Sotkamo Silver is looking to become one of the few precious metal operators using X-ray Transmission (XRT) ore sorting technology at its project in Kainuu, central Finland.
The project, which IM visited as part of the Finland Mine Safari for analysts and investors organised by Mining Finland this week, is expected to start production in the March quarter, ramping up to initial capacity of 350,000-450,000 t/y from a combined open-pit and underground operation.
At this rate, the mine should produce some 1.1 Moz/y of payable silver over a seven year life at all-in cash cost of around $9/oz, along with significant amounts of zinc, lead and gold within three saleable concentrates (Ag-Au-Pb, Zn-Ag and pyrite-Ag).
In order to cut processing costs and improve grades, Sotkamo is looking to install an Outotec-TOMRA XRT ore sorter at the operation. The machine, which will be acquired through a low interest loan from a Business Finland subsidiary, will be used after two-stage crushing (jaw and cone crushers) has taken place and the rock is some 30-70 mm in size.
Test work in 2015 on some 2,200 kg of Sotkamo samples showed the silver content from low-grade ore increased some 1.9x to 116 g/t Ag, while the average silver content in ore was boosted 1.43x to 195 g/t Ag. Silver recovery from low-grade ore samples was 88%, while average ore-grade recoveries were 89%.
In addition to this, about 60% of the rock previously classified as low-grade ore was removed as gangue with the XRT trial at TOMRA’s facility in Germany, with some 43% of rock reporting as gangue from the average grade ore samples.
CEO Dr Timo Lindborg said the sorting technology did not locate the silver within the ore, but recognised lead. This would enable the company to upgrade the silver grade within ore reporting to subsequent grinding, flotation and filtration processes.
During the site visit, IM saw the mill building being constructed, witnessed pre-stripping of the openpit and works on the tailings facility, and saw the already-completed decline down to 350 m depth.
The company is currently in the process of selecting a mining contractor to carry out both openpit (conventional truck and shovel) and underground (longitudinal bench and fill, using cemented backfill where necessary) works.
The 2017 technical report envisaged using electric-hydraulic production drilling jumbos, diesel LHDs (remote controlled where possible) and diesel dump trucks underground for mining the 20 m high, 60-70 m long and 5-30 m wide stopes, along with backhoe excavators and articulated or dump trucks for open-pit mining.
Sotkamo Silver expects to extend the 2.6 km-long decline, which was constructed by Outokumpu back in 1993, down to the 750 m level in year three, while it is also weighing up the use of both solar and wind power at the operation further down the line.