Ball Mills

A row of ball mills at Ashio Copper Mine runs along the length of the milling area, each drum mounted between heavy steel pedestals with drive gears at one end and discharge housings at the other. The drums are the riveted-steel construction of an earlier industrial era, the fasteners hand-driven in long rows along the cylinder seams. Each mill is sized for continuous operation with several tonnes of steel balls inside, tumbling the ore against itself as the drum rotates. The drives are belt-fed from a line shaft above, the belts now hanging slack or removed entirely. Ore residue has built up around the discharge ends in pale grey crusts. Light coming through the side windows falls across the row at an angle.

The ball mills at Ashio reduced crushed ore to the powder consistency required for the flotation cells downstream. Each mill ran for years between major overhauls, with the steel balls topped up regularly as they wore down. The plant's operational life under Furukawa modernisation began in 1877. The Excavation Department closed in 1973, and the mills stopped at the same time. The line shaft drive arrangement in this row dates from an earlier generation of the plant; later mills elsewhere in the works were direct-drive. The mills have stood untouched since closure.