This report describes the proposed 10 Mtpy expansion of the Niobec niobium operation, presenting a new process plant design while documenting the existing historical processing route.
Report context
The December 2013 technical report for the Niobec Expansion outlines a proposed 10 million tonnes per year (Mtpy) processing plant to replace the existing facility. The report details a completely new plant design while providing the historical operating process currently used at Niobec, which was developed from early pilot plant programs and later modified by mill personnel.
Processing route
Existing Historical Process
All run-of-the-mine ore is crushed to 38 mm and fed to rod mills, ball mills, and classification circuits where the ore is ground to 80% passing 105 µm. The ore is deslimed in two stages of cycloning, and the underflow is sent for conditioning prior to carbonate flotation. The carbonate concentrate is sent to tails. The carbonate flotation rougher and cleaner tails are cycloned in two stages to change the process water and then sent to pyrochlore rougher flotation. The rougher concentrate is sent to five stages of cleaner cells, followed by pyrite flotation to remove sulfur, hydrochloric acid leaching to remove phosphorus, and then dried to produce a concentrate with less than 0.2% moisture before being converted into ferroniobium.
Proposed Design for 10 Mtpy Expansion
A completely new processing plant facility will need to be constructed. Conventional crushing and grinding using rod mill and ball mill will be replaced by a semi-autogenous grinding (SAG) mill and ball mill. The design of the recovery process will be based on scale adjustments from the existing facilities in the majority of cases where possible.
Crushing, Stockpile and Reclaim: Primary crushing is done underground with jaw type crushers before being hoisted to head frame bins with 2,000 tonnes (2 x 1,000 tonnes) capacity. Apron feeders followed by sacrificial belt conveyors (2) located under the head frame bins transfer the ore to the stockpile feed belt conveyor. The 180,000 tonnes capacity (30,000 tonnes live) stockpile is covered to control dust emissions. Ore is reclaimed from the stockpile with variable speed apron feeders (3) located in the reclaim tunnel. Each feeder can supply the entire feed to the mill (1,250 t/h) but can be operated simultaneously.
Grinding: The grinding circuit is composed of one SAG mill (12,000 kW) and one ball mill (12,000 kW). Both mills are equipped with twin pinions and motors. The SAG mill discharge is screened, with oversize recirculated to the SAG feed via a belt conveyor. The undersize is combined with the ball mill discharge and pumped to the classification circuit. The target grind is 105 microns. The grinding circuit product feeds the desliming circuit.
Desliming: The desliming circuit is designed to remove fine material before flotation in two steps of cyclone classification. The overflow of the primary desliming cyclones feeds the second cyclone stage. Overflow from the second stage is sent to the tailings thickener. Underflows from both stages are combined and feed the carbonate flotation.
Carbonate Flotation: After desliming, the slurry is conditioned in two agitated tanks in series. The carbonate flotation has one rougher step and two cleaning steps. The flotation concentrate consists of very fine calcite particles and medium size apatite. The carbonate flotation tailings are sent to the tailings thickener.
Dewatering: After carbonate flotation, the slurry is sent to cyclones in an arrangement similar to the desliming circuit. The slurry is first diluted and pumped to primary dewatering cyclones. The overflow is pumped to secondary dewatering cyclones. Overflow from the second stage is sent to the tailings thickener. Underflows from the two stages are combined, diluted with water, and sent to the next processing step.
Magnetic Separation: The dewatering cyclone underflows are sent to two stages of magnetic separation to remove magnetite mineral.
Pyrochlore Flotation: The non-magnetic material is pumped to conditioning tanks. The slurry is sent to a bank of flotation cells where the pyrochlore is recovered. The rougher concentrate is further upgraded with cleaning stages. The pyrochlore flotation concentrate is sent to sulphide flotation and the tails are sent to the tailings pump box.
Sulphide Flotation and Leaching: The cleaned pyrochlore concentrate contains pyrite and phosphate minerals which need to be removed by flotation and a leaching stage.
Drying and Packaging: The final pyrochlore concentrate is pumped to disk filters and sent to a propane counter current dryer. The dried product is stored into bins.
Converter: After the pyrochlore concentrate is dried, it is transferred to the converter where the material is transformed into ferroniobium (standard grade) using an aluminothermic reaction on a batch basis. Raw materials include pyrochlore concentrate, aluminium, a source of iron oxide (hematite or mill scale), quick lime and sodium nitrate. The total batch size contains 7,000 kg of raw materials to produce 2,500 kg of ferroniobium. After mixing, the reactor is placed under fume collection and the reaction is ignited. The reaction is strongly exothermic and once ignited cannot be stopped. It takes five minutes to liquefy all material at a temperature of 2,200°C. During the following seven to eight minutes, separation of the alloy and slag occurs. After cooling, the slag is brought back underground while the ferroniobium ingot is sampled and stored. Ingots are selected according to chemical analysis and customer requirements, then crushed and classified for packing.
Tailings Disposal: Tailings from the pyrochlore and sulfide flotation report to a pumping system to be pumped to the tailings storage facility through 8 km of piping from the mill. All other tailings are combined, thickened, and then pumped to the tailings management facility in a separate pipeline. The thickener overflow is pumped to the recycled water pond.
Testwork Basis
The report states that the niobium recovery process will stay the same in terms of metallurgy. The report notes that experience of ore processing over the last 37 years will help minimize and accelerate the commissioning time. The proposed design is based on scale adjustments from existing facilities where possible.
Key reported parameters
| Parameter | Units | Design Value | Basis |
|---|---|---|---|
| Mill availability | % | 93.0 | Design |
| Hourly rate | t/h | 1,250 | Design |
| Feed grade | % Nb₂O₅ | 0.47 | Design |
| Recovery – concentrator | % Nb₂O₅ | 51.0 | Design |
| Recovery – converter | % Nb | 97.0 | Design |
| Primary grinding – SAG mill | # | 1 | Design |
| SAG feed size F80 | µm | 150,000 | Design |
| SAG installed power | kW | 12,000 | Design |
| Secondary grinding – ball mill | # | 1 | Design |
| Ball mill final product P80 | µm | 105 | Design |
| Ball mill installed power | kW | 12,000 | Design |
| Final concentrate moisture | % | <0.2 | Historical operating |
| Converter batch size (raw materials) | kg | 7,000 | Design |
| Ferroniobium produced per batch | kg | 2,500 | Design |
| Reaction temperature | °C | 2,200 | Design |
Project website: https://iaac-aeic.gc.ca/050/evaluations/proj/80011
Technical qualifications
The report notes that the proposed expansion involves a completely new processing plant facility and that new approaches will need to be adopted with respect to mill throughput and equipment sizes. The design of the recovery process will be based on scale adjustments from existing facilities in the majority of cases where possible. The report identifies the commissioning of an entirely new plant as posing the highest risk and states that sufficient time will be required to ensure a good transition and continued ferroniobium production. The report notes that in terms of mill feed grade reduction, the risk will come from increasing the concentration ratio required to achieve concentrate grade and to control concentrate quality.
Source: Niobec Expansion – 2013 Technical Report, Section 17 – Recovery Methods, Update on Niobec Expansion December 2013.
