Leaching

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Leaching

• Anam Saeed

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What is Leaching

• Leaching generally refers to the removal of a
  substance from a solid via a liquid extraction
  media.  The desired component diffuses into the
  solvent from its natural solid form.  Examples of
  leaching include the removal of sugar from sugar
  beets with hot water and the removal of nickel
  salts or gold from their natural solid beds with
  sulfuric acid solutions.

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Process

• In a typical leaching operation, the solid
  mixture to be separated consists of particles,
  inert insoluble carrier A and solute B. The
  solvent, C, is added to the mixture to
  selectively dissolve B. The overflow from the
  stage is free of solids and consists of only
  solvent C and dissolved B. The underflow consists
  of slurry of liquid of similar composition in the
  liquid overflow and solid carrier A. In an ideal
  leaching equilibrium stage, all the solute is
  dissolved by the solvent none of the carrier is
  dissolved. The mass ratio of the solid to liquid
  in the underflow is dependent on the type of
  equipment used and properties of the two phases.

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• This leaching is essentially countercurrent. In
  industrial leaching, solvent and solid are mixed,
  allowed to approach equilibrium, and the two
  phases are separated. Liquid and solids move
  countercurrently to the adjacent stages. The
  solvent phase, called the extract, becomes more
  concentrated as it contacts in stagewise fashion
  the increasingly solute-rich solid. The raffinate
  becomes less concentrated in soluble material as
  it moves toward the fresh solvent phase.

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Factors

• There are four important factors that aid in
  leaching  
• Temperature
• Contact Time/Area
• Solvent Selection
• Particle size

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• Temperature is adjusted to optimize solubility
  and mass transfer. 
• Liquid-to-solid contact is essential for the
  extraction to take place and maximize contact
  area per unit volume reduces equipment size.
• Solvent selection plays an important role in
  solubilities as well as the separation steps that
  follow leaching.  Nearly all leaching equipment
  employs some type of agitation to aid in mass
  transfer and to ensure proper mixing. 

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There are many different types of equipment used
for leaching.  Most of these pieces of equipment
fall into one of two categories

• Percolation....."Liquid Added to Solids"      The
  solvent is contacted with the solid in a
  continuous or batch method.  This method is
  popular for in-place ore leaching or large scale
  "heap" leaching.  Popular for extreme amounts of
  solids.

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• Dispersed Solids....."Solids Added to Liquid" The
  solids are usually crushed into small pieces
  before being contacted with solvents.  This is a
  popular leaching method when an especially high
  recovery rate can economically justify the
  typically higher operating cost (Ex/ gold
  extraction)

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Leaching Equipments

• Two types on the basis of nature of solids
• When the solid form an open permeable mass
  throughout the leaching operation. Solvent may be
  percolated through an unagitated bed of solids.
• With impermeable solids or materials that
  disintegrate during leaching, the solids are
  dispersed into the solvent and later separated
  from it.

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Leaching by percolation

• Two types
• Through stationary solid beds
• Through Moving bed
• leaching

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Through stationary solid beds

• Is done in a tank with a perforated false bottom
  to support the solids and permit drainage of the
  solvent.
• In some cases the rate of solution is so rapid
  that 1 passage of solvent is sufficient otherwise
  battery of tanks is used.

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• Extraction battery
• Shanks process

• Diffusion battery

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Moving Bed Leaching
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Dispersed Solid Leaching
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Principles Of Continuous Counter Current Leaching
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Ideal Stages In Counter Current Extraction
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• Equilibrium
• Operating Line
• Constant and Variable underflow
• Number of Ideal stages for constant underflow

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