Pin Mill

Pin mill is a method of comminution that acts by producing particle impact and attrition. Particle size reduction occurs by impaction with the pins and by attrition between pins as the particles travel outwards under the influence of centrifugal force.

Construction and Working of Pin Mill

Pin mills consist of two horizontal steel plates with vertical projections arranged in concentric circles on opposing faces, gradually becoming more closely spaced towards the periphery (Fig.1). As the projections of the two faces intermesh, the feed is introduced at a controlled rate to the milling chamber through the center of the stator. Subsequently, it is propelled through intermeshing rings of rotor and stator pins by centrifugal motion. The passage between the pins leads to size reduction by both impact and attrition. Consequently, the material is collected in the annular space surrounding the disks and then passes to a separator. Additionally, the large volumes of air drawn through the mill are discharged through the separator. The final particle size achieved in a pin mill is governed by the rotor speed, solids feed rate, and air flow rate through the mill.

Pin mill
Fig. 1: pin mill

Smaller particles can be generated by maximizing the rotor tip speed and minimizing both product feed and air flow rate.

Advantages and Disadvantages Of Pin Mill

The absence of screens and gratings provides a clog-free action. Moreover, this type of milling is typically able to achieve a smaller average particle size than wet rotor-stator milling. Furthermore, the machine is suitable for grinding soft, non-abrasive powders, and the low milling temperatures permit heat-sensitive materials to be processed. Additionally, the fineness of the grind may be varied by the use of disks with different dispositions of pins. As the equipment scale is increased, maintaining rotor tip speed is one reliable way to achieve milled particle sizes comparable to small-scale results.

Reference:

  • Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd.