Chemical precipitation

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== Chemical precipitation ==
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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.
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== Hydroxide precipitation ==
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The precipitation of metal hydroxides such as for example Al(OH)<sub>3</sub>, Fe(OH)<sub>3</sub> and Zn(OH)<sub>2</sub> 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 (CaCO<sub>3</sub>), slaked lime (Ca(OH)<sub>2</sub>), sodium hydroxide (NaOH) or ammonia (NH<sub>3</sub>).
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[[Image:Solubility diagram for some metal hydroxides.png|thumb|Solubility diagram for some metal hydroxides]]
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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.
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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<sup>2+</sup> 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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[[Image:Eh-pH diagram of Fe-H2O.png]]
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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.
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Formation of a separable solid substance from a solution. This is achieved by converting the substance into an insoluble form by chemical reaction or by changing the composition of the solvent to reduce the solubility of the substance in it.
Formation of a separable solid substance from a solution. This is achieved by converting the substance into an insoluble form by chemical reaction or by changing the composition of the solvent to reduce the solubility of the substance in it.

Revision as of 08:18, 24 August 2007

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).

Solubility diagram for some metal hydroxides
Solubility diagram for some metal hydroxides

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 ??. Image:Eh-pH diagram of Fe-H2O.png 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.


Formation of a separable solid substance from a solution. This is achieved by converting the substance into an insoluble form by chemical reaction or by changing the composition of the solvent to reduce the solubility of the substance in it.

Chemical precipitation is used to remove dissolved metals from solutions, such as process wastewaters containing toxic metals. Soluble metal ions are converted to insoluble form by chemical reaction with a precipitating reagent. The solid particles formed by this reaction are removed from solution by settling and/or filtration.

The effectiveness of a chemical precipitation process is dependent on several factors which include:

  • type and concentration of metal ions present in solution
  • precipitant used
  • reaction conditions (pH, temperature, etc.)
  • presence of other constituents that may inhibit the precipitation reaction.

Metal ions such as zinc can be precipitated out of solution by addition of sodium hydroxide:

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

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