Caving Mining Methods

Caving Mining Methods

Generally, caving methods extract ore, using the principle of fracturing the ore body and the rocks around it, inducing breaks in the whole strata and hence drawing the manageable ore, under more or less controlled conditions. There should therefore be no drilling and blasting, cutting or ploughing required.

Sublevel Caving

In this method, the stope is divided into sublevels, fairly close to each other, normally with 8-15m vertical interval between each two adjacent ones. These sublevels are called sublevel drifts or production drifts.

Production is carried out from the sublevel drifts by means of long holes above the end of the production drifts. Drilling can be done independently of blasting and loading. Several sublevels can therefore be drilled at the same time and before production starts.

Blasting in each sublevel starts at the hanging wall side and retreats towards the footwall. Large ore bodies can be divided into panels, each of which containing a number of stopes. The inter-relationship between both developments and ore production from different stopes and even between panels is more important than most other methods. Hence a high degree of pre-production design work is an integral requirement of the method.

Blasting the holes breaks the ore and the surrounding area and causes the mineral to fall into the production drift. The broken ore is loaded from the end of the production drift and dumped into the ore pass.

The rock in the hanging wall also breaks and fills the voids created in the ore body in a continuous cave. In the production drift, this is only realized as the dilution of the loaded mineral gradually increases.

At some point, the dilution becomes economically too high and loading stops. Next holes (between 1-3 rounds) are then blasted.

Some of the ore therefore inevitably remains in the caved area (ore loss) and cannot be recovered. Since the ore is brought down into the production drift by some waste rock, dilution becomes another inevitable occurrence in the method.

In a typical stope given the total mineral loaded, dilution is 15-35% and ore loss is 15-25%.

Application

- The method is applicable in steeply dipping ore bodies, or in other types of reserves with a large vertical dimension.

- Minimum ore competence level required for the method is that it can stand mainly unsupported in the production drifts. Occasional supports can be used in the drifts.

- The hanging wall must follow the ore extraction in a continuous cave.

- The surface must allow subsidence.

- Dilution and ore loss are the main factors that influence the use or otherwise of sublevel caving.

- The method is preferred for ores that can be easily separated from the waste.

Developments

1)The major part of the development work is the drivage of the production drifts. But up to 20% o the ore can be won from these.

2)A system of ore passes must be driven to connect the drifts to the main level.

If the preparation of the drifts is considered as development, then the amount of development required by the method is substantial. However, this is a repetitive operation and can be done efficiently with modern equipment.

The rest of developments are simple and straightforward.

Production

Production is accomplished by drilling and blasting, using long upward holes from one sublevel to the one above.

Due to repetitive operation, it can use mechanization to a high degree.

Mineral Transport

In the drifts, mineral transport consists of loading the ore, transporting and dumping it down the ore passes. These conditions favor the use of LHD. Different sizes of these suit different circumstances. Slushers are also occasionally used.

The size (mainly length) of the drift is often decided by the optimum distance for a particular type of a loader. The efficiency of this operation can be high since loaders can be kept in continuous operation by transferring them from one sublevel to another.

Conclusions

Sublevel caving is a systematic method and uses regular layouts and working procedures.

Development, production drilling and loading are carried out on separate levels, all independent of each other. Also, a number of faces are always available for production.

These factors make the method highly mechanizable throughout and “really” the method resembles an industrial process. The secret is regularity.

Waste dilution and ore loss are the disadvantages that cannot always be accepted. A lot of research work has been done to minimize these.

Advanced Sublevel Caving

General Description

The original application of the method started where the ground was too weak even to stand around a narrow heading without support. Heavily timbered headings were therefore driven in the ore body and when the timber was removed at the end of the heading, the mineral would cave in and then slushed out.

When the ore was too diluted, the loading would stop and the next round of supports removed. This method gave high dilution, low recovery and it was slow; but it was the only way to mine such ore bodies at that time.

As an advancement of the method, it has in more recent years, been applied to stronger ore bodies that need drilling and blasting. So the method is not really caving as regards the ore itself but since it relies upon the hanging wall rock to be caved, it is named sublevel caving.

Sublevels are established between 8-15m or even more commonly 8-12m interval. These are accessed by a ramp system. A haulage drift is also established below the stopes to serve several stopes.

The sublevels are accessed from the haulage drift through a raise, near the contact between ore and the footwall rock. When a sufficient number of sublevels are driven, production starts.

In the beginning when only a small area is mined, caving does not occur continuously but it soon starts after blasting.

Sequence of working, accurate planning and its implementation are important.

Application

1) Rock Characteristics

The most important problem in sublevel caving is to control and minimize dilution. The ore is surrounded by dilution on 3 sides, therefore strong brows and good fragmentation is required to gain control of dilution.

The ore should be strong enough:

-To stand without much support (or any support at all), remembering the somewhat large amount of developments.

-To provide strong enough brows.

-For the holes to stay open until loaded with explosive.

The waste rock must be weak enough to cave.

2) Size, Shape, Dip

These all have a strong effect on the application and the layout of the method. A vertical dip is the best.

A medium dip of 60 degrees is satisfactory though not ideal at all.

A flat dip is most unsatisfactory unless the ore body is wide.

If the vertical dimension is not much, then the amount of development work required per ton of ore mined is higher, since a limited number of sublevels can be attempted.

On the top sublevel, about half of the ore cannot be recovered because it reaches the angle of repose and cannot be reached since the waste from the top dilutes it.

In the case of a vertical dip there is always another level below to recover what was left above. Therefore in the more highly inclined ore bodies the recovery is higher, but for a flat dip recovery can be very low unless of course there is a substantial vertical distance.

For the medium dip although some ore is lost when loading, but recovery is relatively good.

So, best conditions for sublevel caving are fairly compact ore, weak walls and steep dip.

Advantages

The advantages of sublevel caving are claimed to be that:

1)It can be applied to variety of rock condition - Quite flexible.

2)It is flexible as regards irregularity of the ore and low width, down to about 3.5 - 4m.

3)Good safety records since all operations take place in a drift size heading that can be well supported.

4)High mechanizability.

5)Activities can be specialized, hence simplifying training of personnel and reducing their number.

6)Much less pillars are left – it is even sometimes called “pillarless mining”. The method has been used to recover pillars left by other methods.

Disadvantages

The major disadvantage of the method is dilution and its control.

Dilution control includes:

-Support of the broken zone

-Good drilling and blasting practices

-An organization for strict draw control.

Relatively high development cost can be considered a disadvantage.

Design and Dimensions

Sublevel Height

In theory the more the distance between sublevels, the better. In practice some factors influence this.

1)Dip of the ore body- If dip is vertical there is almost no constraint. As the dip becomes less, the height has to be reduced in order not to draw rock from the hanging wall.

2)Drilling- Factors influencing or limiting the sublevel height are:

-Hole deviation increases with hole length i.e with sublevel height.

-Drilling costs increase with hole length.

-If the ore is weak then it may be difficult to keep long holes open until charging is completed.

-Long holes often decrease fragmentation. This decreases recovery and increases dilution. They also increase the risk of blockage occurring in the broken area with its potentially disastrous consequences.

Sublevel heights of 9-11m are most common. Maximum hole lengths of 15-18m also apply.

Attempts have been made to increase sublevel height, but they usually have been reduced later.

3)Production drifts and pillars- There is a relationship between sublevel height and optimum pillar width (i.e spacing between production drifts). The higher the sublevel distance the larger the pillars. If these pillars are large, the chance of obtaining a continuous flow decreases.

Also the production drifts width is a factor. The wider the production drift, the less the vertical distance between sublevels should be, in order to limit energy release.

Ideal situation, for stability, is equal width of production drifts and pillars. But this is usually uneconomical and in practice pillar should be less.

As regards height, a 9m high sublevel usually means 6m wide pillar. Or it can be increased to 7.5m, if the stability of the area is not maintained.

4) Size and Shape of Production Drifts

This has an important bearing on the draw. The drift should be as wide as possible while giving good support to the brow.

For optimum draw, the back should be flat. In this case ore will be drawn evenly across the width of the drift’s ceiling. If the roof is arched, then ore may be moved to the centre instead of the sides and waste may come in before ore is finished.

If the drift has to be arched for support reasons, then the production drifts have to be closer.

When loading is being done, some ore inevitably remains in the end of the drift because loaders cannot reach them. The quantity of

this increases as the height of the drift increases. Therefore the drift height should be as low as possible, usually limited by the size of drilling equipment and ventilation considerations. Drifts of 3-3.5m high are not uncommon.

Brow Support

This is a critical factor in the performance of sublevel caving. If the brows are strong and intact, recovery can be good and if not, loading can be affected seriously.

If the brow collapses, ore floods the drift and covers the next row or rows of holes to be blasted. The holes can sometimes be dug out but are mostly lost.

The brow may over-break by blasting, as a result of which ore is dropped further back into the drift than planned. Loaders will therefore have difficulty in reaching the ore, since waste covers them from the front. In these cases, it may be necessary to use single ring blasts.

Technical Requirements

1)Mining Sequence- A well ordered mining sequence is essential to the method in order to avoid occurrences such as:

-Undermining areas still in production

-Drilling holes where there are unblasted holes present (i.e. loaded with explosives)

But the most important reason remains to be to maintain the regular and continuousdown flow of the mined mineral and the strata above and around the ore body.

2)Development and Support-The method involves driving a lot of development headings, with 15% or more of the ore being won from these. Also, one or more ramps are required to provide access for men and machines to the sublevels. These machines include drilling machines and loaders such as LHD or any other trackless equipment.

Ramps should be made at a maximum inclination of 20% although 18% is preferred. They should be flattened off at turnoffs to the sublevels. Minimum practical inside radius on spiral ramps is about is about 12m.

Sublevel headings should be supported adequately for their

relatively short life. The areas of intersection between

haulage drifts and production drifts need particular care.

The most critical areas for support are the brows. If the

ground is strong, extra support may not be required. But if it

is medium, then rockbolts, shotcrete, grouting, timber or steel

bars may be needed. However, the brow must be held up.

3)Drilling and Blasting- Holes should be carefully aligned and drilled accurately.

The charging and blasting of the ore must be done with much care. It is essential that holes are loaded right to the top.

If holes are not carefully drilled, loaded and blasted, a solid bridge may be left of the ore above and only the lower part of the area above may be caved down.

Usually several rings are required before the formation of a bridge becomes apparent. This may cause great problems, such as considerable loss of ore, coarse fragmentation etc.

The consequence of these could be:

-High dilution

-Total blockage

4)Loading- It is necessary to use a loader that can positively dig into the pile. The depth to which the loaders can dig into the pile determines the number of rings that drilling and blasting can be done in any round.

Perhaps diesel powered LHD’s are best since their weight

and power allows them to dig into the ore well. The most

common LHD used is the 4m^3 type.

When loading, it is important to alternate from one side of

the heading to the other side, in order to keep an even draw.

Good floor is essential for the trackless equipment to operate efficiently, but since

the life of heading is short, it is not justified to concrete the

floor. Prepared (crushed and screened) rock (waste) from the

surface, spread on the floor and compacted, makes a good

enough floor surface.

It is important to blast hang-ups in the strata above as soon as

they occur, to ensure continuous flow of the broken ore.

5)Ventilation- In this method through ventilation in the active mining area is not possible.

On a conventional sublevel, an auxiliary fan is installed at the entrance of the production drift. Ducting is used to take clean (or relatively clean) air to the end of the drift, where mining operations are carried out.

6)Drainage- Good drainage is required in the production drifts (and haulage roadways) in order that the floor stays even and equipment can operate efficiently. Headings should be driven at, for example, 3% to the rise to facilitate this.

Equipment may be large, comparing to the size of the heading. Hence ditches on the sides may not stay open. Instead, the floor of the heading should be made so that the center forms a ditch.

Considerable water can be generated by drilling machines, plus natural water that can present delays in production operations. If needed, holes can be drilled to the lower sublevel for water to be transferred there for handling.

Draw Control

Perhaps the most important control in this and other caving methods, is to decide when to cease loading.

The point at which loading stops and the next round of holes is blasted is call the cut-off point.

If loading stops before the true cut-off point then the achieved recovery is below the maximum figure.

If, on the other hand, loading stops later than the cut-off-point, then the achieved dilution is higher than the minimum achievable.

The first parameter to be established is the theoretical grade (cut-off grade) at which the loading should be stopped. This is an economic calculation and depends on all types of costs, price of the ore etc.

Cut-off point is derived from the cut-off grade and given to the loading personnel. But even having an accurate cut-off point in hand, it is not easy to decide when the loading operation should cease. There are two possible methods to do this:

-Visual

-Sampling and assaying

Visual Method

In this case it must be possible to visually distinguish between the ore and waste. It is more usual to give the operator the percentage of ore in a given piece of sample required, as opposed to the cut-off grade.