Learn French: Ces hommes étaient non pas des esclaves, mais des ouvriers libres. - These men were not slaves, but free workers.

Using non pas directly before the noun emphasizes a strong, direct contrast, much like saying 'not X, but rather Y' in English. It forcefully corrects an assumption. While n'étaient pas des esclaves, mais... is grammatically correct, étaient non pas... highlights the contrast much more strongly.

Normally, des changes to de in a negative sentence. Why does it remain des esclaves here?

There are two reasons! First, the verb is être (were), and the rule that changes des to de never applies to être. Second, non pas negates the noun phrase itself rather than the verb, so standard verb negation rules do not trigger anyway.

Where exactly does non pas have to be placed in this structure?

It must be placed immediately before the specific element you are rejecting. Here, the contrast is between the nouns des esclaves and des ouvriers libres, so non pas sits right before the first noun phrase to set up the mais (but) that introduces the correct one.

Could we place the adjective before the noun and say mais des libres ouvriers?