A method and system for green preparation of alpha-hemihydrate gypsum by normal pressure low temperature salt medium method

Abstract

The application discloses a kind of normal pressure low temperature salt medium method green preparation alpha-hemihydrate gypsum method and system.The method uses lithium chloride solution as crystal transformation medium, and is mixed with gypsum raw material to form suspension under normal pressure, by controlling the synergic coupling relationship of lithium chloride concentration and reaction temperature, trigger the abnormal kinetics phase transition of dihydrate gypsum to alpha-hemihydrate gypsum at 80 DEG C.Under the reaction, after solid-liquid separation, alpha-hemihydrate gypsum solid phase is washed by boiling water to remove adsorbed salt, and the mother liquor and washing mother liquor are recycled after being concentrated and impure.The application can use single-stage or multi-stage cascade crystallization process, and use crystallization water to gradually dilute and cooperate temperature gradient to improve, to ensure efficient crystallization.The application breaks through the problems of large energy consumption of traditional high temperature or autoclave process and heavy pollution of organic solvent method, and has excellent impurity tolerance and conversion efficiency, to provide a green and general technical route for high-value clean use of industrial solid waste and natural gypsum resources.

Description

Technical Field

[0001] This invention relates to the field of α-hemihydrate gypsum preparation, and specifically to a method and system for preparing α-hemihydrate gypsum using gypsum in an inorganic salt-water system under normal pressure and low temperature conditions. Background Technology

[0002] Gypsum is a widely used traditional cementitious material, its main component being calcium sulfate. Hemihydrate gypsum (CaSO4·½H2O) is an intermediate phase product formed from dihydrate gypsum (CaSO4·2H2O) through a dehydration process. Depending on its crystal structure and preparation conditions, it can be divided into two crystal forms: α-hemihydrate gypsum and β-hemihydrate gypsum. α-Hemihydrate gypsum exhibits high crystal integrity, with crystals appearing as short columnar or dense granules. After hydration, it possesses characteristics such as high strength, low water demand, and a dense structure. Its dry compressive strength can reach 25-50 MPa, far exceeding... β Type hemihydrate gypsum (usually below 10 MPa) is therefore called high-strength gypsum.

[0003] Currently, there are three main methods for preparing α-hemihydrate gypsum industrially: autoclaving, hydrothermal methods, and hybrid methods. Autoclaving involves placing dihydrate gypsum in an autoclave and dehydrating it under saturated steam to convert it into α-hemihydrate gypsum. Hydrothermal methods involve mixing dihydrate gypsum with a crystallizing agent and water to form a suspension, followed by heating for conversion. Hybrid methods combine the first two methods. However, these traditional methods have many technical limitations in industrial production. Autoclaving is a relatively traditional method for preparing α-hemihydrate gypsum. For example, the method described in patent CN100345788C involves mixing chemical gypsum with a crystallizing agent, extruding it, and then autoclaving it in saturated steam at 135-160℃ for 1-3 hours to convert dihydrate gypsum into α-hemihydrate gypsum. Although this method is relatively simple, it suffers from problems such as uneven conversion, difficulty in controlling crystal morphology, and large fluctuations in product strength. Especially for industrial by-product gypsum, such as phosphogypsum and desulfurized gypsum, the impurities they contain significantly affect the conversion process and quality stability. In addition, the autoclaving process has a long production cycle, usually requiring 2-10 hours, high energy consumption, and strict requirements for raw material pretreatment. Chemical gypsum usually needs to be dried before subsequent processing, which increases production costs and environmental burden.

[0004] To overcome the drawbacks of high-pressure operation, researchers have developed methods for crystallization under atmospheric pressure. One type is the atmospheric pressure salt solution method, the core of which utilizes an aqueous solution of a specific inorganic salt (such as calcium chloride, magnesium chloride, potassium chloride, sodium sulfate, etc.) as the crystallization medium to achieve the conversion of dihydrate gypsum to α-hemihydrate gypsum under atmospheric pressure and near-boiling point temperature (usually 90-100℃). For example, a related patent from Zhejiang University (CN1569642A) discloses a method using a composite salt solution (CaCl2, MgCl2, KCl) and reacting at 90-95℃ for 2-4 hours. Another type is the atmospheric pressure alcohol-water solution method. CN120097374A describes a method that, in an ethylene glycol-water mixed solution, achieves efficient and controllable conversion of desulfurized gypsum to α-hemihydrate gypsum with different morphologies under mild atmospheric pressure by controlling three key variables: inorganic salt concentration, type and concentration of organic crystallization agent, and reaction solid content. Compared to acidic solution methods, this method is less corrosive and releases no acidic gases. However, the organic solvent system contains 30-60% ethylene glycol, which, although recyclable, still presents issues of volatility, cost, and energy consumption during recovery. It relies on specific crystallization agents, requiring the use of organic complexing agents such as EDTA / EGTA / PDTA / HDTA, which are expensive. The introduction of nitrogen may limit the product's use in biological or building materials fields. The reaction time is still relatively long, with some products requiring over 20 hours, limiting production capacity. There is a risk of side reactions, such as the presence of polymorphic potassium gypsum impurities in Example 23, which affects product purity.

[0005] Therefore, in view of the above-mentioned long-term high-temperature crystal transformation process, the purpose of this patent is to provide a more efficient and energy-saving crystal transformation solution, overcome the above-mentioned defects, and complete the efficient crystal transformation at a lower temperature and in a shorter time, so as to achieve energy saving, consumption reduction and improved production efficiency. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of long reaction cycles, high energy consumption, organic solvent pollution, and insufficient process flexibility in existing α-hemihydrate gypsum preparation processes, and to provide a green method and system for the preparation of α-hemihydrate gypsum using a low-temperature, atmospheric-pressure salt medium method. This invention is based on the extraordinary crystal transformation kinetics triggered by the strong hydration of lithium ions, achieving efficient crystal transformation at low temperatures and atmospheric pressure through the synergistic coupling control of concentration and temperature. It is compatible with both single-stage and multi-stage process modes, and is equipped with a graded mother liquor recycling system, possessing the comprehensive advantages of high conversion efficiency, low energy consumption, environmental friendliness, and low operating costs.

[0007] The technical solution to the problem solved by this invention is as follows:

[0008] Using lithium chloride aqueous solution as the crystal transformation medium, the core mechanism driving the dehydration phase transition of gypsum dihydrate lies in the synergistic effect of two effects: first, the strong hydration of lithium ions significantly reduces the water activity of the system, disrupting the phase equilibrium of gypsum dihydrate and providing a thermodynamic driving force for the phase transition; second, relying on the specific ion and ion pair effect under high salt conditions, the dehydration reaction kinetics at the solid-liquid interface of gypsum dihydrate crystals are significantly enhanced, greatly increasing the phase transition rate. Based on the above mechanism, this invention has defined the stepwise synergistic coupling law of concentration and temperature through a large number of experiments: when the mass percentage of lithium chloride is 20%~30%, a reaction temperature of 70℃~80℃ is matched to supplement the driving force of the phase transition and ensure efficient crystal transformation; when the mass percentage of lithium chloride is increased to 30%~50%, the synergistic contribution of strong lithium ion hydration and ion effect dominates, and high-speed phase transition can be achieved in a wide temperature range of 30℃~80℃. In addition, to reduce the operating cost of the system, the amount of other soluble inorganic salts that can be doped or tolerated in the crystal transfer medium shall not exceed 30% of the total mass of the solute, forming a mixed high-salt system. Under the synergistic effect of ion and ion pair effects, the system can still maintain excellent crystal transfer performance.

[0009] The first aspect of this invention is to provide a green method for preparing α-hemihydrate gypsum using a low-temperature salt medium under normal pressure, comprising the following steps:

[0010] (1) Ingredients: Under normal pressure, gypsum raw materials are mixed with lithium chloride aqueous solution as a crystallization medium to form a suspension;

[0011] (2) Crystallization reaction: By controlling the synergistic coupling relationship between the concentration of lithium chloride aqueous solution and the reaction temperature, the dihydrate gypsum in the suspension undergoes a phase change and is converted into α-hemihydrate gypsum. The crystallization temperature does not exceed 80℃.

[0012] (3) Separation and washing: The suspension after the reaction is completed is subjected to solid-liquid separation to collect the solid phase of α-hemihydrate gypsum and the mother liquor of crystallization. The obtained solid phase of α-hemihydrate gypsum is washed with boiling water while hot, and then the product of α-hemihydrate gypsum is obtained by solid-liquid separation and drying.

[0013] (4) Mother liquor recycling and regeneration: part of the crystallization mother liquor is recycled back to the preparation step of the suspension, and the other part of the crystallization mother liquor and washing mother liquor are transported to the salt solution regeneration system. After concentration, dehydration and impurity removal, the regenerated salt solution is recycled to the preparation step of the suspension.

[0014] Furthermore, the mass percentage of the lithium chloride aqueous solution is 20%~50%; the synergistic coupling relationship between the concentration and the reaction temperature satisfies:

[0015] When the mass percentage of lithium chloride is 20%~30%, the reaction temperature is controlled at 70℃~80℃;

[0016] When the mass percentage of lithium chloride is 30%~50%, the reaction temperature is controlled at 30℃~80℃.

[0017] Furthermore, the crystal transformation medium also includes other soluble inorganic salts doped or contained in the raw materials, wherein the other soluble inorganic salts are selected from chlorides, nitrates or sulfates of alkali metals or alkaline earth metals, and their doping mass does not exceed 30% of the total mass of the solute in the crystal transformation medium solution.

[0018] Furthermore, the gypsum raw material is selected from natural gypsum, desulfurized gypsum, phosphogypsum, or mixtures thereof.

[0019] Furthermore, the gypsum raw material has a particle size distribution of 1μm-2mm. The particle size of the raw material is positively correlated with the total crystallization time. The smaller the particle size, the shorter the crystallization induction period and the complete conversion time.

[0020] Furthermore, when using desulfurized gypsum or phosphogypsum, deep purification and washing are performed before the reaction to control the P2O5 content in the gypsum raw material to <0.1%, F - Content <0.05%.

[0021] Furthermore, the solid-liquid separation is carried out by hot filtration or centrifugation; after separation, the α-hemihydrate gypsum product is washed with boiling water, and the washing mother liquor generated is concentrated and then recycled together with the crystallization mother liquor.

[0022] Furthermore, step (2) can be a single-stage crystal transformation process or a ladder-type crystal transformation process with at least two stages in series. Along the material flow direction, the concentration of lithium chloride aqueous solution in each stage of the crystal transformation process decreases step by step, and the reaction temperature increases step by step. The crystal water released by the dehydration of gypsum dihydrate enters the liquid phase and gradually dilutes the crystal transformation medium. By gradually increasing the reaction temperature, the synergistic coupling effect of concentration and temperature is maintained, thus ensuring the crystal transformation reaction rate.

[0023] Furthermore, the stepped crystal transformation process is a multi-stage series process; in the first-stage crystal transformation process, the lithium chloride mass percentage is 35%~50%, and the reaction temperature is 30℃~60℃; in the final-stage crystal transformation process, the lithium chloride mass percentage is 20%~30%, and the reaction temperature is 70℃~80℃; the concentration gradient between adjacent stages is matched with the temperature gradient.

[0024] A second aspect of the invention is to provide a system for implementing the method, comprising, sequentially along a process flow, connected via a closed pipeline:

[0025] Salt solution preparation unit: used to prepare and preheat lithium chloride aqueous solution;

[0026] Atmospheric pressure crystal conversion unit: equipped with temperature control and stirring system, it can adopt single-stage or multi-stage series crystal conversion reactor. The inner surface of the atmospheric pressure crystal conversion reactor is polished to reduce heterogeneous crystal nucleation and scale deposition on the equipment wall.

[0027] Solid-liquid separation and drying unit: used for product separation, washing, and drying;

[0028] Mother liquor recycling and regeneration unit: includes pipelines for direct reuse of crystallization mother liquor, and washing mother liquor concentration and regeneration device. The mother liquor recycling and regeneration unit is also connected by a bypass salt separation component for evaporation-cooling coupled crystallization of the multi-element high-salt mother liquor accumulated in the system to separate impurity salts and recover lithium chloride.

[0029] Advantages and beneficial effects of the present invention:

[0030] 1. Compared to the reaction cycle of conventional inorganic salt systems, which requires several hours or even tens of hours, the lithium-containing synergistic system of this invention exhibits an extremely fast phase transition rate, and can complete 100% conversion in as little as a few minutes (e.g., 4 minutes), shortening the industrial conversion cycle by tens to hundreds of times, significantly shortening the production cycle and increasing production capacity, and has extremely outstanding substantive features.

[0031] 2. Compared with other high-temperature and high-pressure reaction systems, the lithium salt synergistic system of this invention has the advantage of being able to completely dehydrate at temperatures below 80°C, and even achieve rapid and efficient conversion at room temperature by adjusting the concentration of the lithium salt solution. It does not require high-pressure steaming equipment or near-boiling-point high-temperature heating, which reduces heat source consumption and equipment pressure resistance costs, and also eliminates the safety hazards of high-pressure processes.

[0032] 3. This invention is compatible with single-stage intermittent production and multi-stage continuous cascade crystal transformation processes, and can be flexibly configured according to production capacity and product grade; the cascade process can match the natural process of dehydration and release of crystal water in dihydrate gypsum, and maintain a stable crystal transformation rate through the synergistic effect of concentration and temperature gradients, thereby reducing salt consumption and washing load while ensuring conversion efficiency.

[0033] 4. This invention adopts a pure inorganic brine crystallization system, which completely avoids the risk of volatilization of organic solvents and the problem of organic reagent residues; with the matching graded mother liquor recycling and regeneration system, the main crystallization mother liquor can be directly reused, and only part of the mother liquor and washing liquid are concentrated and impurities removed. The lithium salt recovery rate is high, the system operating cost is low, and it meets the requirements of green production. Attached Figure Description

[0034] Figure 1 This is a process flow diagram of the preparation of α-hemihydrate gypsum by the atmospheric pressure low temperature salt medium method described in this invention;

[0035] Figure 2The XRD diffraction patterns of raw material gypsum dihydrate and product α-hemihydrate gypsum in Example 2 of this invention;

[0036] Figure 3 The DSC thermal analysis spectra of raw material dihydrate gypsum and product α-hemihydrate gypsum in Example 2 of this invention;

[0037] Figure 4 This is the in-situ Raman spectrum of the crystallization reaction process in Example 2 of the present invention;

[0038] Figure 5 This is a graph showing the conversion rate changes of dihydrate gypsum and the product α-hemihydrate gypsum during the crystallization reaction process in Example 2 of the present invention. Detailed Implementation

[0039] Example 1:

[0040] Prepare 450g of a 50% LiCl aqueous solution, maintain the temperature at 30±0.5℃, stir at 300rpm, and add 50g of gypsum (purity ≥95%, average particle size D). 50 =80μm, particle size distribution 10μm-200μm), forming a suspension with a solid content of about 10wt%.

[0041] α-Hemihydrate gypsum was first detected at 29 minutes, and complete conversion occurred at 840 minutes (the characteristic peak of dihydrate gypsum disappeared). After hot rinsing with boiling water (solid-liquid mass ratio 1:1), ICP-OES analysis showed a Li content of 18 ppm in the solid phase. XRD analysis showed no shift in the characteristic peaks of hemihydrate gypsum or lithium salts, indicating that Li... + It does not enter the crystal lattice structure to form a stable solid phase, but mainly exists in a soluble state or a surface adsorbed state. It can be realized by washing with boiling water. + The basic complete removal and recycling.

[0042] Example 2:

[0043] Prepare 450g of a LiCl aqueous solution with a mass fraction of 40%, control the temperature to 50±0.5℃, stir at 300rpm, add 50g of gypsum (same as in Example 1), and form a suspension with a solid content of about 10wt%.

[0044] α-Hemihydrate gypsum was first detected in just 3 minutes, and complete conversion occurred within 60 minutes (the Raman characteristic peak of dihydrate gypsum disappeared). After hot rinsing with boiling water (solid-liquid mass ratio 1:1), ICP-OES analysis showed a Li content of 15 ppm in the solid phase with no significant enrichment. XRD analysis showed no shift in the characteristic peaks of hemihydrate gypsum or lithium salts, indicating that Li... + It does not enter the crystal lattice structure to form a stable solid phase, but mainly exists in a soluble state or a surface adsorbed state. It can be realized by washing with boiling water. +The basic complete removal and recycling.

[0045] Example 3:

[0046] Prepare 350g of a LiCl aqueous solution with a mass fraction of 20%, control the temperature to 80±0.5℃, stir at 300rpm, add 150g of gypsum (same as in Example 1), and form a suspension with a solid content of about 30wt%.

[0047] α-Hemihydrate gypsum was initially detected at 3 minutes, and complete conversion occurred at 130 minutes (the Raman characteristic peak of dihydrate gypsum disappeared). After hot rinsing with boiling water (solid-liquid mass ratio 1:1), ICP-OES analysis showed a Li content of 12 ppm in the solid phase with no significant enrichment. XRD analysis showed no shift in the characteristic peaks of hemihydrate gypsum or lithium salts, indicating that Li... + It does not enter the crystal lattice structure to form a stable solid phase, but mainly exists in a soluble state or a surface adsorbed state. It can be realized by washing with boiling water. + The basic complete removal and recycling.

[0048] Example 4:

[0049] Prepare 450g of a mixed salt solution with a mass fraction of 25%, wherein LiCl accounts for 80% of the total mass of the solute and NaCl accounts for 20%. Control the temperature to 65±0.5℃, stir at 300rpm, and add 50g of gypsum (specifications same as in Example 1) to form a suspension with a solid content of about 10wt%.

[0050] α-Hemihydrate gypsum was first detected at 18 minutes, and complete conversion occurred at 125 minutes (the Raman characteristic peak of dihydrate gypsum disappeared). The product was then washed with boiling water (solid-liquid mass ratio 1:1) while hot, and the Li content in the solid phase was detected by ICP-OES. + and Na + The total content was 10 ppm, with no obvious enrichment. XRD analysis showed no shift in the characteristic peaks of hemihydrate gypsum or characteristic peaks of sodium and lithium salts, indicating that Na... + Li + It does not enter the crystal lattice structure to form a stable solid phase, but mainly exists in a soluble state or a surface adsorbed state. It can be realized by washing with boiling water. + The basic complete removal and recycling.

[0051] Example 5:

[0052] A two-stage crystallization process under normal pressure was employed: First, 300g of a 40% LiCl aqueous solution was prepared in the first-stage crystallization reactor at 30°C and stirred at 300rpm. Then, 200g of natural dihydrate gypsum (same specifications as in Example 1) with a solid content of 30wt% was added. α-Hemihydrate gypsum was first detected after 54 minutes. The reaction proceeded to a conversion rate of approximately 45% (approximately 100 minutes). The first-stage reaction solution was then rapidly transferred to the second-stage crystallization reactor, where the temperature was controlled at 50°C and the stirring was 300rpm. Complete conversion was achieved in the second-stage reactor after 42 minutes (the Raman characteristic peak of the dihydrate gypsum disappeared). Compared to the single-stage 40% LiCl solution at -30°C process (622 minutes), the total reaction time was shortened by approximately 77%. After hot rinsing with boiling water (solid-liquid mass ratio 1:1), the Li content in the solid phase was detected by ICP-OES. + The content was 20 ppm, with no obvious enrichment. XRD analysis showed no shift in characteristic peaks of hemihydrate gypsum or lithium salt characteristic peaks, indicating that Li... + It does not enter the crystal lattice structure to form a stable solid phase, but mainly exists in a soluble state or a surface adsorbed state. It can be realized by washing with boiling water. + The basic complete removal and recycling.

[0053] Example 6:

[0054] Based on the process conditions of Example 2 (40% LiCl solution, 50±0.5℃, stirring at 300 rpm), five mother liquor recycling experiments were conducted, controlling the solid content to be 10 wt%. After each reaction, the solid phase was separated by vacuum filtration while hot, and 100% of the crystallization mother liquor was reused. The product was washed hot with boiling water (solid-liquid mass ratio 1:1), and the Li content in the solid phase was detected by ICP-OES. + Ca 2+ and SO4 2- The content of impurities and the total mass of impurities in the mother liquor were measured. The results of mother liquor recycling are as follows:

[0055] (1) The mother liquor was circulated for the first time. The specifications of the gypsum raw materials were the same as in Example 1. α-hemihydrate gypsum was detected for the first time after 3 minutes. The conversion was completed after 60 minutes. The content of α-hemihydrate gypsum was 99.3%. The total amount of impurities in the mother liquor was 2.3 g / L.

[0056] (2) The mother liquor was recycled for the second time. The specifications of the gypsum raw materials were the same as in Example 1. α-hemihydrate gypsum was detected for the first time after 3 minutes. The conversion was complete after 60 minutes. The content of α-hemihydrate gypsum was 99.3%, and the total amount of impurities in the mother liquor was 3.5 g / L.

[0057] (3) The mother liquor was circulated for the third time. The specifications of the gypsum raw materials were the same as in Example 1. α-hemihydrate gypsum was detected for the first time after 3 minutes. The conversion was completed after 59 minutes. The content of α-hemihydrate gypsum was 99.4%. The total amount of impurities in the mother liquor was 5.2 g / L.

[0058] (4) The mother liquor was circulated for the fourth time. After grinding, the gypsum raw material (specifications as in Example 1) was D 50 The particle size was 50 μm, with a particle size distribution of 1 μm-100 μm. α-hemihydrate gypsum was first detected after 2 minutes, and complete conversion occurred after 52 minutes. The α-hemihydrate gypsum content was 99.0%, and the total impurities in the mother liquor were 6.4 g / L.

[0059] (5) For the fifth time in the mother liquor recycling, the gypsum raw material used was desulfurized gypsum from a power plant. It was first washed and purified. After washing, the purity of the gypsum was tested to be 94.6%. D 50 The particle size was 80 μm, with a particle size distribution of 1 μm-150 μm. α-hemihydrate gypsum was first detected after 3 minutes, and complete conversion was achieved after 62 minutes. The α-hemihydrate gypsum content was 99.0%, and the total impurities in the mother liquor were 8.3 g / L.

[0060] Comparative Example 1:

[0061] A 25% (w / w) LiCl aqueous solution was prepared, and the temperature was controlled at 60 ± 0.5 °C. The mixture was stirred at 300 rpm. The gypsum raw material specifications were the same as in Example 1. After 720 minutes of reaction, no conversion to α-hemihydrate gypsum occurred.

[0062] Comparative Example 2:

[0063] A 40% (w / w) LiNO3 aqueous solution was prepared, and the temperature was controlled at 80°C with stirring at 300 rpm. The gypsum raw material specifications were the same as in Example 1. After reacting for 6 hours under these conditions, no effective conversion from dihydrate gypsum to α-hemihydrate gypsum was observed. This result indicates that although the LiNO3 system has high solubility and strong ion migration ability at 80°C, a single high-concentration LiNO3 aqueous solution system cannot replace LiCl.

[0064] Figure 1 This is a process flow diagram of the atmospheric pressure salt medium method for preparing α-hemihydrate gypsum according to the present invention. The process system is arranged sequentially along the material flow direction as follows: a salt solution preparation unit, an atmospheric pressure single-stage / multi-stage crystallization system, a solid-liquid separation-washing-drying unit, and a salt solution regeneration system. The atmospheric pressure single-stage / multi-stage crystallization system can be configured as a single-stage reaction or a multi-stage series mode. In the two-stage series mode, it is divided into two reaction stages: a high-concentration low-temperature crystallization unit and a low-concentration high-temperature crystallization unit. The process flow is as follows: the salt solution preparation unit receives intermittently replenished inorganic salt and regenerated salt solution, prepares the lithium chloride crystallization medium, and then sends it to the atmospheric pressure crystallization system; the dihydrate gypsum raw material is introduced into the first-stage unit of the atmospheric pressure crystallization system, sequentially undergoing the main crystallization in the high-concentration low-temperature stage and the deep transformation in the low-concentration high-temperature stage, completing the transformation of dihydrate gypsum into... α - Crystal transformation of hemihydrate gypsum; the slurry after crystal transformation is subjected to solid-liquid separation, boiling water washing, and drying treatment to obtain... α- Hemihydrate gypsum products. The mother liquor is recycled in a graded cycle: most of the crystallization mother liquor generated from solid-liquid separation is directly recycled back to the salt solution preparation unit for reuse, and a small portion of the mother liquor is sent to the salt solution regeneration system; all the washing mother liquor generated by the washing unit is sent to the salt solution regeneration system, and after concentration and removal of soluble salt impurities, the regenerated salt solution is returned to the salt solution preparation unit for recycling, and the water removed and the enriched impurities are discharged for treatment.

[0065] Figure 2 The XRD diffraction patterns of raw material gypsum dihydrate and product α-hemihydrate gypsum in Example 2 of this invention are shown. The top of the image shows the measured diffraction peaks of the sample, while the middle and bottom peaks represent those of the raw material gypsum dihydrate and the standard card, respectively. The main diffraction peak position of the raw material gypsum dihydrate is shown in Figure 2. θ The diffraction peaks at approximately 11.80°, 20.85°, 23.50°, and 29.25° are basically consistent with the standard diffraction peaks of CaSO4·2H2O, indicating that the main phase of the raw material is calcium sulfate dihydrate. After dehydration treatment, the characteristic diffraction peaks of the sample changed significantly, and the positions of the main diffraction peaks were similar to those of CaSO4·2H2O. α - The standard diffraction peaks of hemihydrate gypsum are basically consistent, at 2 θ Distinct characteristic peaks were observed at approximately 14.68°, 25.36°, 29.62°, 31.24°, and 49.30°, and no characteristic peaks of gypsum dihydrate were found, indicating that the crystallization reaction was complete and the main phase of the product was α-hemihydrate gypsum.

[0066] Figure 3 The raw material dihydrate gypsum and the product in Example 2 of this invention α - The DSC thermal analysis spectrum of hemihydrate gypsum shows that the raw gypsum exhibits dual endothermic peaks at 148℃ and 180℃, corresponding to the temperature at which the gypsum crystal water is removed. The hemihydrate gypsum product shows only an endothermic peak at 165℃ after 60 minutes, indicating that the dihydrate gypsum has completely disappeared, corresponding to the process of the hemihydrate gypsum losing the last 0.5 units of crystal water. Figure 3 It is obvious that there is no exothermic peak at 300–350℃, confirming that the crystallization product is α-hemihydrate gypsum.

[0067] Figure 4 This is the in-situ Raman spectrum of the solid phase during the crystallization reaction process in Example 2 of this invention. Because Raman spectroscopy is most sensitive to 1008 cm⁻¹... -1 (CaSO4·2H2O) and 1016cm -1The characteristic peaks of (CaSO4·0.5H2O) are highly sensitive; therefore, these two characteristic peaks are used as the basis for determining the crystalline phase of calcium sulfate in the slurry. As shown in the figure, with the progress of the reaction, the characteristic peak of dihydrate gypsum gradually decreases, while the related characteristic peaks of α-hemihydrate gypsum gradually increase and tend to stabilize. The appearance of the α-hemihydrate gypsum peak at 3 min indicates that dihydrate gypsum has begun to transform into α-hemihydrate gypsum. Between 3 min and 60 min, the characteristic peaks of gypsum and hemihydrate gypsum coexist, with the gypsum peak intensity gradually weakening and disappearing, while the hemihydrate gypsum peak intensity gradually increases, indicating the dynamic transformation of the two crystalline phases. After 60 min, only the hemihydrate gypsum characteristic peak remains, thus obtaining the stable α-hemihydrate gypsum crystalline phase.

[0068] Figure 5 The crystallization reaction process in Example 2 of this invention is based on the Raman spectrum at 1008 cm⁻¹. -1 (CaSO4·2H2O) and 1016cm -1 The graph shows the trend of the conversion rate of the two crystalline phases in the solid phase, calculated from the peak intensity of the characteristic peak of (CaSO4·0.5H2O). As can be seen from the graph, with the progress of the reaction, the content of dihydrate gypsum gradually decreases at 3 min, indicating the start of crystal transformation, while the conversion rate of α-hemihydrate gypsum continuously increases, indicating that dihydrate gypsum is continuously undergoing a crystal transformation to α-hemihydrate gypsum. In the initial stage of the reaction (3–20 min), dihydrate gypsum dissolves rapidly and crystal nuclei form, and the conversion rate of α-hemihydrate gypsum increases rapidly, indicating a fast reaction rate. Subsequently, as the reaction continues, the amount of convertible dihydrate gypsum in the system gradually decreases, the crystal transformation rate decreases, and the conversion curve tends to flatten. After 60 min of reaction, the conversion rate of α-hemihydrate gypsum tends to stabilize, indicating that the crystal transformation reaction gradually reaches equilibrium, and the conversion of dihydrate gypsum to α-hemihydrate gypsum is basically complete.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A green method for preparing α-hemihydrate gypsum using a normal pressure, low temperature, and salt medium method, characterized in that, Includes the following steps: (1) Ingredients: Under normal pressure, gypsum raw materials are mixed with lithium chloride aqueous solution as a crystallization medium to form a suspension; (2) Crystallization reaction: By controlling the synergistic coupling relationship between the concentration of lithium chloride aqueous solution and the reaction temperature, the dihydrate gypsum in the suspension undergoes a phase change and is converted into α-hemihydrate gypsum. The crystallization temperature does not exceed 80℃. (3) Separation and washing: The suspension after the reaction is completed is subjected to solid-liquid separation, and the contents are collected. α - Hemihydrate gypsum solid phase and crystallization mother liquor, obtained α - The hemihydrate gypsum solid phase is washed with boiling water while hot, followed by solid-liquid separation and drying to obtain... α - Hemihydrate gypsum products; (4) Mother liquor recycling and regeneration: part of the crystallization mother liquor is recycled back to the preparation step of the suspension, and the other part of the crystallization mother liquor and washing mother liquor are transported to the salt solution regeneration system. After concentration, dehydration and impurity removal, the regenerated salt solution is recycled to the preparation step of the suspension.

2. The method according to claim 1, characterized in that, The lithium chloride aqueous solution has a mass percentage content of 20%~50%; the synergistic coupling relationship between the concentration and the reaction temperature satisfies: When the mass percentage of lithium chloride is 20%~30%, the reaction temperature is controlled at 70℃~80℃; When the mass percentage of lithium chloride is 30%~50%, the reaction temperature is controlled at 30℃~80℃.

3. The method according to claim 1, characterized in that, The crystal transformation medium also includes other doped soluble inorganic salts, which are selected from chlorides, nitrates or sulfates of alkali metals or alkaline earth metals. The sources of these salts can be actively added or carried in the raw materials, and their doping mass does not exceed 30% of the total mass of the solute in the crystal transformation medium solution.

4. The method according to claim 1, characterized in that, The gypsum raw material is selected from natural gypsum, desulfurized gypsum, phosphogypsum, or mixtures thereof.

5. The method according to claim 4, characterized in that, The gypsum raw material has a particle size distribution of 1μm-2mm. The particle size of the raw material is positively correlated with the total crystallization time. The smaller the particle size, the shorter the crystallization induction period and the complete conversion time.

6. The method according to claim 4, characterized in that, When using desulfurized gypsum or phosphogypsum, deep purification and washing are performed before the reaction to control the P2O5 content in the gypsum raw material to <0.1%, F - Content <0.05%.

7. The method according to claim 1, characterized in that, The solid-liquid separation is carried out by hot filtration or centrifugation; after separation, the α-hemihydrate gypsum product is washed with boiling water, and the washing mother liquor generated is concentrated and then recycled together with the crystallization mother liquor.

8. The method according to claim 1, characterized in that, Step (2) can be a single-stage crystal transformation process or a ladder-type crystal transformation process with at least two stages in series. Along the material flow direction, the concentration of lithium chloride aqueous solution in each crystal transformation process decreases step by step and the reaction temperature increases step by step. The crystal water released by the dehydration of gypsum dihydrate enters the liquid phase and gradually dilutes the crystal transformation medium. By gradually increasing the reaction temperature, the synergistic coupling effect of concentration and temperature is maintained, thus ensuring the crystal transformation reaction rate.

9. The method according to claim 8, characterized in that, The stepped crystal transformation process consists of multiple stages connected in series; in the first-stage crystal transformation process, the lithium chloride mass percentage is 35%~50%, and the reaction temperature is 30℃~60℃; in the final-stage crystal transformation process, the lithium chloride mass percentage is 20%~30%, and the reaction temperature is 70℃~80℃; the concentration gradient between adjacent stages is matched with the temperature gradient.

10. A system for implementing the method according to any one of claims 1 to 9, characterized in that, The process flow includes, in sequence, connections via closed pipelines: Salt solution preparation unit: used to prepare and preheat lithium chloride aqueous solution; Atmospheric pressure crystal conversion unit: equipped with a temperature control and stirring system, using a single-stage or multi-stage series crystal conversion reactor. The inner surface of the atmospheric pressure crystal conversion reactor is polished to reduce heterogeneous crystal nucleation and scale deposition on the equipment wall. Solid-liquid separation and drying unit: used for product separation, washing, and drying; Mother liquor recycling and regeneration unit: includes pipelines for direct reuse of crystallization mother liquor, and washing mother liquor concentration and regeneration device. The mother liquor recycling and regeneration unit is also connected in bypass to a salt separation component, which is used to perform evaporation-cooling coupled crystallization on the multi-element high-salt mother liquor accumulated in the system to separate impurity salts and recover lithium chloride.

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