Lead recovery from scrap cathode ray tube funnel glass by hydrothermal sulphidisation

Abstract

This research focused on the application of the hydrothermal sulphidisation method to separate lead from scrap cathode ray tube funnel glass. Prior to hydrothermal treatment, the cathode ray tube funnel glass was pretreated by mechanical activation. Under hydrothermal conditions, hydroxyl ions (OH^–) were generated through an ion exchange reaction between metal ions in mechanically activated funnel glass and water, to accelerate sulphur disproportionation; no additional alkaline compound was needed. Lead contained in funnel glass was converted to lead sulphide with high efficiency. Temperature had a significant effect on the sulphidisation rate of lead in funnel glass, which increased from 25% to 90% as the temperature increased from 100 °C to 300 °C. A sulphidisation rate of 100% was achieved at a duration of 8 h at 300 °C. This process of mechanical activation and hydrothermal sulphidisation is efficient and promising for the treatment of leaded glass.

Keywords

E-waste cathode ray tube funnel glass hydrothermal sulphidisation recycling

Introduction

Cathode ray tubes (CRTs) have been widely used as a video display component of both televisions and computers, representing a significant and challenging fraction of the end-of-life electronics waste stream (Lee and Hsi, 2002; Lee et al., 2004; Li and Wen, 2006; Poon, 2008). Because of their volume and toxicity, obsolete CRTs pose a major concern in Waste Electrical and Electronic Equipment (WEEE) recycling (Babbitt et al., 2009; Nnorom et al., 2011).

Waste CRT has been estimated at 300,000 metric tonnes in Western Europe. About 100,000 t of scrap screens were collected in 2011 in France (Lecler et al., 2015). In the USA, a citizen averagely generated 2.77 kg of CRT equipment each year between 1980 and 2010 (Schumacher et al., 2014). In China, the implementation of the old-for-new home appliance replacement programme has caused the amount of e-waste to increase rapidly; in 2014, the estimated waste home appliances amounted to 113.78 million units, with computer monitors and TV sets accounting for approximately 57% of the total electronic waste (China Household Electric Appliances Research Institute, 2014). It was assumed that approximately 5.5 billion waste CRTs were generated in the year 2015 (Pant, 2013). By weight, CRT glass accounts for more than 50% of a computer monitor or a TV set (Andreola et al., 2005; Menad, 1999; Musson et al., 2000). Many studies (Jang and Townsend, 2003; Musson et al., 2000; Spalvins et al., 2008) have assessed the toxicity of CRT glass and determined that, owing principally to the high lead concentration in its funnel glass, CRT glass presents a clear risk of environmental pollution and should be safely disposed of.

The disposal of funnel glass is a major challenge in waste CRT glass recycling. It is generally agreed that the best option for recycling CRT glass is to use it in the manufacture of new CRTs (glass-to-glass recycling) (Heart, 2008; Lee and Hsi, 2002). Global mass flow analysis indicates that CRT glass demand is decreasing as CRTs are being rapidly replaced by flat panel screens. The CRT glass supply will begin to exceed demand in approximately 2015 (Gregory et al., 2009). Thus, treating funnel glass to remove lead is a safe and thorough way to avoid polluting the environment. It is impossible to completely extract lead from funnel glass by conventional acid leaching methods, owing to its extremely stable structure (Méar et al., 2007; Yot and Méar, 2009). This challenge has stimulated the development of a number of technologies, such as reduction, mechano-chemical processes, subcritical and ultrasonical pretreatment; all have recently been proposed as viable methods for extracting lead from CRT funnel glass (Chen et al., 2009; Lu et al., 2013; Miyoshi et al., 2004a; Sasai et al., 2008; Saterlay et al., 2001; Zhang et al., 2013). In our previous research, we showed that mechanical activation was a preferable pretreatment process for inducing the changes in CRT funnel glass structure and significantly enhancing its solubility (Yuan et al., 2012, 2013). Lead is present mainly in sulphide form (PbS) in the original ore (galena); if the lead in CRT funnel glass could be converted into lead sulphide, it could then be separated from funnel glass economically with existing technological metallurgical processing methods, such as flotation.

Hydrothermal sulphidisation has attracted much attention as a method for converting lead from lead-containing solid wastes, such as sludge and molten incineration fly ashes, into lead sulphide (Kuchar et al., 2007; Liang et al., 2012). However, little work has been reported on the application of this method to treat CRT funnel glass. This research aims to introduce a novel way to convert the lead contained in funnel glass after activation into lead sulphide to realise its detoxification.

Materials and methods

Materials used

CRT funnel glass was obtained from the Henan Ancai Gaoke Corporation. Its chemical composition, which was determined using X-ray fluorescence spectroscopy (SXF-1200, Shimadzu, Japan), showed that it consisted of SiO₂ (53.90 wt%), PbO (23.10 wt%), K₂O (7.59 wt%), Na₂O (5.81 wt%), CaO (3.06 wt%), Al₂O₃ (3.03 wt%) and minor amounts of other oxides. Blocks of funnel glass were broken and sieved to achieve a particle dimension range of 124~178 μm; the funnel glass particles were then mechanically activated using a planetary ball-mill apparatus (P-7, FRITSCH, Germany) at the rotational speed of 500 r/min for 2 h under ambient atmosphere. The raw material-to-ball mass ratio is 1:19. These milling parameters were proved to be optimal for the dissolution of CRT funnel glass in our previous research (Yuan et al., 2012). The analytical reagent of sulphur was the sulphide used in this research work.

Hydrothermal sulphidisation of the activated samples

All hydrothermal sulphidisation experiments were carried out in a 100-mL stainless steel autoclave with a turbine impeller operating at a constant stirring rate of 700 r/min. A mix of 2.0 g mechanically activated CRT funnel glass, 0.15 g sulphur and 50 mL distilled water were charged into the autoclave, which was sealed. The autoclave was purged with nitrogen to remove residual air. The hydrothermal sulphidisation reaction proceeded at temperatures of 100 °C–300 °C from 0 to 8 h, and then cooled to room temperature with tap water. After treatment, solid–liquid separation was performed by vacuum filtration with a 0.45-µm pore-sized filter paper. The precipitate was collected and washed with carbon disulphide to remove the residual sulphur, and finally dried at 50 °C for 24 h in a vacuum oven. The experimental process was shown in Figure 1.

Figure 1.

Schematic diagram of mechanical activation and hydrothermal sulphidisation of scrap CRT funnel glass.

Analysis of the sulphidisation samples

Scanning electron microscopy (SEM) (JSM-6460LV, Japan) and X-ray diffraction (XRD) (D8 ADVANCE, Germany) using copper Kα radiation with a step size of 0.02° and a recorded range for 2θ from 10° to 60° (λ = 1.5604 Å) were used to observe the morphology of the sulphidisation products and to investigate their crystallographic composition. The content of lead sulphide in the sulphidisation samples was measured using an automatic measuring sulphur instrument (WDL-9000B, China). The percentage of lead in funnel glass sulphidised to lead sulphide was calculated according to:

X = \frac{C_{a}}{C_{b}} \times 100 %

(1)

where X (%) is the sulphidisation rate, C_a the amount of lead sulphide generated (mol) and C_b the initial amount of lead in the funnel glass (mol).

Results and discussion

Characterisation of scrap CRT funnel glass after mechanical activation

Figure 2 shows the morphologies and XRD patterns of CRT funnel glass samples before and after mechanical activation. The raw material presents glass particles, however, the mechanically activated sample is very fine with the surface cotton-like. They are in a state of agglomeration, which is a typical phenomenon for mechanically activated samples. The median particle sizes (D₅₀) for raw material and activated sample are 191.5 μm and 13.4 μm, respectively. From XRD patterns of CRT funnel glass samples shown in Figure 2, both of them were still amorphous and exhibited a typical glass phase pattern with a glass hump. According to the obtained result (Yuan et al., 2012), the physico-chemical changes after mechanical activation, including chemical breakage and defects formation in the glass inner structure, contribute to the easy dissolution of the activated sample in solution.

Figure 2.

SEM images and XRD patterns of scrap CRT funnel glass samples before and after mechanical activation.

Morphology analysis on the prepared samples after hydrothermal treatment

After hydrothermal treatment, the obtained samples turn black, indicating that a sulphidisation reaction has occurred. Figure 3 shows the SEM images of the samples after hydrothermal sulphidisation at different temperatures for the duration of 2 h. It is clear that the reaction temperature has a significant effect on the surface properties of sulphidisation samples. When temperature was at or below 180 °C, the hydrothermal sulphidisation samples were in a particle structure, and a small amount of lead sulphide was found according the XRD analysis. However, with an increase in temperature, the surface of the sample began to change to a loosely layered structure; for the sample treated at 300 °C for 2 h, the surface appeared almost entirely as a layered structure. This suggests that nucleation and crystal growth occur during hydrothermal sulphidisation treatment. The layered structure may be a mixture of several kinds of crystals, which can be determined by XRD analysis. With hydrothermal treatment, the alkali and alkaline earth cations (potassium, sodium, magnesium, calcium, etc.) are removed through ion exchange process, and the CRT funnel glass is converted to a chemically active layered compound (Miyoshi et al., 2004b).

Figure 3.

SEM images of samples hydrothermally treated for 2 h at different temperatures. (A) 100 °C; (B) 140 °C; (C) 180 °C; (D) 220 °C; (E) 260 °C; and (F) 300 °C.

XRD analysis for the hydrothermal sulfidisation samples

XRD patterns of samples hydrothermally treated at 300 °C for different durations are shown in Figure 4. Lead sulphide crystal (PbS) can be clearly seen in the patterns, indicating that PbS was generated during the hydrothermal sulphidisation process. Sulphidisation reactions of lead in mechanically activated CRT funnel glass, converted to lead sulphide under hydrothermal conditions, are presented in equations (2)–(5). Lead ions in mechanically activated CRT funnel glass are easily diffused from the network under hydrothermal conditions. Through the ion exchange process, the dissolution of glass network modifiers, such as alkali and alkaline earth ions, proceeds with water to form hydroxyl ions (OH^–). Between sulphur and hydroxyl ions, a disproportionation reaction proceeds to generate the sulphur ion (S^2–), as shown in equation (4). The sulphidisation reaction is accomplished by S^2– and Pb²⁺ being converted to lead sulphide under hydrothermal conditions (equation (5)).

\equiv Si - O - Pb - O - Si \equiv + {2 H}_{2} O \to 2 \equiv Si - O - H + {Pb}^{2 +} + {OH}^{－}

(2)

\equiv Si - O - A + H_{2} O \to \equiv Si - O - H + A^{+} + {OH}^{－}

(3)

4 S + {6 OH}^{－} \to {2 S}^{2 -} + S_{2} O_{3}^{2 -} + {3H}_{2} O

(4)

{Pb}^{2 +} + S^{2 -} \to PbS ↓

(5)

Figure 4.

XRD patterns of samples after hydrothermal treatment for different times at 300 °C.

where A is alkali and alkaline earth ions, such as potassium, sodium, magnesium and calcium.

Figure 4 shows that, as reaction time increased, the diffraction peak intensity of PbS increased slightly as well, a result that validated the formation and growth of PbS. According to Scherrer’s equation, the mean grain size of PbS (1 h sample) was around 96.3 nm, while that of PbS (8 h sample) increased to larger than 100 nm. Along with PbS diffraction peaks, other diffraction peaks were observed for the mechanically activated CRT glass after hydrothermal sulphidisation treatment; these were attributed to orthoclase (KAlSi₃O₈). For the sample hydrothermally treated for 1 h, just a small diffraction peak corresponding to orthoclase at around 21° was observed. When duration reached 2 h or more, however, other new diffraction peaks (corresponding to orthoclase) could be clearly observed. Its content gradually increased as reaction duration increased, indicating that the dissolution of mechanically activated CRT funnel glass was achieved. The result is in agreement with that reported for utilising subcritical water to treat lead silicate glass and soda–lime–silicate glass by hydrogen (Miyoshi et al., 2004a, 2004b).

Effect of reaction time and temperature on sulphidisation

Figure 5 shows the independent effects of reaction temperature and duration on the hydrothermal sulphidisation rate for CRT funnel glass. For the sample hydrothermally treated at 300 °C, the sulphidisation rate increased from 80% to 100% when reaction duration increased from 1 h to 8 h. Prolonged reaction duration can enhance the sulphidisation rate slightly. As shown in Figure 5, a higher reaction temperature has an obvious effect on the sulphidisation rate, which increased almost linearly with the increase in temperature. For the sample treated for 2 h, the sulphidisation rate was clearly observed to increase from 25% at 100 °C to 88% at 300 °C. For CRT funnel glass, the higher temperature results in an increase in stored energy, and in the breakage of chemical bonds and metal ions with high activation energy. This response indicates that hydrothermal temperature can significantly improve the dissolution rate and diffusive coefficient of lead ions from funnel glass.

Figure 5.

Independence of reaction temperature and time on sulphidation rate.

The sulphidisation agent used in this research was elemental sulphur with a melting point of 119 °C. Sulphur is insoluble in water under ambient atmosphere; however, its solubility increases with an increase in temperature (Deng and Chen, 1990). The dissolved sulphur has a disproportionation reaction in solution, which proceeds very slowly, as it is an endothermic reaction significantly affected by temperature. As seen in Figure 5, when the temperature was below the melting point of sulphur, the disproportionation reaction of elemental sulphur was slight; but with an increase in temperature to above the melting point, the reaction proceeded quickly.

Generally, alkaline compounds are added to a solution to facilitate the disproportionation reaction. However, owing to the easy dissolution of mechanically activated CRT funnel glass, no additional alkaline compound was used in this research. An increase in temperature accelerated the dissolution of funnel glass and rate of ion exchange, leading to an increased hydroxyl ions concentration. The resulting hydroxyl ions can accelerate the disproportionation reaction of the dissolved sulphur to generate sulphur ions. Hence, an increase in temperature rapidly increased the sulphidisation rate.

Lead in CRT funnel glass can be converted to lead sulphide with high efficiency through a hydrothermal sulphidisation reaction, and the resulting PbS particles can be recovered using traditional flotation technology. A process consisting of mechanical activation/hydrothermal sulphidisation/flotation is feasible and promising for the treatment of CRT funnel glass. It is believed that such a process may also be used to treat other leaded glass with high chemical stability. The separation of lead sulphide obtained from glass residue will be carried out in our future work.

Conclusions

Lead in mechanically activated CRT funnel glass can easily be converted to lead sulphide through a hydrothermal sulphidisation reaction. During this process, there is no need for additional alkaline compounds to accelerate the sulphur disproportionation reaction, and hydroxyl ions (OH^–) could be generated through ion exchange; that is one advantage of the method proposed in this research. It is clear that with an increase in the reaction temperature and duration, the sulphidisation rate of lead increased, reaching almost 100% under optimal experimental conditions. The combined process, mechanical activation + hydrothermal sulphidisation + flotation, can realise lead recovery from CRT glass with high efficiency, which is feasible for the industrial application. We think this process can be applied in galena ore mine, where it has pulverisation and flotation equipment. It is also promising for the treatment of other lead-containing wastes.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financially supported, in part, by the National Natural Science Foundation of China [No.21407105, No.21177069], the Shanghai Municipal Natural Science Foundation [No.14ZR1416700], SSPU Key Disciplines Subject [XXKYS1404] and the Shanghai Zhangjiang Special Development Fund [No.201310-PD-JQ-B2-009].

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