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CONDISI CERMERLANG - Mining & Leaching

At this stage, the ore reserves are shallow, about 5 m, and the most suitable mining method is open mining  using backhoe-type excavators and dumptucks. There is no need to use
explosives to assist in the excavation operations. The ores, both primary and secondary, are easily recognisable  visually from the barren material at the excavation site. There is no
overburden, a small but varying amount of barren material needs excavated to enable complete access to the ores. The mine layout is shown on Plan No PMC/KCSB/SJ/001 and the
process in the Process Flow Chart 1209 on page 9.

Development Phase

Development works are constantly been carried out from time to time, the works includes -

•Restoration of the roads leading to the site
•Preparation of new excavation and leaching sites
•Construction of the leaching vats
•Construction of the leach pad
•Construction of the accessory ponds
•Construction of the tailings retention area
•Construction of the gold recovery plant, kongsi, store and laboratory
•Testing and maintenance of equipment and machineries


Schedule of Operations

The operation of the mine is operational with on-going new development. The operation is to mine as much of the ore reserves as quickly as practicable. The lower grade secondary
ore are subjected to heap leaching while the higher grade primary ore to vat leaching.  Both  types  of  leaching operations are to run concurrently though loading, detoxification and
unloading of the leach vats and pad will take place sequentially.  The optimal  leach  time of  25±  days result that each leach cycle lasts about 1 month after allowing for ore loading,
spent ore detoxification and unloading,  thereby  giving a monthly out  put of 5,250 m3 of ore and maximum 5,000 m3 of barren material. Additional prospecting operations are to be
carried out while the mine is in operation.
Major Equipments
Equipment
Units
Type   
Model    
Power
Function
Excavator
2
1.0 m3
Komatsu
Diesel 210
Digging/Lifting
Breaker
1
1.0 m3
Hitachi 200
Diesel 210
Shattering
Dump-Truck
3
3T
China
Diesel 60
Haulage
Shover
1
0.6 m3
Liugong
Diesel 180
Earth Shover
Carbon Column
15
2mx60cm
China
-
Gold Absorption
Generator Set
1
150 KVA
CSR/Lianfei
Diesel 170
Electricity Supply
Excavation Work

All the material that are excavated are rip-able  and therefore it  does not require drilling and blasting.
Materials  recognisable as ore, 5,250 m3 monthly,  is dug out using the excavators  which load it on to
the dumptrucks for haulage to the leach site. Material recognisable as barren, a varying small amount
averaging less than 5,000 m3 per month, is dug out, wherever necessary to gain access to ore, using
the  same excavators and loaded on to the dumptrucks and transported to where it is required for the
construction, maintenance and reinforcement of tracks, bunds and other structures at the mine.

Mine  face  profiles are  to be formed to the specifications below or better, these specifications having
been derived based on soil and rock mechanics principles. The profile summary -

Steepest inclined surfaces : 1 vertical : 1 horizontal
Horizontal bench width, m : 3
Height between benches, m : 5

Haulage

Ores are hauled for  a  maximum  distance of 1.5 km to the leach site. Very little dust is created by the
small dumptrucks and is confined to near the haulage tracks only. The small scale of operations does
not give rise to any traffic problems requiring a dedicated traffic management system.

Inclined haulage tracks are to have slopes which are  less steep than 1 to 7, this slope being obtained
by  increasing  the  length  of incline, if necessary. Besides catering to safety, this measure will ensure
that no undue stress will be imposed on dump-trucks and other machineries, for example the engines,
transmissions and brakes, this will save on  repairs and maintenance costs and prolonging equipment
life.
Plan  No  PMC/KCSB/SJ/002 shows the layout of the area where leaching is
to  be  carried  out.  The  higher grade primary ore is to be leached in the 2
vats  and  the  lower  grade ore, mainly secondary ore and some old alluvial
tailings,  on  the  leach  pad. In both instances, the optimal leach time is 25±
days. The relevant dimensions of these leaching stations
Station
Units
Shape
Area
Depth/Height
Volume
Tonnage
Excavator
2
Rectangular
500 m2
0.75 m nett
375 m3
800 Tons
Breaker
1
Irregular
6,000 m2
0.75 m
4,500 m3
9,600 Tons
Ore Treatment
Vat Leaching

The  vats  are  constructed  from  reinforced  concrete  with leak-proof grouting.  The  ore is deposited by dumptrucks on to each vat and then soaked with a solution of pH 10.5 and
containing 0.025% NaCN pumped from the barren solution pond. Keeping the solution alkaline prevents the formation of HCN which is a toxic gas.

The leach solution is continuously pumped out of the vatto maintain the solution levels in the vats to the sedimentation pond to allow suspended material to settle out. The now clear
solution is pumped into the pregnant solution pond. From time to time, the settled sediment is drained and deposited into the spent ore in the vats or pad for treatment.

Heap Leaching

The pad on which the  heap leaching are  to be carried out is basically  an area   of compacted ground surrounded by a low bund built according to the specifications on page 5. It is
constructed as shown in the unscaled section shown on page 10 in the manner and sequence -


The  area  over  which  the pad lies is sloped  to between 3 and 5° off the horizontal with its lowest point near its northeast corner and compacted by earthmoving equipment running
over it. This  comparatively  low  degree  of  compaction  is adequate since the thickness of ore to be laid on it does is only 0.75 m which imposes a ground pressure of only 0.12 bar
which is only about 6% of that imposed by a motorcar tyre.
A  system of  perforated  100 mm ø perforated PVC  pipes  covering  the  entire  footprint of the pad is laid on this compacted
ground and so connected as to drain to the northeast corner. This pipe system constitutes the leakage detection system. Any
water draining out of it  is  frequently  sampled for CN- ion content, any presence of which indicates failure of the impermeable
layers above it whereupon the entire contents of pad are  immediately detoxified with the use of Ca(OCl)2 solution and placed
temporarily on the tailings retention area to allow the impermeable layers to be inspected and repaired.

A 300 mm thick  layer  of  alluvial  tailings sand  from  which +10 mm  gravel  has been removed is laid on top of items 1 and 2
above and  graded  to  conform  with the slope in item 1. This sand permits compliance with any deformations without causing
failure in the geo-membrane to be laid above it.

A 1 mm thick tough plastic geo-membrane  is laid on top of the sand layer. Joints in the membrane are heat-welded according
to the manufacturer's specifications to ensure strength and durability.

A 500 mm thick layer  of  soft  impermeable clay is laid on top of the geomembrane. This layer reinforces the impermeability of
the geo-membrane  and  absorbs  any  deformations  to  protect  the  geo-membrane against damage when spent ore is later
removed from the pad. This membrane covers an area slightly larger than the  pad and excess sections at its margins are laid
over the bund mentioned earlier.

A pregnant solution receiving system of perforated 100 mm ø perforated PVC pipes is laid atop the soft clay layer. As with the
leakage detection system, these pipes are  connected to  drain  to  the northeast corner. A valve is incorporated into the PVC
pipe receiving pregnant solution from this system which can be turned off if necessary. This pipe leads to the sedimentation
tank.

A layer of ore up to 1 m thick is placed on top of the membrane and pregnant solution pipes.

A system of 25 to 50 mm ø irrigation PVC pipes equipped with sprinkler nozzles is laid on top of the ore for distributing a dilute
sodium cyanide solution over the ore on the leach pad. These pipes are connected to a trunk pipe leading from a pump
installed at the barren solution pond.

A remove-able plastic sheet canopy is then placed over the ore to keep rain water out so as to prevent dilution of the solution
and eliminate the need to deal with excess water.

After  the  pad  has  been  constructed  and  all  the necessary tests have been carried out to prove the absence of leakages,
NaCN  solution  at the  dilution and alkalinity mentioned on page 4 is pumped from the barren solution pond through the pipes
to  the sprinklers  which  spray  the  solution over the ore. As the solution percolates through the ore, it dissolves out the gold
contained in the ore.  When  it descends to the top surface of the  impervious soft clay layer above  the geo-membrane at the
bottom,  the  now  pregnan t solution has nowhere to go except enter the pregnant solution receiving pipe system to drain out
through them to  the  sedimentation  pond  where any sediment settles out and from where the clear solution is transferred to
the pregnant solution pond.

Gold Adsorption & Recovery

The pregnant solution is pumped from its pond through a series  of 15 activated  carbon columns  to allow the gold contained
therein to deposit on the surfaces of the carbon. The now barren solution flows out of the last column into the barren solution
pond where its pH value and NaCN content are restored with the addition, wherever necessary, of water, Ca(OH)2 and NaCN.
Thereafter, it is pumped separately to the leach vats and leach pad, thereby completing the closed circuit for the NaCN
solution in the leaching circuit.

Whenever  laboratory  tests  on  samples  drawn  periodically show that it is appropriate to do so, the gold-laden carbon in the
adsorption  columns  is  washed  with  acid  and  is  stripped  of the gold deposited on it with an elution process. The carbon is
leached with a solution containing NaOH 1% andNaCN 0.1% at a  temperature  of 100°C in a circuit different from the leaching
circuit mentioned earlier which is  referred  to  as  the  leaching  circuit to distinguish it from the elution circuit described in this
paragraph. The pregnant solution or eluate from this new circuit is then subjected to electrolysis or electrowinning  to produce
a gold sludge which is then dried and smelted to produce a  core  which  also  contains some silver leached from the ore. The
elution circuit also takes the form of a closed circuit. Alternatively, the carbon can be combusted to recover the gold which can
then be smelted.

The leach solution in the elution circuit is constantly monitored and its strength is maintained by addition of, wherever
necessary, water, NaOH and NaCN.

The stripped carbon is removed from the columns and re-activated in a regenerator before being re-loaded into the columns.
Any loss of water in both cyanide circuits through evaporation, etc is made up for with water from rain water collected from the
roofs over the pregnant ans barren solution ponds.

Safety Features

The bund enclosing the leach pad is built to the specifications -

Top width : 3 m minimum
Height : Up to 3 m
Side slopes : 1 vertical to 1.5 horizontal or flatter
Material : Selected barren material
Building method : The building material is laid down layer by layer from dumptrucks and
compacted by dumptrucks running on it

The bund is also covered by the leach pad plastic membrane to protect it against erosion and to prevent contamination of the
leaching circuit.

The entire treatment area, i.e. the area in which the leach  pad, the  settling pond, the adsorption unit and their ancillaries are
located, is enclosed by a ditch which drains into the water tank. The ditches are lined with concrete or plastic membrane again
for protection against erosion and contamination. The ditch dimensions are -

Bed width : 1 m
Depth : 1.2 m
Side slopes : 1 vertical to 1.5 horizontal or flatter

All the tanks and ponds are completely waterproof and leak-proof.

All pipes used for the  conveyance  of NaCN solution can be of PVC since this material does not suffer from corrosion and the
pumping pressures are low. All pipe joints are leak-proof.

The minimum free-board  for  all ponds,  ditches and impounding bunds is 1 m under normal conditions. Since the ditches are
provided with a depth of l.2 m, this free-board  is easily achieved as the circulation rate  of leaching solution is less than 1,000
litres per minute.  A  layer of  solution  100 mm  thick  flowing  at this  rate  on a bed l.2 m wide will only achieve a very low flow
velocity of less than 0.14 ms-1 at this high circulation rate.  The  leach  pad,  the  ponds and the ditches are all sheltered from
rain water by plastic sheeting.  The  pumps  in  the  solution  circuits  are  all backed by stand-by units of equal capacity in the
event of failure. As mentioned on pages 4 and 5,  drainage  of  pregnant  solution  out  of  the leach pad can be stopped by a
control valve if and when required.  All  the  above provisions will ensure that no spillage  will  take  place  even under extreme
weather conditions coinciding with complete top canopy cover failure.

Tailings Retention Area

The tailings area can be created progressively at the location  shown  on  Plans  No TAH/KCSB/SJ/001 and 002. Improvement
is effected by building bunds on the lower  sides  and using the toes of the hill slopes on the higher sides. The bunds used for
the tailings areas are built to the same specifications  for the leach pads.
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