Dynamic image based shape analysis of hard and lignite coal particles ground by laboratory ball and gyro mills

• Particle shapes of lignite and hard coal by ball and gyro mills were compared.
• Shape characterization was performed by 3-D (real-time) image analysis.
• At least 6400 particles/sample were measured at the confidence level of 99%.
• Ball milled particles had the higher aspect ratios and the lower circularity.
• Gyro milled particles had the higher circularity and the lower aspect ratio.

A new real-time 3D dynamic image analysis system was used to measure the particle shapes of different type of coal samples (lignite and hard coals) ground by different mills (ball and gyro mills). Shape factors such as circularity, ellipse aspect ratio (EAR), bounding rectangle aspect ratio (BRAR) and Feret aspect ratio (FAR) described the results. At least 6400 particles were measured for each mill product indicating the accuracy of the results achieved with 99% statistical significance. Although experiments were performed on different ranked coal samples, there is a clear difference in the particle shapes generated by the grinding methods tested. Results indicated that the ball mill ground particles had the highest aspect ratios (EAR, BRAR, and FAR) with the lowest circularity. However, the gyro mill produced particles with highest circularity along with the lowest aspect ratios. Thus ball milled particles had the highest elongated particles with the lowest rounded particles. On the other hand, gyro mill products included the highest rounded particles and the lowest elongated particles. This relative difference in shape was attributed to the difference in grinding action involved in the ball and gyro mills. Results will find applications in various coal characterization, preparation, and utilization processes.

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