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A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles
Journal Pre-proofs A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles Shaoqi Zhou, Xuexia Wang, Xiangning Bu, Mengdie Wang, Bairui An, Huaizhi Shao, Chao Ni, Yaoli Peng, Guangyuan Xie PII: DOI: Reference:
S1350-4177(19)31559-7 https://doi.org/10.1016/j.ultsonch.2020.105005 ULTSON 105005
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
3 October 2019 6 February 2020 6 February 2020
Please cite this article as: S. Zhou, X. Wang, X. Bu, M. Wang, B. An, H. Shao, C. Ni, Y. Peng, G. Xie, A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles, Ultrasonics Sonochemistry (2020), doi: https://doi.org/10.1016/j.ultsonch.2020.105005
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3
A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles
4 5
Shaoqi Zhou, Xuexia Wang, Xiangning Bu*, Mengdie Wang, Bairui An, Huaizhi Shao,
6
Chao Ni, Yaoli Peng, Guangyuan Xie,
1 2
7
Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education,
8
School of Chemical Engineering and Technology, China University of Mining and
9
Technology, Xuzhou 221116, Jiangsu, China
10
Abstract
11
In this paper, a novel flotation technique that combines nano-scale bubbles generated by
12
hydrodynamic cavitation (HC) and carrier flotation is proposed to promote the flotation
13
efficiency of a high-ash (43%) ultra-fine coal sample (45 µm). We investigated the
14
mechanism by which cavitation bubbles enhance the separation efficiency of carrier
15
flotation using focused beam reflectance measurements, polarizing microscopy, and
16
extended Derjaguin–Landau–Verwey–Overbeek theory. The carrier particles (polystyrene
17
(PS)) and fine coal were pre-treated in a venturi tube and then floated in a laboratory
18
mechanical flotation cell. The flotation results indicate that the presence of cavitation
19
bubbles significantly improved the carrier flotation performance of high-ash ultra-fine coal.
20
This improvement was attributed to the presence of highly hydrophobic PS, which creates
21
additional gas nuclei in the flotation system. The nano-bubbles, which were produced by
22
the venturi tube and adhered to the fine coal particle surfaces, were conducive to the
23
agglomeration of fine coal particles into large aggregates. Moreover, the nano-bubbles
24
functioned as “bridges” of interaction between the carrier particles and large aggregates of
25
fine coal particles. This paper mainly focused on the effect of carrier (PS) and HC on high-
26
ash fine coal. The influence of different HC intensities on carrier (PS) flotation was
27
discussed. Two models for the interactions between the coal particles, nano-bubbles, and
28
PS during cavitation were proposed and were proved using the E-DLVO theory.
29
Keywords:
30
Hydrodynamic cavitation; Nano-bubble; Carrier flotation; Ultra-fine particle; Aggregate
31
1 Introduction
32
Coal is an important fossil energy resource worldwide, especially in China. China’s energy
33
mix is dominated by coal, but coal resources have not been used effectively, especially
34
those of fine coal [1, 2]. As fine particles have low collision efficiencies with gas bubbles
35
and float slowly, a large number of fine particles are discharged in tailings without effective
36
separation [2, 3]. To solve the problem of fine coal separation, scholars have conducted
37
significant research into two main methods: increasing the apparent size of particles, and
38
reducing the size of bubbles [3].
39
In the first method, there are several ways to increase the apparent particle size, such as
40
agglomerate flotation, selective flocculation, oil flotation, and carrier flotation. Carrier
41
flotation can be regarded as hydrophobic flocculation between finer and coarser
42
hydrophobic particles, which are then recovered via conventional flotation [4]. Carrier
43
flotation has been successfully used as an effective method for recovering fine particles.
44
Fuerstenau, et al. [5] found that compared to conventional flocculation-flotation, the yield
45
of fine hematite using coarse hematite as a carrier was 10% higher. Ateşok, et al. [6] used
46
coarse coal with good buoyancy as a carrier for the flotation of fine coal with poor
47
buoyancy. The study showed that the best flotation effect was achieved at a carrier ratio of
48
2%. Zhang, et al. [7] used polystyrene (PS) particles (90–150 μm) as carriers in the flotation
49
of Smithsonite (d50 = 10.25 μm), which improved the recovery of the latter.
50
Among the methods for reducing bubble size, hydrodynamic cavitation (HC) is one of the
51
least expensive and most energy-efficient ways to generate nano-bubbles. Nano-bubbles
52
produced using HC can improve the flotation performance of fine particles and reduce
53
reagent consumption [8]. Further, they can promote the agglomeration of fine ore and
54
improve its flotation efficiency [9-11]. The behavior and formation of nano-bubbles on
55
smooth surfaces have been reviewed [12, 13]. In the tapered convergence zone of the
56
venturi, liquid flow is accelerated as the pipe diameter narrows. The flow rate of liquid in
57
the throat is higher than that in the inlet, and the pressure is lower than that in the inlet,
58
which leads to cavitation[8].
59
Nano-bubbles are gas cavities with diameters less than 1 μm and are co-produced with
60
larger bubbles using HC [14]. Nano-bubbles have the advantages of large specific surface
61
area, high concentration, long stability, and strong hydrophobicity. When fine minerals are
62
pre-treated with nano-bubbles, the latter are adsorbed on the surfaces of the former, which
63
reduces the induction time between the large bubbles and ultra-fine coal particles in the
64
flotation machine and improves collection efficiency [15, 16]. Meanwhile, the bubbles on
65
the surfaces of the fine particles promote the aggregation of the latter and form bubble-
66
particle aggregates, thereby improving the apparent particle size and increasing collision
67
efficiency [17]. Finally, the flotation velocity of fine particles can be significantly increased.
68
Zhou, et al. [9] proved that particle aggregation is influenced by particle hydrophobicity,
69
number of nano-bubbles generated, and hydrodynamic forces resulting from HC. Calgaroto,
70
et al. [15] demonstrated that a venturi tube can improve the probability of hydrophobic
71
particle aggregation and bubble-particle collision in the production of nano-bubbles by HC.
72
Madanshetty, et al. [18] examined the promotion effect of solid particles on cavitation by
73
utilizing PS microparticles.
74
Polystyrene is a synthetic aromatic polymer made from styrene monomers and has a
75
general chemical formula of (C8H8)n. The long hydrocarbon chain with alternating carbon
76
centers attached to phenyl groups renders PS hydrophobic, which makes it difficult to
77
disperse in water [7]. Hence, PS particles float on the slurry surface during flotation, and
78
cannot function well as carriers. To solve this problem, we combined HC with carrier
79
flotation of high-ash fine coal in this study, and achieved good results. The change in
80
particle size was observed using focused beam reflectance measurements (FBRMs) and
81
polarizing microscopy (PM). Two models of contact between the particles and bubbles
82
were proposed, and the interaction between them was calculated based on the extended
83
Derjaguin–Landau–Verwey–Overbeek (E-DLVO) theory.
84
2 Experimental
85
2.1 Materials
86
The coal samples were procured from Xianyang, Shanxi province, China. Fig. 1 shows the
87
XRD patterns of the coal sample. The main mineral of the coal sample is quartz. A wet
88
sieve was used to obtain a fraction of 45 μm (43% ash content). Polystyrene granules were
89
purchased from Shunjie Plastic Technology Co., China. The mean size of the PS particles
90
was about 125 μm. To ensure that the particle size of PS was greater than that of the sample
91
coal, the PS particles were screened through a 74 μm sieve. Both ultra-fine coal and PS
92
were dispersed into alcohol (5 g/L) and then measured using the FBRM G400 particle size
93
analyzer (Mettler-Toledo Ltd., Redmond, WA, USA). Fig. 2 presents the diagram of
94
FBRM test system. The chord length can be defined as a line segment whose endpoints are
95
located on the outer surface of any shape [19]. Fig. 3 gives the particle size distributions of
96
ultra-fine coal and PS obtained at 2, 4, 6, and 8 min. The particle size distribution of coal
97
is below 45 μm and PS is mainly above 100 μm, there is a clear boundary between the
98
chord length of coal and PS. Kerosene and sec-octyl alcohol were used as the collector and
99
frother, respectively.
100 101
Fig. 1. XRD patterns of coal
102
103 104 105
Fig. 2. Diagram of FBRM test system
106 107
Fig. 3. Chord length distribution of coal and PS (a: coal, b: PS)
108
2.2 Pre-treatment and flotation procedure
109
A standard laboratory RK/FD-II sub-aeration flotation cell with 1.5 L volume (Wuhan
110
Rock Crush & Grand Equipment Manufacture Co., Ltd) was used in the conventional
111
flotation experiments. Fig. 4 shows the physical diagram of flotation machine. The impeller
112
speed, air flow rate, and flotation time were 1900 r/min, 0.25 L/h, and 3 min, respectively.
113
The effects of collector and frother dosages on the flotation performance of the high-ash
114
coal sample were investigated to obtain an optimal condition. A detailed description of the
115
working process of the mechanical flotation cell is reported in the literature [20].
116 117
Fig. 4. Diagram of flotation machine
118 119
In the carrier flotation tests, PS was used as the “carrier” to further promote the flotation
120
performance of the coal sample. It was difficult to disperse the PS particles in water because
121
of their strong hydrophobicity. Hence, they were pre-treated in an HC system with a throat
122
velocity of 16.50 m/s (Fig. 5). The HC system consisted of a peristaltic pump (TL00-700M),
123
venturi tube, beaker, and 82 # tube. The narrowest part of the venturi was 3 mm in diameter.
124
The average velocity in the venturi tube was calculated according to the narrowest cross
125
section area, and this was designated as the throat velocity [11]. The flow rate of the
126
peristaltic pump at different speeds and throat velocities of the venturi tube are shown in
127
Table 1. The coal particles were then mixed with the PS particles pre-treated for flotation
128
(Fig. 6a). According to the conventional flotation test results, the collector and frother
129
dosages for carrier flotation were set at 3000 and 1500 g/t, respectively. The froth samples
130
were collected at 0.5, 1, 2, and 3 min. After the final froth sample was collected, the
131
machine was stopped. The froth samples and tailings were screened using a 45 μm wet
132
sieve. The other conditions of the carrier flotation tests were identical to those of the
133
conventional flotation tests.
134
Table 1. Venturi tube throat velocity at different peristaltic pump speeds (throat diameter,
135
3 mm) Peristatic pump flow (L/min) 7.00 6.89 6.60
136
Throat velocity (m/s) 16.50 16.25 15.56
137 138 139
Fig. 5. Physical diagram of HC system A detailed description of the novel flotation technique combining an HC system and carrier
140
flotation is given in Fig. 6(b). In this method, the ultra-fine coal particles with particle size
141
less than 45 μm (60 g, -45 μm) were mixed with PS particles (10 g, +74 μm) in 500 mL
142
ultrapure water, and kerosene (225 μL) was added to the mixture, which was then treated
143
to the HC system for 5 min. Before HC pretreatment, the mixture needs to be stirred with
144
glass rod for 1 min to disperse. Next, the mixture was floated in the flotation cell using a
145
flotation procedure identical to that of carrier flotation. The influence of throat velocity
146
(HC intensity) on the separation efficiency of the novel flotation technique was explored.
147
To prove the strengthening effect of HC on the flotation technique developed, the ultra-
148
fine coal samples (60 g) individually mixed with 500 mL ultrapure water. Then the mixture
149
was pre-treated in the HC system for 5 min (Fig. 6(c)), and then floated in conditions
150
identical to those of carrier flotation.
151 152 153 154
Fig. 6. Different pre-treatment and flotation processes using HC system (a: carrier flotation; b: novel flotation; c: pre-treatment of coal particles)
155
The Fuerstenau upgrading curve is a good tool to estimate the flotation separation
156
efficiency [21-25]. Bu, et al. [26] used the curve to compare the separation efficiencies of
157
fine graphite achieved via flotation column and flotation machine. This curve can be fitted
158
using the following equation with a single tunable parameter.
159
R1,C
160
R1,C
161
R2,T
162
where R1,C is the recovery of combustible material in the concentrate, R2,T is the recovery
163
of ash material from tailings. Ac, Ar and At are the ash content of concentrate, raw coal and
164
tailings, respectively. γc and γt is the yield of the concentrate and tailings, respectively. α is
165
the separation efficiency coefficient which could be calculated by MatLab software. Ash
166
contents for froth samples (concentrates) and the tailing were determined according to the
100 R2, T
,
(1)
c (100 Ac )
(2)
100 1 100 Ar
t At 100 Ar
(3)
167
literature [27]. There is no separation if α = 1. When 0 < α < 1, the process of upgrading
168
takes place in the tailings. When α > 1, the concentrates are enriched with valuable minerals.
169
The higher the value of α, the better is the separation result. Separation is most desirable
170
especially when α = 0 or ∞. Therefore, in the present study, we used the Fuerstenau
171
upgrading curves to compare the flotation performances of the different processes of pre-
172
treatment and flotation.
173
2.3 Zeta potential measurement of particles
174
Zeta Plus (Brookhaven, US) was used to measure the zeta potential distributions of coal
175
and PS. First, 0.1 g each of coal and PS were weighed and dispersed in 100 mL of deionized
176
(DI) water. The pH was adjusted using 0.1 mM HCl and 0.1 mM NaOH. Finally, after 1
177
day of precipitation, the supernatants of different pH values were used to measure the zeta
178
potential. In the process of measurement, each sample was tested five times and the average
179
was taken as the zeta potential value. The results are shown in Fig. 7.
180 181 182
Fig. 7. Zeta potential distributions of coal sample and PS at different pH values
183
2.4 Size and zeta potential measurements of nano-bubbles generated via HC
184
The size and zeta potentials of the nano-bubbles were measured using Zeta Plus
185
(Brookhaven, US). Firstly, ultrapure water with 0.1 mM NaCl was treated for 5 min in the
186
HC system at different throat velocities (16.50, 16.25, and 15.56 m/s). Meanwhile,
187
ultrapure water untreated via HC (throat velocity of 0 m/s) was used as the control group.
188
Finally, 1 mL of water was withdrawn from each sample to measure the sizes and zeta
189
potentials of the nano-bubbles immediately. In the process of measurement, each sample
190
was tested five times and the average was taken as the finalvalue.
191
2.5 Focused beam reflectance measurement and Polarizing microscopy
192
Focused bean reflectance measurement was used to measure the change in particle size
193
distribution before and after HC. A total of 12 g of the coal sample and 2 g of PS particles
194
were added to a beaker (500 mL), to which 200 mL of tap water was added, and the mixture
195
was pre-conditioned using a glass rod. Finally, the sample in the beaker was tested via
196
FBRM. During the test, the mixture continuously stirred by magnetic stirrer. When the
197
curve was stable, the sample was withdrawn and treated in the HC system. After 5 min, the
198
sample was monitored via FBRM again. The particle size distribution was recorded until
199
the curve was stable.
200
A polarizing microscope (Sunny Optical-Instrument, Zhejiang, China) was utilized to
201
observe the particle behavior after HC. The PM and FBRM analyses were carried out
202
simultaneously. Following treatment in the HC system, a small number of samples were
203
extracted from the beaker and placed on a glass slide until the water evaporated, and the
204
samples were observed under the microscope.
205
3 Results and discussion
206
3.1 Conventional flotation
207
Fig. 8 shows the effect of collector dosage on the conventional flotation performance. With
208
increase in the dosage of collector, the concentrate yield first increased and then there was
209
a slightly decreased. At a collector dosage of 3000 g/t, the yield of concentrate was the
210
highest (41.99%), ash content of concentrate was 18.74%, and recovery of combustible
211
material was the highest (59.75%). At a collector dosage of 3000 g/t, the experiments were
212
conducted by varying the frother dosage (500, 1000, 1500, and 3000 g/t). The results are
213
presented in Fig. 9. As the frother dosage increased, the concentrate yield first decreased
214
and then increased. At frother dosages of 1500 and 3000 g/t, there was no significant
215
difference between the yields of concentrate but the dosage doubled. At a frother dosage
216
of 1500 g/t, the ash content (18.74%) was obtained. Although the ash content was smaller
217
at a frother of 1000 g/t, the yield of concentrate was very low. Hence, in the next flotation
218
experiment, the dosages of collector and frother were set at 3000 and 1500 g/t, respectively.
219 220 221
Fig. 8. Effect of collector dosage on conventional flotation performance (1500 g/t frother dosage, 3 min flotation time)
222 223
Fig. 9. Effect of frother dosage on conventional flotation performance (3000 g/t collector
224
dosage, 3 min flotation time)
225
3.2 Carrier flotation combined with nano-bubbles
226 227
Table 2. Mean size of nano-bubbles produced at different throat velocities Venturi tube throat velocity (m/s)
16.50
16.25
15.56
0
Mean size of nano-bubbles (nm)
229.65
242.28
261.17
none
228 229
Table 2 shows the nano-bubble sizes as measured using Zeta Plus. No nano-bubbles were
230
detected in the ultrapure water not treated in the HC system. However, in the water samples
231
treated in the HC system, bubble size distribution was detected (i.e., HC produced nano-
232
bubbles), and the mean size of the nano-bubbles generated by the venturi tube decreased
233
gradually with increased throat velocity.
234 235 236
Fig. 10 Bubble size distribution at different throat velocity
237
Fig. 10 shows the bubble size distribution at different throat velocity. With the decrease of
238
cavitation strength, the size distribution of nano-bubble not much of a change. Li, et al.
239
[11] proved that the increase of throat velocity had little effect on the size of micron bubbles,
240
but significantly increased the number of bubbles generated. When the velocity of liquid
241
flow in venturi throat increases, the pressure inside the flow decreases, and the increase in
242
pressure difference is conducive to the formation of nanobubbles[11, 28].
243
Figure 11 shows the results of carrier flotation combined with nano-bubbles. In the
244
experimental results of carrier flotation coupled with cavitation bubbles, the value of α
245
reduced from 4.07 to 3.67 as the venturi throat flow velocity decreased from 16.50 m/s to
246
15.56 m/s. As mentioned in the literature [29], the critical velocity for cavitation of water
247
without additional gas injection is 10–15 m/s. Greater throat velocity significantly
248
enhances cavitation probability [30].
249 250 251
Fig. 11. Synergistic test of HC and carrier flotation at different throat velocities
252
In Fig. 12, the separation efficiencies of conventional flotation (process 1: without HC,
253
without PS), flotation of ultra-fine coal pre-treated via HC (process 2: without PS), carrier
254
flotation (process 3: only PS pre-treated by HC), and carrier-nano-bubble flotation (process
255
4: both ultra-fine coal and PS were pre-treated by HC) are presented. The separation
256
efficiencies (α) of process 1 (conventional flotation) and process 2 (flotation of ultra-fine
257
coal pre-treated via HC) were 2.62 and 3.62, respectively. The flotation efficiency in the
258
case of ultra-fine coal pre-treated using HC was better than that of conventional flotation.
259
The test results proved that the nano-bubbles generated by HC enhanced the flotation
260
efficiency of ultra-fine coal. This was consistent with the results of Zhou, et al. [9]. Nano-
261
bubbles can be used as bridges to connect fine particles and promote ultra-fine coal
262
agglomeration.
263
Figure 12 shows that when PS was introduced as a carrier, the flotation separation
264
efficiency (α) of ultra-fine coal was promoted from 2.62 (conventional flotation) to 2.81.
265
The enhancement of ultra-fine particle flotation by carriers was perhaps mainly owing to
266
the agglomeration of the carriers with the hydrophobic fine particles. This promoted the
267
separation efficiency of fine particles of hydrophobic and hydrophilic materials. By
268
combining the use of carrier and aggregation of ultra-fine particles induced by HC (process
269
4), the enrichment efficiency of fine particles (α = 4.07) was significantly improved over
270
those of other processes.
271
272
273 274 275 276 277
Fig. 12. Fuerstenau upgrading curves of different processes of pre-treatment and flotation (process 1: conventional flotation; process 2: ultra-fine coal pre-treated via HC, throat velocity of 16.50 m/s; process 3: carrier flotation (PS, pre-treated via HC, used as carrier particles, throat velocity of 16.50 m/s); process 4: both PS and ultra-fine particles were pre-treated via HC, throat velocity of 16.50 m/s)
278
3.3 Focused beam reflectance measurement and Polarizing microscopy analysis
279
Fig. 13 shows the variation in the count of ultra-fine particles before and after HC. Prior to
280
the FBRM measurements, both the PS and ultra-fine particles were pre-treated using HC
281
(like in process 4), and the variations in count were observed via FBRM. The count of
282
ultra-fine particles decreased gradually with increase in throat velocity. This result was
283
consistent with the trend of enrichment efficiency of fine coal particles at different throat
284
rotational speeds, as seen in Fig. 11. The reduced fine particles may form aggregates with
285
the PS particles through hydrophobic flocculation, or aggregates among the fine particles
286
may be formed under the action of HC. To explore the mechanism of fine particle reduction,
287
the counts of ultra-fine coal particles before and after cavitation pre-treatment (like in
288
process 2) were explored. Fig. 14 shows that the number of ultra-fine particles in the
289
presence of nano-bubbles and PS particles was significantly lower than that in the presence
290
of nano-bubbles alone. Nano-bubbles can assist ultra-fine coal particles in forming
291
aggregates. The PS particles, pre-treated via HC, functioned as carriers and interacted with
292
the ultra-fine coal particles. Li, et al. [30] proved that treated silica particles can provide
293
solid surfaces with higher hydrophobicity, the tensile strength[31] required for bubble
294
formation decreases, and cavitation becomes easier. In addition, the hydrophobic surface
295
characteristics enhance the voids immersed in the gap, which further promotes nucleation
296
at low tensile strengths. Therefore, PS particles with high hydrophobicity can enhance the
297
nano-bubble production achieved using HC. These nano-bubbles can further promote
298
hydrophobic aggregation between the ultra-fine coal particles. At the same time, nano-
299
bubbles are preferentially formed on the surfaces of hydrophobic particles. Solid particles
300
with rough and hydrophobic surfaces are known to promote bubble formation in liquids
301
[32, 33]. The presence of tiny pockets of undissolved gas in crevices on mineral particles
302
assists cavitation because of the expansion of these gas pockets under negative pressure
303
[34, 35]. Therefore, it can be expected that the nano-bubbles attached to the PS particles
304
can be used as bridges between the ultra-fine coal particles (or aggregates) and carrier
305
particles. The mechanism is demonstrated in Fig. 16 (Model Ⅰ). Fig. 15 presents the
306
photographs of the PM measurements of process 4. The PM results indirectly indicate that
307
the ultra-fine coal particles adhere to the carrier particles through nano-bubbles. The
308
mechanism of particle action in carrier flotation is shown in Fig. 16 (Model Ⅱ).
309
310
311 312 313 314
Fig. 13. Variation in count of ultra-fine particles in process 4 at various throat velocities (δ stands for the reduction in count.)
315 316 317 318
Fig. 14. Counts of ultra-fine particles in processes 2, 3, and 4 (16.50 m/s throat velocity in HC system)
319 320 321
Fig. 15. PM test results
322 323 324 325
Fig. 16. Two models of interaction between particles and nano-bubbles during cavitation (Model Ⅰ: carrier flotation combined with HC pre-treatment; Model ⅠⅠ: PS particles used as carrier)
326
3.4 Interaction between coal and bubbles calculated based on E-DLVO theory
327
The calculation of the force between particles and bubbles has been reported in the
328
literature [36-38]. To explain the two models of agglomeration between the particles and
329
nano-bubbles theoretically, E-DLVO calculations were carried out. According to the E-
330
DLVO theory, the total interaction energy between the particles is equal to the sum of van
331
der Waals energy (EV), double electric layer interaction energy (EE), and hydrophobic
332
interaction energy (EH) [37, 39]. The van der Waals energy can be calculated using the
333
following formula:
334
EV
A132 R1 R2
6 H R1 R2
(5)
,
335
where R1, R2 represent the radii of the two spheres, and H represents the distance between
336
the spheres. In this work, the radius of the nano-bubble is 3.94 nm. A132 represents Hamaker
337
constant,
338
A132
339
where A11 and A22 are the Hamaker constants of the two spheres in vacuum, and A33 is the
340
Hamaker constant of water in vacuum.
341
The double electric layer interaction energy is calculated using the following equations: 2 R R (7) EE 0 1 2 12 2 2 2 1 2 2 p q , R1 R2 1 2
342
343 344
and
its
value
can
be
calculated
using
A11 A33 A22 A33 ,
1 exp H p ln , 1 exp H q ln 1 exp H ,
the
following
formula: (6)
(8) (9)
346
0.304 , (10) cNacl where 𝜑1 𝑎𝑛𝑑 𝜑2 represent the surface potentials of spheres 1 and 2, respectively. 𝜀0 and 𝜀
347
are the values of absolute permittivity in the dispersion medium (78.5 F/m) and vacuum
348
(8.854 × 10-12 F/m), respectively[40]. κ−1 is the Debye length in a 0.1 mM NaCl solution.
349
The hydrophobic interaction energy is calculated as follows:
345
1 =
351
K132 R1 R2 , (11) 6 R1 R2 H where K132 is the hydrophobic constant of the two spheres in the medium and is calculated
352
as follows:
353 354
K132 K131 K 232 , (12) where K131 [41] represents the hydrophobic constant between the nano-bubbles in water,
355
and K232 [42] represents the hydrophobic constant between the nano-bubbles and coal
356
particles in water.
357
Table 3. Parameters used in E-DLVO theory calculation
350
EH
Sphere
Aii (×10-20 J)
Zeta potential (×10-3 V)
Radius (×10-6 m)
Coal particles (1)
6.07
-36.09
22.5
Nano-bubbles (2)
0
-36.20
0.114
Water (3)
4.84
―
―
Hamaker constant (×10-20 J)
Hydrophobic force constant (×10-20 J) K131:1.74
A132: 4.84
K232:1.00 K132:4.17
358
The E-DLVO interaction energies of the two models are shown in Fig. 17. The absolute
359
value of the total energy between nano-bubbles and nano-bubbles is always greater than
360
that between nano-bubbles and ultra-fine coal particles. The energy between the nano-
361
bubbles and ultra-fine coal particles is always negative, which represents attraction.
362
Therefore, the coupling of carrier and nano-bubbles facilitates the formation of aggregates.
363
Hence, process 4 is better than the other processes.
364 365
Fig. 17. E-DLVO interaction energies of different models
366
4 Conclusion
367
The experimental results showed that the coupling of PS carrier and HC improved the
368
flotation of high-ash fine coal. The nano-bubbles generated on the surfaces of the
369
hydrophobic particles during cavitation promoted the aggregation of ultra-fine coal
370
particles, the attachment between ultra-fine coal particles and PS particles. Moreover, the
371
flotation results improved with cavitation intensity. The absolute value of total energy
372
between nano-bubbles and nano-bubbles (Model Ⅰ) is always greater than that between
373
nano-bubbles and ultra-fine coal particles (Model Ⅱ). Model Ⅰ could significantly improve
374
the flotation efficiency of ultra-fine coal.
375
Acknowledgement
376
The authors gratefully acknowledge the financial support from the Project funded by
377
China Postdoctoral Science Foundation (No. 2019M652024).
378
5 References
379 380 381 382 383 384 385
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cavitation for the generation of microbubbles, Minerals Engineering, 132 (2019) 268-274. [31] M. Li, Influence of Venturi Tube Geometry and Particle Properties on the Hydrodynamic Cavitation for Fine Particle Flotation, (2017). [32] W.A. Gerth, E.A. Hemmingsen, Heterogeneous nucleation of bubbles at solid surfaces in gassupersaturated aqueous solutions, Journal of colloid interface science, 74 (1980) 80-89. [33] W.L. Ryan, E.A. Hemmingsen, Bubble formation in water at smooth hydrophobic surfaces, Journal of colloid interface science, 157 (1993) 312-317. [34] H. Flynn, Physics of acoustic cavitation in liquids, Physical acoustics, 1 (1964) 57-172. [35] Y. Finkelstein, A. Tamir, Formation of gas bubbles in supersaturated solutions of gases in water, AIChE journal, 31 (1985) 1409-1419. [36] J. Piñeres, J. Barraza, Energy barrier of aggregates coal particle–bubble through the extended DLVO theory, International Journal of Mineral Processing, 93 (2011) 14-20. [37] R.H. Yoon, The role of hydrodynamic and surface forces in bubble–particle interaction, International Journal of Mineral Processing, 58 (2000) 129-143. [38] B.S. Aksoy, M. Richey, Davis, H. Gerald, Luttrell, Hydrophobic forces in free thin films of water in the presence and absence of surfactants, (1997). [39] D. Li, W. Yin, Q. Liu, S. Cao, Q. Sun, C. Zhao, J. Yao, Interactions between fine and coarse hematite particles in aqueous suspension and their implications for flotation, Minerals Engineering, 114 (2017) 74-81. [40] C. Li, Y. Xing, X. Gui, Y. Cao, Enhancement of oxidized coal flotation by preconditioning with positivecharged microbubbles, International Journal of Coal Preparation and Utilization, (2017) 1-11. [41] B.S. Aksoy, Hydrophobic forces in free thin films of water in the presence and absence of surfactants, in, Virginia Tech, 1997. [42] R.-H. Yoon, S. Ravishankar, Long-range hydrophobic forces between mica surfaces in dodecylammonium chloride solutions in the presence of dodecanol, Journal of colloid interface science, 179 (1996) 391-402.
468 469 470 471 472 473 474 475
Highlights
476 477 478 479 480 481 482 483
Declaration of interests
Combined cavitation bubbles and carrier flotation to upgrade ultra-fine particles. Utilized FBRM and PM techniques to directly observe the strengthening mechanisms. Applied E-DLVO theory to evaluate the interaction energy for the novel process.
☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
484 485 486 487 488
S1350-4177(19)31559-7 https://doi.org/10.1016/j.ultsonch.2020.105005 ULTSON 105005
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
3 October 2019 6 February 2020 6 February 2020
Please cite this article as: S. Zhou, X. Wang, X. Bu, M. Wang, B. An, H. Shao, C. Ni, Y. Peng, G. Xie, A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles, Ultrasonics Sonochemistry (2020), doi: https://doi.org/10.1016/j.ultsonch.2020.105005
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3
A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles
4 5
Shaoqi Zhou, Xuexia Wang, Xiangning Bu*, Mengdie Wang, Bairui An, Huaizhi Shao,
6
Chao Ni, Yaoli Peng, Guangyuan Xie,
1 2
7
Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education,
8
School of Chemical Engineering and Technology, China University of Mining and
9
Technology, Xuzhou 221116, Jiangsu, China
10
Abstract
11
In this paper, a novel flotation technique that combines nano-scale bubbles generated by
12
hydrodynamic cavitation (HC) and carrier flotation is proposed to promote the flotation
13
efficiency of a high-ash (43%) ultra-fine coal sample (45 µm). We investigated the
14
mechanism by which cavitation bubbles enhance the separation efficiency of carrier
15
flotation using focused beam reflectance measurements, polarizing microscopy, and
16
extended Derjaguin–Landau–Verwey–Overbeek theory. The carrier particles (polystyrene
17
(PS)) and fine coal were pre-treated in a venturi tube and then floated in a laboratory
18
mechanical flotation cell. The flotation results indicate that the presence of cavitation
19
bubbles significantly improved the carrier flotation performance of high-ash ultra-fine coal.
20
This improvement was attributed to the presence of highly hydrophobic PS, which creates
21
additional gas nuclei in the flotation system. The nano-bubbles, which were produced by
22
the venturi tube and adhered to the fine coal particle surfaces, were conducive to the
23
agglomeration of fine coal particles into large aggregates. Moreover, the nano-bubbles
24
functioned as “bridges” of interaction between the carrier particles and large aggregates of
25
fine coal particles. This paper mainly focused on the effect of carrier (PS) and HC on high-
26
ash fine coal. The influence of different HC intensities on carrier (PS) flotation was
27
discussed. Two models for the interactions between the coal particles, nano-bubbles, and
28
PS during cavitation were proposed and were proved using the E-DLVO theory.
29
Keywords:
30
Hydrodynamic cavitation; Nano-bubble; Carrier flotation; Ultra-fine particle; Aggregate
31
1 Introduction
32
Coal is an important fossil energy resource worldwide, especially in China. China’s energy
33
mix is dominated by coal, but coal resources have not been used effectively, especially
34
those of fine coal [1, 2]. As fine particles have low collision efficiencies with gas bubbles
35
and float slowly, a large number of fine particles are discharged in tailings without effective
36
separation [2, 3]. To solve the problem of fine coal separation, scholars have conducted
37
significant research into two main methods: increasing the apparent size of particles, and
38
reducing the size of bubbles [3].
39
In the first method, there are several ways to increase the apparent particle size, such as
40
agglomerate flotation, selective flocculation, oil flotation, and carrier flotation. Carrier
41
flotation can be regarded as hydrophobic flocculation between finer and coarser
42
hydrophobic particles, which are then recovered via conventional flotation [4]. Carrier
43
flotation has been successfully used as an effective method for recovering fine particles.
44
Fuerstenau, et al. [5] found that compared to conventional flocculation-flotation, the yield
45
of fine hematite using coarse hematite as a carrier was 10% higher. Ateşok, et al. [6] used
46
coarse coal with good buoyancy as a carrier for the flotation of fine coal with poor
47
buoyancy. The study showed that the best flotation effect was achieved at a carrier ratio of
48
2%. Zhang, et al. [7] used polystyrene (PS) particles (90–150 μm) as carriers in the flotation
49
of Smithsonite (d50 = 10.25 μm), which improved the recovery of the latter.
50
Among the methods for reducing bubble size, hydrodynamic cavitation (HC) is one of the
51
least expensive and most energy-efficient ways to generate nano-bubbles. Nano-bubbles
52
produced using HC can improve the flotation performance of fine particles and reduce
53
reagent consumption [8]. Further, they can promote the agglomeration of fine ore and
54
improve its flotation efficiency [9-11]. The behavior and formation of nano-bubbles on
55
smooth surfaces have been reviewed [12, 13]. In the tapered convergence zone of the
56
venturi, liquid flow is accelerated as the pipe diameter narrows. The flow rate of liquid in
57
the throat is higher than that in the inlet, and the pressure is lower than that in the inlet,
58
which leads to cavitation[8].
59
Nano-bubbles are gas cavities with diameters less than 1 μm and are co-produced with
60
larger bubbles using HC [14]. Nano-bubbles have the advantages of large specific surface
61
area, high concentration, long stability, and strong hydrophobicity. When fine minerals are
62
pre-treated with nano-bubbles, the latter are adsorbed on the surfaces of the former, which
63
reduces the induction time between the large bubbles and ultra-fine coal particles in the
64
flotation machine and improves collection efficiency [15, 16]. Meanwhile, the bubbles on
65
the surfaces of the fine particles promote the aggregation of the latter and form bubble-
66
particle aggregates, thereby improving the apparent particle size and increasing collision
67
efficiency [17]. Finally, the flotation velocity of fine particles can be significantly increased.
68
Zhou, et al. [9] proved that particle aggregation is influenced by particle hydrophobicity,
69
number of nano-bubbles generated, and hydrodynamic forces resulting from HC. Calgaroto,
70
et al. [15] demonstrated that a venturi tube can improve the probability of hydrophobic
71
particle aggregation and bubble-particle collision in the production of nano-bubbles by HC.
72
Madanshetty, et al. [18] examined the promotion effect of solid particles on cavitation by
73
utilizing PS microparticles.
74
Polystyrene is a synthetic aromatic polymer made from styrene monomers and has a
75
general chemical formula of (C8H8)n. The long hydrocarbon chain with alternating carbon
76
centers attached to phenyl groups renders PS hydrophobic, which makes it difficult to
77
disperse in water [7]. Hence, PS particles float on the slurry surface during flotation, and
78
cannot function well as carriers. To solve this problem, we combined HC with carrier
79
flotation of high-ash fine coal in this study, and achieved good results. The change in
80
particle size was observed using focused beam reflectance measurements (FBRMs) and
81
polarizing microscopy (PM). Two models of contact between the particles and bubbles
82
were proposed, and the interaction between them was calculated based on the extended
83
Derjaguin–Landau–Verwey–Overbeek (E-DLVO) theory.
84
2 Experimental
85
2.1 Materials
86
The coal samples were procured from Xianyang, Shanxi province, China. Fig. 1 shows the
87
XRD patterns of the coal sample. The main mineral of the coal sample is quartz. A wet
88
sieve was used to obtain a fraction of 45 μm (43% ash content). Polystyrene granules were
89
purchased from Shunjie Plastic Technology Co., China. The mean size of the PS particles
90
was about 125 μm. To ensure that the particle size of PS was greater than that of the sample
91
coal, the PS particles were screened through a 74 μm sieve. Both ultra-fine coal and PS
92
were dispersed into alcohol (5 g/L) and then measured using the FBRM G400 particle size
93
analyzer (Mettler-Toledo Ltd., Redmond, WA, USA). Fig. 2 presents the diagram of
94
FBRM test system. The chord length can be defined as a line segment whose endpoints are
95
located on the outer surface of any shape [19]. Fig. 3 gives the particle size distributions of
96
ultra-fine coal and PS obtained at 2, 4, 6, and 8 min. The particle size distribution of coal
97
is below 45 μm and PS is mainly above 100 μm, there is a clear boundary between the
98
chord length of coal and PS. Kerosene and sec-octyl alcohol were used as the collector and
99
frother, respectively.
100 101
Fig. 1. XRD patterns of coal
102
103 104 105
Fig. 2. Diagram of FBRM test system
106 107
Fig. 3. Chord length distribution of coal and PS (a: coal, b: PS)
108
2.2 Pre-treatment and flotation procedure
109
A standard laboratory RK/FD-II sub-aeration flotation cell with 1.5 L volume (Wuhan
110
Rock Crush & Grand Equipment Manufacture Co., Ltd) was used in the conventional
111
flotation experiments. Fig. 4 shows the physical diagram of flotation machine. The impeller
112
speed, air flow rate, and flotation time were 1900 r/min, 0.25 L/h, and 3 min, respectively.
113
The effects of collector and frother dosages on the flotation performance of the high-ash
114
coal sample were investigated to obtain an optimal condition. A detailed description of the
115
working process of the mechanical flotation cell is reported in the literature [20].
116 117
Fig. 4. Diagram of flotation machine
118 119
In the carrier flotation tests, PS was used as the “carrier” to further promote the flotation
120
performance of the coal sample. It was difficult to disperse the PS particles in water because
121
of their strong hydrophobicity. Hence, they were pre-treated in an HC system with a throat
122
velocity of 16.50 m/s (Fig. 5). The HC system consisted of a peristaltic pump (TL00-700M),
123
venturi tube, beaker, and 82 # tube. The narrowest part of the venturi was 3 mm in diameter.
124
The average velocity in the venturi tube was calculated according to the narrowest cross
125
section area, and this was designated as the throat velocity [11]. The flow rate of the
126
peristaltic pump at different speeds and throat velocities of the venturi tube are shown in
127
Table 1. The coal particles were then mixed with the PS particles pre-treated for flotation
128
(Fig. 6a). According to the conventional flotation test results, the collector and frother
129
dosages for carrier flotation were set at 3000 and 1500 g/t, respectively. The froth samples
130
were collected at 0.5, 1, 2, and 3 min. After the final froth sample was collected, the
131
machine was stopped. The froth samples and tailings were screened using a 45 μm wet
132
sieve. The other conditions of the carrier flotation tests were identical to those of the
133
conventional flotation tests.
134
Table 1. Venturi tube throat velocity at different peristaltic pump speeds (throat diameter,
135
3 mm) Peristatic pump flow (L/min) 7.00 6.89 6.60
136
Throat velocity (m/s) 16.50 16.25 15.56
137 138 139
Fig. 5. Physical diagram of HC system A detailed description of the novel flotation technique combining an HC system and carrier
140
flotation is given in Fig. 6(b). In this method, the ultra-fine coal particles with particle size
141
less than 45 μm (60 g, -45 μm) were mixed with PS particles (10 g, +74 μm) in 500 mL
142
ultrapure water, and kerosene (225 μL) was added to the mixture, which was then treated
143
to the HC system for 5 min. Before HC pretreatment, the mixture needs to be stirred with
144
glass rod for 1 min to disperse. Next, the mixture was floated in the flotation cell using a
145
flotation procedure identical to that of carrier flotation. The influence of throat velocity
146
(HC intensity) on the separation efficiency of the novel flotation technique was explored.
147
To prove the strengthening effect of HC on the flotation technique developed, the ultra-
148
fine coal samples (60 g) individually mixed with 500 mL ultrapure water. Then the mixture
149
was pre-treated in the HC system for 5 min (Fig. 6(c)), and then floated in conditions
150
identical to those of carrier flotation.
151 152 153 154
Fig. 6. Different pre-treatment and flotation processes using HC system (a: carrier flotation; b: novel flotation; c: pre-treatment of coal particles)
155
The Fuerstenau upgrading curve is a good tool to estimate the flotation separation
156
efficiency [21-25]. Bu, et al. [26] used the curve to compare the separation efficiencies of
157
fine graphite achieved via flotation column and flotation machine. This curve can be fitted
158
using the following equation with a single tunable parameter.
159
R1,C
160
R1,C
161
R2,T
162
where R1,C is the recovery of combustible material in the concentrate, R2,T is the recovery
163
of ash material from tailings. Ac, Ar and At are the ash content of concentrate, raw coal and
164
tailings, respectively. γc and γt is the yield of the concentrate and tailings, respectively. α is
165
the separation efficiency coefficient which could be calculated by MatLab software. Ash
166
contents for froth samples (concentrates) and the tailing were determined according to the
100 R2, T
,
(1)
c (100 Ac )
(2)
100 1 100 Ar
t At 100 Ar
(3)
167
literature [27]. There is no separation if α = 1. When 0 < α < 1, the process of upgrading
168
takes place in the tailings. When α > 1, the concentrates are enriched with valuable minerals.
169
The higher the value of α, the better is the separation result. Separation is most desirable
170
especially when α = 0 or ∞. Therefore, in the present study, we used the Fuerstenau
171
upgrading curves to compare the flotation performances of the different processes of pre-
172
treatment and flotation.
173
2.3 Zeta potential measurement of particles
174
Zeta Plus (Brookhaven, US) was used to measure the zeta potential distributions of coal
175
and PS. First, 0.1 g each of coal and PS were weighed and dispersed in 100 mL of deionized
176
(DI) water. The pH was adjusted using 0.1 mM HCl and 0.1 mM NaOH. Finally, after 1
177
day of precipitation, the supernatants of different pH values were used to measure the zeta
178
potential. In the process of measurement, each sample was tested five times and the average
179
was taken as the zeta potential value. The results are shown in Fig. 7.
180 181 182
Fig. 7. Zeta potential distributions of coal sample and PS at different pH values
183
2.4 Size and zeta potential measurements of nano-bubbles generated via HC
184
The size and zeta potentials of the nano-bubbles were measured using Zeta Plus
185
(Brookhaven, US). Firstly, ultrapure water with 0.1 mM NaCl was treated for 5 min in the
186
HC system at different throat velocities (16.50, 16.25, and 15.56 m/s). Meanwhile,
187
ultrapure water untreated via HC (throat velocity of 0 m/s) was used as the control group.
188
Finally, 1 mL of water was withdrawn from each sample to measure the sizes and zeta
189
potentials of the nano-bubbles immediately. In the process of measurement, each sample
190
was tested five times and the average was taken as the finalvalue.
191
2.5 Focused beam reflectance measurement and Polarizing microscopy
192
Focused bean reflectance measurement was used to measure the change in particle size
193
distribution before and after HC. A total of 12 g of the coal sample and 2 g of PS particles
194
were added to a beaker (500 mL), to which 200 mL of tap water was added, and the mixture
195
was pre-conditioned using a glass rod. Finally, the sample in the beaker was tested via
196
FBRM. During the test, the mixture continuously stirred by magnetic stirrer. When the
197
curve was stable, the sample was withdrawn and treated in the HC system. After 5 min, the
198
sample was monitored via FBRM again. The particle size distribution was recorded until
199
the curve was stable.
200
A polarizing microscope (Sunny Optical-Instrument, Zhejiang, China) was utilized to
201
observe the particle behavior after HC. The PM and FBRM analyses were carried out
202
simultaneously. Following treatment in the HC system, a small number of samples were
203
extracted from the beaker and placed on a glass slide until the water evaporated, and the
204
samples were observed under the microscope.
205
3 Results and discussion
206
3.1 Conventional flotation
207
Fig. 8 shows the effect of collector dosage on the conventional flotation performance. With
208
increase in the dosage of collector, the concentrate yield first increased and then there was
209
a slightly decreased. At a collector dosage of 3000 g/t, the yield of concentrate was the
210
highest (41.99%), ash content of concentrate was 18.74%, and recovery of combustible
211
material was the highest (59.75%). At a collector dosage of 3000 g/t, the experiments were
212
conducted by varying the frother dosage (500, 1000, 1500, and 3000 g/t). The results are
213
presented in Fig. 9. As the frother dosage increased, the concentrate yield first decreased
214
and then increased. At frother dosages of 1500 and 3000 g/t, there was no significant
215
difference between the yields of concentrate but the dosage doubled. At a frother dosage
216
of 1500 g/t, the ash content (18.74%) was obtained. Although the ash content was smaller
217
at a frother of 1000 g/t, the yield of concentrate was very low. Hence, in the next flotation
218
experiment, the dosages of collector and frother were set at 3000 and 1500 g/t, respectively.
219 220 221
Fig. 8. Effect of collector dosage on conventional flotation performance (1500 g/t frother dosage, 3 min flotation time)
222 223
Fig. 9. Effect of frother dosage on conventional flotation performance (3000 g/t collector
224
dosage, 3 min flotation time)
225
3.2 Carrier flotation combined with nano-bubbles
226 227
Table 2. Mean size of nano-bubbles produced at different throat velocities Venturi tube throat velocity (m/s)
16.50
16.25
15.56
0
Mean size of nano-bubbles (nm)
229.65
242.28
261.17
none
228 229
Table 2 shows the nano-bubble sizes as measured using Zeta Plus. No nano-bubbles were
230
detected in the ultrapure water not treated in the HC system. However, in the water samples
231
treated in the HC system, bubble size distribution was detected (i.e., HC produced nano-
232
bubbles), and the mean size of the nano-bubbles generated by the venturi tube decreased
233
gradually with increased throat velocity.
234 235 236
Fig. 10 Bubble size distribution at different throat velocity
237
Fig. 10 shows the bubble size distribution at different throat velocity. With the decrease of
238
cavitation strength, the size distribution of nano-bubble not much of a change. Li, et al.
239
[11] proved that the increase of throat velocity had little effect on the size of micron bubbles,
240
but significantly increased the number of bubbles generated. When the velocity of liquid
241
flow in venturi throat increases, the pressure inside the flow decreases, and the increase in
242
pressure difference is conducive to the formation of nanobubbles[11, 28].
243
Figure 11 shows the results of carrier flotation combined with nano-bubbles. In the
244
experimental results of carrier flotation coupled with cavitation bubbles, the value of α
245
reduced from 4.07 to 3.67 as the venturi throat flow velocity decreased from 16.50 m/s to
246
15.56 m/s. As mentioned in the literature [29], the critical velocity for cavitation of water
247
without additional gas injection is 10–15 m/s. Greater throat velocity significantly
248
enhances cavitation probability [30].
249 250 251
Fig. 11. Synergistic test of HC and carrier flotation at different throat velocities
252
In Fig. 12, the separation efficiencies of conventional flotation (process 1: without HC,
253
without PS), flotation of ultra-fine coal pre-treated via HC (process 2: without PS), carrier
254
flotation (process 3: only PS pre-treated by HC), and carrier-nano-bubble flotation (process
255
4: both ultra-fine coal and PS were pre-treated by HC) are presented. The separation
256
efficiencies (α) of process 1 (conventional flotation) and process 2 (flotation of ultra-fine
257
coal pre-treated via HC) were 2.62 and 3.62, respectively. The flotation efficiency in the
258
case of ultra-fine coal pre-treated using HC was better than that of conventional flotation.
259
The test results proved that the nano-bubbles generated by HC enhanced the flotation
260
efficiency of ultra-fine coal. This was consistent with the results of Zhou, et al. [9]. Nano-
261
bubbles can be used as bridges to connect fine particles and promote ultra-fine coal
262
agglomeration.
263
Figure 12 shows that when PS was introduced as a carrier, the flotation separation
264
efficiency (α) of ultra-fine coal was promoted from 2.62 (conventional flotation) to 2.81.
265
The enhancement of ultra-fine particle flotation by carriers was perhaps mainly owing to
266
the agglomeration of the carriers with the hydrophobic fine particles. This promoted the
267
separation efficiency of fine particles of hydrophobic and hydrophilic materials. By
268
combining the use of carrier and aggregation of ultra-fine particles induced by HC (process
269
4), the enrichment efficiency of fine particles (α = 4.07) was significantly improved over
270
those of other processes.
271
272
273 274 275 276 277
Fig. 12. Fuerstenau upgrading curves of different processes of pre-treatment and flotation (process 1: conventional flotation; process 2: ultra-fine coal pre-treated via HC, throat velocity of 16.50 m/s; process 3: carrier flotation (PS, pre-treated via HC, used as carrier particles, throat velocity of 16.50 m/s); process 4: both PS and ultra-fine particles were pre-treated via HC, throat velocity of 16.50 m/s)
278
3.3 Focused beam reflectance measurement and Polarizing microscopy analysis
279
Fig. 13 shows the variation in the count of ultra-fine particles before and after HC. Prior to
280
the FBRM measurements, both the PS and ultra-fine particles were pre-treated using HC
281
(like in process 4), and the variations in count were observed via FBRM. The count of
282
ultra-fine particles decreased gradually with increase in throat velocity. This result was
283
consistent with the trend of enrichment efficiency of fine coal particles at different throat
284
rotational speeds, as seen in Fig. 11. The reduced fine particles may form aggregates with
285
the PS particles through hydrophobic flocculation, or aggregates among the fine particles
286
may be formed under the action of HC. To explore the mechanism of fine particle reduction,
287
the counts of ultra-fine coal particles before and after cavitation pre-treatment (like in
288
process 2) were explored. Fig. 14 shows that the number of ultra-fine particles in the
289
presence of nano-bubbles and PS particles was significantly lower than that in the presence
290
of nano-bubbles alone. Nano-bubbles can assist ultra-fine coal particles in forming
291
aggregates. The PS particles, pre-treated via HC, functioned as carriers and interacted with
292
the ultra-fine coal particles. Li, et al. [30] proved that treated silica particles can provide
293
solid surfaces with higher hydrophobicity, the tensile strength[31] required for bubble
294
formation decreases, and cavitation becomes easier. In addition, the hydrophobic surface
295
characteristics enhance the voids immersed in the gap, which further promotes nucleation
296
at low tensile strengths. Therefore, PS particles with high hydrophobicity can enhance the
297
nano-bubble production achieved using HC. These nano-bubbles can further promote
298
hydrophobic aggregation between the ultra-fine coal particles. At the same time, nano-
299
bubbles are preferentially formed on the surfaces of hydrophobic particles. Solid particles
300
with rough and hydrophobic surfaces are known to promote bubble formation in liquids
301
[32, 33]. The presence of tiny pockets of undissolved gas in crevices on mineral particles
302
assists cavitation because of the expansion of these gas pockets under negative pressure
303
[34, 35]. Therefore, it can be expected that the nano-bubbles attached to the PS particles
304
can be used as bridges between the ultra-fine coal particles (or aggregates) and carrier
305
particles. The mechanism is demonstrated in Fig. 16 (Model Ⅰ). Fig. 15 presents the
306
photographs of the PM measurements of process 4. The PM results indirectly indicate that
307
the ultra-fine coal particles adhere to the carrier particles through nano-bubbles. The
308
mechanism of particle action in carrier flotation is shown in Fig. 16 (Model Ⅱ).
309
310
311 312 313 314
Fig. 13. Variation in count of ultra-fine particles in process 4 at various throat velocities (δ stands for the reduction in count.)
315 316 317 318
Fig. 14. Counts of ultra-fine particles in processes 2, 3, and 4 (16.50 m/s throat velocity in HC system)
319 320 321
Fig. 15. PM test results
322 323 324 325
Fig. 16. Two models of interaction between particles and nano-bubbles during cavitation (Model Ⅰ: carrier flotation combined with HC pre-treatment; Model ⅠⅠ: PS particles used as carrier)
326
3.4 Interaction between coal and bubbles calculated based on E-DLVO theory
327
The calculation of the force between particles and bubbles has been reported in the
328
literature [36-38]. To explain the two models of agglomeration between the particles and
329
nano-bubbles theoretically, E-DLVO calculations were carried out. According to the E-
330
DLVO theory, the total interaction energy between the particles is equal to the sum of van
331
der Waals energy (EV), double electric layer interaction energy (EE), and hydrophobic
332
interaction energy (EH) [37, 39]. The van der Waals energy can be calculated using the
333
following formula:
334
EV
A132 R1 R2
6 H R1 R2
(5)
,
335
where R1, R2 represent the radii of the two spheres, and H represents the distance between
336
the spheres. In this work, the radius of the nano-bubble is 3.94 nm. A132 represents Hamaker
337
constant,
338
A132
339
where A11 and A22 are the Hamaker constants of the two spheres in vacuum, and A33 is the
340
Hamaker constant of water in vacuum.
341
The double electric layer interaction energy is calculated using the following equations: 2 R R (7) EE 0 1 2 12 2 2 2 1 2 2 p q , R1 R2 1 2
342
343 344
and
its
value
can
be
calculated
using
A11 A33 A22 A33 ,
1 exp H p ln , 1 exp H q ln 1 exp H ,
the
following
formula: (6)
(8) (9)
346
0.304 , (10) cNacl where 𝜑1 𝑎𝑛𝑑 𝜑2 represent the surface potentials of spheres 1 and 2, respectively. 𝜀0 and 𝜀
347
are the values of absolute permittivity in the dispersion medium (78.5 F/m) and vacuum
348
(8.854 × 10-12 F/m), respectively[40]. κ−1 is the Debye length in a 0.1 mM NaCl solution.
349
The hydrophobic interaction energy is calculated as follows:
345
1 =
351
K132 R1 R2 , (11) 6 R1 R2 H where K132 is the hydrophobic constant of the two spheres in the medium and is calculated
352
as follows:
353 354
K132 K131 K 232 , (12) where K131 [41] represents the hydrophobic constant between the nano-bubbles in water,
355
and K232 [42] represents the hydrophobic constant between the nano-bubbles and coal
356
particles in water.
357
Table 3. Parameters used in E-DLVO theory calculation
350
EH
Sphere
Aii (×10-20 J)
Zeta potential (×10-3 V)
Radius (×10-6 m)
Coal particles (1)
6.07
-36.09
22.5
Nano-bubbles (2)
0
-36.20
0.114
Water (3)
4.84
―
―
Hamaker constant (×10-20 J)
Hydrophobic force constant (×10-20 J) K131:1.74
A132: 4.84
K232:1.00 K132:4.17
358
The E-DLVO interaction energies of the two models are shown in Fig. 17. The absolute
359
value of the total energy between nano-bubbles and nano-bubbles is always greater than
360
that between nano-bubbles and ultra-fine coal particles. The energy between the nano-
361
bubbles and ultra-fine coal particles is always negative, which represents attraction.
362
Therefore, the coupling of carrier and nano-bubbles facilitates the formation of aggregates.
363
Hence, process 4 is better than the other processes.
364 365
Fig. 17. E-DLVO interaction energies of different models
366
4 Conclusion
367
The experimental results showed that the coupling of PS carrier and HC improved the
368
flotation of high-ash fine coal. The nano-bubbles generated on the surfaces of the
369
hydrophobic particles during cavitation promoted the aggregation of ultra-fine coal
370
particles, the attachment between ultra-fine coal particles and PS particles. Moreover, the
371
flotation results improved with cavitation intensity. The absolute value of total energy
372
between nano-bubbles and nano-bubbles (Model Ⅰ) is always greater than that between
373
nano-bubbles and ultra-fine coal particles (Model Ⅱ). Model Ⅰ could significantly improve
374
the flotation efficiency of ultra-fine coal.
375
Acknowledgement
376
The authors gratefully acknowledge the financial support from the Project funded by
377
China Postdoctoral Science Foundation (No. 2019M652024).
378
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Highlights
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Declaration of interests
Combined cavitation bubbles and carrier flotation to upgrade ultra-fine particles. Utilized FBRM and PM techniques to directly observe the strengthening mechanisms. Applied E-DLVO theory to evaluate the interaction energy for the novel process.
☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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