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topic 57351

Cadmium / Cyanide Bath Maintenance



A discussion started in 2011 but continuing through 2018

June 14, 2011

Q. We are currently running a cyanide bath cad plating line, and have been having trouble keeping our solution parameters in spec. We recently started precipitating out sodium carbonates (as they were creeping above 60 g/L) by cooling buckets of the solution to around 1 - 2° C. Once we began doing this, we found that it became very difficult to maintain our cadmium oxide levels (getting down around 19 or 20 g/L). Presently, we do not agitate or filter the solution (we stir it thoroughly prior to plating or taking bath samples), but will be purchasing a pump in the near future to perform both of these functions.

I was told by a reputable source that cadmium oxide shouldn't be added to the tank unless making up a new solution, and that the anodes should provide the supply of cadmium ions necessary to balance the tank. I have since seen several other specs (as well as postings here) that recommend adding cadmium oxide when the levels get low.

Is the fix as easy as adding the cadmium oxide? Is this what we need to do to maintain our solution parameters on a regular basis? So far, when the cad levels drop, we have been plating sheet stock until our levels come back, then beginning production again. We recently changed out our anodes, with the concern that the older ones had become passive.

Our solution specifications are:
Cadmium Oxide = 21 - 42 g/L
Sodium Cyanide = 87 - 150 g/L
Sodium Carbonate = 15 - 59 g/L
NaCN / Cd = 2.8 - 6.0

Our tank temperature sits around 75° F (24° C). Should we operate it at a higher temp? ASM seems to recommend 80 - 85° F. Should we also measure sodium hydroxide?

Any other pieces of advice for improved operation of this process would also be greatly appreciated.

Thanks in advance,

Nathan Sanz
Manufacturing Engineer - Sidney, BC, Canada


June 23, 2011

A. Hi Nathan,

When I was involved with cadmium plating it was normal practice to add cadmium oxide if the cadmium levels were low. We would also look at the anode configuration if the cadmium content continued to fall and the anode area increased as necessary to try and maintain the cadmium levels without having to make additions. When you add the cadmium oxide it will have more of an effect than you expect as your free cyanide will drop and you sodium hydroxide will increase. It takes some playing with to get the balance right, but a couple of good books such as the Canning Handbook or the Electroplating Engineering Handbook contain some good advice.

We always monitored the sodium hydroxide as well. We also did hull cells when there was suspect quality in the plating that couldn't be traced to any of the cleaning operations (a good quality control test).

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom



June 13, 2013

Q. I'm having trouble with the sodium hydroxide level (too high) and sodium carbonate level (too high) in my cyanide cadmium plating tanks (barrel and tank). The specification for NaOH is 1.6 - 4.0 oz/gal for both (result is 5.1 for tank and 6.6 for barrel). The specification for Na2CO3 is <8.0 oz/gal result is 11.0 oz/gal for tank and 10.9 oz/gal for barrel) I've tried chemical removal using calcium hydrated lime which did nothing but increase NaOH. Freezing is not an option.Tank sizes are about 2500 gallons for tank and 175 gallons for drum.
Please help if you can.

Michael Blair
- Birmingham, Alabama, USA


June 13, 2013

Q. I posted earlier, but entered incorrect data. I'm having trouble with high NaOH in both baths (5.1 oz/gal in 2500 gallon tank and 6.6 oz/gal in 175 gallon tank - range is 1.6 - 4.0 oz/gal), low pH in both baths (11.7 in 2500 gallon tank and 11.6 in 175 gallon tank - range is 12 - 14), and high Na2CO3 in both baths (11.0 oz/gal in 2500 gallon tank and 10.9 oz/gal in 175 gallon tank - range is <8.0 oz/gal). I've tried to precipitate Na2CO3 out using calcium hydrated lime which only increased the NaOH. Freezing out isn't an option.
Please help if you can.

Michael Blair [returning]
- Birmingham, Alabama, USA



March 22, 2018

Q. I do not see an answer to the situation addressing how to reduce a high NaOH concentration exceeding the control limits. I have inherited a new LHE Cadmium solution make up that did not fully take into consideration of the increase from the cadmium Oxide. This is an Aerospace company with Boeing and Safron specification requirements. Besides decant, is there a treatment to reduce the NaOH concentration in an LHE cadmium solution.
Thank you.

Tim Aish
Landing Gear Overhaul - Longwon, China



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