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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