This report describes proposed processing routes for the Nkout iron ore project based on a May 2012 Preliminary Economic Assessment and metallurgical testwork.
Report context
This report is dated September 2013 and presents a Mineral Resource estimate for the Nkout project prepared by SRK Consulting. The processing section summarises recovery methods previously described in a Preliminary Economic Assessment (PEA) dated 28 May 2012, prepared by AMC and filed on SEDAR on 11 July 2012. No preliminary feasibility study or update to the PEA had been prepared to account for subsequent Mineral Resource estimates, including the 28 May 2013 estimate. The PEA was regarded by Afferro as current. The PEA is preliminary in nature and includes Inferred Mineral Resources that are considered too speculative geologically to have economic considerations applied to them for categorisation as Mineral Reserves.
Processing route
Overview
Process definition work was carried out on the 35 Mtpa case only. The process requirement for lower production rates was identical, but with smaller scaled components and lower throughput rates. Based on metallurgical testwork undertaken at the time of the PEA, three flowsheets were indicated for the Nkout plant feed: DSO (direct shipping ore) based on crushing and screening; SAP (saprolite) based on gravity separation supplemented with magnetic separation; and Magnetite BIF based on magnetic separation.
DSO
DSO testwork indicated that an acceptable grade product could be produced by removing the finer size fractions from the as-received plant feed. No detailed process design was undertaken, but a possible flowsheet was described. Key process units included: initial size reduction of the ROM plant feed using a mineral sizer; scrubbing through a trommel with an aperture of 35–40 mm; screening using a double deck screen with top aperture matching the upper size limit for sinter fines (of the order of 8 mm) and bottom aperture likely of the order of 0.2–1.0 mm; an intermediate fraction (-8 +0.2–1.0 mm) reporting to the product stockpile; a cone crusher for oversize material; crusher product screened at 8 mm with oversize returning to the crusher; double deck screen undersize reporting to a dewatering cyclone to separate ultrafine material (-10–25 μm) for thickening and disposal in the tailings storage facility; and dewatering cyclone oversize thickened and stored separately in an intermediate storage facility with potential for processing through the SAP circuit.
The testwork at the time of the PEA had not definitively demonstrated the potential for production of a lump product from the DSO, so the flowsheet was configured for sinter fines only. For the PEA, the plant was sized for a plant feed rate of 11 Mtpa, with a corresponding production rate of 8 Mtpa of sinter fines product. At this throughput, the circuit was expected to consist of a single process line.
Saprolite
SAP testwork indicated that an acceptable grade product could be produced by a combination of gravity separation on the coarser size fractions combined with magnetic separation on the finer size fractions. Key process units included: initial crushing using a gyratory crusher; a crushed ROM stockpile for blending; grinding using an autogenous grinding (AG) mill in closed circuit with a screen with aperture of the order of 1–2 mm; gravity separation using spirals producing concentrate, tailings, and a middlings stream; desliming shown ahead of the spiral circuit; spiral concentrate thickened and filtered using a vacuum belt filter; spiral middlings reground using a ball mill; reground spiral middlings and desliming cyclone overflow combined for magnetic separation consisting of low intensity magnetic separation (LIMS) followed by medium intensity magnetic separation (MIMS); potential to include a further stage of wet high intensity magnetic separation (WHIMS); spiral tailings and magnetic separation tailings combined and thickened ahead of disposal; and magnetic separation concentrates combined, thickened, and filtered using a pressure filter before being added to the spiral concentrate.
SAP testwork was conducted on particle sizes up to 6.7 mm. The appropriate unit process for gravity separation for sizes from approximately 1 mm up to 6.7 mm is jigs, but the mass yields achieved in the testwork for these coarser sizes were low. Based on the mine plan, SRK understood the design capacity of the SAP circuit would be 16 Mtpa of product for the 35 Mtpa production case. The SAP plant was sized for a plant feed rate of approximately 50 Mtpa. The flowsheet represented a single process line, and in practice the circuit was considered likely to consist of two or three such lines.
Magnetite BIF
Magnetite BIF testwork was based on magnetic separation only. For both samples tested, the results indicated the potential for production of a high grade (>68% Fe) pellet feed concentrate. One sample also indicated potential for a lower grade (65% Fe, 5% SiO₂) coarser product for sinter plant blend feed. The mine plan called for parallel processing of SAP and BIF in the initial years followed by conversion to 100% BIF processing.
Key process units included: initial crushing using a gyratory crusher; a crushed ROM stockpile; grinding using an AG mill in closed circuit with a screen with aperture of the order of 1 mm; three stages of sequential magnetic separation using LIMS with a grinding stage (ball mill in closed circuit with cyclones) between each stage; the particle size achieved from the second grinding stage would support production of the coarse concentrate product from the second LIMS stage (of the order of 80% -150 μm); tailings from each magnetic separation stage combined and thickened ahead of disposal; and final magnetic separation concentrate thickened and filtered using a pressure filter.
Based on the 35 Mtpa mine plan, the BIF circuit would support an initial production rate of 20 Mtpa of product, increasing to 35 Mtpa through conversion of the SAP circuit. For the PEA, the BIF plant was sized for a plant feed rate of approximately 50 Mtpa to support the initial 20 Mtpa case. The BIF circuit was considered likely to consist of three process lines.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| DSO plant feed rate (design) | Mtpa | 11 | PEA design |
| DSO sinter fines production rate (design) | Mtpa | 8 | PEA design |
| DSO circuit lines (design) | – | 1 | PEA design |
| SAP product rate for 35 Mtpa case (design) | Mtpa | 16 | PEA design based on mine plan |
| SAP plant feed rate (design) | Mtpa | ~50 | PEA design |
| SAP circuit lines (estimated) | – | 2–3 | PEA estimate |
| BIF initial product rate (design) | Mtpa | 20 | PEA design based on mine plan |
| BIF ultimate product rate (design) | Mtpa | 35 | PEA design |
| BIF plant feed rate (design) | Mtpa | ~50 | PEA design |
| BIF circuit lines (estimated) | – | 3 | PEA estimate |
| DSO trommel screen aperture | mm | 35–40 | Design |
| DSO double deck screen top aperture | mm | ~8 | Design |
| DSO double deck screen bottom aperture | mm | 0.2–1.0 | Design (to be finalised) |
| DSO dewatering cyclone cut size | μm | 10–25 | Design |
| SAP/AG mill screen aperture | mm | 1–2 | Design |
| BIF/AG mill screen aperture | mm | ~1 | Design |
| BIF coarse concentrate grind size | % -150 μm | ~80 | Design |
| Magnetite BIF high grade concentrate | % Fe | >68 | Testwork |
| Magnetite BIF lower grade concentrate | % Fe | 65 | Testwork |
| Magnetite BIF lower grade concentrate SiO₂ | % | 5 | Testwork |
Project website: https://www.srk.com/en/publications/maximising-drilling-to-define-the-mineral-resource-at-the-nkout-iron-ore-project
Technical qualifications
The PEA upon which the processing description is based is preliminary in nature. It includes Inferred Mineral Resources that are considered too speculative geologically to have the economic considerations applied to them that would enable them to be categorised as Mineral Reserves, and there is no certainty that the PEA will be realised. No preliminary feasibility study or update to the PEA had been prepared to take into account subsequent Mineral Resource estimates up to and including the 28 May 2013 estimate. DSO testwork had not definitively demonstrated the potential for production of a lump product. Further magnetic separation testwork for the SAP circuit was required. Detailed estimates of water requirements for operating the mine and process plant were not generated for the PEA.
Source: Nkout MRE – Main Report, September 2013, Section 17 Recovery Methods.
