implementing strategies to improve mill capacity, with case studies

Implementing Strategies to Improve Mill Capacity, with Case Studies.Presented byNic Barkhuysen

From Recent Advances in Mineral Processing Plant Design; 2009; Malhotra, Taylor, Spiller, and LeVier, Editors; p. 433-443.

OverviewFine wet screening definition, mechanism, and rules of thumbDevelopment of high capacity multiple feed point screening machines and screen surfacesThree case studies describing the application of fine screens in closed circuit grinding

DefinitionsScreening: the process of separating particles according to size

Fine screening: separation of particles ranging from 10 mm (3/8 inch) to 38 microns (400 mesh)

Mechanism and rules of thumbDifferent for wet and dry screeningIn fine wet screening, the undersize particles are transported through the screen openings by the fluid; vibration promotes fluid throughput and oversize conveyanceReasonable efficiency is obtained at about 20% solids by volume (80% water)Wet screening process is essentially complete when most of the fluid has passed through the openings, and this usually occurs within a relatively short screen length

Multiple feed point machines1977 - Multifeed screening machineTypically 1.5 to 2 times more capacity than an equivalent single feed point machine

Multiple feed point machines1977 - Multifeed screening machineTypically 1.5 to 2 times more capacity than an equivalent single feed point machine2001 - Stack Sizer screening machineCapacity generally equivalent to 3 to 4 Multifeed machines

Stack Sizer ScreenUp to 5 vibrating screen decks operating in parallelTypically fed from a single or multiple stage flow distribution system, depending upon the number of machines

Stack Sizer ScreenUp to 5 vibrating screen decks operating in parallelTypically fed from a single or multiple stage flow distribution system, depending upon the number of machinesDriven by two 1.9 kW (2.5 HP) vibrating motors rotating in opposite directionsCapacity typically ranges from 100 to 350 t/h per machine depending upon screen panel opening, mass recovery to oversize, amount of nearsize particles, particle shape, and slurry viscosity

Urethane screen surfacesOpenings as fine as 75 microns (200 mesh)High open area (35 to 45% actual)Capacity similar to fine woven wireLong life (6 to 12 months is common)Rarely blinds due to slotted openings and tapered relief anglesUnique technology developed by Derrick

The application of fine screens in closed circuit grindingResearch work over many decades has demonstrated the benefits of improved classification in grinding circuits, butUp until recently, screens have rarely been considered as the primary classifier in mill circuitsWith the availability of high capacity screens and long life, non-blinding screen surfaces, it is now practical to consider screens to close grinding circuits, as illustrated by the following case studies

Three case studies demonstrating the use of screens in closed circuit grindingPhosAgro OJSC Apatit (phosphate mine in Russia)OJSC KMAruda (iron ore mine in Russia)Sociedad Minera El Brocal S.A.A. (lead-zinc mine in Peru)(Fourth case study in paper: Minera Cerro Lindo (copper, lead, and zinc mine in Peru)

PhosAgro OJSC ApatitPhosphate mine located north of the Artic Circle in northwestern Russia

PhosAgro OJSC ApatitPhosphate mine located north of the Artic Circle in northwestern RussiaCrushed ore is fed to multiple parallel lines of single stage ball mills in closed circuit with single or double stage hydrocyclones

PhosAgro OJSC ApatitPhosphate mine located north of the Artic Circle in northwestern RussiaCrushed ore is fed to multiple parallel lines of single stage ball mills in closed circuit with single or double stage hydrocyclonesCircuit product from all lines is combined and pumped to the flotation plants

PhosAgro OJSC ApatitDesired to improve classification efficiency, grinding efficiency, and mill production ratesScreening tests were conducted in 96-97 with a Multifeed screen but screens were eliminated as an option due to the high number of screens required and limited space availableWith the development of the Stack Sizer, Apatit wanted to take another look at the use screensContinuous pilot-scale closed circuit grinding tests were conducted in 2005 with single-deck Stack Sizer screen

Single deck Stack Sizer (screen width restricted to simulate proper loading for full-scale operation)

PhosAgro OJSC ApatitEncouraging pilot test results justified the next phase of the investigation, the conversion of one mill line to screensTo minimize the cost, Mill 24 chosen as it was the smallest line

Mill 24 grinding circuit - cyclone classification

New Feed(crushed ore)95 t/h

Overflow44% solids95 t/h

Underflow 80% solids409 t/h

Water72 m3/h

Flotation

Cyclone feed 504 t/h

77%solids

Water 48 m3/h

Ball Mill

3.6 m x 4.0 m

(430% Circulating Load)

Hydrocyclones

PhosAgro OJSC ApatitEncouraging pilot test results justified the next phase of the investigation, the conversion of one mill line to screensTo minimize the cost, Mill 24 chosen as it was the smallestCyclones were replaced with 4 Stack Sizer screens with 390 m urethane panels

Mill 24 grinding circuit - screen classification

New Feed(crushed ore)124 t/h

Four (4) Stack Sizer Screens

Undersize34% solids124 t/h

Oversize 82% solids167 t/h


Flotation

Screen feed291 t/h

Ball Mill

3.6 m x 4.0 m

Benefits of improved classification

Comparison of circuit products

Chart3

100100

93.290

82.373.6

73.162.4

55.747.3

46.538.2

Hydrocyclone Overflow

Stack Sizer Undersize

Particle Size, microns

Weight Percent Passing

Comparisons

Sociedade Minera El Brocal S.A

MicronsCyclone FeedCyclone UnderflowCyclone Overflow

% Pass% Pass% Pass

50072.471.698.9

30062.359.996.2

21249.245.091.7

15036.328.485.4

7520.511.467.7

MicronsStack Sizer FeedStack Sizer OversizeStack Sizer Undersize

% Pass% Pass% Pass

50069.522.298.4

30057.111.585.2

21252.810.578.0

15042.18.768.9

7538.18.256.3

Milpo - Compania Minera Cerro Lindo

=


% Pass% Pass% Pass

500100.0100.0100.0

300100.0100.0100.0

21263.952.194.4

15046.131.085.4

7522.58.458.9


% Pass% Pass% Pass

50091.088.0100.0

30075.354.999.3

21259.431.190.6

15047.820.575.0

7530.49.947.0

Compania Minera Condestable


% Pass% Pass% Pass

50078.064.096.0

30055.239.394.4

21242.727.081.9

15035.621.477.0

7525.915.153.3


% Pass% Pass% Pass

50077.063.098.0

30059.131.895.1

21248.419.685.2

15039.613.472.8

7527.98.752.0

OJSC Apatit


% Pass% Pass% Pass

40072.665.6100

32061.152.993.2

20043.83482.3

16034.524.673.1

10022.213.655.7

7117.710.346.5


% Pass% Pass% Pass

40064.036.8100.0

32051.422.390.0

20038.912.873.6

16032.09.062.4

10023.65.847.3

7119.04.538.2

0.427.434.40

0.3238.947.16.8

0.256.26617.7

0.1665.575.426.9

0.177.886.444.3

0.07182.389.753.5

36.0342688363.20

48.677.710

61.0658788887.226.4

68.046676519137.6

76.3617429894.252.7

81.0144756395.561.8

Comparisons

10099.26

94.410456713490.63

85.391053832375.04

72.026094709759.46

58.898824916447.01

48.301601210937.92

42.948445747333.59

38.605117642729.23

Overground Materialin Cyclone Circuit




Weight Percentage Passing

Grinding Circuit Product Size Distribution CurveMilpo Unit - Minera Cerro Lindo S.A.A.

Cerro Lindo Comparisons

99.9399.67

99.4895.09

97.8385.2

93.8272.75

87.1761.64

76.9152

66.2

54.12

Underground Materialin Cyclone Circuit






Grinding Circuit Product Size Distribution CurveCompania Minera Condestable

Silica Sand

100100

98.999.9

96.2498.04

91.6692.16

85.3885.16

77.378

67.768.9

59.8662.38

55.8656.26

51.64

49.28

46.98

Overground Material in Cyclone Circuit

Hydrocyclone Overrflow



Weigth Percent Passing

Grinding Circuit Product Size Distribution Curve Sociedad Minera El Brocal





Compania Minera Milpo S.A.A. - Cerro LindoCompania Minera Condestable

Control ParametersHydrocyclone D-26Stack SizerClassificationHydrocyclone DS-26Stack Sizer

Tonnange t/hr242275Ciculating Load243124

Ball charge %3737Classification Efficiency49.880.3

RPM15.515.5Mill DischargeHydrocyclone DS-26Stack Sizer

Critical Speed76.876.8% - 200 mesh25.927.9

Mill MotorHydrocyclone D-26Stack Sizer% solids7370

HP 3000Flotation FeedHydrocyclone DS-26Stack Sizer

Voltage39614139% - 200 mesh53.352

Amperage319316P80 microns196185

Kw21832200

Energy Comsumption (Kw-h/Ton)77

grams of steel/Kw-h778696

ClassificationHydrocyclone D-26Stack Sizer

Circulating Load244108

Classification Eficiency5879

Mill DischargeHydrocyclone D-26Stack Sizer

% - 200 mesh23.5847.01

% solids82.578.7

Flotation FeedHydrocyclone D-26Stack Sizer

% - 200 mesh5547

P80 microns140.5160

Satck Sizer ScreenHydraulic Classifier

Dimensions1.2 m wide x 1.5 m long2.6 m dia. upstream classifier

Feed Rate (t/h)120120

40300

PhosAgro OJSC ApatitFinal product grade from flotation circuit is similar, even though the flotation feed is coarserApatit is now moving forward with plans to upgrade two large mill lines in the newer grinding plant, each requiring 10 Stack Sizers, confident that the mill production rates will increase by a minimum of 30 percent

OJSC KMArudaIron ore mine in RussiaRun of mine ore is about 34% Fe2 stages of grinding each consisting of 2 ball mills operating in parallelPrimary mills closed with spiral classifiersSecondary mills closed with cyclonesEach grinding stage followed by magnetic separation

Secondary grinding with cyclone classification

Second Stage Wet Magnetic Separation

Tailings

Concentrate

Hydrocyclones

Third Stage Wet Magnetic Separation

Concentrate

Ball Mill

Ball Mill

Tailings

Magnetic concentrate from primary grinding circuit

OJSC KMArudaTwo Stack Sizer screens with 100 micron urethane panels installed in the secondary grinding circuit in place of the cyclonesGoal was to maintain the same final product gradeSubsequent testing and optimization resulted in the shut down of one of the two secondary ball mills

Secondary grinding with screen classification

Ball Mill

Magnetic concentrate from primary grinding circuit

Second Stage Wet Magnetic Separation

Undersize

Oversize

Tailings

Concentrate

Two (2) Stack Sizer Screens

Third Stage Wet Magnetic Separation

Concentrate

Tailings

Mineral Processing Journal, 2009, No. 2.

Sociedad Minera El Brocal S.A.A.Lead / zinc concentrator located in Peru at 4,300 meters above sea levelRun of mine ore is 2.4% leadThree rod mills in open circuit feed three ball mills in closed circuit with hydrocyclones; cyclone overflow goes to flotationCyclone underflow is 10 to 15% lead due to high difference in specific gravityCirculating load typically ranges from 350 to 400%Considerable slime losses before flotation

Rod Mill

Rod Mill

Rod Mill

Ball Mill

Overflow138 t/h

483 t/h

Ball Mill

621 t/h

Ball Mill

7 x 12

7 x12

7x12

8 x 10

8 x 10

8 x 10

New Feed 138 t/h

Flotation


Hydrocyclones

Sociedad Minera El Brocal S.A.A.Required additional capacity, so considered adding a fourth ball millFollowing full-scale tests, two Stack Sizer screens with 500 micron urethane panels were installed in place of the cyclones

Initial ImprovementsTwo of the three ball mills were shut downCapacity increased 10%Lead recovery increased about 9 percent due to reduction in slime losses before flotationScreen oversize is less than 2% lead (recall, cyclone underflow was 10-15% lead)

To take advantage of increased grinding capacityCrushing circuit was modified to increase capacity and reduce production of fines Rod mill speed was increased to achieve better throughputThree additional Stack Sizer screens were installedA second ball mill was put back into operationInstalled additional flotation and filtration capacityConcentrator throughput increased over 75%

Five (5) Stack Sizer Screens

Ball Mill

Flotation

Rod Mill

Rod Mill

Rod Mill

Ball Mill

147 t/h (60% Circulating Load)

392 t/h

7 x 12

7 x 12

7 x 12

8 x 10

8 x 10

New Feed 245 t/h

245 t/h

Comparison of circuit products

Chart2

100100

98.999.9

96.2498.04

91.6692.16

85.3885.16

77.378

67.768.9

59.8662.38

55.8656.26

51.64

49.28

46.98





Comparisons

Sociedade Minera El Brocal S.A


% Pass% Pass% Pass

50072.471.698.9

30062.359.996.2

21249.245.091.7

15036.328.485.4

7520.511.467.7


% Pass% Pass% Pass

50069.522.298.4

30057.111.585.2

21252.810.578.0

15042.18.768.9

7538.18.256.3

Milpo - Compania Minera Cerro Lindo

=


% Pass% Pass% Pass

500100.0100.0100.0

300100.0100.0100.0

21263.952.194.4

15046.131.085.4

7522.58.458.9


% Pass% Pass% Pass

50091.088.0100.0

30075.354.999.3

21259.431.190.6

15047.820.575.0

7530.49.947.0

Compania Minera Condestable


% Pass% Pass% Pass

50078.064.096.0

30055.239.394.4

21242.727.081.9

15035.621.477.0

7525.915.153.3


% Pass% Pass% Pass

50077.063.098.0

30059.131.895.1

21248.419.685.2

15039.613.472.8

7527.98.752.0

OJSC Apatit


% Pass% Pass% Pass

40072.665.6100

32061.152.993.2

20043.83482.3

16034.524.673.1

10022.213.655.7

7117.710.346.5


% Pass% Pass% Pass

40064.036.8100.0

32051.422.390.0

20038.912.873.6

16032.09.062.4

10023.65.847.3

7119.04.538.2

0.427.434.40

0.3238.947.16.8

0.256.26617.7

0.1665.575.426.9

0.177.886.444.3

0.07182.389.753.5

36.0342688363.20

48.677.710

61.0658788887.226.4

68.046676519137.6

76.3617429894.252.7

81.0144756395.561.8

Comparisons

10099.26

94.410456713490.63

85.391053832375.04

72.026094709759.46

58.898824916447.01

48.301601210937.92

42.948445747333.59

38.605117642729.23





Weight Percentage Passing

Grinding Circuit Product Size Distribution CurveMilpo Unit - Minera Cerro Lindo S.A.A.

Cerro Lindo Comparisons

99.9399.67

99.4895.09

97.8385.2

93.8272.75

87.1761.64

76.9152

66.2

54.12

Underground Materialin Cyclone Circuit






Grinding Circuit Product Size Distribution CurveCompania Minera Condestable

Silica Sand

100100

98.999.9

96.2498.04

91.6692.16

85.3885.16

77.378

67.768.9

59.8662.38

55.8656.26

51.64

49.28

46.98










Grinding Circuit Product Size Distribution CurveOJSC Apatit

Compania Minera Milpo S.A.A. - Cerro LindoCompania Minera Condestable

Control ParametersHydrocyclone D-26Stack SizerClassificationHydrocyclone DS-26Stack Sizer

Tonnange t/hr242275Ciculating Load243124

Ball charge %3737Classification Efficiency49.880.3

RPM15.515.5Mill DischargeHydrocyclone DS-26Stack Sizer

Critical Speed76.876.8% - 200 mesh25.927.9

Mill MotorHydrocyclone D-26Stack Sizer% solids7370

HP 3000Flotation FeedHydrocyclone DS-26Stack Sizer

Voltage39614139% - 200 mesh53.352

Amperage319316P80 microns196185

Kw21832200

Energy Comsumption (Kw-h/Ton)77

grams of steel/Kw-h778696

ClassificationHydrocyclone D-26Stack Sizer

Circulating Load244108

Classification Eficiency5879

Mill DischargeHydrocyclone D-26Stack Sizer

% - 200 mesh23.5847.01

% solids82.578.7

Flotation FeedHydrocyclone D-26Stack Sizer

% - 200 mesh5547

P80 microns140.5160

Satck Sizer ScreenHydraulic Classifier

Dimensions1.2 m wide x 1.5 m long2.6 m dia. upstream classifier

Feed Rate (t/h)120120

40300

CIA Condestable S.A.Copper and Gold PeruRequired additional capacity, from 4100 tpd to 5500 tpd.

Replace Cyclones with Stack SizersTM.

ImprovementsCirculating load reduced, 204% to 96%

Classification efficiency increased 62% to 85%

Capacity increased 4100 tpd to 5500 tpd (34%)

Lower energy consumption

Economic BenefitsInstalled cost of one 5 deck Stack Sizer US$300k

Economic Margin US$2,630k (17% production increase)

Investment Recovery 1.4 months.

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