EFFECT OF MANGANESE ADDITIVES ON THE KINETICS OF THE CRYSTALLIZATION PROCESS OF AMMONIUM SULFATE

I. N. Nozhkina, V. M. Vityugin

(Presented by the scientific seminar of the Department of General Chemical Technology)

Earlier studies [1] established the effect of soluble salts of \( \mathrm{Mn}^{+2} \), \( \mathrm{Cu}^{+2} \), \( \mathrm{Co}^{+2} \), \( \mathrm{Ni}^{+2} \), etc., on the crystallization process of ammonium sulfate. Depending on the type and concentration of the impurity cation in the solution, the quantitative yield of the solid phase, its granulometric composition, and the habit of the crystals change. In addition, the kinetics of crystallization, which reflects the regularities of the development of the process over time under the influence of the impurity, is of considerable interest for production.

As a microadditive to ammonium sulfate, the sulfate salt of manganese was used. Crystallization was carried out from a suspension (s:l = 1:30), whose liquid phase was a solution of ammonium sulfate saturated in 6% sulfuric acid and whose solid phase was crystalline ammonium sulfate with a particle size of \((-0.25 + +0.15\ \mathrm{mm})\). The process was carried out under isothermal conditions (\(t = 32^\circ\mathrm{C}\)) during the chemisorption of ammonia by sulfuric acid in a bubbling-type apparatus with a capacity of 0.5 l. The concentrations of ammonia, 0.05 g/l, and sulfuric acid, 6%, were kept constant throughout all experiments. The amount of impurity introduced into the solution was 0.1% of the amount of ammonium sulfate in the suspension. The crystalline product formed over a certain time was weighed after filtration and analyzed for granulometric composition.

Fig. 1. 1) Crystallization of ammonium sulfate in the presence of 0.1% manganese. 2) Crystallization without additive.

Fig. 2. 1) Crystallization of ammonium sulfate in the presence of 0.1% manganese. 2) Crystallization without additive.

The change in dispersion and in the yield of solid salt over the crystallization time in the presence of the impurity and without it is presented in Figs. 1 and 2. Analyzing the data obtained, it should be noted that the character of the dependence of the yield of crystalline product on time in both cases is rectilinear. The mathematical expression of this dependence has the form: \(y=a+bx\), where the intercept of the equation is equal to the segment cut off by the straight line on the ordinate axis and corresponds to the amount of the initial solid phase of ammonium sulfate in the suspension. However, in the case of crystallization with a manganese additive, the yield of the solid phase increases with time somewhat faster than without the impurity, which is evidently explained by dehydration of ammonium sulfate under the influence of the manganese ion, which has a greater degree of hydration than the ammonium ion. The heat of hydration for the \(\mathrm{NH}_4^+\) ion, in \(\frac{\text{kcal}}{\text{g-ion}}\), is 78; for \(\mathrm{Mn}^{+2}\), 449 [2].

In the presence of a manganese additive, a more intensive coarsening of the crystalline product with time is also observed than during crystallization without the impurity. The yield of the coarse fraction \(+0.25\ \text{mm}\) in the first case after 60 min was 91.0%; in the second, over the same time, 81.3%. The dependence of coarsening of the crystalline product on time is curvilinear in character and mathematically may be expressed by an equation of the form \(y=ax^b\). In logarithmic coordinates this dependence acquires a rectilinear character (Fig. 2).

According to our studies of EPR spectra, ammonium sulfate crystals contain isomorphously substituted manganese, which indicates the presence of an anomalous solid solution, the cause of whose formation is evidently the structural analogy of the macrocomponent and of the complex salt formed by the micro- and macrocomponents [3]. This explains the fact that the impurity, practically without changing the parameters of the crystal lattice of ammonium sulfate, changes the external habit of its crystals. Morphological studies showed that with increasing crystallization time the yield of the coarse fraction increases owing to the elongation of prismatic ammonium sulfate crystals, which are unstable to mechanical action and therefore are readily ground down. In the presence of the manganese impurity, however, crystals of a stable form are formed, more isometric and not prone to destruction.

Thus, as the investigations carried out have shown, the soluble manganese impurity accelerates both the crystallization process and the process of coarsening of ammonium sulfate with time.

References

1. I. N. Nozhkina, V. M. Vityugin. Effect of trace additions of salts of certain metals on the crystallization of ammonium sulfate in the saturator process. Izvestiya TPI, vol. 175, 1971.

2. K. P. Mishchenko, G. M. Poltoratskii. Problems of thermodynamics and the structure of aqueous and nonaqueous electrolyte solutions. “Khimiya,” 1968.

3. E. M. Ioffe. Studies in the field of anomalous mixed crystals. ZhNKh, vol. 3, issue 1, 1958.