We 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.
Machine DesignThere are many types of agitator mills available on the market. Their designs and sophistication vary signiﬁcantly. Fundamentally however, they all operate on the same basic principle. A liquid (product) is pumped through an agitated, ﬂuidized bed of beads. Four main basic types can be identiﬁed.
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.
CoolingAs 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.
Machine Power and SizeMills are designed to process required volumes of product, to a speciﬁed quality, during a particular time scale. This deﬁnes the ‘production capacity’ of the mill and is related to power and size of the machine. The machine must provide sufﬁcient energy to achieve the desired grinding effect. It must also have sufﬁcient capacity to accommodate the required ﬂow rates of the product. The maximum production capacity (M) can be estimated on the basis of the speciﬁc energy requirement (EM) for the application and the converted diving power P- PO.
M (t/h) = P - PO (KW)
where M = mass flow rate; P = power consumption; PO = idle power; EM = specific energy.
For example, if in a mill a power of P – PO = 250 KW is converted and the specific energy requirement is EM = 50 KWh/t, the net production capacity is M(t/h) = 5 t/h (dry material).
From the above equation, it follows that maximum production capacity is reached if (1) the power input to the grinding chamber is maximized and (2) the specific energy requirements of the product are minimized.
Developments in agitator mills have tended to maximize the intensity of the milling action, high power in limited volume, with maximized flow rates. The graph in Figure 3 highlights the evolution in mill power and flow rate, per unit volume of mill, for various machine designs.
The grinding efﬁciency is improved by increasing the rotation speed of the discs, as more energy is applied to the system. This can be demonstrated in Figure 4, which shows that, as the tip speed increases so the level of achieved grind improves. There is a practical limit to this process, due to the fact, that the wear rates for both beads and machine parts also begins to dramatically increase, as detailed in Figure 5.
ConclusionAs with many aspects of operating bead mills, there is a compromise limiting mill speed to provide adequate productivity without incurring excessive cost due to increased wear. To accommodate mill wear issues, various materials are incorporated as discs and linings: steel, plastics, ceramics, etc. Each of these has merit in particular applications and has consequences for correct media choice.
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