Anhydrous calcium sulfate whisker prepared based on low-magnesium rare earth gypsum slurry as well as preparation method and application of anhydrous calcium sulfate whisker

Abstract

The invention belongs to the technical field of whisker preparation, and particularly relates to an anhydrous calcium sulfate whisker prepared based on low-magnesium rare earth gypsum slurry and a preparation method and application thereof. According to the invention, the calcium sulfate whisker is prepared by using a waste material, namely low-magnesium rare earth gypsum slurry, produced by a rare earth plant as a raw material and adopting a normal-pressure acidification method. And the supersaturated calcium sulfate solution is obtained by regulating and controlling process parameters. And filtering while the solution is hot, aging the filtrate, and filtering again after aging for a period of time to obtain filter residues which are the calcium sulfate whiskers. The residual filtrate is used as acidifying agent mother liquor for circulation. By utilizing the low-magnesium rare earth gypsum slurry, on one hand, the environmental pollution caused by direct discharge of the industrial byproduct gypsum slurry is reduced, and the pressure of rear-end treatment in the rare earth smelting process is relieved; on the other hand, the use amount of water in the calcium sulfate whisker production process is reduced, and the utilization rate of water resources is increased. The method is of great significance in protecting the environment, realizing sustainable utilization of resources and improving the economic benefits of enterprises.

Description

Technical Field

[0001] The invention belongs to the technical field of whisker preparation, and particularly relates to anhydrous calcium sulfate whisker prepared based on low-magnesium rare earth gypsum slurry, and a preparation method and application thereof. Background Art

[0002] Rare earth gypsum is a type of industrial by-product gypsum produced in the process of extracting rare earth elements from rare earth minerals. It is generally a gray-white powdery solid. With the rapid development of science and technology in my country, rare earth elements are increasingly used in the fields of electronic information, new energy, aerospace and medical fields. However, the main source of rare earths is still rare earth ores. With the increasing application of rare earth elements in various fields, the scale of extracting rare earth elements from rare earth ores is also expanding, and the associated solid waste production is also gradually increasing. In the rare earth hydrometallurgical process, the problem of solid waste disposal has not yet been properly solved. Rare earth gypsum contains a large amount of substances such as sulfur, calcium and magnesium. If accumulated for a long time, it will cause great harm to the environment. Low-magnesium rare earth gypsum slurry is an industrial waste directly produced by the magnesium removal process in the rare earth hydrometallurgy process. Its water content is 40-50%, and Mg 2+ The content is 1-3%, and its chemical composition is shown in Table 1. If this type of slurry is discharged directly, it will not only cause irreversible and serious damage to the ecological environment such as soil and water bodies, but will also further increase the burden on the ecosystem and destroy the ecological balance.

[0003] Table 1

[0004] Anhydrous calcium sulfate whiskers are a single-crystal fibrous material composed of calcium sulfate (CaSO4) that contains no water of crystallization in its crystal structure. The orderly arrangement of the crystal structure results in a uniform distribution of chemical bond energy within the whiskers, relatively stable chemical properties, and resistance to chemical reactions in general acidic and alkaline environments. They can withstand corrosion from acid and alkaline solutions of a certain concentration, making them suitable for a variety of complex chemical environments. The crystal structure of anhydrous calcium sulfate whiskers is relatively denser, and the interatomic bonding force is stronger, giving them higher strength and hardness, excellent mechanical properties, and the ability to improve the material's stiffness, strength, toughness, wear resistance, and other properties. Because anhydrous calcium sulfate whiskers do not contain water of crystallization, they do not need to undergo a water of crystallization removal process at high temperatures, resulting in a more stable structure. They can be used in high-temperature ceramics, refractory materials, and other high-temperature industrial applications. Anhydrous calcium sulfate whiskers have a weaker water adsorption capacity and are more suitable for applications that are sensitive to humidity, such as electronic packaging materials and precision instrument components.

[0005] Therefore, developing a technical solution that takes into account both the recycling and utilization of low-magnesium rare earth gypsum slurry and the preparation of anhydrous calcium sulfate whiskers has become a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention addresses the problems existing in the prior art and, based on the inherent characteristics of low-magnesium rare earth gypsum slurry, develops a method for preparing anhydrous calcium sulfate whiskers that can efficiently recycle low-magnesium rare earth gypsum slurry, is energy-saving and environmentally friendly, and is easy to achieve large-scale industrial production.

[0007] To achieve the above objectives, the first object of the present invention is to provide a method for preparing anhydrous calcium sulfate whiskers based on low-magnesium rare earth gypsum slurry. This method solves the problem of low calcium sulfate whisker yield in a sulfuric acid medium due to the common ion effect by optimizing process conditions, and directly produces anhydrous calcium sulfate whiskers with stable properties. The following technical solution is adopted:

[0008] The invention discloses a method for preparing anhydrous calcium sulfate whiskers based on low-magnesium rare earth gypsum slurry. The method uses low-magnesium rare earth gypsum slurry as raw material, sulfuric acid or nitric acid as acidifying agent, and adopts a normal pressure acidification method to prepare the anhydrous calcium sulfate whiskers.

[0009] The atmospheric pressure acidification method involves reacting a strong acid (H2SO4, HNO3) with calcium-containing raw materials (such as desulfurized gypsum and rare earth gypsum) under atmospheric pressure. By controlling reaction conditions such as temperature, stirring speed, and solution concentration, the resulting calcium sulfate precipitates as whiskers. This is essentially a dissolution-crystallization-dehydration process. Compared to other methods for producing calcium sulfate whiskers, the hydrothermal method requires high temperature and high pressure, placing strict demands on the high-pressure resistance and airtightness of the reaction equipment. The atmospheric pressure acidification method, on the other hand, can be carried out under atmospheric pressure, eliminating the need for specialized equipment such as autoclaves. This reduces equipment costs and operational complexity, resulting in relatively low energy consumption and equipment maintenance costs during the production process, making it easier to implement on a large-scale industrial scale. The microwave method requires microwave equipment and has certain dielectric properties requirements for the reactants and solvent. The atmospheric pressure acidification method is relatively straightforward, requiring only the mixing of the raw materials and the acid at a specific temperature for reaction. It does not require specialized microwave equipment or consideration of the dielectric properties of the materials, making it easier to master and implement. The microemulsion method typically requires the addition of surfactants to control whisker growth, and the preparation process requires high raw material purity, resulting in relatively high production costs. The atmospheric pressure acidification method has a wide range of raw materials, which are relatively inexpensive and have a simple production process, making it more cost-effective in large-scale production.

[0010] Moreover, considering the Mg content in ordinary rare earth gypsum 2+ The content of Mg is high, and the excess Mg 2+ Will be with Ca 2+ Competitive adsorption on the surface of the crystal nucleus leads to the dispersion of whisker nucleation sites and uneven grain size, which in turn causes the prepared calcium sulfate whiskers to increase the interfacial stress of the organic matrix during application, thereby reducing the mechanical properties of the material.

[0011] In order to solve this technical problem, the present invention adopts Mg2+ Low-magnesium rare earth gypsum with less content, using a small amount of Mg 2+ This method promotes the growth of calcium sulfate whiskers, resulting in anhydrous calcium sulfate whiskers with excellent mechanical properties. Using sulfuric acid or nitric acid at a defined concentration as an acidifier, the H⁺ reacts with the alkaline components (such as rare earth carbonates) in the rare earth gypsum, disrupting the gypsum crystal lattice and promoting the dissolution of the low-magnesium rare earth gypsum. Nitric acid can also form soluble complexes with certain metal ions (calcium, rare earth, or magnesium ions, among other trace metal ions), promoting the dissolution of the low-magnesium rare earth gypsum.

[0012] Furthermore, the sulfuric acid is 2-3.5 mol / L, and the nitric acid is 1-3 mol / L.

[0013] It is worth noting that due to SO4 2- The common ion effect of Ca 2+ The concentration of hydrogen ions in the solution decreases, thereby reducing the solubility of rare earth gypsum. Therefore, to avoid the common ion effect on gypsum solubility and prevent the reaction from being affected by insufficient hydrogen ion dissociation due to low acidulant concentration, the present invention limits the concentration of sulfuric acid to 2-3.5 mol / L and the concentration of nitric acid to 1-3 mol / L.

[0014] Furthermore, the specific steps of the preparation method include:

[0015] (1) Ball milling: The rare earth gypsum slurry was mixed with deionized water for ball milling pretreatment, and mechanical ball milling was performed for 30 minutes;

[0016] (2) Water bath dissolution: Take an appropriate amount of pretreated low-magnesium rare earth gypsum slurry and dissolve it in an acidifier, heat it in a water bath to prepare a supersaturated calcium sulfate solution, and set the rotor speed to 400 r / min;

[0017] (3) Aging: After the reaction is completed, vacuum filtration is performed while hot; the filtrate after solid-liquid separation is naturally cooled and allowed to age. The aging process is carried out in a closed system;

[0018] (4) Filtration and drying: After aging, vacuum filtration is performed again to obtain a filter cake, which is washed with anhydrous ethanol and dried in a constant temperature drying oven to obtain anhydrous calcium sulfate whiskers; the filtrate after filtration is collected, the concentration is determined, and it is recycled as the acidifier mother liquor.

[0019] Furthermore, in the step (1), the volume ratio of rare earth gypsum slurry to deionized water is 1:5, and the rotation speed of the mechanical ball mill is 200-400 r / min.

[0020] It is worth noting that the present invention can release calcium sulfate particles wrapped by impurities by mixing low-magnesium rare earth gypsum slurry with deionized water and performing ball milling for physical crushing, while promoting the separation of impurities and calcium sulfate, thereby improving the reactivity of the raw materials. Ball milling refines the rare earth gypsum particles and increases their specific surface area. The slurry particles after ball milling have better dispersion, reduce local concentration gradients, accelerate the dissolution rate of ions in the subsequent acid hydrolysis process, and ensure that the Ca 2+ and SO4 2- The concentration quickly reaches a supersaturated state, promoting uniform nucleation. Compared to traditional methods for pulverizing solid gypsum, the liquid medium (water) acts as a buffer and force transmission medium, refining particles through the synergistic effects of shear and impact forces. Water reduces local temperature rise, resulting in a more uniform distribution of extrusion and impact energy. This prevents lattice distortion caused by localized high temperatures during the pulverization of solid gypsum, as well as uneven whisker growth or heterogeneous nucleation caused by incomplete dissolution of large particles, thereby improving the consistency of whisker morphology.

[0021] Furthermore, considering that a high solid-to-liquid ratio can lead to excessive supersaturation of the solution, causing disordered nucleation and the formation of short, thick crystals, while a low solid-to-liquid ratio can lead to insufficient ion concentration and slow whisker growth, an appropriate solid-to-liquid ratio ensures sufficient dispersion of rare earth gypsum particles while avoiding the hindrance of ion diffusion due to excessive viscosity. This makes it easier to form a homogeneous solution in the fine-particle gypsum system after mechanical water grinding, thereby achieving the production of high-purity and high aspect ratio whiskers. Therefore, the present invention limits the solid-to-liquid ratio of the rare earth gypsum slurry and deionized water blend system to 1:5.

[0022] In some embodiments, the grinding balls are made of agate.

[0023] Furthermore, in step (2), the solid-liquid ratio of the acidified mixed system is 1 g:(50-70) mL, the water bath heating temperature is 50-80 °C, and the reaction time is 10-25 min.

[0024] Furthermore, in step (3), the temperature of the vacuum filtration is 55-80°C, and the standing aging time is 4 h.

[0025] It is worth noting that, in the present invention, after the reaction in step (2) is completed, hot filtration is immediately performed. The purpose is to maintain a high temperature of the solution, so that calcium sulfate exists in the solution in a supersaturated state and the undissolved gypsum is separated from the solution so that the solution can be aged later. The most important thing is that the supersaturated solution of calcium sulfate whiskers crystallizes and grows at the same time. The driving force of this process is the temperature reduction process from the filtration temperature to room temperature, which ultimately ensures the uniformity of the whiskers. The conventional method of cooling to room temperature filtration will cause the crystallized calcium sulfate whiskers to mix with the undissolved gypsum, which significantly reduces the yield of calcium sulfate whiskers. At the same time, the calcium sulfate whiskers will clog the filter paper pores after precipitation, slowing the filtration rate and making it impossible to separate the two. When filtering while hot, the viscosity of the solution is low, the fluidity is good, the filtration resistance is small, and the solution can pass through the filter paper quickly, thereby increasing the filtration speed and shortening the filtration time. When filtering while hot, the impurities on the surface of the calcium sulfate whiskers have not yet been fully adsorbed on the whiskers. By filtering quickly, the amount of impurities adsorbed on the whisker surface can be effectively reduced, thereby improving the purity and yield of the calcium sulfate whiskers.

[0026] Furthermore, the present invention limits the aging process to a closed system because the water in the solution is volatile after heating. If it is exposed to air during the cooling process, the H + The concentration increases, thus changing the pH value of the system. Fluctuations in acidity may affect the dissolution-crystallization equilibrium of calcium sulfate, resulting in uneven whisker growth rate or the generation of impurities. At the same time, avoid external impurity contamination. In an open environment, dust and CO2 in the air may enter the solution. CO2 in the air reacts with Ca 2+ The reaction generates a small amount of CaCO3, and impurity particles may become heterogeneous nucleation sites, compromising the integrity of the whisker structure. Maintaining the aging process in a closed device through physical isolation ensures pH stability in the nitric acid medium, a key process optimization measure for preparing high-aspect-ratio, high-purity calcium sulfate whiskers.

[0027] Furthermore, the constant temperature drying temperature in step (4) is 80°C~500°C.

[0028] Furthermore, when the acidifying agent is sulfuric acid, the constant temperature drying in step (4) is a single drying; when the acidifying agent is nitric acid, the constant temperature drying in step (4) also includes a secondary high-temperature calcination.

[0029] It is worth noting that the type of acid will affect the crystal form of calcium sulfate whiskers. Calcium sulfate whiskers prepared with sulfuric acid are dried at 80 °C to obtain anhydrous calcium sulfate whiskers, while calcium sulfate whiskers prepared with nitric acid are dried at 80 °C to obtain dihydrate calcium sulfate whiskers, which require a second high-temperature calcination (500 °C) to be converted into anhydrous calcium sulfate whiskers.

[0030] A second object of the present invention is to provide an anhydrous calcium sulfate whisker prepared by a preparation method as described above.

[0031] A third object of the present invention is to provide an anhydrous calcium sulfate whisker as described above for use as an additive in high-temperature ceramics, glass, rubber, plastics, aluminum alloys, titanium alloys, or in applications such as papermaking, coatings, asphalt, and adsorbents.

[0032] Compared with the existing technology, the present invention uses low-magnesium rare earth gypsum slurry produced by a rare earth plant as raw material, and solves the problem of low yield of calcium sulfate whiskers in a sulfuric acid medium due to the common ion effect by optimizing process conditions such as the concentration of the acidifier and the solid-liquid ratio of the acidified mixing system. Anhydrous calcium sulfate whiskers with stable properties are directly prepared using low-concentration sulfuric acid as the acidifying medium. In addition, the process for preparing calcium sulfate whiskers using nitric acid as a medium has been expanded, overcoming the problem of whisker agglomeration caused by excessively high local SO4²⁻ concentrations when preparing calcium sulfate whiskers using the sulfuric acid method, better balancing the anion concentration in the solution, and providing new ideas for the preparation process of calcium sulfate whiskers. Compared with similar circulation process preparation methods, the present invention has a relatively low reaction temperature, is easy to implement in a normal pressure environment, has simple process conditions, and is highly efficient and energy-saving. This invention directly utilizes rare earth gypsum slurry, a waste material from rare earth plants. This reduces the drying and weathering of the gypsum, preventing the potential soil and water pollution caused by the release of harmful substances during weathering. This effectively protects the ecological environment and provides a practical solution for the green and sustainable development of the rare earth industry. Furthermore, it avoids the large amount of water evaporation caused by the drying step in traditional processes, significantly improving water utilization. This not only reduces dependence on external water resources and lowers production costs, but more importantly, it also meets the current social demand for water resource conservation and sustainable utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative work.

[0034] Figure 1 The present invention is a process flow chart for preparing anhydrous calcium sulfate whiskers based on low-magnesium rare earth gypsum slurry.

[0035] Figure 2 The following are scanning electron micrographs of anhydrous calcium sulfate whiskers prepared in Examples 1-16 of the present invention.

[0036] Figure 3The XRD patterns of anhydrous calcium sulfate whiskers prepared in Examples 1-16 of the present invention are shown in FIG. 1 , where Examples 1-16 are shown from top to bottom.

[0037] Figure 4 These are scanning electron microscope images of anhydrous calcium sulfate whiskers prepared in Examples 17-19 of the present invention, wherein from left to right are Examples 17-19.

[0038] Figure 5 These are the XRD patterns of anhydrous calcium sulfate whiskers prepared in Examples 17-19 of the present invention, wherein from top to bottom are Examples 17-19. DETAILED DESCRIPTION

[0039] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0040] The term "embodiment" is used herein specifically to describe any embodiment as "exemplary," and should not be construed as superior or preferable to other embodiments. Performance indicators in the embodiments of this application were tested using conventional testing methods in the art, unless otherwise specified. It should be understood that the terms used in this application are intended solely to describe specific implementations and are not intended to limit the disclosure herein.

[0041] Unless otherwise specified, the technical and scientific terms used herein have the same meanings as commonly understood by ordinary technicians in the technical field to which this application belongs; other experimental methods and technical means not specifically specified in this application refer to experimental methods and technical means commonly used by ordinary technicians in this field.

[0042] In order to better illustrate the content of this application, numerous specific details are provided in the specific examples below. It should be understood by those skilled in the art that this application can be implemented without certain specific details. In the examples, some methods, means, instruments, equipment, etc. well known to those skilled in the art are not described in detail in order to highlight the main purpose of this application.

[0043] Under the premise of no conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solutions belong to the contents disclosed in the embodiments of this application.

[0044] The present invention discloses an anhydrous calcium sulfate whisker prepared based on low-magnesium rare earth gypsum slurry, a preparation method and application thereof, and belongs to the technical field of whisker preparation. The present invention uses low-magnesium rare earth gypsum slurry, a waste material produced by a rare earth plant, as a raw material, and adopts a normal pressure acidification method to prepare calcium sulfate whiskers. By regulating process parameters such as acid concentration, solid-liquid ratio, reaction temperature, and reaction time, a supersaturated solution of low-magnesium rare earth gypsum is obtained. Subsequently, the solution is filtered while hot, and the filtrate is aged. After aging for a period of time, it is filtered again, and the filtered residue is the calcium sulfate whisker. The present invention utilizes low-magnesium rare earth gypsum slurry, which, on the one hand, reduces the environmental pollution caused by the direct discharge of industrial by-product gypsum slurry and alleviates the pressure of back-end processing in the rare earth smelting process; on the other hand, it reduces the amount of water used in the production process of calcium sulfate whiskers and improves the utilization rate of water resources. It is of great significance to protecting the environment, realizing the sustainable use of resources, and improving the economic benefits of enterprises.

[0045] For a better understanding of the present invention, the present invention is further specifically described below through the following examples, but it should not be understood as limiting the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above invention content are also considered to fall within the scope of protection of the present invention.

[0046] Example 1

[0047] A method for preparing anhydrous calcium sulfate whiskers based on low-magnesium rare earth gypsum slurry:

[0048] (1) Ball milling: The rare earth gypsum slurry was mixed with deionized water for ball milling pretreatment. The solid-liquid ratio of the pretreated low-magnesium rare earth gypsum slurry was 1:5 by volume, and the mechanical ball milling was carried out for 30 minutes.

[0049] (2) Water bath dissolution: Take 2 mL of the pretreated low-magnesium rare earth gypsum slurry in a beaker and add 100 mL of 2 mol / L sulfuric acid. At this time, the solid-liquid ratio of the acidified mixed system is 1:50 g / mL. Place the beaker in a constant temperature water bath, set the water bath heating temperature to 50 °C, the reaction time to 10 min, and the rotation speed to 400 r / min. At this time, the solution is a supersaturated solution of calcium sulfate.

[0050] (3) Aging: After the reaction is completed, vacuum filtration is performed while the temperature is hot at 55-80 °C; the filtrate after solid-liquid separation is naturally cooled and allowed to age for 4 hours. The aging process must be kept in a closed system to prevent volatilization.

[0051] (4) Filtration and drying: After aging, vacuum filtration is performed again to obtain a filter cake. The filter cake is washed with anhydrous ethanol and dried in a constant temperature drying oven at 80°C for 12 hours to obtain anhydrous calcium sulfate whiskers. The filtrate is collected and 1 mL is taken out as a liquid for titrating the calcium ion content. The remaining solution is added with sulfuric acid and then recycled as the mother liquor.

[0052] Example 2

[0053] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the solid-liquid ratio of the acidified mixed system is 1:60 g / mL, the reaction temperature is 60 °C, and the reaction time is 15 min.

[0054] Example 3

[0055] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the solid-liquid ratio of the acidified mixed system is 1:65 g / mL, the reaction temperature is 70 °C, and the reaction time is 20 min.

[0056] Example 4

[0057] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the solid-liquid ratio of the acidified mixed system is 1:70 g / mL, the reaction temperature is 80°C, and the reaction time is 25 min.

[0058] Example 5

[0059] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 2.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:55 g / mL, the reaction temperature is 70 °C, and the reaction time is 25 min.

[0060] Example 6

[0061] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 2.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:60 g / mL, the reaction temperature is 80 °C, and the reaction time is 20 min.

[0062] Example 7

[0063] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 2.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:65 g / mL, the reaction temperature is 50 °C, and the reaction time is 15 min.

[0064] Example 8

[0065] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 2.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:70 g / mL, the reaction temperature is 60°C, and the reaction time is 10 min.

[0066] Example 9

[0067] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3 mol / L, the solid-liquid ratio of the acidified mixed system is 1:55 g / mL, the reaction temperature is 80 °C, and the reaction time is 15 min.

[0068] Example 10

[0069] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3 mol / L, the solid-liquid ratio of the acidified mixed system is 1:60 g / mL, the reaction temperature is 70°C, and the reaction time is 10 min.

[0070] Example 11

[0071] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3 mol / L, the solid-liquid ratio of the acidified mixed system is 1:65 g / mL, the reaction temperature is 60°C, and the reaction time is 25 min.

[0072] Example 12

[0073] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3 mol / L, the solid-liquid ratio of the acidified mixed system is 1:70 g / mL, the reaction temperature is 50°C, and the reaction time is 20 min.

[0074] Example 13

[0075] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:55 g / mL, the reaction temperature is 60 °C, and the reaction time is 20 min.

[0076] Example 14

[0077] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:60 g / mL, the reaction temperature is 50°C, and the reaction time is 25 min.

[0078] Example 15

[0079] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:65 g / mL, the reaction temperature is 80 °C, and the reaction time is 10 min.

[0080] Example 16

[0081] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 1, except that in step (2), the concentration of the acidifying agent sulfuric acid is 3.5 mol / L, the solid-liquid ratio of the acidified mixed system is 1:70 g / mL, the reaction temperature is 70 °C, and the reaction time is 15 min.

[0082] Table 2 Main process parameters of Examples 1-16

[0083] The whisker length and aspect ratio of the anhydrous calcium sulfate whiskers prepared in Examples 1-16 were respectively measured and calculated, and the results are shown in Table 3. In addition, the anhydrous calcium sulfate whisker samples prepared in Examples 1-16 were characterized, and the scanning electron microscope images are shown in Table 3. Figure 2 As shown, the XRD pattern is Figure 3 It should be noted that Figure 2 1-16 correspond to the scanning electron micrographs of the anhydrous calcium sulfate whiskers prepared in Examples 1-16, respectively.

[0084] Table 3

[0085] It can be clearly seen from the data in Table 2-3 that when the concentration of sulfuric acid is 2 mol / L, the solid-liquid ratio is 1:65 g / mL, the reaction temperature is 70 ℃, and the reaction time is 20 min, the length and aspect ratio of the prepared calcium sulfate whiskers are the largest. Figure 3 It can be seen that all 16 groups of examples are anhydrous calcium sulfate whiskers (PDF#99-000-1496). 2- The common ion effect of sulfuric acid concentration will cause the rare earth gypsum dissolution equilibrium reaction to reverse, and Ca 2+In the solution, it will be reduced, thereby reducing the solubility of rare earth gypsum. Therefore, in order to avoid the influence of the common ion effect on the dissolution of gypsum, the concentration of sulfuric acid should be reduced appropriately. However, if the concentration of sulfuric acid is too low, sulfuric acid will not be able to dissociate enough H + Therefore, the present invention limits the concentration of sulfuric acid to 2-3.5 mol / L.

[0086] Example 17

[0087] A method for preparing anhydrous calcium sulfate whiskers based on low-magnesium rare earth gypsum slurry:

[0088] (1) Ball milling: The rare earth gypsum slurry was mixed with deionized water for ball milling pretreatment. The solid-liquid ratio of the pretreated low-magnesium rare earth gypsum slurry was 1:5 by volume, and the mechanical ball milling was carried out for 30 minutes.

[0089] (2) Water bath dissolution: Take 10 mL of the pretreated low-magnesium rare earth gypsum slurry in a beaker and add 100 mL of 1 mol / L nitric acid to the slurry. The acidified mixture has a solid-liquid ratio of 1:70 g / mL. Place the slurry in a constant temperature water bath, set the water bath heating temperature to 50 °C, the reaction time to 20 min, and the rotation speed to 400 r / min. At this point, the solution is a supersaturated solution of calcium sulfate.

[0090] (3) Aging: After the reaction is completed, vacuum filtration is performed while hot; the filtrate after solid-liquid separation is naturally cooled and allowed to age for 4 hours. The aging process must be kept in a closed system to prevent volatilization.

[0091] (4) Filtration and drying: After aging, vacuum filtration is performed again to obtain a filter cake. The filter cake is washed with anhydrous ethanol and dried in a constant temperature drying oven at 80°C for 12 hours to obtain dihydrated calcium sulfate whiskers. The dihydrated calcium sulfate whiskers are placed in a muffle furnace and calcined at 500°C for 3 hours to obtain anhydrous calcium sulfate whiskers. The filtrate is collected and 1 mL is taken out from it as a liquid for titrating the calcium ion content. The remaining solution is supplemented with sulfuric acid and recycled as the mother liquor.

[0092] Example 18

[0093] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 17, except that in step (2), the solid-liquid ratio of the acidified mixed system is 1:60 g / mL, and the concentration of the acidifying agent nitric acid is 2 mol / L.

[0094] Example 19

[0095] The preparation method proposed in this example is substantially the same as the preparation method proposed in Example 17, except that in step (2), the solid-liquid ratio of the acidified mixed system is 1:50 g / mL, and the concentration of the acidifying agent nitric acid is 3 mol / L.

[0096] The whisker length and aspect ratio of the anhydrous calcium sulfate whiskers prepared in Examples 17-19 were respectively measured and calculated, and the results are shown in Table 4. In addition, the anhydrous calcium sulfate whisker samples prepared in Examples 17-19 were characterized, and the scanning electron microscope images are shown in Table 4. Figure 4 As shown, the XRD pattern is Figure 5 shown.

[0097] Table 4

[0098] It can be seen from Table 4 that when the concentration of nitric acid is 1 mol / L, the length and aspect ratio of the whiskers are the largest. Figure 5 As can be seen from the figure, the products are all anhydrous calcium sulfate whiskers (PDF#99-000-1496). - The planar triangular structure of SO4 2- The shielding effect will increase the solubility of rare earth gypsum, and since sulfuric acid is a dibasic acid, the second dissociation constant is low, so the H + It is not as much as in nitric acid, so sulfuric acid dissolves less rare earth gypsum.

[0099] In addition, since the boiling point of nitric acid is 83°C, dihydrate calcium sulfate whiskers can exist stably at 70°C and cannot be dehydrated. The boiling point of sulfuric acid is 338°C, which allows sulfuric acid to react at high temperatures. The high temperature environment promotes the formation of anhydrous calcium sulfate whiskers. 2- As a multidentate ligand, it can form stable complexes with metal ions. 2+ Has a high charge density and is easy to react with SO4 2- This coordination action will occupy the coordination site of calcium ions, reducing the interaction between water molecules and Ca 2+ During the growth of calcium sulfate whiskers, the Ca 2+ Will be exposed to the solution, when SO4 2- When the concentration is high, SO4 2- It preferentially combines with these calcium ions to form a surface coordination structure. 2- A dense monolayer of SO4 is formed on the surface of the whiskers 2- , whose strong coordination hinders water molecules from approaching Ca 2+ , thereby promoting the formation of anhydrous calcium sulfate whiskers.

[0100] Thus, the present invention adopts normal pressure acidification method, takes sulfuric acid / nitric acid as medium, and low-magnesium rare earth gypsum slurry is that raw material prepares anhydrous calcium sulfate whisker.Solved the problem that taking sulfuric acid as medium prepares calcium sulfate whisker productive rate is not high due to the common ion effect at present, expanded the method for preparing anhydrous calcium sulfate whisker by nitric acid.The anhydrous calcium sulfate whisker prepared by this method not only can be applicable to common rubber, plastics etc., also can be applicable to high temperature resistant environment, as high temperature ceramics, high temperature glass etc., widened the application field of calcium sulfate whisker, for the development of related industry provides new technical support and material selection.

[0101] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing anhydrous calcium sulfate whiskers based on low-magnesium rare earth gypsum slurry, characterized in that: Anhydrous calcium sulfate whiskers were prepared by atmospheric pressure acidification method with low-magnesium rare earth gypsum slurry as raw material and 2-3.5 mol / L sulfuric acid or 1-3 mol / L nitric acid as acidifying agent.

2. The preparation method according to claim 1, characterized in that The specific steps include: (1) Ball milling: The low-magnesium rare earth gypsum slurry was mixed with deionized water for ball milling pretreatment, and mechanical ball milling was performed for 30 minutes; (2) Water bath dissolution: Take an appropriate amount of pretreated low-magnesium rare earth gypsum slurry and dissolve it in an acidifier to obtain an acidified mixed system, and heat it in a water bath to prepare a supersaturated calcium sulfate solution; (3) Aging: After the reaction is completed, vacuum filtration is performed while hot; the filtrate after solid-liquid separation is naturally cooled and allowed to age. The aging process must be carried out in a closed system; (4) Filtration and drying: After aging, vacuum filtration is performed again to obtain a filter cake, which is washed with anhydrous ethanol and dried in a constant temperature drying oven to obtain anhydrous calcium sulfate whiskers; the filtrate after filtration is collected, the concentration is determined, and it is recycled as the acidifier mother liquor.

3. The preparation method according to claim 2, characterized in that In the step (1), the volume ratio of low-magnesium rare earth gypsum slurry to deionized water is 1:5, and the rotation speed of the mechanical ball mill is 200-400 r / min.

4. The preparation method according to claim 2, characterized in that In the step (2), the solid-liquid ratio of the acidified mixed system is 1 g:(50-70) mL, the water bath heating temperature is 50-80 °C, and the reaction time is 10-25 min.

5. The preparation method according to claim 2, characterized in that In the step (3), the temperature for vacuum filtration while hot is 55-80°C, and the static aging time is 4 hours.

6. The preparation method according to claim 2, characterized in that The constant temperature drying temperature in step (4) is 80°C to 500°C.

7. The preparation method according to claim 6, characterized in that When the acidifying agent is sulfuric acid, the constant temperature drying in step (4) is a primary drying; when the acidifying agent is nitric acid, the constant temperature drying in step (4) includes a primary drying at 80°C and a secondary high temperature calcination at 500°C.

8. the anhydrous calcium sulfate whisker that the preparation method described in claim 1-7 obtains.

9. The application of anhydrous calcium sulfate whiskers as claimed in claim 8, wherein The anhydrous calcium sulfate whiskers are used as additives for high-temperature ceramics, rubber, plastic, glass, aluminum alloy, titanium alloy, or in the fields of papermaking, coating, asphalt, adsorbent, etc.

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