Can you provide a summary of how the DO test was conducted? I know a lot of factors affect DO in leaching tanks.

You raised a valid query on DO meter calibration.
Soluble sulfides are cyanimides’ as well as O2 consumers; Pb(NO3)2 + S2- = PbS + 2NO3-; S2- + CN- = SCN-; S2- + 2O2 = SO42-

Should you have significant levels of soluble sulfides in your slurry, you might see an improvement in soluble DO & free cyanide levels following Pb+2 addition; if you are adding large quantities of the Pb salt, you might observe a slight reduction in pH as the Pb salt is the product of a weak base-strong acid.

What is the pH of the basic slurry (lime added prior to the Pb salt) & how much Pb (NO3)2 are you adding in lb/st ore?

The measurements were taken lie we do every day in knowing the Do levels in tanks. I looked at the chemical equation of the reaction and I think I found something interesting. For the drop in pH, it was 11.58 prior to the lead nitrate addition and per the quantity dropped to as low as 4.72

Is the pH measurement in: a lab beaker or in the circuit where the Pb+2 solutions are meeting the slurry as a point source and mixing has not occurred or further away in the circuit following reaction of the Pb w/S? It is unfathomable to witness such low pH under scenario unless you were adding copious quantities of concentrated acid into the system. A word of caution: be very aware of HCN evolution.

You have not responded to the quantity of Pb(NO3)2 added or the DO levels in your tanks (with & w/o Pb+2 addition); assuming this was a bench scale test in the lab & if you'd like corroboration of your dosages, send me your Pb add calculations (Slurry wt, wt% solids, Pb(NO3)2 solution conc. RXN time etc.).

Among reagents that improve cyanidation, oxygen and lead nitrate are known to enhance gold recovery and lower cyanide consumption, especially in the presence of sulphide minerals. Cyanidation in some plants indicated that the level of lead nitrate addition varies from one location to another. There is no obvious relation between the composition of the ore and the addition rate. It was found that lead nitrate is detrimental to the process when not added properly it can inhibit the kinetics of dissolution of gold and increase the cyanide consumption. Research on the electrochemical aspects of lead nitrate suggests that its addition increases the dissolution kinetics of gold, even in the absence of sulphides.

The benefits of using lead nitrate in the treatment of a gold ore containing no sulphide and has also been reported that its influence was especially noticeable at low cyanide levels and that there was no advantage at high cyanide addition rates. These conclusions are contradicted by other publications. To understand the effect of lead nitrate in presence of different sulphide minerals at various addition levels! Work was carried out on synthetic ores containing high purity pyrite, pyrrhotite and chalcopyrite and on two gold ores currently processed. The two gold deposits selected had low sulphide content with an average gold content and a high sulphide with a low gold content, respectively.

Lead nitrate has a different effect than lime addition. It was that increasing the lead nitrate concentration initially increased cyanide consumption from 0–100 grt lead nitrate. And then reduce it back to 1.13 kgrt at 800 grt lead nitrate. Cyanide consumption caused by the formation of thiocyanates was thus about 30% and copper was the main cyanide consumer. Since copper dissolution increased slightly with lead nitrate 50 ppm. It means that lead nitrate attenuated cyanide consumption, but its action was only related to iron sulphides passivation. It was found in Part 1 of this report that lead nitrate was ineffective in decreasing copper dissolution and had no significant effect on iron sulphide minerals. That lead nitrate works on pyrite by decreasing cyanide consumption. The difference in the nature of the results may be related to the amount of pyrite, the length of the pre-leaching, and the amount of lead nitrate added.

The gold content in the leach residue was higher by about 0.15 grt in absence of lead nitrate. This value does not seem to be very high. However, it represents a substantial amount of money on an annual basis. As an example, a plant processing 300 000 t/year would have an additional revenue of CAN$675000 by getting an extra recovery of 0.15 grt Au.

That is excellent. Now it means that sometimes the nitrate can consume cyanide and other times not. But all depending on the conditions that prevails at the time or is it that it comes down to the ore type being treated in the plant. Also can it be that the concentration of the lead nitrate should have an optimum?

With the test work, i did not measure anything, just wanted to know if the nitrate could help in the DOs. But i will do another test work to really know why the pH dropped massively. 

If they show me the Lix-Merril L circuit and the point of adding, i can help.

To set the record straight, there are no 'redox' effects occurring w/Pb(NO3)2 or the nitrate anion at basic cyanidation pH as 'claimed' by some respondents. Solubilized sulfide species are precipitated by Pb+2 as PbS, alleviating cyanide loss as thiocyanate; slurry DO levels are improved because of sulfide removal w/Pb+2 (see reactions in an earlier communication); investigate redox potentials for nitrate, Pb, sulfides, cyanide to cyanate etc before you jump to conclusions based on word of mouth alone. Oxidative properties of nitrates such as ammonium nitrate are generally temperature induced and under combustible situations the ammonium ion is used to complex base metal species such as Cu, Zn, Co etc. I'd strongly recommend accessing legitimate publications, itemizing issues that might encounter favourable results w/Pb(NO3)2, developing a testing matrix & subsequently performing the experimentation otherwise your haphazard efforts may lead to nothing. Some publications below for direction:

http://is.gd/7lHqMv
http://is.gd/ouo4ON

Published Ksp of sulfides are in DI water at 25C solubility’s increase under alkaline pH, aeration & cyanide matrices; cyanide slurries are buffered w/several 1000-ppm of lime. H+ or acid formation w/low to medium Pb(NO3)2 (salt of a weak base-strong acid) additions (50-200-g/t ore) undergo neutralization under the basic conditions; if the pH of the mixed slurry were to fall below 9.3, 50% of the cyanide would be lost with exponential losses at lower pH so NO Au dissolution.

I think you said same points to escalate the subject at hand. Nevertheless, we have learnt a lot from this discussion. I will do all the recommended test works to know what really lead to the pH drop and also the increased in the DOs. But all submissions are well appreciated.

Thanks a lot. But if anyone has some publications, they can send to me to help me out with this subject.