Grinding and Dispersing Nanoparticles
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[edit] Introduction
According to a report released by the National Science and Technology Council (NSTC) "Nanoscience and Nanotechnology generally refer to the world as it works on the nanometer scale, say, from one nanometer to several hundred nanometers.1" If we interpret this to mean that a nanoparticle can range in size from one to 700 nanometers, then we can say that most of the components processed on bead mills fall into the area of nanotechnology.
Most pigments used in inks and coatings, for example, have a primary particle size from at least 0.02 microns (μm), or 20 nanometers, up to 200 nanometers. For this article, let us assume that the nanoparticles desired are less than 200 nanometers. Our goal, then, will be to identify the required operating conditions for a bead mill to grind and disperse particles to that size.
[edit] Particle Size Control
Evidence from lab tests show that the particle size achieved from a bead mill is a direct function of the media size used for the grinding process. One rule of thumb is that the average particle size quickly achievable in a bead mill is about 1/1000th the size of the grinding media. Graph 1 illustrates this point:
We see that rapid particle size reduction occurs and the curve plateaus at around 0.5μm for the 0.5mm nominal media, and the curve starts to plateau around 1.5 to 2μm for the 2mm nominal media.
[edit] Current Industrial Applications
The smallest bead size regularly used commercially is 200 to 300 microns. These media are used primarily in the pigment manufacturing and ink industry for fine grinding and dispersion of pigments such as Phthalocyanine blue and green and Carbon Black. There is much information in this area that can be learned by searching patents. For example, US patent #5,500,331 discusses the use of media less than 100μm to grind various dyes.
It is interesting to note that using beads smaller than 50μm do not appear to have as great an effect on particle size reduction as do 50 and 75μm beads. This indicates that, at some point, media size and feed particle size become increasingly important. One measure of efficient grinding and dispersion is that the feed particle size should have a d90 (meaning 90 percent of the particles are less than) one-tenth the media size. For example, when using 100μm beads, the d90 should be about 10 microns.
In these markets in North America, there at least 50 bead mills, manufactured by NETZSCH Fine Particle Technology (http://grinding.netzschusa.com/), operating with the 200-μm media in either steel or ceramic material. These machines range from lab equipment up to 300 horsepower machines. Worldwide, we can probably double this figure again.
This is a significant amount of capacity for this small media and this type of process. But how does it work, and what should a bead mill manufacturer consider for a more efficient design?
[edit] Bead Separation Parameters
In the modern bead mill, removing the grinding media from the slurry is achieved by centrifugal separation. Figure 1 shows a horizontal disc mill with a classifying rotor for this purpose. The beads are retained to the left and separation occurs at the discharge end by centrifugal force. This concept is covered in US patent #4,620,673. If we rely on the classic bead mill design of filtering the beads from a slurry using some type of screen inserted into the chamber or mounted in the end or wall of the chamber, the screen will become blocked with layers of media, pressure in the vessel will rise to an unacceptable level and the overpressure safety device will shut down the mill.
Centrifuging the media out of the slurry is far more efficient, and allows continuous operation of the mill. This is achieve by controlling the forces at hand, making sure that the separation force created by the centrifuging of the rotor exceeds the flow force of the product traveling through the mill.
[edit] Key Factors
The key factors that affect bead separation:
1. Product viscosity. The viscosity of a slurry inside a bead mill is higher than its final viscosity because grinding media is present. If the material is very viscous, the high power needed to agitate the grinding media limits the speed at which the mill can run. This limits the separation system speed. The high viscosity also forces the media toward the discharge screen. The usual solution is to apply more power to the mill volume, but this requires additional mechanical considerations. There is a limit to how much power can be applied to an existing shaft design.
2. Product flow rate. If flow velocity is higher than separation velocity, the beads are taken to the screen. This can cause blockages that slow, or even stop, the grinding process.
3. Product density. If a high-density slurry is processed, the mass creates a drag effect that counteracts the centrifugal force of the separator.
4. Bead size and density. Centrifugal force is a function of mass multiplied by the square of the velocity. High viscosity slurries require beads of greater mass to increase the centrifuging force and overcome the drag force of the product flow. For example, using a 1mm Yttrium Stabilized Zirconium Oxide (YTZP) bead that has a density of 6 gm/cm3 will more than double the separation force created by 1mm glass media with a density of 2.6 gm/cm3, given a constant rotor speed.
5. Bead charge and porosity. Graph 2 shows the calculated interstitial space between grinding media at various charge levels. For example, if a 2mm bead is used at 60 percent bead charge, the calculated space is approximately 400μm. If we use 100μm beads at a 95 percent bead charge, the space is about 1.5μm. What must be accounted for is the effect of hydraulic pressure due to resistance to flow created by the tighter spacing, or filtering effect, of the smaller grinding media and higher charge.
6. Agitator speed. High rotor speed increases centrifugal force, but certain factors limit how fast the rotor can turn, such as installed power, wear, and the mechanical seal. These shortcomings can be eliminated, but engineering modifications may be costly.
7. Dimensions of the separation system. This includes the distance between the screen and rotor and the length of the screen. The slots have to be precise in this application. Normal screen designs do not have the open surface area necessary to allow flow through the mill without creating a high velocity through the screen slots, which may result in poor bead separation and, eventually, screen blocking.
[edit] Selecting A Bead Mill for Small Media
Of the two types of bead mill that can use this small media, favored is the high-energy pin mill over the disc mill. Horizontal disc mills are limited in this application because the large volume of media required in the mill, coupled with the viscosity of the bead/slurry dispersion, requires tremendous power at high rotor speeds.
Two other issues make the high-energy pin mill a more attractive choice. Passing product through the large media volume required by the disc mill increases the likelihood of contamination. In addition, charging a 20-liter mill with 125μm YTZP beads would cost around $43,000 – about the same price as the mill.
Also notable: these two machines have the same installed horsepower. A 20-liter disc mill has 25 or 30 hp, and so does a 10-liter pin mill. The energy input available on the pin mill enables the machine to run at higher tip speeds. This higher tip speed increases the centrifugal compression of the media, which decreases the gaps between the beads and increases the filtration effect. The result is a tighter, finer particle size distribution.
Graph 3 illustrates the increase in grinding efficiency and bead compression while grinding phthalo blue pigment. The same slurry is processed using 250μm steel beads in a disc mill and a pin mill. Three passes through the disc mill at a total residence time of 11.7 minutes and a specific energy consumption (Espec) of 406 kilowatt hours per ton do not produce as fine a particle size as circulation grinding on a pin mill at 4.3 minutes of residence time and 256 kWhrs/ton Espec. The larger size of the separation system allows much higher flow rates through the pin mill versus the disc mill.
Screen open surface area is another area for examination. For example, a 20-liter mill using a 100 μm screen has about 12 cm2 screen open surface area – about the same open surface area as a 1" pipe. But the 10-liter pin mill has about 30 cm2 open surface area, or about the same area as a 2" pipe.
One advantage of a high-energy pin mill for this application is that, as the machines are scaled up, important parameters like constant energy input to mill volume is linear, while the geometry of the agitator rotor and separation system are constant. Also, available rotor tip speed is faster, due to mechanical limitations on small machines.
To summarize the mill selection points, recommended are the following features:
- Low mill volume with high energy input
- Low volume of media required
- High energy for high rotor speed
- Large separation system
- High centrifugal force
- Greater screen open surface area for reduction in flow-through velocity
- Low length-to-diameter ratio
- Reduction in hydraulic compression
- Lower drag force on media
These features are currently available in a high-energy pin mill design. So the question becomes, does this process work?
[edit] Examples of bead milling with small media
Graph 4 shows Titanium Dioxide dispersion and the difference between 0.5mm Zircon beads and 0.1mm Tungsten Carbide (WC) beads. A coarser feed slurry is passed through the disc mill, and some particle size reduction occurs. We then take this slurry and pass it once through a pin mill with 100μm WC spheres. This produces a dramatic difference in the particle size reduction, down to a mean size of 0.2μm, about the desired range for TiO2. When we process longer by circulating, we see further reduction to about 0.15 μm. This test was run to evaluate claims in Patent #5,407,464, in which it is claimed that using WC beads will grind TiO2 and various other mineral and organic powders to 100 percent less than 100 nanometers very rapidly.
Graph 5 illustrates dispersion of a fairly hard grinding synthetic organic pigment. Product specifications were: roughly 35 percent solids; viscosity ~ 500 centipoise; 6 percent dispersant on pigment solids. Residence time in the bead mill was about one minute. Particle size reduction is again very rapid down to primary particle size, but no significant reduction occurs past this point; so we are not grinding the primary pigment particle, at least according to our particle size analysis.
Graph 6 shows dispersion of alumina using 300-400μm Zircon beads and 125μm YTZP beads. Tip speed of the mill was about 11 meters per second. Flow rate was 0.6 liter per minute. In this case, grinding the alumina to a 100-nanometer particle size occurs rapidly, and the ultimate particle size was 87 nanometers (d50).
More graphs would only reinforce this conclusion: Particle size reduction is a function of a material’s properties. If a material is soft and easily friable, then particle size reduction to sub-100-nanometer size is possible. For harder materials, further particle size reduction may not occur. Like most of the tests we do, each application is very product-specific. This is why lab tests must be run – to determine whether materials can be processed.
[edit] Conclusions
Grinding with very small media is possible, and the process parameters are presented in this article with sufficient detail for using less than 200μm grinding media. Demonstrated is the idea that it is possible to operate a bead mill with 100μm beads, at least on a lab scale.
[edit] Further considerations
- Cost of the grinding media. The best media now available are Zircon and YTZP beads. Tungsten carbide beads are experimental and cause severe wear on certain construction materials, but that problem can be solved. Zircon media is less expensive than YTZP, but the contamination rate will be higher. Typically, materials reduced to nano size are high-value, so contamination is a primary concern. Also, capital cost of the media is high, and can escalate if an operator drops the bead charge on the plant floor.
- How practical is it really? Does the time saved by using 200-micron beads outweight the additional cost and potential difficulty? For example, if we can make a product on a 10-liter mill in one-fourth the time needed by a 60-liter mill, but media costs are higher process conditions more difficult, what is the break-even point for the tradeoff?
- This process works on the lab scale. For scale-up purposes, is a company willing to invest in $24,000 for beads to test on pilot scale?
- Patents. The information we derive from patents indicates that, on a very small scale, the process can be made to work. But these patents cover this type of process and this has to be addressed by a manufacturer interested in using a patented process.
[edit] References
Czekai, David A. US Patent # 5,500,331 (March 19, 1996); Eastman Kodak Company.
Kaliski, Adam F. US Patent #5,407,464 (April 18, 1995); Industrial Progress, Inc.
McLaughlin, John R. “Bead Size and Mill Efficiency,” Ceramic Industry, December 1999, Page 34.
McLaughlin, John R. US Patent #5,704,556 (Jan. 6, 1998); No Assignee.
Vernardakis, Theodore G. “Pigment Dispersion,” Coatings Technology Handbook, 1991, Page 529-550.
[edit] Notes
Written by Harry Way, Technical Director, NETZSCH Fine Particle Technology, Exton, PA
1 “Nanotechnology: Shaping the World Atom by Atom,” National Science and Technology Council literature, not dated.
[edit] See also
[edit] External links
Related articles can be downloaded as PDFs at http://grinding.netzschusa.com/press-room/articles.html.
