Solid-phase reactions during the heat treatment of extremely coarse raw meal in the kiln

The reactivity of extremely coarse feed mixture consisting of clay with a low content of clay minerals (30%) and high content of quartz and feldspar (65-70%) with grain size mostly under 500 μm (sometimes greater) and the maximum grain size of the solid component of the raw slurry up to 1.6 mm was examined in the preparatory zones of the kiln of ø 5 × 185 m. The sample temperature in the 91st meter of the preparatory zone of the kiln was about 700 °C. Through the solid phase reactions SiO2 binds into silicates in granules of 0-1 mm (41.77%), 1-3 mm (9.13%) and > 3 mm (8.98%). Since at 700°C an almost complete assimilation of the produced CaO takes place, it should be assumed that for the granules of 1-3 and > 3 mm the amount of CaO for binding of SiO2 into belite is about 7-9% of its total content in the mixture. This corresponds to the extent of calcium carbonate calcination at 700°C. A significantly higher degree of decarbonization of 0-1 mm granules in the 91 m of the ø 5 × 185 m kiln, as compared with 1-3 and > 3 mm fractions suggests that 0-1 mm fraction consists predominantly of the material removed by the flue gases from the calcining zone. A comparison of the amount of the bound SiO2 associated with the results of quantitative X-ray analysis of the raw mix by fractions makes it possible to establish that it is grains of quartz and feldspar smaller than 40 μm that are capable of solid phase reactions in the range of 700-1000°C. In addition, these data allow us to estimate the amount of SiO2, which remains chemically free in the decarbonization zone at 900-1000°C. The quantitative X-ray diffraction and thermodynamic studies confirm the ideas of Jander and Hoffmann that by means of solid-phase reactions in the CaO-SiO2 system at ratios of CaO to SiO2 < 2, capable of solid-phase reactions, for example, equal to 1 : 1 and 3 : 2, first C2S is formed, which with SiO2 assimilation turns to CS or C3S2.

Author: P. Loerke

Section: Science and production

Keywords: extremely coarse raw meal, fraction, solid phase reactions, qualitative X-ray analysis, free energy