Stevan P. Dimitrijević 1, Silvana B. Dimitrijević 2, Zoran M. Stević 3, Biserka T. Trumić 2, Aleksandra T. Ivanović 2

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1 DOBIJANJE BEZKADMIJUMSKIH LEGURA SREBRA ZA TVDRO LEMLJENJE RECIKLAŽOM ELEKTRONSKOG ŠKARTA OBTAINING OF THE CADMIUM-FREE SILVER BRAZING ALLOYS FROM E-SCRAP Stevan P. Dimitrijević 1, Silvana B. Dimitrijević 2, Zoran M. Stević 3, Biserka T. Trumić 2, Aleksandra T. Ivanović 2 1 Innovation Center Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade, Serbia 2 Mining and Metallurgy Institute Bor, Zeleni bulevar 35, Bor, Serbia 3 Technical Faculty Bor, University of Belgrade, VJ 12, Bor, Serbia 1

2 Abstract Silver brazing alloys with Ag-Cu-Zn composition are environmentally friendly and replacement for silver cadmium bearing alloys. Since e-waste has these three metals in the composition and that some of the parts as silver plated contactors are consisted mainly of them, they can be recovered and further processed in aim to increase the profitability of recycling. Process is combination of hydrometallurgy and electrorefining of the copper. Final products are obtained by use of pyrometallurgy. Finall products are alloys within tolerances allowed by EN 1044 standard. The global quantity of e-waste reached over 40 million tones!! (in the 2 nd decade of the XXI century) Discarded PCBs (totally) is only 3% to the total e-waste generated worldwide. BUT, PCBs are one of the most usable materials for: precious metals and copper recovery from e-waste. In metal fraction of PCBs: 49.0% Cu and 21.8% Zn 2

3 What about silver? Silver has much lower content in e-waste. However, several sources have significant concentrations of this metal: Dynamic-RAM modules and photovoltaic modules were found to be waste with high levels of silver. In one ton of raw mobile phone waste, 3.5 kg of silver and 130 kg copper exist as recoverable metals. Silver plated brass contacts is special species of e-waste with exceptional high content of silver (about 2.5 wt. %) 3

4 Is the (even) silver cheap? Not so, but relatively it is. Source: Value of the Ag from the sources with the highest conc. is just similar to the Cu or up to twice higher. Usually it is just 5 to 10 times higher then zinc. 4

5 Valorization of the recycling products In this research a new approach is used to obtain valuable materials from electronic scrap. With a use of same equipment and just materials from recycling. Cadmium-free brazing alloys with high silver content are selected, as economically and environmentally beneficial. Ternary alloys of the Ag-Cu-Zn system and quaternary alloys of the Ag-Cu-Zn-x system (x could be Sn, Ni and In) are the most used for cadmium bearing silver brazing alloys replacement from toxicological reasons. All materials needed for the production of the alloys are constituents of the PCBs and other disposed electronic equipment. The objective of this research was to determine optimal technology for recycling of e-waste in the aim to obtain high silver brazing alloys and process parameters for melting and casting of these alloys. 5

6 Metal recovery The overall process of pure metal recovery includes the following processes: smelting and casting (into anodes), electrolytic refining (copper recovery), hydrometallurgical treatment in the aim of Ag and Zn recovery. Simplified block diagram of the process is shown in following figure Flow sheet of e-waste processing 6

7 Obtaining cadmium-free silver brazing alloys Silver brazing alloys of the system Ag-Cu-Zn and are used in various industries. They are environmentally friendly since do not contain toxic heavy metals like cadmium and lead. Silver brazing alloys Ag 60 Cu 26 Zn 14, Ag 43 Cu 37 Zn 20, Ag 40 Cu 30 Zn 30 and Ag 25 Cu 52.5 Zn 22.5 were obtained. All these alloys correspond in composition to commercial alloys, and alloy Ag 60 Cu 26 Zn 14 belongs to the EN 1044:1999 standard (designated as AG 202). Technology is based on the use of pure metals and two consecutive melting and casting processes of ingot metallurgy. Induction melting was performed with vacuum casting to minimize losses of zinc. 7

8 As an alloy with highest content of silver Ag 60 Cu 26 Zn 14 was used as example for the technology process. Table 2 Composition of Ag 60 Cu 26 Zn 14 alloy after the 1 st melting-casting process and the 2 nd Composition of the alloy Ag, wt.% Cu, wt.% Zn, wt.% After the first stage After the second stage

9 Pyrometallurgy process of silver brazing alloys production Figure 2 a) Surface of the Ag 60 Cu 26 Zn 14 alloy after the first melting-casting Figure 2 b) Surface of the Ag 60 Cu 26 Zn 14 alloy after the second melting-casting and annealing at 600 C In Fig. 2 a) surface of the Ag 60 Cu 26 Zn 14 alloy after the first phase of production is shown. Rough structure with very noticeable porosity is visible. Fig. 2 b) shows polished surface of the final alloy after the second melting-casting process and homogenization annealing (600 C, 24 h). It's easy to notice homogenous structure. Precise composition is obtained with deviations that are allowed by standard (SRPS EN 1044:2008; ±1% for Ag and Cu, and ±2%). 9

10 CONCLUSION This study was carried out to investigate possibility to obtain silver brazing alloys from selected e-waste with the use of same equipment as for recycling process. Recovery rates were of over: 98% for Ag, 95% for Cu and 91% for Zn. The process is combination of pyro-, electro- and hydrometallurgy. Cu with 99.97% purity, Ag with 99.99% and Zn with 99.98% were obtained. Two consecutive melting and casting processes were required to produce alloys with compositions which correspond to the standards for brazing alloys. After the first ingot metallurgy process, lower zinc content was observed. High zinc evaporation has influenced the porous structure of obtained alloys. The repeated melting and casting process resulted with precise compositions and lower porosity of the alloys. After annealing process, very homogenous samples of the alloys were produced. Acknowledgements This research was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia through Projects TR34033 and TR

11 Thank you for your attention! 11