## 3 Results and Discussion  
### Features of the Obtaining Process of MgO from Serpentinite  
The advantage of the bisulfate method of serpentinite reprocessing provides the possibility of creating a closed-loop technological process, where all reagents are recovered. This process was first proposed in [22]. In contrast to the complex chemical formulas used in this work, we introduced simplifications that make it possible to better understand the discussed cyclic process. If we present the formula of serpentinite in the form of a ratio of the main macrocomponents, oxides of magnesium, silicon, and iron, in accordance with the composition presented in Table 1, then the process of decomposition of serpentinite with ammonium bisulfate can be presented as reaction (5). \[ \begin{array}{l}3\ \text{MgO}\times 2\ \text{SiO}_{2}\times 0.04\ \text{Fe}_{3}\text{O}_{4}+6.4\ \text{NH}_{4}\text{HSO}_{4}\to 2\ \text{SiO}_{2}\times\text{H}_{2}\text{O}\downarrow+3\ \text{Mg}(\text{NH}_{4})_{2}( \text{SO}_{4})_{2}+\\ 0.08\ \text{FeNH}_{4}(\text{SO}_{4})_{2}+0.08\ \text{Fe}(\text{NH}_{4})_{2}(\text{SO}_{4})_{2}+ \text{H}_{2}\text{O}+0.08\ (\text{NH}_{4})_{2}\text{SO}_{4}\end{array}  \tag{5}\]  
In reaction (5), in contrast to [22], it was taken into account that in concentrated solutions containing sulfate and ammonium, Mg (II) sulfate and Fe (II) and Fe (III) sulfates form double salts with ammonium sulfate [23, 24]. From the mixture obtained in accordance with reaction (5), it is possible to separate the silicic acid phase (as well as the undecomposed part of serpentinite, which takes place under the usually used real conditions), but this separation is possible only at elevated temperature, since the solubility of double magnesium sulfate and ammonium decreases sharply with temperature decreasing. Therefore, the separation in the work presented by us is carried out at temperatures above 90 \({}^{\circ}\)C. In this case, a simple cooling operation allows most of the magnesium to be removed from the system in the form of Boussingaultite, reaction (6). \[ \begin{array}{l}3\ \text{Mg}(\text{NH}_{4})_{2}(\text{SO}_{4})_{2}+18\ \text{H}_{2}\text{O}\to 3\ \text{Mg}(\text{NH}_{4})_{2}(\text{SO}_{4})_{2}\times 6\text{H}_{2}\text{O} \downarrow\end{array}  \tag{6}\]  
This process, similar to recrystallization, significantly reduces the problems of further purification of the magnesium product. However, it should be taken into account that all Tutton salts (double sulfates formed by two and singly charged cations) with a monoclinic crystal structure are capable of cocrystallization; therefore, [PERSON]'s salt Fe (Fe(NH\({}_{4}\))\({}_{2}\)(SO\({}_{4}\))\({}_{2}\times 6\)H\({}_{2}\)O), like a similar manganese salt, can pollute the Boussingaultite. That is why at the stage of purification of a magnesium compound not an ammonia solution is used, but a mixture of an ammonia solution and ammonium carbonate, since it is impossible to achieve quantitative precipitation of manganese hydroxide at pH 8. Within the framework of the processes presented for macrocomponents, the purification of double magnesium and ammonium sulfate from iron occurs in the following simple way, reaction (7). \[ \begin{array}{l}0.08\ \text{Fe}(\text{NH}_{4})_{2}(\text{SO}_{4})_{2}+0.16\ \text{NH}_{4}\text{OH}+0.04\ \text{H}_{2}\text{O}+0.02\ \text{O}_{2}\to 0.08\ \text{Fe}(\text{OH})_{3}\downarrow+0.16\ (\text{NH}_{4})_{2}\text{SO}_{4}\end{array}  \tag{7}\]  
The Fe (III) double salt (ferric ammonium album) predominantly remains in the filtrate after the Boussingaultite is separated. The filtrate is purified from iron by the reaction (8). \[ \begin{array}{l}0.08\ \text{Fe}\text{NH}_{4}(\text{SO}_{4})_{2}+0.24\ \text{NH}_{4}\text{OH}\to 0.08\ \text{Fe}(\text{OH})_{3}\downarrow+0.16\ (\text{NH}_{4})_{2}\text{SO}_{4}\end{array}  \tag{8}\]  
A very important problem is the problem of magnesium precipitation from double ammonium magnesium sulfate.