international journal of minerals, metallurgy and
TRANSCRIPT
International Journal of Minerals, Metallurgy and Materials Accepted manuscript, https://doi.org/10.1007/s12613-019-1948-9 © University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Influence of coarse tailings on flocculation settlement
Shi Wang1), Xiao-jun Wang1), Qiu-song Chen2), Xue-peng Song1), Jian-chun Qin3), and Yu-xian Ke1)
1) School of Resources and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou
341000, China;
2) School of Resources and Safety Engineering, Central South University, Changsha 410083, China;
3) Department of Personnel, Guangxi University of Science and Technology, Liuzhou 545006, China
Abstract: The composition of tailing particles in mines plays a key role in the flocculation settlement of slurries.
To study the influence of coarse particle tailings (CPT) on the flocculation settlement of tailings slurries (TS), static
flocculent settling tests, scanning electron microscopy observations, and laser particle size analyses were conducted
using the tailings obtained from a copper mine. The results demonstrate that (i) in the accelerated and free settling
process, CPT did not directly settle at the bottom of graduated cylinders; instead, they were netted by the flocculent
structures (FSs) and settled together more quickly. The CPT accelerate the rapid settlement of TS; the acceleration
effect is more obvious when the CPT content is greater than 50 wt%. (ii) The most appropriate flocculant unit
consumption (FUC) is 20 g·t−1, and no substantial increase is observed in the flocculant settling velocity with an
increase in the flocculant because the effective FSs did not substantially change and thus did not lead to a notable
increase in the settling velocity of the solid–liquid interface (SLI). (iii) In the effective settling space of the
thickening facility, free water quickly flowed from the pores of FSs, which is reflected in the period from 0 to 1 min.
Keywords: tailings slurry; particle size distribution; flocculent structures; flocculating sedimentation; solid–liquid
interface.
1. Introduction
Adding flocculants to deep cone thickeners to thicken and dehydrate the tailings slurries (TS)
with low concentrations transported from dressing plants can effectively improve the settling
efficiency of the TS and prevent overflow of water caused by mixing [1-3]. This technology has
been widely applied here and abroad. However, because the ore and rock properties of different
mines differ, and because large differences exist in the ore processing technologies, the particle
size distributions of the tailings produced by the dressing plants vary considerably [4]. Therefore,
the particle size composition and the surface physicochemical characteristics of different particles
also differ. Additionally, the process of inducing flocculation by combining particles and
flocculant molecules is complex. The strength and fractal dimension of the flocculation mesh
structure within the flocs are also substantially different. Moreover, different TS exhibit different
flocculation settlement behaviors; i.e., the finer the tailings, the slower the settling velocity [5, 6].
Several flocculation settlement phenomena in mines have demonstrated that the particle size
distribution of unclassified tailings strongly influences the actual sedimentation [7, 8]; e.g., the
settling velocity of tailings slurries in gold mines is usually lower than that in iron mines under the
same conditions.
The unclassified tailings are a type of a heterogeneous group of particles with a wide range of
particle sizes. Furthermore, the bridging effect of macromolecular flocculant is mainly targeted at
tailing particles with a particle size less than 30 μm [9]. When the particle size of the tailings is
greater than 30 μm, their participation in bridging becomes difficult. Tailing particles smaller than
400 mesh (37 μm) are defined as fine particle tailings (FPT), whereas those larger than 400 mesh
(37 m) are defined as coarse particle tailings (CPT). The FPT are bridged with the macromolecular
flocculant to form larger floccules in the full tailings slurry, whose structure is loose, amorphous,
and interconnected but not very stable [7, 10]. A large amount of space and several tiny networks
exists inside the floccules, which indicates the presence of FS. The floccules contain a large
amount of liquid, which results in the density of the flocculation being similar to that of the liquid
itself. Thus, the settling velocity is low. The CPT sink quickly by overcoming resistance with their
own gravity.
The interaction mechanism between different flocculant molecules and particles is
overwhelmingly complex. Autier et al. used a scanning electron microscopy (SEM) and laser
particle size analysis (LPSA) to analyze the influence of polycarboxylic acid on the particle
dispersion characteristics in cement slurry [11]. They identified the different types of particles and
their mesostructural organization, along with the particle transfer that can occur between the
different granulometric classes [11]. Lee et al. demonstrated that the particle-binding bridges
enhanced flocculation and aggregated kaolinite particles in large, easily settleable flocs, whereas
the polymer-binding bridges increased the steric stabilization by developing polymer layers that
covered the kaolinite surface [12]. Bürger et al. developed a mathematical model for batch and
continuous thickening of flocculated suspensions in vessels with various cross-sections [13]. Zou
et al. studied the influence of anionic polyacrylamide (APAM) on the surface free energy of coal
particles and kaolinite particles, and established that APAM changed the value of the potential
energy of interaction between particles but did not change the state of attraction/repulsion of the
total potential energy of interaction between particles [14]. Benn et al. proposed an experimental
system to provide high-fidelity sedimentation data for the sedimentation modeling of flocculated
systems and used turbulent pipe flow flocculation to offer aggregate size monitored in-line [15].
Kazzaz et al. used poly(acrylic acid) (PAA) to analyze the flocculation of aluminum oxide
particles with various sizes (0.06–0.6 μm) to investigate the effect of the aluminum oxide particle
size on the flocculation effectiveness of PAA [16]. Garmsiri et al. studied the effects of grain size,
grain size reduction, and solution ageing of an anionic high-molecular-weight flocculant on its
preparation, and the results indicated that, for smaller grain sizes, a shorter ageing was required to
achieve a certain settling rate [17]. Ng et al. used a turbidity testing method to confirm the
flocculation of fine hematite particles with poly-N-isopropylacrylamide, which deslimed the
surface of the coarser particles [18].
Although numerous experiments and theoretical analyses have been conducted on the
microscopic mechanism of action between admixtures and particles, the influence of particle size
on the flocculation and settlement of TS has rarely been studied. In the present study, the
relationship between the CPT composition, flocculant unit consumptions (FUC), and settling
velocity of SLI was determined on the basis of the flocculation and settlement test rules of TS
with different CPT compositions in a copper mine. In addition, APAM, which is widely used in
mines, was used in this study. The SEM test was used to observe the particle size distribution in
the flocculating area in the accelerated and free settling process (AFSP), the LPSA was used to
measure the particle distribution in the flocculation area, and the influence of the CPT
composition on the change in the SLI settling velocity of the TS is discussed. The present study
provides important background information for ensuring rapid thickening of TS and realizing
continuous mine backfilling.
2. Experiments
2.1. Experiment materials
(1) Unclassified tailings
The unclassified tailings were obtained from a copper mine in Jiangxi Province, Jiujiang City,
China. The particle size distribution of these tailings was analyzed using a Winner 2000 laser
particle size analyzer. A pycnometer was used to measure the specific gravity, a small relative
density meter was used to measure the unit density, and the pH of the unclassified tailings slurry
with an original mass concentration of ~35 wt% was measured onsite using a METTLER
TOLEDO pH meter. The results are presented and depicted in Table 1 and Fig. 1, respectively.
The average particle size of the total tailings was 49 μm, and the tailings with a particle size
greater than 37 μm accounted for 63.6 wt%; the original slurry was alkaline (see Table 1 and Fig.
1). The composition of the main elements in the unclassified tailings was determined by X-ray
fluorescence spectrometry (see Table 2).
Table 1. Physical properties of the main tailings
Property index
Density (t·m−3)
Bulk density (t·m−3)
Average size (μm)
Porosity (vol%)
Content < 20 μm (wt%)
Content > 74 μm (wt%)
pH
Value 2.97 1.37 49 53.87 31.5 21.6 11
1 10 1000
20
40
60
80
100
Dis
trib
utio
n / w
t%
Particle size / μm
Fig. 1. Particle size distribution of unclassified tailings
Table 2. Composition of the main elements of the unclassified tailings (wt%)
Element Si Ca Al Mg Fe S
Content 33.02 15.68 2.56 1.82 10.37 4.55
Element Pb Mn F K P Cu
Content 0.0095 0.085 0.080 0.37 0.049 0.065
The unclassified tailings are dried and dehydrated, and then divided into two parts using the
standard top-impact-type vibrating screen: CPT (>400 mesh, ≥37 μm) and FPT (<400 mesh, <37
μm). The reassembled tailings slurries (RTS) were adjusted according to the mass concentrations
of the CPT after measurement, which were 90, 80, 70, 60, 50, and 40 wt%. The RTS were then
labeled from z-1 to z-6 in order and mixed separately. The particle size distribution of the RTS is
shown in Fig. 2.
1 10 100
0
20
40
60
80
100
Dis
trib
utio
n / w
t%
Particle size/ μm
Z-1 Z-2 Z-3 Z-4 Z-5 Z-6
Fig. 2. Particle size distribution of built-up tailings
(2) Flocculant
Following a series of flocculant selection tests, APAM with a molecular weight of 10 million,
obtained from Xinyu Chemical Co., Ltd., Zhengzhou, China, was found to exhibit good
flocculation and sedimentation performance for tailings obtained from the copper mine in Jiangxi
Province. An appropriate amount of converted flocculant solution, which needs to be added to the
unclassified tailings slurry as required, was calculated before the test. The addition of the APAM
solution can be defined as follows [19]:
W610
x sx
n x
C V JM
, (1)
where Mx is the APAM addition in mL; CW is the mass concentration of the unclassified tailings
slurry in wt%; V is the volume of the unclassified tailings slurry in mL; Jx is the optimal APAM
dosage in g·t−1; γn is the APAM concentration in wt%; ρs is the density of the unclassified tailings
slurry in g·cm−3; and ρx is the density of APAM in g·cm−3.
(3) Water
Domestic water was used as the test water.
2.2. Experimental process
CPT
FPT
37 μm
The height change rules of the SLI were recorded after the flocculating sedimentation tests of
the RTS with different CPT compositions. The flocculation area was solidified by the silicate
substitution method, and its microstructure was observed by SEM. Meanwhile, the TS in the
flocculation area was extracted and then the change rules of the particle size distribution were
analyzed using the Winner 2000 laser particle size analyzer. The test process is illustrated in Fig. 3.
Fig. 3. Diagram of the test process
2.2.1. Flocculating settlement test
The flocculating settlement test was conducted in a graduated cylinder as follows:
(1) One gram of APAM was weighed and placed in a graduated cylinder containing 1000 mL
of water. The mixture was stirred at 40 r·min−1 for 60 min using an electric agitator, and the
APAM was prepared with a mass concentration of 0.1 wt%.
(2) An appropriate amount of the combined tailings and water was electronically weighed,
and the RTS were prepared with mass concentrations of 15 wt% in six different 1000 mL
graduated cylinders, which were labeled ZJ-1 through ZJ-6. The density of each slurry was
approximately 1.11 t·m−3.
(3) APAM solution (1.67 mL) was transferred to the six graduated cylinders according to the
FUC, which was 10 g·t−1.
(4) The solutions were stirred with a glass rod for 30 s, and the height of the SLI of the RTS
was recorded at intervals of 0, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 8, 12, and 15 min.
(5) Step (2) was repeated, and the following three tests were performed. The APAM solution
with 3.34, 5.01, and 6.68 mL was transferred to six graduated cylinders according to the FUC,
which was 20, 30, and 40 g·t−1, respectively. Step (4) was then repeated.
Extract TS after15 s
RTS
1 cm
SEM
Grading analysis
Liquor area
Flocculation area
Solidification
Drying
2.2.2. SEM
Slurries ZJ-1 through ZJ-6 were again prepared. The APAM solution, whose FUC was 20
g·t−1, was added to the graduated cylinders containing the slurries, and then the flocculating
settlement test was conducted. The slurry 1 cm below the SLI was rapidly extracted from the RTS
using pipettes with long tubes 15 s after sedimentation and then transferred to two small beakers.
An appropriate amount of liquid sodium silicate (Na2SiO3) was added to one of the beakers, and
the distribution of the tailing particles was solidified in the process of flocculation settlement
according to its permeability and air hardening property. Finally, the samples were sprayed with
gold and their microstructures were observed using an XL30W/TMP scanning electron
microscope [20-23].
2.2.3. LPSA tests
The slurry in the other beaker was dried. Tailings samples were collected at four points in
this beaker, and their particle size distribution was measured by LPSA. The average of the results
was determined, and the changes in particle size in the flocculation area of different RTS were
recorded.
2.3. Evaluation index
To analyze the change rules of the velocity of the SLI in the RTS, the maximum settling
velocity, maxU , which is equal to max{ 0.25v , 0.5v , …， Syv }, was defined, where Sy
v (cm·s−1) is
the sedimentation velocity at Sy (i.e., 0, 0.25, 0.5, …. 15 min), and y = 1, 2, …, 12, corresponding
to the times of Sy recorded. If Syv > 0.1 cm·s−1, the slurry undergoes the rapid settlement process
(RSP) within Sy . If 0.01 < Syv < 0.1 cm·s−1, the slurry undergoes the slow settlement process
(SSP) during Sy . The formula is as follows:
1S S
S1S S
-
-
y y
y
y y
h hv
(2)
where Syh is the height of the SLI when the setting time is Sy cm. The average sinking speed was
defined as within 0.5, 1, and 2 min, which is represented by 0.5U , 1U , and 2U , whose values are
0 0.52 -（ ）h h , 0 1-h h , and 0 2- 2（ ）h h cm·s−1, respectively.
The change rules of the relative reduction, ijkd , of maxU , 0.5U , 1U , and 2U were analyzed,
where i indicates the time (max, 0.5, 1, and 2 min, respectively), j represents the FUC (10, 20, 30,
and 40 g·t−1, respectively), and k represents the CPT content (90, 80, …, 40 wt%, respectively).
The formula for ijkd is as follows:
10
10
-100% ij ijk
ijkij
v vd
v (3)
where ijkv is the settling velocity when the content of CPT is k and the FUC is j at time i.
3. Results and discussion
3.1. Flocculating setting test
The relationship between the height of SLI in RTS and time is drawn when different FUCs
were added to RTS after processing test data, as shown in Fig. 4.
0 2 4 6 8 10 12 14
5
10
15
20
25
30
35 (a)
Hei
ght o
f S
LI
/ cm
Time / min
ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6
0 2 4 6 8 10 12 14
5
10
15
20
25
30
35(b)
Hei
ght o
f S
LI
/ cm
Time / min
ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6
0 2 4 6 8 10 12 14
5
10
15
20
25
30
35(c)
Hei
ght o
f S
LI
/ cm
Time / min
ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6
0 2 4 6 8 10 12 14
5
10
15
20
25
30
35(d)
Hei
ght o
f S
LI
/ cm
Time / min
ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6
Fig. 4. Sedimentation curves of SLI: (a) 10 g·t−1; (b) 20 g·t−1; (c) 30 g·t−1; and (d) 40 g·t−1
By analyzing Fig. 4, we determined the change values of maxU , 0.5U , 1U , and 2U for
various CPT; the results are shown in Fig. 5.
0.0
0.1
0.2
0.3
0.4
0.5
4050607080
(a) Umax
U0.5
U1
U2
Set
tling
vel
ocit
y / (
cm·s
-1)
Content of CPT / wt%
90
0.1
0.2
0.3
0.4
0.5
0.6
0.7
4050607080
（b） Umax
U0.5
U1
U2
Set
tlin
g ve
loci
ty （
cm·s
-1）
Content of CPT / wt%90
0.1
0.2
0.3
0.4
0.5
0.6
0.7
4050607080
(c) U
max
U0.5
U1
U2
Set
tlin
g ve
loci
ty /
(cm
·s-1)
Content of CPT / wt%90
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
4050607080
(d) Umax
U0.5
U1
U2
Set
tlin
g ve
loci
ty /
(cm
·s-1)
Content of CPT / wt%90
Fig. 5. Different average sedimentation velocities of RTS: (a) 10 g·t−1; (b) 20 g·t−1; (c) 30 g·t−1; and (d) 40 g·t−1
The change rules of the ijkd of maxU , 0.5U , 1U , and 2U with the reduction of CPT in RTS
for different FUCs were obtained from Fig. 5; the results are shown in Fig. 6.
0
20
40
60
80
4050607080
(a)
Dis
trib
utio
n / w
t%
Content of CPT / wt%
10 g·t-1
20 g·t-1
30 g·t-1
40 g·t-1
90
0
20
40
60
80
10 g·t-1
20 g·t-1
30 g·t-1
40 g·t-1
4050607080
（b）
Dis
trib
utio
n /
wt%
Content of CPT / wt%
90
0
20
40
60
80
100
10 g·t-1
20 g·t-1
30 g·t-1
40 g·t-1
4050608090
(c)
Dis
trib
utio
n / w
t%
Content of CPT / wt%
70
0
20
40
60
10 g·t-1
20 g·t-1
30 g·t-1
40 g·t-1
4050607080
(d)
Dis
trib
utio
n / w
t%
Content of CPT / wt%90
Fig. 6. Sedimentation velocity reduction of different RTS: (a) Umax; (b) U0.5; (c) U1; and (d) U2
Three main following conclusions were obtained from Figs. 4–6.
(1) The CPT considerably influences the settling velocity of SLI in RTS; however, when its
content is less than 50 wt%, the influence evidently weakened. First, when the FUC was 20 g·t−1,
maxU decreased from 0.651 to 0.242 cm·s−1, and the CPT content decreased from 90 to 40 wt%,
as shown in Fig. 5(b). maxU decreased with increasing FPT content; that is, CPT considerably
accelerated the settling velocity. Second, the settling velocity drastically slowed when the content
of FPT in RTS exceeded a certain value. When the FPT content was greater than 50 wt%, maxU
was 0.33 cm·s−1, which was 47.2 wt% the maximum value of 0.70 cm·s−1, as shown in Fig. 5.
When the FPT reached a certain value, the sedimentation velocity increased slowly and the
flocculant alone could not achieve a rapid sedimentation velocity. Third, the values of maxU , 0.5U ,
1U , and 2U all decreased with decreasing CPT content; in addition, the longer the settling time,
the smaller the settling velocity. When the CPT content decreased to 50 wt%, the content of
"coarse" particles continued to decrease as well; however, the sedimentation velocity did not
substantially decrease, as shown in Fig. 5. For the same CPT content, the FUC was higher and ijkd
was smaller; that is, when the content of FPT in the RTS was low, more flocculants were added
and a more violent sedimentation reaction was observed, as shown in Fig. 6.
(2) The optimal FUC was approximately 20 g·t−1, and the sedimentation result did not
considerably improve with increasing amount of flocculant. First, when the FUC was increased
from 10 to 40 g·t−1, the maxU of ZJ-1 took the values 0.462, 0.651, 0.677, and 0.703 cm·s−1; thus,
its sedimentation velocity was high; the other RTS exhibits similar properties, as shown in Fig. 5.
However, when the amount of FUC was more than 20 g·t−1, maxU increased moderately, as shown
in Figs. 4 and 5. Second, when the value of FUC was 10 g·t−1, with the decrease of the CPT
content, the order of sedimentation velocity changed constantly, which was maxU > 1U > 0.5U >
2U → maxU > 1U > 2U > 0.5U → maxU > 2U > 1U > 0.5U . That is, the sedimentation velocity
of RTS for a less CPT gradually decreased at the later setting section. When the values of FUC
were 20, 30, and 40 g·t−1, the order of sedimentation velocity was maxU > 0.5U > 1U > 2U . That
is, the longer the time, the lower the average settlement velocity; in addition, the settlement rules
of the three tests were similar, as shown in Fig. 5. Third, the settlement rules of the RTS whose
FUC values were 20, 30, and 40 g·t−1 were the same but were absolutely different from the
settlement rules of the RTS whose FUC was 10 g·t−1. In addition, the longer the settling time, the
closer the value of ijkd in different RTS, as shown in Fig. 6. Fourth, when the values of FUC were
20, 30, and 40 g·t−1, the average values of the ijkd of maxU , 0.5U , 1U , and 2U in RTS were 75–95
wt%, 55–80 wt%, 50–70 wt%, and 45–60 wt%, respectively, of the value of ijkd in RTS, whose
FUC was 10 g·t−1; the rate of increase of ijkd was substantially reduced, as shown in Fig. 6.
(3) The promoting effect of CPT on the settlement velocity was reflected in the period of 0–1
min. First, when the value of FUC was 10 g·t−1, the RSP of slurries ZJ-1 through ZJ-5 occurred
within 0–2 min; however, when the value of FUC was 20–40 g·t−1, the RSP of slurries ZJ-1
through ZJ-6 occurred within 0–1 min, as shown in Fig. 4(a)–(d). The RSP of different RTSs was
varied according to the amount of flocculant. Second, when the value of FUC was 10 g·t−1, the
SSP of slurries ZJ-1 through ZJ-5 occurred within 2–5 min; the settling velocity was then slowly
reduced to zero. However, when the value of FUC was 20–40 g·t−1, the SSP of slurries ZJ-1
through ZJ-6 occurred within 1–3 min, as shown in Fig. 4(a)–(d). The SSP of different RTSs also
varied according to the amount of flocculant.
3.2. SEM
The 800× SEM images of different RTSs in the flocculation area in the AFSP are shown in
Fig. 7.
Fig. 7. SEM micrographs of different RTSs: (a) ZJ-1; (b) ZJ-2; (c) ZJ-3; (d) ZJ-4; (e) ZJ-5; and (f) ZJ-6
Fig. 7 shows that, when the content of FPT in RTS was only 10 wt%, several CPT evidently
existed in the flocculation area in the AFSP (see Fig. 7a). Thus, these CPT did not directly settle at
the bottom of the cylinder; they instead intertwined with the flocs as a whole. With the increase of
the FPT content in the RTS, the CPT in the flocculation area gradually decreased (Fig. 7b–f); thus,
the influence of CPT on flocculation settlement gradually weakened. When the content of CPT in
the RTS reached 50 wt%, a marginal amount of CPT existed in the flocculation area (Fig. 7f).
Meanwhile, the influence of CPT on flocculation settlement was no longer evident; the flocs
considerably influenced the sedimentation velocity. The experimental results are consistent with
those mentioned in conclusion (1).
3.3. Particle size distribution
100μm
100μm 100μm
100μm 100μm
100μm
CPT
FPT
CPT
FPT
FPT
CPT
FPT
CPT
FPT
CPT FPTCPT
FPT
(a) (b)
(c)
(e)
(d)
(f)
The size gradation curves of tailings in the flocculation zone of different RTSs are shown in
Fig. 8, and the content of CPT in the flocculation zone of RTS and that of the original CPT are
compared in Fig. 9.
1 10 100
0
2
4
6
8
Dis
trib
utio
n / w
t%
Particle size / μm
ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6
Fig. 8. Particle size distribution in the flocculation zone of RTS
0
20
40
60
80
14.821.3
31.938.4
46.3
SJ-6SJ-5SJ-4SJ-3SJ-2
Con
tent
of
CP
T /
wt%
RTS
Original data Flocculation data
SJ-1
55.9
Fig. 9. Variation of CPT content in flocculation area
Figs. 8 and 9 show that the content of CPT in the ZJ-1 flocculation zone varied from 90 to
55.9 wt% and that the CPT contents in the flocculation zones of ZJ-2–ZJ-6 were also relatively
reduced. In addition, the extent of reduction gradually decreased such that the content of CPT of
ZJ-6 was reduced by 25.22 wt%. A correlation was observed between the CPT in the flocculation
zone and that in the RTS because CPT were involved in the sedimentation process. The particle
size in the ZJ-1–ZJ-6 flocculation zones with the largest proportion of different RTS gradually
decreased from 79.00 to 17.19 μm, whereas the settlement velocity of ZJ-1–ZJ-6 decreased
successively, as shown in Fig. 5. That is, the settlement velocity of slurries was directly
proportional to the particle size in the flocculation zone; in addition, the greater the CPT content in
20.8017.1930.46
44.5979.00
36.90
the flocculation area, the faster the slurries settled. The maximum proportion of the particles in the
flocculation zone of ZJ-1–ZJ-6 increased from 5.57 to 7.25 wt%, the content of FPT gradually
reached the maximum value, and the settling velocity of the slurries gradually become stagnant. In
summary, CPT positively influenced flocculation and sedimentation. The experimental results are
consistent with those mentioned in conclusion (1).
4. Mechanism analysis
4.1. Influence process of CPT
The loose flocs with reticular structures form by bridging FPT and macromolecular
flocculant; these FS are formed rapidly via mixing at the beginning of the tests [24]. In the AFSP,
CPT settle rapidly because of gravity; in addition, most of them are netted to form a whole with
flocs when they were being blocked, pulled, and captured by the FS. They both then settle
together, accelerating the sedimentation process [25]. The influence of CPT content on the settling
velocity of SLI is shown in Fig. 10.
(a) (b)
CPT FPT FS
Fig. 10. Settling process of TS with different CPT contents: (a) more CPT; (b) less CPT
CPT and flocs form a whole and then settle together; when the content of CPT is high, the
settling velocity of CPT plays a key role, accelerating the fast settling of flocs (Fig. 10a). However,
when the content of CPT is low, the settling velocity of flocs is at a maximum, and a small amount
of CPT cannot match the rapid settlement of several flocs (Fig. 10b). The tests show that, when
the content of CPT of this copper mine exceeded 50 wt%, the flocculation settlement velocity of
the tailing slurry was rapid. Fig. 1 shows that the actual CPT content in the unclassified tailings of
this copper mine was 63.6 wt% and that the tailings exhibited good settlement.
4.2. Effective flocculation structures
When the FUC added is different, the sedimentation process of RTS for a flocculation
settling time of 30 s is shown in Fig. 11. The FS formed by the combination of flocculants and
FPT will vary with the addition of flocculant in a certain tailing slurry. When the FUC added is
low, some free PFT still exist in the tailing slurry, which cannot be flocculated and form sufficient
FS; this scenario leads to a high solids content in the liquor area and to low sedimentation velocity
(Fig. 11A) [26]. When sufficient FUC is sufficient, almost all of the FPT collectively form the FS;
thus, the liquor area contains less FPT and the sedimentation velocity is high (Fig. 11B). When the
FUC added is high, excess flocculants may exist in the flocs or may be free in the aqueous
solution, whereas the FS that is conducive to accelerating the settlement velocity exhibits no
obvious change. That is, the “effective flocculation structures” are basically the same as the FS
when sufficient FUC is added; at this moment, the sedimentation velocity does not increase
dramatically (see Fig. 11C and D) [27]. The test results show that the most appropriate amount of
FUC of tailing slurry in this copper mine is 20 g·t−1.
Fig. 11. Settling process of TS for different FUC
(A–10 g·t−1; B–20 g·t−1; C–30 g·t−1; D–40 g·t−1)
4.3. Effective settlement space
The AFSP is completed within a brief period of 1 min after flocculant has been added to
different RTS according to the test conclusion (3); the accelerating effect of CPT on settlement is
also reflected in this process. The average settling velocity of the SLI at this stage can reach 0.4
cm·s−1. Thus, the height of the free settling zone in the actual concentration facility determines the
settling time—specifically, the size of the effective settlement space. In this space, CPT causes the
flocs to settle rapidly; in addition, the free water in the lower section rapidly spills from the pores
of the FS [28]. Meanwhile, at the lower section of the thickening facility, the pore water between
A B C D
the particles is slowly squeezed out because of gravity; however, the floc water is protected by FS
and it is difficult to be extruded. Thus, the settling velocity is low in this situation.
In conclusion, an appropriate amount of CPT in the tailing slurry can ensure a rapid SLI
settling speed according to the aforementioned mechanism analysis. Thus, the results suggest that
mines should ensure a certain amount of CPT in their tailings. When the FUC added is very high,
the free macromolecular flocculant in water is difficult to remove, which will affect the recycling
of actual water resources in mines. Thus, an appropriate amount of flocculant is recommended to
thicken tailings.
To improve the settlement velocity of TS, an appropriate amount of flocculant should be
added according to the actual conditions in engineering. In addition, the content of CPT (>37 μm)
should be greater than 50 wt%. The fast settling speed of TS will lead to faster growth of the
thickness of the compression layer in deep cone thickeners, which increases the risk of rake
blockage. Thus, the torque of deep cone thickeners should be increased [29, 30].
5. Conclusions
(1) The SEM images show that CPT do not directly settle at the bottom of the graduated
cylinder because of the FS in the AFSP; instead, they are netted by the floccules to form a whole
and then settle rapidly. CPT accelerates the rapid settlement of TS. In addition, the minimum
maxU can reach 0.441 cm·s−1 when the CPT content exceeds 50 wt%, and the acceleration effect
is evident.
(2) The most appropriate FUC is 20 g·t−1; the rate of increase of maxU is less than 10 wt%,
which is attained for an increased amount of flocculant. This result is observed because the
"effective flocculation structure" does not substantially change at this time and the excess
flocculant molecules are floating free in the water; thus, no evident increase in the settling velocity
of the SLI is observed.
(3) In the effective settling space of the thickening facility, free water flowed from the pores
of the FS rapidly; in addition, the CPT caused the flocs to settle rapidly, which is reflected in the
period from 0 to 1 min, and considerably influenced the settling velocity of the SLI.
(4) The main factor restraining the settling velocity of the solid–liquid interface (SLI) is the
floccules, which are formed by the FPT and flocculant during flocculation and settlement. The
increase in the CPT content positively affects the sedimentation velocity, indicating that the
settlement velocity can be governed by controlling the CPT content in engineering.
Acknowledgments: The authors would like to thank the financial support provided by the
National Key R&D Program of China (2017YFC0804601), the National Natural Science
Foundation of China (51804134, 51804135), the Natural Science Foundation of Jiangxi Province
(20181BAB216013), the Program of Qingjiang Excellent Young Talents, Jiangxi University of
Science and Technology, and the Doctoral Startup Fund of Jiangxi University of Science and
Technology (jxxjbs17011).
Conflicts of Interest: The authors declare no conflicts of interest.
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中图分类号：TD853；二级学科：采矿工程。