A method for preparing hydroxyapatite ultrafine powder and its application as a mulch film

Abstract

This invention relates to the field of waste resource utilization and functional polymer materials technology, and discloses a method for preparing hydroxyapatite ultrafine powder and its application as a mulch film. The method includes the following steps: S1, raw material pretreatment and purification: physical and chemical purification of waste animal bones to obtain pure inorganic aggregate; S2, atmosphere-controlled calcination: high-temperature calcination of the pure inorganic aggregate in a mixed atmosphere containing water vapor and carbon dioxide to obtain hydroxyapatite lumps; S3, multi-stage crushing and online surface modification: ultrafine crushing of the hydroxyapatite lumps through intermediate crushing and air jet milling, and online surface modification during the collection of crushing products to obtain surface-modified hydroxyapatite ultrafine powder. This invention obtains high-purity, highly dispersible hydroxyapatite ultrafine powder by combining atmosphere-controlled calcination with online surface modification technology, and improves its application performance in biodegradable mulch films.

Description

Technical Field

[0001] This invention relates to the field of waste resource utilization and functional polymer materials technology, specifically a method for preparing hydroxyapatite ultrafine powder and its application as a mulch film. Background Technology

[0002] Hydroxyapatite (Ca 10 (PO4)6(OH)2) is the main inorganic component of human and animal bones, possessing excellent biocompatibility and bioactivity. Utilizing waste animal bones as raw materials to prepare hydroxyapatite not only achieves resource utilization of waste but also yields valuable products applicable to biomedicine, food additives, and functional fillers in polymer materials. Therefore, developing an efficient, environmentally friendly technology for preparing high-performance hydroxyapatite powder has significant economic and environmental implications.

[0003] Currently, the conventional method for extracting hydroxyapatite from waste animal bones typically includes steps such as raw material pretreatment, high-temperature calcination, and mechanical crushing. In the pretreatment stage, high-temperature cooking is usually used to remove most of the fat, and strong alkalis (such as sodium hydroxide or sodium bicarbonate) or strong acid solutions are used to hydrolyze and remove organic matter such as collagen from the bones. After purification, the resulting inorganic aggregate is placed in equipment such as a muffle furnace and calcined at high temperature in an air atmosphere to burn off residual organic matter and improve the crystallinity of the product. Finally, the calcined lumps are crushed using methods such as ball milling or air jet milling to obtain powder of the desired particle size. If surface modification is required, the crushed powder is usually collected and then mixed with a modifier in a separate mixing device.

[0004] While existing technologies can prepare hydroxyapatite from animal bones, some inherent technical limitations remain. Regarding product purity and structural integrity, the strong acid or alkali chemicals used in the pretreatment stage inevitably erode and damage the crystal structure of hydroxyapatite while removing proteins. More critically, during high-temperature calcination in air, the hydroxyl groups (-OH) in the hydroxyapatite lattice are easily lost under high temperature and low water vapor pressure, leading to uncontrollable transformations into other phases such as tricalcium phosphate (TCP). This not only reduces the phase purity of the final product but also affects its physicochemical stability. In terms of powder dispersion, when hydroxyapatite is pulverized to the micron or submicron level, its specific surface area increases dramatically, resulting in extremely high surface energy. This makes the primary particles highly prone to spontaneous aggregation. In conventional offline modification processes, the time difference and material transfer between pulverization and surface modification provide sufficient time for these highly reactive primary particles to form hard, difficult-to-disperse agglomerates. Subsequent modifiers can only coat the outer surface of these aggregates and cannot act on the primary particles inside. Therefore, when they are added to the organic polymer matrix as fillers, they will still exist in the form of aggregates and cannot achieve uniform dispersion, thus severely weakening the mechanical properties of the composite material. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing hydroxyapatite ultrafine powder and its application as a mulch film. This method solves the problems of low product phase purity and easy agglomeration of ultrafine powder caused by the limitations of calcination process and modification method when preparing hydroxyapatite from waste animal bones in existing technologies.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The first aspect of this invention provides a method for preparing hydroxyapatite ultrafine powder, the method comprising the following steps:

[0008] S1. Raw Material Pretreatment and Purification: This step aims to thoroughly remove organic impurities such as fat and protein from waste animal bones to obtain the high-purity inorganic precursors required for subsequent calcination. The specific processing flow is as follows:

[0009] First, the bones are degreased by steaming at 120-135℃ for 2.0-3.0 hours, using high temperature and high pressure hydrolysis to melt the fat and separate it from the bone.

[0010] Subsequently, enzymatic deproteinization is performed. Under mild conditions of 45-55℃ and pH 8.5-9.5, a complex protease preparation at 0.5-1.0% of the dry weight of the defatted aggregate is added to the aggregate for hydrolysis for 4.0-6.0 hours. This enzymatic hydrolysis process utilizes the specific catalytic action of proteases on collagen to efficiently degrade it into soluble peptides. Compared with traditional strong acid and strong alkali methods, this method is milder, avoids damage to the apatite structure, and the hydrolysis products can be recycled as resources.

[0011] Next, an alkaline washing deep purification treatment is carried out. A sodium hydroxide or sodium bicarbonate solution with a concentration of 1.0-2.0 mol / L is added to the hydrolyzed aggregate for washing to saponify and remove residual stubborn fats and proteins.

[0012] Finally, a rinsing process is carried out. The aggregates that have undergone cleaning are washed with water until the pH value of the washing water reaches 6.5-7.5 to ensure that all chemical residues are removed and pure inorganic aggregates are obtained.

[0013] S2. Controlled Atmosphere Calcination: This step is crucial for the transformation of bioapatite into highly crystalline hydroxyapatite. The pure inorganic aggregate is calcined at 750-800℃ for 2.0-3.0 hours. Its innovation lies in the continuous introduction of a mixed atmosphere containing water vapor and carbon dioxide into the furnace during the holding period.

[0014] The water vapor atmosphere provides abundant hydroxyl groups (-OH) to the lattice of hydroxyapatite at high temperatures, effectively inhibiting its transformation into other phases such as tricalcium phosphate (TCP) due to dehydration, thus ensuring the integrity and high purity of the final product's chemical structure. Carbon dioxide reacts with free calcium oxide (CaO) that may be generated during calcination, preventing the final product from being strongly alkaline and further improving phase purity. In the mixed atmosphere, the volumetric flow ratio of water vapor to carbon dioxide is preferably 3:1 to 5:1.

[0015] S3. Multi-stage crushing and online surface modification: This step aims to efficiently prepare hydroxyapatite lumps into ultrafine powders with excellent dispersibility.

[0016] First, intermediate crushing is carried out by ball milling the hydroxyapatite lumps for 2.0-4.0 hours to obtain intermediate powder.

[0017] Subsequently, the intermediate powder is subjected to ultrafine grinding and classification by air jet milling. The intermediate powder is fed into the air jet mill at a grinding pressure of 0.6-0.9 MPa. The powder is then ground to the micron or submicron level by the impact, collision and shearing action of the high-speed airflow. The powder is then dynamically classified by a multi-stage cyclone separator, which can accurately collect products with a specific particle size range.

[0018] The key innovation of this step lies in online surface modification: the powder, which has been dynamically graded and separated and possesses high surface energy and activity, is directly fed into a fluidized bed reactor without any intermediate storage. It is then contacted with an atomized surface modifier at a temperature of 60-70°C for 20-30 minutes. This integrated process utilizes the fresh, active surface of the powder immediately after generation, improving the coating efficiency and adhesion of the modifier, and fundamentally avoiding the hard agglomeration problem of ultrafine powders caused by storage.

[0019] The surface modifier used can be an ethanol solution of stearic acid, and its addition amount is 1.0% to 2.5% of the weight of the hydroxyapatite ultrafine powder to be modified. The carboxyl end of stearic acid can be firmly bonded to the surface of hydroxyapatite, while its hydrophobic long carbon chain faces outward, thereby changing the powder surface from hydrophilic to hydrophobic.

[0020] A second aspect of this invention provides a hydroxyapatite ultrafine powder, which is prepared by any of the methods described in the first aspect of this invention. Benefiting from the unique preparation method described above, this hydroxyapatite ultrafine powder has the following structural and performance characteristics: its core is highly crystalline hydroxyapatite, and its surface is uniformly coated with an organically modified layer composed of stearic acid molecules. This powder has a precisely controllable particle size, with a particle size range controllable within 1-3 μm. This structural feature enables it to exhibit excellent dispersibility and interfacial compatibility in organic polymer matrices.

[0021] The third aspect of this invention provides the application of hydroxyapatite ultrafine powder in the preparation of geological film structures.

[0022] Specifically, the hydroxyapatite ultrafine powder described in the second aspect of this invention is used as a multifunctional filler to prepare functional biodegradable mulch films, and its application method is as follows:

[0023] The surface-modified hydroxyapatite ultrafine powder, in 15-40 parts by weight, is melt-blended with 30-45 parts of polylactic acid and 30-45 parts of polybutylene adipate / terephthalate, and then the mixture is blown into a film to prepare a mulch film.

[0024] In this application, the role of hydroxyapatite ultrafine powder is reflected in:

[0025] Because its surface is hydrophobically modified, it can be uniformly dispersed in the polymer matrix as a reinforcing phase, thereby adjusting the mechanical properties of the mulch film.

[0026] After the mulch film reaches the end of its service life and undergoes biodegradation, it can degrade into mineral elements such as calcium and phosphorus, providing nutrients for the soil.

[0027] Its porous structure can adsorb some heavy metal ions in the soil, thus playing a certain role in soil remediation. Therefore, this application organically combines waste resource utilization, material functionalization, and agricultural environmental protection.

[0028] This invention provides a method for preparing hydroxyapatite ultrafine powder and its application as a mulch film. It has the following beneficial effects:

[0029] 1. This invention, through a systematic purification process, especially the introduction of an enzymatic protein decomposition treatment step, can efficiently remove organic matter from bones under mild conditions, avoiding the damage to the hydroxyapatite structure caused by traditional strong acid and strong alkali methods; combined with subsequent atmosphere-controlled calcination, under the protection of a mixed atmosphere of water vapor and carbon dioxide, the transformation of hydroxyapatite into other phases at high temperatures is effectively suppressed, thereby obtaining a hydroxyapatite product with high phase purity, good crystallinity, and excellent color.

[0030] 2. This invention integrates three unit operations—ultrafine grinding, dynamic classification, and surface modification—into a single online processing system. After the powder is pulverized to the ultrafine level by an air jet mill, it directly enters a fluidized bed for surface modification without the need for offline drying or other intermediate steps. This integrated process utilizes the high surface energy and chemical activity of the powder immediately after its formation, significantly improving the coating efficiency and binding strength of the surface modifier, fundamentally solving the technical problem of hard agglomeration of ultrafine powders during storage and transportation.

[0031] 3. The surface-modified hydroxyapatite ultrafine powder prepared by the method of this invention changes its surface from hydrophilic to hydrophobic, improving its interfacial compatibility with organic polymer matrices such as polylactic acid and poly(butylene adipate / terephthalate). Therefore, during the preparation of composite mulch films, this ultrafine powder can be uniformly dispersed in the polymer melt as a functional filler, avoiding the degradation of material mechanical properties and the generation of defects caused by filler agglomeration. Attached Figure Description

[0032] Figure 1 This is a process flow diagram of the preparation method of the present invention;

[0033] Figure 2 This is a detailed flowchart illustrating the raw material pretreatment and purification steps of the present invention.

[0034] Figure 3 This is a schematic diagram of the equipment connection for the multi-stage crushing and online surface modification steps of the present invention. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0036] Please see the appendix Figure 1 -Appendix Figure 3 :

[0037] Preparation example:

[0038] This preparation example describes a method for preparing a surface modifier, which includes the following steps:

[0039] Accurately weigh 80.0g of stearic acid (analytical grade) and prepare 1000mL of anhydrous ethanol;

[0040] Place the weighed stearic acid in a clean beaker and add about 800 mL of anhydrous ethanol;

[0041] Place the beaker in a magnetically stirred water bath, set the water bath temperature to 50°C and heat and stir until the stearic acid solid is completely dissolved, forming a clear and transparent solution;

[0042] Stop heating and allow the solution to cool naturally to room temperature;

[0043] Transfer the cooled solution to a 1000 mL volumetric flask, wash the beaker several times with a small amount of anhydrous ethanol, and transfer the washing solution into the volumetric flask. Finally, dilute to the mark with anhydrous ethanol and shake well.

[0044] This yields an 8.0% (w / v) stearic acid ethanol solution. The solution should be sealed and stored for later use.

[0045] Example 1:

[0046] Preparation of S1, hydroxyapatite ultrafine powder:

[0047] Raw material pretreatment and purification: Take 100 kg of primary crushed waste bovine bones and process them as follows:

[0048] Degreasing treatment by cooking: Add bone pieces and water to a high-pressure reactor at a weight ratio of 1:3, heat to 130°C, maintain at this temperature for 2.5 hours, and then separate and remove the upper layer of floating oil.

[0049] Enzymatic deproteinization treatment: The defatted aggregate is transferred to an enzymatic hydrolysis reactor, water is added to adjust the solid-liquid ratio, and the pH of the system is adjusted to 9.0 using dilute hydrochloric acid, sodium hydroxide, or sodium bicarbonate solution. After heating to 50°C, 0.8 kg (equivalent to 0.8% of the dry weight of the aggregate) of a compound protease preparation is added, and the reaction is carried out at 50°C and pH 9.0 for 5.0 hours.

[0050] Alkaline washing deep purification treatment: Add the enzymatically hydrolyzed aggregate to a 1.5 mol / L sodium hydroxide or sodium bicarbonate solution and react at 90°C for 1.5 hours.

[0051] Rinsing treatment: The aggregate treated with alkali was repeatedly rinsed with deionized water until the pH of the effluent was 7.0, yielding pure inorganic aggregate. The aggregate was then dried at 105°C to constant weight.

[0052] S2, Atmosphere-controlled calcination:

[0053] The dried, pure inorganic aggregate was placed in an atmosphere tube furnace and heated to 780°C at a rate of 8°C / min. It was held at this temperature for 2.5 hours, during which steam was introduced at a flow rate of 4 L / min and carbon dioxide at a flow rate of 1 L / min, with a steam-to-carbon dioxide volume ratio of 4:1. After calcination, the aggregate was cooled to room temperature with the furnace to obtain hydroxyapatite lumps.

[0054] S3, Multi-stage grinding and online surface modification:

[0055] Intermediate crushing: The obtained hydroxyapatite lumps are fed into a ball mill and ball milled for 3.0 hours to obtain intermediate powder.

[0056] Airflow milling for ultrafine grinding and online surface modification: Intermediate powder is continuously fed into an integrated airflow milling-online modification system. Ultrafine grinding is performed at a grinding pressure of 0.8 MPa. After classification by a multi-stage cyclone separator, powder with the required particle size directly enters the fluidized bed reactor.

[0057] Simultaneously, the prepared surface modifier was uniformly sprayed into the fluidized bed, with the addition amount controlled at 1.8% of the weight of the hydroxyapatite (SHI) powder to be modified. Online coating modification was carried out at 65°C for 25 minutes. Finally, surface-modified hydroxyapatite ultrafine powder was obtained.

[0058] Preparation of functional biodegradable mulch films or fully biodegradable mulch films:

[0059] Weigh the following components by weight: 2 parts polylactic acid (PLA), 63 parts polybutylene adipate / terephthalate (PBAT or PBST), 10 parts polypropylene carbonate, 5 parts plant fiber (cotton stalk, bamboo, reed, rice husk or hemp), 15 parts surface-modified hydroxyapatite ultrafine powder, 3 parts plasticizer, 0.7 parts lubricant, 1 part dispersant, 0.3 parts chain extender, and 0.5 parts compatibilizer.

[0060] All solid raw materials, except for the plasticizer, were vacuum dried at 80°C for 65 minutes. Then, the plasticizer was mixed in evenly, and the mixed material was fed into a twin-screw extruder. The temperature of each section of the extruder was set to 130°C-140°C-145°C-140°C-135°C, and the screw speed was 200 rpm. The material was melt-blended, extruded, and pelletized to obtain composite material masterbatch, which was then vacuum-packaged.

[0061] After vacuum drying the masterbatch at 80°C for 1 hour, it is fed into a single-screw blow molding machine. The temperature of the blow molding machine body and the die head is set to 170°C. By controlling the blow ratio to 3.0 and the traction speed to 6m / min, a film with a thickness of 0.012mm is blown into shape. The finished product is obtained by winding it up.

[0062] Example 2:

[0063] Preparation of S3, hydroxyapatite ultrafine powder:

[0064] Raw material pretreatment and purification: The basic steps are the same as in Example 1, and the process parameters are as follows: cooking and defatting treatment is carried out at 125°C for 3.0 hours; enzymatic deproteinization treatment is carried out at 55°C and pH 9.5 for 6.0 hours, and the amount of compound protease preparation added is 1.0% of the dry weight of aggregate; alkaline washing deep purification treatment uses a 2.0 mol / L sodium hydroxide or sodium bicarbonate solution and reacts at 95°C for 2.0 hours; rinsing is carried out until the pH of the effluent is 7.2.

[0065] S2. Atmosphere-controlled calcination: The basic steps are the same as in Example 1, but the process parameters are as follows: heat up to 800℃ and hold for 3.0 hours, and control the volume flow ratio of water vapor to carbon dioxide to be 5:1.

[0066] S3. Multi-stage grinding and online surface modification: The basic steps are the same as in Example 1, and the process parameters used are: ball milling for 4.0 hours; air jet milling pressure of 0.9 MPa; online surface modification is carried out at 70°C with a residence time of 30 minutes, and the amount of surface modifier added is 2.5% of the powder weight.

[0067] Preparation of functional biodegradable mulch films or fully biodegradable mulch films:

[0068] Weigh the following components by weight: 4 parts polylactic acid (PLA), 75 parts polybutylene adipate / terephthalate (PBAT or PBST), 20 parts polypropylene carbonate, 10 parts plant fiber (cotton stalk, bamboo, reed, rice husk or hemp), 25 parts surface-modified hydroxyapatite ultrafine powder, 5 parts plasticizer, 0.9 parts lubricant, 3 parts dispersant, 0.5 parts chain extender, and 0.7 parts compatibilizer.

[0069] The subsequent masterbatch preparation and blow molding process are the same as in Example 1, and a film with a thickness of 0.015 mm is finally produced.

[0070] Comparative Example 1: Compared with Example 1, the difference is that in the controlled calcination of hydroxyapatite ultrafine powder, the pure inorganic aggregate was calcined in a static air atmosphere at the same temperature (780°C) and holding time (2.5 hours), instead of in a mixed atmosphere of water vapor and carbon dioxide. All other steps and parameters were exactly the same as in Example 1.

[0071] Comparative Example 2: Compared with Example 1, the difference is that in the preparation of hydroxyapatite ultrafine powder, online surface modification was not performed. Instead, the unmodified hydroxyapatite ultrafine powder, after being pulverized and graded by an air jet mill, was first taken out and then added to a separate high-speed mixer for offline mixing and modification with an equal amount of surface modifier prepared in Example 1 at the same temperature (65°C) and time (25 minutes). All other steps and parameters were exactly the same as in Example 1.

[0072] Comparative Example 3: Compared with Example 1, the difference is that the hydroxyapatite ultrafine powder used in the preparation of the functional biodegradable mulch film or biodegradable mulch film is the powder in Example 1 that has not undergone online surface modification treatment. The weight proportions of each component in the mulch film formulation and all other preparation process parameters are exactly the same as in Example 1.

[0073] Test Example 1:

[0074] To verify the performance of the hydroxyapatite ultrafine powder prepared by the method of the present invention and its application in biodegradable or biodegradable mulch films, the following performance tests were conducted on the powder samples and mulch film samples obtained in Examples 1-2 and Comparative Examples 1-3.

[0075] The testing methods include powder performance testing and mulch film performance testing.

[0076] Powder performance testing includes:

[0077] Particle size distribution test: A laser particle size analyzer was used for determination. 0.5 g of the powder sample was added to 50 mL of anhydrous ethanol and dispersed in an ultrasonic cleaner for 5 minutes to prepare a uniform suspension. The suspension was pumped into the measuring cell using the instrument's built-in circulating injection system. Testing began when the shading rate reached 10%-15%. Particle size distribution data were recorded, and the median particle size (D50) was taken as the result. Each sample was tested three times, and the average value was taken.

[0078] Whiteness test: An automatic whiteness meter was used for measurement. An appropriate amount of the powder sample to be tested was taken and pressed into a dense circular sample with a diameter of 30 mm and a thickness of 5 mm using a tablet press under a pressure of 20 MPa. After calibrating the instrument with a standard white plate, the sample was placed in the test port for measurement. Five points were measured at different locations on the sample, and the average whiteness value was taken as the final result.

[0079] Surface wettability test: Measurements were performed using a contact angle meter. The powder sample prepared in step 1.2 was used. The sample was placed horizontally on the instrument's sample stage. Using a microsyringe, a 3 μL drop of deionized water was applied to the sample surface. Two seconds after the water droplet contacted the sample surface, an image of the droplet was captured using the instrument's built-in camera system, and the static contact angle between the droplet and the sample surface was calculated using software. Measurements were taken at five different locations for each sample, and the average value was recorded.

[0080] The performance test of plastic film includes:

[0081] Mechanical property testing: The mulch film samples were tested using a universal testing machine according to GB / T1040.3-2006 standard. The mulch film samples were longitudinally cut into dumbbell-shaped specimens with a width of 15 mm and a length of 150 mm. The tensile rate of the testing machine was set to 50 mm / min.

[0082] Clamp the specimen at both ends onto the fixture, start the testing machine to apply tension until the specimen breaks. Record the maximum load at specimen breakage and the change in distance between the markings, and calculate the tensile strength and elongation at break of the material accordingly. Test 5 valid specimens for each type of mulch film sample, and take the average value.

[0083] Test results: The samples prepared in Examples 1-2 and Comparative Examples 1-3 were tested according to the above method, and the results are summarized in Table 1.

[0084] Table 1: Performance test results of each embodiment and comparative sample

[0085]

[0086]

[0087] From Table 1, we can obtain:

[0088] The hydroxyapatite ultrafine powder and its mulch film products prepared by the method of this invention show significant differences in key performance indicators. Data from Comparative Example 1 shows that the whiteness value of the powder obtained by calcination in air is significantly lower than that of Example 1. This is attributed to the lack of water vapor protection at high temperatures, which causes partial dehydration of the hydroxyapatite and possible side reactions with trace impurities in the matrix, resulting in impure phases and poor color in the product. The mixed atmosphere of water vapor and carbon dioxide used in this invention provides a chemical environment for the stable existence of hydroxyapatite, ensuring high purity and excellent whiteness of the product.

[0089] The unmodified powder in Comparative Example 3 exhibited significant hydrophilicity (contact angle only 25.8°). Although Comparative Example 2 underwent offline modification, its median particle size was much larger than that of Example 1, indicating that the ultrafine powder with high surface energy had undergone irreversible hard agglomeration during the crushing, storage, and modification processes. The online modification process employed in this invention coats the powder surface instantly upon generation, utilizing its highly active fresh surface to effectively prevent particle agglomeration, thereby obtaining powder with small particle size and uniform surface hydrophobicity (contact angle 115.6°).

[0090] The mulch films prepared in Examples 1 and 2 exhibited high tensile strength and elongation at break. This is because the hydroxyapatite powder, after online modification, was uniformly dispersed in the polymer matrix at a size close to the original particles, forming a good interfacial bond and effectively transferring stress. In contrast, the agglomerated particles in Comparative Example 2 and the hydrophilic particles in Comparative Example 3 became stress concentration points and weak links in the material due to poor compatibility with the polymer matrix, resulting in a severe weakening of the mechanical properties of the mulch film.

[0091] Test Example 2:

[0092] To further verify the effect of the method of the present invention on the phase purity of hydroxyapatite, the chemical composition of the powder samples prepared in Example 1 and Comparative Example 1 was analyzed.

[0093] Test method:

[0094] Sample digestion: Accurately weigh approximately 0.1 g of the powder sample to be tested and place it in a clean 50 mL beaker. Add 5 mL of 5% (v / v) high-purity nitric acid solution and cover with a watch glass. Place the beaker on a hot plate and heat gently at 80°C until the sample is completely dissolved and the solution is clear and transparent. Stop heating and cool the solution to room temperature. Quantitatively transfer the cooled solution to a 100 mL volumetric flask. Wash the beaker several times with deionized water, transferring the washings into the volumetric flask as well. Finally, dilute to the mark with deionized water and mix well.

[0095] Instrument Testing and Calculation: Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) was used for determination. First, standard working curves were plotted using pre-prepared calcium and phosphorus standard solutions. Then, the sample solutions prepared in step 1 were introduced into the instrument for testing, and the characteristic emission spectral intensities of calcium (Ca) and phosphorus (P) were recorded. Each sample solution was tested in triplicate.

[0096] The mass concentrations of calcium and phosphorus in the solution were calculated based on the standard working curve. Then, based on the molar masses of the two elements (Ca: 40.08 g / mol, P: 30.97 g / mol), their molar concentrations were calculated separately, and the final calcium-to-phosphorus molar ratio (Ca / P) in the powder sample was obtained.

[0097] Test results:

[0098] The samples prepared in Example 1 and Comparative Example 1 were tested according to the above method, and the results are summarized in Table 2 below.

[0099] Table 2: Chemical composition test results of powder samples from Example 1 and Comparative Example 1

[0100] Test object Calcium to phosphorus molar ratio (Ca / P) Example 1 1.65 Comparative Example 1 1.53

[0101] From Table 2, we can obtain:

[0102] The calcium-to-phosphorus molar ratio of the powder obtained in Example 1 was 1.65, which is very close to the theoretical stoichiometry of hydroxyapatite (1.67). In contrast, the calcium-to-phosphorus molar ratio of the powder obtained in Comparative Example 1 was 1.53, which deviated significantly from the theoretical value and was closer to the stoichiometry of tricalcium phosphate (1.50).

[0103] In the air atmosphere of Comparative Example 1, the partial pressure of water vapor is extremely low. The structural hydroxyl groups (-OH) in the hydroxyapatite lattice are easily removed at high temperatures, leading to crystal structure instability and a conversion to tricalcium phosphate, which has a lower stoichiometric ratio. This conversion process is irreversible, resulting in a final product that is a mixture of hydroxyapatite and tricalcium phosphate, macroscopically manifested as a decrease in the calcium-to-phosphorus molar ratio.

[0104] The method employed in this invention artificially creates a high water vapor partial pressure environment by introducing a specific flow ratio of water vapor and carbon dioxide into the calcination system. According to the principle of chemical equilibrium, this high water vapor partial pressure environment effectively inhibits the dehydroxylation reaction of hydroxyapatite, thereby protecting the integrity of its crystal structure. Therefore, this method can produce hydroxyapatite products with high phase purity and chemical composition closer to theoretical values, laying a material foundation for subsequently obtaining high-whiteness, high-performance powder materials.

[0105] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing hydroxyapatite ultrafine powder, characterized in that, Includes the following steps: S1. Raw material pretreatment and purification: physical and chemical purification treatment of waste animal bones to obtain pure inorganic aggregates; S2. Atmosphere-controlled calcination: The pure inorganic aggregate is calcined at high temperature in a mixed atmosphere containing water vapor and carbon dioxide to obtain hydroxyapatite blocks. S3. Multi-stage crushing and online surface modification: The hydroxyapatite lumps are subjected to intermediate crushing and air jet milling for ultra-fine pulverization, and online surface modification is carried out during the collection of pulverized products to obtain surface-modified hydroxyapatite ultrafine powder.

2. The method for preparing hydroxyapatite ultrafine powder according to claim 1, characterized in that, In step S1, the pretreatment and purification of raw materials specifically involves sequentially processing the waste animal bones as follows: Degreasing treatment by steaming: The bones are steamed at 120-135℃ for 2.0-3.0 hours; Enzymatic deproteinization treatment: Add 0.5-1.0% of a compound protease preparation by dry weight of the aggregate to the cooked aggregate for hydrolysis; Alkaline washing deep purification treatment: Add a sodium hydroxide or sodium bicarbonate solution with a concentration of 1.0-2.0 mol / L to the hydrolyzed aggregate for washing; Rinsing treatment: The aggregate after cleaning treatment is washed with water until the pH value is 6.5-7.5 to obtain the pure inorganic aggregate.

3. The method for preparing hydroxyapatite ultrafine powder according to claim 2, characterized in that, In step S1, the enzymatic dehydrogenation of protein is specifically carried out at a temperature of 45-55°C and a pH of 8.5-9.5 for 4.0-6.0 hours.

4. The method for preparing hydroxyapatite ultrafine powder according to claim 3, characterized in that, In step S2, the atmosphere-controlled calcination includes the following steps: The pure inorganic aggregate is calcined at 750-800℃ for 2.0-3.0 hours, and the mixed atmosphere is continuously introduced into the furnace during the holding period to obtain hydroxyapatite blocks.

5. The method for preparing hydroxyapatite ultrafine powder according to claim 4, characterized in that, In the mixed atmosphere, the volume flow ratio of water vapor to carbon dioxide is 3:1-5:

1.

6. The method for preparing hydroxyapatite ultrafine powder according to claim 1, characterized in that, In step S3, the multi-stage crushing and online surface modification includes the following steps: Intermediate crushing: The hydroxyapatite lumps are ball-milled for 2.0-4.0 hours to obtain intermediate powder; Airflow milling and classification: The intermediate powder is fed into an airflow mill under a pulverizing pressure of 0.6-0.9 MPa and dynamically classified by a multi-stage cyclone separator to obtain hydroxyapatite ultrafine powder. Online surface modification: The powder separated by dynamic classification is directly fed into a fluidized bed reactor and contacted with a surface modifier at a temperature of 60-70℃ for 20-30 minutes to obtain surface-modified hydroxyapatite ultrafine powder.

7. The method for preparing hydroxyapatite ultrafine powder according to claim 6, characterized in that, The surface modifier used is prepared by dissolving stearic acid as a solute in ethanol solvent, and the amount of the surface modifier added is 1.0% to 2.5% of the weight of the hydroxyapatite ultrafine powder to be modified.

8. The method for preparing hydroxyapatite ultrafine powder according to claim 6, characterized in that, The surface-modified hydroxyapatite ultrafine powder obtained by the multi-stage cyclone separator and the online surface modification has a particle size range of 1-3 μm.

9. A hydroxyapatite ultrafine powder, characterized in that, It is obtained by the method for preparing hydroxyapatite ultrafine powder according to any one of claims 1 to 8.

10. The application of a hydroxyapatite ultrafine powder as described in any one of claims 1-8 in the preparation of a geological membrane structure.

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