Cementing with Zinc didn't go as planned.

[Nouran Alaa]

Hi Everyone
I know Zinc isn't the preferred method for cementation on this Forum but it was the available metal at the time and copper wasnt available.

I did have a thread before about major gold losses and with your help I was able to recover most of the gold lost and was left satisfied with the results. I used Dave's when in doubt cement it out as a reference and Hoke's book as a guide.

The first time we recovered with zinc was a great success ( Beginners luck I guess!), the second time wasnt a charm.



I had two water tanks (each 1000 liters)
The first was suspected to have 10.5 grams of gold (21 Carat) and the second was suspected to have 20.5 grams of gold (21 Carat)

Before working on any of the tanks they were filtered to remove any gold that was precipitated at the bottom and all the other waste from cotton or gloves were burnt and recovered as waste.

There was an unnecessary step that was made by the technician which is adding Casutic Soda to the tanks believing it will create Gold Hydroxide which will be insoluble. Needless to say this was of no gain and I tested the PH of the water and it was 1, so I assume the caustic soda was of no use and much greater quantities should hav been used.

The First Tank:

I tested with stannous chloride (which I made sure is working by testing on a stock containing gold) and it gave a negative.
I suspected and tested with Hydrazine which gave a positive by forming a light brown cloud (indicating a few amount of gold).

1/2 a kilo of Zinc Powder was added gradually with HCL and the bubbling effect was achieved as described in Hokes book and the tank water did turn green indicating the drop of the metal.
I'm aware that powder was not the best option but my resources were very limited, the tank was stirred by a rod for a while but i do not have a pump or air flow to mix properly as described by Dave.
The tank was left to sit for two days , then filtered and then the filter papers were treated with slightly diluted aqua regia and left in the acids for a day and half and then precipitated with SMB and left to precipitate for three hours then filtered and the filter paper was smelted and it gave no gold.

The Second Tank:

Same procedure was followed
I tested with stannous chloride (which I made sure is working by testing on a stock containing gold) and it gave a negative.
I suspected and tested with Hydrazine which gave a positive by forming a dark brown cloud (indicating a decent amount of gold).

more than 1/2 a kilo of Zinc Powder was added gradually with HCL and the bubbling effect was milder than described in Hokes book and the tank water did not turn green despite adding HCL and Zinc but the tank was filled to the max so I had to stop adding the HCL.

The tank was left to sit for two days , then filtered and then the filter papers were treated with slightly diluted aqua regia and left in the acids for a day and half and then precipitated with SMB and left to precipitate for three hours then filtered and the filter paper was smelted and it gave no gold.


(I'll attach the pictures of both tanks)

There is now a third tank which holds the acid water used in the zinc process its very turbid and I couldnt test it properly with hydrazine due to its turbidity.


after all the process was completed:
The first tank tested negative for gold with Hydrazine
The second tank tested positive for gold with Hydrazine
Stannous chloride keeps providing negative results.


I'm not sure how I can go from here and if at this point its possible to recover the gold.


These pictures are all taken today and there is a new sediment forming at the bottom



Tank 1



Tank 2

Tank 3 ( after the use of zinc the water and acids filtered )

 

[Yggdrasil]

Hi Everyone
I know Zinc isn't the preferred method for cementation on this Forum but it was the available metal at the time and copper wasnt available.

I did have a thread before about major gold losses and with your help I was able to recover most of the gold lost and was left satisfied with the results. I used Dave's when in doubt cement it out as a reference and Hoke's book as a guide.

The first time we recovered with zinc was a great success ( Beginners luck I guess!), the second time wasnt a charm.



I had two water tanks (each 1000 liters)
The first was suspected to have 10.5 grams of gold (21 Carat) and the second was suspected to have 20.5 grams of gold (21 Carat)

Before working on any of the tanks they were filtered to remove any gold that was precipitated at the bottom and all the other waste from cotton or gloves were burnt and recovered as waste.

There was an unnecessary step that was made by the technician which is adding Casutic Soda to the tanks believing it will create Gold Hydroxide which will be insoluble. Needless to say this was of no gain and I tested the PH of the water and it was 1, so I assume the caustic soda was of no use and much greater quantities should hav been used.

The First Tank:

I tested with stannous chloride (which I made sure is working by testing on a stock containing gold) and it gave a negative.
I suspected and tested with Hydrazine which gave a positive by forming a light brown cloud (indicating a few amount of gold).

1/2 a kilo of Zinc Powder was added gradually with HCL and the bubbling effect was achieved as described in Hokes book and the tank water did turn green indicating the drop of the metal.
I'm aware that powder was not the best option but my resources were very limited, the tank was stirred by a rod for a while but i do not have a pump or air flow to mix properly as described by Dave.
The tank was left to sit for two days , then filtered and then the filter papers were treated with slightly diluted aqua regia and left in the acids for a day and half and then precipitated with SMB and left to precipitate for three hours then filtered and the filter paper was smelted and it gave no gold.

The Second Tank:

Same procedure was followed
I tested with stannous chloride (which I made sure is working by testing on a stock containing gold) and it gave a negative.
I suspected and tested with Hydrazine which gave a positive by forming a dark brown cloud (indicating a decent amount of gold).

more than 1/2 a kilo of Zinc Powder was added gradually with HCL and the bubbling effect was milder than described in Hokes book and the tank water did not turn green despite adding HCL and Zinc but the tank was filled to the max so I had to stop adding the HCL.

The tank was left to sit for two days , then filtered and then the filter papers were treated with slightly diluted aqua regia and left in the acids for a day and half and then precipitated with SMB and left to precipitate for three hours then filtered and the filter paper was smelted and it gave no gold.


(I'll attach the pictures of both tanks)

There is now a third tank which holds the acid water used in the zinc process its very turbid and I couldnt test it properly with hydrazine due to its turbidity.


after all the process was completed:
The first tank tested negative for gold with Hydrazine
The second tank tested positive for gold with Hydrazine
Stannous chloride keeps providing negative results.


I'm not sure how I can go from here and if at this point its possible to recover the gold.


These pictures are all taken today and there is a new sediment forming at the bottom

View attachment 63974View attachment 63975

Tank 1

View attachment 63976View attachment 63977

Tank 2
View attachment 63978
Tank 3 ( after the use of zinc the water and acids filtered )

Yes, Zinc is not the best solution in most cases,
it will however after its use have concentrated your values alongside other elements into a much more manageable size.

I had two water tanks (each 1000 liters)
The first was suspected to have 10.5 grams of gold (21 Carat) and the second was suspected to have 20.5 grams of gold (21 Carat)

How did you arrive at these conclusions?

I suspected and tested with Hydrazine which gave a positive by forming a light brown cloud (indicating a few amount of gold).

Is this a clear indication or may it be other metals?
As far as I understand, Hydrazine drops most all metals and as such is almost as Cementing with Zinc.

1/2 a kilo of Zinc Powder was added gradually with HCL and the bubbling effect was achieved as described in Hokes book and the tank water did turn green indicating the drop of the metal. 500g of Zinc will be a gross excess I think. Why add extra HCl? You only need the solution to be acidic.
I'm aware that powder was not the best option but my resources were very limited, the tank was stirred by a rod for a while but i do not have a pump or air flow to mix properly as described by Dave. Powder is the best option as when done correct reduces the risk of contamination.
The tank was left to sit for two days , then filtered and then the filter papers were treated with slightly diluted aqua regia and left in the acids for a day and half and then precipitated with SMB and left to precipitate for three hours then filtered and the filter paper was smelted and it gave no gold. Why treat with dilute AR? Just go warm AR and drop it as usual, the volume should be very small and manageable.

Comment in Bold inside quote

Can you describe the Hydrazine test for Gold in detail?

Carefully siphon off the liquid in the tanks, making sure not to disturb the sediments and add the sediments and liquid to a new and smaller tank.
Repeat after a day or two until you have concentrated the sediment to a thicker layer.
Dissolve the layer in HCl/Peroxide and drop with SMB, Ascorbic acid, Oxalic acid or other preferred method.

As to turbidity, it may be suspended particles, add Sulfuric acid and heat the solution to a simmer, this should agglomerate the particles.

 

[Nouran Alaa]

Yes, Zinc is not the best solution in most cases,
it will however after its use have concentrated your values alongside other elements into a much more manageable size.


How did you arrive at these conclusions?

I keep track of each refining cycle and its specific loss and where the water and acids where placed in which tank

Is this a clear indication or may it be other metals?
As far as I understand, Hydrazine drops most all metals and as such is almost as Cementing with Zinc.

I'm not sure if the color was indicative of other metals, but i tested before adding the zinc and after filtering the zinc

Comment in Bold inside quote

Can you describe the Hydrazine test for Gold in detail?

I add a 2-3 ml on top of the tank, it forms a white cloud like mucus on top of the tank, white indicates no gold and brown indicates gold.

Carefully siphon off the liquid in the tanks, making sure not to disturb the sediments and add the sediments and liquid to a new and smaller tank.
Repeat after a day or two until you have concentrated the sediment to a thicker layer.
Dissolve the layer in HCl/Peroxide and drop with SMB, Ascorbic acid, Oxalic acid or other preferred method.

I'm a bit worried that my gold is still disolved in the water and hence scared to throw any water away.
The tanks were already filtered twice, the second time yielding no Gold which is concerning.

As to turbidity, it may be suspended particles, add Sulfuric acid and heat the solution to a simmer, this should agglomerate the particles.

is it safe to add the sulphuric acid and then heat? the water might contain smb, HCL, Zinc, Urea.
Also its around 400 liters of liquids
should i heat gradually each 50-100 liters till I'm done with the tank fully.
a side note the temperature where i live is 35-40 Celsius.


I also failed to mention that I did in fact heat the aqua regia for around 20 mins at the beginning and the dilution was to follow Hoke's guidelines and steps

 

[Yggdrasil]

I keep track of each refining cycle and its specific loss and where the water and acids where placed in which tank

I'm not sure if the color was indicative of other metals, but i tested before adding the zinc and after filtering the zinc

I add a 2-3 ml on top of the tank, it forms a white cloud like mucus on top of the tank, white indicates no gold and brown indicates gold.

I'm a bit worried that my gold is still disolved in the water and hence scared to throw any water away.
The tanks were already filtered twice, the second time yielding no Gold which is concerning.

is it safe to add the sulphuric acid and then heat? the water might contain smb, HCL, Zinc, Urea.
Also its around 400 liters of liquids
should i heat gradually each 50-100 liters till I'm done with the tank fully.
a side note the temperature where i live is 35-40 Celsius.


I also failed to mention that I did in fact heat the aqua regia for around 20 mins at the beginning and the dilution was to follow Hoke's guidelines and steps

Hokes guidelines include evaporating to a syrup, which for the most part is impractical for these volumes.
Is Gold the only substance giving a brown color with Hydrazine?
Maybe there is no more Gold in there?
Can it have been caught up earlier in the process?
Fallen out, stolen or something else all together?
The negative from Stannous is indicative that there is very little Gold left if at all.

How do you measure the pH?

For the turbidity, filtering it with a good filter will catch that but it may take a time to filter this much liquid.

 

[Nouran Alaa]

Hokes guidelines include evaporating to a syrup, which for the most part is impractical for these volumes.
Is Gold the only substance giving a brown color with Hydrazine?
Maybe there is no more Gold in there?
Can it have been caught up earlier in the process?
Fallen out, stolen or something else all together?

As far as I'm aware nothing was stolen, there are cctv cameras and i was present throughout the whole process

I'll have to check on the hydrazine not giving color with other metals

The negative from Stannous is indicative that there is very little Gold left if at all.

I had a tank before that i restored 14 grams of gold from and it kept showing negative with stannous chloride
When tested with Hydrazine it showed positive
And this is why I'm suspecting there's still gold

How do you measure the pH?

I have the multicolored litmus paper

For the turbidity, filtering it with a good filter will catch that but it may take a time to filter this much liquid.

I have a filter with a diaphragm pump so filtering and time arent an issue


Can u please explain what is the purpose of the sulfuric acid?

 

[Yggdrasil]

As far as I'm aware nothing was stolen, there are cctv cameras and i was present throughout the whole process

I'll have to check on the hydrazine not giving color with other metals

I had a tank before that i restored 14 grams of gold from and it kept showing negative with stannous chloride
When tested with Hydrazine it showed positive
And this is why I'm suspecting there's still gold

I have the multicolored litmus paper

I have a filter with a diaphragm pump so filtering and time arent an issue


Can u please explain what is the purpose of the sulfuric acid?

We have noticed when we have very fine precipitate that do not settle well, boiling in dilute Sulfuric will let it agglomerate better.

The theory is that it aquire a coating that hinders it to agglomerate.
By using weak hot Sulfuric this coating is removed and the particles get cleaner and settles well.

 

[4metals]

I did have a thread before about major gold losses and with your help I was able to recover most of the gold lost and was left satisfied with the results. I used Dave's when in doubt cement it out as a reference and Hoke's book as a guide.

Welcome back to the forum. I am glad we were able to help you in the thread you mentioned above. It is unfortunate that you dropped out of sight once you had the answers you needed leaving that thread incomplete. Usually we prefer members who have been helped to acknowledge that what suggestions we offered worked for them. This helps others who follow the thread to see how your particular situation worked out for you. Without follow up we are left wondering what happened. In the future we would appreciate it if you would follow up with the results attained from the suggestions you receive here.

 

[4metals]

I suspected and tested with Hydrazine which gave a positive by forming a light brown cloud (indicating a few amount of gold).

This is not a definitive test, hydrazine is a strong reducing agent and is not selective for gold so it will reduce other metals (base metals) in solution. It is quite possible you thought you had gold in solution but actually did not.
The real question is why is the gold remaining in solution after precipitation? My suspicion is you did not sufficiently de-NOx the solution and re-dissolved some Gold or you did not clean out under the filter plate when you transferred the solution from the precipitating tank to the waste tank. If done properly the quantities of Gold you are missing should remain on the filter as Gold metal and not remain in solution as Gold chloride.

 

[anarxi]

We have noticed when we have very fine precipitate that do not settle well, boiling in dilute Sulfuric will let it agglomerate better.

The theory is that it aquire a coating that hinders it to agglomerate.
By using weak hot Sulfuric this coating is removed and the particles get cleaner and settles well.

It is suggested that there are iron ions there, which is why sulfuric acid is effective for coagulation...

 

[4metals]

There was an unnecessary step that was made by the technician which is adding Casutic Soda to the tanks believing it will create Gold Hydroxide which will be insoluble.

Adding caustic soda to a solution suspected of having gold in it will not selectively drop gold without an ability to precisely control the pH and even at that it is not a method of recovery. The caustic can and will also drop other metals in the waste as hydroxides and you will have a mess on your hands. Where did you hear about using caustic to drop Gold from solution?

In the first thread you posted you were reining karat gold, is this still the material you are refining? If it is karat there is no reason for there to be iron in solution with the methods you are using.

 

[4metals]

1/2 a kilo of Zinc Powder was added gradually with HCL

The addition of the Hydrochloric Acid essentially negated the ability of the zinc to replace Gold in solution. For successful cementation you need to have metallic zinc (or metallic copper) to replace the Gold in solution. By dissolving the zinc, you negated the effect the zinc has by its position in the electromotive series.

Agitation, and continuous vigorous agitation, is required to keep the zinc suspended in solution, only then, when a metallic zinc contacts a gold salt molecule will the exchange take place. That happens much more efficiently when the solution is moving. The equipment you have will allow you to cement solution with excellent agitation in the same tank you precipitate gold in, just rinse it well when you are done.

 

[Nouran Alaa]

Welcome back to the forum. I am glad we were able to help you in the thread you mentioned above. It is unfortunate that you dropped out of sight once you had the answers you needed leaving that thread incomplete. Usually we prefer members who have been helped to acknowledge that what suggestions we offered worked for them. This helps others who follow the thread to see how your particular situation worked out for you. Without follow up we are left wondering what happened. In the future we would appreciate it if you would follow up with the results attained from the suggestions you receive here.

I'm really sorry for this. I'll make sure to update the thread posted before.

And I would like to pay a special thanks to you 4metals, I still go back to your replies every once n a while to make sure I'm still heading in the right direction.

 

[Nouran Alaa]

The addition of the Hydrochloric Acid essentially negated the ability of the zinc to replace Gold in solution. For successful cementation you need to have metallic zinc (or metallic copper) to replace the Gold in solution. By dissolving the zinc, you negated the effect the zinc has by its position in the electromotive series.

Agitation, and continuous vigorous agitation, is required to keep the zinc suspended in solution, only then, when a metallic zinc contacts a gold salt molecule will the exchange take place. That happens much more efficiently when the solution is moving. The equipment you have will allow you to cement solution with excellent agitation i

This is not a definitive test, hydrazine is a strong reducing agent and is not selective for gold so it will reduce other metals (base metals) in solution. It is quite possible you thought you had gold in solution but actually did not.
The real question is why is the gold remaining in solution after precipitation? My suspicion is you did not sufficiently de-NOx the solution and re-dissolved some Gold or you did not clean out under the filter plate when you transferred the solution from the precipitating tank to the waste tank. If done properly the quantities of Gold you are missing should remain on the filter as Gold metal and not remain in solution as Gold chloride.

.

After revising the cctv cameras and letting the solutions settle for another week. We did realize that we had an issue with filtration. The technician removed the sealing from above the filter paper and then rinsed the precipitated gold once more, which lead to some gold leaking in the solution.

The main issue we did filter the water twice and nonetheless no gold was retrieved.

I assumed we might have an issue with the smelting crucibles and I crushed them and sent them for analysis and it turned out it has around 13 grams of entrapped gold.

I'm currently working on retrieving them.


I did not still evaporate or add sulfuric acid. I assumed it would be better if i tackle all other aspects in the refinery lab before working again on the water tanks.

n the same tank you precipitate gold in, just rinse it well when you are done.

 

[Nouran Alaa]

We have noticed when we have very fine precipitate that do not settle well, boiling in dilute Sulfuric will let it agglomerate better.

The theory is that it aquire a coating that hinders it to agglomerate.
By using weak hot Sulfuric this coating is removed and the particles get cleaner and settles well.

I still didnt try the sulfuric acid, as I mentioned to 4 metals I kept the water tanks for the past week to settle and check if there is any ppt forming in the bottom of the tank. Unfortunately nothing settled down.

I did contact a chemistry engineer who works in a refinery and he suggested I neutralize the water completely to Ph7 and this will render all metals insoluble. I was not sure of the chemistry behind this.

But i guess the sulfuric acid makes more sense.

I'll keep u updated to what happens next.

I'm sorry if my re[lies are not in a timely manner, I was very busy last week with work.

 

[4metals]

The technician removed the sealing from above the filter paper and then rinsed the precipitated gold once more, which lead to some gold leaking in the solution.

The filter trolley you have is an excellent filter unit. The difference between their trolley and a Buchner funnel is the Buchner relies on 2 things, one is the smooth porcelain around the outer edge for the filter paper to seal, and the other is a strong vacuum source.
The Italimpianti filter generates it's own vacuum using the double diaphragm pump. Those pumps do not generate the vacuum required to seal the paper well enough so the rubber tube you fit in under the lip provides the seal. Make sure that the length of rubber tube is long enough for the 2 ends to meet snugly after they are in place because any liquid takes the path of the least resistance. One big difference between a Buchner funnel and the Italimpianti filter trolley is when you filter with a Buchner funnel you have a glass vacuum jug which fills up with the filtered liquid. That means lifting a 20 liter (5 gallon) jug which with acid is about 50 pounds (22.5 kilo's) to empty it. This is usually done with gloves on a wet or damp glass vessel. Considering todays gold price and how much gold containing aqua regia from refining karat scrap is in that jug, it is likely worth over $180,000. Always a scary situation for me. The Italimpianti filter trolley has a discharge hose on the double diaphragm pump so all of the filtered liquid is pumped and not lifted to the next step. Much safer.

 

[4metals]

assumed we might have an issue with the smelting crucibles and I crushed them and sent them for analysis and it turned out it has around 13 grams of entrapped gold.

Usually refined gold is poured out of a dedicated crucible and no flux is used. If, after pouring, the crucible is inspected, any beads clinging will be easy to spot and take out. Only when I poured shot through a shot box did I ever have some gold solidify and not go all the way to shot. That was always in the shot box and never in the pouring crucible. And even at that it was a few easily retrieved grams.

But if you use flux, that is a different story. If you are using flux, what type do you use and why are you using it?

 

[4metals]

I assumed we might have an issue with the smelting crucibles and I crushed them and sent them for analysis and it turned out it has around 13 grams of entrapped gold.

When crucibles are crushed and sifted the gold can be in 2 places. On the screen of the sifter where beads of gold will be trapped and in the powdered fraction where you cannot weigh it but can quantify it by fire assay. Generally molten metal, especially refined metals that are pure, exhibit a large degree of surface tension which is why most metals will flow out of a crucible if they are not hung up as small beads in the thick slag.

 

[Nouran Alaa]

Usually refined gold is poured out of a dedicated crucible and no flux is used. If, after pouring, the crucible is inspected, any beads clinging will be easy to spot and take out. Only when I poured shot through a shot box did I ever have some gold solidify and not go all the way to shot. That was always in the shot box and never in the pouring crucible. And even at that it was a few easily retrieved grams.

But if you use flux, that is a different story. If you are using flux, what type do you use and why are you using it?

The first issue we encountered was when we used a new set of crucibles and I assume they were of a very bad quality.

The flux we use is borax.

We don't use flux in general when smelting the pure gold, but when we are smelting the gold in powder form (the brown powder) we usually smelt with the filter paper so no gold is lost on the filter paper. The technician smelting suggested adding flux because of the filter paper and that he wont be able to smelt without it, a suggestion that might have a caused some of the issues we are facing now.

We were using 2 crucibles at the time, so we crushed one of and made it into very smooth dust and sent a sample for fire assay and it turned out after calculation that it contains 13 grams of gold (21 carats).

Do you suggest smelting the pure gold with the filter paper or do you advise against this? and if its smelted with filter paper is borax necessary?

 

[Nouran Alaa]

When crucibles are crushed and sifted the gold can be in 2 places. On the screen of the sifter where beads of gold will be trapped and in the powdered fraction where you cannot weigh it but can quantify it by fire assay. Generally molten metal, especially refined metals that are pure, exhibit a large degree of surface tension which is why most metals will flow out of a crucible if they are not hung up as small beads in the thick slag.

When we faced the losses, I inspected the crucibles from the inside and removed the cap around the graphite to inspect if there is any gold trapped or holes in the crucible.

The technician used one of the crucibles we had to smelt any gold that is not pure gold and he did use big amounts of borax which damage the crucible significantly.

and as I mentioned in the other reply he did use (in much lesser quantities) borax to smelt pure gold.

 

[Nouran Alaa]

The filter trolley you have is an excellent filter unit. The difference between their trolley and a Buchner funnel is the Buchner relies on 2 things, one is the smooth porcelain around the outer edge for the filter paper to seal, and the other is a strong vacuum source.
The Italimpianti filter generates it's own vacuum using the double diaphragm pump. Those pumps do not generate the vacuum required to seal the paper well enough so the rubber tube you fit in under the lip provides the seal. Make sure that the length of rubber tube is long enough for the 2 ends to meet snugly after they are in place because any liquid takes the path of the least resistance. One big difference between a Buchner funnel and the Italimpianti filter trolley is when you filter with a Buchner funnel you have a glass vacuum jug which fills up with the filtered liquid. That means lifting a 20 liter (5 gallon) jug which with acid is about 50 pounds (22.5 kilo's) to empty it. This is usually done with gloves on a wet or damp glass vessel. Considering todays gold price and how much gold containing aqua regia from refining karat scrap is in that jug, it is likely worth over $180,000. Always a scary situation for me. The Italimpianti filter trolley has a discharge hose on the double diaphragm pump so all of the filtered liquid is pumped and not lifted to the next step. Much safer.

Yes we do snug the sealing and make sure the filter paper is in place.

But after removing the seal, the technician washed the seal over the filter paper when the diaphragm pump was still on and in suction mode.


I would like to point something out, in my country unfortunately we do not have many skilled technicians and even those with many years of experience, if not supervised and instructed properly, they might make some decisions that are not the most adequate. For example washing the gold while the seal is removed and then washing the seal as well on top of the gold, when the filter paper is not sealed in place which defies the purpose of sealing and filtering.
I do take blame for not noticing this step, a lesson that was learned the hard way.
Luckily all the water is still reserved and will be filtered again properly to ensure that whatever escaped the filter paper can be returned.