Using butyl diglyme in gold recovery

I am using a Saturated Salt/Nitric Acid process for leaching gold from a low sulfur refractory ore. Several pre-treatment steps of the head ore are necessary before proceeding to the final step where butyl diglyme is added to the pregnant solution. Depending on which instructions I follow (there seem to be several--including a shake for a minute or stirring at temperature motion for a set time BEFORE adding the Oxalic acid), I don't always get a good phase break and usually end up with a foaming gel-like substance in the butyl. It will reduce somewhat over time or if I break the bubbles with a glass stir rod, or use a "poor man's centrifuge (swinging my arm in an circlular for several seconds), but will never completely go away. If I drain it off, I'm afraid of losing possible values. I do not have access to instruments to identify the substance. When I use dilute HCl to wash the butyl, the foam will reduce further but not completely.

I have four questions. 1 - Any idea what the foam may be? 2 - Do I need to get rid of it? 3 - If necessary, how can I get rid of it? 4 - Can I just leave it in the butyl and add the oxalic acid?

Do you use any soaps or surfactants in your pretreatment? Are any other organics involved in the pretreatment stages?

The word for the foamy transition layer is emulsion. The tendency to emulsify increases with the removal of electrolytes and solvents.

Solutions to the problem involve changing your pretreatment process to eliminate the emulsion culprit, heating the dry ore to destroy organics prior to DBC treatment, or destruction of the culprit via chemical treatment. Two common chemical techniques to destroy the undesirable emulsion layer are 'salting out' with sodium chloride and acidification with muriatic acid. Mechanical means of removing the problem transition layer are a centrifuge (as you mentioned), vacuum filtration (for stubborn emulsions), or using a separatory funnel to drain the lower layer, then drain the emulsion layer into a separate separatory funnel, then drain the upper ether layer into the funnel with the clean lower layer. Once the emulsion is separated from the bulk of the other layers, it can be extracted with more DBC in another separatory funnel which may break the emulsion. A very low tech and time consuming solution is to simply allow the transition layer to settle in the separatory funnel until the layers are completely separated. This could take a long time depending on the reason for the emulsion.

I hope this helps.

Steve

Thanks, Steve, for your response. I have been using a multi-step wet pre-treatment process which does reduce the emulsion, BUT the chemicals are costly and values are lost during each stage of the pretreatment. When I use SSN directly on head ore without any pretreatment, the emulsion in the butyl diglyme is so thick it only phase breaks slowly to a point. With all the emulsion that remains, it useless to process the pregnant butyl with oxalic. (BTW - the emulsion appears to be bubbles--the large ones eventually break but the smaller bubble remain intact. At one time I thought the problem was the presence of silicates and/or silicic acid. I'm not so sure now.) As you suggested, I have waited a full 24 hours for the emulsion to resolve--without success. I haven't yet used additional DBC for fear I might harm the pregnant butyl and no instructions. (I use 200 ml DBC for 600 ml pregnant leach). I have also tried reducing the emulsion during the dilute HCl wash to remove iron. That really doesn't work too well, either. So far, I haven't found a way to reduce the emulsion in the DBC in preparation for the oxalic acid. I recall reading somewhere that a surfactant (not specified) could resolve the problem, but I'm afraid anything I add to the pregnant butyl might destroy the values and change the chemistry. If there is any more information can provide, a site you can refer me to, or another process to try? I would be truly appreciative.

The first time I tried SSN and BDG I shook so vigorously (like preparing a drink in a cocktail shaker), I got an emulsion so thick it took 3 days to separate. Subsequently, I successfully mixed using a fairly vigorous rocking motion and got separation in a matter of minutes.

If you're having problems with a foam, it could be for a number of reasons:

1) Too vigorous shaking, as above. (I would use a separatory funnel to separate the 3 phases, let the foam sit for as long as it takes and continue on as normal with the other two phases.)

2) At least part of the foam is crud. [Note to mods and admins: crud is not a four-letter expletive; it's the correct technical term for this situation.] Crud is a gunky mixture of insolubles and your organic fraction. You can minimize crud by a) carefully filtering your aqueous phase and, b) carefully scrubbing your pregnant SSN (at least twice) with dilute HCl. I posted a Distribution Coefficient graph somewhere on the forum from which you can deduce the ideal pH range of HCl for scrubbing. (And possibly the ideal nitric molarity of your SSN before BDG separation. The graph is for HCl molarity, but may also be relevant for nitric???)

3) Approach of Phase Inversion. The speed of phase separation is a factor of the difference in the specific gravities (SG) of the respective phases ie the greater the difference, the faster the separation. The SG of BDG is lower than that of water, but increases with gold loading and can be greater that the SG of your aqueou phase at maximum loading. Thus as the loading of gold in BDG increases, phase separation become more difficult.

Follow-up to my original post. It appears that NaOH in the pretreatment process contributes GREATLY to the emulsion problem. No amount of washing with dilute HCl will eliminate the emulsion. The use of hydroxides in the pretreatment process is a necessary step to prepare the ore for SSN.

I would tend to blame the hydroxides for the foaming. I use BDG to good affect and never experience the foaming problem.

I have found that after you have a pregnant solution, it's best to dilute it before adding BDG. In doing so I get a better phase separation in less time. BDG is an oily solvent that doesn't really like water, so like oil and water they readily separate. Even better if there is more water. When you have a highly acidic solution and then fraction the BDG by shaking it, it takes awhile for the BDG to accumulate again. But when more water is involved, it almost pushes the small globs of BDG together, faster. Almost like trying to hold air under water, the deeper you go the more it wants to fight, and instead makes it's way to the surface as fast as it can as soon as you release it. BDG in more dilute solutions attempts to make it's way to the surface faster.

It also helps when you are doing your HCl washes if you dilute the HCl (if it's 36%-37% HCl) by half, or (if the HCl you are using is 15%) by a quarter. I do a 50/50 solution of HCl to DI H2O and have found it works best for cleaning the BDG. Also, when you add water to the solution before BDG, the BDG will extract less other metals as it extracts to gold, so your HCl washes end up being much cleaner, which of course means high purity.

I do sometimes end up with a little foam. In those rare cases when I do, I simply drain the separatory funnel, add water, then a little fresh BDG. It readily accepts and absorbs the bubbles, then I add it to my BDG solution.

Scott

Scott, thanks for your reply. Not being a chemist, it appears I am not correctly phrasing the question, or perhaps I just misunderstand the replies I am receiving about my emulsion problem. I have been diluting my pregnant solution with an equal amount of RO water (in the belief it helps get rid of the organics). That definitely helps cut down on the emulsion.

What I don't think I've made clear is that the emulsion in the butyl after heating/mixing with the preg sol does not float. The BDG stays on top. When I drain the separatory funnel, the emulsion comes out first. I've followed suggestions to drain several times into other funnels. The narrow neck at the bottom of the funnel does break up the emulsion, but this is time consuming. When you and others talk about "drain[ing] the funnel, add water, then a little fresh BDG" I suspect I haven't presented my problem well BECAUSE the emulsion comes out first before the clean buty. Obviously, the emulsion contains a lot of BDG but is released when constricted through the narrow neck of the funnel. I imagine that a centrifuge might also correct the problem.

I've been using 5% HCl wash (diluted from a 32% solution). You appear to be using a much stronger HCl wash. If I use the higher concentration might I be more successful in removing the emulsion and cleaing out the trash metals?

BTW - my ore has a very high concentration of iron. On other posts I have asked if anyone knows of an iron oxidizer that would remove the iron from the SSN. (This is part of the reason for my wet pre-treatment of the ore.) Years ago when the SSN process was demonstrated to one of my associates he observed a thick collection of colloids hanging out like a cloud in the middle of the preg sol. When this gelatinous collection of colloids was removed, the amount of iron was greatly reduced without affecting the gold. The Rocky Ledge Mining web site advertises an "iron oxidizer", but they don't respond to my e-mails. This "collector" appears to be very inexpensive. Any ideas?