Cascabel Project — Alpala Copper-Gold-Silver Deposit

This article details the proposed mineral processing route for the Alpala copper-gold-silver deposit, based on the Preliminary Economic Assessment (PEA) for the Cascabel Project in northern Ecuador.

Report context

The Cascabel Project, located in northern Ecuador, hosts the Alpala Copper-Gold-Silver Deposit. The processing information presented here is drawn from the Preliminary Economic Assessment (PEA) report, document number 653060-0000-BA00-RPT-0003rJ. The report describes proposed processing facilities for four mine production scenarios: Case 1 (40 Mt/a), Case 2a (50 Mt/a slow ramp-up), Case 2b (50 Mt/a fast ramp-up), and Case 3 (60 Mt/a). The information presented is a proposed design based on testwork.

Processing route

Mineral characterisation

The dominant copper mineral in the Alpala deposit is chalcopyrite with accessory amounts of bornite. Chalcopyrite forms free grains from approximately 1 µm to 500 µm in altered host rock. Partial chalcopyrite rimming of pyrite, as well as chalcopyrite and bornite inclusions in pyrite exist, from below 10 µm in size. Grains of free gold exist, ranging in size from approximately 1 µm to 20 µm. In most cases the gold is included in chalcopyrite, bornite or pyrite.

Comminution

The grinding circuit is proposed as a conventional SAG mill / ball mill / pebble crusher (SABC) configuration for treatment of competent ores at elevated throughput. It is noted that the pebble crusher may be required only in later years.

Underground ore will be transferred to the coarse stockpile conveyor which deposits the crushed material onto the coarse ore stockpile. A provision will be made for future installation of partial secondary crushers and screening should higher competency material present itself later in the mine life. Coarse ore will be reclaimed from the stockpile by apron feeders installed in each of the reclaim tunnels, with each one supplying one of the independent grinding lines at a rate principally defined by SAG mill demand. Rock quality will be measured by a camera-based particle size analyser that is connected to a grinding expert control system.

Crushed ore will be reclaimed from the coarse ore stockpile, via variable speed apron feeders, directly to the SAG mill feed conveyor which will transport the ore to the SAG mill feed chute. Process water will also be added to the SAG mill, via the SAG mill feed chute. The variable speed SAG mill will operate with a nominal ball charge of 10% v/v. The 125 mm steel balls will be loaded into the SAG mill ball storage hopper via a front-end-loader (FEL). The SAG mill feed rate will be controlled to optimise throughput and flotation feed grind size by the plant's expert control system.

The SAG mill product with a top size of 85 mm will discharge from the mill to a screen. The oversize will be screened and washed over a double deck, vibrating screen. Oversize from the screen decks will discharge to the pebble recycle conveyor which feeds the pebble crusher. The pebble crusher will be operated to maximise power draw and to maintain a set chamber fill level, typically 60%. Crushed pebbles will be transferred to a surge bin before being fed proportionately to the SAG mill feed conveyors via belt feeders. Undersize from the screens will be combined in a pump box and will be transferred to two cyclone feed hoppers.

Comminution lines for both Phase 1 and Phase 2 will feed pebbles back to a common pebble recycle conveyor. SAG mill pebbles will return to the SAG mill feed conveyor.

Each secondary grinding circuit will consist of two ball mills operating in parallel and in closed circuit with cyclone clusters. The discharge from the SAG mill discharge pump box and ball mill trommel screen underflow will discharge into a cyclone feed hopper, one per ball mill line. Each cyclone cluster will be fed by a cyclone feed pump. During the initial Phase 1 ramp-up, only one ball mill-cyclone cluster may be required. The ball mill discharge is screened via a trommel screen. The slurry from the cyclone feed hopper is pumped to a dedicated cyclone cluster. The cyclone overflow will gravitate to a combined cyclone overflow stilling box. Cyclone underflow from each cluster will return to its respective ball mill for additional grinding.

Flotation

The copper flotation circuit will consist of rougher flotation with rougher concentrate regrind, three stages of cleaning and a cleaner-scavenger.

Each grinding circuit line will have its own dedicated roughing circuit and each line will be identical for each phase. Copper concentrate is the only product that will be produced. Flotation reagents will be added to the slurry at various stages. Concentrate from the rougher flotation circuit will be collected in a concentrate launder and will gravitate through a static, dual-fin metallurgical sampler to the rougher concentrate hopper. Tailing from the rougher flotation circuit will gravitate through a static, dual-fin metallurgical sampler to the final tailing launder.

The combined rougher and cleaner scavenger concentrate will be pre-classified, via concentrate regrind cyclone clusters. Cyclone underflow constitutes the feed to the concentrate regrind mill. The regrind section consists of a series of stirred vertical regrind mills arranged in a conventional closed-circuit configuration with cyclones to produce the required feed size to optimise concentrate grade and recovery.

Regrind cyclone overflow will be combined with second and third cleaner tailing and cleaned in the first cleaner bank. Milk of lime will be added to the cleaner flotation circuit to depress pyrite. The first cleaner flotation bank will be followed by cleaner scavenger flotation bank. Concentrate from the first cleaner flotation circuit will be pumped to the second cleaner flotation bank. Tailing from the first cleaner flotation circuit will feed the cleaner scavenger flotation circuit.

The cleaner scavenger concentrate will be combined with rougher concentrate for regrinding. The cleaner scavenger tailing will report to the final tailing launder, where it will be combined with rougher tailings and will flow by gravity to the tailings thickeners. It is noted that future optimisation may see the cleaner scavenger tailings stream treated to produce a pyrite concentrate via flotation to improve primarily gold recovery and secondary copper recovery.

Concentrate from the second cleaner flotation circuit will be pumped to the third cleaner flotation bank. Tailing from the second cleaner flotation circuit will feed the first cleaner bank. Concentrate from the third cleaner flotation circuit (final copper concentrate) will be pumped to the copper concentrate dewatering circuit. Tailing from the third cleaner flotation circuit will feed the first cleaner bank.

Concentrate dewatering

Final concentrate will report to the concentrate thickening area trash screen. The undersize from the trash screen will report to the flotation concentrate thickener. The final concentrate thickener underflow will be pumped to an agitated filter feed tank which will provide surge capacity for the concentrate pipeline. Concentrate will be pumped in batches to the port filtration facility.

At the port site, the pumped copper concentrate will be stored in agitated tanks which will provide surge capacity for the pressure filters. The concentrate will be pumped from the filter feed tanks to the concentrate pressure filters. The filtered concentrate will be discharged to a filter cake transfer conveyor and conveyed to the concentrate storage area. Filtrate from the concentrate filter will be pumped in batches back to the Cascabel plant site.

Tailings disposal

The combined rougher flotation tailing and cleaner scavenger tailing will report to the tailing thickener. Flocculant will be added in multiple locations in the final tailing thickener feed well due to the high slurry flow rate. Final tailing thickener underflow will be pumped to the tailings storage facility. The TSF decant water pumps on a floating pontoon will reclaim the decant water from the TSF and will be pumped to the tailings reclaim water pond. The options of producing final paste or filtered tailings is currently being reviewed and will be assessed in more detail during the Pre-Feasibility Study.

Reagents and consumables

The primary collector, Potassium Amyl Xanthate (PAX), will be delivered in 1,000 kg bags and will be mixed at the required concentration of 20% w/v. The secondary collector, di-thio phosphate, will be delivered as liquid at a concentration of 95% w/v in IBC-boxes or ISO-containers. Frother will be delivered as liquid at a concentration of 99% w/v in IBC-boxes or ISO-containers. Quicklime will be delivered to site in a tanker and will be pneumatically conveyed to the lime storage silo.

Concentrate flocculant (Magnafloc 10) will be delivered in 25 kg bags. Tailing flocculant (Magnafloc 10) will be delivered in 1,000 kg bags. SAG mill grinding media of 125 mm diameter steel balls will be used. Ball mill grinding media of 65 mm diameter balls will be used.

Key reported parameters

Technical qualifications

The processing information presented in this article is derived from a Preliminary Economic Assessment (PEA). The report states that the Cascabel flow sheet is assembled from unit processes used commonly throughout the minerals processing industry, and the configuration of the unit processes have been optimised to provide the most energy efficient flow sheet, with enhanced metallurgical performance. The design criteria and the associated mass balances are used to derive capital cost estimates and schedules for operating requirements such as power, reagents and consumables. The options of producing final paste or filtered tailings is currently being reviewed and will be assessed in more detail during the Pre-Feasibility Study. The adopted TSF solution for this PEA is conservative and there are ongoing studies to optimise the best TSF solution for the project. Design values are the nominal hourly or average flows multiplied by the appropriate design factors and do not relate to annual production nor are they cumulative to represent a metallurgical balance.

Source: Cascabel Project, Northern Ecuador, Alpala Copper-Gold-Silver Deposit, Preliminary Economic Assessment (PEA), Sections 1 and 17.