Petrographic modelling of Egyptian limestones for quicklime manufacture

Abstract

The main objective of this article was to construct a mathematical model for the optimum petrographic variables that affect quicklime production along with others (physico-mechanical and chemical) during Egyptian limestone calcination at different firing conditions. Limestone microstructure is a key player during its calcination. Eighteen limestone technological samples representing the limestones in ten occurrences were petrographically analysed through the microscopic examination of a large number of thin sections in detail. The description and the semi-quantitative measures of the petrographic variables were determined by the aid of the descriptive and comparison charts. The data obtained permitted the delineation of 14 petrographic compositional and textural variables which, in turn, led to the recognition of different petrographic facies. The limestone technological samples were fired at eight firing conditions (900°C and 1,000°C for 15, 30, 60 and 120 min) in an electric muffle. After firing, the obtained quicklime was ground and chemically analysed for free lime determination. The graphical presentations of the relationships between the independent (petrographic) and the dependent (free lime measures) variables declared different behaviours with different strength (correlation) of each petrographic variable on the rate of lime liberation. To recognise the petrographic compositions for the optimum lime yield at each firing condition, statistical (mathematical) models were constructed. The obtained petrographic and free lime data sets were statistically analysed by applying the stepwise regression method. This method examines the relationships between each dependent variable (free lime at eight firing conditions) and the independent variables (14 petrographic measures). This treatment of data led finally to eight mathematical models which involve the more important compositional and textural variables which affect the rate of lime liberation at each firing condition (temperature and soaking time). The relations between the observed and expected cumulative probabilities at all firing conditions proved that the acquainted models require further enhancement to be more applicable. This is quite acceptable especially when it is known that the chemical and physico-mechanical properties are also key players in the quicklime manufacture. Microstructure studies are also recommended for the fired limestone (lime) for testing these models where the type and amount of each petrographic variable could be determined pre- and post-firing.