Zadra and DIY elution system

[oro-del-sur]

I’m working on a small scale stripper for gold bearing carbon and would like to share as I go along. I also need more advice. We have a small scale mining operation in Costa Rica and are building our first lixiviation plant. I made a previous post showing my first stripper test in a Pyrex dish to see if I could elute and deposit gold onto steel wool using a 6v battery charger (which seemed to work). That thread is here: https://goldrefiningforum.com/phpBB3/viewtopic.php?f=44&t=27600&p=291363#p291363

Since then I have read a fair amount of info about Zadra and also pressurized carbon elution. I understand the theory and some basics but finding detailed information in the public domain, the actual nuts and bolts of how to set up and operate a small system is scarce. All the info I found discusses industrial scale - tonnage - sophisticated and expensive systems. Our ultimate need will be for 50-100 kg of carbon and a system I can put together myself that doesn’t cost many thousands of dollars. I will eventually hire someone with more experience to operate the system and perform carbon assays etc. but in the meantime I need to learn develop a system we can deploy.

One thing to note - given that carbon elution only considers stripping from traditional cyanide adsorption we are operating in experimental territory with a non traditional lixiviant - due to environmental restrictions our strips are from one of the cyanide derivative products called Eco-Goldex (Canada). We will leach milled head ores and also process a mountain of tailings from previous artisanal works (our own) which contain traces of mercury.


In this post I show a bit larger system I made which I will use for practice. The black steel tank and plastic EW cell shown here hold a total of 25 liters of solution - capacity for 2-3 kg of carbon. The carbon sits on a fixed screen divider in the black tank keeping it suspended inside the tank about half way up. The solution is heated in the bottom section of the tank with two 110v heating elements and maintains the temp at 70-80 C. The flow from the small mag pump is controlled with a valve, and another valve to drain the tank. The frames for the two anodes and single cathode are made of 1/4” round stainless, anodes are heavy gauge stainless mesh. The coffee filter is to catch any loose solids so they don’t pass into the EW cell.


What I’ve learned so far (non pressurize elution):

Solution makeup is 1% caustic soda and .1% - .2% cyanide

Up to 20% isopropyl alcohol addition can enhance elution but successful elution can also be achieved without alcohol.

De-mineralized water is best, calcium and magnesium from natural water sources can be problematic.

Soak / wash the carbon for a couple of hours in 3%-5% HCL is often necessary to dissolve the junk, making for a cleaner strip of the gold-cyanide complex.

A non pressurized elution strip can take up to 48 hours. Pressured systems can achieve it in less than 10 hours. Ideal flow rate is 2-3 bed volumes x hour.

Electro-winning onto a steel wool cathode is most practical, proper voltage is between 2-4 volts. Amperage is determined by actual resistance in the electro cell i.e. the solution itself and all the metal components. The 6v / 1 amp motorcycle battery charger was tested previously in 4 liters of solution with a carbon rod anode and achieved deposition of gold onto the cathode without burning up the steel wool - so I’ll use it in subsequent tests as long as the results allow it.

To harvest the gold, the steel wool is typically digested with sulfuric acid.



What I don’t know:

How to determine the amount of solution required per batch.

How much steel wool is needed per gram of gold anticipated.

Alcohol - In my previous post Deano responded and mentions that evaporation is an issue with the alcohol and elution will cease without the organic component (alcohol). But I read elsewhere that elution can be accomplished without any alcohol. So does Deano mean the elution will cease altogether or just slow down?

What is a practical method to determine the alcohol level?

What are all the important operating considerations when using alcohol (aside from the vapor hazard) ?

I just bought 5 gallons of “Mosstanol” from an industrial supply which is 60% ethanol and 40% isopropyl. I read that ethanol is more effective versus isopropyl. Is this true?

Can the carbon pre-wash be done with dilute nitric acid?

Can the steel wool be digested with nitric instead of sulfuric?

How do I determine what rectifier to get for the 50kg-100kg system ?

I understand the correct spacing is 1.5” more or less between the anodes and cathodes but I notice that “hairs” on the steel wool make contact with the ss mesh anode. To solve this I was thinking to wrap the steel wool cathode in a cloth mesh (mosquito netting). I assume this will not affect anything (?). Is there a better way?

Thanks in advance!

 

[FrugalRefiner]

due to environmental restrictions our strips are from one of the cyanide derivative products called Eco-Goldex (Canada).

Are you able to buy potassium ferrocyanide? If so, it will probably be easier to work with than Eco-Goldex.

Dave

 

[oro-del-sur]

due to environmental restrictions our strips are from one of the cyanide derivative products called Eco-Goldex (Canada).

Are you able to buy potassium ferrocyanide? If so, it will probably be easier to work with than Eco-Goldex.

Dave

Hey Dave, no not able to get ferrocyanide outright. I believe mostly because of the "green advertising and promotion" and that the Eco Goldex company put on a workshop here it opened up a "loophole" for us to import and use it...the actual ingredients were not not scrutinized which would reveal Eco Goldex is a cyanide derivative.

 

[FrugalRefiner]

That's a shame. I was considering experimenting with Eco-Goldex. I looked into the make up of the product, and eventually contacted Deano about it. He explained that at its heart, Eco-Goldex is a potassium ferrocyanide based leach, with all the other crap thrown in to make it a proprietary product. But that other crap complicates the recovery from the leach, as confirmed by other members who have used it or made contact with Eco-Goldex's staff.

I wonder if there is an effective way to refine the Eco-Goldex to eliminate the crap prior to using it in the leach.

I hope Deano will be along to answer your other questions.

Dave

 

[oro-del-sur]

There is an outfit not far from here that has a first world lab (AA) and spent a couple months testing their ores with some of these alternative lixiviants. They told me the Goldex product gave them very positive results in their bottle roll tests. I've spoken with John Guo the owner of Goldex in Canada and he promises to back up his product with tech support and to contact him directly with whatever issues you might have, claims much of the bad publicity is warranted but from certain products, not Goldex. We did a few bottle rolls of our own with 1 kg samples and got insignificant gold in the tail assay so I know the product will dissolve gold. I don't have enough experience yet to make further claims but will soon hire someone to help me make a final analysis on our ore - carbon loading and recovery etc...all of which I hope to collaborate with John at Goldex. I would love to talk to anyone who has experience with the Goldex product...if you know anyone please put me in contact.

 

[anachronism]

due to environmental restrictions our strips are from one of the cyanide derivative products called Eco-Goldex (Canada).

Are you able to buy potassium ferrocyanide? If so, it will probably be easier to work with than Eco-Goldex.

Dave

Dave. Eco goldex is a Ferricyanide leach not a Ferrocyanide leach. The chemistry between the two is subtlety but fundamentally different and the reasoning behind why they can ship it to people.

It's also at the core as to why its a load safer for people who don't understand cyanide.

 

[Deano]

When playing with carbon stripping and electrowinning systems the obvious route to take is a direct scale up or down of previously proven technology.

There is a mass of information on stripping and electrowinning on the internet on all versions of stripping plants.

Pretty much all of these systems were developed at small scale before being run and optimised at larger scale, but the reverse can also be done.

The key role in elution is played by temperature, in a Zadra system not much happens below 90C.

Pressure Zadra is run around 110C but can vary depending on the subtleties of the system.

Pressure Zadra is much faster than unpressurised Zadra, depending on the operating conditions around 12 hours versus 48 hours.

The strip is also more complete but with plant carbon you are looking at 30 versus 50ppm residual gold, time and thus cost is the major factor for use.

All pre-leaches and steel wool digestions are done with HCl, apart from getting a better result the cost is way cheaper.

If you are running an alcohol type elution system and the alcohol level drops, so will the efficiency of the strip.

The bulk of industry plants are Zadra of one form or another, there is always a reason why the industry preference is used be it cost or safety.

Pretty much all of your other questions are answered in the literature.

The reason why the alternative leaches are not used commercially is, apart from the price of the active cyanide component being raised by the cost of the additives, the additives will dissolve many other metals which cyanide alone will not dissolve.

These metals will adsorb onto carbon but are not desorbed by a cyanide strip, so after a few cycles your carbon is clogged with metals which not only prevent gold loading onto the carbon but also prevent the loaded gold from being effectively stripped.

Deano

 

[oro-del-sur]

Thanks Deano, I've spent many hours searching and reading literature but unable to discover the ratio of solution vs. carbon/gold, how much steel wool per gram of gold and a method to measure the alcohol level. Can you tell me please?

Here in this country HCL and Sulfuric require special permits but nitric is for sale to the public and cheaper and is why I ask the question if it's a reasonable substitute. I'm not trying to reinvent the wheel, just trying to work with what I have at hand.

Other than hire a company to build a really expensive rectifier I prefer to buy one at reasonable cost if I could figure out the parameters. $400-$500 rebuilt USA made rectifiers versus $1800 custom made quoted by a Canadian company specializing in EW from carbon elute. From what I understand it's a simple rectifier to keep volts / amps within a certain range. Nothing fancy.

We propose to deploy the Goldex product only because it offers us an opportunity to leach ores instead of amalgamating whole ores. Cyanide is strictly prohibited and mercury is being phased out so it's literally a matter of survival for our operation and others like ours... assuming we are able to overcome the drawbacks of using the cyanide alternative product.

 

[oro-del-sur]

"These metals will adsorb onto carbon but are not desorbed by a cyanide strip, so after a few cycles your carbon is clogged with metals which not only prevent gold loading onto the carbon but also prevent the loaded gold from being effectively stripped"

So the acid prewash does not rid the carbon of these contaminate metals correct? ...and the next option would be an expensive kiln to regenerate the carbon?

I estimate stripping 40 kg every two or three days or 120 kg x week.

So basically it sounds like carbon replacement every few strips, and to pretty much figure this as an ongoing cost of recovery. Is that right?

 

[Deano]

In field operation you will find that HCl has far superior metal removal from carbon properties than nitric acid, both in terms of time and efficiency.

HCl washes will remove some of the adsorbed metals but for practical use you are looking at days of contact rather than hours.

Also you will need higher HCl levels.

A regen kiln will only re-activate exposed carbon, it will not remove loaded metals from the carbon.

Basically you get only one strip from the carbon then you have to dump it, depending on the contaminants being loaded.

There are no standard ratios of strip solution etc as each mine is subject to its own peculiar contaminants.

When stripping you get the base metals, especially copper, desorbing first and thus being loaded onto the steel wool first.

In some cases you might run the circuit for 24 hours just to get rid of the bulk of the copper, however you will still be eluting gold during this time so you cannot just throw the copper product away.

The strip times etc are totally dependent on the metals present in your mine and which of these metals and at what levels you have put them onto carbon.

All the above can only be found out by doing the test work with your own material, there are no general numbers to plug in.

Always remember that these leaches are being sold to make money for the promotors, your interests are well down the line.

I would, in your case, investigate the possibility of accessing either ferro or ferricyanide availability.

Always keep in mind that all of the other leach formulations are going to be either or both of the above with added metal solubilising agents to keep the mystery going.

Deano

 

[oro-del-sur]

Deano, thanks for the excellent detail. Although very enlightening it’s disheartening. I’m not even sure if I can run a profitable recovery under those conditions but will take the experiment as far as I can. It seems to me the companies that produce these products would go out of business in short order for lack of repeat customers, unless their repeat customers are not using CIP?

So does excessive contaminants adsorbing onto the carbon require higher than normal quantities of carbon in the CIP?

If the carbon is a one-time strip scenario is elution even the best option? Would ashing the carbon not be an alternative, maybe even more practical? Let’s say in our case maybe 80-100 kg every two days.

Without a “green disguise” my guess is the straight ferro / ferri cyanide product would be scrutinized and require special import license and then a restriction on deployment. The mystery of the Eco-Goldex is basically what allows us to even consider leaching ores.

Why do you suggest ferro/ferri-cyanide over potassium cyanide?


Thinking outside the box…

What about acid prewash to whatever point is practical, then strip the carbon with AR or SSN? I’ve experimented off and on with SSN and it’s very aggressive and fast, although not at all selective, attacks all metals even iron pyrites. Heating at 70 C doubles the reaction. About a month ago I tested a hand full of loaded carbon with SSN and it instantaneously put gold into solution (showing dark purple / black on a stannous stain test). Truthfully speaking I didn’t set the test up properly and have not repeated it although I did a 3 gram assay on the carbon tails after several hours of soaking and two rinses and there was still an unexceptable amount of gold left on the carbon to call it any kind of success. I’m going to play around a bit more with this, maybe slow gravity drip thru a column but use fresh SSN after each pass or every other pass. What I couldn’t figure out was if the solution reached a saturation point or stops desorbing for some reason, or maybe the gold was reabsorbing back on to the carbon? I also did it in reverse, put some new carbon in a pregnant solution of SSN and the carbon immediately adsorbed the gold, the liquor turned clear instantly around pH 2 or so if I remember correctly.

 

[Deano]

The first preference for leaching is always straight cyanide.

The ferro/ferri cyanides are also straight cyanides but usually have no prohibitions on their use, transport or storage.

This is an anomaly which I expect would be remedied if they become popular in mining recovery.

Aqua regia and SSN will strip low levels of metal from carbon but the great bulk of the gold remains on the carbon.

Usually you will get one good cyanide strip of the carbon and then you are on the path of rapidly diminishing returns.

Ion exchange resins will perform better but you will still run into the problem of lockup of metals in the resin as well as the much higher resin cost.

CIP was developed to overcome the cost, both capital and operating, of running a zincing plant.

You can use zinc to recover gold from these leaches but the recovery will require several zincing stages to get good overall results.

The problem results from the use of chemical oxidants in the leach so when you zinc out you will also be re-leaching some of the zinced out gold.

You will also bring out some of the base metals which will require extra aciding of the precipitate.

Generally ashing only recovers around half the gold on the carbon unless extreme measures are applied in the baghouse section.

Deano

 

[BIKAKA]

hi everyone!
I have so excitedly jumped onto this thread as its one of my motives to be here on the forum.
Im ofcourse new hr!.I work as a local Gold miner in TANZANIA(AFRICA) and hv been prospecting for about 4yrs now.
As you guys knw things are stil slow in Africa.Local Mining largely depends on amalgamation as 'the one and only means to get gold from its ore'.
wat we actually get is not worth the time and effort with most miners ending up with nothing up of couple years of prospecting.
The problem we have here in Tzn isnt the gold or cost to extract the ore even in our small scale but the right technigues to enable us as local miners extract gold and fully benefit.The mercury we use only gives us 30-40% as local miners with 70% being left in tailings which remain at where we take ore for amalgamation.
thnx to ths forum atleast i can find way to change things here around!

 

[PCZim]

Hi

Also jumping late onto this thread. I have just built a 180kg carbon capacity elution machine for my mine. If you are still active here I’m happy to give you any tips or advice.

 

[Abdullah68]

Hi

Also jumping late onto this thread. I have just built a 180kg carbon capacity elution machine for my mine. If you are still active here I’m happy to give you any tips or advice.

Hi bro
Can you send your design please

 

[Eg.refiner]

Hi

Also jumping late onto this thread. I have just built a 180kg carbon capacity elution machine for my mine. If you are still active here I’m happy to give you any tips or advice.

Please share your experience

 

[shrewdly]

Please share your experience

Hi Deano: I see that I'm a latecomer to is this discussion. Some years ago I was the mill foreman in a 350 MTPD designed hard rock gold mine. We used the carbon in pulp process. We stripped our carbon in steel columns at a Ph of 12 to 12.5 using Sodium Hydroxide at a temperature 99 degree C and without cyanide. Initially we were using 0.25% cyanide solution but we found that it did not significantly increase our rate of extraction and caused fouling of the carbon, so we stopped using it in our EW circuit. The two critical variables that we found were the temperature of the strip and the Ph. Of the two temperature was the most important, a drop of 5 degrees would increase strip times by 10 to 12 percent. We would not experience any drop in strip time until the ph dropped below 11.6 to 11.8. We treated the spent carbon with nitric acid rather than HCL. We found that the chlorine would form insoluble Silver Chloride with any silver still in the stripped carbon fouling the carbon or we would loose the silver to tails. We used a 3.5 VDC and the amperage varied, we simply watched the cell periodically looking for off gassing (usually hydrogen) or the appearance of other metals forming on Steel wool cathodes and adjusted the amperage accordingly. We reused our stripped carbon and regenerated it after every third or fourth strip. P.S. With a few simple design changes we got the mill throughput up to 625MTPD.

 

[Deano]

As you say, temperature is the greatest single effect in stripping. Higher pH will give you less residual metal on the carbon at the end of the strip.
Cyanide is added to the strip solution in order to remove silver present on the carbon, this allows you to pretreat the carbon with HCL which is cheaper and more efficient as a tramp metal removal agent than HNO3.
The number of cycles run on the carbon before regeneration is determined by the activity level of the carbon, the number of cycles for a particular mine is affected by the base metals present in the ore and dissolved solids in the water supply.
As each mine varies in the ore composition and its water supply there is no overall general number for strips before activation.
As the carbon comes out of the regen kiln it is screened to remove fines and then fed into a water container to quench.
Attempting to acid wash after regeneration will lower the carbon activity, not recommended.

Deano