A method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum.

Abstract

This invention discloses a one-step method for preparing aragonite-type calcium carbonate by mineralizing CO2 with gypsum, comprising: adding purified gypsum to deionized water, then adding NH3·H2O solution to form a suspension and heating it, introducing CO2 gas with a concentration of 10%~99.9%, controlling the CO2 flow rate and stirring rate, and simultaneously adding Mg. 2+ The reaction proceeds; the solid product is separated by filtration and dried to obtain aragonite-type calcium carbonate. Studies have found that this invention, at 70℃ and Mg... 2+ Concentration 5.00×10 −3 Aragonite whiskers with an aspect ratio of 10:1 can be directly synthesized in one step under conditions of mol / L, CO2 flow rate of 400 mL / min, and stirring rate of 250 rpm. The product purity exceeds 95%, and the gypsum conversion rate reaches over 99%. This not only realizes the controllable preparation of aragonite and elucidates its formation mechanism, but also provides an efficient and feasible path for gypsum resource utilization and CO2 emission reduction.

Description

Technical Field

[0001] This invention relates to the field of aragonite-type calcium carbonate preparation, and more particularly to a method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum. Background Technology

[0002] As a highly efficient carbon reduction technology, carbon capture, utilization, and storage (CCUS) can effectively reduce CO2 emissions from industrial production and fossil energy utilization, playing a crucial role in addressing global warming. CO2 mineralization technology, by reacting raw materials containing alkaline ions such as Ca and Mg with CO2 to form insoluble carbonates, achieves stable and permanent CO2 sequestration, making it an effective carbon capture technology. Industrial solid waste, rich in calcium and magnesium elements, such as fly ash, steel slag, carbide slag, and gypsum, has advantages over natural minerals in terms of high activity and low pretreatment costs, making it a viable alternative for CO2 capture and mineralization storage.

[0003] Gypsum is a solid waste generated during the wet-process phosphoric acid production process; approximately 4.5-5 tons of gypsum are produced for every ton of phosphoric acid produced. Currently, global gypsum stockpiles have reached 6 billion tons, with a utilization rate of only 15% to 25%, highlighting the urgent need to address its disposal and utilization challenges. Gypsum's main component is CaSO4·2H2O, with a purity of up to 90%. Under ammonia-rich conditions, it can react with CO2 to transform into CaCO3 and (NH4)2SO4, showing broad application prospects.

[0004] The crystal form of mineralized CaCO3 directly affects its application value. CaCO3 mainly has three crystal forms: aragonite, aragonite, and calcite. Among them, metastable aragonite whiskers exhibit excellent performance in polymer materials, papermaking, and advanced fillers due to their unique high aspect ratio. The directional synthesis of aragonite whiskers can effectively improve the economics of mineralization technology.

[0005] Currently, research on mineralization of solid waste to achieve the control of specific crystal forms such as aragonite often employs indirect mineralization methods. This method separates Ca through a leaching step. 2+ Thus, in a highly controlled synthetic environment, by adding Mg 2+ Using crystal form inducing agents, high-purity, high aspect ratio aragonite-type CaCO3 was prepared. Liu et al. systematically studied the extraction of Ca from desulfurized gypsum. 2+ The leaching process and Ca 2+ Various factors during the mineralization process of the leachate were considered. Under the conditions of pH 9.5, temperature 25 ℃, CO2 flow rate 400 mL / min, stirring speed 1040 r / min and no additives, aragonite-type CaCO3 with a purity of up to 97.17% was obtained. After adding MgCl2, a new aragonite crystal phase appeared in the product and accounted for 83.75%.

[0006] Gu et al. extracted Ca from recycled concrete fine powder using NH4NO3 as the leaching agent. 2+ Aragonite whiskers with a high aspect ratio of 10:1 were successfully synthesized under the conditions of 80 ℃, CO2 flow rate of 500 mL / min, stirring speed of 200 r / min, and addition of 25 mmol MgCl2. However, the indirect mineralization method suffers from problems such as long leaching time, low leaching rate, and high cost, which restricts its large-scale application. In contrast, the direct mineralization method eliminates the need for leaching, has a shorter process, and is more suitable for industrial applications.

[0007] Wang et al. conducted direct gypsum mineralization experiments, using organic amines as alkaline reagents, CO2 absorbents, gypsum dissolution promoters, and CaCO3 crystal form inducers. The results showed that high CO2 removal efficiency (>95%), gypsum carbonation efficiency (>93%), and CO2 capture capacity (>340 g CO2 / kg gypsum) could be achieved. To address the low solubility issue during gypsum mineralization, Amibo TA et al. used 1,4-butanediol as an organic additive. Within the concentration range of 0.0–0.5 mol / L, the CO2 sequestration rate increased from 57.4% to 87.7%, and the calcite content in CaCO3 also increased to 90%. However, these studies mainly focused on the conversion rate of gypsum, and the obtained CaCO3 was mostly in the common calcite crystal form. They did not explore in depth the possibility of directionally synthesizing aragonite whiskers with higher added value through process condition control.

[0008] In direct mineralization methods, based on the order in which CO2 absorption and the mineralization reaction occur, they can be divided into one-step and two-step methods, as shown below:

[0009] One-step method:

[0010] Two-step method:

[0011]

[0012]

[0013] Mineralization processes and reaction conditions have a direct impact on the crystal form of CaCO3 products.

[0014] Therefore, in order to solve the above-mentioned technical problems, it is an urgent technical problem to be solved by those skilled in the art to provide a method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum. Summary of the Invention

[0015] In view of this, the present invention provides a method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum. The present invention uses purified and pretreated gypsum as raw material and studies the influence of different processes and process parameters on the crystal form of CaCO3 product during direct mineralization, thus providing support for the economic efficiency of the mineralization process.

[0016] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0017] A method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum includes the following steps:

[0018] (1) Pulping: Add purified gypsum to deionized water, then add NH3·H2O solution, stir and mix to form a suspension;

[0019] (2) Mineralization reaction: The suspension obtained in step (1) is stirred and heated, and CO2 gas with a concentration of 10%~99.9% is introduced. The CO2 flow rate is controlled at 100~500 mL / min and the stirring rate is 150~750 rpm. Mg is added at the same time. 2+ To react;

[0020] (3) Solid-liquid separation and drying: After the reaction is completed, the solid product is separated by filtration, rinsed, and dried in a forced-air drying oven to obtain aragonite-type calcium carbonate.

[0021] Preferably, in step (1), the amount of purified gypsum added is 30 g, the amount of deionized water added is 90 mL, and the amount of NH3·H2O solution with a mass fraction of 25% added is 30 mL.

[0022] Preferably, the ratio of the amount of purified gypsum to deionized water and 25% NH3·H2O solution in step (1) is 1g:3mL:1mL.

[0023] Preferably, the reaction temperature in step (2) is controlled at 30~70 ℃.

[0024] Preferably, the Mg in step (2) 2+ Concentration range: 1.25~6.25×10 -3 mol / L.

[0025] Preferably, the CO2 flow rate in step (2) is 100 mL / min to 500 mL / min.

[0026] Preferably, the stirring rate in step (2) is 150 rpm to 750 rpm.

[0027] Preferably, the drying conditions in step (3) are drying at 80 ℃ for more than 4 hours.

[0028] Preferably, in step (3), the water is rinsed 2 to 3 times with deionized water.

[0029] The present invention achieves the following technical effects compared to the prior art:

[0030] Research has found that this invention is effective at 70 °C with Mg. 2+ Concentration 5.00×10 -3 Aragonite whiskers with an aspect ratio of 10:1 can be directly synthesized in one step under conditions of mol / L, CO2 flow rate of 400 mL / min, and stirring rate of 250 rpm. The product purity exceeds 95%, and the gypsum conversion rate reaches over 99%. This not only realizes the controllable preparation of aragonite and elucidates its formation mechanism, but also provides an efficient and feasible path for gypsum resource utilization and CO2 emission reduction.

[0031] This invention uses purified gypsum directly as raw material, eliminating the need for pre-leaching for calcium extraction, simplifying the process and reducing costs. Simultaneously, it achieves resource utilization of industrial solid waste gypsum and CO2 mineralization and sequestration, combining environmental and economic benefits. By adjusting process parameters and Mg... 2+ The addition amount can be used to directionally prepare high-purity, high aspect ratio aragonite-type calcium carbonate whiskers, resulting in higher product added value. This can meet the application needs of polymer materials, papermaking, and advanced fillers, providing a reliable process solution for the industrial-scale production of aragonite-type calcium carbonate. Attached Figure Description

[0032] Figure 1 The XRD pattern of gypsum, the raw material of this invention;

[0033] Figure 2 The XRD pattern of CaCO3 mineralized by the two-step method of this invention;

[0034] Figure 3 This is a TG curve of CaCO3 mineralization using the two-step method of this invention;

[0035] Figure 4 The XRD patterns of the mineralized CaCO3 products at different temperatures according to this invention are shown below.

[0036] Figure 5 These are SEM images of mineralized CaCO3 products under different crystal form inducing agent conditions according to the present invention; wherein,

[0037] (a) No additives; (b) Na2CO3; (c) MgSO4; (d) STPP;

[0038] Figure 6 The XRD patterns of mineralized CaCO3 products with different additives according to the present invention are shown below.

[0039] Figure 7 The XRD pattern of the mineralized CaCO3 of this invention is shown below.

[0040] Figure 8 The TG curve of mineralized CaCO3 in this invention;

[0041] Figure 9 This is a graph showing the change in gypsum conversion rate over time at different reaction temperatures according to the present invention;

[0042] Figure 10 This is the XRD pattern of the mineralized CaCO3 of this invention; wherein,

[0043] (a) The effect of different temperatures; (b) The effect of different additives; (c) Different Mg 2+ The effects of the amount added; (d) the effects of different CO2 flow rates; (e) the effects of different stirring rates (A = aragonite, C = calcite, V = vaterite, G = gypsum);

[0044] Figure 11 These are scanning electron microscope (SEM) images of CaCO3 products obtained by the next-step mineralization method at different temperatures according to the present invention; wherein,

[0045] (a) 30 ℃; (b) 40 ℃; (c) 50 ℃; (d) 60 ℃; (e) 70 ℃;

[0046] Figure 12 For the different Mg of the present invention 2+ Scanning electron microscope images of mineralized CaCO3 at various concentrations; among them,

[0047] (a) 1.25 × 10 -3 mol / L; (b) 2.50 × 10 -3 mol / L; (c) 3.75 × 10 -3 mol / L; (d) 5.00×10 -3 mol / L; (e) 6.25 × 10 -3 mol / L;

[0048] Figure 13 These are scanning electron microscope (SEM) images of the mineralized CaCO3 products under different CO2 flow rates according to the present invention; wherein,

[0049] (a)100 mL / min; (b) 200 mL / min; (c) 300 mL / min; (d) 400 mL / min; (e) 500 mL / min;

[0050] Figure 14 These are SEM images of the mineralized CaCO3 products under different stirring rates according to the present invention; wherein,

[0051] (a) 150 rpm; (b) 250 rpm; (c) 450 rpm; (d) 750 rpm;

[0052] Figure 15 A comparison of the XRD patterns of CaCO3 mineralized by the two methods of this invention;

[0053] Figure 16 These are characterization spectra of samples taken at different reaction times in the study of the formation mechanism of mineralized aragonite in this invention; wherein,

[0054] (a) XRD pattern of mineralized CaCO3 over time (10–80°); (b) XRD pattern of mineralized CaCO3 over time (19.5–32.4°); (c) FTIR pattern of mineralized CaCO3 over time; (d) XRD pattern of mineralized CaCO3 over time. Detailed Implementation

[0055] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0056] Example 1:

[0057] I. Raw Material Characterization

[0058] To avoid the influence of impurities, purified gypsum was used in this study, and its ICP and XRD characterization results are shown in Table 1 and 2. Figure 1 As shown, the raw materials used in this experiment have high purity, with CaSO4·2H2O content exceeding 95%.

[0059] Table 1: Elemental composition of gypsum (%)

[0060] II. Mineralization Experiment

[0061] 1. Two-step mineralization experiment

[0062] The two-step mineralization method first uses NH3·H2O to absorb CO2, and then adds purified gypsum to the NH3·H2O solution that has absorbed CO2 to carry out the mineralization reaction. The absorption and mineralization processes are carried out in steps, which makes it easier to control, as shown in Equations 2 and 3.

[0063] Based on the ratio of CO2 to NH3·H2O, the product of NH3·H2O absorbing CO2 can be either NH4HCO3 or (NH4)2CO3 solution. The final product of NH3·H2O absorbing CO2 to supersaturation is NH4HCO3. Therefore, in this experiment, NH4HCO3 solution is directly used to prepare the mineralization experiment. The specific steps are as follows:

[0064] Add 30 g of purified gypsum to a mixed solution of 90 mL deionized water and 15 mL 25% NH3·H2O. Heat the solution to the specified temperature using a stirring heater. Then add 15 g of NH4HCO3. After reacting for a certain time, filter to separate and wash the solid product. Place the product in an 80 ℃ forced-air drying oven and dry for 12 h before collecting it for later use.

[0065] The effects of two-step mineralization reaction parameters on the crystal form of CaCO3 products were studied by changing temperature, liquid-to-solid ratio, and reaction time to determine the optimal conditions for two-step mineralization and to explore the regulatory effect of different additives on the crystal form of CaCO3. Different additives were added to the solution before adding NH4HCO3.

[0066] 2. One-step mineralization experiment

[0067] First, purified gypsum is added to an NH3·H2O solution to form a slurry, and then CO2 is introduced for mineralization. This couples CO2 absorption and gypsum mineralization into a single reaction, as shown in Formula 1. The specific steps are as follows: 30 g of purified gypsum is added to 90 mL of deionized water, followed by 30 mL of 25% NH3·H2O solution. The mixture is stirred and heated to a specified temperature, and 99.9% CO2 is introduced. After reacting for a certain time, the solid product is filtered to separate it. The solid product is washed 2-3 times with deionized water, and then dried in a forced-air drying oven at 80 ℃ for 12 h. The product is then collected and characterized.

[0068] Change temperature, Mg 2+ The effects and mechanisms of mineralization reaction parameters such as concentration, stirring speed, and CO2 flow rate on the morphology of calcium carbonate products were systematically studied. The reaction temperature was controlled within the range of 30–70 °C. Mg 2+ Concentration 1.25~6.25×10 -3 The concentrations were mol / L, the CO2 flow rates were 100, 200, 300, 400, and 500 mL / min, and the stirring rates were 150, 250, 450, and 750 rpm.

[0069] 3. Characterization of CaCO3 products

[0070] The crystal composition of the CaCO3 product was characterized using a Rigaku Ultima IV X-ray diffractometer (Cu target) with a scan step of 0.02° and a scan range of 10° to 90°. Scanning electron microscopy (SEM) (Thermo Scientific Apreo 2C, operating voltage 10 kV) was used. The percentage content of CaCO3 in the mineralized product was determined by thermogravimetric analysis (TG) under the following conditions: heating from 20 °C to 1000 °C at a heating rate of 10 °C / min, with N2 as the protective and purging gas. The content of each phase of CaCO3 was calculated using Gsas software based on the Rietveld refinement method.

[0071] result:

[0072] Two-step mineralization

[0073] 1. Orthogonal experiment

[0074] Since the two-step mineralization process is easier to control step by step, an orthogonal experiment was first conducted to investigate the effects of reaction temperature, liquid-solid ratio and reaction time on the gypsum conversion rate. The results are shown in Table 2.

[0075] Table 2: Results of the orthogonal experiment on the two-step mineralization method of gypsum

[0076] The two-step mineralization process is an endothermic reaction, resulting in lower gypsum conversion rates at low temperatures. Range analysis indicates that temperature is the key factor affecting CaCO3 conversion. Under the experimental conditions used in this study, conversion rates exceeding 99% were achieved when the reaction temperature was above 20 °C, while the effects of reaction time and liquid-solid ratio were relatively weak. Based on these results, to reduce energy consumption in the subsequent (NH4)2SO4 concentration and crystallization stage, an optimized liquid-solid ratio of 4:1 was selected. Extending the reaction time can improve gypsum conversion, but its effect is limited after 90 min. Considering both conversion rate and production efficiency, 90 min was chosen as the optimal reaction time.

[0077] 2. Two-step analysis of mineralization products

[0078] First, CaCO3 crystals were prepared under the initial optimized conditions of reaction temperature 30 ℃, liquid-solid ratio 4:1, and reaction time 90 min. The XRD pattern and TG curve are shown below. Figure 2 and Figure 3As shown, the CaCO3 prepared by the two-step mineralization method is pure calcite phase, consistent with the literature results. In calculating the purity (%) of CaCO3 using the TG curve, it was assumed that the product lost free water before 300 ℃, and the decomposition and weight loss phase of CaCO3 occurred between 550 and 850 ℃, resulting in a calculated purity of 99.5%.

[0079] Two-step mineralization process regulation

[0080] 1. Reaction temperature

[0081] Orthogonal experiments show that temperature has a significant impact on the two-step mineralization reaction. Under optimized liquid-to-solid ratio and reaction time conditions, the mineralization reaction temperature was further increased to investigate its effect on the crystal form of the mineralization products. Figure 4 The XRD patterns of CaCO3 prepared under different temperature conditions are shown. It is clear that the reaction temperature does not affect the crystal form of the product; even when the reaction temperature is increased to 70 °C, no aragonite phase formation was observed. This is because the two-step mineralization process is a highly supersaturated system, and the mineralization products tend to form stable calcite.

[0082] 2. Additives

[0083] Adding crystallization inducers during the reaction process is a common method to control the crystal form of the product. The morphology and XRD patterns of the CaCO3 products obtained by adding different crystallization inducers in the two-step mineralization process are shown below. Figure 5 and Figure 6 As shown. Na₂CO₃ is added to the mineralization system to increase the ion activity product (Ion Activity Product). It can be observed that adding Na₂CO₃ did not change the crystal form of CaCO₃, which remained pure-phase calcite. Furthermore, the size of the CaCO₃ particles decreased after the addition of Na₂CO₃. Figure 5 (b) The particle size is approximately 0.5–2 μm. Under initial high supersaturation, the nucleation rate is much higher than the crystallization rate, resulting in a reduction in particle size. When the additive is MgSO4, the crystal form remains calcite, but the individual particle size becomes even smaller, approximately 300–500 nm. Figure 5 c) This is because of the small ionic radius of Mg 2+ The slowing of calcite growth is caused by the substitution of Ca lattice sites. When sodium tripolyphosphate (STPP) is added, XRD patterns show that the CaCO3 is mainly composed of aragonite, containing small amounts of calcite and unconverted gypsum. The phosphate ions (PO4) in STPP... 3- ) can interact with Ca 2+ Formation of soluble complexes reduces the amount of free Ca in the solution. 2+The concentration slows down the nucleation rate of CaCO3, and the chain anions of STPP selectively adsorb onto specific crystal faces of CaCO3 to inhibit calcite growth, so that the final product can be CaCO3 with aragonite as the main phase.

[0084] One-step mineralization

[0085] The results above show that during the two-step mineralization process, Ca... 2+ With CO3 2- Under highly supersaturated conditions, the product tends to form stable calcite, making it difficult to achieve directional synthesis of aragonite even with changes in process parameters. Therefore, this study further investigates a one-step mineralization process. By continuously introducing CO2 into a gypsum slurry containing NH3·H2O, CO2 capture and mineralization are simultaneously achieved, maintaining a low CaCO3 supersaturation level in the system and thus providing a suitable reaction environment for aragonite formation.

[0086] Figure 7 and Figure 8 The XRD patterns and TG analysis curves of the mineralized products are shown below. It can be observed that under the conditions of 30 ℃, liquid-to-solid ratio of 4:1, CO2 flow rate of 400 mL / min, reaction time of 60 min, and stirring rate of 250 r / min, the CaCO3 prepared by the one-step mineralization method is a mixed phase of aragonite and calcite, with aragonite being the dominant phase. The purity of the mineralized CaCO3 was calculated to be 99.6% based on the TG curve.

[0087] One-step mineralization process regulation

[0088] In the CaCO3 precipitation process, chemical reaction, crystal nucleation, and crystal growth are the three main steps. Different mineralization processes can yield CaCO3 with different morphologies and grain sizes. Aragonite crystals are unstable, meaning that aragonite synthesis requires strict control of reaction conditions. Furthermore, the key to the application of aragonite lies in its aspect ratio, which is determined by the synthesis method and process parameters. Therefore, to achieve precise control over aragonite, this study conducted a series of optimization experiments, focusing on the morphological evolution of mineralized CaCO3 under different parameters (such as reaction temperature, time, additives, carbon dioxide flow rate, and stirring rate).

[0089] 1. Reaction temperature

[0090] While keeping other conditions constant, the effect of reaction temperature (30, 40, 50, 60, 70 ℃) on the morphology of mineralized CaCO3 was investigated. The conversion rate of gypsum at different temperatures as a function of time is shown in the figure. Figure 9As shown, the gypsum conversion rate reaches 99.5% at 30 ℃ in 40 min. As the temperature increases, the gypsum conversion rate accelerates, and the mineralization reaction can be completed within 20 min when the temperature reaches 50 ℃ or above.

[0091] Figure 10 Tables a and 3 show the XRD patterns and quantitative composition of CaCO3 crystals at different temperatures. Since the main crystalline phase of the gypsum mineralization product is unstable aragonite in aqueous solution and easily transforms into calcite, to avoid an increase in calcite content in the product due to excessive aging time, CaCO3 prepared in the shortest time required for gypsum to reach the highest conversion rate was selected for determination.

[0092] At lower temperatures (30~50 °C), aragonite is the main crystalline phase. Figure 10 a and Figure 11 (a~c) The calcite diffraction peak is located at 2θ = 29.40° [(104)], and the aragonite diffraction peak is located at 2θ = 21.00° [(004)], 24.90° [(110)], 27.05° [(112)] and 32.78° [(114)]. The aragonite phase appears at 60 °C (2θ = 26.21° [(111)]), and its diffraction intensity is significantly enhanced at 70 °C, and a needle-like crystal morphology with a high aspect ratio can be observed. Figure 11 (d~e). This result indicates that aragonite crystal forms can be obtained under these temperature conditions, which is consistent with the literature's conclusion that aragonite can form in similar temperature ranges.

[0093] Quantitative phase analysis showed that the aragonite content increased from 6.0% at 60 °C to 36.3% at 70 °C, indicating that high temperature promoted aragonite crystallization. Aragonite generally has higher solubility than calcite and tends to form calcite at lower temperatures; however, high temperature reduced the solubility difference between aragonite and calcite, and the increased ion migration rate provided both thermodynamic driving force and favorable kinetic conditions for aragonite formation. Table 3 shows that the calcite content increased with increasing temperature, consistent with the high-temperature accelerated calcite growth rate reported by Rodriguez-Blanco et al. The calcite content decreased at 70 °C compared to 60 °C, attributed to the greater conversion of aragonite into aragonite, thus reducing the net accumulation of calcite. Therefore, to obtain the best aragonite preparation effect, the reaction temperature should be controlled above 70 °C.

[0094] Table 3: Quantitative composition (wt%) of mineralized CaCO3 at different temperatures

[0095] 2. Reaction time

[0096] Experiments have demonstrated that high-temperature conditions can promote aragonite formation. While maintaining a high temperature of 70 °C and keeping other conditions constant, the effect of reaction time (20, 30, 40, 50, 60 min) on the composition of mineralized CaCO3 was investigated. Table 4 shows the quantitative composition of CaCO3 prepared at different reaction times under 70 °C conditions. The CaCO3 product prepared at the optimal conversion rate of 20 min already contained 36.3% aragonite structure. However, in the subsequent 40 min, the aragonite content only increased from 36.3% (20 min) to 38.3% (60 min), confirming the kinetically constrained Ostwald ripening process (i.e., a natural transformation process of small particle dissolution and large particle growth, and a very slow rate in this system). These results indicate that in the absence of a crystal form inducer, aragonite is difficult to completely transform into the more thermodynamically stable aragonite and calcite in a short time; furthermore, simply extending the reaction time cannot effectively obtain high-purity aragonite products.

[0097] Table 4: Quantitative composition (wt%) of mineralized CaCO3 at different times

[0098] 3. Additives

[0099] It is foreseeable that extending the aging time under high temperature conditions can yield a higher aragonite content, but it will inevitably be accompanied by the simultaneous growth of calcite, and the time cost is too high. Therefore, while keeping other conditions unchanged, different crystal form inducers (Na2CO3, MgSO4, STPP) were added to the solution before CO2 was introduced to explore their effects on the composition of CaCO3 products. Figure 10 b shows the XRD patterns of CaCO3 prepared by mineralization at 70 °C with different crystal form inducers. It can be clearly seen that aragonite becomes the dominant crystalline phase in the CaCO3 product with the addition of MgSO4. Similar to the results obtained by the two-step method, the CaCO3 product synthesized after adding STPP also mainly exists in the aragonite crystal form, but due to PO4… 3- The adsorption of Mg inhibits the mineralization reaction, resulting in residual gypsum raw materials in the product. Based on these results, Mg was added... 2+ It can effectively promote the preparation of high-purity aragonite whiskers.

[0100] 4.Mg 2+ Added amount

[0101] Existing research has shown that calcite formation can be achieved by using magnesium-based additives, which can inhibit calcite growth. Figure 10 c and Table 5 show the different Mg values ​​respectively. 2+XRD patterns and quantitative composition of CaCO3 generated under specific addition conditions. (In the absence of Mg) 2+ In the sample, aragonite and spherulite were the main crystalline phases, accounting for 38.3% and 45.9%, respectively. Mg was added. 2+ Subsequently, the contents of aragonite and calcite gradually decreased, and aragonite became the main crystalline phase, indicating that Mg... 2+ The addition of Mg can inhibit calcite formation and promote the transformation of unstable aragonite into aragonite. 2+ The concentration is 5.00 × 10⁻⁶. -3 At mol / L, the proportion of aragonite phase reached a peak of 94.8%, but further increases in Mg... 2+ Up to 6.25×10 -3 On the contrary, mol / L inhibits the formation of aragonite, reducing its content to 90.2%.

[0102] Table 5: Different Mg 2+ Quantitative composition (wt%) of mineralized CaCO3 at different addition levels

[0103] The morphology of CaCO3 was also affected by Mg. 2+ Significant effect of dosage ( Figure 12 With Mg 2+ With the increase of [amount], its morphology changes from a short, thick rod-like shape to a long, thin needle-like shape. At 5.00 × 10 [units / sizes]... -3 mol / L Mg 2+ At a concentration of [missing information], a needle-like aragonite structure with an aspect ratio of approximately 10:1 was observed. Conversely, at higher concentrations (6.25 × 10 [missing information]), [missing information]... -3 At concentrations of Mg (mol / L), aragonite growth was actually inhibited, resulting in coarse rod-like shapes. Santos et al. found in their study that high concentrations of Mg... 2+ It can promote CO2 absorption efficiency, which to some extent increases the supersaturation of the system, making calcite grow more preferentially. Mg 2+ It plays a key promoting role in the crystallization process of aragonite; however, excessive Mg... 2+ This will significantly inhibit its growth kinetics and phase transformation process. Therefore, Mg in the system 2+ The concentration of aragonite needs to be controlled within a suitable range in order to achieve the best aragonite synthesis effect.

[0104] 5. CO2 flow rate

[0105] XRD patterns and quantitative composition of CaCO3 generated under different CO2 flow rates are as follows: Figure 10As shown in d and Table 6, the results indicate that the aragonite content in the product gradually increases with increasing CO2 flow rate. Scanning electron microscopy (SEM) images show that relatively regular needle-like aragonite clusters can be formed when the CO2 flow rate is above 400 mL / min. At low CO2 flow rates, the crystallization rate decreases, particle growth is slow, and the particles are short and coarse rod-shaped. Particle size analysis also shows that the smallest particle size was measured in the sample with the highest aragonite content (400 mL / min). Quantitative composition results indicate that high-purity aragonite-type CaCO3 (approximately 95%) was prepared under the condition of 400 mL / min CO2.

[0106] Table 6: Quantitative composition (wt%) of mineralized CaCO3 under different CO2 flow rates

[0107] 6. Stirring speed

[0108] Figure 10 Tables e and 7 show the XRD patterns and quantitative compositions of CaCO3 prepared at different stirring speeds. The results indicate that as the stirring speed gradually increases to 250 rpm, the aragonite content increases to 94.8%. However, with further increases in the stirring speed, the calcite content gradually increases; for example, at 750 rpm, the calcite content increases from 5.2% at 250 rpm to 9.2%.

[0109] Table 7: Quantitative composition (wt%) of mineralized CaCO3 at different stirring rates

[0110] Excessive stirring speed will lead to a significant increase in amorphous products and calcite. Figure 14 c~d), and form aragonite clusters with low aspect ratios. Increased stirring speed leads to Mg 2+ The system cannot inhibit calcite growth for extended periods, and high-speed stirring accelerates the dissolution of the precursor, making it easier for the system to form stable calcite crystals. A stirring speed of 250 rpm can more stably generate regular, high aspect ratio aragonite whiskers; stirring too fast or too slow will lead to an increase in calcite. The stirring speed affects the mass transfer process but does not significantly affect the crystal morphology, although it can prevent CaCO3 agglomeration.

[0111] 7. Comparison of mineralized CaCO3 particle size

[0112] The particle size distribution results of the two mineralization methods are shown in Tables 8 and 9. Compared with the particle size distribution results of the one-step method, the CaCO3 particles prepared by the two-step method have significantly smaller particle sizes. This is attributed to the higher supersaturation of the two-step system, which leads to a faster nucleation rate, and rapid nucleation usually results in a smaller grain size. Furthermore, the test data of the one-step method further indicate that the reduction in particle size in the product is positively correlated with the increase in aragonite phase content.

[0113] Table 8: Particle size test results of the two-step orthogonal experiment

[0114] Table 9: Particle size distribution of CaCO3 mineralized by one-step method

[0115] 8. Stability analysis of mineralized CaCO3

[0116] To compare the stability of calcite and aragonite in aqueous solution, dynamic phase transformation analysis experiments were conducted on the two crystalline forms of CaCO3 in aqueous solution. The stability of different crystalline forms of CaCO3 was studied, and the results are shown in Table 10. After sand milling in aqueous solution for different times, the mineralized CaCO3 with different initial crystalline compositions showed that aragonite rapidly transformed into calcite at 25 °C, with a conversion rate of 100% within 2 hours; aragonite maintained approximately 95% phase purity within 4 hours, and the formation of calcite was negligible. The outstanding stability of aragonite allows it to withstand mechanical processing to refine the particle size without undergoing a phase transformation, confirming its practical value as a functional filler.

[0117] Table 10: Quantitative composition of mineralized CaCO3 as a function of milling time

[0118] 9. Analysis of Aragonite Formation Mechanism

[0119] Figure 15 The effects of different mineralization pathways and temperatures on the crystal form of CaCO3 were compared. Mg was added before CO2 gas was introduced. 2 + To a concentration of 5.00 × 10⁻⁶ -3 mol / L. At 70 °C, the two-step method, due to the extremely high initial supersaturation, although the product was still mainly calcite, showed characteristic peaks of aragonite in the final product, proving that Mg... 2+ This process inhibits calcite growth. In contrast, the one-step method maintains a lower and more stable supersaturation by controlling the CO2 addition rate. Under these conditions, high temperature and Mg... 2+The synergistic induction effect was fully utilized, significantly promoting the nucleation of aragonite while inhibiting calcite.

[0120] The XRD and FTIR results of the evolution of CaCO3 crystal form over time during the one-step synthesis under the optimized reaction pathway and process conditions obtained in this study are as follows: Figure 15 and Figure 16 As shown, the characteristic peak corresponding to gypsum in the XRD pattern (2θ = 11.70°[(020)]) gradually weakens as the reaction proceeds and disappears completely at 20 min, indicating that the gypsum has been completely transformed at this time.

[0121] Combining the magnified XRD pattern and FTIR spectrum, it can be clearly seen that the characteristic peaks of aragonite first appeared in the system when the reaction had only proceeded for 5 minutes (XRD: 2θ = 26.22° [(111)] and 27.22° [(021)]; FTIR: 854 cm⁻). 1 The out-of-plane bending vibration peak was observed at this point, and calcite and aragonite were not detected in the system at this time. This absolute dominance of aragonite nucleation in the early stage is mainly attributed to the presence of Mg in the system. 2+ The crystal structure guiding effect. From the perspective of crystal coordination chemistry, the oxygen atom in the calcite structure is composed of two Ca atoms. 2+ Coordination (coordination number 6), Mg with a small ionic radius 2+ It readily enters calcium lattice sites, inducing strong lattice distortion and thus inhibiting calcite growth; Ca in the aragonite structure 2+ High coordination number (9) requires larger cation size, Mg 2+ It is difficult for aragonite to be incorporated. Therefore, driven by both the inhibition effect and high temperature, aragonite preferentially forms nuclei.

[0122] As the mineralization reaction proceeds, Ca 2+ The gradual consumption of CaCO3 caused fluctuations in supersaturation and the Mg / Ca ratio, and some CaCO3 began to precipitate through lower-energy pathways. At this point, metastable aragonite and a small amount of calcite were observed in the system. In the later stages of the reaction and during the aging process (20 min to 60 min), the metastable aragonite underwent Ostwald aging. change A dissolution-recrystallization process occurs. At high temperatures and with Mg... 2+ Under continuous induction, aragonite gradually transforms into the aragonite crystal form. Figure 16 (d) Upon complete reaction, through optimization of process conditions and the synergistic effect of additives, high-purity aragonite ultimately becomes the dominant crystalline phase of the product. Based on the above research results, the aragonite formation process can be divided into the following four stages:

[0123] Phase 1 (0-5 min) - Reaction Preparation: After CO2 is introduced into the solution, the pH of the system gradually decreases and tends to stabilize. Gypsum begins to slightly dissolve, releasing Ca. 2+ The supersaturation of the system increases slowly. During this stage, XRD did not detect the CaCO3 phase; the system mainly consisted of CO3. 2- To prepare for enrichment.

[0124]

[0125]

[0126]

[0127] The second stage (5-10 min) - dominant nucleation of aragonite: When supersaturation reaches a critical point, the mineralization reaction begins. Mg 2+ This stage plays a crucial role in crystal form guidance, strongly inhibiting the nucleation and growth of calcite. Driven by a high temperature of 70 °C, aragonite overcomes the energy barrier and preferentially nucleates.

[0128] The third stage (10-20 min) - formation of polycrystalline phases: As the reaction continues, a large number of aragonite nuclei are formed, consuming CO3 in the local area. 2- and Ca 2+ The differences in dissolution equilibrium and diffusion rate on the surface of gypsum particles cause fluctuations in the Mg / Ca ratio in local areas. In this unstable ionic environment, some solutes crystallize into calcite and metastable aragonite via lower energy pathways, forming a multiphase system in which aragonite, aragonite, and a small amount of calcite coexist.

[0129] Fourth stage (20-60 min) - Crystal transformation: When the gypsum is completely consumed, the supersaturation of the system decreases and tends to equilibrium. According to Oswald's ripening theory, the metastable aragonite in the aqueous phase dissolves, and under high temperature and Mg... 2+ Under the synergistic regulation of [unclear], it re-precipitates onto the surface of the existing aragonite seed crystals. After 60 minutes of aging, the aragonite is completely transformed into acicular aragonite with excellent aspect ratio.

[0130] This invention systematically compares the process routes of one-step and two-step mineralization of gypsum. A one-step method was successfully used to prepare calcite whiskers from purified gypsum, and Mg was introduced... 2+ As a crystal form inducer, the formation mechanism of the aragonite phase was investigated in depth. The main conclusions are as follows:

[0131] (1) Using a two-step mineralization process, under the conditions of 30 °C, liquid-solid ratio of 4:1 and reaction time of 90 min, the gypsum conversion rate can reach more than 99%, and the purity of the obtained CaCO3 is 99.5%. XRD characterization shows that it is pure phase calcite.

[0132] (2) A one-step mineralization process was adopted, at 70 °C, a liquid-to-solid ratio of 4:1, and Mg 2+ Concentration 5×10 -3 Under the conditions of mol / L, CO2 flow rate of 400 mL / min, stirring rate of 250 r / min, and reaction time of 1 h, CaCO3 with an aragonite content of about 95% can be synthesized, and its crystal aspect ratio is about 10:1.

[0133] (3) Reaction temperature, Mg 2+ Adjusting parameters such as concentration and CO2 flow rate can effectively control the phase composition of the final CaCO3 product.

[0134] (4) The formation process of aragonite can be divided into four stages, among which the nucleation stage plays a key role in aragonite growth, and its main body is formed in this stage. After the reaction is completed, the remaining aragonite, under the influence of temperature and Mg... 2+ Under the combined action of these factors, it is transformed into aragonite through a dissolution-recrystallization process.

[0135] This study achieved the high-value transformation of gypsum solid waste and simultaneously mineralized and fixed CO2, providing a feasible technical path for the large-scale disposal of gypsum and the preparation of functional calcium carbonate products, and has good prospects for industrial application.

[0136] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum, characterized in that, Includes the following steps: (1) Pulping: Add purified gypsum to deionized water, then add NH3·H2O solution, stir and mix to form a suspension; (2) Mineralization reaction: The suspension obtained in step (1) is stirred and heated, and CO2 gas with a concentration of 10%~99.9% is introduced. The CO2 flow rate is controlled at 100~500 mL / min and the stirring rate is 150~750 rpm. Mg is added at the same time. 2+ To react; (3) Solid-liquid separation and drying: After the reaction is completed, the solid product is separated by filtration, rinsed, and dried in a forced-air drying oven to obtain aragonite-type calcium carbonate.

2. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, In step (1), the amount of purified gypsum added is 30 g, the amount of deionized water added is 90 mL, and the amount of NH3·H2O solution with a mass fraction of 25% added is 30 mL.

3. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, In step (1), the ratio of purified gypsum to deionized water and 25% NH3·H2O solution is 1g:3mL:1mL.

4. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, The reaction temperature in step (2) is controlled at 30~70 ℃.

5. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, The Mg mentioned in step (2) 2+ Concentration range: 1.25~6.25×10 -3 mol / L.

6. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, The CO2 flow rate in step (2) is 100 mL / min to 500 mL / min.

7. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, The stirring rate in step (2) is 150 rpm to 750 rpm.

8. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, The drying conditions in step (3) are drying at 80 ℃ for more than 4 hours.

9. The method for preparing aragonite-type calcium carbonate by one-step mineralization of CO2 using gypsum according to claim 1, characterized in that, In step (3), rinse with deionized water 2 to 3 times.

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