Adding a new mobile screening option has improved efficiency and reduced cartage costs at Horokiwi Quarries Ltd in Wellington.
The company’s purchase of a new Metso ST272 mobile screening plant from Mimico has reduced double handling of waste material.
Quarry foreman Gary Clouston says by adding the mobile screen, the company needs one less dumptruck carting waste. “Before the ST272 arrived, we were carting rock 700m from the new block down to the primary plant, then carting the 65-down material we don’t want back up the hill to the waste area.
“The new block is producing a lot of fines, so there was a lot of double handling. The trip from the primary plant up to the waste area is a 15-20 minute round trip. By screening out the 65-down material near the new face and taking it straight to the waste area, we can take one truck off the job.”
Because there is no room to screen at the face, the Metso mobile screening plant is positioned 250m away, and 2000-3000 tonnes of rock is stockpiled there for a single operator to ‘walk through’ with a 45 tonne excavator and the Metso screen.
The Metso two-screen process produces three products: 600mm down to 110mm for the primary plant and some for sale as decorative rock, 110mm down to 65mm rock for sale or further processing, and 65mm-down to go to waste.
The company trialed two screening plants for the job, and settled on the Metso. It was the right size for the job, it had superior ground clearance for working on uneven ground, and the product belts were a lot higher, which allowed bigger stockpiles and more room for loaders to work. Mimico were also prepared to rent the machine to Horokiwi for a month, then take the rental cost off the purchase price.
“That made the difference, because we were able to make sure the machine would do everything we wanted before we decided to purchase.
“Mimico’s service staff made some adjustments to the plant to suit the material we are working with. Now the plant is pretty well set up the way we want it.”
The plant is processing 250-300 tonne an hour, but Gary says the operator can’t feed the plant flat out because the new rock is not as clean as other material at the quarry.
A 45 tonne excavator has been feeding the ST272 but Gary says a 35 tonner would probably be a perfect match for the brown rock being handled.
Horokiwi is one of Wellington’s two biggest quarries, producing a full range of roading and concrete chip, basecourse, sand, plus landscaping and decorative materials. The greywacke quarry has three fixed processing plants – primary, sand and basecourse.
Other Metso plant on site includes the Barmac primary crusher, an HP200 cone crusher and C100 jaw crusher.
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