Question: We are still using a cyanide zinc electrolyte and keep having problems with the zinc content. We came across the question of what the surface area ratio between anode and cathode must be.
Answer: As far as we know, there is no general rule. It mainly depends on the ratio of NaOH to NaCN and zinc to NaCN.
The ratio of zinc to NaCN is between 1:2 and 1:2.5, while the ratio of NaOH to NaCN is between 3:1 and 1.5:1; as a rule, the anode ratio is between 1:1 and 1:2, but can deviate significantly from this in some cases or vary depending on the zinc content.
At around 12 g/L zinc, the metal distribution is very good. As the zinc content increases, the current yield improves, reaching its maximum at around 24 g/L. The metal content is controlled by the anode surface area. If the anode surface area is too large, the metal content increases.
If the zinc content increases, some of the zinc anodes are replaced by steel anodes. Billet anodes with a steel core are recommended. The anodic current density should be less than 3 A/dm2. A constant mixture of steel and zinc anodes is not recommended, as there is a risk that only some of the anodes will work due to different anode polarization, which can result in an unfavourable metal distribution.
The sodium hydroxide content is normally 75 g/L. A higher current yield results from the higher sodium hydroxide concentration. Concentrations above 90 g/L sodium hydroxide lead to a deterioration in the appearance of the zinc coating.
It is therefore not unusual for the anode area to fluctuate, as it is adjusted individually based on the zinc content. Depending on the throughput, bath volume and metal content, corrections are made to the anodes once a week.
