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Powder & Bulk Solids, May 2014

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www.PowderBulkSolids.com 29 M a y 2 0 1 4 possible, with the added benefit of being energy efficient and eco- nomical. Properties of Steam That Enhance the Jet Milling Process Three key factors make steam flu- idized bed jet milling successful and commercially viable: 1. Steam can be provided to a jet mill at very high pressures compared to air. At higher grind- ing pressures, higher jet speeds can be attained. At a pressure of 100 BAR absolute, the jet speed exceeds 1200 m/s. Compared to a near maximum of 600 m/s when using air, the kinetic energy at the nozzles is approximately four times higher. This increased grinding energy results in much finer size reduction in the milling process, therefore finer particles are available to the classifier. 2. As a medium for dynamic classification of particles, steam allows a finer cut size than air. The properties of steam, which has a lower dynamic viscosity and lower density than air, as well as a higher speed of sound than air, makes possible a higher flow velocity within a classifier wheel. Thus the acceleration forces act- ing on the material being sepa- rated is higher and the possible cut size is finer. In a jet mill, this means the particle size distribu- tion of the product is finer. The graphic below represents the forces at work in the classifi- cation process. 3. Higher jet energy, as men- tioned above, can also result in significant increases in capacity and improved energy efficiency. Jet speeds of 1200 m/s and the increase of the kinetic impact energy can also lead to a signifi- cant increase in mill throughput compared to air operation with comparable finesse. The increase in throughput is proportional to the increase in kinetic energy. For materials of the same par- ticle size and with equivalent gas flow rates, throughput rates are increased by two to three times depending on the friability of the materials. (See tables at the top of the next page.) Using Additives to Enhance the Jet Milling Process As with any grinding process, whether it is a wet or dry process, fine particles have the tendency to re-agglomerate. The finer the particles are, the greater the risk of agglomeration occurring. In a dry grinding process when an air classifier is used to control the final particle size, this tendency has a negative effect on capacity and process efficiency. Classifiers do not recognize individual par- ticles unless they are discrete. Large agglomerate, consisting of many fine particles, appears to the classifier as one large par- ticle and is therefore rejected as oversize to be ground again. A fluidized bed jet mill is essentially a closed system. When the clas- sifier rejects this agglomerate of particles it remains in the jet mill to be ground finer resulting in a reduced capacity. This process of grinding and re-agglomeration will continue unless some type of additive is used to modify the surface of the particles so they no longer re-agglomerate. The positive effects of using a dispersing agent include an in- crease in capacity (at the same fineness) of three to four times, Rotation Peripheral Velocity (V u ) Drag Force (f d ) Mass Force (f m ) d r cut size η c gas viscosity V r radial velocity r classification radius ρ s material density V u peripheral velocity 18 constant d r = Classification Radius (r ) Effective forces of classification Radial Velocity (V r ) s X V 2 u ρ 18 X η c X V r X r Visit us at the Powder Show booth #1021 buntingmagnetics.com (800) 835-2526 ©2014 Bunting® Magnetics Co. Complete Protection Protect your equipment and keep your product pure. quickTRON™ 05 Metal Detector Cost effective metal detector for the examination of bulk material in gravity free-fall applications. Pneumatic HF Series Drawer Magnet Designed for separation in enclosed flow lines and equipped with the most powerful magnetic cartridge to handle a wide range of separation tasks. Visit us at the Powder Show booth #1837 www. .com Check out our Size Reduction Equipment Zone at PowderBulkSolids.com ES425541_PB1405_029.pgs 04.18.2014 05:54 UBM black yellow magenta cyan

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