Soft‑Rock (Underground) Mining: Selection Methods

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CHAPTER 29 [7.4]

SoftRock (Underground) Mining: Selection Methods

Antonio Nieto

INTRODUCTION The soft rocks usually are part of the sedimentary minerals classification, which is subdivided into clastic, organic, and chemical. Examples of the soft-rock ores include coal, metalliferous shales, oil shales, potash, salt, trona, and possibly kimberlites. Where coal, metalliferous shales, potash, and trona occur as economic ores, they are typically laterally extensive beds in a nearly horizontal inclination but with, at most, a shallow dip angle. This differentiation is key, because it enables the application of large-scale mechanization to the mining process. The economy of scale that results from mechanization is often the determinant factor for economic success. As the capabilities of mechanical cutting expand into more demanding applications, the possibility exists that ores previously considered hard-rock deposits, such as the platinumbearing reefs in southern Africa, may be cut instead of blasted.

Coal has its origins in the accumulation of plant debris that becomes buried by sediments. Through a process dependent on time, burial depth, and chemical transformation, the plant debris becomes coal. Therefore, coal is classified as an organic sedimentary mineral. Coal varies in quality from lignite to anthracite, with sub-bituminous and bituminous ranked as intermediate in the progression and the most commonly mined types worldwide.

Coal is primarily used for electricity generation and steelmaking and is commonly referred to, respectively, as steam coal and coking coal. Coal with attributes such as an appreciable free-swelling index (FSI) is used to make coke, which is used in primary production of steel from iron ore. The scarcity of such coals elevates their value compared to steam coal.

Trona is a carbonate mineral of sodium used to form soda ash, used in glassmaking and other industrial processes, including baking soda. It occurs naturally in a few locations worldwide as laterally extensive evaporate deposits, suitable for underground mining methods. It is a moderate value ore and competes with an alternative process that synthesizes the same product from chemical feedstocks.

Potash is the name loosely applied to a variety of potassium salts, particularly potassium chloride, which are encountered in laterally extensive evaporate deposits found worldwide. These

are often associated with intermixed or stratigraphically adjacent halite (sodium chloride). Potassium is key to plant growth, and potash is mainly used as fertilizer, although it is also used to produce soaps, ceramics, and drugs, among others.

Coal, potash, trona, and salt are the principal soft-rock ores and, within limits, share similar production methods that focus on economies of scale. The most common mining techniques for soft-rock ores are longwall, room-and-pillar (R&P), and stope-and-pillar. For water-soluble minerals, solution mining is an alternative. The process of properly selecting an underground mining method for a particular ore deposit is critical to the ultimate success of the operation. An improperly selected method will increase costs, lower productivity, create unnecessary hazards, and reduce resource recovery. Due to the complex nature of ore bodies, no two mines are completely alike, and all operations must adapt to the particular conditions of their deposits.

ORE DEPOSIT CHARACTERISTICS Numerous considerations must to be recognized when selecting the best method to mine a soft-rock ore deposit. Some of the considerations are based on ore deposit characteristics favorable to the mining method being considered:

? Ore strength ? Host rock strength ? Deposit shape ? Deposit dip ? Deposit size ? Deposit thickness ? Deposit grade ? Ore uniformity ? Deposit depth

Other characteristics are a function of mining method:

? Operating cost ? Capital cost and development timing ? Production rate ? Mechanization ? Selectivity and flexibility

Antonio Nieto, Associate Professor, Mining Engineering Department, Penn State University, PA, USA

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Table 29-1. Ore strength definitions

Relative Strength Very weak Weak Moderate Strong Very strong

Example Material

Compressive Strength, psi

Coal

32,000

? Health and safety ? Environmental effects

Ore Strength The material properties of the ore often drive mine design decisions. Although there are many mechanical properties, compressive strength is often discussed as an indicative characteristic inferring structural performance and suitability for mechanical cutting. Mining methods such as R&P and stope-and-pillar depend on the strength of the ore rock to support the roof and overburden in order to create a structurally stable excavation. In soft-rock applications, the relative strength of the ore is often weak, with a compressive strength less than 6,000 psi. This low strength is generally associated with a low to moderate specific energy of cutting (kilowatt-hour/ton). This allows the application of mechanized cutting and loading, which is elemental to the success of many modern mines. As the ore strength increases, the options for mechanical cutting are reduced, and the application productivity declines while costs increase. There is a marked difference between the cost and productivity performance of mechanized cutting versus drilling and blasting in the majority of soft-rock applications, with mechanized methods decidedly preferred.

Table 29-1 gives the strength designations and ranges of values based on the compressive strength of the material.

It is important to note that the strength and mechanical properties of a rock are significantly affected by fracturing and planes of weakness in the deposit. Fracturing is characterized by small discontinuities in the rock mass and may be caused by heat, vapor expansion (as in porphyry deposits), depositional conditions (i.e., slickensides), or tectonic movement (faults). Cleat is a fracture system ordinarily observed in coal. Two different fracture directions are typically present: face cleat (primary direction) and butt cleat (secondary direction). During exploration, the degree of fracturing should be quantified and utilized to reduce ore structural properties, potentially leading to smaller openings, larger pillars, and increased ground control costs. Limited fracturing may be a positive factor for some mining methods, because it promotes caving, lowers blasting requirements, and aids mechanical cutting. However, excessive fracturing can have a negative influence on ground control, water, and gas inflows.

Host Rock Strength The strength of the rock enclosing the ore is also an important driver in mining method selection. Temporary and permanent openings must be developed either in the host rock, in order to access the ore, or with the host rock as roof (back or hanging wall) or floor (footwall) for the ore openings (entries or crosscuts). To execute an appropriate design, the material properties must be understood. The behavior of the roof and floor can be

Table 29-2. Deposit shape definitions

Deposit Type Tabular Lenticular Massive

Shape

Width

Flat

Thin to moderate

Flat, elliptical Thin to moderate

Any

Thin to thick

Extent Horizontal Horizontal Horizontal and vertical

Table 29-3. Deposit orientation definitions

Inclination Category Low Moderate Fairly steep Steep

Dip Angle, degrees 0?5 5?25

25?45 45?90

pivotal in the success of mechanized mining systems. Floors that become muddy and easily rutted can disable production and send maintenance costs skyrocketing.

It is inaccurate to assume that the ore and host material will have the same characteristics, so each must be independently characterized by geomechanical testing.

Deposit Shape Ore deposits are classified into two broad categories: tabular and massive. A tabular deposit is flat and thin, and has a broad horizontal extent. This classification typically refers to materials formed by sedimentation. Similar in shape to tabular ore bodies, lenticular deposits are shaped like lenses and lack the large areal extent of most tabular deposits. Most methods designed to exploit tabular deposits may be adapted to mine lenticular ones. The ore materials must often be of higher value than applications such as coal, because production costs are generally higher but reserve tonnages are lower.

A massive deposit may possess any shape. The ore is often distributed in low concentrations over a wide area with varying horizontal and vertical extents. Frequently, the difference between ore and waste may be a function of grade rather than rock type. Massive deposits may be unpredictable and require a considerable exploration investment in order to document and fully understand the resource. For the purposes of mining method selection, massive deposits are often accompanied by a more specific clause like "massive with large vertical extent." These additions are necessary because the shape of a massive deposit is variable and may be unsuitable for certain mining methods. The deposit shape definitions are summarized in Table 29-2.

Deposit Dip Dip is defined as the angle of inclination of a plane measured downward, perpendicular to the strike direction. The deposit dip is more relevant to tabular ore bodies than massive ones, although it may sometimes be a consideration for the latter. Deposit dips are categorized and defined in Table 29-3.

Both flat-lying soft-rock ore beds, and near-vertical ore veins may be classified as tabular, but the mining methods used to exploit them are dramatically different. Several methods are highly dependent on gravity for material flow and cannot function in flat-lying deposits. Alternatively, low working slopes are a key factor in the application of mechanization for cutting and loading as well as material haulage by rubbertired, rail, or conveyor-belt methods.

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Table 29-4. Tabular deposit thickness definitions

Thin (small) Moderate Fairly thick Thick (large)

Deposit Thickness (T) T < 5 ft

5ft < T < 12 ft 12 ft < T < 20 ft

T > 20 ft

Table 29-5. Ore grade definitions

Grade Low Moderate High

$/ton 10?50 50?250 >250

Deposit Size The volumetric size of an ore body must also be considered. Several of the methods discussed in this chapter rely on large deposits with long mine lives to justify their high initial capital costs and promote economies of scale. Other methods simply do not work efficiently in ore bodies, which are either too large or too small. Deposit size is characterized subjectively by the terms small, medium, and large. As a generalization, large ore deposits have tens to hundreds of million cubic yards of ore and suggest mine lives in the 10- to 50-year range.

Deposit Thickness Deposit thickness refers to the ore thickness of tabular deposits. Thickness plays an important role in opening stability and may prevent certain equipment from functioning efficiently or mining methods from being effective. The deposit thickness (nominally the mining extraction height) definitions are listed in Table 29-4. These definitions are most relevant to mechanical cutting and loading applications, such as longwall or continuous miner R&P. The thickness ranges roughly correlate with the types of equipment available to implement a mining system and the cost/productivity that might be expected.

Deposit Grade Grade is discussed in terms of the amount/value of recoverable/salable material in a unit weight or volume of in-place mineral resource. Where it becomes economically viable to produce the mineral resource, the in-place resource becomes ore. As such, the end-outcome economics of different mining methods may vary the amount of ore that an in-place mineral resource may yield. A gold ore may contain as low as 0.1 oz/ton and still be economic, whereas iron ore grades may approach 60% by weight. Coal is generally characterized by its attributes--that is, energy content (Btu/lb); percentage of ash, moisture, and sulfur; FSI; and so forth. Some mining methods with high operating costs necessitate high-grade ores in order to be economic. Large-scale methods may be suitable for large, low-grade deposits, such as bituminous coals. Ore grades are categorized subjectively and must be investigated on an individual site basis. Ore grade definitions are provided in Table 29-5. Value estimates associated with the classifications give some relative sense of the range involved.

Ore Uniformity The uniformity of the ore in the mineral deposit must be considered, as poor uniformity may render some mining methods

unviable. It is undesirable to excavate subeconomic material, unless it is necessary to reach ore or create necessary infrastructure, such as belt-conveyor galleries. A mineral deposit may be segmented by faults, subeconomic mineral occurrence, or legal/environmental issues. Some mining methods are well suited to flexibility because they can selectively extract specific sections of a deposit without disrupting the overall operation. An example of this is the case where an R&P coal mine adapts the panel geometry while in panel to reflect new findings about unsatisfactory coal quality, adverse roof conditions, or insufficient coal thickness.

Other methods, such as longwall mining, limit selectivity and must produce at least some amount of material leading to equipment advance in order to continue to the panel's intended end. Faults with significant displacements compared to the bed thickness can seriously disrupt longwall or R&P operations. In some areas, igneous or sedimentary materials may be injected into tabular deposits, such as dolerite dykes in coal seams, and create impediments to mechanized cutting and loading. An inconsistent feed of material may disrupt processing plant performance or require blending, rehandling, or disposal of mined material. These situations can be anticipated and minimized with a thorough knowledge of the ore body's uniformity. Ore uniformity designations are

? Variable, ? Moderate, ? Fairly uniform, and ? Uniform.

Deposit Depth Another deposit-related consideration that impacts mining method selection is ore deposit depth relative to the surface. Shallow deposits are generally more suited for surface mining. Deeper deposits may require progressively greater ground control measures (increased costs), larger pillar sizes (lower recovery), or decreased applicability of some mining methods in order to ensure safety and sustainability. Commonly applied variations of R&P or longwall mining occur over deposit depth ranges from 250 to 3,500 ft. The definition of shallow/ moderate/deep is relative depending on the value of the ore and the strength of the material. A deep coal mine might have workings to a depth of 3,500?4,500 ft. Alternatively, a deep gold mine producing from a meta-quartzite reef might have workings to nearly triple that depth. Classification for deposit depths are shallow, moderate, and deep.

MINING METHOD CHARACTERISTICS Every mining method has characteristics that will produce different outcomes based on the ore deposit to be mined. As such, prior to selecting the best mining method, the methods to be applied and their expected outcomes must be clearly understood.

Operating Cost The operating cost of a mine is the cost associated with the production of ore from the primary mining method. The total cost is higher and incorporates items such as depreciation, depletion, taxes, and royalties. The operating cost divided by the number of salable units of production mined creates a metric used to compare efficiency between competing production alternatives--that is, $/ton. When the total cost is the basis of the metric, it can indicate the potential viability of the project

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in total. In mining, the operating cost is composed of fixed and variable expenses. Variable expense totals change in proportion with activity, such as roof-control cost ($/ft) that typically accumulates with the amount of entry development. In comparison, fixed costs, such as labor and ventilation, stay relatively constant over a moderate range of activity variation. Some methods are labor intensive or may require a large quantity of materials in order to operate, thereby necessitating valuable ores to compensate for the greater price of extracting them. Other methods cost little once implemented but have high initial capital costs. These methods, such as longwall mining, may be able to excavate large low-grade deposits economically.

Capital Cost and Development Timing Initial capital cost is defined as the amount of investment needed before the mine begins to generate revenue. A small quarry excavating an outcropping limestone bed has little capital cost because it can start extracting ore almost immediately with little investment in equipment. Alternatively, a deep potash mine might have to sink one or more shafts beyond a depth of 3,000 ft, build a surface plant, and implement a mechanized mining equipment fleet to produce the first salable ton of product. Thus, first production may come after several years and tens to hundreds of million dollars have been committed.

Higher capital costs are frequently associated with long development or start-up times. Equipment manufacturers often have wait times of months or even years before assembly and delivery of new equipment. Typically, this equipment is customized for the mine-specific application.

Production Rate The production rate of a mine is highly dependent on the mining method. A high production rate can accommodate a large market and may overcome low-value ore if operating costs are low. The ability to stockpile and blend ores of varying grade in order to maintain a consistent feed to the mill is typically advantageous. Higher production is generally more desirable because mines are rarely opened in areas where selling more product is disadvantageous. The economics of mines that can sell product up to the limit of their production capacity are drastically different than mines that can produce at levels above what their markets can consume. In the latter case, production enhancement proposals readily embraced by the former case, intended to distribute fixed costs over a larger total production, are rejected, and the focus sharpens on costs contributing to the fixed component of operating cost.

Mechanization Mechanization is a critical element of a modern mine. Utilizing machines to perform production tasks is much safer and more efficient, in cost or production performance, than using manual labor. To justify a large capital investment in equipment, it is common to need a longer mine life and thus a larger ore body. Highly mechanized mining is safer than less mechanized methods because fewer workers will be needed and thus the overall hazard exposure will be lower. Several methods lend themselves to a high degree of mechanization, including longwall and continuous miner R&P methods.

Selectivity and Flexibility Selectivity and flexibility can significantly contribute to the success of a mining method. It is generally valid to assume that mining conditions, market prices, and technology will

change over the course of a mine's life, so the mining method must be adaptable to these fluctuations. Sacrificing optional alternatives in any mining method is not desirable unless there is compelling reason to do so. If commodity prices were to drop substantially, a portion of the ore in a massive deposit may become uneconomic to mine. If the mining method is able to bypass the low-grade sections and continue mining economic material, the mine will continue to be successful.

Health and Safety The safety and health of a mine's workers should be the top priority of every operator. Several methods are inherently safer than others, because the openings are more stable or personnel are less likely to be subjected to hazardous conditions. Although no modern methods are considered to be unsafe, it bears mentioning that specific health and safety concerns are often mitigated by the mining method selection. Longwall mining is recognized as the safest method of mining applied to soft-rock deposits.

Environmental Effects The largest environmental impacts of an underground mine typically fall into three categories: subsidence, groundwater, and atmospheric emissions. Subsidence is defined as the sinking of the surface above mine workings as a result of material settling into the voids created by mineral extraction. It is contentious in urban or suburban areas where it can affect homes, schools, and roads. The surface subsidence created by modern longwall mines is largely predictable in its timing and magnitude, in contrast to the unpredictable outcomes associated with some R&P mines. In this way, longwall subsidence is less hazardous to human-made surface structures, because impacts occur soon after mining and rarely change much after initial stabilization. This allows remediation of surface damage in a time contemporary with mining.

Most areas with a history of mining also have developed legal processes to address damage from mining-induced subsidence. High-extraction mining methods will foreseeably induce surface subsidence. If selected, provisions must exist to mitigate or remedy damages.

Water impacts may arise by accidental causes. Acidgenerating rock of multiple types in excavated ore, waste, or overlying strata may produce acid mine drainage. Water produced by the rock mass and mining process must be afforded appropriate controls, as it will be necessary to keep the mine drained. In all cases, strict controls must be effectively applied to mitigate groundwater or surface-water impacts by miningrelated water discharges.

Air quality in underground mines is typically affected by the natural liberation of mine gases (i.e., methane [CH4], hydrogen sulfide [H2S], and carbon dioxide [CO2]), blasting by-products and equipment emissions (i.e., nitrogen oxides [NOx], sulfur oxides [SOx], and diesel particulate matter), and mineral dust from ventilation fans. Generally, exposure and emission thresholds exist for these emissions and are strictly applied. In the case of coal dust and methane, special precautions are followed to avoid the hazards of fire and explosion.

Zero harm is a sustainability principle applied by the foremost mining enterprises in the context of health, safety, environment, and communities where mines in their portfolios actively operate. It is an acknowledged goal that communities will be forever improved because of the global and local activities of these mining enterprises.

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