The Mount Hope Project processing route comprises primary crushing, SAG mill-ball mill grinding, molybdenite flotation with up to seven cleaning stages, and roasting of concentrate to technical-grade molybdenum trioxide.
Report context
This technical report, dated 15 January 2014 (Revision 0), describes a proposed vertically integrated processing facility for the Mount Hope Project, designed by M3 Engineering for EMLLC. The deposit is a classic molybdenum porphyry, typified by the deposit at Climax, Colorado, with molybdenite (MoS₂) mineralization developed within porphyritic igneous rocks and in Vinini hornfels.
Processing route
Primary crushing and coarse ore storage
Run-of-mine sulfide ore will be trucked from the mine and dumped directly into a dump pocket feeding a 60-inch by 89-inch gyratory crusher with an 800-hp motor. A hydraulically operated, pedestal-mounted rock breaker will be installed at the dump pocket. Primary crushed ore will be withdrawn by a variable-speed pan feeder and conveyed to a conical coarse ore stockpile with approximately 66,000 tons of live storage and 212,000 tons of total storage. A reclaim tunnel beneath the stockpile will feed four variable-speed pan feeders (two operating, two standby) discharging to the SAG mill feed conveyor.
Grinding and classification
Ore will be ground in a SAG mill-ball mill (SAB) primary grinding circuit. The SAG mill, measuring 36 feet by 17 feet effective grinding length, driven by an 18,100 hp (13.5 MW) wraparound gearless mill drive, will operate in closed circuit with a trommel screen and a vibrating screen, both with slotted openings of 11 mm. Two ball mills, each 22 feet by 36.5 feet effective grinding length with dual low-speed synchronous motors totalling 6,500 hp each (9.7 MW total) and variable frequency drives, will operate in parallel with hydrocyclones. The target grind for the grinding circuit is 80% finer than 150 µm. A pebble crusher is planned for future installation, approximately in Year 8 of production, in anticipation of harder ore being mined.
Flotation circuit
The flotation circuit will consist of rougher flotation, first cleaner and first cleaner scavenger flotation, concentrate regrind, and second through sixth cleaner flotation stages. A flotation column will be included as a contingent seventh cleaning stage in case the sixth cleaner concentrate fails to meet final grade or impurity specifications. The flowsheet was modified from locked-cycle test results, sending tailing from the third cleaning stage back to regrind rather than returning each stage tailing to the previous stage.
Rougher flotation will comprise two rows of cells, each row dedicated to one ball mill line. Rougher concentrate will feed the first cleaner circuit, while first cleaner tailing will proceed to first cleaner scavenger flotation. Concentrate from the first cleaner scavenger, combined with third cleaner tailing, will report to the concentrate regrind thickener. Regrinding will be performed in a ball mill in closed circuit with hydrocyclones, with a target grind of 80% finer than 25 µm. Cleaner stages from second through sixth will operate in series, with each stage tailing returning to the previous stage feed.
Concentrate dewatering, leaching, and drying
Flotation concentrate will report to a thickener. Thickened concentrate will be stored in four agitated stock tanks, each sized for 24 hours of production. Approximately 80% of the time, concentrate will be within specification and will bypass the leach circuit to be filtered directly. For out-of-specification concentrate, the leach process will reduce impurities such as copper, lead, and zinc through dissolution in hot ferric chloride-hydrochloric acid solution at approximately 180 °F, using six agitated tanks operating in series. Leached concentrate will be filtered, washed with hot water, and filtered again. Filter cake from either route will be dried using Holo-Flite-type dryers, with heat supplied via hot thermal oil.
Concentrate roasting and off-gas treatment
Two multiple-hearth roasters operating in parallel will convert dried molybdenite concentrate (MoS₂) to molybdenum trioxide (MoO₃), the main component of technical-grade molybdenum oxide (TMO). Each roaster will have twelve hearths, with propane gas fired burners installed on all hearths except Hearth 5 and Hearth 7. TMO discharging from Hearth 12 will pass through water-cooled screw conveyors to a lump breaking circuit comprising hammer mill and vibrating screen in closed circuit.
Roaster off-gas will be treated through water spray coolers, dry cyclones, dry electrostatic precipitators, then a wet gas scrubber (venturi and froth column), and finally a wet electrostatic precipitator to remove sulfur dioxide, particulates, and acid mists. Scrubber effluent will discharge to three oxidation/neutralization tanks operating in series.
Tailing dewatering and storage
Tailing from rougher flotation and first cleaner scavenger flotation will be distributed to two parallel thickeners. Thickener underflow will be pumped through two parallel pipelines to a tailing storage facility, with hydrocyclone stations for classification. Hydrocyclone underflow (coarse sand) will be used for centerline embankment construction of the tailing dam. Water will be reclaimed from the tailing storage facility and returned to the process water tank.
Process control
The control system will be highly automated with extensive on-line sampling and analysis of grinding particle size and various flotation streams.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Plant nameplate capacity | 66,688 tpd / 24,340,938 tpy | Design |
| Mill availability (excluding primary crushing) | 92% | Design |
| Primary crushing area availability | 75% | Design |
| Design ore grade (80th percentile) | 0.085% fltMo | Design |
| Mill cutoff grade | 0.045% fltMo | Design |
| First 34 years average grade | 0.076% fltMo | Design |
| Life-of-mine grade | 0.070% fltMo | Design |
| Design flotation recovery (at 80th percentile grade) | Approximately 90% | Predicted |
| Life-of-mine overall recovery | 88.8% fltMo | Design |
| Stockpile grade range | 0.034% to 0.045% fltMo | Design |
| Primary crusher feed F80 | 400 mm | Design |
| Primary crusher product P80 | 150 mm | Design |
| Crushing work index | 12.3 kWh/t | Design |
| SAG mill product P80 | 2.8 mm | Design |
| SAG Index SPI (tested) | 44 min | Testwork |
| SAG Index SPI (design) | 49 min | Design |
| Ball mill feed F80 | 3,000 µm | Design |
| Ball mill product P80 (final grind target) | 150 µm | Design |
| Bond Ball Mill Work Index (tested) | 10.7 kWh/st | Testwork |
| Bond Ball Mill Work Index (design) | 11.8 kWh/st | Design |
| Regrind circuit target P80 | 25 µm | Design |
| Rougher flotation time | 31 min (lab: 14 min) | Design |
| First cleaner flotation time | 25 min (lab: 10 min) | Design |
| Cleaner scavenger flotation time | 25 min (lab: 10 min) | Design |
| Second to sixth cleaner flotation time (each) | 12.5 min (lab: 5 min) | Design |
| SAG mill dimensions | 36 ft x 17 ft EGL, 19 ft F/F | Design |
| SAG mill drive | 18,100 hp (13.5 MW) wraparound GMD | Design |
| Ball mill dimensions (each) | 22 ft x 36.5 ft EGL, 37 ft F/F | Design |
| Ball mill drive (each) | 6,500 hp (9.7 MW total), VFD | Design |
| Concentrate leach temperature | Approximately 180 °F | Design |
| Coarse ore stockpile live storage | 66,000 tons | Design |
| Coarse ore stockpile total storage | 212,000 tons | Design |
Project website: https://miningdataonline.com/property/1914/Mount-Hope-Project.aspx
Technical qualifications
The process design criteria presented are based on a feasibility-level study conducted by M3 Engineering for EMLLC. The report states that the design ore grade of 0.085% fltMo represents the 80th percentile grade of ore blocks meeting the mill cutoff grade of 0.045% fltMo. The MetSim mass balance simulations were run for design (0.085% fltMo, 89.48% recovery), low (0.04% fltMo, 86.26% recovery), and high (0.12% fltMo, 91.18% recovery) cases. Overall flotation recovery at design grade is predicted at approximately 90%, while life-of-mine recovery is 88.8%. The flowsheet and flotation cell sizing were based on SGS locked-cycle testwork results. The report notes that the availability factors used (92% except 75% for primary crushing) are defined as estimated actual run time of equipment, including both mechanical availability and use of mechanical availability factors.
*Source: Mount Hope Project , 2014 Technical Report, Sections 17.1 through 17.14*
