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In the process for electrolysis for zinc, three things must be done:

  1. Leaching the ZnO
  2. Purifying the ZnSO4 by means of cementation
  3. Performing the electrolysis

However, step 2, as it is presented by Wikipedia, is quite vague. In particular, it says:

The purification process utilizes the cementation process to further purify the zinc. It uses zinc dust and steam to remove copper, cadmium, cobalt, and nickel, which would interfere with the electrolysis process.

However, when looking it the definition of cementation, a crude summary of what happens is that one metal gets oxidized in order to reduce the other ion. I am assuming that this means that the zinc sulfate is getting reduced. However, what is reducing it? What equations are involved? I apologize if this question does not make sense, as chemistry is not particularly my strong suit, but I believe that this has gone unanswered online so far.

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  • $\begingroup$ I took the liberty of adding the relevant hyperlink. $\endgroup$ – Ivan Neretin Oct 19 '17 at 21:15
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As with nearly all STEM-related Wikipedia pages, this one reads quite clear and concise... that is, if you already know at least 75% of the stuff involved. Otherwise it may lead you into the deepest muddle, so you did the right thing by asking this question.

Follow the logic. What is electrolysis used for? It reduces a metal (in particular, zinc) from its salts. You don't reduce what's already been reduced. This means zinc was not reduced before the step 3. In particular, it was not reduced during the cementation. Still, cementation implies that one metal is reduced by another. Which of the two is zinc if we've just concluded that zinc does not get reduced on this step?

Since the naturally found compounds are rarely pure, zinc sulfate after the step 1 contains a few percent of other metals (copper, nickel, etc.), also in the form of sulfates. Should we electrolyze it now, these metals will get reduced along with our zinc and remain in it as impurities, which we don't want. Worse yet, they are mostly less active than zinc, so they will reduce before it and thus concentrate in the produced metal. How do we get rid of them?

Now comes the idea: let's reduce them. We vaguely remember this can be done with a more active metal. But it must not be too active, otherwise it will reduce zinc as well. Also, the oxidized form of this new metal must not spoil our $\ce{ZnSO4}$. Just what metal can that be?

Guess what?

It is zinc. Yes, we add metallic zinc (just a few percent, I remind) and it reduces the impurities as follows: $$\ce{CuSO4 + Zn -> Cu + ZnSO4}$$ The only soluble byproduct is $\ce{ZnSO4}$, which does not upset us one bit.

So it goes.

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  • $\begingroup$ Wow, this is the perfect answer. Thanks so much! $\endgroup$ – Eames Oct 19 '17 at 21:57

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