Differential recrystallization method for a-type semi-hydrated gypsum, and pressure vessel

Abstract

A differential recrystallization method for an α-type semi-hydrated gypsum, and a pressure vessel. The differential recrystallization method comprises: adding both an industrial by-product gypsum raw material containing attached water and a first crystal modifier to a pressure vessel, and performing a constant-pressure reaction under stirring conditions; and after the constant-pressure reaction is finished, discharging steam in the pressure vessel, wherein the vaporized first crystal modifier is entrained by the steam; introducing heat-source steam having a pressure higher than the internal pressure of the pressure vessel into the pressure vessel, wherein the heat-source steam carries a second crystal modifier and a dispersant, and a stirring procedure during the reaction process comprises first performing stirring for 0.4-0.6 h and then stopping stirring; after the reaction is finished, discharging steam in the pressure vessel, wherein the vaporized second crystal modifier and dispersant are entrained by the steam; and finally, introducing heat-source steam into the pressure vessel to perform drying until generated α-type semi-hydrated gypsum is completely dried. The pressure vessel comprises a housing (1), wherein a heat-source steam pipe coil (2) is disposed outside the housing (1), and heat-source steam for providing reaction heat is introduced into the heat-source steam pipe coil (2); a stirring shaft (3) is horizontally disposed inside the housing (1), and a plurality of stirring blades (4) are disposed perpendicular to the stirring shaft (3); and a discharge port (5) is provided at the bottom of the housing (1), and a feed port (6) and a steam exhaust port (7) are provided at the top of the housing (1).

Description

Differential recrystallization method for α-type hemihydrate gypsum and pressure vessels Technical Field

[0001] This invention relates to the field of calcined gypsum processing technology, and in particular to a differentiated recrystallization method and pressure vessel for α-type hemihydrate gypsum using industrial by-product gypsum as raw material. Background Technology

[0002] Alpha-type hemihydrate gypsum, also known as high-strength gypsum, has a strength more than three times that of building gypsum. Since the preparation of alpha-type hemihydrate gypsum requires formation in a saturated steam medium or aqueous solution, the current consensus is that it is formed through the dissolution and recrystallization of dihydrate gypsum. Specifically, during heat treatment in a saturated steam medium or liquid water medium, dihydrate gypsum first undergoes dehydration. If conditions are suitable, one and a half water of crystallization can be extracted from the dihydrate gypsum lattice, forming hemihydrate basal crystals. In an environment surrounded by liquid water, these basal crystals quickly dissolve in the liquid phase. When the concentration of hemihydrate gypsum in the liquid phase reaches supersaturation, the liquid hemihydrate gypsum rapidly crystallizes, forming coarse and dense alpha-type hemihydrate gypsum.

[0003] The reason why α-type hemihydrate gypsum products have higher strength than β-type hemihydrate gypsum products is mainly due to their larger crystal particles and significantly smaller specific surface area (approximately 2.5 to 5 times smaller) compared to β-type hemihydrate gypsum particles. This greatly reduces the water-to-gypsum ratio and increases the density of the hydrated gypsum products. Experiments show that the morphology of α-type hemihydrate gypsum crystals has a significant impact on the standard consistency water requirement, causing the standard consistency to fluctuate between 30% and 80%. The fine needle-like crystal morphology is the worst, requiring the most standard consistency water and resulting in the lowest strength. Conversely, the coarse, short columnar or near-cubic crystal morphology is the best, as its smaller specific surface area allows for a reduction in the standard consistency water requirement, leading to increased density and strength of the product.

[0004] Therefore, to achieve a better crystal morphology, various types of crystal form modifiers are often added during production to improve the crystal morphology of α-type hemihydrate gypsum. Different crystal form modifiers are adsorbed at the interfaces of the gypsum crystals. Consequently, the differential effects of different crystal form modifiers result in significant differences in the morphology, size, and properties of hemihydrate gypsum crystals. However, for α-type hemihydrate gypsum crystals, if large crystals of the same shape are formed, larger pores will form between the crystals, thus affecting the density of the α-type hemihydrate gypsum after hydration into dihydrate gypsum, and consequently affecting the strength of α-type hemihydrate gypsum products.

[0005] To produce gypsum products with excellent strength, hardness, and wear resistance, it is necessary to reduce porosity and increase density. This requires improving the gradation of α-type hemihydrate gypsum particles, i.e., rationally preparing hemihydrate gypsum particles of various sizes to fill the original particle voids and minimize porosity. Currently, the common method is to further grind the dried α-type hemihydrate gypsum to adjust its particle size to the optimal gradation. However, this grinding method increases the number of processing steps and production costs, and also reduces the volume of the hemihydrate gypsum particles, resulting in an increase in their specific surface area. Therefore, based on the principle that crystal form modifiers can improve the crystal morphology of α-type hemihydrate gypsum under certain conditions, this application proposes a differentiated recrystallization technology for α-type hemihydrate gypsum and a corresponding production device. Summary of the Invention

[0006] The purpose of this invention is to provide a differentiated recrystallization method for α-type hemihydrate gypsum and a pressure vessel. By employing a differentiated recrystallization method during the production process, the α-type hemihydrate gypsum becomes denser and has fewer gaps when hydrated into dihydrate gypsum.

[0007] To achieve the above objectives, according to one aspect of the present invention, a method for differential recrystallization of α-type hemihydrate gypsum is provided, comprising:

[0008] Step 1, first crystallization;

[0009] a. Add the industrial by-product gypsum raw material containing attached water and the first crystal modifier into a pressure vessel, stir the materials and heat them;

[0010] b. The water adhering to the industrial by-product gypsum in the pressure vessel is converted into steam. The steam pressure in the pressure vessel is maintained at 0.30-0.34 MPa and the temperature at 130-140°C. The reaction is carried out under constant pressure for 1-2 hours with continuous stirring.

[0011] c. After the constant pressure reaction is completed, the steam in the pressure vessel is discharged and the vaporized first crystal corrector is carried out by the steam;

[0012] Step two, second crystallization;

[0013] a. Introducing heat source steam into a pressure vessel at a pressure higher than the internal pressure of the pressure vessel, the heat source steam carrying a second crystal modifier and a dispersant;

[0014] b. Maintain the steam pressure in the pressure vessel at 0.30-0.34 MPa and the temperature at 130-140°C, and react under constant pressure for 1-2 hours. The stirring procedure during the constant pressure reaction is to stir for 0.4-0.6 hours first, and then stop stirring.

[0015] c. After the constant pressure reaction is completed, start stirring again to remove the steam from the pressure vessel and carry out the vaporized second crystal modifier and dispersant.

[0016] Step 3, drying;

[0017] A steam heat source is introduced into the pressure vessel, and the drying temperature in the pressure vessel is maintained at 105°C to 115°C until the generated α-type hemihydrate gypsum is completely dry.

[0018] In a preferred embodiment, the heat source steam is saturated steam with a pressure of 0.9 MPa to 1.0 MPa and a temperature of 180°C to 195°C.

[0019] In a preferred embodiment, in step one b, the steam pressure is 0.32 MPa, the temperature is 135°C, and the constant pressure reaction time is 1.5 hours.

[0020] In a preferred embodiment, in step one c, the first crystal corrector in the pressure vessel is discharged in two stages. First, steam is discharged to reduce the steam pressure in the vessel to 0.1 MPa, and the discharged steam carries away a portion of the vaporized first crystal corrector. Then, heat source steam with a pressure higher than the internal pressure of the pressure vessel is introduced into the pressure vessel, and steam is discharged again to reduce the steam pressure in the vessel to 0.1 MPa, and the remaining vaporized first crystal corrector is discharged.

[0021] In a preferred embodiment, in step twoa, the amounts of the second crystal modifier and the dispersant added are 0.1% and 0.01% of the mass of the industrial by-product gypsum raw material in the pressure vessel, respectively.

[0022] In a preferred embodiment, in step three, the drying temperature is maintained at 110°C.

[0023] In a preferred embodiment, after the α-type hemihydrate gypsum is completely dried, it is discharged from the stirring reaction pressure vessel, and particles larger than 50 mesh are screened out. The remaining particles become the α-type hemihydrate gypsum finished product with uniform gradation formed by differential crystallization.

[0024] According to another aspect of the present invention, a pressure vessel for the above method is provided, comprising a shell, a heat source steam coil disposed outside the shell, heat source steam for providing heat for the reaction being passed through the heat source steam coil; a stirring shaft is disposed laterally inside the shell, and a plurality of stirring blades are disposed perpendicular to the stirring shaft; a discharge port is opened at the bottom of the shell, and a feed port and a steam vent are opened at the top of the shell.

[0025] In a preferred embodiment, the top of the shell is connected to a crystal correction agent replenishment device, which includes a crystal correction agent storage tube. The two ends of the crystal correction agent storage tube are respectively connected to the pressure vessel shell and a steam source, and the top of the crystal correction agent storage tube is provided with a crystal correction agent addition port.

[0026] In a preferred embodiment, an axial flow fan for exhaust is provided on the housing.

[0027] The crystals obtained by the method described in this invention have different shapes such as short columnar, long columnar, and hexagonal due to differentiated crystallization, and the crystal sizes vary and the gradation is good. As a result, when this type of α-hemihydrate gypsum is hydrated into dihydrate gypsum, it is more compact and has fewer gaps. Compared with the type of α-hemihydrate gypsum produced from the same grade of raw materials, the differentiated recrystallized type of α-hemihydrate gypsum has higher strength, generally more than 30%.

[0028] The α-type hemihydrate gypsum generated by this invention does not require grinding, so the original crystal correction agent film on its crystal surface is still intact, thereby passivating the hydration reaction between water and α-type hemihydrate gypsum. Objectively, this results in a relatively long setting time for this type of α-type hemihydrate gypsum, about one hour, which is just right for preparing gypsum-based self-leveling mortar and plastering mortar. This eliminates the need to add retarders, water-reducing agents, and adhesive powder when preparing these two types of building mortar, thereby reducing production costs. Attached Figure Description

[0029] The accompanying drawings, which are provided to further illustrate the invention and form part of this application, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention.

[0030] Figure 1 is a structural schematic diagram of the pressure vessel of the present invention;

[0031] Figure 2 shows the crystal corrector replenishment device of the present invention;

[0032] Figure 3 is an electron microscope image of the α-type hemihydrate gypsum crystals obtained by the present invention.

[0033] In the figure, 1-shell, 2-heat source steam coil, 3-stirring shaft, 4-stirring blade, 5-discharge port, 6-feed port, 7-steam exhaust port, 8-crystal corrector replenishment device, 9-crystal corrector storage pipe, 10-crystal corrector addition port, 11-first valve, 12-second valve, 13-third valve. Detailed Implementation

[0034] The differential recrystallization method for α-type hemihydrate gypsum provided in this invention includes a first crystallization, a second crystallization, and drying. The implementation of the above steps involves an improved pressure vessel and a crystal correction agent replenishment device.

[0035] As shown in Figure 1, the pressure vessel includes a shell 1, a heat source steam coil 2 is provided on the outside of the shell 1, and heat source steam for providing heat for the reaction is passed through the heat source steam coil 2; a stirring shaft 3 is arranged horizontally inside the shell 1, and multiple stirring blades 4 are arranged perpendicular to the stirring shaft 3; a discharge port 5 is opened at the bottom of the shell 1, and a feed port 6 and a steam exhaust port 7 are opened at the top of the shell 1.

[0036] In this invention, industrial by-product gypsum raw material containing attached water and a first crystal modifier are added into the pressure vessel through the feed inlet. The stirring shaft 3 and stirring blades 4 are used to provide continuous or intermittent stirring conditions.

[0037] In this invention, the external heat for the reaction is provided by heat source steam. The heat source steam is preferably saturated steam with a pressure of 0.9 MPa to 1.0 MPa and a temperature of 180°C to 195°C. The heat source steam is introduced into the heat source steam coil 2, and then the heat source steam coil 2 conducts heat to the inside of the pressure vessel to heat the reaction material inside the pressure vessel.

[0038] A support is provided at the bottom of the casing 1. Valves are provided on the feed inlet 6, discharge outlet 5, and exhaust outlet 7 respectively.

[0039] In addition, an axial flow fan is also provided on the shell 1. The axial flow fan and the exhaust port 7 constitute a dry exhaust system for quickly discharging steam from the pressure vessel. Since the discharged steam contains a certain amount of α-type hemihydrate gypsum, it will form adhesive clumps on the damp blades of the axial flow fan, affecting the operation of the axial flow fan blades. Therefore, an anti-sticking layer mainly composed of polytetrafluoroethylene (PTFE) is coated on the blades of the axial flow fan to prevent the formation of adhesive clumps.

[0040] In the second crystallization process, a second crystal modifier and a dispersant need to be added. Therefore, the present invention designs a crystal modifier replenishment device that can deliver the second crystal modifier and dispersant into the pressure vessel by steam when there is pressure inside the pressure vessel.

[0041] As shown in Figure 1, the crystal correction agent replenishment device 8 is located on the top of the shell 1 and communicates with the inside of the pressure vessel. As shown in Figure 2, the crystal correction agent replenishment device 8 includes a crystal correction agent storage tube 9, with its two ends connected to the pressure vessel shell and a steam source, respectively. The top of the crystal correction agent storage tube 9 is provided with a crystal correction agent addition port 10.

[0042] A first valve 11 is provided at the crystal correction agent addition port 10, a second valve 12 is provided at the end of the crystal correction agent storage pipe 9 connected to the steam source, and a third valve 13 is provided at the end of the crystal correction agent storage pipe 9 connected to the pressure vessel shell 1.

[0043] The differential recrystallization method for α-type hemihydrate gypsum provided by some typical embodiments of the present invention includes the following steps.

[0044] Step 1, first crystallization.

[0045] a. Add the industrial by-product gypsum raw material containing attached water and the first crystal modifier into a pressure vessel, stir the materials and heat them.

[0046] The content of attached water is about 13%. The industrial by-product gypsum raw material and the first crystal modifier are loaded into the pressure vessel at the same time. Meanwhile, the stirring shaft 3 drives the stirring blades 4 to start continuously stirring the material, and the material in the stirring reaction pressure vessel is heated by the steam heating coil.

[0047] The first corrective agent is mainly a surfactant and an organic acid, such as sodium dodecylbenzenesulfonate at 0.05% of the mass of industrial by-product gypsum raw material and succinic acid at 0.01%.

[0048] b. The water adhering to the industrial by-product gypsum inside the pressure vessel is converted into steam. The steam pressure inside the pressure vessel is maintained at 0.30-0.34 MPa and the temperature at 130-140°C. The reaction is carried out under constant pressure for 1-2 hours with continuous stirring.

[0049] After the materials are added, close the valve at inlet 6. As the pressure vessel is continuously heated, the water adhering to the industrial by-product gypsum is converted into steam, generating steam pressure inside the vessel. When the internal steam pressure reaches approximately 0.32 MPa and the temperature reaches approximately 135°C, the pressure inside the pressure vessel is controlled at 0.30-0.34 MPa and the temperature at 130-140°C by controlling the supply of steam heat source. The reaction is carried out at constant pressure for 1-2 hours under continuous stirring. Preferably, the pressure inside the stirred pressure vessel is maintained at approximately 0.32 MPa by controlling the supply of steam heat source, and this constant pressure is maintained for 1.5 hours.

[0050] c. After the constant pressure reaction is completed, the steam in the pressure vessel is discharged, and the vaporized first crystal corrector is carried out by the steam.

[0051] In a preferred embodiment, this step involves discharging the first crystalline corrector from the pressure vessel in two stages. First, steam is discharged to reduce the steam pressure inside the vessel to 0.1 MPa, carrying away a portion of the vaporized first crystalline corrector. Then, heat source steam, with a pressure higher than the internal pressure of the pressure vessel, is introduced into the pressure vessel. Steam is discharged again to reduce the steam pressure inside the vessel to 0.1 MPa, discharging the remaining vaporized first crystalline corrector.

[0052] Step two, second crystallization.

[0053] a. Introducing heat source steam into a pressure vessel at a pressure higher than the internal pressure of the pressure vessel, the heat source steam carrying a second crystal modifier and a dispersant.

[0054] The second crystal modifier is a sulfate, such as aluminum sulfate. Sodium hexametaphosphate can be used as the dispersant. The amounts of the second crystal modifier and the dispersant added are 0.1% and 0.01% of the mass of the industrial by-product gypsum raw material in the pressure vessel, respectively.

[0055] In this step, following the direction of the heat source steam flowing towards the stirred reaction pressure vessel, the steam valves (second and third valves) at both ends of the crystal correction agent replenishment device are opened sequentially. Heat source steam, exceeding the internal pressure of the stirred reaction pressure vessel, is introduced into the pressure vessel. This steam carries the second crystal correction agent into the stirred reaction pressure vessel, initiating a second crystallization (recrystallization) process that differs from or is different from the crystal growth direction of the first α-type hemihydrate gypsum. The first and second crystal correction agents influence the crystals from different perspectives.

[0056] b. Maintain the steam pressure inside the pressure vessel at 0.30-0.34 MPa and the temperature at 130-140°C, and conduct the constant-pressure reaction for 1-2 hours. During the constant-pressure reaction, the stirring procedure is to stir for 0.4-0.6 hours first, and then stop stirring. After stirring stops, the pressure inside the pressure vessel needs to be prevented from dropping below 0.3 MPa by supplying steam through the heat source steam coil.

[0057] In a preferred embodiment, the stirring procedure during the constant pressure reaction is to stir for 0.5 hours, then stop stirring for 1 hour, for a total constant pressure reaction time of 1.5 hours.

[0058] In this step, stirring is continued for 0.5 hours under the conditions of steam pressure of 0.30-0.34 MPa and temperature of 130-140°C inside the pressure vessel, and then stirring is stopped for one hour. This creates a temperature gradient from the center to the shell inside the pressure vessel, causing the activity of the second crystal modifier to vary under different temperature conditions. This results in different growth rates of α-type hemihydrate gypsum crystals from the center to the shell inside the pressure vessel, as well as differences in growth direction compared to the second crystal modifier. Under the dual conditions of different crystal modifiers and different temperature effects, differentiated crystallization of α-type hemihydrate gypsum is formed, ultimately resulting in α-type hemihydrate gypsum with differentiated recrystallization.

[0059] c. After the constant pressure reaction is complete, start stirring again to release the steam from the pressure vessel, which will carry away the vaporized second crystal modifier and dispersant. When the steam pressure in the pressure vessel drops to zero, start the drying and venting device to quickly remove the remaining water vapor from the pressure vessel.

[0060] Step 3: Drying.

[0061] A steam heat source is introduced into the pressure vessel, and the drying temperature in the pressure vessel is maintained at 105°C to 115°C until the generated α-type hemihydrate gypsum is completely dry.

[0062] In a preferred embodiment, the pressure vessel is maintained at a drying temperature of 110°C.

[0063] Finally, after the α-type hemihydrate gypsum is completely dried, it is discharged from the stirring reaction pressure vessel. Particles larger than 50 mesh are screened out, and the remaining particles become the α-type hemihydrate gypsum finished product with uniform gradation formed by differential crystallization.

[0064] The differential recrystallization method for α-type hemihydrate gypsum claimed in this invention will be further clearly and completely described below through some relatively specific embodiments. Example 1

[0065] S1: Industrial by-product gypsum raw material containing attached water and the first crystal modifier are simultaneously loaded into the pressure vessel through the feed inlet 6. The stirring shaft 3 drives the stirring blades 4 to continuously stir the material. The material in the pressure vessel is heated by the heat source steam coil 2. The heat source steam used for heating is saturated steam with a pressure of 0.9 MPa and a temperature of 180°C.

[0066] S2: After the material is added, close the valve at the feed inlet 6. The water adhering to the industrial by-product gypsum inside the pressure vessel is converted into steam. When the steam pressure inside the vessel reaches 0.32 MPa and the temperature reaches 135°C, the pressure inside the pressure vessel is then controlled to maintain at 0.32 MPa by controlling the supply of steam heat source. The pressure is maintained at 0.32 MPa for 1.5 hours.

[0067] S3: Open the pressure relief valve at the pressure vessel exhaust port 7 to release the steam inside the pressure vessel, reducing the steam pressure inside the pressure vessel to 0.1 MPa. At the same time, the released steam will carry away the vaporized first crystal corrector. Then, close the pressure relief valve at the exhaust port 7.

[0068] The second valve 12 and the third valve 13 at both ends of the crystal correction agent replenishment device 8 are opened sequentially to input heat source steam higher than the internal pressure of the pressure vessel into the pressure vessel, so that the pressure inside the pressure vessel is 0.32 MPa. Then, the pressure reducing and steam venting valve of the exhaust port 7 is opened again to release steam, reducing the steam pressure inside the pressure vessel to 0.1 MPa, thereby further discharging the remaining vaporized first crystal correction agent, and then the pressure reducing and steam venting valve is closed.

[0069] S4: Add 0.1% of the weight of the industrial by-product gypsum raw material as a second crystal corrector and 0.01% of the dispersant through the first valve 11 of the corrector addition port 10 on the crystal corrector replenishing device 8, and then close the first valve.

[0070] S5: Sequentially open the second and third valves at both ends of the crystal correction agent replenishment device to input heat source steam higher than the internal pressure of the pressure vessel into the pressure vessel. This steam carries the second crystal correction agent into the stirred reaction pressure vessel to carry out a second crystallization (recrystallization) that differs from or is different from the crystal growth direction of the first α-type hemihydrate gypsum. When the steam pressure inside the pressure vessel reaches 0.32 MPa, sequentially close the third valve 13 and the second valve 12.

[0071] S6: After stirring for 0.5 hours at a steam pressure of 0.32 MPa inside the pressure vessel, stirring is stopped for 1 hour to form α-type hemihydrate gypsum with differentiated recrystallization. After stirring is stopped, the pressure inside the pressure vessel is prevented from dropping below 0.3 MPa by adjusting the supply of steam from the heat source steam coil 2.

[0072] S7: After stopping stirring for one hour, restart stirring and heating, and at the same time open the pressure relief valve of the pressure vessel exhaust port 7 to release the steam pressure in the pressure vessel and discharge the vaporized second crystal corrector and dispersant; when the steam pressure in the pressure vessel drops to zero, start the drying exhaust device to quickly discharge the remaining water vapor in the reactor.

[0073] S8: By controlling the supply of steam heat source to the heat source steam coil 2, maintain the drying temperature at 110°C in the pressure vessel until the generated α-type hemihydrate gypsum is completely dried.

[0074] S9: The dried α-type hemihydrate gypsum is discharged from the pressure vessel, and particles larger than 50 mesh are screened out. The remaining particles become α-type hemihydrate gypsum products with uniform gradation formed by differential crystallization.

[0075] Figure 3 shows a crystal electron microscope image of the differential recrystallization of α-type hemihydrate gypsum obtained in Example 1. The finished α-type hemihydrate gypsum has short columnar, long columnar and hexagonal shapes, with different crystal sizes and good gradation. Example 2

[0076] S1: Industrial by-product gypsum raw material containing attached water and the first crystal modifier are simultaneously loaded into the pressure vessel through the feed port 6. The stirring shaft 3 drives the stirring blades 4 to continuously stir the material. The material in the pressure vessel is heated by the heat source steam coil 2. The heat source steam pressure is 1.0 MPa and the temperature is 195°C saturated steam.

[0077] S2: After the material is added, close the valve at the feed port 6. The water adhering to the industrial by-product gypsum inside the pressure vessel is converted into steam. When the steam pressure inside the vessel reaches 0.30 MPa and the temperature reaches 130°C, the pressure inside the pressure vessel is then controlled to maintain at 0.30 MPa by controlling the supply of steam heat source. The pressure is maintained at 0.30 MPa for 1.5 hours.

[0078] S3: Open the pressure relief valve at the pressure vessel exhaust port 7 to release the steam inside the pressure vessel, reducing the steam pressure inside the pressure vessel to 0.1 MPa. At the same time, the released steam will carry away the vaporized first crystal corrector. Then, close the pressure relief valve at the exhaust port 7.

[0079] The second valve 12 and the third valve 13 at both ends of the crystal correction agent replenishment device 8 are opened sequentially to input heat source steam higher than the internal pressure of the pressure vessel, so that the pressure inside the pressure vessel is 0.30 MPa. Then, the pressure reducing and steam venting valve of the exhaust port 7 is opened again to release steam, reducing the steam pressure inside the pressure vessel to 0.1 MPa, thereby further releasing the remaining vaporized first crystal correction agent. Then, the pressure reducing and steam venting valve is closed.

[0080] S4: Add 0.1% of the weight of the industrial by-product gypsum raw material as a second crystal corrector and 0.01% of the dispersant through the first valve 11 of the corrector addition port 10 on the crystal corrector replenishing device 8, and then close the first valve.

[0081] S5: Sequentially open the second and third valves at both ends of the crystal correction agent replenishment device to input heat source steam higher than the internal pressure of the pressure vessel. This steam carries the second crystal correction agent into the stirred reaction pressure vessel, implementing a second crystallization (recrystallization) that differs from or is different from the crystal growth direction of the first α-type hemihydrate gypsum. When the steam pressure inside the pressure vessel reaches 0.30 MPa, sequentially close the third valve 13 and the second valve 12.

[0082] S6: After stirring for 0.5 hours under a steam pressure of 0.30 MPa inside the pressure vessel, stirring is stopped for 1 hour to form α-type hemihydrate gypsum with differentiated recrystallization. After stirring is stopped, the pressure inside the pressure vessel is prevented from dropping below 0.3 MPa by adjusting the supply of steam from the heat source steam coil 2.

[0083] S7: After stopping stirring for one hour, restart stirring and heating, and at the same time open the pressure relief valve of the pressure vessel exhaust port 7 to release the steam pressure in the pressure vessel and discharge the vaporized second crystal corrector and dispersant; when the steam pressure in the pressure vessel drops to zero, start the drying exhaust device to quickly discharge the remaining water vapor in the reactor.

[0084] S8: By controlling the supply of steam heat source to the heat source steam coil 2, maintain the drying temperature at 105°C in the pressure vessel until the generated α-type hemihydrate gypsum is completely dried.

[0085] S9: The dried α-type hemihydrate gypsum is discharged from the pressure vessel, and particles larger than 50 mesh are screened out. The remaining particles become α-type hemihydrate gypsum products with uniform gradation formed by differential crystallization. Example 3

[0086] S1: Industrial by-product gypsum raw material containing attached water and the first crystal modifier are simultaneously fed into the pressure vessel through the feed inlet 6. The stirring shaft 3 drives the stirring blades 4 to continuously stir the material. The material in the pressure vessel is heated by the heat source steam coil 2. The heat source steam pressure is 1.0 MPa and the temperature is 190°C saturated steam.

[0087] S2: After the material is added, close the valve at the feed port 6. The water adhering to the industrial by-product gypsum inside the pressure vessel is converted into steam. When the steam pressure inside the vessel reaches 0.34 MPa and the temperature reaches 140°C, the pressure inside the pressure vessel is then controlled to maintain at 0.34 MPa by controlling the supply of steam heat source. The pressure is maintained at 0.34 MPa for 1.5 hours.

[0088] S3: Open the pressure relief valve at the pressure vessel exhaust port 7 to release the steam inside the pressure vessel, reducing the steam pressure inside the pressure vessel to 0.1 MPa. At the same time, the released steam will carry away the vaporized first crystal corrector. Then, close the pressure relief valve at the exhaust port 7.

[0089] The second valve 12 and the third valve 13 at both ends of the crystal correction agent replenishment device 8 are opened sequentially to input heat source steam higher than the internal pressure of the pressure vessel, so that the pressure inside the pressure vessel is 0.34 MPa. Then, the pressure reducing and steam venting valve of the exhaust port 7 is opened again to release steam, reducing the steam pressure inside the pressure vessel to 0.1 MPa, thereby further releasing the remaining vaporized first crystal correction agent. Then, the pressure reducing and steam venting valve is closed.

[0090] S4: Add 0.1% of the weight of the industrial by-product gypsum raw material as a second crystal corrector and 0.01% of the dispersant through the first valve 11 of the corrector addition port 10 on the crystal corrector replenishing device 8, and then close the first valve.

[0091] S5: Sequentially open the second and third valves at both ends of the crystal correction agent replenishment device to input heat source steam higher than the internal pressure of the pressure vessel into the pressure vessel. This steam carries the second crystal correction agent into the stirred reaction pressure vessel to carry out a second crystallization (recrystallization) that differs from or is different from the crystal growth direction of the first α-type hemihydrate gypsum. When the steam pressure inside the pressure vessel reaches 0.34 MPa, sequentially close the third valve 13 and the second valve 12.

[0092] S6: After stirring for 0.5 hours at a steam pressure of 0.34 MPa inside the pressure vessel, stirring is stopped for 1 hour to form α-type hemihydrate gypsum with differentiated recrystallization. After stirring is stopped, the pressure inside the pressure vessel is prevented from dropping below 0.3 MPa by adjusting the supply of steam from the heat source steam coil 2.

[0093] S7: After stopping stirring for one hour, restart stirring and heating, and at the same time open the pressure relief valve of the pressure vessel exhaust port 7 to release the steam pressure in the pressure vessel and discharge the vaporized second crystal corrector and dispersant; when the steam pressure in the pressure vessel drops to zero, start the drying exhaust device to quickly discharge the remaining water vapor in the reactor.

[0094] S8: By controlling the supply of steam heat source to the steam coil 2, maintain the drying temperature at 115°C in the pressure vessel until the generated α-type hemihydrate gypsum is completely dried.

[0095] S9: The dried α-type hemihydrate gypsum is discharged from the pressure vessel, and particles larger than 50 mesh are screened out. The remaining particles become α-type hemihydrate gypsum products with uniform gradation formed by differential crystallization.

[0096] The scope of protection claimed by this invention is not limited to the specific embodiments described above. For those skilled in the art, this invention can have various modifications and alterations. Any modifications, improvements, and equivalent substitutions made within the concept and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A differential recrystallization method for α-type hemihydrate gypsum, characterized in that, include: Step 1, first crystallization; a. Add the industrial by-product gypsum raw material containing attached water and the first crystal modifier into a pressure vessel, stir the materials and heat them; b. The water adhering to the industrial by-product gypsum in the pressure vessel is converted into steam. The steam pressure in the pressure vessel is maintained at 0.30-0.34 MPa and the temperature at 130-140°C. The reaction is carried out under constant pressure for 1-2 hours with continuous stirring. c. After the constant pressure reaction is completed, the steam in the pressure vessel is discharged and the vaporized first crystal corrector is carried out by the steam; Step two, second crystallization; a. Introducing heat source steam into a pressure vessel at a pressure higher than the internal pressure of the pressure vessel, the heat source steam carrying a second crystal modifier and a dispersant; b. Maintain the steam pressure in the pressure vessel at 0.30-0.34 MPa and the temperature at 130-140°C, and react under constant pressure for 1-2 hours. The stirring procedure during the constant pressure reaction is to stir for 0.4-0.6 hours first, and then stop stirring. c. After the constant pressure reaction is completed, start stirring again to remove the steam from the pressure vessel and carry out the vaporized second crystal modifier and dispersant. Step 3, drying; A steam heat source is introduced into the pressure vessel, and the drying temperature in the pressure vessel is maintained at 105°C to 115°C until the generated α-type hemihydrate gypsum is completely dry.

2. The method according to claim 1, characterized in that: The heat source steam is saturated steam with a pressure of 0.9 MPa to 1.0 MPa and a temperature of 180°C to 195°C.

3. The method according to claim 1 or 2, characterized in that: In step 1b, the steam pressure is 0.32 MPa, the temperature is 135°C, and the constant pressure reaction time is 1.5 hours.

4. The method according to claim 3, characterized in that: In step 1c, the first crystal corrector in the pressure vessel is discharged in two stages. First, steam is discharged to reduce the steam pressure in the vessel to 0.1 MPa, and the discharged steam carries away part of the vaporized first crystal corrector. Then, heat source steam with a pressure higher than the internal pressure of the pressure vessel is introduced into the pressure vessel, and steam is discharged again to reduce the steam pressure in the vessel to 0.1 MPa, and the remaining vaporized first crystal corrector is discharged.

5. The method according to claim 1 or 4, characterized in that: In step 2a, the amounts of the second crystal modifier and dispersant added are 0.1% and 0.01% of the mass of the industrial by-product gypsum raw material in the pressure vessel, respectively.

6. The method according to claim 5, characterized in that: In step three, maintain the drying temperature at 110°C.

7. The method according to claim 6, characterized in that: After the α-type hemihydrate gypsum is completely dried, it is discharged from the stirring reaction pressure vessel. Particles larger than 50 mesh are screened out, and the remaining particles become the α-type hemihydrate gypsum finished product with uniform gradation formed by differential crystallization.

8. A pressure vessel for use in any one of claims 1 to 7, characterized in that, The device includes a shell, an external heat source steam coil, and heat source steam for providing heat for the reaction. Inside the shell, a stirring shaft is arranged horizontally, and multiple stirring blades are arranged perpendicular to the stirring shaft. A discharge port is opened at the bottom of the shell, and a feed port and a steam exhaust port are opened at the top of the shell.

9. The pressure vessel according to claim 8, characterized in that: The top of the shell is connected to a crystal correction agent replenishment device, which includes a crystal correction agent storage tube. The two ends of the crystal correction agent storage tube are respectively connected to the pressure vessel shell and a steam source, and the top of the crystal correction agent storage tube is provided with a crystal correction agent addition port.

10. The pressure vessel according to claim 8 or 9, characterized in that: An axial flow fan for exhaust is installed on the casing.

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