The Quest for Nanotechnology and the Evolution of Wet and Dry Milling Processes

What is Nanotechnology?
There are a couple of definitions, or more appropriately, descriptions of Nanotechnology.
    
Nanotechnology is science, engineering, and technology conducted at the nano-scale, which is about 1 to 100 nanometers. Nanotechnology (sometimes shortened to "nanotech") is the manipulation of matter on an atomic, molecular, and supramolecular scale. The earliest, widespread description of nanotechnology referred to the particular technological goal of precisely manipulating atoms and molecules for fabrication of macroscale products, also now referred to as molecular nanotechnology. A more generalized description of nanotechnology was subsequently established by the National Nanotechnology Initiative, which defines nanotechnology as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers.
    
Nanometer sized particles are desirable in many industries, including active pharmaceuticals, pigments, technical ceramics, crop protection, new energy, and electronics. The obvious benefit of particles in the nanometer range is to improve performance of existing products or formulations due to the increased surface area that will be available.
    
However, new products can also be developed by processing in the nanometer range. Innovation has driven many ceramics industry researchers to look to nanoparticles – materials ground finer than 200 nanometers (nm) – to enhance product performance or unlock new applications for ceramic materials. Pharmaceutical scientists have enhanced the performance of drug compounds to improve dissolution, solubility, and therefore bioavailability, resulting in more effective compounds that are more cost efficient, and most importantly, with less risks and side effects for the patient.
    
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As a company specialized in size reduction to this scale, we didn’t invent or create nanotechnology – we enable it!
    
Traditionally, producing sub-micron sized particles has only been possible through wet media milling. Recent developments in dry grinding technology, specifically fluidized bed jet mills, have enabled the production of nanoparticles through a jet milling method using steam. Considering the advantages and applications of each method described below will enable the producing company to choose the most appropriate method and equipment to achieve the desired results. Let’s take a look at both technologies and how they evolved to enable “Nano.”
    
What is Media Milling?
Media milling is a process wherein a charge of grinding media (steel or ceramic balls, cylinders, or fine media) is accelerated in either a rotating cylinder or drum (traditional tumbling ball mill) or a stationary vertical or horizontal vessel with a rotating shaft. Media mills can be either a wet or dry process. However, higher fineness is achieved when using wet milling.
    
In each of the media mill types described below, successively finer grinding media can be used. The capability of a system to reach a certain fineness is directly related to the size of the grinding media.

Ball Mills
Ball mills, the simplest form of media mills, are rotating cylinders filled with grinding media. Ball mills employ steel or ceramic spherical grinding media that can range from ½ to several in. in dam, cylinders (Cylpebs) of similar dimensions, flint pebbles, or media of the same material that is being ground (autogenous grinding). In some applications, rods may even be used. The ball mill rotates on its horizontal axis so that the media cascades causing size reduction by impact and sheer forces. Feed material size for ball mills is usually less than 1 in., and they are effective to produce a particle size range of 5-500µ, in some cases as fine as 1µ, but this is usually the limit. When mills are emptied, the slurry is discharged through a grate which retains the grinding media in the mill while allowing the product to pass.

Ball mills work well with brittle, hard materials, and can mill and blend materials at the same time. They are not suitable for elastic, fibrous, or ductile materials.  

Ball mills can be very large, 5-6 m in dam, and even larger with input power up to 20 megawatts. The largest mills are used in mining operations.

Attritor Mills
In attritor mills, smaller grinding media is employed ranging from 1/8 to about 3/8 in. The most common media types are stainless steel, chrome steel, tungsten carbide, or ceramic. There are two basic processes for attritor mills: batch and continuous. Typical feed material size is below 2 mm.

In a batch attritor, the material to be ground and the grinding media are placed in a stationary, jacketed grinding tank. The media and suspension are agitated by arms mounted on the shaft, rotating at high speed, exerting impact and shear forces on the particles, resulting in size reduction and excellent dispersion. Attritors, like ball mills, can create high-intensity mixing or blending of materials, whether introduced together into the mill or added during the process. Premixing is not necessary, but can be beneficial in introducing a well dispersed and wetted material to the process. Premixing can reduce processing time and result in less wear in the mill. While a batch attritor is not a continuous process, there is a pump that keeps material circulating from a bottom discharge and back to the top of the stationary tank. Media is retained in the mill by a screen or grate at the discharge. Circulation aids in maintaining batch uniformity and controlling cycle time.
    
During processing, the batch can be evaluated for fineness, solids content, chemistry, or other parameters that may be critical to the process. Because the mill is an open tank, adjustments can be made to these parameters, as well as determine when the process reaches its end point. After the end point is reached, the batch can be discharged via the same pump used for circulation.
    
Batch attritors can be used to process very hard-to-grind materials such as silicon carbide, tungsten carbide, and some metals. Less abrasive or hard materials can also be successfully processed, such as paints and coatings, inks, minerals, chocolate, resins, cellulose, carbon black, pigments, and dyes. Typical batch attritors can be as large as 500-600 gal, with slurry volume about half the total tank volume.
    
A continuous attritor is similar, but will usually have a slightly larger vertical length over dam than a batch type. This is to control the residence time of the suspension in the mill in order to meet a specific fineness. In the case of a continuous attritor, a well-made premix is critical to the process. Continuous attritors may also use grinding media smaller than batch attritors, as small as 0.4 mm for higher density beads. In this type, the slurry is pumped through the mill from the bottom and discharges from the top of the tank. The media is primarily retained by grids at the bottom inlet and top discharge. A continuous attritor can be a single pass process, multiple pass through a single mill, or passes through several mills using finer media in succession to reach finer particle size distributions. The advantage of a continuous attritor is that the mill size is not the limitation of batch size, therefore a much larger batch can be processed with a lower investment in equipment.
    
There are also some attritors used in a circulation process with a larger holding tank than the mill volume and higher flow rate through the mill, but with a total residence time sufficient to reach the particle size target. The advantages with this type of process are better temperature control and a narrower particle size distribution.

Agitated Small Media Mills
Agitated small media mills are the final link in the evolutionary chain to reach nanometer particle size distributions and will be covered more thoroughly (by small media, we are not referring to the size of the mill, but the size of the media). This type consists of a vertical or horizontal grinding chamber, an agitator that is a rotating shaft equipped with agitator elements, a drive motor, and a media separator (located at the mill’s discharge). The agitator elements are typically disks or pins. The grinding chamber is filled with grinding media up to 95 percent of the mill volume. The grinding media can be made from materials such as stainless steel and glass, as well as advanced ceramic materials such as yttrium-stabilized zirconium oxide and cerium-stabilized zirconium oxide, and can range from as large as 10 mm dam to as small as 30 microns diam. The grinding media charge is activated by the rotation of the agitator shaft to create mechanical hydraulic shearing and particle impact. In agitator bead mills, the forces tear apart the solids suspended in a suspension as they are pumped through the grinding chamber.

In operation, a premix suspension containing the coarse material is pumped through the mill from a feed tank. The material flows into the grinding chamber and downward into the spaces between the grinding media. The agitator rotates at typical tip speeds between 4 and 20 m/s. The media move around the chamber and impart impact, compression, and shear forces to the suspended particles, fracturing or dispersing them. The suspension can be recirculated multiple times (known as high flow recirculation) with each pass having a short residence time in the mill chamber (approximately 30 seconds) until reaching the end product fineness specification, or pass only once (passage mode) through the mill to a product tank with a longer residence time in the mill chamber (1–2 minutes).

Each mode has advantages and disadvantages. The main advantage of passage mode is simplicity for those applications where the end particle size can be reached in a single pass or at most two passes. However, there’s no guarantee that every particle passes through the mill’s highest-energy zones; therefore the final particle size distribution (PSD) may be wider than desired.

There are two variations of passage mode--pendular and serial mode--to potentially address this issue. Pendular mode ensures that more of the particles pass through the mill’s highest-energy zones. Using a high flow rate and two or more passes, the required particle size and a steeper PSD may be reached with a lower total residence time. This mode’s higher flow rate also results in less material heating, but the material is handled two or more times, which is undesirable in some applications.

The serial mode allows the use of two mills with different grinding media sizes a larger size in the first mill takes a coarse feed material to a size that allows the next mill to use finer media to reach the final desired particle size. In this way two-step grinding is accomplished in a single process.

If the material requires more than two or three passes, the high flow circulation mode may be the best option. In this mode, all particles ultimately pass through the mill’s highest-energy zones and achieve the steepest PSD and finest particle size. The circulation mode’s high flow rate also gives the material a short residence time, keeping both the material and the mill cooler and allowing accurate control of the material temperature.

Factors influencing the ultimate particle size:
* Formulation of the premix (solids content and viscosity)
* Quality of the premix (particle size distribution and oversize particles)
* The grinding media used (bead size and density)
* Media filling level in the mill
* Agitator speed
* Flow rate through the mill

Factors that are monitored during operation
* Motor power consumed
* Discharge temperature of the suspension
* Inlet pressure of the suspension to the mill
* Cooling water temperature and flow

Depending on the material to be ground and the objective or end-use of the resulting product, one of two types of media milling processes may be chosen. In comminution, particles are ground within the slurry by high-pressure shearing and impact forces to break apart the actual particles. In de-agglomeration, the small particles that are joined together are broken apart and separated without changing their primary size or structure. In some cases, both comminution and de-agglomeration are used on a single product.
    
As mentioned, the size of the grinding media has a direct relationship to the size of the finished product. As a rule of thumb, the final median size will be approximately 1/1000 the media diam. So, to reach a median particle size of 100 nanometers, a grinding media dam of 100 microns is used. Media as small as 30 microns is sometimes used to reach a median particle size less than 30 nanometers.
  
With very small grinding media, the separation process becomes more critical. In ball mills and attritors, the grinding media is retained in the mill by physical interference of a screen or grate. This is not feasible, or even possible, when using the finest media.  
    
When using media smaller than 200 microns, and considering that some slurries can increase in viscosity during milling, the media can be transported all the way to the separator screen by the suspension’s flow forces though the mill, causing screen blockage. In such a case, the best media separator is a classifying rotor. Generally, the centrifugal forces it generates ensure media separation from the suspension. This is quite similar to air classification in a dry process, except that the classifier in a dry process is employed to separate coarse from fine product fractions. In a wet media mill, the coarse fraction is the grinding media.

There are several variations, and recent advancements, of centrifugal media separation systems, but the graphic demonstrates the effectiveness of this design to retain media in the mill. This is a significant contributing factor in the capability to use the fine grinding media needed to enable milling into the nanometer size range.
    
Agitator small media mill chambers range in size from 15 ml in pharmaceutical development mills to 50,000-l mills used in mining and precious metal recovery processes.

Advantages and Disadvantages of Media Milling
There are many advantages to media milling. The primary reason to select media milling is that the process can produce uniform particle size distributions in the micron and submicron (or nanometer) range. Dry-milled materials have the tendency to agglomerate after processing, or when later added to liquid, will also tend to agglomerate. This can be avoided when initially mixed with the liquid carrier and processed in a wet media mill. Wet milling encapsulates the dry particle, surrounding it with liquid and preventing re-agglomeration. Further, and long-term stabilization of the suspension using either electrostatic charge control or long-chain molecules can be achieved.
    
There are disadvantages as well. One is contamination. A media milling process produces contamination due to wear of the grinding media and internal mill surfaces. This can be mitigated by selecting the proper wear-protection measures (wear protection of the mill and high-quality grinding media) and by adopting processing conditions to fit the requirement, without overgrinding. If a material simply requires de-agglomeration, a low-energy grinding process may be all that is needed. High-energy milling using high agitator speed (as used in primary grinding or comminution), will only create wear. Low agitator tip speeds significantly reduce wear and energy use. The other disadvantage comes when a material is ultimately used as a dry powder. Once wet grinding is completed, an energy intensive drying step is needed to complete the process. A dry process can be an advantage in these cases.

Dry Milling Technology to Produce Nanometer Particle Size Distributions
Only recently has a dry process been able to consistently produce particle sizes in the nanometer range with a steep particle size distribution. This is now done with fluidized bed jet mills using superheated steam instead of compressed air. This too has been an evolution in grinding technology from simple spiral and loop jet mills, to opposed jet mills, and the most effective jet milling technology to date - fluidized bed jet mills. A further advance in this technology is the use of superheated steam to both increase energy input into the milling process and enable the separation of particles in the nanometer range.

Spiral Jet Mills  
Spiral jet mills were first used in the 1930s to enhance the dry milling process to reach particle size distributions with median particle size in the range of 1-10 microns, and in fact used steam as the grinding gas. Spiral jet mills are known for simple construction and simple operation without moving parts. Size reduction is accomplished by particle to particle and particle to wall collisions. Control of the particle size is mainly a function of a free vortex classification flow. Free vortex classification occurs when particles are introduced into a circumferential airstream. The heavier (coarser) particles remain on the outer periphery of the flow stream influenced by mass force created by centrifugal forces, while the lighter (finer) particles are drawn to center by drag force (effect of the fluid stream – usually air) and exit the mill from the centrally located outlet with the air.
    
The grinding process occurs while particles are circulating near the peripheral wall of the mill. There they are accelerated by grinding gas nozzles located on the peripheral wall. The acceleration results in the aforementioned particle to particle and particle to wall collisions. As the particles are reduced in size they migrate with the gas flow towards the central outlet and ‘spiral’ out of the mill.
    
Since there is no active classification in the mill to control the coarse particles, the particle size distributions tend to be wide, yet high fineness can be achieved in the median size. In order to keep the oversize particles to a minimum, or to reach a given median size, there was a tendency to create a high percentage of fine particles by overgrinding.
    
When processing hard or abrasive materials, significant wear can occur due to contact with the wall and in these cases, hardened or ceramic materials are used for protection.

Loop Jet Mills
A further step was taken in the development of the loop jet mill with the goal of improving the sharpness of the “cut” – control of the coarse fraction. Like the spiral jet mill, there are no moving parts in the loop jet mill, and size reduction is a function of particle to particle and particle to wall collisions. Loop jet mills have their grinding nozzles located just after the feed inlet. In the same manner as the spiral jet mill, the coarser particles circulate on the outer wall, while the finer ground particles migrate to the inner wall. Here there is a difference in that the finer particles migrating to, and circulating on, the inner wall follow the inner wall surface and change direction as they exit the mill. There is also an externally adjustable ‘barrier’ inside the mill to help control the migration of coarse particles to maintain their flow on the outer periphery until they are fine enough to exit the mill.

Classification
The following mills all use internal dynamic air classification to control the upper particle size limit of the distribution. The following is a description of that process.
    
Classification is the separation of particles according to their settling velocity in a gas or other fluid. In powder processing using a dynamic air classifier it is the separation of particles according to the effect of dynamic forces on the particles. There are two primary dynamic forces of air classification acting on the particles. The first is mass force. This is the force exerted on a particle by gravity, inertia, or centrifugal force. In this case it is centrifugal force generated by the classifier wheel. Mass force has a greater influence on coarse particles. The second is drag force. This is the force exerted on a particle by the surrounding fluid medium. In the case of dry classification, the fluid is a gas. Drag force has a greater influence on fine particles. There are also certain material parameters affecting air classification. These are material density, particle shape, and particle size. Gas parameters affecting air classification are the gas viscosity and gas density.
    
As described in the graphic, higher density particles tend to classify finer. Therefore once would expect a material such as tungsten carbide to have a finer cut point than calcium carbonate at identical process conditions. Particle shape also is a factor, although it is less predictable. A flaky or high aspect ratio particle may present itself in any orientation affecting its aerodynamic performance in the gas flow. For instance, a rod-like material can present itself perpendicular to the direction of the gas flow and is classified as a coarser particle. If that same particle is presented in the direction of the flow, it will perform as a finer particle. The density of the fluid is also a factor. A higher density gas (example - ambient air) compared to a lower density gas (example - steam) will exert a greater influence on a particle carrying it to the fines discharge, and resulting in a coarser cut point.

Opposed Jet Mills
In opposed jet mills there is finally an integration of a dynamic forced vortex air classifier with an opposed jet mill. This design allows control of the classification cut point independent of the airflow or the feed rate. Feed material is introduced into the mill in the proximity of the classifier. If there are fine particles present in the feed stream, they may exit the system through the dynamic classifier wheel. Coarse particles are rejected by the classifier and fall through the coarse outlet of the classifier into a split stream where they are mixed with high-pressure grinding gas and accelerated into the grinding zone. In the grinding zone they impact with particles from the opposing stream. The expanded airflow carries the particles again to the classifier where the process is repeated.
    
While there is constant feed and constant discharge of product, there is also an internal circulation of coarse or partially ground material in the mill. As the demand for fineness increases, the internal circulating load far exceeds the actual production rate.
    
There are some advantages to this design, including active control of the particle size, which results in higher efficiency, improved product quality, and a steeper particle size distribution. But there are also several deficiencies. One is high wear on the nozzles as both air and feed material pass through. Another is the long classifier shaft that can exhibit critical speed issues. A third is the balancing of classified coarse fraction into equal streams before mixing with the high-pressure grinding gas. Still, for its time, it was a significant improvement over jet mills that came before it.

Fluidized Bed Jet Mills
The fluidized bed jet mill offers several improvements over the opposed jet mill. The material is ground in a fluidized bed by particle to particle impact only. There is virtually no impact velocity against the mill wall and much less wear. Only gas flows through the nozzles significantly reducing wear. The classifier is in closer proximity to the grinding zone. Mechanically and operationally, the classifier is a much more stable compact design. There is also a more effective classifier provided by the high end suppliers of fluidized bed jet mills. There are differences in the approaches that the manufacturers take in classifier design, but most are effective in their own right. Typically the particle size distribution in a fluidized bed jet mill is much finer and much steeper than the other jet mills, including the opposed jet mill, described above.
    
While fluidized bed jet mills, operating with ambient temperature or hot gas are better than those that preceded them, they are still not the dry process needed to grind consistently into the nanometer size range. That was the target for the development of fluidized bed jet mills using superheated steam.

Jet Mills Using Superheated Steam
The demand for finer dry powder products in the submicron or nanometer scale has led to increased use of technology using superheated steam as the grinding gas. Superheated steam as the grinding gas in jet mills has been used for many decades in the spiral or loop jet mills described above and more recently in fluidized bed jet mills.
    
There are several key factors that make this process viable. Steam can be provided to a jet mill at high pressures compared to air. At higher grinding pressures, higher jet speeds can be attained. For example, at 100 BAR absolute, the jet speed exceeds 1200 m/s, compared to 600 m/s when using air, the kinetic energy in the mill is substantially higher with a proportional increase in capacity.
    
Steam allows a finer cut size than air by reducing the drag force conveying particles from the mill. In a jet mill, this means the particle size distribution of the product is finer.
    
Steam jet mills of all types are successfully used in commercial applications from ceramic materials, printing applications to advanced energy processes. On the other hand, steam jet grinding cannot be used for products that are sensitive to high temperatures, such as active pharmaceuticals and organic materials. However, any inorganic material not adversely affected by high temperatures, and where fine particle sizes are desired, may be suitable for steam-jet milling. Extensive testing has been performed on aluminum oxide, barium titanate, ceramic pigments, glass frits, graphite, rice ash, silicon carbide, talcum, and zirconium oxide, to name a few.
    
One last advantage: Steam jet milling is greener than conventional air jet milling. As is well known, steam is the driving force of almost all energy production worldwide. In 2015, about 86% of the electrical energy in the U.S. was generated by large power plants using fossil or nuclear fuel. Large power plants operate on average with a degree of primary energy efficiency of around 40%. Transformation and line losses cause an additional loss of about 10%. Therefore when the electricity arrives at your plant, it has a degree of efficiency (compared to the primary energy) of about 36%. When you factor in compressor efficiency, which is about 45%, the overall energy is only about 16% from primary energy to kinetic (grinding) energy in the mill. By using steam directly, the process becomes two or three times more energy efficient. Grinding with steam is greener.

The Future of Nanotechnology
The needs of companies developing materials in the nanometer size range can be met with either wet media mill or dry jet mill technology. The process and end use are factors that lead to the decision which is best for the application.
    
In some cases, steam jet milling is more energy-intensive than media milling and its use would add additional costs to the product. Although many materials are suitable for steam jet milling, some substances cannot withstand the heat of the process. And when the finished product is needed to be wet or in a solution, it may be more cost-effective to reduce its size using wet media milling rather than steam jet milling. However, when a dry end product is needed, the advantage may be with steam jet milling.
    
Wet milling technology also continues to develop and several new designs are available today that were not available even one year ago. These designs offer improved media separation allowing the use of smaller grinding media. Smaller media enables a finer particle size distribution. Improved separation of media gives flexibility to process materials with higher solids and viscosity. Better cooling efficiency allows more energy input into milling process resulting in higher production rates.  
    
Both technologies can apply to ceramics, alternative energy materials, optical glass, pigments, coatings and industrial minerals markets to name just a few. Both are viable technologies, with advantages and disadvantages, and in some rare cases, wet media milling with small media mills and dry grinding with a steam jet mill may be considered, tested and found to be equally successful! The engineer then has an interesting choice to make!

Stephen Miranda is sales director, Netzsch Premier Technologies LLC, Exton, PA. For more information, call 484) 879-2020 or visit www.netzsch.com.

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