frothability characterization of residual organic solvents

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Minerals Engineering 20 (2007) 241–251

Frothability characterization of residual organic solvents

Yunkai Xia *, Felicia F. Peng

Department of Mining Engineering, West Virginia University, Morgantown, WV 26505, USA

Received 28 May 2006; accepted 12 September 2006Available online 3 November 2006

Abstract

Organic solvents were evaluated through the determination of froth volume and bubble collapse rate in a froth column meter undervarious solvent dosages and aeration rate levels. There is a non-linear relationship between the froth volume and the gas flow rate forpolyglycol ether type solvents. It is inappropriate to apply the concept of linearity retention time to describe the frothing characteristicsof polyglycol ethers. Two new parameters are derived to characterize the foam stability and non-persistency of the frothers. The newparameters are initial dynamic froth index (IDFI) and initial dynamic froth collapse rate (IDCR). IDFI represents the froth volume for-mation properties, and can be used to relate to the ultimate recovery, Rm, and the flotation rate constant, K. IDCR describes the non-persistency of the froth, and can be used to relate to the quality of flotation concentrates. IDFI value depends on chemical structure andmolecular weight of a solvent. Increasing solvent molecular weight and number of hydrophobic groups increases the IDFI value. Bothultimate recovery and flotation rate constant can be maintained at a desirable level, if the solvent dosage is applied in the amount to givethe same product value of IDFI and solvent dosage. The selection of potential frothers from solvents tested is made by using IDFI andIDCR and comparing with laboratory fine coal flotation performance.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Flotation frothers; Flotation kinetics; Froth bubbles

1. Introduction

In flotation, when mineral particle surface has been ren-dered hydrophobic by the use of a collector, the particle-bubble attachment and froth persistence depends uponfrothing characteristics of a frother. The frothability ofthe frother, therefore, plays an important role in enhancingflotation efficiency.

The recognization that flotation process is a complexinteractive system, individual chemical frother specie hasrather well defined optimal feed particle sized and rate offlotation has lead to extensive frother blending. Manyinvestigations have been taken on frother blending. Itwas found that the impact of the blended frothers on totalrecovery is noticeably greater than any changes possible bychanging the collector dosages (Caillierts and Bradshaw,

0892-6875/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2006.09.001

* Corresponding author. Tel.: +1 412 983 7738; fax: +1 412 429 9801.E-mail address: [email protected] (Y. Xia).

1996; Klimpel, 1999). It is a common practice to use frothermixture in order to get optimized flotation results. Mixtureof alcohols is commonly used as tailored frother for differ-ent type of ores. Mixed C4–C7 alcohols plus hydrocarbonoil are designed for copper–molybdenite ores (Hansenand Klimpel, 1985). Mixing of propylene ethers and alco-hols is also a trend. The frothers consisting of propyleneether oxide and aliphatic alcohols was reported to be effec-tive in recovering very coarse mineral particles. Reactinghighly branched alcohols with propylene oxides canimprove selectivity over fine particle (Klimpel, 1988). Thecomplexity of flotation system and a trend of using frothermixture or chemical by-product challenge the methodo-logies to evaluate the frothability of a frother using anappropriate measure or index. Unfortunately, the scientificfundamentals involved in froth formation and stability inthe presence of minerals is not yet sufficiently developedso that the predication of behavior of a frother or frothermixtures can be made easily in any given mineral flotationsystem.

mailto:[email protected]

242 Y. Xia, F.F. Peng / Minerals Engineering 20 (2007) 241–251

A good frother must achieve a delicate balance betweenbalance and non-persistency. Ideally, the froth should pro-duce a high froth volume to get high mineral recovery whilemaintaining some bubble coalescence or collapse rate toguarantee concentrate grade feasible.

Froth height has been used as one of important param-eters to evaluate frothability of a frother in liquid phase. Afrother with strong frothing ability tends to produce a highand stable froth. However, there are some limitations inusing froth height only to explain some phenomena in coaland mineral flotation: (1) Some frothers can not produce ahigh froth, but the good flotation performance in practiceis observed. (2) The froth height changes with frotherdosage and the measured froth height has been reportedat the arbitrary frother concentration. (3) For two frothswith the same froth height, they might have differentbubble size distribution, coalescence rate and further adifferent flotation performance. (4) Froth height is a quan-titative parameter, it is not a material constant index whichis independent of frother dosages. The froth height alsocan not be correlated well with the quality of froth prod-ucts because of complicated effect of drainage by bubblecoalescence.

During coalescence, the weakly attached particles wouldbe rejected first. It can be postulated, therefore, the coales-cence would be beneficial for gangue rejection from thefroth. There are many factors that can influence the bubblecoalescence in froth. These main affecting parameters orvariables include bubble size distribution, frother dosage,aeration rate, and hydrophobic particle content in pulpetc. There exists an optimized bubble size distribution ofgetting good flotation results (O’Connor et al., 1990; Yoonand Luttrell, 1989). The frother concentration affects coa-lescence through influencing initial bubble size and frothvolume. Szatkowski and Freyberger (1985) observed thatincreasing of MIBC concentration beyond some levelaffected the average diameter of bubbles to a much lessdegree. In multi-hole sparger experiment, Cho and Las-kowski (2001) observed that, at low frother concentrations,the bubble size is much larger indicating coalescence as amain mechanism determining the size. Coalescence can beprevented at frother concentrations exceeding the criticalcoalescence concentration (CCC), over which the coales-cence is not sensitive to increasing of frother concentration.With the increase of frother dosage, however, gangueentrainment increased sharply. Thus, increasing of frothstability by addition more frothers causes a reduction inthe extent of bubble coalescence and gangue rejection atthe bottom of the froth.

Froth volume and bubble coalescence are importantparameters influencing the quality of froth. With a reason-able high froth volume and a high hydrophobic particlerecovery is expected. But a high froth is always accompa-nied by a relative low bubble coalescence rate; a high recov-ery is reached at sacrifice of some flotation selectivity.Coordination between froth height and bubble coalescence

is necessary to get an optimized flotation results. However,there is no simple correlation between coalescence inhibi-tion and froth height. Different methodologies have beendeveloped to compare and evaluate the frother perfor-mance. The procedures were summarized and evaluatedby Dey and Bhattacharya (1998).

Conventionally, batch flotation is used to make a com-parative analysis of frother performance. The results arecompared in terms of valuable mineral recovery and flota-tion kinetics (Hansen and Klimpel, 1985; Smar et al.,1994). Using Klimpel’s methodology, the interactionsbetween frothers and collectors, collector dosage, particlesize on the frothers, etc can be studied. Nevertheless, resultsof batch flotation tests have been used as a basis to clarifythe different frothers with respect to their qualities.

Several frothing parameters and criteria have been pro-posed to evaluate frothers. Sun (1952) used a columnfrothmeter to measure the frothability of various alco-holic type frothers. The criteria developed by Sun includefrothability index (FI) and stability index (SI). FI isdefined as the ratio of the froth volume of a test frotherto that of a reference frother. Usually the frothability ofindividual frother is related to its solution concentrationdue to various surface activities. Some frothers can exhi-bit good frothing properties at an arbitrary chosen con-centration, whilst others show negligible frothability.Diacetone alcohol, for example, does not produce a highvolume of froth at a given concentration range (Malysaet al., 1987). In fact, the flotation results demonstratedthat it is a good frother, which produced sufficient stabil-ity of the froth under the flotation conditions (Laskowski,1993). Although FI is a useful index for evaluating differ-ent frothers directly, it is not related to any dynamicphysical–chemical properties of the system being inves-tigated, and therefore it might lead to an incorrectconclusion.

Malysa et al. (1987) proposed a frothing parameterknown as dynamics frothability index (DFI) based on frothretention time, Rt. Rt is equal to the ratio of the froth vol-ume to gas flow rate. DFI is defined as a limiting slope ofRt when the frother concentration approaches zero. Theproduct of DFI and frother concentration is derived toprovide the information of the frothing properties of thefrother under dynamic steady state conditions (Dey andBhattacharya, 1998). DFI requires a constant value of Rtfor a given frother and concentration, which is specificallyobtained from the slope of froth volume (V) versus gas flowrate (G) linear curve. It has been observed that not all thefrothers exhibit the linear V–G relationship (Johansson andPugh, 1992). Thus it is inappropriate to arbitrarily select alinear section of the froth V–G curve to represent the entiresystem.

In this study, we were emphasized on frother evaluationonly. Two improved parameters for evaluation of frothvolume (V) versus gas flow rate (G) non-linear curve havebeen reported and measured elsewhere (Peng and Xia,

Y. Xia, F.F. Peng / Minerals Engineering 20 (2007) 241–251 243

2002). They are initial dynamics frothability index (IDFI)and initial dynamic collapse rate (IDCR). The objectivesof this study are to evaluate the frothing characteristicsof some solvents using frothing parameters developed,and to identify the potential of these solvents as the flota-tion frothers. The following tasks are conducted to achievethe goals: (1) to measure the froth volume and collapse rateat various gas flow rate and solvent concentration usingcolumn frothmeter; (2) to apply the new frothing parame-ters and criteria for solvents tested frothability determina-tion; (3) to measure the flotation performance of fine coalusing various solvents; and (4) to build correlation betweenthe frothing parameters developed and fine coal flotationperformance.

2. Experiments

2.1. Solvents and apparatus for frothability evaluation

The solvents tested in this evaluation are supplied byUnion Carbide Research Corporation, South Charleston,WV and are summarized in Table 1. Among fourteen sol-

Table 1Solvents identification numbers and components

Solvent ID Name

1 UNFLITEREDTAFT OXO MIXTURE

2 METHYL AMYLALCOHOL

3 BUTOXYTRIGLYCOL

4 ETHOXYTRIGLYCOL

5 HEXYL CARBITOLSOLVENT

6 ECOSOFT(TM) SOLVENT PH

7 FILTEREDTAFT OXO MIXTURE

8 ECOSOFT(TM) SOLVENTPB

9 ECOSOFT(TM) SOLVENT PE

10 TEXAS CITY OXOMIXTURE

11 BUTYL CARBITOL(TM) SOLVENT

12 ACETONE POLYMER (CDTA REGULATED)13 DOW FROTH 25014 DOW FROTH 150

vents used in this evaluation, seven of them are glycolethers. Solvents 3, 4, 5 and 11 are almost pure glycol ethersand Solvents 6, 8 and 9 are the mixtures of glycol ethers.Solvents 1, 7 and 10 are alcohol mixtures, while Solvent12 is acetone polymer. The reference frothers selected areMIBC (Solvent 2), Dow Froth 250 (Solvent 13) and DowFroth 150 (Solvent 14).

The schematic diagram of a froth column meter is shownin Fig. 1. The column is made of 51 mm ID and 1.5 mheight plexiglass cylindrical tube. The froth is generatedby aerating a solution of solvent using a fritted glass spar-ger at the bottom of froth column meter. The fritted glasshas a diameter of 20 mm and pore size of 40–60 lm. Thecompressed air from the building is used in the experiment.

To start the test, the froth column is filled with 2 l ofdistilled water. The initial height of liquid in the column,Hi, is recorded. A small amount of compressed air is thenintroduced to flush out the water trapped in the frittedglass disc. Predetermined amount of solvent (0–300 ppm)is added to the distilled water from the top of column.The flowmeter is set to a predetermined air flow raterange (0–2.5 l/min). When the froth height reaches the

Component

Isobutanol <=50%,2-ethyl-3-hydroxyhexyl ester, butanoic acid etc.Methyl isobutyl carbinol,(CH3)2CHCH2CH(OH)CH3

100% 102.18 mg/molTriethylene glycol monobutyl etherCH3CH2CH2CH2O(CH2CH2O)3H85–100% 206.3 g/molTriethylene glycol monoethyl etherCH3CH2O(CH2CH2O)3H85–90% 178 g/molDiethylene glycol monohexyl etherCH3CH2CH2CH2CH2CH2O(CH2CH2O)2H97–100% 190.3 g/molGlycol ethers mixtureCH3CH2CH2CH2CH2CH2O(CH2CH2O)3HTriethylene monohexyl ether 40–85%Polyethylene glycol monohexyl ether 5–25%Diethylene glycol monohexyl ether <25%Isobutanol <=50%,2-ethyl-3-hydroxyhexyl ester, butanoic acid etc.Glycol ethers mixturePolyethylene glycol monobutyl ether 98–99%Glycol mixturePolyethylene glycol monoethyl ether 50–90%Triethylene glycol monoethyl ether <25%2-Ethyl hexyl alcohol, octyl alcohol, butanol, 2methyl butanol etc.Diethylene glycol monobutyl etherC3HCH2CH2CH2O(CH2CH2O)2H99–100%NACH3O(CHCH3CH2O)4HCH3O(CHCH3CH2O)2H

Pressuregauge

Shut-offvalve

Airregulator

Drieriteabsorbenttrap

Sulfuricacidtrap

Watertrap

Moisturetrap

Flowmeter

Frittedglass disc

Plexiglassfroth column

Needlevalve

Fig. 1. Schematic diagram of froth column meter.


equilibrium, the total froth height (maximum height offroth), Hf, is recorded. The froth volume, which is frothheight, H, times the cross sectional area of the column,A, is measured to determine the frothability of the solu-tions of solvents at different concentration levels and aera-tion rates. As illustrated in Fig. 2, the froth height, H, is the

Fig. 2. Froth and liquid interf

summation of froth and bubbles trapped in the liquid perunit cross sectional area of column, or the differencebetween total froth height, Hf, and initial liquid height, Hi.

H ¼ H f � H i ð1Þ

For measuring the froth collapse rate or bubble coales-cence rate, the aeration is turned-off, while simultaneouslystarting to record the changes of the froth height with timeuntil the froth layer disappears. The bubble coalescenceproceeds gradually from the top of froth layer to the inter-face of air and solution. The froth collapse rates as a func-tion of solvent concentrations and aeration rates aredetermined. When the air flow is shut off, there is a residualair remained in the solution. The residual air will continueto support the froth layer until the air completely evolvesoff the column. Five repetitive runs are made for each sol-vent concentration level and airflow rate. All the experi-ment is conducted at a room temperature of 20 �C.

2.2. Coal sample and fine coal flotation

The coal used in this study is Pittsburgh seam coal fromBentleyville, Green County, PA. It is an appropriate coalfor reagent evaluation and comparisons because of its typ-ical flotation characteristics of Bituminous coal. The rawcoal as received is air dried and crushed by a jaw crusherand then screened through a 5 mm sieve. The �5 mm size

ace in froth column meter.

Distiller water

(No frother)

0.0 0.5 1.0 1.5 2.0 2.5

Aeration rate, L/min

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Frot

h he

ight

, cm

Fig. 3. Froth height as a function of gas flow rate in distilled/deionizedwater.

0

4

8

12

16

20

Frot

h he

ight

, cm

0.0 0.5 1.0 1.5 2.0 2.5


MIBC

121 ppm

75 ppm

52 ppm

24 ppm

12 ppm

6 ppm

Fig. 4. Froth height as a function of gas flow rate at various solventconcentrations for Solvent 2 (MIBC).


fraction is further ground to be used in flotation feed. Thehead ash of flotation feed is 26.35%.

Denver laboratory flotation machine (model D-12) witha 2000 ml flotation cell is used for the fine coal flotation. A180 g coal sample is conditioned in 1000 ml distilled waterat impeller speed of 1200 rpm for 6 min. After initial con-ditioning, the collector, kerosene, is added and conditionedfor 3 min. The frother is then added followed by 1 min con-ditioning period and addition of 800 ml distilled water.Further conditioning for 1 min, the air valve and the pad-dles are then turned on simultaneously for flotation. Thevolumetric air flow rate kept at is 1.42 l/min/l cell volumeand the impeller speed at 1200 rpm.

The froth is collected at intervals of 20, 40, 60, 120, and180 s. The combustible material recoveries and ash con-tents of clean coal products as a function of time are deter-mined for different collector and frother dosages. Theperformance of fine coal flotation is evaluated using flota-tion rate constant and the ultimate recovery. The followingexpression proposed by Huber-Panu et al. (1976) is used todescribe the combustible material and ash recovery of flo-tation product as a function of time.

R ¼ Rm 1� 1

Ktð1� expð�KtÞÞ

� �� ð2Þ

where R represents the cumulative material recovery ofsome component in the product at time t, Rm is the ulti-mate (equilibrium) recovery of some component at infiniteflotation time, and K is rate constant of the recovery.

The flotation tests were evaluated using frothing rateand the ultimate coal recovery as the principle criteria ofmerit. To determine the optimal frothing model parameter,a generalized parameter estimation computer program wasused. The criteria used for estimation of parameter valuesare the derivations at a given concentration betweenobserved and calculated combustible coal recovery. Conju-gate gradient method is used to approach the minimizationin above sum of squares.

3. Results and discussion

3.1. Initial dynamical frothability index (IDFI)

The relationship between the froth height and gas flowrate for the distilled/de-ionized water is shown in Fig. 3.The results show a stable retention time value (the slope),which is independent of the gas flow rate and the geometryof the measuring column. Fig. 4 illustrates that the systemcontaining MIBC solution also has the linear relationship.However, as noticed in Fig. 5, the plots of the froth heightand the gas flow rate for polyglycol ether type solvent showa typical non-linear relationship for Solvent 3 (Butoxytri-glycol ether). It is observed that the slope depends uponthe gas flow rate and solvent concentration. The frothheight increases with gas flow rate, but the increase is notproportional to the corresponding increase in gas flow rate.The growth of froth height slows down with increasing gas

flow rate until the froth height reaches a plateau, an ulti-mate froth height, H0. Therefore, it is inadequate to applythe concept of linearity of retention time proposed by Mal-ysa et al. (1987) and used by Johansson and Pugh (1992) todescribe the frothing characteristics of polyglycol ethers

0

5

10

15

20

25

Frot

h he

ight

, cm

0.0 0.5 1.0 1.5 2.0 2.5


196 ppm

Butoxytriglycol

34.8 ppm

78.3 ppm130.5 ppm

8.7 ppm17.4 ppm

Fig. 5. Froth height as a function of gas flow rate at various solventconcentrations for Solvent 3 (Butoxytriglycol ether).

Solvent 1

Solvent 4

Solvent 5

Solvent 2

Solvent 3

4

6

8

10

12

14

16

18

Initi

al r

etet

nion

tim

e, s

0 50 100 150 200 250

Frother concentration, ppm

Solvent 6

Solvent 7

Solvent 8

Solvent 10

Solvent 9

5

10

15

20

25

30

35

40

45In

itial

ret

entio

n tim

e, s

0 50 100 150 200 250

Frother concentration, ppm

Fig. 6. Initial retention time, IRT, as a function of solvent concentrations.


type solvents. A mathematical model is introduced to takeinto account of non-linear V–Q relationship at various sol-vent concentrations for each solvent.

AH ¼ AH 0ð1� EXPð�KaQÞÞ ð3Þ

where A is cross sectional area of the froth column; H isfroth height at aeration rate Q; H0 is ultimate froth height;Q is aeration rate; Ka is a rate constant. H0 denotes the lim-iting height value; its value depends upon concentrations.AH represents the froth volume. Eq. (3) reflects the changesof froth volume with the aeration rate.

The instantaneous retention time, Rt, is then defined as

Rt ¼ dðAHÞdQ

¼ AdHdQ¼ AH 0Ka expð�KaQÞ ð4Þ

when Q! 0, Rt approximates the maximum value AH0Ka.Initial retention time, IRT, is defined to characterize thesolution with a given concentration only, but not soluteitself.

IRT ¼ AH 0Ka ð5Þ

A typical plot for the dependencies of IRT values on thesolvent concentration is presented in Fig. 6 for Solvents 1–10. IRT increases with solvent concentration. If concentra-tion is over some level, its IRT starts leveling off. It alsoobserved that some alcohol type solvents, such as Solvent2 (MIBC), Solvent 7 (alcohol mixture) and Solvent 1, canreach their initial retention time limit at a lower concentra-tion than the Polyglycol ether type solvents.

IRT represents the longest retention time during aera-tion in column froth meter at a given solvent concentra-tion. However, IRT is not connected with dynamicproperties of frothing. Thus, IRT values characterize only

the solution with a given concentration but not the solventitself. IRT is used instead of RT to compute DynamicFroth Index for Polyglycolethers. The IDFI index wasdefined as the limiting slope of the IRT versus concentra-tion curve for C! 0.

IDFI ¼ oðIRTÞoC

� �C¼0

ð6Þ

It fulfills the demands of the material constant searched.The expression

IRT� 2:6615 ¼ IRT1½1� expð�KdCÞ� ð7Þ


was fitted to experimental values of IRT for various con-centrations. Here, IRT1 is the limiting IRT values forC! 0. C is the solvent concentration, and Kd is a constant.The quantity of 2.6615 on the left side of equation is thevalue of Rt obtained for distilled water used in our mea-surements. The fitting was done by the least squaremethod.

IDFI ¼ oðIRTÞoC

� �C¼0

¼ IRT1Kd ð8Þ

Therefore, the IDFI values can be defined on the basisof a complete set of experimental data not only on the basisof the limiting values for C! 0.

In coal flotation, the froth dosage, C, is very low(25 � 50 g/ton),

IRT� 2:6615 ¼ IRT1½1� expð�KdCÞ�when C! 0,

expð�KdCÞ � 1� KdC

IRT� 2:6615 � IRT1KdC ¼ IDFI�Cð9Þ

At low solvent concentration, the product of IDFI andC represents the initial retention time difference of frothin a solution and that in pure water. IDFI*C gives outformation on the frothing properties of the solution stud-ied, under dynamic steady-state conditions. The IRT1,Kd and IDFI values for different solvents are shown inTable 2.

From Table 2, IDFI for the solvents investigateddecreases in the following order: Filtered TAFT OXOMixture (Solvent 7) > Unfiltered TAFT OXO Mixture(Solvent 1) � Hexyl Carbitol Solvent (Solvent 5) > MIBC(Solvent 2) > Butoxytriglycol (Solvent 3) � EcosoftTM Sol-vent PH (Solvent 6) � EcosoftTM Solvent PB (Solvent8) > Butyl Carbitol (Solvent 11) � EcosoftTM Solvent PE(Solvent 9) > Ethoxytriglycol (Solvent 4) � Dow Froth250 (Solvent 13) � TEXAS CITY OXO Mixture (Solvent10) > Dow Froth 150 (Solvent 14) > Aceton Polymer (Sol-vent 12).

Table 2Values of IRT1, Kd and IDFI for solvents

Solvent IRT1, s Kd(·10�2), ppm�1 IDFI, s dm3 g�1

1 15.25 3.61 549.02 6.27 7.45 467.13 15.08 2.44 367.84 7.55 2.40 181.05 11.98 4.50 538.66 9.32 3.94 367.07 7.54 15.55 1166.28 45.63 0.77 352.99 7.06 3.53 249.210 21.49 0.70 150.411 16.74 1.62 270.712 3.57 0.67 23.913 14.21 1.11 158.314 15.09 0.58 87.5

3.2. Factors affecting IDFI values

3.2.1. Molecular weight

It has been known that increase in the length of thehydrophobic group of the frothers results in higher froth-ability. Introduction of branching or unsaturated chainsinto the hydrophobic group of the frother increases solubil-ity in water and leads to loosely packed froth.

The dependence of IDFI on the solvent molecularweight is shown in the Table 3 for the solvents havingclearly known chemical structures. The results show that,if two solvents belong to the same frother type, and havesimilar molecular structure, increasing in the molecularweight, especially in hydrophobic groups, will result in ahigher IDFI value. Solvents 3 and 4 are Triethylene glycolethers, Solvent 3 has a higher IDFI value than Solvent 4because of two more –CH2– group in Solvent 3. Similarly,diethylene glycol ethers Solvent 5 has a higher IDFI thanSolvent 11 because of more –CH2– group in Solvent 5.Solvent 13 has a higher IDFI value than Solvent 14(Dow Froth 250) because of two more hydrophobic group–CHCH2CH2O (PO) in solvent 13.

The IDFI values of two frothers who come fromdifferent families will show great difference even thoughthey have same molecular weights. Although Solvent 5has a lower molecular weight (190.3) than Solvent 3(206.3), it still shows a higher IDFI than Solvent 3. Thehydrophobic property difference of different frothers maycontribute to the gap of their IDFI values. The range ofIDFI on molecular weight for frothers from different fam-ily can be overlapping when their molecular weights areclose or same.

3.2.2. Chemical structure effectIn general, in polyglycolethers, the hydrophobic end

includes butylene oxide (BO) and propylene oxide (PO).The hydrophilic groups include ethylene oxide (EO) andhydroxyl –OH. The relative length of hydrophobic andhydrophilic ends in polyglycol ethers can be modified bychanging the number of corresponding groups, and thusto alter its frothing properties. Increasing the number ofhydrophobic groups PO and –CH2 will increase the IDFIvalues of frothers. This can be used to explain that Solvent3 has a higher IDFI value than Solvent 4, Solvent 13 thanSolvent 14, and Solvent 5 than Solvent 11. On the contrary,increasing number of hydrophilic groups EO and –OH willdecrease the IDFI value of frothers. There are more EOgroups in molecule of Solvent 6 than that in Solvent 5.Although Solvent 6 has a higher average molecular weightthan Solvent 5, it has a lower IDFI value than Solvent 5.Increasing number of PO group results in an increase inhydrophobicity of a solvent. However, this effect may beweakened by the combination of PO with a strong hydro-philic group. These results illustrate that the IDFI value ofa solvent is dependent on its molecular weight and chemi-cal structure.

Table 3IDFI values for several test solvents

Solvent no. Chemical structure Molecular weight HLB IDFI

2 CH3CHCH3CH2COHCH3 102 6.1 467.13 CH3CH2CH2CH2O(CH2CH2O)3H 206.3 7.99 367.84 CH3CH2O(CH2CH2O)3H 178.0 8.94 81.05 CH3CH2CH2CH2CH2CH2O(CH2CH2O)2H 190.3 6.71 538.611 CH3CH2CH2CH2O(CH2CH2O)2H 162.3 7.66 270.713 CH3O(CHCH3CH2O)4H 260 7.83 158.314 CH3O(CHCH3CH2O)2H 146 8.13 87.5


3.2.3. Hydrophilic–lipophilic balance (HLB)

The value of hydrophilic–lipophilic balance (HLB) wasan index or a scale to measure the hydrophilic–lipophilicbalance of a solvent (Laskowski, 1993). The HLB valueof a solvent with a known chemical structure can be calcu-lated from the empirical values assigned to the groups byusing the expression, HLB = (hydrophilic group num-bers) + (lipophilic group numbers) + 7.

The HLB results are given in Table 3, which includesonly the solvents having clearly defined chemical struc-tures. Lower the solvent HLB value means more hydro-philic characteristics of the solvents and less solubility inwater.

HLB values for flotation frother used in coal and min-eral flotation generally falls between 6 and 11 and for mostof good frothers the HLB values were close to 6. HLBvalue alone is not sufficient to characterize the solvent forflotation frother (Laskowski, 1993). HLB number has itslimitation in comparing and selecting frothers. For exam-ple, some frothers with different molecular weights mayhave similar HLB values. As shown in Table 3, for polygly-col ether type solvents, there is no significant difference intheir HLB values over a wide range of molecular weight.Solvent 3, Solvent 13, and Solvent 14 all have the HLBvalue about 8.0. In fact, they show great difference in theirfrothing properties. Two solvents with similar HLB valuesmight have very different molecular weights and areasoccupied by molecules. The solvent with larger molecularweight will generate a more persistent froth and muchhigher surface viscosity. In actual froth flotation, thefrother with higher molecular weight provides higherrecovery with lower selectivity. A lower HLB value meansstronger ability for a solvent to lower surface tension atgas/water interface and hence results in better hydrationof the bubbles, reduces the rate of bubbles coalescence,and leads to appearance of finer bubbles in the presenceof frother.

HLB value can not describe structure effect of frothmolecular either. It may be difficult to calculate HLB num-ber of some frothers with aromatic hydrocarbons, contain-ing one or more side chains, aromatic rings, or unsaturatedbonds. Some branched alcohols are known to be less sur-face active than the corresponding straight chain alcohols.Long EO chains are coiled in aqueous phase and forms abulky froth. All these effects cannot at present be properlyaccounted for in the HLB calculation. Some of the solvents

provided for this study are the mixture of solvents, orunknown in chemical structure. Therefore, no correlationof HLB and their molecular weight is performed in thisstudy.

For all types of tested solvents, IDFI value decreaseswith HLB values. With same HLB numbers, an aliphaticalcohol type frother will tend to show a lowest IDFI value,while aliphatic alcohol has a lower IDFI value than corre-sponding branched alcohol. For a triethylene glycol ethertype frother to reach same HLB number as that of a poly-propylene glycol ether type frother (Dow Froth), it is nec-essary to add more hydrophobic group –CH2– to its chain.The long hydrophobic chains can promote densely packedfoam in the gas/water interface and thus results in a higherIDFI value. Introducing more hydrophilic EO groups inmolecular chain will not only increase the HLB numberbut also cause chain to be coiled and are not fully extendedin aqueous phase, this help to form a loosely packed frothand hence a lower IDFI value.

3.3. IDCR (Initial dynamical collapse rate)

Froth height begins to decrease due to bubble coales-cence once the aeration is stopped. Froth collapse rate isdetermined by plotting the froth volume or height versuselapsed time. Fig. 7 shows typical plot of froth height, H,as a function of elapses time, t, for distilled water and Sol-vent 3 (Butoxytriglycol ether) at various solvent concentra-tions. The delay for froth collapse occurs due to theresidual gas remained in the solution continuing to supportthe froth when the gas flow is stopped. Froth collapse rateis described by the equation shown below.

H ¼ Hmax expð1:0� KCðt � aÞÞ ð10Þ

where H is froth height, Hmax is the froth height when aer-ation is totally stopped, KC is the bubble collapse rate con-stant, t is the bubble collapsing time and a is the inductiontime. The non-linear searching technique is used to esti-mate the value of a and KC, starting from using all the datapoints.

Initial collapse rate, ICR, is a limiting slope of –dH/dt

for t! a. ICR is defined in Eq. (11)

ICR ¼ � dHdðt � aÞ

��t¼a

¼ HmaxKC ð11Þ

Table 4Values of ICR1, Kk and IDCR for solvents

Solvent ICR1, cm/s Kk, ppm IDCR, cm dm3 s�1 g�1

1 1.3050 0.0193 25.142 3.1686 0.0241 76.363 1.2357 0.0960 118.64 1.2473 0.0088 11.015 1.1845 0.0887 105.16 1.0313 0.0451 46.57 1.6643 0.0487 80.978 0.8044 0.1414 112.09 0.8062 0.0740 59.210 2.5092 0.0083 20.6411 0.8435 0.1182 99.6112 0.7782 0.0069 5.3713 1.0093 0.0957 96.6914 0.9593 0.0100 9.56

Time, s

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

Frot

h he

ight

, cm

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Solvent 3 (Butoxytriglycol Ether)

Distilled Water8.9 ppm17.4 ppm78.3 ppm130.5 ppm

Fig. 7. Froth collapse rate as a function of time for Butoxytriglycol etherand distilled water.

100

80

40

20

Pittsburgh Seam Coal

60

Solvent 2 (MIBC)rate constant

Solvent 13 (Dow froth 250)combustible recoverySolvent 13 (Dow froth 250)rate constant

Solvent 2 (MIBC) combustible recovery

kerosene: 0.5 lbs/ton

0 2 4

IDFI*C, s

0

3

6

9

12

15

Rat

e co

nsta

nt, s

-1

Ulti

mat

e co

mbu

stib

le r

ecov

ery,

%

1 3 5

Fig. 8. Ultimate combustible material recovery and flotation rate constantas a function of IDFI*C.


Then changes of ICR with frother concentration, C, isillustrated as

ICR� ICRW ¼ ICR1½1:0� expð�KkCÞ� ð12Þ

where, ICR1 is limit of IRT and Kk is the concentrationrate constant.

To evaluate the coalescence ability of the froth by a sol-vent, a new parameter, Initial dynamics collapse rate(IDCR), is derived. IDCR represents the changes of initialcollapse rate (ICR) with solvent concentration (C) for con-centration approaches zero.

IDCR ¼ dðICRÞdC

� �C!0

¼ ICR1C ð13Þ

ICR� ICRW � ICR1KkC ¼ IDCR�C ð14Þ

where ICRW has an approximated value of 0.7654, itequals to the measured initial collapse rate of froth in dis-tilled water. IDCR tells the froth collapsing ability of a sol-vent and it is independent of frother concentration. Theproduct of IDCR and C represents the initial collapse ratevalue difference between the solvent in a solution and inpure water, IDCR*C provides the information of the frothcollapsing properties for the solvents studied under steady-state conditions. ICR1, KC and IDCR values for the sol-vents tested are given in Table 4.

3.4. Coal flotation and frothability indexes

3.4.1. Coal floatability, IDFI and IDFI*C

The relationship between ultimate combustible materialrecovery and coal flotation rate constant versus IDFI*C

value for Solvent 2 (MIBC) and Solvent 13 (Dow froth250) are presented in Fig. 8. For each Solvent, it can beseen that the ultimate combustible material recovery and

flotation rate constant are the same disregarding the differ-ent solvent types, provided the amount of the solvents wereapplied to produce the same initial retention time IDFI*C

value. In Fig. 9, the relationship between ultimate combus-tible material recovery and coal flotation rate constant, andIDFI was presented for the solvents tested. The resultsshows that, increasing IDFI value, the flotation rate con-stant increases, and reaches its maximum values of6.3 s�1 at about IDFI value of 350 sdm�3 g�1, and thendecreases further, while the ultimate combustible materialrecovery decreases from 95% to 85%, and seems to remainconstant at 85% further.

IDFI sdm-3g-1

0 200 400 600 800 1000 1200

Flot

atio

n ra

te c

onst

ant,

s-1

0.0

2.0

4.0

6.0

8.0

10.0

Ulti

mat

e co

mbu

stib

le r

ecov

ery,

%

0

20

40

60

80

100

Flotation rate constant, s-1

Ultimate combustible material recovery,%

Fig. 9. Ultimate combustible material recovery and flotation rateconstants as a function of IDFI value. 1 14131211109876432 5

Clean coal ash recovery,% Combustible recovery, %

0

20

40

60

80

100

Cle

an c

oal c

ombu

stib

le r

ecov

ery,

%

0

20

40

60

80

Cle

an c

oal a

sh r

ecov

ery,

%

100

Solvents

Fig. 11. Flotation performances of fine coal using various solvents.

IDCR, cmdm3s-1g-1

0 20 40 60 80 100 120

Cle

an c

oal a

sh, %

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Kerosene: 1.60 lbs/tonFrother: 0.375 lbs/ton

Kerosene: 0.50 lbs/tonFrother: 0.3250 lbs/ton

Fig. 10. Clean coal ash and initial dynamic collapse rate (IDCR).


3.4.2. Clean coal ash and IDCR

Fig. 10 illustrates that relationship between clean coalash and initial dynamic collapse rate, IDCR. The cleancoal ash is observed to decrease with IDCR value becausea higher bubble coalescence and dissipated rate (non-per-sistent froth). IDCR can be used to determine a trend ofchanges of ash content of clean coal concentrate. Althougha solvent with IDFI value of 250–500 s dm3 g�1 gives ahigh combustible material recovery, it might result in ahigh clean coal ash if the solvent selected corresponds toa high IDCR value (persistent froth). The referencefrothers used are two most commonly use frothers inpractice including MIBC and DF250. Solvent 2 (MIBC)

has a IDFI value of 467 s dm3 g�1 and IDCR value of76.36 cm dm3 s g�1, while Solvent 13 (DF250) has a IDFIvalue of 495 s dm3 g�1 and IDCR value of 96.6 cm dm3 s g�1.These two frothers have strong frothability and have per-sistent froth in flotation. The clean coal ash is achievedusing MIBC and DF250. From Figs. 8 and 9, a goodfrother candidate should have a IDFI value in the rangeof 250–500 s dm3 g�1, and a value of IDCR less than20 cm dm3 s g�1 to achieve a high clean coal yield and alow clean coal ash Fig. 11.

4. Conclusions

The evaluating of fourteen solvents is conductedthrough the measurement of froth volume in a columnfroth meter and the batch test of fine coal flotation. Themajor conclusions derived from this work are presentedas follows:

(1) For water and alcohol type frothers, there is a linearrelationship between froth height and aeration ratebecause the froths consist of large loosely packedbubbles, which will show a constant retention time.Polyglycol ether type frothers can produce denselyknit froth formed by small bubbles; there exists a typ-ical non-linear relationship between froth height andaeration rate.

(2) For polyglycol ether type frothers, considering the wayin which froth retention time varies with aeration timeeven at a constant frother concentration, Initialdynamical froth index, IDFI, is used instead of reten-tion time to illustrate the frothability of frother. IDFI


is defined as the limiting slope of the initial reten-tion time vs. concentration where concentrationapproaches zero. The product of IDFI and frotherconcentration gives the estimated initial retention timedifference of froth between that in liquid and that inpure water. It gives the frothing properties of the solu-tion studied under dynamic steady-state conditions.

(3) IDFI value depends not only on molecular weight,but also on molecular structure of solvents. Solventsin the same family with a higher molecular weight orwith a lower hydrophilic–lipophilic balance value(HLB) will result in a higher IDFI value. Increasingnumber of hydrophobic groups PO and –CH2 willincrease IDFI values.

(4) The ultimate combustible recovery and kinetic rateconstant are constant provided the frothers areapplied in amounts assuring the same initial retentiontime (IDFI*C) value. Increasing the IDFI*C valuemay increase the fine coal recovery and kinetic rateconstant. However, an excessive high IDFI*C value,may lead to reduction in selectivity.

(5) IDCR is the indication of froth non-persistency,which can be used to estimate the clean coal ash con-tent. To select the potential frothers among the sol-vent testes requires a balance between IDFI andIDCR values.

(6) A solvent (Solvent 3, 8 or 11) with an IDFI value of250–350 yields a high coal recovery, but less selectiv-ity. A solvent (Solvent 5, 6, 9 or 10) with an IDFIvalue slightly higher than 350 or less than 250 willyield a reasonably high combustible recovery whilekeeping good selectivity. A solvent, such as 1, 4, 7or 12, with a too low or too high IDFI value tendsto produce froths, which will yield higher ash contentof clean coal product by sacrificing too much combus-tible recovery. Various solvents might be blended toachieve desired properties for better flotation perfor-mance through improved recovery and rate constant.

References

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Cho, Y.S., Laskowski, J.S., 2001. Effect of flotation frothers on bubble sizeand foam stability, SME Annual Meeting, Denver, Colorado, USA.

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frothability characterization of residual organic solvents

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