Grinding and comminution. Gabrie Meesters

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1 5 Grinding and comminution Gabrie Meesters

2 Overview Single particle breakage Energy requirements Design considerations Equipment types 2

3 Introduction Why crush/grind particles? To release valuable minerals from ores To increase the specific surface area To produce particles of certain size/shape Used in many industries: Mineral industry Cement, etc Also paint, chocolate 3

4 Introduction 5% of ALL electricity generated is used for grinding Efficiency of grinding processes 1-5% Unfortunately models too empirical 4

5 Introduction There are many ways to break a particle: 5 Main mechanisms tension and shear

6 Single Particle Breakage Two ways in which a particle can break: Brittle: elastic failure Stress Strain yield failure Ductile: Stress Strain 6

7 Single Particle Breakage Hooke s Law: Stress: Strain: Y=stiffness or elasticity tensor change in length as a fraction or percentage of total length See Wikipedia on Hooke s Law Y F A x L 0 7

8 Single Particle Breakage Amount of energy (per unit volume) required to stress a particle: W Fdx F A YA YAx W xdx L0 2L Per unit volume: W ' 0 Yx 2L Y A Y 2 YA x L Y 8

9 Single Particle Breakage How to predict strength of a particle? Idealized predictions over/under predict strength by several orders of magnitude! 9

10 Single Particle Breakage Real life is a bit more complicated.. Failure brought about by cracks: Crack No stress No stress Stress concentration at end of crack 10

11 Single Particle Breakage Stress concentration factor (Inglis 1913, Griffith 1921): L= crack length, R=radius of crack tip All real materials have cracks! K 1 2 L R 11

12 Single Particle Breakage Critical failure through crack propagation Minimum crack length for a given stress What happens when there are cracks longer than that minimum length?? Excess energy is released -> other cracks start to propagate 12

13 13 Particle Fracture

14 14 Stress on Particles

15 Predicting Energy Requirements Three postulates for predicting energy requirements: Rittinger, Kick, Bond Highly empirical Quite old, but still widely used Varying levels of success 15

16 Predicting Energy Requirements Rittinger (1867): Energy required is proportional to amount of new surface created. Feed particles of size x 1, product particles of size x x x C E x x k k R p v s 16

17 Predicting Energy Requirements Kick (1885) Energy requirement is related to ratio feed size/product size: E C K x ln 1 x 2 Usually under predicts for fine grinding 17

18 Predicting Energy Requirements Bond (1952) Based on large amount of experimental data EB WI X 2 X X 1, X 2 : sieve size through which 80% passes (in µm) 1 18

19 Predicting Energy Requirements Bond (1952) W I in kwh/short ton (1 short ton=2000 lbs) Ranges from 1 to 80 Bond s equation most often used of the three presented. 19

20 Predicting Energy Requirements EXERCISE: A Material is crushed from 25 mm to 7 mm. This requires 20 kj/kg. How much energy is required to grind the same material from 25 mm to 3.5 mm using a)rittinger s law, b)kick s law and c)bond s Law 20

21 Predicting Energy Requirements Rittinger: C R = (kj/kgmm) Energy required = 47.8 (kj/kg) Kick: C K =15.7 (kj/kg) Energy required = 30.9 (kj/kg) Bond: W I =355.4 (kj/kg µm -1/2 ) Energy required = 37.6 (kj/kg) x x C E R 2 ln x x 1 C E K X X W E I B 21

22 Predicting Energy Requirements Rittinger, Kick and Bond combined: 22

23 Grinding equipment How to choose grinding equipment? Rely on common sense! Take into account: Feed size Desired product size Feed composition/chemical structure Desired production rate 23

24 24 Grinding machines and feed particle size

25 Grinding equipment General considerations regarding particle properties (1): Toughness: Tough materials are difficult to break/shatter Deform rather than break Lowering temperature may change material from tough to brittle (polymers) Use a cutting action rather than compressive forces 25

26 26 Grinding machine and the matreial hardness properties

27 Grinding equipment General considerations regarding particle properties (2): Cohesivity/Adhesivity: Small and/or wet materials difficult to break. Dry before-hand OR use wet grinding (only for certain types of grinding equipment At intermediate moisture contents very low chances of successfully reducing particle size 27

28 Grinding equipment General considerations regarding particle properties (3): Melting/Softening point: Large proportion of energy added converted to heatmaterial may melt or undergo transition from brittle to tough Organic/Biological(food) materials may degrade at higher temperatures Use wet grinding (good heat transfer, liquid may serve a heat sink) Use jet milling: high gas-flowrates may serve as heat sink 28

29 Grinding equipment General considerations regarding particle properties (4): Flammable/explosive materials: Use inert medium (e.g. nitrogen) Specifically design equipment to withstand extremely high pressures Operate outside of explosion-risk conditions 29

30 Grinding equipment Wear/Abrasion: The material of which the crushing equipment is made exposed to the same stresses as the feed material! Use harder materials for the equipment than the feed material High maintenance cost (almost the same as the energy cost) Contamination of product Not well understood 30

31 Grinding equipment Crushers Blake crusher: 31

32 32

33 Grinding equipment Impact mills: Hammer mill: 33

34 34

35 Grinding equipment Tumbling mills: Ball and rodd mills: 35

36 36

37 Grinding equipment Jet mills: Opposed jet mill: 37

38 38

39 39 Roller crushers and roller mills

40 40 Cutting Mills

41 41 Ball Mills

42 42 Wet Media Mills

43 43 High Intensity mill (Buhler)

44 44 Impact pin mills

45 45 Impact mills

46 46 Others

47 47 Grinding circuits (1)

48 48 Grinding circuits (2)

49 49 Summary