circulation pump at the top of the distillation column, 9 is the top reflux pipe, 10 is the top discharge pipe, and R1 is the melt crystallizer (heat medium includes steam, heat transfer oil, molten salt, electric heating, etc.). Medium; the refrigerant is circulating water, low-temperature cold water, frozen brine, ethylene glycol aqueous solution, propylene glycol aqueous solution, etc. 11, 12, 13, and 14 are molten crystallization discharge pipelines, V1, V2, and V3 are molten crystallization product receiving tanks, 15, 16, and 17 are molten crystallization product conveying pipelines, 18, 19, 24, 25, 26, 27, 28, and 29 are heat medium conveying pipelines, 20 and 21 are refrigerant conveying pipelines, and 23 and 24 are hot and cold medium conveying pipelines.

[0040] The present invention will be further described below with reference to the embodiments:

[0041] Comparative Example 1

[0042] Separate distillation:

[0043] The initial mixed rings (D3 accounts for 6.8 wt%, D4 accounts for 83.6 wt%, D5 accounts for 9.5 wt%, and the remainder is impurities) were fed into column 1 for separation and purification of D3. The process parameters of column 1 were: feed temperature: 173℃, column bottom temperature: 184℃, reflux ratio: 2~4. D3 was separated from the top of column 1 with a purity of ~98.0%.

[0044] The bottom material of column 1 enters column 2 for the next step of separation and purification of D4; the process parameters of column 2 are: feed temperature: 184℃, bottom temperature: 211.5℃, reflux ratio: 2~4; D4 is separated from the top of column 2 with a purity of ~99.8%.

[0045] Examples 1-2

[0046] D3 distillation + melt crystallization:

[0047] Examples 1 and 2 both involve melting and crystallizing the D3 obtained from the distillation of Comparative Example 1; the specific process parameters for Example 1 are shown in Table 1:

[0048] Table 1

[0049]

[0050] The D3 obtained in Example 1 and Comparative Example 1 was detected by gas chromatography, and the results are as follows: Figure 2 As shown, Figure 2 The above are gas chromatograms of D3 finally obtained in Example 1 and Comparative Example 1 of the present invention. The upper curve is the analytical result of D3 finally obtained after purification in Example 1, and the lower curve is the analytical result of D3 finally obtained after purification in Comparative Example 1.

[0051] The specific process parameters for Example 2 are shown in Table 2:

[0052] Table 2

[0053]

[0054] Examples 3-4

[0055] D4 distillation + melt crystallization:

[0056] Examples 3 and 4 both involve melting and crystallizing the D4 obtained from the distillation of Comparative Example 1; the specific process parameters for Example 3 are shown in Table 3.

[0057] Table 3

[0058]

[0059] The D4 obtained in Example 3 and Comparative Example 1 was detected by gas chromatography, and the results are as follows: Figure 3 As shown, Figure 3 The above are gas chromatograms of D4 finally obtained in Example 3 and Comparative Example 1 of the present invention. The upper curve is the analytical result of D4 finally obtained after purification in Example 3, and the lower curve is the analytical result of D4 finally obtained after purification in Comparative Example 1.

[0060] The specific process parameters for Example 4 are shown in Table 4:

[0061] Table 4

[0062]

[0063] Comparative Examples 2-3

[0064] D4 distillation + melt crystallization:

[0065] Comparative Examples 2 and 3 both involved melting and crystallizing the D4 obtained from the distillation of Comparative Example 1; the specific process parameters for Comparative Example 2 are shown in Table 5.

[0066] Table 5

[0067]

[0068] The specific process parameters for Comparative Example 3 are shown in Table 6:

[0069] Table 6

[0070]

[0071] Comparative Example 4

[0072] Individual melt crystallization:

[0073] The initial mixed rings (D3 6.8 wt%, D4 83.6 wt%, D5 9.5 wt%, with the remainder being impurities) were directly subjected to melt crystallization to extract D3. The process parameters are shown in Table 7.

[0074] Table 7

[0075]

[0076] The crystallization mother liquor obtained in Comparative Example 4 (D3 accounts for 5.8%, D4 accounts for 84.2%, D5 accounts for 9.8%, and the remainder is impurities) was directly subjected to melt crystallization to extract D4. The process parameters are shown in Table 8.

[0077] Table 8

[0078]

[0079] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for purifying cyclic siloxanes, characterized in that, Includes the following steps: S1) The cyclic siloxane mixture is purified by distillation of hexamethylcyclotrisiloxane, and the purified hexamethylcyclotrisiloxane is obtained at the top of the distillation column and the bottom product is obtained at the bottom of the distillation column. S2) The distillation purified material of hexamethylcyclotrisiloxane obtained in step S1) is subjected to melt crystallization of hexamethylcyclotrisiloxane in a molten state to obtain hexamethylcyclotrisiloxane; The specific process of melting and crystallizing the hexamethylcyclotrisiloxane is as follows: crystallizing is carried out by cooling the temperature at a rate of 0.1℃ / h to 3℃ / h to 5℃ to 20℃ below the melting point of the hexamethylcyclotrisiloxane, and then dynamic sweating is carried out at a rate of 0.1℃ / h to 3℃ / h within the range of 59℃ to 66℃.

2. A method for purifying cyclic siloxanes, characterized in that, Includes the following steps: a) The cyclic siloxane mixture is purified by distillation of hexamethylcyclotrisiloxane to obtain the overhead product at the top of the distillation column and the bottom product at the bottom of the distillation column. b) The bottom material of the column described in step a) is purified by distillation of octamethylcyclotetrasiloxane to obtain purified octamethylcyclotetrasiloxane at the top of the distillation column. c) The distilled and purified material of octamethylcyclotetrasiloxane obtained in step b) is subjected to melt crystallization of octamethylcyclotetrasiloxane in a molten state to obtain octamethylcyclotetrasiloxane. The specific melting and crystallization of the octamethylcyclotetrasiloxane involves: cooling the temperature at a rate of 0.1℃ / h to 3℃ / h to crystallize at 5℃ to 20℃ below the melting point of the octamethylcyclotetrasiloxane, and then performing dynamic sweating at a rate of 0.1℃ / h to 3℃ / h within the range of 14℃ to 19℃.

3. The purification method according to claim 1 or 2, characterized in that, The cyclic siloxane mixture comprises 5 wt% to 10 wt% of hexamethylcyclotrisiloxane, 80 wt% to 85 wt% of octamethylcyclotetrasiloxane, and 8 wt% to 13 wt% of decamethylcyclopentasiloxane.

4. The purification method according to claim 1 or 2, characterized in that, In step S1) or step a), the process parameters for the distillation purification of hexamethylcyclotrisiloxane are: gauge pressure of -0.01 MPa to 0.05 MPa and reflux ratio of 1 to 10.

5. The purification method according to claim 1 or 2, characterized in that, In step S1) or step a), the process parameters for the distillation purification of hexamethylcyclotrisiloxane are: feed temperature of 170℃~175℃ and bottom temperature of the column of 180℃~185℃.

6. The purification method according to claim 1, characterized in that, In step S2), the melting and crystallization of the hexamethylcyclotrisiloxane specifically involves: cooling to 43℃~59℃ at a rate of 0.1℃ / h~3℃ / h and holding at that temperature for 1 h~2 h to crystallize, and then performing dynamic sweating at a rate of 0.1℃ / h~3℃ / h within the range of 59℃~66℃.

7. The purification method according to claim 2, characterized in that, In step b), the process parameters for the distillation purification of octamethylcyclotetrasiloxane are: gauge pressure of -0.01 MPa to 0.05 MPa and reflux ratio of 1 to 10.

8. The purification method according to claim 2, characterized in that, In step b), the process parameters for the distillation purification of octamethylcyclotetrasiloxane are: feed temperature of 180℃~185℃ and bottom temperature of the column of 210℃~215℃.

9. The purification method according to claim 2, characterized in that, In step c), the melting and crystallization of the octamethylcyclotetrasiloxane specifically involves: cooling to -2℃ to 13℃ at a rate of 0.1℃ / h to 3℃ / h and holding at that temperature for 1 h to 3 h to crystallize, and then performing dynamic sweating at a rate of 0.1℃ / h to 3℃ / h within the range of 14℃ to 19℃.

10. A purification apparatus for using the purification method according to any one of claims 1 to 9, characterized in that, It includes a distillation unit and a melting and crystallization unit; The distillation unit includes a packed column, a bottom reboiler, and a top condenser; The melt crystallization unit includes a melt crystallizer connected to the packed tower.

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