Whole ore leaching was selected as the base case processing option for the Copperstone project based on 97 percent gold recovery, elimination of smelter charges, and superior economic results relative to flotation alternatives.
Kerr Mines Inc. commissioned new metallurgical testwork for the Copperstone deposit through Resource Development Inc, with the dual purpose of confirming prior metallurgical results and evaluating processing options. Testing compared three process routes: flotation producing a gold concentrate, and two additional options that produce a doré bar as the final saleable product. The ability to produce a doré bar was identified as considerably advantageous from an offtake revenue perspective compared with selling a gold concentrate.
Test results indicated that whole ore leaching achieved 97 percent gold recovery, while the other processes achieved recoveries of 88 to 90 percent. Based on economic and technical analysis, whole ore leaching was selected as the preferred processing option. Although operating costs for whole ore leaching were the highest of the three options, the increase in recoveries and elimination of smelter charges made this option economically superior. The whole ore leach process was selected as the base case scenario, with the process flow sheet described with reference to Carbon-in-Pulp.
Processing is planned for 840 tons per day on a 5-day per week schedule, which coincides with the 7-day per week mining plan. The process uses the existing crushing circuit, with ore conveyed to the ball mill where it is ground to P80 minus 200 mesh in a closed-circuit arrangement. The whole ore leach process can also be readily modified to incorporate a SART process to produce copper as a by-product and reduce cyanide consumption in the leach process.
The existing flotation plant will be modified, with some equipment eliminated as additional testwork and process plans dictate. Based on gravity testing and Work Index testing, certain existing equipment will not be needed and will be removed from the circuit, including the Knelson concentrator and rod mill. The new process will use the existing crushing circuit conveyed to the ball mill, and the design criteria specify that the plant can operate two days a week extra when required, or downtime will be utilized for maintenance.
The current plan is to construct the leach plant to the north of the existing flotation plant. Startup water for the plant is estimated at 350 gpm, with operating consumption estimated at 52 gpm. Ongoing testing indicates that intermittent copper in the ore feed may necessitate precipitation of copper through additional processing steps, which would recycle free cyanide back to the leach circuit and reduce overall cyanide consumption. This would keep overall cyanide consumption for gold ore leaching at or below design parameters. Testing for this is ongoing and would likely necessitate one additional 9 ft by 9 ft leach tank and a small filter press. The presence of intermittent copper in the ore feed may also necessitate the need for one additional leach tank and a small filter press.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Gold recovery, whole ore leach | 97 | % | Test result |
| Gold recovery, other options | 88 to 90 | % | Range across two flotation options |
| Design tonnage | 600 | tpd/week | Based on 5-day per week plant operation |
| Design tonnage per 5 days | 840 | tpd | Equivalent to 35 tph |
| Plant operation | 5 | days/week | Mine operates 7 days per week |
| Plant availability | 100 | % | Design criterion |
| Run-of-mine ore | P80 of 4 | inch | Feed specification |
| Secondary crusher product | P80 of 3/8 | inch | Feed specification |
| Ball mill Bond Work Index average | 14 | Not stated | Closed size of 100 mesh |
| Ball mill Bond Work Index maximum | 18 | Not stated | Closed size of 100 mesh |
| Ball mill Bond Work Index low | 11 | Not stated | Closed size of 100 mesh |
| Ball mill product P80 | 200 | mesh | For Options 1 and 2 |
| Ball mill product P80 | 150 | mesh | For Option 3 |
| Ball mill discharge P80 | 150 | mesh | Whole ore leach design |
| Leach tank diameter | 9 | ft | 9 ft by 9 ft high with agitators |
| Leach tank height | 9 | ft | 9 ft by 9 ft high with agitators |
| Number of leach tanks | 6 | count | Additional tank may be required for SART integration |
| Tailing thickener diameter | 20 | ft | Unit area of 3 ft²/st/24hrs |
| Tailing thickener unit area | 3 | ft²/st/24hrs | Design criterion |
| Tailing throughput | 840 | tpd | Design criterion |
| Startup water | 350 | gpm | Estimated |
| Operating water consumption | 52 | gpm | Estimated |
| Abrasion index values | 0.0259 to 0.359 | Not stated | Range from testwork |
Overview
The Copperstone project involves reopening and modifying an existing processing facility to treat ore through a whole ore leach flowsheet. The project required removal of some existing equipment and installation of additional equipment specific to the whole ore leach process. This included removing a rod mill, Knelson concentrator, jig, Knelson screen, and associated pumps, along with a dryer and flotation cells, and adding leach tanks and a thickener. The design criteria established a plant availability of 100 percent, with grinding to P80 of 150 mesh for the whole ore leach option.
The project site location for the whole ore leach plant is to the north of the existing flotation plant. The existing equipment list included gravity concentration equipment, which the testwork indicated would not be needed because the deposit does not have free coarse gold. Testwork also provided data for Bond Ball Mill Work Index testing and abrasion testing, with samples representing both softer and harder material.
Key Process Stages
The existing crushing circuit will be used, with ore conveyed to the ball mill. Grinding will occur in closed circuit to achieve the target product size. The whole ore leach process will use Carbon-in-Pulp technology for gold recovery, with six leach tanks each 9 ft in diameter by 9 ft high. The leached residue will be sent to a 20 ft diameter tailing thickener with a unit area of 3 ft²/st/24hrs at a throughput of 840 tpd.
The process has provision for SART integration to treat intermittent copper in the ore feed. The SART process would precipitate copper and recycle free cyanide back to the leach circuit. This would reduce overall cyanide consumption and keep it at or below design parameters. Ongoing testwork is evaluating this option, which would require one additional leach tank and a small filter press.
The flotation plant will be modified, with equipment eliminated as additional testwork and process plans dictate. If flotation of copper from leach tails is proven economical, some existing equipment may be retained. As testwork progresses and required processing is finalized, the elimination of surplus equipment will be defined.
The existing equipment list includes jig and centrifugal gravity equipment for recovery of coarse gold, but current ongoing testwork indicates the deposit does not have free coarse gold. Some equipment will be removed from the circuit, including the Knelson concentrator and rod mill.
Additional Interesting Data and Summary
The project economics for whole ore leaching were superior despite having the highest operating costs, due to higher recovery and elimination of smelter charges. The block flow sheet for the whole ore option notes that CIP refers to Carbon-in-Pulp. The SART process integration is at early stages of evaluation, with copper not currently classified as a recoverable mineral resource according to any standard. Metallurgical testing for economic extraction of copper was ongoing but incomplete, and the study provided no certainty that copper extraction would be economically viable.
The list of major equipment with specifications and projected throughput was provided in the technical report. The additional equipment required for the whole ore leach process included leach tanks, thickener, and ancillary equipment. The removal of equipment and the projected throughput of existing equipment were specified in the report tables. The planned process plant layout was shown in Figure 17-2, with the leach plant constructed to the north of the existing flotation plant.
A review of the existing plant equipment identified that the rod mill, Knelson concentrator, jig, Knelson screen, and associated pumps were not required for the new flowsheet. These were removed from the circuit to be replaced by the whole ore leach equipment. The existing flotation cells would be modified or eliminated as required by the final process design. The plant power requirements and major consumables were detailed in the technical report tables.
Key Processes
- Whole ore leaching with Carbon-in-Pulp gold recovery
- Crushing and grinding in existing circuit with minor modifications
- Closed-circuit ball mill grinding to P80 of 150 mesh
- SART process integration for copper by-product recovery and cyanide reduction
- Existing flotation plant modification with equipment removal
- Tailing thickening in a 20 ft diameter thickener
- Removal of rod mill, Knelson concentrator, jig, and associated equipment
Source: Kerr Mines Inc. NI 43-101 Technical Report, Copperstone Project Recovery Methods, May 18, 2018.
Project website: Kerr Mines Inc., Copperstone Project NI 43-101 Technical Report
