Gold recovery through cyanidation with activated carbon

[sohwohn]

The mining industry standard cyanide level is 0.1% sodium cyanide in solution, this is around 500ppm actual CN^-.

You can play with lesser cyanide concentrations in order to minimise base metal dissolution and relatively enhance the gold dissolution.

High copper ores can be run with cyanide levels at 0.01% or even lower.

As the cyanide level is lowered the protective alkalinity will also lower so you will have to add lime or caustic for pH regulation.

Induction furnaces are not useful for carbon ashing, you need to have an air flow through the carbon with the temperature around 650C. Ashing needs to be complete. Get yourself an electric muffle furnace and some porcelain dishes for the ashing.

Usually you monitor the metals in solution values to see how the leach is going, get yourself a good second hand AAS unit to do this with.

Selling loaded carbon is usually an opportunity for dodgy assay methods to be used, much smarter to do your own stripping and then reuse the carbon, that way you maintain control over the gold.

Carbon in leach levels are usually around 5 grams per litre, much higher and attrition of the carbon becomes a major issue. Gentle agitation is needed.

Deano

Yes! We're ordering muffle furnace, and checking the price of AAS unit. We have muffle furnace and AAS unit in our another factory (Jewerly manufacturor) but not in this small lab.

Regarding the cyanide cocentration, I guess I had been doing too wrong. I will run some more experiments on the Cu-1% ore with much lower concentrations.

Here's a tricky question I wonder: Regardless of the matrix metals, does the increase of cyanide concentration induce the cyanidation speed? I know I may seem greedy and matrix metals can mostly like NOT TO BE IGNORED, but I just wonder.

 

[nickvc]

I know I’m not Deano but as you increase the amount of cyanide in your solution it does speed up recovery at first but it also increases the amount of base metals taken into solution so eventually slowing the reaction down and making recovery harder, the idea is to make your solution more selective, if you can keep the solution moving allowing fresh solution to wash over the material it will help increase the speed of dissolution.

 

[sohwohn]

I know I’m not Deano but as you increase the amount of cyanide in your solution it does speed up recovery at first but it also increases the amount of base metals taken into solution so eventually slowing the reaction down and making recovery harder, the idea is to make your solution more selective,

Ohh..! Got it! Thank you!

if you can keep the solution moving allowing fresh solution to wash over the material it will help increase the speed of dissolution.

Is it because the Au dissolution rate decreases while concentration of gold cyanide increases? Or is it because the 'fresh solution' has guaranteed concentration of free cyanide (like no matter how long time goes by, it stays at certain concentration)?

 

[nickvc]

By moving the solution continually you expose the gold to fresh solution which in turn can dissolve more gold, it’s similar to heating AR it helps the dissolution by making the molecules move faster.

 

[sohwohn]

By moving the solution continually you expose the gold to fresh solution which in turn can dissolve more gold, it’s similar to heating AR it helps the dissolution by making the molecules move faster.

You mean to stir the solution, right? I do!

 

[anachronism]

Stirring is a good start however Nick's referring to a constant movement without the need to physically do it.

People have discussed an inert bucket arrangement on a cement mixer with a variable speed motor as an option. A sharp eyed friend of mine (John) who is a member on here in the UK found this in a scrap yard a couple of weeks ago and paid £100 for it. It works. Not sure whether the bucket is Stainless Steel or Titanium yet but I'll know next week when it's up here and XRFed. Nickvc will laugh at me for having one of these. A lot.

 

[sohwohn]

Stirring is a good start however Nick's referring to a constant movement without the need to physically do it.

People have discussed an inert bucket arrangement on a cement mixer with a variable speed motor as an option. A sharp eyed friend of mine (John) who is a member on here in the UK found this in a scrap yard a couple of weeks ago and paid £100 for it. It works. Not sure whether the bucket is Stainless Steel or Titanium yet but I'll know next week when it's up here and XRFed. Nickvc will laugh at me for having one of these. A lot.

From this angle I can't tell what this is..

Maybe due to the fact that English is not my mother language, I didn't mean I would stir the solution with my precious hands. We have something like this:

 

[anachronism]

That's a good start- a little fast though - then again if you have a vortex it will aerate the solution and introduce oxygen into the liquor which will provide the oxidant you need without the need to add m-NBSS.

I've rotated the picture above so it makes more sense. Apologies I hadn't had coffee when I originally posted it.

 

[sohwohn]

That's a good start- a little fast though - then again if you have a vortex it will aerate the solution and introduce oxygen into the liquor which will provide the oxidant you need without the need to add m-NBSS.

I've rotated the picture above so it makes more sense. Apologies I hadn't had coffee when I originally posted it.

I thought faster the better. If too slow, some powders stay at bottom. This is faster than what I need to avoid that though.

This is what we use for grinding:


This is a small flotation unit but I seldom use 'cause it's too difficult to actually achieve something by using it. I feel like it's too advanced for me:


This is my working area and that metal box is the XRF unit:


This is our induction furnace which will soon be replaced by a muffle furnace (yay!) in a month:


This is our furnace cup:

 

[anachronism]

Thanks for the extra info. My experience is related to ewaste and not mining so RenoChris and Deano will have a much more valid input than me.

 

[sohwohn]

I've came here for about 3-4 months. The first month I spent all time tuning the XRF unit and making use of it. These show some of my work:

 

[sohwohn]

I've been trying flotation and cyanidation for the past 2 weeks.

My last run:
Ore powders used: 300g grinded to -74nm (fire-assayed: 50ppm Au, and 250ppm Pt)
Water used: 1200cc tap water
pH adjustment: Na2CO3 used, pH 11-12
Cyanide used: 25g (Conc: 2%) for 2 hours
AC: 20g for 2 hours
Result: .37g ashes, 4556ppm Au, implying roughly 1.69mg Au

The 300g should contain 15mg Au so the recovery rate is about 11.2%. I'm not certain about if this is the way to calculate but this is my way.

My current run:
Ore powerders used: 240g grinded
Water used: 1500cc tap water
pH adjustment: Na2CO3 used, pH 11-12
Cyanide used: 1.5g (Conc: 0.1%) for 16 hours
AC: 5g for 8 hours, then another 5g for 8 hours

I'm putting this much of water because we don't have titration equipment yet.. So I never know when the cyanide would be used up. If solid-liquid ratio is made to be more sensible like 1:2, then I probably need to add 0.6g of cyanide which I somehow feel it'll very soon used up.

 

[sohwohn]

This is another run I tried earlier:
900g ore powders
2L tap water
30g cyanide (1.5%)
cyanidation 8 hours
pre-treatment with H2O2 4 hours and bleach 4 hours (I found this on a Chinese website but it didn't work)
carbon absorption 4 hours
10g AC used and .29g ashes recovered

Recovery rate was still 11.2% but I got Pd and Ag.

If I recall right, I got 4.9mg Au and .6mg Pd and .4mg Ag in .29g ashes. Fe was about 1.7% and I got Ti, Ni, Cu, Pb, Zn each lower than 1%

 

[sohwohn]

Thanks for the extra info. My experience is related to ewaste and not mining so RenoChris and Deano will have a much more valid input than me.

We tried using cyanide to extract Pd in TWC and we got pretty good result.

2000g sample powders were used (Pd grade: 800ppm)
Bleach and H2O2 were added
It took 4 hours for pre-treatment and 11 hours for cyanidation. 24 hours for AC absorption.

40g AC used ad 1.2g ashes recovered. The 1.2g ashes got 61% Pd and very little Pt, no Rh.

So from 1.6g Pd we extracted 0.732g and recovery rate was around 45.75%.

 

[Mezira]

You have two possibilities for adsorbing gold values onto activated carbon.

The first is where you are adsorbing from a clean liquor with no fine solids present.

In this case you can get very good gold recoveries by ashing the carbon and smelting the ash.

If you have fine ore particles present in the liquor you will get some of these particles caught in the carbon pores and they will be present in the ash no matter how diligently you wash the carbon before ashing.

These residual particles will cause gold losses of up to 50% of the gold if you smelt the ash.

The only way to get high recovery from such ashes is by leaching the gold with either cyanide or aqua regia.

Other gold leaches will also give high recoveries but are a pain to use apart from the pH7 hypochlorite leaches for which you need Eh/pH meters, the meter expense tends to put people off these leaches which is a shame as they are probably the safest leaches for both small and large operators provided basic safety precautions are taken.

Deano

mr deano is honorable .. on testing with this cyanide do we need to measure dissolved oxygen and its ORP meter ...
2. whether AC ash in aqua regia then in SNcl2 wear test

 

[Deano]

I am assuming that you are referring to recovery of gold from the ash from activated carbon, you did not make this clear.

If you recover the gold from the ash with cyanide the simplest way is to make up a leach solution of 1 gram per litre sodium or potassium or calcium cyanide using a pH meter and caustic soda to have a pH around 11.

Stir the ash in the solution for a couple of hours and then filter.

The gold can be gotten from the liquor by electrowinning on to steel wool or by zincing.

For small batches of ash you will not need to worry about D O levels, the action of stirring will keep enough oxygen in the liquor.

You do not need an ORP meter for cyanide leaching, just a pH meter.

If you are digesting the gold in the ash with aqua regia you simmer the mixture of ash and acid until the fumes turn from brown to white, takes around 30 minutes.

After cooling and filtering the gold can be precipitated from the clear liquor with the precipitant of your choice.

Usually the gold is settled over night and then filtered, there will be a very low level of gold residual in the liquor, usually less than a half ppm.

The cost of getting this residual gold is more than the gold is worth.

Deano

 

[sohwohn]

Since it's been Chinese New Year recently, I did not have time to update my progress on this forum.

I went to the mining-site 2 weeks before CNY. The whole licensed-area takes about 0.88km².

There're some tunnels (deepest 300m) which consists mostly sulfides with Gold.

The highest Gold grade is about 300-400ppm (at 300m deep) and lowest is about 20-30ppm (at 100m deep).

Our team won't focus on the tunnels though.

We've found on top of the main vein there's oxidized Gold (around 3-7ppm).

The volume of this oxidized Gold is much larger than the main vein.

It takes lower costs (as well as lower risks) to process this low grade oxidized Gold than high grade Gold surrounded by sulfides.

Anyway, something here I need to clarify..

Before I head to the mine, I found that the chemical I was using wasn't actually Cyanide.

It was mainly composed by Thiourea (with some secret ingredients) and it 'only acts like' Cyanides.

Our people always call the process 'Cyanidation' but it's not actually Cyanidation.

I don't want to get people confused with my experimental results.

For reference, the chemical we're using is named 'Jinchan'.

 

[sohwohn]

For more information about Jinchan, please wait for me collecting more data to make a conclusion.

By far, what I can tell is that it's more selective than Cyanides. The price is similar with NaCN and it consumes more. Though, the processed solution can be re-used by adding less Jinchan with much cheaper NH4HCO3. The overall cost is similar with Cyanides. Pollution-wise, Jinchan is much much more environmental-friendly and safe.

The effectiveness, as far as I have tested, can reach a little bit fast than Cyanides, while collecting above 97% Gold and around 50-60% Silver. If done with pre-treatments, it can also dissolve Pd with unknown effectiveness.

However, I have only done like 20-30 tests. I feel like not enough to optimise all the factors. So I cannot conclude 'how good' this Jinchan is. There must be pros and cons.

We will drive to the mining-site tomorrow, so I suppose I can make more experiments in the following 1-2 weeks. Eventually, I need to do the tests though. I am going to post the detailed collected data with conclusion about effectiveness using Jinchan, maybe in mid-March or late-March.

 

[Eg.refiner]

For more information about Jinchan, please wait for me collecting more data to make a conclusion.

By far, what I can tell is that it's more selective than Cyanides. The price is similar with NaCN and it consumes more. Though, the processed solution can be re-used by adding less Jinchan with much cheaper NH4HCO3. The overall cost is similar with Cyanides. Pollution-wise, Jinchan is much much more environmental-friendly and safe.

The effectiveness, as far as I have tested, can reach a little bit fast than Cyanides, while collecting above 97% Gold and around 50-60% Silver. If done with pre-treatments, it can also dissolve Pd with unknown effectiveness.

However, I have only done like 20-30 tests. I feel like not enough to optimise all the factors. So I cannot conclude 'how good' this Jinchan is. There must be pros and cons.

We will drive to the mining-site tomorrow, so I suppose I can make more experiments in the following 1-2 weeks. Eventually, I need to do the tests though. I am going to post the detailed collected data with conclusion about effectiveness using Jinchan, maybe in mid-March or late-March.

Please i need more information about how to use the jinchan ( best EH ,PH for leaching gold ) and best PH for carbon adsorption ?