Ultrafine nano calcium carbonate with particle size less than 20 nm based on acc precursor and preparation method thereof

Abstract

This invention relates to the field of nano-calcium carbonate preparation, specifically disclosing a method for preparing ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor, comprising the following steps: S1, preparing calcium hydroxide slurry from calcium oxide; S2, introducing CO2 gas into the calcium hydroxide slurry to carry out the reaction; S3, subjecting the calcium hydroxide slurry treated in step S2 to a first-stage cross-flow microfiltration treatment to obtain a first permeable phase and a first retained phase; S4, subjecting the first permeable phase obtained in step S3 to a second-stage cross-flow microfiltration treatment to obtain a second permeable phase and a second retained phase; S5, cyclic reaction; S6, carbonation reaction completed to obtain ultrafine nano-calcium carbonate with a particle size of less than 20 nm. This invention requires no complex additives and only requires process adjustments to prepare ultrafine nano-calcium carbonate products with regular crystal morphology, low agglomeration, high product purity, and a particle size of less than 20 nm.

Description

Technical Field

[0001] This invention relates to the field of calcium carbonate preparation technology, specifically to an ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor and its preparation method. Background Technology

[0002] Fumed silica can meet the reinforcement needs of high-end applications, but it suffers from drawbacks such as high processing difficulty, a tendency to cause severe agglomeration, and a decrease in substrate toughness due to high filler modulus. Nano-calcium carbonate, on the other hand, has advantages such as good processing fluidity, a much lower tendency to agglomerate than fumed silica, and low substrate modulus with good toughness retention after filling. If nano-calcium carbonate can be prepared with a particle size of less than 20 nm and a specific surface area greater than 95 m², it could be a significant breakthrough. 2 / g of nano-calcium carbonate is expected to be a better replacement for fumed silica, while addressing the processing and performance limitations in fumed silica applications.

[0003] Currently, the particle size of commonly available nano-calcium carbonate on the market is mostly between 50nm and 100nm, with a specific surface area of ​​only 16 to 40m². 2 The specific surface area of ​​calcium carbonate is much lower than that of fumed silica, therefore it can only be used as a semi-reinforcing filler and cannot replace fumed silica. Existing technologies have also conducted extensive research on calcium carbonate products with particle sizes less than 20 nm.

[0004] For example, Chinese invention patent CN112479242A discloses a method for preparing nano-calcium carbonate with a particle size of less than 20 nm, including the following steps: first, preparing a crude calcium hydroxide slurry; then, sieving and aging the crude calcium hydroxide slurry to obtain a refined calcium hydroxide slurry; then, subjecting the refined calcium hydroxide slurry to multiple carbonization processes, followed by dehydration and drying to obtain nano-calcium carbonate with a particle size of less than 20 nm. However, this technology relies on additives such as hydrolyzed polyacrylonitrile amine salt to reduce viscosity and inhibit crystal growth during multiple carbonization processes, which not only introduces impurities and results in low purity, but also leads to severe agglomeration of the prepared nano-calcium carbonate in practical applications.

[0005] Furthermore, Chinese invention patent CN111606344A discloses a method for preparing high specific surface area nano-calcium carbonate, including the following steps: adding urea and ammonia to a calcium hydroxide suspension to obtain slurry A; heating slurry A and then introducing a mixture of carbon dioxide and nitrogen gas; after reaction, slurry B is obtained; slurry B is further heated and a composite modifier is added for heat preservation reaction to obtain a nano-calcium carbonate suspension; finally, the nano-calcium carbonate suspension is filtered and freeze-dried to obtain nano-calcium carbonate. This method claims to be able to prepare ultrafine calcium carbonate with a particle size of less than 20 nm. However, due to the high pH value of the calcium hydroxide suspension and the weakly alkaline nature of the urea raw material solution, if ammonia is added to lower the pH value of the slurry to 8-9 in actual production, the mass fraction of the calcium hydroxide suspension must be maintained at a low level, which will lead to low production efficiency and prevent industrial-scale production, thus limiting its practical application.

[0006] Therefore, the present invention aims to develop an ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor and a preparation method thereof, so as to prepare a high-purity, high-quality ultrafine nano-calcium carbonate product with a particle size of less than 20 nm suitable for industrial mass production. Summary of the Invention

[0007] The technical problem solved by this invention is to provide an ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor and a preparation method thereof, so as to prepare a high-purity, high-quality ultrafine nano-calcium carbonate product with a particle size of less than 20 nm that is suitable for industrial mass production.

[0008] The technical problem solved by this invention is achieved by the following technical solution:

[0009] The preparation method of ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor includes the following steps:

[0010] S1. Prepare calcium oxide with a BET specific surface area of ​​30-40 m². 2 / g of calcium hydroxide slurry;

[0011] S2. Introduce CO2 gas into the calcium hydroxide slurry and monitor the conductivity of the calcium hydroxide slurry in real time. When the conductivity of the calcium hydroxide slurry shows a first decrease and then recovers, stop introducing CO2 gas.

[0012] S3. The calcium hydroxide slurry treated in step S2 is subjected to a first-stage cross-flow microfiltration treatment to obtain a first permeable phase and a first retained phase.

[0013] S4. The first permeable phase prepared in step S3 is subjected to a second-stage cross-flow microfiltration treatment to obtain a second permeable phase and a second retained phase. The second retained phase is the ACC precursor.

[0014] S5. Mix the first truncated phase from step S3 and the second permeated phase from step S4 to prepare a calcium hydroxide slurry. Then repeat steps S2, S3, and S4 in sequence to obtain the ACC precursor solution, and cycle 3 to 5 times. Combine the ACC precursor solution truncated in the cycle steps and the ACC precursor solution truncated in step S4 for later use.

[0015] S6. After merging the first truncated phase and the second permeated phase obtained in the last cycle of step S5, adjust the slurry, then introduce CO2 gas and monitor the conductivity of the slurry in real time. When the conductivity of the slurry also shows the first decrease and then recovers, stop the gas supply immediately. Add the ACC precursor merged in step S5 to the slurry, and then continue to introduce CO2 gas to carry out the carbonation reaction. After the carbonation reaction is completed, ultrafine nano calcium carbonate with a particle size of less than 20nm is obtained.

[0016] Further, in step S1, the calcium oxide is a highly active calcium oxide with an MgO content ≤0.8%; after the calcium oxide reacts with water to obtain a crude calcium hydroxide slurry, the crude calcium hydroxide slurry is purified and aged to obtain the calcium hydroxide liquid.

[0017] Furthermore, in step S1, the activity of the highly active calcium oxide is 320~380 ml / 4 mol / L.

[0018] Furthermore, in step S2, the calcium hydroxide slurry is adjusted to a mass fraction of 10-12% before CO2 gas is introduced, with a CO2 volume concentration of 28-33%; the temperature is maintained at 12-18℃ during the reaction.

[0019] Furthermore, in step S2, the calcium hydroxide slurry is 7~9 ml. 3 The CO2 gas injection rate is 2000~3000m. 3 / h.

[0020] Furthermore, in steps S3 and S4, the temperature during the microfiltration process is 12~18℃.

[0021] Furthermore, in step S3, the first-stage cross-flow microfiltration treatment uses a 1µm polypropylene hollow fiber microfiltration membrane system with an operating pressure of 0.05~0.1MPa and a cross-flow rate of 0.8~1.0m / s.

[0022] Furthermore, in step S4, the second-stage cross-flow microfiltration treatment uses a 100kDa polyvinylidene fluoride ultrafiltration membrane, with an operating pressure of 0.3~0.5MPa and a cross-flow rate of 1.0~1.2m / s.

[0023] Furthermore, in step S6, the slurry concentration after conditioning is 8-10%; the CO2 gas introduction rate is 1000-1200 m / s. 3 / h; the temperature is maintained at 12~18℃ during the reaction.

[0024] A method for preparing ultrafine nano-calcium carbonate with a particle size of less than 20 nm, comprising the method described above, wherein the nano-calcium carbonate has a particle size of less than 20 nm and a specific surface area greater than 100 m². 2 / g, purity greater than 97%.

[0025] Beneficial effects: The ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor and the preparation method described in this invention can achieve the separation, enrichment and transformation of ACC (amorphous calcium carbonate) by adjusting the process, thereby preparing ultrafine nano-calcium carbonate products with regular crystal morphology, less agglomeration, high product purity and particle size of less than 20 nm.

[0026] The present invention relates to ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor and its preparation method. The preparation is simple, convenient, and the nodes are easy to control. It does not require the addition of a large amount of auxiliary agents, has low preparation cost, and is suitable for industrial mass production.

[0027] The ultrafine nano-calcium carbonate with a particle size of less than 20 nm described in this invention has a regular crystal morphology, minimal agglomeration, and a specific surface area greater than 100 m². 2 With a purity greater than 97%, it can replace fumed silica to a certain extent to meet the reinforcement needs of substrates in high-end fields, and has great practical value. Attached Figure Description

[0028] Figure 1 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Example 1 of the present invention.

[0029] Figure 2 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Example 2 of the present invention.

[0030] Figure 3 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Example 3 of the present invention.

[0031] Figure 4 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Example 4 of the present invention.

[0032] Figure 5 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Comparative Example 1 of the present invention.

[0033] Figure 6 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Comparative Example 2 of the present invention.

[0034] Figure 7 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Comparative Example 3 of the present invention.

[0035] Figure 8 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Comparative Example 4 of the present invention.

[0036] Figure 9 This is a scanning electron microscope image of the nano-calcium carbonate prepared in Comparative Example 5 of the present invention. Detailed Implementation

[0037] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0038] Example 1

[0039] The preparation method of ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor includes the following steps:

[0040] S1. Highly active calcium oxide with an MgO content of 0.6% and an activity of 350 ml / 4 mol / L was selected and added to tap water at 75°C at a mass ratio of 1:5 to obtain a high-temperature digestion reaction, resulting in a crude calcium hydroxide slurry. After impurity removal by a hydrocyclone separator, the crude calcium hydroxide slurry was sent to an aging tank for 6 hours to obtain a BET specific surface area of ​​35 m². 2 / g of calcium hydroxide slurry;

[0041] S2. After impurities are removed by passing the calcium hydroxide slurry through a 325-mesh vibrating screen, it is conveyed to a slurry mixing tank. Water is added to the mixing tank to dilute it to a mass fraction of 11%. Subsequently, the calcium hydroxide slurry is conveyed to a carbonization synthesis reactor, which is filled with 8m³ of water. 3 Calcium hydroxide slurry, at 2500m 3 The lime kiln gas with a volume concentration of 30% was introduced at a rate of / h, and the conductivity was monitored in real time. When the conductivity of the calcium hydroxide slurry showed a first decrease and then recovered, the gas supply was stopped immediately. The temperature was controlled at 15℃ throughout the process.

[0042] S3. The calcium hydroxide slurry treated in step S2 is passed into a first-stage cross-flow microfiltration device for first-stage cross-flow microfiltration treatment. The first-stage cross-flow microfiltration uses a 1µm polypropylene hollow fiber microfiltration membrane system. The operating pressure during microfiltration is 0.08MPa, the cross-flow rate is 0.9m / s, and the temperature is maintained at 15℃ during the treatment. After cross-flow, a first permeable phase and a first retained phase are obtained. The first permeable phase consists of ACC precursor and ionic state. The first retained phase of the mixed solution is calcium hydroxide solid filter cake, which is then transferred to the slurry mixing tank.

[0043] S4. The first permeable phase obtained in step S3 is introduced into a second-stage cross-flow microfiltration device for second-stage cross-flow microfiltration treatment. The second-stage cross-flow microfiltration device uses a 100kDa PVDF ultrafiltration membrane. Preferably, in this embodiment, a commercially available surface-inert modified PVDF ultrafiltration membrane is used. The operating pressure of the second-stage cross-flow microfiltration treatment is 0.4MPa, the cross-flow rate is 1.1m / s, and the temperature is maintained at 15℃ during operation. During the treatment, every 30 minutes of continuous operation, it is backwashed with 11℃ pure water for 1 minute to avoid clogging, and the backwash pressure is 0.2MPa. In this step, the second permeable phase obtained is ion-containing. The aqueous solution is used as the second permeate phase, which is then transferred to the mixing tank via a centrifugal pump. The second retained phase is the ACC precursor solution, which is transferred to the ACC storage tank via a shear-free diaphragm pump, then sealed and stored under nitrogen atmosphere at a constant temperature of 15°C to prevent the ACC precursor from converting into calcite calcium carbonate crystals.

[0044] S5. After the first truncated phase in step S3 and the second permeated phase in step S4 are mixed by high-speed shearing in a slurry mixing tank, water is added to dilute and adjust the slurry to a concentration of 11% calcium hydroxide slurry. The calcium hydroxide slurry is controlled at 15°C. Then, steps S2, S3 and S4 are repeated sequentially to obtain the ACC precursor solution, and the cycle is repeated 3 times in total. The ACC precursor solution truncated in the cycle steps and the ACC precursor solution truncated in step S4 are combined and stored in an ACC storage tank for later use.

[0045] S6. Combine the first retained phase and the second permeated phase obtained in the last cycle of step S5, and then shear at high speed. Add water to adjust the slurry concentration to obtain a 9% slurry. Control the slurry temperature at 15°C, and then transport the slurry to the carbonization synthesis reactor. Then, at 1100m... 3 Lime kiln gas with a volume concentration of 30% was introduced at a rate of / h, and the conductivity was monitored in real time. The gas supply was stopped immediately after the conductivity of the slurry showed its first decrease and then recovered. The ACC precursor solution from the ACC storage tank was added to the slurry, and then the gas supply was continued at 1100 m³ / h. 3 The lime kiln gas with a volume concentration of 30% is introduced at a rate of / h to carry out the carbonization reaction. The temperature is controlled at 15℃ throughout the process until the pH of the slurry reaches 6.8, at which point the carbonization reaction is complete. After the reaction is completed, the slurry is dehydrated and dried.

[0046] The scanning electron microscope image of the ultrafine nano-calcium carbonate product prepared in this embodiment is shown below. Figure 1 As shown, the particle size of the products is less than 20 nm, the average particle size is 18.2 nm, and the specific surface area is 102 m². 2 / g, purity 97.9%, product agglomeration is extremely slight.

[0047] Example 2

[0048] The preparation method of ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor includes the following steps:

[0049] S1. Highly active calcium oxide with an MgO content of 0.5% and an activity of 330 ml / 4 mol / L was selected and added to tap water at 80℃ at a mass ratio of calcium oxide to water of 1:4.5 for high-temperature digestion reaction to obtain a crude calcium hydroxide slurry. After impurity removal by a hydrocyclone separator, the crude calcium hydroxide slurry was sent to an aging tank for 5 hours to obtain a BET specific surface area of ​​32 m². 2 / g of calcium hydroxide slurry;

[0050] S2. After impurities are removed by passing the calcium hydroxide slurry through a 325-mesh vibrating screen, it is conveyed to a slurry mixing tank. Water is added to the mixing tank to dilute it to a mass fraction of 10%. Subsequently, the calcium hydroxide slurry is conveyed to a carbonization synthesis reactor, which is filled with 8m³ of water. 3 Calcium hydroxide slurry, at 2200m 3 The lime kiln gas with a volume concentration of 28% was introduced at a rate of / h, and the conductivity was monitored in real time. When the conductivity of the calcium hydroxide slurry showed a first decrease and then recovered, the gas supply was stopped immediately. The temperature was controlled at 13℃ throughout the process.

[0051] S3. The calcium hydroxide slurry treated in step S2 is passed into a first-stage cross-flow microfiltration device for first-stage cross-flow microfiltration treatment. The first-stage cross-flow microfiltration uses a 1µm polypropylene hollow fiber microfiltration membrane system. The operating pressure during microfiltration is 0.07MPa, the cross-flow rate is 0.85m / s, and the temperature is maintained at 13℃ during the treatment. After cross-flow, a first permeable phase and a first retained phase are obtained. The first permeable phase consists of ACC precursor and ionic state. The first retained phase of the mixed solution is calcium hydroxide solid filter cake, which is then transferred to the slurry mixing tank.

[0052] S4. The first permeable phase obtained in step S3 is introduced into a second-stage cross-flow microfiltration device for second-stage cross-flow microfiltration treatment. The second-stage cross-flow microfiltration device uses a 100kDa PVDF ultrafiltration membrane. Preferably, in this embodiment, a commercially available surface-inert modified PVDF ultrafiltration membrane is used. The operating pressure of the second-stage cross-flow microfiltration treatment is 0.35MPa, the cross-flow rate is 1.05m / s, and the temperature is maintained at 13℃ during operation. During the treatment, every 30 minutes of continuous operation, it is backwashed with 10℃ pure water for 1 minute to avoid clogging. The backwashing pressure is 0.2MPa. In this step, the second permeable phase obtained is ion-containing. The aqueous solution is used as the second permeate phase, which is then transferred to the mixing tank by a centrifugal pump. The second retained phase is the ACC precursor solution, which is transferred to the ACC storage tank by a shear-free diaphragm pump, and then sealed and stored under nitrogen gas. The storage conditions are a constant temperature of 13°C to prevent the ACC precursor from converting into calcite calcium carbonate crystals.

[0053] S5. After the first truncated phase in step S3 and the second permeated phase in step S4 are mixed by high-speed shearing in a slurry mixing tank, water is added to dilute and adjust the slurry to a concentration of 10% calcium hydroxide slurry. The calcium hydroxide slurry is controlled at 13°C. Then, steps S2, S3 and S4 are repeated sequentially to obtain the ACC precursor solution, and the cycle is repeated a total of 4 times. The ACC precursor solution truncated in the cycle steps and the ACC precursor solution truncated in step S4 are combined and stored in an ACC storage tank for later use.

[0054] S6. Combine the first retained phase and the second permeated phase obtained in the last cycle of step S5, and shear them at high speed. Add water to adjust the slurry concentration to obtain an 8% slurry. Control the slurry temperature at 13°C, and transport the slurry to the carbonization synthesis reactor. Then, at 1050m... 3 Lime kiln gas with a volume concentration of 28% was introduced at a rate of / h, and the conductivity was monitored in real time. The gas supply was stopped immediately after the conductivity of the slurry showed its first decrease and then recovered. The ACC precursor solution from the ACC storage tank was added to the slurry, and then the gas supply was continued at a rate of 1050 m³ / h. 3 The lime kiln gas with a volume concentration of 28% is introduced at a rate of / h to carry out the carbonization reaction. The temperature is controlled at 13℃ throughout the process until the pH of the slurry reaches 6.6, at which point the carbonization reaction is complete. After the reaction is completed, the slurry is dehydrated and dried.

[0055] The scanning electron microscope image of the ultrafine nano-calcium carbonate product prepared in this embodiment is shown below. Figure 2 As shown, the particle size of the products is less than 20 nm, the average particle size is 17.5 nm, and the specific surface area is 105 m². 2 / g, purity 98.9%, no obvious aggregation.

[0056] Example 3

[0057] The preparation method of ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor includes the following steps:

[0058] S1. Highly active calcium oxide with an MgO content of 0.7% and an activity of 370 ml / 4 mol / L was selected and added to tap water at 70°C at a mass ratio of calcium oxide to water of 1:5.5 for high-temperature digestion reaction to obtain a crude calcium hydroxide slurry. After impurity removal by a hydrocyclone separator, the crude calcium hydroxide slurry was sent to an aging tank for 7 hours to obtain a BET specific surface area of ​​38 m². 2 / g of calcium hydroxide slurry;

[0059] S2. After impurities are removed by passing the calcium hydroxide slurry through a 325-mesh vibrating screen, it is conveyed to a slurry mixing tank. Water is added to the mixing tank to dilute it to a mass fraction of 12%. Subsequently, the calcium hydroxide slurry is conveyed to a carbonization synthesis reactor, which is filled with 8m³ of water. 3 Calcium hydroxide slurry, at 2800m3 The lime kiln gas with a volume concentration of 32% was introduced at a rate of / h, and the conductivity was monitored in real time. When the conductivity of the calcium hydroxide slurry showed a first decrease and then recovered, the gas supply was stopped immediately. The temperature was controlled at 17℃ throughout the process.

[0060] S3. The calcium hydroxide slurry treated in step S2 is passed into a first-stage cross-flow microfiltration device for first-stage cross-flow microfiltration treatment. The first-stage cross-flow microfiltration uses a 1µm polypropylene hollow fiber microfiltration membrane system. The operating pressure during microfiltration is 0.09MPa, the cross-flow rate is 0.95m / s, and the temperature is maintained at 17℃ during the treatment. After cross-flow, a first permeable phase and a first retained phase are obtained. The first permeable phase consists of ACC precursor and ionic state. The first retained phase of the mixed solution is calcium hydroxide solid filter cake, which is then transferred to the slurry mixing tank.

[0061] S4. The first permeable phase obtained in step S3 is introduced into a second-stage cross-flow microfiltration device for second-stage cross-flow microfiltration treatment. The second-stage cross-flow microfiltration device uses a 100kDa PVDF ultrafiltration membrane. Preferably, in this embodiment, a commercially available surface-inert modified PVDF ultrafiltration membrane is used. The operating pressure of the second-stage cross-flow microfiltration treatment is 0.45MPa, the cross-flow rate is 1.15m / s, and the temperature is maintained at 17℃ during operation. During the treatment, every 30 minutes of continuous operation, it is backwashed with 12℃ pure water for 1 minute to avoid clogging. The backwashing pressure is 0.2MPa. In this step, the second permeable phase obtained is ion-containing. The aqueous solution is used as the second permeate phase, which is then transferred to the mixing tank via a centrifugal pump. The second retained phase is the ACC precursor solution, which is transferred to the ACC storage tank via a shear-free diaphragm pump, then sealed and stored under nitrogen atmosphere at a constant temperature of 17°C to prevent the ACC precursor from converting into calcite calcium carbonate crystals.

[0062] S5. After the first truncated phase in step S3 and the second permeated phase in step S4 are mixed by high-speed shearing in a slurry mixing tank, water is added to dilute and adjust the slurry to a concentration of 12% calcium hydroxide slurry. The calcium hydroxide slurry is controlled at 17°C. Then, steps S2, S3 and S4 are repeated sequentially to obtain the ACC precursor solution, and the cycle is repeated a total of 5 times. The ACC precursor solution truncated in the cycle steps and the ACC precursor solution truncated in step S4 are combined and stored in an ACC storage tank for later use.

[0063] S6. Combine the first retained phase and the second permeated phase obtained in the last cycle of step S5, and then shear at high speed. Add water to adjust the slurry concentration to obtain a 10% slurry. Control the slurry temperature at 17°C, and then transport the slurry to the carbonization synthesis reactor. Then, at 1150m... 3Lime kiln gas with a volume concentration of 32% was introduced at a rate of / h, and the conductivity was monitored in real time. The gas supply was stopped immediately after the conductivity of the slurry showed its first decrease and then recovered. The ACC precursor solution from the ACC storage tank was added to the slurry, and then the gas supply was continued at a rate of 1150m³ / h. 3 The lime kiln gas with a volume concentration of 32% is continuously introduced at a rate of / h to carry out the carbonization reaction. The temperature is controlled at 17℃ throughout the process until the pH of the slurry reaches 6.7, at which point the carbonization reaction is complete. After the reaction is completed, the slurry is dehydrated and dried.

[0064] The scanning electron microscope image of the ultrafine nano-calcium carbonate product prepared in this embodiment is shown below. Figure 3 As shown, the particle size of the products is less than 20 nm, the average particle size is 16.8 nm, and the specific surface area is 110 m². 2 / g, purity 98.8%, specific dispersibility.

[0065] Example 4

[0066] The preparation method of ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor includes the following steps:

[0067] S1. Highly active calcium oxide with an MgO content of 0.4% and an activity of 340 ml / 4 mol / L was selected and added to tap water at 85°C at a mass ratio of 1:4 to obtain a high-temperature digestion reaction, resulting in a crude calcium hydroxide slurry. After impurity removal by a hydrocyclone separator, the crude calcium hydroxide slurry was sent to an aging tank for 4 hours to obtain a BET specific surface area of ​​33 m². 2 / g of calcium hydroxide slurry;

[0068] S2. After impurities are removed by passing the calcium hydroxide slurry through a 325-mesh vibrating screen, it is conveyed to a slurry mixing tank. Water is added to the mixing tank to dilute it to a mass fraction of 11%. Subsequently, the calcium hydroxide slurry is conveyed to a carbonization synthesis reactor, which is filled with 8m³ of water. 3 Calcium hydroxide slurry, at 2600m 3 The lime kiln gas with a volume concentration of 31% was introduced at a rate of / h, and the conductivity was monitored in real time. When the conductivity of the calcium hydroxide slurry showed a first decrease and then recovered, the gas supply was stopped immediately. The temperature was controlled at 16℃ throughout the process.

[0069] S3. The calcium hydroxide slurry treated in step S2 is passed into a first-stage cross-flow microfiltration device for first-stage cross-flow microfiltration treatment. The first-stage cross-flow microfiltration uses a 1µm polypropylene hollow fiber microfiltration membrane system. The operating pressure during microfiltration is 0.06MPa, the cross-flow rate is 0.8m / s, and the temperature is maintained at 16℃ during the treatment. After cross-flow, a first permeable phase and a first retained phase are obtained. The first permeable phase consists of ACC precursor and ionic state. The first retained phase of the mixed solution is calcium hydroxide solid filter cake, which is then transferred to the slurry mixing tank.

[0070] S4. The first permeable phase obtained in step S3 is introduced into a second-stage cross-flow microfiltration device for second-stage cross-flow microfiltration treatment. The second-stage cross-flow microfiltration device uses a 100kDa PVDF ultrafiltration membrane. Preferably, in this embodiment, a commercially available surface-inert modified PVDF ultrafiltration membrane is used. The operating pressure of the second-stage cross-flow microfiltration treatment is 0.38MPa, the cross-flow rate is 1.1m / s, and the temperature is maintained at 16℃ during operation. During the treatment, every 30 minutes of continuous operation, it is backwashed with 11℃ pure water for 1 minute to avoid clogging, and the backwash pressure is 0.2MPa. In this step, the second permeable phase obtained is ion-containing. The aqueous solution is used as the second permeate phase, which is then transferred to the mixing tank via a centrifugal pump. The second retained phase is the ACC precursor solution, which is transferred to the ACC storage tank via a shear-free diaphragm pump, then sealed and stored under nitrogen atmosphere at a constant temperature of 16°C to prevent the ACC precursor from converting into calcite calcium carbonate crystals.

[0071] S5. After the first truncated phase in step S3 and the second permeated phase in step S4 are mixed by high-speed shearing in a slurry mixing tank, water is added to dilute and adjust the slurry to a concentration of 11% calcium hydroxide slurry. The calcium hydroxide slurry is controlled at 16°C. Then, steps S2, S3 and S4 are repeated sequentially to obtain the ACC precursor solution, and the cycle is repeated a total of 4 times. The ACC precursor solution truncated in the cycle steps and the ACC precursor solution truncated in step S4 are combined and stored in an ACC storage tank for later use.

[0072] S6. After merging the first retained phase and the second permeated phase obtained in the last cycle of step S5, the mixture is sheared at high speed, and water is added to adjust the slurry concentration to obtain a 9% slurry. The slurry temperature is controlled at 16°C, and the slurry is transported to the carbonization synthesis reactor, and then conveyed at 1080m... 3 Lime kiln gas with a volume concentration of 31% was introduced at a rate of / h, and the conductivity was monitored in real time. The gas supply was stopped immediately after the conductivity of the slurry showed its first decrease and then recovered. The ACC precursor solution from the ACC storage tank was added to the slurry, and then the gas supply was continued at a rate of 1080 m³ / h. 3 The lime kiln gas with a volume concentration of 31% is introduced at a rate of / h to carry out the carbonization reaction. The temperature is controlled at 16℃ throughout the process until the pH of the slurry reaches 6.9, at which point the carbonization reaction is complete. After the reaction is completed, the slurry is dehydrated and dried.

[0073] The scanning electron microscope image of the ultrafine nano-calcium carbonate product prepared in this embodiment is shown below. Figure 4 As shown, the particle size of the products is less than 20 nm, the average particle size is 18.0 nm, and the specific surface area is 10³ m². 2 / g, purity 97.9%, low degree of aggregation.

[0074] Compare with Example 1

[0075] The preparation method of nano-calcium carbonate using the technology described in CN112479242A includes the following steps:

[0076] Limestone with a magnesium oxide content of 0.15% is added to a vertical kiln and calcined at 1000℃ to obtain calcium oxide. Then, tap water at 55℃ is added at a mass ratio of calcium oxide to water of 1:5 to carry out a digestion reaction to obtain calcium hydroxide crude slurry.

[0077] The crude calcium hydroxide slurry was passed through vibrating screens of 120 mesh, 200 mesh and 325 mesh in sequence, and then aged for 36 hours to obtain refined calcium hydroxide slurry.

[0078] After adjusting the mass fraction of calcium hydroxide slurry to 13% and the temperature to 14℃, it was added to a carbonization kettle equipped with a cooling coil. Then, lime kiln gas generated from the vertical kiln was introduced, with a carbon dioxide volume concentration of 28% and a gas flow rate controlled at 4500 m³ / s. 3 / h, until the reaction system becomes a gel state, then stop the flow of lime kiln gas, and add 0.06% of hydrolyzed polyacrylonitrile amine salt of dry weight of calcium carbonate in the reaction system. The number average molecular weight of hydrolyzed polyacrylonitrile amine salt is 3500 g / mol. Continue stirring for 3h to reduce viscosity and obtain the first-stage calcium carbonate slurry.

[0079] The temperature of the first-stage calcium carbonate slurry was adjusted to 18℃, and then lime kiln gas was introduced. The volume concentration of carbon dioxide in the lime kiln gas was 28%, and the gas flow rate was controlled at 3000 m³ / s. 3 / h, until the reaction system becomes a gel state, then stop the flow of lime kiln gas, and add 0.02% of hydrolyzed polyacrylonitrile amine salt by dry weight of calcium carbonate in the reaction system. The number average molecular weight of hydrolyzed polyacrylonitrile amine salt is 3500 g / mol. Continue stirring for 1h to reduce viscosity and obtain the second-stage calcium carbonate slurry. Repeat the above steps once to obtain the final-stage calcium carbonate slurry.

[0080] The temperature of the final-stage calcium carbonate slurry was adjusted to 18℃, and then lime kiln gas was introduced. The volume concentration of carbon dioxide in the lime kiln gas was 28%, and the gas flow rate was controlled at 1200 m³ / s. 3 The process continues until the pH of the reaction system is below 7.5, at which point the flow of lime kiln gas is stopped, and the mixture is dehydrated and dried to obtain ultrafine calcium carbonate.

[0081] The scanning electron microscope image of the ultrafine nano-calcium carbonate product prepared in this comparative example is shown below. Figure 5 As shown, the product has an average particle size of 19.3 nm and a specific surface area of ​​98 m². 2 / g, purity 89.6%, product agglomeration is severe.

[0082] Compare with Example 2

[0083] In this comparative example, in step S1, low-activity calcium oxide with an MgO content of 0.6% and an activity of 280 ml / 4 mol / L was selected for the digestion reaction. The BET specific surface area of ​​the calcium hydroxide slurry obtained in this step was 22 m². 2 / g, the rest is the same as in Example 1.

[0084] The scanning electron microscope image of the ultrafine nano calcium carbonate product in this comparative example is shown below. Figure 6 As shown, the product has a particle size of 39.7 nm and a specific surface area of ​​63 m². 2 / g, purity 96.7%, no obvious aggregation.

[0085] Compare with Example 3

[0086] In this comparative example, step S5 is repeated once, and the rest is the same as in Example 1.

[0087] The scanning electron microscope image of the ultrafine nano calcium carbonate product in this comparative example is shown below. Figure 7 As shown, the product has an average particle size of 28.3 nm and a specific surface area of ​​86 m². 2 / g, purity 97.9%, no obvious aggregation.

[0088] Compare with Example 4

[0089] In this comparative example, the ACC precursor in step S4 was stored without nitrogen at 20.5°C, and the rest was the same as in Example 1.

[0090] The scanning electron microscope image of the ultrafine nano calcium carbonate product in this comparative example is shown below. Figure 8 As shown, the product has an average particle size of 48.0 nm and a specific surface area of ​​48 m². 2 / g, purity 97.4%, no obvious aggregation.

[0091] Compare with Example 5

[0092] In this comparative example, the initial reaction temperature in steps S2 and S6 was set to 20°C, and the process was not temperature-controlled; the rest was the same as in Example 1.

[0093] The scanning electron microscope image of the ultrafine nano calcium carbonate product in this comparative example is shown below. Figure 9 As shown, the product has an average particle size of 40.6 nm and a specific surface area of ​​56 m². 2 / g, purity 97.5%, no obvious agglomeration.

[0094] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing ultrafine nano calcium carbonate based on ACC precursor with a particle size less than 20 nm, characterized in that, The method comprises the following steps: S1, preparing calcium oxide into calcium hydroxide slurry with BET specific surface area of 30~40m 2 / g; S2, CO2 gas is introduced into the calcium hydroxide slurry, and the conductivity of the calcium hydroxide slurry is monitored in real time; when the conductivity of the calcium hydroxide slurry decreases for the first time and then recovers, the introduction of CO2 gas is stopped; S3, the calcium hydroxide slurry treated in step S2 is subjected to first cross-flow microfiltration treatment; the first cross-flow microfiltration treatment adopts a 1 µm polypropylene hollow fiber microfiltration membrane system, and the operation pressure is 0.05-0.1 MPa, and the cross-flow rate is 0.8-1.0 m / s; after the treatment, a first permeate phase and a first retentate phase are obtained; S4, the first permeate phase prepared in step S3 is subjected to second cross-flow microfiltration treatment; the second cross-flow microfiltration treatment adopts a 100 kDa polyvinylidene fluoride ultrafiltration membrane, and the operation pressure is 0.3-0.5 MPa, and the cross-flow rate is 1.0-1.2 m / s; after the treatment, a second permeate phase and a second retentate phase are obtained, and the second retentate phase is an ACC precursor; S5, the first retentate phase in step S3 and the second permeate phase in step S4 are mixed to prepare a calcium hydroxide slurry, and then steps S2, S3 and S4 are repeated in sequence to obtain an ACC precursor solution, and the cycle is repeated for 3-5 times; the ACC precursor solution retained in the cycle step and the ACC precursor solution retained in step S4 are combined and used as needed; S6, the first retentate phase and the second permeate phase obtained in the last cycle in step S5 are combined and then mixed to prepare a slurry, CO2 gas is introduced, and the conductivity of the slurry is monitored in real time; when the conductivity of the slurry decreases for the first time and then recovers, the introduction of CO2 gas is immediately stopped; the ACC precursor combined in step S5 is added to the slurry, and then CO2 gas is continuously introduced to perform a carbonization reaction; after the carbonization reaction is completed, ultrafine nano calcium carbonate with a particle size of less than 20 nm is obtained.

2. The method of claim 1, wherein the ACC precursor-based ultrafine nanoscale calcium carbonate has a particle size of less than 20 nm. In step S1, the calcium oxide is high-activity calcium oxide with a MgO content of ≤0.8%; after the high-activity calcium oxide is subjected to a digestion reaction with water, a calcium hydroxide crude slurry is obtained; the calcium hydroxide crude slurry is subjected to impurity removal and aging to obtain the calcium hydroxide slurry.

3. The method for preparing ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor according to claim 1, characterized in that, In step S1, the activity degree of the high-activity calcium oxide is 320-380 ml / 4 mol / L.

4. The method according to claim 1, wherein the ACC precursor-based ultrafine nano-sized calcium carbonate having a particle size of less than 20 nm is characterized in that, In step S2, after the calcium hydroxide slurry is mixed to a mass fraction of 10-12%, CO2 gas is introduced; the CO2 volume concentration of the CO2 gas is 28-33%; and the temperature is maintained at 12-18 ℃ during the reaction.

5. The method according to claim 4, wherein the ACC precursor-based ultrafine nano-sized calcium carbonate having a particle size of less than 20 nm is prepared by the process of: (1) preparing a solution of calcium ions and carbonate ions; (2) adding a dispersant to the solution; (3) adding a precipitant to the solution; and (4) drying the precipitate. In step S2, the introduction rate of the CO2 gas is 2000-3000 m³ / h.

6. The method of claim 1, wherein the ACC precursor-based ultrafine nano-sized calcium carbonate has a particle size of less than 20 nm. In steps S3 and S4, the temperature during the microfiltration treatment is 12-18 ℃.

7. The method for preparing ultrafine nano-calcium carbonate with a particle size of less than 20 nm based on ACC precursor according to claim 1, characterized in that, In step S6, after the slurry is mixed, the slurry concentration is 8-10%; the introduction rate of the CO2 gas is 1000-1200 m³ / h; and the temperature is maintained at 12-18 ℃ during the reaction.

8. A method for preparing ultrafine nano calcium carbonate having a particle size of less than 20 nm, characterized in that, The nano calcium carbonate is prepared by the method in any one of claims 1-7, has a particle size of less than 20 nm, a specific surface area of greater than 100 m² / g, and a purity of greater than 97%.

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