STUDY OF THE BATCH PREPARATION TECHNOLOGY FOR BORON CARBIDE PRODUCTION

V. M. Vityugin, V. A. Lotov, V. A. Trofimov

(Presented at the scientific final conference of the Chemical-Technological Faculty)

At present, the technology for producing boron carbide consists of the following: pure boric acid in the powdered state (particle size—1 mm) is mixed with dry petroleum coke (particle size 1–0 mm) in a ratio of 76–78:22–24, respectively.

The batch is mixed mechanically and, in order to avoid dusting and segregation, is moistened with 2–3% water. The moistened batch, together with return sinter, is charged directly into an electric-arc furnace in zones with temperatures up to 1500°C. The inner zones of the furnace have a temperature of 2600–2800°C. During the smelting of boron carbide in electric-arc furnaces, boron-containing raw material is lost owing to the volatilization of boric acid with water vapors during its dehydration, and also owing to the mechanical entrainment of boron vapors. The mechanically added water also contributes significantly to steam formation.

Since boric acid decomposes most intensely in the temperature range 149–176°C, it is natural that at temperatures on the order of 1000–1500°C the process of boric-acid decomposition will proceed very violently and in a comparatively short time. All this adversely affects the yield of boron carbide, while the amount of return material increases.

On the basis of techno-economic considerations, the above suggests preliminary dehydration of boric acid at low temperatures, as was already indicated earlier in the literature [1].

Undoubtedly, preliminary dehydration of boric acid will have a positive effect on the boron-carbide smelting process. But the process of preliminary dehydration is associated with certain undesirable operations, such as subsequent crushing and grinding of the sinter (sponge) in ball mills, screening through sieves, and batch preparation. Dehydrated boric acid does not eliminate segregation of the batch and the associated deterioration of the boron-carbide smelting process.

A more effective method of preparing the batch is needed. Such a method will apparently be granulation of the batch followed by drying under certain temperature regimes, so that during drying of the granules dehydration of the boric acid takes place.

To verify this assumption, laboratory studies were carried out on the dehydration of powdered and granulated boric acid, with reference to the modern process of prepara-

In preparing the charge, the powdered boric acid was additionally moistened with a certain amount of water, while the granulated boric acid was first dried at a temperature of 105°C.

Next, both powdered and granulated boric acid were heated from 20 to 1000°C at an external pressure of 740 mm Hg. The results of the experiments (see the graphs) indicate the advantages of dried granulated boric acid. As can be seen from the graphs, the weight losses upon heating of moistened powdered boric acid and dry granulated acid differ.

In dry granulated boric acid the weight loss is only 0.27 g. In both cases, the change in weight practically ceases at a temperature of 850–900°C.

It should be pointed out that the loss of weight in powdered boric acid proceeds more vigorously than in granulated acid, which in the future will probably make it possible to carry out drying and dehydration of the corresponding charge for smelting boron carbide under a more severe temperature regime.

Fig. 1. Graph of the dependence of the weight loss of boric acid on temperature. Initial sample weight — 1 g; 1 — \(W = 10\%\); 2 — \(W = 5\%\); 3 — granulated \(H_3BO_3\).

Analyzing the present state of charge preparation and taking into account the results of the investigation, the following technological scheme for charge preparation may be proposed: boric acid and carbonaceous material, in a definite ratio, are introduced into runners, where they are thoroughly mixed; during mixing, a certain amount of water is added to the mixture in order to obtain a molding mass of definite consistency. This mass is then fed to an extruder, where considerable compaction and preliminary granulation of the mass take place; then, to prevent the granules from sticking together, they are sent for rolling in an inclined pan, into which dry charge is fed. From the pan the moist granules go to a belt radiation dryer and then to a receiver. The use of radiation drying is due to the fact that the granulated charge must be dried in a monolayer, where high drying efficiency can be achieved.

Granulated charge undoubtedly has a number of advantages over the usual charge, namely: it has a uniform granulometric composition, promotes better gas permeability, prevents segregation of the charge components, and provides better contact between the carbonaceous and boron-containing materials, which makes it more reactive.

At present, studies are being carried out to select a rational technology for charge granulation and the optimal drying regime. The use of granulated charge in boron carbide production will undoubtedly yield a significant economic effect and improve working conditions.

REFERENCES

1. S. S. Kiyarisov, E. D. Gurevich. The Present State of Boron Carbide Production and Its Application, 1965.