Rio Tinto | Minerals and Metals for the world

Grasberg is scheduled to become an underground mine in about ten years' time and studies are in progress at Bingham Canyon to compare the feasibility of an underground operation below the existing open pit with the alternative of an even larger open pit.

As John O'Reilly, Rio Tinto's head of Technology, points out: "Projections based on current reserves show that in about 15 years' time Rio Tinto could be producing more copper from underground mines than from open pit mines."

Wherever the geology is favourable, Rio Tinto employs a particularly efficient underground mining method called block caving. Essentially it involves undercutting a huge block of ore so that it collapses progressively into a pre-prepared system of funnels called drawbells. Broken ore flows through the drawbells to loading points (drawpoints) in a network of horizontal tunnels that is connected to an ore handling system. In modern mines the broken ore is retrieved and transported by load haul dump units (LHDs). It will usually go through underground crushers before being transported to the surface.

To give an idea of scale, Palabora plans to produce 30,000 tonnes of ore a day by this method. The first undercut - deeper ones may follow - is 480m below the bottom of the depleted open pit and some 30,000m2 in area. Above it, and now caving from the bottom upwards, are about 240 million tonnes of ore. That astonishing figure reflects the size of the Palabora orebody, which has already yielded 900 million tonnes of ore through the open pit. It represents about 20 years of ore supply to the mine's treatment plant.

Block caving harnesses two of nature's free gifts - the force of gravity and stresses inherent in Earth's crust. It can be used on any orebody that is sufficiently massive and fractured, provided that the ore can be induced to cave and fragment in useful and predictable ways. It has been applied to large scale extraction of various metals and minerals, sometimes in thick beds of ore but more usually in steep to vertical masses.



Grasberg is scheduled to become an underground mine in about ten years' time and studies are in progress at Bingham Canyon to compare the feasibility of an underground operation below the existing open pit with the alternative of an even larger open pit. As John O'Reilly, Rio Tinto's head of Technology, points out: "Projections based on current reserves show that in about 15 years' time Rio Tinto could be producing more copper from underground mines than from open pit mines." Wherever the geology is favourable, Rio Tinto employs a particularly efficient underground mining method called block caving. Essentially it involves undercutting a huge block of ore so that it collapses progressively into a pre-prepared system of funnels called drawbells. Broken ore flows through the drawbells to loading points (drawpoints) in a network of horizontal tunnels that is connected to an ore handling system. In modern mines the broken ore is retrieved and transported by load haul dump units (LHDs). It will usually go through underground crushers before being transported to the surface. To give an idea of scale, Palabora plans to produce 30,000 tonnes of ore a day by this method. The first undercut - deeper ones may follow - is 480m below the bottom of the depleted open pit and some 30,000m2 in area. Above it, and now caving from the bottom upwards, are about 240 million tonnes of ore. That astonishing figure reflects the size of the Palabora orebody, which has already yielded 900 million tonnes of ore through the open pit. It represents about 20 years of ore supply to the mine's treatment plant. Block caving harnesses two of nature's free gifts - the force of gravity and stresses inherent in Earth's crust. It can be used on any orebody that is sufficiently massive and fractured, provided that the ore can be induced to cave and fragment in useful and predictable ways. It has been applied to large scale extraction of various metals and minerals, sometimes in thick beds of ore but more usually in steep to vertical masses.
PAGE 5 OF 8

Review is published by Rio Tinto, 6 St James' Square, London SW1Y 4LD, England Telephone +44 (0)20 7930 2399 Editor: Cherry DeGeer