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- Jan 10, 2019 -

Electrowinning is one of the steps of complex metallurgical flowsheets, and a significant modification of one unit operation often affects all the others. The successful application of Dimensionally Stable Anode (DSA) for chlorine evolution is crucial for the developeraent of chloride hydrometallurgy. Use of DSA on a commercial scale for the electrowinning of nickel and cobalt from chloride electrolytes has been established [1]. Falconbridge, in Norway, also converted the electrorefining process to winning by chloride route and substituted with DSA. However, most other commercial electrowinning processes are carried out in sulphate electrolytes with conventional anodes like lead/lead alloy and the anodic reaction is the evolution of oxygen, an electrode reaction well-known for its irreversibility. Hence the development of an improved oxygen evolving anode has become a must. In the present day context of escalating cost of energy, reduction in cell voltage coupled with increase in the stability of the anode will result in considerable savings in energy. Moreover, the stability of such anodes could improve the purity of the cathode product. Relatively low cost of ruthenium oxide coating and its success in chlor alkali industry suggest its application in sulphate electrolyte, although available thermodyamic data indicatjp that it is likely to be unstable in the potential range required for oxygen evolution from acid solution. Preliminary laboratory tests have been attempted in sulphate electrolyte for electrowinng of Cu, Zn, Ni, Co and Mn using DSA [2], graphite based - and ceramic based - (Ru02-Ti02) anodes. It has been reported that this results in lowering of the overpotential by 100 mV and can contribute to about 3% energy saving in the electrowinning. However, investigation [3] on the effect of chemical composition and surface morphology revealed that the active layer of ruthenium oxide was being corroded over a wide range of pH. Again Platinum-clad anodes with titanium, tantalum and niobium have been tested and found that loss of platinum was too high for its commercial applications. On the other hand, multilayer coatings of electrodeposited precious metals on titanium developed at Inco Research and Development Centre, were evaluated both in accelerated life test and in a test with simulated anolyte conditions of a nickel electrowinning cell. The expected life was around 20,000 hrs for multilayer coatings with a Ru-Ir alloy. Coatings comprised of Pd interlayer and Ru-Ir active layer are anticipated to deliver longer service life at 55°C. Yet, in the electrodeposition of nickel from sulphate bath with sulphur containing brighteners, the anodic dissolution increased by atleast one order of magnitude. The complexing properties of noble metals with organic additives are attributed to be the reason for such corrosion [4-6]. As iridium oxide is corrosion resistant in acids, it has attracted the attention of researchers to utilise the same singly or in admixture with other less expensive oxides for oxygen evolution. Accordingly, attempts have been made to test the Ir02 coated anode in laboratory copper winning cells. The initial operating potential was about 300 mV lower than that of a lead anode under the same condition; it increased linearly at the rate of 200 mV per year. This was correlated to the rate of electrochemical corrosion of the coating [ 7 ]. To make it more cost effective, it is heavily doped or mixed with less/least active oxides of higher chemical stability like Ti02, ZrOj, Ta205, Sn02, Mn02 [8-11]. The guiding principle for choice of anode material is a compromise between excellent activity, good stability, low anode potential and less cost. Though the "mixedoxides" coatings of Ir and Ta have been reported to be advantageous [9], the present study (cf.Fig.7.1) shows that it is inferior to Ti/(Ir-Co) anode system as tested under electrowinning condition.