A method for preparing high-purity vaterite based on alcohol amine-phosphogypsum synergistic system

Abstract

This invention relates to the field of solid waste resource utilization and carbon dioxide mineralization, and discloses a method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system. The method includes the following steps: pulverizing and drying phosphogypsum, reacting it fully with a multidentate chelating alkanolamine solution, and then separating the solid and liquid phases to obtain purified phosphogypsum; mixing the purified phosphogypsum with a monoethanolamine solution, introducing a mixed gas containing CO2, and carrying out a mineralization reaction under stirring; after the reaction, separating the solid and liquid phases to obtain aragonite solid and liquid phases; washing and drying the aragonite solid to obtain aragonite; this invention uses a multidentate chelating alkanolamine as a phosphogypsum impurity remover, achieving "fluorine dissolution and calcium retention," with a fluorine removal rate ≥90% and a calcium sulfate loss rate ≤5%; during the mineralization process, monoethanolamine is used to inhibit the conversion of aragonite to calcite to obtain high-purity aragonite, and the obtained aragonite has a spherical morphology and uniform particle size distribution. This invention has no high-temperature calcination or acidic waste liquid throughout the entire process, and has significant advantages such as low energy consumption, high stability, and synergistic resource utilization.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization and carbon dioxide mineralization, specifically a method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system. Background Technology

[0002] Aragonite is a metastable crystal form of calcium carbonate, possessing high specific surface area, good biocompatibility, and adsorption properties, and is widely used in biomedicine, food additives, and high-grade coatings. However, there are three major pain points in the current preparation of aragonite: (1) The biggest technical bottleneck in the traditional preparation of aragonite lies in its thermodynamic metastable characteristics. At room temperature, it is very easy to transform into thermodynamically stable calcite, and usually requires the addition of crystal form control agents (such as Mg²⁺). + (1) Citrate) and its stability is difficult to guarantee; (2) The utilization rate of calcium source is low and the cost of traditional calcium source (calcium chloride, calcium hydroxide) is high; (3) The CO2 conversion efficiency is insufficient and the process is complicated and energy consumption is high.

[0003] Existing alkanolamine methods are mainly used for CO2 capture, which require high-temperature regeneration (energy consumption ≥800kJ / mol CO2). There are few studies on the synergistic effect of defluorination and aragonite crystal form regulation. As a solid waste of phosphorus chemical industry (annual emissions exceed 80 million tons), phosphogypsum has a resource utilization rate of less than 40%. Existing technologies mostly use high-temperature calcination (≥800℃) to extract calcium source, which is energy-intensive and easily causes secondary pollution. More importantly, phosphogypsum typically contains 2%-3% fluoride, mainly in the form of insoluble calcium fluoride (CaF2), which has extremely low solubility (approximately 0.0016 g / 100 mL at 18°C). Traditional physical purification methods such as water washing and flotation are insufficient in removing insoluble fluoride (<30%). While acid leaching can improve the defluorination rate, it introduces a large amount of acidic waste liquid, causing secondary pollution. Lime neutralization can precipitate some fluoride ions, but it is difficult to effectively treat the already stable CaF2 crystals, resulting in excessive fluoride content in phosphogypsum resource products, which seriously restricts its application in high-value fields such as building materials and pharmaceuticals. Therefore, developing a "low-energy-consumption, high-stability, and resource-synergistic" aragonite preparation technology to achieve efficient removal of insoluble fluoride from phosphogypsum is of significant practical importance. Summary of the Invention

[0004] This invention aims to solve the problems of unstable crystal form, low calcium source utilization, insufficient CO2 conversion efficiency, and complex processes in the preparation of aragonite. This invention provides a method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system, comprising the following steps: (1) After pulverizing and drying the phosphogypsum, it is reacted with a multidentate chelating alcohol amine solution and then the solid and liquid are separated to obtain the impurity-free phosphogypsum.

[0005] (2) Mix the impurity-removed phosphogypsum with monoethanolamine solution evenly, introduce a mixed gas containing CO2 and carry out a mineralization reaction under stirring. After the reaction is completed, solid and liquid phases are separated to obtain aragonite solid and liquid phase. After washing the aragonite solid, dry it to obtain aragonite.

[0006] (3) After adding monoethanolamine to the liquid phase and adjusting the pH, it is reused in step (2) for continued use.

[0007] Preferably, in step (1), the liquid-to-solid ratio of the multidentate chelating alkanolamine solution to phosphogypsum is (8~15):1, in mL:g; and the mass fraction of the multidentate chelating alkanolamine solution is 1%~2%.

[0008] Preferably, the multidentate chelating alcohol amine in step (1) is one or more of diethanolamine, triethanolamine or N-methyldiethanolamine mixed in any proportion.

[0009] Preferably, the reaction conditions in step (1) are stirring at 40~80℃ for 10~60 min.

[0010] More preferably, the reaction conditions in step (1) are 40~60℃ and the stirring time is 10~30 min.

[0011] Preferably, in step (2), the liquid-to-solid ratio of monoethanolamine solution to impurity-removed phosphogypsum is (3~9):1, in mL:g; the mass fraction of the monoethanolamine solution is 20%~40%.

[0012] Preferably, in step (2), the volume fraction of carbon dioxide in the mixed gas is 10%~20%, the flow rate of the mixed gas is 100~300 ml / min, the stirring speed is 300~800 rpm, the mineralization reaction temperature is 20~40℃, and the final pH of the mineralization reaction is 7.5~8.5.

[0013] Preferably, in step (2), the washing is performed 2 to 4 times with anhydrous ethanol or a 95% ethanol solution.

[0014] Preferably, in step (3), the amount of monoethanolamine added is 20% to 30% of the liquid phase mass, and the pH is adjusted to 11.5 to 12.5.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) Achieving efficient and selective removal of sparingly soluble fluorides from phosphogypsum: This invention innovatively utilizes multidentate chelating alkanolamines to overcome the technical bottleneck of separating calcium sulfate and calcium fluoride in phosphogypsum. Furthermore, this invention controls the multidentate chelating alkanolamines to be at a low concentration (1%~2%), at an appropriate temperature (40~60℃), and under short stirring time (10~30 min), thereby enhancing the utilization of the advantages of CaF2 dissolution kinetics—the slow dissolution of CaF2 continuously complexes and removes Ca²⁺. + Driven by the process, the rapid dissolution of CaSO4 quickly reaches a dynamic equilibrium. Thus, this invention achieves selective impurity removal through "fluoride dissolution and calcium retention," with a fluoride removal rate ≥90% and a calcium sulfate loss rate <5%. This overcomes the challenge of low removal efficiency (<30%) for sparingly soluble fluorides in existing technologies, while also preventing significant loss of calcium source, providing high-quality raw materials for subsequent high-purity aragonite preparation.

[0016] (2) Low-energy consumption and low-carbon emission preparation process of aragonite: In the impurity removal step, the present invention uses multi-toothed chelating alkanolamine for selective defluorination, and in the mineralization step, it uses monoethanol to specifically stabilize the aragonite crystal form. The two alkanolamines work synergistically but with functional differentiation. High-purity aragonite with a purity of ≥95% and a room temperature stability of ≥30 days can be prepared without the addition of inorganic crystal form control agents. This significantly simplifies the process and increases the added value of the product. At the same time, the present invention uses an alkanolamine-carbon dioxide mineralization system, which can realize the preparation of aragonite at room temperature and pressure without the need for traditional high-temperature calcination (≥800℃) or high-temperature regeneration (≥800 kJ / molCO2) processes. It directly uses industrial carbon dioxide waste gas as a carbon source to realize the mineralization and fixation of CO2, forming a synergistic coupling of "phosphogypsum resource utilization-carbon capture and utilization", which has good environmental benefits and carbon emission reduction benefits.

[0017] (3) High calcium source utilization and recycling of alcohol amines: This invention uses phosphogypsum as a calcium source to replace traditional high-cost chemical raw materials such as calcium chloride and calcium hydroxide. The calcium source utilization rate can reach more than 95%, realizing the high-value utilization of solid waste from phosphorus chemical industry. In addition, this invention achieves the recycling of alcohol amine solution by adding monoethanolamine to the liquid phase after solid-liquid separation and adjusting pH, which significantly reduces reagent consumption and operating costs, forming a closed-loop process that conforms to the concepts of green chemical industry and circular economy.

[0018] (4) High purity and high stability of aragonite products: The aragonite prepared by this invention has a purity of ≥95%, effectively stabilizes the crystal form, and solves the technical problem of easy transformation of the crystal form into calcite in the traditional preparation of aragonite. The single system itself has the function of regulating the crystal form of aragonite, without the need to add additional crystal form control agents, which simplifies the process, improves the purity of the product, and broadens the application prospects of the product in high value-added fields such as biomedicine, food additives, and high-grade coatings.

[0019] (5) Simple process and easy to scale up industrially: The process of this invention is simple, the equipment requirements are low, and the operating conditions are mild (normal temperature, normal pressure, near neutral pH). It is easy to realize continuous production and industrial scale-up. Compared with the existing high-temperature calcination extraction of calcium source or complex acid leaching process of phosphogypsum, this invention avoids secondary pollution problems such as high-temperature equipment corrosion and acidic waste liquid treatment, and has good industrial application prospects and economic benefits. Attached Figure Description

[0020] Figure 1 This is a flowchart of the preparation process for high-purity aragonite based on the synergistic system of alkanolamine-phosphogypsum proposed in this invention.

[0021] Figure 2 These are scanning electron microscope (SEM) images of the morphology of the spheroidal aragonite prepared in Example 1 and the comparative example. Detailed Implementation

[0022] 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.

[0023] Example 1 A method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system, the specific steps of which are as follows: (1) Selective removal of impurities from phosphogypsum: Phosphogypsum was crushed to a particle size of 30-50 μm and dried at 120℃ for 12 h. 100 g of dried phosphogypsum was weighed and mixed with 1000 mL of 1.5% diethanolamine solution (liquid-solid ratio 10:1, unit mL:g). The mixture was placed in a 50℃ water bath and stirred at 400 rpm for 20 min. After stirring, the mixture was vacuum filtered. The filter cake was washed three times with deionized water to obtain 82.3 g of phosphogypsum with impurities removed. The removal rate of insoluble fluoride was 92.5%, and the loss rate of calcium sulfate was 3.8%, achieving selective removal of impurities by "fluorine dissolution and calcium retention".

[0024] (2) Directional mineralization of aragonite: 82.3 g of purified phosphogypsum obtained in step (1) was mixed with 500 mL of a 25% monoethanolamine solution (liquid-solid ratio 6:1, unit: mL:g), and placed in a reactor equipped with a gas distributor. A mixture of CO2 and N2 gas (CO2 volume fraction in the mixed gas was 15%) was introduced at a flow rate of 200 mL / min, a stirring speed of 500 rpm, and a reaction temperature of 25℃. The pH of the slurry was monitored in real time, and the reaction was stopped when the pH dropped to 8.0. A white solid and a liquid phase were obtained by solid-liquid separation. The white solid was washed three times with anhydrous ethanol and dried under vacuum at 60℃ for 4 h to obtain the aragonite product.

[0025] (3) Fresh monoethanolamine (25% of the initial mass of the liquid phase) was added to the liquid phase obtained in step (2), and the pH was adjusted to 12.0 (initial value) with a 30% sodium hydroxide solution. The solution was reused for the next batch of mineralization reaction. After 5 reuses, the purity of the aragonite was still above 96%. The calcium source utilization rate was calculated to be 96.2%, and the CO2 mineralization fixation rate was 91.5%.

[0026] The scanning electron microscope image of the aragonite prepared in this embodiment is as follows: Figure 2 As shown in (a), it can be seen from the figure that the aragonite prepared by the method of the present invention has a spherical morphology and a uniform particle size distribution (the scale bar of the image is 500 nm).

[0027] Example 2 A method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system, the specific steps of which are as follows: (1) Selective removal of impurities from phosphogypsum: Phosphogypsum was crushed to a particle size of 30-50 μm and dried at 120℃ for 12 h. 125 g of dried phosphogypsum was weighed and mixed with 1000 mL of 2% triethanolamine solution (liquid-solid ratio 8:1, unit mL:g). The mixture was placed in a 40℃ water bath and stirred at 400 rpm for 60 min. After stirring, the mixture was vacuum filtered. The filter cake was washed three times with deionized water to obtain 106.2 g of phosphogypsum with impurities removed. The removal rate of insoluble fluoride was 90%, and the loss rate of calcium sulfate was 4.2%, achieving selective removal of impurities by "fluorine dissolution and calcium retention".

[0028] (2) Directional mineralization of aragonite: 106.2 g of impurity-removed phosphogypsum obtained in step (1) was mixed with 320 mL of 40% monoethanolamine solution (liquid-solid ratio 3:1, unit is mL:g) and placed in a reaction vessel with a gas distributor. A mixture of CO2 and N2 gas (the volume fraction of CO2 in the mixed gas was 20%) was introduced. The gas flow rate was 300 mL / min, the stirring speed was 300 rpm, the reaction temperature was 20℃, and the pH of the slurry was monitored in real time. When the pH dropped to 8.5, the reaction was stopped. The solid and liquid phases were separated to obtain a white solid and a liquid phase. The white solid was washed three times with anhydrous ethanol and dried under vacuum at 60℃ for 4 h to obtain the aragonite product.

[0029] (3) Fresh monoethanolamine (30% of the initial mass of the liquid phase) was added to the liquid phase obtained in step (2), and the pH was adjusted to 12.5 (initial value) with a sodium hydroxide solution with a mass concentration of 30%. The solution was reused for the next batch of mineralization reaction. After 5 reuses, the purity of the aragonite was still above 97%. The calcium source utilization rate was calculated to be 95.2%, and the CO2 mineralization fixation rate was 92.0%.

[0030] The spherical aragonite prepared in this embodiment has a spherical morphology and a uniform particle size distribution.

[0031] Example 3 A method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system, the specific steps of which are as follows: (1) Selective removal of impurities from phosphogypsum: Phosphogypsum was crushed to a particle size of 30-50 μm and dried at 120℃ for 12 h. 67 g of dried phosphogypsum was weighed and mixed with 1000 mL of N-methyldiethanolamine solution with a mass fraction of 1.5% (liquid-solid ratio of about 15:1, unit mL:g). The mixture was placed in an 80℃ water bath and stirred at 400 rpm for 10 min. After stirring, the mixture was vacuum filtered. The filter cake was washed three times with deionized water to obtain 57.8 g of phosphogypsum with impurities removed. The removal rate of insoluble fluoride was 92.5%, and the loss rate of calcium sulfate was 3.8%, achieving selective removal of impurities by "fluorine dissolution and calcium retention".

[0032] (2) Directional mineralization of aragonite: 57.8 g of impurity-removed phosphogypsum obtained in step (1) was mixed with 500 mL of monoethanolamine solution with a mass fraction of 20% (liquid-solid ratio of about 9:1, unit is mL:g), and placed in a reaction vessel with a gas distributor. A mixture of CO2 and N2 gas (the volume fraction of CO2 in the mixed gas was 10%) was introduced, the gas flow rate was 100 mL / min, the stirring speed was 800 rpm, the reaction temperature was 40℃, the pH of the slurry was monitored in real time, and the reaction was stopped when the pH dropped to 7.5. The solid and liquid phases were separated to obtain a white solid and a liquid phase. The white solid was washed three times with anhydrous ethanol (liquid-solid ratio of 4:1 each time), and dried under vacuum at 60℃ for 4 h to obtain the aragonite product.

[0033] (3) Fresh monoethanolamine (20% of the initial mass of the liquid phase) was added to the liquid phase obtained in step (2), and the pH was adjusted to 11.5 (initial value) with a 30% sodium hydroxide solution. The solution was reused for the next batch of mineralization reaction. After being reused 5 times, the purity of the aragonite was still above 95%. The calcium source utilization rate was calculated to be 93.4%, and the CO2 mineralization fixation rate was 90%.

[0034] Comparative Example 1 In comparison, this comparative example differs from Example 1 in that the diethanolamine in step (1) is replaced with monoethanolamine, while the remaining steps are the same as in Example 1.

[0035] In this comparative example, due to the loss of a large amount of calcium source in the impurity removal step, the yield of aragonite was reduced by 35%. In addition, due to the interference of residual sulfate in the filtrate, the product contained a small amount of gypsum impurities. XRD showed that the purity of aragonite was reduced to 89.5%, and it was mixed with calcite phase (about 8%).

[0036] The scanning electron microscope image of the aragonite prepared in this comparative example is shown below. Figure 2 As shown in (b), it can be seen from the figure that the calcium carbonate prepared in this comparative example is a loose spherical aragonite with an uneven particle size distribution (the scale bar of the image is 500 nm).

[0037] Comparative Example 2 In comparison, this comparative example differs from Example 1 in that monoethanolamine in step (2) is replaced with diethanolamine; all other steps are the same as in Example 1. The scanning electron microscope image of the aragonite prepared in this comparative example is shown below. Figure 2 As shown in (c), it can be seen from the figure that the calcium carbonate prepared in this comparative example has an irregular blocky morphology and no typical spherical structure, which cannot meet the application requirements of spheroidal aragonite (the scale bar of the image is 500 nm).

[0038] Comparative Example 3 In comparison, the only difference between this comparative example and Example 1 is that monoethanolamine in step (2) is replaced with triethanolamine, while the rest of the steps are the same as in Example 1.

[0039] The scanning electron microscope image of the aragonite prepared in this comparative example is shown below. Figure 2 As shown in (d), it can be seen from the figure that the calcium carbonate prepared in this comparative example is acicular aragonite, which has poor thermal stability and cannot meet the application requirements of spheroidal aragonite (the scale bar of the image is 500 nm).

[0040] Comparative Example 4 In comparison, the only difference between this comparative example and Example 1 is that monoethanolamine in step (2) is replaced with piperazine, while the rest of the steps are the same as in Example 1.

[0041] The scanning electron microscope image of the aragonite prepared in this comparative example is shown below. Figure 2 As shown in (e), it can be seen from the figure that the calcium carbonate prepared in this comparative example is calcite with uneven particle size distribution, which cannot meet the application requirements of aragonite (the scale bar of the image is 500 nm).

[0042] Table 1 As can be seen from Table 1 and the experimental data above, the purity of the aragonite prepared by the method described in this invention is better than that prepared by the comparative example. The reasons are as follows: First, in the impurity removal step of phosphogypsum, this invention uses a multidentate chelate alcohol amine for impurity removal. The differential regulation of the dissolution equilibrium of CaSO4 and CaF2 by the multidentate chelate alcohol amine achieves selective impurity removal of "fluorine dissolution and calcium retention". Second, in the mineralization step, this invention innovatively uses high-concentration monoethanolamine (MEA) as a solvent and crystal form directing agent. It utilizes its unique molecular structure to achieve selective stabilization of the aragonite crystal form. The MEA molecule is small in size (compared to diethanolamine, triethanolamine, etc.), and its primary amine structure makes it exhibit a linear extended conformation in solution. It can preferentially adsorb onto the (001) crystal face of aragonite and occupy active growth sites, forming a steric hindrance layer, inhibiting the orientation aggregation and dissolution-recrystallization process between aragonite grains, thereby blocking its transformation path to calcite. In contrast, diethanolamine (DEA) and triethanolamine (TEA) have multiple hydroxyethyl branches in their molecules, resulting in greater steric hindrance and making it difficult for them to effectively embed into the aragonite crystal face; at the same time, their strong chelating ability readily binds with Ca²⁺. + The formation of stable cyclic complexes interferes with the orientational growth of calcium carbonate crystal nuclei, resulting in products that are mixed crystals of calcite and aragonite or amorphous calcium carbonate.

[0043] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-purity aragonite based on an alkanolamine-phosphogypsum synergistic system, characterized in that: The specific steps of the method are as follows: (1) After pulverizing and drying the phosphogypsum, it is reacted fully with a multidentate chelating alcohol amine solution and then separated into solid and liquid to obtain impurity-free phosphogypsum; (2) Mix the impurity-removed phosphogypsum with monoethanolamine solution evenly, introduce a mixed gas containing CO2 and carry out a mineralization reaction under stirring. After the reaction is completed, solid and liquid are separated to obtain aragonite solid and liquid phase. The aragonite solid is washed and dried to obtain aragonite. (3) After adding monoethanolamine to the liquid phase and adjusting the pH, it is reused in step (2) for continued use.

2. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: In step (1), the liquid-to-solid ratio of the multidentate chelating alkanolamine solution to phosphogypsum is (8~15):1, in mL:g; the mass fraction of the multidentate chelating alkanolamine solution is 1%~2%.

3. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: The multidentate chelating alcohol amine mentioned in step (1) is one or more of diethanolamine, triethanolamine or N-methyldiethanolamine mixed in any proportion.

4. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: In step (1), the reaction conditions are stirring at 40~80℃ for 10~60 min.

5. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: In step (2), the liquid-to-solid ratio of monoethanolamine solution to impurity-removed phosphogypsum is (3~9):1, in mL:g; the mass fraction of the monoethanolamine solution is 20%~40%.

6. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: In step (2), the volume fraction of carbon dioxide in the mixed gas is 10%~20%, the flow rate of the mixed gas is 100~300 ml / min, the stirring speed is 300~800 rpm, the mineralization reaction temperature is 20~40℃, and the final pH of the mineralization reaction is 7.5~8.

5.

7. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: In step (2), washing is performed using anhydrous ethanol or a 95% ethanol solution.

8. The method for preparing high-purity aragonite based on the alkanolamine-phosphogypsum synergistic system according to claim 1, characterized in that: In step (3), the amount of monoethanolamine added is 20% to 30% of the liquid phase mass, and the pH is adjusted to 11.5 to 12.5.

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