STUDY OF GRANULATION OF QUARTZ CHARGE FOR SILICON CARBIDE PRODUCTION

V. M. VITYUGIN, Yu. F. LUTSENKO, A. S. BOGMA, V. A. PROKHOROVICH

(Presented by the scientific seminar of the Department of the Chemical-Technology Faculty)

The production of silicon carbide, which is in very short supply in the USSR, is at present faced with two difficulties, namely, the insufficiently high quality of the raw material and the low-productivity batch technological process in resistance furnaces.

The essence of the first difficulty lies either in the insufficient purity of quartz materials or in their extremely fine-grained character. The reserves of pure coarse-grained quartzites in the USSR are very limited and, moreover, they are intended for more highly qualified special consumption. The purification of less pure coarse-grained quartzites leads to their comminution. Thus, the production of silicon carbide, at least in the near future, will be based on fine-grained quartz raw material.

The production of silicon carbide from fine-grained quartz raw material disrupts the technological process and reduces the yield of finished product. Thus, for example, at the Tashkent Carborundum Plant, the replacement of coarse-grained quartzites from the Mugodzhar deposit with fine sands from the Tashlinsk deposit led to intensive “flaring” of the resistance furnaces and to a decrease in their productivity for green silicon carbide by 20%, with a simultaneous increase in the consumption of table salt to 11%.

From the foregoing, the advisability is evident of finding cheap and effective methods for lumping fine-grained quartz raw material for the production of silicon carbide.

An analysis of possible ways of effective and economical lumping of quartz sands without any additives indicates that these methods are unpromising.

If quartz sand is given a definite granulometric composition by grinding and classification, then by mechanical pressing it can be converted into a monolithic briquette. However, carrying out this process would require enormous energy expenditures at negligible productivity.

A second way of lumping quartz sands without additives could be fusion into a monolith by heating to 1800–1900°C, followed by crushing of the melt. But in this case as well, technical difficulties would result in an excessively high cost of the final product.

A more realistic method appears to be the agglomeration of quartz sands without additives, by granulating the raw sand with subsequent strengthening of the granules during the carburization process itself, owing to the high temperatures developed in core furnaces. In this case the raw granules must be given strength sufficient only for transportation, mixing with charge materials, and charging the charge into the furnace.

However, this is not an easy task, since the screen composition of quartz sands sent for carburization is completely unsatisfactory from the standpoint of granulation because of the low content of fine grains; additional grinding leads to an increase in the cost of the process and to contamination of the raw material with iron.

Thus, the problem arises of granulating quartz sand with some kind of additives. The search for possible ways of agglomerating quartz sands with additives is strictly limited by the choice of such additives. The objective of the present study was to select additives and to develop a rational technology for granulating quartz sand on the basis of these additives.

As materials for granulation under laboratory semi-industrial conditions, averaged samples of Tashlino quartz sands, pure table salt, and concentrates of sulfite-alcohol stillage of grade LKBT from the Solikamsk paper mill were used.

In some experiments, crushed Tashlino sand and marshallite were used as the quartz material.

The characteristics of the materials used are given in Table 1.

Table 1

Characteristics of quartz materials

Granulation of the materials was carried out on a pan granulator with a diameter of 1 m, with smooth adjustment of the rim height from 90 to 150 mm, of the angle of inclination of the pan from 30 to 60°, and of the rotation speed from 15 to 25 revolutions per minute.

As was to be expected, granulation of moistened quartz sand did not give positive results. Partial crushing of the sands to increase their lump-forming ability noticeably improves its granulation; however, the strength of the granules obtained is completely insufficient.

Finer grinding of the quartz sand, bringing the content of the 0.1 mm fraction in it up to 70%, ensures normal lump-forming—

capacity, but increases the cost of the quartz raw material and leads to contamination with iron.

A more realistic path in this direction appears to be the use, as additives to the quartz raw material, of natural dust-like quartz, for example marshalite. The granulatability of marshalite is exceptionally high. The reserves of marshalite in the USSR are sufficient to support the production of silicon carbide; however, because of its insufficient purity it can be used only in the form of limited additives. As can be seen from Table 2, to ensure normal granulatability of Tashla sand, an addition of marshalite in an amount of at least 30% is necessary. The silica content in this mixture will be no more than 97%, which is unacceptable for the production of green silicon carbide.

If a simple and inexpensive scheme were developed for beneficiation of natural dust-like quartz to obtain concentrates containing at least 99% silica, the question of the granulatability of the raw material could be considered solved.

Unfortunately, at present we do not have a reliable technology for beneficiating marshalite and similar materials.

In obtaining green silicon carbide, common salt is usually introduced into the quartz–carbon charge in an amount of from 6 to 10%. The addition of fine-grained common salt facilitates the granulatability of quartz sand, but, as can be seen from Table 2, a sufficient effect is obtained when no less than 20% common salt is introduced.

A combined addition to the quartz sand of 10% common salt with a particle size of less than 0.1 mm and 20% marshalite ensures normal granulatability of the mixture, but the silica content in the quartz portion of the mixture will be no more than 97.8%.

A more effective solution to the problem of granulating quartz sand proved to be the direction involving the use of organic additives—active and sufficiently pure binding substances—while simultaneously improving the granulation regime.

As the binder, ash-free SSB concentrates were used in the form of aqueous solutions of various density. SSB concentrates have a high adhesive capacity with respect to quartz material and are also excellent plasticizers. In addition, this is a comparatively inexpensive material. When an aqueous SSB solution with a density of 1.17–1.2 was added to a mixture of quartz sand and common salt, SiO2 : NaCl = 9 : 1, in an amount of 6–8%, high plasticity of the mass was achieved; however, direct feeding of such a mass into the granulator caused the granules to stick together into excessively large lumps. Reducing the amount of SSB concentrate in the mixture or lowering its density did not make it possible to obtain sufficiently strong granules on a pan granulator.

To ensure normal granulation of the mixture, it was fed into the granulator by a screw feeder through a screen with holes 3–7 mm in diameter. At the same time, dry quartz sand was fed onto the pan of the granulator in order to prevent the mass from sticking together and adhering to the pan. With this technology, several series of enlarged experiments were carried out to select the optimal granulation conditions. As a result of the tests performed, the following optimal granulation regime was established:

Charge composition

Table 2

Effect of additives on the granulometric composition of quartz sand

Note: Tests for dropping of granules were carried out from a height of 0.5 m onto a concrete slab.

Granulator Operating Conditions

Plate inclination angle — 45°
Rim height — 110 mm
Plate rotation speed — 19 rpm

Conclusions

1. The article has considered methods for agglomerating quartz sands from the Tashlinskoye deposit, with an evaluation of their practical application.

2. The optimal composition of the granulated charge and the optimal operating conditions of the granulator have been established.

3. The materials of the article may be used in practical work at the Tashkent and Volzhsky abrasive plants.

References

1. V. A. Rybakov, V. M. Vityugin, V. V. Karlin, and V. A. Prokhodovich. Ways of improving silicon carbide production. Abrasives, 5, 3–5, Moscow, 1963.

2. V. A. Alferov. Compacted charge in silicon carbide production. Abrasives, 15, 13, Moscow, 1949.