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Caustic cyanide carbon pre soak prior to elution for removing copper (4 replies and 10 comments)
A paper was authored and presented at SME (Pre-Print 93-230) on this subject and should contain the information you are requesting. Abstract shown below:
| American Girl Mine is located in the Cargo Muchacho Mountains in the southeastern Imperial Valley of Southern California. Production from the current open pit mine and heap leach recovery system began in early 1989. Previous mining and gold recovery took place commencing in 1987 at the Padre Madre site, situated adjacent to the American Girl site to the southwest. In mid-1990, a 790-tonne-per-day underground gold mine and mill went into production. Approximately 6,500 troy ounces of gold per month are recovered from the combined open pit underground mine operation. Copper in the form of Azurite [Cu3(CO3)2(OH)2] and Malachite [Cu2CO3(OH)2] are abundant in the mined ores (Henshaw, 1942). The high solubility of these minerals in caustic cyanide solutions has a negative impact on carbon adsorption and electrowin processes. This paper presents the results of monitoring programs, process chemistry, and control changes introduced at American Girl Mine to abate production complications in the refinery and measures taken to minimize solution gold losses in the adsorption circuit(s). |
Thanks Craig,
hopefully I cut out the correct sections on the topic of COLD STRIPPING.
Prior to hot pressure stripping, each tonne of mill or heap carbon (from the No.1 or head tank) is subjected to an intensive ambient temperature cyanide strip, a so-called cold strip. The cold strip cycle lasts for approximately 1.5 hours with cyanide solution at 30-38 kg/mt (60-75Ib/ton) passing up-flow through the carbon bed, at a rate of 0.5 bed volumes/minute with recycle of the solution for a total of two to three bed volumes. Cold strip solution is wasted to the barren pond. Following information published about the Mount Morgan mine in Queensland, Australia, and pre-elution methods for selectively removing copper as [Cu(CN)3 2-] and [Cu(CN)4 3-], an ambient temperature or cold strip cycle was placed in the circuit prior to hot
pressure stripping. The intent is to remove copper from the loaded carbon before passing the carbon to hot strip where the copper would elute with the gold and silver. Copper is removed from the carbon by eluting the [Cu(CN)3 2-],
[Cu(CN)4 3-] complexes created in the higher concentration cyanide solution. By removing the copper here substantial amounts of copper will not report to electrowin and deposit on the cathodes. Additional copper renders the gold and silver very difficult to remove during pressure washing and continued processing.
Use of ambient temperature high concentration sodium cyanide solution to drive off copper loaded onto activated carbon was instigated early in the process investigations. As stated in the circuit description (above) there were sufficient information sources as to the validity of a high cyanide pre-soak to remove copper prior to hot pressure strip (Muir, LaBrooy, Cao, 1989, Randol, 1987). A complicated arrangement was implemented, in which each tonne of carbon (from either mill or heap circuit) was cold stripped 3 times for 5 to 5-1/2 hours per cycle. This was an excessive cyanide consumer, and was eventually optimized to a single batch cold strip for 1-1/2 to 2 hours per mill or heap tonne. Figures 5,6,7 show the response of copper on activated carbon from the circuit to the cold stripping and hot stripping processes. Also shown is copper in the cyanide rinse effluent from cold strip.
At this point copper became the primary focus, and all problem-solving efforts focused on copper. Copper-to-gold ratios were checked and demonstrated to be at a critical level (in excess of 600 Cu: 1 Au) in the head column and increasing to
very high levels (several thousand to one) in the tail column.
Cyanide levels in the heap leach barren solution were scrutinized to determine if anything could be done to reduce the leaching rate of copper and maintain favorable conditions for gold leaching.
Cyanide-to-copper concentrations would be required in excess of 3:1 respectively. Current levels are already approximately 0.38 to 0.45 kg NaCN/mt (0.75 to 0.90 Ib NaCN/ton) solution, in excess of those at heap operations where copper is not a significant problem. When copper levels in solution rise to 800 ppm, the cyanide requirements would be nearly 2400 ppm or over 2 kg/mt (4lbs per ton). The thickness of the heap pile contributes to lowering of the cyanide through dissolution of cyanide soluble minerals, as previously described.
With these cyanide requirements in mind, little hope was held for reducing cyanide to limit copper dissolution while maintaining proper cyanide for gold leaching.
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Homestake was able to remove up to 40,000 ppm of copper on carbon from Grisly Gulch reclaim water prior to gold recovery The copper strip was conducted at < 150F without loss of gold into the copper strip solution. You can check the copper content of strip solution by AA to determine an end point. At another property in Mexico, excessive cyanide usage from soluble copper was mediated by doing an acid strip of the cyanide with recover of HCN into a strong caustic solution, which could then be recycled.
I use 1g/l NaCN with 10 g/l NaOH pre-soak for at least 5 hours ambient temperature (it can be longer), then wash with water and elute. Copper will come off in the pre-soak and only traces of gold like around 2.5ppm Au which is negligible. Don't return the pre-soak solution in the process water as carbon will load again on carbon. Detox with ferrous sulphate before dispose it in tail dump.
Yves Mazzon
In addition try to keep your leach solutions at a relatively high pH (11) and high cyanide. Higher copper cyanide complexes don't load that readily on carbon. I have been experimenting with ammonia addition too with good response. By adding ammonia you can reduce the cyanide consumption. I think ammonia has good chelating properties on copper. You have to play with the additions until you get it right. There are no hard rules.
Regards,
Yves
Thanks Yves, I need to know more about ammonia addition.
In what quantities of ammonia to water or sands according to your experience is effective. I know it differs with copper, gold and mineralogical content of the ore but did you come up with some sort of average quantity of ammonia to be added. My ore contain an average of 600 ppm (0.06%)
My carbon tails very fast.
Please help.
I will try 600ml/t concentrated ammonia and see what happen with your recovery and cyanide consumption. Ammonia will also boost your pH which is good too.
600ml/t of solution?
Which amomonia is that, Ammonium nitrate or Ammonium sulfate?
Thanks.
Ammonia is ammonia NH4OH a liquid at 30% with an horrible smell, others are ammonium salts
Hello Gents,
This conversation has taught me a lot. Thank you. The cold strip process is duly noted.
My challenge is that the elution is not stripping the carbon entirely.
There is some 400 grams of Copper and 200 grams of gold left in the carbon (500kgs) after the elute. The elute is being done at a company offering such services. The Gold won to the electrowinning cell is about 300 grams.
We aren't able to use Cyanide in the elution process by law here.
What is being done incorrectly in the elution process?
Thanks kindly
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Anyone with the knowledge of cold caustic cyanide carbon soak prior to elution for removing copper?
What are the quantities for caustic soda, cyanide and water and how long should be the soak?
I have 600kg carbon and copper load is 1.8%.
I use zadra elution process.
Please help.
Thank you