How to quantify particle strength

Transcription

1 8 How to quantify particle strength Wear and Breakage Gabrie Meesters

2 Why measure particle strength? Understanding of: Granulation Product quality Product design Dust formation 2

3 Outlook Introduction Particle properties and strength Measuring particle damage Breakage mechanisms Compression tests Impact tests Shear tests Fluid bed tests 3

4 Particle damage Problems: Change in Physical properties difficulties in subsequent handling particle size distribution, surface area, flowability, bulk density Generation of fine particles Dust Dust explosions, health of employees, emissions to the environment, loss of valuable material. 4

5 Particle Strength Particles have property distributions Sampling issues Properties not only on chemical composition (like fluids and gases) but also on morphology Data not available in handbooks!! You have to measure it 5

6 Examples particle damage in industry Filling of hoppers and containers Screw conveyors Pneumatic conveyers Fluidized beds Cyclones Stirred vessels 6

7 Outlook Introduction Particle properties and strength Measuring particle damage Breakage mechanisms Compression tests Impact tests Shear tests Fluid bed tests 7

8 Particle Strength We cannot predict damage without a tester 8

9 Particle properties and strength How does morphology: Particle size Particle shape Particle porosity Particle surface influence particle strength? 9

10 Particle properties and strength 1) Particle size In general smaller particles are more difficult to break. Small particle contain fewer flaws, so the chance of fatal fracture is smaller. 10

11 Particle properties and strength 2) Particle Shape The shape of a particle influences the distribution of stresses. Rough edges are likely to break off. 11

12 Particle properties and strength 3) Particle porosity X-ray tomography image of an alumina particle 1. Porous particles are more flexible than similar sized non-porous particles. 2. Less rigid than non-porous particles 12

13 Particle properties and strength 4) Particle surface hardness The surface hardness affects the resistance to fracture. 13

14 Outlook Introduction Particle properties and strength Measuring particle damage Breakage mechanisms Compression tests Impact tests Shear tests Fluid bed tests 14

15 Particle Strength in relation to particle properties 15

16 Measuring particle damage We can measure the extend of damage by: Particle size Particle size distribution Particle shape and shape distribution Particle density and porosity Surface area Surface hardness 16

17 Measuring particle damage 1) Particle size The particle diameter is a meaningful parameter. However the particle diameter is not straight forward. From: M. Rhodes, Introduction to particle technology, Wiley1998. A A A 17

18 Measuring particle damage 2) Particle size distributions Obtained by: Sieving Automated Image analysis systems Laser diffraction measurements 18

19 Measuring particle damage 3) Particle shape and shape distributions Assessment of the sphericity of the daughter particle Particleperimeter _ p Perimeterofequivalentcircle This is done by different image analysis techniques There are lots of way to determine the shape of particles 19

20 Measuring particle damage 4) Porosity and porosity distributions Measure displaced volume by pycnometry pore 1 granule solid Calculate porosity Other techniques: Mercury intrusion porosimetry X-ray microtomography 20

21 Measuring particle damage 4) Surface area Important parameter for powders. Catalyst Absorbing agents Active compounds in Pharmaceutical applications Measured by air permeability and nitrogen adsorption (BET) 21

22 Measuring particle damage surface hardness Name Indentor type measures charcteristic Rockwell Diamond cone; steel ball Penetration depth Fast, but sensitive Brinell Steel ball diameter Vickers, Knoop Diamond pyramid Indentation surface Often used, microscale Mohs Compares with standard minerals Talcum = 1 Diamond = 10 22

23 Outlook Introduction Particle properties and strength Measuring particle damage Breakage mechanisms Compression tests Impact tests Shear tests Fluid bed tests 23

24 24 Breakage Mechanisms

25 Compression tests Applications: Pharmaceutical Industry, Tablet testing Mineral processing Iron ore pellets 25

26 Feel judgement tests Used in pharmaceutical and steel industries. Amount of pressure required to break the agglomerate between thumb and fingers. Also the loudness of snap when the agglomerate breaks is used as a guide 26

27 Monsanto tablet tester Barrel containing a spring and indicator that moves along a graduated scale. Pressure is applied by turning a knob until the tablet breaks 27

28 Pfizer hardness tester Hand held device similar to pliers Tablet crushed between the jaws of the instrument Breaking force recorded on dial indicator 28

29 Strong Cobb tester An air pump drives a plunger onto the tablet set on anvil A dial calibrated in units of pressure records final breaking force 29

30 Erweka tablet hardness tester Force is applied by means of a loaded beam Load is varied by a sliding weight and stops when tablet fractures 30

31 Electrical compression tester To measure crushing strength of small particles Load-deformation curves of particles are measured The value of the load and extent of deformation at fracture are measures of particle s strength and ductility End-point of the test is visual observation that the pellet has crushed 31

32 Hydraulic compression tester To study fracture of glass spheres The value of the load and extent of deformation at fracture are measures of particle s strength and ductility Particle Load-deformation curves of particles are measured End-point of the test is visual observation that the pellet has crushed 32

33 3 point bend test A series of bars or tablets is made The stress strain curve is measured 33

34 New developments Combine stress strain measurements with sound and video playback Accoustic emission and playback 34

35 Multi particle compression testers Compression Strength of most particles is variable Large number of particles should be tested in order to obtain representative statistics 35

36 36 Impact tests

37 Single particle impact tests Number of impacts before fracture is the friability index Size, shape, orientation and history determine particle strength. 37

38 Particle free fall impact apparatus Apparatus is under vacuum to avoid drag effects Each test involves dropping 100 particles, one at a time and measure particle size distribution 38

39 Particle free fall impact apparatus Used to impact particles at high velocity Greater the impact, greater the damage Changing the impact angle from 90 to 45 decreased the damage to the particles Disadvantages: Too much variation in particle velocity for a given air velocity Unable to control the orientation of the projected particles 39

40 Rotating arm impact apparatus Developed due to problems in Particle freefall impact apparatus-> v impact not constant! Particle Consists of a cantilevered arm which struck struck particles at high velocity Damage measured by counting number of cracked particles 40

41 Drop impact test A weight is dropped from a fixed height on to a tablet, the weight is increased until the tablet breaks Final weight is used as index of tablet strength Alternatively weight is kept constant and drop height varied 41

42 Drop shatter tests Most common type of multi-particle impact tests Consists of a box containing Particles which are dropped on a steel plate Cumulative weight of particles smaller than is measured. 42

43 Jet impingement test To simulate impacts that occur in pneumatic conveying Sample of particles is transported by air up a vertical tube The sample impacts a deflector plate and falls down to a collection point The material is recycled for 10 minutes 43

44 44 Shear testers

45 Direct shear tests Simple and can be used for constant stress and constant strain testing Jennike shear cell Permits very limited movement Direct double-shear ring 45

46 Annular shear test Permits infinite strain Difficult sample preparation Variation of stress with radial position 46

47 47 Fluidised bed tests

48 Fluid bed tester Direct relevance to applications where particles are used under fluidizing conditions Believed to simulate pneumatic and hydraulic transport Hydrate decomposition and thermal shock can accelerate the attrition 48

49 Spouted bed tests Metal plate Particles are entrained by the fluid and conveyed to the plate in the form of a fountain Air inlet 49

50 General Conclusions Particle degradation is difficult to measure. All test methods are highly empirical. Within one type of industry test methods are highly standardised. But are not generally applicable. In most testers the number of events is neglected hence fatigue is neglected 50

51 The Ideal strength tester Attrition Tester Abrasion Tester Compression Tester Damage All these testers should cause controlled fatigue to the particles. Events Force 51

52 Each application requires a different tester!!! 52

53 Particle breakage mechanisms 53

54 New testers Attrition: W.J. Beekman, measurement and the mechanical strength of granules and agglomerates. Ph.D. thesis TU Delft. R. Pitchumani, Breakage characteristics of particles and granules. Ph.D. thesis TU Delft. Abrasion: This work 54

55 Repeated Impact tester Typical impact velocity 200 granules at desktop scale 1m/s - 5m/s 55

56 What happens inside the box? n cycles E kin mv imp The intensity is the transferred kinetic energy. Combine the intensity with number of events This allows comparison between test and process 56

57 Test Granules: PuroxS TM Material: Sodium Benzoate Size fraction 630 mm 980 mm Produced by a fluidised bed process, hence layered structure Cauliflower structure Picture: SEM picture of a Sodium Benzoate granule 57

58 RIT results The test granules are subjected to different number of collisions: Both round off phase of the granules and size reduction 58

59 RIT results Initially the highest rate of attrition 59

60 PTS results 20 m/s: Initially highest Attrition rate 40 m/s: Initially lower attrition rate 60

61 Intensity and number of events 5 kj/kg 20 m/s 28 bends 30 m/s 13 bends 40 m/s 7 bends 61

62 Attrition measurements in RIT for coated particles HPMC coating Thickness Fraction undamaged [%] % HPMC 50% HPMC 75% HPMC 100% HPMC Methyl Cellulose (MC) coating log time [s] Fraction undamaged [%] % MC 125% MC 150% MC 175% MC 200% MC 225% MC log time [s] 62

63 Attrition: Different coating polymers Coating type 120 Fraction undamaged M C MC HPMC/HEC/C PVA log time [s] 63

64 Abrasion testing Before abrasion testing After abrasion testing 64

65 Attrition vs abrasion testing Before testing Abrasion testing Attrition testing 65

66 The Abrasion tester r box f= 3,6 and 9 s -1 R arm F=3 s -1 F c R mv arm 2 r box 66

67 Experimental F arm = 3s -1 f box = 6s -1 67

68 Literature Powder testing guide, L. Svarovski, British material handling board, Elsevier applied science, 1987 Introduction to Particle Technology, Martin Rhodes, John Wiley & Sons, 1998 The Science and Engineering of Granulation processes, J. Litster, Particle Technology series, Kluwer Academic Publishers,

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