A method for preparing alpha-calcium sulfate hemihydrate whiskers by in-situ induced crystallization of chemical high-salinity wastewater

Abstract

The application provides a method for preparing alpha-hemihydrate calcium sulfate whiskers by in-situ induced crystallization of chemical high-salinity wastewater, and belongs to the technical field of comprehensive utilization of water resources and salt resources. The method uses chemical high-salinity wastewater as raw material, concentrates the wastewater to a proper ion concentration after impurity removal treatment, and then performs slurry preparation. NaCl in the high-salinity wastewater is used as a crystallization induction medium, and in-situ normal-pressure induced crystallization is performed to obtain a crystal slurry. Then, solid-liquid separation is performed on the crystal slurry to obtain a whisker filter cake. After washing and drying, the whisker filter cake is used to obtain finished alpha-hemihydrate calcium sulfate whiskers. The whole process is operated at normal pressure and low temperature, and high-pressure special equipment is not needed, so that investment and energy consumption are saved. NaCl in the high-salinity wastewater is directly used as the crystallization induction medium, ion balance and efficient cooperation in the system are realized, the mother liquor is recycled in a closed loop, and no salt-containing wastewater is discharged. Through precise regulation and control of the external morphology of the crystals by using a compounded crystal habit modifier, uniform needle-shaped whiskers with a length-diameter ratio of more than 60 are produced, and liquid-phase disposal and high-value utilization of the chemical high-salinity wastewater are realized.

Description

Technical Field

[0001] This invention belongs to the field of comprehensive utilization technology of water and salt resources, and specifically relates to a method for preparing α-hemihydrate calcium sulfate whiskers by in-situ induced crystallization of high-salt chemical wastewater. Background Technology

[0002] The aforementioned high-salinity chemical wastewater refers to the concentrated liquid produced by wastewater from industries such as coal chemical, petrochemical, fine chemical, printing and dyeing, pharmaceutical, pesticide, coking, metallurgy, and phosphate chemical after zero-discharge treatment. It may also include residues from chemical production processes, washing solutions, and material washing liquids, typically with a TDS > 35000 mg / L (sodium sulfate and sodium chloride content ≥ 90%). With societal development, the sources of high-salinity chemical wastewater are increasing, and the volume is growing annually. A key challenge is how to achieve high-value recycling and reuse.

[0003] The traditional approach is to treat the high-salinity wastewater using a segmented process. The first segment is "near-zero discharge," with the main process being: chemical wastewater → pretreatment → biochemical treatment → advanced oxidation → hardening and impurity removal → dual-membrane method → ​​high-salinity wastewater. The second segment is "salt separation and crystallization," which is an extension of the first segment. The main process is: high-salinity wastewater → hardening and impurity removal → advanced oxidation → activated carbon adsorption → salt separation (membrane method, cold method, and thermal method combination) crystallization → production of industrial sodium sulfate and sodium chloride → mother liquor drying → disposal of impurities.

[0004] High-salt chemical wastewater comes from a wide range of sources, is diverse in type, and contains complex toxic and harmful components. It is characterized by its high treatment difficulty, high cost, and environmental hazards, possessing both environmental and resource attributes. The first paragraph's "near-zero discharge" and the second paragraph's "salt separation and crystallization" only separate sodium sulfate and sodium chloride. The average cost of treating one ton of salt is 1000-1200 yuan, while sodium sulfate and sodium chloride only sell for 50-200 yuan per ton. Clearly, this is a losing proposition. In Northwest China, the market for sodium sulfate and sodium chloride is even more bleak, with some even receiving subsidies instead. Therefore, the market urgently needs to solve this problem.

[0005] Using sodium sulfate from high-salinity wastewater as a sulfur source and added calcium chloride as a calcium source to prepare high-value-added α-hemihydrate calcium sulfate whiskers is a suitable way to turn harm into benefit, waste into treasure, and achieve high-value utilization.

[0006] Calcium sulfate whiskers (CSW, commonly known as gypsum whiskers) are one-dimensional needle-like single-crystal inorganic functional materials formed by the directional crystallization of calcium sulfate. The mainstream high-value-added product in industrial production is α-hemihydrate calcium sulfate whiskers. It belongs to the monoclinic crystal system, with regular atomic arrangement and very few crystal defects. α-Calcium sulfate hemihydrate whiskers possess many excellent physical and chemical properties, including high strength, high modulus, high toughness, high insulation, wear resistance, high temperature resistance, acid and alkali resistance, corrosion resistance, good infrared reflectivity, easy surface treatment, easy polymer compounding, and non-toxicity.

[0007] α-Calcium sulfate hemihydrate whiskers combine the advantages of reinforcing fibers and ultrafine inorganic fillers, and can be used as reinforcing and toughening agents or functional fillers in resins, plastics, rubbers, coatings, paints, papermaking, asphalt, friction and sealing materials; they can also be used directly as filter materials, thermal insulation materials, fire-resistant and heat-insulating materials, infrared reflective materials, and high insulation materials for covering wires.

[0008] Currently, the conventional processes for preparing α-calcium sulfate hemihydrate whiskers on the market mainly include high-pressure hydrothermal synthesis, atmospheric pressure acidification, microemulsion, ion exchange, and microwave-alcohol-water methods. However, there are no domestic cases of in-situ preparation of α-calcium sulfate hemihydrate whiskers using high-salt chemical wastewater. The main reasons are that traditional processes involve high investment and operating costs, and the incomplete removal of impurities from high-salt wastewater also limits market adoption. In conclusion, the aforementioned conventional processes for preparing α-calcium sulfate hemihydrate whiskers all have shortcomings regarding the resource utilization of high-salt wastewater.

[0009] For example, the high-pressure hydrothermal synthesis method involves mixing dihydrate gypsum with water to form a slurry, which is then placed in a high-pressure, sealed reactor. Under saturated steam pressure of 120-180℃ and 0.2-1.0MPa, the dihydrate gypsum dissolves and recrystallizes, directionally generating α-calcium sulfate hemihydrate whiskers. High temperature and pressure provide the crystallization driving force, with small amounts of magnesium chloride and organic acids acting as crystal habit modifiers, resulting in a low salt content in the system. Advantages include intact whisker crystals with few defects, aspect ratios reaching 90-200, and high purity. However, the high-pressure equipment incurs high investment, annual inspection, and maintenance costs, and requires strict safety control. It is suitable for preparing whiskers from pure gypsum but not for high-salt wastewater scenarios.

[0010] For example, the atmospheric pressure acidification method works by heating a gypsum suspension in an acidic system of dilute sulfuric acid (pH 2-4) at 90-100°C under atmospheric pressure. The strong acid increases the solubility of the gypsum, causing it to precipitate fibrous calcium sulfate whiskers. This process does not require a high-concentration NaCl induction medium. Its advantages include simple equipment, operation at atmospheric pressure, and low initial investment. However, it also has significant disadvantages: firstly, the highly acidic system is extremely corrosive, leading to high costs for waste acid disposal and significant environmental pressure; secondly, the whiskers are uneven in size and prone to agglomeration, with aspect ratios generally ranging from 30-60, resulting in low-grade products; and thirdly, it generates large amounts of acidic wastewater containing hydrochloric acid.

[0011] Furthermore, the microemulsion method employs an oil-in-water reverse microemulsion, where surfactants encapsulate nanoscale "water-core microreactors." This method involves precisely controlled nucleation and growth within a small, confined space. Surfactants selectively adsorb calcium sulfate onto lateral crystal planes, inhibiting radial broadening and forcing crystal growth along the c-axis in a one-dimensional manner. However, it has drawbacks: cyclohexane, n-hexane, and kerosene constitute 60-70 wt% of the system, providing a hydrophobic continuous medium. Each ton of finished product consumes a large amount of cyclohexane and CTAB, resulting in high overall production costs—more than twice that of salt-induced and hydrothermal methods. These technological limitations prevent large-scale industrial production.

[0012] Furthermore, in the ion exchange method, weakly acidic to strongly acidic cation exchange resins are selected and pre-converted to... Saturated resin, with resin skeleton channels acting as a slow-release calcium source + confined space template: resin releases slowly. , in solution Diffusion enters the resin pores, causing exchange precipitation. The advantages are normal pressure and low temperature, simple equipment, high product purity, and uniform morphology; however, the disadvantages are obvious: the resin loading is limited, the output per reactor is only in the kilogram range, it is impossible to build a continuous production line of tens of thousands of tons, and there is no value for scale-up; it cannot utilize the high-salt wastewater of NaCl & Na2SO4 from coal chemical industry, and does not have the value of "using waste to treat waste" for resource recovery. Finally, the microwave-alcohol-water method is an auxiliary process for the small-scale, intermittent preparation of α-hemihydrate calcium sulfate whiskers in the laboratory. It uses a mixed solvent of alcohol (ethylene glycol, glycerol) and water to control water activity, and microwaves replace traditional heat conduction heating to accelerate crystal transformation. However, single-batch production capacity is extremely low, lacking continuous production capability. The microwave reaction equipment has limited volume, with each reactor containing only a few hundred milliliters to a few liters, allowing only kilogram-level pilot production in the laboratory and making it impossible to build an industrial continuous production line.

[0013] Therefore, it is evident that providing a method for the resource-based and high-value utilization of high-salt chemical wastewater, simplifying processes, reducing costs, achieving cleaner production, and realizing liquid-phase treatment has become an urgent problem for those skilled in the art. Summary of the Invention

[0014] The objective of this invention is to provide a method for preparing α-hemihydrate calcium sulfate whiskers by in-situ induced crystallization from high-salt chemical wastewater. This method has an optimized treatment process, low treatment cost, and can meet the excellent resource utilization requirements of turning waste into treasure.

[0015] The objective of this invention is achieved by providing a method for preparing α-calcium sulfate hemihydrate whiskers through in-situ induced crystallization from high-salt chemical wastewater, comprising the following steps: (1) Pretreatment for impurity removal: After the chemical high-salt wastewater is pumped into the equalization tank for conditioning and equalization, it is pumped into a special impurity removal device to deeply remove fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals. (2) Concentration and brine preparation: The product water from step (1) is evaporated and concentrated to obtain brine. The evaporated condensate is pumped into a storage tank, while the brine is pumped into a preparation tank. The ion concentration, pH and temperature of the brine are adjusted under low-speed stirring. Then, additives are added and low-speed stirring is continued for 30 minutes to form a suspension slurry. (3) Induced crystallization: The suspension slurry from step (2) is pumped into the induced crystallization reactor. Under the action of additives, the reactor is stirred at low speed to carry out the atmospheric pressure induced crystallization reaction. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank. The crystal slurry formed is discharged from the bottom outlet of the reactor. (4) Whisker post-processing: the crystal slurry produced in step (3) is separated into solid and liquid, washed and dried to obtain α-hemihydrate calcium sulfate whiskers. The salt-containing mother liquor produced by solid-liquid separation is returned to the inlet water conditioning tank in step (1).

[0016] In this invention, the method uses sodium sulfate from high-salinity wastewater as a sulfur source and added calcium chloride as a calcium source. Sodium chloride in the high-salinity wastewater serves as the fundamental crystallization induction medium, inducing crystallization in situ under the action of a crystal habit modifier to prepare high-value-added calcium sulfate whiskers. This is a suitable pathway for turning waste into treasure and achieving high-value utilization. It overcomes the drawbacks of hydrothermal synthesis, atmospheric pressure acidification, and microemulsion methods. Furthermore, due to thorough impurity removal, it improves yield and ensures product quality. Automated continuous operation facilitates industrial production, resulting in significant economic and social benefits.

[0017] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following technical solutions.

[0018] As a preferred technical solution of the present invention, in step (1), the high-salt chemical wastewater is a concentrated liquid after the wastewater generated by industries such as coal chemical, petrochemical, fine chemical, printing and dyeing, pharmaceutical, pesticide, coking, metallurgy, and phosphorus chemical has been treated by a zero-discharge process. The main indicators of the high-salt chemical wastewater are TDS≥35000mg / L, sodium sulfate and sodium chloride account for ≥90% of the wastewater, pH=1-14, and COD≤15000mg / L.

[0019] As a preferred technical solution of the present invention, in step (1), the high-salt chemical wastewater is pumped into an equalization tank for conditioning and then into a dedicated impurity removal device to deeply remove fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals; the impurity removal operation process is as follows: ① using chemical and adsorption methods to remove fluoride, silicon, phosphate, and inert organic matter; ② using reduction methods to remove carbonate, bicarbonate, nitrate, and nitrite; ③ using adsorption methods to remove heavy metals and potassium ions; so that fluoride ions F- ≤1 mg / L, silicon (SiO2) ≤20 mg / L, carbonate and bicarbonate CO3 2- &HCO3 - ≤1000mg / L, phosphate PO4 3- (Total phosphorus, as P) ≤2 mg / L, nitrate and nitrite NO3 - &NO2 - ≤500mg / L, potassium ions K + ≤50mg / L, inert COD≤30mg / L, heavy metals≤1mg / L.

[0020] As a preferred technical solution of the present invention, in step (1), the special impurity removal device is a customized device, and the acid used to adjust the pH during the impurity removal process is a dilute sulfuric acid or dilute hydrochloric acid with a mass fraction of 15%-30%, and the alkali used is a sodium hydroxide solution with a mass fraction of 15%-30%.

[0021] As a preferred technical solution of the present invention, in step (2), the evaporation and concentration is carried out using a falling film MVR device, so that the TDS of the mixed brine of sodium sulfate and sodium chloride is 150g / L-200g / L; the flow-through components of the falling film MVR device are made of corrosion-resistant materials such as titanium and 2205 duplex steel, which can cope with the working conditions of highly corrosive wastewater containing fluoride and high chloride ions.

[0022] As a preferred technical solution of the present invention, in step (2), the slurry mixing tank is a vertical atmospheric pressure jacketed mixing slurry mixing tank with an integral heat insulation jacket. It is an atmospheric pressure device with a design pressure of 0-0.1 MPa, a carbon steel cylinder, and a PTFE lining. The slurry mixing adjustment under low-speed stirring conditions refers to: ① The low-speed stirring is controlled at 140-180 r / min, preferably 160 r / min; ② Adjust the chloride ion concentration in the brine so that =0.85-1.7 mol / L; ③ Add calcium chloride to adjust the ratio of calcium ions to sulfate ions in the brine, so that... The molar ratio is 1.05-1.1:1; ④ Add additives (crystallization modifiers) at a rate of 0.04% to 0.08% of the total mass of the brine; By adjusting the above ①②③④, a suspension slurry that meets the crystallization conditions is obtained.

[0023] As a preferred technical solution of the present invention, in step (2), the adjustment of pH and temperature refers to controlling pH to 4.5-6, using dilute sulfuric acid with a mass fraction of 5-10%, and controlling the temperature to 85-90℃.

[0024] As a preferred technical solution of the present invention, in step (3), the induced crystallization reactor refers to a jacketed, corrosion-resistant reactor, preferably an atmospheric pressure enamel-lined or carbon steel PTFE-lined jacketed reactor. The reactor is heated by one of steam heating, heat transfer oil circulation heating, or electric heating, preferably steam heating. The reactor is cooled by medium-assisted cooling, and the cooling medium is water, ethylene glycol, or frozen brine. The operating pressure of the reactor is 0-0.01 MPa, the slurry preheating temperature is 85-90℃, the reaction temperature is controlled at 98-103℃, the reaction heating rate is 1-2℃ / min, the reaction time is controlled at 2.5-5h, and the pH of the reaction slurry is 4.5-6, preferably 5-5.5. The low-speed stirring is controlled at 100-150 r / min, preferably 120-130 r / min. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank, and the crystal slurry is discharged from the bottom outlet.

[0025] As a preferred technical solution of the present invention, in step (3), the additive is a crystal habit modifier, specifically a compound agent of quaternary ammonium salt and inorganic salt. The quaternary ammonium salt includes hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, tetradecyltrimethylammonium bromide, and tetradecyltrimethylammonium chloride, and the inorganic salt includes copper chloride CuCl2 and magnesium chloride. Aluminum chloride (AlCl3), preferably hexadecyltrimethylammonium bromide (CTAB) and magnesium chloride. The compound is formulated with CTAB, the dosage of which is controlled at 0.01% to 0.03% of the total brine mass. The dosage should be controlled at 0.03% to 0.05% of the total mass of brine.

[0026] As a preferred technical solution of the present invention, in step (4), the equipment used for solid-liquid separation of the crystal slurry includes a vertical conical centrifuge, a horizontal single-stage pusher centrifuge, a horizontal double-stage pusher centrifuge, a horizontal spiral sedimentation centrifuge, and a heated PTFE-lined horizontal spiral centrifuge, preferably a heated PTFE-lined horizontal spiral centrifuge. The solid-liquid separation, washing, and drying of the crystal slurry include the following continuous operation process: ① The crystal slurry output temperature is ≥90℃, and the whole process is heated and insulated. The pipes and separation equipment are insulated. The heated PTFE-lined horizontal spiral centrifuge has a constant temperature of 90℃ in the machine body jacket. The crystal slurry enters the drum, and the centrifugal force realizes the solid phase sedimentation of the whiskers. It is continuously squeezed to form a filter cake. The salt-containing mother liquor overflows and is collected into the mother liquor storage tank. The salt-containing mother liquor is returned to the water inlet regulating tank in step (1). ② The centrifuge has a built-in countercurrent washing channel, which uses 90℃ deionized hot water to spray the filter cake, washing away the NaCl adsorbed on the surface of the whiskers and controlling the residual chloride ions in the finished product. ≤0.1%; ③ The wet calcium sulfate whisker filter cake is fed into a hot air drying device, which includes a multi-stage belt hot air dryer, a multi-layer belt hot air dryer, an airflow dryer, a vacuum rake dryer, a fluidized bed dryer, and a disc dryer. Preferably, it is a multi-layer belt hot air dryer. The drying hot air temperature is controlled at 80-110℃. The first stage temperature is controlled at 80-90℃ to remove free water, and the second stage temperature is controlled at 100-110℃ to remove adsorbed water, so that the moisture content of the finished product (hemihydrate calcium sulfate whiskers) is ≤0.5%. The equipment operates under closed negative pressure, the hot air is recycled, and the tail gas is sprayed and condensed to recover salt. If high-temperature resistant anhydrous calcium sulfate whiskers are required, a high-temperature calcination device is added to the rear end of the belt dryer to control the calcination temperature at 500-600℃.

[0027] The present invention provides a method for preparing α-hemihydrate calcium sulfate whiskers by in-situ induced crystallization of high-salt chemical wastewater. This method features a streamlined process route and reasonable investment and operating costs, exhibiting good economic efficiency and operability. The method boasts outstanding originality in its process route, optimizes the entire treatment process, and achieves low treatment costs, thus meeting the excellent requirements of resource utilization that transforms waste into resources. Attached Figure Description

[0028] Figure 1 The process flow diagram for preparing α-hemihydrate calcium sulfate whiskers by in-situ induced crystallization of high-salt chemical wastewater provided by the present invention is shown. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in further detail below. However, it should be understood that the following embodiments are used to explain the invention and are not intended to limit the scope of the invention.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0031] Please see Figure 1 The present invention provides a method for preparing α-hemihydrate calcium sulfate whiskers by in-situ induced crystallization from high-salt chemical wastewater, which includes the following concise and highly operable technical control requirements.

[0032] In step (1): the pretreatment for impurity removal involves pumping the high-salt chemical wastewater into a regulating tank for conditioning and equalization, and then pumping it into a special impurity removal device to deeply remove fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals. ① Chemical and adsorption methods are used to deeply remove fluorine, silicon, phosphate, and inert organic matter; ②The reduction method is used to remove carbonate, bicarbonate, nitrate and nitrite ions; ③ Adsorption method is used to remove potassium ions and heavy metals; Through the processes described above ①②③, the fluoride ion F - ≤1 mg / L, silicon (SiO2) ≤20 mg / L, carbonate and bicarbonate CO3 2- &HCO3 - ≤1000mg / L, phosphate PO4 3- (Total phosphorus, as P) ≤2 mg / L, nitrate and nitrite NO3 - &NO2 - ≤500mg / L, potassium ions K + ≤50mg / L, inert COD≤30mg / L, heavy metals≤1mg / L; In step (2): concentration and brine preparation, the product water from step (1) is evaporated and concentrated to obtain mixed brine. The evaporated condensate is pumped into the storage tank, while the brine is pumped into the preparation tank. Under low-speed stirring, the ion concentration, pH and temperature of the brine are adjusted, and then additives are added. Continue low-speed stirring for 30 minutes. The specific control indicators are as follows to obtain the suspension slurry.

[0033] ① The TDS of the mixed brine of sodium sulfate and sodium chloride obtained after concentration is 150-200 g / L; ② The slurry preparation should achieve the desired chloride ion concentration Cl - =0.85-1.7 mol / L; ③ The pulp preparation should achieve the desired calcium-to-sulfur ratio. The molar ratio is 1.05-1.1:1; ④ Add a crystal habit modifier at a dosage of 0.04% to 0.08% of the total mass of the brine; ⑤ Stir the slurry tank at low speed, controlling the rotation speed at 140-180 r / min, and maintain the temperature at 85-90℃ throughout the process; ⑥ After feeding is completed, the pH is tested and adjusted to the optimal range of 4.5-6 using 5-10% dilute sulfuric acid. In step (3): Induced crystallization, the suspension slurry obtained in step (2) is pumped into the induced crystallization reactor. Under the action of additives, the reactor is stirred at low speed to carry out the atmospheric pressure induced crystallization reaction. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank. The crystal slurry formed is discharged from the bottom outlet of the reactor. The specific control indicators are as follows.

[0034] ① A PTFE-lined vertical atmospheric pressure jacketed reactor with a condenser and reflux device, the operating pressure of the reactor is 0~0.01MPa; ② The pH of the reaction slurry is 4.5-6, preferably 5-5.5; ③ Introduce low-pressure steam into the jacket, stabilize the temperature inside the reactor at 98-103℃, maintain slight reflux and gentle boiling, increase the reaction temperature at a rate of 1-2℃ / min, and control the reaction time at 2.5-5h. ④ Stir at a low speed of 100-150 r / min in the reactor, preferably 120-130 r / min, to avoid breaking the growing slender whiskers; ⑤ When the whisker aspect ratio is stable at 60-100 and there is no large amount of granular gypsum sediment, it can be discharged. In step (4): whisker post-processing, the crystal slurry produced in step (3) is separated into solid and liquid, washed and dried to obtain α-hemihydrate calcium sulfate whiskers. The salt-containing mother liquor generated by solid-liquid separation is returned to the influent conditioning tank in step (1). The specific control indicators are as follows.

[0035] ① The entire process is heated and kept at ≥85℃ to prevent α-hemihydrate calcium sulfate whiskers from absorbing water and reversing into short and coarse dihydrate gypsum. ② Ensure that the crystal slurry is transported to the PTFE-lined horizontal screw centrifuge at a temperature of 98°C through a heat-traced and insulated pipeline; ③ The centrifuge has a built-in countercurrent washing channel, through which 90℃ deionized hot water is sprayed onto the wet whiskers to wash away adsorbed sodium chloride impurities on the surface, controlling the residual chloride ions (Cl) in the finished product. - ≤0.1%; ④ The wet filter cake of calcium sulfate whiskers with a moisture content of 25% to 32% is continuously discharged and sent to the drying section; ⑤ The wet whisker filter cake is fed into a multi-layer belt hot air dryer, with segmented temperature control at 80-110℃. The first stage is controlled at 80-90℃ to remove free water, and the second stage is controlled at 100-110℃ to remove adsorbed water, so that the moisture content of the finished product (hemihydrate calcium sulfate whiskers) is ≤0.5%. ⑥ If high-temperature resistant anhydrous calcium sulfate whiskers are required, a calcination device at 500-600℃ should be added after drying to remove the water of crystallization.

[0036] The advantages of the method of the present invention compared with existing technologies are further summarized as follows: 1. It adopts a unique impurity removal technology, which is a continuous liquid-phase process that is highly efficient and reduces impurity removal costs; 2. The entire process is carried out under normal pressure liquid phase treatment, with a short process, high stability, strong adaptability, and strong resistance to shock. 3. By utilizing sodium chloride in the wastewater as the most basic crystallization induction medium in this process, ion balance and efficient synergy within the system are achieved; 4. It highlights both environmental protection attributes and resource attributes, achieving the goals of resource utilization and high-value utilization.

[0037] This invention takes into account the characteristics of high-salt chemical wastewater and opens up a complete liquid-phase treatment path to address the current bottlenecks in high-salt wastewater treatment, turning harm into benefit and waste into treasure.

[0038] To further understand the present invention, the present invention will be further described in detail below with reference to the water quality of high-salt wastewater from a certain coal chemical plant, using Examples 1 to 3 as examples.

[0039] As shown in Table 1 below, the impurities account for 9.5%, and sodium sulfate and sodium chloride account for 90.5%.

[0040] Table 1:

[0041] Example 1: As Figure 1 As shown, a method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater includes the following steps: Step (1), pretreatment for impurity removal: The high-salt chemical wastewater is pumped into an equalization tank for conditioning and then into a dedicated impurity removal device for deep removal of fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals. The impurity removal process is as follows: ① Removing fluoride, silicon, phosphate, and inert organic matter using chemical and adsorption methods; ② Removing carbonate, bicarbonate, nitrate, and nitrite using a reduction method; ③ Removing heavy metals and potassium ions using an adsorption method; so that fluoride ions F - ≤1mg / L, silicon (SiO2) ≤20mg / L, CO3 2- &HCO3 - ≤1000mg / L, phosphate PO4 3- (Total phosphorus, as P) ≤2 mg / L, nitrate and nitrite NO3 - &NO2 - ≤500mg / L, potassium ions K + ≤50mg / L, inert COD≤30mg / L, heavy metals≤1mg / L; the acid used to adjust the pH during the impurity removal process is 15% dilute sulfuric acid, and the alkali used is 15% sodium hydroxide solution. Step (2), concentration and brine preparation: The product water from step (1) is evaporated and concentrated to obtain mixed brine. The evaporated condensate is pumped into a storage tank, while the brine is pumped into a preparation tank. Under low-speed stirring, the ion concentration, pH and temperature of the brine are adjusted. Then, additives are added, and low-speed stirring is continued for 30 minutes. The specific control indicators are as follows to obtain the suspension slurry.

[0042] ① The TDS of a certain coal chemical high-salt wastewater was 67.754 g / L, which was concentrated to TDS=150 g / L using a falling film MVR device; ② The slurry preparation should achieve the desired chloride ion concentration Cl - =0.85mol / L; ③ The pulp preparation should achieve the desired calcium-to-sulfur ratio. The molar ratio is 1.05:1; ④ Add additives (crystallization modifiers) at a rate of 0.04% of the total brine mass; ⑤ Stir the slurry tank at low speed, controlling the rotation speed at 140 r / min, and maintain the temperature at 85-90℃ throughout the process; ⑥ After feeding is completed, the pH is tested and adjusted to pH=4.5 using 5% dilute sulfuric acid. Step (3), Induced crystallization: The suspension slurry obtained in step (2) is pumped into the induced crystallization reactor. Under the action of additives, the reactor is stirred at low speed to carry out the atmospheric pressure induced crystallization reaction. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank. The crystal slurry formed is discharged from the bottom outlet of the reactor. The following indicators are controlled.

[0043] ① A PTFE-lined vertical atmospheric pressure jacketed reactor with a condenser and reflux device, the operating pressure of the reactor is 0-0.01 MPa; ② The pH of the reaction slurry is 4.5-6, preferably pH 5; ③ Introduce low-pressure steam into the jacket, stabilize the temperature inside the reactor at 98-103℃, maintain slight reflux and gentle boiling, increase the reaction temperature at a rate of 1℃ / min, and control the reaction time at 2.5h. ④ Stir at a low speed of 100-150 r / min, preferably 120 r / min, inside the reactor to avoid breaking the growing slender whiskers due to low shear. ⑤ Once the whisker aspect ratio remains stable at >60 and there is no large amount of granular gypsum sediment, the material can be discharged; Step (4), whisker post-processing: the crystal slurry produced in step (3) is separated into solid and liquid, washed and dried to obtain α-hemihydrate calcium sulfate whiskers. The salt-containing mother liquor generated by solid-liquid separation is returned to the influent conditioning tank in step (1) and the following indicators are controlled.

[0044] ① The entire process is heated and kept at ≥85℃ to prevent α-hemihydrate calcium sulfate whiskers from absorbing water and reversing into short and coarse dihydrate gypsum. ② Ensure that the crystal slurry is transported to the PTFE-lined horizontal screw centrifuge at a temperature of 98°C through a heat-traced and insulated pipeline; ③ The centrifuge has a built-in countercurrent washing channel, through which 90℃ deionized hot water is sprayed onto the wet whiskers to wash away adsorbed sodium chloride impurities on the surface, controlling the residual chloride ions (Cl) in the finished product. - ≤0.1%; ④ The wet filter cake of calcium sulfate whiskers with a moisture content of 25% to 32% is continuously discharged and sent to the drying section; ⑤ The wet whisker filter cake is fed into a multi-layer belt hot air dryer, with segmented temperature control of 80-110℃. The first stage temperature is controlled at 80℃ to remove free water, and the second stage temperature is controlled at 100℃ to remove adsorbed water, so that the moisture content of the finished product (hemihydrate calcium sulfate whiskers) is ≤0.5%.

[0045] Example 2: As Figure 1 As shown, a method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater includes the following steps: Step (1), pretreatment for impurity removal: The high-salt chemical wastewater is pumped into an equalization tank for conditioning and then into a dedicated impurity removal device for deep removal of fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals. The impurity removal process is as follows: ① Removing fluoride, silicon, phosphate, and inert organic matter using chemical and adsorption methods; ② Removing carbonate, bicarbonate, nitrate, and nitrite using a reduction method; ③ Removing heavy metals and potassium ions using an adsorption method; so that fluoride ions F - ≤1mg / L, silicon (SiO2) ≤20mg / L, CO3 2- &HCO3 - ≤1000mg / L, phosphate PO4 3- (Total phosphorus, as P) ≤2 mg / L, nitrate and nitrite NO3 - &NO2 - ≤500mg / L, potassium ions K + ≤50mg / L, inert COD≤30mg / L, heavy metals≤1mg / L; the acid used to adjust the pH during the impurity removal process is 15% dilute sulfuric acid, and the alkali used is 15% sodium hydroxide solution. Step (2), concentration and brine preparation: The product water from step (1) is evaporated and concentrated to obtain mixed brine. The evaporated condensate is pumped into a storage tank, while the brine is pumped into a preparation tank. Under low-speed stirring, the ion concentration, pH and temperature of the brine are adjusted. Then, additives are added, and low-speed stirring is continued for 30 minutes. The specific control indicators are as follows to obtain the suspension slurry.

[0046] ① The TDS of a certain coal chemical high-salt wastewater was 67.754 g / L, which was concentrated to TDS=200 g / L using a falling film MVR device; ② The slurry preparation should achieve the desired chloride ion concentration Cl - =1.7mol / L; ③ The pulp preparation should achieve the desired calcium-to-sulfur ratio. The molar ratio is 1.1:1; ④ Add additives (crystallization modifiers) at a rate of 0.08% of the total brine mass; ⑤ Stir the slurry tank at low speed, controlling the rotation speed at 180 r / min, and maintain the temperature at 85~90℃ throughout the process; ⑥ After feeding is completed, the pH is tested and adjusted to pH=6 using 10% dilute sulfuric acid; Step (3), Induced crystallization: The suspension slurry obtained in step (2) is pumped into the induced crystallization reactor. Under the action of additives, the reactor is stirred at low speed to carry out the atmospheric pressure induced crystallization reaction. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank. The crystal slurry formed is discharged from the bottom outlet of the reactor. The following indicators are controlled.

[0047] ① A PTFE-lined vertical atmospheric pressure jacketed reactor with a condenser and reflux device, the operating pressure of the reactor is 0~0.01MPa; ② The pH of the reaction slurry is 4.5-6, preferably 5.5; ③ Introduce low-pressure steam into the jacket, stabilize the temperature inside the reactor at 98-103℃, maintain slight reflux and gentle boiling, with a reaction heating rate of 2℃ / min and a reaction time of 5h. ④ Stir at a low speed of 100-150 r / min in the reactor, preferably 130 r / min, to avoid breaking the growing slender whiskers due to low shear. ⑤ Once the whisker aspect ratio remains stable at >60 and there is no large amount of granular gypsum sediment, the material can be discharged; Step (4), whisker post-processing: the crystal slurry produced in step (3) is separated into solid and liquid, washed and dried to obtain α-hemihydrate calcium sulfate whiskers. The salt-containing mother liquor generated by solid-liquid separation is returned to the influent conditioning tank in step (1) and the following indicators are controlled.

[0048] ① The entire process is heated and kept at ≥85℃ to prevent α-hemihydrate calcium sulfate whiskers from absorbing water and reversing into short and coarse dihydrate gypsum. ② Ensure that the crystal slurry is transported to the PTFE-lined horizontal screw centrifuge at a temperature of 98°C through a heat-traced and insulated pipeline; ③ The centrifuge has a built-in countercurrent washing channel, through which 90℃ deionized hot water is sprayed onto the wet whiskers to wash away adsorbed sodium chloride impurities on the surface, controlling the residual chloride ions (Cl) in the finished product. - ≤0.1%; ④ The wet filter cake of calcium sulfate whiskers with a moisture content of 25% to 32% is continuously discharged and sent to the drying section; ⑤ The wet whisker filter cake is fed into a multi-layer belt hot air dryer, with segmented temperature control of 80-110℃. The first stage temperature is controlled at 90℃ to remove free water, and the second stage temperature is controlled at 110℃ to remove adsorbed water, so that the moisture content of the finished product (hemihydrate calcium sulfate whiskers) is ≤0.5%.

[0049] Example 3: As Figure 1As shown, a method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater includes the following steps: Step (1), pretreatment for impurity removal: The high-salt chemical wastewater is pumped into an equalization tank for conditioning and then into a dedicated impurity removal device for deep removal of fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals. The impurity removal process is as follows: ① Removing fluoride, silicon, phosphate, and inert organic matter using chemical and adsorption methods; ② Removing carbonate, bicarbonate, nitrate, and nitrite using a reduction method; ③ Removing heavy metals and potassium ions using an adsorption method; so that fluoride ions F - ≤1mg / L, silicon (SiO2) ≤20mg / L, CO3 2- &HCO3 - ≤1000mg / L, phosphate PO4 3- (Total phosphorus, as P) ≤2 mg / L, nitrate and nitrite NO3 - &NO2 - ≤500mg / L, potassium ions K + ≤50mg / L, inert COD≤30mg / L, heavy metals≤1mg / L; the acid used to adjust the pH during the impurity removal process is 15% dilute sulfuric acid, and the alkali used is 15% sodium hydroxide solution. Step (2), concentration and brine preparation: The product water from step (1) is evaporated and concentrated to obtain mixed brine. The evaporated condensate is pumped into a storage tank, while the brine is pumped into a preparation tank. Under low-speed stirring, the ion concentration, pH and temperature of the brine are adjusted. Then, additives are added, and low-speed stirring is continued for 30 minutes. The specific control indicators are as follows to obtain the suspension slurry.

[0050] ① The TDS of a certain coal chemical high-salt wastewater was 67.754 g / L, which was concentrated to TDS=175 g / L using a falling film MVR device; ② The slurry preparation should achieve the desired chloride ion concentration Cl - =1.275mol / L; ③ The pulp preparation should achieve the desired calcium-to-sulfur ratio. The molar ratio is 1.075:1; ④ Add additives (crystallization modifiers) at a rate of 0.06% of the total brine mass; ⑤ The mixing tank is stirred at low speed, with the rotation speed controlled at 160 r / min, and the temperature is maintained at 85~90℃ throughout the process; ⑥ After feeding is completed, the pH is tested and adjusted to pH=5.3 using 7.5% dilute sulfuric acid. Step (3), Induced crystallization: The suspension slurry obtained in step (2) is pumped into the induced crystallization reactor. Under the action of additives, the reactor is stirred at low speed to carry out the atmospheric pressure induced crystallization reaction. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank. The crystal slurry formed is discharged from the bottom outlet of the reactor. The following indicators are controlled.

[0051] ① A PTFE-lined vertical atmospheric pressure jacketed reactor with a condenser and reflux device, the operating pressure of the reactor is 0~0.01MPa; ② The pH of the reaction slurry is 4.5-6, preferably 5.25; ③ Low-pressure steam is introduced into the jacket to stabilize the temperature inside the reactor at 98-103℃, maintaining slight reflux and gentle boiling. The reaction heating rate is 1.5℃ / min, and the reaction time is controlled at 3.75h. ④ Stir at a low speed of 100-150 r / min in the reactor, preferably 125 r / min, to avoid breaking the growing slender whiskers; ⑤ When the whisker aspect ratio is stable >60 and there is no large amount of granular gypsum sediment, it can be discharged; Step (4), whisker post-processing: the crystal slurry produced in step (3) is separated into solid and liquid, washed and dried to obtain α-hemihydrate calcium sulfate whiskers. The salt-containing mother liquor generated by solid-liquid separation is returned to the influent conditioning tank in step (1) and the following indicators are controlled.

[0052] ① The entire process is heated and kept at ≥85℃ to prevent α-hemihydrate calcium sulfate whiskers from absorbing water and reversing into short and coarse dihydrate gypsum. ② Ensure that the crystal slurry is transported to the PTFE-lined horizontal screw centrifuge at a temperature of 98°C through a heat-traced and insulated pipeline; ③ The centrifuge has a built-in countercurrent washing channel, through which 90℃ deionized hot water is sprayed onto the wet whiskers to wash away sodium chloride impurities adsorbed on the surface, controlling the residual chloride ions Cl- in the finished product to ≤0.1%; ④ The wet filter cake of calcium sulfate whiskers with a moisture content of 25% to 32% is continuously discharged and sent to the drying section; ⑤ The wet whisker filter cake is fed into a multi-layer belt hot air dryer, with segmented temperature control of 80-110℃. The first stage temperature is controlled at 85℃ to remove free water, and the second stage temperature is controlled at 105℃ to remove adsorbed water, so that the moisture content of the finished product (hemihydrate calcium sulfate whiskers) is ≤0.5%.

[0053] The technical indicators of calcium sulfate whiskers prepared from the resource recovery of high-salt coal chemical wastewater using the above-described processing methods (Examples 1 to 3) are shown in Table 2 below. Table 2:

[0054] As can be seen from Table 2 above: ① In terms of process control, the product quality of Examples 1, 2, and 3 all meet or exceed the requirements of the national standard for calcium sulfate whiskers (GB / T 35471-2017). For easy comparison, please refer to the data listed in Table 3 below; ② When the control indicators of the process flow are taken as the median value, such as in Example 3, the product quality is better.

[0055] Table 3:

[0056] The above description is merely a specific embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater, characterized in that: Includes the following steps: (1) Pretreatment for impurity removal: After the chemical high-salt wastewater is pumped into the equalization tank for conditioning and equalization, it is pumped into a special impurity removal device to deeply remove fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals. (2) Concentration and brine preparation: The product water from step (1) is evaporated and concentrated to obtain brine. The evaporated condensate is pumped into a storage tank, while the brine is pumped into a preparation tank. The ion concentration, pH and temperature of the brine are adjusted under low-speed stirring. Then, additives are added and low-speed stirring is continued for 30 minutes to form a suspension slurry. (3) Induced crystallization: The suspension slurry from step (2) is pumped into the induced crystallization reactor. Under the action of additives, the reactor is stirred at low speed to carry out the atmospheric pressure induced crystallization reaction. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank. The crystal slurry formed is discharged from the bottom outlet of the reactor. (4) Whisker post-processing: the crystal slurry produced in step (3) is separated into solid and liquid, washed and dried to obtain α-hemihydrate calcium sulfate whiskers. The salt-containing mother liquor produced by solid-liquid separation is returned to the inlet water conditioning tank in step (1).

2. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (1), the high-salt chemical wastewater is a concentrated liquid produced by wastewater from industries such as coal chemical, petrochemical, fine chemical, printing and dyeing, pharmaceutical, pesticide, coking, metallurgy, and phosphorus chemical after being treated by a near-zero discharge process. The main indicators of the high-salt chemical wastewater are TDS≥35000mg / L, sodium sulfate and sodium chloride content in the wastewater≥90%, pH=1-14, and COD≤5000mg / L.

3. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (1), the high-salt chemical wastewater is pumped into an equalization tank for conditioning and then into a dedicated impurity removal device for deep removal of fluoride, silicon, carbonate, bicarbonate, phosphate, nitrate, nitrite, potassium ions, inert organic matter, and heavy metals; thus, F - ≤1mg / L, Si≤20mg / L, CO3 2- &HCO3 - ≤1000mg / L,PO4 3- ≤2mg / L, NO3 - &NO2 - ≤500mg / L,K + ≤50mg / L, COD≤30mg / L, heavy metals≤1mg / L.

4. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (1), the special impurity removal device is a customized device. The acid used to adjust the pH during the impurity removal process is a dilute sulfuric acid or dilute hydrochloric acid with a mass fraction of 15%-30%, and the alkali used is a sodium hydroxide solution with a mass fraction of 15%-30%.

5. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (2), the evaporation and concentration are carried out using a falling film MVR device, so that the TDS of the mixed brine of sodium sulfate and sodium chloride is 150g / L-200g / L; the flow-through components of the falling film MVR device are made of corrosion-resistant materials such as titanium and 2205 duplex steel, which can cope with the conditions of highly corrosive wastewater containing fluoride and high chloride ions.

6. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (2), the mixing tank is a vertical atmospheric pressure jacketed mixing tank with an integral insulation jacket. It is an atmospheric pressure device with a design pressure of 0-0.1 MPa, a carbon steel cylinder, and a PTFE lining. The phrase "adjusting the mixing under low-speed stirring" refers to: ① The low-speed stirring is controlled at 140-180 r / min, preferably 160 r / min; ② Adjust the chloride ion concentration in the brine so that =0.85-1.7 mol / L; ③ Add calcium chloride to adjust the ratio of calcium ions to sulfate ions in the brine, so that... The molar ratio is 1.05-1.1:1; ④ Add additives (crystallization modifiers) at a rate of 0.04% to 0.08% of the total mass of the brine; By adjusting the above ①②③④, a suspension slurry that meets the crystallization conditions is obtained.

7. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (2), the pH and temperature are adjusted to control the pH to 4.5-6, the pH is adjusted using dilute sulfuric acid with a mass fraction of 5-10%, and the temperature is controlled at 85-90℃.

8. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (3), the induced crystallization reactor refers to a jacketed, corrosion-resistant reactor, preferably an atmospheric pressure enamel-lined or carbon steel PTFE-lined jacketed reactor. The reactor is heated by one of steam heating, heat transfer oil circulation heating, or electric heating, preferably steam heating. The reactor is cooled by medium-assisted cooling, and the cooling medium is water, ethylene glycol, or chilled brine. The reactor operating pressure is 0–0.01 MPa, the slurry preheating temperature is 85–90°C, the reaction temperature is controlled at 98–103°C, the reaction heating rate is 1–2°C / min, the reaction time is controlled at 2.5–5 h, and the pH of the reaction slurry is 4.5–6, preferably 5–5.

5. The low-speed stirring is controlled at 100–150 r / min, preferably 120–130 r / min. The supernatant in the reactor is discharged from the overflow port on the upper side and pumped into the storage tank, and the crystal slurry is discharged from the bottom outlet.

9. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (3), the additive is a crystal habit modifier, specifically a compound of quaternary ammonium salt and inorganic salt. The quaternary ammonium salt includes hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, tetradecyltrimethylammonium bromide, and tetradecyltrimethylammonium chloride. The inorganic salt includes copper chloride (CuCl2) and magnesium chloride. Aluminum chloride (AlCl3), preferably hexadecyltrimethylammonium bromide and magnesium chloride. The compound is formulated with CTAB, the dosage of which is controlled at 0.01% to 0.03% of the total brine mass. The dosage should be controlled at 0.03% to 0.05% of the total mass of brine.

10. The method for preparing α-calcium sulfate hemihydrate whiskers by in-situ induced crystallization from high-salt chemical wastewater according to claim 1, characterized in that: In step (4), the equipment used for solid-liquid separation of the crystal slurry includes a vertical conical centrifuge, a horizontal single-stage pusher centrifuge, a horizontal double-stage pusher centrifuge, a horizontal spiral sedimentation centrifuge, and a heated PTFE-lined horizontal spiral centrifuge, preferably a heated PTFE-lined horizontal spiral centrifuge. The solid-liquid separation, washing, and drying of the crystal slurry include the following continuous operation process: ① The crystal slurry output temperature is ≥90℃, and the whole process is heated and insulated. The pipes and separation equipment are insulated. The heated PTFE-lined horizontal spiral centrifuge has a constant temperature of 90℃ in the machine body jacket. The crystal slurry enters the drum, and the centrifugal force realizes the solid phase sedimentation of the whiskers. It is continuously squeezed to form a filter cake. The salt-containing mother liquor overflows and is collected into the mother liquor storage tank. The salt-containing mother liquor is returned to the water inlet regulating tank in step (1). ② The centrifuge has a built-in countercurrent washing channel, which uses 90℃ deionized hot water to spray the filter cake, washing away the NaCl adsorbed on the surface of the whiskers and controlling the residual chloride ions in the finished product. ≤0.1%; ③ The wet calcium sulfate whisker filter cake is fed into a hot air drying device, which includes a multi-stage belt hot air dryer, a multi-layer belt hot air dryer, an airflow dryer, a vacuum rake dryer, a fluidized bed dryer, and a disc dryer. Preferably, it is a multi-layer belt hot air dryer. The drying hot air temperature is controlled at 80-110℃. The first stage temperature is controlled at 80-90℃ to remove free water, and the second stage temperature is controlled at 100-110℃ to remove adsorbed water, so that the moisture content of the finished product is ≤0.5%. The equipment operates under closed negative pressure, the hot air is recycled, and the tail gas is sprayed and condensed to recover salt. If high-temperature resistant anhydrous calcium sulfate whiskers are required, a high-temperature calcination device is added to the rear end of the belt dryer to control the calcination temperature at 500-600℃.

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