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!