If i understood your statement well, you are saying your gold dissolution from the ore is actually sufficient, that’s why your gold conc. increases with time hence your oxygen rate is also sufficient since it’s directly proportional to dissolution. Now the only thing that can cause your curve to flatten (clearly showing that your dissolution reaction is being retarded) can be gold precipitation out of solution just like you figured. I think what you need to do is pay attention to other minerals present in your ore which by my understanding the mineral putting gold out of solution could be zinc. If zinc is therefore present, you need to prevent its room for precipitation by altering your cyanide concentration as well as your gold particle size which will lead to fast dissolution thereby reducing your residence time and not giving zinc a chance to act.

There are a number of things that could cause this behaviour:

Very high gold grades in solution - above 4000ppm. It is possible to precipitate sodiumaurocyanide as a salt - NaAu(CN)2 at high levels of gold and sodium in solution. This is made worse by high cyanide and caustic levels. The remedy is to increase the solution volume and/or reduce the caustic addition.

There might be preg robbing carbon in the concentrate. It is only likely if your main ore is highly preg robbing and the effect on the concentrate is usually much less than with the ore because the gravity concentrator will usually reject light carbon species. Also high cyanide reduces the loading efficiency of carbon.

Gold can be loosely absorbed onto some minerals, usually clays. This is a type of weak preg robbing. Weakly absorbed gold can be washed off the solids in the subsequent wash stage. In an ILR the washing is very thorough due to the positive mixing of the solids; in an Acacia there is no positive mixing so you are relying on getting an even distribution of solution through the fixed solids bed. Comparing wash and preg solution grades and volumes will identify if this is happening.

In some cases you can get gold plating out on steel scats. With an Acacia the solution is passed through a fixed bed of solids and you can get uneven distribution resulting in zones where the reagents are depleted; this is a bigger problem for larger units. In an ILR you can get the same effect only if the drum is stopped for a long time (due to a power outage, for example) but it normally re-dissolves quickly once the drum restarts.
I should declare that I am Manager Chemical Technology and Detox at Gekko Systems, and have been responsible for much of the development of the ILR.

What are your assay values of the residue after 8 hours? Is the gold content still high? The problem could be your residence is too much. If your ore contains preg robbing carbonaceous material, your leached gold might be reabsorbing onto your ore.

I think sampling for your profile at such huge time interval of 4 hours will not help to identify the problem popping out. I think you should start sampling at one hour intervals and maximum 2 hours to really ascertain the happenings in the reactor and solving it going forward.

I call this a Nano hydroxyl effect in solution, which is a in and out effect, in effect your solution holds Nano clusters of gold In the Nano range of 1x 10 minus 6 to 1 x10 minus 9th power This hextrogensis solute phase is undetectable at times sense the fact all the gold Is not being released from the clays which are primarily silica which is a good for gold you can’t see at 35 microns this regresses to gold at 7.0 micron you have to have a solution to release this small particles of gold at this physical state. example of this is gold braze used in the aircraft industry the gold basement particle size is powdered Au to be placed on Rolls Royce RB211 blades .Titanium blades the Au is at cosmetic fine but with proper calculations all gold Is removed by electro plating at 140 volts pushing 90 amps collecting lbs of gold from a KCN solution. It’s the particle size that s giving you trouble.

Potential preg-robbers first concern. Also as mentioned the high head grade, if this is due to an abundance of large particles you are not going to recover it quickly. This needs to be understood in your ore characteristics. Do you do any pre-concentration to recover free gold? Is it necessary? Etc.

In addition to potential presence of preg robbers you may also need to check your pH and cyanide strength as your leaching progresses to make sure conditions are favourable for gold ions to remain in solution as leach cycle progresses. Some acidic minerals are slow leaching but will ultimately impact your pH and gold complex stability.

If you have capacity to hold solution and electrowin you may want to try a two stage leach cycle. Transfer the high grade after the first 6 hours then do a low cyanide strength leach for say 8 hours. You can then compare overall dissolution against a one stage 14 hour leach.

Like he said its testing with pre bench table top testing, if you don't do this for maximization of removal you have not achieve 100% recovery time, temperature, and most if all pre cleaning for to get the majority of light particles is critical point and it’s not being taught in the universities at this time. This starts at 45 microns and regresses over into where most of your gold is that is at 35 microns collected in the clays.

The reasons behind the observations in your intensive cyanidation circuit can be one of a number. In order to more clearly diagnose your challenge a bit more information will be required:

•Do you have cyanide, gold, copper, sulphide concentration values? (I assume that you were talking about a solution profile).

•Do you have organic carbons in your solids? This can lead to preg-robbing.

•Sulphides in the solids can lead to preg-borrowing. How much sulphide do you have in the solids? Do you know the sulphide type?

•Some base metals such as zinc can precipitate the gold at low free cyanide tenors. Do you know how much zinc is in your solids?

This information will assist us to give you better advice.

You are right the free gold occurrence creates a situation where you are basically polishing the gold, because in order for it to be dissolved the residence time would go on and on. Pre-concentration would be vital if this is the case. Not only that but if free gold is available you should remove it early as a management practice and to improve recoveries.

How is the pre-concentration going to be done, because I know its feeds on Knelsons or falcons concentrate. Also i believe the input from him is the key. You have to clarify at high ppm of Au and then continue the leaching for a significant time like the 8 hours or even for 6 hours, this will help nullify the effects of potential preg-robbers. And even the downwards drop in the dissolved gold concentration.

Your response is overwhelming! I will draw from all your recommendations and find a suitable solution!

You have to see beyond 35 microns this is where the silica holds in the gold particles at 35 microns that which you can’t see beach Procedures followed by proper instrumentation application should solve your problem.

To answer your questions about the pre-concentration of ores, you are right about using a Knelson or Falcon.

Test work should be carried out to define the ore characteristics, particulate sizing etc. many points of interest will arise from this, such what fraction free gold is most abundant, recovery dynamics and the like. For example in some cases the finer gold may be lost in some cases because of its natural occurrence such as is it nuggets, flake etc. if it is not set correctly.

Also of note and something to look at is the concentration of material to be treated. In some cases, the ore can be pre-concentrated for treatment in terms of separating waste material from actual gold bearing material to minimise the costs and process requirements. For example a standard CIP/CIL circuit and also some acacia circuits treat all material fed through the plant after comminution, we aim to remove none gold bearing material to minimise the plant footprint and in doing so cut the operating costs of doing so. So instead of treating say 100 tons of ore you only need to treat say 10 tons for arguments sake. So you have saved on your cost of reagents required already. There are many ways to approach this.

Always remember the ore dictates the terms and that needs to be understood. The approach can be an astronomical saving and value addition to the process if done correctly with solid understanding of the ore characteristics.

Its people like you who see the preliminary application in reference to metallics you can see starting at 300 microns to 100 microns to that of s human hair at 75 microns all metallic d degreasing to that which you cannot see at 45 microns now at this point I can truthfully say this is a MAJORITY of the gold and other precious metals are at 35 microns. THIS IS WHERE MAJORITY OF ALL GOLD is.

Have you checked the dissolved oxygen levels (DO). I have experienced in some cases when high the grade gold solutions are in the presence of steel (ball scats and chips collected by the gravity circuit) then the gold can precipitate onto the steel. This occurs in a low oxygen environment. You may have to throw in more Leachwell (at a cost) or add oxygen. Just another thing to consider on top of all the other useful advice provided already.

It would help us get your situation in perspective if you could let us know:

Maximum gold concentration in solution (ppm)
Cyanide initial addition (kg/t concentrate)
Leach accelerant addition (kg/t concentrate)

I think this has an interesting submission there, let’s know those parameters and see. Also it seems we have similar problem with ours and this discussion has called for more investigation and attention into it. The main reasons should be laid bare for some of us who are trying to understand the whole process.

Our parameters are as below:

Max gold concentrations we have seen are 1500 ppm and the latest drop in concentration that we have experienced was from 1090 ppm after the fourth hour to 680 ppm after the 14th hour.
The initial cyanide concentration for our reactor is in the range of 50 to 60 kg/t concentrate (cyanide addition is fixed while concentrate can vary between 6 to 8 tons).
Leach aid is in the range of 0.8 to 1.3 kg/t concentrate.

In our solution profile we check for gold, copper and cyanide only. With the batches in question, the cyanide at the end of leach would have been fully depleted (from 2.5 % to only 0.04%) suggesting that something is definitely consuming the cyanide afterwards. The copper flattens out pretty quickly (after 4 to 6 hours) and the maximum concentration we have seen is 50 ppm. We are currently not analyzing for zinc and definitely will lobby for that.

I terms of sulphides, the ore is a mixture of pyrite and pyrrhotite with sulphide sulphur content in the range of 2 to 4%. Organic carbons are well below 0.05% and highly unlikely to remain in the concentrate after the Knelsons.

We are not currently checking for DO in the reactor but that is something to look at also.

If gold precipitation is an issue and leaching ceases at 6-8 hrs then consider reducing your leach time to that 6-8 hrs. You can then look at the issue in greater detail. Is it possible that is all you need to leach your gold to or is overall gold recovery at that point too low?

I have heard of gold re-precipitation as cyanide (and DO) levels fall to very low levels so you may have to consider stage addition of reagents.

You are running out of cyanide in your leach. You definitely need to increase your cyanide addition. Try to get about 1% NaCN remaining at the end of the leach. (0.04% is too low with reactive minerals present.) .

The most likely consumer is pyrrhotite which means you probably also needs to increase leach aid to satisfy the oxygen demand. Getting the leach aid addition right could be tricky since major overdosing will affect the electrowinning and it sounds like your mineralogy and concentrate mass is variable.

You need to keep the leach conditions aggressive throughout the leach. Remember intensive leaching is using a sledgehammer to crack a nut.

Looking at the nature of the circuit and the purpose for its existence, it will be a violation to leach to a time and stop and use another reagent again. Also the idea of stage addition of cyanide, is it feasible and will top management accept that practice. Can you be specific on the leach aid that can affect electrowining when overdosed? I know of caustic but i cannot tell of any other.

Please check for Arsenic as well in your material, DO is also vital!

What role is Arsenic likely to play? Cyanide consumption or Gold precipitation?

Anyone who have any project in site of CIL, Crusher or grinding or detox or etc. concerning metallurgy at its procedure?

Arsenic does not form stable complexes with cyanide, and consequently the presence of cyanide in solution does not appreciably affect the stability of the metal species formed. Under the conditions applied for gold leaching, arsenic sulphides decompose to arsenate and arsenate, with a proportion of each depending on the solution composition and pH.

Dissolution of these minerals has a detrimental effect on gold (and silver) extraction and in some cases this effect is severe. This effect is thought to be due to the formation of a passivating layer of arsenic on the gold surface. Decomposition of these minerals is strongly related to pH, with their solubility increasing with increasing pH.

Antimony also has a similar effect, and because by nature it generally occurs in pockets you may find that antimony in your ore is the problem periodically. There is a lot to think about and look for.

Further to your comments, I understand that leaching the gravity gold concentrates containing +10% arsenic and they prevent soluble arsenic from entering the EWC solutions by simply adding lime into the leach reactor, which precipitates the arsenic. This also negates the effect on the gold leaching kinetics. Maybe someone from that site can confirm?

A big excess of Leach aid or leachwell in solution at the end of the leach can delay electrowinning. Essentially it needs to be reduced at the cathode before gold will plate. In the ILR we generally only use leachaid as an initial boost or if oxygen or peroxide is not available so we rarely encounter this. I think the main issue for the Acacia is when the amount of gold or sulphides in the concentrate varies widely day to day.

I think your first action must be to increase cyanide addition until you have significant cyanide at the end of the leach. Then increase leachaid as necessary. One problem with using leachaid rather than oxygen or peroxide is that DO is not a good indicator of activity. I'm not sure if there is an effective way to measure the remaining leachaid in solution. .

If As is soluble - then during leach it can result in a large oxygen demand due to the oxidation of As to Arsenite(AsIII) and Arsenate(AsV). High levels of Arsenic in solution can also affect electrowinning. By adding lime in the ILR we precipitate most of the As Calcium Arsenite (AsIII) which reduces the oxygen demand significantly as well as removing the problem in electrowinning. As solubility is increased with high caustic levels so the idea is to avoid adding any caustic. Adding lime in the Acacia reactor is not possible.

Antimony if present is more difficult. Generally you need to keep the pH as low as possible to minimise solubility. 

My comment relates to whether you have completed a leach cycle and have achieved the maximum recovery at the earlier stage. If that were the case then it would make sense to cease leaching at this time and hence not have to suffer the re-precipitation problem, i.e. get maximum gold as early as you can. If you haven't completely leached the gold then you obviously need to address the precipitation issue. That may require some staged addition of your reagents. Not knowing your particular circuit I'm unaware as to why you add more reagents later after the intensive cyanidation (which I assume is for your gravity concentrate) unless you mean adding the reagents into the CIP process, which is a separate issue.

So it’s only gold dissolution that is affected by arsenic? Surely the presence of high dose of caustic in the ILR should take care of that. And I think the manufacturers of the technology took that into perspective. Nevertheless, it is relevant that in such a circuit, we pay good attention to these arsenic and antimony. You talked about the antimony as well, may i know which of the two poses severe effects on the circuit output. Also will increasing pH also help curb the effects of the antimony?

I just asked a friend who used to work in and he also confirmed lime was used to that effect. My big question is what is the leachaid you are talking about? Also what will too much cyanide have in the EW, caustic and peroxide as well?

Caustic is the main reagent for pH control and aids stripping in electrowining, how then must it be removed completely from the circuit, mind you, the circuit installed in my plant does not use lime but caustics.

If you can access this paper it may help. It details a study by CANMET on this problem http://is.gd/j9aLd5

It turns out that one of the major contributors was temperature which most likely killed the leachaid towards the end. We were leaching at 55 degrees and dropped to 50 degrees and saw a huge improvement.

In addition as per recommendations, cyanide was periodically increased to maintain above 1% concentration at the end of leach. Leaching time was further cut to 10 hours so everything seems to be back to normal for now.