Implementing Strategies to Improve Mill Capacity, with Case Studies.

1. Implementing Strategies to Improve Mill Capacity, with Case Studies. Presented by Nic Barkhuysen From Recent Advances in Mineral Processing Plant Design; 2009; Malhotra, Taylor, Spiller, and LeVier, Editors; p. 433-443.

2. 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

3. 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)

4. 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

5. Multiple feed point machines • 1977 - Multifeed screening machine • Typically 1.5 to 2 times more capacity than an equivalent single feed point machine

7. 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

10. 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

12. 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

14. 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

15. 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

16. 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)

17. PhosAgro – OJSC Apatit • Phosphate mine located north of the Artic Circle in northwestern Russia

19. 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

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

23. PhosAgro – OJSC Apatit • 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

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

26. PhosAgro – OJSC Apatit • 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

27. Mill 24 grinding circuit - cyclone classification

28. PhosAgro – OJSC Apatit • 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

30. Mill 24 grinding circuit - screen classification

31. Benefits of improved classification

32. Comparison of circuit products

33. PhosAgro – OJSC Apatit • 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

35. 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

36. Secondary grinding with cyclone classification

37. 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

38. Secondary grinding with screen classification

39. Benefits of improved classification

40. Mineral Processing Journal, 2009, No. 2.

41. 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

44. 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

46. 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)

47. 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%

49. Benefits of improved classification

50. Comparison of circuit products