# Why does gold react with cyanide and sodium hydroxide

Why does gold react with cyanide and sodium hydroxide and what is the chemical reaction's impact on gold's atomic structure and bonding?

What I think is since in the 'flotation' process, when the water, chemicals and other substances are added, gold isn't affected and neither is the pyrite, but when the cyanide and caustic acid is mixed with the slurry, it separated both substances, then the oxygen or 'air' is added into the mix, so then the gold rises to the top and frothing over the slurry. I'm not sure if i'm correct, but I don't understand how it's related to the atomic structure and bonding of gold? This process is from (Super Pit - Kalgoorlie exploration mine)

Can someone please explain how the separation of gold from the mixed compounds is related to its atomic structure and bonding? Is it to do with the 'Elution' process? Thank you in advance!

• possible duplicate of Gold's atomic structure and bonding in relation to the separation techniques used – user4076 Mar 3 '14 at 9:14
• I am uncertain if this question is a duplicate. This question wants to know the chemistry behind the process. – Ben Norris Mar 3 '14 at 11:26
• I made sure that it not resemble my previous question, I request a much more intricate answer – Samir Mar 4 '14 at 0:18

Flotation and treatment of gold with $\ce{NaCN}$ in the presence of air are two distictive steps!

Flotation yields fine particles of gold (and pyrite) at the surface of a flotation tank, attached to foamy bubbles while the rest of the minerals sink to the bottom of the tank.

Flotation does not rely on differences in specific density, but on hydrophobicity!

In the cyanide treatment, the following process - similar to what Uncle Al described for the seawater case in his answer - occurs:

Solid gold is oxidized, the cations are bound by cyanide anions and form a water-soluble complex:

$$\ce{4Au + 8NaCN + O2 + H2O -> 4Na[Au(CN)2] + 4NaOH}$$

In order to reobtain solid gold, the $\ce{Au(I)}$ in the cyano complex $\ce{[Au(CN)2]^-}$ has to be reduced to $\ce{Au(0)}$ again. This can be done either by adding zinc dust, according to

$$\ce{2Na[Au(CN)2] + Zn -> Na2[Zn(CN)4] + 2Au}$$

or electrochemically.

Look at half-cell redox potentials. Gold should be chemically inert in water. Now, add a coordinating ligand and a mild oxidizing agent. Gold complexes are deep thermodynamic holes. Saltwater and air will eat gold, giving chloroaurate complexes. Cis-platin was accidentally discovered when "inert" platinum anodes in cell culture met saline and metabolic ammonia.

Gold often looks like a halogen (relativistic diddling of s- and d-orbital energies given probability near the nucleus). Touching gold with aluminum can over time gives "purple plague" (aluminum-gold intermetallics at microchip circuit connections). Gold and beryllium give brittle crumbly AuBe. Gold is only inert under select circumstances.

• Samir asked about "flotation", which means he does not know any basics. – Georg Mar 3 '14 at 19:14
• You don't know about gold beneficiation that involves pulverizing, chemical conditioning agents followed by intense agitation and air sparging of the agitated ore slurry to produce a mineral-rich foam. It works for pyritic and other sulfide gold in which the mineral matrix is grabbed. Cyanidation and cyanide heap leaching are entirely different. – Uncle Al Mar 3 '14 at 22:52
• I don't quite understand this answer, predominantly due to my lack of knowledge in this field. What I really want to know is how to relate the separation process (Flotation), to gold's atomic structure and bonding. Thank you! – Samir Mar 4 '14 at 0:21
• Samir, flotation does not work on gold as such. It works on minerals that occlude gold. Do your chair parade - look it all up in Google, engineering and chemistry. – Uncle Al Mar 4 '14 at 19:29
• @UncleAl "chair parade"? I am going to borrow that! :) – user4076 Mar 5 '14 at 7:04