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Common Purification Reactions of Cobalt

Cobalt is a valuable metal with a variety of uses, especially in the manufacture of alloys for high-tech applications like aircraft turbines, powerful permanent magnets, cutting tools and surgical implants. In nature, however, it is typically found in combination with various impurities, so it must be purified and isolated before it can be of use.

Zone Melting
- Common impurities in cobalt include iron, nickel, copper, zinc and lead, among others. One approach to purification is zone melting in which cobalt is pulled through a heater so that a molten zone travels down the cobalt, collecting impurities in the process. Unfortunately, this approach is ill-suited to removal of iron and nickel, so it's best as the last step in the process, following use of other techniques like anion exchange chromatography and electrowinning.
Anion Exchange Chromatography
- In anion exchange chromatography, a solution containing the cobalt and the impurities flows through a column packed with an inert polymer material, which has been chemically treated so it features positively-charged groups that bind negatively-charged ions flowing through the column. In a solution of hydrochloric acid, the chloride ions can bind to metal ions to create a complex with an overall negative charge. An appropriate concentration of hydrochloric acid ensures the cobalt ions form complexes with a negative charge and remain bound in the column while the other ions flow through in the solvent. Subsequently adding a hydrochloric acid solution of a different concentration will remove the cobalt ions from the column so they can be retrieved.
Electrowinning
- Electrowinning is an electrochemical process that uses an electric current to oxidize cobalt ions and deposit cobalt on an electrode. Electric current is passed through a solution of cobalt ions with +2 positive charge together with sulfate or chloride. The electrodes may be made either of lead or titanium coated with a corrosion-resistant metal. Lead electrodes are employed in conjunction with cobalt sulfate since this process will deposit cobalt oxyhydroxide on the lead electrode. Coated titanium electrodes, by contrast, are used in conjunction with a cobalt chloride solution.
Considerations
- One drawback to the use of cobalt chloride solutions is the release of chlorine gas, which is highly toxic; bagging the anode (the electrode where oxidation takes place) helps keep the chlorine contained. Anion exchange chromatography, electrowinning and zone melting are often employed together and in that order, with each step removing some of the impurities remaining from the last. One way to test the purity of the product is by measuring its residual resistivity, the resistance it offers to electric current at very low temperatures.

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