Towards a parameter characterising attrition

Powder Technology 106 Ž1999. 37–44 www.elsevier.comrlocaterpowtec

Towards a parameter characterising attrition A.U. Neil a , J. Bridgwater b

b,)

a Alexander and Associates, Empire House, Empire Way, Wembley Park, Middlesex HA9 0EW, UK Department of Chemical Engineering, UniÕersity of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK

Received 11 June 1997; received in revised form 24 February 1999; accepted 24 February 1999

Abstract Two particulate systems, heavy soda ash and a tetra-acetyl-ethylene-diamine agglomerate, have been subjected to attrition in a fluidised bed, a screw pugmill and an annular shear cell. Each of these devices has a different mechanical action on the materials. The attrition in each piece of equipment was characterised using a function developed by Gwyn and, for each material, one characterisation parameter was found to be independent of the mechanical action of the equipment. In addition, a recent treatment of material behaviour developed by Neil and Bridgwater was also found to characterise data from the annular shear cell. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Attrition; Breakage; Shear cell; Fluidisation; Screw

1. Introduction Attrition of particulate solids is common to a wide range of industries and can lead to environmental pollution and to the loss of expensive materials. In the majority of cases, attrition has economic and technological implications and, when not required, is to be minimised if not avoided wherever possible. Occasionally, however, attrition can be beneficial as in the removal of impermeable scale formed on the surface of reacting particles. It is believed that bulk degradation caused when particulate materials undergo motion occurs in high strain regions known as failure zones, Roscoe w1x. Work by Delaplaine w2x, Platonov and Poltovak w3x, and Takahashi and Yanai w4x established that failure zones exist physically in the bulk of materials processed in various pieces of equipment. An empirical model was put forward by Gwyn w5x resulting from his work with injecting high velocity air jets into cylindrical fluidised-beds filled with initially singlesized particles. The resulting attrition was found to be described by WAtm

Ž 1.

where W is the mass that has undergone attrition at time t and m is an empirical constant. )

Corresponding author. Tel.: q44-1223-334798; Fax: q44-1223334796; E-mail: [email protected]

This function has been extensively examined by Paramanathan and Bridgwater w6x. More recently Neil and Bridgwater w7x, using an annular shear cell to examine the attrition occurring in a material failure zone, were able to develop what is believed to be a material property of attrition. These workers found that the empirical Gwyn function in partnership with the Schuhmann weight–size distribution function could be used to describe the attrition of the largest size fraction. Their study went further and proposed that the Gwyn parameter ‘m’ could be taken as a material property of degradation. The purpose of this work is to make a further assessment of the extent to which the Gwyn parameter Ž m. as used by Neil and Bridgwater represents a material property of attrition and, as such, to assess whether it is independent of the equipment that causes the degradation.

2. The Gwyn formulation Previous studies have shown that the Gwyn function can be used to characterise the attrition of particulate materials. Gwyn w5x studied the degradation of mono-sized silica–alumina catalyst particles in a fluidised bed attrition test apparatus. He found that the feed particles suffered both surface abrasion producing fines within a narrow size range and, to a lesser extent, particle fracture into interme-

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A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44

38

diate-sized fragments. The attrition of narrow sized particles was found to be represented by: x s Kt m

Ž 2.

where x is the mass fraction degraded and K a constant dependent of the initial particle size. Gwyn experimentally verified that the form of Eq. Ž2. describes the attrition of materials with an initially narrow size distribution. Gwyn proposed that K represents the severity of attrition and the initial attritability of a material, while m deals with the change in material attritability with time. Thus, the two parameters are cited to be descriptive of both the material and the attrition process. However, the most recent work of Neil and Bridgwater w7x in studies using a well-defined annular cell shows that m is a constant for a given material and that the constant of proportionality k is related to the normal stress. It was further found that m could be usefully split such that m s fb

Ž 3.

where f is a material property of attrition and b a term describing the rate of material degradation.

3. Materials The two materials selected for study were agglomerates of heavy soda ash and TAED Žtetra-acetyl-ethylene-diamine.. The physical characterisations for these materials are collated in Table 1. The following is a summary of the methods used to determine these.

fracture occurs when a brittle material is stressed beyond its failure tensile stress, a plastic material suffers plastic or permanent deformation. In studying the stress state in an elastic body subjected to diametrical compression, Hiramatsu and Oka w8x described the failure stress, sc , by

sc s

0.9F0

Ž LŽ d . .

2

Ž 4.

for spherical and irregular shaped particles, and Goodman w9x by

sc s

2 F0

p dL Ž d .

Ž 5.

for cylindrical samples. Here F0 is the average load at failure, LŽ d . is the distance between the loading points, equal to the sample diameter d for cylindrical or spherical particles. These formulae are used here. The instrument used to apply the load, thus, evaluating F0 , was an Instron Tester Model 1195. The Instron was operated at loading rates from 2 to 5 mmrmin. Values of sc were obtained by this simple procedure; it is recognised that it does not deal with the issues of crack formation and propagation. No dependence on loading rate could be observed for any of the materials. In each case, 20 particles were tested, the standard deviations of each distribution being consistently about 20% of the mean. The data cited in Table 1 were obtained at a loading rate of 2 mmrmin. 3.3. Minimum fluidisation Õelocity

3.1. Size fractions Feed materials of narrow size fraction are used in this study. However, the bulk materials are usually supplied with a wide size distribution which then requires separation into the different size fractions. Dry sieving using BS 410 wire-woven square aperture meshes agitated in an Inclyno sieve shaker was employed to obtain the size fractions. The choice of sieving was based primarily on its simplicity.

As the flow of a gas through a bed of particles is increased, a flow is reached at which the force on the bed due to the pressure drop in the gas balances the weight of particles. The superficial velocity of the gas through the bed is then termed the minimum fluidisation velocity. This property is determined by measuring the pressure drop across the bed Žusing the apparatus described below, Fig. 1a. until the point where additional fluid does not further affect the pressure drop.

3.2. Single particle failure stress Single particle compression breakage strength varies with size and material properties. It is known that while

The materials were separately subjected to attrition in a laboratory fluidised bed, a screw pugmill and an annular shear cell, selected to reveal the various equipment mechanisms by which degradation occurs.

Table 1 Some properties of the materials studied

Size fraction Žmm. Bulk density Žkgrm3 . Single particle failure stress ŽMPa. Minimum fluidisation velocity Žmrs.

4. Experimental procedure

Heavy soda ash

TAED

0.36–0.50 960 19

0.50–0.71 800 7.2

1.0–1.6 800 1.0

0.26

0.27

0.56

4.1. Laboratory scale fluidised bed The apparatus is shown in Fig. 1a and the range of experimental conditions are collated in Table 2. The exper-

A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44

39

Fig. 1. Attrition apparatus: Ža. fluidised bed with a distributor of flat sintered plate with a nominal sinter size of 250 mm; Žb. screw pugmill; Žc. annular shear cell.

imental procedure is summarised in four steps. First, fluidise a sample of material for a given time at a specific superficial air velocity. Second, riffle a sample of material after fluidisation and perform, by sieving, a weight–size analysis on representative samples. Third, further refluidise the total sample for different times at the same superficial air velocity. Fourth, operating at different superficial air velocities, repeat the first three steps. Table 2 Summary of experimental conditions

4.2. Screw pugmill The pugmill is shown in Fig. 1b and the experimental conditions are summarised in Table 2. The inside diameter of the barrel is 54 mm and the cylindrical core of the screw is of 20 mm diameter. The length of the cylindrical section of the barrel is 436 mm. The experimental procedure involved four stages. First, feed a sample of material into the hopper and convey at a selected speed, recording the time taken for the sample of material to traverse the length of the pugmill. This is defined as a single pass. Second, riffle the conveyed sample and perform, by sieving, a weight–size analysis on a representative sample. Third, subject the same total sample mass to a different number of passes at the same pugmill speed, performing weight–size analysis at each stage. Finally, repeat the previous steps using a new sample mass operating the pugmill at a new speed.

Heavy soda ash

TAED

Size fraction Žmm.

0.36–0.50

0.50–0.71

1.0–1.6

Fluidised bed Sample mass Žkg. Superficial gas velocity Žmrs.

1.82 0.49

1.40 0.82

1.52 0.68

Screw pugmill Sample mass Žg. Time for single pass Žs. Screw speed Žrpm.

450 200 10

400 43–200 14–10

380 200 10

4.3. Annular shear cell

32 2–15

The annular shear cell design is shown in Fig. 1c. The experimental procedure is well-documented ŽNeil and Bridgwater w10x., but for completeness is summarised as follows. First, fill the cell annulus with a material mass

Annular shear cell Sample mass Žg. Normal stress ŽkPa.

20 2–20

20 2–15

A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44

40

that corresponds to a five to six particle diameter failure zone depth. Second, apply a normal load and rotate the cell for a specific number of revolutions at a cell speed of 5 rpm. Third, empty all the degraded material from the cell and perform weight–size distribution analysis, by sieving. Finally, repeat the whole procedure using fresh samples at different normal loads and number of cell revolutions. The experimental conditions are summarised in Table 2.

5. Results and discussion The mass–size distribution created by attrition was described by the Schuhmann function, namely W s WT

d

ž / dT

G

Ž 6.

where d is the particle diameter, d T the initial particle diameter and G an exponent characterising the size distribution. Size analysis is performed by sieving and the initial material is close-sized and is all held on a sieve of size d T . W is the mass of degradation product having a size less than d and WT is the mass of degradation product having a size less than d T . The study by Neil and Bridgwater w7x has shown this function to be a useful means of describing the weight–size distribution resulting from the attrition of a narrow size fraction feed. Characterisation was found to be satisfactory provided the top sieve cuts Žtypically one or two w7x., which show systematic deviation, were not included in the least squares fit of ln W vs. ln d. By extrapolation to the initial particle size d T , a value of WT is obtained. It was shown, in the earlier study, that treating data in this way compensates for the well-documented inefficiencies associated with sieving. The fractional

degradation of the initial feed size fraction Ž x . is defined by the ratio of WT to the mass of the material at the start of experiment. In the fluidised bed experiments the relationship between x and time t is found for both materials to be characterised by the Gwyn function ŽFig. 2.. The Gwyn parameters are summarised in Table 3. In all cases, a linear relationship is found between ln x and ln t. The smaller TAED particles degrade more rapidly at the higher superficial velocity. The larger TAED particles degrade at a lower rate, perhaps because the group ŽUrUmf . is reduced by both a reduction in U and an increase in Umf . Heavy soda ash is substantially more resistant to attrition. The data for the materials tested with the screw pugmill have again been characterised using the Schuhmann weight–size distribution and the Gwyn function. The Gwyn function plots are given in Fig. 3 and the Gwyn parameters K and m are summarised in Table 3. TAED of the smaller size degrades more rapidly at the higher screw speed with the coarse feed being significantly less damaged. The form of presentation again yields a linear relationship with the lines for all the experiments using TAED being parallel. The attrition data from the annular shear cell were also characterised by the Schuhmann weight–size distribution and the Gwyn function. Fig. 4 shows results for heavy soda ash at normal stresses from 2.7 to 17.7 kPa, and TAED from 2.7 to 13.8 kPa Žinitial size 0.50–0.71 mm. and from 2.7 to 10.5 kPa Žinitial size 1.0–1.6 mm.. In all cases, the Schuhmann size distribution was satisfactory as was the Gwyn formulation of the attrition kinetics. The parameters K and m are summarised in Table 3. Again all the experiments comply with the Gwyn formulation. For TAED, the larger particles are now the more readily damaged, especially at high stress. All experiments at

Fig. 2. Gwyn plot from fluid bed attrition Ž t in s..

A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44 Table 3 Summary of attrition data using the Gwyn approach Material

Parameters from Gwyn function m Žy.

K r10y3 Žsym .

Fluidised bed Heavy soda ash Ž0.36–0.50 mm. TAED Ž0.50–0.71 mm. TAED Ž1.0–1.6 mm.

ŽUr Umf . 1.9 1.8 3.0 1.2

0.87 0.52 0.47 0.50

0.0048 1.2 15 5.9

0.90 0.50 0.46 0.51

0.37 21 46 1.6

Screw pugmill Screw speed Žrpm. Heavy soda ash Ž0.36–0.50 mm. 10 TAED Ž0.50–0.71 mm. 10 14 TAED Ž1.0–1.6 mm. 10 Annular shear cell Normal stress ŽkPa. Heavy soda ash Ž0.36–0.50 mm. 2.7 8.3 17.7 TAED Ž0.50–0.71 mm. 2.7 8.3 13.8 TAED Ž1.0–1.6 mm. 2.7 7.2 10.5

0.72 0.88 0.80 0.51 0.52 0.52 0.47 0.46 0.43

3.1 7.1 22 9.4 35 59 46 94 140

whatever stress showed the same gradient in the presentation.

41

The data in Table 3 show that there is evidence to support the hypothesis that, over the conditions examined, m is approximately constant for the heavy soda ash and m is again approximately constant for each of the size fractions of TAED. The values of m also differ little between the two feed size fractions of TAED. This is consistent with findings by Neil and Bridgwater w7x where a wide range of materials were studied and where it was found, inter alia, that the Gwyn function could be used to characterise the attrition of particulate materials in the annular shear cell. For each material in the cell, the value of the Gwyn parameter m was again independent of normal stress and the initial narrow size distribution. In this study, three different pieces of experimental equipment have been used to study the degradation of particulate materials yet the value of m for a given material is approximately constant. Each piece of equipment has its own mode of operation and it is now worth examining these.

5.1. Fluidised bed Excluding attrition from phenomena such as chemical reaction or thermal changes, three principal distinct attrition processes occur during fluidisation. These processes have been extensively examined by workers such as Seville et al. w11x and Mullier et al. w12x. In both cases, the researchers examined the causes of attrition during the fluidization of various agglomerates. For our purpose, these attrition processes are summarised as follows. First, there is grid jet attrition where at the holes in the plate jets issue at high velocity resulting in some particles experienc-

Fig. 3. Gwyn plot from pugmill attrition Ž t in s..

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A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44

Fig. 4. Gwyn plot from annular shear cell attrition Ž t in s. with normal stress ŽkPa. as parameter.

ing fracture from impact. Such grid jet attrition extends only a short way into the bed. Secondly, there is bubbling bed attrition where bubbles rising through the bed cause bulk flow patterns that result in particles suffering surface abrasion. Lastly, there is the ejection of particles from the bed surface in which bubbles burst at the bed surface throwing particles upwards. On re-entry into the bed, particles may degrade by colliding with other particles and the vessel wall. 5.2. Screw pugmill Particulate materials can suffer extensive degradation when being transported by screw conveyors. With screw conveyors it is well-established that attrition results, in the main, from crushing Žshearing. between the screw blades and the casing wall. During the screwing action the clearance distance and angle between the screw blade and the casing wall varies between some maximum and minimum. To a much lesser extent, bulk attrition also results from inter-particle motion. The dependence on size observed for TAED is a likely manifestation of the clearance between the screw and the wall, with gaps of one or two particles especially promoting breakage w13x.

to isolate a material failure zone and to study the extent of bulk material attrition under different stresses and strains; it may well be a good analogue for attrition in hoppers and bunker reactors. The greater breakage with the larger TAED is likely to be linked to the greater number of defects in the larger particles. The physical action of the three pieces of equipment on particulate materials are significantly different. In cases studied here, the resulting attrition defined in terms the fractional degradation of a material Ž x . can be characterised by the empirical Gwyn function. An implication is that the extent of attrition is generated by some underlying effect which is probably the specific energy input. In summary, we have: m Žmean using data from all three equipment types. 0.83

Range

0.50

0.46–0.52

0.47

0.43–0.51

5.3. Annular shear cell

Heavy soda ash Ž0.36–0.50 mm. TAED Ž0.50–0.71 mm. TAED Ž1.0–1.6 mm.

0.72–0.90

It is argued that the macroscopic motion of particulate materials and the subsequent particle degradation may be partly explained by the existence of failure zones. Within the strained zones, relative particle–particle motion occurs which causes particles to interact and thus degrade by both surface abrasion and fracture. The annular shear cell as an attrition test apparatus has been used by different workers

We now examine the extent to which the annular shear cell attrition data for the materials is characterised using the effective work input ŽEWI. used by Neil and Bridgwater w7x for the annular shear cell. For a given material, normal stress and attrition time can be adjusted to maintain WT as constant. We note that for a given material the slope

A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44 Table 4 Evaluation of EWI parameter f Mass attrited Žg.

43

empirical constant. We thus have the fraction of material degraded given by a function of the form

Condition Ž1.

Condition Ž2.

st ŽkPa.

st ŽkPa.

t Žs.

f Žy.

t Žs.

HeaÕy soda ash (0.36 – 0.50 mm) 1.5 2.7 60 1.5 2.7 60 2 2.7 115 2 8.3 16 2.5 2.7 185 4 8.3 42 6 8.3 76 Average

8.4 17.7 17.7 17.7 17.7 17.7 17.7

11 3.5 5.0 5.0 8.0 13 23

0.672 0.664 0.602 0.654 0.601 0.648 0.636 0.64

TAED (0.50 – 0.71 mm) 1.5 2.7 1.5 2.7 2 8.3 5 8.3 9 8.3 Average

144 144 21 70 168

8.3 13.8 13.8 13.8 13.8

11 4 6 22 53

0.437 0.455 0.406 0.439 0.441 0.43

TAED (1.0 – 1.6 mm) 10 2.7 12 2.7 12 2.7 15 7.2 17 7.2 Average

60 100 100 34 45

xsf

7.2 7.2 10.5 10.5 10.5

12 20 10 18 24

0.609 0.610 0.590 0.593 0.600 0.60

of a plot for ln x vs. ln d is constant. The approach proposes that the work promoting particle degradation is a fraction of the total work input. The fraction of work that causes attrition is proportional to t fy1 where f is an

s tf

ž /

Ž 7.

sc

normalising the stress by sc . Noting characterisation of breakage by the single particle failure stress sc , the occurrence of the breakage of a particle should be related to the ratio srsc . Thus a special case of Eq. Ž7. is of the form

s

xsA

b

tf

ž / sc

Ž 8.

f is an empirically determined exponent, calculated by:

f s ln

s2

t1

ž / ž / s1

ln

t2

Ž 9.

where the subscripts denote the conditions of stress and time at which there is some predetermined value of x. For the materials studied in this treatment, values of f were found which are summarised in Table 4. Logarithmic plots of x against wŽ srsc . t f x are shown in Fig. 5 where the term EWI represents wŽ srsc . t f x and b is the gradient. The product of the empirically determined parameter f , evaluated at constant material attrition, together with b ,

Fig. 5. EWI plot for attrition in annular shear cell with normal stress ŽkPa. as parameter.

44

A.U. Neil, J. Bridgwaterr Powder Technology 106 (1999) 37–44

may be compared with the Gwyn parameter m. The information below shows there to be general consistency.

7. Lists of symbols

EWI treatment Ž fb . Heavy soda ash Ž0.36–0.50 mm. 0.81 TAED Ž0.50–0.71 mm. 0.50 TAED Ž1.0–1.6 mm. 0.41

A F0 G K

Gwyn function m Žaverage. 0.83 0.50 0.47

The EWI treatment incorporates stress as a variable and has been used to characterise the attrition in the annular shear cell. The Gwyn function, while not incorporating stress as a variable, has also been used to characterise the attrition from the annular shear cell. It is therefore reasonable to infer that the EWI treatment can be used to characterise bulk material degradation caused by equipment other than the annular shear cell since the values of m do not vary between pieces of equipment. The dimensions of EWI are t f ; when greater insight has been obtained, time t should be replaced by the shear strain. An overall implication, therefore, is that from this direct comparison between the EWI treatment and the Gwyn treatment it should be possible to gain a greater quantitative understanding of attrition when the bulk materials experience stresses during processing.

U Umf W WT d dT m t x

proportionality constant in the EWI treatment Žg. average load at failure ŽN. exponent in Schuhmann size distribution proportionality constant in the Gwyn formulation Žsym . distance between loading points for particle of diameter d superficial gas velocity in fluidised bed Žmrs. minimum fluidisation velocity Žmrs. mass degradation Žg. estimated overall mass degradation Žg. diameter of sample Žm. largest sized particle Žmm. exponent in the Gwyn formulation Žy. attrition time Žs. fractional mass degradation Žy.

Greek b f s sc

letters logarithmic rate term of material degradation Žy. parameter of attrition Žy. normal stress ŽNrm2 . single particle failure stress ŽNrm2 .

6. Conclusions

Acknowledgements

The purpose was to determine whether the Gwyn approach can be used to characterise the physical attrition of particulate materials irrespective of the equipment that causes the attrition. To this end, three particulate systems, heavy soda ash Ž0.36–0.50 mm., TAED Ž0.50–0.71 mm. and TAED Ž1.0–1.6 mm. were subjected to attrition using a laboratory fluidised bed, a screw pugmill, and an annular shear cell. For each particulate system, the Gwyn function was found to characterise successfully the extent of attrition caused by the three different pieces of equipment. Additionally, for any one material, the value of the Gwyn m parameter differs little between equipment. For the annular shear cell attrition, the EWI approach was used to characterise the attrition of the three particulate systems. Further, equating the Gwyn and the EWI functions support the finding from previous work that the Gwyn parameter m may be split into a material parameter of attrition f and rate of attrition b . The extent to which the Gwyn and EWI treatments can be used to develop material properties of attrition, independent of equipment, now needs to be examined more thoroughly.

Thanks are due to Unilever Research, Port Sunlight Laboratory for supporting this study and for permitting its publication.

LŽ d .

References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x

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