EFFECT OF ALKALI METAL HYDROXIDES ON THE GRANULATION OF ACTIVE MAGNESIUM OXIDE

I. N. Lantsman, P. D. Khalfina, V. M. Vityugin, N. F. Stas,
M. S. Lantsman, Z. A. Gafarova

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

To obtain sorbents based on metal oxides, methods of precipitating hydroxides and carbonates in an aqueous medium in the form of amorphous gels are most often used; adsorption-active oxides are obtained from these gels by thermal decomposition. However, such a technological process is associated with difficulties in producing large batches of sorbents, difficulties connected with filtration of the gels and their washing.

There are data indicating that magnesium oxide active in the adsorption of acid gases can be obtained by thermal decomposition of commercial basic magnesium carbonate, \( \mathrm{Mg(OH)_2 \cdot MgCO_3} \), with preliminary introduction of alkalis [1].

In our work, the effect of alkali metal hydroxides on the molding process and on the technological and adsorption properties of magnesium carbonate granules was investigated. Basic magnesium carbonate of grade “ch” has the following characteristics: maximum capillary moisture capacity—232%, minimum capillary moisture capacity—80%, maximum hygroscopicity—65.1%, and the caking coefficient is 0.098, i.e., this powder belongs to very weakly caking materials.

Table 1

Effect of the quality and quantity of introduced alkalis on the properties of magnesium carbonate granules

The molding of the prepared charges was carried out on a laboratory pan granulator with a pan diameter of 300 mm, a pan inclination angle of \(40^\circ\), and a rotation speed of 30 rpm; the granulation time was 10 min. The strength of the finished granules was determined by compression between two plates, one of which was the pan of a balance, until the first crack appeared. The sieve composition of the granules was determined by sieving through screens, considering the 1–3 mm fraction suitable. Sodium, potassium, lithium, rubidium, and cesium hydroxides were used as additives.

The first stage of the work

the influence of the quality and quantity of the alkalis introduced on the mechanical properties of the resulting magnesium carbonate granules (Table 1). The hydroxides were introduced in the form of 20% solutions.

The data obtained show that the introduction of all the alkalis selected for study in amounts up to 20% leads to substantial strengthening of the dry granules. The maximum strength is exhibited by granules obtained with the introduction of 20% KOH. The maximum yield of the suitable fraction of granules 1–3 mm in diameter, with sufficient strength (1.5 kg/gr), was obtained with the introduction of 20% KOH. Thus, for all the alkalis introduced, from the point of view of the strength characteristics of the granules, the optimum additive is 20% of these hydroxides. When the amount of alkali introduced is increased, the strength of the resulting granules decreases.

In the course of the work it was noted that the strength of the granules depends on the concentration of the alkali being introduced. To clarify this question, a series of experiments was carried out, the results of which are summarized in Table 2.

The optimum concentration selected was 30%.

Table 2

Effect of the concentration of the solution of the alkali introduced on the properties of granules when alkali is added in an amount of 20% relative to dry magnesium carbonate

For the final decision on the choice of the alkali to be introduced, the adsorption properties of the granules were determined on a static weight apparatus based on McBain balances. Decomposition was carried out under nonisothermal conditions at a heating rate of 10–15° per minute in a vacuum of $10^{-3}$–$10^{-4}$ mm Hg. Heating was carried out to 350°; at this temperature the sample was held to constant weight. After decomposition was completed, dry carbon dioxide was passed through the granules at a rate of 1 l/min for 2 hours; then the sample was regenerated and moist carbon dioxide was also passed through for 2 hours. The results of the adsorption tests of the granules obtained are given in Table 3.

In terms of adsorption properties, the best are granules with the addition of 20% KOH.

The investigations carried out revealed the plasticizing action of alkalis in the wet state and their strengthening action during drying of the granules, as a result of the formation of a crystallization structure in the pores of the granule. However, because the binder formed may itself participate in adsorption–desorption processes, it is necessary in the future to study the action of additives that plasticize but are indifferent to the adsorption of acid gases.

Table 3

Adsorption properties of granules based on magnesium carbonate with the addition of 20% hydroxides

LITERATURE

1. L. V. Colombo, E. C. Mils. U.S. Patent No. 3557011, class 252—189, of 19.I.71.