A method for preparing hemihydrate calcium sulfate from calcium sulfate dihydrate

Abstract

The present invention relates to the technical field of the preparation of calcium sulfate hemihydrate, and provides a method for preparing calcium sulfate hemihydrate from calcium sulfate dihydrate. The specific method comprises the following steps: S1, impurity removal of calcium sulfate dihydrate; S2, uniformly mixing the calcium sulfate dihydrate after impurity removal with an additive, then placing the mixture in a high-pressure container, and reacting at 0.1-2 MPa and 120-160 °C for 20-30 min; the components of the additive include tungstate or molybdate, sulfosuccinate, nano-aluminum oxide and water; S3, after the reaction is completed, drying the reactant at 100 °C - 120 °C to obtain calcium sulfate hemihydrate. In the additive of the present invention, tungstate or molybdate and sulfosuccinate are combined with nano-aluminum oxide. By adjusting the crystal growth environment, more nucleation points are provided, the crystal growth path is directionally controlled, and the by-product crystal phase is inhibited, so as to achieve the purpose of improving the aspect ratio and mechanical strength of α-type gypsum.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of calcium sulfate hemihydrate, and particularly relates to a method for preparing calcium sulfate hemihydrate from calcium sulfate dihydrate. Background Art

[0002] In the production process of citric acid, the fermentation method is mainly used. By adding lime milk (calcium hydroxide), calcium citrate precipitate is formed, and then sulfuric acid acidification is carried out to generate citric acid, while by-product gypsum is produced. The main component of this by-product gypsum is calcium sulfate dihydrate (CaSO4·2H2O). If it is directly stacked, it will occupy land and cause environmental problems. Using the citric acid by-product gypsum as raw material and producing building materials products such as calcium sulfate hemihydrate (CaSO4·0.5H2O) through appropriate treatment can not only realize the comprehensive utilization of resources, but also reduce environmental pollution.

[0003] Calcium sulfate hemihydrate has two crystal forms, α-type and β-type. At present, there are mainly three methods for producing α-type calcium sulfate hemihydrate: autoclave method, hydrothermal method and atmospheric pressure salt solution method. The invention patent with the publication number of CN105948547A discloses a method for preparing α-type calcium sulfate hemihydrate from calcium sulfate dihydrate. The admixtures used in this method have complex components. Among them, organic salts and inorganic salts will undergo ionization reactions, generating unnecessary precipitates or intermediate products, interfering with the normal formation of crystals, and the α-type calcium sulfate hemihydrate finally produced has low strength and a small aspect ratio. Therefore, a method for preparing calcium sulfate hemihydrate with higher strength is needed. Summary of the Invention

[0004] In view of this, the present invention provides a method for preparing high-strength and large-aspect-ratio calcium sulfate hemihydrate from calcium sulfate dihydrate.

[0005] The technical solution of the present invention is realized as follows: On the one hand, the present invention provides a method for preparing calcium sulfate hemihydrate from calcium sulfate dihydrate, including the following steps:

[0006] S1, impurity removal of calcium sulfate dihydrate: Since the raw material calcium sulfate dihydrate is derived from the by-product gypsum in the production process of citric acid, impurities need to be removed in order to obtain calcium sulfate hemihydrate with higher purity;

[0007] S2, the calcium sulfate dihydrate after impurity removal is mixed evenly with an additive, and then placed in a high-pressure container and reacted at 0.1-2 MPa and 120-160 °C for 20-30 min; the components of the additive include tungstate or molybdate, sulfosuccinate, nano-aluminum oxide and water;

[0008] S3, after the reaction is completed, the reactant is dried at 100 °C - 120 °C to obtain calcium sulfate hemihydrate.

[0009] Tungstates and molybdates regulate the crystallization kinetics, promote the directional growth of specific crystal planes, and increase the aspect ratio of gypsum whiskers. The principle is as follows: The ions of tungstates and molybdates are adsorbed on the crystal interface or growth front, which can affect the energy distribution of the crystal, induce one-dimensional growth, and simultaneously reduce the crystal extension behavior in other directions, thus realizing the formation of slender whiskers.

[0010] Sulfosuccinate, as a surfactant, can improve the uniformity of crystal growth and inhibit particle aggregation. Principle: The sulfosuccinate molecule has both hydrophilic and hydrophobic properties. Its sulfonic acid group is easily adsorbed on the growth surface of gypsum crystals, such as on certain side crystal planes, and the whiskers extend from the uniaxial direction by means of a stable growth plane.

[0011] Nano-aluminum oxide, as a nucleating agent, increases the consistency of crystal morphology and improves the mechanical strength of the final product. Principle: Nano-aluminum oxide particles are dispersed in the liquid phase to provide more nucleation points, which can evenly distribute the geometric centers of crystal formation, so that the generated crystals have a better aspect ratio and regular morphological characteristics.

[0012] The combination of tungstate or molybdate, sulfosuccinate and nano-aluminum oxide provides more nucleation points, can directionally control the crystal growth path, inhibit the by-product crystal phase, and increase the aspect ratio and mechanical strength of α-type gypsum.

[0013] Based on the above technical solutions, preferably, the dosage of the additive is 5%-10% of the weight of calcium sulfate dihydrate after impurity removal. Calculated according to 100% by weight percentage, the components of the additive include 0.05% - 0.2% of tungstate or molybdate, 0.2% - 0.3% of sulfosuccinate, 0.05% - 0.3% of nano-aluminum oxide, and the balance is water.

[0014] Based on the above technical solutions, preferably, the method for removing impurities from calcium sulfate dihydrate is: mixing calcium sulfate dihydrate with dilute acid, grinding it finely and then filtering, washing the precipitate 2-3 times with water and then filtering and drying to obtain the intermediate product of calcium sulfate dihydrate after impurity removal.

[0015] Based on the above technical solutions, preferably, the dilute acid is dilute hydrochloric acid, and the concentration of the dilute hydrochloric acid is 7v% - 10v%.

[0016] Based on the above technical solutions, preferably, the additive further includes an amino acid, and the dosage of the amino acid is 0.3% - 0.4% of the total weight of the additive.

[0017] Based on the above technical solutions, preferably, the amino acid is one or a combination of glycine, glutamic acid, lysine and aspartic acid.

[0018] Amino acids further adjust the catalyst system, strengthen the directional guidance during crystal growth, and inhibit the generation of impurities. Principle: The amine group (-NH2) and carboxyl group (-COOH) structures on amino acid molecules can chemically complex with the crystal surface, preferentially adsorb specific crystal planes, and induce a more uniform whisker morphology. Different types of amino acids (such as glycine, glutamic acid, etc.) will play an inhibitory or promoting role in the growth of different directions of the crystal, thereby enhancing the controllability of the aspect ratio and size distribution of the whiskers.

[0019] Interaction between nano-aluminum oxide and amino acids: On the one hand, it increases the nucleation points and improves the uniformity of crystal distribution; on the other hand, the selective adsorption of amino acids on the crystal growth surface ensures the consistency of the macroscopic morphology.

[0020] Glycine can provide an excellent crystal adsorption basis, regulate the anisotropic growth of crystals, delay lateral growth to a certain extent, and thus promote the formation of whiskers with a high aspect ratio; due to its simple molecule, it can also play a basic role in assisting in stabilizing the shape of the whiskers.

[0021] Glutamic acid has an additional carboxyl group in its side chain and is a typical acidic amino acid. Its multi-carboxyl structure can preferentially bind to certain crystal surfaces through stronger adsorption force, selectively inhibit the crystal from growing in a specific direction, and enhance the regulation ability of the aspect ratio. The carboxyl group in its side chain also has a buffering effect on the pH of the reaction solution and the crystal growth rate, improving the morphological uniformity of the whiskers.

[0022] Aspartic acid is similar to glutamic acid, with a carboxyl group in its side chain and is an acidic amino acid. Aspartic acid rich in carboxyl groups enhances its coordination adsorption ability and has a high selective adsorption effect on the crystal surface; it can more effectively inhibit the radial growth of whiskers, enhance its aspect ratio, and improve the distribution consistency of whiskers.

[0023] Lysine has an amine group in its side chain and is a basic amino acid. The amine group can undergo electrostatic adsorption with sulfate radicals on the crystal surface, thereby stabilizing the surface structure of the whiskers and preventing lateral growth. Due to the basicity of lysine, it can neutralize acidity in the local reaction environment, avoid out-of-control crystal morphology, and enhance mechanical strength at the same time.

[0024] The four amino acids have the following synergistic effects: Glycine, as a simple molecule, provides a general crystal adsorption and morphology optimization effect. Glutamic acid and aspartic acid, as acidic amino acids, play a directional induction role by preferentially adsorbing specific crystal surfaces through the strong coordination of carboxyl groups. Lysine improves the local crystal growth environment through the action of the amine group, inhibits disordered growth, increases the aspect ratio of the whiskers, and enhances the strength and morphological stability of the whiskers.

[0025] On the basis of the above technical solutions, preferably, the additive further includes microcrystalline cellulose, and the dosage of the microcrystalline cellulose is 0.2% to 0.4% of the total weight of the additive.

[0026] Microcrystalline cellulose provides a skeletal support for the crystals, further improving the product stability and regularity. Principle: Microcrystalline cellulose forms a support network in the reaction medium, reducing the collapse or adhesion of whisker crystals during growth, and ultimately making the product more regular. At the same time, cellulose assists the whiskers to form aggregates, enhancing the strength of the product.

[0027] On the basis of the above technical solutions, preferably, the tungstate is sodium tungstate or ammonium metatungstate, and the molybdate is sodium molybdate or ammonium paramolybdate.

[0028] On the other hand, the present invention also provides a hemihydrate calcium sulfate, which is prepared by the above method.

[0029] The method for preparing hemihydrate calcium sulfate from dihydrate calcium sulfate according to the present invention has the following beneficial effects compared with the prior art:

[0030] The tungstate or molybdate, sulfosuccinate ester in the additive of the present invention is combined with nano-aluminum oxide. By adjusting the crystal growth environment, more nucleation points are provided, the crystal growth path is directionally controlled, and the by-product crystal phase is inhibited, so as to achieve the purpose of increasing the aspect ratio and mechanical strength of α-type gypsum.

[0031] Amino acids further adjust the catalyst system through the amine groups and carboxyl groups on their molecules, strengthen the directional guidance during crystal growth, enhance the controllability of the aspect ratio and size distribution of the whiskers, and inhibit the generation of impurities.

[0032] Microcrystalline cellulose provides a skeletal support for the crystals, reducing the collapse or adhesion of whisker crystals during growth, and further improving the product stability, regularity and strength. Detailed implementation manners

[0033] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0034] Example 1

[0035] The method for preparing hemihydrate calcium sulfate from dihydrate calcium sulfate in this example includes the following steps:

[0036] S1, Impurity removal of calcium sulfate dihydrate: 3000 g of calcium sulfate dihydrate is mixed with 3500 mL of dilute hydrochloric acid with a concentration of 8 v%, ground in a ball mill (less than 100 mesh), filtered, the precipitate is washed 3 times with water and then filtered to retain the filter residue, and the calcium sulfate dihydrate intermediate product after impurity removal is obtained after drying.

[0037] S2, Take 2000 g of the calcium sulfate dihydrate after impurity removal and add it to a high-pressure reactor, add 160 g of the additive and mix evenly, then place it in a high-pressure vessel and react at 0.8 MPa and 170 °C for 25 min; the additive is calculated according to 100% by weight, including 0.15% of sodium tungstate, 0.26% of sulfosuccinate, 0.25% of nano-aluminum oxide, and the balance is water.

[0038] S3, After the reaction is completed, place the reactant in a blast dryer and treat it at 110 °C for 25 min, and hemihydrate calcium sulfate is obtained after drying.

[0039] Example 2

[0040] The additive in this example adds amino acids on the basis of Example 1, specifically as follows:

[0041] The method for preparing hemihydrate calcium sulfate from calcium sulfate dihydrate in this example includes the following steps:

[0042] S1, Impurity removal of calcium sulfate dihydrate: 3000 g of calcium sulfate dihydrate is mixed with 3500 mL of dilute hydrochloric acid with a concentration of 8 v%, ground in a ball mill (less than 100 mesh), filtered, the precipitate is washed 3 times with water and then filtered to retain the filter residue, and the calcium sulfate dihydrate intermediate product after impurity removal is obtained after drying.

[0043] S2, Take 2000 g of the calcium sulfate dihydrate after impurity removal and add it to a high-pressure reactor, add 160 g of the additive and mix evenly, then place it in a high-pressure vessel and react at 0.8 MPa and 170 °C for 25 min; the additive is calculated according to 100% by weight, including 0.15% of sodium tungstate, 0.26% of sulfosuccinate, 0.25% of nano-aluminum oxide, 0.15% of glutamic acid + 0.15% of aspartic acid + 0.1% of lysine, and the balance is water.

[0044] S3, After the reaction is completed, place the reactant in a blast dryer and treat it at 110 °C for 25 min, and hemihydrate calcium sulfate is obtained after drying.

[0045] Example 3

[0046] The additive in this example adds microcrystalline cellulose on the basis of Example 2, specifically as follows:

[0047] The method for preparing hemihydrate calcium sulfate from calcium sulfate dihydrate in this example includes the following steps:

[0048] S1, Impurity removal of calcium sulfate dihydrate: 3000 g of calcium sulfate dihydrate is mixed with 3500 mL of dilute hydrochloric acid with a concentration of 8 v%, ground fine by a ball mill (less than 100 mesh), filtered, the precipitate is washed 3 times with water and then filtered to retain the filter residue, and after drying, the intermediate product of calcium sulfate dihydrate after impurity removal is obtained.

[0049] S2, Take 2000 g of the calcium sulfate dihydrate after impurity removal and add it to a high-pressure reactor, add 160 g of additives and mix evenly, then place it in a high-pressure container and react at 0.8 MPa and 170 °C for 25 min; the additives are calculated according to 100% by weight, including 0.15% of sodium tungstate, 0.26% of sulfosuccinate, 0.25% of nano-aluminum oxide, 0.15% of glutamic acid + 0.15% of aspartic acid + 0.1% of lysine, 0.28% of microcrystalline cellulose, and the balance is water.

[0050] S3, After the reaction is completed, the reaction product is placed in a blast dryer and treated at 110 °C for 25 min, and after drying, calcium sulfate hemihydrate is obtained.

[0051] Example 4

[0052] The method for preparing calcium sulfate hemihydrate from calcium sulfate dihydrate in this example includes the following steps:

[0053] S1, Impurity removal of calcium sulfate dihydrate: 3000 g of calcium sulfate dihydrate is mixed with 3500 mL of dilute hydrochloric acid with a concentration of 7 v%, ground fine by a ball mill (less than 100 mesh), filtered, the precipitate is washed 2 times with water and then filtered to retain the filter residue, and after drying, the intermediate product of calcium sulfate dihydrate after impurity removal is obtained.

[0054] S2, Take 2000 g of the calcium sulfate dihydrate after impurity removal and add it to a high-pressure reactor, add 120 g of additives and mix evenly, then place it in a high-pressure container and react at 0.13 MPa and 130 °C for 30 min; the additives are calculated according to 100% by weight, including 0.05% of sodium tungstate, 0.23% of sulfosuccinate, 0.25% of nano-aluminum oxide, 0.25% of glutamic acid, 0.1% of aspartic acid, 0.35% of microcrystalline cellulose, and the balance is water.

[0055] S3, After the reaction is completed, the reaction product is placed in a blast dryer and treated at 100 °C for 30 min, and after drying, calcium sulfate hemihydrate is obtained.

[0056] Example 5

[0057] The method for preparing calcium sulfate hemihydrate from calcium sulfate dihydrate in this example includes the following steps:

[0058] S1, Impurity removal of calcium sulfate dihydrate: Mix 3000 g of calcium sulfate dihydrate with 3500 mL of dilute hydrochloric acid with a concentration of 9v%, grind it in a ball mill (less than 100 mesh), filter, wash the precipitate 3 times with water, filter to retain the filter residue, and dry to obtain the intermediate product of calcium sulfate dihydrate after impurity removal.

[0059] S2, Take 2000 g of the calcium sulfate dihydrate after impurity removal and add it to a high-pressure reactor. Add 100 g of additives and mix evenly. Then place it in a high-pressure container and react at 0.1 MPa and 120 °C for 22 min; the additives are calculated according to 100% by weight, including 0.15% of ammonium metatungstate, 0.28% of sulfosuccinate, 0.15% of nano-aluminum oxide, 0.1% of glutamic acid + 0.1% of aspartic acid + 0.05% of lysine + 0.05% of glycine, 0.2% of microcrystalline cellulose, and the balance is water.

[0060] S3, After the reaction is completed, place the reactant in a blast dryer and treat it at 120 °C for 10 min, and dry to obtain calcium sulfate hemihydrate.

[0061] Example 6

[0062] The method for preparing calcium sulfate hemihydrate using calcium sulfate dihydrate in this example includes the following steps:

[0063] S1, Impurity removal of calcium sulfate dihydrate: Mix 3000 g of calcium sulfate dihydrate with 3500 mL of dilute hydrochloric acid with a concentration of 10v%, grind it in a ball mill (less than 100 mesh), filter, wash the precipitate 2 times with water, filter to retain the filter residue, and dry to obtain the intermediate product of calcium sulfate dihydrate after impurity removal.

[0064] S2, Take 2000 g of the calcium sulfate dihydrate after impurity removal and add it to a high-pressure reactor. Add 200 g of additives and mix evenly. Then place it in a high-pressure container and react at 0.18 MPa and 140 °C for 24 min; the additives are calculated according to 100% by weight, including 0.2% of ammonium metatungstate, 0.2% of sulfosuccinate, 0.3% of nano-aluminum oxide, 0.35% of lysine, 0.28% of microcrystalline cellulose, and the balance is water.

[0065] S3, After the reaction is completed, place the reactant in a blast dryer and treat it at 115 °C for 14 min, and dry to obtain calcium sulfate hemihydrate.

[0066] Example 7

[0067] The method for preparing calcium sulfate hemihydrate using calcium sulfate dihydrate in this example includes the following steps:

[0068] S1, Impurity removal of calcium sulfate dihydrate: 3000 g of calcium sulfate dihydrate is mixed with 3500 mL of dilute hydrochloric acid with a concentration of 8 v%, ground fine by a ball mill (less than 100 mesh), filtered, the precipitate is washed 3 times and then filtered to retain the filter residue, and the impurity-removed calcium sulfate dihydrate intermediate product is obtained after drying.

[0069] S2, Take 2000 g of the impurity-removed calcium sulfate dihydrate and add it to a high-pressure reactor, add 180 g of additives and mix evenly, then place it in a high-pressure container and react at 2 MPa and 160 °C for 20 min; the additives are calculated according to 100% by weight, including 0.15% of sodium tungstate, 0.3% of sulfosuccinate, 0.05% of nano-aluminum oxide, 0.18% of glutamic acid + 0.17% of lysine, 0.4% of microcrystalline cellulose, and the balance is water.

[0070] S3, After the reaction is completed, the reactants are placed in a blast dryer and treated at 118 °C for 11 min, and calcium sulfate hemihydrate is obtained after drying.

[0071] Comparative Example 1

[0072] Compared with Example 1, Comparative Example 1 lacks sodium tungstate in the additives, and the rest is the same as Example 1.

[0073] Comparative Example 2

[0074] Compared with Example 1, Comparative Example 2 lacks nano-aluminum oxide in the additives, and the rest is the same as Example 1.

[0075] Comparative Example 3

[0076] Compared with Example 1, Comparative Example 3 lacks sulfosuccinate in the additives, and the rest is the same as Example 1.

[0077] Comparative Example 4

[0078] Compared with Example 1, the dosage of the additives in Comparative Example 4 exceeds the limited range (5% - 10%), and the specific dosage is 15%, and the rest is the same as Example 1.

[0079] Comparative Example 5

[0080] Compared with Example 2, the dosage of amino acids in Comparative Example 5 exceeds the limited range (0.3% - 0.4%), and the specific dosage is 0.5%, 0.35% of glutamic acid, 0.15% of aspartic acid, and the rest is the same as Example 2.

[0081] Comparative Example 6

[0082] Compared with Example 3, the dosage of microcrystalline cellulose in Comparative Example 6 exceeds the limited range (0.2% - 0.4%), and the specific dosage is 0.5%, and the rest is the same as Example 3.

[0083] The calcium sulfate hemihydrate prepared in the embodiment and the comparative example is α calcium sulfate hemihydrate, and its strength is tested with reference to the industry standard JC / T 2038-2010, and the results are as follows:

[0084]

[0085] As shown in Table 1, it can be seen from Examples 1-3 that the aspect ratio and strength of calcium sulfate hemihydrate are significantly improved after adding amino acids (see Example 2). This is because glutamic acid and aspartic acid are adsorbed on the crystal surface through the strong coordination of the carboxyl group, which plays a directional induction role, thereby improving the aspect ratio of the whiskers. Lysine improves the local crystal growth environment through the action of the amine group, inhibits disordered growth, improves the aspect ratio of the whiskers, and enhances the strength and morphological stability of the whiskers. The strength is greatly improved after adding microcrystalline cellulose (see Example 3). This is because microcrystalline cellulose provides skeleton support for the crystals, which reduces the collapse or adhesion of the whisker crystals during the growth process, thereby improving the strength of the product.

[0086] It can be seen from Comparative Examples 1-3 that the lack of any one of sodium tungstate, nano-alumina and sulfosuccinate will affect the aspect ratio and strength of calcium sulfate hemihydrate. This is because tungstate or molybdate, sulfosuccinate and nano-alumina have a synergistic effect. Nano-alumina provides more nucleation points, allowing the crystals to be evenly distributed; sulfosuccinate inhibits particle aggregation, and the ions of tungstate and molybdate are adsorbed on the crystal interface or growth front, inducing its one-dimensional growth, while reducing the crystal extension behavior in other directions, thereby achieving the formation of slender whiskers.

[0087] Comparative Examples 4-6 show that the amount of additives, amino acids, and microcrystalline cellulose is not the more the better. Excessive use of tungstate / molybdate or nano-alumina will lead to too many nucleation points in the system, and the crystals cannot fully carry out directional growth, and the whiskers cannot form a mesh skeleton that is interwoven with each other, and the aspect ratio is reduced, and the system density and mechanical properties are reduced. When the amount of amino acids exceeds the standard, an overly thick adsorption layer will be formed, which hinders the extension of the crystal in a specific direction. In addition, excessive amino acids will cause the whiskers to be unable to be interwoven into a network skeleton due to poor directionality, and the dry compressive strength will not increase but decrease. When the amount of microcrystalline cellulose is too much, the viscosity of the system will be significantly increased, the diffusion performance in the solution will deteriorate, the nucleation rate of calcium sulfate in the system will be reduced, the full extension of the crystal will be inhibited, and the aspect ratio will be reduced; when the amount of microcrystalline cellulose is too large, its strong water absorption will expand to form a large amount of gel, and more cellulose pores will remain in the product, resulting in a decrease in the overall density of the system and a significant reduction in strength. Therefore, it is the key to achieve the best aspect ratio and compressive strength performance to reasonably control the amount within the design range.

[0088] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing hemihydrate calcium sulfate from calcium sulfate dihydrate, characterized in that: It includes the following steps: S1. Impurity removal of calcium sulfate dihydrate; S2. The calcium sulfate dihydrate after impurity removal is mixed evenly with an additive, and then placed in a high-pressure container, and reacted for 20 - 30 min under the conditions of 0.1 - 2 MPa and 120 - 170 °C; the components of the additive include tungstate or molybdate, sulfosuccinate, nano-aluminum oxide and water; S3. After the reaction is completed, the reactant is dried at 100 °C - 120 °C to obtain calcium sulfate hemihydrate.

2. The method for preparing hemihydrate calcium sulfate from calcium sulfate dihydrate according to claim 1, wherein: The dosage of the additive is 5% - 10% of the weight of the calcium sulfate dihydrate after impurity removal.

3. A method for preparing hemihydrate calcium sulfate using calcium sulfate dihydrate as claimed in claim 1, characterized in that: Calculated by 100% of weight percentage, the components of the additive include 0.05% - 0.2% of tungstate or molybdate, 0.2% - 0.3% of sulfosuccinate, 0.05% - 0.3% of nano-aluminum oxide, and the balance is water.

4. A method for preparing hemihydrate calcium sulfate using calcium sulfate dihydrate as claimed in claim 1, characterized in that: The method for impurity removal of calcium sulfate dihydrate is: the calcium sulfate dihydrate is mixed with dilute acid and then ground finely, filtered, the precipitate is washed with water 2 - 3 times and then filtered, and dried to obtain the intermediate product of calcium sulfate dihydrate after impurity removal.

5. A method for preparing hemihydrate calcium sulfate using calcium sulfate dihydrate as claimed in claim 1, characterized in that: The additive further includes amino acid, and the dosage of the amino acid is 0.3% - 0.4% of the total weight of the additive.

6. The method for preparing hemihydrate calcium sulfate from calcium sulfate dihydrate according to claim 5, wherein: The amino acid is one or a combination of glycine, glutamic acid, lysine and aspartic acid.

7. A method for preparing hemihydrate calcium sulfate using calcium sulfate dihydrate according to claim 6, characterized in that: The additive further includes microcrystalline cellulose, and the dosage of the microcrystalline cellulose is 0.2% - 0.4% of the total weight of the additive.

8. A method for preparing hemihydrate calcium sulfate using calcium sulfate dihydrate as described in claim 1, characterized in that: The tungstate is sodium tungstate or ammonium metatungstate, and the molybdate is sodium molybdate or ammonium paramolybdate.

9. A calcium sulfate hemihydrate, characterized in that, Prepared by using the method according to any one of claims 1 - 8.

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