Chemical Reactions of Zircon Decomposition by Calcium Carbonate

P. G. Usov, E. P. Solomatina

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

In recent years, many new zircon deposits have been explored in the Soviet Union, among them the Tugan deposit of Tomsk Oblast.

On the basis of experimental studies carried out by the staff of the basic laboratory for mineral raw materials of the Tomsk Polytechnic Institute [1], optimal conditions are recommended for the process of opening zircon concentrate from the sands of the Tugan deposit by sintering it with chalk. The present communication includes studies on the chemistry of the interaction of zircon with calcium carbonate, which is a continuation of the work already done. Only after this question has been clarified will it be possible to proceed to the study of the kinetics of the process of zircon decomposition by calcium oxide and to give theoretically substantiated technological recommendations.

In the work we used zircon concentrate from the Tugan deposit with a content of $\mathrm{ZrSiO_4}$ of 96.97%, of natural grain size, which is 0.074 mm, and chemically pure calcium carbonate. The use of $\mathrm{CaCO_3}$ is due to its stability under ordinary conditions in an air atmosphere and to the fact that the calcium oxide obtained upon dissociation is the most reactive.

Depending on the ratio of calcium oxide and zircon and on the sintering conditions, the decomposition products contain calcium zirconate and calcium silicates, free calcium oxide [2], calcium orthosilicate, a solid solution of $\mathrm{CaO}$ in $\mathrm{ZrO_2}$ of monoclinic or cubic modification, glassy constituents [3], calcium zirconosilicates, and zirconium dioxide [4]. Analysis of the cited literature showed that the conclusions of some authors regarding the chemistry of the process of interaction of zircon with calcium oxide are contradictory, especially when zircon is sintered with reduced amounts of $\mathrm{CaO}$, which are of interest for the technology of zirconium compounds.

We set ourselves the task of studying the behavior of zircon from the deposit under investigation when sintering it with 1, 2, and 3 moles of calcium oxide in the temperature range 900–1300°C. The temperatures were chosen so that the reactions possible under these conditions would proceed at an appreciable rate. After thorough mixing of the initial components, tablets 14 mm in diameter and 2 mm thick were pressed from the charge under a pressure of 700 kg/cm²; these were fired in an electric furnace with carborundum hea-

heaters. Holding at the final firing temperature was two hours.

To study the products of the interaction of zircon with calcium oxide, high-temperature X-ray phase and petrographic methods of analysis were used. X-ray phase analysis was carried out on a URS-50u diffractometer in Cu-Kα radiation with a high-temperature attachment. The attachment is a furnace with a platinum-rhodium heater, making it possible to perform X-ray studies during heating up to 1500°C. X-ray patterns were recorded from semicylindrical specimens pressed from powders of the mixtures under study. Recording during exposure was carried out continuously by constant reversing within a specified interval of angles \(2\Theta\). As a result of comparing the principal indices of the X-ray patterns of the initial components and possible interaction products, the interval \(2\Theta = 24–34^\circ\) was selected; in this interval, at the initial moment, two diffraction maxima corresponding to zircon and \(\mathrm{CaCO_3}\) are clearly recorded. When the specimen is heated, the diffraction pattern changes, and differently depending on the ratio of the initial components. The experimental data are presented in Table 1.

Table 1

Phase composition of the products of interaction of calcium oxide with zircon according to high-temperature X-ray phase analysis

Analyzing the experimental data, we propose the following chemistry of the process of zircon decomposition by CaO.

The interaction of zircon with calcium oxide at their molar ratio in the mixture occurs according to the reaction:

\[ \mathrm{ZrSiO_4 + CaO = CaSiO_3 + ZrO_2.} \]

The presence of the ternary compound \(\mathrm{Ca_3ZrSi_2O_9}\) indicates that the final interaction products are not formed immediately, but sequentially through a series of intermediate reactions according to the scheme proposed by Kordyuk [5].

\[ \mathrm{2CaO + ZrSiO_4 = Ca_2SiO_4 + ZrO_2;} \]

\[ \mathrm{ZrO_2 + CaO = CaZrO_3;} \]

\[ \mathrm{Ca_2SiO_4 + ZrSiO_4 = 2CaSiO_3 + ZrO_2;} \]

\[ \mathrm{2CaSiO_3 + CaZrO_3 = Ca_3ZrSi_2O_9;} \]

Fig. 1. Microphotograph of a mixture of zircon with calcium carbonate, 400×: a—unfired; b—fired at 1100°C; c—fired at 1300°C.

\[ \mathrm{Ca_3ZrSi_2O_9 + ZrSiO_4 = 3CaSiO_3 + 2ZrO_2;} \]

\[ \mathrm{CaO + ZrSiO_4 = CaSiO_3 + ZrO_2.} \]

When zircon is sintered with two moles of calcium oxide, the final products are \(\mathrm{CaZrO_3}\) and \(\mathrm{Ca_3ZrSi_2O_9}\); the interaction proceeds according to the reaction:

\[ \mathrm{ZrSiO_4 + 2CaO = 0.5CaZrO_3 + 0.5Ca_3ZrSi_2O_9.} \]

The interaction of zircon with three moles of calcium oxide is described by the reaction:

\[ \mathrm{ZrSiO_4 + 3CaO = CaZrO_3 + Ca_2SiO_4.} \]

The presence of the ternary compound of composition \(\mathrm{Ca_3ZrSi_2O_9}\) can be explained by the occurrence of side reactions caused by the fact that \(\mathrm{Ca_2SiO_4}\) and \(\mathrm{CaSiO_3}\) do not coexist with \(\mathrm{ZrSiO_4}\), i.e., along with the reaction forming \(\mathrm{CaZrO_3}\), the following reactions proceed:

\[ \mathrm{2Ca_2SiO_4 + ZrSiO_4 = Ca_3ZrSi_2O_9 + CaSiO_3;} \]

\[ \mathrm{3CaZrO_3 + 2ZrSiO_4 = Ca_3ZrSi_2O_9 + 4ZrO_2.} \]

Microscopic examination of the samples after heat treatment, carried out by the immersion method, confirmed the results of the high-temperature X-ray phase analysis. It showed that the interaction of zircon with calcium carbonate begins in the temperature range \(900\text{—}1000^\circ\mathrm{C}\), independently of the ratio of the initial components in the mixture, and is manifested in the appearance of reaction rims around the zircon grains. At the same time, the zircon crystals retain their habit but acquire a brown coloration. Kukolev [6] explains the acquisition of anisotropy by defects in the structural lattice caused by dissociation of zircon.

With increasing firing temperature, the picture changes depending on the ratio of the initial components in the mixture, which indicates differences in the process of interaction of zircon with calcium oxide. In the sample from a mixture with the ratio \(\mathrm{ZrSiO_4 : CaO = 1 : 1}\), fired at \(1300^\circ\mathrm{C}\) with a 2-hour hold, the crystalline phase is represented by cubic \(\mathrm{ZrO_2}\), pseudowollastonite, and the ternary compound \(\mathrm{Ca_3ZrSi_2O_9}\). Samples from mixtures with the ratio \(\mathrm{ZrSiO_4 : CaO = 1 : 2}\) and \(1 : 3\), after the same heat treatment, consist of a loose anisotropic mass that is the product of the interaction, \(\mathrm{CaZrO_3}\).

Petrographic analysis revealed a definite sequence in the sintering process of zircon with calcium oxide, as is seen in Fig. 1 a, b, c. The interaction of zircon with calcium oxide proceeds in the solid phase in layers, from the surface of each zircon crystal. Loosening and spalling of the reaction product occur because the compounds that form have a larger molecular volume than zircon. This also explains the reason for the high rate of the reaction in a comparatively low temperature range: the reaction product does not form a dense layer around the grains; by spalling, it continuously exposes clean surfaces of the zircon crystal.

At present, we are continuing studies of the kinetics of solid-phase reactions between calcium oxide and zircon.

REFERENCES

1. A. K. Beloglazov, M. M. Nolle, V. M. Vityutin. Report on topic 508/59 “Study of the decomposition of zircon concentrate,” Tomsk, 1959.

2. A. Sh. Vaks, E. A. Pepelyaeva. Collection of scientific papers of Giredmet, Technology, vol. 1, 546, 1959.

3. G. N. Voronkov, E. I. Medvedovskaya. Proceedings of the State Research Electroceramics Institute, No. 2, 109, 1957.

4. R. A. Kordyuk, N. V. Gulko. Doklady of the Academy of Sciences of the USSR, 142, No. 3, 639, 1962.

5. R. A. Kordyuk. Collection of scientific papers, UNIIIO, issue 7, 213, 1963.

6. G. V. Kukolev. Chemistry of Silicon and Physical Chemistry of Silicates. Vysshaya Shkola Publishing House, Moscow, 1966.