I’m somewhat familiar with ball mills (machines for crushing solids into powder using a tumbling drum also containing balls). But I can’t see how this particular item can be a ball mill. It’s old, removed from service, and built into a sort of nonsensical sculpture outside a museum. Here’s a guide showing which part of the sculpture I’m talking about:
https://www.hmdb.org/m.asp?m=131183
And here are some other photos of the sculpture, giving much clearer pictures of the ball mill:
I think ball mills are generally rotating steel tubes, horizontal or a little bit tilted, with powerful means to turn them slowly (like a cement truck), and means to fill or empty them, either in batches or continuously. The tube length is usually bigger than the diameter. There are also ball mills that make the tube orbit around to add centrifugal force to the gravity already acting on the balls.
The ball mill I’m talking about here clearly has rotating parts, but not a long tube. It also has what look like spring loaded arms pressing radially inward, if I see it right. I spent a while walking around it trying to make sense of it. Museum staff, at least the folks I found, knew nothing. Googling ball mill images gives me nothing even close.
Is it some special kind of ball mill? Is it misidentified? Help assuage my burning curiosity!