The mining industry needs effective separation methods to recover fine metal and mineral particles from different streams formed during the mining and ore beneficiation processes and to treat various process waters. Filtration of very fine particles (smaller than 10 µm) is challenging due to blockage of the filter media. Traditional filtration systems need to be cleaned or even replaced when the filter media is blocked. This consumes time and money and increases the environmental load in many ways. There are some membrane filtration applications using backflushing as self-cleaning, but they are less suitable for the required scales of the mining industry.

The objective of this study was to investigate how well the filtrate flux could be restored by the self-cleaning method of the Sofi Filter®, that uses ultrasound and backwashing, in the filtration of fine magnetite at two different feed concentrations. The Sofi Filter® is a high velocity filter 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 0.3 µm.

Magnetite powder (D₅₀ 2 µm) was mixed with water to form suspensions of 50 and 500 mg/l. The magnetite suspensions were filtered in dead-end mode with the self-cleaning Sofi Filter® with stainless steel wire mesh filter elements of nominal pore sizes of 0.3–5 µm. The self-cleaning is conducted by ultrasound and then backflushing the detached solids out of the system with filtrate, thus forming the reject. The applied feed pressure during filtrations was 1bar on average. The flux was over 30 m³/(m²h) in the beginning of the filtration.

The flux decline during filtration of magnetite was rapid; the flux dropped as much as 91 % during the filtration of magnetite at 500 mg/l feed concentration (Fig. 1). The higher the feed concentration, the faster and greater the flux decline was during filtration.

The self-cleaning was able to restore the flux fully (Table I). Even with the higher feed concentration the initial capacity was restored by self-cleaning. Particle retention during the filtrations was 40–60 % at 50 mg/l feed concentration and 50–85 % at 500 mg/l feed concentration. Elements with smaller pore sizes yielded higher retention, as is to be expected. Low feed concentration combined with very fine particles is a true separation challenge, which can be observed as relatively low retention values achieved from the filtration with 50 mg/l feed concentration.