## 4 Conclusions  
A direct current plasma-chemical reactor (PChR) was designed and experimentally tested for solid waste treatment. After experimental investigations to determine process efficiency and quality, it was found that plasma-chemical reactors are practical and have easy-to-use control for regulating temperature, production quantity, and time. In this study, an air plasma torch with a power of 50 kW was developed and used for industrial zeolite and clay waste treatment. The electrical and energy characteristics of the plasma torch and plasma-chemical reactor were investigated. After the plasma treatment, the selected waste was converted into an environmentally friendly melt with much lower volume levels, by approximately 10-12 times, compared to the initial material. The investigation results showed that the behavior of the DC arc burning in plasma-chemical reactors with ambient air is very complex. The voltage-current characteristics in the air flow were measured in the arc current range of \(I\) = 50-200 A and at different free arc lengths. The VCCs depend on the arc-burning regime (laminar or turbulent); the arc power increases with the arc current and length. The outflow gas temperature decreases significantly compared with the gas temperature in the volume of the plasma-chemical reactor. At arc lengths greater than 100 mm and arc currents less than 150 A, a significant part of the energy is transferred by convection. At an axial distance of up to 30 mm, the temperature profiles rise in shape and depend on the operating time of the plasma torch. When the axial distance exceeds 30 mm, the reactor's wall temperature stabilizes, and the exhaust gas temperature depends mainly on the arc current. The proportion of energy transferred by electrons or forced convection to the anode depended on the arc length. The results obtained from the plasma-chemical reactor showed that the ambient temperature in the reactor's entire volume is stable. The reactor prototype could be used in solid, hardly melting waste treatment processes.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline Q\({}_{\text{L}}\) & Q\({}_{\text{P}}\) & Q\({}_{\text{R}}\) & Q\({}_{\text{P}}\) + Q\({}_{\text{R}}\) & Q\({}_{\text{P}}\) + Q\({}_{\text{R}}\) & Time, & Q\({}_{\text{R}}\) & Amount of Melt, & Specific Heat Flux q, \\
**kW** & **kW** & **kW** & **kW** & **kW** & **s** & **kWh** & **kg** & **kWh/(kg-s)** \\ \hline \multirow{2}{*}{37.1} & \multirow{2}{*}{6.3} & \multicolumn{3}{c}{Clay} & \multirow{2}{*}{21.6} & \multirow{2}{*}{0.53} & \multirow{2}{*}{21.52} & \multirow{2}{*}{6.1} & \multirow{2}{*}{3.53} \\  & & & & & & & & \\ \multirow{2}{*}{40.0} & \multirow{2}{*}{6.8} & \multirow{2}{*}{10.8} & \multirow{2}{*}{17.6} & \multirow{2}{*}{32.4} & \multirow{2}{*}{0.67} & \multirow{2}{*}{26.8} & \multirow{2}{*}{5.2} & \multirow{2}{*}{5.15} \\ \hline \end{tabular}
\end{table}
Table 3: Energy consumption based on the amount of feedstock.  
**Author Contributions:** Methodology, V.V.; software, V.G. and R.U.; validation, D.G., R.U., and Z.K.; formal analysis, R.U. and V.G.; investigation, V.V., R.K., and R.U.; resources, R.U.; data curation, R.U., V.G., D.G., Z.K. and R.K.; writing--original draft preparation, V.V., R.K., and R.U.; writing--review and editing, V.V. and V.G.; visualization, V.G. and R.U.; supervision, V.V.; project administration, V.V. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This project has received funding from the Research Council of Lithuania (LMTLT), agreement No. P-LU-22-66.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.