This report describes the designed process route for recovering nickel and cobalt from lateritic ore using pressure acid leaching and hydrogen reduction.
Report context
This technical report on the Ambatovy Project was prepared in 2018. The process for the recovery of nickel and cobalt was designed by DMSA, which was acquired by Sherritt in 2007. The report documents the designed processing route from ore preparation through to metal powder and briquette production.
Processing route
Ore Preparation Plant
The ore preparation circuit is designed to process the limonite (ferralite) and LMS fractions of the orebody. Small amounts of saprolite material are fed to the circuit, where the nickel‑rich skins are scrubbed off the competent high‑magnesium boulders.
Run‑of‑mine ore is screened through a grizzly before being fed to two autogenous scrubbers in series. First scrubber discharge material is screened, and undersize material is forwarded to the ore thickener. The remainder goes to the second scrubber for continued treatment and liberation of ore fines. Oversize material from the second scrubber (+8 mm) is rejected; a middling stream is stockpiled for future treatment. The ore thickener prepares consistent ore slurry pulp density for feed to the slurry pipeline. Thickener underflow at about 38–42% solids is slightly diluted with water for pipeline feed.
A 600 mm diameter slurry pipeline, approximately 220 km in length, transports the ore from the Property to the Processing Plant.
Leach and Sulphide Precipitation Plant
All steps of the leach and sulphide precipitation process have been successfully commercialised in other facilities, and all equipment has been proven at the Ambatovy Project.
Ore Slurry Thickening
Ore slurry from the pipeline is received in two thickeners, which ensure a consistent feed slurry pulp density to the acid leaching circuit. Excess water reports as thickener overflow and is used as part of the plant water supply.
Pressure Acid Leaching
The pressure acid leaching section consists of five parallel trains. Thickened ore slurry is fed to the pressure leach autoclaves through three stages of direct heating, with inter‑stage pumping. The first two stages use flashed steam from the autoclave let‑down system; the third stage uses steam from the power plant/acid plant steam header. In the agitated horizontal autoclaves, the slurry is contacted with sulphuric acid at approximately 260°C and held for up to 75 minutes to complete the leaching of nickel and cobalt and to precipitate iron from solution. Leached slurry is discharged through three stages of flashing.
Slurry Neutralisation and Solids Wash Circuit
Cooled pressure acid leach discharge slurry is partially neutralised with limestone in the slurry neutralisation circuit before advancing to the six‑stage countercurrent decantation (CCD) wash circuit. Leach residue and other solids are discharged from the last thickener and sent to tailings neutralisation. Clarified raw leach liquor is drawn off from the first wash stage as overflow.
Raw Liquor Neutralisation
Clarified leach solution from the CCD circuit is further neutralised in two parallel solution neutralisation circuits. This step precipitates ferric iron and other impurities to produce a solution suitable for recovery of nickel and cobalt as a clean mixed sulphide intermediate product. Additional heat supplied from the ore leach flash circuit optimises operating efficiency. Solids are separated from the neutral solution; a portion is recycled as seed material while the balance goes to the CCD circuit.
Sulphide Precipitation
Product liquor from raw liquor neutralisation is mixed with nickel sulphide seed material and then contacted with fresh and recycled hydrogen sulphide in two parallel medium‑temperature and pressure circuits. Reactions precipitate nickel and cobalt as mixed sulphide solids, leaving a solution largely free of those metals. The product slurry is flashed to sub‑atmospheric pressure; flash gas is cooled and steam condensed. The resulting hydrogen sulphide gas is recycled. Degassed slurry goes to a thickener, and product solids are recovered and washed in a two‑stage countercurrent wash circuit.
Seed material, prepared from a blend of fine precipitated sulphides and ground sulphides, minimises scaling of reactor vessels and piping. The parallel circuits allow continued operation while one circuit is being descaled.
Tailings Handling
Tailings from the CCD circuit consist principally of leach residue with some gypsum and precipitated metal hydroxides. Tailings and sulphide precipitation discharge solution are further treated in a two‑stage neutralisation circuit to remove remaining soluble metals. A portion of magnesium is also precipitated. The resulting tailings slurry is forwarded to the tailings management facility (TMF). A neutral water stream is decanted from the TMF; a portion is used in the Processing Plant and the balance is discarded via ocean disposal.
Nickel and Cobalt Refinery
All steps of the refining process have been successfully commercialised in other facilities, and equipment has been proven at the Ambatovy Project.
Sulphide Leach and Impurity Removal
Mixed sulphides are leached in water using an overpressure of oxygen, producing a high‑strength nickel‑cobalt sulphate solution with minor amounts of zinc, copper, and iron. The leaching process has two stages. Unleached solids from the first stage progress to the second stage; second‑stage leach residue is recycled back, and iron‑containing tailings are bled from the circuit.
Combined leach solution is treated with ammonia to raise pH and precipitate soluble iron, which is separated using a thickener and pressure filters. Iron residue containing small amounts of co‑precipitated nickel and cobalt is recycled to the pressure acid leach stage. Iron‑free solution is contacted with zinc sulphide to remove copper to trace levels. The resulting zinc‑copper sulphide is separated and either sold or stockpiled. The solution is polish‑filtered before solvent extraction.
Solvent Extraction and Zinc Precipitation
Zinc and cobalt are removed from the nickel solution in successive stages of solvent extraction with Ionquest 290 extractant. Zinc is removed first, with extraction at a pH of approximately 3.5. The loaded organic is stripped with dilute sulphuric acid. Zinc is precipitated from the strip solution using hydrogen sulphide. A portion of the resulting zinc sulphide is recycled to copper cementation; the balance is sold.
Cobalt is removed from the zinc‑free solution at a pH of approximately 5.5. Loaded organic is scrubbed with dilute cobalt solution and then stripped with dilute sulphuric acid. The high‑strength cobalt strip solution is treated with zinc sulphide, if needed, and reports to the cobalt reduction area.
Nickel and Cobalt Reduction
Separate nickel and cobalt solutions are treated by hydrogen reduction to produce pure metal powder products. Nickel and cobalt sulphate solutions are adjusted with ammonia and ammonium sulphate, then contacted with hydrogen at elevated temperature and pressure to precipitate the metals. Resulting slurries are flashed to atmospheric pressure; metal powders are separated, washed, and dried. Powders may be sold as is, or briquetted and sintered in a reducing atmosphere furnace.
End Solution Treatment
End solutions from nickel and cobalt reduction are treated with hydrogen sulphide to precipitate trace metals, which are recycled as mixed sulphide to the sulphide leach. Ammonium sulphate is crystallised from the metals‑free solution in a triple‑effect vacuum crystallisation plant.
Utilities and Major Materials
The mine and ore preparation plant use diesel‑fired electrical generators for power. The Processing Plant uses coal‑fired boilers to produce power and process steam. At full production coal consumption is estimated at 430,000 t per year. Water for the mine and ore preparation plant is sourced from the Mangoro River; water for the Plant Site is sourced from the Ivondro River. At full production, estimated water requirement is approximately 4,000,000 t per year for the mine and 8,000,000 t per year for the Plant Site. Major bulk materials required are sulphur and limestone: 690,000 t per year of sulphur and 1,700,000 t per year of limestone at full production.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Pressure acid leach temperature | Approximately 260°C | Design |
| Pressure acid leach residence time | Up to 75 minutes | Design |
| Number of pressure acid leach trains | 5 | Design |
| Ore slurry pipeline diameter | 600 mm | Design |
| Ore slurry pipeline length | Approximately 220 km | Design |
| Ore thickener underflow solids | 38–42% | Design |
| Coal consumption (full production) | 430,000 t per year | Estimated |
| Limestone consumption (full production) | 1,700,000 t per year | Estimated |
| Sulphur consumption (full production) | 690,000 t per year | Estimated |
| Mine water requirement (full production) | Approximately 4,000,000 t per year | Estimated |
| Plant Site water requirement (full production) | Approximately 8,000,000 t per year | Estimated |
Project website: https://sherritt.com/sherritt-closes-transaction-to-restructure-ambatovy-joint-venture/
Technical qualifications
The report is based on the designed process route as developed by DMSA and Sherritt. The report states that all steps of the leach and sulphide precipitation process, and all steps of the refining process, have been successfully commercialised in other facilities and equipment has been proven at the Ambatovy Project. The report does not provide historical operating data or testwork results for the parameters listed. No production performance, economics, or current status information is included in the cited sections.
Source: CSA Global Report R501.2018, *Ambatovy Project – NI 43-101 Technical Report*, Section 17: Recovery Methods.
