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 Leaching and its kinetics


What is Leaching? 

➢ Also known as liquid-solid extraction.

 ➢ Two phases are in intimate contact and the solute or solutes can diffuse from the solid to the liquid phase. 

➢ The process of extracting a substance from a solid material that has come into contact with liquid. 

➢ The liquid that enters will separate the components originally in the solid.

 Examples of Leaching: 1) Extracting tea from tea leaves. 2) Production of vegetable oil using hexane. 3) Copper salt leaching from copper ores. 4) Production of nickel from ore.


 Kinetics of Leaching: 


 The above figure shows the concentration profile when a mineral surface dissolves in a leaching medium. Subscripts  S and B refer to interface and bulk respectively. The situation is simple for complete dissolution.  

However when a mineral decomposes, and dissolves only partially, then a new solid residue appears. Leaching would continue if this new phase is porous. This situation is shown in figure 4.13. Subscripts  I and II refer to the mineral residue and residue-solution interfaces, respectively. This figure may be considered as the more general case.  

     


 

The kinetic step that determines the rate of leaching could be one or a combination of the following: 

1) The diffusion of the reagent R from the bulk of the solution to the solid surface  II 

2) The diffusion of the reagent R through the porous reaction product layer. 

3) The reaction of the reagent with the mineral surface I  to form a soluble metal species. 

4) The diffusion of product metal species M or other reaction products P through the porous reaction product layer in the outward direction. 

5) The diffusion of product metal species M (or other products P) away from the surface. 

When there is no porous reaction product layer, then steps 2 and 4 would, of course, be absent. 

If we know the rate determining step, then, the appropriate techniques required for speeding up the leaching reaction can be adopted. Such techniques would depend on the surface area exposed to the leaching action. Finely ground material, which has a large surface area, would, therefore, generally leach faster, for example, if either diffusion step 1 or diffusion step 5 is rate-controlling, then leaching can be speeded up by stirring the aqueous medium. However if the chemical reaction step (step 3) is rate controlling, agitation would have no effect. The effect of concentration of the reactants and products on the surface reaction would depend on the order of the reaction. Obviously as the temperature increases, all the foregoing steps would be accelerated. 

By 

Vijaykumar M.

Metallurgy and Materials Engineering

VNIT, Nagpur

  

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