training manual for cil & elution-final (reviewed)
TRANSCRIPT
Training manual for CIL & Elution
Technical Details:
CIL:
CIL Feed slurry density ~40% Solids
Number of CIL tanks 6
CIL Tank volume 2400 m3
Carbon concentration 10 g/l
Leach CIL residence time ~24hours (~4hours in each
tanks)
Tailing screening:
Tail screen mesh 0.8mm × 0.8mm
Tail screen flux 60m3/m2/hr
Acid Washing:
Acid wash flow rate 2 Bed volumes per hour
Acid wash strength 3 % Hydrochloric acid
Acid wash pH 2-3
Acid wash time 4 hours
Rinsing time ~2 hours (depends on pH)
Loaded vessel volume 30m3
Acid make up tank volume 28m3
Elution:
Elution column volume 24m3
Elution Batch size 24m3 of carbon
Elution circuit type ZADRA
Elution flow rate 2 Bed volumes per hour
Eluate solution 2% NaOH and ~ 1% NaCN
Eluate pH 12-13
Elution cycle time 12-18 hours
Elution temperature ~130o C
Eluted carbon values < 100g Au/t of carbon
Regeneration:
Kiln regeneration temperature ~700-750o C
Carbon regeneration rate 500kgs/hour
Kiln operating schedule 18 hours/ batch
Regeneration time at max temp ~ 6mins
Kiln rotating speed 0.5 rpm with VFD (Full
speed-5rpm)
Reagents:
Lime % 65% as CaO
Lime addition 1-3kg/t of mill feed
Cyanide strength 30-33% NaCN
Cyanide addition 200-300g/t of mill feed as 100%
Cyanide
Carbon (delivery) 600kg (bag) with 0.53 t/m3 density
Carbon addition 50g/t of mill feed
HCl strength 30-33% HCl
HCl addition 350kgs of 100% HCl per acid
wash
Caustic strength 47% NaOH
Caustic Addition 250-450kg/ elution
CIL (Carbon in Leach)
The gold from the solids of the slurry feed can been leached by means
of two methods CIP (Carbon in Pulp) & CIL(Carbon in Leach)
methods.
Difference between CIP & CIL:
In carbon in pulp methods, we have separate tanks for leaching
and adsorption of gold on to Carbon. Hence Leaching takes place
separately in a set of Leach tanks and Adsorption of gold on to carbon
takes place in a set of Adsorption tanks. Hence there is no need for
inter-stage screens in CIP method. If we look at the condition required
for Leaching the gold from solids and adsorption of gold on to carbon,
both need close control on parameter so that both can takes place
simultaneously. Hence in CIP method, the importance of maintaining
the parameters is not as critical as in CIL. Whereas in CIL(Carbon in
Leach), as both leaching and adsorption taking place simultaneously
in same tanks, its very essential to maintain the process parameters
so closely to achieve maximum efficiency of Leaching and Adsorption.
CIL Circuit components:
Trash screens:
The overflow slurry stream from the mill cyclone feeds the CIL
circuit via the Thickeners. Before entering the leaching circuit, all the
wood fiber, cloth, plastic, rocks from cyclone blowouts and other trash
material must be removed from the slurry. If trash is not removed it
may block the CIL interstage screen causing tank to overflow and also
cause problems in the elution and carbon reactivation circuit. The
cyclone overflow is fed to two trash screens of mesh size 0.6mm ×
0.5mm, the undersize particles reports to the Thickener and the over
sized trash material is collected and discarded.
Thickener:
Slurry form the trash screens flows into the thickener feed
distribution box and depending on which thickener is online the slurry
is distributed to the center of the thickener. The slurry in the
thickener is flocculated to settle the solid particles to the bottom and
densify the slurry. The slurry is thickened to 50-60% solids, and the
thickener underflow reports to the CIL stock tank through leach feed
splitter box where the auto sampler has been installed.
CIL Tanks:
The major component of CIL circuit is the CIL tanks. There are
6 tanks, out of which the first tank called the Stock tank is especially
for cyanadization reaction and 5 tanks for leaching and
adsorption(CIL). Each tanks has a capacity of 2400 m3 each, and
operating at a slurry density of 50% solids, giving a resident time of
4hrs in each tank and total of 24hrs in CIL (for all the 6 tanks
together).
The tanks are positioned in two staggered rows. The tanks are
interconnected with open launders and underflow pipelines with plug
valves. The underflow pipelines are designed in a way such that any
tank in the system may be bypassed, while the circuit continues to
operate with reduced volume and resident time.
The slurry from the thickener underflow is pumped to a splitter
box from where the feed is coming to stock tank. Cyanide solution is
added to the stock tank and provisions are provided to dose cyanide
on stock tank, CIL-1 and 2 as per requirement. The tanks are agitated
by twin impellers with a speed of 17rpm and the oxygen is supplied
from the compressed plant air through lances down the hollow
agitator shafts. The air is injected as a jet of bubbles which are
sheared by the slurry flow, giving good oxygen dissolution within the
slurry.
Slurry flows by gravity and difference in the RD through the
underflow pipes, from the overflow launder from each tank preceded
by an interstage screen that prevents the advance of carbon with the
slurry. The barren slurry from the final tank of the CIL circuit flow to
the tailing screen where the fine carbon is screened and pumped back
to CIL by fine carbon system. The barren slurry from the tail screen
underflow report to the slimes dam through residue tank.
Regenerated and virgin carbon is added to the final tank of the
circuit and the carbon is moved counter currently to the flow of slurry
by vertical spindle carbon transfer pump. The flow of slurry is in the
sequence from Stock to Tank-1, then tank-1 to 2, 2 to 3, 3 to 4 and 4
to 5, whereas the flow of carbon is in couter current to the slurry and
the flow of carbon is in the sequence from Tank-5 to tank-4, tank-4 to
tank-3, then tank-3 to 2 and tank-2 to 1. From tank-1 the carbon
loaded with the gold is pumped to the loaded vessel through the
loaded carbon screen where the slurry gets separated from the
carbon by spraying of water. The slurry which underflows through the
loaded carbon screen returns to CIL tank 1.
Interstage Screen:
Inter-stage screens are placed in each of the CIL tanks except
stock tank to retain the carbon in the tank, as the circuit operates
with carbon being moved counter-current to slurry flow.
The screens are cylindrical and are placed just prior to the
slurry exit launder. Wiper blades with a dedicated drive motor system
are installed to keep the screen surface free from carbon build-up. If
the wiper blades fail, then carbon is carried or forced onto the screen
surface by the slurry flow. This impedes the flow of slurry and may
cause the tank to overflow.
The screens may also become holed due to damage or
deterioration. To check whether the carbon are passing screen, the
slurry sample is collected from the overflow launder and filtered over
the mesh to check for any carbon present in it. The screen will also
become pegged with near sized carbon and other material such as
small rocks and need to be removed, cleaned or replaced regularly to
prevent tank overflows.
Carbon transfer pump:
To facilitate the counter current movement of carbon, each CIL
tank has a carbon transferring pump. The pumps are run on a batch
schedule as required to maintain the desired carbon concentration in
the tanks. The carbon from CIL-1 is pumped to the loaded carbon
screen once after the carbon in the vessel is dropped to elution
column and starting the next batch of loaded vessel. The slurry
underflow from the loaded screen is returned to CIL-1
Tailings screen:
The barren slurry from the CIL comes to tailings screen with the
screen mesh size of 0.8mm × 0.8mm, where the fine carbon is
screened and sent to fine carbon system. The barren slurry is sent to
slime dam through residue tank. The carbon may be present in the
tailings slurry due to the following reasons:
1. Carbon has abraded over the time and is fine enough to pass the
Interstage screen
2. The interstage screen in holed
3. The seal between the launder and the screen has deteriorated
or is not seated properly, allowing carbon to pass
Fine carbon system:
The fine carbon system has a fine carbon collection tank which is fixed
with a pump to pump the fine carbon to the fine carbon screen on top
of CIL, where the fine carbon is segregated and collected separately
in a jumbo bag to elute it separately.
CIL Concepts:
In CIL tanks, the two main basic steps are taking place:
1. Leaching of gold from the solids of slurry by
Cyanidation
2. Adsorption of gold from the solution on to Activated
carbon
The above steps takes place through sequential steps and let’s see
them in detail:
1. Leaching of Gold from Solids:
Initially the ore and Slimes dam sand mixture is grinded in mills
and slurry with 80% of -75µm size particles are pumped to thickener
from mill sump through cyclones followed by trash screens to remove
wood chips, rubber and any undesired particles.
The slurry with less RD (Relative Density) of around 1100-1200 is
pumped to thickeners from cyclones, and the less RD slurry is
densified in the Thickener to desired RD (1500-1700). The slurry is
pumped to CIL Stock tank with or without flushing water and the
slurry is received for leaching in the stock tank where RD is
maintained in the range of 1450-1600.
The slurry until it reaches the stock tank, there is no changes
taking place chemically from mills to thickener. The chemical process
starts to takes place from stock tank onwards and continues until the
gold is smelted. Hence maintaining the parameters in CIL & Elution is
very essential for plant efficiency.
The Leaching process involves dissolving the solid gold particles
into solution using a process known as cyanidation. Initially the gold is
present in the solids phase and by leaching the slurry, the gold is
dissolved by oxygen and cyanide and brought to solution phase. The
gold in the solution is adsorbed on to carbon and remaining barren
slurry is reported to tailings. The leaching takes place as per Elsner’s
reaction:
4Au- + 8 CN- + O2 + 2H2O -------- 4 Au(CN)-2 + 4 OH-
Gold+Free cyanide ion +Oxygen+Water Gold Cyanide Complex
ion+Hydroxyl ion
As per the reaction, for dissolving the gold in the solution we
need cyanide and oxygen.
Above mention is the overall reaction of gold dissolution:
1. The Gold in the solids of the slurry will react with cyanide and
form a complex ion, as gold is a noble metal it cannot be easily
dissolved in the solution without the addition of cyanide. Hence
cyanide is the prime factor for leaching without which gold
cannot be leached.
2. This stable Gold cyanide complex ion dissolves in the solution
and now the gold in the solids has been dissolved by oxygen and
cyanide to solution.
Hence the gold in the solids has been leached(dissolved) and brought
in to solution in the leaching step.
2. Adsorption of Gold on to Activated Carbon:
Adsorption is a term used to describe the attraction of a mineral
compound to the surface of another material. Activated carbon is used
to absorb the gold out of solution. The cyanide ion forms very strong
complexes with gold, it is the gold cyanide complex that is loaded onto
the carbon. The cation (Ca2+ from the lime CaO added before milling)
forms a bond with the negatively charged gold cyanide ion which is
then absorbed onto the carbon particle as per ion-pair adsorption
theory as shown below.
Once after the carbon is loaded with enough gold, the
carbon is pumped to loaded vessel through carbon screen to wash the
slurry and load only the clean carbon. The carbon is washed with acid
and rinsed before dropping to elution column to elute the gold from
the carbon.
Factors that affects the Efficiency and rate of leaching the gold
through Cyanidation process:
1. Size of particles-grind
2. Dissolved oxygen content
3. Free cyanide concentration
4. Slurry pH
5. Slurry density
6. Resident time
7. Agitation
8. Temperature
1. Size of particles-grind:
Leaching is a surface reaction and the dissolution takes place on
the gold that is exposed to surface of the solid, hence more the
particle size is finer, more of gold is exposed to the surface. Otherwise
gold will be encapsulated inside solids and cannot be exposed to
cyanidation. Generally, 80% of -75µm size particles will be ideal for
leaching maximum gold out of the slurry and throwing minimum gold
in solids to the residue.
2. Dissolved Oxygen content:
Oxygen is very essential for leaching, as it increase the rate of
dissolution of gold by cyanide. The cyanide in the solution reacts with
the gold to form a stable Gold cyanide complex ion. Oxygen to CIL
tanks has to be spurge through the agitator gear box. As we are not
using pure oxygen, we use compressed air, which contains 21%
oxygen. Hence if we close the plant air supply to the tanks, then
leaching rate will be reduced and the leaching efficiency will be
reduced considerably leading to more gold in residue solids reporting
to the tailings. So opening the air supply to the tanks are essential,
not for the sake of agitation or bringing up the solids for increasing
the RD, but for increasing the leaching rate to takes place.
3. Free cyanide concentration
Increasing cyanide concentration drives the cyanidation
reaction and hence there must be sufficient free cyanide ions in
solution to dissolve all the gold, otherwise gold will be lost to tailings.
The cyanide consumption will be less for non-refractory ores which
are oxidized and contains quartz and silicates. Whereas refractory
ores which are rich in sulphides (Pyrrhotite, Chalcopyrite etc) are
called cyanide consumers, cyanocides and cyanicides will need more
cyanide for leaching. Hence running the CIL Stock tank at cyanide
ppm of 200-300 is essential for effective and efficient leaching.
4. Slurry pH:
pH modification is achieved by adding lime to the mill feed,
which makes the slurry alkaline. The pH level in the tanks has to be
monitored regularly to avoid formation of HCN gas and to avoid
excess cyanide consumption.
When sodium cyanide is added to water, the cyanide portion of
the molecule dissociates from sodium part as shown below:
Depending on the pH of the slurry, the cyanide can react with the
hydrogen in the water to form deadly hydrogen cyanide gas, as shown
below.
Hence maintaining the pH in the stock tank in the range of 10.3 to
10.5 can prevent the formation for HCN gas and excess cyanide
consumption.
Impact of low pH:
1. If pH is lower than 10, HCN gas formation will be favored, and
the cyanide will be lost as gas causing a danger environment to work
and also increased the cyanide consumption.
Impact of high pH:
1. Also if the pH is more than 10.5, the calcium in the lime
precipitates and it blinds the carbon by filling the pores in the carbon
and only fewer sites available for gold adsorption on to the carbon.
Increased lime consumption.
5. Slurry Density:
The slurry density is a important parameter, which cannot be
maintained too high and too low also. Hence the R.D should be
optimized around 1500. The exact value to be obtained by real time
experience based on our plant operation, as the exact RD requirement
differs based on the Ore & raw materials properties and its nature
Impact of running at higher RD:
a. Decreased mixing efficiency as a result of increased
viscosity and decreased energy input per unit of mass of
slurry
b. Physical binding of the carbon surfaces and pores by the
fine ore particle
c. Reduced solution-Carbon ratio at higher slurry densities
which reduced the gold adsorption rate onto carbon
Impact of running at lower RD:
d. The residence time is based on the volumetric flow of the
slurry, as the percentage solids decreases, the total
volume increases and residence time decreases, leading to
incomplete leaching of gold by cyanide.
e. The reagent consumption will be maximum with
decreased slurry density, as smaller volume of solution
per unit mass of material cannot be obtained
f. If the slurry density is too low then the carbon particles
may not stay in suspension, and sink to the bottom of the
tanks.
Hence running at either lower RD or higher RD will not favours the
CIL & Adsorption efficiency; hence optimum RD is required in CIL.
6. Resident time:
Resident time in the CIL circuit is the time taken for the slurry
to flow through the tanks, and is an important operational
consideration. The longer the gold particles are in contact with the
cyanide in the slurry the more gold that will be leached. Resident time
is determined by the volume of the tanks, the slurry flow rate and the
slurry density.
If the Slurry RD is more, the flow rate pumping to CIL will be
less for the same amount of tonnes from the mills when compared
with pumping at low slurry RD. Reason is, Volume will decrease with
increase in RD. If the flow rate is more the resident time will be less.
Hence at very low RD, the resident time will be less and whereas the
resident time will be more if the RD is high, but we need to consider
the impact of running at high RD stated in Slurry density section.
Hence Slurry RD is to be maintained at optimum values.
7. Agitation:
Effective agitation allows the reactants to intimately mix and
prevents the solids from settling out and bogging the tanks. Agitation
also ensure that the gold cyanide complex ions forming on the surface
of a gold particle are removed into the wider solution to allow access
on the gold particles surface for more unreacted cyanide ions to leach
more gold from the particle. The agitator runs at a speed of 17 rpm
(revolution per minute).
8. Temperature:
Higher temperatures will increase the rate of gold dissolution; it
is not economical to heat the slurry. High temperatures also reduce
the capacity of carbon to absorb gold and lower the solubility of
oxygen in the slurry. Hence Leaching and adsorption is conducted at
ambient temperatures.
Factors affecting the efficiency and rate of Adsorption
of Gold on to Carbon:
1. Time
2. Foulants
3. Gold concentration
4. Carbon Activation
5. Slurry density
6. Agitation
7. Temperature
8. pH
1. Time:
The longer the carbon is in contact with the slurry the more gold it
absorbs. However, although at first the gold cyanide adsorption
takes place very quickly, it will slow down as more gold is loaded
onto the carbon. The reason is the gold concentration gradient will
reach a equilibrium condition that the amount of gold in the
solution is equal to the amount of gold in the carbon. Adsorption
will be quick if the difference is more, hence the adsorption is
quick in the beginning.
2. Foulants:
Activated carbon is subject to ‘fouling’ with inorganic and
organic matter. Fouling means, materials other than the valuable
metal is adsorbed or absorbed onto the carbon, decreasing the
number of ‘active sites’ available for adsorption of the valuable metal.
This reduces the carbon’s activity (the ability to absorb gold).
It is not possible to prevent fouling altogether. Salts, other
metals and organic matter are invariably present in the ore and water
supplies. It is possible however, to minimize the degree of fouling by
ensuring no Foulants are added to the pricess unnecessarily (eg. Over
shooting the pH, which means more calcium ions, oils, grease etc).
Foulants are removed from the carbon during acid washing and
carbon reactivation. Inorganic Foulants such as calcium, silica,
magnesium, other salts, other metals and reagents are removed by
acid washing. While organic Foulants such as oils, grease and fats,
are removed by high temperature thermal reaction in the kiln.
3. Gold Concentration:
The rate of gold adsorption and the loading capacity of the
carbon increases with increasing gold concentration in solution.
Hence the rate of adsorption is fast in the beginning and the rate
slows down after the gold concentration in the carbon increases.
4. Carbon Activation:
The carbon in the CIL circuit should be ‘Activated Carbon’. The
ability of carbon to adsorb gold is called its Activity. Only the
activated carbon can load more gold on to it. The Foulants in the
carbon will reduce the gold adsorption capacity and rate onto carbon.
The carbon activation is done at high temperature of above 700o C.
High temperature maintained in the kiln will burn off some of the
organic matter and provide the pores in the carbon which have been
blocked by organic and inorganic Foulants.
5. Slurry Density:
The rate of gold cyanide adsorption decreases with increasing
slurry density as there is reduced solution-Carbon ratio at higher
slurry densities.
However, if the slurry density gets too low then the carbon particles
may not stay in suspension, and sink to the bottom of the tanks.
6. Agitation:
The agitation is essential for loading the gold on to the carbon
as it makes the gold cyanide complex formed to have mobility and
better access to the available carbon in the tank. And if the agitation
is too much in the tank, then the carbon attrition will increase and the
carbon breaks into smaller particle. As the carbon particles size
becomes very small, and then it has the chance to pass through the
inter-stage screen and report in the tailings. Hence to avoid the loss
of carbon, the agitation is set at the optimum with the agitators
running at a speed of 17 rpm (revolution per minute).
7. Temperatures:
The adsorption rate increases slightly with increasing
temperature, however the leaching efficieny is reduced. Hence leach
and adsorption is conducted at ambient temperature.
8. pH:
The gold adsorption on to the carbon is more effective at low
pH, but in practical in CIL circuit, maintaining the CIL tanks at low
pH will leads to formation of HCN gas. That is the reason we should
not over shoot the pH above 10.5, which reduce the gold loading rate
and efficiency. Hence the pH should be maintained in the range of
10.3 to 10.5.
Elution:During the CIL process, gold is leached from the ore using an
alkaline cyanide solution. The resulting gold cyanide complex ions are
then concentrated and separated from the slurry by adsorbing onto
activated carbon. The loaded carbon is removed from the CIL circuit
and taken to the loaded vessel where the loaded carbon is acid
washed to removes inorganic Foulants from the carbon before the
elution to achieve high elution rate and efficiency. Elution is the next
step in the process, whereby the adsorption of the gold cyanide
complex onto carbon is reversed and the gold is desorbed from the
carbon into a pregnant eluate solution. The gold from the high gold
concentration eluate solution is removed by the process called
Electrowinning onto steel wool cathodes.
Elution involves removing the gold from the activated carbon by
reversing the adsorption process that occurs in the CIL circuit. Using
high temperature and pressure and treating the carbon with a
portable water solution with caustic and cyanide concentration, the
gold cyanide complex can be induced to desorbs from the carbon and
return to solution. The desorption process is also referred to as
‘Elution’ or ‘Stripping’.
In the CIL circuit, adsorption of gold onto activated carbon is
most effective at low temperature, low cyanide concentration, low pH
and high gold concentration in solution. By simply reversing these
conditions, elution of gold from carbon occurs.
The main factor that makes desorption or stripping is
temperature. If the temperature of the solution and carbon mixture is
increased, the gold will readily desorb from the carbon into the
solution. Hence temperature is the most important variable in the
elution process and temperature of 120-125o C is necessary to achieve
most effective and optimum elution performance.
Caustic is necessary for eluting the carbon from the gold.
Usually the loaded carbon will have the gold in the form of calcium
dicyanoaurate, since calcium is divalent, it is strongly bonded to the
carbon, at high concentration of caustic in the eluate, sodium ions
displaces the calcium and forms a less strongly bonded sodium
cyanoaurate which can be easily eluted from the carbon as per the
reaction below
But, at reduced temperature and reduced Sodium ions, the ions
further dissociates to AuCN.
Formation of AuCN is not desirable, as it is difficult is elute and
decrease the elution rate and efficiency, thereby increasing the
elution time.
The other requirements of elution process are:
1. Caustic strength (1.8 to 2.2%)
2. Low ionic strength of solution (low level of salts in the water)
3. Cyanide concentration (0.5-1%)
4. optimum flow rate of solution through the carbon, 2-3 bed
volume per hour
5. Low gold concentration in the solution
Elution is the actual gold removal stage. Portable water (low ionic
strength) is pumped through the column at high temperature (120-
125oC) and pressure (200-400kPa). Temperature increases with
pressure, hence high pressure is used to increase the temperature
further. Hence high pressure is used as the gold loading capacity of
carbon is reduced with increasing temperature.
Importance of elution parameters:
Temperature:
Required temp: 120 - 125 oC
If the temp is low, the elution efficiency and rate is decreased,
and when the temp is above 130 deg C, it favors the formation of
AuCN which then slows down the elution process
Flow rate:
Required flow rate: 2-3 bed volume
If the flow rate is less, then the resident time will be more and it
will elute the base metals like Ni, Cu, Fe etc. whereas if the flow rate
is more, then the elution will be incomplete due to insufficient
resident time, also it affects the overall efficiency of the electro-
winning process.
Effect of pH or Free Caustic:
Required pH: 12-13
Required Free Caustic strength: 1.5-1.9%
Impact of pH:
If the pH is lesser than 12, it has the effect of keeping the base
metal in the carbon itself, but this low pH will not favor gold
deposition in Electrowinning as predominant anode reaction is the
oxidation of water to oxygen which results in a decrease in eluate pH
adjacent to the anode. Stainless steel anodes will corrode if pH falls
below 11.5. Anode corrosion generates Fe and Cr ions. These ions in
particular can significantly inhibit gold reduction kinetics due to
formation of an insoluble chromium hydroxide layer on the cathode,
further reducing current efficiency.
If the pH is higher than 13, then almost all the base metals will
be eluted from the carbon and they interfere in the fineness in the
gold bullion, reducing its purity considerably.
Free Caustic Strength:
The caustic strength cannot be reduced below 1.5% as the
current did become unstable and fluctuate severely resulting in poor
deposition of gold.
If the caustic strength is high, this means more ionic strength
which will have negative effect on the elution and also leads to
wastage of reagents.
Cyanide Concentration:
Required: 0.7-1%
High cyanide concentration is required to drive the desorption
of gold from the carbon. Also it increase the elution rate and
efficiency, but the experiment results shows there is not much impact
of cyanide concentration on elution.
Low ionic strength:
Low ionic strength water (no dissolved salts) is used to enable the
gold to be stripped from the carbon. The loading capacity of activated
carbon for gold increases in the presence of on such as Ca2+ (calcium)
and Mg2+ (magnesium). Hence, desorption of gold from carbon is
favored by condition off low ionic strength solution, ie., the absences
of ions such as Ca2+and Mg2+.
Low gold concentration:
The low gold concentration in the solution also aids the desorption of
gold. If the concentration of gold in the solution that is coming back
from smelt house is low, then there exist a concentration gradient
between the eluate solution and the gold in the loaded carbon. The
elution rate increases if the difference between these concentrations
is more.
Acid washing:
Acid washing the loaded carbon takes place in two sequential
steps, they are:
1. Acid washing with 3-10% (pH of 1-3) for 4 to 5hrs
2. Rinsing with portable water for 2 hrs (until the pH is reached
around 7.5)
In acid washing, a dilute hydrochloric acid solution of 3-10% is
circulated by pumping the dilute acidic water from the HCl makeup
tank to the loaded vessel. The acid dissolves inorganic Foulants such
as calcium carbonate, magnesium and sodium salts, fine ore minerals
such as silica, and fine iron.
Elution Circuit components:
Elution Column:
The elution column is 9m high by 1.8m diameter mild steel
rubber lined pressure vessel (rate to 350kPa), having a high length to
diameter ration of 5, which enables solution to flow evenly through
the bed of carbon without short-circuiting or ‘Tunneling’.
Also the flow through the column from bottom enables a even and
uniform flow to ensure proper elution.
The column has a volume of 24m3 (Bed volume of 16.5m3)
which can hold approximately 15tonnes of carbon. The outer surface
of the column is coated with high temperature resistant paint to
prevent heat loss during the elution. With the capacity of elution
pump and its flow meter, two bed volumes per hours is ensured.
Plate heat exchanger and Thermic Oil heat exchanger:
Plate heat exchanger is used to heat the solution entering the
column and at the same time cool the solution going to the smelt
house; hence it acts as both cooler and heater.
Thermic Oil heat exchanger is a device used to transfer heat
from one fluid medium to another via thin metal plates. The fluid
never contact each other, oil is the medium used to transfer heat to
the eluate solution. The oil is heated by means of electrical heaters
(24 heaters per bank, there are two banks of heater, one is used as
stand by heater). The temperature for the elution solution is given set
point and based on the set point the 3-way valve opens, closes and
regulated the oil flow to the thermic oil heat exchanger to maintain
the set temperature.
Carbon Cycle:
The loaded carbon is removed from the CIL circuit and taken to
the loaded vessel where the loaded carbon is acid washed to removes
inorganic Foulants from the carbon before the elution to achieve high
elution rate and efficiency. Foulants reduce the carbon activity, and
hence gold adsorbing efficiency and capacity too. Carbon is only
partly reactivated by the removal of inorganic Foulants (precipitated
salts, mineral matter etc) in the acid washing cycle.
Organic Foulants such as Oil are unaffected by acid and must be
removed by thermal reactivation. Thermal activation (regeneration)
simply involves heating the carbon in the presence of steam to 750
deg C in a gas fired reactivation kiln. The combination of high
temperature and the steam environment vaporizes the organic
Foulants, returning activity to the carbon. The reactivated carbon is
returned to the circuit and the adsorption, elution (desrption) and
reactivation cycle start anew.
Carbon Reactivation Theory:
Carbon Fouling:
Carbon fouling is the build up of organic and inorganic
substance on carbon, which detrimentally affects gold adsorption.
Fouling results in a decrease in the rate of and loading capacity of
gold adsorption onto carbon, and may also adversely affects the
efficiency of desorption (elution) processes.
Fouling occurs when:
Undesirable orgnic or inorganic species are adsorbed onto the
carbon surface, taking up active sites, which would otherwise be
available for gold adsorption.
Inorganic salts are precipitated onto the carbon surface,
blocking active sites.
Solid particles such as fine silica, or precipitates are physically
trapped in carbon pores, restricting access to gold bearing
solution
Inorganic Foulants are those elements and compounds/molecules
other than those composed of carbon. However, inorganic substance
include carbon oxides, metal carbonates and hydrogen carbonates,
but excluded all organic carbon compounds such as alcohols, esters,
hydrocarbons, oils. Fats etc.
Examples of inorganic Foulants include calcium carbonate (CaCO3),
magnesium hydroxide (Mg(OH)2), iron cyanide (Fe(CN)6) and silica
(SiO2).
Whereas, organic Foulants included diesel fuel, lubrication oils,
greases and fine vegetation/plant matter.
It is not possible to prevent fouling altogether, as salts and organic
matter are invariably present in the ore and water supplies. It is,
however, possible to minimize the degree of fouling by ensuring no
Foulants are added to the process unnecessarily (eg. Oils, grease etc)
Inorganic Foulant Removal
Most of the inorganic foulants are removed in the acid washing stage
of the elution cycle, whereby the precipitated/adsorbed salts are
dissolved in hydrochloric acid (HCl) and then rinsed from the carbon.
The HCl will readily dissolve almost (70-90%) of the inorganic species,
but the adsorbed gold complex is unaffected. Silver and copper
cyanide complexes are also not removed
by HCl.
Organic Foulants Removal
Thermal reactivation is used to remove organic Foulants, by
subjecting the carbon to temperatures in the order of 650-750 oC in a
steam environment.
The high temperature burns off some of the organic matter whilst
reaction with the steam removes the rest. Steam also serves to keep
the reactivation system oxygen free (to prevent the carbon burning)
and is involved in the chemical formation of active sites within the
carbon.
Thermal Reactivation
Organic Foulant Types
Organic foulants are categorised into three main types:
Type I : Volatile (easily vaporised) organic compounds,
which are weakly adsorbed to active adsorption sites.
Type II: Organic compounds not sufficiently volatile for
thermal desorption, which
require higher temperatures for thermal decomposition (cracking)
and/or those compounds which are tenaciously bound to surface
sites.
Type III: Carbon residues remaining in the pores from the
cracking of type III compounds.
These carbonaceous residues are similar (but not entirely the same) to
the base
activated carbon. These residues are selectively removed from the
activated
carbon using high temperatures in a steam environment. In reality,
many organic foulants will display combinations of types I, II & III
behavior.
Thermal Reactivation Stages
The following steps usually occur during thermal reactivation inside
the kiln:
The kiln heaters are segregated as 5 zones, IA, IB, IIA, IIB and III.
There are 9+9=18heaters in zone-I, 6+6=12 heaters in zone-II and 6
heaters in Zone-III.
Drying – <200 oC
Carbon enters the kiln tube at approximately 60 oC and 25% moisture.
As initial heating occurs, highly volatile Foulants are vaporised. As the
carbon temperature passes through 100 oC any moisture remaining
inside the carbon pores will boil and be released as gas.
Vaporization – 200 - 500 oC
As the temperature rises to 200-500 oC in about the first metre of the
tubes, volatile ‘Type I’ Foulants are vaporized.
Pyrolysis - 500-700 oC
During this stage the cracking or pyrolysis (the decomposition of a
substance by the action of heat) of non-volatile (type II) foulants
occurs and ‘Type III’ foulants are deposited.
Removal of Pyrolised Residues - > 750 oC
The last metre of the kiln is operated at 750 oC. The steam generated
selectively oxidizes and vaporises the pyrolised (Type III) residues.
The steam creates an inert (oxygen deficient) atmosphere which
prevents the activated carbon from burning. The steam is also thought
to be responsible for generating fresh active sites on the carbon.
Carbon Cooling and Discharge
If the hot (750 oC) carbon were to enter the atmosphere the oxygen in
the air would react with the carbon causing burning and damage of
the carbon surface. The carbon discharges out of the kiln and is
quenched in water to prevent prolonged exposure to oxygen and loss
of activity. It is flushed with the water and carried away into CIL-5
Factors Affecting Thermal Reactivation Efficiency
Temperature
Temperature is one of the most important parameters in the
reactivation of carbon for the adsorption of gold. Too low a
temperature will not give adequate foulant vaporisation and hence
effective reactivation will not occur. On the other hand, if the
temperature is too high (>750oC) the carbon may degrade or become
weakened. If the temperature is not maintained different at different
zones then the clinker formation is observed, the unburnt and charred
carbon coagulates to form a solid mass called clinker.
Residence Time
If the residence time is too low then removal of the organic foulants
will be insufficient. Residence time is an important consideration in
the instance that a kiln tube becomes blocked. The rate of kiln
throughput is determined by the output at the discharge end, which is
set at a fixed speed. If a tube becomes blocked then the carbon will
simply travel faster through the remaining tubes to compensate and
hence carbon residence time is reduced. Therefore it is important to
monitor the tubes to ensure adequate residence times. A blocked
tube, viewed through the furnace observation port, will appear ‘red’
whilst the others ‘black’. A blocked tube should be cleaned prior to
the commencement of the next regeneration cycle to prevent damage
to the tube and to maintain the appropriate residence time.
Feed Carbon Contaminants
Carbon feed to the kiln must be free from grit, plastic and trash
materials for optimum operation. Continued periods of physically dirty
feed will cause blockages and malfunctions.