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Size Reduction
Published in Enrique Ortega-Rivas, Unit Operations of Particulate Solids, 2016
Figure 4.4 shows a hammer mill, an equipment that contains a high-speed rotor turning inside a cylindrical case. The rotor carries a collar bearing a number of hammers around its periphery. By the rotating action, the hammers swing through a circular path inside the casing containing a toughened breaker plate. Feed passes into the action zone with the hammers driving the material against the breaker plate and forcing it to pass through a bottom mounted screen by gravity when the particles attain a proper size. Reduction is mainly due to impact forces, although under choking conditions, attrition forces can also play a part in such reduction. The hammers may be replaced by knives, or any other devices, as to give the mill the possibility of handling tough, ductile, or fibrous materials. The hammer mill is a very versatile piece of equipment, which gives high reduction ratios and may handle a wide variety of materials from hard and abrasive to fibrous and sticky. Hammer mills provide good performance, high capacity, and need little space, apart from attaining the highest reduction relations for single comminution units. Their disadvantages are the elevated noise and vibration they cause, and the accelerated wearing of their hammers and hardened walls.
Melt Pelletization and Size Reduction
Published in Isaac Ghebre-Sellassie, Charles Martin, Feng Zhang, James DiNunzio, Pharmaceutical Extrusion Technology, 2018
Christopher C. Case, Albrecht Huber, Kathrin Nickel
The result is a product shape called “flakes.” Depending on desired flake size, different pin breaker types are available, which include a pin crusher and a teeth crusher. If smaller fragments are desired, a teeth crusher is useful; otherwise a pin crusher is used. Depending on the end product requirements, the flakes can be further processed through, for example, milling and tableting. Therefore, an integration of a mill is possible so that the flakes can be milled in-line (Figure 9.17). Different mills are available that are used for different particle sizes. A conical sieve mill can be integrated for sizes up to 150 μm, while a hammer mill can be used for smaller sizes, down to 50 μm.
Crushing and Grinding
Published in Ko Higashitani, Hisao Makino, Shuji Matsusaka, Powder Technology Handbook, 2019
A hammer mill is an impact type of mill, employing a high speed rotating disk, to which a number of hammer bars are fixed, and they are swung outwards by centrifugal force.50, 51 The material is fed in, either at the top or at the center, and is thrown out centrifugally and crushed by being beaten between the hammer bars or against plates fixed around the periphery of the cylindrical casing. The material is beaten until it is crushed small enough to fall through the screen which forms the lower portion of the casting. Since the hammer bars are hinged the presence of any hard material does not cause damage to the equipment. The machine is suitable for the crushing of both brittle and fibrous materials, and in the latter case, it is usual to employ a screen with cutting edges. The mill is also suitable for hard materials but since a large amount of fine dust is produced, it is advisable to employ positive pressure lubrication to the bearings in order to prevent the entry of dust. The size of the product is regulated by the opening of the screen and the rotational speed. The tip velocity of the hammer in the mill is normally operated from 15 to 50 m/s, however, it is often driven up to 40 to 100 m/s for high speed type, and in some cases, 120 to 150 m/s for an exceptional type. There are several types of the screen and its openings provided, and for example, the shape of the opening is circle [xs (mm)], the relation between the mean size (xm) of the particles passing through this screen and its maximum diameter (xmax) is expressed as xm = (xmax/6) − (xmax/8). When the relation like xm < (xmax/6) − (xmax/8) is measured, then it would be regarded that the mill performance seems to be preferable. There are a number of similar mills, such as pin-type and bar-type with small as well as large scales, rotating in both vertical and horizontal directions.
Grinding characteristics of waste printed circuit boards in hammer mill using population balance model
Published in Geosystem Engineering, 2021
Wonjae Lee, Seungsoo Park, Jaikoo Park
Waste printed circuit boards (PCBs) are artificial materials that have a unique multilayer lamellar structure of copper and other materials. It is essential to select an appropriate grinding machine to liberate the copper (Li et al., 2004; Park et al., 2018). Impacting is known to be an effective grinding method for separating the artificially formed layers of waste PCBs (Duan et al., 2010). A hammer mill is a grinding machine that uses rapidly rotating hammers for impacting and screening discharge samples; it is regarded as the industry standard for waste PCBs (Ogunniyi et al., 2009). Impact-driven ground waste PCBs can be liberated at particle sizes finer than 0.6 mm (Guo et al., 2011). Therefore, it is essential to grind waste PCBs into appropriate particle sizes (0.15–0.60 mm). Although several studies have been conducted on the hammer milling operation, studies focusing on waste PCBs are limited (Ämmälä et al., 2018; Austin et al., 1979; Dey, Dey & Das, 2013; Hajratwala, 1982; Jindal & Austin, 1976; Kwon et al., 2014; Shi et al., 2003; Vanarase et al., 2015).