Preliminary Plasticization of Glass Batch before Granulation
Abstract Generated abstract
This study examines how preliminary moistening and temperature controlled plasticization affect the granulation of a window glass batch rich in soda and sodium sulfate. Using a production batch from the Anzhero-Sudzhensk Glass Plant, the authors varied water addition and batch temperature before laboratory pan granulation, relating granule formation to dissolution, dispersion, and hydrate formation of alkaline components. The results indicate that introducing most of the working moisture before granulation and heating the batch to about 50 to 55°C improves plasticization, shortens granulation time, lowers final moisture, and increases granule density and strength, while excessive temperature worsens results. Industrial trials at the plant are reported to confirm the laboratory findings.
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PRELIMINARY PLASTICIZATION OF GLASS BATCH BEFORE GRANULATION
L. G. Lotova, V. M. Vitogin
(Presented by the scientific seminar of the Department of General Chemical Technology)
It is known that glass batches for melting window glass include, along with coarse-dispersed, nonplastic materials—sand, dolomite, feldspar, etc.—finely dispersed components that are active with respect to water. In a moistened batch during granulation, partial dissolution and dispersion of soda and sodium sulfate occur, and at the same time hydration reactions of these salts proceed with the release of heat. Depending on the amount of water and the temperature of the batch, anhydrous salts may pass into various crystalline-hydrate forms, binding a certain number of water molecules (one, seven, ten). (Table 1 gives the possible crystalline hydrates, their heats of formation, and regions of stability [1, 2, 3].)
Table 1
Heats of formation and region of stability of crystalline hydrates of soda and sodium sulfate
| Crystalline hydrates of soda and sodium sulfate | Region of stability, °C | Heat of formation, kcal/mol of substance |
|---|---|---|
| $\mathrm{Na_2CO_3 \cdot 10H_2O}$ | $<32$ | 21.80 |
| $\mathrm{Na_2CO_3 \cdot 7H_2O}$ | $32 \ldots 35, 37$ | 16.31 |
| $\mathrm{Na_2CO_3 \cdot H_2O}$ | $>35—37$ | 3.38 |
| $\mathrm{Na_2SO_4 \cdot 10H_2O}$ | $<32.4$ | 18.29 |
The plasticity of the batch, and consequently its capacity for granule formation, changes substantially. Granulation of glass batches with an increased content of alkaline components by the generally accepted method for dispersed materials, i.e., with moistening directly on the granulator, as a rule leads to the production of excessively wet (from 17 to 27%) and insufficiently strong granules, with a simultaneous decrease in the productivity of the granulators. This is due to the fact that dissolution, dispersion, and crystallization, which cause plasticization of the batch and proceed over time, do not have time to be completed on the working surfaces of the granulators.
In the present work, the influence of the plasticization effect, which depends on the moisture content and temperature of the batch, on its granulation was investigated. In
Characteristics of granulate obtained from the charge under different conditions
Table 2
| No. | Amount of binder introduced into the charge before granulation, % | Charge temperature at the moment of granulation, °C | Granulate temperature at the moment of softening, °C | Minimum granulation time, min | Granulate moisture, % | Average granule density, g/cm³ | Average strength of granules after 3 days of formation, kg/cm² | Visual characteristics of the granulate |
|---|---|---|---|---|---|---|---|---|
| 1 | 6 | 30—33 | 28 | 6 | 18—20 | 1.55 | 1.4 | The granules are large, spherical, and pour out well. Charges of these granules form a cake by the 3rd minute of firing. |
| 2 | 6 | 37—40 | 30 | 5—6 | 18 | 1.6 | 1.5 | The granules form not only earlier than in the previous experiments, but also [[unclear: continuation of description]] poorly. |
| 3 | 8 | 37—40 | 32 | 4 | 16—16.5 | 1.62 | 1.5 | The granules form not only earlier than in the previous experiments, but also [[unclear: continuation of description]] poorly. |
| 4 | 8 | 50—55 | 34 | 3 | 15 | 1.73 | 1.57 | Granules of medium size (7—10 mm). |
| 5 | 10 | 37—40 | 34 | 4 | 15.3 | 1.68 | 1.54 | The granules are flattened, well compacted, and require additional processing. They give sizes of 5—8 mm. |
| 6 | 10 | 50—55 | 34 | 2—2.5 | 14 | 1.78 | 1.61 | The granules are flattened, well compacted, and require additional processing. They give sizes of 5—8 mm. |
| 7 | 12 | same | same | 1.5 | 13.5 | 1.9 | 1.62 | The granules are flattened, well compacted, and require additional processing. They give sizes of 5—8 mm. |
| 8 | 14 | same | 34 | 2 | 14.5 | 1.87 | 1.59 | The granules are flattened, well compacted, and require additional processing. They give sizes of 5—8 mm. |
Note: increasing the temperature of the mass during charging above 60°C worsened the granulation results.
As the object of study, the production batch of the Anzhero-Sudzhensk Glass Plant was used, consisting of sand—54.6%, dolomite—16.52%, feldspar—6.52%, soda—16.07%, sodium sulfate—5.36%, and fuel oil—0.93%. The components were dosed according to the sequential scheme generally adopted at the plant: sand, fuel oil, water, soda, sodium sulfate, feldspar, dolomite. Mixing was carried out for 4 min.
During batch preparation, the amount of water and the temperature were varied, these being factors affecting the process of formation of crystalline hydrates in the batch. The additional amount of water required for granule formation was supplied directly onto the batch during granulation by means of a nozzle (water temperature—30–35°C).
The temperature of the mass at the initial moment during granulation was maintained within 32–35°C. Granulation was carried out in a laboratory-type pan granulator (pan diameter—0.3 m, angle of inclination—46°, rotation speed—32 rpm). To maintain the temperature regime during granulation, the granulator pan was heated to 32–35°C before each experiment.
The results of the investigations are presented in Table 2.
Glass batches with a significant content of alkaline components (\(\mathrm{Na_2CO_3}\), \(\mathrm{Na_2SO_4}\)) can be successfully granulated at a temperature of 30–32°C and a moisture content of 13–14%. The best granule formation is achieved when 70 to 85% of the total working moisture is introduced into the batch before it is fed to the pelletizer. Raising the temperature to 50–55°C promotes better plasticization of the batch, which can be explained by an increase in the degree of dispersion of the soluble components due to the formation of soda monohydrate. The time required for granule formation is thereby significantly reduced, and the quality (density, strength) of the granulate improves owing to an increase in the compaction time, reduced moisture content, and a stronger structure of the crystalline lattice. The strength and density of granules obtained from batches that had been preliminarily moistened but held at a lower temperature (37–40°) are lower, because the crystalline bridges ensuring the structure of the granule are represented by higher crystalline hydrates of soda and sodium sulfate (7–10 hydrates).
The practice of granulating glass batch in the industrial installation of the Anzhero-Sudzhensk Glass Plant confirmed the data obtained under laboratory conditions.
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