COMMENTARY — Japan’s zinc-lead Toyoha mine a study of

Up on the island of Hokkaido, almost in the northern-most point of Japan, the Toyoha mine is in full swing.

The zinc and lead mining operation is a study of extremes, especially during winter months as the sub-zero temperatures and metres-thick snow conflict with the heat and steam emerging from the heart of this mountain mine through hot springs and mine ventilation shafts.

Workers can’t ignore the contrast when walking to the mine dressed up for winter, only to strip down to a T-shirt and shorts to spend the duration of the shift at temperatures of up to 70 C. Sweaty and exhausted after work, they once again have to face the sub-zero gusts from the cooling air intake as they leave the mine.

Situated in the middle of a hot springs area, the Toyoha mine encounters extreme, in situ rock temperatures exceeding 120 C in the new development areas at a depth of 600 metres below surface.

Cooling air is blown down to the mine at a rate of 20,000 cubic metres per minute through a set of central service shafts. The air is then dispersed back to the surface through the surrounding shafts and raises. Further cooling is provided by water sprays in the high-temperature areas. During the summer season, even though taken from the nearby river, the water has to be cooled by refrigeration to ensure that workers are comfortable. All water piping is covered with insulating materials.

The main mining method is sublevel stoping of the 2-to-10-metre-wide orebody, which dips at a 70 angle.

The stoping holes at the sublevel drifts are drilled either in a parallel fashion downwards or slightly fanned, to a maximum length of 30 metres. The in situ ore averages 11.5% zinc, 2.5% lead and 200 grams silver per tonne. The hoisting is currently at a level of 40,000 tonnes per month. Toyoha is drifting an average of 550 metres of development tunnels and 100 metres of raises in a month.

— From a recent issue of “Tamrock News.”