Optimization of Production Processes

Optimization of Production Processes in Grinding Systems

1. Introduction

In discussions with experts, colleagues and customers, it is repeatedly stated that the facility's production capacity is a decisive factor in the design of grinding and dispersion processes in the paints and coatings and inks industries, for example for new investments. Stirred bead mills are used for many products. Since the result of size reduction is essentially a function of a product's specific energy, the highest possible power input to the stirred bead mill is regarded in most cases as the decisive operating parameter. The argument that a mere increase in power intensity and thus power input to the mill also results in an increase in specific energy requirement is frequently rejected with the claim that it is acceptable to have higher energy costs compared to the product's price. In the following article, a simple example for the size reduction of calcium carbonate will first be used to demonstrate the effect of various parameters on power input, specific energy requirement and production capacity.   Subsequently, how an established and tested pin grinding system was modified will be explained, and the operating parameters specially adapted for the paints and coatings and inks industry to facilitate higher production capacities and simultaneously increase efficiency will be described. The results of field tests will be used to discuss the potential increase in production capacity.

2. Effect of Various Operating Parameters on Production Capacity and Specific Energy Requirement During Size Reduction in Stirred Bead Mills

The size reduction and operating behavior in stirred bead mills has been investigated in depth by Stehr [1] and Weit [2]. It has been found that a comprehensive factor in size reduction behavior is specific energy, namely the energy supplied to the grinding chamber relative to the mass or volume of the grinding product (solid) (see Equation (1)). Research conducted with grinding media of different sizes has shown that, in addition to specific energy, the size of the grinding bead has a significant effect on size reduction in stirred bead mills (Joost [3], Schwedes [4], Thiel [5], Bunge [6], Mankosa et al. [7], Stadler et al. [8] and Roelofsen [9]). Adjustment of the size of grinding beads according to the grinding task provides a degree of significant reduction in specific energy requirement. For discontinuous size reduction of limestone, Kwade [10] also noted that the peripheral speed of the stirrer and the density of the grinding medium have a systematic effect on the correlation between specific energy and product fineness. As an expression of the kinetic energy of grinding beads, Kwade [10] defined the stress energy of the grinding medium SEGM as a function of grinding medium density GM, grinding bead diameter dMK and the peripheral speed of the stirrer (see Equation (2)). During investigation of the effect of operating parameters, grinding bead density, the peripheral speed of the stirrer disk and grinding medium size, it was shown that with fixed specific energy input, there is an optimal stress energy at which the greatest product fineness can be achieved. These effects should be clarified with a simple example. Calcium carbonate was selected for two reasons: 1. For pigmented systems, simple evaluation of test results through particle size distribution is not sufficient, since in order to obtain suitable product quality, additional quality properties such as brightness, transparency and color intensity must collectively match. 2. Additionally, varying loading conditions can mean that for the same product quality characterization for paints, coatings and inks, the formulation also needs to be modified. However, the correlations between the operating parameters discussed in this example can qualitatively be transferred to any material system. At least three test settings are required to determine an optimum. The optimal stress energy range required to advance size reduction is not always reached. In terms of operational reliability or target product output, in most cases using even smaller grinding beads does not appear practical. In this example, a commercial limestone was initially processed in circulation mode in the same mill with identical grinding beads and at three different peripheral speeds of the stirrer: 7 m/s, 10.5 m/s and 14 m/s. Each time, samples were analyzed with a Malvern MasterSizer 3000 after specific energy inputs of 0.03 kWh/kgFS, 0.07 kWh/kgFS and 0.1 kWh/kgFS (see table below). The results of the test are shown in Figure 1, where d90 is plotted as a function of specific energy input. A target value of 6 μm was defined for d90. This situation shows that with the grinding medium used, the desired target value can be achieved with the lowest peripheral speed of the stirrer and the lowest specific energy consumption. However, due to the very low power input at these settings, the resulting production capacity is also at its lowest level (see Figure 4). [caption id="attachment_143113" align="aligncenter"] Figure 1: Size reduction of limestone with different peripheral speeds of the stirrer.[/caption] This finding is confirmed when considering the stress energies of the grinding medium for the tests performed (see Figure 2). The goal now is to achieve the highest production capacity with the lowest energy requirement. For this purpose, the peripheral speed of the stirrer shaft is increased and the grinding medium size is calculated for conditions where the same stress energy exists per grinding bead impact (see simple calculation below). Now if 1.0 mm grinding beads with a diameter greater than 1.7 mm are used at fixed grinding medium density and 14 m/s peripheral stirrer shaft speeds, the size reduction results plotted as a function of specific energy are comparable (see Figure 3). [caption id="attachment_143115" align="aligncenter"] Figure 2: Size reduction results as a function of stress energy of
grinding beads.[/caption] [caption id="attachment_143116" align="aligncenter"] Figure 3: Size reduction of limestone with different peripheral stirrer shaft
speeds, with additional variation of grinding bead size.[/caption] In view of possible production rates, it is clear that a higher production capacity can only be achieved by increasing power input due to the increase in the peripheral speed of the stirrer shaft and thus the kinetic energy of the grinding medium. However, this higher production capacity is associated with increased specific energy requirement. In contrast, through simultaneous optimization of grinding bead size, production capacity can be increased to an extraordinary degree without requiring higher energy input (see Figure 4).

3. Optimization of Existing Pin Grinding System

Compared to the ZETA pin grinding system patented in 1991, several significant upgrades have been made with the NEOS grinding system (see Figure 5): • The cooling water flow in the double jacket of the grinding chamber has been optimized. Additionally, a ceramic material with high wear resistance and maximum thermal conductivity is now standard for the interior of the grinding chamber. • The stirrer shaft has been designed such that, on the one hand, product exchange on the existing cooling surfaces will be improved due to flow-related optimization. On the other hand, attention has been paid to the availability of maximum cooling surface for additional rotor cooling. Improvement in cooling efficiency is an important consideration in the new design of the grinding system because in most cases the power input to the grinding chamber is initially limited by the product's temperature limits. • The gap between the grinding chamber wall and the pins on the stirrer shaft has been reduced. As a result, more intense acceleration of very small grinding beads is possible. This results in significantly higher energy density and thus higher possible power input to the grinding chamber. As a result, the same grinding bead size, grinding medium density and stirrer shaft speed means that these grinding beads have higher average kinetic energy.   Additionally, this leads to higher shear stresses on the product, particularly in the case of shear-thinning products, resulting in lower product viscosity in the machine. This results in better product exchange near the grinding chamber wall and near the stirrer shaft, which results in better heat dissipation from the grinding chamber. • The diameter of the screen in the grinding medium separation screen system has been enlarged. This significantly increased the separation area of the screen. Together with the viscosity effect previously described, this more efficiently prevents accumulation of grinding beads and coarse product particles in the screen. Due to these new features, it has become possible to use smaller grinding beads at significantly higher product yield rates during grinding or dispersing of various products. [caption id="attachment_143118" align="aligncenter"] Figure 5: Basic representation of the
NEOS grinding system[/caption] With the same grinding medium filling mass, use of smaller grinding media means that the number of grinding beads and thus the number of impacts or stresses per unit time increases exponentially. Higher potential product output leads to an increase in the number of circulation passes per grinding time and thus a narrower residence time distribution of product particles in the grinding chamber. Due to the shorter residence time of the product during a passage from product feeding to the grinding chamber to product discharge and correspondingly lower energy input, product temperature control is facilitated provided the grinding circuit is equipped with cooled batch tanks. • Additionally, the stirrer shaft geometry has been modified to improve circulation of the grinding medium without compression in the grinding medium separation system area, even at very high product yield rates. This does not mean that higher production capacity can be achieved with similar installed motor power and grinding chamber sizes. Due to the ability to use smaller grinding media and higher product yield rates, lower energy requirement also accompanies various products. This leads to reduced wear on the grinding medium, reduced stress on machine components and thus reduced downtime, maintenance and spare parts costs relative to the product volume produced.

4. Results of Field Tests with Real Customer Products

In collaboration with Siegwerk Druckfarben AG & Co. KgaA, field tests were conducted with various real products. By switching from the ZETA grinding system to the NEOS grinding system, it was possible to increase the power input to the grinding chamber of the machine in a very wide range between 18% and 143% for all products, although the potential increase in power input depends on several criteria. Important factors here, in addition to the machine's installed motor power, are the temperature limit, viscosity and the product's rheological behavior. With the ZETA grinding system, depending on the viscosity of shear-thinning products, it was often not possible to reach the power limit of the machine for various products. In contrast, with the NEOS grinding system, it was always possible to reach the power limit of the mill before reaching the product's temperature limit. (see Figure 6) This can of course be explained on the one hand by the machine's cooling capacity, but on the other hand also by the viscosity effect discussed previously. Due to higher shear stress and consequently lower viscosity of the product in the mill, there is better product exchange near the grinding chamber wall and near the stirrer shaft, which results in better heat dissipation from the grinding chamber. [caption id="attachment_143119" align="aligncenter"] Figure 6: Performance Increase Achieved by Transition from ZETA 25 (LMZ 25) to NEOS 20
grinding system for various customer products [11][/caption]However, Schönstedt [12] showed that modification of the formulation may also be necessary depending on the product. Depending on the mill technology used, for example changes in product viscosity, color intensity, transparency or brightness may occur. Therefore, to achieve the standard specifications, in the simplest case it is necessary to change the solvent concentration. However, in some cases, changing only the active binder substance yielded the desired result.  

5. Summary

Field tests showed that through the use of a new grinding system, there are various increases in power input and thus production capacity can be achieved. When combined with modification of the mill's operating parameters and/or product formulation, specific energy requirement can be reduced to a significant degree even with constant product quality. Here, important operating parameters are primarily grinding medium size and product output through the mill. The decisive factors for the increase in production capacity that can be achieved are predominantly product properties such as viscosity, rheological behavior and temperature limits. Furthermore, it was demonstrated that when operating parameters are optimized or when loading conditions on the product change due to increased power density in the mill, partial adjustment of the formulation is necessary.       References [1] N. Stehr, Zerkleinerung und Materialtransport in einer, Rührwerkskugelmühle. Thesis, TU Braunschweig, 1982 [2] H. Weit Betriebsverhalten und Maßstabsvergrößerung von Rührwerkskugelmühlen. Thesis, TU Braunschweig, 1987 [3] B. Joost, A. Kwade Feinstzerkleinerung, Rührwerksmühlen. Das Keramiker - Jahrbuch 1996, pp. 23-38 [4] J. Schwedes, A. Kwade, H.-H. Stender, Hochschulkurs Zerkleinern und Dispergieren mit Rührwerkskugelmühlen Umdruck zum Hochschulkurs vom Institut für Mechanische Verfahrenstechnik und Kwade + Schwedes Zerkleinerungstechnik, Braunschweig, 2008 [5] J. P. Thiel, Energiebedarf und Durchsatzverhalten der Kohlenasszerkleinerung in einer Rührwerkskugelmühle. Thesis, TU Braunschweig, 1993 [6] F. Bunge, Mechanischer Zellaufschluß in Rühr¬werkskugelmühlen. Dissertation, TU Braunschweig, 1992 [7] M. J. Mankosa, G. T. Adel, R. H. Yoon, Effect of media size in stirred ball mill grinding of coal. Powder Technology 49, 1986, pp. 75-82 [8] R. Stadler, R. Polke Chemie, J. Schwedes, F. Vock, Naßmahlung, Rührwerkskugelmühlen.- Engineer - Technics, 62, 907-915, 1990 [9] D. P., Roelofsen Entwicklungen auf dem Gebiet des Pigmentdispergierens mit Rührwerkskugelmühlen. Farbe + Lack 97, 1991, 3, pp. 235-242 [10] A. Kwade, Autogenzerkleinerung von Kalkstein, Rührwerkmühlen. Thesis, TU Braunschweig, 1996, ISBN 3-8265-2082-3 [11] H.-H. Stender, Initial industry test results of packaging ink production with NETZSCH Neos technology, presentation, Dispersion Days, 15-17 May 2017, Selb [12] B. Schönstedt, Application of new bead mill technology to regular production, presentation, Dispersion Days, 15-17 May 2017, Selb     Dr. Stefan Mende Büşra Örs Head of Technical and Scientific Communications NETZSCH     Büşra Örs Technical Sales Engineer NETZSCH