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Merrill Crowe Circuit Problems (3 replies and 1 comment)
Charlie, a few questions:
Do you also run a gravity circuit in this plant?
Is the grade you mention in the solids or solution?
What is the screen size of the solids?
What is the pH of the tanks?
What is the NaCN content in the tanks.
Have you done an assay in the solids from your clarifier when it is cleaned?
Have you done an assay on your tailings filter discharge.
For troubleshooting of this type my experience is that chemistry OR mineralogy alone cannot provide all the facts needed to determine the cause of the problem; one needs to apply both. I would first let a mineralogist compare solids of low (tank 1 to 4) and high grade (tank 5). This will show if the high gold is due to contamination with carbon or is it a slug of refractory ore passing through the plant. For example in high gold solids, the gold particles may be trapped inside pyrite and not accessible to cyanide solution for leaching. The problem may be solved by separating the solids, obtaining pyrite cons and then breaking down the pyrite by pyro OR hydro techniques before leaching. Low or no recovery in Merrill Crowe may be a result of low gold in the cyanide solution due to refractory nature of ore. Check if mining is being done or was done recently in a refractory part of your ore body.
I would add to Bob's list of questions: What is the gold solution values in tanks 1 to 5? If you are leaching the ore, the values in solution should be steadily increasing and the highest value going into the MC circuit. No reaction with Zn is unusual. Are you using extra fine grade zinc? What are the pH, turbidity and O2 levels of your clarified pregnant solution? What are the O2 levels in your leach tanks?
Sorry for the late reply. We have turned the plant off and have flushed out the entire plant. We believe it was carbon in the plant that have caused the issue. Only yesterday were we able to turn the plant back on. I should have more results later today.
The grade I have mentioned of 26 g/t was from the solids in tank 5. We have checked the pH and it was at 11. The Knelson Concentrator has been running an hour and stopping for 30 minutes to flush the KC. We are still working on working the KC constantly but with the low feed of 2.5 g/t it is making it difficult to keep the density. The NaCN in the tanks are from 450-600 ppm. We have not done assays from the clarifier but from the filter it was mostly the same of 1.6 g/t. The material has been recirculating for about a week and a half. We have had the plant turned off but the leach circuit running. The reason was to decrease the amount of Au in the solids.
The plant was turned on yesterday so we should have some numbers ready today evening or tomorrow morning.
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We are experiencing an unusual issue at our plant. Firstly the Merrill Crowe is still not recovering. The solid in the leach tank are very unusual. Leach tank 1-4 the grades are 1-1.5 g/t but in leach tank (the last) 5 it is 26 g/t. We believe that there was carbon in the tailings, which is causing this issue.
Since our Merrill Crowe was not recovering we have had our lab do some tests and there is no reaction with the zinc. Our chemists have said that carbon should not cause this issue. We are having the lab do test work on a different ore to see if it is the feed that is causing this issue.
Please let us know of your thoughts.