AMC's 2018 Hermosa property resource and Taylor deposit PEA update for Arizona Mining Inc. plans the plant 80 km southeast of Tucson, with 60% of filtered tailings sent to backfill.
The Technical Report, Hermosa Property Mineral Resource and Taylor Deposit PEA Update, Santa Cruz County, Arizona, was prepared by AMC Mining Consultants (Canada) Ltd. for Arizona Mining Inc. and filed in 2018. Its recovery methods section sets the design criteria for crushing, grinding, flotation and dewatering at a 10,000 st/d (9,072 t/d) lead, zinc and silver plant for the Taylor deposit. The plant site is 80 km (50 miles) southeast of Tucson and 13 km north of the Mexican border.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Report | Hermosa Property Mineral Resource and Taylor Deposit PEA Update | Not applicable | AMC for Arizona Mining Inc., 2018 |
| Throughput | 10,000 st/d (9,072 t/d) | Not applicable | Lead, zinc and silver |
| Primary crushing | Jaw crusher to P100 243 mm (P80 117 mm) | Not applicable | From a nominal 20 inch feed |
| Live stockpile | About 10,000 st (9,072 t) | Not applicable | Reclaim tunnel with belt feeders |
| SAG grind | P80 2,191 | µm | SAG closed with a 10 mm screen |
| Flotation feed | 80% minus 150 | µm | Ball mill with hydrocyclones |
| Lead flotation | 8 x 50 m³ roughers, regrind, three cleaner stages | Not applicable | Not applicable |
| Zinc flotation | 8 x 50 m³ roughers, regrind, two cleaner stages | Not applicable | Zinc rougher tails are the final tails |
| Tailings | Filtered; 60% to backfill, rest to dry stack | Not applicable | Not applicable |
Overview
The plant uses a primary jaw crusher, a SAG mill closed with a vibrating screen, and a ball mill closed with hydrocyclones, followed by differential flotation into separate lead and zinc concentrates. Both concentrates are thickened, filtered and stored for truck shipment. Zinc rougher tailings form the final tailings. They are thickened and filtered, and 60% go to the backfill plant while the rest goes to a dry-stack tailings storage facility. The plant keeps separate lead and zinc process water streams, as well as fresh and potable water.
Key Process Stages
Primary crushing. Rear-dump trucks bring run-of-mine ore to the crushing area, where a front-end loader feeds a dump hopper and grizzly ahead of the jaw crusher. Two mobile rock breakers serve the crusher and the ROM stockpile. The crusher reduces ore from up to 500 mm to about 243 mm. A belt scale, tramp magnet and metal detector sit on the discharge conveyor.
Stockpile and reclaim. Crushed ore is stockpiled on the ground over a reclaim tunnel, with about 10,000 st of live storage. A front-end loader moves material from the dead to the live area when needed. Variable-speed belt feeders and a transfer conveyor feed the SAG mill, and wet dust collectors serve the stockpile and tunnel.
Grinding. The SAG mill discharges onto a double-deck screen with 10 mm bottom openings, and oversize returns to the mill by conveyor. The target SAG product is P80 2,191 µm. The ball mill and hydrocyclones produce flotation feed at 80% minus 150 µm, which is monitored by an on-stream analyzer and a particle size monitor. Zinc sulphate and sodium cyanide are added at the ball mill.
Lead flotation. Eight 50 m³ tank rougher cells produce a concentrate that is reground in a closed cyclone circuit. It is then cleaned in first cleaner, first cleaner scavenger, second cleaner and third cleaner cells. The third cleaner concentrate goes to the lead concentrate thickener. Lead rougher and cleaner scavenger tails go to zinc conditioning.
Zinc flotation. Eight 50 m³ rougher cells produce a zinc concentrate that is reground and cleaned in four first cleaner cells, eight first cleaner scavenger cells and three second cleaner cells, then thickened. Zinc rougher tails go to the tailings thickener.
Dewatering and tailings. The lead and zinc concentrates are thickened and filtered separately. Tailings are thickened and filtered, then split between paste backfill (60%) and the dry-stack facility.
Additional Interesting Data and Summary
On-stream analyzers sample cyclone overflow, rougher concentrates and tailings in both circuits for metallurgical control. Air compressors and dryers serve the crushing, grinding and flotation areas, and cranes are provided for mill and flotation maintenance.
In summary, the 2018 Taylor PEA is a conventional 10,000 st/d SAG-ball and sequential lead-zinc flotation design, notable for putting all its tailings through filtration so that most can return underground as backfill and the rest can be dry stacked.
Key Processes
- Jaw crushing and coarse stockpile reclaim
- SAG milling with screen and ball milling with hydrocyclones to 150 µm
- Lead rougher flotation, regrind and three-stage cleaning
- Zinc rougher flotation, regrind and two-stage cleaning
- Concentrate thickening and filtration
- Tailings filtration for paste backfill and dry stacking
Source: Technical Report, Hermosa Property Mineral Resource and Taylor Deposit PEA Update, Santa Cruz County, Arizona, USA, AMC Mining Consultants (Canada) Ltd. for Arizona Mining Inc., 2018.
Earlier report summaries
Earlier versions of this profile, based on older technical reports, are retained below for source traceability. Dates and figures reflect each report’s own effective date and are superseded by the current information above.
Hermosa Project — 2014 Technical Report
The 2014 technical report describes a proposed processing facility designed to treat mineralized material from the Upper Silver and Manto Oxide zones for recovery of silver, gold, copper, zinc, and manganese.
Report context
This technical report, dated 17 January 2014 and identified as M3-PN120103 Revision 0, presents the proposed processing design for the Hermosa Project. The processing facility is designed to treat mineralized material from two zones: the Upper Silver (LAg) mineralized zone and the Manto Oxide Zone. The report outlines unit operations including crushing, magnetic separation, reduction kilning, grinding, agitation leaching, counter-current decantation, Merrill-Crowe, solvent extraction and electrowinning, sulfidization acidification recycling and thickening (SART), and wet high intensity magnetic separation (WHIMS) with electrowinning to produce manganese metal.
Processing route
Proposed design basis
The plant is designed to process 5 million tons per year of Manto mineralized material through the crushing plant and magnetic separation circuit, with 3 million tons per year of the magnetic product through reduction kilning, grinding, and silver, zinc, copper and manganese recovery processes. Upper Silver material will be processed at 1.875 million tons per year through crushing, grinding, agitation leaching, counter-current decantation, and Merrill-Crowe sections only.
Crushing and magnetic separation
Run of mine mineralized material will be fed to a primary gyratory crusher, followed by secondary cone crusher and tertiary high pressure grinding roll crusher to reduce material to 100% minus 1.7 millimeters. Fine crushed material passes through magnetic separation screens and a magnetic separator to produce magnetic and non-magnetic fractions. The non-magnetic fraction reports to tailings while the magnetic fraction feeds the reduction kiln stockpile.
Manganese reduction and grinding
Magnetic concentrate is calcined in a direct fired horizontal rotary reduction kiln using natural gas. Kiln discharge is water quenched to form slurry, dewatered in a thickener, and fed to a two-stage grinding circuit consisting of a ball mill in closed circuit with hydrocyclones for the first stage and a vertical mill in closed circuit with hydrocyclones for the second stage. Final grind size is 80% minus 30 microns.
Silver and gold recovery
Silver and gold leaching occurs in two stages using sodium cyanide solution, with reagent supplied from both recovered cyanide and new make-up solution. Each leach stage includes absorption towers for hydrogen cyanide gas recovery. The first stage uses two leach tanks in series; the second stage uses four leach tanks in series. Pregnant leach solution is separated from solids using a five-stage counter-current decantation (CCD) circuit, with the first three thickeners for pregnant solution recovery and the last two for cyanide solution recovery.
Silver and gold are recovered from pregnant solution using Merrill-Crowe technology, which includes clarification by filtration, deaeration to reduce dissolved oxygen below 0.2 ppm, precipitation with metallic zinc dust, and collection by plate-and-frame filter presses. Precipitate is retorted at 650°C for mercury removal, then melted in an induction furnace and cast into Doré bars.
Zinc and copper recovery
Zinc is recovered from Merrill-Crowe barren solution by solvent extraction and electrowinning (SX/EW). The solvent extraction plant consists of one train of mixer-settler units with two extraction stages, four organic scrubbing stages, and two stripping stages operating in series. Zinc is plated onto aluminum cathode blanks in electrowinning cells arranged in two parallel banks. Copper is recovered from the SX raffinate using SART technology, which precipitates copper as copper sulfide (Cu₂S) filter cake and recovers cyanide as hydrogen cyanide gas for recycle to leaching.
Manganese recovery
Manganese is recovered from detoxified cyanide leach tail using wet high intensity magnetic separation (WHIMS) in a rougher stage followed by two cleaner stages. WHIMS concentrate is leached with sulfuric acid and ammonium sulfate in a series of three agitated tanks, followed by zinc precipitation with sodium hydrosulfide. Pregnant manganese solution is separated from solids using a three-stage CCD circuit, clarified by filtration, and fed to electrowinning cells where electrolytic manganese metal (EMM) is plated onto Type 316 stainless steel cathodes.
The EMM process design criteria include: cell potential 4 to 4.5 volts; cathode current density approximately 50 ASF; anode current density approximately 90 ASF; electrolyte temperature controlled to 35°C; catholyte pH 7.5-8; anolyte pH 1.0-1.5 with 35-50 gpl H₂SO₄; and assumed current efficiency of 65 percent. Energy consumption for electrolysis alone is estimated at approximately 4 kWh/lb EMM.
Water systems
Fresh water will be supplied from three wells for fire water, potable water, gland seal water, mine road dust control, crusher dust suppression, reagent mixing, and process water make-up. Process water is sourced from gypsum thickener overflow, manganese concentrate thickener overflow, tailing storage facility reclaim, and fresh water make-up.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Manto material processing rate (crushing and magnetic separation) | 5,000,000 | tons per year | Design |
| Manto magnetic product processing rate | 3,000,000 | tons per year | Design |
| Upper Silver material processing rate | 1,875,000 | tons per year | Design |
| Crushed product size | 100% minus 1.7 | mm | Design |
| Final grind size | 80% minus 30 | microns | Design |
| Silver recovery (Manto) | 79 | % | Design |
| Silver recovery (LAg) | 46 | % | Design |
| Gold recovery (Manto and LAg) | 90 | % | Design |
| Manganese recovery (Manto) | 28 | % | Design |
| Copper recovery (Manto) | 61 | % | Design |
| Zinc recovery (Manto) | 8 | % | Design |
| Manganese concentrate grade (WHIMS) | Approximately 35 | % Mn | Assumption |
| Strong electrolyte manganese concentration | 35 | gpl Mn | Design |
| Lean electrolyte manganese concentration | 15 | gpl Mn | Design |
| Catholyte pH | 7.5-8 | – | Design |
| Anolyte pH | 1.0-1.5 | – | Design |
| Anolyte sulfuric acid concentration | 35-50 | gpl H₂SO₄ | Design |
| Electrolyte temperature | 35 (95°F) | °C | Design |
| Cell potential | 4-4.5 | volts | Design |
| Cathode current density | Approximately 50 | ASF | Design |
| Anode current density | Approximately 90 | ASF | Design |
| Assumed current efficiency | 65 | % | Design assumption |
| Energy consumption (electrolysis) | Approximately 4 | kWh/lb EMM | Estimate |
| Annual EMM production rate | 55,000 | short tons | Design |
| Soluble manganese loss (CCD) | Approximately 7 | % | Assumption |
| Manganese recovery from cyanidation residue | 68 | % | Design |
| Deposition rate | 1.00 lb EMM per 442.54 amp hours | – | Design |
| Daily deposit thickness | Approximately 2.1 (0.0833-inch) | mm | Design |
| Cathode center-to-center spacing (preliminary) | 4 (101.6) | inches (mm) | Design |
| Dissolved oxygen after deaeration | Less than 0.2 | ppm | Design |
Project website: https://www.aztechcouncil.org/south32-hermosa-project/
Project website: https://www.patagoniaalliance.org/arizona-mining-inc-wildcat-silver/
Project website: https://www.patagoniaalliance.org/take-action-hermosa-mining-proposal-arizona-mining-wildcat-silver/
Technical qualifications
The report notes that the manganese products industry is secretive with no significant contributions to technical literature since the 1970s. Preparation of the study relied largely on personal communications with individuals familiar with current Chinese technology, equipment selection, and operating practices. Process design criteria for manganese electrowinning were developed from considerations and recollections of actual operating conditions in a commercial EMM plant by one of the qualified persons.
Reagent consumption rates for full scale plant operation were estimated from test results when available; when test results were not available, estimates were based on experience from similar processes.
Potential manganese losses identified for future laboratory examination include: a bleed stream for impurity buildup control; removal of Mn/Mg/NH₄ sulfate by crystallization; and anode corrosion into cell bottom sludge.
Source: Hermosa Project Form 43-101F1 Technical Report, 17 January 2014, Revision 0, Sections 17 Recovery Methods, 17.1-17.15.
