Tumbling Mill Grinding Devices

The various mining machine including grinding devices used in the industry are distinguished in terms of the manner by which energy is introduced into the system and in terms of capacity and particle transport into and out of the mill. Each device is characterized with respect to particle size range, design relationships, wear, and efficiency of energy utilization. For some grinding devices, well-defined design relationships have not been established, and in these cases detailed data on typical installations are given when available.

Similarly, wear data for some grinding devices are not always available, especially for the more recently developed mills that have not been used extensively on an industrial scale. In addition, the wear characterization is difficult to generalize because it is highly dependent both on the nature of the feed and the materials of construction. The efficiency characterization is the free-crushing efficiency and represents that portion of the total energy consumption that would be required for single-particle fracture under slow compression.

Ball Mill

Intermediate and fine size reduction by grinding is frequently achieved in a ball mill in which the length of the cylindrical shell is usually 1 to 1.5 times the shell diameter. Ball mills of greater length are termed “tube mills,” and when hard pebbles rather than steel balls are used for the grinding media, the mills are known as “pebble mills.” In general, ball mills can be operated either wet or dry and are capable of producing products on the order of 100 μm. This duty represents reduction ratios as great as 100.

The ball mill, an intermediate and fine-grinding device, is a tumbling drum with a 40% to 50% filling of balls (usually steel or steel alloys; Figure1). The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture. Very large tonnages can be ground with these devices because they are very effective material handling devices. The feed can be dry, with less than 3% moisture to minimize ball coating, or a slurry can be used containing 20% to 40% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, autogenous mills, or semiautogenous mills. Regrind mills in mineral processing operations are usually ball mills, because the feed for these applications is typically quite fine. Ball mills are sometimes used in single-stage grinding, receiving crusher product. The circuits of these mills are often closed with classifiers at high-circulating loads.

Fig1. Cutaway of ball mill with chute feeder and grate discharge

There are three principal forms of discharge mechanism. In the overflow ball mill, the ground product overflows through the discharge end trunnion. A diaphragm ball mill has a grate at the discharge end (Figure 1). The product flows through the slots in the grate. Pulp lifters may be used to discharge the product through the trunnion, or peripheral ports may be used to discharge the product.

The mill liners used are constructed from cast alloy steels, wear-resistant cast irons, or polymer (rubber) and polymer metal combinations. The mill liner shapes often recommended in new mills are double-wave liners when balls less than 2.5 in. are used and single-wave liners when larger balls are used. Replaceable metal lifter bars are sometimes used. End liners are usually ribbed or employ replaceable lifters.

The typical mill-motor coupling is a pinion and gear. On larger mills two motors may be used, and in that arrangement two pinions drive one gear on the mill. Synchronous motors are well suited to the ball mill, because the power draw is almost constant. Induction, squirrel cage, and slip ring motors are also used. A high-speed motor running 600 to 1,000 rpm requires a speed reducer between the motor and pinion shaft. The “gearless” drive has been installed at a number of locations around the world.

Autogenous/Semiautogenous Mills

Autogenous and semiautogenous mills represent a relatively new type of tumbling mill that, under certain conditions, can replace size reduction equipment used for secondary crushing as well as primary and final grinding. Basically, the breakage mechanism is similar to that found in other tumbling mills. The unique feature of this device is that the coarse ore particles themselves are used as the grinding media, not unlike a pebble mill in which the pebbles are generated naturally from the ore body. In this regard, autogenous grinding is to be applied to ores with necessary characteristics.

The autogenous mill itself is a coarse-grinding device, consisting of tumbling drum with a 25% to 40% volume filling of ore. Metallic or manufactured grinding media is not used. Autogenous mills are fed run-of-mine ore or primary crusher product that is –10 in. Inside the mill, large pieces break into smaller pieces a few inches in size. These natural pebbles act as the grinding media in the autogenous mill. The main modes of breakage are thought to be impact breakage and abrasion.

Semiautogenous milling results when a small amount of steel balls, 3% to 20% of mill volume, is added to the mill charge. The addition of a small ball charge to an autogenous mill changes the nature of the mill performance considerably. In this case, major design modifications may be required to carry the additional charge. Generally, the addition of a ball charge increases the mill capacity significantly but increases operating costs for balls and power (Figure 2).

Fig2. Semiautogenous mill

Many circuit configurations are possible, but essentially the autogenous mill is operated as a single-stage primary mill, or it can be followed by secondary pebble or ball milling. The autogenous mill is often operated in closed circuit with a trommel screen or external vibrating screen classifying the discharge. Circulating loads are low compared with those in ball mill circuits, because autogenous mills do not benefit from high-circulating loads in the same way ball mills do. Intermediate crushers are sometimes used to crush the largest pieces in the recycle stream.

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