## 4 Discussion  
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 \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Fundamental Study: Thermal Conversion**} & \multirow{2}{*}{**Industrial Trial: Reduction Potential**} \\  & **Characterization** & \\ \hline Experiment & Conversion & Co-injection of plastic materials + coal \\ \hline Equipment & Drop tube furnace-Single particle burner & Zinc fuming furnace \\ \hline Particle size & 1–6 mm & Less than 10 mm \\ \hline Material tested & PE, PUR, SRM & Coal, PE, PUR, SRM \\ \hline \multirow{4}{*}{Highlights} & PE has the shortest conversion time (only volatiles, as hydrocarbon), PUR has the longest conversion time (both volatiles and char), SRM shows a mixture of these behaviors. & It is possible to lower the coal injection rate and substitute it with plastic injection; steam generation increased, which suggested that part of the plastic goes to gas phase. \\ \cline{1-1}  & The conversion characteristic helps in understanding the plastic materials in the fuming process; parameters such as conversion time and product plays an important role in the efficiency of plastic materials. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of experiments and highlights of results. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reductant** & **Coal** & **Coal and PE** & **Coal and PUR** & **Coal and SRM** \\ \hline Average rate (wt \%/min) & 0.087 & 0.064 & 0.079 & 0.063 \\ Standard deviation & 0.017 & 0.005 & 0.009 & 0.003 \\ Average rate (wt \%/min)/Molar content of (C + H\({}_{2}\)) & 0.0096 & 0.0060 & 0.0064 & 0.0056 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average Zn reduction rates and standard deviations. 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. [PERSON] [23] studied the injection phenomena in the zinc fuming furnace in Boliden Ronnskar by tuyere back-pressure measurement. Bubbles were reported to be formed, which later dispersed into small bubbles as they moved upward. [PERSON] 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. [PERSON] 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 \(\upmu\)m have enough momentum to penetrate into the slag. [PERSON] et al. [25] used a computational fluid dynamic (CFD) model to describe the same zinc fuming furnace that [PERSON] studied. Their result also indicated that a fraction of coal penetrated the slag; however, there is debate about the role of penetrated particles.