This report describes the processing facilities at the Jerritt Canyon Gold Mine, covering historical operations, the current whole-ore roasting route, and the associated circuit details.
Report context
The Jerritt Canyon Gold Mine, located in Nevada, has been in operation since 1981. This 2021 Technical Report, dated April 30, 2021, was prepared by SLR for First Majestic Silver Corp. as the project operator. The processing facilities are described at design, permitted, and actual 2020 operating rates.
Processing route
Historical processing
Mining and processing by the Freeport-FMC JV commenced at Jerritt Canyon in 1981. The initial processing facility treated mildly refractory, carbonaceous, preg-robbing mineralized material. Alkaline chlorination was used to passivate the carbonaceous constituents to reduce preg-robbing during cyanidation. When chlorine consumption increased due to higher sulfide sulfur levels, the operating cost of the chlorination circuit increased. Roasting was selected as an alternative oxidation process capable of treating both carbonaceous and sulfide refractory material. The whole-ore roaster was commissioned in 1989, and from 1989 to 1997 the two plants operated in parallel: the chlorination circuit treated less refractory material at 3,500 stpd to 4,200 stpd until 1997, while the more refractory carbonaceous shale was processed in the roaster at a design rate of 4,000 stpd. Pilot plant testing of the oxygen-roasted calcine by Dorr Oliver (1987) achieved an average gold recovery of 88%.
Current processing plant
The processing facilities are designed to operate at 4,500 stpd with 90% operating availability and are permitted at 6,000 stpd. During 2020, the plant operated at an average rate of 2,265 stpd. The facilities include primary crushing, ore drying, secondary and tertiary crushing, dry grinding, roasting, thickening, carbon-in-leach (CIL), carbon stripping, carbon reactivation, electrowinning, electrowinning sludge refining, an oxygen plant, cooling pond, water evaporation pond, and a tailings impoundment.
Primary crushing
Run-of-mine mineralized material is crushed to minus six inches using a 36 in. by 42 in., 200 hp jaw crusher.
Drying
Crushed material from primary crushing is fed through a 100 st surge bin and a 48 in. by 20 in. apron feeder to a propane-fired rotary dryer. The dryer is 14 ft by 60 ft, driven by a 350 hp variable speed motor at two to four revolutions per minute, with 300 internal lifters. A Hauck Starjet Sj750 Ratiomatic burner is fitted at the feed end. Feed rate is controlled to maintain discharge moisture below one percent. Discharge air passes through two pulse-jet baghouses (810 bags each) and then a 56 ft dual-bed wet scrubber with 15 ft of Tri-Mer packing per bed for particulate and mercury removal. Baghouse product is pneumatically conveyed by an FLSmidth Fuller-Kinyon pump directly to the roaster feed bin.
Fine crushing
Dried material is conveyed to a 5 ft by 12 ft double-deck vibrating screen with ¾ in. top and ¼ in. bottom panels. Oversize feeds a 4.25 ft standard Omnicone crusher; minus ¼ in. material bypasses the crusher and reports to the 2,000 st fine ore bin (FOB). Approximately 30% of dryer discharge is minus ¼ in. Secondary crusher product is sent to a 100 st tertiary feed bin with two discharge points, each feeding a 6 ft by 16 ft double-deck screen. Oversize is crushed in 4.25 ft short head Omnicone crushers. Minus ¼ in. material bypasses to the fines conveyor and then to the FOB. A north and a south baghouse serve the fine crushing circuit; the north baghouse uses a puker system and the south baghouse an FLSmidth Fuller-Kinyon pump to convey collected fines to the roaster feed bin.
Grinding
Ore from the FOB feeds a 14.5 ft by 18.5 ft ball mill driven by a 2,750 hp synchronous motor. The circuit was designed to reduce material from P100 ¼ in. to P100 35 mesh (500 µm). The mixed ball charge is approximately 40% by volume, with makeup ball sizes of 2.5 in., 2.0 in., and 1.5 in. Discharge passes through grates to an air slide and a bucket elevator that feeds an Osepa air classifier. The circulating load is approximately 300%. Classifier fines report to separator cyclones where about 88% of fines are removed from the air stream and collected in the product bucket elevator feed air slide. The remaining 12% of entrained fines in the air stream report to a 700 hp, 131,000 acfm separator fan; 66% to 75% of this stream is recycled to the air classifier and 25% to 33% is bled to the classifier baghouse. The classifier baghouse fan (125 hp) pulls 44,800 acfm through the bag filters, with captured dust transported via air slide and a 60 hp bucket elevator to the roaster feed bin.
Roasting
Roaster feed is lifted approximately 135 ft by a bucket elevator and discharged through an air slide and disengaging bin into a fluidized feed distributor. The roaster system consists of two identical, side-by-side trains. Each train is permitted for 125 dry stph, but normal operating conditions process 90 to 100 dry stph. The two-stage, countercurrent, fluid-bed roasters use relatively pure oxygen for fluidization and oxidation. Pulverized coal is added because the mineralized material lacks sufficient fuel value to sustain combustion. First-stage temperature is 1,030°F to 1,050°F; retention time is one minute per 4 in. water column (WC), for example 24 minutes at 95 in. WC in the first stage and 16 minutes in the second stage at 65 in. WC. Second-stage discharge temperature is approximately 900°F. Calcine discharges to quench tanks for cooling.
Gas handling
The gas handling circuit maintains a negative differential pressure inside the roaster to prevent dust and toxic gas release. Off-gas from the roaster first-stage secondary cyclone is processed to remove particulate, sulfur dioxide, and mercury vapour before clean gas is discharged through the roaster stack.
Carbon-in-leach
Cooled roaster quench slurry is pumped to a 100 ft diameter thickener. Overflow goes to a cooling pond for recycle to the roasters; underflow passes across a vibrating trash screen with oversize recycled to primary crushing. Trash screen undersize, approximately P80 200 mesh, feeds the first of six CIL tanks. Milk of lime and liquid sodium cyanide are added in the first CIL tank, with cyanide addition possible in subsequent stages. Each tank holds 20 g/L of coconut shell 6 by 12 mesh activated carbon retained on 18-mesh screens. Slurry flows by gravity from tank 1 to tank 6. Loaded carbon is advanced countercurrent to slurry flow using carbon advance pumps, from CIL tank 6 toward tank 1 and then to the strip circuit. Slurry from CIL tank 6 passes through a 30-mesh safety screen before an in-line Heath & Sherwood sampler cuts a tailings sample, prior to the tailings flowing to the tailings storage facility.
Carbon strip circuit
Activated carbon is stripped in six-ton batches using heat and pressure. Strip solution temperature is 280°F at 50–60 psi, with a flow rate of approximately 40 gpm and an average strip time of 12 to 14 hours. Pregnant strip solution reports to two electrowinning cells (150 ft³ and 50 ft³) operating in series.
Refinery
Electrowinning cells are cleaned on a batch basis; filtered sludge is dried in a mercury retort for 48 hours at up to 1,150°F to remove mercury prior to smelting. Dried sludge is mixed with flux (borax, niter, soda ash, silica) and smelted in an induction furnace. Doré bars of approximately 1,000 oz, containing about 90% to 95% gold, are poured and shipped off site for refining. Mercury is collected from the retort condenser in one-ton flasks; approximately five to six tons of mercury are produced annually and shipped to a third-party storage company.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Design processing rate | 4,500 stpd | Design |
| Permitted processing rate | 6,000 stpd | Permit |
| 2020 average plant throughput | 2,265 stpd | Actual |
| Design operating availability | 90% | Design |
| Chlorination plant throughput (1989–1997) | 3,500–4,200 stpd | Historical |
| Roaster design throughput (1989 onward) | 4,000 stpd | Design |
| Roaster design recovery (oxygen, not air) | 87%–90% | Design |
| Pilot plant calcine gold recovery (Dorr Oliver, 1987) | 88% average | Testwork |
| Primary crusher product | Minus 6 in. | Design |
| Dryer dimension | 14 ft × 60 ft | Design |
| Dryer motor | 350 hp variable speed | Design |
| Dryer rotation | 2–4 rpm | Design |
| Dryer discharge moisture target | <1% | Operating |
| Ball mill | 14.5 ft × 18.5 ft, 2,750 hp | Design |
| Grinding product size | P100 35 mesh (500 µm) | Design |
| Ball charge volume | ~40% | Design |
| Makeup ball sizes | 2.5, 2.0, 1.5 in. | Design |
| Grinding circulating load | ~300% | Design |
| Separator fan | 700 hp, 131,000 acfm | Design |
| Classifier baghouse fan | 125 hp, 44,800 acfm | Design |
| Roaster trains | 2 identical side-by-side | Design |
| Roaster permit per train | 125 dry stph | Permit |
| Roaster normal operating rate per train | 90–100 dry stph | Actual |
| Roaster first-stage temperature | 1,030–1,050°F | Operating |
| Roaster second-stage discharge temperature | ~900°F | Operating |
| Roaster retention time | 1 min per 4 in. WC | Design |
| CIL tanks | 6 in series | Design |
| CIL carbon concentration | 20 g/L coconut shell 6×12 mesh | Design |
| CIL carbon retention screen | 18 mesh | Design |
| CIL feed size (P80) | 200 mesh | Design |
| Strip batch size | 6 tons carbon | Design |
| Strip temperature | 280°F | Design |
| Strip pressure | 50–60 psi | Design |
| Strip flow rate | ~40 gpm | Design |
| Strip time | 12–14 hours | Design/Average |
| EW cells | Two: 150 ft³ and 50 ft³ | Design |
| Retort temperature | 1,150°F | Design |
| Retort time | 48 hours | Design |
| Doré weight | ~1,000 oz | Typical |
| Doré gold content | 90%–95% | Typical |
| Annual mercury production | 5–6 tons | Typical/Actual |
Project website: https://www.firstmajestic.com/projects/exploration-development/jerritt-canyon/
Project website: https://miningrecord.com/recommencement-of-exploration-drilling-at-jerritt-canyon/
Technical qualifications
The report provides design, permitted, historical, and actual operating data where indicated. Values presented as design (e.g., 4,500 stpd) are not necessarily demonstrated by sustained operation. Historical throughputs (e.g., chlorination at 3,500–4,200 stpd, roaster at 4,000 stpd) show earlier operating periods. Testwork results from the 1987 Dorr Oliver pilot plant are noted as pilot-plant-scale. The 2020 average throughput of 2,265 stpd indicates that actual performance may differ substantially from design. The report notes that some mineralized materials are reported to have zero gold recovery without oxidation, and that process performance depends primarily on roaster performance. The report also identifies arsenic and mercury as the main deleterious elements and describes installed gas cleaning and mercury collection systems.
Technical report title
Jerritt Canyon Gold Mine, SLR Project No: 233.03396.R0000, NI 43-101 Technical Report, April 30, 2021, Section 17.0 Recovery Methods and subsections.
