The Thor Lake Project technical report dated August 25, 2011, describes a proposed mineral processing operation involving a concentrator at Thor Lake and a hydrometallurgical facility near Pine Point, with all metallurgical design based on testwork completed in 2009.
Report context
The August 25, 2011 technical report for the Thor Lake Project (Project #1714), prepared for Avalon Rare Metals Inc., presents a pre-feasibility level process design based on metallurgical testwork completed in 2009. The flotation and hydrometallurgical plant design data were provided by a consultant to the company, collated from testwork completed at SGS Lakefield Research Limited in 2009. The grinding circuit design was based on test data provided separately in 2009.
Processing route
Basis
The flotation and hydrometallurgical plant process described in the report is based on metallurgical design data provided by a consultant to the company, which in turn were collated from testwork completed in 2009. The grinding circuit design is based on test data from a separate 2009 study. The process design criteria developed from these data are presented in the report as a principal design criteria table.
Proposed process facilities
The proposed process comprises crushing, grinding, and flotation plants located at Thor Lake, plus a hydrometallurgical facility near Pine Point on the south shore of Great Slave Lake. The facility would initially process mineralized material mined at a rate of approximately 1,800 tpd in the first year, ramping up to 2,000 tpd from the second year onwards.
Crushing and grinding
The proposed crushing plant at Thor Lake is sized for the ultimate tonnage and is located in the mine, designed to reduce rock from run-of-mine size to -15 mm. Crushed material would be stored in a fine ore bin excavated in the rock and conveyed up the mine access incline to a rod mill–ball mill grinding circuit. The report notes that the grinding circuit would require the addition of a second ball mill to handle expansion tonnage.
Magnetic separation and flotation
Ground ore would be conditioned, then de-slimed in a series of three hydrocyclones before being pumped to a magnetic separation circuit. This circuit comprises a first magnetic separator, a regrind mill to process the concentrate, and a cleaner magnetic separator. Non-magnetic product would be pumped to a thickener.
Thickener underflow would be diluted and conditioned ahead of rougher-scavenger flotation. Scavenger tails would initially be sent to a tailings storage facility but would be processed for paste backfill production for the mine after the initial couple of years of operation. Flotation concentrates would be cleaned in four counter-current stages to produce a cleaner concentrate, which would be subjected to gravity separation, then thickened and dewatered in a filter press. Gravity tailings would be reground and returned to rougher flotation.
Dewatering and concentrate handling
Dewatered concentrate would be conveyed to special containers able to hold 40 t of concentrate. Filled containers would be stored until concentrate transportation is scheduled, at which time they would be taken across Great Slave Lake to the dock at Pine Point and transported to the hydrometallurgical facility.
Hydrometallurgical facility
At the proposed hydrometallurgical facility, full concentrate containers would be stored and retrieved as required, then placed in a thaw shed. The concentrate would be thawed and dumped into a reclaim system conveying material into the hydrometallurgical plant.
Concentrate would be “cracked” using a combination of acid baking, caustic cracking, and leaching, using sulphuric acid and sodium hydroxide as the primary reagents. The solid residue from the cracking system would be combined with other waste streams and sent to the hydrometallurgical tailings storage facility. The solution arising from the cracking process would be subjected to double salt precipitation, solution pre-treatment, and solvent extraction processes to isolate the values. Products would be precipitated as basic salts, processed, and dried to yield hydrated oxides, which would be packaged for shipment to markets. Products would be trucked to Hay River for on-shipment by rail.
Reagents and sulphuric acid plant
The proposed hydrometallurgical process plant would consume significant quantities of reagents brought to site by rail to Hay River and then by truck to the plant. Sulphuric acid would be produced in a 700 tpd capacity double-contact, double absorption plant from elemental sulphur. Excess heat from the sulphuric acid plant would be used in the hydrometallurgical process in the thaw shed, to evaporate process solutions, and in the product driers.
Limestone would be quarried near Enterprise and crushed and ground to -44 µm for use as a neutralizing reagent in the process. Some limestone would be calcined to lime for neutralizing purposes. Other reagents, such as sodium hydroxide, fuel, and solvent extraction reagents, would be stored at site as needed.
Most of the hydrometallurgical complex, including the acid plant, is designed and constructed for the ultimate tonnage equivalent to 2,000 tpd of flotation concentrate. Units requiring duplication or addition for expansion tonnage include the thaw shed, acid bake and caustic cracking facilities, and some of the product driers.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Processing rate | 730,000 | tpa | Design |
| Processing rate | 2,000 | tpd | Design (year 2 onwards) |
| Processing rate, initial | 1,800 | tpd | Design (year 1) |
| Feed grade | 2.84 | % ZrO₂ | Design |
| Feed grade | 1.50 | % TREO | Design |
| Feed grade | 0.39 | % HREO | Design |
| Feed grade | 0.37 | % Nb₂O₅ | Design |
| Feed grade | 0.040 | % Ta₂O₅ | Design |
| Flotation plant operating time | 8,000 | hr/a | Design |
| Flotation plant processing rate | 91.2 | tph | Design |
| Underground crusher product | 15 | mm (100% passing) | Design |
| Grinding circuit | Rod and ball mill | , | Design |
| Final grind | 38 | µm (80% passing) | Design |
| Slimes-free non-magnetics | 18 | % feed | Design |
| Final concentrate mass | 18 | % feed | Design |
| Recovery to final concentrate, ZrO₂ | 89.7 | % of feed | Testwork basis |
| Recovery to final concentrate, TREO | 79.5 | % of feed | Testwork basis |
| Recovery to final concentrate, HREO | 79.5 | % of feed | Testwork basis |
| Recovery to final concentrate, Nb₂O₅ | 68.9 | % of feed | Testwork basis |
| Recovery to final concentrate, Ta₂O₅ | 63 | % of feed | Testwork basis |
| Hydrometallurgical plant operating time | 7,582 | hr/a | Design |
| Hydrometallurgical processing rate | 17.4 | tph | Design |
| Acid bake temperature | 250 | °C | Design |
| Acid addition | 700 | kg/t concentrate | Design |
| Caustic crack temperature | 600 | °C | Design |
| Net caustic addition | 140 | kg/t concentrate | Design |
| Post double salt precipitation SX feed rate | 83 | m³/h | Design (expansion throughput) |
| Recovery to final products, ZrO₂ | 90 | % of concentrate | Testwork basis |
| Recovery to final products, TREO | 93 | % of concentrate | Testwork basis |
| Recovery to final products, HREO | 93 | % of concentrate | Testwork basis |
| Recovery to final products, Nb₂O₅ | 80 | % of concentrate | Testwork basis |
| Recovery to final products, Ta₂O₅ | 50 | % of concentrate | Testwork basis |
| Sulphuric acid plant capacity | 700 | tpd (100% acid) | Design |
Project website: https://avalonadvancedmaterials.com/nechalacho/
Technical qualifications
The report identifies several limitations and notes regarding the process design. The flotation and hydrometallurgical recoveries are stated as being based on metallurgical design data collated from testwork completed in 2009, with the grinding circuit design based on separate test data from 2009. The report notes that the crushing plant is designed for expansion tonnage, while the grinding circuit would require the addition of a second ball mill to handle expansion tonnage, along with additional flotation cells, gravity separation units, and filters. All solvent extraction units are sized for expansion, with some driers added for expansion. The report also notes that the hydrometallurgical complex, including the acid plant, is designed and constructed for the ultimate tonnage equivalent to 2,000 tpd of flotation concentrate.
Source: Avalon Rare Metals Inc. – Thor Lake Project, Project #1714, Technical Report NI 43-101, August 25, 2011, Sections 1 and 17.
