PILOT-SCALE TESTING OF THE EFFECT OF MAGNESIUM AND MANGANESE MICROADDITIVES ON THE PROCESS OF AMMONIUM SULFATE CRYSTAL ENLARGEMENT IN A SATURATOR*

I. N. Nozhkina, L. M. Zhitkova, V. M. Vittogin

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

The size of ammonium sulfate crystals is one of the most important indicators of its quality. Under the new GOST, first-grade ammonium sulfate must contain no less than 70% of crystals larger than 0.25 mm. However, at many coke-chemical plants, and in particular at the Kemerovo plant, the production of first-grade ammonium sulfate is hampered by overloading of the saturators.

In the period from 1966 to 1970, a joint team of the Department of General Chemical Technology of the Tomsk Polytechnic Institute and the Kemerovo Coke-Chemical Plant carried out studies aimed at developing a simple technology, accessible for production, for enlarging ammonium sulfate in the saturator process. In selecting a method for enlarging ammonium sulfate crystals, primary attention was given to the method of microadditives, since it does not require substantial changes in the technological scheme or operating regime of existing production and is not associated with significant capital expenditures or operating costs. In addition, the choice of method was influenced by the fact that, in recent years, fertilizers with microadditives—stimulators of plant growth—have been used increasingly widely in world agricultural practice [3].

Microelements introduced into the soil together with the main fertilizer not only increase the resistance of plants, entering into their composition, but also, by being present in the soil, contribute to the development of microorganisms beneficial to plant life. Among those vitally important for

* The industrial tests were carried out with the participation of employees of the Kemerovo Coke-Chemical Plant: L. I. Zlatina, V. E. Smolyakov, A. A. Shtein, A. V. Kuzebanova, and G. I. Trondina.

Among the microelements required by plants are, in particular, divalent ions of manganese, magnesium, nickel, copper, and others.

The theoretical foundations of the activating effect of microadditions of ions of the listed metals on the growth of ammonium sulfate crystals under saturator-process conditions have not yet been sufficiently studied. It is known that crystal growth occurs in the region bounded by the solubility curve and the supersaturation curve. The wider this region, the greater the possibility for crystal growth. It has been established [1] that the change in the solubility of ammonium sulfate is 0.09% per \(1^\circ\mathrm{C}\). Crystallization of ammonium sulfate begins at a temperature \(3\text{–}4^\circ\mathrm{C}\) lower than that corresponding to the state of saturation. The limiting supersaturation of the solution is about 0.3%. This already small region of crystal growth is reduced in the presence of industrial impurities in the saturator solution. At the same time, it is known [2] that introducing small amounts of divalent-metal ions, including magnesium, manganese, nickel, and cobalt ions, into the mother liquor leads to a change in the critical supersaturation of the solution, promoting crystal growth.

At the first stage of the investigations, we tested under laboratory conditions a large number of different microadditions of metal salts, mainly in the form of soluble sulfates. These experiments [4,5] established that, under saturator-process conditions, microadditions of nickel, manganese, cobalt, and magnesium sulfates show the greatest activity. Taking into account the relatively high cost and scarcity of nickel and cobalt salts, magnesium and manganese salts were used for subsequent experiments under laboratory and industrial conditions.

At the stage of laboratory investigations, the magnesium addition was introduced in the form of a magnesium sulfate solution, while manganese was introduced both in the form of a solution of the pure salt of manganese sulfate and in the form of a solution of partially reduced pyrolusite in sulfuric acid and manganese-containing alkalis from the production of purpurin from alizarin (a waste product of an aniline-dye plant, ADP). To prepare the model saturator brine, crystalline ammonium sulfate from the Kemerovo coke-chemical plant, with a particle size of less than 0.25 mm, was used. The results of the investigations, presented in Table 1, showed that when a reagent-grade manganese salt was used as the addition in an amount of 0.2%, the yield of the coarse fraction increased by 18%; when a solution of partially reduced pyrolusite in sulfuric acid was used,

Table I

Effect of \(Mg^{2+}\) and \(Mn^{2+}\) ions on the sieve composition of ammonium sulfate

acid was 28%. Analysis of the mother liquor and of the solid product showed that approximately 5% of the additive introduced passes into the crystals. The manganese content in ammonium sulfate averaged 90 mg/kg.

For conducting the initial production tests, the available magnesium chloride hexahydrate crystalline hydrate and manganese sulfate pentahydrate were used. Sampling was carried out in the sulfate department of the Kemerovo Coke-Chemical Plant. Additions of magnesium and manganese salts were introduced into the saturator through the return-liquor ladle periodically, every 30 minutes, until the optimum concentration of magnesium and manganese ions in the solution was reached. Subsequent additions were fed with the aim of maintaining a constant optimum concentration of these ions.

The continuously maintained concentration of additives, calculated as magnesium ion, was 0.5%, and as manganese ion, 0.2% of the amount of solid ammonium sulfate in the saturator. Corrective additions after reaching the normal concentration were, for magnesium, 0.05%, and for manganese, 0.005%. Before being fed into the saturator, the magnesium and manganese salts were dissolved in mother liquor. The size analysis of ammonium sulfate crystals was carried out after each hour of operation. Ammonium sulfate for granulometric analysis was sampled from the belt conveyor after the centrifuge or after the dryer. The technological operating conditions of the saturators before the experiments and during the tests were practically not changed. The temperature in the saturators was maintained within 56–62°C, and the acidity was 5–6%. The duration of the tests with the magnesium chloride additive was 6 hours, and with the manganese sulfate additive, 20 hours. As follows from the results of the short-term tests, introduction of additives into the saturator process ensures production of grade I ammonium sulfate. Thus, when a manganese salt was used as the additive, the yield of the coarse fraction increased from 52.8% to 81.7% (average value over 14 hours of testing). However, in order to recommend this method of enlarging ammonium sulfate for introduction into production, longer tests were needed. It was necessary to establish the effectiveness of the additive on the crystallization of ammonium sulfate in the presence of temporary deviations from the normal operating conditions of the saturator department. Such tests were carried out at the Kemerovo Coke-Chemical Plant in November–December 1969 according to a procedure analogous to the previous tests. The results of the month-long tests are presented in Table 2.

Table 2

Results of industrial tests

The data from the pilot-scale tests (Table 2) unquestionably indicate an improvement in the particle-size distribution of ammonium sulfate crystallized in the presence of a manganese salt. The effective action of the additive on the crystallization process of ammonium sulfate appeared on the 4th day after manganese was introduced into the cycle; when the manganese concentration in the solution reached 0.0002%, its content in the crystalline product was 0.007%. For saturator No. 4, the yield of the +0.25 mm fraction averaged 36.46% over two shifts before the additive was introduced. Throughout the entire test period, the yield of the coarse fraction fluctuated within the range of 63–76%. For saturator No. 1, the yield of the +0.25 mm fraction before the optimum manganese concentration in the solution was reached averaged 57.5%; during crystallization with the optimum additive it was 72–88%.

Thus, the prolonged pilot-scale tests carried out showed that, under shop operating conditions and in the presence of temporary deviations from the normal operating regime of the saturators, the addition of manganese helps improve the particle-size distribution of ammonium sulfate.

The advisability of combining the proposed method with other methods of intensifying the crystallization process of ammonium sulfate is evident. It is hardly expedient to base ammonium sulfate production on additions of magnesium or manganese in the form of pure salts. The raw material for obtaining the magnesium additive may be magnesite concentrate or calcined magnesia. Before being introduced into the saturator, these materials can be relatively easily converted into magnesium sulfate by treating them with mother liquor or sulfuric acid. The manganese additive is most rationally prepared from pyrolusite concentrate reduced to manganese oxide. Pyrolusite can be reduced in coke ovens, in steel containers of the type usually used for box coking. It is also expedient to use magnesium and manganese sulfates, which are by-product wastes from other industries, in the sulfuric-acid leaching of ash-bearing rocks in sulfuric acid production.

References

1. D. S. Petrenko. Production of Ammonium Sulfate. Moscow, 1966.
2. R. Kern. Crystal growth and adsorption. In: Crystal Growth, Vol. 8, “Nauka,” 1968.
3. M. V. Katalymov. Microelements and Microfertilizers. Moscow–Leningrad, 1965.

1. V. M. Vitogin, I. N. Nozhkina. Purification and crystallization of ammonium sulfate. Collected reports of the 5th All-Union Conference on the Technology of Inorganic Substances. Kiev, 1966.

2. V. M. Vitogin, I. N. Nozhkina, O. A. Fuks. Purification and crystallization of ammonium sulfate. Proceedings of the 1st Scientific Conference of the Tomsk Branch of the Mendeleev All-Union Chemical Society. Tomsk University Press, 1969.