Optimizing the Grinding Process

 

Holistic Approach

Tests were conducted to demonstrate the different effects of grinding parameters on power input, energy efficiency and production capacity. The new grinding technology reduces specific energy requirements while allowing increases in power input and thus production capacity. In discussions with experts, colleagues and customers, it was repeatedly stated that in the design of grinding and dispersion processes in the paints and coatings and inks sector, production capacity of the facility is the determining factor, particularly for new investments. Most products use bead mills. Because the grinding achieved is essentially a function of a product's specific energy, providing bead mills with the maximum possible power input is considered the most critical operating parameter in most cases. This article will first use a simple example of calcium carbonate grinding to demonstrate the effect of various parameters on power input, specific energy requirements and production capacity. Subsequently, it will be explained how an approved pin grinding system was modified to simultaneously increase efficiency and facilitate reaching higher capacities, and how operating parameters were adjusted specifically for the paints and coatings and inks sector. Results from field tests will be used to explain potential increases in production capacity.

Effect of Operating Parameters on Production Efficiency

The grinding and operational behavior of bead mills has been thoroughly investigated by Stehr [1] and Weit [2]. It has been shown that specific energy is a general factor indicating grinding behavior, namely the energy supplied to the grinding chamber relative to the mass of the ground product (solid) (see Equation 1).

Summary Results

• Tests were conducted to demonstrate the different effects of grinding parameters on power input, energy efficiency and production capacity. • The important operating parameters are fundamentally bead size and product throughput through the mill. Production capacity increases depend essentially on product properties such as viscosity, rheological behavior and temperature limits. • With the use of the new grinding system, various power input and thus production capacity increases can be achieved. Specific energy requirements can be significantly reduced even while maintaining constant product quality. • When optimizing operating parameters or increasing power density in the mill, some adjustments to formulation were necessary due to changed stress conditions on the product.

Here:

E [Wh] = energy ESpek [kWh/kg] = specific energy related to mass P [W] = power P0 [W] = no-load power msolid [kg] = product mass cm [-] = solid mass concentration susp [kg/s] = suspension mass flow Research conducted with different bead sizes showed that, in addition to specific energy, grinding bead size has a significant effect on grinding in bead mills [3-9]. Changes in bead size provide significant reductions in specific energy requirements at certain grinding targets. Regarding discontinuous limestone grinding, Kwade [10] also noted that the peripheral speed of the stirrer shaft and bead density have systematic effects on the correlation between specific energy and product fineness. As an expression of the kinetic energy of grinding beads Ekin, Kwade [10] defined the stress energy of bead SEGM as a function of bead density pGM, bead diameter dGM and the peripheral speed of the stirrer vt (see Equation 2). During research into the effects of grinding bead density, stirrer disk peripheral speed and bead size operating parameters, it was demonstrated that an optimum stress energy exists at which the highest product fineness can be achieved with constant specific energy input. • In pigmented systems, a simple evaluation of test results in terms of particle size distribution does not match real requirements, because to obtain suitable product quality, properties such as brightness, transparency and color intensity must be matched in combination. • Furthermore, changes in stress conditions mean that formulation changes are necessary to produce the same product quality for paints, coatings and inks. However, the correlations between operating parameters discussed in this example can be qualitatively transferred to any material system. To find an optimum, the minimum of three test setups must be determined. The range of optimum stress energy required to improve grinding is not always achievable. From the perspective of operational reliability or target product throughput, using smaller grinding beads often appears inadvisable. In this example, commercial limestone was initially ground in the same mill with the same grinding beads, in recirculation mode operation, and at three different peripheral speeds on the stirrer shaft: 7 m/s, 10.5 m/s and 14 m/s. Sample particle sizes were analyzed each time at specific energy inputs of 0.03 kWh/kg solid, 0.07 kWh/kg solid and 0.1 kWh/kg solid (see Table 1) using "MasterSizer 3000".  

Table 1: Effect of rotor speed vt on specific energy efficiency SEGM when grinding limestone and reducing bead size dGM (in the fourth group of results).

Maximizing Throughput with Optimal Energy Efficiency

Test results showing final particle size d90 as a function of specific energy input are presented in Figure 1. A d90 value of 6 μm was defined as the target value. This showed that with the beads used, the target value could be reached at the lowest stirrer peripheral speed and with the lowest specific energy consumption. However, due to the very low power input at these settings, production capacity was also at its lowest (see Figure 2). This finding is confirmed when the stress energies of the beads are taken into account for the tests conducted (see Figure 2). The goal is now to achieve the highest production capacity with the lowest energy requirement. For this evaluation, the linear speed of the mill's stirrer is increased and the same stress energy calculation created by bead collision is performed to determine bead size. This calculation is shown in Figure 3. If this time, at constant bead density, grinding beads 1.0 mm in diameter instead of 1.7 mm and 14 m/s peripheral agitator shaft speeds are used, the grinding results become comparable when plotted as a function of specific energy (see Figure 4). It is clearly seen that higher production capacity can be obtained by increasing power input due to the increase in kinetic energy. However, this higher production capacity is associated with an increase in specific energy requirements. In contrast, with simultaneous optimization of grinding bead size, production capacity can be increased to extraordinary levels without requiring higher energy input (see Figure 5).

Optimization of Existing Pin Grinding System

A series of significant upgrades were realized with the "Neos" grinding system compared to the "Zeta" pin grinding system patented in 1991 (see Figure 6). The flow of coolant in the double jacket of the grinding chamber has been optimized. In addition, a ceramic material with high wear resistance and maximum thermal conductivity has now become standard for the inner lining of the grinding chamber. The stirrer shaft was designed so that on one hand, product exchange on cooling surfaces would be improved through flow optimization. On the other hand, attention was paid to having maximum cooling surface available for additional rotor cooling. Improving cooling efficiency is a fundamental aspect of the new design of the grinding system because in most cases the power input to the grinding chamber is fundamentally limited by the temperature limits of the product. The gap between the pins on the grinding chamber lining and stirrer shaft has been reduced. As a result, it became possible to accelerate very small grinding beads more intensely. This provided for much higher energy density and thus higher power input to the grinding chamber. As a result, with the same grinding bead size, bead density and stirrer shaft speed, the grinding beads had higher average kinetic energy. Furthermore, this leads to higher shear stress on the product, and this results in a lower-viscosity product within the machine, particularly in products that are thinned by shear forces. The diameter of the screen in bead separation has been enlarged. This significantly increased the separation area of the screen. Combined with the viscosity-reducing effect, this more effectively prevents buildup of grinding beads and thick product particles in the separation screen.

Benefits of the Revised Machine Setup

These new features made it possible to use smaller grinding beads at much higher product throughput rates during grinding or dispersion of various products. Using smaller beads at the same bead filling ratio means the number of grinding beads, and thus the number of contacts or stress cycles per unit time, increases exponentially. Higher potential product throughput leads to higher cycle numbers per grinding time and thus a narrower residence time distribution of product particles in the grinding chamber. Shorter residence time in a single pass through the grinding chamber from product inlet to product outlet and thus lower energy input due to shorter residence time makes it easier to control the product temperature energy; however, the grinding circuit must be equipped with cooled product tanks. In addition, the stirrer shaft geometry was modified to improve bead mobility even at very high throughputs without compressing the beads against the screen section. This means that higher production capacity can be achieved with comparable installed motor power and grinding chamber sizes. Thanks to the ability to use smaller beads and higher product throughput rates, energy requirements are also lower for many different products. Furthermore, this leads to reduced wear on the beads, less stress on machine components and thus lower downtime, maintenance and spare parts costs relative to product volume produced.

Results from Field Tests with Real Customer Products

Field tests were conducted with various real products in cooperation with Siegwerk Druckfarben. By transitioning from the original "Zeta" grinding system to the new grinding system, it was possible to increase the power input to the mill's grinding chamber across a wide range of 18% to 143% for all products; however, the potential increase in power input is a function of a number of criteria. In addition to the machine's installed motor power, important factors here are the product's temperature limit, viscosity and rheological behavior. In the original grinding system, it was generally not possible to reach the machine's power limit for various products depending on the viscosity of strongly shear-thinned products. In contrast, with the new grinding system it was always possible to reach the mill's power limit before reaching the product's temperature limit (see Figure 7).

Figure 7: Performance increase achieved by transitioning from "Zeta 25" (LMZ 25) to "Neos 20" grinding system for various customer products [11].

This is undoubtedly explained on one hand by the machine's cooling capacity but on the other hand also by the viscosity-reducing effect already discussed. Due to higher shear stress on the product in the mill and the resulting lower viscosity, there is better product exchange near the mill chamber wall and near the stirrer shaft, which results in better heat dissipation from the grinding chamber. However, Schönstedt [12] has shown that modification of the formulation may also be necessary depending on the product. Depending on the mill technology used, changes may occur for example in product viscosity, color intensity, transparency or brightness. To obtain the standard specifications, therefore, at minimum it is necessary to change the solvent concentration. In some cases, however, modification of only the binder produces the desired result.

Constant Quality with Lower Energy and Higher Output

Field tests demonstrated that various power input and thus production capacity increases can be achieved with the use of the new grinding system. By modifying the mill's operating parameters and/or product formulation, specific energy requirements can be significantly reduced even with constant product quality. The important operating parameters here are fundamentally bead size and product throughput through the mill. The determining factors for achievable production capacity increases are fundamentally product properties such as viscosity, rheological behavior and temperature limits. Furthermore, it was shown that when optimizing operating parameters or increasing power density in the mill, some formulation adjustments are necessary due to changed stress conditions on the product. Dr. Stefan Mende - Manager, Technical and Scientific Communications - Netzsch-Feinmahltechnik GmbH

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