Eliminating short circuiting to
improve feedwell functionality
Un-collected fines short circuiting to the
overflow
Jerold Johnson and Antonio Accioly of WesTech Engineering,
Inc. discuss an improved feedwell design for thickening
he feedwell is a key component of any
thickener. Improving its function, to
convert the horizontal momentum of the
feed to a vertical flow and promote flocculation
of the suspended solids, has been the focus of
recent designs. Traditional feedwell designs did
not allow for those functions to be
accomplished effectively. Short circuiting flows
carry the solids into the thickener prematurely,
before all the suspended solids have been
captured or properly flocculated. These flows
generate such force that solids are carried to the
overflow launder before settling can occur and
cause uneven distribution in the thickener bed.
To account for uneven distribution and short
circuiting of unsettled solids, operators have
traditionally added more flocculant or installed
larger sedimentation vessels. Neither solution is
cost effective. In addition, uneven deposition of
coarse solids can generate a “sand bar” to one
side of the thickener floor, causing torque
spikes as the rake arm slows in those areas.
Modern feedwell designs offer significant
improvements to overflow clarity, eliminate
short circuiting, and increase overall thickener
effectiveness.
T
feed flow exiting the feedwell increases.
However, the faster the feed velocity, the more
zxZXzxshear that is present to break up the
floccules. The AMIRA study, using CFD,
determined the potential for floccule generation
for several of the generic feedwells in the
industry. The below image is based on data from
the P266 report.
Floccule generation from AMIRA P266 report
Handling variable feed velocity
The conditions required for good flocculation in
the feedwell were part of a privately-funded
study by CSIRO and the AMIRA P266 thickener
projects. The study showed that the ability to
capture the solids and generate floccules that
have the desired settling rate is highly affected
by throughput, and inlet velocity is a crucial
factor.
As the entrance velocity decreases, premature
P8 International Mining | JUNE 2017 Supplement
Velocity range
across the mix area
in the outer
feedwell