Ball Charges

The chief factors determining the size of grinding balls are fineness of the material being ground and maintenance cost for the ball charge. A coarse feed requires a larger baU than a fine feed. 

Material and Ball Charges The load of a grinding medium can be expressed in terms of the percentage of the volume of the mill that it occupies i.e., a bulk volume of balls half filhng a mill is a 50 percent ball charge. The void space in a static bulk volume of balls is approximately 41 percent. Since the medium expands as the mill is rotated, the ac tual running volume is unknown. 

Ball charge equal to 35-50 percent of the mill volume gives the maximum capacity. 

Size, ft. Ball charge, tons Hp. to run Mill speed, r.p.m. Capacity, tons/24 hr. (based on medium-hard ore) ... 

The second class of grinding equipment is used to prepare dispersions. Typical of this class are ball and pebble mills, ultrasonic mills, and attrition mills. Solids, eg, sulfur, antioxidants, accelerators, and zinc oxide, are generally ground on this equipment (see Size reduction). Ball mill action is assisted in some mills by a combination of dispersion circulation by an external pump and mechanical oscillation of an otherwise fixed nonrotary mill chamber. Where ball mill chambers are rotated it is necessary to experimentally establish an optimum speed of rotation, the size and weight of the ball charge, and ensure the mills do not overheat during the grinding period. 

Ball mills. Ball charge is 315/ton. Price includes liner, motor, drive, and guard. 

A ball mill having an inside diameter of 12 ft and an inside length of 14 ft is to be used to grind a copper ore. Measurement shows that the distance between the top of the mill and the leveled surface of the ball charge is 6.35 ft. What is the weight of balls in the mill What is the critical speed of the mill (the speed at which the centrifugal force on a ball in contact with the mill wall at the top of its path equals the force due to gravity) At what percent of critical speed should the mill operate ... 

A high volume of powder feed produces a cushioning effect whereas small sample volumes cause a loss of efficiency and abrasive wear of the mill parts. The amount of material to be milled in a ball mill may be expressed as a material-to-void ratio (ratio of the volume of material to that of the void in the ball charge). As the amount of material is increased, the efficiency of a ball mill is increased until the void space in the bulk volume of ball charge is filled then, the efficiency of milling is decreased by further addition of material. 

Size, ft Tons/24 h hp Weight mill, lb Weight ball charge, lb... 

The lifting coefficient d can be estimated from results obtained for ball charges in tube mills (Figure 104 ). For the typical granulation drum loadings of ( = 0.1 to 0.3, 6 is approximately constant. 

The grinding media charge can be expressed as a percentage of the mill volume, that is, a mill half-full of media contains a 50% charge. The void space present between a static ball charge is approximately 41%. The quantity of material in a mill can be expressed as the ratio of mill volume to the volume of the voids in the ball load, otherwise known as the material-to-void ratio (M/Vratio). In practice, the M/Vratio is kept close to 1, while grinding media charging varies from 20 to 50%. 

The passage of the balls through the heat exchanger is a random process, but provided there are sufficient balls in the system then the frequency of balls passing through each tube will be sufficient to maintain clean surfaces. Eimer [1985] demonstrates how the ball frequency/ (balls/A) may be determined. If the ball charge is 10% of the number of parallel tubes in the exchanger then... 

Eimer [1985] concludes that in the extreme case, i.e. with continuous operation but with of the normal ball charge would result in a ball frequency of 0.5 balls// and would be considerable deviation from the typical value of 12 balls// given by Equation 15.2 and is not recommended. 

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