Chromium Salt Precipitation-Zinc Exchange for Thallium Recovery
In the production process of thallium compounds, thallium ion is enriched in the solution by chromium salt precipitation-zinc exchange method. In this method, an aqueous solution of thallium ions is brought into contact with an aqueous slurry of zinc hydroxide. Chromium ion in the solution then precipitates out of solution as hydrated chromium oxide and zinc ion balances the charge by precipitating out of the solution as hydrated zinc oxide. The hydrated zinc oxide then exchanges the protons of thallium ions for protons from the zinc hydroxide, thus transferring the thallium ions from the aqueous solution to the solid phase. The resulting solid complex is easily separated from the liquid phase by filtration, and the desired thallium compound can then be isolated from the solid complex.
In general, the chromium salt precipitation-zinc exchange technique is used to recover thallium that is present in low concentrations, typically less than 0.5M. To use this method, the solution containing the thallium must first be adjusted to a pH of 3-4. The addition of chromium salt then causes the thallium ion to precipitate as hydrated chromium oxide. Zinc hydroxide is then added and precipitates out of solution as connected hydrated zinc oxide, which exchanges protons with the solution to take up the thallium ion.
The zinc hydroxide slurry is then separated from the thallium-laden solid phase by filtration, and the thallium can be extracted from the solid phase by treatment with hydrochloric acid or other appropriate solvent. The resulting solution containing the thallium salt then can be used in further applications, such as electrolysis to yield the desired thallium compound.
The chromium salt precipitation-zinc exchange technique provides a simple and effective method for recovering thallium. The technique is relatively easy to perform and achieves a high level of selectivity for thallium ion. However, like any extraction technique, it has some downsides. The first is cost; chromium salts, zinc hydroxide and hydrochloric acid are all relatively expensive reagents. Additionally, the process can take a relatively long time to complete, as the precipitation and subsequent exchange steps take numerous hours. Finally, if the thallium concentration in the starting solution is too high, the chromium salt may precipitate all at once, leading to rapid separation of the solution, making the technique more difficult to implement.
Despite these drawbacks, the chromium salt precipitation-zinc exchange technique has proven itself to be a useful and practical method for recovering thallium from aqueous solutions. The ease of implementation and high level of selectivity make it an attractive option for those who need to recover thallium in low concentrations.
