As the world’s population steadily increased (see Table 1.1), so did the demand for converting raw materials into valuable commodities such as medicines, minerals, and pigments. By the start of the 20th century, these and other commodities were being produced in volumes of tons (see Table 1.2).
Along with the spectacular increases in population and consumption came an ever — increasing demand for size reduction. Consider copper as an example. In 1900, 1 ton of copper could be produced from 50 tons of ore because the ore averaged 2% copper, but, by 2000, more than 100 tons of copper ore were required because the average copper content was less than 1%. As a consequence, the amount of grinding required has more than doubled, because more grinding is required to liberate the copper minerals, which are smaller in the lower-grade ores than in the higher-grade ores that are no longer available.
Throughout the history of size reduction, innovation has been driven mainly by technological advances associated with energy and with new materials. The only way to meet the increasing demands for flour and metals for thousands of years was to increase the number of small machines being used and the number of people working them. Only when larger amounts of energy became available from renewable resources such as water and wind could higher capacity, more-efficient machines be built. The effect was dramatic-more energy per machine meant more flour and less effort by human muscles. Both effects dramatically improved human quality of life.
Higher power and higher productivity per machine also meant higher stresses on the grinding surfaces. Hard, abrasion-resistant volcanic stones, which were suitable for soft grains and even for hard, abrasive ores at low feed rates, were used as the surfaces exposed to wear in size-reduction machinery until better wear-resistant materials were developed. When more energy became available and new machines and higher ore feed rates were introduced, better materials were developed to withstand the wear.