Dear all,
If powdered CaF2 (Fluorite) is mixed with powdered CaCO3 (Calcite) the decarbonation temperature (in air) will be substantially lower compared to calcite.
What is the exact reaction taking place here? As decarbonisation will occur at a temperature around 600 degC is it a solid state reaction, is it due to fluor emission from CaF2?
Hello Fredrik,
Actually, there is some evidence that thermal decomposition of calcium carbonate mixed with calcium fluoride in fact shifts to higher temperatures due to the formation of a CaCO3.CaF2 eutectic liquid phase whose temperature of formation occurs within the temperature range of CaCO3 decomposition.
CaCO3(s) + CaF2(s) ---> CaCO3.CaF2(l)
http://www.springerlink.com/content/k7m24j874828r181/
Another paper confirms this, stating that although CaCO3.CaF2 forms at temperatures below 600 degC, it does not decompose and release its CO2 until above 850 deg C.
CaCO3.CaF2(l) ----> CaO.CaF2(s) + CO2 (g)
See;- http://www.sciencedirect.com/science/article/pii/S0304389406000720
Regards,
Ted.
Ted Krapkat:
thermal decomposition of calcium carbonate mixed with calcium fluoride in fact shifts to higher temperatures
Exactly. If you want to lower this temperature, you must to mix pure carbonate with acidic oxides like SiO2 or clay minerals (Taylor). We found, that after pressing the raw mix, containing carbonates, on 10-12% decreased total enthalpy decomposition of CaCO3.
Silastman
Hello Silastman,
Yes, SiO2 and clay minerals react with CaO and remove it quickly from the equation...
CaCO3 --> CaO + CO2
...so speeding up the reaction in accord with Le Chatelier's principle and thereby reducing the effective decarbonation temperature to below ~700degC instead of 850degC.
Several other compounds such as sulphates, P2O5 and transition metal oxides (eg Cr2O3, SnO2, ZnO and particularly CuO) also reduce decarbonation temperature of calcite.
Regards,
Ted.