Abstract
There are two main equipment types for wet grinding applications; bead mills and basket mills. There has been a long-standing debate in the industry about superiority of these mills. These equipment are key to the productions processes which are subject to wet grinding.
So understanding the basics of wet grinding is very critical for selection of right equipment. This article defends the idea that these two mills are not actually a competitor but are suitable for different wet grinding applications. Hence, how to find select right mill type for right application have been discussed.
Although basics of wet grinding is not the subject of this article, one should have information on these basics to select these kind of equipment.
1. Introduction
Wet grinding in bead mills is a key process step in the production of coatings. The requirements for quality in inks and paints have become significantly increased. [1] These days markets expect extremely high vivid ink colors and long service life for coatings.
Manufacturers spend a lot of time researching and choosing best raw materials and milling equipment for their products in terms of components, viscosities, particle size distributions (before and after the dispersing/milling process), and other properties.
[2] As new application of coatings are raising, number of pigments employed are increasing.
[3,4] As a result,manufacturers have to deal with new formulation with different characteristics. [5] This issues are ending up with debates about choosing right equipment for the individual processes.
A total understanding of the grinding and dispersion process in bead and basket mills is a key to right mill selection. Despite the fact that basics of grinding and dispersion process it not a subject in this article, addressing to the difference of bead mills and baskets mills is impossible without this information.
Bead type, size, machine technology, product properties are all vital parts of the process and a very brief information about these are also will be a part of this article. Basket mill’s design fundamental is to utilize smaller process chamber in larger container that holds the premix under process.
(Figure 1) It must be cooled to control the heat caused by the grinding and mixing operation.
Basket mills are also makes use of a separate agitator shaft and pin or disk-style agitators.
Conversely, a bead mills is consisting of a shaft with a disc or pin type rotor. (Figure 2). All products passes through the process chamber almost homogeneously. Because of the smaller process chamber, cooling effect can be more effective.
Figure 1. Simplified schematic of a basket
mill process chamber
Figure 2. Simplified schematic of a full volume
bead mill process chamber
2. Beads
When the size, density and quality of the grinding beads are coordinated to the dispersion task and machine technology, the efficiency of the process can be improved and the quality of the suspension increased. To disperse inks or paints, bead mills or basket mills are generally used.
Due to physical friction between the beads, the particle agglomerates of color pigment are basically broken up; the primary particulates are wetted and then evenly distributed in the liquid carrier. [1]
2.1. Bead Size
Smaller beads have certain advantages. When an operative uses smaller beads in its mills, actually this operative increasing the quantity of the beads used in the process chamber. Thus increasing the total surface area of all beads and contact possibilities with particle agglomerates.
Figure 3. Enlarged sizes of grinding beads for comparison purposes [1]
Table 1. How bead density and diameter are related to product viscosity for efficient milling (guidelines only: energy input and local shear rate in the mill may change viscosity significantly) [1]
3. Specific Energy
Specific energy is one of the most important criteria for the wet grinding. Specific energy can give us an idea about the grinding behavior of the product as well as a chance to compare efficiency of the milling systems.
For a single-pass operation of a mill, Em is defined as; [2] where P is the total power consumption of the mill (kW); P0 is the idle power (power consumption of the empty mill-kW); and m’ (t/h) is the throughput rate of product through the mill.
Em (specific energy-kWh/t) can now easily calculated from the measured data from a mill. For recirculation mode operation, the appropriate equations are. [2]
4. Particle Size Distribution
Particle size distribution is one of the most important properties of a wet grinding product. [6] Quality of the product is mostly depended on particle size and its distribution. Several different techniques have been developed for the measurement of the particle size distributions.
These techniques include, for instance sieving, measurement of settling velocity of particles, laser diffraction and direct image analysis. The diameters of D10, D50, D90 practically mean that 10%, 50% and 90% of the particles in the sample are smaller than these particle diameters.
[7]An important parameter for color properties such as gloss, tinting strength and color intensity, is smaller pigment particle sizes adjusted to specific particle size distributions. [8] Importance of the particle size distribution can be easily understood by studying Figure 4 and Figure 5.
5. Innovations of Bead Mills
Compression of the beads in the front section of the process chamber has been a problem for bead mills. But this problems are being sorted out by new innovations. Bühler’s Cenomic™ full-volume bead mill incorporates patented solution that allow higher flow without compression of beads.
This allows a reduction of the specific energy requirement, a narrow particle size distribution and a higher product quality with lower grinding costs. [9] Table 2 illustrates how grinding time can be remarkably reduced.
A shorter grinding time not only results in a more efficient production process but also realizes savings in labor costs. In addition, the chart clearly states the decrease in the mass-specific energy consumption only by using the right equipment.
Table 2. Comparison of grinding non-toluene and non-MEK PU-based gravure ink
The table points out that even with highly increased product flow rates, the same or even better dispersion quality in terms of fineness, gloss and color strength can be achieved using full-volume bead mill. The reduced grinding time per batch has a direct impact on minimizing the grinding costs.
But most important are the savings that result from the cutback in manpower, the depreciation and utilities. Furthermore, not only was the grinding time reduced but also the specific energy amount per batch was diminished which again results in energy costs savings.
Figure 4. Comparative microscope analysis – results
after grinding with basket mill [10]
Figure 5. Comparative microscope analysis – results
after grinding with bead mill [10]
6. Conclusion
Basket mills utilizes a relatively small process chamber in a large container. As a results of this every pigment cannot pass through this chamber evenly. Therefore operating times are significantly increased to reach required target fineness and homogeneous particle distribution.
On the other hand, every single pigment particle pass through the process chamber of bead mill almost evenly also required power for a bead mill is significantly lower than a basket mill. When cooling taking into account, basket mills’ surface area for heat exchange is very low compared to its volume.
Beads mill’s cooling jacket surrounds all over its process chamber and it utilizes a process chamber of 10, 30, 60 liters typically instead of typical container volumes of between 200 and 1000 liters for a basket mill. Thus, a better cooling efficiency expected from bead mills.
One of the most important aspect when studying wet grinding is bead size. Basket mills are quite limited when it comes to reducing bead size. While relatively low bead sizes can easily be handled by a bead mill.
Table 3. Comparison of properties of bead mill and
basket mill
One can easily say that basket mills are not an optimal mill for products such as pigment pastes, top coats and other added value products after studying this article.
Besides it can be considered as an alternative for more simple wet grinding processes such as primer, base coats. But even with these products bead mills are still a remarkable alternative to a basket mill.
References
1. Kern, Norbert. Optimizing the wet grinding process. European Coatings Journal (06-2016).
2. Dr. Stalder, Bernhard. Scalability of agitated bead mills. Paint & Coatings Industry Magazine. (04-2014)
3. F. Brenzikofer. The global market for organic high performance pigments. H.M. Smith (Ed.), High Performance Pigment. (2002)
4. G. Buxbaum. Introduction to inorganic high performance pigments. H.M. Smith (Ed.), High Performance Pigment. (2002)
5. Ali, Muhammed and Ling, Long. Optimization and analysis of bead milling process for preparation of highly viscous, binder-free dispersions
of carbon black pigment. Progress in Organic Coatings 119. (2008)
6. Y. Wang, E. Forssberg. Product size distribution in stirred media mills. Miner. Eng. 13. (2000)
7. T. Kinnarinen, R. Tuunila, M. Huthanen, A. Hakkinen, P. Kejik, T. Sverak. Wet grinding of CaCO3 with a stirred media mill: Influence of
obtained particle size distributions on pressure filtration properties. Powder Technology 273. (2015.)
8. Weber, Udo. The effect of grinding media performance on millinh a water-based color pigment. Chem. Eng. Technol. (2010)
9. Traber, Mark. Bead mill offer many solutions. Paint & Coatings Industry Maganize. (04-2016)
10. Grinding & Dispersion Academy, Technical Seminar: Customer Training by Bühler. Dubai. (October 16, 2018)
