Combined Fe2O3 and CaCO3 Additives To Enhance the

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Combined Fe2O3 and CaCO3 Additives to Enhance the Immobilization of Pb in Cathode Ray Tube Funnel Glass Ying Zhou, Chang-Zhong Liao, and Kaimin Shih ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03979 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Sustainable Chemistry & Engineering

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Combined Fe2O3 and CaCO3 Additives to Enhance the

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Immobilization of Pb in Cathode Ray Tube Funnel Glass

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Ying Zhou a, Changzhong Liao a,b, Kaimin Shih a,*

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a

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HKSAR, China

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b

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Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong,

Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management,

Guangdong Institute of Eco-Environmental Science & Technology, Guangzhou, China

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Ying Zhou, E-mail: [email protected] Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, HKSAR, China Changzhong Liao, E-mail: [email protected] Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, HKSAR, China Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management, Guangdong Institute of Eco-Environmental Science & Technology, Guangzhou, China Dr. Kaimin Shih, E-mail: [email protected] Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, HKSAR, China *Corresponding author: Dr. Kaimin Shih, E-mail: [email protected]; Tel: +852 28591973; Fax: +852 25595337.

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ACS Paragon Plus Environment

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Abstract

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Cathode ray tube (CRT) funnel glass has posed an increasing threat to the environment due to its

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rapid replacement by new technology in recent years. In this study, a well-control thermal scheme

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was applied for synthesizing a specific crystalline phase, PbFe12O19, for Pb immobilization when

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reusing CRT funnel glass as raw materials for the ceramics industry. The Fourier Transform

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Infrared Spectroscopy results show that introduction of CaCO3 facilitated the breakage of

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strongly-connected bonds between –O-Si-O– and –Pb-O–, which were firmly linked in the glass

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network. The X-ray diffraction results demonstrate that 30 wt.% CaCO3 loading effectively

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facilitated the transformation of Pb in CRT funnel glass to the stable-phase PbFe12O19. A higher

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sintering temperature increased Pb transformation efficiency while a longer dwelling time only

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showed a slight increase in PbFe12O19 formation. The prolonged toxic characteristic leaching

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procedure results show a substantial improvement in the acid resistance (approximately 2 mg/L) of

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the thermally-treated product with 30 wt.% CaCO3 loading and sintering under 1000°C for 5 h

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compared to the original CRT funnel glass (500 mg/L). The results of this study demonstrate that

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incorporation of CaCO3 and Fe2O3 into CRT funnel glass can effectively promote Pb

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immobilization and provide a new strategy for stabilizing waste CRT funnel glass.

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Keywords: CRT funnel glass; lead immobilization; magnetoplumbite; leaching behavior;

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Rietveld Quantitative XRD.

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2


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Introduction

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With the rapid development of new technologies, the amount of electrical and electronic

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equipment (EEE) has increased substantially in recent years. The types of EEE available on the

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market are also rapidly changing and are constantly being replaced by new products.1 Because the

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waste electrical and electronic equipment (WEEE) often contains large quantities and high levels

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of recalcitrant toxic chemicals (e.g., heavy metals, persistent organic pollutants), it poses a

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tremendous threat to the environment.2 A typical type of WEEE, cathode ray tubes (CRTs), has

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been widely used in televisions, personal computers, and monitors in recent decades.3 Although

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they are relatively recent display technologies, their large volume and high power consumption

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have led to their gradual replacement by new display technologies, such as liquid crystal displays

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and plasma display panels.4 The new technologies have come to occupy the market because of

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their excellent display performance and greater environmental friendliness than traditional CRT

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screens. According to the United Nation University (UNU), the global quantity of CRT screen

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waste generation in 2016 was around 6.3 million sets.5 With the obvious disadvantages of CRTs,

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production ceased in 2007 in developed countries. However, in developing countries, the decline

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of CRTs has occurred more slowly and has resulted in accumulated waste CRTs. Thus, there is an

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urgent need to address this problem.6 In addition, a survey found that the number of CRT devices

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used in U.S. households was underestimated and stressed the need for effective solutions to handle

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waste CRT glass.7 However, direct stacking or dumping of waste CRT glass in domestic refuse

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landfills was forbidden by authorities in 2000.8 Without proper treatment, the waste glass will be

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affected by the acid environment leading to the increase of soil, water, and atmospheric pollution

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to threaten human health.9 Thus, proper treatment techniques must be explored to determine

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whether CRT glass can be of any beneficial uses before landfilling.10,11

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CRT glass comprises more than half the weight of a television set or computer monitor,

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including the panel glass, funnel glass, and neck glass. The funnel glass poses the greatest threat to

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the environment, because it contains a relatively high Pb content (20-30 wt.%), even higher than

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the natural Pb minerals. CRT funnel glass is used as a protective barrier against X-ray and other

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radioactive rays. The Pb atoms in the funnel glass are included in the glass network, which mainly 3



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consists of –O–Si–O– and –O–Si–O–Pb–O– bonds.12,13 Therefore, removal or extraction of the

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lead cannot be easily realized under normal temperatures and pressures with traditional methods.14

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Closed-loop recycling and open-loop recycling are the two main methods used in the CRT

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recycling industry.15,16 In closed-loop recycling, discarded CRT glass is used to produce new CRT

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screens, whereas in open-loop recycling, the Pb is removed from the waste screens and the glass is

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then recycled for building and metallurgical purposes.17,18 However, because the production of

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CRTs has significantly decreased, closed-loop recycling is no longer feasible. Therefore, an

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increasing number of open-loop methods have been developed in recent decades, including

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thermal reduction, mechanical activation, and hydrometallurgical processes.19

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In recent years, solidification/stabilization processes have been used to immobilize hazardous

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metals and have achieved good performance in producing stabilized products.20,21 Studies have

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shown that the incorporation of Fe2O3 into PbO or Pb-containing sludge can enhance the

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formation of PbFe12O19, and this phase performed excellently in leaching tests under an acid

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environment.22 Dolomite, slag, and municipal solid waste have been used as precursors for the

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generation of glass ceramics from end-of-life CRT glass.23 However, CRT funnel glass is a

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high-energy bond lead glass. Thus, large amounts of energy are needed to break the

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three-dimensional vitreous structure of the glass and release the Pb atoms from the glass

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network.24,25 Recent studies on the Pb immobilization of waste CRT funnel glass focused on employing

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the mechanochemical methods for encapsulating the hazardous Pb into the glass matrix to realize Pb

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stabilization.26 However, the immobilization effect was in general insufficient, due to the lack of strong

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chemical bonding mechanisms for metals. Few studies have examined the devitrification process of

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CRT glass, and the phase that contributes to the crystallization behavior is still unknown.

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Regardless the purposes of beneficially using the CRT glass containing products or achieving a

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more sustainable landfilling practice, the well-stabilized Pb in product materials is the key for

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successful applications. Because the silicon-oxygen bond plays an important role in linking the Pb

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atoms and oxygen atoms, we attempted to break the bond structure in the glass first and to then

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release the lead oxides from the system to further achieve Pb immobilization. In this study, CaCO3

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was used to break the bonds, and Fe2O3 was used to stabilize the lead oxides in the sintering

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process. We investigated the phase change processes under various thermal conditions and the 4


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effects of the CaCO3 loading amount, sintering temperature, and processing time on Pb

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immobilization in this treatment strategy. The leaching procedures were conducted in an acid

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environment to quantify the stabilization efficiency. Our method demonstrated promising results in

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stabilizing the Pb in CRT funnel glass through the synergetic effects of CaCO3 and Fe2O3.

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Experimental Section

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Materials

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CRT funnel glass from color monitors was collected from an electronic recycling center in Hong

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Kong. The glass was crushed into small pieces using a crusher, ball-milled into powder (around

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20µm, determined by Laser Particle Size Analyzer (Mastersizer 3000, Malvern Instruments))

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using wet scrubbing technology, and dried at 120°C for 24 h before conducting the experiments.

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The chemical composition of the CRT funnel glass was determined using X-ray fluorescence

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(XRF, JSX-3201z, JEOL), and the results are shown in Table 1. The reaction chemicals used in

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this study included Fe2O3 (purity > 99.0 wt.%) and CaCO3 (purity > 99.0 wt.%).

124 125

Synthesis of Samples

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Different amounts of CRT funnel glass, Fe2O3, and CaCO3 were homogenized by mortar grinding

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and pressed into pellets (about 20 mm in diameter) using a press machine to ensure consistent

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compaction. The weight fractions of the samples are listed in Table 2. The pellets were heated at a

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heating rate of 10°C/min to target temperatures ranging from 700°C to 1100°C in a

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high-temperature furnace (Nabertherm Inc.) and then cooled to room temperature at a rate of

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10°C/min. The dwelling times at the target temperatures ranged from 1 to 10 h. To study the effect

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of CaCO3 loading on the Pb stabilization, the amount of CaCO3 used varied from 10 to 60 wt.%.

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The weight loss after the thermal treatment was also recorded.

134 135

Material Characterizations

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After thermal treatment, the pellets were cooled to room temperature and ground into powder for

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X-ray diffraction (XRD) analysis. The mineral phase transformation during the sintering process 5



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was identified using XRD data obtained from a Bruker D8 Advance X-ray powder diffractometer

139

equipped with Cu Kα radiation and operated at 40 kV and 40 mA with a LynxEye detector. The 2θ

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scan range covered from 15° to 80°, with a step size of 0.02° and a scan speed of 0.5 s per step. A

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Rietveld refinement method using 15 wt.% CaF2 as the internal standard was used to quantify the

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contents of amorphous phase and crystalline phase in the sintered products via the Topas V4.2

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program (Bruker AXS). The sintered samples were also subjected to scanning electronic

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microscopy (SEM) characterization coupled with energy dispersive X-ray spectroscopy (EDX)

145

analysis. The samples used for SEM-EDX were ground by diamond-based pastes (with decreasing

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grain sizes) and observed with a Hitachi S-3400 SEM under variable-pressure mode at 20 kV. The

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infrared spectra of the thermally treated samples were measured in the range of 3000 to 450 cm-1

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by the standard KBr pellet method using a Fourier transform infrared (FTIR) spectrometer

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(Spectrum 100 Optica FT-IR Spectrometer), the pellet was designed by blending the sample and

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KBr with a ratio of 1:100.

151 152

Leaching Performance

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To examine the stability of the sintering products in an acid environment, the leachability of the

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final products was determined using a modified U.S. EPA SW-846 Method 1311 toxic

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characteristic leaching procedure (TCLP). In this leaching test, an extraction fluid

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(glacial acetic acid) with a pH of 4.93 ± 0.05 was selected. The liquid-to-solid ratio of each

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leaching vial was 20 mL/g. The leaching vials were filled with 0.5 g powder sample and 10 ml of

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extraction fluid and rotated end-over-end at 60 rpm from 3 h to 21 d. At the end of each agitation

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period, the leachates were centrifuged and filtered with 0.45-µm syringe filters. The ion

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concentrations (Pb and Fe ions) of the leachates were measured by inductively coupled plasma

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optical emission spectrometry (Optima 8000, Perkin Elmer). In addition, the obtained ion

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concentrations were normalized by the percentages of CRT glass added to the mixture. All

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leaching experiments were conducted in triplicate.

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Results and Discussion

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Incorporation Mechanisms of Fe2O3 and CaCO3 into the Glass System 6


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Different proportions of CRT funnel glass, Fe2O3, and CaCO3 were homogenized and heated

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under 1000°C for 5 h (Table 2). For the untreated CRT funnel glass, the XRD pattern had no

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diffraction peaks, indicating that it was a highly amorphous material. The XRD pattern of the

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sintered CRT with Fe2O3 (GF) only showed hematite (Fe2O3; ICDD PDF 89-2810) as the

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crystalline phase in the sample (Figure 1b). This result indicates that Fe2O3 is stable under these

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thermal conditions and that no interaction between the PbO and Fe2O3 was triggered in the tested

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system. In addition, the use of a higher stoichiometric ratio, higher sintering temperature, and

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nanosized Fe2O3 did not initiate any interaction (Figure S1, Supporting Information). This finding

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can be explained by the robust PbO4 structure of lead glass, due to its ionic field strength and

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coordinated state.27 In a tetra-coordinated state, the Pb in the CRT funnel glass is more difficult to

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crystalize and cannot react with the Fe2O3.

178 179

For the CRT funnel glass combined with CaCO3, reactions between the glass and CaCO3 occurred

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at 1000°C. As shown in Figure 1c, the main crystalline phases of the final products were

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wollastonite (CaSiO3; ICDD PDF 84-0654), tridymite (SiO2; ICDD PDF 75-0638), and litharge

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(PbO; ICDD PDF 05-0561). These results demonstrate that the lead glass in the CRT transformed

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into new crystalline phases. The mechanism of the CaCO3 incorporation was also investigated

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with FTIR spectroscopy. The results in Figure 2 show that the most intensive group of bands is

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located at approximately 1000 cm-1, which corresponds to the Si-O stretching vibration. The 470

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cm-1 band can be attributed to the bending vibration of O-Si-O and can be clearly seen in the CRT

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and CRT-Fe2O3 samples.28,29 Similar band locations in the spectra of the CRT and CRT-Fe2O3

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samples indicate that the Fe2O3 had no effect in breaking the glass network of the CRT funnel

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glass. In contrast, the FTIR spectra of the glass incorporated with CaCO3 show two obvious bands

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broken into small peaks for the CRT-CaCO3 and CRT-Fe2O3-CaCO3 samples. This indicates that

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the introduction of CaCO3 led to the breaking of the Si-O-Si bonds during thermal treatment. In

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addition, because the band at around 1440 cm-1 can be attributed to the stretching vibration of C-O,

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the FTIR results show that some residual carbonates remained in the CRT-CaCO3 and

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CRT-Fe2O3-CaCO3 samples.

195 196

The Pb atoms in CRT funnel glass are usually firmly fixed by the glass network. As a network 7



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modifier of the funnel glass, Pb plays an important role in the lead silicate network, with silicon

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and lead surrounded by oxygen in tetrahedral coordination. The introduction of CaCO3 served to

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break or weaken the bonds between the lead-oxygen clusters and silicon-oxygen clusters in the

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lead glass. PbOn clusters and SiO2 were thus released from the glass structure. At temperatures

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higher than 800°C, the SiO2 generated by the bond breaking process immediately reacted with the

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CaCO3.30 The main reactions between the CRT funnel glass and CaCO3 can be illustrated as

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follows:

204 205 206 207 208 209

2+ Ca ∆ − − − − − − + → − − − − − − +

(1)

O ∆

− − − − − − + → + +

(2)

O

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For the samples with three initial compounds, magnetoplumbite-PbFe12O19 was observed in the

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XRD pattern in the sample of CRT-Fe2O3-CaCO3 due to the potential reaction of:

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∆

+ →

(3)

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In the above mentioned system, the main crystalline phases are Fe2O3, PbFe12O19, Ca2SiO4, and

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SiO2. With the introduction of CaCO3, the structure of the CRT funnel glass first broke to produce

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silicon oxide, lead oxides (PbOn clusters), and some other small molecules. Thus, when heated to

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1000°C, the lead glass first melted, then the bonds linking the –Si-O– and –O-Pb-O– broke, and

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the lead oxides were no longer confined by the glass network. Free PbOn clusters were thus able to

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react with the Fe2O3 to form the stable phase of PbFe12O19. As shown in Figure 1d, the formation

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of PbFe12O19 led to the reduction of Fe2O3 due to its incorporation into the PbFe12O19 structure.

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Effect of CaCO3 Loading amount on Pb immobilization

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To study and further optimize the stabilization effect of combining CRT funnel glass with Fe2O3

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and CaCO3, the effects of different operational parameters (CaCO3 loading amount, sintering

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temperature, and dwelling time) on the Pb immobilization were investigated. According to the 8


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stoichiometric calculations and XRF results, the ratio of CRT funnel glass to Fe2O3 was fixed to

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1:1 for all samples in the sintering process. The CaCO3 loading amount ranged from 10 to 60 wt.%,

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the sintering temperature ranged from 700°C to 1100°C, and the dwelling time was from 1 h to 10

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h. The stable phase PbFe12O19 was successfully quantified using the Rietveld refinement with

229

CaF2 as the internal standard, and the refinement results showed that the calculated profile was

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well fitted to the experimental data, which can be seen in Fig. S2 (Supporting Information) and the

231

relevant refinement results were shown in Table S1 (Supporting Information).

232 233

Figure 3 shows the XRD patterns from the CRT-Fe2O3-CaCO3 mixtures with different CaCO3

234

loading amounts sintered at 1000°C for 5 h. When 10 wt.% CaCO3 was added, only Fe2O3 and

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CaSiO3 were observed in the XRD results, which means that the bonds between the

236

silicon-oxygen and lead-oxygen in the funnel glass began to break down with the incorporation of

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the CaCO3. However, under this condition, the crystalline PbFe12O19 content was still very limited,

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potentially due to the insufficient CaCO3 and the incomplete reaction. When the CaCO3 was

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increased to 20 wt.%, characteristic peaks of magnetoplumbite (PbFe12O19; ICDD PDF 84-2160)

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were first detected. When the CaCO3 was increased to 30 wt.%, the dominant phases were

241

hematite, magnetoplumbite, and rankinite (CaSi3O7; ICDD PDF 76-0623). Further increases of

242

CaCO3 loading continuously promoted the formation of PbFe12O19. The increase of PbFe12O19 can

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thus be attributed to the phase transformation initiated by CaCO3. However, when the amount of

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CaCO3 was increased to 50 wt.%, only CaFe2O4 and Ca2SiO4 were observed in the XRD pattern

245

and no Pb-containing crystalline phase was found. Increasing the CaCO3 to 60 wt.% only

246

enhanced the formation of Ca2Fe2O5 and Ca2SiO4, without resulting in any Pb-containing

247

crystalline phase in the thermal treatment process. The reaction between the lead oxides and Fe2O3

248

was limited at a high CaCO3 content, potentially due to the lower Pb content or the competition

249

between calcium oxide and lead oxide. The PbFe12O19 phase only appeared in the samples with 30

250

wt.% and 40 wt.% CaCO3 (Figure 3).

251 252

Effect of Sintering Temperature on Pb immobilization

253

The sample with 30 wt.% CaCO3 was used to further study the effect of the temperature on the Pb 9



254

stabilization, because our previous work suggested that the immobilization was enhanced with the

255

co-existence of Fe2O3 and PbFe12O19 in the system (Figure S3, Supporting Information). Before

256

investigating the transformation process of Pb in CRT funnel glass, the mass balance was first

257

examined to evaluate the PbO volatilization due to the sintering process. The weight of the sample

258

with 30 wt.% CaCO3 was reduced by 7.6 wt.% after 5 h sintering at 700°C and by nearly 14 wt.%

259

at 1100°C (Figure S4, Supporting Information). Considering the weight loss due to the conversion

260

of CaCO3 into CaO, the total weight lost for the sample of 30 wt.% CaCO3 should be 13.2 wt.%, if

261

no other mass is lost from the system. Therefore, this confirmed that there was no significant PbO

262

volatilization, if any, after 5 h of sintering at 800°C to 1100°C. To quantify the phase composition

263

of the sintered samples containing CRT funnel glass, Rietveld refinement was conducted with an

264

internal standard spiked in the sample. As shown in Figure 4, substantial CaCO3 was decomposed

265

at 700°C and magnetoplumbite was initiated at 800°C. Although the amount of PbFe12O19

266

continued to increase as the sintering temperature increased, the sample with 30 wt.% CaCO3

267

began to melt at 1100°C. Although significant amount of Pb in CRT funnel glass was transformed

268

into the crystalline PbFe12O19, approximately 35% amorphous phase was still existed in the final

269

product which sintered at 1000°C for 5h. The results from Rietveld refinement indicated that the

270

thermal-treated product is the combination of vitrification and crystallization process.

271 272

Effect of Dwelling Time on Pb immobilization

273

The formation of magnetoplumbite when the sample was heated to 1000°C for 1 h to 10 h was

274

further observed with quantitative XRD in the sample with 30 wt.% CaCO3. Figure 5 summarizes

275

the quantitative results for magnetoplumbite formation as a function of the dwelling time and

276

Table S2 (Supporting Information) shows the quality of refinement analyses. At 1000°C, the

277

PbFe12O19 content was increased by prolonging the dwelling time, although this method is less

278

effective than increasing the CaCO3 loading and the sintering temperature. The level of residual

279

iron oxide decreased with the longer dwelling time. The increased efficiency of the Pb

280

transformation in the CRT funnel glass with a longer heating time indicates that more energy is

281

needed to break the coordination bonds and overcome the diffusion barrier in this system.

282

Compared to the amorphous content in the samples with different dwelling times, a considerable 10


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283

amount of glass remained in the samples and was not greatly affected by the dwelling time.

284 285

To observe the Pb distribution in the thermally treated products, the SEM technique coupled with

286

EDX capability was used to characterize the sample microstructures. The heat-treated products

287

were polished using decreasing grain sizes of diamond paste before conducting SEM

288

characterization. Figure 6 shows the backscattered electron image of the sample with 30 wt.%

289

CaCO3 and sintered at 1000°C for 5 h. Five distinct regions were randomly distributed reflecting

290

the different dominant phases in the sample.

291 292

The results of the EDX analyses of different regions are shown in Table S3 (Supporting

293

Information), in which the bright white-color regions (Points 4 and 5) indicate Pb-rich regions, the

294

gray-color parts are iron oxides (Point 2), the dark-color areas are the Ca-Fe-Si oxide compound(s)

295

(Point 3), and the even darker matrixes are calcium silicates (Point 1). The Fe2O3 EDX results

296

shown in the gray areas reflect the incomplete thermal reaction process. In some of the bright

297

white-color regions, strong peaks of Pb and Fe can be observed, although small quantities of Ca

298

and Si may also be observed, potentially due to the influence of the residual glass.

299 300

Leaching Performance of the Products

301

To determine the efficiency of the Pb immobilization, raw CRT funnel glass and samples with 30

302

wt.% and 40 wt.% CaCO3 were chosen to conduct the prolonged leaching experiments to compare

303

the leaching performance of the products after thermal treatment. The 30 wt.% and 40 wt.%

304

CaCO3 pellet samples were first heated to 1000°C for 5 h, ground to power, re-pelletized, and

305

sintered at 1000°C for 5 h again to ensure a complete and homogeneous reaction. Then, the

306

powder samples were dried at 105°C for 24h, and similar particle sizes were observed among the

307

samples (Table S4, Fig. S5, Supporting Information). The initial pH of the leachate was adjusted

308

to 4.93 ± 0.05, and the sampling process was performed from 3 h to 21 d. After the 21-day

309

leaching period, the concentrations of the leached metals (Pb and Fe) in the leachates were

310

measured and normalized by the weight fractions of the added CRT funnel glass. Figure 7 shows

311

the concentrations of Pb ions in the leachates of the three products. In the first 3 h of the leaching

312

reaction, the Pb leached from the untreated CRT funnel glass to produce more than 400 mg/L in 11



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313

the leachate. In contrast, the Pb concentrations in the leachates of the samples with 30 wt.% and

314

40 wt.% CaCO3 were only 0.43 mg/L and 0.31 mg/L, respectively. After 2 d of leaching, the Pb

315

concentration increased to about 450 mg/L in the leachate of the CRT funnel glass, whereas the 40

316

wt.% CaCO3 sample showed around 10 mg/L Pb concentration and Pb concentration in the

317

leachate of the 30 wt.% CaCO3 sample remained very low.

318 319

At the end of the leaching period, the Pb concentration in the leachate of the untreated CRT glass

320

was nearly 500 mg/L. The Pb concentration in the leachate of the 40 wt.% CaCO3 sample

321

remained stable at 40 to 50 mg/L after several days of leaching reaction. The 30 wt.% CaCO3

322

sample demonstrated the best resistance to the acid leaching and remained approximately 2 mg/L

323

throughout the 21-day leaching period, which is well below 5 mg/L- the Pb limit from the criteria

324

of US EPA 40 CFR 261.24. Thus, this sample was two orders of magnitude less leachable than the

325

untreated CRT funnel glass. This result indicates that the formation of the crystalline PbFe12O19

326

phase enhanced Pb immobilization during the leaching process. As a more chemically durable

327

phase, PbFe12O19 is generated through incorporating Pb in the CRT glass network into the

328

crystalline structure. The PbFe12O19 crystalline structure has stronger resistance to acidic attack

329

due to the more energetic bonds, and thus less metal was leached into the leaching fluid, which is

330

consistent with the observations in the literature. The cation-proton exchange process can be used

331

to describe the interaction between the PbFe12O19 crystalline structure and the acidic solution as

332

follows:

333

+ 38 → + 12 + 19

(4)

334

The coexistence of Fe2O3 in the system may also have hindered the Pb leaching from the glass

335

structure. Comparison of the leaching performance of the 30 wt.% CaCO3 and 40 wt.% CaCO3

336

samples showed that there was a continuous increase in the Pb concentration of the leachates of

337

the 40 wt.% CaCO3 sample, which did not have crystalline Fe2O3 coexisting in the system. In

338

contrast, the 30 wt.% CaCO3 sample, which had Fe2O3 coexisting in the system, showed very

339

limited Pb leachability. The very low Fe concentration in the leachate of the sample with 30 wt.%

340

CaCO3 loading indicate that the Fe concentration in the leachate was likely controlled by the

341

re-precipitation of amorphous Fe(OH)3 on the surface which acted as a second barrier for Pb

342

leaching out.31,32 12


Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


343 344

Conclusion

345

The immobilization of the Pb in waste CRT glass was substantially enhanced by the introduction

346

of CaCO3 and Fe2O3 along with thermal treatment. The vitreous network of the CRT funnel glass

347

was first broken by the interaction with CaCO3 and then further stabilized by the Fe2O3 in the

348

system. The CaCO3 loading and sintering temperature were the most influential operational

349

parameters. The XRD analysis shows that 30 wt.% CaCO3 was the optimal dosage to achieve

350

PbFe12O19 formation. The sintering temperature significantly affected the phase transformation

351

and distribution of the products containing CRT funnel glass. The formation of PbFe12O19 was

352

initialized at 800°C and increased at higher temperatures until the sample melted at around

353

1100°C. The results of the leaching experiments reflected the excellent acid resistance of the Pb

354

immobilized product, particularly for the 30 wt.% CaCO3 sample due to the robust PbFe12O19

355

phase and the coexistence of crystalline Fe2O3 in the product. Therefore, the results of this study

356

provide a new strategy for reliable immobilization of the Pb in CRT glass by incorporating CaCO3

357

and Fe2O3 in the thermal treatment. It should be further noted that the compositions of CRT funnel

358

glass may vary, and the amounts of glass formers (or glass modifiers) and breaking agents need to

359

be carefully controlled in the reaction systems. The reaction pathways of multi-phase systems will

360

require further investigations to optimize the processing parameters.

361 362

Acknowledgements

363

This study was funded by the Research Grants Council of Hong Kong (Projects 17212015,

364

C7044-14G, and T21-771/16R), and GDAS’ Special Project of Science and Technology

365

Development (2017GDASCX-0834).

366

13



Page 14 of 33

367

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Captions of Tables and Figures:

455 456

Table 1. Chemical composition of the CRT funnel glass.

457

Table 2. Weight fractions of the CRT-Fe2O3-CaCO3 system used in the study.

458

Figure 1. XRD patterns of the (a) untreated CRT glass, (b) 1000°C 5-h treated sample with 50

459

wt.% CRT funnel glass-50 wt.% Fe2O3, (c) 1000°C 5-h treated sample with 50 wt.% CRT funnel

460

glass-50 wt.% CaCO3 and (d) 1000°C 5-h treated sample with 33 wt.% CRT funnel glass-33 wt.%

461

Fe2O3- 34wt.% CaCO3.

462

Figure 2. FTIR spectra of the (a) untreated CRT glass, (b) 1000°C 5-h treated sample with 50

463

wt.% CRT funnel glass-50 wt.% Fe2O3, (c) 1000°C 5-h treated sample with 50 wt.% CRT funnel

464

glass-50 wt.% CaCO3 and (d) 1000°C 5-h treated sample with 33 wt.% CRT funnel glass-33 wt.%

465

Fe2O3- 34wt.% CaCO3.

466

Figure 3. XRD patterns of samples with different CaCO3 amounts sintered at 1000°C for 5 h.

467

Samples GFC10-60 are of 10-60 wt.% CaCO3 loading amounts.

468

Figure 4. (a) XRD patterns generated from the sample 35 wt.% CRT funnel glass-35 wt.%

469

Fe2O3-30 wt.% CaCO3 sintered at 700°C to 1100°C for 5 h, together with (b) their quantitative

470

phase distributions, calcium silicates include CaSiO3, Ca2SiO4 and Ca3Si2O7.

471

Figure 5. (a) XRD patterns generated from the sample 35 wt.% CRT funnel glass-35 wt.%

472

Fe2O3-30 wt.% CaCO3 sintered at 1000°C for 1 to 10 h, together with (b) their quantitative phase

473

distributions, calcium silicates include Ca2SiO4 and Ca3Si2O7.

474

Figure 6. Backscattered electron image of the 35 wt.% CRT funnel glass-35 wt.% Fe2O3-30

475

wt.% CaCO3 sample sintered at 1000°C for 5 h.

476

Figure 7. Concentrations of Pb leached from the CRT funnel glass and products with 35 wt.%

477

CRT funnel glass-35 wt.% Fe2O3-30 wt.% CaCO3 and 30 wt.% CRT funnel glass-30 wt.%

478

Fe2O3-40 wt.% CaCO3 sintered at 1000°C for 10h (normalized by weight percentage).

479 480 481 482 483 17



Page 18 of 33

Table 1 Chemical composition of the CRT funnel glass.

484 Component

SiO2

PbO

K2O

Na2O

BaO

CaO

SrO

Al2O3

MgO

ZnO

Wt.%

57.0

17.7

7.77

5.90

2.75

2.55

2.41

2.23

1.48

0.21

485 486 487 488

Table 2 Weight fractions of the CRT-Fe2O3-CaCO3 system used in the study. Sample Name GF GC GFC GFC10 GFC20 GFC30 GFC40 GFC50 GFC60

489

Weight Fraction (wt.%) CRT funnel glass (G)

Fe2O3 (F)

CaCO3 (C)

50 50 33 45 40 35 30 25 20

50 —— 33 45 40 35 30 25 20

—— 50 34 10 20 30 40 50 60

Note: G=CRT Funnel Glass, F=Fe2O3 and C=CaCO3, numbers indicated CaCO3 loading amount.

490 491 492 493 494

18


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495 496

Figure 1. XRD patterns of the (a) untreated CRT glass, (b) 1000°C 5-h treated sample

497

with 50 wt.% CRT funnel glass-50 wt.% Fe2O3, (c) 1000°C 5-h treated sample with

498

50 wt.% CRT funnel glass-50 wt.% CaCO3 and (d) 1000°C 5-h treated sample with 33

499

wt.% CRT funnel glass-33 wt.% Fe2O3-34wt.% CaCO3.

500 501 502 503 504 505 506 507 508 509 510 511 512 19



513 514

Figure 2. FTIR spectra of the (a) untreated CRT glass, (b) 1000°C 5-h treated sample

515

with 50 wt.% CRT funnel glass-50 wt.% Fe2O3, (c) 1000°C 5-h treated sample with

516

50 wt.% CRT funnel glass-50 wt.% CaCO3 and (d) 1000°C 5-h treated sample with 33

517

wt.% CRT funnel glass-33 wt.% Fe2O3-34wt.% CaCO3.

518

20


Page 20 of 33

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


519 520

Figure 3. XRD patterns of samples with different CaCO3 amounts sintered at 1000°C

521

for 5 h. Samples GFC10-60 are of 10-60 wt.% CaCO3 loading amounts.

522 523 524 525 526

21



527 528

Figure 4. (a) XRD patterns generated from the sample 35 wt.% CRT funnel glass-35

529

wt.% Fe2O3-30 wt.% CaCO3 sintered at 700°C to 1100°C for 5 h, together with (b)

530

their quantitative phase distributions, calcium silicates include CaSiO3, Ca2SiO4 and

531

Ca3Si2O7.

532 533

22


Page 22 of 33

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


534 535

Figure 5. (a) XRD patterns generated from the sample 35 wt.% CRT funnel glass-35

536

wt.% Fe2O3-30 wt.% CaCO3 sintered at 1000°C for 1 to 10 h, together with (b) their

537

quantitative phase distributions, calcium silicates include Ca2SiO4 and Ca3Si2O7.

538

23



539 540

Figure 6. Backscattered electron image of the 35 wt.% CRT funnel glass-35 wt.%

541

Fe2O3-30 wt.% CaCO3 sample sintered at 1000°C for 5 h.

542 543 544 545

24


Page 24 of 33

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


546 547

Figure 7. Concentrations of Pb leached from the CRT funnel glass and products with

548

35 wt.% CRT funnel glass-35 wt.% Fe2O3-30 wt.% CaCO3 and 30 wt.% CRT funnel

549

glass-30 wt.% Fe2O3-40 wt.% CaCO3 sintered at 1000°C for 10h (normalized by

550

weight percentage).

551 552 553 554

25



Table of Contents (TOC)

555 556

557 558 559 560

Synopsis

561 562

A cost-effective and reliable Pb immobilization strategy was developed through transforming CRT

563

waste glass into environmentally-friendly materials.

26


Page 26 of 33

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Figure 1. XRD patterns of the (a) untreated CRT glass, (b) 1000°C 5-h treated sample with 50 wt.% CRT funnel glass-50 wt.% Fe2O3, (c) 1000°C 5-h treated sample with 50 wt.% CRT funnel glass-50 wt.% CaCO3 and (d) 1000°C 5-h treated sample with 33 wt.% CRT funnel glass-33 wt.% Fe2O3- 34wt.% CaCO3. 287x201mm (300 x 300 DPI)



Figure 2. FTIR spectra of the (a) untreated CRT glass, (b) 1000°C 5-h treated sample with 50 wt.% CRT funnel glass-50 wt.% Fe2O3, (c) 1000°C 5-h treated sample with 50 wt.% CRT funnel glass-50 wt.% CaCO3 and (d) 1000°C 5-h treated sample with 33 wt.% CRT funnel glass-33 wt.% Fe2O3- 34wt.% CaCO3. 287x201mm (300 x 300 DPI)


Page 28 of 33

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. XRD patterns of samples with different CaCO3 amounts sintered at 1000°C for 5 h. Samples GFC10-60 are of 10-60 wt.% CaCO3 loading amounts. 287x201mm (300 x 300 DPI)



Figure 4. (a) XRD patterns generated from the sample 35 wt.% CRT funnel glass-35 wt.% Fe2O3-30 wt.% CaCO3 sintered at 700°C to 1100°C for 5 h, together with (b) their quantitative phase distributions, calcium silicates include CaSiO3, Ca2SiO4 and Ca3Si2O7. 238x350mm (300 x 300 DPI)


Page 30 of 33

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Figure 5. (a) XRD patterns generated from the sample 35 wt.% CRT funnel glass-35 wt.% Fe2O3-30 wt.% CaCO3 sintered at 1000°C for 1 to 10 h, together with (b) their quantitative phase distributions, calcium silicates include Ca2SiO4 and Ca3Si2O7. 238x350mm (300 x 300 DPI)



Figure 6. Backscattered electron image of the 35 wt.% CRT funnel glass-35 wt.% Fe2O3-30 wt.% CaCO3 sample sintered at 1000°C for 5 h. 108x75mm (300 x 300 DPI)


Page 32 of 33

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Figure 7. Concentrations of Pb leached from the CRT funnel glass and products with 35 wt.% CRT funnel glass-35 wt.% Fe2O3-30 wt.% CaCO3 and 30 wt.% CRT funnel glass-30 wt.% Fe2O3-40 wt.% CaCO3 sintered at 1000°C for 10h (normalized by weight percentage). 287x201mm (300 x 300 DPI)


Combined Fe2O3 and CaCO3 Additives To Enhance the

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