Study of the Effect of Preliminary Heat Treatment of the Raw Material Charge on the Formation of Chrome Fused Alumina
Abstract Generated abstract
This study addresses nonuniformity and chromium oxide segregation in the conventional arc furnace production of chrome fused alumina from alumina and chromium oxide powders. The authors investigate whether preliminary heat treatment of a charge containing 98.5% alumina and 1.5% chromium oxide can introduce chromium into the corundum lattice before melting, using X-ray diffraction and visible-region spectrophotometry to characterize fired tablets. The results indicate that interaction begins appreciably at about 600°C, while near-complete restructuring occurs between 1000°C and 1100°C, with X-ray patterns at 1000°C showing only corundum reflections rather than separate chromium oxide lines. The work supports preliminary heat treatment as a practical means to reduce segregation and excess chromium oxide consumption in chrome electrocorundum production.
Full Text
STUDY OF THE EFFECT OF PRELIMINARY HEAT TREATMENT OF THE RAW MATERIAL CHARGE ON THE FORMATION PROCESS OF CHROMIUM ELECTROCORUNDUM
V. M. Vityugin, V. V. Koyain, V. I. Vereshchagin,
G. F. Ivanov, N. I. Poldubiyak
(Presented by the scientific seminar of the Department of General Chemical Technology)
The most widespread method at present for producing chromium electrocorundum is melting, in arc electric furnaces, a mixture consisting of finely ground powders of alumina and chromium oxide.
However, the existing method of producing chromium electrocorundum has a number of substantial shortcomings, the principal one being the nonuniformity of the finished product, caused by partial segregation of the reacting components in the furnace bath. This, in turn, leads to a low yield of standard product, excessive consumption of chromium oxide, and complicates the primary sorting of the block.
X-ray analysis of the product obtained by the factory technology shows that it is not pure. Traces of chromium oxide are found in it—the lines \(d - 3.62\); \(d - 2.67\)—and of free chromium \(d - 2.05\) (see Table 1).
The objective of the present study was to develop a method for producing chromium electrocorundum that would make it possible to completely eliminate segregation of chromium oxide in the furnace bath and to avoid excessive (relative to the calculated amount) consumption of chromium oxide.
It is known from the literature [1] that corundum with chromium oxide very readily forms a continuous series of solid solutions with melting temperatures from \(2050^\circ\text{C}\) to \(2130^\circ\text{C}\). The ease of formation of solid solutions in the system \(\mathrm{Cr_2O_3—Al_2O_3}\) is due to the proximity of the ionic radii \(\mathrm{Al^{3+}} = 0.5\,\text{Å}\) and \(\mathrm{Cr^{3+}} = 0.6\,\text{Å}\) [2]. As a result, when small amounts of \(\mathrm{Cr^{3+}}\) are introduced into the crystal lattice of corundum, no deformation of its parameters should occur. Indeed, calculation of the parameters of the corundum crystal lattice in the industrial product, carried out for the reflection plane (234), gives no deviation from the parameters of the crystal lattice of pure \(\alpha\)-corundum, for which \(a = 5.14\,\text{Å}\) [3].
This allowed us to try heat treatment of a mixture of aluminum oxide with chromium oxide at temperatures below the melting point of the charge, for preliminary introduction of chromium oxide into the crystal lattice of corundum. The products thus obtained were subjected to X-ray analysis and to analysis on an SF-10 spectrophotometer.
Experimental Part
From a charge containing 98.5% \(\mathrm{Al_2O_3}\) and 1.5% \(\mathrm{Cr_2O_3}\), tablets were pressed under a pressure of \(50\,\text{kg}/\text{cm}^2\), which were then fired for 1 hour in an electric furnace.
During firing, as a result of the incorporation of chromium oxide into the crystal lattice of alumina, the color of the material changed from light green to light pink, characteristic of chromic corundum, passing through all intermediate shades.
X-ray analysis of the product obtained at a temperature of 1000°C did not reveal any reflections other than those belonging to corundum. At the same time, in a mixture of the same amount of components that had undergone heat treatment separately, reflection maxima of Cr₂O₃ are recorded: \(d\)—2.65; \(d\)—1.8; \(d\)—1.67 [3] (see Table 1).
Table 1
| \(hKl\) reflection of \(\alpha\)-Al₂O₃ | Alumina treated at 1000°C: I | Alumina treated at 1000°C: \(d/n\) (Å) | Product of joint treatment of alumina with 1.5% Cr₂O₃ at 1000°C: I | Product of joint treatment of alumina with 1.5% Cr₂O₃ at 1000°C: \(d/n\) (Å) | Mixture of aluminum oxide and 1.5% chromium oxide treated separately at 1000°C: I | Mixture of aluminum oxide and 1.5% chromium oxide treated separately at 1000°C: \(d/n\) (Å) | Chromic corundum, melted at \(t = 2400^\circ\)C (1% Cr₂O₃): I | Chromic corundum, melted at \(t = 2400^\circ\)C (1% Cr₂O₃): \(d/n\) (Å) |
|---|---|---|---|---|---|---|---|---|
| — | — | — | — | — | — | — | 0.8 | 5.51 |
| — | — | — | — | — | 0.5 | 4.79 | 0.5 | 4.79 |
| — | — | — | — | — | — | — | 0.4 | 3.62 |
| — | — | — | — | — | — | — | 1.0 | 3.49 |
| 101 | 6 | 3.42 | 6.1 | 3.43 | 5.5 | 3.42 | 5 | 3.42 |
| — | — | — | — | — | 0.5 | 3.24 | 0.6 | 3.25 |
| — | — | — | — | — | 0.7 | 2.65 | 1.3 | 2.67 |
| 112 | 9 | 2.53 | 9.3 | 2.53 | 8.3 | 2.53 | 7.7 | 2.53 |
| 101 | 5 | 2.36 | 5 | 2.36 | 4.5 | 2.36 | 1.7 | 2.36 |
| 102 | 10.0 | 2.07 | 10.0 | 2.07 | 10.0 | 2.07 | 7.2 | 2.07 |
| — | — | — | — | — | — | — | 2.0 | 2.05 |
| — | 0.4 | 1.95 | 0.3 | 1.95 | — | — | — | — |
| — | — | — | — | — | 0.5 | 1.8 | — | — |
| 202 | 5.3 | 1.73 | 5.6 | 1.73 | 5.6 | 1.73 | 2.5 | 1.73 |
| — | — | — | — | — | 0.6 | 1.67 | — | — |
| 123 | 9.8 | 1.595 | 9.6 | 1.597 | 10.0 | 1.595 | 9 | 1.595 |
| — | 0.5 | 1.542 | 0.5 | 1.542 | 0.5 | 1.535 | 0.5 | 1.535 |
| 233 | 1.3 | 1.51 | 1.4 | 1.51 | 1.6 | 1.51 | 0.9 | 1.51 |
| 103 | 4.4 | 1.4 | 4.3 | 1.4 | 4.8 | 1.4 | 5.7 | 1.4 |
| 112 | 6.3 | 1.37 | 6 | 1.37 | 6.6 | 1.37 | 10.0 | 1.37 |
| — | — | — | — | — | 0.4 | 1.329 | 0.3 | 1.33 |
To establish the temperature at which chromium oxide begins to interact with aluminum oxide and to find the optimum temperature of preliminary heat treatment of alumina with an addition of 1.5% Cr₂O₃, studies of the products obtained were carried out on an SF-10 spectrophotometer in the visible region of electromagnetic radiation. For sharper isolation of the reflection band 5.40 Å, an Hg 5760 Å light filter was used. The results of the experiments are summarized in Table 2.
Table 2
| No. | Heat-treatment temperature, °C | Reflection coefficient, in %, at reflection band 5.40 Å |
|---|---|---|
| 1 | 400 | 50.0 |
| 2 | 500 | 50.0 |
| 3 | 600 | 51.0 |
| 4 | 700 | 56.5 |
| 5 | 800 | 66.0 |
| 6 | 900 | 72.0 |
| 7 | 1000 | 73.0 |
| 8 | 1100 | 73.0 |
From the results of these experiments it is evident that appreciable interaction of chromium oxide with alumina begins already at a temperature of 600°C.
Practically complete restructuring of the substance occurs in the temperature range from 1000°C to 1100°C. The investigations carried out provide a basis for the practical use of preliminary heat treatment of the charge in the production of chromic electrocorundum.
LITERATURE
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N. A. Toropov et al. Phase Diagrams of Silicate Systems, 188–189, Publishing House of the Academy of Sciences of the USSR, Leningrad, 1965.
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U. D. Kingery. Introduction to Ceramics, 70, Publishing House for Construction Literature, Moscow, 1967.
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L. I. Mirkin. Handbook of X-ray Structural Analysis of Polycrystals, Publishing House of Physical and Mathematical Literature, Moscow, 1961.