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How auxiliary tank impeller configuration impacts

efficiency of solvent extraction process

Tank agitator designs are critical to the                 Figure 1
    efficiency of solvent extraction processes
in the mining industry. For optimum efficiency,      Typical solvent extraction system loop.
they need to be designed to optimize droplet
size distribution and minimize entrainment           bottom and four standard baffles). For practical
losses, while minimizing power input. SPX Flow       reasons, the laboratory tests used an equal
evaluated a number of different auxiliary tank       parts organic and aqueous solution of soybean
agitator designs to establish which performed        oil and water and so results were qualitative.
best in terms of time for phase separation, and      The first tank was set up with a typical impeller
shear and energy usage comparison. The results       configuration for solvent extraction processes
are detailed in this article.                        using three down-pumping, high-solidity axial
                                                     impellers that have large, wide blades. The
    Solvent extraction is a process whereby          other three tanks used one, two and three up-
a substance is removed from one solution by          pumping hydrofoil impellers (Lightnin type
dissolving it into another immiscible solvent in     A540). Hydrofoil impellers are considered low
which it is more soluble. It is widely used in the   solidity impellers, as they have narrow blades.
mining and minerals industry to extract minerals     These tend to have a lower power draw than the
from bodies of ore. Solvent is pumped onto the       high solidity blades at equal speed and diameter.
ore body and leaches the desired mineral out.        Testing was repeated three times for each
This solution is then pumped to first and second     impeller design to confirm consistent results.
stage extractors where the mineral is transferred
from the aqueous phase to the organic phase.             Average phase separation times across
Each extractor and stripper in the process will      testing showed the system using three hydrofoil
usually have a primary pump tank and at least        impellers gave the quickest process results with
one auxiliary tank.                                  91 percent of the solution separated after 120
                                                     seconds. Two hydrofoils achieved these results
    The primary pump tank mixes fluids to            in 151 seconds. The three high solidity axial
ensure contact between the organic and aqueous       impellers took 191 seconds and, finally, the single
phases for mass transfer. The auxiliary tank then    hydrofoil impeller took 217 seconds to achieve
blends and maintains dispersion for further mass     the 91 percent separation level. The conclusion of
transfer to a settler. The design of a primary       testing showed that the three hydrofoil impeller
pump tank agitator has a huge impact on the          configuration offered significant savings in terms
overall solvent extraction process. If it does not   of processing time, while minimizing power
produce a uniform blend of the mixture, the          input. Minimizing power input leads to lower
performance of the auxiliary tank will also be       operating costs. Not only that, the auxiliary tank
compromised. The auxiliary tank agitator needs       design with the shortest phase separation time
to provide gentle flow with as little shear as       can also correspond to a decrease in entrainment
possible in order to reduce entrainment losses.      losses.
Test results have shown that there is a significant
difference in tank system performance based              Computational fluid dynamics (CFD)
on the configuration of the impellers within the     modelling was used to determine turbulent eddy
auxiliary tank.                                      dissipation and link power dissipation with the

    Different customers may be interested in
different performance criteria for the auxiliary
tank agitator. For some, it may simply be
a question of cost. For others, faster phase
separation or reduced power levels may be top
process goals. For many, it is a balance between
all of the above in the aim to get the best
return on investment. SPX Flow evaluated four
auxiliary tank impeller configurations to see
which designs offered customers the benefits
they were seeking.

    The tests compared equally sized tanks (445
mm diameter, 0.069 m3 volume tanks with a flat

www.miningengineeringmagazine.com	                   Mınıng engıneerıng    november 2016 1
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