Carbon In Leach done. clarifications need

Aeon13

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Good day to all the members of this helpful forum.

I would like to thank this forum and the members for the valuable knowledge that is being shared here.

I learned so much already and I am still learning so much every time I come and read.

I have come again to seek knowledge from you guys. I apologize for this will be a long post because I will try to explain every detail for you guys to understand.

Lately, I tried the Carbon In Leach process to recover gold from very finely ground ores.

Let me explain why these ores are very fine. In the process of milling, the ores here are ground for about 100 microns. It is done by milling it in a rod mill with water for more than an hour. When the miners here take the ground ore out, it goes to container but the water with it goes to another.

The water has with it the very fine ores and after some time they will settle and these are the ones that I bought to process it in a Carbon In Leach tank.

The CIL tanks here I suppose are not industry standards because they were build by small scale miners by the help of someone who has knowledge.

This is how the CIL process works here:
1. we put the ore with water and quicklime in the tank. The tank is mechanically agitated with air purged inside using air compressor.

2. we let the lime, ore and water mix for about 4 hours without cyanide for what they say as "oxidation".I don't know if this is really required but I was not able to read anything about this step. Still I followed what they say.

3. we add cyanide and let the mix run for about 48 hours. Then add again additional cyanide after 48 hours.I think we are putting so much cyanide here because they are not titrating. I tried to titrate using the method I read here and If I am correct the concentration was 3 g/L NaCN.

4. then activated carbon is added and let it mix for about 48 hours also.

5. the tank will be released and the carbon will be recovered using a screen.

6. carbon elution is not used here. Instead, ashing is the processed used. the carbon will be ashed which will take about 12 hours or more.

7. the ash will then be mixed with borax and will be smelted using a torch (compressed air- LPG set up).

8. the metals will all combine in a button and will be refined further for the pure gold.

I decided to read about the CIL process here in the forum as well as the internet because I was not contented with the explanations and the steps in the process done here. I started reading about cyanide concentrations, titration, activated carbon loading,leach time,proper pH, oxygen concentration ,size of ground ore,temperature and much more. I understood these are the factors for a successful leach.

These are the ideas which I understood with my readings and I am adding some questions about them for me to have a much clearer understanding.

1. The ores are very fine that they flow with water and has a consistency of a mud when concentrated, Is it better this way for the cyanide to liberate gold particles?

2. I read that the pH must be in the range of 10.5 - 12. Is this the proper range of pH of the solution with cyanide already mixed? Adding lime or caustic soda will increase the pH, what if I added too much and had the pH higher than I want, how can I decrease the ph?

3. I did titrate the solution but this is my first time and I am not that sure of my readings. I followed the titration using silver nitrate and potassium iodide as an indicator. I understood the unit will be g/L. That is grams per Liter of the whole solution(whole CIL tank?)?

I read somewhere that the concentration used is 0.05 to 1.25 g/L cyanide. My reading here is 3 g/L. this is too much but is there an effect on the leaching?

4. After cyanide leaching, carbon is added. Is it safe to say that there are no gold/silver remaining if the concentration of free cyanide is not changing? Adding carbon will not affect the free cyanide concentration right?

5. Is there any other test in the solution if gold is still present except for stannous test?

6. I find ashing very lengthy and I feel that small gold particles might fly with the air blown. Do you think elution using hydrochloric acid and precipitating with zinc dust is much better? ( I have not read about this HCL elution thoroughly but I am on my way)

7. I read about proper oxygen concentration and how important oxygen in the reaction of leaching. I also searched on how to measure oxygen concentration using titration but I am having a difficult time finding these chemicals that is required, do you have any other options on measuring dissolved oxygen or is it safe to say that the air purged by the compressor is enough supply?

8. I read that many CIL processes use 50% solid slurry density. How can I practically measure this state of density? (I don't have any instruments for this measurement) What will happen if I added too much liquid and made it 30-40% solid?

Any answers/suggestions/recommendations will be greatly appreciated.

I believe that many heads are better than one or two. This is the reason for me asking questions here because I know that many of the members have ideas about any questions herein.

Again thank you so much and more power to the members as well as the forum.
 

Deano

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What you are running is not a CIL process but more of a carbon after leach process similar to a CIP adsorption circuit.

Many of the Philippine ores contain appreciable quantities of sulfides. These sulfides will slime during fine milling and so will be concentrated in the finer fraction you are using.

Unless bottle roll tests compared with aqua regia digests are carried out you do not know what the starting gold grade is and therefore you will have no idea what sort of recovery you are getting with cyanide.

These bottle roll tests will also tell you the required leaching time and the required cyanide level.

Even more important you will have no idea what level of gold is not being leached but which is possible to leach with various process options.

If you are buying these slimes for processing it is vital that you have someone carry out these tests to know what you are actually getting for your money and how you should be processing them.

It is always a matter of luck whether a particular sulfide ore will leach in cyanide, generally if your sample will leach then other samples from the same orebody will also leach.

Unless your ore is a cyanicide (cyanide destroyer) type the standard industry starting concentration is 500 ppm cyanide which approximately equals 1000 ppm sodium cyanide or 1 gram per litre.

If you know your tank volume you can weigh out the cyanide addition at the rate of a kilo of cyanide per cubic metre of water.

Your step 2 may be an attempt to put a passivating layer on the sulfide particles if they do actually act as cyanicides but it seems a bit of a stretch.

Re your questions

1 In slimes leaching the leaching performance is best if the pulp density is kept low, say around 20 to 30%. Unfortunately this affects the carbon contact in that in order for the carbon to be supported in the pulp without carbon destroying levels of agitation you need pulp densities around 50% solids.

2 The pulp pH should be in the range 11 - 12, especially when leaching sulfide slimes. Any oxidation of the sulfides will lower the pH. If you add too much alkali all that will happen is that the leaching rate will slow down, you do not want to be playing acid addition games to optimise your pH.

pH is measured on a logarithmic scale so it takes 10 times the addition of alkali to go from 11 to 12 than it does from 10 to 11. If you get the pH up to 13 then you are not paying any attention to what your alkali additions are doing.

3 Titration of the pulp liquor will give you the cyanide level of that liquor which is different to the whole pulp. All that you are interested in is the cyanide level of the pulp liquor. Don't try to overthink everything.

People will refer to cyanide level in their own understanding of what the phrase means. To some in means the level of the added cyanide as sodium cyanide, to others it means the level of the actual cyanide ion which is about half the level of the sodium cyanide level.

Technically you refer to the cyanide ion, this is what you are titrating.

When doing cyanide additions you are in the right area if you aim for 500 ppm cyanide which is equivalent to 1 gram per litre sodium cyanide.

This is also approximately equivalent to 1 gram per litre calcium or potassium cyanide.

If you go too high with your cyanide level you will lose financially as there is no gain in gold recovery from the higher cyanide level but your cyanide losses will be high.

4 Fresh carbon added to the pulp will adsorb cyanide from the pulp. The amount and rate of adsorption depends on the condition of the carbon and the parameters of the pulp. Your free cyanide level will always alter with time, the degree of alteration will depend on both the leach conditions and the ore characteristics.

5 Most mines have an AAS on site for solution readings, it doesn't have to be a spiffy new model, get a cheap second hand unit. Without this you really do not know what you are leaching or putting onto the carbon. Any other method is just a guesstimate.

6 HCl elution only removes some of the base metals from the carbon, it does not remove the gold.

Gold elution from carbon only occurs with heat, usually with high caustic and cyanide levels in the strip solution.

7 Dissolved oxygen (DO) levels are usually measured with a small hand held meter,fairly cheap. This also applies to cyanide concentrations.

Usually compressed air for agitation does not come in a fine enough bubble stream to really keep the DO levels high enough for leaching. It helps if you take an air bleed hose and use this to run a bubbler similar to a fish tank bubbler to get a stream of fine bubbles for oxygen adsorption in the pulp.

8 Pulp density may be measured by taking a known weight of the pulp, say around 1 kg and then slowly evaporating the water content.

If you start with 1 kg of pulp and end up with 500g of dry solids then you had 50% solids in your pulp.

Smaller quantities can be used but the precision drops with the smaller sample.

Usually you filter out the bulk of the water before drying, the larger the sample the less any solids losses on the filter paper are proportionally.

Small samples will need the filter paper to be weighed and added into the drying stage.


As a general observation the carbon should be pre-attritioned before being added to the pulp.

Any ashing procedure carried out in an air stream will suffer losses from air borne ash.

Any attempt to smelt the ash directly will suffer high losses.

The method you are proposing is about the most difficult to carry out with minimal losses that there is.

Vat leaching will simplify the system and give you higher recoveries at each stage.

Deano
 

Aeon13

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Sir Deano! I am glad to hear from you. You are the one again that took time and effort to help me on my questions. It's always an honor learning from you.

People will refer to cyanide level in their own understanding of what the phrase means. To some in means the level of the added cyanide as sodium cyanide, to others it means the level of the actual cyanide ion which is about half the level of the sodium cyanide level.

I have this understanding that I am titrating to get the concentration of free cyanide in the pulp. I am thinking that measuring the concentration a few hours after putting cyanide and comparing this reading after 24 hours will show if the free cyanide will change. For example, the first reading is 3 g/L and after 24, I titrated and has 1 g/L,this will show that something is consuming cyanide (most probably copper,silver and gold). Now after another 6 hours and the cyanide concentration is the same as 1 g/L, is it safe to say that there is no more left to consume the free cyanide(no more copper,silver or gold)?


Most mines have an AAS on site for solution readings, it doesn't have to be a spiffy new model, get a cheap second hand unit.

I will read first to understand this equipment and will try to find it to use.


HCl elution only removes some of the base metals from the carbon, it does not remove the gold.

Gold elution from carbon only occurs with heat, usually with high caustic and cyanide levels in the strip solution.

Thanks for correcting me. Would you prefer then caustic and cyanide elution compared to ashing sir Deano? because if you will, I will read and understand this process for me to apply it.


Dissolved oxygen (DO) levels are usually measured with a small hand held meter,fairly cheap. This also applies to cyanide concentrations.

What is name of this device that measures Dissolved oxygen and cyanide? I hope this device is here in our country. Another lesson learned on the bubbles.


Pulp density may be measured by taking a known weight of the pulp, say around 1 kg and then slowly evaporating the water content.

There is this procedure they are doing that they get 1 liter of the pulp and weigh it. They target 1.4-1.6 kilogram as they base the proper density on this weight. What do you think about this procedure?


As a general observation the carbon should be pre-attritioned before being added to the pulp.

I will have to read about this pre-attrition sir.


Vat leaching will simplify the system and give you higher recoveries at each stage.

The problem with Vat leaching of slimes sir Deano is that we have to dry it and mix coarser sand to it for the liquid on the vat leach to penetrate and pass until the bottom. These slimes are of mud like consistency and water will have a hard time passing through it without drying and mixing coarser material. This is a problem specially now that it is rainy season here.

That is why I tried CIP process for we put the slimes directly on the tanks and no drying is needed.



Thanks again sir Deano.
 

Deano

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You cannot use the cyanide level as an indicator of gold leaching completion. Unless you have an extremely rich ore the leaching of say 5ppm gold from an ore in itself will not cause a great enough change in the cyanide level to be detectable.

Keep in mind that 1 gram per litre cyanide equals 1,000 grams per cubic metre. If you are leaching at 50% solids then you have 1,000 grams of cyanide for every ton of ore. If you use up say 5 grams of cyanide leaching the gold in that ore you will still have 995 grams of cyanide remaining.

This equals 0.995 grams per litre cyanide, best of luck trying to titrate to this accuracy.

It would be expected that all cyanide accessible gold would have leached in 24 hours from a minus 100 micron ground ore.

You do the bottle roll tests to confirm the timing, as you are talking about a fine fraction of the milled ore you would expect the leach time to be substantially shorter.

The corresponding aqua regia assay will give you the total leachable gold, the difference, if any, between the aqua regia and cyanide bottle roll tests will generally be attributable to gold locked in sulfides.

If there is an appreciable quantity of gold locked in sulfides it may be worth you getting the tailings from the coarse milled fraction and recovering the sulfides from these.

The sulfide concentrate from the above step can then be treated to recover the gold.

Making and running a carbon strip circuit is full of traps, Much experience is needed to do so properly.

You really need to off-side someone who is using such a unit to learn what to do, this is not done quickly.

You then have to build and operate the electrowin circuit to recover the gold from the strip solution.

Overall the carbon stripping process will require you to have a lot of knowledge on how to build and operate the circuit with a fair upfront cost for the equipment.

For small scale it is better to do the ashing followed by a cyanide leach of the ash and zinc the gold from this solution.

I misunderstood your question about pulp density, I thought you were after a simple method not dependent on specialist equipment.

Mines world wide use pulp density scales, they are accurate enough for most ores.

It is usual to get a scale with the pulp density printed on the dial so that there is no mistaking what the correct value is.

One of several problems using CIP on this fine a material is that you will have a high level of fines trapped in the carbon pores.

These fines act as a blanket during ashing leading to incomplete ashing.

People try to get around this problem by either doing a lot of rabbling of the carbon during ashing leading to high ash losses or by forcing high air flows through the ashing carbon, again leading to high ash losses.

It is way more efficient to mix the fines with coarser sand and vat leach, you are then putting clear liquor onto your carbon and avoiding the fines problem.

As a bonus you will recover gold from the coarse fraction which was not gotten in the original leach.

Even better you can take the lower grade mullock fraction of the ore which was never processed, hammer mill it and put this also into your vat leach.

You will be making money from any material which has recoverable gold of greater than 0.5 grams per ton.


Deano
 

Aeon13

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Lots of things to learn.

This equals 0.995 grams per litre cyanide, best of luck trying to titrate to this accuracy.

-yeah I was wrong about my assumption on using the concentration of cyanide to detect gold leaching completion.

Then I guess I will have to learn two things here sir Deano. 1. Bottle roll test 2. Aqua regia assay
I have found this topic on my search here and it is of your own post. I will read those and please bear with me if I have follow up questions sir Deano. :)

For small scale it is better to do the ashing followed by a cyanide leach of the ash and zinc the gold from this solution.

- please explain this further sir. After ashing then I leach again with cyanide then precipitate with zinc dust?

I misunderstood your question about pulp density, I thought you were after a simple method not dependent on specialist equipment.

-you understood it right the first time sir Deano. What I was saying is that they get a liter of the pulp and weigh it in a normal weighing scale (not even digital), then they estimate the right density by getting 1.4 to 1.6 kilograms weight of the pulp.

It is way more efficient to mix the fines with coarser sand and vat leach, you are then putting clear liquor onto your carbon and avoiding the fines problem.

As a bonus you will recover gold from the coarse fraction which was not gotten in the original leach.

Even better you can take the lower grade mullock fraction of the ore which was never processed, hammer mill it and put this also into your vat leach.

You will be making money from any material which has recoverable gold of greater than 0.5 grams per ton.

- I will take your suggestion in consideration sir Deano. I will have to find a site for the vat leach and the processing area.

Thank you very much again for the information sir Deano! :)
 

Deano

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You are using the finest fraction of the ore to leach gold from.

This will ensure that any carbon used in CIL will have high levels of very fine ore particles in the pores.

These particles will slag high levels of gold if you try to smelt them directly.

The only way to recover high levels of gold from the ash is to redigest the ash in either aqua regia or cyanide.

After settling and filtration you recover the gold from the clean liquors.

Aqua regia will give you a higher gold recovery from the ash but the ease of using cyanide balances this out.


Deano
 

Aeon13

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The only way to recover high levels of gold from the ash is to redigest the ash in either aqua regia or cyanide.

- Do I have to crush the ashed carbon into a powder before digesting it in either AR or cyanide? What will happen to the ashes when redigested with AR and cyanide?

Aqua regia will give you a higher gold recovery from the ash but the ease of using cyanide balances this out.

I will have to try both for me to see the most effective way in my situation here.


Just a little rewind from your past post sir Deano.
If you add too much alkali all that will happen is that the leaching rate will slow down
- I am just curious on how too much alkali will slow down leaching rate. What levels of pH does this happen?

Thanks sir Deano!
 

Deano

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The ash from carbon does not need any pretreatment before leaching.

The fine ore particles are filtered out from the leached ash so that you have a clean liquor to recover the gold from.

Usually not a large quantity of solids but very fine particles which make for slow filtering.

The leach rate with cyanide slows down from pH 10 up to pH 14.

At pH 9.5 you have approximately half of your leach cyanide as HCN with the other half as CN- which is the part which actually complexes with gold.

At pH 11 pretty much all of your cyanide is CN- so you cannot get much improvement in the leach rate from HCN to CN- conversion.

As you increase the pH further you introduce hydroxyl ions (OH-) into solution in large quantities.

These ions have two effects, the first is to solubilise many metals which would not normally complex directly with cyanide and thus allow these metals to form cyano complexes. This reduces the amount of CN- ions in solution to complex with the gold.

The second effect is to form layers of these OH- ions around the gold particles which slow down the access of the cyanide to the gold.

The more OH- ions you have the higher the pH and the greater the slowing down of the gold leaching.

pH 11 is used pretty well universally as the standard commercial operating pH because it gives a relatively fast gold leach rate with a fair safety margin from the lower pH ranges which generate HCN gas and represent a danger to personnel.

Deano
 

Aeon13

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I think the most viable way for me now is to cyanide leach the ashes. I have no supply for SMB right now to to take the aqua regia way.

If I will do cyanide leach. Then pH 9.5 to 11 will be the best? What concentration of free cyanide will be used then sir Deano? Is 1 gram per liter alright?

As I understand, the optimal pH for any cyanide leaching process will be 9.5-11. Is this true may it be CIL,Vat leach or any other leach using cyanide sir Deano? I am asking this because I am having a Vat leach for crushed ore and I am thinking to use the optimal pH that you are explaining. Most of the miners here use too much alkali (lime).

Your explanation on how pH plays a critical role cyanide leaching is very useful for me. Thanks for sharing sir Deano!
 

Deano

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Please read what I post, not what you want to read.

Fot safety sake you run the cyanide leaches at pH 11, other pHs mentioned were as part of the explanation of leaching rates, definitely not a recommendation for actual use.

1 gram sodium cyanide per litre is used for all forms of leaching with the general exception of heap leaching where the losses would be greater and the need for faster leaching is not there.

Titration for free cyanide levels was what the industry used before the introduction of hand held cyanide meters, these are pretty much universal now.

Sodium metabisulfite is usually cheaply available at supermarkets and home brewing shops where it is sold as a disinfectant and preservative.

Deano
 

Aeon13

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Thanks for clarifying sir Deano.

Please read what I post, not what you want to read.

please let me know what part of my post are you referring sir Deano, I am sure I am reading your post,but maybe I misunderstood something.

I apologize if my questions are annoying sometimes for you maybe sir Deano. I am just stating things as I have understood them for me to also show you if I understand them correctly because English is not my first language.

Fot safety sake you run the cyanide leaches at pH 11

I will sir Deano. Thanks


Titration for free cyanide levels was what the industry used before the introduction of hand held cyanide meters, these are pretty much universal now.

I have never seen anyone use this device here. I searched for this hand held device but unfortunately I cannot see anyone selling this in the Philippines. I will be trying to get one from abroad if I can.

Many thanks again sir Deano! :)
 

Deano

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The last sentence of my post was


"pH 11 is used pretty well universally as the standard commercial operating pH because it gives a relatively fast gold leach rate with a fair safety margin from the lower pH ranges which generate HCN gas and represent a danger to personnel."

You then asked about using pH in the range of 9.5 to 11.

As I also said, at pH 9.5 you have a lot of HCN generated, this is a safety hazard as well as being less efficient than pH 11 leaching.

I only get annoyed when something relating to safety is ignored.

Apart from the safety aspect, every body has to learn processing, the idea of this forum is to aid in this learning in a safe manner.

If I was not happy to help I would not bother writing these posts.

Deano
 

Aeon13

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As I also said, at pH 9.5 you have a lot of HCN generated, this is a safety hazard as well as being less efficient than pH 11 leaching.

I only get annoyed when something relating to safety is ignored.

My bad sir Deano. I apologize.

If I was not happy to help I would not bother writing these posts.

Yes I know that you are of good intention of teaching and even I think sometimes that my explanation is annoying ,you are still sharing your knowledge. Priceless knowledge.

And I understand that it is of your good intention in focusing on safety first. This is all appreciated because I know that you are also helping me to be out of danger.


I will be using all these knowledge which you shared for our super fine ore and I will update here about my progress.

I will be asking questions again sir Deano because I know that as I learn and apply, I will still encounter some challenges/ideas/processes which will need a scientific explanation and not the explanation that people here are giving ("beacause this is what the old timers are doing").

Thanks again and take care.
 

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