## 3 Results and Discussion
### Recrystallization of Alum for the Removal of Impurities  
Another aim of this work, as noted above, was to study the stage of dissolution and purification of ammonium alum (stage 6 in Figure 2) in order to try to carry out their deep purification from iron by relatively simple methods. One such method is recrystallization. The initial alum (portion of alum with high content of impurities used to make their behavior more noticeable) was redissolved in hot water to which various additives were added, as described in Section 2, and recrystallization was carried out when the solutions were cooled to room temperature. Additives used include: sodium thiosulfate, aluminum grain, and phosphoric acid. Tables 4 and 5 show the data obtained after such experiments. As can be seen from the presented data, sodium thiosulfate is the best agent that allows for a relatively deep purification of alum from iron during their recrystallization. Probably, this property of thiosulfate is associated with its ability to reduce a significant part of ferric iron to ferrous iron in a solution of aluminum ammonium alum. This leads to a decrease in iron ammonium alum impurities which can co-crystallize with the main product. The control experiment was carried out with TS-alum by the same procedure. According to the X-ray fluorescence spectra shown in Figure 8, it is clearly seen that, compared with the original alum, the recrystallization product is significantly cleaner in terms of the content of iron impurities. The entire amount of these impurities accumulates in the residual solution after recrystallization, which is also clearly seen from the EDXRF spectra for the sample obtained by drying such a solution. Table 6 shows quantitative characteristics of the content of various impurities for the listed samples, obtained using the data of analyses of their solutions with the ICP-AES instrument. The content of iron in recrystallized aluminum alum is less than 0.01% by weight. In terms of alumina, the iron oxide impurity would be 0.06% instead of 0.8, i.e., 12-fold iron removal is observed. It is obvious that, at the subsequent stages of separation of the technological process for obtaining aluminum hydroxide shown in Figure 2, the proportion of iron impurities will decrease. However, even without taking this option into account, a multiple reduction in the content of impurity iron in alum by means of their recrystallization in the presence of sodium thiosulfate makes it possible to fundamentally reduce the cost of final deep purification from iron by such expensive methods as iron sorption on chelating sorbents or extraction with special organic reagents. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **Al** & **Fe** & **K** & **Na** & **P** & **S** \\ \hline Initial alum & 5.75 & 0.40 & 0.51 & 0.58 & 0.03 & \\  & & Crystallization products & & & & \\ No additives & 5.71 & 0.12 & 0.10 & 0.00 & 0.00 & 12.18 \\ 1\% Na\({}_{2}\)S\({}_{2}\)O\({}_{3}\cdot\)2H\({}_{2}\)O & 5.94 & 0.01 & 0.11 & 0.02 & 0.00 & 10.97 \\ Al & 5.72 & 0.11 & 0.11 & 0.01 & 0.00 & 10.68 \\ 6.3\% H\({}_{3}\)PO\({}_{4}\) & 5.92 & 0.01 & 0.08 & 0.01 & 0.29 & 10.92 \\
0.27\%H\({}_{3}\)PO\({}_{4}\) & 5.87 & 0.04 & 0.10 & 0.00 & 0.01 & 12.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Composition of alum (mass %) after the recrystallization experiments. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Additive** & **Al** & **Fe** & **K** & **Na** & **P** &
\begin{tabular}{c} _AlFe in alum_ \\ _AlFe in solution_ \\ \end{tabular} \\ \hline No additives & 23.03 & 2.17 & 2.27 & 0.05 & & 11 \\
1\% Na\({}_{2}\)S\({}_{2}\)O\({}_{3}\cdot\)2H\({}_{2}\)O & 28.92 & 0.07 & 1.79 & 0.08 & & 402 \\ Al & 27.84 & 1.67 & 1.92 & 0.09 & & 17 \\
6.3\% H\({}_{3}\)PO\({}_{4}\) & 6.25 & 0.08 & 0.63 & 0.04 & 0.08 & 75 \\
0.27\% H\({}_{3}\)PO\({}_{4}\) & 9.69 & 0.26 & 6.78 & 0.15 & 0.10 & 37 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Distribution coefficients for the components in solid and liquid phases in equilibrium.