I guess you are looking for the same concentration of cyanide in the tank 8 that the tank 1. While you increased the concentration of NaCN in tank 1 is necessary to find the right dosage for the process. Other important issues is the dosage of oxygen or peroxide dosing tanks, agitation, pH control, residence time in each tank, sampling points.

Usually in the last tanks NaCN concentration is less than the first tanks but should not be very high variability as is your case. 

Reading your post, it sounds like you are trying to deal with the symptom of a higher dose rate at tank#1 which I assume to be stage 1 of leach/cil train. I should think the first question you ought to be asking is that "is the current dose the optimum dose rate?" And one can always attain this via a series of bottle roll tests at various [NaCN] ppm with 300 ppm max. The reason you are considering circulating the exit stream of tank#8 which I also assume to be the tail tank of your leach/cil train could be twofold:

Compliance issues with the cyanide regulatory organs
Higher cyanide consumption and hoping to re-use the circulated ppm's
It could be that you have no efficient dispersion of air or oxygen and consequently poor usage of NaCN.

The discharge from tank #8 should go into a clarifier to separate solids and liquids. The clear liquids would be pumped back to tank #1 and the solids from the clarifier fed to a belt filter (preferably with a rinse to remove residual cyanide) or a plate-and-frame filter. The cake from the belt filter or plate-and-frame filter goes to tailings stacking and the liquids recycled.

Rather than add so much cyanide to tank #1, you would probably be better off adding smaller quantities of cyanide to the first 3 tanks to maintain the minimum required concentration of cyanide, and then allow the cyanide concentration to tail off to your last tank.

I want technique to recycle the cyanide from the tailing to use it again.

Instead of pumping the clarified cyanide solution to pulp your feed material in tank #1, you can use it in your milling circuit if you use an appropriate ball mill.

You mentioned valid points, 180ppm free cyanide in the tailings is double the normal. 100 maximum!

So look at why so much, seems the high cyanide concentration in your tailings is just a symptom of another issue.
Some places will install a thickener after the last tank to recover process water and reagents.

This water will be used for make-up to the mill etc. If you are still leaching in the last tank and have a low recovery rate then I would need further information to eliminate the problem, then all symptoms will disappear.

What do you normally do with your pulp from the tailings?
It is important to note whilst reducing cyanide concentration in the last tank through controlled dosing in tank 1 is a theoretical possible solution, there are some ores which a preg borrowing and this will make it impossible to operate at lower cyanide concentration in your leach train.

The other question to ask -is your entire solid sufficiently leached? If your tails solids are very low in gold (leached properly) you could possibly reduce the amount you use in the front tank as you have more than sufficient cyanide left over in the tail end. When the tails solids start to go up in gold value, increase the cyanide (a light dose), maybe in the middle of the circuit!

That’s what I was getting at, they should see all in the samples and Monthly GIC's and sampling! It seems a symptom of another issue.

True, but he hasn't been too forthcoming on that.
I have been in a situation where I have had to recycle the tails back through the leach tank due to extremely high Free Cyanide in the tails. But it also meant reducing the feed rate a tad as well into the leach tank to accommodate the tails. Feed was a concentrate of 12g/t so we eventually used up the cyanide.

Good time to blend some low grade into the circuit, a lot of licenses don't allow in excess of 100 ppm. It could be anything, from grades to grinds, maybe add a mixing box prior to tank addition, where cyanide is added can sometimes change things as well.

At a feed of 12glt do you have a gravity circuit before leaching? Just interest sake!

We did have a very large gravity circuit before the leach circuit as well as an ILR.

My friends depending on our ore and bottle roll test we get the optimum concentration of cyanide is 300 ppm to get the optimum recovery. But if we can recycle the cyanide from the tailing, we can recycle whatever we added to the first tank. So my question depending upon your experiences there's technology for that.

Look for Cyanisorb or AVR processes.
http://is.gd/ZrOGQx

Or you may read the SGS revue
http://is.gd/Ib3MpT

If the header tank concentration is fixed then the issue is to use the tailings water as make up water in the milling section. This will help you reduce dosage in the first tank (saving cyanide) and also help you save water, but it will be interesting to know what you are doing with your tailings at the moment.

You are one of the lucky people to have such high grade. I guess the initial feed before ILR was very high. And what kind of concentrators were you having and were you happy with their efficiency? Because really 12g/t puts a lot of pressure on the leaching section.

You should make some test in laboratory with battle roll test. Because of; some impurities affect your leach recovery at tailing solution. If it not affects the solution, you will use make up water in grinding area or CCD wash water.

What is your leach/cil DO & CN profile like? Do you have the CN speciation at the tail end? I understand that 180 ppm you made mention of is most likely titratable CN which could suggest a preg-borrowing issue because even your bottle roll test have suggested you cannot achieve optimum recovery at [NaCN] below 300 ppm. Have you perhaps considered the use of a high shear reactor such as AACHEN, they tend to do wonders with troubling ores and can reduce CN consumption significantly.

Use tailing thickener to recover this water in cyanide process water tank then pump it to the tank 01 again which will help you to decrease your cyanide addition in front of leaching circuit.

Also, you can decrease the target in the first tank to be less than 300 ppm but you should do some bottle roll test and re-leachable gold at the last tank.

I am a late contributor to this discussion but unless I have missed something he is only trying to reduce CN consumption and not necessarily improve Au extraction. Thus I can only suggest;

Reduce the CN concentration at Tank #1. If this affects Au extraction, which I think it will, then I suggest;

Installing a tailings thickener that will recover clear CN bearing liquor that can be recycled to the grinding circuit.

Unless the thickener is already in place then I suspect (dependent upon slurry flows and cyanide costs) my suggestion #2 would not be economic. I have operated Au circuit leach tailings thickeners and I have to say the gains from recycling the CN bearing thickener overflow is very hard to assess in real life (easy on paper). The most effective means to recover the CN is to use, as suggested above, the Acidification and volatilization approach but this would be very expensive and introduces its own OH&S problems.

So, I think that recycling is the approach least likely to be successful in reducing CN consumption. therefore the better approach is to improve leach conditions by improving the leach kinetics either by;

Size reduction but could be expensive better mixing with shear reactor, better leach tank design and better agitation possible Oxygen addition but this also contributes to CN consumption and contributes to operating costs.

Obviously, I haven't provided a full answer and probably have reinforced the need for more information/investigation.

Well you can use a thickener to wash and recycle the clear solution tank one solution which contains cyanide, followed you can filter the under flow of the thickener, the clear solution containing cyanide and recycle again to the tank 1.

I'd agree with you, reducing the cyanide feed is easier than recycling. The reason for using CIL/CIP is that sieve separation is much cheaper than filtration (as used in Merrill Crowe, often 2 stage filtration) or Thickening. Single stage thickening has only moderate success, so one probably needs counter current decantation, which as you says ads to the cost.

I'd suggest this ONLY if you can incorporate the cost into a paste tailings disposal system. Two questions remain as to why and where the cyanide is being used. If 300ppm is ideal then I'd suggest changing single point addition to at least 3 stages. 150ppm in first tank 75ppm additional to second tank and a further 40ppm to the centre tank, (tank 3 if 6 in a row). Try this in bottle roll or stirred tank lab test before testing on full plant. This way you may find that 265ppm does the work of 300 with an equivalent reduction in tailings cyanide level.

If I recall aright Ma'aden has a high copper content in the ore? If true then it may be that copper is leaching in the cyanide, causing high cyanide reading of "inactive cyanide". If so you may want to do either a low cyanide wash ahead of leaching (recycle "cyanide" is if good for this), or simply a weak cyanide mix. You can then discard this OR use SART to recover the copper and free the cyanide for reuse.

The answer will be in the test work, all ores differ so the Laboratory is the key to finding the right adaption for your ore.

Good point that is exactly what I recommended. Several smaller additions of NaCN into the first 3 tanks is my standard, and it helps reduce wasting NaCN dissolving copper too because low concentrations of CN- over longer periods of time favors dissolution of gold and silver over copper.

When I was in fourth year, i did the final year project at Barrick which have the same problem as yours.

Title: cyanide time leach test work.

The experiment was performed at 3 level
250, 270, and 300 ppm of CN, at leaching time of 18, 20, and 24hrs.

Therefore, i observe that the problem was on pH controlled, which results into fractuation of cyanide remain un-dissolved in the tailing.

Recommendation: 250 ppm of CN is enough to get at least 89% Au recovery at retention time of 20hrs. If and only the pH is maintained at 10.5 throughout the process.

One of the ways to recycle the cyanide is to thicken the tailings and return the cyanide bearing solution at around 180ppm in your case. This cannot be fed to the no1 tank as dilution will occur - you will have to check the water balance. Alternatively the water can be returned as dilution water to the milling circuit but this will mean that you are milling and cyanide with partial gold dissolution -some mines do this but you will have a large inventory of dissolved gold in the thickeners if you use them. The downside is that any overflow or a positive water balance will result in gold losses.

I am assuming the CN levels are free or titratable (silver nitrate) levels. Trying to recycle will be far more complicated and costly than the returns would provide. I concur with them who questions the high CN levels in the circuit; i am used to below 100 but would not want to be below 80ppm (ore dependent!). The easiest means of reducing cyanide consumption is to undergo a plant programme of slowly reducing the Tank #1 levels and monitor the levels in Tank #8 as well as the all-important gold extraction. You may find that by dropping levels in Tank #1 to, say, 100ppm, and maintaining at 100ppm for the next two or three tanks the extractions will remain high, the final tank concentrations will drop to well below 100ppm and cyanide consumption will also drop.

I would suggest to you to follow what has said about splitting the cyanide addition on the 3 first CIL tanks. If the goal is not reached, I suggest to you to start looking at the thickening option to remove the excess liquor (test work to be done in order to see how much water you can remove) and recycle it directly to the mill feed. Filtration can be a second choice (depending on the test work results). A trade-off can be done to compare between filtration and thickening (OPEX, CAPEX)