Dealing with fine particles in flotation plants - some possibilities

Fine particles behave differently during flotation process. However there is no intrinsic lower limit of particles for separation by flotation process as we know flotation technique is applied to recover fine oil droplets and even ions. Hence the lower limit of the size of particles for separation by flotation technique differs from case to case.

As minerals are ground to finer and finer sizes their surface identity get lost due to high charge on their surfaces and become slow floating (eg 20 µm galena particles) or may not float at all ( eg phosphate mineral particles). All particles suspended in water are bound by a layer of water which moves as a part of the particle which is known as lyosphere. For the same percent solids, the water in the lyosphere increases as the particle size decreases, may see from Figs 1a and 1b in the attachment. And the thickness of water layer (Lyosphere) around the particle increases as the particle size decreases and hence slurries of fine particles are more viscous. Fine particles have low momentum (due to low mass) and hence the probability of effective bubble particle encounter reduces.

We note (Froth Flotation, 50th Anniversary Volume) that “ we should not create a problem which we can’t solve”. In other words excess grinding of minerals should be avoided in the first place. [1] Coming to 20 µm galena particles that float slowly, adding longer chain (Heptyl or Hexyl) xanthates in the scavenger feed box may help in recovering them. Gaudin reported (principles of Mineral Dressing) that heptyl xanthate improved the recovery of un activated sphalerite (100 to 600 mesh). This might be true in the case of galena too. [2] May note the following, “ Industrial applications have previously confirmed that applying greater power to flotation slurries yields significant improvements in fine particle recovery. However, recovery of the coarser size class favors a different flotation environment. An improvement in the kinetics of the fine and coarse size classes, provided there is no adverse metallurgical influence on the intermediate size ranges, is obviously beneficial to the overall recovery response. Managing the local turbulent kinetic energy dissipation, and hence the power imparted to the slurry, offers the benefit of targeting the particle size ranges exhibiting slower kinetics. FLSmidth recently introduced the practical implementation of this concept. In principle, it decouples flotation regimes where fine and coarse particles exhibit preferential recovery. In the case of naturally aspirated machines (Wemco®), it is referred to as Hybrid Energy Flotation™ and incorporates at least three phases:  Standard flotation machines (standard energy input, rpm, rotor size/type) at the beginning of the row, where flotation is typically froth phase limited and operational, and set-up parameters have a limited influence on the recovery.  Higher-powered flotation machines (high rpm, high power rotor size/type) at the end of the row to improve fine particle recovery.  Lower-powered flotation machines (low rpm, low power rotor size/type) to enhance coarse particle recovery. “ from, D Govender, D Lelinski and F Traczyk, “ HYBRID ENERGY FLOTATION™ – ON THE OPTIMIZATION OF FINE AND COARSE PARTICLE KINETICS IN A SINGLE ROW” , IMPC 2012. The above suggests that higher rotor speed in the scavenger cells might also help in recovering 20 µm galena particles. High rotor speed may harm flotation of coarse particles.
[3] Generating fine bubbles might increase the probability of particle bubble encounter.

Electro flotation that generate fine bubbles by electrolysis of water is one possibility. However this may not be possible in the existing plants. Alternatively dissolved air flotation (which is used to recover fine oil droplets in the water may come to rescue. Dissolving air in water by compressed air and injecting the water containing dissolved air into the existing flotation cells is possible. May see details here:

In the case of sulfide mineral the surfaces of fine particles may get oxidised faster. Sulfadisation (by adding Sodium Sulfide) may be required in case of surface oxidation of galena.

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