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Highspeed (over 300 RPM) impact pulverizerMediumspeed (between 70 RPM and 300 RPM) ball and raceLowspeed (under 70 RPM) ball tube mill Highspeed impact pulverizer Coal is pulverized by the impact of hammers on large pieces of coal, attrition (grinding) of smaller pieces on each other, and the grinding surface of the mill.