Located near Indonesia’s highest peaks in the island’s interior and accessible via a twisting road with very steep gradients, Grasberg is an extreme example of how the search for resources calls for determination and ingenuity. Some of that ingenuity came from the teams work on a key part of Grasberg’s infrastructure.
The operators of Grasberg, PT Freeport Indonesia (PTFI), called on us to design, implement, and oversee the commissioning of a paste backfill plant at Grasberg’s Big Gossan mine. As a part of the Grasberg mining complex, the largest gold mine and the second-largest copper mine in the world, the Grasberg mine site offered only a small patch of level ground, which would be occupied by the mine’s mill. This meant that the paste plant would have to fit underground, inside the mountain.
The copper/gold ore body is being extracted by the blast hole stoping method, in which stopes are excavated to remove the ore. To safely mine the adjacent stopes, the mined stope must be refilled with paste backfill, made up of classified dewatered tailings, water and cement. The paste backfill cures to a strength that supports the rock overhead, so mining can continue. This means that having a regular, reliable supply of paste backfill is essential to continued production at Big Gossan — regardless of the challenging location.
Underground paste plant calls for thinking outside the box
Many above-ground paste plants are designed to fit inside a building on the surface, containing all the components of the plant. For this underground construction, the team determined that keeping everything together in one large excavation would not be feasible, based on the need to keep the excavated spaces compact yet functional, and to not over stress the rock overhead.
Accordingly, the paste plant at Big Gossan is divided into several parts. These are fitted into a series of underground cavities, including one for the series of three disc filters, separate vertical spaces for the paste batch mixing system and binder storage areas, and another horizontal space for the entry-way housing the paste pumps, electrical room and control room. The paste plant infrastructure includes a range of tunnel sizes, including vertical components and cut outs that link the whole system together.
It is a challenge for large vehicles to access the mine owing to the constrictive mine access tunnels and the steepness of the road leading to the mine. This meant a change in the usual way of delivering cement, an essential component of paste. Rather than using large bulk transport trailers with 40 tonne capacity, Freeport and us chose to rely on smaller isotainers carrying 25 tonnes of cement. This called for the operations to carefully schedule and manage traffic, to allow for a smooth supply of cement to the paste plant.
One of the benefits of the paste plant’s underground location is that it could be placed above most of the orebody. This allows the paste to be distributed primarily through a gravity feed, with little need for pumping. As the mine expands, it will become necessary to operate piston pumps to deliver paste to the stopes at the outer extents of the mine.
The tailings produced by the mill that feeds the Big Gossan paste plant tend to be finer than is optimal for paste. Tailings containing too high a percentage of small particles or fines require a prohibitively large volume of cement, in terms of cost, to create a paste that has the required structural rigidity and load-bearing capacity. To avoid excessive use of cement, and the associated cost, our design included a cyclone that separates out some of the fines in the tailings before the remaining tailings are used for processing.
The initial commissioning brought the plant online and operating at partial capacity for the first few years, meeting the needs of the mine at the time. The team later returned to support our client in bringing the plant up to its full design capacity of 300 tonnes of paste per hour.
Our team’s underground design for the paste plant allows the Big Gossan mine to operate efficiently and maintain the necessary stability despite the challenging mountainous terrain found in the remarkable Grasberg minerals district.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.