The demand for iron, copper, and other strategic metals during World War II was enormous, and their resulting production depleted the available reserves of high-grade ores. After the war, large volumes of metals and cement were needed to rebuild industry and infrastructure, and a large increase in grinding capacity was required. In the United States, an increasing amount of iron had to come from taconites, which are very hard, abrasive siliceous magnetite ores that contain 20%-30% iron. Magnetite extraction involved rod and ball milling with interstage magnetic separation, and the concentrates were 80% passing 38 |rm. The early plants used mills that were 3.2 and 3.8 m in diameter, and even then there was a hint of a problem with rod tangling, which became significant in later plants that used mills up to 4.9 m in diameter. It became apparent that 4.54.9 m was the limiting range for rod mill diameters because of rod tangling. No limitations for ball mill diameters were encountered in iron ore concentrators.
Expansion of copper ore mining began in Arizona and Chile in the mid-1950s and accelerated in the 1960s worldwide. Single-stage ball mill and two-stage rod mill-ball mill circuits were both used. Rod mill growth stopped at 4.9 m, but ball mill growth continued up to 7.7 m in single-stage circuits and as the second stage in SAG mill-ball mill circuits. Efficiency problems, which started at about 5.8 m, have never been explained satisfactorily. However, because the efficiency issues vary in magnitude, they are probably caused by a combination of scale-up factors, ore size and grindability, and the total mill throughput. In both the mining and cement industries in the 1960s and 1970s, increased demand was seen for larger grinding mills that drew more power and had higher grinding capacities. Rod mill diameters increased to 4.9 m, ball mill diameters rose to 6.5 m, and multiple-compartment mills increased to 4.5 m in diameter and 13 m long. There were even more spectacular increases in the sizes of large-diameter, short — length autogenous and SAG mills (Table 7.2).
In the 1950s, the use of hydrocyclones for classification changed the design of grinding circuits almost overnight, allowing for the use of larger diameter mills. The rise of hydrocyclones is discussed in Chapter 9. Briefly, hydrocyclones displaced rake and spiral classifiers in the 1960s, which was timely because mechanical classifiers had just about reached their size limit. Further use of them with larger mills would have required the installation of parallel units requiring expensive space and involving complex materialhandling problems. Hydrocyclones solved these problems and reduced the capital cost.