An analysis of mill grinding noise

Department (Received

of Metallurgical July-11,

Engineering,

1983 ; iti revkd

Uniuersity

form February

of Missouri-Rolls, MO (U.S.A.) 8.1984)

SUMMARY

The noise generated by a ball mill during a batch grinding operation is .investigated and the results-show that, for a given ore, the noise levels may vary- with time of grind, ore charge weight and mill speed. The role of the ore in absorbing noise energy is suggeste.d as a possible control variable for the grinding process and as an indicator of product size distribution. The relationship between ore types and mill noise is also examined and, under conditions of wet grinding, it is demonstrated that mill noise analysis can indicate ore type and may have a possible use as a grindability-type parameter.

INTRODUCTION

The fine breakage of ore particles by rod and ball milling is an extremely inefficient process with less than 1% of the energy input contributing to the actual particle breakage [l] _ Most of the energy is converted to heat with a small proportion giving rise to mill noise. In the past, there has been an interest in utilising the noise level of a mill to control the feed rate [Z, 31 based on the ‘electric ear’ of Hardinge [4] _ In recent years, there has been an upsurge in interest in mill sound, with special emphasis on acoustic emission spectra [5,6]. An analysis of the sound spectrum emanating horn a mill must bc capable of isolating those parts of the spectrum due to particle breakage from the other background sources. This, of course, is a very difficult task and is an area in which there is considerable research effort, especially in Russia. An alternative approach is to monitor noise changes in a mill which will result from changes in ore type or size rather than from the changes in the noise levels due to the mill system itself. 0032-5910/85/$3.30

This project investigates this -alternative approach by monitoring noise level variations in a laboratory batch grinding mill. The aims of the research are to characterize particle breakage and identify ore type in terms of some noise level variation with time_

THEORETICAL

CONSIDERATIONS

The fust step in this project was to consider the noise generated in a laboratory batch mill. If a laboratory mill is run without an ore charge, the resulting noise levels reflect mainly the ball/liner collisions. The introduction of ore into the mill will reduce the noise level as the ore particles prevent the noise generating ball/liner collisions. The ability of ore particles to prevent these collisions will be a fun&ion of the particle size and number of particles present in the mill. Hence, the noise level variations during a batch grinding operation should indicate particle size and number, i.e. degree of grinding_ It is suggested that particles above a certain size, dependent upon the ball size, will be capable of preventing the noisy steel/steel collisions. Initially, in a batch grind, the number of such particles would increase as each large particle breaks into several smaller particles still of sufficient size to be effective in blocking the ball/liner collisions. Thus, for a time governed by the mill and charge conditions, the overall noise will decrease. Eventually, however, the size of many particles will be insufficient to withstand the ball/liner collisions and steel/steel contact will be reestablished. At this point, the mill noise level will increase and further grinding will tend to permit the noise levels to return towards the levels of a mill/ball system without an ore charge. Thus, the manner of particle breakage

will affect

the noise variations

Cl fievier

Sequoia/Printed

within the mill. in The Netherlands

a4

Breakage of coarse particles will reduce mill noise rapidly, whilst production of fine particles would not influence noise levels to the same extent. Alternatively, the production of fine particles with adherent properties could dramatically affect the noise generated in the mill. Thus, the ore character could be rtiflected in the noise variations. In a rubber-lined mill, the ball/liner collisions do not contribute to the mill noise significantly and the mill is considerably quieter. However, grinding is still achieved and therefore, of course, mill noise does not necessarily reflect size reduction but is more an indication of the number of noise-generating ball/liner collisions. This does not preclude mill noise in a steel-lined mill from being a possible indicator of mill grinding performance, as particle breakage, whilst blocking ball/liner collisions, should represent a relatively constant proportion of the overall breakage, from all collisions, within a mill. The available energy of a mill for grinding is governed largely by the mill speed, ball charge and ball size. Again these parameters are reflected to some extent by noise levels and it is suggested that the reduction from mill/ball-only noise levels caused by the introduction of ore could be an indicator of coarse breakage and type of breakage_ If the reductions in sound pressure levels (dB) are calculated as sound power level, using the reference value of lo- I2 W and then summed over the grinding time, the resultant parameter will be related to the sound energy absorbed by the ore_ This ‘absorbed sound energy’, whilst only a minute fraction of the energy input to the mill, is a measure of energy and as such may well be a parameter that reflects the grinding process.

frequency, to be taken each second with a losecond average being logged four times each minute. In addition, several tests utilized a General Radio Real Time Analyzer to determine the overall frequency response of the system, and to determine the appropriate fiequencies to be investigated. Initially, a series of tests were run to investigate the noise produced by the miIl as a function of ball size and miIl feed. The ball size ranges adopted were -12.7 +6.35 mm, -19.0 +12.7 mm, -25.4 +19.0 mm, and +25.4 mm. The mill noise variaton with time was then investigated using a 9.9 kg charge of mixed balls and 500 g of -6.6 +4.6 mm dolomite_ The mill noise level was monitored continuously for 32 min and the ore was sized at discrete time intervals. To examine feed character, four materials were utilized as mill charge and these were dolomite, chert, fluorspar and hematite. The materials were fed to the mill in 0.5,l.O or 2.0 kg charges of -6.6 +4.6 mm. The size distributions of grinding products were determined using a Tyler Sieve series down to 149 pm and a Rotap shaker. In the later tests, wet grinding was undertaken and here the pulp charged to the mill contained 50% solids by weight.

RESULTS AND DISCUSSION Initial tests were aimed at determining the appropriate frequencies for the investigation of mill noise. Figure 1 illustrates a typical frequency response for the conditions of mill and mill/ball charge operations. It is apparent that above a frequency of 2 kHz the noise levels produced are basically a

E_YPERIMENTAL A IO in X 8 in steel laboratory miIl with a charge of 9.9 kg of cast steel balls (1 - 3 cm dia.) formed the basis of the experimental equipment. This mill was run at speeds of 55 75 rpm (58 - 80% critical) and the noise levels produced were monitored with a General Radio Precision Sound Level Meter linked to an Apple computer via an Isaac data acquisition. The computer program provided for 13 sound pressure level samples, at a preset

- . .025

.05

.iO

.iO

A-0

FREQUENCY

Fig.

1. Mill

sound

frequency

.80

I:6 (KHz)

spectrum.

85

function of ball/liner and ball/ball collisio& The effect of .mill speed on -the-frequency: spectrum.was found to merely alter the sound pressure -levels whilst leaving the spectrum shape unchanged. By ~lining the mill with a thin rubber sheet, ball/liner collisions were effectively eliminated, and it was evident that ball/ball collisions account for very little of the generated noise in the mill/ball system. Figure 2 illustrates the sound pressure level response for single-size ball/mill operation over the ball size 6.25 to 25.4 mm for mill speeds of 55,65 and 75 r-pm. The frequency of sound measurement for these tests was 8 kHz, which, fiorn Fig. 1, appeared to be indicative of the ball behavior_ It is interesting to note that the largest balls give a lower sound pressure level than the two preceding sizes and this is due to the frequency (8 kHz) selected for the test. Figure 3 illustrates the variation in sound pressure level at 8 kHz with the number of balls in the mill at speeds of 55,65 and 75 rpm. It can be seen that the noise level increased initially with the number of -19 +12.7 mm balls and mill speed, but flattened when the ball number reached approximately 100. This can be explained initially by an increase in the number of ball/liner collisions as ball number increased and then a leveling of noise due to the effect of ball/ball collisions hindering the occurrence of the noise-producing ball/liner collisions. Similar results were obtained using balls of the other three size ranges. Typical results of the tests to examine the effect of ore on the mill/ball noise production are shown for 2 kg feeds in Fig. 4, from which the following points emerge. Initially, the sound pressure level drops with time for all conditions and this represents the ore particles preventing the ball/liner collisions. Overall, the mill noise levels are lower with ore charge weight, as expected, and higher mill speeds tend to accentuate this effect. The horizontal lines on the plot in Fig. 4 represent the ball/mill system without ore and it can be seen that the noise reduction, from the mill/ball level, increases with mill speed, as the effect of ore is to dampen the increased collisions of ball. and liner at the higher speeds. The mill/ball-only lines on Fig. 4 indicate higher noise levels and hence higher available energies with increasing mill speed. In general, the noise reduction increased slowly

i

A

i

b

SIZE

BALL

Fig. 2. Mill sound variation at 8 kHz with ball size. A = -12-7 +6.3 mm, B = -19.0 +12.7 mm, C = -25.4 +19 mm, D = +25.4 mm.

-

75 65 55

-----

061

0

lb0

2bo

NUMBER

OF

rpm

Lbo

3bo

sbo

-19.0+12.7mmBALLS

Fig. 3_ Mill sound variations with ball number mill speed.

;I]] 0

#)4

*

80

____:_l_:-_--___

1 0

1

Q

TIME

and

_-------

I

&R:6NO

Fig. 4. Mill sound variations mill speed for a 2 kg feed.

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1

’

‘2’8

r

32

t:,NZ)

with grind time and

with time but above 65 rpm a reverse for the 1 kg and 2 kg charges was noted_ This could be explained by sufficient grinding having taken place to remove the collision-blocking particles. Figure 5 illustrates the 2 kg charge size dist-ribution uersus time rlsts where the weight of dolomite of size less than 0.59 mm is represented against time of grind. It is realized that plotting grind size as a single parameter is not truly representative but, in terms of the size distribution of the ground dolomite, the size param eter does tend to

86

0

0

4

8 TIME

12 OF

16 GRIND

20 2r (MIN)

29

32

Fig. 5. Mill grind size variations with time and n.Ill speed for a 2 kg feed_

reflect the overall distribution. The weight of material ground to less than 0.59 mm increases with time at a rate dependent upon the charge weight, although in the first half of the grind the 1 kg sample produces more fine material than the 2 kg. This is possibly due to an excess of coarse feed particles in the larger charge protecting each other Tom breakage. An analysis of the data illustrated in Figs. 4 and 5 provides little assistance in attempting to relate the mill noise to the degree of grinding and it is necessary to examine other derived p ammeters. To facilitate this requirement, the sound pressure level readings have been converted to a power parameter (milliwatts) and plotted against time_ Figure 6 illustrates this relationship for 75 r-pm, and similar relationships exist for the lower speeds. As indicated earlier, the area under the plots of sound power versus time represent energy and therefore the areas under the plots were calculated as ‘absorbed sound energy’_ Figure 7 plots the ‘absorbed sound energy’ values against the weight of material less than 0.59 mm for the 32 min grind times at each of the three mill speeds. It can be seen that the No ore -75

401

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v,

x-L_

0

,

O

-2.

Kg

Ore charge

___

4

e TIME

12 OF

1

16 GRIND

I.0 Kg

5

u-l m *300

200 WEIGHT(g)

LESS

‘THAN

0.59 mm

Fig. 7. Absorbed sound energy variation with fineness of grind for dolomite.

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energy increased with- -material -ground for each of the ore charges, but no overall rela-tionship is apparent. If Fig. 7: is replotted with the ‘absorbed sound energy’ expressed as a percentage of the available sound energy (ie. ball/mill only), then.Fig. 8 results. In this case, there appears to be an overall relationship for the dolomite with % absorbed sound energy increasing with fineness of grind. To investigate the effect of bail size, similar tests to the above were carried out using 2 kg of single size range balls with 500 g of dolomite. Table 1 presents the absorbed sound energy and grinding data for the twelve tests. The ‘% absorbed sound energy’ plots for the data in Table 1 are given in Fig. 9 and again a similar overall relationship to that in Fig. 8 is obtained. The next step in this initial study was to investigate the absorbed sound energy criterion for a variety of materials. Four ores were selected and these included dolomite, chert, fluorspar and hematite. The grinding tests using 1 kg, 600 cm3 packed volume and

I

20

I

24 ( MIN)

2’8

I-

32

Fig. 6. Variation of mill sound power with grind time and feed charge weight at a mill speed of 75 rpm.

20, 0

200

400 WEIGHT(g)

600 LESS

000 THAN

1000 0.59mm

I200

Fig. 8. Per cent absorbed sound energy plotted as a function of fineness of grind for various dolomite feed charge weights and mill speeds.

-: -.

87.

_

TABLE

1

,.

Effect of ball size on mill sound levels 2 k& of b&, 500 g of -6.6 +4.3 mm dolomite, 32 min grind .Size A= -12.7 +6.3 mm; B = ‘19.0.+12.7 mm; C = -25.4 +19.0 mm; b = +25.4 mm

Speed (rpm)

55

Ball size

A.

B

C

D

A

B

C

D

A

B

C

D

298

557

653

442

403

733

768

569

531

966

1162

803

Absorbed

sound energy

(mW min) 70Absorbed sound

65

94.2

95.2

96.6

97.7

75

95.2

96.2

97.3

97.8

94.3

97.2

98.0

96.6

energy

mm material (9)

-0.59

110

196

315

391

142

237

380

447

221

314

100

+ I

.

8

0

100 WEIGHT

*

c z

a

200 (g)

c

+ B *A

300 LESS

X0

P

00

445

No ore

1

l

426

0

Ball Size

403 THAN

--k-Chert ---Dolomite F1 uorspar 500 0.59

600

mm

Fig. 9. Per cent absorbed sound energy plotted as a

function of fmenw of grind for a 0.5 kg dolomite feed charge found with yarious sized 2 kg ball charges and mill speed.

finally 300 cm3 actual volume of ore gave inconsistent results over the four materials. An examination of the results and the mill/ ball system indicated that the problem lay in the production of fine material which could coat the balls and mill liner and reduce the sound pressure levels produced_ This reduction far exceeded that expected horn the weight of fine particles produced by grinding. Whilst hematite and chert had similar grinding characteristics, hematite produced a tie coating which caused large-scale noise reduction but chert produced virtually no coating particles_ To overcome the problem, the final tests were undertaken with a pulp consisting of 50% solids by weight. One kg of each of the four materials was ground at 65 and 75 rpm for 10 min to test the ‘absorbed energy’ parameter as a indicator of grinding in terms of the production of material less than 0.59 mm. The sound pressure level uersus time plots are illustrated in Fig. 10 for 75 rpm and the appropriate data

I

I

l234567d9 GRIND

I

IO TIME

(MINI

Fig. 10. Sound level variations with grind time for

the wet grinding of four feed materials at 75 rpm.

are reproduced in Table 2. Figure 11 gives ‘% absorbed sound energy’ data graphically for the wet grinds and it can be seen that the relationship is similar to that illustrated earlier, although an overall relationship is not evident. Thus, for a given mill speed, ‘% absorbed energy’ tends to be indicative of the degree of coarse grinding of the four feed materials_ Chert and hematite absorb least energy and exhibit least coarse grinding, whilst the opposite is true for the fluorspar and dolomite_ Thus, the ‘% absorbed sound energy’ parameter appears to loosely represent grinding characteristics and further work is necessary to establish a relationship between this parameter and conventional parameters such as grindability. It has been suggested [7, 81 that pulp rheology can play a significant part in the grinding process. It is readily apparent that a relationship should exist between the pulp viscosity and the noise produced by a ball striking the liner wall in the presence of a pulp film. With a lower viscosity, the ball will have a greater available energy to impact the

-

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:.. ..-

TABLE

2

..

65 C

F

123

412

(g)

,...

D 306

C

H 209

Absorbed sound energy (mW mm)

10.6

19.3

17.6

5%absorbed sound energy

33

58

53

28

Work index (kWb/sh. ton)

13.6

11.3

17.0

dolomite t 0 *

8.9

fluorspar * 0

I 0

cherl

zloco

he%lite

100

0

WEIGHT(g)

LESS

:

;

:

:

75

Speed (rpm) Material material

...

‘_:

‘..

Variation of wet mill grinding sound energies with feed type 1 kg of ore, 9.9 kg of balls, 1 litre of water, -10 min of grind F = Fluonpar D = Dolomite H = Hematite C = Chert

-0.59 mm

;

THAN

0.59

mm

Fig. 11. Per cent absorbed sound energy plotted as a function of fineness of grind for 1 kg of each of four different charge materials ground at 65 and 75 rpm.

wall or to cause particle breakage. Further, it has been demonstrated 17, S] that the specific breakage rate constants may be utilised to represent the breakage of particles within a mill and this offers better representivity than the simple parameter used in this research. Hence, a future step in investigating a possible use of mill noise as a contiol param eter will be to examine the relationship linking pulp rheology, mill noise and particle breakage as described by breakage rate constants_

CONCLUSION

The experimental data presented in this paper represents only a brief pilot study carried out in a short time period with a small grant. The aim of the project was to investigate mill noise variations as a possible indicator of grinding performance and it has been demonstrated that such a relationship can be

9.5

F

113

:

449

D 300

H. ...

209

28.4

41.9

40.5

27-3

44

65

63

42

established for a given material under given conditions. The ‘absorbed sound energy’ parameter has been shown to represent fineness of grind for tests involving variations in feed charge weight, mill speed and ball size. It has also been shown that relative grinding characteristics between materials can be inferred from noise level variations if wet grinding is utilized. An extremely large amount of data has been collected during the project and it is suggested that mill noise level variations have a possible role in furthering the understanding and control of the milling process. It is realized that there are many simplistic assumptions in this brief study but it is felt that these are justified, considering the preliminary nature of the investigation_

ACKNOWLEDGEMENTS

The author would like to thank the U.S.D.I. for providing the grant which made this pilot study possible and trusts the information provided in this report will be of interest in further investigations. In addition, Dr. A. Cummings of Mechanical Engineering, U.M.R., must be acknowledged for his advice and assistance with regard to noise measurement and interpretation_

REFERENCES 1 L. G. Austin and Ft. Ft. Klimpel, 56 (1964) 1%

Ind. Eng. C&m.,

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P. Harrington, Trak. A.tM_E.,

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Pi G. 6&-r ‘270.(1980)

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