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Gold electrowinning and amperage problems (3 replies and 1 comment)
Clearly some conditions you haven't mentioned have changed if your friends are truly seeing half the rate as previously. First, check the current-measuring apparatus they are using. In cases like this, a change in a variable that doesn't seem significant is often the culprit. Go over the recorded conditions and grill your friends to learn if ANYTHING is different about the latter runs compared with what they expected from previous experience. For instance, have the sources/qualities of any reagents changed? I take it the 91C temperature is an increase over that previously used, or is that the standard temperature? Basically, when they measure amperage, they are looking at the rate of a chemical reaction. If 25 amps is the highest current they can obtain from their apparatus, all other things being equal, then there is some step that limits the overall rate of reaction. If other steps are affected by something that slows their rates, then (within limits) the reaction rate and therefore the current should not change. But if the rate-limiting step is slowed down, the overall rate at which gold is reduced and therefore the measured current will slow accordingly. If the current measurements are accurate, then you need to uncover the chemical step that limits the overall rate. Is it the rate at which the gold is eluted from the carbon? In which case, you should focus on the conditions that slow that rate. Other steps might include the rate at which the cyano-aurate ions can migrate to the reducing electrode or others I'm not aware of. Some more considerations come to mind:
If the active surface area(s) of the electrodes have decreased, the maximum current they can support could be adversely affected. If the electrodes themselves are operating at max amperage, then the available electrode surface area will limit the amperage and rate of gold deposition. Do you see any evidence of fouling of the electrode materials? If this is the explanation, the yield of the reaction may not be any different; in that case, it will take 2x as long to win the same amount of product. On the other hand, if the gold and other ions are somehow "tied up", current flow will also be less; furthermore, measuring the yield when the reaction has completed (no more gold deposited) may show that not all the gold has been won. For obvious reasons, this is the more serious problem of the 2. Have you tried swapping the electrodes with new or previously successful ones?
I wish I could offer you a snap answer, but this is not my area of expertise. The above are what came to my mind, which has a long history of general chemical troubleshooting. Good hunting.
Why is the low current a problem? What your friends are most concerned with is the yield in gold. If it takes twice as long to reach the same yield as before (at half the amperage), then in 86 hrs they will have the same yield as they would at full amperage. The energy consumed over 96 hrs. will be the same (roughly) as in 48 hrs. The problem in that case is the loss in the plant's throughput. A lower yield, on the other hand, means they will be discarding gold when they uncharge their cells - big problem. Since you don't say what has happened to their yields as a result of their "problem", it's harder to help you diagnose your problem.
Also, why do they add cyanide together with the caustic when all they are trying to do is increase the pH? It seems unlikely that adding caustic will effect their yield, as long as the pH is held constant. On the other hand the cyanide participates in the process in (at least) two places: cyanide competes with the carbon in binding the gold, and increased cyanide concentrations will have the effect of increasing the rate and yield of eluting the gold into solution. On the other hand, the free cyanide competes with the reducing electrode in plating pure gold out of solution, so increasing the cyanide concentration will decrease the rate and yield of gold winning. If the rate and yield elution from the carbon are already 100% of that previously, then the net effect of adding more cyanide may be to decrease the overall rate (and amperage) and yield (and profits) of the process.
Hie
Its a serious problem you have there.I do project management of Vat plants ,low and high pressure elution machines.
At the previous processes we have worked on and installed,we have commissioned them at 25kgs caustic soda and current goes up to 50 amps,these are half a ton machines.
Again its wise to check for lose connections and your measuring instruments before you add more caustic.
The role of caustic soda is conductivity not only pH and definition of condhctivity is the ability to conduct electric current.
I hope you find this useful.I am in Tanzania
Some friends are operating a small carbon elution plant. Their gold electrowinning processes 300kg of carbon per month. They have low-pressure boilers and usually leave the carbons in for 48 hours per elute. They test the pH strength every morning and every evening and it usually reads between 12.5 to 13. We add 3kgs caustic and 3kgs cyanide if not satisfied with the pH. This month they have been having problems with our amperage, it usually runs at around 48 to 50 amps, but this time they could not seem to get the amps higher than about 25 amps. The temps run at about 91 which is meant to be correct for low-pressure vessels. Any suggestions will be appreciated. Thank you.