Q.Why is zinc not extracted from zinc oxide through reduction using CO?

Step 1 – The temperature required for ZnO reduction

Reduction of zinc oxide to zinc metal requires a fairly high temperature, of the order of 1673 K, since zinc has a relatively high affinity for oxygen and its oxide is fairly stable.

Step 2 – What the Ellingham diagram shows at this temperature

We saw (Exercise 5) that the C,CO and CO,CO$_2$ lines cross at about 983 K: below this, CO is the better reducing agent; above it, carbon (C) itself becomes the more effective reducing agent, since the C,CO line continues to fall steeply (large positive $\Delta S$ for $2C+O_2\rightarrow 2CO$) while the CO,CO$_2$ line rises. At the ~1673 K needed to reduce ZnO, we are well above this crossover, so carbon is thermodynamically the stronger reducing agent, not CO.

Step 3 – Why CO specifically fails here

Using CO to reduce ZnO at this high temperature would not be thermodynamically as favourable as using carbon directly — the C,CO line lies below the Zn,ZnO line in this temperature region (making direct carbon reduction feasible), whereas CO's reducing power (governed by the CO,CO$_2$ line) is comparatively weaker at these elevated temperatures.

Step 4 – Industrial practice …

Note the high temperature (~1673 K) needed to reduce ZnO is above the C/CO crossover (~983 K), so per the Ellingham diagram carbon — …

• Treating CO as always the 'standard' reducing agent regardless of temperature — its effectiveness relative to carbon is strictly temperature-dependent per the Ellingham diagram. …