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Ind. Eng. Chem. Res. 2004, 43, 1890-1893
Development of a Technology for Silicon Production by Recycling Wasted Optical Fiber Masaru Ogura,*,† Indra Astuti,† Takeshi Yoshikawa,‡ Kazuki Morita,‡ and Hiroshi Takahashi† Departments of Chemical System Engineering and Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan
A process for the recycling of wasted optical fiber is proposed in this work. By reducing optical fiber, which consists of highly purified silica with a plastic coating, high-purity silicon is obtained. For the preliminary approach for this purpose, the reduction of the silica inside the optical fiber into silicon was carried out using an arc-plasma furnace. As a result, silicon was formed without the addition of carbon as a reducing agent. However, because the coating material was thermally decomposed and gasified in the early stage of the reaction, the quantity of carbonaceous compounds present as the reductant was deficient, resulting in the formation of silicon carbide or silicon monoxide as byproducts. Further addition of carbon showed no significant effect on the yield of silicon but increased the amount of silicon carbide. In contrast, the silicon carbide was found to improve the yield of silicon when it was recycled back into the reactant medium with the unreacted optical fiber. Introduction Because recent growing interest and opportunities in cyber-communication with large volumes of information have prevailed all over the world, the prospect of practical communication has advanced rapidly in the past few decades.1 For these developments, optical fiber manufactured with highly purified silica glass has been utilized because of its advantage in the least loss and attenuation of optical signals.2 In 2001, the quantity of optical fiber manufactured in Japan was about 30 million km, corresponding to ca. 750 rounds of the earth, and the quantity is still increasing every year. Optical fiber is known to consist basically of highly purified silica glass at purity levels of above 99.999 999 999 999% (below part-per-trillion-level impurities), with a small amount of germanium oxide in the core, and the glass part is covered by a plastic coating, which is made of resin. Mainly because of the degradation of the coating resin, communication cables involving optical fiber have to be replaced approximately every 20 years. This waste optical fiber is currently buried because of the dangerousness from the fineness of the glasses. In addition to this waste optical fiber, huge amounts of purified glasses, equal to the amounts of fibered products while being less purified than the fibered products but much more highly purified than conventional optical glasses, actually remain in fibermanufacturing factories. A recycling process for highpurity silica is greatly needed to reduce such kinds of industrial wastes. In terms of energy prospects, further development in solar energy utilization is required, and many researchers have engaged in developing solar cell technology.3 For solar cells, the most common and available resource is a silicon-based material.4 At present, solar cells are * To whom correspondence should be addressed. Address: 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan. Fax: +813-5800-3806. E-mail:
[email protected]. † Department of Chemical System Engineering. ‡ Department of Materials Engineering.
fabricated using semiconductor-grade silicon, because of its purity up to 99.999 999 999%, resulting in high costs of the current solar cell systems. In Japan, recent progressive government programs to provide some monetary incentives for purchasing a solar cell system (100,000 yen per kilowatt) and to enable the selling of excess electricity to electric companies (20 yen per kilowatt-hour) can help electricity customers to install built-up types of solar cells on the roofs of houses and buildings. Nevertheless, an additional few million yen must be expended to finance the system. Therefore, lowcost solar-grade silicon is needed to supply lower-cost solar cell systems.5 If the cost of solar cells can be considerably reduced through the use of new and inexpensive source materials,6,7 solar cell systems would become much more available to most citizens not only in Japan but in other countries as well. Worldwide attention has been attracted to the development of a low-cost, lower-impurity, low-energy, high-volume, and commercially feasible process for the production of highpurity silicon.8,9 In our program on these issues, a process for the recycling of waste optical fiber is proposed, in which the optical fiber is reduced into silicon in an arc-plasma furnace. Initially, the yield of silicon from the optical fiber is considered for this preliminary discussion. Experimental Section Optical fiber with a resin coating (not a wasted one) was supplied by a fiber manufacturer. Before being used in the experiments, the fiber was cut into small pieces of about 1 cm in length. Typically, 2 g of the fiber pieces was adopted as the sample size for use in the reduction by arc. The experimental apparatus for the arc treatment is illustrated in Figure 1. This module is composed of a transparent plastic dome with a 30-cm inner diameter, an arc torch at the top of the dome, a handmade carbon crucible with a 5-cm inner diameter at the bottom of the dome, and an exit for gaseous compounds. A transfer-type arc-plasma torch with an
10.1021/ie0304255 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/16/2004
Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004 1891
Figure 1. Diagram of the experimental apparatus for arc treatment used in this study.
Figure 2. XRD pattern of the residue after arc treatment.
adequate electric power source obtained from Koike Sanso Kogyo Co., Ltd, was used. Ar gas was flowed inside the torch at a rate of 0.5 L/min, and arc-plasma was fired up from the top of the torch with the Ar stream to the carbon crucible attached to ground. Before generation of the arc, the gaseous phase inside the dome was replaced with Ar. The current for the arc was increased to 50 mA, and the period of firing was 7 min at a maximum. During the arc treatment, Ar was kept flowing; therefore, the total pressure in the dome chamber was constant at atmospheric pressure. The optical fiber used in this study and the products after the arc treatment were characterized qualitatively and quantitatively by use of chemical analyses, TG, XPS, and XRD. Thermodynamic equilibrium calculations were performed with Outokumpu HSC Chemistry software (version 4.01, Outokumpu Research Oy, Pori, Finland) for the assumption of adequate reaction conditions to obtain silicon from silica with high conversion and selectivity. Results and Discussion Figure 2 shows a typical XRD pattern of the residue after the arc treatment. Diffraction signals attributable to silicon as well as silicon carbide were found under the experimental conditions used. The hump observed in the 2θ range from 20° to 30° is due to amorphous phases, one of which is the silica inside the optical fiber that remained intact during the arc reaction. XP spectra of the residues indicate that silicon monoxide was also formed in the same reaction. From these results, the reduction of silica contained in the optical fiber was found to occur and to give silicon, silicon monoxide, and silicon carbide.
Figure 3. Equilibrium compositions of silicon compounds.
Thermodynamic equilibrium simulations were carried out to investigate adequate reaction conditions. The effects of the amount of carbon on the equilibrium compositions are summarized in Figure 3. Mixing carbon to silica at an equimolar ratio resulted in the formation of silicon monoxide as a silicon compound at temperatures above 1500 °C, as could occur through the following reaction10
SiO2 + C f SiO + CO
(1)
When the molar ratio was increased to 10, silicon was observed to form, along with silicon carbide in a majority
1892 Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004
Figure 4. Effect of the arc current on the yields of silicon (O) and silicon carbide (b).
to silicon monoxide. Finally, only silicon was obtained at the carbon/silica mixing ratio of 100.
SiO2 + 2C f Si + 2CO
(2)
Generally included in the simulation, silicon carbide was found to form as a silicon compound at temperatures lower than 2000 °C, and silicon was gradually formed at increasing temperature.
SiO2 + 3C f SiC + 2CO
(3)
SiO + 2C f SiC + CO
(4)
SiC f Si + C
(5)
SiO + C f Si + CO
(6)
As for the formation of silicon carbide, reactions 3 and 4 are well-known as carbothermal reductions for the preparation of silicon carbide by use of conventional electric furnaces below 1800 °C.11 Temperatures above 2000 °C are required to form silicon, as has been suggested in the literature.12-14 The relative intensities of the silicon and silicon carbide peaks in the X-ray diffraction data changed as the arc current was increased, as illustrated in Figure 4. The intensity for silicon increased with increasing arc current up to 50 mA. Temperatures higher than 2000 °C in a wider area would result from an increase in the arc power. On the other hand, the intensity for silicon carbide increased and reached a constant value, suggesting that some of the silicon is generated via silicon carbide at increasing reaction temperature. From the XRD data, quantitative analyses were also carried out by using calibrations for silicon and silicon carbide. The yield of silicon was a maximum of 10% from the silica in the optical fiber. In the early stage of the reaction of optical fiber in the arc furnace, gasification was found to occur, and an oily compound was deposited on the wall of the dome chamber. This means that the resin coated on the silica was pyrolyzed into gaseous compounds at lower temperatures. From thermogravimetric analyses, the content of carbon in the optical fiber was 39.0 wt %, and that of silica was 39.7 wt %; therefore, the molar ratio of the optical fiber utilized in this study was SiO2/C )
Figure 5. Effect of the addition of carbon on the yields of silicon (O) and silicon carbide (b).
1:5. This carbonaceous compound was assumed to be sufficient as a reducing agent for silica. However, after pyrolysis of the coating resin at 800 °C, the content of carbon decreased to 18 wt %, that is, the molar ratio is reduced to SiO2/C ) 1:0.88, meaning that the reducing agent is insufficient to form significant amounts of silicon from the consideration of the simulation results. Addition of a reducing agent is needed to obtain more silicon and to increase the selectivity for silicon against silicon carbide. Activated carbon or graphite was added to the reaction media by mixing with the optical fiber, and the reaction was promoted as shown in Figure 5. Addition of the activated carbon to the reaction media enhanced the reaction into silicon carbide, and the yield and selectivity for silicon carbide increased with increasing amount of carbon added. No significantly positive effect was observed on the yield of silicon, and in fact, the addition of carbon gave rise to a slight decrease in the yield. The same results were obtained upon use of graphite as a carbon source. These data indicate that there must be a significant gradient in temperature in the arc flame. At the center of the arc flame, theoretically, the temperature reaches to a few ten-thousands of degrees Celsius, which is high enough to give silicon as the single product among silicon compounds. In this work, however, no heat insulator was used in the chamber, and the crucible was located next to the ambient conditions; therefore, the temperature seems too low to provide reduction energy to form silicon. The formed silicon carbide was recycled back into the reaction center, where the higher-temperature zone existed, as a carbon source for the optical fiber that remained unreacted. The amount of silicon produced was increased as a result of this mixing with silicon carbide, as shown in Figure 6. Decomposition of silicon carbide into silicon and carbon never occurred under the conditions used in this study, indicating that the silicon carbide could further react with the silica or silicon monoxide to give silicon, as suggested in the following reaction scheme
SiO2 + SiC f Si + SiO + CO
(7)
SiO + SiC f 2Si + CO
(8)
These reactions occur at lower temperatures than does
Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004 1893
proved by increasing the arc current. However, because the coating material was thermally decomposed and gasified in the early stage of the reaction, the quantity of carbonaceous compounds available to function as a reductant was deficient, resulting in the formation of silicon carbide or silicon monoxide as byproducts. Further addition of carbon increased the amount of silicon carbide formed, but appeared not to have a significant effect on the yield of silicon. The silicon carbide byproduct, however, improved the yield of silicon when it was recycled back into the reactant medium with the optical fiber. In this sense, a promising recycling process for wasted optical fiber was proposed in this work.
Figure 6. Effect of the addition of silicon carbide on the yield of silicon.
Note Added after ASAP Posting. This article was originally released ASAP on 3/16/04. In paragraphs 2 and 3 of the Introduction section, 14.9% and 11.9% were changed to 99.999 999 999 999% and 99.999 999 999%, respectively. The correct version was posted on 3/22/04. Literature Cited
reaction 2.15,16 Further studies are required for conclusive evidence, but adequate mixing of the products with the remaining reactants appears to enhance the silicon yield and decrease the selectivity to the byproducts. From the aspect of chemical processing, this system could be improved according to the reduction mechanism reported in detail elsewhere.17-19 As a whole, the reaction mechanism of the carbothermal reduction of silica has already been discussed and clarified in those reports. Another point for practical usage is the reproducibility of this purified processing. The conventional arc-plasma process is known as a dirty process. Thus, we must determine the origin of the impurity found in the produced silicon, although it is generally accepted that most impurities come from the raw silica material, or ore. Many literature studies3-7 can be found that report the impurities in silicon to be Fe, Al, and so on, which can be recognized from the raw ore. Furthermore, literature investigations concerning the utilization of pure raw materials for silicon are available. Thus, the impact of the use of waste optical fiber must be significant in the sense of a high-purity raw material, as well as in the sense of recycling. The work presented in this report was limited to a preliminary discussion, but the results concerning the use of a complete-mixing arc reactor or a fluidized arc reactor should be published in the near future. Conclusions The reaction of optical fiber with a coating resin in an arc-plasma furnace without addition of carbon as a reducing agent was investigated. The experimental results obtained in this study confirm the formation of silicon and indicate that the yield of silicon was im-
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Received for review May 19, 2003 Revised manuscript received February 9, 2004 Accepted February 18, 2004 IE0304255
