Bear Creek-Minig Corporation.pdf

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Environment &Sustainability

J07.82.13.02Revision Number 0

Bear Creek Mining Corporation

Feasibility StudySanta Ana ProjectPuno, PerNI 43-101 Technical Report

Prepared for:Bear Creek Mining Corporation1050 625 Howe StreetVancouver, BC V6C 2T6 Canada

Prepared by:Ausenco Vector

I d d t Mi i C lt t I


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23.2 Mineral Processing

Heap Leaching Consulting in coordination with Ausenco-Vector prepared the design criteria for themetallurgical process considering information related to plant capacity and operating conditions.The information used in the design criteria was obtained from field work, metallurgical testingprograms, and leach pad design prepared by Vector. Some data was assumed from experience onsimilar projects and from mathematical calculation.

The basic process consists of material from the open pit transported to the crushing plant, which will

comprise two crushing stages and one classification stage. The final crushed ore product willconsist of 80 percent passing 19mm and will be conveyed to the temporary stockpile. The reclaimsystem will consist of one fixed conveyor stockpile to withdraw material from the stockpile anddeliver onto trucks. Trucks will transport the ore from the stockpile to the heap leach pad, which willreceive 10,000 tpd of ore containing 53 g/t of silver. A sodium cyanide solution will be drip irrigatedon the heap to dissolve silver minerals. The estimated dissolution of silver is 70% and the pregnantsolution will flow to the Merrill-Crowe plant to produce a precipitate, which will be smelted toproduce a silver/gold dor bar.

23.2.1 Flowsheets

The project process flowsheet development considered the design criteria, field work, leach paddesign, water system distribution and electric system distribution as inputs; plus, smelting, reagentsystem preparation, effluents detoxification, water distribution and plant facilities for a heap leachingprocess to recover precious metals. The Merrill-Crowe process was selected as the optimumalternative to recover the precious metals as a precipitate. The precipitate will be sent to theretorting stage, and finally will be smelted to produce a Dore bar. The process flowsheet developedfor the feasibility study is provided as Figure 23.17 (full flow sheet in Figure 24.2).

23.2.2 Mass Balance

The parameters determined in the design criteria were used to calculate the mass balance of theprocess: leaching, Merrill-Crowe Plant, smelting, reagents handling, effluent detoxification process,water distribution, and plant facilities. The information obtained was used to select the appropriateequipment and flow streams for the metallurgical process as illustrated in Figure 23.17.

23.2.3 Piping and Instrumentation

Based on the information obtained on the leaching process, mechanical area, piping andinstrumentation, the piping and instrumentation diagrams were developed. The diagrams includemechanical equipment, instrumentation equipment, piping, valves, effluent detoxification plantequipment, water distribution and plant facilities.

23.2.4 Production Plan

The production plant was prepared using the mine plan prepared by IMC and consists of periods ofthree months until the ore reserves are exhausted and the additional time required to reachmaximum dissolution of silver bearing minerals. The metallurgical recovery of silver by the leaching

i 70% ith l hi i d f 120 d f th i l h l d t t l l h ti


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23.2.5.1 CrushingRun of Mine Ore (ROM) will be delivered by 63t haul trucks to the dump pocket of a single jawcrusher located to the west of the pit. The installation was designed to allow one truck to approachthe dump pocket. The discharge from the dump pocket will flow by gravity to the apron feeder. Theapron feeder will discharge onto a grizzly with a 200 mm nominal spacing. The feeder will beemployed to feed the primary crusher. The primary crusher and grizzly feeder undersize productswill be sent to a vibrating screen in an open circuit with a cone crusher. The cone crusher will beoperated with an approximate 20-mm closed side setting to ensure that the final screen undersizeproduct will be 80% passing 19-mm. The screen undersize product will be dropped onto an

overland conveyor provided with a belt scale. A simplified flowsheet is provided as Figure 23.17.

A hydraulic rock breaker will be installed to clear crusher blockages caused by oversized rocks.Chains are to be installed on the base of the crusher dump pocket before operations commence toallow break up of impacted rocks prior to maintenance around the top of the crusher. The primaryand secondary crusher service pedestal crane will be located adjacent to the crusher to serviceequipment located around and within the crusher structure. A metal detector and belt magnet at thedischarge conveyor that feeds the screen will be used as tramp metal protection system.

Coarse ore from the overland conveyor will be fed to the coarse ore stockpile with a live capacity ofapproximately 6-hrs (3,000t) and dead capacity of one day. The desired design basis for thestockpile is to provide sufficient ore capacity to allow for one day down time. The conical coarse orestockpile will not be covered. The reclaim system will consist of one fixed conveyor stockpile towithdraw material from the stockpile and deliver onto trucks and a radial stacker building the coarseore.

The parameters used to design the crushing circuit are listed below,

Total capacity, 10,000 t/d

Capacity, 555 t/hr

Maximum particle size, 24-inch

Final product, 80% passing -inch (19-mm)

Low Energy Impact Test, 10.3 kw-hr/t

BBWI, 17.1 kw-hr/t

Abrasion index, 0.0539

Specific Gravity, 2.43

Bulk density, 1.65

Ore moisture, 3-4%

Operation 18 hr per day


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Figure 23.17Santa Ana Heap Crushing Flowsheet


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23.2.6 Ore Transport

Crushed ore will be transported by 63t trucks to the leach pad and dumped into leach cells. Thematerial will be placed on the pad in 7m high lifts and pushed to the final placement with a dozer inaccordance with the stacking plan developed for the project heap leach pad. Prior to leaching thetop of the leach cell will be ripped by a dozer to ensure proper infiltration of the leach solutions.

23.2.7 Irrigation System

The design considers cells of approximately 50m x 120m (6,000 m2

) with their respective flow linesand drip irrigation system considering as initial point the distribution of manifolds, which will befabricated with schedule 40 steel pipe of 300mm diameter. There will be four distribution pipes of150mm diameter that will be connected with an irrigation system.

The drip irrigation system comprises HDPE pipes of 150mm diameter and Lay Flat hoses of100mm installed with hoses of 16mm diameter and drips of 4 litres per hour and separated by63 cm.

23.2.8 LeachingThe leaching operation is a hydrometallurgical process of precious metals dissolution. The cyanidesolution is spread on the top of the ore heap using drip emitters and percolates through the pile.Particles of silver and gold are dissolved. The pregnant solution flows to and collects in thepregnant pond. The nominal flow of cyanide solution is 10 l/hr/m

2 and the design is based on

11 l/hr/m2.

23.2.8.1 Cyanide Solution Pumping

The pH of the cyanide solution will be adjusted between 10.5 to11 and the cyanide concentration

will be adjusted to 1000ppm. Also, 4ppm of antiscalant will be added. The solution will be pumpedfrom the barren solution tank to the leach pad by means of one horizontal centrifugal pump. Theprocess design includes two pumps; one on-line and the other on standby. Each pump has anadjustable-speed drive that varies the speed of the pump motor.

Once the solution is delivered to the pad, the main solution pipeline will be divided in two parts toborder the leach pad. Within the active leach cells irrigation systems will be installed. The solutionthat feeds the barren tanks will come from the Merrill-Crowe process and the concentration ofcyanide will be adjusted to 1000 ppm.

The process water required will be taken from the barren pond, storm water pond or fresh make-upwater according to the requirement of the operation. One submergible pump will be installed in eachpond.

23.2.8.2 Pregnant Solution Collection

The pregnant solution that percolates through the leach pad will be collected in a pregnant pond


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The floating vertical pumps will be installed on barges equipped with a special maintenance system.The barges will be steel platforms supported on floating polyethylene.

23.2.9 Merril l-Crowe Plant

The Merrill-Crowe process recovers silver and gold from the pregnant solution in form of precipitate.Zinc powder is added to the clarified and deaerated pregnant solution to precipitate preciousmetals.

The design has considered a Merrill-Crowe plant to treat 571 m3/hr of pregnant solution containing

silver and gold. The capacity of the plant assures the treatment of 10,000 tpd of silver ore from thepit. The precipitation process comprises the following stages: clarification, deaeration andprecipitation with zinc dust. A simplified process flowsheet is provided as Figure 23.18.

23.2.9.1 Clarification

The unclarified pregnant solution will be pumped to the clarifiers at the rate of 571 m3/hr. The

design considers two centrifugal pumps, one operating and other in standby. Each pump has anadjustable-speed drive that varies the speed of the pump motor. One of the pumps is on-line whilethe other is a standby spare. Clarifiers are three leaf pressure filters, two operating and one in

standby.

When one filter is saturated, the flow of solution will be sent to the third filter. At this moment theother filter will be cleaned and prepared to operate. This operation will be practiced in determinedperiods in order to assure the constant feed of pregnant solution to the clarifiers and avoidunexpected problems. It has been estimated that each filter will be cleaned 1 to 2 times daily. Theresultant solids will be collected and recycled to the leach pad by a pumping system.

Prior to the operation of the clarifying filter a pre-coat layer will be applied, this will be reinforcedwith the addition of body feed. During the filtration cycle, solid particles will be retained and thesolution leaving the filter will contain 1 Nephelometric Turbidity Units (NTU) of suspended solids.

When one filter is out of operation, it will be cleaned by pumping barren solution or process water.In this way, solids and remaining pre-coat will be removed.

23.2.9.2 Deaeration

The objective is to remove dissolved oxygen from the clarified solution. The elimination of oxygen isimportant in the precipitation process because an excess of oxygen will oxidize zinc particles andthe efficiency of the precipitation process is affected. Other negative aspect is the presence ofexcessive quantities of suspended solids in the solution, which may passivate the surface of zincparticles.

A vacuum tower will be employed as deaeration unit and will be equipped with control levels,distributor, support and fill. The vacuum is required to maintain a high vacuum within the tower andremoves the oxygen from the solution. The clarified and deaerated solution is withdrawn from thebottom of the tower by a liquid sealed pump which prevents re entry of air


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Figure 23.18Process Flowsheet


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23.2.9.3 PrecipitationZinc dust is used to precipitate precious metals by means of an electrochemical reaction. Leadnitrate is added to catalyse the reaction. Adequate zinc dust is required to provide a coherentdeposit on the filter surface through which the silver bearing solution passes, to make certain thatsufficiently close contact is made with the zinc particles to obtain maximum precipitation efficiency.Zinc dust, lead nitrate and sodium cyanide solution will be added in an emulsifying tank.

The solution containing the precipitate will be pumped by means of liquid sealed centrifugal pumpsto the filter presses. The design considers two pumps with variable speed, one operating and other

in standby and its pumping capacity is 571 m

3

/hr of solution. The precipitate will be collected inthree filter presses, equipped with 55, 1200 mm x 1200 mm plates, and designed to treat 191 m3/hr

of solution. Two filters will operate and one will be in standby. The process flowsheet presented asFigure 23-17 indicates a forth filter press may be added to the system in the future.

When one filter is saturated, the flow of solution will be sent to the third filter while the saturatedfilter is discharged, cleaned and prepared to operate. This operation will be practiced in determinedperiods in order to assure the constant feed of solution to the filters and avoid unexpectedproblems. The resultant solids will be collected, dried, and sent to the retorting stage.

Before discharging the filter, air will be injected to reduce the moisture content of the precipitate.The solution from the filters will be recycled to the barren solution tank.

23.2.10 Smelting Process

The smelting process has been designed to produce Silver/Gold Dore bars with high silver contentfrom the precipitates obtained through the Merrill-Crowe process. The smelting process includesretorts to eliminate mercury, an induction furnace, a gases collection system and slag treatment.

23.2.10.1 Retorting

The precipitate from the filter press contains mercury, which comes from the leaching process and itis recovered in the Merrill-Crowe plant. The design considers three retorts, which operate followingheating and cooling stages. Table 23.6 details the retort process.


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Table 23.6Retorting Process

DescriptionTemperature

(co)

Time(hours)

Initial heating 0 to 300 1.5

Maintain temperature 300 2

Final heating 300 to 540 3

Maintain temperature 540 4

Cooling 100 5.5

Total time 16

During the retorting process, air enters the retorts and collects all the gases. These gases will go toa first condenser equipped with a water cooling system and metallic mercury is precipitated to themercury recovery tank. The gases report to a second condenser to assure the completeprecipitation of mercury. The system includes a column loaded with granular activated carbon to

collect any remaining mercury vapour. The retorting process will allow the release of gases withoutmercury content to the atmosphere. A gas extractor system will help to remove the gases.

23.2.10.2 Smelting

An induction furnace will be used to smelt the precipitate. Upon completion of the retorting process,the precipitate is dried and mixed with fluxes such as potassium nitrate, manganese dioxide, silica,sodium carbonate and fluorite. The charge is loaded into the crucible. The smelting process takesapproximately 3 hours. The molten charge is poured in ingots and the final product of this process isa dor bar with high content of silver.

The dor bars will be deposited in a safe room and will be transported to the market following aschedule, which will be determined with taking the silver production into account.

The smelting process area includes a gas collection system to collect all the gases produced duringthe smelting process.

23.2.11 Chemical Reagents

The chemical reagents that will be used in the process are lime, sodium hydroxide, sodium cyanide,

antiscalant, lead nitrate, and detoxification reagents (hydrogen peroxide, copper sulphate) todestroy cyanide and precipitate some compounds in the cyanide detoxification process. Thereagents will be stored in three separated areas; one for lime, located near the leach pad; one forcyanide and sodium hydroxide, located near the process plant; and the last one will be used tostore all the other reagents.

23.2.11.1Lime Addition


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sodium hydroxide are added to a mixing tank to achieve a concentration of 30 percent. Thepreparation of sodium hydroxide will be completed in batches. A pump will be used to add sodiumhydroxide solution to the sodium cyanide preparation tank.

When the cyanide destruction plant operates, the sodium hydroxide solution will be pumped to theplant.

23.2.11.3 Sodium Cyanide

Sodium cyanide will be received and stored as briquettes in 1-tonne bags. Process water and

sodium cyanide briquettes are mixed to achieve a concentration of 25 percent. A service hoist isused to discharge the contents of the bags through a charge chute and into an agitating mix tankwith process water. The design considers two sodium cyanide preparation tanks.

Pumps transfer sodium cyanide from the mix tank to the storage tank. Any spillage in the cyanidearea is recovered in a sump and pumped to the mix tank. Circulating pumps provide a distributionloop for sodium cyanide use in the leaching circuit and Merrill-Crowe plant. Sodium cyanide ispumped through this loop at a controlled pressure. The unused solution returns to the storage tank.The design considers two pumps with variable speed with a capacity of 20 m

3/hr of solution.

23.2.11.4 Antiscalant

Antiscalant will be used to avoid the formation of precipitates in the pipes, equipment andaccessories of the process plant. The antiscalant solution will be added in the barren solution tank,pregnant pond and storm water pond. The addition of antiscalant will be permanent in the first twopoints.

The concentration of antiscalant must be maintained between 5 and 10 ppm in the leaching andpregnant solutions.

23.2.11.5 Lead Nitrate

Process water and lead nitrate are mixed to achieve a concentration of 5 percent. The mixing tankwill have a capacity of 0.36 m

3. The solution will be sent to the emulsifying tank.

23.2.11.6 Zinc Dust

The zinc dust feed system comprises one bin and one screw feeder, which feeds the material to theemulsifying tank. The maximum capacity of the feeder will be 30.5 kg/hr and the estimatedconsumption of zinc dust is 17 to 25 kg/hr.

23.2.11.7 Precoat

Process water and diatomaceous earth are mixed to achieve a concentration of 0.3 to 2 percent.The mixing tank will have a capacity of 13.3 m

3. The solution will be pumped to the clarifier filters or

filter presses to form a layer of 1.5 to 10-mm on the filtration medium. The solution will recirculatebetween the pre-coat tank, clarifier filter or filter press.


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23.2.11.10 Copper SulphateProcess water and copper sulphate are mixed to achieve a concentration of 5 percent. The mixingtank will have a capacity of 200-m

3. The solution will be pumped to the storage tank, which will have

two pumps for supply to the effluent detoxification plant.

23.2.11.11 Reagent Requirements

Lime and Sodium

Lime and Sodium Cyanide are the major reagents used in the heap leach Merrill-Crowe circuit.

Table 16.9 presents the primary reagent consumptions projected for the p80 19mm crush size.

Table 23.7Primary Reagent Consumption

Reagent Consumption Addition Trim

Lime (@ 80% CaO) 3.0 kg/t CrushingHeap or Barren

Solution

Sodium Cyanide 0.4 kg/t Barren Solution Barren Solution

The lime consumption was estimated from the test program. The p80 19-mm columns had pHcontrol problems at the initial addition rate of 2 kg/t. The 3.0 kg/t addition will assure that the heapwill maintain the pH at 10.5 or above. The sodium cyanide requirement was derived from thecolumn tests. Heap leach consumptions run from to

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the column consumptions.

Other Reagents

The zinc powder requirement of 1g Zn/1g Ag was confirmed in tests at McClelland Laboratories,and utilized in the feasibility study.

Operating Merrill-Crowe plants with the same tenor of silver in solution report requirements as lowas 0.6g Zn/1g Ag.

The reagent consumption was determined using information from metallurgical tests, stoichiometriccalculation and experience on similar projects.

23.3 Infrastructure

23.3.1 Power

Electrical power to the Santa Ana facilities will be provided from the Peruvian grid (SistemaElectrico Interconectado Nacional - SEIN) by a new line to be built connecting the existing Pomatasubstation in the town of Pomata to the Santa Ana substation. In addition, there will be 10kVprimary lines and distribution substations


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60kV bars expansion of the existing Pomata substation and installation of connectionequipment for the 60kV line to the Santa Ana substation;

60kV transmission line from the existing Pomata substation to the Santa Ana substation,about 48.55 km of a three-phase circuit with two guard wires;

A new substation for the Santa Ana mining operation, which includes the installation of atransformation cell (step-down cell) 60/10/4.16kV - 5/5/1.7MVA (ONAN), 6.25/6.25/2.13(ONAF) and distribution cells in 10kV;

Communication systems; and

Emergency power will be provided by diesel generators.

23.3.2 Access Road

In order to develop the Santa Ana project, it will be necessary to improve the existing mine accessroad between the existing interstate highway and the project site. Additional mine site roads willalso be necessary for daily operation of the mine.

In general terms, the roads included in this study have a total width of 7,2m including a diversion

ditch. The slopes along these access roads are generally intended to be kept between 4 and 10%.Due to the relative steepness of the property some roadway sections will require slopes as great as15% or more. Further details and technical data on the access road development are provided inthe DFS Engineering Design Report (Ausenco Vector, 2010).

23.3.2.1 Mine Access Road

Access to the project site is proposed as improvements to an existing rural road that comes fromthe town of Huacullani and continues south, passing through the proposed Santa Ana mine site topermit the mobilization of heavy equipment and materials along the entire 4.3km length. This road

improvement is relatively inexpensive as it gets closer to Huacullani, as the existing ground surfaceis very flat and the road alignment does not present many horizontal curves. This road improvementincreases in costs as it approaches the mine site, where the ground is steeper and mainly rocky.

23.3.2.2 Main Haul Road

The main haul road runs from the end of the mine access road and has a total length of 1.3km untilit joins the perimeter access road of the leach pad. Its location and alignment are strategic sincecomplementary haul roads to the open pit and crusher join this access as well as the access roadthat goes to the offices and camp site.

23.3.2.3 Auxiliary Access Road

These roads are divided into two alignments: the first auxiliary access road has a length of 2.4 kmstarting from station 0+620 of the main haul road and finishing where the general offices arelocated. The second auxiliary access road has a total length of 2.6 km and connects the west sideof the leach pad perimeter access road with the acid water treatment plant, after passing throughthe general offices and camp site


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