Rubber/metal grates for gold mine

Paul Boughton

Faced with weekly emergency cleaning maintenance when its Metso 32’x32’ SAG mill suffered severe pegging problems, including up to 95 per cent of the passing area blocked by balls, a gold mine in Peru turned to Weir Minerals for help. In extreme cases, a torch was needed to burn the material caught in the grates.

The materials experts at Weir Minerals developed a solution that featured a Vulco rubber/metal grate and achieved approximately 60 per cent longer life than metal grates during field trials. Jointly owned by Newmont Mining Company, Buenaventura Group and the World Bank, the Gold Mill project at Yanacocha is the largest gold mine in South America.

With two plants, the operation extracts gold utilizing the Merrill Crowe process, using two refineries, three plants with carbon-in-column (CIC) processing and one concentrator plant to process the sulfide gold ore.
The Gold Mill project started in April 2008, with an estimated production of 14,000 TMD and its initial layout included: two grizzly and jaw crushers, one SAG mill 32’x32’, one cyclone feed pump (Warman MCRTM), one cyclone cluster with nine Cavex hydrocyclones, six leaching tanks and eight Warman AHPP high pressure multi-stage pumps.

The materials experts at Weir Minerals developed a solution that incorporated a Vulco rubber/metal grate. As part of the design, a metal core which was embedded into a rubber moulded body and reinforced with steel inserts for additional wear resistance. The Vulco rubber provided flexibility so the balls could pass through the slots unencumbered.  

In designing a solution, the Weir Minerals engineers faced several challenges. The new hybrid design needed the same overall dimensions so it could be installed on the pulp lifter vanes with the extremes of the grate plate remaining in cantilever.

An innovative metal core was designed by combining structural steel with high chrome steel; a mill model was developed and the design was submitted to a finite element analysis (FEA) to determine the mechanical integrity and structural strength it possessed.

Due to the composite grate design, along with the insert reinforcement existing in the central lifter bar, the fastening system was changed from bolting through the grate to a new system that screwed studs into the back of the grate. The nuts that support the studs on the back have adequate freedom so that the grate installation, with the studs in position, is accomplished without any risk of blockage or interference.

The prototype was then installed for a field trial. The design was reviewed four times during the trial. In the end, the new patent pending rubber/metal grate design achieved a final life of 235 days, approximately 60 per cent longer than the life of the metal grates.

Other benefits of the newly designed rubber/metal grate over the incumbent metal grate design include:

•  The new hybrid rubber/metal grates can be installed in 14 minutes, less than 50 per cent of the time required to install a metal grate
•  Lower energy costs because the rubber/metal grates are about 45 per cent lighter than metal
•  Increased throughput with unobstructed grate openings.  
•  Lower maintenance costs because no extra mill shutdowns are required to clean the grates, which take about nine hours per month.  
•  Improved operational safety because rubber does not 'impact weld' like metal during regular mill operation so the grates can be removed easier.  As a result, the grates are easier to remove when needed. 

While helping solve specific issues faced by Yanacocha Gold Mine in Peru, Weir Minerals developed an innovative solution that helped reduce a pegging issue while driving customer satisfaction. The new hybrid rubber/metal grates are now offered by Weir Minerals for other applications. 

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