【発明の詳細な説明】[Detailed description of the invention] 本発明はイオン交換膜法塩化アルカリの電解に
おいて、循環塩水中に蓄積してくる塩素酸塩を触
媒を用いて水素ガスと反応させて分解除去するこ
とにより塩素酸塩の蓄積を防止する方法に関する
ものである。 更に詳しくは、イオン交換膜法塩化アルカリ電
解において主循環塩水の全部又は一部を抜き出し
て、触媒を充填した塔の上部又は下部に供給し、
該触媒を充填した塔の下部に水素ガス又は水素ガ
スを含むガスを供給し、循環塩水中の塩素酸塩と
水素ガスとを触媒を介して応させて分解すること
により、循環塩水中の塩素酸塩の蓄積を防止する
方法に関するものである。塩化アルカリの電解方
法としては、隔膜法、水銀法及びイオン交換膜法
が知られている。 隔膜法では、OH-の陽極室への逆拡散を防ぐ
ために塩水を陽極室より陰極室に流しつつ電解を
行う。このために、陽極室で副生した塩素酸塩は
陰極液とともに塩水系外に出るので、塩水中に塩
素酸塩が蓄積することはない。 水銀法では、汞和槽で副生した塩素酸塩は陰極
アマルガムにより還元分解を受けるので、循環塩
水中に蓄積する塩素酸塩はそれ程多くなく、循環
塩水中の塩素酸塩濃度は10〜30ｇ／程度であ
る。 一方、イオン交換膜法では陽イオン交換膜を隔
膜として使用するが、OH-イオンの逆拡散を完
全に阻止する理想的な陽イオン交換膜は存在せず
隔膜法及び水銀法と同様に陽極室に塩素酸塩が副
生する。副生した塩素酸イオンは陰イオンであ
り、かつイオン半径が比較的大きいので陽イオン
交換膜を経て陰極室へ拡散することは極めて困難
である。従つて、イオン交換膜法電解では、塩水
は循環使用されるにつれて循環塩水系内で塩素酸
塩が次第に蓄積増加する。少量の塩素酸塩の蓄積
はイオン交換膜法電解に悪影響をおよぼさない
が、過度の塩素酸塩の蓄積は、陽極室の塩化アル
カリ濃度の低下を招き塩素ガス純度を低下させる
ばかりでなく、陽イオン交換膜を経て陰極室に拡
散透過する塩素酸イオンの量も無視できなくな
る。また、塩素酸塩の過度の蓄積は金属陽極のコ
ーテイングの劣化を早める要因ともなる。 このような欠点をもたらす塩素酸塩の過度の蓄
積を防止するために、イオン交換膜法電解におい
ては塩酸等の酸を加えることにより陽極液のPHを
3.5以下に低下させて塩素酸塩の生成を押える方
法又は主循環塩水の一部を抜き出し、過剰の塩酸
を加えて塩素酸塩を分解し、主循環塩水系に回収
して塩素酸塩の蓄積を防止する方法等が採用され
ている。 前者については、塩酸の添加量は陰極室から陽
極室へ拡散してくるOH-イオンのほぼ中和相当
量、即ちほぼアルカリ電流効率損失分を過不足な
く加えることが必要である。この際塩酸が不足し
た場合は塩素酸塩が副生し塩水系内に蓄積するこ
とになる。また過剰の場合は陽極室内水素イオン
濃度が増加する結果、陽イオン交換膜の膨潤及び
アルカリ金属イオンの選択透過性の低下を招き、
アルカリ電流効率を低下させるおそれがあるばか
りでなく、陽極室の過度の酸性は通常陽極として
使用される貴金属コーテイングしたチタン陽極の
耐久性を損う恐れがあり、更に塩水精製時の戻り
塩水中和用アルカリの過大な使用をもたらす。後
者にあつては、塩酸の添加量は電解槽の陽極室で
生成する塩素酸塩の量を分解すればよく、前者に
比べて少い。しかし、塩素酸塩は比較的安定な化
合物であり塩素酸塩を塩酸を用いて分解すると
き、塩素酸塩の分解率を高めるために過剰の塩酸
を加えて塩素酸塩を分解するので、その程使用塩
酸量を少なくすることができない。特に塩水中の
塩素酸塩の濃度を低く保つとき、又は電流効率が
低いときには、多量の塩酸を必要としたときには
前者より多くの塩酸を必要とする場合もある。ま
た、この方法は前者と同様に電解槽に塩酸を添加
することになるので、陽イオン交換膜及び／又は
陽極の耐久性を損う恐れがあり、塩水中和用アル
カリの過大な使用をもたらす。 本発明者らは陽イオン交換膜法塩化アルカリの
電解において生成する塩素酸塩と水素ガスの存在
下で触媒と接触せしめる分解方法に関して種々実
験検討を試みた結果、塩素酸塩を含む陽極室から
の塩水の全部又は一部を触媒を充填した触媒層の
上部又は下部に供給し、触媒層の下部より水素ガ
スを供給して塩水中の塩素酸塩と水素ガスとを触
媒を介して反応させ分解し、分解液を電解槽へ又
は主塩水供給経路に回収し、これを電解槽へもど
すことにより全塩水系の塩素酸塩を容易に一定濃
度以下に維持できるのみならず、従来法を採用し
た場合に伴う各種欠点を完全に免れることがで
き、しかも水素ガスは電解槽から発生するものを
用いることができるので、運転コストが大幅に節
減できることを見出して本発明を完成した。 本発明によれば、電解槽へ塩酸を添加しないの
で、電解槽の電流効率損失分に相当する塩酸量又
は塩素酸塩分解工程に添加する塩酸量を厳密に管
理する必要はない。陽極で副生した塩素酸塩は分
解工程で容易に分解除去されるので、塩素酸塩の
塩水系内への蓄積は防止される。本発明者らの検
討結果によれば、塩素酸塩の分解は触媒に吸蔵さ
れた水素ガスが触媒表面に存在する塩素酸塩と次
のように反応して分解される。 NaClO3＋3H2→NaCl＋3H2O この反応速度は触媒の吸蔵水素ガス量及び反応
温度に依存するが、特に触媒の吸蔵水素ガス量が
分解反応を支配する。例えば、触媒の水素ガス吸
蔵量は反応温度が10℃上がると約２〜３倍にな
る。また、塩素酸塩の濃度が２倍になつても反応
速度にあまり変化しない。触媒の種類によつて反
応速度に多少の違いがある。触媒としては鉄、ニ
ツケル、コバルト、白金、ルテニウム、パラジウ
ム、ロジウム、イリジウム等の第族金属及びそ
の酸化物の中から選定される１種又は２種類を主
体とするものが使用される。分解反応温度は25℃
以上であれば分解反応は進むが、なるべく高いこ
とが望ましく、電解槽の戻り塩水から一部を抜き
出し、この温度で分解を行うのが経済上有利であ
る。通常この温度は80〜95℃であるが更に加熱し
ても良い。陽極液中の塩素酸塩の濃度が過大であ
ると塩化アルカリ濃度が低下して、陽極塩素ガス
純度が低下すると共に陰極液中の塩素酸塩濃度が
上昇する原因となるので、特に塩化アルカリ塩水
系では停電等により塩水系の温度が低下すると、
塩素酸塩が晶出して配管閉塞等の原因になるの
で、陽極液中の塩素酸塩の濃度は通常25ｇ／以
下に維持するのが望ましい、より好ましくは10
ｇ／以下である。塩水中の塩素酸塩濃度を一定
値に保持するためには、陽極室における塩素酸塩
の生成量と、分解反応による分解量が等しくなる
ように決める必要がある。このために塩素酸塩分
解系に流す塩水量は分解系の分解率を高めて主循
環塩水量の２〜30％、好ましくは２〜10％であ
る。 塩素酸塩分解系の触媒層は、分解反応速度が極
めて大きいので小容量でよく、連続方式又はバツ
チ方式を採用することができる。また塩素酸塩分
塩後の塩水は主循環塩水系に回収する。塩素酸塩
の分解反応器はバツチ方式の場合は容器に触媒を
適当量入れて、そこに塩素酸塩濃度の高い塩水を
主循環系から抜き出して入れ、水素ガスでバブリ
ングして塩素酸塩を分解して分解液を主循環系に
もどすことにより、主塩水循環系の塩素酸塩の濃
度を一定に保つことができる。 連続式の場合は、塩素酸塩の分解触媒を充填し
た触媒層に主塩水循環系から抜き出した塩素酸塩
濃度の高い塩水を上昇流又は下降流で連続的に供
給し、水素ガスを容器の下部より連続的に供給
し、容器内で水素と触媒と塩水を接触させ、塩水
中の塩素酸塩を連続的に分解する。塩素酸塩分解
後の塩水は主塩水循環系に連続的に回収され、余
剰の水素は空放又は再使用される。かようにして
主塩水循環系の塩素酸塩の濃度を一定に保つこと
ができる。このように本発明の特徴は塩素酸塩分
解触媒を充填した触媒層に主塩水循環系の塩水の
全部又は主塩水循環系の塩水の一部を抜き出して
バツチ又は連続的に供給し、水素を容器の下部よ
りバツチ又は連続的に供給して塩素酸塩を分解
し、塩素酸塩分解後の塩水を主塩水循環系にもど
すことにより主塩水循環系の塩素酸塩の蓄積を防
止することにある。更に本発明では、塩素酸塩を
分解するために塩酸を必要とせず電解槽から発生
する水素を有効に利用することができ、運転操作
も非常に簡単である。 以下に実施例により更に本発明を説明するが本
発明はこれに限定されるものではない。実施例 １ 第１表に示す触媒100c.c.を500c.c.ビーカーに入
れ、それに塩素酸塩17.2ｇ／（ClO-3として）
を含有する飽和塩化ナトリウム塩水を300c.c.入れ、
５分間水素ガスでバブリングして塩水中の塩素酸
塩の濃度を測定し、第１表に併記する結果を得
た。 The present invention relates to a method for preventing the accumulation of chlorate in alkali chloride electrolysis using an ion exchange membrane method by decomposing and removing the chlorate that accumulates in circulating brine by reacting it with hydrogen gas using a catalyst. It is something. More specifically, in ion exchange membrane method alkaline chloride electrolysis, all or part of the main circulating brine is extracted and supplied to the upper or lower part of a column packed with a catalyst,
Hydrogen gas or a gas containing hydrogen gas is supplied to the lower part of the tower filled with the catalyst, and the chlorate in the circulating brine and hydrogen gas are reacted and decomposed through the catalyst, thereby decomposing the chlorine in the circulating brine. The present invention relates to a method for preventing acid salt accumulation. As methods for electrolyzing alkali chloride, the diaphragm method, the mercury method, and the ion exchange membrane method are known. In the diaphragm method, electrolysis is performed while flowing salt water from the anode chamber to the cathode chamber to prevent back diffusion of OH- to the anode chamber. For this reason, chlorate produced as a by-product in the anode chamber exits the salt water system together with the catholyte, so chlorate does not accumulate in the salt water. In the mercury method, chlorate by-produced in the hydration tank is reductively decomposed by the cathode amalgam, so the amount of chlorate accumulated in the circulating brine is not so large, and the chlorate concentration in the circulating brine is 10 to 30 g. / degree. On the other hand, in the ion-exchange membrane method, a cation-exchange membrane is used as a diaphragm, but there is no ideal cation-exchange membrane that completely prevents back diffusion of OH- ions. chlorate is produced as a by-product. Since the by-produced chlorate ions are anions and have a relatively large ionic radius, it is extremely difficult for them to diffuse into the cathode chamber through the cation exchange membrane. Therefore, in ion-exchange membrane electrolysis, as salt water is recycled, chlorate gradually accumulates in the circulating salt water system. Although a small amount of chlorate accumulation does not adversely affect ion-exchange membrane electrolysis, excessive chlorate accumulation not only reduces the alkali chloride concentration in the anode chamber, but also reduces the chlorine gas purity. , the amount of chlorate ions that diffuse and permeate into the cathode chamber via the cation exchange membrane cannot be ignored. Excessive accumulation of chlorate can also accelerate the deterioration of metal anode coatings. In order to prevent excessive accumulation of chlorate, which causes such drawbacks, in ion-exchange membrane electrolysis, the pH of the anolyte is adjusted by adding an acid such as hydrochloric acid.
3.5 or less to suppress the formation of chlorate, or extract a portion of the main circulation brine, add excess hydrochloric acid to decompose the chlorate, and recover it to the main circulation brine system to prevent chlorate accumulation. Methods are being adopted to prevent this. Regarding the former, it is necessary to add just enough hydrochloric acid to approximately neutralize the OH- ions diffusing from the cathode chamber to the anode chamber, that is, approximately the amount equivalent to the loss of alkaline current efficiency. If there is a shortage of hydrochloric acid at this time, chlorate will be produced as a by-product and will accumulate in the salt water system. In addition, if the amount is excessive, the hydrogen ion concentration within the anode increases, resulting in swelling of the cation exchange membrane and a decrease in permselectivity for alkali metal ions.
In addition to potentially reducing alkaline current efficiency, excessive acidity in the anode chamber may impair the durability of the precious metal-coated titanium anode normally used as an anode, and may also reduce the hydration of return brine during brine purification. resulting in excessive use of alkali. In the latter case, the amount of hydrochloric acid added is sufficient to decompose the amount of chlorate produced in the anode chamber of the electrolytic cell, and is smaller than in the former case. However, chlorate is a relatively stable compound, and when chlorate is decomposed using hydrochloric acid, excess hydrochloric acid is added to increase the decomposition rate of chlorate. It is not possible to reduce the amount of hydrochloric acid used. Particularly when the concentration of chlorate in the salt water is kept low or when the current efficiency is low, more hydrochloric acid may be required than the former. In addition, like the former method, this method involves adding hydrochloric acid to the electrolytic cell, which may impair the durability of the cation exchange membrane and/or anode, resulting in excessive use of alkali for saline water hydration. . The present inventors have attempted various experimental studies on a decomposition method in which chlorate generated in the cation exchange membrane electrolysis of alkali chloride is brought into contact with a catalyst in the presence of hydrogen gas. All or part of the brine is supplied to the top or bottom of the catalyst bed filled with a catalyst, and hydrogen gas is supplied from the bottom of the catalyst bed to cause the chlorate in the brine to react with the hydrogen gas via the catalyst. By decomposing, collecting the decomposed liquid into the electrolytic cell or the main salt water supply route, and returning it to the electrolytic cell, it is possible to easily maintain the chlorate concentration in the entire salt water system below a certain level, and also to adopt the conventional method. The present invention was completed based on the discovery that the various disadvantages associated with such cases can be completely avoided, and that hydrogen gas generated from an electrolytic tank can be used, resulting in a significant reduction in operating costs. According to the present invention, since hydrochloric acid is not added to the electrolytic cell, there is no need to strictly control the amount of hydrochloric acid corresponding to the current efficiency loss of the electrolytic cell or the amount of hydrochloric acid added to the chlorate decomposition step. Since chlorate produced as a by-product at the anode is easily decomposed and removed in the decomposition process, accumulation of chlorate in the salt water system is prevented. According to the study results of the present inventors, chlorate is decomposed by hydrogen gas occluded in the catalyst reacting with chlorate present on the catalyst surface as follows. NaClO3 +3H2 →NaCl+3H2 O Although the reaction rate depends on the amount of hydrogen gas stored in the catalyst and the reaction temperature, the amount of hydrogen gas stored in the catalyst particularly dominates the decomposition reaction. For example, the amount of hydrogen gas stored in the catalyst increases approximately 2 to 3 times when the reaction temperature increases by 10°C. Furthermore, even if the concentration of chlorate is doubled, the reaction rate does not change much. There are some differences in reaction rates depending on the type of catalyst. As the catalyst, one or two selected from Group metals such as iron, nickel, cobalt, platinum, ruthenium, palladium, rhodium, and iridium and their oxides are used as the main catalyst. Decomposition reaction temperature is 25℃
If the temperature is higher than this, the decomposition reaction will proceed, but it is desirable that the temperature be as high as possible, and it is economically advantageous to extract a portion of the salt water returned from the electrolytic cell and perform the decomposition at this temperature. Usually this temperature is 80 to 95°C, but it may be heated further. If the concentration of chlorate in the anolyte is too high, the alkali chloride concentration will decrease, causing a decrease in the anodic chlorine gas purity and an increase in the chlorate concentration in the catholyte. In the system, when the temperature of the salt water system drops due to a power outage, etc.
Since chlorate crystallizes and causes piping blockages, it is generally desirable to maintain the concentration of chlorate in the anolyte at 25 g/lower, more preferably 10 g/l or less.
g/ or less. In order to maintain the chlorate concentration in the salt water at a constant value, it is necessary to set the amount of chlorate produced in the anode chamber equal to the amount decomposed by the decomposition reaction. For this purpose, the amount of brine flowing into the chlorate decomposition system is 2 to 30%, preferably 2 to 10%, of the amount of main circulation brine to increase the decomposition rate of the decomposition system. The catalyst layer for chlorate decomposition has an extremely high decomposition reaction rate, so only a small capacity is required, and a continuous method or a batch method can be adopted. The brine after chlorate salting is recovered to the main circulation brine system. If the chlorate decomposition reactor is a batch method, an appropriate amount of catalyst is placed in a container, and salt water with a high chlorate concentration is taken out from the main circulation system and put there, and hydrogen gas is bubbled through it to decompose the chlorate. By decomposing and returning the decomposed liquid to the main circulation system, the concentration of chlorate in the main salt water circulation system can be kept constant. In the case of a continuous type, brine with a high chlorate concentration extracted from the main brine circulation system is continuously supplied in an upward or downward flow to a catalyst bed filled with a chlorate decomposition catalyst, and hydrogen gas is supplied to the catalyst bed filled with a chlorate decomposition catalyst. Hydrogen is continuously supplied from the bottom, and hydrogen, catalyst, and salt water are brought into contact with each other in the container, and chlorate in the salt water is continuously decomposed. The brine after chlorate decomposition is continuously collected into the main brine circulation system, and excess hydrogen is released or reused. In this way, the concentration of chlorate in the main brine circulation system can be kept constant. As described above, the feature of the present invention is that all of the brine in the main brine circulation system or a portion of the brine in the main brine circulation system is extracted and supplied in batches or continuously to the catalyst bed filled with a chlorate decomposition catalyst, and hydrogen is then supplied to the catalyst bed filled with a chlorate decomposition catalyst. The chlorate is decomposed by supplying it batchwise or continuously from the bottom of the container, and the brine after chlorate decomposition is returned to the main brine circulation system, thereby preventing the accumulation of chlorate in the main brine circulation system. be. Furthermore, the present invention does not require hydrochloric acid to decompose chlorate, making it possible to effectively utilize the hydrogen generated from the electrolytic cell, and the operation is very simple. The present invention will be further explained below with reference to Examples, but the present invention is not limited thereto. Example 1 100 c.c. of the catalyst shown in Table 1 was placed in a 500 c.c. beaker, and 17.2 g of chlorate/(as ClO-3 ) was added to it.
Add 300 c.c. of saturated sodium chloride brine containing
The concentration of chlorate in the salt water was measured by bubbling hydrogen gas for 5 minutes, and the results shown in Table 1 were obtained.
【表】 なお、本実施例で用いた各触媒は次の組成の活
性成分を比表面積1000〜1300m2／ｇの活性炭に担
持したものである。RuO2系 RuO2 100％ （重量％以下同じ） 担持率 ２％PdO系 PdO 100％ 担持率 1.5％白金ブラツク系 白金ブラツク 100％ 担持率 ５％白金系 白金 100％ 担持率 ７％ニツケル系 ニツケル 40％ 鉄 60％ 担持率 ３％ ここで担持率は触媒担体重量に対する活性成分
重量の百分率をいう。実施例 ２ 実施例１と同じ酸化ルテニウム系の触媒100c.c.
を500c.c.のビーカーに入れ、それに塩素酸塩濃度
（ClO-3として）17.2ｇ／、11.7ｇ／、5.2ｇ／
を含有する塩水を300c.c.入れ、５分間水素ガス
でバブリングして、塩水中の塩素酸塩の濃度を測
定し、第２表の結果を得た。[Table] Each of the catalysts used in this example had an active component having the following composition supported on activated carbon having a specific surface area of 1000 to 1300 m2 /g. RuO2 system RuO2 100% (Same weight percentage below) Support rate 2% PdO system PdO 100% Support rate 1.5% Platinum black system Platinum black 100% Support rate 5% Platinum system Platinum 100% Support rate 7% Nickel system Nickel 40 % Iron 60% Supporting rate 3% Here, the supporting rate refers to the percentage of the weight of the active ingredient to the weight of the catalyst carrier. Example 2 The same ruthenium oxide catalyst as in Example 1, 100 c.c.
into a 500c.c. beaker, and the chlorate concentration (as ClO-3 ) was 17.2g/, 11.7g/, 5.2g/
The concentration of chlorate in the salt water was measured by bubbling with hydrogen gas for 5 minutes and the results shown in Table 2 were obtained.
【表】実施例 ３ 内径32mm高さ1500mmのガラスチユーブに実施例
１と同じ酸化ルテニウム系触媒を1000mmの高さに
充填した反応容器の上部から塩素酸塩濃度17.2
ｇ／（ClO-3として）を含有する飽和食塩水を
60℃の温度で４／Hrの割合で連続的に供給し、
容器下部より連続的に抜き出しながら容器下部よ
り水素ガスを80／Hrで連続的に供給し、容器
上部より空放する運転を100日間実施した。容器
下部から流出した塩水中の塩素酸塩濃度（ClO-3
として）を運転経過日数と共に測定し第３表の結
果を得た。[Table] Example 3 A glass tube with an inner diameter of 32 mm and a height of 1500 mm was filled with the same ruthenium oxide catalyst as in Example 1 to a height of 1000 mm.The chlorate concentration was 17.2 from the top of the reaction vessel.
g/(as ClO-3 ) of saturated saline containing
Continuously supplied at a rate of 4/Hr at a temperature of 60℃,
An operation was carried out for 100 days in which hydrogen gas was continuously supplied from the bottom of the container at a rate of 80/hr while being continuously extracted from the bottom of the container, and the hydrogen gas was released from the top of the container. Chlorate concentration (ClO-3
) was measured along with the number of days of operation, and the results shown in Table 3 were obtained.
【表】【table】