The June 2021 preliminary economic assessment for enCore Energy's New Mexico uranium project selects radiometric sorting, single-stage agitated acid leaching for 20 hours and a yellowcake circuit, after comparing heap leaching and in-situ options.
The Marquez-Juan Tafoya Uranium Project lies in the Grants Uranium Mineral District of northwest New Mexico, in McKinley and Sandoval Counties. enCore Energy Corporation commissioned the Preliminary Economic Assessment (PEA), written by Douglas L. Beahm of BRS Inc. and Terence P. McNulty of McNulty and Associates, with a date of June 9, 2021. The planned mine life is 15 years. The report sizes the processing plant at 1,000 t/d of ore, half the 2,000 t/d conventional plant that Bokum Resources designed in the 1970s. Its recovery section reviews that earlier design and then lays out four options: heap leaching, agitated tank leaching, in-situ leaching and a shared solution treatment plant. The PEA evaluates agitated leaching.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Plant size evaluated | 1,000 | t/d | Of ore |
| 1977 Ross plant design | 2,000 | t/d | Bokum Resources |
| Sorter feed size | Minus 2.5 / plus 1 | inch | Undersize minus 1 inch bypasses |
| Single-stage leach time | 20 | hours | Ross design was 12 hours, two stages |
| Optimum leach temperature | 50 | °C | 122 °F |
| Agitated leach tanks | 6 | tanks | Followed by six CCD thickeners |
| U3O8 recovery, heap leaching | 80 | % | Table 17.1 |
| U3O8 recovery, agitated leaching | 95 | % | Table 17.1 |
| CAPEX, heap leaching | 28,770 | US$ thousand | Table 17.1 |
| CAPEX, agitated leaching | 29,960 | US$ thousand | Table 17.1 |
| OPEX, heap leaching | 9,140 | US$ thousand/y | Table 17.1 |
| OPEX, agitated leaching | 12,270 | US$ thousand/y | Table 17.1 |
| Mine depth to mineralization | 1,800 to 2,000 | ft | Shaft stops 100 to 200 ft above |
Overview
Ore is crushed and sorted underground. The more radioactive fraction and the fine screen undersize are hoisted, while the less radioactive rejects return to the stopes as backfill. On surface, the sorter concentrate is rod milled and leached in dilute sulphuric acid in six agitated tanks, with tailings washed in six counter-current decantation (CCD) thickeners. Pregnant leach solution (PLS) goes to clarification, solvent extraction and ammonium diuranate (ADU) precipitation, and yellowcake is dried and drummed for shipment.
Key Process Stages
Underground selective handling and sorting
Ore is trammed from the stopes to a transfer conveyor feeding a primary jaw crusher surge bin and a double deck vibrating screen. Undersize at about minus 1 inch goes straight to the skip. Oversize between minus 2.5 inch and plus 1 inch goes to a radiometric ore sorter with selectable discrimination sensitivity. Fine material is removed first because sorting efficiency falls sharply for fragments with a mean diameter below about 1 inch (25 mm). The less radioactive fraction is rejected and returned to stopes as backfill, and the more radioactive fraction goes to the skip with the undersize. The report gives three reasons for sorting: the reject is ideal backfill, hoisting uses less electricity, and less tailings solids and evaporation pond influent are produced. Wet venturi scrubbers capture dust from crushing and screening, and the slurry goes to a partitioned shaft sump and then to a surface thickener.
Agitated leaching
This is the route with the most information, including 1977 and 1978 laboratory reports by Hazen and the 1977 Ross report for Bokum. Underground crushing and sorting removes the semi-autogenous grinding mill but keeps a rod mill for final grinding to grain liberation size. A DSM-type sieve bend on rod mill discharge keeps oversize, slow-leaching particles out of the leach. The report recommends single-stage leaching for 20 hours instead of the Ross design of two stages and an interstage thickener at 12 hours with about 80 g/L free acid and sodium chlorate as the oxidant. Larger tanks remove a thickener and much of the piping and pumping. Tailings go through six CCD thickeners.
Heap leaching option
Sorter concentrate would go by conveyor to a radial stacker that builds a pile of 25 to 30 feet. The pad would have a welded HDPE primary liner, 12 to 18 inches of gravel with perforated collection piping, a second HDPE sheet and 12 inches of fine soil. PLS would go to clarification, solvent extraction and precipitation. The authors lack column test data, so they provisionally recommend agitated leaching, which Hazen tested thoroughly with very high uranium extractions.
In-situ leaching
Kerr-McGee's limited tests gave mostly negative results, with poor response to an alkaline carbonate lixiviant, overlying and underlying strata more permeable than the mineralized sandstone, very low permeability in the sandstone itself and extreme depth. The authors accept those conclusions, but note that modern solution management has improved and that the existing shaft stops only 100 to 200 feet above mineralization, which could allow shallow injection and extraction wells from a new access level.
Solution treatment and yellowcake
The same plant serves every leaching option. PLS is clarified and passed through a sand filter, and ammonia precipitates ADU, which is thickened. Ammonium sulphate is added to the first thickener underflow to remove sodium, and the repulped slurry is thickened again. The underflow is dewatered in a centrifuge before drying. A rotary vacuum dryer replaces the classic multiple hearth roaster that Ross specified, to cut worker exposure. Yellowcake goes into reinforced steel drums with locking clamps, is weighed, stored on a dock and trucked to buyers or to a uranium hexafluoride conversion plant.
Additional Interesting Data and Summary
Groundwater at Marquez Canyon is about 100 °F because of geothermal influence from Mt. Taylor. Uranium minerals dissolve faster when warm, and Hazen's tests found 50 °C (122 °F) near optimum, so using mine water as process water cuts heating costs for both heap and tank leaching. The effect is smaller for a heap, which cools quickly in the open.
Costs were built from the late-2020 Mining Cost Service database, escalated to June 2021 with the U.S. Consumer Price Index, and scaled at a 0.6 exponent where needed. The tailings impoundment estimate assumes the site work done by Bokum Resources is still serviceable. The authors note that the project is medium-size in pounds but small in ore treatment rate, so full economies of scale are not achieved.
Key Processes
- Underground crushing and screening
- Radiometric ore sorting with rejects returned as backfill
- Rod milling with a sieve bend
- Single-stage agitated sulphuric acid leaching
- Counter-current decantation
- Solution clarification and ADU precipitation
- Rotary vacuum drying of yellowcake
Source: Marquez-Juan Tafoya Uranium Project, 43-101 Technical Report, Preliminary Economic Assessment, June 9, 2021.
Project website: Marquez-Juan Tafoya Uranium Project
