A high-strength α-hemihydrate gypsum, its preparation method, and a composite crystal form regulator thereof.

Abstract

This invention discloses a high-strength α-hemihydrate gypsum, its preparation method, and a composite crystal form regulator, relating to the technical field of industrial wastewater resource utilization and high-performance gypsum material preparation. The composite crystal form regulator includes: malic acid, citric acid, tartaric acid, disodium EDTA, and polyacrylic acid-acrylamide-AMPS copolymer. The preparation method of the high-strength α-hemihydrate gypsum includes the following steps: S1: In a metathesis reaction system of calcium chloride and sodium sulfate, the composite crystal form regulator is added to generate a slurry containing dihydrate gypsum crystals with an average particle size ≥100μm and a specific surface area ≤0.8m² / g; S2: The composite crystal form regulator and residual chloride ions are removed using at least a two-stage countercurrent hot water washing process to obtain dihydrate gypsum with a chloride content ≤150ppm; S3: The dihydrate gypsum is steam-pressurized in a pure water medium to obtain α-hemihydrate gypsum with a chloride content ≤100ppm and a 2h compressive strength ≥55 MPa.

Description

Technical Field

[0001] This invention belongs to the technical field of industrial waste liquid resource utilization and high-performance gypsum material preparation, and in particular relates to a method for preparing low-chlorine residual high-strength α-hemihydrate gypsum from soda ash ammonia stripping waste liquid and its composite crystal form regulator. Background Technology

[0002] The ammonia stripping waste liquid from the soda ash industry contains large amounts of CaCl2 (70-130 g / L) and NaCl (40-90 g / L). Direct discharge would cause serious environmental pollution and resource waste. Reacting this waste liquid with sodium sulfate can generate gypsum dihydrate (CaSO4·2H2O), which can then be transformed into high-strength α-hemihydrate gypsum (CaSO4·0.5H2O) through a phase change, thus achieving resource utilization of the waste liquid.

[0003] However, gypsum products prepared by existing processes have high chlorine residue (typically >0.1%), leading to reduced product strength, easy efflorescence, and equipment corrosion, thus limiting their application in precision casting, high-end building materials, and medical fields. Traditional methods for preparing α-hemihydrate gypsum (such as autoclaving and salt solution methods) struggle to effectively remove chloride ions when treating chlorine-containing wastewater, as chloride ions are easily trapped in the crystal lattice or adsorbed onto the surface.

[0004] Furthermore, existing crystal form regulators (such as citric acid or malic acid alone) have limited effects, resulting in small dihydrate gypsum particles with large specific surface areas, low washing efficiency, and difficulty in reducing chlorine residue to below 0.05%, making it difficult to meet the JC / T 2038-2010 α50 standard. Therefore, developing a process that can simultaneously achieve the preparation of large-particle, low-specific-surface-area dihydrate gypsum, efficient dechlorination, and high-quality α-hemihydrate gypsum conversion has significant industrial application value. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-strength α-hemihydrate gypsum, its preparation method and a composite crystal form regulator, which can effectively reduce the chlorine content of the product, improve its strength and realize the high-value utilization of soda ash waste liquid.

[0006] To achieve the above objectives, the present invention provides a composite crystal form regulator comprising the following components: malic acid, citric acid, tartaric acid, disodium EDTA, and polyacrylic acid-acrylamide-AMPS copolymer.

[0007] Furthermore, the mass percentage content of each component is as follows: malic acid 30%-40%, citric acid 20%-30%, tartaric acid 10%-15%, disodium EDTA 5%-10%, and polyacrylic acid-acrylamide-AMPS copolymer 15%-25%.

[0008] Furthermore, the molar ratio of acrylic acid:acrylamide:AMPS in the polyacrylic acid-acrylamide-AMPS copolymer is 1:0.5:0.3, and the molecular weight is 8000-15000 Da.

[0009] To achieve the above objectives, the present invention also provides a method for preparing high-strength α-hemihydrate gypsum, comprising the following steps: S1: In the metathesis reaction system of calcium chloride and sodium sulfate, the above-mentioned composite crystal form regulator is added. Through the multiple coordination chelation and selective crystal face adsorption of the regulator, the nucleation of dihydrate gypsum crystals is inhibited and crystal growth is promoted, thereby increasing the relative growth rate of

[001] crystal faces and generating dihydrate gypsum crystals with an average particle size ≥100 μm and a specific surface area ≤0.8 m² / g, thus obtaining a dihydrate gypsum crystal slurry; S2: After solid-liquid separation of the dihydrate gypsum crystal slurry obtained in step S1, a at least two-stage countercurrent hot water washing process is adopted, controlling the temperature at 60-80℃ and the pH at 6.5-7.5, to remove the composite crystal form regulator and residual chloride ions, thereby obtaining dihydrate gypsum with a chloride content ≤150ppm; S3: The dihydrate gypsum is subjected to a washing process at 120-160℃ and 0.2-0.4 Alpha-hemihydrate gypsum with a chlorine content ≤100ppm and a 2h compressive strength ≥55 MPa was obtained by steam pressure conversion in pure water medium at MPa for 3-5 hours.

[0010] Furthermore, the amount of the composite crystal form regulator added is 0.2-0.6 wt% of the theoretical yield of gypsum dihydrate.

[0011] Furthermore, in step S2, the liquid-to-solid ratio for each stage is 2.5-3.5:1, the washing time for each stage is 10-20 minutes, the removal rate of the control agent in the washing process is ≥99.5%, and there is no residual organic matter on the surface of the dihydrate gypsum after washing.

[0012] Furthermore, the α-hemihydrate high-strength gypsum obtained in step S3 has a short columnar crystal morphology with an aspect ratio of 1.0-1.5 and a specific surface area of ​​0.4-0.7 m² / g.

[0013] Furthermore, the CaCl2 in step S1 comes from the waste liquid of soda ash distillation for ammonia, the waste liquid of hydrochloric acid wet process for phosphoric acid production, the waste liquid of salt production bittern, and produced water from oil and gas fields. Alternatively, the mother liquor after filtering the dihydrate gypsum crystals after reaction crystallization in step S1 can be recycled, wherein the CaCl2 content is 70-130 g / L and the NaCl content is 40-90 g / L.

[0014] To achieve the above objectives, the present invention also provides a high-strength α-hemihydrate gypsum, which is prepared by the above preparation method. The α-hemihydrate gypsum has a short columnar crystal morphology with an aspect ratio of 1.0-1.5 and a specific surface area of ​​0.4-0.7 m² / g.

[0015] Furthermore, the product whiteness is ≥92%, and the linear expansion rate is ≤0.12%.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: The high-strength α-hemihydrate gypsum, its preparation method, and the composite crystal form regulator provided by the present invention, through the synergistic effect of the five-element composite crystal form regulator, wherein malic acid, citric acid, and tartaric acid interact with Ca² through multidentate coordination. + Combined, the nucleation rate of dihydrate gypsum crystals is controlled and selectively adsorbed onto the {111} crystal plane, promoting growth in the

[001] direction to form large-particle short columnar crystals; disodium EDTA chelates trace metal impurities, reducing lattice defects; polyacrylic acid-acrylamide-AMPS copolymer provides steric hindrance and selective adsorption, realizing the preparation of large-particle low specific surface area dihydrate gypsum and significantly reducing chloride ion inclusion; the regulator and chloride ions are efficiently removed through at least two-stage countercurrent washing; high-strength, low-chlorine α-hemihydrate gypsum is obtained through autoclaving. The product has a chloride content ≤0.03% and a strength ≥55 MPa, meeting the JC / T 2038-2010 α50 grade or higher standard, and can be used for precision casting molds, high-end building materials and medical fixation materials, while realizing the resource utilization of soda ash waste liquid, etc. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the preparation process of high-strength α-hemihydrate gypsum in an embodiment of the present invention; Figure 2 This is a SEM image of the synthesized large-particle dihydrate gypsum in Example 1 of the present invention; Figure 3 This is a SEM image of the synthesized α-hemihydrate gypsum in Example 1 of this invention; Figure 4 This is a SEM image of the synthesized large-particle dihydrate gypsum in Example 2 of the present invention; Figure 5 This is a SEM image of the synthesized α-hemihydrate gypsum in Example 2 of the present invention; Figure 6 This is a SEM image of the synthesized large-particle dihydrate gypsum in Example 3 of the present invention; Figure 7 This is a SEM image of the synthesized α-hemihydrate gypsum in Example 3 of the present invention; Figure 8 This is a SEM image of the synthesized large-particle dihydrate gypsum in Example 4 of the present invention; Figure 9 This is a SEM image of the synthesized α-hemihydrate gypsum in Example 4 of this invention; Figure 10 This is a SEM image of the synthesized large-particle dihydrate gypsum in Example 5 of the present invention; Figure 11 This is a SEM image of the synthesized α-hemihydrate gypsum in Example 5 of the present invention; Figure 12 This is a SEM image of the synthesized large-particle dihydrate gypsum in Example 6 of the present invention; Figure 13 This is a SEM image of the synthesized α-hemihydrate gypsum in Example 6 of the present invention; Figure 14 This is a SEM image of the synthesized gypsum dihydrate in Comparative Example 1 of this invention; Figure 15 This is a SEM image of the synthesized α-hemihydrate gypsum in Comparative Example 1 of this invention. Detailed Implementation

[0018] The technical solution of the present invention will be described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0019] The reaction mechanism involved in this invention is as follows: Calcium chloride and sodium sulfate undergo a metathesis reaction in the aqueous phase to generate gypsum dihydrate precipitate, while simultaneously releasing sodium chloride. The organic acids (malic acid, citric acid, tartaric acid) in the composite crystal form regulator react with Ca²⁺ through multidentate coordination. + Combined, the nucleation rate of crystals is regulated and selectively adsorbed onto the {111} crystal face, promoting growth in the

[001] direction to form large-particle short columnar crystals; this regulation effectively inhibits the nucleation of gypsum dihydrate and promotes crystal growth, resulting in a significant increase in the final average particle size from 35 μm in the comparative example to ≥100 μm in the example (an increase of nearly 3 times). At the same time, the relative growth rate of the

[001] crystal face is increased, achieving optimization of anisotropic growth kinetics, and finally obtaining large-particle low specific surface area crystals with an average particle size ≥100 μm and a specific surface area ≤0.8 m² / g, effectively reducing chloride ion inclusion and surface adsorption; disodium EDTA (disodium ethylenediaminetetraacetate) strongly chelates trace metal impurities (such as Fe³⁺). + Mg² + The reaction process reduces lattice defects. The polyacrylic acid-acrylamide-AMPS (2-acrylamido-2-methylpropanesulfonic acid) copolymer provides steric hindrance and electrostatic repulsion, further inhibiting high-energy crystal face growth and reducing chloride ion inclusion. After the reaction, the product undergoes at least two countercurrent hot water washing stages to remove the regulator and soluble chloride salts, followed by a pressure phase change (120-140℃, 0.2-0.4 MPa) to convert it into α-hemihydrate gypsum. The resulting product has low chloride content and high strength.

[0020] This invention provides a composite crystal form regulator comprising the following components: malic acid, citric acid, tartaric acid, disodium EDTA, and a polyacrylic acid-acrylamide-AMPS copolymer. Further, the mass percentage content of each component is: malic acid 30%-40%, citric acid 20%-30%, tartaric acid 10%-15%, disodium EDTA 5%-10%, and polyacrylic acid-acrylamide-AMPS copolymer 15%-25%. In a preferred embodiment, the molar ratio of acrylic acid:acrylamide:AMPS in the polyacrylic acid-acrylamide-AMPS copolymer is 1:0.5:0.3, and the molecular weight is 8000-15000 Da. The copolymer contains multiple functional groups including carboxyl, amide, and sulfonic acid groups, which selectively inhibit the growth of high-energy crystal faces through the synergistic effect of electrostatic repulsion and steric hindrance, while simultaneously chelating trace metal impurities and improving crystal purity. The molecular weight of the polyacrylic acid-acrylamide-AMPS copolymer is controlled within the range of 8000-15000 Da. This ensures rapid adsorption on the crystal face and effective steric hindrance while avoiding the risk of increased viscosity and bridging flocculation caused by high molecular weight, and at the same time ensures a washing removal rate of ≥99.5%. This molecular weight range has been experimentally optimized and can effectively suppress the nucleation rate and regulate the synergistic control of growth in the

[001] direction.

[0021] Furthermore, the polyacrylic acid-acrylamide-AMPS copolymer was synthesized via free radical copolymerization. Specifically, acrylic acid, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) monomers were dissolved in deionized water at a molar ratio of 1:0.5:0.3. After deoxygenation by purging with nitrogen, ammonium persulfate initiator was added, and the reaction was stirred at 60–70°C for 4–6 hours. After the reaction was completed, the product was purified by dialysis (molecular weight cutoff 5000 Da) and freeze-dried to obtain a polyacrylic acid-acrylamide-AMPS copolymer with a molecular weight of 8000–15000 Da.

[0022] Please see Figure 1 This invention provides a method for preparing high-strength α-hemihydrate gypsum, comprising the following steps: S1: In the double decomposition reaction system of calcium chloride and sodium sulfate, the above-mentioned composite crystal form regulator is added. Through the multiple coordination chelation and selective crystal face adsorption of the regulator, the nucleation of dihydrate gypsum crystals is inhibited and crystal growth is promoted, the relative growth rate of

[001] crystal face is increased, the number of nuclei is reduced, and the

[001] direction is preferentially grown, so that dihydrate gypsum crystals with an average particle size ≥100 μm and a specific surface area ≤0.8 m² / g are finally generated, and dihydrate gypsum crystal slurry is obtained, which effectively reduces chloride ion inclusion and surface adsorption; CaCl2 comes from the waste liquid of soda ash ammonia stripping, the waste liquid of hydrochloric acid wet phosphoric acid production, the waste liquid of salt bittern production, and the produced water of oil and gas fields, or the mother liquor after solid-liquid separation of dihydrate gypsum crystal slurry in step S2 is recycled to realize the resource utilization of the salt gypsum system, wherein the CaCl2 content is 70-130 g / L, the NaCl content is 40-90 g / L, and the chloride content of the product after conversion is stable ≤100ppm. Preferably, the amount of the composite crystal form regulator added is 0.2-0.6 wt% of the theoretical yield of gypsum dihydrate. The theoretical yield of gypsum dihydrate refers to the theoretical mass (g) of gypsum dihydrate obtained by completely converting the actual content of CaCl2 in the waste liquid into gypsum dihydrate according to stoichiometry. Theoretical yield of gypsum dihydrate (g) = m CaCl2 ×172 / 111≈m CaCl2 ×1.551 The polyacrylic acid-acrylamide-AMPS copolymer contains multiple functional groups including carboxyl, amide, and sulfonic acid groups. It selectively inhibits the growth of high-energy crystal faces through the synergistic effect of electrostatic repulsion and steric hindrance, while chelating trace metal impurities to improve crystal purity.

[0023] S2: After solid-liquid separation of the dihydrate gypsum crystal slurry obtained in step S1, a countercurrent hot water washing process with at least two stages is used to remove the composite crystal form regulator and residual chloride ions. The temperature is controlled at 60-80℃, pH at 6.5-7.5, and the liquid-to-solid ratio at each stage is 2.5-3.5:1, preferably 3:1; the washing time at each stage is 10-20 min. The countercurrent washing process is as follows: fresh hot water enters from the last stage washing tank, contacts the dihydrate gypsum with the lowest impurity concentration, and then flows countercurrently to the previous stage, finally discharging wastewater containing regulators and chloride ions from the first stage. This completely removes the composite crystal form regulator and residual chloride ions, obtaining low-chlorine dihydrate gypsum with a chloride content ≤150ppm; the removal rate of the regulator in the washing process is ≥99.5%, and there is no residual organic matter on the surface of the dihydrate gypsum after washing, ensuring that the subsequent autoclaving process is not interfered with by impurities and that the crystal integrity is maintained.

[0024] S3: Dihydrate gypsum is steam-pressurized in a pure water medium at 120-160℃ and 0.2-0.4 MPa for 3-5 hours to obtain α-hemihydrate gypsum with a chloride content ≤100ppm and a 2-hour compressive strength ≥55 MPa, meeting or exceeding the JC / T 2038-2010 α50 grade standard. It is suitable for precision casting molds, high-end building materials, or medical fixation materials. The obtained α-hemihydrate high-strength gypsum has a short columnar crystal morphology with an aspect ratio of 1.0-1.5 and a specific surface area of ​​0.4-0.7 m² / g. No regulator residue is left during the conversion process, and the product has a whiteness ≥92% and a linear expansion rate ≤0.12%.

[0025] Example 1 Take 200 mL of soda ash ammonia stripping waste liquid containing 111 g / L CaCl2 and approximately 46.8 g / L NaCl, pH 6.8, and add a composite crystal form regulator with the following mass percentages: malic acid 35%, citric acid 25%, tartaric acid 12%, disodium EDTA 8%, and polyacrylic acid-acrylamide-AMPS copolymer 20%, molecular weight 12000 Da. The amount of regulator added is 0.40 wt% of the theoretical yield of gypsum dihydrate. Under stirring at 45℃ and 400 rpm, slowly add an equimolar amount of 1.0 mol / L Na2SO4 solution at a rate of 5 mL / min, and react for 40 min to generate a gypsum dihydrate crystal slurry.

[0026] After solid-liquid separation of the dihydrate gypsum crystal slurry, the chlorine content of the dihydrate gypsum was reduced to 0.12% after two-stage countercurrent hot water washing. The low-chlorine dihydrate gypsum was then subjected to autoclaving at 130℃ and 0.3 MPa in pure water for 4 hours to obtain α-hemihydrate gypsum with a chlorine content of 0.025%, a 2-hour compressive strength of 58.3 MPa, a whiteness of 93.5%, and a linear expansion rate of 0.09%, meeting the JC / T 2038-2010 α50 grade standard.

[0027] Example 2 Take 150 mL of hydrochloric acid wet-process phosphoric acid production waste liquid containing 88.8 g / L CaCl2 (and 70 g / L NaCl), and add a composite crystal form regulator with the following mass percentages: malic acid 32%, citric acid 28%, tartaric acid 15%, disodium EDTA 5%, and polyacrylic acid-acrylamide-AMPS copolymer 20% (molecular weight 12000 Da). The amount of regulator added is 0.35 wt% of the theoretical yield of gypsum dihydrate. Add an equimolar amount of Na2SO4 at 50℃ and 300 rpm, and react for 35 min to generate a gypsum dihydrate crystal slurry.

[0028] After solid-liquid separation of the dihydrate gypsum crystal slurry, it underwent two-stage countercurrent washing, controlled at 65℃, pH 6.8, and a liquid-to-solid ratio of 3:1, with a chlorine content of 0.11%. SEM analysis showed the product to be large-particle, short columnar crystals with an average particle size of 105 μm and a specific surface area of ​​0.68 m² / g. Figure 4 As shown.

[0029] After steam-pressing low-chlorine dihydrate gypsum in pure water at 125℃ and 0.25 MPa for 4.5 h, α-hemihydrate gypsum was obtained with a chlorine content of 0.023%, a compressive strength of 56.8 MPa after 2 h, a whiteness of 92.8%, and a linear expansion rate of 0.10%, which meets the JC / T 2038-2010 α50 grade standard.

[0030] Example 3 Take 250 mL of bittern waste liquid from a salt plant containing 133.2 g / L CaCl2, 57.1 g / L MgCl2, and 52.6 g / L NaCl. Add a composite crystal form regulator with the following mass percentages: malic acid 38%, citric acid 22%, tartaric acid 10%, disodium EDTA 10%, and polyacrylic acid-acrylamide-AMPS copolymer 20% (molecular weight 14000 Da). The regulator is added at 0.55 wt% of the theoretical yield of gypsum dihydrate. Add an equimolar amount of Na2SO4 at 42℃ and 500 rpm, and react for 45 min to generate a gypsum dihydrate crystal slurry.

[0031] After solid-liquid separation of the dihydrate gypsum crystal slurry, it underwent four-stage countercurrent washing, controlled at a temperature of 75℃, pH 7.2, and a liquid-to-solid ratio of 3:1. The chlorine content of the dihydrate gypsum was 0.09%. SEM analysis showed that the product consisted of large, short columnar crystals with an average particle size of 125 μm and a specific surface area of ​​0.55 m² / g. Figure 6 As shown.

[0032] After steam-pressure conversion of low-chlorine dihydrate gypsum in pure water at 135℃ and 0.35 MPa for 3.5 h, α-hemihydrate gypsum with a chlorine content of 0.028% and a compressive strength of 59.2 MPa after 2 h was obtained. Figure 7 As shown, the whiteness is 94.1% and the linear expansion rate is 0.08%, which meets the JC / T 2038-2010 α50 grade standard.

[0033] Example 4 Take 180 mL of produced water from an oil and gas field containing 99.9 g / L CaCl2 and 87.7 g / L NaCl. Add a composite crystal form regulator with the following mass percentages: malic acid 30%, citric acid 30%, tartaric acid 13%, disodium EDTA 7%, and polyacrylic acid-acrylamide-AMPS copolymer 20% (molecular weight 12000 Da). The regulator is added at 0.25 wt% of the theoretical yield of gypsum dihydrate. Under stirring at 48℃ and 350 rpm, slowly add an equimolar amount of 1.0 mol / L Na2SO4 solution at a rate of 5 mL / min. React for 38 min to generate a gypsum dihydrate crystal slurry.

[0034] After solid-liquid separation of the dihydrate gypsum crystal slurry, it underwent three-stage countercurrent washing, controlled at a temperature of 68℃, pH 6.5, and a liquid-to-solid ratio of 3:1. The chlorine content of the dihydrate gypsum was 0.14%. SEM analysis of the product showed that it consisted of large, short columnar crystals with an average particle size of 98 μm and a specific surface area of ​​0.78 m² / g. Figure 8 As shown.

[0035] After steam-pressure conversion of low-chlorine dihydrate gypsum in pure water at 128℃ and 0.28 MPa for 4 h, α-hemihydrate gypsum was obtained with a chlorine content of 0.026% and a compressive strength of 55.9 MPa after 2 h. Figure 9 As shown, the product has a whiteness of 92.3% and a linear expansion rate of 0.11%, meeting the JC / T 2038-2010 α50 grade standard.

[0036] Example 5 Take 220 mL of soda ash ammonia stripping waste liquid containing 122.1 g / L CaCl2 and 40.9 g / L NaCl, and add a composite crystal form regulator with the following mass percentages: malic acid 36%, citric acid 24%, tartaric acid 11%, disodium EDTA 9%, and polyacrylic acid-acrylamide-AMPS copolymer 20% (molecular weight 12000 Da). The amount of regulator added is 0.48 wt% of the theoretical yield of gypsum dihydrate. Under stirring at 40℃ and 450 rpm, slowly add an equimolar amount of 1.0 mol / L Na2SO4 solution at a rate of 5 mL / min, and react for 42 min to generate a gypsum dihydrate crystal slurry.

[0037] After solid-liquid separation of the dihydrate gypsum crystal slurry, it underwent two-stage countercurrent washing, controlled at a temperature of 72℃, pH 7.5, and a liquid-to-solid ratio of 3:1. The chlorine content of the dihydrate gypsum was 0.10%. SEM analysis showed that the product consisted of large, short columnar crystals with an average particle size of 118 μm and a specific surface area of ​​0.61 m² / g. Figure 10 As shown.

[0038] After steam-pressure conversion of low-chlorine dihydrate gypsum in pure water at 132℃ and 0.32 MPa for 3.8 h, α-hemihydrate gypsum was obtained with a chlorine content of 0.024% and a compressive strength of 57.6 MPa after 2 h. Figure 11 As shown, the product has a whiteness of 93.8% and a linear expansion rate of 0.09%, meeting the JC / T 2038-2010 α50 grade standard.

[0039] Example 6 Take 160 mL of a mixed waste liquid containing 83.3 g / L CaCl2 (a 1:1 mixture of soda ash waste liquid and salt bittern, containing 47.6 g / L MgCl2 and 58.4 g / L NaCl), and add a composite crystal form regulator with the following mass percentages: malic acid 34%, citric acid 26%, tartaric acid 14%, disodium EDTA 6%, and polyacrylic acid-acrylamide-AMPS copolymer 20%, with a molecular weight of 10000 Da. The amount of regulator added is 0.30 wt% of the theoretical yield of gypsum dihydrate. Under stirring at 52℃ and 280 rpm, slowly add an equimolar amount of 1.0 mol / L Na2SO4 solution at a rate of 5 mL / min, and react for 36 min to generate a gypsum dihydrate crystal slurry.

[0040] After solid-liquid separation of the dihydrate gypsum crystal slurry, it underwent four-stage countercurrent washing, controlled at a temperature of 78℃, pH 7.0, and a liquid-to-solid ratio of 3:1, with a chlorine content of 0.08%. SEM analysis showed the product to be large-particle, short columnar crystals with an average particle size of 130 μm and a specific surface area of ​​0.52 m² / g. Figure 12 As shown.

[0041] After steam-pressure conversion of low-chlorine dihydrate gypsum in pure water at 138℃ and 0.38 MPa for 3.2 h, α-hemihydrate gypsum with a chlorine content of 0.022% and a compressive strength of 59.8 MPa after 2 h was obtained. Figure 13 As shown, the product has a whiteness of 94.5% and a linear expansion rate of 0.07%, meeting the JC / T 2038-2010 α50 grade standard.

[0042] Table 1 Summary of main process parameters and results for Examples 1-6 Comparative Example 1 Without adding any complex crystal form regulators, 200 mL of soda ash ammonia stripping waste liquid containing 111 g / L CaCl2 was added to an equimolar amount of Na2SO4 solution at 45℃ and 400 rpm, and the reaction was allowed to proceed for 40 min. The resulting dihydrate gypsum particles were fine, with an average particle size of 35 μm and a specific surface area of ​​2.1 m² / g. Figure 14 As shown, the chlorine content is 0.28% after four stages of washing.

[0043] After autoclaving, the α-hemihydrate gypsum exhibited a compressive strength of only 42.6 MPa after 2 hours, a chloride content of 0.12%, a whiteness of 89.2%, and a linear expansion rate of 0.18%, all significantly lower than the product of this invention. The crystal morphology was irregularly flaky, and SEM revealed numerous fine crystal agglomerations, such as... Figure 15 As shown.

[0044] Comparative Example 2 Using only malic acid (0.40 wt% of the theoretical yield of gypsum dihydrate) as a regulator, all other conditions were the same as in Example 1. The resulting gypsum dihydrate had an average particle size of 62 μm, a specific surface area of ​​1.4 m² / g, a chlorine content of 0.19% after washing, and a regulator removal rate of only 96.2% (residue affected subsequent conversion).

[0045] After autoclaving, the α-hemihydrate gypsum exhibited a compressive strength of 48.9 MPa after 2 hours, a chlorine content of 0.08%, a whiteness of 90.5%, and a linear expansion rate of 0.15%. The product performance was superior to Comparative Example 1 but significantly lower than the five-component composite regulator system of this invention, demonstrating the necessity of the synergistic effect of multiple components.

[0046] Comparative Example 3 Take 200 mL of soda ash ammonia stripping waste liquid containing 111 g / L CaCl2 (approximately 1.0 mol / L) and approximately 46.8 g / L NaCl (approximately 0.8 mol / L), with a pH of 6.8. Add a ternary regulator system with the following mass percentages: 35% citric acid, 8% disodium EDTA, and 20% polyacrylic acid-acrylamide-AMPS copolymer (molecular weight 12000 Da). The total amount of regulator added is 0.40 wt% of the theoretical yield of gypsum dihydrate. Under stirring at 45℃ and 400 rpm, slowly add an equimolar amount of 1.0 mol / L Na2SO4 solution at a rate of 5 mL / min, and react for 40 min to generate gypsum dihydrate slurry.

[0047] After two-stage countercurrent hot water washing, the chlorine content of the dihydrate gypsum decreased to 0.14%. The low-chlorine dihydrate gypsum was then subjected to autoclaving at 130℃ and 0.3 MPa in pure water for 4 hours to obtain α-hemihydrate gypsum with a chlorine content of 0.07%, a 2-hour compressive strength of 49.3 MPa, short columnar crystals (length-to-diameter ratio 1.2), a whiteness of 91.7%, and a linear expansion rate of 0.13%.

[0048] Table 2 Summary of main process parameters and results for Comparative Examples 1-3 Comparative results show that the composite crystal form regulator provided by the present invention, through the synergistic effect of multiple components, is significantly superior to the process with a single regulator or no regulator, and has obvious advantages in controlling large particles with low specific surface area, eliminating chloride ions and improving the compressive strength of the product.

[0049] In summary, the method for preparing low-chlorine-residue, high-strength α-hemihydrate gypsum provided by this invention achieves large-particle, low-specific-surface-area morphology control of dihydrate gypsum crystals through an innovative five-element composite crystal form regulator system, effectively reducing chloride ion inclusion and adsorption. Combined with efficient at least two-stage countercurrent washing, complete removal of the regulator is ensured. Finally, autoclaving yields a high-strength α-hemihydrate gypsum product with extremely low chloride content and excellent strength. This process is simple, mild, and low-cost, suitable for the resource utilization of soda ash ammonia stripping waste or salt gypsum systems, and has promising industrial application prospects and significant economic and environmental benefits.

Claims

1. A composite crystal form regulator, characterized in that, It includes the following components: malic acid, citric acid, tartaric acid, disodium EDTA and polyacrylic acid-acrylamide-AMPS copolymer, with the following mass percentage content: malic acid 30%-40%, citric acid 20%-30%, tartaric acid 10%-15%, disodium EDTA 5%-10% and polyacrylic acid-acrylamide-AMPS copolymer 15%-25%.

2. The composite crystal form regulator according to claim 1, characterized in that, The polyacrylic acid-acrylamide-AMPS copolymer has a molar ratio of acrylic acid:acrylamide:AMPS of 1:0.5:0.3 and a molecular weight of 8000-15000 Da.

3. A method for preparing high-strength α-hemihydrate gypsum, characterized in that, Includes the following steps: S1: In the metathesis reaction system of calcium chloride and sodium sulfate, the composite crystal form regulator described in claim 1 or 2 is added. Through the multiple coordination chelation and selective crystal face adsorption of the regulator, the nucleation of dihydrate gypsum crystals is inhibited and crystal growth is promoted, thereby increasing the relative growth rate of [001] crystal faces and generating dihydrate gypsum crystals with an average particle size ≥100 μm and a specific surface area ≤0.8 m² / g, thus obtaining dihydrate gypsum crystal slurry; S2: After solid-liquid separation of the dihydrate gypsum crystal slurry obtained in step S1, a countercurrent hot water washing process with at least two stages is adopted to control the temperature at 60-80℃ and the pH at 6.5-7.5, to remove the composite crystal form regulator and residual chloride ions, and obtain dihydrate gypsum with a chloride content ≤150ppm. S3: Dihydrate gypsum is steam-pressed in pure water at 120-160℃ and 0.2-0.4 MPa for 3-5 hours to obtain α-hemihydrate gypsum with a chlorine content ≤100ppm and a 2h compressive strength ≥55 MPa.

4. The preparation method according to claim 3, characterized in that, The amount of the composite crystal form regulator added is 0.2-0.6 wt% of the theoretical yield of gypsum dihydrate.

5. The preparation method according to claim 3, characterized in that, In step S2, the liquid-to-solid ratio for each stage is 2.5-3.5:1, the washing time for each stage is 10-20 minutes, the removal rate of the control agent in the washing process is ≥99.5%, and there is no residual organic matter on the surface of the dihydrate gypsum after washing.

6. The preparation method according to claim 3, characterized in that, The α-hemihydrate high-strength gypsum obtained in step S3 has a short columnar crystal morphology with an aspect ratio of 1.0-1.5 and a specific surface area of ​​0.4-0.7 m² / g.

7. The preparation method according to claim 3, characterized in that, The CaCl2 in step S1 comes from the waste liquid of soda ash ammonia stripping, the waste liquid of hydrochloric acid wet phosphoric acid production, the waste liquid of salt bittering, and the produced water of oil and gas fields. Alternatively, the mother liquor after solid-liquid separation of the dihydrate gypsum crystal slurry in step S2 is recycled, wherein the CaCl2 content is 70-130 g / L and the NaCl content is 40-90 g / L.

8. A high-strength α-hemihydrate gypsum, characterized in that, The α-hemihydrate gypsum is prepared by the preparation method according to any one of claims 3-7, and the crystal morphology is short columnar with an aspect ratio of 1.0-1.5 and a specific surface area of ​​0.4-0.7 m² / g.

9. The high-strength α-hemihydrate gypsum according to claim 8, characterized in that, The product has a whiteness of ≥92% and a linear expansion rate of ≤0.12%.

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