criterios

3.7

then stage . Total environa period of 32the mine of 1O t\1.rorun1emat work

estímate. and values proof such costs . million to $250 mnne).

EXPLOITATION : MINING METHODS

101

Production openings excavated in the mineral deposit tend to have names ·que to the type of mine and locale (surface or underground) and to the mmodity mined (coal, metal, or nonmetal)-in part because these kinds of · ing have evolved with terminology distinctions the world over. On the -urface, for example, we excavate pits or cuts in all kinds of mineral deposits, but underground we call production openings rooms or longwalls in coal mines and stopes in noncoal mines. A review of Section 1.4 reminds us of the present predominance of surface mining over underground insofar as tonnage output is concerned. What the future holds is not clear, although the trend since the 1940s has been wward surface mining (Pfteider, 1973a). That trend, however, has nearly eveled out since the early 1970s, and indications are that it may reverse. Certainly all the signs favor a gradual resumption of underground mining, at east in the United States (e.g., deeper deposits being discovered and mined, ow-grade, near-surface deposits becoming more marginal). Nevertheless , ~at prophecy has proven erroneous befóre; and thanks to massive, more efficient surface mining machines, the trend back to underground miningmay be postponed. Specific arguments favoring one class or method of mining over another are best saved for later chapters. The method chosen for exploitation distinguishes the final stage in the life of a mine. Selection of the mining method is the very crux of the exploitation process and probably the key engineering decision made also in mine development. The procedures presented for development earlier in this cbapter now need to be reviewed and expanded in view of exploitation objectives and the procedure for the choice of mining method fully explained.

·,, 1'

r •

¿_

3.7

EXPLOITATION: MINING METHODS

Method Selection

in economic on the surface. 4, the chief JOO'UCIJIOn. In the are employed, secondary ones operations in

..

í

The cardinal rule ofmine exploitation is to selecta mining method that best matches the unique characteristics (natural, geologic, environmental, etc.) of the mineral deposit being mined, within the limits imposed by safety, technology, and economics, to yield the lowest cost and return the maximum profit. Let us now examine the factors which govern method selection (Morrison and Russell, 1973; Boshkov and Wright, 1973). Factors in Selection l. Spatial characteristics of deposit. These factors are probably the most

important determinant, because they largely decide the choice of surface vs. underground mining and affect the production rate, the method of materials handling, and the layout of the mine in the ore body .

102

2.

3.

4.

5.

STAGES OF MINING: DEVELOPMENT AND EXPLOITATION

a. Size (dimensions, especially height or thickness) b. Shape (tabular, lenticular, massive, irregular) c. Attitude (inclination or dip) d. Depth (mean and extreme values , stripping ratio) Geologic and hydrologic conditions. The geologic characteristics of both the mineral deposit and adjacent country rock (host material) influence method selection, especially choices between selective and nonselective methods and extent of support required for ground control underground. Hydrology affects drainage and pumping requirements, both surface and underground. Mineralogy governs mineral processing requirements. a. Mineralogy and petrography (sulfides vs. oxides) b. Chemical composition (primary, by-product minerals) c. Deposit structure (folds, faults, discontinuities, intrusions) d. Planes of weakness (joints, fractures , cleavage in mineral, cleats in coal) e. Uniformity, alteration, weathering (zones, boundaries) f. Groundwater and hydrology (occurrence, flow rate, water table) Geotechnical (soil and rock mechanics) properties. Again, both ore and waste are involved. The mechanical properties of the materials comprising the deposit and country rock (and soil, if overburden) are the key factors in selecting the equipment in a surface mine and choosing among the classes of methods (unsupported, supported, and caving) if underground. a. Elastic properties (strength, modulus of elasticity, Poisson's ratio, etc .) b. Plastic or viscoelastic behavior (flow, creep) c. State of stress (original, modified by mining) d . Consolidation, compaction, and competence (ability of opening to stand unsupported) e. Other physical properties (specific gravity, voids, porosity, permeability , moisture content) Economic considerations. Ultimately, economics determines the success of a mining venture . These factors govern the choice of method because they affect output, investment, cash flow, payback period. and profit. a. Reserves (tonnages and grades) b. Production rate (output per unit time) c. Mine life (operating period for development and exploitation) d. Productivity (output per unit of labor and time-e.g., tons or tonnes/employee-shift) e. Comparative mining costs of suitable methods Technological factors. The best match between natural conditions and mining method is sought. While a particular method may not be

a. Ground contru. b. Subsidence, or c. Atmospheric ity control) d. Work force

3.7

EXPLOITATION: MINING METHODS

103

ruled out in mining, it may have adverse effects on subsequent operations (e.g., processing, smelting). a. Mine recovery (portion of deposit actually extracted) b. Dilution (amount of waste produced with ore) c. Flexibility of method with changing conditions d. Selectivity of method to distinguish ore and waste e. Concentration or dispersion of workings f. Capital, labor, and mechanization intensities Environmental concerns. Not only the physical environment but the social-political-economic climate is involved. a . Ground control to maintain integrity of openings b. Subsidence, or caving effects on the surface Atmospheric control (ventilation, quality control, heat and humidity control) d. Work force (recruitment, training, health and safety, living, community conditions) elines and Procedure. The basic objective in selecting a method to a particular mineral deposit is to design an exploitation system that is ost suitable under the actual circumstances (Hamrin, 1982). Experi-~ plays a major role in decision making, which of necessity involves a _ measure of judgment. Reaching an optimal solution, however, is conly facilitated by the use of quantitative and engineering evaluation, ding operations research techniques, aided by computerized informaand data processing. -e are now in a better position to understand the planning model utilized evelopment and method selection in Section 3.2 (Folinsbee and Clarke, : . Engineering evaluation is carried on at three levels, in the manner -ted in Figure 3.2. First, in the conceptual study, the physical character-s and output quantities of a number of mining methods, layouts, and ""'IDS are assessed. Next, in the engineering study, the preceding con-- are quantified and compared, resulting in firm designs and costs. Fi: . in the detailed design study, drawings and specifications for construc·or the preferred method are prepared. The result is a final engineering rt on which investment decisions, equipment purchases, anda construc-chedule are based. sorne cases, the conditions attached to a proposed mine project are . ., distinctive, and the choice of a mining method may seem obvious. In cases, several candidate methods appear equally suitable. Engineering uation is essential in either situation, however, and shortcuts are not to lerated. Although one or more standard methods may be applicable, an al solution may require modifying or hybridizing an existing method. and better mining methods ha ve evolved in this way . Few mines can d to follow the textbook slavishly or copy exactly the solutions devised