On Some Features of Determining the Agglomerability Index of Disperse Materials Containing Water-Soluble Components
Abstract Generated abstract
This paper examines why standard moisture-capacity methods can give erroneous agglomerability indices for disperse materials containing substantial water-soluble components. Using a high-boron glass batch as an example, the authors compare conventional thermal drying with low-temperature isothermal dehydration and argue that dissolution, chemical reactions, and crystalline hydrate formation remove part of the water from active participation in granulation and distort measured moisture capacities. The proposed procedure dries capillary-saturated and molecularly saturated samples in a desiccator at 20 to 22 degrees C, below hydrate decomposition temperatures. For the studied batch, this method yields moisture-capacity values and an optimum pelletizing moisture of 10.1 percent, consistent with laboratory and production observations.
Full Text
Proceedings
of the Tomsk Polytechnic Institute
named after S. M. Kirov
Vol. 234 1974
On Some Features of Determining the Agglomerability Index of Disperse Materials Containing Water-Soluble Components
V. M. Vityugin, L. G. Lotova, L. N. Serdyuk
(Submitted by the scientific seminar of the Department of General Chemical Technology)
Studies by the Department of General Chemical Technology have established that the ability of a material to undergo wet aggregation is determined by its characteristic moisture capacities: maximum molecular moisture capacity (MMMC) and maximum capillary moisture capacity (MCMC). The agglomerability index \((K)\) can in this case be calculated from equation (1).
\[ K=\frac{W_{\mathrm{MMMC}}}{W_{\mathrm{MCMC}}-W_{\mathrm{MMMC}}}, \tag{1} \]
where \(W_{\mathrm{MMMC}}\) is the moisture content of the material corresponding to the maximum molecular moisture capacity, in percent; \(W_{\mathrm{MCMC}}\) is the moisture content corresponding to complete capillary saturation of the free layer of disperse material, in percent. As a consequence of this equation, the relation for determining the optimum moisture content for pelletizing follows (2).
\[ W_{\mathrm{opt}}^{p}=W_{\mathrm{MCMC}}-W_{\mathrm{MMMC}} \tag{2} \]
However, the indicated dependence is well confirmed when evaluating the agglomerability of insoluble disperse materials.
In developing the technology for granulating high-boron glass batches, the use of this method leads to erroneous conclusions. Thus, for an industrial glass batch for glass of grade S49-1, the values of \(W_{\mathrm{MMMC}}\) and \(W_{\mathrm{MCMC}}\), determined under standard conditions, proved to be, respectively, 23.6 and 49.0%. The agglomerability index according to equation (1) was 0.93, while the optimum moisture content for pelletizing \((W_{\mathrm{opt}}^{p})\), calculated from relation (2), proved equal to 25.3%. At the same time, direct determination of the optimum moisture content for pelletizing, both under laboratory and under production conditions, ranged within 10–11%. The reason for the discrepancy evidently lies in the specific properties of the disperse materials composing the glass batch and, mainly, in the nature of the interaction between water and the batch components.
The glass batch studied had the following component composition:
| Component | Size fraction | Content |
|---|---|---|
| sand | 0.25–0.14 mm | 40.28% |
| soda ash, calcined | 0.25–0.08 mm | 10.92% |
| boric acid | 0.25–0.05 mm | 37.10% |
| alumina | 0.08 mm | 0.85% |
| sodium nitrate | 0.25–0.05 mm | 9.75% |
| common salt | 1.0–0.08 mm | 1.6% |
It is evident from the composition of the batch that the amount of water-soluble components is about 59%. At the same time, some of the water-soluble components—for example, soda (\(\mathrm{Na_2CO_3}\))—are capable of forming polyhydrate crystalline hydrates, while boric acid and sodium salts enter into chemical interaction with the formation of crystalline hydrates of sodium borates. Crystalline-hydrate water is, naturally, removed from active participation in the process of granulating the material and should not be taken into account in calculations of the agglomerability index and the optimum moisture content for pelletizing. At the same time, in practice, when determining moisture capacities and especially when determining \(W_{\mathrm{mkv}}\), during the drying of the moistened material the dissolution of components and chemical interaction are intensified; decahydrate carbonates melt with the release of crystallization water; the structure of the batch changes, which must be reflected to a significant degree in the moisture-capacity indices (\(W_{\mathrm{mmv}}, W_{\mathrm{mkv}}\)).
From all that has been said, it follows that the method for determining moisture capacities for a disperse material with a high content of soluble components must exclude the process of thermal drying. To eliminate the factor of decomposition of crystalline hydrates when determining moisture capacities, drying of samples of capillary-saturated and molecularly saturated batch should be carried out by isothermal dehydration in a desiccator at a temperature of 20–22° C. Concentrated sulfuric acid may be used as the dehydrating agent.
Analysis showed that, for the above-mentioned batch, the values of the maximum molecular and maximum capillary moisture capacities determined under these conditions were, respectively, 14.5% and 24.6%; the calculated agglomerability index (\(K\)) was 1.4, and the optimum working moisture content was 10.1%, which agrees with the experimental data.
Conclusions
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In evaluating the agglomerability of disperse materials containing water-soluble components, it is necessary, when determining the indices of maximum molecular and maximum capillary moisture capacities, to take into account the process of chemical interaction and the formation of crystalline hydrates.
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Dehydration of samples of disperse materials with a high content of soluble components, when determining the maximum molecular and maximum capillary moisture capacities, should be carried out at temperatures lower than the decomposition temperatures of the crystalline hydrates.