MOLECULAR THERMODYNAMICS OF MOLTEN-SALT EVAPORATION .4. THE EVAPORATION OF MOLTEN CARBONATES IN ATMOSPHERES CONTAINING CO2 AND WATER-VAPOR

The evaporation of the molten alkali carbonates Li2CO3, Na2CO3, K2CO3, and the mixed alkali carbonate melts (Li/K)(2)CO3, and (Li/Na)(2)CO3 have been investigated by the transpiration method using as transport media oxidizing gas mixtures containing CO2, H2O and O-2, and reducing gas mixtures without oxygen containing additionally hydrogen and carb on monoxide. The rate of transpiration of species containing alkali me tal ions emanating from the pure and mixed carbonate melts, investigat ed in the temperature range from 1023 to 1143 K, increases with the ra tio of the partial pressures of water vapour and carbon dioxide accord ing to (p(H2O)/p(CO2))(1/2). This is due to evaporation of alkali hydr oxide, MeOH, generated in the melts by hydrolysis of the carbonate spe cies (CO32- + H2O <-> 2OH(-) + CO2). An enhanced volatility of alkali in reducing atmospheres could not be detected. Hydrolysis and evaporat ion seem to be in fast, established equilibrium on the time scale of t he transpiration experiment after establishing liquid/gas equilibrium for several hours (contact and residence rime of the transporting gase s: 1 to 10 s). Investigation of mixed (Li/K)(2)CO3 melts shows that po tassium hydroxide possesses a higher vapour pressure than lithium hydr oxide in the temperature range from 650 degrees C to 750 degrees C, wh ich is relevant for molten carbonate fuel cells. Above (Li/Na)(2)CO3 m elts the vapour pressure of LiOH is seizeably lower than above Li2CO3, and the NaOH pressure is more than a factor of five lower than p(LiOH ) but slightly enhanced compared to the NaOH pressure above Na2CO3. Th e effective vaporization enthalpies of the alkali hydroxide above Li2C O3, Na2CO3 and K2CO3 are 310, 260 and 200 kJ/mol. Additional investiga tion of the pure alkali carbonate melts by Knudsen mass-spectrometry a nd Knudsen effusion/deposition as well as vapor transpiration with dry carbon dioxide of the pure solid carbonates reveal only for potassium carbonate evaporation of the molecule K2CO3 with seizable vapour pres sures, resulting at 910 degrees C in p(K2CO3) approximate to (2+/-1) . 10(-6) bar. The effective sublimation enthalpy of K2CO3 from Knudsen mass-spectrometry amounts to approximately 300 kJ/mol.