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"
