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Journal of Cleaner Production 199 (2018) 891e899
Contents lists available at ScienceDirect
Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
Green and efficient utilization of waste ferric-oxide desulfurizer to
clean waste copper slag by the smelting reduction-sulfurizing process
Zhengqi Guo, Jian Pan*, Deqing Zhu**, Feng Zhang
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 December 2017
Received in revised form
13 July 2018
Accepted 21 July 2018
Available online 23 July 2018
This research proposed an efficient, innovative and environmentally friendly technology named smelting
reduction-sulfurizing process, which uses one waste (waste ferric-oxide desulfurizer) to treat another
(waste copper slag). In the new process, the waste ferric-oxide desulfurizer was employed not only as a
sulfurizing agent to sulfurize and collect the copper lost in copper slag, but also as a reductant to reduce
the magnetite to “FeO” and thus improve slag fluidity. It was revealed that 90.81% Cu was recovered and
enriched in copper matte under the smelting conditions. The matte contained 15.87% copper, 20.25% S
and 49.56% Fe, which can be returned to the copper smelting process as a feeding. Meanwhile, the
removal rate of hazardous elements, such as Ni, Pb, Zn, As, Sb, Bi and Hg, from the initial copper slag by
the new process was also determined, and the elimination rate of those elements was all over 90%.The
leaching toxicity was used to further evaluate the environmental impact of the cleaned slag, indicating
that the concentrations of toxic element ions in the leachate are all much lower than the thresholds,
which confirmed that the cleaned slag with trace toxic elements is safe and harmless.
© 2018 Elsevier Ltd. All rights reserved.
Keywords:
Waste copper slag
Waste ferric-oxide desulfurizer
Slag cleaning
Copper recovery
1. Introduction
Copper is one of important base metals required for various
applications in modern industries, and nearly 80% of copper in the
word is produced by pyro-metallurgical process (Potysz et al., 2015;
Gbor et al., 2000). Approximately 30 million tons of smelting slag is
estimated to be produced and dumped around the word annually
(Shen and Forssberg,2003; Heo et al., 2013). Typically, the slag
contains not only valuable metals such as Cu, Co, Ni, Pb, Zn, but also
many undesirable hazardous metals like As, Sb, Bi, Hg (Shibayama
et al., 2010). Unfortunately, of these, more than 80% is dumped
directly in many countries without treatment, which not only
wastes the limited resources extravagantly but also occupies
precious land and even poses potentially damages to the ecological
environment (Gorai et al., 2003; Chen et al., 2016).
In recent years, extensive researches have been carried out to
clean the copper slag. Basically, they can be classified into two main
categories, namely hydrometallurgical process and pyro-
* Corresponding author. Peace Building, School of Minerals Processing & Bioengineering Central South University, 410083 Changsha, Hunan, PR China.
** Corresponding author. Peace Building, School of Minerals Processing & Bioengineering Central South University, 410083 Changsha, Hunan, PR China.
E-mail addresses: guuozqcsu@csu.edu.cn (Z. Guo), pjcsu@csu.edu.cn (J. Pan).
https://doi.org/10.1016/j.jclepro.2018.07.203
0959-6526/© 2018 Elsevier Ltd. All rights reserved.
metallurgical process (Krishna et al., 2015; Al-Jabri et al., 2002).
Hydrometallurgical processes are effective methods to extract
metals from copper slags including flotation, leaching, roasting
followed by leaching and bioleaching (Subrata et al., 2015; Li et al.,
2008; Sandeep et al., 2015). But it is very difficult to treat the waste
water generated from that, and its production efficiency is limited.
However, pyro-metallurgy, as a widely used technology in today's
production of base and precious metals, is mainly used for slag
treatment (Huiting and Forssberg, 2003; Hughes, 2000; Jung et al.,
2016; Tang et al., 2015; Zhou et al., 2015). Moreover, if the pyrometallurgical process is employed to clean the slag, the slag may
be treated at its molten stage once it is discarded from the furnace
before it cools down (Dirk et al., 2008; Guo et al., 2016a, b; Guo
et al., 2018). Thus, processing in the molten condition to recover
the valuables is more energy efficient. Maweja et al. had attempt to
clean the copper smelting slag from a water-jacket furnace by direct
reduction (Maweja et al., 2009). But more coal were used to obtain
the higher metals recovery, resulting in unacceptably high iron
content in matte phase. Mikhail and Matusewicz reported that
pyrite concentrate, as a collector phase, was employed to improve
cleaning of copper slag (Mikhail and Webster, 1992; Matusewicz
and Mounsey, 1998) in sulfurizing process. Indeed, it is an effective way to clean slag to produce low grade matte. However, the
excessive addition of pyrite concentrate inevitably causes higher
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
cost. Moreover, the iron content in the matte produced in this
process is too high, leading to a difficulty in further converting
process. Li et al. reported the gypsum was used as a sulfurizing
agent to sulfurize and recover copper and cobalt from coppercobalt smelting slag (Li et al., 2017; Jeong et al., 2016), however,
the removal of toxic elements was neglected. Therefore, this
investigation is set to develop an eco-friendly and cost-effective
process for selective extraction of valuable metals from copper
slag and maximized removal of toxic elements simultaneously.
Besides, we have noticed that waste ferric-oxide desulfurizer(WDS), as a kind of solid residue, is generated from coal-derived
gas industry with high temperature H2S removal process (Ren et al.,
2010). With the rapid development of the coal-derived gas industry, iron oxides desulfurizer has been attracted more attention
due to their low cost and high desulfurization performance (Liu
et al., 2013). Concomitantly, the more and more waste ferricoxide desulfurizer is produced, which is under considerable pressure of green treatment and effective utilization. Unfortunately, it
has been rarely reported to process waste ferric-oxide desulfurizer
with high efficiency and environment protection so far. Hao et al.
reported that WDS can be applied in sulfuric acid production area
due to its high sulfur content (Hao and Wang, 1992). Qian reported
that WDS could be recycled and prepared to iron oxide red, which is
an excellent pigment and has fine qualities such as covering power,
coloring power, heat resistance, resistance to solvent, acid fastness,
innocuity and anti-rust (Qian, 2010). The WDS Generally, it primarily contains iron sulfide (FeS, Fe2S3), sulfur (S2), calcium carbonate(CaCO3), calcium sulfate(CaSO4), carbon (C) and etc (Hao and
Wang, 1992). Thus, it bears the potential to be used as a sulfurizing
agent, flux, or reductant that may play important roles in cleaning
process of smelting slag replacing the traditional sulfurizing agent,
such as pyrite and calcium flux.
In this study, a novel method named smelting reductionsulfurizing process was proposed to recover copper and eliminate
other hazardous metals simultaneously from copper slag. In this
new slag cleaning process, the waste ferric-oxide desulfurizer was
employed as a sulfurizing agent to sulfurize and capture the metals
into matte phase from the slag at molten stage. Then the separation
between matte and secondary slag took place in the crucible by
density difference at high temperature, with the heavier matte
dropping to the bottom of the crucible. The matte phase carrying
valuable metals and hazardous elements would be further processed for refinement. And the cleaned slag containing less toxic
elements has the potential to be utilized as an eco-friendly, clean
functional material, which can avoid land occupancy and minimize
environmental risks.
2. Experimental
2.1. Materials
2.1.1. Copper slag
Copper slag used in this study was collected from Tongling Nonferrous Metals Group Holding Co., Ltd, Anhui, China. Chemical
compositions of copper slag are shown in Table 1. It can be seen
from that the copper slag contains 2.71 wt.% Cu, 0.48 wt.% Ni, and
some toxic elements, including Pb, Zn and As, are also observed in
it. Phase compositions of the copper slag were determined by X-ray
diffraction (XRD)analyses and the result are shown in Fig. 1. It indicates that the used slag is mainly composed of magnetite and
fayalite. The distribution of copper in associated minerals in
smelting slag were carried out based on solubility difference of
various phases in solvent and results are shown in Table 2. As can be
seen from that 50.55% of copper is present in the form of copper
sulfide, 24.35% of copper exists in metallic copper and the proportion of copper in oxide form entrapped in slag is as high as
25.09%.
2.1.2. WDS
The chemical composition of the WDS from Baosteel Group
Corporation is shown in Table 3. WDS mainly comprises 32.33 wt.%
S, 17.91 wt.% CaO and 7.89 wt.% Fe, which was used in this work as a
replacement of sulfurizing agent and flux. In addition, it also contains 5.82% C, which might be suitable to substitute some of the
reductant.
The XRD patterns shown in Fig. 2 confirms that the WDS consists of crystalline gypsum, pyrite, sulfur with calcite and some
unidentified minor phases. In smelting reduction-sulfurizing process, sulfur, existing in CaSO4, FeS2 and S2, would transfer into and
fix in the metal sulfides through contacting and reacting with metal
oxides. Meanwhile, CaCO3, as a kind of fluxes, can decrease the
viscosity of the molten slag by breaking its silica bonds, which can
accelerate the settling of matte particles and minimize the metal
losses with mechanical entrapment of matte (Guo et al., 2016a, b).
2.1.3. Reductant
Blind coal, which was crushed and screened to a size
of 1.0 mm, served as the reductant. Its ash chemical composition,
proximate analysis and ash fusibility analysis were determined by
GB/T212e2008 and GB/T219-2008, and results are shown in
Tables 4 and 5, respectively.
2.2. Experimental methods
2.2.1. Smelting reduction-sulfurizing process
All samples were dried, ground and sieved to yield a particle size
below 1.0 mm prior to the smelting process. In the smelting
reduction-sulfuring step, for each test, 200 g ground slag and a
M
M-Fe3O4
Magnetitie
F-FeSi2O4
Fayalite
F
Intensity/Counts
892
F
F
10
20
30
F
F
F
F
M F
M
F
MF
40
F
M
F
50
Two-theta/deg
M
M
F
60
70
80 34
35
36
Two-theta/deg
Fig. 1. XRD patterns of the copper slag.
Table 1
Chemical analysis of copper smelting slag (Wt.%).
Element
Fetotal
Ni
Cu
Pb
Zn
As
SiO2
Al2O3
CaO
MgO
Cr
S
P
LOI
Copper slag
39.56
0.48
2.71
0.68
1.93
0.043
26.61
3.71
2.50
1.05
0.04
0.83
0.01
0.98
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
893
Table 2
Distribution of copper in associated minerals (mass fraction, %).
Mineral
Copper oxide
Metallic copper
Copper sulfide
Combined copper oxide
Cutotal
Content
Fraction
0.63
23.24
0.66
24.35
1.37
50.55
0.05
1.85
2.71
100.00
Table 3
Chemical analysis of WDS (Wt.%).
Element
Fetotal
Cr
SiO2
Al2O3
CaO
MgO
C
S
P
LOI
WDS
7.89
0.01
0.86
0.31
17.91
0.21
5.82
32.22
0.01
38.02
G-CaSO4Gypsum
G
Intensity/Counts
I-FeS2
G
Sulfur
S-S2
G
10
C
&
6
G
I,
,
G
Pyrite
C-CaCO3 Calcite
6
20
G,
,
I , ,G
30
40
50
Two-theta/deg
S
60
70
80 27
28
29
30
Two-theta/deg
Fig. 2. XRD patterns of the waste ferric-oxide desulfurizer.
Table 4
Main chemical compositions of blind coal ash/wt.%.
Fe2O3
SiO2
Al2O3
CaO
MgO
P
S
0.757
1.24
0.36
1.12
0.06
0.0006
0.58
certain amount of coal (0e8%) and WDS (0e20%) were mixed
evenly and then charged into a 200 ml corundum crucible. Subsequently, the corundum crucible was put in a vertical tube furnace
(as shown in Fig. 3) and heated to fixed temperature with 10 C/min
heating rate under high purity nitrogen (N2) atmosphere, and then
kept at that temperature (1200 C-1400 C) for various durations
(0.5 h-4 h) to allow for homogenization of the melt. After the
smelting reduction-sulfurizing, the molten slag was cooled to room
temperature under the protection of N2, and then it was removed
from the furnace. Subsequently, the crucible was broken to separate
matte and cleaned slag very carefully. Finally, both cleaned slag and
matte were weighed and chemically analyzed by ICP-AES, AAS
(Atomic Absorption Spectroscopy) and chemical titration.
Test methods of the content of total iron (TFe), ferrous iron(Fe2þ)
and metallic iron(MFe) were shown in papers (Meng and
Wang,1980; Huang, 1989). The content of Fe3þ was calculated by
mass balance of iron.
Fig. 3. Schematic diagram of the experimental apparatus.
The chemical species analysis method for determining the
copper phases is based on solubility difference of different phases
in solvent. Addition of appropriate inhibitors can effectively inhibit
dissolution of specific species, and the detail method can be seen in
some references (Liu et al., 2015; Xing and Liu, 2014; Kun, 1974).
2.2.2. Toxicity characteristic leaching procedure (TCLP) test
The cleaned slag was crushed to <9.5 mm for toxicity characteristic leaching procedure (TCLP, USEPA, 2017) (U.S. EPA, Part 261).
The TCLP test of cleaned slag was carried out in a buffered acetic
acid under the conditions of pH 4.93 ± 0.05,100 g cleaned slag obtained from smelting reduction-sulfurizing process, liquid/solid
ratio 20, stirring speed 30 ± 2 rpm, leaching time 18 ± 2 h and
temperature 22.3 ± 3 C. After leaching, the leachates were filtered
by glass fiber filter with pore of 0.6e0.8 mm. The leaching liquor
were detected for potential toxic content by inductively coupled
plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer,
Optima 3000).
Table 5
Proximate and fusibility analysis results of soft coal.
Ash fusibility analysis/ C
Proximate analysis/%
Mad
Aad
Vad
FCad
DT
ST
HT
FT
12.98
4.49
30.41
52.12
1332
1376
1450
1469
(Mad:Moisture;Aad:Ash;Vad:Volatile Matter ;FCad:Fix Carbon;DT:Distortion Temperature;ST:Soften Temperature;HT:Hemispherical Temperature;FT:
Flow Temperature).
894
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
2.2.3. Characterization
Crystalline mineral compositions of samples were revealed by
X-ray powder diffraction (XRD, RIGAKU, D/Max-2500). Optical
microscopy (Leica DMRXP) and scanning electron microscopy
(SEM, JEOL, JSM-6490LV) with energy dispersive spectroscopy
(EDS, JEOLJSM-6490LV) were employed to identify the phases of
samples while the combination of latter two techniques was
applied to determine the elemental quantitative of each phase
(Chen et al., 2017a, b).
likely reduction reactions are as follows:
Fe3O4(s)þC
(s) ¼ 3FeO(l)þCO(g)
(6)
DGq ¼ 31.52e0.045T (KJ/mol)
(7)
3Fe3O4(s)þFeS(l) ¼ 10FeO(l)þSO2(g)
(8)
DGq ¼ 85.43e0.053T (KJ/mol)
(9)
And DGq also can be shown as follows:
3. Results and discussion
DGq ¼ -InKq
3.1. Thermodynamic analysis
As can be seen from Table 2, some copper dissolved in slag as the
form of oxidation state. In order to simplify the reaction mechanism
of reduction-sulfurizing process, Cu2O was used to represent copper oxides dissolved in slag. Fig. 4 shows the variation in the
standard free energy(DGq) with the temperature for the sulfuring
reduction of copper oxides and iron oxides. It can be seen from that
the standard free energy variation for the reactions 1e5 is negative
over the temperature range of 1100e1400 C, indicating that the
sulfurizing reaction is thermodynamically possible. Hence, it is
possible to sulfurize the copper oxides in the slag through adding
the WDS.
Cu2O(l)þCaSO4(s)þ4C(l) ¼ Cu2S(l) þCaO(s)þ4CO(g)
(1)
FeO(l)þCaSO4(s)þ4C(l) ¼ FeS(l)þCaO(s)þ4CO(g)
(2)
2Cu2O(l) þ1.5S2(g) ¼ Cu2S(s)þSO2(g)
(3)
4FeO(l)þ3S2(g) ¼ 4FeS(l)þ2SO2(g)
(4)
Cu2O(l) þFeS(l) ¼ Cu2S(l) þFeO(l)
(5)
In addition, Fe3O4 in the slag also plays a critical role to affect the
cleaning efficiency of copper slag. Generally, low content of Fe3O4 is
beneficial to decreasing the viscosity and improving the fluidity of
molten slag (Sun and Huang, 1992; Cui., 1978; Davenport et al.,
2002). Hence, the reduction treatment of copper slag to decrease
the content of Fe3O4 is necessary by adding reductants in view of
some magnetite existing in the initial slag (as seen Fig. 1). The most
(10)
Where: DGq is the Gibbs free energy of the reaction (9);
Kq is the equilibrium constant of the reaction (9);
Kq can be expressed by that:
kq ¼
a10
Feo PSO2
(11)
a3Fe3o4 aFeS
Through the equations (9), (10) and (11):
.
DGq
a3Fe3o4 aFeS ¼ a10
Feo PSO2 e
(12)
Where: aFe3O4 is the activity of Fe3O4 in molten slag;
aFeO is the activity of FeO in molten slag;
aFeS is the activity of FeS in molten slag;
PSO2 is the partial pressure of SO2 in smelting process;
DGq is the Gibbs free energy of the reaction (9);
e is the nature constant, 2.71828.
When the temperature, aFeO,PSO2 is fixed, the relationship
between aFe3O4 and aFeS can be expressed by combining the equations (9)e(12). Therefore, the relationships between a(FeS) and
a(Fe3O4) in the molten slag with different value of a(FeO) were
calculated by Eq. (12). In case of SO2 partial pressure of 102 atm,
the relationships between a(FeS) and a(Fe3O4) in the molten slag with
different value of a(FeO) were calculated by Eq. (12), and the results
are present in Fig. 5. It can be seen from that with an increase in
a(FeS), the a(Fe3O4) is decreased significantly, which means magnetite
is readily reduced to FeO by WDS. Therefore, the addition of reductants (Coals and WDS) can minimize the amount of Cu lost into
slag by decreasing the content of magnetite in smelting slag and
1.0
-20
a(FeO)=0.35
a(FeO)=0.40
a(FeO)=0.45
a(FeO)=0.50
a(FeO)=0.55
-40
0.8
-80
a(Fe3O4)
Ƹ*©.-PRO
-60
-100
-120
-140
-160
-180
-200
600
Eq.(1)
Eq.(2)
Eq.(3)
Eq.(4)
Eq.(5)
800
0.6
0.4
0.2
1000
1200
1400
Temperatureć
Fig. 4. Variation of standard Gibbs free energy DG of several reactions with temperature obtained by thermodynamic calculations using HSC 6.0 software.
0.0
0.0
0.2
0.4
a(FeS)
0.6
0.8
Fig. 5. Relationship between a(Fe3O4) and a(FeS) by thermodynamic calculations using
HSC 6.0 software.
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
3.2. Smelting reduction-sulfurizing process for Cu recovery
As shown in Table 1, copper is the main valuable metal in copper
slag, which should be considered first for recovery. Hence, the effects of smelting reduction-sulfurizing condition on recovery of
copper were explored systematically.
The effect of WDS addition on grade and recovery rate of Cu was
investigated and results are shown in Fig. 6(a). It is clearly seen that
with an increase of dosage of WDS from 0 to 15%, the recovery of Cu
is increased significantly from 41.64% to 91.67%, Cu grade of matte
and cleaned slag is elevated gradually from 12.21% to 15.87%,
1.69%e0.31%, respectively. When the dosage of WDS is over 15%,
the recovery and grade of copper keep constant. It implied that the
addition of WDS is favorable to recover copper entrapped in initial
smelting slag, which plays an important role in the reductionsulfurizing process. The reasons for the great influence of WDS on
smelting reduction-sulfurizing process might lie in two aspects. On
one hand, the WDS, as a sulfurizing agent, can ensure sulfuration of
all copper existing in the oxide form within the initial slag so that it
enters the matte phase thoroughly. On the other hand, the addition
of WDS enables to provide enough collector phase(matte) to clean
the slag, which allows to separation from slag easily. However,
excessive WDS would sulfurize more “FeO” into matte phase,
resulting in a decrease of copper grade of matte. Over consideration
of grade and recovery, an optimum dosage of WDS addition was
95
20
(a)
90
90
10
Recovery of Cu
Cu grade in matte
Cu grade in cleaned slag
50
40
20
0
5
10
15
(c)
85
80
Recovery of Cu
Cu grade in matte
Cu grade in cleaned slag
70
18
95
16
90
14
85
10
1.6
0.8
0.4
55
0.0
Temperature/
1350
1400
1.6
1.2
60
50
1.2
1300
Recovery of Cu
Cu grade in cleaned slag
Cu grade in matte
65
0.0
60
1250
70
55
65
1200
12
75
0.4
12
75
15
80
0.8
0.4
0.0
0
20
Dosage of WDS/%
Recovery of Cu/%
1.2
0.8
30
90
1.6
Grade of Cu/%
Recovery of Cu/%
70
60
18
85
15
2
4
Dosage of coke/%
6
8
(d)
16
14
Grade of Cu/%
Recovery of Cu/%
Recovery of Cu/%
80
(b)
Recovery of Cu
Cu grade in matte
Cu grade in cleaned slag
80
75
10
1.6
70
1.2
65
0.8
60
55
0.0
12
Grade of Cu/%
100
recommended as 15%.
In addition, the reductive atmosphere is very important to
smelting reduction-sulfurizing process. In order to improve copper
recovery through adjusting the reaction atmosphere, the effect of
coal dosage on recovery and grade of copper were conducted and
the results are show Fig. 6(b). It indicates that the copper recovery
increases obviously with the addition of coal from 0 to 3% and
performs the best of 90.81% at 3% coal dosage, then maintains
constant when the dosage of coal increases to 4.5%. Accordingly, the
grade of copper in cleaned slag and matte present a similar tendency to the copper recovery with the increase of coal dosage.
However, when the coal dosage elevates 8%, each index deteriorates sharply.
It is implied that an appropriate reductive atmosphere is
beneficial for generation of matte, while excessive enhancement of
reductive atmosphere would be detrimental to slag cleaning. In
smelting reduction-sulfurizing process, with the addition of coal,
the “Fe3O4” in the slag is gradually reduced to “FeO” and thus the
content of FeO increases, leading to the improvement of fluidity of
the molten slag. It is also essential for the liberation and settling of
mechanically entrained copper matte and metallic copper inclusions. However, excessive addition of coal would cause that the
“FeO” is further reduced to metallic “Fe”, resulting in high viscosity
and poor fluidity of the molten slag. Therefore, 3% coal dosage is
suitable for cleaning the copper slag.
Undoubtedly, smelting temperature has a significantly effect on
the recovery of valuable metal. Fig. 6(c) shows the effect of smelting
Grade of Cu/%
improving the fluidity of molten slag.
895
0.4
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0
4.5
Duration/h
Fig. 6. Effect of process parameters on grade and recovery of Cu. a-Effect of WDS dosage on grade and recovery of Cu (Smelting reduction-sulfurizing at 1300 C for 2 h with 3% coke
addition); b- Effect of coke dosage on grade and recovery of Cu (Smelting reduction-sulfurizing at 1300 C for 2 h with 15% WDS); c- Effect of smelting temperature on grade and
recovery of Cu (Smelting reduction-sulfurizing for 2 h with 15% WDS and 3% coke); d- Effect of smelting duration on grade and recovery of Cu (Smelting reduction-sulfurizing at
1300 C with 15% WDS and 3% coke).
896
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
anions in the slag, decrease the viscosity and improve the fluidity of
slag, which contributes to collision, aggregation and separation of
matte particles. What is more, increasing the temperature also
weakens interfacial tension between matte and slag, leading to a
decrease in the dissolution and mechanical inclusion of matte in
slag. Consequently, from a practical point of view, the smelting
temperature is fixed at 1300 C in the subsequent experimental
series.
The effect of smelting duration on copper recovery and grade is
shown in Fig. 6(d), which indicates that copper recovery and grade
of matte increases as prolonging duration from 0.5 h to 2.0 h while
the copper grade of cleaned slag decreases accordingly, and then
keep steady. It is implied that insufficient smelting duration cannot
ensure sulfurizing reaction of Cu2O carry out thoroughly, resulting
in the poor cleaning efficiency of smelting slag. Moreover, the full
setting of the matte also requires enough time. Thus, the smelting
time should be performed for 2 h.
Removal rate/%
100
80
60
40
20
0
Ni
As
Sb
Pb
Zn
Bi
Hg
Elements
Fig. 7. Removal rate of hazardous metals with various dosages of WDS. (Smelting
reduction-sulfurizing at 1300 C for 2 h with 3% coke).
Percent of element present
in different phases/%
The removal rate of various toxic elements in copper smelting
slag, especially hazardous elements such as Ni, Pb, Zn, As, Sb, Bi, Hg,
was determined and calculated according to Eq. (13), and the results are shown in Fig. 7.
Matte phase
Slag phase
Vopor phase
100
80
3.3. Removal of hazardous elements (Ni, Pb, Zn, As, Sb, Bi, Hg)
100%
60
40
Where Rt is the removal rate, M1 is the mass of cleaned slag, g1 is
the content of relative metal in the cleaned slag, M2 is the mass of
initial waste copper slag, g2 is the content of relative metal in the
initial slag.
As can be seen from Fig. 7, the dosage of WDS is believed to have
a prominent effect on the removal of Ni, As and Sb. As increasing
the dosage of WDS from 0 to 15%, the removal rate of Ni, As and Sb
obviously ascends from 43.21% to 98.94%, 45.56%e96.81% and
52.23%e95.51%, respectively. The results indicate that the higher
dosage of WDS is beneficial to eliminate those metals, which could
be because Ni, As and Sb minerals in copper slag were reduced and
sulfurized to metal state or sulfide, and then transferred and settle
down to matte phase. However, WDS seems to have a light influence on the eliminate of Pb, Zn, Bi and Hg. The removal rate of those
metals is above 80% even though smelting reduction without WDS.
During the smelting reduction process, the bismuth, lead, zinc and
mercury minerals maybe reduced into vapors of Bi, Pb, Zn and Hg
due to their low vaporization temperature. Hence, they can be
readily collected by cooling the exhaust gas.
In order to further investigate the eliminate behaviors of hazardous metals in reduction-sulfuring process, the element distribution in different phase and chemical compositions of the initial
copper slag, matte and cleaned slag are shown in Fig. 8 and Table 6,
respectively.
20
6
5
4
3
2
1
0
Ni
As
Sb
Pb
Elements
Zn
Bi
Hg
Fig. 8. Elements distribution in different phases (slag phase-cleaned slag). (Smelting
reduction-sulfurizing at 1300 C for 2 h with 15% WDS and 3% coke).
temperature on copper recovery was investigated and results. As
can be seen from that, the copper recovery prominently ascends
firstly and then changes slightly as the smelting temperature
increasing from 1200 C to 1300 C. Meanwhile, the copper grade of
matte maintains around 15% while the copper grade of cleaned slag
decreases gradually from 1.25% to 0.31% with the temperature
elevating from 1200 C to 1300 C. Afterwards, the copper recovery
and grade keep constant with further increasing the temperature.
As is well-known, high temperature can break bonded silicate
1
M1 g1
M2 g2
Rt ¼
(13)
Table 6
Chemical analysis of initial copper smelting slag, matte and cleaned slag.
Elements
Cu
Ni
Pb
Zn
As
Sb
Bi*
Hg*
Initial copper slag
Matte
Cleaned slag
2.71
15.87
0.31
0.48
3.13
0.006
0.68
0.84
0.002
1.93
0.80
0.009
0.088
0.543
0.003
0.036
0.23
0.002
45
87.22
1.20
31
4.36
4.29
Fe
SiO2
Al2O3
CaO
MgO
P
S
Cl
Cr
K
Na
Ti
V
39.56
49.56
37.18
26.61
1.56
29.87
3.71
0.69
3.98
2.5
0.21
5.34
1.05
0.033
1.26
0.01
0.13
0.042
0.83
20.25
1.74
0.04
0.025
0.048
0.04
0.15
0.015
0.58
0.064
0.69
0.24
0.293
0.005
0.049
0.017
0.147
0.049
0.008
0.01
Bi*, Hg*(1 106), Others (1 102).
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
were calculated according to the conservation of mass. Combined
with Table 6, it is found from Fig. 8 that, besides Cu, other elements,
including Ni, As, Sb and S, were also enriched into the matte phase.
In addition, the majority of Pb, Zn, Bi and Hg were removed by
reduction and volatilization, which agrees well with the results
present in Fig. 7.
Consequently, the matte can be returned to the corresponding
smelting process for further recovery of these valuable elements,
and the cleaned slag containing FeO, SiO2, CaO and Al2O3 and trace
impurities may be reused as a raw material for cements.
The vapor phase percent (VPP) is calculated by based on the
mass balance of chemical composition between origin copper slag,
matte and cleaned slag. And the calculated equation is shown as
follow:
VPP ¼
1
M1 g1 M3 g3
M2 g2 M2 g2
100%
(14)
Where VPP is the vapor phase percent, M1 is the mass of cleaned
slag, g1 is the content of relative metal in the cleaned slag, M2 is the
mass of initial waste copper slag, g2 is the content of relative metal
in the initial slag, M3 is the mass of matte, g3 is the content of
relative metal in the matte.
Thereinto, the percent of relevant elements present in gas phase
3.4. Leaching toxicity of cleaned slag
The cleaned slag was achieved under the optimum conditions of
smelting temperature 1300 C, smelting duration 2 h, 15% WDS and
3% coal. The toxicity of cleaned slag was also determined by Toxicity
Characteristic Leaching Procedure (TCLP) test and the results are
present in Table 7. The concentrations of toxic element ions in the
leachate are all much lower than the thresholds prescribed in
USEPA, which implies the cleaned slag obtained by the new process
is harmless and can be used as raw material for safe cement
production.
Table 7
TCLP test results of cleaned slag(mg/L).
Elements
Cu
Ni
Pb
Zn
As
Sb
Bi
Cleaned slag
Toxicity thresholds
70.95
100.00
0.97
1.00
0.28
5.00
1.39
70.00
0.38
0.50
0.15
NS
ND
NS
NS: not stated in regulation, ND: not detected.
(a)
F
F
15% WDS
Intensity/Counts
M
F
F
F
F
F
F FF
M
10% WDS
F
5% WDS
M
M
F FMF
F
34
20
30
40
50
Two-theta/deg
(b)
33
32
2+
3+
31
Ratio of Fe /Fe in the cleaned slg
10
2+
30
29
28
0
5 10 15 20
Dosage of WDS
16
60
70
0.28
(c)
12
10
8
6
4
2
0
5 10 15 20
Dosage of WDS
F
80 34
14
0
F
F M
Withou WDS
M
F
9 LVFRVLW\RIVODJ3Dg
6
F
M
Content of Fe in the cleaned slag
F
M
F
F
F
F
F
F
897
35
M
F
36
Two-theta/deg
(d)
0.24
37
ć
ć
ć
ć
0.20
0.16
0.12
0.08
0
5 10 15 20
Dosage of WDS/%
Fig. 9. XRD patterns and content of Fe2þ and Fe3þ in the cleaned slag. (a-XRD patterns, b-content of Fe2þ, c-ratio of Fe2þ/Fe3þ, d-viscosity of slag, Smelting reduction-sulfurizing at
1300 C for 2 h with 3% coke).
898
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
3.5. Characterization of resultants
In order to further reveal the behavior of WDS in smelting
process, the crystalline compositions, and microstructural characterization of cleaned slag and matte were analyzed using XRD, and
optical microscopy combined with SEM-EDS, respectively, and the
results are shown in Figs. 9e12. It was observed that from Fig. 9
with an addition of WDS in smelting reduction-sulfuring process,
the peaks of fayalite are sharply intensified, while that of magnetite
are weaken, even disappeared in the slag, which is caused by the
reduction of magnetite to “FeO” by WDS and coal. Meanwhile, the
content of Fe3þ and Fe2þ, and the ratio of Fe2þ/Fe3þ were analyzed
and calculated, the results shown in Fig. 9 (b)-(d) further confirms
that WDS is capable to decrease the content of Fe3O4 while increase
the content of “FeO” in the slag, which is beneficial to improving the
futility of molten slag. The theoretical viscosity of slags at various
temperatures calculated by FactSage 7.0 (as shown in Fig. 9(d)),
further confirmed that the addition of WDS can decrease the viscosity of molten slag. The general expression for free-setting velocity of matte particles in smelting process can be shown as follow:
Vs ¼ 2gr 2 ðrm rs Þ=9m
(15)
Where: g is the gravitational acceleration, m/s2; r 2 is the diameter
of matte particles, m; rm is the density of matte, Kg/m3; rs is the
density of smelting slag, Kg/m3; m is the viscosity of the molten slag,
Pa‧s.
As can be seen from Eq. (15), with a decrease of the viscosity of
the molten slag, the free-setting velocity of matte particles through
the molten slag is dramatically increased. This may ultimately
Fig. 10. Optical micrographs(a) with corresponding SEM-EDS results(b) of cleaned
slag.
Intensity/Counts
M
S
S- FeS
W-FeO
C-CuFe2S3
M-Cu
S
S
C
W
S
W
C
10
20
30
40
50
60
Two-theta/deg
Fig. 11. XRD patterns of copper matte.
70
80
Fig. 12. Optical micrographs(a) with corresponding SEM-EDS results(b) of matte.
result in a lower entrainment of matte particles in the slag and a
higher recovery of copper. In addition, the slag after being cleaned
primarily contains fayalite phases. These results are also in good
accordance with the thermodynamic analysis.
In order further confirm the phase and element compositions of
the cleaned slag, optical microscope technique were employed. As
shown in Fig. 10, there are three mineral phases can be obviously
detected in it: entrapped matte, fayalite and magnetite. Comparing
with the initial slag, the ratio of magnetite in cleaned slag are
markedly decreased, while that of fayaite phase is correspondingly
increased, which matches well with the XRD results shown in
Fig. 9(a). Moreover, the rare slight bright spots, as the matte phase,
are still observed in the slag. Nevertheless, the amount of matte
particles entrapped in cleaned slag has seen a dramatic decline,
which also offers stronger evidence that copper lost in initial copper slag can be efficiently recovered via the new process.
On the other hand, the XRD patterns of matte obtained by the
new process present in Fig. 11 shows that the main mineral phases
are iron sulphide (FeS), wustite and cubanite(CuFe2S3), and the
copper, as the minor phases, is also detected in matte. It suggests
that most of copper in the initial slag were sulfurized and collected
as the form of sulfide in the matte phase by WDS. These results
were further verified by optical micrographs with corresponding
SEM-EDS of matte. As shown in Fig. 12(a), four mineral phases are
identified: matte, wustite, matte and alloy phase. The phase compositions of the matte obtained from Fig. 12 are basically consistent
with those from the XRD patterns shown in Fig. 11. Especially, it is
noted that for the point 3, it represents the alloy phase. The EDS
analysis results shows that it contains 4.14% As, 1.43% S, 2.65% Pb,
15.65% Sb, 10.23% Fe, 44.41% Ni and 21.19% Cu. It is implied that
some toxic elements, such as As, Pb, Sb and Ni, were also reduced
and sulfurized together with copper and iron oxide and then
transferred into matte phase during cleaning process. As a result,
the content of those hazardous elements in the cleaned slag is
decreased obviously, which further confirms that the removal of
Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899
hazardous elements can be realized and the cleaned slag was
successfully achieved by the new process (as shown in Figs. 10 and
11). Therefore, this smelting reduction-sulfurizing process is very
effective to clean the copper slag.
4. Conclusions
Based on the experimental results above, it can be concluded
that the smelting reduction-sulfurizing process is an effective
technology to clean the copper slag and recover the valuable metals
from that. Our main findings can be summarized as follows:
(1) Waste ferric-oxide desulfurizer applied as a sulfurizing agent
to treat the copper slag is efficient and environmentally
friendly, which enables the copper oxides dissolved in initial
slag to transform copper sulfide and then enriched in matte
phase. Meanwhile, it also can decrease the content of
magnetite in slag and improve slag fluidity. Hence, the recovery of copper is increased significantly and the copper lost
in slag as mechanical entrapment is decreased
correspondingly.
(2) Under the optimum conditions of smelting temperature
1300 C, smelting duration 2 h, 15% WDS and 3% coal, 90.81%
Cu was recovered and enriched in copper matte, which can
be returned to the copper smelting process as a feeding.
(3) The elimination rates of hazardous elements, such as Ni, As,
Pb, Zn, Sb, Bi and Hg from waste copper slag via the new
process were over 90%, by which a cleaned slag with trace
toxic elements was achieved. The leaching toxicity of the
cleaned slag is much lower than the environmental limit
regulatory of USEPA, confirming it is harmless and can be
used as the raw material for production of cement.
Acknowledgements
The authors wish to express thanks to the National Key Technology R&D Program of China (grant no. 2013BAB03B04) for its
financial support of this study, which supplied us with the facilities
and funds needed to complete the experiments.
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