Cleaning Flotation

The cleaning-flotation cells at Ashio Copper Mine sit on a separate level of the processing plant, set up to re-treat the froth concentrate from the primary flotation circuit. Each cell is a smaller steel tank than the rough flotation cells, with the same agitator-and-froth-skimmer arrangement built in. The cells run in series along the floor, the output of one feeding the input of the next. Walkways pass between the rows. The tank walls carry the same pale blue-green residue as the rough cells, the dried mineral coatings holding to the steel where the slurry sat. Overhead a length of pipework runs along the underside of the floor above, carrying reagent feeds to the cells below.

Cleaning flotation is a secondary stage in the flotation circuit that upgrades the copper concentrate by re-running it through smaller cells with controlled reagent additions. Each pass increases the copper content of the froth and reduces the residual waste rock. Ashio's cleaning-flotation circuit ran alongside the rough cells through the operational life of the works under Furukawa modernisation from 1877 to the Excavation Department's closure in 1973. The cells have been still since. The residue inside them is the last slurry that passed through, fixed in place by decades of evaporation and drying.