PLANT DESCRIPTION INTI RAYMI S.A. (KORI KOLLO) NEWMONT MINING by vow16147

VIEWS: 96 PAGES: 19

									                        PLANT DESCRIPTION
                   INTI RAYMI S.A. (KORI KOLLO)
                 NEWMONT MINING CORP.- BOLIVIA

1.0   GENERAL PROCESS DESCRIPTION

      A primary gyratory crusher receives run-of-mine ore and reduces it to 100%
      minus 200 x 380 mm. The product is conveyed to a coarse ore stockpile,
      approximately 40.7 m high and 106 m diameter containing approximately 88,000
      tonnes total and approximately 15,000 tonnes live capacity.

      Ore withdrawn from the stockpile is fed to a semi-autogenous grinding (SAG)
      mill, which discharges through a screen to a mill sump. Screen oversize is
      conveyed to the SAG mill feed, while the undersize is pumped to the ball mill
      sump. This product is pumped to a group of hydrocyclones in closed circuit with
      two ball mills in parallel. Cyclone underflow returns to the ball mill feed, and the
      overflow gravitates to a vibrating trash screen and is pumped to leach.

      The leach circuit consists of six leach stages and six leach/adsorption (CIL)
      stages, comprising a total nominal residence time of 31.7 hours. Cyanide is added
      to the first leach tank, and air is added to all tanks. Activated carbon removed
      from the first CIL tank is washed on a vibrating screen and transferred to the acid
      wash column in the stripping area.            Carbon is advanced continuously,
      countercurrent to the slurry flow, and stripped carbon is added to the last CIL
      tank. A safety screen prevents loss to tails of any carbon which has bypassed the
      intertank screens.

      After acid washing of the carbon, the gold and silver are solubilized in a Zadra
      stripping system. The precious metals are recovered from the strip solution by
      electrowinning, and gold-silver precipitate is filtered, dried, mixed with flux and
      smelted.

      The screened CIL tailing slurry gravitates to a high rate thickener for water
      recovery. Thickener underflow is pumped to the tailings dam, while the overflow
      gravitates to the process water tank.

2.0   CRUSHING, STOCKPILE AND RECLAIM

      General Description

      Run-of-mine ore is delivered to the dump pocket by 52.6t rear dump trucks. The
      ore passes through the crusher into a surge hopper from whence it is withdrawn
      by apron feeder onto the stockpile feed conveyor. A rock breaker is provided to
      break down any oversize material in the dump pocket.
3.0   GRINDING AND CLASSIFICATION

      3.1 General Description

            Ore withdrawn from the stockpile is fed to a semi-autogenous grinding
            (SAG) mill, which discharges over a trommel screen to a sump. The
            screen oversize is recirculated via conveyor to the mill feed. Screen
            undersize is pumped to the ball mill discharge sump, the product from
            which is pumped to a nest of hydrocyclones in closed circuit with two ball
            mills. Cyclone underflow is split to the ball mill feed chutes, and the
            overflow gravitates via trash screens to the leach circuit feed pump.


      3.2 Grinding Operating Schedule

            Days/year                           365
            Days/week                           7
            Shifts/day                          3
            Hours/shift                         8
            Availability annual                 91%
            Available time                      7,972 h/y
            Design grinding rate                641 t/h

      3.3 SAG Mill

            New feed rate (dry)                 641 t/h
            Lime addition                       SAG mill feed conveyor

            Feed particle size
                   F100                         200 mm x 380 mm
                   F80                          150 mm

            Discharge grate aperture            15.88 x 50.8 mm
            Pebble ports                        None
            Trommel aperture, design            12.5 mm x 50.9 mm
                    minimum                     10 mm
            Ball load, normal                   5%
                    operating                   10%
                    maximum                     15%
            Circulating load                    25%
                    design                      35%
                    solids flow                 224 t/h
            Mill discharge
                    hopper capacity, min.       45 seconds
                                                7.0 m3
                     pump, type                 Centrifugal slurry


                                                                                    2
                              number               2 – 1 op., 1 s/b
                              control              Both variable speed
                              flow rate            561 m3/h

      3.4 Ball Mills

             New feed rate                         641 t/h
             Feed particle size, F80               1100 microns
                                                   1500 microns

             Product (cyclone overflow)
                    size distribution
                      P80 product                  92 microns
                          sulfides                 70 microns
                      P50 product                  38 microns
                      % passing 75 microns         72.4 microns

             Ball load                             37%

             Circulating load
                    previous                       300%
                    design                         200%

             Mill discharge
                     hopper capacity, min          45 seconds
                                                   27.9 m3
                       pump, type                  Centrifugal slurry
                              number               2 – 1 op., 1 s/b
                              control              Both variable speed
                              flow rate            2,236 m3/h

4.0   TRASH SCREENING

      General Description

      Cyclone overflow gravitates to the screens to remove tramp oversize, wood pulp
      and other trash. Screen undersize flows to the leach circuit feed pumps via a two
      stage sampling system, while the oversize gravitates to containers for disposal as
      necessary.

5.0   LEACHING AND ADSORPTION

      General Description

      Circuit is a hybrid CIL type with six leach stages and six leach/adsorption stages.
      Total residence time is 31.7 hours. Tanks are mechanically agitated and air is


                                                                                       3
      provided to each. Bypasses are provided to allow any tank to be taken out of
      service without disrupting the operation.

6.0   TAILING THICKENING

      General Description

      CIL tailings flow by gravity from the carbon safety screen underflow to the
      thickener feed collection box. The majority of plant makeup water is provided
      from the mine deep wells, with river water providing the balance of the
      requirement. Both streams are added to the CIL tailings in the thickener
      collection box. The combined streams flow by gravity to the thickener feed
      well/deaeration tank where the solids are automatically diluted to approximately
      15% by weight to maximize settling characteristics. Diluted flocculant solution is
      also added at this point. Thickener overflow gravitates to the process water surge
      tank, from where it is pumped to the low and high pressure usage points, while
      the underflow is pumped to the tailings dam. An additional pump is provided for
      recycling the thickener underflow.

7.0   ACID WASH, DESORPTION, ELECTROWINNING AND
      BULLION SMELTING

      General Description

      Carbon will be stripped by the pressure Zadra method in 21.5 tonne batches.
      Loaded carbon gravitates from the recovery screens to a surge hopper, and is acid
      washed in a column. After rinsing to neutral pH the carbon is transferred to one
      of two parallel elution columns, and eluted in a hot caustic cyanide solution to
      remove the gold and silver. These are recovered by electrowinning in eight
      parallel cells, and the barren solution recirculated to the elution column. The
      metal is washed from the cathodes, and the sludge filtered, dryed, mixed with flux
      and direct smelted to produce doré.

8.0   CARBON REGENERATION

      General Description

      The design has provision to regenerate a third of the carbon inventory at
      maximum capacity. Eluted carbon is transferred from the column to the
      regeneration kiln feed hopper. The kiln is operated as required and regenerated
      carbon is discharged into water in the storage/quench hopper. The remainder of
      the carbon bypasses the kiln to the quench hopper. Fresh carbon is also added to
      the hopper and the stripped, regenerated and fresh carbon is transferred to the
      leach/adsorption circuit by a combination eductor-pump system.




                                                                                      4
9.0     WATER SYSTEMS

        General Description

        The majority of plant make-up water is provided from the mine deep wells, with
        the remainder being provided by submersible pumps and a pipeline from the
        Desaguadero river. The existing pumping and natural clarifying system will be
        utilized and treated to provide water for specific uses within the plant and site
        infrastructure. No water is reclaimed from the tailings system.


10.0 PRINCIPLES OF CONTROL

      10.1 GENERAL PHILOSOPHY

      All operations throughout the plant will be controlled by a central Programmable
      Logic Controller (PLC, 02-XE-20), located in the main control room. Control is
      enacted through a series of individual screens, which cover all the operating sections
      of the plant. These are full color graphic screens that mimic the plant processes, and
      display all pertinent process, mechanical and electrical information. They have the
      following functions available through the keyboard:

            o Start and Stop functions for all drivers.

            o Access and monitor all process flow diagrams and instrumentation loops
              for the entire plant – except the Crushing Plant which can only be
              monitored.

            o Access and monitor flows, densities, levels, and other variables at selected
              points in the process.

            o Access and monitor a sequence-of-events recorder to study previous
              alarms and other historical data.

        Three additional computers and graphic display screens (02-XE-21/23) are
        provided, all of which are capable of operating the plant. Two are located in the
        main control room, and the third is positioned in the desorption area to
        specifically control the elution circuit. A fourth system (01-XE-12) is located in
        the crusher control room, but is not provided with a graphic screen panel. Text
        information only is used at this control station for the crushing system. However,
        operations can be monitored on graphic screens in the main control room. The
        four computer packages are known as “Man Machine Interface Panels”, or MMIs.
        These communicate along the same network into the main plant PLC, and with
        the exception of the crusher MMI, are each capable of running all plant operations
        from any of the screens.



                                                                                          5
A number of smaller PLC´s are also provided as part of the vendor equipment
packages. There are all connected into main PLC, and controlled from the screen.
The package equipment PLC´s provided are:

   o One for each mill, SLC 501

   o Flocculant mixing system

   o Carbon regeneration kiln

All the equipment, except for the regeneration kiln is currently programmed to be
operated by the main PLC. The regeneration kiln has been left to be controlled by
the operator at that location.

There will be a Local/Auto selector switch and Start/Stop push button unit
(Local Control Station, LCS) located adjacent to each drive throughout the plant.
These are mounted in a common box and each Stop button incorporates lock-off
capability. The three reclaim feeders each have (in addition to the stop and start
buttons and the selector switch) a speed “pot” for varying the feeder speed in the
Local mode.

10.2. AREA CONTROL

The plant is divided into seven areas for control purposes. Each area is
summarized below and the relevant screens are listed in the appropriate sections.

Area 01 – Crushing

Control of the equipment is from the Crusher MMI located in the crushing control
room, and for maintenance at the LCS. Crushing operations are also monitored in
the main control room.

Area 02 – Reclaim, Grinding and Classification

Control of the Reclaim, Grinding, Classification and Trash Screening equipment
is from the main Plant Control Room. The significant difference between the
motors of the SAG and Ball mills is that drives are not equipped with LCSs.
However, each are equipped with a Local Stop Button.

Area 03 – Leach and Carbon-in-Leach (CIL)

Control of the Leach equipment is from the main Plant Control Room, or from the
respective LCS.




                                                                                6
Area 04 – Thickening and Process Water

Control of the Thickening/Tailings and Process Water equipment is from the main
Plant Control Room, or from the respective LCS.

Area 05 – Carbon Acid Wash, Desorption and Regeneration, and Metal
Refining

The control of the Desorption and Regeneration equipment is from the “Operator
Station” located in the Desorption area, or from the respective LCS.

Area 06 – Reagents

Control of the Reagents equipment is from the main Plant Control Room, or the
respective LCS.

Area 07 – Plant Services

Control of the Plant Services equipment is from the main Plant Control Room, or
the respective LCS.

All control loops are discussed in the following sections. The major operating
philosophy is that all loop control functions are undertaken and that the status of
all drives and flows are monitored through the PLC for each area. When the plant
is operating normally, control of all the drivers will be in Auto to ensure that each
system status is capable of being monitored in the main control panel. Normally,
the only time a drive will be run in Local will be for maintenance purposes.


10.3 OPERATOR STATION CONTROLS

The keyboards provided are standard full size IBM PC type cursor keys, and a
number keypad. The cursor is moved quickly around the screen by the trackball,
however, the cursor keys can also be used for full plant control. The left hand
button on the trackball pad is the Enter key. The cursor is positioned on the piece
of equipment and the Enter key pressed. This will surround the object with a
green box, and it becomes “active”, or available for commands. An additional
press of the Enter key will bring up a pop up list of commands along with the
function key required for each command. Pressing the correct function key issues
the command desired. The operator can move the green box to the object desired
by either pressure cursor keys directly or pressing control key while pressing the
cursory keys. The latter method cursors through every controlled object on the
screen in a set pattern while the former method cursors the box directly to the
object nearest in direction according to the cursor key pressed.




                                                                                   7
The second method of operation requires learning all control view commands.
Pressing Alt key and the “C” key simultaneously brings up the command line,
then typing in a command and pressing enter issues the command directly. This
method will only be used by management personnel and operators specially
trained in its use.

The MMI in the crushing area has an integrated keyboard with function keys
covered by a sealed membrane. Operation of the MMI is by pressing the desired
function key according to commands listed on the screen. If the controlled device
is stopped, pressing its function key will start it by issuing a start command to the
PLC, if the selector switch on the device is in Auto mode. Pressing the same
function key will stop the device.

Several overview screens of the operation are used for various operating
functions. Examples of these screens which follow, are:

   o All screens can be accessed from the main menu screen – ACCESO.

   o The totalized plant throughput tonnages are listed on a shift basis,
     combined with the main water demands are given on a summary screen –
     TOTAL.

   o The status of each of the process control loops are listed, and each can be
     altered on a summary screen – PIDTUNE.

10.4 STANDBY POWER SYSTEM

A diesel driven 750 kW generator set serves as the standby power source for the
critical drives. The unit will be configured to allow certain equipment in the wet
plant area to be operated, and in the event of loss of normal utility power, the
standby generator will sense this loss and start after a specified time period –
typically 10 seconds. The emergency unit substations in the main plant electrical
room and in the desorption electrical building will then open their main 380V
secondary breakers 380V and close their standby 380V breakers in that order, and
power will then be fed to unit substations. Interlocks are provided to ensure that
both secondary breakers on each unit substation cannot be closed at the same
time.

       On return of normal utility power the unit substations will transfer to
       normal power after a pre-determined time to ensure that power return is
       long lasting. The generator will then go into a pre-determined cool-down
       cycle before shutting off.

       The drives that can be run from the standby power system are:

               Thickener 04-TH-01 rake drive and lift.



                                                                                   8
              Thickener Recirculation Pump (04-PP-17).

              Electrowinning Rectifiers (05-RE-01/02).

              Strip Solution Pumps (05-PP-36/37).

              Plant Air Compressor (07-PP-01/02 or 03).

              High Pressure Air Dryer (07-DR-02).

              Miscellaneous HVAC.

              Major tank agitators (Leach, and CIL) will be cycled on for ten
              minutes and then off for half hour through the control system logic
              while on standby power. Four agitators will be cycled at a time.

              Water Treatment Plant (07-WP-01).

              Selected heat tracing.

              Selected lighting and small power.

              Computers and PLC´s in the Main Plant and the Desorption areas.

10.5   CRUSHING CIRCUIT

PROCESS DESCRIPTION

The crushing circuit receives run-of-mine (ROM) ore and crushes it to a nominal
size of 150 mm and conveys the resulting material to an open stockpile.

The system consists of an ore receiving hopper, a hydraulic rock breaker for
oversize material, a gyratory crusher (01-CR-01), an apron feeder (01-FE-01), a
stockpile feed conveyor (01-CV-01), dust collection (01-DC-01) and suppression
systems, and related instrumentation.

ROM ore is dumped directly from the ore haul trucks into the ore receiving
hopper which is situated directly above the gyratory crusher. The hopper is
equipped with a hydraulic rock breaker (01-RB-01) to break any oversize material
and to clear blockages in the crusher feed cavity.

The ore passes through the crusher into a surge pocket, and withdrawn by a
constant speed apron feeder onto the stockpile feed conveyor. This conveys and
lifts the ore to an elevated discharge point to form an open stockpile with a live
capacity of 15,000 tonnes.


                                                                                9
Dust created by truck dumping is suppressed by water sprays in the ore receiving
hopper. Water sprays are also employed to suppress dust at the discharge end of
the skirting on the stockpile feed conveyor and at the discharge end of the feed
conveyor as the ore falls onto the stockpile. Dust is collected at the discharge of
the apron feeder and in two locations near the loading point on the stockpile feed
conveyor by a positive collection system. The collected dust is discharged onto
the stockpile feed conveyor near the end of the skirted section at the water spray
location.

10.6 GRINDING CIRCUIT

PROCESS DESCRIPTION

The grinding circuit receives primary crushed ore from the stockpile and reduces
it to a nominal 75 microns to prepare it for leaching. The circuit consists of
stockpile reclaim, ore conveying, SAG milling with recycle of oversize to the feed
conveyor, ball milling in closed circuit with classifying cyclones, removal of trash
and oversize particles, and automatic sampling of the leach feed slurry.

Ore is reclaimed from the coarse ore stockpile by three variable speed apron
feeders (02-FE-02/03/04) which discharge onto the SAG mill feed conveyor (02-
CV-02). The feed conveyor is equipped with a weightometer (02-WT-01) which
totals the quantity of mill feed as well as allows the proportioning of pebble lime
to the feed conveyor and grinding water to the SAG mill. Ground ore of
approximately 15 mm in size discharges the SAG mill onto a trommel screen
equipped with water sprays. The oversize material discharges from the screen
onto the oversize recycle conveyor (02-CV-03) which returns the material via a
transfer tower onto the SAG mill feed conveyor. The tower is designed to allow
for the future installation of a pebble crusher if it is determined to be necessary.

Trommel screen undersize is collected in the SAG mill discharge pump box (02-
PB-01) and is pumped (02-PP-01, 02) to the ball mill discharge pump box. The
design has provision for the addition of SAG mill cyclones in the future if it is
determined to be necessary.

The ball mill discharge products are collected in a common pump box (02-PB-02)
and pumped (02-PP-03, 04) to a cluster of classifying cyclones which produce the
final grinding circuit product in the overflow stream and recycle the coarse
product in the underflow stream to the two ball mills operating in parallel. The
cyclone underflow launder is designed so that a portion of the cyclone underflow
slurry can be directed to the underflow recycle pump box (02-PB-03) and be
pumped (02-PB-03) to the SAG mill feed chute. This option will be used if the
recirculating load on the ball mills is too high, and operation in this mode will put
the SAG mill in a “semi-closed circuit” configuration.




                                                                                  10
The cyclone overflow slurry is screened on three vibrating trash screens to
remove trash and tramp oversize from the slurry prior to leaching. The oversize
material is collected in a box for disposal. Screen undersize material is collected
in a launder and flows into the leach feed pump box via two stages of automatic
sampling before being pumped (02-PP-08, 09) to the leach/CIL circuit.

10.7 LEACH/CARBON-IN-LEACH (CIL) CIRCUIT

PROCESS DESCRIPTION

The objectives of the circuit are to dissolve the gold and silver contained in the
ground ore, and to adsorb these from the slurry onto activated carbon particles
added to the leached pulp. The ore slurry at the correct pH flows by gravity
through the trash screens (02-SC-01/03) and the primary and secondary samplers
(03-SA-04/01), and is pumped from the grinding circuit into the first leach tank.

The Kori Kollo plant has been designed with 12 agitated tanks to provide a
nominal 32 hours of retention time. Six leach tanks (03-TK-01/06) and six
carbon-in-leach tanks (03-TK-07/12) are installed, each 15.85 m in diameter and
16.46 m in height, arranged in a zig-zag with a drop of 0.2 m between each tank.
The pattern allows any individual tank to be bypassed. The leach tanks do not
contain carbon and the slurry overflows from one tank to the next. The carbon is
added and retained in the CIL tanks by three intertank screens per tank (03-SC-
10/21, 23/28). These 1.1 m diameter by 1.7 m high wedge wire screens are
continuously wiped to prevent buildup of carbon and to allow the pulp to pass
through to the next tank. Carbon is advanced continuously from tank to tank in
the opposite direction (from TK-12 to TK-07) to the movement of the ore pulp by
means of submersible pumps (03-PP-10/14) located at the pulp level in the tanks.
In this way the carbon advancing up the circuit becomes progressively more
loaded with previous metals. It is transferred from the first CIL tank (TK-07) as
required by the transfer pump (03-PP-15) to the loaded carbon recovery screens
(03-SC-08, 09) where it is washed from the pulp and stored in the surge hopper
(05-HP-01) prior to the desorption sequence (See Section 7.0).

Carbon addition to the last CIL tank occurs continuously as regenerated, stripped
or new carbon is pumped from the desorption section to the carbon sizing screen
(03-SC-07) located on the platform between tanks 11 and 12. This is a double
deck screen which removes carbon fines and all particles that would flow through
the intertank screen prior to entering the circuit.

The barren pulp leaving the last CIL tank flows by a launder to the carbon safety
screens (03-SC-04/06) and from there to the thickening and tailing disposal
systems (Section 6.0).




                                                                                11
10.8 TAILINGS, THICKENING AND PROCESS WATER SYSTEMS

PROCESS DESCRIPTION

The function of these systems is to dewater the CIL tailings to approximately 50%
solids by weight prior to deposition in the tailings dam, and provide process water
for uses at various positions around the plant. The tailings are automatically
sampled in two stages (04-SA-02/03) before flowing to the collection box (04-
CB-01). The raw water makeup for the plant process water requirements is added
at this point from the mine dewatering system, supplemented by the Desaguadero
river water pumping (13-PP-67, 71, 72) system. This procedure, along with the
thickening action, washes a portion of the cyanide from the tailings slurry. The
recovered cyanide values in the thickener overflow solution report to the process
water tank with the majority of the water, while the thickened slurry is pumped to
the tailings disposal area.

The dilute slurry flows horizontally into the thickener feed well which acts both
as the deaeration vessel, and as the addition point for dilution water. This is
required to maximize the settling rate of the slurry, and the solids concentration
actually entering the thickening zone is 15-17% by weight. Diluted flocculant is
also added at this point, delivered from the flocculant system (described in
Section 9). The thickened underflow is pumped to the tailings dam by a two stage
stage pumping system (04-PP-21/24) and the tailings pipeline and dam
distribution system.

The thickener overflow gravitate to the process water tank (04-TK-13), from
where it is pumped into the high (04-PP-26/27) and low (04-PP-29/30) pressure
distribution systems.

10.9 CARBON DESORPTION CIRCUIT

PROCESS DESCRIPTION

The Kori Kollo plant is provided with two pressure Zadra stripping systems to
remove gold and silver from carbon. Thirty three tonnes of carbon a day is
treated in two 21.5 tonne, parallel systems. The circuit is complicated by both
columns being capable of simultaneous operation, with the consequence that more
intermediate stages are required to complete the sequence for both.

10.9.1 Carbon Recovery from CIL

Slurry containing loaded carbon is continuously withdrawn from the first CIL
tank (03-TK-07) and pumped over the Loaded Carbon Recovery Screens (03-SC-
08, 09). The ground ore slurry flows through the screen and is returned to fifth
leach tank. Loaded carbon is thoroughly washed and gravitates to the Carbon
Surge Hopper (05-HP-01). Carbon recovery from CIL is continuous at the design


                                                                                12
concentration of 4 g/1. A level switch in the hopper will shut off the loaded
carbon withdrawal sequence once the correct batch size has been collected.

10.9.2 Acid Washing

The carbon surge hopper is located above the acid wash column (05-CM-01)
allowing the carbon to transfer by gravity from the hopper to the column. The
operator initiates the loading sequence from the control panel (05-WP-03) when
the batch is ready, opening the valve between the hopper and column, to
commence transfer. The residual water in the column flows through the top
screens to the tailings collection box.

The column contents are saturated with 3% w/v hydrochloric acid solution.
Treated water is pumped to the column, and, once flowing, pump (06-PP-55)
delivers concentrated hydrochloric acid into the header. The two flows combine
to yield the required acid solution strength, and the cycle runs until sufficient acid
solution has been introduced into the column to cover the carbon. The valves and
pump then close down in reserve order to that at the start of the sequence. This
step requires 0.6 Bed Volumes (BV) or 27 m3 of solution. (A bed volume is
defined as the volume occupied by the carbon bed in the column. This is 45.7 m3
for the 21.5 tonne batch. When in operation, 60% of this volume is occupied by
solution).

The purpose of the acid washing is to give the acid solution time to react with and
remove any acid-soluble deposits from the loaded carbon prior to stripping. If not
removed, these deposits may reduce the activity of the carbon and impair its
ability to absorb gold once it is returned to the CIL circuit. To ensure thorough
mixing and complete reaction of the acid with the carbon, the solution in the
column is circulated for a period of one hour. The circulation pump (05-PP-31,
32) commences flow through the manifold and column. The sequence is reversed
at the end of the cycle.

Residual acid solution must be thoroughly washed from the carbon to prevent the
formation of hydrogen cyanide (HCN) gas during the next stages of elution. The
acid solution is rinsed from the carbon by the addition of treated water, and
discharged to the tailings collection box (04-CB-01). During this step HCN will
be released for a short time prior to neutralization with the tailings slurry. This is
contained in the box and vented to atmosphere. At this time a light will flash in
this area to inform operators that the rinsing step is in progress.

10.9.3 Elution

The rinsed carbon at neutral pH is pressure transferred to the elution column by
softened water. During this step the residual water in the column will flow to the
tailings launder.




                                                                                   13
The transfer water is flushed from the column into the strip solution tank, and
reagents are added manually to replace the volume discarded during the solution
discard (Step #25) of the previous cycle. At the same time the temperature of the
solution, carbon, column and associated pipework is increased. The strip solution
is brought up to temperature (80°C) by being pumped (05-PP-36, 37) from the
tank (05-TK-20) through the Preheat Heat Exchanger (05-XH-01, 02), the Final
Heat Exchanger (05-XH-03, 04) and back to the tank. This step is used when the
system has not been operating continuously, and if required, both pumps can be
used to accelerate heat up. The Strip Solution Heater and circulation water pump
heats the strip solution in a closed loop through the final heat exchanger.

After reagent makeup strip solution continues to recirculate through the column
and back to the strip solution tank via the strip solution pump. Heat is recovered
from the hot solution exiting the column in the Preheat Heat Exchanger (05-HX-
01), and brought up to temperature by the Final Heat Exchanger prior to entering
the column manifold. Flow through the Preheat Heat Exchanger serves two
purposes:

   o It reduces the overall heating entry requirements.

   o It cools the displaced strip solution preventing it from flashing or boiling
     on discharge to the electrowinning circuit or solution bleed destination.

The temperature of the strip solution continues to rise during this step until the
design temperature of 135°C is reached. This takes approximately 2 hours, and
continues until the end of the step. A time period of 50 BV (25 hours) has been
allowed for elution to ensure complete recovery of gold and silver.

The elution cycle has been completed, and cold water (softened) is pumped via
the strip solution pump (05-PP-36) into the column to displace the final BV of
solution. This corresponds to the bleed volume required per cycle. After cooling
to below boiling point in the preheat exchanger, this solution is directed either to
the cyanide mixing tank, or the first CIL tank (03-TK-07). Cooling is necessary
to prevent flashing during discharge of carbon and also solution from the column.

Barren carbon is pressure transferred from the column to the kiln regeneration
screen (05-SC-22) with softened water via the strip solution pump.

10.9.4 Regeneration

Approximately one third of the designed daily carbon movement can be
regenerated. The dewatered barren carbon gravitates to the kiln feed hopper (05-
HO-02), which is a split design allowing the kiln to be bypassed as required. The
feed to the kiln is withdrawn by a vibrating wedge wire screen feeder. A fan and
ducting draws the offgas from the kiln over the screen, predrying the carbon. It
enters the kiln feed distribution hopper, and passes down the vertical heating


                                                                                 14
tubes. Direct heat is applied by gas burners on the outside of the tubes. A small
quantity of water is added to the carbon as it exits the tubes which generates
additional steam to ensure the temperature required for efficient regeneration is
achieved. Flowrate through the kiln is controlled by a vibrating feeder at the exit
of the tubes, and the hot regenerated carbon is immediately “quenched” by
dropping into the water contained in the carbon quench hopper (05-TK-21). The
carbon that is not regenerated is withdrawn from the kiln feed hopper by the
bypass feeder (05-FE-05), directly into the quench hopper.

10.10 REFINING CIRCUIT

PROCESS DESCRIPTION

The first stage of the circuit occurs during the elution sequence (described in
Section 7.0), where the gold and silver that had been removed from the carbon is
deposited onto stainless steel cathodes in two banks of electrowinning cells, each
located in parallel, (05-EC-01/04 and 05/08 for the two elution circuits). The
solution is fed continuously during the elution cycle to the bank of cells and after
passage through is returned to the strip solution tank (05-TK-20) from the barren
eluate tank (05-TK-44) by pump (05-PP-97/98).

Each cell has 18 cathodes and 19 anodes. The cathodes consist of pads of knitted
stainless steel wood onto which the gold and silver are deposited as in metallic
form, while the anodes are expanded stainless steel mesh plates. These are
located alternately across the cells, and the solution flows in the transverse
direction to the electrodes. A direct current voltage is applied between the
cathodes and anodes by a rectifier (05-RE-01/02) which causes the gold and silver
in the solution to be deposited on the cathodes.

The number of cathodes selected for each will maximize the removal of metal
from the solution each pass through the cells, and 90 – 95 % of the metals present
in solution are expected to be recovered on the cathodes each pass.

The deposited gold and silver is recovered from the cells after each cycle by hand.
A high pressure, low volume water spray (05-XM-34) removes the metal from
each individual cathode into the bottom of the cell, where the accumulated metal
is washed into the sludge collection hopper (05-PB-05). The dilute sludge is
dewatered in a plate and frame pressure filter (05-FL-10), and the dewatered
sludge is collected and dryed in an oven (05-DR-02). The dried metal is stored in
a vault (05-SD-01) prior to the final step in the gold/silver production process is
the production of doré bars. The dried sludge is charged into the rotary furnace
combined with appropriate quantities of fluxes. The extent and quantity of these
will be determined according to the pressure of undesirable metals (particularly
copper) present.




                                                                                 15
The unwanted metals will chemically combine with the fluxes when the metals
are in the molten stage and will flow off as part of the slag phase during the
bullion pouring.

10.11 REAGENTS

PROCESS DESCRIPTION

       10.11.1 Lime

       Lime is added to the SAG mill feed to achieve an alkaline pH (greater than
       10.5) in the grinding circuit, and from there in the leach/CIL circuit. This
       is a requirement to prevent the disassociation of cyanide into the gaseous
       form, which occurs at a pulp pH below 10.0. The process water added to
       the ore contains residual cyanide recovered from the tailings thickener and
       fresh cyanide solution is added to the leach circuit.

       The lime is provided by “pebble lime”, delivered in bulk by tanker trucks.
       This is pneumatically transferred to the storage silo (06-XM-07), which is
       equipped with a dust collector vent. This allows the transport air to be
       vented from the silo dust free after depositing the lime.

       Lime is extracted from the silo by a bin activator when discharge is
       required.

       10.11.2 Cyanide

       Cyanide is used to dissolve gold and silver contained in the ground ore in
       the leach/CIL circuits. It is delivered in 1000 kg bags in briquette form
       and dissolved in a mixing tank (06-TK-30, 06-AG-14) to a predetermined
       solution concentration. After mixing is complete, the solution batch is
       transferred to the Cyanide Day Tank (06-TK-31) by pumps (06-PP-43,
       44). Cyanide solution is added to the leach circuit from a ring main
       system. The solution is continuously pumped (06-PP-41, 42) in a loop
       over the leach tanks, returning back to the top of the day tank. The
       solution is added in the required volumes at the demand points by control
       valves. This loop also provides the cyanide required for the strip solution
       make up.

       10.11.3 Flocculant

       Flocculant is used to accelerate the settling rate of the solids feeding into
       the tailings thickener (04-TK-14). It is delivered in powder form in 500 kg
       bags and prepared in an automatic mixing system (06-XM-06). The
       powder is added to an enclosed feed hopper, metered out by a screen
       feeder and pneumatically conveyed into the mixing/aging tank via an



                                                                                 16
     educator.   Treated water is added to the eductor and to the tank as
     required.

     The eductor disperses the flocculant into the water, and the dilute solution
     (0.2 – 0.4 % flocculant by weight) is “aged” for approximately one hour to
     complete dissolution. This is achieved by slow agitation in the tank.
     After complete the batch is transferred by pump into the holding tank (06-
     TK-37), from where it is metered as required into the thickener feed well
     by variable speed pump (06-PP-61, 62)

     10.11.4 Sodium Hydroxide

     Sodium hydroxide solution (caustic) is primarily used in the
     desorption/electrowinning circuit. It provides the conductivity in the
     solution required to enable electro deposition of gold and silver on the
     cathodes. Its secondary use is to provide the protective pH (>12.5)
     required to safely mix cyanide. It is delivered in 200 kg drums in flake
     form, and is mixed in batches in the mix/storage tank (06-TK-32).

     10.11.5 Hydrochloric Acid

     Hydrochloric acid is used in the acid wash cycle of the desorption circuit
     to dissolve any precipitated salts that may have deposited on the carbon. It
     is delivered in drums as concentrated solution (32% by weight), and is
     transferred by drum pump (06-PP-58) into the storage tank (06-TK-34).


10.12 PLANT SERVICES

PROCESS DESCRIPTION

     10.12.1 Water Circuits

     Raw water is used in the plant at several positions and is sourced from two
     locations – the mine dewatering system (deep wells), and the Desaguadero
     River (13-PP-67, 68, 71, 72). Plant make up water is added to the
     thickener feed in the tailings collection box at a nominal demand of 580
     m3/h. River water is also used in various locations around the plant at a
     nominal demand of 90 m3/h and is treated according to the use. The
     treatment process commences with an existing pump station and pre
     settling channel and pond system which allows settlement of some of the
     suspended solids. This water is then pumped to the water storage tank
     (07-TK-43) providing surge capacity prior to water treatment circuit.

     It is pumped (07-PP-91) to the circuit (07-WP-01), which settles the
     suspended solids in a plate type (Lanella) thickener, after the addition of



                                                                              17
appropriate reagents. The water overflows into one of two sand filters in
parallel to complete the treatment, and is transferred to the storage tank
(07-TK-42) by pump (07-PP-77, 78).

The storage tank distributes the water to the required locations around the
site by a gravity ring main, and the lower portion of the tank is reserved as
the supply to the fire water system.

Points of usage around the process plant for treated water are:

           •   Potable water treatment

           •   Water softening system

           •   Crushing plant dust suppression sprays

           •   High pressure service water supply

           •   Reagent make up

           •   Gland water distribution

           •   Acid wash column

           •   Warehouses and truckshop

Potable water is stored in a tank (07-TK-40) located by the water
treatment package, from where it is distributed by gravity. The water
flowing to the tank is chlorinated by injection of hypochlorite according to
the water flow and the addition rates required.

Softened water, mainly used in the desorption circuit, is produced by ion
exchange (07-WP-02), and is stored in a surge tank (07-PP-65, 66), for
carbon transfer to and from the elution columns, and as strip solution
make up. Minor uses are as make up supply to the cooling water, and as
the spray water used in the cooling water heat exchangers (07-HX-05, 06).

The cooling water circuit is a pressurized system which provides and
distributes water in closed circuit to cool the lubricating oil of the crusher,
rock breaker, SAG and Ball mills, and high and medium pressure air
compressors. The water is circulated (07-PP-80, 81) through the cooling
heat exchangers (07-HX-05, 06) and around the distribution loop. The
return water flows into the pressurizing system, comprised of an air
separator and compressions tank. Make up into the circuit is by the glycol
make up tank (07-TK-45) and pump (07-PP-84).



                                                                            18
10.12.2 Air Circuits

Compressed air is provided in the plant by two systems. High pressure
plant air is produced by rotary screw type compressors (07-CP-01/03), and
is used for the instrument air and all plant services.

Air for leaching is produced by centrifugal type compressors (07-CP-
05/07). The primary distribution system to the tanks is by upcomers and
non-return valves located at specific points below the agitators in each of
the leach and CIL tanks.




                                                                        19

								
To top