Method for preparing high-whiteness cubic nano calcium carbonate at low temperature

Abstract

The invention discloses a method for preparing high-whiteness cubic nano calcium carbonate at low temperature. The method comprises the following steps: (1) performing low-temperature control on the whole carbonization process; (2) a carbonization-whitening integrated system is adopted, and a coumarin fluorescent whitening agent is added in the carbonization stage; the cubic nano calcium carbonate with high whiteness, high dispersion, uniform particle size and high crystal form regularity is prepared by the two means. Experimental results show that the carbonization stage is subjected to low-temperature control in the whole process, and the addition of the coumarin fluorescent whitening agent has remarkable beneficial effects of improving the whiteness, the crystal form regularity and the dispersity of the nano calcium carbonate and narrowing the particle size distribution. The cubic nano calcium carbonate crystal prepared by the method is regular in morphology and narrow in particle size distribution, and has high whiteness and high dispersity.

Description

Technical Field

[0001] This invention discloses a novel method for preparing cubic nano-calcium carbonate with high whiteness and excellent crystal morphology and dispersion at low temperature. Background Technology

[0002] Nano-calcium carbonate is an ultrafine inorganic filler with a particle size between 1 and 100 nm. It combines the conventional advantages of calcium carbonate with the unique surface effects, small size effects, and quantum size effects of nanomaterials. With the maturation of technologies such as critical carbonization, biotemplation, in-situ modification, and composite functionalization, it has been endowed with superior activity in terms of photothermal resistance, magnetic properties, mechanical properties, chemical properties, and catalytic properties, making it a novel inorganic functional material with broad application value in high-end application scenarios such as new energy, electronic information, biomedicine, high-grade paper, automotive paint, high-performance plastic products, and environmental governance.

[0003] Currently, the preparation methods for nano-calcium carbonate are mainly chemical methods, which include carbonation, metathesis, precipitation, emulsion, and sol-gel methods. Among these, carbonation is the mainstream industrial production process for nano-calcium carbonate, but it suffers from problems such as long carbonation time, poor quality stability, irregular shape, uneven particle size distribution, easy alkali return, and severe particle agglomeration, affecting the application of nano-calcium carbonate in downstream products. Therefore, there is an urgent need to further improve and optimize the production process and develop a new production process for nano-calcium carbonate with regular crystal form, narrow particle size distribution, and good dispersibility.

[0004] Whiteness is one of the key indicators for evaluating the quality of nano-calcium carbonate, affecting not only powder quality but also market price and downstream applications. Papermaking, water-based coatings, and toothpaste generally require a whiteness of ≥93%, while high-end art paper and UV wood coatings even require ≥96%. my country's mineral deposits are mainly low- to medium-grade limestone, with high levels of impurities such as Si, Mg, Fe, and Mn. Nano-calcium carbonate products produced using conventional carbonation processes from this type of lime have low whiteness and do not meet national standards. Currently, research on whitening nano-calcium carbonate is relatively limited. The commonly used whitening method is the redox method, but the strong reducing agents added in this process are prone to side reactions, leading to excessive consumption of reagents and affecting product quality. If the product is not washed and dried promptly, it can yellow. Therefore, new whitening technologies need to be developed. Summary of the Invention

[0005] To address the above problems, this invention proposes a novel method for preparing cubic nano-calcium carbonate with high whiteness and excellent crystal morphology and dispersion at low temperature, comprising the following steps: S1: Calcium oxide is obtained by high-temperature calcination of limestone; S2: Weigh a certain amount of calcium oxide and add it to water at 40~80℃ for digestion reaction. After the reaction is completed, it is matured to obtain calcium hydroxide slurry. S3: After the matured calcium hydroxide slurry is slurried, it is placed into the carbonization reactor. A crystal form control agent is added and stirred evenly. CO2 gas is then introduced. After the gelation phenomenon occurs, a dispersant is added to continue the reaction. In the later stage of the reaction, a whitening agent is added (i.e., in an alkaline or weakly alkaline environment, such as when the pH is around 7-12). The reaction ends when the pH drops below 6.5. S4: Heat the slurry from step S3 to 60~90℃, start stirring, and carry out the aging reaction under heat preservation conditions; S5: After the aging reaction is completed, a surfactant is added to activate the reaction, followed by filtration, washing, and drying to obtain high-whiteness cubic nano-calcium carbonate.

[0006] Preferably, in step S1, the limestone contains ≥85% calcium carbonate, ≤0.5% magnesium, and has a particle size of 15~30mm.

[0007] Preferably, in step S1, the limestone is calcined at a temperature of 950~1200℃ for 4~8 hours.

[0008] In step S2, the activity of calcium oxide is controlled at t 60 ≤60s, water-cement ratio of 6~10:1, aging temperature of 60~90℃, aging time of 8~16h.

[0009] Preferably, after digestion in step S2, the reaction slurry is sieved through an 80-mesh sieve to obtain primary calcium hydroxide slurry, and after maturation, it is sieved through a 200-mesh sieve to obtain secondary calcium hydroxide slurry.

[0010] The concentration of the calcium hydroxide slurry in step S3 is in the range of 4% to 12%, the initial carbonization temperature is 10 to 30°C, and the CO2 concentration is 30 to 40%.

[0011] Step S3 uses a high and low temperature cycle integrated machine (DC / GDX-10 / 30) to control the temperature throughout the process, ensuring that the maximum reaction temperature is ≤40℃, and uses a pH meter to monitor the carbonization reaction process throughout.

[0012] Preferably, in step S3, the stirring speed is 1000-2500 r / min and the CO2 gas flow rate is 1.0-6.0 ml / min; The crystal form control agent used in step S3 is either sucrose or glucose.

[0013] Preferably, the amount of crystal form control agent used in step S3 is 0.1% to 0.5% of the dry weight of calcium hydroxide.

[0014] The dispersant used in step S3 is at least one of the following sodium salts: trisodium citrate, sodium tartrate, sodium dodecylbenzenesulfonate, sodium hexametaphosphate, sodium pyrophosphate, sodium silicate, sodium alginate, sodium polyacrylate, sodium salt of styrene-maleic anhydride copolymer, and sodium carboxymethyl cellulose.

[0015] Preferably, the amount of dispersant used in step S3 is 0.1% to 4% of the dry weight of calcium hydroxide.

[0016] The coumarin-based fluorescent whitening agent used in step S3 is at least one of 7-amino-4-methylcoumarin, 7-diethylamino-4-methylcoumarin, 6,7-dihydroxycoumarin, 4,5,7-trihydroxy-3-phenylcoumarin, 3-cyano-7-hydroxy-4-methylcoumarin, 5,7-dihydroxy-4-methylcoumarin, and 5,7-dihydroxy-4-phenylcoumarin.

[0017] Preferably, the amount of whitening agent used in step S3 is 0.1% to 3% of the dry weight of calcium hydroxide.

[0018] The surfactant used in step S5 is a saponified sodium fatty acid / boronic acid ester composite modifier.

[0019] Preferably, the sodium fatty acid used in step S5 is at least one of sodium stearate, sodium arsenate, sodium oleate, sodium palmitate, and sodium myristate.

[0020] Preferably, the borate ester used in step S5 is at least one of diethanolamine borate, triphenyl borate, tris(trimethylsilyl)borate (TMSB), and distearate isopropyl borate (LD-100P).

[0021] Preferably, the amount of surfactant used in step S5 is 2% to 8% of the dry weight of calcium carbonate.

[0022] Preferably, the activation reaction temperature in step S5 is 80~90℃.

[0023] A novel method for preparing cubic nano-calcium carbonate with high whiteness, regular crystal shape, and excellent dispersibility at low temperature is disclosed. The method includes the aforementioned preparation method, which can obtain cubic nano-calcium carbonate with high whiteness, high dispersion, regular crystal shape, and narrow particle size distribution, thereby improving quality and efficiency.

[0024] A novel method for preparing cubic nano-calcium carbonate with high whiteness and excellent crystalline dispersion at low temperature, as described above, can be applied to high-end application scenarios such as new energy, electronic information, biomedicine, high-grade paper, automotive paint, high-performance plastic products and environmental governance.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention, based on the traditional industrial intermittent bubbling carbonization process, controls the entire carbonization process at low temperature and adds coumarin-based fluorescent whitening agents in the later stage of carbonization to achieve an integrated carbonization-whitening process.

[0026] The cubic nano-calcium carbonate crystals prepared by this invention have regular morphology and narrow particle size distribution, and also have high whiteness and high dispersibility. The whiteness is greater than 95%, the particle size distribution is narrow (controllable within 30-70 nm, and the particle size range is less than 20, and further less than 10), and the particles are uniformly dispersed and few in number at the same magnification, such as less than 10, less than 6, or less than 4. Attached Figure Description

[0027] Figure 1 The images are TEM images of different products, where a, b, c, d, e, and f are TEM images of Examples 1 to 6 at a magnification of 50 nm, respectively.

[0028] Figure 2 The images are TEM images of different products, where g, h, i, and j are TEM images of Comparative Example 1, Comparative Example 2, Comparative Example 3, and the domestically sold product Orchid SP200 at a magnification of 50 nm, respectively. Detailed Implementation

[0029] The present invention will now be described in more detail. The present invention can be implemented in various forms, and is not limited to the embodiments described herein.

[0030] Example 1 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 950℃ for 4 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 2500ml of 60℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 60℃ for 8 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the calcium hydroxide slurry concentration to 12%, put it into the carbonization reactor. Add 0.2% glucose by dry weight of calcium hydroxide, stir evenly, and turn on the high and low temperature integrated machine to 0℃ cold circulation. After the slurry temperature drops to 15℃, start to pass CO2 gas at a flow rate of 4.0 ml / min. After the gelation phenomenon occurs, add 1% trisodium citrate by dry weight of calcium hydroxide to the system to continue the reaction. When the reaction reaches 50% and the pH drops to 11-12, add 0.2% 7-amino-4-methylcoumarin and continue the reaction until the pH drops below 6.5, which indicates the end of the reaction. S4: Heat the slurry from step S3 to 60°C, start stirring, and age it for 2 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 2% sodium stearate and 2% diethanolamine borate by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0031] Example 2 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 3000ml of 60℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 70℃ for 12 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the concentration of calcium hydroxide slurry to 12%, put it into the carbonization reactor, add 0.2% sucrose by dry weight of calcium hydroxide, stir evenly, turn on the high and low temperature integrated machine to -10℃ cold circulation, after the slurry temperature drops to 20℃, start to pass CO2 gas, the gas flow rate is 4.0 ml / min, after the gel phenomenon occurs, add 2% sodium dodecyl sulfonate by dry weight of calcium hydroxide to the system to continue the reaction, when the reaction reaches 50% and the pH drops to 11-12, add 0.5% 7-amino-4-methylcoumarin until the pH drops below 6.5, indicating the end of the reaction; S4: Heat the slurry from step S3 to 60°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 3% sodium lauryl carbonate and 2% diethanolamine borate by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0032] Example 3 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 8 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 3500ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 12 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the calcium hydroxide slurry concentration to 10%, put it into the carbonization reactor. Add 0.2% sucrose by dry weight of calcium hydroxide, stir evenly, and turn on the high and low temperature integrated machine to -20℃ cold circulation. After the slurry temperature drops to 25℃, start to pass CO2 gas at a flow rate of 3.0 ml / min. After the gel phenomenon occurs, add 2% sodium polyacrylate by dry weight of calcium hydroxide to the system to continue the reaction. When the reaction reaches 70% and the pH drops to 7-9, add 0.5% 6,7-dihydroxycoumarin and continue the reaction until the pH drops below 6.5, which indicates the end of the reaction. S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 2% sodium palmitate and 2% tris(trimethylsilyl)borate (TMSB) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano-calcium carbonate.

[0033] Example 4 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 3500ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 12 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the calcium hydroxide slurry concentration to 10%, put it into the carbonization reactor, add 0.2% sucrose by dry weight of calcium hydroxide, stir evenly, turn on the high and low temperature integrated machine to -20℃ cold circulation, after the slurry temperature drops to 25℃, start to pass CO2 gas, the gas flow rate is 2.0 ml / min, after the gel phenomenon occurs, add 2% sodium hydroxymethyl cellulose by dry weight of calcium hydroxide to the system to continue the reaction. When the reaction reaches 80% and the pH drops to 8-10, add 1% 5,7-dihydroxy-4-phenylcoumarin and continue the reaction until the pH drops below 6.5, which indicates the end of the reaction; S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 2% sodium oleate and 2% tris(trimethylsilyl)borate (TMSB) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano-calcium carbonate.

[0034] Example 5 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 3500ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 12 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the concentration of calcium hydroxide slurry to 8%, put it into the carbonization reactor, add 0.4% sucrose by dry weight of calcium hydroxide, stir evenly, turn on the high and low temperature integrated machine to -20℃ cold circulation, after the slurry temperature drops to 25℃, start to pass CO2 gas, the gas flow rate is 2.0 ml / min, after the gel phenomenon occurs, add 2% sodium hydroxymethyl cellulose by dry weight of calcium hydroxide to the system to continue the reaction. When the reaction reaches 80% and the pH drops to 8-10, add 1% 7-diethylamino-4-methylcoumarin and continue the reaction until the pH drops below 6.5, which indicates the end of the reaction; S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 2% sodium palmitate and 2% distearate isopropyl borate (LD-100P) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0035] Example 6 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 4000ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 16 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the calcium hydroxide slurry concentration to 8%, put it into the carbonization reactor, add 0.4% sucrose by dry weight of calcium hydroxide, stir evenly, turn on the high and low temperature integrated machine to -20℃ cold circulation, after the slurry temperature drops to 20℃, start to pass CO2 gas, the gas flow rate is 2.0 ml / min, after the gel phenomenon occurs, add 2% sodium salt of styrene-maleic anhydride copolymer by dry weight of calcium hydroxide to the system to continue the reaction. When the reaction reaches 80% and the pH drops to 8-10, add 1% 7-diethylamino-4-methylcoumarin and continue the reaction until the pH drops below 6.5, which indicates the end of the reaction. S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 3% sodium stearate and 2% distearate isopropyl borate (LD-100P) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0036] To verify the effects of low-temperature control and coumarin-based brighteners on the whiteness, dispersibility, and crystal morphology of nano-calcium carbonate powder, the following three comparative cases were conducted based on Case 6: Comparative Example 1 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 4000ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 16 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the calcium hydroxide slurry concentration to 8%, put it into the carbonization reactor, add 0.4% sucrose by dry weight of calcium hydroxide, stir evenly, and after the slurry temperature drops to 20℃, start to pass CO2 gas at a flow rate of 2.0 ml / min. No temperature control is performed during the reaction. After the gelation phenomenon occurs, add 2% sodium salt of styrene-maleic anhydride copolymer by dry weight of calcium hydroxide to the system to continue the reaction until the pH drops below 6.5, which indicates the end of the reaction. S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 3% sodium stearate and 2% distearate isopropyl borate (LD-100P) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0037] Comparative Example 2 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 4000ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 16 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the concentration of calcium hydroxide slurry to 8%, put it into the carbonization reactor. Add 0.4% sucrose by dry weight of calcium hydroxide and stir evenly. After the slurry temperature drops to 20℃, start passing CO2 gas at a flow rate of 2.0 ml / min. No temperature control is performed during the reaction. After gelation occurs, add 2% sodium salt of styrene-maleic anhydride copolymer by dry weight of calcium hydroxide to the system and continue the reaction. When the reaction reaches 80% and the pH drops to 8-10, add 1% 7-diethylamino-4-methylcoumarin and continue the reaction. The reaction ends when the pH drops below 6.5. S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 3% sodium stearate and 2% distearate isopropyl borate (LD-100P) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0038] Comparative Example 3 S1: Take 1 kg of limestone and calcine it in a muffle furnace at 1000℃ for 6 hours to obtain calcium oxide; S2: Weigh 500g of calcium oxide and slowly add it to 4000ml of 70℃ water for digestion reaction. The reaction ends when the temperature of the slurry drops below 50℃. The slurry is then sieved through an 80-mesh sieve to obtain the first-grade calcium hydroxide slurry. After being kept at 80℃ for 16 hours, the slurry is sieved through a 200-mesh sieve to obtain the second-grade calcium hydroxide slurry. S3: After adjusting the calcium hydroxide slurry concentration to 8%, put it into the carbonization reactor, add 0.4% sucrose by dry weight of calcium hydroxide, stir evenly, turn on the high and low temperature integrated machine to -20℃ cold circulation, after the slurry temperature drops to 20℃, start to pass CO2 gas, the gas flow rate is 2.0 ml / min, after the gel phenomenon occurs, add 2% sodium salt of styrene-maleic anhydride copolymer by dry weight of calcium hydroxide to the system to continue the reaction until the pH drops below 6.5, which indicates the end of the reaction; S4: Heat the slurry from step S3 to 80°C, start stirring, and age it for 4 hours under heat preservation conditions; S5: After the aging reaction is completed, the slurry is heated to 80°C, and 3% sodium stearate and 2% distearate isopropyl borate (LD-100P) by dry weight of calcium carbonate are added for activation reaction. Then, the mixture is filtered, washed, and dried to obtain nano calcium carbonate.

[0039] Table 1 shows the whiteness test data of Examples 1-6, Comparative Examples 1-3, and the domestically available product Orchid SP200 nano-calcium carbonate powder.

[0040]

[0041] Currently, the whiteness of nano-calcium carbonate products in the domestic market is generally between 92% and 94%. Comparing the whiteness test data of Examples 1-6, Comparative Examples 1-3, and the domestically available Orchid SP200 nano-calcium carbonate powder in Table 1, it is evident that the whiteness of Comparative Examples 1, 3, and Orchid SP200 without added whitening agents is generally between 92% and 93%. However, the products of Examples 1-6 and Comparative Example 2 with added whitening agents exhibit high whiteness, with whiteness values ​​≥95%, and Example 6 even reaching a whiteness of 96.8%, showing a significant performance improvement. These data indicate that adding coumarin-based fluorescent whitening agents during the carbonization stage of nano-calcium carbonate has a significant and crucial impact on improving product whiteness. Comparing the whiteness data of Comparative Examples 1 and 3, and Comparative Examples 2 and 6, it was found that Comparative Examples 3 and 6, which underwent low-temperature control throughout the carbonization process, exhibited higher whiteness, with a whiteness increase of at least 1.5 points. This demonstrates that low-temperature control during the carbonization stage has a beneficial effect on improving whiteness.

[0042] In summary, the experimental results show that coumarin-based fluorescent whitening agents have an excellent effect on improving the whiteness of nano-calcium carbonate powder and play a key role. Combined with low-temperature control throughout the carbonization process, the whiteness of the product continues to improve.

[0043] Table 2 shows the TEM test data of Examples 1-6, Comparative Examples 1-3, and the domestically available product Orchid SP200 nano-calcium carbonate powder.

[0044]

[0045] Figure 1 a, b, c, d, e, f, g, h, i, and j are TEM images of Examples 1-6, Comparative Examples 1-3, and the domestically sold product Orchid SP200 at a magnification of 50 nm, respectively.

[0046] Combine Table 2 and Figure 1Comparing the microstructures of nano-calcium carbonate in Examples 1-6, Comparative Examples 1-3, and Orchid SP200, it is evident that Example 6 exhibits the best performance, displaying a regular cubic crystal morphology with clear boundaries, a smooth surface, high crystallinity, almost no crystal defects, large crystal size, narrow particle size distribution (controllable within 30-70 nm), and good dispersion. This is rare and exceptional for the current microstructure of nano-calcium carbonate in the domestic market, providing a solution and effective technical support for addressing the problems of irregular shape, uneven particle size distribution, and severe particle agglomeration in current domestic products. Comparing the TEM images of Comparative Examples 1 and 2 synthesized without temperature control, and Comparative Examples 3 and Examples 1-6 synthesized with low temperature control, the following two conclusions were drawn: (1) The crystal morphology of Comparative Examples 3 and Examples 1-6 synthesized with low temperature control was significantly better than that of Comparative Examples 1 and 2 synthesized without temperature control. In particular, the dispersibility of the products was significantly improved, and the regularity of the crystal form was improved and the particle size distribution was narrower. This confirms that the low temperature control of the entire carbonization process of nano-calcium carbonate has a key and decisive influence on the preparation of highly dispersible particles; (2) Under the same conditions, the cubic crystal form of Comparative Examples 2 and Examples 1-6 with added whitening agent was more regular, the particle size distribution was narrower, and the dispersion effect was better. This confirms that the addition of coumarin fluorescent whitening agent also has a good effect on promoting the directional growth of nano-calcium carbonate particles and improving the regularity of the crystal.

[0047] In summary, based on the whiteness data and crystal morphology analysis above, it is easy to see that in the production process of nano-calcium carbonate, the synergistic effect of low-temperature control during the carbonation stage and the addition of coumarin-based fluorescent whitening agents has outstanding advantages in improving product whiteness, dispersibility, crystal morphology regularity, and reducing particle size distribution, thus confirming the significant beneficial effects of this invention.

[0048] In summary, this is a novel method for preparing cubic nano-calcium carbonate with high whiteness and excellent crystal morphology and dispersion at low temperatures.

[0049] The nano-calcium carbonate prepared by this invention can be applied to high-end application scenarios such as new energy, electronic information, biomedicine, high-grade paper, automotive paint, high-performance plastic products and environmental governance.

[0050] This invention is only illustrated through the above-described embodiments to illustrate the detailed preparation process of the invention, but it is not limited to the fact that the invention must rely on the above-described detailed process to be implemented. Any improvements to the invention, equivalent substitutions of the raw materials of the product, addition of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of this invention.

Claims

1. A method for preparing high-whiteness cubic nano-calcium carbonate at low temperature, characterized in that, Includes the following steps: S1: Calcium oxide is obtained by high-temperature calcination of limestone; S2: Weigh a certain amount of calcium oxide and add it to water at 40~80℃ for digestion reaction. After the reaction is completed, it is matured to obtain calcium hydroxide slurry. S3: After the matured calcium hydroxide slurry is slurried, it is placed into the carbonization reactor, crystal control agent is added, and after stirring evenly, CO2 gas is introduced. After the gelation phenomenon occurs, dispersant is added to continue the reaction. Whitening agent is added in the later stage of the reaction. The reaction ends when the pH drops below 6.

5. S4: Heat the slurry from step S3 to 60~90℃, start stirring, and carry out the aging reaction under heat preservation conditions; S5: After the aging reaction is completed, a surfactant is added to activate the reaction, followed by filtration, washing, and drying to obtain high-whiteness cubic nano-calcium carbonate.

2. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 1, characterized in that, The concentration of calcium hydroxide slurry in step S3 is in the range of 4% to 12%, the initial carbonization temperature is 10 to 30°C, the CO2 concentration is 30 to 40%, the stirring speed is 1000 to 2500 r / min, the CO2 gas flow rate is 1.0 to 6.0 ml / min, and a high and low temperature circulation integrated machine is used for full-process temperature control to control the maximum reaction temperature to ≤40°C. A pH meter is used to monitor the reaction process throughout.

3. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 1, characterized in that, The crystal form control agent used in step S3 is either sucrose or glucose, and the amount used is 0.1% to 0.5% of the dry weight of calcium hydroxide.

4. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 1, characterized in that, The dispersant used in step S3 is at least one of trisodium citrate, sodium tartrate, sodium dodecylbenzene sulfonate, sodium hexametaphosphate, sodium pyrophosphate, sodium silicate, sodium alginate, sodium polyacrylate, sodium salt of styrene-maleic anhydride copolymer, and sodium carboxymethyl cellulose, and the amount used is 0.1% to 4% of the dry basis mass of calcium hydroxide.

5. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 1, characterized in that, The whitening agent used in step S3 is a coumarin-based fluorescent whitening agent, and the total amount used is 0.1% to 3% of the dry weight of calcium hydroxide.

6. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 5, characterized in that, Coumarin-based fluorescent whitening agents are at least one of 7-amino-4-methylcoumarin, 7-diethylamino-4-methylcoumarin, 6,7-dihydroxycoumarin, 4,5,7-trihydroxy-3-phenylcoumarin, 3-cyano-7-hydroxy-4-methylcoumarin, 5,7-dihydroxy-4-methylcoumarin, and 5,7-dihydroxy-4-phenylcoumarin.

7. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 1, characterized in that, The surfactant used in step S5 is a saponified sodium fatty acid compound borate ester modifier, and the amount used is 2% to 8% of the dry weight of calcium carbonate.

8. The method for preparing high-whiteness cubic nano-calcium carbonate at low temperature according to claim 7, characterized in that, The saponified sodium fatty acid used in step S5 is at least one of sodium stearate, sodium arsenate, sodium oleate, sodium palmitate, and sodium myristate; the borate ester used is at least one of diethanolamine borate, triphenyl borate, tris(trimethylsilyl)borate (TMSB), and distearyloxyisopropylborate (LD-100P).

9. A high-whiteness cubic nano-calcium carbonate, characterized in that, Including nano-calcium carbonate obtained by the preparation method according to any one of claims 1-8.

10. The application of the high-whiteness cubic nano-calcium carbonate of claim 9 in new energy, electronic information, biomedicine, high-grade paper, automotive paint, high-performance plastic products or environmental remediation.

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