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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