Volume 2 Number 3 Dennis R. Dinger 1 January 2004
An Update
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The topic in this issue applies to all who must perform milling in their routine processes.
Three Modes of Milling
There are three modes of ball milling that cover the range of solids contents. They each have their benefits, and they each produce different results. If you must perform ball milling in your operation, I think this is a valuable topic.
The three are low solids ball milling (which is essentially equivalent to dry ball milling), high solids ball milling, and extremely high solids ball milling. They could also be described by the terms first-order milling, non-first-order milling, and perfect-packing-milling. Most milling experts would probably tell you that there are only two types of milling -- the first two in each list. I am not a milling 'expert,' so I have added the third category. With these three modes, one can preferentially mill the coarse particles, one can mill all particle sizes, or one can mill those particles which don't fit well into the packing structure, respectively. Let's consider them one-at-a-time.
First-Order Milling
Whenever you pick up a book on the subject of milling (which is also known as comminution), if the author tells you the conditions for most efficient milling, this is usually the mode that he or she will describe. It is very suspect when an author tells you to, "Do it this way for most efficient milling!" ... especially when he/she has no idea what you are trying to do, and therefore, has absolutely no idea how you are defining efficiency!
First-order milling happens to be the mode in which the largest volume of milling research has been done over the years. Which industry performs the most milling in routine production practice? Which industry has done the most milling research? The answer is the mineral processing industry! Milling by ceramic industries is a small fraction of the total tonnage milled throughout the world in the mineral processing industries. Since they have done most of the milling, they have also done most of the research and written most of the how-to books on the subject. And their idea of 'most efficient' is the efficiency they present.
What is their idea of efficiency? According to the mineral processing industries, an efficient system mills coarse particles to 100% passing a certain largest particle size, while producing minimal fines, for the least amount of energy. What do they consider to be fines? Anything smaller than about 200mesh (smaller than about 74μm) is a fine powder. This means that most ceramic systems (there are a few exceptions) deal totally with fines. Therefore, we might ask what the mineral processing industries even know about the milling of fines? Not very much. Some have studied it, but relatively few -- and only for relatively unique applications.
I'm sure I've stepped on a few toes by making such a claim, but in my experience, these statements are generally accurate. There are exceptions, of course, and to those of you who are in the mineral processing fields and who do know something about milling fines, I apologize.
You see ... the mineral processing industries don't like fines because they are difficult to recover in their various beneficiation (cleaning) processes. Years ago, we toured a coal cleaning facility in upstate Pennsylvania. The engineer giving the tour asked if we knew where in the process we could find the cleanest coal that they produced? The answer was, and he gestured over his shoulder and out the window, that the cleanest coal was in the settling pond out back. They could clean it well, but then they couldn't separate it easily from the water, so it ended up out back in the settling pond. They had tons and tons of fine, clean coal out there where it could not be recovered easily, nor used. So to minimize this problem, milling was done in a way to minimize the production of the fines. If you don't produce the fines in the first place, you minimize the tonnage of fines out back in the settling ponds.
Why the term first-order? On disappearance plots where the amount of feed material remaining is plotted versus time as milling proceeds, the most efficient milling produces linear behaviors. Cumulative percent finer than (CPFT) plots versus size on log-log axes also produced linear distributions. The solids content in a mill, the amount of media, the amount of feed powder in the mill, the mill rpm, etc., are all well-defined to mill in this way.
The question is this: Is this the goal of most ceramic milling operations? Maybe, sometimes, it is. In many cases, however, it isn't. But if you pick up a milling book and they are willing to define the parameters to be used for 'the most efficient' milling -- this is usually what you will get.
Non-First Order Milling
The second mode of milling is non-first order milling. Disappearance plots of non-first order systems are no longer straight lines. They generally curve away from the first-order plots and milling is slower. But what is happening? When a particle is broken, the smaller particles are considered to be progeny particles. In non-first order milling, the progeny particles tend to be broken again and reduced further in size before all of the largest feed particles are eliminated. Instead of milling preferentially the largest particles (because they are the largest and slowest moving -- like a large, slow football player who is relatively easy to catch and tackle) as in first-order milling, some of the large particles AND some of the progeny particles are milled with each comminution event in non-first order milling.
Compared to first-order milling, this is less efficient (if you agree with their definition of efficiency.) In terms of the product particle size distribution, more and more fines will be produced with each comminution event in non-first order milling than would have been produced by first-order milling conditions. This type of milling allows one to change the shape of the particle size distribution curve in ways that cannot be done by first-order milling.
How do you achieve non-first order milling? To do this, you make adjustments in the opposite direction from those recommended to produce the 'most efficient' milling. We found that by putting more powder into a mill than the recommended, optimum amount, or by putting more media in the mill than recommended, or by milling at slower rpms than recommended, or by raising the solids content of the feed suspension (especially if milling aids or deflocculants were also added) non-first order milling could be achieved.
The results were that we could change the shape of the particle size distribution curves in one milling step. Using the same feed material, we could produce a whole variety of particle size distribution curves in the same ball mill. Time of milling and energy of milling changed, but if we wanted 100% less than 400mesh, we could achieve a variety of particle size distribution curves by varying the milling parameters in this way. The mineral processing engineers may not want to mill any progeny particles before all feed particles have been impacted and removed, but we did. And it worked! We were able to tailor our particle size distributions by adjusting the various mill parameters and milling conditions in ways not recommended by the big kids who wrote the textbooks.
Perfect-Packing Milling
This is a bad title, but I couldn't think of a better one. But it does appropriately label this type of milling. First-order milling preferentially mills the coarsest particles. Non-first-order milling mills all sizes. Perfect-packing milling preferentially mills all particles which don't pack well.
We stumbled on this during the coal slurry project. If you want a system of particles that packs well, this is the mode of packing you want to use. It is an extreme version of the non-first order case. Milling needs to take place at the highest possible solids content or in the most crowded system possible. This doesn't mean it works in dry systems (which would be the highest solids contents possible) because dry milling is similar to low solids wet milling. Ball mills are not the best type of mill in which to do this, but they can work. It is a balancing act, though. If the solids content is too high, media and powder will be glued to the mill walls and all comminution will stop. If the solids content is too low, milling that is more consistent with the first-order and non-first order modes described will occur.
This became clear to us one day when we were watching a calendaring operation. The calendaring process passes a system of particles between two huge rollers under great pressure. It isn't actually a milling process -- its purpose is to form a thin tape of material. But when the feed particles are coarse, the gap between the rollers is narrow, and the available pressure behind the rollers is great, comminution can take place. Usually, these systems are run at high solids contents so the tape will hold together as it exits the rollers. In this environment, this perfect-packing mode of comminution results. How do we know? Systems of particles that were passed through calendars were re-slurried and the particle size distributions were measured. The distributions produced were those that packed almost perfectly.
Why does this occur? How does this occur? The answer is relatively simple. In highly dense systems of particles, the stresses created by comminution events are transmitted through all particles that touch. As particles fall into pores within the structure, they are not exposed to the stresses from the comminution events. So only the particles that don't pack well and don't fit into the packing structure are exposed to comminution stresses. As particles break and move about, new particles fit into pores between the particles, and new particles (that don't pack well) are exposed to the comminution event stresses because they form the structures which transmit the stresses from particle to particle.
As mentioned earlier, this is an extreme set of conditions, but if dense packing is required, this is a way to achieve the desired size distribution.
When All Else Fails
When all else fails, it is always possible to mix several different particle size distributions together to achieve the desired body distribution. If this procedure is used, the particular milling mode used to produce the distributions to be mixed is less important.
Our experience has shown that the higher the solids content used during the milling operation, the better the packing system. For example, if we want an excellent packing system of particles in a 60% slurry, milling at 60% will work, but if we mill at 70% solids and then dilute to 60% for process use, its viscosity will be lower than the viscosity of the slurry milled directly at 60%. If we can mill at 80% solids and then dilute to 60% for process use, its viscosity will be lower than either the slurries milled at 70% or 60%.
Conclusions
If your goal is to ball mill body to the desired particle size distribution in one step, rather than to produce several products and mix them together, the milling mode, defined by the milling conditions, is very important.
Next time you are required to set up or adjust a milling operation, consider these three modes of milling: (1) Preferentially mill the coarse, (2) Mill all particle sizes, or (3) Mill particles that don't pack well. These correspond to first-order, non-first-order, and perfect packing milling, respectively. You probably won't find this third mode of milling in any textbooks, so you will need to experiment with it. The three modes are relatively easy to understand, however, and the results gained by thinking this way may be of great value to you and your company.
Any Job Openings?
Since the economy has been poor, and consulting has been really sloooowwwwww, I am beginning a search for a full-time job for myself either in academia or in industry. If any of you know of a position which calls for someone with my abilities, please send me an e-mail with the information. Consulting has been a great opportunity to work with many different people in many different industries and I will continue consulting as opportunities arise. But unless this new year brings a marked increase in opportunities, I will have no alternative but to take another job.
All of you who subscribe to this E-zine should have a pretty good idea what I know and what I can do. Other appropriate details of my qualifications are located on my web site. Any leads or suggestions you can give me will be appreciated.
I will keep you posted.
Thanks.
Miscellany
Please continue to send suggested topics for future columns. Thanks!
Until next time ...
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