A method for preparing zinc sulfate heptahydrate based on spatially induced directional crystallization

Abstract

This invention proposes a method for preparing zinc sulfate heptahydrate through spatially induced directional crystallization via a zinc-substrate rolled structure. The method involves rolling zinc sheets into zinc coils, utilizing the geometric constraints between the zinc coil layers in conjunction with the high-speed shearing of the reaction liquid to guide seed crystals to grow along a specific direction, yielding high-purity, uniformly morphologically uniform ZnSO4·7H2O crystals. This method achieves ultra-low impurity residue while exhibiting breakthrough structural stability, with a product purity reaching 99.99%. Environmental testing shows that the material retains over 98% of its initial performance during long-term use, successfully solving the stability problems caused by lattice distortion, impurity residue, and industrial scale-up in traditional processes, providing a reliable guarantee for high-end material applications.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of inorganic material synthesis and crystal engineering, specifically relating to a method for preparing high-purity zinc sulfate heptahydrate (ZnSO4·7H2O) through spatially induced directional crystallization via a zinc substrate coiled structure, applicable to the controllable synthesis of zinc salt materials. Background Technology

[0002] Currently, the industrial preparation of zinc sulfate heptahydrate mainly employs methods such as evaporation crystallization, cooling crystallization, and a combination of sodium sulfide precipitation and crystallization. However, these technologies have significant drawbacks: evaporation crystallization is prone to polyhydrate coexistence and dendrite formation due to localized temperature unevenness; cooling crystallization is limited by temperature control precision, leading to nucleation explosions and excessively wide particle size distribution; sulfidation introduces sulfur pollution and results in excessive impurity residues; and ion exchange processes experience an adsorption efficiency decrease of over 60% at high zinc ion concentrations. Products prepared by existing methods generally suffer from disordered orientation, crystal defects, impurity entrapment, uneven hydration, and high energy consumption, resulting in insufficient product purity and solubility fluctuations exceeding 30%, making it difficult to meet the demands of high-end applications.

[0003] This technology innovatively achieves ultra-low impurity residue and high stability in materials through a spatially induced directional crystallization system. Furthermore, this method effectively blocks the co-crystallization pathway of impurity ions; simultaneously, based on a crystal plane control strategy, it generates a dense single-crystal structure, resulting in a product purity exceeding 99.99%, thus overcoming the technical bottlenecks of orientation disorder, crystal defects, impurity residue, and industrial scale-up in traditional processes. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method for preparing zinc sulfate heptahydrate by achieving spatially induced directional crystallization through a zinc substrate curled structure. By utilizing geometric constraints to guide the growth of seed crystals along a specific direction, high-purity and uniformly morphological ZnSO4·7H2O crystals are obtained.

[0005] This invention adopts the following technical solution: a method for preparing zinc sulfate heptahydrate based on space-induced directional crystallization, comprising the following steps:

[0006] S1: Anneal the zinc foil to obtain a zinc substrate with a highly preferred orientation of Zn(002) crystal plane;

[0007] S2: The zinc sheet is rolled into a zinc coil. The spacing between adjacent layers in the zinc coil is less than 2.2 mm; the height of the zinc coil is less than 50 mm. The height of the zinc coil can be achieved by selecting zinc foil of appropriate width, or by cutting during winding. A suitable zinc coil height can ensure the formation of sufficient (002) orientation seed crystals without hindering subsequent orientation growth.

[0008] S3: Immerse the zinc coil in dilute sulfuric acid at 5-10°C with a concentration of 1.2±0.05 mol / L and stir for 6-10 hours; the stirring speed is 100~300 rpm; stirring creates laminar shear between the layers inside the zinc coil, forming a large number of (002) oriented seed crystals in the limited space of the coiled structure, allowing them to grow with high orientation on the outside of the zinc coil.

[0009] S4: Concentrate the reaction solution, then crystallize by gradient cooling to obtain directionally grown ZnSO4·7H2O.

[0010] Generally, the annealing procedure in step S1 is as follows: In step S1, the zinc foil is annealed at 420°C for 4 hours under an argon atmosphere at a rate of 3°C / min, and after natural cooling, a zinc substrate with a Zn(002) crystal plane ratio of ≥90% and a surface roughness Ra≤10 nm is obtained.

[0011] To ensure crystallization quality, the concentration process in step S4 was carried out using a rotary evaporator under a vacuum of -0.08 MPa and a water bath temperature of 55±1°C. The concentration endpoint was determined by a solution density of 1.32±0.02 g / cm³ (25°C). The gradient cooling rate was 0.05°C / min.

[0012] The aforementioned zinc coil is formed by precisely winding zinc sheets using CNC winding technology to ensure appropriate interlayer spacing, thereby constraining the formation of seed crystals. Specifically, the operation of rolling annealed zinc sheets into coils with a diameter of 12±0.2 mm involves using a high-precision servo winding machine (positioning accuracy ±0.01 mm) to spirally wind the zinc foil onto a polished stainless steel mandrel under a constant tension of 10-15 N, strictly controlling the coil diameter to 12±0.2 mm and the interlayer overlap rate to 5-10%. The winding process is carried out in a clean environment, using a constant linear speed mode (0.5-1 m / min) to maintain uniform stress. After winding, the coil is left to stand for 2 hours to release internal stress. This coiled structure, through precise geometric constraints, ensures that the interlayer spacing is no greater than 2.2 mm, for example, controlled between 1.6-2.2 mm, providing an ideal spatial confinement environment for subsequent directional crystallization reactions. Its strict dimensional control and porosity management are crucial to ensuring the directional growth of crystals.

[0013] Compared with existing technologies, the advantages of this invention are: Based on a space-induced directional crystallization system, this technology achieves ultra-low impurity residue while exhibiting breakthrough structural stability. Through a unique crystallization path control technology, the co-crystallization channels for impurities are blocked; combined with a precise crystallographic energy control strategy, a highly dense single-crystal structure is constructed, ensuring that the product purity remains consistently above 99.99%. Tests show that the material retains over 98% of its initial performance during long-term use, successfully solving the stability problems caused by orientation disorder, lattice distortion, impurity residue, and industrial scale-up in traditional processes, providing a reliable guarantee for high-end material applications. Attached Figure Description

[0014] Figure 1 A schematic diagram of the preparation process of zinc sulfate heptahydrate;

[0015] Figure 2 The XRD and SEM images of zinc sulfate heptahydrate obtained in Example 1 of this invention are shown below.

[0016] Figure 3 The XRD and SEM images of zinc sulfate heptahydrate obtained in Example 2 of this invention are shown.

[0017] Figure 4 The XRD pattern of zinc sulfate heptahydrate obtained in Comparative Example 1 of this invention;

[0018] Figure 5 This is the SEM image of zinc sulfate heptahydrate obtained in Comparative Example 2 of this invention;

[0019] Figure 6 This is the SEM image of zinc sulfate heptahydrate obtained in Comparative Example 3 of this invention;

[0020] Figure 7 The XRD pattern of zinc sulfate heptahydrate obtained in Comparative Example 4 of this invention is shown. Detailed Implementation

[0021] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0022] In this invention, 1.2 ± 0.05 mol / L means that the concentration can fluctuate by 0.05 mol / L from the base of 1.2 mol / L.

[0023] Example 1

[0024] This invention discloses a preparation process for zinc sulfate heptahydrate, comprising the following preparation methods:

[0025] 1. Zinc substrate pretreatment

[0026] (1) Take high-purity zinc foil with a purity of 99.99% (thickness 0.2 mm) and cut it into strips of 120±0.3 mm using a CNC cutting machine. The surface flatness error is ≤0.1 mm.

[0027] (2) Place the sample in deionized water (resistivity ≥18 MΩ·cm) and anhydrous ethanol (HPLC grade) for ultrasonic cleaning (frequency 40 kHz, power 120 W) for 10 minutes each to remove surface oxides and organic contaminants.

[0028] (3) Purge the surface of the zinc sheet with high-purity nitrogen until completely dry, and place it in a vacuum drying oven (10⁻³ Pa) for later use.

[0029] 2. Annealing process

[0030] The pretreated zinc sheet was placed horizontally in a tube annealing furnace (argon purity ≥ 99.999%), heated to 420°C at a uniform rate of 3°C / min, held at the temperature for 4 hours, and then the heating system was turned off and allowed to cool naturally to room temperature.

[0031] 3. Precision forming of zinc coils

[0032] (1) Using a CNC winding machine (positioning accuracy ±0.01 mm), the annealed zinc sheet is wound into a spiral structure with a diameter of about 12 mm under constant tension (5.0 ± 0.2 N).

[0033] (2) In the coiled zinc coil, the spacing between two adjacent layers is 1.6-2.2 mm and the height is 50 mm.

[0034] 4. Confined reaction crystallization

[0035] (1) Prepare a 1.2±0.05 mol / L dilute sulfuric acid solution and pre-cool it to 10±0.5°C;

[0036] (2) Immerse the zinc coil in 200 mL of pre-cooled dilute sulfuric acid, place it in a constant temperature reaction vessel (temperature control accuracy ±0.1°C), maintain the system temperature at 8±0.2°C, and react for 10 hours under magnetic stirring at 100 rpm;

[0037] (3) After the reaction is completed, the solution is concentrated using a rotary evaporator under vacuum of -0.08 MPa and water bath temperature of 65±1°C. The concentration endpoint is determined by the solution density reaching 1.32±0.02 g / cm³ (25°C).

[0038] (4) Gradient cooling crystallization was carried out at a cooling rate of 0.05°C / min to obtain oriented ZnSO4·7H2O.

[0039] XRD analysis revealed that the sample exhibited significantly enhanced diffraction peaks with intensities far exceeding the standard reference values, and other characteristic peaks matched the standard spectrum. Combined with the absence of impurity peaks, this indicates that the sample is high-purity zinc sulfate heptahydrate.

[0040] Example 2

[0041] This invention discloses a preparation process for zinc sulfate heptahydrate, comprising the following preparation methods:

[0042] 1. Zinc substrate pretreatment

[0043] (1) Take high-purity zinc foil with a purity of 99.99% (thickness 0.2 mm) and cut it into strips of about 120 mm using a CNC cutting machine. The surface flatness error is ≤0.1 mm.

[0044] (2) Place the sample in deionized water (resistivity ≥18 MΩ·cm) and anhydrous ethanol (HPLC grade) for ultrasonic cleaning (frequency 40 kHz, power 120 W) for 10 minutes each to remove surface oxides and organic contaminants.

[0045] (3) Purge the surface of the zinc sheet with high-purity nitrogen until completely dry, and place it in a vacuum drying oven (10⁻³ Pa) for later use.

[0046] 2. Annealing process

[0047] The pretreated zinc sheet was placed horizontally in a tube annealing furnace (argon purity ≥ 99.999%), heated to 450°C at a uniform rate of 5°C / min, held at the temperature for 6 hours, and then the heating system was turned off and allowed to cool naturally to room temperature.

[0048] 3. Precision forming of zinc coils

[0049] (1) Using a CNC winding machine (positioning accuracy ±0.01 mm), the annealed zinc sheet is wound into a spiral structure with a diameter of about 12 mm under constant tension (5.0 ± 0.2 N).

[0050] (2) In the coiled zinc coil, the spacing between two adjacent layers is about 1.5~2mm and the height is 40mm.

[0051] 4. Confined reaction crystallization

[0052] (1) Prepare a 1.2±0.05 mol / L dilute sulfuric acid solution and pre-cool it to 5±0.5°C;

[0053] (2) Immerse the zinc coil in 200 mL of pre-cooled dilute sulfuric acid, place it in a constant temperature reaction vessel (temperature control accuracy ±0.1°C), maintain the system temperature at 8±0.2°C, and react for 6 hours under magnetic stirring at 300 rpm;

[0054] (3) After the reaction was completed, the solution was concentrated using a rotary evaporator under vacuum of -0.08 MPa and water bath temperature of 55±1°C. The concentration endpoint was determined by the solution density reaching 1.32±0.02 g / cm³ (25°C).

[0055] (4) Gradient cooling crystallization was carried out at a cooling rate of 0.05°C / min to obtain oriented ZnSO4·7H2O.

[0056] XRD analysis revealed that, Figure 3 As shown, the sample exhibits significantly enhanced diffraction peaks with intensities far exceeding the standard reference values, and all other characteristic peaks match the standard spectrum. Combined with the absence of impurity peaks, this indicates that the sample is high-purity zinc sulfate heptahydrate.

[0057] Comparative Example 1

[0058] Same as Example 1, except that the annealed zinc foil is directly placed in dilute sulfuric acid for confined reaction crystallization, without undergoing a curling process.

[0059] As can be seen from the XRD pattern, the curling treatment can induce the crystal to crystallize in a directional manner in the space domain. The preferred orientation of the crystal without the curling treatment is destroyed, and the grains exhibit a multi-directional random arrangement.

[0060] Comparative Example 2

[0061] Similar to Example 1, the only difference is that no magnetic stirring was used during the confined reaction, and the reaction was allowed to stand for 8 hours. Comparative analysis of SEM images clearly shows that the introduction of magnetic stirring significantly improved the uniformity of the reaction system. Under stirring conditions, the reactants were thoroughly mixed, mass transfer efficiency was improved, resulting in a narrower grain size distribution and highly uniform morphology in the product. In contrast, the product obtained from the static reaction exhibited significant size inhomogeneity.

[0062] Comparative Example 3

[0063] Similar to Example 1, the only difference is that the interlayer spacing of the zinc coil is 5 mm. SEM image analysis clearly shows that with an interlayer spacing of 5 mm, the grain size is relatively large, indicating that a suitable interlayer spacing has a spatial confinement effect on the grains.

[0064] Comparative Example 4

[0065] Same as Example 1, except that the height of the zinc coil is 70 mm. SEM image analysis clearly shows that a zinc coil height of 70 mm results in a relatively large grain size, indicating that a suitable height promotes uniform grain nucleation and growth.

[0066] Comparative Example 5

[0067] Same as Example 1, except that the magnetic stirring speed was 500 rpm. XRD analysis of the obtained product is shown below. Figure 7 Comparison with the XRD results of the products obtained in Examples 1 and 2 shows that when the stirring speed is too fast during crystallization, the XRD image will show the characteristic of preferred orientation weakening: the intensity of the originally significantly enhanced crystal plane diffraction peaks is significantly reduced, while the intensity of other characteristic peaks is relatively increased. This indicates that the disordered nucleation caused by high-speed stirring inhibits the directional growth of the crystal, causing the grains to exhibit a multi-orientation random distribution.

Claims

1. A method for preparing zinc sulfate heptahydrate based on space-induced directional crystallization, characterized in that, Includes the following steps: S1: Anneal the zinc foil to obtain a zinc substrate with a highly preferred orientation of Zn(002) crystal plane; S2: The zinc sheet is rolled into a zinc coil, wherein the spacing between two adjacent layers in the zinc coil is less than 2.2 mm; the height of the zinc coil is less than 50 mm. S3: Immerse the zinc coil in a dilute sulfuric acid solution at 5-10°C and stir for 6-10 hours; the stirring speed is 100-300 rpm. S4: Concentrate the reaction solution, then crystallize by gradient cooling to obtain directionally grown ZnSO4·7H2O.

2. The method according to claim 1, characterized in that: In step S1, the zinc foil is annealed at 420-450°C for 4-6 hours under an argon atmosphere at a rate of 3-5°C / min, and then naturally cooled to obtain a zinc substrate with a Zn(002) crystal plane ratio of ≥90%.

3. The method according to claim 1, characterized in that: The concentration process described in step S4 is carried out using a rotary evaporator under vacuum conditions of -0.08 MPa and water bath temperature of 55~65°C.

4. The method according to claim 1, characterized in that: In step S4, the gradient cooling rate is 0.05°C / min.

5. The method according to claim 1, characterized in that: The concentration of the dilute sulfuric acid solution in step S3 is 1.2 ± 0.05 mol / L.

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