Block Cave Mining

Overview

Applications

Block-caving is a large scale production mining method applicable to low grade, massive orebodies with:

1. Large dimensions both vertically and horizontally.
2. A rock mass that behaves properly, breaking into manageable size blocks.
3. Surface which is allowed to subside.

These rather unique conditions limit block-caving applications to special mineral deposits. Looking at world-wide practise, we find block-caving used in iron ore, low grade copper and molybdenum mineralisations, and diamond-bearing kimberlite pipes. Large tonnage produced by each individual mine makes block-caving mines real heavy-weights when compared to other mines. Currently, the Philex Sto. Tomas II porphyry copper-gold mine, which appears very similar in nature to the Lubo-Boneng orebody has been developed successfully as a block cave mining property 40 aerial kilometers south of the Lubo-Boneng site.

Description

Block caving is based on gravity combined with internal rock stresses, to fracture and break the rock mass in pieces, which can be handled by miners. The drilling and blasting required for ore production is minimal, while development volume is massive. "Block" refers to the mining layout, which divides the orebody in large sections, or blocks, with areas of several thousand square meters.

Caving of the rock mass is induced by undercutting the block. The rock section underneath the block is fractured by blasting, which destroys its ability to support overlaying rock. Gravity forces, in the order of millions of tons, act on the block. Fractures spread to affect the whole block. Continued pressure breaks rock into smaller pieces, passing the drawpoints, where the ore is handled by LHD-loaders.

Development

Development for block-caving applying conventional gravity flow involve:

1. The undercut, where the rock mass underneath the block is fractured by longhole blasting.
2. Drawbells underneath the undercut, gathering rock into finger raises.
3. Finger raises, collecting rock from draw-bells to the grizzlies.
4. A grizzly level, where oversize blocks are caught, and treated.
5. A lower set of finger raises, channelling ore form grizzlies to chutes for loading.
6. The lowermost level prepared for haulage and chute loading.

The finger raises are arranged as branches of a tree, gathering ore from a large area at the undercut level, further channelling material to chutes at the haulage level.

Openings underneath the block are subject to high internal rock stresses. Drifts and other openings in the block caving mine are excavated with minimum cross sections. Still, heavy concrete lining and extensive rock bolting is necessary, to secure the integrity of mine drifts and drawpoint openings.

Production

After completion of the undercut, the rock mass above begins fracturing. The blocks are gathered by draw-bells/crates and funnelled down through finger raises. The intention is to maintain a steady draw from each block. Miners keeping records of volumes extracted from individual draw-points. Theoretical, no drilling and blasting is required for ore production. In practise, it is often necessary to assist the rock mass fracturing, by longhole drilling and blasting in wide spaced patterns.

Boulders which must be treated with drilling and blasting, is a frequent disturbance. Large blocks cause hang-ups in the cave, which are difficult and dangerous to tackle.

Ore handling

Original block-caving techniques rely up to 100 % on gravity, to deliver ore from the cave, into rail cars. The ore funnelled through a system of finger raises and ore passes, ending trough chutes at the main haulage level. As chute loading requires controlled fragmentation, rock has to pass the grizzly before entering the ore pass. The grizzly-man, with his sledgehammer, used to be the bottleneck in old-style block-caving mines. Today, hydraulic breakers are installed to treat boulders, and with LHD loaders assisting ore handling, dealing with oversize rock becomes more efficient. Today, block-caving mines have adapted trackless mining, using LHD-loaders to handle the cave in drawpoints. As a consequence a ventilation level is added to development preparations, to clear the production level from diesel exhaust. The LHD loaders are able to handle large size rock, oversize boulders blasted in drawpoints.

Summary

Block caving is an economical and efficient mass-mining method, where rock conditions are favourable. The behaviour of the rock mass, and conditions for caving are difficult to predict, when planning a block-caving mine. The extensive development required, and time lag before production start, are also factors to consider, when block-caving is compared to other methods.