Ball Mills Before the gold could be extracted, the rock was turned to powder. Depending on the size of the steel balls inside the mill, the rock would be reduced to a certain size. So, multiple mills were usually used in stages. Similar Images ...based on the tags: collapsing, doors, gears, machinery, mills... These were some of the most attractive shops of all the mines in the area. It’s no wonder Hanna Mining wanted to use them as their center of operations in the Iron River district. An automatically closing door, in case of fire or flood in the engine compartment. In a strange loft next to the brewhouse are these twin kettles, which seem much older than the main kettles in the brewhouse. When a big motor rusted free of its ceiling mount, it smashed onto this workbench. Hunter and the Hoist House. These stairs lead to the balcony. This might have been part of the Pioneer Pellet Plant. It looks to be a ball mill, which pulverizes ore by spinning it with thousands of ball bearings. There big filters helped the mill sort through the flour, for additional milling, for example. There is a cool old air compressor in the corner of the powerhouse.