Many factors are involved in the selection of grinding media. Included are ore hardness, feed size, product size pulp density, pulp level, mill size, mill speed and feed rate. Media selection is important because the cost of media is a major expense in grinding.

 

Significant savings are available by optimizing the use of grinding media. Media diameter can be selected so as to provide for the best mill performance. Media shape is fairy well established as being either a sphere or a cylinder. Other shapes have not been economically justified.

 

The use of small diameter media is favored because it results in a large surface area to weight ratio. Also, liner wear decreases as media diameter decreases. However, there is a limit to how small grinding media diameter can become. For one thing, smaller diameter media are more expensive per unit weight. As the ore becomes larger and harder, the media need to be of a larger diameter to grind. Also, the smaller the media, the sooner it reaches reject size. This is especially true in the case of a rod mill where rods break up below 1” to 1 ½” in diameter. In fact, some operators remove rods at 1” (or at about the size where they are expected to break up) along with broken rod pieces to increase grinding efficiency. New rods are seldom smaller than 2” diameter. At larger mill diameters and/or at higher mill speeds, the grinding media are moving at a higher velocity and it becomes possible to use smaller diameter media to achieve the same momentum or grinding power.

 

 

While various technologies have been develop to arrive at a mathematical relationship to determine mill media size, comparison of a grinding application with an existing application is the best way to assure selection of a media charge which will be satisfactory. In establishing an initial charge be certain the initial size is sufficiently large to achieve the grind. Too large an initial size may result in higher than optimum media wear until the media are partially worn. However too small an initial sizing may result in lowering the capacity of the mill since a small ball is less likely to fracture a large piece of ore than is a large ball due to its having less momentum. As time goes on, some experimentation can be done b y adding media of different diameter than the initial top size. The tables opposite contain data on grinding media and provide examples showing how to determine the weights needed of each size of rods and balls required for a predetermined weights needed of each size of rods and balls required for a predetermined weight of charge. The criteria used in the example ensures that the ball and rod charges will be made up of approximately equal surface areas of each size.

 

It is difficult to give figures on media consumption due to the many variables involve. Rods will be consumed at a rate ranging from 0.2 lb/ st on soft, easily ground material up to 2 lb. per st on harder material. Steel consumption of balls is spread out over an even greater range. An indication as to the consumption of balls is spread out over an even greater range. And indication as to the consumption of media can be obtained from power consumed in grinding. Balls or rods will often wear at a rate of about 1 lb for each 6 or 7 kwh consumed per st of ore. Liner consumption is generally about 10%-15% of the media consumption. A significant portion of the metal wear in grinding can be attributed to corrosion. Dry grinding metal consumption is appreciably less than that of wet grinding partly because corrosion is minimize. However, the material ground dry is typically softer and less abrasive than wet a true comparison.

 

Choice of materials used in grinding media are significant. Grinding balls should be made of forged steel, or cast steel having the same resistance to breakage. Rods should be hot rolled, hot sawed or sheared with standard tolerances and machine straightened.