Training Manual for CIL & Elution-Final (Reviewed)

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 Eluted carbon values

~130o C < 100g Au/t of carbon

Regeneration: Kiln regeneration temperature~700-750o C Carbon regeneration rate Kiln operating schedule

500kgs/hour 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 tank3, 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 14501600. 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 (120125oC)

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 – 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.