2008/04/01 

When Palabora Copper Mine’s open pit reached its final economic depth in 2002 after 38 years of operation, owners Rio Tinto undertook the development of an underground mine to work the ore remaining below the pit bottom. This $465 million initiative, which was planned to add up to 20 years to the life of the mine and significantly boost the regional economy, was made possible by technological improvements – not least the geo-stabilisation products and techniques of BASF Construction Chemicals SA.

“The shift to underground operations began in the last years of open-pit production,” explains Gavin Sims, site manager at Palabora for BASF Construction Chemicals SA, “and in 1999 we started supplying the mine with specialised concrete admixtures to assist in the stabilisation of the shafts and tunnels for the block cave operation – the highest HOD (height of draw) and the ‘hardest’ rock to be caved worldwide.”

The underground operation consisted of the installation of the main service shaft and 1,280m-deep production shaft. The first block cave is situated approximately 420m below the final pit bottom and the development consists of 33km of tunnels, four crusher stations and the ore handling infrastructure. All construction work is now completed. Normal rehabilitation (ie - support and repair work in the tunnels and on the roadways) is ongoing.

Explains Dave Wright, mining operations manager at Palabora: “Block cave mining requires that all of the mining of the tunnels is done ‘up front’. Ideally, for the following life of mine no re-support will be required and the mine can run unaffected. Normal mining operations with secondary blasting will take place and some bucket damage to the shotcrete is expected. In general, however, the shotcrete applied needs to stay in place and perform. To this end, BASF CC SA was involved from the start of the project to assist with their expertise.”

Timing was of the essence. Notes Wright: “Rio Tinto works to a very demanding safety strategy and this is stringently implemented at the underground mine at Palabora. The use of robotic machines to place and spray the shotcrete while full mining operations were ongoing required that the supply of cementitious products had to at times wait several hours for shaft time before being transported to the work place. Chemicals to retard the setting and then accelerate the cement curing were thus essential. Another requirement was that these materials be non caustic and useable by underground contractors without risk to their person.”

Shaft and tunnel stabilisation

The transition to underground production at Palabora saw output fall from 82 000 tons a day to less than 20 000 t/d in 2003. Problems were experienced with fragmentation in the block cave and the handling of oversize ore at breakpoints, yet a higher ore grade has assisted to right the imbalances. Today production has increased to an output of 34 000 t/d, meeting and overtaking projected targets. Production is dependant in large measure on the safety of the working environment.

Explains Wright: “The geology is a carbonatite ore body with dolerite dykes and some faulting present. BASF CC SA shotcrete mixes have been used extensively in creating the footprint of the mine, as well as in the area where all of the ore is drawn out of the drawpoints. Additive chemicals have also been used to stabilise segments of the shaft. BASF is now doing maintenance work where needed by operations.”

For the shotcrete, a mix of Meyco SA 160 (Alkali-Free Accelerator), Delvocrete Stabiliser, Polyheed T804 (superplasticiser), and the Meycofib HPP fibres were used. Meyco TCC 735, concrete improving admixture, was also used to improve the quality of the shotcrete, such as bonding, impermeability, i.e., enhanced durability.

Explains Sims: “The shotcrete is batched on the surface, transported underground in agitating vehicles (agicars) and sprayed onto the shaft and tunnel walls in 50mm and 100mm thicknesses for support. The shotcrete was also previously sent down the shaft via a slick line (an 8-inch steel pipe) to reduce the logistics load in the shaft.

Adds Wright: “Support in the form of 2.4m full column resin anchored rock bolts and cable anchors has been augmented in certain areas where abutment stresses exist with both fibre reinforced shotcrete and mesh reinforced shotcrete. Shotcrete has been used extensively in the underground where a high level of areal support is required, given the mines life and the exposure of persons. All the access ways from the shaft to the workings that transect the micaceus pyroxinites have been sprayed where the rock surface weathers and stability becomes an issue if left unsupported.”

Roads

A six-month road refurbishment and repair project has been initiated at the mine. Explains Sims: “With heavy duty machinery weighing approximately 40 tons each using the roadways around the mine, repairs and upgrades are essential every three to four years. The life of the roadway is also affected by the presence of water in the area, abrasion from vehicles driving on it and the rock fragments that fall from the 3200 buckets trammed every day by the operations.”

BASF CC SA products used on the road repairs include Rheobuild ESP 25, a water reducing agent providing high early strength with a water-cement ratio of .35, a superplasticiser and a multidose retarder. “The admixtures are customised to suit logistics, a slower curing ‘50mpa in 28 days’ mix being used for less critical roads and a ‘70mpa in 14 days’ mix used for those in regular use,” says Sims. “However, the road repair project has been ongoing for two to three months and it is early days yet,” he adds. “We are still busy strength testing mix designs to find the optimal cost/benefit ratio for the mine’s roadways.”

The 1km roadway has been divided into eight segments of varying lengths and the team is currently on section three. The project is expected to be completed by early 2007.

BASF CC SA competes in an open tender on the price/performance of their products. Says Visser Schade, service and rehabilitation superintendent at Palabora: “BASF staff are highly professional and go the extra mile to ensure projects are completed successfully. They maintain a good relationship with mine management and operations, ensuring we are informed and assisted whenever necessary. Their products, in particular on the road refurbishment project, have proven superior to competitive solutions. They meet the demands of an extreme environment effortlessly and have in fact delivered more than promised.”

Commitment to excellence

Dedicated BASF personnel are stationed at the mine and the company also undertakes remote support. BASF CC SA has its own infrastructure on site, including consignment stock and spare parts. A BASF onsite testing rig (Round Panel Rig) is stationed at Palabora to ensure conformance of shotcrete and other concrete mix designs to the required performance standards.

To advise the mine, BASF’s onsite personnel make use of the company’s considerable experience in mining environments around the world, as well as its in-depth knowledge of the chemical composition of construction materials. “Safety is critical in this dangerous environment and a high priority at Palabora,” says Sims. “We are available on a 24x7 basis to the mine and aim to solve any problems before crises arise. We do a lot of consulting, trouble shooting and fault finding and assist to optimise the design of the concrete mixes used for stabilisation in different parts of the mine.”

BASF CC SA also trains mine staff in the use of its products, especially the shotcrete, providing theoretical and practical implementation courses. A competency certificate is then awarded to successful attendees.

“This onsite service is testimony to both BASF CC SA’s commitment to its client’s success and our own efforts to ensure we match the highest standards,” says Sims. “We are well aware when dealing with concrete and chemical admixtures that the environment and aggregates used will influence outcomes. As meeting safety and production standards are critical in mining, we strive to always find means to improve on the minimum requirements where possible.”