TWI866937B - Method for producing monodispersed ag powder - Google Patents

Abstract

The present invention discloses a method for producing silver powder, which comprises a silver salt reduction step (S2), which comprises: a reaction solution production step (S21) of producing a first reaction solution containing silver ions, ammonia (NH ₃ ) and an organic acid alkali metal salt and a second reaction solution containing a reducing agent; and a precipitation step (S22) of allowing the first reaction solution and the second reaction solution to fall freely from the air and react to obtain silver powder; the silver powder can be precipitated by free falling in the air to obtain a monodisperse silver powder with a size (SEM size) of 0.3-1.3 μm.

Description

Method for producing monodisperse silver powder

The present invention relates to a method for manufacturing silver powder and a conductive slurry containing the silver powder. The silver powder is contained in the conductive slurry, and the conductive slurry is used for electronic components such as electrodes for solar cells or internal electrodes for multi-layer capacitors, and conductor patterns for circuit boards.

Silver has been widely used as an electrode material in the field of electrical electronics due to its inherent high conductivity and oxidation stability. In particular, the development of printed electronics technology that can directly produce circuits of the desired shape has enabled the industry to achieve rapid development in the form of conductive pastes that powder silver and process it into paste or ink. Conductive pastes using silver powders are used not only in traditional conductive electrodes such as through-holes, die bonding, and chip components, but also in a variety of applications such as plasma display panels (PDPs), solar cell front and back electrodes, and touch screens. Their application areas continue to expand and their usage is increasing.

Prior art used a wet reduction process to produce silver powder by using silver nitrate aqueous solution and ammonia water to produce a silver-ammine complex aqueous solution and then adding an organic reducing agent to the solution. The silver powder is used to form electrodes or circuits such as chip components, plasma screens, and solar cells.

Among them, silver powder used in solar cell electrodes often agglomerates and has a wide distribution of particle sizes due to uneven nucleation and reaction rate differences during synthesis. This will cause defects in the printing process after slurry manufacturing due to broken wires and short circuits between electrodes.

Generally speaking, in dumping reactions, the generation is uneven due to the inconsistency between the reducing agent input rate and the composition, nucleation-growth, and the silver mirror reaction of the reactor outer wall. To remedy this, dispersants such as PVP and gelatin are used, but this will increase the organic content of the powder and cause problems such as reduced electrode characteristics.

Prior art literature

Patent Literature

1. Japanese Patent Publication No. 2001-107101 (April 17, 2001).

The present invention aims to solve the above-mentioned problems. The purpose of the present invention is to provide a manufacturing method that can remove the agglomeration of silver powder and produce submicron fine powder in large quantities and meet economic benefits.

The purpose of this invention is not limited to the purpose mentioned above. Other purposes not mentioned above can be clearly explained from the following records.

The present invention discloses a method for producing silver powder, which includes a silver salt reduction step (S2), which includes: a reaction solution production step (S21) of producing a first reaction solution containing silver ions, ammonia (NH3) and an organic acid alkali metal salt and a second reaction solution containing a reducing agent; and a precipitation step (S22) of allowing the first reaction solution and the second reaction solution to fall freely from the air and react to obtain silver powder.

Moreover, the precipitation step (S22) supplies the first reaction liquid and the second reaction liquid at a specific height of the reaction tank through a supply pipeline with adjustable flow rate, allowing the first reaction liquid and the second reaction liquid to fall freely and react.

Moreover, the height (H) at which the first reaction liquid and the second reaction liquid are supplied is more than 3m from the bottom of the reaction tank, and the reaction temperature of the reaction tank is 30°C to 50°C.

Moreover, the organic acid alkali metal salt includes a salt formed by one or more organic acids selected from the group consisting of acetic acid (CH ₃ COOH), formic acid (CH ₂ O ₂ ), oxalic acid (C ₂ H ₂ O ₄ ), lactic acid (C ₃ H ₆ O ₃ ), citric acid (C ₆ H ₈ O ₇ ), fumaric acid (C ₄ H ₄ O ₄ ), citric acid (C ₆ H ₈ O ₇ ), butyric acid (C ₄ H ₈ O ₂ ), propionic acid (CH ₃ CH ₂ COOH) and uric acid (C ₅ H ₄ N ₄ O ₃ ) and one or more metals selected from the group consisting of lithium (Li), sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg).

Furthermore, when the silver ions are added to a 500 g/L silver nitrate (AgNO ₃ ) aqueous solution, the organic acid alkali metal salt is added at a ratio of 300 to 600 g with respect to 1600 ml of the 500 g/L silver nitrate (AgNO ₃ ).

Furthermore, the present invention discloses a silver powder manufactured according to the manufacturing method, wherein the SEM size (D _SEM ) of the silver powder is 0.3 to 1.3 μm, and the PSA size (D ₅₀ ) is 0.1 to 2.0 μm.

Moreover, the span value of the silver powder calculated according to the following formula is less than 1.0.

Span value=(D90-D10)/D50

(Here, D90, D10 and D50 refer to the particle sizes corresponding to 90%, 10% and 50% of the maximum value in the cumulative distribution of solid particle sizes, respectively.)

Furthermore, the degree of aggregation of the silver powder calculated as the ratio (D ₅₀ /D _SEM ) of the PSA size (D ₅₀ , μm) to the SEM size (D _SEM , μm) was 1.7 or less.

The present invention can precipitate silver powder by free falling in the air to obtain monodisperse silver powder of 0.3~1.3μm size (SEM size), which can prevent agglomeration. By adjusting the amount of organic acid and alkali metal salt added and the reaction temperature, even if fine powder of about 0.3μm size is produced, it can maintain monodisperse and prevent agglomeration.

Figure 1 shows a schematic diagram of the process of the precipitation step of an embodiment of the present invention.

Figure 2 is a SEM photograph of the silver powder of the first embodiment.

Figure 3 is a SEM photograph of the silver powder of the second embodiment.

Figure 4 is a SEM photograph of the silver powder of the third embodiment.

Figure 5 is a SEM photograph of the silver powder of the fourth embodiment.

Figure 6 is a SEM photograph of the silver powder of the first comparative example.

Figure 7 is a SEM photograph of the silver powder of the first comparative example.

Before explaining the present invention, the terms used in this specification are only used to explain specific embodiments and are not used to limit the scope of the present invention as defined by the scope of the patent application. Unless otherwise defined, the technical terms and scientific terms used in this specification have the same meanings as those generally understood by those with ordinary knowledge in the technical field to which the present invention belongs.

Throughout this specification and the scope of the patent application, unless otherwise specifically mentioned in the sentence, the term "comprise, comprises, comprising" means including the mentioned items, steps or a series of items and steps, and is not used to exclude any other items, steps or a series of items and steps.

On the other hand, unless explicitly indicated otherwise, various embodiments of the present invention may be combined with other embodiments. In particular, any feature indicated as being preferred or advantageous may be combined with another feature or features indicated as being preferred or advantageous. The following describes the embodiments of the present invention and their effects in conjunction with the drawings.

In one embodiment of the present invention, the silver powder production method adjusts the free fall reaction in the air and the amount of oxalic acid added to produce silver powder, thereby being able to obtain a monodisperse silver powder of 0.3~1.3μm.

The method for producing silver powder in one embodiment of the present invention includes a silver salt production step (S1), a silver salt reduction step (S2), a refining step such as filtering and washing (S3), a surface treatment step (S4) and a post-treatment step (S5). The method for producing silver powder of the present invention must include a silver salt reduction step (S2), and the remaining steps can be omitted.

The silver salt production step (S1) of an embodiment of the present invention is a step of treating silver (Ag) in the form of ingots, scraps, or fine particles with an acid to produce a silver salt solution containing silver ions (Ag ⁺ ). The present invention can directly produce a silver salt solution through the silver salt production step (S1), or can use commercially available silver nitrate (AgNO ₃ ), silver salt complex, or silver intermediate solution for subsequent steps.

The silver salt reduction step (S2) of an embodiment of the present invention is a step of reducing silver ions and precipitating silver particles by allowing the silver salt solution and the reducing solution to react in a free-falling manner in the air. Specifically, it includes the following steps: a reaction solution manufacturing step (S21) of manufacturing a first reaction solution containing a silver salt solution, ammonia and an organic acid alkali metal salt and a second reaction solution containing a reducing agent; a precipitation step (S22) of obtaining silver powder by allowing the first reaction solution and the second reaction solution to react in a free-falling manner in the air.

In one embodiment of the present invention, the reaction solution preparation step (S21) adds ammonia and an organic acid alkali metal salt to a silver salt solution containing silver ions and stirs and dissolves them to prepare a first reaction solution. More specifically, an organic acid alkali metal salt is added to a silver salt solution containing silver ions and the pH is adjusted with ammonia to prepare the first reaction solution.

The present invention does not limit the silver ions as long as they are in the form of silver cations. For example, it may be silver nitrate (AgNO ₃ ), a silver salt complex or a silver intermediate. Preferably, silver nitrate (AgNO ₃ ) is used. The following description is made using silver nitrate (AgNO ₃ ) containing silver ions as an example. The following description is made based on 1600 ml of 500 g/L silver nitrate (AgNO ₃ ) as a standard to describe the contents of other components.

The organic acid alkali metal salt can be, for example, a salt formed by one or more organic acids (mono-diluted fatty acids) selected from the _group consisting of acetic acid (CH _{3 COOH} ), formic acid (CH _{2 O 2} ), oxalic acid (C ₂ H _{2 O 4 ), lactic acid (C 3 H 6 O 3 ) ,} citric acid (C ₆ H ₈ O ₇ ), fumaric acid (C ₄ H ₄ O ₄ ), citric acid (C 6 H ₈ O ₇ ), butyric acid (C ₄ H 8 O 2 ), propionic acid (CH ₃ CH ₂ COOH) and uric acid (C ₅ H ₄ N ₄ O ₃ ) and one or more metals selected from the group consisting of lithium (Li), sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg). Preferably, potassium oxalate (C ₂ K ₂ O ₄ ) is used, and it may be optionally mixed with potassium sulfide for use.

The organic acid alkali metal salt can be added at a ratio of 300 to 600 g relative to 1600 ml of the 500 g/L silver nitrate (AgNO ₃ ). When the organic acid alkali metal salt is added within the above range, the shrinkage rate can be increased, and the size of the precipitated silver powder can be controlled, and the control is supported by the experimental examples described below. When the amount of the organic acid alkali metal salt added deviates from the above range, the pH of the first reaction solution will decrease, resulting in a significant decrease in the reaction rate. Therefore, it is difficult to ensure the height for free fall in the air in the precipitation step described below, and as a result, the problem of uneven growth of silver particles will occur, similar to the case of dumping the reaction at the bottom of the reaction tank. Furthermore, if the amount of the organic acid alkali metal salt added is adjusted within the above range, the particle size of the precipitated silver powder can be adjusted. If the amount of the organic acid alkali metal salt added exceeds the above range, the powder aggregates and a silver powder of uniform size cannot be obtained.

Ammonia (NH ₃ ) can be used in the form of an aqueous solution. For example, when a 25% aqueous ammonia solution is used, 2000 to 3000 ml of the 25% aqueous ammonia solution can be added to 1600 ml of 500 g/L silver nitrate (AgNO ₃ ). As mentioned above, ammonia in the present invention has the function of adjusting pH.

If 25% aqueous ammonia solution is added to 1600ml of 500g/L silver nitrate (AgNO ₃ ), silver ions may not be completely reduced or it may be difficult to form uniform particle distribution. If 25% aqueous ammonia solution is added to 1600ml of 500g/L silver nitrate (AgNO ₃ ), the pH may be increased to improve the sphericity or monodispersity of the powder, but the organic content of the produced silver powder is higher than the required standard, and carbon aggregates after producing the conductive slurry, resulting in reduced conductivity. The ammonia includes derivatives thereof.

In the subsequent precipitation step, a faster reduction rate is required if the reaction is carried out by free fall in the air. The ammonia has the function of controlling pH and reduction rate. If the ammonia content is less than the above content, the reduction rate is reduced and the reaction rate is slowed down, which may cause the particles to grow unevenly in the lower part of the reaction tank. If the content exceeds the above content, the reaction rate is too fast and the second reaction liquid is captured in the powder, resulting in a slight increase in organic matter (~1.5%).

The first reaction liquid can be made into an aqueous solution by adding silver ions, organic acid alkali metal salt, and ammonia solution to a solvent such as water and stirring to dissolve, or it can be made into a slurry form.

The reaction liquid preparation step (S21) of one embodiment of the present invention also prepares a second reaction liquid containing a reducing agent.

The reducing agent may be one or more selected from the group consisting of alkanolamine, hydroquinone, hydrazine and formalin, preferably hydroquinone. At this time, the reducing agent may be contained in 300 to 500g relative to 1600ml of 500g/L silver nitrate contained in the first reaction solution. If the ratio of the reducing agent relative to 1600ml of 500g/L silver nitrate is less than 300g, the silver ions may not be completely reduced, and if the ratio of the reducing agent relative to 1600ml of 500g/L silver nitrate is used in excess of 500g, the organic matter content will increase.

If the content of the reducing agent is lower than the above range, the silver ions cannot be completely reduced. Therefore, the reducing agent should be included in an amount that can completely reduce the silver ions. The concentration of the reducing agent contained in the second reaction solution can be adjusted to adjust the reduction rate.

For example, the reduction rate can be increased by increasing the concentration of the reducing agent or decreased by decreasing the concentration of the reducing agent. The second reaction liquid containing the reducing agent can be prepared into an aqueous solution state with a concentration of 5% or less, that is, the reducing agent is added to a solvent such as water and then stirred and dissolved.

The precipitation step (S22) of an embodiment of the present invention is a step of allowing the first reaction liquid and the second reaction liquid to react to obtain silver powder, and the first reaction liquid and the second reaction liquid produced in the reaction liquid production step (S21) can react in a free fall in the air. As mentioned above, when the silver powder is obtained by reacting in a free fall in the air, an appropriate amount of reaction liquid is uniformly and continuously reacted during the free fall of the reaction liquid in the air to prevent agglomeration between particles and improve dispersibility.

More specifically, the precipitation step (S22) of the present invention can utilize the reaction solution tank and reaction tank shown in FIG. 1 to precipitate silver particles by free fall in the air. In the reaction solution manufacturing step (S21), the first reaction solution and the second reaction solution can be manufactured in the first reaction solution manufacturing tank and the second reaction solution manufacturing tank, respectively.

The reaction liquid produced in each tank is supplied to the reaction tank through a supply pipeline with adjustable flow rate. When the reaction liquid is sprayed into the reaction tank through a nozzle with a diameter of 15Φ, it is preferably supplied at a flow rate of 3.8L/min to 4.5L/min.

After the reaction liquid is transferred to the reaction tank, it is supplied to the inside of the reaction tank through a nozzle and falls from the air to make the first reaction liquid and the second reaction liquid react. The height (H) to which the first reaction liquid and the second reaction liquid are supplied is dropped from the bottom of the reaction tank at a height of more than 3m. When the drop is performed from the air at a height lower than 3m, the silver powder produced will agglomerate. Preferably, when the drop is performed from the air at a height of 5m to 7m, better monodisperse particles can be obtained.

The reaction tank is adjusted to a reaction temperature of 30°C to 50°C and then reacted. The size of the precipitated silver powder can be controlled by adjusting the reaction temperature to the above range, and the control is supported by the experimental examples described below. Increasing the reaction temperature will increase the density of the powder surface, increase the degree of crystallization, and affect the coating degree of the coating. When the content of the organic acid, alkali, and metal salt added to the first reaction solution is the same, increasing the reaction temperature can reduce the particle size of the precipitated silver powder.

Through the precipitation step (S22) of the present invention, a monodisperse silver powder of 0.3~1.3μm in size (SEM size) can be obtained, which can prevent agglomeration. By adjusting the amount of organic acid and alkali metal salt added and the reaction temperature, it is possible to maintain monodispersity and prevent agglomeration when producing fine powder of about 0.3μm in size.

The refining step (S3) of an embodiment of the present invention includes the following steps, namely, the step of separating and washing the silver powder dispersed in the silver powder dispersion obtained at the lower end of the reaction tank through the silver salt reduction step (S2) by filtering or the like (S31). More specifically, after the silver particles in the silver powder dispersion are allowed to settle, the supernatant of the dispersion is removed and filtered by a centrifugal separator, and the filter material is washed with pure water. The washing process can completely remove the washing water that has washed the powder.

Moreover, the refining step (S3) of an embodiment of the present invention may also include a drying and crushing step (S32) after washing. Here, the moisture content may be less than 10%, but the present invention is not limited thereto.

The surface treatment step (S4) of an embodiment of the present invention is a step of hydrophobizing the hydrophilic surface of the silver powder, which can be selectively implemented. If the silver powder has a hydrophilic surface, its properties will change due to moisture and surface oxidation during long-term storage, which will have a significant impact on its compatibility with organic solvents and the final printing properties when it is made into a conductive slurry. At this time, the surface treatment agent can use a single or multiple compounds in the form of salt or emulsion.

For example, a surface treatment agent containing octadecyl amine is added to the silver powder obtained after filtration to give the silver powder hydrophobicity. For example, octadecyl amine can be included in 0.01 to 0.1 parts by weight (for example, 0.03 parts by weight) relative to 100 parts by weight of silver nitrate. After that, the silver powder can be obtained through filtering, washing, drying, and grinding. When the silver powder is surface treated, the powder must be well dispersed to fully perform the surface treatment. When the water content is low, the dispersion efficiency is poor, so it is better to perform the surface treatment with a certain amount of water content (for example, 70~85%).

The post-processing step (S5) of an embodiment of the present invention may include the following processes: a grinding process to disperse the dried and agglomerated silver powder obtained after the surface treatment, and a grading process to remove the coarse powder. As an example, the grinding process can be performed using a device such as a jet mill at a certain air pressure (e.g., 0.4 kgf) and a supply speed (e.g., 30 to 60 g/min), but the present invention is not limited thereto.

The SEM size (D _SEM ) of the silver powder produced by the silver powder production method according to an embodiment of the present invention is 0.3 to 1.3 μm, the PSA size (D ₅₀ ) is 1.0 to 2.0 μm, the span value measured in the experimental example described below is less than 1.0, and the degree of agglomeration (D ₅₀ /D _SEM ) measured in the experimental example described below is less than 1.7.

According to the manufacturing method of the present invention, as shown in the following embodiments, fine silver powder with a SEM size less than 0.3 μm and monodispersed in the SEM image can also be produced.

The present invention also discloses a conductive slurry comprising silver powder manufactured according to an embodiment of the present invention. More specifically, the conductive slurry of the present invention comprises silver powder manufactured according to the present invention, a glass medium and an organic carrier and is suitable for forming a solar cell electrode.

The conductive slurry composition of the present invention can be added with generally known additives such as dispersants, plasticizers, viscosity regulators, surfactants, oxidants, metal oxides, metal organic compounds, etc. as needed.

The present invention also discloses a method for forming a solar cell electrode by applying the conductive slurry onto a substrate and then drying and firing the conductive slurry, and a solar cell electrode manufactured according to the method. Of course, in addition to using a conductive slurry containing silver powder having the above-mentioned characteristics, the substrate, printing, drying and firing in the solar cell electrode forming method of the present invention can adopt the methods commonly used in the manufacture of solar cells. As an example, the substrate can be a silicon wafer.

Implementation examples and comparative examples

(1) First embodiment

In the first reaction tank, 1600 ml of 500 g/L silver nitrate, 380 g of potassium oxalate, and 2560 ml of ammonia (concentration 25%) were added to 15840 g of pure water at room temperature and stirred to prepare the first reaction solution. On the other hand, in the second reaction tank, 400 g of hydroquinone was added to 20000 g of pure water at room temperature and stirred to prepare the second reaction solution.

Next, the reaction liquids were transferred to the reaction tanks using a chemical pump from WILO, a Korean pump manufacturer, at a flow rate of 3.8L/min to 4.5L/min. The silver powder dispersion containing silver powder was recovered at the bottom of the reaction tank after being sprayed (falling from the air) by a nozzle. The silver powder was precipitated by the first reaction liquid and the second reaction liquid falling from the air and reacting. At this time, the reaction temperature of the reaction tank was 35°C, and the falling height of the first reaction liquid and the second reaction liquid was 6m.

After the silver particles in the obtained silver powder dispersion are precipitated, the supernatant of the dispersion is removed and filtered using a centrifuge, and the filter material is washed with pure water. After that, the washing water is removed so that the moisture content is less than 10%. After that, a surface treatment agent is added to adjust the moisture content to 70 to 85%, and the final silver powder is obtained after drying and grinding.

(2) Second embodiment

Silver powder was obtained by the same method as in the first embodiment, except that the falling height of the first reaction liquid and the second reaction liquid was 3m.

(3) Third embodiment

Silver powder was obtained by the same method as in the first embodiment except that the reaction temperature of the reaction tank was 50°C.

(4) Fourth embodiment

Silver powder was obtained by the same method as in the first embodiment, except that 570 g of potassium oxalate was added to the first reaction solution and the reaction temperature of the reaction tank was 50°C.

(5) First comparison example

Silver powder was obtained by the same method as in the first embodiment, except that the falling height of the first reaction liquid and the second reaction liquid was 2m.

(6) Second comparative example

Silver powder was obtained in the same manner as in the first embodiment, except that a silver powder dispersion containing the following silver particles was obtained, wherein the silver particles were precipitated by adding the entire second reaction liquid to the first reaction liquid in a beaker at a reaction temperature of 35°C (by pouring) and stirring for another 10 minutes after the addition was completed.

Implementation example

(1) SEM size measurement of silver powder

For the silver powder produced according to the embodiment and comparative example of the present invention, the diameter of 100 powders was measured using a scanning electron microscope manufactured by JEOL, and the average value was obtained and the SEM size (μm) was measured and listed in the following Table 2. In addition, Figures 2 to 7 show the SEM images of the silver powder produced according to the embodiment and comparative example.

(2) PSA (Particle size analysis) measurement of silver powder

50 mg of silver powder produced according to the embodiment and comparative example of the present invention was added to 30 ml of ethanol and dispersed in an ultrasonic washer for 3 minutes. The PSA size (μm) was measured using a particle size distribution measuring device based on laser diffraction (S3500, Microtrac). The results are listed in Table 2 below.

(3) Span value measurement

For the silver powder produced according to the embodiments and comparative examples of the present invention, the span value defined as follows was calculated using the measured particle size distribution.

Span value = (D90-D10)/D50

(Here, D90, D10 and D50 refer to the particle sizes corresponding to 90%, 10% and 50% of the maximum value in the cumulative distribution of solid particle sizes, respectively.)

A smaller span value indicates a narrower distribution of particle sizes, which can be considered as producing silver powder of uniform size.

(4) Cohesion measurement

In order to evaluate the agglomeration degree of the produced silver powder, the ratio of the PSA size (D ₅₀ , μm) to the SEM size (D _SEM , μm) (D ₅₀ /D _SEM ) was calculated. For particles that are polydispersed due to light scattering, the smaller the difference between the PSA particle size analyzed for one particle and the particle size measured by SEM photography, the better the dispersion.

As shown in the results of Table 2 and Figures 2 to 5, when silver particles are precipitated in an air free fall method at a height of more than 3m (the first to third embodiments), a monodisperse silver powder with a low span value and agglomeration degree can be obtained. By increasing the reaction temperature and the amount of potassium oxalate added (the fourth embodiment), a monodisperse powder of about 300μm can be obtained. In the fourth embodiment, the particle size is very fine, so the surface energy is high, and when the particle size analysis is performed, the powder will agglomerate and thus cause the span value and agglomeration degree measurement values to be somewhat higher. However, as shown in Figure 5, in the absence of large agglomeration, mass production can be carried out in the same manner as the first to third embodiments. Moreover, when the falling height is lower than 3m (first comparative example), the PSA size, span value, and cohesion degree cannot be measured due to powder coagulation as shown in Figure 6. When the silver powder is precipitated by tilting the beaker (second comparative example), a higher span value and cohesion degree are shown as shown in the results of Table 2 and Figure 7, and a coagulated silver powder is obtained.

The features, structures, and effects illustrated in the aforementioned embodiments can be combined or modified in other embodiments by those with ordinary knowledge in the technical field. Therefore, the relevant contents of these combinations and modifications should also be interpreted as belonging to the scope of the present invention.

Claims (5)

A method for producing silver powder includes a silver salt reduction step (S2), wherein the silver salt reduction step includes: producing a silver ion, ammonia (NH ₃ ) and a first reaction liquid containing an organic acid alkali metal salt and a second reaction liquid containing a reducing agent; and a precipitation step (S22) of allowing the first reaction liquid and the second reaction liquid to fall freely from the air and react to obtain silver powder, wherein the precipitation step (S22) supplies the first reaction liquid and the second reaction liquid to each other at a specific height of a reaction tank through a supply pipeline with an adjustable flow rate so that the first reaction liquid and the second reaction liquid are allowed to fall freely and react, the height (H) at which the first reaction liquid and the second reaction liquid are supplied is 6m to 7m from the bottom of the reaction tank, the SEM size (D _SEM ) of the silver powder is 0.3 to 1.3μm, and the PSA size (D ₅₀ ) is 0.1 to 2.0 μm, and the degree of agglomeration of the silver powder calculated by the ratio of the PSA size (D ₅₀ , μm) to the SEM size (D _SEM , μm) (D ₅₀ /D _SEM ) is 1.7 or less. The method for producing silver powder as described in item 1 of the patent application, wherein the reaction temperature of the reaction tank is 30°C to 50°C. The method for producing silver powder as described in claim 2, wherein the organic acid alkali metal salt is selected from acetic acid (CH ₃ COOH), formic acid (CH ₂ O ₂ ), oxalic acid (C ₂ H ₂ O ₄ ), lactic acid (C ₃ H ₆ O ₃ ), citric acid (C ₆ H ₈ O ₇ ), fumaric acid (C ₄ H ₄ O ₄ ), citric acid (C ₆ H ₈ O ₇ ), butyric acid (C ₄ H ₈ O ₂ ), propionic acid (CH ₃ CH ₂ COOH) and uric acid (C ₅ H ₄ N ₄ O ₃ ) and one or more metals selected from the group consisting of lithium (Li), sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg). The method for producing silver powder as described in claim 3, wherein when the silver ions are added to a 500 g/L silver nitrate (AgNO ₃ ) aqueous solution, the organic acid alkali metal salt is added at a ratio of 300 to 600 g to 1600 ml of the 500 g/L silver nitrate (AgNO ₃ ). The method for producing silver powder as described in item 1 of the patent application, wherein the span value of the silver powder calculated according to the following formula is less than 1.0: span value = (D90-D10)/D50, wherein D90, D10 and D50 refer to the particle sizes corresponding to 90%, 10% and 50% of the maximum value in the cumulative distribution of solid particle sizes, respectively.