On the Selection of Modifiers for the Crystallization Process of Ammonium Sulfate
Abstract Generated abstract
This paper examines the selection of soluble salt modifiers for enlarging ammonium sulfate crystals in the saturator process of coke-chemical production. Using concepts of ionic force field and impurity effects on surface energy and crystal habit, the authors test sulfate salts of several metal cations during polythermal crystallization and evaluate product quality by the yield of crystals larger than 0.25 mm. The results indicate that coarsening improves only up to an optimum range of ion force-field values, since stronger deformation effects can change the crystal habit and increase dispersity. Laboratory and industrial tests identify manganese, magnesium, copper, cobalt, and nickel ions as effective modifiers, with magnesium and manganese increasing the coarse fraction by 15 to 20 percent in plant trials.
Full Text
ON THE SELECTION OF MODIFIERS FOR THE AMMONIUM SULFATE CRYSTALLIZATION PROCESS
I. N. Nozhkina, V. M. Vityugin
(Submitted by the scientific seminar of the Department of GCT)
The problem of improving the conditions for growing coarse-crystalline ammonium sulfate in the saturator process of coke-chemical production is currently being solved mainly by the use of technical methods (stirring, etc.). At a number of plants, in mass crystallization, the introduction of modifiers—soluble salts—is practiced for the purpose of enlarging the crystals of the solid salt [2]. The widespread application of this method is hindered by its weak theoretical development. The selection of appropriate additives and the determination of their effective concentration in solution are reduced to investigation by the method of successive approximations.
As is known, the nature of the influence of impurities is determined simultaneously by several factors. The chemical affinity of an impurity with the crystallized substance promotes the formation of complex and double salts [1]. The ability of an impurity to be adsorbed by the surface of a crystal leads to a selective change in the growth rate of the faces of the crystal [3]. In general, however, the mechanism of the influence of an impurity depends on the nature of its ions; therefore the magnitude of the force field of an ion may serve as an indicator in the selection of modifiers for enlarging crystalline products.
A quantity that approximately characterizes the force field of an ion is the generalized moment \(m\) [4], whose numerical value is determined chiefly by two properties of the molecule: its mean electric moment, the charge of the ions, the dipole moment of the molecule, and their sizes. For ionic and metallic liquids, the generalized moment is equal to
\[ m = \frac{e^2}{r}, \]
where
\(e\) is the ion charge,
\(r\) is the ion radius.
According to Semenchenko’s molecular theory of surface phenomena [4], the greater the radius of the generalized moment of the main substance and of the impurity for melts of salts and metals, the stronger the influence of the impurity on crystal size. If \(M\) of the impurity molecules is greater than \(M\) of the main substance, the dispersity of the crystalline precipitate decreases; if it is smaller, it increases. The nature of this effect is expressed in the change in the surface energy of the crystal under the influence of the impurity. When an impurity that reduces the free surface energy of the crystal is introduced, the size of the latter decreases. The increase in the degree of dispersity grows in parallel with the surface activity of the given impurity, which, in turn, increases with a decrease in the charge of the impurity ion and an increase in the magnitude of its radius.
The relationship between the effectiveness of the action of an impurity and the nature of its ions has also been established for the crystallization of salts from aqueous solutions [5]. It is manifested in a change in the habit of the crystal under the action of the impurity, depending on the magnitude of the force field of its ion. The action of ions of a soluble impurity in this case reduces to increasing the rate of orientation of nuclei, which leads to a decrease in aggregation and ultimately to the formation of more ordered structures. According to Tillmans’ data [5], the effect of impurities is the stronger, the greater the magnitude of the force field of their ions. This quantity is characterized by the author as the ionic potential and the capacity for deformation of the crystal:
\[ F=\frac{z^e}{r^2}, \]
where \(r\) is the ion radius,
\(z^e\) is the ion charge.
According to the propositions set forth above, the dispersity of crystalline ammonium sulfate should decrease when salts are used as impurities whose molecular force-field magnitude is higher than the force-field magnitude of \((NH_4)_2SO_4\). To verify this proposition, sulfate salts of \(Fe^{3+}\), \(Mg^{2+}\), \(Ni^{2+}\), \(Co^{2+}\), \(Cu^{2+}\), \(Mn^{2+}\), \(K^+\), \(Na^+\), and \(Cr^{3+}\) were studied. In addition, the choice of additives was governed by the advisability of using them in agriculture as micronutrient fertilizers. Crystallization of ammonium sulfate in the presence of 0.2% additives was carried out under polythermal conditions by cooling a suspension of an ammonium sulfate solution saturated at \(60^\circ C\) and its solid salt from 65 to \(24^\circ C\). Crystallization was conducted under conditions of constant stirring with a mechanical stirrer at a speed of 120 rpm for two hours.
Table I
Yield of the coarse fraction and shape of ammonium sulfate crystals as a function of the magnitude of the cation force field
| Additive ion | $NH_4^+$ | $K^+$ | $Na^+$ | $Cd^+$ | $Mn^{2+}$ | $Cu^{2+}$ | $Co^{2+}$ | $Ni^{2+}$ | $Mg^{2+}$ | $Fe^{3+}$ | $Cr^{3+}$ |
|---|---|---|---|---|---|---|---|---|---|---|---|
| $10^2$ | — | 3.58 | 4.87 | 9.25 | 10.55 | — | 11.45 | — | 12.17 | — | — |
| $10^6$ | 3.65 | 3.80 | 4.92 | 9.0 | 11.5 | 12.54 | 14.2 | 15.17 | 15.7 | 31.9 | 33.9 |
| Yield of the $+0.25$ mm fraction | 84.1 | 88.2 | 91.4 | 97.7 | 96.6 | 99.4 | 98.1 | 96.2 | 92.9 | 83.5 | 85.8 |
| Crystal shape | Flat elongated | Transition to prismatic form | Transition to prismatic form | Transition to prismatic form | Transition to prismatic form | Transition to prismatic form | Transition to prismatic form | Transition to prismatic form | Transition to prismatic-bipyramidal | Transition to prismatic-bipyramidal | Transition to prismatic-bipyramidal |
The index characterizing the quality of the solid product in terms of particle-size composition was the yield of crystals larger than 0.25 mm. The experimental results are presented in Table I.
The effect of impurities on the coarsening process of ammonium sulfate increases only up to a certain element—copper. This is explained by the fact that, in this case, the impurity has a dual effect on the dispersion of ammonium sulfate. On the one hand, as the magnitude of the force field of the impurity ion increases, the dispersion of the crystalline product of the ion decreases; on the other hand, the increasing ability of the impurity ion to deform leads to a change in the habit of the crystal of the principal salt. For ammonium sulfate this is manifested in the transition of flat, broad-tabular crystals into prismatic, prismatic-bipyramidal, and acicular ones. Depending on the form assumed by the crystal under the action of the impurity, the size of the crystalline product may increase or decrease. For ammonium sulfate, the formation of the prismatic-bipyramidal crystal form under the action of \(Cr^{3+}\) and \(Fe^{3+}\) ions is accompanied by enrichment of the crystal with new faces and by an increase in the dispersion of the solid salt.
The impurity ions with an optimum value of the ion force field have the greatest potential for improving the particle-size composition of ammonium sulfate. As studies of a wide range of impurities have shown, these include the ions \(Mn^{2+}\), \(Mg^{2+}\), \(Cu^{2+}\), \(Co^{2+}\), and \(Ni^{2+}\). The sulfuric-acid salts of these ions were tested by us as impurities in the isothermal crystallization of ammonium sulfate under laboratory conditions while maintaining a regime close to that of the saturator. The yield of the coarse fraction increased by 12–16%. The effectiveness of the action of impurities selected with allowance for the magnitude of the ion force field on the coarsening process of ammonium sulfate was confirmed by industrial tests carried out at the Kemerovo Coke-Chemical Plant. Under the influence of magnesium and manganese impurities, the content of the coarse fraction (\(+0.25\) mm) in the solid product increased during the tests by 15–20%.
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