A Question about Activated Carbon

[Aeon13]

Good day to all!

It's good to be back reading and learning again thru this forum.

I have a question about activated carbon guys. I am currently using it to recover PMs from cyanide solutions.

I tried this method of intensive cyanidation for concentrates that I bought. It is based on this book "The
Chemistry of Gold Extraction" by Marsden and House. On pages 269-271 it states this ACACIA process. This uses 15-25 g/L NaCN, 3-4 g/L NaOH, and 2-10 g/L of soluble solid oxidant (I use Hydrogen peroxide). I have some success on this process but I am wondering on some things.

Since I use Hydrogen peroxide as an oxidant not the solid one that the book stated, and I don't have an oxygen meter, I am wondering if I am using too much oxidant.

Is too much oxygen affect the recovery of activated carbon?

I learned here that too much oxygen will hinder recovery using zincdust.

I would also like to ask if there is an ideal pH in which activated carbon will recover PMs.


Hope to hear from you.

Thanks guys!

 

[Deano]

Activated carbon adsorption of precious metals is not affected by oxygen levels or pH, it may be affected by competition for loading sites if the leaching conditions favour leaching of base metals in quantity.

Gross excesses of peroxide will consume cyanide above the levels consumed in normal leaching but you are having to be pretty heavy with the peroxide to the point where the process is not economic.

The Acacia reactor is used for treatment of concentrates, the high cyanide levels employed require the use of additional oxidants.


Deano

 

[Aeon13]

Activated carbon adsorption of precious metals is not affected by oxygen levels or pH, it may be affected by competition for loading sites if the leaching conditions favour leaching of base metals in quantity.

Gross excesses of peroxide will consume cyanide above the levels consumed in normal leaching but you are having to be pretty heavy with the peroxide to the point where the process is not economic.

The Acacia reactor is used for treatment of concentrates, the high cyanide levels employed require the use of additional oxidants.


Deano

Thanks for answering my questions sir Deano. I am at ease now knowing that carbon adsorption will not be affected by pH and oxygen level of the solution.

Please let me explain my the process that I am doing in order to be criticized and commented. I am not sure if what I am doing (especially the order of the steps in adding NaCN and Hydrogen peroxide) is proper.

1. I regrind the concentrates for 4 hours on a rod mill.

2. I put the ground concentrates on a 200 liter plastic drum which has an agitator.

3. I add water and start to agitated as well as add NaOH to adjust the pH to 12.

4. I add Sodium cyanide.

5. I add hydrogen peroxide.

6. Let the agitator run for about 24-48 hours.

7. Sample the concentrates using manual panning.

8. Leave the solution overnight for settling.

9. Siphon the solution on a separate plastic drum.

10. Add activated carbon to this separate drum and mix thru an improvised way I just made and let it run for 48 hours.

11. Collect the carbon for ashing then I return the solution to the original drum with the settled concentrates still on it.

12. Ash the carbon and smelt.

13. Agitate again the returned solution for 24 hours and back to step 8.

14. Once I notice the gold I recover I discard the concentrates and grind new concentrates using the same solution just recycled(adding NaCN and oxidant).

 

[Deano]

Usually smelting ashed carbon gives a substantially poorer gold return than leaching the ash.

Comparisons between splits of loaded carbon using the two methods show greater returns from the leaching method whether aqua regia or cyanide is used.

If cyaniding is used then the gold is recovered by contacting the cyanide leach with fresh carbon and ashing this carbon.

The gold loadings on this fresh carbon will be high so only small quantities of carbon need to be used, the small amount of ash from the second carbon is digested in aqua regia and the gold is dropped with any sulfite.

The precipitated gold is simmered in 50% HCl until the gold clumps, usually takes around 1 hour or less depending on how much base metals are present.

The clean dry gold is then smelted without flux.

Deano

 

[Aeon13]

Usually smelting ashed carbon gives a substantially poorer gold return than leaching the ash.

Comparisons between splits of loaded carbon using the two methods show greater returns from the leaching method whether aqua regia or cyanide is used.

If cyaniding is used then the gold is recovered by contacting the cyanide leach with fresh carbon and ashing this carbon.

The gold loadings on this fresh carbon will be high so only small quantities of carbon need to be used, the small amount of ash from the second carbon is digested in aqua regia and the gold is dropped with any sulfite.

The precipitated gold is simmered in 50% HCl until the gold clumps, usually takes around 1 hour or less depending on how much base metals are present.

The clean dry gold is then smelted without flux.

Deano

Thanks sir Deano.

At the moment I am smelting the ashed carbon directly because I am not using aqua regia and I am not familiar with leaching the ashed carbon with cyanide. I hope to improve my process soon.

You stated that

The gold loadings on this fresh carbon will be high so only small quantities of carbon need to be used

-- If I understood correctly, the first ashed carbon will be leached using cyanide and use a second batch of carbon to adsorb gold again.

Do you have any idea how much gold/PMs will activated carbon be able to adsorb? For instance the ratio of a gram of activated carbon to PMs?

Thanks again!

 

[Deano]

The level of gold adsorbed on to carbon is dependent on several variables.

If the carbon is brand new it will load more gold than re-used carbon.

The brand new, just off the shelf carbon will load more than carbon which has been sitting in storage for months.

The more base metals in the ash the less carbon will load per unit volume of carbon.

If all of the stars align then you may get around 20 kg gold per ton of carbon, substantially higher loadings are possible if, with much care, the cyanide liquor is acidified with HCl to < pH3 before contacting the fresh carbon.

If in any way you are concerned about handling acid cyanide solutions then I recommend that you use aqua regia digest as the alternative leach for the ash.

The ash can also be treated with pH 7 hypochlorite leach with 20% common salt added.

As with the aqua regia solution, after filtration the gold can be precipitated with sulphite additions.

The cost of a ton of new carbon is way less than the losses in direct smelting of the ash.

Deano

 

[Aeon13]

The level of gold adsorbed on to carbon is dependent on several variables.

If the carbon is brand new it will load more gold than re-used carbon.

The brand new, just off the shelf carbon will load more than carbon which has been sitting in storage for months.

The more base metals in the ash the less carbon will load per unit volume of carbon.

If all of the stars align then you may get around 20 kg gold per ton of carbon, substantially higher loadings are possible if, with much care, the cyanide liquor is acidified with HCl to < pH3 before contacting the fresh carbon.

If in any way you are concerned about handling acid cyanide solutions then I recommend that you use aqua regia digest as the alternative leach for the ash.

The ash can also be treated with pH 7 hypochlorite leach with 20% common salt added.

As with the aqua regia solution, after filtration the gold can be precipitated with sulphite additions.

The cost of a ton of new carbon is way less than the losses in direct smelting of the ash.

Deano

Thanks for these information sir Deano. I will be trying these methods in small batches for me to be familiar.

Again thank you!

 

[kumbirai.mhere]

The ash can also be treated with pH 7 hypochlorite leach with 20% common salt aded

How do you implement this method and for how long

 

[Deano]

Make up an approximate solution of 1% hypochlorite, calcium pool dry type or supermarket basic sodium type liquid offering with no added detergents or fragrances or speciality cleaning products.
Both types have high pH for stability under storage so you will need to drop the pH to around 7 with HCl.
For fastest leaching you need around 4% chloride in the solution, this can be added as common salt, do not use iodised types.
If you have high silver levels in your ash and you wish to recover this silver you need to have at least 10% chloride present in the leach, by adding 20% salt you will reach at least this level.
Run the leach under agitation, keep in mind that this leach will be aggressive towards metals so all wettable surfaces should be of plastic.
Usually an hour or so will dissolve the gold but if you have high gold and base metal levels you should filter the pulp at 1 hour and then re-leach with fresh solution for another hour and see what recovery you get for this stage.
Due to the limitless possible variations in levels of gold and base metals I cannot specify any particular time frame.
You now have to get the chlorine out of the filtered solutions, the easiest way is to acidify the solutions to less than pH 3 and heat in a glass beaker.
This step must be done in a fume cupboard or outdoors, keep in mind that the chlorine has to end up somewhere and that it is toxic to breathe.
A responsible approach is to run the fumes through caustic soda solution thus making your next leach solution..
With the chlorine gone you can precipitate with your preferred precipitant.
Keep in mind that you will always have some level of residual chlorine no matter how hard you try to remove it all.
This means that some of the added precipitant is consumed by the chlorine and a higher than usual amount of precipitant will be needed.
Metabisulfite is pretty cheap in commercial 20 or 25 kg bags so the need to add extra is not a major drama unless you have really done a pretty ordinary chlorine removal step.
Deano

 

[kumbirai.mhere]

Make up an approximate solution of 1% hypochlorite, calcium pool dry type or supermarket basic sodium type liquid offering with no added detergents or fragrances or speciality cleaning products.
Both types have high pH for stability under storage so you will need to drop the pH to around 7 with HCl.
For fastest leaching you need around 4% chloride in the solution, this can be added as common salt, do not use iodised types.
If you have high silver levels in your ash and you wish to recover this silver you need to have at least 10% chloride present in the leach, by adding 20% salt you will reach at least this level.
Run the leach under agitation, keep in mind that this leach will be aggressive towards metals so all wettable surfaces should be of plastic.
Usually an hour or so will dissolve the gold but if you have high gold and base metal levels you should filter the pulp at 1 hour and then re-leach with fresh solution for another hour and see what recovery you get for this stage.
Due to the limitless possible variations in levels of gold and base metals I cannot specify any particular time frame.
You now have to get the chlorine out of the filtered solutions, the easiest way is to acidify the solutions to less than pH 3 and heat in a glass beaker.
This step must be done in a fume cupboard or outdoors, keep in mind that the chlorine has to end up somewhere and that it is toxic to breathe.
A responsible approach is to run the fumes through caustic soda solution thus making your next leach solution..
With the chlorine gone you can precipitate with your preferred precipitant.
Keep in mind that you will always have some level of residual chlorine no matter how hard you try to remove it all.
This means that some of the added precipitant is consumed by the chlorine and a higher than usual amount of precipitant will be needed.
Metabisulfite is pretty cheap in commercial 20 or 25 kg bags so the need to add extra is not a major drama unless you have really done a pretty ordinary chlorine removal step.
Deano

Noted. I managed to ash 400kg of spent carbon from cyanide solutions streams(carbon assay value of 62g/t) and got two 20 litres of ashes. Is it possible to recover gold by aqua regia. As it is tedious to put it on hot plate. I was thinking of soaking the ashes in aqua regia for days. What advice would you give? I am performing this in the countryside area. Forest settings and with PPE

 

[Deano]

Aqua regia is usually run hot in order to solubilise some of the base metals present in your material.
If run cold there is a good chance that some base metals will remain in native form and react with some of the dissolved gold.
Longer term room temperature leaches using aqua regia usually fail because the aqua regia loses its reactivity and hence gold dissolving power with time.
Usually I would use cyanide to dissolve the gold in the ash but if you do not wish to, or are prohibited by regulations, from using cyanide then the only realistic leach system for a large ash volume is pH 7 hypochlorite with added salt.
Deano

 

[kumbirai.mhere]

For fastest leaching you need around 4% chloride in the solution, this can be added as common salt, do not use iodised types

Why is iodised salt not preferred? All I see is iodised salts in my local supermarkets

 

[Yggdrasil]

Why is iodised salt not preferred? All I see is iodised salts in my local supermarkets

The Iodine may be interacting in ways we do not want.

 

[kumbirai.mhere]

The Iodine may be interacting in ways we do not want.

But iodine used to dissolve gold to as it is a halogen. I thought it would be a plus

 

[Yggdrasil]

But iodine used to dissolve gold to as it is a halogen. I thought it would be a plus

Different and it may counter the Hypochlorite.

 

[Deano]

Iodine will dissolve metals which chlorine will not under the conditions of leaching being employed.
This may give you extra problems down the line regarding total recovery of precious metals and the purity of these metals.
Look in the cooking section of supermarket for non-iodised salt, usually available as coarser salt crystals.
Deano

 

[Shark]

I get it here at most stores as non iodized sea salt.