A method for preparing alpha-hemihydrate gypsum stable at atmospheric pressure

Abstract

The application relates to a method for preparing alpha-hemihydrate gypsum stable at normal pressure, which comprises the following steps: S1, preparing a crystal transformation liquid containing a crystal transformation agent, a morphology adjusting agent and a diffusion control agent, pH=2-12; S2, adding dihydrate gypsum raw materials into the crystal transformation liquid, performing a crystal transformation reaction under the action of the crystal transformation agent at 50-150 DEG C and under a pressure of less than or equal to 0.5 MPa, keeping the dihydrate gypsum raw materials relatively static in the crystal transformation liquid during the reaction, controlling the crystal growth speed of alpha-hemihydrate gypsum by utilizing the slow diffusion speed of solutes in the relatively static crystal transformation liquid, and obtaining large-particle alpha-hemihydrate gypsum crystals in the crystal transformation liquid, the reaction time being 0.1-200 h; S3, rapidly separating solid from liquid, washing by using a washing liquid containing a hydration inhibitor, and drying at normal pressure. The application controls the crystallization conditions of alpha-hemihydrate gypsum, obtains large-particle alpha-hemihydrate gypsum, reduces the hydration speed of the gypsum at normal pressure, and reduces the process cost.

Description

Technical Field

[0001] This invention relates to the field of preparation technology of specific crystalline gypsum, and specifically to a method for preparing α-hemihydrate gypsum stable under normal pressure. Background Technology

[0002] Industrial by-product gypsum, such as titanium gypsum and desulfurization gypsum, is produced in large quantities. In order to realize its resource utilization, liquid phase method, dry method or semi-dry method is often used to convert industrial by-product gypsum into high-strength, high-performance and high-value-added hemihydrate gypsum. It can then be further applied to abrasive gypsum, high-grade medical gypsum and other fields, so as to realize the high-value-added transformation of gypsum resources.

[0003] Hemihydrate gypsum, prepared from dihydrate gypsum, is divided into α-hemihydrate gypsum and β-hemihydrate gypsum. Dihydrate gypsum is dehydrated in a saturated steam medium or certain acidic and saline media to produce α-hemihydrate gypsum; while dehydration in an unsaturated steam environment produces β-hemihydrate gypsum. Generally, α-hemihydrate gypsum is considered to have higher strength and is classified as high-strength gypsum, while β-hemihydrate gypsum has lower strength and is used as building gypsum. Crystal form modifiers include organic and inorganic substances. The principle of crystal form modification, for the growth of α-hemihydrate gypsum crystals, involves introducing an impurity ion that adsorbs onto a specific crystal face of the gypsum crystal, affecting the normal growth along that facet direction, thereby altering the growth habit of the original hemihydrate gypsum crystal. Various foreign ions in natural gypsum and various chemical gypsum raw materials can have different effects on the growth of α-hemihydrate gypsum crystals. Therefore, modifiers for different gypsums require extensive testing to be selected.

[0004] The formation mechanism of α-hemihydrate gypsum is currently inconclusive. It is generally believed that α-hemihydrate gypsum is formed through the dissolution and recrystallization of dihydrate gypsum. Specifically, during heat treatment, dihydrate gypsum first undergoes dehydration. Under suitable conditions, one and a half molecules of water of crystallization are removed from the dihydrate gypsum crystal lattice, forming hemihydrate gypsum precursor crystals. Surrounded by liquid water, these precursor crystals quickly dissolve in the liquid phase, increasing the concentration of hemihydrate gypsum in the liquid phase. When the concentration of hemihydrate gypsum reaches saturation, it rapidly crystallizes, forming coarse-to-dense α-hemihydrate gypsum crystals.

[0005] Currently, there are two main methods for preparing α-hemihydrate gypsum: the autoclaving method and the hydrothermal method. The hydrothermal method is further divided into the pressurized aqueous solution method and the atmospheric pressure salt solution method. The autoclaving method (pressurized steam method) is a process that dehydrates dihydrate gypsum in saturated steam to prepare α-hemihydrate gypsum, and it was the earliest method to achieve industrialized production of α-hemihydrate gypsum. The pressurized aqueous solution method involves adding finely ground dihydrate gypsum to an aqueous solution containing a crystal-transforming agent to prepare a suspension. The suspension is then pressurized and heated while continuously stirred, followed by filtration, drying, and grinding to obtain α-hemihydrate gypsum. The specific process of the atmospheric pressure salt solution method involves adding finely ground dihydrate gypsum to a solution of inorganic salt or acid of a certain concentration, adding a small amount of crystal-transforming agent, stirring evenly, heating to above 95℃, and maintaining the temperature for 4-6 hours to convert it into α-hemihydrate gypsum.

[0006] The atmospheric pressure salt solution method has advantages such as mild preparation conditions and easy control of product quality. This method can stably produce α-hemihydrate gypsum, but the produced α-hemihydrate gypsum particles are small, only about 60-80 μm. α-hemihydrate gypsum of this size can only be filtered by sedimentation centrifugation or filter press, and cannot be used with filtration centrifuges that have specific requirements for particle size, especially horizontal filtration centrifuges. Furthermore, because the α-hemihydrate gypsum particles are small and have a high water content (around 20% after separation), the subsequent drying process consumes a lot of heat energy.

[0007] Furthermore, α-hemihydrate gypsum is thermodynamically unstable under normal pressure and can only exist for a long time at temperatures above 107°C. At normal pressure, the boiling point of water is 100°C, and α-hemihydrate gypsum produced by existing processes will also reversely transform into dihydrate gypsum at 100°C, thus becoming deactivated. Since α-hemihydrate gypsum easily hydrates into dihydrate gypsum under normal pressure, the hydration rate generally shows a negative correlation with particle size (smaller particle size means faster hydration, and larger particle size means slower hydration). α-hemihydrate gypsum with a particle size of around 60-80 μm only stabilizes in boiling water for 2-3 minutes under normal pressure. This cannot meet the time requirements of conventional washing-filtration-drying processes in industry, necessitating the use of rapid drying and rapid filtration methods to treat α-hemihydrate gypsum. However, this method has significant drawbacks, including high equipment costs and high energy consumption. The core reason why α-hemihydrate gypsum products are prone to hydration under normal pressure is that the existing processes produce α-hemihydrate gypsum with excessively small grain size and excessively rapid hydration rate. In order to solve the problem of easy hydration of α-hemihydrate gypsum, it is necessary to propose an improved scheme for the preparation method of α-hemihydrate gypsum to obtain α-hemihydrate gypsum with larger particle size. Summary of the Invention

[0008] (a) Technical problems to be solved

[0009] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method for preparing α-hemihydrate gypsum stable under normal pressure. By controlling the crystallization conditions of α-hemihydrate gypsum, large-particle-size α-hemihydrate gypsum crystals are obtained, thereby slowing down the hydration rate of α-hemihydrate gypsum under normal pressure, and obtaining α-gypsum products that can be washed and dried under normal pressure. This method solves a series of technical problems caused by the excessively small crystal size and excessively fast hydration rate of α-hemihydrate gypsum produced by the existing process.

[0010] (II) Technical Solution

[0011] This invention provides a method for preparing α-hemihydrate gypsum stable under normal pressure, comprising:

[0012] S1. Prepare a crystallization solution containing a crystallization agent, a morphology modifier, and a diffusion control agent, and with a pH of 2-12;

[0013] S2. Add the dihydrate gypsum raw material to the crystallization solution. Under the action of the crystallization agent, carry out the crystallization reaction at 50-150℃ and pressure ≤0.5MPa. During the reaction, keep the dihydrate gypsum raw material relatively static in the crystallization solution. Take advantage of the slow diffusion rate of solute in the relatively static crystallization solution to control the crystal growth rate of α-hemihydrate gypsum. The reaction time is 0.1-200h. Large α-hemihydrate gypsum crystals are obtained in the crystallization solution.

[0014] S3. Rapid solid-liquid separation, followed by washing with a washing solution containing a hydration inhibitor, and then drying under normal pressure; rapid solid-liquid separation means that the solid-liquid separation time is controlled within 0.5 hours.

[0015] According to a preferred embodiment of the present invention, in S1, the crystallization agent is one or more of the following: potassium, sodium, ammonium, magnesium, calcium chloride, sulfate, nitrate, acetate, and perchlorate, with a concentration of 0.01-10.0 mol / L.

[0016] The rapid solid-liquid separation can be achieved using a sieve through one of the following methods: centrifugal filtration, pressure filtration, or atmospheric pressure filtration. The sieve material is either metal or non-metal, and the short side width of the sieve openings is between 20 and 500 micrometers. The sieves can be stacked in layers of 1 to 10. According to a preferred embodiment of the present invention, in S1, the diffusion control agent is one or more of alcohols, aldehydes, and ketones with a boiling point higher than 100°C at atmospheric pressure, and the concentration is 0.001-10.0 mol / L. Examples include butanol, cyclohexanol, furfural, ethylene glycol, cyclohexanone, glycerol, pentaerythritol, furfuryl alcohol, and propylene glycol. Preferably, the diffusion control agent is one or more of glycerol, pentaerythritol, furfural, furfuryl alcohol, glycerol, propylene glycol, and ethylene glycol, and the concentration is 0.01-8.0 mol / L.

[0017] According to a preferred embodiment of the present invention, in S1, the morphology modifier is one or more of the following: chlorides, sulfates, double sulfates, dicarboxylic acids, aminocarboxylic acid complexing agents, tartaric acid, citric acid, gluconic acid, tannic acid, stearic acid, and lactic acid, with a concentration of 0.01-10.0 mol / L.

[0018] The aminocarboxylic acid complexing agent is one or more of ethylenediaminetetraacetic acid, ethylenediaminemalonic acid, diethylenetriaminepentaacetic acid, aminotriacetic acid, hydroxyethylethylenediaminetriacetic acid, ethylene glycol bis(α-aminoethyl) ether tetraacetic acid, and 1,2-cyclohexanediaminetetraacetic acid.

[0019] The dicarboxylic acid is one or more of oxalic acid, malonic acid, succinic acid, and phthalic acid.

[0020] According to a preferred embodiment of the present invention, in S2, the mass ratio of dihydrate gypsum raw material to crystallization solution is 10-0.001; the dihydrate gypsum raw material is a raw material with a dihydrate gypsum content of more than 90%, which can be natural gypsum, industrial by-product gypsum, or industrial by-product gypsum or natural gypsum that has undergone chemical or physical purification to obtain dihydrate gypsum. Using high-purity dihydrate gypsum raw material can avoid the influence of impurities, thereby ensuring product purity and higher added value.

[0021] According to a preferred embodiment of the present invention, in step S2, the dihydrate gypsum raw material is kept relatively stationary in the crystallization solution during the reaction. Specifically, the relative velocity between the dihydrate gypsum raw material and the crystallization solution is controlled to be below 0.5 m / s; the reaction temperature is 80-120℃, the pressure is ≤0.2 MPa, the reaction time is 1-20 h, and the pH of the reaction system is controlled at 4-10. The pH adjustment method is as follows: the pH value is adjusted using inorganic acid and inorganic base. The inorganic acid is one or more of nitric acid, hydrochloric acid, sulfuric acid, and perchloric acid, and the inorganic base is one or more of ammonia, sodium hydroxide, potassium hydroxide, calcium oxide, calcium hydroxide, and barium hydroxide. Preferably, the relative velocity between the dihydrate gypsum raw material and the crystallization solution is controlled to be below 0.05 m / s.

[0022] According to a preferred embodiment of the present invention, in S2, during the reaction, a circulating liquid guide pipe is inserted into the stack layer of dihydrate gypsum raw material. The circulating liquid guide pipe circulates the crystal transformation liquid from the outside into the stack layer of dihydrate gypsum raw material. The circulating liquid guide pipe is a porous pipe, and an isolation layer covers the outside of the circulating liquid guide pipe. This isolation layer does not allow the crystal transformation liquid in the circulating liquid guide pipe to directly contact the dihydrate gypsum raw material, but allows the crystal transformation liquid to exchange solutes with the dihydrate gypsum raw material through diffusion. In the circulating liquid guide pipe, the crystal transformation liquid flows through external circulation. Through diffusion and heat transfer, a more stable, uniform, and controllable crystal transformation reaction is achieved in the stack layer of dihydrate gypsum raw material, resulting in α-hemihydrate gypsum crystals with more uniform size, thereby obtaining a more uniform crystal transformation effect.

[0023] Preferably, the isolation layer is a fibrous mesh material made of organic and / or inorganic materials that has a certain obstructive effect on the crystallization liquid, preferably one or more of filter cloth, gauze, cotton cloth, and metal mesh, and the number of layers used is 1-100.

[0024] According to a preferred embodiment of the present invention, in S3, the mother liquor produced by rapid solid-liquid separation (sieve aperture of 20-500 micrometers, preferably 80-150 micrometers), after adjusting its concentration, is replenished with a crystallizing agent as needed and returned to S2 as a crystallizing solution for recycling. The concentration adjustment is achieved through one or more of evaporation, dilution, and membrane concentration. The rapid solid-liquid separation can be achieved using a filtration centrifuge, preferably a horizontal filtration centrifuge (a screen-type centrifuge) capable of rapid drying.

[0025] According to a preferred embodiment of the present invention, in step S3, the large-particle α-hemihydrate gypsum crystals generated by rapid solid-liquid separation are partially returned to step S2 and added to the crystallization solution together with the dihydrate gypsum raw material for reaction; during the reaction, these large-particle α-hemihydrate gypsum crystals serve as seed crystals for continued growth, thereby obtaining α-hemihydrate gypsum crystals with even larger particle sizes.

[0026] According to a preferred embodiment of the present invention, in S3, the hydration inhibitor in the washing liquid is one or more of organic monohydric alcohols and organic dihydric alcohols, with a concentration of 0.01-5.0 mol / L; preferably, the hydration inhibitor is one or more of ethanol, methanol, propanol, butanol, ethylene glycol, and propylene glycol.

[0027] According to a preferred embodiment of the present invention, in S3, the washing time with the washing liquid is 0.1-1.0 h, and the drying time is 0.1-1.0 h.

[0028] According to a preferred embodiment of the present invention, step S3 further includes: crushing the large α-hemihydrate gypsum crystals in the dried product once or multiple times to crush them into powdered α-hemihydrate gypsum with a predetermined particle size. Preferably, the particle size of the granular α-hemihydrate gypsum is first crushed to 50-125 micrometers, and then further pulverized by crushing, ball milling, grinding, etc., to meet the requirements of downstream products. The present invention first obtains large α-hemihydrate gypsum crystals, making them easier to dry and avoiding hydration. After drying, they can be crushed to the required particle size.

[0029] (III) Beneficial Effects

[0030] This invention enables the full utilization of dihydrate gypsum and also has the following technical effects:

[0031] (1) This invention is an improved scheme of low-cost atmospheric pressure salt solution method. By using the low-cost salt solution method to recycle and prepare α-hemihydrate gypsum, considerable economic benefits can be achieved.

[0032] (2) This invention can obtain α-hemihydrate gypsum with larger particle size. The larger the particle size, the more suitable it is for separation using conventional centrifuges or large-aperture screens, making separation and washing easier during the preparation process and helping to reduce production costs. The larger the particle size of α-hemihydrate gypsum, the lower the moisture content, which can reduce the energy consumption for subsequent drying. The larger the particle size of α-hemihydrate gypsum, the slower the hydration rate, which slows down the hydration rate of α-hemihydrate gypsum under normal pressure, resulting in α-hemihydrate gypsum that can be washed and dried under normal pressure. Therefore, the process of this invention is suitable for washing and drying using conventional process equipment. Even if the washing and drying time is long, it will not cause a large amount of hydration of α-hemihydrate gypsum.

[0033] (3) Preferably, the present invention uses raw materials with a gypsum dihydrate content of more than 90%, and the α-hemihydrate gypsum prepared has good morphology, high strength, high whiteness, and few impurities, and has broad market prospects. Attached Figure Description

[0034] Figure 1 This is a flowchart of the method for preparing α-hemihydrate gypsum stable under normal pressure according to the present invention.

[0035] Figure 2 A schematic diagram of the circulating liquid guide pipe structure inserted into the pile layer of dihydrate gypsum raw material.

[0036] Figure 3 This is a SEM image of the α-hemihydrate gypsum prepared in Example 1 of the present invention. Detailed Implementation

[0037] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] To address the issue of easy hydration of existing α-hemihydrate gypsum products under normal pressure, this invention proposes a technical solution that, by controlling the crystallization conditions of α-hemihydrate gypsum, can obtain large-sized α-hemihydrate gypsum crystals. Generally, larger crystal particles exhibit better stability and lower reactivity, effectively slowing down the hydration rate of α-hemihydrate gypsum under normal pressure. Simultaneously, the use of a hydration retarder (hydration inhibitor) during washing allows the hydration retarder (hydration inhibitor) to be adsorbed onto the surface of the α-hemihydrate gypsum, further reducing the hydration rate under normal pressure. This results in a stability time of over 1 hour for large-particle α-hemihydrate gypsum under normal pressure. This stable α-hemihydrate gypsum can be dried using conventional drying equipment, significantly reducing process difficulty and equipment requirements. It eliminates the need for expensive and energy-intensive dryers, thus lowering process costs.

[0039] The α-hemihydrate gypsum crystals produced according to the technical solution of the present invention have a size of more than 150 μm and can be processed by a filter centrifuge, especially a horizontal filter centrifuge that can quickly spin dry; the larger the particle size of the α-hemihydrate gypsum, the lower its moisture content after filtration, which can effectively reduce the moisture content of the α-hemihydrate gypsum product after filtration and further reduce the energy consumption of the α-hemihydrate gypsum drying process.

[0040] The basic scheme of this invention is as follows: Gypsum dihydrate raw material is added to a crystallization solution containing a crystallization agent, a morphology modifier, a diffusion control agent, and a pH of 2-12. Under the action of the crystallization agent, a crystallization reaction is carried out at 50-150°C and a pressure not exceeding 0.5 MPa. During the reaction, the solid is kept relatively stationary in the solution. Taking advantage of the slow diffusion rate of the solute in the relatively stationary crystallization solution, the crystal growth rate of α-hemihydrate gypsum is controlled. The reaction time is controlled to be 0.1-200 h, and large-particle α-hemihydrate gypsum crystals are obtained in the crystallization solution. After the reaction, large-particle α-hemihydrate gypsum crystals are obtained through rapid solid-liquid separation. The crystals are then washed with a washing solution containing a hydration inhibitor (hydration delay agent), and dried under normal pressure to obtain the product.

[0041] During the reaction, a circulating liquid guide pipe is inserted into the pile layer of dihydrate gypsum raw material. For example... Figure 2 As shown, the circulating liquid guide pipe 1 circulates the crystal transformation liquid from the outside into the stack layer 2 of the dihydrate gypsum raw material. The circulating liquid guide pipe 1 is a porous pipe, and an isolation layer 3 covers the outside of the circulating liquid guide pipe 1. The isolation layer 3 does not allow the crystal transformation liquid in the circulating liquid guide pipe to directly contact the dihydrate gypsum raw material, but allows the crystal transformation liquid to exchange solutes with the dihydrate gypsum raw material through diffusion. In the circulating liquid guide pipe 1, the crystal transformation liquid flows through external circulation. Through diffusion and heat transfer, a more stable, uniform, and controllable crystal transformation reaction of the dihydrate gypsum raw material is achieved, resulting in α-hemihydrate gypsum crystals with more uniform size, thereby obtaining a more uniform crystal transformation effect. Preferably, the isolation layer 3 is a fibrous mesh material made of organic and / or inorganic materials that has a certain obstructive effect on the crystal transformation liquid. It is preferably one or more of filter cloth, gauze, cotton cloth, and metal mesh, and the number of layers used is 1-100.

[0042] The circulating liquid guide tube 1 can be a vertical circulating liquid guide tube, a horizontal circulating liquid guide tube, or a circulating liquid guide tube in any other direction, or any combination of vertical, horizontal, and inclined circulating liquid guide tubes 1. The liquid flow within the circulating liquid guide tube 1 can be achieved by either end being open, with the flow of the solution through a pressure difference, or by one end being closed and connected to an inlet pipe, allowing the solution to flow through the inlet pipe, or any other method that enables liquid flow within the circulating liquid guide tube.

[0043] The following description, in conjunction with preferred embodiments of the present invention, provides further details.

[0044] Example 1

[0045] 4.68 kg of sodium chloride (crystallization agent) was added to a 20 L reactor, and the volume was adjusted to 20 L with water to prepare a 4 mol / L brine solution. 2 g of succinic acid (morphology modifier) ​​and 4 g of strontium chloride (morphology modifier) ​​were added, along with 0.1 mol / L glycerol (diffusion control agent). The pH was adjusted to 6-8 using ammonia. 8 kg of gypsum dihydrate (gypsum content greater than 92%) was added, and after thorough soaking, a slurry was obtained. The reaction temperature was controlled at 97℃, the pressure at 0.2 MPa, and the reaction was carried out at a constant temperature for 8 hours, during which the reaction was allowed to stand. After the reaction, the slurry was removed, filtered, and centrifuged. The solids were then washed with a 95℃ aqueous solution containing 0.01 mol / L ethylene glycol to obtain α-hemihydrate gypsum and a circulating solution. The α-hemihydrate gypsum was dried at 100℃ to obtain 6.74 kg of hemihydrate gypsum. Relevant tests were performed on the high-purity α-hemihydrate gypsum. Testing revealed that the product has a regular and complete crystal morphology, exhibiting a distinct hexagonal prism shape, with a whiteness of 97%, a purity greater than 98%, an average particle size of 150 μm, and a stable time of approximately 1.25 hours under normal pressure.

[0046] Example 2

[0047] 80 kg of water was added to a 100 L reactor, along with 3.0 mol / L magnesium chloride (a crystallization agent), 10 g of stearic acid (a morphology modifier), 1 g of tannic acid (a morphology modifier), and 0.5 mol / L ethylene glycol (a diffusion control agent). The pH was adjusted to 6-8 using ammonia. 40 kg of gypsum dihydrate (containing more than 93% gypsum) was added and thoroughly soaked to obtain a slurry. The reaction temperature was controlled at 97℃ and maintained for 8 hours, followed by static reaction. After the reaction, the slurry was removed, filtered, and centrifuged. The solids were then thoroughly washed with hot water containing 0.05 mol / L butanol at 95℃ to obtain α-hemihydrate gypsum and a circulating solution. The α-hemihydrate gypsum was dried at 100℃ to obtain 34 kg of α-hemihydrate gypsum. Relevant tests were performed on the high-purity α-hemihydrate gypsum. Testing revealed that the product has a regular and complete crystal morphology, exhibiting a distinct hexagonal prism shape, with a whiteness of 97.2%, a purity greater than 98%, an average particle size of 160 μm, and a stability time of approximately 0.9 hours under normal pressure.

[0048] Example 3

[0049] Prepare a 20L aqueous solution containing 3.5mol / L potassium chloride (a crystallization agent), add 2g of disodium ethylenediaminetetraacetate (an aminocarboxylic acid complexing agent, a morphology modifier), and 10g of pentaerythritol (a diffusion control agent). Adjust the pH to between 6 and 8 using ammonia water, then add 8kg of gypsum dihydrate (gypsum content greater than 93%). After thorough soaking, obtain a slurry. Control the reaction temperature at 98℃ and react at this constant temperature for 6 hours, then allow the reaction to stand. After the reaction is complete, remove the slurry, filter and centrifuge it, then thoroughly wash the solids with hot water containing 0.06mol / L ethylene glycol at 95℃ to obtain α-hemihydrate gypsum and a circulating solution. Dry the α-hemihydrate gypsum at 100℃ to obtain 6.68kg of hemihydrate gypsum. Perform relevant tests on the high-purity α-hemihydrate gypsum. Testing revealed that the product has a regular and complete crystal morphology, exhibiting a distinct hexagonal prism shape, with a whiteness of 97.5%, a purity greater than 98%, an average particle size of 200 μm, and a stability time of approximately 1.0 hour under normal pressure.

[0050] Example 4

[0051] A 20L aqueous solution containing 0.02mol / L potassium chloride (a crystallization agent) and 4.5mol / L ethylene glycol (a diffusion control agent) was prepared. 5g of nitric acid (an aminocarboxylic acid complexing agent, a morphology modifier) ​​was added, and the pH was adjusted to 6-8 using ammonia water. 8kg of gypsum dihydrate (with a content greater than 93%) was added and thoroughly soaked to obtain a slurry. The reaction temperature was controlled at 97.5℃, and the reaction was maintained at this temperature for 7 hours, with the relative flow rate between the solid and liquid controlled below 0.3m / s. After the reaction, the slurry was removed, filtered, and centrifuged. The solids were then thoroughly washed with hot water containing 0.1mol / L ethylene glycol at 95℃ to obtain α-hemihydrate gypsum and a circulating solution. The α-hemihydrate gypsum was dried at 100℃ to obtain 6.61kg of hemihydrate gypsum. Relevant tests were performed on the high-purity α-hemihydrate gypsum. Testing revealed that the product has a regular and complete crystal morphology, exhibiting a distinct hexagonal prism shape, with a whiteness of 97.7%, a purity greater than 98%, an average particle size of 200 μm, and a stability time of approximately 1.25 hours under normal pressure.

[0052] The α-hemihydrate gypsum produced in the existing technology mostly has a particle size of only 60-80μm, and its stability time in boiling water under normal pressure is only 2-3 minutes, which cannot meet the time requirements of conventional washing-filtration-drying processes in industry.

[0053] This invention controls the conversion rate of dihydrate gypsum and reduces the crystallization rate of α-hemihydrate gypsum by controlling reaction conditions using crystallization agents, diffusion control agents, and a relatively static state between the solid and liquid phases, thereby obtaining large-particle α-hemihydrate gypsum crystals. Utilizing the stability of these large-particle α-hemihydrate gypsum crystals, the invention achieves a stability time exceeding 1 hour under normal pressure, significantly reducing the processing difficulty of α-hemihydrate gypsum, lowering the investment costs and energy consumption of drying and filtration equipment, and drastically reducing the cost of α-hemihydrate gypsum preparation, resulting in products with unparalleled advantages. This invention also utilizes a technique for preparing and separating α-hemihydrate gypsum from dihydrate gypsum raw materials in a salt solution. When the dihydrate gypsum content in the raw material is above 90%, it can effectively and significantly reduce the current industry cost of α-hemihydrate gypsum preparation, and the product exhibits high purity, high whiteness, and low impurities, showing broad commercial application prospects.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing α-hemihydrate gypsum stable under normal pressure, characterized in that, It includes: S1. Prepare a crystallization solution containing a crystallization agent, a morphology modifier, and a diffusion control agent, and with a pH of 2-12; The crystallization agent is one or more of the following: chlorides, sulfates, nitrates, acetates, and perchlorates of potassium, sodium, ammonium, magnesium, and calcium, with a concentration of 0.01-10.0 mol / L; The diffusion control agent is one or more of the organic compounds, such as alcohols, aldehydes, and ketones, with a boiling point above 100℃ under normal pressure, and a concentration of 0.001-10.0 mol / L; The morphology modifier is a combination of succinic acid and strontium chloride, a combination of stearic acid and tannic acid, disodium ethylenediaminetetraacetate, or aminotriacetic acid, and the concentration of the morphology modifier is 0.01-10.0 mol / L; S2. Add the dihydrate gypsum raw material to the crystallization solution. Under the action of the crystallization agent, carry out the crystallization reaction at 80-98℃ and pressure ≤0.2MPa. During the reaction, keep the dihydrate gypsum raw material relatively stationary in the crystallization solution and control the pH of the reaction system at 4-10. Utilize the characteristic that the diffusion rate of solute in the relatively stationary crystallization solution is slower, thereby controlling the crystal growth rate of α-hemihydrate gypsum. The reaction time is 1-20h, and large-particle α-hemihydrate gypsum crystals are obtained in the crystallization solution. Specifically, keeping the dihydrate gypsum raw material relatively stationary in the crystallization solution means controlling the relative velocity between the dihydrate gypsum raw material and the crystallization solution to below 0.5m / s. During the reaction, a circulating liquid guide pipe is inserted into the stack layer of gypsum dihydrate raw material. The circulating liquid guide pipe circulates the crystal transformation liquid from the outside into the stack layer of gypsum dihydrate raw material. The circulating liquid guide pipe is a porous pipe, and an isolation layer covers the outside of the circulating liquid guide pipe. This isolation layer does not allow the crystal transformation liquid in the circulating liquid guide pipe to directly contact the gypsum dihydrate raw material, but allows the crystal transformation liquid to exchange solutes with the gypsum dihydrate raw material through diffusion. The isolation layer is a fibrous mesh material made of organic and / or inorganic materials that has a certain obstruction effect on the crystal transformation liquid. S3. Rapid solid-liquid separation: washing with a washing solution containing a hydration inhibitor, followed by drying under normal pressure to obtain α-hemihydrate gypsum crystals with a size of 150 μm or larger; the rapid solid-liquid separation is performed with a solid-liquid separation time controlled within 0.5 hours; the hydration inhibitor is ethylene glycol, butanol, or propylene glycol, and its concentration in the washing solution is 0.01-0.1 mol / L.

2. The method according to claim 1, characterized in that, In S2, the mass ratio of gypsum dihydrate raw material to crystallization solution is 10-0.001; the gypsum dihydrate raw material is a raw material with a gypsum dihydrate content of more than 90%.

3. The method according to claim 1, characterized in that, In step S3, the large α-hemihydrate gypsum crystals generated by rapid solid-liquid separation are partially returned to step S2 and added to the crystallization solution along with the dihydrate gypsum raw material for reaction. During the reaction, these large α-hemihydrate gypsum crystals continue to grow as seed crystals, thereby obtaining α-hemihydrate gypsum crystals with even larger particle sizes.

4. The method according to claim 1, characterized in that, S3 also includes crushing the large gypsum crystals in the dried product once or multiple times to break them into powdered α-hemihydrate gypsum with a predetermined particle size.

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