The prefeasibility study for the Soto Norte Project describes a 2,750 tonne per day processing plant with gravity gold, copper flotation, and pyrite flotation circuits designed to treat ore from the underground mine and purchased mill feed.
Report context
This report is dated effective August 18, 2025, and presents the prefeasibility study for the Soto Norte Project located in Santander, Colombia. The processing route described herein is based on metallurgical testwork programs undertaken between 2009 and 2018, with samples representative of the mineral resource and mineral reserve estimates. The processing flow sheet differs from the previous 2021 feasibility study by reducing throughput from 7,000 tpd to a nominal 2,750 tpd and maximum 3,500 tpd, adding a separate receiving facility for purchased mill feed, including a ball mill for finer grinding, adding a gravity gold recovery circuit, and including a paste fill plant.
Processing route
Feed preparation and transfer to surface
Ore will be crushed underground to a maximum size of 150 millimetres and transferred to a surface conveyor bin. An apron feeder will extract the ore for transport by rope conveyor to the process plant. The ore discharges onto a cleaner conveyor fitted with a hammer sampler for metallurgical testwork and a cross belt magnet to remove tramp metal. Low quality ore can be directed to a separate stockpile via a diverter chute and blended as required.
Mill feed purchased from contract mining partners will be delivered by truck to a secondary run of mine stockpile with approximately 40 tonne capacity. A secondary vibrating feeder transfers the feed onto a conveyor fitted with a weightometer and metal detector for discharge onto an impact crusher. The crusher discharges onto a transfer conveyor fitted with a hammer sampler for head grade analysis and metallurgical accounting.
The combined run of mine ore is transferred to a final 12,000 tonne live capacity stockpile above the process plant tunnel. Two apron feeders fitted with a cross belt magnet, metal detector, and end belt magnet withdraw ore onto a conveyor discharging into the SAG mill.
Grinding and gravity concentration
Primary grinding occurs in a closed circuit, single stage 1,305 kilowatt SAG mill. The milled material reports to a single deck vibrating classifying screen. SAG oversize material greater than 20 mm is transferred to a pebble crusher surge bin, then extracted by a vibrating feeder to the pebble crusher, with crushed pebbles recycled to the SAG mill. SAG undersize material less than 20 mm discharges onto a cyclone feed hopper and is pumped to the primary cyclone cluster for classification.
The cyclone underflow reports to a screen for further classification. Screen oversize greater than 2 mm is recycled to a 2,237 kilowatt ball mill operating in closed circuit with a hydrocyclone cluster for regrinding. The ball mill grinds material to a P 80 of 106 microns. Cyclone overflow is pumped to the sequential flotation circuit.
Screen undersize less than 2 mm reports to a Knelson gravity concentrator circuit. Recovered concentrate is upgraded on a shaking table, pumped to a gold concentrate filter press feed tank, then dewatered in a filter press. The filter cake is discharged into a product bin for collection and transport to port. Gravity circuit rejects and screen undersize are recycled to the ball mill. The gravity concentrator circuit receives a material load equivalent to the fresh feed of the milling circuit.
Sequential copper and pyrite flotation and regrind
Hydrocyclone overflow from the SAG mill and gravity circuit tailings are pumped to the copper rougher flotation tank. Slurry is pumped into a bank of four forced air rougher flotation cells with additions of lime slurry, frother, and copper collector reagents. The low grade copper concentrate is pumped to the copper concentrate regrind circuit for further liberation before copper cleaning. Copper rougher flotation tailings report to the pyrite conditioning tank and feed the pyrite rougher flotation circuit.
The copper rougher concentrate is reground in open circuit with a hydrocyclone cluster. A three stage copper cleaning operation includes a copper cleaner flotation circuit, copper recleaner flotation circuit with forced air mechanical flotation cell tanks, and a copper tertiary cleaner flotation circuit with a Jameson flotation cell. Slurry from the regrind mill circuit flows into three copper cleaner flotation cells, with tailings recirculated to the copper rougher flotation circuit and concentrate pumped to two copper recleaner flotation cells. The recleaner concentrate is pumped to the Jameson cell, while recleaner tailings return to the copper regrind mill circuit. Jameson cell concentrate is pumped to the copper concentrate thickener, and tailings are pumped back to the copper recleaner flotation circuit.
Copper rougher tailings and pyrite cleaner tailings report to the pyrite conditioning tank and are pumped into seven pyrite rougher flotation cells operating in series. Pyrite rougher tailings are sent to a dedicated tank feeding both the tailings thickener and pulse back filters. Low grade pyrite rougher concentrate is sent to a pyrite regrind circuit consisting of a single regrind mill operating in open circuit with a hydrocyclone cluster. Regrind mill product is combined with fine cyclone overflow and pumped to five pyrite cleaner flotation cells. Cleaner tailings are recirculated to the pyrite rougher flotation cells, while concentrate reports to three pyrite recleaner flotation cells. Pyrite recleaner tailings report to the pyrite regrind mill cyclone fill tank, and pyrite recleaner concentrate reports to the pyrite concentrate thickener.
Concentrate dewatering
Copper and pyrite concentrates are sent to a single dedicated high rate thickener and a horizontal plate pressure filter press. Concentrate thickener overflow reports to an overflow tank with process water recirculated to the flotation circuit. Concentrate underflow is thickened to 55 to 60% solids, pumped to an agitated filter feed tank, then to the concentrate filter press and dewatered to produce a filter cake with 9% moisture content. Filtrate is collected in a concentrate filtrate tank and pumped back to the concentrate thickener overflow tank. Concentrate filter cake is discharged into a dedicated bin with bottom screw feeders, placed on a concentrate conveyor fitted with a weightometer, and transported to port.
Thickened tailings dewatering and storage
The tailings thickening circuit consists of a tailing thickener feed box and a high rate thickener to thicken final tailings to 55 to 60% solids. Thickener overflow is directed to an overflow tank then pumped to the water treatment plant. Thickened underflow is pumped to an agitated tailings transfer tank, then to the tailings filter feed tank and three parallel tailings filter presses producing a tailings cake with 15% moisture content. Filter cake is discharged onto conveyors for stacking and compaction at the filtered tailings facility. Filtrate is collected in a tailings filtrate tank, pumped to a reservoir tank, then passed through a water treatment plant.
Pyrite rougher flotation tailings are pumped to the tailings thickener for pressure filtration prior to stacking or to the paste fill plant. For paste filling, tailings slurry is pumped to pulse back filters to reduce water content to 65 to 67% solids, then pumped onto the rope conveyor for transfer to a discharge tank at the mine portal. Filtrate from the pulse back filters is sent to the tailings thickener overflow tank.
Reagents and consumables
Powder lime is delivered in bulk road tankers and unloaded into a storage silo at the lime preparation plant. Liquid copper collector is received in containers and pumped to the copper rougher flotation conditioning tank. Liquid frother is transferred to a day tank for distribution throughout the copper circuit excluding tertiary flotation, and throughout the pyrite circuit. Pellet form pyrite collector is delivered in bags or boxes and unloaded into a feed bin equipped with a dust filter. The pyrite collector is mixed with raw water and pumped to the pyrite rougher and cleaner flotation cells. Pellet form gangue depressant is delivered in bags and unloaded into a feed bin, then mixed with water for depression in the copper cleaner and pyrite cleaner flotation cells. Liquid pyrite activator is delivered in bulk road tankers, stored, and pumped to the pyrite flotation circuit. Bulk bagged flocculants are stored in bins, mixed with raw water, and pumped via an inline mixer.
Power, water, and air
Estimated power requirements are 13 MW at the process plant, supplied from the Palos 115 kV substation via a 34 km long 115 kV, 45 MVA capacity overhead line to a new substation at the process plant, where voltage is stepped down to 13.8 kV. Standby and emergency power is provided by a 3.125 MW diesel generator station at the process plant, a 630 kW diesel generator at the filtered tailings facility, a 250 kW diesel generator at the water intake plant, and a 250 kW diesel generator at the emergency ponds.
Construction water will be sourced from La Baja Creek and Suratá River. During operation, the main source of water for the underground mine will be from groundwater, and the main source for processing will be from the Suratá River. Approximately 96.5% of total water requirements will be reused, recycled, and returned, with net water use estimated at 3.5%. The Suratá River will supply approximately 2.8 litres per second of net make up water for the processing plant, representing 0.22% of the average flow at the planned water access point and 0.08% of the average flow at Bucaramanga, 55 km downstream.
Water from mine dewatering, seepage from the filtered tailings facility, and process water streams report to seven water treatment plants. These include a 0.68 litre per second plant for potable water at the mine, a 0.51 litre per second plant for camp waste water, a 43.18 litre per second plant for waste water from the underground mine and paste plant, a 0.34 litre per second plant for potable water at the processing plant, a 34.12 litre per second plant for thickener bleed water, a 29.84 litre per second plant for tailings filtration water, and a 0.13 litre per second plant for potable water at the filtration plant. Treated water from plants at the mine will discharge at La Baja Creek, and treated water from other plants will discharge to the Suratá River.
Three low pressure air blowers provide flotation cells with air for particle attachment. Three air compressors provide high pressure air for plant instruments and general service points. Filter press systems have dedicated compressor systems for cake blowing and drying.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Nominal plant feed throughput | 2,750 tpd | Design |
| Maximum plant feed throughput | 3,500 tpd | Design |
| Purchased mill feed capacity | 750 tpd | Design |
| Annual operating days | 365 days | Design |
| Operating shifts | Two 12-hour shifts | Design |
| Operating availability | 91.3% | Design |
| SAG mill power | 1,305 kW | Design |
| Ball mill power | 2,237 kW | Design |
| Grind size | P 80 of 106 microns | Design |
| Gravity gold recovery | Up to 15% | Design |
| Copper concentrate target grade | 16% | Testwork based |
| Non-sulphide waste in concentrates | Restricted to 10% | Design |
| Concentrate filter cake moisture | 9% | Design |
| Tailings filter cake moisture | 15% | Design |
| Paste fill solids content | 65 to 67% | Design |
| Average gold recovery | 92.8% | Design |
| Average silver recovery | 88.8% | Design |
| Average copper recovery | 92.8% | Design |
| Process plant power | 13 MW | Design estimate |
| Power requirement per annum | 13 MW | Current prefeasibility study |
| Power requirement per annum | 39 MW | 2021 feasibility study |
| Net water use | 3.5% of total | Design |
| Suratá River net make up water | 2.8 litres per second | Design |
Project website: https://aris-mining.com/operation/soto-norte/
Technical qualifications
The metallurgical testwork programs supporting the processing flow sheet were undertaken between 2009 and 2018 using samples representative of the mineral resource and mineral reserve estimates. The target grade of 16% for the copper concentrate is based on testwork results as well as considerations for maximizing gold content and reducing penalty element concentrations. Potential penalty elements considered in the economic analysis include arsenic, bismuth, cadmium, antimony, and likely zinc. The power requirements estimated in the current prefeasibility study are estimated at 13 MW per annum, compared to the 39 MW estimated in the 2021 feasibility study.
Source: Soto Norte Project , 2025 Technical Report, Prefeasibility Study for the Soto Norte Project, Santander, Colombia, sections 17.1, 17.2, 17.3, 17.4, and 17.5, effective August 18, 2025.
