Requirements on mill-balls

The unit consumption of steel ball, which are applied to crushing of mineral ore with average durability under the Bond at 12-15 unit, is 1.18-1.5 kg depending on features of a specific mill and dressing mode.


Basic requirements on grinding balls: 

  1. To keep the original size as long as possible.
  2. Not getting heated inside the mill.
  3. To have as much specific weight as possible within the fixed size. 
  4. The shape of ball should have as much as possible full contact with the material to be crushed. 
  5. It should be practicable to have as much as possible material to enter between the grinding balls. 
  6. The surface of balls should be as dense as possible not to allow any material to stick to the surface of the ball. 
  7. Stronger the material of the balls, weaker its crushing capacity coefficient. In order to have good impact and good crushing result it is required to have heavy weight. 


Abrasion of grinding balls.

Grinding balls in the course of their use are reduced irreversibly and speed of this abrasion depends on quality of a material of which they are made. But hardness is not a only indicator of wear resistance, the alloy structure is more important factor. At a hardness increased in 2 times the wear resistance increases no more than by 20 percent, at the same time shock resistance falls approximately on as much percent. The principle of metal deterioration sharply differs from a principle fragile bodies crumbling. As of its form, a metal is a body deformed enough under the impact of external actions, so in a mill it works under influence much smaller, than a breaking force. Therefore, abrasion of metal basically depends only on surface interaction.

At calculation of steel balls’ abrasion it is inevitably required to consider a loss of the balls, which have been worn out till is minimum permissible sizes, per one ton of a finished powder. The given sizes should be established depending on the scheme of grinding and a material to be crushed. 
Ball wearing basically depends on it abrasion by crushed material, impact at falling from the maximum height of lifting and mutual collision. Thus, wear resistance of steel balls in a mill depends, first from a shock resistance and heat stability, and secondly from ability to maintain interaction of a crushed material (rotation, friction). 

Such characteristics as hardness, tensile strength and compression resistance have indirect influence. During rotation of a mill, a running cycle so-called “lifting-falling of balls” occurs in 1.5 times faster than a speed of rotation of the mill. The ball, which has worked in a mill within 1 month, takes up about 1 million blows. After a certain time, when they reached shock endurance limit, the balls start to heat up. Therefore, the life of balls work is defined besides shock resistance and ability to maintain interaction of a crushed material, also by endurance limit (in our case shock endurance limit).


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