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Acid addition to Zadra process to reduce pH (3 replies and 2 comments)
Hi CA
I've in the past also tried to find the reasons for the NaOH in eluant - and mostly it seems as if the role it plays is more to do with protecting the electrodes in EW, providing a conductive solution and safeguarding your CN concentration than to do with the actual elution process. That said, Marsden and House have a figure in chapter 7 (Fig 7.15) showing the relationship of elution rate with [OH-], it appears there is a relationship but not as strong as that with [CN-]. They also in Section 7.1.3.4 suggest that the hydroxide ions displace gold cyanide ions on the carbon - so you may need to play with free cyanide concentrations to get them to perform this role in the reduced concentration of OH-.
I suspect the only way to tell would be to give it a try - drop the NaOH concentration/addition rate by 10%, measure your solution profile (versus a solution profile done at the current conditions) and see what kind of change this induces, and if you can limit the formation of zinc hydroxide.
Other factors that could be causing this - are you keeping your free cyanide levels up during cementation? And have you checked that the stores folk haven’t changed zinc supplier, and you have less surface area? Unlikely with shavings I know, but it has happened to me in the past with dust. I am sure you have checked on the other factors – dissolved oxygen levels (are you running the solution over the zinc hot? not an issue if so), adding lead can make a significant difference to conventional MC, but probably not so much to a hot eluate, while As, Sb, Cu, Ni and Co will also mess you around. You are bleeding the spent (barrens) out to leach to keep the zinc below around 0.05 g/L?
If you are seriously considering changing the pH using an acid, you will need to do this under strictly controlled conditions to capture any HCN generated – and you would want to use a mild acid, possibly even something as innocuous as CO2 (although the reaction with Ca in the solution may end up being problematic - or helpful, you can never tell!). However you add the acid there will be points of high concentration where you are going to have HCN formed – I would be more inclined to reduce the NaOH and see what the tests show.
Good luck!
Thank you for your reply Colette.
I wasn't sure if the NaOH levels for Zadra were set in stone or not. After reading some of the other posts on cyanide-free elution, I became quite confused as to what chemicals are actually critical to successful elution.
I will reduce our NaOH levels by 10% and see if that gets our pH levels below 12. Yes, we are running hot eluate over the zinc, so I am not worried about dissolved oxygen. If we do end up having to use acid, we will be sure to keep the pH high enough to avoid HCN formation!
This is a small pilot plant that we are doing. On a daily basis, we are only trying to strip 25g Au and 60g Ag from approximately 7 kilos of carbon. We digest our zinc shavings with dilute H2SO4. Because zinc is difficult for us to come by out here in the middle of nowhere, are there and recommended methods for precipitating the digested zinc back out of solution?
Thanks!
Hi again CA
I suspect that getting the zinc back out of solution for reuse is going to be complicated and expensive - in the MC circuits I have worked on we just bled it out through the leach. The easiest way of recovering it would be precipitating zinc as a sulphide, but you are going to have loads of other metal cations in there that are also going to precipitate out, and things start escalating really quickly in terms of how to separate the metals and ultimately get zinc dust. Someone who is more electrochemistry oriented may be able to give you an idea of whether or not you could refine it out by electrowinning at the tenors you are dealing with and with the contaminants likely to be present – I don’t think it will be simple, but it definitely would be fun to try.
Re elution - I've been to loads of sites that have COMPLETELY changed the strip solution recipe - it can creep up and bite you in the proverbial, in which case I always suggest going back to the basic recipe, but other sites run without adding cyanide, and even without NaOH, quite successfully. I tend to prefer keeping the [CN-] as per M&H's levels, and running the NaOH to a pH rather than a g/L. The CN- seems to be the most important, and plays a big role as far as I can tell in "popping" the aurocyanide complex off of the carbon and taking its place. M&H suggest that the OH- ions also do this, but the [CN-] seems to have a far more significant effect.
Just in terms of adding pH - sorry, I wasn't suggesting you would let it drop below 9.8'ish - more that, no matter how careful you are to keep the pH at the correct levels in the bulk of solution, you will still have HCN generation because of localised micro-pockets of low pH, no matter how well you stir the solution. So just be wary - I made this mistake in a lab once and still bear the mental scars of the lambasting I got from my supervisor. I subsequently have used CO2 to drop solution pH when working with NaHS (which has very similar behaviour to NaCN), and it has worked reasonably well. You however face the additional challenge of dealing with a hot solution, so I reckon the easiest and safest route to start with is by dropping that NaOH addition and seeing what happens. With it being on a pilot plant at least you won't have the production superintendent wanting to lynch you if things go slightly pear-shaped (which they shouldn't).
Let us know how you go, it would be good to see if this helps – there are a few other things you can try, but I reckon you have identified your most likely cause.
Thanks for the great information; I never considered the micro-pockets of low pH. I will let you know how we make out over the next week. It doesn't sound like zinc precipitation is something we are going to pursue. In the end, we may have to put in a small electrowinning circuit due to the local unavailability of zinc. Again, thank you very much.
I think you should review the level of alkalinity in the leaching process. If the level is very high, it is possible that a high content of OH ions during the precipitation process can generate a high consumption of zinc, and also to produce the hydration of zinc. For example, when calcium hydroxide reacts with zinc, the reaction produces calcium zincate and some hydrogen can be released.
The calcium zincate can be soluble if there a considerable quantity of cyanide, otherwise, it can be in suspension acting as a coating on the metallic zinc surface. Probably, your process contents sodium zincate. This condition affects negatively the precious metals precipitation on zinc surface.
In general, it could be necessary to increase or reduce the level the alkalinity to control the formation of coatings. The change should be practiced step by step in the leaching process.
We are using the atmospheric Zadra process with 10g/L NaOH and 1-2g NaCN/Lat about 90 deg. C. to strip our carbon. We do not have access to electrowinning cells or a traditional Merrill Crowe circuit, so we are passing the heated eluate over zinc shavings. The pH of our eluate is over 12.
Mardsen and House in The Chemistry of Gold Extraction (pg. 379) note that in reference to zinc cementation, "at pH values >12 the precipitation rate has been found to drop sharply due to excessive hydrogen evolution." Furthermore, "solution pH also has a marked effect on the formation of zinc hydroxide, which can inhibit precipitation by the formation of an insoluble layer on the zinc."
We are having low recoveries from our zinc precipitation, could the above mentioned reactions be the culprits? If so, would the introduction of an acid to lower the pH to, let's say, 11.5 adversely affect the elution? We are recycling our eluate, so the acid would be returned to the process.
I can not find a good explanation on exactly what the roll of the NaOH is during elution and if lowering the pH or adding an acid would affect its function.
Thanks for any thoughts.