Minerals processing notes GRINDING.pptx

Mineral processing grinding notes

1. MINERAL PROCESSING

2. 4.0 Grinding

3. Learning outcomes have a knowledge and understanding of; factors influencing operation & be able to quantify the effect of these factors, if possible the function of classification equipment in close-circuit grinding the effect of the separation efficiency of the classification equipment on mill performance & subsequent processing Different mill types available

4. grinding is the final stage of comminution is performed in rotating cylindrical vessels known as tumbling mills mills contain grinding media which are large, hard & heavy vs ore particles usually performed wet although dry process is possible

5. Cross section of tumbling mill

6. Drive System

7. Tumbling Mill

8. Liners

9. Milling (Modes of breakage) Compression breakage Surface abrasion by rubbing Self breakage Impact cracking Impact spalling Surface chipping

10. mill speed Definition Critical mill speed Speed at which charge starts to centrifuge n c = 42.3/(D-d) 0.5 with n c in rpm D = mill diameter d = particle diameter

11. Charge volume Definition percentage of the cross sectional area of the mill occupied by balls/rods and their interstices

12. Ball mill operation Large balls needed to crush large ore pieces Hard ore requires larger balls Finer grinding requires larger surface area per unit mass  smaller balls  attrition milling surface effect

13. Ball mill operation Usually used in closed-circuit grinding

14. Ball mill operation Closed-circuit grinding prevent over grinding increase throughput Usually 250% circulating load circulating load = mass of solids in class underflow * 100 mass of solids in feed

15. Ball mill operation Slurry density Viscous enough to cover balls If H 2 O is increased reduces number of smaller particles to be milled reduces number of effective impacts Even more H 2 O - slurry not viscous enough to cover balls - power draw decreases

16. Marcy Scale

17. Ball milling

18. Rod Milling

19. AG/ SAG-milling

20. Autogenous and Semi- Autogenous milling Autogenous : self generated/ produced the use of ore lumps as grinding media Main advantages lower capital cost ability to treat wide range of ore types Disadvantages Ore type must be amenable to milling Control of feed properties

21. Autogenous and Semi- Autogenous milling Key variables Feed size Ore hardness Mill speed Ball size (SAG) Ball loading (SAG)

22. Autogenous and Semi- Autogenous milling Feed size Grinding media comes from ore change in feed size distribution  change in grinding size distribution

23. Autogenous and Semi- Autogenous milling Feed size AG-milling large enough rocks for impact breaking of smaller sizes In general AG mill performs better with coarser feeds

24. Autogenous and Semi- Autogenous milling Feed size SAG mills no need for large rocks for impact breaking big rocks become burden Feed size impact greater on AG-milling than SAG-milling

25. Autogenous and Semi- Autogenous milling Ore hardness Harder ore requires coarser feed size AG milling Too soft ore  not large enough quantities of larger grinding media produced  throughput decreases Too hard ore  difficult to break critical size  throughput decreases

26. Autogenous and Semi- Autogenous milling Circuit-configuration lower throughput finer product cyclone cut must not be too small  large recirculating loads

27. Autogenous and Semi- Autogenous milling Circuit-configuration Significant increase in throughput critical size fraction 25-50 mm removed less grinding media (cm 2 /g)  coarser product

28. Single stage milling

29. Double stage milling

30. Milling in Practice Single stage milling Lowest capital and working costs Simplicity Exceptions Fine ore : F80 < 80 mm Hard component in ore, build up of critical size Economy of scale - single large mill Risk ?

31. Control variables

32. COST OF GRINDING

33. SIZE REDUCTION & POWER RANGES

34. WET/ DRY GRINDING