Shrinkage Stoping
Figure shows the general approach. Means of extraction is overhead drilling and blasting. The method extracts the ore and leaves the stope empty.
Ore is extracted in horizontal slices starting from the bottom of the stope and advancing upwards. Part of the ore remains in the stope to provide a working platform and in order to support the walls.
The blasted ore increases its volume by about 70%. Therefore 30-40% of the ore must be drawn continuously to provide enough space for working. This is called Shrinkage Draw.
When overhead mining reaches the planed height, drilling and blasting is ceased and the remaining 60-70% of the broken ore is drawn. This is called Final Draw.
Small ore bodies can be mined as one stope and large ore bodies can be divided into several stopes with pillars left between them in both horizontal and vertical directions to maintain the stability of the area.
In this method, it is most common that the pillars left in place as sills, between the stopes etc, are recovered after mining the stope.
Applications
The method can be used in ore bodies with these characteristics:
1)Steep or near vertical ore bodies. The dip must exceed the angle of repose.
2)The ore body must be firm and relatively competent.
3)The hanging wall and foot wall must be stable. Broken ore at best diminishes dilution.
4)The ore body must have relatively regular boundaries.
5)The ore must not be affected by storage in the stope.
Developments
Shrinkage requires these developments:
1)A haulage or transport drift, driven along the bottom of the planned stope.
2)Crosscuts are made from transport drift to the bottom of the stope.
3)Finger raises or cones are developed upwards from the crosscuts into the stope.
4)Undercut of some 5-10m in height should be made across the whole area of the stope bottom.
5)A raise is provided from the transport drift, through the undercut and to the main level above. This provides access and ventilation air to the stope.
The bottom of the stope can be simplified by eliminating the finger raises and using draw points and crosscuts for loading.
Production
Drilling and blasting are the means of production, performed as overhead or overhand operation.
The rough surface in the stope prevents the use of mechanized equipment. The standard method of drilling is therefore the use of hand-held drills (pusher legs). Occasionally some long horizontal holes can be drilled.
Mineral Transport
Conventional system is to load the broken ore directly into the rail mounted mine cars (through the chutes at the bottom of the stope).
Today, loaders are more effective when used in a drawpoint loading design.
Conclusions
When there were few loading machines available, shrinkage was one of the most attractive methods, mainly because ore could be dropped directly into the mine cars, as opposed to hand loading.
Today, this is of little importance and hence shrinkage has been largely replaced by other methods.
Disadvantages include:
-It is a labor-intensive method. It also requires skilled labor.
-Working conditions are often difficult and dangerous.
-Production capacity is limited. Drilling long holes is impossible, stope dimensions are limited and mechanizability is negligible.
-Most of the broken ore remains in the stope for a long time.
Therefore: Under similar conditions, sublevel stoping, sublevel
caving and even cut-and-fill are used and they result in
considerable economic advantage.
However, shrinkage stoping has not been completely eliminated and it is applied in the cases of small-scale operations with low investment in machinery and developments.
Shrinkage Stoping – Advanced
Introduction
Shrinkage is referred to any method in which the broken ore is temporarily left in the stope to:
-Provide a working platform
-Support the walls
Since ore swells when broken, it is necessary to shrink it by drawing some of the broken ore from the bottom of the stope - hence the name.
When final draw has taken place, the stope can be filled, or it can be left empty. If it is to be filled, it can be done after the final draw is completed or simultaneously with the final draw.
Conventionally the method uses overhand or overhand stoping methods. There are many variations in details, most of them decided in the stope.
Stope dimensions
Shrinkability is assessed in terms of:
-Dip
-Width
-Regularity along the dip
Dip is ideally 70-90 degrees. As the dip falls bellow 70, the
shrinkage draw starts to favor the hanging wall side therefore
an even poorer working surface is provided. This problem is
more acute in wide stopes.
Also, when the dip decreases, the support offered to the
hanging wall becomes less, being zero at the angle of repose.
Generally, dips below 45-50 are not shrinkable at all. Shrinkage is probably the most sensitive method to dip and in situations where dip is less than 60 or even 70, many problems begin to occur. The main reason is the requirement for a steady flow of the broken ore to the bottom of the stope.
Width- The minimum width is fixed by the space required for
working, usually at least 1m. Shrinkage is used in veins of down to 0.5m, but on one or both sides, wall rock is also mined and
separated later.
Narrow veins are susceptible for blockage in the stope and the bridging action. Problems of irregular draw or incomplete draw may be caused in these situations.
Maximum width is about 30m, but the best width is perhaps
between 3-20m.
The ideal width depends on:
-Competence of the ore
-Ability of the hanging wall to stand unsupported
Stability of the stope back (roof) is perhaps the most important safety factor in shrinkage method. This is the reason why the method requires competent ore more than any other underground mining method.
Regularity of the dip along the stope is another requirement of the
method. There must be no obstruction to the flow of ore from
the working platform to the bottom of the stope.
Gentle or local irregularities can be accepted if the local dip is not less than 45-50 degrees.
Off dip hanging wall and foot wall pieces can generally be mined selectively without affecting the flow of ore. There is therefore some degree of selective mining possibility inherent to the method.
If small rolls are encountered, these can usually be smoothed over but if they are large, then a sublevel should be established with draw points etc.
Ground Conditions
The only support given to the walls is broken ore, which is dynamic too. It is therefore not a reliable support to the walls. Hence the walls must be strong enough to stand somewhat unsupported.
Dilution by means of local sloughing (sheding) is prevented by this dynamic support but stope closure is not. To prevent stope closures, leaving pillars is the most effective remedy.
Another remedy is to use rock bolts for the walls.
The ore itself must be strong enough to stand totally unsupported. Some artificial temporary supports can be used but of course it is expensive and troublesome.
Physical or mineralogical characteristics of the ore may sometimes restrict the application of shrinkage stoping. Examples include:
i) Some cohesive ores, when broken, may pack or cement
together. This probability increases in the stope where these
conditions are present:
-Ground water
-Wall pressure
-Chemical reactions
If this happens, regular draw of the ore is impaired. Difficult or incomplete draw may then be the result.
ii)Pyritic ores that when exposed to air oxidize rapidly and may generate much heat, increasing the chance of spontaneous combustion.
iii)Sulfide ores that when exposed to air oxidize and hence processing the ore becomes difficult, or the recovery rate decreases.
iv)Ores that may emit gases that impose constraints on ventilation system. Examples include uranium minerals giving off radon gas.
In most cases, these problems can be minimized by:
-Limiting the size of the stope
-Minimizing the duration of mining activities in each stope
-Drawing he broken ore quickly after the completion of mining activities.
Merits of shrinkage stoping
Merits of the method have to be compared with other methods applicable in similar conditions, i.e. sublevel stoping and cut-and-fill.
Shrinkage is unique in that at least 2/3 of the output is tied up in the stope. This can be an important economic disadvantage.
This disadvantage can, however, sometimes be turned into an advantage. During the final draw, stopes can act as bunkers and therefore production level can be varied easily and cheaply to suit the varying price of the ore in the market.
Comparing to sublevel stoping, its disadvantages are:
1)It is a higher cost method
2)It needs a more competent ore body
3)It is more labor intensive, especially skilled labor
4)Less mechanizable, therefore less productive
5)It has a poorer safety record
And its advantages are:
1)It is applicable with less stable wall rocks with less dilution
2)It is more selective
3)More applicable with irregularities of the ore and its boundaries
4)Lower initial capital required in machinery
5)Less development
Development Design
Haulage levels are driven at the vertical intervals of between about 30m or less or 200m or more.
Vertical spacing depends on:
i)Dip: steeper dips allow vertical spacing to be more
ii)Width: In narrow veins, the vertical spacing is limited
iii)Regularity: If the boundaries are regular, the spacing could be more
In all these cases, if the vertical spacing is excessive, then the problem of irregular flow of mineral occurs.
In these cases, the ore body is mined from more sublevels, each with its own drawpoints.
In any case, the maximum stope height is usually fixed by draw control considerations and it seldom exceeds 75-100m.
Spacing between the drawpoints is 5-15m. Closer spacing needs more capital expenditure on development but:
i)A more even surface is created
ii)Leaves less ore in the stope after the final draw
Wider spacing can be carried out when:
-The sill of the stope is worked later.
-When safe cleaning of the stope is possible for example when remote control LHD’s are used.
Typical of shrinkage method is uneven surface, even when close spacing of drawpoints is practiced. It is therefore often necessary to do some hand work, to smooth the working surface or minor drilling to smooth the roof.
Starting the method depends on the design, but the normal system is to start from the bottom and proceed upwards into the stope through the drawpoints. Sill pillar is left in the stope bottom.
Production
The method advances upwards maintaining a relatively flat ceiling. In some cases other forms may be made: wavy, stepped back or inclined. The main factors affecting this could be:
-Drawpoint location
-Vein width
-Stope access
-Ventilation scheme
-Machinery
In practice, this is decided after the face is opened and actually during mining of the stope, by the production team.
In normal circumstances, drilling up holes is preferred - it is simpler and more straightforward and therefore conventional.
After drilling, operations are carried out in this order:
-Explosive loading
-Shrinkage draw
-Blasting
-Barring down
Some of these (not much) could be done during drilling to save time. Different operations in this method are not independent of each other. This is a main disadvantage of shrinkage stoping.
Horizontal holes may give equal or better results if the stope is wide enough to allow a fairly sizable round. In such cases, fragmentation may be poorer. It is thought to be a safer drilling method, since the action of the drill loosens the rock above. The lower number of drill holes being drilled in each round is the main drawback of horizontal drilling method.
Mechanizability is limited because of the limited space, uneven and loose floor and roof and the possibility drilling of short holes only. Only hand drills or occasionally small jumbos are used in this method. Shrinkage is perhaps of lowest level of mechanizabilty amongst all underground mining methods.
Walls support in the stope can be accomplished by using rock bolts or leaving pillars. If pillars are left in the stope, they should be as small as possible and to be left in vertical succession between drawpoints so that the flow of ore downward is not impaired.
Temporary support of the face is occasionally done by using rock bolts, wire mesh, shotcrete or by full scale grouting.
Such materials must be of a type that break easily in the pile and therefore do not hinder ore draw. Rock bolts may cause difficulty in mineral processing plant, for example in crushing or milling.
Unlike sublevel stoping, the shrinkage draw is an essential part of the mining process. It can be done after blasting or after drilling. The two possible systems are:
-Blast-Shrink-Drill
-Blast-Drill-Shrink
In practice the second system is preferred since the working surface produced by blasting is smoother than that provided by shrinking. This is even more true when blasting is controlled in a way that lower parts of the pile are filled with more ore resulted by blasting.
As a result of shrinking, subsidence cones are produced. Therefore drilling pattern and timing of the blasting should be arranged in a way that the broken ore fills these cones.
This is particularly true in narrow stopes and stopes with lower dip than ideal. An uneven surface is also produced when wider spacing of drawpoints is practiced. In these cases, perhaps the preferred cycle should be adopted.
In practice, the broken ore in the stope may lie at an angle, considerably higher than the angle of repose, due to such factors as compaction, cementing of the ore pieces, wall squeezing and generally space confinement.
A serious problem exists when bridging or hang ups develop during shrinking the ore. If such voids occur and are undetected, they may move during drilling by the influence of water and vibration of the drills. In such cases, miners may be swallowed.
Therefore shrinking must be done by authorized personnel and observed carefully.
If hang ups occurs occasionally, the problem is normally solved by pouring water down. But if they occur regularly, the drawpoints design should be modified.
There are many detailed variations to the method. For example, blast hole shrinkage is where a raise is driven upwards and horizontal holes are drilled into the ore.
Final Draw
The final draw of some 60% o the ore is normally started as soon as mining terminates.
It is necessary that a small cat way is constructed in the footwall about 2m below the ore boundary, to gain access to the stope in case such problems as irregular draw are encountered.
Water at the pressure of 5-10 kg/cm2 through nozzles of 20-30 mm is the most effective way of pushing the ore down, to ensure maximum recovery.
Final draw can be done in two different ways:
1)Drawing ore from all drawpoints simultaneously. In this case the problem of bridging becomes more probable, especially in narrow stopes.
This method is preferred if dilution from the hanging wall is an expected problem. The method is compulsory if simultaneous filling of the
stope is to be carried out.
2)Finishing one drawpoint and start with the next one.
This method, particularly when aided by water jets, pushes the ore to the active drawpoint and achieves maximum recovery. It also minimizes the problem of slushing.
In any case, it must be noted that, during the final draw, steep cones may develop again, with the angle much more than the angle of repose. This is more acute in narrow stopes and when water is not used.
After final draw, the stope can be:
a)Left empty
b)Filled with development waste or mill tailing. Coarse waste material poured into the stope simultaneously with the final draw, has in practice proved an effective ground control measure, but of course, at the cost of some dilution and decreased recovery.
Productivity and Costs
Productivity and costs in shrinkage method are more variable than in any other underground method.
Least favorable conditions would be hard ore and narrow and irregular veins. In these cases productivity could be as low as 10 tons per production man shift. Costs could be $30-40 per ton.
In most favorable conditions, productivity could be as high as 200 tons per production man shift and cost of some $3-4 per ton.
In unfavorable conditions, where mechanizability is limited, productivity and costs are directly related to the skill of miners. In such cases, piece work contracting is the most effective system.
Generally, productivity and costs are very sensitive to stope dimensions and ore hardness.
In more favorable conditions, where larger and more regular stopes can be adopted and some mechanization can be applied, then machine performance and availability are the key factors. In this case, labor specialization is the rule.