Separation of ultrafine particles at mineral processing plants is a painful challenge. In some cases, a notable part of the concentrate, i.e. the product, is lost because of needs to grind finer to liberate and thus generating very fine particles down to one single micrometers. These valuable particles escape conventional processing. Fines may also cause issues in tailings thickening, from where water can either be discharged or purified and recycled back into mineral processing. Advanced filtration offers a potential solution for these challenges. However, conventionally filtration is challenging for ultrafine particles due to blinding of the filter medium and the subsequent loss of capacity in filtration.

The objective of this work was to investigate the separation of ultrafine particles from the overflow water of a tailings thickener at Oulu Mining School (OMS) using advanced fine particle filtration. The Sofi Filter® is a high velocity filtration unit which can be operated either in cross-flow or dead-end modes using most commonly stainless-steel filter elements with nominal pore sizes down to 0.3µm. The Sofi Filter® uses unique ultrasonic self-cleaning technology and is designed for separation of ultrafine mineral particles.

Ores from two mines, a polymetal mine and a silver mine, were used in concentration tests at OMS. Different pH adjusting chemicals were used in froth flotation leading to four different tailings thickener overflow waters for each ore. Each water was filtered with three filter elements having micron ratings between 0.3 and 20µm. The overflow waters were characterised by TSS and PSD measurements and filtration performance was evaluated by measured flux and calculated solid reductions.

The self-cleaning technology worked well for the particles in these overflow waters. The composition of the waters varied for TSS and PSD, which in some cases clearly had impact on the filtration capacity and solid reduction rate. TSS values in the overflow waters were 18–82mg/L. Median particle sizes were below 10µm for the water from polymetal ore processing and 9–97µm for the waters from silver ore processing. Fluxes from below measurement accuracy to over [...] were measured with solid reductions varying all the way from 0 to 100%, generally filter elements with larger pore size giving higher flux values than elements with smaller pore sizes.