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CLOSE THIS BOOKTools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)
B. Underground mining
VIEW THE DOCUMENTB.1. Definition
VIEW THE DOCUMENTB.2. Existing situation and problem areas
VIEW THE DOCUMENTB.3. Organizational measures
VIEW THE DOCUMENTB.4 Environmental and health aspects

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

B. Underground mining

B.1. Definition

Underground mining includes all aspects of raw mineral extraction by man assisted by the use of technical aids. In addition to the activities involving mining and haulage, it also includes exploration and provision of the necessary infrastructure as well as all measures for the miner's safety. Included among these are:

- drilling

- drainage

- blasting

- ventilation

- loading

- lighting

- haulage

- roof support.

The frequently-used small-scale mining method in developing countries, characterized as a shallow digging or excavating (cateo), can be regarded as a transitory form of open-pit mining.

Deposit exploration in small-scale mining in Latin America and other developing countries is performed using underground mining development methods (tunnelling), due to the comparatively high cost of core drilling.

B.2. Existing situation and problem areas

Small-scale mining in the developing countries extracts raw ores from extremely varying deposit types. The following ore-deposit types can be considered suitable for small-scale mining methods:

- alluvial or placer deposits,

- oxidation zones,

- magmatic hydrothermal vein ore deposits with defined veins (which, however, frequently contain complex mineralizations as a result of extreme telescoping),

- pegmatite veins,

- low-sulphide gold-quartz-veins,

- veins with high-grade gold-containing sulphides (which can be enriched into a sulphide concentrate by flotation),

- pneumatolytic and metasomatic deposits.

In can generally be stated that in small-scale mining the individual mineralized parts or excavations are of small dimension. The mine buildings are sometimes so small that the use of technical improve meets in the form of standardized, mechaninized mining equipment is impossible. An example of this is the large tungsten deposit Kami in Bolivia, where numerous small veinlets and associated difficulties in mechanizing the operation have led to a predominantly manual extraction of the ore.

Besides the purely technical problems accompanying nonmechanized mining, small-scale mines, particularly the cooperative mining operations (span: Cooperativas Mineras), also encounter a multitude of organizational difficulties, namely;

- inappropriate extracting/ mining methods,
- low degree of division of labor,
- lack of coordinated efforts.

The organizational problems are especially apparent in the structure devided in "cuadrillas", typically four-man mining teams in the cooperatives. At the Cooperativa Minera Progreso in Kami, it could be observed that every cuadrilla in the cooperative was given the right to mine the portion of the deposit extending above or below of 15 meters drift. This results in unplanned, irregular and totally varying mining activities which have limited advance rates due to the lack of ventilation, supports, etc.

These technical and organizational deficits are responsible for the especially low specific performance rates characteristic of small-scale mining in developing countries. As a result, many small mines, although rich in ore, must be classified as economically marginal operations.

These economical inefficiencies lead to further problems specific to small-scale mining:

- Insufficient safety measures. Deficient cash availability has caused mine operators to save on expenses, particularly in the areas of ventilation and support, as well as in supplying safety equipment for miners.

- The economic problems of miners' families force the women and children to work in the mines (see photo). While women, due to tradition and religious beliefs, are only allowed to work above ground, i.e. in the beneficiation processing activities, children as young as 10 - 12 years old are already working underground mining the ore. These children frequently work in extremely small holes which are inaccessible to adult miners.

- The high exploitation costs and related costs incurred in poorly-organized, manual small-scale underground mining force the operators to more selectively mine only the high-grade zones in the vein. This screening-out method of mining (= highgrading) which follows only the rich portions of ore veins is a form of destructive exploitation which can lead to substantial macro-economic damages. In areas where poorer deposits become inaccessible or are abandoned due to destructive exploitation of rich ores, a later mining becomes technically difficult or if not impossible. Even under changing economic conditions (as for example higher world market prices for mineral resources), the deposits that have been destroyed through exploitation mining practices still may not be mineable. This situation applies only to unorganized small-scale mining of large deposits; the unique macro-economic value of small-scale mining lies in its ability to adapt to small deposits which could not be mined by any other organinized form of mining.

A further goal of the handbook "Tools for Mining" is to help solve the problems associated with small-scale underground mining. Recommended work-organization improvements are presented below which can benefit small-scale mining operators without requiring additional investment costs:

- lowering the cut-off grade,
- extending the life-span of the deposits,
- improving work conditions (increased safety, elimination of child labor practices),
- improving mine productivity,
- increasing incomes, and
- stimulating the economy through job security.

B.3. Organizational measures



B.3.1 USE OF GRAVITY TO REDUCE HANDLING

Small-scale mining frequently employs inefficient loading and transport methods. Loose material is rehandled a number of times through reloading, redumping, relocating. Particularly primitive and unproductive are the mining methods practiced in the small-scale mining cooperatives where the miners are organized into small mining teams (cuadrilla) which work from the haulage level downward. In these mines, hoists are the standard form of ore transport (see photo, Technical Outline 9.1), sometimes being found every 15 - 20 meters. This is a situation in which changes in mining procedures in order to increase production, listed below as a three-fold concept, are not only logical but also necessary:

1. A reorganization which incorporates division of work duties (job specialization) should be established.

2. A mining procedure should be chosen which employs gravity to increase the efficiency of loading and transport activities.

3. Shaft haulage should be centralized where possible; this involves planning and driving of haulage drifts for the transport of raw ore to the haulage shaft or blind shaft.

A planned loading procedure through the use of loading platforms, raise chutes and bunkers can significantly increase mine productivity and reduce loading and transportation costs. Furthermore, a centralization of shaft haulage can simplify a mechanization of the hoisting equipment.

B.3.2 BACKFILL WITH HAND-PICKED ROCKS

Another method for reducing haulage costs is the hand-sorting of waste rocks underground for further use as packing or backfill material in the excavations. In deposits where portions of unmineralized hanging or foot wall also need to be mined, hand sorting can significantly decrease the volume of raw-ore to be transported. Although hand-sorting in small-scale underground mines in developing countries is a frequent occurrence (see photo, bottom), the sorted-out waste material is not always used in the excavations for backfill, but rather hauled separately out of the mine and deposited on the surface. A change in this practice could contribute significantly to lowering transport costs, improving safety at the mining face and, especially in the small manually-operated mines, alleviating drift and shaft haulage activities. Aside from these, a systematic back-filling can also contribute towards improving the ventilation in the mine, for example by filling in old man (abandoned) workings and thereby preventing short circuits in the ventilation flow.

B.3.3 DIVISION OF LABOR IN UNDERGROUND MINING

One basic organizational deficiency in small-scale mining is the frequent lack of work specialization. Especially the cooperatives' "cuadrilla" work procedures repeatedly lead to difficulties due to the parallellism or duplication of work performed by these small mining teams. As a result, a continuous working operation is not possible, and due to economic and organizational necessities, the work activities are limited to a few critical areas. Mining, haulage and beneficiation are performed sequentially, and other essential tasks are negelected for the present time; as a result, work activities such as development of deposits (even where this is possible internally inside the ore-body structure), timbering and maintenance of galleries (see photo) are not performed.

This has the following effects:

- lack of safety in the mining operations


- a steadily worsening mineral-reserves situation which further lowers the ability of the operations to receive credits and further impedes potential investment through exploration funds (e.g. from the Fondo Nacional de Exploracion Minera, FONEM, Bolivia).

These problems can be countered by a systematic division of labor in the mining operations. This normally requires, however, that the existing distrust first be eliminated. This lack of confidence has been the primary cause of failure so far for numerous projects which attempted to promote a cooperative work system, despite the fact that a concept incorporating rotating job responsibilities not only contains components for specialized training, but also most ideally encompasses the cooperative idea.

Furthermore, a system of work specialization could also facilitate essential planning and coordination activities such as ventilation, supply of energy, mine planning and mine safety.

The introduction of work specialization should include the negociation of personnel salaries based on performance or productivity.

B.3.4 COST REDUCTION IN DRILLING, BLASTING, MAULING, CRUSHING

Depending upon deposit geology and existing mechanization and equipment both on the surface and underground, possibilities exist for reducing costs for drilling, blasting, hauling and crushing. The following relationships regulate potential savings within these cost categories:

fewer drill holes per drilling round (lower drilling costs) produces coarser material (higher crushing costs), stronger explosives (higher blasting costs) results in fewer and/or smaller drill holes (lower drilling costs), electrical milk-second detonator (higher detonator costs) yields finer-grained material (lower loading and crushing costs).

In mines with defined veins without impregnation zones, coarser mined materials can make the hand-sorting or backfilling work easier. Optimization possibilities are dependent on the specific mineralization conditions and the technical capabilities of the mine operation.

B.3.5 SELECTING AN APPROPRIATE STOPING METHOD

The primary deficiency in underground mining operations is the lack of mine planning. As a rule, a type of exploitation mining in the form of irregular excavating or room-and-pillar mining is practiced without any prior planning. This results in lower recovery, lack of safety, and adverse macroeconomic effects due to a partial destruction of the deposits. The different mining methods can be classified according to the type of mine development and support and roof-control measures as follows:

Table: Main Classification of Mining Methods

Mining Method

Roof Control


with pillars

with backfilling

with roof caving

Longwall type (50 m and longer advancing face)


longwall mining
inclined cut-and-fill mining
overhand stoping
cut-and-fill stoping

longwall mining

drift type (2-4 m face width, gallery driving)


drift stoping
cut-and-fill stoping
cross cut stoping

cross cut stoping

room-and-pillar type (drifts separated by pillars which are mined in retreat)


pillar mining
cross-cut stoping

pillar mining
sublevel caving
cross cut stoping

panel type (large axially-expanding rooms extending to mine limits boundary)

panel mining
room-and-pillar mining
breast stoping


panel mining
room and pillar stoping
stall pillar stoping
glory hole
sublevel stoping

block type(excavation chamber not open or visible)


block caving with
square sets

block caving

In the following sections, mining methods are presented (according to Stoces) which, under the special conditions of small-scale mining in the Andean region, contribute to lowering costs, increasing productivity, improving the use of resources (through higher recovery) and decreasing the effort required to extract the ore (consequently increasing mine safety).

Pillar mining


Fig.: Development of pillar mining in an inclined deposit. Source: Stoces

Pillar mining is characterized by irregular forms and arrangements of the excavation chambers, determined by the characteristics of the deposit, the chambers being separated by pillars of varying shapes to support the roof.

It can be applied in deposits with competent mineral and host rock.

Room-and-pillar method


Fig.: Room-and-pillar method. Source: Stoces

This mining method is characterized by the development of parallel headings which resemble long drifts in their form and dimensions. The width of the headings depends upon the competence of the host rock and can reach 10 meters; the height can total up to 3 meters.

The individual headings are laid out either parallel to each other, or either perpendicularly or diagonally crossing each other. Support pillars are left between the headings to support the roof. The roof and floor of the headings usually correlate with the hanging and foot walls of the vein being mined; in some cases, however, the mineralization may extend beyond the upper and lower heading boundaries.

This mining method can be applied in flat or slightly-inclined deposits with competent ore and country-rock.

Panel mining


Fig.: Panel mining. Source: Stoces

Within the deposits, only the narrow, long panels are mined, the valuable mineral contained in the support pillars between panels is left unmined. The deposit is normally developed by a main gallery from which other drifts branch off. These drifts are then widened into panels, leaving a stretch of unwidened drift between the main gallery and the panel for support reasons.

This method of mining is characterized by the construction of panels of regular, mostly rectangular shape. These panels are usually larger than headings, being developed according to preplanned, defined measurements.

Support pillars are left between the panels, consisting of either a solid wall (without cross-cuts), or a row of singular pillars (separated by cross-cuts connecting adjacent panels), depending on the method of ecavation employed.

In gently-dipping deposits, either the hanging wall and foot wall, or portions of the mineralized ore itself, form the roof and floor of the panels. In steeply-dipping or massive deposits, the mineralization can extend beyond the chamber boundaries in all directions. The panels can be mined by various methods, for example, a full advance to the final dimension, or with overhand or bench stoping, with or without backfilling or roof caving.

Panel mining can be applied in thick and massive deposits with competent mineral and host rock regardless of dip.

Shrinkage stoping

The blasting is performed from small chambers in the roof of the stope itself which are sunk from the overlying drifts.


Fig.: Shrinkage stope. Source: Stoces

With this procedure, the extracted ores are stored in the excavation chamber for the duration of mining of the individual slopes. The advantages of shrinkage stoping lie in the fact that no support measures are required and recovery is very high. Shrinkage stoping in small-scale mining in the Andean region is particularly suitable where local conditions permit only seasonal execution of certain processing steps; for example where there is a lack of processing water during the dry season, so that raw-ore beneficiation can only be performed in those months with sufficient rainfall.

Sub-level stoping

Sub-level stoping is an irregular form of panel mining.

This method is characterized by the blasting of large chambers, varying in size depending upon the structure and stability of the deposit and the host rock, and therefore, contrary to panel mining, not precisely defined prior to mining. The excavation chamber must be designed so that gravitational forces alone enable the blasted ore to slide down out of the chamber. Only in rare exceptions (for example, in particularly competent host rock) in only slightly inclined deposits, can a scraper be installed to assist in removing the blasted ore from the chamber.

The stoping, contrary to that in the panel stoping method, does not occur within the chamber itself, but rather at the chamber perimeter, either from horizontal sublevels or through long drillholes, since for safety reasons the chamber may not be entered.

The sublevel stoping can be performed either with roof-caving or with backfilling. It is applicable in steeply-diping deposits of lesser or greater thickness, and in flatter, more massive deposits where a required minimum stope height of around 15 meters can be realized.

A sufficient host-rock stability is important since the stopes can only be worked as long as they remain open. Due to the specified minimum sizes of the chambers and the corresponding greater degree of mechanization, sublevel stoping cannot be considered suitable for small-scale mining.


Fig.: Sublevel stoping. Source: Stoces.

Sublevel stoping (sublevel widening and sublevel caving). When competent ore is being mined from sublevel drifts, then mining from the lower sublevels can proceed.

Cut-and-fill stoping


Fig.: One-Sided cut-and-fill stoping of overhand faces with brace support. Source: Stoces.

This type of stoping is defined primarily according to the type of advance and not the shape of the excavated chamber. The overhand stope, which aside from bench stoping is the oldest form of mining, is characterized by the arrangement of the overhand-stope faces in a step-like pattern of advance whereby each stope cuts into the roof of the preceding stope. The floor of the stope generally is constructed with backfill' although in rare cases square sets are employed for chamber support.

Bench stoping (Underhand stoping)


Figure

Bench stoping is sometimes employed for mining smaller regions of deposits which lie below the haulage level where it would be uneconomical to develop additional levels.


Fig.: Bench (or underhand) stoping. Source: Stoces


Fig.: underground bench stoping or glory-hole mining in a steeply-dipplig coal mine in Checua Region, Cundinamarca, Colombia.

This mining method is the graphic opposite of overhand stoping. Here also, the type of development rather than cavity shape characterizes this mining method. The step-like stoping advances in such a manner that each face mines the floor of the preceding one.

In more massive deposits, the bench stoping develops into an underground glory-hole mining without backfill. It is applicable in deposits of smaller thickness and steep dip, and also as underground glory-hole mining in more massive deposits.

Inclined cut-and-fill mining is differentiated from the regular cut-and-fill method only by the inclined position of the face, which occurs as a result of applying this stoping method in steeply dipping deposits.

This method is only applied in steeply-dipping seamlike deposits of smaller thickness.

Inclined cut-and-fill mining


Fig.: Example of double-sided inclined cut-and-fill mining. Source: Stoces

Sub-level caving


Fig.: Sub-level caving. Source: Stoces

This form of stoping is characterized by the drifting of underground sub-levels, aligned underneath each other, separated by vertical distances of two to three times the height of the roadway. Mining progresses, as the name implies, from the top downwards, followed by caving which automatically also advances downward from sublevel to sub-level.

At each sublevel, the mineral is mined by a two-step form of "small panel mining" as follows:

advance mining involving the driving of individual parallel headings (similar to drifts in height and width), which is then immediately followed by retreat mining whereby the in-situ mineral above the sub-level is mined and the pillars between the drifts simultaneously weakened to the furthest extent possible. The stoping can be performed either sequentially, one sublevel after the next, or simultaneously with several staggered sublevels in deposits of sufficient thickness.

The sub-level caving method is predominantly applied in steeply inclined deposits, of smaller or greater thickness, or in rare cases in thick flat deposits.

A comparison of the various mining methods with regard to their technical and economic characteristics is presented below:

In any case, the application of a systematic mining method leads to a reduction in costs and improved mine saftey compared to the visual mining methods currently being used. The selection of one of the above-mentioned mining methods must give serious consideration according to the deposit characteristics.

Table: Compasion of the essential mining parameters of the major mining methods

Mining Costs:

Productivity per man:

(low)

panel mining

(high)

panel mining

­

pillar mining

­

pillar mining

|

abandoned pillar mining

|

abandoneeed pillar mining

|

shrinkage stoping

|

shrinkage stoping

|

overhand cut-and-fill

|

overhand cut-and-fill

|

bench stoping

|

sublevel caving

¯

sublevel caving

¯

inclined cut-and-fill

(high)

inclined cut-and-fill

(low)

bench stoping

Recovery

Preparation:

(high)

cut-and-fill mining

(low)

panel mining

­

shrinkage stoping

­

pillar mining

|

sublevel caving

|

abandoneeed pillar mining

|

pillar mining

|

bench stoping

|

abandoned pillar mining

|

cut-and-fill mining

|

panel mining

|

shrinkage stoping

¯


¯

sublevel caving

Timber Consumption:

Ore Dilution:

(low)

panel mining

(low)

panel mining

­

abandoned pillar mining

­

abandoned pillar mining

|

pillar mining

|

pillar mining

|

shrinkage stoping

|

shrinkage stoping

|

bench stoping

|

cut-and-fill mining

|

cut-and-fill mining

|

bench stoping

¯

inclined cut-and-fill mining.

¯

sublevel stoping(high)

(high)

sublevel caving

(high)

Number of drills


B.3.6 DEVELOPMENT OF FURTHER PORTIONS OF THE DEPOSIT

A factor worth considering for increasing the economically mineable reserves is the possibility of developing parallel mineable seams or areas of the deposit. In the small-scale mining industry in developing countries, exploration of the mined areas of the deposit is only performed where very massive seams or veins are being mined. Only in a few mines are the mine workings designed to accommodate mining of parallel seams or veins. This would be especially favorable, from an economic-geology point of view, for operations where steeply to moderately-inclined seams or veins are being mined, since the steep country-rock layers between the mineralized seams or veins could be penetrated by horizontal cross-cuts. Furthermore, cross-cuts could be driven through the country-rock simultaneously with the ongoing mining activities.

From a mining perspective, the development of parallel seams offers the following advantages:

- simplification of ventilation
- centralization of haulage
- reduction of exploration and extraction costs
- avoidance of water supply and drainage problems.

The mining of parallel seams or veins also permits a postponement of development at greater depths, which characteristically encounters substantial technical difficulties such as advancing into water-bearing levels, higher mining costs due to greater ground pressure, or higher transport costs due to longer haulage distances.

Without exception, the development of the mine should begin with the upper seams or veins, especially in flat or moderately-inclined strata. The mining of underlying seams occurs only after mining and caving of the upper seams has been completed. Only in this way can damage to overlying mineable seams be avoided, caused by fracturing or caving of the roof of the underlying mined seams which results in the overlying seams becoming incompetent and therefore unsuitable for mining. A fracturing and caving of the exposed rock surfaces also in large mined stopes as well can affect the competency of massive country-rock over a distance of several hundred meters. As a result, complete portions of the overlying veins or seams can fracture or cave, rendering them in any event unsuitable for mining. Given this fact, the mining of only one mineralization, for example the thickest seam, can under certain conditions cause major irreversible damage to the economy as a whole.

On the one hand, mine operators should be motivated through consulting efforts to design their mine workings to accommodate parallel mining activities, even if this results, under the circumstances, in temporary economic disadvantages such as postponement in the mining of explored sections. On the other hand, it remains to be investigated whether a small revolving fund with pre-financing capabilities for the purpose of developing the cross-cuts traversing the country-rock could offer sufficient support to the mines in their exploration activities.

B.4 Environmental and health aspects

Mining activities adversely affect the environment both underground and on the surface by polluting air and water.

a) Air Pollution: Contamination of the mine air in small-scale mining of non-iron metallic ores in developing countries is not, as a rule, due to natural causes. Radon emission from host rock and natural radioactivity which occurs, for example, in uranium mining, firedamp gas from methane emission which occurs in coal mining, or CO2 blow outs which occur in salt mining can be disregarded. The main causes of mine-air contamination are man-made, produced by gas emissions from mechanized diesel equipment and vehicles, by oil aerosols generated by direct oiling of compressed-air equipment, and also by blasting fumes. As a result of the explosive reaction of blasting materials, highly toxic nitrous gases are released. To solve these air-quality problems, artificial ventilation is employed, which in small-scale mines in developing countries is often employed insufficiently and operated inadequately. Standard values for minimum air volume should be incorporated here according to the specifications applicable in Europe:

6m³ / man × min

plus

3-6m³/ PS × min

for diesel equipment underground.

In addition, high dust levels further contaminate the mine-air. Quartz-containing country-rock is particularly problematic, in that the respirable quartz fines cause the lung disease silicosis. These respirable dusts are generated during drilling and blasting activities. Wet drilling, wearing of masks, and sprinkling of blasted muck are attempts to minimize these problems. In general, growing mechanization increases dust levels and the associated health hazards.

b) Water Pollution: Contrary to mine-air pollution underground, pollution of mine water directly affects the above-ground ecosystem. Almost without exception, the vein deposits in smaller non-ferrous metal ore mines contain more or less high proportions of sulphide ore minerals or other accompanying minerals. In permeable zones of the vein mineralization, soluble sulfate compounds are formed through oxidation processes (partially stimulated by microbial calatytic reactions); in combination with water these compounds form sulphuric-acidic mine water. The pH-value of this acidic water can reach levels below pH 2. Besides being acidic and containing high levels of sulfates, these waters form solutions containing high levels of heavy metals, some of which are toxic. Furthermore, these waters may also be contaminated with oil from diesel-operated equipment and lubrication of compressed-air machine-tools. One lifer of oil poisons one million lifers of water. This polluted water becomes hazardous when it ends up on the surface or when it comes into contact with the ground water. Serious impacts on unstable, vulnerable ecosystems, for example in the semi-arid high Andean region, cannot be ruled out. Surface water not only serves as processing water for mining and beneficiation, but is also used as a source for drinking water and for irrigation purposes.

Quantitative statements regarding the degree of environmental impact cannot be made since measurement values of pollution levels outside regions of greater population density in developing countries are not available. Measures to alleviate this deficiency are greatly needed.

In addition, general deficiencies are apparent in terms of work safety, namely:

- noise protection during drilling or other mining and transport activities is rare

- safety shoes and helmets (see photo) are not standard equipment

- no safety measures are provided during personnel transport

- safety measures during blasting operations (for example, detonating fuses are too short, etc.) are lacking

- lighting is inadequate (e.g. candles).

The cause for this deplorable state of affairs is not the negligence or mentality of the miners but rather the result of economic pressures.

Increased mine productivity and improvements in ore beneficiation should, above all, also place priority on the implementation and financing of miner health and safety measures.

c) Destruction of Trees and Forests: Lumbering for purposes is one of the major causes of massive destruction of forests in Latin America and elsewhere. This can be countered by application of cheaper, reusable support elements (individual props, such as railroad ties; see technical chapter).

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