Prospecting and Quarrying
The techniques used in prospecting for and extracting limestone arc similar to those for many other hard rocks and have been fully described elsewhere [1-4]. This chapter, therefore, only seeks to give an appreciation of the processes involved.
Limestone is extracted by both surface quarrying and under-ground mining operations. However, economic and safety considerations result in almost all limestone being extracted by quarrying, which also recovers a higher proportion of the rock. Some examples of successful limestone mines are given in section 3.7.
A small tonnage of limestone is used in the production of dimension stone. The process is outlined in section 7.
Before looking for a suitable deposit in the field, consideration needs to be given to:
a) establishing the minimum acceptable size and quality of the deposit,
b) collecting the geological and other related information that is already available,
c) obtaining details of government regulations and restrictions on the extraction of minerals, and
d) which locations might be particularly favourable from the viewpoints of geography and infrastructure.
Size and quality. The minimum acceptable size of the deposit is related to the expected maximum extraction rate and the minimum life of the site required to give an acceptable return on the investment. When estimating the maximum extraction rate, allowance should be made for losses in quarrying, stone processing and (where appropriate) lime production. Where investment is low, or where a mobile plant is used, a relatively short life might be acceptable. Where investment is high and includes fixed plant and perhaps lime kilns, a life of over 40 years is usually sought. The quality of limestone required depends on the uses to which it will be put (see chapters 7 to 12 and 14). tain adequate natural, or man-made exposures, some exploratory core drilling may be required to provide the necessary information. Where drilling is not feasible, digging shallow trenches perpendicular lo the strike of the beds may provide useful information and samples, although the surface rock may be weathered and, therefore, unrepresentative of the under-lying deposits.
Having established that the deposit is sufficiently large and of a suitable quality, evidence should generally be obtained to establish the structure of the deposit by the use of core drilling. This should include:
a) the dip and strike of the bedding,
b) the thickness of the beds,
c) the physical and chemical qualities of the beds,
d) the extent of mineralisation,
e) the type and intensity of any folding, 0 the extent of jointing and faulting,
g) the extent of solution and weathering effects, h) the effective level of the water table.
Recommended procedures for core drilling [5. 6] should be followed and adequate samples should be taken for assessment by standard testing procedures (chapter 6).
If the interpretation of the initial results is favourable, the geologist may recommend that additional cores be drilled to clarify the nature and extent of particular geological features (such as major faults) and/or to determine the hydro-geology of the area.
2.3 Interpretation of Field and Test Results
The prospecting report should give an estimate of the workable volume of the deposit, making due allowance for:
a) overburden removal,
b) the profiles of the perimeter faces,
c) sterilisation of deposits by access ramps and any permanent plant,
d) tipping of sub-standard rock, including fault material,
e) environmental, land-use and planning constraints.
The report should also recommend where to start quarrying and how to develop the quarry. It should highlight the major geological features and how they might affect the development. It should assess the physical and chemical qualities of the various beds in the deposit in the context of the proposed end-uses and indicate whether selective quarrying might be necessary. In addition, it should address the question of reinstatement when the quarry is worked out, which is likely to be a condition of planning consent.
The quarrying process can be divided into five operations – overburden removal, drilling, blasting, loading and hauling to the processing plant.
Historically, these operations were very labour intensive. However, mechanisation, which started in the late 1940′s in Europe, and is currently being applied in most of the developing countries of the world, has dramatically reduced labour requirements. This section, therefore, concentrates on mechanised quarrying practices.
3.2 Overburden Removal
The thickness of overburden can vary from less than 1 m to tens of metres. Indeed, if the thickness is considerable, the costs of overburden removal can make the development of an open-cast quarry uneconomic and may force the developer to consider mining (see section 3.7). The overburden generally consists of top-soil and sub-soil, but may also include rock overlying the limestone.
Disposal of the overburden can be a significant operation. Top-soil should be handled and stored in such a way as to preserve its fertility and permit its later use in landscaping schemes [7,8]. Sub-soils and over-lying rock may be tipped, in which case the lips must be designed to be stable and have adequate drainage [9,10]. In some situations, it may be possible to sell part of the overburden as in-fill.
Overburden may be removed as an on-going operation, or in campaigns. In many countries, soil is best removed in summer, when it is drier, able to bear the weight of earth-moving equipment and is in a better condition for handling and storage.
The capital, operating and maintenance costs of primary fragmentation of the rock arc low in relation to those of the subsequent operations to produce saleable limestone products. It is essential, therefore, to design the drilling and blasting operations to produce optimum fragmentation of the rock and a rock pile profile suitable for the loading equipment .
The pattern of drilling depends on the properties of the rock, on the geological structures (e.g. bedding planes), on the diameter of the drill holes and on the required rock pile profile. Holes may be inclined at up to 20° to the vertical (Fig. 1) and positioned to give controlled spacing and burden (Fig. 2). Inclined holes generally:
a) produce a safer, more stable face, allowing higher benches to be blasted safely,
b) reduce the incidence of “toes” (stumps of unbroken rock at the foot of the face) and
c) result in a better rock pile profile for loading .
The optimum height of the quarry faces depends on a number of site-specific factors. In many countries, the regulatory authorities recommend a maximum height of 15 m for safe working.
3.5 Secondary Breaking
Secondary breaking is required when boulders are produced which are too large to load or to feed into the crushing plant, and when toes are left by primary blasting. Over-sized boulders arc broken by three techniques — hydraulic hammers, drop-balls and secondary blasting.
There are two types of secondary blasting. In “pop” blasting, a hole is drilled into the boulder or toe, high explosive is placed in the hole, and stemming is rammed into the drill hole to contain the force of the explosive. In “plaster” blasting, the high explosive is placed on the surface of the rock and fired. While pop blasting breaks the rock more effectively than plaster blasting and creates less noise, it does produce more “fly rock”.
Some limestones, notably the softer chalks, can be extracted by ripping. While ripping is not widely used, it is considerably cheaper than drilling and blasting. Various techniques are used, depending on the nature of the deposit. Some soft rocks can be extracted by hydraulically operated shovels and backhoes. Somewhat harder limestones can be broken by a ripper tooth fitted to the rear of a tractor-dozer.
Underground mining of limestone uses the “room and pillar” technique. As mining costs per tonne of saleable product are generally much higher than those of quarrying, there have to be special circumstances to justify operating in this way.
Haulage is a major variable cost in producing limestone products. The most widely used vehicle for hauling rock from the quarry face to the primary crusher is the rigid-bodied dump truck, which is available in sizes ranging from 15 to over 150 tonnes capacity. Articulated dump trucks are more manoeuvrable and are often favoured in smaller quarries.
5 Current TVends in Quarrying
A relatively recent trend is to install in-pit crushers (Fig. 7). These may be fed directly from the quarry face by rubber-tyred shovels in a load-and-carry operation. The crushed rock is transported to the processing plant by conveyers, which generally have lower capital and operating costs than dump trucks.
6 Dimension Stone
Marble and other limestones are used in relatively small tonnages as dimension stone for ornamental purposes such as facing buildings and tombstones.
The production of dimension stone is a specialised operation. It is cut from shallow benches using either a channelling machine or a wire saw. A vertical cut, some 3 m deep and 15 to 30 m long, is made about 1 m behind the face of the bench. The base of the block is then cut, using wedges to support the cut section. The block is then sawn at convenient intervals, typically 1 m, to produce smaller blocks with dimensions of about 1 x 1 x 3 m. The smaller blocks are transported to a workshop for further cutting, or shaping, and finishing.
Last update: January 15, 2012
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