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This paper has been presented at IPMC ITB, Bandung, Indonesia.

PT. Agincourt Resources (PTAR) is a prominent mining enterprise in Indonesia, specializing in exploration and mining of precious metals, notably gold and silver. The primary operating site of the company is in the Martabe Gold Mine, situated in the Batangtoru village in North Sumatra, Indonesia. As of September 30, 2023, the Martabe mine boasts substantial mineral resources, comprising 6.2 million ounces of gold and 59 million ounces of silver. Initially operating at a capacity of 3-4 million tonnes in 2011, the Martabe Gold Mine underwent strategic plant optimization and upgrades. These enhancements have elevated the plant's current capacity to more than 7 million tonnes of ore per year. Consequently, the mine is now capable of producing over 200,000 ounces of gold and 1-2 million ounces of silver annually.

PTAR has undertaken numerous initiatives to continuously enhance operations, focusing on increasing throughput and achieving an optimal comminution product size. Key projects include upgrades to the Pulp Discharge System, the addition of a secondary crushing circuit in 2017, and the installation of a Vertimill® VTM 4500 in 2021. PTAR remains dedicated to exploring new strategies to optimize site efficiency and maximize production outcomes. The pulp discharge upgrade by Î÷¹ÏÊÓƵ highlights PTAR’s commitment to continuously improving operations and maintaining high standards of mining practice.

Since the first rubber SAG Discharger trial in 2015, PTAR and Î÷¹ÏÊÓƵ have been in close collaboration to improve the design. With a total of four major design changes have been implemented over the last eight years as illustrated in Figure 1 below, each of the design improvements has brought positive impact to the operation.

Conversion from steel to rubber discharger

Initial changes proposed in the first generation in 2015 aimed to minimize the number of liner components and reduce weight. The main objective of the trial was to upgrade fast-wearing feed end and shell liners without adding extra mill weight. Î÷¹ÏÊÓƵ achieved this by designing the new pulp discharger to be made of rubber, leveraging rubber's lower density compared to steel.

To significantly cut mill reline time, the new pulp discharger is designed in larger pieces to reduce the number of liner components. However, due to size limitations at the feed end trunnion opening, the largest Î÷¹ÏÊÓƵ rubber discharger piece weighs only 756 kg, below the onsite liner handler's 1.5-ton capacity. The decrease in discharger components from 64 to 42 pieces reduced the reline time by approximately nine hours.

The switch from steel to rubber discharger reduced weight by 56 tonnes, allocated to upgrading the fast-wearing feed end outer and shell lining. This liner upgrade cut reline frequency from five to three times per year, saving the operation 96 hours of reline-related downtime, equivalent to an additional revenue potential of USD 5 million annually.

Conversion from radial to curved outer pulp discharger

The central function of the discharge system in the mill is to allow an unrestricted flow of slurry and ground solids from the mill. The system consists of grates, discharge systems and a discharge cone. All selected sizes of these parts must be designed to work together in an optimal way that ensures all material is discharged from the mill. Insufficient flow of material due to the design of discharge systems and pan depth, narrow passages or an inadequate grate open area will limit the capacity. This typically causes problems like reduced throughput, back-flow and recirculation, which in turn can cause a higher wear rate, slurry pooling, increased maintenance and downtime.

The larger the mill diameter, the longer the distance the material must travel before being discharged into the trunnion. A curved discharge system design, directing material towards the mill center earlier, facilitates quicker pan emptying and refilling before the next pass. This expeditious material removal reduces backflow, optimizing energy for grinding fresh ore, enhancing wear life and potentially increasing throughput while reducing power consumption. When the radial discharge system was in use, a high wear area on the inner discharger occurred only on the leading surface of the vane. However, after conversion to curve discharge system, a new high wear area was observed to occur on the trailing side of the vane. This change in wear behavior is an indication that the SAG mill is discharging the material faster.

Ceramic lining to improve wear life

The first rubber pulp discharge system installed in Martabe SAG Mill in 2016 had all the wearing surfaces lined with rubber. Since the elimination of backflow, which extended the service life of the curved discharge system, the wear on the inner discharger became the driver for discharger set replacement. After collating enough wear data from the first two installations, design upgrade was incorporated to the third installation in 2017 with a ceramic layer added to the edge of the long vanes, where the most aggressive wear occurred.

Ceramic as a hard and high wear resistant material was added on the vane where the wear was mostly attributed to abrasion. The result of this ceramic application was an increase in the service life of the whole discharger set from 13.5 months to ~22 months. As a result, the total tonnes being processed have increased. Table 1 below shows the historical record of the tonnage achieved and the service life by the individual sets of the pulp discharger system installed in the Martabe SAG mill.

Ceramic upgrade to HD64

The ceramic panel used in the third generation of rubber discharger is the classic lightweight, medium duty panel, which has proven to be effective in providing extra wear life to the inner discharger. However, it was noticed that when the ceramic beads had lost more than 50% of their thickness due to wear, it tends to dislodge from the rubber vane. As a solution to this issue, the classic ceramic panel was replaced with a more heavy-duty HD64 ceramic panel that provides a thicker wearing surface and better resistance from ceramic dislodgment due to the T-shaped ceramic profile which gives interlocking effect.

Summary

With the close collaboration between PTAR and Î÷¹ÏÊÓƵ to continuously improve the SAG discharge system design, the operational aims were realised, with positive impacts being:

• A total of four major design changes have been conducted over the last eight years, each one improving the design.
• Annual plant availability has increased from 92% to 93%.
• The reduction in the number of discharger components from 64 to 42 has resulted in a reduction in pulp discharger reline time by approximately nine hours.
• The conversion from a Steel to a Rubber Discharger has resulted in a 56 tonnes weight reduction, which was then allocated to upgrade the fast-wearing Feed End Outer and Shell Lining. The result of this liner upgrade is a reduction of reline frequency from five to three times per year, saving the operation a total of 96 hours of reline related downtime, equalling additional revenue of USD 5 Million per year.
• The increase in the service life of the entire discharger set from 13.5 months to ~22 months has led to an increase in the total tonnes being processed.

Operational data

The conversion from steel radial to rubber radial discharger resulted in weight reduction. Conversion of rubber radial to curved design then further optimized and improved the efficiency of the SAG Mill. Based on the data provided during the SAG discharger installation period, the Martabe SAG circuit maintained a throughput typically ranging between 425 and 600 tph on both periods. The frequency distribution for power draw, mill weight and mill rate are shown in Figure 2. With a similar mill rate on both periods, the SAG power draw in steel radial period is typically varying between 5,500-6,250 kW, while in the rubber curved period was lower, typically varying between 5,000-5,750 kW. The mill weight also reduced from the steel radial period, varying between 250-300 tons, while in the rubber curved period varying between 225-275 tons. This shows that the SAG has been more effective in terms of specific energy to grind the ores, similar mill rate with lower power consumption.