**4. Conclusions and outlook**

The study shows the dependency of the rate of recovery on the condition in an arbitrary manner. The recovery rate has a different approach with each parameter. The results show that the maximum percentage of metals recovered at 80°C, 0.05 mm thickness, 3 hours of contacting time, 80 rpm shaking speed, and PCB sample concentration of 0.5 g/ml�<sup>1</sup> .Under this condition, the resultwas obtained with 97.59% of copper, 96.29% of lead, 94.66% of tin, and 96.64% of zinc, respectively. It is the most effective recovery condition for this experiment. However, targeted extraction of a particular metal can be made possible by varying a particular parameter only. In such a way, the percentage recovery of that particular metal can be increased with negligible loss. In such extractions, the other heavy metals are retained in the sample or less amounts of other metals are separated. Disposal of such residuals also causes environmental issues. Since this type of extraction is promoted in order to reduce the environmental problems caused by these kinds of heavy metals, the targeted extraction of a particular metal is not advisable, even if it is economically profitable. It was concluded that the combination of aqua regia (HCI and HNO3 and HCI and H2SO4) leaching is an effective and economic way for the recovery of copper from leached solution. According to studies, modifying the dissolving of metal ions in the reagents increases the rate of leaching, but this raises the total cost and results in the introduction of additional chemicals into the atmosphere. As a result, attempts will be made in the future to resolve these issues. Only a few field trials have been performed, and more systematic studies are needed to decide the best conditions for using aqueregia as a leaching medium.
