This report details the hydrometallurgical processing route at the Moa Project, covering the PSA Plant in Moa and the refinery in Fort Saskatchewan, as described in the 2023 technical report.
Report context
The Moa Project , 2023 Technical Report, dated March 2023, describes the nickel and cobalt laterite mining and processing operation. The project includes a mixed sulphide intermediate production facility adjacent to the mine site at Moa, and a refinery in Fort Saskatchewan, Canada. Processing includes high pressure acid leach and sulphide precipitation at the mine site, with refining to metal conducted at the Canadian facility.
Processing route
Slurry Preparation Plant
Run-of-mine ore is fed to the Slurry Preparation Plant (SPP) where the +0.8 mm size fraction is recovered as a slurry of ore and water in log-washers. Size separation uses vibrating screens and a cylindrical scrubber to yield an overall limonite recovery in excess of 95%. The product slurry at approximately 25% solids is transported by gravity through a pipeline to the ore thickener plant.
Ore Thickening
The ore thickener plant thickens the slurry in the underflow to approximately 43% to 45% solids and returns overflow water to the SPP. Five conventional thickeners and one high rate thickener are used. The conventional thickeners have rake drives that handle high loading and allow significant ore inventory in the thickener beds, typically five to ten days of plant feed.
Pressure Acid Leaching
The underflow slurry is preheated with 15 psig steam to 82°C, then pumped through a direct contact steam heater to increase the temperature to 245°C. The heated slurry flows into high pressure, vertical, Pachuka-type reactors where it contacts 98% sulphuric acid to extract nickel and cobalt into solution. Leached slurry is cooled through indirect slurry coolers, recovering 15 psig steam, then sent to a flash tank. Leach extraction of nickel and cobalt is around 94%, controlled by acid addition to maximise financial performance rather than metal extraction.
Wash Circuit and Neutralisation
The cooled leached slurry flows by gravity to a counter-current decantation (CCD) wash circuit to separate the raw liquor from the residue. The leach residue is sent to a tailings pond with a water recovery circuit that returns water to the CCD plant as wash water. The raw liquor is treated with hydrogen sulphide in a pipeline reactor to reduce Cr⁶⁺ to Cr³⁺, Fe³⁺ to Fe²⁺, and precipitate copper. The liquor is then neutralised with limestone mud to reduce free acid concentration and increase solution pH to approximately 2.3 in mechanically stirred atmospheric reactors. Gypsum solids formed from the reaction of limestone mud and sulphuric acid are removed in thickeners. A portion of the gypsum underflow is recycled back to the neutralisation stage as seed, and the remaining underflow is pumped to the CCD circuit.
Sulphide Precipitation
Product liquor is preheated with flashed steam from the leach plant flash tanks, then heated to 125°C using 15 psig steam recovered by the slurry coolers in the leach plant. The heated liquor is pumped into mechanically agitated autoclaves where nickel and cobalt are precipitated as a mixed sulphide product using hydrogen sulphide. The mixed sulphide slurry is cooled through a flash tank and thickened. The thickened slurry is washed, filtered and bagged for shipment to the Cobalt Refinery Company Inc. in Fort Saskatchewan, Canada.
Refinery Processing
Mixed sulphides produced at the PSA Plant are received at the refinery in Fort Saskatchewan, where commercially pure nickel and cobalt metal products are produced. The mixed sulphide material is combined with ammonium sulphate liquor and leached under relatively mild oxidising conditions. The majority of the nickel, cobalt and copper dissolves in the leach step. The metal-containing leach solution is separated from the residue, which is washed, and the leach solution is directed to a nickel-cobalt separation step.
Cobalt is separated from the leach solution as a cobalt salt. The nickel-rich solution is directed to copper removal, then nickel is recovered as a metal product. The cobalt salt is directed to cobalt recovery.
Copper Removal
Dissolved copper is removed by lowering the solution pH, precipitating copper as a copper sulphide. The copper sulphide is separated from solution, washed and shipped to a custom smelter for recovery of a pure copper product.
Nickel Recovery
The copper-free solution is sent to an oxydrolysis adjustment step. Following oxydrolysis, nickel metal is precipitated in autoclaves using hydrogen as the reducing agent. Nickel powder is precipitated batch-wise in repeated cycles until the nickel particles grow to sufficient size. The nickel powder is washed and dried, then either packaged as powder or compacted into briquettes.
Cobalt Recovery
The cobalt salt is purified in multiple processing steps to remove nickel, then dissolved in a process solution. The solution undergoes a conversion step to adjust the cobalt chemistry into a suitable form for metal recovery. Cobalt metal is precipitated in autoclaves using hydrogen as the reducing agent. The cobalt powder is precipitated batch-wise in repeated cycles until the cobalt particles grow to sufficient size, then washed, dried, and either packaged as powder or compacted into briquettes.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Plant feed solids in SPP product slurry | Approximately 25% | Design |
| Ore thickener underflow solids | 43% to 45% | Design |
| Leach temperature | 245°C to 260°C | Design |
| Leach extraction of nickel and cobalt | Around 94% | Design |
| Neutralisation pH | Approximately 2.3 | Design |
| Sulphide precipitation temperature | 125°C | Design |
| 2022 production of nickel and cobalt in mixed sulphides | 32,496 t | Actual |
| LoM average processing cost | US$48.60/t treated | Estimate |
| LoM average nickel cash cost | US$4.64/lb | Estimate |
| Average annual processing capital | US$14,773 thousand | Estimate |
Project website: https://sherritt.com/operations/metals/
Technical qualifications
Operating cost estimates in the base case show actual unit costs averaged over the period 2019-2021, applied to volumes in each forecast period of the LoM. Estimates are expressed in first quarter 2023 United States dollars, without provision for escalation, with an exchange rate of US$0.76/CAD applied.
The NETF tailings facility is predicted to be full by the end of 2023, with tailings then sent to the Area 22 Phase 3 TSF due for completion prior to 2024, and anticipated to have capacity until mid-way through 2026. Moa West TSF, in pre-feasibility study phase, is expected to provide tailings storage beyond the current LoM, but there is risk Moa West may not be developed in time for continuity after Area 22 Phase 3 is full.
The report notes that the Moa Project slurry preparation and processing plant capital in Years 1-5 includes the NSPP under construction and an intensive programme of refurbishment for the PSA Plant. Infrastructural capital includes Area 22 TSF expansion and construction of the Moa West TSF.
Source: Moa Project , 2023 Technical Report, March 2023, Sherritt International.
