A method and application for preparing calcium sulfate whiskers using waste acid from titanium dioxide.

Abstract

This invention discloses a method and application for preparing calcium sulfate whiskers using waste acid from titanium dioxide production, specifically relating to the field of calcium sulfate preparation technology. The method includes raw material pretreatment, segmented induced nucleation, gradient temperature ripening, and post-treatment. This invention operates entirely under atmospheric pressure, eliminating the need for high-temperature, high-pressure hydrothermal equipment, resulting in low safety risks, low equipment investment, and significantly reduced energy consumption. The segmented temperature control and segmented feeding process parameters have a wide window and good robustness, are compatible with conventional chemical reactor operations, and are easy to scale up and achieve continuous production. It possesses significant cost advantages and industrialization prospects, and is particularly suitable for the field of radiant cooling.

Description

Technical Field

[0001] This invention relates to the field of calcium sulfate preparation technology, specifically to a method and application for preparing calcium sulfate whiskers using waste acid from titanium dioxide. Background Technology

[0002] Radiative cooling is a zero-energy passive cooling technology that relies on the dual-spectral characteristics of materials: high reflectivity in the solar spectrum (0.3-2.5μm) and high emissivity in the atmospheric infrared window (8-13μm). This allows for efficient passive heat dissipation without consuming additional energy, and it has broad application prospects in building energy conservation, outdoor equipment, and thermal management coatings. Calcium sulfate whiskers, as an inorganic single-crystal fiber material, possess intrinsically high whiteness, high infrared emissivity, excellent chemical stability, and fiber reinforcement properties. They can construct an efficient light scattering network within radiative cooling coatings, making them an ideal choice for high-performance radiative cooling fillers.

[0003] Currently, the industrial production of calcium sulfate whiskers mostly uses natural dihydrate gypsum as raw material and is prepared by hydrothermal synthesis. This method requires a closed reactor under high temperature and high pressure, which not only requires high pressure resistance of the equipment and high energy consumption, but is also mostly an intermittent operation, making it difficult to achieve continuous and large-scale production. At the same time, long-term and large-scale mining of natural gypsum will lead to excessive consumption of mineral resources, which does not meet the requirements of green and sustainable development.

[0004] To achieve the resource utilization of industrial solid waste, the preparation of calcium sulfate whiskers using waste acid from the sulfuric acid process of titanium dioxide production has become an important research direction for the preparation of low-cost, green functional materials. Chinese patent CN116876069A discloses a method for directly preparing α-hemihydrate gypsum whiskers from waste acid of titanium dioxide. Although this method achieves the resource conversion of waste acid, it still relies on high-temperature and high-pressure hydrothermal conditions, resulting in large equipment investment, high energy consumption, and complex processes. Furthermore, the obtained whiskers have an aspect ratio of only 32.4-73.1 and poor morphological uniformity, failing to meet the core requirements of high aspect ratio and high scattering efficiency for radiation refrigeration fillers.

[0005] Huang Jia et al. prepared hemihydrate calcium sulfate whiskers using a hydrothermal method with titanium gypsum as raw material, but this still requires high temperature and high pressure, and the crystal phase is easily mixed and the morphology is difficult to control. Gan Yongle et al. first prepared titanium gypsum from titanium dioxide waste acid and then synthesized whiskers hydrothermally. The process was lengthy, the impurity removal was incomplete, and the product purity and optical properties were poor. Pan Yi et al. used a one-step method under normal pressure to prepare anhydrous calcium sulfate whiskers. The product had an extremely low aspect ratio and was short rod-shaped or granular, which could not build an effective light scattering network and could not meet the requirements for radiation cooling.

[0006] Atmospheric pressure acidification is a preferred alternative to hydrothermal methods due to its mild reaction conditions, lack of high-pressure equipment, low production cost, and ease of industrial-scale continuous production. However, existing atmospheric pressure acidification processes still have many drawbacks: Nai Xueying et al. used a one-step atmospheric pressure acidification method, which is simple but difficult to control supersaturation, resulting in uneven whisker morphology; Xie Qing et al. used a one-step atmospheric pressure acidification method with phosphogypsum, but the resulting whiskers had an aspect ratio of only 3-8 and extremely poor light scattering ability; Lü Zhihui et al.'s research confirmed that temperature, acid concentration, and feed rate are highly coupled in atmospheric pressure systems, and even small fluctuations in parameters can lead to uncontrolled whisker morphology. Meanwhile, Fe in titanium dioxide waste acid... 3+ Fe 2+ Iron-based impurities are difficult to remove effectively and easily adsorb onto the crystal nucleus surface, interfering with the directional growth of crystals. Conventional one-step feeding can easily cause local oversaturation and trigger explosive nucleation, resulting in uneven whisker size, low aspect ratio, and poor light scattering performance. Related studies have shown that the morphology and regularity of the filler directly determine the solar reflectivity and infrared emissivity, and existing process products are difficult to meet the optical performance requirements of high-performance radiative cooling.

[0007] Therefore, existing technologies cannot simultaneously achieve efficient removal of impurities from titanium dioxide waste acid, mild preparation under normal pressure, controllable growth with high aspect ratio, and synergistic improvement of high optical performance, which severely limits the large-scale application of calcium sulfate whiskers in the field of high-performance radiative cooling. Summary of the Invention

[0008] Therefore, this invention provides a method and application for preparing calcium sulfate whiskers using waste acid from titanium dioxide, in order to solve the problems in the prior art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: According to a first aspect of the present invention, a method for preparing calcium sulfate whiskers using waste acid from titanium dioxide is provided, the method comprising the following steps: 1) Raw material pretreatment: Take the waste acid of titanium dioxide produced by the sulfuric acid process, remove iron-based impurities through a combination of reduction pretreatment and concentration and refining process to obtain a refined dilute sulfuric acid solution with a total iron ion concentration ≤3g / L. 2) Segmented induced nucleation: Under normal pressure, calcium source solution is added dropwise to the refined dilute sulfuric acid solution in two stages. The first stage adds 20% to 40% of the total calcium source solution and induces uniform crystal nuclei to form at a first preset temperature. After heat preservation, the second stage adds the remaining 60% to 80% of the calcium source solution and achieves directional one-dimensional crystal growth at a second preset temperature. 3) Gradient temperature ripening: After the droplet addition is completed, a segmented gradient cooling and holding ripening process is adopted to stabilize the crystal morphology; 4) Post-processing: Calcium sulfate whiskers are obtained by solid-liquid separation, washing and drying.

[0010] Furthermore, in step 1), the reduction pretreatment includes: adding elemental iron powder to the titanium dioxide waste acid, and reducing the Fe content in the system at a temperature ranging from room temperature to 70°C. 3+ Reduced to Fe 2+ After reduction, the insoluble residue is removed by filtration, and then concentrated by negative pressure evaporation to control the total iron ion concentration in the refined dilute sulfuric acid solution to ≤3g / L.

[0011] Furthermore, the calcium source is any one or more of calcium chloride, calcium hydroxide, and calcium oxide, and is prepared into an aqueous solution of calcium source with a mass concentration of 5% to 15%.

[0012] Furthermore, in step 2), the first preset temperature is 65-75°C, and the second preset temperature is 65-75°C.

[0013] Furthermore, in step 2), before adding the calcium source solution, a crystal form regulator is added to the refined dilute sulfuric acid solution; the crystal form regulator is one or more of magnesium chloride, citric acid, and disodium EDTA, and the amount added is 0.03% to 0.5% of the mass of the refined dilute sulfuric acid solution.

[0014] Furthermore, the gradient temperature curing process in step 3) includes: lowering the system temperature to 50-60°C and curing at a constant temperature for 2-4 hours.

[0015] Furthermore, in step 4), the washing process involves first washing with a dilute salt solution, followed by rinsing with deionized water; the drying temperature is 90–120°C.

[0016] According to a second aspect of the present invention, a calcium sulfate whisker prepared by the method described above is characterized in that the calcium sulfate whisker is calcium sulfate with 0.67% water of crystallization.

[0017] Furthermore, the diameter of a single calcium sulfate whisker is 0.5-3 μm, and the aspect ratio is 50-100. The whisker whiteness is ≥92%, the emissivity of the 8-13 μm atmospheric infrared window is ≥93%, and the full-spectrum solar reflectivity is ≥95%.

[0018] According to a third aspect of the present invention, the application of calcium sulfate whiskers as described in claim 8 in the field of radiative cooling is provided.

[0019] The present invention has the following advantages: This invention utilizes a combined pretreatment process of "iron powder reduction + concentration and refining" to remove Fe from titanium dioxide waste acid. 3+ Reduced to Fe 2+This method utilizes the characteristic that ferrous sulfate's solubility significantly decreases under high temperature and high acid conditions, causing it to crystallize out as FeSO4·H2O, which can then be effectively separated by hot filtration. Compared to the crude treatment methods in existing technologies that rely solely on dilution or simple filtration, this method can stably control the total iron ion concentration in the refined acid solution to below 3 g / L, eliminating Fe ions at the source. 2+ / Fe 3+ The interference with whisker nucleation and directional growth ensures the purity of the crystal nucleus and the orderliness of subsequent one-dimensional crystal growth.

[0020] This invention overcomes the technical bottlenecks of existing atmospheric pressure acidification methods, which often result in excessive local supersaturation, explosive nucleation, and uneven whisker size due to the "one-step feeding" method. By employing a segmented induction strategy—"partial feeding to induce nucleation, heat preservation and aging to form uniform seed crystals, and then secondary feeding for directional growth"—the nucleation and growth stages are effectively separated, suppressing secondary nucleation. Combined with a "gradient cooling aging" process, whiskers are fully developed, defects are repaired, and lateral coarsening is suppressed at low temperatures, ultimately resulting in the stable production of high-quality calcium sulfate hemihydrate whiskers with diameters of 0.5–3 μm and aspect ratios of 50–100, exhibiting uniform size distribution and regular morphology.

[0021] The calcium sulfate whiskers prepared by the method of this invention have a whiteness ≥92%, an emissivity ≥93% in the 8–13 μm atmospheric infrared window band, and a full-spectrum solar reflectivity ≥95%. These excellent intrinsic optical properties enable the whiskers to construct an efficient light scattering network in the coating matrix, while simultaneously achieving high solar reflectivity and high infrared radiation heat dissipation, making them an ideal functional filler for preparing high-performance radiation-cooling composite coatings.

[0022] This invention operates entirely under normal pressure, eliminating the need for high-temperature, high-pressure hydrothermal equipment, resulting in low safety risks, minimal equipment investment, and significantly reduced energy consumption. The segmented temperature control and segmented feeding process parameters offer a wide window of opportunity and good robustness, making it suitable for conventional chemical reactor operations. It is easily scaled up and can be implemented for continuous production, demonstrating significant cost advantages and promising industrialization prospects.

[0023] This invention uses titanium dioxide waste acid, a waste product from the sulfuric acid process for titanium dioxide production, as the core raw material. Through resource-based transformation, it prepares high-performance functional materials, which not only significantly reduces the raw material cost of calcium sulfate whiskers, but also provides a new path for the harmless treatment and recycling of waste acid in the titanium dioxide industry, in line with the strategic requirements of green chemistry and sustainable development. Attached Figure Description

[0024] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0025] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0026] Figure 1 This is a sample of calcium sulfate whiskers and its SEM image provided in Embodiment 1 of the present invention; wherein, a-sample image; b-SEM image; Figure 2 This is a SEM image of calcium sulfate whiskers provided in Embodiment 2 of the present invention; Figure 3 This is a SEM image of calcium sulfate whiskers provided in Example 3 of the present invention; Figure 4 SEM images of calcium sulfate whiskers provided in Example 4 and Comparative Example 9 of the present invention, wherein a-Example 4, b-Comparative Example 9; Figure 5 This is a SEM image of calcium sulfate whiskers provided in Embodiment 5 of the present invention; Figure 6 This is a SEM image of a calcium sulfate product provided in Comparative Example 1 of the present invention; Figure 7 This is a SEM image of a calcium sulfate product provided in Comparative Example 2 of the present invention; Figure 8 This is a SEM image of a calcium sulfate product provided in Comparative Example 3 of the present invention; Figure 9 This is a SEM image of a calcium sulfate product provided in Comparative Example 5 of the present invention; Figure 10 This is a SEM image of a calcium sulfate product provided in Comparative Example 6 of the present invention; Figure 11 This is a SEM image of a calcium sulfate product provided in Comparative Example 7 of the present invention. Detailed Implementation

[0027] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Unless otherwise specified in the embodiments of this invention, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products; different manufacturers and models of raw materials do not affect the implementation of the technical solution or the achievement of the technical effect of this invention.

[0029] The titanium dioxide waste acid used in the embodiments and comparative examples of this invention is the hydrolysis waste acid liquid generated during the sulfuric acid process of titanium dioxide production in titanium dioxide plants. The initial sulfuric acid concentration in the hydrolysis waste acid liquid is approximately 2.8 mol / L, Fe... 2+ Concentration approximately 49.08 g / L, density approximately 1.31 g / mL. Elemental iron powder (purity ≥98%); calcium source (calcium chloride, calcium hydroxide, calcium oxide, analytical grade); crystal form regulator (magnesium chloride, citric acid, disodium EDTA, analytical grade); dilute salt solution (5% calcium chloride solution); deionized water.

[0030] Example 1 1) Raw material pretreatment: Take 1000mL of titanium dioxide waste acid, add 8g of elemental iron powder, control the temperature at 60℃, stir and react for 30min, and then add Fe... 3+ Reduced to Fe 2+ The insoluble residue was removed by filtration, and the solution was concentrated to 500 mL by negative pressure evaporation to obtain a refined dilute sulfuric acid solution with a total iron ion concentration of 2.2 g / L. 2) Segmented induced nucleation: Add 0.15g of magnesium chloride (crystal form regulator, the amount added is 0.03% of the mass of the refined dilute sulfuric acid solution) to the purified dilute sulfuric acid solution. After stirring and dissolving, control the temperature at 70℃ (both the first and second preset temperatures are 70℃). Use a precision dropping funnel to add 10% calcium chloride solution (calcium source) dropwise in segments, with a total addition of 500mL. In the first segment, add 20% (100mL) at a dropping rate of 1.5L / min and keep warm for 20min to induce uniform crystal nucleus formation. In the second segment, add the remaining 80% (400mL) at a dropping rate of 1.5L / min and continue to keep warm for 30min to achieve directional one-dimensional crystal growth. 3) Gradient temperature ripening: After the addition is completed, the system temperature is lowered to 55℃ and ripened at a constant temperature for 3 hours to stabilize the crystal morphology; 4) Post-processing: Solid-liquid separation is achieved by vacuum filtration. The mixture is first washed three times with a 5% calcium chloride dilute salt solution, then rinsed twice with deionized water, and finally dried in a 110℃ forced-air drying oven for 2 hours to obtain calcium sulfate whiskers.

[0031] like Figure 1 As shown in figures a and b, the figures depict numerous slender, fibrous whiskers that are dispersed and do not aggregate, with smooth surfaces and no adhering particles. In Mg... 2+ With Cl - In the coexisting system, SEM images showed that whiskers with better morphology could be obtained, confirming the synergistic effect between the two. This conclusion has important guiding significance for the targeted control of practical waste acid systems.

[0032] Example 2 1) Raw material pretreatment: Take 1000mL of titanium dioxide waste acid, add 10g of elemental iron powder, control the temperature at 70℃, stir and react for 25min, and then add Fe... 3+ Reduced to Fe 2+ The insoluble residue was removed by filtration, and the solution was concentrated to 400 mL by negative pressure evaporation to obtain a refined dilute sulfuric acid solution with a total iron ion concentration of 1.8 g / L. 2) Segmented induced nucleation: Add 0.8g of citric acid (crystal form regulator, the amount added is 0.2% of the mass of the refined dilute sulfuric acid solution) to the purified dilute sulfuric acid solution. After stirring and dissolving, control the temperature at 65℃ (both the first and second preset temperatures are 65℃). Use a precision dropping funnel to add 5% calcium hydroxide solution (calcium source) dropwise in segments, with a total addition of 720mL. In the first segment, add 40% (288mL) at a dropping rate of 1.0L / min and keep warm for 30min to induce uniform crystal nucleus formation. In the second segment, add the remaining 60% (432mL) at a dropping rate of 1.0L / min and continue to keep warm for 25min to achieve directional one-dimensional crystal growth. 3) Gradient temperature ripening: After the addition is completed, the system temperature is lowered to 50℃ and ripened at a constant temperature for 4 hours to stabilize the crystal morphology; 4) Post-processing: Solid-liquid separation is achieved by vacuum filtration. The mixture is first washed three times with a 5% calcium chloride dilute salt solution, then rinsed twice with deionized water, and finally dried in a 90℃ forced-air drying oven for 3 hours to obtain calcium sulfate whiskers.

[0033] SEM, such as Figure 2 As shown, the product consists of slender, fibrous whiskers with a regular morphology, and one-dimensional directional growth can still be achieved even with the presence of citric acid, a crystal form regulator. Compared to Example 1, this example uses a chloride-free calcium source, resulting in slightly inferior whisker surface smoothness and size uniformity, which, from a comparative perspective, confirms the role of Cl... - The synergistic effect of mineralization on improving the surface morphology of whiskers.

[0034] Example 3 1) Raw material pretreatment: Take 1000mL of titanium dioxide waste acid, add 9g of elemental iron powder, control the temperature at 50℃, stir and react for 40min, and then add Fe... 3+ Reduced to Fe 2+ The insoluble residue was removed by filtration, and the solution was concentrated to 600 mL by negative pressure evaporation to obtain a refined dilute sulfuric acid solution with a total iron ion concentration of 2.8 g / L. 2) Segmented induced nucleation: Add 1.5g of disodium EDTA (crystal form regulator, the amount added is 0.25% of the mass of the refined dilute sulfuric acid solution) to the purified dilute sulfuric acid solution. After stirring and dissolving, control the temperature at 75℃ (both the first and second preset temperatures are 75℃). Use a precision dropping funnel to add 15% calcium chloride solution (calcium source) dropwise in segments, with a total addition of 360mL. In the first segment, add 30% (108mL) at a dropping rate of 2.0L / min and keep warm for 25min to induce uniform crystal nucleus formation. In the second segment, add the remaining 70% (252mL) at a dropping rate of 2.0L / min and continue to keep warm for 35min to achieve directional one-dimensional crystal growth. 3) Gradient temperature ripening: After the addition is completed, the system temperature is lowered to 60℃ and ripened at a constant temperature for 2 hours to stabilize the crystal morphology; 4) Post-processing: Solid-liquid separation is achieved by vacuum filtration. The mixture is first washed three times with a 5% calcium chloride dilute salt solution, then rinsed twice with deionized water, and finally dried in a 120℃ forced-air drying oven for 1.5 hours to obtain calcium sulfate whiskers.

[0035] SEM, such as Figure 3 As shown, the product is a fibrous whisker with a regular morphology, indicating that disodium EDTA can also be used as an effective crystal form regulator. Under higher temperature conditions (75℃), combined with a segmented induction process, one-dimensional directional growth of whiskers can be achieved.

[0036] Example 4 1) Raw material pretreatment: Take 1000mL of titanium dioxide waste acid, add 8.5g of elemental iron powder, control the temperature at 60℃, stir and react for 35min, and then add Fe... 3+ Reduced to Fe 2+ The insoluble residue was removed by filtration, and the solution was concentrated to 450 mL by negative pressure evaporation to obtain a refined dilute sulfuric acid solution with a total iron ion concentration of 2.0 g / L. 2) Segmented induction of nucleation: The temperature was controlled at 68℃. A 10% calcium chloride solution was added dropwise in segments using a precision dropping funnel, with a total addition volume of 550 mL. The first segment added 25% (137.5 mL) at a dropping rate of 1.2 L / min and kept at the temperature for 25 min to induce the formation of uniform crystal nuclei. The second segment added the remaining 75% (412.5 mL) at a dropping rate of 1.2 L / min and kept at the temperature for another 30 min to achieve directional one-dimensional crystal growth. 3) Gradient temperature ripening: After the addition is completed, the system temperature is lowered to 58℃ and ripened at a constant temperature for 3.5 hours to stabilize the crystal morphology; 4) Post-processing: Vacuum filtration, wash three times with 5% calcium chloride dilute salt solution, rinse twice with deionized water, and dry in a 120℃ forced-air drying oven for 2.5 hours to obtain calcium sulfate whiskers.

[0037] SEM, such as Figure 4 As shown in Figure a, the product is a fibrous whisker with a smooth surface and regular morphology. In this example, no crystal form regulator was added; one-dimensional growth of the whiskers was achieved solely through segmented induced nucleation and gradient temperature ripening processes. Compared with Example 1, the addition of Mg... 2+ As a crystal form regulator, it can be further adsorbed onto the sides of whiskers to inhibit radial growth and significantly increase the aspect ratio. However, its concentration has a clear upper limit; if it is too high, it will lead to whisker coarsening (e.g., Figure 4 (as shown in b), therefore, the amount of crystal form regulator added needs to be controlled to be 0.03% to 0.5% of the mass of the refined dilute sulfuric acid solution.

[0038] Example 5 1) Raw material pretreatment: Take 1000mL of titanium dioxide waste acid, add 10g of elemental iron powder, control the temperature at 65℃, stir and react for 30min, and then add Fe... 3+ Reduced to Fe 2+ The insoluble residue was removed by filtration, and the solution was concentrated to 500 mL by negative pressure evaporation to obtain a refined dilute sulfuric acid solution with a total iron ion concentration of 2.5 g / L. 2) Segmented induced nucleation: Without adding crystal form regulators, the temperature was controlled at 72℃. A 12% calcium chloride solution was added dropwise in segments using a precision dropping funnel, with a total addition volume of 480mL. The first segment added 35% (168mL) at a dropping rate of 1.8L / min and kept at this temperature for 20min to induce uniform crystal nuclei. The second segment added the remaining 65% (312mL) at a dropping rate of 1.5L / min and kept at this temperature for another 30min to achieve directional one-dimensional crystal growth. 3) Gradient temperature ripening: After the addition is completed, the system temperature is lowered to 55℃ and ripened at a constant temperature for 3 hours to stabilize the crystal morphology; 4) Post-processing: Vacuum filtration, first wash with 5% calcium chloride dilute salt solution 3 times, then rinse with deionized water 2 times, and dry in a 105℃ forced-air drying oven for 2 hours to obtain calcium sulfate whiskers.

[0039] SEM, such as Figure 5As shown, the product is a fibrous whisker with a regular morphology. This embodiment successfully prepared whiskers even without a crystal form regulator, using different segmentation ratios (35% / 65%), and at a slightly higher temperature (72°C), further verifying the universality and parameter robustness of the segmented induced nucleation and gradient temperature ripening process itself. Together with Example 4, it demonstrates that even without the addition of a crystal form regulator, the core process of this invention can still stably obtain high-quality whisker products with an aspect ratio of over 50.

[0040] Comparative Example 1 The only difference from Example 1 is that: in step 1), no iron powder is added, no reduction pretreatment and concentration to remove iron are performed, and the waste titanium dioxide acid is diluted to the same sulfuric acid concentration as in Example 1 before being used directly.

[0041] The remaining steps are the same as in Example 1.

[0042] SEM, such as Figure 6 As shown, the product morphology deteriorated, almost losing its fibrous structure, and completely transformed into a poorly crystallizable anhydrous phase, clearly indicating that Fe... 2+ It is the most critical and harmful impurity that needs to be strictly controlled during whisker growth.

[0043] Comparative Example 2 The only difference from Example 1 is that in step 2), the step of adding materials in stages and changing the temperature is not performed. Instead, all 500 mL of calcium chloride solution is added at one time at a uniform rate, and the temperature is kept constant at 70°C throughout the process. After the addition is completed, the mixture is stirred and kept at 70°C for 3.5 hours (the total reaction time is equivalent to that of Example 1).

[0044] The remaining steps are the same as in Example 1.

[0045] SEM, such as Figure 7 As shown, the product is a mixture of short fibers and granules with uneven size distribution. One-step feeding results in an inability to separate and control the nucleation and growth processes, leading to continuous generation of new nuclei under persistently high supersaturation, thus limiting the space for whisker growth.

[0046] Comparative Example 3 The only difference from Example 1 is that the reaction temperature in steps 2) and 3) is controlled at 95-100°C.

[0047] The remaining steps are the same as in Example 1.

[0048] SEM, such as Figure 8 As shown, the product, as revealed by SEM, appears as a mixture of short rods and particles. The high temperature and atmospheric pressure resulted in an excessively rapid reaction rate, severe explosive nucleation, and loss of the anisotropic growth characteristics of the crystals.

[0049] Comparative Example 4 The only difference from Example 1 is that in step 2), the first and second preset temperatures are both controlled at 35–40°C, and in step 3), the ripening temperature is reduced to 25–30°C. The remaining steps are the same as in Example 1. SEM images of the product show a mixture of short rods and particles with incomplete whisker development. The excessively low temperature results in insufficient driving force for crystal growth, making it difficult to achieve sufficient one-dimensional directional growth.

[0050] Comparative Example 5 The only difference from Example 1 is that the dropping rate of the first and second stages in step 2) is increased to 8-10 mL / min (small-scale test to simulate excessively fast feeding).

[0051] The remaining steps are the same as in Example 1.

[0052] Product SEM, such as Figure 9 The particles appear as fine needle-like structures with obvious agglomeration. An excessively rapid feeding rate leads to extremely high instantaneous local supersaturation, resulting in an excessive number of crystal nuclei due to explosive nucleation, which leaves insufficient subsequent material supply to support adequate growth.

[0053] Comparative Example 6 The only difference from Example 1 is that: in step 3), cooling and maturation are not performed, and after the ingredients are added, the temperature is kept constant at 70°C and stirred for 3 hours.

[0054] The remaining steps are the same as in Example 1.

[0055] Product SEM, such as Figure 10 It appears as fibrous whiskers, but some surfaces show dissolution pits and fractures, resulting in decreased dimensional uniformity. The lack of a low-temperature ripening stage makes it impossible to repair crystal defects, and lateral coarsening is obvious.

[0056] Comparative Example 7 The only difference from Example 1 is that the sulfuric acid concentration of the purified dilute sulfuric acid solution in step 1) is adjusted to 0.3-0.5 mol / L.

[0057] The remaining steps are the same as in Example 1.

[0058] Product SEM, such as Figure 11 It appears as irregular flake-like or plate-like particles, with no fibrous products. Low acid concentration leads to a weak common ion effect, increased calcium sulfate solubility, insufficient crystallization driving force, and difficulty in forming one-dimensional directional growth.

[0059] Comparative Example 8 The only difference from Example 1 is that the sulfuric acid concentration of the purified dilute sulfuric acid solution in step 1) is adjusted to 4.5–5.5 mol / L. The remaining steps are the same as in Example 1.

[0060] The product, as observed by SEM, consists of coarse, uneven particles with a rough surface. Excessive acidity inhibits the dissolution-recrystallization process of calcium sulfate, hindering crystal growth and preventing its effective development into fibrous whiskers.

[0061] Comparative Example 9 The only difference from Example 1 is that in step 2), the amount of magnesium chloride added is 2.5g, which is 0.5% of the mass of the refined dilute sulfuric acid solution, much higher than the 0.03% in Example 1. The remaining steps are the same as in Example 1.

[0062] Product SEM, such as Figure 4 b appears as short, thick rods with noticeably coarsened whiskers and a significantly decreased aspect ratio. Mg 2+ At excessively high concentrations, the adsorption selectivity of Mg on each crystal face of the whisker decreases, inhibiting not only lateral growth but also axial growth, leading to overall whisker coarsening and a decrease in aspect ratio. This confirms that Mg... 2+ As a crystal form regulator, it has a clear upper limit concentration and needs to be controlled within the range of 0.03% to 0.5% of the mass of the refined dilute sulfuric acid solution.

[0063] Test Example 1 XRD analysis: The phase composition of the products of each example and comparative example was analyzed by X-ray diffractometer (Cu Kα radiation, λ=1.5406Å), with a scanning range of 2θ=5°-80°.

[0064] SEM morphology analysis: The microstructure of the product was observed using a scanning electron microscope. The diameter and length of at least 100 whiskers were counted using image analysis software, and the average aspect ratio and diameter distribution range were calculated.

[0065] The results are shown in Table 1.

[0066] Table 1. Phase and morphology characterization results of products from each example and comparative example.

[0067] Table 1 shows that the products obtained in Examples 1-5 were all calcium sulfate hemihydrate (CaSO4·0.67H2O). The characteristic XRD diffraction peaks corresponded perfectly to the standard cards, with no impurity peaks and good crystallinity. The whisker diameters of the five examples ranged from 0.8 to 2.5 μm, with an average aspect ratio of 68 to 92. Example 1, in Mg... 2+ With Cl -Under synergistic effect, a maximum aspect ratio of 92 was achieved, with whiskers exhibiting slender, fibrous characteristics, dispersed without aggregation, and smooth surfaces free of particle adhesion. Examples 4 and 5, without the addition of crystal form regulators, had aspect ratios of 72 and 68, respectively, yet still yielded high-quality whiskers with regular morphology, verifying the universality of the segmented induced nucleation and gradient temperature ripening core processes. Examples 2 and 3, using citric acid and disodium EDTA as crystal form regulators, achieved aspect ratios of 85 and 78, respectively, indicating that various crystal form regulators can be effectively used.

[0068] Comparative Example 1, without iron removal pretreatment, produced a multiphase mixture that completely lost its fibrous structure, with an aspect ratio <10, clearly demonstrating the severe inhibitory effect of iron-based impurities on whisker growth. Comparative Example 2, using a traditional one-step feeding method, failed to separate nucleation and growth, resulting in uneven product size with an aspect ratio of only 40–48. Comparative Example 3, with an excessively high reaction temperature (95–100℃), resulted in explosive nucleation, leading to a mixture of short rod-shaped particles with an aspect ratio of only 15–20. Comparative Example 4, with an excessively low reaction temperature (35–40℃), lacked sufficient driving force for crystal growth, resulting in incomplete whisker development and an aspect ratio of only 20–30. Comparative Example 5, with an excessively rapid feeding rate, caused excessively high local supersaturation, triggering explosive nucleation. The product was needle-shaped and significantly agglomerated, with an aspect ratio of only 35–45. Comparative Example 6, without gradient temperature ripening, failed to repair crystal defects, resulting in surface dissolution pits and fractures, with an aspect ratio of only 50–55. In Comparative Example 7, the sulfuric acid concentration was too low, resulting in calcium sulfate dihydrate, which was irregularly shaped and lacked fibrous morphology. In Comparative Example 8, the sulfuric acid concentration was too high, resulting in coarse and uneven products with rough surfaces and aspect ratios of only 20–28. In Comparative Example 9, Mg… 2+ Excessive addition (0.5%) resulted in significant coarsening of the whiskers, which became short and thick rods, and the aspect ratio decreased to 30-40.

[0069] Test Example 2 Whiteness test: The blue light whiteness (R457) of the whisker powder was tested using a whiteness meter.

[0070] Infrared emissivity test: The integral emissivity of the whiskers in the 8-13 μm band was tested using a Fourier transform infrared spectrometer.

[0071] The results are shown in Table 2.

[0072] Table 2. Whiteness and infrared emissivity of products from each example and comparative example.

[0073] As shown in Table 2, the calcium sulfate whiskers prepared in Examples 1-5 all have a whiteness ≥92.5% and an integrated emissivity ≥0.932 in the 8-13 μm atmospheric infrared window, with Example 1 being the best, achieving a whiteness of 94.5% and an emissivity of 0.945. This indicates that the whiskers prepared by the method of this invention possess excellent intrinsic optical properties and fully meet the core performance requirements for functional fillers in the field of high-performance radiative cooling.

[0074] Comparative Example 1, without iron removal pretreatment, had residual iron impurities resulting in a product whiteness of only 78.5% and an emissivity of only 0.85, representing a decrease of 16 percentage points and approximately 10% respectively compared to Example 1. This further verifies the necessity of iron powder reduction-concentration iron removal pretreatment for ensuring the optical quality of whiskers. Comparative Examples 2-9 all showed varying degrees of decrease in whiteness and emissivity. Comparative Example 7 (too low sulfuric acid concentration, product was calcium sulfate dihydrate) had a whiteness of only 80.2% and an emissivity of 0.86; Comparative Example 8 (too high sulfuric acid concentration) had a whiteness of only 83.5% and an emissivity of 0.87; Comparative Examples 3 and 4 had whiteness and emissivity below 89% and 0.88 respectively; and Comparative Example 9 (Mg²⁺) showed a decrease in whiteness and emissivity. + The whiteness and emissivity (in excess) were 88.5% and 0.90, respectively. The data indicate that crystal phase purity, morphological regularity, and size uniformity are key factors determining the optical performance of whiskers, and any loss of control over the process parameters will lead to a significant deterioration in optical performance.

[0075] Test Example 3 Optical performance testing of the coating: Calcium sulfate whiskers prepared in each example and comparative example were used to prepare radiation-cooling coatings. The coating preparation method was as follows: The whiskers and TiO2 powder were surface-modified with KH-570 silane coupling agent to achieve hydrophobicity; PDMS prepolymer and crosslinking agent were mixed at a mass ratio of 10:1 to obtain the matrix material; modified whiskers and TiO2 were added at a mass ratio of whiskers:TiO2:matrix material = 15:10:100, stirred until a uniform paste was formed, diluted with an equal mass of n-hexane, and ultrasonically dispersed for 2 hours; the slurry was coated onto a clean aluminum sheet with a wet film thickness of 300 μm and cured at 80℃ for 2 hours to obtain the coating sample. The solar reflectance of the coating in the 0.3–2.5 μm wavelength range (according to ASTM E903 standard) was tested using a spectrophotometer, and the infrared emissivity of the coating in the 8–13 μm wavelength range (according to ASTM E408 standard) was tested using a Fourier transform infrared spectrometer.

[0076] Outdoor cooling test: On a clear, cloudless day, place the coated sample (10cm×10cm) and a black anodized aluminum plate of the same size side by side as a reference sample, with the back of the sample tightly attached to insulating foam. Use a T-type thermocouple to connect a data acquisition instrument to monitor the temperature in real time, and record the maximum temperature drop ΔT1 during peak solar irradiance and the temperature drop ΔT2 during the evening period.

[0077] The results are shown in Table 3.

[0078] Table 3 Optical properties and cooling effects of coatings in each embodiment and comparative example

[0079] As shown in Table 3, after the calcium sulfate whiskers prepared in each example and comparative example were made into radiation-cooling coatings, the coatings of Examples 1-5 all had a solar reflectance ≥95.0%, an infrared emissivity ≥93.8%, a maximum temperature drop ΔT1 ≥3.5℃ during peak solar irradiation (Example 1 achieved the best result of 4.2℃), and a temperature drop ΔT2 ≥8.3℃ during the evening period (Example 1 achieved the best result of 9.1℃). The data fully demonstrate that the whiskers prepared by the method of the present invention can construct an efficient three-dimensional light scattering network within the coating, achieving excellent daytime radiation-cooling effect.

[0080] Comparative Example 1 (without iron removal) had the worst coating performance, with a solar reflectance of only 85.2% and a temperature drop of only 0.8℃, verifying the decisive influence of iron powder reduction-concentration iron removal pretreatment on the final application performance. Comparative Examples 2 (one-step feeding) and 6 (no gradient temperature change) showed reduced coating reflectances to 92.0% and 93.0%, respectively, and temperature drops to 2.5℃ and 3.0℃, respectively, confirming that segmented induced nucleation and gradient temperature ripening are the core processes for obtaining high-optical-performance whiskers. Comparative Example 3 (too high temperature) showed a temperature drop of only 2.0℃, and Comparative Example 4 (too low temperature) showed a temperature drop of only 2.1℃. These two examples, through bidirectional temperature deviation, verified the rationality of the process window of 65–75℃ nucleation temperature and 50–60℃ ripening temperature. Comparative Example 5 (too fast feeding) showed a temperature drop of only 2.2℃, verifying the importance of slow, segmented feeding for controlling supersaturation. Comparative Example 7 (acid concentration too low) showed a temperature drop of only 1.2℃, while Comparative Example 8 (acid concentration too high) showed a temperature drop of only 1.5℃, confirming that the sulfuric acid concentration needs to be strictly controlled within a moderate range. Comparative Example 9 (Mg... 2+ The excessive amount of coating reduced the reflectivity to 92.0% and the temperature drop to 2.6℃, further confirming that the crystal form regulator needs to be controlled within a reasonable concentration range (0.03% to 0.5%) to achieve the best effect.

[0081] The temperature drop of each comparative coating differs from that of the examples, which is completely consistent with the decreasing trend of whisker aspect ratio and morphological uniformity in Table 1. This indicates that the microstructure regularity and size uniformity of the whisker filler are the core factors determining the radiation cooling performance of the coating.

[0082] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing calcium sulfate whiskers using waste acid from titanium dioxide production, characterized in that, The method includes the following steps: 1) Raw material pretreatment: Take the waste acid of titanium dioxide produced by the sulfuric acid process, remove iron-based impurities through a combination of reduction pretreatment and concentration and refining process to obtain a refined dilute sulfuric acid solution with a total iron ion concentration ≤3g / L. 2) Segmented induced nucleation: Under normal pressure, calcium source solution is added dropwise to the refined dilute sulfuric acid solution in two stages. The first stage adds 20% to 40% of the total calcium source solution and induces uniform crystal nuclei to form at a first preset temperature. After heat preservation, the second stage adds the remaining 60% to 80% of the calcium source solution and achieves directional one-dimensional crystal growth at a second preset temperature. 3) Gradient temperature ripening: After the droplet addition is completed, a segmented gradient cooling and holding ripening process is adopted to stabilize the crystal morphology; 4) Post-processing: Calcium sulfate whiskers are obtained by solid-liquid separation, washing and drying.

2. The method according to claim 1, characterized in that, In step 1), the reduction pretreatment includes: adding elemental iron powder to the titanium dioxide waste acid, and reducing the Fe content in the system at a temperature ranging from room temperature to 70°C. 3+ Reduced to Fe 2+ After reduction, the insoluble residue is removed by filtration, and then concentrated by negative pressure evaporation to control the total iron ion concentration in the refined dilute sulfuric acid solution to ≤3g / L.

3. The method according to claim 1, characterized in that, The calcium source is any one or more of calcium chloride, calcium hydroxide, and calcium oxide, and is prepared into an aqueous solution of calcium source with a mass concentration of 5% to 15%.

4. The method according to claim 1, characterized in that, In step 2), the first preset temperature is 65-75℃, and the second preset temperature is 65-75℃.

5. The method according to claim 1, characterized in that, In step 2), before adding the calcium source solution, a crystal form regulator is added to the refined dilute sulfuric acid solution; the crystal form regulator is one or more of magnesium chloride, citric acid, and disodium EDTA, and the amount added is 0.03% to 0.5% of the mass of the refined dilute sulfuric acid solution.

6. The method according to claim 1, characterized in that, The gradient temperature curing process in step 3) includes: reducing the system temperature to 50-60℃ and curing at a constant temperature for 2-4 hours.

7. The method according to claim 1, characterized in that, In step 4), the washing process involves first washing with a dilute salt solution, followed by rinsing with deionized water; the drying temperature is 90–120°C.

8. A calcium sulfate whisker prepared by the method according to any one of claims 1 to 7, characterized in that, The calcium sulfate whiskers are calcium sulfate with 0.67% water of crystallization.

9. The calcium sulfate whiskers according to claim 8, characterized in that, The diameter of a single calcium sulfate whisker is 0.5-3 μm, and the aspect ratio is 50-100.

10. An application of calcium sulfate whiskers as described in claim 8 in the field of radiative cooling.

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