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.
