SELECTING MILL MEDIAS

By Ivan Quackenbush (Updated in 2005 by Tom Weiss)

Ever since the first sand mill was run, whatever the failure, it was blamed on the media.  Actually, most sand mills will operate efficiently with a wide range of medias, and once a choice is made that suits the operator and the dimensions of the mill, the media becomes a minor variable.  By itself, media seldom is the cause of trouble.

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

1. How heavy it is, (specific gravity, typically in grams per cubic centimeter - g/cm³)
2. How big it is, (diameter, usually expressed in millimeters or mesh size)
3. 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. 

Let’s take a look at each of these factors.  Size is the easiest to handle, so we get that out of the way first.

Mill Media Diameter - Size Does Matter!

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.  For instance, the commonly used nickel slotted screen has a hole that has a 0.014” width, so a minimum diameter for the media would be 0.021” diameter.  The welded bar screen with a 0.018” nominal slot width would require a 0.027” minimum, and the open-top batch mills using the 0.024” screen call for a bead with a minimum diameter of 0.036”

Medias are screened to a range of diameters, not to a single diameter.  The size may be stated as a range, for example 14-18 U.S. Mesh, or to an average diameter such as 1.0 mm.  Where the range is expressed, as in this example, the smallest particle is the controlling particle.  Where the size is shown as an average diameter, it may be assumed that the smallest particle will be within 20% of the stated diameter, and these dimensions are close enough for our considerations.  A chart comparing various grading systems is available on-line at www.quackco.com. 

The mathematics of spheres gives the volume as 0.523598D³ (where “D” is the diameter), and from this it is seen that by the time a sphere is worn down by one-third, or equal to the orifice size, that over two-thirds of its volume is gone.  Thus, if the smallest bead chosen on the basis of 1.5 times the hole diameter is finally worn down to that hole diameter, it has given up over two-thirds of its volume, and has reached the end of its life.           

What actually happens, and what the operator sees when the bead bed has worn to nearly this small discard diameter, is not so much a leaking thru the screen as a plugging of the screen.  Normal wear in a sand mill brings the beads all to a similar size, and when this plugging occurs the operator is signaled that it is time to change the charge.  It is seldom worthwhile to salvage the discard bead by screening. 

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.  (ORWIG: “The Quickie”, etc., Official Digest, September 1954).  Just a small reduction in the diameter of the bead makes a large change in the number of beads in a given volume. (See “Packing of Spheres” etc., Do Ik Lee, JPT Vol 42, No. 550, November 1970 PP579).

If there is any choice, then, it seems obvious that we work with the smallest bead possible.

The smallest commonly used media is the 20-30 mesh sand mentioned in the early patents, and on which much of the original mathematics was based.  This sand and the corresponding glass bead are actually graded over 18-25 mesh commercial screens to eliminate undersize particles.  This size and density of media is capable of dispersing almost any pigment into and sensible vehicle system, however, the environmentally-dictated trend toward higher-solids bases, and the increasing use of mills with short gaps or small screen areas, has increased the use of larger and less efficient bead sizes, or the more efficient higher density ceramic media. 

Where a mill has a separating device too large to use sand, a larger media is necessary, and sand mill medias are available to about 3.0 mm (6-8 US mesh) diameter.  The same factors apply, however, and the smallest usable bead is the most efficient on the basis of number of particles per unit volume.  There are indeed a few types of dispersions that are better done with large medias.  Typical of these are certain suspensions of water-dispersed dyes, some large-particle ore and oxide dispersions, and high-solids bases.

Mixing mill media diameters is not a good idea, as the media tends to wear quickly.  Most users will “top-off” a mill during use to maintain the proper media volume, so invariably there usually is a mix of diameters in a production mill.  This is OK to do, but keep in mind that the media will wear more quickly after the mill is “topped-off”.

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, according to work reviewed by Fehrenbach and Draper  (Modern Casting May 1968,”Determination of Particle Sizes of Sand Grain”-etc.) and others.  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) or more, according to experience in many plants.  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 weight of the vertical column of such media is low, which reduces the abrasion of bead in low-viscosity materials, adding to the life of the media.  Such medias are, for the most part, glasses, which are, like the sand, inert, clear, and with invisible clear wear products.  They are, in general, not abrasive and do not cause excessive mill wear.  

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. 

An available but not very common media is aluminum oxide pellets such as Coors Type “M” with a density of about 3.8 g/cm³ followed by zirconium silicate beads at 3.8 to 4.1 g/cm³ approximately.  More recently, 95% pure zirconium oxide (5.5 g/cm³) and yttrium stabilized zirconia (5.95 g/cm³) media have appeared in the market.  These higher density media can be expected to have more mass and do more work, of course, but will pack in low-viscosity materials, and are generally recommended only for long runs of lubricant, viscous materials with little wash-up between runs.  They are not suitable for general-purpose use in most commercial mills, but do find specific areas where their unique properties are at an advantage, such as heavy inks, or high-solids bases.   

Finally, steel or 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, although the so-called shot mills are especially adapted for their use.  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.   

None of the medias presently offered will simply “break up” in a sand mill.  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.  If truly smashed media appears, it is not the normal action of the mill, which has caused the breakage, and other reasons must be found, although the bead will get first blame.

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.

By these tokens, then, no media must be discarded on the basis that it will break up in normal operation, for none will break up during normal operation of the mill.  One exception is the zirconium silicate media called, “ER-120A”.  We normally recommend that zirconium silicate users select the pre-conditioned ER-120S, ER-120S (Narrow), or Quackenbush QBZ-58A, which requires no pre-conditioning.

Glasses have the very considerable advantage of not being abrasive, and they are available in a wide range of sizes.  The wear products are invisible.  The greatest disadvantage is their wear rate.  Whatever is done to sand to make glass of it, softens it, so glass inherently softer than sand.  ALL glasses are about the same hardness and will all wear about the same rate in the products we must consider here.  Glass can be surface hardened, but in doing so strains are set up which are otherwise disadvantageous.  Glass is amorphous and homogeneous, so the initial surface condition is of no importance, since there is always new surface being exposed.  Glass can be made by casting, as many European glasses are, giving them exceptional sphericity; they can be blown, which is inexpensive and which can produce small beads, but which can in some cases allow bubble inclusion; or they can be made in the so-called solid state process where exceptional crush strengths are possible.  All the glasses offered for sand mill use are more than spherical enough for the purpose. 

Ceramics have the advantage of very long wear life (when used properly and under the right conditions) and high strength.  When used improperly, as with very low viscosity mill bases or by having excessively long wash up times with thin solvents or water, ceramics have the drawback of being aggressively abrasive.  This is highly undesirable because of the cost of replacing mill parts, and can result in discoloration because of metallic pickup.  Special mill configurations are often necessary to allow use of these medias, especially where the denser ones are used.  Like steel based media, heavy ceramics are of most use in viscous, long run, materials where there is some lubricity in the mill base. 

Shots are usually iron or steel.  Austenitic stainless steels are not recommended, as they have a tendency to gall and wear rapidly, the wear products being layers of flakes.  Other stainless steels are generally the chrome-irons, which are resistant to some corrosives.  Stainless steel media tends to be very costly.  For most applications, we do not recommend the use of stainless steel media due to the problems listed above.  It is generally a much better choice to formulate the mill base for use with a ceramic bead. 

One example of these the so-called burnishing balls, which were originally generated as reject ball bearing balls.  They are by nature extremely hard, and since they are polished, can be immediately placed into use in even relatively light colors.  They are quite expensive, as compared to the other generally available shots.

Most shots have an oxide coating, which disappears rapidly when exposed to mill forces, and they quickly take on a polish.  It is almost impossible to generate a spherical shot without some off-shapes, and once formed, not all of these can be separated from the spheres, although most mill shots have the greater part of them removed.  They are not nearly the problem that off-shape lighter medias are, for the very high density of the shot tends to keep the media down in the mill away from the discharge point.  Another naturally occurring phenomenon is that the off-shapes, which break off into small pieces, tend to go to the bottom of the mill, and cause little trouble.

Hardness in shot can be a trap.  Hardness is desirable for long wear life, but relative hardness in steel mills with steel shot becomes critical.  The chrome-iron burnishing balls and the very hard iron shots are just that much harder (at 65-72 Rockwell C) to be abrasive to mill parts and themselves, than the softer steels (40-50 Rockwell C) which do the same dispersing job and last many hours without grinding themselves up or abrading mill parts.  Media replacement is cheaper than mill replacement, so we sacrifice the media.  The milling action is exactly the same whether the hard, expensive balls or the softer steel shots are used.

In the hands of very careful formulators and operators, (and somewhat prove a point) shots have been used as far as light primrose yellows, but never successfully in whites without discoloration.  There is no real need to go this far, of course, for the shot mill or sand mill with shot generally considered for use in darker colors, primers, inert dispersions or other products where graying-off is acceptable.  The cost is low on soft shots, and the life fully satisfactory. 

Claims for exceptional dispersing ability for some particular media may be heard frequently, but must be considered carefully.  Three feet down in a sand mill, and going past it at 25 mile an hour in the dark, the impeller cannot tell whether the bead is clear, or yellow, or white.  All it feels is the size and the weight.  Size (number of particles and impingement points) and density (effect per impingement or shear instant) are the things that affect dispersion, assuming that velocity is constant.  A good formulator, or a ready good mill operator, can make any media look better by playing the other variables available as discussed in “Bugs In Your Bead Mill”, and the operator and formulator can do well with just about any media if they try.

There may also be claims that one of several examples of a particular material, such as one glass bead or another, will wear much longer.  Actually, within a family, most medias have similar wear lives.  Most glasses are about the same hardness and wear at similar rates.  However, an easy way of increasing wear life is to make a slight increase in the diameter.  Example: let us build a mill with a screen 0.66 mm in opening diameter.  The smallest bead is recommended is 1.5 times larger, or 1 mm diameter.  The 1 mm bead has a volume of 0.5236 cubic millimeters.  It wears down to the hole size, or 0.66 mm diameter, when it has a residual volume of only 0.1575 cubic millimeters.  Along comes Blowhard Bead Works with a “1.0 mm” bead that is actually only 0.2 mm oversize, or 1.2 mm diameter. The volume of this bead is 0.904 cubic millimeters, so instead of being 3.3 times its throwaway size, it is 5.7 times its throwaway size, and the difference is the “longer life” that seems to marvelous in the ad.  HOWEVER, in increasing the diameter by this “negligible” amount, the number of particles per unit volume (or in the mill) has dropped almost 50%, with attendant loss of dispersing ability!

Choosing a media or medias for your sand mill operations is not really difficult.  Your products may fall into a range where some special media is necessary, but almost thirty years of operation and investigation into sand mills shows that:

1. Simple and cheap sand and glass beads are still able to make most finished products if formulated properly and used in a mill with well trained operators.
2. The cost of mill maintenance becomes a major factor when using abrasive medias except in extremely heavy products or in mills especially made to resist abrasion.
3. Excessively large medias are only necessary where operator capabilities and training are less than desirable, or when breakup of agglomerates outside the normal size range is encountered.
4. Seldom is the media itself the cause of mill problems.  A good, simple beaker-type sand mill for the laboratory for checking material qualities, a formulator who understands the mechanics of the mill, and an operator trained to read the reactions of the mill are much more important than jumping from media to media.

If you have any questions or need help selecting the best media for your applications, contact Tom Weiss at Quackenbush Company.  We look forward to hearing from you!

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Copyright 2005 by Quackenbush Co., Inc.