## 4 Discussion
While [PERSON] et al. suggested that the zinc fuming process was essentially controlled by the amount of coal that can be entrained in the slag, [PERSON] et al. reported that the entrained coal particles had less influence on the overall fuming. [PERSON] 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 Ronnskar 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/CO\({}_{2}\) and H\({}_{2}\)/H\({}_{2}\)O, and it could also include volatiles. Reduction in the slag bath occurs when the oxides come into contact with the reducing gases CO/H\({}_{2}\) 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. [PERSON] et al. [27] studied the total conversion time for single bituminous coal particles with particle sizes of 38-45 \(\upmu\)m 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 8a 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/CO\({}_{2}\), while the gases that are produced through the devolatilization of PE materials are mainly hydrocarbons (C\({}_{n}\)H\({}_{m}\)) [28]. 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 tuning 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.