Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

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CIL Gold recovery problem (3 replies)

6 years ago
firebird 6 years ago

Hi everyone. I am now engaged in a CIL circuit. I tried to calculate my final gold bullion recovery after smelting but I cant. The problem is I had a total of 76kg gold fed to the grinding for 11 days.in the final circuit of the CIL a total of 4kg of gold reported to the tailing both in solid and soln. My loaded carbon for the 11 days was 63kg. The barren carbon and the barren soln contained at most 2kg. The final gold bullion was 91kg. The problem comes here now. Look at the Au fed to the grinding and the final Au recovered. At the first I thought may be the gold was locked up from the previous and added to the 11 days, but look at the loaded carbon...this can't be. And it cant be lost through preg-robbing. You saw the tailing assay. Do you think our lab assay is correct or are we taking uniform samples to the laboratory?  What am I missing?

6 years ago
Zolani 6 years ago

Hi Firebird,

According to my knowledge, the reasons you can get more gold than what you put in are as follows:

  1. Poor metal accounting 
  2. Sampling and sample preparation techniques
  3. Incorrect Lab results
  4. Previously locked gold released    

I think you also need to use CIL feed for you calculation instead of mill feed grade. Are your flow meters calibrated an working correctly? How much gold was locked previously compared to now? What is the frequency of your sampler and how are samples prepared for assay? Are you using weighted average or just assays as they are for your metal accounting? 

If all of the above asked question are done correctly, then I don't know.

One thing I know for sure we cant create gold, hope you find solution to your problem.

6 years ago
Bhaduri 6 years ago

Adding to Zolani's insight, were there changes to your CIL density incl. rheology, viscosity, pH during the 11-trial days as compared to previous operations...if so, you may have recovered some settled loaded C from your tanks...check your shift operator CIL tank gpL's for confirmation...or less likely, the alchemist stone submerged in one of your tanks is transmuting C into Au?

Greg Henderson
6 years ago
Greg Henderson 6 years ago
1 like by David

Reconciling feed gold with back-calculated gold based on change in inventory + dore gold + tails gold can be tricky as sampling of feed in a representatives manner is notoriously difficult, especially if there is a coarse gold component and the mill feed is primary crushed.  As Zolani pointed out, regular sampling of CIL feed should give a good indication of CIL feed, and provided the sample taken at each sampling interval is the same amount, the 24 hour average should be reliable.

The most accurate method is the back-calculation approach, i.e. change in carbon inventory (gold in circuit) + dore' + tails gold, as most of the gold for the accounting period will be in gold dore' and on carbon (usually a week's worth depending on inventory) so the effect of tails gold on the calculation is minimised.  Over a one month the accounting improves even further.

Is the 63 kg on carbon the change in inventory or gold stripped?  If the latter, you really need to establish the gold on carbon at the beginning and end of each accounting period as changes in head grade or carbon movement practices can alter the loading.  In this case, an increase of gold on carbon may have resulted which has is not reflected in carbon stripped. 

Back to the mill feed assay issue, only fire assay will give a complete representation of gold content and if nuggetty gold is present there will be reproducibility problems in assays.  Screened fire assays are a way of resolving this.  If there is significant gold locked in silica you may be over-stating the "recoverable" gold content as fire assay uses pulverised sample and recovers all contained gold through fluxing and cupellation.

Carbon sampling in CIL tanks is also tricky, make sure you sample from various points and depths to get a good average carbon measurement in the tank as this is an important measurement to make in determining change in gold in circuit.


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