Fluxing reactions of sulfates and carbonates in cement clinkering. II. The system CaCO3-K2CO3: Fid-Bau Portal

Fluxing reactions of sulfates and carbonates in cement clinkering. II. The system CaCO3-K2CO3

Arceo, H.B. / Glasser, F.P.

The system CaCO3-K2CO3 has been investigated using 25 compositions. Three solid binary phases were encountered: CaK2(CO3)2, Ca2K2(CO3)3 and a third having the most probable composition (2K2CO3)(5CaCO3). Three eutectics were located by thermal methods. In a CO2 atmosphere at 1 bar pressure, melts did not lose significant CO2. Taking the results of this study, together with other recent investigations of carbonate-sulfate systems, it is apparent that calcium and potassium carbonates exhibit regions of comparatively low melting point, 750 deg C to 820 deg C in the case of K2CO3 and CaCO3. Moreover, such melts are relatively stable; in a CO2 atmosphere, decomposition with loss of carbon dioxide occurs only slowly. Fluxing systems based on molten carbonates, and possibly sulfates, are potentially operative in the cooler zones of long kilns and in the suspension preheat zones of modern, energy efficient kilns. The role of low-temperature liquid formation in the ultimate production of calcium silicates is as yet unknown. But these melts have relatively low viscosities and surface tensions and are good solvents for a variety of oxides. Hence their transient existence during the initial stages could affect the main clinkering reactions. The existence of low melting point compositions does have another and perhaps less favourable consequence; the melts contribute to ring formation in the long rotary kiln and to particle agglomeration in suspension preheaters. The formation of low melting point eutectics with alkali salts also helps to remove alkali from kiln atmospheres. This may be desirable to control emissions, but is les desirable in limiting the alkali content of the resulting clinker.