Two Rivers: moving with the times
Unforgiving underground mining conditions prompted management at Two Rivers Platinum mine to replace its traditional load haul dumpers fleet with hydrostatic machines, writes Leon Louw.
Two Rivers Platinum (TRP) mine officially started producing platinum in 2007. Trail mining got underway at the mine three years earlier and today, the mine churns out about 3.5 megatons of platinum group metals (PGM) per year. The mine comprises two decline shafts — the Main Decline and the North Decline — and a processing plant. The Main Decline is currently operating at about 190 000 tonnes a month and the North Decline at almost 110 000 tonnes a month. TRP mines platinum, palladium, rhodium, iridium, gold, and nickel, with chrome being the by-product of its waste. Platinum constitutes about 52% of the mine’s final product.
Even though TRP is located on the eastern limb of the Bushveld Igneous Complex, which is well known for its harsh underground mining environment, the operation consistently produces exemplary results. The mine, located near the town of Steelpoort in Mpumalanga, is partly owned by African Rainbow Minerals (ARM) (55%) and Impala Platinum (45%). The mine is regarded as one of the top underground platinum projects in South Africa. A sharp increase in chrome prices resulted in the mine’s attributable headline earnings increasing by 32% to R205-million for the six months ended 31 December 2016. Increased tonnages milled, combined with an improved plant recovery to 88%, resulted in PGM ounces increasing by 5%. Of the 1.75 million tonnes milled, 58 689 tonnes were toll treated at ARM’s Modikwa mine, close to Burgersfort, as part of TRP’ working capital reduction initiatives.
According to Kevin Reynders, managing director at equipment manufacturer Rham Equipment, the success at TRP can largely be attributed to the willingness of management — especially the engineering department — to engage with manufacturers and suppliers in developing modern technology. “The team at Two Rivers has always been technology-driven, and I would definitely say that this is one of the main reasons why they are so successful,” says Reynders.
TRP acquired two Rham HD20 load haul dumpers (LHD) for a trial run in 2010, after the original fleet of LHDs did not perform as expected. The Rham LHDs are diesel-driven hydrostatic machines, and TRP is in the process of converting its entire fleet to Rham equipment. It currently has 19 HD20s operating underground in a conventional bord-and-pillar mine. Although the HD20 is extremely efficient in difficult underground conditions, both Reynders and Andre Heydenrych, engineering leader at TRP, agree that the final product is still a work in progress. The HD20s at TRP are the result of ongoing research and improvements made by both the Rham team headed by Reynders, and the TRP engineering team, led by Heydenrych and Andre Crouse, general engineering superintendent at TRP. Through ongoing collaboration and a strong partnership between the equipment manufacturer and the mine, the LHD has morphed into a machine best adapted to the specific conditions at TRP. But, it wasn’t all smooth sailing since 2010.
“Our most serious challenge at the beginning with the original, low profile fleet was the exorbitant cost of drivetrains,” says Heydenrych. “Due to the drivetrain design on those LHDs, we were losing a lot of axles, so we started looking at alternative options,” he adds.
The entire engineering team believed hydrostatic-driven machines could be a solution, but at that stage, hydrostatic LHDs were not a common product. The concept was known for drill rigs and bolters, even for utility vehicles, but not much was known about hydrostatic drives or LHDs. In a hydrostatic drive, the motor drives a variable displacement pump that is hydraulically connected to a hydraulic motor driving the axle via a gearbox. The speed is controlled by changing the displacement volume of the axial pump. The power train consists of a closed loop hydraulic transmission, parking brakes, two-stage gear box, and drive lines.
When Rham presented the idea to Heydenrych and his team, the engineering leader admits they were apprehensive. “We were all very sceptical about the working principles, but our constraints at that stage forced us to consider all alternative suggestions and solutions,” says Heydenrych. That is when the mine agreed to give the first two machines a test run, and after its performance was acceptable in the unique underground conditions of TRP, the mine decided to purchase the first two machines. Although the results were not bad at that stage, according to Heydenrych, Crouse and the team felt that certain components needed enhancement. “The John Deere engine package used in the HD20 during those initial stages was not the ideal solution for the inhospitable conditions that the mining team encountered 530m underground, although it worked perfectly in other mines,” says Heydenrych.
As a result, Heydenrych, Crouse, Reynders, and all other stakeholders started looking at solutions and after numerous tests and research, decided to replace the John Deere with a Deutz 1013 engine, which improved the underground performance slightly. However, the Rham LHD still did not perform as well in TRP as it was doing at Impala Platinum’s number 14 shaft, were it was also deployed. Finally, after serious contemplation and more research, the Deutz engine was replaced by a Cummins QSB7. Since then, the HD20 has outperformed the rest of the loading fleet, both from a mean time to failure and mean time to repair perspective. Apart from the 19 Rhams, there are 10 other LHDs in operation at TRP.
Comparing apples with apples
“The big positive about TRP is that the mine uses a number of different equipment, so they have the true results — they can compare apples with apples. The mine can use two different machines in the same section and then compare the results,” says Reynders.
Meanwhile, the same machine with an air-cooled Deutz engine (TRP’s Deutz engines were water-cooled) is performing exceptionally well at Impala. This has a lot to do with the geological and vastly different mining conditions. Although the stoping width at both Impala and TRP is between 1.9m and 2.3m on average, the dip angles at Impala are extremely flat compared to those at TRP. In addition, Heydenrych is of the opinion that the footwall conditions at Impala are far better and flatter than at TRP. “We have a rolling reef ore body that the machine needs to negotiate,” says Heydenrych.
Any miner knows that it is essential to get the best cut and, as a result, the required grade through the processing plant. It is standard practice to follow the reef and, therefore, it is a requirement that the machine must be powerful, flexible, and robust enough to negotiate the reef. Hence, the further development work that TRP undertook to increase the horsepower and give the machine extra breakout force, extra loading ability and, from an efficiency point of view, to move from a five tonner to a seven tonner. “By increasing the loading capacity to seven tonnes, it enabled us to reduce the fleet composition from three machines in each half level to two, while still maintaining good production results,” says Heydenrych. All the new Rham HD20 LHDs are seven-tonne machines.
Rham and TRP are currently working on a new machine, which Reynders and Heydenrych refer to as an intelligent machine, where all data will be available on an android device and in a control room on surface, and which will be fully integrated and compliant with the new safety and collision avoidance regulations for underground mines.
Improved equipment efficiency
The new LHD, according to Heydenrych, has not made a significant difference in terms of time and the pace of mining. However, it has made a substantial impact on the overall equipment efficiency. The major difference has been the cost and the frequency of changing major components on the conventional machines versus the Rham hydrostatic equipment. When the conventional LHDs were still in operation, Heydenrych says the engineering team was changing components like transmissions, talk converters, and axles on a regular basis.
“We were changing axles on our LHDs every 800–1 000 hours and that was excessive. We tried everything, including ratio changes on the axles and on the transmissions to try to alleviate the problem. Although the team achieved some level of success, costs were running away with us at that stage,” explains Crouse. The bottom line is that the efficiencies of the original LHDs were extremely poor. In 2007, the average availability was 67% on the loading fleet at TRP. Today — six and a half years after the hydrostatic machines were introduced — the average availability is 93%, which is a massive improvement. That is well above the international benchmark, which is 85%. In general, according to Reynders, conventional drivetrain machines are available almost 88% of the time, whereas hydrostatic LHDs operate at an average of 94% availability. “A Rham hydrostatic machine has a power end — a diesel end — with a main hydraulic pump. The hydraulic pump drives four hydraulic motors on each of the wheel sets. A conventional machine will have a normal engine, talk convertor, transmission, and then a propshaft driven to the axles,” explains Reynders. The first Rham hydrostatic LHD was built in 1996.
Despite adverse mining conditions, a rolling ore body, undulating footwall, and variations on the dip angle, the machine has been robust and has performed admirably in this hard rock application. Compared to an opencast mine, where dump trucks will reach 45 000–60 000 hours, the Rham machine continuously loads ore for 15 000 hours before it needs to be refurbished. “Underground conditions are tough and abrasive, and there is constant contact with the hanging wall and with the front-end rear end belly section on the footwall. In addition, there is always stress on the articulation underground, which is not what you experience with surface equipment,” says Robert Alcaraz, owner of Rham.
Photos courtesy of ARM Two Rivers
Perfecting the mining cycle
The fleet composition in any particular section at TRP is one rig, one bolter, two LHDs, and two utility vehicles: one for materials and lubrication and the other for emulsion, explosives, and charging up practices. According to Heydenrych, synchronising the mine cycle is essential. The cycle includes a bolter that needs to support between two and three panels on a shift. The rig needs to drill and blast the two or three panels that were supported on the previous shift, and the LHD needs to load the two or three panels that were blasted by the previous shift. The material is taken from the face to the tipping position; there is a conveyor that transports the ore from the section to the conveyor through an ore pass. The tipping distance from the face to the tipping position is about 80–90m, which is maintained at all times. “For a mechanised mine this is key — we don’t have any rail-bound equipment, scraper winches, manual rock drill operators, or hand drill operators. We have to transport via a conveyor system onto an ore pass that holds a capacity of 400t search capacity onto a main belt. The main belt takes it through a main decline conveyor system to surface and there it is tipped into a silo, which then feeds into the plant. The rigs and bolters used at TRP are all standardised Sandvik Equipment, the drill rigs are all DD211s, and the bolters DS210Ls. The blasting is a centralised system supplied by Sasol, but TRP does all the blasting activities in-house,” says Heydenrych.
Expansions increase life of mine
The life of TRP was recently increased significantly with the addition of the Kalkfontein block. The expansion will now allow the mine to deepen its operation, which was not possible before. According to Heydenrych, the geology at TRP remains a challenge. He says the mine’s drilling practices allows management to anticipate changes in the geology or potential difficulties, which enables them to devise plans to accommodate such changes. “For example, the North Shaft is nearing a massive fault line, which shifts the reef horizon up with 40m, and then 200m further with another 30m. There are spirals and declines that we need to blast so that we can get to the table or block to continue mining. There are plans in place though, and we have already started development work,” says Heydenrych. Potholes and rolling reefs are further challenges that need to be addressed on a continuous basis.
According to Reynders, the main reason why the partnership between Rham and TRP has worked, and why the mine has been successful, is that there is collaboration. “The mine works together as a team; all of them take ownership and the mine’s structures work. Management can tell us exactly what each machine has done. The mine has traceability and data for every single aspect of the operation and that helps an OEM like Rham in a big way,” says Reynders. “This is the reason we originally deployed our prototypes at Two Rivers. The mine can give you any information that you need. The entire mine works together,” concludes Alcaraz.
Mining methods at TRP
Access to the underground workings is through a main conveyor decline system developed from surface, as well as a chairlift decline for human transport. The conveyor/service and chairlift declines have separate surface portal entries. Due to poor, highly weathered hanging wall, the first leg of the conveyor decline was developed on strike. On intersection with the UG2, the service decline branches off from the main decline’s first leg and it is developed on the plane of the UG2 on true dip (approximately eight degrees). The chairlift decline has been developed from surface at 34.5 degrees in the direction of true dip until intersection with the UG2. The chairlift decline is then developed on true dip on the plane of the UG2, parallel with the service decline. The conveyor decline is rectangular in cross-section (3m high and 6.5m wide), and has been developed in the centre and 20–25m below the two reef declines, namely the service and chairlift declines. The service and chairlift declines are 22m apart, measured from their respective centres.
Reef production at Two Rivers is through a fully mechanised room and pillar stoping method. A mining section consists of eight 12m rooms, with pillar sizes increasing with depth below surface. In the shallow areas of up to 100m below surface, the pillars are, for instance, 6m × 6m in size. These rooms are mined mainly on strike, except when re-establishing sections affected by geological disturbances (faults, dykes, and potholes).
Source: Mabuza, M. 2007. Two Rivers Platinum Mine: the orebody, the mining method — a perfect match. The Journal of the Southern African Institute of Mining and Metallurgy, volume 107, pp.43–50.