This report describes a proposed processing plant design for the Obotan Gold Project based on a conventional single-stage crushing, SAG and ball milling, and carbon-in-leach flowsheet.
Report context
This National Instrument 43-101 Technical Report for the Obotan Gold Project, prepared for Adansi Gold Company (GH) Ltd, presents the process design and recovery methods for the proposed treatment plant. The report references historical processing data from Resolute Mining Ltd operations at the Obotan region from 1998 to 2002 and incorporates results from recent metallurgical testwork.
Processing route
Summary of Proposed Flowsheet
The Project Plant design is based on a typical single stage crushing, SAG and ball milling circuit (SABC), carbon in leach (CIL) flowsheet. It includes single stage jaw crushing to a surge bin with reclaim from a dead stockpile and open circuit SAG mill, feeding cyclones that in turn operate in a closed circuit with a ball mill. A pebble crusher will receive scats from the SAG mill, crush them and return them to the SAG for further grinding. The hydrocyclones will achieve the final product size of P80 106 μm. A gravity circuit will be utilised to treat a portion of the cyclone feed stream to recover coarse free gold from the recirculating load. The milled product will gravitate to a trash screen before entering a pre-leach conditioning tank. A seven stage carbon-in-leach (CIL) circuit will be used to leach and adsorb gold from the milled ore onto activated carbon. An AARL elution circuit will be used to recover gold from loaded carbon. Cyanide in the CIL tailings will be detoxified using the SO2/Air process subsequent to thickening. The detoxified tailings are then pumped to the tailings storage facility.
The report notes that this process flow sheet is well known in industry, and is relatively low risk as it has historically been proven a successful processing route for Obotan region ores during Resolute Mining Ltd operations of 1998 to 2002.
Design Basis
The milling capacity is designed for 3,000,000 tpa of primary ore, with a maximum crushing and wet plant capacity of 3,840,000 tpa to allow for higher throughput when treating soft oxides. Mechanical availability of 85% for the crushing plant and 91.3% for the wet plant is assumed, supported by crushed ore storage and standby equipment in critical areas. Sufficient automated plant control is specified to minimise the need for continuous operator interface while allowing manual override and control when required.
Crushing Circuit
The crushing circuit has been designed as a single stage jaw crusher with intermediate surge capacity. A run of mine (ROM) grizzly aperture was selected based on the vendor recommended maximum ore feed size to the crusher. The primary jaw crusher has been sized on the basis of operating at 85% utilisation at a feed rate in excess of the mill feed requirement, assuming 60% of the vibrating grizzly split goes to oversize, and at a P80 of 150 mm. This product size was selected as it is suitable for SAG milling and minimises material handling issues, which may have become significant in a multistage crushing plant when treating tropical ores with high clay content.
The crushing circuit has been designed to operate at a treatment rate in excess of the mill feed rate. Crushed ore will report to a surge bin designed to smooth out fluctuations in the mill feed rate caused by crusher feed surges. The excess crushed ore will be stockpiled so that reclaimed ore can be fed to the process plant using a front end loader via an emergency ore feed bin during periods of crusher maintenance. All conveyor and equipment chutes have been designed to accommodate tropical clays and minimise circuit downtime, duct blockages and hang-ups.
A crushed ore surge bin with a nominal capacity of 250 tonne is specified. The surge bin overflow will be conveyed to a dead stockpile. Ore from the dead stockpile will be reclaimed by front end loader to feed the mill during periods when the primary crusher is offline.
Grinding and Classification
The milling circuit incorporates a primary SAG mill and secondary Ball mill in closed circuit with a cyclone bank and a pebble crusher. The SAG Mill, Ball Mill and pebble crushing circuit was selected in place of a single stage SAG mill and pebble crusher as it offered greater operating flexibility while also producing a cyclone overflow product with less variability in product size and production rate. The mills were designed with similar size drives to minimize the requirements for spares.
The process design criteria specify a grated primary SAG mill with a recycle pebble crusher and a secondary overflow Ball mill with a cyclone bank and associated infrastructure to produce a cyclone overflow product with P80 size of 106 µm.
Large diameter cyclones have been selected for the classification duty to reduce the potential for spigot blockages occurring from coarse SAG mill discharge material. While this might limit operational flexibility, the report states it provides the most robust operating circuit.
Gravity Concentration
The gravity recovery circuit has been designed based on a 40% gold recovery to gravity concentrate. Gravity concentration for removal of coarse gold will be from dedicated ports on the cyclone feed distributor. The recovered gravity gold concentrate will be leached in an intensive leach reactor. A separate electrowinning circuit has been provided for the intensive cyanidation solution to assist with metallurgical accounting and eliminate any potential impact on the operation of the carbon elution and electrowinning circuit.
Leach and Carbon Adsorption Circuit
A CIL circuit incorporating seven stages with a preconditioning tank is specified. The main drivers for the design of the CIL circuit are leach recovery and tailings solution grade. The overall tank volumes have been sized based on the greater volume required to give a minimum leach residence time.
Elution and Goldroom Operations
An AARL desorption circuit with electrowinning and gold smelting to produce doré bars is specified.
Tailings Treatment and Disposal
Cyanide destruction will be achieved using the SO2/Air process. Tailings thickening and pumping to a tailings storage facility (TSF) is specified.
Reagents and Process Services
Associated reagents and O2 generation by PSA (pressure swing adsorption) are specified. Process water supply is included in the design.
Key reported parameters
| Parameter | Unit | Design Value | Basis |
|---|---|---|---|
| Milling capacity (primary ore) | tpa | 3,000,000 | Design |
| Maximum crushing and wet plant capacity | tpa | 3,840,000 | Design (for soft oxides) |
| Crushing plant mechanical availability | % | 85 | Design assumption |
| Wet plant mechanical availability | % | 91.3 | Design assumption |
| Primary crusher product size (P80) | mm | 150 | Design |
| Final grind size (cyclone overflow P80) | μm | 106 | Design |
| Gravity gold recovery to concentrate | % | 40 | Design basis |
| CIL stages | number | 7 | Design |
| Surge bin nominal capacity | tonne | 250 | Design |
| ROM grizzly oversize split | % | 60 | Design assumption |
| Crusher utilisation | % | 85 | Design basis |
| Historical operations period | years | 1998 to 2002 | Historical (Resolute Mining Ltd) |
Project website: https://www.mining.com/pmi-gold-corporation-agreement-to-purchase-contiguous-mining-lease-with-the-obotan-gold-project/
Technical qualifications
The report states that the combined SAG and Ball milling capacity has been based on a nominal treatment rate of 3.0 Mtpa of primary ore with characteristics as per those seen in previous metallurgical test work and confirmed by recent variability testing. The crushing circuit and wet plant has been designed to treat up to approximately 3.8 Mtpa of ore, under the assumption that oxide ores will have properties resembling oxide ores treated by Resolute Amansie Ltd. Historically, these ores have had lower milling work indices than the Nkran primary ore, which would allow an increased mill throughput for a given mill size.
The report does not provide actual plant performance data, testwork results, or operating parameters from the proposed plant as it has not been constructed or commissioned at the time of the report. All processing parameters are design criteria unless otherwise noted as historical operating data. No economic analysis, mineral resource or reserve estimates, or project ownership information is included in this processing section. The report does not specify the current status or development stage of the project beyond the process design presented.
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Source: Adansi Gold Company (GH) Ltd, Obotan Gold Project, National Instrument 43-101 Technical Report, Reference: 11906 0367:P:ck Revision A, Section 17 Recovery Methods.
