مرکزی صفحہ Russian Journal of Non-Ferrous Metals Arsenic Removal from Cu–As-Containing Filter Cakes by Na2CO3 Leaching

Arsenic Removal from Cu–As-Containing Filter Cakes by Na2CO3 Leaching

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60
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Russian Journal of Non-Ferrous Metals
DOI:
10.3103/S1067821219040175
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July, 2019
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Self-Propagating High-Temperature Synthesis of a Thin-Layer CuO–B–Glass System

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ISSN 1067-8212, Russian Journal of Non-Ferrous Metals, 2019, Vol. 60, No. 4, pp. 372–379. © Allerton Press, Inc., 2019.

METALLURGY
OF NONFERROUS METALS

Arsenic Removal from Cu–As-Containing Filter Cakes
by Na2CO3 Leaching
Jian-Hui Wua, Xian-Peng Zhanga, Bo Donga,*, Jun Wub, Xiao-Song Chena, and Si-Lei Chena
aSchool

of Metallurgy and Environment, Central South University, Changsha, Hunan, 410083 China
bDepartment of Chemistry, Imperial College London, London SW72AZ, England
*e-mail: dongbo0214@163.com
Received February 25, 2019; revised April 12, 2019; accepted April 29, 2019

Abstract—In the present study, a process for selective removal of arsenic from Cu–As-containing filter
cakes using Na2CO3 leach was examined. When decreasing the weight percentage of arsenic in Cu–Ascontaining filter cakes, environmental problems can be eliminated while valuable metals will be recycled.
The effect of α(Na2CO3) (where α represents excess coefficient), temperature, L/S ratio and leaching time were
investigated. the optimal leaching conditions were achieved as follows: α(Na2CO3) = 2.7, L/S ratio 6 : 1 (mL/g),
temperature 75°C, leaching time 3 h, where the leaching efficiency were 96.47% for As, 0.41% for Cu with
final As and Cu concentrations of 11.72 and 0.02 g/L respectively. Further analysis showed that the Cu content in the leach residue increased to 51.42% with CuS as the major Cu phase. Notably, only 0.92% of As
remained in the leach residue. The leaching solution was treated by sodium persulfate oxidation and arsenic precipitation which eliminated As 99%, leaving 0.5 mg/L As in the solution. Therefore, the proposed
process is a viable method for the effective separation of Cu and As in Cu–As-containing filter cakes.
Broadly, the results of this paper provide a reference for the treatment of arsenic sulfide from nonferrous
metallurgy cyclic leaching process.
Keywords: Cu–As-containing filter cakes, sodium carbonate, arsenic removal, selective leaching, scorodite,
recovery
DOI: 10.3103/S1067821219040175

1. INTROD; UCTION
Arsenic, one of the toxic elements that threaten the
health of human being and environment, is an
unwanted hazardous waste generated from nonferrous
pyrometallurgical industries in smelting slag, dust,
anode slime and so on, especially copper smelting
[1‒3]. During the copper smelting process, large
amounts of acidic wastewater are formed by the gaseous emission containing SO2. Cu, As and other
impurities are washed with diluted acid. Subsequently,
the As, Cu, and other heavy metals in the acidic wastewater are removed by the sulfide precipitation method,
which generates the Cu–As-containing filter cakes [4].
Cu–As-containing filter cakes not only represent a
potential risk of environmental contamination but also
are a secondary resource, which contains a large number of valuable metals, such as copper, bismuth and so
on [5]. Therefore, the development of innovative treatment technologies for Cu–As-containing filter cakes
is currently a very critical and vital priority to obtain
the maximum economic and environmental benefits.
The treatment of As-containing filter cakes to
decline valueless arsenic while recycling valuable elements from nonferrous pyrometallurgical industries

has been developed. Methods for arsenic removal can
be roughly classified as pyrometallurgy, hydrometallurgy, and combination of hydrometallurgical and
pyrometallurgical processes according to the characteristics of current technologies [6–9]. The pyrometallurgy, such as redox roasting, volatilization roasting
or eliminating arsenic in a vacuum, is a relatively traditional method, the As in residue is recovered in the
form of white arsenic. Although pyrometallurgical
processes are short and convenient, arsenic removal
efficiency is found to be relatively negative, the
increased costs and more stringent environmental standards also restrict the expansion of this approach [10].
In addition, As2O3 is not suitable for solidification and
disposal. The hydrometallurgy mainly includes hot
water leaching, acid leaching, alkali leaching, chloridization leaching, mechanical activation leaching, flotation and pressure leaching [11], then the arsenic is
separated and precipitated from the leaching liquid by
lime, ferric salt and sodium sulfide [12–18]. The
hydrometallurgical procedure has the superiority of
low-cost, no secondary pollution, efficient and high
removal rate. Alkaline leaching of removing arsenic in
As-containing filter cakes has been extensively
revealed in the past decades [19]. Yuhu Li [20] has dis-

372

ARSENIC REMOVAL FROM Cu–As-CONTAINING FILTER CAKES

covered arsenic removal using a mixed sodium
hydroxide-sodium sulfide (NaOH–Na2S) leach,
more than 90% As was extracted under the optimum
conditions. Kunhong Gu [21] utilized sodium hydroxide-hydrogen peroxide (NaOH–H2O2) to treat arsenic-bearing waste ash, the leaching rate of arsenic was
as high as 98.17%. Moreover, sodium hypochlorite
(NaClO) [22], sodium sulfide (Na2S) [23], mixed
sodium hydroxide-sodium thiocyanate (NaOH–
NaHS) [24, 25] was reported as leaching reagent elsewhere in literature.
In the previous study, mixed sodium-hydroxidesodium sulfide (NaOH–Na2S) or NaOH solution alkaline leaching has broad application to extract arsenic
from As-containing byproducts. There are some obvious defects of this method that the pH value of leachate,
around 12, could not only produce additional industrial
wastewater limiting economic production but also
affect the follow-up process. Furthermore, sodium-sulfide is easy to absorb moisture and difficult to store in an
atmospheric environment. It is difficult to control the
distribution of S2– in the solution, and the local S2–
concentration increase will result in a large consumption of sodium sulfide besides.
In this paper, the Na2CO3 alkaline leaching is first
adopted to remove arsenic from Cu–As-containing
filter cakes. The influence of processing parameters
including α(Na2CO3), temperature, L/S ratio and
leaching time from Cu–As-containing filter cakes
have been investigated systematically in this study.
2. EXPERIMENTAL
2.1. Materials
Cu–As-containing filter cakes used in this study
were provided by Northern copper Co., Ltd. The main
chemical compositions of the ore were given in Table 1.
As shown in this table, the contents of copper and arsenic are 44.60 and 9.40 wt %, respectively. According to
the X-ray diffraction (XRD, D/Max-2550 X-ray diffractometer with CuKα radiation from 10° to 80°) pattern of Cu–As-containing filter cakes shown in Fig. 1,
the dominant phases were CuS, As2S3. The surface
morphology analyses were obtained in Fig. 2 by using a
JSM-6360LV spectrometer coupled with energy-dispersive X-ray spectroscopy (EDX-GENESIS60S).
2.2. Methods and Conditions
The leaching experiments were conducted in a
500 mL three-neck round bottom flask fitted with an
overhead mechanical stirrer. For each leaching experiment, according to the pre-determined liquid-solid
ratio, sodium carbonate at a pre-determined excess
coefficient was transferred into the reactor, which was
then heated to the desired temperature through the
digital homoeothermic water bath before adding the
Cu–As-containing filter cakes. After a certain period
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373

500
CuS
As2S3
S

450
400
Intensity

350
300
250
200
150
100
0

10

20

30

40 50
2θ, deg

60

70

80

90

Fig. 1. The X-ray diffraction pattern of Cu–As-containing
filter cakes.

50 μm
Fig. 2. The scanning electron microscopy image of
Cu‒As-containing filter cakes.

of time, the mechanical stirrer and the digital
homoeothermic water bath were both switched off.
The leach slurries were then filtered and washed several times with deionized water. All solutions were collected for subsequent analysis. In the meantime, the
leach residues were dried in an oven at 100°C for 6 h,
weighed. Finally, Elemental analysis of the residue
samples was conducted using X-ray fluorescence
spectrometer (XRF, XRF-1800, Shimadzu, Japan)
and filtered leaching liquor was conducted using
Table 1. The main chemical analysis of Cu–As-containing
filter cakes
Element

Cu

S

As

Content, wt % 44.60 32.61 9.40

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Fe

Pb

Sb

3.24

0.33

0.22

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JIAN-HUI WU et al.

As
Cu

80

60 //
3

The probable main reactions in the leaching process
are as follows:
Contents of metals
in leach residues, wt %

Leaching efficiency, %

100

52
48
44
/
12
8
4
0
1.5

2

2.0
2.5
α(Na2CO3)

3.0

0
2.0
2.5
α(Na2CO3)

3.0

Fig. 3. Effect of α(Na2CO3) on the leaching efficiency of
As, Cu (conditions: L/S = 6/1, t = 3 h, T = 90°C).

atomic emission spectrometer (ICP-AES, IRIS
IntrepidIIXSP, Thermo Electron Corporation, USA).
In the leaching tests, all chemicals were of analytical reagent grade and were used as precursors without
further purification. The effects of α(Na2CO3), temperature, L/S ratio and leaching time were investigated. Only one of the above parameters was allowed
to vary while all others were fixed.
The removal efficiency of arsenic can be calculated
as Eq. (1)

M leach residues ωleach residues
(1)
× 100%,
M raw materialωraw material
where RMe is the removal ratio (%), Mleach residues are the
weights of arsenic in the leach residues (g), ωleach residues
are the contents of arsenic in leach residues (wt %),
Mraw material is the weights of raw material (g), ωraw material
is the contents of metals in raw material (wt %).
The loss efficiency of copper can be calculated as
Eq. (2)
RMe =

RMe =

Cleaching solutionVleaching solution
M raw materialωraw material

× 100%,

(3)

2As 2S3 + 3CO32 − → AsO33− + 3AsS33− + 3CO2 ↑.

(4)

Firstly, the single carbonate leaching of Cu–Ascontaining filter cakes was carried out under the following conditions: α(Na2CO3) of 2.7, the reaction
temperature of 80°C, the leaching time of 2 h, the liquid-to-solid ratio of 6 : 1 (mL/g). Under these conditions, the contents of arsenic and copper in the leachate were 10.49, 50.06 mg/L, respectively. There was a
possibility of dissolution loss of metallic copper in
Cu–As-containing filter cakes while arsenic was
dissolved during the experiment. It’s universally
acknowledged that metal sulfide precipitation was an
important method for recovering valuable metals due
to the low solubility of metal sulfide precipitates.
Therefore, sodium thiosulfate, superior to sodium sulfide, should be added to enrich copper.

1
1.5

2As 2S3 + 2CO32 − → AsO2− + 3AsS2− + 2CO2 ↑,

(2)

where RMe is the loss ratio (%), Cleaching solution is the
concentrations of copper in the leaching solution
(g/L), Vleaching solution is the volume of leaching solution (L), Mraw material is the weights of raw material (g),
ωraw material is the contents of metals in raw material
(wt %).
3. RESULTS AND DISCUSSION
3.1. Alkaline Leaching
3.1.1. Effects of leaching system. Cu–As-containing filter cakes will dissolve in the carbonate system.

Na 2S2O3 + H2O = Na 2SO4 + H2S,

(5)

(6)
Cu 2 + + S2– = CuS.
Thus, the mixture or single carbonate leaching
were selected depending on the content of soluble
copper in the Cu–As-containing filter cakes. Single
carbonate leaching system was selected in this experiment.
3.1.2. Effects of α(Na2CO3). The effects of α(Na2CO3)
on the removal of As and the loss of Cu are shown in
Fig. 3. It is found that the removal of arsenic increased
from 70.47 to 96.34%. The content of arsenic in the
leachate decreased from 6.44 to 0.573% when the
α(Na2CO3) increased from 1.35 (90 g/L) to 3.15
(210 g/L), while the loss of Cu decreased gradually
from 2.62 to 0.47%. The solubility of arsenic raised
with the α(Na2CO3) increased. However, the concentration of arsenic in the leaching solution declined
slightly when alkalinity continued to increase. On the
contrary, S2– in leachate kept releasing as the increase of
sodium carbonate content. Therefore, the α(Na2CO3)
of 2.7 (180 g/L) was selected for the rest of the leaching
experiments.
3.1.3. Effects of temperature. The effects of temperature on the extraction of arsenic, as shown in Fig. 4,
indicated that the removal of arsenic increased from
48.44 to 97.83% and the content of arsenic decreased
from 8.86 to 0.30% because the solubility of arsenic in
leaching solution increased with the increase of temperature. While the loss of Cu decreased gradually
from 1.75 to 0.45% and then increased to 0.84% when
the temperature of increased from 30 to 90°C. The
release of sulfur ion was an exothermic process, thus
excessive temperature reduced the concentration of
Cu2+. Therefore, the temperature of 75°C was selected
for the rest of the leaching experiments.

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ARSENIC REMOVAL FROM Cu–As-CONTAINING FILTER CAKES

100

As
Cu

40 //
3

50
40
30
20
10
0
30

2

45 60 75 90
Temperature, °C

1
0
30

45

60
75
Temperature, °C

85
80 //
4
3
2
1
0

4
3
2
1
0

1.5
0
1

2

2

3
Time, h

3
Time, h

4

4

5

5

350
CuS

300
50
40
30
20
10 //
3.0
1.5
1.5
0

250

5:1

200
150
100
50

4:1 5:1 6:1 7:1 8:1
L/S ratio

4:1

80 //

50
40
30
20
10 //
4.5
3.0

Fig. 5. Effect of leaching time on the leaching efficiency of
As, Cu (conditions: α(Na2CO3) = 2.7, L/S = 6/1,
T = 75°C).

Intensity

90

85

1

As
Cu
Contents of metals
in leach residues, wt %

Leaching efficiency, %

95

90

90

Fig. 4. Effect of temperature on the leaching efficiency of
As, Cu (conditions: α(Na2CO3) = 2.7, L/S = 6/1, t = 3 h).

100

Contents of metals
in leach residues, wt %

60

Leaching efficiency, %

80

As
Cu

95
Contents of metals
in leach residues, wt %

Leaching efficiency, %

100

375

6:1
L/S ratio

7:1

0
0

8:1

Fig. 6. Effect of L/S ratio on the leaching efficiency of As,
Cu (conditions: α(Na2CO3) = 2.7, t = 3 h, T = 75°C).

20

30

40 50
2θ, deg

60

70

80

90

Fig. 7. The X-ray diffraction pattern of 2nd residue.

3.1.4. Effects of leaching time. The effects of leaching time on the removal of As and the loss of Cu are
shown in Fig. 5. It is found that the removal of arsenic
increased from 87.46 to 99.48% and the content of
arsenic decreased from 2.65 to 0.16%, while the loss of
Cu decreased gradually from 0.37 to 0.27% with the
leaching time increased from 1 to 5 h. Therefore, the
leaching time of 3 h was selected for the rest of
the leaching experiments.
3.1.5. Effects of L/S ratio. The effects of the L/S
ratio on the removal of As and the loss of Cu are shown
in Fig. 6. It is found that the removal of As increased
from 88.39 to 98.2% and the content of As decreased
from 1.33 to 0.41%, while the loss of Cu decreased
gradually from 1.09 to 0.28% under the L/S ratio of
4 to 8. The loss of arsenic from leaching residue
increased in the process of liquid-solid separation due
to the high viscosity of the solution and the difficult
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10

distribution of S2– when the liquid-solid ratio was low.
Therefore, the L/S ratio of 6 was selected for the rest
of the leaching experiments.
3.1.6. Optimum conditions and residue characterization. Based on the results of factorial experiments,
the optimum conditions were determined as follow:
α(Na2CO3) of 2.7, leaching time of 3 h, leaching temperature of 75°C, and the liquid-to-solid ratio of 6 : 1
(mL/g). The optimum experiment was repeated three
times, and the results were listed in Table 2. The average removal efficiency of As was 96.47% and the average loss rate of Cu was 0.41%.
The XRD pattern listed in Fig. 7 of leach residue
showed that the main phase in the leach residue was
CuS, and no phase containing arsenic was detected,
which proved that the removal of arsenic from Cu–Ascontaining filter cakes was expected. Giving consideration to a better economic value and lower arsenic

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JIAN-HUI WU et al.

Table 2. The results of optimum leaching experiments
Leaching efficiency, %
Experiment no.
Cu
As
1
2
3

0.40
0.45
0.37
0.41

Average

Leaching residue, wt %

95.89
96.71
96.82
96.47

Cu

As

50.71
51.71
51.83
51.42

0.957
0.902
0.893
0.917

Table 3. The second leaching experimental results
Leaching solution, g L –1

Additional Na2CO3, g L–1

Cu

10
40
70
100

0.23
0.053
0.046
0.054

content, the leach residue, in which the content of
copper was 51.42%, could be recycled as raw material
to a copper smelting plant.
3.1.7. Circulating process for arsenic removal. The
recycling process of arsenic-containing leaching solution can reduce the amount of wastewater for subsequent steps, and fully utilize unreacted sodium carbonate in the leachate to achieve energy saving and
emission reduction. Circulating process for arsenic
removal was carried out using the arsenic-containing
leaching solution generated from the leaching tests
under the previously determined optimum conditions.
The concentration of arsenic, copper in the first
leaching solution were 11.72, 0.0169 g/L. At normal
temperature, the saturated concentration of arsenic
(mainly arsenate and thioarsenite) in the alkaline
solution is about 30 g/L.
The concentration of soda in the leachate was
about 110 g/L after theoretical analysis. For the sake of
optimum concentration of sodium carbonate in the
Table 4. The third and fourth leaching experimental results
–1
Leaching Leaching solution, g L
times
Cu
As

Third
Fourth

0.2657
0.5003

Arsenic removal
efficiency, %

28.147
31.361

As

Arsenic removal
efficiency, %

21.81
22.18
23.11
24.43

84.17
90.94
95.89
97.91

circulating process, a series of experiments were carried out by varying Na2CO3 concentration from 10 to
100 g/L, while leaching time, leaching temperature,
and liquid-to-solid ratio were fixed at 3 h, 75°C, 6 : 1
(mL/g), respectively. The second leaching experimental results were shown in Table 3. According to experimental results of circulation leaching of Cu–As-containing filter cakes, the arsenic removal efficiency
could be achieved in 95.89% when 70 g/L Na2CO3 was
added and kept almost constant with the increase of
extra Na2CO3 concentration. The amount of soda
added in the circulating process and the remaining soda
in the leachate almost made the excess coefficient
achieve 2.7, which was consistent with the optimal conditions obtained from the first alkaline leaching.
The maximum cycle index of the leachate was
explored on the basis of the conclusions obtained from
the above experiments, and 70 g/L Na2CO3 was added
to the leachate each time. The experimental details
were shown in Table 4, from which it was concluded
that although sufficient sodium carbonate was supplied, the arsenic content in the leachate became saturated, and the leaching rate was greatly reduced when
the leachate was recycled for the third time.
3.2. Oxidation of As(III) to As(V)

89.76
68.94

Table 5. Chemical composition of sulphur from the oxidation process

In the leaching solution, Arsenic mainly exists in
the form of AsS33− and AsO33− . Sodium persulfate is a
strong oxidizing agent that can oxidize AsS33− and
AsO33− to AsO34− as expressed by reactions (7), (8).
3−

2−

Element
S0

As

S

Cu

Fe

Si

AsS3 + 8OH + S2O8

wt %

wt %

wt %

wt %

wt %

= AsO4 + 3S↓ + 8SO4 + 4H2O,

Content

0.015

99.40

0.12

0.07

0.395

3−

–

(7)

2−

AsO33− + 2OH– + S2O82 − = AsO34− + 2SO42 − + H2O. (8)

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ARSENIC REMOVAL FROM Cu–As-CONTAINING FILTER CAKES

Figure 8 shows the XRD pattern of the sulphur
after acid washing from the oxidation process. Chemical analysis carried out on the pickling residue indicated that the formed precipitate mainly consisted of
S0 (99.40%) with traces of As, Cu, and Fe. Element S0
from the oxidation process could be applied as industrial sulphur because the purity of sulphur met industrial standards. Furthermore, oxidation produced less
sulphur due to side reaction (9), hence there was no
necessity to consider the probable adverse effects of
sulphur.

S0 + 8OH– + 3S2O82 − = 7SO24 − + 4H2O.

800
700
600
500
400
300
200
100
0
–100

S

Intensity

Therefore, the factors that will affect the oxidation
results, including the molar ratio of sodium persulfate
to trivalent arsenic, reaction temperature and time,
were investigated. In our study, As(III) can be completely oxidized to As(V) using 3 times the stoichiometric amount of sodium persulfate in 2 h at room
temperature. The oxidized residue sulphur was washed
by 2 mol/L diluted sulfuric acid with a liquid-solid
ratio of 1 : 1.

377

0

10

20

30

40 50
2θ, deg

60

70

80

90

Fig. 8. The X-ray diffraction pattern of the element S0 from
the oxidation process.

350
Scorodite

300

(9)

Crystalline scorodite has been considered a suitable
medium for the stable immobilization and long-term
storage of arsenic as this mineral combines low leaching rates with high arsenic content. This work utilizing
the arsenic-containing solution obtained from previous progress, scorodite was acquired to decline As(V)
in the aqueous phase. Pentavalent arsenic could turn
into scorodite with good crystallinity in Fe/As the
molar rate of 2 at 75°C for 7 h when pH value was 1.5.
Table 6 reports experimental details of the final filtrate
before and after precipitation. It’s observed that arsenic removal efficiency has reached about 99% via the
formation of scorodite with trace metal impurities.
The XRD characterization results of the scorodite precipitate at optimum conditions are shown in Fig. 9. It
could be seen from Fig. 9 that the precipitates displayed an ideal crystallinity. Table 7 indicates the
results of environmental leaching tests of the dried
precipitate samples. The TCLP tests showed that the
arsenic concentration was 2.01 mg L–1 and low concentration of Cu and Fe after 7 days of treatment.
Therefore, the scorodite synthesized from this study
had a safe leachability.
The final sodium sulfate couldn’t be returned to
the precipitation process owing to the negative effects
of the formation of scorodite. Figure 10 displayed
XRD patterns of unfavorable product Na3Fe(SO4)3
when circulating sodium sulfate. Therefore, sodium
sulfate should be crystallized to produce sodium sulfide, which could be disposed of the Cu–As-containing filter cakes.
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200
150
100
50
0
10

20

30

40
2θ, deg

50

60

70

Fig. 9. The X-ray diffraction pattern of scorodite precipitate obtained at a Fe/As molar ratio of 1.5 at 75°C for 7 h
in pH 1.5.

1400
Na3Fe(SO4)3

1200
1000
Intensity

3.3. Precipitation of Arsenic

Intensity

250

800
600
400
200
0

–200
0

10

20

30

40 50
2θ, deg

60

70

80

90

Fig. 10. The X-ray diffraction pattern of by-product when
circulating sodium sulfate.
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JIAN-HUI WU et al.

Fig. 11. Proposed arsenic removal scheme.

3.4. Proposed Arsenic Removal Scheme
Based on the experimental results, a proposed scheme
of the process was shown in Fig. 11. First, Cu‒As-containing filter cakes were leached by Na2CO3 at atmospheric pressure for separation of arsenic and copper,
and the leaching solution was once again subjected to
the leaching process until arsenic (mainly arsenate and
thioarsenite) in the alkaline solution reached saturated. Second, As(III) was oxidized to As(V) through
further addition of sodium persulfate to the filtrate
then solid/liquid separation to obtain the As(V)-solution and S0 solid. Finally, arsenic was precipitated in
Table 6. Analytical data of the final filtrate before and after
precipitation

Before
After

As

Cu

Fe

mg/L

mg/L

mg/L

0.75

0.93

0.38

0.69

49.33%

25.81%

As

Cu

Fe

2.01

0.03

1.10

28420
0.28

Removal efficiency

99%

Table 7. The results of TCLP test
Element
Content, mg L–1

the form of scorodite via the addition of Fe2(SO4)3 and
sulphuric acid.
4. CONCLUSIONS
The removal of arsenic as an impurity in Cu–Ascontaining filter cakes by leaching in Na2CO3 media
was investigated in this work. The results obtained and
discussed herein can be summarized as follows.
(1) Base on the results of factorial experiments, the
optimized leaching conditions were established as follows:
α(Na2CO3) = 2.7, liquid–solid ratio 6 : 1 (mL/g),
leaching temperature 75°C, leaching time 3 h. 96.47%
of arsenic was removed from the Cu–As-containing
filter cakes. Under the optimum conditions, the average leaching efficiencies of arsenic, copper were 96.47
and 0.41%, respectively.
(2) Circulating process for arsenic removal was carried out by varying Na2CO3 concentration while keeping the other variables constant. It was conducted that
the arsenic removal efficiency could be achieved in
95.89% when 70 g/L Na2CO3 was added and kept
almost constant with the increase of extra Na2CO3
concentration.
(3) The main chemical composition in leach residue
was CuS, and the content of arsenic was less than 1%.
The leach residue could be applied as raw material to a
copper smelting plant.

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(4) As(III) could be completely oxidized to As(V)
using 3 times the stoichiometric amount of sodium
persulfate in 2 h at room temperature. Highly pure elemental sulphur was generated during the oxidation
procedure.
(5) As(V) was precipitated to scorodite with good
crystallinity in Fe/As the molar rate of 2 at 75°C for
7 h when pH value is 1.5. The TCLP tests indicated
that the scorodite synthesized from this study had a
safe leachability (2.01 mg L–1 after 7 days).
CONFLICT OF INTEREST
The authors claim that they have no conflict of interest.

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