dobijanje bezkadmijumskih legura srebra za tvdro lemljenje ... · pdf file5. mkoiee, beograd,...

5. MKOIEE, Beograd, 12. i 13. oktobar 2017 5th ICREPS, Belgrade, 12th and 13th October 2017
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DOBIJANJE BEZKADMIJUMSKIH LEGURA SREBRA ZATVDRO LEMLJENJE RECIKLAOM ELEKTRONSKOG KARTA
OBTAINING OF THE CADMIUM-FREE SILVER BRAZING ALLOYS FROM E-SCRAP
Stevan P. Dimitrijevi1, Silvana B. Dimitrijevi2, Zoran M. Stevi3, Biserka T. Trumi2, Aleksandra T. Ivanovi2
1 Innovation Center Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
2 Mining and Metallurgy Institute Bor, Zeleni bulevar 35, 19210 Bor, Serbia
3 Technical Faculty Bor, University of Belgrade, VJ 12, 19210 Bor, Serbia

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Abstract
Silver brazing alloys with Ag-Cu-Zn composition are environmentally friendly and replacement for silvercadmium bearing alloys. Since e-waste has these three metals in the composition and that some of theparts as silver plated contactors are consisted mainly of them, they can be recovered and furtherprocessed in aim to increase the profitability of recycling. Process is combination of hydrometallurgy andelectrorefining of the copper. Final products are obtained by use of pyrometallurgy. Finall products arealloys within tolerances allowed by EN 1044 standard.
The global quantity of e-waste reached over 40 million tones !!(in the 2nd 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

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

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Is the (even) silver cheap?Not so, but relatively it is.
Source: https://www.lme.com/Metals/Precious-metals/LME-Silver
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.

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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 andenvironmentally beneficial. Ternary alloys of the Ag-Cu-Zn system and quaternary alloys ofthe Ag-Cu-Zn-x system (x could be Sn, Ni and In) are the most used for cadmium bearingsilver brazing alloys replacement from toxicological reasons.
All materials needed for the production of the alloys are constituents of the PCBs andother disposed electronic equipment. The objective of this research was to determineoptimal technology for recycling of e-waste in the aim to obtain high silver brazing alloysand process parameters for melting and casting of these alloys.

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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

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Obtaining cadmium-free silver brazing alloys
Silver brazing alloys of the system Ag-Cu-Zn and are used in variousindustries. They are environmentally friendly since do not contain toxicheavy metals like cadmium and lead.
Silver brazing alloys Ag60Cu26Zn14, Ag43Cu37Zn20, Ag40Cu30Zn30 andAg25Cu52.5Zn22.5 were obtained. All these alloys correspond in compositionto commercial alloys, and alloy Ag60Cu26Zn14 belongs to the EN 1044:1999standard (designated as AG 202).
Technology is based on the use of pure metals and two consecutive meltingand casting processes of ingot metallurgy. Induction melting was performedwith vacuum casting to minimize losses of zinc.

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As an alloy with highest content of silver Ag60Cu26Zn14 was usedas example for the technology process.
Table 2 Composition of Ag60Cu26Zn14 alloy after the 1st and the 2nd
melting-casting process
Composition of the alloy Ag, wt.% Cu, wt.% Zn, wt.%
After the first stage 61.21 26.31 12.48
After the second stage 60.53 25.76 13.71

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Pyrometallurgy process of silver brazing alloys production
Figure 2 a) Surface of the Ag60Cu26Zn14 alloy
after the first melting-casting
Figure 2 b) Surface of the Ag60Cu26Zn14 alloy after
the second melting-casting and annealing at 600 C
In Fig. 2 a) surface of the Ag60Cu26Zn14 alloy after the first phase of production isshown. Rough structure with very noticeable porosity is visible.Fig. 2 b) shows polished surface of the final alloy after the second melting-castingprocess and homogenization annealing (600 C, 24 h). It's easy to noticehomogenous structure. Precise composition is obtained with deviations that areallowed by standard (SRPS EN 1044:2008; 1% for Ag and Cu, and 2%).

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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 TechnologicalDevelopment of the Republic of Serbia through Projects TR34033 and TR34024.

Thank you for your attention!
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dobijanje bezkadmijumskih legura srebra za tvdro lemljenje ... · pdf file5. mkoiee, beograd,...

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