Powder Technology 353 (2019) 20–26

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Powder Technology

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

Determination of oxidation properties and flotation parameters

of low-rank coal slimes
Yuchu Cai a,b,⁎, Meili Du a,b,⁎, Shuili Wang a, Lei Liu a,b
a
School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, Shaanxi, China
b
Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, MLR, Xi'an 710021, Shaanxi, China

a r t i c l e i n f o a b s t r a c t

Article history: The oxidation properties and flotation parameters of low-rank coal slimes from Shuilian mine were researched in
Received 26 November 2018 this paper. Because coal slimes remain piled up for long periods outdoors, they are highly susceptible to oxida-
Received in revised form 30 April 2019 tion, which makes it difficult for them to undergo flotation. Based on elemental analysis, Fourier transform infra-
Accepted 9 May 2019
red spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), different characteristics of fresh and
Available online 11 May 2019
oxidized coal slimes were compared. The results showed that coal slimes collected from Shuilian mine were char-
Keywords:
acterized by high ash content, fine mud texture, low calorific value, and susceptibility to serious sliming. After
Coal slimes oxidization, the O element content increased, whereas that of C and H elements decreased. Therefore, many
Oxidation oxygen-containing functional groups formed on coal slimes surface, such as C–O–C, C=O, and O=C–O, which
Oxygen-containing functional groups imparted strong hydrophilicity, thereby reducing the flotation effects. Thus, with the aim to improve the flotation
Flotation conditions effects, we investigated the influence of flotation conditions, such as impeller speed, pulp concentration and air
flow rate, on flotation effects. Based on our experiments, optimal flotation conditions were determined as
follows: impeller speed, 1800 r/min; pulp concentration, 60 g/L; and air flow rate, 0.25 m3/(m2·min).
© 2019 Elsevier B.V. All rights reserved.

1. Introduction oxygen-containing collector (α-furanacrylic acid) is used to improve

clean coal yield and reduce the ash content. The high flotation
Coal is the main source of energy in China, which unsurprisingly recovery resulting from this process is attributed to the formation of hy-
controls the national economy. During coal washing, a by-product drogen bond between α-furanacrylic acid and the oxidized coal surface
known as coal slimes are produced. However, these initial coal slimes [13]. He et al. used a new collector (NC) to achieve efficient flotation of
within high ash content and low thermal power cannot be directly oxidized coal slimes, and reported higher yield of clean coal with lower
used as fuel [1,2]. Once the ash content decreases and the thermal ash content; additionally, the authors also achieved higher combustible
power improves, it will be used as fuel to power plant [3–7]. In this matter recovery rate [14]. In general, not only oxidization properties,
way, the energy will be make full use. And large volumes of coal slimes but also the sliming extent affects flotation characteristics. Slimes
piled up inside the coal plants for long periods will be exhausted to im- cover the surface of minerals, which prevents the direct contact
prove the plant environment [8]. Therefore, it is necessary to investigate between collectors and air bubbles [15]. Ni et al. found that slimes
the process of flotation in-depth to better utilize coal slimes. But com- would decrease the flotation recovery of coal particles at a high mass
prising fine particles of mined coal and water, coal slimes are essentially proportion of slimes particles [16]. Chen et al. developed a mechanism
fine mud that are known to cause sliming, which increases flotation dif- for enhancing low-rank coal flotation using a cationic surfactant
ficulty [9,10]. [didodecyldimethylammonium bromide (DDAB)] in the presence of
Factors that affect flotation are determined by raw material proper- an oily collector [17]. To sum up, the oxidization properties change the
ties [11]. Because coal slimes are piled up outdoors for long periods, they floatability of coal slimes and kinetic parameters [6]. However, from
are directly exposed to the effects of wind, sun (heat), and rain. As a re- previous studies in this area, flotation properties can be improved by
sult coal slimes may be subjected to oxidation, which at times can be se- utilizing various chemical methods [18–20].
rious [8]. Many studies have investigated the effects of oxidization on Many methods are also available to improve flotation performance
flotation [12]. To improve the flotation effects of oxidized coal, an from a physical aspect. Flotation design parameters such as flow regime,
gas holdup, bubble size and its distribution, mixing characteristics, and
⁎ Corresponding author at: School of Chemistry and Chemical Engineering, Xi'an
carrying capacity were investigated to discuss the flotation mechanism
University of Science and Technology, Xi'an 710054, Shaanxi, China. [21]. By employing simultaneous ultrasonic treatment in flotation, coal
E-mail addresses: caiyuchu@126.com (Y. Cai), duml@xust.edu.cn (M. Du). slimes flotation performance was demonstrated to be improved with

https://doi.org/10.1016/j.powtec.2019.05.017
0032-5910/© 2019 Elsevier B.V. All rights reserved.
 Y. Cai et al. / Powder Technology 353 (2019) 20–26 21

increased combustible recovery and decreased ash values [22,23]. Fur- Table 2
thermore, an improved oily bubble flotation method combining model- Screening results.

ing and optimization of technological parameters was used to enhance Size/mm Weight/g Yield/% Ash/% Positive Negative
the flotation properties of low/medium-rank coal samples. Following accumulation accumulation
treatment with the aforesaid method, coal samples (low/medium Yield/% Ash/% Yield/% Ash/%
rank) displayed better flotation efficiency. Besides, the method allowed
+0.500 8.43 4.24 35.28 4.24 35.28 100.00 40.34
for higher combustible matter recovery at a much smaller collector dos- −0.500 − 0.250 16.84 8.47 24.79 12.71 28.29 95.86 40.53
age [24,25]. Zhang et al. [26] found that coal separation was maximum -0.250 − 0.125 14.06 7.07 31.12 19.78 29.30 87.39 42.05
at 70% feeding position relative height of the shallow tank heavy- -0.125-0.074 32.86 16.52 34.18 36.30 31.52 80.32 43.00
medium separator and 75% flow rate, a method that successfully −0.075 − 0.045 68.44 34.61 43.07 70.91 37.16 63.80 45.28
-0.045 58.06 29.19 47.91 100.00 40.34 29.19 47.91
reduced the volume of coal slimes from the source (coal plant). At low Total 198.69 100.00
pulp densities, the reflux flotation cell was used to achieve fast coal flo-
tation, mainly because the volumetric feed flux was increased to nearly
10 times the standard feed level [27]. Wang and Wang improved the flo- Using the XPS PEAK software, the content of functional groups was an-
tation effects of oxidized or ultrafine coal particles by changing the flow alyzed quantitatively under pre-set parameters according to the PEAK
pattern of air supply [28]. The aforesaid methods identified ways to fitting method. The wide-range scanning resolution was 1 eV and the
improve the flotation performance by changing flotation conditions. scanning energy was 100 eV, whereas the narrow-range scanning reso-
In this work, properties of coal slimes from Shuilian mine were ana- lution was 0.05 eV and the scanning energy was 20 eV. In addition, the
lyzed. By proximate analysis and size distribution analysis, they were vacuum degree of analysis room was 5 × 10−8 Pa, and the excitation
determined to have high ash content and low calorific value. They light source was a monochromatic aluminum anode target (Al Kα).
were characterized as fine mud with serious sliming potential. We ob-
tained its oxidation properties and describe its flotation mechanism
based on elemental analysis, FTIR, and XPS results. To improve the flota- 2.3. Flotation experiments
tion performances of coal slimes, effects of flotation conditions, such
as impeller speed, pulp concentration, and air flow rate factors, were To determine the conditions that favor flotation of coal slime, the
investigated, and the best flotation conditions were determined. Our re- influence of flotation parameters, such as the impeller speed, pulp con-
sults are of great significance for oxidized coal slimes to improve their centration, and air flow rate, on the flotation effects was investigated. A
flotation effects. 1.5-L XFD flotation cell was used in these experiments. The collector
dosage (X2) and the foaming agent dosage (sec-octyl alcohol) were
2. Experiments 800 g/t and 300 g/t, respectively.

2.1. Samples
3. Results and discussion
Both fresh coal slimes (immediately obtained from production) and
oxidized coal slimes (exposed to the weather for long time) were col- 3.1. Elemental analysis
lected from Shuilian mine (Shaanxi Binchang Mining Group Co., LTD),
and sealed in container for future use. Before the experiments, the Table 3 presents the results of elemental analysis of fresh and oxi-
method of quarter division was adopted to select the coal slimes sample. dized coal slimes. Compared with the elemental composition of fresh
Results of proximate analysis and calorific value of these coal slimes are coal slimes, no obvious changes in N and S elements were noted in
presented in Table 1, which indicate that these coal slimes had high ash oxidized coal samples. By contrast, the content of C in oxidized samples
content and low calorific value. Therefore, these could not be directly reduced by about 6% and H by about 1.8%; the content of O increased by
used as fuel. about 7%. This difference can be attributed to the changes in the oxygen-
The grain size of coal slimes was determined by wet screening with containing functional groups on the molecular side chains of coal slimes.
200-g coal weight at 20 °C experimental temperature (see Table 2 for
the results). From Table 2, it can be concluded that nearly 63.7% of par-
ticles were b 0.075 mm; moreover, their ash content was relatively high 3.2. Surface functional groups analysis of coal slimes
at this point. Therefore, we deduce these particles are highly susceptible
to sliming. The FTIR spectrum of oxidized coal slimes is shown in Fig. 1. From
the figure it can be concluded that after oxidization, some hydrophobic
2.2. Surface functional groups of coal slimes functional groups were formed, such as –CH2– (represented by the an-
tisymmetric stretching vibration peak or symmetric stretching vibra-
First, elemental analysis was performed using an automatic elemen- tion) at wave numbers of 2923 and 2855 cm−1, –CH3 at the wave
tal analyzer (Elementar Vario EL III) to understand the proportion of number of 1380 cm−1, and C–H (represented by symmetric angular vi-
various elements in slimes, such as C, H, N, and S. Second, samples bration absorption peaks) at the wave number of 907 cm−1. Besides,
were subjected to FTIR analysis (NICOLET iZ10), and their infrared spec- many oxygen-containing functional groups, such as –OH (represented
tra were measured with scanned area between 4000 and 400 cm−1, 32 by the stretching vibration peak) at the wave number of 3680 cm−1,
scan times, 4 cm−1 resolution, and KBr substrate. Finally, an X-ray pho- were produced. In addition, there was also hydrogen bonding resulting
toelectron spectroscopy analyzer (ESCALB 250Xi) was used to deter- from resonance between NC=O and –OH at the wave number of
mine the composition of O, C, Si, Al, and other elements in coal slimes 1605 cm−1.
before and after oxidation. C1s was chosen as reference for calibration.
Table 3
Table 1 Elemental analysis of coal slimes before and after oxidation.
Proximate analysis results of coal slimes.
Element Cdaf/% Hdaf/% Ndaf/% Odaf/% St,d/%
Mad/% Aad /% Vad /% FCad/% Qnet，v，ad/(MJ/kg)
Fresh coal slimes 77.12 5.83 1.21 14.83 1.01
2.78 41.59 21.42 34.21 17.85 Oxidized coal slimes 71.58 4.07 1.47 21.91 0.94
22 Y. Cai et al. / Powder Technology 353 (2019) 20–26

Fig. 1. Infrared spectra of oxidized coal slimes.

3.3. XPS analysis

Fig. 2(a)and (b) show the XPS wide-range scanning results of fresh
and oxidized coal slimes, respectively. From Fig. 2(a) and (b), it could
be seen that the surface of coal slimes was dominated by C and O ele-
ments, whose contents were significantly higher than other elements.
Meanwhile, the composition of C in the fresh coal slimes was higher
than that (47.35% vs. 42.44%) of oxidized coal slimes, whereas that of
O was lower (18.67% vs. 24.65%). These results are consistent with
those of elemental analysis.
The O element in coal slimes can not only combine with C element to
form oxygen-containing functional groups, which exist in the form of
organic oxygen, but also combine with Si or Al to form quartz or silicate
minerals (inorganic oxygen), respectively. Therefore, the C element was
chosen for peak fitting. For analysis using XPS PEAK, the fitting parame-
ters were set as follows: fitting range, (281–290) ± 0.5 eV; Shirley base-
line; ratio of Lorenz–Gosby (L-G), 0. The binding energy ranges of C1s
were as follows: C–C/C–H, 284.6 eV; C–O–C/C–OH, 285.8–286.3 eV;
C=O, 287.3–287.6 eV; and O=C–O, 289.0–289.2 eV.
The final fitting results of C1s PEAK are shown in Fig. 3(a) and (b). Fig. 2. XPS wide-range scanning results.

Based on the fitting parameters and peak area, the contents of func-
tional groups after fitting were obtained and these are presented in 0.25 m3/(m2·min) in the method for the batch flotation testing of coal
Table 4. From Fig. 3 and Table 4, it can be seen that the relative content slimes. Therefore, during researching the effects of these factors to flota-
of oxygen-containing functional groups in coal slimes presented an ob- tion results, the impeller speed (1800 ± 600 r/min) and air flow rate
vious increase following oxidization (e.g., C–O–C increased by 2%, C=O (0.25 ± 0.1 m3/(m2·min)) range were determined. About the pulp con-
increased by 5%, and O=C–O increased by 1.3%). The content of C=O in- centration, the range of 40 g/L to 100 g/L was selected mainly because
creased the most, which indicates that the functional groups on coal the coal slimes in these experiments were of fine size, which would
slimes surface not only experienced oxidation, but also had higher oxi- lead to agglomeration phenomenon when the pulp concentration was
dation degree during outdoor storage. too high.
After oxidization, many oxygen-containing functional groups were First, the experiments were designed to study the influence of the
formed on coal slimes surface. These oxygen-containing functional impeller speed on flotation effects. The impeller speed determines not
groups show polarity, thus making the surface hydrophilicity of coal only the dispersing degree of the agent in the pulp, but also the size of
slimes stronger. Therefore, it is difficult for the bubbles to be mineral- the bubbles. For the experiments, the pulp concentration was set as
ized, which worsens the floatability of coal slimes. As a result, the flota- 60 g/L and air flow rate as 0.25 m3/(m2·min). Flotation results under
tion effects decreased. different impeller speeds are shown in Fig. 4(a) and (b).
From Fig. 4(a) and (b), it can be concluded that with the increase of
impeller speed, both the recovery rate of combustible matter and the
3.4. Determination of flotation parameters yield of clean coal increased first and then decreased, whereas the ash
of clean coal showed a trend of gradual increase. When the impeller
To improve flotation effects of oxidized coal slimes, we investigated speed was 1800 r/min, the clean coal yield was 42.89% and the recovery
the influence of factors favoring flotation, such as impeller speed, pulp rate of combustible matter was 61.33%, both reaching the maximum.
concentration, and air flow rate, on the flotation effects. According to Within a reasonable range (b1800 r/min), the higher impeller speed
GB-T4757-2001, the impeller speed, the pulp concentration and air could not only effectively separated fine mud from high ash and com-
flow rate are respectively determined as 1800 r/min, 100 g/L and bustible matter, but also separated it from coal slimes particles. The
 Y. Cai et al. / Powder Technology 353 (2019) 20–26 23

Fig. 4. Influence of impeller speed on flotation effects.

Fig. 3. XPS PEAK fitting results.

hydrophobicity of coal particles surface thus became strong. Besides, difficult for the mineral pulp surface to form a stable foam layer. There-
the higher mixing speed could effectively promote the dispersion of fore, the yield of clean coal and flotation perfection would decrease,
collecting agent in the pulp to form small droplets that adsorbed on whereas ash content increases. On the other hand, there were many mi-
the surface of coal slimes particles. As a result, the effects of flotation cropores on the surface of oxidized coal slimes. The higher impeller
would be improved. In addition, with the increase of impeller speed, speed made the water-filled coal particle surface to form hydrated
the bubbles generated were small, dense, and foamy, with a stable film and reduced the hydrophobicity of the coal particle surfaces.
foam layer. We found that the smaller and more stable the bubble Because clean coal ash content increases with impeller speed, the higher
size, the more efficient the flotation. However, when the impeller impeller speed would cause fine mud to be wrapped around the surface
speed exceeded 1800 r/min, with the increase of impeller speed, the of coal slimes particle to form a reunion phenomenon. Thus, thee fine
flotation indexes decreased. The reasons for this phenomenon were as mud would exist in the foam layer, which causes difficulty in separating
follows: on the one hand, high-intensity impelling would cause the minerals and combustible matter. Eventually, the ash content of clean
coal slimes particles adsorbed on the bubbles to fall off, making it coal showed an increasing trend.

Table 4
C1s fitting data of fresh and oxidized coal slimes.

Fitting Functional Central binding Half-peak bandwidth/eV Area/cps·eV Relative content/%

range/eV groups energy/eV
Fresh coal Oxidized coal Fresh coal Oxidized coal Fresh coal Oxidized coal
slimes slimes slimes slimes slimes slimes

C–C/C–H 284.60 1.37 1.57 17,190.27 12,324.23 49.26 41.34

281–290 C–O–C/C–O–H 285.80 2.06 2.11 14,134.83 12,533.68 40.50 42.05
C=O 287.80 1.66 1.79 2806.62 3892.60 8.04 13.06
O=C-O 289.20 1.39 1.38 767.94 1059.43 2.20 3.55
24 Y. Cai et al. / Powder Technology 353 (2019) 20–26

Second, we studied the influence of pulp concentration on flotation flotation, and so did the ash content in clean coal. When the pulp con-
effects. In general, the floating pulp concentration is 60 g/L–120 g/L in centration was higher than 60 g/L, the flotation clean coal yield, the re-
preparation plants. The specific parameters are determined according covery of combustible matter, and flotation perfection began to decline
to the particle-size distribution and mineral types of raw coal. Because significantly. This was because the interference degree between coal
the coal slimes pulp used in this test was of fine size and had serious particles and air bubbles was also increasing. Therefore, according to
mud deposition, and the minerals were mainly composed of clay, the the comprehensive indexes, although excessive pulp concentration
pulp concentration should not be too high. Therefore, the concentration could improve the treatment quantity of primary flotation, it would af-
range of flotation was chosen in the range of 40 g/L–100 g/L. The impel- fect the quality of clean coal and reduce the flotation effects. As a result,
ler speed of the flotation machine was fixed at 1800 r/min and the air when the fine mud content in the original coal slimes is higher, the
flow rate was 0.25 m3/(m2·min). Fig. 5(a) and (b) reflects the flotation lower pulp concentration should be selected.
effects under different pulp concentration. Finally, the influence of air flow rate on flotation effects was studied.
In the experiments, the clean coal yield increased with the increase In these experiments, the impeller speed was set as 1800 r/min and
of pulp concentration. It could be seen that when the pulp concentration pulp concentration as 60 g/L. The flotation effects under different air
increased, the reagent concentration would also increase in pulp per flow rates are shown in Fig. 6(a) and (b). From Fig. 6(a) and (b), it
unit volume relatively. Thus, more coal particles in the pulp were able could be concluded that with the increase of air flow rate, both the
to establish contact with bubbles, thereby improving the clean coal clean coal yield and the combustible matter recovery rose, and reached
yield. With the increase of pulp concentration, the ash content in the flo- the maximum value at 0.25 m3/(m2·min). As the number of bubbles in
tation clean coal initially increased first firstly but then decreased. When pulp increased with the air flow rate, the collision probability between
the pulp concentration was low, the clean coal might mix with tail coal bubbles and clean coal would be improved to increase the clean coal
due to the shortage of regent dosage. Furthermore, the content of high- yield. However, when the air flow rate of the flotation machine was
ash fine mud attached to the air bubbles did not reduce, so the clean coal too high, the bubbles within a large volume would reduce the mineral-
ash was relatively high during flotation. As the pulp concentration in- ization degree of the pulp. Further, the recovery rate of combustible
creased gradually, the content of high ash materials decreased during matter in flotation would decrease. In addition, the higher air flow

Fig. 5. Influence of pulp concentration on flotation effects. Fig. 6. Influence of air flow rate on flotation effects.
 Y. Cai et al. / Powder Technology 353 (2019) 20–26 25

of clean coal reaches 42.89%. This is mainly because the increase of

pulp concentration leads to more reagent in pulp per unit, which results
into more coal slimes particles contacting with bubbles. Another factor
is air flow rate; with the increase of air flow rate, the number of bubbles
increase. All the flotation indexes will increase first and then decrease.
When air flow rate is 0.25 m3/(m2·min), the clean coal yield reaches
the maximum value. However, if the air flow rate is oversized to pro-
duce bubbles of larger diameter, the flotation indexes worsen. To sum
up, for the oxidized coal slimes from Shuilian mine, the best flotation
conditions are determined as follows: impeller speed, 1800 r/min;
pulp concentration, 60 g/L; and air flow rate, 0.25 m3/(m2·min).

Acknowledgments

This work was supported by National natural science foundation of

China (No. 41672154). We also thank SWAN Editorial Services for im-
proving the language of this paper.
Fig. 7. Ash content comparison between initial coal slimes and clean coal.
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