Download to read offline
Minerals processing notes GRINDING.pptx
- 1.
- 2.
- 3. Learning outcomes have aknowledge 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 thefinal 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 oftumbling mill
- 6.
- 7.
- 8.
- 9. Milling (Modes ofbreakage) Compression breakage Surface abrasion by rubbing Self breakage Impact cracking Impact spalling Surface chipping
- 10. mill speed Definition Critical millspeed Speed at which charge starts to centrifuge nc = 42.3/(D-d)0.5 with nc 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-circuitgrinding 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 H2O is increased reduces number of smaller particles to be milled reduces number of effective impacts Even more H2O - slurry not viscous enough to cover balls - power draw decreases
- 16.
- 17.
- 18.
- 19.
- 20. Autogenous and Semi-Autogenousmilling 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-Autogenousmilling Key variables Feed size Ore hardness Mill speed Ball size (SAG) Ball loading (SAG)
- 22. Autogenous and Semi-Autogenousmilling Feed size Grinding media comes from ore change in feed size distribution change in grinding size distribution
- 23. Autogenous and Semi-Autogenousmilling 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-Autogenousmilling 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-Autogenousmilling 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-Autogenousmilling Circuit-configuration • lower throughput • finer product • cyclone cut must not be too small large recirculating loads
- 27. Autogenous and Semi-Autogenousmilling Circuit-configuration Significant increase in throughput critical size fraction 25-50 mm removed less grinding media (cm2/g) coarser product
- 28. Single stage milling 2x SAG Mill 22’ D x 33’ L 5 MW 220 tph 250 tph Mill Silo
- 29. Double stage milling P80= 75um Mill Silo 2 X SAG Mills 16’ D x 30’ L 3 MW 220 tph 250 tph Mill Silo 2 x Ball Mills 14’ D x24 ‘ L 3 MW
- 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.
- 32.
- 33. SIZE REDUCTION &POWER RANGES
- 34.