Exploring ball size distribution in coal grinding mills

• BSD affects milling efficiency.
• Ball top up policy in turn affects BSD.
• Wear rate models can predict BSD for a given top up policy.
• Fine BSD generally enhances milling rate for uncontaminated coal feed.

Tube mills use steel balls as grinding media. Due to wear in the abrasive environment it is necessary to charge new balls periodically to maintain a steady balanced ball charge in the mill. The amount and ball size distribution in this charge, as well as the frequency with which new balls are added to the mill, have significant effects on the mill capacity and the milling efficiency. Small balls are effective in grinding fine particles in the load, whereas large balls are required to deal with large particles of coal or stone contaminant. The steady state ball size distribution in the mill depends on the top-up policy.The effect of the ball size distribution on the milling rate of coal has been measured as a function of ball size distribution. The change in ball size distribution as affected by wear and ball top-up policy has been modelled. From this a best ball top-up policy can be recommended that will ensure a close approximation to the desired steady-state ball size distribution that gives the required PF size distribution for the selected mill demand.

Optimising PF production by controlling ball size distribution.Figure optionsDownload as PowerPoint slide