The term powder mill is usually used for a mill that manufactures blackpowder, a type of gunpowder.
A powder mill could be driven by wind or water power, and contained rollers for grinding the ingredients of gunpowder together, as well as presses and tumbling barrels and sieves for compacting, granulating, and grading the powder. Powder mills were built with an eye towards mitigating the danger of explosion, usually with one or more walls built purposely weak so that the force of an accidental explosion could be directed towards an open field or a river.
In 1802, the DuPont family started their industrial enterprises in the United States by building the Eleutherian Mills on the Brandywine Creek in Delaware. This was the start of large-scale gunpowder production in America, supplanting what had been primarily a cottage industry.
The place where the finished powder was stored was called a powder magazine.