Passivation

From BioMineWiki

Passivation is the spontaneous formation of a hard non-reactive surface film. A passivating layer is a thin layer which stabilises and protects the surface of a component from environments which might induce changes in characteristics.

In metallurgy, there are good and bad sides to passivation.

Applications

Passivation can be used to protect a material. For example, metals such as titanium, aluminium and stainless steel are protected from corrosion by a passivating oxide layer.

May decrease leaching efficiency

In leaching passivation may decrease the leaching rate by creating diffusion barriers. There are several theories concerning the nature of the inhibiting layer:

A passivating layer in a bioleaching environment may consist of:

• ferric iron precipitates
  • jarosites (pH<3).
  • ferric oxides and hydroxides (pH>3) ^{[1] [2]}
• Elemental sulfur
• Intermediate sulfide layer

How to avoid passivation?

• Stirring may remove a passivating layer by shear forces.
• Catalysts. Cl^--ions promote the formation of a more porous sulfur layer ^[3]

References

1. ↑ Klauber K, Parker A, Bronswijk W and Watling HR. 2001. Sulphur speciation of leached chalcopyrite surfaces as determined by X-ray photoelectron spectroscopy. International Journal of Mineral Processing 62:65-94
2. ↑ Stott MB, Watling HR, Franzmann PD and Sutton D. 2000. The role of iron-hydroxy precipitates in the passivation of chalcopyrite during bioleaching. Minerals Engineering 13:1117-1127
3. ↑ Kinnunen, P.H.-M. and Puhakka, J.A. 2004. Chloride-promoted leaching of chalcopyrite concentrate by biologically-produced ferric sulfate. Journal of Chemical Technology and Biotechnology, 79: 830 – 834.