The Agnes Mine GEOCOAT ® Project

The Agnes orebodies were first exploited in 1893. The present owners and operators, African Pioneer Mining (APM) (a subsidiary of Metallon Resources PLC), acquired the property in 2002 from Cluff Mining PLC (Cluff). The refractory sulphide ore is mined by underground methods and a conventional milling and flotation plant produces a gold bearing sulphide concentrate. In 2000, Cluff signed a license agreement with GeoBiotics for the use of the GEOCOAT® heap biooxidation process for treatment of the concentrate. Column biooxidation testwork conducted at SGS Lakefield Africa Pty. confirmed the amenability of the concentrate to biooxidation, and APM prepared a feasibility study based on the GEOCOAT® process. Design and construction followed, with commissioning starting in the first quarter of 2003. The following table provides the general design criteria for the GEOCOAT® facility at the Agnes Mine.

A double liner system of synthetic geomembranes ensures that the process solution is fully contained. Figure 1 is a general view of the pad during liner installation. A leak detection system is installed between the liners to give an early indication of potential solution loss and allow remedial action before any discharge to the environment occurs.

Figure 1. Lining System At Agnes – HDPE 1.5 and 2.0 mm

Figure 2. Air Supply Fan and Distribution System

Three low-pressure fans are installed at intervals along the edge of the heap and provide process air via a system of HDPE headers, subheaders, and perforated stringers. The stringers are buried in a one-meter deep layer of crushed rock to assist in the uniform distribution of the air. One of the fans and the air distribution headers and piping are shown in Figure 2. The rock layer also provides a path for the leach solution draining from the heap to reach the pad liner and the solution trench.

Support rock was initially prepared by crushing and screening waste rock from an waste old dump. Recycled support rock is recovered from the trommel and fed to a horizontal conveyor which runs along side the heap. This conveyor transfers the support substrate via a tripper to a series of “grasshopper” conveyors, which in turn feed the heap stacking conveyor. The grasshoppers and stacker operate on the surface of the drain rock layer and the heap is stacked to a height of 6 meters above this surface. The “moving slot” method is used to stack and reclaim the heap in an “on-off” configuration. Stacking occurs on the advancing face of the slot and oxidized material is reclaimed from the opposite retreating face. Once sufficient area has been stacked, solution distribution piping is installed and irrigation of the heap is started. Solution is applied via sprinklers at a rate of 10-30 liters per square meter per hour. Figure 3 is a general view of the Agnes GEOCOAT® heap and materials handling system. Figure 4 shows the surface of the heap with solution application in progress.

Figure 3. Agnes Plant Stacking And Reclaim

Figure 4. Heap Surface Showing Steam Rising

Solution is recirculated to the heap via a lined solution pond A stainless steel pump delivers the solution from the pond to the heap. A portion of the circulating solution is bled off to maintain the iron concentration within design limits. The bleed stream is pumped to the neutralization circuit, a series of agitated tanks where flotation tailings are added to neutralize excess acid and precipitate iron. Flotation tailings at Agnes contain carbonate minerals and provide an inexpensive and convenient source of neutralizing material. The neutralized solution, containing the precipitated iron, is pumped to a tailings impoundment. A separate impoundment is provided for the cyanide residue to ensure that no cyanide is returned to the GEOCOAT® circuit via the process water recycle. Cyanide and its decomposition products are highly toxic to the bacteria.

Support rock with its oxidized concentrate coating is reclaimed from the heap by a front-end loader and conveyed to a trommel where the concentrate is separated (Figure 5). The concentrate slurry underflow from the trommel screen is pumped to a stainless steel high-rate thickener. Thickener underflow is transferred to the pH adjustment tank, after secondary screening for rock chip removal, where lime is added to raise the pH in preparation for cyanidation. The concentrate slurry is then pumped to the CIL plant located adjacent to the GEOCOAT® heap. The washed support rock is returned to the radial stacker for recoating with fresh concentrate. Substrate losses are made up by the addition of fresh rock.

Figure 5. Concentrate Recovery Trommel

Commissioning Issues and Flowsheet Evolution

The original as-built GEOCOAT® flowsheet for the Agnes operation is shown in Figure 6. This is a traditional refractory gold pretreatment flowsheet employing biooxidation as the oxidation step. The flotation concentrate is delivered to the site, coated onto the support rock and stacked in the GEOCOAT® heap. After a suitable period of biooxidation, the coated rock is reclaimed and the concentrate neutralized and subjected to CIL for gold recovery. However, due to a wide variety of circumstances the flowsheet has evolved to that shown in Figure 7. This flowsheet is non-traditional in the sense that biooxidation takes place on the flotation concentrate after an initial cyanide leach. Since the Galaxy concentrate has a high baseline cyanide gold recovery of 60-70% there were several economic benefits in removing the cyanide leachable gold prior to biooxidation.

Figure 6. Original Agnes Flowsheet

Figure 7. Modified Agnes Flowsheet

The original Agnes flowsheet was not modified because it was flawed but as a result of circumstances related to the mining difficulties leading to cash flow issues at the mine. Additionally there were some teething problems associated with the GEOCOAT® plant which also exacerbated the cash flow situation.

The critical issue that lead to the alteration of the flowsheet was the fact that the mine failed to ramp-up to its design production in the expected timeframe. This lack of ore lead to the robbing of the GEOCOAT® heap inventory in order for the mine to maintain cash flow. It was always expected that the heap inventory would quickly be refilled but the ramp-up period took much longer than expected. Once the GEOCOAT® heaps were empty the mine started to direct cyanide leach the concentrate producing enough cash flow to cover their costs. At this point the flotation concentrate CIL tailings were discarded containing 15-25 g/t of gold. Furthermore, the intermittent nature of the feed of concentrate to the GEOCOAT® plant during commissioning also made it difficult to commission the plant properly and isolate the commissioning issues.

After a period of several months it was decided that a return to the original flowsheet was going to be unlikely so GeoBiotics and Agnes embarked on a program to try and recover the balance of the CIL tailings gold through the GEOCOAT® plant. A comprehensive test program was undertaken to determine if the CIL tails could be decontaminated to allow biooxidation to progress. The test program successfully isolated the conditions under which the tailings could be treated. Basically the tailings would require a series of wash stages and an acid pretreatment before biooxidation could proceed.

There were several issues associated with the operation of the GEOCOAT® plant that were revealed during commissioning that required attention to optimize biooxidation. The main problems within the plant were the unexpected level of carbonates in the concentrate and support rock and control of the coating system. The carbonate issue within the support rock originated due to a change in support rock characteristics from that tested in the initial design work. The additional carbonate in the concentrate was the result of pulling the float circuit harder to increase gold recovery to the concentrate.

The high carbonate content of the concentrate was resolved by the addition of an acid pretreatment stage but the support rock issue was more difficult to rectify. The wholesale replacement of the support rock was not an economic option so operational controls needed to be developed to minimize its impact. It was necessary to control the irrigation pH at a lower level to allow for support acid consumption. A bulk acid system was retrofit to the plant to facilitate better pH control of the solution pond and to allow lower the unit cost acid to be utilized. The coating issue was basically a migration problem resulting from poor control of the concentrate coating density and uneven concentrate distribution. The coating system was modified through the addition of a “coating contactor” which facilitates a more uniform distribution of concentrate on the support substrate and a density control loop. Additionally, a new high density concentrate thickener was added as part of the acid pretreatment stage replacing the existing undersized floatation concentrate thickener.

GeoBiotics continues to have a significant presence at the Agnes plant working closely with APM to further the GEOCOAT® development. A planned expansion of the Agnes mine is underway to bring the Princeton orebody on-stream and to treat this highly refractory concentrate directly via GEOCOAT® as per the original design.