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Under normal circumstances, the diameter of steel balls used in ball mills ranges from Φ20 mm to Φ125 mm (for example, small balls of 40 mm and 60 mm, medium balls of 80 mm, large balls possibly 100 mm or 120 mm). For extra-large ball mills, steel balls with diameters of Φ130 mm to Φ150 mm may be used.

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Normally, there is no need to add small balls during grinding operations, because during normal operation of the ball mill, reasonable friction between steel balls, between steel balls and ore, and between steel balls and the mill liner will gradually wear down the larger balls into smaller ones — i.e., large balls are ground down to medium balls, and medium balls to small balls.

The proportion of different-sized steel balls varies, but when the ball mill is first commissioned, it is necessary to use a mix of large, medium, and small balls. The initial addition should be 80 % of the maximum recommended ball charge specified by the manufacturer. This is because after installation, the large and small gears of the ball mill need to mesh properly, and the processing capacity must be increased gradually. After the ball mill has run continuously and normally for two to three days, check the gear meshing condition; once everything is normal, open the mill’s access port and add the remaining 20 % of the steel balls.

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(1) High-Manganese Steel: Steel balls made of this material have good toughness, excellent workability, and low cost. Their main feature is that under relatively large impact or contact stress, the surface layer rapidly undergoes work hardening, with a work-hardening index 5–7 times higher than other materials, significantly improving wear resistance.

(2) Low-Carbon Alloy Steel Balls: Made from low-carbon alloy, these balls offer good toughness at a low price. Under the same working conditions, their service life is more than double that of low-chromium cast balls.

(3) High-Chromium Cast Iron: A wear-resistant material with excellent abrasion resistance, but it has lower toughness and is prone to brittle fracture during use; it is also expensive.

(4) High-Carbon High-Manganese Alloy Steel: These steel balls are mainly alloy structural steels containing elements such as chromium and molybdenum. They have high hardness and good toughness. Under the same working conditions, their service life is more than double that of high-manganese steel balls.