A method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum.

Abstract

This invention provides a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 using desulfurized gypsum. The method includes the following steps: First, desulfurized gypsum is mixed with water to obtain a mixture. Ammonia and serine are added to the mixture to obtain a suspension. Second, CO2-containing flue gas is introduced into the bottom of the suspension until the pH of the reaction system reaches approximately 7. The flue gas is then stopped, and solid-liquid separation is performed to obtain a solid product containing a high proportion of aragonite-type calcium carbonate and an ammonium sulfate solution containing heavy metal ions. Finally, the heavy metals in the ammonium sulfate solution are removed, and crystallization is performed to obtain fertilizer-grade ammonium sulfate crystals. This invention features a simple process, yields high-purity aragonite-type calcium carbonate as the solid product, and the prepared ammonium sulfate crystals can be directly used as nitrogen fertilizer in agricultural production. This achieves efficient treatment and resource utilization of desulfurized gypsum, while also capturing and utilizing some CO2 from the flue gas.

Description

Technical Field

[0001] This invention belongs to the field of solid waste treatment technology, specifically relating to a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum. Background Technology

[0002] Desulfurization gypsum is a byproduct of the limestone-gypsum wet desulfurization process for coal-fired flue gas. Its main component is CaSO4·2H2O, and it is a typical industrial solid waste. Developing new technologies for the resource utilization of desulfurization gypsum from coal-fired power plants and broadening the pathways for its resource utilization are of great significance.

[0003] CO2 resource utilization technologies mainly include chemical conversion, biological conversion, energy development, and mineralization. Among these, mineralization refers to simulating the mineralization process in a natural environment, converting some industrial solid waste into stable carbonates through carbonation reactions. Using desulfurized gypsum to mineralize CO2 into calcium carbonate not only fixes CO2 in stable carbonates but also effectively reduces adverse environmental impacts, thus achieving the resource utilization of CO2.

[0004] Ammonium sulfate has long been used as a fertilizer in agricultural production. Its nitrogen content is 21.2%, which is significantly higher than that of ammonium bicarbonate (17.7% nitrogen). Compared to the unstable ammonium bicarbonate, ammonium sulfate has better chemical stability and is less prone to decomposition during application. Stable fertilizer effect means that it can provide crops with nutrients for a longer period of time, thus improving fertilizer utilization.

[0005] To address this, Chinese patent CN107583466A discloses a method for directly converting wet desulfurization gypsum into ammonium sulfate and calcium carbonate for recycling, thereby improving the treatment efficiency of desulfurization gypsum and realizing its resource-based recycling. Chinese patent CN112441607A discloses a method for recycling desulfurization gypsum. This method first adds water to the desulfurization gypsum to obtain a solid-liquid mixture of desulfurization gypsum and water. Ammonia water is then added, and CO2 is introduced. After stirring and reacting at room temperature and pressure, the solid precipitate is filtered and dried to obtain light calcium carbonate, which can be used as a raw material for flue gas desulfurization, with ammonium sulfate as a byproduct. This achieves comprehensive utilization of desulfurization gypsum while saving limestone required for flue gas desulfurization.

[0006] It should be noted that the above method has two shortcomings: First, the calcium carbonate crystal forms obtained from the mineralization reaction of desulfurized gypsum include calcite and aragonite crystal forms, but the proportion of aragonite crystal form is relatively low, and it is only used as a flue gas desulfurizing agent; while aragonite crystal form calcium carbonate has advantages over other crystal forms, such as large specific surface area, high dispersibility, uniform spherical size distribution, and porous or hollow structure, and has better prospects and application value in many fields. Second, the national standard GBT535-2020 for fertilizer-grade ammonium sulfate requires that the content of arsenic and cadmium elements be less than or equal to 10 mg / kg. The ammonium sulfate by-product of the mineralization reaction method provided by the above patent still contains some heavy metals carried by the desulfurized gypsum raw materials, which will lead to excessive heavy metal content in the ammonium sulfate fertilizer, which seriously limits the application of the by-product ammonium sulfate as fertilizer.

[0007] Based on this, a method for mineralizing CO2 using desulfurized gypsum is provided, which can effectively remove heavy metals from the by-product ammonium sulfate fertilizer while increasing the proportion of aragonite-type calcium carbonate in the product. This is of great significance for the high-value resource utilization of desulfurized gypsum and is also a technical problem that urgently needs to be solved. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizers using desulfurized gypsum to mineralize CO2.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum, comprising the following steps:

[0010] S1. Mix desulfurized gypsum with water to obtain a mixture. Add ammonia and serine to the mixture. The amount of serine added is 20% to 40% of the mass of desulfurized gypsum. Mix to obtain a suspension.

[0011] S2. Pass CO2-containing flue gas into the bottom of the suspension and react for a certain time until the pH of the reaction system is 6.7~7.2. Stop passing the flue gas and separate the reaction product into solid and liquid to obtain a solid product containing a high proportion of aragonite-type calcium carbonate and an ammonium sulfate solution containing heavy metal ions.

[0012] S3. Add FeSO4 and ammonia to the ammonium sulfate solution containing heavy metal ions to make the pH of the reaction system 8.0~8.5, separate the solid and liquid, and obtain the precipitate and the purified ammonium sulfate solution; the purified ammonium sulfate solution is crystallized to obtain fertilizer-grade ammonium sulfate crystals.

[0013] The general idea of ​​the method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 using desulfurized gypsum provided by this invention is as follows:

[0014] To increase the proportion of aragonite-type crystals in the calcium carbonate product, this invention adds serine as a crystal form regulator to the mixture of desulfurized gypsum and water. The addition of serine not only plays a leading role in increasing the proportion of aragonite-type calcium carbonate in the product, but also actively promotes the application of the ammonium sulfate crystals as fertilizer.

[0015] On the one hand, serine in the reaction system will electrostatically attract calcium ions from the ionization of CaSO4·2H2O, the main component of desulfurized gypsum, to form a certain complex, which will then react with carbonate ions to form aragonite-type calcium carbonate. However, with the increase of serine dosage, serine will competitively adsorb onto the reactants or reaction intermediates, reducing the calcium carbonate ions generated by the dissolution and ionization of carbon dioxide and the calcium ions ionized from CaSO4·2H2O. 2+ The resulting calcium carbonate precipitate reduces the purity of calcium carbonate in the solid product of the desulfurized gypsum mineralization reaction. Therefore, considering the above reasons, this invention determines that the amount of serine added should be 20% to 40% of the mass of the desulfurized gypsum.

[0016] On the other hand, serine is more soluble in water than other types of crystal form regulators, allowing it to enter ammonium sulfate crystals through the washing process of the product, thus playing a positive role as a fertilizer component. When ammonium sulfate carrying amino acids enters the soil, the amino acids can be directly absorbed by plant roots and soil microorganisms, serving as a nitrogen source and increasing soil enzyme activity. This plays a crucial role in improving soil nutrient composition, regulating soil microbial activity, and promoting plant growth, while also helping to alleviate soil acidification and inhibit salinization. Furthermore, serine plays a key role in plant growth, participating in cell tissue differentiation, promoting germination, and regulating various physiological behaviors of plants, including innate immunity, growth and development, and stress tolerance.

[0017] Further, in step S1, the desulfurized gypsum contains 20-25 mg / kg of cadmium and 20-25 mg / kg of arsenic. Typically, the heavy metals contained in desulfurized gypsum include chromium, arsenic, lead, nickel, and cadmium. Except for cadmium and arsenic ions, the content of other heavy metals is generally in the range of 4-5 mg / kg. Considering that the national standard GBT535-2020 for fertilizer-grade ammonium sulfate has relatively strict requirements for cadmium and arsenic ions (As≤10 mg / kg, Cd≤10 mg / kg), cadmium and arsenic ions require special treatment in this invention.

[0018] In step S3 of this invention, FeSO4 and ammonia are added to an ammonium sulfate solution containing heavy metal ions, and the pH of the reaction system is controlled within a suitable range to remove heavy metal ions from the ammonium sulfate crystals. Arsenic ions are primarily removed through ferrite and colloidal precipitation after the addition of FeSO4 to the system, while cadmium ions are removed by forming hydroxide precipitates through pH control. During this process, other heavy metal ions with lower concentrations (lead, chromium, etc.) are also removed through hydroxide precipitation.

[0019] Furthermore, in the desulfurized gypsum and water mixture of step S1, an increase in the solid-liquid ratio leads to a decrease in the amount of water, thereby reducing the volume of the mixture. Correspondingly, the exothermic energy per unit volume of solution also increases. Studies have found that in the reaction system of this invention, increasing the reaction temperature is detrimental to increasing the proportion of aragonite-type calcium carbonate. Preferably, in step S1, the mass ratio of desulfurized gypsum to water is controlled at 1:(5~6) to ensure a high proportion of aragonite-type calcium carbonate in the product.

[0020] Preferably, the desulfurized gypsum is pretreated before being mixed with water: the desulfurized gypsum is dried at 40-50°C for 12-24 hours to remove its free water, and then ground and sieved to obtain powdered desulfurized gypsum.

[0021] Furthermore, in step S1, the mass of NH3 contained in the ammonia water affects the reaction conversion rate of the desulfurized gypsum carbonation reaction. Appropriately adding excess ammonia water will promote the forward reaction and increase the purity of calcium carbonate in the product. Preferably, the mass ratio of desulfurized gypsum to NH3 contained in the ammonia water is 1:(1~1.5), and the mass concentration of the ammonia water is 25%~28%.

[0022] Preferably, in step S1, the serine is L-serine with a purity of not less than 99.9%.

[0023] Preferably, in step S2, the CO2-containing flue gas is introduced into the bottom of the suspension through an air bubble. Studies have shown that the placement of the air bubble ensures that the CO2-containing flue gas is introduced into the reaction solution system at a uniform speed, which further helps to increase the proportion of aragonite-type calcium carbonate in the generated product.

[0024] Further, in step S2, the mass ratio of desulfurized gypsum to CO2 in the flue gas is 1:(1.2~1.5). The mass of CO2 is calculated by multiplying the volume of the flue gas by the proportion of CO2 in the flue gas.

[0025] Furthermore, in the reaction system of step S2, as the reaction temperature increases, the rate of ion movement in the reaction solution system increases, while the solubility of CO2 in the solution system decreases, leading to an increase in CO3 in the system. 2- A decrease in concentration will prevent the reaction with serine to form a complex that can not be electrostatically attracted, thus preventing the formation of spheroidal calcium carbonate. Considering both energy consumption and experimental results, the optimal reaction temperature is 20-30°C.

[0026] Furthermore, in step S2, the reaction is carried out under stirring conditions. After carbon dioxide is introduced into the system through an air bubble stone, the stirring at a certain speed ensures that CO2 is introduced into the reaction solution more uniformly and at a more consistent rate. This facilitates the reaction between the serine complex and carbonate ions to form aragonite-type calcium carbonate, increasing the proportion of aragonite-type calcium carbonate in the product. In addition, the stirring rate primarily affects the carbonation reaction by accelerating the reaction rate; a lower stirring rate will correspondingly prolong the reaction time. However, an excessively high stirring rate can lead to ammonia evaporation and incomplete reaction, and does not guide the formation of a high proportion of aragonite-type calcium carbonate. Preferably, the stirring speed in step S2 is 800~1200 rpm.

[0027] Furthermore, step S2 also includes drying, crushing and sieving the solid product containing a high proportion of aragonite-type calcium carbonate.

[0028] In some preferred embodiments, the solid product obtained in step S2 contains calcium carbonate with a purity of 89%–91%, and the proportion of aragonite-type calcium carbonate in the calcium carbonate is 92%–96%. Compared to other crystal forms, aragonite-type calcium carbonate has a larger specific surface area, better dispersibility, and a more uniform spherical size distribution, giving it significant advantages in improving the filling performance of inks, plastics, paper, coatings, and other products, as well as enhancing product flowability and gloss. Furthermore, aragonite-type calcium carbonate is composed of porous or hollow micro / nano-scale particles with a larger specific surface area and excellent biodegradability and biocompatibility. Its decomposition is mild and its rate can be controlled, making it a safer drug carrier. It can also load a higher amount of drug compared to other crystal forms of calcium carbonate, and it is more competitive in terms of price compared to polymer nanoparticles, liposomes, and other carriers.

[0029] Furthermore, in step S3, the amount of FeSO4 added is controlled according to the arsenic ion content in the ammonium sulfate solution, so that the arsenic ions react with Fe... 2+ The mass ratio is 1:(2~4). In this step, arsenic ions are precipitated by ferrite, while cadmium ions and other heavy metal ions are removed by precipitation of hydroxides under pH conditions of 8.0~8.5.

[0030] Furthermore, in step S3, the crystallization process is carried out by evaporation crystallization at a temperature of 130~180℃, within which ammonium sulfate crystals will not undergo crystal transformation. Preferably, the evaporation crystallization temperature is 140~160℃.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] (1) The method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 using desulfurized gypsum provided by the present invention prepares aragonite-type calcium carbonate and fertilizer-grade ammonium sulfate through the mineralization reaction of desulfurized gypsum. The preparation process is simple and easy to operate. While increasing the proportion of aragonite-type calcium carbonate in the product, it effectively removes heavy metals from the by-product ammonium sulfate fertilizer. It is of great significance for the high-value resource utilization of desulfurized gypsum and the resource utilization and storage of CO2.

[0033] (2) The method provided by the present invention for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 using desulfurized gypsum directly uses power plant desulfurized gypsum as raw material, without the need for pretreatment steps, and the source of CO2 in the mineralization reaction is flue gas emitted by coal-fired power plants, which can effectively save costs, improve economic efficiency, and realize the capture and resource utilization of some CO2 in flue gas; the process conditions of this method are mild and can be carried out at room temperature and pressure, and the reaction liquid product can be used as ammonia fertilizer after heavy metal ion removal and evaporation crystallization, which has certain economic benefits and prospects for promotion and application. Attached Figure Description

[0034] Figure 1 This is a schematic flowchart of a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 using desulfurized gypsum, as provided by the present invention.

[0035] Figure 2 This is a SEM image of the aragonite-type calcium carbonate obtained in Example 1 of the present invention;

[0036] Figure 3 This is a SEM image of the calcium carbonate prepared in Comparative Example 1 of this invention. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0039] This invention provides a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum, as shown in the schematic diagram below. Figure 1 As shown, the method includes the following steps:

[0040] Step 1: Dry the desulfurization gypsum from the coal-fired power plant at 40-50℃ for 12-24 hours to remove free water, and grind it into powder; take a certain amount of desulfurization gypsum powder and mix it with water at a mass ratio of 1:(5-6) to obtain a mixture; add ammonia water with a mass concentration of 25%-28% and L-serine (analytical grade) to the mixture; the mass ratio of NH3 contained in the desulfurization gypsum and ammonia water is 1:(1-1.5); the amount of serine added is 20%-40% of the mass of desulfurization gypsum, and mix to obtain a suspension.

[0041] Step 2: Under normal pressure and a temperature of 20-30℃, control the magnetic stirring speed at 800-1200 rpm, and uniformly and evenly introduce CO2-containing flue gas into the bottom of the suspension through an air bubble stone. Allow the reaction to proceed for a certain time until the pH of the reaction system reaches 6.7-7.2, then stop introducing the flue gas. During the above reaction, the mass ratio of desulfurized gypsum to CO2 in the flue gas is controlled at 1:(1.2-1.5). Separate the reaction product into solid and liquid components to obtain a solid product containing a high proportion of aragonite-type calcium carbonate and an ammonium sulfate solution containing heavy metal ions. Dry, crush, and sieve the solid product containing a high proportion of aragonite-type calcium carbonate.

[0042] Step 3: Add FeSO4 and ammonia (mass concentration 25%~28%) to the ammonium sulfate solution containing heavy metal ions to adjust the pH of the reaction system to 8.0~8.5. The amount of FeSO4 added is determined based on the arsenic ion content in the ammonium sulfate solution, so that the arsenic ions react with FeSO4. 2+ The mass ratio is 1:(2~4), solid and liquid are separated to obtain precipitate and purified ammonium sulfate solution; the purified ammonium sulfate solution is evaporated and crystallized at 130~180℃ to obtain fertilizer grade ammonium sulfate crystals.

[0043] The present invention will be further described below with reference to specific embodiments, but these are not intended to limit the scope of the invention.

[0044] The desulfurized gypsum used in the embodiments and comparative examples of this invention is desulfurized gypsum produced by coal-fired power plants. It is dried at 40°C for 24 hours to remove free water, and then ground and sieved to obtain powdered desulfurized gypsum. The main chemical components of this desulfurized gypsum (by weight percentage) are: SO3 38%, CaO 36.4%, SiO2 2.65%. Testing showed that the cadmium content in this desulfurized gypsum is 23.25 mg / kg, and the arsenic content is 22.10 mg / kg.

[0045] Example 1

[0046] This invention provides a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum. The method includes the following steps:

[0047] Step 1: Take 20g of desulfurized gypsum and add 113mL of deionized water to prepare a solid-liquid mixture. Then add 19.8mL of ammonia water (25wt%) and 4g of L-serine (analytical grade) to the solid-liquid mixture and mix well to obtain a suspension.

[0048] Step 2: Under normal temperature (25℃) and pressure conditions, the suspension was thoroughly mixed by stirring at a rate of 1000 r / min, and then simulated coal-fired flue gas (15% carbon dioxide, 85% nitrogen) was introduced at a flow rate of 6 L / min. The introduction of simulated coal-fired flue gas was stopped when the pH of the solution reached 6.8. During the above reaction process, the mass ratio of desulfurized gypsum to CO2 in the flue gas was controlled at 1:1.2. The reaction products were filtered to obtain solid and liquid products. The solid product was aragonite-type calcium carbonate, which was dried at 40℃ and then crushed and sieved to obtain aragonite-type calcium carbonate samples of suitable particle size.

[0049] Step 3: Determine the arsenic ion content in the liquid product, according to the relationship between arsenic ions and Fe. 2+ The mass ratio is 1:2.2. Add an appropriate amount of FeSO4 to the liquid product and add a certain amount of ammonia to adjust the pH of the solution to 8.1, so that the heavy metal ions are converted into the corresponding hydroxides and precipitated. Then, the purified ammonium sulfate solution is obtained by solid-liquid separation. Then, the solution is evaporated and crystallized at 150℃ to obtain ammonium sulfate crystals. After drying at 60℃ for 3 hours, fertilizer-grade ammonium sulfate product is obtained.

[0050] Figure 2 This is a SEM image of the aragonite-type calcium carbonate obtained in this embodiment. Figure 2 It can be seen that the obtained spheroidal calcium carbonate exhibits a good spherical or ellipsoidal shape, with a relatively smooth surface, and no rhombic calcite-type calcium carbonate appears.

[0051] Example 2

[0052] This invention provides a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum. The method includes the following steps:

[0053] Step 1: Take 20g of desulfurized gypsum and add 120mL of deionized water to prepare a solid-liquid mixture. Then add 25mL of ammonia water (25wt%) and 6g of L-serine (analytical grade) to the solid-liquid mixture and mix well to obtain a suspension.

[0054] Step 2: Under normal temperature (25℃) and pressure conditions, the suspension was thoroughly mixed by stirring at a rate of 1200 r / min, and then simulated coal-fired flue gas (15% carbon dioxide, 85% nitrogen) was introduced at a flow rate of 7 L / min. The introduction of simulated coal-fired flue gas was stopped when the pH of the solution reached 6.9. During the above reaction process, the mass ratio of desulfurized gypsum to CO2 in the flue gas was controlled at 1:1.3. The reaction products were filtered to obtain solid and liquid products. The solid product was aragonite-type calcium carbonate, which was dried at 40℃ and then crushed and sieved to obtain aragonite-type calcium carbonate samples of suitable particle size.

[0055] Step 3: Determine the arsenic ion content in the liquid product, according to the relationship between arsenic ions and Fe. 2+ A suitable amount of FeSO4 was added to the liquid product at a mass ratio of 1:3, and a certain amount of ammonia water (mass concentration of 25wt%) was added to adjust the pH of the solution to 8.2, so that the heavy metal ions were converted into the corresponding hydroxides and precipitated. The purified ammonium sulfate solution was then obtained by solid-liquid separation. The solution was then evaporated and crystallized at 140℃ to obtain ammonium sulfate crystals. After drying at 70℃ for 2 hours, fertilizer-grade ammonium sulfate product was obtained.

[0056] Example 3

[0057] This invention provides a method for preparing high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum. The method includes the following steps:

[0058] Step 1: Take 20g of desulfurized gypsum and add 125mL of deionized water to prepare a solid-liquid mixture. Then add 27mL of ammonia water (25wt%) and 8g of L-serine (analytical grade) to the solid-liquid mixture and mix well to obtain a suspension.

[0059] Step 2: Under normal temperature (25℃) and pressure conditions, the suspension was thoroughly mixed by stirring at a rate of 900 r / min, and then simulated coal-fired flue gas (15% carbon dioxide, 85% nitrogen) was introduced at a flow rate of 6 L / min. The introduction of simulated coal-fired flue gas was stopped when the pH of the solution reached 7.0. During the above reaction process, the mass ratio of desulfurized gypsum to CO2 in the flue gas was controlled at 1:1.4. The reaction products were filtered to obtain solid and liquid products. The solid product was aragonite-type calcium carbonate, which was dried at 45℃ and then crushed and sieved to obtain aragonite-type calcium carbonate samples of suitable particle size.

[0060] Step 3: Determine the arsenic ion content in the liquid product, according to the relationship between arsenic ions and Fe. 2+The mass ratio is 1:3.5. Add an appropriate amount of FeSO4 to the liquid product and add a certain amount of ammonia water (mass concentration of 25wt%) to adjust the pH of the solution to 8.3, so that the heavy metal ions are converted into the corresponding hydroxides and precipitated. Then, the purified ammonium sulfate solution is obtained by solid-liquid separation. Then, the solution is evaporated and crystallized at 140℃ to obtain ammonium sulfate crystals. After drying at 100℃ for 1 hour, fertilizer-grade ammonium sulfate product is obtained.

[0061] Comparative Example 1

[0062] The difference between this comparative example and Example 1 is that in step 1, serine was not added as a crystal form regulator, while other conditions and operations remained unchanged.

[0063] Figure 3 This is a SEM image of the calcium carbonate product obtained in Comparative Example 1. Figure 3 It can be seen that the calcium carbonate products contain some aragonite-type calcium carbonate and rhombic calcite-type calcium carbonate, and the surface of the aragonite-type calcium carbonate is relatively rough, with certain impurities present.

[0064] Comparative Example 2

[0065] The difference between this comparative example and Example 1 is that in step 1, 4g of glutamine was used to replace serine as the crystal form regulator, while other conditions and operations remained unchanged.

[0066] Comparative Example 3

[0067] The difference between this comparative example and Example 1 is that in step 1, the amount of serine is adjusted to 10g, while other conditions and operations remain unchanged.

[0068] Comparative Example 4

[0069] The difference between this comparative example and Example 1 is that FeSO4 and ammonia were not added to the liquid product in step 3. Instead, the liquid product was directly evaporated and crystallized to obtain ammonium sulfate crystals. Other conditions and operations remained unchanged.

[0070] Tests and Results Discussion

[0071] The products calcium carbonate and ammonium sulfate obtained in the above embodiments and comparative examples were tested, and the results are shown in Table 1 below:

[0072] Table 1

[0073]

[0074] As can be seen from the above table,

[0075] Comparative document 1 did not add serine as a crystal form regulator, comparative document 2 used other types of crystal form regulators, and comparative document 3 used excessive serine. In all three comparative examples, the proportion of aragonite-type calcium carbonate in the final products was significantly lower than in Example 1. This indicates that in this invention, using an appropriate amount of serine as a crystal form regulator plays a dominant role in increasing the proportion of aragonite-type calcium carbonate in the product. Furthermore, comparative document 4 directly evaporated and crystallized ammonium sulfate solution, resulting in ammonium sulfate crystals with cadmium and arsenic ion contents of 15 mg / kg and 17 mg / kg, respectively, which do not meet the relevant requirements of the national standard GB / T535-2020 for fertilizer-grade ammonium sulfate.

[0076] In contrast, the calcium carbonate products obtained in Examples 1-3 of this invention have a purity of 89%~91%, with the proportion of aragonite-type calcium carbonate reaching 92%~96%. The ammonium sulfate crystals obtained also contain cadmium and arsenic content below 10 mg / kg, meeting the relevant requirements of the national standard GB / T535-2020 for fertilizer-grade ammonium sulfate. Furthermore, the serine carried in the ammonium sulfate crystals prepared by this invention can serve as a nitrogen source, synergistically working with fertilizer-grade ammonium sulfate to play a more positive role in improving soil enzyme activity, enhancing soil nutrient composition, and promoting plant growth and development.

[0077] The above are merely preferred embodiments of the present invention and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made based on the content of this specification should be included within the protection scope of the present invention.

Claims

1. A method for preparing a high-proportion aragonite-type calcium carbonate and ammonium sulfate fertilizer by mineralizing CO2 with desulfurized gypsum, characterized in that, Includes the following steps: S1. Mix desulfurized gypsum with water to obtain a mixture. Add ammonia and serine to the mixture. The amount of serine added is 20% to 40% of the mass of desulfurized gypsum. Mix to obtain a suspension. S2. Pass CO2-containing flue gas into the bottom of the suspension and react for a certain time until the pH of the reaction system is 6.7-7.

2. Stop passing the flue gas and separate the reaction product into solid and liquid to obtain a solid product containing a high proportion of aragonite-type calcium carbonate and an ammonium sulfate solution containing heavy metal ions. S3. Add FeSO4 and ammonia to the ammonium sulfate solution containing heavy metal ions to make the pH of the reaction system 8.0-8.5, and separate the solid and liquid to obtain the precipitate and the purified ammonium sulfate solution; the purified ammonium sulfate solution is crystallized to obtain fertilizer-grade ammonium sulfate crystals.

2. The method according to claim 1, characterized in that, In step S1, the desulfurized gypsum contains 20-25 mg / kg of cadmium and 20-25 mg / kg of arsenic.

3. The method according to claim 1, characterized in that, In step S1, the mass ratio of desulfurized gypsum to water in the mixture is 1:(5-6).

4. The method according to claim 1, characterized in that, In step S1, the mass ratio of desulfurized gypsum to NH3 contained in ammonia water is 1:(1-1.5).

5. The method according to claim 1, characterized in that, In step S1, the serine is L-serine with a purity of not less than 99.5%.

6. The method according to claim 1, characterized in that, In step S2, CO2-containing flue gas is introduced into the bottom of the suspension through a bubble stone.

7. The method according to claim 1, characterized in that, In step S2, the mass ratio of desulfurized gypsum to CO2 in flue gas is 1:(1.2~1.5).

8. The method according to claim 1, characterized in that, In step S2, the reaction temperature is 20–30°C; the reaction is carried out under stirring conditions, and the stirring speed is 800–1200 rpm.

9. The method according to claim 1, characterized in that, In step S3, the amount of FeSO4 added is controlled according to the arsenic ion content in the ammonium sulfate solution, so that the arsenic ions react with Fe... 2+ The mass ratio is 1:(2~4).

10. The method according to claim 1, characterized in that, In step S3, the crystallization process is carried out by evaporation crystallization, and the evaporation crystallization temperature is 130-180℃.

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