[Johannesburg, 29 July 2016]: The Wirtgen Group, global leader in mobile machine solutions for road construction and rehabilitation, as well as mining and mineral processing, has recently conducted several large-scale field tests on material degradation. Findings confirm that Wirtgen surface miners offer significant advantages in the reduction of contamination experienced in normal open cast mining methods.

In the coal mining industry, the breakage of coal occurs throughout production, from extraction at the face to end use. While some of this breakage is intentional, such as during extraction and crushing, breakage occurring during transportation, stockpiling, sizing or washing is not desired. Generally speaking, breakage behaviour depends heavily on geology, but mining technology of today offers the option to reduce the amount of fines generated during production to maximize coal recovery for optimised operation as well as minimizing contamination of mined coal to increase yields.

Most coal mines measure the particle size distribution (PSD) of their plant feed to obtain information about the suitability of the feed for their processes, especially with regards to fines content. However, few mines know precisely where these fines come from and even fewer measure the fines content at the face to compare it with the plant feed data and to optimise the connecting processes.

According to Calvin Fennell, Wirtgen SA Business Development Manager, “There are a number of challenges associated with failing to optimise this connecting process. The cost of washing coal fines is higher because of the intensive processes used and the product losses that occur, all resulting in a lower rate of recovery. With increased losses, more tailings must be suitably disposed of and coal that does not meet the customer’s size requirements cannot be sold. Furthermore, coal mines have the tendency to retain moisture, which can cause problems in the downstream process.”

“On the other hand, optimising fines content and reducing contamination of the run-of-mine (ROM) coal offers numerous savings and benefits, such as a lower level of respirable and airborne dust, increasing workplace safety, reducing the risk of coal dust explosions and improving yields,” comments Fennell.

In an effort to serve its customers better, Wirtgen recently conducted a number of large-scale field tests on material degradation. The company found that its surface miners offer significant advantages when it comes to minimizing contamination by selectively mining coal seams to separate the ore from the waste.

Wirtgen surface miners

A Wirtgen surface miner is a crawler-mounted mining machine with a rotating cutting drum for rock penetration mounted at the centre of gravity, a configuration that ensures that the full weight of the miner machine can be transformed into rock penetration force. The cutting drum transfers the material onto a conveyor belt from which it is directly loaded onto a dump truck.

The machine mines layer by layer down to the required depth and the cutting depth can vary according to seam thickness. “Even thin seams of just 10 cm thick can be mined and precisely separated from the layers above and below. This level of precision makes for a cost-effective and more environmentally sensitive approach to mining of mineral deposits without any drilling or blasting,” Fennell notes.

PSD field tests

In its many field tests over the years on PSD and material degradation, coal and sedimentary ore from eight different pits were analysed to obtain their PSD. Close to 8000 tons of material was screened to compare the material produced by surface miners versus conventional mining methods.

In one coal mine, Wirtgen surface miners operated alongside dozers that employed the conventional rip-and-stack method and in view of the coal prices at the time, the mine was optimising every step of the operation. This included optimising the process for target size material, enabling most of the material to be processed in the cheaper coarse circuit of the processing plant. Reducing crushing costs was also an objective.

“Once again, the test proved the suitability of the Wirtgen surface miner for this type of operation,” says Fennell, explaining, “The machine delivered coal with a lower fines level and more target size material than the conventional dozer rip-and-stack method. In fact, out of the 1500 t/h of coal delivered from the dozer to the processing plant, 225 t/h were fines smaller than 2 mm, but with the surface miner, this figure is closer to 185 t/h. This means that 22% less coal had to be washed in the fines circuit and could be washed in the cheaper coarse circuit. In terms of fines smaller than 1 mm, by using the Wirtgen surface miner, the plant could process 33% less fine material.”

The Wirtgen surface miner delivers more than 70% target size coal (2-40 mm), while the dozer ranges at less than 58%. Savings are also enjoyed in the crushing stage, where only 17% of the material from the surface miner has to be crushed, as opposed to more than 26% when processing dozer coal, resulting in more fines that have to be processed.

In another trial location with sedimentary ore deposit mined using surface miners as well as the conventional drill-and-blast method, the surface miner was able to continually feed material with a < 1 mm fines level as low as 15% to the processing plant. The normal plant feed (includes material of drill-and-blast and surface miner operation) contained 25% and more fines < 1 mm.

Impact of rehandling

Rehandling contributes significantly to material degradation occurring during the mining process. Simulating this process, Wirtgen conducted a study to establish the amount of material degradation that occurs during rehandling.

For simulation purposes, 80 tons of material was loaded by a wheel loader and run through a screening plant several times. With every throughput, the amount of fine material increased significantly and during five test cycles in coal, the amount of material < 4 mm increased from 19% to 26%, representing an increase of 34%. A similar result was found with sedimentary ore in that the fine fraction increased by 24% during five test cycles.

“None of the steps in the simulation (loading, transport to the screen deck, sizing on the screen deck itself) involved the high material stress levels that would be induced, for example, by a dozer moving on stockpiled material,” comments Fennell. “Nevertheless, significant material degradation was measured during the simulation, leaving one to assume that the material degradation that takes place during stockpiling results in an even higher increase of the fine fractions,” he adds.

Keeping in mind that even this level of ‘soft’ rehandling causes significant material degradation, it is important to minimise the number of rehandling steps, each of which contribute to processing costs.

“This is where Wirtgen surface miners offer a two-fold advantage: Material mined with a surface miner has not been blasted, but cut out of the ground by the rotating cutting drum, which means that the level of fines is already low; and as the material is loaded for transport, rehandling is kept to a minimum, contributing further to a low level of fines. With a view of the complete mining operation, it is clear that a surface miner simplifies mining operations and reduces the number of process steps tremendously, resulting in immediate cost savings,” Fennell says.

The Wirtgen surface mining concept is no longer an unknown in the southern African mining sector. Surface mining has proven to be a viable alternative in South African open-cast mines, most especially in coal mines, where these machines really prove their worth. “Wirtgen SA is engaged in extensive talks with various mining companies about solutions for their greenfields and existing mines,” remarks, Fennell, concluding, “Wirtgen surface miners are set to revolutionise the open-cast mining industry in southern Africa, as it has done across the world, by addressing the need for safer, more environmentally friendly, effective and efficient mining by improving yields.”