PURIFICATION OF CORUNDUM FROM IRON OXIDES BY TREATMENT IN THE GAS PHASE

P. G. Usov, A. T. Vrlia

(Presented by the scientific seminar of the Department of Silicate Technology)

At present, the principal industrial method for fine grinding of calcined alumina and corundum is vibromilling with the use of steel balls [1, 2]. In such grinding, considerable iron is abraded; in the form of various oxides it is firmly retained on the surface of the highly dispersed corundum particles. Iron in any form is one of the most harmful impurities in alumina intended for the production of high-quality dielectric materials; therefore it is removed by treatment with a solution of hydrochloric acid. The method consists of the following operations: alumina in a vessel is poured over with a hydrochloric-acid solution and stirred; the suspension is heated in a water bath to 70–80°C, after which the solution is decanted, and the dense residue is washed with distilled water [3]. This method has a number of disadvantages:

1) high labor intensity when working with large batches of material,

2) a large loss of finely dispersed alumina fractions.

To purify corundum from iron oxides, we tested a method of treating dry powder with a gas mixture of carbon monoxide and carbon tetrachloride. In the dry treatment method, iron in the form of chlorides is removed as a gas phase, and the treated material does not require washing with water. The chlorination method is used in the production of rare metals, in obtaining iron chlorides from oxides, and in the chemical purification of kaolin [4,5]. Electric-fused white corundum with a grain size below 3 μm, ground in the dry state in an M-10 vibromill with steel balls, was treated. The amount of abraded iron in it was 1.15% by weight of the material. The corundum, in a quartz boat, was placed in the quartz tube of a reactor, which was heated to the specified temperature in a tubular furnace with silite heaters. After the specified temperature had been reached, a gas mixture of CO₂ + CCl₄ was blown into the tube containing the corundum. The gas leaving the reactor was passed through absorbing solutions to capture the chlorides of iron and aluminum (Fig. 1). In the solution, the amount of iron and aluminum chloride carried out from the reaction zone (from the corundum) was determined.

Figure 2 shows the dependence of the degree of conversion of iron oxides in the sample of material into chlorides, of the iron extracted from corundum in the form of FeCl₃, on temperature. The chlorination time at each tempera-

Fig. 1. Diagram of the laboratory setup for studying the process of enriching corundum from iron oxides in the gas phase: 1) and 2) absorption bottles with distilled water acidified with chemically pure HCl and chemically pure HNO₃; 3) quartz reactor tube; 4) tubular electric furnace with silit heaters; 5) platinum–platinum-rhodium thermocouple; 6) millivoltmeter; 7) quartz boat; 8) absorption bottle with CCl₄; 9) absorption bottle with CCl₂.

The holding time was maintained at 15 minutes. In the temperature interval from 300 to 600°C, the degree of conversion of the iron oxides contained in the corundum increases almost linearly with increasing temperature. In the temperature range 600–800°C, the percentage of iron converted to chloride is a practically constant value, equal to 40%. This phenomenon is due to the fact that the rate of conversion on the linear portion of the curve is determined mainly by the vapor pressure of FeCl₃. The formation of FeCl₃ may be represented by the following reactions:

\[ \mathrm{Fe_2O_3 + 6Cl_2 = 4FeCl_3 + 3O_2,} \tag{1} \]

\[ \mathrm{3O_2 + 3C = 3CO_2.} \tag{2} \]

Summing (1) and (2), we obtain

\[ \mathrm{2Fe_2O_3 + 3CCl_4 \rightleftarrows 4FeCl_3 + 3CO_2.} \tag{3} \]

Fig. 2. Increase in the degree of conversion of iron oxides in the weighed sample of material to chlorides with increasing temperature.

The second stage of reaction (2) shifts equilibrium (3) to the right, which agrees well with the work of V. I. Spitsyn [7]. Carbon has a catalytic effect in the reaction, substantially lowering the onset of interaction of iron oxides with chlorine [7]; thus, already at a temperature of 300°C the degree of conversion of iron oxides to chloride reaches 12.8%, whereas the vapor pressure of FeCl₃ becomes equal to atmospheric pressure only at 320°C [8]. Therefore, it may be assumed that the second factor affecting the rate of the chemical reaction in the temperature range 300–600°C is the degree of dissociation of carbon tetrachloride. In the second stage, in the temperature zone 600–800°C, the process is mainly limited by the chlorination time. As the time during which the CO₂ + CCl₄ gas mixture is passed through the reactor is increased, the weight losses of the charge of the material being treated increase substantially and can reach 7.16% when it is chlorinated for 60 min.

Fig. 3. Increase in the weight loss of corundum powder as a function of chlorination time at a temperature of 850°C.

The exponential character of the curve expressing the dependence of the material’s weight loss on chlorination time (Fig. 3) indicates the predominant role of diffusion in the process of removing iron chloride vapors. Removal proceeds more intensively when the thickness of the layer of the material being treated in the boat is decreased, or when the powder is stirred during chlorination. Beginning at a temperature of 600°C, aluminum oxide starts to be chlorinated. The amount of Al₂O₃ lost by the charge in the form of AlCl₃ at 600°C is 0.474%, which agrees with the data of other authors [7]. When the enriched powder is heated to 700°C and above, FeCl₂ begins to appear in the waste gas, as indicated by the green coloration of the absorbing solutions. This phenomenon usually occurs when, after the dissociation of CCl₄ in the reactor, a large amount of carbon appears that has not entered into the reaction.

It is likely that, in this case, the reactions proceed according to the following scheme:

\[ \mathrm{CCl_4 \to C + 2Cl_2,} \tag{4} \]

\[ \mathrm{C + CO_2 \rightleftarrows 2CO,} \tag{5} \]

\[ \mathrm{Fe_2O_3 + CO \rightleftarrows 2FeO + CO_2,} \tag{6} \]

\[ \mathrm{FeO + Cl_2 + CO \rightleftarrows FeCl_2 + CO_2.} \tag{7} \]

Fig. 4. Dependence of the weight loss of powder of the enriched material on temperature at a chlorination time of 15 min.: 1) charge of white corundum powder weighing 4 g; 2) charge of white corundum powder weighing 20 g; 3) charge of chromium corundum weighing 20 g.

The formation of FeCl₂ under analogous conditions is also noted by other authors [4], who determined that, in the temperature range 700–730°C, the total amount of iron chlorides evolved contains 25% FeCl₂. Since the vapor pressure of FeCl₂ reaches 760 mm Hg only at 1012–1026°C [8], the decrease in the increase of the weight loss of the enriched material at temperatures exceeding 600°C may be explained by an increase in the amount of FeCl₂ formed (Fig. 4). The appearance of ferrous iron chlorides does not make it possible to completely purify corundum from ground-in iron, since at high temperatures the losses of aluminum oxide increase substantially, which is highly undesirable. When corundum powder heated to 500°C is chlorinated for 40 min, the content of iron impurity in it decreases from 1.15% to 0.06%, which is quite acceptable for the production of a high-quality dielectric based on aluminum oxide.

Conclusions

1. Purification of corundum from iron by treatment with a gaseous mixture is more practical technologically. This method can be continuous and fully mechanized.

1. Intensive removal of iron impurities during corundum beneficiation occurs in the temperature range of 500–600°C. During 40 min of treatment with the gas mixture $\mathrm{CO_2 + CCl_4}$, the iron content in corundum decreases to 0.06%.

2. Raising the chlorination temperature above 600°C leads to the removal of the highly dispersed fraction of corundum and can substantially reduce the quality of its purification because of the formation of a significant amount of $\mathrm{FeCl_2}$.

REFERENCES

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