Freeport McMoRan Copper & Gold recently started operation of an overburden handling system from Krupp Fördertechnik GmbH, Essen, at its Grasberg opencast mine in Irian Jaya, Indonesia. The system includes one of the world's two biggest gyratory crushers and a transport crawler with a load-bearing capacity 50 percent higher than any previously known crawler. Krupp Fördertechnik's engineers designed a number of innovations into the system to adapt it to the geographical location of the mine and the on-site conditions.

Situated around three degrees south of the equator, the Grasberg mine has some of the world's largest gold and copper deposits. The ratio of overburden to usable minerals is three to one. With overburden volumes growing in line with increasing mining depths, in 1997 Freeport decided to install a continuous handling system, comprising a semi-mobile crushing plant with a 63-114 gyratory crusher, three belt conveyors, a semi-mobile stacker/spreader and a T1250 transport crawler.

The gyratory crusher reduces overburden to conveyable size at a rate of up to 10,000 metric tons per hour. It is of similar design to a Krupp Fördertechnik crusher which Freeport already put into operation in 1998. Together, these two units are the biggest crushers of their type in the world.

The 160 meter long stacker dumps the overburden to a depth of 400 meters from its 127 meter long boom. Dumping from this height means that the stacker can be operated for a relatively long time from the same position without having to be slewed or relocated, a fact which prompted Freeport to decide in favor of a semi-mobile system without its own travel and slewing mechanism. The stacker superstructure is supported on a gantry. When required, the transport crawler moves beneath the gantry and moves the reclaimer to a new position.

This semi-mobile solution provides considerable cost benefits to the customer: as the weight of the stacker must not exceed the load-bearing capacity of the crawler, Krupp Fördertechnik engineered the entire boom support structure from lightweight tubes braced by horizontal and vertical ropes. The overall weight saving achieved in this way versus conventional systems was more than 50 percent. Also, the stacker was lower in price and costs less to run and service.

Despite its load-bearing capacity of 1,250 metric tons, the transport crawler is only 180 millimeters higher than the previous biggest crawler which had a capacity of 850 tons. The installed engine capacity and chain transmission are around twice as high as on an 850 ton crawler. Among other reasons, the design had to be kept compact because the crawler is also to be used to reposition existing older crushers, making it necessary to adapt the outer shape to the systems already in use in the mine.

The fact that the Grasberg mine is located at around 4,000 meters above sea level represented a further challenge to Fördertechnik's engineers: to allow transportation through narrow tunnels on the way to the site, they divided the crusher ring segments into half shells and designed the steel structure for the lattice girder boom of the stacker to split both lengthways and crossways into segments. The platform of the transport crawler was also delivered to the mine in segments and the 60 millimeter thick plates then welded together under difficult on-site conditions at 4,000 meters.