Efficient utilization of waste plastics as raw material for metallic iron and syngas production by combining heat treatment pulverization and direct reduction
Introduction
Plastics, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly (vinyl chloride) (PVC), have been used in modern life in large quantity due to its suitable physical and chemical properties as well as low production cost. These plastics are widely used in many important everyday applications such as clothing, household appliances, automotive products, electrical and electronic equipment, and even aerospace. In fact, more and more plastics has been produced and consumed in recent years (Lopez et al., 2018; Hahladakis et al., 2018; Silveira et al., 2018). For example, China has become one of largest plastics production and consumption markets in the world and has consumed more than 80 million tons plastics per year, only 30 % of which can be recycled. Large quantities of plastics will be discarded and associated with other municipal solid waste.
However, waste plastics will cause serious environmental problems, such as contaminating earth, groundwater, and marine environments as well as emission of greenhouse gases, such as CO2 and CH4 (Campbell et al., 2000), and formation of microplastics (Andrady, 2017). In addition, the inadequate management of waste plastics leads to the loss of valuable natural resources, as the plastics are produced by nonrenewable petroleum and natural gas (Lopez et al., 2018). Recycling and effective utilization of waste plastics other than landfill is an urgent issue. Lots of methods have been tried for the disposal of waste plastics, such as reuse, recycling and thermochemical conversion. Despite the environmental benefits from reuse and recycling, the amount of waste plastics for reuse is very small and recycled polymer possibly suffers from inferior quality (Hong and Chen, 2017). Thermochemical conversion roughly includes combustion, pyrolysis, and gasification, which can efficiently recycle energy, fuels and chemicals from waste plastics, and has aroused increasing concern by global researchers (Lopez et al., 2018; Cooke et al., 2003; Onwudili and Williams, 2016; Song et al., 2019).
Due to the waste plastics contains high of carbon and hydrogen, some researchers and steel companies have utilized it as the heat supply and reducing agent for iron ore reduction in place of coal and coke to some extent by several well developed technological paths (Murakami and Kasai, 2011; Sahajwalla et al., 2009; Asanuma et al., 2000; Babich et al., 2016; Kim et al., 2002; Collin et al., 1997), which are similar to thermochemical conversion regime and can realize the efficient waste recycling in large quantity, smaller pollution risk, CO2 emission mitigation and production cost reduction. However, only 3 % of waste plastics can be used in the ironmaking process as a raw material, either in the production of coke or by direct injection into the raceway of a blast furnace (Murakami and Kasai, 2011). Therefore, some researchers in metallurgical field extend their attention to the utilization of waste plastics as the reducing agent in the iron ore/carbon composite pellet reduction to produce metallic iron as well as recycling H2 and CO gases. The composite pellet technology has lower requirements for the physical and chemical properties of reducing agent. Then, the waste plastics can be more easily utilized.
Generally, pulverization of raw materials is a critical initial step for most of the industrial reaction system and it can improve the transportation, reaction rate and composition homogeneity of the raw materials. However, all the published papers on waste plastics utilization in composite pellet merely used the commercial powdery plastics without consideration of the pulverization of waste plastics, which may hinder the industrial application of the technology. In this paper, the authors focus on a novel method for the pulverization of waste plastics by low temperature heat treatment of mixed pulverized coal and waste plastics (Li et al., 2007; Asanuma et al., 2014; Murai et al., 2015), which is more easily realized at industrial scale and is designed for the blast furnace injection processing technology. In this paper, we try to utilize the pulverized heated mixture as the reducing agent of composite pellet for the production of metallic iron and syngas in a flexible and well developed direct reduction facility, such as rotary hearth furnace (McClelland and Metius, 2003), for the first time.
The changing of waste plastics pulverization method and physical property of the newly obtained reducing agent may obviously influence the reduction of the composite pellet, which will be systematically investigated at laboratory scale for the first time. The results obtained in the present work can provide reference for design of a more efficient and massive processing technology incorporating ironmaking. The flow sheet of proposed technology can be schematically illustrated in Fig. 1. The waste plastic film is cut into pieces, mixed with pulverized coal, and then heated in an inert atmosphere. The heated material is ground to obtain a suitable particle size, mixed with iron ore fine, and pelletized. The obtained carbon-containing pellets are heated in a reduction device, and the iron ore is reduced by the pulverized heated mixture, thereby obtaining the metallized pellet, which mainly consist of metallic iron, and the flue gas rich in CO and H2. Metallized pellets can be used as the charge for steelmaking, and the flue gas can be used for heating or chemical synthesis after filtration.
Section snippets
Experimental
This paper will mainly focus on the thermoplastics, especially those most often fabricated into disposable items which are discarded in short time after being purchased. A wasted polyethylene (PE) thin film for packaging was used and cut into square pieces of 20 mm length. In order to understand its behavior during the heating process, the decomposition of the PE was conducted by Thermogravimetry and Differential Scanning Calorimetry (TG-DSC) with a heating rate of 10 °C /min under the protection
Properties of heated coal-waste plastics mixture
The fine coal powders will adhere to the surface of the PE pieces after mixing together of the two materials, whose morphology is shown in Fig. 3. This is an automatic phenomenon occurs in the natural world, which is essentially caused by the interaction force and energy between fine particle and solid surface. Particle locating inside the solid is in an equilibrium state due to offset of all the forces from the surrounding particles. However, the particles locating in the solid surface layer
Conclusions
(1)The fine coal powder can adhere to the surface of PE film plastic by mixing. The mixture can be easily pulverized by heating treatment at the temperature above 250 °C for 20 min. The heated mixture has better reactivity with CO2 than original coal due to the addition of plastics.
(2)The reduction rate of composite pellets with different reducing agents increases with the temperature increasing. The heated mixture pellet has smaller reduction starting temperature and bigger reduction rate than
Notes
The authors declare no competing financial interest.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by the National Natural Science Foundation of China (No.51804024) and the Fundamental Research Funds for the Central Universities (No.FRF-TP-18-008A2).
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