## 3 Results and Discussion  
### Neutralisation Method  
#### 3.1.1 Laboratory Tests  
Chemical composition of AMD from the Robule Lake, (mg L\({}^{-1}\)): Al--209.8; Fe--322.6; Mg--893.1; Mn--90.8; Zn--12.8; Cu--34.7; Co--0.87; S--2376.2; Sr--1.7; Ni--0.413; As--0.004; Se--0.008 and Cd--0.039. Site measurement indicated: pH--2.87; ORP--793 mV and flow rate--3489 L min\({}^{-1}\). Concentration of Pb, Cr and Cs ions were below the sensitivity limit of used analytical method, (\(\upmu\)g L\({}^{-1}\)): Pb \(<\) 0.1; Cr \(<\) 0.5 and Cs \(<\) 0.1. The main point of AMD purification by the neutralization method using lime milk is the conversion of soluble metal forms into insoluble ones. The increased pH allows reactions between the metal ions present in the wastewater and hydroxide ions to form the following precipitates: Al(OH)\({}_{3}\); Fe(OH)\({}_{3}\); Co(OH)\({}_{2}\); Ni(OH)\({}_{2}\); Cu(OH)\({}_{2}\); Pb(OH)\({}_{2}\); Fe(OH)\({}_{2}\); Zn(OH)\({}_{2}\); Mg(OH)\({}_{2}\), etc. Table 3 shows the results of neutralization tests during the first neutralization step on different pH values: 3, 3.5 and 4. Based on the results for each element starting concentration and concentration after neutralization at different pH values (Table 3), a removal degree for each element was calculated. At pH 3.5 and pH 4 Fe removal degree had the values of 96.20 mass % and 99.70 mass %, respectively, while about 15 mass % of Fe was removed at pH 3. Those data revealed the maximum Fe conversion into the insoluble hydroxide form occurred at pH 4. Thus, it was confirmed that Fe could possibly be separated from AMD at pH 4 during the first neutralization step. Cu removal degree at different pH values is almost constant (8.99 mass %, 10.06 mass %, and 9.37 mass % for pH 3, 3.5, and 4, respectively). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Element**} & \multicolumn{4}{c}{**Neutralizer**} \\ \cline{2-7}
**Neutralization from** & **Fe** & **Mn** & **Cu** & **Zn** & **Cd** & **Co** & **Ni** & **2.5 Mass \% Line Milk** \\ \cline{2-7}
**pH Start Value of 2.78** & \multicolumn{4}{c}{**Concentration, mg L\({}^{-1}\)**} & \multicolumn{4}{c}{**Consumption, mL L\({}^{-1}\)**} \\ \hline start & 322.6 & 90.8 & 34.7 & 12.8 & 0.040 & 0.87 & 0.41 & - \\ to pH 3 & 274.2 & 62.9 & 31.6 & 12.2 & 0.041 & 0.86 & 0.42 & 3.5 \\ to pH 3.5 & 12.3 & 62.9 & 31.2 & 12.1 & 0.041 & 0.87 & 0.42 & 20.8 \\ to pH 4 & 1.0 & 62.7 & 31.5 & 12.0 & 0.041 & 0.86 & 0.42 & 25.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Concentration of some elements in AMD after the first neutralization step. Figure 3 shows a dependence of pH on Zn, Cd, and Co removal degree. Mn concentration was reduced during the neutralization at different pH values, and removal degree was about 30 mass % in any case. It was confirmed that a removal degree of Zn, Cd, and Co was between 1 and 6 mass % regardless of pH. These elements are originally minor component and generally have a higher solubility at around pH 4. Thus, it is considered that removal of these elements was caused by the co-precipitation with Fe hydroxides. Ni concentration was not changed during the neutralization on pH up to 4 (Table 3). Lime milk consumption is lower for 17 mass % during the AMD neutralization up to pH 3.5 in comparison with lime milk consumption during the AMD neutralization up to pH 4 (Table 3). Concentration of some elements after the second neutralization step of real AMD is presented in Table 4.