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Printed circuit board waste as a source for high purity porous silica
Accepted Manuscript Printed Circuit Board Waste as a Source for High Purity Porous Silica Alireza Bazargan, Damien Bwegendaho, John Barford, Gordon McKay PII: DOI: Reference:
S1383-5866(14)00524-3 http://dx.doi.org/10.1016/j.seppur.2014.08.026 SEPPUR 11947
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 November 2013 15 August 2014 19 August 2014
Please cite this article as: A. Bazargan, D. Bwegendaho, J. Barford, G. McKay, Printed Circuit Board Waste as a Source for High Purity Porous Silica, Separation and Purification Technology (2014), doi: http://dx.doi.org/ 10.1016/j.seppur.2014.08.026
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Printed Circuit Board Waste as a Source for High Purity Porous Silica
Alireza Bazargan 1,2 Damien Bwegendaho 1 (second author, equal contribution as first author) John Barford 1 Gordon McKay 1,3 *
1
Department of Chemical and Biomolecular Engineering, Hong Kong University of
Science and Technology, Clearwater Bay, Hong Kong.
2
Department of Chemical Engineering and Biotechnology, New Museums Site,
Pembroke Street, University of Cambridge, Cambridge, CB2 3RA, UK
3
Division of Sustainable Development, College of Science, Engineering and Technology,
Hamad bin Khalifa University, Qatar Foundation, Doha, Qatar * corresponding author: Tel: (+852)23588412 Email: [email protected]
1
Abstract The electronic waste stream is one of the fastest growing streams of waste in the world. Printed circuit boards which are found in all electronic equipment are composed of a mixture of various metallic and non-metallic constituents. In this manuscript, the nonmetallics have been treated with acids followed by thermal treatment aiming at the production of high purity porous silica. The results show the possibility of obtaining SiO2 with specific surface areas (BET) as high as 300 m2/g and purity higher than 99% (X-ray fluorescence, molar %). The as-prepared silica is comparable to industrially sold samples. The study has successfully demonstrated the possibility of using these unwanted waste electronic materials for the synthesis of products with higher value and utility.
Keywords: E-waste; Printed circuit board; Amorphous silica; Urban mining; WEEE;
2
1. Introduction With advancements in the electronic world almost occurring on a day-to-day basis and increased availability of products to the public, it is not surprising to see a staggering increase of electronic wastes over the past several decades. According to a United Nations Environment Program (UNEP) alert bulletin in 2005, 20 to 50 million tons of ewastes are produced every year with an estimated annual increase rate of 4% [1]. Miniaturization of electronic devices such as mobile phones due to technological advancement does not slow down the growth of the waste stream, as the effect has been counter-balanced by increased sales, particularly in developing countries. The production of electrical and electronic devices is expected to be the fastest-growing sector in industrialized countries even with implemented environmental regulations [2]. The rapid growth is due to the economic growth in the industry, shorter lifespans of electronic devices, and the development of diversified electronic related products in recent years [3]. In general, e-wastes consist of more than a thousand different substances [4]. These substances can be categorized into the general categories of metals and non-metals. Manufacturing mobile phones and personal computers consumes 3% of the gold, 2.5% of the silver, and 12% of the palladium mined annually world-wide. 15% of cobalt mined annually across the globe is also incorporated into these devices mainly as a component of the Li-ion battery [5]. In Hong Kong, the amount of Waste Electric and Electronic Equipment (WEEE) generated has been increasing at a steady rate of approximately 2% per year. The amount of WEEE production per person is about 10 kg per year. With a population of over 7 million people, this amounts to more than 70 million kg of e-waste per year. Citizens of Hong Kong are heavy e-waste-producers, even when compared to Americans who produce about 7 kg of e-waste per person per year [6]. For India, South Africa, and
3
Brazil this number is approximately 0.4, 1.3, and 2 kg per person per year, respectively [3,7].
A Printed Circuit Board (PCB) is the generalized term used for the platform upon which microelectronic components such as semiconductor chips and capacitors are mounted. PCBs are used to support the electronic components as well as to connect them using conductive pathways, tracks or signal traces etched from copper sheets laminated onto them. In literature, a PCB is also referred to as a printed wiring board (PWB) or etched wiring board. A PCB populated with electronic components is sometimes referred to as a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). Before PCBs can be processed, they must be disassembled and separated from the body of the waste electronic equipment. Based on the fact that WEEE comes in various different shapes and sizes, it is difficult to develop a mechanized process for disassembling PCBs from all WEEE. Thus, at present, manual disassembly is the most practical option. There are several research projects underway for developing automated and semi-automated processes for removing the different components from PCBs [8–10]. It should be noted that the components of circuit board assemblies are held on to the board via solder; so effective recovery of solder via electrochemical or thermal methods would in turn lead to the separation of the components from the board itself [11–15]. An alternative could be to use molten salts to dissolve glasses and oxides, and to destroy plastics present in wastes without oxidizing the most valuable metals [16]. The removal of valuable metals such as palladium [17], gold [18], and silver [19] from WPCBs have been studied. Due to its large content, the separation of copper has always been a popular target for researchers [20–24].
4
In a review in 2009, Guo et al. [25] reported that although the major economic driving force for recycling PCBs in the metallic fraction, the non-metallic fraction makes up approximately 70% weight of the PCBs. Thermosetting resin matrix composites, thermoplastic matrix composites, and concrete and viscoelastic materials [26] were identified as the main physical recycling routes of the non-metallic fraction. Supercritical fluids depolymerization, hydrogenolytic degradation, as well as other thermo-chemical processes have also been tested [27–31]. In one creative study, acrylonitrile–butadiene– styrene waste plastic has been co-recycled with the nonmetal particles from WPCBs to manufacture composites [32]. The recovery of oils and other compounds from printed circuit board pyrolysis has recently been reported [33–35]. Low-value applications such as being used as fillers have also been investigated [36,37]. Our research group has previously used the non-metallic fraction of WPCB for the production of a novel adsorbent for wastewater treatment [38–40]. In the current paper, for the first time, a simple process for the production of amorphous porous silica from the non-metallic fraction of WPCB is reported for the first time. The porous silica can in turn be used in countless applications [41–43].
2. Materials and Methods The non-metallic fraction of the printed circuit boards was collected from Total Union PCB Recycle Ltd. The waste printed circuit board (WPCB) material consists of a powdered mixture of cured thermosetting resins, glass fiber (cellulose paper), ceramics, brominated flame retardants, residual metals and other additives [25]. This paper is concerned with the removal of the residual metals and volatile fraction of the nonmetallic WPCB. Hydrochloric acid (37%), nitric acid (70%), and sulfuric acid (95%) were purchased from Sigma Aldrich and diluted to the desired molar concentration. The e-waste was treated with various acids for different durations of time and temperatures in a round-bottom 5
flask. Typically 4.0 g of WPCB was used in a 100 ml solution. The treated sample was then filtered, washed, and dried. In order to remove the bromine and volatile fraction, subsequent heat-treatment was carried out in a muffle furnace. The Materials Characterization and Preparation Facilities (MCPF) at the Hong Kong University of Science and Technology assisted in the X-Ray Fluorescence Spectroscopy (XRF) and X-ray diffraction (XRD) analyses. A JEOL JSX-3201Z model automatic sequential XRF was used. The XRD patterns were obtained via a Philips powder diffraction system (model PW 1830) using a Cu Ka source operating at 40 keV at a scan rate of 0.025 s−1. Nitrogen adsorption-desorption isotherms were measured using a Quantachrome Autosorb-I Surface Area Analyzer. The dried samples were outgassed at 150 oC followed by nitrogen sorption at -251 oC. The Brunauer–Emmet–Teller (BET) and t-plot equations were used for calculating the total surface area and the micropore volume respectively. The total pore volume was calculated as the volume of nitrogen adsorbed at P/P0 = 0.9814. The mesopore volume could subsequently be calculated as the difference between the total and micropore volumes. The BJH method was used for discerning the pore size distribution.
3. Results and Discussion In order to obtain an initial understanding of the non-metallic fraction of the WPCB, characterization was carried out. The elemental carbon, hydrogen, nitrogen, and sulfur (CHNS) and X-ray florescence (XRF) analyses data are shown in Tables 1 and 2, respectively. Note that the XRF data is relative (sums to 100% even though it does not include lighter elements such as carbon and oxygen), while the CHNS is an absolute mass fraction of the WPCB. Table 1 – CHNS elemental analysis of the non-metallic fraction of WPCB [44] 6
Elem men nt W Weightt %
C 21 1
H 0.1 1
N 0..6
S 0
O Otheers (byy difffereence) 78 8.3
RF eelem mentaal an nalyysiss (reelattivee) o of th he n non n-m metaallicc fraactiion of WP PCB B Taablee 2 ––XR EElem men nt
Al
Si
S
C Ca
TTi
FFe
C Cu
B Br
B Ba
M Molar %
8
51 1
1
3 30
<<1
<<1
3
5
<1
he rrelaativeely higgh b bromin ne ccon nten nt iss du ue tto tthe exiisteencee of brrom minaateed fllam me Th retardan nts.. Mateerials w with h lo ow fflam mmabiilityy are reequ uireed ffor P PCB Bs d duee to thee risk o of peraatures du uring asseemb bly and d op perratio on of tthe bo oard d. A Althouggh pho osp phorrou us rissingg temp orr nittroggen n baased d fire rretaardantts aare avaailable as an altern natiive and d po ost-ind dusstriaalized co ounttriees aare graaduallyy moving tow warrds end dingg halogen nateed firee reetarrdan nts in PCB Bs, brrom minaated d fire rretaardantts are sstill ovverw wheelm minggly preedomin nan nt o on a glo obaal sccalee [4 45]. So omee off the m mosst co ommo on ttypees o of b brom min nateed fflam me rettard dants includee po olyb brom nateed bip phen nyl,, po olyb bromin nateed dip phen nyl eth hers, aand min heexab bro omo ocycclod dod deccanee. FFigu ure 1 show ws tthee mostt comm mon n brrom minaated fllam me retardan nt u used d in n prrintted circcuitt bo oards, nameely ttetrrabrom mob bisp pheenol A (also kno own n ass 2’,6 6,6’-tetrabro omo o-4,,4’--isopro opyylideeneedip pheenol orr mo oree sim mplly TTBBPA))[46 6]. 2,2
Figgure 1 1- Sttruccture o of ttetrrabrrom mob bisp phenoll A
Nitriic aacid d treeatmeent 3.1 N 7
Silica is only soluble in HF. Hence theoretically, it should be possible to remove the remnant metals in the non-metallic fraction of the WPCB through reactions with acids and the formation of soluble salts. Since metal nitrates are known to be ubiquitously soluble, the first acid to be tested was nitric acid. Table 3 shows the effect of temperature on the metal removal. As temperatures rise from 50 to 70 oC a significant decrease in calcium concentration is seen. Further increase of the temperature, enhances the calcium removal but to a lesser extent. Table 3- The effect of temperature on metal removal by 4 M HNO3 treatment for 1 hour, (XRF molar %). Treatment temperature Element
50 oC
70 oC
90 oC
105 oC
110 oC
Al
-
-
-
-
-
Si
80
90
92
93
94.5
S
<1
<1
<1
-
-
Ca
10.5
4
3
2-3
<1
Ti
1
1
1
1
1
Fe
<1
<1
-
-
-
Cu
-
-
-
-
-
Br
4-5
4-5
4-5
3-4
4
Ba
<1
<1
-
-
-
Solid yield
75%
72%
71%
69%
66%
Since the slurry boils at 110 oC, the temperature cannot be increased further under atmospheric pressure. Visibly, the titanium is persistent after the nitric acid treatment. The resistance of titanium towards removal with nitric acid is an indication that it is probably present in the form of titanium dioxide. TiO2 is used as a semiconductor and in Bragg-stack style dielectric mirrors due to its high refractive index [47]. The literature shows that titanium dioxide is insoluble in water, hydrochloric acid, nitric acid, dilute 8
sulfuric acid, and organic solvents. However, it dissolves slowly in HF and in “hot concentrated” sulfuric acid [48]. Noting that the reactivity of titanium dioxide towards an acid is dependent on the temperature at which the TiO2 has been calcined. If the TiO2 is freshly precipitated without high temperature heat treatment, it may be dissolved in hydrochloric acid. However, high temperature treatment (900 oC) will make it insoluble [49]. The removal of metals increases with the increase of acid concentration to a certain extent. Predictably, lower concentrations of acids lead to less removal of metals. As shown in Table 4, even at near boiling conditions, or with long reaction times, a 2 M solution was not strong enough for adequate calcium removal. Above 4 M, the further increase of concentration has no positive effect on metal leaching; for example, the XRF results of 4 M, 6 M, and 8 M HNO3 treatments are almost identical. Nonetheless, it should be noted that as the acid concentration increased above 4 M to 6M and 8M, there was a volume increase of the final solid product. The volume increase is mild after 6M treatment; but 8M treatment increases the volume of the residue at least 3 fold. This is an interesting observation when taking into account the mass of the solid yield which remains more or less the same.
Table 4- XRF molar elemental analysis after 2 M HNO3 acid treatment Treatment conditions Element
90oC for 1hr
105oC for 1hr
90oC for 6hr
Al
-
-
-
Si
81
84
90
S
<1
-
<1
Ca
12
9
5
Ti
1
1
1
Fe
<1
<1
<1
9
Cu
-
-
-
Br
4-5.
4-5
4-5
Ba
<1
<1
-
Solid yield
77%
75%
74%
Figure 2 shows the remaining calcium content within the waste after acid treatment of different molarities and durations. The temperature is fixed at 90 oC. Unsurprisingly, higher molarities and reaction times are more successful for calcium removal. Other metals are not shown because save for titanium which is not removed at any appreciable degree, calcium is the most persistent component. In tandem, as seen in Figure 3, the silica molar percentage in the remaining solids increases as the leaching reactions become more severe and more calcium is removed. The observed trends would logically lead us to believe that the most pure silica content can be obtained if long treatments are employed with 4 M Nitric Acid. Experiments showed that with a long reaction time of 24 hours, minute amounts of titanium could also be removed (XRF relative molar percentages of 95% Si, 0.7% Ti, and 4.3% Br with a solid yield of 59%).
Calcium content (XRF molar %)
30 25 20 6 hr
15
1 hr
10 5 0
0
1
2
3
4
HNO3 molarity
10
5
Figure 2- Calcium removal from WPCB with various HNO3 molarities and reaction durations at 90 oC
Silica content (XRF molar %)
100 95 90 85 80 75
1 hr
70
6 hr
65 60
0
1
2
3
4
5
HNO3 molarity
Figure 3- XRF data showing relative silica content after various HNO3 treatments of WPCB 3.2 Hydrochloric acid treatment Hydrochloric acid was also tested for leaching the metals from the non-metallic fraction of the WPCB. Since HCl can give off chlorinated gases during the process, less emphasis was placed on using this acid. Nonetheless, the data obtained shows that HCl is a slightly more effective acid for dissolving the residual metals from the heterogeneous WPCB. Table 5 shows the XRF molar percentages following 4 M HCl treatment at 90 oC. Table 5- XRF molar percentage of elements in the WPCB after 4 M HCl treatment at 90 o
C Treatment duration Element
1 hr
3 hr
11
6 hr
Al
-
-
-
Si
93
94
94
S
1
1
<1
Ca
<1
-
-
Ti
1.3
1.25
1
Fe
-
-
<1
Cu
-
-
-
Br
4
3-4.
4
Ba
-
-
-
68%
67%
69%
Solid Yield
The minor variations in the percentages can be the result of experimental error. Observably, 1 hour treatment of the solid with 4 M HCl is adequate to obtain solid residues similar to those obtained by 4 M HNO3 after 3 - 6 hours of treatment. The comparison between HCl and HNO3 treatment becomes more distinct when observing the data of 2 M treatment by the different acids for 6 hours. Table 6 shows this comparison. Table 6- XRF molar percentage comparison of acid treatment for 6 hours at 90 oC 2 M HNO3
2 M HCl
Al
-
-
Si
89
93
S
<1
1
Ca
5
1.3
Ti
1
1
Fe
<1
-
Cu
-
-
Br
4.5
4
Ba
-
<1
12
Solid Yield
74%
67%
Two possible explanations which are not mutually exclusive are put forth for the more powerful leaching capability of HCl. Firstly, the more effective removal of calcium at lower concentrations with HCl might be due to the size of the Cl- anion compared to NO3-. Since the heterogeneous waste is thought to be composed of a calciumaluminosilica matrix [50] where Ca is embedded into the silica medium, the removal of the calcium would require penetration of anions into the matrix. Hence, chlorine ions would more easily reach the target metals and subsequently form ligands for their removal. Secondly, the higher acidic strength of HCl compared to HNO3 could be responsible for stronger leaching. Generally, the strength of an acid is its ability to lose protons. When an acid completely ionizes (dissociates) in water leaving no non-ionized acid, it is considered ‘strong’. HA(aq) → H+(aq) + A−(aq) Equation 1
The acid dissociation constant (Ka) also known as the acid-ionization constant is a quantitative measure which can show the strength of an acid in solution. It is in fact the equilibrium constant of Equation 1 and is calculated as: ି ା Equation 2
Where [HA], [A−] and [H+] are equilibrium concentrations in mol/L. Because Ka values span many orders of magnitude, they are mostly reported by their logarithmic constant pKa: 13
pK ୟ logଵ K ୟ Equation 3
Larger values of pKa mean less dissociation of the acid. Therefore, weak acids have positive pKa values whereas acids with pKa less than -2 are said to be strong. If pKa values and pH are not known, several rules of thumb such as electronegativity of the anion, the atomic radius of the anion, and the charge of the species could be used to estimate acidic strength. For our case, when we compare the pKa values of HCl at -6.3 and HNO3 at -1.64 we can clearly see the higher acidic strength of hydrochloric acid [51].
3.3 Sulfuric acid treatment Contrary to nitric and hydrochloric acid, the employment of sulfuric acid for calcium removal was unsuccessful. Nevertheless, the titanium could be removed. Table 7 shows the XRF analysis of the residue after 4 M H2SO4 treatment at 90 oC for 6 hours. For the aim of silica production from the non-metallic fraction of printed circuit board waste, treatment with sulfuric acid is not recommended. Note that the sulfur added to the sample is stable. It is speculated that the sulfur is either in the form of precipitated calcium sulfate, or embedded into the calcium aluminosilicate matrix as it cannot be removed even after heat treatment. Incineration at 700 oC for 3 hours removes the Br completely and reduces the solid yield to 65%, but the sulfur persists. Table 7- XRF relative molar percentages of elements after 4 M H2SO4 acid treatment for 6 hours at 90 oC Element Molar XRF %
Al -
Si 61
S 20.5
Ca 14
Ti -
14
Fe -
Cu -
Br 3.5
Ba <1
The comparison of the results from three different acid treatments can be seen in Figures 4 and 5.
Figure 4- XRF molar percentage of various elements before and after acid treatment of the non-metallic fraction of WPCB. The acid concentration is 4 M and the reaction duration is 6 hours at 90 oC.
Figure 5- XRF data of the lesser metallic impurities before and after acid treatment of the non-metallic fraction of WPCB. The acid concentration is 4 M and the reaction 15
duration is 6 hours at 90 oC. Observably, the barium is not removed by the sulfuric acid. We speculated that this is because the Ba reacts with the sulfuric acid to form BaSO4 which is more-or-less insoluble in all conventional solvents.
3.4 Thermal treatment Regardless of acid concentration or reaction duration, the bromine content of the heterogeneous waste is not altered. Due to the nature of the bromine content, it was speculated that thermal treatment of the acid-treated WPCB, would result in its removal. This speculation was confirmed with experimentation. The final silica content of the products after heating at 600 oC for 1 hour is presented in Figure 6. Silica with purity above 99% (XRF molar %) was obtained.
Figure 6- Silica content after acid treatment and heating at 600 oC for 1 hour The temperature was found to have a considerable effect on the specific surface areas of the resulting silica samples. Table 8 orders the samples by their specific surface areas (BET) from highest to lowest. The loss of BET surface area of SiO2 samples at higher temperatures can be attributed to an agglomeration effect [52]. Figure 7 shows the 16
mass loss of the acid-treated sample in the presence of air as measured by TGA. Note that when the temperature is ramped above 500 oC, after constant weight is reached, no further loss of volatiles is witnessed. The XRD spectrum (not shown here) confirms the amorphous nature of the as-prepared silica. A faster heating rate and longer heating durations lead to minimal decrease in BET specific surface area values. The solid yield after the thermal treatment is 45-55% of the acid-treated material. Figure 8 shows the pore size distribution of the most porous sample, with a corresponding pore volume of 0.17 mL/g. Table 8- Specific surface areas of various samples Heat Acid Yield treatment HNO3 Acid molarity treatment treatment after acid temperature treatment (oC) (M) temp (oC) time (hr) 4 90 6 64% 500 4 90 6 64% 500 4 90 6 64% 500 4 90 6 64% 500 6 90 1 68% 600 8 90 1 68% 600 4 106 1 68% 600 4 106 1 68% 600 4 70 24 59% 600 4 106 1 68% 700 4 90 24 59% 700 4 90 24 59% 800 4 106 1 68% 800 4 90 24 59% 900
17
Heat treatment Ramping duration rate (hr) (oC/min) 3 2 5 2 5 7 3 50 3 50 3 50 3 2 3 50 3 50 3 50 3 50 3 50 3 50 3 50
BET specific surface area (m2/g) 300.2 299.5 299.2 298.5 246.0 234.0 210.1 184.2 134.6 72.7 40.3 32.9 24.6 21.1
Figure 7- Thermo gravimetric analysis of the acid-treated sample for the production of porous silica.
35 30
Percentage
25 20 15 10 5 0 Under 6
6-8
8 - 10
10 - 12 12 - 16 16 - 20 20 - 80 Over 80 Pore size (nm)
Figure 8- Pore size distribution of the obtained silica, as calculated by the BJH method. The micropore surface area and volume are 181.8 m2/g and 0.08 mL/g, respectively.
4. Conclusion 18
Acid treatment combined with thermal treatment has proven to be a simple yet effective method for the production of porous silica from the non-metallic fraction of printed circuit board waste. Nitric, hydrochloric, and sulfuric acids were used. Regardless of the choice of acid, although metals such as aluminum and iron were removed more easily, traces of calcium persisted. Hydrochloric acid was the most effective acid used, followed closely by nitric acid. However, sulfuric acid was not effective for the removal of residual metals such as calcium from the WPCB. In addition, sulfuric acid treatment led to the contamination of samples with large amounts of sulfur impurities. After acid-treatment, thermal treatment was used to remove the bromine and other volatiles from the samples. Higher heat treatment temperatures and longer durations lead to more crystalline SiO2 with lower specific surface areas. The best porous silica sample showed relative XRF molar purities above 99% and BET surface areas as high as 300 m2/g. In the future, studies should be carried out to discern the reaction pathways occurring during the acid treatment step. Kinetics data should be collected and evaluated. In addition, the spent acid could be analyzed for the possibility or regeneration or reuse. Finally, analysis of gaseous products during thermal treatment is required to confirm the details of the emerging gases.
19
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24
Graphical abstract
25
*Highlights
Highlights
* Printed circuit board waste has been used for the first time for silica production * Concentrated sulfuric acid is not as effective as nitric or hydrochloric acid treatment * The as-prepared silica is porous with specific surface areas exceeding 300 m2/g * The purity of the obtained silica can be higher than 99% * If the treatment temperature is too high the surface area will diminish due to agglomeration
S1383-5866(14)00524-3 http://dx.doi.org/10.1016/j.seppur.2014.08.026 SEPPUR 11947
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 November 2013 15 August 2014 19 August 2014
Please cite this article as: A. Bazargan, D. Bwegendaho, J. Barford, G. McKay, Printed Circuit Board Waste as a Source for High Purity Porous Silica, Separation and Purification Technology (2014), doi: http://dx.doi.org/ 10.1016/j.seppur.2014.08.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Printed Circuit Board Waste as a Source for High Purity Porous Silica
Alireza Bazargan 1,2 Damien Bwegendaho 1 (second author, equal contribution as first author) John Barford 1 Gordon McKay 1,3 *
1
Department of Chemical and Biomolecular Engineering, Hong Kong University of
Science and Technology, Clearwater Bay, Hong Kong.
2
Department of Chemical Engineering and Biotechnology, New Museums Site,
Pembroke Street, University of Cambridge, Cambridge, CB2 3RA, UK
3
Division of Sustainable Development, College of Science, Engineering and Technology,
Hamad bin Khalifa University, Qatar Foundation, Doha, Qatar * corresponding author: Tel: (+852)23588412 Email: [email protected]
1
Abstract The electronic waste stream is one of the fastest growing streams of waste in the world. Printed circuit boards which are found in all electronic equipment are composed of a mixture of various metallic and non-metallic constituents. In this manuscript, the nonmetallics have been treated with acids followed by thermal treatment aiming at the production of high purity porous silica. The results show the possibility of obtaining SiO2 with specific surface areas (BET) as high as 300 m2/g and purity higher than 99% (X-ray fluorescence, molar %). The as-prepared silica is comparable to industrially sold samples. The study has successfully demonstrated the possibility of using these unwanted waste electronic materials for the synthesis of products with higher value and utility.
Keywords: E-waste; Printed circuit board; Amorphous silica; Urban mining; WEEE;
2
1. Introduction With advancements in the electronic world almost occurring on a day-to-day basis and increased availability of products to the public, it is not surprising to see a staggering increase of electronic wastes over the past several decades. According to a United Nations Environment Program (UNEP) alert bulletin in 2005, 20 to 50 million tons of ewastes are produced every year with an estimated annual increase rate of 4% [1]. Miniaturization of electronic devices such as mobile phones due to technological advancement does not slow down the growth of the waste stream, as the effect has been counter-balanced by increased sales, particularly in developing countries. The production of electrical and electronic devices is expected to be the fastest-growing sector in industrialized countries even with implemented environmental regulations [2]. The rapid growth is due to the economic growth in the industry, shorter lifespans of electronic devices, and the development of diversified electronic related products in recent years [3]. In general, e-wastes consist of more than a thousand different substances [4]. These substances can be categorized into the general categories of metals and non-metals. Manufacturing mobile phones and personal computers consumes 3% of the gold, 2.5% of the silver, and 12% of the palladium mined annually world-wide. 15% of cobalt mined annually across the globe is also incorporated into these devices mainly as a component of the Li-ion battery [5]. In Hong Kong, the amount of Waste Electric and Electronic Equipment (WEEE) generated has been increasing at a steady rate of approximately 2% per year. The amount of WEEE production per person is about 10 kg per year. With a population of over 7 million people, this amounts to more than 70 million kg of e-waste per year. Citizens of Hong Kong are heavy e-waste-producers, even when compared to Americans who produce about 7 kg of e-waste per person per year [6]. For India, South Africa, and
3
Brazil this number is approximately 0.4, 1.3, and 2 kg per person per year, respectively [3,7].
A Printed Circuit Board (PCB) is the generalized term used for the platform upon which microelectronic components such as semiconductor chips and capacitors are mounted. PCBs are used to support the electronic components as well as to connect them using conductive pathways, tracks or signal traces etched from copper sheets laminated onto them. In literature, a PCB is also referred to as a printed wiring board (PWB) or etched wiring board. A PCB populated with electronic components is sometimes referred to as a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). Before PCBs can be processed, they must be disassembled and separated from the body of the waste electronic equipment. Based on the fact that WEEE comes in various different shapes and sizes, it is difficult to develop a mechanized process for disassembling PCBs from all WEEE. Thus, at present, manual disassembly is the most practical option. There are several research projects underway for developing automated and semi-automated processes for removing the different components from PCBs [8–10]. It should be noted that the components of circuit board assemblies are held on to the board via solder; so effective recovery of solder via electrochemical or thermal methods would in turn lead to the separation of the components from the board itself [11–15]. An alternative could be to use molten salts to dissolve glasses and oxides, and to destroy plastics present in wastes without oxidizing the most valuable metals [16]. The removal of valuable metals such as palladium [17], gold [18], and silver [19] from WPCBs have been studied. Due to its large content, the separation of copper has always been a popular target for researchers [20–24].
4
In a review in 2009, Guo et al. [25] reported that although the major economic driving force for recycling PCBs in the metallic fraction, the non-metallic fraction makes up approximately 70% weight of the PCBs. Thermosetting resin matrix composites, thermoplastic matrix composites, and concrete and viscoelastic materials [26] were identified as the main physical recycling routes of the non-metallic fraction. Supercritical fluids depolymerization, hydrogenolytic degradation, as well as other thermo-chemical processes have also been tested [27–31]. In one creative study, acrylonitrile–butadiene– styrene waste plastic has been co-recycled with the nonmetal particles from WPCBs to manufacture composites [32]. The recovery of oils and other compounds from printed circuit board pyrolysis has recently been reported [33–35]. Low-value applications such as being used as fillers have also been investigated [36,37]. Our research group has previously used the non-metallic fraction of WPCB for the production of a novel adsorbent for wastewater treatment [38–40]. In the current paper, for the first time, a simple process for the production of amorphous porous silica from the non-metallic fraction of WPCB is reported for the first time. The porous silica can in turn be used in countless applications [41–43].
2. Materials and Methods The non-metallic fraction of the printed circuit boards was collected from Total Union PCB Recycle Ltd. The waste printed circuit board (WPCB) material consists of a powdered mixture of cured thermosetting resins, glass fiber (cellulose paper), ceramics, brominated flame retardants, residual metals and other additives [25]. This paper is concerned with the removal of the residual metals and volatile fraction of the nonmetallic WPCB. Hydrochloric acid (37%), nitric acid (70%), and sulfuric acid (95%) were purchased from Sigma Aldrich and diluted to the desired molar concentration. The e-waste was treated with various acids for different durations of time and temperatures in a round-bottom 5
flask. Typically 4.0 g of WPCB was used in a 100 ml solution. The treated sample was then filtered, washed, and dried. In order to remove the bromine and volatile fraction, subsequent heat-treatment was carried out in a muffle furnace. The Materials Characterization and Preparation Facilities (MCPF) at the Hong Kong University of Science and Technology assisted in the X-Ray Fluorescence Spectroscopy (XRF) and X-ray diffraction (XRD) analyses. A JEOL JSX-3201Z model automatic sequential XRF was used. The XRD patterns were obtained via a Philips powder diffraction system (model PW 1830) using a Cu Ka source operating at 40 keV at a scan rate of 0.025 s−1. Nitrogen adsorption-desorption isotherms were measured using a Quantachrome Autosorb-I Surface Area Analyzer. The dried samples were outgassed at 150 oC followed by nitrogen sorption at -251 oC. The Brunauer–Emmet–Teller (BET) and t-plot equations were used for calculating the total surface area and the micropore volume respectively. The total pore volume was calculated as the volume of nitrogen adsorbed at P/P0 = 0.9814. The mesopore volume could subsequently be calculated as the difference between the total and micropore volumes. The BJH method was used for discerning the pore size distribution.
3. Results and Discussion In order to obtain an initial understanding of the non-metallic fraction of the WPCB, characterization was carried out. The elemental carbon, hydrogen, nitrogen, and sulfur (CHNS) and X-ray florescence (XRF) analyses data are shown in Tables 1 and 2, respectively. Note that the XRF data is relative (sums to 100% even though it does not include lighter elements such as carbon and oxygen), while the CHNS is an absolute mass fraction of the WPCB. Table 1 – CHNS elemental analysis of the non-metallic fraction of WPCB [44] 6
Elem men nt W Weightt %
C 21 1
H 0.1 1
N 0..6
S 0
O Otheers (byy difffereence) 78 8.3
RF eelem mentaal an nalyysiss (reelattivee) o of th he n non n-m metaallicc fraactiion of WP PCB B Taablee 2 ––XR EElem men nt
Al
Si
S
C Ca
TTi
FFe
C Cu
B Br
B Ba
M Molar %
8
51 1
1
3 30
<<1
<<1
3
5
<1
he rrelaativeely higgh b bromin ne ccon nten nt iss du ue tto tthe exiisteencee of brrom minaateed fllam me Th retardan nts.. Mateerials w with h lo ow fflam mmabiilityy are reequ uireed ffor P PCB Bs d duee to thee risk o of peraatures du uring asseemb bly and d op perratio on of tthe bo oard d. A Althouggh pho osp phorrou us rissingg temp orr nittroggen n baased d fire rretaardantts aare avaailable as an altern natiive and d po ost-ind dusstriaalized co ounttriees aare graaduallyy moving tow warrds end dingg halogen nateed firee reetarrdan nts in PCB Bs, brrom minaated d fire rretaardantts are sstill ovverw wheelm minggly preedomin nan nt o on a glo obaal sccalee [4 45]. So omee off the m mosst co ommo on ttypees o of b brom min nateed fflam me rettard dants includee po olyb brom nateed bip phen nyl,, po olyb bromin nateed dip phen nyl eth hers, aand min heexab bro omo ocycclod dod deccanee. FFigu ure 1 show ws tthee mostt comm mon n brrom minaated fllam me retardan nt u used d in n prrintted circcuitt bo oards, nameely ttetrrabrom mob bisp pheenol A (also kno own n ass 2’,6 6,6’-tetrabro omo o-4,,4’--isopro opyylideeneedip pheenol orr mo oree sim mplly TTBBPA))[46 6]. 2,2
Figgure 1 1- Sttruccture o of ttetrrabrrom mob bisp phenoll A
Nitriic aacid d treeatmeent 3.1 N 7
Silica is only soluble in HF. Hence theoretically, it should be possible to remove the remnant metals in the non-metallic fraction of the WPCB through reactions with acids and the formation of soluble salts. Since metal nitrates are known to be ubiquitously soluble, the first acid to be tested was nitric acid. Table 3 shows the effect of temperature on the metal removal. As temperatures rise from 50 to 70 oC a significant decrease in calcium concentration is seen. Further increase of the temperature, enhances the calcium removal but to a lesser extent. Table 3- The effect of temperature on metal removal by 4 M HNO3 treatment for 1 hour, (XRF molar %). Treatment temperature Element
50 oC
70 oC
90 oC
105 oC
110 oC
Al
-
-
-
-
-
Si
80
90
92
93
94.5
S
<1
<1
<1
-
-
Ca
10.5
4
3
2-3
<1
Ti
1
1
1
1
1
Fe
<1
<1
-
-
-
Cu
-
-
-
-
-
Br
4-5
4-5
4-5
3-4
4
Ba
<1
<1
-
-
-
Solid yield
75%
72%
71%
69%
66%
Since the slurry boils at 110 oC, the temperature cannot be increased further under atmospheric pressure. Visibly, the titanium is persistent after the nitric acid treatment. The resistance of titanium towards removal with nitric acid is an indication that it is probably present in the form of titanium dioxide. TiO2 is used as a semiconductor and in Bragg-stack style dielectric mirrors due to its high refractive index [47]. The literature shows that titanium dioxide is insoluble in water, hydrochloric acid, nitric acid, dilute 8
sulfuric acid, and organic solvents. However, it dissolves slowly in HF and in “hot concentrated” sulfuric acid [48]. Noting that the reactivity of titanium dioxide towards an acid is dependent on the temperature at which the TiO2 has been calcined. If the TiO2 is freshly precipitated without high temperature heat treatment, it may be dissolved in hydrochloric acid. However, high temperature treatment (900 oC) will make it insoluble [49]. The removal of metals increases with the increase of acid concentration to a certain extent. Predictably, lower concentrations of acids lead to less removal of metals. As shown in Table 4, even at near boiling conditions, or with long reaction times, a 2 M solution was not strong enough for adequate calcium removal. Above 4 M, the further increase of concentration has no positive effect on metal leaching; for example, the XRF results of 4 M, 6 M, and 8 M HNO3 treatments are almost identical. Nonetheless, it should be noted that as the acid concentration increased above 4 M to 6M and 8M, there was a volume increase of the final solid product. The volume increase is mild after 6M treatment; but 8M treatment increases the volume of the residue at least 3 fold. This is an interesting observation when taking into account the mass of the solid yield which remains more or less the same.
Table 4- XRF molar elemental analysis after 2 M HNO3 acid treatment Treatment conditions Element
90oC for 1hr
105oC for 1hr
90oC for 6hr
Al
-
-
-
Si
81
84
90
S
<1
-
<1
Ca
12
9
5
Ti
1
1
1
Fe
<1
<1
<1
9
Cu
-
-
-
Br
4-5.
4-5
4-5
Ba
<1
<1
-
Solid yield
77%
75%
74%
Figure 2 shows the remaining calcium content within the waste after acid treatment of different molarities and durations. The temperature is fixed at 90 oC. Unsurprisingly, higher molarities and reaction times are more successful for calcium removal. Other metals are not shown because save for titanium which is not removed at any appreciable degree, calcium is the most persistent component. In tandem, as seen in Figure 3, the silica molar percentage in the remaining solids increases as the leaching reactions become more severe and more calcium is removed. The observed trends would logically lead us to believe that the most pure silica content can be obtained if long treatments are employed with 4 M Nitric Acid. Experiments showed that with a long reaction time of 24 hours, minute amounts of titanium could also be removed (XRF relative molar percentages of 95% Si, 0.7% Ti, and 4.3% Br with a solid yield of 59%).
Calcium content (XRF molar %)
30 25 20 6 hr
15
1 hr
10 5 0
0
1
2
3
4
HNO3 molarity
10
5
Figure 2- Calcium removal from WPCB with various HNO3 molarities and reaction durations at 90 oC
Silica content (XRF molar %)
100 95 90 85 80 75
1 hr
70
6 hr
65 60
0
1
2
3
4
5
HNO3 molarity
Figure 3- XRF data showing relative silica content after various HNO3 treatments of WPCB 3.2 Hydrochloric acid treatment Hydrochloric acid was also tested for leaching the metals from the non-metallic fraction of the WPCB. Since HCl can give off chlorinated gases during the process, less emphasis was placed on using this acid. Nonetheless, the data obtained shows that HCl is a slightly more effective acid for dissolving the residual metals from the heterogeneous WPCB. Table 5 shows the XRF molar percentages following 4 M HCl treatment at 90 oC. Table 5- XRF molar percentage of elements in the WPCB after 4 M HCl treatment at 90 o
C Treatment duration Element
1 hr
3 hr
11
6 hr
Al
-
-
-
Si
93
94
94
S
1
1
<1
Ca
<1
-
-
Ti
1.3
1.25
1
Fe
-
-
<1
Cu
-
-
-
Br
4
3-4.
4
Ba
-
-
-
68%
67%
69%
Solid Yield
The minor variations in the percentages can be the result of experimental error. Observably, 1 hour treatment of the solid with 4 M HCl is adequate to obtain solid residues similar to those obtained by 4 M HNO3 after 3 - 6 hours of treatment. The comparison between HCl and HNO3 treatment becomes more distinct when observing the data of 2 M treatment by the different acids for 6 hours. Table 6 shows this comparison. Table 6- XRF molar percentage comparison of acid treatment for 6 hours at 90 oC 2 M HNO3
2 M HCl
Al
-
-
Si
89
93
S
<1
1
Ca
5
1.3
Ti
1
1
Fe
<1
-
Cu
-
-
Br
4.5
4
Ba
-
<1
12
Solid Yield
74%
67%
Two possible explanations which are not mutually exclusive are put forth for the more powerful leaching capability of HCl. Firstly, the more effective removal of calcium at lower concentrations with HCl might be due to the size of the Cl- anion compared to NO3-. Since the heterogeneous waste is thought to be composed of a calciumaluminosilica matrix [50] where Ca is embedded into the silica medium, the removal of the calcium would require penetration of anions into the matrix. Hence, chlorine ions would more easily reach the target metals and subsequently form ligands for their removal. Secondly, the higher acidic strength of HCl compared to HNO3 could be responsible for stronger leaching. Generally, the strength of an acid is its ability to lose protons. When an acid completely ionizes (dissociates) in water leaving no non-ionized acid, it is considered ‘strong’. HA(aq) → H+(aq) + A−(aq) Equation 1
The acid dissociation constant (Ka) also known as the acid-ionization constant is a quantitative measure which can show the strength of an acid in solution. It is in fact the equilibrium constant of Equation 1 and is calculated as: ି ା Equation 2
Where [HA], [A−] and [H+] are equilibrium concentrations in mol/L. Because Ka values span many orders of magnitude, they are mostly reported by their logarithmic constant pKa: 13
pK ୟ logଵ K ୟ Equation 3
Larger values of pKa mean less dissociation of the acid. Therefore, weak acids have positive pKa values whereas acids with pKa less than -2 are said to be strong. If pKa values and pH are not known, several rules of thumb such as electronegativity of the anion, the atomic radius of the anion, and the charge of the species could be used to estimate acidic strength. For our case, when we compare the pKa values of HCl at -6.3 and HNO3 at -1.64 we can clearly see the higher acidic strength of hydrochloric acid [51].
3.3 Sulfuric acid treatment Contrary to nitric and hydrochloric acid, the employment of sulfuric acid for calcium removal was unsuccessful. Nevertheless, the titanium could be removed. Table 7 shows the XRF analysis of the residue after 4 M H2SO4 treatment at 90 oC for 6 hours. For the aim of silica production from the non-metallic fraction of printed circuit board waste, treatment with sulfuric acid is not recommended. Note that the sulfur added to the sample is stable. It is speculated that the sulfur is either in the form of precipitated calcium sulfate, or embedded into the calcium aluminosilicate matrix as it cannot be removed even after heat treatment. Incineration at 700 oC for 3 hours removes the Br completely and reduces the solid yield to 65%, but the sulfur persists. Table 7- XRF relative molar percentages of elements after 4 M H2SO4 acid treatment for 6 hours at 90 oC Element Molar XRF %
Al -
Si 61
S 20.5
Ca 14
Ti -
14
Fe -
Cu -
Br 3.5
Ba <1
The comparison of the results from three different acid treatments can be seen in Figures 4 and 5.
Figure 4- XRF molar percentage of various elements before and after acid treatment of the non-metallic fraction of WPCB. The acid concentration is 4 M and the reaction duration is 6 hours at 90 oC.
Figure 5- XRF data of the lesser metallic impurities before and after acid treatment of the non-metallic fraction of WPCB. The acid concentration is 4 M and the reaction 15
duration is 6 hours at 90 oC. Observably, the barium is not removed by the sulfuric acid. We speculated that this is because the Ba reacts with the sulfuric acid to form BaSO4 which is more-or-less insoluble in all conventional solvents.
3.4 Thermal treatment Regardless of acid concentration or reaction duration, the bromine content of the heterogeneous waste is not altered. Due to the nature of the bromine content, it was speculated that thermal treatment of the acid-treated WPCB, would result in its removal. This speculation was confirmed with experimentation. The final silica content of the products after heating at 600 oC for 1 hour is presented in Figure 6. Silica with purity above 99% (XRF molar %) was obtained.
Figure 6- Silica content after acid treatment and heating at 600 oC for 1 hour The temperature was found to have a considerable effect on the specific surface areas of the resulting silica samples. Table 8 orders the samples by their specific surface areas (BET) from highest to lowest. The loss of BET surface area of SiO2 samples at higher temperatures can be attributed to an agglomeration effect [52]. Figure 7 shows the 16
mass loss of the acid-treated sample in the presence of air as measured by TGA. Note that when the temperature is ramped above 500 oC, after constant weight is reached, no further loss of volatiles is witnessed. The XRD spectrum (not shown here) confirms the amorphous nature of the as-prepared silica. A faster heating rate and longer heating durations lead to minimal decrease in BET specific surface area values. The solid yield after the thermal treatment is 45-55% of the acid-treated material. Figure 8 shows the pore size distribution of the most porous sample, with a corresponding pore volume of 0.17 mL/g. Table 8- Specific surface areas of various samples Heat Acid Yield treatment HNO3 Acid molarity treatment treatment after acid temperature treatment (oC) (M) temp (oC) time (hr) 4 90 6 64% 500 4 90 6 64% 500 4 90 6 64% 500 4 90 6 64% 500 6 90 1 68% 600 8 90 1 68% 600 4 106 1 68% 600 4 106 1 68% 600 4 70 24 59% 600 4 106 1 68% 700 4 90 24 59% 700 4 90 24 59% 800 4 106 1 68% 800 4 90 24 59% 900
17
Heat treatment Ramping duration rate (hr) (oC/min) 3 2 5 2 5 7 3 50 3 50 3 50 3 2 3 50 3 50 3 50 3 50 3 50 3 50 3 50
BET specific surface area (m2/g) 300.2 299.5 299.2 298.5 246.0 234.0 210.1 184.2 134.6 72.7 40.3 32.9 24.6 21.1
Figure 7- Thermo gravimetric analysis of the acid-treated sample for the production of porous silica.
35 30
Percentage
25 20 15 10 5 0 Under 6
6-8
8 - 10
10 - 12 12 - 16 16 - 20 20 - 80 Over 80 Pore size (nm)
Figure 8- Pore size distribution of the obtained silica, as calculated by the BJH method. The micropore surface area and volume are 181.8 m2/g and 0.08 mL/g, respectively.
4. Conclusion 18
Acid treatment combined with thermal treatment has proven to be a simple yet effective method for the production of porous silica from the non-metallic fraction of printed circuit board waste. Nitric, hydrochloric, and sulfuric acids were used. Regardless of the choice of acid, although metals such as aluminum and iron were removed more easily, traces of calcium persisted. Hydrochloric acid was the most effective acid used, followed closely by nitric acid. However, sulfuric acid was not effective for the removal of residual metals such as calcium from the WPCB. In addition, sulfuric acid treatment led to the contamination of samples with large amounts of sulfur impurities. After acid-treatment, thermal treatment was used to remove the bromine and other volatiles from the samples. Higher heat treatment temperatures and longer durations lead to more crystalline SiO2 with lower specific surface areas. The best porous silica sample showed relative XRF molar purities above 99% and BET surface areas as high as 300 m2/g. In the future, studies should be carried out to discern the reaction pathways occurring during the acid treatment step. Kinetics data should be collected and evaluated. In addition, the spent acid could be analyzed for the possibility or regeneration or reuse. Finally, analysis of gaseous products during thermal treatment is required to confirm the details of the emerging gases.
19
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Graphical abstract
25
*Highlights
Highlights
* Printed circuit board waste has been used for the first time for silica production * Concentrated sulfuric acid is not as effective as nitric or hydrochloric acid treatment * The as-prepared silica is porous with specific surface areas exceeding 300 m2/g * The purity of the obtained silica can be higher than 99% * If the treatment temperature is too high the surface area will diminish due to agglomeration