Granulating pan

W.B. Pietsch, Granulating pan operating variables and their effect on granulate properties, Aufbereit.-Tech., 7 (4) (1966) 177—191. 

As with balling drums, it is most important to select the correct rotational speed of the granulation pan. It is defined as the percentage of critical speed that can be calculated by (see Figure 112)... 

Klatt " estimated the rotational speed of granulation pans as a function of diameter by evaluating experimental data (Figure 114) and obtained... 

The angle of repose also somewhat influences the critical speed of a granulation pan ... 

Typically, dry potassium nitrate is pulverized in a ball mill. Sulfur is milled into cellular charcoal to form a uniform mix in a separate ball mill. The nitrate and the sulfur—charcoal mix are screened and then loosely mixed by hand or in a tumbling machine. Magnetic separators may be used to ensure the absence of ferrous metals. The preliminary mix is transferred to an edge-mimer wheel mill with large, heavy cast iron wheels. A clearance between the pan and the wheels is required for safety purposes. The size of this gap also contributes to the density of the black powder granules obtained. Water is added to minimize dusting and improve incorporation of the nitrate into the charcoal. The milling operation requires ca 3 to 6 h. 

Pan granulation of ammonium nitrate also has been developed commercially (16,17). Essentially water-free melt (<0.5%moisture) is sprayed onto a cascading bed of fines (usually cmshed recycle) ia a tilted, rotatiag pan. Production of 18—20 t/h is common. Product made ia this manner is said to require ao conditioning. The pan is an excellent size classifier, making it easy to produce granules of any desired size from 1 to 11 mm diameter. 

Advantages of pan granulation are ease of granule size control, hardness of granule, and simplicity of pollution control. 

O. Skauh, "Pan Granulation of Ammonium Nitrate and Urea," paper presented at the 168th National Meeting of the Mmerican Chemical Society, Adantic City, N.J., 1974. 

Variations of the simple pan shape include (1) an outer reroU ring which allows granules to be simultaneously coated or densified without further growth, (2) multistepped sidewalls, and (3) a pan in the form of a truncated cone (Capes, Par ticle Size Enlat gement, Elsevier, 1980). 

In the pharmaceutical industiy, pan granulators are still widely used for coating application. Pans are suitable for coating only relatively large granules or tablets. For smaller particles, the probability of coalescence is too high. 

Inspection of this equation shows that in the course of the snowballing growth the size distribution curves at various times are simply shifted toward the right on the pellet size scale without any change in their shape, as demonstrated by Capes (C2) for sand pellets snowballed in a pan granulator (Fig. 13). 

Thus, during product and process development, even though compression batch weights are often defined in terms of the capacities of blenders, granulators, dryers, etc., there is sufficient justification, when the product is to be coated, to keep in mind the capacities of the coating pans that will be used. 

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