INVESTIGATION OF THE INTERACTION OF AMMONIUM DICHROMATE WITH ALUMINA

V. M. VITYUGIN, V. V. KOYIN, N. I. PODDUBNYAK, L. V. SOKOLOV

(Presented by the scientific seminar of the Department of GCT)

Abrasive materials, widely used for mechanical processing in many branches of industry, are of great importance in the development of modern technology.

Along with the abrasive materials produced earlier, new ones are now also being manufactured, possessing improved quality. Such new abrasives include alloyed electrocorundums and, in particular, chrome electrocorundum [1].

The production of this abrasive material consists in melting, in electric-arc furnaces, a mixture consisting of alumina (99%) and chromium oxide (1%) [2].

However, when using a charge containing alumina and chromium oxide, incomplete interaction occurs, and as a result a rather heterogeneous product is obtained, in which, along with chrome electrocorundum, white electrocorundum and other impurities (metallic chromium, iron, carbides, etc.) are present. Thus, for example, a mineralogical analysis [3] of a product obtained at the Yurga abrasive plant shows that it contains 52.01% chrome electrocorundum, 35.65% white electrocorundum, and 12.34% other impurities.

The reasons for such heterogeneity of the finished product lie in the imperfection of preparation of the initial charge, as well as in its segregation during transportation and charging, caused by the different particle sizes and densities of aluminum and chromium oxides.

One of the effective methods of combating segregation of bulk materials entering a raw-material mixture is its wet granulation. However, experiments on granulating a charge consisting of alumina and chromium oxide showed that the water-physical properties of the material do not make it possible to obtain sufficiently strong granules without the introduction of appropriate binding substances. From this standpoint it would be advisable to replace chromium oxide in the charge with some soluble compound containing chromium, which, on the one hand, would promote a uniform distribution of chromium oxide over the surface of the alumina, and, on the other hand, crystallization of this compound during drying would ensure strengthening of the granules.

Below are presented the results of an experimental testing of this proposal, carried out under laboratory conditions on production alumina. As the soluble chromium-containing compound,

ammonium dichromate was used. The cost of chromium oxide and ammonium dichromate, recalculated for the required amount of chromium oxide, is practically the same.

For the most uniform possible distribution of ammonium dichromate over the surface of the alumina, the amount and concentration of its solution must be taken in accordance with the value of the maximum molecular moisture capacity of the batch used (24.2%) (4), since only under this condition does it appear possible to obtain a completely uniform film of solution on the surface of each individual alumina particle. Introducing a solution in an amount greater than the maximum molecular moisture capacity may lead to substantial redistribution of ammonium dichromate in the batch due to diffusive mass exchange during drying; and when the amount of solution is less than the maximum molecular moisture capacity, complete wetting of the active surface of the alumina will not be achieved.

A second, no less important factor influencing the choice of solution concentration is the optimum granulation moisture content, which, in accordance with the water-physical properties of the given system, also corresponds to the value of the maximum molecular moisture capacity of the alumina.

It is known from the literature [5] that corundum with chromium oxide very readily forms a continuous series of solid solutions with a melting temperature from 2050 to 2130°C. The ease of formation of solid solutions in the system $\mathrm{Cr_2O_3—Al_2O_3}$ is due to the closeness of the ionic radii $\mathrm{Al^{3+}} = 0.5\ \mathring{\mathrm A}$ and $\mathrm{Cr^{3+}} = 0.6\ \mathring{\mathrm A}$ [6]. Consequently, when small amounts of $\mathrm{Cr^{3+}}$ are introduced into the crystal lattice of corundum, deformation of its parameters should not occur. Indeed, calculation of the parameters of the corundum crystal lattice in the industrial product, carried out in an X-ray structural study using the reflection plane (234), gives no deviation from the parameters of the crystal lattice of pure $\alpha$-corundum, for which $a = 5.14\ \mathring{\mathrm A}$ [7].

This made it possible to test thermal treatment of the batch at temperatures significantly below the melting temperature of the components, for the preliminary introduction of chromium oxide into the crystal lattice of alumina.

Experimental part

As the starting batch, a mixture containing 99% aluminum oxide and 1% chromium oxide or ammonium dichromate (also in an amount of 1% recalculated as chromium oxide) was used. In the case of the use of ammonium dichromate, the batch was preliminarily heated in a muffle furnace to 300°C for 1 hour, with stirring, in order to decompose the ammonium dichromate and convert it into chromium oxide. During heat treatment of the mixture, as a result of the introduction of chromium oxide into the crystal lattice of the alumina, the color of the material changes in the first case from gray-green to light pink, characteristic of chromic corundum, and in the second case from yellowish-brown to light pink.

To establish the optimum temperature of interaction of chromium oxide with aluminum oxide, the products obtained were studied on an SF-10 spectrophotometer in the visible region of electromagnetic radiation. In order to distinguish more sharply the reflection band at $5040\ \mathring{\mathrm A}$, an Hg $5760\ \mathring{\mathrm A}$ light filter was used. The results of the experiments are given in Tables 1 and 2.

As follows from the experimental results, interaction of chromium and aluminum oxides in the solid phase begins already at a temperature of 500–600°C.

Practically complete assimilation of chromium oxide and restructuring of the material occur in the temperature range of 1000–1100°C.

Table 1

Results of the study on an SF-10 batch composed of Al₂O₃ and Cr₂O₃, fired for 1 hour at various temperatures

Table 2

Results of the study on an SF-10 batch composed of Al₂O₃ and (NH₄)₂Cr₂O₇, fired for 1 hour at various temperatures

The results of kinetic studies of the processes of interaction of the mixture components are presented in Tables 3 and 4.

Table 3

Determination of the optimal heat-treatment time for a batch containing chromium oxide

Consequently, the optimal heat-treatment time for a batch containing chromium oxide at a temperature of 1000°C is 40 min, and at 1100°C, 10 min. For a batch containing ammonium dichromate, these values are respectively 45 and 5 min.

Table 4

Determination of the optimal heat-treatment time for a charge containing ammonium bichromate

REFERENCES

1. M. V. Kamentsev. Artificial Abrasive Materials. MASHGIZ, Moscow, 1950.

2. A. S. Polubelova, V. N. Krylov, V. V. Karlin, and I. S. Efimova. Production of Abrasive Materials. Mashinostroenie Publishers, Leningrad, 1968.

3. Edited by N. M. Strakhov. Methods for the Study of Sedimentary Rocks. Vol. 1, Goskhimizdat, Moscow, 1957.

4. A. F. Lebedev. Soil and Ground Waters. Academy of Sciences of the USSR, 1936.

5. N. A. Gorbunov. Diagrams of the State of Silicate Systems. Academy of Sciences of the USSR, 1965.

6. U. D. Kingery. Introduction to Ceramics. Publishing House for Construction Literature, Moscow, 1967.

7. L. I. Nirkin. Handbook of X-ray Structural Analysis of Polycrystals. FIZMAT, Moscow, 1961.