Article pubs.acs.org/est
Innovated Application of Mechanical Activation To Separate Lead from Scrap Cathode Ray Tube Funnel Glass Wenyi Yuan,† Jinhui Li,*,† Qiwu Zhang,‡ and Fumio Saito‡ †
School of Environment, Tsinghua University, Beijing 100084, China Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
‡
S Supporting Information *
ABSTRACT: The disposal of scrap cathode ray tube (CRT) funnel glass has become a global environmental problem due to the rapid shrinkage of new CRT monitor demand, which greatly reduces the reuse for remanufacturing. To detoxificate CRT funnel glass by lead recovery with traditional metallurgical methods, mechanical activation by ball milling was introduced to pretreat the funnel glass. As a result, substantial physicochemical changes have been observed after mechanical activation including chemical breakage and defects formation in glass inner structure. These changes contribute to the easy dissolution of the activated sample in solution. High yield of 92.5% of lead from activated CRT funnel glass by diluted nitric acid leaching and successful formation of lead sulfide by sulfur sulfidization in water have also been achieved. All the results indicate that the application of mechanical activation on recovering lead from CRT funnel glass is efficient and promising, which is also probably appropriate to detoxificate any other kind of leaded glass.
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INTRODUCTION The cathode ray tube (CRT) was widely used as a video display component of both televisions and computers.1−3 Therefore, the used personal televisions and computers with CRT4−6 reach as high as 22% of ten of categories of waste electrical and electronic equipment (WEEE) and 58% in regulated e-waste respectively, in Europe and U.S.A.7,8 It is anticipated that in China, the amount of discarded e-waste will increase rapidly due to the old-for-new home appliance replacement program that was implemented in 2009. Up to December ninth 2010, totally 31.109 million units of waste household appliances were collected.9 CRT glass consists of three primary parts: the front panel made of barium−strontium glass, the funnel made of lead silicate glass containing approximately 20 wt % PbO and neck glass 40 wt % PbO.10 The high content of lead in funnel glass remains a concern because it can seriously pollute the environment.11 The assessments using the Toxicity Characteristic Leaching Procedure (TCLP)12−14 have confirmed that funnel glass is a hazardous waste. The CRT glass supply will begin to exceed the demand of CRT manufacturing in approximately 2015.15 In order to find solutions for safely disposing of scrap CRT glass, some approaches have been proposed for recycling the scrap CRT glass to manufacture other products,16−20 such as foam glass, ceramic glaze, and clay brick. However, the potential health risks associated with lead-containing materials would preclude above-mentioned recycling.4 Since lead atoms are tightly entangled in the glass network21 that it cannot be fulfilled under ordinary temperature and pressure conditions, some technologies have recently been developed for removing the lead in order to detoxify waste funnel glass,22−25 such as reduction process, ultrasonical and subcritical technology. © 2012 American Chemical Society
The mechanical activation method has received much attention for potential applications in wide ranges based on the triggered physicochemical changes such as phase transformations, strain, structural defects, amorphization, and even direct reaction.26 A large number of publications in the field of environmental science have been available including treatment of radioactive wastes, mechanochemical decompositions of toxic substances, and development of metallurgical processes.27−34 Sasai, R.35 has reported lead extraction from leadsilicate glass using Na2EDTA reagent with an aid of wet grinding operation. In this study, we have investigated the changes of physicochemical properties affected by the mechanical activation of funnel glass treated with a typical dry grinding procedure. As a result, disruption of glass network and subsequent easy dissolutions of alkali and lead cations from the disrupted glass network have been observed. This communication reports particularly the possible applications of traditional metallurgy by dilute acid leaching and sulfidization-flotation to the funnel glass based on the mechanochemical activation.
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EXPERIMENTAL SECTION Materials Used. The sample of CRT funnel glass was obtained from the Henan Ancai Gaoke Corporation. The funnel glass block was broken into small pieces, pulverized with a ball mill, and then sieved, in order to achieve a powder with a
Received: Revised: Accepted: Published: 4109
December 8, 2011 February 9, 2012 March 3, 2012 March 4, 2012 dx.doi.org/10.1021/es204387a | Environ. Sci. Technol. 2012, 46, 4109−4114
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particle dimension range of 124−178 μm. The glass powder obtained was dried at 105 °C for 24 h. The chemical composition, determined by using X-ray fluorescence spectroscopy (SXF-1200, Shimadzu, Japan), showed that the funnel glass consisted basically of SiO2 (53.90 wt %), PbO (23.10 wt %), K2O (7.59 wt %), Na2O (5.81 wt %), CaO (3.06 wt %), Al2O3 (3.03 wt %), and other oxides in minor amounts. Nitric acid and sulfur powder were supplied from Wako Pure Chemical Industries, Ltd. Japan. Activating Operation. The funnel glass powder with the setting range described above was mechanically activated using a planetary ball-mill apparatus (P-7, FRITSCH, Germany). Four grams of the sample was put into a zirconia pot of 45 mL inner volume with 7 zirconia balls of 15 mm in diameter, and the mill was run for 120 min (15 min grinding and 15 min pause in turn) under ambient atmosphere at rotational speeds of 100, 300, 500, and 700 rpm, respectively. Characterization of the Samples. The particle size and specific surface areas of glass powder particles before and after the mechanical activation were individually analyzed using a MICROTRAC Particle Size Analyzer (Microtrac MT3300, Japan) and the BET method (ASAP2010, Micromeritics, USA). The morphology of the samples was observed by the scanning electron microscopy (SEM HATICHI 6600, Japan). Electron spin resonance (ESR) measurement were conducted at room temperature on a JEOL FA-200 (X-band) with modulation, and the g-value in ESR spectra was defined by the equation of hν = gβH, where h is the Planck’s constant, ν is the spectrometer frequency, β is the Bohr magneton, and H is is the magnitude of the laboratory applied magnetic field at resonance. Dissolution in Distilled Water and Diluted Acid Solution. Two grams of the prepared samples was agitated in 40 mL of distilled water at room temperature, and pH measurements were conducted continuously for 40 min. On the other hand, 2 g of prepared samples was agitated in 40 mL of distilled water heated at 95 °C for 120 min. After filtration, the filtrate was analyzed by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Optima 3300SYS, Japan) for lead concentration. Acid leaching of the activated samples was also conducted by agitating 0.5 g samples into 75 mL of 3 mol/L nitric acid solution heated at 95 °C for 120 min. After leaching, the solid−liquid separation was performed by vacuum filtration. Concentrations of lead, aluminum, and silicon in the filtrate were measured by ICP-OES analysis. Sulfidization of the Activated Samples in Solution. Sulfidization of the activated glass samples was carried out by agitating 0.15 g of element sulfur with 2 g of the glass sample in 40 mL of distilled water at 95 °C for 120 min. After treatment the sulfidized samples were separated by vacuum filtration, dried at 105 °C for 24 h, and analyzed by X-ray diffraction (XRD, RINT 2500, Rigaku, Japan) to identify sulfide formation.
Figure 1. Changes in pH values of the suspended solution with the prepared samples at room temperature for various time periods. Error bars represent the standard error of the mean for three replicates.
resulted in an increase in pH value when the sample was dispersed in water. When the glass is exposed to water, the preferential dissolution of glass network modifiers such as alkali and alkaline earth oxides (Si−O−A) is known to proceed through an ion exchange reaction with water to form hydroxyl ions (Si− OH + A+ + OH−), which will lead to an increase in the pH value of water, as illustrated in the following formula1 ≡Si−O−A + H2O → ≡Si−O−H + A+ + OH−
(1)
The pH value of the raw material was determined to be less than 7, indicating that it was difficult for the alkali cations of potassium and sodium inside the network of glass to diffuse out to the surface. After activation, the obtained pH values over 11.00 at the activating speed of 500 rpm indicates the easy diffusion of alkali cations to the surface to produce an increased hydroxyl ion concentration. Not only alkali cations, the easy dissolution of lead cation from the activated sample has also been observed as the results shown in Figure 2. Although the lead dissolution of the raw material under these conditions proceeded slightly, with the
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RESULTS AND DISCUSSION Metal Recovery from Activated Samples by the Leaching Process. Figure 1 shows the changes in the measured pH value within 40 min of agitation time. Compared with the raw material, where the pH value was below 7.00 (specifically, 6.54), the pH value for the activated samples increased greatly with the rotational speed of activating operation, stabilizing at values of 9.28, 10.75, 11.24, and 11.34, activated at 100, 300, 500, and 700 rpm, respectively. An increase in activation intensity (rotational speed) clearly
Figure 2. Changes in lead concentration dissolved in hot water with rotational speed of activating operation. Error bars represent the standard error of the mean for three replicates. CRT funnel glass activated at 0 rpm is raw material. 4110
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increase in rotational speed of the activating operation the lead concentration from the activated sample was observed to increase, reaching a value more than four times higher than that for the raw material. The result that both alkali cations and lead cations were easily dissolved even in distilled water from silica network implies that with the activated sample great changes could occur in the cations surrounding the silica network after mechanical activation. Since the dissolution of activated samples in distilled water was found to be relatively easy compared with raw material, solubility in acid solution is quite expectable. Figure 3 shows
Figure 4. XRD patterns of raw material and activated samples with different rotational speeds after sulfidization. CRT funnel glass activated at 0 rpm is raw material.
intensity was found to increase with the increase in the speed of the activating operation. Such high reactivity can be attributed to the changes rendered in the glass sample by the mechanical activationnamely, the easy diffusion of lead ions from the network, shown in Figure 2, and high pH of the solution from alkali cation diffusion, shown in Figure 1. The chemical properties of dissolved sulfur, in particular in alkaline solution, are believed to facilitate such reaction, and it is expected that this connection will become clear with further research. The wastewater generated by this process mainly containing sulfur ion is treated by chemical precipitation method in this study. Sulfur ion is removed by reacting with precipitating agent, ferrous sulfate (FeSO4), to generate ferric sulfide. Water-soluble sulfide of sodium is normally used for sulfide preparation; however, sulfide is usually expensive. Using elemental sulfur to react with the activated glass is both economically and environmentally friendly, increasing the likelihood of practical application. We have checked the flotation operation to concentrate the formed galena, and separation was easily obtained. Further experiments are being planned to find out the best conditions for sulfidization and subsequent flotation, in hopes of finding an engineering application. Direct reaction by grinding sulfur with CRT glass may also be worth considering. Analysis on the Physicochemical Changes of Prepared Samples after Mechanical Activation. Figure 5 shows the SEM images of the raw material (A) and the activated sample (B) for 2 h at the rotational speed of 500 rpm. In order to clearly show the surface morphology of the activated sample, the SEM image of smaller particle was chosen. Compared with the raw material, where large particles presented a natural rough surface on the order of several hundred micrometers, the activated sample is very fine, with the surface cotton-like, in a state of agglomerationa typical phenomenon observed with mechanically activated samples. The initial sample has a small specific surface area (SSA, 0.05 m2/g) and a big median particle size (D50, 191.5 μm). The increase in SSA and decrease in median particle size are observed with the increase of rotational speed to 500 and 300 rpm. The values of maximum SSA and minimum median particle size are 5.37 m2/g and 13.4 μm, respectively. Further
Figure 3. Recovery rates of main elements Si, Pb, and Al dissolved from CRT funnel glass activated at different rotational speeds. Error bars represent the standard error of the mean for three replicates. CRT funnel glass activated at 0 rpm is raw material.
the recovery rates of the three main elements of lead, aluminum, and silicon, dissolved from the samples activated at different rotational speeds. It is well-known that silicate minerals as well as silica-glass in general are difficult to dissolve in acid solution even with the aid of heating, compared with minerals of carbonates, oxides, or other types. The recovery rates of lead, aluminum and silicon from the raw glass sample under the experimental conditions were 1.2, 0.0, and 0.3%, respectively. Obviously the data shown in Figure 6 illustrate a rapid increase in the recovery rates of lead, with the rates of 92.5, 55.0, and 3.5% for lead, aluminum, and silicon, respectively dissolved the sample activated at 500 rpm. In addition to the high recovery rate of lead from the modifier side, over half of the aluminum was leached out from the completely insoluble state on the glass-network-forming side. This result also indicates that the local structure of the glass sample was altered by the mechanical activation. Feasibility of Sulfidization of Activated Samples in Solution. In addition to acid leaching, flotation is well-known as a traditional method of mineral processing, particularly for sulfide minerals such as Pb, Cu, Zn, and Fe. If the lead in glass could be sulfidized into sulfide, the well-known methods of minerals processing and metallurgy for mineral galena (PbS) could be used to recycle the glass. The possibility of treating the activated glass sample with sulfur to form lead sulfide was investigated, and the results are shown in Figure 4. For the raw material, no peaks of sulfide were observed in the XRD pattern. Peaks of lead sulfide were observed, however, for the activated samples, even with the use of elemental sulfur, and the peak 4111
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Figure 5. SEM images of CRT funnel glass: (A) raw material and (B) activated sample at the speed of 500 rpm.
≡Si−O−A + mechanical activation → ≡Si−O· + A+
extension of rotational speed, to 700 rpm, does not cause evident changes in these two values. It is clearly important to understand what has happened with the activated sample and the relationship between the activation-induced structural changes and the enhanced dissolution. Although various changes under activation have been observed with glass sample, analysis by ESR as shown in Figure 6 was chosen to for discussion just because that in the
(3)
≡Si· + O2 → ≡Si−O−O·
(4)
≡Si−O· + 1/2O2 → ≡Si−O−O·
(5)
The increase in the signal intensity indicated the increase in the amount of mechano-radicals produced from the mechanically activated samples, which also indicated that the numbers of Si− O and Si−O−A bonds breakage increased. It indicated that higher rotational speed of milling supplied higher energy to break chemical bonds. NBOHC and peroxy radical are the most important structural point defects in silicate glass,41 which are generated during the process of mechanical activation. Structural energy barrier model developed by McGrail et al.42 describes the kinetics of ion exchange from silicate glass. Ion exchange rate was governed by the equation: r = ωexp(−Ex/ RT), where ω is the pre-exponent term, Ex is the energy barrier, R is the universal gas constant, and T is temperature. The energy barrier Ex, is the sum of Eb (bond energy of the metal A on the Si−O−A site) and Es (elastic strain energy associated with the distortion of the glass network). The exponent exp(−Eb/RT) accounts for the release of metal ion from Si− O−A site, and the exponent exp(−Es/RT) accounts for the successful motion within glass network.38 Compared with raw material, activation-induced point defects in activated CRT funnel glass (breakage of Si−O-A bonds) means the existence of free metal ion (unbound metal ion) releasing from activated samples, in which Eb decreased compared with raw material. To summarize, CRT funnel glass is a synthetic material, not a natural mineral, and is not easy to use traditional mineral processing methods to deal with it. Mechanical activation of minerals could cause the structure defect, increase in internal energy, and disordering to promote the leaching process. This work has clarified the changes in CRT funnel glass triggered by mechanical activation with a dry activating operation and has demonstrated the feasibility of using traditional mineral processing and metallurgical methods to deal with the troublesome issue of lead recovery. Mechanical activation seems to be a favorable process for the treatment of CRT funnel glass prior to leaching with significantly enhanced lead recovery through mechanical activation-induced changes in microstructure and glass network bonding. To compare with other current methodologies, mechanical activation used to recover lead from CRT funnel glass is with high efficiency and simple process, which could dispose of CRT funnel glass with traditional hydrometallurgical extraction process. More detailed information, with specific investigations of acid leaching and sulfidization-flotation, will be discussed in subsequent reports. A process consisting of first sulfidization-flotation and then
Figure 6. Variation of ESR spectra of raw material, activated samples with different rotational speeds and leaching residue (leaching residue is for 500 rpm activated sample after leaching in HNO3 solution).
field of study of mechanical activation the ESR method has proved to be very useful36 and we had studied the effect radicals in mechanochemical decomposition of chlorinated polymers.37,38 Raw material does not exhibit radical signal. Radical signals have been clearly found from mechanically activated samples, and the signals disappeared for leaching residue. The ESR signals are attributed to radicals, which are nonbridging oxygen hole center (NBOHC, ≡Si−O·) and the peroxy radical (≡Si−O−O·), where · depicts an unpaired spin residing on oxygen.39,40 When CRT funnel glass, with the framework structure of a SiO4 tetrahedron, is broken by mechanical activation, scissions of chemical bonds, ≡Si−O− Si≡ and ≡Si−O−A, take place to generate E′ center (≡Si·) and NBOHC. Due to the mechanical activation process carried out under air atmosphere, part of the E′ center and NBOHC radicals produced on the freshly formed surface reacted easily with the oxygen molecules and changed to peroxy radicals, as shown by the following equations ≡Si−O−Si≡+mechanical activation → ≡Si· + ·O−Si≡ (2) 4112
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dilute acid leaching would be recommended. It is believed that such mechanical activation could be also used to treat other leaded glasses and tough issues, particularly industrial wastes with compositions or structures different from the known natural ones.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedure, measured data, and reaction mechanism between activated funnel glass with sulfur in solution. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-10-6279 4143. Fax: +86-10-6277 2048. E-mail:
[email protected]. Corresponding author address: Tsinghua University, Sino-Italia environmental energy building, Rm. 805, Haidian district, Beijing 100084, China. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was financially supported by the Environmental Protection Public Welfare Project (2011467035), the National Nature Science Foundation of China (21177069) and the Short-time Study Abroad Program for PhD Candidates from Tsinghua University. The authors are grateful to Huabo Duan and Bo Yang for the help with experiments.
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