Weba Chute Systems and Kwatani have come together to design and install ore silo chutes at a South Africa gold mine to reduce mill wear and other processing challenges caused by the uncontrolled flow of mined material into the mills.

The solution from Weba and Kwatani, a leading local manufacture of vibrating screens and feeders, must also deal with frequent large-size material as the mine has no crushing stage before the milling circuit, Weba said.

According to Weba Chute Systems Technical Advisor, Alec Bond, the over-feeding of material through the existing manually operated chutes is causing regular “mill vomit” in the mine’s four mills. The inconsistent feed exacerbates wear on mill bearings as the material’s weight shifts forwards and backwards inside the mill.

The waves of material causing the “vomit” carry insufficiently milled material out of the mill, including large chunks of rock. This leads to problems for the downstream mineral processing facilities, including inefficient recovery in flotation cells and even blockages in pumps, according to Bond.

“The challenge starts with the existing chutes needing constant supervision and control by operators, being opened and closed with a chain block device,” he says. “Our solution was to design a robust, self-controlling chute and feeder system that would ensure an even flow of material into the mills.”

He explained that the mine’s existing system has no means of closing the silo outlet; any maintenance at the chute area requires the emptying of the silo and the stoppage of the mill. Each of the four silos has three outlet chutes.

Weba Chute Systems Designer, Wesley Hunkin, says: “We therefore added a spile bar arrangement which seals off the silo. The Weba chute, which is choke fed, is placed under this installation. This allows the feed rate to be controlled by the Kwatani feeder, which has been integrated into the chute design.”

The vibrating action of the feeder controls the tonnage and feed rate to the mill, keeping the flow constant. New mounting structures have been designed to accommodate each chute and feeder. There will also be civils works below the silo to provide a solid foundation that absorbs vibrations from the feeder, according to the companies.

A serious challenge is over-sized rocks in the ore feed, which can be up to 800 mm in size. This makes it important for chute designs to accommodate the worst-case scenario of chutes choking, says Hunkin.

He highlighted that the flow of material is also controlled to prevent direct impact onto the conveyor belt feeding the mills, and to ensure central loading onto the centre of the belt.

“If the material from the feeder is biased to the one side, our chute brings everything to the centre of the conveyor,” he says. “This enhances the consistency of material flow into the mill.”

Bond emphasised that the customer motivated for a concept change to address the challenges being experienced with the silo feed.

“Given our materials handling experience, design expertise and high-quality local manufacturing facility, we were able to work closely with the customer and with Kwatani to turn this new concept into reality,” he said.

“Our solution promises direct savings in terms of mill bearings, as well as less mill downtime. There will also be significant gains in terms of recovery rates in the plant if the flow and size of milled material can be improved.”