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My question can also be generalized as, why are some sulfide ores (like $\ce{ZnS}$) roasted first to generate oxides, and only then reduced in a blast furnace or similar furnace?

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    $\begingroup$ What process would you suggest for the direct reduction? It is not that we have a cozy interface with a nice red button "Reduce". $\endgroup$ – Ivan Neretin Feb 2 '17 at 8:09
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The standard free energy of formation of most of the sulphides are larger than those of $\ce{CS2}$ and $\ce{H2S}$. Therefore, these sulphides are most stable than those with carbon or hydrogen. So, neither carbon not hydrogen can reduce metal sulphides to metals.

But standard free energies of formation of oxides are much lower than those of $\ce{SO2}$. Therefore, oxidation of metal sulphides to metal oxides is thermodynamically favorable. Also, the oxides are easily reducible.

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