Block caving is an underground mining method that permits the bulk mining of large, relatively low-grade bodies of ore. As a sublevel mining process, block caving involves the controlled collapse of ore from under its own weight into chutes or draw points using gravity.
In terms of cost and rate of production, bulk mining of this kind yields a substantial return per ton and has a higher rate of production than other underground methods. In order for a block caving operations to be successful, the rock mass must be able to fracture naturally once an undercut has been applied.[1] In addition, enough horizontal room must be provided for the ore body to cave in freely without compromising the strength of the surrounding waste rock, as separating ore from fallen waste rock or muck is not economically feasible.[2] Block caving is commonly used in the large-scale extraction of metals and minerals situated in thick ore beds in a steep or vertical rock mass. It may also be used in converting an open-pit mine operation to an underground mine operation.
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[edit] Process
Block caving works in a manner similar to the way sand passes through an hourglass. The first step involves establishing an undercut in the caved area in addition to a number of draw points, collections points, or tunnels underneath the ore body to be mined. These cuts essentially weaken the ore body, causing it to “cave” in and then be drawn off or funneled through draw points.[3] Simply put, what block caving really involves is undermining the ore body.
A number of factors go into making the first undercut so that caving can commence. Enough space must be provided at the bottom of the ore for the overhanging rock mass to become unstable and fracture. The initial cutting process is known as “belling” out the ore and involves making a number of closely spaced sequential crosscuts just below the actual ore zone.[4]
To start, a series of blocks 197 to 492 feet (60 to 150 m) are made on a side in all three directions. Haulage tunnels are then dug out underneath the ore. At intervals of 98 feet (30 m), finger raises from 10 to 20 feet (3 to 6 m) in diameter connect the haulage tunnel to another series of crosscuts. Through these crosscuts, motor scrapers and Load haul dump trucks (LHDs) transport the ore back to a central haulage level. The cuts end up resembling a checkerboard pattern.[5]
Ore falls through the finger raises intersecting with the cross drifts below. The raises are driven up into the ore in multiple directions sometimes with four or more raises intersecting at any one point in a cross drift. Theses raises serve two purposes. Firstly, they provide ample space to blast out the entire underside of the ore block. Secondly, they later function as a funnel that draws in the blasted ore. Gravity works on the blasted ore, which continues to fall off from the underside of the ore body. It then gets pulled down and funneled through the raises.[6] The broken off ore is collected by wheel loaders, motor scrapers, or LHDs and crushed in an underground crusher before being conveyed up through a shaft to the surface for further processing.[7] The actual speed at which the rock falls and funnels through the raise depends on how fast rock is removed from the tunnel.[8] This is why control of the “caving line” becomes critical in a block caving operation.
[edit] Pros and Cons
One of the marks left behind from a block caving operation is a depression in the surface. Though not very deep due to being partly filled with waste rock, the depression results in surface subsidence and ground settling. The surface area vulnerable to settling is also usually larger in circumference than the actual mined cave block.[9] Other disadvantages include development costs in preparing the cave block for production, improper maintenance of the cave line, and cost of managing and sometimes reinforcing the draw points through which ore and waste rock are funneled. On the contrary, block caving is a technique gaining popularity. It is currently being considered for many operations worldwide, is hailed as cost-effective, and has a fairly solid safety standing.[10]
[edit] Equipment Used
[edit] References
- ↑ Block Caving. Mining Lift.2008-10-07.
- ↑ Block Caving. AMEBC. 2008-10-07.
- ↑ Block Caving. AMEBC. 2008-10-07.
- ↑ Block Caving. AMEBC> 2008-10-07.
- ↑ Block Caving Description. Mining Life. 2008-10-07.
- ↑ Block Caving. Mining Life. 2008-10-07.
- ↑ Block Cave Mining. Resolution Copper. 2008-10-07.
- ↑ Block Cave Mining. Resolution Copper. 2008-10-07.
- ↑ Block Cave Mining. Resolution Copper. 2008-10-07.
- ↑ Block Caving. AMEBC. 2008-10-07.
