A method for preparing high-strength gypsum based on phosphogypsum with an atmospheric pressure salt solution

Abstract

The present invention relates to a method for preparing phosphogypsum-based high-strength gypsum using an atmospheric-pressure salt solution, belonging to the field of high-value comprehensive utilization of phosphogypsum, and solving the problems of long reaction time, complex process, and high production cost in the preparation of phosphogypsum-based high-strength gypsum by the atmospheric-pressure salt solution method in the prior art. A method for preparing phosphogypsum-based high-strength gypsum using an atmospheric-pressure salt solution includes the following steps: (1) Pour nitrate, a crystal conversion agent, and deionized water into a container in sequence and stir until completely dissolved to obtain a salt solution system; (2) After heating the salt solution system to the hydrothermal reaction temperature, add phosphogypsum for hydrothermal reaction; (3) After the hydrothermal reaction is completed, immediately perform suction filtration on the hydrothermal reaction system; (4) Wash and dry the filter cake obtained by suction filtration in step (3); (5) Collect the filtrate obtained by suction filtration in step (3) for recycling. The efficient, green and waste-free production of phosphogypsum-based high-strength gypsum is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of high-value comprehensive utilization of phosphogypsum, and particularly to a method for preparing phosphogypsum-based high-strength gypsum by an atmospheric-pressure salt solution. Background Art

[0002] Phosphogypsum is the main by-product generated in the wet production of phosphoric acid and phosphate fertilizers, and is also one of the solid wastes with the largest emissions in the chemical industry. Its main component is calcium sulfate dihydrate, and it contains a small amount of free acid, phosphate, fluoride, organic matter, and trace elements. Statistics show that the current global stockpile of phosphogypsum has exceeded 6 billion tons, and it continues to grow at a rate of about 200 million tons per year. As the world's largest producer of phosphogypsum by-products, China's cumulative stockpile has exceeded 830 million tons. The large stockpile of phosphogypsum not only occupies land area, but also causes certain damage to water, soil, atmosphere, and biological environment, increasing the environmental load around the stockpile site. Long-term accumulation can also pose potential risks to the health of humans and animals through the food chain, severely restricting the sustainable utilization of phosphate rock resources. Therefore, it is very important to seek comprehensive and safe utilization ways and technologies for phosphogypsum. One of the ways to utilize phosphogypsum resourcefully is to dehydrate it into short-columnar α-hemihydrate gypsum (also known as high-strength gypsum). Using phosphogypsum to prepare high-strength gypsum can not only be used in high-end fields such as preparing high-strength gypsum blocks, ceramic molds, sculpture decoration, and medical treatment, but also reduce the harm caused by phosphogypsum to soil and environment, and promote the solid waste resource utilization of phosphogypsum.

[0003] Common methods for preparing high-strength gypsum include the autoclave method, the pressurized aqueous solution method, and the atmospheric-pressure salt solution method. The autoclave method is only applicable to massive gypsum and is not suitable for powdered by-product gypsum. Both the autoclave method and the pressurized aqueous solution method require high temperature and high pressure conditions, and the process is relatively complex, resulting in high production energy consumption and costs. The atmospheric-pressure salt solution method has received wide attention due to its high product quality, mild conditions, and low requirements for raw materials. As a new theory, it provides a new direction for the application and production of high-strength gypsum. As an electrolyte, the salt solution can reduce the activity of water, thereby increasing the solubility of calcium sulfate dihydrate, and promoting the nucleation and growth of crystals under the action of a crystal conversion agent to obtain α-hemihydrate gypsum with good crystal form. Currently, the atmospheric-pressure salt solution method is mainly carried out in inorganic acids or chloride salt solutions represented by CaCl2. In order to improve the reaction rate, the medium concentration is often very high. This will not only seriously corrode the metal unit, but also require treatment of the wastewater after the reaction, increasing the difficulty and production cost of the process. Summary of the Invention

[0004] In view of the above analysis, the embodiments of the present invention aim to provide a method for preparing phosphogypsum-based high-strength gypsum by an atmospheric-pressure salt solution, so as to solve at least one of the problems such as long reaction time, high reaction energy consumption, complex process, and high production cost in the existing method for preparing phosphogypsum-based high-strength gypsum by an atmospheric-pressure salt solution.

[0005] An embodiment of the present invention provides a method for preparing high-strength gypsum based on phosphogypsum using an atmospheric-pressure salt solution, which includes the following steps:

[0006] (1) Pour nitrate, a crystal conversion agent, and deionized water into a container in sequence and stir until completely dissolved to obtain a salt solution system;

[0007] (2) After heating the salt solution system to a preset temperature, add phosphogypsum for hydrothermal reaction;

[0008] (3) Immediately perform suction filtration on the hydrothermal reaction system after the hydrothermal reaction ends;

[0009] (4) Wash and dry the filter cake obtained from the suction filtration in step (3);

[0010] (5) Collect the filtrate obtained from the suction filtration in step (3) for recycling.

[0011] Specifically, in step (1), the nitrate is magnesium nitrate; the crystal conversion agent is a dibasic organic acid, including one or any mixture of succinic acid, glutaric acid, and adipic acid.

[0012] Further, in step (2), in the hydrothermal reaction system, the mass fraction of the nitrate is 30 - 35% wt.

[0013] Preferably, in step (2), in the hydrothermal reaction system, the mass fraction of the crystal conversion agent is 0.01 - 1.0 wt%.

[0014] Exemplarily, in step (2), in the hydrothermal reaction system, the addition amount of dry-based phosphogypsum is 6 - 15 wt%.

[0015] Further, the preset temperature in step (2) is 85 - 120 °C.

[0016] It should be noted that when the hydrothermal reaction temperature is 85 °C, the hydrothermal reaction time is ≤ 2 h.

[0017] Preferably, in step (4), for the drying treatment, the drying temperature is 45 - 60 °C.

[0018] Further, in step (5), add an appropriate amount of nitrate, crystal conversion agent, and deionized water to the filtrate for recycling.

[0019] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0020] 1. The present invention adds magnesium nitrate. On the one hand, as a salt medium, it increases the driving force of reaction crystallization through the salt effect; on the other hand, it inhibits the growth of α-hemihydrate gypsum crystals to a certain extent and synergistically regulates the morphology of high-strength gypsum with the organic dibasic acid crystal conversion agent;

[0021] The hydrothermal reaction is carried out at a relatively low reaction temperature and in a relatively mild reaction environment with pH > 1, and the reaction time is controlled within 2 h, saving reaction time, reaction energy consumption and reaction cost;

[0022] Meanwhile, effective regulation of the microstructure of high-strength gypsum can be achieved by adding a small amount of magnesium nitrate and trace organic dibasic acid, reducing the chemical agent cost required for production.

[0023] 2. The reaction system of nitrate + organic dibasic acid is selected in the present invention. During the preparation of high-strength gypsum in an atmospheric-pressure salt solution, the original environment of the solution can be directly utilized without regulating the pH of the solution, simplifying the production steps.

[0024] 3. The drying temperature of the high-strength gypsum sample in the present invention is 45 - 60 °C. On the one hand, it avoids the problem of excessive dehydration of the high-strength gypsum sample caused by high temperature. On the other hand, it reduces the reaction energy consumption and saves the production cost.

[0025] 4. The magnesium nitrate solution after the reaction in the present invention is recycled and further in-depth research is carried out on the recycling performance of the magnesium nitrate solution; avoiding the discharge of waste liquid and realizing waste-free and clean production of the process; and the recycling of the salt medium can greatly reduce the production cost of preparing high-strength gypsum.

[0026] 5. The phosphorus gypsum-based high-strength gypsum prepared in the present invention has good crystal morphology, with coarse grains and short columnar shape. The aspect ratio of the crystal is between 1 and 2.25, the average length is 28 - 35 μm, and the dried compressive strength reaches 29.74 MPa, meeting the industry standard of "α-Type High-Strength Gypsum" (JC / T 2038-2010), with high cost performance and wide applications.

[0027] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the subsequent specification, and some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained from the content specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings are only for the purpose of showing specific embodiments and are not considered as a limitation to the present invention. Throughout the drawings, the same reference signs represent the same components.

[0029] Figure 1 is the technical roadmap of the present invention;

[0030] Figure 2 is the morphology diagram of the phosphorus gypsum of the present invention;

[0031] Figure 3 This is the morphology diagram of the high-strength gypsum sample in Embodiment 1 of the present invention. Detailed implementation manners

[0032] The preferred embodiments of the present invention will be specifically described below with reference to the accompanying drawings. The accompanying drawings form a part of this application and are used together with the embodiments of the present invention to explain the principles of the present invention, rather than to limit the scope of the present invention.

[0033] A specific embodiment of the present invention discloses a method for preparing phosphogypsum-based high-strength gypsum from an atmospheric-pressure salt solution, which includes the following steps:

[0034] (1) Pour nitrate, a crystal conversion agent, and deionized water into a container in sequence and stir until completely dissolved to obtain a salt solution system;

[0035] (2) After heating the salt solution system to a preset temperature, add phosphogypsum for hydrothermal reaction;

[0036] (3) Immediately perform suction filtration on the hydrothermal reaction system after the hydrothermal reaction ends;

[0037] (4) Wash and dry the filter cake after the suction filtration in step (3);

[0038] (5) Collect the filtrate after the suction filtration in step (3) for recycling.

[0039] It should be noted that the conversion kinetics of preparing high-strength gypsum from phosphogypsum is mainly affected by the conversion driving force, and the conversion driving force is the solubility difference between the two phases of CaSO4·2H2O and CaSO4·0.5H2O. In the present invention, magnesium nitrate is added. As an electrolyte, magnesium nitrate can reduce the activity of water, and the oxygen atom in the nitrate ion can form a hydrogen bond with the hydrogen atom in the water molecule, generating a significant salt effect, which helps the efficient conversion of phosphogypsum.

[0040] The salt medium is selected as nitrate, which overcomes the corrosion problem of the chloride salt system and the interference of the common ion effect of sulfate, and nitrate itself has a significant salt effect, effectively reducing the activity of water, making the solubility of CaSO4·2H2O and CaSO4·0.5H2O form a difference, which is conducive to the precipitation of α-hemihydrate gypsum. The salt effect in the high-valent cation salt is not obvious and is not conducive to the dissolution of phosphogypsum. In the present invention, Mg(NO3)2 is used. Mg(NO3)2 can promote the formation of short-columnar α-hemihydrate gypsum at high concentrations and does not react with CaSO4, and there is no common ion effect.

[0041] In a salt solution, if the salt concentration is too low, the effect of reducing the water activity is not obvious, resulting in the inability to precipitate α-hemihydrate gypsum; however, if the salt concentration is too high, on the one hand, the salt medium cannot be fully dissolved, causing waste of the reagent, and on the other hand, the phosphogypsum will rapidly transform into α-hemihydrate gypsum. At this time, the α-hemihydrate gypsum has many defects, resulting in poor comprehensive performance; therefore, in the hydrothermal reaction system, the mass fraction of nitrate is controlled to be 30 - 35% wt.;

[0042] In a possible design, in the hydrothermal reaction system, the mass fraction of nitrate is 30% wt.

[0043] Furthermore, the crystal modifier described in step (1) is a dibasic organic acid, including one or any mixture of succinic acid, glutaric acid, and adipic acid; in the hydrothermal reaction system, the mass fraction of the crystal modifier is 0.01 - 1.0 wt%;

[0044] Utilize the two carboxyl groups in the dibasic carboxylic acid to form a stable cyclic complex with two Ca on the end face of α-hemihydrate gypsum, inhibiting the growth of the end face of α-hemihydrate gypsum, thereby controlling the aspect ratio of α-hemihydrate gypsum; 2+ Form a stable cyclic complex, inhibit the growth of the end face of α-hemihydrate gypsum, thereby controlling the aspect ratio of α-hemihydrate gypsum;

[0045] Ca on the end face of α-hemihydrate gypsum 2+ The distance between them is a fixed value. In order to form a stable cyclic complex, organic acids with shorter carbon chains such as succinic acid, glutaric acid, and adipic acid are selected;

[0046] Adding too little crystal modifier will make the regulation effect not obvious, but adding too much crystal modifier will, while inhibiting the growth of the end face of α-hemihydrate gypsum, also inhibit the growth of the side face, weakening the crystal transformation effect;

[0047] Preferably, in the hydrothermal reaction system, the mass fraction of the crystal modifier is 0.2% wt;

[0048] On the other hand, magnesium nitrate can regulate the morphology of hemihydrate gypsum together with the dibasic organic acid, enabling the addition of a small amount of dibasic organic acid to achieve a more excellent regulation effect on the morphology of hemihydrate gypsum;

[0049] In a possible design, in the hydrothermal reaction system, the mass fraction of nitrate is 30% wt, and the mass fraction of the crystal modifier is 0.2% wt.

[0050] It should be noted that during the dissolution process of these reagents such as nitrate and crystal modifier, it needs to be carried out under acidic conditions. When the solution is alkaline, a large amount of OH in the solution - will react with Ca 2+Calcium hydroxide, which is slightly soluble, is formed and adheres to the surface of phosphogypsum, inhibiting the dissolution of phosphogypsum and affecting the reaction rate. In this invention, a reaction system of nitrate + organic dibasic acid is selected. During the preparation of high-strength gypsum in an atmospheric-pressure salt solution, the original environment of the solution (pH > 1) can be directly utilized without adjusting the pH of the solution, which can simplify the production steps.

[0051] In step (2), when the salt solution reaches the preset temperature, phosphogypsum is added and heating and stirring are continued. The preset temperature is 85 - 120°C. Its main function is to increase the difference in solubility between calcium sulfate dihydrate and calcium sulfate hemihydrate, making it easier for phosphogypsum to dissolve and crystallize into hemihydrate gypsum. As the temperature increases, the reaction time for the conversion of phosphogypsum to hemihydrate gypsum shortens.

[0052] In a possible design, the preset temperature is 85°C.

[0053] In step (2), the container containing the salt solution can be placed in an oil bath for hydrothermal reaction. The hydrothermal reaction temperature is 85 - 120°C; the hydrothermal reaction temperature affects the solubility difference between CaSO4·₂H₂O and CaSO4·0.5H₂O; if the reaction temperature is too low, the solubility difference is small, and α-hemihydrate gypsum cannot be easily precipitated, resulting in difficulty in dehydration and crystal transformation of phosphogypsum; but if the reaction temperature is too high, α-hemihydrate gypsum precipitates rapidly, resulting in an imperfect crystal structure and even further dehydration to form anhydrous gypsum.

[0054] It should be noted that in this invention, Mg(NO3)₂ is used as the salt medium, increasing the reaction crystallization driving force. Therefore, the hydrothermal reaction can be maintained at a relatively low temperature to ensure the normal precipitation of α-hemihydrate gypsum.

[0055] Exemplarily, the phosphogypsum used in step (2) is phosphogypsum concentrate after flotation impurity removal and whitening treatment, and the purity of CaSO4·₂H₂O is not less than 90%; the addition amount of dry-based phosphogypsum is 6 - 15 wt% of the total mass of the hydrothermal reaction system.

[0056] If the addition amount of phosphogypsum is too low, there will be too few reaction materials to form an effective conversion driving force; but if the addition amount is too high, the slurry cannot form effective stirring at a certain rotation speed, resulting in some slurry being unable to react or even solidify.

[0057] In a possible design, the addition amount of dry-based phosphogypsum is 12 - 15 wt% of the total mass of the hydrothermal reaction system.

[0058] Furthermore, when the hydrothermal reaction temperature in step (2) is 85°C, the hydrothermal reaction time is ≤2h; if the hydrothermal reaction time is too short, the dissolved Ca 2+ and SO4 2-It is impossible to crystallize to form hemihydrate gypsum, or the generated α-hemihydrate gypsum crystal nuclei cannot further grow into complete crystals, resulting in the incomplete conversion of phosphogypsum into α-hemihydrate gypsum. However, if the hydrothermal reaction time is too long, some products may further dehydrate, reducing the reaction rate and conversion rate.

[0059] It should be noted that in this invention, Mg(NO3)2 is used as the salt medium, which increases the reaction crystallization driving force, so the hydrothermal reaction time is further shortened.

[0060] Furthermore, in step (3), the end of the hydrothermal reaction is determined according to the crystal water content of the product (whether it reaches the theoretical crystal water content of hemihydrate gypsum, 6.21%). After the hydrothermal reaction ends, the hydrothermal reaction system needs to be filtered immediately to avoid the precipitation of calcium sulfate dihydrate due to temperature drop.

[0061] Specifically, in step (4), it is washed successively with boiling water and absolute ethanol. Among them, the function of boiling water washing is to prevent the precipitation of calcium sulfate dihydrate due to temperature reduction, and it can also remove soluble impurities in phosphogypsum. The function of absolute ethanol washing is to dissolve impurities to improve the product purity and facilitate the crystal growth of the product. In order to prevent the precipitation of calcium sulfate dihydrate and thoroughly wash away the impurities in the product, the number of times of boiling water washing is 2 times, and the number of times of absolute ethanol washing is 1 time. After the filter cake is washed, it is dried, and the drying temperature is 45-60°C.

[0062] Preferably, the drying temperature is 60°C. Low-temperature evaporation removes the attached water on the surface of the sample while preventing the excessive dehydration of the product hemihydrate gypsum to anhydrous gypsum due to too high drying temperature.

[0063] Furthermore, when the filtrate in step (5) is recycled, appropriate nitrates, crystal conversion agents and deionized water are added to the filtrate to prepare a salt solution with a certain mass fraction, and it is recycled to step (1).

[0064] The salt medium hardly participates in the dehydration and crystal conversion reaction of phosphogypsum, so in the whole reaction process, the consumption of the salt medium is very limited, and the salt solution can be recycled to avoid waste liquid discharge.

[0065] The nitrates and crystal conversion agents added during recycling should be appropriate. If the addition amount is too small, phosphogypsum cannot react completely and the product morphology is difficult to guarantee; if the addition amount is too large, the conversion rate of phosphogypsum is too fast and the crystal conversion agent cannot form selective adsorption, resulting in defects in the performance of the generated α-hemihydrate gypsum. The addition amounts of nitrates and crystal conversion agents need to be determined according to the loss amounts of the two under specific preparation conditions.

[0066] The main basis for the supplementary addition amounts of nitrate and crystal conversion agent is to measure the volume of the salt solution before and after the reaction, then use methods such as ion chromatography or titration to measure the main ion content in the salt solution after the reaction, calculate the loss of salt medium and crystal conversion agent content during the reaction process, and finally add the calculated dosage of the loss agent and deionized water to make the solution volume equal to the initial solution volume;

[0067] If nitrate is not added, its filtrate can generally be recycled about 2 times; adding an appropriate amount of magnesium nitrate and glutaric acid after the reaction can achieve continuous recycling.

[0068] In a possible design, in the initial hydrothermal reaction system with a magnesium nitrate mass fraction of 30%, when recycling each time, magnesium nitrate needs to be added again, and the mass fraction of magnesium nitrate in the initial reaction system is 7%.

[0069] The reaction system of nitrate + organic dibasic acid is selected in the present invention. During the preparation of high-strength gypsum from an atmospheric-pressure salt solution, the original environment of the solution is directly utilized without adjusting the pH of the solution; the reaction crystallization driving force is increased through the salt effect of magnesium nitrate, and the reaction time is controlled within 2 h, saving the reaction time; magnesium nitrate and the organic dibasic acid crystal conversion agent synergistically regulate the morphology of high-strength gypsum, and the addition amounts of the two agents can be reduced simultaneously; the magnesium nitrate solution after the reaction is recycled and treated, reducing the reagent cost required for production and avoiding the discharge of waste liquid.

[0070] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments.

[0071] The reagents used in the embodiments of the present invention are all of analytical purity.

[0072] Example 1

[0073] This example provides a method for efficiently recycling and preparing phosphogypsum-based high-strength gypsum from an atmospheric-pressure salt solution, and the method includes:

[0074] The total mass of the reaction system in this example is 150 g, where the mass fraction of magnesium nitrate is 30%, the mass fraction of glutaric acid is 0.2%, the mass fraction of dry-base phosphogypsum is 15%, and the remaining component is deionized water;

[0075] 77.84 g of magnesium nitrate hexahydrate, 0.3 g of glutaric acid and deionized water were successively poured into a beaker to prepare a solution; after the magnesium nitrate and glutaric acid were completely dissolved, the pH of the solution was 1.39. Since the solution was acidic, the pH was not adjusted. Then, the beaker was placed in an oil bath and heated with stirring; when the solution reached the preset temperature of 85 °C, pre-treated dry-based phosphogypsum concentrate accounting for 15 wt% of the total mass of the hydrothermal reaction system was added, and heating and stirring were continued. The beaker was sealed with a plastic wrap and subjected to a hydrothermal reaction at 85 °C for 2 h; after the reaction ended, the hydrothermal reaction system was immediately filtered by suction; the filter cake after suction filtration was washed twice with boiling water and once with absolute ethanol, and then placed in an oven at 60 °C for low-temperature drying treatment; the filtrate after suction filtration was collected for secondary utilization. The dried samples were tested for the water of crystallization content and analyzed for the microscopic morphology. The results showed that the water of crystallization content of the sample obtained from the hydrothermal reaction of phosphogypsum for 2 h was 5.73%, close to the theoretical water of crystallization content of high-strength gypsum, which was 6.21%; the morphologies of the samples were all short columns, with an average length of 28.62 μm, an average aspect ratio of 2.02, and a dried compressive strength of 29.74 MPa.

[0076] Example 2

[0077] This example provides a method for efficiently recycling an atmospheric pressure salt solution to prepare high-strength gypsum based on phosphogypsum, and the method includes:

[0078] The total mass of the reaction system in this example was 150 g, where the mass fraction of magnesium nitrate was 30%, the mass fraction of adipic acid was 0.2%, the mass fraction of dry-based phosphogypsum was 15%, and the remaining component was deionized water;

[0079] 77.84 g of magnesium nitrate hexahydrate, 0.3 g of adipic acid and deionized water were successively poured into a beaker to prepare a solution; after the magnesium nitrate and adipic acid were completely dissolved, the pH of the solution was 1.53. Since the solution was acidic, the pH was not adjusted. Then, the beaker was placed in an oil bath and heated with stirring; when the solution reached the preset temperature of 85 °C, pre-treated dry-based phosphogypsum concentrate accounting for 15 wt% of the total mass of the hydrothermal reaction system was added, and heating and stirring were continued. The beaker was sealed with a plastic wrap and subjected to a hydrothermal reaction at 85 °C for 2 h; after the reaction ended, the hydrothermal reaction system was immediately filtered by suction; the filter cake after suction filtration was washed twice with boiling water and once with absolute ethanol, and then placed in an oven at 60 °C for low-temperature drying treatment; the filtrate after suction filtration was collected for secondary utilization. The dried samples were tested for the water of crystallization content and analyzed for the microscopic morphology. The results showed that the water of crystallization content of the sample obtained from the hydrothermal reaction of phosphogypsum for 2 h was 5.44%, close to the theoretical water of crystallization content of high-strength gypsum, which was 6.21%; the morphologies of the samples were all short columns, with an average length of 26.36 μm, an average aspect ratio of 2.04, and a dried compressive strength of 29.63 MPa.

[0080] Example 3

[0081] This embodiment provides a method for efficiently recycling and preparing high-strength gypsum based on phosphogypsum in an atmospheric-pressure salt solution, and the method includes:

[0082] (1) Conversion of phosphogypsum crystals

[0083] In this embodiment, the total mass of the initial reaction system is 150 g, in which the mass fraction of magnesium nitrate is 30%, the mass fraction of glutaric acid is 0.2%, the mass fraction of dry-based phosphogypsum is 15%, and the remaining components are deionized water;

[0084] " Pour 77.84 g of magnesium nitrate hexahydrate, 0.3 g of glutaric acid and deionized water into a beaker in sequence to prepare a solution; after the magnesium nitrate and glutaric acid are completely dissolved, the pH of the solution is 1.38. Since the solution is acidic, the pH is not adjusted. Then, place the beaker in an oil bath and heat with stirring; when the solution reaches the preset temperature of 85 °C, add the pre-treated dry-based phosphogypsum concentrate accounting for 15 wt% of the total mass of the hydrothermal reaction system and continue heating with stirring. Seal it with a plastic wrap and conduct a hydrothermal reaction at 85 °C for 2 h; after the reaction ends, immediately perform suction filtration on the hydrothermal reaction system; wash the filter cake after suction filtration twice with boiling water and once with absolute ethanol, and then place it in an oven at 60 °C for low-temperature drying treatment; collect the filtrate after suction filtration for secondary utilization. The crystallized water content test and microscopic morphology analysis were carried out on the dried sample. The results show that the crystallized water content of the sample after 2 h of hydrothermal reaction of phosphogypsum is 6.26%, close to the theoretical crystallized water content of high-strength gypsum, which is 6.21%; the morphology of the sample is all short columns, its average length is 30.24 μm, the average aspect ratio is 2.05, and the drying compressive strength is 29.5 MPa.

[0085] (2) Solution recycling

[0086] Recycle the filtrate in the above step (1). Before each utilization, the amounts of magnesium nitrate and glutaric acid added to the filtrate are 7% and 0.2% respectively in terms of mass fraction in the initial hydrothermal reaction system, and add deionized water to make the volume of the solution the same as that of the original solution. Since the solution is acidic, the pH does not need to be adjusted. Other test operations are the same as those in step (1), and the hydrothermal reaction time is still 2 h. The results show that under different filtrate times, the crystallized water contents of the samples after 2 h of hydrothermal reaction of phosphogypsum are 6.30% (the first time), 6.18% (the second time), 6.38% (the third time) and 6.26% (the fourth time) in sequence. The crystallized water contents are all close to the theoretical crystallized water content of hemihydrate gypsum, which is 6.21%; the morphology of the samples is basically all short columns, and their average lengths are 31.78 μm, 29.59 μm, 30.14 μm and 28.41 μm in sequence, and the average aspect ratios are 2.21, 2.02, 2.14 and 2.08 in sequence.

[0087] The above test data show that with the increase in the number of cycles, there is basically no difference in the dehydration conversion rate of phosphogypsum and the morphology of the produced high-strength gypsum, and the salt solution can be recycled.

[0088] Comparative Example 1

[0089] This embodiment provides a method for efficiently recycling an atmospheric-pressure salt solution to prepare phosphogypsum-based high-strength gypsum, which includes:

[0090] The total mass of the reaction system in this embodiment is 150 g, where the mass fraction of magnesium nitrate is 30%, the mass fraction of dry-based phosphogypsum is 12%, and the remaining components are deionized water;

[0091] Pour 77.84 g of magnesium nitrate hexahydrate and deionized water into a beaker in sequence to prepare a solution; after the magnesium nitrate is completely dissolved, the pH of the solution is 2.1. Since the solution is acidic, the pH is not adjusted. Then, place the beaker in an oil bath and heat and stir; when the solution reaches the preset temperature of 98 °C, add the pre-treated dry-based phosphogypsum concentrate accounting for 12% of the total mass of the hydrothermal reaction system and continue heating and stirring. Seal it with plastic wrap and carry out a hydrothermal reaction at 98 °C for 1 h; after the reaction is completed, immediately filter the reaction system by suction; wash the filtered cake with boiling water twice and with absolute ethanol once, and then place it in an oven at 60 °C for drying treatment; collect the filtrate after suction filtration for secondary use. The crystallized water content test and microscopic morphology analysis were carried out on the dried samples. The results show that the crystallized water content of the sample with 1 h of phosphogypsum crystal conversion is 5.50%, close to the theoretical crystallized water content of high-strength gypsum, which is 6.21%; the morphology of the samples is all short columnar, with an average length of 27.72 μm and an average aspect ratio of 2.87.

[0092] Comparative Example 2

[0093] This embodiment provides a method for efficiently recycling an atmospheric-pressure salt solution to prepare phosphogypsum-based high-strength gypsum, which includes:

[0094] The total mass of the reaction system in this embodiment is 150 g, where the mass fraction of magnesium nitrate is 30%, the mass fraction of dry-based phosphogypsum is 12%, and the remaining components are deionized water;

[0095] 77.84 g of magnesium nitrate hexahydrate and deionized water were successively poured into a beaker to prepare a solution; after the magnesium nitrate was completely dissolved, the pH of the solution was 2.1. Since the solution was acidic, the pH was not adjusted. Then, the beaker was placed in an oil bath and heated with stirring; when the solution reached the preset temperature of 85 °C, pre-treated dry-based phosphogypsum concentrate accounting for 12% of the total mass of the hydrothermal reaction system was added, and heating and stirring were continued. The beaker was sealed with plastic wrap and subjected to a hydrothermal reaction at 85 °C for 1.5 h; after the reaction ended, the reaction system was immediately filtered by suction; the filter cake after suction filtration was washed twice with boiling water and once with absolute ethanol, and then placed in an oven at 60 °C for drying; the filtrate after suction filtration was collected for secondary use. The dried samples were tested for the water of crystallization content and analyzed for the microscopic morphology. The results showed that the water of crystallization content of the sample with 1.5 h of phosphogypsum crystal transformation was 5.64%, close to the theoretical water of crystallization content of high-strength gypsum of 6.21%; the morphologies of the samples were all short columnar, with an average length of 37.63 μm and an average aspect ratio of 3.59.

[0096] Comparative Example 3

[0097] This example provides a method for efficiently recycling an atmospheric pressure salt solution to prepare high-strength gypsum based on phosphogypsum, and the method includes:

[0098] In this example, the total mass of the reaction system was 150 g, where the mass fraction of magnesium nitrate was 40% and the mass fraction of dry-based phosphogypsum was 12%, and the remaining component was deionized water;

[0099] 103.79 g of magnesium nitrate hexahydrate and deionized water were successively poured into a beaker to prepare a solution; after the magnesium nitrate was completely dissolved, the pH of the solution was 1.25. Since the solution was acidic, the pH was not adjusted. Then, the beaker was placed in an oil bath and heated with stirring; when the solution reached the preset temperature of 85 °C, pre-treated dry-based phosphogypsum concentrate accounting for 12% of the total mass of the hydrothermal reaction system was added, and heating and stirring were continued. The beaker was sealed with plastic wrap and subjected to a hydrothermal reaction at 85 °C for 1 h; after the reaction ended, the reaction system was immediately filtered by suction; the filter cake after suction filtration was washed twice with boiling water and once with absolute ethanol, and then placed in an oven at 60 °C for drying; the filtrate after suction filtration was collected for secondary use. The dried samples were tested for the water of crystallization content and analyzed for the microscopic morphology. The results showed that the water of crystallization content of the sample with 1 h of phosphogypsum crystal transformation was 5.39%, close to the theoretical water of crystallization content of high-strength gypsum of 6.21%; the morphologies of the samples were all short columnar, with an average length of 20.26 μm and an average aspect ratio of 2.83.

[0100] Comparative Example 4

[0101] This example provides a method for efficiently recycling an atmospheric pressure salt solution to prepare high-strength gypsum based on phosphogypsum, and the method includes:

[0102] The total mass of the reaction system in this embodiment is 150 g, among which the mass fraction of magnesium nitrate is 30%, the mass fraction of dry - based phosphogypsum is 15%, and the remaining component is deionized water;

[0103] Pour 77.84 g of magnesium nitrate hexahydrate and deionized water into a beaker in sequence to prepare a solution; after the magnesium nitrate is completely dissolved, the pH of the solution is 2.1. Since the solution is acidic, the pH is not adjusted. Then, put the beaker into an oil - bath pot for heating and stirring; when the solution reaches the preset temperature of 85 °C, add the pre - treated dry - based phosphogypsum concentrate accounting for 15% of the total mass of the hydrothermal reaction system and continue heating and stirring. Seal it with plastic wrap and carry out a 1.5 - h hydrothermal reaction at 85 °C; after the reaction ends, immediately filter the reaction system by suction; the filter cake after suction filtration is washed twice with boiling water and once with absolute ethanol respectively, and then put into an oven at 60 °C for drying treatment; the filtrate after suction filtration is collected for secondary utilization. The dried samples are tested for the crystal water content and analyzed for the microscopic morphology. The results show that the crystal water content of the sample with 1.5 - h phosphogypsum crystal transformation is 6.19%, close to the theoretical crystal water content of high - strength gypsum, which is 6.21%; the morphologies of the samples are all short columns, with an average length of 37.51 μm and an average aspect ratio of 2.86.

[0104] Comparative Example 5

[0105] This embodiment provides a method for efficiently recycling an aqueous salt solution to prepare high - strength gypsum based on phosphogypsum, and the method includes:

[0106] The total mass of the reaction system in this embodiment is 150 g, among which the mass fraction of magnesium nitrate is 30%, the mass fraction of dry - based phosphogypsum is 15%, and the remaining component is deionized water;

[0107] Pour 77.84 g of magnesium nitrate hexahydrate and deionized water into a beaker in sequence to prepare a solution; after the magnesium nitrate is completely dissolved, the pH of the solution is 2.1. Add sodium hydroxide solution to adjust the pH of the solution to 3, and then put the beaker into an oil - bath pot for heating and stirring; when the solution reaches the preset temperature of 85 °C, add the pre - treated dry - based phosphogypsum concentrate accounting for 15% of the total mass of the hydrothermal reaction system and continue heating and stirring. Seal it with plastic wrap and carry out a 2 - h hydrothermal reaction at 85 °C; after the reaction ends, immediately filter the reaction system by suction; the filter cake after suction filtration is washed twice with boiling water and once with absolute ethanol respectively, and then put into an oven at 60 °C for drying treatment; the filtrate after suction filtration is collected for secondary utilization. The dried samples are tested for the crystal water content and analyzed for the microscopic morphology. The results show that the crystal water content of the sample with 2 - h phosphogypsum crystal transformation is 6.20%, close to the theoretical crystal water content of high - strength gypsum, which is 6.21%; the morphologies of the samples are all short columns, with an average length of 39.44 μm and an average aspect ratio of 2.99.

[0108] Comparative Example 6

[0109] This embodiment provides a method for efficiently recycling an atmospheric-pressure salt solution to prepare high-strength phosphogypsum-based gypsum, and the method includes:

[0110] The total mass of the reaction system in this embodiment is 150 g, where the mass fraction of magnesium nitrate is 30%, the mass fraction of glutaric acid is 1.3%, the mass fraction of dry-based phosphogypsum is 15%, and the remaining component is deionized water;

[0111] 77.84 g of magnesium nitrate hexahydrate, 2 g of glutaric acid, and deionized water are poured into a beaker in sequence to prepare a solution; after the magnesium nitrate and glutaric acid are completely dissolved, the pH of the solution is 1.05, and then the beaker is placed in an oil bath for heating and stirring; when the solution reaches the preset temperature of 85°C, 15% of the total mass of the hydrothermal reaction system of pretreated dry-based phosphogypsum concentrate is added and heating and stirring continue, and it is sealed with plastic wrap for a 2-hour hydrothermal reaction at 85°C; after the reaction ends, the reaction system is immediately subjected to suction filtration; the filter cake after suction filtration is washed twice with boiling water and once with absolute ethanol, and then it is placed in an oven at 60°C for drying treatment; the filtrate after suction filtration is collected for secondary utilization. The crystallized water content test and microscopic morphology analysis are carried out on the dried sample. The results show that the crystallized water content of the sample with 2-hour phosphogypsum crystal conversion is 5.93%, close to the theoretical crystallized water content of 6.21% of high-strength gypsum; the morphology of the sample is all short columnar, with an average length of 36.49 μm and an average aspect ratio of 2.76. [[ID=I0]]

[0112] Comparative Example 7

[0113] This embodiment provides a method for efficiently recycling an atmospheric-pressure salt solution to prepare high-strength phosphogypsum-based gypsum, and the method includes:

[0114] The total mass of the reaction system in this embodiment is 150 g, where the mass fraction of magnesium nitrate is 5%, the mass fraction of glutaric acid is 0.2%, the mass fraction of dry-based phosphogypsum is 12%, and the remaining component is deionized water;

[0115] 12.97 g of magnesium nitrate hexahydrate, 0.3 g of glutaric acid, and deionized water are poured into a beaker in sequence to prepare a solution; after the magnesium nitrate and glutaric acid are completely dissolved, the pH of the solution is 2.84, and then the beaker is placed in an oil bath for heating and stirring; when the solution reaches the preset temperature of 85°C, 12% of the total mass of the hydrothermal reaction system of pretreated dry-based phosphogypsum concentrate is added and heating and stirring continue, and it is sealed with plastic wrap for hydrothermal reaction, and the result is that the phosphogypsum does not undergo dehydration conversion as detected.

[0116] Comparative Example 8

[0117] This embodiment provides a method for efficiently recycling an atmospheric-pressure salt solution to prepare high-strength phosphogypsum-based gypsum, and the method includes:

[0118] In this embodiment, the total mass of the reaction system is 150 g, among which the mass fraction of magnesium nitrate is 30%, the mass fraction of glutaric acid is 0.2%, the mass fraction of dry-based phosphogypsum is 12%, and the remaining component is deionized water;

[0119] 77.84 g of magnesium nitrate hexahydrate, 0.3 g of glutaric acid and deionized water are poured into a beaker in sequence to prepare a solution; after the magnesium nitrate and glutaric acid are completely dissolved, the pH of the solution is 2.1, and then the beaker is placed in an oil bath for heating and stirring; when the solution reaches the preset temperature of 75 °C, 12% of the total mass of the hydrothermal reaction system of pretreated dry-based phosphogypsum concentrate is added and heating and stirring are continued, and it is sealed with a plastic wrap and subjected to hydrothermal reaction at 75 °C. As a result, it is detected that the phosphogypsum does not undergo dehydration conversion.

[0120] Table 1 Preparation parameters and sample properties of hemihydrate gypsum in the embodiment and comparative examples

[0121]

[0122]

[0123] Table 2 Performance of filtrate recycling in Example 3

[0124] Number Average crystal length μm Average length-to-diameter ratio First reuse 31.78 2.21 Second reuse 29.59 2.02 Third reuse 30.14 2.14 Fourth reuse 28.41 2.08

[0125] It can be seen from Comparative Examples 1-5 that only adding nitrate solution and not adding dibasic organic acid cannot achieve good regulation of the morphology of α-hemihydrate gypsum at a relatively low hydrothermal reaction temperature and a relatively mild reaction environment (pH > 1); it can be seen from Comparative Example 6 and Examples 1 and 3 that the excessive addition of organic acid will inhibit the growth of the side surface while inhibiting the growth of the end surface of α-hemihydrate gypsum, weaken the crystal transformation effect, and is not conducive to the morphology of hemihydrate gypsum; it can be seen from Comparative Example 7 that when the concentration of magnesium nitrate is too low, the salt effect of nitrate cannot be fully exerted, and at a relatively low hydrothermal reaction temperature, phosphogypsum cannot be converted into α-hemihydrate gypsum; it can be seen from Comparative Example 8 that when the hydrothermal reaction temperature is too low, phosphogypsum cannot react and transform into α-hemihydrate gypsum.

[0126] It can be seen from Table 2 that for the recycling of the filtrate in Example 3, with the increase of the number of filtrate recycling times, there is basically no difference in the dehydration conversion rate of phosphogypsum and the morphology of high-strength gypsum generated, indicating that the salt solution can be recycled.

[0127] In summary, the present invention utilizes the salt effect of magnesium nitrate to control a reasonable concentration of the salt medium, enabling the dehydration conversion of phosphogypsum at a relatively low hydrothermal reaction temperature, saving reaction time and reducing reaction energy consumption. Magnesium nitrate and organic dibasic acids are used together to regulate the morphology of hemihydrate gypsum, achieving a more excellent regulation effect on the morphology of hemihydrate gypsum with the addition of a small amount of dibasic organic acids, without the need to adjust the pH of the reaction system. The process is simple, the reaction environment is relatively mild, and the dosage of chemicals is reduced, further saving costs.

[0128] The present invention recycles and utilizes the magnesium nitrate solution after the reaction, avoiding the discharge of waste liquid and achieving waste-free and clean production of the process. The recycling of the salt medium can greatly reduce the production cost of preparing high-strength gypsum.

[0129] The high-strength gypsum based on phosphogypsum prepared by the present invention has good crystal morphology, with coarse grains and short columnar shapes. The aspect ratio of the crystals is between 1 and 2.25, and the dried compressive strength reaches 29.74 MPa, meeting the industry standard of "α-Type High-Strength Gypsum" (JC / T 2038-2010). It has a high cost performance and wide applications.

[0130] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing short columnar phosphogypsum-based high-strength gypsum using a normal pressure salt solution, characterized in that: The steps include: (1) pouring nitrate, a crystal-changing agent, and deionized water into a container in sequence and stirring until completely dissolved to obtain a salt solution system; the nitrate is magnesium nitrate; The crystal-changing agent is a dibasic organic acid, including one of glutaric acid and adipic acid, or any mixture thereof; In the salt solution system, the mass fraction of the nitrate is 30wt%, and the mass fraction of the dibasic organic acid is 0.2wt%; wherein the total mass of the reaction system is 150g, the mass of magnesium nitrate hexahydrate is 77.84g, and the mass of the dibasic organic acid is 0.3g; (2) After heating the salt solution system to the hydrothermal reaction temperature, phosphogypsum is added to carry out a hydrothermal reaction; the hydrothermal reaction temperature is 85°C, and the hydrothermal reaction time is ≤2h; In the hydrothermal reaction system, the amount of phosphogypsum added is 15wt%; (3) After the hydrothermal reaction is completed, the hydrothermal reaction system is immediately filtered; (4) washing and drying the filter cake obtained after filtration in step (3); (5) collecting the filtrate after filtration in step (3) and recycling it; when recycling the filtrate, adding an appropriate amount of nitrate, a crystal-transforming agent and deionized water to the filtrate to prepare a salt solution with a certain mass fraction; in an initial hydrothermal reaction system with a mass fraction of 30% of magnesium nitrate, the mass fraction of magnesium nitrate to be added again during each recycling is 7% of the initial reaction system; Under a relatively mild reaction environment with a low reaction temperature and pH>1, the reaction time is controlled to less than 2 hours, saving reaction time, reaction energy consumption and reaction cost. The prepared phosphogypsum-based high-strength gypsum has good crystal morphology, coarse grains and short columnar shape, a crystal aspect ratio between 2.02 and 2.05, and a drying compressive strength of 29.74 MPa.

2. The method according to claim 1, characterized in that The drying temperature in step (4) is 45-60°C.

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