Tumbling mills are built for either continuous or batch operation. In continuous mills, feed enters one end and broken product leaves the other. In batch mills, the material to be ground is charged to the mill, and the mill is closed and run until the material is ground. The mill is then stopped, the load in the mill is dumped out, and another load is put into the mill. Ball mills use steel, cast iron, or sometimes ceramic balls as the grinding media. Long ball mills called “tube mills” consisted of rotating drums partially filled with rocks or pebbles that were lifted to the top of the drum, then tumbled freely to the bottom. The tumbling grinding media would break the material being fed to the drum to the desired fineness. The fineness of the material discharged from the drum could be made coarser or finer by increasing or decreasing the feed rate to the drum. Before the development of the cyanidation process for gold extraction and the flotation process for sulfide mineral extraction, a few installations of small tumbling mills ground metallic mineral ores.
Tube mills were used to grind gold ore in mines on the Rand in South Africa in the early 1900s. The first mills used local pebbles for grinding media, but grinding pebbles imported from Belgium and Normandy were found to be harder and have a longer service life than the local ones. When the cost of these imported pebbles became prohibitively high, the South Africans began testing pebbles removed from their ores. After World War II, they began to test autogenous grinding, in a process that became known as primary autogenous grinding and led to secondary autogenous grinding, which uses cast iron or forged steel grinding media instead of pebbles extracted from the ore. The heavier weight of the iron and steel ball increased the power drawn by the mills, which in turn increased the mill capacity. By the 1930s, many of the South African gold mines were using ball mills, which led to the use of large-diameter, long tube mills with open discharge ends at mill speeds to 90%-95% of critical speed. The discharge ends of the mills were carried on shoe bearings fitting around the mill shell. These types of mills are still used in the gold mining areas of South Africa.
The early tube mills used to grind mineral ores had diameters of 0.9-1.5 m with the length about twice the diameter. By the end of World War I, the diameter of ball mills had increased to 2.4 m, and by the end of World War II the diameter of ball mills was 3.2 m. The size of the drive motors had increased from 10 to 600 kW, and the two — and three-compartment ball mills used in the cement industry required 933-kW motors. As the 21st century began, mill diameters had increased to 6.7 m with motor sizes up to 15 MW.
At the same time that pebble milling started in South Africa, gold and silver mines in Mexico, Honduras, Australia, and the United States started using pebble mills to grind their ores. These mines began using pebbles from their ores and relatively quickly converted to cast iron and forged steel balls. The native copper ore mines in the Lake Superior mining district of the United States began regrinding the tailings that still contained copper with pebble mills, which were eventually converted to ball mills.
Just before World War I, tumbling mills using rods 150 mm shorter than the length of the mill shell as grinding media were introduced. These rod mills could be fed ore that was approximately 12 mm larger than the feed to ball mills and could grind ore in open circuit to about 10 mesh, which made good ball-mill feed. Rod mills replaced double-roll crushers and covered the finer end of the range of product sizes available from cone
crushers. Rod-mill ball-mill circuits improved the power efficiency of grinding circuits. After World War II, rod-mill ball-mill circuits were installed in new plants processing ferrous and nonferrous ores, but, beginning in the 1970s, two factors slowed the installation of rod mills. First was the availability of quality rods that would have an economic life as grinding media and would not bend or break prematurely. The grinding rods in a rod mill must stay straight and be parallel with the length of the mill. If rods bend or break prematurely, they disrupt the action of the rod charge and cause the rods to bend and disrupt the parallel rotation of the other rod. The rod charge becomes a tangled mess of bent and broken rods. The length of the rods should be at least 1.4 times the diameter of the rod mill to prevent tangling. The limit on the availability of quality rods was 6.7 m, which limited the diameter of rod mills to 4.57 m. Better quality rods became available late in the 20th century. The second factor was the use of primary autogenous and semiautogenous rod mills to replace cone crushers and rod mills in size-reduction circuits.