STUDY OF THE EFFECT OF ADDITIVES ON THE KINETICS OF THE INTERACTION OF ZIRCON WITH CALCIUM CARBONATE

P. G. USOV, E. P. SOLOMATINA

(Presented by the scientific seminar of the inorganic departments of the KhTF)

A large number of studies [1—4] have been devoted to the influence of various additives in the process of decomposition of zircon by calcium carbonate. On the basis of an analysis of the cited literature, we set ourselves the task of investigating the effect of \(\mathrm{Na_2CO_3}\) and \(\mathrm{CaCl_2}\), introduced in amounts of 1, 3, 5, and 8% by weight of the charge, on the interaction of zircon with calcium carbonate at ratios in the mixture \(\mathrm{ZrSiO_4 : CaO = 1 : 1,\ 1 : 2,\ 1 : 3}\), which is of the greatest industrial importance.

In the work we used zircon concentrate with a \(\mathrm{ZrSiO_4}\) content of 97.5% from the sands of the Tugan deposit and chemically pure reagents: calcium carbonate, sodium carbonate, and calcium chloride. The use of calcium carbonate is due to its stability under ordinary conditions in an air atmosphere and to the fact that the calcium oxide obtained upon its dissociation is the most reactive. The introduction of additives in amounts of 1, 3, 5, and 8% by weight of the charge is explained by the fact that, as has been established in many works, for substantial intensification of reactions in crystalline mixtures, an additive content of 1—3% in them is sufficient.

Of great importance in studying the influence of additives is the method of preparing the charge; thorough mixing of the constituent components is necessary in order to achieve close contact of the additive with the crystalline mixture. Taking this into account, the components of the charge with additives were mixed in the slip state with an electric stirrer for 30 minutes. The charges were then dried in air and on a sand bath.

The optimum amount of additive introduced, exerting an effective influence on the interaction of zircon with calcium oxide at different stoichiometric ratios of the components in the mixture, was determined by differential thermal analysis. Thermograms were recorded on a Kurnakov pyrometer using, as the standard, corundum calcined at \(1200^\circ\mathrm{C}\). The recording conditions were kept constant and were as follows: rate of temperature rise \(10^\circ\) per minute, thermocouples \(Pt/Pt,\ Rh\), additional resistance 500 ohms. The thermograms obtained are presented in Fig. 1.

On the thermograms of mixtures of zircon with calcium carbonate, one endothermic effect is observed at \(960^\circ\mathrm{C}\), corresponding to the dissociation of calcium carbonate; moreover, the area of this effect noticeably increases with increasing \(\mathrm{CaCO_3}\) content in the mixture. When \(\mathrm{Na_2CO_3}\) is introduced as an additive to the initial mixtures, on the thermograms

Fig. 1, a. Thermograms of mixtures of zircon with calcium carbonate.
1a with additions of $\mathrm{Na_2CO_3}$
1b with additions of $\mathrm{CaCl_2}$

(Fig. 1a) a second endothermic effect appears at $840^\circ\mathrm{C}$, corresponding to the dissociation of soda. It was noted that the presence of this effect with an increase in the $\mathrm{CaCO_3}$ content in the charge is recorded at a higher content of the introduced additive. Common to all the mixtures studied is that, with an increase in the amount of introduced $\mathrm{Na_2CO_3}$, a decrease is observed in the area of the endothermic effect of $\mathrm{CaCO_3}$ dissociation and its shift into the region of lower temperatures; moreover, these changes are most substantial at the molar ratio of the components in the mixture.

In the thermograms of mixtures of zircon with calcium carbonate with additions of $\mathrm{CaCl_2}$ (Fig. 1b), a sharp decrease in the area of the endothermic effect of $\mathrm{CaCO_3}$ decarbonization and its shift toward lower temperatures, approximately by $40$–$100^\circ$, depending on the amount of additive introduced, is also noticeable. In addition, in the thermograms, upon introduction of $\mathrm{CaCl_2}$, two endothermic effects are observed at $560$–$580^\circ$ and $600$–$620^\circ\mathrm{C}$. To determine the nature of these peaks, thermograms were recorded

Fig. 1, c. Thermograms of mixtures of zircon with calcium carbonate.

Curves:
a — $\mathrm{ZrSiO_4:CaO}=1:1$,
b — same $=1:2$,
c — same $=1:3$.

1 — thermograms of mixtures without additions
2 — $1\%$ $\mathrm{Na_2CO_3}$, $1\%$ $\mathrm{CaCl_2}$
3 — $3\%$ same, $3\%$ same
4 — $5\%$ same, $5\%$ same
5 — $8\%$ same, $8\%$ same

pure $\mathrm{CaCl_2 \cdot 6H_2O}$ and a mixture of $\mathrm{CaCl_2}$ with calcium carbonate. On the first, two endothermic effects are noted at 40 and 220°C, corresponding to dehydration, and on the second, endothermic effects at 560–600°C. It may be assumed that the presence of these effects on the thermograms of the mixtures studied is due to the formation of a double compound which, with the additive, gives a low-melting eutectic that promotes acceleration of the dissociation of $\mathrm{CaCO_3}$, and thereby also the direct interaction of zircon with calcium oxide.

The decrease in the endothermic effect (960°) upon introducing additives into the mixtures can be explained as follows. The interaction of zircon with $\mathrm{CaO}$ begins in the temperature range 900–1000°C and is accompanied by heat absorption. In the presence of additives in the mixtures that accelerate this interaction, this process coincides with the process of dissociation of $\mathrm{CaCO_3}$, accompanied by heat release. Thus the exothermic effect is superimposed on the endothermic one, with a decrease in the area of the latter and its shift into the region of lower temperatures. Consequently, from the degree of decrease in the area of the endothermic effect at 960°C, one can judge the course of the interaction of zircon with calcium oxide and the effectiveness of the additive introduced. The presence of minimum points at 8% $\mathrm{Na_2CO_3}$ and 5% $\mathrm{CaCl_2}$ on the curves of the change in the area of the endothermic effect with change in the percentage content of additive in the mixture (Fig. 2) indicates the effectiveness of the action of a definite amount of additive under the given conditions of the process.

Fig. 2. Change in the relative area of the endothermic effect of $\mathrm{CaCO_3}$ dissociation upon introduction of additives.
2a — $\mathrm{Na_2CO_3}$, 2b — $\mathrm{CaCl_2}$ for mixtures
1 — $\mathrm{ZrSiO_4 : CaO = 1 : 1}$; 2 — $\mathrm{ZrSiO_4 : CaO = 1 : 2}$; 3 — $\mathrm{ZrSiO_4 : CaO = 1 : 3}$

The kinetics of the interaction of zircon with calcium oxide at the ratio $\mathrm{ZrSiO_4 : CaO = 1 : 2}$ in the presence of 1% $\mathrm{CaCl_2}$ as an additive was studied on specimens 14 mm in diameter and 2 mm high. The specimens were pressed on a hydraulic press under a pressure of 750 kg/cm² and then fired in an electric furnace with silicon-carbide heating elements at temperatures of 1200, 1250, 1300, and 1350°C with holding times from 0 to 2 hours at 20-minute intervals. The firing temperature was controlled with a $\mathrm{Pt/Pt, Rh}$ thermocouple. The fired specimens were cooled in a desiccator.

over anhydrous calcium chloride in order to avoid carbonation of free calcium oxide. The content of free CaO in the samples after heat treatment was monitored; on this basis, curves were determined for the dependence of the amount of CaO bound by zircon on time.

For describing the experimental data, a power equation of the following form proved satisfactory:

\[ \lg \frac{a}{a-x}=B\cdot Z^n . \]

The rate of binding of CaO at any moment can be expressed as:

\[ \frac{dx}{dz}=\frac{n(a-x)^2}{a\cdot Z\cdot 2.3}, \]

where

\(a\) is the amount of CaO bound by the beginning of the holding period, in %,

\(x\) is the amount of CaO bound over a certain time interval \(Z\), in %,

\(n\) is determined as the tangent of the angle of inclination of the straight line \(\frac{a}{a-x}—Z\) to the abscissa axis.

The reaction-rate constant was determined from the expression \(K=\frac{dx}{dz}\cdot Z\); the calculated values are given in Table 1. As can be seen from Fig. 3, gra-

Table 1

Values of the rate constants of the reactions of interaction of zircon with calcium carbonate

Fig. 3. Dependence of \(\lg K\) on \(1/T^\circ K\).
1 — mixture \(\mathrm{ZrSiO_4:CaO}=1:2\) without additives;
2 — the same, but with 1% \(\mathrm{CaCl_2}\).

the dependences of \(\lg K\) on \(1/T^\circ\) for charges without an additive and with an additive have the form of straight lines over the entire temperature interval in which the reactions were studied.

On the basis of the obtained values of the reaction-rate constants at different temperatures, the values of the activation energy of the process of binding CaO with zircon were calculated from the Arrhenius equation. The calculated values of \(E\), kcal/mol, are given in Table 2.

Table 2

Values of \(E\), kcal/mol

Conclusions

1. Of the investigated amounts of the introduced additives \(\mathrm{Na_2CO_3}\) and \(\mathrm{CaCl_2}\), the introduction of 8% \(\mathrm{Na_2CO_3}\) and 5% \(\mathrm{CaCl_2}\) by weight of the charge has the most effective influence.

2. From the kinetic point of view, the presence of 1% \(\mathrm{CaCl_2}\) in the charge increases the rate of the reaction of zircon with calcium oxide up to a temperature of 1250°C.

3. From the energetic point of view, the introduction of 1% \(\mathrm{CaCl_2}\) into the charge significantly lowers the activation energy, thereby causing an acceleration of the process of opening up zircon concentrate during its sintering with chalk.

References

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