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.