DEVELOPMENT OF A METHODOLOGY FOR ASSESSING THE AGGLOMERABILITY OF FINELY GROUND MATERIALS

A. S. BOGMA, V. M. VITOGIN

(Presented at the scientific final conference of the Faculty of Chemical Technology)

Despite the increased importance and introduction into industrial production of the aggregation of finely ground materials by the granulation method, there is still no generally accepted methodology for assessing their agglomerability. It has merely been accepted as one of the basic conditions that solid fine-grained materials acquire sufficient agglomerability for granulation when they contain 60–80% of grains smaller than 60 microns [1].

The agglomerability of materials determines the degree of their spontaneous aggregation when wetted with liquid and the strength of the lumps that form. Consequently, the efficiency of granulation depends to a considerable extent on the moisture content of the charge being granulated. For the granulation process to proceed satisfactorily, a definite relationship must be maintained between the values of moisture content and the moisture capacity of the materials being granulated; this relationship determines the index of the optimum moisture content of the charge being granulated.

According to investigations by Yuzhgiprocement [2], the optimum moisture content of a material in the granulation process is close to the indices of maximum molecular moisture capacity. However, such a conclusion does not fully encompass the assessment of the lump-formation process, since the maximum molecular moisture capacity determines only the strength of raw granules. Under real conditions, in order to accelerate the lump-formation process, considerably more moisture is introduced into the material than corresponds to the value of the maximum molecular moisture capacity. For the rapid formation of granule nuclei, the moistening of the material is brought close to the value of its capillary moisture capacity. Thus, the assessment of the agglomerability of fine-grained materials must be carried out with account taken of the indices of maximum molecular and capillary moisture capacities.

The principal influence on the maximum molecular and capillary moisture capacities of materials, apart from their nature, is exerted by the shape of the grains, their size, and the particle-size distribution, i.e., the specific surface area of the material and the packing density of its grains in a layer. In this case, the value of the specific surface area of the material will determine the maximum molecular moisture capacity, while the sieve composition will determine the capillary moisture capacity. With finer grinding, the number of possible contacts between solid particles and water increases, more thin films of water are formed, which causes an increase in the maximum molecular moisture capacity and in the optimum moisture content.

As already noted above, as an indicator for assessing the agglomerability of materials one should take the ratio of the values of the maximum molecular and capillary moisture capacities. A quantitative expression of this indicator, as a first approximation, may be represented as follows:

\[ K = W_{\mathrm{m}} \frac{1}{W_{\mathrm{k}} - W_{\mathrm{m}}}, \]

where

\[ \begin{aligned} K &\text{ — indicator of the agglomerability of the material;}\\ W_{\mathrm{k}} &\text{ — capillary moisture capacity of the material;}\\ W_{\mathrm{m}} &\text{ — maximum molecular moisture capacity of the material.} \end{aligned} \]

It follows from the formula that the higher the value of the maximum molecular moisture capacity and the smaller the difference between the values of the capillary and maximum molecular moisture capacities, the higher the degree of agglomerability of the material.

Experimental Part

For an experimental verification of the validity of the expression for the agglomerability indicator through the moisture capacities of granulated materials, averaged samples of agglomerable materials with different degrees of granulatability were specially selected as the objects of study. These were Tashly quartz sand (more than 92% of grains of size 0.2–0.5 mm), which does not granulate under ordinary conditions; marshalite (\(\mathrm{SiO_2} = 92\%\)), containing more than 83% of grains smaller than 0.063 mm and agglomerating well. For comparison, samples of a well-agglomerating iron-ore concentrate from the Sokolovsko-Sarbaiskoe deposit, with a size below 0.1 mm, were also studied [3].

In determining capillary moisture capacity, the material under study, at air-dry natural moisture, was poured, according to the method of R. N. Pitin [4], into glass cylinders 100 mm high and 50 mm in diameter, with a mesh bottom covered with filter paper. The cylinders containing the material were placed in a cuvette with water so that the water level exceeded the lower boundary of the material layer by 1–2 mm, and this level was maintained throughout the entire experiment.

After complete saturation of the material layer with capillary water, samples were taken from the middle portion of the layer height for moisture determination.

The maximum molecular moisture capacity of the material was determined by the method of moisture-capacity media [5]. In this case, samples of the materials were taken after their saturation with capillary water.

The results of experiments to determine the moisture capacities of the materials studied (as the true capillary moisture capacity, the moisture obtained as the difference between the values of the maximum capillary moisture capacity, determined experimentally, and the maximum molecular moisture capacity was taken) are given in Table 1 and in Fig. 1.

It is evident from Table 1 that the lowest value of the agglomerability indicator, as was to be expected, is found for Tashly quartz sand. The main reason for this lies in the monodispersity of the sand and, consequently, in its small specific surface area and the insignificant number of contacts between grains.

To increase the dispersity of Tashly sand, marshalite was added in various amounts. It is interesting to note that, with an increase in the marshalite content in the mixture, the maximum molecular moisture capacity increases linearly, while the change in capillary moisture capacity passes through a minimum (Fig. 1). Obviously, with such a burden composition the densest packing of particles is ensured (which is confirmed ...).

bulk density of the charge) and thereby the diameter of the pores occupied by capillary water is reduced.

Table 1

The agglomerability indices for well-granulated marshalite and iron-ore concentrate are almost equal. To establish the minimum value of the agglomerability index at which the granulation process will proceed satisfactorily under production conditions, two series of experiments were carried out.

At first, the agglomerability of the materials under study was determined by the following method: material in an air-dry state was poured into a 100 ml beaker. Into the middle layer of the material, 0.5 ml of water was introduced by means of a syringe over 5 sec. One minute after the water had been introduced, the material under study was carefully poured onto a tray and the lump that had formed was removed from it. After this, the average size of the lump, its weight, the magnitude of the crushing load, and its moisture content were determined.

The results of the experiments in this series showed that, even with an addition of 20% marshalite to the sand, the strength of the lumps formed increases by more than a factor of three in comparison with the crushing load for one lump of quartz sand and averages 98.5 grams. The moisture content of the lumps in this case drops sharply and almost approaches the value of the maximum molecular moisture capacity (Fig. 1). With an increase in the marshalite addition, the strength of the lumps increases to

Fig. 1. Dependence of moisture capacity on the composition of the agglomerated charge.

1. True capillary moisture capacity; 2. Maximum molecular moisture capacity; 3. Moisture content of lumps.

200–230 grams (with a 90% marl content), while the values of the moisture content of the lumps noticeably approach the moisture-capacity indices of the charges.

For a more detailed assessment of the agglomerability of the materials studied, tests were carried out on a laboratory disk granulator with a diameter of 0.3 m. The mechanical strength of the granules obtained was evaluated by the magnitude of the crushing load and by the number of safe drops of granules from a height of 100 mm onto a concrete slab.

Indeed, as expected, quartz sand moistened to the optimum moisture content did not granulate at all. The granulation process begins satisfactorily with an addition of 30% marl, but the granules obtained are still weak and do not withstand drop tests (Table 2). Normal granulation of the quartz mixture begins with an addition of 40% marl, i.e., at an agglomerability index approximately equal to unity.

Table 2

Effect of marl on the granulatability of quartz sand

Thus, the experimental data obtained confirm the possibility of classifying dispersed materials according to agglomerability indices through their moisture capacity.

Conclusions

1. A methodology has been developed for assessing the agglomerability of fine-grained materials, on the basis of which a first version of a classification of dispersed materials by agglomerability has been proposed.

2. It has been established that readily granulated materials have an agglomerability index of 4 and higher. Satisfactory granulatability of materials occurs at an agglomerability index on the order of unity.

3. It has been shown that by changing the indices of the capillary and maximum molecular moisture capacity of materials, it is possible to regulate the granulation process.

References

1. Rumpf. Fundamental principles and methods of granulation, Chemistry and Chemical Technology (Inorganic Chemistry), No. 1, 64, 1959.

2. L. A. Bernstein, M. B. Frenkel. Granulation of cement raw-material mixtures by dry and wet preparation methods. Moscow, Gosstroiizdat, 1959.

3. V. M. Vityugin, P. N. Dokuchaev. On the question of choosing a rational technology for agglomerating Sokolovsko-Sarbaisk concentrates, Mining Industry of Kazakhstan, No. 3, 81–83, 1961.

4. R. N. Pitin. Increasing the weight of coal with microadditives of hydrocarbon liquids (collected papers). Publishing House of the USSR Academy of Sciences, 1947.

5. A. M. Vasiliev. Study of the physical properties of soil. State Publishing House of Moldavia, 1952.