Cantung — Recovery Methods Report

This report details the recovery methods for the Cantung tungsten-copper milling facility, covering crushing, grinding, flotation, gravity separation, and concentrate handling.

Report context

This report describes the recovery methods for the Cantung tungsten-copper milling facility, including proposed design targets from the Life of Mine Plan (LOMP), historical operating data, and testwork information. The Mill Processing Facilities at Cantung comprise primary crushing and coarse ore storage installations, secondary and tertiary crushing, fine ore storage, a general gravity and flotation building with offices and a maintenance shop, a backfill preparation building (inactive), reagents and supplies storage building, and an assay lab.

Processing route

Primary Crushing

Ore is handled from the stockpile by a loader or directly dumped from a haulage truck into a 30-ton receiving bin equipped with a 42 inch by 10 ft apron feeder, which feeds a 42 inch by 48 inch jaw crusher. The jaw crusher is set to produce a nominal five inch crushed product. A conveyor transports crushed ore into a 1,000-ton capacity coarse ore bin, which acts as a surge bin for the secondary crushing circuit.

Secondary and Tertiary Crushing

A vibratory feeder and a conveyor feed a 4¼ ft (secondary) standard cone crusher set at 1 inch. The crushed ore is discharged via conveyor to a vibrating screen equipped with a 7/16 inch by four inch slotted screen. Oversize feeds a 4¼ ft (tertiary) short-head cone crusher set at 3/8 inch. The tertiary crusher discharge combines with the secondary crusher discharge to feed the screen in a closed-circuit recycle. Screened undersize (minus 7/16 inch) product is conveyed to two fine ore bins ahead of the grinding mills in the concentrator. Dust is controlled by the use of a wet scrubber with the discharge effluent returning to the mill.

Grinding, Classification and Talc Flotation

The grinding circuit consists of a 9 ft diameter by 12 ft long rod mill powered by a 450 hp motor, a 7 ft diameter by 10 ft long ball mill powered by a 200 hp motor, and a 6 ft by 6 ft ball mill powered by a 150 hp motor. The rod mill is fed from two fine ore bins via belt conveyors that discharge ground ore slurry into a sump along with the discharge from the ball mill for pumping to vibrating screens. Screen oversize drops by gravity to a pump that transports the slurry to a dewatering screen, which removes excess water prior to further grinding of the solids in the ball mills. Ball mill discharges join rod mill discharge for presentation to the vibrating screens. Water removed by the dewatering screen is reused for screen feed dilution. Particles larger than the screen openings circulate through the ball mill until they are sufficiently reduced in size to pass through the screen openings.

Screen undersize drops by gravity to a pump which transports the slurry to a set of cyclones that classifies according to particle size. Particles larger than the cyclones cut point drop by gravity to the sands bulk sulfide flotation unit operation. Particles smaller than the cut point feed the slimes bulk sulfide flotation unit operation.

Sulfide Flotation

After conditioning with appropriate promoters, collectors, depressants, and frothers, underflow from the size separation cyclones is directed to flotation cells to remove sulfide minerals which would be deleterious in the downstream scheelite recovery processes. Concentrates can be directed to a copper separation circuit or rejected to tails depending on copper content. Tailings are pumped to the scheelite gravity recovery circuit.

Overflow from the cyclones is directed to the talc flotation cells to remove sulfide minerals in the same manner as in the sands sulfide flotation operation. Concentrates can be directed in the same manner. Tailings are pumped to the scheelite flotation recovery circuit as the particle size is too small for effective gravity recovery.

Gravity Separation

Sands bulk sulfide flotation tails are distributed to triple deck shaking tables and spirals for rougher concentration of scheelite. Concentrate from these tables and spirals are cleaned on single deck tables then pumped to a final sulfide mineral removal flotation step, then to a dewatering classifier before going to the roaster. Rougher table tails join tails and middlings from other tables to undergo desliming with a cyclone. The cyclone overflow containing particles too small for effective gravity recovery and excess water goes to the scheelite flotation recovery circuit after thickening.

Cyclone underflow, containing particles that remain amenable to gravity separation but are considered to be locked minerals as middlings that require further size reduction to liberate scheelite from gangue, is directed by gravity to a 5 ft diameter by 8 ft long ball mill powered by a 150 hp motor. The ball mill discharges to a pump that transports the particles in slurry to another flotation step for removal of liberated sulfide minerals. Concentrate from this flotation step can be directed in the same manner as concentrates from the sands and slimes bulk sulfide flotation operations. Tailings are pumped to triple deck tables and spirals for scavenging of gravity recoverable scheelite. The concentrate from these tables joins the concentrates from the coarse gravity cleaner tables for final sulfide removal and high temperature drying. Tailings from this fine scheelite gravity recovery operation go to scheelite flotation recovery.

The gravity circuit can be adjusted to produce a range of concentrate grades. Higher grade increases gravity circuit losses which increases the volume of flotation concentrate.

Scheelite Flotation

Flotation feed consists of material that is too fine for gravity separation. This material is first thickened in three parallel 40 ft diameter three tray stacked thickeners. Thickener underflow is first passed through a third sulphide flotation stage. Concentrate reports to tails, and the tails feeds three agitated conditioner tanks where a pH modifier, depressants, collectors and a frother are added. The slurry stream then passes down a bank of six 5 m³ agitated forced air tank type flotation cells. The first two cells are roughers, the next two can direct concentrate as rougher or scavenger and the last two are scavengers.

Rougher concentrate is pumped to the scheelite first and second cleaners which is a set of naturally aspirated conventional mechanical flotation cells. Scavenger concentrate is returned to the rougher feed, along with first cleaner tail, by pump. The second cleaner concentrate is the final concentrate. Second cleaner tails return to the first cleaner feed. The final concentrate is thickened in a 20 ft diameter thickener, filtered, dried, and bagged for shipment to markets. Dried concentrate is weighed and packed in two-ton tote bags. Bagged concentrate is stored in a covered shed while awaiting shipment.

Drying and Magnetic Separation

The gravity concentrate typically contains 60% to 70% WO₃. It is dewatered in a spiral classifier before entering the roaster. The roaster is a propane multiple hearth type with rotating rakes to move material from an upper hearth to a lower hearth and finally to discharge. Material discharging from the dryer is transported in two water jacketed screw type conveyors in series which cool the material prior to downstream transport and processing. Discharge from the second screw conveyor is transported vertically in a bucket elevator to a screen that rejects undesirable material that is recycled via the primary crusher. Undersize then reports to the first stage of dry magnetic separation that consists of two roll type separators in parallel. Each unit has three rolls, one rougher roll that produces a magnetic reject straight away, then two scavenger rolls in series that produce a non-magnetic final product and a magnetic reject that is passed on the second stage of dry magnetic separation.

The second stage is a unit with a rare earth magnet and three electromagnetic units fixed over a moving belt. A non-magnetic final product joins the first stage non-magnetics to be bagged for shipment to markets. The magnetic reject joins the first stage reject and is then returned to main process via the scheelite regrind mill. Dried concentrate is weighed and packed in two ton tote bags. Bagged concentrate is stored in covered areas while awaiting shipment.

Concentrate Handling and Storage

Dried concentrate is packed in two-ton tote bags for shipment to the customers. Bagged concentrate is stored in covered areas before being shipped. The tote bags are handled with a small forklift.

Copper Circuit

The copper recovery circuit can take all or some of the bulk sulfide concentrates from the sands and slimes bulk sulfide flotation and secondary sulfide flotation cells. Only the sands bulk sulfide flotation concentrate is cycloned to separate out material already finer than the requisite optimum particle size. The cyclone underflow containing the material coarser than optimum passes through a 4 ft by 4 ft ball mill powered by a 40 hp motor. Mill discharge joins the new bulk sulfide concentrate to be cycloned again. Particles larger than optimum are recirculated through the ball mill until they are reduced in size sufficiently to pass the cyclone’s cut size gate and pass on to further processing. The cyclone overflow is passed on to a bank of forced air conventional mechanical flotation cells, known as the first cleaners.

A chalcopyrite specific collector is added to the head end of the bank of cells which promotes chalcopyrite recovery. The pH is taken to approximately 12 by a lime slurry, Ca(OH)₂ in water. The lime acts as a depressant for the undesirable pyrite and pyrrhotite in the bulk sulfide concentrate but has no effect on the desirable chalcopyrite.

The tails from the first cleaner cells go to final tails and out. The first cleaner concentrate drops by gravity to the second and third cleaners. Tails from the second cleaner are fed back to the head end of the first cleaners and second cleaner concentrate feeds the head end of the third cleaner. The third cleaner tails feed the head end of the second cleaners. The third cleaner concentrate is the final concentrate.

Final concentrate is thickened and subsequently dewatered on a belt filter then bagged for shipment. Filtered concentrate is weighed and packed in two-ton tote bags. Bagged concentrate is stored in covered areas while awaiting shipment. Sulphuric acid and coagulant are added to the tails as it leaves the mill to aid in solids settling. Supernatant from No.5 Pond is pumped to a waste water treatment facility (WWTF) for removal of suspended solids before discharge to the environment.

Wastewater Treatment Facility

The WWTF uses the Actiflo process, designed and built by Veolia Water Solutions and Technologies. The process uses the addition of a coagulant and a polymer, aided by a micro-sand, which is used as a seed for flocculants formation and ballast. The water is then clarified through lamella clarifiers before discharge to the environment. The ballasted sludge is separated from the micro-sand by way of cyclones, sludge returns to tailings pond 5, and the cleaned sand is returned to the head end of the process.

The discharge to environment is limited to a maximum of 4,500 to 8,000 cubic meters per day, depending on the flow in the Flat River.

Assay Laboratory

There is an on-site assay laboratory, which has been used for sample preparation and assaying services for the mine and mill. The laboratory QA/QC testing with other labs is monitored and managed by the Geology Department. Reference materials and control samples are used in each assay run in the lab.

Mill Recoveries

Recovery and grade is back-calculated daily using mill tails assay and weight (mill feed weight in less concentrate weight out) plus concentrates weights and grades to develop a total WO₃ mass in (total out in tails plus total out in product is total in). Total weight of WO₃ out divided by mill feed weight in is the back-calculated mill feed grade. Recovery is then total weight of WO₃ in concentrates divided by total calculated WO₃ in. As a check the sampled mill feed assay is checked against the back calculated feed assay. If a significant discrepancy is noted, conveyor weightometer calibration and sampling systems are immediately checked.

Key reported parameters

Technical qualifications

The LOMP target numbers for processing tonnage and recoveries are stated to “appear to be obtainable based on projected ore metallurgical characteristics and past performance attained by the operation.” The report notes that the backfill preparation building is inactive. No information is provided on the current status of the mill, ownership, economic performance, or links to external resources.

Source: Cantung , Recovery Methods Report, relevant sections.