Preparation method of superfine silver powder

Abstract

The invention discloses a preparation method of superfine silver powder, which relates to the technical field of metal powder materials and comprises the following steps: step S1, dissolving ingredients; s2, preparing a seed crystal; step S3, carrying out double dropwise addition reaction; s4, performing suction filtration and powder washing; s5, carrying out wet treatment; step S6, coating modification; step S7, jet milling; and S8, drying and grading. The particle size D50 of the product prepared by the method is less than or equal to 185 nm, the dry powder moisture is less than or equal to 0.13%, the burn out rate at 538 DEG C is less than or equal to 0.68%, the wet powder solid content is greater than or equal to 85%, and the product is uniformly dispersed without obvious agglomeration and has excellent comprehensive performance; the problems of large particle size, prominent agglomeration, unstable performance and the like are effectively avoided, the requirements of high-end fields such as new energy batteries and high-end coatings are met, the process is controllable, and the method is suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of metal powder materials technology, and in particular to a method for preparing ultrafine silver powder. Background Technology

[0002] Ultrafine silver powder, due to its excellent electrical conductivity, thermal conductivity, and chemical stability, has wide applications in fields such as new energy battery electrodes, high-end electronic pastes, and conductive coatings. As downstream industries move towards higher performance, the requirements for silver powder particle size and dispersibility are becoming increasingly stringent. However, due to its large specific surface area and high surface energy, ultrafine silver powder is prone to agglomeration during synthesis and post-processing, leading to a decline in its practical application performance.

[0003] In existing technologies, the preparation of ultrafine silver powder mostly adopts chemical reduction methods, but the following problems exist: In the traditional single-drop addition process, the reaction between silver salt solution and reducing agent will generate acidic substances such as nitric acid, which will cause the pH value of the reaction system to fluctuate continuously, making it difficult to stably control the growth rate of silver powder and resulting in uneven particle size distribution; there is a lack of effective particle size guiding mechanism, making it difficult to stabilize the silver powder particle size at the nanoscale, and the agglomeration phenomenon is aggravated when the particle size is too small; the selection and use of dispersants are unreasonable, and agglomeration cannot be fundamentally inhibited; the post-processing process is simple and has not formed a systematic dispersion and stabilization scheme, resulting in insufficient product dispersibility and stability.

[0004] To address the aforementioned problems, invention patent document CN105345023B discloses a method for preparing highly dispersible ultrafine silver powder. The method involves first preparing a silver nitrate solution, then adding an aqueous solution of hydrazine hydrate to the silver nitrate solution and mixing thoroughly to obtain a mixture. This mixture is then reacted at 60℃~90℃ for 30~60 min. The reacted material is separated and washed to obtain ultrafine silver powder. The obtained ultrafine silver powder is added to a mixed solution of anhydrous ethanol and water to obtain a silver powder slurry. This slurry is then ultrasonically crushed in an ultrasonic crusher for 30s~60s. The resulting silver powder slurry is filtered and dried to obtain highly dispersible ultrafine silver powder. In this invention, silver nitrate is first reduced with hydrazine hydrate to obtain ultrafine silver powder. After washing, the ultrafine silver powder is then subjected to a second treatment using anhydrous ethanol and water as the dispersion medium and ultrasonically crushed. After drying, the final ultrafine silver powder has smaller particle size, more uniform particle size, and better dispersibility. However, this method relies solely on the physical dispersion effect of ultrasonic disruption and fails to form a synergistic solution from the aspects of pH stability control of the reaction system, particle size orientation growth guidance, and silver powder surface energy regulation. As a result, the silver powder is prone to secondary agglomeration during subsequent drying, storage, and application, and the particle size is difficult to stably control below 200nm. This method cannot meet the stringent requirements of high-end new energy batteries, electronic ceramics, and other fields for long-term dispersion stability and precise particle size of ultrafine silver powder.

[0005] Therefore, developing a method for preparing ultrafine silver powder that can achieve nanoscale particle size control, high dispersibility, and high stability is of great practical significance. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing ultrafine silver powder. Through a systematic approach that combines multiple technologies, the problems of easy agglomeration, difficult particle size control, and poor dispersibility of ultrafine silver powder are solved.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is: a method for preparing ultrafine silver powder, comprising the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water and stir to dissolve to obtain a silver salt solution. Control the concentration of the silver salt solution to be 0.5-2 mol / L. Step S2, Seed preparation: Take 5-10 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 3-5, react for 10-30 min to prepare silver seed crystals, the particle size of the silver seed crystals being 50-100 nm; Step S3, Double Drop Reaction: Add deionized water to the reaction vessel, heat to 30-60℃, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 2-6, add a superdispersant during the drop addition process, and continue to keep the reaction at the temperature for 30-60 minutes after the drop addition is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 40-60%, and perform wet grinding for 10-30 minutes; Step S6, Coating Modification: Add coating agent to the slurry from Step S5, heat to 40-80℃, stir and react for 60-120 minutes, and control the coating content to 3-15‰. Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.3-0.8 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 50-60℃ for 2-4 hours, pass it through a 200-400 mesh sieve, grade it and put it into storage to obtain ultrafine silver powder product.

[0008] Preferably, the silver salt in step S1 is one or a mixture of two of silver nitrate and silver sulfate.

[0009] Preferably, the reducing agent in step S2 is one or a mixture of ascorbic acid, formaldehyde, and hydrazine hydrate.

[0010] Preferably, the molar ratio of the reducing agent to the silver salt in step S2 is (1.2-2.0):1.

[0011] Preferably, the alkaline solution in step S3 is either sodium hydroxide solution or potassium hydroxide solution, with a concentration of 0.5-3 mol / L; the double dropping rate is 5-20 mL / min.

[0012] Preferably, the superdispersant in step S3 is one or a mixture of polycarboxylate, polyurethane, and polyolefin polymer superdispersants.

[0013] Preferably, the amount of seed crystal used in step S3 is 0.1-1% of the final silver powder mass.

[0014] Preferably, the amount of the superdispersant used in step S3 is 0.5-5‰ of the final silver powder mass.

[0015] Preferably, the coating agent in step S6 is one or a mixture of organosilanes, fatty acid salts, and titanate coupling agents.

[0016] Preferably, the inlet air temperature of the spray drying in step S7 is 150-200℃ and the outlet air temperature is 80-100℃.

[0017] Due to the application of the above technical solution, the present invention has the following beneficial effects: (1) The method for preparing ultrafine silver powder disclosed in this invention adopts a double drop addition process, in which silver salt solution and alkaline solution are added simultaneously to effectively counteract the acidic substances generated in the reaction and maintain the pH stability of the reaction system. This solves the problem of uneven particle size caused by pH fluctuations in the traditional single drop addition process and ensures the stability of product quality.

[0018] (2) The method for preparing ultrafine silver powder disclosed in this invention introduces seed crystal guidance technology. By using pre-made nanoscale silver seed crystals to guide the growth of silver powder, the particle size of silver powder is precisely controlled, and nanoscale silver powder with D50≤185nm can be stably prepared.

[0019] (3) The method for preparing ultrafine silver powder disclosed in this invention involves adding a polymer superdispersant during the reaction process, combined with wet grinding and coating modification in the post-treatment, to synergistically inhibit agglomeration from three dimensions: "reaction inhibition - physical dispersing - surface modification", significantly improving the product dispersibility and the particles are in a uniformly dispersed state.

[0020] (4) The preparation method of ultrafine silver powder disclosed in this invention has excellent comprehensive performance, with particle size D50≤185nm, dry powder moisture ≤0.13%, burn loss rate at 538℃ ≤0.68%, wet powder solid content ≥85%, tap density and specific surface area meet the requirements of downstream applications, and are suitable for high-performance fields such as new energy batteries and high-end coatings; the process is mature and controllable, and can realize large-scale production, with good economic benefits and application prospects. Detailed Implementation

[0021] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0022] Example 1 A method for preparing ultrafine silver powder includes the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water, stir and dissolve to obtain a silver salt solution, and control the concentration of the silver salt solution to be 0.5 mol / L; Step S2, Seed preparation: Take 5 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 3, react for 10 min to prepare silver seed crystals, the particle size of the silver seed crystals is 50 nm; Step S3, Double Drop Reaction: Add deionized water to the reaction vessel, heat to 30°C, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 2, add a superdispersant during the drop addition process, and continue to keep the reaction at the temperature for 30 min after the drop addition is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 40%, and perform wet grinding for 10 minutes; Step S6, Coating Modification: Add coating agent to the slurry from step S5, heat to 40°C, stir and react for 60 minutes, and control the coating content to 3‰; Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.3 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 50℃ for 2 hours, pass it through a 200-mesh sieve, grade it and put it into storage to obtain the ultrafine silver powder product.

[0023] The silver salt mentioned in step S1 is silver nitrate; the reducing agent mentioned in step S2 is ascorbic acid; the molar ratio of the reducing agent to the silver salt in step S2 is 1.2:1; the alkaline solution mentioned in step S3 is sodium hydroxide solution with a concentration of 0.5 mol / L; the double drop addition rate is 5 mL / min; the superdispersant mentioned in step S3 is Disuper S9 comb-shaped polyurethane superdispersant; the amount of seed crystals mentioned in step S3 is 0.1% of the final silver powder mass; the amount of superdispersant mentioned in step S3 is 0.5‰ of the final silver powder mass; the coating agent mentioned in step S6 is silane coupling agent KH560; the inlet air temperature of the spray drying in step S7 is 150℃, and the outlet air temperature is 80℃.

[0024] Example 2 A method for preparing ultrafine silver powder includes the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water and stir to dissolve to obtain a silver salt solution, controlling the concentration of the silver salt solution to be 0.9 mol / L; Step S2, Seed preparation: Take 6 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 3.5, react for 15 min to prepare silver seed crystals, the particle size of the silver seed crystals is 60 nm; Step S3, Double Drop Reaction: Add deionized water to the reaction vessel, heat to 40°C, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 3, add a superdispersant during the drop addition process, and continue to keep the reaction at the temperature for 40 min after the drop addition is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 45%, and perform wet grinding for 15 minutes; Step S6, Coating Modification: Add coating agent to the slurry from step S5, heat to 50°C, stir and react for 80 minutes, and control the coating content to 5‰. Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.4 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 53℃ for 2.5 hours, pass it through a 250-mesh sieve, grade it and put it into storage to obtain the ultrafine silver powder product.

[0025] The silver salt in step S1 is silver sulfate; the reducing agent in step S2 is formaldehyde; the molar ratio of the reducing agent to the silver salt in step S2 is 1.4:1; the alkaline solution in step S3 is potassium hydroxide solution with a concentration of 1.5 mol / L; the double-dropping rate is 10 mL / min; the superdispersant in step S3 is Disuper S9 comb-shaped polyurethane superdispersant; the amount of seed crystals in step S3 is 0.4% of the final silver powder mass; the amount of superdispersant in step S3 is 2.5‰ of the final silver powder mass; the coating agent in step S6 is silane coupling agent KH570; the inlet air temperature of the spray drying in step S7 is 160℃, and the outlet air temperature is 90℃.

[0026] Example 3 A method for preparing ultrafine silver powder includes the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water and stir to dissolve to obtain a silver salt solution, controlling the concentration of the silver salt solution to be 1.3 mol / L; Step S2, Seed preparation: Take 7 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 4, react for 20 min to prepare silver seed crystals, the particle size of the silver seed crystals is 75 nm; Step S3, Double Drop Reaction: Add deionized water to the reaction vessel, heat to 45°C, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 4, add a superdispersant during the drop addition process, and continue to keep the reaction at the temperature for 45 min after the drop addition is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 50%, and perform wet grinding for 20 minutes; Step S6, Coating Modification: Add coating agent to the slurry from step S5, heat to 60℃, stir and react for 90 min, and control the coating content to 11‰. Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.6 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 55℃ for 3 hours, pass it through a 300-mesh sieve, grade it and put it into storage to obtain the ultrafine silver powder product.

[0027] The silver salt mentioned in step S1 is silver nitrate; the reducing agent mentioned in step S2 is hydrazine hydrate; the molar ratio of the reducing agent to the silver salt in step S2 is 1.7:1; the alkaline solution mentioned in step S3 is sodium hydroxide solution with a concentration of 2 mol / L; the double drop addition rate is 13 mL / min; the superdispersant mentioned in step S3 is Disuper S9 comb-shaped polyurethane superdispersant; the amount of seed crystals mentioned in step S3 is 0.6% of the final silver powder mass; the amount of superdispersant mentioned in step S3 is 3.5‰ of the final silver powder mass; the coating agent mentioned in step S6 is silane coupling agent KH550; the inlet air temperature of the spray drying in step S7 is 180℃, and the outlet air temperature is 90℃.

[0028] Example 4 A method for preparing ultrafine silver powder includes the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water and stir to dissolve to obtain a silver salt solution, controlling the concentration of the silver salt solution to be 1.8 mol / L; Step S2, Seed preparation: Take 9 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 4.5, react for 25 min to prepare silver seed crystals, the silver seed crystals having a particle size of 90 nm; Step S3, Double Drop Reaction: Add deionized water to the reaction vessel, heat to 55°C, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 5, add a superdispersant during the drop addition process, and continue to keep the reaction at the temperature for 55 minutes after the drop addition is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 55%, and perform wet grinding for 25 minutes; Step S6, Coating Modification: Add coating agent to the slurry from step S5, heat to 75°C, stir and react for 110 min, and control the coating content to 13‰. Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.7 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 58℃ for 3.5 hours, pass it through a 350-mesh sieve, grade it and put it into storage to obtain the ultrafine silver powder product.

[0029] The silver salt mentioned in step S1 is silver nitrate; the reducing agent mentioned in step S2 is ascorbic acid; the molar ratio of the reducing agent to the silver salt in step S2 is 1.9:1; the alkaline solution mentioned in step S3 is sodium hydroxide solution with a concentration of 2.8 mol / L; the double drop addition rate is 18 mL / min; the superdispersant mentioned in step S3 is Disuper S9 comb-shaped polyurethane superdispersant; the amount of seed crystals mentioned in step S3 is 0.9% of the final silver powder mass; the amount of superdispersant mentioned in step S3 is 4.5‰ of the final silver powder mass; the coating agent mentioned in step S6 is silane coupling agent KH570; the inlet air temperature of the spray drying in step S7 is 190℃, and the outlet air temperature is 95℃.

[0030] Example 5 A method for preparing ultrafine silver powder includes the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water, stir and dissolve to obtain a silver salt solution, and control the concentration of the silver salt solution to 2 mol / L; Step S2, Seed preparation: Take 10 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 5, react for 30 min to prepare silver seed crystals, the particle size of the silver seed crystals is 100 nm; Step S3, Double Drop Reaction: Add deionized water to the reaction vessel, heat to 60°C, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 6, add a superdispersant during the drop addition process, and continue to keep the reaction at the temperature for 60 min after the drop addition is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 60%, and perform wet grinding for 30 minutes; Step S6, Coating Modification: Add coating agent to the slurry from step S5, heat to 60℃, stir and react for 120 min, and control the coating content to 15‰. Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.8 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 120℃ for 4 hours, pass it through a 400-mesh sieve, grade it and put it into storage to obtain the ultrafine silver powder product.

[0031] The silver salt mentioned in step S1 is silver nitrate; the reducing agent mentioned in step S2 is ascorbic acid; the molar ratio of the reducing agent to the silver salt in step S2 is 2.0:1; the alkaline solution mentioned in step S3 is sodium hydroxide solution with a concentration of 3 mol / L; the double drop addition rate is 20 mL / min; the superdispersant mentioned in step S3 is Disuper S9 comb-shaped polyurethane superdispersant; the amount of seed crystals mentioned in step S3 is 1% of the final silver powder mass; the amount of superdispersant mentioned in step S3 is 0.5% of the final silver powder mass; the coating agent mentioned in step S6 is silane coupling agent KH550; the inlet air temperature of the spray drying in step S7 is 200℃, and the outlet air temperature is 100℃.

[0032] Comparative Example 1 This example provides a method for preparing ultrafine silver powder, which is basically the same as that in Example 5. The difference is that the seed preparation step is not performed, and the remaining silver nitrate solution is added dropwise in the reaction stage without the addition of a superdispersant. During the reaction, the pH of the system fluctuates naturally with the reaction (fluctuation range 1-3). The wet treatment and coating modification steps are omitted. After filtration and washing, the filter cake is directly spray-dried and air-jet pulverized.

[0033] Comparative Example 2 This example provides a method for preparing ultrafine silver powder, which is basically the same as that in Example 5. The difference is that the seed preparation step is omitted, and the silver nitrate solution is added dropwise in the reaction stage. The pH is adjusted by manually adding alkali solution intermittently (the adjustment frequency is 10 min / time, and the pH fluctuation range is 2-5). Only the wet processing step is retained, the coating modification step is omitted, and the amount of superdispersant is adjusted to 0.4‰ of the final silver powder mass.

[0034] Comparative Example 3 This example provides a method for preparing ultrafine silver powder, which is basically the same as that in Example 5. The difference is that the seed preparation step is not performed, and only silver nitrate solution and deionized water are added in the double drop reaction stage (no alkali solution is added). The pH of the reaction system is initially adjusted to 4, and the pH is not maintained during the reaction. In addition, ordinary stearic acid is used as the coating agent, and the coating content is controlled at 2‰.

[0035] Performance tests were conducted on the products of Example 5 and Comparative Examples 1-3. The test items included particle size D50, dry powder moisture content, burn loss rate (dried in air at 538°C for 1 hour), wet powder solid content, and dispersibility. The test results are shown in Table 1.

[0036] Table 1 Item Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 Particle size D50 (nm) 185 1520 650 480 Dry powder moisture (%) 0.13 0.82 0.43 0.35 Burn-off rate (%) 0.68 3.56 2.05 1.68 Wet powder solid content (%) 85 62 72 78 Dispersion Uniform dispersion, no obvious agglomeration Severe agglomeration, particle caking Partial agglomeration, uneven dispersion Local agglomeration, strong particle association As shown in Table 1, Example 5, through a multi-technology combination of "seed guidance + double-drop pH stabilization control + super-dispersant synergy + wet processing + special coating agent modification," prepared ultrafine silver powder with a particle size D50 as low as 185nm, dry powder moisture ≤0.13%, burn-off rate ≤0.68%, wet powder solid content ≥85%, and uniform dispersion without obvious agglomeration, fully meeting the technical specifications. In contrast, the comparative examples, lacking core technologies such as seed guidance, pH stabilization control, and complete dispersion / coating processes, resulted in significantly larger particle sizes (480-1520nm), excessive moisture and burn-off rates, and prominent agglomeration. This fully demonstrates that the integrated technical solution of the present invention can effectively solve the key problems of difficult particle size control, easy agglomeration, and unstable performance of ultrafine silver powder in traditional processes, and the product is suitable for high-end application needs.

[0037] The above embodiments are only for illustrating the technical concept and features of the present invention. Their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing ultrafine silver powder, characterized in that, Includes the following steps: Step S1, Ingredient Dissolution: Mix the silver salt raw material with deionized water and stir to dissolve to obtain a silver salt solution. Control the concentration of the silver salt solution to be 0.5-2 mol / L. Step S2, Seed preparation: Take 5-10 vol% of the silver salt solution prepared in step S1, add a reducing agent and adjust the pH to 3-5, react for 10-30 min to prepare silver seed crystals, the particle size of the silver seed crystals being 50-100 nm; Step S3, Double Dropping Reaction: Add deionized water to the reaction vessel, heat to 30-60℃, first add the silver seed crystals prepared in step S2, then simultaneously add the remaining silver salt solution and alkaline solution, maintain the pH of the reaction system at 2-6, add a superdispersant during the dropping process, and continue to keep the reaction at the temperature for 30-60 minutes after the dropping is completed. Step S4, Filtration and Washing: Filter the reaction product from step S3 and wash it with deionized water until the conductivity of the filtrate is <50μS / cm. Step S5, Wet treatment: Add the washed filter cake to deionized water, stir and disperse to form a slurry with a solid content of 40-60%, and perform wet grinding for 10-30 minutes; Step S6, Coating Modification: Add coating agent to the slurry from step S5, heat to 40-80℃, stir and react for 60-120 minutes, and control the coating content to 3-15‰. Step S7, Airflow milling: The coated slurry is spray-dried to obtain dry powder, which is then subjected to airflow milling at a pressure of 0.3-0.8 MPa. Step S8, Drying and Grading: Dry the pulverized powder at 50-60℃ for 2-4 hours, pass it through a 200-400 mesh sieve, grade it and put it into storage to obtain ultrafine silver powder product.

2. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The silver salt mentioned in step S1 is one or a mixture of two of silver nitrate and silver sulfate.

3. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The reducing agent mentioned in step S2 is one or a mixture of ascorbic acid, formaldehyde, and hydrazine hydrate.

4. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The molar ratio of the reducing agent to the silver salt in step S2 is (1.2-2.0):

1.

5. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The alkaline solution mentioned in step S3 is either sodium hydroxide solution or potassium hydroxide solution, with a concentration of 0.5-3 mol / L; the double dropping rate is 5-20 mL / min.

6. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The superdispersant mentioned in step S3 is one or a mixture of polycarboxylate, polyurethane, and polyolefin polymer superdispersants.

7. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The amount of seed crystals used in step S3 is 0.1-1% of the final silver powder mass.

8. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The amount of the superdispersant used in step S3 is 0.5-5‰ of the final silver powder mass.

9. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The coating agent mentioned in step S6 is one or more of organosilanes, fatty acid salts, and titanate coupling agents.

10. The method for preparing ultrafine silver powder according to claim 1, characterized in that, The inlet air temperature of the spray drying process in step S7 is 150-200℃, and the outlet air temperature is 80-100℃.