Volume 2 Number 11 Dennis R. Dinger 1 September 2004
An Update
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The topic of the main article in this issue is along the same lines as the article in the previous issue -- using one type of machine to do the work of a different type of machine.
Mixing Using a Ball Mill
The previous article dealt with the use of extruders as mixers. Whereas extruders aren't typically used as mixers, ball mills frequently are used as mixers. So we will consider the use of ball mills as mixers in this article.
Ball mills happen to be one of the unit operations to which I was referring when I said that I always preached that engineers should use equipment for the tasks for which they were designed. "Mixers are for mixing! Ball mills are for ball milling!" etc.
There are three divisions of this topic that we will consider: using ball mills (containing media) as mixers, using ball mills (without media -- that is, empty) as mixers, and the practical use of ball mills as mixers and holding tanks.
Using Ball Mills (Containing Media) as Mixers
Milling
The average mixer, and even intense varieties of mixers such as high intensity dispersers (HID), do not cause particle sizes to be reduced by milling events. Mixers are designed to create convective flow to quickly produce homogeneity of distribution within tanks, but mixers do not create impacts which are strong enough to cause comminution. Even under HID conditions, impacts of particles with impellors are insufficiently intense to break or fracture particles. At best, impellors impact particles and cause them to impact other particles -- but this is not intense enough to cause comminution.
Weak agglomerates can be broken apart by HID but research has shown that surface areas do not increase during deagglomeration, even under HID conditions. When particles are actually broken by comminution events, new surfaces are created, and measured surface areas should increase. Research has also shown that strong agglomerates cannot be deagglomerated by HID mixing. When particles must be broken, or strong agglomerates must be deagglomerated, the unit operation of choice is the ball mill.
So it must be remembered that whenever ball mills are used for 'mixing,' comminution (i.e., milling) is taking place. Ball mills are designed to cause the media to tumble and crash into one another -- which breaks particles that are caught in the impact areas between media. The larger the diameter of ball mills, the stronger are the stresses which are generated which cause comminution. This happens in two ways: (1) when mills are running at rpms approaching (but less than) mill critical speed, media are launched into the air as they approach the top of the mill and they free fall until they impact the bed near the bottom of the mill; and (2) the depth of media increases the stresses between media and mill lining in the bottoms of the mills.
Let's reconsider these two points separately:
(1) Consider the end of a rotating ball mill as the face of a clock. When the mill is running in the counterclockwise direction and its rpm is slightly less than critical speed, balls will be launched around 2 o'clock (i.e., they will fall away from the wall); they will fly past the 12, 11, 10, and 9 o'clock positions; and they will fall down to about the 7-8 o'clock where they will impact the other balls in the mill charge. As mill diameters increase, momentum of these impacts increases because the balls fall greater distances before impacting other media in the bottom of the mill.
Critical speed is the rpm at which everything in the mill centrifuges and no tumbling or comminution occurs. The Round-Up rides at local carnivals are designed around their critical speed. These rides allow people to stand against the outside wall of the ride, which is a large cylindrical cage. As the ride begins, the rpm increases up to the appropriate level (that is, past the critical speed), and then the axis around which the cylinder is rotating is raised from a vertical orientation to nearly a horizontal orientation. In this final position, riders are lying almost horizontal (looking up) at the bottom of each revolution and are lying almost horizontal (looking down) at the top of each revolution of the ride. If the ride was not spinning fast enough, riders at the top would fall out of their positions, but since the ride is spinning faster than the critical speed, riders are pinned by centrifugal force against the wall. In this case, the ride must spin at rpms above the critical rpm to pin riders against the walls before the axis of rotation can be tipped.
If ball mills are run at such rpms (above their critical speeds) milling will not happen. Above the critical speed, balls, fluid, and powder will be spun for the length of milling time, but no milling will occur. Rpms just below critical rpms will cause intense cataracting of the media, fluid, and powder in the mill. That is, media will hug the wall until they are launched near the top to fly across the mill and fall to the bottom (as described above), thereby producing comminution events which reduce particle sizes.
(2) From a static point of view, the larger the diameter of the ball mill, the greater will be the pressures created in the bottom of the mill due to the mass of media in the mill. Most ball mills are filled to the center-line with media -- so they are half full of media. As mill diameters increase, the depth of media increases, and static stresses which cause comminution increase -- simply because the depth of media is deeper in larger mills.
The main point to remember when mills are used for mixing is that comminution is occurring. That is the purpose for which mills were designed, and that is what happens inside mills. Larger mills and denser media increase the level of comminution that occurs. A small laboratory mill with polymer (low density) media will produce the least comminution, and large production mills with steel media (such as those used in the mineral processing industries which can be 15' diameter and 40' long, or larger) will produce the most comminution.
When mixing is desired, but it is performed in ball mills, particle size distributions (PSDs) of the powders will be changing. When mixing occurs in mixing tanks, however, PSDs do not change.
Low Shear Mixing
When the first point above (that ball mills produce comminution) is ignored and ball mills are used as mixers, at best, the mills can produce low shear mixing. Mixer impellors are designed to bat particles around, produce high levels of convection, and quickly improve the homogeneity of distribution of the suspension or fluid in the tank. Ball mills, however, are designed to trap particles between media where they can be crushed.
In continuous ball mills, as well as batch ball mills, mixing and distribution of particles throughout the mill are relatively slow. Mills weren't designed to distribute particles quickly and uniformly throughout the load. Mills were designed to trap and break particles.
Yes, mixing occurs in ball mills, but it is relatively low shear mixing, and it occurs relatively slowly.
Using Ball Mills (Without Media) as Mixers
Milling
Someone had the idea that if mills break particles because they contain grinding media, removal of the media should eliminate the problem. Generally speaking, this is true. But unless impellors or mixing blades are also included in the mills, mixing will be low shear -- on par with low speed impellors in holding tanks.
Settling
When the grinding media (the balls) are removed from ball mills, the greatest remaining benefit is that such mills prevent settling. Some companies make bodies that contain relatively dense powders, or that are relatively low solids contents, or that are generally unstable due to low solids contents and lack of dispersant additives due to process requirements. How does one keep such bodies mixed? How does one prevent settling?
Actually, settling is not prevented in such mills. Settling is allowed to happen, but since the mills are tumbling, particles remain generally mixed and suspended. If such mills are filled, but then allowed to sit without tumbling, settling will occur. In some cases, it is very difficult to redisperse particles after they have settled. Some particles settle into hard packs which are almost impossible to redisperse. Additives, solids contents, and PSDs that might prevent such hard packs are frequently incompatible with later processing requirements.
Tumbling mills containing no media do a nice job of preventing the effects of settling.
Attrition
Doesn't comminution occur in ball mills running without media? Yes -- attrition occurs. As particles collide, sharp corners and sharp edges are slowly removed with time. Attrition also occurs in conventional holding tanks simply because particles are colliding with each other. Attrition, however, is a very slow process which occurs in most cases over periods of days.
Practical Use of Ball Mills as Mixers and Holding Tanks
Finally, we need to look at this topic from a practical point of view. When mixing or holding tanks are needed, but not available, ball mills can be used. The considerations presented above should be taken into account when using ball mills as mixers.
Some processes routinely put all batch ingredients into ball mills for batch mixing operations. In many cases, such techniques have been used successfully for years. It can't be denied that mills can be used as mixers, but since ball mills are not equivalent to mixing tanks, my recommendation is that they should not be considered to be equivalent processes. If a long series of process adjustments has shown that two steps can be combined into one by mixing and milling in one step, that is good. But new process designs should not immediately employ ball mills as substitutes for mixers -- especially when mixers, mixing tanks, and holding tanks are available.
Miscellany
Please continue to send your ideas or questions for future topics. Thanks. Until next time ...
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