Melting of Chromium Electrocorundum with Ammonium Dichromate
Abstract Generated abstract
This paper addresses compositional nonuniformity in chromium electrocorundum produced from alumina and solid chromium oxide, attributing it to imperfect mixing and segregation during smelting. It proposes replacing solid chromium oxide with an aqueous ammonium dichromate solution, which decomposes during drying and heating to form finely dispersed chromium oxide uniformly distributed on alumina particles, provided the charge moisture remains below the alumina molecular moisture capacity. Industrial experimental smelting using this charge preparation method indicated improved control of chromium oxide content, reduced chromium oxide consumption to about 1 percent or less, and no increase in smelting time or electrode consumption. The experimental block yielded a more homogeneous product, with the reported proportion of pink grains increasing by 22 percent compared with the existing plant process.
Full Text
SMELTING OF CHROMIUM ELECTROCORUNDUM WITH AMMONIUM DICHROMATE
N. I. Poddubnyak, G. S. Frolova, V. V. Kolyin, V. M. Vityutin
(Presented by the scientific seminar of the Department of General Chemical Technology)
One of the reasons causing nonuniformity in the grain products of chromium electrocorundum (CE), obtained at the Yurginsk Abrasive Plant, is the very imperfect method of preparing the initial charge from alumina and chromium oxide, which does not make it possible to obtain a mixture completely homogeneous in composition. This, in particular, causes considerable segregation of the raw materials during smelting of the block.
The most reliable method making it possible to obtain a charge practically homogeneous in composition is to replace solid chromium oxide with an aqueous solution of ammonium dichromate.
During subsequent drying and heating of the charge to \(185^\circ\mathrm{C}\), ammonium dichromate decomposes with the formation of very finely dispersed chromium oxide according to reaction I:
\[ (\mathrm{NH}_4)_2\mathrm{Cr}_2\mathrm{O}_7 = 2\mathrm{CrO}_3 + 2\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O}(\text{vapor}) - 64.5\ \text{kcal}, \]
\[ 2\mathrm{CrO}_3 = \mathrm{Cr}_2\mathrm{O}_3 + 1.5\mathrm{O}_2 - 5.6\ \text{kcal}, \]
\[ 2\mathrm{NH}_3 + 1.5\mathrm{O}_2 = \mathrm{N}_2 + 3\mathrm{H}_2\mathrm{O}(\text{vapor}) + 151.3\ \text{kcal}. \]
Thus, the overall reaction for the decomposition of ammonium dichromate has the following form:
\[ (\mathrm{NH}_4)_2\mathrm{Cr}_2\mathrm{O}_7 = \mathrm{Cr}_2\mathrm{O}_3 + \mathrm{N}_2 + 4\mathrm{H}_2\mathrm{O}(\text{vapor}) + 81.2\ \text{kcal}. \]
The chromium oxide released as a result of the reaction is found to be very uniformly distributed over the entire external as well as internal surface of the alumina particles.
To obtain a charge practically uniform in composition, it is necessary that its moisture content be below the value of the maximum molecular moisture capacity for alumina, i.e., 27%.
The use of ammonium dichromate makes it possible to reduce the amount of chromium oxide introduced into the charge to 1% of the weight of the alumina. For this purpose, for each ton of charge it is necessary to take 16.6 kg of ammonium dichromate and 990 kg of alumina.
The industrial smelting of an electrocorundum block carried out with ammonium dichromate used as the source of chromium, and subsequent separation of the block according to the plant procedure, gave the following results:
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For analysis, the \(+0.16\) mm fraction was used, as containing the largest amount of impurities.
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From the table it may be concluded that the use of an aqueous solution of ammonium dichromate as the source of chromium oxide makes it possible to increase the yield of pink grains in the product by 22%, and consequently to improve the quality of the electrocorundum.
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The use of ammonium dichromate did not increase the duration of block smelting in comparison with the plant technology and did not cause increased electrode consumption.
On the basis of the work performed, the following conclusions may be drawn:
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It has been established that replacing solid chromium oxide with an aqueous solution of ammonium dichromate makes it possible to obtain a charge that is practically homogeneous in composition.
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The use of ammonium dichromate solution also makes it possible to regulate more precisely the chromium oxide content in the finished product and thereby reduce the consumption of chromium oxide for the production of electrocorundum to 1% or less.
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Carrying out an experimental smelting of a charge block prepared with an aqueous solution of ammonium dichromate shows that the corundum obtained is more homogeneous in composition and contains 22% more pink grains than the product smelted by the existing plant technology.
Table
| Grains | Before beneficiation | Before beneficiation | After beneficiation | After beneficiation |
|---|---|---|---|---|
| Grains | plant block | experimental block | plant block | experimental block |
| Chromium electrocorundum | 62.80 | 81.80 | 63.80 | 85.70 |
| White electrocorundum | 28.58 | 8.90 | 29.50 | 9.20 |
| Impurities | 8.62 | 9.30 | 6.70 | 5.10 |
References
- A. A. Shidlovskii, S. A. Oranzhereev. ZhKh, vol. 26, no. 1, p. 25, 1953.
- S. I. Mitrofanov. Investigation of Ores for Beneficiation. Metallurgizdat, 1954.