Study of the Effect of Heat Treatment in a Fluidized Bed of Dispersed Chromium Electrocorundum on the Efficiency of Its Magnetic Beneficiation
Abstract Generated abstract
This study examines whether oxidative heat treatment of finely dispersed chromic electrocorundum in a fluidized bed can improve subsequent magnetic beneficiation compared with treatment in a muffle furnace. Samples of 40 to 70 micron material were roasted under previously established optimal conditions and then separated magnetically, while the stability limits of the fluidized bed were also determined. The fluidized bed existed over a narrow gas velocity range, from 0.28 to 0.5 m/s, and produced a magnetic fraction yield 1.2 to 1.4 times higher than muffle treatment, with slightly reduced carryover of chromium spinels into the magnetic fraction. The results suggest that fluidized bed oxidative treatment can intensify impurity removal and improve the grain composition of chromium spinel products.
Full Text
Study of the Effect of Heat Treatment in a Fluidized Bed of Disperse Chromic Electrocorundum on the Efficiency of Its Magnetic Beneficiation
A. S. Bogma, V. M. Vityugin, I. P. Chashchin, L. V. Sokolov
(Presented by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
Powder products of chromic electrocorundum (ChE), produced by the block process at the Yurga Abrasive Plant, have a nonuniform composition. In the finished ChE fractions, particles of white electrocorundum and various impurities are present in significant quantities; among these, metallic iron and chromium, chromium oxide, and carbides make up the greatest mass. As the dispersity of the powder increases, the amount of impurities grows.
One means of improving the quality of ChE powder products is oxidative roasting followed by magnetic separation. During oxothermal treatment, certain nonmetallic impurities, in particular particles of metallic chromium, pass into a magnetically susceptible form and, during magnetic separation, are removed into the magnetic fraction, which makes it possible to obtain a purer ChE product.
Previously, we carried out heat treatment of samples in a muffle furnace with natural air circulation, and experimentally established the optimal conditions for roasting and magnetic beneficiation of ChE powders: a temperature of 500°C, a treatment duration of 30 minutes, and subsequent separation at a magnetic-field strength of 3500 oersteds. In order to determine the possibility of further intensification of the oxothermal treatment of ChE powders, their heat treatment under optimal conditions was carried out in a fluidized bed [1].
Samples of the material studied (particle size 40–70 microns) were loaded into a metal conical thermal chamber (lower diameter 55 mm, upper diameter 120 mm, chamber height 280 mm). Glass fiber was used as the gas-distribution grid. For visual observation of the process in the chamber, an inspection window was provided on its side surface. Air, heated to the required temperature in a tubular electric furnace, was supplied beneath the gas-distribution grid; the spent air was removed from the chamber by a vacuum pump. Dust carryover was determined visually; for this purpose, the spent air first passed through a dust-settling glass chamber.
The first stage of the investigations was devoted to clarifying the overall pattern of the behavior of the ChE bed under the action of a stationary flow of heat carrier and to determining the range of velocities for stable existence of the fluidized bed. It was experimentally established that the transition velocity to the fluidized state is minimal and amounts to 0.28 m/sec, while the particle carryover velocity is 0.5 m/sec. In the interval of these velocities, the fluidized bed was in a clearly pronounced spouting regime.
Samples that had undergone heat treatment in the fluidized-bed chamber were subjected to magnetic separation on a laboratory single-roller separator. The quantitative composition of the samples studied was determined by microanalytical analysis. The experimental results are given in Table 1.
Table 1
Yield of the magnetic fraction and microanalytical analysis of the magnetic fractions of chromium spinels
| Item | Heat treatment of chromium-spinel samples in a muffle furnace | Heat treatment of chromium-spinel samples in a fluidized bed |
|---|---|---|
| Yield of magnetic fraction, % | 0.163--0.174 | 0.197--0.244 |
| FeCr, Mn, Fe, carbides, % | 66.11 | 66.73 |
| Fe$_2$O$_3$, % | 7.82 | 9.12 |
| Cr$_2$O$_3$, % | 1.74 | 1.51 |
| Chromium spinels, % | 24.33 | 22.64 |
As can be seen from Table 1, during exothermic treatment of chromium-spinel powders in a fluidized bed, the yield of the magnetic fraction is 1.2--1.4 times greater than during heat treatment of chromium-spinel samples in a muffle furnace, while the carryover of chromium spinels into the magnetic fraction decreases by 1.69%. This is explained by the specific nature of exothermic treatment of chromium-spinel powders in a fluidized bed, namely that, because of the high abrasiveness of the material, intensive abrasion of impurities occurs. The considerable content of iron oxides in the magnetic fraction is evidently due to the magnetization of hematites during oxidative roasting in the temperature range 450--550°C.
Conclusions
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It has been established that the fluidized bed of finely dispersed chromium spinels has a narrow range of existence velocities: the first critical velocity is 0.28 m/s, and the second critical velocity is 0.5 m/s.
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It is more expedient to carry out oxidative treatment of granular chromium-spinel products in a fluidized bed, which, on the one hand, intensifies the process and, on the other hand, makes it possible to improve the grain composition of chromium spinels.
References
- P. G. Romankov, N. B. Rashkovskaya. Drying in the Fluidized State. Leningrad, Khimiya Publishing House, 1968.