Rocking around the clock - platinum mining in South Africa

Materials World magazine
,
5 Feb 2012
Stockpile, feed silo and conveyors at Mogalakwena North platinum mine

Processing platinum ore into metallic powder is a highly complex task. It requires a huge amount of machinery and energy, and efficiency improvements can result in significant cost savings. Tim Probert visited the recently commissioned Mogalakwena North platinum mine in South Africa to find out how Anglo American has improved output at the largest single stream platinum concentrator in the world.

Platreef ore is tough stuff – hard and variable. If it was not the largest source of platinum group metals (PGM) in the world, it would perhaps be better left alone. The Platreef is part of northern South Africa’s Bushveld Complex, which also contains the Merensky Reef and the Upper Group 2 Reef. Unlike the other reefs, which are narrow, usually less than one metre thick and mined underground, open-pit methods are used to mine the Platreef, which varies between five and 90 metres in thickness.

Anglo Platinum has been mining platinum at Mogalakwena, formerly named Potgietersrust, since 1993. Mining Platreef platinum ore at Mogalakwena, 200 miles north of Johannesburg, is a piece of cake. Daily blasts at the open-cast mine break open the Platreef to extract the ore. Then the hard work of processing this metres-thick rock into millimetresthin metallic powder begins.

The concentration of platinum, or head grade, in Platreef ore is significantly lower than other South African reefs. It varies between 2.2 and 3.5 grammes per tonne, compared to the five grammes per tonne typical of the Marensky Reef near Rustenburg. Based on a typical conversion rate of 20%, it requires as much as 40 tonnes of Platreef ore to produce just 28 grammes of platinum.

New pit and concentrator

In 2006, with the original Sandsloot pit approaching the end of its life, Anglo American, owners of Anglo Platinum, decided to invest in a new pit and concentrator, named Mogalakwena North. Anglo Platinum designed the concentrator to be the world’s largest single stream platinum concentrator, with an ore processing capacity of 600,000 tonnes per month.

In order to achieve such a high capacity with a high-risk, single stream plant – one where all the ore undergoes primary milling and then secondary milling in sequence – Anglo Platinum needed some ground-breaking technology. Having suffered throughput problems due to the extreme hardness and variable quality of Platreef ore, they explored methods to improve the platinum recovery rate and operational efficiency with the new facility at Mogalakwena North.

Ultimately, the company decided against the traditional four-stage crushing process used at its other concentrators and instead took the decision to replace the third and fourth crushing stages with a high-pressure grinding roll (HPGR) crusher. Usually the preserve of copper mining, this is the first time an HPGR crusher has been used in platinum mining.

Anglo Platinum claims several other firsts for Mogalakwena North, which was commissioned in 2009. The plant is running 900–1,000 tonnes of ore per hour into the mill, a world best for platinum, according to section engineering manager Natalie Fourie. Mogalakwena North also has the biggest primary gyratory crusher in the world, weighing 480 tonnes with an 18 metre diameter and 1MW motor.

The concentrator also sees the first use by Anglo Platinum of gearless mill drives (GMD), in this instance made by Swiss engineering firm ABB. The drives are powered by a 17.5MW motor, five times more powerful than a similarly-sized throughput mill, says Fourie.

At a diameter of eight metres Mogalakwena North’s GMDs were the largest installed in the world, but they have since been superseded by a 12-metre diameter drive in Australia. Mogalakwena North also has the biggest single stream centrifugal blower installation in Africa and the biggest mill discharge pumps in South Africa.

Concentrating process

The freshly blasted rock is loaded by gigantic hydraulic shovels, again the world’s largest, onto trucks for transport to the primary crusher. All material tipped directly from the trucks into the primary crusher has to be smaller than one square metre. Material from the primary crusher goes through secondary crushing until it is less than 65 millimetres thick.

From there the ore goes through tertiary crushing by the HPGR crusher, supplied by ThyssenKrupp Polysius. Unlike normal jaw crushers that strike the rock or cone crushers that rotate, HPGRs use two 100-tonne rolls covered with studs 25mm in diameter and 35mm in length.

The rolls, each powered by a 2.8MW motor, turn at 20rpm, with one fixed in position while the other moves horizontally to adjust the gap. The crushing force is exerted hydraulically on the moving roll, with pressurised nitrogen acting as a spring. The initial gap is set to accept the largest particle size in the feed and thereafter the pressure is adjusted hydraulically to maintain interparticle crushing in the area between the rolls.

Fourie says the HPGR is working extremely well. ‘It gives a very fine product that gives us a lot more flexibility in milling,’ she explains. ‘A normal tertiary crusher would not be able to reduce the size of the ore to just eight millimetres.’

Fourie says the novel usage of an HPGR crusher for platinum concentrating has not been without problems. ‘The HPGR is a highly sophisticated machine that has a great deal of interlocks. When it decides not to play nicely, I have sleepless nights. If the rolls are not exactly parallel or the pressures are not exactly equal, the machine will simply refuse to start up.’

Due to various problems at Mogolakwena North, including frequent ore conveyor belt breakdowns, problems with the GMDs and HPGR crusher, it has taken Anglo Platinum nearly three years to achieve the plant’s stated throughput capacity of 600,000 tonnes per month.

‘Few engineers contracted to work with Amplats have experience of GMDs or HPGRs. But if I have a problem with a conveyor belt, I can call 20 people. If we have a problem with an HPGR, I have to get hold of the original equipment manufacturer (OEM). As this is the first utilisation of HPGRs with hard rock mining, the OEM is also going through a learning process. It’s a lesson learned for the whole of Anglo American. We now get visits from Anglo American engineers from around the world to learn how to use an HPGR.’

From the HPGR crusher, the platinum slurry is fed to the GMD, in which steel balls grind the material. The primary milling grind is rated at 55% at <75 microns – the secondary grind is rated at 80% at <75 microns. Grinding the material in this way exposes the platinum and other precious metals so they can react with the reagents in the flotation chamber and disperse into individual materials.

Fourie says the GMD, used for the first time by Anglo Platinum, has been a success. ‘The flexibility cannot be underestimated,’ she says. ‘As it has fewer mechanical moving parts, the mill can be slowed down and sped up like a dimmer switch. It’s proven to be more reliable than standalone motors.’

Again, however, using novel technology has not been without problems. ‘At the whiff of moisture, the motor trips to avoid catastrophic failure,’ says Fourie. ‘We’ve had to make modifications to the outside of the GMD in order to enable exterior washing and reduce the likelihood of slurry clogging.’

After milling, the slurry is then placed in flotation cells for separating, while the waste material falls into a trough, ready for disposal.The valuable concentrate is thickened and then filtered at high pressure to remove water. Before being transported to Anglo Platinum’s smelter in Polokwane, 64 kilometres away, the fine powder is finally put through an IsaMill that grinds the material to less than 75 microns. By now the powder has a concentration of 60 grammes per tonne, compared to the three grammes per tonne contained in the freshly-blasted ore.

Mogalakwena North produces 11,000–12,000 ounces of platinum each month. Platinum accounts for around 50% of Mogalakwena North’s total output, with palladium accounting for 40% and all other minerals, including gold, copper, rhodium, ruthenium, iridium, nickel and cobalt the final 10%.

Power supply problems

It is estimated the HPGR provides Anglo Platinum with an energy saving of 15–20% versus four-stage conventional crushing. When Mogalakwena North alone consumes a colossal 33,000MWh of electricity per month, this is no small amount.

Fourie says the mine’s power supplies can be highly unstable. South Africa’s state power utility, Eskom, is contracted to supply 11kV, but this can occasionally drop to 10.8kV or increase to 11.2kV. As concentrators become ever more highly automated, the plant’s equipment is sensitive to fluctuations in power voltage and more likely to trip.

Mogalakwena North suffered six to eight trips per month until voltage ride-through technology was installed. This allows the GMDs, which are particularly sensitive to changes in power quality, to keep rotating until they catch up with the power supply. Some are unavoidable when the voltage dips too low for the concentrator to keep operating, says Fourie, but the site now suffers just two trips per month on average.

In 2008, South Africa was struck by a near two-week blackout, affecting platinum production at Mogalakwena for several days. Anglo Platinum, which operates 11 mines and nine concentrators in South Africa, had to shut down a number of concentrators in order to give priority to its smelters, which are not easily shut down and restarted. Since 2008 blackouts have not occurred, but the company continues to hold weekly meetings with Eskom to discuss potential power supply problems.

Anglo Platinum has a contract that requires Eskom to give notice of power outages that may affect platinum production, with financial penalties for failure. Should Eskom reduce Anglo Platinum’s power to 75% of load or lower, it must choose whether to reduce capacity at its concentrators or shut operations completely at designated units. However, because Mogalakwena is an open-cast mine and not as energy-intensive as underground mining, unlike others it is able to keep running through power outages.

Also in place is a rolling five-year infrastructure and electricity plan between Anglo American and Eskom, which sets out future power demand. The miner has to keep within 10% of the agreed demand and so far, says Fourie, the two companies have been aligned in terms of power supply and demand.

Rising input costs

Eskom is to increase electricity prices by 27% in 2012, having imposed a 25% hike the previous year. After signing an unfavourable deal with BHP Billiton, Eskom is wary of entering into long-term power contracts and Anglo Platinum will be subject to Eskom’s programme of significant price rises in the coming years.

Steel costs have also risen 17% year on year, but Fourie says Anglo Platinum will endeavour to stay on a flat unit cost for three years, so it is under considerable pressure to cut costs in other areas. Yet the input cost rises are making Anglo Platinum more efficient, she says. ‘You would think it would be impossible to cope with these increases, but we are managing. We have streamlined our buying to a just-in-time process to reduce warehousing and increased our maintenance intervals where possible in order to reduce contracting costs. We have also reduced the volume of reagents used in the flotation process.’

Anglo Platinum plans to produce platinum at the site for at least another 60 years. Eventually the mine’s three pits will all join up. Once this happens (scheduled for 2020) Mogalakwena will be the largest man-made excavation in the world. Mogalakwena appears to be the jewel in Anglo Platinum’s crown, despite the hardness of Platreef ore.