Silver Cyanide Solution Turning Dark Brown
We are a decorative silver plating shop servicing the Hotel banquetware industry. We use a type III cyanide silver solution with an organic brightener system. The bath is 750 gallons, We use continuous filtration through a 1 micron bag filter at 60 gpm. We Carbon treat the solution 8 hrs. every two weeks.The solution is 5 years old with the following concentrations:
Silver metal 4.3 oz/gal
Free Sodium cyanide 10.5 oz/gal
Sodium carbonate 8.5 oz/gal
Operating temp 72 f
Issue- The bath has turned a dark brown in color, the brightness has dropped, the tendency to burn has increased. (when we add cyanide the burning increases).We have tried micro-filtration in the lab to clear up the solution (supplier says it is carbon dust) but that did not clear it up.Steve P
- San Diego, California, USA
Check the pH of your solution. It should be above 12. Add sodium or potassium hydroxide if the pH is low.
Silver solution of 750 gallon is a bigger volume, why don't you take out 1 litre solution and filter it manually on Whatman filter and check, if the solution gets cleared off then it means its contaminated with some solid particles which require more adequate filtration, also check in hull cell by adding more sodium cyanide and addition of brightener , I am sure it will solve your brightness and burning problem, silver metal - 30 gms/ltr, sodium cyanide--120 gms/ltr, carbonate 20- 25 gms /ltr is best combination.
I am on my way to check the pH, As far as hull cells go, we have run 100's.We are running them at 1 amp for 10 min. Adding cyanide blows out the top end, and it requires a 5x make-up addition of brightener to clean up 50% of the of the panel(high end) . The only way to get a good panel is to cut the sample by at least 50%, add back the silver and cyanide and bring in a ton of brightener.We have tried many techniques in the lab to try and filter out the "Dark Color" but I believe the solution to be free of particulate. We are in the process of freezing out some carbonate to see if that improves performance and or color.
I appreciate the responses and look forward to any more suggestions.Steve P
Metal Finishing - San Diego, California
It would be nice to save this big old $25,000 workhorse.
Here's a few more ideas:
1)What have you been using to treat your carbonates? If you use nitrates, a 5 year build up might be the problem. I know of no way to remove nitrates other than dilution, with subsequent build up of the other ingredients. You might try some Hull cell panels, using some dilution and additions.
If you use hydroxides, accurately test the pH. Use closely bracketing pH standards. I found an interesting page in an old British analysis book, concerning the Hull cell testing of conventional silver cyanide solutions that were similar to your formulation. The make-up pH was 11.5. At a pH of 13, the Hull cell showed rough, burnt deposits, over most of the panel. I know of no way of lowering the pH without killing everybody. Maybe someone else could give us a safe way of doing this. Maybe some sort of simple electrodialysis membrane could be used safely. To see if the pH is the problem, you might adjust a sample to pH 11.5, UNDER A FUME HOOD, with some sort of compatible acid - if there is any. Heat the solution up a little and stir for a few hours, in order to drive off the dissolved HCN. Cool and perform Hull cell tests under the hood.
2) I've suspected all along that your problem is organic breakdown products from 5 years of brightener additions and from such things as rack coatings. The dark color seems to be the key. I never believed in the "carbon dust" theory. I assume, since you are measuring your "carbon treatment" in hours, that you are either packing a filter with carbon or using carbon cartridges. This may not be enough. As I understand it, carbon filters only pick up short-chain organics and leave the longer-chain varieties behind. These build up and can eventually ruin the deposit, in various ways. You may need more power in your organics removal.
The only way to fix your bath may be a traditional, hot, peroxide, carbon treatment. Carbon treating precious metals baths is only done as a last resort, because about 10% of the silver or gold gets hung up with the carbon and must be sent out for refining. Try it on, say,
1000 mL samples in the lab, first. Make the first trial a little strong, just to see if it works. You might:
A) Heat 1000 mL of analyzed solution to 120 deg F.
B) With stirring, add about 3-5 mL of 30% (100 volume) hydrogen peroxide. Check the temperature.
C) Maintain temperature at 120 and stir for 1 hr.
D) Add about 2.5 grams of powdered activated carbon.
E) Stir and maintain temp. for 1 hour, or more.
F) With stirring, add about a gram of filter aid. Diatomateous earth works OK.
G) Cool and filter well
H) Analyze and adjust silver and cyanide. Analyze carbonates and compare. Analyze brighteners, if you can.
I) Run Hull cell panels and adjust brightener(s).
J) If this works, repeat with more or less temperatures, concentrations, and times, if you wish. The more heat and peroxide, the more cyanide will break down to carbonates. The more carbon you use, the more silver it will adsorb.
If this works and, if you decide to go full scale, you have to do it right. You HAVE to do it in a separate treatment tank. The treatment tank should be located away from the plating tank, since even traces of the easily airborne carbon dust in the plating tank can cause roughness problems, sometimes long term. After treating, the filter must remove all traces of carbon before the solution is returned to the plating tank. To make filtering easier, let everything settle overnight and then start filtering near the top. Make sure everything is ready before you start.
According to one of my books, the chilling out of sodium carbonate, at 3 deg C, won't lower it below 8 oz/gal. You're only at 8.5 now - why go to the expense?. Did you prove this in the Hull cell? If you can afford it (and, find it), barium cyanide is the best way to remove carbonates. Maybe you'll find it's time to retire the old gal.Chris Owen
- Houston, Texas
I really appreciate you taking the time to give such a detailed answer.The pH issue seems to be of debate. Pat had recommended raising the PH over 12 where as your info is to lower to 11.5. The pH of the solution is @ 12.7. Raising it from there did not alter the Hull cell. It seems to me that it may be possible to safely lower the PH with Sulfamic or sulfonic acid. Has anyone tried this ? As for dilution, That, of course, improves the panel but not the color. Working with such a large volume makes any treatment very costly. After the peroxide, carbon treatment, we are able to filter out the "Dark Color". Ending color is a fairly bright orange, not the straw color I would have expected to see. Has anyone heard of "bad " cyanide turning the solution orange ? Anyways, Even after the peroxide treatment and the color lightening up, the Hull cell panels still did not improve to much degree. Still burning half the panel at .5 amps.
In talking with my brightener supplier, They now tell me that the "organic" brightener system has Selenium in it. Could it be that the selenium has built up to a very high level and is the bad guy ? I am getting very discouraged with my suppliers tech support. Feel like I am on an island.
We will keep chuggin along...the great thing about plating is you never stop learning.......
Thank you all for any and all of your advice.Stephen P
Metal Finishing - San Diego, California
It's 20 years since I last did any silver plating, and the memory is fading fast, I used to keep the carbonate low and control the hydroxide(I can't remember at what strength) as this gave a harder deposit and better contact resistance, but as the carbonate level went up,the solution would go darker but not brown, and I got your problems of sluggish plating, if I raised the voltage to compensate it would burn, To reduce the carbonate, I added slaked lime, Ca(OH)2 This precipitated calcium carbonate and put hydroxide back. When the lime is added it needs vigorous stirring to be effective.
Keep chuggin along.Steve K
hard chrome plating - Cheshire, UK
There may not be any way to lower the pH safely. No matter what the source of hydrogen ion, acid or otherwise, HCN gas will still form. A big problem is that when the HCN gas forms, it is not eliminated immediately. It dissolves somewhat in the solution and is slowly expelled. Heat, stirring, and a fan across the top speed it up, but who's to say when the HCN is gone. You may be able to lower the pH by running stainless anodes for a while. Of course, you will waste silver. Being insoluble, the anode would split water, giving off oxygen gas and feeding hydrogen ion into the solution, thus hopefully lowering the pH. I still think that HCN gas will build up in the solution. Another problem might be that, whatever method you use, the hydrogen would preferentially combine with with the cyanide instead of lowering the pH - which would be bad.
From your work, it seems that the dark color was produced by organic contamination. The way I interpret your post, this helped a little. Maybe the CT still wasn't strong enough. Before you dismiss CT completely, you might run another test - doubling up on the CT chemicals and times and heating to 150 deg F.
I notice, in the CRC Handbook of Chemistry & Physics [link is to info about book at Amazon], that most selenium compounds are yellow, red, or orange. Coincidence? Maybe the orange color has been there for a while, but was masked by the dark color. Maybe the color was produced by oxidation of the Se, during CT, and the subsequent formation of different selenium compounds. High selenium could very well be the culprit. This may be why the dilution was successful.
It doesn't sound like you're getting enough help from your supplier. With a 750 gal. tank, in one shift you're using about 1000+ tr ozs of silver a month and lots of hi-dollar brightener. Plus, you have a little refining done. You deserve a lot more attention. I would strongly suggest you contact some other silver bath suppliers, such as Technic or Enthone. Lea Ronal is another one. It's good to get at least 2 fresh opinions. I think that the Enthone Silverex 400 still uses antimony as a brightener, which produces a little harder deposit than the selenium. I don't know what Technic uses, but I know that their products are excellent - I have used many of them, but I haven't tried their silver. Tell them about all your problems and your attempts and see what they say. Tell them that you want to "convert" your present bath to their brightener system. Get them a sample, somehow quickly, and the chances are excellent that one of them can solve your problem. These big companies have big customer service labs with personnel capable of finding out anything. This way, you will know where you stand. You might also find that one of these companies will give you such a good deal, in installing a brand new bath, that you can't refuse. At least, you'd be back in business. Keep plugging. There's always an answer.Chris Owen
- Houston, Texas
I have a couple of questions. What are you using for anodes? Are you using steel anode baskets? Are you using a silver strike before this bath? any possibility of dragging in contamination from your strike bath?
The pH is certainly high enough now, but was it ever lower than 11? If it was lower and you used steel anode baskets then possibly the black color is ferricyanide.
I also agree there is no safe way to lower the pH.
As for selenium, it is a common brightener for silver is effective at concentrations of less than 5 ppm. Your supplier should be able to do a selenium analysis and give you a recommendation if it is too high or too low.
St Paul, Minnesota
Thanks all for your continued support, Some added info, The pH has never been below 12, Part of our Air Quality permitting requires this and it is monitored on a weekly basis. In lab tests lowering the carbonate with calcium nitrate did eliminate a lot of the burning but did not clear up the color. Also, with any addition of brightener the burning returns. An interesting observation; The silver anode is polarizing in the hull cell and sloughing off black "stuff".Conductivity of the solution seems very poor. We use silver bits in a PVC anode basket.(titanium contacts)in our tank, I am thinking the dark color may be from the anodes( although they are bagged). I will be examining this next. Our supplier is one you all have heard of but has not seen our scenario before and would prefer us to replace the bath.
Thanks Again,Steve P
Metal Finishing - San Diego, California
Please let me know your findings we have a Technic silver E bath with the same problem. Can not seem to figure it out.
I agree also with you about tech support, getting a lot of different answers. Please let me know of findings.
- St. Pete, Florida
March 11, 2008
Just read your postings, seems like you have been having the same problem I have just inherited. I work for a company that uses a silver cyanide bath (E-type) but I will not say the manufacture of the bath. I just got the plating shop as part of my duties and have the same kind problem. The silver bath is a dark orange color and the plating is coming out a very satin off white. There is a strong brown tint to the plating.
I have been running hull cells' adding the (E)brightener to the solution. As I add the brightener to the bath, the plating seems to burn (black coating) on the low density areas at normal plating amperage. I have to plate at 1/2 the amp hours I normally do. I have been getting around this by adding silver cyanide and Potassium cyanide to the bath. It helps a little but I'm loosing ground quickly. Our pH is 11.73
Please let me know your problem works out and how you solve the problem.
- Sacramento, California, USA
April 11, 2011
I have not read all the answers in here, but the concentration of carbonate is way to high. You have to freeze the carbs out of the bath, lowering the temperature to below 40 °F. The carbs will then crystallize and you can filter the liquids out. Adjust the cyanide concentration and you are way to go again :)
- Trondheim, Norway