This article summarises the processing methods and historic plant performance for the Minera Tres Valles copper project as described in the March 2018 technical report.
Report context
The Minera Tres Valles Copper Project is located in the Salamanca area of the Coquimbo Region, Chile. The March 29, 2018 technical report documents the existing heap-leach and solvent extraction-electrowinning (SX-EW) process, historic plant performance from 2010 to October 2017, and a proposed chloride leach process modification.
Processing route
Current crushing and agglomeration
Ore is stockpiled above the crusher and processed through a primary jaw crusher, secondary cone crusher, tertiary crusher, and a quaternary crusher. The target final granularity is 80 percent passing 6.4 millimetres (one-quarter inch). Crushed material is agglomerated with water and acid to commence leaching. The agglomerated material is trucked to the heap leach area and stacked by front-end loaders.
Heap leaching
A grid of hoses with drippers is placed over the heaped material. The first phase of intensive leaching recovers oxide ore during an initial three-month period using constant and high acid-concentration irrigation. The product of this first leaching stage is the "oxide-pregnant leach solution" (oxide-PLS), which has high copper content and is accumulated in a pond at the bottom of the leach pads. For the subsequent six months the material is leached on an on-off basis with lower acid concentration to extract copper from sulphides including chalcocite, covellite, and some bornite. The resulting sulphide-PLS is stored in another pond.
Solvent extraction and electrowinning
Oxide and sulphide PLS are pumped to the solvent extraction plant where organic resins capture copper ions over several stages. After the stripping stage the highly concentrated electrolyte is sent to the electrowinning plant where an electrolysis process deposits metallic copper onto steel plates. The plates are sent to a semi-automatic stripping machine where pure copper sheets (cathodes) are separated. Cathodes are then stored in finished goods inventory.
Historic plant performance
The crushing plant reached its maximum nameplate capacity in September 2012 after improvements and adjustments from 2011 to the first half of 2012. It then surpassed nameplate capacity for the next eight months, maintaining permanent production over 5,500 tonnes per day. The maximum performance was reached in April 2013 with an average of 6,164 tonnes per day. This production was achieved with only three crushing stages, before the inauguration of the quaternary crusher.
The quaternary crusher installation was planned from the beginning of the project and was authorised in the original environmental licence. The rationale was to decrease the granulometry of the stacked mineral to 80 percent passing 6.4 millimetres, as metallurgical tests indicated an increase in recovery of 2 to 11 percent at this size. The higher recovery would mainly be obtained from fine-disseminated chalcocitic mineral from deeper parts of the Don Gabriel open pit, with some gains also expected from Papomono. The second rationale was to increase nameplate capacity to compensate for lower than planned grade from the Papomono mine, mainly due to the Cumbre open pit low grade. The selected equipment has a capacity of 400 tonnes per hour.
Commissioning of the quaternary crusher took place in August 2013, producing 3,505 tonnes in a single shift, which demonstrated that the new installed capacity at full operation would be higher than 7,000 tonnes per day. The report notes that operating at this rate may require a revision to existing operating permits.
In November 2012, 1,331 tonnes of cathodes were produced, reaching 89 percent of the electrowinning plant capacity. The total cathode capacity had not yet been achieved because 2012, the last year of full production, had the lowest average copper grade at 1.0 percent total copper. The report states that results show the plant is capable of operating at maximum capacity in all stages: crushing, agglomeration, leaching, PLS pumping, extraction, and electrowinning.
Proposed chloride leach process
In 2015 MTV undertook a pilot project on site to perform several tests and simulations with MTV resource, looking for best-possible operational parameters. The pilot plant included 12 three-metre-high columns, 12 mini-columns of one metre height, and four 20 by 20 metre pads for heap leaching. Campaigns were driven by the potential use of different salt oxidant agents (chloride-medium leaching). The use of salts as oxidant agents in copper leaching is not new in Chile; since the beginning of this century several mines have adopted the use of salt water in heap leaching or different types of salts in agglomeration.
MTV engaged ProPipe S.A. of Santiago to undertake a Preliminary Feasibility Study of the chloride leaching process. There are no changes to the crushing plant; feed is processed in four stages to reduce size to 80 percent passing 6.4 millimetres.
The crushed feed is fed by belt to an agglomeration system where water, sulfuric acid, and salt are added in solid state at a dosage of 18 kilograms per tonne NaCl for sulphides. This stabilises fine particles and generates sulfation of mineralogical species containing copper. The agglomerated material is transported and deposited in leach pads where heaps are kept at rest for 30 days and kept wet to prevent upper layers from losing moisture. After 30 days the pads are irrigated with raffinate solutions (acid solutions) to extract contained copper, with recoveries around 90 percent. The leaching process includes chemical leaching stages for 30 to 90 days for oxidised feed and 120 to 150 days for sulphide feed with salt, in order to fully benefit the mixed ore resource. Independent solutions are considered for these two circuits.
Leaching is carried out by first applying an irrigation solution called raffinate-oxide and subsequently for the sulphide leaching stage an irrigation solution called raffinate-sulphide. Copper-rich solutions (oxide PLS and sulphide PLS) are concentrated and cleaned using solvent extraction. A high concentration copper solution (rich electrolyte) is obtained and processed by electrowinning to obtain high quality copper cathodes.
The design considers treating a maximum flow rate of 400 cubic metres per hour of rich solution generated in the leaching of oxidised copper (PLS-oxide), with average concentration between 2.2 and 3.7 grams per litre Cu and 0.4 to 0.5 grams per litre H2SO4, and a maximum flow rate of 800 cubic metres per hour of rich solution generated in the leaching of sulphide copper (PLS-sulphide), with estimated average concentration between 1.8 and 3.5 grams per litre Cu and 0.5 grams per litre H2SO4.
To separate the circuits of solutions in the leaching of oxides and sulphides, it is considered necessary to operate in parallel with mixer-decanter equipment with a total design flow of 400 cubic metres per hour each. The initial configuration considered four extraction stages (E1-oxide, E2-sulfur, E3-sulfur, E4-oxide) and a re-extraction stage (S). As operations progressed, it was decided to modify an extraction stage and transform it into washing. Due to the increase in chlorine associated with the addition of salt, an additional washing step (L) has been considered, giving a configuration of 1 E0x + 2Sulf + 1S + 2L. This achieves chlorine control regardless of the efficiency of the coalescer system. A facilities configuration was also provided to implement a layout with a single washing stage, 2EOx + 2Esulf + 1S + 1L, which would allow increasing the flow rate to SX if required to increase production beyond the current nominal capacity of 18,500 tonnes of copper per year.
The solvent extraction plant will use conventional mixer settlers. Controlled drop size agitators will be used to avoid ultra-fine drops. Since the organic reagent is highly selective to copper, impurities in the leaching solution return to the circuit until equilibrium is reached for each. Three organic coalescers are considered, two operating and one stand by, to control the level of chlorine transferred to the organic from long-term reuse of the reagent.
The loaded organic passes to a coalescer to remove entrained aqueous raffinate solution, then to the washing stage and a second coalescer, then to the copper discharge or re-extraction stage where the organic reagent delivers copper to an aqueous solution with high acid and copper content, generating the advance rich electrolyte transferred to electrowinning.
The concentration of reagent in the calculated organic phase less than or equal to 20 percent v/v allows efficient extraction of 90 percent of copper from the rich solutions (PLS-oxide and PLS-sulphide) from the leaching area. The SX plant also has a system to clean entrainment of organic phase through electrolyte filters. There is a recovery system for organic trapped from waste (crud) through a mechanical rupture plant using a tricanter centrifuge to separate the three phases (organic, aqueous, and solid).
Upgrades to accommodate chloride leach
Upgrades to the existing plant to accommodate the chloride leach process are described as modest. A salt metering and transportation system was designed for addition of salt in the existing agglomerating drum. The dosing system consists of a volumetric screw type dosser fed from a hopper mounted below floor level of the projected storage building. The feeder discharges onto a high slope conveyor belt at 36 degrees, which transports the salt to the existing ore feed box in the agglomeration drum.
In solvent extraction a new mixer-settler will be required to operate as a washing stage, for which a new decanter with primary and secondary mixers with identical characteristics to existing equipment was designed. All piping related with the mixer and connections to existing lines is considered in the design.
In the tank farm area, three coalescers will be incorporated: one primary, one secondary, and a third as stand by, with all piping elements and pumping systems required for operation also designed. All equipment will be installed within spill containment and its location was designed in an available area between the existing tank farm facilities and the cathode storage.
Power and water requirements
Based on historic consumptions and projecting a situation of plant operating close to maximum capacity, MTV water consumption should be around 34,600 cubic metres per month, representing a flow between 13 and 17 litres per second. This is far below the existing water rights and pumping water capacity of MTV.
Sulfuric acid consumption is estimated to be in the order of 39,000 tonnes, which represents a saving of up to 40 percent in relation to the current acid-leach process.
The consumption of salt is projected to a range between 15,000 and 41,000 tonnes per year, averaging 30,000 tonnes per year.
MTV is connected to the Chilean power grid and operates under a take-or-pay contract under a specific minimum consumption with KDM Energia, a local bio-gas power producer.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Crushing plant nameplate capacity | > 5,500 tpd | Historic maximum sustained operation, 2012-2013 |
| Historic maximum throughput (three-stage crushing) | 6,164 tpd (average, April 2013) | Historic operating data |
| Quaternary crusher capacity (single shift commissioning) | 3,505 tonnes (August 2013) | Historic operating data |
| Quaternary crusher design capacity | 400 t/h | Design |
| Target crushed product size | 80% < 6.4 mm (1/4 inch) | Design |
| Cathode production (November 2012) | 1,331 tonnes | Historic operating data; 89% of EW plant capacity |
| EW plant nominal capacity | Not stated in relation to 1,331 tonnes | Historic operating data indicates 89% achieved |
| Chloride leach salt dosage (sulphides) | 18 kg/t NaCl | Proposed design |
| Chloride leach recovery target | ~ 90% | Proposed design / testwork |
| Chloride leach PLS-oxide flow | 400 m³/h max | Proposed design |
| Chloride leach PLS-sulphide flow | 800 m³/h max | Proposed design |
| PLS-oxide Cu concentration | 2.2 – 3.7 g/L | Proposed design |
| PLS-sulphide Cu concentration | 1.8 – 3.5 g/L | Proposed design |
| Chloride leach nominal capacity | 18,500 t Cu / year | Proposed design |
| Water consumption (near max capacity) | ~ 34,600 m³/month (13-17 L/s) | Historic consumption projection |
| Sulfuric acid consumption estimate | ~ 39,000 tonnes | Estimated; up to 40% saving vs current acid-leach |
| Salt consumption projection | 15,000 – 41,000 t/year (average 30,000 t/year) | Projection for proposed process |
Project website: https://www.bnamericas.com/en/features/minera-tres-valles-comes-under-pressure-after-serious-environmental-charges-in-chile
Technical qualifications
The report notes that the cathode production summary covering 2010 through October 31, 2017 does not account for additional recoverable copper available in the plant as of that date. The historic production data shows the period before the quaternary crusher was commissioned, and the report states that operating at rates above 7,000 tonnes per day may require a revision to existing operating permits. The chloride leach process is based on a Preliminary Feasibility Study and pilot plant testwork; the recoveries and other performance parameters are described as design values and testwork results rather than demonstrated commercial performance.
Source: Minera Tres Valles Copper Project, Salamanca, Coquimbo Region, Chile, Technical Report / Form 43-101F1, March 29, 2018, Sections 17.0 through 17.5.
