Minerals Engineering 21 (2008) 883–888

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Minerals Engineering

journal homepage: www.elsevier .com/locate /mineng

On-line froth acoustic emission measurements in industrial sites

Carlos Vanegas, Peter Holtham *

Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly, Queensland, Australia

a r t i c l e i n f o

Article history:Received 3 December 2007Accepted 15 April 2008Available online 2 June 2008

Keywords:Froth flotationFlotation frothersProcess instrumentationFlotation froths

0892-6875/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.mineng.2008.04.007

* Corresponding author.E-mail address: p.holtham@uq.edu.au (P. Holtham

a b s t r a c t

A sensor for froth acoustic emission detection, capable of isolating the external noise produced bymachinery in the concentrator, has been recently developed and tested. The froth acoustic emission tech-nique involves the use of ultrasonic sensors to measure the elastic energy released by film rupture duringbubble coalescence in industrial flotation cells. The signal emitted by the sensor has been proven torespond to changes in froth conditions. This signal, the acoustic emission activity of the froth phase, pro-vides an alternative method to measure the stability of the froth. This device has been recently tested inindustrial size flotation cells, measurements were performed in different cells along a flotation bankwhere differences in the froth phase between cells can be appreciated in steady state. The sensor has alsobeen tested in a pilot cell (3 m3), operating with different frothers and concentrations. The response of thesensor to changes in metallurgical performance are shown in this paper and compared with other instru-mentation outputs.

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1. Introduction

Froth stability has been recognised as an important factor in theperformance of the froth phase of a flotation cell. A very stablefroth, one in which there is low coalescence and collapse rates, pro-duces high recoveries as particles remain strongly attached to sta-ble bubbles. However, stable froths also result in lower grades dueto entrainment and low drainage. They may also cause problems inpumping. On the other hand, a very unstable froth often yieldsundesirably low recoveries due to high particle detachment anddrainage. Because of these problems at the extremes of froth stabil-ity, it has been shown by several researchers that there is a rela-tionship between froth stability and flotation performance aswell as ranges of froth stability for optimum flotation results (Bar-bian et al., 2006, 2005; Tsatouhas et al., 2006).

In order to determine the optimum range of froth stability for agiven cell requires froth stability to be able to be measured. How-ever, froth stability measurement is challenging because of thetransient nature of the froth and the lack of appropriate instrumen-tation. Current methods provide both direct and indirect indicatorsof froth stability. Indirect indicators can be obtained from manipu-lation of output variables provided by froth vision systems such asfroth speed and froth surface bursting (Barbian et al., 2005; Ven-tura-Medina and Cilliers, 2002). Direct methods include the frothstability column technique, which provides output indicators suchas, b, the fraction of air remaining in the froth, obtained by height–time measurements of rising froth in a column (Barbian et al.,

ll rights reserved.

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2005) and t1/2, the froth half-life time, obtained by height–timemeasurements of froth decay in a column (Tsatouhas et al.,2006). The froth stability column has been shown to be a practicaland low cost means of assessing froth stability. However, the diffi-culty lies in automating this method to provide on-line informa-tion for use in control of a flotation circuit.

This paper describes a novel method that overcomes this diffi-culty by using a sensor which takes advantage of acoustic eventsemitted by the froth phase.

2. Background

When bubbles coalesce their junction film ruptures, producingan instantaneous and short oscillation in acoustic pressure. Thisacoustic pressure is mainly concentrated in the audible frequencyrange; however, because coalescence events last a short time (lessthan 20 ms), a significant amount of energy can be detected atultrasonic frequencies in what we have termed froth acousticemissions (FAE). The authors have designed a sensor (patent pend-ing), the JK froth acoustic emissions detector (JK-FAED), to capturethese events in the froth while at the same time isolating the envi-ronmental noise which is concentrated at lower frequencies.

The JK-FAED system consists of a probe which is immersed inthe froth phase and a PC for signal processing. The probe containsa piezo-electric ultrasonic transducer and specialist embeddedelectronics for signal amplification. The amplified signal is thensent to a pc and digitised by means of a National Instrumentsacquisition card. The digitised signal is further processed using asimple program designed in Lab View. The program filters the sig-nal with a band-pass filter and then extracts the root mean square

Table 2Froth depths and sensor location along the Zn scavenger bank at Century

Scavengercell number

5 6 7 8 9

Froth depth(cm)

22 20 27 27 25 25 20 20

Sensorlocationaboveinterface(cm)

7 5 7 12 5 10 5 10

Experimentlabel

Scv5-7

Scv6-5

Scv7-7

Scv7-12

Scv8-5

Scv8-10

Scv9-5

Scv9-10

884 C. Vanegas, P. Holtham / Minerals Engineering 21 (2008) 883–888

(RMS) values in 1 s cycles. These 1 s cycle RMS values are storedand displayed in the pc. Further processing of the stored signalcan be done off-line if required.

3. Experiments with the JK-FAED in industrial sites

The JK-FAED was tested at two sites, namely Zinifex’s Centurylead–zinc concentrator in Australia and at an Anglo Platinum con-centrator in South Africa.

At the Anglo Platinum concentrator, the sensor was tested inseveral cells in different parts of the circuit where cameras of afroth vision system were also in place. This site has clear differ-ences in stability of the froth between the rougher and the slowcleaner banks. The froth of the rougher bank has bubbles that aremore mineralised than the later sections of the circuit; in contrast,the froth of the slow cleaner bank shows bubbles with transparentwindows, an indication of a lower amount of solids in the froth.The rougher bank operates with deep froth levels (around 50 cm)and the slow cleaner cell with shallower froth levels (around35 cm). It has been observed that froth recoveries in the slow clea-ner section are lower than in the rougher section, probably causedby the lower froth stability. In order to monitor the froth phase indetail, SmartFroth cameras(supplied by StoneThree) were locatedon top of the froth close to the weir, providing froth velocity asthe main output. The JK-FAED was located close to the area of frothbeing monitored by the camera, and immersed 20 cm into thefroth. Data was acquired over 10 min intervals, with several re-peats, for the first rougher cell and the fifth cell of the slow cleanerbank.

The sensor was also tested along the secondary rougher bank inorder to observe differences in the acoustic signals down the bank.For these measurements the sensor was positioned 20 cm belowthe froth surface; however, as each cell has a different froth levelthe location of the sensor in respect to the froth phase interfaceis different for all the experiments. Table 1 shows the froth levelsand location of the sensor in the cells.

At Century, a variety of experiments were performed, includingsurveying a bank with the JK-FAED, data acquisition in a single cellfor a 12 h period and a series of trials with different frother typesand concentrations in a 3 m3 cell. The pilot cell work was con-ducted in conjunction with the AMIRA project P541-B. In first in-stance, the JK-FAED was placed in various cells down the zincscavenger bank. The acquisition time varied between 5 and10 min per cell. The probe was immersed in some cells at two dif-ferent froth depths to see how the sensor signal changed depend-ing on the position in the froth phase. Table 2 summarises thesensor position and froth depths for this survey.

Since it was unknown how the probe would cope immersed inthe froth phase for longer periods of measurement, the JK-FAEDwas left for a 12 h period (overnight) in the first cell of the Zn scav-enger bank. The acquired data was compared to some other instru-mentation outputs of the plant.

Finally, a series of experiments was carried out in a 3 m3 cell fedby the rougher tails, with a zinc grade of about 7%. During these

Table 1Froth depth and sensor location across the secondary rougher bank at AngloPlatinum’s concentrator

Cell number SR1(cm)

SR3(cm)

SR4(cm)

SR5(cm)

SR7(cm)

Froth depth 43 32 32 30 39Sensor location above the

interface23 12 12 10 19

experiments feed rate and air rate were kept constant for variousfrother types and concentrations at fixed froth depths.

4. Results

4.1. Anglo Platinum’s concentrator

The cells selected for froth monitoring were the first roughercell (receiving the plant feed) and the fifth cell of the slow cleanerbank (which processes slow floating material).

Fig. 1 shows three measurements of 1 s cycle rms values ofacoustic emissions acquired for 2 min. One of these measurementswas made outside the flotation cell, which shows the backgroundnoise, clearly indicated by a non-changing signal. This characteris-tic is the main advantage of the sensor: its ability to isolate envi-ronmental noise caused by machinery in a flotation concentrator.The other two measurements were made by immersing the probein the froth phase of two different cells. It has been show before(Tsatouhas et al., 2006) that froth stability is lower in the latterstages of flotation where slow floating components are present,and this is confirmed by our results shown in Fig. 1. The acousticemission signal from the first rougher cell has a lower value overalland less ‘‘spikes” than the signal from the slow cleaner bank, whichin contrast has higher values and more erratic behaviour. Theacoustic signal is caused mainly by the coalescence of bubbleswithin the froth; therefore, the first rougher cell has a lower coa-lescence rate producing a more stable froth than the slow cleanercell. It can be said that stable froths produce lower acoustic emis-sion signals with less spikes, interpreted as a constant coalesce ratein time. However, it seems that in cases where froth stability islower the coalescence activity is not constant but erratic and inspecial cases cyclic as shown later in this paper.

Fig. 1. Time graph of 1 s cycle rms values of acoustic emission signals acquiredoutside a flotation cell (background noise-single line) and inside the froth phase oftwo flotation cells of different banks.

Fig. 2. Froth acoustic (FA) and velocity (FV) measurements in two cells in differentflotation banks. (a) PR1: primary rougher cell 1; (b) SC5: slow cleaner cell 5. (c)Summary of the three outputs (the standard deviation has been magnified fivetimes).

Fig. 3. (a) Average signals obtained every minute for different cells in the secondaryrougher bank. (b) Summary of the overall average and standard deviation values.

C. Vanegas, P. Holtham / Minerals Engineering 21 (2008) 883–888 885

Froth velocity is also different for the two cells of interest. Fig. 2shows a comparison of the froth vision and froth acoustic data.Several sets of data were acquired, each measurement lasting10 min; in Fig. 2 every point represents the average signal pro-cessed every minute. During the survey period conditions werekept constant, hence these results represent steady statebehaviour.

The first rougher cell has a constant and higher velocity than theslow cleaner cell. The acoustic emission and froth velocity signalsseem to provide a similar diagnosis, that is at higher froth veloci-ties the acoustic emission signal is lower (probably there is lesscoalescence); on the other hand slower froth produced a higheracoustic emission signal (probably caused by higher coalescencedue to increase in drainage and reduction of the froth overflow).However, there are other factors that have not been analysed andmight affect froth velocity such as air rate and froth structure.

It is proposed that the stability of the froth is not just inverselyproportional to the acoustic emission signals (represented by theaverage) but also to its erratic behaviour (represented by the stan-dard deviation). Therefore, these two values, average and standarddeviation, are chosen as the main outputs of the JK-FAED system.Further experiments, not presented in this paper, are being carriedout to confirm the relationship between froth acoustic emissionand froth stability.

The JK-FAED was also tested down the secondary rougher bankas shown in Fig. 3. In this case, there is no complementary froth vi-sion data that allows comparisons to be made.

This bank does not show a trend (upwards or downwards) inacoustic emissions down the bank. Generally, it is expected thatthe stability of the froth decreases down the bank as a responseto the decreasing amount of solids entering the froth phase. Thistrend is not present, possibly because the location of the sensorwas fixed at 20 cm from the froth surface for all cells. However,the cells had different froth depths, thus the location of the sensorwith respect to the pulp froth interface was different for each cell.It seems this is too important a factor to allow for comparisons be-tween different cells to be made. Considering this location effect,measurements down the bank in another operation are reportednext.

4.2. Century – Zinifex concentrator

As in the previous site, the JK-FAED was also tested down a flo-tation bank, in this case the zinc scavenger bank. In cells 7, 8 and 9two measurements at different vertical positions were performed.

Fig. 4. JK-FAED measurements along the Zn scavenger bank (a) time signals. (b)Summary of average and standard deviation signals.

Fig. 5. Comparison of plant instrumentation and JK-FAED in a zinc scavenger cell.

Table 3Low and high values of various signals of relevance in the Zn scavenger cell

Zincgradea

(%)

CollectorSIPX(l/min)

FrotherW22(l/min)

FrotherMIBC(l/min)

ActivatorCuSO4

(l/min)

Low values 7.5 0.79 0.60 4.43 32High values 6.4 3.05 0.80 4.37 35

a Zinc grade of rougher tails which accounts for most of the feed flow of the cell.

886 C. Vanegas, P. Holtham / Minerals Engineering 21 (2008) 883–888

The nomenclature used in Fig. 4 is the same as expressed in Ta-ble 2; the left side of Fig. 4 a and b, corresponds to signals acquiredcloser to the froth phase interface (about 5 cm) and the right sideto signals acquired closer to the froth surface (10 cm above thefroth interface). For this case the expected trend of decreasing frothstability down the bank is observed for cells 6–9, with froth acous-tic emission signals increasing as coalescence rate increases. In thisbank, frother is added on the top of cell 5, close to the impeller in-side the cone crowder. Fig. 4b suggests that the frother is not beingmixed perfectly in this cell and is carried to the next cell (Scv6)where it arrives better mixed, increasing froth stability. Seamanet al. (2004) showed that bubble load of attached material de-creased along the bank, decreasing the amount of attached mate-rial entering the froth phase and consequently solidsconcentration in the froth phase. The trend in the zinc scavengebank agrees with Seaman’s findings and supports the hypothesisthat solids concentration in the froth enhances its stability (Johans-son and Pugh, 1992).

It is interesting to note that the trend is maintained for mea-surements at different vertical locations in the froth; however,the acoustic emission activity decreases closer to the surface. It isimportant to note that the JK-FAED only reacts to activity occurringimmediately in front of the probe, and this could give an indicationof where exactly most of the material is being released due to bub-ble coalescence along the froth.

As indicated earlier, the JK-FAED was tested overnight in thefirst zinc scavenger cell and its data compared to other instrumen-tation on site. Fig. 5 shows the JK-FAED signal simultaneously plot-ted with other plant signals which have been scaled to identify

interrelated trends. Table 3 shows the high and low values of theplant instrumentation signals for ease of interpretation.

According to Table 3 and Fig. 5 most of the instrumentation sig-nals remained constant for the period of observation with theexception of collector addition (SIPX), which increased substan-tially after midnight.

In Fig. 5, it can be seen that the JK-FAED signal decreased duringthe monitored period. The zinc grade of the rougher tails stream,which accounts for most of the feed entering the cell being moni-tored varied slightly around the 7% mark, but no trend can beclearly established and related to the JK-FAED signal. During thesame period the collector addition was increased significantly.However, there was no metallurgical sampling to confirm thatthe decrease in acoustic emission corresponds to an increase infroth stability caused by an increase in solids entering the froth.

In order to provide an output proportional to froth stability, theoriginal JK-FAED signal has been inverted and scaled, this new out-put signal is plotted simultaneously with the SIPX dosage signal inFig. 6. Even though there is no clear relationship between the twosignals, this test suggests that the JK-FAED sensor is capable ofuninterrupted functioning for periods of time up to a day.

Finally, tests in a 3 m3 flotation cell were performed with differ-ent frother types, MIBC, Frother B and Frother C. Experiments werecarried out at different concentrations and froth depths. For MIBCthe froth depths were 20, 30 and 40 cm. However, due to the highfroth carrying capacity at high concentrations of frothers B and C,the flotation cell was not able to maintain a shallow froth; conse-quently, experiments were performed at 25, 30 and 40 cm. For thesake of brevity and clarity, only results at 30 cm for MIBC areshown here, since similar trends are observed for the otherfrothers.

Fig. 7 shows JK-FAED signals for MIBC at different concentra-tions, at a froth depth of 30 cm. For all the experiments performedin this cell, the JK-FAED was kept at a constant position of 13 cmbelow the surface.

As expected, increasing the concentration for all frothers resultsin a lower FAE signal or a higher stability of the froth. There arehowever differences in the amplitude of the signals between fro-

Table 4JK-FAED and metallurgical response to changes in frother concentration and type

Frother MIBC F

Concentration (g/t) 101 208 389 1JK-FAED average (mV rms) 57.1 49.4 36.7 4JK-FAED StDev (mV rms) 14.5 6.3 4.9 7Water recovery (%) 7 21 42 1Overall zinc recovery (%) 54 66 75 4Grade zinc concentrate (%) 35 24 15 3

Fig. 6. Comparison between collector dosage signal and the JK-FAED inverted output.

Fig. 8. JK-FAED and metallurgical response to changes in frother (MIBC)concentration.

Fig. 7. JK-FAED signals for MIBC at different concentrations.

C. Vanegas, P. Holtham / Minerals Engineering 21 (2008) 883–888 887

thers that seem to be related to water carrying capacity, as shownin Table 4.

It is interesting to note that at lower frother concentrationsthere is an oscillation in the sensor signal. The period of this oscil-lation is approximately 1 min and cannot be attributed directly tothe cell mechanics. The oscillation diminishes as the frother con-centration increases, suggesting that it is linked to the coalescencebehaviour in the froth.

The oscillatory coalescence process in the froth phase with thepresence of particles could be explained as follows. As bubbles riseand coalesce, solids are detached and enter the plateau borders in adownward flow; this is the period of high coalescence or high JK-FAED values. Accumulation of previously detached solids in thebottom of the froth phase consequently stabilises this part of thefroth, which results in a reduction of coalescence and increasingthe upward froth flow, corresponding to the period of low coales-cence or low JK-FAED values. At this point more solids are leavingthan entering the froth; therefore coalescence is promoted againuntil it reaches a peak with solids being liberated into the plateauborders, starting a new cycle. Increasing frother concentration re-duces coalescence in the high period, thus reducing the amplitudeof the cycle. When higher amounts of frother are introduced thecoalescence process becomes more constant in time. The cycle pre-viously described is also visible at the concentrate overflow, and ischaracterised by a wave like behaviour.

Overall, increasing frother concentration increases frother sta-bility by reducing its coalescence. This results in higher recoverybut at the same time higher entrainment, due to increased waterrecovery, which ultimately reduces the concentrate grade. Thistrend is clearly shown in Fig. 8 for the MIBC case.

There is a clear correlation between the JK-FAED signal andwater recovery which is dependant on froth coalescence. Fig. 9shows the inverted JK-FAED signal being linearly related to waterrecovery, demonstrating the effectiveness of the JK-FAED in pro-ducing an on-line measurement of froth stability.

rother B Frother C

02 226 362 116 232 3542.8 39.2 25.2 42.9 29.2 23.3.5 5.6 2.9 6.6 3.9 2.50 17 55 17 42 683 50 75 51 69 831 25 13 23 13 9

Fig. 9. FAED inverted signal vs. water recovery for different frother types andconcentrations.

888 C. Vanegas, P. Holtham / Minerals Engineering 21 (2008) 883–888

5. Conclusions

The JK-FAED offers a new method for on-line measurement offroth stability by giving a signal which is directly caused by coales-cence in the froth phase. The amplitude of the JK-FAED and its var-iability (standard deviation) are inversely proportional to thestability of the froth.

The JK-FAED responds well to changes in the froth, showingfroth stability can be increased by increasing the frother concentra-tion and the amount of solids entering the froth, as is well estab-lished in the literature.

Thanks to its small size the JK-FAED can provide a localisedmeasurement in cells of any size which makes it ideal for frothmapping, and potentially useful in cell design.

In order to allow for comparisons between different cells, it isproposed to perform measurements at the same location in thefroth measured from the pulp froth interface.

As the JK-FAED measurements are made inside the froth, theinstrument offers complementary information to froth vision sys-tems that can only track superficial changes.

In this paper it has been shown that the JK-FAED is particularlyuseful in case studies in plants wishing to characterise the frothphase of its flotation cells.

Acknowledgements

The authors wish to gratefully acknowledge Anglo Platinum andZinifex (Century Concentrator) for the support offered by theiroperations, UCT researchers for the coordination of the Anglo Plat-inum’s concentrator field trip, Stone Three for providing frothvelocity data from their SmartFroth system, and the AMIRAP541B project for the trials with different frothers in Century, espe-cially Dong Tang for providing the mass balance and metallurgicaldata.

References

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