A method for recycling silicone rubber scraps

Abstract

The invention relates to a method for recycling silicone rubber scraps, comprising the following steps: (1) crushing the silicone rubber scraps, and then adding silicon dioxide and ferrous sulfate to grind; (2) dehydrating the crushed silicone rubber scraps, adding the diatomite zeolite composite catalyst and sulfonic acid, and catalyzing the silicone rubber scraps to slowly decompose into crude DMC gas; (3) cooling and refluxing the DMC gas, and obtaining D4 and D5 through dehydration, alkali rearrangement, activated carbon adsorption, and rectification; D4 and a capping agent are injected into a silicone oil reaction kettle, and the temperature is increased and reacted to obtain finished silicone oil; (4) high-temperature calcination and impurity removal of solid residues to obtain white carbon black; the method of the invention is used to treat silicone rubber scraps under mild reaction conditions and short time, and the DMC recovery rate is between 97.1% and 98.1%. The additives silicon dioxide and diatomite of the invention are both raw materials of white carbon black and can be converted into white carbon black under the action of high temperature, without introducing too much waste, which makes subsequent treatment difficult.

Description

Technical Field

[0001] The invention relates to a method for recycling silicone rubber scraps and belongs to the field of recycling waste silicone rubber materials. Background Art

[0002] Silicone rubber materials have excellent comprehensive properties and are widely used in aerospace, electronics, medical, new energy vehicles and other fields. The increase in demand and consumption of silicone rubber has resulted in a large amount of waste such as scraps and defective products in the production process. At the same time, the end of the life of silicone rubber used for insulators in the power grid field has generated a large amount of waste silicone rubber materials.

[0003] The main methods for recycling waste silicone rubber include chemical cracking, thermal cracking, ultrasonic cracking and physical crushing. Chemical cracking includes acid-catalyzed cracking and base-catalyzed cracking. Acid-catalyzed cracking is to use the acid commonly used for siloxane polymerization as a catalyst to depolymerize siloxane under certain conditions and convert it into a rubber material that can be processed and reused for the production of silicone rubber raw rubber. Acid-catalyzed cracking (concentrated sulfuric acid catalysis) is a common method for silicone rubber cracking and is also a method commonly used by most factories in my country. Compared with other methods, this method has a higher safety factor. Wu Shixiong used concentrated sulfuric acid with concentrations of 1-10% and 3-20% and dodecylbenzenesulfonic acid mixed acid to catalyze the cracking of waste silicone rubber collected after crushing and washing. However, the cracking of silicone rubber catalyzed by concentrated sulfuric acid has the following disadvantages: (1) the equipment has high corrosion resistance requirements, which will increase industrial production costs; (2) the recovery rate of DMC in this method is unstable and has a large fluctuation range; (3) the DMC recovery rate of this method is low, for example, only 40-50% of the leftover materials of buttons can be recovered; (4) this method will produce acidic waste liquid and waste residue, which will cause secondary pollution and increase environmental protection costs. Summary of the invention

[0004] The invention provides a method for recycling silicone rubber scraps, which solves the problems of pollution and low DMC recovery rate in treating silicone rubber waste by an existing acid catalytic cracking method (concentrated sulfuric acid catalysis).

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A silicone rubber scrap cracking catalyst, the cracking catalyst is a diatomite zeolite composite catalyst, and the preparation method is: diatomite and concentrated sulfuric acid are mixed in a volume ratio of 1:1 for reaction for 2-3 hours, then the sulfuric acid is washed away to obtain acidified diatomite, then sodium aluminate, sodium hydroxide and deionized water are added in sequence, mixed and blended, roasted, cooled and ground, deionized water is added to make the water-cement ratio 8:1, aged for 24 hours at room temperature with stirring at 500-600r / min, then crystallized at 120°C for 2 hours, filtered, washed to a pH value of 8-9, dried at 100°C, and ground into powder to obtain the catalyst.

[0007] Furthermore, preferably, the weight ratio of the diatomaceous earth, sodium aluminate, sodium hydroxide and deionized water is 10:3:8:20.

[0008] The present invention also provides a method for recycling silicone rubber scraps, comprising the following steps:

[0009] (1) Grinding the silicone rubber scraps into particles of 50-80 mesh, then adding silicon dioxide and ferrous sulfate, grinding into particles with a particle size of 100-150 mesh, and setting aside;

[0010] (2) Add the crushed silicone rubber to a reactor and heat it to 100°C to vacuum dehydrate. After dehydration, add diatomaceous earth zeolite composite catalyst and sulfonic acid, stop vacuuming, and heat it to 120-150°C. The silicone scraps are slowly decomposed into crude DMC gas;

[0011] (3) The DMC gas is cooled and refluxed, and then crude oil and solid residue are obtained. The crude oil is precipitated, and the oil residue is removed. The oil residue is returned to step (2) to continue to participate in the reaction. The crude oil after separation of the oil residue enters the rearrangement reactor, and is dehydrated, rearranged with alkali, adsorbed on activated carbon, and distilled to obtain D4 and D5. D4 and the capping agent are injected into the silicone oil reactor, heated to 100°C for dehydration, and an alkaline catalyst is added to react for 3-4 hours. After the reaction is completed, it is introduced into a thin film evaporator for dehydration, and the temperature is maintained at 200°C to remove low molecular weight compounds until no fraction is distilled out, and then cooled to room temperature and filled to obtain the finished silicone oil;

[0012] (4) The solid residue is calcined at high temperature and impurities are removed to obtain white carbon black.

[0013] Furthermore, preferably: the amount of silicon dioxide added is 0.5-1% of the weight of the silicone rubber scraps, the amount of ferrous sulfate added is 0.03-0.05% of the weight of the silicone rubber scraps; the amount of diatomaceous earth zeolite composite catalyst added is 1-1.5% of the weight of the silicone rubber scraps, and the amount of sulfonic acid added is 0.3-0.5% of the weight of the silicone rubber scraps.

[0014] Further, preferably: the dehydration and alkali rearrangement are specifically as follows: the crude oil enters the rearrangement reactor, the temperature is raised to 100° C., the water is removed in vacuum, 1.5-2% of the weight of the crude oil is added with potassium hydroxide, the temperature is raised to 150° C., the unqualified products rearranged before 150° C. are released, the temperature is continued to be raised to 220° C., and the product DMC after 150° C. is collected until no fraction is distilled out.

[0015] Furthermore, preferably: the activated carbon adsorption is specifically as follows: adding 1% activated carbon by weight of DMC into DMC, stirring at 600r / min for 3 hours, filtering and transferring to a fine DMC storage tank.

[0016] Further, preferably: the capping agent is one or more of hexamethyldisiloxane, octamethyltrisiloxane and decamethyltetrasiloxane mixed in any ratio.

[0017] Furthermore, preferably: the alkaline catalyst is potassium hydroxide, and the addition amount is 2-3%.

[0018] Furthermore, preferably: the step (4) is specifically as follows: the solid residue is calcined at 800-900°C for 3-5h to reduce the black color to white, and impurities are removed with a sieve to obtain white carbon black.

[0019] Beneficial effects of the present invention:

[0020] The method of the present invention is used to treat silicone rubber scraps under mild reaction conditions and in a short time, and the DMC recovery rate is between 97.1% and 98.1%. Most of the additives of the present invention are silicon dioxide and diatomaceous earth, both of which are raw materials for white carbon black and can be converted into white carbon black under the action of high temperature without introducing too much waste, which makes subsequent treatment difficult. DETAILED DESCRIPTION

[0021] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The silicone rubber scraps used in the embodiment of the present invention are the scraps of key-type silicone rubber.

[0023] Example 1-11 Investigating the Effect of Silicon Dioxide and Ferrous Sulfate on the Decomposition of Silicone Rubber Scrap

[0024] A method for recycling silicone rubber scraps comprises the following steps:

[0025] (1) The silicone rubber scraps were crushed into particles of 50-80 mesh, and then silicon dioxide and ferrous sulfate were added and ground into particles of 100-150 mesh in size for later use. The specific amounts of silicon dioxide and ferrous sulfate added are shown in Table 1.

[0026] (2) Add the crushed silicone rubber particles to a reactor and heat to 100°C for vacuum dehydration. After dehydration, add 1.5% dodecylbenzenesulfonic acid and 4% 98% concentrated sulfuric acid, stop vacuuming, heat to 220°C, and the silicone scraps slowly decompose into crude DMC gas, and catalyze the reaction for 3 hours;

[0027] (3) The DMC gas is cooled and refluxed, and then crude oil and solid residue are obtained. The crude oil is precipitated, and the oil residue is removed. The oil residue is returned to step (2) to continue to participate in the reaction. The crude oil after the oil residue is separated enters the rearrangement reactor, the temperature is raised to 100° C., the water is removed in vacuo, and potassium hydroxide (2% by weight of the crude oil) is added, and the temperature is raised to 150° C. The unqualified products rearranged before 150° C. are discharged, and the temperature is continued to be raised to 220° C. The product DMC after 150° C. is collected until no fraction is distilled out;

[0028] Then, 1% activated carbon by weight of DMC is added to DMC, and the mixture is stirred at 600r / min for 3 hours, and filtered to a fine DMC storage tank; D4 and D5 are obtained by distillation, and D4 and hexamethyldisiloxane are injected into a silicone oil reactor, and the mixture is heated to 100°C for dehydration, and potassium hydroxide is added in an amount of 2%, and the mixture is reacted for 4 hours. After the reaction is completed, the mixture is introduced into a thin film evaporator for dehydration, and the temperature is maintained at 200°C to remove low molecular weight compounds until no fraction is distilled out, and then the mixture is cooled to room temperature and bottled to obtain finished silicone oil;

[0029] (4) The solid residue is subjected to high-temperature calcination and impurity removal to obtain white carbon black; the step (4) specifically comprises: the solid residue is calcined at 900° C. for 4 hours to reduce the black color to white, and the impurities are removed by a sieve to obtain white carbon black.

[0030] The preparation method of the diatomite zeolite composite catalyst is as follows: diatomite and concentrated sulfuric acid are mixed and reacted for 3 hours in a volume ratio of 1:1, then the sulfuric acid is washed away to obtain acidified diatomite, and then sodium aluminate, sodium hydroxide and deionized water are added in sequence, mixed and blended, roasted, cooled and ground, deionized water is added to make the water-cement ratio 8:1, aged for 24 hours at room temperature with stirring at 600r / min, then crystallized at 120°C for 2 hours, filtered, washed to a pH value of 8-9, dried at 100°C, and ground into powder to obtain the catalyst, wherein the weight ratio of diatomite, sodium aluminate, sodium hydroxide and deionized water is 10:3:8:20.

[0031] The DMC recovery rate was calculated, and the specific results are shown in Table 1. The DMC recovery rate calculation formula is as follows:

[0032] DMC recovery rate (%) = DMC liquid weight / silicone rubber scrap weight * 100;

[0033] DMC liquid refers to the product obtained by separating the crude oil from the oil residue and entering the rearrangement reactor, heating it to 100°C, and removing the water in vacuum. It is a colorless and transparent liquid.

[0034] Table 1 DMC recovery results of different embodiments

[0035] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Silicon dioxide / % 0 0.3 0.5 0.8 1.0 1.2 Ferrous sulfate / % 0 0 0 0 0 0 DMC recovery rate / % 56.8 63.4 71.6 73.5 74.2 74.5 Example 6 Example 7 Example 8 Example 9 Example 10 Embodiment 11 Silicon dioxide / % 0.8 0.8 0.8 0.8 0.8 0.8 Ferrous sulfate / % 0.01 0.02 0.03 0.04 0.05 0.06 DMC recovery rate / % 76.1 78.3 83.1 84.1 85.1 85.3

[0036] In the process of pulverizing silicone rubber waste, silicon dioxide and ferrous sulfate are added. As a carrier, silicon dioxide can effectively improve the grinding effect, making the rubber powder particles smaller, which is conducive to catalysis; on the other hand, it can effectively improve the dispersibility of ferrous sulfate catalyst and enhance the catalytic effect. Ferrous sulfate is a transition metal salt, which forms a synergistic effect with silicon dioxide catalyst to improve the activity and selectivity of the cracking reaction; at the same time, ferrous sulfate can form Fe3+ under acidic conditions, further promoting the breakage of rubber molecular chains, which can effectively increase the decomposition rate. At the same time, silicon dioxide can eventually be recycled as white carbon black without adding new impurities.

[0037] As shown in Table 1, compared with Example 1 in which no silicon dioxide and ferrous sulfate were added, the DMC recovery rate increased by 11.6%-31.1% when silicon dioxide was added alone; compared with the addition of silicon dioxide alone, the DMC recovery rate increased by 3.5%-16.0% when silicon dioxide and ferrous sulfate were added. Adding silicon dioxide and ferrous sulfate at the same time can effectively promote the decomposition of silicone rubber and improve the DMC recovery rate.

[0038] Examples 12-16 Investigate the effects of different catalysts on the decomposition of silicone rubber scraps

[0039] The method is basically the same as Example 1, except that: (1) the amount of silicon dioxide added is 0.8% and the amount of ferrous sulfate added is 0.05%. (2) The cracking catalyst is different, as shown in Table 2.

[0040] Table 2 DMC recovery results of different catalysts

[0041] Example 12 Example 13 Embodiment 14 Embodiment 15 Example 16 Dodecylbenzenesulfonic acid / % 0.5 0 0.3 0.5 0.8 98% concentrated sulfuric acid / % 1.5 0 0 0 0 Diatomite zeolite composite catalyst / % 0 2 1.7 1.5 1.2 Cracking temperature / ℃ 150 150 150 150 150 DMC recovery rate / % 51.3 75.2 93.4 97.8 96.5

[0042] The present invention adopts a diatomite zeolite composite catalyst as a main catalyst and sulfonic acid as an initiator, and has a good catalytic cracking effect: the zeolite in the diatomite zeolite composite catalyst has an acidic site, which can promote the breaking of the silicon-oxygen bond in the silicone rubber waste, thereby realizing the cracking of the waste, and these acidic sites accelerate the progress of the chemical reaction by providing an active center. Diatomite has good adsorption performance, can adsorb and fix the catalyst, improve the dispersibility and stability of the catalyst, and thus improve the catalytic efficiency. The diatomite zeolite composite catalyst combines the effects of acidic catalysis and physical adsorption, so that the catalytic cracking process has both the promoting effect of the chemical reaction and the strengthening effect of physical adsorption. The diatomite zeolite composite catalyst can significantly improve the cracking efficiency of silicone rubber waste, reduce the energy required for the reaction, and accelerate the reaction rate. Diatomite, as a carrier, can improve the thermal stability and mechanical strength of the zeolite catalyst and extend the service life of the catalyst. At the same time, diatomite is finally used as a raw material for white carbon black, and no new impurities will be added. Sulfonic acid is a strong acid with high acidity, which can provide protons to attack the silicon-oxygen bonds in the silicone rubber molecules, thereby promoting the breaking of these bonds. This acid catalysis is a key step in the cracking reaction. Sulfonic acid can achieve efficient cracking at lower temperatures, reducing the undesirable side reactions that may be caused by high temperatures. At the same time, sulfonic acid can also accelerate the reaction rate, thereby shortening the time required to reach reaction equilibrium.

[0043] As shown in Table 3, the method of the present invention, using a combination of diatomite zeolite composite catalyst and sulfonic acid, can achieve good catalytic cracking effect on the basis of a relatively low catalyst (2%). Compared with the combination of dodecylbenzenesulfonic acid and 98% concentrated sulfuric acid, the DMC recovery rate increased by 82.1-88.1%, and compared with the use of diatomite zeolite composite catalyst alone, the DMC recovery rate increased by 46.7%.

[0044] Examples 17-22 Investigating the Effect of Different Catalytic Temperatures on the Decomposition of Silicone Rubber Scrap

[0045] It is basically the same as Example 15, except that the pyrolysis temperature is different, as shown in Table 3.

[0046] Table 3 DMC recovery results of different catalysts

[0047] Embodiment 17 Embodiment 18 Embodiment 19 Embodiment 20 Embodiment 21 Embodiment 22 Cracking temperature / ℃ 100 110 120 130 140 150 DMC recovery rate / % 45.3 62.1 92.4 95.6 96.8 97.6

[0048] As shown in Table 3, the method of the present invention can achieve catalytic cracking of silicone rubber waste at a relatively low temperature. When the temperature is 120°C, the DMC recovery rate reaches 92.4%. As the temperature rises, the catalytic cracking speed becomes faster and the DMC recovery rate increases. When the temperature reaches 140-150°C, the DMC recovery rate does not change much. Therefore, the cracking temperature is more suitable between 120-150°C, and the optimal choice is 140°C.

[0049] Examples 23-28 Investigate the effect of different catalytic temperatures on the decomposition of silicone rubber scraps

[0050] Basically the same as Example 21, except that the lysis time is different, see Table 4 for details.

[0051] Table 4 DMC recovery results of different catalysts

[0052] Embodiment 23 Embodiment 24 Embodiment 25 Embodiment 26 Embodiment 27 Embodiment 28 Time / h 0 1 2 3 4 5 DMC recovery rate / % 0 56.8 91.5 97.9 98.1 98.3

[0053] As can be seen from Table 4, the method of the present invention can achieve catalytic cracking of silicone rubber waste in a relatively short time. When the temperature is 140°C, the DMC recovery rate reaches 56.8% at 1h. As time increases, the DMC recovery rate increases, reaching 91.5% at 2h and 97.9% at 3h. Thereafter, the DMC recovery rate has almost no significant change. Therefore, the cracking time is more suitable between 2-3h, and the optimal choice is 3h.

[0054] Embodiment 29

[0055] A method for recycling silicone rubber scraps comprises the following steps:

[0056] (1) The silicone rubber scraps are crushed into particles of 50-80 mesh, and then silicon dioxide and ferrous sulfate are added and ground into particles of 100-150 mesh in size for later use. The addition amount of silicon dioxide is 0.8% and the addition amount of ferrous sulfate is 0.05%.

[0057] (2) Add the crushed silicone rubber particles to a reactor and heat to 100° C. to vacuum dehydrate. After dehydration, add 0.5% dodecylbenzenesulfonic acid and 1.5% diatomaceous earth zeolite composite catalyst, stop vacuuming, heat to 140° C., and the silica gel scraps slowly decompose into crude DMC gas, and the catalytic reaction lasts for 3 hours;

[0058] (3) The DMC gas is cooled and refluxed, and then crude oil and solid residue are obtained. The crude oil is precipitated, and the oil residue is removed. The oil residue is returned to step (2) to continue to participate in the reaction. The crude oil after the oil residue is separated enters the rearrangement reactor, the temperature is raised to 100° C., the water is removed in vacuum, and potassium hydroxide (2% by weight of the crude oil) is added, and the temperature is raised to 150° C. The unqualified products rearranged before 150° C. are discharged, and the temperature is continued to be raised to 220° C. The product DMC after 150° C. is collected until no fraction is distilled out;

[0059] Then, 1% activated carbon by weight of DMC is added to DMC, and the mixture is stirred at 600r / min for 3 hours, and filtered to a fine DMC storage tank; D4 and D5 are obtained by distillation, and D4 and hexamethyldisiloxane are injected into a silicone oil reactor, and the mixture is heated to 100°C for dehydration, and potassium hydroxide is added in an amount of 2%, and the mixture is reacted for 4 hours. After the reaction is completed, the mixture is introduced into a thin film evaporator for dehydration, and the temperature is maintained at 200°C to remove low molecular weight compounds until no fraction is distilled out, and then the mixture is cooled to room temperature and bottled to obtain finished silicone oil;

[0060] (4) The solid residue is subjected to high-temperature calcination and impurity removal to obtain white carbon black; the step (4) specifically comprises: the solid residue is calcined at 900° C. for 4 hours to reduce the black color to white, and the impurities are removed by a sieve to obtain white carbon black.

[0061] The preparation method of the diatomite zeolite composite catalyst is as follows: diatomite and concentrated sulfuric acid are mixed and reacted for 3 hours in a volume ratio of 1:1, then the sulfuric acid is washed away to obtain acidified diatomite, and then sodium aluminate, sodium hydroxide and deionized water are added in sequence, mixed and blended, roasted, cooled and ground, deionized water is added to make the water-cement ratio 8:1, aged for 24 hours at room temperature with stirring at 600 r / min, then crystallized at 120°C for 2 hours, filtered, washed to a pH value of 8-9, dried at 100°C, and ground into powder to obtain the catalyst.

[0062] The above method was repeated 10 times, and the DMC recovery rate was between 97.1% and 98.1%. The method of the present invention has good stability.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for recycling silicone rubber scraps, characterized in that: The following steps are involved: (1) Grind the silicone rubber scraps into 50-80 mesh particles, then add silicon dioxide and ferrous sulfate, grind to particles with a particle size of 100-150 mesh, and set aside; (2) Add the crushed silicone rubber scraps into the reactor and heat it to 100°C to vacuum dehydrate. After dehydration, add diatomaceous earth zeolite composite catalyst and sulfonic acid, stop vacuuming, and heat it to 120-150°C. The silicone rubber scraps are slowly decomposed into crude DMC gas; (3) The DMC gas is cooled and refluxed to obtain crude oil and solid residue. The crude oil is precipitated and the oil residue is removed. The oil residue is returned to step (2) to continue to participate in the reaction. The crude oil after separation of the oil residue enters the rearrangement reactor. After dehydration, alkali rearrangement, activated carbon adsorption, and distillation, D4 and D5 are obtained. D4 and the capping agent are injected into the silicone oil reactor and heated to 100°C for dehydration. An alkaline catalyst is added to react for 3-4 hours. After the reaction is completed, it is introduced into a thin film evaporator for dehydration. The temperature is maintained at 200°C to remove low molecular weight compounds until no fraction is distilled out. Then it is cooled to room temperature and filled to obtain the finished silicone oil. (4) The solid residue is calcined at high temperature and impurities are removed to obtain white carbon black; The diatomite zeolite composite catalyst is prepared by: mixing diatomite and concentrated sulfuric acid in a volume ratio of 1:1 for reaction for 2-3 hours, then washing away the sulfuric acid to obtain acidified diatomite, then adding sodium aluminate, sodium hydroxide and deionized water in sequence, mixing and blending, roasting, cooling and grinding, adding deionized water to make the water-cement ratio 8:1, aging for 24 hours at room temperature under stirring at 500-600r / min, then crystallizing at 120°C for 2 hours, filtering, washing to a pH value of 8-9, drying at 100°C, and grinding into powder to obtain the catalyst; the weight ratio of the diatomite, sodium aluminate, sodium hydroxide and deionized water is: 10:3:8:

20.

2. A method for recycling silicone rubber scraps according to claim 1, characterized in that: The amount of silicon dioxide added is 0.5-1% of the weight of the silicone rubber scraps, the amount of ferrous sulfate added is 0.03-0.05% of the weight of the silicone rubber scraps; the amount of diatomaceous earth zeolite composite catalyst added is 1-1.5% of the weight of the silicone rubber scraps, and the amount of sulfonic acid added is 0.3-0.5% of the weight of the silicone rubber scraps.

3. The method for recycling silicone rubber scraps according to claim 1, characterized in that: The dehydration and alkali rearrangement are specifically as follows: the crude oil enters the rearrangement reactor, the temperature is raised to 100°C, the water is removed in vacuo, 1.5-2% of the weight of the crude oil is added with potassium hydroxide, the temperature is raised to 150°C, the unqualified products rearranged before 150°C are released, the temperature is continued to be raised to 220°C, and the product DMC after 150°C is collected until no fraction is distilled out.

4. The method for recycling silicone rubber scraps according to claim 1, characterized in that: The activated carbon adsorption is specifically as follows: adding activated carbon 1% by weight of DMC into DMC, stirring at 600r / min for 3 hours, and filtering to a fine DMC storage tank.

5. The method for recycling silicone rubber scraps according to claim 1, characterized in that: The capping agent is one or more of hexamethyldisiloxane, octamethyltrisiloxane and decamethyltetrasiloxane mixed in any ratio.

6. The method for recycling silicone rubber scraps according to claim 1, characterized in that: The alkaline catalyst is potassium hydroxide, and the addition amount is 2-3%.

7. The method for recycling silicone rubber scraps according to claim 1, characterized in that: The step (4) is specifically as follows: the solid residue is calcined at 800-900°C for 3-5h to reduce the black color to white, and impurities are removed with a sieve to obtain white carbon black.