A Different Perspective on Selecting Wet Grinding Equipment

Summary

The two most fundamental pieces of equipment for wet grinding applications are bead mills and basket mills. A long-standing debate in industry has centred on which of these mills is superior. These pieces of equipment occupy a critical position in processes requiring wet grinding. For this reason, it is important to have basic knowledge of wet grinding in order to select the correct equipment. This article argues that these two mills are not actually competitors, but rather address different wet grinding applications. Therefore, the discussion will focus on how to select the right equipment for the right application. While the principles of wet grinding applications are not the subject of this article, knowledge of the relevant principles is necessary for the selection of such equipment.

1. Introduction

Wet grinding with bead mills is a critical process stage for production such as paints. Quality requirements in inks and paints have increased significantly. [1] Nowadays, market expectations for highly brilliant colours and paints include long service life. Manufacturers spend considerable time researching and selecting the best raw materials and grinding equipment suitable for their products' composition, viscosity, particle size distribution (before and after dispersion and grinding processes) and other properties. [2] As new applications for paints increase, the number of pigments used also rises. [3,4] As a result, manufacturers encounter new formulations with different characteristics. [5] These challenges give rise to discussions regarding each manufacturer selecting equipment suitable for their own process. Fully understanding grinding and dispersion systems is very important for correct mill selection. While the basic principles of grinding and dispersion are not the subject of this article, without this knowledge it would be impossible to interpret the differences between bead mills and basket mills. For this reason, brief information about bead type and size, machine technology of bead mills and basket mills, and product properties—which are important components of the process—is also included in this article. The basic operating principle of a basket mill involves a small process chamber located within a large vessel that holds the pre-mix during processing. (Figure 1) The heat generated during grinding and mixing operations must be controlled by cooling. Basket mills have a separate mixer configuration with pin or disk-type mixers. In contrast, bead mills consist of a shaft with a disk or pin type rotor. (Figure 2) All product passes through the process chamber in a nearly homogenized manner. Due to the relatively small process chamber, the cooling effect is much more efficient.   Figure 1. Simple schematic representation of the process chamber of a basket mill                     Figure 2. Simple representation of a full-volume bead mill process chamber

2. Beads

The size, density and quality of grinding beads, when coordinated with the dispersion function and machine technology, can improve process efficiency and increase suspension quality. To disperse inks or paints, bead mills or basket mills are typically used. Through friction between beads, particle agglomerates of colour pigments are simply broken; primary particles are wetted and evenly distributed in the liquid carrier. [1]

2.1. Bead Size

Small beads have distinct advantages. When an operator uses smaller beads in their mills, that operator effectively increases the number of beads in the process chamber as well. This increases the total surface area of all beads and improves the probability of collisions with particle agglomerates. Figure 3. Enlarged dimensions of grinding beads for comparison purposes

2.2. Bead Quality

As an energy-intensive process, friction between grinding beads during wet grinding is relatively severe. If grinding beads made from low-quality material are selected, wear occurs and the rough surfaces of the beads can damage machine components. Using beads made from high-quality material is important for obtaining high-quality product. [1] High-quality beads wear more uniformly, making it easier to ensure the required product quality while extending machine life. On the other hand, high-quality beads may not always be necessary. For some processes, the process chamber may be lined with polyurethane plastic. In such applications, rough bead surfaces do not cause damage. In addition, material costs are always a significant factor. For example, for a process chamber with 1,000 litre capacity, bead cost can be very important.

Table 1. Relationship between bead density and diameter with product viscosity for efficient grinding (guidance only; energy input and local shear values can change viscosity significantly.) [1]

Product viscosity range η [mPA.s]	Minimum bead density ρ [g/cm³]	Bead diameter, min-max range
0.2≤η≤50	1.0 (polymeric)- 6.0 (e.g. "Draison" yttrium/cerium)	30-300 μm
20≤η≤200	1.0 (polymeric)- 6.0 (e.g. "Draison" yttrium/cerium)	50-500 μm
100≤η≤800	3.6 (ceramic)- 6.0 (e.g. yttrium/cerium)	0.3-1.0 mm
800≤η≤2000	≥6.0 (ZrO₂ – ceramic)	0.8-1.5 mm
1500≤η≤20,000	6.0- 7.8 (chrome steel)	1.5-3.0 mm
η≥20,000	Viscosity too high for bead grinding

3. Specific Energy

Specific energy is one of the most important criteria for wet grinding. With specific energy, we can gain insight into the grinding behaviour of the product, and we also have the opportunity to compare the efficiency of grinding systems. For a mill operating with pass operation, Em is defined as follows [2]. P is the total power consumption of the mill (kW); P₀ is idle power (power consumption of the empty mill, kW); and ṁ (t/h) is the product output quantity in the mill. Em (specific energy-kWh/t) can now be easily calculated from data measured from the mill. For operation in circulation mode, the appropriate equation would be as follows. [2]

4. Particle Size Distribution

Particle size distribution also represents one of the most important characteristics of a wet-ground product. [6] Product quality depends largely on particle size and distribution. Various techniques have been developed to measure particle size distribution. Some of these techniques include filtration, measurement of particle settling rate, laser diffraction method and direct image analysis. D10, D50, D90 diameters respectively mean that 10%, 50% and 90% of the particles in the sample are smaller than the given diameters. [7] Important parameters of colour properties such as brightness, colour tone and colour intensity are adjusted with specific particle distributions and small pigment particle sizes. [8] The importance of particle size distribution can be easily understood by examining Figures 4 and 5.

5. Innovations in Bead Mills

Bead jamming in front of the process chamber has always been a problem for bead mills. However, this problem has been solved with new innovations. Bühler's full-volume chamber bead mill Cenomic™ has a patented solution that provides higher flow rates without bead jamming. This reduces specific energy requirements, provides a narrow particle distribution and helps achieve high product quality. Moreover, grinding costs are reduced. [9] Table 2 shows how significantly grinding time can be reduced. Reduced grinding time not only provides more efficient production process. At the same time it provides a reduction in labour costs. Additionally, the table shows that mass-based specific energy consumption can be reduced by using the correct equipment. Table 2. Grinding comparison for an engraving ink without toluene and MEK

Grinding equipment	Bühler Cenomic™3	Standard industry mill
Grinding bead	Zirconium oxide, 0.8-1.0 mm	Zirconium oxide, 1.0-1.2 mm
Process chamber volume	20.8 L	18.5 L
Process parameters
Flow rate for 1st pass	675 kg/h	370 kg/h
Flow rate for 2nd pass	460 kg/h	270 kg/h
Power consumption P0	12 kW	10 kW
Particle size after 2nd pass (GM value)	8 μm	10 μm
Colour intensity (ΔE)	0.46	0.51
Brightness	41.2	44.3
Opacity (%)	31.5	29.4
Summary
Net productivity	274 kg/h	156 kg/h
Specific energy requirement	42 kWh/t	67 kWh/t

Results of very high flow rate: • 43% reduction in grinding time • 37% reduction in mass-based specific energy consumption > Reduction in total specific energy cost

Table 2 shows that by using a full-volume chamber bead mill, despite increasing product flow rates, the same or even higher dispersion quality can be achieved (in terms of particle size, brightness and colour intensity). However, most importantly, there are reductions in labour costs, depreciation and electricity expenses. In addition, not only is grinding time shortened, but also the specific energy amount per batch is reduced, resulting in energy cost savings. Figure 4. Microscope analysis for comparison – results after grinding with basket mill [10]     Figure 5. Microscope analysis for comparison – results after grinding with bead mill [10]

6. Conclusion

Basket mills use a relatively small process chamber within a large vessel. As a result, not every pigment can pass through the process chamber an equal number of times. Therefore, the operating times required to achieve target particle size precision and homogeneous particle distribution increase significantly. On the other hand, in a bead mill almost every pigment passes through the process chamber an equal number of times, and the power required by bead mills to do the same job is considerably lower than that of basket mills. Considering cooling, the cooling surface area of basket mills is quite small relative to their volume. The cooling jackets of bead mills surround the entire process chamber and use process chambers such as 10, 30, 60 litre compared to typical basket vessel volumes of 200 and 1,000 litres. For this reason, better cooling performance from bead mills can be expected. When working on wet grinding, one of the most important considerations is bead diameter. When it comes to bead diameter, basket mills are quite limited. In contrast, bead mills can work with very small bead sizes. Table 3. Comparison of characteristics of bead mill and basket mill In conclusion, based on the work conducted in this article, it can be stated that a basket mill is not the optimal mill for pigment paste, finish coat paints and other value-added products. However, it is an alternative for simple wet grinding processes such as primer and base coat paints. Nevertheless, for such products, bead mills can still be significant alternatives to basket mills. Aybars Kaya Regional Sales Manager (Turkey) – Grinding and Dispersion Unit Bühler Sales and Service Ltd. Co.    

References

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