PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV

Vol. 175　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1971

THE EFFECT OF MICROADDITIONS OF SALTS OF CERTAIN METALS ON THE CRYSTALLIZATION OF AMMONIUM SULFATE IN THE SATURATOR PROCESS

I. N. NOZHKINA, V. M. VITYUGIN

(Submitted by the scientific summary conference of the Faculty of Chemical Technology)

The directives for the five-year plan for the development of the national economy for 1966–1970 provided that by the end of the five-year period fertilizers were to be produced only in granulated and noncaking forms. The caking of a product, as is known, is influenced to a considerable extent by its particle-size composition. However, ammonium sulfate, currently obtained by the saturator method at most coke-chemical plants, has a fine-crystalline structure. Thus, the content of the fraction smaller than 0.25 mm in ammonium sulfate from the Kemerovo Coke-Chemical Plant averages 34%, whereas according to the GOST standard scheduled for 1967, the content of this fraction must not exceed 8% for the higher grades of this product.

In addition to the fact that a fine-crystalline product cakes during storage, it is also easily blown out of the soil when applied as fertilizer. Therefore, in recent years considerable attention has been given in the production of this product to the problem of improving the particle-size composition of ammonium sulfate. The existing measures for coarsening ammonium sulfate, however, require definite modifications of the technological scheme and therefore have not found wide application.

The production of ammonium sulfate of good particle-size composition in a saturator is hindered by the following causes: a) the large amount of impurities present in the mother liquor; b) insufficient time for crystal growth in the saturator cycle; c) insufficiently intensive stirring of the suspension in the saturator.

The influence of all these causes can to a considerable extent be eliminated by introducing into the system microadditives in the form of soluble salts of a number of metals.

For each salt in solution there exists a metastable region, bounded on the solubility diagram by the solubility curve and the supersaturation curve, within which spontaneous formation of crystallization nuclei does not occur, and only the growth of existing crystals takes place. By expanding the metastable region, it is thereby possible to improve the particle-size composition of the crystallized product. The introduction of foreign ions into the system expands the metastable crystallization region [1].

It is known from the literature that in Japanese practice, for obtaining coarse-crystalline ammonium sulfate, the following has found application:

a method of enlarging crystals by introducing additions of chromium salts in an amount of 0.01–0.02% of the weight of ammonium sulfate [2, 3]. One of the methods of enlarging crystalline ammonium sulfate at English plants [4] is the use of aluminum salts as additives. Large and stable ammonium sulfate crystals are obtained when an aluminum salt is introduced into the system in an amount of 0.7% by weight of the product being crystallized. The effect of salts of a number of metals on the crystallization of ammonium sulfate from caprolactam liquors was studied by the Czech researchers Nyvlt and Vaclavů [5, 6].

In the present study, the effect of magnesium, chromium, aluminum, and iron salts on the particle-size distribution of ammonium sulfate during crystallization was determined. In addition, the task was set of selecting the most suitable form of the additive compound for introducing it into the production cycle of the saturator process.

Ammonium sulfate from the Kemerovo coke-chemical plant, with a particle size of less than 0.25 mm, was used as the starting raw material. The procedure for carrying out the experiments was as follows. At a temperature of 60°C, a suspension was prepared from a saturated solution of ammonium sulfate and its crystals with a S:L ratio of 1:30. The suspension was mixed by means of a mechanical stirrer at a speed of 90 rpm for two hours. The acidity of the entire mass was 6% and was close to the acidity of the plant mother liquors. The initial crystallization temperature was determined by the temperature regime of the saturator. Microadditives were introduced into the suspension in the form of a soluble salt of each metal. Upon completion of crystallization, the product was filtered, dried, and screened into fractions. Preliminary tests established the optimum crystallization time and stirring rate of the suspension. The ratio of liquid and solid phases was chosen taking into account the possible existence of such a ratio under production conditions. Crystallization of ammonium sulfate took place by cooling the suspension from an initial temperature of 60°C to 24°C. The results of the experiments are given in Table 1.

The studies showed that the introduction of additions of the salts MgCl₂, Cr₂(SO₄)₃, Al₂(SO₄)₃, Fe₂(SO₄)₃ not only improves the particle-size distribution of the crystalline product, but also increases its overall yield. The best results in enlarging ammonium sulfate were obtained with additions of magnesium and chromium salts.

Along with the fact that the additives introduced intensify the crystallization process of ammonium sulfate and improve its particle-size distribution, they substantially change the crystal form. During crystallization of ammonium sulfate in the presence of magnesium salts, strong crystals of tabular and prismatic form are formed, whereas the presence of chromium salts in the system leads to the formation of brittle ammonium sulfate crystals of needle-like form.

Taking into account the fact that chromium salts increase the yield of the total crystalline product, while the introduction of magnesium salts significantly improves its particle-size distribution and promotes the growth of stable crystals, tests were carried out on the use of both salts together as an additive in the crystallization of ammonium sulfate. One of the results of such a study is placed in Table 1.

Analyzing the results obtained, it should be noted that under production conditions it is most advisable to use magnesium chloride, as the most effective owing to its better solubility and its lower molecular weight in comparison with magnesium sulfate. Analytical studies showed that 25% of the additive introduced into the system remains in the cycle; therefore,

the implementation of a method for enlarging ammonium sulfate with the aid of an addition of magnesium chloride salt will not require large expenditures of the latter.

When ammonium sulfate is crystallized in the presence of 0.05% \(MgCl_2\) (calculated as the \(Mg\) ion from the total ammonium sulfate), the resulting crystalline product contains up to 78% of the coarse fraction (\(> 0.5\) mm). Under production conditions, however, such an amount of the coarse fraction is not required, and to obtain ammonium sulfate with 92% of crystals \(< 0.25\) mm, a consumption of 0.01–0.02% magnesium chloride will be sufficient. Thus, the introduction into the system of additives in the form of \(Mg\) and \(Cr\) salts will promote the production of higher grades of \((NH_4)_2SO_4\).

Conclusions

1. The presence of \(Mg\), \(Cr\), \(Al\), and \(Fe\) salts intensifies the crystallization process of ammonium sulfate and improves its particle-size distribution.

2. It is most expedient to use magnesium chloride salt as an additive in an amount of 0.01–0.02% by weight of the total ammonium sulfate, or jointly the salts \(MgCl_2\) and \(Cr_2(SO_4)_3\).

3. The magnesium chloride solution may be fed directly into the saturator cycle (into the circulating vessel or the reverse-current vessel).

4. The \(Mg\) and \(Cr\) salts introduced into the soil with ammonium sulfate may serve as microfertilizers for agricultural crops.

Literature

1. J. Mullin. Recent Advances in Crystallization (translation). Moscow, 1962.

2. Kokubo Makato, Sasaki Dzoti. Method for increasing the size of crystalline ammonium sulfate. RZhKh, 5L113, 1964.

3. Kokubo Makato, Sasaki Dzoti. Method for improving ammonium sulfate crystals. RZhKh, 5L112, 1964.

4. Mitchel J. Crystallization of ammonium sulfate. RZhKh, 7K104, 1961.

5. Nývlt Jaroslav, Václavu Vladimir. Determination of the parameters of ammonium sulfate crystallization. RZhKh, 14K107, 1962.

6. Nývlt Jaroslav. Effect of impurities on the crystallization of ammonium sulfate. RZhKh, 15L10, 1963.