Waste Management 76 (2018) 671–678

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Waste Management

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Lead extraction from Cathode Ray Tube (CRT) funnel glass: Reactionmechanisms in thermal reduction with addition of carbon (C)

https://doi.org/10.1016/j.wasman.2018.04.0100956-053X/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: School of Environment, Guangzhou Key Laboratory ofEnvironmental Exposure and Health, and Guangdong Key Laboratory of Environ-mental Pollution and Health, Jinan University, Guangzhou 510632, China.

E-mail address: wf1984@jnu.edu.cn (F. Wang).

Xingwen Lu a,b, Xun-an Ning a, Da Chen d, Kui-Hao Chuang c, Kaimin Shih b, Fei Wang b,d,⇑a School of Environmental Science and Engineering and Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, ChinabDepartment of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative RegioncDepartment of Safety Health and Environmental Engineering, Central Taiwan University of Science and Technology, Taichung 406, Taiwan, ROCd School of Environment, Guangzhou Key Laboratory of Environmental Exposure and Health, and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University,Guangzhou 510632, China

a r t i c l e i n f o

Article history:Received 3 January 2018Revised 27 March 2018Accepted 6 April 2018Available online 9 April 2018

Keywords:Cathode Ray Tube (CRT)Funnel glassThermal reductionQuantitative X-ray diffractionLead recovery

a b s t r a c t

This study quantitatively determined the extraction of lead from CRT funnel glass and examined themechanisms of thermally reducing lead in the products of sintering Pb-glass with carbon in the pre-heated furnace. The experimentally derived results indicate that a 90.3 wt% lead extraction efficiencycan be achieved with 20 wt% of C addition at 950 �C for 3 min under air. The formation of viscoussemi-liquid glass blocked the oxygen supply between the interaction of C and Pb-glass, and was highlyeffective for the extraction of metallic Pb. A maximum of 87.3% lead recover was obtained with a C toNa2CO3 ratio of 1/3 at 1200 �C. The decrease of C/Na2CO3 ratio enhanced the metallic lead recovery byincreasing the glass viscosity for effective sedimentation of metallic lead in the bottom. However, withthe further increase of temperature and treatment time, re-vitrification of lead back to silicate-glassmatrix was detected in both Pb-glass/C and Pb-glass/C/Na2CO3 systems. The findings indicated that withproper controls, using C as an inexpensive reagent can effectively reduce treatment time and energy,which is crucial to a waste-to-resource technology for economically recovering lead from the wasteCRT glass.

� 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Cathode Ray Tubes (CRTs) have been extensively used for morethan 70 years. The number of discarded CRTs in televisions andcomputer monitors has drastically increased because of thereplacement of CRT by advanced flat panel products (Chen et al.,2010). In North America, approximately 163,420 computers andtelevisions became obsolete every day in 2006, and the cost ofproper disposal or recycling reached as high as $10.8 billion bythe year 2015 (Kim et al., 2009). In Europe, 50,000–150,000 tonsof end-of-life CRTs were collected each year, and this flux is notexpected to decrease in the upcoming years (Singh et al., 2016a).In China, more than 32 million units of waste televisions and 37million units of waste computers, as well as 43.11 million tons ofCRT glasses were generated in 2013 only (Singh et al., 2016b).

Additionally, approximately 6 million discarded TVs and 10 millioncomputers are stockpiled each year (Xu et al., 2012).

Waste CRTs are mostly generated from panel and funnel glassesaccounting for about two-thirds of the whole weight of the device(Andreola et al., 2005). The front part of CRTs is called panel glass,which is made of barium-strontium glass free of lead (Andreolaet al., 2005; Musson et al., 2000). The rear part of the CRTs is calledfunnel glass, made of silica and lead oxide (�20 wt% of PbO)(Musson et al., 2000; Andreola et al., 2007). CRTs are usually incin-erated and dumped in landfills as municipal solid waste (MSW) inmost countries, even developed countries (Jang and Townsend,2003; Tian and Wu, 2016; Needhidasan et al., 2014). Unfortu-nately, lead can leach out from the landfilled CRT funnel glasses,which may seriously contaminate the environment and endangerhuman health (Liu et al., 2011; Turbini et al., 2001). Thus, thedevelopment of new technology to detoxify and recycle CRT glassis strongly needed.

Generally, closed-loop and open-loop recycling are two princi-pal ways of recycling CRT glass (Singh et al., 2016a; Herat, 2008).In closed-loop recycling, waste CRT glass is commonly used asraw material to manufacture new CRTs (Herat, 2008; Gregory

672 X. Lu et al. /Waste Management 76 (2018) 671–678

et al., 2009). However, the increasing supply of secondary glass hasexceeded the continuously declining demand of new CRT glass as aresult of their replacement by new display technologies (Singhet al., 2016a; Herat, 2008; Gregory et al., 2009; Kang andSchoenung, 2006). Some applications have been proposed for theuse of recycled CRT glass in the manufacture of clay bricks, decora-tive tiles, nuclear waste encapsulation, foam glass, ceramic glazeand fluxing agent for open-loop recycling (Ling and Poon, 2012;Dondi et al., 2009; Ling and Poon, 2011; Andreola and Barbieri,2007). However, a limitation of these techniques is the potentialthreat to environmental and human health associated with lead-containing products.

Development of techniques to extract lead from funnel glass isessential to safe disposal of waste CRTs (Singh et al., 2016c). Manytechnologies have been developed to extract lead from CRT funnelglass, including pyrometallurgy, mechanochemical method,enhanced leaching and others (Singh et al., 2016a; Tian and Wu,2016; Singh et al., 2016c). Operational parameters and productproperties of various methods used for extracting lead from CRTfunnel glass are summarized in Table 1. Among the advanced leadextraction technologies, pyrometallurgy was proven to be an effec-tive way to extract lead from Pb-glass matrix (ICER, 2003; Lu et al.,2013; Xing and Zhang, 2011; Chen et al., 2009; Okada andYonezawa, 2013; Okada, 2015; Okada et al., 2015; Lv et al., 2016;Veit et al., 2015). Since lead containing PbO3, PbO4, and PbO6 poly-hedra are firmly encapsulated between SiO4 tetrahedrons to form

Table 1Operational parameters and product properties of various methods used for extracting lea

Technology Reagents Temperature(C)

Pyrometallurgy Al (ICER, 2003) 1400SiC/TiN (Yot and Méar, 2009) 950

Mg, Fe2O3 (Wang and Zhu, 2012) >2000

Fe (Lu et al., 2013) 700

Poly(vinyl chloride), Ca(OH)2 (Grause et al.,2014a, 2014b)

1000

CaCl2 (Erzat and Zhang, 2014) 1000

C (Xing and Zhang, 2011; Chen et al., 2009) 1000

C, Na2CO3 (Okada and Yonezawa, 2013;Okada, 2015; Okada et al., 2015)

1000

C, lead slag (Lv et al., 2016) 1200

C (Veit et al., 2015) 800Mechanochemical

treatment2-[Bis(carboxymethyl)amino], acetic acid,ferric sulfate (Singh et al., 2016b)

Roomtemperature

Na2EDTA, ferric sulfate (Sasai et al., 2008) Roomtemperature

S (Yuan et al., 2012, 2013, 2015) Roomtemperature

Enhanced leaching Nitric acid (Miyoshi et al., 2004) 355

Nitric acid, ultrasound (Saterlay et al., 2001) Roomtemperature

Serratia plymuthica and EDTA (Pant et al.,2014)

37

Thermal reduction+ acid leaching

Na2CO3, Na2S (Hu et al., 2015) 1300

C, HNO3(5 mol/L) (Xing et al., 2016) 1200

B2O3, HNO3(5 mol/L) (Xing et al., 2017) 1000

N.A. Not avaiable.ER: Extraction ratio (%).RR: Recovery ratio (%).

the glass network (Méar et al., 2006a, 2006b), the extraction of leadfrom the complex network structure often involves high energyconsumption to break the structure of lead silicate and separatelead from funnel glass. However, the usage of reducing agent canchange the structure of three-dimensional glass and extract Pbfrom PbOx polyhedron via redox reaction at relative low tempera-ture with less time consumption. Thus, the development of cost-effective processes is necessary and meaningful to recover Pb fromCRT funnel glass for environmental protection and resourcerecovery.

This work used C as a reducing reagent to extract the metalliclead from glass matrix at temperatures of 600–1000 �C in air. Thesintering conditions (temperature, holding time, reagent dosage)were optimized to find the proper parameters for lead extraction.Furthermore, Na2CO3 was used to recovery lead from CRT funnelglass. Reaction mechanisms in thermal reduction with additionof C and Na2CO3 were examined by quantitatively determiningthe phase transformation during the reductive reaction of CRTfunnel glass with C.

2. Materials and methods

2.1. Materials

Funnel glass from color computer monitors was collected inHong Kong and crushed into small pieces (around 3 cm) with the

d from CRT funnel glass.

Pressure Time Extraction ratio (ER, %) orRecovery ratio (RR, %)

Product Properties

1 atm 1.5 h ER, 50 Metallic lead, glass1 atm 1–4

hER, 40 Foam glass, metallic lead

1 atm 1min

ER, 90 Refractory materials, leadoxide

1 atm 30min

ER, 58 Metallic lead, glass

1 atm 2 h ER, 99.9 PbCl2, calcium silicate-containing residues

600 ±50 Pa

2 h RR, 99.1 PbCl2

500–2000 Pa

1 h RR, 96 Porous residue, nano-lead

1 atm 60min

RR, 92 Metallic lead

1 atm 60min

ER, 97 Metallic lead, calciumsilicateslag

1.3 kPa, 18 h ER, 92 N.A.1 atm 12 h RR, 99.8 PbSO4, silica powder

1 atm 20 h RR, 99 PbSO4, silica powder

1 atm 2 h ER, 92.5 Glass, PbS

24 MPa 18–22 h

ER, 93 Crystalline alkaline-earthlead silicate, orthoclase

1 atm 1 h ER, 90 Pb2+

1 atm 12days

N.A. Pb-EDTA

0.75MPa

1 h ER,100 and RR 43.3 sodium silicate, PbS, PbSO4

1 atm 30min

ER, 94.8 Pb2+, SiO2 glass

1 atm 30min

ER, 99.8 Pb2+, SiO2 glass

Fig. 1. XRD patterns of the Pb-glass/C of 1/0.1 mixtures thermally treated in heatingrates of 5, 10, 20, 30 and 40 �C/min, and the pre-heated furnace at 950 �C for 3 minunder air atmosphere.

X. Lu et al. /Waste Management 76 (2018) 671–678 673

coating fully removed by the wet scrubbing method (Lee and Hsi,2002). The cleaned funnel glass particles were then dry ball milledand sieved to collect particles smaller than 80 lm. The powderobtained was dried at 105 �C for 24 h. The chemical compositionof the glass powder was examined by XRF (JSX-3201z, JEOL) to beSiO2 (51.72%), PbO (21.50%), K2O (8.88%), Na2O (5.70%), CaO(3.63%), Al2O3 (3.03%), MgO (2.98%) and others (2.55%). Carbonpowder with a particle size of 4–8 lm was obtained from Aldrichfor use as the reducing agent. Na2CO3 was also purchased fromAldrich and used as the fluxing agent in the thermal reductionprocess.

2.2. Thermal reaction

The experiments were carried out by heating the mixture ofwaste funnel glass and reducing agent C with or without fluxingagent Na2CO3 in air atmosphere. Samples weighed with differentratios of funnel glass, C and Na2CO3 were homogenized by ballmilling. The well-mixed powders were transferred into a 30-mLalumina crucible and combusted in a high temperature furnace.The powders were heated at 600–1200 �C with temperatureincreasing rates of 5, 10, 20, 30 and 40 �C/min. Powder mixtureswere directly put into the pre-heated furnace. The door of the fur-nace was opened slightly in a fraction, and the solid sample wasquickly placed into the furnace before the temperature dropped.The retention time of thermal treatment in the furnace was con-trolled at 3–120 min under air condition. After thermal treatment,the samples were allowed to cool naturally to room temperature.After quenching, the solid samples were transferred from thecrucible onto weighing papers for weight analysis. Because theRietveld quantitative analysis can only determine the phase com-position of each chemical in the mixture, the product weight ofeach sample would be used to calculate the actual weight of eachcompound. Finally, the samples were ground into powder with aparticle size of less than 10 lm for XRD analysis.

2.3. X-ray diffraction

Phase transformation during thermal reduction process wasidentified by the X-ray powder diffraction technique. The X-raydiffraction data from the powder samples were collected on a Bru-ker D8 Advance X-ray powder diffractometer equipped with a CuKa radiation and a LynxEye detector. The diffractometer was oper-ated at 40 kV and 40 mA, and the 2h scan range was 10� to 80�witha step size of 0.02� and a scan speed of 0.3 s/step. Phase identifica-tion was performed using Eva XRD Pattern Processing software(Bruker Co. Ltd.) by matching the powder XRD patterns with thoseretrieved from the Powder Diffraction File (PDF) database pub-lished by the International Centre for Diffraction Data (ICDD).The crystalline phases found in the products included lead (Pb;PDF #65-2873), lithargite (PbO; PDF #05-0561), and aluminum(Al; PDF #04-0787).

2.4. Rietveld refinement

A quantitative refinement method using 25% CaF2 as the inter-nal standard (De La Torre et al., 2001; Magallanes-Perdomo et al.,2009; Rendtorff et al., 2010) was used to quantify the amorphouscontent in the samples. XRD scans were conducted from 2h = 10�to 110�, with a step width of 2h = 0.02� and a sampling time of0.5 s per step. The Rietveld refinements for phase quantificationwere processed by the TOPAS (version 4.0) program. Structuremodels of the crystalline phases were obtained from the ICDDdatabase. Figs. S1–S3 in the Supplementary Information (SI)present the Rietveld refinement plots of the products sintered fromthe C/Pb-glass and Na2CO3/C/Pb-glass systems, and the

corresponding reliability values, indicating the good refinementquality achieved using this analytical scheme, are provided in SITables S1–S4.

3. Results and discussion

3.1. Effect of heating rate

To investigate the influence of heating rate on lead extraction, amixture of CRT Pb-glass and C with a Pb-glass/C mass ratio of 1/0.1was heated with a temperature increasing rate of 5, 10, 20, 30 or 40�C/min, respectively, and in the pre-heated furnace at 950 �C for 3min under air atmosphere. As shown in Fig. 1, the signal of metallicPb was significantly increased with the increase of temperatureheating rate. Glass transition temperature (Tg) of CRT glass wasreported at around 480 �C (Méar et al., 2006a, 2006b); thus thePb-glass quickly melts into a viscous semi-liquid at 950 �C in thepre-heated furnace. This highly viscous matrix blocks the oxygensupply during the interaction between C and Pb-glass, and is highlyeffective for inhibiting C oxidation and metallic lead extraction.Thus, strong signals of Pb were observed in the XRD pattern of ther-mally treated product. In contrast, the peak intensity of Pb was lowin the products heated with a slower heating rate (5, 10 and 20 �C/min) due to the slower transformation of solid to liquid, whichcould not prevent the oxidation of C. Without the inhibition of oxy-gen in the air, the C would be quickly combusted other than reduc-ing Pb in PbOx polyhedra into metallic lead.

To evaluate the lead extraction efficiency with different heatingrates, quantitative phase compositions of the heated products weredetermined by diffraction data and Rietveld analysis. Using theresults of the quantitative XRD analysis, an extraction ratio (ER)index of metallic lead was defined by Eq. (1) to indicate the extrac-tion efficiency:

ERð%Þ ¼ mT � wtPb þwtPbO � MWPb

MWPbO

� �� �=ðmCRT �wt0PbÞ

� ��100%

ð1Þ

Fig. 2. Effect of the heating rate on the Pb extraction efficiency. The extraction ratio(ER) values are plotted for the mixtures with mass ratio of Pb-glass/C of 0.1/1 at950 �C for 3 min.

Fig. 3. Effect of temperature, treatment time and Pb-glass/C mass ratio on Pbextraction efficiency. Extraction ratio (ER) values were obtained for Pb-glass/Csamples with different mass ratios thermally treated at 600–1000 �C for 3–120 min.

674 X. Lu et al. /Waste Management 76 (2018) 671–678

where mT is the weight of the treated products; wtPb and wtPbO arethe amount of metallic lead and PbO in the treated sample, respec-tively; MWPb is molecular weight; mCRT represents the weight ofthe CRT funnel glass used for the treatment; and wt’Pb is 19.9 wt%based on the XRF result .

Fig. 2 summarized the ER values of C on CRT glass as the func-tion of heating rate at 950 �C for 3 min. Substantial increase of Pbextraction efficiency was observed with the increase of heatingrate, and the lead extraction ratios increased significantly to 65%in the heating product. This finding clearly demonstrated that apre-heated atmosphere was the optimized condition for leadextraction.

The significant formation of metallic lead in the preheated fur-nace was probably due to the quick fusion of the mixtures into aviscous semi-liquid at 950 �C. Such a reaction would block the oxy-gen supply during the interaction between C and effectively inhibitthe oxidation of C. The thermal reduction of lead in the glass withaddition of C can be revealed by the reaction equation described inEq. (2):

BSiAOAPbA + AOA + C(0) ! Pb(0) + BSiAOA + ACAOA ð2ÞIn the reductive melting process, C reacts with O- in 3-

dimensional viscous network for reducing Pb2+ in glass networks.The result also shows the benefit of using this new method forindustrial operations, because the continuous processing (materi-als directly entering the constant-temperature zone withoutslowing temperature ramping) and short reaction times are usuallypreferred.

3.2. Effect of temperature, treatment time and C dosage

Extraction of lead from CRT funnel glass was reported to bestrongly dependent on treatment parameters (Lu et al., 2013;Xing and Zhang, 2011; Chen et al., 2009; Okada and Yonezawa,2013; Okada, 2015; Okada et al., 2015; Lv et al., 2016). To obtainthe optimal parameters for the efficient recovery of lead, differentdoses of the reducing agent (C) were tested under temperatures of600–1000 �C for 3–120 min. Fig. 3(a) summarizes the obtained ERvalues as the function of temperature with a Pb-glass/C mass ratioof 1/0.1. At temperatures of 600–950 �C, substantial increases of Pbextraction efficiency were observed and the lead extraction ratiosincreased significantly to 65%. However, the ER value dramaticallydecreased when temperature increased from 900 �C to 1000 �C.The results of the quantitative XRD revealed the most effective

temperature for lead extraction from glassy networks was around950 �C. The significant growth of metallic Pb at 950 �C is probablydue to the intensive lead diffusion across the glass-C interface thatoccurs in the crystallization of metallic lead (Rawlings et al., 2006),together with the higher mobility of C in the viscous glass at highertemperatures (Kushima et al., 2009). However, the dramaticdecrease at a higher temperature (1000 �C) may be attributed tothe potential transformation of the lead crystals back to the glassymatrix or the volatilization of metallic lead at higher temperatures.

To evaluate the metal evaporation under the test conditions,samples with a Pb-glass/C mass ratio of 1/0.1 were weighed before

Fig. 5. Metallic lead recovery ratios under different thermal treatment times forCRT/C/Na2CO3 samples (mass ratio of Pb-glass/C = 1/0.1) treated at 950–1200 �C for1 h.

X. Lu et al. /Waste Management 76 (2018) 671–678 675

and after thermal treatment at 800–1000 �C and were subjected toelemental characterization using the X-ray fluorescence (XRF)technique (Table S5 SI). According to the sample weight and XRFdata, the loss of Pb due to evaporation was determined to be lessthan 1 wt%. This finding indirectly indicates that the amorphiza-tion of lead occurs at higher temperatures (1000 �C or above), sug-gesting the extracted lead transforms back to the glass product.Fig. 3(b) summarizes the quantified results of the lead extractionefficiency as a function of treatment time of 3–120min. At 700 �C,the lead extraction efficiency initially increased before 30 min ofheating, but then gradually decreased with a prolonged heatingtime. At higher temperatures of 950 and 1000 �C, negative relation-ships between the lead extraction efficiency and thermal treat-ment time were found. The findings suggested two key reactionmechanisms of lead during the thermal reduction process: thereduction of lead by C with a short treating time as described byEq. (2) and the re-vitrification of lead back to silicate-glass matrixwith a prolonged treating time as defined by Eq. (3).

BSiAOASiB + Pb(0) ! BSiAO�Pbþ + �SiB ð3ÞSimilar observation was previously reported (ICER, 2003; Lu

et al. 2013), which suggest that the incorporation of lead into theglass matrix is due to the oxidation of lead under a long reactiontime and further melting. The lead extraction pathways duringthe extraction process were shown in Fig. 4. Pathway I representsthe main reactions for extraction metallic lead from Pb-glass andthe formation of crystal lead at 600–950 �C for 3 min heating;pathway II shows higher temperatures (950–1000 �C) and longerheating time initiate the re-vitrification of lead back to silicate-glass matrix.

Fig. 3(c) illustrates the lead extraction with different doses ofthe reducing agent under a treatment of 950 �C for 3 min. Theresults revealed that lead extraction was generally enhanced byan increase in C. At 950 �C, the lead extraction efficiency increasedwith the increasing Pb-glass/C weight ratio (1/0.01–1/0.2), andreached a maximum efficiency of 90% with a Pb-glass/C weightratio of 1/0.2. However, when Pb-glass/C mass ratio increased from0.2/1 to 0.3/1, no further efficiency enhancement was observed.Thus, the use of a Pb-glass/C mass ratio of 1/0.2 is more effectiveand economic for Pb extraction. The efficiency of interfacialreaction is usually influenced by the encountering rate betweenthe reactant molecules (Kushima et al., 2009). More addition of Cin the mixture will provide sufficient contact frequency withPb-glass reactants, which will further enhance the reduction rate.

In this section, it was identified that the addition of 20 wt% C toPb-glass and treatment at 950 �C for 3 min resulted in the best leadextraction performance. Compared to other methods, this methodcan be conducted in air with a shorter treating time and without

Fig. 4. The extraction (I) and vitrification (II) pathways of metallic Pb by heating the mixfrom PbOx polyhedron in glass matrix and the formation of Pb crystals at 600–950 �C forglass matrix initiated by higher temperatures (950–1000 �C) and longer heating time (5

the facility of a high temperature (above 1000 �C) to obtain metal-lic lead in the product. Furthermore, direct placement of the mix-ture of C and Pb-glass into the heated furnace (without the needfor slow temperature ramping) is beneficial as it allows for the con-tinuous processing preferred in industrial operations. Therefore,with proper controls, using C as an inexpensive reagent can effec-tively reduce treatment time and energy, which may allow for anew opportunity to more economically extract lead from wasteCRT glass.

3.3. Recovery of Pb with the addition of Na2CO3

To further recover metallic Pb from CRT funnel glass, Na2CO3

was used as a fluxing agent added into Pb-glass/C system forsettling down the recovered metallic Pb to the bottom of productsduring the melting process (Table S6 in SI). Initially, the mixtureswere heated at 950 �C (the optimal temperature for lead extractionof Pb-glass/C), but the recovery efficiency of metallic lead wasquite low (<20%). The low recovery efficiency may be caused bythe reduced encountering rate between the reactant molecules(i.e. C and Pb-glass) and the addition of Na2CO3 in the system. Tofind out the optimal temperature for recovering metallic lead fromCRT Pb-glass/C/Na2CO3 system, the effects of the Na2CO3 dosage(Na2CO3/C of 1/1 to 4/1) and temperature (950–1150 �C) on thelead recovery efficiency were studied and shown in Fig. 5.

tures of CRT Pb-glass and C. Pathway I represents the main reactions for freeing Pb3 min heating; pathway II shows the re-vitrification of Pb crystal back into silicate-–120 min).

Fig. 6. XRD patterns of the residual glass and lead products recovered from C/Na2CO3 of 1/3 products heated at 950–1100 �C for 1 h.

676 X. Lu et al. /Waste Management 76 (2018) 671–678

The lead recovery efficiency was firstly enhanced by decreasingC/Na2CO3 to 1/3, and reduced with the further decrease of C/Na2CO3 to 1/4. Because of the decrease in glass viscosity with theincrease of Na2CO3 dosage, the increase in the sedimentation rateof the metallic lead particles was achieved by decreasing C/Na2CO3

from 1/1 to 1/3 (Okada and Yonezawa, 2013; Okada, 2015). How-ever, with further increase of Na2CO3 dosage to C/Na2CO3 of 1/4,the efficiency of interfacial reactions influenced by the encounter-ing rate between the reactant molecules (such as: C and Pb-glass)(Kukukova et al., 2009) was greatly reduced, leading to the reduc-tion of lead recovery efficiency. Thus, the use of C/Na2CO3 massratio of 1/3 was more cost-effective for lead extraction.

The maximum recovery efficiency of metallic lead was only 20%at 950 �C but increased to 87.3% when the temperature wasincreased to 1200 �C. Because both glass transition temperatureof CRT funnel glass (�480 �C) (Méar et al., 2006a, 2006b) and melt-ing temperature of Pb (327.5 �C) are quite lower than the treatingtemperature, the Pb-glass and reduced Pb quickly melt into viscoussemi-liquid at 950–1200 �C in the pre-heated furnace. The viscos-ity of the glass decreases with increasing temperature, resultingin an increase in the sedimentation rate of the metallic lead parti-cles (Okada and Yonezawa, 2013; Okada, 2015). However, with the

Fig. 7. The reduction (I) and sedimentation (II) pathways for recovering metallic Pb by hPb from PbOx polyhedron and the formation of Pb crystals. By melting CRT Pb-glass andmetallic Pb and facilitated the formation of Pb crystals. Pathway II reveals the sedimentviscosity of the glass liquid and lead to the effective sedimentation of Pb crystals.

further increase of temperature to 1200 �C, a slight decrease of therecovery efficiency was observed. As discussed previously, highertemperature would facilitate the transformation of the metalliclead crystals back to the glassy matrix due to the oxidation of leadunder high temperature melting (ICER, 2003; Lu et al.,2013).

After the reduction melting at 1200 �C, 87.3% of the lead in thePb-glass was recovered and the residual lead was left in the glassmatrix after reduction melting. To confirm the formation of resid-ual lead, the settled lead was separated from the glass samples forXRD analysis. Fig. 6(a) shows XRD patterns of the residual glasstreated at temperatures of 950–1100 �C. More residual metalliclead was observed in the samples sintered at low temperatures.The results may suggest that the lower viscosity of glass matrixin 950–1000 �C leads to the decrease of lead sedimentation rate.With the temperature increasing, the reduced intensity of leadsignal in the residual glass suggested that higher temperatures(1100–1200 �C) facilitate the settling down of metallic lead, buteasily lead to the transformation of metallic lead back into theglass matrix. The schematic diagram for the recovery of metalliclead with the addition of Na2CO3 in Pb-glass/C mixtures was illus-trated in Fig. 7. At 950–1000 �C, Pb-glass, reduced metallic lead andNa2CO3 melt into viscous semi-liquid in the pre-heated furnace.

eating the mixtures of Pb-glass/C/Na2CO3. Pathway I demonstrates the reduction ofNa2CO3, the highly viscous liquid formed, which inhibited the oxidation of C and

ation of metallic Pb in the bottom of crucible. The melting of Na2CO3 increased the

X. Lu et al. /Waste Management 76 (2018) 671–678 677

The highly viscous matrix blocks the oxygen supply during theinteraction between C and Pb-glass, and is highly effective ininhibiting the oxidation of C and metallic Pb. In addition, theadding of Na2CO3 increased the viscosity of the glass, leading toan increase in the sedimentation rate of the metallic lead particles.However, further temperature increase may facilitate the re-vitrification of lead back to silicate-glass matrix.

To identify the purity of recovered lead, Fig. 6(b) shows the XRDplots of metallic Pb recovered from C/Na2CO3 of 1/3 at tempera-tures of 950–1100 �C. Both metallic lead and aluminum werefound in the recovered lead samples. As the content of Al2O3 inCRT funnel glass was reported to be �3 wt%, aluminum wasreduced by C and settled down together with lead at higher tem-peratures. To obtain the pure metallic lead, the separation of Alfrom the recovered products or the modified extraction methodsfor specific extraction of Pb are required.

4. Conclusions

Two reaction pathways (extraction and sedimentation) forrecovering metallic Pb from CRT glass by thermal reductionmethod were provided. In extraction pathway, 90.3% of Pb wasextracted from PbOx polyhedron in glass matrix by the additionof C. By directly putting the materials into constant-temperaturezone, CRT glass quickly melted into viscous semi-liquid, whichinhibited the oxidation of C and facilitated the extraction of metal-lic Pb. In sedimentation pathway, 87.3% of Pb was recovered andsettled down in the bottom with the addition of Na2CO3. However,the vitrification of lead back to the glass matrix under the longertreatment time and higher temperature was observed. Thus, theoptimal parameters for recovering metallic Pb from CRT glass weresuggested. Compared with current methods, this study introducesa lead extraction technique by directly putting the materials intoconstant-temperature zone without slow temperature ramping(continuous processing), which is highly beneficial for industrialoperations.

Acknowledgements

This research was supported by National Natural ScienceFoundation of China (Project 41503087, 21637001) and Hong KongResearch Grants Council (Projects 17206714, 17212015, C7044-14G).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at https://doi.org/10.1016/j.wasman.2018.04.010.

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