implementing strategies to improve mill capacity, with case studies. presented by nic barkhuysen...

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.

Overview

• Fine wet screening – definition, mechanism, and rules of thumb

• Development of high capacity multiple feed point screening machines and screen surfaces

• Three case studies describing the application of fine screens in closed circuit grinding

Definitions

• Screening: 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 thumb• Different for wet and dry screening• In fine wet screening, the undersize particles are

transported through the screen openings by the fluid; vibration promotes fluid throughput and oversize conveyance

• Reasonable 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 machines

• 1977 - Multifeed screening machine– Typically 1.5 to 2 times more capacity than an

equivalent single feed point machine

Multiple feed point machines

• 1977 - Multifeed screening machine– Typically 1.5 to 2 times more capacity than an

equivalent single feed point machine

• 2001 - Stack Sizer screening machine– Capacity generally equivalent to 3 to 4

Multifeed machines

Stack Sizer Screen• Up to 5 vibrating screen decks operating in parallel• Typically fed from a single or multiple stage flow

distribution system, depending upon the number of machines

Stack Sizer Screen• Up to 5 vibrating screen decks operating in parallel• Typically fed from a single or multiple stage flow

distribution system, depending upon the number of machines

• Driven by two 1.9 kW (2.5 HP) vibrating motors rotating in opposite directions

• Capacity 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 surfaces

• Openings as fine as 75 microns (200 mesh)

• High open area (35 to 45% actual)

• Capacity similar to fine woven wire

• Long life (6 to 12 months is common)

• Rarely blinds due to slotted openings and tapered relief angles

• Unique technology developed by Derrick

The application of fine screens in closed circuit grinding

• Research work over many decades has demonstrated the benefits of improved classification in grinding circuits, but…

• Up until recently, screens have rarely been considered as the primary classifier in mill circuits

• With 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 grinding

• PhosAgro – 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 Apatit

• Phosphate mine located north of the Artic Circle in northwestern Russia



• Crushed ore is fed to multiple parallel lines of single stage ball mills in closed circuit with single or double stage hydrocyclones



• Crushed ore is fed to multiple parallel lines of single stage ball mills in closed circuit with single or double stage hydrocyclones

• Circuit product from all lines is combined and pumped to the flotation plants

• Desired to improve classification efficiency, grinding efficiency, and mill production rates

• Screening 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 available

• With the development of the Stack Sizer, Apatit wanted to take another look at the use screens

• Continuous 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)

• Encouraging pilot test results justified the next phase of the investigation, the conversion of one mill line to screens

• To 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% solids

95 t/h

Underflow 80% solids

409 t/h

Water

72 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

• Encouraging pilot test results justified the next phase of the investigation, the conversion of one mill line to screens

• To minimize the cost, Mill 24 chosen as it was the smallest

• Cyclones were replaced with 4 Stack Sizer screens with 390 μm urethane panels


Mill 24 grinding circuit - screen classification

New Feed(crushed ore)

124 t/h

Four (4) Stack Sizer Screens

Undersize34% solids

124 t/h

Oversize 82% solids

167 t/h

Flotation

Screen feed291 t/h

Ball Mill


3.6 m x 4.0 m

Benefits of improved classification

ParameterCycloneCircuit

ScreenCircuit

Capacity (t/h) 95 124

Circulating load (%) 430 135

Circulating load (% passing 160 µm) 24.6 9.0

P80 (µm) 190 240

Comparison of circuit products

30

40

50

60

70

80

90

100

10 100 1000

Particle Size, microns

Wei

gh

t P

erce

nt

Pas

sin

g

Hydrocyclone Overflow Stack Sizer Undersize


• Final product grade from flotation circuit is similar, even though the flotation feed is coarser

• Apatit 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 KMAruda

• Iron ore mine in Russia

• Run of mine ore is about 34% Fe

• 2 stages of grinding each consisting of 2 ball mills operating in parallel

• Primary mills closed with spiral classifiers

• Secondary mills closed with cyclones

• Each grinding stage followed by magnetic separation

Secondary grinding with cyclone classification

Ball Mill

Ball Mill

Magnetic concentrate from primary

grinding circuit

Second Stage Wet Magnetic

Separation

TailingsConcentrate

Hydrocyclones

Third Stage Wet Magnetic

Separation

Concentrate

Tailings

OJSC KMAruda

• Two Stack Sizer screens with 100 micron urethane panels installed in the secondary grinding circuit in place of the cyclones

• Goal was to maintain the same final product grade

• Subsequent 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

TailingsConcentrate

Two (2) Stack Sizer Screens Third Stage Wet Magnetic

Separation

Concentrate

TailingsUndersize

Oversize



ScreenCircuit

Capacity (t/h) 120 140 to 150

Power draw (kW) 1260 630

Circulating load (%) 210 to 220 130 to 160

Circulating load size (% minus 71 µm) 51.0 22.9

Circulating load grade (%Fe) 65 36 to 45

Final concentrate size (% minus 71 µm) 92.5 82.0

Final concentrate grade (%Fe) 65.85 66.00

Final concentrate iron recovery (%) 82.2 84.6

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 level• Run of mine ore is 2.4% lead• Three rod mills in open circuit feed three ball mills

in closed circuit with hydrocyclones; cyclone overflow goes to flotation

• Cyclone underflow is 10 to 15% lead due to high difference in specific gravity

• Circulating load typically ranges from 350 to 400%• Considerable slime losses before flotation

Rod Mill

Rod Mill

Rod Mill

Ball Mill

Overflow

138 t/h

483 t/h

Ball Mill

Ball Mill

621 t/h

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 mill

• Following full-scale tests, two Stack Sizer screens with 500 micron urethane panels were installed in place of the cyclones

Initial Improvements• Two of the three ball mills were shut down

• Capacity increased 10%

• Lead recovery increased about 9 percent due to reduction in slime losses before flotation

• Screen oversize is less than 2% lead (recall, cyclone underflow was 10-15% lead)

To take advantage of increased grinding capacity…

• Crushing circuit was modified to increase capacity and reduce production of fines

• Rod mill speed was increased to achieve better throughput

• Three additional Stack Sizer screens were installed• A second ball mill was put back into operation• Installed additional flotation and filtration capacity• Concentrator throughput increased over 75%

Ball MillRod Mill

Rod Mill

Rod Mill Ball Mill

147 t/h (60% Circulating Load)

7 x 12

7 x 12

7 x 12

8 x 10

8 x 10

Flotation

392 t/h

New Feed 245 t/h

Five (5) Stack Sizer Screens

245 t/h



ScreenCircuit

Capacity (t/h) 138 245

Circulating load (%) 350 60

Circulating load (% Pb) 10 - 15 < 2.0

P80 (µm) 122 234

Comparison of circuit products

40

60

80

100

10 100 1000 10000

Particle Size, microns

We

igh

t P

erc

en

t P

as

sin

g

Hydrocyclone Overflow Stack Sizer Undersize

CIA Condestable S.A.Copper and Gold Peru

• Required additional capacity, from 4100 tpd to 5500 tpd.

• Replace Cyclones with Stack SizersTM.

Improvements• Circulating load reduced, 204% to 96%

• Classification efficiency increased 62% to 85%

• Capacity increased 4100 tpd to 5500 tpd (34%)

• Lower energy consumption •

Economic Benefits• Installed cost of one 5 deck Stack Sizer

US$300k

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

• Investment Recovery 1.4 months.

Thank you for your time

• Over 900 Stack SizersTM in production world wide

• Questions?

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