Preparation method of type II anhydrous gypsum

Abstract

The invention relates to a preparation method of type II anhydrous gypsum. The preparation method comprises the following steps: mixing an industrial by-product gypsum raw material with type II anhydrous gypsum seed crystal to obtain a mixture; autoclaving the mixture at the temperature of lower than 200 DEG C to obtain intermediate powder; and drying the intermediate powder at a drying temperature lower than the autoclaved temperature to obtain the type II anhydrite. According to the preparation method, high-temperature calcination or addition of an inducer and a catalyst is not needed, no acidic raw material is added, and the preparation method has the characteristics of low energy consumption, environment optimization and high conversion rate.

Description

Technical Field

[0001] The present invention relates to the technical field of resource utilization of industrial solid waste, specifically to the resource utilization of industrial by - product gypsum, and particularly to a preparation method of type II anhydrous gypsum. Background Art

[0002] Type II anhydrous gypsum, also known as insoluble anhydrite, is the final product that is stable at room temperature after high - temperature calcination and dehydration of gypsum dihydrate, hemihydrate gypsum, and type III anhydrous gypsum. Type II anhydrous gypsum is mainly used as a building material in gypsum - based self - leveling mortar, plastering gypsum, new cementing materials, etc. It is also widely used in fields such as plastic fillers and paper fillers in addition to the building materials field.

[0003] Currently, there are mainly two methods for preparing type II anhydrous gypsum from industrial by - product gypsum:

[0004] One is to dehydrate the by - product gypsum under the condition of concentrated acid to generate type II anhydrous gypsum. For example, CN115849427A discloses a method for preparing type II anhydrous gypsum from high - silicon phosphogypsum. By adding phosphogypsum to a wet - process phosphoric acid solution with an acid concentration of 30% - 40% P2O5 and a mixed solution of hydrofluoric acid, and dropping sulfuric acid solution at a certain temperature and stirring time, type II anhydrous gypsum is prepared. This scheme prepares larger - particle type II anhydrous gypsum and reduces the silicon content in phosphogypsum, but there is a problem of difficult treatment of waste acid.

[0005] The other is to prepare type II anhydrous gypsum by high - temperature calcination method. For example, CN113307522A provides a device and method for producing type II anhydrous gypsum from industrial by - product gypsum. By an effective calcination unit at 700°C - 800°C, the crystal water in gypsum dihydrate is removed to obtain a crude product of type II anhydrous gypsum. Then, through the control of temperature and residence time by a crystal form controller, precise step - by - step crystal control can be achieved, and a type II anhydrous gypsum product with a proportion of AⅡU crystal form ≥ 98wt.% can be obtained. This scheme can obtain type II anhydrous gypsum with higher purity and can effectively reduce the impurities in industrial by - product gypsum through calcination, but there is a problem of high energy consumption. Summary of the Invention

[0006] Aiming at the deficiencies existing in the prior art, the purpose of the present invention is to provide a preparation method of type II anhydrous gypsum to solve the technical problems of difficult treatment of waste acid and high energy consumption existing in the background art.

[0007] To achieve the purpose of this invention, the following technical solutions are adopted in the present invention:

[0008] The present invention provides a preparation method of type II anhydrous gypsum, and the preparation method includes the following steps:

[0009] Mix the industrial by - product gypsum raw material with type II anhydrous gypsum seeds to obtain a mixture;

[0010] Steam - press the mixture under the condition of lower than 200 °C to obtain intermediate powder;

[0011] Dry the intermediate powder at a drying temperature lower than the steam - pressing temperature to obtain type II anhydrous gypsum.

[0012] In the present invention, the type II anhydrous gypsum seeds have high activity and high fineness. Under the conditions of steam - pressing and drying, the type II anhydrous gypsum seeds act as crystal nuclei to induce the growth of type II anhydrous gypsum in the industrial by - product gypsum raw material, and finally convert the industrial by - product gypsum into type II anhydrous gypsum.

[0013] In the present invention, the industrial by - product gypsum raw material includes one or a combination of at least two of flue gas desulfurization gypsum, phosphogypsum, citric acid gypsum, salt gypsum, monosodium glutamate gypsum, copper gypsum, fluorogypsum, titanium gypsum, nickel gypsum, chromium gypsum, boron gypsum, mirabilite gypsum, tartaric acid gypsum or lactic acid gypsum. Typical but non - limiting combinations are, for example, desulfurization gypsum and phosphogypsum, phosphogypsum and fluorogypsum, citric acid gypsum and boron gypsum, etc. Preferably, it is phosphogypsum.

[0014] The present invention adopts a method of first steam - pressing and then drying to prepare type II anhydrous gypsum. The steam - pressing temperature and the drying temperature are both controlled within 200 °C, thereby greatly reducing energy consumption, reducing pollution, simplifying the overall process flow, and having a high conversion rate.

[0015] Furthermore, appropriate temperature and process coordination can ensure the stable properties of the grown gypsum and finally obtain an effective product. The powder of the present invention completes a rapid switch between the two processes of steam - pressing and drying, so as to keep the temperature of the intermediate powder higher than or equal to the drying temperature from the end of steam - pressing to the start of drying, and quickly enter the drying program, so that the two processes are closely connected, and the excess free water in the powder can be quickly removed without re - combining with the effective components in the powder, thereby obtaining type II anhydrous gypsum with a high conversion rate.

[0016] In the present invention, the fineness of the type II anhydrous gypsum seeds will affect its activation degree, thereby affecting the conversion rate of the prepared type II anhydrous gypsum. The larger the fineness, the larger the surface area of the gypsum seeds, providing more active sites, and the higher its activation degree. On the contrary, the smaller the fineness, the lower its activation degree.

[0017] Furthermore, the fineness of the type-II anhydrous gypsum seed crystal is 200 - 1500 mesh, for example, it can be 200 mesh, 300 mesh, 400 mesh, 500 mesh, 600 mesh, 700 mesh, 800 mesh, 900 mesh, 1000 mesh, 1100 mesh, 1200 mesh, 1250 mesh, 1300 mesh, 1350 mesh, 1400 mesh, 1450 mesh or 1500 mesh, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0018] Furthermore, the fineness of the type-II anhydrous gypsum seed crystal is 750 - 1500 mesh.

[0019] In the present invention, the mass ratio of the industrial by-product gypsum raw material to the type-II anhydrous gypsum seed crystal in the mixture affects the conversion time and conversion rate of the prepared type-II anhydrous gypsum. When the mass ratio of the industrial by-product gypsum raw material in the mixture is too high, it will lead to too long conversion time and too low conversion rate of the type-II anhydrous gypsum; when the mass ratio of the type-II anhydrous gypsum seed crystal in the mixture is too high, it will also reduce the production efficiency and affect the conversion rate of the type-II anhydrous gypsum.

[0020] Furthermore, in the said mixture, the mass ratio of the industrial by-product gypsum raw material to the type-II anhydrous gypsum seed crystal is (80 - 95):(20 - 5), for example, it can be 80:20, 81:19, 82:18, 83:17, 84:16, 85:15, 86:14, 87:13, 88:12, 89:11, 90:10, 91:9, 92:8, 93:7, 94:6 or 95:5, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0021] Furthermore, the mass ratio of the industrial by-product gypsum raw material to the type-II anhydrous gypsum seed crystal is (80 - 90):(20 - 10), and further preferably (80 - 85):(20 - 15).

[0022] Furthermore, the autoclaving reaction method includes autoclaving after piling up the said mixture in an autoclaving container. The autoclaving container can be an autoclave.

[0023] In the present invention, the stacking thickness of the mixture of the industrial by-product gypsum raw material and the type-II anhydrous gypsum seed crystal in the autoclaving container will affect the conversion efficiency. Too large stacking thickness will lead to a decrease in the conversion efficiency, and the smaller the stacking thickness, the higher the conversion efficiency, but too small stacking thickness will lead to a decrease in the production efficiency.

[0024] Further, the thickness of the stacked mixture is 5 - 30 cm, for example, it can be 5 cm, 6 cm, 7 cm, 8 cm, 9 cm, 10 cm, 11 cm, 12 cm, 13 cm, 14 cm, 15 cm, 16 cm, 17 cm, 18 cm, 19 cm, 20 cm, 21 cm, 22 cm, 23 cm, 24 cm, 25 cm, 26 cm, 27 cm, 28 cm, 29 cm or 30 cm, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0025] Further, the thickness of the stacked mixture is 10 - 30 cm.

[0026] In the present invention, the temperature and time of the autoclave reaction are important factors affecting the conversion rate of type II anhydrous gypsum obtained. If the temperature of the autoclave reaction is too high, the energy consumption required for production is high; if the temperature of the autoclave reaction is too low, the conversion rate of type II anhydrous gypsum decreases; if the time of the autoclave reaction is too long, the energy consumption increases; if the time of the autoclave reaction is too short, the conversion rate decreases.

[0027] Further, the temperature of the autoclave reaction is 160°C - 190°C, for example, it can be 160°C, 170°C, 180°C or 190°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable. In the present invention, the preparation of type II anhydrous gypsum is achieved under the medium and low temperature conditions of 160°C - 190°C, which can balance energy conservation and conversion rate.

[0028] Further, the temperature of the autoclave reaction is 170°C - 190°C, and more preferably 180°C - 190°C.

[0029] Further, the time of the autoclave reaction is 2 - 6 h, for example, it can be 2 h, 3 h, 4 h, 5 h or 6 h, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0030] Further, the time of the autoclave reaction is preferably 3 - 6 h, and more preferably 3 - 5 h.

[0031] Further, the temperature of the drying is 130°C - 160°C, for example, it can be 130°C, 140°C, 150°C or 160°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0032] Further, the time of the drying is 1 h - 5 h, for example, it can be 1 h, 2 h, 3 h, 4 h or 5 h, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0033] Further, the drying time is 2h to 4h.

[0034] Further, the preparation method of the type II anhydrous gypsum includes the following steps:

[0035] Mix an industrial by - product gypsum raw material with a mass ratio of (80 - 95):(20 - 5) and type II anhydrous gypsum seeds with a fineness of 200 - 1500 meshes, stack the mixture in an autoclave, and autoclave at 160°C - 190°C for 3 - 6h to obtain an intermediate powder. Dry the intermediate powder at a drying temperature lower than the autoclaving temperature to prepare type II anhydrous gypsum.

[0036] The numerical ranges described in the present invention not only include the point values exemplified above, but also include any point values between the above - mentioned numerical ranges not exemplified. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) In the preparation method of type II anhydrous gypsum provided by the present invention, the raw materials only include industrial by - product gypsum raw materials and type II anhydrous gypsum seeds. No inducer and catalyst need to be added during the whole preparation process. The process is simple, no acid solution is used, no secondary pollution is generated, and it is environmentally friendly.

[0039] (2) The present invention prepares type II anhydrous gypsum by the autoclaving method, without high - temperature calcination, saving energy consumption and having a low preparation cost.

[0040] (3) The conversion rate of the type II anhydrous gypsum prepared by the present invention can be as high as 98.7%, with extremely low loss. Further, the acquisition cost of type II anhydrous gypsum is low, and the type II anhydrous gypsum obtained in this application has high quality and extremely high market competitiveness. Detailed implementation manners

[0041] The technical solutions of the present invention will be further described below through specific implementation manners. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations to the present invention.

[0042] Example 1

[0043] This example provides a preparation method of type II anhydrous gypsum, and the preparation method includes the following steps:

[0044] Mix phosphogypsum and type II anhydrous gypsum seeds with a fineness of 1500 mesh evenly at a mass ratio of 80:20 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave and keep it at a temperature of 190 °C for 6 hours; quickly take it out of the autoclave, put the powder into an oven at 160 °C and bake for 4 hours to prepare type II anhydrous gypsum, where the conversion rate of type II anhydrous gypsum is 98.7%.

[0045] Example 2

[0046] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0047] Mix flue gas desulfurization gypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 80:20 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave and keep it at a temperature of 180 °C for 6 hours; quickly take it out of the autoclave, put the powder into an oven at 150 °C and bake for 2 hours to prepare type II anhydrous gypsum, where the conversion rate of type II anhydrous gypsum is 96.3%.

[0048] Example 3

[0049] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0050] Mix citric acid gypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 80:20 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave and keep it at a temperature of 190 °C for 3 hours; quickly take it out of the autoclave, put the powder into an oven at 160 °C and bake for 2 hours to prepare type II anhydrous gypsum, where the conversion rate of type II anhydrous gypsum is 96.8%.

[0051] Example 4

[0052] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0053] Mix fluorogypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 80:20 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave and keep it at a temperature of 190 °C for 4 hours; quickly take it out of the autoclave, put the powder into an oven at 160 °C and bake for 2 hours to prepare type II anhydrous gypsum, where the conversion rate of type II anhydrous gypsum is 97.9%.

[0054] Example 5

[0055] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0056] Mix titanium gypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 80:20 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave, keep it at a temperature of 190 °C for 5 h; quickly take it out of the autoclave, put the powder into an oven at 160 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 98.3%.

[0057] Example 6

[0058] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0059] Mix borax gypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 85:15 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave, keep it at a temperature of 180 °C for 3 h; quickly take it out of the autoclave, put the powder into an oven at 130 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 95.7%.

[0060] Example 7

[0061] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0062] Mix tartaric acid gypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 85:15 to obtain a mixture; put the mixture with a thickness of 30 cm into an autoclave, keep it at a temperature of 180 °C for 4 h; quickly take it out of the autoclave, put the powder into an oven at 140 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 96.1%.

[0063] Example 8

[0064] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0065] Mix mirabilite gypsum and type II anhydrous gypsum seeds with a fineness of 1250 mesh evenly at a mass ratio of 85:15 to obtain a mixture; put the mixture with a thickness of 25 cm into an autoclave, keep it at a temperature of 170 °C for 6 h; quickly take it out of the autoclave, put the powder into an oven at 160 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 94.8%.

[0066] Example 9

[0067] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0068] Mix the salt gypsum and type II anhydrous gypsum seeds with a fineness of 750 mesh in a mass ratio of 90:10 evenly to obtain a mixture; put the mixture with a thickness of 15 cm into an autoclave and keep it at a temperature of 170 °C for 4 h; quickly take it out of the autoclave, put the powder into an oven at 150 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 93.3%.

[0069] Example 10

[0070] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0071] Mix the copper gypsum and type II anhydrous gypsum seeds with a fineness of 1000 mesh in a mass ratio of 90:10 evenly to obtain a mixture; put the mixture with a thickness of 20 cm into an autoclave and keep it at a temperature of 170 °C for 5 h; quickly take it out of the autoclave, put the powder into an oven at 150 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 93.7%.

[0072] Example 11

[0073] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0074] Mix the nickel gypsum and type II anhydrous gypsum seeds with a fineness of 500 mesh in a mass ratio of 90:10 evenly; put the mixture with a thickness of 10 cm into an autoclave and keep it at a temperature of 170 °C for 3 h; quickly take it out of the autoclave, put the powder into an oven at 140 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 80.7%.

[0075] Example 12

[0076] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0077] Mix the chromium gypsum and type II anhydrous gypsum seeds with a fineness of 500 mesh in a mass ratio of 90:10 evenly to obtain a mixture; put the mixture with a thickness of 5 cm into an autoclave and keep it at a temperature of 160 °C for 4 h; quickly take it out of the autoclave, put the powder into an oven at 140 °C and bake for 2 h to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 46.1%.

[0078] Example 13

[0079] This example provides a method for preparing type II anhydrous gypsum, and the preparation method includes the following steps:

[0080] Mix the lactic acid gypsum and type II anhydrous gypsum seeds with a fineness of 200 mesh in a mass ratio of 95:5 evenly to obtain a mixture; put the mixture with a thickness of 5 cm into an autoclave, keep it at a temperature of 160 °C for 3 hours; quickly take it out of the autoclave, put the powder into an oven at 130 °C and bake for 2 hours to prepare type II anhydrous gypsum powder, where the conversion rate of type II anhydrous gypsum is 39.6%.

[0081] Comparative Example 1

[0082] This comparative example provides a preparation method of type II anhydrous gypsum. Except that the temperature of the autoclave reaction is 150 °C, the rest is exactly the same as that of Example 1. The conversion rate of type II anhydrous gypsum prepared in this comparative example is 20.36%.

[0083] Comparative Example 2

[0084] This comparative example provides a preparation method of type II anhydrous gypsum. Except that the temperature of the autoclave reaction is 200 °C, the rest is exactly the same as that of Example 1. The conversion rate of type II anhydrous gypsum prepared in this comparative example is 98.83%.

[0085] Record the parameters and conversion rates of the above-mentioned various examples and draw them into Table 1 as follows.

[0086] Table 1

[0087]

[0088]

[0089] It can be seen from Table 1 that the combination of the process parameters in Example 1 obtains the second highest conversion rate. According to the comparison of the conversion rates of type II anhydrous gypsum in Example 1 and Comparative Example 2, it can be known that under the same other conditions, when the temperature of the autoclave reaction increases, the conversion rate of type II anhydrous gypsum can increase. In the comparative example, it reaches 98.83%, that is, it increases by 0.13% compared with Example 1, but the increase amplitude is limited. However, this requires consuming less but too high preparation energy consumption, which is not conducive to cost control in industrial production.

[0090] According to the comparison of the conversion rates of type II anhydrous gypsum in Example 1 and Comparative Example 1, it can be known that under the same other conditions, when the temperature of the autoclave reaction is too low, the conversion rate of type II anhydrous gypsum drops extremely significantly, only being 20.36%.

[0091] The above comparison shows that the autoclave reaction temperature is an important factor in the implementation effect of the present invention, and the higher the autoclave temperature, the higher the conversion rate.

[0092] Comparing Examples 3 and 4, it can be seen that under the same other conditions, the longer the autoclave time, the higher the conversion rate.

[0093] Comparing Comparative Examples 6 and 7, it can be seen that under the condition that other conditions are the same, the higher the drying temperature, the higher the conversion rate.

[0094] The present invention realizes the preparation of type II anhydrous gypsum through an autoclaving method, a medium and low temperature preparation method. In the preparation method of the present invention, the preparation raw materials only include industrial by-product gypsum raw materials and type II anhydrous gypsum seeds. No inducer and catalyst need to be added during the whole process, and no acid solution needs to be introduced, so no secondary pollution is generated. Moreover, the present invention does not require high-temperature calcination, greatly reducing the preparation cost. It can be seen from the examples of the present invention that by using different parameters of the examples of the present invention, the conversion rate of type II anhydrous gypsum prepared while taking into account energy consumption can be as high as 98.70%, which is an economical and efficient production method.

[0095] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for preparing type II anhydrous gypsum, characterized in that, The preparation method comprises the following steps: Mix the industrial by-product gypsum raw material with type II anhydrous gypsum seeds to obtain a mixture; Autoclave the mixture at a temperature below 200 °C to obtain intermediate powder; Dry the intermediate powder at a drying temperature lower than the autoclaving temperature to obtain type II anhydrous gypsum.

2. The preparation method according to claim 1, characterized in that, Before drying the intermediate powder, the temperature of the intermediate powder is maintained at a temperature higher than or equal to the drying temperature from the end of autoclaving to the start of drying.

3. The preparation method according to claim 1 or 2, characterized in that, The fineness of the type II anhydrous gypsum seeds is 200 - 1500 mesh, preferably 750 - 1500 mesh.

4. The preparation method according to any one of claims 1-3, characterized in that, In the mixture, the mass ratio of the industrial by-product gypsum raw material to the type II anhydrous gypsum seeds is (80 - 95):(20 - 5), preferably (80 - 90):(20 - 10), and further preferably (80 - 85):(20 - 15).

5. The preparation method according to any one of claims 1-4, characterized in that, The autoclaving includes autoclaving the mixture after piling it up in an autoclaving container; Preferably, the autoclaving container is an autoclave reactor; Preferably, the thickness of the piled-up mixture is 5 - 30 cm, preferably 10 - 30 cm.

6. The preparation method according to any one of claims 1-5, characterized in that, The autoclaving temperature is 160 °C - 190 °C, preferably 170 °C - 190 °C, and further preferably 180 °C - 190 °C.

7. The preparation method according to any one of claims 1-6, characterized in that, The autoclaving time is 2 - 6 h, preferably 3 - 6 h, and further preferably 3 - 5 h.

8. The preparation method according to any one of claims 1 to 7, characterized in that, The drying temperature is 130 °C - 160 °C, preferably 140 °C - 160 °C, and further preferably 150 °C - 160 °C.

9. The preparation method according to any one of claims 1-8, characterized in that, The drying time is 1 h - 5 h, preferably 2 h - 4 h.

10. The preparation method according to any one of claims 1-9, characterized in that, The preparation method comprises the following steps: Mix the industrial by-product gypsum raw material with a mass ratio of (80 - 95):(20 - 5) and type II anhydrous gypsum seeds with a fineness of 200 - 1500 mesh, pile up the mixture in an autoclaving container, autoclave it at 160 °C - 190 °C for 3 - 6 h to obtain intermediate powder, and dry the intermediate powder at a drying temperature lower than the autoclaving temperature to prepare type II anhydrous gypsum.

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