You can try multipoint cyanide control suggest tank 1,3 and 5 but remember you do need residual cyanide to not influence the gold loading kinetics in the system, tails cyanide levels of 100 is not bad. Do you have copper in the ore?

110 ppm free cyanide in the tails for free milling ore is not good. One should expect 50-60ppm.You will have to try to keep the other leach parameters constant and reduce cyanide dosage whilst monitoring the recovery to find the optimum dosage. There will not be a given dosage to satisfy your request because your ore is unique and will have to establish the optimum yourself. Also, please bear in mind that high DO level will allow to reduce your cyanide requirement as some of the cyanide consumers will be oxidized.

A good idea is that you can build a curve Leaching Kinetics Vs Leaching Assays. According to this you can modify parameters cyanide concentration. Remember that if you do this round of testing, the parameters should be similar in both cases.

There is insignificant copper and silver in the ore. Yes I think the stage addition the possible option to reduce the cyanide consumption close to the limit. In the process of running cyanide optimization we have reduced from 190 to 165 ppm without identified any gold recovery reduction (baselines daily leach profile on solid, solution and carbon on each leach tank). But compared to the reference, "the advance in gold ore processing" G. Deschenes pages, oxide should be able to run at concentration 90 - 150 ppm. I think as your advice, there is still plenty of space to go.

This issue might be complex: technical and economic considerations may be needed for a good solution. In lots of times this is a problem of short residence time into the tanks, then we need more cyanide concentration to increase the leach recovery since in some conditions It’s actually impossible to extend the leach tanks volumes or the pulp density (for other relating issues).

In this case the problem becomes more economic because this high cyanide concentration requires a high detox reagents (hydrogen peroxide, H2SO4, SMBS, Copper sulfate) concentration.

High Do level can be the solution Yes, but still it depends and only test works and cyanide control suggest tank can lead for an optimum dosage.

If there is no re-leachable gold at the end of circuit you can drop the cyanide target in the first tank but if you still have re-leachable gold you have to investigate why this not leaching . You may need to check the pH if it is suitable for your circuit or need to adjust it or may be something else.

Carry out two daily bottle roll tests on your tails. One with no cn added and one with +50ppm and see if there is any benefit in having more cyanide. Track this on a daily spreadsheet for life of mine and calculate the additional gold recovery versus cost of cyanide.

When there is no benefit, decrease slightly (10ppm)
When the benefits outweigh the cost, increase
When it is neither benefit or dis-benefit keep it stable.

Average results over a couple of days and look for trends rather than responding to spikes unless they are dramatic.

Judging from the ore being treated, carry out bottle roll or leach test on the leach feed, optimizing the cyanide concentration (dropping it from 165) using the plant's residence time or varying that and measuring each CN concentration used, pH used against the level of recovery and residual cyanide, that should give a good head start although certain factors can also be considered.

As he says if you’re CN at the CIP tails falls to under 30ppm gold will re-adsorb on the solids and be lost so 50ppm makes a safe discharge limit.

As says all ores differ only you can do the test work to see what the kinetics is for your ore so do a few bottle roll tests and see what suits your ore.

Yes, cost and benefit analysis must be used. I just reconsider the plant is dynamic, adjustment have to be made periodically based on the historian data and test work.

I think 50 ppm in the last leach tank will be good for the contingency in the last leach tank.

The previous suggestions are relevant. The free cyanide concentration depends of the size of gold grains in the ore. If there is a little bit of sulphides, it may preg-rob gold if the free cyanide is too low. Determine the effect of free cyanide on the leaching profile and retention time at the lab scale. You may also reduce cyanide concentration by increasing the DO.

Best option is installing multi tank control can tone it down to any set-point and have full control, not only control on head value. Just check your copper else you may be surprised.

Based on my experience with relatively clean ores, you can run the front end of the CIL circuit in the 50 mg/L to 100 mg/L free cyanide range. Having an on stream cyanide analyzer and control system allows you to operate at the lower end of this range.

The Leaching tank#1 free cyanide range is about 75ppm and adds to 75ppm adsorption tank#3 or #4 if required. If the CIL tails is contain less than 50ppm free CN, it would be easily detoxification for WAD CN. Those all for oxidized ore!

From my experience you can drop cyanide dosage in free milling ores considerably, but you will need to account for a range of factors - leachable gold in the tails, DO and pH profile, residence times, cost of detox, and maybe some others. Automated cyanide dosing is always a good idea; it saves on average about 20% of cyanide. You can contact me, if you need further information on cyanide analysers, both free and WAD. Staged addition will also further reduce cyanide consumption, especially if the DO is lower in the first tank because of oxygen demand of the ore.

You already have some knowledgeable comments from some experienced operators, most of which I concur with. If I can add the following:

It's often best to be conservative with your cyanide set point, to allow for the possibility of disturbances to your leach conditions or feed presented to the leach. It is very easy to lose small (or large) amounts of additional gold to tail, some of which may not be measurable due to the typically high variations in tails assays. A bit of extra cyanide is cheap, when compared to the potential for gold losses.

The exact optimisation of cyanide levels is challenging because the sampling statistics usually dictate a requirement for a large number of data points to make confident comparisons of small variations in recovery between test conditions. In addition, it is easy to lose partially leached larger gold particles to tail without them even being detected, especially if there is no gravity circuit. In the event that this happens, they appear as minor 'spikes' occurring randomly in your plant tails. Yet, these are very real losses of gold. You must be very careful to evaluate your data correctly in this respect, and often it's a good idea to do some vario-graphie to check for this possibility. The feed grade/ recovery profile for each major ore type must also be taken into account in this evaluation.

Experimental options for consideration include randomised paired trials at different cyanide levels, taking additional samples for supporting leach work in the lab, or just using your operational data - but generally more data will allow you to achieve outcomes faster.

Note that it is best to also optimise your oxygen, density and pH in parallel with efforts to optimise cyanide.

All of the above takes time and effort, and is usually considered an ongoing effort through life of mine with varying degrees of intensity. It is a good idea to implement ongoing supplementary monitoring of the leach tails in addition to the routine operational samples so that data can be readily available at any time that the recoveries come into question.

The optimum cyanide concentration is best determined by test work. This should also be done keeping a close eye on ore variation and factors that enhance leach kinetics.

You may also want to look at the way you add Cyanide. In some cases the unit operation side of the system affect more than the unit process. More kinetics issue than stoichiometric. Do we add the cyanide to tank effectively? Just a thought!

No other way than doing a bottle roll test as the ores are different run it at different CN conc. with the same plant conditions, residence time, check cyanide conc. and recovery at the end of the test there are some ores which are preg borrowing, where at very low cyanide levels gold re adsorb temporal on the ore and lost in the solid tails.

The suggestions of stage addition, daily bottle roll tests and a residual free cyanide tail of 50 ppm are in line with standard practice and will guide you in the right direction. I am happy to hear that there are not copper minerals or other cyanide consumers. But you have not told us what your current gold recovery is. It should be in the mid or high 90 percent level. If not then other factors such as grind size and gold particle size may well play a part.

You are very fortunate to have such a friendly ore. There are obviously no cyanide consumers in your feed (yet, anyway). If you put an online cyanide analyser and controller you could control the levels efficiently. I don't understand why you would want to consider stage addition if you are not currently seeing a drop off in cyanide of any significance in the leach train. Keep reducing the cyanide level in the first tank until you get around 60 ppm in the last one and that should be sufficient. You do not mention recovery but I would imagine for the type of ore I suspect you have it would be +95% and probably higher and should this be the case I believe you have sufficient cyanide in the system. Keep an eye on recovery and oxygen levels.

Bear in mind the ore may change if you are in an open pit and the mine gets deeper so you may need to modify your addition of reagent at that point. If it is primary ore now then there may be some cyanide consumers come up from time to time.

Based on experience, it came to know that the cyanide can be maintained in the last leach tank. However this can be further brought down by on stream cyanide & control system.

First I must say you are one of the fortunate people to still be treating such forgiving ore. The cost of cyanide prompts for urgent and efficient ways to significantly bring down its consumption without the compromise of recovery which you haven't shared. There is a unit in use in many plants around the world called AACHEN reactor which ensures efficient transfers of oxygen, higher gold recovery and low cyanide consumption.