Materials Transactions, Vol. 52, No. 7 (2011) pp. 1462 to 1470
#2011 The Japan Institute of Metals
Recovery and Concentration of Precious Metals
from Strong Acidic Wastewater
Hisayoshi Umeda1;2; * , Atsushi Sasaki2 , Kunihiko Takahashi2 ,
Kazutoshi Haga1 , Yasushi Takasaki3 and Atsushi Shibayama1
1
Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan
Yokohama Metal Co., Ltd., Sagamihara 252-0132, Japan
3
International Center for Research and Education on Mineral and Energy Resources, Akita University, Akita 010-8502, Japan
2
Generally, trace precious metals remaining in wastewaters generated from the refining process of precious metals are not recovered, due to
a relatively high processing cost as well as various technical problems. Recovery of precious metals from wastewaters is very important for the
conservation of resources and the protection of environment. However, wastewaters containing a large amount of ammonium ion (NH4 þ ) cannot
be treated by general neutralization operation, due to formation of metal ammine complexes with increasing pH. In this study, the possibility of
recovering precious metals and other valuable metals from wastewaters by various traditional metallurgical processes such as cementation,
neutralization and reduction, were investigated. A recovery of 99% Copper (Cu), 96% Palladium (Pd), and 85% Gold (Au) by cementation using
Iron (Fe) powder, and 99.6% Cu, 99.5% Pd by cementation using Aluminum (Al) powder was achieved. However, complete recovery of all
valuable metals by a one-step cementation process was not possible. On the other hand, precious metals and other valuable metals including
Copper and Indium, etc., were precipitated by combining neutralization, deammoniation and reduction processes. Results showed that the
recovery of Platinum (Pt) in the reduction process was improved by adding deammoniation step. Finally, precious metals are concentrated in the
crude copper metal by fusion process. The recovery of Au, Ag, Pd was more than 91%, and that of Pt was about 71%.
[doi:10.2320/matertrans.M2010432]
(Received December 24, 2010; Accepted April 5, 2011; Published May 25, 2011)
Keywords: wastewater, precious metals, cementation, neutralization, deammoniation, reduction, fusion
1.
Introduction
Precious metals such as Gold (Au), Silver (Ag), Platinum
(Pt) and Palladium (Pd), etc., are utilized in various
manufacturing fields including jewellery, electronics and
dental industries.1,2) In recent years, especially in the
emerging countries, the demand for precious metals has
increased with the significant growth of economy. However,
it is difficult to economically or technologically acquire these
precious metals due to the small amount of supply by specific
producing countries in unevenly distributed production
areas.3–5) There is a growing anxiety about securing a stable
supply, and therefore, the development of recycling technologies is very important to utilize resources efficiently.
Examples of scrap wastes with high precious metal contents
that can be recovered are shown in Fig. 1.
Several recovery techniques including; leaching,6,7) cementation,8,9) precipitation,10) solvent extraction11–14) and
biological methods15–17) for the precious metals from scrap
materials have been developed over the years. Generally,
scrap materials containing precious metals with relatively
high concentrations over 1% are treated by precipitation
methods. A schematic flowsheet for the precipitation method18) to recover Au, Pt, Pd precious metals from scrap
materials is shown in Fig. 2. Various types of both alkali and
acidic solutions are used throughout precious metal recovery
process which eventually report as process wastewater,
strongly acidic and contain a large amount of ammonium
ion (NH4 þ ).
*Graduate
Student, Akita University. Corresponding author, E-mail:
[email protected]
However, such wastewaters contain precious metals not
recovered from precipitation methods with concentrations
around 10 mg/L, and other valuable metals with concentrations ranging from several mg/L to more than 10,000
mg/L. The recovery of precious metals and other valuable
metals in the wastewater by general neutralization operation
is usually difficult due to the formation of metal ammine
complex with increasing pH.
The objective of this work was to recover the precious
metals and other valuable metals that remain in such
wastewaters containing a large amount of ammonium ion
(NH4 þ ) by using the traditional hydrometallurgical processes
such as cementation, neutralization and reduction.
2.
Experimental
2.1 Wastewater sample
The strongly acidic (pH 0.15) wastewater sample used in
this experiment was collected from the recycling process of
precious metals such as Au, Ag, Pt and Pd, etc. Quantitative
analysis of the wastewater is given in Table 1. The concentration of Au, Ag and Pt in this wastewater varied from 10 to
20 mg/L. On the other hand, the concentration of Pd in this
sample was higher than that of usual wastewater. The
wastewater also contained many other metals such as nickel
(Ni), lead (Pb), tin (Sn), bismuth (Bi), and so on. In the study
of precipitation method (‘‘Cementation’’ and ‘‘Neutralization’’), we focused on five metals, namely; gold (Au),
platinum (Pt), palladium (Pd), copper (Cu), and indium (In).
Cu was considered due to its high concentration (12,293
mg/L) in this sample, and the other four elements are
expensive metals that are found only in rare amount.
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater
1463
Scrap Materials
Precipitate
(AgCl)
dissolution
Aqua regia
Concentration/
Denitrification
HCl
Crude-Au
Au-Recovery
(Reduction)
Na2SO3
Pt-Recovery
(Precipitation)
(K2PtCl6)
H 2O 2
Crude-Pt
Pd-Recovery
(Precipitation)
(PdCl2(NH3)2)
NH3
(1) Discarded electronic parts (CPU, etc.)
Crude-Pd
KCl
HCl
Wastewater
Fig. 2
Flowsheet for recovering Au-Pt-Pd by means of precipitation.
Table 1
The composition of waste water.
(1) Elements (mg/L)
(2) Used Jewellery (Ring, Chain)
Au
Pt
Pd
Ag
Cu
Fe Pb Bi Ni Sn Cr Al
Zn
In
11.3 20.9 183.1 10.9 12,293 244 111 277 642 122 44 166 4,375 1,008
(2) Others (mg/L)
(3) Used dental alloy
Fig. 1
2.2
Example of scrap materials containing precious metals.
Preliminary test — Precipitation method for metallic ions in wastewater
2.2.1 Cementation
Separation of dissolved metals in the wastewater by
cementation was performed by the addition of iron (Fe),
aluminum (Al), and zinc (Zn) metallic powders into 250 mL
of wastewater (in a 300 mL beaker) and stirred continuously
with a magnetic stirrer. Mole ratio of metallic powder and Cu
(metallic powder/Cu in wastewater) was 1 and 2, so each
metallic powder was added at 0.2 and 0.4 mol/L respectively.
5 mL of samples were drawn from the solution at different
Cl
NO3 NH4 þ
8:3 104
9:5 104
4:8 104
times between 10 to 360 min. The samples were properly
diluted, and each metallic ion in solution was analyzed by
using an ICP-AES equipment.
2.2.2 Neutralization
For evaluation of neutralization effect, 100 mL of wastewater was put in a 300 mL beaker and was stirred using a
magnetic stirrer, while solution pH was adjusted using a
5 mol/L sodium hydroxide (NaOH). After adjusting pH to
target pH (pH 2–12), the solution was stirred for 15 min, after
which stirring was stopped and left overnight for precipitation to occur. The treated solution was filtered under reduced
pressure by using a 5C filter paper, and then the filtrate was
properly diluted and analyzed by using an ICP-AES equipment for evaluation of metal ion concentrations.
2.2.3 Reduction of filtrated water generated from neutralization
In general, neutralization process is utilized to recover
heavy metals such as copper, etc. Therefore, the reduction
process was investigated to recover valuable metals such as
precious metals remaining in the filtrated water from
neutralization process.
Solution sample for reduction test was obtained as follows;
first, deammonization for the removal of ammonium ions
(NH4 þ ) in the filtrated water (non-deammoniated water)
1464
H. Umeda et al.
Table 2 The composition of samples obtained for reduction experiments.
Reduction conditions
Precious metals concentration (mg/L)
Au
Pt
Pd
Deammoniation
4.2
15.3
136.8
Non-deammoniation
5.8
12.9
131.5
from neutralization at pH 6 was performed with the addition
of NaOH and heating. This process is usually referred to as
the ‘‘Ammonia stripping method’’.19) To evaluate efficiency
of the reduction process, tests were performed both with and
without deammoniation process and results obtained are
given in Table 2.
100 mL of wastewater sample (deammoniated or nondeammoniated) was put into a 300 mL beaker and was stirred
by using a magnetic stirrer. 3 mL of sodium borohydride
(2.6 mol/L-NaBH4 solution) was added while solution pH
was monitored using a pH-meter. After boiling the sample
for 15 min, it was removed from heater and left overnight,
followed by filtration under reduced pressure using a 5C
filter paper. The filtrated water was properly diluted and
prepared for analysis of each metallic ion in solution by ICPAES.
2.3
Wastewater treatment by combining precipitation
and fusion
The recovery of precious metals and other valuable metals
were investigated by combining precipitation and fusion
processes, where fusion was performed to decrease the
volume of the precipitate.20) Precipitation method was
selected from the results of preliminary cementation, neutralization and reduction tests discussed above.
Starting wastewater solution sample for the combined
precipitation and fusion tests was a 3,000 mL solution, details
provided in Table 1. The precipitate obtained from the
optimum conditions was charged into a graphite melting pot
with 50 g of flux (borax) and heated in a high frequency
induction furnace under air atmosphere. The surface temperature of fusion was measured by using an infrared radiation
thermometer. The sample in the melting pot was taken out in
a mold after maintaining temperature at 1300 to 1700 C for
30 min and then cooled rapidly. Metallic fraction and crushed
slag were dissolved using HNO3 solution or aqua regia, and
then these solutions were properly diluted.
Distributions of each element in the metal and slag
fractions were finally determined from analysis of these
solutions by using ICP-AES equipment.
3.
Results and Discussion
3.1 Cementation
3.1.1 Effect of cementation time
The effect of cementation time on the recovery of Cu, In,
Au, Pd and Pt in the wastewater sample during addition of Fe,
Al and Zn metal powders was investigated and results
obtained are shown in Fig. 3. Mole ratio of the metal powders
(cementation agents) to Cu content in the wastewater
(cementation agent/Cu) was fixed at 2. (Cementation agent
was added at 0.4 mol/L).
Figure 3 shows recovery behavior of the different elements. Complete recovery of Cu was achieved within 3–6 h
by using Fe and Al powders. Maximum recovery of Au was
obtained in less than 30 min and did not change up to 6 h.
Also, the recovery of Pd reached maximum (>90%) within
1–3 h by using Fe and Al powders and remained constant up
to 6 h. Pt recovery was only about 20% during cementation
time of 6 h.
In general, it is difficult to recover indium by cementation
method because standard electrode potential of indium is
lower than that of other elements such as precious metals.
However, indium was recovered by using Zn powder, unlike
Fe and Al powders. It was also found that the pH of solution
increased during the cementation tests, depending upon
the type of the cementation agent being used. The initial pH
of solution was 0.15. However, the pH of solutions at the
end of cementation with Fe, Al and Zn increased to 1.2, 3.5
and 5.6, respectively. A final solution pH of 5.6 with Zn
powder suggests that the recovery of indium in solution
occurs as indium hydroxide precipitation due to increasing
pH.
When Zn powder was used as a cementation agent, pH of
the solution was greatly increased, compared to the case of
using Fe and Al powder. More work is needed to clarify this
behavior. Due to the complex composition of wastewater,
this reaction mechanism is still being investigated.
Results given in Fig. 3 indicate that the optimum cementation time is 6 h, due to the complete recovery of copper with
high concentration (12,293 mg/L). At 6 h cementation time,
the recovery of 99.5% Cu (Al power), 98.2% In (Zn powder),
73.4% Au (Fe powder), 99.5% Pd (Al powder) and 24.9% Pt
(Zn powder) were achieved.
3.1.2 Effect of cementation agents
The effect of different cementation agents; Fe, Al and Zn
metal powders (added at 0.2 and 0.4 mol/L) on the recovery
of Cu, Au, In, Pd and Pt in the wastewater during cementation
tests performed for 6 h is shown in Fig. 4.
With Fe metal powder addition, Cu recovery reached over
95% for 0.2 mol/L and over 99% when Fe concentration was
increased to 0.4 mol/L. Au recovery reached over 80% with
0.2 mol/L Fe addition but decreased slightly to 70% when Fe
addition was increased to 0.4 mol/L. Pd recovery was also
very high above 90% for both Fe concentrations. However,
indium and Pt recoveries were extremely low.
With Al metal powder additions, over 99% Cu was
recovered with Al concentration of 0.4 mol/L and reduced
by half when Al was reduced to 0.2 mol/L. Au recovery
remained consistent at near 70% under both 0.2 and 0.4
mol/L Al addition. Pd recovery was extremely high at over
98% for Al addition at 0.2 mol/L and over 99% at Al addition
of 0.4 mol/L. Like in Fe metal powder addition, In and Pt
recovery was again very low at below 5 and 20% for the two
metals respectively.
Cementation with 0.4 mol/L Zn metal powder when
compared against Fe and Al metal powders, a very high
recovery of indium at over 99% was achieved. Recoveries
of other metals remained lower compared to Fe and Al
powders.
Pure Pt metal has high standard potential and can be
recovered by cementation on the electro chemical principle.
(1) Cu
100
90
80
70
60
50
40
30
20
10
0
In - recovery (%)
Cu - recovery (%)
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater
Fe
Al
Zn
0
100
200
300
(2) In
100
90
80
70
60
50
40
30
20
10
0
Fe
Al
Zn
0
400
Pd - recovery (%)
Au - recovery (%)
(3) Au
Fe
Al
Zn
0
100
200
300
400
200
300
400
(4) Pd
100
90
80
70
60
50
40
30
20
10
0
Fe
Al
Zn
0
100
200
300
400
Reaction time, t /min
Reaction time, t /min
Pt - recovery (%)
100
Reaction time, t /min
Reaction time, t /min
100
90
80
70
60
50
40
30
20
10
0
1465
(5) Pt
50
40
30
20
10
0
0
Fe
Al
Zn
100
200
300
400
Reaction time, t /min
Fig. 3 Effect of type of cementing agent (Fe, Al and Zn) on recovery of various metals during cementation.
However, in this cementation test, the recovery of Pt was
only about 20% and this might be due to formation of
complex ions, which has low standard electrode potential
making it difficult for cementation. In the cementation
condition with 0.4 mol/L Zn powder, the recovery ratio of
indium was more than 99%. This behavior might be due to
the increase of pH value (refer to section 3.1.1 Effect of
cementation time).
When 0.2 mol/L or 0.4 mol/L of Fe powder was added to
the wastewater, the recovery ratio of Cu, Au and Pd were
relatively high in both experimental conditions. Also, the
highest recovery ratio of Au was achieved by using Fe
powder as a cementation agent. Therefore, comparative
tests of cementation agents confirmed usefulness of Fe
powder.
3.1.3 Summary of cementation
Results from cementation section showed that Cu, Au and
Pd can be recovered by using Fe and Al powder, but Fe
powder was more effective in recovering of Au than Al
powder. Also, the optimum addition amount of cementation
agent and cementation time is 0.4 mol/L and 6 h, respectively, due to the complete recovery of copper with high
concentration (12,293 mg/L). Indium was recovered by
using Zn powder (added at 0.4 mol/L), unlike Fe and Al
powder. Under the cementation conditions tested (0.2 and
0.4 mol/L Fe, Al, Zn powder addition, 6 h), the recovery of
Pt was only about 20%.
3.2 Neutralization by using NaOH
Figure 5 shows recovery of metals dissolved in the
wastewater at different pH conditions from pH 2 to pH 12
(pH adjusted with NaOH). Complete indium recovery was
achieved at pH 4 and Cu recovery reached over 95% between
pH 5 and 6 but decreased to below 10% when pH was further
increased to over pH 7. The recovery of Pd remained
constant at around 12% under all pH conditions. Au and Pt
recoveries reached maximum at around pH 7 of 50% and
30% for the two metals respectively but decreased at higher
pH conditions due to re-dissolution. Following are the factors
that might influence the amount of re-dissolution. There are
large quantities of ammonium ions (NH4 þ ) in wastewater
used in this experiment. Therefore, NH4 þ becomes NH3 with
increasing pH. For example, Cu forms an ammonia complex
species and thus dissolves again.21)
1466
H. Umeda et al.
Cu
Au
In
Recovery (%)
(2) Al-powder addition:
0.4mol/L
Pd
0.2mol/L
Pt
0.4mol/L
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
Cu
In
Au
Pd
Pt
1 2 3 4 5 6 7 8 9 10 11 12 13
pH
Fig. 5 Behavior of each metallic element during neutralization by using
NaOH.
Cu
Au
In
(3) Zn-powder addition:
Recovery (%)
0.2mol/L
Metal recovery (%)
Recovery (%)
(1) Fe-powder addition:
100
90
80
70
60
50
40
30
20
10
0
Pd
0.2mol/L
Pt
(1) Fe-Cementation
(2) Al-Cementation
(0.4 mol/L, 360 min)
(0.4 mol/L, 360 min)
0.4mol/L
100
90
80
70
60
50
40
30
20
10
0
Cu
In
Au
Pd
Pt
0
20
40
60
80
100
20
In
Pd
60
80
100
(4) NaOH-Neutralization
(0.4 mol/L, 360 min)
Au
40
Metal recovery (%)
(3) Zn-Cementation
Cu
(pH: 6.0, 15 min)
Pt
Cu
In
Fig. 4 Effect of type of metallic powder on recovery of various elements at
the end of cementation.
3.3
Comparison between cementation and neutralization
Both cementation and neutralization processes investigated for the recovery of Cu, In, Au, Pt, Pd and their results
discussed in Figs. 4 and 5 are summarized in Fig. 6.
According to Fig. 6, it was found that all valuable metals
cannot be recovered in one-step process such as cementation
or neutralization.
In comparing cementation and neutralization, recovery of
Cu and indium by neutralization was faster than that of
cementation. Complete recovery of the two metals can be
achieved at pH 6. However, the recovery of precious metals
(Au, Pd, Pt) is extremely low and continue to remain in the
filtered water. Therefore, the reduction process was investigated to recover precious metals remaining in the filtrated
water from neutralization process.
3.4
0
Metal recovery (%)
Reduction of filtrated water generated from neutralization by using NaBH4
In general, hydrazine (N2 H4 ) is often utilized to recover
precious metals in chemical industries. However, Pt in this
Au
Pd
Pt
0
20
40
60
80
Metal recovery (%)
100
0
20
40
60
80
100
Metal recovery (%)
Fig. 6 Comparison of recovering valuable metals between cementation
and neutralization process.
sample (Table 2) was not recovered by using hydrazine. On
the other hand, the recovery of Pt was improved by using
sodium borohydride (NaBH4 ). Therefore, NaBH4 was used
as reduction agent. In addition, the effect of deammoniation
during NaBH4 -reduction process on the recovery of precious
metals was investigated and the results are shown in Fig. 7.
Complete recovery of Au was achieved under both
conditions (deammoniation and non-deammoniation) and
across all pH range. Complete recovery of Pd was achieved
only under the deammoniation condition. Also, recovery of
Pd increased with rising of pH value under non-deammoniation condition. The recovery ratio of Pd reached over 95%
at pH 12, but was very low at pH below 4. The recovery of Pt
under deammoniation condition was constant at 70% across
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater
(b) Without deammoniation
Recovery (%)
Recovery (%)
(a) With deammoniation
100
90
80
70
60
50
40
30
20
10
0
Au
Pd
Pt
1467
100
90
80
70
60
50
40
30
20
10
0
Au
Pd
Pt
1 2 3 4 5 6 7 8 9 10 11 12 13
1 2 3 4 5 6 7 8 9 10 11 12 13
pH
pH
Fig. 7 Effect of deammoniation on recovery of precious metals during reduction by using NaBH4 . (Reduction of filtrated water generated
from neutralization.)
all pH range, but under non-deammoniation condition, Pt
recovery gradually increased from 10% at pH 3 and reached
40% at pH 12. In the case of Pd and Pt, under nondeammoniation condition, the recovery ratio of these precious metals increased with rise of pH value. This behavior
might be due to the fact that ammonium ions in the sample
were removed during reduction at the relatively high pH
value.
According to Fig. 7, it was found that Au and Pd remaining
in the filtrated water of neutralization were recovered by
reduction process under deammoniation condition. Also, the
recovery of Pt from the filtrated water was improved,
compared to the reduction under non-deammoniation condition. Au, Pd and Pt can be recovered in any pH value, but it
is expected that the reduction apparatus is damaged in the
case of using strong acidic or alkali solutions. Therefore, it is
most suitable that the pH value is controlled at around 7 for
the reduction process.
3.5
Wastewater treatment — combination of neutralization, reduction and fusion
According to Fig. 6 and Fig. 7, it was found that Cu and In
can be recovered by neutralization at pH 6, and the filtrated
water from neutralization can be treated for recovery of Au,
Pd and Pt by reduction at pH 7. Therefore, the recovery of
precious metals and other valuable metals were investigated
by combining neutralization, deammoniation, reduction and
fusion processes.
The flowsheet of wastewater treatment is shown in Fig. 8.
This process consists of three steps; (1) neutralization, (2)
reduction (under deammoniation condition) and (3) fusion.
3.5.1 Behaviour of each metallic element during neutralization and reduction
The results of neutralization and reduction are shown in
Fig. 9. When wastewater was treated by neutralization at
pH 6, Fe, Pb, Bi, Sn, Cr, Al, In were completely recovered,
and about 96% of Cu was also recovered. On the other hand,
recoveries of Au, Pt, Ag, Pd, Ni, Zn were consistently low at
below 30%, where Pt exhibited the lowest recovery of only
14.7%. After neutralization, the filtrated water was treated by
deammoniation and reduction. The pH value of reduction
was controlled at 7.5. As a result, Au, Ag, Pd which remained
Wastewater
NaOH
Flux (borax)
Neutralization
Filtration
Precipitate
Fusion
Metal
(Precious metal
contained)
Filtrated water
Slag
NaOH
Deammoniation
Filtration
Precipitate
Filtrated water
NaBH4
Reduction
Filtration
Precipitate
Final wastewater
Fig. 8 Flowsheet of wastewater treatment. (Combination of Neutralization, Reduction and Fusion.)
in solution were also recovered. Also the recovery of Pt in
the treated water was 78.3%. About 50% of Zn remained in
solution.
The precipitate recovered by reduction contained a large
amount of Au, Ag, Pd and Pt. And therefore, this precipitate
can be treated in refining process of precious metals.
However, about 20–30% of Au, Ag, Pd and Pt are lost to
precipitate during neutralization for Cu and In recovery. In
this study, due to the complete recovery of all valuable
metals, both precipitates that had been recovered by
neutralization and reduction were mixed and melted in the
final step of the process, referred to as fusion.
3.5.2 Behaviour of each metallic element during fusion
The weight of each precipitate obtained from neutralization and reduction were 95.6 g and 30.0 g. These precipitates
and 50 g of flux (borax) were melted at 1300 to 1700 C. As a
result, separation of phase occurred by taking advantage of
the difference in specific gravity. A glass-shaped slag phase
was formed in the upper part, while a metal phase was formed
in the lower part.
1468
H. Umeda et al.
(1) Precious metals
Cu
Pt
Fe
Ag
Pb
Pd
Bi
20
40
60
80 100
Cu
5000
Intensity (a.u.)
Au
0
6000
(2) Other metals -1
0
20
Recovery (%)
40
60
4000
3000
2000
80 100
1000
Recovery (%)
0
(3) Other metals -2
(4) Other metals -3
Ni
Al
Sn
Zn
Cr
In
0
20
40
60
80 100
20°
0
20
40
60
80°
<Concentrate (Crude copper)>
Recovery (%)
<Wastewater>
Grade
Content
Recovery
(%)
Wastewater
(mass%)
MetalÞ
(mass%)
Wastewater
(mg)
MetalÞ
(mg)
Au
0.0010
0.09
38.2
36.5
95.5
Pt
0.0018
0.11
58.5
41.6
71.1
Ag
0.0010
0.09
36.5
33.4
91.5
Pd
0.0157
1.32
555.1
519.1
93.5
MetalÞ
(mass%)
Wastewater
(g)
MetalÞ
(g)
Grade
Content
Recovery
(%)
Cu
1.029
80.1
38.11
31.43
82.5
Fe
0.021
0.78
0.70
0.31
43.8
Pb
0.010
0.20
0.37
0.08
21.0
Bi
0.026
1.66
0.87
0.65
75.3
Ni
0.056
4.69
1.90
1.84
96.6
Sn
0.011
0.68
0.33
0.27
80.5
Cr
Al
0.004
0.015
0.00
0.00
0.13
0.50
0.00
0.00
0.0
0.0
Zn
0.394
1.08
13.19
0.42
3.2
In
0.090
0.00
3.13
0.23
7.4
Metal was obtained from fusion process.
Table 3 shows both the recovery of various metals and the
composition (grade) of metal obtained from fusion process.
Also, Fig. 10 shows the X-ray diffraction pattern of metal
obtained from fusion. It can be seen that metal (concentrate)
obtained from this experiment contained mainly Cu and other
precious metals.
Recovery
Au : 38.2 mg
Processing
Au : 36.5 mg
95.5 %
Pt : 58.5 mg
Neutralization
Pt : 41.6 mg
71.1 %
Ag : 36.5 mg
(Deammoniation)
Reduction
Ag : 33.4 mg
91.5 %
Pd : 555.1 mg
Fusion
Pd : 519.1 mg
93.5 %
Table 3 Recovery of various metals and the composition (grade) of metal
obtained from fusion process.
Þ
70°
60°
<Exhaust gas>
80 100
Fig. 9 The recovery ratio of various elements at the end of neutralization
(pH of solution: 6.0) and reduction processing (pH of solution: 7.5) in the
wastewater treatment.
Wastewater
(mass%)
50°
Fig. 10 X-ray diffraction pattern of metal obtained by fusion.
1st treatment (Neutralization)
2nd treatment (Reduction)
(B) Other
metal
40°
2θ
Recovery (%)
(A) Precious
metal
30°
<Wastes>
< Slag>
< Final wastewater>
Au : 0.99 mg
2.6 %
Au : 0.72 mg
1.9 %
Ag : 0.98 mg
2.7 %
Pt : 10.9 mg
18.7 %
Fig. 11 Experimental results of precious metals recovery from initial
wastewater to the concentrate by wastewater treatment.
3.5.3 Material balance in the wastewater treatment
Figure 11 shows the recovered amount of precious metals
(such as Au, Pt, Ag, Pd). In addition, the material balance of
this wastewater treatment is shown in Fig. 12. This experiment was performed in air atmosphere, and maximum
melting temperature was 1700 C. As a result, the oxidation
and volatilization reaction occurred, and then, each metallic
element, unlike precious metals, was distributed to the slag
and the air. On the other hand, most of precious metals were
distributed to the Cu-metal (concentrate).22)
The main purpose of this experiment was to recover
precious metals. Au, Ag, Pt, Pd were absorbed and recovered
by a Cu-metal (concentrate). The recovery of Au, Ag, Pd was
more than 91%, whereas that of Pt was about 71%. On the
other hand, according to the preliminary test, when deammoniation of the wastewater was not performed, recovery of
Pt was about 20%. And about 70% of Pt remained in the
wastewater. Therefore, it is thought that recovery of Pt rises
due to the deammoniation of the wastewater. Relation
between the reduction of Pt and the deammoniation are
being investigated now.
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater
<Material Balance>
Wastewater
Au
< Recycling process >
< Conventional Cu-smelting >
Scrap
materials
Raw materials
Pt
Cu-matte process
Precious
metals
Ag
Chemical
treatment
Converter
Pd
Processing
1469
Cu
Wastewater
Anode furnace
Neutralization
Electrolytic refining
Fe
Pb
Neutralization
Bi
Ni
(Deammoniation)
Final
wastewater
Reduction
Slag
Fusion
Sn
Reduction
Cr
Concentrate
(Cu-metal)
Al
Fusion
Zn
(Precious metals
contained)
In
< Wastewater treatment >
0
20
40
60
80
100
Distribution (%)
Electrolytic
copper
Anode slime
Chemical
treatment
Residue
Precious
metals
< Refining process >
Fig. 13 Separation of precious metals from concentrate (Cu-metal) by the
conventional copper smelting process.
Concentrate (Metal, Crude copper)
Slag
Treated wastewater (Final wastewater)
Other (Exhaust gas, etc.)
Fig. 12 Distribution of various elements at the end of the wastewater
treatment. (Material balance.)
Also, Fig. 12 shows that the content of Cr and Al in slag is
high, the value is 98% and 82%, respectively. On the other
hand, the reduction ratio of Zn is low. Therefore, about 40%
of Zn remained in wastewater. In addition, ‘‘Other’’ in Fig. 12
shows the vaporization, due to the fact that the fume was
exhausted during the fusion process. The fume, which was
recovered by a plate made of ceramics, was white powder.
The white powder was dissolved in hydrochloric acid, and
then the acidic solution was properly diluted. After that,
analysis of acidic solution was performed by ICP-AES and
the results confirmed the presence of Cu, Pb, Zn, and In, etc.
3.5.4 Separation of precious metals from concentrate
(Cu-metal)
The concentrate obtained from the wastewater treatment in
this experiment was crude copper containing some precious
metals. In general, precious metals in the crude copper and
scrap materials, etc., can be recovered as a by-product of the
conventional copper smelting process.3,23) Figure 13 shows
the flowsheet for separation of precious metals from
concentrate (Cu-metal) obtained by wastewater treatment.
The concentrate is treated by converter and a refinement
furnace in the conventional copper smelting process, and
then, the concentrate is recovered as an anode. After that, the
anodes are sent to electrolytic refining process of copper.
Finally, ‘‘electrolytic copper (grade of 99.99%)’’ and slime
(the precipitate) containing precious metals are recovered.
The slime is then sent to refining process for recovering
precious metal. Au, Ag, Pt and Pd are recovered by
precipitation or solvent extraction. In recent years, it seems
that solvent extraction is utilized as compared to precipitation
due to the fact the process is simple and effective.24)
4.
Conclusions
Recovery of precious metals and other valuable metals
that remain in the wastewater containing a large amount
of ammonium ions (NH4 þ ) was investigated by using some
traditional hydrometallurgical processes such as cementation, neutralization and reduction. Following are the main
results of this experimental work.
(1) Precious metals and other valuable metals cannot be
recovered by one-step process such as cementation or
neutralization, and therefore, combining some processes is
necessary to recover all valuable metals completely.
(2) When wastewater was treated by neutralization at
pH 6, Fe, Pb, Bi, Sn, Cr, Al, In were completely recovered,
and about 96% of Cu was also recovered. On the other hand,
Au, Pt, Ag, Pd, Ni, Zn recovery remains below 30%, with
Pt lowest at 14.7%.
(3) After neutralization, the filtrated water was treated by
deammoniation and reduction. The pH value of reduction
was controlled at 7.5. As a result, Au, Ag, Pd which remained
in solution were also recovered. In addition, the recovery
ratio of Pt in treated water was 78.3%. About 50% of Zn
remained in solution.
(4) Wastewater was treated by combining neutralization,
deammoniation, reduction and fusion. Finally, precious
metals were concentrated in the metallic fraction which
mainly contains copper. The recovery of Au, Ag, Pd was
more than 91%, and that of Pt was about 71%.
(5) During fusion, it was found that vaporization of some
metals (such as Cu, Pb, Zn, and In) occurred. However, these
metals can be recovered by using dust collector.
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