Paint & Coatings Industry - March 2009 - (Page 54) Milling-Media Review: Bead Milling Operating Parameters W Machine Design Machine Design Volume Power Speed e can identify four distinct areas of consideration; these relate to the machine or mill, the method of operation, the formula of product processed and the milling media used. For each of these, there are a number of main parameters that affect the milling process, as detailed in Table 1. In this issue, we focus on matters particularly related to the machine itself. Future reviews will selectively target the other parameters for mill operation mode, product formula and milling media. on the same basic principle. A liquid (product) is pumped through an agitated, fluidized bed of beads. Four main basic types can be identified. Disc Mill Annular Gap Mill Pin/Peg Mill Basket Mill There are many types of agitator mills available on the market. Their designs and sophistication vary significantly. Fundamentally however, they all operate TABLE 1 | Parameters that effect the milling process. Operation Flow Rate Process Mode Temperature Communition Deagglomeration Milling Time Formula Feed Particles Viscosity Solid Content Chemistry Media Type Size Fill Rate For the first three, the distinctions can certainly be less well defined than the simplified perspective given. The machines also can have different orientation; they can be aligned vertically or horizontally, for example. Each type of configuration has particular benefits in specific applications. No single machine design is perfect for every single process demand. Two important considerations that have a major effect on final designs are cooling and bead separation. Cooling As work is carried out inside the mill, there is an accompanying increase in product temperature. This can have a detrimental effect on the product and needs to be controlled. This control is achieved by the inclusion of a cooling jacket. The heat exchanged (Q) can be expressed as: Q = (K/X)*A*ΔT, where K is the thermal conductivity of material; X is thickness; A is the contact area; and ΔT is temperature difference. The contact area is therefore an important consideration in mill design (Figure 1). The mill lining also has a great effect. Table 2 details the conductivity for typical lining materials. FIGURE 1 | Mill cooling design. Bead Separation Bead separation keeps the bead within the mill during operation. The mechanism for separation can vary from static screen arrangements to dynamic gaps and to centrifugal separators. These three generic designs are detailed in Figure 2. In the static separation screen arrangement, the bead is retained as the gaps are maintained at approximately one third of the size of the smallest bead. In the dynamic gap design, this ratio is maintained although there is an additional action of sweeping the beads clear of the gap due to the rotation of the outer disc. For situations of high product flow, designs incorporating a centrifugal action are adopted. Here the bead is accelerated by discs away from the separation screen. If the centrifugal force (FC ) is greater than the drag force (F DRAG ), then there is good separation. Horizontal Mill: Cooling Annular Gap: Cooling FIGURE 2 | Generic bead separation designs. 2 - Dynamic Gap Separator 3 - Centrifugal Separator 1- Static Separation Screen FDRAG ∞ (1/d ) . . V(FLOW) Viscosity d Screen Length V(FLOW) Flow rate FC ∞ m . ( V2 / r ) m Bead mass V Velocity r Bead diameter Product Flow Bead Flow FDRAG FC By Dr. Paul Hassall, Saint-Gobain ZirPro | SEPR, Le Pontet Cedex, France 54 MARCH 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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