Coarse reduction

Crushers are slow-speed machines for coarse reduction of large quantities of solids. The main types are jaw crushers, gyratory crushers, smooth-roll crushers, and toothed-roU crushers. The first three operate by compression and can break large lumps of very hard materials, as in the primary and secondary reduction of rocks and ores. Toothed-roll crushers tear the feed apart as well as crushing it they handle softer feeds like coal, bone, and soft shale. 

In sohd—sohd separation, the soHds are separated iato fractions according to size, density, shape, or other particle property (see Size reduction). Sedimentation is also used for size separation, ie, classification of soHds (see Separation, size separation). One of the simplest ways to remove the coarse or dense soHds from a feed suspension is by sedimentation. Successive decantation ia a batch system produces closely controUed size fractions of the product. Generally, however, particle classification by sedimentation does not give sharp separation (see Size MEASUREMENT OF PARTICLES). 

Crushers and Roller Mills. In this equipment group, stress is applied by either cmshing single particles or a bed of particles between two sohd surfaces. In general, most machines are used for coarse and medium-size reduction, with the exception of the high pressure roUer mill which can achieve extremely fine particle distributions. 

Roll mills are employed in a wide variety of applications for medium-coarse down to fine size reduction. By far the most prevalent use is in flour milling where banks of roU mills, beginning with coarse profiles and ending with smooth roUs, are used in series with sifting machines to extract the white flour from the associated bran. 

Tungsten tetrachloride [13470-14-9], WCl, is obtained as a coarse, crystalline, deHquescent soHd that decomposes upon heating. It is diamagnetic and maybe prepared by the thermal-gradient reduction of WCl with aluminum (10). 

Ahoys having 15 to 30 wt % Co and very coarse WC grain sizes (up to 20 -lm) have replaced steel roUs in the production of hot-roUed steel rods. Carbide roUs are also weU suited for the cold reduction and finishing of strip products in Sendzimir mills where rigidity and dimensional stabhity are particularly important. The compositions used in these apphcations have a 5.5 wt % Co and medium WC grain size (1—2 p.m). 

Flour and Feed Meal The roller mill is the traditional machine for grinding wheat and lye into high-grade flour. A typical mill used for this purpose is fitted with two pairs of rolls, capable of making two separate reductions. After each reduction the product is taken to a bolting machine or classifier to separate the fine flour, the coarse produc t being returned for further reduction. Feed is supphed at the top, where a vibratoiy shaker spreads it out in a thin stream across the full width of the rolls. 

In order to produce coarse-grained tantalum or niobium powder it is recommended to perform the reduction of molten K2TaF7 or K2NbF7 with sodium or potassium in the presence of tantalum or niobium metal particles, which are added to aid nucleation [582]. It is reported that tantalum powder with an average particle size of 2.7-4.2 pm was obtained at a yield of 90.1-94%. 

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