Fractal assessment of finely ground limestone for flue gas desulphurisation

Industrial minerals, those utilised for their physical properties and chemical content, e.g., limestone, often employ comminution during their preparation. A study was performed to investigate if the type of pulveriser used to comminute limestone, a grade suitable for flue gas desulphurisation, influenced its morphology and subsequent behaviour. Samples of the limestone were pulverised by three different devices a hammer mill, rolls crusher and ball mill. A narrow size fraction was removed from each of the products by elutriation. These fractions were then subjected to sulphuric acid digestion tests, to simulate the desulphurisation process. The tests revealed that the amount of acid consumed, i.e., limestone utilisation, varied with the pulverisers under study, with the hammer mill having the highest limestone utilisation. From SEM micrographs of the limestone fragments their overall geometric shape and fractal dimension were determined, using image analysis procedures. The fractal dimension is a useful descriptive parameter for characterising boundaries that take on a randomly rugged appearance. Using the classical geometric shape factors, aspect ratio and circularity, one could not distinguish between the three comminution products. However, fractal analysis was sensitive enough to differentiate between the fragments produced by the different pulverisers. Hammer milling produced the most rugged particles followed by rolls crushing and ball milling respectively.