Back Forty Project: Integrated Flotation and Cyanidation Flowsheet for Base and Precious Metals

The Back Forty Project in Michigan, USA, is a development-stage polymetallic property targeting gold, silver, zinc, and copper. Owned by Goldgroup Mining Inc., the project is documented in a 2023 Preliminary Economic Assessment with processing facilities designed to handle ore from both open pit and underground operations.

Critical Data

Overview

The Back Forty processing plant is designed to run 24 hours per day, 365 days per year. The flowsheet combines flotation stages for base metal recovery with cyanidation stages for precious metals. Three separate facilities make up the plant: a crushing facility with primary, secondary, and tertiary crushers; a coarse ore silo with feeding and reclaim systems; and a main processing building that houses grinding, flotation, leaching, dewatering, and cyanide recovery circuits. Some equipment, including leach tanks, cyanide destruction tanks, and pre-leach thickeners, sits outdoors near the main building.

The mill was sized using specific energy calculations performed by Orway Mineral Consultants. Flotation cells were sized using a scale-up factor of 2.5 from laboratory locked-cycle test residence times.

Key Process Stages

Ore comes from the open pit during early years and from the underground mine in later years. Trucks haul ore to the crushing circuit and dump onto a static grizzly with 750 mm openings. A hydraulic rock breaker handles oversized rocks. Material collects in the crushing plant feed hopper where rocks at 95% solids are metered at 138 t/h to a vibrating grizzly feeder. Oversize goes to a jaw crusher. The crusher product and grizzly undersize transfer by gravity to a sacrificial conveyor, then to a primary classification screen on top of the mill feed silo. That screen removes material above 9.5 mm and returns it to a secondary classification screen. The secondary screen sends rock above 23 mm to a secondary cone crusher and rock below 23 mm to a tertiary cone crusher. Material passing the primary screen at minus 9.5 mm feeds the silo. Secondary and tertiary crusher products recirculate to the primary screen.

Ore from the crushing circuit moves to a 2,500-tonne capacity ore silo with 24-hour residence time. Apron feeders reclaim the ore and discharge onto the ball mill feed conveyor at 113 t/h. The single-stage grinding circuit uses a ball mill with 3,200 kW installed power, 4.88 m diameter, and 100 mm grinding media. Lime and process water are added to reach 72% feed density. Feed size is 9.5 mm (F80), and target product size is 50 µm (P80). The ball mill operates in closed circuit with a cyclone cluster. Cyclone underflow recirculates to the mill. Cyclone overflow goes through a trash screen then to copper flotation. The ball mill has an automated grinding media feeder.

Copper flotation uses a pre-aeration stage with two 10 m³ cells, six rougher cells at 10 m³ each, and four cleaner stages: four first cleaner cells at 5 m³, two first cleaner scavenger cells at 5 m³, two second cleaner cells at 5 m³, and one third cleaner cell at 5 m³. Fresh feed enters at 31% solids. The regrind mill is set to a target product size of 15 µm (P80). A cyclone cluster of ten cyclones feeds the regrind circuit. Cyclone overflow flows by gravity to the cleaner circuit. Cyclone underflow goes to the regrind mill. The regrind mill has 1,475 kW installed power and uses 150 mm diameter cyclones. An on-stream analyzer provides live metal grade information for 16 flotation streams. A particle size analyzer monitors size distribution for copper rougher feed, copper cleaner feed, and zinc cleaner feed. The target copper concentrate grade is 18% Cu.

Zinc flotation takes the copper rougher tails and copper first cleaner scavenger tails as feed. The circuit has two rougher conditioning cells at 20 m³, seven rougher cells at 20 m³, two first cleaner conditioning cells at 10 m³, four first cleaner cells at 10 m³, four second cleaner cells at 5 m³, and two third cleaner cells at 5 m³. The target zinc concentrate grade is 55% Zn. Zinc rougher tails go to the pre-leach thickener.

Copper concentrate from third cleaner flotation is pumped to a 4 m diameter thickener. Underflow at 65% solids goes to a single horizontal press filter that reduces moisture to 8%. The filter discharge is stockpiled and trucked to an offsite rail loadout for shipping. Zinc concentrate from third cleaner flotation goes to a 10 m diameter thickener. Underflow at 65% solids goes to two horizontal filter presses that reduce moisture to 8%. One zinc concentrate filter operates for the first four years. A second unit comes online in year five for increased production. Zinc filter discharge is also stockpiled and trucked to the rail loadout.

Zinc rougher tails are pumped to a 14 m diameter pre-leach thickener where flocculant is added. Thickened slurry at 67% solids is mixed with lime, cyanide-bearing solution recycled from the SART process, and new cyanide make-up. The selected sodium cyanide concentration is 1 g/L. Slurry flows through four leach tanks with 24-hour residence time. Each tank holds 1,434 m³, has a 55 kW agitator, and is aerated. Slurry then gravity feeds to a filter feed tank and is pumped to three horizontal press filters. Wash water circulates through the press filters to remove residual cyanide and precious metals. Discharged cake at 15% residual humidity goes by conveyor to a bin, then is trucked to the dry stack tailings facility. Filtrate goes to the reverse osmosis system.

The reverse osmosis unit operates under high pressure. It separates incoming water into a low-salt permeate stream at about 60% of feed flow and a high-salt retentate stream at about 40% of feed flow. The membrane rejects over 95% of all ions including SCN-, CNO-, and cyanide into the retentate. Antiscalants are dosed upstream to minimize scaling. The retentate goes to desaturation reactors with 45-minute hydraulic retention time. Ferric at 5 to 10 ppm and flocculant are added to enhance solid formation. A clarifier separates solids, with underflow recirculated as seed or sent to a filter press. Clarifier overflow goes to the SART circuit.

The SART process uses a primary reactor where sulfuric acid and a sulphidization agent are added to precipitate metal sulfides. Clarifier underflow goes to a press filter that dewaters solids to 8% moisture. Saleable metal concentrate is bagged and sold to a smelter. Clarifier overflow containing HCN goes to a second reactor where lime neutralizes the solution and converts HCN back to sodium cyanide. Gypsum precipitates and is removed. The recovered cyanide solution goes to the Merrill-Crowe plant.

Precious metal recovery uses Merrill-Crowe zinc dust cementation. The nominal solution feed rate is about 100 m³/h. Pregnant solution goes through clarifying disk filters to reduce suspended solids below 5 mg/L. Filtrate enters a de-aeration tower where dissolved oxygen is removed under vacuum. De-aerated solution is mixed with zinc dust slurry and pumped to precipitate filters. Filtrate reports to the barren solution tank and is recycled to the leaching circuit as a cyanide source. Precipitate cake is loaded into a mercury retort, then mixed with fluxes and smelted in an electric furnace. Gold and silver are cast as doré ingots.

Cyanide destruction uses the SO2/Air process in two tanks in series with 85-minute design residence time. The feed comes from dry stack press filter wash water and clean RO permeate. Sodium metabisulfite and copper sulfate are mixed in before the first tank. Compressed air sparges both tanks. Effluent is pumped to the water treatment pond.

Additional Interesting Data and Summary

The process plant requires 52 workers total: 10 salaried staff and 42 hourly workers. The operations department accounts for 31 people, including a mill manager, metallurgists, technicians, supervisors, operators, laborers, and loader operators. The maintenance department employs 21 people, including supervisors, technicians, planners, mechanics, electricians, and instrument technicians.

Reagent dosing rates include lime at 5.48 kg/mt, sodium cyanide at 0.677 kg/mt, copper sulfate at 0.488 kg/mt, sodium metabisulfite at 0.68 kg/mt, sulfuric acid at 4.31 kg/mt, and zinc sulfate at 0.321 kg/mt. Other reagents include flocculant at 0.04 kg/mt, MIBC frother at 0.031 kg/mt, and antiscalant at 0.026 kg/mt.

Water services include a process water tank at 580 m³ with 1.5-hour residence time, a fresh water tank at 215 m³ with 1.5-hour residence time, and a gland water tank at 80 m³ with 3-hour residence time. Excess process water goes to the wastewater treatment plant. Fresh water comes from the contact water basin.

Key Processes

• Crushing: Three-stage crushing circuit with jaw crusher, secondary and tertiary cone crushers, and classification screens
• Grinding: Single-stage ball mill in closed circuit with cyclones, targeting 50 µm P80
• Copper flotation: Rougher, regrind, and four cleaner stages with 15 µm regrind target
• Zinc flotation: Rougher and three cleaner stages fed by copper circuit tails
• Concentrate dewatering: Thickening and pressure filtration to 8% moisture for both copper and zinc concentrates
• Cyanide leaching: Four agitated leach tanks with 24-hour residence time at 1 g/L NaCN
• Solution dewatering: Pressure filtration producing dry stack tailings at 15% humidity
• Reverse osmosis: Membrane system concentrating cyanide and gold into retentate
• SART: Cyanide recovery with copper and zinc precipitation as saleable concentrate
• Merrill-Crowe: Zinc dust cementation for precious metal recovery at 99.5% efficiency
• Cyanide destruction: SO2/Air process in two stages with 85-minute residence time

Source: NI 43-101 Preliminary Economic Assessment on the Back Forty Project, Michigan, USA, November 15th, 2023. Project website: Back Forty Project