Method for preparing hemihydrate gypsum powder from phosphogypsum by means of atmospheric salt solution process, and hemihydrate gypsum powder

Abstract

The present invention relates to the technical field of waste recovery, and provides a method for preparing a hemihydrate gypsum powder from phosphogypsum by means of an atmospheric salt solution process, and a hemihydrate gypsum powder. The method comprises: preparing a slurry from a mixed liquid of a salt and a crystal modifier and purified phosphogypsum, wherein the crystal modifier is a betaine compound with pH responsiveness; and adjusting the pH value of the slurry on the basis of the isoelectric point of the crystal modifier and the required length-diameter ratio of hemihydrate gypsum crystals, subjecting the slurry to a constant-temperature dynamic reaction, and filtering same, thereby obtaining a hemihydrate gypsum powder, wherein when the pH value of the slurry is greater than the isoelectric point of the crystal modifier, the length-diameter ratio of the hemihydrate gypsum crystals increases, and when the pH value of the slurry is less than the isoelectric point of the crystal modifier, the length-diameter ratio of the hemihydrate gypsum crystals decreases. By using the betaine compound as the crystal modifier, hemihydrate gypsum crystals with different length-diameter ratios can be obtained by adjusting the pH value of the slurry, and therefore the regulation and control of the crystal morphology of hemihydrate gypsum are achieved and exhibit a certain pattern.

Description

Method for preparing semi-hydrated gypsum powder by using phosphogypsum normal pressure salt solution method and semi-hydrated gypsum powder Technical Field

[0001] The present disclosure relates to the technical field of waste recycling, and in particular to a method for preparing semi-hydrated gypsum powder using a normal-pressure salt solution method of phosphogypsum and the semi-hydrated gypsum powder. Background Art

[0002] Phosphogypsum is a byproduct of phosphate fertilizer production. Its primary component is CaSO₄·2H₂O, exceeding 80% by mass. Because it contains certain impurities, such as SiO₂, F, P, and organic matter, these impurities hinder its comprehensive utilization. The rapid development of the phosphate fertilizer industry has resulted in the production and discharge of large quantities of phosphogypsum, yet its resource utilization rate is low. Currently, the primary method for disposing of phosphogypsum is stockpiling, which occupies significant land and causes environmental pollution.

[0003] The properties of hemihydrate gypsum are closely related to its morphology. Particle size, shape, and particle size distribution significantly influence its performance and applications. Spherical or low-aspect-ratio hemihydrate gypsum crystals exhibit high strength and injectability, making them well-suited for use in building materials and as bone cement. High-aspect-ratio hemihydrate gypsum crystals (such as whisker-shaped and linear) exhibit excellent thermal stability, strong chemical resistance, and good compatibility with materials such as polymers and ceramics, making them suitable as reinforcements for composite materials.

[0004] The methods for preparing hemihydrate gypsum from phosphogypsum mainly include: roasting method, autoclaving method, pressurized aqueous solution method, and normal pressure salt (alcohol) solution method.

[0005] The roasting method is to dehydrate dihydrate gypsum in a dry environment to produce hemihydrate (anhydrous) gypsum. The product has low purity, difficult to control quality, and is unstable; the autoclave method uses saturated steam as a solvent, and the dihydrate gypsum is pressurized and heated in an autoclave to be converted into hemihydrate gypsum. This preparation process is simple and has a large output, but it has high energy consumption, the reaction process is difficult to control, and the high content of β-hemihydrate gypsum leads to unstable product quality; the pressurized aqueous solution method has relatively complex process conditions, low production efficiency, and long preparation time, resulting in high production energy consumption and cost; the atmospheric pressure salt (alcohol) solution method is a new process developed in recent years. It is currently in the stage of laboratory research to semi-industrial testing. Compared with the autoclave method and the pressurized aqueous solution method, the atmospheric pressure salt (alcohol) solution method does not require a pressure vessel, has the advantages of atmospheric pressure, low synthesis temperature, low energy consumption, and high production efficiency, and has good application prospects.

[0006] The atmospheric pressure salt (alcohol) solution method for preparing hemihydrate gypsum allows for easy control of the morphology and strength of the gypsum crystals by adjusting the type and amount of the salt (alcohol) medium and crystal-transforming agent, reaction temperature, liquid-to-solid ratio, pH, etc. Existing control strategies employ different crystal-transforming agents to manipulate the morphology of gypsum crystals, but the effects are limited and lack regularity.

[0007] In view of this, the present disclosure is proposed.

[0008] Summary of the Invention

[0009] The purpose of the present disclosure is to provide a method for preparing semi-hydrated gypsum powder by using a normal-pressure salt solution method of phosphogypsum, which can use the same crystal-changing agent to adjust the pH value of the slurry to control the final crystal morphology, and the control effect is obvious and shows a certain regularity.

[0010] The present invention aims to provide a semi-hydrated gypsum powder with controllable morphology and high production efficiency.

[0011] The present disclosure is achieved as follows:

[0012] In a first aspect, the present disclosure provides a method for preparing hemihydrate gypsum powder using a saline solution method at atmospheric pressure, comprising:

[0013] A mixture of salt and a crystal-changing agent is mixed with purified phosphogypsum to prepare a slurry, wherein the crystal-changing agent is a betaine compound having pH responsiveness;

[0014] According to the isoelectric point of the crystal-changing agent and the required aspect ratio of the hemihydrate gypsum crystals, the pH of the slurry is adjusted with an alkali agent or an acid agent, the slurry is subjected to a constant temperature dynamic reaction, and filtered to obtain the product; wherein, when the pH value of the slurry is greater than the isoelectric point of the crystal-changing agent, the aspect ratio of the hemihydrate gypsum crystals increases, and when the pH value of the slurry is less than the isoelectric point of the crystal-changing agent, the aspect ratio of the hemihydrate gypsum crystals decreases.

[0015] In an optional embodiment, the isoelectric point of the crystal-transforming agent is 5-8.

[0016] In an optional embodiment, the pH of the slurry is adjusted in the range of 2-10.

[0017] In an optional embodiment, the betaine compound is selected from a combination of one or more of carboxylic acid type betaine and sulfonic acid type betaine, and the betaine compound has the following general formula: R1N + (CH3)2R2X - ;

[0018] Wherein, R1 is a substituted or unsubstituted C1-C12 alkyl straight chain, R2 is a substituted or unsubstituted C2-C12 alkyl straight chain; and X- is a carboxylate or sulfate.

[0019] In an optional embodiment, the substituent in R1 includes an amino group, a hydroxyl group, or a thiol group; the substituent in R2 includes an amino group, a hydroxyl group, or a thiol group.

[0020] In an optional embodiment, the carboxylic acid type betaine includes CH3(CH2) 11 N + (CH3)2(CH2)2COO - 、N + (CH3)3(CH2) 12 COO - or CH3(CH2)5N + (CH3)2(CH2)5COO - .

[0021] In an optional embodiment, the sulfonic acid type betaine includes CH3(CH2) 11 N + (CH3)2(CH2)3SO3 - .

[0022] In an optional embodiment, the amount of the crystal-changing agent added is 0.3 to 1% of the dry mass of calcium sulfate dihydrate in the purified phosphogypsum.

[0023] In an optional embodiment, the content of calcium sulfate dihydrate in the purified phosphogypsum is 80-98%, the soluble phosphorus content is ≤0.1%, and the soluble fluorine content is ≤0.03%.

[0024] In an optional embodiment, the concentration of the salt in the mixed solution is 0.3 to 1.5 mol / L.

[0025] In an alternative embodiment, the salt comprises a combination of any one or more of sodium sulfate, calcium nitrate, magnesium sulfate and calcium chloride.

[0026] In an optional embodiment, the solid content of the slurry is 60-80%.

[0027] In an optional embodiment, the stirring speed of the constant temperature dynamic reaction is 180 to 250 r / min.

[0028] In an optional embodiment, the alkaline agent includes any one or more combinations of sodium hydroxide, potassium hydroxide, calcium hydroxide and calcium oxide.

[0029] In an optional embodiment, the acidic agent includes any one or more combinations of hydrochloric acid and sulfuric acid.

[0030] In an optional embodiment, the solid matter is washed, dried and ground after the filtration.

[0031] In an alternative embodiment, the washing comprises washing with boiling water.

[0032] In an optional embodiment, the temperature for drying the solid matter is 110-120°C.

[0033] In an optional embodiment, the preparation method of the purified phosphogypsum includes: first washing the phosphogypsum with water to a pH of 6.5 to 7.0, then drying and sieving to obtain gypsum powder, preparing the gypsum powder, quicklime and water into a slurry and mixing it for 24 to 48 hours, and drying to obtain the purified phosphogypsum.

[0034] In an optional embodiment, the mass ratio of the gypsum powder, the quicklime and the water is 100:0.5-2:10-20.

[0035] In an optional embodiment, the screening includes passing the dried phosphogypsum through a square hole sieve with a pore size of 0.2-0.4 mm.

[0036] In an optional embodiment, the temperature for drying the slurry is 40-50°C.

[0037] In a second aspect, the present disclosure provides a semi-hydrated gypsum powder, which is prepared by the method for preparing semi-hydrated gypsum powder by the phosphogypsum normal pressure salt solution method as described in any of the aforementioned embodiments.

[0038] The present disclosure has the following beneficial effects: When only one crystal-transforming agent (betaine compound) is used in the hemihydrate gypsum crystals prepared according to the present disclosure, the pH value of the slurry can be adjusted to obtain hemihydrate gypsum crystals with different aspect ratios, thereby achieving control over the crystal morphology of the hemihydrate gypsum. The control principle is as follows: when the pH value of the liquid slurry is lower than the isoelectric point of the crystal-transforming agent, the crystal-transforming agent is positively charged, equivalent to a cationic surfactant, and tends to adsorb on the {010} crystal planes on the sides of the hemihydrate gypsum crystals, allowing the hemihydrate gypsum to grow into a one-dimensional whisker shape along the c-axis. When the pH value of the slurry is higher than the isoelectric point of the crystal-transforming agent, the crystal-transforming agent is negatively charged, equivalent to an anionic surfactant, and tends to adsorb on the {111} crystal planes of the hemihydrate gypsum crystals, resulting in restricted growth along the c-axis of the hemihydrate gypsum, a decrease in the aspect ratio of the crystals, and a cylindrical shape. The method for producing hemihydrate gypsum powder from a phosphogypsum atmospheric pressure salt solution provided by the present disclosure demonstrates significant control effects when regulating crystal morphology, and exhibits a pattern in which the aspect ratio of the resulting hemihydrate gypsum crystals decreases as the pH of the slurry increases. Therefore, the pH of the slurry can be adaptively adjusted based on the desired aspect ratio of the hemihydrate gypsum crystals. Compared to amino acid-based crystal-modifying agents, the amphiphilic betaine molecules provided by the present disclosure possess greater proton acceptance and donation capabilities and charge stability, resulting in greater sensitivity in crystal form control. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0040] FIG1 is a SEM image of hemihydrate gypsum crystals obtained at pH=2.1 in Example 5;

[0041] FIG2 is a SEM image of hemihydrate gypsum crystals obtained when pH=5.2 in Example 5;

[0042] FIG3 is a SEM image of hemihydrate gypsum crystals obtained when pH=8.3 in Example 5. DETAILED DESCRIPTION

[0043] To make the purpose, technical solutions, and advantages of the embodiments of the present disclosure more clear, the technical solutions in the embodiments of the present disclosure are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer were followed. Reagents or instruments used where the manufacturer is not specified are conventional products that can be purchased commercially.

[0044] The present disclosure provides a method for preparing hemihydrate gypsum powder by using a phosphogypsum normal pressure salt solution method, which comprises:

[0045] A mixture of salt and a crystal-changing agent is mixed with purified phosphogypsum to prepare a slurry, wherein the crystal-changing agent is a betaine compound with pH responsiveness; the pH of the slurry is adjusted according to the isoelectric point of the crystal-changing agent and the aspect ratio of the desired hemihydrate gypsum crystals, the slurry is subjected to a constant temperature dynamic reaction, and filtered to obtain; wherein, when the pH value of the slurry is greater than the isoelectric point of the crystal-changing agent, the aspect ratio of the hemihydrate gypsum crystals increases, and when the pH value of the slurry is less than the isoelectric point of the crystal-changing agent, the aspect ratio of the hemihydrate gypsum crystals decreases. In the present disclosure, a betaine compound with pH responsiveness is used as a crystal-changing agent, and only one crystal-changing agent is used to achieve the control of the crystal morphology by adjusting the pH value of the slurry, and the control effect is obvious, showing a rule that the higher the pH of the slurry, the smaller the aspect ratio of the obtained hemihydrate gypsum crystals.

[0046] Specifically, the method disclosed herein comprises the following steps:

[0047] S1. Phosphogypsum pretreatment.

[0048] The phosphogypsum is first washed with water to a pH of 6.5 to 7.0, then dried and sieved to obtain gypsum powder. The gypsum powder, quicklime and water are prepared into a slurry and mixed for 24 to 48 hours, and then dried to obtain purified phosphogypsum.

[0049] The mass ratio of gypsum powder, quicklime, and water is 100:0.5-2:10-20. Sieving includes passing the dried phosphogypsum through a square mesh sieve with a pore size of 0.2-0.4 mm. The slurry is dried at a temperature of 40-50°C.

[0050] The content of calcium sulfate dihydrate in the purified phosphogypsum is 80-98%, the soluble phosphorus content is ≤0.1%, and the soluble fluorine content is ≤0.03%.

[0051] In some typical embodiments, the mass ratio of gypsum powder, quicklime, and water can be, for example, any one of 100:0.5:10, 100:1:15, 100:1.5:20, 100:2:15, 100:0.8:17, or 100:0.5:20, or a range between any two thereof. The mixing time can be, for example, any one of 24 hours, 30 hours, 36 hours, 42 hours, or 48 hours, or a range between any two thereof. The drying temperature can be, for example, any one of 40° C., 42° C., 45° C., 46° C., 48° C., or 50° C., or a range between any two thereof.

[0052] S2. Prepare slurry.

[0053] After the mixture of salt and crystal-changing agent is heated to a preset temperature (95-100°C), purified phosphogypsum is added to prepare a slurry.

[0054] The salt includes but is not limited to any one or more combinations of sodium sulfate, calcium nitrate, magnesium sulfate and calcium chloride.

[0055] The betaine compound is selected from a combination of one or more of carboxylic acid type betaine and sulfonic acid type betaine. The betaine compound has the following general formula: R1N + (CH2)2R2X - wherein R1 is a substituted or unsubstituted C1-C12 alkyl straight chain, R2 is a substituted or unsubstituted C2-C12 alkyl straight chain, and X- is a carboxylate or sulfate. The substituents in R1 include amine, hydroxyl, or thiol; and the substituents in R2 include amine, hydroxyl, or thiol.

[0056] Specifically, carboxylic acid-based betaines include CH3(CH2) 11 N + (CH3)2(CH2)2COO - 、N + (CH3)3(CH2) 12 COO - or CH3(CH2)5N + (CH3)2(CH2)5COO -Sulfonic acid type betaine includes CH3(CH2) 11 N + (CH3)2(CH2)3SO3 - .

[0057] In the present disclosure, the mixture of salt and crystal-transforming agent is heated in advance to achieve a better mixing effect of the two, and then the purified phosphogypsum is added to make it easier to disperse into a slurry.

[0058] In the present disclosure, the amount of the crystallization agent added is 0.3-1% of the dry weight of calcium sulfate dihydrate in the purified phosphogypsum. The salt concentration in the mixed solution is 0.3-1.5 mol / L. The solid content of the final prepared slurry is 60-80%.

[0059] In some typical embodiments, the amount of the crystallization agent added is any one of 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1% of the dry weight of calcium sulfate dihydrate in the purified phosphogypsum, or a range between any two of them. The salt concentration in the mixed solution is any one of 0.3 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.3 mol / L or 1.5 mol / L, or a range between any two of them. The solid content of the final prepared slurry is any one of 60%, 65%, 68%, 70%, 72%, 75%, 78% or 80%, or a range between any two of them.

[0060] S3. Adjust the pH value to obtain the product.

[0061] The pH of the slurry is adjusted according to the isoelectric point of the crystal-transforming agent and the aspect ratio of the desired hemihydrate gypsum crystals, and the slurry is subjected to a constant temperature dynamic reaction at a stirring speed of 180 to 250 r / min. The solid matter is filtered out, washed with boiling water, dried at a temperature of 110 to 120° C., and then ground to obtain the product.

[0062] Among them, the isoelectric point of the crystal-transforming agent is 5-8.

[0063] Alkaline agents include, but are not limited to, any one or more combinations of sodium hydroxide, potassium hydroxide, calcium hydroxide, and calcium oxide. Acidic agents include, but are not limited to any one or more combinations of hydrochloric acid and sulfuric acid.

[0064] In some typical embodiments, the pH of the slurry is adjusted within a range of 2-10, and the pH of the slurry is any one of 2, 3, 4, 5, 6, 7, 8, 9, or 10, or a range between any two of them. The stirring speed can be, for example, any one of 180 r / min, 200 r / min, 220 r / min, or 250 r / min, or a range between any two of them. The drying temperature can be, for example, any one of 110° C., 115° C., or 120° C., or a range between any two of them.

[0065] The features and performance of the present disclosure are further described in detail below with reference to the embodiments.

[0066] Example 1

[0067] This embodiment provides a betaine crystal conversion agent, the molecular structure of which is CH3(CH2) 11 N + (CH3)2(CH2)3SO3 - .

[0068] The preparation method comprises: weighing N,N-dimethyldodecylamine and dissolving it in anhydrous tetrahydrofuran, heating it at 50°C to dissolve it. Then, 1,3-propane sultone is added dropwise, wherein the ratio of N,N-dimethyldodecylamine to 1,3-propane sultone is 1:1.2. After the addition is complete, the temperature is raised to 60°C and the reaction is carried out for 12 hours. After centrifugation, a white precipitate is obtained, which is washed three times with ether and dried in a vacuum drying oven to obtain the product with a yield of 81% and an optical purity of 99.2%.

[0069] 1 H-NMR(d-DMSO): δ0.82~0.84(t,3H),1.24~1.29(m,18H),1.74~1.77(m,2H) ),,2.26~2.28(m,2H),2.71~2.73(t,2H),3.30(s,6H),3.38~3.41(t,4H),.

[0070] Example 2

[0071] This embodiment provides a betaine crystal conversion agent, the molecular structure of which is CH3(CH2) 11 N + (CH3)2(CH2)2COO - .

[0072] The preparation method comprises weighing N,N-dimethyldodecylamine and dissolving it in anhydrous tetrahydrofuran. The mixture is heated to 50°C and dissolved, followed by dropwise addition of β-propiolactone, wherein the molar ratio of N,N-dimethyldodecylamine to β-propiolactone is 1:1.1. After the addition is complete, the mixture is stirred at 50°C for 12 hours. After centrifugation, a white precipitate is obtained, which is then washed three times with ether and dried in a vacuum drying oven to obtain the product with a yield of 81% and an optical purity of 99.2%.

[0073] 1 H-NMR (d-DMSO): δ0.82~0.84(t,3H),1.24~1.29(m,18H),1.74~1.77(m,2H),2.86~2.88(t,2H),3.22(t,2H),3.32(s,6H),3.67(t,2H),.

[0074] Example 3

[0075] This embodiment provides a betaine crystal conversion agent, the molecular structure of which is CH3(CH2)5N + (CH3)2(CH2)5COO - .

[0076] The preparation method comprises: weighing N,N-dimethylhexylamine and 6-bromohexanoic acid, dissolving them in anhydrous ethanol, adding potassium iodide as a catalyst, introducing nitrogen, reacting at a constant temperature of 35°C for 72 hours, filtering the reaction liquid, drying the filter residue, dissolving it in water, and extracting it three times with ether. After drying the aqueous phase, the product is obtained with a yield of 68% and an optical purity of 99.6%.

[0077] The molar ratio of N,N-dimethylhexylamine to 6-bromohexanoic acid is 1:1.2, and the amount of potassium iodide added is 10% of the mass of N,N-dimethylhexylamine.

[0078] 1 H-NMR(d-DMSO): δ0.82~0.84(t,3H),1.28~1.32(m,8H),1.72~1.75(m,4H) ), 3.30 (s, 6H), 3.48~3.51 (t, 4H), 1.52~1.55 (m, 2H), 2.87~2.91 (t, 2H).

[0079] Example 4

[0080] This embodiment provides a betaine crystal-transforming agent, the molecular structure of which is N + (CH3)3(CH2) 11 COO - .

[0081] The preparation method comprises slowly adding a mixed aqueous solution of sodium hydroxide and trimethylamine dropwise to an aqueous solution of methyl bromododecanoate at 0-5°C over approximately 3 hours, continuing the reaction with stirring and maintaining the temperature for 24 hours, and then stopping the reaction. Hydrochloric acid (36%) is added to neutralize the mixture to a pH of 5, and the mixture is purified by centrifugation, ultrafiltration, and resin purification to obtain a pure product with a yield of 79% and an optical purity of 99.5%.

[0082] The molar ratio of trimethylamine to methyl bromododecanoate was 1.2:1. In the mixed solution, the concentration of trimethylamine was 15 wt %, the concentration of sodium hydroxide was 28 wt %; and the concentration of the aqueous solution of methyl bromododecanoate was 1 wt %.

[0083] 1 H-NMR (d-DMSO): δ1.26~1.30(m,14H), 1.52~1.55(t,2H), 1.68~1.71(t,2H), 2.41~2.43(t,2H), 3.28~3.30(t,2H), 3.35(9H,s).

[0084] The isoelectric point of the betaine crystal-transforming agents prepared in Examples 1-4 was tested. The testing method included adjusting the pH of the crystal-transforming agent solution with an aqueous NaOH solution or an aqueous HCl solution, measuring the conductivity (λ) of the solution at different pH values ​​using a zeta potential meter, and plotting a pH-λ curve. The isoelectric point is reached when the curve reaches a minimum, indicating that the number of positive and negative ions in the solution is equal.

[0085] Please refer to Table 1 for the test results:

[0086] Table 1. Isoelectric points of the crystal-transforming agents prepared in Examples 1 to 4

[0087] Example 5

[0088] A method for preparing semi-hydrated gypsum powder by using a phosphogypsum normal pressure salt solution method, comprising the following steps:

[0089] (1) Phosphogypsum pretreatment

[0090] The phosphogypsum was repeatedly washed with tap water until its pH reached 6.8. The pretreated phosphogypsum was dried and passed through a 0.3 mm square mesh sieve. 0.5 wt% quicklime and 10% water were then added to prepare a slurry. After pretreatment for 48 hours, it was dried at 45°C to obtain purified gypsum. The source and physical and chemical properties of the phosphogypsum were as follows: phosphogypsum from a phosphate fertilizer plant in Hubei Province. The phosphogypsum was gray-black in color, with a whiteness of 22.71, a free water content of 25.71%, a total phosphorus content of 1.524%, a soluble phosphorus content of 0.4986%, a total fluorine content of 0.22%, of which the soluble fluorine content was 0.08%, an organic matter content of 0.05%, and a leachate pH between 1 and 2. The source and physical and chemical properties of the phosphogypsum used in subsequent examples were the same as those in this example.

[0091] The soluble phosphorus, soluble fluoride, and calcium sulfate dihydrate content of the phosphogypsum were tested according to the GB / T 23456-2009 standard for "Phosphogypsum." The purified gypsum obtained contained 92% calcium sulfate dihydrate, 0.051% soluble phosphorus, and 0.015% soluble fluoride.

[0092] (2) Preparation of slurry

[0093] An aqueous solution having a sodium sulfate concentration of 0.7 mol / L and a crystallization agent prepared in Example 1 at a dosage of 1% of the dry mass of calcium sulfate dihydrate in the purified gypsum was prepared. The solution was poured into a reactor and heated to 97° C., and purified gypsum was added to prepare a slurry with a solid content of 60%.

[0094] (3) Adjust the pH value to obtain the product

[0095] 3 mol / L KOH solution or concentrated sulfuric acid is added to adjust the pH of the slurry to 2-10, and then a constant temperature dynamic reaction is carried out with a stirring speed of about 200 r / min. The product after the final reaction is quickly filtered, washed with boiling water, and dried at 120°C. After grinding, α-hemihydrate gypsum powder is obtained.

[0096] By adjusting the pH of the slurry, hemihydrate gypsum crystals with different aspect ratios (ratio of length to cross-sectional diameter) can be obtained.

[0097] The method for determining the pH of the slurry includes: extracting the slurry and filtering it with medium-speed qualitative filter paper, placing a precision pH meter into the filtered solution, letting it stand at room temperature for 15 minutes, and reading the reading with the pH meter.

[0098] The testing method of the aspect ratio includes: observing the crystal morphology and size of hemihydrate gypsum using a JEOL JSM-6490LV scanning electron microscope, measuring the length and diameter of the crystals using Image-Pro Plus image analysis software, and calculating the average aspect ratio.

[0099] The above method is used in the subsequent examples when measuring the pH of the slurry and testing the aspect ratio.

[0100] The aspect ratio test results of this embodiment are shown in Figures 1, 2, 3 and Table 2:

[0101] Table 2: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Example 5

[0102] Example 6

[0103] A method for preparing semi-hydrated gypsum powder by using a phosphogypsum normal pressure salt solution method, comprising the following steps:

[0104] (1) Phosphogypsum pretreatment

[0105] The phosphogypsum was repeatedly washed with tap water until its pH value was 6.8. The pretreated phosphogypsum was dried and passed through a 0.3 mm square hole sieve. Then, 2 wt% quicklime and 20% water were added to prepare a slurry. After pretreatment for 24 hours, it was dried at 45°C to obtain purified gypsum.

[0106] The content of calcium sulfate dihydrate in the obtained purified gypsum is 93%, the content of soluble phosphorus is 0.038%, and the content of soluble fluorine is 0.013%.

[0107] (2) Preparation of slurry

[0108] An aqueous solution with a sodium sulfate concentration of 0.3 mol / L, a calcium chloride concentration of 0.5 mol / L, and a crystallization agent prepared in Example 2 at a dosage of 1% of the dry basis mass of calcium sulfate dihydrate in the purified gypsum was prepared. The solution was poured into a reactor and heated to 97°C. The purified gypsum was added to prepare a slurry with a solid content of 60%.

[0109] (3) Adjust the pH value to obtain the product

[0110] 3 mol / L KOH solution or concentrated sulfuric acid is added to adjust the slurry's pH to 2-10. A constant temperature dynamic reaction is then carried out with a stirring speed of approximately 200 rpm. The final product is quickly filtered, washed with boiling water, dried at 120°C, and ground to obtain α-hemihydrate gypsum powder. Hemihydrate gypsum crystals with varying aspect ratios can be obtained by adjusting the slurry's pH. See Table 3 for details:

[0111] Table 3: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Example 6

[0112] Example 7

[0113] A method for preparing semi-hydrated gypsum powder by using a phosphogypsum normal pressure salt solution method, comprising the following steps:

[0114] (1) Phosphogypsum pretreatment

[0115] The phosphogypsum was repeatedly washed with tap water until its pH value was 6.8. The pretreated phosphogypsum was dried and passed through a 0.3 mm square hole sieve. Then, 1 wt% quicklime and 20% water were added to prepare a slurry. After pretreatment for 24 hours, it was dried at 45°C to obtain purified gypsum.

[0116] The content of calcium sulfate dihydrate in the obtained purified gypsum is 93%, the content of soluble phosphorus is 0.045%, and the content of soluble fluorine is 0.011%.

[0117] (2) Preparation of slurry

[0118] An aqueous solution having a calcium nitrate concentration of 1 mol / L and a crystal-changing agent prepared in Example 3 at a dosage of 1% of the dry mass of calcium sulfate dihydrate in the purified gypsum was prepared. The solution was poured into a reactor and heated to 95° C., and purified gypsum was added to prepare a slurry with a solid content of 80%.

[0119] (3) Adjust the pH value to obtain the product

[0120] 3 mol / L KOH solution or concentrated sulfuric acid is added to adjust the slurry's pH to 2-10. A constant temperature dynamic reaction is then carried out with a stirring speed of approximately 200 rpm. The final product is quickly filtered, washed with boiling water, dried at 120°C, and ground to obtain α-hemihydrate gypsum powder. Hemihydrate gypsum crystals with varying aspect ratios can be obtained by adjusting the slurry's pH. See Table 4 for details:

[0121] Table 4: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Example 7

[0122] Example 8

[0123] The difference from Example 7 is that step (2) uses the crystal-transforming agent prepared in Example 4, the solid content of the slurry is 70%, and the drying temperature in step (3) is 110°C. By adjusting the pH of the slurry, hemihydrate gypsum crystals with different aspect ratios can be obtained. Please refer to Table 5 for details:

[0124] Table 5: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Example 8

[0125] Example 9

[0126] A method for preparing semi-hydrated gypsum powder by using a phosphogypsum normal pressure salt solution method, comprising the following steps:

[0127] (1) Phosphogypsum pretreatment

[0128] The phosphogypsum was repeatedly washed with tap water until its pH value reached 6.8. The pretreated phosphogypsum was dried and passed through a 0.3 mm square hole sieve. Then, 1 wt% quicklime and 15% water were added to prepare a slurry. After pretreatment for 48 hours, it was dried at 45°C to obtain purified gypsum.

[0129] The content of calcium sulfate dihydrate in the obtained purified gypsum is 93%, the content of soluble phosphorus is 0.052%, and the content of soluble fluorine is 0.015%.

[0130] (2) Preparation of slurry

[0131] An aqueous solution having a magnesium sulfate concentration of 1.3 mol / L and a crystal-transforming agent prepared in Example 1 at a dosage of 0.5% of the dry mass of calcium sulfate dihydrate in the purified gypsum was prepared. The solution was poured into a reactor and heated to 100° C., and the purified gypsum was added to prepare a slurry with a solid content of 60%.

[0132] (3) Adjust the pH value to obtain the product

[0133] 3 mol / L KOH solution or concentrated sulfuric acid is added to adjust the pH of the slurry to 2-10. A constant temperature dynamic reaction is then carried out with a stirring speed of approximately 200 rpm. The final product is quickly filtered, washed with boiling water, dried at 120°C, and ground to obtain α-hemihydrate gypsum powder. Hemihydrate gypsum crystals with different aspect ratios can be obtained by adjusting the pH of the slurry. See Table 6 for details:

[0134] Table 6: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Example 9

[0135] Example 10

[0136] The difference from Example 9 is that step (2) uses calcium nitrate salt at a concentration of 2 mol / L, and the crystal-transforming agent is the crystal-transforming agent of Example 4, with a doping amount of 0.3%. By adjusting the pH of the slurry, hemihydrate gypsum crystals with different aspect ratios can be obtained. For details, please refer to Table 7:

[0137] Table 7: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Example 10

[0138] Comparative Example 1

[0139] The difference from Example 5 is that no crystal-changing agent is added. The pH of the slurry is adjusted to detect the aspect ratio of hemihydrate gypsum crystals grown at different pH values. Please refer to Table 8 for details:

[0140] Table 8: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Comparative Example 1

[0141] Comparative Example 2

[0142] The difference from Example 5 is that the crystal inverting agent is L-aspartic acid (isoelectric point is 2.98). By adjusting the pH of the slurry, the aspect ratio of the hemihydrate gypsum crystals grown at different pH values ​​was detected. For details, please refer to Table 9:

[0143] Table 9: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Comparative Example 2

[0144] Comparative Example 3

[0145] The difference from Example 5 is that the crystal inverting agent is 12-aminododecanoic acid (isoelectric point is 4.2). By adjusting the pH of the slurry, the aspect ratio of the hemihydrate gypsum crystals grown at different pH values ​​was detected. For details, please refer to Table 10:

[0146] Table 10: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Comparative Example 3

[0147] Comparative Example 4

[0148] The difference from Example 5 is that the crystal-changing agent is succinic acid. The pH of the slurry was adjusted to detect the aspect ratio of hemihydrate gypsum crystals grown at different pH values. Please refer to Table 11 for details:

[0149] Table 11: Aspect ratio of hemihydrate gypsum crystals grown at different pH values ​​in Comparative Example 4

[0150] As can be seen from Tables 1-11 above, the hemihydrate gypsum crystals prepared in the embodiments of the present disclosure, when using only one crystal-transforming agent (betaine compound), can be prepared by adjusting the pH of the slurry to obtain hemihydrate gypsum crystals with different aspect ratios, thereby achieving control over the crystal morphology of the hemihydrate gypsum. The control principle is as follows: when the pH of the liquid slurry is lower than the isoelectric point of the crystal-transforming agent, the positively charged crystal-transforming agent acts as a cationic surfactant and tends to adsorb on the {010} crystal planes on the sides of the hemihydrate gypsum crystals, facilitating the growth of the hemihydrate gypsum into a one-dimensional whisker shape along the c-axis. When the pH of the slurry is higher than the isoelectric point of the crystal-transforming agent, the negatively charged crystal-transforming agent acts as an anionic surfactant and tends to adsorb on the {111} crystal planes of the hemihydrate gypsum crystals, limiting the growth of the hemihydrate gypsum along the c-axis, resulting in a reduced aspect ratio and a cylindrical shape. The method for producing hemihydrate gypsum powder from a phosphogypsum atmospheric pressure salt solution provided by the present disclosure demonstrates significant control effects when regulating crystal morphology, and exhibits a pattern in which the aspect ratio of the resulting hemihydrate gypsum crystals decreases as the pH of the slurry increases. Therefore, the pH of the slurry can be adaptively adjusted based on the desired aspect ratio of the hemihydrate gypsum crystals. Compared to amino acid-based crystal-modifying agents, the amphiphilic betaine molecules provided by the present disclosure possess greater proton acceptance and donation capabilities and charge stability, resulting in greater sensitivity in crystal form control.

[0151] The above describes in detail the optional embodiments of the present disclosure, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple variations of the technical solution of the present disclosure can be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as the contents disclosed by the present disclosure and fall within the scope of protection of the present disclosure. Industrial Applicability

[0152] The hemihydrate gypsum crystals prepared by the present disclosure can be obtained by adjusting the pH value of the slurry to obtain hemihydrate gypsum crystals with different aspect ratios, thereby achieving the regulation of the crystal morphology of the hemihydrate gypsum. The regulation principle is: when the pH value of the liquid phase slurry is lower than the isoelectric point of the crystallization agent, the crystallization agent is positively charged, equivalent to a cationic surfactant, and tends to adsorb on the {010} crystal planes on the sides of the hemihydrate gypsum crystals, making it easy for the hemihydrate gypsum to grow into a one-dimensional whisker shape along the c-axis; when the pH value of the slurry is higher than the isoelectric point of the crystallization agent, the crystallization agent is negatively charged, equivalent to an anionic surfactant, and tends to adsorb on the {111} crystal planes of the hemihydrate gypsum crystals, making it easy for the hemihydrate gypsum to grow along the c-axis and restricting its growth, resulting in a decrease in the aspect ratio of the crystals and a cylindrical shape. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum provided by the present disclosure has a significant regulation effect when regulating the crystal morphology, and shows a rule that the higher the pH of the slurry, the smaller the aspect ratio of the obtained hemihydrate gypsum crystals. Therefore, the pH value of the slurry can be adaptively adjusted according to the desired aspect ratio of the hemihydrate gypsum crystals. Compared to amino acid-based crystal-inverting agents, the amphiphilic betaine molecules provided by the present disclosure have higher proton acceptance and donation capabilities and charge stability, and thus have higher sensitivity in crystal form control.

Claims

1. A method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum, characterized in that, it includes: Mixing a mixed solution of salt and a crystal conversion agent with purified phosphogypsum to prepare a slurry, wherein the crystal conversion agent is a betaine compound with pH responsiveness; According to the isoelectric point of the crystal conversion agent and the aspect ratio of the required hemihydrate gypsum crystals, adjusting the pH of the slurry with an alkali agent or an acid agent, and performing a constant temperature dynamic reaction on the slurry, followed by filtration to obtain the product; wherein, when the pH value of the slurry is greater than the isoelectric point of the crystal conversion agent, the aspect ratio of the hemihydrate gypsum crystals increases, and when the pH value of the slurry is less than the isoelectric point of the crystal conversion agent, the aspect ratio of the hemihydrate gypsum crystals decreases.

2. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the isoelectric point of the crystal conversion agent is 5 - 8.

3. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the adjustment range of the pH of the slurry is 2 - 10.

4. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, The betaine compound is selected from one or a combination of more than one of carboxyl-type betaines and sulfonic acid-type betaines, and the betaine compound has the following general formula: R 1 N + (CH 3 ) 2 R 2 X - ; Among them, R 1 is a substituted or unsubstituted straight-chain C1-C12 alkyl group, and R 2 is a substituted or unsubstituted straight-chain C2-C12 alkyl group; X- is a carboxylate or sulfate group.

5. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 4, characterized in that, R 1 The substituents in R include amino group, hydroxyl group or mercapto group; R 2 The substituents in it include amino group, hydroxyl group or mercapto group.

6. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 4, characterized in that, The carboxy-type betaine includes CH 3 (CH 2 ) 11 N + (CH 3 ) 2 (CH 2 ) 2 COO - 、N + (CH 3 ) 3 (CH 2 ) 12 COO - or CH 3 (CH 2 ) 5 N + (CH 3 ) 2 (CH 2 ) 5 COO - 。 7. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 4, characterized in that, The sulfobetaine includes CH 3 (CH 2 ) 11 N + (CH 3 ) 2 (CH 2 ) 3 SO 3 - 。 8. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the addition amount of the crystal conversion agent is 0.3 - 1% of the dry basis mass of calcium sulfate dihydrate in the purified phosphogypsum.

9. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the content of calcium sulfate dihydrate in the purified phosphogypsum is 80 - 98%, the soluble phosphorus content ≤ 0.1%, and the soluble fluorine content ≤ 0.03%.

10. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the concentration of the salt in the mixed solution is 0.3 - 1.5 mol / L.

11. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the salt includes any one or a combination of more than one of sodium sulfate, calcium nitrate, magnesium sulfate, and calcium chloride.

12. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the solid content of the slurry is 60 - 80%.

13. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the stirring speed of the constant temperature dynamic reaction is 180 - 250 r / min.

14. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, the alkali agent includes any one or a combination of more than one of sodium hydroxide, potassium hydroxide, calcium hydroxide, and calcium oxide.

15. The method for preparing hemihydrate gypsum powder by an atmospheric pressure salt solution method using phosphogypsum according to claim 1, characterized in that, The acid agent includes any one or a combination of more than one of hydrochloric acid and sulfuric acid.

16. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 1, characterized in that, after the filtration, it further includes washing, drying and grinding the solid matter.

17. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 16, characterized in that, the washing includes washing with boiling water.

18. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 16, characterized in that, the temperature for drying the solid matter is 110 - 120 °C.

19. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 1, characterized in that, the preparation method of the purified phosphogypsum includes: first washing the phosphogypsum with water until the pH is 6.5 - 7.0, then drying and sieving to obtain gypsum powder, and preparing a slurry by mixing the gypsum powder, quicklime and water for 24 - 48 h, and drying to obtain the purified phosphogypsum.

20. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 19, characterized in that, the mass ratio of the gypsum powder, the quicklime and the water is 100:0.5 - 2:10 - 20.

21. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 19, characterized in that, the sieving includes sieving the dried phosphogypsum through a square hole sieve with a pore size of 0.2 - 0.4 mm.

22. The method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to claim 1, characterized in that, the temperature for drying the slurry is 40 - 50 °C.

23. A hemihydrate gypsum powder, characterized in that, it is prepared by the method for preparing hemihydrate gypsum powder by the atmospheric pressure salt solution method of phosphogypsum according to any one of claims 1 - 22.

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