Method for preparing ultrahigh-strength dihydrate gypsum from industrial byproduct phosphogypsum and dihydrate gypsum

Abstract

The invention relates to the technical field of ardealite treatment, in particular to a method for preparing ultrahigh-strength dihydrate gypsum from industrial byproduct ardealite and dihydrate gypsum. The invention discloses a method for preparing dihydrate gypsum from industrial byproduct phosphogypsum, which comprises the following steps: mixing mother liquor to be treated and a decalcifying agent at 40-50 DEG C to obtain mixed slurry; and carrying out a decalcification reaction on the mixed slurry at 40-50 DEG C, and after the reaction is finished, filtering and drying to obtain the dihydrate gypsum. The method provided by the invention solves the problem that the desilicication clear mother liquor cannot be efficiently utilized in the traditional process, and overcomes the problems of insufficient product strength and poor performance caused by excessive calcium ions and other impurities.

Description

Technical Field

[0001] The invention relates to the technical field of phosphogypsum treatment, and in particular to a method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum and the dihydrate gypsum. Background Art

[0002] Dihydrate gypsum, also known as gypsum or raw gypsum, is a stable phase in nature. Most industrial by-product gypsum is also mainly composed of dihydrate gypsum, which belongs to this category. They are both the raw materials of dehydrated products and the final products of rehydration of dehydrated products. This final product can also be called recycled gypsum.

[0003] Desiliconization mother liquor is a phosphate calcium nitrophosphate solution after nitric acid decomposes phosphate ore, and is the product of frozen decalcification. Conventional technology can only be used to produce nitrophosphate fertilizers and cannot be used in high quality. In order to increase the value of nitric acid wet-process phosphoric acid and meet the needs of my country's high-quality development under the new situation, it is necessary to expand the high-quality utilization of desiliconization mother liquor.

[0004] To this end, the present application proposes a method for preparing ultra-high strength dihydrate gypsum from industrial by-product phosphogypsum and dihydrate gypsum. Summary of the invention

[0005] In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing ultra-high strength dihydrate gypsum from industrial by-product phosphogypsum, which comprises the following steps: at a specific temperature (40-50°C), a mixture containing a certain proportion of nitric acid (HNO3), phosphoric acid (P2O5) and calcium ions (Ca 2+ ) is mixed with high-purity sulfuric acid (98%) as a decalcifying agent, and the sulfate radical (SO4 2- ) mass content is 2-3%, decalcification reaction is carried out (5-6 hours), followed by filtration and reduced pressure drying (50-60°C), and finally ultra-high strength dihydrate gypsum is obtained, which solves the problem that the desiliconization mother liquor cannot be efficiently utilized in the traditional process, and overcomes the problems of insufficient product strength and poor performance caused by excessive calcium ions and other impurities.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] In a first aspect, the present application provides a method for preparing dihydrate gypsum from industrial by-product phosphogypsum, comprising the following steps:

[0008] The mother liquor to be treated and the decalcifying agent are mixed at 40-50° C. to obtain a mixed slurry; the mixed slurry is subjected to a decalcification reaction at 40-50° C. After the reaction is completed, the mixed slurry is filtered and dried to obtain dihydrate gypsum.

[0009] In some specific embodiments, in the mother liquor to be treated, the content of HNO3 is 10-15% and the content of P2O5 is 5-20% by mass.

[0010] In some embodiments, the decalcifying agent is sulfuric acid.

[0011] In some embodiments, the SO4 in the mixed slurry is detected. 2- When the mass content is 2-3%, stop adding the decalcifying agent.

[0012] In some specific embodiments, the decalcification reaction time is 5-6 hours.

[0013] In some specific embodiments, the mother liquor to be treated has an HNO3 content of 11-14% and a P2O5 content of 10-15% by mass.

[0014] In some specific embodiments, the drying temperature is 50-60° C. under reduced pressure.

[0015] In some specific embodiments, the C a2+ Content <5.5%.

[0016] In a second aspect, the present application provides dihydrate gypsum, including the dihydrate gypsum obtained by the method described in the first aspect.

[0017] In some specific embodiments, the compressive strength of the dihydrate gypsum is ≥70 MPa, and the flexural strength is ≥15 MPa.

[0018] The beneficial effects of the present invention are:

[0019] The method described in the present application converts the calcium ions in the mother liquor into insoluble calcium sulfate precipitates through specific chemical reaction conditions, thereby achieving deep decalcification, which not only effectively removes impurities, but also ensures that the generated dihydrate gypsum crystals have a good growth environment, maintains an appropriate crystallization water content, and avoids the influence of high temperature or excessive acid on the crystal structure.

[0020] The dihydrate gypsum prepared by the method described in this application has a compressive strength of ≥70MPa and a flexural strength of ≥15MPa, while maintaining a relatively low density (about 2.31 to 2.35 g / cm 3 ), dihydrate gypsum not only has excellent physical properties, but also has flaky and irregular microstructure, which is suitable for a variety of high-quality application needs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0022] Figure 1is a scanning electron microscope image of the dihydrate gypsum prepared in Example 1 of the present application;

[0023] Figure 2 This is a scanning electron microscope image of the dihydrate gypsum prepared in Comparative Example 3 of the present application; DETAILED DESCRIPTION

[0024] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments, and the contents mentioned in the implementation modes are not intended to limit the present invention.

[0025] As used herein, "and / or" includes the term of any and all combinations of one or more associated listed items. The terms used herein are only used to describe specific embodiments and are not intended to limit the present invention. As used herein, the singular forms "a", "an", "an" and "the" are also intended to include the plural forms, unless the context clearly indicates otherwise. It is further understood that "including", when used in this specification, specifies the stated features, integers, steps, operations, elements and / or compositions, but does not exclude the existence or addition of one or more other features, integers, steps, operations, elements, compositions and / or combinations thereof.

[0026] Desiliconization mother liquor is a nitric acid calcium phosphate solution after nitric acid decomposes phosphate rock, and is the product of frozen decalcification; desiliconization mother liquor can traditionally only be used to produce nitrophosphate fertilizer and cannot be used in high quality. In order to increase the value of nitric acid wet-process phosphoric acid and adapt to the needs of my country's high-quality development under the new situation, it is necessary to make high-value use of desiliconization mother liquor, such as using desiliconization mother liquor to produce gypsum.

[0027] The present application provides a method for preparing dihydrate gypsum from industrial by-product phosphogypsum, comprising the following steps:

[0028] The mother liquor to be treated and the decalcifying agent are mixed at 40-50° C. to obtain a mixed slurry; the mixed slurry is subjected to a decalcification reaction at 40-50° C. After the reaction is completed, the mixed slurry is filtered and dried to obtain dihydrate gypsum.

[0029] Specifically, in the mother liquor to be treated, the content of HNO3 is 10-15%, the content of P2O5 is 5-20%, and the content of Ca 2+ Content <5.5%.

[0030] For example, in the mother liquor to be treated, the content of HNO3 is 10%, the content of P2O5 is 5%, and the content of Ca 2+ The content is 5.0%;

[0031] Or in the mother liquor to be treated, the content of HNO3 is 15%, the content of P2O5 is 20%, and the content of Ca 2+ The content is 4.0%;

[0032] Or in the mother liquor to be treated, the content of HNO3 is 12%, the content of P2O5 is 10%, and the content of Ca 2+ The content is 4.5%.

[0033] Or in the mother liquor to be treated, the content of HNO3 is 11%, the content of P2O5 is 16%, and the content of Ca 2+ The content is 3.5%.

[0034] Or in the mother liquor to be treated, the content of HNO3 is 14%, P2O5 is 9%, Ca 2+ The content is 3.8%.

[0035] In the present application, the content of N in the mother liquor to be treated is calculated as HNO3, and the HNO3 content is maintained at 10-15%, which helps to maintain the acidic environment of the solution, thereby promoting subsequent chemical reactions. If the HNO3 content is lower than 10%, it may lead to a decrease in the efficiency of the decalcification reaction and affect the formation of calcium sulfate precipitation; if the HNO3 content is higher than 15%, it may cause corrosion risks to the equipment, and excessive acid will increase the treatment cost and the difficulty of waste liquid treatment.

[0036] The content of P in the mother liquor to be treated is calculated as P2O5, and the content of P2O5 is maintained at 5-20%. Phosphate ion is one of the main components in the desiliconization mother liquor, and it participates in the process of forming insoluble phosphates with calcium ions. Maintaining an appropriate P2O5 content can ensure sufficient reactive sites, so that the decalcification agent can act more effectively on the target substance. When the P2O5 content is insufficient, it may lead to poor decalcification effect; while excessive P2O5 content will increase the cost of raw materials and may produce unnecessary by-products.

[0037] Ca in the mother liquor to be treated 2+ Content <5.5%, by controlling Ca 2+ The content does not exceed 5.5% in order to avoid excessive calcium ions in the solution, which is not only not conducive to the formation of the target product dihydrate gypsum, but may also cause co-precipitation of other impurity phases, affecting the purity of the final product. In addition, a higher calcium ion concentration will also inhibit the good growth of dihydrate gypsum crystals.

[0038] Specifically, the decalcifying agent is 98% sulfuric acid.

[0039] In the present application, by using high-purity concentrated sulfuric acid (98% sulfuric acid), the possibility of introducing other impurities can be reduced, and the purity of the final product can be ensured. High-concentration sulfuric acid (98%) has strong acidity and can effectively convert calcium nitrate in the mother liquor to be treated into insoluble calcium sulfate precipitate, thereby achieving deep decalcification.

[0040] Specifically, the SO4 in the mixed slurry is detected 2- When the mass content is 2-3%, stop adding the decalcifying agent.

[0041] In this application, by monitoring SO4 2- The degree of decalcification reaction is judged by the mass content. When it reaches 2-3%, it means that most of the calcium nitrate has been converted into calcium sulfate precipitation. At this time, stopping the addition of decalcification agent can avoid problems caused by excessive addition.

[0042] If SO4 2- A content exceeding 3% means that too much sulfuric acid is introduced into the system, which may lead to excessive sulfate concentration in the solution, affecting the subsequent crystallization process and reducing the quality of dihydrate gypsum crystals.

[0043] If SO4 2- If the content is lower than 2%, it indicates that the decalcification reaction is not fully carried out, and a large amount of calcium nitrate or other calcium compounds remain. Incomplete decalcification will cause the generated dihydrate gypsum to contain more impurities, affecting its mechanical properties and other physical properties, and failing to meet the expected high strength requirements.

[0044] Specifically, the decalcification reaction temperature is 50-60° C. and the time is 5-6 hours.

[0045] In the present application, the temperature is controlled at 50-60°C, and the reactant molecules have enough energy to accelerate the occurrence of the chemical reaction, so that calcium nitrate can react with sulfuric acid more quickly to form calcium sulfate precipitation. If the temperature is too low, part of the calcium nitrate may not be completely converted into calcium sulfate, affecting the quality of dihydrate gypsum in the subsequent steps.

[0046] The reaction time is controlled within 5-6 hours, giving enough time for all soluble calcium ions to combine with sulfate to form insoluble calcium sulfate, thereby achieving deep decalcification. At the same time, long-term reaction helps to reduce residual calcium ions and other impurities, thereby improving the purity and quality of the final product. Insufficient reaction time (<5 hours)

[0047] Specifically, the drying temperature is 50-60° C. under reduced pressure.

[0048] In this application, the process of dehydration under reduced pressure at 50-60°C is a relatively low but gentle dehydration process that is sufficient to promote water evaporation, which can avoid product structure damage or composition changes caused by high temperature and maintain the integrity and stability of dihydrate gypsum crystals. Higher drying temperatures may cause changes in the internal structure of dihydrate gypsum crystals, reduce density, etc.

[0049] At the same time, under reduced pressure conditions, the boiling point of water decreases, and a rapid and thorough drying process can be achieved at a lower temperature.

[0050] The ultra-high strength dihydrate gypsum prepared by the above method can be applied in many fields due to its excellent physical and mechanical properties. In particular, considering its compressive strength ≥70MPa, flexural strength ≥15MPa and suitable density (about 2.31 to 2.35g / cm 3 ) is suitable for the following aspects:

[0051] Building materials: Due to its good mechanical properties and low density, dihydrate gypsum can be used as a high-grade building gypsum material, such as making high-strength gypsum boards, gypsum blocks, etc., which are suitable for walls, ceilings and other structural components.

[0052] Decorative materials: Because its flaky irregular microstructure helps to improve the density and mechanical strength of the material, it can also be used to make various decorative interior products such as sculptures, molds, and line decorations.

[0053] The following examples and comparative examples will further illustrate the method for preparing ultra-high strength dihydrate gypsum from industrial by-product phosphogypsum described in the present application.

[0054] Example 1

[0055] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0056] At 40°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 2%, stop adding the decalcifying agent;

[0057] The mixed slurry was subjected to decalcification reaction at 40°C for 5 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 50°C for 20 hours to obtain dihydrate gypsum.

[0058] Wherein: the mother liquor to be treated has, by mass content, 10% HNO3, 5% P2O5, and 5% Ca 2+ The content is 5.0%.

[0059] Example 2

[0060] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0061] At 50°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 3%, stop adding the decalcifying agent;

[0062] The mixed slurry was subjected to decalcification reaction at 50°C for 6 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 60°C for 24 hours to obtain dihydrate gypsum.

[0063] Wherein: the content of HNO3 in the mother liquor to be treated is 15%, the content of P2O5 is 20%, the content of Ca 2+ The content is 4.0%.

[0064] Example 3

[0065] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0066] At 45°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 2.5%, stop adding the decalcifying agent;

[0067] The mixed slurry was subjected to decalcification reaction at 45°C for 5.5 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 55°C for 22 hours to obtain dihydrate gypsum.

[0068] Wherein: the mother liquor to be treated has, by mass content, 12% HNO3, 10% P2O5, and 10% Ca 2+ The content is 4.5%.

[0069] Example 4

[0070] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0071] At 40°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 3%, stop adding the decalcifying agent;

[0072] The mixed slurry was subjected to decalcification reaction at 40°C for 6 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 50°C for 24 hours to obtain dihydrate gypsum.

[0073] Wherein: the mother liquor to be treated has, by mass content, 11% HNO3, 16% P2O5, and 16% Ca 2+ The content is 3.5%.

[0074] Example 5

[0075] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0076] At 50°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 2%, stop adding the decalcifying agent;

[0077] The mixed slurry was subjected to decalcification reaction at 50°C for 5 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 50°C for 22 hours to obtain dihydrate gypsum.

[0078] Wherein: the mother liquor to be treated has, by mass content, 14% HNO3, 9% P2O5, and 9% Ca 2+ The content is 3.8%.

[0079] Comparative Example 1

[0080] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0081] At 90°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 in the supernatant of the mixed slurry. 2- Mass content, when SO4 2- When the mass content stabilizes at 2%, stop adding the decalcifying agent;

[0082] The mixed slurry was subjected to decalcification reaction at 90°C for 5 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 50°C for 20 hours to obtain dihydrate gypsum.

[0083] Wherein: the mother liquor to be treated has, by mass content, 10% HNO3, 5% P2O5, and 5% Ca 2+ The content is 5.0%.

[0084] Comparative Example 2

[0085] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0086] At 40°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 2%, stop adding the decalcifying agent;

[0087] The mixed slurry was subjected to decalcification reaction at 40°C for 5 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 100°C for 20 hours to obtain dihydrate gypsum.

[0088] Wherein: the mother liquor to be treated has, by mass content, 10% HNO3, 5% P2O5, and 5% Ca 2+ The content is 5.0%.

[0089] Comparative Example 3

[0090] A method for preparing ultra-high-strength dihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:

[0091] At 40°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 2- Mass content, when SO4 2- When the mass content stabilizes at 6%, stop adding the decalcifying agent;

[0092] The mixed slurry was subjected to decalcification reaction at 40°C for 5 hours. After the reaction was completed, it was filtered and the filter cake was dried under reduced pressure at 50°C for 20 hours to obtain dihydrate gypsum.

[0093] Wherein: the mother liquor to be treated has, by mass content, 10% HNO3, 5% P2O5, and 5% Ca 2+ The content is 5.0%.

[0094] The dihydrate gypsum prepared in Examples 1-5 and Comparative Examples 1-3 was subjected to density testing, compressive strength and flexural strength, scanning electron microscope (SEM) observation and thermogravimetric analysis, and the test results are summarized in Table 1.

[0095] Among them: compressive strength and flexural strength are carried out in accordance with GBT 17669.3-1999 "Determination of mechanical properties of building gypsum".

[0096] Table 1

[0097]

[0098] From Table 1, we can see that:

[0099] The dihydrate gypsum prepared in Examples 1-5 exhibited the following properties: density was between 2.31 and 2.35 g / cm 3 The compressive strength is between 70 and 75 MPa, the flexural strength is between 15 and 17 MPa, the crystal water content is about 20.93wt% to 21.05wt%, and the microstructure is irregular flaky.

[0100] It can be seen that the dihydrate gypsum in the examples has a lower density because they contain a certain amount of crystal water (about 21%), which makes its structure relatively loose and its density lower than that of anhydrous calcium sulfate or hemihydrate gypsum.

[0101] In this application, during the decalcification reaction, HNO3, P2O5 and Ca 2 + mass percentage, and the use of high-purity 98% sulfuric acid as the decalcifying agent ensures an effective decalcification process and reduces the presence of impurities.

[0102] At the same time, SO4 2- The mass content is stable at 2%-3%, ensuring sufficient but not excessive sulfate radicals to participate in the reaction, avoiding the influence of excessive acid on crystal growth. In addition, the decalcification reaction temperature is maintained in the range of 40-50°C, and the time is 5-6 hours, providing suitable conditions to promote the good formation of crystals, improve the integrity of crystals and the stability of internal structure.

[0103] Moreover, the drying step adopts a reduced pressure drying method at 50-60°C, which can effectively remove moisture without destroying the crystal structure, thereby maintaining high mechanical properties.

[0104] Therefore, the microstructure of the dihydrate gypsum obtained in Examples 1 to 5 presents a flaky and irregular characteristic (e.g. Figure 1 These plate-like crystals are intertwined and stacked with each other, which helps to improve the density and mechanical strength of the material.

[0105] The dihydrate gypsum prepared in Comparative Example 1 exhibits the following properties:

[0106] Density: 2.56g / cm 3 , compressive strength: 61MPa, flexural strength: 12MPa, crystal water content: 13.24%, microscopic morphology: partially flaky and irregular;

[0107] In Comparative Example 1, the temperature of the decalcification reaction was too high, reaching 90° C. The high temperature caused the decalcification reaction to be too intense, which may have caused local overheating and affected the formation of crystals. At the same time, calcium sulfate may further lose water at high temperatures, causing the crystal water content to decrease to 13.24%, which is lower than Example 1.

[0108] At the same time, the rapid reaction under high temperature conditions may result in the generated crystals being incomplete or in order, which reduces the mechanical strength of the material (compressive strength 61MPa, flexural strength 12MPa) and the density is also high (2.56g / cm 3 ).

[0109] The dihydrate gypsum prepared in Comparative Example 2 exhibits the following properties:

[0110] Density: 2.94g / cm 3 , compressive strength: 34MPa, flexural strength: 8MPa, crystal water content: 0.14%, microscopic morphology: powdery;

[0111] The drying temperature of Comparative Example 2 was too high, reaching 100°C. The excessively high drying temperature almost completely removed all the crystal water (only 0.14% remained), turning the product into anhydrous calcium sulfate, which changed its physical properties and caused the density to increase significantly to 2.94 g / cm 3 .

[0112] At the same time, due to the loss of crystal water, the crystals that should have been flaky turned into powder, which greatly affected the mechanical properties of the material, manifested as extremely low compressive strength (34MPa) and flexural strength (8MPa).

[0113] The dihydrate gypsum prepared in Comparative Example 3 exhibits the following properties:

[0114] Density: 2.62g / cm 3 , compressive strength: 65MPa, flexural strength: 13MPa, crystal water content: 15.79%, microscopic morphology: partially flaky and irregular;

[0115] In Comparative Example 3, SO4 2- The mass content is too high (6%), when SO4 2- When the mass content reaches 6%, it means that too much sulfuric acid has been added, which not only increases the concentration of sulfate ions in the solution, but also may cause co-precipitation of other impurity phases, affecting the purity of the final product.

[0116] Moreover, excessive sulfate may inhibit the good growth of dihydrate gypsum crystals, resulting in some crystals failing to fully develop and presenting a "partially flaky and irregular" morphology (e.g. Figure 2 This in turn affects the overall mechanical properties of the material.

[0117] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present invention is within the protection scope of the present invention.

Claims

1. A method for preparing dihydrate gypsum from industrial by-product phosphogypsum, characterized in that: The following steps are involved: The mother liquor to be treated and the decalcifying agent are mixed at 40-50° C. to obtain a mixed slurry; the mixed slurry is subjected to a decalcification reaction at 40-50° C. After the reaction is completed, the mixed slurry is filtered and dried to obtain dihydrate gypsum.

2. The method according to claim 1, characterized in that In the mother liquor to be treated, the content of HNO3 is 10-15% and the content of P2O5 is 5-20% by mass.

3. The method according to claim 1, characterized in that The decalcifying agent is sulfuric acid.

4. The method according to claim 1, characterized in that: Detection of SO4 in the mixed slurry 2- When the mass content is 2-3%, stop adding the decalcifying agent.

5. The method according to claim 1, characterized in that The decalcification reaction time is 5-6 hours.

6. The method according to claim 2, characterized in that In the mother liquor to be treated, the content of HNO3 is 11-14% and the content of P2O5 is 10-15% by mass.

7. The method according to claim 1, characterized in that The drying temperature is 50-60° C. under reduced pressure.

8. The method according to claim 1, characterized in that: The Ca in the mother liquor to be treated 2+ Content <5.5%.

9. Dihydrate gypsum, characterized in that The invention relates to dihydrate gypsum obtained by the method according to any one of claims 1 to 8.

10. The dihydrate gypsum according to claim 9, characterized in that: The compressive strength of the dihydrate gypsum is ≥70MPa, and the flexural strength is ≥15MPa.

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