Agata Nickel Project — 2011 Technical Report

This prefeasibility study outlines a proposed integrated hydrometallurgical process for treating limonite and saprolite ores from the Agata nickel laterite deposit.

Report context

The 2011 Technical Report for the Agata Nickel Project describes the process plant and associated facilities proposed at a prefeasibility study (PFS) level. Engineering deliverables produced for the study include Process Design Criteria, Process Flow Diagrams (PFDs), Mass and Energy Balance, Equipment List and Layout Drawings. The process flowsheets were developed from the results of high pressure acid leach tests and atmospheric leach tests on composites and individual ore types, the results of settling tests for ore preparation and leach residues, and Ausenco and BWHC's experience with other similar nickel laterite projects.

Processing route

Ore feed and stockpiling

Ore for the project is sourced from the Agata nickel laterite deposit. The laterite profile consists of limonite and saprolite zones. The limonite zone is characteristically iron oxide-rich, with predominant minerals hematite, goethite and clays, and moderate nickel content (<1%). The limonite overlies the saprolite zone, which is less oxidised and magnesium-rich with slightly higher nickel content. Limonite and saprolite ore will be mined and stockpiled separately at the processing facility at Tagpangahoy, in Butuan Bay. Saprolite ore is stockpiled according to magnesium content (low, medium, high) and is processed in batches in the saprolite ore preparation circuit.

Proposed three-circuit leach design

The proposed processing route consists of a high pressure acid leach (HPAL) circuit and parallel atmospheric leach (AL) circuit. Limonite and low magnesium ore will be treated by conventional HPAL. Medium magnesium saprolite ore will be treated by a parallel AL circuit. The combined discharge of the HPAL and AL circuits will then be processed in a Saprolite Neutralisation (SN) circuit followed by counter-current decantation and sections to neutralise excess acid and precipitate iron and aluminium. The SN circuit will involve pre-neutralisation of residual free acid in the combined leach discharge streams using the balance of saprolite ore, consuming much of the free acid while recovering additional nickel and cobalt values.

Ore preparation

Limonite ore, containing mainly fines, requires scrubbing of clay material prior to screening. Scrubbing testwork at Ammtec showed some magnesium rejection and hence nickel upgrade in the product. Clay composites were difficult to break up in the laboratory scrubbing process. Saprolite ore contains a rock component and is processed through a single stage SAG mill. Physical testwork data on ore was limited; comminution circuit design was based on criteria for similar ore preparation processes.

Atmospheric leaching and saprolite neutralisation testwork

The concept of saprolite neutralisation was first established in testwork conducted on Philippine laterite ores in 1998, with recovery of 60–65% of nickel and cobalt contained in high magnesium saprolite ore. Bench-scale testwork at SGS using Agata saprolite ore demonstrated recoveries of 75-77% for nickel and 75% for cobalt are achievable.

Metal recovery

Metal recovery will be by a two-stage mixed hydroxide precipitate (MHP) circuit similar to that operated at the Cawse Nickel Project in Western Australia and at Ravensthorpe Nickel Operation. The product is a filter cake containing nickel and cobalt metal in the form of a mixed hydroxide, bagged and containerised for onward shipment and export. Mixed hydroxide filter cake moisture level is expected to be 40%.

Key design basis and assumptions

All processing from ore preparation to MHP packaging, residue discharge and the sulphuric acid plant will operate on a continuous basis of 24 hours per day, 7 days per week. Allowing for downtime, long term availability is expected to be 87%, giving an operating time of 7,600 hours equivalent to a total plant throughput of 232 tonnes per hour. Most major process equipment is sized with a 15% design allowance. Exceptions include ore preparation, limestone and lime facilities with 33% design allowance, and the autoclave based on a direct 30 minutes residence time.

Autoclave throughput is based on 30% solids in the autoclave feed slurry after direct steam heating. As of August 2011, ore characterisation and scoping level iron/aluminium removal and mixed hydroxide precipitation tests were being performed by SGS Lakefield; results were not available for the PFS. Design criteria for the Fe/Al removal circuit are based on the Ramu Nickel Project pilot plant data. Design criteria for the precipitation circuit are largely based on the Ravensthorpe MHP circuit. Design criteria for the final neutralisation circuit are based on the Gladstone Pacific Nickel Limited Project testwork on manganese removal.

Key reported parameters

Technical qualifications

The design criteria for the Fe/Al removal circuit are based on the Ramu Nickel Project pilot plant data rather than Agata-specific testwork. The design criteria for the precipitation circuit are largely based on the Ravensthorpe MHP circuit. The design criteria for the final neutralisation circuit are based on the Gladstone Pacific Nickel Limited Project testwork on manganese removal. As of August 2011, ore characterisation and scoping level iron/aluminium removal and mixed hydroxide precipitation tests were being performed by SGS Lakefield; the results were not available for the PFS. The plant site location is considered sub-optimal due to poor geotechnical conditions and mountainous terrain resulting in significant risk of land slip failures and high earthworks costs. The ore preparation scrubbing and grinding design should be re-evaluated following a metallurgical testwork programme. Higher recoveries than the PFS assumptions may be possible for saprolite nickel dissolution; ongoing testwork on the Agata ores may improve upon the PFS assumptions.

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Source: Agata Nickel Project , 2011 Technical Report, Section 17 Recovery Methods