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A comparative study of methyl cyclohexanemethanol and methyl isobutylcarbinol as frother for coal flotation

Hangil Park, Junyu Wang, Liguang Wang

PII: S0301-7516(16)30160-0DOI: doi: 10.1016/j.minpro.2016.08.006Reference: MINPRO 2938

To appear in: International Journal of Mineral Processing

Received date: 14 January 2016Revised date: 16 July 2016Accepted date: 11 August 2016

Please cite this article as: Park, Hangil, Wang, Junyu, Wang, Liguang, A com-parative study of methyl cyclohexanemethanol and methyl isobutyl carbinol asfrother for coal flotation, International Journal of Mineral Processing (2016), doi:10.1016/j.minpro.2016.08.006

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A comparative study of methyl cyclohexanemethanol and methyl

isobutyl carbinol as frother for coal flotation

Hangil Park, Junyu Wang, and Liguang Wang*

The University of Queensland, School of Chemical Engineering, Brisbane, Qld 4072

Corresponding author: Liguang.Wang@uq.edu.au

Abstract

Methyl isobutyl carbinol (MIBC), an aliphatic alcohol, is widely used as a frothing reagent in

coal flotation but it has safety concerns owing to its low flash point (approximately 40 °C). In

the present work, we studied a cyclic alcohol, methyl cyclohexanemethanol (MCHM) with a

high flash point (approximately 110 °C) and compared its coal flotation performance with

that of MIBC. A bottom-driven mechanical flotation cell and two coking coals of distinct

floatability, namely A and B, were used. Collectorless flotation tests were carried out with

process water for coal A. Flotation tests with diesel as collector at 50 ppm were carried out

with simulated process water (0.03 M NaCl solution) and highly saline water (0.5 M NaCl

solution), respectively, for coal B. The flotation results showed that MCHM was an effective

alternative to MIBC. The highly saline water produced sufficient frothing, obviating the

necessity of adding MIBC or MCHM. To understand the effect of frother type and

concentration and NaCl concentration on the coal flotation performance, we conducted

surface tension measurement for the frother solutions, characterised the dispersion of air near

bubble sparger, and measured the stabilities of froth, foam, and foam film. It was found that

MCHM was more surface active and more capable of stabilizing froth and foam than MIBC.

Foam film stability measured at a broad range of interface approach velocity followed a bell-

shaped trend and at a given NaCl concentration, the observed peak foam film stability of 15

ppm MCHM was higher than that of 15 ppm MIBC. Increasing NaCl concentration from 0.03

M to 0.5 M had the effect of stabilizing the froth and foam but destabilising the thin foam

film.

Keywords: Coal cleaning, froth flotation, frother, MCHM, foam film

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1 Introduction

Run of mine coals are often beneficiated to remove mineral matter before being sold to

market. Fine (< 1 mm) and ultrafine (< 0.1 mm) coals are usually beneficiated using froth

flotation. In coal flotation, air bubbles were employed in the pulp phase to selectively pick up

the hydrophobic clean coal particles while leaving behind the gangue minerals. Frothing

reagent (frother) is added to produce small and stable bubbles and control froth stability and

mobility. Frother is considered important in the coal flotation as it significantly affects the

kinetic viability and separation efficiency of the flotation. The most widely used frothers in

coal flotation process are aliphatic alcohols and polypropylene glycols (Klimpel and

Isherwood, 1991).

In Australia, methyl isobutyl carbinol (MIBC), an aliphatic alcohol, is the most widely used

frother in coal flotation plants (Firth, 1999). But MIBC has safety concerns owing to its low

flash point (approximately 40 °C ) and high evaporation rate (Pugh, 2007). In addition, MIBC

has been under safety alert from the Australian government (Department of Natural

Resources and Mines, 2015). Hence, there is a pressing need to seek a safer and cost-effective

frother to replace MIBC.

A potential substitute for MIBC is 4-Methyl cyclohexanemethanol (MCHM), a cyclic

alcohol, with a high flash point (approximately 110 ). MCHM has been applied in several

coal flotation plants in the United States to meet the strict regulations of United States’

Environmental Protection Agency, but little information on MCHM and its flotation

performance is available, apart from the original patent (Christie et al., 1990). The patent

claimed that MCHM could achieve similar flotation performance even at a smaller dosage

compared with MIBC. However, the flotation tests reported in the patent utilized de-ionised

water and the reason why MCHM performed better than MIBC was not provided. Recently,

He et al., (2015) noted that MCHM would adsorb on coal and tailing. Similar conclusion was

drawn by Noble et al., (2015), who conducted a plant-wide survey at two coal preparation

plants. However, these two studies addressed neither flotation performance nor interfacial

characteristics.

Recycling the process water or using saline water is an integral part of current coal flotation

practice. The process water or saline water contains a significant amount of inorganic

electrolytes which interact with frother. There has been growing interest in understanding the

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effect of water quality on coal flotation performance. It has been recognized that use of

process water or saline water especially under a turbulent condition can increase flotation

recovery and reduce frother dosage (Craig et al., 1993; Li and Somasundaran, 1993; Ozdemir

et al., 2009; Castro and Laskowski, 2011; Castro et al., 2013; Quinn et al., 2007, 2014). For

instance, Quinn et al. (2007) found that 23,400 ppm (= 0.4 M) NaCl could give a bubble size

and a gas hold up similar to those of 10 ppm of MIBC, suggesting the importance of water

quality on flotation performance and thus the need to investigate the interactions between

frothers and inorganic electrolytes. There are a number of studies that have reported the

interactions between different types of electrolytes and MIBC (Kurniawan et al., 2011; Castro

et al., 2013; Bournival et al., 2014a) but none has addressed such interactions with MCHM.

Hence, one of the aims of the present work is to answer the question as to whether the

MCHM performs better than MIBC in multiple water sources of different quality.

In the present work, we studied MCHM, the cyclic frother, and compared its coal flotation

performance with that of MIBC using two coking coals of distinct floatability, namely A and

B. Coal A is readily floatable without adding any collector, therefore, collectorless flotation

tests were carried out. Flotation of coal B would require diesel as collector and the optimum

collector dosage was 50 ppm, so flotation tests with collector were carried out for coal B,

with two water sources, namely simulated process water (0.03 M NaCl solution) and highly

saline water (0.5 M NaCl solution). We also characterized the dispersion of air near bubble

sparger using high-speed imaging and measured the stabilities of froth, foam, and foam films

at different conditions. The observed difference in coal flotation performance between

MCHM and MIBC was linked to their different interfacial properties and foam-stabilising

effects.

2 Materials and Experimental Methods

2.1 Materials

Diesel (Caltex) was used as collector, and MIBC (98% purity, Sigma-Aldrich, USA) and

MCHM (98% purity, TCI America, USA) were used as frother. Table 1 shows the physical

properties of these frothers. NaCl (99.5% purity, Sigma-Aldrich, USA) was dissolved into

distilled water to prepare the simulated process water and the highly saline water.

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De-ionised water was used throughout the experiments, except for the coal flotation tests that

used the actual process water in the collectorless flotation described in Section 3.1.1 and

distilled water in preparing the simulated process water and highly saline water for the

flotation tests described in Section 3.1.2. Two different coking coal samples from the feed

stream of coal preparation plants in Queensland, Australia were used. The P80 and the ash

content of coal A were 300 μm and 25%, respectively, and those of coal B were 220 μm and

17%, respectively.

2.2 Coal flotation

A bottom-driven mechanical flotation cell (112 × 110 × 145 mm), which was fed with fine

coal slurry (1.2 L slurry), was used. The agitation speed of 1000 rpm and air flow rate of 3

L/min were kept constant throughout the experiments. The froth was scrapped every 15

seconds and four concentrates were collected after cumulative times of 0.5, 1.5, 3.5, and 8.5

min. The ash content of the concentrate and tailing was determined by measuring the mass

difference before and after the combustion of the samples in a furnace at 815 °C for 2 hours.

Collectorless flotation tests were carried out for coal A. The solids content of the coal slurry

fed to the flotation cell was 6 wt%, with aqueous medium being the original process water

from the coal preparation plant where coal A was sampled. The pH of the process water was

7.8 and its electrolytic conductivity was 840 μS/cm (equivalent to 0.008 M NaCl). The

preliminary tests indicated that Coal A was highly floatable and addition of diesel had little

influence on the flotation performance so these flotation tests were carried out with the

addition of a frother but no collector.

Flotation tests with diesel as collector at 50 ppm were carried out for coal B with using two

water sources, simulated process water (0.03 M NaCl solution) and highly saline water (0.5

M NaCl solution), to understand the influence of salinity on the flotation performance. The

solid content of the flotation feed was controlled at 5 wt%, and the aqueous component of the

feed comprised predominantly distilled water and NaCl, with the usage of the original

process water being kept minimal. The NaCl concentration of the simulated process water

was 1,750 ppm NaCl, commensurate with the salinity (electrolytic conductivity 3000 ± 90.18

μS/cm, equivalent to 0.03 M NaCl) of the actual process water with coal B. Also, 29,200 ppm

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NaCl was selected to simulate the highly saline water. Preliminary flotation tests for coal B

using tap water found that the optimum diesel dosage was 50 ppm. Therefore, the diesel

dosage was fixed as 50 ppm throughout all the experiments with coal B.

2.3 Analysis of coal flotation performance

The plot of cumulative yield ( ) or combustible (Rcomb) versus product ash content ( ) has

been widely used to compare and assess coal flotation performance. Some integrated

performance parameters/indices such as EI and SER are also considered useful for evaluating

coal flotation performance, which is simple to use and is related to the financial return from

the coal washing operation (that is, maximising the efficiency index leads to optimum

financial return from a coal cleaning plant). Maximization of EI or SER has been used for

process optimization (Vanangamudi et al 1981; Bhattacharya and Dey, 2010). In the presence

work, EI and SER were calculated using Eqs. [1] and [2] (Swanson et al., 1978; Bhattacharya

and Dey, 2010), respectively:

[1]

where is the overall combustible recovery and is the tailing ash content,

[2]

and

where represents the tailing yield, is the recovery of ash in product concentrate,

is the recovery of combustible in tailing, K is the flotation rate constant, and t is the

flotation time. SER is considered an advanced index for coal flotation performance as it takes

into account flotation rate and the misplacement of non-combustibles in clean coal and loss of

combustible to tailings.

Following the work of Sripriya et al. (2003), Amini et al. (2009), and Vapur et al. (2010), the

selectivity of the separation was estimated from the ratio of the rate constant for combustible

to the rate constant for ash-forming minerals, at the beginning of the batch flotation process

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(time t approaches 0). More specifically, the experimentally obtained flotation recovery-

versus-time data were fitted to the first-order rate equation:

[3]

where Ri is the recovery of the component i (combustible or ash-forming minerals) at time ,

is the ultimate recovery of the component and is the apparent rate constant of the

component. The fitted values for and were used to calculate the modified rate

constant ( ) using the following expression (Xu, 1998; Sripriya et al., 2003):

[4]

The selectivity index (SI) was then calculated using Eq.[5], which is applicable for comparing

and assessing coal flotation efficiency at different conditions (Sripriya et al., 2003; Amini et

al., 2009, Vapur et al., 2010).

[5]

where represents the modified rate constant of the combustible and the

modified rate constant of ash-forming minerals, recovered to the product stream.

In the present work, we determined the optimal frother dosage by considering the following

two criteria:

i) The minimum concentration of frother that yields a maximum value of the SER,

ii) The minimum concentration of frother that reaches a maximum value of SI.

Next, the recovery-grade curves obtained at respective optimum conditions were compared

using the Fuerstenau upgrading curve. Drzymala and Ahmed (2005) derived a number of

mathematical equations that express the Fuerstenau upgrading curve. Among them, Equation

[6] was selected for its simplicity over other mathematical expressions.

[6]

where is the cumulative combustible recovery, represents the cumulative ash

rejection, and is the fitting variable that indicates the effectiveness of coal cleaning (the

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extent of upgrading of the feed coal). Therefore, a greater value of implies better coal

cleaning performance, while indicates no upgrading.

An F-test was carried out to test a null hypothesis that the values of at optimum condition

were equal. Bootstrapping technique was used to undertake the F-test as it is practically

impossible to carry out a large number of repeats of the flotation tests. The bootstrapping was

done using Excel add-in MCSimSolver (Barreto and Howland, 2005) and the details of this

procedure can be found elsewhere (Napier-Munn, 2012).

2.4 Dispersion of air

Gas dispersion characteristics of MIBC and MCHM solutions were compared using a high-

speed camera (Phantom V2011). Bubble swarms at the bottom of the bubble column

(diameter = 6 cm) at different chemical conditions (varying frother type and dosage and NaCl

concentration in the absence of diesel) were compared. A porous plate with pore size at the

range of 40- 100 µm was installed at the bottom of the bubble column to generate the

bubbles. The superficial gas velocity was set to 0.7 cm/s, which was the same as the flotation

test. The frame rate and resolution of the image were set as 22,000 frames per seconds and

1280 x 800 pixels throughout the experiment.

The high-speed camera and a 100 W LED light were placed approximately 70 apart around

the bubble column (see Fig.1a). This configuration allowed a maximum contrast between air

bubbles and the solution since the air bubble would be brighter than the solution (see Fig. 1b).

The gas holdup was estimated by off-line image analysis using a method similar to what was

used by Acuña and Finch (2010), which determines the area occupied by the air bubbles over

the unit area using image analysis. The major difference is that the present technique

determines the grey level of the typical section in the images (see Fig. 1b) to estimate the gas

holdup, therefore it does not require detection and segmentation of bubble boundaries.

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2.5 Froth stability

The froth stability was measured using a modified Bikerman test method (Barbian et al.,

2003). The experimental conditions were kept same as those of the coal flotation tests for

coal B except that a rectangular transparent column (112 × 110 × 250 mm) was mounted on

top of the flotation cell to prevent froth overflowing. The volume (1.2 L) and solid

concentration (5 wt% coal B) of the feed slurry, diesel dosage (50 ppm), air flow rate (3

L/min) and the agitation speed (1000 rpm) were set the same for all the experiments. The

froth height was measured using a laser distance meter (LDM-100, CEM, China).

2.6 Foam stability

The dynamic foam stability was measured using the Bikerman method (Bikerman, 1938).

The same bubble column as described in Section 2.4 was used. The equilibrium foam height

was measured when the foam height reached a constant value and remained unchanged for at

least 3 min. Note that the equilibrium foam height reported in the present work is the

difference between the maximum foam height and the height of the solution without air

supply, which is slightly different from the conventional method of measuring the dynamic

foam stability.

Once the equilibrium foam height was measured, the air supply was cut off to determine the

static foam stability. The time taken to see the appearance of a foam-free liquid surface at the

centre of the foam was measured as an indication of the static foam stability.

2.7 Surface tension

The surface tension isotherm of MCHM solution in the presence of 1,750 ppm NaCl was

measured at 23 °C using the pendant drop method with the Krüss DSA10 Drop Shape

Analysis System.

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2.8 Thin film stability

The thin foam film was formed in a bike-wheel film holder in the presence of 0.1 M NaCl. A

single horizontal foam film was formed in a capillary tube placed in a glass vessel, which in

turn was mounted on an inverted optical microscope (Olympus IX51). The drainage of foam

film was recorded using a video camera (Canon, EOS 550D) and the film life time was

determined off-line.

We measured the stability of the thin foam film at various interface approaching speeds

which were controlled by compressing the film by adjusting air pressure inside the glass

vessel. More details can be found elsewhere (Wang and Qu, 2012).

3 Results and Discussion

3.1 Coal flotation

3.1.1 Collectorless coal flotation

A series of mechanical flotation tests for Coal A in the absence of collector were carried out

to compare the performance of MCHM and MIBC at the concentration range of 3 – 18 ppm,

and the results are shown in Table 2 and Figure 2. The cumulative yield was increased with

increasing frother concentration but the product grade was largely decreased. These trends

were consistent with the observations made by other researchers (Aktas and Woodburn,

1994; Asplin et al., 1998; Qu et al., 2013). The other indices such as combustible recovery,

EI, and SER also increased with increasing frother concentration. Especially, the SER values

steadily increased with increasing frother dosage before levelling off around 12 ppm (see

Figure 2). At a given concentration, MCHM gave seemingly slightly higher SER values

compared with MIBC, and the same appeared to hold for the cumulative yield, product ash

content, and combustible recovery, except for EI. MCHM gave slightly higher or lower EI

values than MIBC, depending on the concentration, but the difference was small.

A t-test (two-sample assuming equal variances) was undertaken to find the minimum

concentration of frother that yields a maximum value of the SER. The results indicated that

the optimum dosages of both frothers were 15 ppm as a further increase in dosage would lead

to no statistically significant difference in cumulative recovery or ash content. At 15 ppm,

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MCHM gave statistically significantly higher combustible recovery and SER value than

MIBC (P = 0.02 and 0.004, respectively), but one can find no statistically significant

differences in cumulative yield (P = 0.12), product ash content (P = 0.07) and EI values (P =

0.11) between MCHM and MIBC. Overall, the collectorless flotation results showed that

comparable optimal performance was achieved at the same concentration, 15 ppm, for MIBC

and MCHM.

Figure 3 shows a steady increase in SI with increasing frother concentration over the

concentration range of 3 – 18 ppm studied. At a given frother dosage, one can see little

difference in the selectivity of separation between MCHM and MIBC. One can also see that

both frothers achieved the maximum value of SI at 18 ppm. Note that the flotation test with

18 ppm of frother was conducted only once; this dosgae was well above the typical dosage of

the MIBC applied in industrial flotatoin operations for Australian coking coal. It is, therefore,

difficult to tell if any statisically significant difference in SI exists bewteen 15 ppm and 18

ppm.

By jointly considering SER and SI and other constraints, one can take 15 ppm as the

optimum frother dosage at which the flotation effiencies of Coal A with MCHM and MIBC

are comparable.

3.1.2 Coal flotation in the presence of collector

Table 3 shows the overall flotation performance of MCHM and MIBC using coal B at a fixed

diesel concentration (50 ppm) in the presence of 1,750 ppm NaCl. As shown, at low

concentrations (i.e., 3 and 7.5 ppm), higher cumulative yield, product ash content,

combustible recovery, EI, and SER values were achieved by MCHM compared to MIBC but

the differences became smaller as the frother concentration was increased to 15 ppm. Note

that at 15 ppm, the SER and EI values of MIBC were higher than those of MCHM mainly

because of the higher product ash content resulted from the use of MCHM. MCHM only gave

a slightly higher tailing ash content (less loss of combustible to tailing) than MIBC.

The SER values in Tables 3 and 4 are also plotted in Figures 4a and 4b. Note that the SER

value of 25 ppm MIBC in the presence of 1,750 ppm NaCl was not shown here, as further

increase in MIBC concentration would allow the flotation performance to level off, resulting

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in no further statistically significant improvement. Figure 4a shows that the maximum SER

value was obtained at 7.5 ppm for MCHM and 15 ppm for MIBC. To test whether there was

a statistically significant difference in cumulative recovery and ash content at the respective

optimum concentration between MCHM and MIBC, a t-test was conducted. It was found that

there was no significant difference in cumulative yield (P-value: 0.36), product ash content

(P-value: 0.33), and combustible recovery (P-value: 0.40).

Fig. 5a shows the SI values of MIBC and MCHM at different frother concentrations in the

presence of 1,750 ppm NaCl. The SI value of MIBC steadily increased with increasing the

dosage. In contrast, the SI value of MCHM increased with increasing the dosage, reaching a

peak at 7.5 ppm before decreasing at higher dosages, which is consistent with the trend of

SER shown in Fig. 4a. In order to find the optimum concentration of both frothers with

respect to the selectivity index (SI), we carried out a t-test (two-sample assuming equal

variances), and the result showed that the optimum dosage of MCHM was 7.5 ppm and that

of MIBC was 15 ppm, which match those obtained based on SER. At the respective optimum

concentrations, the SI values of MCHM and MIBC are comparable (P-value: 0.14).

Comparing Tables 3 and 4, one can see the importance of salinity of water to the flotation

performance. For example, 29,200 ppm NaCl in the absence of frother gave almost the same

flotation performance as 15 ppm of MIBC (with 1,750 ppm NaCl), in consistence with the

finding of Quinn et al. (2007).

In the presence of 29,200 ppm NaCl, increasing frother concentration had, however, little

effect on SER (Fig. 4b) or SI (Fig.5b). The optimum frother dosage was therefore considered

zero. The above-mentioned cumulative yield and ash content were then compared with those

of the 15 ppm MIBC in the presence of 1,750 ppm NaCl. The t-test indicates that there was

no difference in yield while there was a statistically significant difference (P-value = 0.05)

that product ash content obtained with 29,200 ppm NaCl was 0.6 percentage point higher

than 15 ppm of MIBC.

Figure 6 shows the cumulative combustible recovery-versus-ash rejection data and the fitted

Fuerstenau upgrading curves at respective optimum dosage of frothers in the presence of 50

ppm of diesel at two different NaCl concentrations (1,750 ppm and 29,200 ppm NaCl).

Each cluster of three data points represents three independent experimental runs at a given

flotation time. For a given condition, the data were fitted to Eq. [6], and the corresponding c

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value was determined. It turned out that the c values were close to each other, indicating

similar coal cleaning ability. An F-test was undertaken to check a statistical evidence of same

values of c using the bootstrapping technique (the number of replicates was 1000). The

outcomes suggested that all three Fuerstenau upgrading curves of MCHM and MIBC fell

essentially on the same line. These results are consistent with the work of Christie et al.

(1990) who used deionised water for coal flotation tests.

3.2 Dispersion of air

The importance of the gas dispersion properties (i.e., bubble size, number of bubble, gas hold

up, bubble surface area flux) on flotation have been advocated by many researchers (Ahmed

and Jameson, 1985; Gorain et al., 1997; Finch et al., 2000; Yoon, 2000; López-Saucedo et al.,

2012). More specifically, several studies found a close relation between bubble surface area

flux and gas hold up with the flotation performance (Gorain et al., 1997; Finch et al., 2000;

López-Saucedo et al., 2012). In the present work, we used a high-speed camera (Phantom

V2011) to examine whether there is any significant difference in the gas dispersion properties

near the porous plate between MCHM and MIBC.

Figures 7 and 8 show gas dispersion characteristics of both frothers in the presence of 1,750

ppm and 29,200 ppm NaCl. In the presence of 1,750 ppm NaCl, increasing frother dosage

enhanced the gas dispersion as the number of air bubbles was increased while their size was

decreased. Meanwhile, in the highly saline water, adding a small amount of frother such as 3

and 7.5 ppm had little impact on gas dispersion properties.

Figures 9a and 9b show estimated gas holdup at different conditions. In the presence of 1,750

ppm NaCl (Figure 9a), MCHM gave a higher gas hold up than MIBC. The difference in gas

hold up was pronounced at low concentration but increasing the frother concentration

reduced the difference. Figure 9b shows that when the highly saline water (29,200 ppm NaCl)

was used, high gas hold up could be achieved without adding any frother, which is consistent

with the literature (Craig et al., 1993; Castro and Laskowski, 2011; Castro et al., 2013; Quinn

et al., 2007, 2014). Also, the addition of the frother found minor impact on gas hold up in the

presence of 29,200 ppm NaCl, which correlates well with the flotation performance shown in

Section 3.1.2 and visual observation of the bubbles near the sparger (see Fig. 8).

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3.3 Froth stability

In what follows, we measured the dynamic froth stability as it also plays an important role in

flotation (Neethling and Cilliers, 2003; Barbian et al., 2003, 2005). Figure 10 shows that at a

given concentration, MCHM gave more stable froth than MIBC regardless of water quality. It

was also found that at a given frother type and concentration, changing the concentration of

the background NaCl from 1,750 ppm to 29,200 ppm NaCl would increase the maximum

froth height by 5 – 10 cm. That MCHM gave higher froth stability and better gas dispersion

properties than MIBC might account for our observations that higher combustible recovery

was achieved using MCHM compared to MIBC at the same concentration (see Sections 3.1.1

and 3.1.2) and that MCHM required smaller dosage than MIBC to achieve similar optimal

flotation performance (see Section 3.1.2). It also explains why at 29,200 ppm NaCl with no

frother, the flotation attained good separation efficiency similar to that of 15 ppm of MIBC

with simulated process water.

3.4 Foam stability

We also measured the dynamic (Fig. 11a and 11c) and static foam stability (Fig. 11b and 11d)

with MCHM and MIBC in the presence of 1,750 ppm and 29,200 ppm NaCl. Figure 11a and

9c show that at a given frother and NaCl concentration, the foam generated by MCHM is

more stable than MIBC. When comparing two different water qualities (i.e., 1,750 ppm and

29,200 ppm NaCl), a noticeable difference was observed below 25 ppm of frothers but the

difference diminished as frother concentration was increased further. A similar trend was

reported by Bournival et al. (2014b) that addition of 5,840 ppm (= 0.1 M) NaCl greatly

improved dynamic foam stability of 1-pentanol, especially at low concentrations.

Figure 11b and 11d also show the static foam stability (represented by the foam decay time)

at different frother concentrations. The decay time of MCHM foam was longer than MIBC,

which indicates that MCHM is more effective than MIBC in stabilizing foams.

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3.5 Surface tension isotherm and Gibbs elasticity

Figure 12 shows the measured surface tension isotherms of MCHM and MIBC. Also shown

in Fig. 12 are the lines representing the Langmuir–Szyszkowski equation (Eq. [7]), which

was fit to the surface tension data.

[7]

where is the surface tension of the pure water, is the universal gas constant, is the

temperature, is the maximum adsorption density, is the equilibrium adsorption

constant, and is the concentration of frother. The fitted value of MCHM was smaller

than that of MIBC. Further analysis found that each MCHM molecule when closed packed at

the air/water interface would occupy 3 larger than that of MIBC probably because of the

bulkiness of MCHM molecule with its cyclic group. The fitted value of MCHM is higher

than that of MIBC, which is consistent with the fact that the molecular weight of the MCHM

(128.21 g/mol) is larger than that of MIBC (100.16 g/mol). It is clear that MCHM is more

surface active than MIBC.

A close relationship between foaminess (foam formation rate) and the surface elasticity was

reported by many researchers (Małysa et al., 1985, 1991; Laskowski, 2004; Tan et al., 2006).

Hence, we calculated the Gibbs surface elasticity using the following expression:(Wang,

2015).

[8]

Figure 13 shows that the Gibbs surface elasticity of MCHM was higher than MIBC, which is

consistent with the observed difference in foam stability (see Fig. 11).

3.6 Stability of thin foam films

It is believed that the froth and foam stabilities are largely determined by the stability of thin

liquid film (lamellae) inside the froth phase. With bubble bursting at the top of the froth and

the variation in bubble size along the height, one can expect that the interface approaching

speeds of the lamellae throughout the froth phase are not uniform (Wang and Qu, 2012). In

the present work, we measured the stability of the thin liquid films at different interface

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approaching velocities, and the results are shown in Figure 14. As shown, for a given set of

frother dosage and NaCl concentration, the film lifetime increased with increasing the

interface approach speed (excess pressure), reaching a peak at a certain approach speed

before decreasing at higher approach speeds. The bell-shaped curves obtained in the present

work are consistent with those reported by Wang and Qu (2012) and Katsir and Marmur

(2014).

In the presence of 1,750 ppm NaCl, at a given low or intermediate approach speed, MCHM

could produce slightly more stable foam films than MIBC but the difference in film stability

diminished at high interface approach velocities. The higher film stability of MCHM at quasi-

static condition (i.e., low excess pressure such as 1 Pa and 5 Pa) can be attributed to the larger

film size of MCHM (see Fig. 15a), implying that larger volume of the liquid has to be drawn

to reach the critical thickness for film rupture. At the intermediate approaching, the higher

stability of MCHM film than that of MIBC can be attributed to higher Gibbs elasticity of

MCHM which helps to withstand the hydrodynamic corrugation. This argument is supported

by the images of the film immediate before film rupture (see Fig. 15a). For example, MIBC

film is more colourful and brighter than the film generated by MCHM, suggesting that the

rupture thickness of the MIBC film is higher than that of the MCHM film.

Fig. 15b showed that in the presence of 29,200 ppm NaCl, the film generated by MCHM was

more stable than MIBC at intermediate approaching speed but they were almost the same at

low or high approach speed. Comparing Fig. 14a and 14b, at a given frother type, the stability

of the films were decreased at a very high salt concentration, which can be attributed to

complete suppression of double layer repulsion force at 29,200 ppm NaCl.

Compared to MIBC, the higher foam film stability of MCHM is consistent with its higher

froth and foam stabilities achieved when all other conditions were kept the same. With

increasing electrolyte concentration, however, the observed increasing trend of froth and

foam stabilities and air dispersion were not consistent with the decreasing trend of foam film

stability. Similarly, Wang and Qu (2012) pointed out the difference in the trend between thin

free films and turbulent gas-liquid dispersion systems, in the absence of frothers. It is likely

that the interaction between electrolyte and hydrodynamic condition in a flotation machine

cannot be captured in the present thin film studies where a bike-wheel film holder was used

to form the single horizontal foam film.

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The results presented hitherto suggest that what determines froth stability and the separation

efficiency in coal flotation with highly saline water can be irrelevant to surface active species

and there is a need to improve the understanding of the interaction between electrolyte and

hydrodynamic turbulence.

4 Conclusions

This paper compares the coal flotation performance of MCHM with that of MIBC. A series

of mechanical flotation tests were carried out using two different coking coal samples. The

optimum frother dosage was determined by finding the minimum concentration of frother

that yields the maximum value of SER and selectivity index (SI). The collectorless flotation

tests for coal A showed that MIBC and MCHM had the same optimum concentration (i.e., 15

ppm) and comparable optimal flotation performance. Flotation tests with diesel as collector at

50 ppm were carried out with simulated process water for coal B, and the results showed that

the optimum dosage was 7.5 ppm for MCHM and 15 ppm for MIBC. At the respective

optimum concentration, similar flotation performance was made. This flotation performance

was comparable to that of 0.5 M NaCl solution, free of frother, suggesting that addition of

MIBC or MCHM to the highly saline water had little impact on the flotation of coal B. These

flotation tests results suggest that MCHM is an effective alternative to MIBC in coal flotation

systems where the use of MIBC is not suitable.

We investigated the surface tension, gas dispersion and the stabilities of froth, foam, and

foam films to understand the effect of frother type and concentration and electrolyte

concentration on the coal flotation performance. The results show that MCHM is more

surface active than MIBC and in the presence of 1,750 ppm NaCl, at a given concentration,

MCHM gave better gas dispersion and more stable froth and foam than MIBC. It was also

found that increasing electrolyte concentration from 1,750 ppm to 29,200 ppm considerably

improved the froth and foam stabilities and improved the gas dispersion.

Foam film stability measured at a broad range of interface approach velocity followed a bell-

shaped trend and at a given NaCl concentration, the observed peak foam film stability of

MCHM was higher than that of MIBC. In the presence of 1,750 ppm NaCl, MCHM could

produce more stable film than MIBC, especially at low or intermediate approaching speed.

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However, further increase in approaching speed diminished the difference in the film stability.

For a given frother at a given concentration, when the NaCl concentration was increased to

29,200 ppm, the thin film stabilities of both frothers were slightly decreased. This trend was

inconsistent with the increasing trend of froth and foam stabilities and gas dispersion with

increasing electrolyte concentration.

5 Acknowledgements

The authors gratefully acknowledge the financial support from The Australian Coal

Association Research Program (Project C23035). The authors thank Prof. David Williams for

his help with high-speed imaging and Dr. Chunxia Zhao for her assistance with surface

tension measurement.

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Figure Captions

Fig. 1. a) Experimental configuration of the high-speed camera, the LED light and the bubble

column; b) Typical image obtained using the high-speed camera. The square box corresponds

to 150 x 150 pixels where the gas hold up was analysed using off-line image analysis.

Fig. 2. Flotation performance (represented by SER) of MCHM and MIBC with coal A and

original process water in the absence of collector.

Fig. 3. Selectivity index (SI) of flotation of coal A in the original process water with MCHM

and MIBC in the absence of collector.

Fig. 4. Flotation performance (represented by SER) of MCHM and MIBC using coal B with

two different water sources in the presence of 50 ppm of diesel: a) 1,750 ppm NaCl b) 29,200

ppm NaCl.

Fig. 5. Selectivity index (SI) of flotation of coal B with MCHM and MIBC using two

different water sources in the presence of 50 ppm of diesel: a) 1,750 ppm NaCl b) 29,200

ppm NaCl.

Fig. 6. The Fuerstenau upgrading curve of MCHM and MIBC at respective optimum

conditions of MCHM (7.5 ppm) and MIBC (15 ppm) in the presence of 1,750 ppm NaCl. The

curve in the presence of 29,200 ppm NaCl without frother is plotted for comparison.

Fig. 7. Gas dispersion characteristic of MIBC and MCHM in the presence of 1,750 ppm

NaCl. The superficial gas velocity was kept as 0.7 cm/s and the image was taken at 22,000

frames per second.

Fig. 8. Gas dispersion characteristic of MIBC and MCHM in the presence of 29,200 ppm

NaCl. The superficial gas velocity was kept as 0.7 cm/s and the image was taken at 22,000

frames per second.

Fig. 9. Grey level of the images shown in Figures 7 and 8: a) 1,750 ppm NaCl b) 29,200 ppm

NaCl. Each data point represents the average grey level of three consecutive different images

at a time interval of 0.01 s. The lines were drawn at grey levels of the NaCl solutions in the

absence of frother to guide the eye.

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Fig. 10. Maximum froth height at three different concentrations of MCHM and MIBC: a) In

the presence of 1,750 ppm NaCl. b) In the presence of 29,200 ppm NaCl. The experimental

conditions of froth stability tests were the same as those of the flotation tests: coal B, solid

concentration (5 wt %), diesel dosage (50 ppm), air flow rate (3 L/min), and agitation speed

(1000 rpm). The error bars represent one standard error obtained from three independent

experimental runs.

Fig. 11. a) Dynamic foam stability of MIBC and MCHM in the presence of 1,750 ppm NaCl

b) Static foam stability in the presence of 1,750 ppm NaCl c) Dynamic foam stability in the

presence of 29,200 ppm NaCl d) Static foam stability in the presence of 29,200 ppm NaCl.

The foam was generated using a porous plate with the superficial gas velocity being 0.7 cm/s

in the absence of diesel.

Fig. 12. Surface tension isotherms of MCHM and MIBC. The surface tension data of MIBC

are adapted from Qu et al., (2009). The solid line represents Eq. [4] with the fitted and

values shown in the figure.

Fig. 13. Gibbs surface elasticities versus frother concentration. The elasticity was calculated

using Eq. [8].

Fig. 14. Impact of interface approaching velocity on film lifetime in the presence of 15 ppm

MIBC or MCHM. The bike-wheel film holder (inner diameter =0.75 mm) was used and the

interface approaching speed was controlled by compressing the film by adjusting air pressure

inside the chamber beyond an onset pressure for film formation: a) in the presence of 1,750

ppm NaCl, b) in the presence of 29,200 ppm NaCl.

Fig. 15. Effect of interface approach speeds (determined by the excess pressure) on the

images of the thin foam films immediately before rupture: a) in the presence of 1,750 ppm

NaCl b) in the presence of 29,200 ppm NaCl. Also shown in each picture are the film life

time and radius.

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Fig. 1. a) Experimental configuration of the high-speed camera, the LED light and the bubble

column; b) Typical image obtained using the high-speed camera. The square box corresponds

to 150 x 150 pixels where the gas hold up was analysed using off-line image analysis.

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Fig. 2. Flotation performance (represented by SER) of MCHM and MIBC with coal A and

original process water in the absence of collector.

MCHM

MIBC

0

200

400

600

800

1000

1200

1400

1600

1800

3 6 9 12 15 18

SER

Concentration, ppm

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Fig. 3. Selectivity index (SI) of flotation of coal A in the original process water with MCHM

and MIBC in the absence of collector.

0

1

2

3

4

5

6

7

8

9

10

3 ppm 6 ppm 9 ppm 12 ppm 15 ppm 18 ppm

SI

Concentration, ppm

MCHM

MIBC

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Fig. 4. Flotation performance (represented by SER) of MCHM and MIBC using coal B with

two different water sources in the presence of 50 ppm of diesel: a) 1,750 ppm NaCl b) 29,200

ppm NaCl.

0

200

400

600

800

1000

1200

1400

1600

1800

0 ppm 3 ppm 7.5 ppm 15 ppm

SER

Concentration, ppm

0

200

400

600

800

1000

1200

1400

1600

1800

0 ppm 3 ppm 7.5 ppm 15 ppm

SER

Concentration, ppm

MCHM

MIBC NaCl Only

B

MCHM

MIBC

NaCl Only

A1,750 ppm (= 0.03M) NaCl

29,200 ppm (= 0.5M) NaCl

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Fig. 5. Selectivity index (SI) of flotation of coal B with MCHM and MIBC using two

different water sources in the presence of 50 ppm of diesel: a) 1,750 ppm NaCl b) 29,200

ppm NaCl.

0

1

2

3

4

5

6

7

8

9

10

0 ppm 3 ppm 7.5 ppm 15 ppm 25 ppm

SI

Concentration, ppm

MCHM

MIBC

A

0

1

2

3

4

5

6

7

8

9

10

0 ppm 3 ppm 7.5 ppm 15 ppm

SI

Concentration, ppm

MCHM

MIBC

NaCl Only

B

1,750 ppm (= 0.03M) NaCl

29,200 ppm (= 0.5M) NaCl

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Fig. 6. The Fuerstenau upgrading curve of MCHM and MIBC at respective optimum

conditions of MCHM (7.5 ppm) and MIBC (15 ppm) in the presence of 1,750 ppm NaCl. The

curve in the presence of 29,200 ppm NaCl without frother is plotted for comparison.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Cum

bust

ible

reco

very

, %

Ash rejection, %

c = 3.5

c = 3.7

c = 3.6

c = 1.0

MCHM, 7.5ppm, 1,750 ppm(= 0.03M) NaCl

MIBC, 15ppm, 1,750 ppm(= 0.03M) NaCl

29,200 ppm(= 0.5M) NaCl only

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Fig. 7. Gas dispersion characteristic of MIBC and MCHM in the presence of 1,750 ppm

NaCl. The superficial gas velocity was kept as 0.7 cm/s and the image was taken at 22,000

frames per second.

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Fig. 8. Gas dispersion characteristic of MIBC and MCHM in the presence of 29,200 ppm

NaCl. The superficial gas velocity was kept as 0.7 cm/s and the image was taken at 22,000

frames per second.

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Fig. 9. Grey level of the images shown in Figures 7 and 8: a) 1,750 ppm NaCl b) 29,200 ppm

NaCl. Each data point represents the average grey level of three consecutive different images

at a time interval of 0.01 s. The lines were drawn at grey levels of the NaCl solutions in the

absence of frother to guide the eye.

0

20

40

60

80

100

120

0 5 10 15 20 25

Gre

y level, a

.u.

Concentration, ppm

A

0

20

40

60

80

100

120

0 5 10 15 20 25

Gre

y level, a

.u.

Concentration, ppm

B

MCHM

MIBC

MCHM

MIBC

1,750 ppm (= 0.03M) NaCl

29,200 ppm (= 0.5M) NaCl

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Fig. 10. Maximum froth height at three different concentrations of MCHM and MIBC: a) In

the presence of 1,750 ppm NaCl. b) In the presence of 29,200 ppm NaCl. The experimental

conditions of froth stability tests were the same as those of the flotation tests: coal B, solid

concentration (5 wt %), diesel dosage (50 ppm), air flow rate (3 L/min), and agitation speed

(1000 rpm). The error bars represent one standard error obtained from three independent

experimental runs.

MCHM

MIBC

Maxim

um

fro

th h

eig

ht, c

m

0

5

10

15

20

25

30

35

0 5 10 15 20

Concentration, ppm

MCHM

MIBC Maxim

um

fro

th h

eig

ht, c

m

A

B

0

5

10

15

20

25

30

35

0 5 10 15 20

Concentration, ppm

1,750 ppm (= 0.03M) NaCl

29,200 ppm (= 0.5M) NaCl

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Fig. 11. a) Dynamic foam stability of MIBC and MCHM in the presence of 1,750 ppm NaCl

b) Static foam stability in the presence of 1,750 ppm NaCl c) Dynamic foam stability in the

presence of 29,200 ppm NaCl d) Static foam stability in the presence of 29,200 ppm NaCl.

The foam was generated using a porous plate with the superficial gas velocity being 0.7 cm/s

in the absence of diesel.

0

1

2

3

4

0 20 40 60 80 100

Maxim

um

foam

heig

ht, c

m

Concentration, ppm

0

2

4

6

8

10

12

0 20 40 60 80 100

Deca

y t

ime, se

c

Concentration, ppm

MCHM

MIBC

A

C

B

D

0

1

2

3

4

0 20 40 60 80 100

Maxim

um

foam

heig

ht, c

m

Concentration, ppm

0

2

4

6

8

10

12

0 20 40 60 80 100

Deca

y t

ime, se

c

Concentration, ppm

29,200 ppm (= 0.5M) NaCl

MCHM

MIBC

1,750 ppm (= 0.03M) NaCl

MCHM

MIBC

1,750 ppm (= 0.03M) NaCl

MCHM

MIBC

29,200 ppm (= 0.5M) NaCl

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Fig. 12. Surface tension isotherms of MCHM and MIBC. The surface tension data of MIBC

are adapted from Qu et al., (2009). The solid line represents Eq. [4] with the fitted and

values shown in the figure.

0

10

20

30

40

50

60

70

80

1 10 100 1000

Surf

ace

tensi

on, m

N/m

concentration, ppm

KL(M-1) (μmol/m2) Area occupied (A2)

MCHM 1040 4.7 36

MIBC 220 5.1 33

MIBC

MCHM

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Fig. 13. Gibbs surface elasticities versus frother concentration. The elasticity was calculated

using Eq. [8].

0

20

40

60

80

100

1 10 100 1000

EG, m

N/m

Concentration, ppm

MCHM

MIBC

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Fig. 14. Impact of interface approaching velocity on film lifetime in the presence of 15 ppm

MIBC or MCHM. The bike-wheel film holder (inner diameter =0.75 mm) was used and the

interface approaching speed was controlled by compressing the film by adjusting air pressure

inside the chamber beyond an onset pressure for film formation: a) in the presence of 1,750

ppm NaCl, b) in the presence of 29,200 ppm NaCl.

0

1

2

3

4

5

0.1 1 10 100 1000

Film

life t

ime, se

c

Excess pressure, Pa

MCHM

MIBC

0

1

2

3

4

5

0.1 1 10 100 1000

Film

life t

ime, se

c

Excess pressure, Pa

MCHM

MIBC

A

B

1,750 ppm (= 0.03M) NaCl

29,200 ppm (= 0.5M) NaCl

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Fig. 15. Effect of interface approach speeds (determined by the excess pressure) on the

images of the thin foam films immediately before rupture: a) in the presence of 1,750 ppm

NaCl b) in the presence of 29,200 ppm NaCl. Also shown in each picture are the film life

time and radius.

1.0 sec

8 μm

1.5 sec 2.3 sec 2.5 sec 0.8 sec 0.4 sec 0.2 sec

24 μm 31 μm 116 μm 157 μm 338 μm 375 μm

MIBC

MCHM 1.4 sec 2.4 sec 3.1 sec 4.2 sec 3.2 sec 0.4 sec 0.2 sec

μm 30 μm 40 μm 112 μm 169 μm 334 μm 375 μm

1 Pa 5 Pa 10 Pa 50 Pa 100 Pa 500 Pa 1,000 Pa

A

1.0 sec 1.6 sec 2.2 sec 2.5 sec 3.0 sec 0.6 sec 0.1 sec

13 μm 30 μm 46 μm 121 μm 170 μm 335 μm 375 μm

MIBC

MCHM

1.4 sec 1.8 sec 2.5 sec 1.3 sec 1.0 sec 0.9 sec 0.3 sec0.4 sec

16 μm 23 μm 42 μm 111 μm 159 μm 329 μm 375 μm

B

1 Pa 5 Pa 10 Pa 50 Pa 100 Pa 500 Pa 1,000 Pa

1,750 ppm (= 0.03M) NaCl

29,200 ppm (= 0.5M) NaCl

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Table 1: Physical properties of MCHM and MIBC (adapted from TOXNET 2015a and

2015b unless otherwise specified).

MCHM MIBC

Molecular weight, g/mol 128.22 102.18

Density, g/cm3 0.9074 0.8075

Solubility, g/L 1.585 at 23 *

2.024 at 25

16.4 at 25

*He et al. (2015)

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Table 2: Effect of MCHM and MIBC on the flotation of coal A with the original process

water (Mean ± 1 standard error). Note that tests with 3, 6 and 18 ppm of frothers were

conducted only once while the others were repeated at least twice.

Cumulative

Yield(%)

Cumulative

Ash(%)

(%)

EI SER

MIBC MCHM MIBC MCHM MIBC MCHM MIBC MCHM MIBC MCHM

3

ppm

51.2 51.4 6.1 6.2 64.8 65.6 493 506 564 581

6

ppm

58.6 61.4 5.5 5.7 74.7 77.6 747 775 799 875

9

ppm

70.5

0.2

71.7

0.9

7.6

0.1

7.9

0.2

89.1

0.3

91.1

0.7

897

5

890

3

1305

22

1411

34

12

ppm

74.6

0.6

75.4

0.2

8.3

0.2

9.0

0.1

93.5

0.3

94.6

0.0

917

5

885

9

1566

28

1650

7

15

ppm

75.3

0.6

76.5

0.3

8.7

0.1

9.4

0.2

94.7

0.1

95.5

0.1

916

7

877

21

1675

0.07

1724

4

18

ppm

77.1 78.5 8.9 9.9 95.3 96.1 911 840 1698 1735

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Table 3: Effects of MCHM and MIBC on the performance of coal flotation with coal B

in the presence of 1,750 ppm NaCl and 50 ppm of diesel (Mean ± 1 standard error, from

three independent experimental runs).

Cumulative

Yield(%)

Cumulative

Ash(%)

(%)

EI SER

MIBC MCHM MIBC MCHM MIBC MCHM MIBC MCHM MIBC MCHM

0

ppm

50.7

4.3

6.6

56.8

5.0

240

33.5

414

68

3

ppm

57.9

4.0

82.5

1.1

7.6

0.1

8.7

0.5

64.5

4.3

90.6

1.1

259

28.7

586

32.0

516

7

1063

38

7.5

ppm

85.3

1.3

90.7

0.2

8.8

0.11

10.19

0.1

93.4

1.3

97.6

0.1

676

45.3

788

44.6

1180

60

1341

70

15

ppm

90.1

0.4

91.2

0.1

9.6

0.2

10.5

0.1

97.6

0.1

97.9

0.1

807

17.4

752

2.4

1376

31

1296

1

25

ppm

89.7

0.4 9.6

0.1 97.5

0.1 810

16.2 1401

29

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Table 4: Effects of MCHM and MIBC on the performance of coal flotation with coal B

in the presence of 29,200 ppm NaCl and 50 ppm of diesel (Mean ± 1 standard error,

from three independent experimental runs).

Cumulative

Yield(%)

Cumulative

Ash(%)

(%)

EI SER

MIBC MCHM MIBC MCHM MIBC MCHM MIBC MCHM MIBC MCHM

0

ppm

90.8

0.2

10.2

0.1

97.8

0.1

768.6

11.2

1332

7

3

ppm

90.7

0.6

91.5

0.1

10.1

0.0

10.1

0.2

97.8

0.1

98.1

0.1

782.1

3.4

785.2

14.6

1346

37

1323

26

7.5

ppm

90.7

0.3

91.7

0.2

10.2

0.2

10.89

0.1

97.9

0.0

98.2

0.0

781.3

26.3

740.9

2.2

1366

50

1283

8

15

ppm

91.1

0.0

91.5

0.0

10.6

0.1

10.9

0.1

98.0

0.0

98.1

0.0

755.6

10.7

734.0

1.8

1326

12

1281

7

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Highlights

MCHM is a high flash point alternative to MIBC

MCHM is more surface active than MIBC

MCHM largely outperforms MIBC in gas dispersion and foam stabilisation