We now have to separate the copper, cadmium, nickel, cobalt and calcium from the neutral solution'

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We now have to separate the copper, cadmium,
nickel, cobalt and calcium from the neutral
solution. All of these elements are also
dissolved in the solution with the zinc. Because
they are dissolved (ions) and not suspended
particles, gravity separation cannot remove them.
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The neutralised solution is pumped into another
set of reactors - similar to the leaching
reactors. In the first set of reactors the copper
and cadmium are removed. We do this by adding
very fine zinc powder and mixing it with the
neutral solution. The copper and cadmium are
displaced out of solution because the copper and
cadmium are more electro-positive than zinc.
CdSO4 Zn ZnSO4 Cd (in
solution) (Solid) (In
solution) (Solid)
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Because zinc has a stronger tendency to attract
electrons than copper and cadmium, it will
chemically bond and form zinc sulphate. This
causes the copper and cadmium to revert from a
sulphate (ion) to a solid particle. This is what
is meant by the term displacement.
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Zinc powder is added into neutral solution.
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Now that the copper and cadmium are present as
solid particles we must remove them from the
liquid. Unfortunately the particles are very
fine and using a gravity settling method of
separation would take too much time. For this
reason we pump the liquid through a pressure
filter.
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Filter Cloths
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Filtrate - liquid containing zinc, cobalt, nickel
and calcium goes to the second stage reactors to
remove the cobalt and nickel.
Filter cake - the solid cadmium and copper
trapped in the cloths is collected in drums for
further processing.
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The cobalt and nickel is removed in the same way
that cadmium and copper are removed.
Fine zinc dust is added into the solution which
causes the cobalt and nickel to precipitate out
as solid particles. The solid particles are
removed using another pressure filtration system.
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As a final means of removing any residual nickel
and cobalt particles the solution is then put
into a super-settler. This works in similar
fashion to a thickener.
The liquid that is overflowed out the top of the
settler now only contains zinc and calcium - we
are almost there!
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Ferrite Pond
The settled solids in the super-thickener
(underflow) are pumped from the bottom of the
super-settler to the ferrite ponds.
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The solubility of calcium is proportional to the
temperature of the solution in which it is
dissolved. Lowering the solution temperature
reduces the super-saturation point. As a result,
by reducing the temperature of the solution, the
saturation point is lowered causing calcium to
precipitate out of solution as solid particles.
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The calcium precipitates and crystallizes as
gypsum.
The gypsum is gravity separated from the solution
in a clarifier, which operates like the
super-settler. The solid gypsum is then pumped
to the Gypsum pond.
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What we are left with now is called zinc sulphate
solution. All unwanted elements have now been
removed. The only problem is that we still do
not have any zinc metal. We now have to use an
electrolysis process to extract the zinc as a
metal.
Zinc sulphate solution (contains 165gL of zinc)
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Summary

• In the Purification Process the unwanted
  copper,
• cadmium, cobalt, nickel and calcium is
  removed.
• Copper and cadmium is removed first.
• Nickel and cobalt is then removed and pumped to
  the
• zinc ferrite pond.
• Calcium is precipitated out of solution as
  gypsum and
• pumped to the gypsum pond.