Thermodynamic Characterization of Reactions in the CaCO3-CaO-ZnSiO4 System
Abstract Generated abstract
This paper presents a thermodynamic analysis of reactions in the CaCO3, CaO, ZrSiO4 system in order to estimate the sequence of formation and relative stability of zirconium-containing compounds. Isobaric potential changes were calculated for proposed reactions at ZrSiO4:CaO ratios of 1:1, 1:2, and 1:3 over 500 to 1600 K, using literature thermochemical data and approximate methods for missing constants. The calculations indicate that reaction between CaCO3 and ZrSiO4 is theoretically possible from about 520 K, and they establish stability sequences for the studied compositions. Calcium zirconate is inferred to form initially in all mixtures, while high-temperature stability depends on composition, with ZrO2 favored at 1:1, ZrO2 and CaZrO3 comparable at 1:2, and CaZrO3 with Ca3ZrSi2O9 favored at 1:3.
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THERMODYNAMIC CHARACTERIZATION OF REACTIONS IN THE SYSTEM CaCO₃—CaO—ZrSiO₄
P. G. Usov, E. P. Solomatina
(Presented by the scientific seminar of the Department of Silicate Technology)
In studying reactions in mixtures of solid substances, in order to establish the sequence of formation of various compounds, the thermodynamic description of the corresponding reactions is of great importance. The aim of thermodynamic analysis is to study the regularities in the change of the isobaric potential and, on this basis, to compile thermodynamic sequences for the formation of compounds.
According to literature data, in the system CaCO₃—CaO—ZrSiO₄ the formation of the following compounds is most probable: CaZrO₃, ZrO₂, Ca₃ZrSi₂O₉, Ca₂ZrSi₄O₁₂. To evaluate the thermodynamic sequence of formation of these compounds, we considered the dependence of \(\Delta Z\) of the reactions on the composition of the initial mixtures. Values of \(\Delta Z\) were analyzed for ratios of the initial mixtures ZrSiO₄:CaO = 1:1, 1:2, 1:3 in the temperature interval 500—1600°K.
The thermochemical data required for the calculation were taken from literature sources [3, 4]. The missing thermal constants were calculated by an approximate method: \(\Delta H_{298}\)—by the modified method of C. A. Shukarev’s isoatoms [1, 2], \(S_{298} = \sum S_{298}\) of oxides, suitable for determining the thermal constants of silicates, zirconates, titanates.
The use of approximate calculation methods makes it possible to obtain thermodynamic constants with an accuracy of 3—5%, which is quite acceptable for practical use. The calculation of reactions, proceeding from simple thermodynamic data, was carried out according to the scheme proposed by Medlov-Petrosyan [3].
The \(\Delta Z\) values of the following reactions were calculated:
I. At the ratio ZrSiO₄ : CaO = 1 : 1
\[ 1.\ \mathrm{ZrSiO_4 + CaCO_3 = ZrO_2 + CaSiO_3 + CO_2;} \]
\[ 2.\ \mathrm{ZrSiO_4 + CaCO_3 = CaZrO_3 + SiO_2 + CO_2;} \]
\[ 3.\ \mathrm{ZrSiO_4 + CaCO_3 = \frac{1}{4}(Ca_2ZrSi_4O_{12}) + \frac{1}{2}CaO + \frac{3}{4}ZrO_2 + CO_2;} \]
\[ 4.\ \mathrm{ZrSiO_4 + CaCO_3 = \frac{1}{3}(Ca_3ZrSi_2O_9) + \frac{1}{3}SiO_2 + \frac{2}{3}ZrO_2 + CO_2;} \]
II. At the ratio ZrSiO₄ : CaO = 1 : 2
\[ 5.\ \mathrm{ZrSiO_4 + 2CaCO_3 = Ca_2SiO_4 + ZrO_2 + 2CO_2;} \]
\[ 6.\ \mathrm{ZrSiO_4 + 2CaCO_3 = CaZrO_3 + CaSiO_3 + 2CO_2;} \]
\[ 7.\ \mathrm{ZrSiO_4 + 2CaCO_3 = \frac{1}{4}(Ca_2ZrSi_4O_{12}) + \frac{3}{2}CaO + \frac{3}{4}ZrO_2 + 2CO_2;} \]
\[ \text{8. }\quad \mathrm{ZrSiO_4}+2\mathrm{CaCO_3} = \frac{1}{2}(\mathrm{Ca_3ZrSi_2O_9})+\frac{1}{2}\mathrm{CaZrO_3}+2\mathrm{CO_2}; \]
III. At the ratio \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:3\)
\[ \text{9. }\quad \mathrm{ZrSiO_4}+3\mathrm{CaCO_3} = \mathrm{ZrO_2}+\mathrm{Ca_3SiO_5}+3\mathrm{CO_2}; \]
\[ \text{10. }\quad \mathrm{ZrSiO_4}+3\mathrm{CaCO_3} = \mathrm{CaZrO_3}+\mathrm{Ca_2SiO_4}+3\mathrm{CO_2}; \]
\[ \text{11. }\quad \mathrm{ZrSiO_4}+3\mathrm{CaCO_3} = \frac{1}{4}(\mathrm{Ca_2ZrSi_4O_{12}})+\frac{3}{4}\mathrm{ZrO_2}+\frac{5}{2}\mathrm{CaO}+3\mathrm{CO_2}; \]
\[ \text{12. }\quad \mathrm{ZrSiO_4}+3\mathrm{CaCO_3} = \frac{1}{2}(\mathrm{Ca_3ZrSi_2O_9})+\frac{1}{2}\mathrm{CaZrO_3}+3\mathrm{CO_2}+\mathrm{CaO}. \]
The results obtained are summarized in Table 1 and presented in Fig. 1 a, b, c.
The results of the calculations of \(\Delta Z\) for the compositions made it possible to compile series of the thermodynamic stability of the compounds. These series are presented in the form of the corresponding Table 2, in which the value of the isobaric potential increases from left to right, and the stability correspondingly decreases.
Table 2
Thermodynamic sequence of stability of compounds of the system
\(\mathrm{CaCO_3—CaO—ZrSiO_4}\)
| Ratio \(\mathrm{ZrSiO_4}:\mathrm{CaO}\) | Thermodynamic series of stability, beginning with the maximally stable compound in the interval \(1100—1600^\circ\mathrm{K}\) |
|---|---|
| 1:1 | \(\mathrm{ZrO_2},\ \mathrm{Ca_2ZrSi_4O_{12}},\ \mathrm{CaZrO_3},\ \mathrm{Ca_3ZrSi_2O_9}\) |
| 1:2 | \(\mathrm{ZrO_2},\ \mathrm{CaZrO_3},\ \mathrm{Ca_2ZrSi_4O_{12}},\ \mathrm{Ca_3ZrSi_2O_9}\) |
| 1:3 | \(\mathrm{CaZrO_3},\ \mathrm{Ca_3ZrSi_2O_9},\ \mathrm{Ca_2ZrSi_4O_{12}}\) |
Analysis of the curves obtained makes it possible to draw the following general conclusions. Theoretically, the onset of the reaction between \(\mathrm{CaCO_3}\) and \(\mathrm{ZrSiO_4}\) proves possible beginning at \(520^\circ\mathrm{K}\).
At the ratio \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:1\), \(\mathrm{ZrO_2}\) has the greatest thermodynamic probability of formation; it is also the stable compound in the high-temperature region.
At the ratio \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:2\), the primary product of formation is \(\mathrm{CaZrO_3}\), but in the temperature interval \(1100—1600^\circ\mathrm{K}\), \(\mathrm{ZrO_2}\) and \(\mathrm{CaZrO_3}\) have equal thermodynamic stability; therefore, the existence of \(\mathrm{CaZrO_3}\) and \(\mathrm{ZrO_2}\) in this temperature interval will be determined by kinetic factors. At the ratio \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:3\), the primary products, and those stable in the high-temperature region, are \(\mathrm{CaZrO_3}\) and \(\mathrm{Ca_3ZrSi_2O_9}\).
It should be noted that the initial temperature of formation of calcium zirconate in all the mixtures studied is lower than the temperature of formation of the other compounds. Therefore it may be assumed that, irrespective of the ratio of the starting components in the mixture, the primary process is the formation of \(\mathrm{CaZrO_3}\), which subsequently enters into interaction with excess starting components with the formation of other compounds.
REFERENCES
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V. B. Glushkova. In: Silicates and Oxides in the Chemistry of High Temperatures. Publishing House of the Academy of Sciences of the USSR, 1963.
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S. A. Shchukarev. Scientific Notes of Leningrad State University, Chemical Series. Issue 7, No. 79, 197, 1945.
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V. I. Babushkin, G. M. Matveev, O. P. Mchedlov-Petrosyan. Thermodynamics of Silicates. Stroiizdat, Moscow, 1965.
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Edited by Glushko. Thermodynamic Properties of Individual Substances. Moscow, Publishing House of the Academy of Sciences of the USSR, 1962, vols. 1, 2.
Change in \(\Delta Z\) with temperature for reactions in the system \(\mathrm{CaCO_3—CaO—ZnSiO_4}\)
Table 1
\(\Delta Z\), kcal/mol, at temperature, K
| Reaction no. | 500 | 600 | 700 | 800 | 900 | 1000 | 1100 | 1200 | 1300 | 1400 | 1500 | 1600 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2800 | −2200 | −5900 | −9400 | −12800 | −16100 | −19800 | −23700 | −23700 | −23800 | −23000 | −23000 |
| 2 | 21900 | 16100 | 13500 | 9700 | 5900 | 1800 | −2100 | −5000 | −5000 | −6000 | −6000 | −6500 |
| 3 | 24400 | 20300 | 16300 | 13200 | 9000 | 5000 | 2700 | −5700 | −7500 | −8300 | −9000 | −9700 |
| 4 | 25800 | 22700 | 19300 | 15700 | 12000 | 9200 | 6100 | −1200 | −1400 | −1600 | −1900 | −2190 |
| 5 | 20700 | 16800 | 11700 | 8000 | 4000 | −24500 | −27300 | −33450 | −33600 | −33900 | −34500 | −34500 |
| 6 | 24000 | 17000 | 9700 | 2100 | −5200 | −12300 | −19500 | −26300 | −26600 | −26900 | −27100 | −27700 |
| 7 | 48500 | 44700 | 33500 | 25000 | 19600 | 11400 | 44500 | −2900 | −3400 | −3750 | −4500 | −4500 |
| 8 | 48500 | 40700 | 33500 | 25000 | 19600 | 11400 | 3600 | −1100 | −1260 | −1400 | −1550 | −1700 |
| 9 | 42000 | 31000 | 20300 | 10000 | −300 | −11000 | −21600 | 7000 | 8800 | 12700 | 15500 | 18350 |
| 10 | −3500 | 16200 | 44600 | 64000 | 85000 | −16000 | −27000 | −36300 | −36750 | −37200 | −37300 | −38600 |
| 11 | — | — | 50600 | 37000 | 27200 | 16400 | 7900 | 6000 | 8000 | 10000 | 13000 | 14000 |
| 12 | — | — | 50600 | 37000 | 26200 | 11100 | 6500 | −450 | −250 | −350 | −300 | −250 |
Fig. 1. Dependence of \(\Delta Z\) of reactions in the \(\mathrm{CaCO_3}—\mathrm{CaO}—\mathrm{ZrSiO_4}\) system on temperature in the range \(500—1600^\circ\mathrm{K}\) for the ratios:
a) \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:1\);
b) \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:2\);
c) \(\mathrm{ZrSiO_4}:\mathrm{CaO}=1:3\).