ATTRITOR
How Attritor Works
Wet Milling Attritor - Batch Type
Wet Milling Attritor - Continuous
Dry Milling Attritor - Batch & Cont.
Recirculation Series
Laboratory Attritor
Grinding Media

GRINDING MEDIA

SELECTION OF MEDIA

There are only three real factors controlling the choice of media: 

  • How heavy it is, (specific gravity, typically in g/cm3)
  • How big it is, (diameter, usually expressed in millimeters or mesh size)
  • What it is made of (strength, wear life, abrasiveness, inertness, cost)

Of these, only size and density control its ability to disperse, the other factors being secondary.  In other words, for a given size and given density, the material of which the bead is made makes no difference in its ability to disperse. 

 
Mill Media Diameter

Every small-media mill has some separating device for keeping the media in the mill and letting the paste out.  The size of the opening in that device can be obtained from the mill manufacturer.  The minor dimension, whether it be a slot, hole, gap, or whatever other orifice, is the controlling dimension, as most medias are at least roughly spherical.  As a rule of thumb, a media particle with a minor diameter at least 1.5 times this minor orifice diameter is about the smallest practical size. 
It is obvious that the smaller the bead, the more beads per unit volume.  Again as obviously, the more beads in the sand mill, the more points of contact and the more work can be done. 

 
Mill Media Density

After making the selection on size, some selection must made on density.  Sand has a density of approximately 2.65 g/cm3,  Sand, which was the most common media in the paint and ink industry in sand mills for some time, has sufficient mass to disperse most pastes using mills with peripheral velocities of 2,200 FPM (feet per minute)  Using sand or a synthetic media with a density in this basic low range, where possible, does have advantages.  The cost of such medias is low. 

The media next to sand in density is the high-strength glass bead, usually a straight soda-lime type, with specific gravity of 2.7 to 2.75 g/cm3.  The dispersing ability of this bead can be considered the same as sand, assuming the size to be the same. 

Finally, steel iron shots in the over-7.1 g/cm3 specific gravity range are quite common, and fortunately, can be used in almost any commercial mill.  They are especially useful in inks, very viscous pastes, and for fast processing of primers or other products in large volumes.  They cannot be used for clears or white, or any product where discoloration or iron contamination would be objectionable.
Very generally, then, the choice of media density is based on type of material to be dispersed.

Many plants keep one mill on shot or heavy bead for blacks, primers, etc., or one on large glass for whites and yellows, and find it much more efficient than trying to do everything with one mill and one media.  The general-purpose glass media are a high effective and long-proven media for production of a very wide range of dispersions.

The third factor controlling choice of media is, of course, the material of which it is made.  Certain points pertain to all, one of the most important being crush strength.   

There is NO force in a normally operating mill, which is great enough to break up even the weakest, sand.  A foreign object or a broken disc in a mill will break up any media, and pass thru a feed pump will do the same, as well as destroy the pump. 

Besides those just mentioned, two other sources of media “failure” have found by experience. One of these is that when the mill has been freshly charged, with whatever media, the new charge has a tendency to “scour” the old media from disc hubs, stabilizers, corners, etc., generating a lot of fine media particles, which take some time to rinse from the bed. The other is the puzzling phenomenon of the scalloped disc or impeller.  The normal flow around a sand mill disc is smooth and laminar with little axial component.  When the disc wears to a point where there are definite valleys in the flat surfaces, and scallops in the smooth peripheral surfaces, this laminar flow is interrupted.  When this occurs on the bottom discs of a vertical mill where the beads are most tightly packed, it seems to generate enough force to actually break any kind of bead.  In the batch-type mill the same pattern forms and the same wear or break-up factor enters as well as a readable drop-off in dispersing ability.

 

Rules for selecting large media

Rule 1: The media must not "float" in the product. Viscosity and density action must not be impaired.

Rule 2: The media specific gravity MUST NOT be greater than that of the mill liner UNLESS the liner is rubber or urethane

Rule 3: The media must not destroy the product value through color change, abrasive contamination or chemical reaction.

Rule 4:
The media should promote highest throught per unit of time.

If the product requires dispersion to a particle size of 1-4 microns and more than one pass through the media mill is probable, then for best efficiency, at least 2 mills would be required, each with a different size media.

The first pass through the mill should be made with a larger bead to work on the large pre-mix agglomerates. After the first pass through a media mill, the large agglomerates should now be smaller and a smaller bead used in order to increase the contact points.

 
Zirconia
Alumina Grinding
Glass Beads
ZIRCONIA
Alumina Grinding Media
Glass Beads
     
Stainless Steel
Carbon Steel
Chrome Steel
Stainless Steel
Carbon Steel
Chrome Steel
 
 
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