Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction 2017-03-23T09:50:58+00:00
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Analyze for CN- (11 replies)

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

Wondering if it is possible to test for CN- only as opposed to free cyanide (HCN and CN-) in conventional CIL leaching, as a way to justify maintaining high pH in the circuit?

My understanding is that at a pH of 9.3, 50% of the free cyanide is HCN, which eventually evolves into the atmosphere, which in turn would require a constant high dosage of NaCN to maintain set points. Economically, would it be a sound argument that the cost of the extra CaO to get a high pH would be offset by the decrease in NaCN usage (which is more costly)?

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

First step to minimize Cyanide usage is to run the [CN] at realistic PPM values. I still have to find a conventional gold CIL circuit needing more than 300 ppm [CN]. Running at elevated pH is costly and impacts negatively on your leach kinetics. Typical WA plants have high salinity water which makes high pH even more undesirable as it needs lots of lime to achieve higher pH levels. What [CN] ppm concentrations are you running at?

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

We aim for 250-300ppm depending on ore type. It's true that we would need a lot of lime to get pH upwards of 9.6 (approx. 6.5kg/t for our process), but I would like to think this would result in much lower cyanide dosages whilst maintaining the 250ppm in the tanks.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

Is the mine you are hoping to optimize in WA? If so, it has generally been found that you cannot offset the increased cost of cyanide consumption versus the lime cost of achieving a pH higher than 10! The hyper salinity buffers against increases in pH. I unfortunately don't have the papers to link to, but if you have a look in the literature there is a fair bit out there investigating this peculiarity. As far as I am aware there is no way of analyzing the two species separately outside of specialized labs, Paul Breuer is usually my first stop when researching cyanide analysis, worth looking up his papers to see if he has anything along these lines. I have found that my best results in optimization of cyanide come from close monitoring and stage addition, as well as a pragmatic approach to recovery.

OberstGruppen
1 year ago
OberstGruppen 1 year ago

Lime can seldom get you into the pH ranges needed for cyanidation or gold electrowinning, you should be using caustic soda (NaOH) for pH control

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

As far as I am aware, all WA operations have historically used lime for pH control in their CIL operations and don't see that changing anytime soon. It would be great to get access to the literature you mentioned, so could you please post the links when you get a chance?

Should have clarified too, the aim is to try and increase the pH from 9.2 to about 9.6-9.7. Would that be too high to start affecting other parameters like slurry density or DO levels?

Yes I think with better historical tracking I could probably be able to derive some kind of conclusion. Say, compare monthly consumption rates of lime vs. cyanide at desired pH over time? Obviously other factors like ore type and water quality might affect that.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

The primary article that I would use is by R Perry et al, "Low pH cyanidation of Gold" - if you look it up on Science Direct, you will see citing and reference articles which can lead you further. The cost studies are unfortunately not in the public domain, but reproducing them will provide you with an interesting exercise if you are looking for a short term project to finalize your graduate training program? NaOH and working out the economics of using NaOH if you can do some lab work. If this is possible I would also recommend investigating the effect of the two reagents on leach kinetics/extent (Ca has a role in gold leaching, have a peek in Marsden and House) and the silica in solution concentrations, together with the latter’s implications on downstream processing, particularly if you have carbon regeneration.

I have so far only seen one operation consider using NaOH for pH adjustment in cyanide leach, and that was in a very different high value application.

Your query regarding slurry density is, I think, really referring to slurry viscosity? Yes, the lime can have a significant impact here. Broadly speaking (I am emphatically not a specialist in this field) slurry will be most viscous at a pH when its particles are at their point of zero charge. Depending on your mineralogy, this could be anywhere on the pH spectrum – so the really annoying answer to your question is “it depends”. In oxide ores I have certainly seen a tendency for the viscosity to increase in a very pronounced fashion with increased pH, to be honest in sulphides I have never paid any attention to this.

DO is a function of a number of things including viscosity, as the latter increases you are going to struggle to maintain the DO. Highly unlikely but an increase in temperature of slurry will also drive down the DO, so if you are adding copious amounts of lime slurry at elevated temperature this may also not help your cause.

Rahil Khan
1 year ago
Rahil Khan 1 year ago

Adding onto the dialogue, excerpts from "Desperate times call for desperate measures-the use of sea water in mineral processing", Sustainable Mining Conference, WA, Aug 2010, are included below:

Kalgoorlie Consolidated Gold Mines (KCGM) operates the Kalgoorlie ‘Super Pit’, one of Australia’s largest gold mines. KCGM uses about 12 000 ML of water each year. Fresh water is piped to the mine, some 600 km from Perth, but the allocation is insignificant. So, KCGM uses hyper saline paleochannel waters, containing 30 to 200 g/L dissolved element. The paleochannel water has, in some cases, low pH down to three. The hypersaline water is used in the plants and also to transport tailing slurries (URL-1).

Perry et al (1999) reports that in gold processing when cyanidation is carried out in fresh water a pH of 10.5 is easily achieved and 95 per cent of the cyanide is present as the cyanide ion. When hypersaline water is used, in areas such as the Kalgoorlie Goldfields of Western Australia, magnesium in the water forms Mg(OH)2 buffering the pH and forcing plants to operate at pH values around 9.0, therefore more cyanide is present as hydrogen cyanide. Two problems associated with this are that hydrogen cyanide can be lost to the atmosphere increasing cyanide consumption and presenting a safety problem and that aqueous hydrogen cyanide may not be as effective in leaching gold as the cyanide ion.

It suggested that closed leach tanks can reduce cyanide consumption by minimizing HCN gas loss. They conclude that the leaching of gold in a closed vessel could be conducted at a low pH or even at the natural pH of the pulp. The natural pH of the pulp is dependent on the acidity of the process water and the acid generating or consuming power of the ore. The scope to reduce cyanide consumption by using closed tanks, especially when operating in saline water, as suggested by Costello et al (1991) depends on the ability of aqueous hydrogen cyanide to leach gold. A reduction in pH from 10.5 to 8 will reduce the cyanide ion concentration from 95 to

5.8 per cent. A 16-fold increase in cyanide usage would be required to maintain the leaching rate if aqueous hydrogen cyanide did not contribute.

Ore leached in high magnesium containing hypersaline process water showed increased gold dissolution in cyanide at low pH. Reducing the pH caused more cyanide to be consumed by the pulp but it did not significantly increase the amount lost to the atmosphere.

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

As is it mentioned the equilibrium between CN and HCN shows that CN and HCN at pH 9.4 is 50%. The standard procedure to measure free cyanide is at pH 12. Adding 3 drops of NaOH 0.1% using Rhodanine as indicator!

The equilibrium is reversible but some HCN is release from the solution which aided with the agitation. So, if the cyanide is titrated at operation pH (9.8 to 10.8) the cyanide concentration will be lower than the right value and we are pushed to add more cyanide to keep at the number we set.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

The extent of HCN measured by silver nitrate titration at the sample pH is dependent also on any buffering ions in the solution that can shift the equilibrium during the measurement. Titration pH also influences the extent of cyanide complexed with zinc being measured .

Likewise due to the equilibrium, the HCN may be available depending on buffering to partially replace losses in CN- (but also have some HCN losses). The CN- concentration is what is required for gold leaching and operations typically run with this in excess to oxygen, such that the leaching reaction is not cyanide ion limited. As stated above you need to evaluate and balance lime addition vs. cyanide to find the optimum. In hypersaline waters with Mg present the optimum tends to be lime addition up to the MgO ppt point around pH 9 and cyanide such that the gold leach rate is not cyanide ion limited (HCN + CN- typically 50-100% higher than the cyanide concentration used if leaching at higher pH).

Sandeep Bisht
1 year ago
Sandeep Bisht 1 year ago

Lime is cheaper than NaOH and can easily give you a high pH (up to 10.5). Check your leach profile to ensure that you are not over dosing CN.

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

Thanks for all your input, really appreciate you taking your time to help out. Will need to digest all that has been said and look into it a little further, and if I come up with a reproducible conclusion with my testwork, will be glad to report back.


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