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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
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
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
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
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
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
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)
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
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
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
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
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
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