Rod mill

The raw ROM (run of mine) ore is reduced in size from boulders of up to 100 cm in diameter to about 0.5 cm using jaw cmshers as weU as cone, gyratory, or roU-type equipment. The cmshed product is further pulverized using rod mills and ball mills, bringing particle sizes to finer than about 65 mesh (230 p.m). These size reduction (qv) procedures are collectively known as comminution processes. Their primary objective is to generate mineral grains that are discrete and Hberated from one another (11). Liberation is essential for the exploitation of individual mineral properties in the separation process. At the same time, particles at such fine sizes can be more readily buoyed to the top of the flotation ceU by air bubbles that adhere to them. 

Many different pulverizers are used by the industry. For sand-sized particles, hammer, cage, and rod mills are utilized for soft stone, roUer mills are preferred for dust and very fine sizes, ball, tube, pebble, rod, and compartmented mills are used. 

There are several methods of preparing ore for beneficiation after it arrives at the plant site (Fig. 2). (/) The ore is transferred to rod mills,... 

Sodium sihcate (41°Bh, 1 3.22 ratio Na20 Si02) is added in the milling operation to disperse the slime, mosdy kaolin. Dispersion also aids the grinding process. The rod mill serves to grind the ore to 0.833 mm (—20 mesh) or to the point where mica, quart2, feldspar, and iron minerals are Hberated. Cyclones, or rake, hydrauhc, or other types of classifiers, are used after grinding to produce coarse and fine mica fractions that are treated separately. 

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. 

Cobbers. Magnetic dmms used in cobbing services are designated to obtain maximum rejection of a nonmagnetic product and maximum recovery of the iron mineral. Typically, cobbers are appHed on a rod mill discharge product. Because the objective is to obtain maximum capacity, these dmms are 914 or 1219 mm in diameter and incorporate wear covers on the dmm shells to take the wear introduced by the relatively coarse feed size. 

Grinding. Further size reduction is achieved by several stages of grinding with water to produce an ore slurry. Rod mills are frequently used for the first stage, in which the ore may be reduced to <3 mm. To reduce the size of the rodmill product so that about 75% is less than 0.25 mm in size, two stages of ball mills in series maybe used. 

Tumblei with agglomerate breaker See Sec. 20 Tumbling Mills, for ball mill, rod mill, and vibratory pebble mill which will accomplish mixing along with size reduction. 

A graded charge of rods results from wear in a rod mill. Rod diameter may range from 10 to 2.5 cm (4 to 1 in), for example. A new rod load usually is patterned after a used one found to give good results. The maximum length of a rod mill appears to be 20 rt, because longer rods tend to twist and bend. 

Swaroop et al. [Powder Technology, 28, 253-260 (Mar.-Apr. 1981)] found that the material holdup is higher and the vessel dispersion number Dl/L (see subsection on Continuous Mill Simulation) is lower in the rod mill than in the ball mill under identical dimensionless conditions. This indicates that the known narrow-product-size distribution from rod mills is partly due to less mixing in the rod mill, in addition to different breakage kinetics. 

Feed and Discharge Feed and discharge arrangements for ball and rod mills depend on their mode of operation. Various feed and discharge mechanisms are shown in Fig. 20-34. 

FIG. 20-56 Ball- and rod-mill circuit. Simplified flow sheet of the Cleveland-Chffs Iron Co. Republic mine iron-ore concentrator. To convert inches to centimeters, multiply hy 2.54 to convert feet to centimeters, multiply hy 30.5. (Johnson and Bjotne, Milling in the Americas, Gordon and Breach, New York, 1964.)... 

A flow sheet for one iron ore process is shown in Fig. 20-56. For the grinding of softer copper ore the rod mill might be eliminated, both coarse-crushing and baU-milhng ranges being extended to fill the gap. 

Primaiy crushers used are jaw, gyratoiy, impact, and toothed roll crushers. Impact mills are hmited to hmestone and softer stone. With rocks containing more than 5 percent quartz, maintenance of hammers may become prohibitive. Gyratoiy and cone crushers dominate the field for secondaiy crushing of hard and tough stone. Rod mills have been employed to manufacture stone sand when natural sands are not available. 

Frequently ball and tube mills are combined into a single machine consisting of two or three compartments, separated by perforated-steel diaphragms and charged with grinding media of different size. Rod mills are hardly ever used in cement plants. The compartments of a tube mill may be combined in various circiiit arrangements with classifiers, as shown in Fig. 20-59. 

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 bah mihs in closed circuit with screens. Hourly capacity of a 1.2- by 3-m (4- by 10-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). 

The classifier described above works very well with coarse particles where exact splits are not needed. Typical applications are in connection with ball or rod mills for reduction to particle sizes between 8- and 20-mesh. These classifiers have high capacities they lift coarse solids for return to the mill, so that auxiliary conveyors and elevators are not required. There are other types of classifier that must be usual for close separations with fine particles. One such device is the centrifugal classifier its action bears a strong resemblance to that of the crossflow classifier, but the settling is greatly accelerated by the substitution of centrifugal force for gravitational force. 

The cathodes removed from the electrolytic cell are the primary product of the copper producer and contain >99.99% copper. These may be sold to wire-rod mills as cathodes or processed further to a product called rod. In manufacturing rod, cathodes are melted in a shaft furnace and the molten copper is poured onto a casting wheel to form a bar suitable for rolling into a 3/8-in.-diameter continuous rod. This rod product is shipped to wire mills, where it is extruded into various sizes of copper wire. 

The flowsheet used in this plant (Figure 19.8) consists of a rod mill-ball mill grinding system and a copper rougher-scavenger flotation circuit, followed by two cleaning stages. Initially, the plant used a rake classifier, but now the rake classifiers have been replaced by cyclones. 

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