For decades, yellow metal earthmoving machines have been the trusted workhorses on mining sites.
However, GRAHAM MITCHELL-INNES of African Mining and Crushing believes that choosing in-pit crushers and conveyors as opposed to earthmoving trucks in open pit mines gives companies a competitive edge. GERARD PETER reports.
First off, Mitchell-Innes states that he doesn’t believe that conveyors are a solution for every mine, stating however that conveyors could be a viable cost solution for most mines.
“When it comes to OpEx and CapEx costs, conveyors come in considerably lower than yellow metal,” he explains.
He cites that installed power on a haul truck is significantly higher than a conveyor as a case in point.
“While it’s true that you have to carry a conveyor, remember, you also have to carry a truck. Now if for example, a truck with Gross Vehicle Mass of 250t only has a 140 t payload and the truck itself weighs 110 t, you need a fair amount of power to drive its own mass, notwithstanding the amount of fuel that is needed to power such a vehicle, he states.
Furthermore, the longevity of a conveyor, for the most past, will outlast that of a truck, according to Mitchell-Innes.
“For example, a large haul truck will last about 60 000 hours which is about seven to nine years, assuming it has been correctly maintained. Plus, you have to factor in rebuild during that lifespan.
"A typical certified rebuild will set you back about 70% the amount as a new vehicle. On the other hand, a conveyor can run for about 20 years with far less costly maintenance requirements.”
Building the argument for conveyors
Regarding OpEx, Mitchell-Innes adds that less manpower is required to operate a conveyor.
What’s more, having less trucks on a mine also improves the safety aspect of its operations, potentially reducing accidents, fatalities and injuries. In addition, using conveyors uses less fuel and this reduces the impact on the environment.
When drawing direct comparisons between the two modes of moving ore from the pit, ramp grade is an important factor to consider. Mitchell-Innes explains:
“Most trucks can operate in pits ranging between 8% and 10% angle declines. Conveyors however can move ore comfortable at angles of up to 28% which offers enormous benefits to well established pits that are a few hundred metres in depth.
"Also, haul trucks need a ramp length of 1 000 m if a pit is 100 m deep while a conveyor only needs a ramp length of 386 m for a pit of the same depth.”
Meanwhile, those pundits who advocate the use of yellow metal argue that downtime is kept to a minimum when using this mining methodology. For example, if one haul truck breaks down, there are other vehicles still in operation, whereas if a conveyor breaks down, all production stops.
However, Mitchell-Innes believes that there are a number of ways to mitigate a complete stop in production.
“For example, if you are running a 3 000 tph conveyor belt, you can reduce this risk by investing 50% more in your CapEx and operating two 1 500 tph conveyors.
"This 50% increase is still about 50% less than the yellow metal CAPEX. You could also add intermediate stock piles (ISPs) to create redundancy. ” he says.
However, in order for conveyors to work effectively, you need to have in-pit crushing in order to crush material to a size that can be conveyed. “You can’t really convey a -800 mm rock,” explains Mitchell-Innes.
“As such, you need a mobile in-pit crushing system to complete the process. All of this does cost money, however it still comes in substantially lower than investing in yellow metal.”
That said, pit conveyors also require special design for each operation which translates into an expense. In addition, they do also require maintenance.
However, Mitchell-Innes points out that maintenance is mainly required on the rollers – the conveyor belt itself requires minimal maintenance.
This is because a belt only wears where material is transferred onto the conveyor. In the case of a long conveyor, he points out, this means that new material only touches it minimal times per hour, so there is little wear on the belt.
So, which mines are best suited to use conveyor belts? According to Mitchell-Innes, it all depends on the design, life and geology of the mine.
Of course, the big question remains: Will conveyors supersede yellow metal?
Mitchell-Innes believes that mines will move towards having a hybrid of the two solutions.
“We will use haul trucks for flexibility around the pits, however for a standard haul, in-pit crushing and conveying makes a lot of financial sense,” he explains.
“This also translates into cost savings such as longer lifespan of yellow metal because there is less wear and tear because they no longer have to run out fully loaded on ramp inclines.
As such, I don’t believe that yellow metal will be completely replaced, but by using conveyors, mines can save on CapEx and become more efficient,” he concludes.