Two Rivers Platinum Mine, located near the town of Steelpoort in Mpumalanga rates among the most successful platinum mines on the Eastern Limb of South Africa’s Bushveld Complex. Production from the mine’s underground operations has ramped up to well beyond the planned capacity of 225,000 tonnes per month (tpm) and the concentrator plant milled 230,000 tpm during the second quarter of 2008.
The mine has enjoyed an exceptional operating performance but still believes it can improve its current ounce output. To this end Two Rivers has identified key projects to optimise particularly the performance of the concentrator plant. A secondary crusher, planned for completion in April 2009, will reduce plant feed from -300 mm to -30 mm according to Nico Muller, the mine’s general manager.
“The secondary crusher will result in improved milling rates as well as notable cost reductions. We will be able to reduce the ball size in the primary mill and replace the mill’s steel liners with rubber steel compound liners. This will also reduce noise levels and create an improved operating environment. The installation of the secondary crusher will facilitate a future DMS plant as the mine expects increased dilution in latter years from its mechanised bord and pillar mining operations.”
Two Rivers also plans to improve current concentrator plant recoveries through enhancements to the cleaner flotation circuit. This will be achieved by August 2009. Two Rivers mines the UG2 reef, which consists of the UG2 as well as the UG2A leader that is separated by some 15 to 20 cm of pyroxenite waste. The grade distribution assimilates a typical binomial distribution with grade peaks towards the centre of the UG2 and particularly towards the bottom contact. This bottom loading combined with the undulating nature of the footwall contact necessitates the mining of an average of 25 cm of footwall pegmatoid to ensure optimum content recovery.
The operation accesses the UG2 orebody via two decline shaft systems, the Main Portal and the North Portal, which mine some 180,000 tpm and 60,000 tpm respectively. Mining costs are R340/t and Two Rivers produces platinum at a cost of R8,160/oz. Its total UG2 resource is 57.82 million tonnes at a grade of 4.67 g/t (6E) giving a total mineable reserve of 40.59 million tonnes at a grade of 4.13g/t (6E) subsequent to applying the various discount factors. At current mining rates the reserve equates to a life of mine of 14 years.
“The two metre mining width and orebody gradient of 😯 – 10O provides an ideal platform for mechanised mining operations,” Grinaker-LTA contract mining division director MI Botha says.
Mining contractor, Grinaker-LTA Mining Contracting has been involved with the Two Rivers project since inception. In 2002 Grinaker-LTA was commissioned to sink a 60 metre shaft from which a 600 tonne bulk sample was taken for metallurgical test work at Mintek. The contractor participated in the initial mine planning and design and commenced trial mining operations in 2004 to verify key financial and technical parameters used in the mine’s feasibility study. Official project release was approved in 2005 and the treatment plant was commissioned one month ahead of schedule in August 2006. The plant design capacity of 225,000 tpm was achieved in the second quarter of 2007. At that time the mine had developed a 1.2 million tonne stockpile, to account for the mining ramp-up that was still in progress. The R1.3 billion capital project closed off 10% under budget.
The total scope of work awarded to Grinaker-LTA since the inception of the project amounts to R800 million. “Our initial scope was focussed on the development of the Main Portal decline and the establishment of the capital footprint,” Botha says. “This included the ledging, conveyor systems, mining infrastructure, power, piping, pumping, chairlift, communications and equipping and the initial stoping.
“We then continued with the stoping work and today we operate six crews on the Main Portal, each of which produces approximately 20,000 tpm.” Overall, the mining contractor is responsible for around 135,000 tpm at the Main Portal with the balance being produced by Two Rivers teams. The Main Portal decline has advanced some 1.3 kilometres.
Grinaker-LTA Mining Contracting has subsequently also developed the North Portal, which began in April 2007. The contractor is in the process of completing the initial capital scope of work and is well advanced with the rampup from stoping operations. Current output at the North Portal is well above the planned rate of 60,000 tpm.
Two Rivers owns the mining fleet used across the six sections mined by Grinaker-LTA Mining Contracting at the Main Portal. This includes seven single boom Atlas Copco drill rigs, six Axera 126 LP Robolt roof-bolters and 16 LHDs. The five tonne capacity LHDs are a mix of Sandvik, GHH and Boart Longyear units, and Botha says the mine achieves a machine availability of about 85%. The LHDs emerge from underground daily, while the drill rigs and bolters come to surface once a week. The construction of an underground workshop commenced in August 2008.
The Grinaker-LTA contract has historically been renegotiated on an annual basis, but due to the success achieved to date both parties agreed to a two year agreement starting July 2008.
There have been a number of contributing factors to the success of the Two Rivers operation, according to mine management. “We have been blessed with a relatively homogenous orebody that is well suited to tried and tested mechanised mining methods,” Muller says. “We were also afforded the opportunity to test important mine design and financial parameters extensively during the initial trial mining campaign. A key added advantage from the trial mining campaign was the resultant ore stockpile created, which reduced ramp-up pressure during initial capital construction. This allowed for the installation of a high quality infrastructure backbone to support future mining operations.”
A salient feature of the infrastructure design at Two Rivers is the placement of the main dip conveyor in the footwall, as opposed to the industry norm of placing the conveyor on reef. The major advantage is that it decouples the strike and dip conveyor ore handling infrastructure, offering increased operational flexibility. The ore passes joining the strike and dip belts allow for a full shift’s production buffer. Furthermore, during the initial capital development phase, the footwall decline acted as an exploration platform from which drilling could be carried out to the reef plane to optimise the on reef excavations layouts.
In addition to the infrastructure elements designed into the mine from the outset, Grinaker – LTA Mining Contracting’s extensive experience in mechanised mining combined with its strong engineering capabilities in design and construction of key infrastructure has been a strategic asset. “The engagement of Grinaker- LTA Mining Contracting at the very start of the project and its direct involvement in the design stage enabled the mining contractor to gain full insight into the project requirements,” Muller says. “Its participation in the scoping and scheduling of the overall extraction strategy as well as the detailed mining activities was instrumental in completing the initial capital footprint on schedule.”
The mine has also benefited from a constructive relationship between its joint venture owners, African Rainbow Minerals (55%) and Impala Platinum (45%). Both shareholders had implemented previous projects in the Eastern Limb and offered invaluable experience and insights. “A major benefit to our operation has been the strong alignment between the two shareholders as well as the decentralised management approach adopted,” says Muller. “The empowerment of mine management through delegation of authority has promoted a strong attitude of ownership at operational level.
” The project has benefited from sound relationships between various key stakeholders. This was achieved through an inclusive approach in which contributions from all role players have been valued. A relatively low turnover in key positions, as well as a stable workforce, has resulted in the development of skills and experience as well as an increase in trust between stakeholders. The original mine management and contractor teams are still present, as is the NUM union structure elected during the project phase.
Although the mine was originally geared as a pure trackless operation, conventional mining methods subsequently have been introduced in areas where re-establishment through geologically affected areas is required. This strategy ensures optimal use of the capital intensive trackless fleet. With a well developed infrastructure and sound operational performance, Two Rivers is positioned favourably to make use of future expansion potential. Expansion options include a third UG2 decline, exploitation of the Merensky reef that is situated 140 metres above the UG2 on the Two Rivers property, as well as possible reserves across its current boundaries.