EP0445516B1 - Process and apparatus for the production of silver nitrate - Google Patents

Process and apparatus for the production of silver nitrate Download PDF

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Publication number
EP0445516B1
EP0445516B1 EP91100949A EP91100949A EP0445516B1 EP 0445516 B1 EP0445516 B1 EP 0445516B1 EP 91100949 A EP91100949 A EP 91100949A EP 91100949 A EP91100949 A EP 91100949A EP 0445516 B1 EP0445516 B1 EP 0445516B1
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nitric acid
chamber
silver
concentration
cathode
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German (de)
French (fr)
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EP0445516A1 (en
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Alexander Przybilla
Hans Horst Joachim Dr. Dipl.-Chem. Schoberth
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Doduco Solutions GmbH
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Doduco GmbH and Co KG Dr Eugen Duerrwaechter
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25BELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00Electrolytic production of inorganic compounds or non-metals

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  • the subject of the invention is a method for producing silver nitrate by dissolving silver in nitric acid.
  • the formation of the nitrogen oxides makes it necessary to post-treat the exhaust gas, the nitrogen oxides being broken down into nitrogen and oxygen by thermal decomposition or being converted into nitric acid in gas scrubbers with the addition of oxygen.
  • the nitric acid formed can be fed back into the process.
  • the silver nitrate is obtained by evaporation from the nitric acid solution, during which it crystallizes out.
  • the silver nitrate is obtained by evaporation from the nitric acid solution, during which it crystallizes out.
  • the invention is based on the object of specifying a method for producing silver nitrate in which fewer nitrogen oxides are produced than previously.
  • the silver nitrate is produced by an electrodialytic process: two are produced by one Anion exchange membrane separate chambers formed; in one chamber there is dilute nitric acid in which anodically polarized silver is immersed, for example in the form of a silver plate; In the other chamber there is more concentrated nitric acid, into which a silver-free cathode, which is made of V2A steel, for example, is immersed.
  • a cold, highly dilute nitric acid is used, which merely has the task of ensuring a certain electrolytic conductivity.
  • concentration of silver nitrate in the dilute nitric acid increases, the anion exchange membrane being an obstacle for the silver ions which want to migrate into the more concentrated nitric acid.
  • a small proportion of the silver ions migrate into the through osmosis more concentrated nitric acid and is deposited as sludge on the cathode. This process reduces the silver nitrate yield of the process and can lead to the formation of nitrogen oxides which are undesirable per se, but the amount of which is less by one to three orders of magnitude than in the known process.
  • Electrodialysis is ended when the yield drops below a predetermined value, which is associated with the consumption of the silver anode.
  • the dilute nitric acid is then treated in a manner known per se to obtain the silver nitrate enriched therein, e.g. evaporated.
  • the deposition of silver on the cathode and its subsequent conversion with nitric acid to silver nitrate can be drastically reduced again with the formation of nitrogen oxides by providing a third chamber between the chamber containing the anode and the chamber containing the cathode (Hereinafter also referred to as the bipolar chamber in which nitric acid is present in a higher concentration than in the chamber in which the anode is contained.
  • the third chamber is separated from both neighboring chambers by an anion exchange membrane.
  • the nitric acid can be present in a higher concentration than in the chamber with the anode, but this does not have to be the case; rather, a low nitric acid concentration in the catholyte is sufficient to maintain the necessary ionic conductivity nitric acid concentration, in particular from a concentration that begins with the If the nitric acid concentration in the third chamber matches, then the nitric acid concentration in the catholyte gradually decreases by electrodialysis in favor of the nitric acid concentration in the third chamber.
  • nitrate ions from the more concentrated nitric acid migrate through the anion exchange membrane into the anolyte (this is the electrolyte in the chamber containing the anode), where they form silver nitrate with the anodically dissolved silver.
  • the silver ions migrate through osmosis into the concentrated nitric acid in the third chamber, but cannot be converted there with the formation of nitrogen oxides; on the contrary, this could only happen in the nitric acid in which the cathode is immersed.
  • the silver ions would have to overcome a further anion exchange membrane, but this would only be possible after the electrolyte had been enriched with silver ions.
  • the losses in yield of this improved method which is the subject of claims 2 and 3, are only low, nitrogen oxides are only produced in very small amounts.
  • the concentration of the initially higher concentrated nitric acid gradually decreases. If the anolyte is removed in order to obtain the silver nitrate by evaporation, the initially more concentrated nitric acid, which has now decreased in concentration, can be used as anolyte for the next process run and replaced by fresh, concentrated nitric acid. This double use has a beneficial effect on the cost of the process.
  • Electrodialysis is preferably carried out with current densities of 5 to 50 A / dm2 at a temperature between 5 ° C. and 60 ° C. If the current density remains below 5 A / dm2, the yield is low and the process is not particularly economical . However, the yield cannot be increased arbitrarily by increasing the current density. Above a current density of 50 A / dm2, one has to reckon with the fact that the anion exchange membrane is damaged and becomes porous.
  • the working temperature should not be below 5 ° C, otherwise the electrical conductivity in the electrolytes will be too low.
  • the working temperature should also not exceed 60 ° C to avoid that the silver is chemically dissolved by the hot nitric acid.
  • the electrodialysis is expediently ended when the concentration of the silver ions in the catholyte exceeds 2 g / l. Since the concentration of the silver ions in the catholyte increases with the concentration of the silver ions in the anolyte, a criterion for the termination of the electrodialysis can also be formed from the concentration of the silver ions in the anolyte; in the event of the method, as stated in claim 2 or 3, in which the silver ions have to pass through two anion exchange membranes before they can reach the cathode, the electrodialysis is preferably ended when the concentration of the silver ions in the anolyte is 400 g / l exceeds.
  • anolyte is dilute nitric acid; a certain minimum concentration is required to ensure the necessary ion conductivity and to prevent hydrolysis.
  • an anolyte is used which contains 25 to 100 g HNO3 / l.
  • the catholyte in the case of the process variant according to claim 2 or 3:
  • the catholyte preferably contains 10 to 100 g HNO3 / l; the nitric acid in the catholyte only serves to maintain the conductivity.
  • nitric acid is used to store nitrate ions for the process; their concentration is not critical; it should be greater than the concentration in the anolyte and is preferably between 40 and 300 g HNO3 / l.
  • a device can be used which is known in a similar structure from US-A-3,795,595 for the production of tin and lead salts and an anode chamber filled with an acid with a tin or lead anode and one with an acid Has filled cathode chamber with a cathode, for example made of stainless steel, the chambers being separated by an anion exchange membrane.
  • a device with a corresponding structure is used for the purposes of the invention, with the proviso that the Anode consists of silver, the cathode contains no silver and nitric acid is filled in as acid.
  • the concentration of nitric acid in the anode chamber is lower than in the adjacent chamber, which can be the cathode chamber, but which is preferably a chamber free of electrodes, which is arranged between the cathode chamber and the anode chamber and is separated from both by an anion exchange membrane (claims 11 and 12). With such a structure, particularly little nitrogen oxides are formed.
  • An advantageous further development of the invention is characterized in that several such modular devices are combined to form larger systems. This can be done by arranging several anode chambers and cathode chambers in an alternating sequence and separating them from each other by an anion exchanger membrane, but preferably by arranging several anode chambers and cathode chambers in an alternating sequence, each with the interposition of a chamber free of electrodes (bipolar chamber) and the chambers are separated from each other by an anion exchange membrane.
  • a vessel 1 is divided into three chambers 3, 4 and 5 by two mutually parallel anion exchange membranes.
  • the middle chamber 4 contains a silver anode 8 and nitric acid concentrated as anolyte.
  • the chambers 3 and 5 arranged on both sides of the middle chamber 4 each contain a cathode 9 made of V2A steel and nitric acid diluted as a catholyte.
  • silver is anodically dissolved in the chamber 4 and an equivalent amount of nitrate ions migrates from the chambers 3 and 5 through the two anion exchange membranes 2 into the middle chamber 4.
  • the device shown in Figure 2 differs from that shown in Figure 1 in that between the anode chamber 4 and the cathode chambers 3 and 5 of electrodes-free chambers 10 and 11 (bipolar chambers) are arranged, which contain the concentrated nitric acid, while the cathode chambers 9 and the anode chamber 8 each contain dilute nitric acid.
  • a direct voltage is applied between the anode 8 and the cathodes 9, silver anodically dissolves in the anode chamber 4.
  • An equivalent amount of nitrate ions migrates from the bipolar chambers 10 and 11 through the anion exchange membranes 2 into the anode chamber 4 and an equivalent amount of hydrogen ions is discharged at the cathodes 9 and escapes as molecular hydrogen.
  • the anode chamber 4 contains 8 15 l of dilute nitric acid in a concentration of 35 g HNO3 / l in addition to the silver anode.
  • the two cathode chambers 3 and 5 each contain 20 l of dilute nitric acid in a concentration of 20 g HNO3 / l in addition to the cathodes 9.
  • the two bipolar chambers 10 and 11 each contain 20 l of concentrated nitric acid in a concentration of 220 g HNO3 / l. Electrodialysis is carried out at a temperature of 30 ° C and a current density of 30 A / dm2 and is terminated when the concentration of silver ions in the anolyte reaches 400 g / l.
  • the concentration of silver ions in the catholyte is then less than 2 g / l.
  • the amount of nitrogen oxides released is less than 1/100 of the amount that would result from working with the conventional method (dissolving silver in hot nitric acid).
  • the anolyte is removed in order to obtain the silver nitrate.
  • the silver nitrate can be obtained in a conventional manner, e.g. by evaporation.
  • the device shown in FIG. 3 differs from that in FIG. 2 in that it additionally contains a further anode chamber 14, a further cathode chamber 13 and two further bipolar chambers 20, 21, so that double the amount of silver nitrate can be produced with this arrangement in the device according to FIG. 2.
  • FIG. 3 shows that by inserting further modules formed from an anode chamber, a cathode chamber and two bipolar chambers, larger systems can be built up according to the desired production capacity.

Description

Die Erfindung hat ein Verfahren zum Erzeugen von Silbernitrat durch Auflösen von Silber in Salpetersäure zum Gegenstand.The subject of the invention is a method for producing silver nitrate by dissolving silver in nitric acid.

Es ist bekannt, Silbernitrat dadurch herzustellen, dass man Silber in heißer Salpetersäure auflöst. Dabei läuft die folgende Reaktion ab:



        4 Ag + 6 HNO₃ → 4AgNO₃ + NO + NO₂ + 3 H₂O



Aus der Reaktionsgleichung ist ersichtlich, dass man - bezogen auf die eingesetzte Silbermenge - mit einem Überschuss an Salpetersäure arbeiten muss, weil zunächst aus einem Drittel der eingesetzten Salpetersäure Stickoxide entstehen, welche die Lösung als Abgas verlassen, soweit nicht ein Teil des entstehenden Stickstoffdioxids mit dem in der Lösung anwesenden Wasser wiederum zu Salpetersäure und Stickstoffmonoxid umgesetzt wird. Die Entstehung der Stickstoffoxide macht es erforderlich, das Abgas nachzubehandeln, wobei die Stickstoffoxide durch thermische Spaltung in Stickstoff und Sauerstoff zerlegt oder in Gaswaschanlagen unter Sauerstoffzusatz zu Salpetersäure umgesetzt werden. Die gebildete Salpetersäure kann erneut dem Prozess zugeführt werden.
It is known to produce silver nitrate by dissolving silver in hot nitric acid. The following reaction takes place:



4 Ag + 6 HNO₃ → 4AgNO₃ + NO + NO₂ + 3 H₂O



The reaction equation shows that - based on the amount of silver used - one has to work with an excess of nitric acid, because nitrogen oxides initially form from a third of the nitric acid used, which leave the solution as waste gas, unless a part of the nitrogen dioxide formed is in turn converted into nitric acid and nitrogen monoxide with the water present in the solution. The formation of the nitrogen oxides makes it necessary to post-treat the exhaust gas, the nitrogen oxides being broken down into nitrogen and oxygen by thermal decomposition or being converted into nitric acid in gas scrubbers with the addition of oxygen. The nitric acid formed can be fed back into the process.

Aus der salpetersauren Lösung wird das Silbernitrat durch Eindampfen gewonnen, wobei es auskristallisiert. Dabei fallen ebenfalls erhebliche Mengen an Stickoxiden in der Abluft an, die auf eine der erwähnten Arten aus der Abluft wieder entfernt werden müssen. Das ist mit erheblichem Aufwand verbunden.The silver nitrate is obtained by evaporation from the nitric acid solution, during which it crystallizes out. There are also significant amounts of nitrogen oxides in the exhaust air, which must be removed from the exhaust air in one of the ways mentioned. This is associated with considerable effort.

Der Erfindung liegt die Aufgabe zugrunde, ein Verfahren zum Erzeugen von Silbernitrat anzugeben, bei welchem weniger Stickoxide entstehen als bisher.The invention is based on the object of specifying a method for producing silver nitrate in which fewer nitrogen oxides are produced than previously.

Diese Aufgabe wird gelöst durch ein Verfahren mit den im Patentanspruch 1 angegebenen Merkmalen. Vorteilhafte Weiterbildungen der Erfindung sind Gegenstand der abhängigen Ansprüche.This object is achieved by a method with the features specified in claim 1. Advantageous developments of the invention are the subject of the dependent claims.

Erfindungsgemäß wird das Silbernitrat durch ein elektrodialytisches Verfahren erzeugt: Es werden zwei durch eine Anionenaustauscher-Membran getrennte Kammern gebildet; in einer Kammer befindet sich verdünnte Salpetersäure, in welche anodisch gepoltes Silber eintaucht, z.B. in Gestalt einer Silberplatte; in der anderen Kammer befindet sich höher konzentrierte Salpetersäure, in welche eine silberfreie Kathode, welche beispielsweise aus V2A-Stahl besteht, eintaucht. Nach Anlegen einer Gleichspannung zwischen Kathode und Anode wird in der verdünnten Salpetersäure Silber anodisch gelöst und eine äquivalente Menge Nitrationen wandert durch die Anionenaustauscher-Membran aus der Kammer mit der konzentrierten Salpetersäure in die Kammer mit der verdünnten Salpetersäure und bildet dort mit den in Lösung gegangenen Silber-Ionen eine Silbernitratlösung. Eine äquivalente Menge Wasserstoff-Ionen wird an der Kathode unter Bildung von molekularem Wasserstoff entladen, der die Lösung verläßt. Stickstoffoxide entstehen bei dieser Reaktion zunächst keine und demgemäß wird auch kein Salpetersäureüberschuss benötigt. Vielmehr kommt man im Anolyten, in welchem sich die Silberanode befindet, mit einer kalten, stark verdünnten Salpetersäure aus, die lediglich die Aufgabe hat, eine gewisse elektrolytische Leitfähigkeit sicherzustellen. Im Laufe des Verfahrens steigt die Konzentration des Silbernitrates in der verdünnten Salpetersäure an, wobei die Anionenaustauscher-Membran ein Hindernis für die Silber-Ionen darstellt, welche in die höher konzentrierte Salpetersäure wandern wollen. Mit steigender Silberkonzentration in der verdünnten Salpetersäure wandert aber doch ein geringer Anteil der Silber-Ionen durch Osmose in die höher konzentrierte Salpetersäure und wird als Schlamm an der Kathode abgeschieden. Dieser Vorgang verringert die Silbernitratausbeute des Verfahrens und kann zum Entstehen von an sich unerwünschten Stickoxiden führen, deren Menge jedoch um eine bis drei Größenordnungen geringer ist als beim bekannten Verfahren.According to the invention, the silver nitrate is produced by an electrodialytic process: two are produced by one Anion exchange membrane separate chambers formed; in one chamber there is dilute nitric acid in which anodically polarized silver is immersed, for example in the form of a silver plate; In the other chamber there is more concentrated nitric acid, into which a silver-free cathode, which is made of V2A steel, for example, is immersed. After applying a direct voltage between the cathode and anode, silver is anodically dissolved in the dilute nitric acid and an equivalent amount of nitrate ions migrates through the anion exchange membrane from the chamber with the concentrated nitric acid into the chamber with the dilute nitric acid and forms there with the silver that has dissolved -Ion a silver nitrate solution. An equivalent amount of hydrogen ions is discharged at the cathode to form molecular hydrogen which leaves the solution. No nitrogen oxides are initially formed in this reaction and accordingly no excess nitric acid is required. Rather, in the anolyte in which the silver anode is located, a cold, highly dilute nitric acid is used, which merely has the task of ensuring a certain electrolytic conductivity. In the course of the process, the concentration of silver nitrate in the dilute nitric acid increases, the anion exchange membrane being an obstacle for the silver ions which want to migrate into the more concentrated nitric acid. However, with increasing silver concentration in the dilute nitric acid, a small proportion of the silver ions migrate into the through osmosis more concentrated nitric acid and is deposited as sludge on the cathode. This process reduces the silver nitrate yield of the process and can lead to the formation of nitrogen oxides which are undesirable per se, but the amount of which is less by one to three orders of magnitude than in the known process.

Die Elektrodialyse wird beendet, wenn die Ausbeute unter einen vorgegebenen Wert absinkt, was mit dem Verbrauch der Silber-Anode einhergeht. Die verdünnte Salpetersäure wird dann zur Gewinnung des darin angereicherten Silbernitrates in an sich bekannter Weise behandelt, z.B. eingedampft.Electrodialysis is ended when the yield drops below a predetermined value, which is associated with the consumption of the silver anode. The dilute nitric acid is then treated in a manner known per se to obtain the silver nitrate enriched therein, e.g. evaporated.

In vorteilhafter Weiterbildung des Grundgedankens der Erfindung läßt sich die Abscheidung von Silber an der Kathode und seine anschließende Umsetzung mit Salpetersäure zu Silbernitrat unter Bildung von Stickstoffoxiden nochmals drastisch verringern, indem man zwischen der die Anode enthaltenden Kammer und der die Kathode enthaltenden Kammer eine dritte Kammer vorsieht (nachfolgend auch als bipolare Kammer bezeichnet, in welcher sich Salpetersäure in höherer Konzentration befindet als in jener Kammer, in welcher die Anode enthalten ist. Die dritte Kammer ist von beiden benachbarten Kammern durch eine Anionen-Austauschermembran getrennt. In der Kammer, welche die Kathode enthält, kann die Salpetersäure in höherer Konzentration vorliegen als in der Kammer mit der Anode, doch muss das nicht so sein; es genügt vielmehr im Katholyt eine geringe Salpetersäurekonzentration zur Aufrechterhaltung der nötigen Ionen-Leitfähigkeit. Geht man jedoch im Katholyten zunächst von einer höheren Salpetersäurekonzentration aus, insbesondere von einer Konzentration, die anfänglich mit der Salpetersäurekonzentration in der dritten Kammer übereinstimmt, dann nimmt die Salpetersäurekonzentration im Katholyten durch die Elektrodialyse allmählich ab zugunsten der Salpetersäurekonzentration in der dritten Kammer. Im Verlauf der Elektrodialyse wandern Nitrat-Ionen aus der höher konzentrierten Salpetersäure durch die Anionenaustauscher-Membran in den Anolyten (das ist der Elektrolyt in der die Anode enthaltenden Kammer) und bilden dort mit dem anodisch in Lösung gegangenen Silber Silbernitrat. Mit steigender Konzentration der Silber-Ionen wandert ein Teil der Silber-Ionen durch Osmose in die konzentrierte Salpetersäure in der dritten Kammer, kann dort aber nicht unter Bildung von Stickstoffoxiden umgesetzt werden; das könnte vielmehr erst in der Salpetersäure geschehen, in welche die Kathode eintaucht. Dazu müssten die Silber-Ionen aber eine weitere Anionenaustauscher-Membran überwinden, was aber erst nach einer Anreicherung des Elektrolyten mit Silber-Ionen möglich würde. Die Ausbeuteverluste dieses verbesserten Verfahrens, welches Gegenstand der Ansprüche 2 und 3 ist, sind nur noch gering, Stickstoffoxide entstehen nur noch in kleinsten Mengen.In an advantageous development of the basic idea of the invention, the deposition of silver on the cathode and its subsequent conversion with nitric acid to silver nitrate can be drastically reduced again with the formation of nitrogen oxides by providing a third chamber between the chamber containing the anode and the chamber containing the cathode (Hereinafter also referred to as the bipolar chamber in which nitric acid is present in a higher concentration than in the chamber in which the anode is contained. The third chamber is separated from both neighboring chambers by an anion exchange membrane. In the chamber which contains the cathode contains, the nitric acid can be present in a higher concentration than in the chamber with the anode, but this does not have to be the case; rather, a low nitric acid concentration in the catholyte is sufficient to maintain the necessary ionic conductivity nitric acid concentration, in particular from a concentration that begins with the If the nitric acid concentration in the third chamber matches, then the nitric acid concentration in the catholyte gradually decreases by electrodialysis in favor of the nitric acid concentration in the third chamber. In the course of electrodialysis, nitrate ions from the more concentrated nitric acid migrate through the anion exchange membrane into the anolyte (this is the electrolyte in the chamber containing the anode), where they form silver nitrate with the anodically dissolved silver. With increasing concentration of the silver ions, some of the silver ions migrate through osmosis into the concentrated nitric acid in the third chamber, but cannot be converted there with the formation of nitrogen oxides; on the contrary, this could only happen in the nitric acid in which the cathode is immersed. To do this, however, the silver ions would have to overcome a further anion exchange membrane, but this would only be possible after the electrolyte had been enriched with silver ions. The losses in yield of this improved method, which is the subject of claims 2 and 3, are only low, nitrogen oxides are only produced in very small amounts.

In beiden Verfahrensvarianten nimmt die Konzentration der anfänglich höher konzentrierten Salpetersäure allmählich ab. Wenn man den Anolyten entnimmt, um durch Eindampfen das Silbernitrat zu gewinnen, kann man die anfänglich höher konzentrierte, inzwischen in ihrer Konzentration abgefallene Salpetersäure für den nächsten Verfahrensdurchlauf als Anolyt verwenden und durch frische, konzentrierte Salpetersäure ersetzen. Diese zweifache Verwendung wirkt sich günstig auf die Kosten des Verfahrens aus.In both process variants, the concentration of the initially higher concentrated nitric acid gradually decreases. If the anolyte is removed in order to obtain the silver nitrate by evaporation, the initially more concentrated nitric acid, which has now decreased in concentration, can be used as anolyte for the next process run and replaced by fresh, concentrated nitric acid. This double use has a beneficial effect on the cost of the process.

Vorzugsweise arbeitet man bei der Elektrodialyse mit Stromdichten von 5 bis 50 A/dm² bei einer Temperatur zwischen 5° C und 60° C. Bleibt man mit der Stromdichte unterhalb von 5 A/dm², dann ist die Ausbeute gering und das Verfahren nicht besonders wirtschaftlich. Man kann die Ausbeute jedoch nicht beliebig steigern, indem man die Stromdichte erhöht. Oberhalb einer Stromdichte von 50 A/dm² muss man damit rechnen, dass die Anionenaustauscher-Membran Schaden nimmt und porös wird.Electrodialysis is preferably carried out with current densities of 5 to 50 A / dm² at a temperature between 5 ° C. and 60 ° C. If the current density remains below 5 A / dm², the yield is low and the process is not particularly economical . However, the yield cannot be increased arbitrarily by increasing the current density. Above a current density of 50 A / dm², one has to reckon with the fact that the anion exchange membrane is damaged and becomes porous.

Es ist ein wesentlicher Vorteil des erfindungsgemäßen Verfahrens, dass man mit verhältnismässig kalter Salpetersäure arbeiten kann. Die Arbeitstemperatur sollte jedoch nicht unter 5° C liegen, weil sonst die elektrische Leitfähigkeit in den Elektrolyten zu gering wird. Die Arbeitstemperatur sollte aber auch nicht mehr als 60° C betragen, um zu vermeiden, dass das Silber durch die heiße Salpetersäure chemisch gelöst wird.It is an essential advantage of the method according to the invention that one can work with relatively cold nitric acid. However, the working temperature should not be below 5 ° C, otherwise the electrical conductivity in the electrolytes will be too low. The working temperature should also not exceed 60 ° C to avoid that the silver is chemically dissolved by the hot nitric acid.

Wann man die Elektrodialyse beendet, hängt im wesentlichen davon ab, wieviel Stickstoffoxide man im Abgas zulassen will. Zweckmässigerweise beendet man die Elektrodialyse, wenn die Konzentration der Silber-Ionen im Katholyt 2 g/l übersteigt. Da die Konzentration der Silber-Ionen im Katholyt mit der Konzentration der Silber-Ionen im Anolyt steigt, kann man auch aus der Konzentration der Silber-Ionen im Anolyt ein Kriterium für die Beendung der Elektrodialyse bilden; im Falle des Verfahrens, wie es im Anspruch 2 oder 3 angegeben ist, bei welchem die Silber-Ionen zwei Anionenaustauscher-Membranen durchwandern müssen, ehe sie zur Kathode gelangen können, beendet man die Elektrodialyse vorzugsweise, wenn die Konzentration der Silber-Ionen im Anolyt 400 g/l übersteigt.When you stop electrodialysis essentially depends on how much nitrogen oxides you want to allow in the exhaust gas. The electrodialysis is expediently ended when the concentration of the silver ions in the catholyte exceeds 2 g / l. Since the concentration of the silver ions in the catholyte increases with the concentration of the silver ions in the anolyte, a criterion for the termination of the electrodialysis can also be formed from the concentration of the silver ions in the anolyte; in the event of the method, as stated in claim 2 or 3, in which the silver ions have to pass through two anion exchange membranes before they can reach the cathode, the electrodialysis is preferably ended when the concentration of the silver ions in the anolyte is 400 g / l exceeds.

Für den Anolyten benötigt man lediglich eine verdünnte Salpetersäure; eine gewisse Mindestkonzentration ist erforderlich, um die nötige Ionen-Leitfähigkeit zu gewährleisten und um eine Hydrolyse zu unterbinden. Vorzugsweise verwendet man einen Anolyten, der 25 bis 100 g HNO₃/l enthält. Ähnliches gilt für den Katholyten im Fall der Verfahrensvariante gemäss Anspruch 2 oder 3: In diesem Fall enthält der Katholyt vorzugsweise 10 bis 100 g HNO₃/l; die Salpetersäure im Katholyten dient lediglich der Aufrechterhaltung der Leitfähigkeit.All you need for the anolyte is dilute nitric acid; a certain minimum concentration is required to ensure the necessary ion conductivity and to prevent hydrolysis. Preferably an anolyte is used which contains 25 to 100 g HNO₃ / l. The same applies to the catholyte in the case of the process variant according to claim 2 or 3: In this case, the catholyte preferably contains 10 to 100 g HNO₃ / l; the nitric acid in the catholyte only serves to maintain the conductivity.

Die höher konzentrierte Salpetersäure dient der Bevorratung von Nitrat-Ionen für das Verfahren; ihre Konzentration ist nicht kritisch; sie soll größer sein als die Konzentration im Anolyten und beträgt vorzugsweise zwischen 40 und 300 g HNO₃/l.The higher concentration of nitric acid is used to store nitrate ions for the process; their concentration is not critical; it should be greater than the concentration in the anolyte and is preferably between 40 and 300 g HNO₃ / l.

Zum Durchführen des erfindungsgemäßen Verfahrens kann eine Vorrichtung verwendet werden, die in ähnlichem Aufbau aus der US-A-3,795,595 zum Herstellen von Zinn- und Bleisalzen bekannt ist und eine mit einer Säure gefüllte Anodenkammer mit einer Zinn- bzw. Bleianode sowie eine mit einer Säure gefüllte Kathodenkammer mit einer Kathode z.B. aus rostfreiem Stahl hat, wobei die Kammern durch eine Anionenaustauschermembran getrennt sind. Gemäss Anspruch 10 wird für Zwecke der Erfindung eine Vorrichtung mit entsprechendem Aufbau verwendet, und zwar mit der Maßgabe, dass die Anode aus Silber besteht, die Kathode kein Silber enthält und als Säure Salpetersäure eingefüllt wird. Dabei ist die Konzentration der Salpetersäure in der Anodenkammer geringer als in der benachbarten Kammer, bei der es sich um die Kathodenkammer handeln kann, bei der es sich aber vorzugsweise um eine von Elektroden freie Kammer handelt, welche zwischen der Kathodenkammer und der Anodenkammer angeordnet ist und von beiden durch je eine Anionenaustauscher-Membran getrennt ist (Ansprüche 11 und 12). Bei einem solchen Aufbau entstehen besonders wenig Stickstoffoxide.To carry out the method according to the invention, a device can be used which is known in a similar structure from US-A-3,795,595 for the production of tin and lead salts and an anode chamber filled with an acid with a tin or lead anode and one with an acid Has filled cathode chamber with a cathode, for example made of stainless steel, the chambers being separated by an anion exchange membrane. According to claim 10, a device with a corresponding structure is used for the purposes of the invention, with the proviso that the Anode consists of silver, the cathode contains no silver and nitric acid is filled in as acid. The concentration of nitric acid in the anode chamber is lower than in the adjacent chamber, which can be the cathode chamber, but which is preferably a chamber free of electrodes, which is arranged between the cathode chamber and the anode chamber and is separated from both by an anion exchange membrane (claims 11 and 12). With such a structure, particularly little nitrogen oxides are formed.

Eine vorteilhafte Weiterbildung der Erfindung ist dadurch gekennzeichnet, dass man mehrere solcher Vorrichtungen in Modulbauweise zu größeren Anlagen zusammenfügt. Das kann dadurch geschehen, dass man mehrere Anodenkammern und Kathodenkammern in abwechselnder Aufeinanderfolge anordnet und jeweils durch eine Anionenaustauscher-Membran voneinander trennt, vorzugsweise jedoch dadurch, dass mehrere Anodenkammern und Kathodenkammern in abwechselnder Aufeinanderfolge jeweils unter Zwischenfügung einer von Elektroden freien Kammer (bipolare Kammer) angeordnet und die Kammern jeweils durch eine Anionenaustauscher-Membran voneinander getrennt sind.An advantageous further development of the invention is characterized in that several such modular devices are combined to form larger systems. This can be done by arranging several anode chambers and cathode chambers in an alternating sequence and separating them from each other by an anion exchanger membrane, but preferably by arranging several anode chambers and cathode chambers in an alternating sequence, each with the interposition of a chamber free of electrodes (bipolar chamber) and the chambers are separated from each other by an anion exchange membrane.

Zwei Ausführungsbeispiele der Erfindung sind schematisch in den beigefügten Zeichnungen dargestellt.

Figur 1
zeigt eine Vorrichtung mit drei aufeinanderfolgenden Elektrolytkammern,
Figur 2
zeigt eine Vorrichtung mit fünf aufeinanderfolgenden Elektrolytkammern, und
Figur 3
zeigt eine Vorrichtung mit neun aufeinanderfolgenden Elektrolytkammern.
Two embodiments of the invention are shown schematically in the accompanying drawings.
Figure 1
shows a device with three successive electrolyte chambers,
Figure 2
shows a device with five successive electrolyte chambers, and
Figure 3
shows a device with nine successive electrolyte chambers.

Alle drei Figuren zeigen die Vorrichung in einem Vertikalschnitt. Gleiche oder einander entsprechende Teile sind in den verschiedenen Figuren mit übereinstimmenden Bezugszahlen bezeichnet.All three figures show the device in a vertical section. The same or corresponding parts are identified in the different figures with the same reference numerals.

Bei der in Figur 1 dargestellten Vorrichtung ist ein Gefäß 1 durch zwei zueinander parallele Anionenaustauscher-Membranen in drei Kammern 3, 4 und 5 unterteilt. Die mittlere Kammer 4 enthält eine Silberanode 8 und als Anolyten konzentrierte Salpetersäure. Die zu beiden Seiten der mittleren Kammer 4 angeordneten Kammern 3 und 5 enthalten jeweils eine Kathode 9 aus V2A-Stahl und als Katholyten verdünnte Salpetersäure. Nach Anlegen einer Gleichspannung zwischen der Anode 8 und den Kathoden 9 geht in der Kammer 4 Silber anodisch in Lösung und eine äquivalente Menge Nitrat-Ionen wandert aus den Kammern 3 und 5 durch die beiden Anionenaustauscher-Membranen 2 in die mittlere Kammer 4. Zugleich wird eine äquivalente Menge Wasserstoff-Ionen an den Kathoden 9 entladen und entweicht als molekularer Wasserstoff.In the device shown in FIG. 1, a vessel 1 is divided into three chambers 3, 4 and 5 by two mutually parallel anion exchange membranes. The middle chamber 4 contains a silver anode 8 and nitric acid concentrated as anolyte. The chambers 3 and 5 arranged on both sides of the middle chamber 4 each contain a cathode 9 made of V2A steel and nitric acid diluted as a catholyte. After a direct voltage has been applied between the anode 8 and the cathodes 9, silver is anodically dissolved in the chamber 4 and an equivalent amount of nitrate ions migrates from the chambers 3 and 5 through the two anion exchange membranes 2 into the middle chamber 4. At the same time discharge an equivalent amount of hydrogen ions at the cathodes 9 and escape as a molecular one Hydrogen.

Die in Figur 2 dargestellte Vorrichtung unterscheidet sich von der in Figur 1 dargestellten dadurch, dass zwischen der Anodenkammer 4 und den Kathodenkammern 3 und 5 von Elektroden freie Kammern 10 und 11 (bipolare Kammern) angeordnet sind, welche die konzentrierte Salpetersäure enthalten, während die Kathodenkammern 9 und die Anodenkammer 8 jeweils verdünnte Salpetersäure enthalten. Beim Anlegen einer Gleichspannung zwischen der Anode 8 und den Kathoden 9 geht in der Anodenkammer 4 Silber anodisch in Lösung. Eine äquivalente Menge Nitrat-Ionen wandert aus den bipolaren Kammern 10 und 11 durch die Anionenaustauscher-Membranen 2 in die Anodenkammer 4 und eine äquivalente Menge Wasserstoff-Ionen wird an den Kathoden 9 entladen und entweicht als molekularer Wasserstoff.The device shown in Figure 2 differs from that shown in Figure 1 in that between the anode chamber 4 and the cathode chambers 3 and 5 of electrodes-free chambers 10 and 11 (bipolar chambers) are arranged, which contain the concentrated nitric acid, while the cathode chambers 9 and the anode chamber 8 each contain dilute nitric acid. When a direct voltage is applied between the anode 8 and the cathodes 9, silver anodically dissolves in the anode chamber 4. An equivalent amount of nitrate ions migrates from the bipolar chambers 10 and 11 through the anion exchange membranes 2 into the anode chamber 4 and an equivalent amount of hydrogen ions is discharged at the cathodes 9 and escapes as molecular hydrogen.

Zahlenbeispiel:Numerical example:

Die Anodenkammer 4 enthält ausser der Silberanode 8 15 l verdünnte Salpetersäure in einer Konzentration von 35 g HNO₃/l. Die beiden Kathodenkammern 3 und 5 enthalten ausser den Kathoden 9 jeweils 20 l verdünnte Salpetersäure in einer Konzentration von 20 g HNO₃/l. Die beiden bipolaren Kammern 10 und 11 enthalten jeweils 20 l konzentrierte Salpetersäure in einer Konzentration von 220 g HNO₃/l. Die Elektrodialyse wird bei einer Temperatur von 30° C und einer Stromdichte von 30 A/dm² durchgeführt und wird beendet, wenn die Konzentration der Silber-Ionen im Anolyten 400 g/l erreicht. Die Konzentration an Silber-Ionen im Katholyten beträgt danach weniger als 2 g/l. Die freigesetzte Menge an Stickoxiden beträgt weniger als 1/100 der Menge, die beim Arbeiten nach dem herkömmlichen Verfahren (Auflösen von Silber in heißer Salpetersäure) entstehen würde.The anode chamber 4 contains 8 15 l of dilute nitric acid in a concentration of 35 g HNO₃ / l in addition to the silver anode. The two cathode chambers 3 and 5 each contain 20 l of dilute nitric acid in a concentration of 20 g HNO₃ / l in addition to the cathodes 9. The two bipolar chambers 10 and 11 each contain 20 l of concentrated nitric acid in a concentration of 220 g HNO₃ / l. Electrodialysis is carried out at a temperature of 30 ° C and a current density of 30 A / dm² and is terminated when the concentration of silver ions in the anolyte reaches 400 g / l. The concentration of silver ions in the catholyte is then less than 2 g / l. The amount of nitrogen oxides released is less than 1/100 of the amount that would result from working with the conventional method (dissolving silver in hot nitric acid).

Nach dem Abschluss der Elektrodialyse wird der Anolyt entnommen, um daraus das Silbernitrat zu gewinnen. Die Gewinnung des Silbernitrates kann auf herkömmliche Weise geschehen, z.B. durch Eindampfen.After the electrodialysis is complete, the anolyte is removed in order to obtain the silver nitrate. The silver nitrate can be obtained in a conventional manner, e.g. by evaporation.

Die in Figur 3 dargestellte Vorrichtung unterscheidet sich von der in Figur 2 darin, dass sie zusätzlich eine weitere Anodenkammer 14, eine weitere Kathodenkammer 13 und zwei weitere bipolare Kammern 20, 21 enthält, so dass mit dieser Anordnung die doppelte Menge Silbernitrat erzeugt werden kann wie in der Vorrichtung gemäß Figur 2. Figur 3 zeigt, dass durch Einfügen weiterer solcher aus einer Anodenkammer, einer Kathodenkammer und zwei bipolaren Kammern gebildete Module größere Anlagen entsprechend der gewünschten Produktionskapazität aufgebaut werden können.The device shown in FIG. 3 differs from that in FIG. 2 in that it additionally contains a further anode chamber 14, a further cathode chamber 13 and two further bipolar chambers 20, 21, so that double the amount of silver nitrate can be produced with this arrangement in the device according to FIG. 2. FIG. 3 shows that by inserting further modules formed from an anode chamber, a cathode chamber and two bipolar chambers, larger systems can be built up according to the desired production capacity.

Claims (14)

  1. Method of producing silver nitrate by dissolving silver in nitric acid, characterized in that diluted nitric acid and nitric acid with a higher concentration are separated by an anion exchange membrane, the silver is introduced into the diluted nitric acid anodically poled and a silver-free cathode is immersed into the nitric acid with the higher concentration.
  2. Method of producing silver nitrate by dissolving silver in nitric acid, characterized in that nitric acid with a higher concentration is separated from a first volume of diluted nitric acid by a first anion exchange membrane and from a second volume of nitric acid by a second anion exchange membrane, that the silver is immersed into the diluted nitric acid in the first volume anodically poled and a silver-free cathode is immersed into the nitric acid in the second volume, the current flow from the cathode to the anode in each case taking place completely through the nitric acid with the higher concentration.
  3. Method as claimed in Claim 2, characterized in that the nitric acid in said second volume is from the outset diluted nitric acid (i.e. its concentration is lower than that of the nitric acid with the higher concentration, into which no electrode is immersed).
  4. Method as claimed in Claim 1, 2 or 3, characterized in that after the removal of the diluted nitric acid enriched with silver nitrate, the nitric acid with the initially higher concentration which has dropped in the meantime, is reused for forming the anolyte and replaced with fresh, concentrated nitric acid.
  5. Method as claimed in any one of the preceding claims, characterized in that the process is carried out with current densities of 5 to 50 A/dm³ at a temperature between 5°C and 60°C.
  6. Method as claimed in any one of the preceding claims, characterized in that the electrodialysis is completed as soon as the concentration of silver ions in the catholyte exceeds 2 g/l.
  7. Method as claimed in any one of Claims 2 to 6, characterized in that the electrodialysis is completed as soon as the concentration of silver ions in the anolyte exceeds 400 g/l.
  8. Method as claimed in any one of the preceding claims, characterized in that an anolyte containing 25 to 100 g HNO₃/l is used.
  9. Method as claimed in any one of Claims 2 to 8, characterized in that a catholyte containing 10 to 100 g HNO /l is used.
  10. Use of an apparatus for producing a metal salt by dissolving the metal in acid, comprising
       a cathode chamber (3, 13) including a cathode (9) not containing the metal to be dissolved,
       an anode chamber (4, 14) including an anode (8) consisting of the metal to be dissolved,
       and at least one anion exchange membrane (2) separating the cathode chamber (3, 13) from the anode chamber (4, 14),
       with the proviso that silver is used as metal and nitric acid is used as acid, the concentration of the nitric acid in the anode chamber (4, 14) being lower than in the adjacent chamber (3, 4; 10, 11; 20, 21).
  11. Use of an apparatus as claimed in Claim 10, wherein the cathode chamber (3, 5, 13) and the anode chamber (4, 14) are separated by two anion exchange membranes (2) defining between them an electrode-free chamber (10, 11; 20, 21) (hereinforth referred to as bipolar chamber), in which the concentration of the nitric acid is higher than in the anode chamber (4, 14).
  12. Use of an apparatus as claimed in Claim 11, wherein the concentration of the nitric acid in the bipolar chamber (10, 11; 20, 21) is also higher than in the cathode chamber (3, 5, 13).
  13. Use of an apparatus as claimed in Claim 10, wherein several anode chambers (4, 14) and cathode chambers (3, 5, 13) are arranged in an alternating sequence and are separated from each other by an anion exchange membrane (2).
  14. Use of an apparatus as claimed in Claim 11 or 12, wherein several anode chambers (4, 14) and cathode chambers (3, 5, 13) are arranged in alternating sequence each with a bipolar chamber (10, 11; 20, 21) inserted therebetween and each separated from each other by an anion exchange membrane (2).
EP91100949A 1990-03-03 1991-01-25 Process and apparatus for the production of silver nitrate Expired - Lifetime EP0445516B1 (en)

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DE4006764A DE4006764A1 (en) 1990-03-03 1990-03-03 METHOD AND DEVICE FOR PRODUCING SILVER NITRATE
DE4006764 1990-03-03

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CN104789982A (en) * 2015-04-09 2015-07-22 上海应用技术学院 Method for preparing silver molybdate with cationic membrane electrolysis method

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AU1403200A (en) * 1998-12-11 2000-07-03 Combrink, De Wet Francois Production of silver salts
CN114959734A (en) * 2021-02-24 2022-08-30 中国科学院上海硅酸盐研究所 Device and method for efficiently preparing hydrogen and silver compounds

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