First of all, let me thank you for the interest in my question. I can use all the help I can get. And if someone has a similar project, I’d be willing to share any information I have or find.
The ore I’m working with is a massive sulfide deposit which is both refractory and carbonaceous. I have been told by many people that this ore has high levels of precious metals, including both gold and platinum. I had a man look at it with a scanning electron microscope and he said he detected high levels of gold but could not identify any particles of metal. Another company I sent a sample to told me there was no metallic gold in the sample; everything was micron gold or smaller. I’ve since learned about submicroscopic particles called ‘clusters’ of precious metal molecules, which are too small to be considered metal. I’ve learned fire assays will often miss micron gold due to its volatile nature, so despite the fact that several assayers have told me there is no gold in this sample I’m optimistic that they are wrong and that the gold is there, just hard to detect and extract.
I have considered using cyanide but I hesitate due to the fact that I live in a residential area and can’t risk the safety of my family or the neighborhood should something go wrong. Plus I suspect it’s illegal to use those chemicals inside the city limits.
I understand my ore needs to be roasted or chemically oxidized to minimize the interference of the sulfur. Initially the crushed ore shows very little interaction with a large common magnet, but after roasting, almost 40% of the ore can be separated magnetically. This tells me the iron sulfide is being converted to magnetite
I’ve learned that precious metals are adsorbed onto carbon, which may be where the gold particles are hiding in my carbonaceous ore. I also know iron will alloy with the precious metals and keep them from being detected using spectrographic analysis. Sometimes the iron encapsulates or coats the gold particles making it extremely difficult to extract the gold chemically. As you know it is common for precious metals which have been dissolved in a leach to be displaced by more active base metals (like iron, copper, lead, etc.) causing the precious metals to precipitate out of the solution and fall back in with the ore material being leached.
I’ve tried several chemical leaches including Aqua-Regia, SSN, and one using Sodium Hypochlorite and NaOH, but they all extract the iron and base metals, regardless of temperature of the leach or the length of time the ore is exposed to the leach.
John (aussco999) hit the nail on the head in his second paragraph above:
“Of the 5 halogens, chlorine and bromine have been used with some success in the past for batch leaching, but they also have their problems. Chlorine when used with an oxidizer or as a gas (nascent), will dissolve all of the precious metals, plus a long list of base metals (see the EMF series, i.e. iron, copper, lead, etc.), and bromine is similar…”
I have heard iodine will not complex iron if you keep the pH >8. I have also read that AuI2- is stable between 0.51 V and 0.69 V and at pH <12. This seems to imply that if the pH is adjusted the iron should not be a problem in the precipitates. The other advantage I’ve read is that iodine will oxidize sulfide minerals in the reaction which means other oxidants do not need to be added to break down the sulfide minerals which may contain the precious metals.
I understand the iodine is initially more expensive but I know it can be precipitated from the leach and reused, making it cost effective as a batch leach. Also there would be no acid fumes with iodine. Unlike Aqua Regia and other chlorine and nitric based leaches, the fact that there would be no off gassing seems like it would be a real advantage in my situation.
Now my question is: Has anyone ever done something like this? If so some information about the correct pH, redox potential, temperature range, iodine/iodide concentration ratio would be very helpful (and save me a lot of time). Any thoughts or ideas would be very much appreciated. Thanks for your input!
