Maximizing wear resistance of balls for grinding of coal

A Brazilian power plant consumes mineral coal with high ash contents to feed four turbines to generate 450 MW. Four ball mills, 4.2 m in diameter, are fed with crushed coal to deliver the material ground to sizes less than 0.075 μm. High wear rates were observed when using forged high-carbon steel balls.A pilot-plant ball mill, with a 0.01 m3 chamber, was used to perform wear tests, comparing the original ball material and seven grades of high chromium cast irons, grinding the same coal as used in the industrial plant. Test pieces were 60-mm balls with different chemical compositions and heat treatments. Balls were weighed after 10-h grinding periods, for up to 7 periods. The mass losses were converted to equivalent diameter losses and regression straight lines were obtained, showing the wear rates of the materials.The wear rates of the cast irons with 25–30% Cr were only 10–20% of the wear rate of the forged steel. Cast irons with 15–18% Cr wore at around 50% of the rate of the steel.Analyses of the coal ashes showed around 40% of quartz as 10–20 μm particles. Examination of the surface of worn balls showed that these particles caused the wear.