Ball mills closed circuit operation

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of operation employs an air stream through the equipment at such a rate that only the appropriately fine material is withdrawn and the rest remains until it is crushed to size.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 For closer size control, all of the crushed material is withdrawn as it is formed and classified externally into product and recycle.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. 

Ball mills usually have double scoop feeder especially with higher loadings, closed circuit operations with a rake or spiral dassifier. Makeup balls can be fed directly in if a spout feeder is used but makeup balls fed into the scoop box cause jamming. Use a central makeup ball feed pipe. 

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. 

Ball mills or tube mills can be operated in closed circuit with external air classifiers with or without air sweeping being employed. If air sweeping is employed, a cyclone separator may Be placed between mill and classifier. (The principles of size reduction combined with size classification are discussea under Characteristics of Size Classifiers. ) Likewise other types of grinding mill can be operated in closed circmt with external size classifiers (Fig. 20-12), as will be described at appropriate places on succeeding pages. However, many types of grinders are air-swept and are so closely coupled with their classifiers mat the latter are termed internal classifiers. 

In modern iustaUatious wet grinding is sometimes accomphshed in ball mills alone, operating with excess water in closed circuit with classifiers and hydroseparators. 

A wet-process plant maldug cement from shale and hmestoue has been described by Bergstrom [Roc/c Prod., 64—71 (June 1967)]. There are separate facilities for grinding each type of stone. The ball mill operates in closed circuit with a battery of Dutch State Mines screens. Material passing the screens is 85 percent minus 200 mesh. The entire process is extensively instrumented and controlled by computer. Automatic devices sample crushed rock, slurries, and finished product for chemical analysis by X-rav fluorescence. Mill circuit feed rates and water additions are governed by conventional controllers. 

In the grinding operation of Fig. 13.3, a ball mill is in closed circuit with a hydrocyclone classifier. The mass flow rates of the classifier feed, oversize, and undersize are denoted by the symbols A, O,... 

Coke The grinding characteristics of coke vary widely. Byproduct coke is hard and abrasive, while certain foundry and retort coke is extremely hard to grind. For certain purposes it may be necessary to produce a uniform granule with minimum fines. This is best accomplished in rod or ball mills in closed circuit with screens. Hourly capacity of a 1.2- by 3-m (4- by lO-ft) rod mill with screens, operating on by-product-coke breeze, was 8.1 Mg (9 tons), 100 percent through No. 10 sieve, and 73 percent on No. 200 sieve power requirement, 30kW(40 hp). 

---