The Aurora processing plant, commissioned in 2015 and expanded in two phases through early 2019, processes mixed saprolite and fresh rock through crushing, single-stage SAG milling, gravity concentration, and carbon-in-pulp leaching.
Report context
This technical report is dated March 31, 2020, and relates to the Aurora Gold Mine, owned by Guyana Goldfields Inc (Project #3184). The report was prepared under National Instrument 43-101 standards. The processing plant at Aurora was commissioned in 2015 and reached commercial production in January 2016. The plant was designed by Sedgman Limited (2013) with modifications made by JDS and the Mine in 2017 and 2018. A plant expansion was executed in two phases: the first phase commenced in the first quarter of 2017 and was completed in early 2018; the second phase commenced in the first quarter of 2018 and was completed in February 2019.
Processing route
Pre-expansion plant configuration
The pre-expansion plant included a single stage primary crushing circuit utilising a jaw crusher, a 15,000 tonne to 20,000 tonne crushed ore stockpile and re-feed bin, and a separate saprolite crushing circuit including a feeder breaker (subsequently removed from circuit). The grinding circuit consisted of a 5,500 kW SAG mill including a pebble crushing circuit. Gold recovery was achieved through a gravity concentration circuit including a centrifugal concentrator and intensive cyanide leaching of the concentrate, followed by a CIP circuit with two leach tanks and six carbon adsorption tanks. The plant also included a four tonne carbon elution circuit, a cyanide detoxification circuit with two tanks, and all required reagents and plant services including a power station, plus water and air services.
Following commercial production and ramp up, the plant achieved sustained production rates of 6,000 to 6,500 tpd.
Expansion design and modifications
A simple expansion plan that included a 5,500 kW ball mill was defined in the Sedgman plan to achieve 10,000 tpd. The plan was later modified to eliminate the ball mill and replace it with a pre-crushing circuit designed to expand the mixed saprolite and fresh rock throughput from 5,000 tpd to 7,500 tpd. Maximum achievable throughput is 8,000 tpd with additional saprolite blending.
First phase expansion
The first phase of expansion included increasing pumping capacity in grinding and tailings circuits, adding a pre-leach thickener, three additional leach tanks, upgrades to the carbon management system, and expansion of the elution circuit. The pre-leach thickener was commissioned and put on standby due to the expansion design not addressing the upstream bottleneck in cyclones or grinding. The pre-leach thickener and leach tanks were installed to achieve better control of slurry density and increased retention time in the leaching circuit with the objective of increasing gold recovery.
Second phase expansion
The second phase of the expansion included the installation of a pre-crushing circuit. This phase allows the processing of mixed saprolite and fresh rock at a rate of 7,500 tpd. The plant expansion included three additional leach tanks, modified inter-stage screens installed in adsorption tanks, replacement of trash and carbon recovery screens, an additional four tonne carbon elution circuit, replacement of tailings pumps and a larger tailings pump box, replacement of cyclone feed pumps and piping, a pre-crushing circuit consisting of a jaw crusher, cone crusher, and associated feeders and conveyors, and one upgraded electrowinning cell.
Crushing fresh rock
ROM ore from the mine is trucked to the ROM pad where it is dumped into the stockpile. The ROM ore stockpile is processed through two circuits: a single stage primary jaw crushing circuit and a pre-crushing circuit consisting of a jaw crusher and a secondary cone crusher.
The fresh rock crushing plant was designed for a nominal throughput of 500 t/h, representing 40% availability for the initial 1.75 Mtpa throughput. This circuit was sized to process the crushing requirements for the initial Stage 2 expansion to reach a throughput of 3.5 Mtpa.
ROM ore can be direct dumped or a Front End Loader (FEL) reclaims fresh rock ore from the ROM stockpile and dumps it into the 320 t capacity dump pocket that feeds the single stage jaw crusher. The dump pocket is protected by a static grizzly that has a nominal 800 mm sizing. Oversize material is rejected to the ROM pad. An apron feeder draws material from the dump pocket and discharges it into the jaw crusher.
The primary jaw crusher has nominal sizing of 1,600 mm x 1,200 mm with a 160 kW motor and was designed to operate with a closed side setting of 150 mm. Primary crushed product is conveyed to the 170 t (live) refeed bin by the jaw crusher discharge conveyor. During periods of milling shutdown, the bin will overflow to the refeed overflow conveyor and is deposited in the 15,000 t to 20,000 t coarse ore stockpile. During crusher shutdown events, material from the stockpile is fed with a FEL to the refeed bin. During normal operation, ore is withdrawn from the refeed bin at a measured rate by the refeed bin apron feeder and discharged to the SAG mill feed conveyor.
Saprolite feeding system
In the initial design, saprolite ore was fed from the ROM stockpiles into the variable speed saprolite feeder breaker via a FEL. The separate feeding system was planned to prevent potential operational blockages of the jaw crushing circuit and provide a controlled blend of feed to the SAG mill. The feeder breaker was designed to break saprolite ore down to a nominal top size of 200 mm. Broken ore then discharged to the Saprolite Conveyor which, in turn, discharges directly to the SAG mill feed conveyor and is blended with the fresh rock to feed the SAG mill at a predetermined blend of fresh rock to saprolite. A belt weightometer is provided to monitor and control the discharge rate from the saprolite feeding system by varying the feeder speed. The feeder breaker was removed from the circuit as it was not effective and subject to plugging.
Pre-crushing circuit
The pre-crushing circuit uses the newly installed jaw crusher and secondary cone crusher to deliver finely crushed rock to the saprolite feeder system. The circuit consists of a Metso C110 jaw crusher and a Metso HP 300 cone crusher that operate in parallel to the original fresh rock crushing circuit. The capacity of the circuit is approximately 2,400 tpd. A FEL transfers ore from the ROM pad to the jaw crusher via an apron feeder. The crushed ore is conveyed to the cone crusher and discharge from the cone crusher is conveyed to the existing, original saprolite feeder circuit and subsequently transferred to the SAG mill feed conveyor.
Grinding
The Aurora milling circuit is a single stage SAG mill operating in closed circuit with hydrocyclones and a pebble crushing circuit. The initial design operated at a nominal ore throughput of 218.8 dry t/h with a mill availability of 93.0% and grinding to a nominal particle size of 68% passing 75 μm. With the newly installed pre-crushing circuit, the mill will process 7,500 tpd of mixed saprolite and fresh rock and maintain the same product size.
Ore discharging from the refeed bin apron feeder and the saprolite conveyor is combined on the SAG mill feed conveyor. The mill feed rate is measured by a weightometer on the conveyor and is controlled to a set point by varying the speed of the apron feeder.
The ore discharges from the SAG mill feed conveyor to the feed chute of the SAG mill and is mixed with process water and cyclone underflow slurry. Slurry from the SAG mill discharges through a trommel screen. Critical size pebbles discharge from the trommel oversize to a pebble crushing circuit. Undersize slurry flows by gravity to the cyclone feed pump box. Slurry is pumped to the cyclone cluster from the pump box. Larger cyclone feed pumps were added during the plant expansion to accommodate the higher flow rates. Cyclone overflow is directed to the leach circuit trash screen and into the leach or CIP circuit. Cyclone underflow is split. A portion of the flow is directed to the gravity circuit and the other portion is returned to the SAG mill feed.
The pebble crushing circuit consists of a pebble crusher feed conveyor that is fitted with a magnet to remove steel grinding balls from the conveyor. A self-cleaning magnet and a metal detector are provided to prevent steel from entering the pebble crusher. Discharge from the pebble crusher can either feed the pebble crusher or alternatively dump into a bypass chute that feeds directly onto the pebble crusher discharge conveyor. The pebble crusher discharge conveyor feeds onto the SAG mill feed conveyor.
The SAG mill is a grate discharge high aspect ratio design, having 5,500 kW installed power with a variable speed drive. Grinding media is loaded into a front end loader for tipping into the refeed bin.
Gravity concentration
The gravity concentration circuit is provided to recover coarse, free gold particles for intensive cyanide leaching. The gravity circuit is designed to recover 30% of the contained gold.
Gravity gold concentration includes a two-stage circuit including a centrifugal concentrator followed by a high intensity cyanidation reactor. Feed to the circuit is provided by a bleed stream taken from the cyclone underflow, which is directed to a single scalping screen. It has a nominal aperture of approximately 2 mm to prevent coarse, oversize material entering the centrifugal concentrator. Oversize from the scalping screen is returned to the SAG mill feed chute. Undersize from the scalping screen passes to an automated 76 cm diameter centrifugal concentrator. The unit is operated on a semi-continuous basis, automatically discharging concentrate approximately every hour. The concentrator tail is rejected to the cyclone feed pump box. The concentrate from the centrifugal concentrators passes to a high intensity cyanidation reactor.
In the high intensity cyanidation reactor, the concentrate is leached using solution containing high cyanide and caustic concentrations in a fluidised bed reactor. Operation is via its own PLC which is integrated with the operation of the centrifugal concentrator. Sodium cyanide is supplied in a liquid form along with the leach aid that is manually added to a hopper located on the reactor skid. Pregnant solution that is produced from the high intensity reactor is stored and processed in a dedicated electrowinning circuit. The residue is washed prior to being returned to the cyclone feed pump box.
The centrifugal concentrator and the high intensity cyanidation unit are located adjacent to the SAG mill feed. The gravity electrowinning circuit is located in the gold room.
Leaching and carbon adsorption
Leaching takes place in a modified CIP-type circuit that initially included two leach tanks that do not contain activated carbon and six adsorption tanks that contain activated carbon for gold adsorption. During the first phase of the plant expansion, three additional leach tanks and the pre-leach thickener were added to the circuit.
The cyclone overflow discharges to the trash screen. Underflow from the trash screen discharges to the 30 m diameter pre-leach thickener or directly to the leach tanks if the thickener is bypassed. A flocculent make-up system was added as part of the installation of the thickener.
Underflow from the thickener is pumped to the first leach tank at a design slurry density of 50% solids by weight. The thickener is on standby and not in use. Cyanide solution is added to the slurry to achieve a cyanide concentration that is controlled by an on-line cyanide analyzer. The target pH of 10.5 is controlled by an automated pH control system by the addition of slaked lime. Lime is slaked in the reagent area and pumped in a distribution loop from the lime distribution tank to Leach Tank No. 1 and return. Overflow from the pre-leach thickener is stored in the process water tank, when in use.
The leach tanks are included in the design to maximize solution grades to the first carbon adsorption contactor and to ensure maximum metal loading to the carbon.
Leaching takes place in five 13.2 m diameter by 13.2 m high leach tanks that operate in series with a combined residence time of approximately 18 hours. The agitators have down-draft air addition from leach blowers to provide air to the slurry. The new leach tanks were also fitted with cone spargers for additional air injection into the circuit. Slurry advances from one leach tank to the next by gravity flow before overflowing to the adsorption circuit.
Gold adsorption onto activated carbon is carried out in six 9.85 m diameter by 11.45 m high adsorption tanks, with a combined residence time of 11 hours. Slurry is pumped through mechanically-swept cylindrical intertank pumping screens that are located in each tank for carbon retention. The interstage screens were converted from 5.5 m² screens to 6.5 m² screens during the plant expansion.
Slurry flows from tank one through tank six before discharging to the cyanide detoxification circuit then discharging over the carbon safety screen into the tailings pump box.
From the first CIP tank one, batches of four tonnes of carbon are transferred to one of the two elution columns in the elution circuit. Regenerated (or new) carbon is loaded into tank 6 and progressively advanced through the adsorption train in countercurrent flow to the slurry (from tank 6 to tank 5 and so on) until the fully loaded carbon is transferred to the elution circuit from tank 1. Interstage carbon movement is carried out using extended spindle, recessed impeller pumps mounted above each of the adsorption tanks.
Acid wash and elution
The elution circuit design is based on split Anglo American Research Laboratory (AARL) elution technology, with acid washing to remove inorganic contaminants prior to elution. The circuit consists of two four-tonne circuits that are operated in parallel.
The elution column is designed for both acid wash and elution and is constructed with stainless steel. Carbon and slurry are pumped from the first adsorption tank to the loaded carbon screen. The slurry is returned to the first adsorption tank, while the carbon flows by gravity to the four tonne elution column. Acid washing with nitric acid commences after the column is filled. On completion of the acid wash cycle, the carbon is rinsed with potable water to ensure residual acid is removed. The water is heated to 90°C using a diesel-fired thermal oil heater that is fitted with primary and recovery heat exchangers to heat the carbon prior to elution. Both the spent acid and rinse solutions are discarded to tailings.
Cyanide and caustic are added to water in the pre-strip tank to produce pre-strip solution. The solution is heated and added to the elution column on completion of the acid rinse stage. Discharge from the elution column at this point is directed to the electrowinning tank.
After the pre-strip tank is empty, primary elution commences. The weak pregnant solution from the water elution stage of the previous strip is heated and added to the elution column. After primary elution is complete, heated potable water is added to the elution column, with the discharge from the column directed to the weak pregnant solution tank to produce weak pregnant solution for the next elution cycle. After completion of the water elution step, the heater is shut down and ambient potable water is added to the column to cool the carbon. Discharge from the column during this step also reports to the weak pregnant solution tank.
The pregnant solution in the electrowinning tank is processed in two sludging electrowinning cells that operate in parallel to recover the precious metals from the pregnant solution. During the plant expansion, one of the smaller electrowinning cells was replaced with a larger electrowinning cell with a larger capacity. Pregnant solution that is produced in the gravity intensive cyanidation reactor is processed in a separate, independent electrowinning cell.
Carbon regeneration
To maintain activity, barren carbon requires periodic thermal regeneration to remove organic contaminants. The regeneration frequency is determined based on operating experience and the carbon loading profiles. The plant is limited by the kiln capacity to the regeneration of one batch of carbon per day.
At the completion of the elution cycle, the carbon is hydraulically transferred to a dewatering screen that is located above the feed hopper of the regeneration kiln. The barren carbon is metered into the high temperature, oxygen free environment within the rotary kiln for a residence time of approximately 15 minutes before discharge into a small quench hopper and then to a Kason sizing dewatering screen prior to being returned to the last adsorption tank. Underflow from the screen that contains carbon fines is discharged to the new system for carbon fines collection. During the plant expansion, a carbon attrition tank for new carbon and carbon fines collection systems for both new and process carbon fines were added.
New carbon is added to the carbon attrition tank along with fresh water to remove fines and prepare the carbon for addition to the carbon adsorption circuit in tank 6 when make-up carbon is needed.
Refining
Precious metal recovery and refining is carried out in the high security area gold room. The gold room is equipped with access control, security mesh to the walls, and video surveillance, incorporating CCTV cameras strategically placed throughout the gold room. An alarm system is installed in the gold room area including passive motion detectors, door proximity switches, and keypad for arming the alarm. The alarm system will be armed whenever the gold room area is not manned.
Sludge from the cathodes in the electrowinning cells is manually removed using high pressure water. The sludge is collected, and excess water is removed by decantation and dried in an oven. The dried sludge is mixed with fluxes and smelted in the gold room furnace. Doré bars are poured in a cascade mould before storage in the gold room vault prior to collection by the bullion security transport company.
Cyanide detoxification
Tailings slurry that discharges by gravity from the final adsorption tank flows over a carbon safety screen to capture any carbon which may have gotten past the interstage screens. The safety screen oversize material is rejected to a dedicated bunker, bagged, and stored for later sale or offsite treatment. Underflow from the safety screen passes to the final tails pump box that supplies the cyanide detoxification feed pumps.
The detoxification circuit reduces the level of weak acid dissociable (WAD) cyanide in the tailings for safe disposal using the sulphur dioxide-air process. Sodium metabisulphite (SMBS) provides the sulphur dioxide and copper sulphate is added as the catalyst for the reaction. Slurry enters the first of two agitated and aerated tanks where it is mixed with SMBS and copper sulphate solutions. The slurry is controlled to a pH between 8.0 and 9.0 by the addition of lime slurry, as required. The slurry overflows from the first to the second reactor before overflowing to the detox hopper. Compressed air, required for the aeration, is provided by dedicated air blowers operating in a duty or standby configuration.
The resulting detoxified slurry is pumped by the tailings pumps for disposal to the tailings management area (TMA). A single stage tailings pump is used to pump the detoxified slurry to the TMA. Larger pumps were installed during the plant expansion to increase the pumping capacity. Deposition of the slurry into the TMA is via peripheral spigotting systems located on the dam faces. Excess water that separates from the settled slurry is recycled back to the process plant by two submersible decant pumps and the intermediary pond for re-use in the process as make-up water.
Reagent storage and distribution
Reagents include grinding media for use in the SAG mill, hydrated slaked lime for use in cyanide circuit, sodium cyanide for use in leach and CIP, intensive cyanide leach in the gravity concentration circuit, and elution, sodium hydroxide for use in the intensive cyanide leach in the gravity concentration circuit, elution, and reagent mixing, nitric acid for use in elution, activated carbon for use in CIP, copper sulphate for use in cyanide detoxification, and SMBS for use in cyanide detoxification. The facilities at Aurora include all infrastructure necessary for reagent storage, mixing, and distribution.
Plant services
Plant services at the Mine include power generation using eight 2.1 MW Cummins QSK60 diesel generators (site power supply), potable water systems, raw water systems, and process water system.
Key reported parameters
| Description | Unit | Design | Current |
|---|---|---|---|
| Plant Feed | tpd | 5,000 | 8,000 |
| Grade | g/t Au | 3.30 | 3.33 |
| Bond Crushing Work Index | kWh/t | 16.2 | 16.2 |
| Bond Rod Mill Work Index | kWh/t | 16.0 | 16.0 |
| Bond Ball Mill Work Index | kWh/t | 14.2 | 14.2 |
| JK Axb Parameter | na | 35.8 | 35.8 |
| Primary Jaw Crusher Feed | tpd | 5,000 | 7,500 |
| Primary Jaw Crusher Capacity | tph | 605 | 605 |
| Primary Jaw Crusher Nominal Product Size P80 | mm | 150 | 150 |
| Primary Jaw Crusher Availability | % | 80 | 80 |
| Pre-crushing Circuit Capacity | tpd | — | 1,000 |
| Pre-crushing Circuit Nominal Product Size P80 | mm | — | 31.75 |
| Pre-crushing Circuit Crusher Availability | % | — | 75 |
| SAG Mill Capacity | tph | 218.8 | 334 |
| SAG Mill Feed Size F80 | mm | 150 | 133 |
| SAG Mill Product Size P80 | µm | 75 | 75 |
| SAG Mill Diameter | m | 7.9 | 7.9 |
| SAG Mill Effective Grinding Length (EGL) | m | 5.4 | 5.4 |
| SAG Mill Motor Size | kW | 5,500 | 5,500 |
| Gravity Concentration Centrifugal Concentrator | number | 1 | 1 |
| Gravity Concentration Intensive Leach Reactor | number | 1 | 1 |
| Leaching Slurry Density | % solids | 49.1 | 50.0 |
| Number of Leach Tanks | number | 2 | 5 |
| Leach Tank Volume | m³ | 3,504 | 10,512 |
| Leach Residence Time | hr | 11.5 | 18 |
| Number of Adsorption Tanks | number | 6 | 6 |
| Adsorption Tank Volume | m³ | 4,938 | 4,938 |
| Adsorption Residence Time | hr | 16 | 11 |
| Number of Cyanide Detoxification Tanks | number | 2 | 2 |
| Cyanide Detoxification Tank Volume | m³ | 458 | 687 |
| Cyanide Detoxification Residence Time | hr | 2 | 1.7 |
Project website: https://www.zijinmining.com/global/program-detail-71743.htm
Technical qualifications
The report states that the cyclone overflow density at 7,500 tpd has been 58% to 62%, indicating an operating condition that differs from the design slurry density of 49.1% to 50.0% solids. The pre-leach thickener was commissioned and put on standby due to the expansion design not addressing the upstream bottleneck in cycl
