Impurity removal and concentration of metal-rich solutions

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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.
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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[[Image:Example.jpg]]
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In the figure the line for each metal ion represents the equilibrium between a 1 M solution of respective metal ion and its hydroxide. On the y-axis the metal ion concentration is given and pH is on the x-axis. At low pH-values the hydroxides are soluble and at higher pH-values the metal ion concentration decreases as the hydroxides are formed. It can be seen that trivalent metal ions can be precipitated at lower pH values compared with the divalent metal ions. Metal ions located close to each other like Fe<sup>2+</sup> and Zn<sub>2+</sub> are difficult to separate and give mixed hydroxide precipitates if both are present during neutralisation. The most difficult metals to precipitate in the diagram are the alkaline earth metals calcium and magnesium.
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Another even more powerful tool for prediction of metal ion behaviour in aqueous solutions is the Eh-pH (Porbaix) diagrams. An example of a Eh-pH diagram for iron is shown in figure ??.
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Chemical precipitation can be used for either solution purification or metal recovery. Eh-pH diagrams is a good tool to visualize the stability areas of metal species in solution depending on the solutions redox potential (Eh) and pH. These diagrams can be used to indicate how different metal ions can be separated from each other by making changes in these variables.

Revision as of 14:37, 28 June 2007

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.

In the figure the line for each metal ion represents the equilibrium between a 1 M solution of respective metal ion and its hydroxide. On the y-axis the metal ion concentration is given and pH is on the x-axis. At low pH-values the hydroxides are soluble and at higher pH-values the metal ion concentration decreases as the hydroxides are formed. It can be seen that trivalent metal ions can be precipitated at lower pH values compared with the divalent metal ions. Metal ions located close to each other like Fe2+ and Zn2+ are difficult to separate and give mixed hydroxide precipitates if both are present during neutralisation. The most difficult metals to precipitate in the diagram are the alkaline earth metals calcium and magnesium.

Another even more powerful tool for prediction of metal ion behaviour in aqueous solutions is the Eh-pH (Porbaix) diagrams. An example of a Eh-pH diagram for iron is shown in figure ??.

Chemical precipitation can be used for either solution purification or metal recovery. Eh-pH diagrams is a good tool to visualize the stability areas of metal species in solution depending on the solutions redox potential (Eh) and pH. These diagrams can be used to indicate how different metal ions can be separated from each other by making changes in these variables.

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