Impurity removal and concentration of metal-rich solutions

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Impurity removal and concentration

In practice, leachates always contain impurities or by-products that have to be removed prior to recovery of wanted metals. In some cases the desired metal(s) is the main constituent in solution but in other cases it can be present at lower concentration than the impurities. Therefore different approaches are needed to be taken to achieve the goal of separating wanted metals from impurities. Methods used for the purification and/or concentration of the aqueous solution include chemical precipitation, solvent extraction, ion exchange and cementation.

Chemical precipitation

Chemical precipitation is used for removal of both impurities and wanted metals from leaching solutions. When the desired metal is precipitated it is usually treated further before a desired product, metal or metal salt, is obtained. Precipitation is achieved by the addition of proper reagents (alkali, salts) and can under correct conditions be done selectively. Examples of chemical precipitations commonly used are hydroxide- and sulphide precipitation.

Hydroxide precipitation

The precipitation of metal hydroxides such as for example Al(OH)_3, Fe(OH)3 and Zn(OH)2 is the most widely used chemical precipitation process in hydrometallurgy. The precipitation is achieved by increasing pH in the solution by the addition of alkaline reagents such as limestone (CaCO3), slaked lime (Ca(OH)2), sodium hydroxide (NaOH) or ammonia (NH3).

To give an indication on the conditions needed to separate different elements by hydroxide precipitation solubility diagrams of metal hydroxides are helpful. In figure ?? a solubility diagram with some of the common metal hydroxides is shown.

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