Vacuum-Gasification-Condensation of Waste Toner To Produce

Research Article pubs.acs.org/journal/ascecg

Vacuum-Gasification-Condensation of Waste Toner To Produce Industrial Chemicals and Nanomaterials Jujun Ruan,*,† Lipeng Dong,† Jiaxin Huang,† Zhe Huang,† Kui Huang,‡ Haili Dong,‡ Tao Zhang,† and Rongliang Qiu*,† †

School of Environmental Science and Engineering, Sun Yat-sen University, 135 Xingang Xi Road, Guangzhou 510275, People’s Republic of China ‡ School of the Environment, Guangxi University, 100 Daxue Road, Nanning 530004, People’s Republic of China ABSTRACT: With the development of information industries, abundant waste toner was produced along with the generation of waste toner cartridges in office works. Waste toner is considered a hazardous material due to the small size (2−7 μm) and components of polystyrene, polyacrylate, Fe3O4, and SiO2. Cancers of blood circulation and digestive systems will be induced if toner is inhaled. According to our knowledge, little information was published about the disposal technology of waste toner. This paper proposes an environmentally-friendly technology to dispose waste toner. Vacuumgasification-condensation was employed to treat waste toner. Industrial chemicals, n-butene gas, and liquid oils of styrene, polystyrene, and acrylic ester were obtained from the pyrolysis of polystyrene and polyacrylate. The organics of waste toner began to be decomposed and gasified when the temperature reached 450 °C. Above 570 °C, most of the polystyrene and polyacrylate were converted to gases and then condensed into oil in the temperatures of 180 and 80 °C. SiO2 and Fe3O4 of waste toner were transformed into nano-Fe3O4 and nano-SiO2 and gathered. The size of the nanoparticles was about 200 nm. This paper provides a recovery technology of waste toner that is environmentally-friendly and high value-added. KEYWORDS: Waste toner, Recovery, Industrial chemicals, Nanoparticle

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INTRODUCTION The production of e-waste has reached about 30 million ton per year in the world. The number in China is about 5 million per year. Developing countries are taking the major responsibility of recycling e-waste.1 Recovering e-waste is a vital social issue in developing countries not only from environmental protection but also from resource recycling.2,3 However, because of having no advanced technologies, serious environmental pollution, such as heavy metals and polycyclic aromatic hydrocarbons, had been caused by treating e-waste in developing countries.4,5 In the year 2008, the Chinese State Council issued Regulations on Recovery Processing of Waste Electrical and Electronic Products to manage the recovery of e-waste and encouraged the development of and employed advanced technologies to treat e-waste. The regulation was implemented in the year 2011. Therefore, new recovery technologies of e-waste are developing.6−8 Due to the damage of functional units, a large number of waste toner cartridges (TCs) had been generated in office works. In developed countries, waste TCs are encouraged to be remanufactured and recycled by ink and toner industrial organizations. Original equipment manufacturers (HewlettPackard and Epson) take this responsibility.9−11 However, in developing countries, there is no detailed way to recycle waste TCs. Waste TCs are always discarded into garbage cans and © 2017 American Chemical Society

treated together with the municipal waste, or waste TCs are passed on to informal street vendors who have no reliable technology to remanufacture waste TCs in good-quality.12 Therefore, legal institutions and regular recycling channels for waste TCs urgently need to be established in developing countries. It is estimated the output of collected waste TCs had reached about 450 tons per year in China.13 Although the government and origin entrusted manufactures had made great efforts on refilling, refurbishing, and remanufacturing, only about 25% waste TCs were translated to the licensed e-waste treatment company. Most of waste TCs were not treated in the proper manner. The main way of treating waste TCs is that they were collected from the normal household garbage and street trader and then were sent to e-waste treatment enterprise. Different types of waste TCs have different compositions. In general, waste TCs contained about 35.0 wt % plastics, 40.0 wt % steel, 12.0 wt % aluminum, 5.0 wt % magnets, and 8.0 wt % waste toner.14 Before the year 2007, there was no proper method of treating waste TCs. Incineration and landfills were not proper methods for treating waste TCs. High quality plastic materials Received: February 2, 2017 Revised: April 12, 2017 Published: April 17, 2017 4923

DOI: 10.1021/acssuschemeng.7b00328 ACS Sustainable Chem. Eng. 2017, 5, 4923−4929

Research Article

ACS Sustainable Chemistry & Engineering

The structure of the vacuum-gasification-condensation furnace was presented as Figure 3. There were three sections (T1, T2, and T3) controlled by different temperatures of 800 °C, 180 °C, and 80 °C, respectively, in the vacuum furnace. Section T1 was used to decompose the organic components of waste toner with high temperature and transform the organics to gases. Section T2 and section T3 were controlled in low temperature to condensate the gases to oil. The mechanical pump and the diffusion pump provided the vacuum condition. They were also responsible for moving the pyrolysis gases from the T1 area to T2 and T3 areas. Then, the pyrolysis gases were cooled into oil in T2 and T3 areas. Smallmolecule gases cannot be condensed in T2 and T3 areas, and they were pumped out and collected. Vacuum-gasification-condensation is a green technology of recovering organic materials from others.24,25 The organic components of waste toner will be decomposed, cooled, and collected as oils and gases. Fe3O4 and SiO2 were the residual. Vacuum-gasificationcondensation was operated under the condition of vacuum. Without the atmospheric molecules disturbance, the reaction rate can be accelerated, and the required temperature is reduced. Compared to traditional pyrolysis, vacuum-gasification-condensation was an energy saving method.26,27 Thermogravimetric Analysis of Waste Toner and Detection of the Oils, Residual, and Small-Molecule Gases. A thermogravimetric analyzer (NETZSCH TG 209) was employed for thermogravimetric analysis of waste toner. N2 was the shielding gas. The rate of temperature increase was 10 °C/min, and the temperature increased from 20 to 720 °C. The pyrolysis oil, residual, and small-molecule gases were detected by the methods of Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and gas chromatography−mass spectrometry (GC-MS).

in waste TCs would be lost in incineration. Meanwhile, the plastic materials of waste TCs were hard to degrade in landfills. Soil and underground water pollution might be induced in a landfill of waste TCs. We proposed combination technology of physical methods to treat waste TCs in previous work.15−17 The technology included closed crushing (configured circulating water cooling system to decrease the crushing temperature and avoid the explosion of waste toner), air current separation, magnetic separation, and eddy current separation. Abundant aluminum, magnetic materials, steels, plastics, and waste toner were recovered. Recovery of waste TCs brought about 1100 dollars/ t. Waste toner is the powder part of waste TCs. The weight of waste toner reached about 36 t per year in China. Waste toner was a granular mixture and was comprised of about 7.0 wt % polyacrylate, 55.0 wt % polystyrene, 3.0 wt % SiO2, and 35.0 wt % Fe3O4.14 So far, there has been no properly treating technology. Waste toner contains 62.0 wt % fine organic particles. It will pollute air, soil, and underground water in the landfill process. Meanwhile, the organic components might bring diseases and cancers to human bodies. Incineration is also not good for treating waste toner.18 About 38.0 wt % of waste toner was Fe3O4 and SiO2. Polyacrylate and polystyrene have high ignition temperatures. Waste toner will be liquefaction but cannot be set on fire in open incineration. Thus, it is difficult to incinerate waste toner. Additionally, incineration of waste toner will lose the resources of polyacrylate and polystyrene. Therefore, the disposing technology of waste toner is a pressing need.

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RESULTS Thermogravimetric Analysis (TGA) of Waste Toner. Before decomposing the organic components by the method of vacuum pyrolysis, thermogravimetric analysis (TGA) of waste toner was performed. The results of TGA of waste toner were presented in Figure 4. The weightlessness peak of the DTG (differential thermal gravity) curve of waste toner appeared at 283 °C. At the beginning, the TG (thermal gravity) curve declined slowly. When the temperature reached 319.4 °C, the rate of weight loss was 2.0 wt %. When the temperature reached 353 °C, the rate of weight loss was 5.01 wt %. Due to having large molecular weight, the long chain organic molecule of waste toner began to decompose at this temperature and caused this weight loss. The highest peak of the DTG curve appeared at 398 °C. It meant the rate of weight loss got the maximum value, and the organic components of waste toner were decomposed rapidly at this temperature. The DTG curve also indicated there was only one weightlessness peak in the whole pyrolysis of waste toner. It meant polystyrene was the main organic component of waste toner. When the temperature was greater than 460 °C, TG and DTG curves became smooth. It meant the rates of weight loss were approximately 0, and the organic components of waste toner were decomposed completely. Therefore, pyrolysis final temperature of waste toner should be greater than 460 °C so as to decompose the organic components of waste toner completely. Figure 4 showed the rate of weight loss of waste toner stabilized at 50.36 wt % when the temperature went from 460 to 700 °C. This result showed the mass proportion of organic components in waste toner was 50.36 wt %. Vacuum-Gasification-Condensation of Waste Toner. About 45.27 g of waste toner was fed into the crucible placed in section T1 of the vacuum furnace. According to the results of TGA of waste toner, the temperature of section T1 was set as

MATERIALS AND METHODS

Seen from the SEM (scanning electron microscopy) picture of waste toner (Figure 1), waste toner was a granular mixture whose particle

Figure 1. (a) SEM of waste toner in 20 μm and (b) SEM of waste toner in 5 μm.

size ranged from 2 to 7 μm. Fe3O4 and SiO2 were wrapped by mixed materials of polyacrylate and polystyrene and presented as particles. Waste toner was a hazardous material. Due to fine particle size, toner was easily inhaled and accumulated in the human body. Styrene in waste toner would improve by about 4% the probability of cancers to humans if it was inhaled.19,20 Meanwhile, if polyacrylate was inhaled and ingested, diseases of the respiratory tract, nausea, and head would be induced.21 Waste polystyrene cannot be degraded by biological progress. If polystyrene was inhaled and ingestion, the digestive system would be badly destroyed.22,23 The Flowchart of the Recovery Process of Waste Toner. After the technologies of crushing, air current separation, magnetic separation, and eddy current separation (see Figure 2), we employed vacuum-gasification-condensation to recycle waste toner. 4924


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Figure 2. Recovery process of waste TCs and waste toner.

Figure 3. Structure of the vacuum pyrolysis furnace.

Figure 4. Nitrogen thermogravimetric curves of the waste toner.

800 °C, and the organics began to pyrolysize. Crucibles placed in sections T2 and T3 were used to collect the pyrolysis oil. The temperatures of section T2 and section T3 were set as 180 and 80 °C. A crucible placed in section T2 was used to collect the oil whose boiling points were greater than 80 °C and lower than 180 °C. A crucible placed in section T3 was used to collect the oil whose boiling points were less than 80 °C. The heating rate of the vacuum pyrolysis was set as 5 °C/ min. The relationship between pressure, pyrolysis time, and temperature during the vacuum-gasification-condensation of waste toner was given in Figure 5. Before the temperature reached 300 °C, there was no pressure change in the furnace. It indicated no moisture evaporation from waste toner. When the temperature increased from 300 to 570 °C, the atmospheric

Figure 5. Relationship between pressure, time, and temperature in vacuum-gasification-condensation of waste toner.

pressure in the furnace increased rapidly. It showed drastically how pyrolysis was proceeding and abundant gases were generated. When the temperature reached 450 °C, the atmospheric pressure continued to increase, but the increase rate became small. During this process, due to having low activation energy, the weak chemical bonds were broken, and part of the organics of waste toner became gases. When the temperature increased to 490 °C, the strong chemical bonds, which had high activation energy, began to break, and the atmospheric pressure increased rapidly again. When the temperature reached 570 °C, the atmospheric pressure reached 4925


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of the absorption spectrum of oil (180 °C) were of greater intensity than that of recovered oil (80 °C). This result showed a number of polymers were not decomposed and presented in the recovered oil at 180 °C. High polymers of acrylic ester and styrene were condensed at the temperature of 180 °C due to their high boiling points. Table 1 and Figure 6 indicated small-molecular polystyrene and styrene monomer were the main components of pyrolysis oil at 80 °C. Meanwhile, acrylic ester and small-molecular polystyrene were the main composition of the pyrolysis oil at 180 °C. In the pyrolysis process of waste toner, a few small-molecular gases were collected. The gases were detected by the method of GC-MS. The results were presented in Table 2.

the highest value. Then, the temperature continued to improve, but the atmospheric pressure decreased. It meant most of the organics of waste toner were decomposed. The pyrolysis gases were pumped to a condensation area, and the gases turned to oils. At last, 21.14 g of oils was obtained. The converted ratio of waste toner to oils was 46.69%. A small amount of smallmolecule gases (0.51 g) was pumped out from the vacuum furnace and collected. The weight of residual particles was 23.63 g. Analyses of the Pyrolysis Products. We used the method of FT-IR to analyze the functional groups contained in waste toner and the pyrolysis oils. FI-IR revealed the molecular structure of the materials. The absorption peaks represented the specific functional groups in the molecular. The corresponding relationship between functional groups and wavenumbers was presented in Table 1. The results of FT-IR analysis of waste toner and the recovered oils were presented in Figure 6.

Table 2. Composition of Pyrolysis Gas of Waste Toner

Table 1. Corresponding Relationship between Wavenumber and Functional Group in Infrared Spectrum wavenumber/cm−1 1725 1489 902 753 697 1598 1151 1373 1025

molecular structure CO stretching group in ester −CH2 −OH in carboxyl benzene ring CC in benzene ring framework in-plane bending benzene ring −CH3

order

retention time

pyrolysis gas

molecular formula

proportion (wt %)

1 2 3 4 5 6 7 8 9 10 11 12

6.209 6.668 7.048 8.032 9.508 11.007 12.853 13.038 13.441 13.800 15.265 15.838

methane ethane ethylene propane propylene n-butane trans-butene n-butene isobutene cis-butene 1,3-butadiene propyne

CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C4H8 C4H8 C4H8 C4H6 C3H4

4.12 1.92 2.01 1.91 4.40 2.32 1.17 75.76 3.97 1.39 0.64 0.39

Twelve kinds of gases were identified. Six kinds of gases had the proportions greater than 2.0 wt %. n-Butene had the largest proportion that accounted for 75.76 wt %. The second and third proportions were propylene and methane which accounted for 4.40 and 4.12 wt %. Although pyrolysis gases account for a rather small proportion of the pyrolysis products of waste toner, the pyrolysis gases contained a large proportion of n-butene. Along with the pyrolysis of waste toner, inorganic particles (23.63 g) were gathered as a large size of solid waste (see Figure 7) and contained litter organic components. The composition of the residual particle was SiO2 and Fe3O4. The residual solid waste is not any more damage to humans. We used the method of scanning electron microscopy (SEM) to observe the elementary composition and microstructure of the residual solid waste. The results were presented as Figure 7 and Figure 8. As seen from Figure 7, the distributions of elements in the surface of the residual solid waste indicated the Fe element was much greater than the Si element. The EDS analysis indicated that solid waste included C, O, Fe, Si, and S elements. C accounted for 43.75 wt % and 60.84 atomic %. O accounted for 29.80 wt % and 31.11 atomic %. Si had the proportions of 0.32 wt % and 0.19 atomic %. A small amount of S element was presented in the residual solid waste and had the proportions of 0.23 wt % and 0.12 atomic %. Fe was the sole metallic element in the residual solid waste. Fe accounted for 25.90 wt % and 7.75 atomic % of the residual solid waste. According to Figure 7, it could be concluded the residual solid waste was comprised of C, Fe3O4, and SiO2. The SEM pictures of the residual solid waste were presented as Figure 8. The results showed the solid waste was clustered by

Figure 6. FT-IR spectrum of waste toner and pyrolysis oils (180 and 80 °C).

Comparing the curves of the absorption spectrum of waste toner and the oils recovered at 180 °C and 80 °C, we found most of the absorption peaks appeared in the same wavenumber but were different in intensity. These results indicated a number of functional groups of organics in waste toner were pyrolyzed and caused the decrease of the intensity. The disappearance of the absorption peak (1373 cm−1) in the curve of the absorption spectrum of oil (80 °C) showed the acrylic ester of waste toner was decomposed completely. The weakening of the absorption peak (1448 cm−1) in the curve of the absorption spectrum of oil (80 °C) indicated part of the benzene ring in waste toner was opened in the pyrolysis process. The absorption peaks (3000−2800 cm−1) in the curve 4926


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Figure 7. Element analysis of the residual solid waste.

bonds having higher activation energy. Chemical bonds having higher activation energy would not be decomposed at the beginning stage of pyrolysis due to insufficient energy. Therefore, according to the chemical bond energy and the pyrolysis products, the pyrolysis routes of the organic compounds of waste toner were concluded in Figure 9.

Figure 8. SEM pictures of the residual solid waste.

the nano-C, nano-Fe3O4, and nano-SiO2 particles. The size of the nanoparticles was about 200 nm.

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DISCUSSION This paper proposed the method of vacuum-gasificationcondensation to recycle waste toner. Waste toner was safely recycled and high value-added recovered. Oils, nano-C, nanoFe3O4, and nano-SiO2 were recovered. In the vacuumgasification-condensation process, organic components of waste toner were decomposed to oil. The decomposition routes of organic components were analyzed. Table 3 gave the bond length and bond energy of chemical bonds in the organic compounds.28 In vacuum-gasification-condensation of waste toner, according to the Arrhenius formula, the chemical bonds having lower activation energy will be decomposed earlier than the chemical

Figure 9. (a) Pyrolysis route of polystyrene and (b) pyrolysis route of polyacrylate.

In the vacuum-gasification-condensation process of polystyrene, the bonds of C−H and C−C were destroyed by high temperature. Analysis results (Figure 9) of pyrolysis routes of the organics of waste toner indicated that the bond of CC was not destroyed. It meant the temperature of 800 °C was not enough to destroy the bond of CC. If the aim of polystyrene pyrolysis was not to destroy the CC bond to obtain smallmolecule gases, according to the results of Figure 5, the pyrolysis temperature of 570 °C was enough to transform polystyrene to oils. In other words, the temperature of 570 °C was enough to separate polystyrene from waste toner. Due to the temperature of 800 °C (the setting temperature of T1 of the vacuum furnace) not being enough to destroy the bond of

Table 3. Bond Length and Bond Energy of Chemical Bonds in Organic Compounds chemical bond

bond length (nm)

bond energy(kJ/mol)

C−C CC C−O CO C−H O−H

0.154 0.134 0.146 0.1235 0.111 0.096

347 620 351 732 414 464 4927


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ACS Sustainable Chemistry & Engineering Notes

CC, the bonds of CO in polyacrylate were also not decomposed. The bonds of C−C and C−H were destroyed. Thus, the products of polyacrylate were mainly small-molecule gases. According to the components of gases in Table 2, it can be conducted that the substituent group (R′) in polyacrylate of waste toner was C4H8. Styrene and polystyrene contained in the pyrolysis oils of waste toner are the important raw materials in plastic industries. In general, styrene can be used to produce butadiene styrene rubber, foam polystyrene, ABS resin, and other engineering plastics for different applications in pharmacy, dye, pesticides, and mineral industries. Polystyrene is often used to manufacture polyfoam products and contribute to catering and building industries. Acrylic ester is an important raw material for producing medical adhesives for suturing in surgery. The above results also showed the recovered oils were a mixture. The oils need to be further purified for obtaining pure raw materials for plastic industries. Supercritical fluid extraction might be the suitable method to purify the mixture oils, and this is our future work. n-Butene is the main component of the recovered smallmolecular gases. It is an important raw material in the chemical industries of manufacturing plasticizers, mineral-dressing agents, herbicides, and spices. The SEM results of the residual solid waste indicated it was mainly comprised of nano-Fe3O4 and nano-C. The size of the nanoparticles was about 200 nm. The mixed nanoparticles should be separated for high value-added reuse. Magnetic separation might be the suitable method to separate nanoFe3O4 from nano-C particles. The reuse of nano-Fe3O4 and nano-C will be our research work in the future. Additionally, the formation mechanism of nano-Fe3O4 and nano-C in the vacuum-gasification-condensation is also worthy to investigate.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (51308488), Scientific and Technological Projects of Guangdong Province (2015B020237005, 2016A020221014), and the Pearl River Star of Science and Technology (201710010032).

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CONCLUSION For developing the recovery technology of waste toner, this paper proposed vacuum-gasification-condensation technology to recycle waste toner of waste TCs. Industrial chemicals, nbutene gas, and liquid oils of styrene, polystyrene, and acrylic ester were obtained from the pyrolysis of polystyrene and polyacrylate. When the temperature was above 570 °C, most polystyrene and polyacrylate were converted to gases and then condensed into oil. SiO2 and Fe3O4 of waste toner were transformed into nano-Fe3O4 (44.65 wt %) and nano-SiO2 (0.85 wt %) and gathered. This paper provided a recovery technology of waste toner that is environmentally-friendly and high value-added.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 20 84113620. Fax: +86 20 84113620. E-mail: [email protected] (J.J.R.). *E-mail: [email protected] (R.L.Q.). ORCID

Jujun Ruan: 0000-0001-8194-2988 Tao Zhang: 0000-0002-4424-0423 Author Contributions

J.J.R. wrote the manuscript text and prepared Figures 1, 2, 3, 7, and 8. L.P.D., Z.H., and J.H. prepared Figures 4, 5, and 6 and Table 2. K.H. and H.D. prepared Table 1 and Table 3. R.L.Q. contributed the apparatus for the testing and improved the language. All authors reviewed the manuscript. 4928


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Vacuum-Gasification-Condensation of Waste Toner To Produce

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