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QM March 2007 www.quarrymanagement.com 19
Well ScrubbedThe selection of washing equipment for processing of clay-contaminated
rock and aggregate
By Mike Burton, managing director, Sepro Mineral Processing Int’l Ltd
Efficient clay removal from ‘as
dug’ gravel and sand is of
increasing importance inmaximizing product value
and mineral resource utilization.
Similar incentives are driving the
reprocessing of high clay content
scalpings stockpiles in hard rock
quarrying operations.Four types of process
equipment are typically used for
clay dispersal:Ore scrubbers: drums that run at
relatively high-speed with heavy-duty high-lift liners.Their intensive
washing action is characterized by
vigorous turbulence and
cataracting of the charge under
the influence of the speed of
rotation and the form of thelifters, which are custom designed
for each application.
Washing barrels: drums that run atrelatively low-to-medium speed,
generally with thinner liners and
lower lifters than scrubbers.Theirwashing action is characterized by
tumbling and rolling of the
material in the toe and body of
the charge.Log-washers: usually comprise twincontra-rotating shafts fitted with
a series of intermeshing angled
blades.Their washing action is
generated by inter-particle
attrition between the blades,
which also transport materialuphill from the feed point to the
discharge point.
Attrition cells: multi-stage tanks,
each fitted with a high-powered
agitator with opposing-flow
blades.Their washing actioninvolves vigorous inter-particle
attrition at high pulp density.
Attrition cells are only suitable
for sand-size feeds, leaving three
process options to be consideredfor new or retrofit aggregate
washing plants. Of these,
traditionally only washing barrelsand log-washers have been
considered for clay removal
applications in UK operations –
until
recently.
Washing
barrels have
been widelyused
historically
but their
relatively
low speedcan tend to
agglomerate,
or fail to
disperse clay
masses that occur in some
deposits, leading to themisconception that rotary-drum
washing equipment is inferior to
log-washers in handling feeds with
high clay contents. Consequently,
log-washers have sometimes been
specified in the expectation of
better performance inapplications with high clay
contents, despite limitations on
feed size and higher maintenance
costs.Overseas experience with
rotary ore scrubbers in
metallurgical ore washing plants
has led to very different
conclusions.These plants typically
process ores with very high claycontents.The viability of such
operations depends on the
efficient dispersal of this clay
content for efficient recovery of the metal ore.The rock and sand
fractions that are rejected mustbe super-clean to avoid the loss
of high-value ore minerals, which
typically represent only 0.01–1.0%
of the plant feed. By using high-
speed scrubbers in these
applications, it has been possibleto obtain a metal recovery rate of
99%, compared with only 60–70%
when using washer barrels or log-
washers, which do not fully
disperse the clays or completely
remove clay coatings from therock and sand particles.
High-speed scrubbers operating
in washing plants for alluvial
precious metals, nickel/cobalt,tropical laterites, diamonds and
similar ores, with clay contents
up to 90% in some cases, have
demonstrated that rotary
scrubbing, when properly applied,
can totally disperse the clays toallow liberation of the very fine
mineral values.Without completedispersion, metal recovery levels
rapidly decline, which has driven
the optimization of the washing
equipment used. Performance inthis type of washing plant, in
various countries, has clearly
demonstrated that high-speed
rotary scrubbers offer an effective
solution to efficient clay removalfrom rock and sand.
Beneficial features of modern,
well-designed, heavy-duty rotary
scrubbers that are applicable to
the processing of gravel, crushedrock and sand with significant claycontamination include:
––an ability to efficiently handle
much coarser feeds than log-
washers or washer barrels,
with plants running
successfully that areprocessing run-of-mine or
primary crushed feeds up to
500mm.This eliminates the
need to scalp the feed ahead
of the washing process andremoves the risks and
operational problems of feeding unwashed scalper
oversize to secondary or
tertiary crushers
Log-washer reject
reprocessing plant
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20 www.quarrymanagement.com QM March 2007
––no de-sanding of feed orcountercurrent operation is
required. Scrubbers operate
efficiently with a simple co-
current flow of solids and
water, irrespective of feed,
clay and fines content ––tolerance of wide fluctuations
in feed rate and quality
––very low maintenance costs
––very high availability, typicallyin excess of 95% on
24 hours-per-day, 365 days-per-year mining operations
––disintegration of weathered
mineral, soft siltstone and
similar deleterious
contaminants ––very clean product surfaces
due to the vigorous washing
action and absence of dead
areas, which are a particular
characteristic of log-washers.
Two examples of successful orescrubber applications are
summarized below:
Bonte Gold Mines,
GhanaBonte Gold Mines previously
operated a 3.0m diameter x 8.0m
long washer barrel to process
350 tonnes/h of terrace alluvial
gold ore containing up to 60%clay interspersed with quartz
boulders up to 600mm in size.
The gold content of
approximately 0.5g/tonne wasintimately associated with the clay
fines. Gold recovery was less than
70% due to a lack of full clay
dispersion and consequent loss of gold particles tied up in clay ball
agglomerates and residual clay
coatings.This machine was
replaced with a Sepro 3.0m
diameter x 8.0m long heavy-duty
ore scrubber, which immediatelyincreased the gold recovery rate
to over 95% and subsequentlymaintained this level at an
increased feed rate of 700
tonnes/h (figs. 1a and 1b).
Moa Nickel SA,
CubaMoa Nickel operated four 72in
log-washers handling up to700 tonnes/h of extremely sticky
and fine lateritic nickel/cobalt ore
with an inherent as-mined
moisture content of 35–40% and
5–15% serpentine rock content.
The feed was scalped at 120mmon a wet vibrating grizzly to
protect the log-washers from
oversize rock, leading to metal
losses in lump clay and coated
rock in the grizzly oversize.Thelog-washer product was screened
at 0.85mm with large volumes of
sprayed water but undispersed
clays in this material carried 30%
of the nickel content in the ore
to reject.After several years of fruitless
experimenting with changes tothe log-washer operation, and
tests by various Canadian
manufacturers and consultants to
improve metal recovery, a 1.2m
diameter x 2.1m long Sepro
scrubber was purchased and usedfor test work on reprocessing the
log-washer product.These tests
showed that intensive scrubbing
increased recovery from 70% to98% and, as a result, a 2.5m
diameter x 6.0m long Seproscrubber was installed as part of
a 300 tonnes/h log-washer reject
reprocessing facility.This plant
was commissioned in 2003 and
metal recovery has been
maintained above a guaranteed98% level ever since (figs. 2a and
2b).
This demonstration of the
benefits of high-speed intensive
scrubbing has led to a current
project to replace the log-washersand expand plant capacity with
the installation of two 3.0m
diameter x 10.0m long scrubbers.
Aggregate
washingThe effectiveness of intensive
scrubbing on typical UK
aggregates has been
demonstrated by two installationswhere 2.1m diameter x 5.0m long
high-speed scrubbers have been
installed in preference to larger
conventional washer barrels.The
first scrubber processes heavilycontaminated as-dug gravel and
handles up to 500 tonnes/h,
producing a clean washed
product.A similar scrubber
installed in 2004 processes
300 tonnes/h of granite primaryscalpings with substantial clay
contamination. In both
installations the machines were
selected in preference to larger
conventional washer barrels, and
both have fully vindicated thisdecision by their performance,
reliability, spare capacity and
very low maintenance costs.
ConclusionHigh-speed intensive ore
scrubbers have been shown
to be extremely effective in fully
dispersing clay coatings andagglomerates to deliver clean
washed rock products, and
will fully repay their consideration
as the equipment of choice for
new or retrofit stone-washingapplications.
Fig. 1a (above):‘As
dug’ gold ore
Fig. 1b (above right):
Clean quartz reject
from the scrubber
Fig. 2a (below): Log-
washer screen oversize
reject feed to a
scrubber
Fig. 2b (below right):
Clean washed reject from the scrubber
Washing & Filter Pressing