Several methods to measure the structures of coal aggregates are compared. Loose and compact coal aggregates were generated through flocculation of ultrafine coal particles (mean volume diameter of 12 μm) under specific shearing conditions. Aggregate structure in terms of mass fractal dimension, Df, was determined using various methods; namely 2D and 3D image analysis, interpretation of intensity patterns from small angle light scattering, changes in aggregation state through light obscuration, and settling behavior. In this study, the measured values of Df ranged from 1.84–2.19 for coal aggregates with more open structures, and around 2.27–2.66 for the compact ones. All of these approaches could distinguish structural differences between aggregates, albeit with variation in Df values estimated by the different techniques. The discrepancy in the absolute values for fractal dimension is due to the different physical properties measured by each approach, depending on the assumptions used to infer Df from measurable parameters. In addition, image analysis and settling techniques are based on the examination of individual aggregates, such that a large number of data points are required to yield statistically representative estimations. Light scattering and obscuration measure the aggregates collectively to give average Df values of the particulate systems; consequently ignoring any structural variation between the aggregates, and leaving possible small contaminations undetected (e.g. by dust particles or air bubbles). Appropriate utilization of a particular method is thus largely determined by system properties and required data quality.
In separati on processes, th e kn ow ledge of parti cle size and density arc often not enough to describe th e settlin g behav iour in a co ncentrated suspension. Therefore, a direc t meth od for the characteri za ti on of the settlin g behav ior of submi cron particles in co nce ntrated suspensions is introduced in a centrifugal fi eld by a manometri c sedimentati on analys is. By mean s of thi s cumulative meth od in a homogeneou s suspension, the analyses o f both the in ter fac ial settlin g rate and th e se ttlin g rate of th e parti cles within th e co nce ntrated suspension arc possible. Thi s permits a
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