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) À À þ Cl NO3 NH4 8:3 Â 104 9:5 Â 104 4:8 Â 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.