EXPERIMENTAL STUDY ON SMELTING OF WASTE SMARTPHONE PCBs BASED ON Al2O3-FeOx-SiO2 SLAG SYSTEM
Youqi Fan1, Yaowu Gu1, Qiyong Shi1, Songwen Xiao2, Fatian Jiang1
1School of Metallurgical Engineering, Anhui University of Technology, Ma’anshan, Anhui, 243002, PR China
2Changsha Research Institute of Mining & Metallurgy, Changsha, Hunan, 410012, PR China Keywords: waste smartphone; PCBs; rare and precious metals; smelting
Abstract
Waste smartphone, as an important type of secondary resource has high content of rare and precious metals. The traditional mechanical process could easily lead to the dispersion and loss of precious metals. In this research, a smelting method using Al2O3-FeOx-SiO2 slag system is proposed to recover the valuable metals in smartphone PCBs. Based on the evaluation of liquidus projection calculated and plotted by Factsage software, reasonable smelting temperatures and slag composition ranges were selected, namely 1300℃-1500℃, 10-15wt%
Al2O3, FeO / SiO2 ratio of 0.8-1.5. Then several lab experiments were conducted, with Cu-Fe- Sn-Ni alloy obtained. The results show that distribution of valuable metals could be controlled by appropriate oxidation of iron. Rare metals primarily enrich in the alloy, and most of active metals like Fe, Al in slag as oxide. Recovery efficiencies of Cu, Ni, Sn, Au, Ag are more than 95wt%.
Introduction
Over the last five years, smartphones have rapidly replaced the traditional mobile phone many times over. Due to its high performance with numerous additional new applications, smartphone sales surpassed one billion units in 2014 [1]. And large quantity of smartphone has reached their end of life. Besides of the basic metals as in mobile phone[2] like copper, nickel, lead, bismuth, lithium and precious metals silver and gold, it entails a relatively high content of special (some critical) and precious metals[3], such as Cobalt, Gallium, Indium, Niobium, Tantalum, Tungsten, Platinum group metals and Rare earths. Thus, recovering such valuable metals is strongly required in terms of effective utilization of resources and environmental conservation. This research will focus on the recovery of printed circuit boards (PCBs) in smartphone.
Many processes have been developed to extract the valuable metals from PCBs. These are largely divided into mechanical, hydrometallurgical and pyrometallurgical processes[4]. In mechanical process, PCBs is usually shred to small pieces and grinded to particles in order to dissociate the organics from metallic components. Then physical separation methods are applied to obtain several concentrates, such as organics, iron and steel, light metals and copper- rich metals, which are sent to special plants for further treatments. This process has been widely adopted in China to recycle the metals of PCBs in traditional household appliances because of its simple equipment, lower cost. Hydrometallurgical process[5] treats materials in aqueous
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
solution using chemical reagents like acid, alkali and salt. Then the valuable metals are separated and purified. This process can produce high purity products, but it is suitable for postprocessing due to the large consumption of reagent and risk of waste water pollution.
Furthermore, the PCBs generally need mechanical pretreatment. In the pyrometallurgical process[6-8], PCBs are smelted with fluxes after simple shredding. A molten bath is formed containing slag and “collector metal” like copper, nikel, lead etc., into which the precious metals dissolve and accumulate.
In either mechanical or hydrometallurgical processes, the precious metals easily disperse in the physical separation[9]. Among the above processes, the pyrometallurgical process is effective for extracting precious metals from smartphone PCBs[10-13]. Based on the chemical composition reported in literatures[14], copper is reasonable as a precious metals collector.
And a difference Al2O3-FeOx-SiO2 slag system is proposed. This study will focus on the chosen of suitable slag region and evaluation of smelting performance using this slag system.
Experimental Raw materials
The waste smartphones were collected from recyclers by internet, including the main types of smartphone in the market, such as Samsung, iPhone, Huawei, etc. The phones were reserved for disassembly and removal of printed circuit boards (PCBs) (Fig.1), which were used in the following mechanical and smelting processing and recovery of valuable metals. Compared with the traditional mobile phone PCBs[15], it is much smaller in volume for smartphone PCBs and lots of components are integrated on the multilayer board. Outside the chips, there are some stainless steel shielding case. The chemical composition of PCBs was shown in Table 1.
Table 1 Chemical composition of PCBs used in this research
Element Cu Fe Al Sn Ni Ag Au SiO2 CaO
Content (wt%) 28.11 12.66 5.72 2.53 2.65 0.168 0.049 9.67 2.42
(a) (b)
Fig.1 Smartphone disassembled
(a) Samsung GALAXY S3; (b) iPhone 4s; (c) PCBs in smartphone (b)
(c)
Experimental procedures
Fig.2 showed the summary flowchart of PCBs processing. Due to the varieties of shapes and metal quantities among different brands and models of smartphone, about 2 kilograms of PCBs were collected. A fine shredder was used together with a steel grate of small perforate size (20 mm). The small plates were then roasted at 700-800℃ in order to partially oxidizing the metallic elements, like aluminum, iron, silicon and so on. A drum vibrating mill was applied to grind the residues in finer particles (particle size < 1mm). After that, the powder were divided into several samples by quartering method and each sample weighted 150g. One of them was sent for chemical composition analysis. The others was mixed with flux ( Fe2O3 or SiO2) according to the target slag composition. The mixtures was hold in corundum crucible and smelted for half hour at 1350~1550℃ in nitrogen atmosphere. When the sample was cooling down to room temperature, alloy and slag samples could be obtained by broken the crucible (Fig.3).
Analysis
X-ray fluorescence (XRF) was used for quantification of the chemical compositions of solid samples. Scanning electron microscopy (SEM) and Energy Dispersive Spectrometer (EDS) were used for detailed composition and phase analysis of. Chemical analysis and inductively coupled plasma (ICP-OES) measurements were used to analyze the composition of alloy and slag.
Fig.2 The proposed flowchart of valuable metals recovery from waste smartphone PCBs
Fig.3 The photograph of smelting product of smartphone PCBs
Alloy
Slag
Determination of Extraction Efficiency
In the present study, the extraction Efficiency of metals EM were defined as:
100%E
M M
M
M A S
A
(1)
where, EM is the extraction ratio of M metal, AM is the weight (g) of M metal concentrated into the molten metal phase (alloy phase), and SM is the weight (g) of M metal remained in the slag phase.
Results and discussion Choice of the slag system
To estimate the melting temperature of the PCBs used in this work, it is need to confirm the slag system by investigating the chemical composition in Table 1. Generally, aluminum, silicon and calcium are easily enter into slag as oxides in smelting. Iron may also partially or completely be oxidized under certain oxidizing atmosphere. So the main composition of slag is composed of aluminum oxide, ferrous oxide (ferric oxide), silicon dioxide and little calcium oxide, namely Al2O3 – FeOx – SiO2 slag system.
The isothermal liquidus lines (1200-1500℃) of the ternary slag system consisting of three main slag components contained in the PCBs is shown in Fig. 4, which is calculated and plotted by the “phase diagram” module of Factsage 6.4. The oxygen partial pressure is set as 10-9atm when the iron exists mainly as ferrous oxide. From Fig.4 it is revealed that the liquid phase area rapidly expands with the increase of temperature and the solubility of aluminum oxide can
Fig.4 The isothermal liquidus lines of the Al2O3 – FeOx – SiO2 slag system ternary slag system (oxygen partial pressure 10-9atm)
10 20
30 40
50 60
70 80
90
10 20 30 40 50 60 70 80 90
10 20
30
40 50
60 70
80
90
SiO2
FeO Al2O3
mass fraction
1200 1250 1300 1350 1400 1450 1500 T oC
1500C 1400C 1300C 1200C Liquid
FeO - Al2O3 - SiO2 - O2 Projection (ASlag-liq), p(O2) = 10-9 atm, 1 atm
reach 20wt% at 1300℃ to 35wt% at 1500℃. The more of aluminum oxide in slag, the closer to “SiO2”corner of liquid phase area. It is reasonable for controlling the smelting temperature at 1300-1500℃. Considering the high aluminum oxide content in PCBs, it is better to choose a slag composition with more Al2O3 solubility. So the displayed dished-line oval region in this figure is the target slag composition region that will be used to calculate the input ratio of PCBs and fluxes.
The dot in the figure (29.4%Al2O3, 44.3%FeO, 26.3%SiO2) represents the weight-percent composition based on the total mass of just the three main slag forming oxides contained in the PCBs used in this study, given all the iron existing in slag as ferrous oxide. Its melting temperature located between 1400-1500℃. The over content of aluminum oxide should be adjusted to the target region by adding some fluxes, especially at lower temperature.
Effect of pre-oxidizing roast and choice of alloy composition
In order to determine the effect of oxygen partial pressure on smelting, two contrast smelting experiments were designed before and after the pre-oxidizing roast of PCBs. Fig.5 showed the photographs of smelting products without pre-oxidizing roast. Although well separation of alloy and slag was obtained, immiscibility of Fe-rich phase and Cu-rich phase was observed in the alloy. Seen from the corresponding chemical compositions of alloy and slag in Table 2 and 3, the iron was partially oxidized into slag while considerable amount of it stayed in the alloy.
It is revealed in Cu-Fe-Sn phase diagram of 1300℃ (Fig.6) that the solubility of iron is less than 10wt% when tin content fall in 5-10wt% (oval region). Or the alloy will easily form two immiscible liquid, namely Cu-rich phase and Fe-rich phase. Consequently, it’s necessary to oxidize more iron to ferrous oxide so as to avoid the immiscibility of alloy. In this research, a pre-oxidizing roast process was introduced before smelting.
Fig.5 Photographs of smelting products and the immiscibility of alloy without pre-oxidizing roast (smelting for 0.5h at 1350℃)
Fe-rich phase F
Table 2 Average chemical composition of the alloy
Element Cu Fe Sn Ni
Content / wt% 49.25 25.24 8.06 5.24 Fe-rich phase
Cu-rich phase Alloy
Slag
Table 3 Chemical composition of the slag
Element FeO SiO2 Al2O3 CaO Sn Cu Ni Content / wt% 22.62 39.42 15.30 8.61 0.041 0.884 0.011
Fig.6 Cu-Fe-Sn computed isothermal section at 1300℃[16]
The effect of temperature and target Al2O3 content on recovery efficiency of valuable metals
Two smelting experiments A and B were carried out respectively at 1350℃ and 1450℃ for 0.5h using the pre-oxidized PCBs as raw material. The target Al2O3 content was set as 10- 15wt%. The product compositions is shown in Table 4. As shown in Table 4, FeO, SiO2, Al2O3
and CaO were the main components for obtained slags. FeO/SiO2 ratio is 1.07 and 1.25, right located in the oval region of Fig.4. The alloy mainly composes of Cu, Ni and Sn, while the Fe content falls down to 1wt% by pre-oxidizing of PCBs.
The mineralogical structure of the slag A was characterized by SEM and EDS analysis (Fig.7).
The result shows that slag A are primary composed of bar-shaped Olivine ( 2FeO·SiO2) and compounds of CFAS. There are some square spinel of Al2O3·FeOx and inclusion dot of alloy.
Table 4 Chemical composition of the alloy and slag
No. Content (wt%)
FeO SiO2 Al2O3 CaO Cu Fe Ni Sn Zn Pb Ag Au A
1350℃
alloy / / / / 86.55 1.10 4.83 3.81 0.76 0.40 0.29 0.05 slag 39.29 31.41 10.37 8.36 0.38 / 0.06 0.08 1.10 0.06 4ppm 1ppm B
1450℃
alloy / / / / 87.84 0.73 5.04 3.51 0.43 0.37 0.26 0.07 slag 35.7 33.16 13.62 7.12 0.68 / 0.12 0.14 1.11 0.09 3ppm 1ppm Note: “/” mean the composition not analyzed
As can be seen from Fig. 8, recovery efficiencies of Cu, Ni and Sn for A are all more than 95wt%
while that of Zn and Pb are only 23wt% and 65wt% (the data may be lower if considering the vaporization). When smelting temperature raise to 1450℃，the recovery efficiency decreases to some extent. It is obvious for Sn, Zn and Pb which may be due to the enhanced vaporization of them at higher temperature. But the change of temperature shows little effluence to precious metals Au and Ag, reaching 99.5wt% for both A and B.
Fig.7 Mineralogical structure of the slag A C-CaO; F-FeOx; A-Al2O3; S-SiO2
99.03 1.59 97.3 95.52 23.63 74.91 99.69 99.5698.2 1.1 94.67 91.38 14.08 63.49 99.73 99.66
0 20 40 60 80 100
Au
Recovery efficiency (wt%)
Element 1350℃
1450℃
Cu Fe Ni Sn Zn Pb Ag
Fig.8 Recovery efficiency of valuable metals in smelting process
Conclusion
In this study, a smelting process is proposed to extract valuable metals contained in waste smartphone PCBs using Al2O3-FeOx-SiO2 slag system. Based on the evaluation of liquidus projection, suitable smelting conditions are selected, namely 1300℃-1500℃, 10-15wt% Al2O3, FeO / SiO2 ratio of 0.8-1.5. by the contrast of the smelting performance, the iron content in alloy should be less than 10wt% to avoid immiscible liquid alloy, which can be realized with pre-oxidizing roast. Meantime, pre-oxidizing roast can also affect the metal distribution by appropriate oxidation of iron. In lab experiments, Cu-Sn-Ni-Fe alloy was obtained, in which most precious metals enriched. And almost all active metals like Fe, Al are oxidized and
Spinel of AF
concentrate in slag typically composed of 10-15wt%Al2O3, 35-40wt%FeOx, 30-35wt%SiO2, 7-9wt%CaO. At 1350℃，Recovery efficiencies of Cu, Ni, Sn are more than 95wt% as well as Au, Ag up to 99.5wt%.
In conclusion, the proposed process with Al2O3-FeOx-SiO2 slag system revealed an acceptable performance for smartphone PCBs recycling. The minor component in slag such as CaO, may further decrease the melting temperature and expand the liquid phase area. Thus this smelting process might be conducted in a lower temperature. Besides the metals concerned in this article, the distribution of other rare metals like In, Ga, Nd, and precious metals Pd, Pt are also worth of researching.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51404004) and the SRTP projects (No. 201410360003), and the authors are very grateful to Analysis and Testing Centre of Anhui University of Technology and Changsha Research Institute of Mining
& Metallurgy, for the sample examination and analysis.
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