Advances in drilling and blasting were always hard won, and the work was onerous and dangerous until well into the 20th century. Objectives of blasting during the 20th century were
■ Before 1960: to make excavation possible
■ 1960-2000: to make excavation efficient
■ After 2000: as part of the overall rock breakage process
Much of the technical progress during the 1960s and 1970s was aimed at increasing productivity by gaining economy of scale. This did not always benefit fragmentation.
Larger loading and haulage equipment were introduced, and extraction developments and draw-points were designed with larger cross sections. Pneumatic-tired machines powered by diesel engines were introduced, providing greater flexibility and permitting the handling of larger rock fragments. From a mining consideration alone the main requirements were that rock fragments were small enough to
■ Avoid choking draw-points or bridging brows
■ Be handled readily by loaders, trucks, and tipples
■ Fit into crushers
Better equipment made larger rocks easier to handle, and this improved mine production rates, but there was still a need for good fragmentation because this improved the ease of flow of the ore and the maintenance of equipment. The effective cost of poor fragmentation can be several times the cost of the blast itself because of problems such as
■ Excessive dilution of the ore
■ Increased secondary blasting
■ Reduced loading rates
■ Difficulties in ore handling and transport
■ Poor performance in the mill comminution system
■ Variable ore quality in concentration (Scott, Chitombo, and Kleine 1993)
In economic terms, if the cost of a blast is $1.30/ton, the extra cost caused by poor fragmentation can be $1.50/ton in grinding and $2.00/ton in concentration.