The Leaching Process

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The Leaching Process
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The next element that we remove is the iron (Fe).
We do this using a Leaching process. Leaching
is a process of mixing the calcine (solid) into a
liquid called spent solution. The spent solution
contains sulphuric acid (H2SO4).
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Zinc Oxide (ZnO)
Sulphuric Acid(H2SO4)
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Zinc Sulphate (ZnSO4)
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The leaching process is carried out in a series
of tanks called leaching reactors. Solid calcine
is mixed with liquid spent solution and
overflowed, as a slurry (runny mud), from one to
reactor to another.
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Reactor Vessel
Airlift line
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As more zinc is dissolved, the slurry neutralises
or becomes less acidic and the rate at which zinc
dissolves slows down. By the last of the
leaching reactors the dissolving of zinc oxide is
completed. However, something else also
happens...
Minor dissolving of zinc
pH1.0
pH2.5
pH3.8
pH4.5
pH5.2
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We add air into the slurry inside the reactors to
create an oxidation reaction.
Oxidising
air
line
Oxidising
air mixed in with circulating slurry
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As result of adding oxidation air the Fe2 iron
changes to Fe3.
Fe2 becomes Fe3
After the pH goes above pH3.7 iron (Fe3)
precipitates as iron hydroxide particles (FeOH3)
pH1.0
pH2.5
pH3.8
pH4.5
pH5.2
Oxidation air
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By the end of the leaching process we have
separated the zinc from the iron by dissolving
the zinc. This slurry is known as neutralised
solution.
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We now have to separate or remove the insoluble
particles (iron hydroxides and silica) from the
liquid phase of the slurry. This is carried out
using a gravity settling process.
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Gravity settling is carried out in large diameter
but shallow tanks called thickeners.
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If you could look inside a thickener, this is
what you would see.
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The gravity settling process is accelerated by
adding a flocculent into the slurry. This acts
like a glue or bonding agent and causes the small
suspended particles to clump and form larger
particles. By increasing the mass of the
suspended particles they settle out more rapidly.

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We have now removed the iron and the silica.
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Summary

• In the Leaching Process the zinc oxide is
  dissolved in
• spent solution.
• Using an oxidation reaction and pH control the
  iron is
• precipitated out as solid iron hydroxide
  particles.
• The solid iron hydroxide and silica particles
  are gravity
• separated out of the liquid, neutralised
  solution.
• All that remains to be removed now is the
  dissolved
• copper, cadmium, cobalt, nickel and calcium.