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"mills" is also used as a crossword clue. If that's what you saw, the answer is often factories (9 letters).
Definitions
Factories or machines that grind or process material.
Word origin
From mills, in reference to mill owners and mill workers.
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Grinding factories”5 letters
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