Open-pit mining

Open-pit mining provides significant proportions of many of the world's major mineral commodities. In hard rock mining, much of the world's annual output of copper, gold and iron ore is won from open-pit operations. Other commodities produced from open-pit mining include diamonds, molybdenum, manganese, lead and zinc, uranium and a variety of industrial minerals, such as borates, talc and specialist clays.

The list of commodities is extensive, although the relative proportions mined underground and on the surface vary from mineral to mineral. Massive tonnages of hard coal and lignite are also produced from surface mines, although the terminology used – open cast or open cut, rather than open-pit – indicates that the technology and engineering concepts used are often markedly different from those employed in the hard rock environment.

 

Open-pit mining differs from quarrying (with the possible exception of iron ore) in that the valuable mineral constitutes only a small proportion of the total tonnage of rock produced. In quarrying, the rock itself is the valuable commodity, with virtually all of the raw rock that is won being processed (often only by crushing and screening) to give saleable products. In open-pit mining, on the other hand, the metal content of the ore produced may be only fractions of a percent, meaning that a huge proportion of the rock produced is effectively waste material.

 

In addition, the ore body geometry may mean that large tonnages of barren rock also have to be mined and transported in order to access the ore in which the valuable mineral (or minerals) makes up only a small proportion. This has to be recovered from the rock matrix through both physical (crushing and grinding) and/or chemical processes such as flotation, meaning that processing costs are higher. This is reflected in metal commodity prices, with a ton of copper or zinc being valued much more highly than a ton of granite or basalt.

Open-pit mining clearly offers a number of economic advantages over underground mining, especially now that mineral-processing technology has advanced to the stage where very low-grade ores can be treated profitably. 100 to 150 years ago, this was not the case, and mining was still much more selective.

However, the introduction of flotation technology for recovering base metal minerals, and the cyanidation of gold (particularly heap-leaching) changed all that, and large-scale open-pit mining has developed steadily since the latter part of the 19th century. While the scale of open-pit operations may have changed, the fundamental concepts have not. The challenge is always to access ore grade mineralization without having to strip excessive amounts of barren waste rock or overburden. An additional constraint is that the definition of ore can change over time as technology advances and commodity markets fluctuate. In some cases, where resources are defined by clear differences in the geology of the ore and waste, this will have little effect. In others, such as with low-grade porphyry copper deposits, a minor reduction in cut-off grade resulting from the introduction of higher capacity mining equipment can lead to an increase in available resource tonnage. The same is true of a long-term rise in the copper price — what may have been considered as waste in the past can now be reclassified as ore.

High performance drilling is key to successful open-pit mining, with two principal areas of operation: blasthole drilling, and drilling presplit holes that help to enhance pit wall stability. These operations have different requirements, in terms of both their aim and the equipment used.

 

Blasthole drilling is an integral part of the production process, and needs large, heavy drill rigs that can produce high meterage of often large diameter holes. These are then charged with bulk explosives, such as ANFO or emulsions, to produce broken rock that can be handled by the loading shovels. Drilling is carried out on a specifically defined grid system, taking into account the relationships between the hole diameter, the burden (the distance between each row of holes) and the spacing (the distance from hole to hole along the bench).

Two main changes have occurred in open-pit mining technology over the past 100 years. The most obvious has been the increase in the scale of operations as higher capacity equipment has been developed. For instance, in the 1920s, a mining shovel with a 5 m³ bucket was exceptional. Today, large open-pits use rope shovels or hydraulic excavators with bucket sizes of ten times that capacity.

 

The other big change which has occurred is the evolution of transport systems. Mines have moved from rail-bound to haul truck and, in an increasing number of cases, to in-pit primary crushing of the ore (but not the waste rock) followed by belt conveyor transportation out of the pit. In addition, in tandem with the growth in loading-shovel capacities, haul trucks have increased in size with the top-of-the-range 45 tons capacity hauler of the 1950s having been replaced by the 400 tons hauler that takes three-or four-pass loading from the current generation of shovels.