Method for preparing calcium carbonate by sealing carbon dioxide with ardealite

Abstract

The invention discloses a method for preparing calcium carbonate by utilizing ardealite to seal carbon dioxide, which comprises the following steps: firstly, removing acidic impurities in ardealite through water leaching pretreatment, and reducing the adjusting difficulty of pH value in subsequent reaction; then carrying out secondary leaching by adopting an ammonium chloride solution with an ultralow solid-to-liquid ratio, efficiently extracting calcium ions and separating impurity elements to form high-grade silicon dioxide leaching residues; finally, leach liquor is subjected to carbon dioxide bubbling carbonation reaction, high-purity calcium carbonate is generated, and meanwhile fixation of carbon dioxide is achieved. In addition, secondary carbonization is promoted by heating and carbonizing filtrate, the conversion efficiency of calcium carbonate is further improved, carbonate ions in the solution are removed, and the cyclic leaching process is optimized. The whole process is carried out under the conditions of normal pressure and low temperature, under the optimized process condition, the conversion rate of calcium ions can reach 100%, the reaction filtrate can be recycled, stepped recycling of impurity elements is achieved, and remarkable economic benefits and environmental friendliness are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of carbon dioxide storage and resource utilization, and in particular to a method for preparing calcium carbonate by using phosphogypsum to seal carbon dioxide. Background Art

[0002] With the acceleration of industrialization and population growth, carbon dioxide (CO2) emissions caused by human activities continue to increase. The accumulation of carbon dioxide in the atmosphere has intensified the greenhouse effect, causing a series of serious ecological problems such as global warming and frequent extreme weather events. Controlling the concentration of carbon dioxide in the atmosphere and achieving carbon neutrality have become key measures in the global climate change mitigation strategy and an urgent challenge facing mankind.

[0003] Among the many carbon capture, utilization and storage technologies, mineral carbonation has attracted widespread attention due to its safety and long-term stability. Mineral carbonation simulates the natural weathering process of silicate rocks and fixes carbon dioxide into stable carbonate minerals by accelerating chemical reactions. This process can not only effectively store carbon dioxide, but also convert carbon dioxide into economically valuable products, such as light calcium carbonate (CaCO3). Under suitable conditions, carbon dioxide can undergo thermodynamic spontaneous reactions with solid wastes containing active calcium or magnesium (such as fly ash, waste gypsum, metallurgical slag and carbide slag) to generate stable carbonate minerals.

[0004] Phosphogypsum (PG) is the main byproduct of wet-process phosphoric acid production. Its annual global production is huge. Every ton of phosphoric acid produced is accompanied by about 4.5 to 5 tons of phosphogypsum, and the annual production is between 100 and 300 million tons. However, currently only about 15% of phosphogypsum is used in the cement industry, agriculture and building materials, and the remaining more than 85% is piled up in large landfills and exposed to weathering for a long time. In addition, phosphogypsum contains harmful impurities such as phosphorus and fluorine, as well as organic matter, and even a few contain radioactive elements, which pose a serious risk to the environment and limit its wider application.

[0005] Given the high content of calcium ions in phosphogypsum, it shows great potential in carbon dioxide sequestration. In recent years, the use of phosphogypsum as raw material for mineral carbonation to sequester carbon dioxide has become a hot topic of research. According to the differences in the process flow, the technology of waste gypsum mineral carbonation to sequester carbon dioxide is mainly divided into two methods: direct method and indirect method. The direct method captures carbon dioxide through an alkaline medium and causes it to react directly with waste gypsum; while the indirect method involves the extraction and purification of active calcium from phosphogypsum, which is then used for carbon dioxide sequestration. Despite the progress made in research, large-scale industrial applications still face many challenges. The direct method has problems such as low carbonation efficiency of phosphogypsum, harsh reaction conditions, low product purity and low added value; the main challenges of the indirect method are the large consumption of chemical reagents, the need to further improve the purity of the carbonization product, and high economic and environmental costs.

[0006] In order to deal with the impurity removal problem and harsh reaction conditions in the traditional carbonation process of phosphogypsum, Ding et al. used ammonium acetate, sodium chloride and ammonium chloride to enrich and separate the impurity ions in phosphogypsum to improve the purity of the carbonation product and reduce the corrosion of the reaction equipment. However, this method still has some limitations, such as the strong acidity of phosphogypsum requires a large amount of ammonia water to adjust the pH value, which increases the consumption of chemical reagents; the high solid-liquid ratio leaching process fails to completely convert phosphogypsum, resulting in its underutilization, and the resulting leaching residue needs further treatment. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for preparing calcium carbonate by using phosphogypsum to seal carbon dioxide, so as to solve the problems that the existing phosphogypsum mineral carbonation to seal carbon dioxide requires a large amount of ammonia water to adjust the pH value, which increases the consumption of chemical reagents; the high solid-liquid ratio leaching process fails to completely convert the phosphogypsum, resulting in its underutilization, and the generated leaching residue needs further treatment.

[0008] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing calcium carbonate by using phosphogypsum to seal carbon dioxide comprises the following steps:

[0010] (1) Water leaching: adding the dried phosphogypsum to ultrapure water and fully immersing it under stirring to remove acidic impurities in the phosphogypsum, the solid-liquid ratio of the water leaching being 2-40 g:700 mL; after the immersion, the solid-liquid separation is performed to obtain leached residue and leached liquid, and then the obtained leached residue is dried to obtain water-leached phosphogypsum;

[0011] (2) ammonium leaching: placing the water-soaked phosphogypsum in a 1-3 mol / L ammonium chloride solution and fully immersing it under stirring at 50-70°C, with the ammonium leaching solid-liquid ratio being 6-7 g:700 mL; after the immersion, the solid-liquid separation is performed to obtain the ammonium-soaked phosphogypsum and the ammonium leaching filtrate;

[0012] (3) Carbonization: under stirring conditions, an ammonia solution is added to the ammonium leaching filtrate to adjust the pH value to 8.8-9. After solid-liquid separation, CO2 is introduced into the filtrate at a rate of 45-50 mL / min for carbonization reaction. After sufficient carbonization, the introduction of CO2 is stopped, and the filtrate is allowed to stand for 30-40 minutes for aging. Solid-liquid separation is performed to obtain calcium carbonate precipitate and carbonized filtrate;

[0013] (4) placing the carbonized filtrate under heating conditions and stirring for 30 to 40 minutes, and obtaining a secondary precipitation of calcium carbonate and a secondary carbonized filtrate after solid-liquid separation;

[0014] (5) drying the calcium carbonate precipitate obtained in step (3) and the calcium carbonate secondary precipitate obtained in step (4) to obtain a calcite calcium carbonate product;

[0015] (6) The secondary carbonization filtrate obtained in step (4) is used to replace the ammonium chloride solution in step (2), and steps (2) to (6) are repeated to perform a cyclic leaching experiment to achieve the recycling of the filtrate.

[0016] Furthermore, in step (1), the water immersion temperature is 10 to 25°C.

[0017] Furthermore, in step (2), the ammonium leaching solid-liquid ratio is 1 g:100 mL.

[0018] Furthermore, in step (4), the heating temperature is 40 to 60°C.

[0019] Furthermore, in step (6), the solid-to-liquid ratio of the water-leached phosphogypsum and the secondary carbonization filtrate in the cyclic leaching experiment is 8g:1000mL.

[0020] Working principle: The method for preparing calcium carbonate by using phosphogypsum to store carbon dioxide in the present invention first removes acidic impurities in PG through water leaching pretreatment, reduces the difficulty of adjusting the pH value in the subsequent reaction, greatly reduces the amount of ammonia water used, and then uses an ultra-low solid-liquid ratio ammonium chloride solution for secondary leaching, efficiently extracts calcium ions and separates impurity elements to form high-grade silicon dioxide leaching residue. The leachate undergoes carbonation reaction by carbon dioxide bubbling to generate high-purity calcium carbonate (CaCO3), and carbon dioxide is fixed at the same time. In addition, the secondary carbonization is promoted by heating the carbonized filtrate, the conversion efficiency of calcium carbonate is further improved, and the carbonate ions in the solution are removed to optimize the cyclic leaching process. The entire process is carried out under normal pressure and low temperature conditions, the conversion rate of calcium ions reaches 100%, the reaction filtrate can be recycled, and the impurity elements are recycled in stages, which solves the problem that the existing technology of carbonation of phosphogypsum minerals to store carbon dioxide requires a large amount of ammonia water to adjust the pH value, the chemical reagent consumption is large, and the leaching process with a high solid-liquid ratio fails to completely convert phosphogypsum, resulting in it not being fully utilized, and the generated leaching residue needs to be further processed. Ultimately, about 598kg of high-purity CaCO3 and 55kg of silicon-rich products were produced from 1000kg of PG, and about 263kg of CO2 was permanently fixed, which has significant economic benefits and environmental friendliness.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. Achieve the dual goals of efficient carbon dioxide storage and resource utilization of phosphogypsum: The present invention removes soluble acidic impurity ions in phosphogypsum through water immersion pretreatment, thereby reducing the amount of ammonia water and impurity influence required for subsequent pH adjustment. At the same time, the water immersion treatment can well retain calcium salts and reduce the loss of calcium salts. Combined with the ultra-low solid-liquid ratio ammonium leaching process, calcium ions can be efficiently extracted while separating impurity elements, and the leachate is used for carbonation and carbon dioxide capture. High-purity calcium carbonate can be effectively prepared under normal pressure and low temperature conditions to achieve permanent fixation of carbon dioxide. At the same time, calcium ions in phosphogypsum can be efficiently extracted and impurity elements can be recovered in a step-by-step manner, so as to prepare high-purity calcium carbonate and high-grade silicon-rich products with high economic value, realize the resource utilization of phosphogypsum, and provide an innovative and efficient solution for carbon dioxide storage and phosphogypsum treatment.

[0023] 2. Significantly reduce costs and have significant economic benefits: Compared with traditional processes, this process reduces the amount of chemical reagents used and lowers the requirements for reaction conditions. At the same time, it realizes the recycling of filtrate and the recycling of ammonium chloride solution, greatly reducing production costs and having significant economic benefits. It provides a new economical and energy-saving method for related industries.

[0024] 3. Environmentally friendly and sustainable: This process avoids the risks to the environment caused by long-term storage of phosphogypsum, reduces greenhouse gas emissions, and at the same time reduces dependence on other chemical reagents and reduces environmental pollution. It is a green and sustainable carbon dioxide storage and resource utilization technology, which is of great significance to environmental protection and sustainable development. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The XRD spectrum of the reaction raw material phosphogypsum of the present invention;

[0026] Figure 2 It is a process flow chart of the present invention;

[0027] Figure 3 This is a graph showing the effect of the number of cycles on the PG cyclic leaching-carbonization process in Example 1 of the present invention;

[0028] Figure 4 The XRD pattern of the precipitate produced after adjusting the pH in step (3) of Example 1 of the present invention;

[0029] Figure 5 This is a curve showing changes in pH value and calcium ion concentration of the water immersion system under the conditions of a solid-liquid ratio range of 0 to 40 g:700 mL in the water immersion reaction of Example 2 of the present invention.

[0030] Figure 6 The pH value and calcium ion concentration change curves under different water immersion temperature conditions in Example 3 of the present invention;

[0031] Figure 7 The changes in calcium ion concentration and pH value when the solid-liquid ratio of ammonium leaching in Example 4 of the present invention is between 0 and 15 g:700 mL;

[0032] Figure 8 This is the XRD spectrum of the phase change of the residue leached out of ammonium leaching in Example 4 of the present invention;

[0033] Fig. 9 The change of calcium ion concentration under different initial pH values ​​of the solution during the carbonization reaction of Example 5 of the present invention;

[0034] Fig.10 The change of pH value under different initial pH values ​​of the solution during the carbonization reaction of Example 5 of the present invention;

[0035] Fig.11 The change of pH value at different carbonization times in Example 6 of the present invention;

[0036] Fig.12 The change of calcium ion concentration at different carbonization times in Example 6 of the present invention;

[0037] Fig.13 XRD spectrum of calcium carbonate prepared by carbonization reaction in Example 6 of the present invention;

[0038] Fig.14 The changes of pH value and calcium ion concentration at different heating temperatures during the heating-promoted secondary carbonization process of Example 7 of the present invention;

[0039] Fig.15 The XRD patterns of the leached residues under different solid-liquid ratios during the cycle of Example 8 of the present invention;

[0040] Fig.16 is the calcium ion concentration in the leachate under different solid-liquid ratio conditions during the cycle of Example 8 of the present invention; DETAILED DESCRIPTION

[0041] The specific implementation modes of the present invention are further described in detail below in conjunction with specific examples.

[0042] The numerical ranges in the present invention are understood to also specifically disclose each intermediate value between the upper and lower limits of the range. Each smaller range between any intermediate value in the stated value or stated range and any other stated value or intermediate value in the stated range is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded in the range.

[0043] Unless otherwise stated, all technical and scientific terms used herein have the same meanings as those of ordinary skill in the art described in the present invention are generally understood. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of conflict with any incorporated document, the content of this specification shall prevail. Regarding "comprising", "including", "having", "containing", etc. used herein, they are all open terms, which means including but not limited to.

[0044] Unless otherwise specified, the experimental methods used in the present invention are all conventional methods.

[0045] Unless otherwise specified, the materials, reagents, etc. used in the present invention can be purchased or synthesized by known methods.

[0046] The quantitative tests in the present invention were all repeated three times, and the results were averaged.

[0047] 1. Preparation of experimental raw materials

[0048] Phosphogypsum (PG): It is a by-product produced during the production of dihydrate wet-process phosphoric acid by Sinochem Chongqing Fuling Chemical Co., Ltd. The chemical composition analysis is shown in Table 1.

[0049] Table 1 Chemical composition of PG

[0050]

[0051] As can be seen from Table 1, the main components of phosphogypsum include 33.49% CaO and 57.87% SO3, indicating that the content of CaSO4·2H2O is about 92%, which is suitable as a raw material for storing CO2 by mineral carbonation method. Impurities include SiO2 (5.48%), P2O5 (0.80%), etc.

[0052] The XRD spectrum of phosphogypsum is as follows: Figure 1 As shown by Figure 1 It can be seen that PG slag mainly contains CaSO4·2H2O, CaHPO4·(H2O)2 and SiO2, which is consistent with the results of its chemical composition analysis. By comparing with the standard card, the characteristic peaks of CaSO4·2H2O and SiO2 correspond to 31.09 and 26.638, respectively. The second strongest peak of CaHPO4·(H2O)2 is located at 23.39, which is very close to the peak of CaSO4·2H2O at 23.387 and is difficult to distinguish, but the intensity of CaHPO4·(H2O)2 is significantly higher near this peak. Considering that the phosphorus content in PG is only 0.80%, when the peak is more obvious, it can be considered that there is a certain amount of CaHPO4·(H2O)2, which is considered as its characteristic peak.

[0053] Before the experiment, phosphogypsum (PG) was dried in an oven at 80°C overnight to remove moisture and then cooled to room temperature for use.

[0054] Chemical reagents: analytical grade ammonium chloride (NH4Cl), ammonia (NH4OH), high purity carbon dioxide (CO2), and ultrapure water with a resistivity exceeding 18.25MΩ·cm.

[0055] Equipment: Jacketed glass reactor, magnetic stirrer, constant temperature blower, pH acidity meter, X-ray diffraction analyzer (XRD), inductively coupled plasma emission spectrometer (ICP), etc.

[0056] 2. Embodiment

[0057] Example 1

[0058] This embodiment provides a method for preparing calcium carbonate by using phosphogypsum to seal carbon dioxide. The process flow is as follows: Figure 2 As shown, the following steps are included:

[0059] (1) Water leaching pretreatment: Take the dried phosphogypsum and add it to ultrapure water, and carry out water leaching reaction at 25°C with stirring, with a solid-liquid ratio of 2g:700mL and a reaction time of 10 minutes. After the reaction is completed, the solid-liquid separation is performed to obtain the leaching liquid and the leaching residue, and the leaching residue is placed in a constant temperature blower at 40°C and dried overnight to obtain water-leached phosphogypsum (W-PG);

[0060] (2) Ultra-low solid-liquid ratio ammonium leaching: 7 g of the water-leached phosphogypsum obtained in step (1) was subjected to leaching reaction with 2 mol / L ammonium chloride solution under stirring, the solid-liquid ratio of ammonium leaching was 1 g:100 mL, the leaching temperature was 60°C, and the reaction time was 60 minutes. After the leaching was completed, the solid-liquid separation obtained ammonium leaching filtrate (AL-solution) and ammonium leached phosphogypsum (A-PG);

[0061] (3) Carbon dioxide bubbling carbonation: The ammonium leaching filtrate obtained in step (2) is transferred to a reactor, and an appropriate amount of concentrated ammonia water is added under stirring to adjust the pH value of the solution to 8.8. After filtering to remove the precipitate, carbon dioxide is introduced into the filtrate at a rate of 45 mL / min at 25° C. for carbonation reaction. After reacting for 10 minutes, the introduction of carbon dioxide is stopped, and an aging stage of 30 minutes is entered. After the aging is completed, calcium carbonate precipitate (P-CaCO3) and carbonization filtrate (C-solution) are obtained.

[0062] (4) Heating to promote secondary carbonization: The carbonized filtrate is heated to 60°C and stirred for 30 minutes to promote the secondary precipitation of calcium carbonate (S-CaCO3). After heating, the calcium carbonate precipitate (S-CaCO3) and the heated solution are separated. P-CaCO3 and S-CaCO3 are placed in a blast constant temperature drying oven at 40°C and dried overnight to obtain a calcite calcium carbonate product.

[0063] (5) Recycling: The solution heated in step (4) was used again in the ammonium leaching process of water-leaching phosphogypsum instead of the ammonium chloride solution in step (2), and a circulation experiment was carried out with a solid-liquid ratio of 8g:1000mL. After the immersion, the solid-liquid separation was performed to obtain the circulating leaching residue (CL-Residue) and the circulating leaching solution (CL-solution), and then the circulating leaching solution (CL-solution) was used for the carbonization experiment, and the other steps remained unchanged. After 10 cycles, the solution can still continue to participate in the cyclic reaction, showing good cyclic stability.

[0064] Example 2

[0065] The present embodiment provides a method for preparing calcium carbonate by sealing carbon dioxide using phosphogypsum, which is mainly the same as that of Example 1, except that the solid-liquid ratio of the water leaching reaction in step (1) is different. The present embodiment determines the pH value and calcium ion concentration change curve of the water leaching system under the condition of the solid-liquid ratio of the water leaching reaction in the range of 0 to 40 g:700 mL.

[0066] Example 3

[0067] The present embodiment provides a method for preparing calcium carbonate by sealing carbon dioxide with phosphogypsum, which is mainly the same as that in Embodiment 1, except that the temperature of the water immersion reaction in step (1) is modified to 10°C, 45°C, and 65°C, respectively, and the pH value and calcium ion concentration change curves under different water immersion temperature conditions are measured.

[0068] Example 4

[0069] The present embodiment provides a method for preparing calcium carbonate by sealing carbon dioxide with phosphogypsum, which is mainly the same as that of Embodiment 1, except that in step (2), the solid-liquid ratio of the ammonium leaching reaction is different. The present embodiment determines the changes in calcium ion concentration and pH value when the solid-liquid ratio of the ammonium leaching is between 0 and 15 g:700 mL.

[0070] Example 5

[0071] This embodiment provides a method for preparing calcium carbonate by using phosphogypsum to seal carbon dioxide, which is mainly the same as that of Embodiment 1, except that the initial pH value of the carbonization reaction in step (3) is different, and the initial pH value is modified to 8.5 and 9 respectively. The changes in calcium ion concentration and pH value under different initial pH values ​​of the solution during the carbonization reaction are measured.

[0072] Example 6

[0073] The present embodiment provides a method for preparing calcium carbonate by sealing carbon dioxide using phosphogypsum, which is mainly the same as that of Embodiment 1, except that the carbonization reaction time in step (3) is different, and the changes in pH value and calcium ion concentration at carbonization reaction times of 9 min, 11 min, 12 min, 13 min, 14 min, and 15 min are measured respectively.

[0074] Example 7

[0075] This embodiment provides a method for preparing calcium carbonate by sealing carbon dioxide using phosphogypsum, which is mainly the same as Example 1, except that the heating temperature in step (4) is different, and the changes in pH value and calcium ion concentration in the heating range of 15 to 90° C. are measured.

[0076] Example 8

[0077] This embodiment provides a method for preparing calcium carbonate by using phosphogypsum to store carbon dioxide, which is mainly the same as that of Example 1, except that the solid-liquid ratio of W-PG and the heated solution after secondary carbonization in the circulation experiment of step (5) is modified to 7g:1000mL, 9g:1000mL, and 10g:1000mL. The XRD spectra of the leached residue and the calcium ion concentration in the leaching solution under different solid-liquid ratios are measured.

[0078] 3. Data Analysis

[0079] Figure 3 The figure is a result diagram showing the effect of the number of cycles on the PG cyclic leaching-carbonization process in Example 1; Figure 3 (a) is the pH change curve during the PG cycle leaching-carbonization process. Figure 3 (a) It can be seen that, except for the ammonium chloride solution, the pH value change trends of the carbonized filtrate are roughly the same; Figure 3 (b) and Figure 3 (c) is the curve of calcium ion changes during PG cycle leaching-carbonization. Figure 3 (b) and Figure 3 (c) It can be seen that with the increase in the number of cycles, the leaching efficiency of calcium ions in the carbonized filtrate decreases. This is because although the carbonization reaction and heating treatment can reduce the calcium ions in the system, the sulfate ions in PG continuously enter the system during the circulation process, resulting in the solubility of PG and the leaching efficiency of calcium ions gradually decreasing under the common ion effect. Figure 3 (d) is the XRD pattern of the CL-Residue. Figure 3 (d) It can be seen that with the increase of the number of cycles, except for the ammonium leaching residue and CL-Residue1 which only contain silicon dioxide, the other leaching residues contain a certain amount of calcium phosphate dihydrate and calcium sulfate dihydrate, and the phase peak intensity of calcium salt is enhanced, which is consistent with Figure 3 (b) and Figure 3 (c) is consistent with the decrease in calcium ion concentration, but by selecting a suitable solid-liquid ratio or performing a second leaching treatment, effective separation of the leached residue can be achieved. Figure 3 (e) and Figure 3 (f) are the XRD spectra of the precipitate obtained after carbonization and the filter residue after heating treatment, respectively. Figure 3 (e) and Figure 3 (f) It can be seen that the calcium carbonate produced by the carbonization reaction and the calcium carbonate produced after the heating treatment are both calcite-type calcium carbonate and contain a small amount of ammonium chloride. This is because the sample is directly dried without being washed with water. The ammonium chloride can be removed by washing with water, thereby obtaining high-purity calcite-type calcium carbonate. Figure 3 (g) is the turbidity time during the carbonization reaction, Figure 3(g) It can be seen that as the number of cycles increases, the time for the solution to become turbid and produce calcium carbonate during the carbonization process is extended. This is because the calcium ion concentration in the system decreases with the increase in the number of cycles, and on the other hand, ammonia is continuously enriched in the carbonization system, delaying the formation of calcium carbonate. Figure 3 (h) is the XRD spectrum of the sample before and after water washing. The test results show that the generated high-purity CaCO3 meets the HG / T2226-2010 Chinese recommended industry standard. Figure 3 (i) is the bar graph of sulfate content, Figure 3 (i) It can be seen that although the sulfate ion increases during the cycle, the increase in sulfate ion is still limited after 10 cycles, indicating that the circulating solution can continue to react. In addition, the impurity elements of PG, in addition to Si, are mainly Al, Fe, and Mg. Figure 3 (j) is the bar graph of impurity element content, Figure 3 (j) shows that due to the high pH value of the system, the contents of Fe, Mg, and P in the solution have not been high, while Al will enter the system during the leaching process and will decrease significantly after carbonization. This may be due to the formation of a complex between Al and ammonium ions, which increases its solubility. However, during the carbonation reaction, Al ions may enter calcium carbonate, and its specific impact needs further study. In summary, from Figure 3 It is not difficult to see that the method of the present invention can effectively realize the reuse of phosphogypsum, and can be used to store carbon dioxide, alleviate the greenhouse effect, and generate calcite calcium carbonate products. Moreover, the carbonized filtrate obtained by the present invention can continue to participate in the cyclic reaction after 10 cycles, showing good cyclic stability.

[0080] Figure 4 This is the XRD pattern of the precipitate produced after adjusting the pH in step (3) of Example 1. XRD analysis confirms that the precipitate is ammonium chloride. This is due to the common ion effect formed by the addition of a large amount of ammonia water and the original ammonium chloride in the solution. This part of the precipitated ammonium chloride can be recycled and used to prepare ammonium chloride solution to achieve recycling.

[0081] Figure 5 The pH value and calcium ion concentration curve of the water immersion system under the condition of solid-liquid ratio range of 0 to 40 g:700 mL in the water immersion reaction of Example 2. Figure 5It can be seen that as the amount of PG added increases, the pH value of the system after water immersion decreases significantly and continues to decrease, indicating that a large amount of hydrogen ions are continuously dissolved into the water. At the same time, the calcium ion concentration in the system first increases rapidly to about 13mmol / L, and then reaches saturation equilibrium. Considering that the pH value changes exponentially, it shows that the water immersion liquid can absorb a large amount of hydrogen ions many times, but the calcium ions no longer increase after reaching saturation, and the calcium ion concentration is very low. Therefore, the acid in PG can be removed by water immersion pretreatment, while the calcium salt of PG is retained; the leaching solution after water immersion can be recycled for leaching PG again. And from the above data, it can also be seen that if the acid is not washed by water immersion, the pH value of the system after direct ammonium immersion will drop significantly, which is not conducive to the subsequent carbonization reaction.

[0082] Figure 6 The pH value and calcium ion concentration change curves under different water immersion temperature conditions in Example 3 are shown in FIG. Figure 6 It can be seen that increasing the leaching temperature is beneficial to the increase of the pH value of the system and the increase of the calcium ion concentration, but the increase in pH means that the solubility of hydrogen ions decreases, and the hydrogen ions cannot be removed well, which is not conducive to the subsequent carbonization experiment. Therefore, the preferred water immersion temperature is 10-25°C.

[0083] Figure 7 The changes in calcium ion concentration and pH value when the ammonium leaching solid-liquid ratio in Example 4 is between 0 and 15 g:700 mL. Figure 8 The XRD pattern of the phase change of the residue leached out of ammonium leaching in Example 4 is shown in FIG. Figure 7 It can be seen that as the amount of W-PG added increases, the calcium salt in the solution gradually approaches saturation. In the newly added W-PG, the calcium ions that can continue to dissolve become limited, so the growth trend of calcium ion concentration gradually slows down. At the same time, the pH value changes from a rapid decline at the beginning to a slow decline. Figure 8 Analysis of the phase changes of ammonium leaching residue shows that when the amount of W-PG added does not exceed 7.0g, the phase in the leaching residue is mainly silicon dioxide. When 7.0gW-PG is added, the calcium ion concentration in the leachate reaches 51.82mmol / L. At this time, the leaching residue is mainly silicon dioxide, and the characteristic peak of calcium sulfate dihydrate is very low. When the amount of W-PG added continues to increase, characteristic peaks of calcium sulfate dihydrate and calcium phosphate dihydrate begin to appear in the leaching residue, and their intensity gradually increases. These results show that under the condition of ammonium leaching solid-liquid ratio of 1g:100ml, most of the calcium salts can be effectively extracted from W-PG, and impurities mainly composed of silicon dioxide can be separated, achieving efficient leaching of calcium ions and high-grade recovery of silicon dioxide. Therefore, we selected a solid-liquid ratio of 1g:100ml as the optimal leaching condition.

[0084] Fig. 9This is the change in calcium ion concentration under different initial pH values ​​of the solution during the carbonization reaction of Example 5. Fig.10 The following is the change of pH value under different initial pH values ​​of the solution during the carbonization reaction of Example 5. Fig. 9 and Fig.10 It can be seen that with the introduction of carbon dioxide, the pH value of the carbonization system shows a downward trend, but the higher the initial pH value, the more moderate the downward trend. This is because the higher the initial pH value in the carbonization system, the more ammonia water is added, and the ammonium-ammonium chloride buffer system formed can hinder the rapid decline of pH value. pH value is crucial to the nucleation and growth of calcium carbonate. When the initial carbonization pH value is 8.8 and 9, the calcium ion concentration is gradually reduced to 0mmol / L with the introduction of carbon dioxide, but when the initial carbonization pH value is 8.5, the carbonization reaction lasts for 40 minutes, and the solution still contains a large amount of calcium ions. The system has a pH value of less than 7, and no longer meets the conditions for calcium carbonate nucleation, so the reaction is stopped. Considering production costs and reagent losses, an initial pH value of 8.8 is a better choice, and the product after carbonization is calcite.

[0085] Fig.11 The changes of pH value at different carbonization times in Example 6 are shown in FIG. Fig.12 The change of calcium ion concentration under different carbonization time in Example 6. Fig.11 and Fig.12 It can be seen that in order to ensure that the introduced carbon dioxide can fully dissolve and react, and to avoid the influence of unreacted carbon dioxide on the carbonate ion content in the system, we let the reaction system stand for 30 minutes after stopping the introduction of carbon dioxide. It was observed that during the carbonation process, the pH value first dropped rapidly, and then showed a small upward trend. The turning point of this change is exactly the time point when carbon dioxide stops being introduced and enters the aging stage. The reason for this phenomenon may be that carbon dioxide dissolves to form carbonic acid and ionizes hydrogen ions, resulting in a decrease in the pH value of the system. When carbon dioxide stops being introduced, the previously generated calcium carbonate may undergo partial reverse reaction and re-equilibrium, resulting in a small increase in the pH value. During the static aging stage, although the pH value of the system did not change significantly, the calcium ion concentration decreased significantly, indicating that during the aging stage, the carbon dioxide that was not fully dissolved and reacted continued to react with the calcium ions in the system to generate calcium carbonate. Therefore, after the carbonation reaction is completed, aging is required to increase the conversion rate of calcium ions. Fig.13 XRD pattern of calcium carbonate prepared by carbonization reaction in Example 6, Fig.13 This indicates that the calcium carbonate obtained is all calcite, and because it is not washed with water, a certain amount of ammonium chloride is mixed in the calcium carbonate.

[0086] Fig.14The changes in pH value and calcium ion concentration at different heating temperatures during the heating-promoted secondary carbonization process of Example 7. Fig.14 It can be seen that as the heating temperature increases, the pH value decreases linearly. This may be due to two reasons. One is that the temperature increases and the ionization of hydrogen ions in the system increases. The other is that heating causes the escape of ammonia, and the escape of ammonia reduces the formation of ammonia and calcium ions, thereby promoting the precipitation of calcium carbonate. In order to verify this idea, the reaction solution was heated, and the heated gas passed through the condenser and sealed with ultrapure water. The results are as follows: Fig.14 As shown, it can be seen that when the temperature is heated to 40°C, the calcium ion concentration has dropped to a low level. After that, the heating temperature is continued to increase, and the calcium ion concentration does not change significantly. The pH value of the absorption solution also reaches the highest value at 40°C, and then continues to decline, while the pH value of the reaction system first slowly decreases and then rapidly decreases, which indicates that it is caused by the volatilization of ammonia in the buffer solution during the heating process. When ammonia begins to volatilize, the calcium ion-ammonia complex decomposes to form calcium carbonate, so the calcium ion content in the system decreases. Since the reaction system is a buffer system, the pH value decreases slowly, and the escaped ammonia causes the pH value of the absorption aqueous solution to continue to increase. With the further escape of ammonia, a large amount of calcium ion-ammonia complex decomposes, and the calcium ion continues to decrease until it stabilizes. The pH value of the reaction system continues to decrease due to the increase in temperature, while the pH value of the absorption aqueous solution begins to slowly decrease after reaching the highest value due to the lack of ammonia volatilization. In this paper, calcium carbonate precipitation can be promoted by increasing the volatilization of ammonia, and the volatilized ammonia can be used for subsequent pH adjustment. From the above research results, it can be seen that by controlling the carbonization time and heating temperature to carry out two calcium carbonate precipitation reactions, not only can the conversion efficiency of calcium carbonate be effectively improved, but also the carbonate ions in the solution can be removed, thereby improving the efficiency of the circulating leaching solution.

[0087] Fig.15 The XRD patterns of the leached residues under different solid-liquid ratios during the cycle of Example 8; Fig.16 is the calcium ion concentration in the leachate under different solid-liquid ratio conditions during the cycle of Example 8. Fig.15 and Fig.16It can be seen that as the solid-liquid ratio of W-PG to C-solution increases, the calcium ion concentration in the leaching system gradually increases and tends to be flat. When the solid-liquid ratio reaches 10g:1000mL, the calcium ion concentration reaches 43.59mmol / L. However, at this solid-liquid ratio, in addition to the peak of silica, the phase peaks of calcium sulfate dihydrate and calcium hydrogen phosphate dihydrate also appear in the leaching residue. Considering that when the solid-liquid ratio is 8g:1000mL, the calcium ion concentration is in a higher range, and only silica is detected in the leaching residue, it shows that the leaching effect under this solid-liquid ratio is more ideal. In addition, the ultra-low solid-liquid ratio helps to reduce the introduction of sulfate and phosphate ions, thereby improving the sustainability of the cyclic leaching-carbonization process. Therefore, 8g:1000mL is selected as the solid-liquid ratio of W-PG to C-solution to optimize the subsequent cyclic leaching-carbonization process.

[0088] In summary, the method of preparing calcium carbonate by using phosphogypsum to seal carbon dioxide in the present invention first adopts PG water immersion pretreatment, followed by ultra-low solid-liquid ratio ammonium immersion treatment, and then the obtained filtrate is used for carbonation reaction and carbon dioxide capture to prepare calcium carbonate, followed by secondary calcium carbonate preparation through a heating process, and the solution after the reaction is circulated and leached again. This innovative process can not only effectively seal carbon dioxide, but also enrich high-grade silica slag and prepare high-purity CaCO3, providing a new solution path for mineral carbonation technology in carbon dioxide sealing and PG resource utilization. The optimized process can ensure that the calcium in PG is completely converted into high-purity calcium carbonate, and all the silica enters the high-grade enriched silica slag. Taking the treatment of 1000kg PG as an example, it is expected to produce about 598kg of high-purity calcium carbonate and 55kg of silicon-rich products, and permanently fix about 263kg of carbon dioxide. These products can replace 598kg of light calcium carbonate and 55kg of active silica on the market for the production of composite materials, with market prices of $145 / ton and $97 / ton respectively. In the limited cycle process, ammonium chloride and ammonia water can be recycled, so without considering energy consumption and equipment costs, it is expected that the process can bring an economic benefit of $119.6, which has significant economic benefits.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing calcium carbonate by using phosphogypsum to store carbon dioxide, characterized in that: The following steps are involved: (1) Water leaching: adding the dried phosphogypsum to ultrapure water and fully immersing it under stirring to remove acidic impurities in the phosphogypsum, the solid-liquid ratio of the water leaching being 2-40 g:700 mL; after the immersion, the solid-liquid separation is performed to obtain leached residue and leached liquid, and then the obtained leached residue is dried to obtain water-leached phosphogypsum; (2) ammonium leaching: placing the water-soaked phosphogypsum in a 1-3 mol / L ammonium chloride solution and fully immersing it under stirring at 50-70°C, with the ammonium leaching solid-liquid ratio being 6-7 g:700 mL; after the immersion, the solid-liquid separation is performed to obtain the ammonium-soaked phosphogypsum and the ammonium leaching filtrate; (3) Carbonization: under stirring conditions, an ammonia solution is added to the ammonium leaching filtrate to adjust the pH value to 8.8-9. After solid-liquid separation, CO2 is introduced into the filtrate at a rate of 45-50 mL / min for carbonization reaction. After sufficient carbonization, the introduction of CO2 is stopped, and the filtrate is allowed to stand for 30-40 minutes for aging. Solid-liquid separation is performed to obtain calcium carbonate precipitate and carbonized filtrate; (4) placing the carbonized filtrate under heating conditions and stirring for 30 to 40 minutes, and obtaining a secondary precipitation of calcium carbonate and a secondary carbonized filtrate after solid-liquid separation; (5) drying the calcium carbonate precipitate obtained in step (3) and the calcium carbonate secondary precipitate obtained in step (4) to obtain a calcite calcium carbonate product; (6) The secondary carbonization filtrate obtained in step (4) is used to replace the ammonium chloride solution in step (2), and steps (2) to (6) are repeated to perform a cyclic leaching experiment to achieve the recycling of the filtrate.

2. The method for preparing calcium carbonate by sequestering carbon dioxide using phosphogypsum according to claim 1, characterized in that: In step (1), the water immersion temperature is 10-25°C.

3. The method for preparing calcium carbonate by sequestering carbon dioxide using phosphogypsum according to claim 1, characterized in that: In step (2), the ammonium leaching solid-liquid ratio is 1g:100mL.

4. The method for preparing calcium carbonate by sequestering carbon dioxide using phosphogypsum according to claim 1, characterized in that: In step (4), the heating temperature is 40 to 60°C.

5. The method for preparing calcium carbonate by sequestering carbon dioxide using phosphogypsum according to claim 1, characterized in that: In step (6), the solid-liquid ratio of the water-leached phosphogypsum and the secondary carbonization filtrate in the cyclic leaching experiment is 8g:1000mL.

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