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Design Highlights

1) Mill Speed - An important design aspect for perfect milling

There is a specific operating speed for most efficient grinding. At a certain point, controlled by the mill speed, the load nearest the wall of the cylinder breaks free and it is so quickly followed by other sections to form a cascading, stream containing several layers of balls separated by material of varying thickness.

The top layers in the stream travel at a faster speed than the lower layers thus causing a grinding action between them. There is also some action caused by the gyration of individual balls and secondary movements having the nature of rubbing or rolling contacts occur inside the main contact line.

It is important to fix the point where the charge, as it is carried upward, breaks away from the periphery of the Mill. We call this the "break point", or "angle of break" because we measure it in degrees. It is measured up the periphery of the Mill from the horizontal.

There are four factors affecting the angle of break:
  1. Speed of Mill
  2. Amount of grinding media
  3. Amount of material
  4. In wet grinding, the consistency or viscosity

The smaller the Mill the faster in RPM it must run to attain critical speed.

For most grinding and dispersing problems, we strive to attain the cascading, sliding action described earlier, and to accomplish this we have found that the most desirable angle of break ranges from 50 to 60 degrees from the horizontal.

The lower range is recommended for most wet grinding operations like paints and soft dry materials, and the higher break point (which provides a more severe grinding action) for most dry materials and wet grinding such hard products as enamel frit and glaze.

It is also known that the grinding action in a larger mill is more severe than in the smaller sizes and, consequently, we are of the opinion that the angle of break should be lower for the larger Mills than for the smaller.

2. Quantity of Media

For the most efficient results, the Mill should be at least half filled with grinding media. Some prefer to go a little beyond the halfway mark to compensate for wear. There is no objection to this and we have been suggesting a limit of about 5 percent.

In steel ball grinding, many especially in the paint industry, are satisfied to run with a smaller ball charge ranging as low as one-third the volume of the mill. They find the smaller charge gives them the required grind within allowable limits of grinding time and the extra space gives them more loading room.

Grinding media should be periodically checked. Reduction in the quantity and size of the grinding media will result in poor grinding. We suggest a maximum schedule of once every six months. In some cases, where abrasive materials are involved, once a month is not too often and, in a few cases, even shorter intervals are indicated.

3) Type of Media
There are three types of grinding media that are most commonly used:
  1. Flint Pebbles
  2. Porcelain Balls - regular and high density
  3. Steel and other metal Balls
Flint Pebbles - These are the oldest type of grinding media in use and they continue to be extremely popular. They can be used with all types of lining .

Porcelain Balls - This is a pure white ceramic material with a dense, highly vitrified body that will not chip or crack in service. They have been immensely improved in quality in recent years and are used exclusively in many industries.

High Density Media - This is the latest grinding media developed for Ball Mills. They are made with high alumina oxide content and have a density 40 to 50% greater than the regular porcelain balls. They are also fired at higher temperature making them

harder and more abrasion resistant.High density media are available in various shapes including spheres, cylinders and ovals resembling the natural flint pebbles.

Steel Balls - Steel balls are unquestionably doing a faster grinding job than any of the other commercially available media. They have proven especially valuable in the paint industry.

The following types of metal balls are commonly used in Ball Mills:
  1. High Carbon - High Manganese Steel .
  2. Cast Nickel Alloy
  3. Stainless Steel
  4. Chilled Iron
  5. Forged Low Carbon Steel
  6. Other, more special types include bronze or brass, aluminum, tungsten carbide, etc.
The following general rules should be carefully adhered to regardless of the type media used.
  1. There should be enough material in the batch to cover the grinding media.
  2. Grinding time must be watched carefully to avoid excessive grinding.
  3. Excessive buildup of heat should be avoided. In paint grinding, this may lower the operating viscosity beyond the critical point.
  4. The smallest grinding media should be employed. These not only reduce the danger of overheating but, as is well known, the smaller grinding media provide faster and better results.
  5. When using extenders, their abrasive nature may cause excessive wear. To avoid this, some operators are able to hold out the extenders until the grinding is almost completed and then add them for the final operation.
4. Size of Media
Probably the most common cause for faulty operation and complaints has been due to the size of grinding media. It is strongly recommended that the smallest feasible grinding media be used in all cases. The optimum size of media should not change with Mill size. If the laboratory small Ball successfully grinds a sample batch in a lab Mill, the same size grinding media will do the best job in a production Mill whether the Mill is one foot or eight feet in diameter.
Small grinding media are recommended because:
  1. They provide many more grinding contacts per revolution than larger media. This results in much quicker grinding action.
  2. They provide smaller voids, limiting the size of particles or agglomerates which can exist there.
  3. They do not create excessive energy which cannot be utilized.
  4. Oversized grinding media frequently develop more grinding energy than is needed for the job. This excess merely builds up heat and wears down the media and lining, introducing contamination in the batch.
  5. The chief disadvantage of the smallest size grinding media is that discharging takes somewhat longer due to increased surface tension in the smaller voids. Almost invariably, however, the reduced grinding time realized by smaller media more that offsets this disadvantage.
5 . Clogging Problems in Ball Mills
Clogging of material in the Mill makes further operation harmful. This is generally caused by moisture of fat, as in oily seeds. Possible remedies include:
  1. Taking the material out and thoroughly dry it.
  2. Adding a dry filler to absorb the excessive moisture while the batch is being ground.
  3. Adding a few pieces of steel angle, bar, or chain which can slide along the Mill surface and scrape off any materials starting to pack.
  4. If the material is packing due to particle size alone, grinding should be stopped prior to this point. The material should then be screened and tailings returned to the mill.
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