Temperature at which calcium carbonate changes to calcium oxide
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Answer:
Calcination reactions usually take place at or above the thermal decomposition temperature (for decomposition and volatilization reactions) or the transition temperature (for phase transitions). This temperature is usually defined as the temperature at which the standard Gibbs free energy for a particular calcination reaction is equal to zero. For example, in limestone calcination, a decomposition process, the chemical reaction is
CaCO3 → CaO + CO2(g)
The standard Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol).[3] The standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or 848 °C.
Examples of chemical decomposition reactions common in calcination processes, and their respective thermal decomposition temperatures include:
CaCO3 → CaO + CO2; 848 °C
Calcination of limestone using charcoal fires to produce quicklime has been practiced since antiquity by cultures all over the world. The temperature at which limestone yields calcium oxide is usually given as 825 °C, but stating an absolute threshold is misleading. Calcium carbonate exists in equilibrium with calcium oxide and carbon dioxide at any temperature. At each temperature there is a partial pressure of carbon dioxide that is in equilibrium with calcium carbonate. At room temperature the equilibrium overwhelmingly favors calcium carbonate, because the equilibrium CO2 pressure is only a tiny fraction of the partial CO2 pressure in air, which is about 0.035 kPa.
At temperatures above 550 °C the equilibrium CO2 pressure begins to exceed the CO2 pressure in air. So above 550 °C, calcium carbonate begins to outgas CO2 into air. However, in a charcoal fired kiln, the concentration of CO2 will be much higher than it is in air. Indeed if all the oxygen in the kiln is consumed in the fire, then the partial pressure of CO2 in the kiln can be as high as 20 kPa.[36]
The table shows that this equilibrium pressure is not achieved until the temperature is nearly 800 °C. For the outgassing of CO2 from calcium carbonate to happen at an economically useful rate, the equilibrium pressure must significantly exceed the ambient pressure of CO2. And for it to happen rapidly, the equilibrium pressure must exceed total atmospheric pressure of 101 kPa, which happens at 898 °C.
Equilibrium pressure of CO2 over CaCO3 (P) vs. temperature (T).[37]
P (kPa) 0.055 0.13 0.31 1.80 5.9 9.3 14 24 34 51 72 80 91 101 179 901 3961
T (°C) 550 587 605 680 727 748 777 800 830 852 871 881 891 898 937 1082 1241
But, three Au/TiO2 catalysts, with the same Au loading and with different particle sizes, were prepared by the deposition–precipitation method followed by calcination at three different temperatures, 473, 573, and 873 K. The mean diameters of Au particles were 2.4, 2.5, and 10.6 nm, respectively. On all the samples the CO adsorption and different CO–O2 interactions were examined by FTIR at 90 K and at room temperature. The higher catalytic activity on CO oxidation found for the samples calcined at 473 and 573 K is related to the higher concentration of step sites over the Au surfaces and to a higher concentration of step sites at the borderline with the support. At 90 K, CO and molecular oxygen are competitively adsorbed on step sites. By CO pre-adsorption on hydrated catalysts, the reaction with O2 gives CO2 already at 90 K, while by oxygen pre-adsorption the reaction is completely inhibited, unless moisture is present in the gas phase.
Hence Parvin, the Au/TiO2 catalyst seems to have the desired effect on the reduction of calcination temperature. Check it out and save energy!