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
1Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan 2Yokohama Metal Co., Ltd., Sagamihara 252-0132, Japan 3International 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 to a relatively high processing cost as well as various technical problems. Recovery of precious metals from wastewaters is very important for the þ conservation of resources and the protection of environment. However, wastewaters containing a large amount of ammonium ion (NH4 ) 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 However, such wastewaters contain precious metals not recovered from precipitation methods with concentrations Precious metals such as Gold (Au), Silver (Ag), Platinum around 10 mg/L, and other valuable metals with concen- (Pt) and Palladium (Pd), etc., are utilized in various trations ranging from several mg/L to more than 10,000 manufacturing fields including jewellery, electronics and mg/L. The recovery of precious metals and other valuable dental industries.1,2) In recent years, especially in the metals in the wastewater by general neutralization operation emerging countries, the demand for precious metals has is usually difficult due to the formation of metal ammine increased with the significant growth of economy. However, complex with increasing pH. it is difficult to economically or technologically acquire these The objective of this work was to recover the precious precious metals due to the small amount of supply by specific metals and other valuable metals that remain in such producing countries in unevenly distributed production wastewaters containing a large amount of ammonium ion 3–5) þ areas. There is a growing anxiety about securing a stable (NH4 ) by using the traditional hydrometallurgical processes supply, and therefore, the development of recycling tech- such as cementation, neutralization and reduction. nologies is very important to utilize resources efficiently. Examples of scrap wastes with high precious metal contents 2. Experimental that can be recovered are shown in Fig. 1. Several recovery techniques including; leaching,6,7) ce- 2.1 Wastewater sample mentation,8,9) precipitation,10) solvent extraction11–14) and The strongly acidic (pH 0.15) wastewater sample used in biological methods15–17) for the precious metals from scrap this experiment was collected from the recycling process of materials have been developed over the years. Generally, precious metals such as Au, Ag, Pt and Pd, etc. Quantitative scrap materials containing precious metals with relatively analysis of the wastewater is given in Table 1. The concen- high concentrations over 1% are treated by precipitation tration of Au, Ag and Pt in this wastewater varied from 10 to methods. A schematic flowsheet for the precipitation meth- 20 mg/L. On the other hand, the concentration of Pd in this od18) to recover Au, Pt, Pd precious metals from scrap sample was higher than that of usual wastewater. The materials is shown in Fig. 2. Various types of both alkali and wastewater also contained many other metals such as nickel acidic solutions are used throughout precious metal recovery (Ni), lead (Pb), tin (Sn), bismuth (Bi), and so on. In the study process which eventually report as process wastewater, of precipitation method (‘‘Cementation’’ and ‘‘Neutraliza- strongly acidic and contain a large amount of ammonium tion’’), we focused on five metals, namely; gold (Au), þ ion (NH4 ). platinum (Pt), palladium (Pd), copper (Cu), and indium (In). Cu was considered due to its high concentration (12,293 *Graduate Student, Akita University. Corresponding author, E-mail: mg/L) in this sample, and the other four elements are [email protected] expensive metals that are found only in rare amount. Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1463
Scrap Materials
Precipitate dissolution Aqua regia (AgCl)
Concentration/ HCl Denitrification
Au-Recovery Crude-Au Na SO (Reduction) 2 3
Pt-Recovery H2O2 Crude-Pt (Precipitation) (1) Discarded electronic parts (CPU, etc.) (K2PtCl6) KCl
Pd-Recovery NH3 Crude-Pd (Precipitation) (PdCl2(NH3)2) 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 (2) Used Jewellery (Ring, Chain) 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) þ Cl NO3 NH4 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 waste- water 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 (3) Used dental alloy target pH (pH 2–12), the solution was stirred for 15 min, after which stirring was stopped and left overnight for precipita- Fig. 1 Example of scrap materials containing precious metals. tion 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 equip- 2.2 Preliminary test — Precipitation method for metal- ment for evaluation of metal ion concentrations. lic ions in wastewater 2.2.3 Reduction of filtrated water generated from neu- 2.2.1 Cementation tralization Separation of dissolved metals in the wastewater by In general, neutralization process is utilized to recover cementation was performed by the addition of iron (Fe), heavy metals such as copper, etc. Therefore, the reduction aluminum (Al), and zinc (Zn) metallic powders into 250 mL process was investigated to recover valuable metals such as of wastewater (in a 300 mL beaker) and stirred continuously precious metals remaining in the filtrated water from with a magnetic stirrer. Mole ratio of metallic powder and Cu neutralization process. (metallic powder/Cu in wastewater) was 1 and 2, so each Solution sample for reduction test was obtained as follows; metallic powder was added at 0.2 and 0.4 mol/L respectively. first, deammonization for the removal of ammonium ions þ 5 mL of samples were drawn from the solution at different (NH4 ) in the filtrated water (non-deammoniated water) 1464 H. Umeda et al.
Table 2 The composition of samples obtained for reduction experiments. Figure 3 shows recovery behavior of the different ele- Precious metals concentration (mg/L) ments. Complete recovery of Cu was achieved within 3–6 h Reduction conditions by using Fe and Al powders. Maximum recovery of Au was Au Pt Pd obtained in less than 30 min and did not change up to 6 h. Deammoniation 4.2 15.3 136.8 Also, the recovery of Pd reached maximum (>90%) within Non-deammoniation 5.8 12.9 131.5 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. from neutralization at pH 6 was performed with the addition In general, it is difficult to recover indium by cementation of NaOH and heating. This process is usually referred to as method because standard electrode potential of indium is the ‘‘Ammonia stripping method’’.19) To evaluate efficiency lower than that of other elements such as precious metals. of the reduction process, tests were performed both with and However, indium was recovered by using Zn powder, unlike without deammoniation process and results obtained are Fe and Al powders. It was also found that the pH of solution given in Table 2. increased during the cementation tests, depending upon 100 mL of wastewater sample (deammoniated or non- the type of the cementation agent being used. The initial pH deammoniated) was put into a 300 mL beaker and was stirred of solution was 0.15. However, the pH of solutions at the by using a magnetic stirrer. 3 mL of sodium borohydride end of cementation with Fe, Al and Zn increased to 1.2, 3.5 (2.6 mol/L-NaBH4 solution) was added while solution pH and 5.6, respectively. A final solution pH of 5.6 with Zn was monitored using a pH-meter. After boiling the sample powder suggests that the recovery of indium in solution for 15 min, it was removed from heater and left overnight, occurs as indium hydroxide precipitation due to increasing followed by filtration under reduced pressure using a 5C pH. filter paper. The filtrated water was properly diluted and When Zn powder was used as a cementation agent, pH of prepared for analysis of each metallic ion in solution by ICP- the solution was greatly increased, compared to the case of AES. using Fe and Al powder. More work is needed to clarify this behavior. Due to the complex composition of wastewater, 2.3 Wastewater treatment by combining precipitation this reaction mechanism is still being investigated. and fusion Results given in Fig. 3 indicate that the optimum cemen- The recovery of precious metals and other valuable metals tation time is 6 h, due to the complete recovery of copper with were investigated by combining precipitation and fusion high concentration (12,293 mg/L). At 6 h cementation time, processes, where fusion was performed to decrease the the recovery of 99.5% Cu (Al power), 98.2% In (Zn powder), volume of the precipitate.20) Precipitation method was 73.4% Au (Fe powder), 99.5% Pd (Al powder) and 24.9% Pt selected from the results of preliminary cementation, neu- (Zn powder) were achieved. tralization and reduction tests discussed above. 3.1.2 Effect of cementation agents Starting wastewater solution sample for the combined The effect of different cementation agents; Fe, Al and Zn precipitation and fusion tests was a 3,000 mL solution, details metal powders (added at 0.2 and 0.4 mol/L) on the recovery provided in Table 1. The precipitate obtained from the of Cu, Au, In, Pd and Pt in the wastewater during cementation optimum conditions was charged into a graphite melting pot tests performed for 6 h is shown in Fig. 4. with 50 g of flux (borax) and heated in a high frequency With Fe metal powder addition, Cu recovery reached over induction furnace under air atmosphere. The surface temper- 95% for 0.2 mol/L and over 99% when Fe concentration was ature of fusion was measured by using an infrared radiation increased to 0.4 mol/L. Au recovery reached over 80% with thermometer. The sample in the melting pot was taken out in 0.2 mol/L Fe addition but decreased slightly to 70% when Fe a mold after maintaining temperature at 1300 to 1700 C for addition was increased to 0.4 mol/L. Pd recovery was also 30 min and then cooled rapidly. Metallic fraction and crushed very high above 90% for both Fe concentrations. However, slag were dissolved using HNO3 solution or aqua regia, and indium and Pt recoveries were extremely low. then these solutions were properly diluted. With Al metal powder additions, over 99% Cu was Distributions of each element in the metal and slag recovered with Al concentration of 0.4 mol/L and reduced fractions were finally determined from analysis of these by half when Al was reduced to 0.2 mol/L. Au recovery solutions by using ICP-AES equipment. remained consistent at near 70% under both 0.2 and 0.4 mol/L Al addition. Pd recovery was extremely high at over 3. Results and Discussion 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 3.1 Cementation recovery was again very low at below 5 and 20% for the two 3.1.1 Effect of cementation time metals respectively. The effect of cementation time on the recovery of Cu, In, Cementation with 0.4 mol/L Zn metal powder when Au, Pd and Pt in the wastewater sample during addition of Fe, compared against Fe and Al metal powders, a very high Al and Zn metal powders was investigated and results recovery of indium at over 99% was achieved. Recoveries obtained are shown in Fig. 3. Mole ratio of the metal powders of other metals remained lower compared to Fe and Al (cementation agents) to Cu content in the wastewater powders. (cementation agent/Cu) was fixed at 2. (Cementation agent Pure Pt metal has high standard potential and can be was added at 0.4 mol/L). recovered by cementation on the electro chemical principle. Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1465
(1) Cu (2) In 100 100 90 90 80 80 70 70 60 60 50 Fe 50 Fe 40 40 Al Al 30 30
Zn (%) In - recovery Zn Cu - recovery (%) Cu - recovery 20 20 10 10 0 0 0 100 200 300 400 0 100 200 300 400 Reaction time, t /min Reaction time, t /min
(3) Au (4) Pd 100 100 90 90 80 80 70 70 Fe 60 60 Al 50 50 Zn 40 Fe 40 30 30 Al Pd - recovery (%) Pd - recovery Au - recovery (%) - recovery Au 20 20 10 Zn 10 0 0 0 100 200 300 400 0 100 200 300 400 Reaction time, t /min Reaction time, t /min
(5) Pt 50 40 Fe Al Zn 30 20 10 0 Pt - recovery (%) Pt - recovery 0 100 200 300 400 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 concentration (12,293 mg/L). Indium was recovered by only about 20% and this might be due to formation of using Zn powder (added at 0.4 mol/L), unlike Fe and Al complex ions, which has low standard electrode potential powder. Under the cementation conditions tested (0.2 and making it difficult for cementation. In the cementation 0.4 mol/L Fe, Al, Zn powder addition, 6 h), the recovery of condition with 0.4 mol/L Zn powder, the recovery ratio of Pt was only about 20%. indium was more than 99%. This behavior might be due to the increase of pH value (refer to section 3.1.1 Effect of 3.2 Neutralization by using NaOH cementation time). Figure 5 shows recovery of metals dissolved in the When 0.2 mol/L or 0.4 mol/L of Fe powder was added to wastewater at different pH conditions from pH 2 to pH 12 the wastewater, the recovery ratio of Cu, Au and Pd were (pH adjusted with NaOH). Complete indium recovery was relatively high in both experimental conditions. Also, the achieved at pH 4 and Cu recovery reached over 95% between highest recovery ratio of Au was achieved by using Fe pH 5 and 6 but decreased to below 10% when pH was further powder as a cementation agent. Therefore, comparative increased to over pH 7. The recovery of Pd remained tests of cementation agents confirmed usefulness of Fe constant at around 12% under all pH conditions. Au and Pt powder. recoveries reached maximum at around pH 7 of 50% and 3.1.3 Summary of cementation 30% for the two metals respectively but decreased at higher Results from cementation section showed that Cu, Au and pH conditions due to re-dissolution. Following are the factors Pd can be recovered by using Fe and Al powder, but Fe that might influence the amount of re-dissolution. There are þ powder was more effective in recovering of Au than Al large quantities of ammonium ions (NH4 ) in wastewater þ powder. Also, the optimum addition amount of cementation used in this experiment. Therefore, NH4 becomes NH3 with agent and cementation time is 0.4 mol/L and 6 h, respec- increasing pH. For example, Cu forms an ammonia complex tively, due to the complete recovery of copper with high species and thus dissolves again.21) 1466 H. Umeda et al.
(1) Fe-powder addition: 0.2mol/L 0.4mol/L 100 100 90 90 Cu 80 80 70 In 60 70 Au 50 60 40 Pd
Recovery (%) Recovery 30 50 Pt 20 40 10 0 30 Cu Au In Pd Pt (%) Metal recovery 20 (2) Al-powder addition: 0.2mol/L 0.4mol/L 10 100 90 0 80 1 2 3 4 5 6 7 8 9 10 11 12 13 70 60 pH 50 40 Fig. 5 Behavior of each metallic element during neutralization by using Recovery (%) Recovery 30 NaOH. 20 10 0 (1) Fe-Cementation (2) Al-Cementation Cu Au In Pd Pt (0.4 mol/L, 360 min) (0.4 mol/L, 360 min)
(3) Zn-powder addition: 0.2mol/L 0.4mol/L Cu 100 In 90 80 Au Pd 70 Pt 60 50 0 20 40 60 80 100 0 20 40 60 80 100 40 Metal recovery (%) Metal recovery (%) Recovery (%) Recovery 30 20 10 (3) Zn-Cementation (4) NaOH-Neutralization 0 (0.4 mol/L, 360 min) (pH: 6.0, 15 min) Cu Au In Pd Pt Cu Fig. 4 Effect of type of metallic powder on recovery of various elements at In the end of cementation. Au Pd Pt
3.3 Comparison between cementation and neutraliza- 0 20 40 60 80 100 0 20 40 60 80 100 tion Metal recovery (%) Metal recovery (%) Both cementation and neutralization processes investigat- ed for the recovery of Cu, In, Au, Pt, Pd and their results Fig. 6 Comparison of recovering valuable metals between cementation and neutralization process. 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. sample (Table 2) was not recovered by using hydrazine. On In comparing cementation and neutralization, recovery of the other hand, the recovery of Pt was improved by using Cu and indium by neutralization was faster than that of sodium borohydride (NaBH4). Therefore, NaBH4 was used cementation. Complete recovery of the two metals can be as reduction agent. In addition, the effect of deammoniation achieved at pH 6. However, the recovery of precious metals during NaBH4-reduction process on the recovery of precious (Au, Pd, Pt) is extremely low and continue to remain in the metals was investigated and the results are shown in Fig. 7. filtered water. Therefore, the reduction process was inves- Complete recovery of Au was achieved under both tigated to recover precious metals remaining in the filtrated conditions (deammoniation and non-deammoniation) and water from neutralization process. across all pH range. Complete recovery of Pd was achieved only under the deammoniation condition. Also, recovery of 3.4 Reduction of filtrated water generated from neu- Pd increased with rising of pH value under non-deammo- tralization by using NaBH4 niation condition. The recovery ratio of Pd reached over 95% In general, hydrazine (N2H4) is often utilized to recover at pH 12, but was very low at pH below 4. The recovery of Pt precious metals in chemical industries. However, Pt in this under deammoniation condition was constant at 70% across Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1467
(a) With deammoniation (b) Without deammoniation 100 100 90 90 80 80 Au Pd 70 70 Pt 60 60 50 50 40 40 Recovery (%) Recovery Recovery (%) Recovery 30 30 20 20 Au Pd Pt 10 10 0 0 1 2345678910111213 1 2345678910111213 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 Wastewater recovery gradually increased from 10% at pH 3 and reached NaOH 40% at pH 12. In the case of Pd and Pt, under non- Neutralization Flux (borax) deammoniation condition, the recovery ratio of these pre- Metal cious metals increased with rise of pH value. This behavior Filtration Precipitate Fusion (Precious metal might be due to the fact that ammonium ions in the sample contained) were removed during reduction at the relatively high pH Filtrated water Slag value. NaOH According to Fig. 7, it was found that Au and Pd remaining Deammoniation in the filtrated water of neutralization were recovered by reduction process under deammoniation condition. Also, the Filtration Precipitate recovery of Pt from the filtrated water was improved, Filtrated water compared to the reduction under non-deammoniation con- NaBH dition. Au, Pd and Pt can be recovered in any pH value, but it 4 Reduction is expected that the reduction apparatus is damaged in the case of using strong acidic or alkali solutions. Therefore, it is Filtration Precipitate most suitable that the pH value is controlled at around 7 for the reduction process. Final wastewater
3.5 Wastewater treatment — combination of neutrali- Fig. 8 Flowsheet of wastewater treatment. (Combination of Neutraliza- tion, Reduction and Fusion.) zation, 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 in solution were also recovered. Also the recovery of Pt in water from neutralization can be treated for recovery of Au, the treated water was 78.3%. About 50% of Zn remained in Pd and Pt by reduction at pH 7. Therefore, the recovery of solution. precious metals and other valuable metals were investigated The precipitate recovered by reduction contained a large by combining neutralization, deammoniation, reduction and amount of Au, Ag, Pd and Pt. And therefore, this precipitate fusion processes. can be treated in refining process of precious metals. The flowsheet of wastewater treatment is shown in Fig. 8. However, about 20–30% of Au, Ag, Pd and Pt are lost to This process consists of three steps; (1) neutralization, (2) precipitate during neutralization for Cu and In recovery. In reduction (under deammoniation condition) and (3) fusion. this study, due to the complete recovery of all valuable 3.5.1 Behaviour of each metallic element during neu- metals, both precipitates that had been recovered by tralization and reduction neutralization and reduction were mixed and melted in the The results of neutralization and reduction are shown in final step of the process, referred to as fusion. Fig. 9. When wastewater was treated by neutralization at 3.5.2 Behaviour of each metallic element during fusion pH 6, Fe, Pb, Bi, Sn, Cr, Al, In were completely recovered, The weight of each precipitate obtained from neutraliza- and about 96% of Cu was also recovered. On the other hand, tion and reduction were 95.6 g and 30.0 g. These precipitates recoveries of Au, Pt, Ag, Pd, Ni, Zn were consistently low at and 50 g of flux (borax) were melted at 1300 to 1700 C. As a below 30%, where Pt exhibited the lowest recovery of only result, separation of phase occurred by taking advantage of 14.7%. After neutralization, the filtrated water was treated by the difference in specific gravity. A glass-shaped slag phase deammoniation and reduction. The pH value of reduction was formed in the upper part, while a metal phase was formed was controlled at 7.5. As a result, Au, Ag, Pd which remained in the lower part. 1468 H. Umeda et al.
(1) Precious metals (2) Other metals -1 6000
Au Cu 5000 Cu Pt Fe 4000 Ag Pb 3000 Pd Bi 2000 Intensity (a.u.) 0 20 40 60 80 100 0 20 40 60 80 100 1000 Recovery (%) Recovery (%) 0 (3) Other metals -2 (4) Other metals -3 20° 30° 40° 50° 60° 70° 80° Ni Al 2θ
Sn Zn Fig. 10 X-ray diffraction pattern of metal obtained by fusion. Cr In
Table 3 Recovery of various metals and the composition (grade) of metal
Grade Content (A) Precious Recovery Þ Þ < Slag> < Final wastewater> metal Wastewater Metal Wastewater Metal (%) (mass%) (mass%) (mg) (mg) Au : 0.99 mg 2.6 % Au : 0.72 mg 1.9 %
Au 0.0010 0.09 38.2 36.5 95.5 Ag : 0.98 mg 2.7 % Pt : 10.9 mg 18.7 % Pt 0.0018 0.11 58.5 41.6 71.1 Ag 0.0010 0.09 36.5 33.4 91.5 Fig. 11 Experimental results of precious metals recovery from initial Pd 0.0157 1.32 555.1 519.1 93.5 wastewater to the concentrate by wastewater treatment.
Grade Content (B) Other Recovery Þ Þ metal Wastewater Metal Wastewater Metal (%) 3.5.3 Material balance in the wastewater treatment (mass%) (mass%) (g) (g) Figure 11 shows the recovered amount of precious metals Cu 1.029 80.1 38.11 31.43 82.5 (such as Au, Pt, Ag, Pd). In addition, the material balance of Fe 0.021 0.78 0.70 0.31 43.8 this wastewater treatment is shown in Fig. 12. This experi- Pb 0.010 0.20 0.37 0.08 21.0 ment was performed in air atmosphere, and maximum Bi 0.026 1.66 0.87 0.65 75.3 melting temperature was 1700 C. As a result, the oxidation Ni 0.056 4.69 1.90 1.84 96.6 and volatilization reaction occurred, and then, each metallic Sn 0.011 0.68 0.33 0.27 80.5 element, unlike precious metals, was distributed to the slag Cr 0.004 0.00 0.13 0.00 0.0 and the air. On the other hand, most of precious metals were Al 0.015 0.00 0.50 0.00 0.0 distributed to the Cu-metal (concentrate).22) Zn 0.394 1.08 13.19 0.42 3.2 The main purpose of this experiment was to recover In 0.090 0.00 3.13 0.23 7.4 precious metals. Au, Ag, Pt, Pd were absorbed and recovered ÞMetal was obtained from fusion process. 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 deam- Table 3 shows both the recovery of various metals and the moniation of the wastewater was not performed, recovery of composition (grade) of metal obtained from fusion process. Pt was about 20%. And about 70% of Pt remained in the Also, Fig. 10 shows the X-ray diffraction pattern of metal wastewater. Therefore, it is thought that recovery of Pt rises obtained from fusion. It can be seen that metal (concentrate) due to the deammoniation of the wastewater. Relation obtained from this experiment contained mainly Cu and other between the reduction of Pt and the deammoniation are precious metals. being investigated now. Recovery and Concentration of Precious Metals from Strong Acidic Wastewater 1469
0 20 40 60 80 100 Fig. 13 Separation of precious metals from concentrate (Cu-metal) by the Distribution (%) conventional copper smelting process.
Concentrate (Metal, Crude copper) Slag 4. Conclusions Treated wastewater (Final wastewater) Recovery of precious metals and other valuable metals Other (Exhaust gas, etc.) that remain in the wastewater containing a large amount þ of ammonium ions (NH4 ) was investigated by using some Fig. 12 Distribution of various elements at the end of the wastewater traditional hydrometallurgical processes such as cementa- treatment. (Material balance.) tion, neutralization and reduction. Following are the main results of this experimental work. (1) Precious metals and other valuable metals cannot be Also, Fig. 12 shows that the content of Cr and Al in slag is recovered by one-step process such as cementation or high, the value is 98% and 82%, respectively. On the other neutralization, and therefore, combining some processes is hand, the reduction ratio of Zn is low. Therefore, about 40% necessary to recover all valuable metals completely. of Zn remained in wastewater. In addition, ‘‘Other’’in Fig. 12 (2) When wastewater was treated by neutralization at shows the vaporization, due to the fact that the fume was pH 6, Fe, Pb, Bi, Sn, Cr, Al, In were completely recovered, exhausted during the fusion process. The fume, which was and about 96% of Cu was also recovered. On the other hand, recovered by a plate made of ceramics, was white powder. Au, Pt, Ag, Pd, Ni, Zn recovery remains below 30%, with The white powder was dissolved in hydrochloric acid, and Pt lowest at 14.7%. then the acidic solution was properly diluted. After that, (3) After neutralization, the filtrated water was treated by analysis of acidic solution was performed by ICP-AES and deammoniation and reduction. The pH value of reduction the results confirmed the presence of Cu, Pb, Zn, and In, etc. was controlled at 7.5. As a result, Au, Ag, Pd which remained 3.5.4 Separation of precious metals from concentrate in solution were also recovered. In addition, the recovery (Cu-metal) ratio of Pt in treated water was 78.3%. About 50% of Zn The concentrate obtained from the wastewater treatment in remained in solution. this experiment was crude copper containing some precious (4) Wastewater was treated by combining neutralization, metals. In general, precious metals in the crude copper and deammoniation, reduction and fusion. Finally, precious scrap materials, etc., can be recovered as a by-product of the metals were concentrated in the metallic fraction which conventional copper smelting process.3,23) Figure 13 shows mainly contains copper. The recovery of Au, Ag, Pd was the flowsheet for separation of precious metals from more than 91%, and that of Pt was about 71%. concentrate (Cu-metal) obtained by wastewater treatment. (5) During fusion, it was found that vaporization of some The concentrate is treated by converter and a refinement metals (such as Cu, Pb, Zn, and In) occurred. However, these furnace in the conventional copper smelting process, and metals can be recovered by using dust collector. then, the concentrate is recovered as an anode. After that, the anodes are sent to electrolytic refining process of copper. REFERENCES Finally, ‘‘electrolytic copper (grade of 99.99%)’’ and slime (the precipitate) containing precious metals are recovered. 1) K. Takahashi, A. Sasaki and H. Umeda: J. MMIJ 123 (2007) 744–746. The slime is then sent to refining process for recovering 2) J. Shibata and A. Okuda: J. MMIJ 118 (2002) 1–8. precious metal. Au, Ag, Pt and Pd are recovered by 3) T. Okabe, H. Nakada and K. Morita: J. Surf. Sci. Soc. Japan 29 (2008) 592–600. precipitation or solvent extraction. In recent years, it seems 4) S. Kondo, A. Takeyama and T. Okura: J. MMIJ 122 (2006) 386–395. that solvent extraction is utilized as compared to precipitation 5) M. Taniguchi: J. MMIJ 122 (2006) 564–572. due to the fact the process is simple and effective.24) 6) J. Shibata, S. Gonomaru and H. Yamamoto: KAGAKU KOGAKU 1470 H. Umeda et al.
RONBUNSHU 27 (2001) pp. 367–372. 16) K. Inoue: J. MMIJ 123 (2007) 59–67. 7) K. Liu, A. Shibayama, T. Suzuki, W. T. Yen and T. Fujita: J. MMIJ 117 17) Y. Shimada, T. Niide, F. Kubota, N. Kamiya and M. Goto: KAGAKU (2001) 221–225. KOGAKU RONBUNSHU 36 (2010) pp. 255–258. 8) S. Ardiwilaga, T. Wakamatsu and Y. Nakahiro: Resourc. Process. 41 18) Y. Nakahiro: Resourc. Process. 40 (1993) 161–165. (1994) 65–71. 19) A. Watanabe: J. Surf. Finish. Soc. Japan 48 (1997) 257–260. 9) W. Zhike, C. Donghui and C. Liang: Miner. Eng. 20 (2007) 581–590. 20) Y. Hirai and Y. Sasaki: Japanese Patent Disclosure H19-92133. 10) K. Hirayama: Proc. MMIJ Fall Meeting (1990) Vol. U, pp. 5–8. 21) H. Sato, Y. Shike, K. Takahashi, H. Nakazawa and Y. Kudo: Proc. 11) S. Nishimura: FLOTATION 31 (1984) 274–286. MMIJ Annual Meeting, (2001) Vol. II, pp. 36–37. 12) A. Okuda, H. Sawai, S. Ichiishi and J. Shibata: J. MMIJ 116 (2000) 22) K. Yamaguchi, H. M. Henao, S. Ueda and K. Itagaki: Proc. MMIJ Fall 929–933. Meeting, (2005) Vol. CD, pp. 247–248. 13) J. Shibata: J. Surf. Finish. Soc. Japan 53 (2002) 641–646. 23) T. Kametani, S. Akagi, F. Sato and K. Toda: Japanese Patent 14) A. Shibayama: J. MMIJ 120 (2004) 406–411. Disclosure H20-31547. 15) Y. Konishi: J. Soc. Powder Technol. Japan 43 (2006) 515–521. 24) A. Toraiwa and Y. Abe: J. MMIJ 116 (2000) 482–492.