Magnetite Mines is preparing to commence a definitive feasibility study at its Razorback iron ore project in South Australia after receiving positive results back from a pre-feasibility study (PFS).
The PFS supports declaration of a maiden ore reserve of 473 Mt based on 12.8 Mt/y plant throughput and 2 Mt/y of high-grade concentrate, but it has opened the door for two other options.
Process plant optimisation, for instance, could see a nominal 15.5 Mt/y feed using three grinding stages, three stage magnetic separation and flotation to generate a premium-grade magnetite concentrate with 67.5-68.5% Fe content. And a “Head Grade Improvement Case”, based on higher mining rates with a head grade upgrade from selective mining or ore sorting, could see around 2.7 Mt/y of high-grade concentrate produced.
Razorback would involve initial capital investment of $429-$506 million for a post-tax internal rate of return of 14-33%. This is based on the range of throughput and concentrate production options, in addition to 62% Fe iron ore prices of either $110/t or $150/t.
Magnetite Mines said preparation for a prompt commencement of a definitive feasibility study is well advanced with further drilling, test work, metallurgical investigation and engineering workplans in progress.
Magnetite Mines Limited CEO, Peter Schubert, said: “The PFS is a significant milestone for the company, and defines our optimised go forward scope, which has been developed following rigorous and methodical testing of various options. The resulting scope meets our objectives of practical scale, capital efficiency, attractive returns, high quality product and an expected low emissions footprint.
“This small-scale start-up allows for a practical development of a long life, high quality business with a targeted date for first ore on ship at the end of 2024.”
The mining strategy involves a simple, small-scale mining operation, using mining contractors at start-up to simplify development and leverage the advantages of low strip ratio and short, flat hauls due to orebody geometry and outcropping nature, it said.
“The potential for selective mining is a key criterion and a simple truck and shovel operation was selected as a flexible, reliable and selective method of resource extraction,” the company said. “Bulk methods such as electric rope shovels, in-pit crushing and conveying and continuous miners were investigated but not selected.”
The selected fleet used a single 350 t excavator as primary unit with wheel loader back-up loading medium class (150-190 t) rear dump trucks. The 350 t excavator class was chosen as the maximum size of excavator that can achieve the 1 m of selectivity required to take advantage of the orebody characteristics. Ancillary gear has been sized to a size class appropriate for the excavator productivity and road geometry.
“During the definitive feasibility study, as further geological drilling and geo-metallurgical testing is undertaken, the fleet mix will be reassessed match capacity requirements once selective mining strategies are finalised,” the company said.
During the PFS, investigations and modelling showed there is significant potential in accelerating mining activities and realising higher plant feed grades, from some combination of accelerated and selective mining, stockpiles strategy and/or ore sorting, the company said.
Magnetite Mines has been investigating the potential application of a NextOre magnetic resonance analyser (MRA) with ore sorting technology to the Razorback resource. The use of the MRA allows for a high throughput, high accuracy bulk sorting application that is typically added to the front-end of a processing flow sheet to divert waste ores away before processing, it said. “This has the effect of improving mining grades by pre-concentrating the ore that will be subject to processing, whilst rejecting significant tonnages of low-grade material to tailings via a diversion method such as a chute flop gate or dead box diverter,” the company added.
In October, the company announced it had entered into an agreement with NextOre to supply a mobile bulk ore sorting plant using a magnetite resonance sensor for a trial of the NextOre technology. While the bulk trial was originally scheduled for later in 2021, NextOre and the company have agreed to reschedule this trial until later in the development schedule to allow for the results of planned infill drilling and metallurgical test work that are part of the planned definitive feasibility study to be incorporated in the bulk trial design, the company said.
To assess the impact of improved head grades in the PFS, meanwhile, results from an ore sorting case have been developed, using an increased mining rate and the block model used for reserves, then applying the previously released ore sorting results to generate improved plant head grades and mass recoveries.
“These results are consistent with the analysis earlier in the year on the discrete mineralised bands of the deposit and the gridded seam model,” it said. “Due to these encouraging results, the go-forward case for Razorback will be based on the higher head grades available from selective mining and ore sorting, which will be investigated further with comprehensive infill drilling of the Razorback orebody planned and designed to inform a selective mining schedule to definitive feasibility study standards.”
For the PFS, in addition to the test work completed as part of the 2013 PFS and additional high resolution DTR (Davis Tube Recovery) test work, a comprehensive mineralogical test program was completed to better understand the mineralogical composition of the Razorback and Iron Peak deposits, complementing the existing data from the previous test work program. This was informed by the results of the 2013 PFS study, which was completed for a two-module processing plant for a total of 6.2 Mt/y, and an optimised business case for a third module bringing it to 9.3 Mt/y.
Designed by the company’s process engineering consultants, the test work was used to improve the flowsheet. The flowsheet in the 2019 scoping study had three stages of grinding, three stages of magnetic separation and a final cleaning stage with a hydro separator producing final magnetite concentrate at a grind size of a P80 of 25 μm. This is a widely used, low risk flowsheet, but has significant power requirements and generates a very fine magnetite concentrate with potential filtration and product use issues, the company said.
The company has now generated a preferred flowsheet and plant layout for the PFS, which has significant advantages in efficiency and separation over the conventional configuration used in the scoping study estimates, it said. The inclusion of fine grinding and flotation allows efficient production of high-quality concentrate. The final scale of the preferred go-forward option is plant feed of approximately 15.5 Mt/y with ability to process up to 20% DTR with a capacity of up to 3.1 Mt/y concentrate.