Research Article pubs.acs.org/journal/ascecg
An Innovative Method for the Extraction of Metal from Waste Cathode Ray Tubes through a Mechanochemical Process Using 2‑[Bis(carboxymethyl)amino]acetic Acid Chelating Reagent Narendra Singh, Jinhui Li,* and Xianlai Zeng State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China ABSTRACT: To minimize electronic waste and reduce the environmental impact of hazardous metals, a new greener approach has been developed for the extraction of lead and barium metals from waste cathode ray tube (CRT) glass. The process steps comprise an initial mechanical activation of CRT funnel glass and then wet ball milling of the funnel glass powder using the metal chelate reagent 2-[bis(carboxymethyl)amino]acetic acid (NTA) and water at room temperature. After the mechanochemical treatment, the desired ≳99 mass % of lead and barium metals contained in the funnel glass powder was extracted as a metal−NTA species and pure nonleaded SiO2 glass material from the funnel glass matrix. The separation was accelerated by the ball-milling atomization and high stability constant of the metal−NTA species. After separation, the maximal amounts of lead and barium ions (∼99 mass %) were obtained as PbSO4 and BaSO4, respectively, with the addition of ferric sulfate. Notably, the developed method does not require heat and does not generate any secondary waste. Therefore, the results show that the developed method is an innovative and sustainable process, which can also probably be applied to purify any other kind of leaded glass. KEYWORDS: Cathode ray tubes, Mechanochemical treatment, Recycling, Lead
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INTRODUCTION The electrical and electronic equipment (EEE) industry is one of the most significant industries in the world, and it has grown progressively in recent years, generated a number of jobs, fostered technological development, and, at the same time, increased the level of generation of e-waste after the end of the life of electronic products.1 According to the estimation by United Nations University, the amount of e-waste will increase from around 41 metric tons (Mt) in 2014 to 47 Mt in 2017.2 Among all the e-waste, CRTs have been significantly used for more than seven decades, and it was the core unit for display in electrical and electronic equipment, such as televisions, computers, and oscilloscopes, because of its mature technology, high reliability, low price, and long lifespan.3,4 In the past, the level of CRT production increased with the demand for televisions and computers. However, at present, CRT technology for televisions and computers is obsolete; the market for new CRTs is declining. Currently, almost no CRT manufacturers use waste CRT glass from end-of-life products in the production of new display devices, and huge volumes of CRT waste enter the waste stream around the world5−9 because CRTs are being replaced mainly by flat panel technologies for video displays. Consequently, numerous discarded CRTs account for >70% of e-waste worldwide.9,10 According to a white paper on the e-waste recycling industry in China, more than 32 million waste televisions and 37 million © XXXX American Chemical Society
waste computers were generated in only 2013 and 43.11 million CRT glasses were generated in 2013.11 Currently, CRTs may have disappeared from the marketplace of the developed nations; however, the number entering the end of life is yet to peak in developed countries, and the demand for CRT devices such as televisions and computer monitors has disappeared.9,10 In Europe alone, e-waste amounted to around 11.6 Mt in 2014, and the amount has increased annually by approximately 3−5%.2,10,12 A CRT is comprised mostly of panel and funnel glass, which represents approximately two-thirds of the whole weight of the device.9 The front part of CRTs is made from nonleaded glass, which includes silica and barium, and is called panel glass. The rear part of the CRTs is made from silica and lead oxide, which contains ∼30% lead oxide, and is called funnel glass.13 Mostly, CRTs consist of a very complex chemical composition of silica materials. The chemical composition of the CRTs is one the major problems of CRT waste management. Its different glass chemical composition significantly changes its characteristics. The presence of lead in the glass protects the X-ray emission, but waste CRTs are considered hazardous waste.13,14 Received: April 26, 2016 Revised: June 30, 2016
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DOI: 10.1021/acssuschemeng.6b00875 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic diagram of the metal removal system for waste CRT glass using chelating NTA.
The disposal of televisions and computers is a very complex issue because of their decreasing life span and the presence of lead metals in the CRT devices. The life span of the CRT devices is short because the devices in which they are used are obsolete and because of the rapidly changing technologies available to the consumer.15,16 Additionally, CRT devices, such as televisions and computer monitors, have been considered an environmental hazard if disposed of improperly17 because they contain large amounts of leaded glass, which is considered toxic. CRTs handled improperly at disposal pose considerable threats to the environment and human beings mainly because of the presence of lead. Various research findings have uncovered the lead contamination of groundwater near CRT landfill sites and informal CRT recycling sites; the studies show that the presence of lead in the CRT is great threat to the environment if it is not properly disposed of.18−21 As lead particles contained in CRT funnel glass are fixed firmly in the glass network, lead extraction is highly unlikely under normal conditions. The structure combination of lead and silica dioxide in the glass is so strongly bonded that it requires a huge amount of energy to melt the silicon dioxide glass, so that usually lead can be removed from the silica glass network.22 In recent years, a few technologies have been developed to remove lead from CRT funnel or leaded glass by using chemicals and thermal technologies.23−26 For example, Mingfei et al. extracted lead from CRT funnel glass through the carbon thermal reduction process, and approximately 94% of lead was removed under the following optimal conditions: 10% carbon, 30 min holding time, and 1000 °C.24 Okada also explored the removal of lead from CRT funnel glass via a reducing and oxidizing atmosphere; the process includes the melting of the funnel glass under a reducing atmosphere and then generation of metallic lead separated and collected through acid leaching.27 A similar method studied by Viet et al. involved the removal of lead from the funnel glass, and the rate of removal of lead was ∼92% under the following conditions: 5% carbon as a reducing agent, 800 °C, vacuum of 1.3 kPa, and thermal processing for 18 h.28
Apart from these technologies, the mechanical activation of minerals represents an important development in several solid processing technologies, such as extractive metallurgy, activation by ball milling in pyrometallurgy, and an enhanced leaching process for several oxide minerals in hydrometallurgy.29 The application of the mechanochemical technique to various e-waste recycling process has been reported. For example, studies by Yuan et al. showed that ∼92.5% of lead could be mechanochemically removed from CRT glass by ball milling and acid leaching.30,31 Similaraly, Sasai et al. reported how to remove Pb atoms from Pb glass powder by using the chelating reagent sodium ethylene diamino tetraacetate (Na2EDTA) during the wet ball-milling process at room temperature.22 However, these techniques have provided good results on a lab scale in practical applications yet are far from being widely used, compared to the established methods, because of the higher cost of operation and the long time period, including the huge amount of chemicals required and also some time required for the treatment of the generated secondary waste. Therefore, it is a necessity for us to develop a new method that allows us to remove the lead from the waste CRT funnel glass by using a nonhazardous chemical. Thus, we conducted an experiment to remove lead and barium metals from the waste CRT funnel glass powder by using a commonly known and nonhazardous chelate reagent NTA through the wet ballmilling process. The use of NTA is similar to that of EDTA, both being chelating agents. In contrast to EDTA, NTA easily biodegrades and is almost completely removed during wastewater treatment. It is used for water softening and as a replacement for sodium and potassium triphosphate in detergents and cleansers.32 In this paper, we conducted a lab experiment to extract the lead and barium metals from CRT funnel glass through the wet ball-milling process using a NTA chelate reagent as a metal ion extractor. Then, we successfully extracted both lead and barium metals in the form of lead−NTA aqueous solutions, including high-purity silica powder collected as a precipitate after filtration, and then with the addition of ferric sulfate by the B
DOI: 10.1021/acssuschemeng.6b00875 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
powdery particles before and after the mechanochemical treatment was observed by scanning electron microscopy (SEM) (JSM-6330F, JEOL). The specific surface area of the funnel glass samples before and after the treatment was examined by the BET method (Tristar II 3020, Micrometrics Instrument Corp.).
chemical substitution method, we successfully recovered lead and barium metals in the form of sulfate (Figure 1).
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MATERIALS AND METHODS
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Sample Preparation. The sample of CRT funnel glass was provided by Henan Ancai Gaoke Corp. Initially, the funnel glass block was broken into small pieces, and then coatings on the glass surface were cleaned by wet scrubbing and an ultrasonic method. The coarse glass block was powdered by using dry ball milling and then sieved; the obtained glass powder with a particle size range of 150−180 μm was dried at 105 °C for 24 h. The chemical composition, determined by using X-ray fluorescence spectroscopy, is given in Table 1. NTA and ferric sulfate were supplied by Beijing Chemical Works China.
RESULTS AND DISCUSSION Mechanochemical Treatment of the Lead Glass Powder. The rate of extraction of lead and barium metals from the treated funnel glass shown in Figure 2 significantly
Table 1. Chemical Composition of the Examined Funnel Glass Determined by X-ray Fluorescence weight (%) SiO2 PbO K2O Na2O Al2O3 CaO SrO BaO MgO other metals total
42.86 35.33 7.66 3.90 2.33 2.32 2.02 1.37 1.22 0.99 100
Activation and Mechanochemical Treatment of the Funnel Glass Powder. The collected sample of funnel glass was initially broken into small pieces and then mechanically activated by using a planetary ball-mill apparatus (P-7, Fritsch). The activation process was performed by repeating the unit operation 10 times in a zirconia pot with an inner volume of 45 mL with seven zirconia balls 15 mm in diameter, which includes milling for 150 min at 500 rpm with an interval of 15 min. The pretreated funnel glass was dried at 105 °C for 24 h. The other chemicals were used in the experiment without any additional treatment In the mechanochemical treatment process, the pretreated funnel glass powder (2.5 g) was placed in a zirconia pot with an inner volume of 45 mL with seven zirconia balls 15 mm in diameter together with a proper amount of NTA (2−3 g) and deionized/distilled water. The samples were treated for different amounts of time and with different water/solid ratios. After the treatment, the obtained activated glass powder sample and zirconia balls were washed with 100 mL of water and then an extracted aqueous solution and the white powder separately by vacuum filtration. The obtained white powder was dried at 105 °C for 24 h. Recovery of Metal by Ferric Sulfate. In this process, 50 mL of the metal−NTA aqueous solution obtained by the mechanochemical treatment and ferric sulfate Fe2(SO4)3 (30−60% pure) were placed in a 100 mL Teflon bottle under the optimal conditions at Fe/metal molar ratios of 0.5−2.0, and then the mixed solution was stirred for 1.0 h at room temperature. After that, the aqueous solution was filtered by vacuum filtration, and the obtained precipitate was dried at 105 °C for 24. Analysis. The funnel glass powder before and after the activation and obtained precipitates were digested in HNO3, HCLO, and HF, and the obtained aqueous solution was quantitatively analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) (OPTIMA 3300DV) for the lead and barium metals. Similarly, the amount of metal in a metal−NTA aqueous solution was quantitatively analyzed by ICP-AES after each treatment. The obtained residues of glassy compounds after each treatment were identified with an X-ray diffractometer over a 2θ range of 20−60° (D8 Advance, Bruker) using Ni-filtered Cu Kα radiation. The morphology of the funnel glass
Figure 2. Dependence of the rate of extraction of the lead and barium metals upon addition of water and NTA for 6 h. The added amounts of NTA were (a) 2.0, (b) 2.5, and (c) 3.0 g.
depends on the water/solid ratio (milliliters to grams), which is shown on the left axis, and the specific surface area, which is shown on the right axis. The result revealed that the wet ballmilling treatment could successfully remove lead and barium metals from funnel glass particles in the presence of the NTA chelating agent. Moreover, the rate of extraction of lead or barium strongly depends on the rate of saturation of the specific surface area that can be easily observed in Figure 2. As shown in the figure, the added amount of NTA and the water/solid ratio have played a significant role in saturation of the specific area of the glass powder. A total of 2.5 g of NTA was added with a >0.4 C
DOI: 10.1021/acssuschemeng.6b00875 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
The SEM images of funnel glass particles are shown in Figure 4. These SEM images were taken after the wet ball-milling treatment in different time period with 3 g of NTA and 4 mL of water. Figure 4a shows that the pretreatment funnel glass particles had a relatively wide-ranging size distribution. Judging by the SEM images of funnel glass treated for 1.0 and 3.0 h, the particle size gradually decreases with an increase in treatment time (Figure 4b,c). This phenomenon of pulverization of the particles would have caused a rapid increase in the rate of extraction of lead and barium metal within 3.0 h. The extent of this phenomenon gradually increases up to 9.0 h of wet ball milling and then slightly decreases, but particle size did not decrease drastically. However, the rate of extraction of lead and barium metals was saturated because the pulverization of the particles was completed at the 9.0 h treatment time by wet ball milling. Similarly, this can be seen as saturation of the specific surface area in Figure 4. Recovery of Silicate Glass. For the recovery of pure silicate glass material shown in Figure 5, the XRD patterns of the solid were recorded after the wet ball milling through vacuum filtration. The diffraction lines of various silicate glasses, including silica oxide, scandium silicate, sodium aluminum, and calcium silicate, are given in Figure 5, which has been identified by X’pert high score software through peak metal analysis. On the basis of this result, we can assume that the lead and barium ions with NTA are replaced by the metal−NTA form in the extracted aqueous solution (Pb−NTA and Ba−NTA). Therefore, the remaining silica material was collected as the precipitate after vacuum filtration. Recovery of Lead and Barium. The XRD patterns of recovered lead and barium metal are given in Figure 6; the XRD patterns are of the solid collected after the addition of ferric sulfate with an Fe/metal mole ratio of 1.5 to the extracted solution and agitation for 1.0 h. In Figure 6, the XRD lines for lead sulfate and barium sulfate are identified in the collected precipitates. This phenomenon shows that the lead and barium ions with NTA are replaced by Fe3+ in the extracted aqueous solution, which forms by dissolution of ferric sulfate in water, and then the eluted ions are collected as the lead sulfate and barium sulfate by the reaction with sulfate. The rates of recovery of lead and barium from the extracted aqueous solution are listed in Table 2. Upon addition of an Fe/ metal ratio of 1.5, both lead and barium ions at more than ∼99 mass % were successfully collected by the ferric sulfate method, which was conducted in the extracted aqueous solution. The obtained results show that the replacement of lead and barium ions with iron ions will be mostly a stoichiometric reaction, mainly because the metal complex stability constant of iron is larger than that of both combined lead and barium together under the natural condition.22 Additionally, when the Fe/metal ratio was
