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topic 18854

Carbonate problems in Copper Cyanide Plating

A discussion started in 2003 but continuing through 2020


Q. I have a lot of problems with a high level of Carbonates in my copper CN bath. Why it grows so fast? How to reduce it?

- Caxias do Sul, RS Brazil



Panjala Mukesh
Panjala Mukesh
     fashion jewelry mfgr.
Hyderabad, India


A. In the cyanide baths, the carbonate can be formed by different ways, but can be formed mostly by absorption of carbon dioxide from the air (reactions 1A and 1B) or by hydrolysis of the cyanide. 2XCN + CO2 + H2O = X2CO3 + 2HCN (1A),X: Na or K 2XOH + CO2 = X2CO3 + H2O (1B), X: Na or K Keep the concentration of sodium carbonate in the bath below of about 75 g/L (or below of about 100 g/L of potassium carbonate) The are two different ways that can be used to reduction of the carbonate concentration: one can be done by cooling the bath and other can be done by precipitation with barium hydroxide.

Jean Vicente Ferrari
- São Paulo, São Paulo State, Brazil


A. Are you using air to agitate the tank , because that will push your carbonates as will a pump that is cavitating . The cooling of the tank will only work if you are using Sodium Cyanide, not Potassium .

Dave Brackenridge
aerospace plating - Germiston, South Africa


A. 1. pH greater to 12

2. Temperature greater than 50 °C

3. CN frees Too much 30 g/L

4. NaOH too much :

Use H2O2 and activated charcoal

universidad - Medellin-Colombia


!! Whenever the topic of freezing out Carbonates arises, I feel compelled to issue a friendly bit of advice.

Throughout my career, I have heard knowledgeable platers talk about high carbonates. They were going to freeze it out by adding dry ice to the tank. Well "DON'T EVER DO IT."

Dry Ice is Carbon Dioxide, which forms Carbonic Acid in solution. This reacts with cyanide in solution and forms poisonous Hydrogen Cyanide gas. Repeat: DON'T DO IT.

Many years ago, someone published a "tip" about using a "Dry Ice Tube" in their bath to rapidly freeze out carbonates. A tube, sealed watertight at the bottom, was filled with dry ice and inserted into the cyanide tank. The carbonate crystals would form on the outside of the tube and could be scraped off and discarded.

Use refrigeration devices to achieve the carbonate crystallization. You may even have a special tank with freezer coils. If you also do hard coat anodizing, you may have surplus cooling available for this purpose.

ed budman
Ed Budman  eb sig
- Pennsylvania
With deep sadness we advise that our friend Ed passed away Nov. 24, 2018

-- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What kind of treatment can I use to reduce carbonates in a Cyanide Copper bath. I don't want use freeze method.


December 1, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Does anybody know how to remove carbonates in a plain cyanide copper bath

Calcium hydroxide how much

Barium salts are any useful?

cooling the bath to 30 degrees?

Daniel Hernandez
plating shop - Bucaramanga Santander Colombia

August 12, 2013

A. For Calcium Hydroxide the reaction is 0.7 ounces of calcium hydroxide removes 1 ounce of carbonate.

The complete procedures for the various methods of carbonate removal are discussed in this paper:

Steven Gorzo
- Calgary, Alberta, Canada

May 15, 2019

Q. We are having an unusual red color on the parts in a copper cyanide bath with 80 amp. We lower amps to 20 and the color is expected but the plating time is long, over 90 minutes in an usual 45 min plating desired time frame
1. pH is 14.7 (how can we lower it?)
2. Potassium carbonate's is 18.8 oz/gal ( We use potassium cyanide, Can we begin introducing sodium cyanide so we are able to freeze carbonates. Will there be or has anybody encounter any issues if we start adding sodium cyanide when we have a copper potassium cyanide bath? The tank volume is 635 gallons) KOH and Rochelle salts are at optimum concentration.

Your response(s) will be greatly appreciated.


Maribel Tirado
- Santa Fe Springs, California USA

Carbonate removal, specifically with the use of Calcium Hydroxide

August 28, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am inquiring of my fellow platers if anyone uses the precipitate method with calcium Hydroxide to remove carbonates from a Sodium Cyanide plating bath. I am looking for an alternative method in lieu of a freeze out. My two concerns using this method:

1. Am I able to return the solution to the bath after filtration with no amounts of calcium left over?
2. Am I able to treat the precipitate (calcium carbonate) in the same way as wastewater since there will be some amount of Cyanide left over after filtration.

I know these are loaded questions and dependent upon how much calcium hydroxide is used. But, what I seek is, essentially, a personal experience using this method for carbonate removal and if it is effective for your plating process and worth trying.

Sadly, there is not much information out there on this process.

Jennifer Farner
shop chemist - Saint George, Utah United States

September 2019

A. Hi Jennifer. We added your question to a thread where the subject has already been discussed. Steven Gorzo suggested looking at

Good luck!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

simultaneous September 10, 2019

Q. Thank you so much for your response, Mr. Mooney, and also for posting this to the thread. The article suggested by Mr. Gorzo is very useful in learning the process of how to precipitate the carbonates using the calcium hydroxide method, although, there was nothing said about what is done with the precipitate (Calcium Carbonate) and if the filtered cyanide solution can be reused in the bath or if there might be any residual calcium left over after filtration. Due to these questions after reading the article, I thought to present this to the forum for those to respond who use the calcium hydroxide process for carbonate reduction in a cyanide bath. Specifically, I am looking for another's personal experience with this method to help answer these questions for me.

Jennifer Farner [returning]
Shop Chemist - Saint George, Utah

September 11, 2019

A. Hi, Jennifer
We have cadmium cyanide and silver cyanide plating bathes. We check carbonates concentration and periodically remove them by precipitation using calcium hydroxide. Both solutions have a minimum limit for carbonates, so addition of calcium hydroxide will be always in deficit. All calcium ions will react with carbonate ions and precipitate, so no calcium will remain in the solution.
After adding calcium hydroxide and intensive mixing let the mixture stand for a weekend. The calcium carbonate will settle down. Now transfer the clear solution to the bath using 10 micron PP filter. Also filter the solution in the bath by circulating it through the new 10 micron filter until it is absolutely clear.
Any way some portion of solution will be lost with the calcium carbonate precipitate, so it will be necessary to check all the components of the solution and to add as needed.
The calcium carbonate precipitate and all washes should be treated as cyanide/cadmium/silver containing waste according with your local ecological law.
Good luck!

Alex Sirota
- Lod, Israel

Carbonate removal from a potassium cyanide bronze bath

June 24, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am a process engineer and we are using a potassium based bronze bath (~6000 gallons) for barrel plating. The finish is matte or semi-bright. After a few weeks we exceeded our carbonate limit of about 260 g/L and started bailing out solution. We are using copper and carbon anodes. I have reviewed a 1953 article by Modjeska but I still have a few basic questions.
1. Does treating with calcium hydrate generate a lot of sludge?
2. What purity of calcium hydrate is needed?
3. Is it ok to have ~ 3% of MgO impurity in the calcium hydrate?
Thank you in advance,

Allen Jones
- Jonesborough, Tennessee, USA

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