Conical screening mill

Poska RP, HillTR, van Schaik JW. 1993. The use of statistical indices to gauge the mixing efficiency of a conical screening mill. Pharm. Res. 10(8) 1248-1251. 

Soon to follow the stone grinder were the roll crusher and lump breaker. While still in use today, the roll crusher is more apt to be found size reducing rocks and other aggregates in a more industrial setting. The lump breaker similarly is also commonly used in the food industry although it is still occasionally used in the pharmaceutical industry for size reducing agglomerates. The pharmaceutical industry demanded a more refined approach to repeatability led to the invention of the hammermill, oscillator, and conical screen mill. 

The conical screen mill provides many benefits over some of the other equipment designs including (Fig. 8) ... 

FIGURE 11 Capacity vs. gap size for granuiated sugar in conical screen mill. 

FIGURE 13 Typical conical screen mill impellers (1609) (A) square arm design, (B) round arm design. 

The ability of comminuting mills to scale-up from lab to production scale is one of the most important characteristics size reduction equipment must offer. Scale-up in conical screen mills is achieved through the maintaining of the tip velocity of the tooling as applied in the pilot scale (Fig. 17). 

Typical conical screen mill applications will best function when flood fed (plug), providing an uniform head pressure to the material in the milling zone thus forcing product to the screen surface and for uniform discharge. However some products will not respond well to this type of set-up (i.e., waxy, heat sensitive) and at times it can be recommended that the unit be controlled fed instead. 

COMPARATIVE ANALYSIS Conical Screen Mill and Hammermill... 

In industry there are many applications where the hammermill and the conical screen mill will overlap, however, in recent years there has been an increased demand for the conical screen mill as they are amore efricient and predictable technology. Developed in 1965, the hammermill has been replaced by conical screen mills particularly in the pharmaceutical industry. The main benefits of the conical screen mill includes low noise, low dust, a tighter particle size distribution, flexibility to mill both wet and dry material, ability to be flood fed and generally a higher capacity due to the 360 discharge available with the conical design. 

Shear-compression Conical screening mill Moderate to coarse Wet, dry granulation Abrasive materials... 

The dual action of conical screening mills (size reduction and mixing) makes this equipment more desirable than the use of traditional oscillators (14,15). 

Figure 3 Conical-screening mill (A) Glatt model GSF 180 and (B) principle of operation.

Figure 4 An integrated pharmaceutical manufacturing facility (high-shear granulator-conical screening mill-fluid bed drier).

Similarly, for conical screening mills, the operating variables affecting particle size distribution are type of impeller, impeller speed, and screen size and type ... 

Material feed rate In contrast to hammer mills, conical screening mills perform with greater efficiency when the comminution chamber is kept relatively full. Underfeeding results in low efficiency and reduced throughput. 

Figure 9 Conical screening mill (Comil) (A) impellers and (B) screens.

Speed The speed of the impeller can affect the particle size of the product. Conical screening mills available have variable- or fixed-speed drives, however, the number of revolutions per minute vary depending on the size of the impeller. It is suggested that one keep the tip speed of the impeller the same on scale-up to achieve the same particle size distribution. 

The effect of changing the dry milling from a hand screen operation to a conical screening mill is shown in Figure 12. Naproxen granulations (0.5 and 4 kg) were... 

---