JP2015071814A - Method of producing silver powder - Google Patents
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本発明は、銀粉の製造方法に関し、特に、爆発性を有する雷銀の生成を抑えて安全に銀粉を製造する方法に関する。 The present invention relates to a method for producing silver powder, and more particularly, to a method for producing silver powder safely by suppressing generation of thunder silver having explosive properties.
電子機器における配線層や電極などの導電膜の形成には、樹脂型銀ペーストや焼成型銀ペーストのような銀粉を含んだ銀ペーストが多用されている。これら銀ペーストは加熱硬化あるいは加熱焼成によって銀粉が連なり、電気的に接続した電流パスを形成する。従って銀ペーストを塗布または印刷してから加熱硬化または加熱焼成することにより、所望のパターンを有する導電膜を形成することができる。 For the formation of conductive films such as wiring layers and electrodes in electronic devices, silver paste containing silver powder such as resin-type silver paste and fired-type silver paste is often used. These silver pastes are linked with silver powder by heat curing or heat baking to form an electrically connected current path. Therefore, a conductive film having a desired pattern can be formed by applying or printing a silver paste and then heat-curing or baking.
例えば、樹脂型銀ペーストの場合は、銀粉、樹脂、硬化剤、および溶剤などからなる銀ペーストを回路パターンや端子形状に印刷し、１００℃〜２００℃で加熱硬化させて配線や電極用の導電膜を形成することができる。一方、焼成型銀ペーストの場合は、銀粉、ガラス、および溶剤などからなる銀ペーストを回路パターンや端子形状に印刷し、６００℃〜８００℃で加熱焼成して配線や電極用の導電膜を形成することができる。 For example, in the case of a resin-type silver paste, a silver paste composed of silver powder, resin, curing agent, solvent, etc. is printed on a circuit pattern or terminal shape, and is heated and cured at 100 ° C. to 200 ° C. to conduct electricity for wiring and electrodes. A film can be formed. On the other hand, in the case of fired silver paste, silver paste made of silver powder, glass, solvent, etc. is printed in a circuit pattern or terminal shape, and heated and fired at 600 ° C to 800 ° C to form a conductive film for wiring and electrodes. can do.
かかる銀ペーストに使用する銀粉の粒径は一般に０.１μｍから数μｍ程度であり、形成する配線の幅や電極の厚さに応じて異なる粒径の銀粉が使用される。銀粉はペースト中で均一に分散させることが望ましく、これにより均一な厚みと幅を有する配線や均一な厚みを有する電極を形成することができる。 The particle size of silver powder used for such silver paste is generally about 0.1 μm to several μm, and silver powder having different particle sizes is used depending on the width of the wiring to be formed and the thickness of the electrode. The silver powder is desirably dispersed uniformly in the paste, whereby a wiring having a uniform thickness and width and an electrode having a uniform thickness can be formed.
上記した銀ペーストに使用する銀粉の製造方法には、アンモニアを用いた湿式還元法が数多く提案されている。アンモニアはアンモニウムイオンとなって銀イオンと安定な錯体を形成するため還元反応を制御しやすく、湿式還元法による銀粉の製法では有効な成分である。さらにアンモニアは一般に薬剤コストが低い上、廃液処理において容易に処理可能な薬剤である。 Many wet reduction methods using ammonia have been proposed as a method for producing silver powder for use in the silver paste described above. Ammonia forms ammonium ions and forms a stable complex with silver ions, so that the reduction reaction is easy to control, and is an effective component in the production of silver powder by the wet reduction method. Furthermore, ammonia is generally a chemical that can be easily treated in waste liquid treatment in addition to a low chemical cost.
例えば特許文献１には、銀アンミン錯体を含む銀溶液に還元剤をその濃度が銀濃度に対して０.６〜１.４反応当量となるように添加し、これにより銀イオンを還元して銀粉を作製する方法が示されている。この特許文献１の方法は、還元剤を含んだ溶液のｐＨを１１から１２に調整し、その酸化還元電位を安定化させてから銀溶液に添加して還元を行っており、これにより従来よりも高濃度の銀を還元できると記載されている。 For example, in Patent Document 1, a reducing agent is added to a silver solution containing a silver ammine complex so that the concentration is 0.6 to 1.4 reaction equivalents relative to the silver concentration, thereby reducing silver ions. A method of making silver powder is shown. In the method of Patent Document 1, the pH of a solution containing a reducing agent is adjusted from 11 to 12, and the oxidation-reduction potential is stabilized and then added to the silver solution for reduction. Is also described as being capable of reducing high concentrations of silver.
しかしながら、銀アンミン錯体は水酸化アルカリの存在下で加熱すると爆発性の雷銀を生成すると言われており、製造工程に銀アンミン錯体を用いる場合は安全性に留意する必要があった。例えば非特許文献１には、雷銀の一種である窒化銀の反応性について記載されている。 However, it is said that the silver ammine complex generates explosive thunder silver when heated in the presence of alkali hydroxide, and it is necessary to pay attention to safety when using the silver ammine complex in the production process. For example, Non-Patent Document 1 describes the reactivity of silver nitride, which is a type of lightning silver.
アンモニアを用いた湿式還元法による銀粉の作製では、還元剤の反応当量が銀に対して不足すると還元反応後に未還元の銀アンミン錯体が残留するため、この銀アンミン錯体を含む溶液が濃縮した場合に雷銀が生成する恐れがある。これは意図的に濃縮する場合に限られず、銀溶液が飛散して乾燥濃縮した場合でも同様に雷銀が生成するリスクがある。また、銀アンミン錯体溶液にアルカリを加えた場合、錯体からアンモニウム配位子が外れることにより雷銀の生成が進行する。従って、アンモニアを用いた湿式還元法では雷銀の生成を抑える安全対策が必要となるが、特許文献１ではそのことについて特に考慮されていない。 In preparation of silver powder by wet reduction method using ammonia, if the reaction equivalent of the reducing agent is insufficient relative to silver, unreduced silver ammine complex remains after the reduction reaction, so the solution containing this silver ammine complex is concentrated There is a risk of lightning silver formation. This is not limited to intentional concentration, and there is a risk that thunder silver will be generated even when the silver solution is scattered and dried and concentrated. Moreover, when an alkali is added to the silver ammine complex solution, generation of thunder silver proceeds by removing the ammonium ligand from the complex. Therefore, in the wet reduction method using ammonia, a safety measure for suppressing the formation of thunder silver is required, but Patent Document 1 does not particularly take this into consideration.
上記したようにアンモニアを用いた湿式還元法による銀粉の製造方法は、爆発性の雷銀が生成するリスクを孕んでおり、銀粉の製造効率を高めたとしても雷銀生成のリスクが回避されていなければ、その製造方法を採用することは困難であった。本発明の発明者らはこのような従来の事情に鑑み、アルカリの共存下において銀アンミン錯体を含む銀溶液が雷銀を生成する条件について鋭意研究を重ねた結果、該銀溶液に含まれる特定の成分の濃度を調整することで雷銀の生成を抑えて安全に銀粉を製造しうることを見出し、本発明を完成するに至った。 As described above, the method of producing silver powder by the wet reduction method using ammonia entails the risk of generating explosive thunder silver, and even if the production efficiency of silver powder is increased, the risk of thunder silver generation is avoided. If not, it was difficult to adopt the manufacturing method. In view of such conventional circumstances, the inventors of the present invention have conducted intensive research on the conditions under which a silver solution containing a silver ammine complex generates thunder silver in the presence of an alkali. It has been found that by adjusting the concentration of the component, silver powder can be produced safely by suppressing the formation of thunder silver, and the present invention has been completed.
即ち、本発明が提供する銀粉の製造方法は、アンモニア溶液に銀塩を溶解してなる銀溶液中の全塩素および全炭酸の合計モル濃度を該銀溶液中の全銀のモル濃度で除した値が１と等しいか又は１より大きく５以下となるように銀溶液を調製する工程と、該銀溶液に還元剤を添加して還元により銀微粒子を析出させる工程とを有する銀粉の製造方法であって、水酸化アルカリの濃度が０.２ｍｏｌ／Ｌを超えない程度に前記銀溶液および／または前記還元剤に水酸化アルカリを添加することを特徴としている。 That is, in the method for producing silver powder provided by the present invention, the total molar concentration of total chlorine and total carbonic acid in a silver solution obtained by dissolving a silver salt in an ammonia solution is divided by the molar concentration of total silver in the silver solution. A method for producing silver powder comprising a step of preparing a silver solution so that the value is equal to 1 or greater than 1 and 5 or less, and a step of adding a reducing agent to the silver solution and precipitating silver fine particles by reduction. The alkali hydroxide is added to the silver solution and / or the reducing agent to such an extent that the alkali hydroxide concentration does not exceed 0.2 mol / L.
上記本発明による銀粉の製造方法においては、水酸化アルカリが、水酸化リチウム、水酸化ナトリウム、および水酸化カリウムの内の１種以上であることが好ましい。また、還元剤は、ヒドラジン、ヒドラジニウム塩、アスコルビン酸、ホルマリン、および酒石酸の内の１種以上であることが好ましい。更に、銀塩が、酸化銀、炭酸銀、塩化銀、および硝酸銀の内の１種以上であることが好ましい。 In the method for producing silver powder according to the present invention, the alkali hydroxide is preferably at least one of lithium hydroxide, sodium hydroxide, and potassium hydroxide. The reducing agent is preferably one or more of hydrazine, hydrazinium salt, ascorbic acid, formalin, and tartaric acid. Further, the silver salt is preferably at least one of silver oxide, silver carbonate, silver chloride, and silver nitrate.
本発明によれば、銀粉の生産性が高く、得られる銀粉の粒径や結晶粒径の制御も容易となる上、爆発性の雷銀の生成を抑制できるので、その工業的価値は極めて高い。 According to the present invention, the productivity of silver powder is high, the grain size and crystal grain size of the resulting silver powder are easily controlled, and the production of explosive thunder silver can be suppressed, so its industrial value is extremely high. .
本発明の銀粉の製造方法は、アンモニアを含む溶液に銀塩を溶解して銀アンミン錯体を含む銀溶液を調製する工程と、該銀溶液に還元剤を添加することで銀イオンを還元して銀微粒子を析出させる工程とを有する銀粉の製造方法において、還元前の銀溶液中の全塩素および全炭酸の合計モル濃度を該銀溶液中の全銀のモル濃度で除した値が１と等しいか又は１より大きく５以下となるように調整する。また、水酸化アルカリの濃度が０.２ｍｏｌ／Ｌを超えない程度に前記銀溶液および／または前記還元剤に水酸化アルカリを添加する。 The method for producing silver powder of the present invention comprises a step of dissolving a silver salt in a solution containing ammonia to prepare a silver solution containing a silver ammine complex, and reducing silver ions by adding a reducing agent to the silver solution. In the method for producing silver powder having a step of precipitating silver fine particles, a value obtained by dividing the total molar concentration of total chlorine and total carbonic acid in the silver solution before reduction by the molar concentration of total silver in the silver solution is equal to 1. Or is adjusted to be greater than 1 and 5 or less. Further, alkali hydroxide is added to the silver solution and / or the reducing agent to such an extent that the concentration of alkali hydroxide does not exceed 0.2 mol / L.
銀アンミン錯体を含む銀溶液は乾燥や加熱により雷銀が生成されるが、上記したように銀溶液中の全塩素、全炭酸、および全銀の濃度を限定することにより当該銀溶液が乾燥あるいは濃縮しても雷銀の生成を抑えることができる。なお、雷銀とは爆発性物質である窒化銀、アミド銀、およびイミド銀などの一般的な総称である。例えば非特許文献１に窒化銀の反応性が示されている。 A silver solution containing a silver ammine complex produces thunder silver by drying or heating, but as described above, the silver solution is dried or reduced by limiting the concentration of total chlorine, total carbonic acid, and total silver in the silver solution. Even if concentrated, generation of thunder silver can be suppressed. Lightning silver is a general term for explosive substances such as silver nitride, amide silver, and imide silver. For example, Non-Patent Document 1 shows the reactivity of silver nitride.
また、銀溶液にアルカリ性物質である水酸化アルカリを添加することにより還元時の酸化還元電位が下がり、還元反応を促進することで還元反応の効率を上げることができる。しかしながら、水酸化アルカリの添加により銀溶液中のアンモニア濃度が下がるので、雷銀が生成されるおそれがある。すなわち、水酸化アルカリの添加により銀アンミン錯体からアンモニウム配位子が外れ、雷銀の生成が促進される。そこで、前述したように、還元時の溶液のみならず還元前の溶液においても水酸化アルカリの濃度が０.２ｍｏｌ／Ｌを超えないようにしている。これにより、雷銀の生成を抑えることと還元反応の促進を両立させることができる。 Further, by adding an alkali hydroxide, which is an alkaline substance, to the silver solution, the redox potential at the time of reduction is lowered, and the efficiency of the reduction reaction can be increased by promoting the reduction reaction. However, the addition of alkali hydroxide lowers the ammonia concentration in the silver solution, which may generate thunder silver. That is, the addition of alkali hydroxide removes the ammonium ligand from the silver ammine complex and promotes the formation of lightning silver. Therefore, as described above, the concentration of the alkali hydroxide does not exceed 0.2 mol / L not only in the solution during the reduction but also in the solution before the reduction. Thereby, suppression of the formation of thunder silver and promotion of a reduction reaction can be made compatible.
以下、本発明に係る銀粉の製造方法を工程毎に詳細に説明する。まず、塩化物塩および／または炭酸塩を必要に応じて溶解させたアンモニア水に２０〜４５℃の温度範囲内にて銀塩を溶解し、得られた銀溶液を該温度範囲内にて保持する。ここで、銀溶液中の全塩素および全炭酸の合計モル濃度を銀溶液中の全銀のモル濃度で除した値が１と等しいか又は１より大きく５以下となるように調製する。 Hereinafter, the manufacturing method of the silver powder which concerns on this invention is demonstrated in detail for every process. First, a silver salt is dissolved within a temperature range of 20 to 45 ° C. in ammonia water in which a chloride salt and / or a carbonate is dissolved as necessary, and the resulting silver solution is maintained within the temperature range. To do. Here, the total molar concentration of total chlorine and total carbonic acid in the silver solution is divided by the molar concentration of total silver in the silver solution so that the value is equal to 1 or greater than 1 and 5 or less.
具体的には、銀溶液に含まれる全塩素のｍｏｌ濃度をＡとし、該銀溶液に含まれる炭酸イオン、炭酸水素イオン、および二酸化炭素分子（以降、これらを合わせて全炭酸と称する）の合計ｍｏｌ濃度をＢとし、該銀溶液に含まれる全銀のｍｏｌ濃度をＣとした時、これらＡ，Ｂ、およびＣが下記式１または式２のいずれかの関係を満たすように調整する。 Specifically, the molar concentration of total chlorine contained in the silver solution is A, and the total of carbonate ions, hydrogen carbonate ions, and carbon dioxide molecules (hereinafter collectively referred to as total carbonic acid) contained in the silver solution. When the mol concentration is B and the mol concentration of all silver contained in the silver solution is C, the A, B, and C are adjusted so as to satisfy either of the following formulas 1 and 2.
［式１］
１＝（（Ａ＋Ｂ）／Ｃ）
［式２］
１＜（（Ａ＋Ｂ）／Ｃ）≦５
[Formula 1]
1 = ((A + B) / C)
[Formula 2]
1 <((A + B) / C) ≦ 5
なお、銀塩の添加だけで上記した２つの式のいずれかを満たすのであれば、塩化物塩および／または炭酸塩の添加は不要である。例えば、銀塩に塩化銀および／または炭酸銀を用いた場合は、理論上式１の条件を満たすことになるので、塩化物塩および／または炭酸塩の添加は基本的に不要になる。 It should be noted that the addition of a chloride salt and / or carbonate is not required if only one of the two formulas described above is satisfied by the addition of a silver salt. For example, when silver chloride and / or silver carbonate is used as the silver salt, the condition of Formula 1 is theoretically satisfied, so that the addition of chloride salt and / or carbonate is basically unnecessary.
上記した全塩素のモル濃度とは、銀溶液中に含まれる塩素および塩素を含む化学物質を塩素原子に換算したモル濃度である。この塩素には、Ｃｌ−で示される塩素イオンやＣｌ２で示される塩素分子さらには塩素の化合物が含まれる。また、全炭酸のモル濃度とは、銀溶液中に含まれる二酸化炭素や炭酸イオン等の炭酸を二酸化炭素に換算したモル濃度である。この炭酸には、銀溶液に溶解した状態の二酸化炭素（ＣＯ２）分子、ＣＯ３ −２で示される炭酸イオン、ＨＣＯ３ −で示される炭酸水素イオン、およびこれらの化合物が含まれる。更に、全銀のモル濃度とは、銀溶液中に含まれる銀の化学種を銀原子に換算したモル濃度である。銀溶液中に含まれる銀は、ほとんどが銀アンミン錯体として存在するものであるが、化学平衡により銀アンミン錯体以外の錯体として存在する銀イオンも含まれる。 The above-mentioned molar concentration of total chlorine is a molar concentration obtained by converting chlorine contained in the silver solution and chemical substances containing chlorine into chlorine atoms. This chlorine includes a chlorine ion represented by Cl − , a chlorine molecule represented by Cl 2 , and a chlorine compound. Moreover, the molar concentration of the total carbonic acid is a molar concentration obtained by converting carbon dioxide such as carbon dioxide and carbonate ions contained in the silver solution into carbon dioxide. The carbonate, in the state dissolved in silver solution carbon dioxide (CO 2) molecules, CO 3 carbonate ion represented by -2, HCO 3 - bicarbonate ion represented by, and includes these compounds. Furthermore, the molar concentration of total silver is a molar concentration obtained by converting the chemical species of silver contained in the silver solution into silver atoms. Most of the silver contained in the silver solution is present as a silver ammine complex, but also includes silver ions present as a complex other than the silver ammine complex due to chemical equilibrium.
銀塩は酸化銀、炭酸銀、塩化銀、硝酸銀、酢酸銀、および臭化銀の内の１種以上を含むことができるが、これらの中では酸化銀、炭酸銀、塩化銀、硝酸銀が比較的安価な原料であるので望ましい。塩化物塩は、限定するものではないが、塩化ナトリウム、塩化カリウム、塩化アンモニウム、および塩化リチウムの内の１種以上を含むことができる。また、炭酸塩は、限定するものではないが、炭酸ナトリウム、炭酸水素ナトリウム、炭酸カリウム、炭酸アンモニウム、および炭酸リチウムの内の１種以上を含むことができる。これら塩化物塩や炭酸塩は、溶解しないものや還元反応中で水酸化物のように沈殿を生じるものは好ましくない。 Silver salts can include one or more of silver oxide, silver carbonate, silver chloride, silver nitrate, silver acetate, and silver bromide, among which silver oxide, silver carbonate, silver chloride, silver nitrate are compared It is desirable because it is an inexpensive material. The chloride salt can include, but is not limited to, one or more of sodium chloride, potassium chloride, ammonium chloride, and lithium chloride. Moreover, although carbonate is not limited, 1 or more types in sodium carbonate, sodium hydrogencarbonate, potassium carbonate, ammonium carbonate, and lithium carbonate can be included. Of these chloride salts and carbonates, those that do not dissolve and those that cause precipitation such as hydroxide during the reduction reaction are not preferred.
銀溶液を調製する具体的な方法としては、所定の量のアンモニア水が張り込まれた反応槽に必要に応じて塩化物塩および／または炭酸塩を投入し、これにより得られるアンモニア溶液を２０〜４５℃に保持して撹拌しながら銀塩を投入して十分に溶解すればよい。アンモニア溶液の温度が２０℃未満では、銀塩の溶解度が下がり生産性が悪くなる。一方、アンモニア溶液の温度が４５℃を超えるとアンモニアの揮発が激しくなり、一旦溶解した銀塩がアンモニアの揮発に伴って析出するため、安定した銀溶液が得られなくなる。得られた銀溶液は、引き続き２０〜４５℃に保持するのが好ましく、３５〜４０℃に保持することがより好ましい。 As a specific method for preparing the silver solution, a chloride salt and / or carbonate is added to a reaction tank filled with a predetermined amount of aqueous ammonia as required, and the resulting ammonia solution is 20%. The silver salt may be added and sufficiently dissolved while stirring at -45 ° C. When the temperature of the ammonia solution is less than 20 ° C., the solubility of the silver salt is lowered and the productivity is deteriorated. On the other hand, when the temperature of the ammonia solution exceeds 45 ° C., the volatilization of ammonia becomes violent, and the silver salt once dissolved precipitates with the volatilization of the ammonia, so that a stable silver solution cannot be obtained. The obtained silver solution is preferably kept at 20 to 45 ° C., more preferably 35 to 40 ° C.
上記したアンモニア溶液への銀塩の投入では、得られる銀溶液中の銀濃度が８０〜１００ｇ／Ｌとなるように投入量を調整することが好ましい。この銀濃度が８０ｇ／Ｌ未満であっても得られる銀粉に問題はないが、１バッチ当たりで得られる銀粉の量が少なくなるため生産性が低下する。一方、銀濃度が１００ｇ／Ｌを超えることは、銀塩の析出が開始することから困難である。 When the silver salt is added to the ammonia solution as described above, it is preferable to adjust the input amount so that the silver concentration in the obtained silver solution is 80 to 100 g / L. Even if the silver concentration is less than 80 g / L, there is no problem with the obtained silver powder, but the productivity is lowered because the amount of silver powder obtained per batch is reduced. On the other hand, it is difficult for the silver concentration to exceed 100 g / L since the precipitation of the silver salt starts.
次に、還元剤に例えばポリビニルアルコールやポリビニルピロリドン等の分散剤を混合する。銀粒子の湿式合成においては、還元剤の添加後短時間で銀塩から核生成が起こり、核が成長すると共に凝集して銀粒子が形成される。従って、核生成時から溶液中に分散剤を存在させることにより、生成した銀粒子表面に分散剤を吸着させて、銀粒子の凝集を防ぐことができる。尚、成長した核は銀粒子内の多結晶組織の各結晶となり、また核が凝集せずそのまま成長すると単結晶の銀粒子となる。 Next, a dispersing agent such as polyvinyl alcohol or polyvinyl pyrrolidone is mixed with the reducing agent. In the wet synthesis of silver particles, nucleation occurs from the silver salt in a short time after the addition of the reducing agent, and the nuclei grow and aggregate to form silver particles. Accordingly, the presence of the dispersant in the solution from the time of nucleation makes it possible to adsorb the dispersant on the surface of the generated silver particles and prevent aggregation of the silver particles. The grown nuclei become crystals of a polycrystalline structure in the silver particles, and when the nuclei grow as they are without agglomeration, they become single crystal silver particles.
このように核成長および凝集を制御して銀粒子の分散性を向上させるため、あらかじめ還元剤に分散剤を混合して還元剤混合液としてから銀溶液に添加するのが好ましい。これにより、少ない分散剤でも核の表面に効率よく分散剤を吸着させることができる。還元剤には一般的な還元剤を用いることができるが、反応速度や薬剤のコストの面から、ヒドラジン、ヒドラジニウム塩、ホルマリン、アスコルビン酸、酒石酸が好ましい。なお、分散剤として用いるポリビニルアルコールやポリビニルピロリドンは還元反応時に発泡する場合があるため、銀溶液または還元剤混合液に消泡剤を添加してもよい。 In order to improve the dispersibility of silver particles by controlling nucleation and aggregation in this way, it is preferable to add a dispersant to a reducing agent in advance to form a reducing agent mixed solution and then add it to the silver solution. Thereby, even with a small amount of dispersant, the dispersant can be efficiently adsorbed on the surface of the core. Although a general reducing agent can be used as the reducing agent, hydrazine, hydrazinium salt, formalin, ascorbic acid, and tartaric acid are preferable from the viewpoint of reaction rate and chemical cost. In addition, since polyvinyl alcohol and polyvinylpyrrolidone used as a dispersant may foam during the reduction reaction, an antifoaming agent may be added to the silver solution or the reducing agent mixed solution.
還元剤への分散剤の添加量は、分散剤の種類および作製する銀粉の粒径により適宜決めればよいが、前述したいずれの分散剤を用いる場合であっても、銀溶液中に含まれる銀１００質量部に対して３〜１０質量部とすることが好ましい。また、銀溶液への還元剤混合液の添加量は、還元剤混合液中の還元剤の量が銀溶液中の銀を全て還元できればよく、そのために必要な最少量とすることがコスト面から好ましい。例えば還元剤がアスコルビン酸の場合、銀溶液中の銀１モル当たり０.２５モルが化学量論量であるので、その添加量は銀１モル当たり０.２５〜１モルとすることが好ましく、０.２５〜０.３５モルがより好ましい。 The amount of the dispersant added to the reducing agent may be appropriately determined depending on the type of the dispersant and the particle size of the silver powder to be produced. Even when any of the above-described dispersants is used, the silver contained in the silver solution It is preferable to set it as 3-10 mass parts with respect to 100 mass parts. In addition, the amount of the reducing agent mixed solution added to the silver solution is not limited as long as the amount of reducing agent in the reducing agent mixed solution can reduce all the silver in the silver solution. preferable. For example, when the reducing agent is ascorbic acid, since 0.25 mol per mol of silver in the silver solution is a stoichiometric amount, the addition amount is preferably 0.25 to 1 mol per mol of silver, 0.25 to 0.35 mol is more preferable.
この還元剤混合液を、前述したように好適には２０〜４５℃で保持されている銀溶液の保持温度以上で且つ添加後の銀溶液の温度が５０℃以下となるように温度調節して、前記温度範囲内に保持した銀溶液に添加する。これにより、銀イオンを還元させて銀微粒子を析出させる。この還元時又は還元前の銀溶液および／または還元剤混合液に０.２ｍｏｌ／Ｌを超えない程度に水酸化アルカリを添加する。水酸化アルカリの添加は還元剤混合液もしくは銀溶液、またはこれらの両方でも構わないが、銀溶液に添加するとアンモニアが揮発し、銀の溶解度が下がり析出する可能性があるため、還元剤混合液に加えるのが好ましい。 As described above, the reducing agent mixed solution is preferably temperature-adjusted so that the temperature is higher than the holding temperature of the silver solution held at 20 to 45 ° C. and the temperature of the silver solution after addition is 50 ° C. or lower. To the silver solution kept within the above temperature range. Thereby, silver ion is reduced and silver fine particles are deposited. Alkali hydroxide is added to the silver solution and / or reducing agent mixed solution before or during the reduction to such an extent that does not exceed 0.2 mol / L. Alkali hydroxide may be added to the reducing agent mixture or the silver solution, or both of them, but if added to the silver solution, ammonia will volatilize and the silver solubility may decrease, causing precipitation. It is preferable to add to.
還元反応による温度上昇は、１バッチで作製する銀粉の量、還元剤の投入速度、銀溶液の撹拌状態、反応槽の温度管理状態（冷却機能の有無や放熱性）などにより異なるが、再現性があるため、あらかじめ試験を行うことでどの程度温度が上昇するか把握することができる。この試験結果に基づいて、還元反応時に銀溶液の温度が２０〜５０℃の範囲内に収まるように運転すればよい。その際、還元による温度上昇が大きい場合には、ジャケット付き反応容器を用いて水冷するなど一般的な温度制御を行えばよい。還元剤混合液を添加した銀溶液は、反応を均一化させると共に銀粒子同士の凝集を防止するため、連続的に撹拌することが好ましい。撹拌方法には特に限定がなく、通常用いられる撹拌装置を用いることができる。 The temperature rise due to the reduction reaction varies depending on the amount of silver powder produced in one batch, the reducing agent charging speed, the stirring state of the silver solution, the temperature control state of the reaction tank (with or without cooling function, heat dissipation), etc., but reproducibility Therefore, it is possible to grasp how much the temperature rises by conducting a test in advance. Based on the test results, the silver solution may be operated so that the temperature of the silver solution is within the range of 20 to 50 ° C. during the reduction reaction. At that time, if the temperature rise due to the reduction is large, general temperature control such as water cooling using a jacketed reaction vessel may be performed. The silver solution to which the reducing agent mixed solution is added is preferably continuously stirred in order to make the reaction uniform and prevent aggregation of silver particles. The stirring method is not particularly limited, and a commonly used stirring device can be used.
このようにして得られた銀微粒子を含んだ懸濁液を一般的な濾過手段で濾過した後、得られた湿潤状態の銀粉を洗浄して乾燥する。洗浄方法には特に限定はないが、例えば湿潤状態の銀粉を水に投入し、撹拌機または超音波洗浄器を使用して撹拌した後、再度濾過して銀粉を回収する方法が好ましい。この場合、水への投入、撹拌洗浄および濾過からなる一連の操作を、複数回繰り返すことがより好ましい。また、洗浄に用いる水は、銀粉に対して有害な不純物元素を含有していない水を使用するのが好ましく、純水の使用が特に好ましい。 The suspension containing the silver fine particles thus obtained is filtered by a general filtration means, and the obtained wet silver powder is washed and dried. Although there is no particular limitation on the washing method, for example, a method is preferable in which wet silver powder is put into water, stirred using a stirrer or ultrasonic cleaner, and then filtered again to recover the silver powder. In this case, it is more preferable to repeat a series of operations consisting of charging into water, stirring and washing, and filtration a plurality of times. Moreover, it is preferable to use the water which does not contain the impurity element harmful | toxic to silver powder as water used for washing | cleaning, and use of pure water is especially preferable.
上記洗浄で得られる湿潤状態の銀粉の水分を蒸発することで乾燥した銀粉が得られる。乾燥方法には特に限定がなく、例えば上記洗浄後の湿潤状態の銀粉をステンレスパッド上に敷き詰め、大気オーブンまたは真空乾燥機などの市販の乾燥装置を用いて、４０〜８０℃の温度で加熱することにより乾燥するのが好ましい。 Dry silver powder is obtained by evaporating the moisture of the wet silver powder obtained by the above washing. There is no particular limitation on the drying method. For example, the wet silver powder after washing is spread on a stainless steel pad and heated at a temperature of 40 to 80 ° C. using a commercially available drying apparatus such as an atmospheric oven or a vacuum dryer. Is preferably dried.
［実施例１］
２８％アンモニア水１ｇに塩化銀６０ｍｇを溶解した後、水酸化ナトリウムを濃度０.０１ｍｏｌ／Ｌとなるように添加して試料１の銀溶液を作製した。この試料１の銀溶液をサンプリングして３５℃で乾燥したところ塩化銀が析出し、雷銀の生成は認められなかった。次に、上記銀溶液にヒドラジンを１.４当量となるように加えたところ、雷銀を生成することなく安全に銀粉を作製することができた。なお、雷銀の生成の有無は、乾燥後に得られた固形分に波長９２０ｎｍ〜９４０ｎｍ、パルスピーク出力５Ｗ、パルス幅２ｍ秒の赤外線レーザを照射して起爆試験を行い、爆発しなければ雷銀は生成していないと判断した。
[Example 1]
After dissolving 60 mg of silver chloride in 1 g of 28% ammonia water, sodium hydroxide was added to a concentration of 0.01 mol / L to prepare a silver solution of Sample 1. When the silver solution of this sample 1 was sampled and dried at 35 ° C., silver chloride was precipitated, and no formation of thunder silver was observed. Next, when hydrazine was added to the above silver solution so as to be 1.4 equivalents, silver powder could be safely produced without producing thunder silver. The presence or absence of thunder silver was determined by irradiating the solid content obtained after drying with an infrared laser having a wavelength of 920 nm to 940 nm, a pulse peak output of 5 W, and a pulse width of 2 msec. Was determined not to have been generated.
［実施例２］
水酸化ナトリウムを濃度が０.０１ｍｏｌ／Ｌに代えてそれぞれ０.０５ｍｏｌ／Ｌおよび０.１ｍｏｌ／Ｌとなるように添加した以外は実施例１の試料１と同様にして試料２および試料３の銀溶液を作製した。また、水酸化ナトリウムを濃度が０.０１ｍｏｌ／Ｌに代えて０.２ｍｏｌ／Ｌとなるように添加し、３５℃に代えて３９℃で乾燥した以外は実施例１の試料１と同様にして試料４の銀溶液を作製した。これら試料２〜４の銀溶液に対して実施例１の試料１と同様に乾燥する実験とヒドラジンを加える実験を行ったところ、いずれも雷銀が生成することなく塩化銀が析出した。このことから、安全に銀粉を作製できることがわかった。
[Example 2]
Samples 2 and 3 were prepared in the same manner as Sample 1 of Example 1 except that sodium hydroxide was added so that the concentration was 0.05 mol / L and 0.1 mol / L instead of 0.01 mol / L. A silver solution was prepared. Also, sodium hydroxide was added so that the concentration became 0.2 mol / L instead of 0.01 mol / L, and dried at 39 ° C. instead of 35 ° C. As in Sample 1 of Example 1, A silver solution of sample 4 was prepared. When a drying experiment and an experiment of adding hydrazine were conducted on the silver solutions of Samples 2 to 4 in the same manner as Sample 1 of Example 1, silver chloride was precipitated without any formation of thunder silver. From this, it was found that silver powder can be produced safely.
［比較例１］
２８％アンモニア水１ｇに、塩化銀６０ｍｇを溶解して、水酸化ナトリウムを０.６ｍｏｌ／Ｌとなるように添加し、３９℃で乾燥したところ雷銀が生成し爆発反応が生じた。この銀溶液は飛散などによって液が乾燥した場合には爆発等の危険性があることがわかった。
[Comparative Example 1]
In 1 g of 28% aqueous ammonia, 60 mg of silver chloride was dissolved, sodium hydroxide was added to a concentration of 0.6 mol / L, and drying at 39 ° C. produced thunder silver and an explosion reaction occurred. It was found that this silver solution has a risk of explosion when the liquid dries due to scattering or the like.
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