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Installation and Optimization of aGravity Circuit at Northgate Minerals

Kemess Mine

Eliab RomanKemess Mines

Michael FullamKnelson Gravity Solutions

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Northgate Minerals Kemess Mine

52,000 tpd copper goldmine located north inNorthern BritishColumbia

Copper75 million

lbs/year Gold300,000

ounces/yr

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Northgate Minerals Kemess Mine

Two main ore types

Supergene, highcopper/native copperore

Hypogene, coppersulphide ore

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Hypogene ore

High pyrite hypogene ore starting to be

processed

Low is 3-5 pyrite ratio typical 4 high is 5-15 typical 7

This will also be similar to Kemess North ore,which is an identified new resource

This ore is fine grained, and requires fine regrindsize to liberate chalcopyrite from pyrite.

This causes losses of free gold in cleaner circuit

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2005 AMTEL Mineralogy Study

Change in ore causedhigh losses of finefree gold in cleanerscavenger tails

Free gold mostly

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Data required for determination of gravity recovery

Target stream was the regrind cyclone underflow

Model gravity recovery in regrind Common to model gravity recovery in primary circuits

Never been done in regrind

Modelling gravity recovery Ore

Classification

GRG unit recovery

Samples sent to Knelson

Rougher concentrate Cyclone feed, underflow, and overflow

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GRG of the Rougher Concentrate

GRG Test of therougherconcentrate

Two stage test

Process as is

Grind to p80 of 38microns, same asregrind circuit

0

10

20

30

40

50

60

70

80

90

100

10 100 1000

Particle Size (microns)

CumulativeG

RG

Value(%)

Stage 1

Stage 1+2

0

20

40

60

80

100

10 100 1000

Particle Size, m

%C

umulativeRecover

Stage 1

Stage 2

Stage 3

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Classification

Samples ofcyclone streamsfor GRGdetermination

GRG partition

curve calculated

Curve showedunusual shape atfine sizes

Likely sampleprocessing issues

0

10

20

30

40

50

60

70

80

90

100

10 100 1000

Particle Size (microns)

GRG

RecoverytoUnderflow

(%)

Measured

Corrected

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GRG Unit RecoveryProblem 1

No hard data on Knelsons operating in regrind

Different from primary applications Fine GRG

Fine gangue

High sg gangue

Large amounts of pyrite (concentrate bed erosion)

Need data

Pilot or full scale?

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Gravity Recovery

Initial modelling using assumed Knelson unit reocveryshowed good potential for meaningful gravity recovery,but. The goal was not simply to recover gold by gravity, it was to reduce losses

in cleaner scavenger tails by using gravity

To quantify benefit, need full scale installation

Decision made to install single Knelson XD-30 VG ontrial

Very modest installation costs No pumping

Add gravity con to final con

Structure required to support concentrator (~$250,000)

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Optimization Program

Large Optimization program carried out jointly byKemess and Knelson

Cone style (six cones styles tested)

Fluidisation water flow (low to high) Cycle time (5-15 minutes)

G force (60-150)

Feed density (undiluted/diluted)

Feed rate (38-134 mtph)

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Results - Optimization Program

Best parameters

Short cycle times

Low range of

fluidisation water flow

High g force (~150g)

Of the operatingparameters, cycletime most

dominant

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5

10

15

20

25

30

35

40

10 100 1000Particle Size (microns)

UnitGRGRecovery(%)

Test 29 - 10 minute

Test 31 - 5 minute

Test 25 - 10 minute

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ResultsCone Styles

Generation 7 Cone

Fully fluidised, highmass yield

Low water flow

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5

10

15

20

25

30

35

40

45

10 100 1000

Particle Size (microns)

UnitGRGRec

overy(%)

Test 21 -G7 90g

Test 22 - G7 150 g

Test 24-G5

Test 25-G5

Test 26-G5

Test 28-G5

Test 29-G5

Test 30-G5

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Gravity Recovery

Gravity Circuit Operating Conditions

One XD-30 operating with G7 cone, and fullyoptimized

Treating 40-50 mpth (~25% of CUF)

Gravity recovery of ~10% of total gold (to regrind)

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Gravity Recovery

Why regrind, and not the primary circuit?

Kemess primary circuit, modeled recovery in 2003:

Four XD-48s at 300-400 mtph each

Four 6 x 16 screens Pumping

Capital investment of several $M

5-9% recovery by gravity

The advantage of being able to model recovery

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Gravity Recovery Benefit

Knelson on/Knelson Off trials

Two sets of on/off trials

Each set consisted of multiple days of Knelson

on/Knelson off, with samples of rougher con andcleaner-scavenger tails collected

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Preliminary Results

1.0-1.8% overall gold recovery benefit at 10% gravity

recovery (by assay) Ore dependent (higher pyrite, higher benefit)

2,700-5,000 ounces/yr

$1.7-3.0 M/year, but shipping penalty of ~$300k/yearfor additional concentrate produced, which is addedto final copper concentrate

Rapid payback

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Quantify Recovery Benefit

Characterize gold in cleaner-scav tails, withand without gravity

Knelson lab unit

AMTEL, mineralogy

Results pending

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Cycle Timethe down side

Shorter is better, but

Produces more con ollutes the copper con Costs money to ship

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Cycle TimeRemedies

Grade biased tofines

Gangue biasedto coarse

Top 50% mass

accounts for 5%gold distribution

SCREENING

Mass (g) Wt % Au g/t %Dist'n Au

1700

1180 0.0 0.00

850 0.0 0.00

600 0.0 0.00

425 0.0 0.0

300 0.0 0.0212 14.9 3.8 1.58 0.1

150 23.9 6.1 3.24 0.2

106 51.0 12.9 7.62 1.1

75 101.6 25.7 13.66 3.9

53 130.0 32.9 28.6 10.4

38 56.5 14.3 136 21.4

25 9.8 2.5 638 17.4-25 6.9 1.7 2373 45.6

Total 394.6 100.0 90.9 100.0

Microns Knelson 30" Conc

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THANK YOU