The production of electrical and electronic equipment (EEE) is one of the fastest-growing sections of human activities [1
]. With the expansion of the social economy, the demand for electronic products has rapidly increased [2
]. During the 1960s, an average family possessed only a few electric devices: typically a radio, television, refrigerator, vacuum cleaner, and washing machine. Today, however, the number of electronic household appliances has greatly increased. This fast growth has led to the fast obsolescence and substitution of electronic devices. Thus, as the growth of EEE increases, the generation of waste electric and electronic equipment (WEEE) increases as well [1
]. WEEE mainly consists of ferrous and nonferrous metals (such as Cu), and a large fraction of WEEE consists of plastic materials (Figure 1
The global estimation of the amount of WEEE is in the range of 50 million tons annually [3
]. While in 2016 about 45 million tons of WEEE were generated globally, this will reach about 52.2 million tons in 2021 [4
]. The composition of WEEE is largely dependent on the type and age of the discarded equipment [5
]. In Europe, the total quantity of plastics used for the production of electric and electronic products is 3 million tons per year. In total, 1.2 million tons of mixed WEEE plastics arise from the separated collection of WEEE in Europe per year [6
Managing WEEE is important from both an environmental and an economic perspective. The processing of WEEE involves removing hazardous parts and shredding magnetic and eddy current separators to retrieve ferrous and non-ferrous metals. The remaining material contains a high fraction of plastics, in addition to some glass, ceramic, and metals (Figure 2
). This heterogeneous fraction, which is called shredder residue material (SRM), is the most problematic fraction from WEEE.
The main plastic contents of SRM are as follows: acrylonitrile butadiene styrene (ABS), used in the casings of phones, microwave and flat screens; polystyrene (PS), used in refrigerators and small household appliances; and polypropylene (PP), used in components inside washing machines and dishwashers, and in the casings of small household appliances. A smaller fraction of plastics is polycarbonate (PC), which is used in housings for information and communication technology equipment (ICT); epoxy polymers are mainly used in printed circuit boards (PCB) [7
The current options to process SRM are deposition in a landfill and incineration [8
]. The options for treating SRM are limited. Another option to treat SRM is pyrolysis, which includes heating the material in the absence of oxygen. Pyrolysis turns the material into a liquid fraction, which can be used as fuel, and a solid fraction, which is ash and can be recycled [9
]. The mechanical recycling of plastic waste is hindered by three main problems: the presence of hazardous substances, the degradation level of the polymer, and the miscibility of the plastics within WEEE. Hazardous substances include lead (Pb), which is used as a plastic stabilizer; cadmium (Cd), which is used as a pigment; and brominated flame retardants (BFR) used to prevent the products from burning. The use of these substances in new products is now restricted, but they can still be found in large amounts in certain waste streams. [7
] Incineration as a method to recover the energy content has been a common practice for Sweden as it provides inexpensive district heating, but it is no longer carried out. Toxic compounds such as dioxins can be released from incinerators, which could be due to the high concentration of antimony and copper present in WEEE fractions, which favors their production [10
One other possible option is the utilization of SRM in metallurgical processes. Some metallurgical processes deal with the production of metals from oxide ores (MeO), where the oxygen is removed through reaction with reductants such as coal or coke. Plastic material contains carbon and hydrogen, which might have the potential to substitute for coal in metallurgical processes, which require reducing conditions [11
]. In addition to production from primary sources like ore, metallurgical processes also deal with secondary sources, such as slag, which is the byproduct of several metallurgical processes. Slag usually contains a significant amount of metal oxides. One method to recover the metals from slag is a fuming process, which involves treating the molten slag by the injection of a reducing agent into the slag bath. During the slag-fuming process, the oxides of zinc and lead are reduced by the injection of a coal‒air mixture into the bath [12
Through the utilization of SRM in nonferrous metallurgy, the plastic fraction can be used for reduction and energy supply. On the other hand, the metal content can be recycled through the processes and returned to the production of EEE. There are, however, very few studies that have examined the utilization of plastic as an alternative reductant [13
]. The possibility of utilizing plastic material as a substitute for the current reductant and energy source has been studied in a full-scale trial at Umicore. These trials were performed with WEEE materials, which contained a high plastic content, with only coke and fuel oil as references. Energy balance calculations revealed that approximately 2/3 of the energy content of the WEEE plastics replaced the consumption of coke and fuel oil. The results showed that approximately 1/3 of the injected WEEE was used to increase the steam production [16
The aim of this study was to study the potential of plastic-containing materials, especially SRM, as a reductant in metallurgical processes, where a zinc-fuming process was used as a case study. Three plastic-containing materials, namely polyethylene (PE), polyurethane (PUR), and SRM, were used. The first step was to compare plastic-containing materials to coal, particularly in terms of thermal conversion characterization. After a fundamental study at the laboratory scale, plastic materials were tested at the industrial scale through injection to the zinc fuming process at the Boliden Rönnskär smelter.
The conversion characteristics and the industrial trial was discussed and the results from the conversion study are used in this section to understand the behavior of plastic materials in the process. Table 4
summarizes the experiments and summarizes the most important findings of the experiments.
The results of the industrial trials indicate the potential of plastic materials as reducing agents. As these materials are injected alongside air to the furnace, they will go through thermal conversion. Conversion under oxidizing atmosphere has shown that these materials have different characterizations, which possibly leads to different interaction in the furnace. No studies have been found on the interaction of plastic materials in the zinc fuming process; however, the interaction of coal has been studied. In this study, the gas injected into the slag bath pushes the slag away and a plume is formed. Slag moves upward and falls back to the opposite direction of the gas injection. Slag starts to circulate, and new slag comes into contact with the gas plume [22
]. Injected particles are carried away by the high-velocity gas jet. Some of the injected particles combust in the plume and generate bubbles, which get trapped in the slag bath and a foam is generated. Degerstedt [23
] studied the injection phenomena in the zinc fuming furnace in Boliden Rönnskär by tuyere back-pressure measurement. Bubbles were reported to be formed, which later dispersed into small bubbles as they moved upward. Richards et al. [24
], who conducted a comprehensive study on the zinc fuming process, also reported that the air‒coal mixture discharges into the bath as a continuous series of bubbles, which, owing to the low modified Froude number, rise directly up the furnace wall.
Richards et al. [24
] also mentioned that particles required to have a certain velocity and a minimum particle size managed to break through the surface tension and penetrate the slag. They assumed that the coal particles travelled at 50–80% of injected gas velocity; thus, particles larger than 0.7 µm have enough momentum to penetrate into the slag. Huda et al. [25
] used a computational fluid dynamic (CFD) model to describe the same zinc fuming furnace that Richards studied. Their result also indicated that a fraction of coal penetrated the slag; however, there is debate about the role of penetrated particles. While Richards et al. suggested that the zinc fuming process was essentially controlled by the amount of coal that can be entrained in the slag, Huda et al. reported that the entrained coal particles had less influence on the overall fuming. Huda et al.’s simulation further predicted that a major portion of the injected coal (50%) was combusted in the tuyere gas column. Finally, it is possible that a fraction of the injected particles bypassed the process and went to the combustion shaft. A study of the fluid flow phenomena in the zinc fuming furnace at Boliden Rönnskär also confirmed that some of the coal bypassed the process and entered the post-combustion, as coal particles were captured in the gas phase over the slag bath [26
All of these phenomena possibly occur in the process; however, their extent depends on the furnace, slag properties, the injection system, etc. During most of the batches in the industrial trial, 90 tons of slag was charged, which fills the furnace up to 1.2 m. Due to the turbulence caused by the gas injection and the foam generated by combustion of coal, the height of the slag bath increased. The height of the bath can oscillate by several meters during the process and can periodically splash a smaller amount to the top of the furnace (8 m). Some fractions of the injected particles combust in the plume, where the extent of the combustion depends on the conversion characteristic of the particles. The product of conversion consists of CO/CO2 and H2/H2O, and it could also include volatiles. Reduction in the slag bath occurs when the oxides come into contact with the reducing gases CO/H2 in the bubbles, i.e., at the interface of the slag/gas phase. The agitation caused by the sloshing and splashing of the slag accelerates the contact between the slag and gas phase. Unreacted particles and gases generated would either fall back into the slag bath due to the flow circulation or leave the slag bath, combust in the combustion shaft, and participate in steam generation.
The extent of conversion in the slag bath depends on the conversion time of particles. Timothy et al. [27
] studied the total conversion time for single bituminous coal particles with particle sizes of 38–45 µm and reported that the total conversion time was 20–40 ms. All the plastic materials studied showed longer conversion times compared to the reported times for the single coal particles. Thus, the extent of conversion of the studied plastic materials at these particle sizes is possibly lower than that of coal during the process. The conversion tests indicated that plastics with smaller particle sizes have a shorter conversion time, so a decrease in particle size would possibly improve the conversion of plastic materials.
PE particles had the shortest conversion time; thus, it is expected that upon injection, PE would convert more compared to other plastics. Figure 8
a shows the expected phenomena occurring by injection of PE. It was assumed that the injection of plastic materials would not affect the behavior of coal in the process. PE and coal particles flow upwards due to the gas, and as they ascend in the gas plume, they can go through combustion. Bubbles and unreacted particles could fall back into the slag through recirculation. The gases that are generated through conversion are more likely to consist of CO/CO2
, while the gases that are produced through the devolatilization of PE materials are mainly hydrocarbons (Cn
]. It has been reported that when methane is used as a reductant, it needs to be externally cracked at high temperatures with oxygen-enriched preheated air to produce a reducing mixture. The slow breaking of carbon‒hydrogen bonds can be the rate-limiting step in the zinc fuming process [29
]. Thus, it is possible that, with the slow decomposition of hydrocarbons, the residence time of bubbles may not be enough to complete the cracking of hydrocarbons to participate in reduction reactions in the slag bath. This could be why PE efficiency in the zinc fuming furnace was not as expected.
PUR has the longest conversion time among the studied plastic materials. PUR was injected in extruded form and broke into different lengths during injection. Smaller particles were expected to have a shorter conversion time, while the larger particles took a longer time for conversion. The released volatiles however, consisted of fewer hydrocarbons due to the presence of a high amount of O in the structure, so some of the volatiles were in the form of CO/CO2
]. Furthermore, PUR has an aromatic structure and could form a char during conversion. Once a particle was entrained in the slag, in addition to volatiles, the C in the char could produce CO by the Boudouard reaction (C + CO2
= 2CO), as shown in Figure 8
b. Finally, a fraction of PUR reached the top of the furnace unreacted or partially reacted; here it oxidizes and participates in steam generation.
Finally, SRM consists of different types of plastic materials that consequently have different conversion times and different conversion characteristics. Conversion studies showed that some the SRM only decompose by devolatilization, while others can also produce char, which could further participate in reactions in the slag bath. SRM consists of different particle sizes, just as it consists of materials with different grindability. Thus, while the conversion of smaller particles might be completed, some fractions could pass through the slag bath unreacted. A schematic illustration of the possible interaction mechanisms of SRM in the process is shown in Figure 9
. The variety of the conversion characteristics and times observed for SRM improves its potential as a reducing agent.
Although this study focused on the utilization of plastic materials in the zinc fuming process, it is possible to utilize them in other metallurgical processes as well. While implementing the plastic materials as a substitute for the conventional reducing agents, such as coal, it must be noted that the most important difference is the number of volatiles developed. The process needs to be designed in such a way that it can utilize the developed volatiles. On the other hand, one of the limitations of utilizing SRM is the ash content, which could contaminate the final product. One of the limitations in this study was that the composition of gas phase was not analyzed. Combustion of SRM could lead to the release of gases such as Cl, which could lead to corrosion in the furnace. Furthermore, to implement the injection of plastic materials, the emission of different gases, such as Br2, Sb, and NOx (from the nitrogen from PUR), should be investigated.
SRM is a complex, heterogeneous material, and its current methods of treatment (deposition or incineration) are not only a waste of resources, but also an environmental hazard. By utilizing this material in metallurgical processes, the plastic fraction could help with reduction and the metal fraction can be recycled and returned to production, which is a step towards a circular economy.