By the middle of the 19th century, the need for better fine-grinding mills had become apparent. Stamps driven by steam engines had limitations on how fine they could grind, particularly on the size of the screens used to remove the desired product from the breaking chamber and keep the oversized material in the chamber. Querns were also sometimes used for fine grinding. As discussed in Chapter 6, the roller mills (a modernized version of the quern) had high maintenance costs when used for grinding abrasive materials and also were unsuitable for wet grinding. These technologies, including the Cornish double-roll crushers, produced a sufficiently consistent product fineness for mineral liberation. At the time, a large volume of cement clinker was being produced, which had to be ground very fine to make finished cement. In addition, deposits of coarse-grained ores were depleted, and the fine-grained ores being mined required finer grinding. Inventors, then, turned their attention toward fine-grinding machines, and tumbling mills were developed.
INTRODUCTION TO TUMBLING MILLS
The tumbling grinding mill was not just an adaptation, it was an invention because it required thinking on a somewhat higher order—there was no prototype. (SME-AIME 1985)
Tumbling mills use horizontal rotating cylinders that contain the grinding media and the particles to be broken. The mass moves up the wall of the cylinder as it rotates and falls back into the “toe” of the mill when the force of gravity exceeds friction and centrifugal forces. Particles are broken in the toe of the mill when caught in the collisions between the grinding media themselves and the grinding media and the mill wall. In tumbling mills the grinding media and particles acquire potential energy that becomes kinetic energy as the mass falls from the rotating shell. Tumbling mills are customarily divided into categories that are mainly defined by the size of the feed particles and the type of grinding media:
■ Ball mills, which use steel or ceramic balls, are mainly used for fine grinding and are divided into two types:
— Tube mills, which usually have a high length:diameter ratio (~6:1) and two compartments separated by a partition
— Single-compartment mills, which have a small length:diameter ratio (~ 1.5:1). Single-compartment ball mills are the best-known form of tumbling mills
■ Pebble mills, which use beach pebbles or small worn rocks, are fine-grinding mills.
■ Rod mills, which use steel rods as grinding media that are 50-100 mm in diameter and extend the length of the mill, are fine crushers.
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■ Autogenous mills, which use the large rocks in the feed to break small pebbles by impact while the large rocks themselves are worn by abrasion until they are small enough to be broken by impact. The word “autogenous” means self-generating— the feed itself continuously creates the grinding media.
■ Semiautogenous (SAG) mills, which are conventional autogenous mills in which 4%-12% of the mill volume is filled with large steel balls because of a lack of a sufficient quantity of grinding media in the feed. The name SAG mill is imprecise but has become accepted through common usage for primary mills, whether fully autogenous or semiautogenous.
■ Secondary autogenous mills, which are mills using pebbles screened from the products from primary autogenous or SAG mills, or secondary crushers. These mills are some times referred to as pebble mills. The length:diameter ratio is similar to pebble mills, and the grinding media are pebbles extracted from the ore.