## 4 Discussion
This could be why PE efficiency in the zinc tuning 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/CO\({}_{2}\)[28]. 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 + CO\({}_{2}\) = 2 CO), as shown in Figure 8b. 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. Figure 8: Schematic illustration of possible phenomena by plastic materials injection: with **(a)** PE and **(b)** PUR in the fuming furnace, unreacted particles will react at the top of the slag bath or in the combustion shaft and participate in steam generation; bubbles and particles recirculate by the motion of the slag. Although this study focused on the utilization of plastic materials in the zinc tuning 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 Br\({}_{2}\), 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.