Method for preparing ultrahigh-strength alpha-semi-hydrated gypsum from industrial byproduct phosphogypsum and alpha-semi-hydrated gypsum
By performing a decalcification reaction at 90-100°C, the desiloxidation mother liquor is converted into ultra-high strength α-semi-water gypsum, which solves the problem that the desiloxidation mother liquor cannot be used in the prior art in a high-quality manner, and realizes efficient utilization of resources and the preparation of high-strength gypsum.
Patent Information
- Application Number
- CN202411943905.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-27
- Publication Date
- 2025-05-06
AI Technical Summary
The existing technology cannot use desiloxidation mother liquor in high quality, resulting in its value being unable to be effectively improved.
The decalcification reaction was carried out at 90-100°C, and the mother liquor to be treated was mixed with the decalcification agent, and Ca2+ was stably converted into α-hemihydrate gypsum. Ultra-high strength α-hemihydrate gypsum was obtained by filtration and drying.
The treatment efficiency of the desiloxidation mother liquor was improved, the cost was reduced, and high-strength α-semi-water gypsum with compressive strength ≥81MPa and flexural strength ≥20MPa was prepared.
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Abstract
Description
Technical Field
[0001] The invention relates to the technical field of phosphogypsum processing, and in particular to a method for preparing ultra-high-strength alpha-hemihydrate gypsum from industrial by-product phosphogypsum and the alpha-hemihydrate gypsum. Background Art
[0002] Hemihydrate gypsum has two forms, α and β, both of which are rhombus crystals, but with different physical properties. α-type hemihydrate gypsum is well-crystallized and solid; β-type hemihydrate gypsum is a flaky crystal with cracks, with very fine crystals and a much larger specific surface area than α-type hemihydrate gypsum; when producing gypsum products, α-type hemihydrate gypsum requires less water than β-type, and the products have higher density and strength; α-type hemihydrate gypsum is suitable for GRG, DIY gypsum products, craft jewelry production, and various precision mold production.
[0003] The α-type hemihydrate gypsum, also known as high-strength gypsum, is usually produced by steaming in an autoclave in a saturated steam medium; the β-type hemihydrate gypsum, also known as building gypsum, is produced by calcining in an open device such as a wok or a rotary kiln.
[0004] Desiliconization mother liquor is the product of calcium nitrophosphate solution after nitric acid decomposes phosphate rock, which is frozen and decalcified. Conventional technology can only be used to produce nitrophosphate fertilizer and cannot be used in high quality. In order to improve 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 improve the high-quality utilization of desiliconization mother liquor.
[0005] To this end, the present application proposes a method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum and α-hemihydrate gypsum. Summary of the invention
[0006] In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum and α-hemihydrate gypsum, which can remove Ca in the mother liquor to be treated by decalcifying at 90-100 ° C. 2+ Stable conversion into α-hemihydrate gypsum solves the problem that conventional technology cannot make high-quality use of desiliconization mother liquor.
[0007] The technical solution adopted by the present invention to solve its technical problem is:
[0008] In a first aspect, the present application provides a method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum, comprising the following steps:
[0009] The mother liquor to be treated and the decalcifying agent are mixed at 90-100° C. to obtain a mixed slurry; the mixed slurry is subjected to a decalcification reaction at 90-100° C. After the reaction is completed, the mixed slurry is filtered and dried to obtain α-hemihydrate gypsum.
[0010] In some specific embodiments, in the mother liquor to be treated, the content of HNO3 is ≥25% and the content of P2O5 is ≥35% by mass.
[0011] In some embodiments, the decalcifying agent is sulfuric acid.
[0012] In some embodiments, the SO4 in the mixed slurry is detected. 2- When the mass content is 4-5%, stop adding the decalcifying agent.
[0013] In some specific embodiments, the decalcification reaction time is 2-4 hours.
[0014] In some specific embodiments, the mother liquor to be treated has an HNO3 content of 25-30% and a P2O5 content of 35-45% by mass.
[0015] In some embodiments, the drying temperature is 130-140°C.
[0016] In some specific embodiments, the Ca in the mother liquor to be treated is 2+ Content <5.5%.
[0017] In a second aspect, the present application provides α-hemihydrate gypsum, including α-hemihydrate gypsum obtained by the method described in the first aspect.
[0018] In some specific embodiments, the compressive strength of the α-hemihydrate gypsum is ≥81 MPa, and the flexural strength is ≥20 MPa.
[0019] The beneficial effects of the present invention are:
[0020] The method described in the present application can stably produce α-hemihydrate gypsum filter cake by controlling the ratio and composition of each raw material of the mother liquor to be treated and the mixed slurry at 90-100°C within a certain range, and then sintering the α-hemihydrate gypsum filter cake to obtain the α-hemihydrate gypsum finished product, thereby improving the treatment efficiency of the desiliconized clear mother liquor and reducing the cost.
[0021] The method described in the present application can prepare high-strength α-hemihydrate gypsum; the compressive strength of the α-hemihydrate gypsum is ≥81MPa, and the flexural strength is ≥20MPa. BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The present invention is further described below in conjunction with the accompanying drawings and embodiments.
[0023] Figure 1 This is a scanning electron microscope image of the α-hemihydrate gypsum prepared in Example 1 of the present application.
[0024] Figure 2This is a scanning electron microscope image of the α-hemihydrate gypsum prepared in Comparative Example 1 of the present application.
[0025] Figure 3 This is a scanning electron microscope image of the α-hemihydrate gypsum prepared in Comparative Example 2 of the present application. DETAILED DESCRIPTION
[0026] 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.
[0027] 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.
[0028] Desiliconization clear liquor is a product of calcium nitric phosphate solution after nitric acid decomposes phosphate rock, which is frozen and decalcified; desiliconization clear liquor can traditionally only be used to produce nitrophosphate fertilizer and cannot be used in high quality. The present invention aims to change this situation, by using desiliconization clear liquor to prepare ultra-high strength α-hemihydrate gypsum, to achieve efficient utilization of resources; in order to improve 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 clear liquor, such as using desiliconization clear liquor to produce gypsum.
[0029] To this end, the present application provides a method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum, comprising the following steps:
[0030] The mother liquor to be treated and the decalcifying agent are mixed at 90-100° C. to obtain a mixed slurry; the mixed slurry is subjected to a decalcification reaction at 90-100° C. After the reaction is completed, the mixed slurry is filtered and dried to obtain α-hemihydrate gypsum.
[0031] Specifically, in the mother liquor to be treated, the content of HNO3 is ≥25%, the content of P2O5 is ≥35%, and the content of Ca 2+ Content <5.5%;
[0032] For example, in the mother liquor to be treated, the content of HNO3 is 25-30%, the content of P2O5 is 35-45%, and the content of Ca 2+ Content <5.5%.
[0033] In one embodiment, the mother liquor to be treated has, by mass content, 25% HNO3, 35% P2O5, and 35% Ca 2+ Content 4.5%.
[0034] Alternatively, in the mother liquor to be treated, the content of HNO3 is 30%, the content of P2O5 is 45%, and the content of Ca 2+ The content is 3.4%.
[0035] Or the mother liquor to be treated has, by mass content, 28% HNO3, 41% P2O5, and 41% Ca 2+ The content is 4.7%.
[0036] It can be understood that in this application, the decalcification reaction is carried out at 90-100°C to ensure that the Ca in the mother liquor to be treated\the desiliconized clear mother liquor is 2+ It can be stably converted into α-hemihydrate gypsum (α-CaSO4·0.5H2O); higher temperature helps to accelerate the reaction rate, but too high temperature may cause other adverse reactions or phase changes.
[0037] The content of N in the mother liquor to be treated\desiliconization clear mother liquor, calculated as HNO3, is required to be ≥25%. A high content of HNO3 can help dissolve certain impurities and promote Ca 2+ release, and provide sufficient acidity to facilitate the formation of α-hemihydrate gypsum crystals; at the same time, if the content of HNO3 is low, it may slow down the Ca 2+ The speed of the conversion to α-hemihydrate gypsum may even result in different types of gypsum phases, such as β-hemihydrate gypsum or other undesirable phases.
[0038] The content of P in the mother liquor to be treated\desiliconized clear mother liquor, calculated as P2O5, is ≥35%: a higher content of P2O5 helps to improve the strength and other physical and chemical properties of the final product. A lower content of P2O5 may lead to poor development of α-hemihydrate gypsum crystals, thereby reducing its application value.
[0039] In the mother liquor to be treated\the mother liquor of desiliconization, Ca 2+ Content <5.5%, by limiting Ca 2+ The initial concentration is to avoid the formation of excessive by-products or affecting the crystallization process of α-hemihydrate gypsum.
[0040] Specifically, the decalcifying agent is sulfuric acid. In the present application, sulfuric acid with a mass content of 98% is selected.
[0041] In this application, the use of high-purity (98%) sulfuric acid can ensure that a sufficient acidic environment is provided during the reaction to promote the Ca 2+The ions precipitate from the mother liquor to form α-hemihydrate gypsum; concentrated sulfuric acid has strong dehydrating and corrosive properties, which helps to more thoroughly remove the calcium component in the solution.
[0042] Specifically, at 90-100°C, add a decalcifying agent to the mother liquor to be treated, stir and mix thoroughly, and detect SO4 in the supernatant of the mixed slurry. 2- Mass content, when SO4 2- When the mass content stabilizes at 4-5%, stop adding the decalcifying agent.
[0043] In this application, by monitoring the SO4 2- The mass content of Ca2+ can be stabilized at 4-5%, which can accurately determine whether the decalcification reaction has reached the expected level. 2+ With SO4 2- The reaction generates CaSO4 precipitate, and as Ca 2+ Gradually removed, SO4 2- The concentration will tend to stabilize; at the same time, ensure that the appropriate amount of decalcifying agent (sulfuric acid) is added to avoid over or under addition.
[0044] Too high SO4 2- Concentration may mean excessive addition of decalcifying agent (sulfuric acid), which not only increases cost but may also introduce unnecessary impurities such as sulfate ions, which may interfere with subsequent crystallization processes and lead to reduced product purity.
[0045] If SO4 2- Too low a concentration indicates incomplete decalcification and unreacted Ca 2+ The residue may cause the generated α-hemihydrate gypsum crystals to be poorly developed, affecting its physical properties (such as strength). In addition, lower SO4 2- The content may also lead to the formation of non-target phase gypsum crystals (such as β-hemihydrate gypsum or other forms), thereby reducing the quality and application performance of the product.
[0046] Specifically, after the addition of the decalcifying agent is stopped, the mixed slurry continues to undergo decalcification reaction at 90-100° C. for 2-4 hours.
[0047] In this application, the reaction is continued at a temperature range of 90-100°C for 2-4 hours in order to provide sufficient time for the decalcification reaction to allow Ca 2+ The ions can be precipitated from the mother liquor as completely as possible to form α-hemihydrate gypsum crystals, and extending the reaction time can increase the thoroughness and consistency of the reaction.
[0048] At the same time, holding time of 2-4h and temperature control at 90-100°C are helpful to control the morphology and size of the generated α-hemihydrate gypsum crystals; holding time of 2-4h gives the crystals more opportunities to grow and improve their structure, thereby obtaining a more ideal crystal morphology (such as regular prisms).
[0049] If the holding time exceeds 4 h, for example 10 h, the excessively long reaction time may lead to unnecessary increase in energy consumption and may induce the formation of gypsum crystals in non-target phases, such as anhydrous gypsum (CaSO4) or other undesirable phases, which will affect the quality and yield of the final product.
[0050] If the holding time is only 1h, the reaction time may be insufficient, which may lead to insufficient decalcification reaction. 2+ Failure to convert into α-hemihydrate gypsum in time will result in reduced product purity and may even lead to the appearance of non-target phases such as β-hemihydrate gypsum, affecting the performance of the final product.
[0051] Specifically, after the decalcification reaction is completed, filtration is performed to obtain a filter cake; the filter cake is dried at 130-140° C. to obtain α-hemihydrate gypsum.
[0052] In the present application, 130-140°C is selected for drying in order to ensure that the moisture in the filter cake is fully removed, while avoiding further chemical changes or phase changes in the α-hemihydrate gypsum due to excessively high temperature; the appropriate drying temperature (130-140°C) helps to maintain the integrity of the α-hemihydrate gypsum crystals and promotes the stabilization of the internal structure of the crystals, thereby improving the mechanical strength and other physical properties of the final product.
[0053] However, too high a temperature may cause excessive water loss in α-hemihydrate gypsum crystals and may even trigger a phase change to form anhydrous gypsum (CaSO4), which will not only change the chemical composition of the product but also affect its application performance, such as reducing compressive strength.
[0054] If the temperature is too low, the moisture in the filter cake may not be effectively removed. The presence of residual moisture will affect the storage stability of α-hemihydrate gypsum and its performance in subsequent use. In addition, excessive moisture may also cause product agglomeration or other quality problems.
[0055] Specifically, the mother liquor to be treated in the present application and the embodiments can be adjusted by adding distilled water, concentrated nitric acid, etc. to meet the corresponding weight content requirements.
[0056] The method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum described in the present application is further described through the following examples and comparative examples.
[0057] Example 1
[0058] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0059] At 100°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 content in the supernatant of the mixed slurry. 2- Mass content, when SO4 2- When the mass content stabilizes at 5%, stop adding the decalcifying agent;
[0060] The mixed slurry was subjected to a decalcification reaction at 100°C for 4 hours. After the reaction was completed, it was filtered and the filter cake was dried at 140°C for 6 hours to obtain α-hemihydrate gypsum.
[0061] Wherein: the mother liquor to be treated has, by mass content, 25% HNO3, 35% P2O5, and 35% Ca 2+ Content 4.5%.
[0062] Example 2
[0063] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0064] At 95°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 4.5%, stop adding the decalcifying agent;
[0065] The mixed slurry was subjected to decalcification reaction at 95°C for 3 hours. After the reaction was completed, it was filtered and the filter cake was dried at 135°C for 5.5 hours to obtain α-hemihydrate gypsum.
[0066] Wherein: the mother liquor to be treated has, by mass content, 30% HNO3, 45% P2O5, and 45% Ca 2+ The content is 3.4%.
[0067] Example 3
[0068] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0069] 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 4%, stop adding the decalcifying agent;
[0070] The mixed slurry was subjected to decalcification reaction at 90°C for 2 hours. After the reaction was completed, it was filtered and the filter cake was dried at 130°C for 5 hours to obtain α-hemihydrate gypsum.
[0071] Wherein: the mother liquor to be treated has, by mass content, 28% HNO3, 41% P2O5, and 41% Ca 2+ The content is 4.7%.
[0072] Example 4
[0073] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0074] At 90°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO in the supernatant of the mixed slurry. 42- Mass content, when SO 42- When the mass content stabilizes at 5%, stop adding the decalcifying agent;
[0075] The mixed slurry was subjected to decalcification reaction at 90°C for 4 hours. After the reaction was completed, it was filtered and the filter cake was dried at 130°C for 6 hours to obtain α-hemihydrate gypsum.
[0076] Wherein: the mother liquor to be treated has, by mass content, 25% HNO3, 35% P2O5, and 35% Ca 2+ Content 4.5%.
[0077] Example 5
[0078] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0079] At 100°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 content in the supernatant of the mixed slurry. 2- Mass content, when SO4 2- When the mass content stabilizes at 4%, stop adding the decalcifying agent;
[0080] The mixed slurry was subjected to decalcification reaction at 100°C for 3 hours. After the reaction was completed, it was filtered and the filter cake was dried at 130°C for 5.5 hours to obtain α-hemihydrate gypsum.
[0081] Wherein: the mother liquor to be treated has, by mass content, 25% HNO3, 35% P2O5, and 35% Ca 2+ Content 4.5%.
[0082] Comparative Example 1
[0083] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0084] At 50°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO42- Mass content, when SO4 2- When the mass content stabilizes at 5%, stop adding the decalcifying agent;
[0085] The mixed slurry was subjected to decalcification reaction at 50°C for 4 hours. After the reaction was completed, it was filtered and the filter cake was dried at 140°C for 6 hours to obtain α-hemihydrate gypsum.
[0086] Wherein: the mother liquor to be treated has, by mass content, 25% HNO3, 35% P2O5, and 35% Ca 2+ Content 4.5%.
[0087] Comparative Example 2
[0088] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0089] At 100°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 content in the supernatant of the mixed slurry. 2- Mass content, when SO4 2- When the mass content stabilizes at 5%, stop adding the decalcifying agent;
[0090] The mixed slurry was subjected to a decalcification reaction at 100°C for 4 hours. After the reaction was completed, it was filtered and the filter cake was dried at 140°C for 6 hours to obtain α-hemihydrate gypsum.
[0091] Wherein: the mother liquor to be treated has, by mass content, 10% HNO3, 15% P2O5, and 15% Ca 2+ Content 4.5%.
[0092] Comparative Example 3
[0093] A method for preparing ultra-high-strength α-hemihydrate gypsum from industrial by-product phosphogypsum comprises the following steps:
[0094] At 100°C, add 98% sulfuric acid to the mother liquor to be treated, stir and mix thoroughly, and detect the SO4 content in the supernatant of the mixed slurry. 2- Mass content, when SO4 2- When the mass content stabilizes at 5%, stop adding the decalcifying agent;
[0095] The mixed slurry was subjected to decalcification reaction at 100°C for 4 hours. After the reaction was completed, it was filtered and the filter cake was dried at 200°C for 10 hours to obtain α-hemihydrate gypsum.
[0096] Wherein: the mother liquor to be treated has, by mass content, 25% HNO3, 35% P2O5, and 35% Ca 2+ Content 4.5%.
[0097] The α-hemihydrate 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.
[0098] Among them: compressive strength and flexural strength are carried out in accordance with GBT 17669.3-1999 "Determination of mechanical properties of building gypsum".
[0099] Table 1
[0100]
[0101]
[0102] From Table 1, we can see that:
[0103] The α-hemihydrate gypsum prepared in Examples 1-5 exhibits the following properties:
[0104] Density: 2.68 to 2.79 g / cm 3 ; Compressive strength: 81 to 91 MPa; Flexural strength: 20 to 24 MPa; Crystal water content: 6.22 wt% to 6.25 wt%, close to the theoretical 6.21 wt%; SEM observation: a large number of prismatic crystals (such as Figure 1 It can be seen that by carrying out the decalcification reaction at 90-100°C and controlling the amount of decalcification agent (sulfuric acid) added, SO4 2- The mass content is stable at 4-5%, thus ensuring the Ca 2+ Highly efficient conversion to α-hemihydrate gypsum; the HNO3 content in the mother liquor to be treated is 25-30%, the P2O5 content is 35-45%, and the Ca 2+ The content is less than 5.5%, which ensures the appropriate proportion of raw materials and promotes the formation of α-hemihydrate gypsum; the drying temperature is 130-140°C, which helps to remove excess moisture and maintain the crystallinity of α-hemihydrate gypsum.
[0105] The α-hemihydrate gypsum prepared in Comparative Example 1 exhibits the following properties:
[0106] Density: 2.59g / cm 3 , compressive strength: 65MPa, flexural strength: 19MPa, crystal water content: 10.89%; SEM observation: a small amount of prismatic crystals and incomplete (such as Figure 2 Therefore, it can be seen that the decalcification reaction of Comparative Example 1 was carried out at 50°C, resulting in a slow reaction rate and failure to fully generate α-hemihydrate gypsum, thereby affecting the crystal morphology and physical properties.
[0107] The α-hemihydrate gypsum prepared in Comparative Example 2 exhibits the following properties:
[0108] Density: 2.83g / cm 3 , compressive strength: 57MPa, flexural strength: 15MPa, crystal water content: 7.21%; SEM observation: there are some prismatic crystals and they are incomplete (such as Figure 2 The HNO content in the mother liquor to be treated in Comparative Example 2 is only 10%, and the P2O5 content is 15%, which is much lower than the standard in the embodiment, which may lead to insufficient acidity during the reaction, which is not conducive to the formation of α-hemihydrate gypsum.
[0109] The α-hemihydrate gypsum prepared in Comparative Example 3 exhibits the following properties:
[0110] Density: 2.32g / cm 3 , compressive strength: 39MPa, flexural strength: 9MPa, crystal water content: 0.13%; SEM observation: no prismatic crystals. It can be seen that the drying temperature of Comparative Example 3 is 200°C, which is much higher than 130-140°C in the embodiment. The high temperature causes the α-hemihydrate gypsum to further dehydrate and form anhydrous gypsum, so the crystal water content is extremely low and the crystal structure is completely destroyed.
[0111] 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 ultra-high-strength α-hemihydrate 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 90-100° C. to obtain a mixed slurry; the mixed slurry is subjected to a decalcification reaction at 90-100° C. After the reaction is completed, the mixed slurry is filtered and dried to obtain α-hemihydrate gypsum.
2. The method according to claim 1, characterized in that In the mother liquor to be treated, the content of HNO3 is ≥25% and the content of P2O5 is ≥35% 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 4-5%, stop adding the decalcifying agent.
5. The method according to claim 1, characterized in that The decalcification reaction time is 2-4h.
6. The method according to claim 2, characterized in that In the mother liquor to be treated, the content of HNO3 is 25-30% and the content of P2O5 is 35-45% by mass.
7. The method according to claim 1, characterized in that The drying temperature is 130-140°C.
8. The method according to claim 1, characterized in that The Ca in the mother liquor to be treated 2+ Content <5.5%.
9. α-hemihydrate gypsum, characterized in that The invention comprises α-hemihydrate gypsum obtained by the method according to any one of claims 1 to 8.
10. The α-hemihydrate gypsum according to claim 9, characterized in that: The α-hemihydrate gypsum has a compressive strength of ≥81 MPa and a flexural strength of ≥20 MPa.