On a Method for Selecting the Most Effective Additives to a Solution in the Growth of Technically Valuable Crystals¹

S. A. Stroitelev

It is known that, in growing technically valuable crystals, it is very often necessary to add co-soluble impurities to the solution. The latter, by altering the course of crystallization, help improve the quality of the crystals. Thus, for example, without the use of special additives in aqueous solution it is not possible to grow large, high-quality crystals of NaCl, KCl, and other alkali halides. Single crystals of the indicated salts are usually grown from the melt, but even after prolonged annealing and gradual cooling they are considerably more defective than crystals grown from aqueous solutions with the use of special impurities. Without the use of special additives it is not possible to grow sufficiently large, high-quality crystals of the sulfate salts of zinc, magnesium, copper, potassium, and others. The method of obtaining high-quality modified metals and alloys is based on the use of special additives to the melt (V. D. Kuznetsov, 1953).

Thus, special additives to a melt or solution are used rather widely in practice for the purpose of improving the quality of the crystalline phase, but until now the selection of the most effective impurities has been entirely empirical and therefore extremely labor-intensive.

Our studies aimed at elucidating the physicochemical nature of the influence of impurities on the crystallization of salts make it possible to draw certain conclusions for developing a method of selecting the most effective additive, one that promotes an improvement in crystal quality (1958). These conclusions are based on the results of many experiments, of which we consider it necessary to present here only the following.

In the experiments, magnesium sulfate salt \((\mathrm{MgSO_4 \cdot 7H_2O})\) was taken as the substance under investigation. A slightly supersaturated solution of this salt was poured in portions of 40 \(\mathrm{cm^3}\) into 15 identical crystallizers. Into each crystallizer there was added the same molecular amount (0.0025 g/mol) of one of the sulfate salts of Al, Cr, Co, Zn, Ni, Fe, Cu, Mn, Na, K, \(\mathrm{NH_4}\), potassium chromic alum, iron-ammonium alum, and borax. In one of the solutions an additional 0.0025 g/mol of \(\mathrm{MgSO_4 \cdot 7H_2O}\) was dissolved. This solution was considered the control. The differences introduced by the unequal number of molecules of crystallization water in the additives were equalized by—

then poured into the corresponding crystallizers. Into all the solutions there was simultaneously introduced a small crystal of epsomite, in order to induce crystallization. The crystallizers were kept under otherwise identical conditions, so that the solutions differed from one another only in the amount of impurities. After three days had elapsed, we separated the grown crystals from the solution and, after drying them with filter paper, weighed them on an analytical balance. The weighing results are summarized in Table 1.

Table 1

Crystallization of epsomite ($\mathrm{MgSO_4\cdot 7H_2O}$) with the participation of an impurity added to the solution in the form of sulfate salts of different cations.

Comparison of the data in Table 1 shows that different impurities have unequal effects on the separation of the salt from solution. All cations that have a higher ionic potential²) and a greater hydration energy than those of the magnesium cation probably promote dehydration of Mg and displace $\mathrm{MgSO_4\cdot 7H_2O}$ from solution. Such cations include: $\mathrm{Al^{\cdots}}$; $\mathrm{Zn^{\cdot\cdot}}$; Co, $\mathrm{Cr^{\cdots}}$, $\mathrm{Ni^{\cdot\cdot}}$. Impurities possessing a lower ionic potential and a lower hydration energy ($\mathrm{NH_4^{\cdot}}$, $\mathrm{K^{\cdot}}$, $\mathrm{Na^{\cdot}}$, $\mathrm{Mn^{\cdot\cdot}}$) promote retention of the salt in solution. Consequently, the results of the experiment make it possible to speak of a change in supersaturation and solubility of the salt under

¹) The value of the hydration energy of $\mathrm{NH_4}$ is given according to K. P. Mishchenko; that of the other cations, according to V. A. Mikhailov and S. I. Drakin.

²) According to Cartledge, the ionic potential is the ratio of the valence of a cation to its radius.

by the action of the co-soluble additive. From Table 1 it is also seen that the size and quality of the crystals increase the more, the larger the amount of \(\mathrm{MgSO_4\cdot 7H_2O}\) that remains in the liquid phase after crystallization. Therefore, judging from the crystallization results, the increase in the solubility of salts can be used as one of the criteria for evaluating the effectiveness of the influence of impurities on crystal quality. Of the additives tested, borax proved to be the best impurity for the \(\mathrm{MgSO_4}\) solution; in its presence the crystals formed were of the highest quality and the largest.

Naturally, for the growth of large, high-quality crystals it is necessary to select the optimal concentration of the impurity. For this purpose a second experiment was carried out, which consisted in adding quantitatively different amounts of borax to four separate portions, \(500\ \mathrm{cm^3}\) each, of a \(\mathrm{MgSO_4\cdot 7H_2O}\) solution supersaturated with 24 g of salt per \(100\ \mathrm{cm^3}\).

During crystallization, owing to the release of heat, the temperature of the solution did not remain constant. At first it rose from the initial temperature \((23^\circ\mathrm{C})\), then, after reaching a maximum, again fell to \(23^\circ\). Systematic observations were made of the change in temperature and the growth of the crystals. The results of the experiment are summarized in Table 2.

Table 2

Crystallization of \(\mathrm{MgSO_4\cdot 7H_2O}\) in the presence of different concentrations of borax in the solution

As can be seen from the data in Table 2, the presence of borax in the solution affected crystallization in a highly varied way. The higher the concentration of borax in the solution, the lower the maximum crystallization temperature, the smaller the weight of the crystalline phase, and the longer the period of increase in the solution temperature. The weight of the crystalline phase changes especially strongly. According to the results of the experiment, it was found that 10 g of borax retained 93 g of \(\mathrm{MgSO_4\cdot 7H_2O}\) in solution. The largest crystals and those of the best quality proved to be the crystals grown at a borax concentration of 1.100 per \(100\ \mathrm{cm^3}\).

It is also necessary to select the optimal supersaturation of the solution for a given crystallization temperature. The very design of the exploratory experiment in this case differs in principle in no way from that set forth above, with the only difference that in each portion of solutions of the salt under investigation, of different supersaturation, the same additive of an effective impurity is introduced. From the quality of the crystals that separate out, one can judge the required optimal supersaturation at the time of…

at a given crystallization temperature and concentration of the effective impurity. Without describing the experiment itself, we note that, at a borax concentration in the solution equal to 1.100 g per 100 cm³, in order to preserve good crystal quality, supersaturation of the solution within 15–20 g of \(\mathrm{MgSO_4 \cdot 7H_2O}\) per 100 cm³ is permissible if the crystallization temperature is within the range 20–25°C. Greater supersaturation, in order to maintain a sufficient growth rate and good crystal quality, requires an increase in the concentration of the impurity.

The method described for selecting an effective impurity for a solution was tested and applied by K. M. Kevroleva (1956) in growing crystals of \(\mathrm{MgSO_4 \cdot 7H_2O}\), \(\mathrm{ZnSO_4 \cdot 7H_2O}\), \(\mathrm{NiSO_4 \cdot 7H_2O}\), \(\mathrm{CuSO_4 \cdot 5H_2O}\), and others. It was established that the effective impurity in growing crystals of the sulfates of Mg, Zn, and Ni from aqueous solution is borax, while in growing crystals of copper sulfate it is glycerin.

The author, together with V. A. Ermolaev, established that an impurity of zinc sulfate has a favorable effect on the growth of potassium sulfate crystals.

The foregoing made it possible to expect that any impurity affecting crystal growth should cause a change in the crystallization process as a whole and, in particular, a change in the supersaturation and solubility of the salt. These expectations were confirmed also in the case of the crystallization of \(\mathrm{NH_4Cl}\) and \(\mathrm{KCl}\). As I. Ya. Melik-Gaikazyan has recently established, \(\mathrm{PbCl_2}\) and \(\mathrm{ZnCl_2}\) increase the solubility of \(\mathrm{KCl}\), while pectin increases the solubility of ammonium chloride in water.

References

1. Kuznetsov V. D. Crystals and Crystallization, State Publishing House of Technical-Theoretical Literature, Moscow, 1953.

2. Kevroleva K. M. Electrical Strength and Certain Properties of Crystalline Hydrates. Dissertation, 1956.

3. Mikhailov V. A. and Drakin I. S. Calculation of the hydration energy of cations. Zhurnal fizicheskoi khimii, 29, 12, 1955.

4. Mishchenko K. P. Solvation of ions in electrolyte solutions. Zhurnal fizicheskoi khimii, 26, 12, 1952.

5. Stroitelev S. A. Investigation of the influence of impurities added to a solution on the crystallization of certain sulfate salts. Scientific Notes of Tomsk State University (in press).

1. Report presented at the Scientific Conference on Dielectrics and Semiconductors on February 6, 1958, in Tomsk. ↩