bc & yukon branch - srk.com mill feed quality issues (2) hardness evident in sag/ag pebbles...
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Adrian Dance & Bob McCarthySRK Consulting (Canada) Inc.
November 26th, 2015
Mill Feed Quality:where did all the waste come from?
BC & Yukon Branch
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Mill feed quality issues (1)
Gradelower for all metals
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Mill feed quality issues (2)
Hardnessevident in SAG/AG pebbles
Complexityfine-grainedblend of oxide/sulphide mineralscontaminated concentratessuitable for pre or multi-stage processing options
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Dilution
Lack of selectivitylarge, open pit equipmentmass underground mining methods
Internal dilutiongrade heterogeneity
Ore vs. wastedo they have similar properties?can they be separated?
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Opportunities to selectively process?
Consider mill feed componentseasy to recover at lower costdifficult to recover at higher cost
incremental cost?• 75% to 85% to 95% recovery• finer grind, expensive reagents
each component• if isolated, is it ‘ore’ or should it be subject to a higher cut-off?
for example• coarse material after blasting & primary crushing
easydifficult
chal
leng
ing
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AG/SAG mill pebbles (1)
Are they worth returning?
JawCrusher
Cyclones
to LeachTanks
Stockpile
SAG Mill
Ball Mill
PebbleCrusher
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AG/SAG mill pebbles (2)
0
5
10
15
20
25
30
35
% P
ebbl
es
SAG Total Load, %18% 20% 22% 24% 26%
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Effect of pebbles (1)
On competent feed% pebbles proportional to loadup to… “1 tph pebbles ≡ 1 tph fresh”
Pebble distributionvery little broken materialwell rounded, coarse pebbles
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Effect of pebbles (2)
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‘Coarse Beneficiation’… exploiting a natural tendency
Classification by sizepreferential grade by size deportment
After two applications of energy…why recombine competent, coarse material with softer, fine material?coarse material requires higher kWh/t to processcompetent material should be evaluated at a higher cut-off grade
“ore” “good” “poor”
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Metal vs. size distribution
Bougainville Copper 1986
100mm50mm
“The Application of Pre-concentration by Screening at Bouganville Copper Limited”, Burns, R., Grimes, A., 1986.
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“Grade Engineering”®(CRC ORE)
88%
5%
7%
36%
26%
38%
“Transforming Mining Productivity with Grade Engineering®, CRC ORE, www.crcORE.org.au
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Deferring difficult material
Coarse screeningeliminate competent componentincrease mill throughput (lower kWh/t)• 15% to 20% higher tonnagepossibly lower grade• suitable for cheaper processing options
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Challenge…how to assess based on drillcore?
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Example drillcore test results (1)
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Example drillcore test results (2)
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Example drillcore test results (3)
“Development of Geometallurgical Laboratory Tests to Characterize Metal Pre-concentration by Size”, Carrasco, C., Keeney, L. & Walters, S. IMPC 2014
bulk
drillcore
RC cuttings
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Example flowsheet(involving ore sorters)
-25mm
-75mm+25mm
+75mmscavenger
30%
30%
cleaner40%
process separately?
high grade
lowest grade
lower grade
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Bulk/particle concentration
Homogenous
Heterogeneous
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Heterogeneity assessment
Analysis of drillcore assays
Spatial assessment
Variograms2.4m and 6.5m< 2% Zn+Pb
2.2m < 2% Zn+Pb
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Summary
Facing more challenging mill feedlower grade, harder, more complexis all of it worth processing?should some of it be separated, deferred or returned to the source?
Optionsupgrade before it hits the grinding circuitupgrade after it exits the AG/SAG mill
What is the cost of ‘optimal’ metal recovery?are we paying a high cost/low efficiency for maximum recovery?