Removing/freezing excess carbonate from cyanide silver plating tanks

A. Hi Naji. That seems to be the subject of this whole thread and its 50 postings --so more words please, so that we can understand where the difficulty lies.

Luck & Regards,

A. Hi Jancie. Carbonates should not be completely removed, but reduced to about 4 ounces/gal (30 g/l).

A proprietary salt for this purpose is 'Decarbonator Salts' available from, for example, W. Canning plc, Birmingham, UK. They are added at a ratio of 1.8 parts by weight for each part of potassium carbonate to be removed. Generic calcium or barium salts can also be used, but I don't have the hands-on knowledge of this to offer specific instructions or points to watch out for (perhaps a reader who silver plates will help us out?)

A. We operate a 40 gal silver cyanide plating bath and had run into carbonate build up in the past. We eliminated the carbonate problem by not returning dragout to the tank and keeping the tank at 70 °F.

I checked our files and found a letter from Arthur Kushner from the Joseph B Kushner Electroplating school. The letter states "...carbonates can be removed from your silver bath by treating with any of the following materials: calcium nitrate, barium cyanide, calcium sulphate, calcium hydroxide, calcium cyanide. If you cannot find the details on these methods, give me a call. " (signed Arthur Kushner) That's all I have. MBK

A. Since I am quoted, I thought I might as well get involved. The whole idea is to remove the carbonate from the silver plating bath. Potassium carbonate is fairly soluble in water (compared to sodium carbonate) so freezing out does not work. Materials such as calcium nitrate, barium cyanide, etc., will react with the potassium carbonate in the plating solution to give you a relatively insoluble carbonate. For example, barium cyanide will react with potassium cyanide to give you barium carbonate and potassium cyanide. Two ounces of barium cyanide will remove about 1.5 ounces of potassium carbonate and reform about 1.25 ounces of potassium cyanide.

A word of caution here. It is a good idea to have some recent analyses of your tank so know where you "are" with regard to cyanide and carbonate concentrations. Test a small batch of your plating solution before charging ahead with the whole tank.

A. We recently did this treatment on a 600 liter silver tank, using Calcium Hydroxide, which is cheapest, but found that the volume of "sludge" produced was large. Additionally we seemed to lose about 10% of our solution. If the sludges are allowed to stand, the solids settle out and solution becomes recoverable from the top. 4 months after treatment we are still recovering solution from the top of the sludges, at present about 100 ml. per day.

The process vendors always recommend high levels of potassium cyanide, up to 150 ml/liter for a silver content of 20 gms/liter. We feel this accelerates carbonate build up, and try to work with a maximum 120 g/L KCN.

Is there any way to recover the solution more quickly?

A. We tried the calcium hydroxide precipitation of potassium carbonate in a silver plating bath. The resulting sludge is excessive. Rinsing the sludge several times with some water helps to recover some of the bath. The hydroxide content increases dramatically, but luckily seems to have no deleterious effect on the baths performance. Lately, we have employed a flow through centrifuge as a means of dewatering the resulting sludge. Our recovery of the plating bath has greatly improved.

A. Dear Jeff, It is very difficult to eliminate potassium cyanide in silver bath. But you can use Barium cyanide (BaCN) but this is very expensive method. That is cheaper to remove a part by the bath and preparing to full volume. Regards, Anders

A. I believe what you need is barium hydroxide. I don't have the recipe handy, but it works quite well to lower carbonate levels in silver cyanide plating baths. It is best to treat and then filter the tank before using again.

A. Jeff and Phil I think Barium hydroxide is wrong. When you reduce potassium cyanide the only way to do it is use Barium cyanide and after you have filled the tank with Barium cyanide you must freeze the solution and the carbonates be some ice over the silver bath. Pump the bath from the bottom. But again it is cheaper to reduce carbonate from potassium cyanide by dumping some gallons of silver bath and restoring it to the right parameters. Regards, Anders S.

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
1999

A. Jeff: We have been very successful using slaked lime (calcium hydroxide) and barium hydroxide to reduce our carbonate levels in our potassium cyanide silver bath.

Q. Daniel

When you say you used slaked lime & barium hydroxide, did you use them as a mixture, or did you try them separately? And what effect did this have?

Thanks

Mark.

A. Hello Doug,
I would hate to see you add 300 plus pounds of calcium nitrate to your tank at one time. I would seek out the advice of your silver supplier. Yes, you can add more than 10# at a time to the tank. I have not ever had to reduce potassium carbonate that much in any bath. Apparently your predecessor did not take too much with him if he let the carbonate level get that high! You may have to break the treatments up into 3 series. Sorry, but some 800 troy oz of silver is too expensive to play with. Good Luck!

A. Hi Andy.

When you put a little carbonate in a silver solution, this carbonate will get the solution a higher leading capability in the silver solution. But then the solution used you will get a higher carbonate because potassium, or if you use sodium cyanide dissolves as a carbonate in the solution as a contamination.

The carbonate is in a new solution about 15 g/l. If the carbonate is more then 120 g/l you should dump approximately half on your solution, and make addition to the right level. You can't freeze potassium cyanide (carbonate) out from the solution because this is very expensive, but if you use sodium cyanide (carbonate), it's no problem to freeze it out.

Regards,

A. Text books say that it is to buffer at high pH.

Andy and Yashawant.

Yashawant is also right but carbonate helps to get a pH over 10 in the solution.

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
2001

A. Hi Benigna.

Why I propose to dumping approximately half of your solution is because it's very expensive to freeze carbonates out from silver solution. If you use potassium carbonates. It's only barium cyanide who can remove carbonates from a potassium cyanides silver solution. But if you use Sodium cyanide in your silver solution you can remove the carbonates. A high carbonate it's a contamination in a silver solution the solution will lose the current effectivity and you get dark deposit. The solution will also be black.

Regards,

A. A bit of an English problem on part of your request. Your problem is carbonate, not carbon. A silver bath that uses sodium cyanide and sodium hydroxide can freeze out carbonates on a very cold coil or possibly a cold finger (if you are a chemist). The solubility of sodium carbonate is that in a cold tank with a near freezing coil or similar apparatus, sodium carbonate crystals will form and temporarily stick to the cold device because you have reached a point beyond the saturation point. This does remove a small amount of trapped bath chemicals, but not bad. The coil is needed as if you just precipitate it out, the flow rate of filtration and the rising temperature will redissolve it. Metal finishing vendors sell these things.
This does not apply for a potassium based bath because the much higher solubility of potassium carbonate at any temperature. For this you are going to have to bail out part of the tank--Not cheap.

A. The Canning Group markets some "decarbonating salts" which I think are some type of barium salt. The use of them is detailed in The Canning Handbook.

It involves simply adding the prepared salts to the solution and leaving for a set length of time. The carbonate is then precipitated out and the solution can be pumped off the top and filtered and reused. We regularly use this method to decarbonate our silver bath.

A. Years ago JWI offered for sale a unit to remove potassium carbonate from plating solutions, I believe the equipment was made in Europe, it was quite expensive at the time. I'm wondering if it is still available.

A. Hi!

You can find appropriate equipment on internet, if you "Google" it with key-word "carbonate removal"!

A. Short of reclaiming the solution and starting over, I have never read of a way to do it. Literature indicates that the potassium salt plates slightly better than the sodium. You could make all of your future adds with sodium rather than potassium and eventually it would be mostly sodium, but I suspect that your need is now.

A. Hi Max,
You can still freeze out carbonates from a potassium cyanide based bath. The carbonates can also be reduced by adding calcium nitrate to the solution. Your silver supplier can give you this process. Sodium cyanide silver solutions are not really used that much in the US anymore. personally, I prefer potassium cyanide based baths. I feel they are easier to work with. Good Luck!

A. Hi, W. They would seem to me to be an F006 plating sludge, which must be disposed of as hazardous waste.

Regards,

A. Hi, Jean-Claude

I believe that freezing only works on sodium cyanide based plating baths, not on potassium based baths. What is the sodium v. potassium situation with your bath?

Regards,

Q. Hi Ted

First thank you (1 year later) for the answer. Our baths only contain potassium. We are now successful with lowering the carbonates with calcium hydroxide (from 135 g/L down to 30 g/L) but are worried about the new level of hydroxide (120 g/L). Those results were obtained in lab and we are actually conducting hull cells on the solutions. My question is actually "Have we just changed one problem for another?"
Thanks for your help

Hi, Jean-Claude.

That's a question I can't help with, but maybe another reader can.

P.S.: Don't feel bad about the delay. The threads are not solely about offering concurrent help to the inquirer, they are also about building reference material for the silent majority who read the postings but don't post themselves due to employer restrictions or other reasons.

Regards,

A. You can use Barium Nitrate to remove Potassium Carbonate from Silver plating solutions. To reduce one gram of Potassium carbonate in the solution add 1.8 grams of Barium Nitrate. Mix well leave for settling 24 hours and then filter it. There is no side effect on the bath. It is not altering any bath composition. Only removes the carbonate. First do it in 5 liter solution in the lab. Do a hull cell. Test the potassium carbonate level reduced in the solution.
Wish you all the best.

A. The best, and quite possibly least expensive way to accomplish this is to send your old bath to a reputable refiner who will pay you for the silver, and then build a new bath, using just enough of the old bath to have an initial carbonate of about 1 oz/gal.

A. Heather,
I agree with Jeffrey that a part replacement is probably the best answer. But I'll offer a reply to your question.

1.8 grams of barium nitrate is required to precipitate 1 gram of potassium carbonate (actually I calculate the value to be 1.9, but that is splitting hairs.) If you prefer to work in oz/gal, then the same factor applies.

Dissolve the barium nitrate in water and add it slowly to the bath with vigorous stirring. I can't put a figure on the volume of barium carbonate. But, the bath should be left standing for several days to allow the barium carbonate to compact.

Finally, decant the bath and remove the sludge. Dispose of the sludge. Remember that the sludge will contain silver and cyanide. Some of the silver cyanide can be recovered adding water to the sludge and allowing the carbonate to settle again. One problem with this recovery procedure is that you gain a lot of extra volume of dilute silver cyanide which is difficult to return to a bath with a low evaporation rate.

A. Did I do this right? 2 pounds in 100 gallons is 0.32 oz/gal per month - not an unreasonable rate of break down into carbonate. Start checking the carbonates each month and see if that is where it is going.

A. Hi Jonathan. The thing I would disagree with is making additions to your whole tank based on theory. I would suggest instead taking a beaker [beakers on eBay or Amazon] of solution, making the calculated addition, letting it settle, filtering it, and trying the solution in a Hull Cell. If it works, go ahead and scale up ... but don't risk turning your plating tank into a cauldron of witches brew by trying to implement suggestions for fixes that sound good but may not actually work :-)

Regards,