Grinding equipment classification

Wet classification is performed by filtration, settling, centrifugation, Sind hydrocyclones. When operated in conjunction with grinding equipment, the wet classification equipment must operate continuously and give a pumpable fluid. This is often accomplished in practice with hydrocyclones because the other methods are unsuitable (e.g., settlers and centrifuges are used for dilute suspensions, filters and screens produce a nonpumpable cake.)... 

The second and third stages of grinding and classification involve even greater choices of equipment, but are relatively more expensive. For small liberation sizes (usually between 100- and 150-mesh, Tyler), wet ball or rod mills are required. For larger liberation sizes (plus 100-mesh, Tyler), wet or dry impactors or cagemills are... 

Fine grinding and air classification make possible the production of some cake flour from hard wheat and some bread flour or high-protein fractions from soft wheat. AppHcation of the process theoretically frees the miller from dependence on different wheats, either hard or soft, that change each crop year. The problem is how to market the larger volume of low protein or starch fractions at prices adequate to justify the installation and operation of the special equipment (46). 

There is a clear need for other size classifiers which combine a high separation efficiency with flexibility and compactness. Hydrocyclones have a small volume, are simple in operation and are standard size classification equipment, for example in closed circuit grinding applications. The recent development of the flat-bottom hydrocyclone, which permits classification in the coarse size range, creates an additional motive to study the use of hydrocyclones for Crystal Size Distribution (CSD) control. Furthermore, throttling of a flat botom hydrocyclone does not necessarily provoke blockage but allows continuous control of its cut size when a controlled throttling valve is used. There is a clear incentive for its use in this application since it may provide an additional process input. 

Fig. 8.3). Vertical gas flow is in turn used to fluidized the milled material out of the grinding chamber. Typically fluidized bed jet mills are fitted with a classification wheel. Based on the rotational speed of the classifier wheel, particles of too large a size have a radial momentum imparted upon them returning these particles back to the grinding zone of the fluidized bed. This type of arrangement will typically lead to much narrower particle size distributions than other types of jet milling equipment. 

To use natural minerals it is necessary to grind them down to a desired particle size distribution. Grinding can be performed with the minerals dry or slurried in liquid. In most laboratories, this process is performed in a batch jar mill while on an industrial scale, continuous comminution equipment is used in conjunction with size classification equipment to recycle the coarse material. Figure 4.1 shows a typical comminution circuit with classification and recycle steps, as well as separation of the mineral from the conve3ring fluid. 

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