Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-04-21T02:33:06+00:00
  • To participate in the 911Metallurgist Forums, be sure to JOINLOGIN
  • Use Add New Topic to ask a New Question/Discussion about Pyrometallurgy and Electrometallurgy.
  • OR Select a Topic that Interests you.
  • Use Add Reply = to Reply/Participate in a Topic/Discussion (most frequent).
    Using Add Reply allows you to Attach Images or PDF files and provide a more complete input.
  • Use Add Comment = to comment on someone else’s Reply in an already active Topic/Discussion.

Fire Assays and Preg-Robbing Organics (9 replies)

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

Fire assays are returning double the Au reading when using a standard aqua regia method. Suggestions are that the preg robbing material could be adsorbing the gold during the acid digest, which would not be transferred to the aqueous phase to organic (DIBK) and hence under-reported. How likely is this scenario, and what would be the best way to check/ deal with this issue?

Alan Carter
1 year ago
Alan Carter 1 year ago

I need more information before I can make any sensible suggestions. What is the matrix containing the gold? What is the approximate level of gold we are talking about? Is the difference between results the same for both heads and tails? Are you finishing your fire assay gravimetrically or by AAS? Very importantly, which method does your gold room agree with? Within your circuit there are lots of opportunities to do a gold balance, with a 100% difference between the two methods it should not be difficult to work out which, if either, method is the most accurate.

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

Some test work done on these ores showed high levels arsenic, with gold locked in ultra-refractory mode in solid-solution with arsenopyrite and other sulphides. Mild preg-robbing is apparent (0.5% organic carbon) and some of this carbonaceous material can be seen floating on the surface of our leaching tanks. The grades range from 3-6g/t. The difference in the assays was apparent in both head (COF) and tails by approx. the same factor(x2). Also, our recent balances indicate there is a possibility gold produced is more than gold reported (based on the COF grade). Hope this helps, and please let me know if you need any more clarification.

Alan Carter
1 year ago
Alan Carter 1 year ago

You have certainly picked a "Doozy" to cut your metallurgical teeth on. I am guessing that you may have more than one problem, but first things first; you must have confidence in your assays. Arsenopyrite and pyrites in general have the potential to disrupt every part of the process including the assays. From what you have said so far I am guessing that your routine assays for process control are done by aqua regia digest, followed by extraction into DIBK for reading by AAS. The presence of arsenic presents potential health issues with a roasting pre-treatment of the ore prior to digestion. 650 degrees C for 2hr. And alternatively you can try adding nitric acid until brown fumes subside and then make your normal aqua regia addition. There aren't too many ways that you can over report gold by fire assay but it is possible. We can go into this if the aqua regia digest doesn't match the fire assay results which should be the most reliable.

I am also guessing you are running high tails and low recoveries. Have a go and report back on your findings.

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

As we don't have fire assay facilities on site we will have a go at adding nitric acid prior to A/R. Will see how we go from there. So from what you have said, I gather the biggest issue with the assay variances could be due to the pyrite presence and not any preg-robbing tendencies the ore might possess?

Also, just some preliminary results I have. Put several samples in the furnace at 800 degree Celsius for over 12hrs, and followed that up with the aqua regia method. Assays were pretty close to the fire assay results. Testwork ongoing!

Alan Carter
1 year ago
Alan Carter 1 year ago

Looks as though we are moving in the right direction! Be mindful of the possibility of arsenic fume when roasting. Also be careful not to over roast the samples as it is possible to sinter Fe and occlude some of the gold. You may have to do a time and temperature trial to determine the optimum roasting conditions. 9x replicates of the same sample, 650 degrees C x 2,4,6 hrs repeat at 725 and 800 degrees C if there is no difference, no harm done, just use the temperature that best suits your lab routine.

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

It would appear that pre-treating with nitric acid only till the brown fumes have subsided still returns lower assays than fire assay or pre-roasting+A/R. Is there a particular reason for this that you would be aware of?

Alan Carter
1 year ago
Alan Carter 1 year ago

Without a full understanding of your ore's mineralogy I would only be guessing. And what you need now are facts. Now that you are gaining confidence in your assays perhaps it is time to start thinking about recoveries which will probably bring us back to mineralogy. I hope I am wrong but I am guessing your recoveries will turn out to be low. Perhaps it is time to start thinking about cyanidation tests and all that that entails.

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

One more question, still on this topic. We have been pre-roasting all our COF and tails samples since the assay anomalies were discovered. Another issue that has risen is that when treating 100% free milling ore, rich in hematite, the ashed samples return lower results than non-ashed. I read somewhere that this would be due to the iron dissolving at the high temps and thereby blinding the previously exposed gold surfaces. How likely would this phenomenon be?

Alan Carter
1 year ago
Alan Carter 1 year ago

It is a possibility, pay close attention to your ashing temperature and time. Higher and longer are not necessarily better.

Please join and login to participate and leave a comment.

BUY Laboratory & Small Plant Process Equipment

We have all the laboratory and plant equipment you need to test or build/operate your plant.

ENTER our Mining Equipment' Store

We Sell EQUIPMENT for all types of Mineral Treatment PROCESSES and Laboratory Testing needs

Have a Mineral Processing QUESTION?

Come in, ask your question

911Metallurgist Community Forums

Talk to other metallurgists and be helped.

Need ENGINEERING Services or Plant TROUBLESHOOTING?

We can IMPROVE ALL PLANTS / Mineral Processing Engineering & LABORATORY Ore Testing

911Metallurgy Engineering

Contact us for process engineering, metallurgical investigations, plant optimization, plant troubleshooting, needs. WE “FIX” METALLURGY.