Results of Testing Granulated Charge in Silicon Carbide Production
Abstract Generated abstract
This paper addresses flare formation and smoke emission in green silicon carbide production at the Tashkent Abrasive Combine, attributed to the poor gas permeability and high evaporation behavior of fine-grained quartz sand charges. It describes a simplified granulation process using quartz sand, common salt, sulfite-alcohol stillage, and carbonaceous material, designed to use existing charge-preparation equipment and to produce a porous, broad-grain-size furnace burden. Industrial experimental campaigns comparing granulated and conventional charges indicated that granulation eliminated flaring, greatly reduced smoke emission, maintained product quality within specifications, and increased the gross silicon carbide output from the furnace by about 10%.
Full Text
On the Results of Testing Granulated Charge in Silicon Carbide Production
V. M. Vitkovich, V. V. Shvetsov, V. A. Lotov
(Presented by the scientific seminar of the Department of General Chemical Technology)
One of the shortcomings of the modern production of green silicon carbide at the Tashkent Abrasive Combine is the flaring of industrial furnaces, which is a consequence of the use of fine-grained Tashkent sands.
Silicon carbide is obtained according to the reaction equation
\[ \mathrm{SiO_2 + 3C = 2CO + SiC} \]
in core furnaces at ratios of 2300–2400°. The use of fine-grained quartz sand causes a decrease in the gas permeability of the charge-material layer and contributes to clogging of the reaction zone. An increase in pressure in the furnace and uneven, channel-like removal of gases from the reaction zone lead to the formation of a flare. The mismatch between the particle sizes of the carbonaceous material and the quartz sand causes the rate of evaporation of silicon dioxide to exceed the rate of its absorption by the carbonaceous part of the raw material. The excess silica is then removed from the furnace space in the form of a flare.
For a number of years, the Department of General Chemical Technology has successfully carried out studies on two-layer granulation of fine-grained quartz raw material, as a result of which technological recommendations were issued for the construction of a pilot-industrial granulation unit. Such a unit was designed by the design group of VolzhskVNIIDASH and then installed at the Tashkent Abrasive Combine.
Semi-industrial tests and experience in operating the granulation unit made it possible to develop a simplified granulation technology aimed at making maximum use of the existing equipment of the charge-preparation section of the silicon carbide shop.
Tashkent sands are monodisperse and, consequently, are difficult-to-granulate materials. Their granulation was carried out with the use of a special binder—sulfite-alcohol stillage. The basis for obtaining granules is a plastic mass formed by mixing quartz sand, common salt, and sulfite-alcohol stillage in runners 5 (Fig. 1). The homogeneous plastic mass is discharged into the receiving hopper of screw press 6 and is forced through a grate with holes 10 mm in diameter. The cylindrical quartz-salt blanks enter the tray-type…
Fig. 1. Diagram of the equipment chain of the granulation installation: 1—tank for storing sulfite-alcohol stillage; 2—storage facility for quartz raw material; 3—storage facility for common salt; 4—stillage measuring tank; 5—edge runners; 6—screw press; 7—granulator; 8—hopper for carbonaceous materials; 9—conveyor belt; 10—receiving hopper; 11—intermediate vessel; 12, 14—hoppers; 13—hopper for storing carbonaceous materials; 15—batching scales; 16, 17, 19—transport belts; 18—charge mixer; 20—container for the finished charge; 21—storage facility for carbonaceous materials.
Granulator 7. Into it, from hopper 8, carbonaceous material is also fed by means of a screw feeder (a mixture of low-ash anthracite and petroleum coke in a ratio of 1:1), with a particle size of 3–0 mm, in an amount of 30% of that required by calculation. In granulator 7, the nuclei of the granules are rounded in a layer of dry carbonaceous material. The granulated material is fed by belt conveyor 9 into receiving hopper 10.
All subsequent operations are carried out on the operating equipment of the charge-mixing section of the silicon carbide shop. Through intermediate vessel 11 and batching scales 15, the granulated material is directed to charge mixer 18. Into the same charge mixer is fed the carbonaceous material metered on scales 14 (the remaining 70%). In the charge mixer, additional rounding of the quartz-salt nuclei of the granules takes place, as well as mixing and averaging of the charge, which is discharged into container 20 and sent for loading into the furnace.
The granulated charge eliminates the shortcomings listed above. On the one hand, a loose porous charge of broad granulometric composition promotes uniform removal of gases throughout the entire reaction zone; on the other hand, the agglomerated quartz material, having a smaller specific surface area, has a lower evaporation rate.
In the course of developing the granulation technology and the technology for charging industrial furnaces, at the final stage we carried out six experimental campaigns for green silicon carbide using a granulated charge obtained on the pilot-industrial granulation installation of the Tashkent Abrasive Combine. Comparative results of furnace operation on granulated and nongranulated charges are given in Table 1.
Industrial tests established that the furnace power factor
Results of experimental heats on granulated charges
Table 1
| No. | Features of charge loading | Data on furnace operation | Electrical regime: initial voltage, V | Electrical regime: final voltage, V | Electrical regime: time under current | Amount of silicon carbide: SiC content, % | Amount of silicon carbide: content of green grains, % | Amount of silicon carbide: product characteristics | Batch yield from furnace, kg |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Under the core—ordinary reaction charge; along the sides and on top—granulated | Weak dust emission toward the end of the campaign; no flaring | 330 | 230 | 25-10 | 98.36 | 80.5 | Ordinary green | 6130 |
| 2 | Same | Same | 320 | 220 | 25-20 | 97.96 | 80.0 | Ordinary green | 6100 |
| 3 | Control, loaded 9 hours after switching on, until the end of the campaign, with ordinary nongranulated charge | Flaring 9 hours after switching on, until the end of the campaign | 310 | 230 | 25-00 | 98.10 | 80.1 | Same | 5630 |
when they are charged as a granulated burden is completely eliminated. The analysis showed that smoke emission from the furnace is reduced 130-fold, i.e., is practically completely eliminated. The residual smoke consists of finely dispersed sodium chloride. In all cases, the final product obtained was of a quality fully corresponding to the technical specifications. It is characteristic that all the campaigns proceeded with an increase in the gross output of silicon carbide from the furnace by an average of 10%.
Conclusions
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At the Tashkent Abrasive Combine, a simplified technology for granulating fine-grained quartz sand has been developed.
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Six experimental campaigns were carried out with partial charging of the furnaces with a granulated burden; in this process the fundamental possibility of eliminating flare formation and smoke emission was demonstrated.