A precise calcination method of building gypsum powder phase composition

Abstract

This invention relates to the field of calcium sulfate cementitious material preparation technology, specifically to a precise calcination method for the phase composition of building gypsum powder. The invention discloses a precise calcination method for the phase composition of building gypsum powder, comprising: washing, drying, and homogenizing industrial solid waste gypsum, and then feeding it into a two-stage series fluidized bed calciner; calculating the partial pressure of water vapor in the dehydration section in real time based on material balance and adjusting the exhaust damper; simultaneously sampling and measuring the water of crystallization every 10 minutes and using feedback to control the calcination temperature, so that the dehydration operation point is locked in the thermodynamically stable zone of β-hemihydrate gypsum; the calcined product is then ground after short-range aging in a fluidized bed aging chamber to obtain building gypsum powder. This invention achieves precise control of phase composition through a dual feedback closed loop of water vapor partial pressure and water of crystallization. The resulting building gypsum powder has a hemihydrate gypsum content ≥90%, soluble anhydrous gypsum ≤6%, small fluctuation in setting time, and good batch consistency, making it a high-quality cementitious base material for lightweight building materials.

Description

Technical Field

[0001] This invention relates to the field of calcium sulfate cementitious material preparation technology, specifically to a precise calcination method for the composition of building gypsum powder. Background Technology

[0002] Building gypsum is typically produced by calcining dihydrate gypsum, with β-hemihydrate gypsum as the main cementing phase. During calcination, insufficient dehydration will leave residual dihydrate gypsum; excessive dehydration will produce soluble anhydrous gypsum. Existing patent literature indicates that the properties of building gypsum are related to the ratio of the three phases: hemihydrate gypsum, soluble anhydrous gypsum, and undried dihydrate gypsum. Phase composition analysis and control are crucial aspects of quality control.

[0003] For example, CN118047551A discloses an energy-saving ultra-low pressure steam calcination gypsum fluidized bed furnace control method. This method uses a superheated steam flow valve, a bypass cold water valve, and a control position temperature to maintain the steam temperature at the control position within ±2℃ of the saturated steam temperature, thus ensuring the gypsum calcination quality. This scheme focuses on the interlocked control of steam temperature and furnace bottom pressure, but does not disclose real-time calculation of bed steam partial pressure based on dehydrated material balance, nor does it disclose closed-loop correction of phase composition using rapid detection of powder crystal water. Due to the lack of these technical features, the existing scheme still struggles to solve problems such as uncontrollable phase composition, detection lag, fluctuations in dehydration driving force, and long aging cycles during the calcination of industrial solid waste gypsum, and cannot achieve the directional conversion and continuous stable production of β-hemihydrate gypsum.

[0004] In summary, how to innovate a new calcination method that can sense and control the partial pressure of water vapor in real time during dehydration, use the water of crystallization as a feedback signal to close the loop and correct the phase composition, and significantly shorten the aging cycle has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a precise calcination method for the phase composition of building gypsum powder, so as to solve the technical problems of uncontrollable phase composition and poor batch stability in the prior art.

[0006] The specific technical solution is as follows: A method for precisely calcining building gypsum powder phase composition includes the following steps: S1: Phosphogypsum is washed with water, filtered, pre-dried, dispersed, and the free water content of the discharged material is controlled to be 2.5%~3.5%, and then homogenized and stored.

[0007] S2: Homogenized material is fed into the calcining furnace, passing through a preheating and free water removal section and a crystal water dehydration section. The operating gas velocity is controlled at 1.8 times the critical fluidization velocity. Based on the feed moisture content, exhaust air volume, and exhaust temperature, the water vapor partial pressure is calculated in real time, and the exhaust damper is adjusted to stabilize the partial pressure within the target range. Simultaneously, the crystal water content is measured every 10 minutes, and the calcination temperature is adjusted accordingly. The calcined product is air-cooled.

[0008] S3: The calcined product is sent to the aging chamber for fluidized bed aging. The gas velocity, chamber temperature and relative humidity are controlled for two-stage aging. During the process, pulsed strong fluidization is used to prevent agglomeration.

[0009] S4: The aged product is ground, homogenized, and packaged to obtain the finished building gypsum powder.

[0010] Further, in S1, the water washing temperature is set to 35~45℃, and the washing time is set to 8~15 minutes; the pressure filtration is required to filter to a moisture content of 22%~28%; the pre-drying conditions are set to a rotation speed of 15~25 rpm, a residence time of 5~8 minutes, and a pre-drying to a moisture content of 5%~8%; the dispersing is set to a rotation speed of 2000~3000 rpm; the control of the free water content of the discharge is set to a hot air temperature of 120~150℃, an air velocity of 10~15 m / s, and a material residence time of 1~3 seconds; the homogenization storage is set to an aeration pressure of 0.2~0.3 MPa, loosening for 3~8 minutes every 3~5 hours, and homogenization storage for 8~16 hours.

[0011] Furthermore, the conditions for the preheating and dewatering section described in S2 are set as follows: temperature 120~140℃, material layer height 350~450mm; and the conditions for the crystal water dewatering section are set as follows: temperature 150~160℃, material layer height 350~450mm, residence time 10~15 minutes.

[0012] Furthermore, in the two-stage aging process described in S3, the conditions for the first chamber are set as follows: gas velocity 0.1~0.2 m / s, chamber temperature 25~35℃, relative humidity 70%~90%, and residence time 2~4 hours; the conditions for the second chamber are set as follows: gas velocity 0.1~0.2 m / s, chamber temperature 22~32℃, relative humidity 50%~70%, and residence time 2~4 hours; the pulsed high-pressure fluidization needs to be automatically triggered every 20~40 minutes, with the gas velocity instantly increasing to 0.5~0.8 m / s and lasting for 30~60 seconds.

[0013] Furthermore, in the grinding described in S4, the filling rate is set to 30%~35%, the cylinder rotation speed is 65%~75% of the critical speed, the grinding speed is set to 100~120 rpm, and the grinding needs to achieve a specific surface area of ​​2800~3200 cm². 2 / g, the milling temperature is set to 50~55℃; the homogenization time is set to 1~3 hours.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) Precise and controllable phase composition: Through the double feedback closed loop of water vapor partial pressure and crystal water, the operating point of the dehydration section is locked in the thermodynamic stability zone of β-hemihydrate gypsum, which effectively inhibits the excessive generation of soluble anhydrous gypsum and makes the phase composition of the product stable and controllable.

[0015] (2) Good batch consistency: Using water of crystallization as a substitute indicator, the feedback correction is rapid every 10 minutes, eliminating the impact of raw material fluctuations on the calcination process and significantly improving the consistency of product performance between different batches.

[0016] (3) Significantly improved production efficiency: fluidized bed aging replaces natural stacking aging, and pulsed strong fluidization anti-caking design greatly shortens the aging cycle and realizes continuous production of calcination-aging.

[0017] (4) Strong adaptability to raw materials: Soluble impurities are removed by water washing pretreatment and combined with water vapor partial pressure adaptive feedforward control, which is suitable for various industrial solid waste gypsum such as phosphogypsum, desulfurized gypsum, and titanium gypsum. Attached Figure Description

[0018] Figure 1 This is a flowchart of a method for precisely calcining building gypsum powder according to the present invention.

[0019] Figure 2 The X-ray diffraction (XRD) spectra of building plaster powder in Examples 1-5 and Comparative Examples 1-4 of this invention are shown. Detailed Implementation

[0020] This invention proposes a precise calcination method for the phase composition of building gypsum powder, such as... Figure 1 The diagram shows a flowchart of a method for precisely calcining building gypsum powder according to the present invention, and the detailed preparation steps are as follows: 1. Raw material processing Industrial solid waste gypsum is washed with water to remove soluble phosphorus, fluorine, and organic impurities. If these impurities remain in the gypsum, they can interfere with dehydration kinetics, causing a shift in the temperature window for hemihydrate gypsum formation during calcination. After washing, the material is pressure filtered and dried to a free water content of 2.5%–3.5%. Removing free water prevents bed temperature fluctuations and uncontrolled water vapor partial pressure due to excessive water evaporation during subsequent calcination stages. Homogenized storage buffers the compositional fluctuations between different batches of raw materials, providing stable feeding conditions for precise subsequent control.

[0021] 2. Precision calcination The homogenized material enters the calcination furnace, passing sequentially through a preheating and free water removal section and a crystal water dehydration section. The reaction direction and product phase of gypsum dihydrate dehydration to form hemihydrate gypsum are determined by both temperature and water vapor partial pressure: if the partial pressure is too low, it easily skips the hemihydrate stage and directly forms soluble anhydrous gypsum; if the partial pressure is too high, gypsum dihydrate remains. Based on this thermodynamic principle, this invention uses online sampling to quickly detect the feed rate, feed moisture content, exhaust air volume, and exhaust temperature. Based on material balance, the water vapor partial pressure in the dehydration section is calculated in real time, and feedforward adjustment is performed using the exhaust damper opening as the actuator. Simultaneously, the crystal water of the powder is measured every 10 minutes, and the calcination temperature is adjusted based on the deviation direction of the crystal water. Feedforward pressure control and feedback temperature adjustment form a dual closed-loop control, locking the operating point within the thermodynamically stable region of β-hemihydrate gypsum.

[0022] 3. Flow aging The calcined product is fed into a two-stage, series-connected aging chamber. The first chamber controls the temperature and relative humidity at a higher level, while the second chamber controls the temperature and relative humidity at a lower level, maintaining a partial pressure gradient between the two stages. The chambers are kept in a microfluidic state, with periodically triggered pulsed strong fluidization to prevent moisture absorption and agglomeration. After aging, the material is discharged from the bottom of the chamber. The principle behind this is that the conversion rate of soluble anhydrous gypsum to hemihydrate gypsum is controlled by the partial pressure of water vapor. The higher the partial pressure, the greater the driving force for water molecules to diffuse into the zeolite water channels in the soluble anhydrous gypsum lattice. The two-stage partial pressure gradient design ensures conversion efficiency while avoiding over-hydration.

[0023] 4. Grinding and Packaging The aged products are ground to a suitable specific surface area to increase particle surface energy and enhance hydration activity. The discharge temperature is controlled to prevent hemihydrate gypsum from being dehydrated again by mechanical heat during grinding to form soluble anhydrous gypsum. After homogenization, the product is packaged to obtain a finished building gypsum powder with stable phase composition and good batch consistency.

[0024] The technical solution designed by this invention to solve the existing problems includes the following key points: 1. Water vapor partial pressure as the main controlling variable in gypsum calcination. Traditional gypsum calcination processes focus solely on temperature control, assuming that maintaining the temperature within the range of 145–165°C will yield β-hemihydrate gypsum. However, practical experience shows that the phase composition of the product still fluctuates significantly within this temperature window. This invention, through thermodynamic analysis, reveals that the equilibrium partial pressure of water vapor P in the gypsum dehydration reaction CaSO4·2H2O⇌CaSO4·0.5H2O+1.5H2O↑ eq The rate of increase is exponential with increasing temperature. According to the principle of phase equilibrium, the direction of the dehydration reaction depends on the actual partial pressure of water vapor, P. H2O With equilibrium partial pressure P eq The relative size, that is, when P H2O <P eq When P is in the right direction (dehydration direction), the reaction proceeds to the right; when P is in the right direction (dehydration direction), the reaction proceeds to the right. H2O>P eq At this time, the reaction proceeds to the left. Simultaneously, the phase of the dehydration product is also affected by P. H2O Control, when P H2O Much lower than P eq At this time, the dehydration reaction of dihydrate gypsum may skip the hemihydrate gypsum stage and directly produce soluble anhydrous gypsum; when P H2O Slightly lower than P eq At that time, gypsum dihydrate is dehydrated according to the stoichiometric ratio to produce β-hemihydrate gypsum.

[0025] Based on the above principles, this invention introduces water vapor partial pressure as the second primary control variable. By online measurement of feed rate F (kg / h), feed free water content W (%), exhaust air volume Q (m3 / h), and exhaust temperature T (°C), and based on material balance and by introducing a temperature correction coefficient ε, the water vapor partial pressure P in the dehydration section is calculated in real time according to the following steps. H2O : (1) Calculate the total water vapor generation m in the dehydration section w (kg / h), the water vapor originates from the water of crystallization released during the dehydration of gypsum dihydrate, and the calculation formula is: m w =F×C×X×k, where C is the mass fraction (%) of dihydrate gypsum in the dry basis material. The phosphogypsum raw material was sampled from the incoming batch and, according to GB / T 5484-2024 "Methods for Chemical Analysis of Gypsum", the crystal water content was determined to be 19.8%, thus the mass fraction of dihydrate gypsum C = 94.5%; X is the decomposition rate (%) of dihydrate gypsum, determined through pre-calibration tests, defined as: X = (mass of dihydrate gypsum in feed - mass of dihydrate gypsum in calcined product) / mass of dihydrate gypsum in feed × 100%. In Example 1, X was calibrated to 82% under the conditions of a dehydration zone temperature of 155℃ and a residence time of 13 minutes. This value is used for calculating the partial pressure of water vapor in the feedforward control model; k is the water volume coefficient generated by complete dehydration of dihydrate gypsum, 0.157, determined by k = 1.5 × M w / M DH ≈0.157 (M) w =18.015 g / mol, M DH =172.17 g / mol) was determined; (2) Convert the amount of water vapor generated into the molar flow rate of water vapor n w The calculation formula is: n w =m w / 18.015; Simultaneously, the measured exhaust air volume Q is converted to the dry basis volumetric flow rate Q0 under standard conditions, and then converted to the dry air molar flow rate n. da The calculation formula is: Q0 = Q × 273.15 / (273.15 + T), n da = (101325 × Q0) / (8.314 × 273.15); (3) Calculate the water vapor mole fraction y. The formula is: y = n w / (n w +n da ); (4) Introduce a temperature correction factor ε (value 0.90~0.94, used to correct non-ideal gas behavior at 150~160℃) to calculate the partial pressure of water vapor P in the dehydration section. H2O (Pa): P H2O =ε×P×y, where P is the absolute pressure of the exhaust gas (Pa, usually the local atmospheric pressure, about 101325Pa).

[0026] After obtaining the real-time water vapor partial pressure through the above calculations, the opening of the exhaust damper is used as the actuator to control the water vapor partial pressure within the target range of 2000~6000 Pa. This target range is based on the following: Pre-calibration experiments (with a fixed dehydration section temperature of 155℃) showed a regular correlation between water vapor partial pressure and the phase composition of the calcined products. When the partial pressure is below 2000 Pa, the dehydration driving force is too strong, resulting in the formation of a large amount of soluble anhydrous gypsum; when the partial pressure is in the 3000~5000 Pa range, the β-hemihydrate gypsum content reaches its peak (≥90%); when the partial pressure is above 6000 Pa, the dehydration driving force is insufficient, leading to an increase in dihydrate gypsum residue. Therefore, this invention, based on controlling the dehydration section temperature at 150~160℃, further locks the water vapor partial pressure within the 2000~6000 Pa range, ensuring that the dehydration driving force is sufficient to complete dehydration within 12~15 minutes, while simultaneously avoiding the formation of soluble anhydrous gypsum due to excessive driving force. Through the above temperature-partial pressure coupling control, the operating point is locked at the center of the thermodynamic stability region of β-hemihydrate gypsum in the temperature-vapor partial pressure phase diagram, realizing the directional conversion of dihydrate gypsum to β-hemihydrate gypsum.

[0027] 2. A dual-feedback closed loop consisting of timed rapid detection of water of crystallization and model self-calibration. Traditional processes rely on offline XRD or chemical methods to determine the three-phase composition, with detection cycles as long as 2.5 hours, making them unsuitable for online feedback control. The theoretical water of crystallization content of β-hemihydrate gypsum (HH) is 6.2%, that of soluble anhydrous gypsum (AⅢ) is close to 0%, and that of dihydrate gypsum (DH) is 20.9%. The measured water of crystallization content of the calcined product is a weighted average of its three-phase composition: water of crystallization = 6.2% × w HH +0%×w AⅢ +20.9%×w DHTherefore, the water of crystallization content can be used as a proxy indicator of phase composition: when the water of crystallization is significantly lower than 6.0%, it indicates an excess of soluble anhydrous gypsum; when the water of crystallization is significantly higher than 6.3%, it indicates residual dihydrate gypsum. Unlike XRD analysis, which requires complex sample preparation, the determination of water of crystallization can be performed using rapid thermogravimetric analysis, which takes only 8-10 minutes from sampling to results, and can keep up with the control rhythm of the calcination process.

[0028] This invention incorporates a timed rapid detection of water of crystallization as a feedback channel: samples are automatically taken every 10 minutes, and the water of crystallization is measured using a rapid thermogravimetric analyzer. The measured value is compared with a target range of 6.0% to 6.3%. When the water of crystallization is below 5.9%, an over-burning tendency is identified, and the temperature of the dehydration section is lowered by 1 to 3°C, while the water vapor partial pressure setting is increased by 0.2 to 0.5 kPa. When the water of crystallization is above 6.3%, an under-burning tendency is identified, and the temperature of the dehydration section is increased by 1 to 3°C, while the water vapor partial pressure setting is decreased by 0.2 to 0.5 kPa. After each adjustment, at least one average material residence time is maintained before the next significant adjustment to prevent oscillation.

[0029] The following describes in further detail a method for precisely calcining a building gypsum powder with a specific phase composition. This does not limit the scope of protection of this application; rather, the scope of protection is defined by the claims. Certain specific details disclosed provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented using other materials, etc., without employing one or more of these specific details.

[0030] Unless the context otherwise requires, the terms “comprising” and “including” in the specification and claims shall be understood as open-ended and inclusive, meaning “including, but not limited to”.

[0031] The terms "implementation," "an implementation," "another implementation," or "certain implementations" used in this specification refer to specific features, structures, or characteristics described in relation to said implementation that are included in at least one implementation. Therefore, "implementation," "an implementation," "another implementation," or "certain implementations" do not necessarily all refer to the same implementation. Furthermore, specific features, structures, or characteristics can be combined in any way within one or more implementations. Each feature disclosed in this specification can be replaced by any alternative feature that provides the same, equivalent, or similar purpose. Therefore, unless otherwise specified, the disclosed features are merely general examples of equivalent or similar features.

[0032] Example 1 Table 1 Raw Material Information Table A method for precisely calcining building gypsum powder phase composition includes the following steps: S1: Take 100 parts of phosphogypsum and send it into a three-stage countercurrent washing tank. Each stage uses 80 parts of washing water at a temperature of 40℃ for 10 minutes to remove soluble phosphorus, fluorine, and organic impurities. After washing, the material is dehydrated to a moisture content of 25% using a belt filter press and then enters a hollow paddle dryer. It is steam-heated at 20 rpm for 6 minutes to pre-dry to a moisture content of 6%. The pre-dried material is then crushed and dispersed by a high-speed disperser at 2500 rpm and then enters a pulse jet dryer. The hot air temperature is 130℃, the air velocity is 12 m / s, and the material residence time is 2 seconds. The free water content of the discharged material is controlled to be 2.5%~3.5%, and the material outlet temperature is not higher than 80℃ to avoid significant dehydration during the pre-drying stage. After drying, the material enters a raw material homogenization silo with a volume of 80 m³. 3 The device is equipped with a rotating material distributor on the top of the silo and a 6-point air-loosening device at the bottom of the silo. The air pressure is 0.25 MPa. Loosening is performed for 5 minutes every 4 hours. After homogenization and storage for 12 hours, it is ready for use. After homogenization, the free water is continuously monitored. If the fluctuation exceeds ±0.5%, the feed moisture parameters of the calcination feedforward model are adjusted.

[0033] S2: The homogenized material is fed into a two-stage series fluidized bed for calcination. The particle size distribution of the homogenized raw material is D. 50 =20~40μm, D 90 =60~80μm, bulk density is 600~800kg / m³ 3 The furnace body is divided into a preheating and dewatering section and a crystallization water dewatering section along the material flow direction. The operating air velocity is controlled at 1.8 times the critical fluidization velocity. The temperature in the preheating and dewatering section is 130℃, the material bed height is 400mm, and the exhaust damper is fully open. The temperature in the crystallization water dewatering section is 155℃, the material bed height is 400mm, the residence time is 13 minutes, and an internal tube heat exchanger indirectly heats the fluidizing air with superheated steam. The pulse cleaning cycle is 3 hours. The feed moisture content was measured to be 3.2% by an online microwave moisture meter, the feed rate was measured to be 5.0t / h by a belt scale, and the exhaust air volume was measured to be 2.64×10⁻⁶. 4 m 3 / h, the thermocouple measured the exhaust temperature to be 162℃. The real-time water vapor partial pressure was calculated according to the material balance formula. The decomposition rate of gypsum dihydrate was taken as the calibration value of 82%, and the temperature correction coefficient ε was taken as 0.92. The calculated water vapor partial pressure was about 4070Pa. Within the target range of 3800~4200Pa, the exhaust damper was maintained at the current opening of 50%. Automatic sampling was performed every 10 minutes. The rapid thermogravimetric analyzer was used to measure the water of crystallization at a heating rate of 20℃ / min. The temperature of the dehydration section was adjusted to 153℃ to suppress further dehydration. After the calcined product was collected by a two-stage cyclone separator and a bag filter, it was air-cooled to 35℃ for later use.

[0034] S3: The calcined product is fed into a two-stage series fluidized bed aging chamber. A slow-temperature material air zone is set at the inlet. After natural heat dissipation to the chamber temperature ≤ 5℃, it enters the main aging zone. The aging chamber adopts low-speed fluidization and is equipped with temperature, relative humidity, dew point and differential pressure detection. The gas velocity in the first chamber is 0.15m / s, the chamber temperature is 30℃, the relative humidity is 80%, the corresponding water vapor partial pressure is 3400Pa, and the residence time is 3 hours. The material temperature is controlled to be at least 2℃ higher than the gas dew point to avoid condensation. This chamber is used to promote the stabilization of a small amount of soluble anhydrous gypsum into hemihydrate gypsum; the second chamber has an air velocity of 0.15 m / s, a chamber temperature of 28℃, a relative humidity of 60%, and a corresponding water vapor partial pressure of 2270 Pa, with a residence time of 3 hours. This chamber is used to reduce surface moisture absorption, homogenize activity, and prevent over-hydration; every 30 minutes, a pulsed strong fluidization is automatically triggered, and the air velocity is instantly increased to 0.6 m / s for 50 seconds to break up moisture-absorbing clumps; the aged product is discharged through a screw conveyor at the bottom of the chamber, and the conversion rate of soluble anhydrous gypsum to hemihydrate gypsum was measured to be 62%.

[0035] S4: The aged product is fed into the closed-loop grinding system of a ball mill, with a filling rate of 32%, a cylinder speed of 70% of the critical speed, a classifier speed of 110 rpm, and ambient temperature closed-loop circulating air. The product is ground to a specific surface area of ​​3050 cm². 2 / g, milling temperature 52℃, after homogenization in the homogenization chamber for 2 hours, packaged to obtain the finished building gypsum powder.

[0036] Example 2 The preparation method is the same as in Example 1, except that: S1: Washing temperature 35℃, washing time 8 minutes; dehydrate to 22% moisture content via belt filter press; speed 15 rpm, residence time 5 minutes, pre-dry to 5% moisture content; speed 2000 rpm; hot air temperature 120℃, air velocity 10 m / s, material residence time 1 second; air pressure 0.2 MPa, loosen for 3 minutes every 3 hours, homogenize and store for 8 hours; S2: Preheating and dewatering section temperature 120℃, material layer height 350mm; crystal water dewatering section temperature 150℃, material layer height 350mm, residence time 10 minutes; S3: First chamber: gas velocity 0.1 m / s, chamber temperature 25℃, relative humidity 70%, corresponding water vapor partial pressure 2217 Pa, residence time 2 hours; Second chamber: gas velocity 0.1 m / s, chamber temperature 22℃, relative humidity 50%, corresponding water vapor partial pressure 1322 Pa, residence time 2 hours; every 20 minutes, pulsed high-pressure fluidization is automatically triggered, the gas velocity is instantly increased to 0.5 m / s, lasting for 30 seconds; S4: Filling rate 30%, cylinder rotation speed 65% of critical speed, classifier rotation speed 100 rpm; grinding to a specific surface area of ​​2800 cm². 2 / g, milling temperature 50℃, packaged after homogenization in homogenization chamber for 1 hour; All other steps are the same.

[0037] Example 3 The preparation method is the same as in Example 1, except that: S1: Washing temperature 45℃, washing time 15 minutes; dehydrate to 28% moisture content via belt filter press; speed 25 rpm, residence time 8 minutes, pre-dry to 8% moisture content; speed 3000 rpm; hot air temperature 150℃, air velocity 15m / s, material residence time 3 seconds; air pressure 0.3MPa, loosen for 8 minutes every 5 hours, homogenize and store for 16 hours; S2: Preheating and dewatering section temperature 140℃, material layer height 450mm; Crystallization water dewatering section temperature 160℃, material layer height 450mm, residence time 15 minutes; S3: First chamber: gas velocity 0.2 m / s, chamber temperature 35℃, relative humidity 90%, corresponding water vapor partial pressure 5061 Pa, residence time 4 hours; Second chamber: gas velocity 0.2 m / s, chamber temperature 32℃, relative humidity 70%, corresponding water vapor partial pressure 3325 Pa, residence time 4 hours; every 40 minutes, pulsed high-pressure fluidization is automatically triggered, the gas velocity is instantaneously increased to 0.8 m / s, lasting for 60 seconds; S4: Filling rate 35%, cylinder rotation speed 75% of critical speed, classifier rotation speed 120 rpm; grinding to a specific surface area of ​​3200 cm². 2 / g, milling temperature 55℃, packaged after homogenization in homogenization chamber for 3 hours; All other steps are the same.

[0038] Example 4 The preparation method is the same as in Example 1, except that: S1: Replace phosphogypsum with desulfurized gypsum. The specific pretreatment steps are as follows: Take 100 parts of desulfurized gypsum with a moisture content of 25%, SO3 content of 45%, and Na... + Content 0.3%, K + The content of soluble impurities in the desulfurized gypsum is 0.2%, and the CaSO3 content is 0.5%. The soluble impurities in the desulfurized gypsum are low, requiring no pre-treatment with water washing. It is directly fed into a hollow paddle dryer, heated by steam at 20 rpm for 6 minutes, pre-drying until the free water content at the discharge is 5%–6%. The pre-dried material is then crushed and dispersed by a high-speed disperser at 2500 rpm, before entering a pulse airflow dryer. The hot air temperature is 130℃, the air velocity is 12 m / s, and the material residence time is 2 seconds, controlling the free water content at the discharge to 2.5%–3.5%, and ensuring the material outlet temperature does not exceed 80℃ to avoid significant dehydration of the dihydrate gypsum during the pre-drying stage. After drying, the material enters a raw material homogenization silo with a volume of 80 m³. 3The device is equipped with a rotating material distributor on the top of the silo and a 6-point air-loosening device at the bottom of the silo. The air pressure is 0.25MPa. Loosening is performed for 5 minutes every 4 hours. After homogenization and storage for 8 hours, it is ready for use. After homogenization, the free water content of the output is continuously monitored. If the fluctuation exceeds ±0.5%, the feed moisture parameters of the calcination feedforward model are adjusted. All other steps are the same.

[0039] Example 5 The preparation method is the same as in Example 1, except that: S1: Replace phosphogypsum with titanium gypsum. The specific pretreatment steps are as follows: Take 100 parts of titanium gypsum with a moisture content of 35%, Fe2O3 content of 2.5%, and residual H2SO4 content of 0.6% (as SO4). 2- (Calculated), pH=4.5. The titanium gypsum has a high content of soluble iron ions and residual acid, so it is sent to a three-stage countercurrent washing tank. Each stage uses 100 parts of washing water, the washing temperature is 45℃, and the washing time is 12 minutes to remove soluble iron ions, residual sulfuric acid, and organic impurities. The washing wastewater is partially reused after neutralization and sedimentation. After washing, the material is dehydrated to a free water content of 28% by a belt filter press and then enters a hollow paddle dryer. It is steam-heated at 22 rpm for 8 minutes to pre-dry to a free water content of 5%~6%. The pre-dried material is then crushed and dispersed by a high-speed disperser at 2800 rpm and then enters a pulse airflow dryer. The hot air temperature is 135℃, the air velocity is 12 m / s, and the material residence time is 2~3 seconds, controlling the free water content at 2.5%~3.5%, and ensuring the material outlet temperature does not exceed 80℃ to avoid significant dehydration of the dihydrate gypsum during the pre-drying stage. After drying, the material enters a raw material homogenization silo with a volume of 90 m³. 3 The device is equipped with a rotating material distributor on the top of the silo and a 6-point air-loosening device at the bottom of the silo. The air pressure is 0.30 MPa. Loosening is performed for 5 minutes every 4 hours. After homogenization and storage for 16 hours, it is ready for use. After homogenization, the free water content of the output is continuously monitored. If the fluctuation exceeds ±0.5%, the feed moisture parameters of the calcination feedforward model are adjusted. All other steps are the same.

[0040] Comparative Example 1 Building gypsum powder is prepared using traditional methods. The preparation method involves: phosphogypsum being naturally dried to a moisture content of 8%–10%, then fed into a single-stage fluidized bed calcination furnace. The temperature of the dehydration section is manually set to 155℃, and the frequency of the exhaust fan is fixed at 50Hz. After the calcined product is naturally piled and aged for 72 hours, it is then ground in a Raymond mill to a specific surface area of ​​2500–2800 cm². 2 / g, to obtain the finished building gypsum powder.

[0041] Comparative Example 2 The preparation method is the same as in Example 1, except that: S2: The step of crystallization water feedback is omitted, and the steps of online moisture monitoring and automatic sampling are not performed. The calcination process adopts fixed endpoint conditions, that is, the temperature of the dehydration section is set to be constant at 155℃ and the material residence time is fixed at 13 minutes. All other steps are the same.

[0042] Comparative Example 3 The preparation method is the same as in Example 1, except that: S2: The step of controlling water vapor partial pressure is omitted, and the exhaust damper is fixed at 50% without adjustment based on the calculated value of water vapor partial pressure; All other steps are the same.

[0043] Comparative Example 4 The preparation method is the same as in Example 1, except that: S3: The fluidized bed aging step is omitted, and the calcined product is naturally piled up and aged for 72 hours instead of 6 hours of fluidized bed aging; All other steps are the same.

[0044] Experimental Example 1 The building plaster powders prepared in Examples 1-5 and Comparative Examples 1-4 were tested: (1) Setting time: Refer to GB / T 17669.4-1999 "Determination of physical properties of building gypsum - neat cement paste", weigh water according to the measured standard consistency water volume and pour it into the mixing bowl. Weigh 200g of sample and pour it into the water within 5 seconds. Stir with a stirring rod for 30 seconds to obtain a uniform slurry. Pour it into the ring mold of the setting time tester. Raise the glass base plate by about 10mm and vibrate it up and down five times to remove air bubbles. Scrape off the overflow slurry so that the slurry is flush with the top of the ring mold. Place the ring mold together with the glass base plate under the instrument's steel needle so that the needle tip contacts the surface of the slurry. Quickly loosen the fixing screw to allow the steel needle to freely insert into the slurry. Repeat this every 30 seconds, changing the insertion point position each time and cleaning and straightening the steel needle. The initial setting time is the time from when the sample comes into contact with water until the steel needle first fails to touch the glass base plate; the final setting time is the time until the steel needle first inserts into the slurry to a depth of no more than 1mm. Take the average of the two test results, accurate to 1min. Each group should have no fewer than 5 valid samples, and the final result should be the average.

[0045] (2) Standard consistency water consumption: GB / T 17669.4-1999 "Determination of physical properties of building gypsum—neat mortar". First, clean and moisten the consistency tester cylinder and glass plate, and place the cylinder vertically on the glass plate. Weigh 300g of sample and pour it into water pre-weighed according to the estimated standard consistency water consumption within 5 seconds. Stir with a stirring rod for 30 seconds to obtain a uniform gypsum slurry. Quickly pour the slurry into the cylinder and scrape off the overflow so that the slurry surface is flush with the top of the cylinder. From the time the sample comes into contact with the water until 50 seconds later, start the instrument to raise the cylinder at a speed of 150mm / s. After the cylinder is lifted, measure the diameter of the test cake expanded in the two vertical directions and calculate the arithmetic mean. Adjust the amount of water added until the diameter of the test cake is equal to 180±5mm. At this time, the ratio of the amount of water added to the mass of the sample is the standard consistency water consumption. Take the average of the two test results, accurate to 1%. At least 5 parallel samples should be tested in each group, and the average value of the results should be taken.

[0046] (3) 2h flexural strength: Referring to GB / T 17669.3-1999 "Building Gypsum - Determination of Mechanical Properties", calculate the water volume and building gypsum volume according to the standard consistency water volume. Pour water into the mixing container, add the sample and let it stand for 1 minute. Stir 30 times in 30 seconds with a stirring rod, and continue stirring slowly until the slurry begins to thicken. Immediately pour it into the mold coated with mineral oil, lift the front end about 10 mm and drop it down. Repeat five times to remove air bubbles. After final setting, remove the mold. The specimen is tested for flexural strength 2 hours after contact with water. The three-point bending method is used for determination: Place the specimen on the two support rollers of the electric flexural testing machine, with the forming surface upright. Keep the loading roller and the support roller at equal distances. The loading speed is 50 N / s. Record the breaking load value. Flexural strength Rf = 3FL / 2bh 2 Where F is the breaking load (N), L is the spacing between the support rollers (mm), and b and h are the width and height of the specimen (mm), respectively. Five parallel samples were tested for each group of samples, and the average value was taken as the final result.

[0047] Experimental Example 2 The phase composition of the building gypsum powders prepared in Examples 1-5 and Comparative Examples 1-4 was determined according to GB / T 36141-2018 "Analysis Method for Phase Composition of Building Gypsum". (1)Determination of the content of adhering water and soluble anhydrous gypsum (AⅢ): Weigh about 5 g of the sample, record the mass as m1, add 5 mL of 95% ethanol solution to moisten the sample, and dry it to constant weight at 50 °C. Record the mass after drying as m2. Calculate the content of adhering water according to the mass difference before and after drying: W = (m1 - m2) / m1 × 100%; when m1 > m2, there is no AⅢ in the sample, and the content of AⅢ is 0; when m1 < m2, there is AⅢ in the sample, and the content of AⅢ is calculated by the following formula: AⅢ = 15.11 × (m2 - m1) / m1 × 100%, where the coefficient 15.11 comes from the molar mass ratio of CaSO4 to CaSO4·0.5H2O multiplied by the stoichiometric coefficient.

[0048] (2)Determination of the content of hemihydrate gypsum (HH): Weigh about 5 g of the sample, record the mass as m3, soak it in sufficient distilled water for more than 5 hours to fully hydrate HH and AⅢ into dihydrate gypsum (DH). Dry it to constant weight at 50 °C, record the mass after drying as m4, and calculate the hydration increment rate c = (m4 - m3) / m3. The content of HH is calculated according to the following formula: When there is adhering water in the sample (m1 > m2): HH = 5.37 × (c + W) / (1 - W) × 100%; when there is no adhering water in the sample (m1 < m2): HH = 5.37 × (c - AⅢ × 0.265) × 100% where the coefficients 5.37 and 0.265 come from the molar mass ratio of CaSO4·0.5H2O to CaSO4·2H2O and the reaction stoichiometric coefficient.

[0049] (3)Determination of the content of DH: Take another 5 g of the sample, record the mass as p1, heat it to constant weight at 200 °C to completely remove the crystal water of DH and HH to become AⅢ, record the mass after heating as p2, and calculate the dehydration rate d = 1 - p2 / p1. When there is adhering water in the sample (m1 > m2): DH = 4.78 × [(d - W) - HH × 0.062] × 100% When there is no adhering water in the sample (m1 < m2): DH = 4.78 × (d - HH × 0.062) × 100% where the coefficients 4.78 and 0.062 come from the molar mass ratio of CaSO4·2H2O to CaSO4 and the reaction stoichiometric coefficient.

[0050] Table 2 Comparison table of experimental results of Examples 1 - ⑤ and Comparative Examples 1 - ④ The experimental results of Examples 1 - ⑤ and Comparative Examples 1 - ④ are shown in Table 2. The building gypsum powder prepared in Example 1 of the present invention has appropriate initial setting and final setting times, a low standard consistency water consumption, and the 2-hour flexural strength reaches the excellent product standard of more than 3.0 grades. It shows that the present invention realizes the precise control of the phase composition of building gypsum powder through the double feedback closed-loop of water vapor pressure and crystal water, and the product has high strength and good batch consistency.

[0051] Example 2: The washing temperature, drying temperature, calcination temperature, and aging time were adjusted to the lower limit of the process range. The initial setting time was slightly shortened, the standard consistency water consumption was reduced, and the 2-hour flexural strength was similar but slightly lower. This is attributed to the lower washing temperature and shorter aging time slightly accelerating the hydration rate of hemihydrate gypsum while slightly reducing water consumption. This example demonstrates that building gypsum powder meeting standard requirements can still be obtained within the lower limit of the process parameter range of this invention. Example 3: The washing temperature, drying temperature, calcination temperature, and aging time were adjusted to the upper limit of the process range. The initial and final setting times were slightly prolonged, the standard consistency water consumption increased, and the 2-hour flexural strength decreased to some extent. This is attributed to the higher calcination temperature enhancing the dehydration driving force, leading to the formation of a small amount of soluble anhydrous gypsum, which has a slight adverse effect on mechanical properties. This embodiment verifies the boundary conditions of the upper limit of the process parameters of the present invention, indicating that controlling the operating point near the midpoint of the range is beneficial to obtaining better performance. In Example 4, replacing phosphogypsum with desulfurized gypsum improved the 2-hour flexural strength and slightly reduced the standard consistency water requirement. This is because the desulfurized gypsum has a lower content of soluble impurities, higher raw material purity, and a more homogeneous phase composition of the hemihydrate gypsum after calcination. This embodiment shows that the method of the present invention is also applicable to desulfurized gypsum and can obtain mechanical properties superior to phosphogypsum. In Example 5, replacing phosphogypsum with titanium gypsum prolonged the initial and final setting times, increased the standard consistency water requirement, and caused a certain degree of decrease in the 2-hour flexural strength. This is because the iron ions and residual sulfuric acid contained in titanium gypsum interfere with the hydration process of hemihydrate gypsum, resulting in a slower hydration rate and increased water requirement. This embodiment shows that the method of the present invention is also applicable to titanium gypsum, but the product performance is slightly lower than that of phosphogypsum-based building gypsum powder.

[0052] Comparative Examples 1-4, lacking key technologies, showed varying degrees of reduced overall performance compared to the examples. Comparative Example 1 employed a traditional process, with phosphogypsum undergoing no pre-treatment by washing, no partial pressure control or crystal water feedback during calcination, a fixed opening of the exhaust damper, and natural stacking for 72 hours of aging. Because soluble impurities such as phosphorus and fluorine in the raw materials were not removed, the partial pressure of water vapor during calcination passively fluctuated with the feed moisture content, resulting in inconsistent dehydration driving forces. This led to an unstable residual ratio of soluble anhydrous gypsum to dihydrate gypsum in the product, and low conversion efficiency of hemihydrate gypsum under natural aging conditions. Consequently, the product strength was low, and the setting time fluctuated significantly. This comparative example demonstrates that without pre-treatment by washing, control of water vapor pressure, crystal water feedback, and fluidized bed aging, it is difficult to consistently obtain high-performance building gypsum powder. Comparative Example 2 omitted the crystal water feedback step, and the calcination temperature was fixed at 155°C without adjustment. Due to the lack of real-time monitoring and temperature correction of crystallization water, deviations in phase composition caused by factors such as fluctuations in feed moisture content and sensor drift could not be corrected in a timely manner. After long-term operation, the content of hemihydrate gypsum gradually decreased, and soluble anhydrous gypsum accumulated, resulting in a significant decrease in product strength compared to Example 1 and increased fluctuations in setting time. This comparative example demonstrates that crystallization water feedback control is a necessary step to maintain the long-term stability of the calcination process. Comparative Example 3 omitted the water vapor partial pressure control step, and the opening of the exhaust damper was fixed at 50%. Because the exhaust volume was not adjusted in real time according to the feed rate, feed moisture content, and exhaust parameters, the water vapor partial pressure in the bed passively changed when the raw material moisture content fluctuated: when the partial pressure was too high, the dihydrate gypsum was not completely dehydrated; when the partial pressure was too low, the hemihydrate gypsum was over-dehydrated to generate soluble anhydrous gypsum, resulting in a decrease in the target hemihydrate gypsum content in the product and a significant increase in performance fluctuations. This comparative example demonstrates that water vapor partial pressure feedforward control is a key means to suppress the excessive formation of soluble anhydrous gypsum. Comparative Example 4 replaced fluidized bed aging with natural stacking aging for 72 hours. Because the water vapor partial pressure control and crystal water feedback in the calcination section remained intact, the initial content and phase composition of hemihydrate gypsum in the calcined product were similar to those in Example 1. However, under natural stacking conditions, the conversion rate of soluble anhydrous gypsum to hemihydrate gypsum was slower. After 72 hours of aging, the residual amount of soluble anhydrous gypsum was still slightly higher than that of the sample aged for 6 hours with fluidized bed aging, resulting in a slight decrease in product strength. This comparative example demonstrates that fluidized bed aging can shorten the aging period from 72 hours to 6 hours while ensuring the conversion effect.

[0053] Table 3. Powder composition of building gypsum in each embodiment and comparative example. Table 3 shows the phase composition determination results of the building gypsum powders prepared according to Examples 1-5 and Comparative Examples 1-4 of this invention, which are three industrial solid waste gypsum raw materials: phosphogypsum, desulfurized gypsum, and titanium gypsum. Figure 2As shown. The building gypsum powder prepared in Examples 1-4 of this invention all have a hemihydrate gypsum content of over 90%, while the contents of soluble anhydrous gypsum and dihydrate gypsum are controlled at low levels, meeting the requirements for superior-grade building gypsum. Among them, desulfurized gypsum has the best phase composition due to its high raw material purity; phosphogypsum is the next best; while in Example 5, titanium gypsum has a hemihydrate gypsum content slightly lower than 90% due to interference from impurities such as iron ions. Comparative Example 1 uses a traditional process, resulting in a significantly lower hemihydrate gypsum content and a significantly higher soluble anhydrous gypsum content, with the worst phase composition. Comparative Example 2 lacks crystal water feedback, and Comparative Example 3 lacks water vapor pressure control; both have lower hemihydrate gypsum contents than the examples, while their soluble anhydrous gypsum contents are significantly higher. Among them, the soluble anhydrous gypsum content in Comparative Example 3 is second only to Comparative Example 1, indicating that water vapor pressure control is a key means to inhibit the formation of soluble anhydrous gypsum. Comparative Example 4, which underwent natural stacking aging, exhibited a better phase composition than Comparative Examples 2 and 3, but its aging period was as long as 72 hours, significantly exceeding the 6 hours of fluidized bed aging as described in this invention. These results demonstrate that this invention, through a dual closed-loop regulation of water vapor partial pressure control and crystal water feedback, significantly increases the content of hemihydrate gypsum, reduces the residues of soluble anhydrous gypsum and dihydrate gypsum, achieves stable and controllable phase composition, and significantly improves aging efficiency.

[0054] In summary, this invention, through dual feedback closed-loop control of water vapor partial pressure and water of crystallization, locks the operating point of the dehydration stage within the thermodynamically stable region of β-hemihydrate gypsum, achieving the directional conversion of industrial solid waste gypsum into high-quality building gypsum powder. Compared with traditional processes and schemes lacking individual control features, this invention significantly improves product strength and batch stability, shortens the aging cycle, and is applicable to various industrial by-product gypsums such as phosphogypsum, desulfurized gypsum, and titanium gypsum, serving as a high-quality cementitious base material for lightweight building materials.

Claims

1. A method for precisely calcining building gypsum powder, characterized in that, Includes the following steps: S1: Gypsum is washed with water, filtered, pre-dried, broken up, and the free water content of the discharged material is controlled to be 2.5%~3.5%, then homogenized and stored. S2: The homogenized material passes through the preheating and dehydration sections for free water and crystal water, with the operating gas velocity controlled at 1.8 times the critical fluidization velocity. The partial pressure of water vapor is calculated in real time based on the feed moisture content, exhaust air volume, and exhaust temperature, and the exhaust damper is adjusted to keep the partial pressure stable within the target range. At the same time, the crystal water is measured every 10 minutes and the calcination temperature is adjusted accordingly. The calcined product is air-cooled. S3: The calcined product is sent into the aging chamber for fluidized bed aging. The gas velocity, chamber temperature and relative humidity are controlled to carry out two-stage aging. During the process, pulsed strong fluidization is used to prevent agglomeration. S4: The aged product is ground, homogenized, and packaged to obtain the finished building gypsum powder.

2. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The gypsum mentioned in S1 is one of phosphogypsum, desulfurized gypsum, or titanium gypsum; the water washing is set at a temperature of 35~45℃ and a washing time of 8~15 minutes; the pressure filtration is required to filter to a moisture content of 22%~28%.

3. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The pre-drying described in S1 is performed under the following conditions: a rotation speed of 15-25 rpm, a residence time of 5-8 minutes, and a moisture content of 5%-8%. The dispersing is performed under the following conditions: a rotation speed of 2000-3000 rpm.

4. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The conditions for controlling the free water content of the discharged material as described in S1 are set as follows: hot air temperature 120~150℃, wind speed 10~15m / s, and material residence time 1~3 seconds.

5. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The homogenization storage described in S1 is set under the following conditions: inflation pressure 0.2~0.3MPa, loosening for 3~8 minutes every 3~5 hours, and homogenization storage for 8~16 hours.

6. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The preheating and dewatering section of S2 is set with a temperature of 120~140℃ and a material layer height of 350~450mm; the crystal water dewatering section is set with a temperature of 150~160℃, a material layer height of 350~450mm, and a residence time of 10~15 minutes.

7. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The two-stage aging process described in S3 has the following conditions: the first chamber has an air velocity of 0.1~0.2m / s, a chamber temperature of 25~35℃, a relative humidity of 70%~90%, and a residence time of 2~4 hours; the second chamber has the following conditions: an air velocity of 0.1~0.2m / s, a chamber temperature of 22~32℃, a relative humidity of 50%~70%, and a residence time of 2~4 hours.

8. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The pulsed high-pressure fluidization described in S3 needs to be automatically triggered every 20 to 40 minutes, instantly increasing the gas velocity to 0.5 to 0.8 m / s for 30 to 60 seconds.

9. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The grinding process described in S4 involves a filling rate of 30% to 35%, a cylinder rotation speed of 65% to 75% of the critical speed, and a grinding speed of 100 to 120 rpm. The desired grinding result is a specific surface area of ​​2800 to 3200 cm². 2 / g, the milling temperature is set to 50~55℃; the homogenization time is set to 1~3 hours.

10. The precise calcination method for the phase composition of building gypsum powder as described in claim 1, characterized in that, The real-time calculation process for water vapor partial pressure is as follows: (1) Calculate the total water vapor generation m in the dehydration section w (kg / h), the water vapor originates from the water of crystallization released during the dehydration of gypsum dihydrate, and the calculation formula is: m w =F×C×X×k, where C is the mass fraction (%) of gypsum dihydrate in the dry basis material; X is the decomposition rate (%) of gypsum dihydrate; and k is the water coefficient 0.157 generated by complete dehydration of gypsum dihydrate. (2) Convert the amount of water vapor generated into the molar flow rate of water vapor n w The calculation formula is: n w =m w / 18.015; Simultaneously, the measured exhaust air volume Q is converted to the dry basis volumetric flow rate Q0 under standard conditions, and then converted to the dry air molar flow rate n. da The calculation formula is: Q0 = Q × 273.15 / (273.15 + T), n da = (101325 × Q0) / (8.314 × 273.15); (3) Calculate the water vapor mole fraction y. The formula is: y = n w / (n w +n da ); (4) Introduce the temperature correction factor ε to calculate the partial pressure of water vapor P in the dehydration section. H2O (Pa): P H2O =ε×P×y, where P is the absolute pressure of the exhaust gas, 101325Pa.

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