WO2000036185A2

WO2000036185A2 - Electrolytic production of silver salts

Info

Publication number: WO2000036185A2
Application number: PCT/IB1999/001961
Authority: WO
Inventors: Roger Leslie Paul
Original assignee: Combrink, De Wet, Francois
Priority date: 1998-12-11
Filing date: 1999-12-09
Publication date: 2000-06-22
Also published as: WO2000036185A3; AU1403200A

Abstract

A process (10) for producing silver nitrate includes treating elemental silver in at least one electrolytic cell (12) which has an electrolyte which includes nitrate ions, thereby producing silver nitrate in solution in the electrolyte. The invention extends to processes for producing silver cyanide and potassium silver cyanide.

Description

PRODUCTION OF SILVER SALTS

THIS INVENTION relates to the production of silver salts. In particular, it relates to a process for producing silver nitrate, to a process for producing silver cyanide, and to a process for producing potassium silver cyanide.

According to a first aspect of the invention, there is provided a process for producing silver nitrate, which includes treating elemental silver in at least one electrolytic cell which has an electrolyte which includes nitrate ions, thereby producing silver nitrate in solution in the electrolyte.

Typically, the electrolytic cell includes an ion exchange membrane separating its anode and its cathode, and thus separating the electrolyte into an anolyte and a catholyte. During the process, Ag ⁺ ions are formed at the anode. At least the catholyte typically includes nitrate ions. However, both the anolyte and the catholyte may include nitrate ions. The ion exchange membrane thus prevents or at least inhibits migration of silver ions from the anode to the cathode, but allows migration of nitrate ions from the cathode to the anode to provide an anolyte comprising dissolved silver nitrate.

The catholyte may be an aqueous nitric acid solution. The catholyte may have a nitric acid concentration of between 3% by mass and 1 8% by mass. Preferably, the nitric acid concentration of the catholyte is between 5% by mass and 1 2% by mass. Typically the nitric acid concentration of the catholyte is between 6% by mass and 1 0% by mass, e.g . 7% by mass.

As mentioned above, the anolyte may also include nitrate ions. Thus, the anolyte may also be an aqueous nitric acid solution. The anolyte may have a nitric acid concentration of between 0,2% by mass and 6% by mass. Preferably, the nitric acid concentration of the anolyte is between 0,5 % by mass and 4 % by mass. Typically, the nitric acid concentration of the anolyte is between 0,8% by mass and 2% by mass, e.g. 1 % by mass.

The ion exchange membrane may be any general purpose anionic exchange membrane such as those commonly used in electrolytic cells for the desalination of water. Examples produced by Ashai Glass Company and sold under the trade name "Selemion" include catalogue names AMT, AMV, ASV, AAV and DSV. An example supplied by Ionic is type MA3475.

Preferably, depending on production capacity requirements, more than one electrolytic cell, e.g . two or three, are used . The electrolytic cells may be arranged in parallel in respect of flow of reactants and products through the process, but may be in series electrically.

The process is typically a batch process and may include, once the electrolysis of the silver has proceeded to a desired extent, withdrawing the anolyte, which includes silver ions and nitrate ions in solution, and neutralising any excess nitric acid . The excess nitric acid in the anolyte may be neutralised by adjusting the pH of the anolyte upwards with a basic medium, e.g. sodium hydroxide or potassium hydroxide. Preferably, the pH of the anolyte is adjusted up to a maximum of eight, to prevent the undesirable formation of Ag₂O. More preferably, the pH of the anolyte is adjusted to be between 6 and 8. The concentration of dissolved silver ions in the anolyte may be between 30g/ and 300g/ f . Preferably, the concentration of dissolved silver ions in the anolyte is between 80g/ and 200g/ , e.g . 1 08g/£ .

The method may include recovering silver nitrate from the anolyte by evaporation or crystallisation followed by filtration.

The process may include recovering any silver which may be present as silver ions in the catholyte, e.g. by means of a cementation stage or an electro-winning stage. The cementation stage may include passing the catholyte through a bed of zinc dust or may include employing steel wool to recover the silver. The electro-winning stage may include passing the catholyte through a perforated expanded graphite cathode to reduce the Ag ⁺ ions to elemental silver, using a titanium anode.

According to a second aspect of the invention, there is provided a process for producing silver cyanide, the process including electrolytically producing silver nitrate in solution as hereinbefore described; and reacting, in solution, the dissolved silver nitrate with an alkali metal cyanide to form silver cyanide.

The process may include agitating the solution while the alkali metal cyanide is being added and until the reaction between the silver nitrate and the alkali metal cyanide has been completed.

The alkali metal cyanide may be sodium cyanide or it may be potassium cyanide. When it is intended to produce silver cyanide only, i.e. when it is not intended to convert the silver cyanide to potassium silver cyanide, the alkali metal cyanide is preferably sodium cyanide. However, if it is intended to convert some or all of the silver cyanide to potassium silver cyanide, the alkali metal cyanide is preferably potassium cyanide. The process may include separating the silver cyanide from the solution, e.g. by precipitation and/or filtration, to provide a nitrate containing waste stream.

The process may include treating the nitrate containing waste stream, to destroy any cyanide which may be present therein. Treating the nitrate containing waste stream may include adding calcium hypochlorite to the waste stream to convert any cyanide which may be present to ammonia, carbon dioxide and nitrogen.

The process may include recovering any silver which may be present in the nitrate containing waste stream prior to treating the waste stream to destroy any cyanide which my be present therein, e.g. by means of a cementation stage or an electro-winning stage as hereinbefore described .

The process may further include washing the silver cyanide, e.g. with demineralised water in a stirred tank reactor, and filtering the silver cyanide out of the water.

The wet silver cyanide may be dried and packed in conventional fashion.

According to a third aspect of the invention, there is provided a process for producing potassium silver cyanide, the process including producing silver cyanide as hereinbefore described; and reacting, in an aqueous medium in a reaction zone, the silver cyanide with potassium cyanide to form dissolved potassium silver cyanide.

The process may include recovering the dissolved potassium silver cyanide from the aqueous medium, e.g. by crystallization. If desired, crystallising the dissolved potassium silver cyanide may be preceded by evaporating some of the aqueous medium. Preferably, the dissolved potassium silver cyanide is crystallised at a maximum temperature of 0°C. More preferably, the maximum temperature is -4°C, e.g . -5 °C. If required, the aqueous medium may be filtered to remove any unreacted silver cyanide prior to crystallising the dissolved potassium silver cyanide.

Recovering the potassium silver cyanide may also include filtering the potassium silver cyanide crystals, once formed, from the aqueous medium to produce a filtrate, e.g. with a pan vacuum filter. The filtrate may be returned to the reaction zone to be used as the aqueous medium in which further silver cyanide is reacted with potassium cyanide.

Periodically, some of the filtrate or aqueous medium may be bled off to recover any silver present therefrom, e.g . by means of a cementation stage or an electro-winning stage as hereinbefore described, and to remove impurities which may build up in the aqueous medium. The filtrate or aqueous medium bled off may be treated to destroy any cyanide present, e.g. with calcium hypochlorite, before passing it to effluent.

The process may include purifying the potassium silver cyanide crystals produced. Purifying the potassium silver cyanide crystals may include washing the crystals with methanol. Purifying the potassium silver cyanide may further include redissolving the potassium silver cyanide crystals in hot demineralised water, and recrystallising the potassium silver cyanide as hereinbefore described . The hot demineralised water may be at a temperature of at least 60 °C. Purifying the potassium silver cyanide crystals may further include once again washing the potassium silver cyanide crystals with methanol.

The process may include drying the potassium silver cyanide crystals in a vacuum oven or the like, and packing it in conventional manner. It is to be appreciated that washing the crystals in methanol also advantageously shortens the drying time of the crystals. The invention will now be described, by way of example, with reference to the single accompanying diagrammatic diagram showing a simplified flow diagram of a process in accordance with the invention for producing silver nitrate, silver cyanide and potassium silver cyanide.

Referring to the drawing, reference numeral 10 generally indicates a batch process in accordance with the invention for producing silver nitrate, silver cyanide and potassium silver cyanide.

The process 1 0 includes three electrolytic cells arranged parallel in respect of the flow of reagents and products in the process, but in series electrically, only one electrolytic cell 1 2 being shown in the drawing. The electrolytic cell 1 2 includes an anode compartment 14 and a cathode compartment 1 6 separated by an ion exchange membrane 1 8, which is an Ashai DSV membrane. The anode compartment 14 is provided with an anolyte circulation line 20. The cathode compartment 1 6 is provided with a catholyte circulation line 22. A 2500£ recirculation tank

(not shown) is provided for each of the circulation lines 20, 22, each recirculation tank servicing all three electrolytic cells 1 2. The cathode compartment 1 6 is also provided with a catholyte disposal line 24 and a hydrogen vent 27. The anode compartment 14 is provided with an anolyte drain line 26 leading into a stirred tank neutraliser 28. A potassium hydroxide feed line 30 leads into the neutraliser 28 and a neutralised anolyte line 32 leads from the neutraliser to a precipitator 34.

A potassium cyanide feed line 36 leads to the precipitator 34, and a precipitated silver cyanide line 38 leads from the precipitator 34 to a pan vacuum filter 40. A potassium nitrate solution disposal line 42 leads from the filter 40, and reference numeral 44 refers to a silver cyanide product produced by the process 1 0 of the invention. The process 1 0 further includes a leach tank or reactor 46 into which a potassium cyanide feed line 48 leads. A potassium silver cyanide solution line 50 leads from the reactor 46 to a crystalliser 52 and from the crystalliser 52 to a pan vacuum filter 54. A filtrate line 56 returns from the filter 54 to the reactor 46. A bleed line 57 branches off from the filtrate line 56.

The process 1 0 further includes a second crystalliser 62 connected by a flow line 64 to a pan vacuum filter 66 and by a flow line 60 to the pan vacuum filter 54. A demineralised water feed line 68 leads into the second crystalliser 62. A filtrate line 70 leads from the filter 54 to the reactor 46. Reference numeral 72 indicates a potassium silver cyanide product produced by the process 1 0.

In use, a charge of elemental silver, weighing about 201 ,6kg, in the form of silver granules, is divided up into three equal portions and placed in the anode compartments 1 4 of each electrolytic cell 1 2. Each anode compartment 14 and the anolyte recirculation tank are charged with a 1 % aqueous nitric acid solution as anolyte, the total volume of the anolyte being about 1 865 . The anolyte in each anode compartment 14 is circulated by means of the anolyte circulation line 20, anolyte recirculation tank and a pump (not shown) .

Each cathode compartment 1 6 and the catholyte recirculation tank are charged with a 7 % aqueous nitric acid solution as catholyte, the total volume of the catholyte being about 1 780£ , which makes provision for about 5% excess catholyte. The catholyte in each cathode compartment 1 6 is circulated through the cathode compartment 1 6 by means of the catholyte circulation line 22, catholyte recirculation tank and a pump (not shown) . A direct current of about 1 000A, at a potential difference of about 9V (3V per electrolytic cell 1 2) , is provided by a rectifier (not shown) and passed in series through each electrolytic cell 1 2 using titanium or platinised titanium anodes and cathodes (not shown) . The following reactions take place in each anode compartment 14 and each cathode compartment 1 6 respectively:

Anode: Ag → Ag ⁺ (l) + e

Cathode: H + ^"•^" (!) + e → ¹/₂ H₂(g)

The hydrogen formed in the cathode compartments 1 6 is taken off or vented as hydrogen gas through the hydrogen vents 27 and in order to maintain the electrochemical balance, negatively charged nitrate ions migrate from the cathode compartment 1 6 to the anode compartment 14 through the membrane 1 8, as indicated by arrow 63. Thus, the membrane 1 8 prevents or at least inhibits migration of silver ions from the anode compartment 1 4 to the cathode compartment 1 6, but allows migration of nitrate ions from the cathode compartment 1 6 to the anode compartment 14.

At a cell current of about 1 000A the rate of oxidation of elemental silver is approcimately 1 2 kg per hour. Regular addition of elemental silver is made to the three cells 1 2 to maintain the elemental silver inventory roughly constant. Once the concentration of dissolved silver in the anode compartment 14 has attained a concentration of 1 08 g per liter, the acidic anolyte is transferred via the anolyte drain line 26 to the neutraliser 28, and the catholyte is readjusted to 7% nitric acid strength by adding concentrated nitric acid . The catholyte is periodically drained via the catholyte disposal line 24 for further treatment and disposal, as described in more detail hereunder. Enough potassium hydroxide, in the form of an aqueous solution, is added to the neutraliser 28 by means of the potassium hydroxide feed line 30, under control of a pH controller (not shown), until the pH of the anolyte in the neutraliser 28 has been adjusted upwards so that it is between 6 and 8.

Silver nitrate has thus been produced in a neutralised solution by the process 1 0, and if desired, the silver nitrate may be recovered from the neutralised solution by, e.g . crystallisation or evaporation followed by pan filtration. However, in the process 10, as illustrated, the neutralised anolyte or solution, containing about 1 08g silver ions per litre of anolyte, is transferred from the neutraliser 28 to the precipitator 34 through the neutralised anolyte line 32. About 1 1 6 kg of potassium cyanide is required for the precipitation of the dissolved silver. Potassium cyanide is fed into the precipitator 34 through the potassium cyanide feed line 36 at a rate of about 2 kg per minute, while monitoring the voltage of a silver ion-sensitive electrode (not shown) . A rapid decrease in voltage of about 0,2 V indicates that the precipitation of silver is complete, and the addition of potassium cyanide is stopped . Silver cyanide is formed inside the precipitator 34 and due to its extremely low solubility in water the silver cyanide precipitates out. The solution in the precipitator 34 is agitated by means of an agitator (not shown) until the reaction in the precipitator 34 has reached completion.

The solution and precipitated silver cyanide is transferred from the precipitator to the filter 40 through the precipitated silver cyanide line 38 to separate the precipitated silver cyanide from the solution, which also contains dissolved potassium nitrate. The potassium nitrate containing solution or filtrate is removed through the potassium nitrate solution disposal line 42 for further treatment and disposal, as described in more detail below. If desired, the silver cyanide recovered from the filter 40, as indicated by reference numeral 44, can be washed in a stirred tank reactor (not shown) with demineralised water and re-filtered until all trace elements have been removed to yield a product of desired quality. The silver cyanide is then dried and packed in commercial driers and packers in a dust-free pressurised dark (i.e. UV filtered) environment, since silver cyanide is light sensitive.

The table below gives an analysis of the purity of the silver cyanide produced by the process, compared to industry requirements.

If it is desired to produce potassium silver cyanide, as in the process 1 0, a desired quantity of the silver cyanide is weighed and added into the reactor 46 as indicated by arrow 45. The reactor 46 is filled with demineralised water to the required level. About 1 00 of demineralised water is required for 1 00kg of silver cyanide.

A slightly less than stoichiometric quantity of potassium cyanide is added to the reactor 46 through the potassium cyanide feed line 48, and the silver cyanide is allowed to react with the potassium cyanide to form potassium silver cyanide in solution. The reaction is allowed to reach completion, which is established by visually inspecting that all the reagents have dissolved, and the solution is then transferred from the reactor 46 to the crystalliser 52 by means of the potassium silver cyanide solution line 50. If required, the solution can be passed through an in-line filter (not shown) between the reactor 46 and the crystalliser 52, to ensure that no silver cyanide ends up in the crystalliser 52. The temperature of the solution in the crystalliser 52 is reduced to -5 °C by means of a chiller plant (not shown) to allow the solution to become supersaturated with potassium silver cyanide and to form potassium silver cyanide crystals. The solution is then passed through the filter 54 by means of the potassium silver cyanide solution line 50 to separate the potassium silver cyanide crystals from the solution.

A filtrate is returned from the filter 54 to the reactor 46 by means of the filtrate return line 56, to be used for a next batch of potassium silver cyanide to be produced, instead of demineralised water. Periodically, some of the filtrate is bled off through the bleed line 57 to remove any impurities which have built up in the filtrate.

The potassium silver cyanide separated by the filter 54 is purified by manually washing the crystals with methanol on the filter pan. The crystals are then placed inside the second crystalliser 62 as indicated by reference numeral 60, and the minimum quantity of demineralised water at a temperature of 60°C is added to the crystalliser 62 by means of the water feed line 68. The solution inside the crystalliser 62 is then agitated until all the potassium silver cyanide has dissolved, whereafter the crystals are recrystallised in the crystalliser 62. The potassium silver cyanide is again filtered out of the solution by means of the filter 66 and a filtrate is returned to the reactor 46 by means of the filtrate line 70. The potassium silver cyanide crystals on the filter pan are manually washed again in methanol, whereafter it is dried in a vacuum oven (not shown) to produce the potassium silver cyanide product as shown by reference numeral 72.

The potassium nitrate containing solution from the potassium nitrate solution disposal line 42, the catholyte from the catholyte disposal line 24 and the filtrate from the bleed line 57 are treated in an effluent treatment plant (not shown) .

Any silver ions present in the catholyte periodically removed by means of the catholyte disposal line 24 is recovered in the effluent treatment plant by a cementation stage or an electro-winning stage whereafter the catholyte is neutralised with a cheap base, e.g . limestone and to then put to drain.

The potassium nitrate containing solution removed from the filter 40 by means of the potassium nitrate solution disposal line 42 is similarly treated to recover any silver present by means of the cementation stage or electro-winning stage, and is then treated by adding calcium hypochlorite to convert any cyanide to ammonia, carbon dioxide and nitrogen, whereafter it is put to drain.

The filtrate bled off through the bleed line 57 is treated to recover any silver present using the cementation stage or electro-winning stage, and is then treated with calcium hypochlorite to destroy any cyanide present before it is put to drain.

The cementation stage, if present, typically includes a bed of zinc dust through which the catholyte or solution is passed or may include a stainless steel wool which is used to recover the silver. The electro- winning stage, if present, typically includes a perforated expanded graphite cathode through which the catholyte or solution is passed to reduce the Ag ⁺ ions to elemental silver, using a titanium anode.

It is an advantage of the process 1 0 of the invention, as illustrated, that no NOx is produced during the production of the silver nitrate.

It is a further advantage of the process 1 0 of the invention, as illustrated, that relatively small quantities of dilute nitric acid are required compared to a conventional silver oxidation process using oxygen and strong nitric acid.

It is a further advantage of the process 1 0, as illustrated, that the electrolytic cells are connected electrically in series, thus reducing the rating requirements of the rectifier.

Claims

CLAIMS:

1 . A process for producing silver nitrate, which includes treating elemental silver in at least one electrolytic cell which has an electrolyte which includes nitrate ions, thereby producing silver nitrate in solution in the electrolyte.

2. A process as claimed in claim 1 , in which the electrolytic cell includes an ion exchange membrane separating its anode and its cathode and separating the electrolyte into an anolyte and a catholyte, at least the catholyte including nitrate ions.

3. A process as claimed in claim 2, in which both the anolyte and the catholyte include nitrate ions.

4. A process as claimed in claim 3, in which the anolyte is an aqueous nitric acid solution having a nitric acid concentration of between 0,2 % by mass and 6 % by mass.

5. A process as claimed in claim 4, in which the nitric acid concentration of the anolyte is between 0,8 % by mass and 2 % by mass.

6. A process as claimed in any one of claims 2 to 5 inclusive, in which the catholyte is an aqueous nitric acid solution having a nitric acid concentration of between 3 % by mass and 1 8 % mass.

7. A process as claimed in claim 6, in which the nitric acid concentration of the catholyte is between 6 % by mass and 1 0 % by mass.

8. A process as claimed in any one of the preceding claims, which includes using at least two electrolytic cells, the electrolytic cells being arranged in parallel in respect of flow of reactants and products through the process but in series electrically.

9. A process as claimed in any one of the preceding claims, which is a batch process, and in which, once dissolved silver ions have reached a desired concentration in the anolyte, the anolyte, which includes the dissolved silver ions and dissolved nitrate ions, is withdrawn from the electrolytic cell and any excess nitric acid is neutralised.

10. A process as claimed in claim 9, in which any excess nitric acid is neutralised with a basic medium, so that the anolyte has a pH of between 6 and 8.

1 1 . A process as claimed in claim 9 or claim 10, in which the desired concentration of dissolved silver ions in the anolyte is between 30 g/£ and 300 g/ϊ .

1 2. A process as claimed in claim 1 1 , in which the desired concentration of dissolved silver ions in the anolyte is between 80 git and

200 git .

1 3. A process as claimed in any one of the preceding claims, which includes recovering any silver which may be present as silver ions in the catholyte.

1 4. A process for producing silver cyanide, the process including electrolytically producing silver nitrate in solution as claimed in any one of claims 1 to 1 3 inclusive; and reacting, in solution, the dissolved silver nitrate with an alkali metal cyanide to form silver cyanide.

1 5. A process as claimed in claim 1 4, in which the alkali metal cyanide is potassium cyanide.

1 6. A process as claimed in claim 14, in which the alkali metal cyanide is sodium cyanide.

1 7. A process as claimed in claim 1 4 or claim 1 5 or claim 1 6, which includes separating the silver cyanide from the solution, to provide a nitrate containing waste stream, and treating the nitrate containing waste stream to destroy any cyanide which may be present therein.

1 8. A process as claimed in claim 1 7, in which the nitrate containing waste stream is treated by the addition of calcium hypochlorite to the waste stream, thereby converting any cyanide which may be present to ammonia, carbon dioxide and nitrogen.

1 9. A process as claimed in claim 1 7 or claim 1 8, which includes recovering any silver which may be present in the nitrate containing waste stream, prior to treating the waste stream to destroy any cyanide which may be present therein.

20. A process for producing potassium silver cyanide, the process including producing silver cyanide as claimed in any one of claims 14 to 1 9 inclusive; and reacting, in an aqueous medium in a reaction zone, the silver cyanide with potassium cyanide to form dissolved potassium silver cyanide.

21 . A process as claimed in claim 20, in which the dissolved potassium silver cyanide is recovered from the aqueous medium by crystallization.

22. A process as claimed in claim 21 , in which the aqueous medium is filtered to remove any unreacted silver cyanide which may be present prior to crystallising the dissolved potassium silver cyanide, and in which the dissolved potassium silver cyanide is crystallised at a maximum temperature of 0 °C.

23. A process as claimed in claim 21 or claim 22, in which the potassium silver cyanide crystals, once formed, are filtered from the aqueous medium to produce a filtrate, the filtrate being returned to the reaction zone to be used as the aqueous medium in which further silver cyanide is reacted with potassium cyanide.

24. A process as claimed in any one of claims 21 to 23 inclusive, in which the potassium silver cyanide crystals produced are purified by washing them with methanol.

25. A process as claimed in claim 24, which includes redissolving the potassium silver cyanide crystals in hot demineralised water, and recrystalling the potassium silver cyanide.

26. A process for producing silver nitrate as claimed in claim 1 , substantially as herein described and illustrated .

27. A process for producing silver cyanide as claimed in claim 14, substantially as herein described and illustrated .

28. A process for producing potassium silver cyanide as claimed in claim 20, substantially as herein described and illustrated .

29. A new process for producing silver nitrate or silver cyanide or potassium silver cyanide, substantially as herein described.