Simultaneous Determination of Chromium and Iron in Corundum
Abstract Generated abstract
This paper examines chemical beneficiation of granular chromium electrocorundum contaminated with metallic chromium, iron, carbides, and related acid soluble impurities. Samples were boiled with hydrochloric acid of varying concentrations and treatment times, then separated into enriched corundum and acid extract, with the combined chromium and iron content of the extract determined iodometrically through oxidation, iodide reaction, and thiosulfate titration. The results indicate that hydrochloric acid treatment can markedly improve abrasive quality, with the reported optimum at 10 percent acid for 20 minutes, and microanalytical checks showing a reduction of impurities from 11 percent to 3.5 percent.
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SIMULTANEOUS DETERMINATION OF CHROMIUM AND IRON IN CORUNDUM
A. V. Voznyak, V. V. Koyan
(Presented by the scientific seminar of the Department of General Chemical Technology)
The granular products of chromium electrocorundum (EC) currently obtained contain impurities of metallic chromium and iron, carbides, and other contaminants, the content of which ranges from 3.2 to 11%. The most contaminated is abrasive grain No. 16, containing only 39% EC grains and 11% impurities.
One possible way to improve the quality of EC is chemical beneficiation by treatment with a mineral acid. In such treatment, all impurities capable of dissolving in acids are separated. Hydrochloric acid was chosen as the reagent.
In order to determine the optimum conditions for beneficiation, acid of various concentrations was used: 1, 3, 5, 10, 15, 20, 25%, and the treatment time was 5, 10, 20, 25, 30 minutes.
The procedure for chemical beneficiation is simple and consists of the following: a 10 g weighed portion of EC is treated with 50 ml of hydrochloric acid under boiling in a flask with a reflux condenser.
After completion of the acid treatment, the contents are separated by filtration into enriched corundum and an acid extract. The enriched product is subjected to microanalytical analysis, and the combined content of chromium and iron in the filtrate is determined iodometrically [1]. In this procedure, the combined content of chromium and iron ions is determined indirectly from the amount of sodium thiosulfate used for titration of the liberated iodine.
The course of the determination is as follows: 10 ml of the analyzed solution of the acid extract is neutralized with a 15% potassium hydroxide solution to an alkaline reaction, a 10% hydrogen peroxide solution is added until the color of the solution changes from green to yellow, and the mixture is boiled for 10 minutes to decompose the excess peroxide.
As a result of this treatment, trivalent chromium is oxidized to hexavalent chromium according to the reaction:
\[ 2\mathrm{CrCl}_3 + 3\mathrm{H}_2\mathrm{O}_2 + 10\mathrm{KOH} = 2\mathrm{K}_2\mathrm{CrO}_4 + 6\mathrm{KCl} + 8\mathrm{H}_2\mathrm{O}. \tag{1} \]
Then the solution is cooled, neutralized with sulfuric acid (1:3), and a further 1 ml excess of acid is added. To the solution thus treated, 10 ml of a 12% potassium iodide solution is added:
\[ 2\mathrm{K}_2\mathrm{CrO}_4 + \mathrm{H}_2\mathrm{SO}_4 = \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7 + \mathrm{K}_2\mathrm{SO}_4 + \mathrm{H}_2\mathrm{O}; \tag{2} \]
\[ \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7 + 6\mathrm{KI} + 7\mathrm{H}_2\mathrm{SO}_4 = \mathrm{Cr}_2(\mathrm{SO}_4)_3 + 4\mathrm{K}_2\mathrm{SO}_4 + 7\mathrm{H}_2\mathrm{O} + 3\mathrm{I}_2; \tag{3} \]
\[ \mathrm{Fe}_2(\mathrm{SO}_4)_3 + 2\mathrm{KI} = 2\mathrm{FeSO}_4 + \mathrm{K}_2\mathrm{SO}_4 + \mathrm{I}_2. \tag{4} \]
The free iodine liberated by the reaction is titrated with 0.1 N sodium thiosulfate solution in the presence of starch
\[ \mathrm{I_2 + 2Na_2S_2O_3 = Na_2S_4O_6 + 2NaI.} \tag{5} \]
The data for the simultaneous determination of chromium and iron are given in the table. It is evident from the table that the best results in enriching electrocorundum are obtained when it is treated with 10% hydrochloric acid for 20 minutes.
If a separate determination of the chromium and iron content in acid extracts is required, the Fe ions should first be removed from the solution being analyzed; this can be achieved by extracting them with ether or by precipitating them with cupferron [2].
Table
Amount of sodium hyposulfite used for titrating the solution as a function of the concentration of hydrochloric acid and the duration of enrichment
| Time (min) | 1% hydrochloric acid | 3% hydrochloric acid | 5% hydrochloric acid | 10% hydrochloric acid | 15% hydrochloric acid | 20% hydrochloric acid | 25% hydrochloric acid |
|---|---|---|---|---|---|---|---|
| 5 | 0.0245 | 0.0300 | 0.0332 | 0.0411 | 0.0411 | 0.0411 | 0.0403 |
| 10 | 0.0316 | 0.0443 | 0.0475 | 0.0555 | 0.0522 | 0.0490 | 0.0190 |
| 15 | 0.0300 | 0.0395 | 0.0475 | 0.0617 | 0.0617 | 0.0610 | 0.0617 |
| 20 | 0.0300 | 0.0490 | 0.0507 | 0.0562 | 0.0602 | 0.0602 | 0.0617 |
| 25 | 0.0316 | 0.0475 | 0.5550 | 0.0555 | 0.0602 | 0.0602 | 0.0602 |
| 30 | 0.0364 | 0.0545 | 0.0570 | 0.0570 | 0.0670 | 0.0602 | 0.0602 |
The results obtained for the simultaneous determination of chromium and iron were also confirmed by microanalytical analysis, which showed that, upon acid treatment, the content of impurities in electrocorundum decreases from 11 to 3.5%.
On the basis of the foregoing, the following conclusions may be drawn:
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Treatment of granular electrocorundum products with hydrochloric acid makes it possible to substantially improve the quality of the abrasive materials obtained.
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The best results in the enrichment of electrocorundum with hydrochloric acid are obtained when 10% acid is used and the treatment lasts 20 minutes.
REFERENCES
- G. N. Babachev. Chemistry and Industry, Bulgaria, 4, 36, 145–149, 1964.
- G. Charlot. Methods of Analytical Chemistry. Moscow–Leningrad, “Khimiya,” 1966.