USING OF WASTE PET (POLYETHYLENE TEREPHTHALATE) AS AN ALTERNATIVE REDUCTANT IN IRON AND STEEL INDUSTRY

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1 USING OF WASTE PET (POLYETHYLENE TEREPHTHALATE) AS AN ALTERNATIVE REDUCTANT IN IRON AND STEEL INDUSTRY Gökhan POLATolat, Burak BIROL and Muhlis Nezihi SARIDEDE Department of Metallurgical and Materials Engineering, Yildiz Technical University, Istanbul, Turkey, Abstract Using of plastics in the world increases day by day and the amount of waste plastics increase as a result of high consumption. The long degradation period of plastics in nature cause the plastic wastes to be a major problem for environmental damage and pollution. By recycling of plastics these problems can be eliminated. For this aim plastics can be used as alternative reductant for iron and steel industry instead of coke. Additionally, high production cost and high consumption of coke has always been the primary problem of iron and steel industry. The aim of this study is reduction of costs due to coke consumption in iron making and environmental damage caused by waste plastics. By this aim waste plastics were added to composite pellets as reductant. In this study PET (Polyethylene terephthalate) which is the remarkable percent of plastic wastes, was used due to its high carbon content. In the experiments, composite pellets which contain PET wastes and magnetite iron ore were prepared. After that, composite pellets were reduced at relatively high temperatures during various reduction times in order to investigate its behavior during reduction melting process. The results of experiments showed that reduction rate of iron ore increased as a function of time. Keywords: PET wastes, recycle, composite pellet, iron and steel, reduction 1. INTRODUCTION Because of affordability, ease of application and increasing improvements of plastic features day by day, the amount and application areas of plastics increase. However, as a result of high consumption rates, plastic wastes cause major problems such as long degradation periods in nature [1]. Although there are many types of plastics, PET (Polyethylene terephthalate) is widely used in packing industry and constitutes a considerable part of plastic wastes. In order to eliminate these wastes many researches have been made for recycling like using as polyester fiber, PET sheet and reductant in iron and steel industry. Using of them in ironmaking and steelmaking is one of these concerns and it has been developed in some countries [2, 3, 4, 5]. Traditionally, coke used as a reductant in ironmaking process. PET can be an alternative of coke with its high carbon content which is the major element in the chemical composition of PET. When PET decomposed, two reducing gases, CO and H 2 are produced. Therefore, PET can be used in composite pellets as a reducing agent [3, 6, 7]. It is necessary that to use PET in composite pellets it has to be ground to micron sizes and therefore PET wastes should be crystallized at 250 C [8]. Iron oxide composite pellets generally include iron ore, reductant as a carbon source and a binder. When composite pellets are reduced in a furnace, the following reactions occur [6, 9]; 3Fe 2 O 3 + CO/H 2 = 2Fe 3 O 4 + CO 2 /H 2 O (1) Fe 3 O 4 + CO/H 2 = 3FeO + CO 2 /H 2 O (2) 3FeO + CO/H 2 = 3Fe + CO 2 /H 2 O (3) C + CO 2 = 2CO (Boudouard Reaction) (4) 2CO = [C]+CO 2 (Carburization) (5) Reducing agent using in composite pellets does not have to be high quality like an expensive coke. Although there are some disadvantages of composite pellets such as low mechanical strength, there are some advantages such as using low quality reductant and high reduction rates. Also, composite pellets are not suitable for firing. Because of this reason they cannot be reduced in blast furnace conditions. Thus, rotary

2 hearth furnace can be used to make pig iron-like product. The process which uses rotary hearth furnace is so-called ITmk3 (Iron Making Technology Mark Three). ITmk3 process converts composite pellets to iron-like product and slag at relatively high temperatures 1400 C and 1450 C [10-14]. 2. EXPERIMENTAL To investigate effect of residence time on crystallization of PET wastes, experiments were performed in Ecocell 55 laboratory scale oven, at 250 C for 1, 2, 3 hours. The samples were ground in the ball mill for 1 hour. After that, 500 μm, 250 μm, 125 μm and 100 μm sieves were used to sieve analysis of the ground PET wastes. It was carried out for 15 minutes. The result of the sieve analysis is shown in Figure 1. Fig. 1 Sieve analysis of ground PET wastes It can be seen from the Fig. 1 that residence time in the oven gives the similar results for 1, 2 and 3 hours. Thus, 1 hour was decided as necessary time for crystallization of PET. The needed powder size of PET wastes for pelletizing in this study is -250 μm. Therefore, the powders less than 250 μm was selected for process and the other sizes were returned to grinder to obtain -250 μm grain size. The chemical composition of PET used in process is given in Table 1. Tab. 1 The chemical composition of the PET waste C % H % O % The main constituent of composite pellets was magnetite concentrate. The concentrate screened to -100 μm for pelletizing. The chemical composition of the magnetite ore concentrate is given in Table 2. Tab. 2 The chemical composition of the magnetite concentrate Fe % SiO 2 % Al 2 O 3 % CaO % MgO % S % Mn % Cu % K 2 O % Na 2 O % Pb %

3 To investigate the reduction properties of composite pellets which contain PET, coke or coke and PET in various amounts were prepared. Coke was also ground in ball mill and -100 μm was used. The chemical composition of coke used in process is given in Table 3. Tab. 3 Chemical composition of coke Moisture % Ash % Volatile Matter % Fix Carbon % Sulphur % Three types of mixture were prepared by adding different ratios of PET and coke and then they were prepared in a mixer for 1 hour. These mixtures are as shown in Table 4. Raw Materials Tab. 4 Type and content of green pellet mixtures 0% PET (100% coke) 50% PET + 50% coke 100% PET (0% coke) % % % Magnetite ore Coke PET Binder Total The amount of coke and PET wastes added to composite pellets were determined as stoichiometric ratio according to the following reaction. Fe 3 O 4 + 4C = 3Fe + 4CO For the agglomeration of the fine raw materials 10% (wt.) water was sprayed during pelletizing. Beet molasses was used as a binder. The amount of the binder chosen was 1% percent of total weight of the pellets. Then, this liquid binder was sprayed with the water which onto the mixture during pelletizing. Pelletizing was done by Multipex laboratory scale pelletizing disc which have 30 g/min, 45 o slope and 40 cm diameter. Each pelletizing was carried out approximately for 30 min. The sizes of the pellets were being maintained at about 10 mm. The prepared green pellets were dried at 110 C. After this process, the pellets ready for reduction were obtained. The pellets reduced in Protherm PTF 15/50/250 tube furnace at 1400 C and 1450 C. The weight loss of pellets measured after reduction. Reduction rate was calculated according to weight loss. The weight loss in the pellets was assumed as the binder, volatile matter in the coke and oxygen in the magnetite concentrate. Thus, the reduction rate was calculated according to following formula; % Reduction Rate 3. RESULTS AND DISCUSSION Reduction behavior of the pellets was determined at 1400 C, 1450 C for 10, 20 and 30 min. The reduction rates were obtained depend on residence time and furnace temperatures as shown in Fig. 2 and Fig. 3.

4 Fig. 2 Relationship between reduction rate and reduction time at 1400 C It can be seen in Fig. 2 that reduction rate of each pellet increases with increasing reduction time at 1400 C. Reduction rate of the sample of %0 PET, which include only carbon as reductant, is higher than the other pellets in 10 and 20 min. With increasing ratio of waste PET in the pellets, kinetic of reduction decreases compared to the 100% coke up to 20 min. However, the reduction rates are close to each other when reduction time reached to 30 min. At the end of reduction, approximately 92% reduction rates are obtained for each pellet. Fig. 3 Relationship between reduction rate and reduction time at 1450 C Fig. 3 shows that reduction rate of 0% PET, 50% PET and 100% PET is respectively 99%, 97% and 90% in 10 min. at 1450 C. However, after 10 min., reduction rates decrease to 85-87% at the end of process. This tendency should be a result of re-oxidation at high temperatures for long residence time. Because of these reasons, the optimum reduction period at 1450 C can be chosen as 10 min. Thus, in 10 min., higher reduction rates can be obtained at 1450 C compared to at 1400 C for the composite pellets.

5 4. CONCLUSIONS Using of waste PET as an alternative reductant in iron making gives the following results; 1. The optimum conditions for reduction of composite pellet which include 100% PET wastes are 1450 C and 10 min. 2. Instead of the pellets which include 100% waste PET, the higher reduction rate obtained by the pellets which include waste PET and coke mixture. 3. Reduction of the composite pellets at 1450 C is the more efficient than at 1400 C. 4. The results show that, waste PET can be used as an alternative reductant instead of coke. 5. PET wastes can be recycled by this method, so the negative effects of PET wastes to environment can be diminished. REFERENCES [1] Saridede, M N, Plastic Waste Utilization in Blast Furnace, Metalurji, 138 (2004), (In Turkish) [2] Anabal, F. Y., To Make Profitable of PET Wastes in Industry, Gazi University, 2007, M.Sc Thesis [3] Indian Centre For Plastics in The Environment, 2006, Use of Plastics Waste in Blast Furnace, Management Of Plastics, Polymer Wastes And Bio Polymers And Impact of Plastics on The Eco-System, Volume 4, Issue 2 [4] Caı, J., Yu, G., Lıao H., Qıan K., Zhao P., He, Y., Disposal of Waste Plastics With Traditional Coking Process, Journal Of Iron And Stelz Research, Internatıonal. 2006, 13(1): [5] Achılıas D. S., and Karayannıdıs G. P., The Chemıcal Recycling of PET in The Framework of Sustainable Development, Water, Air, and Soil Pollution, 2004, Focus 4, [6] Fowkes N., The Reduction of Iron Ore Pellets Used In The Fastmet Process, IMA Journal of Mathematics Applied in Business & Industry (1999) 10, [7] Zhang C., Chen S., Miao X., Yuan H., Reduction Experiment of Iron Scale By Adding Waste Plastics, Journal of Environmental Sciences, 21 (1) (2009), [8] Demir, T., and Tincer, T., Preparation and Characterization of Polyethylene Terephthalate Powder-Filled High- Density Polyethylene in The Presence of Silane Coupling Agents, Journal of Applied Polymer Science, 79 (2001), [9] Baolin H., Haiying Z., Hongzhong L., and Qingshan Z., Study on Kinetics of Iron Oxide Reduction by Hydrogen, Chinese Journal of Chemical Engineering, 20(1) (2012) [10] Meissner S., Kobayashi I., Tanigaki Y. and Tacke K. H., Reduction and Melting Model of Carbon Composite Ore Pellets Ironmaking and Steelmaking, 2003 Vol. 30 No. 2, [11] Eloy A., Nogueira A., Mourao M. B., Effect of Slag Composition on Iron Nuggets Formation from Carbon Composite Pellets, Materials Research. 2010; 13(2): [12] Fortını, O. M., and Fruehan R. J., Rate of Reduction of Ore-Carbon Composites: Part I. Determination of Intrinsic Rate Constants, Metallurgıcal And Materıals Transactıons B Volume 36, 2005, [13] Kıkuchı S., Ito S., Kobayashı I., Tsuge O., Tokuda K., ITmk Process, Kobelco Technology Review No. 29, 2010, [14] Carvalho, R. J. D., Netto, P. G. Q., and D'abreu, J. C., Kınetıcs Of Reductıon of Composıte Pellets Containing Iron Ore and Carbon, Canadian Metallurgical Quarterly, Vol. 33, No.3, pp , 1994