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In the metallurgy of $\ce{Pb}$, Galena is first concentrated by froth floatation process. It is then partially roasted to and self reduced to obtain $\ce{Pb}$.

Why is galena only partially roasted(that is in limited oxygen supply)?

My initial thought was that sulphur could further react with $\ce{O2}$ to form $\ce{SO2}$, but in the reaction:

$\ce{3PbS + 5O2(limited)-> 2PbO + PbSO4 + 2SO2\uparrow}$

we get $\ce{PbSO4}$, which I dont think reacts with $\ce{O2}$. Whats the problem here then?

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Your equation is the sum of two different and independent equations, namely $$\ce{2 PbS + 3 O2 -> 2 PbO + 2 SO2}$$ $$\ce{PbS + 2 O2 -> PbSO4}$$ And these equations are no use for producing metallic lead. They are not a moderate oxydation, as the ratio $\ce{O2/PbS = 5/3}$. Producing metallic lead by moderate oxydation requires a reaction based on the equation $$\ce{PbS + O2 -> Pb + SO2}$$ And here the ratio $\ce{O2/Pb = 1 < 5/3}$

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  • $\begingroup$ I'd take ratio of $\ce{2PbO :: 1PbSO4 }$ with a grain of salt. $\endgroup$ – MaxW Jan 26 at 17:41
  • $\begingroup$ @MaxW. You are right. The ratio $\ce{PbO:PbSO4}$ is arbitrary. I don't know why Newbie has chosen $2:1$. There is no reason. $\endgroup$ – Maurice Jan 26 at 20:39

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