The Lawyers Gold-Silver Project processing plant is designed as a 10,600 t/d cyanide leach facility with Merrill Crowe recovery, tailored for a deposit with a high silver/gold ratio.
The Lawyers Gold-Silver Project’s process plant is a 10,600 metric tonne per day (t/d) cyanide leach facility that uses Merrill Crowe precipitation technology rather than carbon adsorption to recover precious metals from solution. The plant design is typical for a precious metals deposit with a high silver/gold ratio. The processing circuit includes crushing, grinding, leaching, Counter Current Decantation (CCD), and a Merrill Crowe circuit to recover gold and silver from solution.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Plant Throughput | 10,600 | t/d | Design throughput |
| Total Silver Recovery | 83.0 | % | Design (Cliff) |
| Total Silver Recovery | 60.6 | % | Alternate case (AGB) |
| Leach Residence Time | 32 | h | Excludes 5 h pre-aeration |
| Pre-aeration Time | 5 | h | Before leach |
| Cyanide Addition | 2,000 | ppm | Through leaching circuit |
| Thickening Flux | 1.00 | t/m²/h | Design flux |
| Mill Power | 15.1 | MW | Instantaneous |
| Mill Power | 121,800 | MWh/year | Annual consumption |
| Process Water Reclaim | 95 | % | Of total process water requirement |
| Freshwater Demand | 5 | % | Of total process water requirement |
| Detox Cyanide Target | 1 to 5 | ppm | After sulphur dioxide/oxygen treatment |
| Process Plant Personnel | 78 | personnel | Total staff and hourly |
Overview
The Lawyers processing plant is a 10,600 t/d facility that treats open pit ore through a conventional precious metals flowsheet. The circuit consists of primary crushing, semi-autogenous grinding (SAG) and ball milling, agitated cyanide leaching, Counter Current Decantation (CCD) for solid-liquid separation, and Merrill Crowe zinc dust precipitation for precious metal recovery. The plant was designed for a high silver/gold ratio feed, which is the primary factor in selecting Merrill Crowe over carbon adsorption. The process design criteria were established for a projected throughput of 10,600 t/d, with total silver recovery indicated at 83.0 percent in the base case (Cliff) and 60.6 percent in an alternate case (AGB).
Key Process Stages
Crushing and Grinding
Mineralized material is moved from the open pit to a primary Metso C160 jaw crusher. The crusher is designed to receive feed with a particle size F100 of 800 mm, meaning all particles in the feed will be smaller than 800 mm. The crushed ore is reclaimed by two apron feeders onto a 1050 mm wide by 80 m long conveyor that transports material to the SAG mill. The SAG mill is 28 feet in diameter. The ball mill circuit operates with a circulating load of 300 percent, with cyclone feed handled by a pair of 16 x 14 Krebs slurry pumps, one operating and one standby.
Gravity Recovery
A centrifugal concentrator is installed within the circulating load of the ball mill. The concentrate produced by the centrifugal concentrator is leached in a Concep Acacia CS3000 intensive leaching unit with a dedicated electrowinning cell. Silver recovery in the gravity circuit is expected to be minimal, which is typical for a precious metals gravity circuit. The electrowinning product from the gravity circuit is melted in the gold room to produce doré bars.
Leaching
Cyanide is added to the grinding and leaching circuit at a concentration of 2,000 ppm. The leach circuit consists of six 16.5 m diameter by 21 m tall agitated leach tanks, providing a total leach residence time of 32 hours, not including the 5 hours of pre-aeration.
Counter Current Decantation
The leaching circuit discharges into a 5-thickener Counter Current Decantation (CCD) circuit. This recovers a clarified pregnant leach solution for the Merrill Crowe circuit and a barren slurry for the tailings. The CCD circuit consists of five 25 m diameter thickeners. Tailings water is added to the slurry in the final CCD thickener, CCD thickener #5. The CCD thickener #5 underflow is pumped to the cyanide detox circuit. The solution from the final CCD thickener is mixed with the slurry feed to the 4th CCD thickener in an agitated mix tank.
Merrill Crowe
Merrill Crowe technology was selected because it is typically used when silver grades are greater than 5 times the gold grade in the feed. The Merrill Crowe process involves producing a clear solution, de-aerating the solution to prevent the zinc cyanide reaction, and then adding zinc dust to precipitate the precious metals. Lead nitrate is added to aid the precipitation process. The circuit selected is a packaged unit from FLSmidth, which consists of a pre-circuit filter to ensure the feed to the circuit is a clear solution, a Merrill Crowe tower which de-aerates the solution and then adds zinc dust. The Merrill Crowe package is rated at 5,723 gallons per minute (GPM).
Detoxification
The chosen detox process uses sulphur dioxide and oxygen to reduce the cyanide concentration in the final CCD thickener underflow to between 1 ppm and 5 ppm. The detox circuit consists of a single 13 m diameter tank. Sulphur dioxide is produced on site by a sulphur burner, which allows elemental sulphur to be shipped to site. The sulphur burner produces sulphur on demand, which reduces the risk of a large, pressurized vessel on site and decreases shipping costs.
Reagent and Consumables
Reagent and consumable consumption rates have been calculated from expected consumptions from the metallurgical testwork.
Tailings
A Tailings Distribution System conveys tailings from the Mill to the Tailings Management Facility (TMF) embankment crest, depositing into the TMF via a series of discharge spigots during normal operating conditions. The system is gravity draining and does not require a pump to convey the tailings to the TMF. Tailings are deposited from a series of discharge spigots on the HDPE pipeline. Tailings must be discharged from one or more spigots to reduce the risk of static pressure build-up in the pipeline. The system is supplied with insulated and heat traced air release valves for safe operation.
Water Management
Water for mill processing is sourced from a combination of reclaim from the TMF, reclaim within the mill circuit from the thickeners, and a freshwater component sourced from dewatering of the open pits or historic underground workings. The reclaim water system consists of two vertical turbine pumps on two floating barges, floating HDPE pipelines that merge into one overland HDPE pipeline, and four centrifugal booster pumps. The reclaim system is designed to meet 95 percent of the total process water requirement year-round, assuming a 5 percent freshwater demand. Two booster station pumps, with 100 percent installed standby capacity, operate concurrently in series to provide the necessary additional total dynamic head (TDH) to deliver the process water to the mill.
Personnel
The process plant is operated continuously throughout the year by 4 operating crews working 12-hour shifts on a two week in / two week out rotation. The process plant total staffing is 78 people, comprising operations, maintenance, and technical services roles.
Additional Interesting Data and Summary
The plant is designed for continuous year-round operation. The process plant total staff is 78 personnel. The plant manager, process superintendent, and metallurgical staff are among the senior roles. The maintenance team totals 28 personnel, and technical services (including assay laboratory staff) totals 18.
Key Processes
- Primary crushing with a jaw crusher.
- SAG and ball mill grinding in closed circuit with cyclones.
- Gravity recovery using a centrifugal concentrator.
- Intensive leaching of gravity concentrate.
- Agitated cyanide leaching with pre-aeration.
- Counter Current Decantation with five thickeners.
- Merrill Crowe zinc dust precipitation with lead nitrate addition.
- Sulphur dioxide and oxygen cyanide detoxification.
- Gravity-drained tailings deposition via discharge spigots.
Source: Lawyers Gold-Silver Project , Preliminary Economic Assessment, report date not stated. Project website: MillMatters Global Circuits.
Editorial Note: The technical source provides a total silver recovery of 83.0 percent in the "Cliff" column and 60.6 percent in the "AGB" column, but does not define which is the base case within the provided text. The original article text was not used to determine facts where the source material contradicted or did not provide support; all facts above are drawn directly from the technical report text provided. The technical report date is not stated in the provided source data.
Project website: Lawyers Gold-Silver Project, Preliminary Economic Assessment
