Print4.3.3: Why Do We Choose One Method over Another?
There are many factors that can affect the choice of a mining method. However, a relatively small number of them will dictate the choice. The others may affect the layout of that method, or other details, but rarely do they eliminate a method from consideration or drive the selection of a method. Let’s take a look at a comprehensive set of factors and understand what they mean. Then, we’ll step back, take a deep breath, and see how uncomplicated it can really be! Here is a comprehensive list with a few annotations to indicate the significance of the factor.
Spatial characteristics of the deposit
These factors play a dominant role in the choice of a mining method because they largely decide the choice between surface and underground mining, affect the production rate, and determine the method of materials handling and the layout of the mine in the ore body.
- size (especially height, thickness, and overall dimensions)
- shape (tabular, lenticular, massive, or irregular)
- attitude (inclination or dip)
- depth (mean and extreme values, stripping ratio)
- regularity of the ore boundaries
- existence of previous mining
Geologic and hydrologic conditions
Geologic characteristics of the ore and surrounding country rock influence method selection, especially choices between selective and nonselective methods, and ground support requirements for underground mines. Hydrology affects drainage and pumping requirements, both surface and underground. Mineralogy governs solution mining, mineral processing, and smelting requirements.
- mineralogy and petrography (e.g., sulfides vs. oxides in copper)
- chemical composition (primary and secondary minerals)
- deposit structure (folds, faults, discontinuities, intrusions)
- planes of weakness (joints, fractures, shear zones, cleavage in minerals, cleat in coal)
- uniformity of grade
- alteration and weathered zones
- existence of strata gases
Geotechnical (soil and rock mechanics) properties
The mechanical properties of ore and waste are key factors in selecting the equipment in a surface mine and selecting the class of methods (unsupported, supported, and caving) if underground.
- elastic properties (strength, modulus of elasticity, Poisson's ratio, etc.)
- plastic or viscoelastic behavior (flow, creep)
- state of stress (pre-mining, post-mining)
- rock mass rating (overall ability of openings to stand unsupported or with support)
- other physical properties affecting competence (specific gravity, voids, porosity, permeability, moisture content, etc.)
Economic considerations
Ultimately, economics determines whether a mining method should be chosen, because economic factors affect output, investment, cash flow, payback period, and profit.
- reserves (tonnage and grade)
- production rate (output per unit time)
- mine life (total operating period for development and exploitation)
- productivity (tons /employee hour)
- comparative mining costs of suitable methods
- comparative capital costs of suitable methods
Technological factors
The best match between the natural conditions and the mining method is sought. Specific methods may be excluded because of their adverse effects on subsequent operations (e.g., processing, smelting, environmental problems, etc.).
- recovery (proportion of the ore that is extracted)
- dilution (amount of waste that must be produced with the ore)
- flexibility of the method to changing conditions
- selectivity of the method (ability to extract ore and leave waste)
- concentration or dispersion of workings
- ability to mechanize and automate
- capital and labor intensities
Safety, Health, Environmental concerns
The physical, social, political, and economic climate must be considered and will, on occasion, require that a mining method be rejected because of these concerns. The impact of one mining method over another method on the environment must be considered. Similarly the ability to provide the highest level of safety and health with one method as compared to a competing method must be considered.
- ground control to maintain integrity of openings
- subsidence, or caving effects at the surface; or affect on ground water system
- atmospheric control (ventilation, air quality control, heat and humidity control)
- availability of suitable waste disposal areas
- workforce (availability, training, living, community conditions)
- comparative safety conditions of the suitable mining methods