TSpace Research Repository tspace.library.utoronto.ca

Nonintrusive Monitoring and Control of

Metallurgical Processes by Acoustic Measurements

Hao-Ling Yu, Leili Tafaghodi Khajavi and Mansoor Barati

Version Post-print/Accepted Manuscript

Citation

(published version) Yu, HL., Khajavi, L.T. & Barati, M. Metall and Materi Trans B (2011) 42: 516. https://doi.org/10.1007/s11663-011-9496-3

Publisher’s statement This is a post-peer-review, pre-copyedit version of an article published in Metallurgical and Materials Transactions B. The final authenticated version is available online at: http://dx.doi.org/10.1007/s11663-011-9496-3

How to cite TSpace items

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace

because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page.

This article was made openly accessible by U of T Faculty. Please tell us how this access benefits you. Your story matters.

1

Non– intrusive Monitoring and Control of Metallurgical Processes by Acoustic Measurements

Luke Yu, Leili Tafaghodi Khajavi, Mansoor Barati*

*) Corresponding author

Assistant professor

University of Toronto, Dept. of Materials Science and Engineering

140 – 184 College Street, Toronto, ON

Canada M5S 3E4

Tel: (416) 978 – 5637

Fax: (416) 978 – 4155

Email: mansoor.barati@utoronto.ca

The feasibility of developing a new on–line monitoring technique based on characteristic

acoustic response of gas bubbles in a liquid has been studied. The method is intended to

monitor the chemistry of the liquid through its relation to the bubble sound frequency. A

low temperature model consisting of water and alcohol mixtures was established and the

frequency of bubbles rising under varying concentrations of methanol was measured. It

has been shown that the frequency of the sound created by bubble pulsation is changed

by the percent of alcohol in water. The frequency drops sharply with the increase in

methanol content up to 20 wt%, after which the decreases is gradual. Surface tension

appears to be one of the critical liquid properties affecting the sound frequency through

its twofold effects on the bubble size and pulsation domain. The strong dependence

between the frequency and liquid composition suggests the feasibility of developing an

acoustic based technique for process control purposes.

I. INTRODUCTION

Today, gas injection into metallurgical baths is a standard practice, employed for thermal and/or chemical

homogenization of the liquid, as well as introduction of reactive gasses to promote the refining reactions. Of the critical

issues in metallurgical refining practices is controlling the chemistry of the melts to achieve desired composition, avoid

excessive use of the refining gases and fluxes, and maximize productivity by treating the melt for shortest time possible.

However, this has proved to be extremely difficult due to the continuous and rather rapid variations in the composition

and properties of the molten metal (or matte). Conventionally, the chemical composition of the melt is controlled by

off-line analysis of withdrawn samples, followed by corrective actions. This approach in monitoring is generally

inefficient because the feedback provided is not quick enough to allow precise and timely adjustments. Moreover, the

sampling practice can be costly and the results are affected by the sampling position and method. The evident

drawbacks of such control approach are: inconsistent analysis, loss in productivity, higher energy consumption, more

production rejects, and economic inefficiency. As a result, the tendency has shifted to online and preferably continuous

monitoring of the process.

It is well established that for many metallurgical melts, the physical properties of the liquid vary significantly

over the treatment process. For example, the sulfur removal or oxygen dissolution associated with the refining reactions

in steelmaking and converting of matte result in substantial variations in the surface tension of the liquid, as illustrated

in

2

Figure 1. Changes in the physicochemical properties of the melt with its chemistry may potentially be

employed to establish indirect monitoring and control of the process.

Figure 1: Variations in the surface tension of (a) Fe–S[1, 2] and (b) Cu–O[3] melts with the dissolved element content .

1E-4 1E-3 0.01 0.1

800

1000

1200

1400

1600

1800

2000

2200

Su

rfa

ce

Te

ns

ion

x1

03 (

N/m

)

Lee and Morita [1]

Keene et. al. [2]

Sulfur content (Mass %)

(a)

-13 -12 -11 -10 -9 -8 -7 -6 -5

800

900

1000

1100

1200

1300

1400

(b)

Su

rfa

ce

Te

ns

ion

x1

03 (

N/m

)

log (pO2

(atm))

1E-4 1E-3 0.01 0.1

800

1000

1200

1400

1600

1800

2000

2200

Su

rfa

ce

Te

ns

ion

x1

03 (

N/m

)

Lee and Morita [1]

Keene et. al. [2]

Sulfur content (Mass %)

(a)

-13 -12 -11 -10 -9 -8 -7 -6 -5

800

900

1000

1100

1200

1300

1400

(b)

Su

rfa

ce

Te

ns

ion

x1

03 (

N/m

)

log (pO2

(atm))

3

The application of acoustic–based techniques in process control is relatively new, with only a few reported

works in metallurgical industry [4-7]. The intended applications vary from controlling the desulfurization process in

ladle to monitoring the refractory lining in blast furnace or other reactors. Although the refractory monitoring attempts

have been successful to some extent, the control of melt chemistry through acoustic measurements is still very

immature and has not been studied in depth. The present study was undertaken to investigate the feasibility of relating

a liquid composition to the characteristics of the bubble sound, emitted during gas injection into the same liquid. Low

temperature system of water–methanol was selected as it allows easier measurements and more precise control of the

process conditions.

It is well established that the oscillation frequency of a bubble is related to the physical properties of the liquid

in which the bubble is formed, and the size of the rising bubble [8, 9]. The following equation was obtained by Lamb

[8] to express the frequency of a ball of one medium oscillating in another medium.

4𝜋𝑓2 = 𝑛(𝑛 − 1)(𝑛 + 1)(𝑛 + 2)𝜎

((𝑛 + 1)𝜌𝑙 − 𝑛𝜌𝑔) 𝑟3 (1)

where 𝑓 is the oscillation frequency, 𝑟 is the radius of the bubble, 𝜎 is the surface tension, 𝜌𝑔 and 𝜌𝑙 are

the densities of the gas and the surrounding liquid respectively, and n represents the type of bubble

oscillation. It is such dependency between bubble sound frequency and the physical properties of the

liquid that is used in the present work as a basis to correlate the bubble sound frequency to the liquid

composition.

II. EXPERIMENTAL WORK

The experiments were performed in a bottom blown rectangular Plexiglas container (10cmwidth , 20cm

length, 80-cm height). The tank was filled with 7 liters of methanol-water mixtures with varying methanol content and

compressed air was introduced from a 2 mm nozzle centred at the bottom. The bubble sound was recorded using two

ME66/K6 microphones (obtained from Sennheiser) placed on the top and side of the tank. The top microphone was

partially isolated from the ambient sound by mounting an antenna–like stainless steel shield around it. Raven Lite 1.0

software was used for recording the sound profile. The background noise was eliminated by comparing and contrasting

the noise profile before the experiment and the recorded sound track during the experiment. Eventually in order to

obtain the frequency peaks, the noise-free sound track was analyzed using Audacity sound editing program with Fast

Fourier Transform (FFT) method.

In addition to the sound recording apparatus, a CCD camera was used to record a video of the rising bubbles.

The video was later analyzed to measure the number of fully detached bubbles during a fixed time interval. Assuming

the bubbles to be spherical, the average bubble size can be calculated by dividing the flow rate to the number of released

bubbles within a specific timeframe. The schematic of the experimental set up is shown in Figure 2.

Figure 2: Schematic representation of the e xperimental setup

gas inlet

flowmeter

Camera

Microphones

4

In some preliminary experiments, the maximum gas flowrate and bath depth that give the highest contrast in

the recorded spectrum were determined. It was found that the bubble sound echo intensity increases with decreasing

the bath height, subsequently larger volumes of the solution would help to decrease the interference from the echo.

The optimum flow rate was determined according to the distinctness of the sound profile while the liquid

volume and methanol concentration were constant. The results of a primary experiment in seven litres of water solution

revealed that at higher flow rate (e.g. greater than 0.75 lpm) the background noise and bubble sounds interfere with

each other due to larger number of bubbles at higher flow rates. The flow rate was fixed at 0.6 lpm where it is easy to

distinguish the background noise and the bubble sound. The relatively small gas flow rate also facilitated detection and

counting of the bubbles in the recorded video. As Figure 3 shows, the frequency of bubble generation increase

substantially after the gas flowrate exceeds ~ 0.8 lpm.

Figure 3: Frequency of bubble generation as a function of gas flowrate.

In order to study the effect of liquid composition (and properties) on the acoustic response, numerous

experiments with different methanol concentrations (0–100 %) were performed at constant gas flow rate (0.6 lpm) and

bath height (35cm).

III. RESULTS AND DESCUSSION

A. Effect of solute content on liquid properties

The previously measured physical properties of water-methanol mixture [10-12] are available at specific

solute concentration. As presented in Appendix I, the values were fitted into equations that are used for evaluation of

the liquid properties in the current analysis.

B. Effect of additive content on bubble size

Bubble size and additive concentration

The average size of the bubbles generated in solutions with different methanol content is shown in Figure 4.

As seen, the bubble size goes through a sharp decrease for an increase in the methanol content up to 20 wt%. Further

increase in the methanol content results in only a slight decrease in the bubble size. It is well established that under the

conditions of fixed gas flowrate, pressure, and temperature, the size of the gas bubbles is predominantly decided by

the properties of the liquid and gas. According to several theoretical and experimental studies, the bubble size is directly

related to the interfacial tension and inversely proportional to the liquid density. More specifically, it has been shown

that the bubble size is a function of 𝜂 = (𝜎 𝜌⁄ )1/3[13-17]. The validity of this relationship was tested for the

experimental results of the present work by plotting bubble diameter against 𝜂 (Figure 5). As seen, in agreement with

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80

100

120

140

Fre

qu

en

cy

of

bu

bb

le g

en

era

tio

n (

se

c-1)

Flowrate (LPM)

5

the previous findings, the bubble diameter follows a linear relationship with 𝜂 for the wide range of the compositions

examined in this study.

Figure 4: The effect of methanol content on bubble diameter

Figure 5: Bubble diameter as a function 𝜂 = (𝜎 𝜌⁄ )1/3

C. Effect of liquid composition on bubble acoustic frequency

Effect of solute concentration on frequency

The measured bubble sound frequency as a function of methanol concentration is shown in Figure 6. As seen,

the frequency gradually decreases as the solute concentration increases, but the variations are not strongly linked. This

is primarily because of the simultaneous variations in the physical properties of liquid and bubble size with methanol

content, both of which affect the bubble sound frequency. According to Minnaert equation provided below [9], at fixed

0 20 40 60 80 100

1.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Bu

bb

le D

iam

ete

r (c

m)

Methanol Content (wt%)

3.0 3.2 3.4 3.6 3.8 4.0 4.2

1.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Bu

bb

le D

iam

ete

r (c

m)

(

R2=0.8409

6

surface tension, the acoustic frequency of a single bubble in an infinity large container is inversely proportional to the

bubble radius and the square root of density.

𝑓 =1

2𝜋𝑟(

3𝛾𝑃𝐴

𝜌)

12 (2)

In this equation, f is the resonant frequency, r is the radius of the bubble, is the proportionality

coefficient, AP is the ambient pressure at place of bubble and is the density of the solution.

Since in metallurgical systems the density variation during refining is negligible (e.g. desulphurization), any

variations in the bubble sound frequency is chiefly originated from the changes in the surface tension and bubble size.

Therefore, in order to investigate the dependence of frequency on these parameters, the frequency is corrected by

eliminating the effect of the liquid density based on Eq. (2). After eliminating the density effect, the corrected frequency

values for methanol-water solutions are significantly lower than the measured frequency and the decrease with

methanol content is more pronounced (Figure 6).

Figure 6: The effect of methanol content on bubble sound frequency

Effect of liquid surface tension on frequency

Surface tension is one of the melt properties that is most affected during refining of metals/mattes (

0 20 40 60 80 100

400

420

440

460

480

500

520

540

560

Measured Frequency

Corrected Frequency

Fre

qu

en

cy

(H

z)

Methanol Content (wt%)

7

Figure 1). Also, surface tension is believed to affect the oscillation characteristics of a bubble through both its

direct effect on the bubble/liquid interface and also its effect on the bubble size (Figure 5), which in turn affects the

frequency through Eq. (2). Therefore, for an acoustic–based process monitoring technique in a system with negligible

variations in density and viscosity (such as liquid steel), strong dependence of the frequency to the surface tension is

warranted. As discussed earlier, the effect of density on bubble sound frequency was eliminated based on Minnaert

equation. In order to investigate the exclusive effect of surface tension on bubble sound frequency, the effect of

viscosity should be eliminated as well. The variations of surface tension and viscosity with methanol content are

provided in

Figure 7. As seen, with increase in methanol content, the viscosity first rises to a maximum at around 40 wt%

and then drops, while the surface tension decreases gradually. Several iso–viscosity compositions in this system can

be identified by drawing a horizontal line that intersects the viscosity profile, consequently two points with identical

values of viscosity can be obtained. The bubble sound frequencies associated with these pair of points are presented in

Table 1 for several iso–viscosity compositions.

1E-4 1E-3 0.01 0.1

800

1000

1200

1400

1600

1800

2000

2200

Su

rfa

ce

Te

ns

ion

x1

03 (

N/m

)

Lee and Morita [1]

Keene et. al. [2]

Sulfur content (Mass %)

(a)

-13 -12 -11 -10 -9 -8 -7 -6 -5

800

900

1000

1100

1200

1300

1400

(b)

Su

rfa

ce

Te

ns

ion

x1

03 (

N/m

)

log (pO2

(atm))

0 20 40 60 80 100

20

40

60

80

Methanol Content (wt%)

Su

rfa

ce

Te

ns

ion

at

25

°C

(m

N.m

-1)

0.0

0.4

0.8

1.2

1.6

2.0

Vis

co

sity

(mP

a.s

)

8

Figure 7: Surface tension and viscosity of methanol–water solutions

Table 1: Effect of surface tension of methanol - water mixture on bubble sound

µ (cS) σ1 (mN.m-1) σ2 (mN.m-1) σ1- σ2 f1 (Hz) f2 (Hz) f1-f2 (Hz)

1.0 71.9 23.3 48.6 540 461 79

1.2 54.0 24.0 30.0 532 472 60

1.3 48.8 24.4 24.4 525 483 42

1.4 45.1 24.8 20.3 524 488 36

1.5 42.7 25.2 17.4 522 493 29

1.6 37.6 26.2 11.4 517 502 15

1.8 33.3 28.0 5.3 517 512 5

It is evident from Table 1 that a decrease in the surface tension results in smaller frequencies. This is expected

according to Eq. (1) that suggests a direct relationship between f2 and surface tension. However, both equations (1)

and (2) show an inverse relationship between the frequency and the bubble size (although not entirely consistent in

degree of dependence), while the bubble size itself becomes smaller when the surface tension decreases. It may then

be discussed that the surface tension plays a twofold role on the sound frequency; on one hand, higher surface tension

favors faster pulsation of the bubble due to the smaller domain of oscillation. On the other hand it results in formation

of larger bubbles that are more resistant against pulsation and decrease the frequency. From the overall effect of surface

tension, it however is clear that the former, direct effect of surface tension, overcomes its secondary effect and increase

the bubble frequency.

0 20 40 60 80 100

20

40

60

80

Methanol Content (wt%)

Su

rfa

ce

Te

ns

ion

at

25

°C

(m

N.m

-1)

0.0

0.4

0.8

1.2

1.6

2.0V

isc

os

ity (m

Pa

.s)

9

Assuming negligible density of air compared to the liquid, frequency should be directly

proportional to 𝑆 = √𝜎 𝑑3⁄ , following Eq. (1).

Figure 8 presents the corrected frequency as a function of 𝑆, where each value of 𝑆 corresponds to a specific surface tension and the associated bubble diameter. The figure shows that as 𝑆 increases with decreasing methanol content, greater frequency values are obtained . A sharp

change in the slope of the graph can be seen at around 𝑆 = 4.2, which indicates slower increase in frequency after this point. According to the experimental results 𝑆 = 4.2 occurs at around 37wt% of methanol content at which the change in the surface tension prof ile of methanol-

water mixture becomes less steep. Moreover, as seen in

Figure 7, the increasing trend of viscosity with methanol content transforms to a decreasing trend at around

the same methanol content. The above changes in surface tension and viscosity of methanol-water mixture are the

possible reasons for the change of the slope in frequency vs. 𝑆 variations.

3.6 4.0 4.4 4.8 5.2 5.6 6.0

400

420

440

460

480

500

520

540

560

Co

rre

cte

d F

req

ue

nc

y (

Hz)

d3

Increasing

methanol content

0 20 40 60 80 100

20

40

60

80

Methanol Content (wt%)

Su

rfa

ce

Te

ns

ion

at

25

°C

(m

N.m

-1)

0.0

0.4

0.8

1.2

1.6

2.0

Vis

co

sity

(mP

a.s

)

10

Figure 8: The effect of the solution surface tension and bubble size on bubble sound frequency

The pulsation of bubble is a phenomenon that is primarily dominated by the resistance of the liquid against

bubble deformation. The liquid elasticity modulus (EM) that can be thought as the inverse of compressibility thus can

influence bubble sound frequency. Figure 9 shows EM for methanol-water mixtures vs. methanol concentration, from

a study by Blaudez et al.[18]. It is apparent that a sudden decrease in EM takes place at around the same methanol

content that the sharp change in frequency values occurred. By increasing the concentration above this specific point,

EM decreases continuously. The drop in EM indicates higher level of compressibility of the mixture, which leads to

easier pulsation of the bubbles or wider domain of pulsation, which in turn results in lower frequency values. After a

specific level, the increase in methanol content results in a sharp decrease in elasticity modulus which causes lower

bubble sound frequency. As the elasticity modulus starts decreasing rapidly with methanol content, the slope of the

graph of frequency vs. S becomes steeper (at around 37wt %).

3.6 4.0 4.4 4.8 5.2 5.6 6.0

400

420

440

460

480

500

520

540

560

Co

rre

cte

d F

req

ue

nc

y (

Hz)

d3

Increasing

methanol content

0 20 40 60 80 100

0.5

1.0

1.5

2.0

2.5

Ea

sti

cit

y M

od

ulu

s (

10

9 N

/m2)

Methanol Content (wt%)

11

Figure 9: Elasticity modulus of the water-methanol system vs. methanol concentration at room temperature[18].

Practical implications

The measurements in methanol–water system show that the dependence of the bubble sound frequency on

liquid composition is sensitive enough to enable prediction of the composition through non–intrusive acoustic

measurements. Therefore, in principle, the application of a technique based on bubble sound to monitor and control

metallurgical refining operations appears to be feasible, considering the substantial changes that take place in the

physical properties of the fluid. A purely theoretical or semi–empirical model that translates acoustic measurements

directly into process parameters (such as chemical composition or a physical property) may not be practically realized

because of the numerous parameters involved and the operation–specific response of the system. However, an

intelligent system that is calibrated over time can produce reliable results, and eventually become independent of

sampling.

IV. CONCLUSION

The sound frequency generated as result of bubble pulsation in water–methanol solutions was successfully

detected by using two microphones at the side and top of the container. The results obtained from the acoustic response

were found to be closely related to the physical properties of the liquid such as density, surface tension and viscosity.

The results from image analysis of the bubbles confirmed that at a constant gas flow rate and liquid volume the size of

the bubbles decreases with increasing methanol content. Moreover the bubble sound frequency decreases with

methanol content. The frequency increases with an increase in √𝜎 𝑑3⁄ while a substantial drop in the degree of

dependence is seen for mixtures containing over 37 wt% methanol. This composition corresponds well to a point where

the compressibility of the liquid becomes dependent and decreases with methanol content. Based on the findings of

this study, the application of an acoustic based technique for controlling the melt chemistry and/or quantifying physical

properties in metallurgical processes appears feasible.

REFERENCES

[1].J. Lee and K. Morita, ISIJ International, 2002, vol. 42, 6, pp. 588-594.

[2].B.J. Keene,K.C. Mills,J.W. Bryant and E.D. Hondros, Canadian Metallurgical Quarterly, 1981, vol.

21, 4, pp. 393-403.

[3].B. Gallois and C.H.P. Lupis, Metallurgical Transactions B (Process Metallurgy), 1981, vol. 12B, 3, pp.

549-57.

[4].A. Sadri,I. Gordon and A. Rampersad, Dnipropetrovsk, pp. 77-85.

[5].X.F. Zhang,A. McLean and I.D. Sommerville, Steelmaking Conference Proceedings, pp. 659-662.

[6].X.F. Zhang, A Study of Bubble Sound Emitted by Injecting Gas into Lquid through a downward

Facing Lance. 1990.

[7].Y. Kostetsky,D. Kukuy,I. Kvasov,V. Khodyachikh,I. Degtyarenko and O. A., METAL 2007, Hradec

nad Moravici, pp. 451-456.

[8].H. Lamb, Hydrodynamics. 1945, New York: Dover Publication, pp.

[9].M. Minnaert, On Musical Air-bubbles and the Sound of Running Water, in Philosophical Magazine.

1933. p. 235-248.

[10].A.V. Wolf, Aqueous Solutions and Body Fluids,. 1966, Hoeberpp.

[11].O. Söhnel and P. Novotny, Densities of Aqueous Solutions of Inorganic Substances. 1985,

Amsterdam: Elsevier, pp.

[12].G. Vhquez,E. Alvarez and J.M. Navaza, Surface Tension of Alcohol + Water from 20 to 50 °C, in

Journal of chemical engineering data. 1995. p. 611-614.

12

[13].M. Iguchi,H. Kawabata,Y. Ito,K. Nakajima and Z. Morita, Continuous Measurement of Bubble

Characteristics in a Molten lron Bath with Ar Gas Injection, in ISIJ International. 1994. p. 980-

985.

[14].J.F. Davidson,A.M.I. Mech and B.O.G. Shuler, Bubble Formation at an Orifice in an Inviscid Liquid,

in Transactions of the Institution of Chemical Engineers. 1960. p. 335-342.

[15].R.Q. Li and R. Harris, Bubble Formation from a Very Narrow Slot, in Canadian Metallurgical

Quarterly. 1993. p. 31-37.

[16].A. Satyanarayan,R. Kumar and N.R. Kuloor, Studies in Bubble Formation—II Bubble Formation

Under Constant Pressure Conditions, in Chemical Engineering Science. 1969. p. 749-761.

[17].A.V. Byakova,S.V. Gnyloskurenko,T. Nakamura and O.I. Raychenko, Influence of Wetting

Conditions on Bubble Formation at Orifice in an Inviscid Liquid Mechanism of Bubble

Evolution, in Colloids and Surfaces A. 2003. p. 19-32.

[18].D. Blaudez,F. Mallamace,N. Micali,S. Trusso and C. Vasi, Il Nuovo Cimento D, 1994, vol. 16, 7, pp.

923-931.

APPENDIX I

The previously measured physicochemical properties of methanol–water solutions [10-12] were fitted into

the following equations and used in the evaluation of the solution properties where needed. In the equations below, 𝑋

is the methanol content in weight percent, 𝜌, 𝜎, and 𝜇 are density, surface tension, and viscosity of the liquid,

respectively.

𝜌 = 9.97 − 1.44 × 10−3𝑋 − 1.77 × 10−6𝑋2 − 4.47 × 10−8𝑋3 (𝑔. 𝑐𝑚–3)

𝜎 = 19.63 + 27.37 exp(− 𝑋 43.11⁄ ) + 25.17 exp(−𝑋 7.11⁄ ) (𝑚𝑁. 𝑚–1)

𝜇 = 9.86 + 3.80 × 10−3𝑋 − 2.34 × 10−4𝑋2 − 6.5 × 10−6𝑋3 + 4.65 × 10−8𝑋4 (𝑚𝑃𝑎. 𝑠)