Closed Circuits

In the moving-bed system of Figure 7, soHd is moving continuously ia a closed circuit past fixed poiats of iatroduction and withdrawal of Hquid. The same results can be obtained by holding the bed stationary and periodically moving the positions at which the various streams enter and leave. A shift ia the positions of the iatroduction of the Hquid feed and the withdrawal ia the direction of fluid flow through the bed simulates the movement of soHd ia the opposite direction. 

The three basic types of size reduction circuits used to produce a fine product ate shown in Eigure 1. The final stages of the grinding circuit are typically operated in closed circuit, at comparatively high circulating loads, so that the material has tittle chance of being broken a second time before it is removed from the circuit by a classifier. Rod mills are operated normally in an open circuit. 

As only a small proportion of the material is in contact with the roUs and friction on the rollers is low, hard materials can be processed with tittle wear. The high pressure action creates a slab of ultrafine particles which usually requires a low speed impact milting system to disagglomerate. Used in closed circuit with such a disagglomerator and an air classifier, such machines can reduce the energy requirement for fine grinding many minerals. 

Closed circuit voltage is the voltage of a cell or battery when the battery is producing current into the external circuit. 

Raw material for dry process plants is ground in closed-circuit ball mills with air separators, which may be set for any desired fineness. Drying is usually carried out in separate units, but waste heat can be utilized directiy in the mill by coupling the raw mill to the kiln. Autogenous mills, which operate without grinding media are not widely used. For suspension preheater-type kilns, a roUer mill utilizes the exit gas from the preheater to dry the material in suspension in the mill. 

FIG. 12-77 Turbo-tray dryer in closed circuit for coutiuiioiis drying with solvent recovery. Wyssmont Company, Inc.)... 

Performance and Cost Data for Turbo-Tray Dryers Performance data for three apphcations of closed-circuit diying are included in Table 12-29. Operating, labor, and maintenance costs compare favorably with those of direct-heat rotating eqmpment. 

TABLE 12-29 Turbo-Tray Dryer Performance Data in Wyssmont Closed-Circuit Operations ... 

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. 

FIG. 1 9-24 Spiral tvpc classifier. W euico S-H 78-in classifiers in closed-circuit rindin operation at St. Joseph Lead Co., Indian Creek plant. ICour lesij Weui/ o Div., EmnnAeoh Coq. )... 

Ores must be ground to a point of complete or nearly complete liberation, Even though this might possiblv be accomplished bv coarse crushing, grinding to finer than 10 mesh in all cases and finer than 48 mesh in most cases is necessaiw prior to Rotation, (Grinding is done in closed circuit with classifiers. 

The work index may be round experimentally from laboratoiy crushing and grinding tests or from commercial miU operations. Some rules of thumb for extrapolating the work index to conditions different from those measured are that for dry grinding the index must be increased by a factor of 1.34 over that measured in wet grinding for open-circuit operations another factor of 1.34 is required over that measured in closed circuit if the product size Xp is extrapolated below 70 [Lm, an additional correction factor is (10.3 + Xp)/l.l45X . Also for a jaw or gyratory crusher the work index may be estimated from... 

FIG. 20-12 Hammer mill in closed circuit with an air classifier. 

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 Milling In closed-circuit miUing the taiUngs from a classifier are mixed with fresh feed and recycled to the miU. Calculations can be based on a material balance and an expUcit sohi-... 

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