Hoppers

A row of steel hoppers at Ashio Copper Mine hangs above one of the processing levels, holding crushed ore between stages of the milling and flotation circuit. Each hopper is a riveted-steel cone narrowing to a discharge gate at its lower point, suspended from the structural steel above on heavy lugs and bolts. The hopper walls are darkened with the bloom of ore dust that accumulates on any vertical surface in a mineral processing plant. A catwalk runs along the top of the row, with access ports cut into each hopper for inspection and cleaning. Below the discharge gates, the next conveyor or chute carries the ore on to its next stage. Light here is filtered through the dust-coated windows on the side of the building.

Hoppers are the buffers between the continuous stages of a mineral processing plant. Crushers and mills run at their own steady rate; flotation cells take their input from the hoppers above, drawing the ore down as the cells demand it. The arrangement decouples the upstream from the downstream and lets each stage be maintained without shutting the whole plant. At Ashio the hoppers carried ore between the mills and the flotation cells across the working life of the works under Furukawa from 1877 to the Excavation Department's closure in 1973. They have been still since.