## 3 Results and Discussion
### Characterization of the Obtained Sludge  
This study was carried out to purify the AMD generated in the area of copper mining activities in the southeast of Serbia. It is also taking the recycling of metals in the AMD into consideration. In this case, Fe and Cu are main components in the AMD. The possibility for re-use of these metals was investigated. From the results of the laboratory test and semi-industrial plant operation, the two-step neutralization process through pH 4 and 7 could remove Fe, Cu, Zn, Co, Ni, and Cd from the AMD water effectively. It was also confirmed Fe and Cu could mainly be recovered at pH 4 and 7, respectively. Table 6 shows a chemical composition of the obtained sludge due to the two-step neutralization process. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Element Mass \%**} & **First Neutralization Step** & **Second Neutralization Step** \\  & **(pH Initial\(-\)pH 4)** & **(pH 4\(-\)pH 7)** \\ \hline Al & 2.73 & 7.13 \\ Cu & 0.15 & 1.24 \\ Fe & 33.52 & 0.47 \\ Ca & 3.07 & 11.77 \\ Mg & 0.015 & 2.79 \\ Mn & 0.26 & 1.67 \\ Zn & 0.013 & 0.50 \\ As & 0.0024 & 0.0014 \\ Cd & 0.000018 & 0.00174 \\ Co & 0.000885 & 0.0374 \\ Ni & 0.000651 & 0.0189 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Chemical composition of the sludge obtained in the two-step neutralization process. Figure 7: Change in Mn concentration before and after the adsorption removal test using 5 g of chitosan. During the second neutralization step, elements concentration reducing have the tendency: Cu > Zn > Co > Ni > Cd > Mn. Manganese concentration has a higher value than maximum allowed value according to the Serbian legislation [24]. The results of the sludge chemical characterization are presented in Table 6.  
The obtained sludge at pH 4 (1 st neutralization step) contained 33.5 mass % of Fe (Table 6). This result indicates that this sludge was mainly iron hydroxide. This supports Fe concentration in the real AMD sample drastically decreased up to pH 4. Especially, the obtained sludge at pH 7 (2 nd neutralization step) mainly contained 1.2 mass % of Cu (Table 6). This Cu grade of sludge is higher than that of the copper ore which has been currently mined. It is expected that the obtained sludge including Cu will be applied as a raw material for Cu metal through some operations such as the acid leaching, solvent extraction and electrochemical methods, and applied in the existing Copper Smelter in Company Serbia Zijin Bor Copper. Based on the requirements of the steel processing plants, sludge obtained on pH 4 will be direct used or used after the appropriate treatment. Consequently, it can be assumed that a combination process including neutralization and adsorption methods is applied to the AMD generated from some copper mines in the southeast Serbia. Figure 8 shows a schematic illustration of the assumed AMD treatment process for metal removal and recovery. The two-step neutralization process with pH 4 and 7 is applied to the real AMD with pH value of less than 3. Through this operation, Fe and Cu are removed and recovered separately. For instance, it is suggested that recovered Fe sludge is available for a raw material for an inorganic flocculant such as ferric polysulfate. The recovered Cu sludge through some treatments will finally contribute to Cu metal production. The treated AMD still contains most of the Mn\({}^{2+}\) ions which has a higher value than the maximum allowed value according to the Serbian legislation. In this case, there are two options for Mn removal. One option is Application of the 3 rd step neutralization to the treated AMD. However, the pH of treated wastewater should be reduced again below the effluent standard. It is also expected that adsorption method using chitosan is applied for Mn removal. This method will be operated below the pH value for the effluent standard. Some improvements may be required for the application of adsorption method to the actual removal of Mn.