Method for preparing nano calcium carbonate and ammonium sulfate from byproduct gypsum

Abstract

The invention belongs to the technical field of solid waste resource utilization, and particularly relates to a method for preparing nano calcium carbonate and ammonium sulfate by using byproduct gypsum. The method comprises the following steps: (1) carrying out impurity removal pretreatment on byproduct gypsum, and then preparing gypsum slurry with the solid content of 10-30wt%; (2) ammonia gas is introduced into deionized water to be dissolved, and an ammonia water system is obtained; and (3) continuously introducing CO2 gas into an ammonia water system in a micro-nano bubble form, simultaneously continuously adding the gypsum slurry, carrying out a stirring reaction, carrying out solid-liquid separation after the reaction is completed, washing and drying a solid product to obtain nano calcium carbonate, and carrying out concentration and crystallization treatment on residual mother liquor to obtain an ammonium sulfate crystal product. According to the method, the CO2 gas in the micro-nano bubble form, the ammonia water solution and the gypsum slurry are subjected to in-situ one-step reaction to prepare the calcium carbonate, and a high-purity nano-scale calcium carbonate product can be obtained. The preparation process is simple and easy to control, and is easy for large-scale application.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to a method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum. Background Technology

[0002] By-product gypsum is a major industrial byproduct generated during the production of phosphorus chemicals, fluorine chemicals, and other chemicals. Its main component is calcium sulfate, and it often contains soluble salts, fine mud, and small amounts of impurities. Due to limited resource utilization pathways, by-product gypsum has long been disposed of through stockpiling or landfilling, which not only occupies a large amount of land resources but also poses environmental risks. Existing methods for preparing calcium carbonate from gypsum resources mostly involve the addition of carbonates or ammonium carbonate, which suffers from problems such as high raw material costs, high ion concentration in the reaction system, large particle size, wide distribution, and low purity of the generated calcium carbonate. For example, the invention patent with publication number CN 116986618 A obtains calcium carbonate and ammonium sulfate products by adding ammonium carbonate solution or ammonia water dropwise to waste gypsum solution and then introducing CO2 for stirring. However, the obtained calcium carbonate product is mainly micron-sized, and the product is prone to contain unreacted desulfurized gypsum, resulting in low product purity. The invention patent with publication number CN 111957281 A synthesizes calcium carbonate and ammonium sulfate rapidly using microwave energy, which can significantly increase the reaction rate and shorten the reaction time. However, this method requires a specially designed microwave reactor, and the reaction speed is too fast, the ion concentration is difficult to control, making it difficult to obtain nano-sized calcium carbonate products. In addition, it is easy to trap impurities, resulting in low purity.

[0003] Nano-calcium carbonate is typically prepared using organic crystal form control agents or complex processes, resulting in high costs and complicated post-processing, which hinders large-scale applications. Furthermore, ammonia and carbon dioxide have low utilization rates in traditional gas-liquid reactions and are prone to emission, impacting economic efficiency and environmental safety. Therefore, there is an urgent need for a new method that, under mild conditions, requires no external addition of solid ammonium carbonate or organic additives, and simultaneously achieves the resource utilization of by-product gypsum and the controllable preparation of nano-calcium carbonate. Summary of the Invention

[0004] In view of the shortcomings and deficiencies of the existing technology, the purpose of this invention is to provide a method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum includes the following steps:

[0007] (1) The by-product gypsum is pretreated to remove impurities and then prepared into a gypsum slurry with a solid content of 10~30wt%;

[0008] (2) Ammonia gas is dissolved in deionized water to obtain an ammonia-water system;

[0009] (3) CO2 gas is continuously introduced into the ammonia system in step (2) through micro-nano bubbles, and gypsum slurry from step (1) is continuously added and stirred. After the reaction is completed, solid and liquid are separated. The solid product is washed and dried to obtain nano calcium carbonate. The remaining mother liquor is concentrated and crystallized to obtain ammonium sulfate crystal product.

[0010] Furthermore, the impurity removal pretreatment in step (1) includes at least one of water washing and grinding and sieving.

[0011] Pretreatment such as washing, grinding and screening is used to remove soluble salts, fine mud and other impurities from the by-product gypsum, so as to obtain pretreated by-product gypsum raw materials and improve the quality of the subsequent nano-calcium carbonate and ammonium sulfate crystal products.

[0012] Further, the pH of the gypsum slurry in step (1) is adjusted to 6.0~6.5.

[0013] By controlling the pH of the gypsum slurry within the range of 6.0 to 6.5, a suitable reaction environment is provided for the subsequent precipitation reaction system, reducing the fluctuation range of the initial alkalinity of the system, which is conducive to the uniformity of the carbonate nucleation process, and ultimately achieving finer particle size and concentrated distribution of the product.

[0014] Furthermore, the ammonia gas mentioned in step (2) is dissolved in deionized water using a micro-nano bubble generator.

[0015] Introducing the gas through a micro-nano bubble generator can increase the dissolution rate and reduce ammonia gas escape.

[0016] Furthermore, in step (3), the amount of CO2 gas introduced is 1 to 1.2 times the molar amount of CaSO4 contained in the gypsum slurry, and the molar amount of ammonia in the ammonia water system is more than twice the amount of CO2 gas introduced.

[0017] By introducing an appropriate excess of CO2, the recovery rate of calcium ions from the by-product gypsum can be ensured. Controlling the molar amount of ammonia to more than twice the amount of CO2 introduced can prevent the formation of ammonium bicarbonate in the later stages, thus ensuring the nucleation efficiency of calcium carbonate. A small excess of ammonium carbonate has high solubility and is easily decomposed by heating, preventing co-crystallization during the subsequent ammonium sulfate concentration and crystallization process, thereby significantly improving the yield and purity of the ammonium sulfate crystal product. The remaining small amount of soluble impurities accumulates in the final concentrate.

[0018] Furthermore, the micro-nano bubble morphology described in step (3) is generated by a micro-nano bubble generator, and the average diameter of the micro-nano bubbles is 200~500nm.

[0019] By introducing CO2 gas in the form of micro-nano bubbles, the efficiency of gas-liquid mass transfer reaction can be improved. This invention achieves the technical effects of improving the efficiency of metathesis reaction, increasing CaCO3 nucleation efficiency, increasing the number of CaCO3 crystal nuclei, and reducing the particle size of CaCO3 precipitates.

[0020] Furthermore, the continuous introduction time of CO2 gas and the continuous addition time of gypsum slurry in step (3) are 15~60 min.

[0021] This invention utilizes the key conditions for obtaining high-purity nano-sized calcium carbonate products: the continuous introduction of CO2 gas in the form of micro-nano bubbles and the simultaneous addition of gypsum slurry for stirring. The principle is as follows: the continuously introduced CO2 gas in the form of micro-nano bubbles efficiently disperses in an ammonia-water system to generate ammonium carbonate in situ, which further reacts with calcium ions, thereby significantly improving the nucleation efficiency of CaCO3, increasing the number of CaCO3 crystal nuclei, and reducing the particle size of the CaCO3 precipitate. Simultaneously, maintaining a stable low concentration of CO2 during the reaction process avoids the formation of ammonium bicarbonate, improving the CaCO3 nucleation efficiency; it also inhibits the rapid growth of CaCO3 crystals, reducing the particle size of the CaCO3 precipitate and minimizing the inclusion of impurities, thus significantly improving product purity. The continuous addition of gypsum slurry for in-situ reaction reduces the content of insoluble calcium sulfate particles in the system, minimizing their impact on CaCO3 crystal precipitation (solid-phase nucleation) and preventing the coating of insoluble calcium sulfate particles. This improves the efficiency of the metathesis reaction, reduces the particle size of the CaCO3 precipitate, and increases the purity of the CaCO3 crystalline product.

[0022] The reaction process involved in this invention is as follows:

[0023] 2NH3 + CO2 + H2O = (NH4)2CO3;

[0024] CaSO4+(NH4)2CO3=(NH4)2SO4 +CaCO3↓.

[0025] Furthermore, the stirring rate of the stirring reaction in step (3) is 500~1000 r / min, and the reaction temperature is 20~30℃.

[0026] Furthermore, after the continuous introduction of CO2 gas and the continuous addition of gypsum slurry in step (3) are completed, the reaction is stirred for another 60 to 90 minutes.

[0027] Maintaining stirring conditions can prevent crystal growth. After adding CO2 gas and gypsum slurry, the reaction continues to allow the reaction system to age fully and complete the crystal transformation.

[0028] Furthermore, the average particle size of the nano-calcium carbonate in step (3) is 40~80 nm, and the purity is >98%.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] (1) This invention uses CO2 gas in the form of micro-nano bubbles to react with ammonia solution and gypsum slurry in an in-situ one-step reaction to prepare calcium carbonate, which can obtain high-purity (purity > 98%) nano-sized calcium carbonate products. Its preparation process is simple and easy to control, and is easy to scale up.

[0031] (2) The method of the present invention can simultaneously achieve efficient and high-quality conversion of by-product gypsum resources, and has significantly improved economic and environmental benefits. Attached Figure Description

[0032] Figure 1 The image shows the XRD pattern of the nano-calcium carbonate product obtained in Example 1. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0034] Example 1

[0035] (1) Nano-sized calcium carbonate and co-produced ammonium sulfate were prepared using gypsum dihydrate, a byproduct of phosphate chemical production, as raw material. Gypsum dihydrate, a byproduct of phosphate chemical production, was selected as raw material. Its main component was CaSO4·2H2O, with a CaSO4 mass fraction of approximately 92.4 wt% and a soluble impurity salt content of approximately 3.1 wt%. The byproduct gypsum was crushed and added to deionized water to prepare a gypsum slurry with a solid content of 20 wt%. After stirring at room temperature for 30 min, the slurry was sieved using a 200-mesh sieve to remove coarse particle impurities. Subsequently, the pH of the slurry was adjusted to 6.5, and the slurry was washed twice with water at a liquid-to-solid ratio of 5:1 each time. After washing, the soluble impurity content in the gypsum slurry was reduced to below 1.0 wt%, and the solid content was adjusted to 20 wt%.

[0036] (2) Add 5 L of deionized water to a closed reaction vessel, start the micro-nano bubble generator to introduce ammonia gas to dissolve it, and obtain an ammonia water system. The molar amount of ammonia gas in the ammonia water system is twice the molar amount of CaSO4 contained in the gypsum slurry.

[0037] (3) CO2 gas was continuously introduced into the ammonia system of step (2) through a micro-nano bubble generator. By adjusting the rotation speed of the micro-nano bubble generator, a CO2 micro-nano bubble dispersion system with an average bubble diameter of about 500 nm was formed in the system. At the same time, the gypsum slurry of step (1) was continuously added and stirred for reaction. The stirring speed was controlled at 500 r / min, the reaction temperature was 30℃, and the continuous introduction time of CO2 gas and gypsum slurry was 30 min. By controlling the introduction rate, the amount of CO2 gas introduced was 1 times the molar amount of CaSO4 contained in the gypsum slurry. After the introduction was completed, the stirring reaction was continued for 60 min. No obvious agglomeration or sedimentation phenomenon occurred in the system during the reaction. After the reaction was completed, solid-liquid separation was performed by high-speed centrifugation at 8000 rpm for 10 min. The obtained solid was washed twice with deionized water and dried at 60℃ for 12 h to obtain a white powdery nano-calcium carbonate product. The remaining mother liquor was analyzed to be an ammonium sulfate solution, and ammonium sulfate product was obtained by further concentration and crystallization.

[0038] The calcium carbonate product obtained in this embodiment was subjected to particle size analysis. The average particle size of the obtained calcium carbonate was approximately 80 nm, and the particle size distribution was concentrated. XRD analysis results ( Figure 1 The results indicate that the product is mainly nano-calcium carbonate. The product purity test result is 98.3%, and the product yield is 98.7% (based on calcium ion recovery rate).

[0039] The purity of the ammonium sulfate product obtained in this example was 98.6%, and the product yield was 91.5% (based on sulfate ion recovery rate).

[0040] Example 2

[0041] (1) Nano-sized calcium carbonate and ammonium sulfate were prepared using hemihydrate gypsum, a byproduct of chemical production, as raw material. Hemihydrate gypsum, a byproduct of fluorochemical production, was selected as raw material. Its main component is CaSO4·0.5H2O, with a CaSO4 mass fraction of approximately 89.6 wt%. The byproduct gypsum was wet-milled for 40 min to reduce its D50 particle size to below 15 μm. Then, deionized water was added to prepare a gypsum slurry with a solid content of 15 wt%. The slurry was heated to 40℃ and stirred. Under these conditions, it was washed once with hot water at a liquid-to-solid ratio of 6:1. After washing, the pH of the slurry was adjusted to 6.0, and the solid content was 15 wt%, resulting in a pretreated activated gypsum slurry.

[0042] (2) Add 6 L of deionized water to a closed reaction vessel, start the micro-nano bubble generator to introduce ammonia gas to dissolve it, and obtain an ammonia water system. The molar amount of ammonia gas in the ammonia water system is 2.5 times the molar amount of CaSO4 contained in the gypsum slurry.

[0043] (3) CO2 gas was continuously introduced into the ammonia system of step (2) through a micro-nano bubble generator. By adjusting the rotation speed of the micro-nano bubble generator, a CO2 micro-nano bubble dispersion system with an average bubble diameter of about 200 nm was formed in the system. At the same time, the gypsum slurry of step (1) was continuously added and stirred for reaction. The stirring rate was controlled at 800 r / min, the reaction temperature was 20℃, and the continuous introduction time of CO2 gas and gypsum slurry was 45 min. By controlling the introduction rate, the amount of CO2 gas introduced was 1.1 times the molar amount of CaSO4 contained in the gypsum slurry. After the introduction was completed, the stirring reaction was continued for 90 min to allow the reaction system to fully age and complete the crystal transformation. After the reaction was completed, solid-liquid separation was performed by membrane filtration. The filter cake was washed once with an ethanol-water mixed solution (volume ratio 1:1) and then vacuum dried at 50℃ to obtain nano-calcium carbonate product. The remaining mother liquor was analyzed to be ammonium sulfate solution, and ammonium sulfate product was obtained by further concentration and crystallization.

[0044] The calcium carbonate product obtained in this embodiment was analyzed by SEM and particle size analysis. The obtained calcium carbonate exhibited a needle-like morphology with an average particle size of approximately 40 nm. XRD analysis showed that the product was mainly nano-calcium carbonate. The product purity test result was 98.3%, and the product yield was 99.2% (based on calcium ion recovery rate).

[0045] The purity of the ammonium sulfate product obtained in this example was 98.7%, and the product yield was 92.2% (based on sulfate ion recovery rate).

[0046] Example 3

[0047] (1) Nano-sized calcium carbonate and co-produced ammonium sulfate were prepared using gypsum dihydrate, a byproduct of phosphate chemical production, as raw material. Gypsum dihydrate, a byproduct of phosphate chemical production, was selected as raw material. Its main component is CaSO4·2H2O, with a CaSO4 mass fraction of approximately 92.4 wt% and a soluble impurity salt content of approximately 3.1 wt%. The byproduct gypsum was crushed and added to deionized water to prepare a gypsum slurry with a solid content of 30 wt%. After stirring at room temperature for 30 min, the slurry was sieved using a 200-mesh sieve to remove coarse particle impurities. Subsequently, the pH of the slurry was adjusted to 6.5, and the slurry was washed twice with water at a liquid-to-solid ratio of 3:1 each time. After washing, the soluble impurity content in the gypsum slurry was reduced to below 1.0 wt%, and the solid content was adjusted to 30 wt%.

[0048] (2) Add 5 L of deionized water to a closed reaction vessel, start the micro-nano bubble generator to introduce ammonia gas to dissolve it, and obtain an ammonia water system. The molar amount of ammonia gas in the ammonia water system is 3 times the molar amount of CaSO4 contained in the gypsum slurry.

[0049] (3) CO2 gas was continuously introduced into the ammonia system of step (2) through a micro-nano bubble generator. By adjusting the rotation speed of the micro-nano bubble generator, a CO2 micro-nano bubble dispersion system with an average bubble diameter of about 300 nm was formed in the system. At the same time, the gypsum slurry of step (1) was continuously added and stirred for reaction. The stirring speed was controlled at 1000 r / min, the reaction temperature was 25℃, and the continuous introduction time of CO2 gas and gypsum slurry was 60 min. By controlling the introduction rate, the amount of CO2 gas introduced was 1.2 times the molar amount of CaSO4 contained in the gypsum slurry. After the introduction was completed, the stirring reaction was continued for 60 min to allow the reaction system to fully age and complete the crystal transformation. After the reaction was completed, solid-liquid separation was performed by high-speed centrifugation at 8000 rpm for 10 min. The obtained solid was washed twice with deionized water and dried at 60℃ for 12 h to obtain a white powdery nano-calcium carbonate product. The remaining mother liquor was analyzed to be an ammonium sulfate solution, and ammonium sulfate product was obtained by further concentration and crystallization.

[0050] The calcium carbonate product obtained in this embodiment was analyzed by particle size analysis. The average particle size of the obtained calcium carbonate was approximately 50 nm, and the particle size distribution was concentrated. XRD analysis results showed that the product was mainly nano-calcium carbonate. The product purity test result was 98.4%, and the product yield was 99.5% (based on calcium ion recovery rate).

[0051] The purity of the ammonium sulfate product obtained in this example was 98.2%, and the product yield was 93.0% (based on sulfate ion recovery rate).

[0052] Example 4

[0053] (1) Nano-sized calcium carbonate and ammonium sulfate were prepared using hemihydrate gypsum, a byproduct of chemical production, as raw material. Hemihydrate gypsum, a byproduct of fluorochemical production, was selected as raw material. Its main component is CaSO4·0.5H2O, with a CaSO4 mass fraction of approximately 89.6 wt%. The byproduct gypsum was wet-milled for 40 min to reduce its D50 particle size to below 15 μm. Then, deionized water was added to prepare a gypsum slurry with a solid content of 10 wt%. The slurry was heated to 40℃ and stirred. Under these conditions, it was washed once with hot water at a liquid-to-solid ratio of 7:1. After washing, the pH of the slurry was adjusted to 6.0, and the solid content was 10 wt%, resulting in a pretreated activated gypsum slurry.

[0054] (2) Add 6 L of deionized water to a closed reaction vessel, start the micro-nano bubble generator to introduce ammonia gas to dissolve it, and obtain an ammonia water system. The molar amount of ammonia gas in the ammonia water system is 2.4 times the molar amount of CaSO4 contained in the gypsum slurry.

[0055] (3) CO2 gas was continuously introduced into the ammonia system of step (2) through a micro-nano bubble generator. By adjusting the rotation speed of the micro-nano bubble generator, a CO2 micro-nano bubble dispersion system with an average bubble diameter of about 400 nm was formed in the system. At the same time, the gypsum slurry of step (1) was continuously added and stirred for reaction. The stirring rate was controlled at 600 r / min, the reaction temperature was 25℃, and the continuous introduction time of CO2 gas and gypsum slurry was 40 min. By controlling the introduction rate, the amount of CO2 gas introduced was 1.1 times the molar amount of CaSO4 contained in the gypsum slurry. After the introduction was completed, the stirring reaction was continued for 75 min to allow the reaction system to fully age and complete the crystal transformation. After the reaction was completed, solid-liquid separation was performed by membrane filtration. The filter cake was washed once with an ethanol-water mixed solution (volume ratio 1:1) and then vacuum dried at 50℃ to obtain nano calcium carbonate product. The remaining mother liquor was analyzed to be ammonium sulfate solution, and ammonium sulfate product was obtained by further concentration and crystallization.

[0056] The calcium carbonate product obtained in this embodiment was analyzed by particle size analysis. The average particle size of the obtained calcium carbonate was approximately 60 nm, and the particle size distribution was concentrated. XRD analysis results showed that the product was mainly nano-calcium carbonate. The product purity test result was 98.5%, and the product yield was 99.3% (based on calcium ion recovery rate).

[0057] The purity of the ammonium sulfate product obtained in this example was 98.5%, and the product yield was 92.7% (based on sulfate ion recovery rate).

[0058] Comparative Example 1

[0059] (1) The preparation process of gypsum slurry is the same as in Example 1.

[0060] (2) Add 5 L of deionized water to a closed reaction vessel, start the micro-nano bubble generator to introduce ammonia gas to dissolve it, and obtain an ammonia-water system. Then, introduce CO2 gas into the aqueous phase through the micro-nano bubble generator and react with ammonia to obtain an ammonium carbonate solution. The molar amount of ammonia gas introduced is twice the molar amount of CaSO4 contained in the gypsum slurry, and the molar amount of CO2 gas introduced is once the molar amount of CaSO4 contained in the gypsum slurry.

[0061] (3) The gypsum slurry from step (1) was continuously fed into the ammonium carbonate solution from step (2) for stirring and reaction. The stirring rate was controlled at 500 r / min, the reaction temperature at 30℃, and the gypsum slurry was continuously fed in for 30 min. After the feeding was completed, the stirring and reaction continued for 60 min. No obvious agglomeration or sedimentation was observed in the system during the reaction. After the reaction was completed, solid-liquid separation was performed by high-speed centrifugation at 8000 rpm for 10 min. The obtained solid was washed twice with deionized water and dried at 60℃ for 12 h to obtain a white powdery nano-calcium carbonate product. The remaining mother liquor was analyzed to be an ammonium sulfate solution, which was further concentrated and crystallized to obtain the ammonium sulfate product.

[0062] The calcium carbonate product obtained in this comparative example was tested by particle size analysis. The average particle size of the calcium carbonate was about 2.5 μm, the product purity was 95.1%, and the product yield was 99.4%.

[0063] The comparison results of Example 1 show that, compared to preparing ammonium carbonate solution first and then performing a metathesis reaction, the present invention uses CO2 gas in the form of micro / nanobubbles for an in-situ one-step reaction, which can significantly reduce the particle size and improve the purity of the calcium carbonate product. This is because the in-situ reaction of CO2 gas in the form of micro / nanobubbles can significantly improve the nucleation efficiency of CaCO3, increase the number of CaCO3 crystal nuclei, and thus reduce the particle size of the CaCO3 precipitate. Simultaneously, maintaining a stable low concentration of CO2 during the reaction process can inhibit the rapid growth of CaCO3 crystals, reducing the particle size of the CaCO3 precipitate while minimizing the inclusion of impurities, thereby significantly improving product purity.

[0064] Comparative Example 2

[0065] (1) The preparation process of gypsum slurry is the same as in Example 1.

[0066] (2) Add 5 L of deionized water to a closed reaction vessel, start the micro-nano bubble generator to introduce ammonia gas to dissolve it, and obtain an ammonia-water system. Then, introduce CO2 gas into the aqueous phase through the micro-nano bubble generator and react with ammonia to obtain an ammonium carbonate solution. The molar amount of ammonia gas introduced is twice the molar amount of CaSO4 contained in the gypsum slurry, and the molar amount of CO2 gas introduced is once the molar amount of CaSO4 contained in the gypsum slurry.

[0067] (3) The ammonium carbonate solution from step (2) was continuously introduced into the gypsum slurry from step (1) for stirring and reaction. The stirring rate was controlled at 500 r / min, the reaction temperature at 30℃, and the continuous introduction time of the ammonium carbonate solution was 30 min. After the introduction was completed, the stirring and reaction continued for 60 min. No obvious agglomeration or sedimentation occurred in the system during the reaction. After the reaction was completed, solid-liquid separation was performed by high-speed centrifugation at 8000 rpm for 10 min. The obtained solid was washed twice with deionized water and dried at 60℃ for 12 h to obtain a white powdery nano-calcium carbonate product. The remaining mother liquor was analyzed to be an ammonium sulfate solution, which was further concentrated and crystallized to obtain the ammonium sulfate product.

[0068] The calcium carbonate product obtained in this comparative example was tested by particle size analysis. The average particle size of the calcium carbonate was about 4.7 μm, the product purity was 90.4%, and the product yield was 99.5%.

[0069] The comparison results from Example 1 show that the reaction method of introducing ammonium carbonate solution into gypsum slurry results in a high content of insoluble calcium sulfate particles in the reaction system, which greatly affects the crystallization and precipitation process of calcium carbonate (CaCO3 crystal nuclei tend to concentrate on the surface of calcium sulfate solid particles). The calcium carbonate precipitate generated by the metathesis reaction tends to coat the calcium sulfate particles, thereby reducing the contact reaction efficiency. At the same time, the particle size of the obtained calcium carbonate product increases significantly, and the purity decreases significantly.

[0070] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum, characterized in that... Includes the following steps: (1) The by-product gypsum is pretreated to remove impurities and then prepared into a gypsum slurry with a solid content of 10~30wt%; (2) Ammonia gas is dissolved in deionized water to obtain an ammonia-water system; (3) CO2 gas is continuously introduced into the ammonia system in step (2) through micro-nano bubbles, and gypsum slurry from step (1) is continuously added and stirred. After the reaction is completed, solid and liquid are separated. The solid product is washed and dried to obtain nano calcium carbonate. The remaining mother liquor is concentrated and crystallized to obtain ammonium sulfate crystal product.

2. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: The impurity removal pretreatment in step (1) includes at least one of water washing and grinding and sieving.

3. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: The pH of the gypsum slurry in step (1) is adjusted to 6.0~6.

5.

4. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: The ammonia gas mentioned in step (2) is dissolved in deionized water using a micro-nano bubble generator.

5. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: In step (3), the amount of CO2 gas introduced is 1 to 1.2 times the molar amount of CaSO4 contained in the gypsum slurry, and the molar amount of ammonia in the ammonia water system is more than twice the amount of CO2 gas introduced.

6. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: The micro-nano bubble morphology described in step (3) is generated by a micro-nano bubble generator, and the average diameter of the micro-nano bubbles is 200~500nm.

7. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: The continuous CO2 gas introduction time and the continuous gypsum slurry addition time in step (3) are 15~60 min.

8. The method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 7, characterized in that: The stirring rate of the stirring reaction in step (3) is 500~1000 r / min, and the reaction temperature is 20~30℃.

9. A method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 8, characterized in that: After the continuous introduction of CO2 gas and the continuous addition of gypsum slurry are completed, continue stirring and reacting for 60-90 minutes.

10. A method for preparing nano-calcium carbonate and ammonium sulfate using by-product gypsum according to claim 1, characterized in that: The average particle size of the nano-calcium carbonate in step (3) is 40~80 nm, and the purity is >98%.

Images