On a New Method for Granulating the Charge for Silicon Carbide Production
Abstract Generated abstract
This paper addresses reduced gas permeability and furnace smoking in silicon carbide production caused by the use of fine-grained quartz raw materials. It proposes a simplified granulation method in which a plasticized quartz sand, salt, and sulfite-alcohol stillage mixture is disintegrated in a drum charge mixer within a dry carbonaceous material layer, with tests examining moisture content, binder amount, and preliminary compaction. Laboratory experiments showed that low initial moisture and adequate binder improved granulometric composition, while preliminary compaction of the plastic mass increased the yield of coarser fractions and reduced ungranulated sand in fines. The study concludes that the method can produce a compositionally uniform, nonsegregating charge with sufficient mechanical strength, making drying before furnace charging unnecessary.
Full Text
A New Method for Granulating Charge for Silicon Carbide Production
V. M. Vityugin, V. V. Shvetsov, V. A. Lotov
(Presented by the scientific final conference of the Faculty of Chemical Technology)
In connection with the transition to fine-grained quartz raw material in the production of silicon carbide, the gas permeability of the charge sharply decreased, which led to intensified smoking of industrial furnaces. To increase the gas permeability of the layer of charge materials and eliminate smoking, it became necessary to agglomerate fine-grained sands. The previously proposed method of charge granulation*) has a number of substantial shortcomings:
- Drying of the granules complicates and increases the cost of the entire granulation process.
- Dry charge segregates strongly, which leads to nonuniformity of its composition in the layer of charge material.
Analysis of the operation of the pilot-industrial granulation installation at the Tashkent Abrasive Products Combine showed that it is possible to substantially simplify the granulation technology.
A mixture of quartz sand, table salt, and sulfite-alcohol stillage plasticized in edge runners, under certain conditions of its preparation, can be such a viscous aggregated system that, when it is fed into a drum charge mixer into a layer of dry carbonaceous material, limited disintegration will occur down to lumps of 3–5 mm in size. Thus, the charge mixer will perform the role of a granulator. During the disintegration of the plastic quartz-salt mixture in the charge mixer, favorable conditions are created for obtaining granules with a wide range of sizes, which may be the decisive factor for forming a uniform, nonsegregating charge.
To verify the possibility of simplifying the technology of charge granulation for silicon carbide production, experimental work was carried out on a laboratory model installation.
The charge for silicon carbide production is calculated according to the modulus:
\[ M = \frac{\%\,C}{\%\,C + \%\,SiO_2}\cdot 100 = 37.5. \]
In accordance with the modulus, a charge of the following composition was used in the work.
) V. A. Rybakov, V. V. Karlin, V. M. Vityugin, V. A. Prokhorovich. Abrasives*, 5, (37), 1963.
Plasticized mass: quartz sand from the Tashla deposit — 2.76 kg, common salt — 0.24 kg, sulfite-alcohol stillage with a density of 1.22 g/cm³ — 0.28–0.3 kg;
carbonaceous material: petroleum coke — 0.9 kg, anthracite — 0.9 kg.
The sieve composition of the starting materials is given in Table 1.
Table 1
Sieve composition of the raw materials
| Material | +2 mm | 2–1 mm | 1–0.5 mm | 0.5–0.25 mm | −0.25 mm |
|---|---|---|---|---|---|
| Mixture of 50% anthracite and 50% petroleum coke | 10.0 | 37.5 | 11.0 | 19.5 | 21.0 |
| Quartz sand | 6.8 | 75.8 | 17.4 | ||
| Common salt | 30.2 | 20.0 | 25.4 | 10.2 | 12.2 |
Plasticization of the quartz-salt mixture was carried out in laboratory runners for 5 min, which corresponds to production conditions. Granulation of the charge was performed in a drum mixer 610 mm in diameter and 300 mm wide, having on the inner peripheral surface 7 transverse baffles 40 mm high. The mixer was provided with a charging hatch for the charge. One of its end faces was covered with a sheet of organic glass, which made it possible visually to evaluate the behavior of the charge during disintegration and the formation of granules. The mixer rotation speed was 40 rpm. After 100 revolutions the mixer was discharged, and the quality of the resulting charge was determined: its sieve composition, bulk weight, and the amount of ungranulated quartz sand contained in the fine classes. In this process the following were studied:
- The effect of the moisture content of the starting materials on the granulometric composition of the charge.
- The effect of the amount of sulfite-alcohol stillage added to the quartz sand to obtain a plastic mass.
- The role of preliminary compaction of the plastic mass before charge mixing.
The experimental data are given in Table 2.
As can be seen from the table, with an increase in the moisture content of the starting materials, the granulometric composition of the charge changes toward an increase in the yield of fine classes. In experiment 5, because of excessive overwetting of the charge, it was not possible to screen the 2–0 fraction. The bulk weight of the charge from drier starting materials is, as a rule, higher, and the amount of ungranulated sand in the fine classes is lower. Thus, a low moisture content of the starting materials promotes the production of a charge of good granulometric composition.
When the amount of binder is reduced from 8.3% to 8%, the yield of fine classes increases, and the percentage of granulation decreases.
The results of the experiments show the fundamental possibility of obtaining a granulated charge by the disintegration method. However, the charge obtained contains many fine fractions (−2 mm). The bulk weight of such a charge practically does not exceed 1.0 g/cm³. Therefore, before treatment in the charge mixer, it is advisable to subject the plastic mass to preliminary compaction so that granulation proceeds by the formation of medium fractions of 10–2 mm.
To determine the role of preliminary compaction of the plastic mass before granulation, the latter was forced by a screw press through a grate with a diameter of 15 mm. The lumps of mass were mixed
Table 2
Characteristics of the granulated charge obtained by the disintegration method
| Test conditions | 8.3% CCB; quartz sand and carbonaceous material without moistening — sieve composition, % | 8.3% CCB; quartz sand and carbonaceous material without moistening — % SiO₂ | 8.3% CCB; quartz sand and carbonaceous material without moistening — sieve composition, % | 8.3% CCB; quartz sand and carbonaceous material without moistening — % SiO₂ | 8.0% CCB; quartz sand moistened with 2% water — sieve composition, % | 8.0% CCB; quartz sand moistened with 2% water — % SiO₂ | 8.0% CCB; quartz sand moistened with 4% water — sieve composition, % | 8.0% CCB; quartz sand moistened with 4% water — % SiO₂ |
|---|---|---|---|---|---|---|---|---|
| +20 | 7.2 | 7.5 | 1.1 | — | ||||
| 20–10 | 12.4 | 14.8 | 9.1 | 9.5 | ||||
| 10–5 | 8.9 | 7.1 | 11.8 | 9.9 | ||||
| 5–2 | 23.8 | 62.0 | 18.0 | 61.9 | 24.2 | 70.9 | 29.3 | |
| 2–0.5 | 38.4 | 49.2 | 41.6 | 40.4 | 44.5 | 49.5 | ||
| 0.5–0.25 | 8.7 | 38.0 | 9.3 | 37.5 | 8.8 | 39.9 | 51.2 | 54.1 |
| −0.25 | 0.9 | 17.2 | 1.7 | 18.3 | 0.6 | 19.1 | ||
| bulk density, kg/dm³ | 1.0 | 1.01 | 0.98 | 0.96 |
with carbonaceous material, were rolled for 1 minute in a pan granulator, and were subjected to charge mixing (experiment 2, Table 3).
Table 3
Characteristics of the granulated charge obtained with preliminary compaction of the plastic mass
| Test conditions | 8% CCB, moistening of quartz sand with 2% water — sieve composition, % | 8% CCB, moistening of quartz sand with 2% water — % SiO₂ | 8% CCB, moistening of quartz sand with 2% water — sieve composition, % | 8% CCB, moistening of quartz sand with 2% water — % SiO₂ |
|---|---|---|---|---|
| +20 | 11.9 | — | ||
| 20–10 | 8.0 | 40.4 | ||
| 10–5 | 18.5 | 4.0 | ||
| 5–2 | 43.0 | 7.1 | ||
| 2–0.5 | 17.2 | 53.0 | 32.3 | 49.6 |
| 0.5–0.25 | 2.2 | 26.8 | 12.9 | 20.0 |
| −0.25 | 1.0 | 5.25 | 3.0 | 4.3 |
| bulk density, kg/dm³ | 1.0 | 1.05 |
In experiment 1, the plastic mass, after compaction in a screw press, was not subjected to rolling in the granulator.
The data in Table 3 show that preliminary compaction of the plastic mass substantially improves the particle-size composition of the charge: the yield of coarse fractions increases, while the sand content in the fine fractions is insignificant.
Rolling the charge in a pan granulator considerably compacts it; the bulk density of such a charge is higher than in the other cases. A characteristic feature of the charge obtained by the disintegration method is its increased moisture content, reaching 4–5%. The moisture increases the adhesion forces between individual particles, which prevents segregation. In all cases, a charge uniform in composition was obtained, and no visible separation of the particles by size was observed.
Conclusions
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A simplified technology for granulating charge for silicon carbide production has been proposed and tested.
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It has been established that drying the granules before charging them into the furnace is not necessary, since their mechanical strength is sufficiently high.
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The granulated charge is uniform in composition and does not segregate.