Closed circuit grinding

Used for closed circuit grinding (particularly regriud circuits) where clean underflow is necessary. Larger pool allows finer separations. Bowl Desiltor has larger pools (and capacities). Relatively expensive. 

Gives very clean sands and has relatively low hydraulic water requirements (0.5 t/t underflow). One of the most efficient single-stage classifiers available for closed circuit grinding and washing. Relatively expensive. 

Hydraulic cylindrical tank classifier Om tp (M-F) Hydraulic form of overloaded thickener. Siphon-Sizer (N-F) uses siphon to discharge underflow instead of rotating rake. 1.0 to 40 1.4 mm to 45 im (25 mm) 1 to 150 Not critical 0.4 to 15 20 to 35 0.75 to 11 Two-product device giving very clean underflow. Requires relatively little hydraulic water (2 t/t solids feed). Used for washing, desliming, and closed circuit grinding. 

Hydraulic cone classifier V°W (M-F) Open cylindrical upper section with conical lower section containing slowly rotating mechanism. 0.6 to 1.6 400 im to 100 im (6 mm) 10 to 120 Not critical 2 to 15. 30 to 50 3 to 7.5 Used primarily in closed circuit grinding to reclassify hydrocycloue underflow... 

The hvdrocvclone has mostly replaced other classifiers in closed-circuit grinding. 

It has been suggested that the circulating load can be calculated by a material balance from size analyses of the feed, fine product, and coarse product of the classifier in a closed-circuit grinding system [Bond, Rock Prod., 41, 64 (January 1938)]. However, since size analyses are subject to error, it is better to use this information to check the size analyses (Vaillant, op. cit.). The appropriate equation is (Dahl, Classifier Test Manual, Portland Cem. Assoc. BuU. MRB-53, 1954)... 

Closed circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to design of air classifiers. Closed circuit is most common with ball and roller mills. 

Figure 2.3. Flow diagram for closed circuit grinding system...

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. 

Closed circuit grinding employs a means whereby only material smaller than a specified size appears in the product. A less precise mode of operation employs an air stream through the equipment at such a rate that only the appropriately fine material is withdrawn and the rest remains until it is crushed to size. Ball mills sometimes are operated in this fashion, and also the ring-roller mill of Figure 12.4(a). For closer size control, all of the crushed material is withdrawn as it is formed and classified externally into product and recycle. The other examples of Figure 12.4 illustrate several such schemes. 

Note CC is closed circuit grinding the ring-roller mill has built-in air classification. 

Cement clinker is manufactured by both the dry process and by the wet process (Fig. 1). In both processes closed-circuit grinding is preferred to open-circuit grinding in preparing the raw materials because, in the former, the fines are passed on and the coarse material returned, whereas in the latter, the raw material is ground continuously until its mean fineness has reached the desired value. The wet process, though the original one, is... 

MATERIAL-BALANCE CALCULATIONS FOR CLOSED-CIRCUIT GRINDING 13.6... 

A closed-circuit grinding system employs a high-efficiency air classifier wherein the classifier feed (i.e., the mill discharge) is exposed to outside air rather than recirculated air, thus reducing the product-cooling load. The system is shown in Fig. 13.4. 

FIGURE 13.4 Closed-circuit grinding system with high-efficiency air classifier (Example 13.10). 

This is significantly lower cooling duty than would be the case with a conventional closed-circuit grinding system. 

FIGURE 4J.9 Schematic of (a) postclassification closed dicuit grinding and (b) preclas-sification closed circuit grinding. 

Rake and Spiral Classifiers Rake-type classifiers such as the Dorrf classifier and spiral types such as the Akinsj have been the workhorses for general-classification problems for half a century, and their names describe the mechanisms installed in sloping-bottom tanks. See Fig. 19-24. Mechanically the devices are powerfully built, and functionally they are versatile and flexible. They were tfie first classifiers used successfully for closed-circuit grinding. Separations as fine as 325 mesh can be accomplished at reduced tonnage rates. 

FIG. 19-24 Spiral type classifier. Wemco S-H 78-in classifiers in closed-circuit grinding operation at St. Joseph Lead Co., Indian Creek plant. Courtesy Wemco Div., Envirotech Corp.)... 

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