About Media Grinding
Mill and Media Types
Horizontal Mill Physics
One might ask the significance of the roundness for media used in horizontal mills. To fully understand this, we must examine the horizontal mill’s internal physical environment and hydraulic flow characteristics. As we all know, the general physical layout of a horizontal mill includes a horizontal cylindrical grinding chamber, in which an agitator shaft with agitator discs is centrally located. The agitator discs set up two tori of laminar flow, one on either side of the disc, and the premix material is fed through these tori. The media is homogeneously distributed in these energy tori, not touching or running into each other. The energy is transferred hydraulically from the disc to the fluid in the mill, causing infinitely small layers of laminar flow, in which the media acts solely as a surface to build shear against. Each gap between media amounts to the analog of a rotor/stator homogenizer. The space between the media is changed by varying media loading levels; the number of gaps is changed by varying the media size. As the product is pumped through this fluidized bed of media, the material is de-agglomerated in proportion to the shear force generated in the gaps. There is, for all practical purposes, no physical attrition in a horizontal media mill. The media tends not to collide in the high energy areas of the mill, just in the low energy areas around the shaft. If media does happen to collide, the chances are against the media colliding with a pigment particle between them. Of course, the probability increases as the solids level approaches 100 percent…but even then, this happens in the low energy are of the mill, so the energy levels are not significant.
The bottom line in media selection for a horizontal mill is to use the smallest, roundest media that you can afford, irrespective of the density and material type of the media.
Vertical Mills – The Basics
A vertical mill is a similar, and in some ways more complex device than the horizontal mill, with a much different set of media concerns, owing to the fact that it works completely differently from a horizontal mill. In a vertical mill, one must deal with gravity, hydrodynamics and media flotation which are not a concern with the horizontal mill.
The average vertical mill comprises a large vertical grinding chamber with a centrally located agitator which has a number of grinding disks mounted on it. On the bottom of the shaft is a massive balance pulley which serves to help center the shaft and to a certain extent direct the flow of the product from the inlet, which is at the bottom of the vessel. The premix material is introduced at the bottom of the mill, is sheared in the various shear zones caused by the rotation of the disks, and exits through a separation device at the top of the grinding vessel. The hubbed disks impart a peculiar flow characteristic in the mill which causes actual media impingement in the high energy area of the mill. This is the major way in which the vertical mill is different from the horizontal. As it is generally not be loaded with as much media as a horizontal mill (as a percentage of the total void volume), this wiping action between the disk flow patterns allows the vertical mill to pick up some efficiency. Your average vertical mill can only be loaded to a 50-60% media load for several reasons. First, due to the negative effects of gravity, it is likely that the agitator shaft drive horsepower will be insufficient to start the media bed in motion and fluidize it completely. Second, if more media is loaded, it is likely to rise when fluidized to the discharge device and either blind or wear it.
The life of the vertical mill began in around 1950, when several engineers at E. I. DuPont de Nemours and Company, Inc. developed a design and a specification for a vertical mill with multiple agitators that combined shear and attrition to do the grinding. A manufacturing license was granted to Chicago Boiler in the USA, and the Red-Head was born. The mill was comprised of a tall, vertical chamber, with a top-entering agitator shaft oriented in the center of the vessel. Pinned or set screwed to the agitator shaft are multiple hardened spoked disks of specific design. The exterior flat disk area is used to accelerate in the mill into multiple layers of fairly orderly laminar flow. These zones are arranged the whole way up the shaft to the discharge device. The spoked open center of the disk allows the media which is carried up to the top of the mill to flow back down to the bottom in the low energy area close to the shaft. Unlike the horizontal mill, these tori intersect, causing considerable attrition in a fairly high energy area of the mill, where the two flow patterns “wipe” across one another. This is what causes the characteristic “herringbone” media flow pattern in a vertical mill.
In order to more completely appreciate the significance of the disk design in the operation of the mill, and your choice of media, let’s explore the physics of the mill more carefully. As the agitator disk spins, the energy state of the mill is transformed from low to high in relationship to the peripheral speed of the grinding disk across a line bisecting its axis. The area between the grinding disk and the shell, then, is the area of highest kinetic energy in the mill. The disk itself couples to the media. The product is accelerated away from the inner ring diameter of the disk toward the grinding vessel wall, and the media goes along for the ride, displaying finite impedance to motion, or inertia, determined jointly by the force of gravity, the “stickiness”, or viscosity of the material under process, and the density of the media. Thus is formed two energy tori for each disk, one “doughnut” above, and one below each disk. The flow inside each torus is ordered from high next to the disk to low in the area of recumbent flow between the disks. The vessel wall forces a transform of velocity which causes the material to lose considerable energy induced by the disk. It is in this area of recumbent flow that media is allowed to move from torus to torus up and down the mill. Compared to a horizontal mill, the recumbent flow area of a vertical mill contains more potential energy, due to the design of the disks. It is for this reason that physical attrition in a vertical mill is an important component in the work that it does.
As the media tumbles from its most highly energized state to its most benign state, it is likely that it will be drawn into the high energy area of another disk, jumping randomly between energy tori, allowing overall random media flow within our laminar flow model. Even though the media flow through the mill is random, the product flow is highly ordered and highly predictable. None of this physical activity would be possible without this peculiar disk design, which maximizes shear within the framework of an inefficient model of shear generation. Remember, this was ground-breaking research in the 1950’s, and the world has never seen the like of even this level of efficiency in the grinding process.
What we have created with the mill disks is a fluidized bed of highly energized media, just like in a horizontal mill. Unlike a horizontal, however, gravity causes the media to return to the bottom of the mill when it loses energy. The space between the media is influenced by the percent media charge in the mill, since the energized beads seek a homeostatic spacing scheme, influenced by the level of energy transfer from the fluid under process. The closer the spaces between the media, the higher the level of shear developed in the mill. Also, the closer the beads are spaced, the more likely they will be to be involved with the wiping impact (causing physical attrition) that is characteristic of vertical mills.
If a vertical mill were to be fitted with a glass shell so that you could see the media flow while the material is under agitation, you would see an even, herringbone pattern of motion, as the media lines up into shear layers and begins to do work. If you were to load two colors of media lines into the mill in two distinct layers, you would see that the two colors would mix and become ostensibly homogenous in the first few seconds of operation. This indicates that the mill will wear the media uniformly, and that the load is fluidized properly.
Some paint manufacturers over the years have charged their mill with two different sizes of the same or different density media. The object here was to use the larger media (often more dense) at the bottom of the shell, and the smaller (often less dense) media at the top. As you know, the efficiency of media varies with its size ratio to the particles being ground. Some have reported picking up some efficiency, since the larger, denser media will tend to segregate to the bottom of the mill where the larger particle size exists. As the product grinds and is moved up the mill, it is worked on by the smaller media, which more nearly mates with the smaller product particle size. There are certain practical limits to this, however. The density and size difference must be kept to a minimum, and this is a largely dependent on the shear viscosity of the premix. In addition, there will be a certain loss of efficiency in the area where the media will mix, as it invariably will. This media mixing causes two problems. First, the larger media will cause a “windage” area on either side of the smaller media, decreasing the actual amount of shear surface in the mill. Also, the denser media, irrespective of size, will tend to wear the less dense media at an accelerated rate. If the larger, denser media is too much of either, it will not fluidize correctly, grinding away the bottom of the grinding chamber.
Go, No-go Shear Model
As opposed to the horizontal mill, not all of the work done in a vertical mill is attributable to shear. For this reason, density makes a little difference when choosing media for a vertical mill. Denser media stores more energy, and when it runs into pigment particles and other media, it releases more energy, generally providing more of a “whack.” However, an indication of the efficiency of attrition in a vertical mill is shown by the fact that this extra energy would be liberated into the mill upon impact as heat. Vertical mills are not known as being product heat control problems, in fact, they are known as being somewhat less of a problem than horizontal mills in this regard. The indication, then, is that very little of the work done is by attrition. All other things being equal then, most of the work is still done by shear. But this puts the vertical mill at an extreme disadvantage, since common knowledge says that you charge a vertical mill to only a 50 or 55 percent media load. Having said this, then, it is obvious that the media will be more distantly spaced in a vertical mill, and less shear will be exerted on the product. Why is it necessary for a vertical mill to run at this lower bead charge?
The primary reason for this is media float and the resultant screen blinding that occurs. If you are one of the intelligent who converted your open vertical mills to run with packing glands, effectively sealing them, you have received a secondary benefit. Your primary benefit is that your open mill is no longer belching VOC’s into your manufacturing area. The secondary benefit is that you can now jack up your media loading and completely flood the screen area with media and product. The increase will not be a milestone, you are safest to start at 65%, but it will yield you considerable increased efficiency where some is sorely needed. All of the other old rules also apply. Read more under Media Selection under Technical Articles.