This report describes the existing sulphide ore processing plant at the Chapada Mine and the planned oxide ore heap leaching facilities, with all data drawn from the July 31, 2014 technical report.
Report context
The Chapada Mine treatment plant is designed to treat sulphide ore at a nominal rate of 60,000 tpd, with process recoveries for copper and gold averaging approximately 80% and 59% respectively from June 2013 to May 2014. Run of mine material from the Suruca mineralization will be treated and incorporated into the system through two separate processes: oxide ore will be processed using conventional heap leaching technology, scheduled to start production in late 2016, and sulphide ore will be processed in the existing plant after some modifications.
Processing route
Sulphide ore
The first step of the process occurs in the primary grinding circuit in two parallel crushing systems. The primary crushing system consists of a roll crusher and secondary jaw crushers. Crushing is followed by grinding in a semi-autogenous mill. The ore is then sent to flotation, thickening, and filtration processes. Tailings are placed in a tailings storage facility, where the embankments are constructed using the coarser material from the grinding plant.
Primary crushing
The comminution process begins with the crushing circuit. Two parallel systems receive the mined ore. The first system, with a capacity of up to 3,500 dmt/hr, comprises an MMD 1000 Sizer and five inch grate for scalping the fine ore. The second system, with a capacity of up to 2,000 dmt/hr, comprises a Metso C160 jaw crusher and a five inch grate for scalping the fine ore. Both systems can perform the primary crushing with a P70 of five inches. The ore processed in the two systems, with a five inch P80, is then transported by conveyor belt to an intermediate stockpile.
Grinding
The grinding circuit is divided into four systems: reclaim ore, primary grinding and pre-classification, pebble crushing, and secondary grinding and classification.
The feeder conveyor belt, measuring 1.4 m wide and 246.8 m in length, powered by a 373 kW motor, carries the ore from the stockpile and discharges on the SAG mill in-feed conveyor. The conveyor runs at a fixed speed and also receives material from the pebble crusher and ball mill. The nominal capacity of this system is 3,500 tph. The conveyor is equipped with a scale for measuring the feed rate of ore to the mill.
The purpose of the primary grinding and pre-classification circuit is to reduce the ore to a size small enough to expose most of the surfaces of mineral particles containing gold. Approximately 80% of the product is less than 1.06 mm.
The pebble crushing circuit fragments material that is too small to be milled efficiently in the SAG mill and too large to be sent to the ball mill. This material could accumulate in the SAG mill, reducing mill capacity. The crushing of this material removes this risk and results in increased efficiency of the SAG mill grinding process.
The secondary grinding and classification circuit further reduces the milled material from the SAG mill to deliver 80% passing 210 microns to the flotation circuit. The secondary grinding process comprises classification by cyclone and grinding by ball mill.
Flotation
The aim of the flotation circuit is to recover over 85% of the copper and 59% of the gold from the plant feed. The flotation circuit collects the copper minerals and gold in a concentrate with a concentrate grade of approximately 26% Cu and 15 g/t Au.
The ore is brought to the flotation process in pulp form with approximately 35% solids. The pulp flows from the cyclone overflows for both primary and secondary grinding circuits to a flow distributor in the flotation circuit.
There are two flotation cell lines, rougher and rougher/scavenger. Each cell line produces two concentrates. The rougher concentrate is produced in the first two cells of each line. Depending on the grade of the concentrate in the third cell, it can be remixed with the rougher or the rougher/scavenger concentrates. The rougher concentrate is sent to the concentrate regrind circuit and the rougher/scavenger concentrate is returned to the primary or secondary regrind circuit. The tailings from the rougher/scavenger system are sent to the final tailings storage facility.
Thickening and filtering
The last step in the sulphide process is thickening and filtration. The column flotation material is thickened and dewatered in a thickener tank which is 13 m in diameter. The thickening process involves passing the pulp, containing 60% solids, through 12 LAROX filter plates which reduce the ore concentrate moisture content to an average of 8%. The average filter output is approximately 45 tph. This is discharged in the concentrate storage shed to be loaded and shipped to customers.
Tailings storage facility
The flotation tailings are pumped to a pressure boosting system consisting of a reservoir box, three pumps, and a security system which monitors the pumping system in case of problems. The tailings are pumped with enough pressure to be processed by a 20 inch hydrocyclone. The fines from the hydrocyclone are deposited in the tailings basin and the coarse material is used for dam construction.
Tailings consisting of approximately 60% solids are deposited into the tailings basin. Water from the basin is recirculated back to the plant. Water percolating through the dam is pumped into the reservoir by a leachate pump circuit.
Oxide ore
Processing oxide from the Suruca deposit is planned.
Crushing and stacking
The crushing circuit for oxide ore consists of two MMD Sizers in series and associated equipment. ROM material from the mine is fed to the first of the MMD Sizers using either dump trucks or front end loaders. The ROM material is unloaded directly to the ROM bin through a 400 mm aperture fixed grizzly screen. Grizzly screen oversize is collected, broken with a mobile rock-breaker and fed to the ROM bin at a later time. Grizzly undersize material is stored in the ROM bin and control fed to the MMD Sizer via apron feeder. Material is pre-screened ahead of the MMD Sizer using a fixed screen with the undersize reporting directly to the crushed product conveyor. Material passing through the first MMD Sizer (625 model) passes directly to the second MMD Sizer (625 model) in series. MMD crusher product then combines with screen undersize and is conveyed to the crushed product stockpile. The crushed stockpile is covered with a plastic cover to prevent the crushed product being exposed to the rain.
Crushed product is then control fed to an agglomeration drum. Prior to the drum, cement is added in a controlled fashion based on the agglomeration drum feed rate and pre-set cement addition rate. In the agglomeration drum, a weak cyanide solution (barren pond solution) is added and mixed to produce agglomerates which are conveyed and stacked. A series of semi-portable conveyors runs the material outside the plastic lined pads. A further set of mobile conveyors transports the material down the centre of the pads to the stacking conveyor. The crushed ore can also be fed to the agglomeration drum using a front end loader when the crushing plant is stopped. The material is transferred to a cross conveyor and finally transfers to the heap leach stacker, which discharges the agglomerated material at a height of eight metres. The agglomerated material is stacked on pads which are prepared using impermeable HDPE plastic. The pads are approximately 100 m wide and 620 m long.
Heap leaching
The heap leach pads have a slight inclination to allow collection of the pregnant solution at one end of the prepared pad. The heap leach pads are stacked from one end to the other, with the mobile conveyors progressively removed as the stacker moves up along the length of the pads. The heap leach pads are divided into three distinct phases: active leaching, rinsing, and idle ore (before leaching and after leaching and rinsing).
Once the stacking process is completed, the agglomerated ore is allowed to dry for a period of one to four days depending on the prevailing climatic conditions. Cyanide distribution lines are installed onto the top surface of the stacked ore. In the high rainfall months, a sprinkler (wobbler) system is used to promote evaporation and during the low rainfall months a drip system is utilized to reduce evaporation rates.
A weak cyanide solution from the barren solution pond at basic pH levels (10 to 12) is then used to leach the gold from the stacked ore. The weak solution filters through the agglomerated ore with the gold inherent in the ore leached to produce a gold rich solution. The gold rich solution collects at the base of the pad and is diverted to the end of the pad and collected in a drainage system. The gold rich pregnant solution runs through a plastic lined drainage system to the pregnant solution pond.
A second drain collects rinse water which is utilised to neutralise the cyanide in the heaps after cyanidation has been completed. The rinse water is directed to the emergency pond for re-use at a later time. A third drain is used to collect rainfall from the top of the heaps. Heaps which have been leached and rinsed are covered with HDPE plastic and the rain water (free of chemicals) is directed to a water collection dam which can be reclaimed during the drier months.
Solution ponds
The process plant for oxide ore consists of six plastic lined HDPE ponds and one water collection pond. The six plastic lined ponds are as follows:
- Pregnant Solution Pond – Receives gold rich solution from the active leaches. This solution is then pumped through the adsorption columns. In instances of high rainfall, the pregnant solution pond overflows in the barren solution pond.
- Barren Solution Pond – Receives solution from the adsorption columns once gold is removed onto activated carbon. Make-up water, either from the emergency pond or water collection dam and cyanide, is added to this pond during the drier months to maintain leaching solution volumes. Solution from the barren pond is pumped to the active leach and to the agglomeration drum using separate pumps. During the wet months during high rainfall periods leaching will cease while gold adsorption will continue. When the barren solution pond is full, solution overflows into the emergency solution pond.
- Carbon Fines Pond – At the completion of the carbon regeneration process, the regenerated carbon is screened to remove fine carbon. The fine carbon is directed to a small plastic lined pond. The overflow from the carbon fines pond runs into the barren solution pond.
- Emergency Solution Pond – The emergency solution pond prevents weak cyanide solutions from entering the environment. During the wet months the pond acts as an emergency containment system during periods of high rainfall events. This solution is then returned to the system for gold recovery. During the dry months, the pond acts as a water reservoir and make-up pond. It receives water from the local river system and from the water collection pond. Water from the emergency pond is added to the barren solution pond using a pontoon pump arrangement in the emergency pond.
- Neutralization Ponds 1 and 2 – At the completion of the active leach process, weak concentrations of cyanide within the heaps must be neutralized prior to reclamation. The ponds are used to leach the inactive leach pads with copper sulphate and hydrogen peroxide. The first pond is used to mix the copper sulphate and hydrogen peroxide. Solution is pumped to the heaps and the solution return flows to the emergency pond. The solution is re-used at a later time.
- Water Collection Pond – The water collection pond or safety dam is a clay lined pond which receives water from the local river system (Rio dos Bois) and also rainfall run-off from plastic lining over the top of the leach pads. The water is reclaimed during the dry seasons and either runs into the environment naturally when full or is pumped/discharged to the environment when required.
Adsorption and elution
Pregnant solution is sampled and then sent to the adsorption feed tank. The solution flows through four adsorption columns in series and flows by gravity from one adsorption column to the next. In each of the columns is 6 x 12 mesh activated carbon at a concentration of approximately 24 g/l in solution or a total of 12 t of carbon.
Barren or eluted carbon is added to the last adsorption column and pumped forward to the next adsorption column in a counter current mode. A sieve is installed on the discharge of each of the adsorption columns to prevent the carbon flowing to the next adsorption column. The total residence time in the adsorption columns is in the order of 25 minutes. The final liquor discharged from the final adsorption column containing a low concentration of gold is sent to the barren solution pond.
Loaded activated carbon is transferred to an acid wash tank where the loaded carbon is washed with a cold dilute hydrochloric acid solution to remove organic salts of calcium and magnesium from the loaded carbon surface prior to the elution process.
After acid washing, the carbon is washed and sent to the elution column to remove gold from the loaded carbon. The atmospheric Zadra process will be used to elute or desorb the contained gold from the loaded carbon. The Zadra desorption process consists of the desorption of the precious metal present in the loaded carbon by washing with a solution of NaCN and NaOH at high temperature (85ºC to 95ºC) and at ambient pressure and requires around 24 hours to complete.
The gold removed from the loaded carbon cools in a flash cell and then reports to the two Mintek electrowinning cells in parallel. Gold in solution is removed onto stainless steel cathodes. The electrowon solution then flows to the solution tank below the electrowinning cells. The solution is then pumped to a gas heater and then continues on to the elution column for further elution. Once the solution returning from the elution column is below 1 ppm gold in solution the elution process is stopped. The eluted or barren carbon is then rinsed and sent to the carbon regeneration process.
Once per week, the stainless steel cathodes are rinsed off with a high pressure washer. The cathode sludge is then filtered, dried in an oven and then transferred to the barring furnace. Fluxes are added to the barring furnace which is then heated to around 1,150°C. Gold is then poured into molds.
Utilities
Eluted carbon is sent to the thermal regeneration kiln to remove inorganic material which has loaded into the pores and onto the surface of the carbon such as naturally occurring oils. The carbon is heated to between 650°C and 750°C in a sealed and steam atmosphere. The regenerated carbon is then screened to remove fine carbon generated during the elution and regeneration process. Carbon oversize is cooled using water in a carbon storage hopper. The carbon fines report to the fine carbon settling pond.
New carbon is added to the process when carbon in circuit stocks drop. Five hundred kilograms bulk bags are used to maintain circuit stocks. Regenerated oversize carbon is then pumped to the last adsorption column.
Hydrochloric acid is used to wash the activated carbon to remove particles of calcium and magnesium. The acid will be received as 1,000 bulk drums and then pumped through to the acid mixing tank where it is mixed with water to a concentration of around 3% w/w HCl. The dilute solution is re-circulated through the acid wash hopper.
Sodium cyanide solution is used in the leaching process and in the desorption process. The sodium cyanide will be stored in one tonne bulker bags and will be mixed with fresh at a concentration of 30% solids w/w. The dosage of sodium cyanide will be in the order of 100 g/t of ore stacked. Any gases generated will be removed through an exhaust fan. To ensure consistent cyanide addition rates and cyanide concentrations reporting to the active leach cells, a cyanide storage tank and dosing pump will be installed. Another pump is installed to dose cyanide into the solution dosing line going to the agglomeration drum. Cyanide will run by gravity to the elution storage tank.
Portland or “High Early” strength cement will be used in the agglomeration process for crushed ore with an average dosage equal to 15 kg/t of ore processed. The cement will be delivered in trucks equipped with air compressors and unloaded directly into two installed cement silos. These silos will be installed above the crushed product discharge conveyor and fed at a controlled onto the discharge conveyor using screw feeders.
Caustic soda or sodium hydroxide is used in the desorption of gold from loaded carbon. The caustic soda will be delivered in 25 kg bags and mixed in a tank and then transferred to a dosing tank. From there, the solution is added to the elution circuit.
Liquefied gas will be used in the elution circuit and the gold room for heating and smelting. The project calls for a tank to be installed in the field to meet this need and to store enough LPG to supply the plant for two weeks continuously.
A fuel station installed in the field will be required for all light vehicles used in the plant, and for mobile plant equipment (front end loaders, excavators, and rock breakers) as well as for the fleet of trucks and other mining equipment including scrapers and tractors. The facilities must have the capacity to supply all equipment for a period of one week continuously.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Plant design capacity | 60,000 tpd | Design |
| Copper recovery | ~80% | Actual, June 2013–May 2014 |
| Gold recovery | ~59% | Actual, June 2013–May 2014 |
| Primary crusher capacity (system 1) | Up to 3,500 dmt/hr | Design |
| Primary crusher capacity (system 2) | Up to 2,000 dmt/hr | Design |
| Primary crushing product size | P70 of five inches | Design |
| Primary grinding product size | 80% passing 1.06 mm | Design |
| Secondary grinding product size | 80% passing 210 microns | Design |
| Flotation copper recovery target | Over 85% | Design |
| Flotation gold recovery target | 59% | Design |
| Concentrate grade | ~26% Cu and 15 g/t Au | Design |
| Feed solids concentration | Approximately 35% | Design |
| Thickener diameter | 13 m | Design |
| Filter plates | 12 LAROX plates | Design |
| Concentrate moisture | Average 8% | Design |
| Filter output | Approximately 45 tph | Design |
| Tailings solids | Approximately 60% | Design |
| Hydrocyclone diameter | 20 inch | Design |
| Agglomeration cement dosage | 15 kg/t ore | Design |
| Agglomeration drum feed height | 8 m | Design |
| Leach pad dimensions | ~100 m wide, 620 m long | Design |
| Carbon mesh size | 6 x 12 mesh | Design |
| Carbon concentration | ~24 g/l (12 t total per column) | Design |
| Adsorption residence time | ~25 minutes | Design |
| Elution temperature | 85ºC to 95ºC | Design |
| Elution duration | ~24 hours | Design |
| Zadra process | Atmospheric | Design |
| Carbon regeneration temperature | 650ºC to 750ºC | Design |
| Cyanide dosage | ~100 g/t ore stacked | Design |
| Cyanide solution concentration | 30% solids w/w | Testwork/planned |
| HCl concentration for carbon wash | ~3% w/w | Design |
| Electrowinning cells | Two Mintek cells in parallel | Design |
| Carbon elution stop criterion | Below 1 ppm gold in solution | Design |
| Barren solution pH range | 10 to 12 | Design |
| Agglomerated ore drying time | 1 to 4 days | Design |
| Pregnant solution pond | Six HDPE ponds plus collection pond | Design |
| LPG storage capacity | 2 weeks continuous supply | Design |
| Fuel station supply capacity | 1 week continuous | Design |
Project website: https://www.lundinmining.com/our-portfolio/operations/chapada/
Technical qualifications
The following limitations are specific to the report:
- The oxide ore processing sections describe planned facilities, with the crushing circuit consisting of two MMD Sizers in series, agglomeration drum, and associated equipment. Sulphide ore will be processed in the existing plant after some modifications.
- Heap leaching of oxide ore from the Suruca deposit is planned, scheduled to start production in late 2016.
- All performance figures for the oxide ore circuit, including recoveries, reagent dosages, and equipment capacities, are presented as design or planned values unless otherwise noted as historical operating data from June 2013 to May 2014 for the sulphide plant.
- The report notes that the process recoveries for copper and gold in the sulphide plant are historical operating averages from June 2013 to May 2014, not long-term guarantees.
- Reagent consumptions, including cyanide dosage and cement addition rates, are stated as design or planned values based on the testwork referenced in the report.
- The report describes the planned use of a front end loader to feed the agglomeration drum when the crushing plant is stopped, and the entry of the loader under the plastic sheet to feed the conveyor underneath.
- The report describes the planned installation of a tank in the field for LPG storage to supply the plant for two weeks continuously, and the planned installation of a fuel station with capacity to supply all equipment for one week continuously.
Source: Yamana Gold Inc. – Chapada Mine, Project #2177. Technical Report NI 43-101 – July 31, 2014. Section 17 Recovery Methods.
