Precipitating gold from cianides solutions with zinc or aluminium

[arthur kierski]

I had a solution of gold cianide and tried to precipitate with zinc powder and did not work.Then i tried with aluminium and it precipitated very well. a brown precipitate wish i washed well and put in nitric to clean the gold--Can someone tell me why zinc did not work and aluminium worked very well?thanks and regards to all.---Arthur Kierski

 

[4metals]

I suspect either the free cyanide content was too low or the pH was too low, possibly both.

This is how I use zinc to recover gold in cyanide solutions

Recovering Gold from potassium or sodium gold cyanide

Required supplies and equipment
​

Zinc dust 325 mesh is commonly available. Make sure the powder is pure zinc with no de-clumping chemicals added. (cab-o-sil)

A slab of zinc metal with a hole drilled in the top edge so it can be tied and hung in a bucket of solution.

Sodium hydroxide, either flake or as 50% caustic liquid

Sodium or Potassium cyanide

pH papers or a pH meter

A strong propeller driven mixer

Process​

If the salts are in powder form they must be dissolved in warm water, if they are in the form of a solution and any salts have dropped out, heat the solution to assure all of the crystallization has dissolved, if necessary add water.

Allow the solution to cool to room temperature.

Check the pH, if it is below 12, raise it with sodium hydroxide, try not to add too much sodium hydroxide, it shouldn’t take a lot to get the solution to pH 12.

Add 5 grams per liter of solution of either sodium or potassium cyanide. This makes the dropping of the gold easier and more complete.

While stirring, slowly add powdered zinc dust. If the zinc dust has clumped up due to humidity, sift it through a flour sifter before adding it. Clumped up zinc dust causes you to use too much and is just a waste. A rule of thumb is one gram of zinc dust for every gram of gold expected. When you first add the zinc dust, you will notice the dark gray zinc dust will appear brown in color. That is because the original addition immediately displaces gold which accounts for the color. Add until all of the precipitate brought up from stirring is gray in color.

Hang a bar of zinc metal in the solution and allow it to settle overnight. The zinc will both drop any minute quantities that may still be in solution and prevent the re-dissolution of any precipitated gold.

Decant the clear solution and filter the balance to collect the solids.

Rinse the collected solids several times with hot water. This is to remove the cyanide from the solids. Collect the rinses and put them with the solution collected earlier.

While a hood is not necessary for the first steps, it doesn’t hurt. The following steps however, MUST BE CARRIED OUT IN A FUME HOOD!

Add a small amount of nitric acid, it will react quickly and agressively. When the reaction dies down, stir the solution and add another small portion of nitric acid. Stir and repeat until there is no longer any reaction when nitric acid is added.

Filter the solution and save the insoluble gold residue. This gold should be refined in aqua regia.

​

 

[Barstool miner]

I suspect either the free cyanide content was too low or the pH was too low, possibly both.

This is how I use zinc to recover gold in cyanide solutions

Recovering Gold from potassium or sodium gold cyanide

Required supplies and equipment
​

Zinc dust 325 mesh is commonly available. Make sure the powder is pure zinc with no de-clumping chemicals added. (cab-o-sil)

A slab of zinc metal with a hole drilled in the top edge so it can be tied and hung in a bucket of solution.

Sodium hydroxide, either flake or as 50% caustic liquid

Sodium or Potassium cyanide

pH papers or a pH meter

A strong propeller driven mixer

Process​

If the salts are in powder form they must be dissolved in warm water, if they are in the form of a solution and any salts have dropped out, heat the solution to assure all of the crystallization has dissolved, if necessary add water.

Allow the solution to cool to room temperature.

Check the pH, if it is below 12, raise it with sodium hydroxide, try not to add too much sodium hydroxide, it shouldn’t take a lot to get the solution to pH 12.

Add 5 grams per liter of solution of either sodium or potassium cyanide. This makes the dropping of the gold easier and more complete.

While stirring, slowly add powdered zinc dust. If the zinc dust has clumped up due to humidity, sift it through a flour sifter before adding it. Clumped up zinc dust causes you to use too much and is just a waste. A rule of thumb is one gram of zinc dust for every gram of gold expected. When you first add the zinc dust, you will notice the dark gray zinc dust will appear brown in color. That is because the original addition immediately displaces gold which accounts for the color. Add until all of the precipitate brought up from stirring is gray in color.

Hang a bar of zinc metal in the solution and allow it to settle overnight. The zinc will both drop any minute quantities that may still be in solution and prevent the re-dissolution of any precipitated gold.

Decant the clear solution and filter the balance to collect the solids.

Rinse the collected solids several times with hot water. This is to remove the cyanide from the solids. Collect the rinses and put them with the solution collected earlier.

While a hood is not necessary for the first steps, it doesn’t hurt. The following steps however, MUST BE CARRIED OUT IN A FUME HOOD!

Add a small amount of nitric acid, it will react quickly and agressively. When the reaction dies down, stir the solution and add another small portion of nitric acid. Stir and repeat until there is no longer any reaction when nitric acid is added.

Filter the solution and save the insoluble gold residue. This gold should be refined in aqua regia.

​

Thank you 4metals this post is very informative. I have had a Hobby of inorganic chemistry since I was 10 years old when I received a chemistry set as a gift I am now 78 years old and I still keep working at it, It is now called Hydrometallurgy . I have never been bored in my life. My library contains several hundreds of books on chemistry and metal refining. I want to repeat a fact of wisdom I once heard " Acknowledging Your Ignorance Is The First Step To Gaining Knowledge"
In gold, mining reduction works that use Cyanide Leaching, the gold-rich solution is first De-Aired before Zinc powder is added to reduce the gold from leachate.

 

[Yggdrasil]

Thank you 4metals this post is very informative. I have had a Hobby of inorganic chemistry since I was 10 years old when I received a chemistry set as a gift I am now 78 years old and I still keep working at it, It is now called Hydrometallurgy . I have never been bored in my life. My library contains several hundreds of books on chemistry and metal refining. I want to repeat a fact of wisdom I once heard " Acknowledging Your Ignorance Is The First Step To Gaining Knowledge"
In gold, mining reduction works that use Cyanide Leaching, the gold-rich solution is first De-Aired before Zinc powder is added to reduce the gold from leachate.

There is another approach to this acknowledge.
The more you learn, the more you understand how little you know.

 

[4metals]

In gold, mining reduction works that use Cyanide Leaching, the gold-rich solution is first De-Aired before Zinc powder is added to reduce the gold from leachate.

Do you have any information on preferred methods for degassing in cyanide refining? I have done this using a vacuum and also by sparging with inert gas (typically argon because we had it for casting Silver bars).

This may be effective for recovering Gold (and Silver) from leachate in mining where the concentrations are low. I have always used zinc to drop Gold from more concentrated solutions and never saw a benefit from degassing.

 

[anachronism]

I suspect either the free cyanide content was too low or the pH was too low, possibly both.

This is how I use zinc to recover gold in cyanide solutions

Recovering Gold from potassium or sodium gold cyanide

Required supplies and equipment
​

Zinc dust 325 mesh is commonly available. Make sure the powder is pure zinc with no de-clumping chemicals added. (cab-o-sil)

A slab of zinc metal with a hole drilled in the top edge so it can be tied and hung in a bucket of solution.

Sodium hydroxide, either flake or as 50% caustic liquid

Sodium or Potassium cyanide

pH papers or a pH meter

A strong propeller driven mixer

Process​

If the salts are in powder form they must be dissolved in warm water, if they are in the form of a solution and any salts have dropped out, heat the solution to assure all of the crystallization has dissolved, if necessary add water.

Allow the solution to cool to room temperature.

Check the pH, if it is below 12, raise it with sodium hydroxide, try not to add too much sodium hydroxide, it shouldn’t take a lot to get the solution to pH 12.

Add 5 grams per liter of solution of either sodium or potassium cyanide. This makes the dropping of the gold easier and more complete.

While stirring, slowly add powdered zinc dust. If the zinc dust has clumped up due to humidity, sift it through a flour sifter before adding it. Clumped up zinc dust causes you to use too much and is just a waste. A rule of thumb is one gram of zinc dust for every gram of gold expected. When you first add the zinc dust, you will notice the dark gray zinc dust will appear brown in color. That is because the original addition immediately displaces gold which accounts for the color. Add until all of the precipitate brought up from stirring is gray in color.

Hang a bar of zinc metal in the solution and allow it to settle overnight. The zinc will both drop any minute quantities that may still be in solution and prevent the re-dissolution of any precipitated gold.

Decant the clear solution and filter the balance to collect the solids.

Rinse the collected solids several times with hot water. This is to remove the cyanide from the solids. Collect the rinses and put them with the solution collected earlier.

While a hood is not necessary for the first steps, it doesn’t hurt. The following steps however, MUST BE CARRIED OUT IN A FUME HOOD!

Add a small amount of nitric acid, it will react quickly and agressively. When the reaction dies down, stir the solution and add another small portion of nitric acid. Stir and repeat until there is no longer any reaction when nitric acid is added.

Filter the solution and save the insoluble gold residue. This gold should be refined in aqua regia.

​

The zinc bar thing is new to me. Thanks, you learn something new every day....

 

[anachronism]

Do you have any information on preferred methods for degassing in cyanide refining? I have done this using a vacuum and also by sparging with inert gas (typically argon because we had it for casting Silver bars).

This may be effective for recovering Gold (and Silver) from leachate in mining where the concentrations are low. I have always used zinc to drop Gold from more concentrated solutions and never saw a benefit from degassing.

Is (edit) the de-airing to remove the oxygen from the liquor and effectively remove the oxidant?

 

[4metals]

Yes it is to remove the oxygen which in turn extends the ability of the cyanide to exist in solution because it does not oxidize the CN. I would guess this is more critical in low concentrations but this is actually a question for @Deano

 

[Barstool miner]

There is another approach to this acknowledge.
The more you learn, the more you understand how little you know.

So True!

 

[anachronism]

Yes it is to remove the oxygen which in turn extends the ability of the cyanide to exist in solution because it does not oxidize the CN. I would guess this is more critical in low concentrations but this is actually a question for @Deano

True. That given the higher the pH the higher the free CN- too.

 

[kurtak]

Then i tried with aluminium and it precipitated very well. a brown precipitate wish i washed well and put in nitric to clean the gold

If you cemented with aluminum you are better served to use HCl rather then nitric

Nitric does not react (dissolve) well aluminum

Kurt