The Stabilization of Waste Funnel Glass of CRT by SiO₂ Film Coating Technique

Abstract

The funnel glass of the CRT monitor contains about 22–28% of lead oxide, of which lead is a highly toxic species and hazardous to the environment. This study proposes a process to form a protective layer of SiO₂ film coating on the funnel glass to reduce the hazardous effect of lead leaching to the environment. The film coating benefits from the advantages of the sol–gel method. There are two key procedures of the stabilization technique, including the alkaline treatment and the formation of SiO₂ coating from TEOS. The results show that the funnel glass powder treated with 10 M NaOH can produce a mushy layer on the surface. The mushy layer, which comprises OH⁻ and water, can promote the formation of the SiO₂ film layer on the surface of funnel glass powder. The conditions of the SiO₂ film coating proposed in this study are: alkaline treatment by 10 M NaOH, the addition ratio of TEOS and funnel glass powder 2: 1, reaction temperature 40 °C, and reaction time 1.3 h. The EDS and ESCA results show that the Pb peak intensity on the surface of funnel glass decreases with the film coating. In the TCLP test, the leaching amount for Pb of the SiO₂ film coated funnel glass powders is 0.7 mg/L, which is far lower than the standard in Taiwan EPA. Based on the experimental results, the formation mechanism of the SiO₂ film layer on the surface of waste funnel glass powder is proposed. This study demonstrates that the SiO₂ film coating is a potentially effective method to solve the problem of the waste funnel glass.

1. Introduction

The rapid development of display technology has generated a large amount of discarded cathode ray tube (CRT) monitors. Previous studies indicate that CRT monitors around the world will be phased out from the market in 2022 [1]. As a result, there will be about 3.2 million tons of waste CRT glass needing to be treated annually in the coming years [1,2].

The traditional CRT monitor consists of three types of glass: panel glass, funnel glass, and neck glass. Panel glass contains about 8–10% barium oxide (BaO) and strontium oxide (SrO). The other two types of glass contain high levels of lead oxide (PbO). The Pb content of funnel glass in a CRT monitor is about 22–28% (Figure 1) [3]. Pb has major toxicological effects for biological systems and environments. Funnel glass is considered to be a hazardous waste [4,5,6,7]. It is estimated that the amount of waste funnel glass generated in Taiwan is about 5000–6000 tons a year. Technology for the treatment of hazardous waste CRT funnel glass, therefore, is needed to solve the environmental problem immediately.

Previous studies indicate that silica can be used to coat the glass surface [8,9,10,11]. In this study, a technology for the treatment of the waste funnel glass by SiO₂ coating is proposed. SiO₂ is selected as the coating material as the major component of funnel glass is SiO₂. Therefore, a better physical and chemical compatibility between coating material and funnel glass can be ensured. The preparation of silica material has attracted considerable attention due to its wild applications [12]. Many synthesis and application technologies have been studied for SiO₂. Among the synthesis techniques of SiO₂, the sol–gel process is the most popular method to prepare SiO₂ [13,14,15]. Tetraethoxysilane and tetramethoxysilane can be used as silica sources [16]. In a typical sol–gel process, the silica is prepared by dispersion and hydrolysis of Si precursor in an ethanol solution to form silanol, which then undergoes condensation (polymerization) to produce silica [17]. The preparation of SiO₂ is affected by the amounts of TEOS, alkaline, water and alcohol, temperature, reaction time, etc. [18].

In this study, the SiO₂ coating on funnel glass was carried out by sol–gel process. Two schemes of SiO₂ stabilization coating were investigated and compared, namely SiO₂ particle coating and SiO₂ film coating. The factors of the coating process were evaluated. The feasibility of application of a SiO₂ coating was further demonstrated by TCLP analysis. A mechanism of SiO₂ coating was proposed based on the experimental results.

2. Materials and Methods

The waste funnel glass was provided by E&E recycling, Inc. (Taiwan). The waste funnel glass was crushed to particle size < 75 μm (200 mesh). All the funnel glass powder was washed with 0.1 M NaOH to remove the residue and impurity on the surface. After the pretreatment, the funnel glass powder was risen with deionized water.

In this study, a SiO₂ protection layer on the surface of the waste funnel glass powders was prepared based on the sol–gel technique. All chemicals were analytical grade, tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, 99%) as a precursor, ethanol (C₂H₅OH) (Sigma-Aldrich, 99.8%) as solvents, and ammonia solution (NH₄OH) (Sigma-Aldrich, 28–30%) as a catalyst.

The SiO₂ protection layer was covered on the powder surface by two different forms, namely particles and film coating (Figure 2).

For the SiO₂ particles coating process (Figure 2a), the SiO₂ particles were prepared first and deposited on the surface of the funnel glass powder. The conditions for the preparation of the SiO₂ particles followed the previous study [19,20,21]. For the preparation of SiO₂ particles, the TEOS (2.1 mL) and C₂H₅OH (31.7 mL) mixture was catalyzed with NH₄OH (9.1 mL). The reaction temperature was 40 °C and the reaction time was 1 h. The SiO₂ particles were added to the funnel glass powder (2 g) and mixed at 40 °C for 1.3 h. The funnel glass powder coated with SiO₂ particles (SiO₂-particle-funnel glass) was dried at 150 °C for 24 h.

For the SiO₂ film coating process (Figure 2b), the funnel glass powder (2 g) was first treated with 0.01–10 M NaOH. After alkaline treatment, the funnel glass powder was mixed with TEOS/ethanol solution. For a typical experiment, the composition of TEOS/ethanol solution was the same as the preparation of SiO₂ particles (i.e., TEOS 2.1 mL and C₂H₅OH 31.7 mL). The reaction temperature was 40 °C and the reaction time was 1.3 h. The funnel glass coated with a layer of SiO₂ film (SiO₂-film-funnel glass) was dried at 150 °C for 24 h.

The TCLP test procedure followed the standard method (NIEA R201.15C) by EPA, Taiwan. A sample of 100 g (particle size of 1 cm or less) was added to 20 mL of acetic acid (0.1 M) in a container. The container was rotated (30 rotations/min) at room temperature for 20 h. The Pb concentration dissolved in the TCLP test was analyzed by atomic absorption spectrometer (AA, Young Lin Instrument, AAS-8010).

The characteristics of the funnel glass powder with SiO₂ coating were analyzed by scanning electron microscope (SEM, Hitachi S-4800), energy dispersive X-ray spectrometer (EDS, Bruker, Quad FQ5060), Total Reflection X-ray Fluorescence Spectrometry (TXRF, Bruker, S2 PICOFOX), and electron spectroscopy for chemical analysis (ESCA, JEOL, JPS-9030).

3. Results and Discussion

3.1. The Characteristics of Waste Funnel Glass Powder

The morphology of the waste funnel glass powder is shown in Figure 3. The image shows that the powder particle is irregular with sharp edges and a rough surface. The waste funnel glass powder samples were scanned with TXRF. The results indicate that the main components are Si, O, Pb and Al. There are also some trace elements (K, Ca, Fe, Zn, Sr, Ba, etc.) in the composition of funnel glass.

According to EDS analysis, the main components include silicon (29.5 atom %), oxygen (59.4 atom %), lead (5.5 atom %), and aluminum (5.6 atom %). The Pb leachability was evaluated based on the toxicity characteristic leaching procedure (TCLP). TCLP is designed to simulate the effect of acidic leaching by the environment in the landfills. Due to the high amount of Pb content, the Pb leachability is about 4352.5 ppm, which is exceedingly higher than the standard of EPA Taiwan (5 mg/L). Therefore, the funnel glass of CRT is considered to be a hazardous waste. In order to reduce the leaching of Pb from funnel glass powder, two schemes of protection coating were proposed in this study, namely SiO₂ particle coating and SiO₂ film coating.

3.2. SiO₂ Particles Coating

The SiO₂ particles were first prepared by sol–gel synthesis. TEOS and ethanol solution was added with ammonia. The molecules of TEOS in the mixture were hydrolyzed to form silanol groups. The silanol groups were polymerized to form SiO₂ particles via nucleation and growth. After the preparation of the SiO₂ particles, they were deposited on the surface of the funnel glass powder according to Figure 2a. Figure 4a shows the deposition of SiO₂ particles on funnel glass powder with a single deposition. It is clear that SiO₂ particles can deposit on the flat surface of glass powder with sparse and uneven distribution. Nevertheless, the surface of the small glass powder is almost completely covered with SiO₂ particles. This may be due to high surface energy at the sharp edges of small glass powder.

In order to obtain complete coverage of the SiO₂ particle layer, the SiO₂ particle deposition procedure was repeated three times. According to the SEM images (Figure 4), the difference of the SiO₂-particle-funnel glass with various numbers of SiO₂ coating layer can be observed. By comparison, the SiO₂ particle coverage had been significantly improved. For three depositions, the funnel glass powder is almost completely covered with a SiO₂ particle layer. However, it is interesting to note that the arrangement of SiO₂ particles appears to be hexagonal, indicating that the SiO₂ particle is closely packed via the nondirectional physical interaction force. Therefore, experimental observation showed that the coated particles can be easily detached.

The effect of a SiO₂ particle coating on the surface of the funnel glass powder was evaluated with surface analysis technique (ESCA), specifically for the Pb component. Figure 5a indicates that the nontreated waste funnel glass powder shows the high intensity of Pb peaks of typical PbO₂. The intensity of Pb peaks decreases as the SiO₂ particle coating thickness increases. The intensity of Pb peaks of the sample with three layers of the SiO₂ coating is significantly reduced (Figure 5b).

Evaluation based on the toxicity characteristic leaching procedure (TCLP) shows that the leaching amount of Pb of the SiO₂-particle-funnel glass with three layers of SiO₂ particle coating is 1838.8 mg/L, which is less than half of that of the nontreated funnel glass. This result indicates that, with sufficient coverage, a SiO₂ particle coating can reduce the leaching of Pb components into the environment. However, the physical deposition of SiO₂ particles is not effective for the further application of SiO₂-particle-funnel glass.

3.3. SiO₂ Film Coating

In order to have an effective SiO₂ layer barrier to prevent the leaching of Pb to the environment, a SiO₂ film coating process was proposed (Figure 2b). This proposed process takes the advantage of the partial dissolution of the surface of funnel glass powder in an alkaline solution followed by the formation of a layer of SiO₂ film via the interaction of TEOS and funnel glass.

3.3.1. Alkaline Treatment

In alkaline treatment, the tested concentrations of NaOH are within 0.01–10 M. The appearance of the waste funnel glass after alkaline treatment is shown in Figure 6. Compared with the nontreated funnel glass powder (Figure 3), the funnel glass surface becomes smooth and the sharp edges of the glass powder disappear after alkaline treatment. This may be due to the dissolution of the funnel glass powder with high surface energy (i.e., fine particle and edge area) in an alkaline solution (Figure 6a,b). After strong alkaline treatment, the surface of funnel glass particles gradually becomes flat and smooth (Figure 6c,d).

Figure 7 shows the weight change of the funnel glass powder treated by various concentrations of NaOH. After drying at 105 °C for 24 h, the weight of the samples treated with 0.01 M and 0.1 M NaOH have reduced by 0.16% and 0.19%, respectively. This may be due to the dissolution of small particles and edge areas as shown from SEM (Figure 6a,b). The dissolution of funnel glass is similar to the previous observation for amorphous silica [22]. The dissolution of silica from funnel glass powder in basic solution follows Equations (1) and (2) [23]:

$$\equiv {\mathrm{SiO}+\mathrm{OH}}^{-}\to \mathrm{SiO}{\left(\mathrm{OH}\right)}^{+}+{\equiv }^{2-}$$

(1)

$$\mathrm{SiO}{\left(\mathrm{OH}\right)}^{+}{+\mathrm{OH}}^{-}{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_4{\mathrm{SiO}}_4$$

(2)

where ≡SiO represents a silicon site at the surface, ≡²⁻represents an oxygen vacancy site at the surface.

Nevertheless, for the samples treated by 1 M and 10 M NaOH, it is observed that there is a formation of the viscous mushy layer on the funnel glass surface. This may be due to the combined effects of dissolution and incorporation of the NaOH aqueous solution. After drying at 105 °C for 24 h, the weight differences of the samples treated by 1 M and 10 M NaOH were increased by 0.22% and 3.29%, respectively. The weight increase is due to the viscous mushy layer containing NaOH and some water. The formed mushy layer was difficult to dry, especially with 10 M NaOH. After extensive drying (105 °C, a week) to remove the water for SEM observation, the surface of the sample treated with 10 M NaOH becomes flat and smooth (Figure 6d) due to the resolidification of the mushy layer. The concentration of 5 M NaOH was also tested. However, the 5 M NaOH alkaline treated funnel glass showed a similar physical appearance as that treated with 1 M NaOH.

The atomic composition on the surface of alkaline treated funnel glass was also analyzed by EDS. The result is shown in

Table 1. The EDS analysis of alkaline treated funnel glass powder (alkaline treatment: solid/solution ratio = 0.2, 25 °C, 1 h).

NaOH Conc.(M)	Element (Atom %)
	Na	O	Si	Pb	Al
Nontreated	-	59.4	29.5	5.5	5.6
0.01	-	59.3	29.7	5.3	5.7
0.1	-	59.5	29.8	5.2	5.5
1	7.9	59.8	25.4	3.4	3.6
10	9.2	60.2	26.0	2.3	2.2

. For NaOH concentration of 0.01 M and 0.1 M alkaline treatment, the main components in the funnel glass powder are oxygen and silicon (with a ratio of Si:O = 1:2), which is the same as that of the nontreated funnel glass. For alkaline treatment with 1 M and 10 M NaOH, the sodium content increases to 7.9 and 9.2 atom %. This result confirms that the mushy layer is formed due to the combination effects of dissolution and incorporation of NaOH aqueous solution.

3.3.2. Film Coating

In the SiO₂ film coating process, the alkaline (10 M NaOH) treated funnel glass powder was added directly into the TEOS–ethanol solution. As pointed out previously, the surface of the funnel glass powder after strong alkaline treatment has a mushy layer containing NaOH and water. This is important for the formation of the SiO₂ film coating.

Figure 8 shows the SEM image of SiO₂-film-funnel glass. The weight ratio for the addition of TEOS to the funnel glass powder is 1, 2, 5, and 10, respectively. With the increasing amount of TEOS, the surface of SiO₂-film-funnel glass becomes smoother, indicating the formation of a SiO₂ film from TEOS. Furthermore, some SiO₂ particles appear on the surface. The amount of SiO₂ particles on the surface increased with the TEOS/funnel glass ratio. This may be due to the high amount of TEOS in the solution phase forming SiO₂ particles, which then deposit on the funnel glass surface.

The Pb content on the surface of SiO₂-film-funnel glass analyzed by ESCA is shown in Figure 9. The results show that the peak intensity of Pb was decreased by the coating of the SiO₂ film layer. The coating effect is dependent on the weight ratio of TEOS to the funnel glass powder. In comparison with the nontreated funnel glass powder (Figure 5a), the Pb peak intensity for the sample coated with TEOS/funnel glass = 1 decreased by 75%. The result at TEOS/funnel glass = 5 is the same as TEOS/funnel glass = 1. The Pb peak intensity for the sample coated with TEOS/funnel glass = 10 is not detectable. According to the ESCA analysis, the coating effect of the SiO₂ film is better than the SiO₂ particles.

The content of O, Si, and Pb on the surface of SiO₂-film-funnel glass is shown in

Table 2. The EDS analysis of SiO₂-film-funnel glass (film coating treatment: 40 °C, 1.3 h).

Weight Ratio (TEOS/Funnel Glass)	Element (Atom %)
	Na	O	Si	Pb	Al
Nontreated	-	59.4	29.5	5.5	5.6
1	-	68.8	30.2	1.0	-
2	-	68.8	30.4	0.8	-
5	-	67.7	31.6	0.7	-
10	-	66.6	32.9	0.5	-

. The weight ratios of TEOS to the funnel glass powder are 1, 2, 5, and 10. The results indicate that the Pb content decreases with an increasing amount of TEOS. The Pb content decreases from 5.5 atom % for nontreated funnel glass to 1.0, 0.8, 0.7, and 0.5 atom % for weight ratios of TEOS to the funnel glass powder equal to 1, 2, 5, and 10, respectively, indicating that the effect of the SiO₂ film coating is dependent on the additional amount of TEOS. The SiO₂-film-funnel glass surface has a better coverage effect of SiO₂ film at a high TEOS/funnel glass ratio. When the TEOS/funnel glass = 10, the Pb content further decreased to 0.5 atom %. This may be due to improved coverage and film thickness of the SiO₂ film. The SiO₂ film coating provides a film layer on the funnel glass surface.

3.4. Toxicity Characteristic Leaching Procedure (TCLP) of SiO₂-Film-Funnel Glass

For practical application, the sample of SiO₂-film-funnel glass was analyzed for the Pb leaching characteristics by the TCLP test. The standard of Pb leachability levels in Taiwan EPA is 5.0 mg/L. The TCLP test results are shown in

Table 3. The result of Pb of TCLP for the funnel glass powder with different preparation processes of SiO₂ (TCLP: solid/solution ratio = 0.2, 25 °C, 20 h).

Sample	Pb (mg/L)
Nontreated	4352.5
SiO2-particle-funnel glass (three layers)	1838.8
SiO2-film-funnel glass (TEOS/funnel glass = 1)	2047.5
SiO2-film-funnel glass (TEOS/funnel glass = 2)	0.7
SiO2-film-funnel glass (TEOS/funnel glass = 10)	0.6

. For the nontreated waste funnel glass, the leaching amount of Pb was 4352.5 mg/L. For the SiO₂-film-funnel glass, the leaching amount of Pb decreased with an increasing amount of TEOS. The result of Pb dissolution of the SiO₂-film-funnel glass with TEOS/funnel glass = 1 was much lower than that of nontreated funnel glass. Nevertheless, the leaching amount of Pb was still significantly higher than the EPA standard. This may be due to the incomplete coverage of funnel glass by the SiO₂ film. For a TEOS/funnel glass ratio equal to 2 and 10, the leaching amount of Pb reduced to 0.7 and 0.6 mg/L, respectively. The surface cover effect for the funnel glass with the SiO₂ film coating (TEOS/funnel glass = 2) was sufficient to provide a barrier for Pb leaching to the environment. The waste funnel glass after stabilization may become a renewable material for further application in compliance with environmental regulations.

3.5. Mechanism of SiO₂-Film Coating

For the SiO₂ film coating, a mechanism of SiO₂ film coating is proposed (Figure 10) based on the above experimental results.

The steps of the mechanism are proposed as following Equations (3)–(7):

Step 1: Alkaline treatment of funnel glass

In a basic solution, the dissolution of funnel glass will drastically increase the concentration of the negatively charged surface species, ≡ Si-O^- [22]. In contact with water, the negatively charged surface can interact with water according to Equation (3) [24].

$${\left(\equiv {\mathrm{Si}-\mathrm{O}}^{-}\right)}_{\mathrm{funnel}\mathrm{glass}}{+\mathrm{H}}_2\mathrm{O}\to {\left(\equiv \mathrm{Si}-\mathrm{OH}\right)}_{\mathrm{funnel}\mathrm{glass}}{+\mathrm{OH}}^{-}$$

(3)

where $\equiv {\mathrm{Si}-\mathrm{O}}^{-}$ and $\equiv \mathrm{Si}-\mathrm{OH}$ represent a part of the SiO₂ framework of funnel glass. Therefore, the surface of funnel glass will be rich with hydroxyl groups, which can interact with silanol groups from TEOS.

Step 2: Diffusion of TEOS from the outer solution to the mushy layer and formation of silanol group

The TEOS diffused from the outer solution undergoes base catalytic hydrolysis with water to form a silanol group in the mushy layer. The formation of the silanol group from TEOS solution follows the mechanism: [20,25]

$$\mathrm{Si}{\left({\mathrm{OC}}_2{\mathrm{H}}_5\right)}_4{+\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{base}(\mathrm{OH}}^{-})-\mathrm{catalyzed}\mathrm{hydrolysis}}{\to }\mathrm{Si}{\left({\mathrm{OC}}_2{\mathrm{H}}_5\right)}_3{\mathrm{OH}+\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$$

(4)

Step 3: Interaction of hydrolyzed TEOS with the alkaline-treated funnel glass

This procedure is similar to the water condensation in the preparation of SiO₂ particles [20,25].

$$\equiv \mathrm{Si}-\mathrm{O}-\mathrm{H}+\mathrm{H}-\mathrm{O}-\mathrm{Si}\equiv \stackrel{\mathrm{water}\mathrm{condensation}}{\to }\equiv \mathrm{Si}-\mathrm{O}-\mathrm{Si}\equiv {+\mathrm{H}}_2\mathrm{O}$$

(5)

The proposed mechanism in this step is as Equation (6):

$$\equiv \mathrm{Si}-\mathrm{O}-\mathrm{H}+\mathrm{O}-\mathrm{H}-\mathrm{Si}{\left({\mathrm{OC}}_2{\mathrm{H}}_5\right)}_3\to \equiv \mathrm{Si}-\mathrm{O}-\mathrm{Si}{\left({\mathrm{OC}}_2{\mathrm{H}}_5\right)}_3{+\mathrm{H}}_2\mathrm{O}$$

(6)

Step 4: Growth of SiO₂ film from the addition of TEOS

In the final stage, a film of silica is formed. The proposed reaction follows the condensation of TEOS to form SiO₂ [20,25].

$$\equiv {\mathrm{Si}-\mathrm{OC}}_2{\mathrm{H}}_5+\mathrm{H}-\mathrm{O}-\mathrm{Si}\equiv \stackrel{\mathrm{alcohol}\mathrm{condensation}}{\to }\equiv \mathrm{Si}-\mathrm{O}-\mathrm{Si}\equiv {+\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$$

(7)

4. Conclusions

Due to the high content of lead (5.5 atom %), the funnel glass in the CRT monitor is considered hazardous waste, based on TCLP. The stabilization of funnel glass is an important issue. This study proposed a process for the stabilization of the waste funnel glass by coating a protective layer of SiO₂. The results show that the SiO₂ coating layer can effectively reduce the leaching of lead from the funnel glass.

Two schemes of SiO₂ stabilization coating were investigated and compared, namely SiO₂ particle coating and SiO₂ film coating. In SiO₂ particle coating, the SiO₂ particles prepared with TEOS precursor by sol–gel synthesis can physically deposit on the surface of funnel glass. With efficient coverage, the leaching result of the TCLP test shows that the coating of the SiO₂ particles can reduce the Pb releasing. Nevertheless, the physical deposition of SiO₂ particles is not effective for further application of SiO₂-particle-funnel glass.

SiO₂ film coating on funnel glass takes the advantage of the partial dissolution of the surface of the funnel glass powder in the alkaline solution followed by the deposition of a layer of SiO₂ film via the interaction of TEOS with alkaline-treated funnel glass. The funnel glass powder treated with NaOH alkaline solution can form a mushy layer containing OH^- and water, which can promote the formation of a SiO₂ film from TEOS on the funnel glass. The SEM image indicates that the SiO₂ film layer prepared with 10 M NaOH treatment can be well coated on the surface of the funnel glass powder. In addition, the TEOS/funnel glass ratio is an important factor in SiO₂ film coating. The Pb content on the surface of funnel glass powder decreases with an increasing amount of TEOS. The coating conditions in this study for the SiO₂-film-funnel glass are alkaline treated with 10 M NaOH and a TEOS/funnel glass ratio equal to 2. The TCLP test shows that the leaching amount for Pb reduces from 4352.5 mg/L for nontreated funnel glass powder to 0.7 mg/L for that treated with a film coating. The test result is far lower than the standard in Taiwan EPA. The study provides an effective stabilization technique for the treatment of waste funnel glass.