Process Optimization in the Paints, Coatings and Inks Industry

Process Optimisation in the Paints, Coatings and Ink Industry: A Combination of Grinding System, Process Parameters and Product Formulation

In discussions with experts, colleagues and customers, it has been noted that in the paints, coatings and ink industry, when designing grinding and dispersion processes, the production capacity of the facility, particularly for new investments, is a determining factor. Bead mills are used in many different products. Since grinding is essentially linked to the specific energy of a product, the maximum possible power input to the bead mill is seen in most cases as the most critical operating parameter. The following article will initially demonstrate, using a simple calcium carbonate grinding example, various parameters that affect power input, specific energy requirement and production capacity. Subsequently, it will explain how the modified pin grinding system was tested and how process parameters were adjusted specifically for the paints, coatings and ink industry to achieve higher production capacities while also improving efficiency. The results of field tests will be used to explain potential increases in production capacity.

1. The Effect of Various Process Parameters on Production Capacity and Specific Energy Requirement in Bead Mills During Grinding

Grinding and operating behaviour in agitated bead mills has been thoroughly investigated by Stehr [1] and Weit [2]. It has been shown that specific energy is a general factor indicating fracture behaviour, i.e., the energy supplied to the grinding vessel in relation to the mass of the ground product (solid) (see Equation 1). Research with different bead sizes has shown that in addition to specific energy, bead size has a significant effect on grinding in mills [3-9]. Changes in bead size provide significant reductions in specific energy requirements.

In the grinding of limestone, Kwade [10] also noted that the peripheral speed of the stirrer shaft and bead density have a systematic effect on the correlation between specific energy and product fineness.

Kwade [10] defined the expression for the kinetic energy of grinding beads as stress energy SEGM, as a function of bead density ρGM, bead diameter dGM and peripheral speed vt of the stirrer (see Equation 2). When investigating the effect of bead density, the linear speed of the stirrer and bead size on operating parameters, it was possible to demonstrate that there is an optimum stress energy at which the highest product fineness can be achieved with constant specific energy input. The effects mentioned above can be clarified with a simple example. To find an optimum, a minimum of three test conditions must be established. 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. Each time samples were analysed with "MasterSizer 3000" at specific energy inputs of 0.03 kWh/kgFS, 0.07 kWh/kgFS and 0.1 kWh/kgFS (see Table 1). Test results showing 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 desired target value could be reached at the lowest stirrer peripheral speed and with the lowest specific energy consumption.

Figure 1. Grinding of limestone at different stirrer linear speeds

However, due to the low power input at these settings, the production capacity was also lowest as a result (see Figure 2). This finding is confirmed when bead stress energies are taken into account for the tests performed (see Figure 2). The goal is to achieve the highest production capacity with the lowest energy requirement. For this evaluation, the linear speed of the mill stirrer is increased and the same stress energy calculation caused by bead collision is performed to determine bead size. The result of this calculation is shown in Figure 3. If this time grinding beads with a diameter of 1.0 mm instead of 1.7 mm are used with constant bead density and stirrer peripheral speeds of 14 m/s, the grinding results are comparable when plotted as a function of specific energy (see Figure 4).

Figure 2. Grinding results as a function of bead stress energy

Figure 3. Grinding of limestone at different peripheral speeds with stirrer shaft and variable bead size

Considering the achievable production rates, it is clear that higher production capacity can only be obtained by increasing power input due to the increase in tip speed of the stirrer shaft and thus the kinetic energy of the beads. However, this higher production capacity is associated with an increase in specific energy requirement. By contrast, simultaneous optimisation of grinding bead size can increase production capacity to extraordinary levels without requiring higher energy input (see Figure 4).

Figure 4. Production capacity as a function of operating parameters

2. Optimisation of the Existing Pin Grinding System

A number of significant improvements have been made with the "Neos" grinding system compared to the "Zeta" pin grinding system patented in 1991 (see Figure 5). • The flow of cooling water inside the double-jacketed grinding vessel has been optimised. Ceramic with high wear resistance and maximum thermal conductivity has also become standard as the internal lining material of the grinding vessel. • Through stirrer shaft flow optimisation, the design takes into account improving product exchange on cooling surfaces on the one hand and ensuring maximum cooling surface area available for additional rotor cooling on the other hand. In the new design, improvement of cooling efficiency is a key feature of the grinding system because in most cases the power input to the grinding vessel is limited by the maximum temperature limits of the product. • The gap between the pins on the grinding vessel lining and stirrer shaft has been reduced. As a result, smaller grinding beads can be accelerated more densely. This provides much higher energy density and thus enables higher power input to the grinding vessel. As a result, with the same bead size, bead density and stirrer shaft speed, grinding beads have higher average kinetic energy. This also results in higher shear stress on the product, and the resulting lower viscosity ensures better product exchange near the grinding vessel wall and near the stirrer shaft, which results in better heat dissipation from the grinding vessel. However, Schönstedt [12] has shown that in particular, depending on the product, changes to the formulation may also be necessary. For example, depending on the mill technology used, changes can occur in product viscosity, colour intensity, transparency or gloss. To obtain standard product specifications, in the simplest case, the concentration of solvent may need to be changed. In some cases, changing only the binder produces the desired result.

Thanks to these new features, it has become possible to achieve much higher flow rates during grinding or dispersion of various products with smaller grinding beads.

At the same bead filling ratio, the use of smaller beads means an exponential increase in the number of grinding beads and thus the number of contacts or stress events per unit time. Higher product throughput results in more circulation cycles per grinding time and thus a narrower residence time distribution of product particles in the grinding vessel. With shorter residence time in a single pass from product inlet to product outlet through the grinding vessel and thus lower energy input, it becomes easier to control the product temperature energy; however, the mill must be equipped with a cooling product tank. In addition, the stirrer shaft geometry has been modified to improve bead mobility without jamming beads into the sieve section even at very high throughputs. This means higher production capacity can be achieved with comparable installed motor power and grinding vessel sizes. The ability to use smaller beads and higher product throughput rates also results in lower energy requirements for many different products. This also leads to reduced wear on the beads, less stress on machine components and thus lower downtime, maintenance and spare parts costs relative to the volume of product produced.

3. Results of Field Tests with Customer Products

Field tests were conducted with various products in collaboration with a global ink manufacturer. By switching from the original "Zeta" grinding system to the new grinding system, it was possible to increase the power input to the mill's grinding vessel for all products over a wide range extending from 18% to 143%; however, the potential increase in power input is a function of a number of criteria. In addition to the installed motor power of the mill, the important factors here are the product's temperature limit, viscosity and rheological behaviour. In the original grinding system, depending on the viscosity of products thinned by shear, it was generally not possible to reach the maximum power limit of the mill for various products. By contrast, with the new grinding system, it was always possible to reach the maximum power limit of the mill before reaching the product's temperature limit (see Figure 6).

Figure 6. Performance improvement achieved by switching from "Zeta 25" (LMZ 25) to "Neos 20" grinding system for various customer products [11]

This can undoubtedly be explained on the one hand by the mill's cooling capacity and on the other hand by the viscosity reduction effect. Due to the higher shear stress on the product in the mill and the resulting lower viscosity, there is better product exchange near the grinding vessel wall and near the stirrer shaft, which results in better heat dissipation from the grinding vessel. However, Schönstedt [12] has shown that in particular, depending on the product, modification of the formulation may also be necessary. Depending on the mill technology used, for example, changes can occur in product viscosity, colour intensity, transparency or gloss. To obtain standard product specifications, in the simplest case, the concentration of solvent may need to be changed. In some cases, changing only the binder produces the desired result.

4. Summary

Field tests have demonstrated that increases in production capacity can be achieved with the new grinding system. By changing the mill's operating parameters and/or product formulation, the specific energy requirement can be significantly reduced even for the same 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 behaviour and temperature limits. In addition, it has been observed that some adjustments to the formulation are necessary when the load conditions on the product change due to optimisation of operating parameters or an increase in power intensity in the mill. Dr. Stefan Mende Manager of Technical and Scientific Communications Netzsch-Feinmahltechnik GmbH    

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