The Parral Tailings Project pre-feasibility study describes a proposed heap leach processing facility designed to retreat old tailings from the Mina la Prieta operation using conventional cyanide leaching, Merrill-Crowe zinc precipitation, and copper acid leaching circuits.
Report context
The Parral Tailings Project is a pre-feasibility study for a proposed processing plant located in Parral, Mexico, 220 km from the state capital city of Chihuahua. The plant is designed to retreat old tailings from three zones (Zone 1, Zone 2A, and Zone 2B) that were produced by the Mina la Prieta silver and base metal mine, which operated periodically from 1629 to 1975. The study documents the recovery methods, process design criteria, and plant description for the proposed operation.
Processing route
Process overview
The proposed plant is designed to process a minimum of 5,000 t/d of re-mined tailings. The extraction process uses conventional heap leaching with sodium cyanide solution. Gold and silver are recovered from cyanide solution using zinc precipitation through the Merrill-Crowe process. The very high silver content of the ore makes this process more cost effective than a carbon adsorption process.
The plant comprises tailings reception with a temporary 9,000 t re-mined tailings stockpile, an agglomeration and stacking circuit, a heap leach circuit, a Merrill-Crowe plant, and a copper-acid leaching and precipitation circuit.
Ore reception
Re-mined tailings are transported by haul trucks from the tailings dump to a temporary ore stockpile with a capacity of 36 hours. A front-end loader adds material from the stockpile into a feed tipping bin designed with a capacity of 100 t. Material is drawn from the tipping bin by a variable speed belt feeder, which discharges at a controlled rate onto the primary screen feed conveyor. The belt feeder operates in a feedback control loop with a mass meter to control feed rate.
A tramp magnet is installed over the primary screen feed conveyor for removal of tramp metal. The primary screen is a single deck screen equipped with a 25 mm aperture sizing deck. Oversize material gravitates into a bunker for waste collection and removal. Undersize material passes onto a mixing belt for further processing. A dust suppression unit is installed in the ore reception area.
Agglomeration circuit
The agglomeration circuit comprises cement and lime addition systems, a mixing belt, and an agglomeration drum. Cement is added as a binder for fine tailings, providing agglomerating strength and permeability to allow a high flow of leach solution through the heap. Cement also provides protective alkalinity during leaching to maintain a leaching solution pH of 11.
The combination of material with cement is mixed on a mixing conveyor belt and reports to the agglomeration drum at a rate of 250 t/h. Inside the drum, the mixture is dampened to 18% moisture by adding barren solution. The flexibility exists to add cyanide solution to the agglomeration drum if required. During agglomeration, clay and fine particles adhere to coarser particles, creating a coating of fines around these particles. On discharge from the drum, the agglomerated ore is conveyed to an impervious leaching pad.
Ore stacking circuit
The agglomerated material is conveyed to a series of grasshopper conveyors using transfer conveyors. Each mobile grasshopper conveyor is 750 m wide by 30 m long and will be positioned to suit the relevant heap leach cell under construction. The grasshopper conveyors feed a 600 m wide by 16.5 m long radial arm stacker with a working radius of 75 m stack width and 10 m stacking height with the stinger retracted. Material will be stacked and allowed to cure for 48 hours prior to cyanide irrigation.
Heap leach circuit
The heap leach area has 4 main solution ponds: a pregnant pond, barren pond, emergency pond, and release pond. Sodium cyanide solution is used as a leachate to irrigate the heap for 60 days. The leachate is applied using a drip irrigation method due to site climate conditions. Testwork indicates an irrigation rate of 10 L/m²/h. Gold, silver, copper, and other metals contained in the ore are dissolved to various degrees by the sodium cyanide solution as it percolates down through the stacked ore and collects at the bottom of the heap as pregnant leach solution.
Cyanide is added to the barren solution from the recovery plant via the irrigation pumps suction lines before the solution is passed through a sand filtering circuit and onto the heap leach pad using variable speed pumps. Barren solution is also pumped to the recovery plant where it is used as dilution and make-up water.
The emergency pond is designed to handle excess runoff water from the heap leach area and to store accumulated excess solution during the rainy season. In extreme rain conditions the emergency pond overflows to a detoxification step to destroy excess cyanide, using hydrogen peroxide and sulphuric acid in the release pond. A pump is provided to recover excess solution from the emergency pond back to the heap leach circuit via the barren pond.
Clarification circuit
The pregnant leach solution (PLS) from the pregnant pond is pumped through clarifying filters to reduce suspended solids prior to zinc addition. The filter elements are covered with polypropylene cloth, pre-coated with a layer of fine silica (diatomaceous earth) to create a bed of filter media to trap very fine particles.
The PLS is pumped through the filter channel into all filter chambers at the same time until they are filled. The filter cake build-up starts when the filtrate is pressed through the cloth by newly fed solution. At the end of the filtration cycle, the filter is drained along with filtered solids which slump by gravity from the filter cloths into the settlement pond. The filter cloths are washed with barren solution, and upon completion of the wash cycle, the filter is put back online and the pre-coating process initiated.
The unwanted solids in the settlement pond are removed via front-end loader and returned to the prepared heap leach cell. Spillage collected in this area is pumped back to the pregnant pond using a submersible pump. The clarified solution reports to the deaeration and zinc precipitation circuit.
Deaeration and zinc precipitation circuit
Deaeration of clarified PLS is accomplished using a packed tower under vacuum. Clarified solution is pumped to the top of the deaeration tower where it is distributed over a bed of packing, providing surface area for thin film formation and release of dissolved oxygen. A vacuum pump reduces the pressure within the vessel, and evolved gases, including oxygen, are exhausted.
Deaerated solution discharges from the tower by gravity into the zinc mixing system. The zinc precipitation circuit comprises a zinc powder storage bin, variable speed drive belt feeder with mass meter, mixing cone, and precipitate pumps. Zinc powder is metered into the deaerated PLS using a VSD belt feeder and is fed into a mixing cone containing cyanide solution. The zinc addition rate is calculated as the stoichiometric amount required to precipitate the precious and base metals in solution plus an excess amount.
Lead nitrate is added to the zinc mixing chamber to aid in recovery of gold precipitate by preventing zinc passivation. The cementation reaction occurs very rapidly, and sufficient retention time is available in the pipeline between the filter press feed pumps and the precipitate filter presses. The key consideration in the design of the filter press feed pumps is the prevention of air ingress, as leakage of air through the pump shaft seal may allow oxygen to enter the system.
Precious metal filtration
Filtration of the silver-gold-copper precipitate is accomplished by 3 plate and frame type filters installed in series. The filters are fed continuously until the pressure differential reaches the maximum recommended operating level or should the flow decrease to unacceptable levels. At the end of the filtration cycle, the filter is taken offline and drained. The press is opened and the filter cake drops onto a dedicated filter cake discharge conveyor. The collected cake gravitates into the copper leach tank. The clear barren solution gravitates into a transfer tank and is pumped back to the barren pond.
Copper leaching circuit
The copper leaching circuit is a batch operation located inside the gold room for security reasons. The circuit comprises an agitated stainless steel conical tank as a reactor with a fume extraction fan, circulation pumps, air blowers, a plate and frame filter press, and a dedicated safety shower. The intention of this circuit is to dissolve all copper contained in the precipitated metals using strong sulphuric acid.
The slurry is circulated until the copper is completely dissolved. The gold-silver precipitate is then filtered out of the solution and discharges onto a tray. The clear copper sulphate solution from the filter gravitates to the neutralisation and copper precipitation circuit.
Neutralisation and copper precipitation circuit
The neutralisation and copper precipitation circuit is a batch operation consisting of 2 agitated neutralisation tanks, circulation pumps, a plate and frame type filter press, and barren solution return pumps. The circuit raises the barren solution pH to between 9 and 10 and precipitates saleable copper out of the solution. The clear solution from the copper leaching circuit is mixed with caustic solution. During the reaction, the copper sulphate solution reacts with caustic to produce a copper hydroxide solid and sodium sulphate solution.
During the process, the mixed solution is circulated through the filter and back to the neutralisation tank until a clear solution is achieved. Thereafter, the filter is bypassed while the cake (copper hydroxide solids) is removed and dropped into the copper storage bunker. The clear solution from the filter gravitates into the barren solution return tank and is pumped back to the barren pond. A sample of the barren return stream is monitored for pH and metal content.
Gold room
The silver-gold cake is dried in a calcine oven. The calcine is then smelted with fluxes into silver-gold doré bullion using the smelting furnace. Provision is made for the storage and weighing of fluxes in the gold room. Bullion bars are cleaned, weighed, and stamped before storage in the strong room prior to despatch.
A 16 mm thick steel plate is cast into the gold room floor in front of the smelting furnace to allow bullion moulds to be tipped out without damaging the concrete floor. The smelting furnace is covered by a fume hood to collect high value dust emitted during a smelt. A single 6-tray calcine oven is installed to provide standby capacity, secure storage capacity, and to allow the concentrate to be dried. Digital electronic scales are provided for weighing fluxes and doré bullion bars.
Reagent services
Sulphuric acid will be supplied to site by a 30 t road tanker and off-loaded into a sulphuric acid storage tank. Sulphuric acid is used as a leachate during the copper dissolution process, maintaining the required low pH. Sulphuric acid is also dosed, if required, into the detoxification plant together with hydrogen peroxide.
Sodium cyanide is delivered to site in 1 t bulk bags. During make-up, the cyanide bag is hoisted to the top of a mechanically agitated sodium cyanide mixing tank and placed into a bag breaker. The bag breaker cuts the bulk bag, discharging the cyanide solids into the mixing tank while it is being filled with barren solution or raw water. The mixer dissolves the briquettes to make a 25% solution of sodium cyanide. On completion of dissolution, the cyanide solution is pumped to the cyanide dosing tank. Sodium cyanide solution is dosed to the barren pond from the cyanide storage tank. Provision has been made to pump sodium cyanide solution to the zinc mixing system and the agglomeration drum if required. Cyanide spillage is collected in a bunded area and pumped back to the cyanide mixing tank.
Caustic soda pearls or flakes are received in 25 kg bags, which are mixed to a solution of 25% strength in a caustic mixing tank. The tank also serves as a storage tank, with dosing to the neutralisation circuit using the caustic pump. The caustic and cyanide tanks are located in an interconnected bunded area. Safety showers are provided at the cyanide and caustic make-up areas, storage, and offloading facilities.
Capacity increase considerations
The throughput of the process plant should be capable of up to a 20% increase with modest additional capital cost. The agglomeration drum is considered to be conservatively sized for the duty, and an increase in throughput should be achievable once operating skills are well developed. The stacking, leach pads, and solution pumping should all be readily upgraded with minimal cost. The clarification filters could be designed to accommodate additional filter plates to increase throughput, and the precious metal filters could similarly receive additional filter plates. The subsequent processes are all batched, and the frequency of batch processes can be readily increased.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Tailings feed rate | 5,000 t/d minimum | Proposed design |
| Heap leach irrigation | 10 L/m²/h | Testwork/design criterion |
| Cyanide leach period | 60 days | Proposed design |
| Agglomeration moisture | 18% | Proposed design |
Project website: https://gogoldresources.com/properties/parral-tailings/
Technical qualifications
The study describes a proposed retreatment facility. The reported capacities and conditions are design criteria rather than confirmed operating results.
Safety and risk assessment
The occupational health and safety system is a 5-stage proactive system including risk and impact assessments, first aid and occupational health service facilities, medical services, training, and inspection and action. The risk assessment process addresses chemical stress, psychological stress, physical stress including noise, lighting and vision, heat and cold extremes, vibration, ventilation, non-ionising radiation, ionising radiation, biological stress, and ergonomical stress.
Risk assessments take full cognisance of material safety data sheet information, survey results, medical information, and general complaints. Occupational health experiences are included in assessments of existing processes, new processes and modifications, pre-commissioning, new plant handover, and job safety analyses and workplace safe work procedures. Occupational hygiene surveys are conducted by appropriately qualified hygienists according to legal requirements, with all instrumentation complying with regulated standards.
Project implementation
The implementation plan is driven by the Project site location, prevalent weather conditions, Mexican construction regulations, and anticipated efficiency levels of construction teams. The most cost effective approach in current market conditions would be a reimbursable type contract with a main contractor responsible for overall project design and management, with turnkey packages for major equipment supply, civil construction, structural steel and plate work fabrication, power supply, and transportation of goods to site.
The main contractor will supervise design and construction of the plant, with mine infrastructure construction carried out by local or Mexican contractors under main contractor supervision. All civil excavation and construction work will use local contractors employing local labour. All fabrication of steelwork and plate work will be carried out by local fabricators and labour under main contractor supervision.
Critical drivers include front-end engineering design confirmation activities, permitting, appointment of main and sub-contractors, refining cost estimates to an accuracy of 10%, and placing orders for standby power generation sets, mining equipment, and construction cranes prior to construction commencement.
The execution programme is divided into 2 phases. Phase 1 covers front end engineering design (FEED) and early works, including detail design, Mexican authority approvals, design optimisation, supplier sourcing, and procurement of long lead items, with completion targeted by mid-2013. Phase 2 covers construction execution and commissioning, with construction activities conducted in accordance with the attached programme. Key drivers include contractor capabilities, procurement of long lead mechanical equipment including the agglomerator, stacker and grasshopper conveyors, and environmental permitting.
