Question: Thank you for answering our question in issue 08/2022[1]. We are still unclear about carbonate formation on the anodes. We disagree on how a higher proportion of zinc anodes affects carbonate formation. Some believe that it is reduced, while others are of the opinion that carbonate formation increases with soluble anodes, as more oxidation takes place.
Answer: Let's look at this in theory. Anode processes are oxidation processes. If there are enough zinc anodes in the electrolyte, the preferred reaction is
Zn - 2e- → Zn2+
The zinc is then recomplexed by free cyanide. Basically, it does not matter whether they have soluble or insoluble anodes, the same number of electrons are always converted and thus the same amount is oxidized. If no or not enough soluble anodes are present, "something else" is oxidized. Three reactions should be mentioned here:
- The oxidation of oxygen, which is available in abundance. For example by water (H2O) or from the hydroxide (OH-)
- Decomposition of cyanide [2]
- Decomposition of organic additives
Both reaction 1, often in combination with reaction 3, and reaction 2 can form carbonates. The lower the proportion of anodes, the higher the anodic current density and the more these reactions will take place. The decomposition of cyanide is an exception with regard to high current densities. It takes place preferentially in the low current density range. Continued on p. 1340
The process is used, for example, to increase the service life of stand sinks. Here, the cyanide is electrolytically oxidized in the bypass, which takes place in the low current density range. This means that from a certain surface area of the soluble anodes, not only does their zinc content increase, but the proportion of cyanide also decreases and the proportion of carbonate increases. This is usually only a relatively small effect. Carbonate formation in the absence of soluble anodes is considerably higher, as all three of the above reactions occur.
Removal of carbonate
Unfortunately, carbonate formation cannot be completely avoided. You can try to minimize the circulation, avoid pumps drawing air, but there are limits to all measures. There are two ways to reduce the carbonate content, apart from dilution:
- Freeze out
- Chemical precipitation
Word cloud on the subject of cyanideIn the past,the electrolyte was pumped into a tub outside the electroplating shop in winter and left to freeze out over the weekend. However, winters are no longer as cold, and there is also the issue of safety. But there are special systems for this [3].
Such systems do not only work with zinc electrolytes, of course, but also with silver, copper (cyanide), zinc-nickel, zinc-iron and other alloy electrolytes. The carbonates are continuously frozen out. A certain amount of medium is removed from the bath without affecting production. This is then cooled down to a precisely defined temperature. The resulting crystals remain on the belt filter - after the fully automatic return of the purified medium to the bath - and then end up in the collection container. The filtered salts can then be disposed of.
Chemical precipitation is somewhat more expensive, but quite efficient. However, the process must be adapted to the respective electrolyte. Precipitation can be carried out with calcium sulphate, calcium hydroxide, barium hydroxide or barium cyanide. A more detailed description, albeit for silver electrolytes, can be found in this article [4].
Literature
[1] From practice - for practice 8/2022; Anode-cathode ratio in cyanide zinc electrolytes
[2] Technology of electroplating - Eugen G. Leuze Verlag, p. 471
[3] https://www.tecga.info/carbolux-d
[4] From practice - for practice 11/2019, Carbonate in silver electrolytes
