$T_1$ and $T_2$, respectively, and were reduced to $1\ \mathrm{cm}^3$ of volume, since during the transition from one temperature to another a certain change in volume occurred. $T_1$ and $T_2$ are absolute temperatures, °K. The activation energy of sintering of corundum with 0.05 wt.% additions of zinc, chromium, and magnesium oxides was found to be equal to:

for the temperature interval 1573–1623°K — 331 kcal/mol;

for the temperature interval 1773–1823°K — 559 kcal/mol.

Conclusions

1. To implement the dynamic method, a special high-temperature dilatometric apparatus (SVDU) was created. With the aid of the SVDU, the dynamics of the sintering rate was followed, and it was established that the selected oxide composition accelerates the densification process of electrocorundum powder during firing; the optimum holding time exceeds 2 hours.

2. The activation energy of sintering of a corundum specimen with 0.05% zinc, chromium, and magnesium was calculated; at 1573–1623°K it is 331 kcal/mol. At temperatures of 1773–1823°K, the activation energy reaches 559 kcal/mol.

3. The limiting shrinkage rate of specimens with small additions of magnesium, zinc, and chromium is observed at a concentration of 0.075 wt.%. Deviation of their content in either direction from the optimum concentration by 0.025% significantly reduces the densification rate of corundum powder during firing.

References

1. J. L. Woolfrey, M. J. Bannister. Methods of nonisothermal investigation of the initial stage of sintering. Glass, Ceramics and Refractories, Express Information, 1973, No. 7.

2. V. A. Ivensen. Kinetics of Densification of Metallic Powders during Sintering. Moscow, Metallurgizdat, 1971.

3. E. V. Degtyareva, I. S. Kainarskii. Doklady Akademii Nauk SSSR, 156, 4, 937, 1964.

SIMULTANEOUS DETERMINATION OF CHROMIUM AND IRON IN CORUNDUM

A. V. Voznyak, V. V. Koyan

The granular products of chromium electrocorundum (EC) currently being 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 grinding powder No. 16, containing only 39% grains of electrocorundum and 11% impurities.

One of the possible ways to improve the quality of electrocorundum is its chemical beneficiation by treatment with a mineral acid. With such treatment, all impurities capable of dissolving in acids are separated. Hydrochloric acid was taken as the reagent.

In order to determine the optimal beneficiation conditions, acid of various concentrations was used: 1, 3, 5, 10, 15, 20, 25%; treatment durations were 5, 10, 20, 25, and 30 min.

The method of chemical beneficiation is simple and consists of the following: a 10 g sample of electrocorundum 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, while in the filtrate the combined content of chromium and iron is determined by the iodometric method /1/. In this method, the combined content of chromium and iron ions is determined indirectly from the amount of sodium thiosulfate used to titrate the liberated iodine.

The course of the determination is as follows: 10 ml of the analyzed solution of the acid extract are neutralized with 15% caustic potassium solution to an alkaline reaction; 10% hydrogen peroxide solution is added until the color of the solution changes from green to yellow, and the solution is boiled for 10 min to decompose the excess peroxide.

As a result of this treatment, trivalent chromium is oxidized to hexavalent chromium according to the reaction

\[ 2CrCl_3 + 3H_2O_2 + 10KOH = 2K_2CrO_4 + 6KOH + 8H_2O. \tag{1} \]

Then the solution is cooled, neutralized with sulfuric acid (1:3), and a further 1 ml excess of sulfuric acid is added.

To the solution treated in this way, 10 ml of 12% potassium iodide solution are added.

\[ 2K_2CrO_4 + H_2SO_4 = K_2Cr_2O_7 + K_2SO_4 + H_2O \tag{2} \]

\[ K_2Cr_2O_7 + 6KI + 7H_2SO_4 = Cr_2(SO_4)_3 + 4K_2SO_4 + 7H_2O + 3I_2 \tag{3} \]

\[ Fe_2(SO_4)_3 + 2KI = 2FeSO_4 + K_2SO_4 + I_2 . \tag{4} \]

The free iodine liberated in the reaction is titrated with 0.1 N sodium thiosulfate solution in the presence of starch

\[ I_2 + 2Na_2S_2O_3 = Na_2S_4O_6 + 2NaI . \tag{5} \]

The data on the simultaneous determination of chromium and iron are presented in the table.

It follows from the table that the best results in enriching EC are obtained when it is treated with 10% hydrochloric acid for 20 min.

If a separate determination of the chromium and iron contents in the acid extracts is necessary, the \(Fe\) ions should first be removed from the solution being analyzed, which can be achieved by extracting them with ether or by precipitating them with cupferron / 2 /.

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 EC decreases from 11 to 3.5%.

Conclusions

1. Treatment of granular EC products with hydrochloric acid makes it possible to substantially improve the quality of the abrasive materials obtained.

2. The best results in enriching EC with hydrochloric acid are obtained when 10% acid is used and the treatment duration is 20 min.

Literature

1. G. N. Babachev. Khimiya i industriya (Bolg.), 4, 36, 145–149, 1964.

2. G. Charlot. Methods of Analytical Chemistry. Moscow–Leningrad, “Khimiya,” 1966.

Table I.

Amount of sodium thiosulfate \((Na_2S_2O_3)\) consumed for titration of the solution as a function of the HCl concentration and irradiation duration