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Recovery and Concentration of Precious Metals
from Strong Acidic Wastewater
Hisayoshi Umeda1;2;*, Atsushi Sasaki2, Kunihiko Takahashi2,Kazutoshi Haga1, Yasushi Takasaki3 and Atsushi Shibayama1
1Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan2Yokohama Metal Co., Ltd., Sagamihara 252-0132, Japan3International Center for Research and Education on Mineral and Energy Resources, Akita University, Akita 010-8502, Japan
Generally, trace precious metals remaining in wastewaters generated from the refining process of precious metals are not recovered, due toa relatively high processing cost as well as various technical problems. Recovery of precious metals from wastewaters is very important for theconservation of resources and the protection of environment. However, wastewaters containing a large amount of ammonium ion (NH4
þ) cannotbe treated by general neutralization operation, due to formation of metal ammine complexes with increasing pH. In this study, the possibility ofrecovering 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 usingIron (Fe) powder, and 99.6% Cu, 99.5% Pd by cementation using Aluminum (Al) powder was achieved. However, complete recovery of allvaluable metals by a one-step cementation process was not possible. On the other hand, precious metals and other valuable metals includingCopper and Indium, etc., were precipitated by combining neutralization, deammoniation and reduction processes. Results showed that therecovery of Platinum (Pt) in the reduction process was improved by adding deammoniation step. Finally, precious metals are concentrated in thecrude 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 variousmanufacturing fields including jewellery, electronics anddental industries.1,2) In recent years, especially in theemerging countries, the demand for precious metals hasincreased with the significant growth of economy. However,it is difficult to economically or technologically acquire theseprecious metals due to the small amount of supply by specificproducing countries in unevenly distributed productionareas.3–5) There is a growing anxiety about securing a stablesupply, and therefore, the development of recycling tech-nologies is very important to utilize resources efficiently.Examples of scrap wastes with high precious metal contentsthat can be recovered are shown in Fig. 1.
Several recovery techniques including; leaching,6,7) ce-mentation,8,9) precipitation,10) solvent extraction11–14) andbiological methods15–17) for the precious metals from scrapmaterials have been developed over the years. Generally,scrap materials containing precious metals with relativelyhigh concentrations over 1% are treated by precipitationmethods. A schematic flowsheet for the precipitation meth-od18) to recover Au, Pt, Pd precious metals from scrapmaterials is shown in Fig. 2. Various types of both alkali andacidic solutions are used throughout precious metal recoveryprocess which eventually report as process wastewater,strongly acidic and contain a large amount of ammoniumion (NH4
þ).
However, such wastewaters contain precious metals notrecovered from precipitation methods with concentrationsaround 10 mg/L, and other valuable metals with concen-trations ranging from several mg/L to more than 10,000mg/L. The recovery of precious metals and other valuablemetals in the wastewater by general neutralization operationis usually difficult due to the formation of metal amminecomplex with increasing pH.
The objective of this work was to recover the preciousmetals and other valuable metals that remain in suchwastewaters containing a large amount of ammonium ion(NH4
þ) by using the traditional hydrometallurgical processessuch as cementation, neutralization and reduction.
2. Experimental
2.1 Wastewater sampleThe strongly acidic (pH 0.15) wastewater sample used in
this experiment was collected from the recycling process ofprecious metals such as Au, Ag, Pt and Pd, etc. Quantitativeanalysis of the wastewater is given in Table 1. The concen-tration of Au, Ag and Pt in this wastewater varied from 10 to20 mg/L. On the other hand, the concentration of Pd in thissample was higher than that of usual wastewater. Thewastewater also contained many other metals such as nickel(Ni), lead (Pb), tin (Sn), bismuth (Bi), and so on. In the studyof precipitation method (‘‘Cementation’’ and ‘‘Neutraliza-tion’’), 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 (12,293mg/L) in this sample, and the other four elements areexpensive metals that are found only in rare amount.
*Graduate Student, Akita University. Corresponding author, E-mail:
Materials Transactions, Vol. 52, No. 7 (2011) pp. 1462 to 1470#2011 The Japan Institute of Metals
2.2 Preliminary test — Precipitation method for metal-lic ions in wastewater
2.2.1 CementationSeparation 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 mLof wastewater (in a 300 mL beaker) and stirred continuouslywith a magnetic stirrer. Mole ratio of metallic powder and Cu(metallic powder/Cu in wastewater) was 1 and 2, so eachmetallic powder was added at 0.2 and 0.4 mol/L respectively.5 mL of samples were drawn from the solution at different
times between 10 to 360 min. The samples were properlydiluted, and each metallic ion in solution was analyzed byusing an ICP-AES equipment.2.2.2 Neutralization
For evaluation of neutralization effect, 100 mL of waste-water was put in a 300 mL beaker and was stirred using amagnetic stirrer, while solution pH was adjusted using a5 mol/L sodium hydroxide (NaOH). After adjusting pH totarget pH (pH 2–12), the solution was stirred for 15 min, afterwhich stirring was stopped and left overnight for precipita-tion to occur. The treated solution was filtered under reducedpressure by using a 5C filter paper, and then the filtrate wasproperly diluted and analyzed by using an ICP-AES equip-ment for evaluation of metal ion concentrations.2.2.3 Reduction of filtrated water generated from neu-
tralizationIn general, neutralization process is utilized to recover
heavy metals such as copper, etc. Therefore, the reductionprocess was investigated to recover valuable metals such asprecious metals remaining in the filtrated water fromneutralization 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)
(1) Discarded electronic parts (CPU, etc.)
(2) Used Jewellery (Ring, Chain)
(3) Used dental alloy
Fig. 1 Example of scrap materials containing precious metals.
dissolution
Scrap Materials
Aqua regia
Concentration/Denitrification
Pt-Recovery(Precipitation)
(K2PtCl6)
Pd-Recovery(Precipitation)(PdCl2(NH3)2)
Precipitate(AgCl)
Crude-Au
Crude-Pt
Crude-Pd
HCl
Na2SO3
H2O2
KCl
NH3
Au-Recovery(Reduction)
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)
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)
Cl� NO3� NH4
þ
8:3� 104 9:5� 104 4:8� 104
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1463
from neutralization at pH 6 was performed with the additionof NaOH and heating. This process is usually referred to asthe ‘‘Ammonia stripping method’’.19) To evaluate efficiencyof the reduction process, tests were performed both with andwithout deammoniation process and results obtained aregiven in Table 2.
100 mL of wastewater sample (deammoniated or non-deammoniated) was put into a 300 mL beaker and was stirredby using a magnetic stirrer. 3 mL of sodium borohydride(2.6 mol/L-NaBH4 solution) was added while solution pHwas monitored using a pH-meter. After boiling the samplefor 15 min, it was removed from heater and left overnight,followed by filtration under reduced pressure using a 5Cfilter paper. The filtrated water was properly diluted andprepared for analysis of each metallic ion in solution by ICP-AES.
2.3 Wastewater treatment by combining precipitationand fusion
The recovery of precious metals and other valuable metalswere investigated by combining precipitation and fusionprocesses, where fusion was performed to decrease thevolume of the precipitate.20) Precipitation method wasselected from the results of preliminary cementation, neu-tralization and reduction tests discussed above.
Starting wastewater solution sample for the combinedprecipitation and fusion tests was a 3,000 mL solution, detailsprovided in Table 1. The precipitate obtained from theoptimum conditions was charged into a graphite melting potwith 50 g of flux (borax) and heated in a high frequencyinduction furnace under air atmosphere. The surface temper-ature of fusion was measured by using an infrared radiationthermometer. The sample in the melting pot was taken out ina mold after maintaining temperature at 1300 to 1700�C for30 min and then cooled rapidly. Metallic fraction and crushedslag were dissolved using HNO3 solution or aqua regia, andthen these solutions were properly diluted.
Distributions of each element in the metal and slagfractions were finally determined from analysis of thesesolutions by using ICP-AES equipment.
3. Results and Discussion
3.1 Cementation3.1.1 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 resultsobtained 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 agentwas added at 0.4 mol/L).
Figure 3 shows recovery behavior of the different ele-ments. Complete recovery of Cu was achieved within 3–6 hby using Fe and Al powders. Maximum recovery of Au wasobtained in less than 30 min and did not change up to 6 h.Also, the recovery of Pd reached maximum (>90%) within1–3 h by using Fe and Al powders and remained constant upto 6 h. Pt recovery was only about 20% during cementationtime of 6 h.
In general, it is difficult to recover indium by cementationmethod because standard electrode potential of indium islower than that of other elements such as precious metals.However, indium was recovered by using Zn powder, unlikeFe and Al powders. It was also found that the pH of solutionincreased during the cementation tests, depending uponthe type of the cementation agent being used. The initial pHof solution was 0.15. However, the pH of solutions at theend of cementation with Fe, Al and Zn increased to 1.2, 3.5and 5.6, respectively. A final solution pH of 5.6 with Znpowder suggests that the recovery of indium in solutionoccurs as indium hydroxide precipitation due to increasingpH.
When Zn powder was used as a cementation agent, pH ofthe solution was greatly increased, compared to the case ofusing Fe and Al powder. More work is needed to clarify thisbehavior. Due to the complex composition of wastewater,this reaction mechanism is still being investigated.
Results given in Fig. 3 indicate that the optimum cemen-tation time is 6 h, due to the complete recovery of copper withhigh 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 Znmetal powders (added at 0.2 and 0.4 mol/L) on the recoveryof Cu, Au, In, Pd and Pt in the wastewater during cementationtests performed for 6 h is shown in Fig. 4.
With Fe metal powder addition, Cu recovery reached over95% for 0.2 mol/L and over 99% when Fe concentration wasincreased to 0.4 mol/L. Au recovery reached over 80% with0.2 mol/L Fe addition but decreased slightly to 70% when Feaddition was increased to 0.4 mol/L. Pd recovery was alsovery high above 90% for both Fe concentrations. However,indium and Pt recoveries were extremely low.
With Al metal powder additions, over 99% Cu wasrecovered with Al concentration of 0.4 mol/L and reducedby half when Al was reduced to 0.2 mol/L. Au recoveryremained consistent at near 70% under both 0.2 and 0.4mol/L Al addition. Pd recovery was extremely high at over98% for Al addition at 0.2 mol/L and over 99% at Al additionof 0.4 mol/L. Like in Fe metal powder addition, In and Ptrecovery was again very low at below 5 and 20% for the twometals respectively.
Cementation with 0.4 mol/L Zn metal powder whencompared against Fe and Al metal powders, a very highrecovery of indium at over 99% was achieved. Recoveriesof other metals remained lower compared to Fe and Alpowders.
Pure Pt metal has high standard potential and can berecovered by cementation on the electro chemical principle.
Table 2 The composition of samples obtained for reduction experiments.
Reduction conditionsPrecious metals concentration (mg/L)
Au Pt Pd
Deammoniation 4.2 15.3 136.8
Non-deammoniation 5.8 12.9 131.5
1464 H. Umeda et al.
However, in this cementation test, the recovery of Pt wasonly about 20% and this might be due to formation ofcomplex ions, which has low standard electrode potentialmaking it difficult for cementation. In the cementationcondition with 0.4 mol/L Zn powder, the recovery ratio ofindium was more than 99%. This behavior might be due tothe increase of pH value (refer to section 3.1.1 Effect ofcementation time).
When 0.2 mol/L or 0.4 mol/L of Fe powder was added tothe wastewater, the recovery ratio of Cu, Au and Pd wererelatively high in both experimental conditions. Also, thehighest recovery ratio of Au was achieved by using Fepowder as a cementation agent. Therefore, comparativetests of cementation agents confirmed usefulness of Fepowder.3.1.3 Summary of cementation
Results from cementation section showed that Cu, Au andPd can be recovered by using Fe and Al powder, but Fepowder was more effective in recovering of Au than Alpowder. Also, the optimum addition amount of cementationagent and cementation time is 0.4 mol/L and 6 h, respec-tively, due to the complete recovery of copper with high
concentration (12,293 mg/L). Indium was recovered byusing Zn powder (added at 0.4 mol/L), unlike Fe and Alpowder. Under the cementation conditions tested (0.2 and0.4 mol/L Fe, Al, Zn powder addition, 6 h), the recovery ofPt was only about 20%.
3.2 Neutralization by using NaOHFigure 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 wasachieved at pH 4 and Cu recovery reached over 95% betweenpH 5 and 6 but decreased to below 10% when pH was furtherincreased to over pH 7. The recovery of Pd remainedconstant at around 12% under all pH conditions. Au and Ptrecoveries reached maximum at around pH 7 of 50% and30% for the two metals respectively but decreased at higherpH conditions due to re-dissolution. Following are the factorsthat might influence the amount of re-dissolution. There arelarge quantities of ammonium ions (NH4
þ) in wastewaterused in this experiment. Therefore, NH4
þ becomes NH3 withincreasing pH. For example, Cu forms an ammonia complexspecies and thus dissolves again.21)
01020304050
0
Reaction time, t /min
Pt
- re
cove
ry (
%)
0102030405060708090
100
0
Reaction time, t /min
Pd
- re
cove
ry (
%)
0102030405060708090
100
0
Reaction time, t /min
Au
- re
cove
ry (
%)
0102030405060708090
100
0
Reaction time, t /min
In -
rec
over
y (%
)
0102030405060708090
100
0
Reaction time, t /min
Cu
- re
cove
ry (
%)
(1) Cu (2) In
(4) Pd(3) Au
(5) Pt
Fe
Al
Zn
Fe
Al
Zn
Fe
Al
Zn
Fe
Al
Zn
Fe Al Zn
100 200 300 400 100 200 300 400
100 200 300 400100 200 300 400
100 200 300 400
Fig. 3 Effect of type of cementing agent (Fe, Al and Zn) on recovery of various metals during cementation.
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1465
3.3 Comparison between cementation and neutraliza-tion
Both cementation and neutralization processes investigat-ed for the recovery of Cu, In, Au, Pt, Pd and their resultsdiscussed in Figs. 4 and 5 are summarized in Fig. 6.According to Fig. 6, it was found that all valuable metalscannot be recovered in one-step process such as cementationor neutralization.
In comparing cementation and neutralization, recovery ofCu and indium by neutralization was faster than that ofcementation. Complete recovery of the two metals can beachieved at pH 6. However, the recovery of precious metals(Au, Pd, Pt) is extremely low and continue to remain in thefiltered water. Therefore, the reduction process was inves-tigated to recover precious metals remaining in the filtratedwater from neutralization process.
3.4 Reduction of filtrated water generated from neu-tralization by using NaBH4
In general, hydrazine (N2H4) is often utilized to recoverprecious metals in chemical industries. However, Pt in this
sample (Table 2) was not recovered by using hydrazine. Onthe other hand, the recovery of Pt was improved by usingsodium borohydride (NaBH4). Therefore, NaBH4 was usedas reduction agent. In addition, the effect of deammoniationduring NaBH4-reduction process on the recovery of preciousmetals was investigated and the results are shown in Fig. 7.
Complete recovery of Au was achieved under bothconditions (deammoniation and non-deammoniation) andacross all pH range. Complete recovery of Pd was achievedonly under the deammoniation condition. Also, recovery ofPd increased with rising of pH value under non-deammo-niation condition. The recovery ratio of Pd reached over 95%at pH 12, but was very low at pH below 4. The recovery of Ptunder deammoniation condition was constant at 70% across
0
10
20
30
40
50
60
70
80
90
100
Cu
Rec
over
y (%
)0.2mol/L 0.4mol/L(1) Fe-powder addition:
0
10
20
30
40
50
60
70
80
90
100
Cu
Rec
over
y (%
)
0.2mol/L 0.4mol/L(2) Al-powder addition:
0
10
20
30
40
50
60
70
80
90
100
Cu
Rec
over
y (%
)
0.2mol/L 0.4mol/L(3) Zn-powder addition:
Au In Pd Pt
Au In Pd Pt
Au In Pd Pt
Fig. 4 Effect of type of metallic powder on recovery of various elements at
the end of cementation.
0
10
20
30
40
50
60
70
80
90
100
1
pH
Met
al r
ecov
ery
(%)
Cu
In
Au
Pd
Pt
2 3 4 5 6 7 8 9 10 11 12 13
Fig. 5 Behavior of each metallic element during neutralization by using
NaOH.
(1) Fe-Cementation (0.4 mol/L, 360 min)
(2) Al-Cementation (0.4 mol/L, 360 min)
(3) Zn-Cementation (0.4 mol/L, 360 min)
(4) NaOH-Neutralization (pH: 6.0, 15 min)
0
Metal recovery (%)
CuIn
AuPdPt
0
Metal recovery (%)
0
Metal recovery (%)
CuIn
AuPdPt
0
Metal recovery (%)
20 40 60 80 100 20 40 60 80 100
20 40 60 80 10020 40 60 80 100
Fig. 6 Comparison of recovering valuable metals between cementation
and neutralization process.
1466 H. Umeda et al.
all pH range, but under non-deammoniation condition, Ptrecovery gradually increased from 10% at pH 3 and reached40% at pH 12. In the case of Pd and Pt, under non-deammoniation condition, the recovery ratio of these pre-cious metals increased with rise of pH value. This behaviormight be due to the fact that ammonium ions in the samplewere removed during reduction at the relatively high pHvalue.
According to Fig. 7, it was found that Au and Pd remainingin the filtrated water of neutralization were recovered byreduction process under deammoniation condition. Also, therecovery of Pt from the filtrated water was improved,compared to the reduction under non-deammoniation con-dition. Au, Pd and Pt can be recovered in any pH value, but itis expected that the reduction apparatus is damaged in thecase of using strong acidic or alkali solutions. Therefore, it ismost suitable that the pH value is controlled at around 7 forthe reduction process.
3.5 Wastewater treatment — combination of neutrali-zation, reduction and fusion
According to Fig. 6 and Fig. 7, it was found that Cu and Incan be recovered by neutralization at pH 6, and the filtratedwater from neutralization can be treated for recovery of Au,Pd and Pt by reduction at pH 7. Therefore, the recovery ofprecious metals and other valuable metals were investigatedby combining neutralization, deammoniation, reduction andfusion 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 neu-
tralization and reductionThe results of neutralization and reduction are shown in
Fig. 9. When wastewater was treated by neutralization atpH 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 atbelow 30%, where Pt exhibited the lowest recovery of only14.7%. After neutralization, the filtrated water was treated bydeammoniation and reduction. The pH value of reductionwas controlled at 7.5. As a result, Au, Ag, Pd which remained
in solution were also recovered. Also the recovery of Pt inthe treated water was 78.3%. About 50% of Zn remained insolution.
The precipitate recovered by reduction contained a largeamount of Au, Ag, Pd and Pt. And therefore, this precipitatecan be treated in refining process of precious metals.However, about 20–30% of Au, Ag, Pd and Pt are lost toprecipitate during neutralization for Cu and In recovery. Inthis study, due to the complete recovery of all valuablemetals, both precipitates that had been recovered byneutralization and reduction were mixed and melted in thefinal 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 neutraliza-tion and reduction were 95.6 g and 30.0 g. These precipitatesand 50 g of flux (borax) were melted at 1300 to 1700�C. As aresult, separation of phase occurred by taking advantage ofthe difference in specific gravity. A glass-shaped slag phasewas formed in the upper part, while a metal phase was formedin the lower part.
0
10
20
30
40
50
60
70
80
90
100
1
pH
Rec
over
y (%
)
Au Pd Pt
0
10
20
30
40
50
60
70
80
90
100
1
pH
Rec
over
y (%
)
AuPdPt
(a) With deammoniation (b) Without deammoniation
2 3 4 5 6 7 8 9 10 11 12 13 2 3 4 5 6 7 8 9 10 11 12 13
Fig. 7 Effect of deammoniation on recovery of precious metals during reduction by using NaBH4. (Reduction of filtrated water generated
from neutralization.)
Wastewater
Filtration
Filtrated water
NaBH4
Precipitate
NaOH
Flux (borax)
Final wastewater
Metal(Precious metal
contained)
Slag
Filtration
Filtration
NaOH
Filtrated water
Precipitate
Precipitate
Neutralization
Deammoniation
Reduction
Fusion
Fig. 8 Flowsheet of wastewater treatment. (Combination of Neutraliza-
tion, Reduction and Fusion.)
Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1467
Table 3 shows both the recovery of various metals and thecomposition (grade) of metal obtained from fusion process.Also, Fig. 10 shows the X-ray diffraction pattern of metalobtained from fusion. It can be seen that metal (concentrate)obtained from this experiment contained mainly Cu and otherprecious metals.
3.5.3 Material balance in the wastewater treatmentFigure 11 shows the recovered amount of precious metals
(such as Au, Pt, Ag, Pd). In addition, the material balance ofthis wastewater treatment is shown in Fig. 12. This experi-ment was performed in air atmosphere, and maximummelting temperature was 1700�C. As a result, the oxidationand volatilization reaction occurred, and then, each metallicelement, unlike precious metals, was distributed to the slagand the air. On the other hand, most of precious metals weredistributed to the Cu-metal (concentrate).22)
The main purpose of this experiment was to recoverprecious metals. Au, Ag, Pt, Pd were absorbed and recoveredby a Cu-metal (concentrate). The recovery of Au, Ag, Pd wasmore than 91%, whereas that of Pt was about 71%. On theother hand, according to the preliminary test, when deam-moniation of the wastewater was not performed, recovery ofPt was about 20%. And about 70% of Pt remained in thewastewater. Therefore, it is thought that recovery of Pt risesdue to the deammoniation of the wastewater. Relationbetween the reduction of Pt and the deammoniation arebeing investigated now.
(1) Precious metals (2) Other metals -1
(3) Other metals -2
1st treatment (Neutralization)2nd treatment (Reduction)
0
Pd
Ag
Pt
Au
Recovery (%)
0
Bi
Pb
Fe
Cu
Recovery (%)
0
Cr
Sn
Ni
Recovery (%)
0
In
Zn
Al
Recovery (%)
20 40 60 80 100 20 40 60 80 100
20 40 60 80 10020 40 60 80 100
(4) Other metals -3
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.
Table 3 Recovery of various metals and the composition (grade) of metal
obtained from fusion process.
(A) PreciousGrade Content
Recovery
metal Wastewater
(mass%)
Metal�Þ
(mass%)
Wastewater
(mg)
Metal�Þ
(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
(B) OtherGrade Content
Recovery
metal Wastewater
(mass%)
Metal�Þ
(mass%)
Wastewater
(g)
Metal�Þ
(g)(%)
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 0.004 0.00 0.13 0.00 0.0
Al 0.015 0.00 0.50 0.00 0.0
Zn 0.394 1.08 13.19 0.42 3.2
In 0.090 0.00 3.13 0.23 7.4
�ÞMetal was obtained from fusion process.
0
1000
2000
3000
4000
5000
6000
20°
2θ
Inte
nsit
y (a
.u.)
Cu
30° 40° 50° 60° 70° 80°
Fig. 10 X-ray diffraction pattern of metal obtained by fusion.
Au : 38.2 mg
Pt : 58.5 mg
Ag : 36.5 mg
Pd : 555.1 mg
<Wastewater>
Processing
Neutralization
(Deammoniation)Reduction
Fusion
Recovery
Au : 36.5 mg 95.5 %
Pt : 41.6 mg 71.1 %
Ag : 33.4 mg 91.5 %
Pd : 519.1 mg 93.5 %
<Concentrate (Crude copper)>
Au : 0.72 mg 1.9 %
Pt : 10.9 mg 18.7 %
< Final wastewater>
Au : 0.99 mg 2.6 %
Ag : 0.98 mg 2.7 %
<Exhaust gas>
< Slag>
<Wastes>
Fig. 11 Experimental results of precious metals recovery from initial
wastewater to the concentrate by wastewater treatment.
1468 H. Umeda et al.
Also, Fig. 12 shows that the content of Cr and Al in slag ishigh, the value is 98% and 82%, respectively. On the otherhand, the reduction ratio of Zn is low. Therefore, about 40%of Zn remained in wastewater. In addition, ‘‘Other’’ in Fig. 12shows the vaporization, due to the fact that the fume wasexhausted during the fusion process. The fume, which wasrecovered by a plate made of ceramics, was white powder.The white powder was dissolved in hydrochloric acid, andthen the acidic solution was properly diluted. After that,analysis of acidic solution was performed by ICP-AES andthe 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 preciousmetals. In general, precious metals in the crude copper andscrap materials, etc., can be recovered as a by-product of theconventional copper smelting process.3,23) Figure 13 showsthe flowsheet for separation of precious metals fromconcentrate (Cu-metal) obtained by wastewater treatment.The concentrate is treated by converter and a refinementfurnace in the conventional copper smelting process, andthen, the concentrate is recovered as an anode. After that, theanodes 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 recoveringprecious metal. Au, Ag, Pt and Pd are recovered byprecipitation or solvent extraction. In recent years, it seemsthat solvent extraction is utilized as compared to precipitationdue to the fact the process is simple and effective.24)
4. Conclusions
Recovery of precious metals and other valuable metalsthat remain in the wastewater containing a large amountof ammonium ions (NH4
þ) was investigated by using sometraditional hydrometallurgical processes such as cementa-tion, neutralization and reduction. Following are the mainresults of this experimental work.
(1) Precious metals and other valuable metals cannot berecovered by one-step process such as cementation orneutralization, and therefore, combining some processes isnecessary to recover all valuable metals completely.
(2) When wastewater was treated by neutralization atpH 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%, withPt lowest at 14.7%.
(3) After neutralization, the filtrated water was treated bydeammoniation and reduction. The pH value of reductionwas controlled at 7.5. As a result, Au, Ag, Pd which remainedin solution were also recovered. In addition, the recoveryratio of Pt in treated water was 78.3%. About 50% of Znremained in solution.
(4) Wastewater was treated by combining neutralization,deammoniation, reduction and fusion. Finally, preciousmetals were concentrated in the metallic fraction whichmainly contains copper. The recovery of Au, Ag, Pd wasmore than 91%, and that of Pt was about 71%.
(5) During fusion, it was found that vaporization of somemetals (such as Cu, Pb, Zn, and In) occurred. However, thesemetals can be recovered by using dust collector.
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