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The reactions occurring during the firing of a calcareous illitic-chloritic clay and of a kaolin clay in the presence of 2% NaCl were studied by gravimetric, thermal, chemical, and X-ray powder diffraction analysis. The NaCl reacted with both clays to form volatile hydrochloric acid. For the kaolin clay, this reaction led to the complete removal of chlorine at about 520°C. For the calcareous illitic-chloritic clay, however, part of the product HCl was volatilized and part promoted the decomposition of carbonate impurities and formed potassium chloride, which volatilized at a higher temperature. The addition of the NaCl resulted in a significant decrease in the amount of residual potassium and a progressive elimination of residual Cl in samples fired above 750°C.

The emission of alkali-element compounds from NaCl-containing calcareous clays used for heavy clay products causes degradation of the refractory lining of tunnel kilns in brickworks. To minimize this degradation, it is proposed to inject steam-saturated air into an appropriate zone of the kiln where the temperature is less than 500°C to enhance the formation and subsequent emission of HCl in this ”low” temperature range, thereby reducing the amount of alkali-element compounds volatilized at higher temperatures.

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