Chemical precipitation with hydroxide

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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. Another even more powerful tool for prediction of metal ion behaviour in aqueous solutions is the Eh-pH diagrams.

Examples of reactions

Zn SO4 (aq) + 2NaOH (aq) → Zn(OH)2 (s) + Na2SO4 (aq)
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