Revision as of 12:49, 23 January 2013

Introduction

Sublevel Caving is one of the most advanced mining methods. This method is usually undertaken when mining the orebody through an open pit is no longer economically viable. In Sublevel Caving, mining starts at the top of the orebody and develops downwards. Ore is mined from sublevels spaced at regular intervals throughout the deposit. A series of ring patterns is drilled and blasted from each sublevel, and broken ore is mucked out after each blast. Sublevel Caving can be used in orebodies with very different properties and is an easy method to mechanize. This method is normally used in massive, steeply-dipping orebodies with considerable strike length, and usually has a high amount of dilution and low recoveries. Thus Sublevel Caving is usually used to mine low-grade, low-value orebodies.

Ore Body Characteristics

•Strong and competent rock with few major structures:

- A competent rock mass allows for more drilling and blasting within each stope. This provides good fragmentation and leaves the caving above very coarse. This results in blasted material finer than the cave, making distinction between ore and waste easier.

• Steeply dipping orebody:

- Keeps the low grade waste further away from the current draw points, keeps dilution low.

•Massive deposit:
- Need a large footprint to minimize dilution levels as most dilution comes from the boundary between ore and waste. A massive deposit minimizes the dilution as a smaller proportion of material is being mined from these boundary areas.
• No weak or weathered materials:
- Can cause problems with muck rushes, over-compaction, and hang-ups.

• Preferably have mineralised waste:

- Keeps the dilution low and allows for overdrawing at drawpoints. The model (Figure 2) below shows the proportion of ore (below the line) and dilution (above the line) as the extraction increases. It can be seen that very high extractions can be achieved depending on the shape of the curve and the grade of the ore and waste. If the waste rock is mineralized more ore can be removed without increasing the dilution factor.

Advantages

• Inexpensive method that yields a large amount of muck
• Highly mechanized process:
- in most cases the drifts and tunnels are sufficiently large enough to introduce large trackless mining equipment
• High efficiency:
- with the repetitive nature of this mining method you can standardize all the mining activities
• High amount of flexibility with production rates
• Because all of the mining activities are executed in or from relatively small openings, sublevel caving is one of the safest mining methods

 Disadvantages

• High level of dilution
• Low recovery
• Ore loss:
- when the extraction limit (point with maximum allowable dilution) is reached, the remaining diluted ore represents an ore loss
- Losses are larger as the inclination of the orebody and footwall is reduced
• Large amount of development required

Mine Development

Sublevel caving operations involve a large amount of development before the mine can go into production. The entire mine must be developed with the exception of the production headings on the lower sublevels. They will be developed after the mine has gone into production since the orebody is mined out from top to bottom.  

1. Production Shaft
2. Internal ramp
3. Sublevel extractions drifts
4. Production Headings
5. Orepasses
6. Main Haulage Level(s)
7. Mine Infrastructure

1. Production Shaft

• Usually Located in the footwall
• Sized based off of the production rate

2. Internal Ramp

• Developed in the footwall
• Grade of around 15% with flattening at sublevel access drifts
• Follows the dip of the orebody
• Allows equipment to travel freely from level to level without the use of the shaft
• Designed to be approximately 5 ft wider and higher than the largest piece of equipment

3. Sublevel Extraction Drifts

• Developed in footwall parallel to the strike of the orebody
• Designed to be approximately 5 ft wider and higher than the largest piece of equipment
• Usually developed every 100 ft

4. Production Headings

Drifts that run the width of the orebody from the sublevel extraction drifts in the footwall to the hangingwall.

Heading Width:
• Developed as wide as possible to get good draw coverage

Heading Spacing:
• Determined based off of the ellipsoid extraction width usually around 60% to 65% of the ellipsoid draw height
• Should be wide enough to provide some interactive draw between headings
• Should be no less than twice the heading width to limit development costs

Flat Back:
• Results in a level zone of moving material
• An arched back will cause more ore to be drawn from the middle of the stope resulting in more dilution from the caved waste rock

Heading Height:
• As low as possible to allow more room for loading blasts but high enough to allow equipment to enter

5. Orepasses

• Located every 100 ft to 200 ft in the sublevel extraction drift
• Used for transporting ore from the sublevels to the main haulage level
• Designed to have a dip of approximately 70° to 80°
• Sized based off of the production rate and orepass spacing
• Grizzlies and rock breakers should be installed at the top of each finger raise to prevent oversize from causing hang-ups

6. Main Haulage Level

• Track drift developed in the footwall parallel to orebody strike
• Used to tram ore from the orepasses to the crushing station and loading pocket for skipping to surface
• Sized based off of the size of the tramming equipment which is based off of the production rate for the mine

7. Mine Infrastructure

• Ventilation Raises – Sized based off of ventilation requirements( 100 CFM per brake horsepower)
• Equipment shops – Sized based off of equipment fleet
• Mine Dewatering system
• Pressurized air and water pipes
• Electrical System
• Refuge Stations