Training Manual for CIL & Elution-Final (Reviewed)

  • Published on
    26-Oct-2014

  • View
    106

  • Download
    3

Embed Size (px)

Transcript

Training manual for CIL & Elution Technical Details: CIL: CIL Feed slurry density Number of CIL tanks CIL Tank volume Carbon concentration Leach CIL residence time tanks) Tailing screening: Tail screen mesh Tail screen flux Acid Washing: Acid wash flow rate Acid wash strength Acid wash pH Acid wash time Rinsing time Loaded vessel volume Acid make up tank volume Elution: Elution column volume Elution Batch size Elution circuit type Elution flow rate Eluate solution 24m3 24m3 of carbon ZADRA 2 Bed volumes per hour 2% NaOH and ~ 1% NaCN 30m3 28m3 2-3 4 hours ~2 hours (depends on pH) 2 Bed volumes per hour 3 % Hydrochloric acid 0.8mm 0.8mm 60m3/m2/hr ~40% Solids 6 2400 m3 10 g/l ~24hours (~4hours in each

Eluate pH Elution cycle time Elution temperature Eluted carbon values Regeneration:

12-13 12-18 hours ~130o C < 100g Au/t of carbon

Kiln regeneration temperature~700-750o C Carbon regeneration rate Kiln operating schedule Kiln rotating speed 5rpm) Reagents: Lime % Lime addition Cyanide strength Cyanide addition Cyanide Carbon (delivery) Carbon addition HCl strength HCl addition wash Caustic strength Caustic Addition 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. 47% NaOH 250-450kg/ elution 600kg (bag) with 0.53 t/m3 density 50g/t of mill feed 30-33% HCl 350kgs of 100% HCl per acid 65% as CaO 1-3kg/t of mill feed 30-33% NaCN 200-300g/t of mill feed as 100% Regeneration time at max temp 500kgs/hour 18 hours/ batch ~ 6mins 0.5 rpm with VFD (Full speed-

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 lets 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 -75m 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 tak